ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06766.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06767.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Mullite (3Al2O3·2SiO2) is becoming increasingly important in electronic, optical, and high‐temperature structural applications. This paper reviews the current state of mullite‐related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid‐solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mol% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for low‐temperature (90% of its room‐temperature strength to 1500°C and displays very high creep resistance. Because of its low dielectric constant, mullite has now emerged as a substrate material in high‐performance packaging applications. Interest in optical applications mainly centers on its applicability as a window material within th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06768.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The structure of 3:2 mullites is investigated by high‐resolution electron microscopy. Attention is paid to the identification of isolated defects, i.e., oxygen vacancies in pure 3:2 mullite and chromium interstitials in chromium‐doped mullite. Models for such defects, including lattice reconstruction effects, are analyzed with the help of image simulati

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06769.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Critical analyses of published reaction studies on kaolinite, and sol‐gel and powder mixtures equivalent to 3Al2O3·2SiO2mullite, were made. Supplementary experiments on reactions of kaolinite at different rates of heating were analyzed. Complete atomic homogeneity of precursors results in formation of tetragonal mullite. Colloidal precursors initially form spinel which reacts with the remaining silica by a diffusion‐nucleation mechanism to form orthorhombic mullite. Precursors with intermediate atomic homogeneity follow both reaction paths. Kaolinite with a layer lattice structure has two‐dimensional atomic homogeneity. Both reaction routes are followed and their degree of interaction is dependent on the crystallinity and impurities of the kao

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06770.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Control over the structure of hybrid (colloidal + molecular) aluminosilicate gels was utilized to demonstrate that precursor chemistry has a direct and controllable effect on the ∼1000°C crystallization of spinel and mullite in molecular precursor systems. Synthesis or preparation conditions leading to the development of a cubic, transition alumina result in the epitactic nucleation of spinel at ∼1000°C in gels that otherwise crystallize directly to mullite at ∼1000°C. Thus, the preference for spinel nucleation in gels derived from solution precursor systems whose chemistries promote formation of transition alumina readily explains the reported inability to obtain substantial mullite yields at ∼1000°C. Isothermal transformation kinetics of colloidal and hybrid gels show that in the absence of direct mullite formation at ∼1000°C, the release of alumina from the spinel‐type crystal structure becomes the rate‐controlling step in the transformation. This necessitates higher temperatures for mullite formation and limits the kinetic enhancement possible with extrinsic increases in mullite nu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06771.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Diphasic gels with Al/Si atomic ratios of 6/1, 3.1/1, 3/1, 2/1, and 1/1 were used to study the effect of precursor composition on mullite formation process and the resulting microstructure. Mullite formation initiated at about 1300°C for all samples, with only some slight differences in temperatures. The mullite formation temperature was a minimum for an Al/Si ratio of 3.1/1 and increased as the Al/Si ratio of the gels increased or decreased. For the 6/1 gel, dissolution of alumina into mullite solid solution was observed after the initial mullite formation. The dissolution process was reversed above 1450°C with the formation of θ‐Al2O3and then α‐Al2O3. Change of microstructures from equiaxed to elongated mullite grain structures was found within a narrow range of composition near the nominal Al/Si ratio

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06772.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Recent studies have shown that the mullitization of diphasic aluminosilicate matrices comprising transitional alumina and amorphous silica occurs via a nucleation and growth process. Nucleation is preceded by a temperature‐dependent incubation period. Following this incubation period, rapid nucleation of mullite occurs, producing about 1.8 × 1011nuclei/cm3, which remains constant throughout the rest of the transformation. Both incubation and mullite growth are thermally activated processes with apparent activation energies of 987 ± 63 and 1070 ± 200 kJ/mol, respectively. The growth rate of mullite grains under isothermal conditions is time dependent. An interpretation of these results is proposed on the basis of the nucleation and growth concepts of LaMer and Dinegar which supports the concept that the growth rate of mullite grains is controlled by the dissolution of transitional alumina into the amorphous ma

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06773.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The surface area and density of aluminosilicate xerogels containing a one‐to‐one Al/Si molar ratio (47 wt% alumina) can be varied dramatically by changing the pore fluid prior to drying. The surface area of ethanol‐washed 47 wt% alumina gels was more than 500 m2/g, while gels dried from the mother liquor (approximately 75 vol% ethanol, 25 vol% water) had less than 1 m2/g surface area. Changes in the physical structure of the dried gels had dramatic effects on subsequent phase evolution and densification behavior during heat treatment. NMR, X‐ray diffraction, and DTA were used to follow the phase evolution of different gels. Differences in the amorphous gel structure were identified using27Al and29Si MAS NMR. Gels of identical composition prepared from the same precursor solutions crystallized to different phases, depending upon the surface area of the gel prior to heating. The high surface material (ethanol washed) formed mullite and amorphous silica, while the low surface area gel (unswashed) crystallized to mullite and cristobalite. These gels were prepared from alkoxide precursors. A low surface area gel with a different degree of chemical homogeneity was prepared by the nitrate method for comparison. Results indicate that the physical structure of aluminosilicate gels, i.e., pore structure and chemical homogeneity, has a dramatic influence on phase ev

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06774.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

NMR study of mullite precursors has shown that local arrangement of Al in samples synthesized by spray pyrolysis differs considerably from the one adopted by samples obtained by polymeric or colloidal routes. Aluminum is tetra‐ and pentahedrally coordinated in the first type of samples but is tetra‐ and octahedrally coordinated in the second ones. Segregation of SiO2and Al2O3is directly produced in colloidal preparation; however, this phenomenon occurs only in polymeric gels when they are heated between 980° and 1100°C. In polymeric samples, thermal treatment at ∼980°C produces the formation of γ‐Al2O3. A similar treatment in spray‐pyrolized powders gives directly 3:2 mullite. From these results, exothermic and expansive effects detected at ∼980°C were ascribed to changes in coordination of Al produced during the atomic rearrangement that accompanies formation of these two phases (γ‐Al2O3or mullite). Above 1200°C, incorporation of Si in the Al‐rich phase induces the formation of 3:2 mullite in polymeric

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb06775.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY