摘要:

Microstructure and bending strength of commercial (MgO,CaO)‐partially‐stabilized zirconia subjected to heat treatments in the temperature range 900° to 1550°C were characterized. The as‐received optimally aged material contained small ellipsoid‐shaped tetragonal (t) precipitates dispersed homogeneously in the cubic (c) grains. Annealing at lower temperatures caused isothermal martensitic transformation to the monoclinic (m) phase. Annealing at higher temperatures caused formation of chevronlike and irregularshaped cluster precipitates, due to stress‐induced coarsening and precipitate impingement. The loss of bending strength was related to the stored strain energy contributed by each type of precipitate morphology, in addition to decrease in the volume fraction o

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07860.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The experiments reported in this paper demonstrate the causes of the failure of monolithic ceramics. The specimens are made of silicon nitride and tested at room temperature. The stress field within the specimen is different for each of four series of tests that have been conducted. Fractographic observations have also been made to identify the causes of the failures. A size effect analysis is performed.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07861.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The mechanism of oxidation resistance in SiC‐coated carbon‐carbon composites containing boron‐based inhibitors has been investigated using thermodynamic calculations, thermogravimetric analysis, and electron microscopy. A model is developed based on the formation of a volatile B2O2sub‐oxide in the interior of the composite which condenses to B2O3upon encountering a locally high oxygen partial pressure in coating cracks. The active‐to‐passive transition for the oxidation of elemental boron has been

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07862.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

This paper describes a process by which highly reactive yttria powder can be sintered to high density at a relatively low firing temperature to form a variety of useful ceramic items. The process utilizes an aqueous polymer to precipitate a yttrium precursor. The yttrium precursor is thermally treated to form a friable oxide ash. After a double‐calcination‐grind procedure, high‐density yttria items such as substrates, crucibles, and nozzles can be fabri

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07863.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The lattice and dislocation diffusion coefficients for Ca2+in single‐crystal MgO have been determined in the temperature range 1743 to 1293 K from the diffusion penetration determined using secondary ion mass spectrometry. The dislocation diffusion coefficients were 4 to 5 orders of magnitude larger than the lattice diffusion coefficients in the same temperature range. The activation energy of Ca2+lattice diffusion was comparable to that of ionic conductivity. The diffusion mechanism of Ca2+is related to Mg vacancie

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07864.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Compared to conventional sintering of Y2O3‐doped ZrO2using a slow heating rate of 10°C/min, microwave sintering and fast firing using a rapid heating rate of about 500°C/min resulted in lower final sintered densities. It is attributed to the residual chlorine in commerical zirconia powders manufactured by the chloride process. By calcining at 1100°C for 1 h to remove residual chlorines from the powder compacts, near full densities (>99% of theoretical) could be obtained by both fast firing and microwave sinte

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07865.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Stress rupture testing was performed in four‐point flexure at 1000°C to determine the effects of Na2SO4‐induced corrosion on the static‐fatigue life of a Ce‐TZP and a MgO‐doped Si3N4. The results showed that the static‐fatigue life of the Ce‐TZP was unaffected by this corrosive environment. However, the static‐fatigue life of a MgO‐doped Si3N4was reduced by the intr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07866.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Experimental evidence shows that the acceptor‐state levels in Sb‐doped positive‐temperature‐coefficient‐type BaTiO3are compensated up to a critical acceptor‐state density. Using the slope of the natural logarithm of the resistivity with respect to 1/T, instead of maximum resistivity as a measure for the acceptor‐state density, it is possible to estimate this critical value. The value obtained (4.2 × 1017m−2) is believed to be the first reported estimate based on experimental data. It is in good agreement with the estimate of 6 × 1017m−2(first reported by Jonker) obtained from the spontaneous polarization of BaTiO3. This shows that the ferroelectric behavior of BaTiO3is indeed a feasible explanation for the low resistivity below the Curie point, as

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07867.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The effects of joining pressure and deformation on the strength and microstructure of diffusion‐bonded SiC were examined using a hot‐press to apply varying amounts of interfacial pressure and a close‐fitting die to restrict deformation. SiC substrates were successfully diffusion bonded at 1950° and 2100°C. Joints which had uniform grain structure across the joint interface and bend strengths up to 300 MPa (44 ksi) were achieved. Pressing pressure was found to be a requirement for producing reasonable joint strength. It was also found that macroscopic deformation (>4% joint area expansion) is not necessary for effective diffusion bonding. Various methods for joining SiC are reviewed with regard to ease of processing, use limitations, and joint s

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07868.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

To investigate the effect of reoxidation on the grain‐boundary acceptor‐state density of reduced barium titanate,n‐doped BaTiO3ceramics are sintered in a reducing atmosphere (2% H2+ 98% N2) and then annealed in oxygen. After annealing at 1150°C for different times, the experimental results show a relationship between temperature‐averaged acceptor‐state density and annealing time asNs=NsoBt1/nwithnbetween 2 and 3. An inherent acceptorstate densityNso= 4.25 × 1012cm−2is obtained with an increase rateB= 4.8 × 1012cm−2. min−1/3, whennreaches 3. The inherent grain‐boundary acceptor states in the reducedn‐doped BaTiO3ceramics are believed not due t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07869.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY