摘要:

A variety of spray pyrolysis (SP) techniques have been developed to directly produce ceramic powders from solutions. This paper reviews the current status of these processes in terms of the process parameters that enable the formation of particles with controlled morphology and composition. A model incorporating solute diffusion in the droplet and solvent evaporation from the droplet surface is presented to establish the critical parameters leading to solid particle formation. The model illustrates that solid particles can be obtained if solutes with high solubility and a large difference between the critical supersaturation and equilibrium concentration are used and if the process is designed to avoid solvent boiling. It is demonstrated that mixed metal oxide, non‐oxide, and composite particles that are solid, hollow, porous, or fibrous can be produced by modifying the precursor characteristics, solution properties, and process parameters. The physical and chemical flexibility of SP processes offers numerous opportunities for the controlled synthesis of advanced ceramic powders and films. However, production rates are limited by the need to produce<5‐μm‐diameter droplets and to avoid subsequent droplet coagulation. Developments in process controls, atomization, and system design are required for wider commercialization of SP‐type p

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04007.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Irradiation of Ti[N(CH3)2]4by the 1.064‐μm line of a pulsed Nd: YAG laser in the presence of TiO2, Al2O3, or Si3N4particles has been found to form amorphous deposits on the oxide particles. The resulting materials can be processed into TiN/TiO2, TiN/Al2O3, or TiN/Si3N4composites with the TiN component on the surface of the particles. The powders have been characterized by Raman spectroscopy and X‐ray powder diffraction studies. The surface analysis of the composites by X‐ray photoelectron spectroscopy and high‐resolution electron microscopy is p

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04008.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A coprecipitation method is described for preparing ferroelectric ceramic powders. The process is based on source solution preparation for coprecipitation after previous calcination of the constituent oxides or carbonates. Rhombohedral PZT 53/47 could be obtained by this method after calcination at 850°C. In addition, PLZT 9/65/35 was prepared by the same method

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04009.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Problems associated with manufacturing Si3N4/SiC‐whisker composites have been overcome by developing selfreinforced Si3N4with elongated β‐Si3N4grains formedin situfrom oxynitride glass. This Si3N4–Y2O3–MgO–SiO2–CaO‐based material has a flexure strength>1000 MPa and fracture toughness>8 MPa·m½. The optimum combination of mechanical properties has been obtained with Y2O3:MgO ratios ranging from 3:1 to 1:2, CaO contents ranging from 0.1 to 0.5 wt%, and Si3N4contents betwe

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04010.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

This paper compares the applicability of a few theoretical models for determining effective elastic moduli, using published experimental data on ceramic materials in a porosity range of 0–40% and on a cellular material with a porosity of about 90%. As the experimental data for the effective Poisson's ratio involve a large scatter, a set of numerical experiments using the finite element method was carried out to obtain the variation of the effective Poisson's ratio with porosity. These variations show that the effective Poisson's ratio approaches 0.25 with increasing porosity, irrespective of the material Poisson's ratio. The effect of pore shapes on the effective elastic moduli and the Poisson's ratio has also been analyzed using FE

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04011.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Wide composition range studies of the IR spectra ofxNa2S + (1 –x)B2S3glasses are reported for the first time. Glasses can be prepared in two composition regions: 0x0.33 in a low‐alkali region and 0.55x0.80 in a highalkali region. The structures of glasses in the former region are dominated by the creation of tetrahedral boron units similar to those observed in the alkali borate glasses. For pure B2S3, a large fraction of six‐membered rings is found, as in glassy B2O3. As alkali is added, the vibrational frequency of the six‐membered ring mode decreases, but not the intensity, and a new band grows in which is used to propose, by analogy with the alkali borate glasses, that the fraction of tetrahedral borons increases with alkali sulfide content. These observations suggest that isolated six‐membered rings persist even in the presence of tetrahedral borons. For the high‐alkali glasses, both the six‐membered rings and tetrahedral boron structures are destroyed in preference to the formation of isolated orthothioborate groups, Na3BS3. The IR spectra of the high‐alkali glasses show a monotonic increase in symmetry and simplicity, indicating an increase in structural simplicity as the orthothioborate composition is approached. For the orthothioborate composition, both glass and polycrystal can be prepared. The IR spectra of the two phases are very similar, with the glass exhibiting a broadened abso

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04012.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Topological features were calculated for three‐dimensional microstructures simulated using a computer model. A method in which the material is divided into small volume elements (SVE method) was developed to measure topological features such as numbers of coordinating faces, edges, and corners on the computer‐generated microstructures. New relationships between different types of topological features of special interest in ceramics were derived based on the computer simulation statistics. These relationships enabled us to calculate the topological properties of a two‐phase system in which one phase has no internal boundaries, such as the pore, liquid, or glass phases in polyerystalline ceramics. Computer simulations allowed averaging those features statistically from large sample sizes, which can be difficult to do in practice. Dependence of topological features, such as the number of coordinating pores around a grain, on particle size distribution and sintered density of the system was studied. Results show that the number of coordinating faces is proportional to the square of particle size, and the number of edges per face is smaller for a wider particle size distribution. Equations are derived to describe the dependence of the number of coordinating faces and number of edges per face on grain size. The simulation shows that closed pores form at a lower sintered density for a wider particle size distrib

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04013.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Films composed of ceramic particles were observed during drying. The films were prepared from 20 vol% aqueous dispersions of α‐alumina and α‐quartz and were free of any organic binder. Conditions for uniform film saturation during drying were established by consideration of a liquid transport model and by direct observation of the drying films. Drying stresses were measuredin situby a substrate deflection method based on an optical interference technique. Simultaneous stress and weight measurements were used to determine stress as a function of saturation. The maximum stress occurred near 100% saturation and was approximately 2 and 1.1 MPa for films produced from 0.35‐and 0.68‐μm particles, respectively. The maximum stress decreased from 2 to 0.9 MPa for films produced from the 0.35‐μm particles when 0.005 wt% surfactant was added to the slurry. The surfactant decreased the liquid surface tension from 72 to 32 dyn/cm. These observations are direct evidence of the effects of capillary tension on the state of stress in a ceramic body. Mechanical properties of the green ceramic films were estimated by use of a linear elastic fracture model. Knowledge of the critical cracking thickness and maximum stress in the film was used to estimate the fracture resistance of the granular film. The fracture resistance values are approximately 0.02 and 0.007 MPa·m1/2for films produced from alumina and silica, respectively. The difference in mechanical behavior of the silica and alumina films is similar to that expected by the difference in Hamaker constants between the

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04014.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Isothermal growth of β‐Si3N4crystals dispersed in an oxynitride glass (Y‐Si‐Al‐O‐N) was studied by electron microscopy after heat treatment at temperatures between 1550° and 1640°C for 1 to 18 h. The β‐crystals exhibited growth striations introduced by intermediate coolings and these striations were used for developing a sophisticated technique for analysis of growth. It was determined that α/β‐transformation and Ostwald ripening can be treated as different kinetic stages of grain growth, while β‐nucleation was found to be insignificant. The mean diameter of the needlelike β‐grains was almost constant during phase transformation, indicating negligible growth of the β‐prism plane; growth was mainly one‐dimensional with the maximum mean length and aspect ratio just at the end of the phase transformation. The growth rate of the β‐basal plane was independent of diameter, indicating interface‐controlled growth. During Ostwald ripening, the length distribution broadened and the aspect ratio of smaller grains decreased. Dissolution of small grains caused an increase in the mean diameter, wh

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04015.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The glass‐ceramic matrix of the well‐known lithium aluminosilicate (LAS)/SiC composite is usually formulated near the spodumene composition. We report here a new composition which is rich in alumina (78 wt%) and lean in silica (21 wt%) and lithia (1 wt%). This formulation offers a new option of converting the glass‐ceramic matrix to a mullite/alumina matrix upon annealing above 1400°C, and hence better creep resistance and other high‐temperature mechanical properties. Using a transient‐phase processing method that we developed previously for the superplastic forming of mullite, we are able to hot‐press a composite containing 30 vol% SiC whiskers at ∼1350°C to achieve full density. Flexural strength measurements up to 1400°C have confirmed the improved high‐temperature strength and creep resistance over conventional LAS. The fracture toughness is also higher than that of LAS. The results suggest that the new composition may be chosen as a better candidate matrix for SiC‐fiber

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb04016.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY