ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05451.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

SiO2, Al2O3, and 3Al2O3.2SiO2powders were synthesized by combustion of SiCl4or/and AlCl3using a counterflow diffusion flame. The SiO2and Al2O3powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al2O3.2SiO2powder produced with a low‐temperature flame was also amorphous and had a similar morphology. However, those produced with high‐temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the fl

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05452.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Fine, homogeneous lead zirconate titanate (PZT) powders have been prepared by the thermal decompostion of metalorganic complexes derived from nitrate solutions using ethylenediaminetetraacetic acid (EDTA) as a complexing agent. It has been shown that nitrate ions accelerate the decomposition of the precursor and the crystallization of the PZT phase initiates at temperatures as low as 250°C. No intermediate phases, other than PbO, were found. The coexistence region of the rhombohedral and tetragonal phases in the sintered ceramic was found to be a little over 1 at.%. The chloride‐EDTA precursor did not form the desired PZT phase, even after calcination at 1000°C. The segregation and loss of lead was observed for this material, due to the formation and evaporation of PbCl2. DTA/TGA, SEM, and XRD were employed to characterize the powders. The use of TEM with nanoprobe energy dispersive X‐ray analysis (EDX), allowed the identification of the phase segregation in the chloride‐EDTA derived powder. A possible reaction mechanism for this phase segregation is su

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05453.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A new ceramic joining technique has been developed that utilizes an exothermic combustion reaction to simultaneously synthesize the joint interlayer material and to bond together the ceramic workpieces. The method has been used to join SiC/SiC composites and dense SiC ceramics using TiC‐Ni powder mixtures that ignite below 1200°C to form a TiC‐Ni joining material. Thin layers of the powder reactants were prepared by tape casting, and joining was accomplished by heating in a hot‐press to ignite the combustion reaction. During this process, localized exothermic heating of the joint region resulted in chemical interaction at the interface between the TiC‐Ni and the SiC ceramic that contributed to bonding. Room‐temperature four‐point bending strengths of joints produced by this method have exce

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05454.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A simple bend stress relaxation (BSR) test has been developed to measure the creep‐related deformation properties of ceramic fibers and whiskers. The test was applied to a variety of commercial and developmental Si‐based fibers to demonstrate test capabilities and to qualitatively evaluate the relative creep resistance of the fibers at 1200° and 1400°C. The implications of these results and the advantages of the BSR test over typical tensile creep tests are dic

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05455.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Experimental observations of the spacings between cracks formed in the matrix in unidirectional fiber‐reinforced brittle materials in a situation where a tensile stress is applied parallel to the lengths of the fibers, and where the fibers have a longer elongation to failure than the matrix, suggest that the distribution of crack spacings does not follow the one expected on the basis of a model where the matrix cracks at a well‐defined single‐valued stress. A simple model which is able to account for this apparent discrepancy between theory and experiment is developed and disc

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05456.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The influence of SiO2addition on the densification and microstructural development of high‐purity Si3N4during hot isostatic pressing (HIP) was studied. During HIP, densification was promoted, but the phase transformation fromα‐Si3N4toβ‐Si3N4was impeded by SiO2. Analysis using a simple model shows that the enhanced densification was mainly due to the viscous flow of SiO2. The microstructure changed remarkably at between 10 and 20 wt% SiO2additions. Analysis of the phase transformation kinetics suggests that the diffusion of Si3N4through SiO2glass is the ratecontrolling step for the transfo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05457.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Preparation of chemically homogeneous cordierite powders of submicrometer size was investigated using alkoxide precursors, synthesized from Mg metal, Al(OBus)3, and partially prehydrolyzed Si(OEt)4. The amount of partial prehydrolysis water in precursor synthesis was changed to study the effects on the hydrolysis products. In the hydrolysis of the diluted solution of the precursor, product morphology was quite sensitive to the precursor, and different morphologies such as gelation, gel precipitation, and powder precipitation were observed. A redispersable powder of submicrometer size was obtained only from the precursor synthesized at a H2O/Si(OEt)4ratio of 1.2. Differences were also observed in minor crystalline phases among calcined hydrolysis products. The alkoxide precursors were characterized using Raman spectroscopy, gas chromatography, and NMR spectroscopy. The differences in hydrolysis products are most likely due to differences in hydrolysis and condensation of different chemical species in precursors.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05458.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The composition, the structure, and some physical properties of boron–carbon–nitrogen solid solutions prepared by chemical vapor deposition (CVD) have been investigated. Both crystalline films and amorphous granular materials resulting respectively from heterogeneous and homogeneous nucleation were characterized by X‐ray diffraction, XPS, RBS, and TEM. The compositions of these single‐phase materials are gathered in two main domains located in the B/N>1 part of the C─B─N composition diagram. It is stated that the carbon‐rich domain results from structural disorder of an ideal C5B2N composition. The thermal behavior of these films indicates that no mass loss can be detected after 1 h at 1700°C but a graphitization and a formation of small amounts of B13C2are observed. Density and preliminary electrical conductivity measurements were

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05459.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Submicrometer SiC (β‐form) powders were synthesized by reacting silica and carbon black at temperatures between 1450° and 1800°C. Simultaneous application of vacuum and mixing provides the condition for full conversion of silica to SiC. It was shown that two different reaction mechanisms are possible, depending on the reaction temperature and the partial pressure of CO. At lower temperatures (below approximately 1400°C), the dominant mechanism for silicon carbide formation involves the solid‐state reaction of silica and carbon. At higher temperatures (above approximately 1400°C), the dominant mechanism is the reaction between gaseous SiO and C. Above 1400°C, the rate of SiC formation is controlled by the rate of SiO formation. In as‐synthesized form, the SiC powders typically contain<0.2 wt% of unreacted silica and free carbon in the range between 6 and 15 wt%. Precise control of partial pressure of CO in the reaction chamber and continuous mixing of the reactants provide the conditions under which the rate of silicon carbide formation may be increased by one order of magnitude. The process is suitable for large‐scale commercial production of SiC, requiring no postfabrication acid leaching or

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05460.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY