首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到10条相似文献,搜索用时 156 毫秒
1.
Chemical Durability of Silicon Oxycarbide Glasses   总被引:2,自引:0,他引:2  
Silicon oxycarbide (SiOC) glasses with controlled amounts of Si—C bonds and free carbon have been produced via the pyrolysis of suitable preceramic networks. Their chemical durability in alkaline and hydrofluoric solutions has been studied and related to the network structure and microstructure of the glasses. SiOC glasses, because of the character of the Si—C bonds, exhibit greater chemical durability in both environments, compared with silica glass. Microphase separation into silicon carbide (SiC), silica (SiO2), and carbon, which usually occurs in this system at pyrolysis temperatures of >1000°–1200°C, exerts great influence on the durability of these glasses. The chemical durability decreases as the amount of phase separation increases, because the silica/silicate species (without any carbon substituents) are interconnected and can be easily leached out, in comparison with the SiOC phase, which is resistant to attack by OH or F ions.  相似文献   

2.
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   

3.
Interfaces of silicon carbide-whisker-reinforced alumina (SiC( w )/Al2O3) composites were examined using high-resolution electron microscopy (HREM). HREM specimens were prepared from the bulk of samples that were previously tested for fracture toughness at 25°, 1000°, 1200°, or 1400°C, in ambient air. The test temperature history served as an independent variable. It was found that the as-received material did not possess a distinct interfacial layer and that the test temperature history (which included a 30°C/min heating and cooling rate, a 30-min soak prior to specimen loading, and a typical test duration of 5–10 min) did not appreciably change the interface thickness at any of the elevated test temperatures.  相似文献   

4.
SiCO glasses prepared from sol–gel precursors via pyrolysis in argon at temperatures ranging from 1000° to 1400°C were studied by transmission electron microscopy (TEM), in conjunction with electron energy-loss spectroscopy (EELS). EELS analysis showed that stoichiometric SiCO glass underwent phase separation, forming SiO2- and SiC-based environments. This process started at ∼1200°C. However, at temperatures >1300°C, precipitation of nanometer-sized SiC particles embedded in vitreous SiO2 was monitored by high-resolution TEM.  相似文献   

5.
Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum-silicon carbide (Al4SiC4) with the specific surface area of 15.5 m2·g−1. The powders nitrided at and above 1400°C for 3 h contained the 2H-phases which consisted of AlN-rich and SiC-rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m2·g−1, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.  相似文献   

6.
Inhibition of cubic-rhombohedral phase transformation and low-temperature sintering at 1000°C were achieved for 10-mol%-Sc2O3-doped cubic-ZrO2 by the presence of 1 mol% Bi2O3. The powders of 1-mol%-Bi2O3–10-mol%-Sc2O3-doped ZrO2 were prepared using a hydrolysis and homogeneous precipitation technique. No trace of rhombohedral-ZrO2 phase could be detected, even after sintering at 1000°–1400°C. The average grain size of the ZrO2 sintered at 1200°C was >2 μm because of grain growth in the presence of Bi3+. Cubic, stabilized Bi-Sc-doped ZrO2 sintered at 1200°C had sufficient conductivity at 1000°C (0.33 S/cm) to be used as an electrolyte for a solid-oxide fuel cell (SOFC) and at 800°C (0.12 S/cm) for an intermediate-temperature SOFC.  相似文献   

7.
The stability of lanthanum orthophosphate (LaPO4) on SiC was investigated using a LaPO4-coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO4 is reduced in the presence of SiC and reacts to form La2O3 or La2Si2O7 and SiO2 as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO2 film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO4 coating. Continuous LaPO4 coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO4 coating becomes morphologically unstable due to free-energy minimization as the grain size reaches the coating thickness, which allows the SiO2 oxidation product to penetrate the coating.  相似文献   

8.
Calcium hexa-aluminate (CaO·6Al2O3) has been prepared from calcium nitrate and aluminum sulfate solutions in the temperature range of 1000°–1400°C. A 0.3 mol/L solution of aluminum sulfate was prepared, and calcium nitrate was dissolved in it in a ratio that produced 6 mol of Al2(SO4)3·16H2O for each mole of Ca(NO3)2·4H2O. It was dried over a hot magnetic stirrer at ∼70°C and fired at 1000°–1400°C for 30–360 min. The phases formed were determined by XRD. It was observed that CaO·Al2O3 and CaO·2Al2O3 were also formed as reaction intermediates in the reaction mix of CaO·6Al2O3. The kinetics of the formation of CaO·6Al2O3 have been studied using the phase-boundary-controlled equation 1 − (1 − x )1/3= K log t and the Arrhenius plot. The activation energy for the low-temperature synthesis of CaO·6Al2O3 was 40 kJ/mol.  相似文献   

9.
The effect of Si3N4, Ta5Si3, and TaSi2 additions on the oxidation behavior of ZrB2 was characterized at 1200°–1500°C and compared with both ZrB2 and ZrB2/SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB2 was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si3N4-modified ceramics increased with increasing Si3N4 content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB2, with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta5Si3, 30 vol% TaSi2, 35 vol% Si3N4, or 20 vol% Si3N4 with 10 mol% CrB2. These materials exceeded the oxidation resistance of the ZrB2/SiC ceramics below 1300°–1400°C. However, the ZrB2/SiC ceramics showed slightly superior oxidation resistance at 1500°C.  相似文献   

10.
Copper phosphate glasses with 40, 50, and 60 mol% CuO in batch were melted in air at 1000°, 1100°, and 1200°C using quartz or alumina crucibles, and the [Cu2+]/[Cutotal] ratio variations with melting time were measured. Glasses were oxidized during melting and reached equilibrium [Cu2+]/[Cutotal] ratios which were independent of melting temperature and identical for the 40 and 50 mol% CuO content glasses. Structural considerations seemed to have determined oxidation-reduction equilibrium rather than an equilibrium redox reaction. Also, the effects of crucible type on the oxidation-reduction balance were examined. It was found that a quartz crucible is more inert and has less effect on the oxidation-reduction equilibrium of glass than an alumina crucible. Crucible contamination and phosphorus vaporization were found to diminish as the CuO content in the batch was increased.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号