Showing papers on "Mullite published in 1969"

Transparent glass-ceramics

Abstract: Glass-ceramic materials may transmit visible light if either of the following conditions are operative: (i) the crystallites of all species are much smaller than the wavelength of visible light, or (ii) the optical anisotropy (birefringence) within the crystals and refractive index difference between crystals and glass are very small. These conditions are achieved in several aluminosilicate glass-ceramic systems. Solid solutions (ss) of β-quartz, spinel, and mullite are the major crystalline phases in these transparent glass-ceramics. The transparent β-quartz solid solution glass-ceramics can be divided into three areas: ultra-low expansion materials, refractory and colourless materials, and high electrical resistivity—low dielectric loss materials. These three material groups, though chemically distinct, are all characterised by high crystallinity, thermal shock resistance, excellent chemical durability, and susceptibility to mechanical strengthening by ion-exchange techniques. The transparent spinel and mullite glass-ceramics contain considerable glassy phase, but nevertheless are characterised by excellent transparency, even after thermal exposure for long periods above 1000° C.

Development of Noncrystalline Material in Subsolidus Reactions Between Silica and Alumina

Abstract: Reactions between corundum and cristobalite in mixed powders and at interfaces between sintered pellets at 1500°C and in various forms of reaction couples at 1550°C were studied by quantitative X-ray diffraction, electron microprobe analysis, and optical microscopy. The results indicate that a noncrystalline product is an important transitional phase in reactions under subsolidus conditions. When corundum is used as a coarse powder (particle size 20 to 30 μm), the amount of noncrystalline material produced can be as much as 30 to 40% of the total reactants, and the amount of mullite is correspondingly small. When corundum is used as a single crystal, the development of a noncrystalline phase is notably increased. Microprobe analyses of coarse powders reacted at 1500°C show a noncrystalline phase of the approximate composition Al2O3·12SiO2. Calculations based on the X-ray analyses indicate a higher proportion of alumina in the noncrystalline material, and it is suggested that alumina-rich zones may form adjacent the corundum surfaces where they are not resolved in the microprobe analysis.

Kinetics and Mechanism of High‐Temperature Reactions of Kaolinite Minerals

Abstract: The kinetics of mullite growth from kaolinite and halloysite at high temperatures were studied by quantitative X-ray diffraction analysis. The evidence in both cases is consistent with a nucleation-controlled model.

Strengthening Sapphire by Compressive Surface Layers

Abstract: Compressive surface layers were formed on sapphire rods. The methods included glazing and quenching, formation of Al2O3–Cr2O3 solid solution surface layers, and formation of mullite and calcium aluminate surface layers. The presence of the compressive stresses was demonstrated by slotted rod tests. Substantial improvements in flexural strength and thermal shock resistance were observed. The highest average flexural strength was obtained by glazing and quenching and was 340 000 psi. The strengths were measured in the ``as‐treated'' and ``as‐treated and abraded'' conditions. The abraded samples retain a substantial portion of the improved strength.

The examination of ceramics by x-ray powder diffraction

Abstract: By using X-ray powder diffraction to identify the crystalline constituents of ceramics, especially porcelains, a considerable amount of information can be obtained from a very small sample The characteristic mineral in hard-paste porcelains, stonewares and highfired earthenwares is shown to be mullite, an aluminium silicate, 3Al2O3 2SiO2, (AS:TM 15–776); where there is a high calcium content aluminium silicates may occur eg anorthite, CaO Al2O3 2SiO2, (ASTM10–379) and gehlenite 2CaO Al2O3 SiO2, (ASTM9–216) Among the soft-paste porcelains, whitlockite, β calcium orthophosphate, Ca3(PO4)2, (ASTM 9–169), is typical of those containing bone-ash, and enstatite, magnesium meta silicate, MgO SiO2 (ASTM7–216) is typical of those containing soapstone The glassy-frit porcelains generally contain a calcium silicate, wollastonite or pseudowol1astonite, CaO SiO2, (ASTM10–487 and 10–486) In addition, silica–either as α quartz or cristobalite–may occur in varying proportions i

A Mossbauer study of the role of iron impurities in the high temperature reactions of kaolinite minerals

Abstract: Mossbauer spectroscopy was used to study the valence and bonding changes of iron in (a) lattice sites, and (b) cation exchange sites during thermal treatment of kaolinite and halloysite. Lattice iron which is initially in octahedral sites occurs in octahedral and tetrahedral sites at the completion of the thermal reactions. At tempera- tures below 650~ some divalent ions in cation exchange positions become trivalent and assume octahedral co-ordination, which they retain until the later stages of mullite formation. The implications of the work on the high temperature reaction mechanism and structural sequence are discussed.

Process for the production of spherical alumina-silica-containing solid particles which are predominantly mullite

Abstract: A process for producing spherical alumina-silica-containing solid particles which comprises introducing a powdered raw material (A) containing alumina and silica as well as a compound selected from the group consisting of magnesia and magnesium compounds capable of forming magnesia under burning conditions to a fluidized bed with the forced circulation flow of the solid particles, in the presence of a solid seed material (B) having a particle diameter one-tenth to two-thirds that of the product particles and consisting essentially of silica, alumina and magnesia; burning said powdered raw material (A) and said seed material (B) in said fluidized bed to coat said seed material (B) with said powdered raw material (A); and thereafter recovering the resulting solid particles which are predominantly mullite.

Infrared Kinetic Study of High‐Temperature Reactions of Synthetic Kaolinite

Abstract: An infrared frequency shift method was used to follow the kinetics of conversion of dehydroxylated synthetic kaolinite to mullite. Comparison of the kinetic and activation thermodynamic data for natural and synthetic kaolinite suggests that the mechanism of high-temperature reaction is similar, although the activation energy of synthetic kaolinite is larger than that of natural kaolinite because of the lack of nucleation-promoting impurity cations in the former. The high-temperature ir data indicate that there is little or no difference in the cation coordination of the phases intermediate to metakaolinite and mullite, but that the principal high-temperature coordination change (probably the accommodation of aluminum in well-defined tetrahedral sites) occurs during the later stages of mullite formation.

X-Ray Study of Induced Mullitization of Clay

Abstract: The influence of alkalies, alkaline earth oxides, Fe2O3 and TiO2 on mullitization of a pure kaolinitic clay was studied by quantitative X-ray diffraction measurements, using γ-alumina as the internal standard Earlier investigations indicate that the yield of mullite varies, within wide limits, with the concentration of the mineralizers The results show that the amounts in which these impurities are generally present in clays have profound influence on the process of mullitization

Process for the preparation of mullite bonded refractory materials

Abstract: A process for making (1) a mullite binder precursor powder, (2) mullite, and (3) mullite bonded refractory materials is disclosed. The contribution to the art lies in the discovery that mullite can be made by firing a dry mixture of silica and aluminum chlorhydroxide at temperatures ranging from 1,050* C. to 1,300* C.

Chemical and Thermal Characteristics of Allophane in Andosols of the Tropics1

Abstract: Profile characteristics, chemical properties, and clay mineralogy of volcanic ash soils in the tropics were studied. The soils were classified as andosols or andepts. The properties studied varied somewhat depending on the pedogenic age. The younger soil had an AC profile and contained amorphous material, tentatively called pro-allophane, mixed with pedogenic intergradient vermiculitic clay, especially in the subsoil. The older soils had thicker AC or ABC profiles with clay fractions dominated by allophane. Whereas pro-allophane was inverted into mullite at 800C, allophane showed the normal thermal reaction without being inverted into mullite. Titration curves of the clay fractions also indicated differences in chemical behavior.

Zeolite a synthesis

Abstract: KAOLIN IS CALCINED AT A SEVERITY SUFFICIENT TO PREPARE AMORPHOUS DE-EXOTHERMED KAOLIN SUBSTANTIALLY FREE FROM MULLITE. A MIXTURE OF ABOUT 4.3 PARTS OF SUCH DE-EXOTHERMED KAOLIN AND 1 PART OF ALUMINA ALPHA TRIHYDRATE IS PREPARED AND DISPERSED IN A SODIUM HYDROXIDE SOLUTION AT AMBIENT TEMPERATURE. THE FRESHLY PREPARED SOLUTION IS HEATED TO ABOUT 100* C. AND MAINTAINED AT HOT AGING CONDITIONS FOR FROM ABOUT 10 TO 20 HOURS. TO PREPARE A PRODUCT COMPRISING SIGNIFICANT AMOUNTS OF ZEOLITE A IN AN ALUMINOSILICATE MATRIX.

Corrosion of Mullite Refractory by Alkaline Vapor

Abstract: Mullite specimens were subjected to corrosion at high temperatures by alkaline (Na2CO3) vapor and many kinds of reaction products were formed containing corundum, nepheline, carnegieite, carnegieite-sodium aluminate solid solution and sodium aluminate.In the first stage of the corrosion process, mullite decomposed into carnegieite and corundum. As the supply of alkaline vapor was continued, the latter gradually changed into sodium aluminate, which in turn dissolved into carnegieite to form carnegieite-sodium aluminate solid solution.In the low content of sodium aluminate, that solid solution could be converted, on cooling, to nepheline structure like as the case with pure carnegieite. With increasing amount of sodium aluminate, it was found to disturb this conversion and to stabilize the solid solution as carnegieite.The final crystalline product by the corrosion was sodium aluminate. However, after a long time of exposure to the alkaline vapor, this phase gradually changed into glassy phase resulting in the wear of mullite refractory.

Hydrothermal reactions and crystallographic relationships found during the reactive hot pressing of kaolinites

Abstract: Water released by dehydroxylation was trapped in clay compacts during reactive hot pressing. The dynamic conditions of water pressure and temperature produced in the compacts modified the high temperature reactions of kaolinite. Kaolinite was converted to pyrophyllite or mullite in the temperature range 600-700~ depending on the applied pressure. Oriented aggregates of clay were used to determine orientational relationships between kaolinite and the high temperature phases. Structural continuity was observed in the reactions through the basal planes of kaolinite and pyrophyllite to the (130) plane of mullite.

Copper base friction material with dispersed spinels

Abstract: 1,105,998. Sintered copper base friction material. BENDIX CORPORATION. Oct. 6, 1966 [Oct.15, 1965], No.44725/66. Headings C7A and C7D. A sintered copper base friction material suitable for use in heavy duty brakes, includes from 0A2 to 25% by weight of a spinel. The spinel may be a chromate or an aluminate spinel and the material may contain Pb, Sn, Sb, Mo, steel fibre, silica, mullite and graphite in addition to the copper. The mixture may be sintered in exothermic gas and thereafter carried prior to fixing to the backing (e.g. by rivetting).

Improvements in or relating to Refractory Articles

Abstract: 1,165,384. Isostatic pressing. G. L. WILLAN Ltd. 1 Sept., 1967 [3 Sept., 1966], No. 39479/66. Heading B5A. [Also in Division F4] A crucible or furnace lining such as bricks or tiles is made from a mixture of a refractory filler and binder by isostatically pressing the material against a former with a pressure of at least 1 ton/sq. inch and firing the material. The former shapes the surface of the crucible or bricks which will contact the working fluid. The refractory filler may be fused alumina, fused magnesia, zirconia, mullite, sillimonite or silica and the binder may be aluminous clay, borax, lime or silicates. The crucible may be pressed in the apparatus shown comprising a polished steel core 1, supported in a perforated canister 2 by a metal framework 3. The canister is lined with a flexible bag 5 and the refractory material 6 is sealed from the pressing fluid by a steel ring 7 which shapes the top of the crucible and a sealing ring 8.