Article

Bonding Mechanism in the Nitrides Ti2AlN and TiN: An Experimental and Theoretical Investigation

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Abstract

The electronic structure of nanolaminate Ti2AlN and TiN thin films has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured Ti L, N K, Al L1 and Al L2,3 emission spectra are compared with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Three different types of bond regions are identified; a relatively weak Ti 3d - Al 3p bonding between -1 and -2 eV below the Fermi level, and Ti 3d - N 2p and Ti 3d - N 2s bonding which are deeper in energy observed at -4.8 eV and -15 eV below the Fermi level, respectively. A strongly modified spectral shape of 3s states of Al L2,3 emission from Ti2AlN in comparison to pure Al metal is found, which reflects the Ti 3d - Al 3p hybridization observed in the Al L1 emission. The differences between the electronic and crystal structures of Ti2AlN and TiN are discussed in relation to the intercalated Al layers of the former compound and the change of the materials properties in comparison to the isostructural carbides.

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... The low intensity of the impurity peaks compared to the Ti 3 AC 2 (A = Al, Si, Ge) phase peaks is due to the fact that these impurity concentrations are very small and that their contributions to X-ray spectroscopy measurements can be disregarded. Similar diffractograms were also found for other thin film MAX phases [16,[24][25][26][27]. ...
... A very similar situation was also disclosed for Ti 4 SiC 3 [64]. When comparing Ti 2 AlC, Ti 2 AlN, TiN [24], and AlN [65] the electronic structure and chemical bonding were found to be considerably different [66,67]. Nitrides have deeper bond regions and therefore stronger bonds. ...
... Thus, the covalent Ti 3d\ \N 2p bonding in TiN is significantly stronger than the Ti 3d\ \C 2p bonding in TiC. By further analyzing the partial DOS for Ti 2 AlC and Ti 2 AlN (Fig. 9), their crystal overlap population data (Section 7) and bond lengths (Section 6), the same difference as in the binaries is observed for Ti 2 AlC [27] and Ti 2 AlN [24]. This general difference tendency is also confirmed by the energy shift in the experimental X-ray emission spectra of Ti 2 AlC in comparison to Ti 2 AlN (Fig. 8). ...
Article
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... The low intensity of the impurity peaks compared to the Ti 3 AC 2 (A = Al, Si, Ge) phase peaks is due to the fact that these impurity concentrations are very small and that their contributions to X-ray spectroscopy measurements can be disregarded. Similar diffractograms were also found for other thin film MAX phases [16,[24][25][26][27]. ...
... A very similar situation was also disclosed for Ti 4 SiC 3 [64]. When comparing Ti 2 AlC, Ti 2 AlN, TiN [24], and AlN [65] the electronic structure and chemical bonding were found to be considerably different [66,67]. Nitrides have deeper bond regions and therefore stronger bonds. ...
... Thus, the covalent Ti 3d\ \N 2p bonding in TiN is significantly stronger than the Ti 3d\ \C 2p bonding in TiC. By further analyzing the partial DOS for Ti 2 AlC and Ti 2 AlN (Fig. 9), their crystal overlap population data (Section 7) and bond lengths (Section 6), the same difference as in the binaries is observed for Ti 2 AlC [27] and Ti 2 AlN [24]. This general difference tendency is also confirmed by the energy shift in the experimental X-ray emission spectra of Ti 2 AlC in comparison to Ti 2 AlN (Fig. 8). ...
Article
Full-text available
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong M\ \C bonds in high-density MC slabs, and relatively weak M\ \A bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity , elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... The bulk and surface properties of TiN and Fe are well studied both experimentally [1,2] and theoretically [2][3][4][5][6][7]. However, the reported research on the TiN/Fe interfaces consists of only few studies of the separation of the TiN(001)/bcc Fe(001) system [8][9][10][11]. ...
... The bulk and surface properties of TiN and Fe are well studied both experimentally [1,2] and theoretically [2][3][4][5][6][7]. However, the reported research on the TiN/Fe interfaces consists of only few studies of the separation of the TiN(001)/bcc Fe(001) system [8][9][10][11]. ...
Article
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By means of first principles density functional theory, we investigate the properties of the TiN(001)/fcc Fe(111) and TiN(001)/bcc Fe(110) interfaces. We demonstrate that along certain directions Fe slides with negligible energy barriers against TiN at both interfaces, whereas sliding along other directions is involved with significant energy barriers. The interface between bcc Fe and TiN has a low energy barrier for sliding along the [110] direction of the TiN lattice, as does sliding along the [010] direction at TiN(001)/fcc Fe(111). For fcc Fe on TiN, a large energy barrier is found for sliding along the [100] direction of the TiN lattice. We show that this phenomenon and the stability of these interfaces are determined by the interplay between N-Fe bonding and Ti-Fe antibonding interactions.
... On the basis of density-functional theory (DFT) and ab initio calculations of the electronic structure [50,60,[72][73][74][75][76][77][78][79][80][81][82][83][84][85][86], as well as relying on the data of novel computation methods, such as the full-potential band-structure method (FP-LAPW) [87,88], pseudopotential plane-wave method (PP-PW) [89], DFT + U [90] and the hybrid functionals [91,92], M 2 AlC phases can be classified into two groups: weakly coupled (M = Sc, Ti, Zr, Hf) and strongly coupled (M = V, Nb, Ta, Cr, Mo, W) nanolaminates, in accordance with the valence electron concentration (VEC) of the transition metal M. There exists common agreement in the literature about the general tendency that the transition metal atoms, more specifically their 3d and 4s orbitals, are the main charge reservoir which always lose electronic charge, while C-, N-or A-element tend to gain charge, upon bond formation. ...
... For this reason, Cui et al. predicted Cr 2 AlC to be more brittle than Cr 2 AlN [94]. X-ray spectroscopic studies have confirmed this tendency in the general difference of the chemical bonding schemes of MAX phase nitrides and carbides [72,[78][79][80][81][82]. ...
Article
Recent research has highlighted the potential of MAX phase properties including its machinability, good mechanical behaviour, high electric and thermal conductivity as well as good oxidation and erosion resistance to a range of applications including electrical contact coatings, high-temperature heating elements, and barrier- and protection coatings in gas-turbine engine. Successful engineering of MAX phases requires an understanding of how the manufacturing conditions and the deposition parameters influence the composition, crystallographic, electronic structure and morphology. It is, in turn, important to know how these determine the anisotropy of physical, mechanical and technological properties of the material. A focus point of this part of the review is the systemic study of the regular trends in the preparation parameter–structure–properties relationships of MAX phase coatings. In particular, the Cr–Al–C (Y) coating synthesised by the HIPIMS technique will be considered and compared with other synthesised MAX phase bulk ceramics and coatings.
... On the basis of density-functional theory (DFT) and ab initio calculations of the electronic structure [50,60,[72][73][74][75][76][77][78][79][80][81][82][83][84][85][86], as well as relying on the data of novel computation methods, such as the full-potential band-structure method (FP-LAPW) [87,88], pseudopotential plane-wave method (PP-PW) [89], DFT + U [90] and the hybrid functionals [91,92], M 2 AlC phases can be classified into two groups: weakly coupled (M = Sc, Ti, Zr, Hf) and strongly coupled (M = V, Nb, Ta, Cr, Mo, W) nanolaminates, in accordance with the valence electron concentration (VEC) of the transition metal M. There exists common agreement in the literature about the general tendency that the transition metal atoms, more specifically their 3d and 4s orbitals, are the main charge reservoir which always lose electronic charge, while C-, N-or A-element tend to gain charge, upon bond formation. ...
... For this reason, Cui et al. predicted Cr 2 AlC to be more brittle than Cr 2 AlN [94]. X-ray spectroscopic studies have confirmed this tendency in the general difference of the chemical bonding schemes of MAX phase nitrides and carbides [72,[78][79][80][81][82]. ...
Article
Part III of this overview presents a summary of the potential multifunctional applications of MAX phase materials. Coatings of these materials have been investigated for a range of uses, such as for: high-temperature electrical contacts, microelectronic layers, magnetic and optical materials, fuel holder protection in the nuclear industry, oxidation, corrosion and erosion protection, bio-compatible material, thermal barriers, protective aerospace coatings and as armour in defence applications. What makes this material useful for many of these applications is its excellent mechanical properties, damage tolerance, self-healing, high-temperature melting point, and its outstanding oxidation, corrosion and abrasion resistance. Particular attention is given to an aircraft engine's design and the related materials challenges. A comparison is made to the currently utilized turbine surface coatings, as well as the motivation behind the usage of these new high performance MAX phase coatings, the nature of their protection/wear and other strengths/weaknesses.
... Apart from these peaks, a double structure peak at~533 eV and 535 eV shown within the range of 530-536 eV in the spectra of O K-edge assigned to the physisorbed O 2 [47], is expected to arise from the chemicals as well as TiO 2 used during the synthesis of the nanocomposites. However, the spectra regions of 530-536 eV in MWCNTs-TiO 2 nanocomposites are recognized as the hybridization of O 2p to Ti 3d states [48]. The degenerate Ti 3d band splits into two peaks, 533 eV and 535 eV corresponding to t 2g and e g splitting owing to the effect of crystal field [48]. ...
... However, the spectra regions of 530-536 eV in MWCNTs-TiO 2 nanocomposites are recognized as the hybridization of O 2p to Ti 3d states [48]. The degenerate Ti 3d band splits into two peaks, 533 eV and 535 eV corresponding to t 2g and e g splitting owing to the effect of crystal field [48]. This splitting is very sensitive to the degree of hybridization and coordination number. ...
Article
The improvement of electronic structure and electrical behaviour of low-density materials have been of great concern. Multiwall carbon nanotube (MWCNT) has been identified as a low-density material, which needs attention to be used mostly as a strengthening phase in lightweight metal lattice composites (MLC). In this work, vaporization and drying process were used to produce TiO 2 incorporated MWCNTs nanocomposites. The incorporation of TiO 2 is to improve electronic system as well as electrical behaviour of MWCNTs in oxidized atmosphere, which is essential in most of MLC processing methods. We have used field-emission scanning electron microscopy Raman spectroscopy, X-ray diffraction and X-ray photoemission spectroscopy techniques for the study of structural and electronic properties; whereas current-voltage technique was used for the study of electrical behaviour of the nanocomposites. The observed change in electrical behaviour of the nanocomposites with improvement of electric/bonding structure indicates possibilities of the material properties be tailored for electronic/electrical devices as well as photo-catalytic activity applications.
