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1.
The vaporization of the system MgO-Cr2 O3 was studied in a vacuum of 10−5 torr (10−3 N/m2 ) at 1500° to 1700°C using the Langmuir and Knudsen methods. It was found that the phases in the system vaporize nearly congruently and the logarithm of the vaporization coefficient, α, of MgCr2 O4 increases linearly with increasing reciprocal temperature. Alpha tends to unity at a temperature near the melting point (2525±23°C). The additivity rule can be applied to the Langmuir vaporization rates on the basis of the surface area ratios of the phases in the 2-phase system MgO-Cr2 O3 . The enthalpies of vaporization of MgCr2 O4 were 695.0 and 549.2 kcaVmol for activated and equilibrium processes, respectively. 相似文献
2.
H. U. ANDERSON 《Journal of the American Ceramic Society》1974,57(1):34-37
The sintering behavior of MgCr2 O4 powder compacts was investigated as a function of temperature, time, and oxygen activity. The results show that MgCr2 O4 cannot be densified to >70% of theoretical density at temperatures up to 1700°C if the oxygen activity exceeds 10−6 atm. The oxygen activity must be decreased to <10−10 atm before densities exceeding 90% of theoretical can be achieved. Weight loss and X-ray data indicated that maximum density occurred at an oxygen activity just above that where MgCr2 O4 becomes unstable. 相似文献
3.
WALTER C. ALLEN 《Journal of the American Ceramic Society》1968,51(9):485-490
The characteristics of spinels in the series MgCr2 O4 -MgFe2 O4 were determined. The plot of cell size vs. molar composition is unusual in series showing complete solid solution because an unusually large deviation from Vegard's law was observed. This deviation is caused by changes in spinel structure with composition and temperature, and an equation was derived which applies a correction in terms of the degree of inversion. The effects of temperature on compositions high in MgFe2 O4 include changes in density and refractive index. Solid solution of forsterite in MgCr2 O4 decreases the cell size to 8.329 A but apparently is less than 1%. Changes in composition caused by vapor loss or by dissociation are small enough that this series is essentially binary below 1400° C. 相似文献
4.
Phase relations in air at 1300°C were determined for the system MgO-Cr2 O3 −Fe2 O3 by conventional quenching techniques. Details of the phase equilibria were established for: (1) the sesquioxide solid solution between Cr2 O3 and Fe2 O3 , (2) the spinel solid solution field between MgCr2 O4 and MgFe2 O4 , and (3) the periclase solid solution field for MgO. Selected tie lines connecting coexisting compositions were established with X-ray diffractometer data. Diffuse reflectance spectra, diffractometer intensity ratios, and lattice parameter measurements were obtained for quenched samples to study the structural inversion in the spinel series MgCr2 O4-MgFe2 O4 . 相似文献
5.
Porous Cr3 C2 grains (∼300 to 500 μm) with ∼10 wt% of Cr2 O3 were prepared by heating a mixture of MgCr2 O4 grains and graphite powder at 1450° to 1650°C for 2 h in an Al2 O3 crucible covered by an Al2 O3 lid with a hole in the center. The porous Cr3 C2 grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m2 /g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr3 C2 grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics. 相似文献
6.
Combustion Synthesis of Metal Chromite Powders 总被引:1,自引:0,他引:1
S. Sundar Manoharan Kashinath C. Patil 《Journal of the American Ceramic Society》1992,75(4):1012-1015
Fine-particle metal chromites (MCr2 O4 , where M = Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn) have been prepared by the combustion of aqueous solutions containing the respective metal nitrate, chromium(III) nitrate, and urea in stoichiometric amounts. The mixtures, when rapidly heated to 350°C, ignite and yield voluminous chromites with surface areas ranging from 5 to 25 m2 /g. MgCr2 O4 , sintered in air at 1500°C for 5 h, has a density of 4.0 g/cm3 . 相似文献
7.
OLAF MULLER RUSTUM ROY WILLIAM B. WHITE 《Journal of the American Ceramic Society》1968,51(12):693-699
Phase equilibria were determined for the systems NiO-Cr2 O3 −O2 , MgO-Cr2 O3 ,-O2 , and CdO-Cr2 O3 −O2 from 450° to above 850° C and at oxygen pressures of from 2 to 3500 atm. Only two intermediate phases were found in the nickel system: NiCrO., (CrVO4 structure) and the spinel NiCr2 O4 . The magnesium and cadmium systems are similar in that they have three analogous phases: the low-temperature α-MgCrO4 and α-CdCrO4 (both with the CrVO4 structure), the high-temperature β-MgCrO4 and β-CdCrO4 (both with the α-MnMoO4 structure), and the spinels MgCr2 O4 and CdCr2 O4 . The cadmium system contains an additional phase, Cd2 CrO5 , which is primitive monoclinic. 相似文献
8.
Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al2 O3 ) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O2 atmosphere using the fine powders prepared by hydrolysis of ZrOCl2 solution. The composites consisting of 25 wt% Al2 O3 and tetragonal zirconia with compositions 4 mol% YO1.5 –4 mol% CeO2 –ZrO2 and 2.5 mol% YO1.5 –5.5 mol% CeO2 –ZrO2 exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al2 O3 and (2.5Y, 5.5Ce)-TZP/Al2 O3 composites was useful to enhance the phase stability under hydrothermal conditions and strength. 相似文献
9.
An isothermal section of the ternary system MgO–Al2 O3 -Cr2 O3 was determined at 1700°± 15°C to delineate the stability field for spinel crystalline solutions (cs). Crystalline solutions were found between the pseudobinary joins MgAl2 O4 –Cr2 O3 and MgCr2 O4 -Al2 O3 , and the binary join MgAl2 O4 -MgO. The first two crystalline solutions exhibit cation vacancy models while the latter can probably be designated as a cation interstitial model. Precipitation from spinel cs may proceed directly to an equilibrium phase, (Al1-x Crx )2 O3 , with the corundum structure or through a metastable phase of the probable composition Mg(Al1-x Cr )26 O40 . The composition and temperature limits were defined where the precipitation occurs via metastable monoclinic phases. The coherency of the metastable monoclinic phase with the spinel cs matrix can be understood by considering volume changes with equivalent numbers of oxygens and known crystallographic orientation relations. Electron probe and metallographic microscope investigations showed no preferential grain boundary precipitation. 相似文献
10.
Hidehiro Yoshida Koji Morita Byung-Nam Kim Keijiro Hiraga Masayasu Kodo Kohei Soga Takahisa Yamamoto 《Journal of the American Ceramic Society》2008,91(5):1707-1710
We investigated the densification of undoped, nanocrystalline yttria (Y2 O3 ) powder by spark plasma sintering (SPS) at sintering temperatures between 650°C and 1050°C at a heating rate of 10°C/min and an applied stress of 83 MPa. In spite of the low sinterability of the undoped Y2 O3 , a remarkable densification of the powder started at about 600°C, and a theoretical density of more than 97% was achieved at a sintering temperature of 850°C with a grain size of about 500 nm. The low temperature SPS is effective for fabricating dense Y2 O3 polycrystals. 相似文献
11.
Akio Kawabata Masaru Yoshinaka Ken Hirota Osamu Yamaguchi 《Journal of the American Ceramic Society》1995,78(8):2271-2273
Chromium (III) oxide (Cr2 O3 ) crystallizes at low temperatures from an amorphous material prepared by adding hydrazine monohydrate to an aqueous solution of Cr(NO3 )3 -9H2 O. Individual particles of Cr2 O3 tend toward a hexagonal morphology above 800°C. Well-densified Cr2 O3 pellets (98.8% of theoretical density) have been fabricated by hot isostatic pressing for 2 h at 1100°C and 196 MPa. Their fracture toughness is 4.4 MPa.m1/2 . The sample annealed in air for 12 h at 1300°C exhibits a high electrical conductivity of 3.6 Ω-1 .m-1 at 700°C. 相似文献
12.
Hot isostatic pressing was studied for densification of reaction-bonded Si3 N4 containing various levels of Y2 O3 . Near-theoretical density was achieved for com positions containing 3 to 7 wt% Y2 O3 . An Si3 N4 -5 wt% Y2 O3 composition had a 4-point flexural strength at 1375°C of 628 MPa and survived 117 h of stress rupture testing at 1400°C and 345 MPa . 相似文献
13.
Masanori Hirano Michio Inagaki Yasunobu Mizutani Kazuhiro Nomura Masayuki Kawai Yasuhisa Nakamura 《Journal of the American Ceramic Society》2000,83(10):2619-2621
Significant improvement in the fracture strength, accompanied by an enhancement in the electrical conductivity, of zirconia polycrystals that were doped with 3–7 mol% Sc2 O3 was obtained by sintering at 1300°C for 1 h in air, followed by hot isostatic pressing (HIP) at 196 MPa at 1300° and 1450°C for 1.5 h in an argon-gas atmosphere. Dense bodies (with an average grain size of <0.5 μm) that were doped with 3.5 mol% of Sc2 O3 showed the highest average fracture strength up to 1770 MPa and an electrical conductivity of 0.08 S/cm at 1000°C. The present zirconia ceramics, which consisted of submicrometer-sized grains of tetragonal phases and were stabilized with 5 and 6 mol% of Sc2 O3 , exhibited high strength (1330 and 1140 MPa, respectively) and good conductivity (0.15 and 0.18 S/cm, respectively); values for both properties were greater than those previously reported. The present HIPed zirconia ceramics, which have excellent properties, are candidates for an electrolyte of planar-type solid oxide fuel cells. 相似文献
14.
The sinter forging behavior of α-Al2 O3 seeded and unseeded nanocrystalline θ-Al2 O3 was investigated as a function of temperature, stress, and strain rate. Seeded samples exhibited the highest degree of plastic deformation during the θ- to α-AI2 O3 phase transformation. As a result, microstructure control, increased densification, and a higher degree of transformation were obtained. A uniform microstructure of 150 nm α-Al2 O3 grains developed, reaching 57% relative density after sintering 1.5 wt%α-Al2 O3 seeded samples for 30 min at 1060°C. When sinter forged at 0.25 mm/min to 63 MPa and 1060°C for 30 min large deformations during the phase transformation increased the relative density to 74%. When the stress was increased to 235 MPa (1060°C, 30 min), 99.7% dense α-Al2 O3 with a grain size of 230 nm was obtained. By increasing the sinter forging temperature to 1150°C, 99.5% relative density was achieved at 190 MPa for 30 min. 相似文献
15.
Masanori Hirano Shinzi Watanabe Etsuro Kato Yasunobu Mizutani Masayuki Kawai Yasuhisa Nakamura 《Journal of the American Ceramic Society》1999,82(10):2861-2864
The microstructure, crystal phase, electrical conductivity, and mechanical strength of less than 7-mol%-Sc2 O3 -doped zirconia ceramics fabricated by comparatively low-temperature sintering at 1200–1300°C for 1 h were investigated. Zirconia ceramics having a uniform microstructure (grain size < 0.5 μm) stabilized with 6 mol% Sc2 O3 showed high electrical conductivity (0.15 S/cm at 1000°C) and high fracture strength (660 MPa). With the increase of Sc2 O3 content from 3.5 to 7 mol%, the grain size, fracture strength, and electrical conductivity at 1000°C changed from 0.2 to 0.5 μm, 970 to 440 MPa, and 0.07 to >0.2 S/cm, respectively. Sc2 O3 -doped zirconia polycrystals with high fracture strength and high electrical conductivity are promising candidates for the electrolyte material of solid oxide fuel cells. 相似文献
16.
High Quality and Yield in Potassium Titanate Whiskers Synthesized by Calcination from Hydrous Titania 总被引:2,自引:0,他引:2
Ningzhong Bao Liming Shen Xin Feng Xiaohua Lu 《Journal of the American Ceramic Society》2004,87(3):326-330
Hydrous titanium dioxide (TiO2 · n H2 O) was used to prepare K2 Ti2 O5 single crystals, K2 Ti4 O9 whiskers and K2 Ti6 O13 whiskers at 820°, 940°, and 1110°C, respectively, by calcination. At T < 820°C, dehydration of hydrous titania, decomposition of potassium carbonate, and reaction between titanate and potassium oxide occurred simultaneously, ending with a crystallization reaction of K2 Ti2 O5 single crystals at 820°C. Subsequently, K2 Ti2 O5 single crystals convert into K2 Ti4 O9 whiskers at 940°C, and K2 Ti4 O9 whiskers further convert into K2 Ti6 O13 whiskers at 1110°C. The reaction temperatures for the generations of these types of potassium titanates were all 10°–40°C lower than the corresponding temperatures when anatase was used as the reactant. The whiskers synthesized in the present study exhibited uniform size, good morphology, and a high yield. 相似文献
17.
Young-Wook Kim Mamoru Mitomo Toshiyuki Nishimura 《Journal of the American Ceramic Society》2002,85(4):1007-1009
Using AlN and RE2 O3 (RE = Y, Yb) as sintering additives, two different SiC ceramics with high strength at 1500°C were fabricated by hot-pressing and subsequent annealing under pressure. The ceramics had a self-reinforced microstructure consisting of elongated α-SiC grains and a grain-boundary glassy phase. High-temperature strength up to 1600°C was measured and compared with that of the SiC ceramics fabricated with AlN and Er2 O3 . SiC ceramics with AlN and Y2 O3 showed the best strength (∼630 MPa) at 1500°C, while SiC ceramics with AlN and Er2 O3 the best strength (∼550 MPa) at 1600°C. 相似文献
18.
