摘要:

The mechanisms of electrophoretic deposition (EPD) are discussed and their shortcomings identified. The kinetics of the processes involved are analyzed for constant‐current and constant‐voltage conditions. A method of determining the Hamaker constant of suspended particles is developed by modeling the relationship between the particle inter‐action energy and the suspension stability. A three‐probe dc technique is used to map the voltage profile around the depositing electrode, and the results are used to explain discrepancies between the calculated and experimentally observed voltage drops during deposition. A mechanism of deposition is proposed based on DLVO theory and particle double‐layer distortion/thinning on application of a dc field to the suspension. Kinetic equations are developed for constant‐current and constant‐voltage EPD using mass balance conditions; these are verified by experiments. After the phenomenon is introduced and discussed, a critique of the application of EPD to the synthesis of ceramic shapes and coat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08929.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

High‐purity alumina powder with submicrometer particle size was colloidally dispersed and classified in either water or ethanol to remove agglomerates from as‐received powder. After the slurries were dried, fine alumina particles cohered to form agglomerates, which were “hard” when formed in a water slurry, and “soft” in an ethanol slurry. Firing of the powder compacts with “hard” agglomerates resulted in inhomogeneous microstructures, and homogeneous microstructures were formed with “soft” agglomerates. The reasons for their formation are discussed and experimentally confirmed. In the case of water slurry, a small amount of transition alumina reacted with water at low pH. After drying, alumina gels were formed and acted as a strong binder between α‐alumina particles to fo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08930.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Preliminary leaching of rice husks with a solution of hydrochloric acid before their combustion at 600°C is shown to be required to obtain relatively pure silica (∼99.5%) with a high specific surface area (∼260 m2/g) that is maintained even after heating at 800°C. Transmission electron microscopy observations indicate that this material has a homogeneous size distribution of nanometric particles. However, if the leaching with HCl is performed on the white ashes obtained by combustion of the rice husks at 600°C, an amorphous silica with the same purity also is obtained, but its specific surface area decreases to 1 m2/g. This behavior is due to a strong interaction between the silica and the potassium contained in the rice husks, which leads to a dramatic decrease of the specific surface area if K+cations are not removed prior to the heat treatment at 600°C. This finding leads to a better understanding of the effect of potassium on the morphology of

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08931.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SiC has been proved chemically unstable at temperatures>1073 K (800°C) in the presence of cobalt, nickel, or iron. The instability results from reactions of cobalt, nickel, or iron with SiC to produce silicides and free carbon. A method of calculation is proposed for thermodynamic analyses of these reactions. The results show that the silicides of cobalt, nickel, and iron are thermodynamically more stable than SiC. The metal–silicon eutectic may provide kinetic condition to the reactions. Calculations also are performed for reactions of SiC with other transition metals. Instabilities of SiC in the presence of those metals are predict

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08932.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The metal–ferrite composite (Fe0.2Co0.8)0.8(Fe2.38Co0.62O4) has been studied by X‐ray diffractometry measurements and high‐resolution transmission electron microscopy. Spinel ferrite occurs in highly crystalline domains 100–150 nm in size, and the iron–cobalt alloy occurs in smaller and less‐crystalline domains (10–20 nm). Both phases are heterogeneous in composition. The metal is embedded in the spinel phase, located near the edges, and overlaid by a poorly crystallized layer or misshapen regions containing small spinel crystals and amorphous phases. By annealing under vacuum up to 800°C, the misshapen regions disappear and the size of the metallic regions increases. The concentration of iron in the metallic regions decreases and their structure changes to face‐centered cubic, while the spinel becomes

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08933.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The CaO–MgO system is used as a prototype system to evaluate the accuracy of several energy and entropy approximations for predicting solid‐state phase diagrams in ionic materials. Configurational disorder between the cations is parameterized with the cluster expansion technique. The vibrational contribution to the free energy is incorporated with a harmonic model that accounts for the dependence of the vibrational density of states on the cation configuration. The CaO–MgO phase diagram can be predicted very accurately with quantum mechanical energy methods, without the use of any adjustable parameters. Published empirical potential parameters for the CaO–MgO system reproduce the qualitative features of the phase diagram but significantly underestimate the solubility limits. Parameters that reasonably reproduce the quantum mechanical results are pr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08934.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Reduced and internally biased oxide wafer (RAINBOW) actuators are fabricated by a controlled reduction of Pb(Zr, Ti)O3‐based piezoelectric material. The reduction process results in a conductive layer composed of an interconnected metallic lead phase and refractory oxides (ZrTiO4, ZrO2, La2O3, etc.). The nature of the reduction is discovered to be the result of a complex volume change leading to a nanoscale interconnected metallic structure. The distribution of phases within the cermet vary within the thickness of the wafer. Within the piezoelectric ceramic phase, the reduction process modifies the grain‐boundary structure to give two distinct types of fracture: transgranular and intergranular. The complexed microstructures of the RAINBOW actuator materials are discussed in relation to their dielectric and piezoelectric propert

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08935.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Using experimental and modeling studies, the effect of field direction on the field‐activated combustion synthesis of SiC was investigated. Field application imposed in a direction perpendicular to wave propagation resulted in enhancement: the wave velocity increased linearly with an increase in field strength. For field application parallel to wave propagation, two cases were considered: confined and nonconfined areas of impacted energy from an ignition source. In the confined case both experimental (using laser ignition) and simulation studies were made. Both showed the establishment of a two‐front combustion with waves moving to opposite sides in a direction perpendicular to the field. In the nonconfined case, initial waves propagated in a direction parallel to the field but were independent of field strength. The results are interpreted in terms of a model in which the current is confined to the combustion zone, adding another source of heat to the combustion process, at a rate of σE2(J·cm

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08936.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

BMT perovskite ceramics were synthesized at low temperature (∼100°C) by chemical processing comprised of the hydrolysis of Ba‐Mg‐Ta alkoxide precursor. FT‐IR and Raman spectroscopic studies of the Ba‐Mg‐Ta ethoxide precursor reveal that barium, magnesium, and tantalum ethoxides are bonded with each other via alkoxy bridging to form a trimetallic alkoxide. The hydrolysis of the Ba‐Mg‐Ta ethoxide with a large amount of water under refluxing results in the direct formation of a crystalline BMT phase with the pseudocubic perovskite structure at a low temperature of ∼100°C. The alkoxide‐derived BMT powder has high sinterability to provide a high‐density sintered body with a density of 94.0–98.0% at a low fir

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08937.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Single crystals of α‐Si3N4several millimeters in diameter and several millimeters long have been grown by chemical vapor deposition. Some of the microstructural and mechanical properties have been evaluated using X‐ray diffracometry, optical and transmission electron microscopy, and high‐temperature microhardness testing. The crystallographic growth direction determines the quality of the crystals, including the density of internal microcracks and the nature and quantity of special boundaries. The measurement of crack lengths associated with microindentations has shown that cleavage in α‐Si3N4is relatively isotropic. Finally, indentation fracture toughness values agree well with theoretical predictions based on recent bond‐energy c

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08938.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY