摘要:

Oxygen ion mobility in the cubic fluorite‐structure phase having the composition Zr0.85Ca0.15O1.85has been directly determined by exchange measurements employing the stable isotope O18and mass spectrometer analysis. Measurements of electrical conductivity have been carried out as a function of both temperature and oxygen pressure. Results indicate that the conductivity is wholly ionic with a transfer number near unity for the oxygen io

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07830.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07831.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Experimental observations of the tetragonal phase transformation behavior in polycrystalline zirconias and the related toughening contribution are presented. An analysis which considers transformation thermodynamics and residual stresses is developed to describe the influence of grain size on tetragonal‐to‐monoclinic transformation temperature. The model is based on the promotion of the transformation by local internal tensile stress concentrations whose effects scale with grain size. The analysis is supported by observations of the martensite start temperature–grain size behavior in polycrystalline tetragonal zirconia containing 12 mol% ceria (12CeTZP). Next, the analysis considers the grain‐size‐dependent behavior of the transformation‐toughening contribution, ΔKT, and the transformation zone size,rT. The tetragonal‐to‐monoclinic (t‐to‐m) formation temperature,Ms, increases with increase in tetragonal grain size,d. Experimental results for zirconia‐12 mol% ceria (12CeTZP) and 2YTZP ceramics illustrate the predicted forms of the grain size dependencies for ΔKTandrT. The analytical model also describes the temperature dependence of the transformation‐toughening contribution ΔKTobserved in 2 mol% yttriadoped polycrystalline te

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07832.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The elastic properties, molar volume, and glass transition temperature (Tg) of rare‐earth‐containing aluminosilicate glasses were investigated in the compositions of SiO2–LnAlO3and SiO2–Ln3/4Al5/4O3, where Ln is Y, La, Nd, Eu, or Yb. The molar volume decreased with decreased ionic size of the Ln3+ion, andTgand elastic moduli increased in the same order. The Yb‐containing glasses showed the highest Young's modulus among all the oxide glasses, even higher than the highest value ever known for glass containing Y2O3, as expected from the smaller ionic radius of Yb3+than that of Y3+. The bulk modulus was found to be almost proportional to the inverse four‐thirds power of the molar volume of glasses in each composition, indicating that Ln3+ions can substitute for each other without changing the glass structure except for the size of the local structure around themselves. From the comparison of these properties, the structural role of rare‐earth ions in these glasses

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07833.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The volatilities of simulated, high‐level nuclear waste glasses have been measured using thermogravimetric analysis (TGA). These volatilities were measured in the region of the glass transition temperature (Tg) of the waste glasses, which is between 450° and 500°C. These data were obtained because the Waste Acceptance Preliminary Specifications require that no foreign materials be released into the canistered waste form upon heating of the canister to this glass transition temperature. In fact, all of the waste glass samples studied actually exhibited a net weight gain upon heating. This weight gain was shown to be due to oxygen uptake through oxidation of FeO. Powdered glass samples did show a small weight loss which was smaller in magnitude than the weight gain and was associated with water desorption. No true volatility was detected to the level of sensitivity (0.01 wt%) of the TGA instrument. This converts to a sensitivity of 330μg/m2of glass surface and a corresponding minimum value of 41 mg of volatiles for each Defense Waste Processing Facility conistered waste form. TGA experiments carried out at higher temperatures (800°C) revealed that organic concentrations in the waste glasses are less than 0.01 wt%. Thus, these results demonstrate that the Defense Waste Processing Facility will be able to comply with the Waste Acceptance Preliminary Specifications on the exclusion of foreign materials from the canistered waste forms, after exposure

