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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
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The effect of oxygen activity on the sintering of high-purity Cr2 O3 is shown. Theoretical density was approached at the equilibrium O2 partial pressure needed to maintain the Cr2 O3 phase ( P o2 =2×10−12 atm). The presence of N2 in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O2 activity, correlated with the changing predominant volatile species in the Cr-O system. 相似文献
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Cr2 O3 and ZrO2 were mixed in various ratios and pressed to form compacts, which were then sintered in carbon powder. Compacts with >30 wt% Cr2 O3 were sintered to densities >98% of true density at 1500°C. This method of sintering in carbon powder can be used to prepare very dense Cr2 O3 -ZrO2 ceramics at a relatively low temperature, (∼1500°C) without additives. 相似文献
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JONG-HEE PARK WAYNE E. KING STEVEN J. ROTHMAN 《Journal of the American Ceramic Society》1987,70(12):880-885
The cation diffusivities in the lattice and along dislocations and grain boundaries have been measured on sintered polycrysals of Cr2 O3 ; and Cr2 Cr2 O3 -0.09 wt% Y2 O3 at1100°C and at the pO2 corresponding to that of Cr/Cr2 O3 equilibrium at that temperature. Results for lattice and dislocation diffusivities in pure Cr2 O3 are in good agreement with previous work. The present results indicate that yttrium additions have negligible effect on lattice and dislocation diffusion. However, grain-boundary diffusion in pure Cr2 O3 is significantly slower than grain-boundary diffusion in Cr2 O3 -0.09 wt% Y2 O3 . The results are discussed in terms of their implications for the reactive-element effect in high-temperature oxidation of chromium-containing alloys. 相似文献
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Self-diffusion of 51 Cr was measured both parallel to and perpendicular to the c axis in single crystals of Cr2 O3 as a function of oxygen partial pressure at 1490° and 1570°C. The oxygen-partial-pressure dependence of the diffusivity indicates that cation self-diffusion occurs by a vacancy mechanism. The values of the self diffusion coefficients determined in this experiment are about 104 times smaller than those previously reported in this temperature range . 相似文献
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Refractory bodies of 65 wt% Al2 O3 were prepared from a mixture of calcined alumina and raw kaolin with the addition of Cr2 O3 up to 15 wt%. The Cr2 O3 addition effectively enhances slag resistance and reduces mullite formation. Petrographic analysis of the refractories after the slag test suggests that Cr2 O3 increases the viscosity of both the glassy phase in the refractory as well as the slag, thereby retarding slag penetration and reaction at elevated temperature. 相似文献
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Yong Zheng Min You Weihao Xiong Wenjun Liu Shengxiang Wang 《Journal of the American Ceramic Society》2004,87(3):460-464
Based on the empirical electron theory of solids and molecules, the valence-electron structure (VES) of the rim phase in Ti(C,N)-based cermets was calculated, and the relationship between the VES and plasticity was determined. The results indicated that the plasticity of the rim phase in a Ti(C,N)-based cermet could be defined using the sum of the n a values for the covalent bonds, and that chromium dissolution in the rim phase improved the plasticity of the rim phase. Moreover, a series of experiments showed that adding Cr2 C3 to a typical Ti(C,N)-based cermet strengthened the interface. Based on those results, a Ti(C,N)-based cermet with added Cr3 C2 was manufactured; the new cermet had more than twice the transverse rupture strength of a typical cermet. 相似文献
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Koichiro Fukuda Satoshi Mori Shinobu Hashimoto 《Journal of the American Ceramic Society》2005,88(12):3528-3530
The crystal structure of Zr2 Al3 C4 was refined by the Rietveld method from conventional X-ray powder diffraction data. The structure was hexagonal (space group P 63 mc , Z =2) with a =0.334680(6) nm, c =2.22394(3) nm, and V =0.215731(6) nm3 , being isomorphous with that of U2 Al3 C4 . The final reliability indices were R wp =8.57%, R p =6.06%, and S =1.32. The crystal showed an intergrowth structure with NaCl-type ZrC slabs separated by Al4 C3 -type Al3 C2 layers. 相似文献
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The weight loss of Cr2 O3 in oxidizing environments (Po2 = 1 to 10−3 atm) at 1200°C was measured. Both hot-pressed and sintered Cr2 O3 pellets were investigated in O2 /Ar gas mixtures, and the dependence of the weight loss on the O2 partial pressure, the gas flow rate, and the total pressure was determined independently. The experimentally determined O2 partial pressure dependence (rate ∝ PO2 3/4 ) corresponds to that expected for the reaction Cr2 O3 (s)+3/2O2 ⇌2CrO3 (g). The flow rate and total pressure dependencies show that mass transport through a gaseous boundary layer is the rate-controlling step in the oxidation/vaporization of Cr2 O3 . Evaporation coefficients for the loss of CrO3 (g) under the experimental conditions were <0.01. 相似文献
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RODNEY D. BAGLEY IVAN B. CUTLER D. LYNN JOHNSON† 《Journal of the American Ceramic Society》1970,53(3):136-141
Alpha alumina with additions of TiO2 sintered more rapidly than "pure" alumina. The rate of initial sintering increased approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained approximately constant or decreased slightly with additional titania. The concentration which produces the maximum rate of sintering is thought to be the solubility limit of TiO2 in Al2 O3 . For alumina particles larger than about 2 μm, the kinetic process was mainly grain-boundary diffusion. With smaller particles, volume diffusion increased. The "solubility limit" increased with decreasing particle size, indicating an excess surface concentration of TiO2 . The data may be interpreted in terms of a region of enhanced diffusion at the grain boundary that increases with TiO2 concentration. With small alumina particles, this region is large enough to become a significant portion of the volume of the particle, and the small particles appear to sinter by volume diffusion kinetics, but the diffusion coefficient corresponds to an enhanced diffusion coefficient. 相似文献
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Augusto O. Kunrath Ivar E. Reimanis John J. Moore 《Journal of the American Ceramic Society》2002,85(5):1285-1290
A microstructural analysis of compounds produced by combustion synthesis coupled with hot pressing, for reactions between titanium, chromium, and carbon, was conducted. The reactions were aimed to produce composites of Cr3 C2 and TiC at three different volume fractions of each carbide (25/75, 50/50, and 75/25). Large amounts of chromium and carbon were found to be in solution in the B1 rock-salt structure of TiC. The materials with 25 and 50 vol% of Cr3 C2 consisted of 100% (Ti,Cr)Css solid solution, while the composition with 75 vol% Cr3 C2 was formed by Cr3 C2 + (Ti,Cr)Css . Some precipitation of Cr3 C2 was achieved by annealing, but a minimum of 20 wt% was always in solution. The 50 vol% Cr3 C2 –50 vol% TiC composition was the most affected by the heat treatments. Discontinuous and general precipitation were observed, depending on the annealing conditions. A TTT-type diagram was plotted for this material. 相似文献
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KE-CHIN WANG LLOYD H. DREGER V. V. DADAPE JOHN L. MARGRAVE 《Journal of the American Ceramic Society》1960,43(10):509-510
The sublimation of chromic oxide, Cr2 O3 , has been observed in vacuum by the Langmuir technique using induction and solar heating. Extensive sublimation did not yield any new phases on the basis of X-ray powder studies, and condensates of Cr2 O3 were always obtained. Flash vaporization and flow experiments in CO or O2 atmospheres and in vacuum indicated no appreciable differences in rates of sublimation. Weight-loss experiments showed that the rate of sublimation was slightly higher than predicted for decomposition to the elements and suggested that small amounts of complex molecules, e.g. CrO and CrO2 , were also present in the equilibrium vapor. 相似文献
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Jin-Seong Kim Ngoc-Huan Nguyen Myung-Eun Song Jong-Bong Lim Dong-Soo Paik Sahn Nahm Jong-Hoo Paik Byung-Hyun Choi Soon-Jae Yu 《International Journal of Applied Ceramic Technology》2009,6(5):581-586
Bi2 O3 was added to a nominal composition of Zn1.8 SiO3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi2 O3 content was <8.0 mol%, a porous microstructure with Bi4 (SiO4 )3 and SiO2 second phases was developed in the specimen sintered at 885°C. However, when the Bi2 O3 content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛr =7.6, and τf =−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi2 O3 sintered at 885°C for 2 h. 相似文献
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Mi-Ri Joung Jin-Seong Kim Myung-Eun Song Jae-Hong Choi Jong-Woo Sun Sahn Nahm Jong-Hoo Paik Byung-Hyun Choi 《Journal of the American Ceramic Society》2009,92(9):2151-2154
Li2 CO3 was added to Mg2 V2 O7 ceramics in order to reduce the sintering temperature to below 900°C. At temperatures below 900°C, a liquid phase was formed during sintering, which assisted the densification of the specimens. The addition of Li2 CO3 changed the crystal structure of Mg2 V2 O7 ceramics from triclinic to monoclinic. The 6.0 mol% Li2 CO3 -added Mg2 V2 O7 ceramic was well sintered at 800°C with a high density and good microwave dielectric properties of ɛ r =8.2, Q × f =70 621 GHz, and τf =−35.2 ppm/°C. Silver did not react with the 6.0 mol% Li2 CO3 -added Mg2 V2 O7 ceramic at 800°C. Therefore, this ceramic is a good candidate material in low-temperature co-fired ceramic multilayer devices. 相似文献