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Structural, Microstructural and Vibrational Characterization of Apatite-Type Lanthanum Silicates Prepared by Mechanical Milling
Abstract
Apatite-type lanthanum silicates have been successfully prepared at room temperature by dry milling hexagonal A-La2O3 and either amorphous or low-cristobalite SiO2. Milling a stochiometric mixture of these chemicals in a planetary ball mill with a moderate rotating disc speed (350 rpm), allows the formation of the target phase after only 3 h although longer milling times are needed to eliminate all SiO2 and La2O3 traces. Thus, the mechanically activated chemical reaction proceeds faster when using amorphous silica instead of low cristobalite as silicon source and pure phases are obtained after only 9 and 18 h, respectively. As obtained powder phases are not amorphous and show an XRD pattern as well as IR and Raman bands characteristic of the lanthanum silicate. The domain size of the as-prepared phases varies gradually with the temperature of post-milling thermal treatment with activation energies of about 26(8) and 52(10) kJ mol-�1K-1 for the apatites obtained from amorphous silica and low-cristobalite, respectively. These values suggest that the crystallite growth is favored when amorphous silica was used as reactant.
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... Ceramics were characterized by TGA an XRD. Raman spectroscopy was used in order to study the shortrange structure according to previous procedure [17,42,43]. The proton insertion and its distribution/effect in the apatite structure have never been studied using Raman spectroscopy. ...
... With both spectrophotometers, the spectra were averaged on the area of the bands between 130 and 600 cm -1 using LabSpec software. Each spectrum was decomposed into a sum of Lorentzian functions (corresponding to the refined bands [17,42,43]) using FOCUS software [48]. ...
... A typical Raman spectrum of oxyapatite is presented in Figure 2, decomposed into a sum of Lorentzian functions using FOCUS software [48] according to previous procedure [17]. The corresponding calculated bands [17,42,43] are given in Table 1. In this work we focus our attention on the wavenumber range below 600 cm -1 which is mostly affected by high pressure treatment. ...
Dense lanthanum silicate oxyapatite samples La9.33 + x(SiO4)6O2 + 3x/2were exposed to high water pressure in autoclave, in order to study the effects of oxygen stoichiometry and treatment duration on the pro-tonation of these materials. TG analyses showed that protonic species were successfully introduced into the bulk of the material, especially for La9.60(SiO4)6O2.4 sample after 84 h of treatment at 550◦C under40 bar water pressure. It was shown that the mass loss compares well with perovskite materials and increases when protonation time rises (0.66% above 600◦C for 408 h), with a time limit beyond which the microstructure is no more stable. Rietveld refinement and Raman spectroscopy studies confirmed some structural modifications which could be linked to the incorporation of protonic species. It was shown that autoclave treatment induced an increase of the cell volume related to an increase of the a and b lattice parameters (enlargement of the characteristic channels of oxyapatite).
... The Raman spectra of the samples in the 100-1000 cm À 1 range are shown in Fig. 2. The bands were assigned to their corresponding modes based on the similar apatites reported in the literature [34][35][36]. The peaks above 350 cm À 1 are assigned to the internal modes of the tetrahedral SiO 4 groups at wavenumbers closely related to those of free species [37], while those below 350 cm À 1 are due to their external modes, and the translation as well as the libration modes of La and Sr [38,39]. ...
... The band 680 cm À 1 is due to the OH surface groups, following the rehydration of the powder after synthesis [40]. The bands at 277 and 188 cm À 1 correspond to the translation modes (T'Sr 2 þ ), and at 159 cm À 1 to the translation mode (T'La 3 þ ) [38,39]. The bands associated to the librational (LSiO 4 ) and translation modes (T'SiO 4 ) are observed at 249 and 212 cm À 1 , respectively [38]. ...
... The bands at 277 and 188 cm À 1 correspond to the translation modes (T'Sr 2 þ ), and at 159 cm À 1 to the translation mode (T'La 3 þ ) [38,39]. The bands associated to the librational (LSiO 4 ) and translation modes (T'SiO 4 ) are observed at 249 and 212 cm À 1 , respectively [38]. Compared to Sr 4 La 6 Fap, the peaks of the Sr 4 La 6 Oxyap spectrum are less intense and broader (Fig. 2b). ...
... This route possesses many drawbacks such as inability in controlling the crystalline growth, composition non-homogeneity and grain size non-uniformity. In order to overcome the aforementioned problems, many research groups investigated the application of alternative synthetic routes such as sol-gel synthesis [22][23][24][25][26] , coprecipitation [27] , glycine-nitrate combustion [28] , freeze drying [4] and high energy ball milling [29] . One of the most successful and versatile synthetic routes for the preparation of ceramic oxides is Pechini method [30,31] , although it has so far found little use in the synthesis of ATLS powders [32] . ...
... In the same spectra the bands of the carbonates have been eliminated while two new bands at 3605 and 657 cm -1 are present. These absorption bands can be attributed to the formation of hydroxide phases in the sample and especially in the vibration of La-OH bonds [29,40,41] . The presence of lanthanum hydroxide refers to moisture absorption by La 2 O 3 phase which is contained as intermediate product in all samples at this temperature range. ...
... An intense absorption band at 1120 cm -1 along with a shoulder at 1200 cm -1 is present in the temperature range from 300 to 900 °C which is drastically reduced after calcination at 700 °C while at 1200 °C is eliminated. These bands can be associated with Si-O bond vibrations in the unattached SiO 2 [29,44] . The existence of SiO 2 is not visible from the XRD patterns indicating an amorphous phase. ...
In this paper we reported the preparation and extensive characterization of La9.83−xSrxSi6O26+δ (0≤x≤0.50) precursors, intermediate and final products. The sintering reactions, the phase formation, the structure as well as the powders' morphology were studied by means of thermogravimetric analysis, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Moreover, the effect of stoichiometry on precursor's structure and morphology as well as on intermediate and final products was reported. As was concluded pure La9.83Si6O26+δ, La9.38Sr0.45Si6O26+δ and La9.33Sr0.50Si6O26+δ could be prepared after sintering at 1400 °C for 20 h while La9.68Sr0.15Si6O26+δ and La9.53Sr0.30Si6O26+δ compounds contained minor traces (<3.5%) of La2Si2O7 secondary phase. Concerning the synthesis, there have been no previous reports on the preparation of pure La9.83Si6O26+δ, La9.38Sr0.45Si6O26+δ and La9.33Sr0.50Si6O26+δ compounds. The final powders consisted of spherical particles and an increase of Sr content seemed to inhibit sintering phenomena. The existence of interstitial oxygen at intermediate crystallographic positions of apatite structure had great effect on SiO4 sub-structure distortion. The increase of Sr content led to a major reduction of interstitial oxygen quantity and the refutation of silicon tetrahedron distortion.
... We observed that at the higher temperatures it becomes sharper. 6,25 A similar situation is observed for the mode located at 989 cm -1 , due to asymmetric stretching vibration of the Si-O-Si bond in isolated SiO 4 tetrahedra of the apatite. 6,25 The band at 475 cm -1 , observed in the spectrum of the powder calcined at 700 °C, is due to the Si-O-Si bending vibration. ...
... 6,25 A similar situation is observed for the mode located at 989 cm -1 , due to asymmetric stretching vibration of the Si-O-Si bond in isolated SiO 4 tetrahedra of the apatite. 6,25 The band at 475 cm -1 , observed in the spectrum of the powder calcined at 700 °C, is due to the Si-O-Si bending vibration. 26 This band transforms at higher temperatures to the band with a fine structure. ...
... Vojisavljevi} et al.: Characterization of the Alkoxide-based Sol-gel ...cm -1 and 376 cm -1 are related to asymmetric and symmetric vibrations in SiO 4 tetrahedra of apatite.6 The La-O modes cannot be easily observed, since the Si-O-Si bonds in the measured spectral range (4000-400 cm -1 ) are much stronger than the La-O bonds.25 According to literature data,6,25 the bands connected with the presence of secondary phases, could be detected at 374 cm -1 ). ...
Vojisavljevi} et al.: Characterization of the Alkoxide-based Sol-gel ... Abstract In this study, we report on the acid-catalysed synthesis of La 9.33 Si 6 O 26 from lanthanum nitrate or acetate and silicon et-hoxide (TEOS) in the ethanol solvent, upon the transition from liquid to amorphous and crystalline phases. The simila-rity of the Fourier transform infrared spectra of the lanthanum-salt solutions and lanthanum-silicon sols indicates that the lanthanum environment is not changed in the reaction of the La-salt with TEOS. In the nitric-acid catalysed synthe-sis, the hydrolysis reaction was almost instantaneous, as a consequence of a higher amount of water in this system, which contributed to a high level of chemical heterogeneity in the product. The acetic acid-based synthesis ensured a good mixing of the reagents at the nanometre level, which gave rise to the formation of the pure La 9.33 Si 6 O 26 powder up-on heating at 900 °C, and single phase ceramics with 94 % relative density after sintering at 1400 °C for 3 h in air, which is 200 °C lower temperature then usually reported for the apatite material.
... In addition to these bands, we note the presence of bands of low intensities around 799, 745 and 699 cm −1 attributed to silica [55], not detected by DRX, probably because of its small amount. This unincorporated silica was replaced in the apatitic structure by carbonates from the reactants. ...
... Indeed, it is the hydrated lanthanum oxide that reacts with silica to give La2Si2O7. It should be noted that Rodriguez-Reyna et al., were able to obtain pure oxyapatite by dry grinding of La2O3 and SiO2 [55]. In the present work, we have observed the formation of Sr3(PO4)2, whether the synthesis is carried out in one or two steps, and it remains present regardless of the grinding time. ...
1. Introduction Apatites belong to a large family of compounds with the general chemical formula M10(XO4)6Y2, where M represents a divalent cation, XO4 an anion group and Y a monovalent anion. These materials crystallize mainly in the hexagonal system with the space group P63/m [1]. Owing to their chemical and thermal stability, apatite-type materials have attracted a lot of interest. They are used as bone substitutes in orthopedics and dentistry [2-4], fluorescent lamp phosphors [5, 6], laser host [6], gas sensors [7] or catalysis materials [8]. Physico-chemical properties are modified and affected by a wide range of cationic and anionic substitutions [9-11]. For example, the simultaneous substitutions of La 3+ and SiO4 4-by Ca 2+ and PO4 3-, result in a new family of compounds called britholites [12, 13]. These compounds are potential candidates for the retention of certain radionucleides [14, 15]. Indeed, the discovery of the Oklo site [16-18] confirmed their ability to retain actinide elements with thermal and chemical stability under radiation conditions. In addition, silicate apatitic compounds have recently emerged as an alternative electrolytic material for solid oxide fuel cells [19-23]. These compounds are usually prepared by solid state reaction at higher temperatures. In addition, several heat treatments are often required to obtain single-phase materials [24-27]. On the contrary, mechano-chemical synthesis involves only a reaction in the solid state at room temperature [28, 29].
... Increased interest in silicon oxyapatites as solid electrolytes and catalysts stimulated research aimed at developing new methods of their preparation. In particular, a number of low-temperature methods were suggested as alternatives to high-temperature solid-phase synthesis, including methods based on condensation from solutions, namely co-precipitation [34][35][36], various modifications of sol-gel method [37][38][39][40][41][42][43][44][45] as well as soft mechanochemical route [46][47][48][49]. Some interesting results on preparation of lanthanum silicate by co-precipitation have been published by Li and Yang [34,35]. ...
... Absorption bands in the range of 698-850 cm -1 of ν 2 and ν 4 C-O bond vibrations in the CO 3 -group disappear, and two symmetrical bands typical for monodentate carbonate groups on the oxide surface are present in the range of 1200-1400 cm À 1 . At the same time in the case of aluminum-modified sample, intense absorption band with maximum at 917 cm À 1 gets a shoulder in the range of 698-850 cm À 1 that should be assigned to Al-O vibrations in the isolated AlO 4 tetrahedra in the apatitetype silicate structure [46]. ...
Peculiarities of genesis of the structure and texture of apatite-type lanthanum silicates, both undoped and doped with aluminum, in the course of co-precipitation followed by sediment thermal treatment were elucidated by using a combination of methods including thermal analysis, XRD, IR spectroscopy, low-temperature adsorption of nitrogen, 27Al and 29Si MAS NMR spectroscopy and TEM with EDX analysis. An amorphous lanthanum silicate with the short-range ordering typical for ortho diorthosilicate was shown to be formed in the course of precipitation, while its crystallization proceeds via rearrangement of the polymeric structure of primary particles during subsequent thermal treatment.
... Redrígues-Reyna et al. reported partial results of their Rietveld refinement of La x (SiO 4 ) 6 O 1.5x¹12 (x = 9.25, 9.42, and 9.5) synthesized by a mechanical milling method. 75) They only reported the lattice constants and pattern index factors (R wp , R B , R exp , etc.). Refinement of the crystal-structure parameters other than the lattice constants from the XRD pattern reported in this study would be impossible because of low peak intensity and large signal-to-noise (S/N) ratio. ...
... Anisotropic thermal displacement of the La1(4f ) site was observed in all samples. However, almost isotropic thermal displacement of the O4(2a) site was reported in the case of La 9.16 (SiO 4 ) 6 O 1. 75 . ...
The discovery of oxygen-ion conductivity and changes in the crystal-structure model of nondoped lanthanum silicate oxyapatites are reviewed from the researches which led to the discovery to current work. The oxygen-ion conductivity of lanthanoid silicate oxyapatites was discovered by Nakayama et al., during development of new oxide lithium-ion conductors. Although samples with compositions of LiRESiO4 (RE = La, Nd, Sm, Gd, Dy) were initially reported to be single phases with the same crystal structure, the accurate crystal structure of LiRESiO4 was not clarified until later. The crystal structure determined from X-ray diffraction patterns of LiLnSiO4 was revised as oxyapatite by another group. The chemical composition of the crystal phases in LiRESiO4 was also revised to RE10Si6O25 and/or RE9.33Si6O24. The discovery of oxygen-ion conductivity in these materials was confirmed when researchers noted that samples of RE10Si6O25, specifically, the samples without lithium, are electrically conducting. Furthermore, very high oxygen-ion conductivity in the direction parallel to c-axis was discovered after single crystals of RE9.33(SiO4)6O2 (RE = Nd, Pr, and Sm) were successfully grown. The crystal structure and defect models were also altered after the discovery of oxygen-ion conductivity. Although numerous reports related to the electrical conductivity of La9.33+x(SiO4)6O2+3x/2 ceramics have appeared in the literature, clear dependences of the total conductivity on the cation nonstoichiometry (x) as well as the sintering temperature are still unclear because of large discrepancies in the reported data. Furthermore, the composition region, where the lanthanum silicate oxyapatite single phase is formed, is also still unclear because of the inconsistencies in the reported results.
... A polynomial baseline was fitted directly to the corrected Raman spectra which were then normalized. For comparison with the partially crystallized glass samples, the Raman spectra of pure Ca 2 Nd 8 (SiO 4 ) 6 O 2 , CaMoO 4 , and Na 2 MoO 4 were also recorded (acquisition of 60 s for apatite and 3 s for CaMoO 4 and Na 2 MoO 4 ), and the vibrational modes were attributed according to the literature [69][70][71][72][73][74][75][76][77] (Fig. 3). Ca 2 Nd 8 (SiO 4 ) 6 O 2 was synthesized by solid state reaction with a first heat treatment during 24 h at 900°C (heating and cooling at 5°C/min) of a mixture of SiO 2 ? ...
... Consequently, at most two absorption bands are expected at low temperature for each kind of neodymium environment, the position of these bands (apatite) and MoO 4 2-(powellite and Na 2 MoO 4 ) tetrahedra. m1: symmetric stretching; m2: symmetric bending; m3: asymmetric stretching; m4: asymmetric bending [69][70][71][72][73][74][75][76][77]. Asterisk external vibrational mode in apatite, involving the network (translation or libration of SiO 4 entities and Nd 3? or Ca 2? ions). ...
The effect of adding Nd2O3, MoO3 and RuO2 separately or simultaneously on the crystallization of a soda lime aluminoborosilicate glass during cooling from the melt or glass heating was studied by DTA, XRD (at room and high temperature), SEM, Raman and optical absorption. Nd2O3 addition strongly reduces liquid-liquid phase separation and crystallization of calcium and sodium molybdates (CaMoO4 (powellite) and Na2MoO4) in Mo-rich compositions as long as Nd3+ ions remain solubilized in the glassy network. This suggests that (MoO4)2- entities and Nd3+ ions are close to each other in the glass structure (Nd3+ ions would prevent the clustering of molybdate entities). The effect of MoO3 addition in Nd-rich compositions is more complex since an increase of the solubility of Nd2O3 is observed whereas the nucleation rate of a Nd-rich silicate apatite (Ca2Nd8(SiO4)6O2) in the bulk of the glass increases as soon as molybdates crystallized. The addition of RuO2 has a nucleating effect on apatite crystallization in the bulk but not on molybdates crystallization.
... The conventional synthesis of ATLS via solid-state reaction requires very long firing cycles at temperatures in the range of 1200-1600°C. The use of low temperature synthesis allows obtaining nanocrystalline powders, which play a crucial role in the functional ceramic preparation (13)(14)(15)(16). It has recently been reported that the mechanical milling of constituent oxides provides the formation of undoped ATLS at room temperature (14)(15)(16). ...
... The use of low temperature synthesis allows obtaining nanocrystalline powders, which play a crucial role in the functional ceramic preparation (13)(14)(15)(16). It has recently been reported that the mechanical milling of constituent oxides provides the formation of undoped ATLS at room temperature (14)(15)(16). Our results have shown that ATLS doped with different cations can also be easily prepared at room temperature in the similar route using a high power planetary ball mill which provides a significant decrease of milling time (17). ...
Cited By (since 1996):1, Export Date: 26 February 2014, Source: Scopus, References: Nakayama, S., Kageyama, T., Aono, H., Sadaoka, Y., (1995) J. Mater. Chem, 5, p. 1801;
... These anions relate to the different coordination of cations located in 4f and 6 h sites and also to the O 2À anions near the packed SiO 4 tetrahedron (O(1)), near the 4f site (O(2)), near the 6 h site (O(3)) and at the center of the 6 h site (O(4)) [16,17]. Recently, silicate oxyapatite with a stable structure at high temperatures has been studied as an electrolyte for a solid fuel cell [18,19]. Hopkins et al. [20] studied the thermal properties of a Ca 2 La 2 (SiO 4 ) 6 O 2 silicate oxyapatite single crystal in 1973; however, no further research was conducted on this material until Qu et al. [21] recently studied the thermal conductivity of gadolinium calcium silicate apatite. ...
... After comparing measured spectra with literature data of oxyapatite and fluorapatite most of the Raman bands and the different vibration mode regions are marked in Fig. 5. The bands below the 350 cm À1 region are all assigned to external lattice modes and bands above the 350 cm À1 range 10 are all assigned to internal modes of SiO 4 tetrahedral units [19,29,30]. ...
Complex rare-earth silicate oxyapatite RE9.33(SiO4)6O2 (RE = La, Nd, Sm, Gd, Dy) ceramics have been synthesized and their thermal conduction characteristics investigated. When evaluated using a steady-state laser heat-flux technique under conditions ranging from room temperature to 1000 °C the materials demonstrated very low thermal conductivities (0.96–1.49 W m–1 K–1), especially Gd9.33(SiO4)6O2, which shows a value of 1.10–1.14 W m–1 K–1 in the measured temperature range. Phonon mean free path and Raman spectra were used to investigate the thermal transfer mechanism. The source of low thermal conductivity was determined to be the strong intrinsic scattering in the crystal cell, which is due to the phonon mean free path being on the inter-atomic level. Furthermore, a connection between the full width at half maximum Raman spectra and the thermal conductivity of RE9.33(SiO4)6O2 ceramics at room temperature was established. The insensitivity of the thermal conduction properties to temperature for RE9.33(SiO4)6O2 ceramics have allowed it to show great potential in high temperature thermal insulation applications.
... Vibration modes y 1 and y 2 are more intense in Raman spectroscopy. 23,24 asymmetric stretching modes (n 3 ) of the tetrahedral SiO 4 unit molecule. In addition, another such intense band at 842.92 cm −1 is attributed to the symmetric bending mode (n 1 ) and the band at 453.24 and 520.95 cm −1 is attributed to the asymmetric bending mode (n 4 ) of the SiO 4 tetrahedral unit. ...
Transition metal doped apatite La10Si6-x Co x O27-δ (x = 0.0; 0.2; 0.8) and La10Si5.2Co0.4Ni0.4O27-δ are synthesized by co-precipitation method followed by sintering. The precursor precipitates and apatite products are characterized by XRD, FTIR, TGA/DTA, Raman Spectroscopy, SEM-EDX and electrochemical impedance spectroscopy. The presence of apatite phase with hexagonal structure is confirmed through the XRD results. The conductivity measurements of the samples sintered at 1000 °C show that the ionic conductivity increases with increasing content of Co2+ doping into apatite that is further increased by co-doping of Ni2+. The Co doped apatite (La10Si5.2Co0.8O27-δ ) exhibited conductivity of 1.46 × 10-3 S cm-1 while Co-Ni co-doped sample (La10Si5.2Co0.4Ni0.4O27-δ ) exhibited highest conductivity of 1.48 × 10-3 S cm-1. The maximum power density achieved is also for Co, Ni co-doped sample i.e., 0.65 W cm-2 at 600 °C. The results represented show that Co and Ni enhances the SOFC performance of apatite and makes it potential electrolyte candidate for solid oxide fuel cell application.
... Due to their simple crystal structure and perfect ionicity, La 10 Si 6 O 27 appears to form outstanding building blocks for the construction of functional nanostructures, or to serve as a suitable model compound for the investigation of surface reactivity on silicates. Its nanostructures are expected to have novel properties superior to their bulk counterparts due to the quantum confinement effect [7][8][9][10]. ...
The rare earth (RE = Eu, Sm, Dy, and Tb) ions doped La10Si6O27 nanophosphor was synthesized by a simple solution combustion method. The prepared La10Si6O27:RE3+ nanophosphors (LNPs) were subjected to diverse technical tools for exploring their structural, optical, morphological, and electrochemical features. The structural analysis using powder X-ray diffraction (PXRD) patterns revealed the hexagonal oxy apatite phase for LNPs with a crystallite size in the range of 25–50 nm, and the equivalent was affirmed by image analysis via transmission electron microscopy (TEM). Utilizing DRS data, the bandgap energy (Eg) values were recorded for LNPs. Cyclic voltametric (CV), electrochemical impedance spectroscopy (EIS), and sensor studies were performed using a modified carbon paste electrode of LNPs. The modified LNP electrodes were found to be highly effective in sensing paracetamol in acidic medium with a quick response time of 3 secs for sensing the drugs at 1 mM concentration. All the RE ions Eu3+, Sm3+, Dy3+, and Tb3+ (5 mol%)-doped LNPs exhibited the most promising electrochemical sensing characteristics.
... ions were identified by comparison with the pure apatites spectra reported in the literature[19][20][21].External modes corresponding to the liberational modes of PO 4 and SiO 4 groups, were revealed in the frequencies range 100-300 cm -1(Figs. 3a and 4a), while, the bands associated to the vibration modes of both tetrahedrons are shown in Figs. 3 and 4. ...
Two series of strontium-lanthanum apatites, Sr 10-x La x (PO 4) 6-x (SiO 4) x F 2 and Sr 10-x La x (PO 4) 6-x (SiO 4) x O with 0 ≤ x ≤ 6, were synthesized by solid state reaction in the temperature range of 1200-1400°C. The obtained materials were characterized by X-ray diffraction, NMR spectroscopy and infrared and Raman measurements were performed to investigate the crystal structure and vibrational active modes. Pure solid solutions were obtained within a limited range of unsubstituted phosphate and silicate apatites. A variation of the lattice parameters was observed, with an increase of a and a decrease of c parameters, related to the radius of the corresponding substituted ions. The bands of Infrared and Raman spectra were assigned to the vibration modes of PO 4 and SiO 4 groups. The comparison of the results of 29 Si MAS NMR analysis with those obtained with the 31 P, suggested that both species occupied the same crystallographic sites.
... However, it was reported that the calcination temperature of 1650 °C for 10 h was used to get the pure phase of La 10 Si 6 O 27 [16] . Further, with the increase in the concentration of dopant, the peaks were found to shift towards the lower angle side corroborating the phenomenon of expansion of unit cell up on the addition of dopant ions [17] . ...
The current work involves the synthesis of a series of Eu³⁺ (1–11 mol%) doped La10Si6O27 nanophosphors (ELNPs) by solution combustion method using oxalyl dihydrozil (ODH) as a fuel for the first time. The PXRD studies revealed the average crystallite size in the range 15–35 nm for ELNPs. DRS studies revealed the energy gap of Eu³⁺ (1–11 mol %) doped La10Si6O27 is ~5.4 eV. The Photoluminescence (PL) studies of the ELNPs showed all the characteristic emission of Eu³⁺ cations and optimal emission intensity was observed for 5 mol % concentration of Eu³⁺ ions. The excitation maxima detected at 360 nm, 379 nm, 391 nm, 400 nm, 413 nm and 462 nm were conformed to transitions from ⁷F0→⁵G3, ⁵G4, ⁵L6, ⁵D4, ⁵D3 and ⁵D2 states of Eu³⁺ ions respectively. The observed intensities of PL transitions clearly indicated great potentiality for the fabrication of blue/near UV-LED excited phosphor for WLEDs. The calculated CIE chromaticity coordinates confirm the red emission and were depicted in the CIE diagram. The paracetamol and penegra materials were effectively sensed by the prepared carbon paste electrode of Eu³⁺ (3 mol %) doped La10Si6O27 as revealed by cyclic voltammetric and EIS studies. All the above results show that the synthesized ELNPs can be a potential candidate for display and electrochemical sensor applications.
... It is also worth noting that although the lattice modes are strongly coupled, contribution of translational motions of light Ca 2+ ions should be more significant for the higher wavenumber modes whereas much heavier La 3+ ions are expected to contribute significantly to the lower wavenumber modes. Based on literature data we expect that the former motion should contribute to the 125-300 cm À1 modes and the latter to the modes observed below 125 cm À1 [22,28,32]. ...
... Furthermore, this peak becomes more intense at higher lanthanum concentrations (Figure 2b). This peak has widely been reported to originate from the SiO 4 42 ion, which comprises part of the lanthanum silicate X-ray photoelectron spectroscopy (XPS) was carried out to gain a more thorough understanding of the chemical formation of the lanthanum silica xerogels. The XPS spectra in Figure 3 displays the O1s region of the xerogel for samples LaSi(0.15) to LaSi(0.50) ...
This work reports on the nanoscale assembly of hybrid lanthanum oxide and silica structures, which form patterns of interfacial dense and porous networks. It was found that increasing the molar ratio of lanthanum nitrate to tetraethyl orthosilicate (TEOS) in an acid catalysed sol-gel process alters the expected microporous metal oxide silica structure to a predominantly mesoporous structure above a critical lanthanum concentration. This change manifests itself by the formation of a lanthanum silicate phase, which results from the reaction of lanthanum oxide nanoparticles with the silica matrix. This process converts the microporous silica into the denser silicate phase. Above a lanthanum to silica ratio of 0.15, the combination of growth and microporous silica consumption results in the formation of nanoscale hybrid lanthanum oxides, with the inter-nano-domain spacing forming mesoporous volume. As the size of these nano-domains increases with concentration, so does the mesoporous volume. The absence of lanthanum hydroxide (La(OH)3) suggests the formation of La2O3 surrounded by lanthanum silicate.
... The La 2 SiO 5 structure is composed of only isolated tetrahedra [SiO 4 ] 4− [40] whereas La 2 Si 2 O 7 is constituted by only pyrosilicates. [41] The spectra corresponding to the apatite compounds are in good agreement with the spectra reported by Lucazeau et al., [35] Rodriguez-Reyna et al. [36] and more recently by Orera et al. [32] Two relatively intense bands are observed near 850 and 385 cm −1 . The first one is followed by high wavenumber components with lower intensity beyond 1000 cm −1 , and the second displays components between 325 and 600 cm −1 . ...
To characterize the local relaxation in the structure of lanthanum silicate oxyapatite materials, six compositions with different cation and oxygen stoichiometries (La8Ba2Si6O26, La9BaSi6O26.5, La10Si5.5Mg0.5O26.5, La9.33SiO26, La9.67SiO26.5 and La9.83Si5.5Al0.5O26.5) were investigated by combining Raman scattering and 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies. Only [SiO4]4− species were evidenced and the hypotheses of [Si2O7]6− and [Si2O9]8− entities were ruled out. Both oxygen excess and cation vacancies induce local distortions in the structure, which leads to nonequivalent [SiO4]4− species, characterized by different 29Si MAS-NMR signals and by splitting of Raman signals. Copyright © 2011 John Wiley & Sons, Ltd.
... The spectroscopic effects of replacing La with Nd are subtle, the general shape and features of the spectra are preserved, although their R value differs. Comparison with Raman spectra of pure apatite ceramics (not shown) confirms that the Raman signature originates from RE-containing apatites (supported by the presence of the vibration of Q 0 units which are found in apatite structures), 26 although some contribution from the glass matrix can be seen at about 1000 cm À1 . The sharp band located between 850 and 860 cm À1 , attributed to Q 0 silicate tetrahedra, shifts toward higher frequencies when neodymium substitutes for lanthanum, as expected when replacing a network modifying cation by one with higher field strength [ Fig. 7(b)]. ...
Lanthanum and neodymium incorporation in simplified high level waste glasses has been investigated for SiO2–B2O3–Na2O–Al2O3–CaO compositions quenched from 1200°C, for varying La/(La + Nd) (atomic) and increasing rare‐earth oxides contents. In this system and beyond the solubility limit, rare‐earths (RE) elements are reported to form apatite phases with the general formula Ca2RE8(SiO4)6O2. In the current study, speciation of these trivalent RE3+ cations in both amorphous network and crystal phases was determined from X‐ray diffraction, scanning electron microscopy, optical absorption at 10 K, Raman spectroscopy, and electron probe microanalysis. It appeared that RE2O3 solubility was higher for La‐rich formulations than for Nd‐rich ones and that an increase in the RE oxide content reduces the connectivity of the network building units through formation of non‐bridging oxygens at the expense of the oxygen bridges. This depolymerization of the glass network did not affect neodymium environment which consisted in silicate tetrahedra. The composition of the apatite crystals was found to be affected by the La/(La + Nd) of the parent glass and deviation from the ideal composition (Ca2RE8(SiO4)6O2) occurred in the neodymium end of the system. It thus appears that both RE2O3 solubility and crystal composition are strongly dependent on the type and crystal chemistry of the RE elements.
... Moreover, the costs associated with raw materials and processing technologies should be as low as possible. Recently, significant oxygen ionic conductivity in combination with moderate thermal expansion and low electronic transport in a wide range of the oxygen chemical potential were reported for lanthanum apatitetype silicates [5][6][7][8][9][10][11][12]. At 500-600 C, the conductivity of optimized compositions is higher than ionic transport of an yttria-stabilized zirconia (YSZ) and quite close to that of gadolinia-doped ceria [10][11][12]. ...
... Similar difficulties have also been reported by other authors and are suggested to be due to very low particle sizes and amorphization. 50,51 In the current case, difficulties also arise because de-convolution of XRD peaks is affected by the close proximity of coexisting tetragonal and monoclinic polymorph reflections after extended milling. ...
The current work assesses the impact of structural differences between stable and metastable ZrO2 precursors on the mechanochemical preparation of BaZrO3. Monoclinic (m-ZrO2) and tetragonal (t-ZrO2) zirconia polymorphs were prepared without stabilizing additives by slow alkaline precipitation. High-energy milling of the individual ZrO2 precursors induced different partial transformations in each case. The as-synthesized m-ZrO2 powders showed partial conversion to the tetragonal polymorph on mechanical activation, reaching about 10% t-ZrO2 after 420 min accompanied by increases in strain. In contrast, the as synthesized t-ZrO2 powders underwent the inverse transformation to the monoclinic phase, producing about 50% m-ZrO2 after 120 min with the liberation of strain. The t-ZrO2 precursor was shown to exhibit the higher reactivity with barium peroxide, yielding significantly earlier formation of barium zirconate under room-temperature mechanosynthesis. The progress of the mechanochemical formation of BaZrO3 has been discussed with respect to the differing behaviour of the ZrO2 precursors upon mechanical activation and associated thermodynamic perspectives.
In the past few decades, Low temperature co-fired ceramics (LTCC) technology with high thermal conductivity had been developed to meet the requirements of high-density integrated devices packaging. This work investigates the effect of LiF sintering agent on the densification, microstructure and thermal conductivity of LTCC material (Li2SiO3 +4 wt% Al2O3-x wt% LiF (x = 1,2,3,4)). The enhanced densification and a high thermal conductivity value of 11.36 W/mK are achieved when adding 2 wt% LiF and sintered at 940 ℃. Besides, the phonon-dispersion relation of Li2SiO3 lattice indicates that the anharmonicity interaction between low-frequency phonons are the primary factor in reducing the thermal conductivity of material. Furthermore, the Raman spectra were used to analyze the phonon scattering mechanism of all samples. The addition of the LiF changes the full width at half maximum (FWHM) which is inversely proportional to the phonon mean free path. And the reduced FWHM of Raman spectra implies higher thermal conductivity of ceramics. Base on the above results, the novel glass-free high thermal conductivity and low sintering temperature Li2SiO3 + 4 wt% Al2O3-2 wt% LiF ceramic was prepared successfully with great potential in high heat conduction LTCC materials. This work could bring new insights for exploring novel high thermal conductivity ceramic materials.
This paper focused on the synthesis and characterization of silicate britholites (Ca4La6(SiO4)6O). Ca4La6(SiO4)6O has been prepared by two synthetic routes: mechanosynthesis and solid-state reaction at 1200°C. The obtained compounds were characterized by powder X-ray diffraction, infrared absorption spectroscopy, solid ²⁹Si Nuclear Magnetic Resonanceand, and thermal analysis. To determine the influence of the synthetic method on the distribution of lanthanum between the two cationic sites, a refinement by Rietveld method was carried out. Using mechanochemical method, Lanthanum was preferentially located in the sites Me (1), while it has a marked preference for the sites Me (2) using heat treatment method. Thermal analysis revealed that the amount of water and carbone dioxyde decreased after milling during 24 h. Further, the compounds were used to remove methylene blue dye from aqueous solution. At equilibrium, the adsorbed amount of methylene blue reached 385 mg. g⁻¹ for the milled sample. For the calcined material, the dye uptake was reduced to 236 mg. g⁻¹. The kinetics data were well described using pseudo-second-order model, suggesting a chemi-sorption process. Regeneration experiments displayed about 93 % and 80 % removal efficiency, for milled and calcined samples, respectively, after four cycles.
The current research work presents the preparation of Samarium (1–9 mol%) doped Lanthanum Silicate (La10Si6O27:Sm³⁺) nanophosphors (LNPs) by means of solution combustion synthesis utilizing Oxalidihydrazide as a fuel. The prepared LNPs were described by Powder X-ray Diffraction (PXRD) studies, Transmission Electron Microscopy (TEM), Diffuse reflectance spectroscopy (DRS), Cyclic Voltammetry (CV), Electrochemical impedance spectroscopy (EIS) and Photoluminescence (PL) studies. The normal crystallite size of oxyapatite hexagonal phase LNPs were in the range of 10–20 nm as determined by Scherrer's technique and the equivalent was confirmed by TEM. Band gap energy was found in the range of 4.8–5.1 eV utilizing DRS. PL investigations of the LNPs show all the characteristic emissions of Sm³⁺ cations related to transitions ⁴G5/2→⁶Hj/2 (j = 5,7,9,11) and optimal emission intensity was observed for 3 mol% addition of Sm³⁺ ions with orange red emission as depicted in the CIE. CV and EIS results uncover the great electrochemical properties of LNPs, which detect effectively Paracetamol and Sildenafil (Penegra) utilizing modified carbon paste electrodes. Results show that the blended LNPs can be a potential candidate for displays, catalytic and electrochemical sensor applications.
Four different rare-earth oxyapatites of Ca2RE8(SiO4)6O2 (RE = Pr, Tb, Ho, Tm) were synthesized using a solution-based method followed by drying, calcination, and high-temperature sintering in air. X-ray powder diffraction and Raman spectroscopy were performed on the synthesized oxyapatites. The RE oxyapatites crystallize in the hexagonal space group P63/m with similar unit cell parameters, increasing linearly with larger RE cations. The unit cell volumes increase linearly whereas the densities decrease nonlinearly with larger RE cations. Raman spectra showed intense bands of the symmetric bending and stretching modes of SiO4 at ~ 400 and 860 cm−1 regions, respectively. The bands generally shifted to higher frequencies with smaller RE cations in the structures. Rare-earth oxyapatites Ca2RE8(SiO4)6O2 (RE = Pr, Tb, Ho, Tm) crystallize in the hexagonal space group P63/m, and their unit cell parameters increase linearly with larger RE cations.
Ships used to transport human beings to interact in life, consuming fuel not small in number. In general,
almost all of the fishing vessels use diesel engine applications and generator sets as a primary energy source.
Conventional Propeller, until now it is still the primary means of ship propulsion. In practice, there are still
large enough energy is wasted in the propeller that is equal to 40% .With utilizing wasted energy from the
fishing boat propeller rotation is converted by the turbine into electrical energy. Electrical energy conversion
results from the motion of this turbine will be used as power sources in generator sets freezer fishing vessels. By
implementing renewable energy sources Ship_Protech (Ship Propeller Technology) is then obtained alternative
sources of electrical energy that is more efficient, innovative, inexpensive and environmentally friendly. The
method we have done in the research is to redesign of the fishing vessel to obtain the appropriate size and
generate new size with Loa = 30.72 m; B = 7.67 m; H = 3.5 m; T = 2.2 m, from the size of the coefficient
between the propeller to rudder leaves 0.8 m and a turbine with a diameter of 0.5 m with a turbine located at
0.1 m from 0.2 m from the propeller and rudder leaf. Turbines are used that kind savorius vertical axis which
has a blade amounted to 5 pieces, the selection of this turbine is based on the effectiveness of catches that would
be generated by the flow of fluid out of the rotation propeller is then converted by a generator in the turbine to
be electrical energy as a substitute material diesel fuel. The turbine power generating rotation of 1.31 rpm and
obtained by 34.3897 HP or 25.6547 KW. With the power generated by the turbine at 25.9547 KW it is enough to
supply gensets with a power of 20.1 KW. Where the generator is used on a ship (Ship Propeller Technology) is
Brand Kohler CH750. The electrical energy produced will supply and stored in the freezer fishing vessels.
Various advantages Ship_Protech (Ship Propeller Technology) is expected to be applied as an alternative
energy solution for diesel fuel in the freezer fishing vessels to meet the electricity needs and preserving fish.
In this work, the Ca4La6-xNdx(SiO4)4(PO4)2O2 (x = 0, 1, 2, 3, 4, 5, 6) apatites were explored for nuclear waste immobilization, and Nd3+ ions were used as a surrogate of radionuclides (such as Am3+, Cm3+ and Pu3+). The synthesized samples conform to P63/m (176) symmetry in the hexagonal system according to the characterization of X-ray diffraction, Raman spectra and Fourier transform infrared spectra. Rietveld analyses indicate that both Ca2+ and Ln3+ (La, Nd) cations are located at M4f and M6h sites, which is different from the previous study. Furthermore, the M6h sites prefer to be occupied by Ln3+ (La, Nd) cations with higher valence. Besides, the content of impurity phase Ca3(PO4)2 reduces from 2.815 wt % to 0 as the incorporation of Nd3+ ions. These results demonstrate that the apatites possess excellent ability to accommodate radionuclides with various valence and radius in M4f and M6h sites. Moreover, we investigated the thermal expansion behavior by high-temperature X-ray diffraction. There is no phase transformation in the range of 298-1173 K, and the Ca4La6-xNdx(SiO4)4(PO4)2O2 apatites exhibit lower thermal expansion coefficient than other candidates extensively studied. Furthermore, the thermal expansion coefficient decreases gradually with the accommodation of Nd3+ ions. All the results suggest that apatite is a promising candidate for nuclear waste immobilization.
The calcium rare‐earth (RE) silicate oxyapatites, Ca2RE8(SiO4)6O2 (RE = Yb, Er, Y, Dy, Nd, Gd and Sm), powders were synthesized by the solid state reaction method and characterized by X‐ray diffraction, Raman spectroscopy and elemental composition analysis. The thermodynamic properties of the oxyapatites have been investigated using high‐temperature oxide melt calorimetry in molten 2PbO‐B2O3 solvent at 805 °C. The energetics of the oxyapatites related to ionic substitution on two crystallographic sites, M(1) and M(2), are discussed. The enthalpy of formation from the oxides becomes more exothermic as the ionic potential decreases or the ionic radius of the rare‐earths increases, which indicates increasing energetic stability in this order.
This work aims at comparing the damage induced in the Nd silicate apatite ceramic (Ca2Nd8(SiO4)6O2) by medium energy (ME) and swift heavy ion (SHI) irradiations to evaluate the effects of nuclear collisions and intense electronic excitations for ME ions and SHI ions respectively. The macroscopic induced changes were studied as a function of the fluence by swelling and hardness measurements, whereas structural modifications were followed by X-ray diffraction, Raman spectroscopy and grazing incidence EXAFS (Nd L3-edge). At ME (1.9–6.75 MeV Au ions), radiation-induced amorphization occurred above 6.22 × 10¹³ Au/cm² associated with a volume expansion of about 8% and a drop of 37% in hardness. At SHI (90 MeV Xe ions or 35 MeV Ar ions), similar macroscopic and structural changes were observed. The electronic stopping power threshold of Nd silicate apatite for amorphization was assessed at about Se = 5 keV/nm. As apatite crystals containing actinides could be present in rare-earths rich nuclear glasses, SHI irradiation with Xe (995 MeV) ions was also used to damage the Nd silicate apatite crystals dispersed in a soda-lime aluminoborosilicate simplified nuclear glass over a depth of about 60 μm, to evaluate the possible formation of cracks in the residual glass due to crystals swelling. In spite of apatite crystals amorphization under SHI irradiation in the glass-ceramics, no cracking was observed in the glassy phase even close to the biggest crystals which could be explained by strain relaxation in the glass due to plastic deformation (creep) induced by SHI ion beam.
The present study deals with the effects of temperature and residence time of calcinations on substitution process. The upgrading of sedimentary phosphorites coming from the Ras-Dhraa deposit which belongs to the Gafsa-Metlaoui basin (Tunisia) was conducted. The characterization of raw and calcined phosphorites and the evolution of rare earth elements and yttrium (REEs+Y) content and associated gangue were evaluated with different techniques (X-ray powder diffraction (XRD), scanning electron microscopy (SEM/EDX), Fourier transform infrared (FT-IR) and inductively coupled plasma mass spectrometry (ICP-MS)). The effect of increasing residence time and temperature of calcinations on phosphate structure were determined. FT-IR spectroscopy indicates that PO4 3-was substituted by SiO4 4-and SO4 2-and these substitutions cause a worsening of phosphorites properties for sulfuric acid production. A temperature of calcination approximately of 900°C and a period of 30 minutes is sufficient, because higher temperature and longer residence time accentuate these substitutions. Indeed, in these conditions we obtain a phosphate with a better grade of P2O5. It permits to increase the phosphate content from 29.58% to 33.07%. Moreover, REEs+Y contents increased by 5%.
Devices used to carry out mechanochemical transformations are described. The key views of the threshold mechanism of ultrafast mechanochemical synthesis in oxide systems with oxide melting points up to 3000 K in energy intensive mills are presented. Data on structural studies of complex oxides obtained by mechanochemical synthesis are analysed. The specific structural features of metastable complex oxides including high structure disorder, high vacancy concentration and inhomogeneous chemical composition are distinguished. It is shown that these features are responsible for the unusual properties of mechanochemically produced nanopowders. It is noted that currently mechanochemical synthesis is becoming a competitive method for the preparation of oxides and nanomaterials.
Nano-scale lanthanum/serpentine composite powders were prepared by high-energy mechanical ball-milling method. The as-prepared powders were heat treated at different temperatures and the as-treated products were characterized by SEM, XRD and FTIR in order to study the effect of nano-scale lanthanum on the thermal phase transformation behavior of ultrafine serpentine powders. Results show that nano-scale lanthanum can present significant catalytic effects on the thermal phase transformation of serpentine powders. With the addition of lanthanum particles, the three characteristic stages and dominant mechanisms for the dehydration reaction of serpentine powders keep unchanged while the corresponding reaction speed and extent can be significantly improved, the structural and thermodynamic stability is reduced, and the interlayer collapse and phase transformation are enhanced. The main reason for the above results is that the lanthanum combines with[SiO4] tetrahedron to form an apatite-type transitional compound, which can promote the active oxygen atoms and free water conduction between the layers of serpentine.
In this study, we report on the acid-catalysed synthesis of La9.33Si6O26from lanthanum nitrate or acetate and silicon ethoxide (TEOS) in the ethanol solvent, upon the transition from liquid to amorphous and crystalline phases. The similarity of the Fourier transform infrared spectra of the lanthanum-salt solutions and lanthanum-silicon sols indicates that the lanthanum environment is not changed in the reaction of the La-salt with TEOS. In the nitric-acid catalysed synthesis, the hydrolysis reaction was almost instantaneous, as a consequence of a higher amount of water in this system, which contributed to a high level of chemical heterogeneity in the product. The acetic acid-based synthesis ensured a good mixing of the reagents at the nanometre level, which gave rise to the formation of the pure La9.33Si6O26powder upon heating at 900°C, and single phase ceramics with 94% relative density after sintering at 1400°C for 3 h in air, which is 200°C lower temperature then usually reported for the apatite material.
Thorium silicate apatite with the formula [Ca3.84Th0.16]F[Ca2.79Th3.21]T(SiO4)6O2·x(H) was synthesized by solid-state reaction, and its structure refined in P63/m from powder X-ray diffraction (XRD) data using the Rietveld method (a = 9.50172(9) Å, c = 6.98302(8) Å, V = 545.98(1) Å(3); R-Bragg = 2.102%). It was found that thorium partitions strongly to the tunnel (T) 6h position rather than the framework (F) 4f site. Fourier transform infrared spectroscopy revealed only SiO4 tetrahedron, with SiO5 and SiO6 groups, sometimes observed in siliceous apatites absent, at least to the limit of detection of this technique. Thermal expansion of the thorium apatite determined by high-temperature XRD from 298-1173 K found Δa (0.87%) dilation to exceed Δc (0.73%) with increasing temperature consistent with other silicate apatites.
This work is devoted to the synthesis and characterization of apatite-type La9,33(SiO4)6O2+3x/2 (-0,2<x<0,27), as electrolyte material for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC) and potentially Proton Conducting SOFC (PC-SOFC).Pure and dense oxyapatite pellets and membranes were prepared from powders synthetized by solid-state reaction and shaped by isostatic pressing or tape-casting process and thermocompression, then sintered at 1550�C. The electrochemical properties of the samples were characterized by complex impedance spectroscopy and O2 and H2 semi-permeation. La2SiO5 et La2Si2O7 phases, which may appear during the oxyapatite synthesis, were prepared following a similar process, in order to study their electrochemical properties. Theses phases were also used in an experiment whose purpose was to elaborate highly c-axis-oriented apatite-type lanthanum silicate polycrystals by combined use of tape casting and reactive diffusion between La2SiO5 and La2Si2O7.In order to study the protonation process of oxyapatite, tests were performed in autoclave with water at 550�C and 50 bars. Thermogravimetric analyses enabled to locate (surface/bulk) and to make a semi-quantitative analysis of inserted protonic species. The longer the protonation time and the higher the oxygen stoichiometry are, the greater the mass loss is. Moreover, structural studies by X ray diffraction and Raman spectroscopy revealed that the cell volume was increased and the channels of the structure enlarged, while the symmetry was modified and the disorder increased inside them. Thus, it appears that the protonic species insertion in oxyapatite takes place preferentially inside the channels of the structure.
Rare-earth doped oxynitrides have attracted much attention as phosphors for white LEDs. A new family of Ce3+ or Eu2+-doped oxynitride phosphors with the apatite structure has been studied: La8+xSr2-x(Si/Ge)6NyO26+x/2-3/2y. The ammonolysis of an apatite oxide precursor has been used as a general method of synthesis, allowing decreasing the nitriding temperature respective to the classical solid state reaction in N2/H2 atmosphere. The luminescence properties of the obtained phosphors have been studied and relationships with the crystalline structure have been drawn.The apatite structure shows several crystallographic sites available for nitrogen as well as two cationic sites for optically active rare-earth ions. Several structural characterization technique have been used (IR, Raman, NMR, Neutron diffraction) and important information has been obtained concerning the nitrogen distribution in the available positions of the crystal structure. Comparison of the luminescent properties with those of compounds reduced under Ar/H2 allowed attributing emissions to the different crystallographic sites available in the structure.Quantum efficiency measurements have been carried out as well as trials for improving the intensity of the luminescent properties.
The tribological properties of serpentine powders (Mg6Si4O10(OH)8, marked as FGM1) and the admixture of 95 wt% serpentine powders + 5 wt% La2O3 nanoparticles (FGM2) as oil additives were comparatively investigated using an optimal SRV-IV friction and wear tester. The worn surface was characterized by scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectrometry (XPS) and the friction mechanism is discussed. The results show that both FGM1 and FGM2 had excellent tribological performance and 0.5 wt% content was the most efficient in reducing friction and wear at 50 N. With the addition of La2O3 nanoparticles, the tribological properties were further improved compared to those of serpentine powders alone. A smoother and more compact tribofilm with iron oxides, silicon oxides, graphite, organic compounds, and La9.33 (SiO4)6O2 was formed on the worn surface and provided further reduced friction and wear.
This work investigates the characterisation and performance of binary lanthanum cobalt doped silica membranes. The prepared membranes resulted in temperature dependent gas transport, a characteristic of molecular sieving membranes. Under reduction and oxidation cycles at 500 °C, the maximum steady state permeance of the membranes reached 1.5×10−7 mol m2 s Pa at 500 °C. It was found that the membranes contained both cobalt oxide and lanthanum silicate phases embedded in the silica matrix. The permeance of all tested gases under redox cycling decreased during the 350 h permeation test, though the reduction was slightly more pronounced for larger gas molecules (H2, CO2 and N2). As a result, He/CO2 permselectivity increased from ~80 at the initial tests to 196 during the redox cycling test, an improvement of 145%. This indicates that the pore sizes ≥2.89 Å (i.e. kinetic diameter of H2) preferentially collapsed/closed instead of the smaller pores available for the permeation of the smaller gas He (dk=2.6 Å). These results are contrary to other silica and metal doped silica membranes which undergo thermal densification and closure of the smaller pore sizes. Therefore, the formation of lanthanum silicates conferred superior structural stability in the silica matrix.
A complex impedance of oxyapatites Ca2−xBaxLa4Bi4(SiO4)6O2 (0≤x≤2) prepared by solid state reaction has been investigated. The formation of apatites has been checked by X-ray diffraction, FTIR, Raman and 29Si MAS-NMR techniques. The electric impedance data indicate that relaxation phenomena are strongly dependent on temperature in the 923-1048 K range. The bulk resistance decreases with increasing temperature, showing a typical negative temperature coefficient of resistance (NTCR). ac-Conductivity measurements have been performed on a wide range of frequencies and temperatures. The complex modulus plots have confirmed the presence of bulk contributions. The complex impedance analysis suggests the presence of non-Debye relaxations that would be associated with correlation on ions motion.
In this paper, (Ba,Ti)-doped lanthanum silicate nano-sized powders have been synthesized via microwave assisted sol-gel auto-combustion route by using TEOS and lanthanum nitrate as the starting materials, citric acid and glycol as chelators. Both the phase compositions and morphologies of the obtained powders have been characterized. The results indicated that: the synthesized nano powders were characterized as fluffy aggregates with the particle size ranging from 50 to 100nm. As the doping contents of Ba increased, the crystalline sizes decreased and the aggregation were deteriorated, while the particle size decreased from 120nm to 80nm and the aggregation between particles were halted as the doping contents of Ti increased.
Lanthanum silicate La9.5Si6O26.25 (LSO) with an apatite-type structure has been synthesized via a sol-gel process. The microstructure and ionic conductivity of such samples have been evaluated as a function of sintering conditions by X-ray diffraction, scanning electron microscopy, Raman spectra and AC impedance spectroscopy. The result shows that dense pellet LSO of relative density higher than 95% with pure apatite phase can be obtained at low sintering temperature of 1500 degrees C. The samples exhibit an increased conductivity with sintering temperature, owing to grain size effect as well as local modulation of the [SiO4](4-) tetrahedra. LSO sintered at 1650 C exhibits the highest ionic conductivity of 2.70 x 10(-2) S cm(-1) at 800 degrees C. Probed by Raman spectroscopy, the local relaxation of the [SiO4]4- tetrahedra in LSO, tuned by sintering temperature, contributes to a fast conduction pathway for interstitial oxide ions accompanying with low activation energy, which results in a high ionic conductivity. (C) 2013 Elsevier B.V. All rights reserved.
Apatite type lanthanum silicates (ATLS) are a new class of electrolyte materials exhibiting good oxygen ion conductivity in the intermediate temperature range. Electrophoretic deposition of aluminium doped ATLS powders from a non-aqueous solvent on a conductive polymer substrate was assessed by carefully adjusting the charging agent and polyelectrolyte addition. A modified electrode was built to be able to deposit on the non-conductive substrate. The optimised suspension was used to assess the feasibility of depositing a thin ATLS electrode powder layer directly on a precompacted non-conductive porous mixed ATLS-NiO anode. The combined use of optimised suspension along with the modified electrode configuration enabled successful production of green half cells.
The apatite-type compounds Ca(2-x)Ba5La6Biz(SiO4)602 (0 < x < 2) have been prepared by high temperature solid-state reaction technique. Samples have been investigated by powder X-ray diffraction (XRD), infrared absorption spectroscopy (IR), Raman scattering spectroscopy (Raman) and 29Si MAS-NMR spectroscopy. IR, Raman and NMR techniques have been used to demonstrate the presence of isolated SiO4 groups and to investigate the cation distribution around the silicate tetrahedra. The analysis of 29Si MAS-NMR spectra indicates that Ca and Ba distribution becomes disordered at intermediate compositions. An impedance analysis has been used to analyze the electrical behavior of samples as a function of frequency at different temperatures. Evidences of temperature-dependent electrical relaxation phenomena have been observed.
Apatite-type oxides ([A(I)4][A(II)6][(BO4)6]O2), particularly those of the rare-earth silicate and germanate systems, are among the more promising materials being considered as alternative solid oxide fuel cell electrolytes. Nonstoichiometric lanthanum silicate and germanate apatites display pure ionic conductivities exceeding those of yttria-stabilized zirconia at moderate temperatures (500-700 °C). In this study, mixed Si/Ge-based apatites were prepared by hydrothermal synthesis under mild conditions rather than the conventional solid-state method at high temperatures. Single-phase and highly crystalline nanosized apatite powders were obtained with the morphology changing across the series from spheres for the Si-based end member to hexagonal rods for the Ge-based end member. Powder X-ray and neutron analysis found all of these apatites to be hexagonal (P63/m). Quantitative X-ray microanalysis established the partial (<15 at%) substitution of La(3+) by Na(+) (introduced from the NaOH hydrothermal reagent), which showed a slight preference to enter the A(I) 4f framework position over the A(II) 6h tunnel site. Moreover, retention of hydroxide (OH(-)) was confirmed by IR spectroscopy and thermogravimetric analysis, and these apatites are best described as oxyhydroxyapatites. To prepare dense pellets for conductivity measurements, both conventional heat treatment and spark plasma sintering methods were compared, with the peculiar features of hydrothermally synthesized apatites and the influence of sodium on the ionic conductivity considered.
La–Si–O thin films with Si/(La + Si) atomic ratios ranging from 0.36 to 0.43 were produced by magnetron sputtering in a reactive Ar/O discharge gas. The as-deposited films have large X-ray diffraction peak characteristic of quasi-amorphous materials and oxygen contents from 29 to 35 at.%. The Apatite-type lanthanum silicate phase was formed in all the as-deposited films upon annealing at 900 °C for 1 h. The lanthanum silicate films obtained by annealing the as-deposited films with lower Si/(La + Si) atomic ratios have a preferential orientation with the c-axis perpendicular to the substrate while low intensity diffraction peaks ascribed to La2Si2O7 phase were detected in the films deposited with higher Si content. The preferentially oriented films have higher activation energy and lower ionic conductivity as the ionic conductivity measurements were performed in the direction perpendicular to the c-axis. The highest ionic conductivity was obtained for the film deposited with a Si/(La + Si) atomic ratio of 0.42, with a value of 1.2 × 10− 2 S·cm− 1 at 750 °C.
We prepared a heterogeneous SiO2-B2O3-Na2O-Al2O3-CaO-La2O3 glass containing 11.76 wt.% AmO2. The composition of this heterogeneous material was determined from that of non-active glasses in which Am was fully substituted by Nd. For trivalent ion concentrations above the solubility limit, crystal formation occurred in the form of apatite-like silicates, this in both active and non-active glasses. Additions of Am+3 and Nd3+ led to depolymerization of the silicate network which is consistent with the modifier role expected from these cations. Thus, we can compare the incorporation mechanisms of neodymium and americium in the glassy network and within the apatite-like crystalline phases.
A series of apatite-type lanthanum silicates La9.33+xSi6O26+1.5x (LSO) are synthesized via a sol–gel process. Differential thermal analysis, X-ray diffraction, transmission electron microscopy, scanning electron microscopy and Raman measurements are performed to examine the phase present and morphology of LSO. It revealed that LSOs with 9.33 + x ≤ 10 calcined at 900 °C are identified as pure oxyapatite phase with an average grain size of 30 nm. LSOs with 9.33 + x ≤ 9.5 sintered at 1600 °C are also identified as single apatite phase. Based on 2 and 4-probe AC impedance spectroscopy measurements, the dependence of calcination temperature and composition on the electrical conductivity of LSOs are evaluated. Samples with composition of La9.5Si6O26.25 and La9.33Si6O26 function as purely ionic conductors, exhibiting high conductivities of 1.6 × 10−2S cm−1 and 1.3 × 10−2 Scm−1 at 800 °C.
Thin films of La and Si with Si/(La + Si) atomic ratios ranging from 0.36 to 0.44 were produced by magnetron sputtering in pure Ar. For all compositions, the apatite-like La9.33Si6O26 phase was formed during annealing in air at 900 °C. A preferential orientation was developed during annealing of the films with higher silicon content while formation of oxide impurities was detected for the films with less silicon. Silicon segregation to the thin film/substrate interface was observed after annealing for thin films with higher Si/(La + Si) atomic ratios. The higher ionic conductivity values were obtained with the films with lower silicon content (2.81 × 10− 3 Scm− 1 at 800 °C for the film with Si/(La + Si) atomic ratio of 0.36). This film presented the lower activation energy Ea (0.94 eV).
The La10−x(SiO4)6O3−1.5x
(9.33 ≤ 10−x
≤ 9.73) apatite series has been prepared and hexagonal single phases were obtained in a narrow compositional range (9.33 ≤ 10−x
≤ 9.60). The room temperature crystal structure of La9.55(SiO4)6O2.32 has been determined from joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data: a
= 9.7257(1)
Å, c
= 7.1864(1)
Å, V
= 588.68(1)
Å3, Z
= 1, RwpN
= 3.2%, RwpX
= 7.7%, RFN
= 1.8%, RFX
= 1.9%. An interstitial site for the extra-oxygen has been determined in the position very recently predicted in a theoretical study using atomistic simulations. The high temperature crystal structures have been obtained from neutron powder diffraction, NPD, collected at 773 and 1173 K showing the thermal evolution of this interstitial site. Previously reported neutron data for La9.60(GeO4)6O2.40 have also been re-analysed establishing the existence, and thermal evolution, of this interstitial site. The electrical results suggest that the samples are oxide ion conductors. The plots of the imaginary parts of the impedance, Z″, and the electric modulus, M″, vs. log(frequency), possess maxima for both curves separated by two decades in frequency. Bulk conductivities have been obtained from the fitting of the complex impedance spectra with the appropriate equivalent circuit. Bulk activation energies have been determined from two Arrhenius plots, one representing the bulk conductivities and the other representing the frequencies of the modulus peak maxima, fmax(M″). A comparative discussion of the two series, La10−x(TO4)6O3−1.5x
(T = Si, Ge), is given.
A procedure for the synthesis of lanthanum oxyapatite using preliminary mechanochemical treatment of La2O3 and SiO2 mixture is described. An amorphous precursor of the oxyapatite is found to be formed by milling in a planetary ball mill. At subsequent heating at 1000°C for 2 h, polycrystalline lanthanum oxyapatite was obtained.
Recent reports have indicated good fast oxide ion conductivity in apatite silicates. In this article we report on the successful low temperature synthesis of the apatite-type lanthanum silicates, La10(SiO4)6O3 and La9.33(SiO4)6O2, via a sol-gel process. The properties of the resulting apatite phases have been characterised by thermal analysis (TGA-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and conductivity measured by both a.c. impedance spectroscopy (IS) and d.c. methods. The apatite phases may be obtained at 800°C. On further reaction at 1400°C, crystallinity improved, with the apatite structure retained. The room-temperature structure is hexagonal, space group P63 or P63/m, with a = 9.722(1), c = 7.182(1)Å for La10(SiO4)6O3 and a = 9.717(2), c = 7.177(1)Å for La9.33(SiO4)6O2, i.e., the cell volume of La10(SiO4)6O3 is a little greater than that of La9.33(SiO4)6O2. Both compositions exhibit high ionic conductivity, although the grain boundary resistance is the dominant feature in the impedance spectrum of both. At 1000K the total conductivity is 10-3Scm-1. In general, the conductivity of La10(SiO4)6O3 is higher than La9.33(SiO4)6O2 indicating that oxygen interstitials may be introduced into the apatite lattice for La10(SiO4)6O3, which may benefit the oxygen ion transportation. The a.c. and d.c. conductivitis are comparable for La10(SiO4)6O3 but not for La9.33(SiO4)6O2.
The behaviour of three different samples of La2O3 on exposure to atmoshperic CO2 and H2O and the influence of the origin is discussed. The thermal evolution of the samples, stabilized in air for months or even years, has been studied by TG, TPD, IR spectroscopy and X-ray diffraction. BET surface areas of the samples were determined from the corresponding nitrogen adsorption isotherms at 77 K. In all three samples, hydration and carbonation occur in bulk. In accordance with our results, lanthana samples stabilized in air would consist of lanthanum hydroxide, La(OH)3, partially carbonated, La2(OH)6−3x
(CO3)x
,(x⪝1). When hexagonal phases of La2O3, obtained by calcining, at 1130 K, the samples stabilized in air were re-exposed, hydration and carbonation levels similar to those observed in the stabilized samples were reached after less than 24 h. Some lanthana samples were soaked in water, at 298 K, and then dried at 380 K. In this way, the evolution of the oxide when treated under similar conditions to those used in both impregnation and ion exchange techniques for preparation of supported metal phases, could be investigated.
In spite of intrinsic limitations, neutron powder diffraction is, and will still be in the future, the primary and most straightforward technique for magnetic structure determination. In this paper some recent improvements in the analysis of magnetic neutron powder diffraction data are discussed. After an introduction to the subject, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed. Next, we discuss the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure. The simulated annealing approach for magnetic structure determination is briefly discussed and, finally, some features of the program FullProf concerning the magnetic structure refinement are presented and discussed. The different themes are illustrated with simple examples.
The broadening of powder pattern peaks has been studied by three methods—Fourier analysis, integral breadth measurements, and variance of the line profiles. The results obtained from the variances are compared with those obtained from the integral breadths and Fourier coefficients. Tungsten filings were prepared at room temperature and their powder pattern peaks were recorded with a Norelco diffractometer using filtered Cu K α radiation. The variances, integral breadths, and Fourier coefficients were calculated with the IBM 7094 computer. The results indicate that the variance is very sensitive to the range of integration s2 — s1 = (2θ 2 — 2θ 1) cos θ0/λ. An error of ± 10% in this range due to the difficulty in choosing the correct background changes the values of the variance significantly and the integral breadth to a lesser extent. However, the same error does not affect the values of the Fourier coefficients. Comparing the particle sizes and strains obtained by the three methods, it was found that the strains agreed remarkably well. The particle size calculated from the variance was smaller (D eW = 150Å) than that evaluated from the initial slope of the Fourier coefficients (D e = 210Å) and from the integral breadths 2D e≃ D I= 430Å.
The silicated apatite Na1.5Sm8.5(SiO4)6FO crystallizes in the hexagonal system, with a=9.4761(6) Å, c=6.9484(4) Å cell parameters, P63/m space group, and Z=1. The samarium distribution over the 4f and 6h positions is not uniform. Due to their polarizability higher than the one of sodium, samarium ions mainly set on the 6h position, but their occupation of this position rate is limited by the valence required for fluorine. The Raman spectra, collected in various polarizations, are consistent with the proposed space group. An attribution of several external modes is given by comparison with the spectra of Ca6Sm2Na2(PO4)6F2, Ag2Pb8(PO4)6, Na2Pb8(PO4)6 and Ca10(PO4)6F2 (fluorapatite).RésuméL'apatite silicatée Na1,5Sm8,5(SiO4)6FO cristallise dans le système hexagonal dans une maille de paramètres a=9,4761(6) Å, c=6,9484(4) Å, avec pour groupe d'espace P63/m, Z=1. La distribution du samarium sur les sites 4f et 6h n'est pas uniforme. Le samarium, plus polarisable que le sodium, occupe préférentiellement le site 6h, mais cette occupation est limitée par l'exigence de la valence de liaison du fluor. Les spectres Raman, enregistrés dans diverses polarisations, sont en accord avec cette structure. L'étude comparée des spectres Raman de ce composé avec ceux de Ca6Sm2Na2(PO4)6F2, Ag2Pb8(PO4)6, Na2Pb8(PO4)6 et Ca10(PO4)6F2 (la fluorapatite), est en outre utilisée pour estimer les mouvements atomiques mis en jeu par plusieurs modes externes.
Raman spectra have been obtained for La2O3dispersed on Al2O3, but only after La2O3coverages reach one theoretical monolayer or higher. Sharp peaks at 104, 191, and 408 cm−1were observed, which are in excellent agreement with previous studies of bulk La2O3; however, two additional broader, but intense bands not previously reported were also observed at 322 and 935 cm−1. These latter two bands disappear at temperatures of 450 K or higher, but they returned with full intensity upon cooling to 300 K. A sample of bulk La2O3(a-form) with a low surface/volume ratio also exhibited weak peaks at these latter two positions. Raman spectra taken with different exciting laser lines showed that the bands at 322 and 935 cm−1can be attributed to a laser-excited luminescence. It is inferred that these luminescence bands are evidence for the presence of oxygen anion vacancies. After exposure to NO at 300 K, two peaks developed at 747 and 1047 cm−1which can be assigned to a surface nitrate species.
Ionic conductivities have been investigated for lanthanoid silicates of the Ln10(SiO4)6O3 solid solution series and related compounds. The activation energy and conductivity at 500 °C were estimated to be 69 kJ mol–1 and 1.8 × 10–4 S cm–1 for La10(SiO4)6O3 and 71 kJ mol–1 and 1.5 × 10–4 S cm–1 for Nd10(SiO4)6O3. The a and c lattice constants of the hexagonal phase decreased with decreasing radius of the Ln3+ ion for Ln10(SiO4)6O3(Ln = La, Nd, Sm, Gd and Dy). The activation energy increased and the conductivity decreased when Ln3+ ions with smaller ionic radii were used. The sole carrier in these materials is the O2– ion.
The silicated apatite Na1.5Sm8.5(SiO4)6FO crystallizes in the hexagonal system, with a=9.4761(6) Å, c=6.9484(4) Å cell parameters, P63/m space group, and Z=1. The samarium distribution over the 4f and 6h positions is not uniform. Due to their polarizability higher than the one of sodium, samarium ions mainly set on the 6h position, but their occupation of this position rate is limited by the valence required for fluorine. The Raman spectra, collected in various polarizations, are consistent with the proposed space group. An attribution of several external modes is given by comparison with the spectra of Ca6Sm2Na2(PO4)6F2, Ag2Pb8(PO4)6, Na2Pb8(PO4)6 and Ca10(PO4)6F2 (fluorapatite).
Electrical properties were investigated for lanthanoid-silicates of Ln10(SiO4)6O3(Ln = La, Nd, Sm, Gd and Nd). The conductivity at 773 K was 2.3 × 104 S cm−1 for Nd10(SiO4)6O3. The sole carrier is the O2− ion, which was determined using an O2 gas concentration cell.
Cubic lanthanum oxide was prepared by the oxidation of lanthanum iodide at 700°C in air atmosphere. The oxide was characterized by X-ray fluorescence analysis, X-ray diffraction, and Fourier-transformed infrared spectroscopy. The cubic La2O3 is most likely a single lanthanum oxide phase containing periodate hydrate and hydroxycarbonate species. The cubic lanthanum oxide is found to be chemically stable even if they are dispersed in water because of the presence of hydroxycarbonate and periodate hydrate species which inhibit the bulk hydroxylation.
In this paper we report an investigation into the relationship between structure and conductivity for the two apatite-type phases, La9.33Si6O26 and La8Sr2Si6O26. High oxide ion conduction has been observed in the former (σ=1.2×10−4 S cm−1 at 700°C, Ea=0.73 eV), whereas the oxide ion conductivity of La8Sr2Si6O26 is low (σ=2.9×10−7 S cm−1 at 700°C, Ea=1.34 eV). Structural refinement of powder neutron diffraction data showed that both compounds have a hexagonal cell (space group P-3; a=b≈9.7, c≈7.2 Å). In addition to the presence of vacancies on the cation sites, the sample La9.33Si6O26 also showed a key difference in the channel oxygen sites compared to La8Sr2Si6O26. For the latter the channel oxygen atoms all appear to be located close to the ideal (0,0,0.25) site, whereas for La9.33Si6O26, ∼14% of these oxygens are displaced to a new position around (0,0,≈0.38). From these studies we can propose that the higher conductivity and lower activation energy for oxide ion conduction in La9.33Si6O26 compared to La8Sr2Si6O26 is due to this observed disorder on the channel sites.
Single crystals of Nd9.33(SiO4)6O2 and Sr2Nd8(SiO4)6O2 oxide ion conductors with the oxyapatite structure were grown without any macroscopic defect by the floating zone method. The oxide ion conductivity of the Sr2Nd8(SiO4)6O2 single crystal varied with the distance from its seed crystal along the growth direction, because of its changing deficiency in Sr content. Its stoichiometric portion had a lower electrical conductivity by about five orders of magnitude at 600 °C than the value for Nd9.33(SiO4)6O2 single crystal. Structural refinement of neutron diffraction data for powdered Nd9.33(SiO4)6O2 single crystal showed that the previous structure analysis was misleading. Cation vacancies were present only at 4f site and its channel oxygen site was fully occupied in space group of P63/m. The oxygen had an anisotropic displacement along the channel in the refined oxyapatite structure.
Atomistic simulations have allowed us to gain fresh insight into the mechanisms of oxygen ion transport in novel apatite silicates, which form part of a new family of ionic conductors for potential fuel cell applications.
Crystallographic studies of La2O3 and the oxides isostructural with it have failed to decide between noncentrosymmetric space group D32(P321) and the centrosymmetric space group D3d3(P3m1) for their structure. The infrared and Raman spectra of La2O3 and Nd2O3 have been obtained and the absence of coincidences between the spectra unequivocally favours the centrosymmetric space group. The vibrational spectra have been analysed using the Wilson FG matrix method and force constants for metal-oxygen and oxygen-oxygen interactions derived, assuming a quadratic central force field.
This paper reports the mechanochemical synthesis and the structural and microstructural characterization of three titanates, A2 Ti2 O7 (A = Y, Gd and Dy), with a pyrochlore-like structure. Starting from stoichiometric mixtures of elemental oxides TiO2 and A2 O3 , single-phase samples of highly disordered pyrochlores were
obtained after milling. Differential thermal analysis of the as prepared powders showed in every case the presence of a single exothermic event at temperatures close to 800C. The evolution of the structure and microstructure of these highly-disordered pyrochlores with temperature was studied by combining XRD and IR and Raman spectroscopies. On heating, both the cation and anion arrays in Y2 Ti2 O7 and Dy2 Ti2 O7 , order by two independent processes. The exothermic events observed in DTA have their origin in the ordering of the anion sublattice, whereas cation ordering progress smoothly with temperature. Gadolinium titanate, Gd2 Ti2 O7 , behaves differently to the other two compounds studied concerning the cation sublattice: no evidence of cation disorder is observed, even in the just-milled sample.
A new process called “mechanical alloying” has been developed which produces homogeneous composite particles with an intimately
dispersed, uniform internal structure. Materials formed by hot consolidation of this powder achieve the long-sought combination
of dispersion strengthening and age-hardening in a high temperature alloy. While the process is amenable to making a variety
of alloys, its first use has been to combine yttrium oxide and gamma prime hardening hr at 1400°F and 15,000 psi for 100 hr
at 1900°F together with excellent sulfidation and cyclic oxidation resistance. From a fundamental standpoint, results show
that the age-hardening dominates the low-temperature strength, dispersion strengthening dominates at high temperature, and
the two are augmentative in the intermediate temperature range 1300° to 1500°F.
Electrical properties of the high oxide ionic conductive ceramic RE10Si6O27 (RE=La, Pr, Nd, Sm, Gd, Dy) sintered at 1700–1800 °C by the use of MgO-stabilized zirconia setter during a sintering have been investigated. The lowest activation energy and the highest conductivity at 200 °C were achieved for Pr10Si6O27 (55.4 kJmol−1) and La10Si6O27 (1.32 × 10−5 S cm−1), respectively. The conductivity of La10Si6O27 at 200 °C was higher by order of 1.5 × 101 times than that (8.82 × 10−7 S cm−1 at 200 °C) of (Bi2O3)0.75(Y2O3)0.25. The three-point bending strength of La10Si6O27 was 100 MPa.
Computer modelling techniques have been used to examine the mechanistic features of oxygen ion transport in the La8Sr2Si6O26 and La9.33Si6O26 apatite-oxides at the atomic level. The potential model reproduces the observed complex structures of both phases, which are comprised of [SiO4] tetrahedral units and La/O channels. Defect simulations have examined the lowest energy interstitial and vacancy sites. The results suggest that oxygen ion migration in La8Sr2Si6O26 is via a vacancy mechanism with a direct linear path between O5 sites. Interstitial oxygen migration is predicted for La9.33Si6O26via a non-linear (sinusoidal-like) pathway through the La3/O5 channel. The simulations demonstrate the importance of local relaxation of [SiO4] tetrahedra to assist in the facile conduction of oxygen interstitial ions. In general, the modelling study confirms that the high ionic conductivity in silicate-based apatites (with oxygen excess or cation vacancies) is mediated by oxygen interstitial migration.
Solid oxide fuel cells have considerable interest in recent years, because of their high efficiency and environmentally friendly nature. Such systems required oxygen-conducting electrolytes and now the most common electrolyte is yttria stabilized zirconia (YSZ). This compound exhibits high oxide ion conductivity at elevated temperatures (850–1000 °C). However, this high working temperature causes problems in terms of materials selection and lifetime. One solution is to develop new oxide ions conductors exhibiting high oxide ion conductivity at intermediary temperatures (700–800 °C). Recent work has identified Ln10−xSi6O26±z (Ln = rare earths) as a good fast oxide ion conductor.
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