摘要:

The phase relations in and physical properties of SiAlON ceramics prepared by the hot isostatic pressing technique or by pressureless sintering with a sintering aid are reviewed. The sintering aid used is an AIN and Al2O3mixture either pure or in combination with Y2O3and/or various rare‐earth oxides. Special attention is paid to the amount and type of phase(s) formed and to how properties such as hardness (HV10), fracture toughness (KIC), and oxidation resistance vary with the sintering aid used for different types of SiAlON materials, i.e., O'‐, almost monophasic α‐, pure β‐, and mixed (α+β)‐Si

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08175.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Structural development of tellurite glasses with the addition of Li2O and Na2O has been studied using infrared, Raman, and X‐ray photoelectron spectroscopies. The increase in intensity of the peak at 755 cm−1in the infrared spectra as compared to the peak at 620 cm−1suggests the transformation of TeO4building units to TeO3pyramids with the addition of alkali oxide. Proposed structural change is further supported by the strong compositional dependence of the 755‐cm−1peak in the Raman spectra as well as by the formation of a shoulder in the O 1speak of X‐ray photoelectron spectra. In contrast to alkali silicate glasses, formation of nonbridging oxygens with the addition of alkali oxide is n

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08176.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

We have prepared various high‐Tcsuperconductors, of the LnBa2Cu3O7‐x(Ln = Y, Eu, Yb) and Bi‐Sr‐Ca‐Cu‐O families, by oxidation of rapidly quenched precursor alloys. In this metallurgical approach, appropriate metallic constituents are first combined into homogeneous alloys by rapid solidification processing. Subsequent oxidation of the as‐obtained precursors leads to formation of the desired superconducting phases. For EuBa2Cu3O7‐x, a notable increase in critical current density is observed, and we attribute this to both the uniform grain size and a more homogeneous composition. In the case of bismuth compounds, the Bi4Sr3Ca3Cu4O16+x(4334) and Bi2Sr2Ca2Cu3O10+x(2223) phases obtained have properties similar to those of their ceramic‐processed counterparts, and there is evidence for accelerated kinetics for the 2223 phase formation. We have also studied effects of additions and substitutions in the Bi‐Sr‐Ca‐Cu‐O system prepared by this method, where passage through the liquid state can ensure good chemical homogeneity for multicomponent alloys. By this means, the substitution limit of bismuth by lead in the 2223 phase is estimated to be about 30 at.%, and there is evidence that lead substitution both accelerates the formation of the 2223 phase and sharpens the superconducting transition. Finally, silver addition to Bi4Sr3Ca3Cu4alloy leads to the production of flexible metallic ribbons, which, after oxidation, form Ag/4334 microcomposites. These superconducting ribbons remain relatively ductile because of the formation of a layered microstructure in which superconducting regions alternate with silver bands aligned a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08177.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Six alumina casting slips with particle‐size distributions varying from 44 to 0.1 μm were examined. Particle packing was calculated using the approach of Andreasen. Viscosity, green density, and pore‐size distribution were measured. It was found that contouring the intermediate size distribution for particles finer than 15 μm provided the most desirable viscosity for slips composed of wide size distributions. For slips containing 50 vol% solids, the lowest viscosity obtained was 196 × 10−3N · s/m2(with a two‐component size distribution), and a green density of 2.52 g/cm3(65% of theoretical) was achieved with a ternary system. These casts had bimodal pore‐size distributions centered around approximatel

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08178.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The electrical conductivity of high‐purity single‐crystal alumina is determined in a temperature range from 400° to 1300°C. By applying an advanced fully guarded threeterminal measurement technique, reliable conductivity measurements are performed to as low as 10−17Ω−1· cm−1. Gas and surface conduction are measured separately and shown to be negligible. High‐purity sapphire exhibits a conductivity of 10−16Ω−1· cm−1at 400°C, two characteristic activation energies of 0.4 and 4.8 eV with increasing temperature, and a conductivity of 3 × 10−8Ω−1· cm−1at 1300°C. The fraction of the current carried by ions is determined by electron probe analysis of the electrodes following a 640‐h transference test at 1200°C with 4 kV/cm field applied. Only 0.3% of the current at 1200°C is carried by ions. A mathematical model of electrical conduction in sapphire is developed which describes sapphire as a wide‐bandgap semiconductor, doped with one dominant donor and one dominant acceptor. The observed conductivity is well described by the model over the enti

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08179.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Various methods to measure fracture toughness using in‐dentation precracks were compared using soda‐lime glass as a test material. In situ measurements of crack size as a function of applied stress allow both the toughnessKcand the residual‐stress factor χ to be independently determined. Analysis of the data showed that stress intensity factors based on classical half‐penny crack shapes overestimate toughness values and produce an apparentR‐curve effect. This is due to a constraint on crack shape imposed by primary lateral cracks in soda—lime glass. Models based on elliptical cracks were developed to account for the crack

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08180.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Conventional shear‐lag analyses of matrix cracking and debonding in uniaxial composites loaded in tension predict that the matrix stress varies only very slowly with position except near existing cracks. It therefore follows that the location of subsequent cracks is very sensitive to minor local variations in matrix strength, leading to significant statistical variation in crack spacing. This question is investigated using a discrete random process model of a composite and by direct experimental measurements of crack spacing. In the limit of a completely homogeneous composite, it is shown that the crack spacing distribution tends to an inverse square distribution between the theoretical maximum spacing and half that value. The random process model recovers this behavior in the limit and exhibits an approximately Weibull distribution of crack spacings when the matrix strength has significant variance. The theoretical predictions are compared with experimental results obtained for a unidirectional ceramic‐matrix composite (SiC fibers in a calcium aluminosilicate matrix). The experimental results exhibit features similar to those predicted by the model and are compatible with a matrix strength whose standard deviation is of the order of 40% of the mean strength. An important point is that, with this magnitude of strength variation, the material exhibits a significant size effect and it is essential to take this into account in estimating the mean crack spacing from the corresponding mean matrix propert

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08181.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A flexural strength of up to 1 GPa was achieved in SiC‐AIN materials and is attributed to a dense, equiaxial grain structure of the 2H(δ) SiC‐AIN solid solution, with a relatively uniform grain size of ∼ 1 μm. The strength was found to decrease with increasing grain size. While the β→α phase transformation and the formation of various metastable polytypes make microstructural control difficult in SiC materials, excellent control is facilitated in SiC‐AIN materials as a result of the stable 2H solid solution. Several mechanisms of grain refinement during the β→ 2H transition were observed, most notably the direct formation of several 2H grains from a single β grain. In addition, grain growth is limited by the diffusion‐controlled nature of the transition. These mechanisms could be utilized to achieve even higher strength values, with potentially higher reliability of the materials in stru

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08182.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The role of an interfacial carbon coating in the heat conduction behavior of a uniaxial silicon carbide nitride was investigated. For such a composite without an interfacial carbon coating the values for the thermal conductivity transverse to the fiber direction agreed very well with the values calculated from composite theory using experimental data parallel to the fiber direction, regardless of the ambient atmosphere. However, for a composite made with carbon‐coated fibers the experimental values for the thermal conductivity transverse to the fiber direction under vacuum at room temperature were about a factor of 2 lower than those calculated from composite theory assuming perfect interfacial thermal contact. This discrepancy was attributed to the formation of an interfacial gap, resulting from the thermal expansion mismatch between the fibers and the matrix in combination with the low adhesive strength of the carbon coating. In nitrogen or helium the thermal conductivity was found to be higher because of the contribution of gaseous conduction across the interfacial gap. On switching from vacuum to nitrogen a transient effect in the thermal diffusivity was observed, attributed to the diffusion‐limited entry of the gas phase into the interfacial gap. These effects decreased with increasing temperature, due to gap closure, to be virtually absent at 100

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08183.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The successful microwave sintering of zirconia demonstrates the necessity to understand both the materials and electromagnetic field aspects of microwave processing. It was difficult to produce crack‐free parts in the multimode microwave furnace employed in this investigation. Nonuniformities in the microwave field, and dielectric properties that increased rapidly with temperature, produced “hot spot” in the parts, which led to differential sintering and subsequent cracking. To produce crack‐free sintered parts, an indirect heating method was developed that eliminated the severe differential heating. Using this indirect heating method, it was demonstrated that the sintering temperature of zirconia could be lowered from 1375° to 1200°C by microwave processing and that the resulting grain size

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb08184.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY