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Nanosintering Mechanism of MgAl2O4 Transparent Ceramics Under High Pressure
Abstract
Nanosintering behaviour of MgAl2O4 ultra-fine powder was investigated under high pressure. We found that the nanosintering mechanism under high pressure was quite different from that under ambient pressure. To shed light on the mechanism, the residual stress, grain size, and boundary situation were investigated using HRTEM and XRD, respectively. Our results show high pressure can restrain grain growth and initiate plastic deformation to eliminate pores and/or additional phases existing in triple junctions of the grains. However, the conventional sintering process is controlled by grain growth to avoid imperfections between grain boundaries when preparing transparent ceramics.
... Of particular interest are nanoceramics of magnesium aluminate spinel (MgAl 2 O 4 ) obtained by this method. [12][13][14][15][16]. Spinel is an attractive matrix for doping with a wide spectrum of ions with unfilled d and f shells [5,17]. ...
... The synthesis temperature was 600 °C, the range of applied pressures was P = 2-9 GPa, and the holding time was 10 min. These synthesis parameters were selected based on the data obtained from synthesis of transparent nanoceramics in studies [12,30]. ...
... The behavior of residual strain parameter with increasing applied pressure is of interest. A similar dependence of strain on the applied pressure was obtained by the J o u r n a l P r e -p r o o f authors of [12]. It should be noted that strain in this case is understood as a complex of factors that distort the lattice parameter a. ...
Optically transparent Mg1-xMnxAl2O4 (x = 0.005; 0.00005) nanoceramics were obtained by thermobaric synthesis. Varying the parameters of thermobaric synthesis leads to a controlled modification of crystallographic parameters, such as the lattice constant, the region of coherent scattering, and residual strain. It is shown that as a result of thermobaric pressing, intrinsic and impurity structural defects are formed, the concentration of which depends on the P-T parameters. Two stages of thermobaric synthesis were found, differing by both crystallographic and optical parameters. In optically transparent nanoceramics, a correlation between the residual strain and a scattering coefficient associated with localization of impurity manganese ions in anti-site positions of spinel was found. Thermobaric synthesis of optical nanoceramics can be used as a method of compressing nanosized objects in an optical spinel matrix while maintaining such objects in a metastable state.
... The synthesis of such ceramics is often carried out by holding the compact from nanopowder at high temperature (up to 1800 K) and relatively low pressure up to 200 MPa (isostatic holding) [1]. Recent studies have shown the possibility of producing ceramics of aluminum-magnesium spinel using the thermobaric hardening method, which implies a synthesis temperature of about 900K, with a pressure of 2-8 GPa [2]. Thermobaric hardening of nanopowder allows to save grains within tens of nanometers, while achieving high transparency of such ceramics. ...
... The physical characteristics of nanoceramics are sensitive to the applied pressure. In [2][3][4], it was shown that the lattice constant and the grain size change with increasing pressure. The observed shifts in the structural characteristics of nano-MgAl 2 O 4 in turn cause a change in the strength of the crystal field acting on impurity defects, which makes it possible to control the optical properties of such defects [5]. ...
... The presence of F + centers in can be explained by a large number of anionic defects on the surface of the nanoparticles forming the grain boundaries and interfaces. Under the action of high pressures, as a result of elastic deformation [2], electrons are emitted from dangling bonds, which are localized on anion vacancies with the formation of F + centers. ...
MgAl2O4 nanoceramics was synthesized by thermobaric hardening. The six narrow lines in the ESR spectrum are the hyperfine structure of the uncontrolled Mn²⁺ impurity. Thermobaric hardening of nanoceramics leads to the formation of paramagnetic F⁺ centers. A change in the hyperfine structure constant (A) of the impurity Mn²⁺ and the formation of an additional signal with g = 4.3, as well as the appearance of a second hyperfine splitting with a similar g- factor, but less A parameter. Appearances of new g-factor and additional A are presumably associated with an increase in the coordination number Mn²⁺, as a result of the incorporated to the octahedral position of Al³⁺.
... 4 Comparing to the approaches [1][2][3][4] which prolonged sintering at high temperatures (above 1000 C) to promote atomic diffusion, boundaries migration, grain growth, and eventually elimination of pores, high-pressure sintering of ceramics has several advantages, namely, (1) lower sintering temperature, (2) shorter sintering time, (3) inhibition of the grain growth, (4) ease of fabrication, (5) less expensive, etc. Prior studies have shown the feasibility of MgAl 2 O 4 , hydroxyapatite, and YAG nanoceramics fabrication by means of this technique as reported earlier. [5][6][7][8] The extreme pressure could initiate the plastic deformation to eliminate pores of the products and enhance the local atomic diffusion at the grain boundaries. 6,8,9 However, it is difficult to monitor the plastic deformation in the nanocrystalline ceramics sintering under high pressure. ...
... [5][6][7][8] The extreme pressure could initiate the plastic deformation to eliminate pores of the products and enhance the local atomic diffusion at the grain boundaries. 6,8,9 However, it is difficult to monitor the plastic deformation in the nanocrystalline ceramics sintering under high pressure. Shock loading applying to micro-sized spherical alumina particles has provided insight into processes for deformation with maximum pressures of 13-26 GPa and a maximum adiabatic temperature about 400 C. 10 The density of the achieved compact was just 90% of theoretical. ...
... This method is based on the measurements of diffraction peaks broadening after correcting for the instrumental broadening and has been extensively applied in the field of high pressure research to study the yield behavior of materials. 6,8,9,[16][17][18][19] The starting grain size of our sample was in range of 2-52 lm. Since peak broadening due to grain size reduction is not significant for grains with dimensions >0.1 lm and we did not observe any obvious grain size reduction after high pressure treatments of the samples, the grain size effects on the XRD peak width were neglected in the data analysis. ...
Plastic deformation of alumina (Al2O3) under high pressure was investigated by observing the shape changes of spherical particles, and the near fully dense transparent bulks were prepared at around 5.5 GPa and 900 °C. Through analyzing the deformation features, densities, and residual micro-strain of the Al2O3 compacts prepared under high pressures and temperatures (2.0–5.5 GPa and 600–1200 °C), the effects of plastic deformation on the sintering behavior of alumina have been demonstrated. Under compression, the microscopic deviatoric stress caused by grain-to-grain contact could initiate the plastic deformation of individual particles, eliminate pores of the polycrystalline samples, and enhance the local atomic diffusion at the grain boundaries, thus produced transparent alumina bulks.
... While it is agreed that the cubic spinel lattice is, in general, intrinsically non-birefringent, such a strict criterion may be relaxed in the present case where the lattices of individual grains are expected to be in non-equilibrium stress states [13]. The very high pressures and low temperatures used in the sintering technique can be expected to strain the lattice and introduce residual stresses ($100-1300 MPa [13]) which can produce stress-induced birefringence in magnesium aluminate spinel [14,15]. ...
... While it is agreed that the cubic spinel lattice is, in general, intrinsically non-birefringent, such a strict criterion may be relaxed in the present case where the lattices of individual grains are expected to be in non-equilibrium stress states [13]. The very high pressures and low temperatures used in the sintering technique can be expected to strain the lattice and introduce residual stresses ($100-1300 MPa [13]) which can produce stress-induced birefringence in magnesium aluminate spinel [14,15]. Typical stress levels in microstructures of optical materials can induce a refractive index change between ±0.01 [16] and thin film deposition experiments (which may be highly strained and/or preferentially oriented) demonstrate that strain can easily vary the refractive index of magnesium aluminate spinel by $2%, from 1.69 to 1.73 [17]. ...
... From Eq. (3), the required residual stress to produce Dn = 0.005 can be calculated as $1532 MPa for diamond and $213 MPa for zinc sulfide. Measured residual stresses in transparent nanocrystalline spinel ceramics produced by high-pressure sintering can range from $100 MPa to 1300 MPa [13], which reach much higher values than that required for zinc sulfide and are only slightly less than those required for diamond to produce Dn = 0.005. The required levels of residual stress creating higher Dn in spinel are probably closer in magnitude to that calculated for diamond, since spinel has a significantly higher elastic stiffness and is a much harder material than zinc sulfide. ...
A response is provided to comments by Krell concerning the validity of the Hall-Petch relationship and the optical transmission in nanocrystalline ceramics discussed in a recent Acta Materialia paper. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
... Nanocrystalline ceramics have outstanding potential for use in structural applications due to their superior mechanical properties, high-temperature resistance (no glass transition), and excellent optical transparency [1]. Thus, there have been extensive studies to improve their mechanical properties [2,3]. So far, the reduction of grain size has been considered as the most effective method to enhance the mechanical properties of polycrystalline ceramics [4,5]. ...
... We also confirmed from the thinnest region (the brighter regions in the channels that all nanoscale pores lie at triple junctions in the grain boundary network (Fig. 2). Based on our literature survey, we confirmed almost all previous TEM work on nanocrystalline ceramics has used bright-field TEM imaging to check the sintering quality of their samples [2,23,[42][43][44][45]. Due to the chemically homogeneous microstructure of most ceramics, one can assume that there is no need to use high-angle annular dark-field imaging for microstructural analysis. ...
To develop transparent materials with superior mechanical properties, nanocrystalline magnesium aluminate (MgAl2O4) spinel with grain sizes ranging from 3.7 to 80 nm has been synthesized by environmentally controlled pressure assisted sintering. In this study, we investigated the microstructure and grain size dependence of the mechanical properties of nanocrystalline MgAl2O4 by performing transmission electron microscopy, nanoindentation, uniaxial micropillar compression, and micro-cantilever bending. Electron microscopy confirmed that the environmentally controlled pressure assisted sintering technique produces a nearly fully dense grain structure with a porosity of less than 1% in larger grain-sized ceramics and observably pore-free grain structures in the smaller grain-sized ceramics. Mechanical characterization revealed that nanoindentation hardness, compressive fracture strength, and fracture toughness each exhibit distinct grain size dependence. Our experimental results and numerical analyses point to a change in dominant strain accommodating mechanisms from dislocation-based plasticity to shear banding as the grain size is reduced, as previously suggested by the literature. Practical implications of the change in strain accommodation mechanisms manifest as the emergence of indentation size effect, weak grain size dependence of hardness and strength, and a ∼2-fold increase in apparent fracture toughness for the smaller grain-sized ceramics.
... In general, it is challenging to fabricate MgAl2O4 transparent nano-ceramics using traditional pressureless sintering techniques [73,78]. Therefore, complex sintering strategies, such as hot pressing, HIP, and SPS, are often employed to develop these ceramics [74][75][76]79]. In 2010, Meir et al. [80] synthesized and densified MgAl2O4 transparent nanoceramics using the SPS method. ...
... In general, it is challenging to fabricate MgAl 2 O 4 transparent nano-ceramics using traditional pressureless sintering techniques [73,78]. Therefore, complex sintering strategies, such as hot pressing, HIP, and SPS, are often employed to develop these ceramics [74][75][76]79]. In 2010, Meir et al. [80] synthesized and densified MgAl 2 O 4 transparent nano-ceramics using the SPS method. ...
Transparent nano-ceramics have an important high-transmittance, material-integrating structure and function and a variety of potential applications, such as use in infrared windows, optical isolators, composite armors, intelligent terminal screens, and key materials of solid-state lasers. Transparent ceramics were originally developed to replace single crystals because of their low fabricating cost, controllable shape, and variable composition. Therefore, this study reviews and summarizes the development trends in transparent nano-ceramics and their potential applications. First, we review the research progress and application of laser nano-ceramic materials, focusing on the influence of controllable doping of rare earth ions on thermal conductivity and the realization of large-scale fabrication technology. Second, the latest research progress on magneto-optical transparent nano-ceramics, mainly including terbium gallium garnet (Tb3Ga5O12, TGG) ceramics and terbium aluminum garnet (Tb3Al5O12, TAG) ceramics, are summarized, and their performance is compared. Third, the research progress of transparent armor nano-ceramic materials, represented by MgAl2O3 and Aluminum oxynitride (AlON), are reviewed. Lastly, the progress in electro-optical transparent nano-ceramics and scintillation transparent nano-ceramics is reported, and the influence of the material-fabrication process on electro-optic effect or luminous intensity is compared. Moreover, the effect of particle diameter on fabrication, the relationship between nano powder and performance, and different sintering methods are discussed. In summary, this study provides a meaningful reference for low-cost and sustainable production in the future.
... While the application of pressure (< 500 MPa) in hot isostatic pressing or spark plasma sintering decreased the sintering temperature to about half the melting point [9][10][11]. Interestingly, high pressure (> 1 GPa) can further decrease the sintering temperature of many high melting transparent ceramics such as alumina [12][13][14], MgAl 2 O 4 [15][16][17], YAG [18,19], cubic boron nitride (cBN), and diamond [20,21] to even 1/3 melting point. The spinel powder can be compressed to transparent bulk even at room temperature and 5 GPa [17], and γ-Al 2 O 3 powder compressed at 5.6 GPa and room temperature can yield a Vickers microhardness of 5.7 GPa at an applied load of 50 gf [22]. ...
... Microstructures of the samples were investigated by transmission electron microscope (TEM) (FEI Tecnai G2 F20 S-TWIN) and scanning electron microscope (SEM) (FEI Inspect F50). [13,16,19]. The relative density of this sample was about 98%, which is inaccurate due to cracks and Ta package. ...
Cold bonding of metals necessitates high normal/frictional loads, or ultrahigh-vacuum treated atomically flat ductile surface. In case of ceramics composed of ionic and/or covalent bonds, the high diffusion barrier of ions or atoms gives them a constraint to bond together at room temperature. However, from this investigation, it has been found that alumina crystals could be well bonded at room temperature under high pressure. Also, it has been noticed that if the pressure is capable of yielding and breaking down alumina crystals, it could then re-bond the fractured crystals by reducing the distance of their surface atoms to meet the condition for bonding. Overall, we demonstrate a process for the cold bonding of alumina under high pressure with grain fracturing and re-bonding.
... A method of producing transparent ceramics of pure AMS (without the addition of LiF) is described in [9]. The properties of such ceramics are largely determined by intrinsic defects (cation and anion sublattices of the MgAl 2 O 4 system) [10]. The method is based on great pressure and simultaneous heating of the sample to temperatures not more than: 500-700°C for 30-60 minutes [9][10][11]. ...
... The properties of such ceramics are largely determined by intrinsic defects (cation and anion sublattices of the MgAl 2 O 4 system) [10]. The method is based on great pressure and simultaneous heating of the sample to temperatures not more than: 500-700°C for 30-60 minutes [9][10][11]. The special features of such ceramics is the absence of LiF additive during synthesis, lower temperature and pressing time, as well as nanosize of crystallites and their narrow size distribution. ...
Nanoceramics of aluminum-magnesium spinel doped with gadolinium ions were obtained. Morphological features of such ceramics were given. Optical absorption spectra were analyzed. There is an intense peak of thermally stimulated exoelectronic emission in opaque nanoceramics. Manufactured ceramics are promising for utilizing as material for electron radiation registration.
... The mechanism of this jump is still unclear. It is possible that the yield strength of the material is reached, and as a result, a sharp increase in the concentration of intrinsic defects with the displacement of manganese centers into oxygen octahedra [41]. ...
Optically transparent nanoceramics of the composition Mg1-xMnxAl2O4(x=0.005) with a grain size of 15–40 nm were obtained by thermobaric synthesis in a toroid-type high-pressure chamber in the range P = 4–9 GPa at a temperature of 600 °C. EPR spectroscopy indicates the presence of divalent manganese ions with a characteristic HFS signal. In addition, the signal from F+ centers were registered in the EPR spectrum. It has been established that in the HFS of impurity Mn2+ there is an additional contribution from manganese centers split in the field of rhombic symmetry. Photoluminescence spectroscopy methods revealed the presence of optically active centers based on Mn3+ and Mn4+. With an increase in the synthesis pressure, the conversion of manganese ions localized in octa- and tetra-positions occurs, accompanied by a change in valence. It has been found that some manganese ions with a valence of 2+ can be localized in an octahedral oxygen environment, occupying anti-site positions with reduced point symmetry. The formation of F-type centers, as well as polyvalent manganese centers, was confirmed by the cathodoluminescence method. Changing the synthesis parameters makes it possible to control the ratio of valence manganese ions in the corresponding sites of the spinel crystal lattice, thereby expanding the methodological base of defect nanoengineering.
... As a transparent ceramic, transparent PcBN has extensive potential applications as window materials in extreme conditions owing to its remarkable performance. Porosity and grain boundaries are the primary factors impacting the optical transparency of PcBN [9][10][11]. Being the second most impregnable material, the hardness and low plasticity of cBN make it difficult for pores to close. ...
... The middle and far infrared band of electromagnetic radiation remain underdeveloped due to the limited amount of transparent materials in these ranges. Among infrared glasses, chalcogenides [1][2][3][4][5], fluorides [5][6][7] of various systems are known, among ceramics -borates [8], aluminummagnesium spinels [9,10], silver and thallium (I) halides [11]. Also, a separate class is crystalline materials, which include crystalline silicon [12], silver and thallium (I) halides [13][14][15][16]. ...
... Sintering of spinel ceramics can be implemented using various technological approaches, such as low-temperature sintering, hot pressing (HP) [15,16], hot isostatic pressing (HIP) [17,18], and Spark Plasma Sintering (SPS) [19][20][21] In recent years, the SPS method has been widely used to consolidate powders of various types of materials. The main reason is that, compared with well-known HP or HIP technologies, high heating rates, exceeding 50 • C/min, can reduce the total processing time when compacting the powder. ...
In the present study, the concentration series of MgAl2O4:Ce3+ ceramics have been fabricated by the Spark Plasma Sintering (SPS) method. Cerium-doping concentration was varied within a range of 0.1–5 wt.%. The prepared ceramics have been tested using the various experimental techniques: X-ray diffraction (XRD), scanning electron microscopy, as well as optical and cathodoluminescence spectroscopy. According to XRD, all synthesized samples are biphasic with structural impurities. The cerium ion concentration effect on the cathodoluminescent characteristics of MgAl2O4:Ce3+ ceramics has been studied in terms of emission intensity and decay time. Before annealing the concentration, quenching is observed. The optimal doping Ce3+ concentration was determined to be 5 wt.% after temperature annealing at 1300 °C. The successfully prepared spinel ceramics could be potentially applying for high-energy electrons detection.
... Fabricating transparent MgAl 2 O 4 ceramics via pressureless sintering is challenging [34,148] and therefore, advanced sintering techniques like hot pressing (HP), hot isostatic pressing (HIP), and spark plasma sintering (SPS) are required for its manufacturing [97,[149][150][151][152]. Sintering aids used to produce transparent spinel ceramics are LiF [153], B 2 O 3 [154] , and CaO [155,156]. ...
Owing to superior properties, i.e. high hardness, high wear resistance, and weight reduction of transparent ceramics (TCs) over glasses, TCs have shown promising tribological potential for applications such as face shields, explosive ordnance visors, windows for aircraft, spacecraft and, re-entry vehicles, electromagnetic windows, laser igniter windows, screens for smartphones and more. Researchers globally have been attracted to explore more about TCs, considering the tremendously increasing demand over different other transparent materials. The optical quality of TCs is mostly characterized by the in-line transmittance, and the effect of various processing parameters on transmittance has already been studied by various researchers. In this review, the current research progress regarding tribological performance of TCs is compiled. TCs with potential in tribological applications include MgAl2O4, Al2O3, AlON, Lu2O3, c-BN, Y2O3, Si3N4, and SiAlON. The relevant strategies to improve the tribological properties, including microstructures and mechanical properties are comprehensively discussed. In addition, the mechanisms of material removal of different transparent ceramics are also presented. It is well observed that surface fracture comprising three stages is found as one of the dominant wear mechanisms during wear. This review aims to provide some meaningful guidelines for development of transparent ceramics with enhanced wear resistance, while identifying the wear mechanisms in particular wear conditions.
... Although the introduction of hBN can play a toughening effect, as a typical layered material, its hardness is far less than that of superhard cBN. As an advanced sintering technology, high-pressure sintering has been developed and used to sinter both superhard ceramics (e.g., diamond and cBN) [24,[33][34][35] and transparent ceramics [36][37][38]. The high pressure (>1 GPa) increases the density, while simultaneously limiting grain growth. ...
... Magnesium aluminate (MgAl 2 O 4 ) is a normal spinel and finds applications in optically transparent ceramics, neutron radiation resistance, refractory, catalyst and catalyst support etc. [35][36][37][38]. There are some literatures on MgAl 2 O 4 based resistive/impedance-based humidity sensors [27][28][29][30][31]. ...
In this paper we have reported on the excellent humidity sensing properties of screen-printed magnesium aluminate (MgAl2O4) microporous, semi-thick film, capacitive sensor in the range of 2%–98% RH. MgAl2O4 nanoparticles were prepared by a facile solid-state reaction route. Screen printing technique was employed to prepare the MgAl2O4 microporous semi-thick film. The binders used in screen-printing generated micropores on the film surface during firing of the film. The as prepared nanoparticles and the thick film were characterized by different sophisticated techniques, viz. X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray (EDX), BET-surface area analysis, laser diffraction-based particle size analysis, and contact angle measurement. Parallel plate capacitive configuration was used whereby the sensor film was interposed between copper bottom and silver top electrode. The sensor exhibited high and stable response up to 98%RH with a lower detection limit of 2%, which is seldom reported. Further we observed low dissipation (<2), fast response (∼66 s) and recovery (∼71 s) times, low hysteresis (∼109 pF at 89% RH), excellent repeatability (at least 15 cycles), and long-term stability (at least 6 months). This sensor is suitable for commercial applications in tea industry, food packaging, hospitals etc.
... Transparent superhard materials are of great interest for scientific and practical applications because of their optical and ultratough mechanical properties in extreme conditions. 1 With a Knoop hardness of $110-140 GPa, polycrystalline diamond is widely known as being the hardest transparent material, 2 but its industrial applications are limited by its high cost and its challenges in being shaped. Since the successful preparation of transparent alumina ceramics in 1957, 3 several transparent ceramics have been synthesized successively, such as Y 2 O 3 , 4-6 MgO, 7 ZrO 2 , 8 MgAl 2 O 4 , 9,10 and cubic Si 3 N 4 (c-Si 3 N 4 ). 1 Of the aforementioned transparent ceramics, c-Si 3 N 4 has the highest hardness of $34 GPa, 1 but that relatively low hardness limits the industrial applications of such transparent ceramics. Therefore, it is necessary to synthesize economical superhard transparent ceramics. ...
Polycrystalline cubic boron nitride (PcBN) has been synthesized at 14 GPa and high temperatures of 1300–2000 °C in a two-stage multi-anvil cell. Sintered PcBN synthesized at 1700–1800 °C and 14 GPa with a grain size of ~200 nm is optically transparent with a transmittance of ~70% at wavelengths of 400–1500 nm and has the Vickers hardness of ~63-69 GPa. Analyses with scanning and transmission electron microscopy reveal that PcBN can be strengthened by introducing nanometer-scale grains and microscopic defects at high pressure and temperature. The optical transparency of the bulk PcBN synthesized at high pressure and temperature can be explained by the very thin intergranular films between grains. The present sintered PcBN is the second-hardest transparent material after diamond and can be used for windows in extreme conditions.
... It also has an acceptable toughness and high hardness [18,19]. The MgAl 2 O 4 spinel as a kind of optical materials has a broad range of applications in different engineering parts, including the medical application (implants, anti-wear coatings), optical lenses, smartphones (anti-wear and shockproof plates), watches (anti-wear and anti-shock lenses with high strength), cars, and etc [20][21][22][23]. The principal aim in the development of MgAl 2 O 4 spinel ceramics is to achieve optical transparency and suitable mechanical properties. ...
In this study, to achieve high transparency of magnesium aluminate spinel, the ready-to-sinter spinel (RSS) powder was sintered by spark plasma sintering (SPS). To prepare the RSS powder, LiF nanopowder was synthesized at diverse temperatures and concentrations on the spinel powder surface (0.7 wt %). The optimal temperature and concentration were achieved to be (60 °C) and (200 g/l), respectively. Then, the spinel body was fabricated by SPS method. The XRD, BET, ICP, FESEM and TEM analyses were applied for the characterization of the prepared powder. Also, the results of TEM and XRD analysis confirmed the presence of the fluoride light element (LiF compound) on the spinel particles surface. Finally, the final strength and density of the spinel body were measured 97.8 MPa and 99.98% respectively, with the transmission of 86.8% at the wavelength of 1100 nm after sintering at 1100 °C.
... In this case, the charge compensation is carried out by the formation of a pair of such defects [Al 3+ ] and [Mg 2+ ] so that the neutrality of the crystal lattice is, in general, kept [14]. In ceramics obtained by the thermobaric treatment, a great amount of antisite defects exist due to the quenching of the nonequilibrium state of the system under high pressure as a result of fast removal of the pressure and temperature [15,16]. The studies of paramagnetic features of single-crystal AMS and ADs showed that such centers do not have a resonance absorption of electromagnetic energy in a magnetic field, but can significantly distort the signal of paramagnetic centers present nearby (for example, F + centers) [13]. ...
The effect of structure and size parameters on the formation of intrinsic and impurity paramagnetic centers in nanoceramics of aluminum–magnesium spinel is studied. The studied samples (grain size ~30 nm) are obtained by thermobaric synthesis. Microcrystalline ceramics and MgAl2O4 single crystal are used as the reference samples. Characteristic paramagnetic centers of Mn2+ (hyperfine structure constant (HFS) A = 82 G) are present in both single crystal and microceramics. In the studied samples of nanoceramics in the initial state, both impurity Mn2+ and intrinsic F+ centers exist. Unlike the nanoceramics, the centers of F+ type in the reference sample appear only after the irradiation with accelerated electrons (130 keV). The parameters of Mn2+ centers in nanoceramics significantly differ on that in microceramics and single crystal. EPR signal of Mn2+ centers in nanoceramics is characterized by two anomalous constant HFS (A1 = 91.21 G, A2 = 87.83 G) caused by two types of octahedrally coordinated manganese ions ([Mn2+] antisite defects). The features of spectral parameters of manganese centers correlate with a decrease in the cell parameter of MgAl2O4 in the nanostructural state. The observed effects are interpreted based on the assumed scheme of [Mn2+] charge compensation by the aluminum antisite defect and F+ center.
... Зарядовая компенсация в данном случае осуществляется путем образования пары таких дефектов [Al 3+ ] Mg 2+ и Mg 2+ ] Al 3+ таким образом, что в целом сохраняется нейтральность в кристаллической решетке [14]. В керамиках, синтезированных в результате термобарической обработки, дефекты катионного перемешивания присутствуют в большем количестве, в следствие закалки неравновесного состояния системы под высоким давлением в результате быстрого снятия давления и температуры [15,16]. Исследования парамагнитных особенностей монокристаллической АМШ с ADs показали, что такие центры не обладают резонансным поглощением электромагнитной энергии в магнитном поле, но могут в значительной степени искажать сигнал присутствующих рядом парамагнитных центров (например, F + -центров) [13]. ...
The influence of structural and dimensional factors on the formation of intrinsic and impurity paramagnetic centers in nanoceramics of aluminum-magnesium spinel is studied. The studied samples (with a grain size of 30 nm) were obtained by thermobaric synthesis. Microcrystalline ceramics and a MgAl2O4 single crystal were used as standards. The single crystal and microceramics contain characteristic Mn2+ paramagnetic centers (hyperfine structure constant (HFS) A = 82 G). In the studied nanoceramic samples in the initial state, both impurity Mn2+ and intrinsic F+ centers are detected. In contrast to nanoceramics, in reference samples centers of the F+ type appear only after irradiation with 130 keV by accelerated electrons. The parameters of the Mn2+ centers in nanoceramics differ significantly from those in microceramics and single crystals. For the Mn2+ center in nanoceramics, the EPR signal is characterized by two anomalous HFS constants (A1 = 91.21 G, A2 = 87.83 G) caused by two varieties of octahedrally coordinated manganese ions (anti-site defects [Mn2+]Al3+). The specific features of the spectral parameters of manganese centers correlate with a decrease in the lattice parameter of MgAl2O4 in the nanostructured state. The observed effects are interpreted based on the proposed charge compensation scheme of [Mn2+]Al3+ with an aluminum anti-site defect and an F+ center.
... High pressure can largely decrease the sintering temperature of high melting point ceramics like alumina [1][2][3][4], MgAL 2 O 4 [5,6], cubic born nitride (cBN) [7][8][9] and even diamond [10,11]. Through high pressure sintering, nano-sized twinning or stacking faults were produced inside grains, and bulk ceramics with excellent mechanical and thermal properties can be obtained. ...
High pressure obviously decreased the sintering temperature of high melting point ceramics and produced bulks with excellent mechanical and thermal properties. In this study, the effect of pressure alone on the densification process of alumina ceramics was demonstrated. The translucent alumina bulks with unexpected hardness were obtained by compressing α-Al2O3 powder under ultra-high pressure at room temperature. Through analyzing microstructure and hardness of cold compressed samples, the results reveal that ultra-high pressure could crush alumina grains under appropriate pressure, and initiate plastic deformation of grains when the applied pressure exceeds the yield strength of alumina. Thus to enhance the hardness of the samples by grain boundary strengthening and plastic deformation at higher pressure.
... Owing to the extreme requirements, the sintering method must be carefully selected, including hot pressure [5], hot isostatic press [6], vacuum sintering [7], spark plasma sintering [8] and microwave sintering [9]. Among all of the sintering methods mentioned above, SPS has attracted more and more attention over last decades because of its advantages, such as low sintering temperature, high heating and cooling rate, fast sintering process and environmental friendly. ...
The effect of heat treatment on the in-line transmittance of BaZr0.5Ce0.3Y0.2O3−δ (BZCY532) ceramics prepared by spark plasma sintering method was investigated. The loss of Ba in transparent BZCY532 ceramics is the key reason for the loss of transmittance during the annealing process. This problem can be effectively alleviated by using a powder bed of BZCY532. Heat treatment atmospheres, wet air and dry air, were also found to be critical for obtaining high quality transparent ceramics. A highly transparent BZCY532 ceramic with the in-line transmittance (Tin) of 71.4% at 2000 nm can be obtained by using SPS method followed by an annealing in powder bed at 1500 °C in wet air.
... High pressure experiments were carried out using a cubic-type mutlianvil high pressure system. 21,22 The Si 2 BC 3 N powders were compacted into cylinders, placed into a BN capsule, and then in a pyrophyllite cell, and finally sintered at 1000°C-1400°C for 30 min under 5 GPa obtaining Si 2 BC 3 N ceramic monoliths. ...
The crystallization behavior of amorphous Si2BC3N monoliths by heating at 1000°C–1400°C and 5 GPa was investigated with the special attention to the nucleation mechanisms of β-SiC and BN(C) phases. Nanoscale puckered structures arising in particle bridging areas were found and its evolution behavior well reflected the nucleation process of nanocrystallites. The temperature-dependent crystallization of amorphous Si2BC3N monoliths at 5 GPa passes through four stages: The material remains amorphous below 1100°C. It undergoes partial phase segregation (1100°C–1200°C), followed by initiation of nucleation (1200°C–1250°C), and then nucleation and growth of β-SiC and turbostratic BN(C) crystallites (>1250°C). The first principles calculation indicates the nucleation precedence of BN(C) phase over β-SiC. BN(C) nucleates preferentially at bridges between ceramic particles causing SiC to concentrate in particle interiors thus forming capsule-like structures.
... Pressure was calibrated against the ram load using the pressurefixed points of Bi, ZnS, and GaAs at room temperature Zou, 2009, Zou et al., 2010. The effect of temperature on pressure was further corrected using the αβ and βγ phase transitions of olivine (Katsura and Ito, 1989;Yamada et al., 2004). ...
High-pressure and high-temperature phase relations for Mg3Cr2Si3O12 have been studied at pressures from 8 to 16 GPa and temperatures of 1200-1800 degrees C using a Kawai-type multianvil apparatus. The low-pressure phase assemblage of MeSiO3 pyroxene + Cr2O3 eskolaite was found to transform into a high-pressure phase assemblage of majoritic knorringite + eskolaite with a negative phase boundary of dP/dT = -0.010 GPa/degrees C. No pure Mg3Cr2Si3O12 knorringite garnet was observed over the entire range of the P-T conditions in the present study, suggesting that pure knorringite garnet may not be a stable phase under these pressure and temperature conditions. This result is inconsistent with earlier studies, where knorringite was reported to be stable at pressures higher than similar to 10 GPa. On the other hand, the present result agrees well with recent results for the synthesis of majoritic knorrineite at pressures of 11 and 14 GPa and at temperatures of 1500-1600 degrees C. Majoritic knorringite becomes more Cr-deficient with increasing pressure, which is similar to the Al-deficient nature of majoritic garnet in the MgSiO3-Al2O3 system.
... It was found that sintering mechanism under high pressure is quite different from that under ambient pressure [20]. It was stated that pressure can restrain grain growth and initiate plastic deformation to eliminate pores and additional phases that can exist in triple junction of grains. ...
Recent improvements in powder synthesis and ceramics sintering made it possible to fabricate high-quality
materials. Of particular interest to this work is the structural and optical characterization of (Y3Al5 O12 , YAG) ceramics fabricated by high-pressure low-temperature technique (HPLT). We study the effect of sintering temperature on structural and optical properties of YAG ceramics fabricated under pressure P = 7.5 GPa. Structural properties studies of sintered ceramic samples were carried out using X-ray diffraction and FTIR techniques. Optical scattering model under the Rayleigh-Gans-Debye approximation has been applied to estimate average grain size for YAG ceramics. It was found that effective scatterer size does not exceed 200 nm. In-line and total transmittance (TT) nonlinear optical response (NLO) of ceramic samples was studied with laser beam self-action technique. The beam of mode-locked Nd:YAG laser with Gaussian spatial profile (42 ps pulse duration, repetition frequency 5 Hz) was used. All studied samples demonstrate photoinduced darkening that depends on the ceramics sintering temperature. Estimated values of imaginary part of NLO cubic susceptibility at wavelength of 1064 nm is Im(χ(3)) ∼ 10−11 esu.
In this paper, the electron beam vacuum coating method was used to coat a SiO2 film on an MgAl2O4 spinel substrate. The thickness of the coating was aimed to be 925 nm based on the physics of the anti‐reflection coatings. Atomic force microscope (AFM) images revealed that the coated silica was 880 nm thick, which is close to the aimed theoretical thickness and had 2.11 nm roughness. It could enhance the transparency of the spinel substrate by being coated on it. The infrared transmittance of the sample coated with SiO2 film in the range of 3700 nm‐4800 nm was measured by a Fourier transform infrared spectrometer and reached 92.5% to 78.5%, which was about 2–4% higher than that of MgAl2O4 spinel. In addition, it was discovered that the bonding force between the coating and the substrate is determined to be about 200 MPa. The results of this study can be used for further precise design and production of anti‐reflection coatings on the transparent materials that need more transparency. This article is protected by copyright. All rights reserved
Multicomponent alloys prepared using different experimental methods under atmospheric pressure have been widely studied. However, research on multicomponent alloys involving methods using high pressures and high temperatures is still in its infancy. In the high-pressure CoCrFeNiMox alloy, the combination of each element atom and the change in the sample crystal structure indicate a relationship between the evolution mechanism of the multi-element alloy microstructure and high pressure. We observe that molybdenum (Mo) atoms enter the medium-entropy alloy (MEA) lattice near its theoretical recrystallization temperature to form a high-entropy alloy (HEA). The diffusion of Mo atoms requires the energy provided by the temperature to overcome the low-entropy effect under high pressure, Mo atom reacts with MEA near its recrystallization temperature to form HEA means the formation of HEA under high pressure follows the specific steps. High pressure inhibits grain growth and generates numerous dislocations in the crystal, thereby preparing the high-pressure CoCrFeNiMox alloy with a high hardness of 3.7 ± 0.12 GPa, which is harder than the CoCrFeNiMox alloy prepared under atmospheric pressure. Additionally, the generalized high-pressure HEA constant is introduced, which is 1 ≥ P ≥ 0.17, proving that high-pressure promotes the synthesis of HEAs. This constant provides a new method for studying the thermodynamic behavior of HEAs under extreme conditions.
Reciprocating pressure-induced phase transition (RPPT) has been proposed as a new approach to synthesize nanostructured bulk materials with clean grain boundary interfaces for structures that undergo reversible pressure-induced phase transitions. The modulation effects on grain size under different cycle numbers of RPPT for InAs were investigated and the initial single-crystal bulk, with a dimensional size of about 30 μm, was transformed into a nanostructure with an average grain size of 7 nm by the utilization of the in situ high-pressure diamond anvil cell (DAC) technique. To verify the DAC findings, compact nanostructured bulk InAs with grain sizes ranging from 2-20 nm (average = 8 nm) and large dimensions (3.2 mm × 3.2 mm × 0.5 mm) was successfully synthesized from single-crystal InAs using a large volume press (LVP). The smaller work function (3.86 eV) and larger bandgap energy (2.64 eV) of the compact nanostructured bulk InAs phase compared to those of single-crystal InAs demonstrated that the nanostructure affected the macroscopic properties of InAs. The findings confirm the feasibility of synthesizing nanostructured bulk materials via RPPT.
This article is devoted to research of the TlBr0.46I0.54 – AgI system' phase diagram as part of the AgBr – AgI – TlBr – TlI isothermal section of the Ag – Tl – Br – I concentration tetrahedron. The study was carried out by means of differential thermal and X-ray diffraction analyzes, on the basis of which homogeneous and heterogeneous regions were obtained. Within these regions as suitable compositions for the synthesis of optical crystals and ceramics as the phases and crystal lattices parameters were determined. It is possible to obtain crystals and ceramics in the range from 0 to 43 mol. % AgI in TlBr0.46I0.54. The research results made it possible to clarify the areas of crystals and ceramics synthesis within the AgBr – AgI – TlI – TlBr isothermal section. Crystals and ceramics based on the TlBr0.46I0.54 – AgI system are intended for wide application in optics and photonics of the visible, infrared and terahertz ranges.
Tough and hard ultrafine-grained B4C-cBN composites were firstly fabricated by high-pressure sintering mixed B4C and cBN nanopowders at 6 GPa and 1700 °C. The phase transition from cBN to hBN is avoided by high pressure during the sintering process. The effects of the cBN content on the densification and mechanical properties of B4C-cBN composites were evaluated. The results indicated that the hardness of the as-fabricated composites increased gradually with the increase of cBN content. The composite composed of 50 wt.% cBN exhibited excellent comprehensive mechanical properties with relative density of 98.6%, density of 2.9 g/cm³, Vickers hardness of 36.2 GPa and fracture toughness of 6.7 MPa m1/2. The introduction of superhard cBN maintained the lightweight and high hardness while enhancing the fracture toughness of the B4C. The main toughening mechanisms were crack bridging, crack deflection and pull-out of homogeneously dispersed cBN grains.
Herein, we report transparent LaGdZr2O7 nano-grained ceramics with an average grain size of 10 nm prepared using the high-pressure sintering method at 5.0 GPa/450 °C. These ceramics showed improved plastic deformation as compared to transparent micro-grained LaGdZr2O7 ceramics. Moreover, the fracture toughness of the nano-grained ceramics reached up to 3.4 MPam1/2, which is 79 % higher than that of the micro-grained samples. This improvement in the fracture toughness of the nano-grained ceramics was associated with the presence of a large number of weak grain boundaries in them.
The in situ axial X-ray diffraction patterns of four ceramic powder samples (MgO, Al2O3, AlN, and cBN) that were compressed in a diamond anvil cell under uniaxial non-hydrostatic conditions were recorded. The microscopic deviatoric stress as a function of the pressure was determined from the X-ray diffraction peak broadening analysis: the curves increased approximately linearly with the pressure at the initial compression stage and then levelled off under further compression. Pressure-induced transparency was observed in all of the samples under compression, and the pressure at the turning point on the curves of the microscopic deviatoric stress versus pressure corresponded to the pressure at which the samples became transparent. Analysis of the microstructural features of the pressure-induced transparent samples indicated that the compression caused the grains to fracture, and the broken grains bonded with each other. We demonstrated that the ceramics’ pressure-induced transparency was a process during which the grains were squeezed and broken, the pores were close between the grains, and the broken grains were re-bonded under compression.
Nanoceramics of Mg1-xMnxAl2O4; x=0.005, 0.00005 was obtained using thermobaric synthesis method. As a result of an increase in the concentration of impurity manganese, the impurity is redistributed over cationic positions, which stimulates lattice compression. The stabilization of the Mn²⁺ and Mn³⁺ states was discovered, due to the relatively low temperatures of thermobaric synthesis (not more than 600 °C). It is shown that impurity manganese ions in nanoceramics can act as indicators of structurally nonequivalent tetra- and octahedral positions.
Pressure is an important thermodynamic parameter in addition to temperature and chemical composition during material sintering process. However, few experimental images intuitively describing the densification process of grains during compression, especially for the high melting point ceramics. Here we observed deformation evolution and mechanical response of spherical alumina particles after high pressure treatment. By analyzing the plastic deformation and grain boundary characters of spherical alumina particles, and the boundary stress and yield stress variation of grains under various pressures and temperatures, a clear picture of the high-pressure powder densification and sintering process is depicted.
Nanoceramics may have different structural and physical properties compared to their coarse-grained counterparts. Here, we report the high-pressure study of micro- and nano-crystalline MgAl2O4 in order to examine the effect of particle size on the structural stability. A reversible pressure-induced phase transition (cubic to tetragonal) is observed in MgAl2O4 nanocrystals under non-hydrostatic pressure at room temperature, in contrast to the previously reported structural transition of MgAl2O4 at high pressure and high temperature. It is also found that the compressed MgAl2O4 microcrystals do not fracture further below 60 nm, suggesting a plastic deformation mechanism transition. MgAl2O4 with a grain size above ∼60 nm exhibits normal cracking behaviors, but shows metal-like plastic deformation behaviors below this critical size. It is implied that combined ductility and strength can be achieved in nanoceramic MgAl2O4.
Luminescent transparent nanoceramics were obtained by thermobaric treatment (TBT) of magnesium aluminium spinel nanopowder. The morphological features were studied by scanning electron microscopy combined with X-ray powder diffraction. Obtained ceramics do not have any agglomerates and pores larger than 100 nm. Crystallites have a high size uniformity. An increase in the lattice constant of nanoceramics compared to the initial powder is observed. Under the TBT, a decrease in the region of coherent scattering due to elastic deformation of crystallites is found. The absence of cracks, large pores, nanosize grains, and high size uniformity reduce light loss in the material, increasing its transparency. Point defects were characterized by photoluminescence and electron spin resonance (ESR) methods. The glow in the 1.8 eV band is caused by the presence of Ti³⁺ impurity ions. An abnormally wide peak with a maximum at 2.4 eV in the photoluminescence spectrum is recorded. This signal is a superposition of the Mn²⁺ ions emission bands and oxygen vacancies (F and F⁺ centres). In the ESR spectrum, signals from impurity ions of iron, titanium, and manganese, as well as an intense signal at g = 2.005 associated with oxygen vacancies in nanoceramics were detected.
In this study, amorphous SiBCN powders were mechanical milled and consolidated by low-temperature sintering at different pressure to provide an insight into the densification and mechanical behavior of dense amorphous SiBCN monoliths. Results show that the mechanical properties undergo a gradual variation closely related to the microstructural evolution. The increased degree of densification with few defects and free volume results in significantly improving Young's modulus and nano hardness in case of high pressure application. The consolidation at relatively low pressure (3 GPa) occurs by heavy deformation with the support of limited short-range diffusion, whereas the densification at higher pressure (5 GPa) is mostly diffusion controlled showing a classic sintering mechanism. At 1000 °C, higher pressure (5 GPa) suppresses crystallization leading to an effective solid-state reaction, while SiBCN monoliths consolidated by lower pressure (3–4 GPa) show a hybrid structure of dominant amorphous phases and few nanocrystals.
Here, we report a high-pressure study of orthorhombic structured β-Sb2O3 (valentinite) by the combination of synchrotron in situ x-ray diffraction and first-principles theoretical calculations at pressures up to 40.5 GPa. Our results reveal that the metastable β-Sb2O3 undergoes an isostructural phase transition at high pressure, yielding a distorted β phase at 7~15 GPa through symmetry breaking and structural distortion as inferred from our XRD analyses and DFT theoretical calculations where pressure-induced elasticity softening is observed at pressures of 7~15 GPa. At pressures higher than 15 GPa, a new high-pressure monoclinic phase is discovered from the current synchrotron x-ray diffraction data. Upon further compression up to ~33 GPa, the monoclinic Sb2O3 starts to lose its long-range order and forms an amorphous component coexisting with the monoclinic one. To further explore the structural instability and understand the origin of pressure-induced phase transitions in β-Sb2O3 upon compression, we have performed first-principles calculations to track the evolution of its phonon velocities, density of states and phonon dispersion curves under high pressure. Our results may play an important role in determining the local structures as well as their structural relationship among sesquioxides.
Traditionally, densification and grain growth are two competing processes in sintering of ceramics. To improve the density, while limiting grain growth at the same time, an ultrahigh pressure (> 1 GPa) is employed here and results in plastic deformation as the dominant densification mechanism during the sintering process. In this way, fully dense boron carbide (B4C) structural ceramics without grain growth is prepared under the pressure of 4.5 GPa at low temperature of 1300 °C in 5 minutes, while showing excellent mechanical properties such as Vickers hardness of 38.04 GPa, Young's modulus of 487.7 GPa, and fracture toughness of 3.87 MPa·m1/2. This study should also facilitate the development of other structural ceramics for practical applications.
Transparent ceramics can be used as laser hosts, infrared (IR) windows/domes, lamp envelopes, and transparent armors. The two key requirements for transparent ceramics are high purity and high density. To ensure high purity, it is necessary to use high-purity precursor powders, while to achieve high-density, special technologies have to be used, such as high-pressure (HP) sintering, high isostatic pressure (HIP) sintering, vacuum sintering, and spark plasma sintering (SPS). In this article, the latest progress in processing, materials, and applications of transparent ceramics is summarized.
There are various sintering techniques that can be used to fabricate transparent ceramics. Conventional sintering techniques include vacuum sintering, hot pressing (HP), and hot isostatic pressing (HIP), while spark plasma sintering (SPS) is more popular than microwave sintering in the new sintering technique category. Every method has its own advantages and disadvantages. Different methods can be combined to offer higher sintering efficiency. The selection of sintering technique is also dependent on materials.
In the present study, we present a novel method to sinter Cr3C2 powders under high pressure without any addittives. The sintering Cr3C2 samples were charaterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), relative density measurements, Vicker’s hardness tests and Fracture toughness tests. The reasults show that Cr3C2 powders could be sintered to be bulk under the conditions of 3-5 GPa, 800-1200 °C and the heat preservation for 15 min. Moreover, the sintering body of Cr3C2 compound with the relative density of 99.84% by simultaneously tuning the pressure-temperature conditions exhibited excellent mechanical properties: a Vickers hardness of 20.3 GPa and a fracture toughness of ~8.9 MPam1/2. These properties were much higher than that by using the previous methods. The temperature condition obtained good mechanical properties in the experiment was about 1/3 lower than that using any other methods owing to the high pressure.
The fabrication of dense amorphous Si–B–C–N monoliths is a processing challenge given that it is hard to avoid crystallization at the sintering temperatures needed to attain full density up to 1900°C for conventional hot pressing and SPS methods. We report here successful densification of amorphous Si2BC3N monoliths achieved by heating at 1100°C and 5 GPa. The relationships between microstructure, types of chemical bonding, and mechanical properties were investigated. The strong amorphous 3-D networks of Si–C, C–B, C-N (sp3), N-B (sp3), and C–B–N bonds provide high densities at high applied pressure and thus amorphous Si2BC3N monoliths show high hardness of 29.4 GPa and elastic modulus of 291 GPa. The amorphous structure is lost with crystallization of β-SiC and BN(C) reducing contributions from Si–C, C-N (sp3), and C–B–N bond networks thereby decreasing mechanical properties.
A new theoretical model is proposed to describe the mechanical properties of bimodal nanocrystalline (BNC) materials. This composite model is comprised of coarse grains evenly distributed in the nanocrystalline (NC) matrix. In this study, we have studied the effect of grain size distribution on the constitutive behavior of BNC materials. During the plastic deformation, effects of nanocracks and dislocation emission from crack tips on the constitutive behavior of BNC materials are also analyzed. Numerical calculations have been carried out according to the model, and it is found that the nanocracks make a positive effect on the strain hardening, and the results show that this model can describe the enhanced strength and strain hardening of BNC materials successfully. The prediction of the bimodal Cu-Ag material is in good agreement with the experimental results.
NRL technology has produced transparent MgAl2O4 ceramics with hardness ~20 GPa and grain sizes less than 30nm; 50 % improvement in hardness and an order of magnitude reduction in grain size than present literature best.
Compressional (VP) and shear (VS) wave velocities of polycrystalline MgAl2O4 spinel have been measured up to 14 GPa and 900 K using ultrasonic interferometry and in situ X-ray diffraction techniques. Here, we observed a weaker pressure dependence in shear modulus (G) for MgAl2O4 spinel, as compared to a stronger ∂G/∂P for magnesium silicate/germanate counterpart. Our first-principles calculations show that the tetragonal shear modulus CS = (C11–C12)/2 decrease with pressures, indicating acoustic mode softening, which further supports our observed experimental results. Using a finite strain equation of state approach the elastic bulk and shear moduli, as well as their pressure and temperature derivatives, are derived from the directly measured velocities and densities, yielding KS0=196.0(9) GPa, G0 = 109.0(4) GPa, ∂KS/∂P = 4.60(9), and ∂G/∂P = 0.58(3) independent of pressure calibration. The temperature derivatives for the bulk and shear moduli were tightly constrained from acoustic velocity measurements as ∂KS/∂T = −0.022(3) GPa/K and ∂G/∂T = −0.014(1) GPa/K. In addition, the mechanism for the unusual pressure effect on the shear modulus in MgAl2O4 spinel has been addressed by the coupling between atomic displacements and shear strains, namely a better accommodation of the AlO6 octahedral distortion and shear strains, as well as the pressure-induced tilting/distortion and/or symmetry changes in MgAl2O4 spinel.
The highly visible and infrared (up to 6 μm) transparent Sr3Al2O6 polycrystalline ceramic was obtained by full crystallization of the corresponding glass composition. The glass synthesis and the direct congruent crystallization processes are described, and the material transparency is discussed in light of its microstructure. This new transparent ceramic exhibits a high density (i.e., complete absence of porosity) and micrometer-scale crystallites with very thin grain boundaries. These microstructural characteristics, inherent to the preparation method, minimize light scattering and demonstrate the advantages of this synthesis route compared to the high-pressure process used for the few reported transparent polycrystalline materials. This Sr3Al2O6 ceramic shows a H = 10.5 GPa hardness, a Er = 150 GPa reduced elasticity modulus, and a 9.6 × 10–6 K–1 thermal expansion coefficient. Such a transparent strontium aluminate ceramic opens the way to a wide range of applications, especially photonics when doped by various doping agents. As examples, the luminescence of Sr3Al2O6:Eu3+ and Sr3Al2O6:Er3+, which show strong emissions in the visible and infrared ranges, respectively, is presented. Moreover, the Sr3Al2O6:Ce3+ material was found to exhibit scintillation properties under X-ray excitation. Interestingly, the analogous Sr3Ga2O6 transparent polycrystalline ceramic material could equally be prepared using the same elaboration method, although its hygroscopicity prevents the preservation of its high transparency under normal conditions. The establishment of the key factors for the transparency of this economical and innovative synthesis method should enable the prediction of new classes of technologically relevant transparent ceramics.
SiC-diamond nanocomposites were synthesized from nanodiamond and nanosilicon powders. A core-shell model of the composite nanocrystals was examined assuming that interatomic distances in the grain interior, the core, and at the surface shell (grain boundaries in nanocrystalline solids) are different. The samples were investigated by x-ray diffraction using synchrotron source. The powder diffractograms were elaborated based on the apparent lattice parameter methodology. The structure of the composites and its dependence on the sintering conditions is discussed. It is shown that as the sintering temperature increases the interatomic distances in the grain cores decrease, while the opposite occurs in the grain shells (forming the grain boundaries). Under some sintering temperature the interatomic distances in the core and in the shell get equal. However, for diamond this happens under different temperature than for SiC, thus internal strains in the composites are unavoidable.
The equation of state and the structural behavior of synthetic MgAl2O4 have been investigated using synchrotron X-ray powder diffraction data collected to 30 GPa at room temperature. The Birch-Murnaghan, Vinet, and Poirier-Tarantola models have been fitted to the observed P-V data. The Birch-Murnaghan equation of state, with V-0 fixed at its experimental value, yields K-0 = 190.8(+/-1.2) GPa, K-0' = 6.77(+/-0.15) and K0"= -0.075 GPa(-1) (implied value). The compression of spinel occurs with a negligible change of the fractional coordinate of oxygen. Therefore the structural shrinking is a function of cell edge shortening alone. The results presented here are compared with those from the literature.
Transparent Mg Al <sub>2</sub> O <sub>4</sub> spinel nanoceramics have been sintered at relatively low-temperature (500–700 ° C ) under high pressure (2–5 GPa ) using a hydrostatic press with high-temperature-calcined nanopowders. The morphology, nanostructure, optical property, and density of ceramics were analyzed by scanning electron microscopy, transmission electron microscopy, UV-VIS-IR transmission spectrum, and Archimedes draining method. The average grain size ( ≪100 nm for all samples sintered) depends on the sintering pressure and temperature. The nanoceramics are highly transparent even though their relative densities are all less than 99%, due to the low or negligible light scattering from the nanosized grains and pores.
The yield strength of diamond is measured under a pressure of 10 gigapascals at temperatures up to 1550 degrees C by the analysis of x-ray peak shapes on diamond diffraction lines in a powdered sample as a function of pressure and temperature. At room temperature, the diamond crystals exhibit elastic behavior with increasing pressure. Significant ductile deformation is observed only at temperatures above 1000 degrees C at this pressure. The differential yield strength of diamond decreases with temperature from 16 gigapascals at 1100 degrees C to 4 gigapascals at 1550 degrees C. Transmission electron microscopy observations on the recovered sample indicate that the dominant deformation mechanism under high pressure and temperature is crystal plasticity.
Application of in situ high pressure powder diffraction technique for examination of specific structural properties of nanocrystals based on the experimental data of SiC nanocrystalline powders of 2 to 30 nrn diameter in diameter is presented. Limitations and capabilities of the experimental techniques themselves and methods of diffraction data elaboration applied to nanocrystals with very small dimensions (< 30 nm) are discussed. It is shown that due to the complex structure, constituting a two-phase, core/surface shell system, no unique lattice parameter value and, consequently, no unique compressibility coefficient can satisfactorily describe the behavior of nanocrystalline powders under pressure. We offer a tentative interpretation of the distribution of macro- and micro-strains in nanoparticles of different grain size.
Using radial x-ray diffraction techniques together with lattice strain
theory, the behavior of boron suboxide (B6O) was
investigated under nonhydrostatic compression to 65.3GPa in a
diamond-anvil cell. The apparent bulk modulus derived from
nonhydrostatic compression data varies from 363GPato124GPa depending
on the orientation of the diffraction planes with respect to the loading
axis. Measurement of the variation of lattice spacing with angle, ψ
, from the loading axis allows the d spacings corresponding to
hydrostatic compression to be obtained. The hydrostatic d spacing
obtained from a linear fitting to data at 0° and 90° is
consistent with direct measurements at the appropriate angle
(ψ=54.7°) to within 0.5%, which suggests that even two
measurements ( ψ=0° and 90°) are sufficient for accurate
hydrostatic equation of state determination. The hydrostatic compression
data yield a bulk modulus K0=270±12GPa and its
pressure derivative K0'=1.8±0.3 . The
ratio of differential stress to shear modulus ranges from 0.021 to 0.095
at pressures of 9.3-65.3GPa . Together with estimates of the
high-pressure shear modulus, a lower bound to the yield strength is
26-30GPa at the highest pressure. The yield strength of B6O
is about a factor of 2 larger than for other strong solids such as
Al2O3 . The ratio of yield stress to shear modulus
derived from lattice strain theory is also consistent with the result
obtained by the analysis of x-ray peak width. This ratio might be a good
qualitative indicator of hardness as it reflects the contributions of
both plastic and elastic deformation.
Despite success of diamond anvil cells in all fields of high-pressure research, there have been continuous efforts to search for new anvil materials that are complementary to diamond but not limited by its cost, availability, and crystal size. In this regard, hexagonal silicon carbide, moissanite, has been found to be an ideal material (Xu and Mao, 2000). It is believed that moissanite anvil cell will open a new window for studies that require large sample volumes as well as stable and accurate temperature measurements. This study reports the yield strength measurements of moissanite at high temperature, which is one of the fundamental properties that define the ultimate performance of this material at high temperature conditions. We use the principles outlined by Weidner et al. (1994) to obtain information of stress and strength in the powder samples from x-ray diffraction signals. Two experiments have been performed at pressures up to 18.3 GPa and temperatures up to 1200 ° C using a DIA-type cubic anvil apparatus and a newly developed "T-Cup" high pressure system. At room temperature, the moissanite crystal behaves elastically with increasing pressure up to 13.7 GPa. At higher pressures applied, the sample is yielded, and the yield strength of moissanite is determined to be 13.6 GPa. Upon heating at 18.3 GPa, significant stress relaxation is observed at temperatures above 400 ° C, and the yield strength of moissanite decreases rapidly from 12.8 GPa at 400 ° C to 4.2 GPa at 1200 ° C. Such behavior will place severe limitations on the use of moissanite as anvil material when external heating is desired for high pressure and temperature experiments. References: Xu and Mao (2000), Science 290, 783-786. Weidner et al. (1994), Geophy. Res. Lett. 21, 753-756.
A model based on Rayleigh–Gans–Debye light-scattering theory has been developed to describe the light transmission properties of fine-grained, fully dense, polycrystalline ceramics consisting of birefringent crystals. This model extends light transmission models based on geometrical optics, which are valid only for coarse-grained microstructures, to smaller crystal sizes. We verify our model by measuring the light transmission properties of fully dense (>99.99%), polycrystalline α-Al2O3 (PCA) with mean crystal sizes ranging from 60 to 0.3 μm. The remarkable transparency exhibited by PCA samples with small crystal sizes (
Commercial corundum powder and a liquid-shaping approach are used for manufacturing complex hollow components and large flat windows of sintered and hot isostatically pressed Al2O3 ceramics having grain sizes of 0.4–0.6 μm at relative densities of >99.9%. High macrohardness (HV10 = 20–21 GPa) and four-point bending strength (600–700 MPa; 750–900 MPa in three-point bending) are associated with a real in-line transmission of 55%–65% through polished plates. The submicrometer microstructure and the optical properties can be retained for use at >1100°C using dopants that shift the sintering temperature to high values without additional grain growth.
Sintering and grain growth of nano-crystalline undoped ZnO has been studied in detail over a wide range of temperature and holding time. Below 800°C, sintering of over 70% theoretical density is not observed, irrespective of particle size. At 900°C for 6h, the nano-crystalline sample sinters to 99% of theoretical density whereas the density for as received sample is 93% of theoretical density. However, at 1300°C or higher, the densification is found to be much faster and after a few hours becomes independent of holding time. Grain growth studies reveal a similar feature of attaining saturation over holding time. The average saturated grain size is found to be ∼1.5 and ∼2.2μm at 800 and 900°C, respectively, while at 1300°C or higher, it is in between 12 and 13μm.
Transparent spinel can be fabricated without any sintering aids by spark plasma sintering (SPS) processing for only a 20 min soak at 1300 °C. For heating rates <10 °C min−1, the spinel exhibits an in-line transmission of 47% for a visible-wavelength of 550 nm and a fracture strength of ∼500 MPa. By means of low heating rate SPS processing, high transmission and strength can be attained mainly by reducing the residual pores to <0.5%.
The optical and mechanical properties of polycrystalline MgAl2O4 spinel make this material of interest for transparent armor and for window and dome applications in the 0.3 micrometers to 5.5 micrometers range. Spinel was briefly produced commercially, and qualified for a range of dome and window applications in the early 1990's. Since 1993 however, there has been no commercial producer and consequently the interest in the application of spinel has waned. This paper summarizes development efforts by Technology Assessment and Transfer (TA&T) to fabricate transparent spinel with high optical quality for both transparent armor, and a selection of window and dome applications. A cooperative research and development agreement between TA&T and the US Army Research Laboratory is focused at optimizing processing parameters to maximize strength and transparency while minimizing the costs for fabrication by the hot-press/HIP approach. Present interest is in fabricating large armor panels of spinel up to 15 inches square and 0.5 inches thick, and in the fabrication of thinner windows and domes with the view to establishing TA&T as a commercial supplier of spinel in the near future.
The use of microwaves to process absorbing materials was studied
intensively in the 1970s and 1980s, and has now been applied to a wide
variety of materials. Initially, success in microwave heating and
sintering was confined mainly to oxide and some non-oxide ceramics; but
recently the technique has been extended to carbide semimetals used in
cutting tools. Here we describe the microwave sintering of powdered
metals to full density. We are able to sinter a wide range of standard
powdered metals from commercial sources using a 2.45-GHz microwave
field, yielding dense products with better mechanical properties than
those obtained by conventional heating. These findings are surprising in
view of the reflectivity of bulk metals at microwave frequencies. The
ability to sinter metals with microwaves should assist in the
preparation of high-performance metal parts needed in many industries,
for example, in the automotive industry.
This work presents a comprehensive study on phase transitions in LiAlO2 system at high pressures and temperatures (0.5–5.0GPa and 300–1873K, respectively), as well as the phase stability for polymeric phases of LiAlO2 in the studied P–T space by X-ray diffraction (XRD). Besides the previously described polymorphic hexagonal α-phase, orthorhombic β-phase and tetragonal δ-phase, a possible new phase of LiAlO2 was observed after the tetragonal γ-LiAlO2 sample was treated at 5.0GPa and 389K. The stable regimes of these high-pressure phases were defined through the observation of coexistence points of the polymeric phases. Our results revealed that LiAlO2 could experience structural phase transitions from γ-LiAlO2 to its polymorphs at lower pressures and temperatures compared to the reported results. Hexagonal α-LiAlO2 with highly (003) preferential orientation was prepared at 5.0GPa and 1873K.
The strength of tungsten was determined under static high pressures to 69 GPa using x-ray diffraction techniques in a diamond anvil cell. Analysis of x-ray diffraction peak broadening and measurement of peak shifts associated with lattice strains are two different methods for strength determination of materials under large nonhydrostatic compressions. Here these methods are directly compared under uniaxial compression in a diamond anvil cell. Our results demonstrate the consistency of the two approaches, and show that the yield strength of tungsten increases with compression, reaching a value of 5.3 GPa at the highest pressure. The obtained yield strength of tungsten is also compared with previous experimental data involving shock wave and static compression measurements, and with theoretical predictions. The high-pressure strength of tungsten is comparable to that of other dense metals such as Re and Mo, and ratio of yield strength to shear modulus is about 0.02 for all these materials between 20 and 70 GPa. The static strength of tungsten is much greater than values observed for W under shock loading but is very similar to values observed under quasi-isentropic loading.
Yield strength is measured at high pressure and temperature using a large volume, high pressure apparatus (SAM85) with synchrotron radiation. A macroscopic deviatoric stress is manifest as a uniform deviatoric strain that is oriented by the geometry of the pressurizing medium. Microscopic deviatoric stress is identified as the elastic broadening of diffraction lines. The deviatoric stress reaches the yield point as evidenced by the uniformity, the saturation, and the temperature dependence of the deviatoric stress. Yield strengths, which correspond to the stress saturation level at a few per cent strain, are determined for NaCl and MgO up to 8 GPa and 1200°C. The results are consistent at room temperature with previous diamond anvil studies and demonstrate the effect of pressure on yield strength. These data demonstrate the feasibility of determining high pressure, high temperature yield strengths for mantle phases.
Two types of nanocalcium aluminate powders containing 70 and 60 wt.% Al2O3 were prepared by thermal decomposition and investigated in terms of mineralogical composition, hydration, mechanical properties and microstructure. The results revealed that S1 is composed mainly of the CA and CA2 phases, while S2 composed of CA and C12A7 phases after heat-treatment at 1000 °C. The maximum crystallite sizes for S2 and S1 were 44 and 52 nm and the minimum ones 12 and 19, respectively. Both samples still have small crystallite size after heat-treatment at 1000 °C. The present phases, i.e. CA, CA2 and C12A7 affect the properties of hydrated and sintered ceramic bodies. S2 hydrated sample achieved higher strength (58.5 MPa) than S1 (52.4 MPa). The higher strength of S2 is ascribed to the presence of CA and C12A7 as a major component, since it reacts rapidly with water. In S1, the poor hydration of CA2 at the early stage of hydration lowers strength after 7 days hydration as compared with S2. Cold crushing strength (CCS) data of the sintered ceramics bodies exhibit high strength of both samples after firing at 1550 and 1450 °C for S1 and S2, respectively. This is due to the formation of a ceramic bond.
The possibility of superplastic tensile deformation in nanocrystalline materials has not yet been realized, largely due to the difficulty in maintaining a fine grain size during the densification of nanocrystalline powders. In this preliminary study, it is demonstrated that it may be possible to retain a nanocrystalline grain size in a 3 mol% yttria stabilized zirconia by sinterforging, which enhances densification without significantly affecting grain growth. In addition, other published data are examined to identify conditions favorable for enhancing densification and limiting grain growth.
Porous hydroxyapatite ceramics with porosity up to 73% have been fabricated by microwave processing at 1150 to 1200 °C for 1 to 5 min. Various porosities in these ceramics have been obtained by using starting materials with different morphology, adjusting green density, changing sintering time and temperature, as well as optionally mixing ammonium carbonate in the hydroxyapatite powder during the consolidation process. Porosity, microstructure, and diametral tensile strength of the porous hydroxyapatite ceramics have been studied.
Collision cascades in MgAl2O4 are investigated using molecular dynamics simulations in order to determine the threshold displacement energies, E-d, and the damage imparted to the lattice at energies of up to 5 keV The value of Ed is determined for MgAl2O4 on each of the Mg, Al and O sublattices for different orientations of the primary knock-on atom (PKA). The lowest Ed required to create permanent defects was for an 0 PKA along the (100) direction with a value of 27.5 eV, while the highest was 277.5 eV along (13 1) for an Mg PKA. Higher energy cascades show that a much wider variety of defects remain after the collisional phase than for similar cascades in MgO but the number of Frenkel pairs produced is smaller. The predominant defects that form are antisite defects on the cation sublattice only and O and Mg split interstitials orientated along the (110) direction. Some Mg-Al split interstitials centred on an Mg site were also observed. However, some more extended defect complexes can also arise which have no well defined structure.
Usage of a new but simple and reactive technique employing metallic aluminum as one of the reactants to produce very high phase-purity magnesium aluminate powder under rather mild experi-mental conditions is described. Low temperature melting of aluminum and subsequent exothermic reaction between molten aluminum and magnesia appeared to have led to the powder with a very high fraction of the spinel phase with small particle size and narrow particle size distribution. This powder upon sintering for 4h at 1600°C led to compacts with density as high as 92% with benign microstructural features. The beneficial effect of slightly off-stoichiometry (9 wt. %) in composi-tions on either side of magnesium aluminate in the starting powders has been discussed.
Spinel precursors have been obtained by sol-gel processing using either magnesium nitrate and chemically modified aluminum alkoxide or a double alkoxide as raw materials. The xerogels have been characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), infrared (IR) spectroscopy, and transmission electron microscopy (TEM). It is proposed that the size of the particles is related to the functionality of the aluminum alkoxide. The use of the double alkoxide allowed pure spinel nanosized materials to be obtained.
A study was conducted, to demonstrate the sintering preparation of nanostructured super-hard B6O-B4C compacts, starting from a mixture of B, B2O3, and B4C at a mild pressure of -3 GPa and high temperatures of 1500-1900 K. The one-step sintering method simplified the synthesis process and significantly reduced the costs involved, while making it possible to manufacture super-hard compacts that were greater than 200 nm in diameter. It was observed that the sintered B 6O-B4C compacts exhibited hardness values that were comparable to industrially manufactured polycrystalline cubic boron nitride (PCBN) materials. It was also found that these materials can be used for high-speed cutting of steel. The characteristics of 6O composites have emerged as better alternatives to polycrystalline diamond (PCD) and PCBN in industrial processes, due to their easier and cost-effective manufacturing processes.
Direct measurement of the average yield strength for individual grain using X-ray diffraction, which avoids the influence of a bulk sample quality on identation or deformation experiments, is presented. This method has been widely used in measuring residual stresses in materials. The technology was extended here to measure the stress in a powder sample during loading. Since stresses are generated by grain-to grain contact, the method has been applied to measure the strength of superhard materials such as diamond.
Synthesis of nanocrystalline yttria powder from Y(NO3)3 solution and ammonia water was investigated. It was found that the precursor precipitate is Y2(OH)5(NO3).H2O. The addition of small amount of ammonia sulfate in yttrium nitrate solution can reduce the agglomeration and particle size of the produced yttria powders. Nanocrystalline yttria powder (60 nm in average size) was obtained by calcining the precursor at 1100 °C for 4 h. Using this yttria powder and a commercial ultrafine Al2O3 powder, fully transparent YAG ceramics was fabricated by vacuum sintering at 1700 °C for 4 h through a solid-state reaction method. It was found that the addition of 0.5 wt.% tetraethyl orthosilicate (TEOS) is suitable for the fabrication of transparent YAG ceramics. If the amount of TEOS is less than 0.05 wt.%, abnormal grain growth occurs, and pores are entrapped in the grains. If the amount of TEOS is more than 3 wt.%, large amount of liquid phase is yielded, leaving some residual inclusions at grain boundaries. These are detrimental for the transparency of YAG ceramics.
Bulknanocrystalline α-Al2O3samples with a relative density >98% and a grain size < 50 nm have been produced by high pressure/low temperature sintering, using a toroidal-type high pressure apparatus. Nanocrystalline (n-) alumina powder with metastable γphase was used as the starting material. During sintering, the γphase transforms to αphase. The transformation temperature decreases from ~1075 °C at ambient pressure to about 460 °C at 8 GPa. Grain growth is limited by the low sintering temperature, and a multiplicity of nucleation events in the parent γ phase at very high pressure creates a nanoscale α grain size. The average grain size of the α-Al2O3 increases from 18 nm in the original powder to only about 49 nm in the sintered compact (98.2% dense). In addition, we found that high pressure could increase the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreased with sintering pressure under the same sintering temperature and time. Due to its high surface area, n-Al2O3 powder readily absorbs chemical species from the environment. Alumina hydrates, formed by the reaction of Al2O3 with chemisorbed OH− species during sintering, had a profound influence on sintering and phase transformation behaviors of n-Al2O3. To control grain size of the transformed α phase, it is essential to eliminate the hydrates before sintering.
Bulk n-TiO2 samples with a relative density as high as 95% and a grain size less than 50 nm were fabricated by hot-pressing at temperatures as low as 400 °C and at pressures up to 1.5 GPa. During hot-pressing, the anatase phase transformed to the rutile phase and the amount of transformation increased with sintering pressure. The grain size in both the anatase and the rutile phase increased with sintering pressure at a constant temperature but the grain size of the transformed phase is always smaller than that of the starting material. We believe that the smaller grain size of the rutile phase is related to multiple nucleation events in the anatase phase during sintering at very high pressure. The average grain size increased from 27 nm in the original powder to only 45 nm in the compact after hot-pressing. Analysis of the grain size and closed porosity by transmission electron microscopy suggested that closed pores at grain boundary triple junctions might also retard the grain boundary migration and thus prevent grain growth. A competing mechanism is also proposed in which the rate of grain growth is controlled by the pressure effect on the bulk diffusion rate and interface energy.
Transparent alumina with a fine grain size (0.27 mu m) was obtained by controlling the heating rate during spark plasma sintering processing. The alumina sintered at 1150 degrees C with a heating rate of 8 degrees C/min has a residual porosity of 0.03% and an in-line transmission of 47% for a wavelength of 640 nm. We show that a low heating rate has an effect on the densification and transparency of alumina for sintering at 1150 degrees C. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
A precursor for Mg–Al spinel has been synthesized via the precipitation method, using ammonium bicarbonate as the precipitant. The precursor was composed of crystalline ammonium dawsonite hydrate [NH4Al(OH)2CO3·H2O] and hydrotalcite [Mg6Al2(CO3)(OH)16·4H2O] phases. The precursor converted to pure spinel phase at ∼900°C via two steps upon calcination: (i) decomposition of hydrotalcite at lower temperatures (400–800°C) and (ii) solid-state reaction between MgO (decomposed from hydrotalcite) and γ-Al2O3 (derived from NH4Al(OH)2CO3·H2O) at higher temperatures (>800°C). The effect of calcination temperature on particle morphology and sinterability of the resultant spinel powders were investigated.
The current knowledge on the microstructure, sintering and creep properties of the nanostructured oxides TiO2, Y2o3 AND zRo2 synthesized by gas condensation in summarized. Size dependent phase stability in nanostructured oxide particles with average diameter in the range from 3 to 20 nm and phase transformations during particle growth are discussed. The influence of various microstructural parameter of the powder, i.e. grain size and degree of agglomeration, on consolidation and sintering is discussed. Plastic deformation by creep processes under compressive and tensile stresses at temperatures below half the melting point is presented.
Commercial nanocrystalline yttrium aluminum garnet (nc-YAG) powders were used for fabrication of dense and transparent YAG by spark plasma sintering (SPS). Spherical 34 nm size particles were densified by SPS between 1200 and 1500 °C using 50 and 100 MPa pressures for 3, 6, and 9 min durations. Fully dense and transparent polycrystalline cubic YAG with micrometer grain size were fabricated at very moderate SPS conditions, i.e. 1375 °C, 100 MPa for 3 min. Increase in the SPS duration and pressure significantly increased the density especially at the lower temperature range. The observed microstructure is in agreement with densification by nano-grain rotation and sliding at lower densities, followed by curvature driven grain boundary migration and normal grain growth at higher densities. Residual nanosize pores at the grain boundary junctions are an inherent microstructure feature due to the SPS process.
We present a comparative study of thermomechanical properties of nano-polycrystalline nickel (nano-Ni) and micrometer-polycrystalline nickel (micron-Ni) by in situ high pressure-temperature (P-T) diffraction experiments. The yield strength of 2.35 GPa for the nano-Ni measured under high-pressure triaxial compression is more than three times that of the micron-Ni value. Contrary to tensile experiments of uniaxial loading, we observe significant work-hardening for the nano-Ni in high-pressure plastic deformation stage, whereas the micron-Ni experiences minor high-pressure work-softening and considerable energy dissipation into heat. The significantly reduced energy dissipation for the nano-Ni during the loading-unloading cycle indicates that the nanostructured materials can endure much greater mechanical fatigue in cyclic loadings. The nano-Ni exhibits steady grain growth during bulk plastic deformation at high-pressure loading, and drastic stress reduction and grain growth occur during the high P-T cycle. Our experiments utilized novel approaches to comparatively study micro- and nanostructured materials revealing recoverability of elastic/plastic deformations, strain corrections by diffraction elasticity ratio, and identifying dominances of stress relaxation, grain growth, and intrinsic residual stresses. The results should be of considerable interest to the fields of materials science, condensed matter, and computational physics.
A key question in nanomechanics concerns the grain size effects on materials' strength. Correct solution to this question is critical to design and tailor the properties of materials for particular applications. The full map of grain sizes-hardness/yield stress relationship in metals has been built. However, for ceramic materials, the similar studies and understandings are really lacking. Here we employed a novel technique to comparatively study the mechanical features of titanium dioxide (TiO(2)) with different crystallite sizes. On the basis of peak profile analysis of the X-ray diffraction data, we determined yield strength for nanocrystalline and bulk TiO(2). Our results reveal a remarkable reduction in yield strength as the grain size decreases from 30-40 microm to approximately 10 nm, providing the only evidence of a strength weakening by nanocrystals relative to their bulk counterparts. This finding infers an inverse Hall-Petch effect, the first of its kind for ceramic materials, and a dramatic strength weakening after the breakdown of classic Hall-Petch relation below a characteristic grain size.
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