... Scabarozi et al. [13] report on correlations of specific measurements of heat and transport in the MAX phase family. In addition, the electronic properties of these materials have been studied theoretically and experimentally [14]. Recently, the physical properties of thin films MAX phases have been reported [10], where the anisotropy of their conductivity is of great interest, but it is difficult to solve this problem experimentally [10]. ...
Article
Full-text available
In this study, we have investigated the structural, electronic, and elastic properties of the M2CdC (M = Ta, Zr, and Hf) MAX phases, using the first-principle methods based on the density functional theory. The calculated formation energies revealed that these compounds are thermodynamically stable in the hexagonal MAX phase. The stability is confirmed by the elastic constants and the conditions of mechanical stability criterion. Also, we have determined the bulk and shear modules of the Young modulus and the Poisson coe cient. The band structures indicate that the three materials are electrically conductive. The chemical bond in M2CdC is covalent-ionic in nature with the presence of metallic character. For the density of states the hybridization peak between M d and C p occurs in the lower energy range. We have found that there is no gap for these materials due to the existence of a maximum peak of DOS around Fermi level.
... As observed in Fig. 3, the valence band edge shifts to higher energy with increasing x. The hybridization at the top of the valence band shows similarities with XES spectra of other nitrides e.g., Ti2AlN [35] and Sc3AlN [36]. ...
Article
Full-text available
The electronic structure, chemical bonding, and interface component in ZrN-AlN nanocomposites formed by phase separation during thin film deposition of metastable Zr 1−x Al x N (x = 0.0, 0.12, 0.26, 0.40) are investigated by resonant inelastic x-ray scattering, x-ray emission, and x-ray absorption spectroscopy and compared to first principles calculations including transitions between orbital angular momentum final states. The experimental spectra are compared with different interface-slab model systems using first principles all-electron full-potential calculations where the core states are treated fully relativistically. As shown in this work, the bulk sensitivity and element selectivity of x-ray spectroscopy enables one to probe the symmetry and orbital directions at interfaces between cubic and hexagonal crystals. We show how the electronic structure develops from local octahedral bond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results in three different kinds of bonding originating from semicoherent interfaces with segregated ZrN and lamellar AlN nanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes place with increasing Al content and affects the bond strength and increases resistivity.
... In the latter case, the experiments can be performed either in emission [x-ray emission spectroscopy (XES)] or in absorption [x-ray absorption spectroscopy (XAS)]. XES and XAS, combined with first-principles calculations, were used to study the electronic structure of Ti 2 AlC [29], Ti 2 AlN [30], Ti 4 SiC 3 [31], Ti 3 AlC 2 , Ti 3 SiC 2 , and Ti 3 GeC 2 [32]. However, studies specifically dedicated to electronic structure anisotropy and polarization-dependent core-level spectroscopies have been limited to V 2 GeC [33], Ti 3 SiC 2 [34], and Ti 3 AlC 2 [35]. ...
Article
Full-text available
The anisotropy of the electronic structure of the MAX phase Cr2AlC has been investigated by electron-energy-loss spectroscopy (EELS) at the C K edge, and x-ray-absorption spectroscopy (XAS) at the Al K, Cr L2,3, and Cr K edges. The experimental spectra were interpreted using either a multiple-scattering approach or a full-potential band-structure method. The anisotropy is found to be small around C atoms because of the rather isotropic nature of the octahedral site, and of the averaging of the empty C p states probed by EELS at the C K edge. In turn, a pronounced anisotropy of the charge distribution around Al atoms is evidenced from polarized XAS measurements performed on textured Cr2AlC sputtered thin films. From the analysis of the XAS data using the multiple-scattering feff code, it is demonstrated that the probed thin film is constituted of 70% (0001) and 30% (101¯3) grains oriented parallel to the film surface. A decomposition of the calculated spectrum in coordination shells allows for the ability to connect XAS fine structures to the Cr2AlC structure. Combining high-resolution data with up-to-date multiple-scattering calculations, it is shown that the crystalline orientations of the grains present in a probe of 100×100 μm2 can be determined from the Cr K edge. Interestingly, it is also revealed that a static disorder is involved in the studied thin films. These findings highlight that, given the overall agreement between experimental and calculated spectra, the Cr2AlC electronic structure is accurately predicted using density functional theory.
... Also, the electronic properties of these materials have been studied both theoretically and experimentally [12]. For MAX-phases in thin film form, the processing and physical properties have been recently reviewed. ...
Article
Full-text available
We have studied the electronic structure and chemical bonding mechanism of nanolayered M 2 SbP with M = Ti, Zr and Hf using the full-relativistic of an all-electron full potential linearized augmented-plane-wave (FP-LAPW) method based on the density functional theory, within the local density approximation scheme for the exchange-correlation potential. Furthermore, we have to calculate the energy of formation for prove the existence of these compounds experimentally. Geometrical optimizations of the unit cell are in good agreement with the available theoretical and experimental data. The bulk modulus of M 2 SbP conserved as Ti is replaced with Zr, and increases by 8.7% as Ti is replaced with Hf, which can be understood on the basis of the increased number of valence electrons filling the p-d hybridized bonding states. The bonding is of covalent-ionic nature with the presence of metallic character. Analyzing the bonding in the binary MP, it can be concluded that this character is essentially conserved in M 2 SbP ternaries.
... It is observed that the r-GO has a wide structure π * peak at ≈529.3 eV, whereas TiO 2 has a double structure with two peaks at ≈530.2 and 532.6 eV, respectively. [45][46][47] In the r-GO/TiO 2 composites, those two peaks of TiO 2 have reduced their intensity by shifting the second peak from 532.6 to 532.0 eV, implying that the structural changes occur after the hybridization of O 2p to Ti 3d states 48 during the formation of r-GO/TiO 2 composite thin film. The Ti 3d splits into two peaks 530.2 (t 2g ) and 532.6 eV (eg) due to crystal field effects. ...
Article
Full-text available
Titanium dioxide (TiO 2 ) and reduced graphene oxide (r-GO) were synthesized separately by the radio frequency reactive magnetron sputtering technique on a Si-substrate (TiO 2 /Si) and the improved Hummers method, respectively. For deposition of r-GO onto TiO 2 , an aqueous solution of r-GO was deposited on the TiO 2 /Si thin film by the spin coating process to fabricate the (r-GO/TiO 2 )/Si composite thin-film. The (r-GO/TiO 2 )/Si composite thin-film is characterized using different spectroscopic techniques to study the effects at the interface of TiO 2 and r-GO in the surface defects, vacancy, incorporation of the different oxygenated moiety, microstructural, surface morphological, electronic, and magnetic performance. Our motivation is to contribute and understand mainly the tuning of the electronic structure and magnetic performance of the (r-GO/TiO 2 )/Si composite structure for the development of future promising optoelectronic and spintronic applications. We found that the magnetic performance is improved due to the change in the electronic properties of r-GO deposited on TiO 2 thin films. Thus, the role of the interfacial defects in the (r-GO/TiO 2 )/Si thin film and the mechanism of the tuning of the electronic structure and magnetic performance are elucidated comprehensively.
... The Ti-O bond is generally thought to be ionic with negligible covalence. Even in the presence of small quantity of Al, Ti atoms bond stronger to the O than Al [17]. Increasing the time of emersion, due to the hydrophilic interaction surface monolayer of amphiphilic water molecules orients close to the surface in order to compensate the ionic surface polarization charge. ...
Conference Paper
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Physical surface properties including surface roughness, topography, morphology, and wettability could influence the implant material behavior, bio-response, bacterial contamination and contact with other bio-active surfaces or fluids. This work investigates microand nanoscale roughness parameters and surface morphology of PVD deposited (Ti,Al,V)N/TiO2 coating using optical microscopy, white light interferometry (WLI), contact profilometer and scanning electron (SEM) microscopy. Results indicated that vacuum oxidized surface possessed medium roughness values, anisotropy in surface texture and irregular morphology. The vacuum oxidation of the nitride maintained the oxide with nano-crystal size and showed pores at the interface between the layers. The effect of prolonged exposure to Ringer-Braun solution droplet on the modified crystalline structure of the oxide was also explained.
... The metal TiN is used as a bottom electrode to evaluate the ferroelectric properties of the organic P(52% VDF-48% TrFE) copolymer films with the different chemical bonding states at the polymer/metal interface and on the metal electrode surface. Since the chemical bonding of TiN contains a mixed contribution of metallic, covalent, and ionic character [25], the surface of metal electrode TiN can be modified into TiN x O y which depends on the RTA annealing in a N 2 ambient. Therefore, TiN can play a role of suitable interlayer between the ferroelectric P(VDF-TrFE) and the metal electrode as well as a role of the bottom electrode. ...
... In this paper, we investigate the anisotropy and the orbital occupation in the electronic structure of singlecrystal Cr 2 GeC (0001) thin films. By applying bulk-sensitive and element-specific soft x-ray absorption (XAS), x-ray emission spectroscopy (XES) and resonant inelastic x-ray scattering (RIXS), we characterize the unoccupied and occupied bands of the containing elements, respectively [17][18][19]. This enables exploring the orbital occupation of the electrons buried several hundred nanometers below the surface. ...
Article
The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr$_{2}$GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal differences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr$_{2}$GeC. The Cr $L_{2,3}$, C $K$, and Ge $M_{1}$, $M_{2,3}$ emission spectra are interpreted with first-principles density-functional theory (DFT) including core-to-valence dipole transition matrix elements. For the Ge $4s$ states, the x-ray emission measurements reveal two orders of magnitude higher intensity at the Fermi level than DFT within the General Gradient Approximation (GGA) predicts. We provide direct evidence of anisotropy in the electronic structure and the orbital occupation that should affect the thermal expansion coefficient and transport properties. As shown in this work, hybridization and redistribution of intensity from the shallow $3d$ core levels to the $4s$ valence band explain the large Ge density of states at the Fermi level.
... The hardness values of TiC x and TiN y decrease with increasing vacancy content [13,15]. In substoichiometric TiN y , the integrated density of states(DOS) demonstrates that Ti(3d)-N(2p) bonding states are reduced in the presence of vacancies, leading to a weakened bond strength, shear strength and hardness [41]. Furthermore, it is predicted that the introduction of 12.5 % of vacancies in TiN y significantly reduces its shear stiffness from 190 GPa to less than 150 GPa [15], agreeing well with the experimental results [42]. ...