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07834.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Colloidal heterocoagulation in aqueous media was used to form coatings of alumina or polystyrene latex particles on alumina spheres and of alumina or latex particles on silicon carbide whiskers. The polyelectrolyte dispersants poly‐(ammonium methacrylate) and poly(ethylenimine) were used to alter the inherent surface potentials of the materials to increase the applicability of this technique. In most cases, the coatings formed were partial monolayers, although thicker coatings of alumina were formed on the SiC due to aggregation of the alumina. The oxide coatings were relatively well adhered while the latex coatings were easily removed in aqueous media. The adherence of the latex coatings was improved by adding low alcohols to the heterocoagulating suspensio

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07835.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The microstructural changes produced by large (38 to 53μm), single‐crystal ZrO2inclusions (0, 0.09, 0.30 volume fractions, based on solid volume) within an Al2O3powder matrix were detailed as a function of constrained densification. Composite powder compacts were produced by pressure filtration for conditions where the Al2O3slurry was either flocced or dispersed. For both conditions, the ZrO2inclusions constrained densification. Microstructural observations for all composites revealed (1) the presence of cracks with large opening displacements between inclusions and (2) large density variations within the matrix. The cracks were most frequent at high volume fraction of inclusions in composites produced from flocced slurries and apparently originated during specimen preparation. Their large opening displacment was a result of matrix densification. Fewer cracks were observed in composites produced from dispersed slurries. Instead, these microstructures were dominated by large variations in matrix density, viz., dense regions surrounding low‐density regions, not consitent with the initial packing density of the matrix powder. The denser regions were formed early in the densification schedule. The lower‐density regions eventually developed into regions containing large, elongated voids as the Al2O3matrix grains became larger with heat‐treatment time. This pore enlargement process was shown to result from the disappearance of necks between originally sintered grains and appeared similar to the thermodynamic instability observed in thin films and constraine

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07836.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Experimental studies conducted in conjunction with a numerical analysis of strain‐rate gradients have established the origin of the damage process that occurs upon the densification of powders containing nonsintering inclusions and reinforcements. The underlying phenomenon is the development of localized compressive strains in the porous matrix, both around high‐aspect ratio reinforcements and between closely spaced reinforcements. These regions of compression densify first and then support grain growth. This process produces nondeformable networks that constrain the shrinkage of the adjacent, porous matrix. The constraint causes desintering and cracklike void formation in the lowdensity regions. The variables shown to be of importance are the volume fraction and aspect ratio of the reinforcements. The process is shown to be sensitive to the green density, such that a high initial density reduces initial damage and lowers the differential in densificat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07837.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

C and SiO2of different types were mixed and heat‐treated (1410° to 1550°C), according to a standardized temperature program, in flowing N2at different pressures. It was found that a starting material combination of C (115 m2/g) and SiO2(50 m2/g) yielded pure Si3N4after about 2 h at 1550°C and 1.3 MPa. By adjustment of the pressure in the range 2 to 6 MPa, irreversible evaporation of SiO(g) as well as formation of nonequilibrium phases was suppressed, and the amounts of residual C and O in the sample were controlled. Even a small amount of CO in the N2was observed to retard the nitridation. Possible explanations are discussed, based on thermodynamic calculations on the Si‐O‐N

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07838.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The work originally performed by Charles and Hillig (C&H) on the chemical stress corrosion cracking of glass is based on the chemical reaction rate theory and restricts the analysis to only the kinetic change at the exact location of the crack tip. As a result, crack sharpening/blunting is predicted when the applied stress lies above/below the static fatigue limit. The present paper extends the investigation within the same physical framework to the geometric change of the entire cavity surface, particularly in the vicinity of the cavity apex region. It has been found that a physical‐property‐dependent parameter (m) exists which exerts a strong influence on the crack‐tip morphology. In the case ofm=mth, wheremthis a thresholdmwhich assumes a value of ∼ 45 for an elliptical cavity having a minor to major axes ratio of 0.01, the current predictions reduce to the C&H results. In general, however,m≠mth, and the single‐valued fatigue limit degenerates into a range of applied stresses under which either enhanced blunting (m>mth) or necking (m100 in a soda–lime

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07839.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY