共查询到20条相似文献,搜索用时 15 毫秒
1.
Hongwu Xu Alexandra Navrotsky M. Lou Balmer Yali Su 《Journal of the American Ceramic Society》2002,85(5):1235-1242
The crystal structures for a suite of substituted pollucites with the compositions CsTi x Al1– x Si2 O6+0.5 x , 0 ≤ x ≤ 1, have been determined from Rietveld analysis of powder synchrotron XRD data. Our results indicate that the pollucite end member (CsAlSi2 O6 ) has a tetragonal structure (space group I 41 / a ), whereas all other compositions are cubic (space group Ia 3 d ). The increased symmetry for the titanium-substituted structures is presumably due to the incorporation of additional O2− anions (needed for compensating the charge imbalance between Ti4+ and Al3+ ), which effectively holds open the expanded cubic framework. In situ cooling experiments of the substituted phase CsTi0.1 Al0.9 Si2 O6.05 reveal a displacive transformation to the tetragonal structure at ∼230 K. This transformation is tricritical in nature and is analogous to the tetragonal-to-cubic transition in pollucite on heating. 相似文献
2.
Jie Fu 《Journal of the American Ceramic Society》1997,80(7):1901-1903
Fast lithium ion conducting glass-ceramics have been successfully prepared from the pseudobinary system 2[Li1+ x Ti2 Si x P3− x O12 ]-AlPO4 . The major phase present in the glass-ceramics was LiTi2 P3 O12 in which Ti4+ ions and P5+ ions were partially replaced by Al3+ ions and Si4+ ions, respectively. Increasing x resulted in a considerable enhancement in conductivity, and in a wide composition range extremely high conductivity over 10−3 S/cm was obtained at room temperature. 相似文献
3.
Sergey Kucheiko Dong-Hun Yeo Ji-Won Choi Seok-Jin Yoon Hyun-Jai Kim 《Journal of the American Ceramic Society》2002,85(5):1327-1329
The microwave dielectric properties of CaTi1− x (Al1/2 Nb1/2 ) x O3 solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO3 . The substitution of Ti4+ by Al3+ /Nb5+ improved the quality factor Q of the sintered specimens. A small addition of Li3 NbO4 (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛr ) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τf ) of −2 ppm/K. Li3 NbO4 as a sintering additive had no harmful influence on τf of ceramics. 相似文献
4.
Interdiffusion coefficients in single-crystal MgO were determined using an MgO-MgAl2 O4 diffusion couple. For a concentration of 1 mol% Al2 O3 in MgO, the interdiffusion coefficient can be expressed as D =2.0±0.2 exp (−76,000±3,000/ RT ) for the MgO-MgAl2 O4 couple. This relation compares well with previous measurements in the MgO-Al2 O3 system. The interdiffusion coefficients, which increased with the mol fraction of cation vacancies, were in the range of 10−8 to 10−10 cm2 s−1 for the concentrations and temperatures studied. Diffusion was enhanced below 1640°C if powdered MgAl2 O4 was used. Self-diffusion coefficients for Al3+ ions in MgO were calculated; Al3+ diffuses faster than Cr3+ in MgO. 相似文献
5.
Subsolidus phase relationships in the Ga2 O3 –Al2 O3 –TiO2 system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO2 (rutile), Ga2−2 x Al2 x O3 ( x ≤0.78 β-gallia structure), Al2−2 y Ga2 y O3 ( y ≤0.12 corundum structure), Ga2−2 x Al2 x TiO5 (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga4−4 x Al4 x Ti n −4 O2 n −2 ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work. 相似文献
6.
The decomposition of Al2(1- x ) Mg x Ti(1+ x ) O5 ceramics in air has been studied between 900° and 1175°C for 0 lessthan equal to x lessthan equal to 0.6. The decomposition temperature versus composition x predicted using a thermodynamic model based on the regular solution approach is in satisfactory agreement with the experimental results. The decomposition kinetics has been studied at 1100°C for x = 0, 0.1, and 0.2 and follows a nucleation and growth mechanism. Random nucleation of the reaction products is hindered by the high elastic stresses that result from the molar volume change related to decomposition because of the small chemical driving force available. Decomposition occurs only at a limited number of sites, probably associated with the presence of impurities and/or glassy phase. The decomposition products grow as nodules formed by an Al2 O3 (+ MgAl2 O4 for x > 0) core and a TiO2 shell. The growth is parabolic for x = 0 and linear for x = 0.1 and 0.2. The rate-controlling step in the decomposition mechanism of pure Al2 TiO5 ( x = 0) is the transport of Al3+ ions through the TiO2 -rutile phase. 相似文献
7.
We have investigated the ZrO2 –AITaO4 system to understand how selected chemical substitutions can be used to control cation-ordering transformations in zirconium titanate based dielectric ceramics. The complete replacement of the Ti content of Zr x Ti2–x O4 by a coupled Al3+ /Ta5+ substitution permits the synthesis of a wide range of isostructural Zr x (Al0.5 Ta0.5 )2–x O4 solid solutions. At high temperatures a disordered α-PbO2 type of structure is formed for 0.375 ≤ x ≤ 1.03. Samples with 0.67 ≤ x ≤ 1.03 undergo a cation-ordering reaction to a structure in which the a and b axes of the parent disordered Cell are doubled. The stabilities of these cation-ordered derivative structures are significantly greater than those of the ordered zirconium titanates. The ordering temperatures are composition dependent with a maximum of 1393°C occurring for Zr0.86 (Al0.5 TaO0.5 )1.14 O4 . The higher transition temperatures also enhance the kinetics of the ordering transition; whereas the pure zirconium titanates require extended annealing to produce complete cation order, fully ordered Zrχ (Al0.5 Ta0.5 )2–χ O4 solid solutions are produced during a normal furnace cool. 相似文献
8.
High-purity polycrystalline MgO and Al2 O3 were thermally grooved at 1500° and 1600°C. Accurate techniques were developed for following the growth of a single groove. For high-purity samples growth kinetics were essentially similar to those reported in the literature but were determined to be controlled by volume diffusion. Specimens for thermal grooving were prepared from Al2 O3 to which transition metal oxides (Fe2 O39 , MnO, and TiO2 ), which are known to accelerate shrinkage and sintering of Al2 O3 powder compacts, had been added; the rate of groove growth was increased remarkably by minor amounts of these additives. Control of partial pressure indicated that Fe2+ and Ti4+ are the species active in promoting groove growth. Substantial evidence was found for volume diffusion as the mechanism controlling groove formation. 相似文献
9.
Titanium-doped α-Al2 O3 exhibits a high-temperature conductivity which is ionic at high oxygen pressures and electronic at low oxygen pressures. Both are isotropic. The temperature dependence of conductivity under conditions where equilibrium with the atmosphere is not maintained indicates both the position of the energy level of titanium (TiAl x ) in the forbidden gap and the temperature dependence of the mobility of the native ionic defects (Al vacancies, V Al m ). Optical absorption responsible for the pink color of the reduced crystals is measured as a function of p o2 and is used to determine concentrations of Ti3+ and Ti4+ . Parameters for the equilibrium constants of the reactions involving electrons by which the composition of Al2 O3 :Ti and undoped Al2 O3 is varied are determined. The chemical diffusion data by Jones et al. are described quantitatively. 相似文献
10.
Alumina powders with varying iron oxide contents were prepared by coprecipitation. The powders were spheroidized by passing them through an oxygen-acetylene flame. The spheres were sized, annealed, and sintered in air and in N2 with 132 ppm O2 . Isothermal studies were combined with constant-rate-of-heating studies to identify the mechanism of sintering and to calculate the diffusion coefficients. The contribution of surface diffusion during initial-stage sintering of Fe-doped Al2 O3 was estimated by combining shrinkage and neck-growth data. The effect of Ti on the sintering rate of Fe-doped Al2 O3 was also studied. Both Fe2+ and Ti4+ ions enhanced the sintering rate of Al2 O3 . A defect model for corundum is proposed to explain the sintering data for transition-metal-ion-doped Al2 O3 . 相似文献
11.
Masayuki Nogami Naoya Hayakawa Naoya Sugioka Yoshihiro Abe 《Journal of the American Ceramic Society》1996,79(5):1257-1261
Samarium ions (Sm2+ ) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na2 O-Al2 O3 SiO2 synthesized by the hydrolysis of Si(OC2 H5 )4 , Al(OC4 H9 )3 , CH3 COONa, and SmCl3 ·6H2 O were heated in air at 500°C, then reacted with H2 gas to form Sm2+ ions. Whereas Al3+ ions effectively dispersed the Sm3+ ions in the glass structure, Na+ ions were not effective. The Al2 O3 -SiO2 glasses proved appropriate for reacting the Sm3+ ions with H2 gas and exhibited the intense photoluminescence of Sm2+ ions. The reaction of Sm3+ ions with H2 in the Al2 O2 -SiO2 glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm2+ ions was achieved within 20 min. 相似文献
12.
ALEXANDRA NAVROTSKY BARRY A. WECHSLER KAREN GEISINGER FRIEDRICH SEIFERT 《Journal of the American Ceramic Society》1986,69(5):418-422
Solution calorimetry of MgAl2 O4 -Al8/3 O4 solid solutions was performed in a molten 2PbO · B2 O3 solvent at 975 K. The results indicate small negative heats of mixing, relative to spinel standard states for both end-members. These data were combined with information on the energetics of the α-γ transition in Al2 O3 and on the MgAl2 O4 -Al8/3 O4 (MgO-Al2 O3 ) subsolidus phase relations to estimate the partial molar entropy of mixing of γ-Al8/3 O4 in the solid solution. This entropy is much less positive than that calculated from several models for the configurational entropy of mixing of magnesium, aluminum, and vacancies on octahedral and/or tetrahedral sites. The data suggest a good deal of local order to be present in the solid solutions, consistent with negative enthalpies of mixing and entropies of mixing far less than ideal configurational values. 相似文献
13.
The rate of formation of NiAl2 O4 by reaction between single crystals of NiO and Al2 O3 can be described by k = 1.1 × 104 exp (−108,000 ± 5,000/ RT ) cm2 /s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity-vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al3+ ions diffuse as [AlNi · VNi ]'pairs. The diffusion coefficient of pairs, Dp , obeys the equation 6.6 × 10−2 exp (−54,000 ± 3,000/ RT ) cm2 /s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al3+ dissolved in the spinel. 相似文献
14.
Daniele Mazza Mario Vallino Guido Busca 《Journal of the American Ceramic Society》1992,75(7):1929-1934
A morphous solids belonging to the systems Al2 O3 –Me2 O (Me = Na, K) and Al2 O3 –B2 O3 were prepared by nitrate decomposition, introducing boron in the form of boric acid. Crystalline metastable solids with pseudotetragonal symmetry were obtained from thermal treatment at 850° to 900°C for the compositions Al6 Mex O(9+0.5 x ) ( x ≅ 1; Me = Na, K) and Al6- x B x O9 (1 x 3). The resultant solids were stable only within a difinite temperature range and transformed, with further treatment increases, into stable equilibrium phases. The structures of the metastable phases were examined by X-ray diffraction and Fourier transform infrared spectroscopy, and both analyses showed a mullite type of framework, inside of which the atomic coordinates were refined in the Pbam (no. 55) space group. The present results indicate that these silica-free mullite structures are stabilized by two different mechanisms: (1) interstitial occupation of bulky cations (Na+ , K+ ) or (2) substitution of B for Al in some of the tetrahedral positions. 相似文献
15.
Karen M. Cruickshank Xiping Jing Graeme Wood Eric E. Lachowski Anthony R. West 《Journal of the American Ceramic Society》1996,79(6):1605-1610
A solid-solution phase with the general formula Ba6-3x Nd8+2x Ti18 O54 , where 0.25(5) ≤×≤ 0.75(5), has been characterized at 1250°C; this phase has been variously described as BaNd2 Ti4 O12 and BaNd2 Ti5 O14 in the literature. Variation in its stoichiometry is accommodated via the cation substitution mechanism, 3Ba2 ⇆2Nd3+ . The location and extent of the solid solution were demonstrated by a combination of phase diagram studies and X-ray diffraction techniques, including lattice parameter measurements and electron microscopy. A combination of techniques was employed due to the insensitivity of secondary phase detection by X-ray diffraction in this system. Using this approach, a second possible solid-solution mechanism, Ba2+ 2Nd3+ ⇆2Ti4+ , is discounted. 相似文献
16.
Bengt Hallstedl 《Journal of the American Ceramic Society》1990,73(1):15-23
A thermodynamic assessment of the quasi-binary system CaO-Al2 O3 has been made using a computerized CALPHAD (calculation of phase diagrams) technique. The actual optimization was carried out with a computer program for the optimization of parameters in thermodynamic models called PARROT. The liquid phase is described by a simple two-sublattice subregular solution model with Ca2+ and Al3+ as cationic species and O2- as anionic species. All solid phases are treated as stoichiometric compounds. A consistent set of parameters describing the system is presented, and numerous comparisons with experimental data are made. There is a lack of accurate data in the high-alumina part of the system, and the importance of a better knowledge of the CaO·6Al2 O3 phase is stressed. 相似文献
17.
Zn-substituted CaCu3 Ti4 O12 ceramics were synthesized by solid-state sintering. Their microstructures and dielectric properties were investigated. Ca(Cu1− x Zn x )3 Ti4 O12 single-phase structures were obtained up to x =0.1, and the Cu+ /Cu2+ and Ti3+ /Ti4+ mixed-valent structure was enhanced with increasing Zn substitution. The giant dielectric response was significantly enhanced by Zn substitution. The dielectric constant increased with increasing x , and a giant dielectric constant plateau as high as ∼9 × 104 was achieved for x =0.1 at 10 kHz, while that for x =0 was ∼3 × 104 . The enhanced giant dielectric response was profoundly concerned with the modified mixed-valent structure. 相似文献
18.
Steady-state creep was studied in hot-forged polycrystalline Al2 O3 (3 to 42 μm) of nearly theoretical density doped with≤1 cation % of Fe, Ti, or Cr. Tests were conducted at stresses between 10 and 550 kg/cm2 at 1375° to 1525°C under O2 partial pressures of 0.88 to 10−10 atm. Except in the 10-μm Fe-doped material tested at very small stresses, slightly nonviscous creep behavior was generally observed. The effects of P o2 on the creep rate indicated that increased concentration of a divalent (Fe2+ ) or quadrivalent (Ti4+ ) impurity in solid solution enhances the creep rate of polycrystalline Al2 O3 . The activation energies for the creep of Fe- and Ti-doped Al2 O3 samples (148 and 145 kcal/mol, respectively) were significantly higher than that for Cr-doped material (114 kcal/mol). Taking into account the effects of Po2 , temperature, and grain size, it was concluded that the steady-state creep of transition-metal-doped Al2 O3 is controlled by cation lattice diffusion. 相似文献
19.
RAM A. SHARMA 《Journal of the American Ceramic Society》1988,71(4):272-276
Partial equilibrium phase diagrams for the systems MgF2 -MgO, MgF2 -CaO, and MgF2 -Al2 O3 were determined by differential thermal analysis. Simple eutectics were observed at 8.5 mol% MgO and 1228°± 3°C in the MgF2 -MgO system, at 7.5 mol% CaO and 1208°± 3°C in the MgF2 -CaO system, and at 2.5 mol% Al2 O3 and 1250°± 3°C in the MgF2 -Al2 O3 system. On the basis of agreements between the activities calculated by the Clausius-Clapeyron equation and Temkin's model using the present data, the eutectic melt consists of Mg2+ , F- , and O2- ions in the MgF2 -MgO system; Mg2+ , Ca2+ , F- , and O2- ions in the MgF2 -CaO system; and Mg2+ , Al3+ , F- , and AlO2¯ ions in the MgF2 -Al2 O3 system. Well-defined long needles of MgO in the MgF2 -MgO system, less defined needles of CaO in the MgF2 -CaO system, and Al2 O3 grains in the MgF2 -Al2 O3 system were observed by optical microscopy. 相似文献
20.
Dong-Wan Kim Kyung Hyun Ko Do-Kyun Kwon Kug Sun Hong 《Journal of the American Ceramic Society》2002,85(5):1169-1172
(1− x )ZnNb2 O6 · x TiO2 ceramics were prepared using both anatase and rutile forms of TiO2 . At a composition of x = 0.58, a mixture region of ixiolite (ZnTiNb2 O8 ) and rutile was observed and the temperature coefficient of resonant frequency (τf ) was ∼0 ppm/°C. We found that although ɛr and τf were comparable, the quality factor ( Q × f , Q ≈ 1/ tan δ, f = resonant frequency) of 0.42ZnNb2 O6 ·0.58TiO2 prepared from anatase and rutile was 6000 and 29 000, respectively. The origin of the difference in Q × f of both samples was investigated by measuring electrical conductivity and by analysis of the anatase–rutile phase transition. The anatase-derived sample had higher conductivity, which was related to the reduction of Ti4+ . It is suggested that the increase of dielectric loss originates from an increase in Ti3+ and oxygen vacancies due to an anatase–rutile phase transition. 相似文献