摘要:

Most of the known cuprate superconductors are shown to belong structurally to a single family and are closely related to each other. The relationships between the structures of cuprate superconductors are reviewed with emphasis on their similarities. Individual details of the structures are ignored and only idealized structures are discussed. A formula is used to describe the basic structures of the cuprate superconductors. Crystal structure analysis leads to three significant insights into the nature of, and requirements for, superconductivity in these phases: (1) A nearly planar sheet of CuO2supports the supercurrent. (2) As the distance between thenACuO2blocks in a crystal structure increases, the critical current density decreases at high reduced temperatures due to the magnetic flux lines becoming unpinned. (3) The strain energy generated by the lattice mismatch between the CuO2layers and the nonsupercon‐ ducting layers controls the oxidation state of the Cu ions, which needs to be about +2.2 (inp‐type superconductors) to get Cooper pair condensat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07953.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Segregation of isovalent solute cations to (001) and (001) free surfaces in cubic metal oxides is investigated using atomistic computer simulations. Solute concentrations are represented by a mean‐field approximation, and equilibrium distributions of solute are calculated by minimizing the free energy. Surface energy effects are found to dominate segregation behavior, even when in competition with misfit strain energy effects. Results are compared with a conventional Langmuir‐McLean (LM) analysis. The two approaches are found to agree well in certain cases, but the LM treatment fails to reproduce important phenomena revealed using the free energy method (i.e., segregation to subsurface atomic laye

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07954.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Experiments are conducted to study the dislocation nucleation conditions at the crack tip in {110}〈110〉 oriented Si single crystals. Specimens with surface cracks are first statically loaded at elevated temperatures for a prolonged period of time to initiate and move dislocations away from the crack tip, then cooled down to room temperature and loaded to fracture to measure the fracture toughness. Fractographic analysis of the fracture surfaces is performed. Distinct wavy patterns on the fracture surface at the initial cleavage crack front are observed, which is attributed to the existence of local mixed mode I/mode III stresses resulting from the inhomogeneous dislocation activity. Confocal microscopy is employed to quantify the fracture surface roughness. The results show that the increase of fracture toughness is directly associated with the increased area of the rough surface, which is characterized by the roughness number or the fractal dimension increment. Our results also demonstrate that dislocation nucleation can occur only at discrete sites. The spacing between these dislocation nucleation sources is of the order of 1 μm. A simple model is developed for the relationship between the fracture toughness and the surface roughness parameters, which is in good agreement with the experimental res

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07955.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Mullite/SiC continuous fiber composites were fabricated by hot‐pressing under different processing conditions. The interfacial shear strength was measured during the pull‐out test, and the effect of fabrication conditions on interfacial debonding behavior was discussed. The debonding length during the pull‐out test was quantitatively evaluated using acoustic emission. The interfacial shear strength was evaluated by stress analysis. The control of interfacial shear strength was achieved by controlling the hot‐press temperature. An increase of load was found during the pull‐out process after complete debonding. In order to explain the increased load, a new model is

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07956.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The reaction sintering of equimolar mixtures of ZnO and A12O3powders was investigated as a function of primary processing parameters such as the temperature, heating rate, green density, and particle size. The powder mixtures were prepared by two different methods. In one method, the ZnO and A12O3powders were ball‐milled. In the other method, the ZnO powder was chemically precipitated onto the A12O3particles dispersed in a solution of zinc chloride. The sintering characteristics of the compacted powders prepared by each method were compared with those for a prereacted, single‐phase powder of zinc aluminate, ZnAl2O4. The chemical reaction between ZnO and A12O3occurred prior to densification of the powder compact and was accompanied by fairly large expansion. The mixing procedure had a significant effect on the densification rate during reaction sintering. The densification rate of the compact formed from the ball‐milled powder was strongly inhibited compared to that for the single‐phase ZnAl2O4powder. However, the densification rate of the compact formed from the chemically precipitated mixture was almost identical to that for the ZnAl2O4powder. The difference in sintering between the ball‐milled mixture and the chemically precipitated mixture is interpreted in terms of differences in the microstructural uniformity of the initial powder compacts resulting from the different preparation p

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07957.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Three dispersants (stearic acid, oleic acid, and poly(12‐hydroxystearic acid)) are compared for their ability to produce low‐viscosity suspensions of zirconia in kerosene. Rheological measurements and sediment packing density measurements show that poly(12‐hydroxystearic acid) is a better dispersant than stearic acid or oleic acid; this is explained in terms of the longer tail of the poly(12‐hydroxystearic acid) surfactant molecule. The amount of dispersant can be optimized to reduce viscosity and yield point of the suspension, and to eliminate thixotropic hysteresis. The use of a dispersion medium of lower viscosity than the dispersant makes it easy to detect when complete monolayer coverage has been achieved. The loss of pseudoplasticity, brought about by a higher degree of deflocculation, can be recovered by increasing the volume fraction of solids of a suspension and this is beneficial in the plastic forming of ceramics. Rheological measurements showed that these suspensions reach a “critical state” above a critical shear stress (τc). This critical state is described by several parameters, i.e., the Bingham yield stress (τb), the plastic viscosity (νPl), and the critical shear rate (γc), which are dependent on the volume fra

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07958.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Electrically induced strain studies of hardened cement pastes were performed using an inductance technique. Investigations were done as a function of electrical history, measurement frequency, moisture content, and initial water to cement ratio (w/c) during hydration. Expansive strains as large as 4.5 × 10−4were observed under field strengths of ∼0.5 kV/cm in specimens which had been saturated with water. Both switchable and nonswitchable strains were observed under cycling with an ac electrical field. Evidence was observed that the switchable strains are due to redistribution of water within the specimen, whereas the nonswitchable strains are related to long‐range water transport toward the surface of the specimen. Investigations as a function of measurement frequency revealed a strong relaxation of the induced strain in the frequency range of 6.67 × 10−3to 1 Hz. It is believed that ions in the diffuse double layer move under an electrical field and that water is transported by electro‐osmosis. Electrically induced strains then develop because of local swelling effects associated with a saturation gradient. Investigations as a function ofw/cratio revealed that the induced strains are related to the volume fraction of gel porosity. Electrically induced strain measurements of porous silica gels with average pore sizes of 25 and 50 Å were then performed. The characteristics of the electrically induced strains were found to be nearly identical to those for portland cement. It is believed that the characteristics of the electrically induced strains are controlled by the entry and removal of physically absorbed water within the gel por

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07959.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Three different CVD SiC fibers were tested for bend stress relaxation (BSR) and tensile creep over a wide range of temperatures, times, and stresses. Primary creep was always observed, even for creep strains on the order of 2%. The BSR and tensile creep results were compared using simple linear viscoelastic principles. It was found that BSR results could predict the same time and temperature dependence as tensile creep; however, BSR‐predicted creep strains usually overestimated the magnitude of tensile creep strain. The time, temperature, and stress dependence were determined for all the fibers for the experimental conditions of this study. Some of the primary creep behavior can be explained by load‐sharing effects between the core and the CVD SiC substrate and some microstructural changes; however, the extent of primary creep cannot fully be accounted for from this w

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07960.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

At 1 atm of SiF4, mullite and SiF4react below 660° 7°C to form A1F3and SiO2. From 660° to 1056°± 5°C, the product is fluorotopaz. Mullite is stable in the presence of 1 atm of SiF4above 1056°C. The transition temperatures at other pressures of SiF4can be calculated from logp(atm) = 11.587 – 10811/T(K) and logp(atm) = 9.9609 – 13238/T(K). The phase diagram shows only gas–solid equilibria, but there is evidence for a metastable melt from which acicular mullite and fluo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07961.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Cell structures developed by convection in glass thick films have been studied with special emphasis on the effects of material parameters and heating conditions on the cell structure. Borosilicate glass films from 10 to 30 μm thick were prepared on alumina substrates by printing and firing at temperatures between 750° and 950°C up to 2 h, and surface flow patterns were observed with an optical microscope under dark‐field illumination. In most cases a flow pattern developed and changed with time, finally reaching a steady state. Cell structures with regular polygons of four to seven sides from 15 to 150 μm across developed, depending on the glass composition. The effects of heating time, temperature, film thickness, and glass chemistry on the cell structure have been examined. From observations of cell structures, the driving force for the convection in glass thick films has been identified to be the gradient in surface tension due to small temperature fluctuations on the surface. Hence, it is concluded that the convection in glass thick films is of Marangoni

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb07962.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY