摘要:

The presence of nontransformable tetragonal (t') phase in plasma‐sprayed ZrO2‐Y2O3coatings enhances the stability of the coating during high‐temperature exposure. In this paper, we describe the importance and procedure for correctly identifying t’phase in X‐ray diffraction patterns of the as‐sprayed and heat‐treated coatings. The results agree with the phase diagram. We also point out that in the heat‐treated materials, the durability should be specified in t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05061.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The undulating grain‐boundary structure formed by chemically induced grain‐boundary migration in lanthanum‐modified‐lead‐zirconium‐titanium ceramics causes a change in the fracture mode and an increase in room‐temperature fracture toughness. Specimens with normal and undulating grain‐boundary structure exhibit intergranular and transgranular fracture, respectively. The grain‐boundary undulation is suggested to be effective for improving high‐temperatu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05062.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The effect of various atmospheres on the long‐term operation of positive temperature coefficient resistors was studied. In most cases where degradation of properties occurred, the operable mechanism appeared to be volatilization of oxygen from grain boundaries due to reduction of the partial pressure of oxygen in the atmosphere around the sample. The effect of CCl2F2, however, was much more pronounced and was attributed to a chemical mechanis

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05063.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Bending strength and Young's modulus of aluminum titanate ceramics at room temperature to 1300°C were examined. Bending strength increased from 62 MPa at room temperature to 280 MPa at 1100°C. Young's modulus also increased, to 99 GPa at 1100°C. These increments were caused by crack healing. In particular, crack cylinderization occurring at 1000° to 1100°C markedly increased the mechanical strength. The thermal‐hysteresis curves also showed healing of grain‐boundar

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05064.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Bridging stresses at the crack interface behind the crack tip are calculated for nontransforming ceramics which show increasing toughness with crack extension (R‐curve behavior). With this approach, bridging stresses can be related analytically to the experimental R curve. Sample calculations for SiC‐whisker‐reinforced Al2O3, with a bridging‐zone length of>0.5 mm, are shown in the present study. The results indicate that the bridging stress has a maximum value within a short distance (<0.01 mm) behind the crack tip and then decreases to 0 or levels off to a finite value at the end of the bridging zone depending upon the nature of the R‐curve

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05065.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

A thin cylindrical powder compact (3‐mm diameter by 20‐mm) with a nominal composition of YBa2Cu3Oxwas sintered in a static temperature gradient of 25°C/mm in air for 12 h. The temperature at the hot end of the sample was maintained at 940°±5°C. Considerable mass transport and chemical demixing occurred along the length of the sample. The results are consistent with the formation of a liquid phase at the hot end of the sample, transport of the liquid down the temperature gradient, and the formation of different eutectic compositions along the length of the sample. The implications for processing high‐Tcsuperconductors in the system Y‐Ba‐Cu‐

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05066.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The procedures used to obtain diffusion coefficients from experimental creep data are analyzed in detail. Using experimental data for polycrystalline alumina, it is demonstrated that, in general, it is impossible to correctly determine the diffusion coefficients from experimental creep data.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05067.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Yttria‐stabilized zirconia crystals were produced by devitrification within a sodium borate glass containing zirconia and yttria. Yttria (3, 6, or 10 mol%) in zirconia was added as part of the starting material of the glass batch. Phase separation and crystallization occurred during the cooling and subsequent heat treatment of the glasses. The zirconia powders obtained after leaching away the sodium borate had surface areas between 16 and 48 m2/g. X‐ray diffraction traces revealed a mixture of monoclinic, tetragonal, and cubic phases, the phase ratios of which depended on the composition of the gl

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05068.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The ac electrical response of ZnO‐based varistors was monitored as a function of frequency (50 Hzf2 MHz) and electric field (60% ElmA/cm2EElmA/cm2, ElmA/cm2in the critical field at 1 mA/cm2) at room temperature, and these data were represented in the complex impedance (Z*) plane. The finite depression angle observed in this plane was rationalized in terms of nonuniformity in the measured reactance between the terminals. The singular nature of the series‐parallel combination of the uniform conducting paths between the electrodes was suggested when this depression angle diminished to 0° at high electric fields (EElmA/

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05069.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

A solution‐derived precursor may be calcined at temperatures>500°C to yield MgNb2O6. Calcining temperatures 800°C result in the familiar, cation‐ordered columbite structure type. Calcining temperatures<800°C result in a previously unobserved, cation‐disordered α‐PbO2structure type. The low‐temperature form of MgNb2O6is a more reactive material than is obtained by conventional solid‐state reaction as evidenced by its small particle size and its ability to react with PbO to form lead magnesium niobate at low

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05070.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY