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1.
Akshoy Kr. Chakraborty 《Journal of the American Ceramic Society》2005,88(1):134-140
Mechanical mixture of γ-Al2 O3 and amorphous SiO2 , and diphasic Al2 O3 /SiO2 gels of three different compositions were synthesized. They were subjected to heat treatment to various temperatures in the range 900°–1600°C. Qualitative X-ray diffraction data show that these diphasic gels do not crystallize to a combined mixture of θ-Al2 O3 and α-Al2 O3 polymorphs at the intermediate stage, prior to mullite formation. Estimated mullite formation data show that the course of its formation from mixed oxides was different from that of diphasic gels. Results are compared with previous findings and the concept of Al–Si spinel formation in the phase transformation of stoichiometric diphasic gel system is substantiated. 相似文献
2.
Phase Transformation of Diphasic Aluminosilicate Gels 总被引:1,自引:0,他引:1
Aluminosilicate gels with compositions Al2 O2 /SiO2 and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC2 H5 )4 . Upon heating the pseudo-boehmite transforms to γ-Al2 O3 around 400°C, then to δ-Al2 O3 at 1050°C, and at 1200°C reacts with amorphous SiO2 to form mullite. Some twinned θ-Al2 O3 forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al2 O3 or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape. 相似文献
3.
Stoichiometric mullite (71.38 wt% Al2 O3 -28.17 wt% SiO2 ) and 80 wt% Al2 O3 -20 wt% SiO2 gels were prepared by the single-phase and/or diphasic routes. Dense sintered bodies were prepared from both sets of gels in the Al2 O3 -SiO2 system. Apparent densities of 96% and 97% of theoretical density were measured for the diphasic (using two sols) mullite samples sintered at 1200° and 1300°C for 100 min, respectively; this compared with 85% and 94% for the single-phase xerogels under the same conditions, and to much lower values for mullite prepared from conventional mixed powders. The microstructure of the mullite pellets from diphasic xerogel precursors is also considerably finer. 相似文献
4.
Interpretation of the Kaolinite-Mullite Reaction Sequence from Infrared Absorption Spectra 总被引:2,自引:0,他引:2
The phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al-containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ-Al2 O3 and SiO2 , before the "950°C" exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel-type phase, which is associated with amorphous SiO2 and some poorly crystalline "primary" mullite, is γ-Al2 03 (crystalline) rather than an Al-Si spinel. There is some evidence, however, that a fraction of the γ-Al2 O3 , may be an Al-Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ-Al2 O3 /amorphous SiO2 reaction and the recrystallization of primary mullite, whereas excess amorphous SiO2 eventually crystallizes as cristobalite. 相似文献
5.
The thermal evolution of a mullite gel of composition 2Al2 O3 ·SiO2 has been investigated. The gel crystallized at 1300°C into an alumina-rich mullite and corundum, instead of single-phase 2Al2 O3 ·SiO2 mullite. The amount of Al2 O3 that dissolved in the mullite structure has been determined in the 1300–1780°C temperature range by measuring the mullite lattice parameters. A maximum limit for the amount of Al2 O3 in solid solution has been observed. Densification of the gel powders has been analyzed up to temperatures of 1780°C. The microstructure of dense materials always showed the presence of residual Al2 O3 particles. 相似文献
6.
James E. Pitchford Robert J. Stearn Anthony Kelly William J. Clegg 《Journal of the American Ceramic Society》2001,84(5):1167-1168
The structure of mullite, which has a composition ranging from 3Al2 O3 ·2SiO2 to Al2 O3 ·2SiO2 , contains ordered oxygen vacancies. Sillimanite, Al2 O3 ·SiO2 , has a similar structure but with no vacancies. The indentation hardness of polycrystalline mullite (3Al2 O3 ·2SiO2 ) was measured from room temperature up to 1400°C and compared with that of single-crystal sillimanite (Al2 O3 ·SiO2 ) up to 1300°C. It was found that both materials show the same variation in hardness with temperature, suggesting that the structures have a similar resistance to plastic deformation, and therefore that the oxygen vacancies in the mullite structure are not the primary cause of mullite's resistance to high-temperature deformation. 相似文献
7.
The cell dimensions of pure triclinic 3CaO·SiO2 and monoclinic 3CaO·SiO2 solid solution (54CaO·16SiO2 ·Al2 O3 ·MgO) were determined and the powder diffraction patterns were indexed by the method of precise measurement of the spacings. The lattice constants are expressed in terms of triclinic or monoclinic cells corresponding to pseudo-orthorhombic cells derived from Jeffery's trigonal cell. The apparent lattice constants for pure 3CaO·SiO2 are a = 12.195 a.u., b = 7.104 au., c = 25.096 a.u., α= 90°, β= 89°44'γ= 89°44'; for 54CaO·16SiO2 .-Al2 O3 MgO, a = 12.246 a.u., b = 7.045 a.u., c = 24.985 a.u., β= 90°04'. Precise lattice constants of Jeffery's monoclinic lattice for 54CaO.-16SiO2 -Al2 O3 ·MgO are derived as a = 33.091 a.u., b = 7.045 a.u., c = 18.546 a.u., β= 94°08'. High-temperature X-ray patterns showed that pure triclinic 3CaO·SiO2 transformed to a monoclinic form at about 920°C. and then to a trigonal form at about 970°C. Monoclinic 54CaO.16SiO2 ·Al2 O3 –MgO transformed to trigonal at about 830°C. These transitions were reversible and reproducible and were accompanied by only slight deformation of the structure forms. 相似文献
8.
Suxing Wu Helen M. Chan Martin P. Harmer 《Journal of the American Ceramic Society》1997,80(6):1579-1582
Mullite and mullite/ZrO2 ceramics were fabricated starting from Si/Al2 O3 and Si/Al2 O3 /ZrO2 powder mixtures, which were mixed and attrition milled with TZP balls in water. Isopressed powder compacts were subjected to a heat treatment in air, during which the Si was oxidized to SiO2 . At } 1410°C, reaction between Al2 O3 and SiO2 occurred, resulting in mullite (3Al2 O3 ·2SiO2 ). Depending on the composition of the starting powders, the end product was either single-phase mullite or a mullite composite. The reaction process was monitored by thermogravimetry and dilatometry. It was found that the microstructure and mechanical properties of the reaction-formed mullite ceramics were significantly improved by ZrO2 additions. 相似文献
9.
Noboru Yoshikawa Atsushi Kikuchi Shoji Taniguchi 《Journal of the American Ceramic Society》2002,85(7):1827-1834
An aluminum/Al2 O3 composite body is produced by a displacement reaction between SiO2 and molten aluminum. The growth rate of the reaction layer possesses negative (anomalous) temperature dependence at 1000–1300 K. This study compared reported reaction-kinetic data and investigated causes for this temperature dependence. The reaction product, Al2 O3 , changed from the γ-/θ-Al2 O3 phase to the α-Al2 O3 phase in this temperature range and α-Al2 O3 became the dominant phase at >1273 K. Isothermal transformation of the γ-/θ-Al2 O3 product phases to the α-Al2 O3 phase was also observed. Morphologies and scales of the Al2 O3 phases change drastically at 1173 K; this transition occurred in a spatially discontinuous manner. Reaction-rate retardation was interpreted in terms of occurrence of the competitive and simultaneous reactions to produce different Al2 O3 phases in this temperature range. It was also found that the hydrogen release from the raw SiO2 and the SiO2 phase transformation were not related to the negative temperature dependence. 相似文献
10.
Thermal reactions in 93% Al2 O3 -7% MgO and 95.8% Al2 O3 -4.2% MgO gels seeded with α-Al2 O3 , MgAl2 O4 , α-Fe2 O3 , and SiO2 , sols were investigated by differential thermal analysis to determine the extent of nucleation catalysis of solid-state reactions. Seeding with α-Al2 O3 lowered the α-Al2 O3 crystallization temperature in these xerogels by 100° to 150°C. Spinel seeds have much less effect on the γ-α transition, and α-Fe2 O3 and SiO2 seeds do not affect it significantly. Isostructural seeding of gels may therefore permit lower ceramic processing temperatures. 相似文献
11.
Thermochemistry of the Aluminas and Aluminum Trihalides 总被引:1,自引:0,他引:1
STEPHEN C. CARNIGLIA 《Journal of the American Ceramic Society》1983,66(7):495-500
Consistent standard free energies of formation of gibbsite, bayerite, boehmite, and diaspore; their respective transition aluminas, α-Al2 O3 ; and AlF3 , AlCl3 , H2 O3 HF, and HCl are compiled from 298.16 to 2100 K from literature review, computations, and estimates. Significant adjustments and additions to earlier compilations are included. Revised analysis is made of the gibbsite-to-α-A12 O3 transition series and of reactions of appropriate aluminas with HF and HCl, comparing them with experimental data. These updated Δ G ° tables should also yield accurate ΔG° data for many other alumina reactions, e.g. with SiO2 , M2 O, MO, etc. 相似文献
12.
The solid solution range of melt-grown mullite was examined by crystal-chemical methods. The maximum Al2 O3 content as determined by EDX was ∼83.6 wt%, 75 mol%, or the nominal composition 3Al2 O3 .SiO2 . For samples of overall composition 81 to 83 wt% Al2 O3 , extra lines indicating crystallographic superstructure appeared in Guinier X-ray patterns. The corresponding TEM microstructure consisted of a mullite matrix finely twinned on (001), the twins being 0.02 to 0.10 μm wide, with oriented exsolution of α-A12 O3 , often twinned, also being present. The analogy between mullite superstructure and that of plagioclase feldspars, as well as the relevance of these findings to the SiO2 -Al2 O3 metastable phase equilibria are discussed. 相似文献
13.
Preparation of Nanometer-Sized α-Alumina Powders by Calcining an Emulsion of Boehmite and Oleic Acid
Chih-Peng Lin Shaw-Bing Wen Ting-Tai Lee 《Journal of the American Ceramic Society》2002,85(1):129-133
This study proposes a method to form ultrafine α-Al2 O3 powders. Oleic acid is mixed with Al(OH)3 gel. The gel is the precursor of the Al2 O3 . After it is mixed and aged, the mixture is calcined in a depleted oxygen atmosphere between 25° and 1100°C. Oleic acid evaporates and decomposes into carbon during the thermal process. Residual carbon prevents the growth of agglomerates during the formation of α-Al2 O3 . The phase transformation in this process is as follows: emulsion →γ-Al2 O3 →δ-Al2 O3 →θ-Al2 O3 →α-Al2 O3 . This process has no clear θ phase. Aging the mixed sample lowers the formation temperature of α-Al2 O3 from 1100° to 1000°C. The average crystallite diameter is 60 nm, measured using Scherrer's equation, which is consistent with TEM observations. 相似文献
14.
Mechanical-Activation-Triggered Gibbsite-to-Boehmite Transition and Activation-Derived Alumina Powders 总被引:2,自引:0,他引:2
Mechanical activation of monoclinic gibbsite (Al(OH)3 ) in nitrogen led to the formation of nanocrystalline orthorhombic boehmite (AlOOH) at room temperature. The boehmite phase formed after merely 3 h of mechanical activation and developed steadily as the mechanical-activation time increased. Forty hours of mechanical activation resulted in essentially single-phase boehmite, together with α-alumina (α-Al2 O3 ) nanocrystallites 2–3 nm in size. The sequence of phase transitions in the activation-derived boehmite was as follows: boehmite to γ-Al2 O3 and then to α-Al2 O3 when flash-calcined at a heating rate of 10°C/min in air. γ-Al2 O3 formed at 520°C, and flash calcination to 1100°C led to the formation of an α-Al2 O3 phase, which exhibited a refined particle size in the range of 100–200 nm. In contrast, the gibbsite-to-boehmite transition in the unactivated gibbsite occurred over the temperature range of 220°–330°C. A flash-calcination temperature of 1400°C was required to complete the conversion to α-Al2 O3 phase, with both δ-Al2 O3 and θ-Al2 O3 as the transitional phases. The resulting alumina powder consisted of irregularly shaped particles 0.4–0.8 μm in size, together with an extensive degree of particle agglomeration. 相似文献
15.
Yoshitoshi Saito Takahiro Takei Shigeo Hayashi Atsuo Yasumori Kiyoshi Okada 《Journal of the American Ceramic Society》1998,81(8):2197-2200
This paper focused on the effects of various phases of SiO2 additives on the γ-Al2 O3 -to-α-Al2 O3 phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO2 , such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO2 , such as quartz and cristobalite. Amorphous SiO2 was considered to retard the γ-to-α phase transition by preventing γ-Al2 O3 particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al2 O3 surface. On the other hand, crystalline SiO2 accelerated the α-Al2 O3 transition; thus, this SiO2 may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO2 additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al2 O3 -to-α-Al2 O3 phase transition. 相似文献
16.
Akira Kato Hisao Yamashita Hiroshi Kawagoshi Shimpei Matsuda 《Journal of the American Ceramic Society》1987,70(7):157-C
Mixtures of La2 O3 and Al2 O3 with various La contents were prepared by co-precipitation from La(NO3 )3 and Al(NO3 )3 solutions and calcined at 800° to 1400°C. The addition of small amounts of La2 O3 (2 to 10 mol%) to Al2 O3 gives rise to the formation of lanthanum β-alumina (La 2 O3 ·11–14Al2 O3 ) upon heating to above 1000°C and retards the transformation of γ-Al2 O3 to α-Al2 O3 and associated sintering. 相似文献
17.
Luis A. M. Scudeller Elson Longo Jose A. Varela 《Journal of the American Ceramic Society》1990,73(5):1413-1416
A graphite chamber was used for the reaction between samples of 45 or 55 wt% alumina and a mixture of metallurgical coke and potassium carbonate. Thermal treatments were conducted at 1000°C. The results suggest that the potassium attack in silica-alumina bricks is controlled by the following reactions: K2 O + SiO2 → K2 O → SiO2 in the glassy matrix; 3(K2 O · 2SiO2 ) + 3Al2 O3 → 2SiO2 · 3(K2 O · Al2 O3 · 2SiO2 ) + 2SiO2 for short times; and K2 O → Al2 O3 · 2SiO2 + 2SiO2 · K2 O · Al2 O3 · 4SiO2 for long times. In 55 wt% alumina bricks containing corundum and tridymite, potassium also attacks those phases forming a glassy phase. The formation of kaliophilite at the matrix/mullite grain interface causes a volumetric expansion of 55.5%, resulting in cracks in the matrix. Because the kaliophilite phase is not in equilibrion with mullite, the former will react with free silica to form leucite that is more thermodynamically stable. 相似文献
18.
Gradient, porous alumina ceramics were prepared with the characteristics of microsized tabular α-Al2 O3 grains grown on a surface with a fine interlocking feature. The samples were formed by spin-coating diphasic aluminosilicate sol on porous alumina substrates. The sol consisted of nano-sized pseudo-boehmite (AlOOH) and hydrolyzed tetraethyl orthosilicate [Si(OC2 H5 )4 ]. After drying and sintering at 1150°–1450°C, the crystallographic and chemical properties of the porous structures were investigated by analytical electron microscopy. The results show that the formation of tabular α-Al2 O3 grains is controlled by the dissolution of fine Al2 O3 in the diphasic material at the interface. The nucleation and growth of tabular α-Al2 O3 grains proceeds heterogeneously at the Al2 O3 /glass interface by ripening nano-sized Al2 O3 particles. 相似文献
19.
Aran Rafferty Anton Clifford Robert Hill David Wood Bisserka Samuneva Margarita Dimitrova-Lukacs 《Journal of the American Ceramic Society》2000,83(11):2833-2838
The nucleation and crystallization of a series of glasses based on 4.5SiO2 ·3Al2 O3 ·1.5P2 O5 ·(5 − z )CaO · z CaF2 with a Ca:P ratio corresponding to apatite were studied. In these glasses, the objective was to investigate the influence of fluorine content and z was varied from 3 to 0. All the glasses studied crystallized to fluorapatite (FAP) and mullite with the exception of the glass containing no fluorine, which crystallized to β-tricalcium phosphate (Ca3 (PO4 )2 ) and anorthite (CaAl2 Si2 O8 ). Glasses that contained sufficient fluorine to form FAP bulk nucleated to give FAP without a nucleation hold. Thermal gravimetric analysis demonstrated a significant weight loss corresponding to the crystallization of mullite, which increased with the fluorine content of the glass and also with decreased particle size. The loss was attributed to volatile SiF4 . The glass transition temperature decreased with increased fluorine content of the glass. 相似文献
20.
The reaction kinetics for the formation of mullite (3Al2 O3 · 2SiO2 ) from sol-gel-derived aluminosilicate gels prepared under various hydrolysis conditions were studied using dynamic X-ray diffraction (DXRD) and differential thermal analysis (DTA). The DXRD experiments showed that the apparent single-phase gels aluminosilicate gels were not completely single-phase gels but a composite of a single-phase gel (which has molecular-scale mixing) and a diphasic gel (which has nanometer-scale mixing). Mullite formation from these composite gels exhibits a two-stage conversion, the first at about 980°C and the second at about 1220°C. Gels prepared by a slower hydrolysis rate tend to have a higher conversion after the first stage, therefore, better molecular-scale mixing. Simultaneous formation of Al-Si spinel and mullite was also observed at 980°C. This coincidence of mullite and spinel formation could explain some of the controversy in the literature concerning the 980°C thermal event. 相似文献