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1.
L. P. Lyashenko I. V. Kolbanev L. G. Shcherbakova E. I. Knerel'man G. I. Davydova 《Inorganic Materials》2004,40(8):833-839
The phase transitions in TiO2–Sc2O3 (40–50 mol % Sc2O3) samples prepared from coprecipitated and mechanically activated oxide mixtures are studied. It is shown that heat treatment below 1000°C leads to the formation of nonstoichiometric, metastable cubic phases with a distorted fluorite structure. With increasing temperature, this structure transforms into a stable fluorite-like structure through (low-symmetry) orthorhombic and hexagonal phases, which is associated with the incorporation of OH groups. Characteristically, the phases studied contain nanoscale regions differing in the degree of ordering, which result from internal stress. The internal stress in mechanically activated phases is shown to be fully relieved by about 1300°C. 相似文献
2.
The phase composition, electrical conductivity, and structural and electrical stability of ZrO2–Sc2O3–Cr2O3 solid electrolytes prepared by solid-state reactions involving three-step firing at 1350, 1850 (vacuum), and 1300°C were studied for compositions along two lines: x(0.91ZrO2 + 0.09Sc2O3)–yCr2O3 (I) andx(0.89ZrO2 + 0.11Sc2O3)–yCr2O3 (II), x + y = 1, y = 0–0.04. The results indicate that the ternary solid solutions withy= 0.01–0.02 retain a cubic structure in a broad temperature range, down to room temperature. This increases the low-temperature (<600°C) conductivity of the solid electrolytes, especially in system II. In both systems, Cr2O3 solubility is about 3 mol %. Stability tests at 900°C for 200 h reduce the conductivity of the solid electrolytes, particularly at the lower Sc2O3 content and in the presence of Cr2O3. The reduction in conductivity is due to the decomposition of the high-temperature tetragonal phase and the formation of a tetragonal phase with a low stabilizer content. 相似文献
3.
The (metastable) tetragonal phase in 3–4 mol% Y2O3-ZrO2 alloys undergoes a transition to the monoclinic form in the 200–300 °C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating. The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic tetragonal transition in ZrO2-1 mol% Y2O3 alloy at 850–750 °C and the same phase change in 2, 3, 4 and 6 mol% Y2O3 compositions at the eutectoid temperature of about 560 °C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol% Y2O3 compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1–12 mol% Y2O3 region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction. 相似文献
4.
Chemical and structural properties of the mixed metal oxides (1–x)Fe2O3+xCr2O3 were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe1–x
Cr
x
)2O3, over the whole concentration region, 0x1. The gradual replacement of Fe3+ with Cr3+ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx0.3–0.9. The samples having the fraction of Cr2O3 in the region from 0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.57Fe Mössbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr2O3 content. The sample having the fraction of Cr2O3 of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr2O3 content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for -Fe2O3 to the spectrum typical for Cr2O3 was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and57Fe Mössbauer spectroscopic results. 相似文献
5.
Fine particles of the amorphous Cr2O3-Fe2O2 solid solutions were prepared by dehydration of coprecipitated hydroxides and their crystallization behavior was studied by differential thermal analysis and X-ray diffraction. The peak temperature for crystallization attained a maximum at a composition near Fe2O3 content of about 60 mol % and the activation energy for crystallization attained a minimum at a composition near Fe2O3 content of about 50 mol % in this quasibinary system. Phase separation occurred in a range of Fe2O3 content from about 35 to 80 mol % in the corundum-type solid solutions heat treated at 600 °C for 2 h. Crystallization behavior was discussed briefly related with phase separation and diffusion in fine particles. 相似文献
6.
L. I. Podzorova A. A. Il'icheva N. A. Mikhailina V. Ya. Shevchenko D. S. Bashlykov G. V. Rodicheva L. I. Shvorneva 《Inorganic Materials》2001,37(1):51-57
The morphology and phase composition of ultrafine ZrO2–CeO2–Al2O3 powders prepared by heat-treating coprecipitated and successively precipitated hydroxides were shown to depend on the precipitation procedure. The relative amounts of tetragonal and monoclinic zirconia in the sol–gel powders can be controlled by adjusting the precipitation conditions. 相似文献
7.
The sintering characteristics of SiO2-36.6 wt % Al2O3 powder, prepared by condensation from high frequency plasma, have been studied and microstructural changes occurring during sintering followed by transmission electron microscopy The as-prepared amorphous powder showed evidence of spinodal decomposition into an Al2O3-rich and SiO2-rich glass consistent with the position of a previously reported metastable miscibility gap in the SiO2-Al2O3 system. Mullite crystallized on an extremely fine scale at 1000° C and gradually coarsened at higher temperatures. Sintering occurred above 1100° C by a viscous flow mechanism with activation energy 87 kcal mol–1 which corresponds to the activation energy for viscous flow of SiO2-Al2O3 glass containing approximately 17 wt % Al2O3. 相似文献
8.
The effect of additive amount on the gas-pressure sintering of silicon nitride is investigated. Silicon nitride containing 0.5 to 10 mol % (SN10) of equimolar Y2O3-Nd2O3 is fired at 1600 to 1900 °C for 4 h in 10 M Pa N2 gas. A small amount of oxide (1 mol %; SN1) is effective for densification as well as a larger amount of oxide (6–10 mol %) when fired at 1900 °C. Composition analysis of sintered specimens indicates that SN1 densifies through a small amount of SiO2-rich liquid-phase, whereas SN10 densifies by way of a large amount of additive-oxide-rich liquid phase. 相似文献
9.
The structure of Al2O3-SiO2 sub-micron powders prepared by oxidation of mixed aluminium-silicon halides in an oxygen-argon high frequency plasma flame has been studied. The powders were completely amorphous up to at least 52 wt % Al2O3 and partially amorphous in the range 52 to 88 wt % Al2O3. The crystalline phase was mullite up to 75 wt % Al2O3 but at higher Al2O3 contents a metastable solid solution of SiO2 in -Al2O3 was observed in addition to mullite. Amorphous particles crystallized to mullite on heating to 1000°C, independently of composition. Extension of glass formation towards the high Al2O3 end of the Al2O3-SiO2 system as the cooling rate is increased and particle size decreased, may be explained by the effect of viscosity on the nucleation rate of mullite from liquid, for Al2O3 contents up to 60 wt %. The viscosity change is relatively small as the Al2O3 content is increased beyond 60% and it is suggested that the change in nucleus-liquid interfacial energy with composition is the predominant factor controlling nucleation rate in this range. At Al2O3 concentrations greater than approximately 80 wt %, -Al2O3 is the phase which nucleates from the melt. A double DTA peak was observed for powders containing more than 80 wt % Al2O3. The lower temperature peak is believed to arise from the formation of mullite from a metastable solution of SiO2 in -Al2O3, and the higher temperature peak from crystallization of mullite from the amorphous phase. The presence of SiO2 in solution in metastable Al2O3 increases the temperature of transformation to -Al2O3 to greater than 1500° C compared with 1230° C for pure Al2O3. 相似文献
10.
A metastable binary phase diagram between SiO2 (cristobalite) and-Al2O3 (corundum) in the absence of any mullite phase is presented. A eutectic is indicated at a temperature of 1260° C and a composition of 18 wt% ( 12 mol%) Al2O3. The liquidi of the proposed metastable system were positioned on the basis of the thermodynamic data calculated from the stable equilibrium diagram of Aksay and Pask [2]. Experimental evidence is also presented. A SiO2-Al2O3 melt containing 80 wt% Al2O3 cooled at a slow rate in sealed molybdenum crucibles shows crystalline Al2O3 plus a glass phase whose composition followed the calculated extension of the stable Al2O3 liquidus to lower temperatures. Compacts of cristobalite—corundum mixtures were fired at subsolidus temperatures to estimate the eutectic temperature experimentally. The proposed metastable phase diagram effectively explains the formation of non-crystalline phases in subsolidus reactions, and microstructure obtained on solidification of high alumina melts. 相似文献
11.
T. N. Kol'tsova 《Inorganic Materials》2001,37(4):378-385
The phase relations in the Y2O3–WO3–CuO system were studied by x-ray diffraction and thermal analysis. The results were used to construct the 800°C section of the phase diagram. Based on the new and earlier data on the liquidus relations, the section through the Y2O3–WO3–CuO phase diagram along the Y2O3–CuWO4join was mapped out. 相似文献
12.
The system Fe2O3-In2O3 was studied using X-ray diffraction,57Fe Mössbauer spectroscopy and infrared spectroscopy. The samples were prepared by chemical coprecipitation and thermal treatment of the hydroxide coprecipitates. For samples heated at 600 °C, a phase, α- (Fe1?x In x )2O3, isostructural with α-Fe2O3, exists for 0?x?0.8, and a phase C-(Fe1?x In x )2O3, isostructural with cubic In2O3, exists for 0.3?x?/1. In the two-phase region these two phases are poorly crystallized. An amorphous phase is also observed for 0.3?x?0.7. For samples heated at 900 °C the two-phase region is wider and exists for 0.1?x?0.8 with the two phases well crystallized. In these samples an amorphous phase is not observed.57Fe Mössbauer spectroscopy of samples prepared at 600 °C indicated a general tendency of the broadening of spectral lines and the decrease of numerical values of the hyperfine magnetic field (HMF) with increasing molar fraction In2O3 in the system Fe2O3-In2O3. The samples prepared at 900 °C, in the two-phase region, are characterized by a constant HMF value of 510 kOe at room temperature. Infrared spectroscopy was also used to follow the changes in the infrared spectra of the system Fe2O3-In2O3 with gradual increase of molar fraction of In2O3. A correlation between X-ray diffraction, Mössbauer spectroscopic and infrared spectroscopic results was obtained. 相似文献
13.
K. Park J.-G. Kim W.-S. Cho 《Journal of Materials Science: Materials in Electronics》2002,13(1):13-19
Rutile TiO2 (a=4.594 å and c=2.958 å) phase was formed on the outer region of Ti powders after oxidation at 600 °C for 1–300 h. Porous (Ba,Sr)TiO3 ceramics were fabricated by adding partially oxidized Ti powders (4–8 vol %) into (Ba,Sr)TiO3 powders, and showed excellent positive temperature coefficient of resistivity (PTCR) characteristics after paste-baking treatment at 580 °C in air. The PTCR characteristics of the porous ceramics were mainly attributed to the adsorption of oxygen at the grain boundaries. The microstructure and electrical properties of the porous (Ba,Sr)TiO3 ceramics containing the partially oxidized Ti powders oxidized at 600 °C for different oxidation times (1–300 h) were investigated. 相似文献
14.
V. V. Zyryanov 《Inorganic Materials》2003,39(11):1163-1171
The phase composition of crystalline mechanochemical synthesis products in the CaO–Sb2O3–Bi2O3 system was determined. Of the known phases in this system, only three could be prepared mechanochemically: Ca2Sb2O5, CaSb2O4, and CaBiO2.5 (fcc). A new metastable phase, "-Bi2O3, with an orthorhombic structure close to that of the high-temperature, fluorite phase -Bi2O3, was obtained by mechanical processing at 30°C. A number of new metastable fluorite solid solutions of binary and ternary oxides were obtained as single-phase powders by mechanochemical synthesis. The mechanochemical yield of primary crystalline products was shown to be several times higher than that of secondary products. A broad composition range was revealed in which perovskite and fluorite phases are in mechanochemical equilibrium. The composition dependence of the lattice parameter of the metastable fluorite phase Bi2 – x
Sb
x
O3 was found to be the opposite of the one predicted by Vegard's law. Metastable mixed oxides undergo phase transformations during heating (starting at 280°C in the case of the ternary perovskite phase). Bi2 – x
Ca
x
O3 – 0.5x
fluorite solid solutions experience a transformation at 400°C, accompanied by oxygen loss. During heating in air, Sb2O3-containing fluorite phases partially stabilize owing to oxidation but, nevertheless, undergo structural transformations above 480°C. The transformation of Sb2 – x
Ca
x
O3 – 0.5x
metastable fluorite solid solutions near 500°C in air is accompanied by the formation of needle-like Sb2O3 crystals. A mechanism is proposed for the extremely rapid growth of such crystals: extrusion of the Sb2O3 resulting from fluorite decomposition in agglomerates through triple junctions of aggregates and through cracks in the surface layer of agglomerates. 相似文献
15.
The reaction mechanism of PbMg1/3Nb2/3O3-PbZrO3-PbTiO3 (PMN-PZT) perovskite phase prepared by the columbite route has been studied in the temperature range from 600 to 800 °C. The effects of heating and cooling rate during the calcination of 3PbO +MgNb2O6+PZT powder mixtures have also been investigated. Nearly pure perovskite phase, 0.9 PMN-0.1 PZTsolid solution with no pyrochlore phase, as determined by X-ray diffraction, could be prepared at 800 °C for 2 H. From DTA/TGA, dilatometry and XRD data the reaction mechanism of PMN-PZT solid solution formation could be divided into three steps: (i) decomposition of columbite (MgNb2O6) by reacting with PbO at 350 to 600 °C (ii) the formation of a B-site-deficient pyrochlore phase Pb2Nb1.33Mg0.17O5.50 at close to 650 °C, and (iii) the formation of perovskite phase PMN-PZT solid solution from the reaction of Pb2Nb1.33Mg0.17O5.50 pyrochlore phase with MgO and PZT above 650 °C. 相似文献
16.
H. Watanabe K. Hirota O. Yamaguchi S. Inamura H. Miyamoto N. Shiokawa K. Tsuji 《Journal of Materials Science》1994,29(14):3719-3723
In compositions having ZrO2/Y2O3=(74.25–71.25)/(0.75–3.75) (mol% ratio) with 25 mol% Al2O3, metastable t-ZrO2 solid solutions crystallize at 780° to 860°C from amorphous materials prepared by the simultaneous hydrolysis of zirconium, yttrium and aluminium acetylacetonates. Hot isostatic pressing has been performed for 1 h at 1130 and 1230°C under 196 MPa using their powders. Two kinds of material are fabricated: (i) perfect ZrO2 solid-solution ceramics and (ii) composites of ZrO2 solid solution and -Al2O3. Their mechanical properties are examined, in connection with microstructures and t/m ZrO2 ratios. Composites with a homogeneous dispersed -Al2O3 derived from solid-solution ceramics result in a remarkable increase of strength. 相似文献
17.
S. F. Horvath M. P. Harmer D. B. Williams M. R. Notis 《Journal of Materials Science》1989,24(3):863-872
Analytical electron microscopy has been used to study the precipitation reactions in sintered samples of 9 mol% La2O3-Y2O3 samples upquenched from the single phase cubic region into the cubic and hexagonal phase field. Samples annealed just inside the two-phase cubic-cubic and hexagonal solvus exhibited predominantly grain boundary precipitation. Small La2O3 rich second phases formed within the first ten minutes and developed into strained, facetted precipitates after 300 min. Intergranular and intragranular precipitation occurred in samples annealed further into the two-phase field. Strained, lathlike La2O3-rich monoclinic precipitates, exhibiting a preferrred orientation in the matrix, appeared as the dominant morphology for long times at temperature. Chemical microanalyses of the strained structures obtained in samples annealed for 300 min revealed La2O3 matrix concentrations in agreement with phase diagram predictions. However, the La2O3 concentrations in the second-phase precipitates were found to be far in excess of the cubic and hexagonal-hexagonal solvus. This discrepancy is believed to arise from a re-equilibration of the second phase in the cubic and monoclinic phase field during quenching. 相似文献
18.
Mixed metal oxides in the system Fe2O3-NiO were prepared by coprecipitation of Fe(OH)3/Ni(OH)2 and the thermal treatment of hydroxide coprecipitates up to 800 or 1100°C. X-ray diffraction showed the presence of -Fe2O3, NiO and NiFe2O4 in samples prepared at 800°C. The oxide phases -Fe2O3, NiO, NiFe2O4 and a phase with structure similar to NiFe2O4 were found in samples prepared at 1100°C. Fourier transform-infrared spectra of oxide phases formed in the system Fe2O3-NiO are discussed. Two very strong infrared bands at 553 and 475 cm–1, a weak intensity infrared band at 383 cm–1 and two shoulders at 626 and 441 cm–1 were observed for -Fe2O3 prepared at 1100°C. NiFe2O4, prepared at the same temperature, showed two broad and very strong infrared bands at 602 and 411 cm–1, while NiO showed a broad infrared band at 466 cm–1. Fourier transform infrared spectroscopic results were in agreement with X-ray diffraction. 相似文献
19.
O. Berger T. Hoffmann W.-J. Fischer V. Melev 《Journal of Materials Science: Materials in Electronics》2004,15(7):483-493
The electrical response of tungsten-oxide thin films as-deposited by electron-beam deposition and annealed (at 350–800 °C for 1–3 h in O2) to NO2, O3 and H2S was studied both experimentally and theoretically. In order to interpret the kinetic characteristics of tungsten-oxide thin films on exposure to different gases, a model based on surface adsorption/desorption processes coupled with bulk diffusion was used. A link between the geometrical and chemical heterogeneities of the tungsten-oxide film surfaces and their performance characteristics as gas sensors was established. It was shown that the nature and amount of surface-adsorption sites in the different nonstoichiometric phases (W
n
O3n–2 or W
n
O3n–1) and WO3 as well as their conduction mechanisms are defined from the phase composition of the film, the crystallographic and electronic structures of the phases, the orientation of the crystallites within the film and the geometrical shape and dimensions of the crystallites. All tungsten-oxide thin films investigated in this work are suitable for detection of very low concentrations of NO2 (0.05–0.5 ppm in N2 and synthetic air), ozone (25–90 ppb) and H2S (3–15 ppm in N2 and synthetic air) at very low working temperatures (80–160 °C). The films annealed at 400 °C for 1–2 h are very selective to ozone at 120–160 °C; the films annealed at 400 °C for 1–3 h and at 800 °C for 1 h are very sensitive to NO2 (in N2). 相似文献
20.
Subsolidus phase equilibria in the MgO-Ga2O3-SiO2 system were studied by sintering 18 compositions. A stable gallium analogue of sapphirine and a metastable mullite-like phase were prepared. Ga-sapphirine melts incongruently to MgGa2O4 and becomes liquid at 1490 ± 5° C. A diagram showing subsolidus phase compatibility at 1390° C is presented. 相似文献