Yasuo Hikichi Toshitaka Ota Keiji Daimon Tomotoshi Hattori Masatoshi Mizuno 《Journal of the American Ceramic Society》1998,81(8):2216-2218
Xenotime-type RPO4 (R = Y, Er, Yb, or Lu) powder was dry-pressed into disks and bars. The disks and bars could be sintered to a relative density of greaterthan equal to98% in air without cracking at 1300° (R = Yb or Lu) or 1500°C (R = Y or Er), depending on the grain size. The linear thermal expansion coefficient (at 1000°C), thermal conductivity (at 20°C), and bending strength (at 20°C) of the xenotime-type RPO4 ceramics were 6.2 10-6 /°C, 12.02 W(mK)-1 , and 95 ± 29 MPa for R = Y; 6.0 10-6 /°C, 12.01 W(mK)-1 , and 100 ± 21 MPa for R = Er; 6.0 10-6 /°C, 11.71 W(mK)-1 , and 135 ± 34 MPa for R = Yb; and 6.2 10-6 /°C, 11.97 W(mK)-1 , and 155 ± 25 MPa for R = Lu. The xenotime-type RPO4 ceramics did not react with SiO2 , TiO2 , Al2 O3 , ZrO2 , or ZrSiO4 , even at 1600°C for 3 h in air, and were stable in aqueous solutions of HCl, H2 SO4 , HNO3 , NaOH, and NH4 OH at 20°C. 相似文献
19.
Jing-Fong Tsai Uong Chon N. Ramachandran Dinesh K. Shetty 《Journal of the American Ceramic Society》1992,75(5):1229-1238
Microstructure, phase stability, and mechanical properties of CeO2 -partially-stabilized zirconia (12 mol% Ce-TZP) containing 10 wt% Al2 O3 and 1.5 wt% MnO were studied in relation to the base Ce-TZP and the Ce-TZP/Al2 O3 composite without MnO. The MnO reacted with both CeO2 and Al2 O3 to form a new phase of approximate composition CeMnAl11 O19 . The reacted phase had a magnetoplumbite structure and formed elongated, needlelike crystals. The MnO-doped Ce-TZP/Al2 O3 composites sintered at an optimum temperature of 1550°C exhibited high strength (650 MPa in four-point bending) and rising crack-growth-resistance behavior, with fracture toughness increasing from 7.6 to 10.3 MPa.In12 in compact tension tests. These improved mechanical properties were associated with relatively high tetragonal-to-monoclinic transformation temperature ( M s =−42°C) at small grain size (2.5 μm), significant transformation plasticity in mechanical tests (bending, uniaxial tension, and uniaxial compression) and transformation zones at crack tips in compact tension specimens. The transformation yield stress, zone size, and fracture toughness were sensitive to the sintering temperature varied in the range 1500° to 1600°C. Analysis of the transformation zones using Raman microprobe spectroscopy and calculation of zone shielding for the observed zones indicated that a large fraction of the fracture toughness (∼70%) was derived from transformation toughening. 相似文献
20.
Microstructure-Mechanical Property Relationships in Hot Isostatically Pressed Alumina and Zirconia-Toughened Alumina 总被引:1,自引:0,他引:1
Dong-Woo Shin K. K. Orr Helmut Schubert 《Journal of the American Ceramic Society》1990,73(5):1181-1188
The rates of densification and the mechanical properties of pure Al2 O3 and ZrO2 -toughened Al2 O3 (ZTA) have been investigated as a function of the temperatures and time schedules used for hot isostatic pressing (HIP) as a postsintering heat treatment for samples which had already been pressureless sintered in air at 1460°C for 45 min. ZTA hot isostatically presed at 1400°C had a finer grain size and a narrower grain size distribution than ZTA hot isostatically pressed at 1600°C. At both HIP conditions, the density which could be obtained was almost the maximum theoretical density. The amount of grinding-induced and fracture-induced monoclinic ZrO2 formed as a result of the tetragonal → monoclinic martensitic transformation in ZTA was higher in the samples hot isostatically pressed at 1400°C. ZTA hot isostatically pressed at 1600°C and 100 MPa had fewer flaws and higher strengths than ZTA hot isostatically pressed at 1400°C for the same time, with a gradual improvement in mechanical properties with increasing HIP time at each of these two temperatures. The best mechanical properties were obtained from ZTA hot isostatically pressed at 100 MPa and 1600°C for 1 h: these specimens had a four-point bend strength of 940 ± 15 MPa at room temperature and 540 ± 15 MPa at 1000°C and an indentation fracture toughness at room temperature of 9.4 ± 0.2 MPa·m1/2 . 相似文献