共查询到20条相似文献,搜索用时 15 毫秒
1.
Md. Hasan Zahir Koji Sato Hiroshi Mori Yuji Iwamoto Mikihiro Nomura Shin-ichi Nakao 《Journal of the American Ceramic Society》2006,89(9):2874-2880
We report here on our thermal and hydrothermal investigations comparing mesoporous γ-Al2 O3 membrane with single and double dopant membranes prepared by the sol–gel method. Improvements in the hydrothermal stability of mesoporous γ-Al2 O3 by transition (Ga3+ ) or rare-earth (La3+ ) cations are discussed, along with the effectiveness of double dopant (Ga3+ –La3+ ). The amounts of Ga2 O3 oxide used varied between 0.0 and 30 mol% and those of La2 O3 between 6 and 15 mol%. The thermal and hydrothermal (up to 75% steam) stability of nine types of membranes fabricated on an asymmetric porous α-Al2 O3 support by means of a dip-coating process was characterized by H2 gas permeation at 500°C. By conducting tests with wide variations in dopant concentrations, material characterizations, and gas permeance performance, we have been able to optimize the key parameters for hydrothermally stable systems. 相似文献
2.
Feng Ye Chunfeng Liu Limeng Liu Yu Zhou Qingchang Meng 《Journal of the American Ceramic Society》2008,91(3):1022-1026
Sc3 + and dual Sc3+ –Lu3+ -doped α-SiAlON compositions were sintered by hot pressing, and the formation behavior, microstructure, and mechanical properties were assessed. It was found that the small cation Sc3+ could not be accommodated into the α-SiAlON structure alone. The addition of Lu2 O3 in the composition induces the Sc3+ cation to enter the α-SiAlON structure, and leads to the production of α-SiAlON with an elongated-grain microstructure. Transmission electron microscopy analysis shows that α-SiAlON grains always contain an α-Si3 N4 core, implicating heterogeneous nucleation to be in present in a mixed lutetium/scandium-doped α-SiAlON system. 相似文献
3.
Refractory Y-α-SiAlON with elongated grain morphology was obtained by utilizing La2 O3 as a densification aid, which resulted in excellent room-temperature and high-temperature strength. Room-temperature strength of 1000 MPa was achieved when La2 O3 was augmented by adding Y2 O3 or removing AlN. With only La2 O3 , a temperature-independent strength of 800–950 MPa was maintained up to 1100°C, then gradually decreasing by 25% when reaching 1300°C. The R-curve measurements of fracture toughness showed relatively little dependence on microstructure, consistent with a strong interface that suppresses grain boundary decohesion. Compared with other densification aids such as SiO2 , Al2 O3 , Sc2 O3 , Y2 O3 , and Lu2 O3 , a finer microstructure was obtained by using La2 O3 . High nitrogen content in the residual La–Si–Al–O–N glass in equilibrium with the nitrogen-rich α-SiAlON is suggested to be the cause of these findings. 相似文献
4.
Akira Kato Hisao Yamashita Hiroshi Kawagoshi Shimpei Matsuda 《Journal of the American Ceramic Society》1987,70(7):157-C
Mixtures of La2 O3 and Al2 O3 with various La contents were prepared by co-precipitation from La(NO3 )3 and Al(NO3 )3 solutions and calcined at 800° to 1400°C. The addition of small amounts of La2 O3 (2 to 10 mol%) to Al2 O3 gives rise to the formation of lanthanum β-alumina (La 2 O3 ·11–14Al2 O3 ) upon heating to above 1000°C and retards the transformation of γ-Al2 O3 to α-Al2 O3 and associated sintering. 相似文献
5.
SHIGERU ITO NOBUO KUBO SHINYA NARIKI NOBORU YONEDA 《Journal of the American Ceramic Society》1987,70(12):874-879
The potassium ions in potassium β-ferrite ((1 + x)K2 O ·11Fe2 O3 ) crystals were exchanged with Na+ , Rb+ , Cs+ , Ag+ , NH4 + , and H3 O+ in molten nitrates or in concentrated H2 SO4 . On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K+ , Rb+ , and Cs+ β-ferrites decomposed to form α-Fe2 O3 at high temperatures of 800° to 1100°C. In addition, H3 O+ , NH4 + , and Ag+ β-ferrites decomposed to form α-Fe2 O3 at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured. 相似文献
6.
La0.8 Sr0.2 Cr0.9 Ti0.1 O3 perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La0.8 Sr0.2 Cr0.9 Ti0.1 O3 shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10−6 /°C in air, 10.5 × 10−6 /°C under a He–H2 atmosphere (oxygen partial pressure of 4 × 10−15 atm at 1000°C), and 10.9 × 10−6 /°C in a H2 atmosphere (oxygen partial pressure of 4 × 10−19 atm at 1000°C). La0.8 Sr0.2 Cr0.9 Ti0.1 O3 perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect. 相似文献
7.
Tzer-Shin Sheu 《Journal of the American Ceramic Society》1994,77(9):2345-2353
The in situ β-Si3 N4 /α'-SiAlON composite was studied along the Si3 N4 –Y2 O3 : 9 AlN composition line. This two phase composite was fully densified at 1780°C by hot pressing Densification curves and phase developments of the β-Si3 N4 /α'-SiAlON composite were found to vary with composition. Because of the cooperative formation of α'-Si AlON and β-Si3 N4 during its phase development, this composite had equiaxed α'-SiAlON (∼0.2 μm) and elongated β-Si3 N4 fine grains. The optimum mechanical properties of this two-phase composite were in the sample with 30–40%α', which had a flexural strength of 1100 MPa at 25°C 800 MPa at 1400°C in air, and a fracture toughness 6 Mpa·m1/2 . α'-SiAlON grains were equiaxed under a sintering condition at 1780°C or lower temperatures. Morphologies of the α°-SiAlON grains were affected by the sintering conditions. 相似文献
8.
Michael Rozumek Peter Majewski Fritz Aldinger 《Journal of the American Ceramic Society》2004,87(4):656-661
The metastable crystal structure of strontium- and magnesium-substituted LaGaO3 (LSGM) was studied at room and intermediate temperatures using powder X-ray diffractometry and Rietveld refinement analysis. With increased strontium and magnesium content, phase transitions were found to occur from orthorhombic (space group Pbnm ) to rhombohedral (space group R [Threemacr] c ) at the composition La0.825 Sr0.175 Ga0.825 Mg0.175 O2.825 and, eventually, to cubic (space group Pm [Threemacr] m ) at the composition La0.8 Sr0.2 Ga0.8 Mg0.2 O2.8 . At 500°C in air and at constant strontium and magnesium content, a phase transformation from orthorhombic (space group Pbnm ) to cubic (space group Pm [Threemacr] m ) was observed. For the orthorhombic modification, thermal expansion coefficients were determined to be α a ,ortho = 10.81 × 10−6 K−1 , α b ,ortho = 9.77 × 10−6 K−1 , and α c ,ortho = 9.83 × 10−6 K−1 (25°–400°C), and for the cubic modification to be αcubic = 13.67 × 10−6 K−1 (500°–1000°C). 相似文献
9.
Kiyoshi Okada Akiyoshi Hattori Yoshikazu Kameshima Atsuo Yasumori Rathindra Nath Das 《Journal of the American Ceramic Society》2000,83(5):1233-1236
The effect of monovalent cation addition on the γ-Al2 O3 -to-α-Al2 O3 phase transition was investigated by differential thermal analysis, powder X-ray diffractometry, and specific-surface-area measurements. The cations Li+ , Na+ , Ag+ , K+ , Rb+ , and Cs+ were added by an impregnation method, using the appropriate nitrate solution. β-Al2 O3 was the crystalline aluminate phase that formed by reaction between these additives and Al2 O3 in the vicinity of the γ-to-α-Al2 O3 transition temperature, with the exception of Li+ . The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phase-transition temperature. Thus, Cs+ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of γ-Al2 O3 . Because the order of the phase-transition temperature coincided with that of the formation temperature of β-Al2 O3 in these samples, suppression of ionic diffusion in γ-Al2 O3 by the amorphous phase containing the added cations must have played an important role in retarding the transition to α-Al2 O3 . Larger cations suppressed the diffusion reaction more effectively. 相似文献
10.
Darko Makovec Zoran Samardija Uro Delalut Drago Kolar 《Journal of the American Ceramic Society》1995,78(8):2193-2197
Incorporation of La3+ into the BaTiO3 lattice was studied on samples of various composition, using quantitative WDS microanalysis (EPMA) in combination with scanning electron microscopy (SEM) and X-ray powder diffractometry (XRPD). Direct determination of solid-solution formulas by microanalysis supports the structure model of a solid solution with La3+ on Ba2+ sites and a deficient Ti4+ sublattice, independent of the starting composition. Solid solution extends on the tie line, which points from BaTiO3 to La4 Ti3 O12 , to the composition of approximately Ba0.70 - La0.38 Ti0.925 ( V "")0.075 O3 . On the basis of these results, the BaTiO3 -rich part of the BaO-La2 O3 -TiO2 phase diagram was constructed. 相似文献
11.
Ya-Wen Li Pei-Ling Wang Wei-Wu Chen Yi-Bing Cheng Dong-Sheng Yan 《Journal of the American Ceramic Society》2002,85(10):2545-2549
Two calcium-doped α-SiAlON compositions (Ca0.6 Si10.2 Al1.8− O0.6 N15.4 and Ca1.8 Si6.6 Al5.4 O1.8 N14.2 ) were prepared by hot pressing at 1600° and 1500°C, respectively, for complete phase transformation from α-Si3 N4 to α-SiAlON. Both samples were subsequently fired at different temperatures for different periods of time to study the grain growth of α-SiAlON. Elongated α-SiAlON grains were developed in both samples at high temperatures. The kinetics of grain growth was investigated based on the variations in length and width of the α-SiAlON grains under different sintering conditions. Different growth rates were found between the length and width directions of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development. 相似文献
12.
Youichirou Kaga Mark I. Jones Kiyoshi Hirao Shuzo Kanzaki 《Journal of the American Ceramic Society》2004,87(5):956-959
A reaction-bonding process, which offers low sintering shrinkage and is a low-cost process, was applied to fabricate Y–α-SiAlON ceramics. The green compacts composed of Si, Y2 O3 , Al2 O3 , and AlN were nitrided and subsequently postsintered. Dense single-phase Y–α-SiAlON with elongated grain morphology could be achieved in the specimen postsintered at 1900°C. The material exhibited high hardness (1850 HV10) and high fracture toughness (5.1 MPa·m1/2 ). 相似文献
13.
Rong-Jun Xie Naoto Hirosaki Mamoru Mitomo Yoshinobu Yamamoto Takayuki Suehiro Naoki Ohashi 《Journal of the American Ceramic Society》2004,87(7):1368-1370
Cerium-doped α-SiAlON (M x Si12−( m + n ) Al m + n O n N16– n ) materials have been prepared by gas-pressure sintering and post-hot-isostatic-press (HIP) annealing, using four powder mixtures of α-Si3 N4 , AlN, and either (i) CeO2 , (ii) CeO2 + Y-α-SiAlON seed, (iii) CeO2 + Y2 O3 , or (iv) CeO2 + CaO. Cerium-containing CeAl(Si6– z Al z )(N10– z O z ) (JEM) phase, rather than Ce-α-SiAlON phase, forms in the sample with only CeO2 , whereas a single-phase α-SiAlON generates in samples with dual doping (CeO2 + Y2 O3 and CeO2 + CaO). On ultraviolet-light excitation, JEM gives one broad emission band with maximum at 465 nm and a shoulder at 498 nm; α-SiAlON shows an intense and broad emission band that peaks at 500 nm. The unusual long-wavelength emissions in JEM and α-SiAlON are due to increases in the nephelauxetic effect and the ligand-field splitting of the 5 d band, because the coordination of Ce3+ in JEM and α-SiAlON is nitrogen enriched. 相似文献
14.
D. H. RIDGLEY H. LESSOFF J. D. CHILDRESS 《Journal of the American Ceramic Society》1970,53(6):304-311
The effects of sintering temperature and cooling rate on the magnetic and crystallographic properties of lithium ferrite were studied. The magnetic moments and lattice parameters increased with increasing sintering temperature; these increases result from correlated oxygen and lithium oxide losses. Either annealing at lower temperatures or slow cooling under O2 causes reoxidation of the Fe2+ formed at higher temperatures with attendant decreases in moment and lattice parameter and gradual precipitation of α-Fe2 O3 as a second phase. The products formed on rapid cooling are equivalent to solid solutions of spinel lithium ferrite with Fe3 O4 , and those formed on slow cooling, to solid solutions of lithium ferrite and γ-Fe2 O3 with precipitation of α-Fe2 O3 . Lithium losses and α-Fe2 O3 precipitate amounts are calculated. The magnetic moment of stoichiometric lithium ferrite at 25°C is 3736±20 G; the lattice parameter at 28°C is 8.3296±0.0005 Å. 相似文献
15.
ERNEST M. LEVIN CARL R. ROBBINS JON L. WARING 《Journal of the American Ceramic Society》1961,44(2):87-91
The phase equilibrium diagram for the system La2 O3 -B2 O3 has been determined experimentally. The compounds La2 O3 -3B2 O3 and La2 O3 -B2 O3 melt congruently at 1141°± 5°C. and 1660°± 15°C, respectively. At 1488°± 5°C, La2 O3 -B2 O3 inverts from the aragonite-type structure to a high-temperature form. Trilanthanum borate, 3La2 O3 B2 O3 , melts incongruently at 1386°± 5°C. to give liquid and La2 O3 . No solid solutions exist in the system. A region of liquid immiscibility exists in the system and extends at 1136°± 5°C. from almost pure B2 O3 to 21.5 mole % La2 O3 . The experimental value for the extent of immiscibility agrees with that calculated from theoretical considerations. A second method for estimating immiscibility in the system is demonstrated, which requires experimentally only the determination of the index of refraction of the modifier-rich liquid. Principles governing immiscibility are discussed. 相似文献
16.
Phase equilibria of the La2 O3 -SrO-CuO system have been determined at 950°C and 10 kbar (1 GPa). Stable phases at the apices of the ternary phase diagram are CuO, La2 O3 , and SrO. Stable intermediate phases are La2 CuO4 in the LaO1.5 -CuO binary and Sr2 CuO3 , SrCuO2 , and Sr14 Cu24 O41 in the CuO-SrO binary. The La2-x Sr x CuO4-δ solid solution is stable where 0.0 ≤ x ≤ 1.3, the La2-x Sr1+x Cu2 O6+δ solid solution is stable where 0.0 ≤ x ≤ 0.2, the La8-x Sr x Cu8 O20-δ solid solution is stable where 1.3 ≤ x ≤ 2.7, the La x Sr14-x -Cu24 O41 solid solution is stable where 0 ≤ x ≤ 6, and the La1+x Sr2-x Cu2 O5.5+δ phase is stable where 0.04 ≤ x ≤ 0.16. The La2 O3 -SrO-CuO phase diagram at 950°C and 10 kbar is almost identical to that determined by other authors at 950°C and 1 atm, in terms of phase stability and solid-solution ranges. 相似文献
17.
Robert C. Pullar Jonathan D. Breeze Neil McN. Alford 《Journal of the American Ceramic Society》2005,88(9):2466-2471
This paper details the investigation of the quality factor ( Q ), dielectric permittivity (ɛr ) and temperature coefficient of resonant frequency (τf ) of the TE01δ mode of the columbite binary niobate ceramics, with the formula MNb2 O6 where M=2+ cation, in relation to their degree of sintering, microstructure and phase composition. The ceramics were made from a mixed oxide preparative route and fired over a range of temperatures from 800° to 1400°C, and most formed the columbite structure. A comprehensive study was made of the niobates containing the transition metal cations M=Mn2+ , Co2+ , Ni2+ , Cu2+ , and Zn2+ , and the group II metal cations M=Mg2+ , Ca2+ , Sr2+ , and Ba2+ . All columbite niobates were found to have ɛr between 17 and 22 and negative τf values between –45 and –76 ppm/°C, and ZnNb2 O6 , MgNb2 O6 , CaNb2 O6 , and CoNb2 O6 had high Q f values of 84 500, 79 600, 49 600, and 41 700 GHz, respectively. The Q f of MgNb2 O6 was found to rise to over 95 000 GHz when heated at 1300°C for 50 h. 相似文献
18.
Masatomo Yashima Yoichi Kawaike Masahiko Tanaka 《Journal of the American Ceramic Society》2007,90(1):272-274
Unit-cell parameters of the α-tricalcium phosphate [TCP; Ca3 (PO4 )2 ] were investigated using high-resolution synchrotron powder diffraction and the Rietveld method. The diffraction experiment was conducted at 29°C at the BL-15XU experimental station of SPring-8, Japan. Precise unit-cell parameters of the α-TCP were obtained; a =12.87271 (9), b =27.28034(8), c =15.21275(12) Å, α=γ=90°, and β=126.2078(4)°. The calculated density of α-TCP (2.8677 g/cm3 ) is smaller than that of β-TCP, indicating the "looser" structure of α-TCP. 相似文献
19.
Improvement of Mechanical Properties and Corrosion Resistance of Porous β-SiAlON Ceramics by Low Y2 O3 Additions 总被引:2,自引:1,他引:1
Jian-Feng Yang Guo-Jun Zhang Ji-Hong She Tatsuki Ohji Shuzo Kanzaki 《Journal of the American Ceramic Society》2004,87(9):1714-1719
Addition of Y2 O3 as a sintering additive to porous β-SiAlON (Si6− z Al z O z N8− z , z = 0.5) ceramics has been investigated for improved mechanical properties. Porous SiAlON ceramics with 0.05–0.15 wt% (500–1500 wppm) Y2 O3 were fabricated by pressureless sintering at temperatures of 1700°, 1800°, and 1850°C. The densification, microstructure, and mechanical properties were compared with those of Y2 O3 -free ceramics of the same chemical composition. Although this level of Y2 O3 addition did not change the phase formation and grain size, the grain bonding appeared to be promoted, and the densification to be enhanced. There was a significant increase in the flexural strength of the SiAlON ceramics relative to the Y2 O3 -free counterpart. After exposure in 1 M hydrochloric acid solution at 70°C for 120 h, no remarkable weight loss and degradation of the mechanical properties (flexural and compression strength) was observed, which was attributed to the limited grain boundary phase, and with the minor Y2 O3 addition the supposed formation of Y-α-SiAlON. 相似文献
20.
An all-alkoxide route to films and nano-phase powders of the La0.5 Sr0.5 CoO3 perovskite is described. To our knowledge, this is the first purely alkoxide-based route to (La1− x Sr x )CoO3 , and it yields phase-pure and elementally homogeneous perovskite at 700°C by heating at 2°C/min. At 700°C, a cubic unit cell was obtained with a c =3.853Å, and after further heating to 1000°C, a rhombohedral cell could be indexed: a r =5.417 Å, αr =59.94°. Ninety to 130 nm thick films of La0.5 Sr0.5 CoO3 were obtained by spin coating. The gel-to-oxide conversion was studied in some detail, using thermo-gravimetric analysis, differential scanning calorimetry, powder X-ray diffraction, IR spectroscopy, and transmission electron microscope equipped with an energy-dispersive X-ray spectrometer. 相似文献