共查询到20条相似文献,搜索用时 31 毫秒
1.
Sintered α-SiC was exposed for 10 h to H2 containing various partial pressures of H2 O ( P H2 O from 5×10−6 to 2×10−2 atm; 1 atm≅105 Pa) at 1300° and 1400°C. Weight loss, surface morphology, and room-temperature flexural strength were strongly dependent on P H2 O . The strength of the SiC was not significantly affected by exposure to dry H2 at a P H2 O of 5×10−6 atm; and following exposure at P H2 O >5×10−3 atm, the strength was even higher than that of the as-received material. The increase in strength is thought to be the result of crack blunting associated with SiO2 formation at crack tips. However, after exposure in an intermediate range of water vapor pressures (1×10−5 < P H2 O <1×10−3 atm), significant decreases in strength were observed. At a P H2 O of about 1×10−4 atm, the flexural strength decreased approximately 30% and 50% after exposure at 1300° and 1400°C, respectively. The decrease in strength is attributed to surface defects caused by corrosion in the form of grain-boundary attack and the formation of pits. The rates of weight loss and microstructural changes on the exposed surfaces correlated well with the observed strength changes. 相似文献
2.
S. Shimada T. Onuma H. Kiyono M. Desmaison 《Journal of the American Ceramic Society》2006,89(4):1218-1225
Isothermal oxidation of dense TiC ceramics, fabricated by hot-isostatic pressing at 1630°C and 195 MPa, was performed in Ar/O2 (dry oxidation), Ar/O2 /H2 O (wet oxidation), and Ar/H2 O (H2 O oxidation) at 900°–1200°C. The weight change measurements of the TiC specimen showed that the dry, wet, and H2 O oxidation at 850°–1000°C is represented by a one-dimensional parabolic rate equation, while the oxidation in the three atmospheres at 1100° and 1200°C proceeds linearly. Cross-sectional observation showed that the dry oxidation produces a lamellar TiO2 scale consisting of many thin layers, about 5 μm thick, containing many pores and large cracks, while H2 O-containing oxidation decreases pores in number and diminishes cracks in scales. Gas evolution of CO2 and H2 with weight change measurement was simultaneously followed by heating the TiC to 1400°C in the three atmospheres. Cracking in the TiO2 scale accompanied CO2 evolution, and the H2 O-containing oxidation produced a small amount of H2 . A piece of single crystal TiC was oxidized in 16 O2 /H2 18 O to reveal the contribution of O from H2 O to the oxidation of TiC by secondary ion mass spectrometry. 相似文献
3.
The effects of heat treatment in Ar-O2 and H2 -H2 O atmospheres on the flexural strength of hot isostatically pressed Si3 N4 were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si3 N4 was exposed at 1400°C to (1) H2 with water vapor pressure ( P H2O ) greater than 1 × 10−4 MPa or (2) Ar with oxygen partial pressure ( P O2 ) of between 7 × 10−6 and 1.5 × 10−5 MPa. However, the strength of the material was degraded when the P H2O in H2 was lower than 1 × 10−4 MPa, and essentially unaffected when the P O2 in Ar was higher than 1.5 × 10−5 MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y2 Si2 O7 layer formed during exposure. 相似文献
4.
The following variables were considered in an investigation of the sintering of uranium dioxide: (1) sintering aids, i.e., small additions of substances that might aid the densification process, (2) soaking temperature, (3) soaking time, (4) furnace atmosphere, and (5) particle size of the uranium dioxide. The incipient sintering temperature, the effect of binders, and the effect of fabricating pressure also were determined. The data obtained in the investigation led to the following optimum conditions for obtaining dense UO2 by cold pressing and sintering: (1) preparing the UO2 as a submicron powder but not fine enough to be pyrophoric, (2) cold pressing at 20 to 30 tons per sq. in. without the use of organic binders, (3) using the following sintering schedule: ( a ) in H2 to 1400°C., (b) in steam or a wet inert gas from 1400° to 1500°C., ( c ) in steam or a wet inert gas at 1500°C. for a 1-hour soak, and (d) cooling in H2 from 1500°C. to room temperature. By following this procedure, pellets of UO2 (3/8 in. in diameter by 1/4 in. high) were obtained that were 97 to 98% of theoretical density. 相似文献
5.
A two-step ion-exchange technique was developed for introducing compressive stresses on the surface of ZrO2 –Al2 O3 composites. In the first step, a thin layer (∼250 μm) of Na-β"-Al2 O3 was formed on the surface of the composite by a vapor-phase process at ∼1400°C. In the second step, Na+ ions were replaced by K+ ions by a heat treatment at ∼385°C for 2 h in a molten KNO3 bath. Replacement of sodium by potassium led to the creation of surface compressive stresses. The flexural strength and Weibull modulus of ZrO2 –Al2 O3 composite were ∼915 MPa and 10, respectively, for the as-sintered samples. By contrast, the flexural strength and Weibull modulus were ∼1140 MPa and 26, respectively, for the ion-exchanged samples. A residual surface compressive stress of ∼480 MPa was measured by a strain-gauge technique in K+ -ion-exchanged samples. The presence of surface compressive stresses also was confirmed using an indentation technique. The technique developed here can be used to introduce compressive stresses on components of virtually any shape. 相似文献
6.
Minako Nagamori Taro Shimonosono Soichiro Sameshima Yoshihiro Hirata Naoki Matsunaga Yoshio Sakka 《Journal of the American Ceramic Society》2009,92(S1):S117-S121
A thin film (60 μm thick) of a gadolinium-doped ceria (GDC) electrolyte was prepared by the doctor blade method. This film was laminated with freeze-dried 42 vol% NiO–58 vol% GDC mixed powder and pressed uniaxially or isostatically under a pressure of 294 MPa. This laminate was cosintered at 1100 °–1500 °C in air for 4–12 h. The laminate warped because of the difference in the shrinkage of the electrolyte and electrode during the sintering. A higher shrinkage was measured for the electrode at 1100 °–1200 °C and for the electrolyte at 1300 °–1500 °C. The increase of the thickness of anode was effective in decreasing the warp and in increasing the density of the laminated composite. The maximum electric power density with a SrRuO3 cathode using 3 vol% H2 O-containing H2 fuel was 100 mW/cm2 at 600 °C and 380 mW/cm2 at 800 °C, respectively, for the anode-supported GDC electrolyte with 30 μm thickness. 相似文献
7.
Phase-transformation-induced compressive surface stresses were introduced into ceria-doped tetragonal zirconia polycrystals by reduction of CeO2 . Four-point-bending strength of sintered ZrO2 containing 12 mol% CeO2 increased from 240 to 545 MPa after it was annealed at 1400°C for 2 h in nitrogen. The strength of the same material hot isostatically pressed in oxygen increased after it was annealed in nitrogen for 2 h at 1500°C from 430 to 595 MPa. 相似文献
8.
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
9.
M. G. M. U. Ismail Zenjiro Nakai Shigeyuki Smiya 《Journal of the American Ceramic Society》1987,70(1):7-C-8
Mullite (3Al2 O3 ·2SiO2 ) of stoichiometric composition was prepared by mixing boehmite sol and silica dispersion and gelling at a pH of 3. Complete mullitization takes place at or above 1300°C. Ultrafine mullite powder prepared by calcining gel at 1400°C and attrition milling could be sintered to >98% (theoretical density) at 1650°C for 1.5 h. The flexural strength of the sintered body at room temperature was 405 MPa and 350 MPa at 1300°C. Only traces of a secondary phase were observed along the grain boundary. 相似文献
10.
Toshiyuki Mori Hiroshi Yamamura Hidehiko Kobayashi Takashi Mitamura 《Journal of the American Ceramic Society》1992,75(9):2420-2426
Effects of the concentration of ZrOCl2 , calcination temperature, heating rate, and the size of secondary particles after hydrolysis on the preparation of high-purity ZrSiO4 fine powders from ZrOCl2 .8H 2 O (0.2 M to 1.7 M ) and equimolar colloidal SiO2 using sol–gel processing have been studied. Mechanical properties of the sintered ZrSiO4 from the high-purity ZrSiO4 powders have been also investigated. Single-phase ZrSiO4 fine powders were synthesized at 1300°C by forming ZrSiO4 precursors having a Zr–O–Si bond, which was found in all the hydrolysis solutions, and by controlling a secondary particle size after hydrolysis. The conversion rate of ZrSiO4 precursor gels to ZrSiO4 powders from concentrations other than 0.4 M ZrOCl2 .8H2 O increased when the heating rate was high, whereupon the crystallization of unreacted ZrO2 and SiO2 was depressed and the propagation and increase of ZrSiO4 nuclei in the gels were accelerated. The density of the ZrSiO4 sintered bodies, manufactured by firing the ZrSiO4 compacts at 1600° to 1700°C, was more than 95% of the theoretical density, and the grain size ranged around 2 to 4 μm. The mechanical strength was 320 MPa (room temperature to 1400°C), and the thermal shock resistance was superior to that of mullite and alumina, with fairly high stability at higher temperatures. 相似文献
11.
Silicon monoxide vapor generated from Si/SiO2 mixed-powder compacts was used with NH3 to synthesize silicon nitride in a tubular flow reactor operated at temperatures in the range of 1300°-1400°C. The ammonolysis of SiO with excess NH3 was very rapid, yielding three different types of silicon nitride at different longitudinal locations in the reactor: amorphous nanophase powder of an average size of about 20 nm, amorphous whiskers of a few micrometers in diameter, and α-polycrystals. The amorphous products were heat-treated for crystallization at temperatures between 1300° and 1560°C in a stream of dissociated NH3 , N2 , or N2 /H2 mixture gas. When dissociated NH3 was used, nanophase powder was crystallized at 1300°C. The yield of nanophase silicon nitride from SiO varied from 13% to 43%, depending on operating conditions. 相似文献
12.
Jiang Li Yubai Pan Yusong Wu Huamin Kou Jingkun Guo 《International Journal of Applied Ceramic Technology》2007,4(3):276-284
High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO2 as additives using α-Al2 O3 -seeded nanocrystalline γ-Al2 O3 powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al2 O3 sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m1/2 , respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m1/2 . 相似文献
13.
Samarium oxide forms a ceramic of medium strength and density (6.0) when it is fired at 1300°C. When it is fired at 1500°C., its density is increased to 7.4 but it loses its stability toward boiling water. X-ray data are given concerning the structure of the oxide as received and at 1300°C. and of the reaction product of Sm203 with water. Gadolinium oxide forms a ceramic of somewhat higher density (7.0) at 1300° than does Sm2 O3 ; at 1500°C. its density is slightly higher (7.6) whereas its stability toward water is unchanged. Data concerning both oxides include density, shrinkage, moduli of elasticity and of rupture, linear thermal expansion, differential thermal analysis, and specific heat as well as X-ray data. Results are given of a brief survey of the reaction products obtained when Sm2 O3 and Gd2 O3 are heated at 1500°C. in equimolecular quantities with Al2 O3 , BaO, CaO, CdO, Fe2 O3 , HfO2 , MgO, SO2 , SrO, ThO2 , and ZrO2 . X-ray data for the following rare-earth oxides as received and after calcination at 1400°C. are given: ceria, praseodymia, neodymia, europia, dysprosia, holmia, erbia, thulia, and ytterbia. 相似文献
14.
Douglas A. Weirauch Jr. Jerold E. Lazaroff P. Darrell Ownby 《Journal of the American Ceramic Society》1995,78(11):2923-2928
The wetting of polycrystalline alumina by a colored, calciamagnesia aluminosilicae glass was found to be dependent on temperatutre between 1300° and 1500°C, but independent of gas atmosphere effects. Neither the oxygen partial pressure, oveer the range of 10-6 to 10-10 Pa, the gas buffer system (Co/CO2 or H2 /H2 O), nor pre-equilibration of the substrate surface with the atmosphere at tge exoperimental temperature before solid-liuid interface formation affected the stable contact angle. An initial drop in contact angle the stable contact angle. An initial drop in contact angle occurring within the first hour is attributed to repaid dissolution of alumina and the formation of a stable glass/alumina interface. The contact angle after an 8-h isothermal hold decreased from 1300° to 1500°C. The solid-liquid interfacial energy, μMS1 , controls the wetting behavior. Changes in μMs1 are attributed to he breakup of the silica network as temperature increases. 相似文献
15.
The modulus of rupture of Al2 O3 -spinel castables containing 20 wt% Al2 O3 -rich MgO-Al2 O3 spinel and 1.36-2.04 wt% CaO generally increases with an increase in both CaO content and temperature from 1300° to 1500°C, but it remains virtually constant from 1000° to 1300°C. Microscopic observation of the castable fired at 1500°C for 3 h reveals the growth of some CA6 crystals out of the Al2 O3 -rich spinel grains in the bonding matrix of the castable. The bond linkage between the CA6 and spinel grains in the matrix is believed to cause both the CaO content and temperature dependence of the hot strength of the Al2 O3 -spinel castables as well as the hot strength enhancement of high-Al2 O3 castables with addition of Al2 O3 -rich spinel. 相似文献
16.
S. C. CARNIGLIA S. D. BROWN T. F. SCHROEDER† 《Journal of the American Ceramic Society》1971,54(1):13-17
The compositions of anion-deficient zirconia and thoria in equilibrium with O2 were measured from 1 to 10−6 atm and 1400° to 1900°C; for ZrO2- x (po2 in atm, and T in °K), log x∼−0.890-[(0.400×104 )/ T ]-[(log p )/6]; for ThO2- x , log x∼−1.870-[(0.340×104 )/ T ]-[(log p )/6]. The ZrO2- x -Zr boundary was located at x=0.014 at 1800°C; thoria was single-phase over the entire range. Consistent results were obtained when O2 /inert gas mixtures were used, but use of H2 /H2 O and CO/CO2 at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic-tetragonal phase change of ZrO2 and the lattice thermal expansion, room-temperature Young's modulus, and strength properties of ZrO2 and ThO2 bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased slightly with departure from stoichiometry. 相似文献
17.
Oxidation behavior of silicon nitride (Si3 N4 ) was investigated in flowing air (2.45 cm/s) containing 10%–50% H2 O at a total pressure of 1.8–10 atm at 1300°–1500°C for 100 h. The oxidation of Si3 N4 progressed with volatilization of the SiO2 scale; it was more enhanced at a high partial pressure of H2 O rather than at high temperature. The total pressure had little effect on the oxidation. In order to avoid the oxidation, Si3 N4 substrate was coated with lutetium disilicate (Lu2 Si2 O7 ) layer through the intermediate SiO2 -rich phase. While the coating layer well suppressed the oxidation in case of small amount of water vapor, it was not sufficiently effective to suppress the oxidation when the water vapor was rich. SiO2 volatilization was observed between the layer and substrate. The flexural strength of the coated Si3 N4 at room temperature was somewhat increased after the oxidation in wet air, while that of the uncoated one was almost unchanged. This increase was attributable to crack healing of the substrate during the oxidation. 相似文献
18.
Measurements of threshold stress intensities for crack growth, K h , of three polycrystalline SiC materials were attempted using interrupted static fatigue tests at 1200°–1400°C. Weibull statistics were used to calculate conservative Kth values from test results. The K th of a chemically vapor deposited β-SiC could not be determined, as a result of its wide variations in strength. The Kth ≥ 3.3,2.2, and 1.7 MPa·m1/2 for an Al-doped sintered α-SiC; and Kth ≥ 3.1, 2.7, and 2.2 MPa·m1/2 for a hot isostatically pressed α-SiC, both at 1200°, 1300°, and 1400°C, respectively. A damage process concurrent with subcritical crack growth was apparent for the sintered SiC at 1400°C. The larger Kth 's for the HIPed SiC (compared to the sintered SiC) may be a result of enhanced viscous stress relaxation caused by the higher silica content and smaller grain size of this material. Values measured at 1300° and 1400°C were in good agreement with the Kth 's predicted by a diffusive crack growth model, while the measured Kth 's were greater than the predicted ones at 1200°C. 相似文献
19.
Polycrystalline bulk samples of Ti3 SiC2 were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10−8 and 2 × 10−5 kg2 -m−4 .s−1 . The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 106 Ω−1 .m−1 , roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10−6 °C−1 , 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti3 SiC2 does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature. 相似文献
20.
Strength of Nicalon Silicon Carbide Fibers Exposed to High-Temperature Gaseous Environments 总被引:1,自引:0,他引:1
The effects of exposures to high-temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H2 with various P H2O for 10 h at 1000° and 1200°C, and (2) air for 2 to 100 h at 800° to 1400°C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO2 layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO2 layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO2 inherent in the fibers. 相似文献