摘要:

The first aerogels were made by S. S. Kistler in 1931. Aerogels are produced by a sol–gel process followed by supercritical drying of the wet gel in an autoclave. Their porosity extends from about 1 to 100 nm, which is the reason why properly made aerogels are highly transparent. Aerogels have a spongelike, open‐pore structure with a large inner surface. Monolithic aerogels are used in Cerenkov detectors in high‐energy physics; several houses have been insulated with layers of translucent granular aerogels for the purpose of passive solar energy usage; opacified aerogel powders are being tested as substitutes for chlorofluoro‐carbon‐blown polyurethane foams and as thermal superinsulations in heat‐storage systems; experiments have been performed with aerogels as catalytic substrates, acoustic impedance matching layers, precursors for high‐quality glasses, containment for fusion fuels, and gas filters; and aerogels are used in radioluminescent light and energy sources. The attraction that these low‐density materials exert on physicists, chemists, and material scientists is recognized from the upsurge of publications on aerogels: about 200 papers are currently published per year. Recently, the periodicalSciencehas rated aerogels as one of the top ten scientific and technologic

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04461.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The fabrication and intergranular‐phase devitrification of silicon nitride densified with rare‐earth (RE) oxide additives has been investigated. The additions of the oxides of Sm, Gd, Dy, Er, and Yb, having high melting points and behaving similarly to Y2O3, were compositionally controlled to tailor a microstructure with a crystalline secondary phase of RE2Si2O7. The lanthanide oxides were found to be as effective as Y2O3in densifying Si3N4, resulting in identical microstructures and densities of 98–99% of theoretical density. The crystallization behavior of all six disilicates was similar, characterized by a limited nucleation and rapid growth mechanism resulting in large single crystals. Complete crystallization of the intergranular phase was obtained with the exception of a thin residual amorphous film which was observed at interfaces and believed to be rich in impurities, the cause of incomplete devitrific

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04462.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The oxidation behavior and microstructure of the oxidized surfaces of RE2Si2O7–Si3N4ceramics were investigated. The high oxidation resistance of these materials at 1400°C is attributed to the minimization of amorphous phases via devitrification to disilicates that are in equilibrium with SiO2, the oxidation product of Si3N4. Crystals of RE2Si2O7grew out of the surface silicate in prefered orientations that were dictated by crystal structure. The morphology of the microstructure of the oxidized surfaces was shown to be partially dependent on the concentration of impurities; the presence of Ca was found to coincide with the growth of Gd2Si2O7and Dy2Si2O7crystals with high aspect rati

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04463.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The flexural strength and creep behavior of RE2Si2O7–Si3N4materials were examined. The retention in room‐temperature strengths displayed by these ceramics at 1300°C was 80–91%, with no evidence of inelastic deformation preceding failure. The steady‐state creep rates, at 1400°C in flexural mode, displayed by the most refractory materials are among the lowest reported for sintered Si3N4. The creep behavior was found to be strongly dependent on residual amorphous phase viscosity as well as on the oxidation behavior of these materials. All of the rare‐earth oxide sintered materials, with the exception of Sm2Si2O7–Si3N4, had lower creep strains than the Y2Si2O7

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04464.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The effect of processing and material parameters on the sintering and coarsening of model composites consisting of a fine‐grained ZnO matrix and coarse, rigid, inert particulate inclusions of ZrO2was investigated. The green composites were formed by two methods: (i) mixing the matrix and inclusion powders in a ball‐mill followed by die‐pressing and (ii) slip casting. For both forming methods, the inclusions caused a significant reduction in the density and the grain size of the composite matrix but had almost no effect on the grain size vs density relationship. The effects of inclusion volume fraction and sintering temperature were somewhat independent of the forming method. However, for the slip‐cast composites, the effect of inclusion size was less severe and the packing of the matrix phase immediately surrounding the inclusion showed some improvement. The sintering kinetics and microstructural observations indicated that two main factors controlled the sintering of these composites: (i) interactions between the randomly distributed inclusion particles that created a constraint on the matrix and (ii) the packing of the matrix, especially in regions immediately surrounding the inclusion pa

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04465.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Composite powders were synthesized by coating coarse ZrO2inclusion particles with a cladding of fine‐grained, crystalline ZnO powder using a chemical precipitation technique. Three different inclusion sizes (1, 3, and 14 μm) were used by selecting the size of the starting ZrO2powder, and the volume fraction of the inclusions was controlled by the amount of ZnO precipitated. The powders were compacted by uniaxial pressing in a die and then sintered at a constant heating rate of 4°C/min to 1500°C. The sintering kinetics were almost independent of the inclusion volume fraction, and of the inclusion size, for inclusion contents up to ∼40 vol%. Furthermore, composites containing up to ∼40 vol% inclusions were sintered to almost full density under the same conditions used for the unreinforced matrix. This is considerably better than the densities obtained for conventionally mixed powders, where a modest inclusion content (<∼ 10 vol%) has been observed to cause a severe reduction in the sintered density of the composite matrix. The kinetic data and microstructural observations are a further indication that the main factors which oppose the free sintering of ceramic particulate composites are processing‐related; these factors are (i) inclusion‐inclusion interactions which constrain the matrix and (ii) the packing of the matrix phase in regions immediately surrounding

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04466.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The effect of temperature and volume fraction on the growth of silver particles in a lead borosilicate glass has been studied. The particle growth obeyed the Ostwald ripening theory and was controlled by diffusion. An anomaly of growth rate coefficient was observed around 700°C and explained in terms of the dissolution behavior of silver. Also, the solubility of silver in glass was evaluated from the normalized rate coefficientKusing volume fraction of silver particles and viscosity of the glass

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04467.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

We investigated the characteristics of calcium phosphate cements (CPC) prepared by an exothermic acid–base reaction between NH4H2PO4‐based fertilizer (Poly‐N) and calcium aluminate compounds (CAC), such as 3CaO · Al2O3(C3A), CaO · Al2O3(CA), and CaO · 2Al2O3(CA2), in a series of integrated studies of reaction kinetics, interfacial reactions, in‐situ phase transformations, and microstructure development. Two groups were compared: untreated and hydrothermally treated CPC specimens. The extent of reactivity of CAC with Poly‐N at 25°C was in the following order: CA>C3A ≫ CA2. The formation of a NH4CaPO4·xH2O salt during this reaction was responsible for the development of strength in the CPC specimens. The in‐situ phase transformation of amorphous NH4CaPO4·xH2O into crystalline Ca5(PO4)3(OH) and the conversion of hydrous Al2O3gel →γ‐AIOOH occur in cement bodies during exposure in an autoclave to temperatures up to 300°C. This phase transformation significantly imp

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04468.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Lead zirconate titanate (PZT) with several Zr/Ti ratios was prepared by the organic solution of citrates of the respective cations. The Rietveld method was applied to perform quantitative analysis of the coexisting phases. The ratio of tetragonal/rhombohedral phases increased with calcination temperature. For the composition where Zr/Ti = 53/47, the tetragonal quantity varied from 25% to 35% for temperatures ranging from 600° to 800°C. For the powder with Zr/Ti = 57/43, only the rhombohedral phase was observed after calcination at 700°C. For the powder with Zr/Ti = 49/51, only the tetragonal phase was observed after calcination at 800°C. The amount of tetragonal phase was controlled in Zr/Ti = 55/45 by seeding the powder with the tetragonal phase of

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04469.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

It has been confirmed that the essential condition for glasses and glass‐ceramics to bond to living bone is the formation of an apatite layer on their surfaces in the body. It has been proposed that a hydrated silica formed on the surfaces of these materials in the body plays an important role in forming the surface apatite layer, but this has not yet been proved. In the present study, it is shown experimentally that a pure hydrated silica gel can induce apatite formation on its surface in a simulated body fluid when its starting pH is increased from 7.2 to 7.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04470.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY