摘要:

Ionic conductivity in phosphate glasses has been known for almost 30 years. Recently these glasses have been shown to exhibit some of the highest ionic conductivities ever reported. In many cases, because of their ease of preparation, low melting points, strong glass‐forming character, and simple composition, phosphate glasses have been studied more than any other ionically conducting glasses. However, no single review has ever been made of these glasses with the purpose of correlating the apparent widely disparate values of conductivity that these glasses exhibit. In this review, the conductivities of phosphate glasses are examined considering new structural studies of them. A systematic comparison of the dependence of the conductivity on glass chemistry reveals that, similar to other glass families (silicates and borates for example), the conductivity maximizes when the cation environments in the glass are minimized in their charge density and maximized in their site proximit

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07788.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The tetragonal tungsten bronze (TTB) type of SrxBa1−xmNb2O6(SBN) (0.5 ≤x≤ 0.75) was synthesized by a mixed‐oxide route. Using the two‐step densification process of pressureless sintering followed by oxygen HIPing, transparent ceramics were successfully fabricated. Microstructural, dielectric, optical, and electrooptic properties were investigated. All single‐phase TTB‐type SBN showed the characteristics of relaxor ferroelectrics. The extinction coefficients of the sample with the highest transparency were calculated from transmission spectra to be 30.4, 5.8, and 2.4 cm−1at 600, 1450, and 2000 nm, respectively. The linear electrooptic coefficients for SBN55,r33andr13, were 46 × 10−12and 21 × 10−12m/V at

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07789.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

During high‐temperature oxidation in air, SiC‐whisker‐reinforced Al2O3—ZrO2composites degrade by the formation of a whisker‐depleted mullite‐zirconia scale. The reaction kinetics have been studied as a function of time and temperature for composites with whiskers preoxidized for different times. The evolution of the microstructure has been investigated by optical, scanning and transmission electron microscopy. Possible reaction mechanisms have been discussed. A model compatible with our observations on Al2O3—ZrO2—SiC and the results reported in the literature for Al2O3—SiC whisker composites is proposed: The oxidation occurs at an internal reaction front. Oxygen diffuses along dislocations and grain boundaries through the mullite scale to react at this front with silicon carbide, thereby forming amorphous silica and graphite. Silica penetrates grain boundaries and further reacts with alumina and zirconia to form mullite and zircon, while the second reaction product, graphite, is oxidized into carbon monoxide when the reaction front moves deepe

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07790.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Composite bioactive glass‐ceramics reinforced with 30 vol% Ti particles were produced by warm‐forging and in situ crystallization. The bend strength of the composite was 87 ± 7 MPa. A fracture mechanical study in air and Ringer's bioactive solution determined crack velocity vs stress intensity factor and the effect of stressing rate on the bend strength. Time to failure of 10 years is predicted for the composite loaded at 10 MPa in Ringer's solution or 50 MPa in air. A disappointing fatigue performance of the relatively tough metal/ceramic composite illustrates that satisfactory short‐time fracture resistance does not guarantee lifetime under fatigue conditions (long‐term fracture res

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07791.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Aluminum titanate powder has been prepared through thermal decomposition of a transparent gel formed from a mixture of titanium butoxide and boehmite sol in acetic acid—butanol medium. The thermogravimetric curve of aluminum titanate precursor gel is characterized by a decomposition temperature extending up to 800°C while the constituent gels of boehmite and hydrous titania independently decompose at lower temperature. The removal of water entrapped in the gel structure heated at the above temperature is further made clear by infrared spectral data. The gel stays in the amorphous state up to 800°C, as revealed from XRD. The sintered aluminum titanate shows that grains with sizes above 2 μm are cracked while smaller ones are intact. When heated for a period of 2 h, the gel decomposes to powders with average particle sizes of 2.7 μm at 900°C and 6.5 μm at

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07792.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Polycrystalline, tetragonal (t′) zirconia samples containing 3 and 4 mol% yttria were fabricated by annealing pressureless‐sintered samples in air at ∼ 2100°C for 15 min. The grain size of these fully tetragonal samples was on the order of 100 to 200 μm. Domain structure of the samples and of a 3‐mol%‐yttria‐doped tetragonal zirconia single crystal was examined by transmission optical microscopy under polarized light and by transmission electron microscopy. The orientations of the domain/colony boundaries were in accord with the predictions of group theory. As‐polished surfaces of polycrystallinet′ materials showed no monoclinic phase even after 1000 h at 275°C in air. By contrast, conventionally yttria‐doped tetragonal zirconia polycrystalline (Y‐TZP) ceramics of grain size>0.5 μm showed substantial transformation. Surface grinding enhanced the resistance to degradation of Y‐TZP but decreased that oft′ materials. Even then, thet′ materials exhibited better resistance to degradation than the Y‐TZP ceramics. Excellent resistance of thet′ materials to low‐temperature aging despite a very large grain size and the opposite effect of grinding on phase stability are all explained on the basis of ferroelastic

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07793.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AIN coatings were prepared by chemical vapor deposition from the AlCl3—NH3—Ar reagent system using an impinging jet reactor in the temperature range of 700° to 1100°C. A mass transfer model and thermodynamic calculations were used to analyze the deposition data. The AIN‐CVD process could be approximated by calculating mass transfer—thermodynamic limits at low AlCl3concentrations. The AIN deposition rate decreased drastically with increasing temperature above 1000°C in agreement with thermodynamic predictions. At high AlCl3concentrations, a surface kinetic mechanism involving AlCl3adsorbed on the deposition surface appeared to be the rate‐limiting step. The AIN deposition rate decreased on increasing the AlCl3concentration or total pressure. The crystalline structure of AIN was strongly influenced by the processing parameters. The AIN coatings became highly crystalline and preferentially oriented with an increase in the AlCl3concentration

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07794.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

A simple approach is presented for the determination of crack‐tip‐bridging stresses in ceramic materials. The technique utilizes the difference in compliance predicted for a crack of known size and that measured. The bridging stresses for an alumina material that displays considerableR‐curve behavior are calculated from measured compliance and crack‐length observations. The results are almost identical with those recently reported in the literature. Additional measurements on a more complex “duplex” ceramic that exhibits a very substantialR‐curve due to a crack‐branching chain reaction are

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07795.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

A thermodynamic modeling of phase diagrams in binary alkali silicate systems is described by using the Gibbs free energy of the phases. Simple models employing regular, quasi‐regular, and subregular solution models were applied to describe the liquid phase. The interaction parameters of the liquid phases were obtained by using the liquidus curves of silica (and subliquidus data) through a multiple linear regression method. The present calculated liquidus curves agree very well with Cs2O—SiO2and Rb2O—SiO2systems. In the K2O—, Na2O—, and Li2O—SiO2systems, the calculations permitted discrimination between sets of data and, based upon the calculation choices, could be correctly made to select the most appropriate phase diagram. Also, the spinodals were calculated where phase separation can occur by a spinodal decomposit

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07796.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Phase relations in the BaTiO3—TiO2system were studied at temperatures above 1300°C in air. Quenching experiments were performed with high‐purity reagents, and a new equilibrium phase diagram was constructed. Results include redetermination of the liquidus boundaries, the eutectic temperature, the melting or decomposition temperatures of the stable compounds in the system, the cubic—hexagonal transition in BaTiO3, and the solid solubility of TiO2in

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07797.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY