ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07574.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07575.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Percolation and Bruggeman's effective media theories, as they apply to the electrical conductivity of composites, are reviewed, and a general effective media (GEM) equation, which combines most aspects of both percolation and effective media theories, is introduced. It is then shown that the GEM equation quantitatively fits electrical resistivity (conductivity) as a function of the volume fraction data for binary composites. The parameters used to fit the experimental data are the electrical resistivities of the two phases, the percolation threshold for the lower resistivity phase (ôc), and an exponentt.Preliminary work, showing how the GEM equation can be used to model the piezoresistivity of composites by postulating that ôcis a function of the independent variable, is also presente

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07576.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Methodology for designing reliable ceramic components requires a precise evaluation and correlation of strengths in different stress states. The present paper compares the merits of the Weibull approach and the multiaxial elemental strength model on an experimental case involving mixed‐mode failure in the presence of bimodal flaw populations (surface and volume flaws). The experimental data were obtained using flexure specimens of Si3N4tested at various spans, with the purpose of enhancing shearing effects. The analysis of data was refined by developing an advanced postprocessor program to finite‐element codes for failure probability determination based upon the Barnett‐Freudenthal approximation of the Weibull approach and the multiaxial elemental strength model. In a second step, the strengths of the specimens exhibiting failures from the two concurrent populations of flaws (intermediate span) were predicted using both approaches from data obtained with different span lengths (long and short spans). Comparison with experimental data showed that the multiaxial elemental strength model is an improvement over the Weibull approach. It also allowed the short‐span bending test to be assessed. Finally, important implications for structural design with ceramics are di

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07577.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The dispersion of diamond particles in superplastic zinc sulfide produces flow softening at low diamond concentrations, and hardening at high concentrations. The minimum in the flow stress is observed at 5 wt% (5.5 vol%) diamond content. The softening can be explained in terms of the grain refinement in the ZnS in the presence of diamond particles. The smaller grain size is shown to reduce the flow stress by enhancing the rate of diffusional creep. The hardening is believed to have been caused by the formation of a network among the diamond particles at high concentrations. The fact that the diamond particle size was approximately onethird the grain size of ZnS may have contributed to the network formation.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07578.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Physical and chemical properties of poly(ethylene glycol) (PEG)‐containing TiO2‐SiO2gel films have been studied relative to their application to the fine‐patterning technique for optical memory disks. PEG added to TiO2‐SiO2gel films decomposed completely at temperatures between 250° and 300°C. The combustion of PEG in the gel films led to decreases in the refractive index and in the hardness of the films, which should result from the increase in porosity of the films. The addition of PEG into the gel films, however, did not adversely influence the weathering resistance of the glass substrates coated with the sol‐gel‐

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07579.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The effects of gas pressure and diluent content on the combustion synthesis of titanium nitride was investigated. Combustion of titanium powder samples containing TiN as a diluent in the concentration range 0 to 60 wt% was carried out in a nitrogen atmosphere at pressures in the range 0.1 to 1.4 MPa. Analysis of the dependence of combustion wave velocity on temperature gave an activation energy of 342 ± 50 kJ. mol−1, which is in good agreement with reported values for the diffusion of nitrogen in titanium nitri

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07580.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Transmission electron microscopy (TEM), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and nitrogen sorption technique (BET) were utilized to characterize the microstructure of a 10:90 wt% colloidal silica/potassium silicate gel as first described by Shoup. Gels in the unsintered state (15% theoretical density) were prepared for microscopy by the techniques of ultramicrotomy, Pt/C replication, and pore casting. Electron microscopic images of the ultramicrotomed thin sections (70 nm) show that the unfired gel possesses three distinct species of pores which are referred to as the micropores, mesopores, and macropores. The average micropore diameter was found to be 4 nm as determined by nitrogen desorption. Quantitative stereological analysis of the ultramicrotomed sections indicated that the average circular and lengthwise dimensions of the cylindrical mesopores were 0.15 and 0.39 μm, respectively. Similarly, this same analysis determined the average spherical macropore diameter to be 0.83 μm. In contrast, MIP results suggested that these gels possessed a unimodal pore size distribution centered around the 0.2‐μm pore size. The discrepancy between MIP and microscopy can be explained by viewing the void space as a pore‐throat network. Experimental evidence for this type of pore geometry was obtained from stereo pairs of Pt/C replicas and thick microtomed sections (0.5 μm) which gave information about particle connectivity and pore casts which depicted the pore connectivity in three dim

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07581.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Cubic BN was synthesized under high‐temperature and ‐pressure conditions from BN powder formed by pressure pyrolysis of borazine below 700°C and 100 MPa. The conversion of BN powder to cubic BN was strongly influenced by the residual hydrogen identified by the BH/BN ratio of IR absorption band. The activation energy for cubic BN synthesis from BN powder‐20 mol% AIN was 46 kJ/mol, when the starting BN was synthesized at 250°C. A mixture of BN powder and cubic BN particles was converted to cubic BN in a 100% yield by heat treatment at 1800°C and 6.5 GPa without any catalyst. The presence of cubic BN particles does enhance the conversion to cubic BN from BN powder. The energy required for the transformation of starting BN to cubic BN in the presence of cubic BN seed was 35

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07582.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The present report explores the use of cross‐polarization (CP) and single‐pulse magic angle spinning nuclear magnetic resonance (MAS‐NMR) as well as normal and surfaceenhanced Raman spectroscopy (SERS), in concert, for characterizing highly crystalline polymorphic silicon carbide ceramics. The combined use of these techniques provides a wealth of information regarding bulk, near‐surface, and surface speciation. Both Raman and SERS are promising techniques for fracture surface characterization of these systems. The application of CPMAS‐NMR and SERS to the study of ceramics is reported for the first time in this inve

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb07583.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY