摘要:

A new method for forming spherical, submicrometer ceramic oxide particles by the hydrolysis of emulsified alkoxide droplets is reported. Emulsions are formed of alkoxide droplets dispersed in an inert, polar solvent. The alkoxide droplets are hydrolyzed to form oxide particles by adding water to the emulsion. It was shown that individual droplets acted as “microreactors” and controlled the powder size, shape, and composition. Both single‐oxide and mixed‐oxide powders were formed by this te

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03694.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Mineral oil lubricated rolling and sliding wear of SiC whisker (SiCw) reinforced Si3N4composite and monolithic Si3N4prepared identically against M2 tool steel were investigated using a cylinder‐on‐cylinder apparatus. Wear of this Si3N4was higher than that of the composite. Wear of the steel against Si3N4was also higher than that against the composite. Relatively larger scale microfracture occurred in the Si3N4than in the composite; more pullout and microchipping of carbide particles were observed in the steel against Si3N4than against the composite. Polishing of the worn surfaces of the steel occurred in both sliding and rolling tests. This was attributed to fine, hard wear debris circulating in the contact area. Spalling was observed in the steel sliding against Si3N4but not in the steel sliding against the compos

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03695.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The large shear component of the tetragonal‐to‐monoclinic transformation in zirconia causes a stress‐induced preferred orientation of the tetragonal and monoclinic variants. The resultant texture, which is dependent on the loading condition, has been analyzed in terms of stress assistance to transformation and experimentally verified in simple tension and compression. Such a preferred orientation is a clear indication of the shear contribution to transformation plasticity and to fracture. Other implications of this analysis are also explored. For example, while the monoclinic texture is obviously most relevant to generating strains and plastic work, it is the tetragonal texture which might be advantageously tailored to enhance transformability and toughness. Transformation texture causes the ratio of {111} monoclinic and tetragonal X‐ray peaks in tensile fracture to be higher than the actual ratio of monoclinic‐to‐tetragonal phase fractions, making it generally unsuitable for estimating phase fraction in stress‐induced t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03696.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The grain growth of donor‐doped BaTiO3prepared from powders with different initial specific surface areas was investigated, and a microscale model was proposed that relates the critical amount of donor‐dopant, which inhibits anomalous grain growth, to the critical free surface area at the phase boundary. The results show that the model proposed can explain the inhibition of anomalous grain growth in donor‐doped BaTiO3and can predict well the ratio of critical concentration to surface area for the system st

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03697.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

There are few data regarding the temperature dependence of elastic and related properties of materials. This paper describes a semiquantitative basis for estimating such properties in oxide materials to temperatures even approaching their melting points. The various elastic constants are shown to correlate with the field strength of the cationic species, and onVo, the apparent atomic volume of oxygen. The value ofVois readily determined from crystallographic data. High‐temperature diamond pyramid hardness measurements are shown to correlate withVoand to provide additional information regarding the structural stability of materials at high temperature. The various refractory oxides are analyzed using these approaches, allowing identification of oxides that appear to be exceptionally strong, stiff, and creep resistant at high temperatur

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03698.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Two‐point bending is a useful method for investigating the mechanical properties of optical fiber and has several advantages when compared to the traditional tensile test. However, the strength of the fiber is usually determined at constant faceplate velocity rather than at constant strain rate as in the uniaxial tensile test, and previous work casts doubt on the comparability of fatigue results obtained using different loading modes and hence on the reliability of the bending technique. This paper presents dynamic fatigue results using a two‐point bend apparatus that can be programmed to operate in constant velocity, constant strain rate, and constant stress rate loading modes. These results, obtained for both bare and polymer‐coated fused silica optical fiber, show no significant differences in the calculated fatigue parameters for the three loading modes and clearly indicate the reliability of the two‐point bend method at constant faceplate velocity. The results, however, show that the obtained value of the stress corrosion susceptibility parameter,n, is dependent on the quantity used to define it, i.e., stress or strain to failure, because of the nonlinear elastic behavior of

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03699.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Characteristics of the sintered compacts and the microstructures and electrical properties of Nb‐doped TiO2varistors with added Ba and Bi were studied at various sintering temperatures ranging from 1250° to 1400°C. Adding both Ba and Bi to Nb‐doped TiO2ceramics resulted in a maximum intergranular phase and a minimum weight loss at 1350°C. In contrast, adding either Ba or Bi alone produced no such maxima and minima. The intergranular phases included mainly a Bi2Ti2O7crystal phase, apt to occur at a triple junction, and a Ba‐rich amorphous phase that surrounded the grains, but discontinuously. The intergranular phases varied consistently with variation in electrical properties. The optimum conditions for the most efficient boundary barrier layer, with the lowest weight loss and the highest resistivity at low frequencies, were 1350°C with both Ba and Bi addition. The highest values for α (∼9.5), Vgb10(∼0.8 V), and EB(∼0.42 e V) support that finding. The effective relative dielectric constant,Keff∼ 20 000, also was obtained under optimum conditions. The single addition of either Ba or Bi, however, produced nearly the opposite results, as discu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03700.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Transformation plasticity in Mg‐PSZ and Y‐TZP ceramics is investigated using a novel stress reversal Hopkinson bar technique recently developed at the University of California–San Diego (UCSD) for dynamic recovery experiments.1The longitudinal and transverse strains are measured by strain gauges mounted on the specimen. The specimens are loaded until transformation reaches saturation. Reloading of the same specimens to higher stress levels does not reveal additional inelasticity. Cuboid specimens which have been loaded initially to attain transformation saturation are then reloaded in a direction perpendicular to the first loading. The second loading produces additional inelasticity and microcracking, indicating the formation of transformation texture in each loading under uniaxial compression. Microscopic observations, and the results of ultrasonic and X‐ray diffraction measurements, as well as micromechanical modeling of the damage evolution are di

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03701.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The surface nature and the composition of AlN powder, as‐received and exposed to binder burnout, were studied using XPS and TEM. The surface of as‐received AlN powder was covered by a thin layer of aluminum oxynitride and oxide mixture. A small portion of residual carbon from binder burnout was bound to oxygen atoms on the AlN powder surface, and the majority of the carbon was amorphous graphitoid carbon which covered the AlN powder surface uniformly. AlN samples were made using tape casting and pressureless sintering. Surface‐carbon‐to‐oxygen ratio of AlN powder after binder burnout was evaluated using XPS. The surface C/O atomic ratios were observed to correlate with the sintering behavior, the composition of the second phase, the second phases distribution, and grain‐boundary composition, as well as thermal conductivity of

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03702.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

This paper describes a new low‐cost, melt infiltration method for manufacturing silicon carbide–silicon (SiC─Si) and graphite–silicon (G─Si) materials for radiant tube applications, and the physical and mechanical characteristics of these materials. Characterization of the microstructures and various physical and mechanical properties of three SiC─Si and one G─Si materials which were produced by this newly developed processing method are discussed in detail. The C‐ring and O‐ring specimens tested in compression showed that at all test temperatures (room to 1350°C), the SiC─Si materials are substantially stronger than the G─Si material, which actually contained 8.5 vol% SiC due to reaction of the graphite with the molten Si during processing. The strengths as determined by the C‐ring and O‐ring tests for each of the materials were similar, thus indicating that the inherent flaws on the inside and outside surfaces of the tubes are similar in severity. Since these materials showed significant deformation when tested at and above 1000°C, the strength of the G─Si material and one of the SiC─Si materials was also measured at room and at elevated temperatures, using a tensile test procedure. Results of the tensile tests illustrate that, due to stress redistribution at 1000°C and above, the C‐ring and O‐ring data overestimate the actual fracture strengths of these materials. The fracture toughness of the G─Si material measured at room temperature and 1000°C was much less than that of the SiC─Si materials. This behavior was explained by the presence of low‐strength graphite grains, which did not inhibit crack p

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03703.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY