Showing papers on "Mullite published in 1987"
TL;DR: In this paper, the alumina-silica phase relations of samples annealed in oxygen and quenched were studied by optical microscopy, image analysis, X-ray diffraction, and the electron microprobe.
Abstract: The alumina-silica phase relations of samples annealed in oxygen and quenched were studied by optical microscopy, image analysis, X-ray diffraction, and the electron microprobe. The solid solution boundaries of mullite changed with increasing temperature and joined at 1890°C and a composition of 77.15 wt% alumina. The melting point of mullite was 189°C with a peritectic between 76.5 and 77.0 wi% alumina.
236 citations
TL;DR: In this article, it was shown that γ-Al2O3 type spinel phase is solely responsible for the 980°C exotherm in the kaolinite-to-mullite reaction series.
Abstract: With the use of differential thermal analysis, X-ray diffraction, and transmission electron microscopic techniques, we showed that γ-Al2O3 type spinel phase is solely responsible for the 980°C exotherm in the kaolinite-to-mullite reaction series. Transmission electron microscopic characterization indicated that the spinel formation is preceded by a phase separation in the amorphous dehydroxylated kaolinite matrix. Chemical analysis of the spinel phase by energy dispersive X-ray spectroscopy revealed a nearly pure Al3O2 composition.
174 citations
TL;DR: In this paper, ZrO2 particles were added to a toughened, whisker-reinforced ceramic matrix, and the results showed that the combination of these multiple toughening agents should result in ceramic composites tougher than (1) that achieved by either mechanism by itself or (2) the sum of two processes.
Abstract: Since the contribution of transformation toughening increases with the loca crack resistance (which is proportional to the toughness of the matrix), ZrO2 particles were added to a toughened, whisker-reinforced ceramic matrix. Analysis revealed that the combination of these multiple toughening agents should result in ceramic composites tougher than (1) that achieved by either mechanism by itself or (2) the sum of the two processes. The toughness of mullite could be increased 1.8-and 2.4-fold with a 20 vol% addition of ZrO2 particles or SiC whiskers, respectively. However, when 20 vol% of both ZrO2 particles and SiC whiskers were added, the toughness was increased at least 3-fold with monoclinic m-ZrO2 and by >5-fold with tetragonal t-ZrO2. The differences in the toughening achieved when t-ZrO2 vs m-ZrO2 is present in the SiC-whisker-reinforced mullite are attributed to differences in their interdependencies upon the whisker reinforcement.
115 citations
TL;DR: In this paper, starting powders containing 72 wt% Al/sub 2/O/sub 3/ and 28 Wt% SiO/Sub 2/2/3/ were prepared by sol-gel methods classified as colloidal and polymeric.
Abstract: Starting powders containing 72 wt% Al/sub 2/O/sub 3/ and 28 wt% SiO/sub 2/ were prepared by sol-gel methods classified as colloidal and polymeric. Compacts fired at 1700/sup 0/C showed significant differences in microstructure. The specimens formed with the colloidal powder had mullite grains of prismatic shape and liquid phase; with polymeric powder, mullite grains were granular with no liquid phase present. It is shown that the mullite grains in the first case are higher in Al/sub 2/O/sub 3/ content, resulting in an excess of SiO/sub 2/ which is the base for the liquid phase. In the second case, the mullite grains have the same Al/sub 2/O/sub 3/ content as the starting powders. The presence of a liquid phase in the first case is considered to be metastable, resulting from the nature of the starting materials and processing conditions employed.
112 citations
TL;DR: In this article, a mixture of boehmite sol and silica dispersion was used for the preparation of 3Al2O3·2SiO2 powder, which was then sintered to 98% (theoretical density) at 1650°C for 1.5 h.
Abstract: Mullite (3Al2O3·2SiO2) of stoichiometric composition was prepared by mixing boehmite sol and silica dispersion and gelling at a pH of 3. Complete mullitization takes place at or above 1300°C. Ultrafine mullite powder prepared by calcining gel at 1400°C and attrition milling could be sintered to >98% (theoretical density) at 1650°C for 1.5 h. The flexural strength of the sintered body at room temperature was 405 MPa and 350 MPa at 1300°C. Only traces of a secondary phase were observed along the grain boundary.
112 citations
TL;DR: There is a close relationship between the occurrence of high-temperature crystalline phases and the colour of clay bricks after being fired under oxidizing conditions at 1000°C as mentioned in this paper.
111 citations
TL;DR: Komarneni et al. as mentioned in this paper used magic-angle-spinning MAS/NMR spectroscopy to study the thermal transformation of metakaolin and high-temperature phases of kaolinite.
Abstract: Thermal transformations of kaolinite were studied by zgsi- and ZVA1- MAS/NMR and ESR techniques. In metakaolin, Si is still dominantly in the Q3 state (three Si atoms bonded to an Si-O4 tetrahedron) and A1 detectable by NMR is in both 4-coordination and 6-coordination. Coordination polyhedra around A1 or Fe replacing A1 are much distorted. Metakaolin crystallizes into y-alumina and mullite exothermically at ~ 980 ~ this crystalliza- tion being preceded by a faint endothermic reaction. The latter is due to segregation of SiO2 and A1203, which results in an increase of Si in the Q4 state and an increase of 6-coordinated A1203 . Kaolin minerals dehydroxylate at ~600~ this process corresponding to an intense endothermic peak on the DTA curve. The resultant anhydrous phase, called metakaolin or metakaolinite, is amorphous to X-rays. y-alumina plus mullite develop from the amorphous phase at ~ 980~ and this reaction is represented by a combination of a faint endotherm and a sharp and intense exotherm. The mechanism of this series of reactions has been studied from various standpoints, as reviewed by Brindley & Nakahira (1959), Tsuzuki (1971) and Brindley (1976). X-ray diffraction, electron microscopy, and X-ray fluorescence, IR absorption and M6ssbauer spectroscopy have all been used to study these reactions. Recent developments in high-resolution magic-angle-spinning MAS/NMR spectroscopy have allowed a new approach to this problem, Komarneni et al. (1985), MacKenzie et al. (1985b) and Brown et al. (1985) being forerunners in the application of this method to metakaolin and high-temperature phases of kaolinite. This paper gives the results of examination of a kaolinite heated to various temperatures, using 29Si- and 27A1-MAS/NMR and ESR techniques. Some new data are presented on the thermal transformation of kaolinite.
79 citations
TL;DR: In this paper, the authors used solid-state NMR with magic-angle-spinning (MAS) combined with X-ray powder diffraction, thermal analysis and 57Fe Mossbauer spectroscopy to study two muscovites of different iron contents.
Abstract: Studies of two muscovites of different iron contents, using solid-state NMR with magic-angle-spinning (MAS) combined with X-ray powder diffraction, thermal analysis and57Fe Mossbauer spectroscopy, suggest that dehydroxylation occurs by a homogeneous rather than an inhomogeneous mechanism, forming a dehydroxylate in which the aluminium is predominantly 5-coordinate. On further decomposition at about 1100° C, the tetrahedral layer and interlayer K+ form a feldspar-like phase similar to leucite (KAISi2O6), the remainder forming a spinel, which, contrary to previous suggestions, appears to contain little silicon. Further heating induces the formation of mullite (AI6Si2OP13), and, in the higher-iron sample, corundum (α-Al2O3), in addition to the feldspar-like phase. The presence of the iron impurity enhances the recrystallization reactions and promotes the conversion of mullite to corundum, which eventually becomes the sole aluminous product in the high-iron sample. In samples fired to higher temperatures, only the tetrahedral aluminium resonance is detectable by27AI NMR, probably because most of the iron is located in either the mullite or corundum phases, in which it broadens the octahedral aluminium resonance beyond detection.
68 citations
TL;DR: In this article, the thermal conductivity and diffusivity of silicon carbide reinforced mullite was shown to increase with the amount of whisker content, and the effect was much greater for vapor-liquid-solid (VLS) whiskers than for rice-hull (RH) whisker.
Abstract: The thermal conductivity and diffusivity of silicon carbide whisker reinforced mullite was shown to increase with whisker content. This effect was much greater for vapor-liquid-solid (VLS) whiskers than for rice-hull (RH) whiskers. This suggests that the thermal conductivity for the VLS whiskers was significantly higher than for the RH whiskers. Due to preferred orientation of the whiskers, thermal conductivity and diffusivity of the composite samples exhibited significant anisotropy.
38 citations
37 citations
TL;DR: In this paper, the chemical composition of mullite was examined by analytical TEM, which showed around 66 mol% Al2O3, which was fairly Al 2O3 rich compared with the bulk composition.
Abstract: The chemical composition of mullite which was termed from 2SiO23Al2O3 xerogel by firing was examined by analytical TEM. Mullite formed at 950°C firing showed around 66 mol% Al2O3, which was fairly Al2O3 rich compared with the bulk composition. The chemical composition of mullite gradually approached the bulk composition as the firing temperature increased to 1400°C and slightly departed again above that firing temperature.
01 Aug 1987
TL;DR: In this paper, a variety of oxidizable but self-protective materials are potential candidates for reinforcing media in ceramic-ceramic composites for use as high-temperature structural components in oxidizing environments.
Abstract: A variety of oxidizable but self-protective materials are potential candidates for reinforcing media in ceramic-ceramic composites for use as high-temperature structural components in oxidizing environments. The oxidation kinetics of 10 to 30 percent dispersions of particles of SiC and MoSi/sub 2/ as well as SiC whiskers in fully dense matrices of alumina, mullite, and various silicates were determined in the temperature range of 1200-1500 C. SiC and MoSi/sub 2/, which form self-protective coatings of silica when heated as bulk pieces, exhibit an increase in their parabolic rate constants of 15-fold when incorporated in alumina or mullite matrices, and of almost 30-fold when incorporated in a strontium silicate matrix. Reaction mechanisms are discussed. 10 references.
TL;DR: In this paper, the thermomechanical relationship between mullite and a lithium aluminosilicate (LAS) glass-ceramic has been studied by a technique of bimaterial dilatometry.
Abstract: The thermomechanical relationship between mullite and a lithium aluminosilicate (LAS) glass-ceramic has been studied by a technique of bimaterial dilatometry The viscoelastic behavior of LAS was found to result in a temperature Trvsl below the glass-transformation temperature Tg at which the interfacial stresses between the materials undergo a reversal in sign The effect of Trvsl on the high-temperature mechanical properties of mullite or SiC fiber-reinforced glass-ceramic composites is discussed It is suggested that fiber clamping by the matrix at temperatures between Trvsl and Tg contributes to the brittle, high-temperature behavior of such composites
Patent•
06 Jul 1987TL;DR: In this paper, a process for preparing mullite, 3Al2 O3.2SiO2, whiskers by forming an intimate, anhydrous mixture of AlF3 and SiO2 powders in about a 12:13 molar ratio was presented.
Abstract: A process for preparing mullite, 3Al2 O3.2SiO2, whiskers by (1) forming an intimate, anhydrous mixture of AlF3 and SiO2 powders in about a 12:13 molar ratio; (2) heating the mixture in an anhydrous, SiF4 atmosphere at about 700° C. to about 950° C. to form barlike crystalline topaz, Al2 (SiO4)F2 ; and (3) heating the barlike crystalline topaz under anhydrous conditions in a SiF4 atmosphere at a temperature of from about 1150° C. to about 1400° C. to produce mullite whiskers. The mullite whiskers produced can be used in ceramic and metal matrices without further chemical treatment.
TL;DR: Des ceramiques de mullite hautement translucides ont ete preparees par filtrage sans pression ni additif, avec elimination des phases secondaires and agrandissement approprie de la taille des grains de mollite as mentioned in this paper.
Abstract: Des ceramiques de mullite hautement translucides ont ete preparees par filtrage sans pression ni additif, avec elimination des phases secondaires et agrandissement approprie de la taille des grains de mullite
Patent•
28 Sep 1987TL;DR: In this paper, a whisker-reinforced zirconia-mullite composite article exhibiting excellent high-temperature modulus of rupture strength and good toughness is provided from batches comprising the free oxides ZrO2, SiO2 and Al2 O3 in combination with SiC whiskers for reinforcement.
Abstract: Whisker-reinforced zirconia-mullite ceramic articles exhibiting excellent high-temperature modulus of rupture strength and good toughness are provided from batches comprising the free oxides ZrO2, SiO2 and Al2 O3 in combination with SiC whiskers for reinforcement. Due to the fine oxide particle sizes attainable and the fact that the mullite is formed in situ in the consolidation process, dense, whisker-reinforced mullite composite articles exhibiting excellent high-temperature strength and toughness may conveniently be obtained.
TL;DR: In this paper, the microstructural and phase evolution of a reaction sintering of zircon-alumina mixtures with MgO and CaO as additives has been investigated by electron microscopy and microanalysis.
Abstract: The microstructural and phase evolution, which leads to final zirconia-mullite tough ceramic composites by using reaction sintering of zircon-alumina mixtures with MgO and CaO as additives, has been investigated by electron microscopy and microanalysis. The results confirm that the reaction, in a first stage, produces zirconia inside an amorphous matrix as well as saphirine and spinel for MgO and anorthite for CaO additions respectively. Finally the glassy phase distribution, the mechanical behaviour and the microstructure of the final reaction sintered compacts are comparatively discussed.
TL;DR: In this paper, the authors have made a characterization of mullite-ZrO2 ceramics by using the Cathodoluminescence (CL) technique, which showed complex spectra with broad and sharp bands and an attempt has been carried out to relate these bands to the main constitutive phases in the samples.
Abstract: Characterization of mullite-ZrO2 ceramics by Cathodoluminescence (CL) technique has been made. The analyses of the CL emission show complex spectra with broad and sharp bands. An attempt has been carried out to relate these bands to the main constitutive phases in the samples. Spatial distribution of the CL is found to be inhomogeneous.
TL;DR: In this article, a tetragonal ZrO/sub 2/polycrystal (Y-TZP)/mullite composite was sintered at 1450/sup 0/ to 1500/Sup 0/C in air to disperse rod-like mullite grains at the grain boundary of Y-ZP and the mechanical and thermal properties were investigated.
Abstract: Yttria-doped tetragonal ZrO/sub 2/ polycrystal (Y-TZP)/mullite composite were sintered at 1450/sup 0/ to 1500/sup 0/C in air to disperse rodlike mullite grains at the grain boundary of Y-TZP and the mechanical and thermal properties were investigated. The aspect ratios of mullite grain were >2. High fracture strength of 1000 MPa and fracture toughness of 12 MPa.m/sup 1/2/ were obtained by dispersing <20 vol% of mullite into Y-TZP. The thermal expansion coefficient of Y-TZP/mullite composites decreased with increasing mullite content. The thermal shock resistance of Y-TZP was greatly improved by dispersion of rodlike mullite grains.
TL;DR: The phase equilibrium of the Al 2 O 3 SiO 2 system was studied in this paper, and the differences in the procedures and experimental set-up employed by the pioneer researchers were analyzed.
TL;DR: In this paper, a complex temperature-controlled exsolution mechanism of iron from mullite was considered, and the annealed samples consisted of mullite as the main compound with minor amounts of glass and sometimes magnetite.
Abstract: Sample disks prepared from Al2O3 (61 wt%), SiO2 (28 wt%), and Fe2O3(II wt%) powders were sintered at 1270° and 1440°C and then annealed between 1300° and 1670°C. The annealed samples consisted of mullite as the main compound with minor amounts of glass and sometimes magnetite. The iron content of the mullites decreases strongly from ∼ 10.5 wt% Fe2O3 at 1300°C to ∼ 2.5 wt% Fe2O3 at 1670°C. A complex temperature-controlled exsolution mechanism of iron from mullite is considered.
TL;DR: In this article, the sinterability of SiO2-Al2O3 powders containing 20, 30, 40wt% Al 2O3, synthesized by hydrolysis of metal alkoxide, was examined.
Abstract: The sinterability of SiO2-Al2O3 powders containing 20, 30, 40wt% Al2O3, synthesized by hydrolysis of metal alkoxide, was examined. The relationship between the chemical composition and the properties, such as thermal, electrical and mechanical properties, of the sintered bodies has been studied. These powders densified to almost full density at 1300°-1350°C for 4h without forming cristobalite. When heated above 1350°C, however, crystallization of cristobalite was recognized which resulted in the increase in thermal expansion of the sintered body due to α-β phase transformation of cristobalite. The densification rate increased with increasing SiO2 content in the powder. The sintered bodies consisted of very fine acicular mullite particles and glassy matrix. The aspect ratio of the mullite particle increased with the decrease in Al2O3 content. The dielectric constants, thermal expansion coefficients and flexural strength of the sintered bodies containing no cristobalite phase were 4.3-5.3 (at 1MHz), 1.8-2.9×10-6/C (30°-400°C) and 124-201MPa, respectively. These values seem to be more advantageous in application for the insulating substrate material than those of conventional alumina ceramics. The above mentioned values increased with the increase in Al2O3 content which determines the amount of mullite phase in the sintered body. Consequently, the optimum property values can be designed by adjusting Al2O3/SiO2 ratio in the starting powder.
TL;DR: In this article, the homogeneous broadening of the 2 E→4 A 2 transition on Cr 3+ ions in high-field sites in a mullite ceramic and in the mullite mother glass are measured and compared.
TL;DR: In this paper, the temperature dependent solubility of titanium, iron and magnesium in mullites coexisting with SiO 2 -rich partial melts in a used chamotte brick was studied with a high-resolving microprobe.
TL;DR: In this article, the microstructural evolution of mullite/zirconia and mullite alumina and zinc/alumina composites after different duration heat treatments at 1570°C has been investigated by scanning electron microscopy (SEM), transmission electron microscope (TEM) and energy dispersive X-ray microanalysis.
01 Jan 1987
TL;DR: In this paper, a silicon carbide whisker reinforcement of ceramic matrices is proposed to increase flexure strength and fracture toughness as compared to the monolithic materials, and the reinforced alumina has been shown to have good resistance to thermal shock, slow-crack growth and creep.
Abstract: Silicon carbide whisker reinforcement of ceramic matrices is currently of great interest because of the potential to use these composites as high temperature structural materials. In particular matrices of alumina and mullite reinforced with 5–60 vol.% whiskers have been shown to have dramatic increases in flexure strength and fracture toughness as compared to the monolithic materials.1,2 In addition, the reinforced alumina has been shown to have good resistance to thermal shock,3 slow-crack growth and creep.5
Patent•
26 Oct 1987
TL;DR: In this paper, a coating is used to prevent oxidation of the silicon carbide whiskers within an alumina matrix by sealing off the exterior of the matrix so as to prevent oxygen transport into the interior.
Abstract: Ceramic composites formed of an alumina matrix reinforced with silicon carbide whiskers homogenously dispersed therein are provided with a protective coating for preventing fracture strength degradation of the composite by oxidation during exposure to high temperatures in oxygen-containing atmospheres. The coating prevents oxidation of the silicon carbide whiskers within the matrix by sealing off the exterior of the matrix so as to prevent oxygen transport into the interior of the matrix. The coating is formed of mullite or mullite plus silicon oxide and alumina and is formed in place by heating the composite in air to a temperature greater than 1200° C. This coating is less than about 100 microns thick and adequately protects the underlying composite from fracture strength degradation due to oxidation.
TL;DR: In this paper, the influence of these factors differs in sign ( + or −) and magnitude and the predominance of some of these causes mullite to melt incongruently in certain experiments while in others it melts congruently due to the predominating reverse factors.
Patent•
24 Jun 1987
TL;DR: The method of making refractory ceramic products in which a slurry is formed of water, powdered alumina, and amorphous fibers containing AL 2 O 3 and SiO 2, is described in this article.
Abstract: The method of making refractory ceramic products in which a slurry is formed of water, powdered alumina, and amorphous fibers containing AL 2 O 3 and SiO 2 , the volume of water in the slurry being sufficient to permit the slurry to flow under vibration but insufficient to permit the slurry to flow without vibration, the fibers having a molecular ratio of Al 2 O 3 to SiO 2 substantially less than 3 to 2 and the molecular ratio of Al 2 O 3 to SiO 2 in the combined slurry being at least 3 to 2, thereater placing the slurry in a liquid absorbing mold and vibrating the slurry to fill the mold, then permitting the mold to absorb a portion of the water from the slurry to form a body, then removing the body from the mold and drying the body, and thereafter heating the body to combine silica and alumina from the fibers and powdered alumina to recrystalize the fibers in the from of mullite.