Article
The role of X deficiency on the mechanical properties of MAX phases was studied by synthesizing Ti2AlN through powder metallurgy in stoichiometric and sub/extra-stoichiometric nitrogen compositions. XRD analyses and ab initio calculations indicate that nitrogen vacancies result in a lattice contraction predominantly along the c-axis. The elastic moduli and intrinsic hardness of substoichiometric Ti2AlN0.9 measured from nanoindentation tests are shown to be slightly smaller than that of Ti2AlN. The key mechanical indexes, bulk (B), shear (G) and Young’s (E) moduli as well as the hardness variation are calculated in density functional theory, and show different responses depending on the concentration of N vacancies. This joint experimental and theoretical study provides a full understanding of the energetics, chemical bonding, electronic structure, and mechanics of the N deficient MAX phases which would increase the application of nitride ceramics.
... As observed in Fig. 3, the valence band edge shifts to higher energy with increasing x. The hybridization at the top of the valence band shows similarities with XES spectra of other nitrides e.g., Ti2AlN [35] and Sc3AlN [36]. ...
Preprint
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The electronic structure, chemical bonding and interface component in ZrN-AlN nanocomposites formed by phase separation during thin film deposition of metastable Zr1-xAlxN (x=0.0, 0.12, 0.26, 0.40) is investigated by resonant inelastic X-ray scattering/X-ray emission and X-ray absorption spectroscopy and compared to first-principles calculations including transitions between orbital angular momentum final states. The experimental spectra are compared with different interface-slab model systems using first-principle all electron full-potential calculations where the core states are treated fully relativistic. As shown in this work, the bulk sensitivity and element selectivity of X-ray spectroscopy enables to probe the symmetry and orbital directions at interfaces between cubic and hexagonal crystals. We show how the electronic structure develop from local octahedral bond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results in three different kinds of bonding originating from semi-coherent interfaces with segregated ZrN and lamellar AlN nanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes place with increasing Al content and affects the bond strength and increases resistivity.
... 22 Furthermore, MAX phase nitrides and carbides show distinct differences in the distribution of the density of states. 23 Thus, the electronic and electrical conductivity/resistivity properties of MAX phases differ significantly between the two types of ceramics. Therefore, it is of importance to test the intercalation of Au into both MAX phases to establish any universality of the substitutional reaction. ...
Article
Thermally-induced intercalation of noble metals into non-van der Waals ceramic compounds presents a method to produce a new class of layered materials. We recently demonstrated an exchange reaction of Au with A layers of MAX phase carbides with plentiful combinations of A and M elements. Here, we report the first substitution of Al with Au in a Ti2AlN MAX phase nitride at an elevated temperature without destroying the original layered structure. These results bolster the generalization of the Au intercalation for the A elements in MAX phases with diverse combinations of M, A, and X elements. Furthermore, we propose crucial factors to achieve the exchange reaction: there should be a chemical potential for the A element to dissolve in or react with noble metals to intercalate; the noble metals should be inert to the initial metal carbides/nitrides; and it is necessary to choose the reaction temperature that allows balanced interdiffusion of the noble metals and A elements.
... This interaction of both metal atoms could result in a local structure similar to that found in ternary laminated structures where Al is intercalated in the cubic structure. 32 ...
Article
The local bonding structure of titanium aluminum nitride (Ti1-xAlxN) films grown by dc magnetron cosputtering with different AlN molar fractions (x) has been studied by x-ray absorption near-edge structure (XANES) recorded in total electron yield mode. Grazing incidence x-ray diffraction (GIXRD) shows the formation of a ternary solid solution with cubic structure (c-Ti1-xAlxN) that shrinks with the incorporation of Al and that, above a solubility limit of x~0.7, segregation of w-AlN and c-Ti1-xAlxN phases occurs. The Al incorporation in the cubic structure and lattice shrinkage can also be observed using XANES spectral features. However, contrary to GIXRD, direct evidence of w-AlN formation is not observed, suggesting a dominance and surface enrichment of cubic environments. For x>0.7, XANES shows the formation of Ti-Al bonds, which could be related to the segregation of w-AlN. This study shows the relevance of local-order information to assess the atomic structure of Ti1-xAlxN solutions.
... First-principle calculations based on the density-functional theory (DFT) [10] have successfully been used to study the MAX phases to obtain microscopic information [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. These studies have included predictions of the structural parameters and stability, formation energies, electronic structures, elastic constants, etc. ...
... The crystal structure of (a) hexagonal Ti 2 AlN and (b) Ti 2 N, where Al was deficient in Ti 2 AlN lattice. Adapted from M. Magnuson et al.[30]. ...
Article
Ti–Al–N thin films were synthesized utilizing a high power impulse magnetron sputtering (HIPIMS) from a Ti2AlN compound target. The deposition temperatures and bias voltages were varied in the range of room temperature (RT) to 450 °C and 0 V to − 70 V, respectively. It was indicated that amorphous films formed at low deposition temperatures of RT and 300 °C, which changed into MAX-phase Ti2AlN films after vacuum annealing at 800 °C for 1 h. Densely packed nano-fibrous crystalline films mainly composing of Ti2AlN MAX phase and tetragonal Ti2N phase were acquired at deposition temperature of 450 °C, which exhibited stable film structure during vacuum annealing at 800 °C. The Ti2AlN–Ti2N composite films exhibited excellent oxidation and corrosion resistance, as compared to (Ti,Al)N film having same Ti/Al ratio and/or TiN film synthesized by a hybrid coating system with HIPIMS and DC pulse magnetron sputtering. The mechanical properties of the Ti2AlN–Ti2N films were also investigated in this work.
... The obtained results of bond length also showed good agreement with the experimental bond length of bulk TiN and HfN structures [40][41][42]. In the proposed system configuration of Ti16-xHfxN16, the bond length of TiN ranges from 1.88 Å to 1.99 Å (with increasing deposition of Hf atoms) which is closer to the experimental value of 2.12 Å [43,44]. Similarly in the case of HfN the obtained range lies between 2.04 Å and 2.10 Å which is again not very far from the experimental value of 2.30 Å [41,45]. ...
Article
The comparative study of structural transformation of two-dimensional TiN monolayer into HfN monolayer is presented in this paper. The structural, electronic, magnetic, and optical properties of the proposed system were analyzed based on the first principles of density functional theory. The obtained results indicate that unlike the conventional monolayer of TiN and HfN, the proposed system configuration of Ti16-xHfxN16(x∈0,4,16) possesses a direct band gap, higher conductivity, improved optical behavior, and magnetic moment. It can be concluded that the transformation of TiN monolayer to HfN monolayer opens up new possibilities for many optoelectronic and biomedical applications.
... The characteristic XAS peaks for the standard TiO 2 powder are shown in figure 3(c) for comparison. The degenerate Ti 3d band splits into t 2g (corresponds to 530.3 eV peak) and e g (532.9 eV peak) bands due to crystal field effects [30]. This splitting is very sensitive to the coordination number and to the extent of the hybridization. ...
Article
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Different oxidation states of titanium nitride thin films, including pure TiN(h 0 0), TiN1−xOx(h 0 0), Ti2O3(0 0 l) and pure anatase TiO2(0 0 l), were prepared by pulsed laser deposition with various oxygen pressures using a TiN target. Elaborative evolutions of the crystal and electronic structures of the obtained films were examined systematically by x-ray diffraction and x-ray absorption spectroscopy. We found that the Ti2O3(0 0 l) film, which was prepared at oxygen pressures , exhibited the maximum room temperature ferromagnetism (RTFM) behaviour. The bound magnetic polaron model is used to clarify the origin of RTFM in these films.
... Thermal expansion measurements are made using high-temperature x-ray diffractions. Also, the electronic properties of these materials have been studied both theoretically and experimentally[12]. For MAX-phases in thin film form, the processing and physical properties have been recently reviewed.[13]Because ...
Article
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We have studied the electronic structure and chemical bonding mechanism of nanolayered M2SbP with M = Ti, Zr and Hf using the full-relativistic of an all-electron full potential linearized augmented-plane-wave (FP-LAPW) method based on the density functional theory, within the local density approximation scheme for the exchange-correlation potential. Furthermore, we have to calculate the energy of formation for prove the existence of these compounds experimentally. Geometrical optimizations of the unit cell are in good agreement with the available theoretical and experimental data. The bulk modulus of M2SbP conserved as Ti is replaced with Zr, and increases by 8.7% as Ti is replaced with Hf, which can be understood on the basis of the increased number of valence electrons filling the p-d hybridized bonding states. The bonding is of covalent-ionic nature with the presence of metallic character. Analyzing the bonding in the binary MP, it can be concluded that this character is essentially conserved in M2SbP ternaries.
... Two peaks at ~533.7 and ~535.1 eV are attributed to the physical absorption of oxygen between 530-536 eV region [40]. These two peaks are assigned to t 2g and e g orbit separation of Ti 3d and Si 3d states [41,42]. The photon energies above 536 eV are signatures of oxygen 2p interaction with Ti 4sp and Si 3sp states [43]. ...
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We have functionalized multiwall carbon nanotubes (MWCNTs) with the composition of SiO 2 and TiO 2 (MWCNTs:TiO 2 :SiO 2) at different Ti:Si stoichiometric ratios (Ti:Si ≈ 6:6 at% and ≈10:10 at%) using the hy-drothermal process. The micro-structural, electronic and electrical properties of the unfunctionalized and functionalized MWCNTs were studied. Changes in surface morphology, degree of hybridization, crystallite structure and bonding structure due to functionalization were studied using field-emission scanning electron microscopy, Raman spectroscopy, x-ray diffraction, x-ray photoemission spectroscopy (XPS) and x-ray absorption near edge structure (XANES) spectroscopy techniques. Memristive and charge storage properties for MWCNTs: TiO 2 :SiO 2 nanocomposites (NCs) are more pronounced on NCs functionalised with high stoichiometric ratio (Ti: Si ≈ 10:10) due to the contribution of Ti 3+ and Si 2p core states as indicated by XPS and XANES results. The tunability of electrical conductivity is shown by an increase in the measured current and semiconducting I-V behaviour of the material as a result of high content of Ti-charge transfer. These observed changes in the electrical behaviour and electronic/bonding structure of the NCs indicate that the material could be useful for electrical/electronic applications and photocatalytic activity.
... There is a hybridisation between 3p and 3d states of Ti atom and 3p states of Si in −2.0 eV to −3.0 eV energy range. Thus, these hybridizations can be evaluated as a result of interactions between covalently bonded atoms [15,38]. ...
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In this study, the structural, electronic, mechanical, lattice dynamical and thermodynamic characteristics of Ti n+1 SiN n (n = 1, 2 and 3) MAX phase compounds were investigated using the first principle calculations. These ternary nitride compounds were found to be stable and synthesisable, and the results on the stability nature of them were also evaluated for the possible a and b phases. a-Ti4SiN3 was found to be the most stable one among these new class of layered MAX phases for which limited works are available in the literature. The band structures, that are essential for the electronic properties, were determined along with the partial density of states (PDOS) indicating the metallic behaviour of these compounds. The polycrystalline elastic moduli were calculated based on the single-crystal elastic constants and the mechanical stabilities were verified. Some basic physical parameters, such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, Debye temperature, and sound velocities, were also predicted. Furthermore, the anisotropic elastic properties were visualised in three dimensions (3D) for Young's modulus, linear compressibility, shear modulus and Poisson's ratio as well as with the calculation of the anisotropic factors. a-Ti 4 SiN 3 phase showed the most isotropic characteristics with minimum deviations. These theoretical values were also used to identify the stiffness and ionic characteristics. The phonon dispersion curves and corresponding PDOS indicated that Ti n+1 SiN n compounds were dynamically stable. Moreover, thermodynamic properties obtained from phonon dispersion curves were investigated in detail. ARTICLE HISTORY Highlights (1) The studied compounds are stable in view of three stability criteria. (2) α-Ti4SiN3 is energetically more favourable. (3) The studied compounds are brittle in nature. (4) α-Ti4SiN3 is almost elastically isotropic.
... Their structure was described as Ti-N block interleaved by Al-layer [36]. The metallic-covalent bonding between Ti and Al is relatively weak and gives rise to the outward diffusion of Al at high temperatures [37]. This behavior reduces the thermodynamic stability at high temperatures resulting in degradation of Ti 2 AlN and enabling the formation of new phases. ...
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In this work, the oxidation behavior of Ti2AlN coating deposited on nickel super alloy IN718 was investigated. The coating was obtained by DC-magnetron sputtering at 540°C and subsequent vacuum annealing at 800°C for 1h. The coating morphology as well as the chemical composition were analyzed using SEM, EDS and XRD, respectively. The XRD results revealed that the coating mainly composed of Ti2AlN MAX phase. Cycling oxidation was performed at 700 °C and 800 °C in air. The XRD and SEM results proved the interaction between substrate and coating and the formation of the quaternary Ti3NiAl2N phase during oxidation at the interface. Due to the Ni diffusion towards the surface, the Ti3NiAl2N phase grew continuously and the Ti2AlN phase decomposed gradually resulting in a coating failure. The results indicate that the oxidation behavior of the coating is essentially controlled by the interdiffusion of Ni from substrate into the coating.
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The effect of post deposition annealing and Al2O3 interlayer introduced between the film and substrate on the nitrogen content in TiN films in TiN/SiO2/Si structures were studied using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. It was established that: i) the structure of the studied films corresponds to TiN1-xOx; ii) annealing the system insignificantly decreases the concentration of the oxygen in the film; iii) an Al2O3 interlayer prevents diffusion of oxygen from SiO2 and supports high nitrogen content in the TiN-film.
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The formation and migration energies of the mono-vacancy, foreign impurities(H, He and O atoms) and interstitial in Ti2AlN have been investigated using the first principles calculations. The results have shown that the mono-vacancy formation in Ti2AlN is energetically most favorable for the Al atom. There are three stable configurations for the foreign impurities, such as the Al-layer triangle center site, the interstitial site(1/3,2/3,0.6578) and the vacancy site. It is also found that the O substitution is easier to be formed. For interstitials, the octahedral interstitial(Ti, N, H or O) and Al-layer triangle interstitial(Al or He) sites are observed in our simulation. Moreover, the H, O and N interstitials are dominant in Ti2AlN. These results could provide a theoretical guideline in the future experiment and the application of Ti2AlN.
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Microstructure, phase composition, and properties of the titanium aluminide diffusion layers produced on Ti6Al4V by Chemical Vapour Deposition (CVD) at various process parameters were investigated. It was found that the layers produced by CVD contain a mixture of the Ti2AlN + TiAl2+Ti3Al phases, while the additional heat treatment step after CVD changes the composition of the layers into the TiN + Ti2AlN + TiAl + Ti3Al mixture. The layers are characterized by high hardness of about 800 HV0.05 after CVD aluminizing and about 850 HV0.05 after CVD aluminizing followed by heat treatment, and by good adhesion to the substrate. Their corrosion resistance is only slightly lower than that of the Ti6Al4V alloy substrate and is fully adequate to a wide range of applications.
Article
Single-crystalline Ti2AlN thin films have been grown on MgO(111) substrates at 750 °C using DC magnetron sputtering from a Ti2Al alloy target in a mixed N2/Ar plasma. X-ray diffraction and transmission electron microscopy confirm epitaxial layered growth of Ti2AlN (0001) on MgO(111). On the basis of the experimental results, ab initio calculations were carried out to study the geometry and electronic structure between Ti2AlN and oxygen/magnesium -terminated MgO polar surfaces. A total of eight interface structures were calculated, and results showed that the adhesion between Ti2AlN (0001) and O-terminated MgO(111) is stronger than that with Mg-terminated MgO(111) surfaces in general. In particular, the largest adhesion energy was found for the interfacial structure with N–Ti–Al–Ti/O–Mg configuration. The stronger adhesion is due to reactive nature of Ti2AlN surface with N–Ti–Al–Ti stacking sequence. In addition, electronic structure calculation results showed that there is charge transferring between Ti2AlN and MgO with directions from Ti2AlN to MgO for O termination and from MgO to Ti2AlN for Mg termination. Moreover, the adhesion energies and charge-transferring amount are independent of the layer thickness of Ti2AlN, indicating that the interaction between Ti2AlN and MgO is rather localized.
Article
A statistical indentation method has been employed to study the hardness value of fire-refined high conductivity copper, using nanoindentation technique. The Joslin and Oliver approach was used with the aim to separate the hardness (H) influence of copper matrix, from that of inclusions and grain boundaries. This approach relies on a large array of imprints (around 400 indentations), performed at 150 nm of indentation depth. A statistical study using a cumulative distribution function fit and Gaussian simulated distributions, exhibits that H for each phase can be extracted when the indentation depth is much lower than the size of the secondary phases. It is found that the thermal treatment produces a hardness increase, due to the partly re-dissolution of the inclusions (mainly Pb and Sn) in the matrix.
Article
Investigations into the electronic properties, elastic properties, and ideal tensile strengths for Ti2AlN and Ti4AlN3 were conducted using first-principles density functional calculations. The electronic band structures and density of states show metallic conductivity in which Ti 3d states dominate for Ti2AlN and Ti4AlN3. Moreover, the hybridization peak of Ti 3d and N 2p lies at a lower energy than that of Ti 3d and Al 3p, which suggests that the Ti 3d-N 2p bond is stronger than the Ti 3d-Al 3p bond. The variations of elastic constants with pressure indicate that Ti2AlN and Ti4AlN3 possess higher mechanical stability in the pressure range 0-100 GPa. By calculating the bulk-modulus-to-shear-modulus ratio and Cauchy pressure, we predict that Ti2AlN and Ti4AlN3 are brittle. We show that the structural failure of these ternary compounds can be ascribed to the breakage of weak Ti-Al bonds under uniaxial tension and that layered structural stability is determined by the strength of the Ti-Al bond under tensile deformation.
Article
This article presents the development of superimposed Ti2AlN MAX phase coatings on cutting tools through reactive co-sputtering technique in dry machining of Ti-6Al-4 V. The effect of varying nitrogen flow rate during the reactive co-sputtering process and its influence on cutting properties has also been discussed. The predictive modeling along with validation of experimental results were carried out using response surface methodology (RSM) and optimized condition for cutting force were obtained using composite desirability function. Machining studies using the superimposed Ti2AlN coatings reduced the cutting force by 23–35 % when compared to commercial coatings. Investigation of cutting mechanics revealed an increase of shear angle by 3–5 % and decrease in friction angle by 20 %. The superimposed MAX phase coatings also reduced the friction coefficient by 34 % during dry turning of Ti-6Al-4 V. The superimposed Ti2AlN coatings were held responsible to improve the performance of the tool by 21–30 % in terms of tool life when compared to commercial tool. It could be inferred that the superimposed Ti2AlN coatings reduced the effect of chemical affinity between the tool and workpiece, eliminating the formation of built-up edge at rake face of the tool.
Chapter
Introduction Response of Quasi-Single Crystals to Compressive Loads Response of Polycrystalline Samples to Compressive Stresses Response of Polycrystalline Samples to Shear Stresses Response of Polycrystalline Samples to Flexure Stresses Response of Polycrystalline Samples to Tensile Stresses Hardness Fracture Toughness and R-Curve Behavior Fatigue Resistance Damage Tolerance Micromechanisms Responsible for High K1c, R-Curve Behavior, and Fatigue Response Thermal Sock Resistance Strain Rate Effects Solid Solution Hardening and Softening Machinability Summary and Conclusions References
Thesis
L'objet de la thèse est l'étude de films monocouches et multicouclies du système (Ti-Al-N) utilisés principalement comme revêtements d'outils de coupe. Le but a été d'étudier la structure et le comportement mécanique d'une série de films Tii-xAlxNy déposés par pulvérisationinagnétron réactive à partir d'une cible TiAl frittée, et de multicouches Tii-xAlxNy/Tii.xAl>;N à différentes périodes. L'étude structurale des films monocouches a montré que le changement du débit d'azote injecté n'avait pas d'influence sur la stoechiométrie en azote ; mais a généré néanmoins une différence de contraintes dans le film qui a joué à son tour un rôle dans l'adhésion et la dureté des dépôts. Pour les fîlms multicouches, deux compositions ont été choisies. Pour les films céramique/céramique et pour deux périodes (12 et 34 nm), les obsen/ations au MET ont montré que la structure multicouche n'existe que pour les 400 premiers nanomètres. Les analyses en EDXS ont montré que cette alternance de couche s'expliquait par la présence d'oxygène piégé dans le circuit d'azote et déposé involontairement. Les propriétés mécaniques de ces revêtements ont été trouvées proches de celles mesurées pour les films monocouches. Pour la composition TiAl/(Ti,Al)N, la structure multicouche a été maintenue tout au long du dépôt mais avec une épaisseur de la couche (Ti,Al)N plus importante que celle de TiAl ; ceci provient du temps nécessaire à la cible pour sa transition du régime empoisonné au régime non empoisonné. Ces films ont révélé une plus faible dureté, en raison des liaisons métalliques (moins dures que les liaisons covalentes), mais ont montré de meilleures résistances à l'usure et adhésion.
Article
In this work, we have studied the structural, electronic, elastic, and thermodynamic properties of the new Max phase’s class Mn2SiC1, Mn3SiC2 and Mn4SiC3 (Mnn+1SiCn with n = 1, 2 and 3), using the linearly augmented plane wave method (FP-LAPW) based on density functional theory. The exchange-correlation potential is treated with the local density approximation LSDA. The formation energies calculated for all compounds showed that these compounds are thermodynamically stable. We found that the ferromagnetic (FM) configuration is more stable than the non-magnetic (NM) one, at their lattice parameters for all three compounds. Cohesive energy confirms the structural stability of all structures. The total magnetic moment increases with an increasing value of n. The band structure indicates that the three materials are electrically conductive. For the density of state, we see that there is no gap for these three materials; they exhibit a metallic nature which results from the fact that the Mn-3d states are dominant at the Fermi level. The peak of hybridization of the Mn–3d, and C–2p states leads to a strong covalent bond than that between the Mn–3d and Si–3p states in the low energy domain. 3p electrons in silicon elements can effectively alter the covalence and iconicity of bonds that govern compressibility, ductility, and even superconducting properties. The chemical bond in three compounds is a combination of covalent, ionic, and metallic nature. The main factors governing the electronic properties are the hybrid states Mn–3d, Si–3p, and C–2p, and the bond (p–d) stabilizes the structure. The elastic constants are calculated and the conditions of the criterion of mechanical stability are checked. In addition, we have calculated the bulk, and shear modulus, Young's modulus, Poisson’s ratio, and anisotropy index. The quasi-harmonic Debye model was used to study the temperature-dependent thermodynamic properties of the Mnn+1SiCn (n = 1, 2, 3) compounds.
Article
Plasma sheath controls the modification of titanium which is found to increase the osseointegration time. In this work a theoretical background is presented for the surface treatment of the Ti as bioimplant. The effect of ion flux and emission of secondary electron from the wall of titanium surface plays a vital role in controlling the flow of heat through the plasma sheath to the titanium metal thereby affecting the TiN bond formation. We have considered N2 plasma in our modeling which hits the titanium surface at appropriate temperature such that the wall emits secondary electrons. This study is done via considering a 1D fluid model with electrons and ions generating a flux relation which is solved computationally. The comparison between hot plasma (α = 1) and cold plasma (α = 0.001) for surface treatment of titanium is presented and found the time required to reach the bond energy of 4.94 eV for TiN bond formation.
Article
The nature and the mechanism of the film interaction with the substrate at the film/substrate interface are still far from being fully understood. Here, we demonstrate an ab initio investigation of the thermodynamic driving force across the initial phase among amorphous and crystalline structures in the film formation of TiN on an alumina substrate. Combining ab initio molecular dynamics (AIMD) and density functional theory (DFT), it provides evidence that the amorphous TiN indeed becomes thermodynamically stable at finite length scales (0.6 nm) as the conductive and adhesive layer on am-Al2O3. In other words, cubic TiN growth is thermodynamically restricted until the thickness reaches above 0.6 nm, and therefore the amorphous TiN film can maintain its conformal coverage under such conditions. The approach presented here can be used for computational design and discovery of new corrosion-resistant alloys and semiconductors, by providing a first-principles framework to search for ways to induce a stable conductive and adhesive TiN layer. Other possible applications are predicted to be in the area of thin-film deposition, especially, searching for predictive pathways for the selection of polymorphs during the deposition process.
Article
The effective work function (eWF) of Al-doped titanium carbide (TiAlC) metal electrodes prepared by atomic layer deposition shows a strong dependence on the underlying gate dielectrics. The eWF of TiAlC on HfO2 shows a low value of 4.2 eV independent of the deposition temperature and process conditions, whereas that on SiO2 shifted to a midgap value of 4.7 eV, and it was sensitive to the process conditions. The mechanism underlying this TiAlC work function dependence on different gate dielectrics is investigated in detail.
Thesis
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Les phases MAX sont des carbures et / ou des nitrures ternaires avec un fort potentiel dans des applications diverses. Cette étude a porté sur deux phases MAX, Ti3SiC2 et Ti2AlC qui sont les plus connues dans ce groupe de matériaux. La première partie de ces travaux était dédiée à l’élaboration des poudres et des matériaux frittés. L’objectif était d’obtenir une variété de matériaux présentant différentes caractéristiques microstructurales, en termes de composition chimique et de taille de grains. Ainsi, des poudres commerciales et synthétisées par SHS ont été densifiées à l'aide de deux techniques de frittage sous charge, i.e. SPS et HP. La deuxième partie du travail a été consacrée à une meilleure compréhension de l’influence de la composition chimique et de la taille des grains sur le comportement thermomécanique des phases MAX. Des informations supplémentaires ont été fournies en couplant deux techniques expérimentales, la flexion quatre points et l’émission acoustique, et en les associant à des observations SEM post-mortems. L’approche expérimentale développée, basée sur la comparaison des réponses mécaniques des matériaux Ti3SiC2 (contenant la phase MAX et des phases secondaires) et de Ti2AlC (phases MAX uniquement), a permis d’approfondir la compréhension des mécanismes de déformation et d’endommagement induits. Il était également montré que les phases sécondaires et la taille de grains influence la manière dont les différents endommagements sont accumulées dans le matériaux. Les résultats d’EA sont fourni les informations supplémentaires sur les type d’endommagements rencontrées et leur chronologie qui résultent avec le comportement nonlinéaire de phases MAX. La dérnière partie de cette thèse a montré que le température de transition fragile-plastique est autour de 1200˚C et que la taille de grains l’abaisse.
Article
The structural, elastic, electronic and magnetic properties of the cubic [Formula: see text] anti-perovskite are investigated by means of the full-potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT). We have used three approximations: the generalized gradient (GGA), the GGA+[Formula: see text][Formula: see text], where [Formula: see text] is on-site Coulomb interaction correction, and the modified Becke–Johnson (mBJ-GGA). The elastic constants [Formula: see text] show that our compounds are ductile and anisotropic. The results obtained for the spin-polarized band structure and the density of states show a half-metallic behavior for the compounds using the GGA, GGA+[Formula: see text][Formula: see text] and mBJ-GGA approaches. These results make [Formula: see text] a promising candidate for spintronics applications.
Article
We investigate sputtering of a Ti3SiC2 compound target at temperatures ranging from RT (no applied external heating) to 970 oC as well as the influence of the sputtering power at 850 oC for the deposition of Ti3SiC2 films on Al2O3(0001) substrates. Elemental composition obtained from time-of-flight energy elastic recoil detection analysis shows an excess of carbon in all films, which is explained by differences in angular distribution between C, Si and Ti, where C scatters the least during sputtering. The oxygen content is 2.6 at.% in the film deposited at RT and decreases with increasing deposition temperature, showing that higher temperatures favor high purity films. Chemical bonding analysis by X-ray photoelectron spectroscopy shows C-Ti and Si-C bonding in the Ti3SiC2 films and Si-Si bonding in the Ti3SiC2 compound target. X-ray diffraction reveals that the phases Ti3SiC2, Ti4SiC3, and Ti7Si2C5 can be deposited from a Ti3SiC2 compound target at substrate temperatures above 850 oC and with growth of TiC and the Nowotny phase Ti5Si3Cx at lower temperatures. High-resolution scanning transmission electron microscopy shows epitaxial growth of Ti3SiC2, Ti4SiC3, and Ti7Si2C5 on TiC at 970 oC. Four-point probe resistivity measurements give values in the range 120 to 450 mucro-Ohm-cm and with the lowest values obtained for films containing Ti3SiC2, Ti4SiC3, and Ti7Si2C5.
Article
Dynamical properties of the two-dimensional Ti2C and Ti2N MXenes were investigated using density functional theory and discussed in connection with their structures and electronic properties. To elucidate the influence of magnetic interactions on the fundamental properties of these systems, the nonmagnetic, ferromagnetic and three distinct antiferromagnetic spin arrangements on titanium sublattice were considered. Each magnetic configuration was also studied at two directions of the spin magnetic moment with respect to the MXene layer. The zero-point energy motion, following from the phonon calculations, was taken into account while analyzing the energetic stability of the magnetic phases against the nonmagnetic solution. This contribution was found not to change a sequence of the energetic stability of the considered magnetic structures of Ti2X (X = C, N) MXenes. Both Ti2X (X = C, N) systems are shown to prefer antiferromagnetic arrangement of spins between Ti layers and the ferromagnetic order within each layer. This energetically privileged phase is semiconducting for Ti2C and metallic for Ti2N. The type of magnetic order as well as the in-plane or out-of-plane spin polarizations have a relatively small impact on the structural parameters, Ti-X bonding length, force constants and phonon spectra of both Ti2X systems, leading to observable differences only between the nonmagnetic and any other magnetic configurations. Nonetheless, a noticeable effect of the spin orientation on degeneracy of the Ti-3d orbitals is encountered. The magnetic interactions affect to a great extent the positions and intensities of the Raman-active modes, and hence one could exploit this effect for experimental verification of the theoretically predicted magnetic state of Ti2X monolayers. Theoretical phonon spectra of Ti2X (X = C, N) MXenes exhibit a linear dependence on energy in the long-wavelength limit, which is typical for a 2D system.
Article
In this research work, 15 vol% Ti2AlN particles reinforced TiAl alloy matrix composite was prepared by an in-situ reactive hot processing method using Ti, Al and TiN powders as raw materials. Transmission electron microscope (TEM) coupled with energy-dispersive spectroscopy (EDS) was utilized to investigate the morphology characteristics of Ti2AlN particles. The crystallographic orientation relationships and interface atomic structures between various shaped Ti2AlN particles and TiAl matrix were investigated by the means of selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). The results show that four kinds of Ti2AlN particles presented in the composite, which include vast majority of elliptical and polygonal Ti2AlN particles with the sizes of 4 μm, few hexagonal Ti2AlN particles with the sizes of 4 μm and small plate Ti2AlN particles with the sizes of 0.4 μm. The elliptical and polygonal Ti2AlN particles have no fixed crystallographic orientation relationship with TiAl matrix, the elliptical and polygonal Ti2AlN/TiAl interfaces are incoherent interphase boundaries composed by many atomic-scale microfacets. The hexagonal and small plate Ti2AlN particles have a fixed orientation relationship with TiAl matrix. Both the hexagonal and small plate Ti2AlN particles have a ledge and terrace coherent interface with TiAl matrix.
Article
This paper outlines general physical issues associated with performing computational numerical simulations of primary point defects in MAX phases Ti2AlN. First-principles solutions are possible due to the development of computational resources of software and hardware. The calculation accuracy is a good agreement with the experimental results. As an important application of our simulations, the results could provide a theoretical guidance for future experiments and application of Ti2AlN. For example, the N mono-vacancy is the most difficult to form. On the contrary, the mono-vacancy formation in Ti2AlN is energetically most favorable for the Al atom. The essence of the phenomena is explained by the calculated density of state (DOS).
Article
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We have grown single-crystal thin films of Ti2GeC and Ti3GeC2 and a new phase Ti4GeC3, as well as two new intergrown MAX-structures, Ti5Ge2C3 and Ti7Ge2C5. Epitaxial films were grown on Al2O3(0001) substrates at 1000 °C using direct current magnetron sputtering. X-ray diffraction shows that Ti–Ge–C MAX-phases require higher deposition temperatures in a narrower window than their Ti–Si–C correspondences do, while there are similarities in phase distribution. Nanoindentation reveals a Young’s modulus of 300 GPa, lower than that of Ti3SiC2. Four-point probe measurements yield resistivity values of 50–200 μΩcm. The lowest value is obtained for phase-pure Ti3GeC2(0001) films.
Article
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We have previously shown that one-particle calculations of X-ray spectra in simple metals give very different results depending on whether the core hole is taken into account or not. Guided by experiment and the dynamical theory of X-ray spectra developed by Nozières and DeDominicis (ND) we have, however, also established the rule that rather realistic spectra can be obtained from a one-particle calculation provided final state wavefunctions are used in the transition matrix elements. In this paper we report on direct numerical evaluations of the dynamical ND theory for several different cases including the case where a bound state appears. In all cases the results support our final state rule. We also give arguments in support of the procedure used to extract threshold exponents and asymmetry indices from X-ray absorption and emission spectra and X-ray photoemission spectra.
Article
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The structural parameters of various Haegg phases (H or M{sub n+1}AX{sub n} phases) are studied experimentally by x-ray and electron spectroscopies, x-ray diffraction, and ab initio full potential as well as full mutiple scattering theoretical calculations. Experimentally it was found that the structure of all ternary compounds analyzed herein are relaxed. The values of the lattice parameters and relaxations obtained from ab initio calculations are in excellent agreement with those deduced from the analysis of the experimental data. The bonding scheme has been analyzed and the charge transfer between constituting atoms determined. It is demonstrated that the strength and electrical transport properties in these materials are principally governed by the metallic planes. For the solid solution (Ti{sub 0.5}Nb{sub 0.5})âAlC, the most salient result is that the basal planes are corrugated, which could explain the solid solution scattering observed in this H phase.
Article
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We have produced pure thin-film single-crystal Ti2AlN(0001), a member of the Mn+1AXn class of materials. The method used was UHV dc reactive magnetron sputtering from a 2Ti:Al compound target in a mixed Ar–N2 discharge onto (111) oriented MgO substrates. X-ray diffraction and transmission electron microscopy were used to establish the hexagonal crystal structure with c and a lattice parameters of 13.6 and 3.07 Å, respectively. The hardness H, and elastic modulus E, as determined by nanoindentation measurements, were found to be 16.1±1 GPa and 270±20 GPa, respectively. A room-temperature resistivity for the films of 39 μΩ cm was obtained.
Article
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The electronic structures of epitaxially grown films of Ti(3)AlC(2), Ti(3)SiC(2), and Ti(3)GeC(2) have been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured high-resolution Ti L, C K, Al L, Si L, and Ge M emission spectra are compared with ab initio density-functional theory including core-to-valence dipole matrix elements. A qualitative agreement between experiment and theory is obtained. A weak covalent Ti-Al bond is manifested by a pronounced shoulder in the Ti L emission of Ti(3)AlC(2). As Al is replaced with Si or Ge, the shoulder disappears. For the buried Al and Si layers, strongly hybridized spectral shapes are detected in Ti(3)AlC(2) and Ti(3)SiC(2), respectively. As a result of relaxation of the crystal structure and the increased charge-transfer from Ti to C, the Ti-C bonding is strengthened. The differences between the electronic structures are discussed in relation to the bonding in the nanolaminates and the corresponding change of materials properties.
Article
In this paper we report on the fabrication and characterization of Nb2AlC (actual Nb:Al:C ratios: 52.5±0.5, 24.0±0.2 and 23.5±0.5 at.%, respectively) and (Ti,Nb)2AlC (actual Ti:Nb:Al:C atomic ratios: 24.4±0.5, 27.3±0.5, 24.0±0.3 and 24.4±0.5 at.%, respectively). Polycrystalline, fully dense, predominantly single-phase samples of Nb2AlC, (average grain size ≈14±2 μm) were fabricated by reactive hot isostatic pressing of Nb, graphite, and Al4C3 at 1600°C for 8 h and 100 MPa. The identical procedure resulted in predominantly single-phase samples - with an average grain size of 45 μm - of (Ti,Nb)2AlC. To obtain finer-grained (≈15±3 μm) samples of the solid solution the powder mixtures were hot pressed at 1450°C for 24 h. The a and c lattice parameters of Al-poor Nb2AlC samples are, respectively, 3.107±0.001 and 13.888±0.001 Å; the corresponding values for the solid solution are 3.077±0.001 and 13.790±0.001 Å. Since the hardness of the solid solution (5.8 GPa) is in between those of Nb2AlC (6.1 GPa) and Ti2AlC, and, at comparable grain sizes, Nb2AlC is stronger, we conclude that no solid solution strengthening occurs in this system. All samples explored in this work are quite damage tolerant and thermal shock resistant. A 300 N Vickers indentation in a 1.5 mm thick, four-point bend bar decreases the strengths by anywhere from 25 to 50% depending on grain size. Quenching in water from 1200°C reduces the four-point flexural strength by 40 to 70%; i.e., it is not catastrophic. Both compositions are strain rate sensitive. Like Ti3SiC2 and ice, and for the same reasons, the compressive stress of the Nb2AlC sample decrease with decreasing strain rate. In contradistinction, and for reasons that are not understood, the strengths of the (Ti,Nb)2AlC samples decrease with increasing strain rates. The grain growth kinetics of Nb2AlC are quite sluggish; no appreciable grain growth was observed even after annealing at 1600°C for 16 h.
Article
The Al L2,3 emission spectra of Al-Mn, Al-Co, Al-Ni and Al-Cu alloys with various composition of transition metals have been measured with a grazing incidence spectrometer. The results show that the Fermi edges of the Al L2,3 emission spectra of the alloys do not markedly shift with the change of phase, and that the energy levels arising from the 3d states of the transition metal and contributing to the formation of the conduction band of the alloy locate at a few electron volts below the Fermi edge in any phases of the aluminum-transition metal alloys.
Article
The Mn+1AXn layered carbide/nitride-derived phases, where M is an early transition metal, A is an A-group element and X is N or C, have an unusual combination of mechanical, electrical and thermal properties. The surface and crystal-chemistries of two members with n=2 and 3 have been investigated by X-ray photoelectron spectroscopy. The results show that the constituent species are characterized by low binding energies, sometimes exceptionally so. The energies are 281.0 281.5 and 396.9 eV, respectively, for C 1s and N 1s, both of which are at, or below, the lowest values measured for carbides and nitrides. Similarly the Ti 2p energies, in the range 454.0 454.7 eV, are comparable to that of Ti metal and TiC, while the energy of the Al 2p, 72.0 eV, is lower by ca. 0.8 eV than that for Al metal. The signature of the XTi6 octahedral units in the stacking sequence is reminiscent of the corresponding units in TiN, and it is found that a decrease in Ti 2p binding energy is correlated with decrease in average X-Ti bond length. The low binding energies of the A-type species, Al and Si, in planar coordination suggest that binding and thus screening may be derived from out-of-plane interactions. Results relevant to oxidation arising from exposure to air have been obtained. Yes Yes
Article
A complete set of d spacings, intensities, and h, k, l indexes for Ti2AlN has been determined from;Y-ray powder diffraction. The lattice parameters are a=2.989(2) Angstrom, c=13.614(5) Angstrom; in good agreement with previous work. The new set of results comprises seven reflections not present in the current Ti2AlN PDF card No. 18-70. Furthermore, a new set of relative intensities are reported that are in better agreement with the calculated values than they are with those listed in the PDF card. (C) 2000 International Centre for Diffraction Data. [S0885-7156(00)00603-5].
Article
Titanium was reactively r.f. sputtered in mixed ArN2 and ArCH4 discharges onto substrates held at 775 K. The films obtained were characterized by scanning electron microscopy and X-ray diffraction and through measurements of the microhardness and electrical resistivity. The composition of the films was determined by Auger electron spectroscopy. The measurements show that the morphology of the deposits to a large extent influences the properties of the films obtained. For TiN coatings the electrical resistivity reaches the bulk resistivity only if coatings with the full bulk density are obtained. The difference observed in the lattice parameter for TiN thin film and bulk samples is explained using a grain boundary relaxation model. It is also shown that the heat of formation of the compounds plays an important role in the formation of carbide and nitride films. A high heat of formation promotes the development of large grains and dense structures.
Article
Transmission electron microscopy (TEM) of aligned, macrograined samples of Ti3SiC2, deformed at room temperature, shows that the deformed microstructure is characterized by a high density of perfect basal-plane dislocations with a Burgers vector of 1/3〈112 0〉. The dislocations are overwhelmingly arranged either in arrays, wherein the dislocations exist on identical slip planes, or in dislocations walls, wherein the same dislocations form a low-angle grain boundary normal to the basal planes. The arrays propagate across entire grains and are responsible for deformation by shear. The walls form as a result of the formation of kink bands. A dislocation-based model, that builds on earlier ideas proposed for kink-band formation in hexagonal metallic single crystals, is presented, which explains most of the microstructural features. The basic elements of the model are shear deformation by dislocation arrays, cavitation, creation of dislocation walls and kink boundaries, buckling, and delamination. The delaminations are not random, but successively bisect the delaminating sections. The delaminations and associated damage are contained by the kink boundaries. This containment of damage is believed to play a major role in endowing Ti3SiC2 and, by extension, related ternary carbides and nitrides with their damage-tolerant properties.
Article
A survey of some more recent results on the structural chemistry of compounds between transition elements and IVb group elements (carbon, silicon, germanium and tin) will be presented. There are essentially two large classes of compounds to be discussed, characterized by uniform structural principles, namely transition element carbides and related phases on the one hand and defect disilicide structure compounds and derivatives on the other.Starting with the problem of carbon ordering in transition element carbides and hydrogen containing carbides which reveal the significance of the octahedral [T6C]-group, numerous complex carbides of the general formula TxMyCz (T = transition element, M = another transition or B-group element) can be explained by means of a few common structural features. Perovskite carbides of formula T3MC, corresponding to the filled up Cu3Au-type or the filled up U3Si-type structures, β-Mn carbides of formula T3M2C, corresponding to the filled up β-Mn-type structure and K-carbides, related to the Mn3Al9Si-type structure are characterized by linking of the [T6C]-groups by corners. H-phase carbides of formula T2MC and carbides having Ti3SiC2-type structure exhibit linking of the [T6C]-groups by edges. A similar mode of linking also occurs for carbides with V3AsC-type or the filled Re3B-type structures, although in some cases such as VCr2C2 the trigonal prismatic [T6C]-group intervenes. Finally, the η-carbides having filled Ti2Ni-type and carbides of formula T5M3C with filled up Mn5Si3-type structure can be regarded as built up by linking of the octahedral [T6C]-groups by faces. The geometrical factor within the carbides is strongly supported by the short T-C-distances in the structural element [T6C], thus the formation and architecture of complex carbides may be understood from a topo-chemical point of view, for example, Ti2GeC (H-phase carbide) consists of the sum of TiC (octahedral group) and TiGe (trigonal prism).The second class of compounds, which are derived from the TiSi2-type structure, also belong to an uniform geometrical principle; however, some influence of the electronic concentration on the defect of the B-group element (Si,Ge) and the cell parameter will be recognized. The peculiar structural principle can be described by a partial lattice of the transition metal atom corresponding almost perfectly to that of the Ti-atoms of the TiSi2-type while the second partial lattice (Si,Ge or Sn) according to the defect of these atoms is expanding in one direction of the generating (110) plane. As a consequence of the mutual interference of T- and B-group element atoms a helicoidal structural element of the respective Si, Ge, etc., atoms results. Thus, the arrangement is characterized by a typical subcell and occasionally by extremely long c-axis. That also means, fairly complex compositions occur such as Mn11Si19, Mo13Ge23, V17Ge31 or Rh39 (Ga0.5Ge0.5)58. The problem of pseudo-homogenous domains of compounds arise in as far as within fairly small regions of composition a split according to different multiple of subcell will be observed, such as Mn11Si19(MnSi1.727); Mn26Si45(MnSi1.730); Mn15Si25(MnSi1.733) and Mn27Si47(MnSi1.741). A similar change in the multiple of subcells that means independent phases, takes place by substituting either the transition element by another or by substituting the B-group element by another B-group element such as Cr37Ge59As4 which corresponds to the Rh10Ga17-type while Cr11Ge19 is isotypic with Mn11Si19. In general, lowering of the overall electron concentration diminishes the defect of the B-group element in the compound.
Article
The ternary compounds Zr2TlC, Zr2PbC, Hf2TlC and Hf2PbC were prepared and examined. The crystal structure of these carbides has been found to be isotypic with Cr2AlC, H-phase.
Article
We studied the electronic structure of TiN and VN by means of band structure calculations and spectroscopic techniques. The band structure calculations show that the bonding in these compounds is mostly covalent. The Fermi level intersects the transition metal 3d bands giving rise to the metallic conductivity observed in these nitrides. The particularly large stability of these compounds is due to filled metal 3d-nitrogen 2p bonding states. The X-ray photoemission (XPS) and the N 1s X-ray absorption (XAS) spectra are related to the occupied electronic states in the valence band and to the unoccupied electronic states in the conduction band respectively.
Article
The electronic structure of the nanolaminated transition metal carbide Ti2AlC has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured Ti L, C K, and Al L emission spectra are compared with calculated spectra using ab initio density-functional theory including dipole matrix elements. The detailed investigation of the electronic structure and chemical bonding provides increased understanding of the physical properties of this type of nanolaminates. Three different types of bond regions are identified: The relatively weak Ti 3d–Al 3p bond 1 eV below the Fermi level and the Ti 3d–C 2p and Ti 3d–C 2s bonds which are stronger and deeper in energy are observed around 2.5 and 10 eV below the Fermi level, respectively. A strongly modified spectral shape of the 3s final states in comparison to pure Al is detected for the intercalated Al monolayers indirectly reflecting the Ti 3d–Al 3p hybridization. The differences between the electronic and crystal structures of Ti2AlC, Ti3AlC2, and TiC are discussed in relation to the number of Al layers per Ti layer in the two former systems and the corresponding change of the unusual materials properties.
Article
Electronic structures of the hexagonal Ti2AlC and Ti2AlN compounds with Cr2AlC-type structure calculated within the full-potential linearized augmented plane-waves formalism are presented. Geometrical optimization of the unit cell are in good agreement with experimental data. The analysis of the site and momentum projected densities of states shows that bonding is due to Ti d-C p (or Ti d-N p) and Ti d-Al p hybridizations. It is found that the intensity of the total density of state at Fermi level is higher for Ti2AlN that has also a higher electrical conductivity. Results are compared to a recent work by Zhou and Sun1 who assume a different crystal structure.
Article
We present sets of special points in the Brillouin zone from which the average over the Brillouin zone of a periodic function of wave vector (e.g., energy, charge density, dipole matrix elements, etc.) can be determined in a simple and accurate way once the values of the function at these points are specified. We discuss a method for generating the special-point sets and apply it to the case of crystals with cubic and hexagonal Bravais lattices.
Article
In this paper we report on the electronic and magnetotransport properties of two Ti-based ternaries, Ti3AlC2 and Ti4AlN3. In order to determine the effective carrier concentrations and their mobilities, the Hall effect, electrical conductivity, thermoelectric power, magnetic susceptibility, and magnetoresistance were measured as a function of temperature between 4 and 300 K and at magnetic fields up to 9 T. For Ti3AlC2, the Hall voltage is a linear function of magnetic field at all temperatures. At the lowest temperatures, the Hall coefficient is small but positive; above 100 K it is negative and drops more or less linearly with temperature. The magnetoresistance of Ti3AlC2 is dominated by a positive quadratic field dependence. The magnetic susceptibility is nearly constant but displays a weak maximum around the temperature at which the Hall effect changes sign (≈100 K). In contrast, the Seebeck coefficient remains positive up to 800 K, with a maximum at 700 K. The results were analyzed within a two-band framework assuming a temperature-independent charge-carrier density and a hole mobility that is slightly smaller than the electron mobility. The model quantitatively accounts for our observations. The resistivity, magnetoresistance, and Hall coefficient of Ti4AlN3, on the other hand, were successfully described within the single-band model, with holes as the dominant charge carriers. This was supported by measurements of the Seebeck coefficient, which is positive and peaks at ≈300 K. The magnetic susceptibility of Ti4AlN3 is also quite temperature independent.
Article
Thin films of the Mn+1AXn-phases Ti2AlC and Ti3AlC2 have been deposited by dc magnetron sputtering. In agreement with the Ti–Si–C system, the MAX-phase nucleation is strongly temperature dependent. At 900 °C epitaxial films of Ti2AlC and Ti3AlC2 were grown, but at 700 °C only a cubic (Ti,Al)C phase was formed. In addition, a perovskite carbide, Ti3AlC was grown at 800 °C. A bulk resistivity of 0.51 μΩ m, 0.44 μΩ m, and 1.4 μΩ m was measured for the Ti3AlC2, Ti2AlC, and Ti3AlC films deposited at 900 °C, respectively. By nanoindentation the hardness and Young’s module was determined for an epitaxial Ti3AlC2 film to 20 GPa and 260 GPa, respectively.
Article
A new tool for analysing theoretically the chemical bonding in solids is proposed. A balanced crystal orbital overlap population (BCOOP) is an energy resolved quantity which is positive for bonding states and negative for antibonding states, hence enabling a distinction between bonding and antibonding contributions to the chemical bond. Unlike the conventional crystal orbital overlap population (COOP), BCOOP handles correctly the situation of crystal orbitals being nearly linear dependent, which is often the case in the solid state. Also, BCOOP is much less basis set dependent than COOP. A BCOOP implementation within the full-potential linear muffin tin orbital method is presented and illustrated for Si, TiC and Ru. Thus, BCOOP is compared to the COOP and crystal orbital Hamilton population (COHP) for systems with chemical bonds ranging from metallic to covalent character.
Article
The electronic structure in the new transition-metal carbide Ti(4)SiC(3) has been investigated by bulk-sensitive soft x-ray emission spectroscopy and compared to the well-studied Ti(3)SiC(2) and TiC systems. The measured high-resolution Ti L, C K, and Si L x-ray emission spectra are discussed with ab initio calculations based on density-functional theory including core-to-valence dipole matrix elements. The detailed investigations of the Ti-C and Ti-Si chemical bonds provide increased understanding of the physical properties of these nanolaminates. A strongly modified spectral shape is detected for the intercalated Si monolayers due to Si 3p hybridization with the Ti 3d orbitals. As a result of relaxation of the crystal structure and the charge-transfer from Ti (and Si) to C, the strength of the Ti-C covalent bond is increased. The differences between the electronic and crystal structures of Ti(4)SiC(3) and Ti(3)SiC(2) are discussed in relation to the number of Si layers per Ti layer in the two systems and the corresponding change of materials properties.
Article
The formation of ternary compounds within the Ti-Al-C system was studied by magnetron sputtering for thin-film deposition and first-principles calculations for phase stability. As-deposited films were characterized with X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM). The hardness and Young's moduli of the material were studied by nanoindentation. Epitaxial and phase-pure films of M(n+1)AX(n) phases Ti3AlC2 and Ti2AlC as well as the perovskite phase Ti3AlC were deposited on Al2O3(00l) wafers kept at temperatures between 800 and 900 degrees C. The only ternary phases observed at low temperatures (300 degrees C) were Ti3AlC and cubic (Ti,Al)C, the latter can be described as a metastable solid solution of Al in TiC similar to the more studied (Ti,Al)N system. The difficulties to form MAX phases at low substrate temperatures were attributed of requirement for a sufficient diffusivity to partition the elements corresponding to the relatively complex crystal structures with long c-axes. While MAX-phase synthesis at 800 degrees C is significantly lower than contemporary bulk sintering processes, a reduction of the substrate temperature towards 300 degrees C in the present thin-film deposition experiments resulted in stacking sequence variations and the intergrowth of (Ti,AI)C.
Article
For narrow band metals the distortions of the form of X-ray and X-ray photoelectron spectra are investigated under the influence of the vacancy on the inner level. A calculation method over the whole energy interval of the distoring influence of the hole field is proposed and realized numerically. The site formalism is developed to describe the X-ray spectra. Equations are obtained in the energetic representation which allow to effect a numerical analysis. Für Schmalbandmetalle werden die Verzerrungen der Form der Röntgen- und Röntgenphoto-elektronenspektren unter dem Einfluß einer Lücke auf das innere Niveau untersucht. Es wird eine Berechnungsmethode über das gesarute Energieintervall des Verzerrungseinflusses des ge-samten Feldes vorgeschlagen und numerisch realisiert. Der Platzformalismus zur Beschreibung der Röntgenspektren wird beschrieben. Gleichungen in der Energiedarstellung werden erhalten, die eine nuraerische Analyse ermöglichen.
Article
Die Struktur von Ti3SiC2 wird aus Einkristallaufnahmen bestimmt. Die Gitterparameter der hexagonalen Zelle sind:a=3,068,c=17,669 undc/a=5,759. Die Titan-Atome besetzen die Punktlagen 2a) und 4f) (zTi=0,135), die Silicium-Atome die Punktlage 2b) und die Kohlenstoff-Atome die Punktlage 4f) (zC=0,5675) in der Raumgruppe D 6h 4 –P63/mmc. Die Struktur gehrt zu den Komplexcarbiden mit oktaedrischen Bauelementen [T 6C].The crystal structure of Ti3SiC2 has been determined by means of single crystal photographs; the lattice parameters of the hexagonal cell were found to be:a=3.068,c=17.669 andc/a=5.759. The titanium atoms occupy the positions 2a) and 4f) (zTi=0.135), the silicon atoms 2b) and the carbon atoms 4f) (zC=0.5675) of the space group D 6h 4 –P63/mmc. The crystal structure type belongs to the class of complex carbides having octahedral groups [T 6C].
Chapter
The essential features of a full potential electronic structure method using Linear Muffin-Tin Orbitals (LMTOs) are presented. The electron density and potential in the this method are represented with no inherent geometrical approximation. This method allows the calculation of total energies and forces with arbitrary accuracy while sacrificing much of the efficiency and physical content of approximate methods such as the LMTO-ASA method.
Article
A formalism to compute x-ray spectra due to core excitations in metals by using single-particle band-structure techniques is presented and illustrated with a detailed calculation of the $K,L$, and $M$ emission and absorption spectra of palladium over 200 eV. Within the muffin-tin approximation for the potential, any spectrum can be factorized into atomiclike and solid-state contributions. The atomiclike factor is the dipole transition strength connecting a core state to a muffin-tin orbital in a free-electron metal. The solid-state factor is proportional to the density of band states with angular momentum determined by the orbital symmetry of the core state and the dipole selection rules. These projected densities of states have been calculated by using a linearized version of the augmented-plane-wave method specifically designed to cover large energy ranges. In particular, the method can describe simultaneously several principal quantum numbers of the eigenstates (e.g., $4d$ and $5d$ for palladium).
Article
A molecular‐orbital energy‐level diagram is constructed and used to interpret the metal‐ion L II,III x‐ray emission and absorption spectra from the oxides, nitrides, carbides, and borides of titanium and vanadium. It is shown that the MO model provides a more accurate description of the experimental results than did the ionic model and crossover transition theory. Specific electronic transitions are assigned to spectral components. Anomalous emission peaks which fall on the low‐energy side of the main L III band are found to originate in molecular orbitals which are primarily associated with ligand 2p and 2s levels.
Article
We present calculations on the physical properties of Ti <sub>4</sub> AlN <sub>3</sub> obtained from first principles, as well as some general observations concerning this group of materials. We further report mechanical data obtained through numerical simulations for some representative phases, and elaborate on the issue of high pressure stability. By examining the density of states of Ti <sub>4</sub> AlN <sub>3</sub>, we conclude that it is a semimetal, and we predict an opening up of the band gap around the Fermi level with increasing pressure. © 2002 American Institute of Physics.
Article
The mechanical properties of (001)‐, (011)‐, and (111)‐oriented MgO wafers and 1‐μm‐thick TiN overlayers, grown simultaneously by dc magnetron sputter deposition at 700 °C in a mixed N 2 and Ar discharge, were investigated using nanoindentation. A combination of x‐ray‐diffraction (XRD) pole figures, high‐resolution XRD analyses, and Auger electron spectroscopy was used to show that all TiN films were single crystals with N/Ti ratios of 1.0±0.05. The nanoindentation measurements were carried out using a three‐sided pyramidal Berkovich diamond indentor tip operated at loads ranging from 0.4 to 40 mN. All three orientations of MgO substrates, as‐received, exhibited identical hardness values as determined using the Oliver and Pharr method. After a 1 h anneal at 800 °C, corresponding to the thermal treatment received prior to film growth, the measured hardness of MgO(001) was 9.0±0.3 GPa. All TiN films displayed a completely elastic response at low loads. Measured hardness values, which decreased with increasing loads, increased in the order (011)≪(001)≪(111). After a 30 s postdeposition anneal at 1000 °C, however, hardness was found to be independent of load except at displacements ≫100 nm where substrate effects were apparent. TiN(001) and (111) films had hardnesses of 20±0.8 and 21±1 GPa, respectively, while data obtained from (011) layers exhibited large scatter due to surface roughness effects. Young’s moduli for annealed samples, calculated from the elastic unloading curves, were found to be 307±15 GPa for MgO (001) and 445±38 and 449±28 GPa for TiN (001) and TiN (111), respectively. © 1996 American Institute of Physics.
Article
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the- different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
Article
The dependence of technically important properties of wear resistant nitride coatings on chemical composition requires advanced methods of compositional analysis to establish optimized fabrication processes. High spatial resolution and chemical bonding specificity are provided by such techniques as Auger electron spectroscopy (AES) in the form of scanning Auger microscopy (SAM) and x‐ray photoelectron spectroscopy (XPS). A survey is given for the study of reactively sputter deposited Ti–N, Hf–N, and Ti–Al–N coatings on high‐speed steel substrates using point, line, and in‐depth distribution analysis by SAM in conjunction with sputter profiling. Capabilities and limitations of quantitative AES to determine the influence of processing parameters on coating composition are presented. XPS gives additional information on chemical bonding and on quantification. This is particularly important in cases of peak overlapping in Auger spectra (e.g., TiN).
Article
The new undulator beamline I511 at MAX-lab, now under commissioning, has been optimized for X-ray emission and photoelectron spectroscopies. Using an SX-700 high flux monochromator the accessible photon energy range is from 90 eV to about 1500 eV. The performance of the undulator agrees very well with the specifications, as shown by measurements using a photodiode. The energy resolution of the monochromator has been checked using absorption measurements in a gas cell. It was found to meet the expectations and exceeds a resolving power of 10 000 at 244 eV. The photon flux as a function of energy has been recorded as well and gives a maximum flux of 3×1013 photons/s/100 mA/0.1% BW. Beamlines I511 and I411 will be the first synchrotron beamlines making use of a so-called beam waist phenomenon, known from laser physics. We show results of ray-tracing calculations to determine the ultimate spot size on the sample location. The endstations to be used at this new beamline and their capabilities will be discussed as an example of the future use of this facility.
Article
A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
Article
Titanium nitride may be used as a hard coating material or as a dielectric in semiconductor devices whose properties for an application depend on the composition of the film. Auger electron spectroscopy is often used as a method for elemental quantification of thin film materials; however, the Auger electron spectrum of TiNx is complicated by the direct overlap of the N KL2,3L2,3 and Ti L3M2,3M2,3 at 387 eV kinetic energy.Using target factor analysis, we have isolated the Auger spectra of nitrogen and titanium from TiNx samples of different stoichiometry. The isolated spectra are then used to fit Auger spectra of titanium nitride samples by linear least squares. The combination of target factor analysis and linear least-squares fitting is demonstrated to be an accurate method for quantification of titanium and nitrogen in TiNx films.
Article
The unusual magnetic behavior of the heavier Ce monopnictides may be understood on the basis of a model Hamiltonian for a system of moderately delocalized f states hybridizing with band states. The parameters entering the theory have previously been taken as phenomenological input. We present a first-principles calculation of the parameters in the model Hamiltonian based on self-consistent, warped–muffin-tin, linear muffin-tin-orbital (LMTO) band structures calculated for CeBi, CeSb, CeAs, and CeP. With the self-consistent potential, we calculate the bands and the band-f hybridization matrix element entering the Anderson lattice Hamiltonian. The band-f hybridization potential is derived from the 4f5/2 resonance in the potential surrounding a Ce site; the f-state energy with respect to the band Fermi energy and the f-f correlation energy U are estimated by averaging f-state eigenvalues of f0, f1, and f2 Ce configurations. The result is used to calculate the anomalous crystal-field splitting of the Ce 4f5/2 manifold predicted by the model Hamiltonian for the Ce monopnictides. Due to the structure of the cubic symmetry group, band-f hybridization has a greater effect on the Γ8 quartet than on the Γ7 doublet of the 4f5/2 manifold, and the reduction of the splitting of the crystal-field levels from that expected on extrapolation from the isostructural heavier rare-earth monopnictides may be understood quantitatively on this basis. Our quantitative results are in good agreement with experimental values. We also calculate the range functions describing the anisotropic magnetic behavior of CeBi and CeSb, in fair agreement with phenomenological parameters fitted to data on those materials.
Article
A full-potential linearized muffin-tin orbital calculation is presented of titanium-carbon systems in a variety of crystallographic forms. The calculated electronic structure, total energies, and equilibrium lattice constants are determined for the ground-state NaCl structure of TiC and for prototype superlattice structures, and these results are discussed in terms of the nature of bonding found in TiC. Similar calculations are also given for WC in two of these crystalline forms, and the differing ground-state structure and equilibrium lattice constants in these two carbide materials are related to the behavior of those metallic d states which are occupied in WC and unoccupied in TiC. The behavior of these one-electron states, which stabilize WC in a simple hexagonal form, is similar to the calculated behavior of associated states in the prototype superlattice Ti-C structures, and these states are found to play a similar role in determining the structural characteristics in these systems. Some of the properties and probable stability of the various crystalline forms are also discussed in terms of our results.
Article
We have studied the structural and elastic properties of TiC, TiN, and TiO by means of accurate first-principles total-energy calculations using the full potential linear muffin-tin orbital method. The calculations are based on the density functional theory and we have used the local-density Hedin-Lundqvist parametrization as well as the generalized gradient approximation proposed by Perdew and Wang for the exchange and correlation potential. The calculated values for the equilibrium volume, bulk modulus, and elastic constants are generally in very good agreement with experiments. At elevated pressures all these compounds are predicted to undergo a structural phase transition from the relatively open NaCl structure into the more dense CsCl atomic arrangement. The predicted transition pressure for TiO can be reached in modern high-pressure laboratories. {copyright} {ital 1996 The American Physical Society.}