摘要:

The uniaxial strength of engineering ceramics is often measured by the well‐known flexure strength test method there is a risk that flexure data are not representative of the properties of fabricated components. Reliability estimates for components based upon statistical extrapolation techniques from flexure data may not be valid. This paper reviews the problem and judges the usefulness of flexure data for design purposes. It is shown that some of the limitations of flexure data apply; to other modes of testing, including direct tension testi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08259.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The grain boundaries in zinc oxide ceramics exhibit different electrical behaviors. This results in separate paths of current flux through the microstructure and also in different breakdown voltages of each path. The paper describes some new methods for characterization of these paths. A galvanic determination is able to show the number of paths and their distribution across the varistor surface. The differences in breakdown voltage are visible using a line scan method. Current images in SEM can detect the paths of current along a varistor surface. Possible reasons for inhomogeneous current flux are inhomogeneous distribution of dopants, insufficient binder burnout, and pressing faults.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08260.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The Phase relations of the system Gd2O3‐Ta2O5in the composition range 50 to100 mol% Gd2O3 was studied by solidstate reactions at 1350°, 1500°, or 1700°C and by thermal analyses up to the melting temperatures. Weberite‐type orthorhombic phase (W2 phase, space group C2221) with the composition of Gd3 TaO7 seems to melt incongruently; at about 2040°C, although this Gd3TaO7Phase was previously reported to melt congruently. A new fluorite‐type cubic phase (F phase, space groupFm3m) was found for the first time above 1500°C in the system. It melts congruently with the composition of about 80mol% Gd2O3at 2318° 3°C. A phase diagram was proposed for the system Gd2O3–Ta2O5in the Gd2

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08261.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The phase relations for the system y2o3–Ta2o5in the composition range 50 to 100 mol% Y2O3have been studied by solid‐state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite‐type orthorhombic phases (W2 phase, space group C2221), fluorite‐type cubic phases (F phase, space groupFm3m)and another orthorhombic phase (O phase, space groupCmmm)are found in the system. The W2 phase forms in 75 mol% Y2O3under 17000C and O phase in 70 mol% Y2O3up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y2O3and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y2O3, and at about 1990°C, 62 mol% Y2O3. A Phase diagram including the above three phases were not identified w

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08262.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The concentration of foreign element (Al, Fe, Mg, and Ti) in alite in portland cement clinker decreases with increasing temparature of crystallization. The constituent phase of alite at ambient temperature changes systematically fromM1toM3and then toT2with decreasing concentration of Al and Fe in solid solution. The distribution of those amphoteric elements between the Ca‐ and Si‐sites in the structure of alite, depending on the concentration of CaO or SiO2in the environmental liquid phase from which alite is precipitated, can be represented by the superposition of (Ca2Me)MeO5and (Me2□)SiO5(Me is Al, Fe and is a vacant Ca

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08263.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Oxygen tracer diffusion coefficients are determined in single‐crystal alumina samples; with differing dopant levels using the gas‐exchange technique. The diffusion direction is parallel to thec‐axis and the ambientPo2is 1 atm (105Pa) for all experiments except a single run with a lowPo2The diffusion is insensitive to both; impurities and ambientPo2The insensitivities are discussed in terms of point‐defect clustering. Prior tracer studies are compared and di

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08264.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The kinetics of formation of a new class of ceramic composite material, zirconium diboride platelet‐reinforced zirconium carbides, are discussed. These materials are prepared by the directed reaction of molten zirconium with boron carbide to form a ceramic material composed of zirconium carbide matrix containing a controlled amount of residual zirconium metal. Results from interrupted growth studies, differential thermal analysis, adiabatic reaction temperature calculations, and kinetic measurements have been used to study the kinetics of the process. The reaction is very fast and proceeds parabolic ally with time with a rate constant between 1.6 X 10‐2and 3.9 X 10‐2cm2/s. The proposed mechanism suggests that when molten zirconium contacts boron carbide, the molten zirconium exothermically reacts with the boron carbide to form a boron‐rich liquid. Further reaction is sustained by the continuous dissolution of the boron carbide as the boron‐rich liquid is drawn into the boron carbide. The product is a zirconium carbiride/zirconium carbide/zirconium composite which homogenizes quickly at the reaction temperature to yield a uniform product microstructure throughout the composite. Two alternative rate‐limiting steps are discussed and the implications of each a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08265.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Superconducting Bi(Pb)‐Sr‐Ca‐Cu‐O powders were prepared by the emulsion‐drying method The powders consisting of the low‐Tcphase (Bi(Pb)2Sr2Ca1Cu2Oy) and small amount of Ca2PbO4were prepared by calcining under low oxygen partial pressure at 750°, 800°C. These calcined powders were used to prepare thick films on single‐crystal MgO(100) substrates using a screen‐printing technique to study the effect of the calcinations temperature and annealing procedure on the high‐Tcphase (Bi(Pb)2Sr2Ca2Cu3Oy)formation. The formation of high‐Tcphase was observed to be dependent on the calcinations conditions. The high‐Tcphase formed very quickly (sintering at 840°C for 4 h), and the best films showed a sharp superconducting t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08266.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

A thermodynamic evaluation of the ZrO2‐MgO system has been developed and combined with previous assessments of the ZrO2–YO1.5and YO1.5–MgO systems to describe the ZrO2–YO1.5‐MgO Systems to describe the ZrO2–YO1.5‐MgO system by means of Bonnier's equation. The calculated results are shown by isothermal and vertical sections, a projection of the liquidus surfaces, and the reaction scheme. Comparisons between calculated and experimental diagrams demonstrate that the calculations satisfactorily account for most of the available expe

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08267.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Various experimental evidence indicates that the porosity of a compact has a pronounced effect on the steady‐state velocity in combustion synthesis of condensed phases, hut no simple and explicit treatment for it is available in the literature. Utilizing an existing model of the effective thermal conductivity in a porous media, a simple model for the steady‐state velocity of the combustion front has been developed. As ob‐ served in experiments, the model also predicts a decrease in combustion velocity with increasing porosity. The developed equation can be helpful in the analysis and interpretation of experimental results in self‐propagating high‐temperature synthesis of conden

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb08268.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY