Sintering Of Mullite Research Articles

Physical chemistry and technology of mullite-corundum refractories for casting special steels

ABSTRACT The article summarises the research on the theoretical foundations of the mullite-corundum refractories technology used in casting special steels by the siphon method. The resistance of these refractories to metal jet casting is achieved with a minimum content of residual silica glass in the bond after firing. To search for the factors providing this condition of work, we focused on the following areas: synthesis and sintering of mullite in the presence of CaO and TiO2 mineralisers; structural properties of invariant compositions of melts of the CaO–Al2O3–SiO2–TiO2 system; physicochemical model of mullite synthesis in the presence of high-silica melt and the kinetics of this transformation; improvement of refractory production technology.

Evaluation of the Influence of MgO and La2O3 on the Fast Sintering of Mullite

The influence of MgO and La2O3 on the fast sintering of mullite was evaluated in this work. High-purity mullite was doped with MgO and La2O3, conventionally sintered, fast sintered, and microwave-sintered in rapid sintering cycles. The results showed that MgO and La2O3 strongly affected the densification of fast sintered samples; and only doped bodies reached high densities when fast sintered. However, the densification behavior of doped samples was dependent on the amount of sintering aids. MgO doped samples presented better densification behavior than La2O3 doped samples. Microwave-processed samples required a balance between power, time and amount of additives to obtain highly dense and homogenous microstructures.

Effects of SiO<sub>2</sub> and Fe<sub>2</sub>O<sub>3</sub> on Morphological of Mullite

Effects of SiO2 and Fe2O3 on morphological of mullite were investigated for a special use related to mullite containing SiO2 and Fe2O3. It was concluded that, when Al2O3/SiO2 ratio near the theoretical component of mullite, the composites cannot been sintered at 1600°C. The normal morphology of mullite presents granule form with Fe2O3 addition, while takes a long column shape with the condition of excessive. SiO2 presenting. Fe2O3 can promote sintering of mullite, as well as growing in needle-like morphology.

Microwave-assisted synthesis and sintering of mullite

Mullitization behaviour of a mixture of clay and alumina as the starting materials was examined by microwave heating of (a) mixed powder and (b) compacted powder samples for different soaking times. X-ray diffraction results showed that in compacted samples mullitization process was completed after 20 min heating with a density of about 87%. Densification and microstructure of samples with different green densities heated in a microwave oven and conventional electric furnace were compared. Results showed that the grain growth of mullite was restricted by microwave heating.

Topaz-Activated Solid-Phase Sintering of Mullite

The solid-phase topaz-activated sintering of mullite from stoichiometric mixtures of pure aluminum and silicon oxides is studied. The role of topaz in the mechanism of synthesis and sintering of mullite is discussed in detail.

Novel Microwave Method for the Synthesis and Sintering of Mullite from Kaolinite

A novel and potentially economical microwave route has been developed for the synthesis and sintering of mullite from the naturally available minerals kaolinite and alumina. Mullite obtained by this route could be sintered to as high as 98% of theoretical density in under 60 min. A detailed structural investigation has been carried out using XRD, IR, and MAS NMR. It is clearly evident from the structural studies that there is a partial conversion of octahedral Al to tetrahedral Al with time during the formation of mullite. Also, it is evident that the exsolved SiO2 reacts with the added Al2O3 to form secondary mullite over primary mullite which is nucleated from the decomposition of kaolinite. A detailed microstructural evolution of this conversion has been followed. A possible microwave−material interaction has been discussed for the rapid formation of mullite.

Microwave-assisted preparation and sintering of mullite and mullite–zirconia composites from metal organics

Decomposition characteristics of a mixture of aluminum-tri-sec butoxide (ASB) and tetraethyl orthosilicate (TEOS) in ethyl acetate solution by microwave pyrolysis was examined. The decomposed product was used as a second-stage precursor in the preparation of pure mullite (3Al 2O 3·2SiO 2). Mullite–zirconia composites were obtained, using the aLtd mixture along with zirconium-iso-propoxide as starting materials. Pellets of mullite and mullite–zirconia composites were sintered in a microwave oven using β-SiC as a secondary heater.

Sintering of Mullite–Zirconia Composite Powder Prepared by Ultrasonic Spray Pyrolysis Technique

Sinterabilities of mullite (3Al2O3·2SiO2–zirconia (ZrO2)composite powders prepared by ultrasonic spray pyrolysis techniques (USPTs) were examined. Starting mullite powders containing ∼18.5 mol% (∼15.0 vol%) of zirconia (ZrO2) were prepared by single-nozzle (SN) and double-nozzle (DN) USPT. In SN-USPT, the composite powder was prepared by spray pyrolysis of a water–ethanol solution in an Al(NO3)3–Si(OC2H5)4–ZrOCl2–YCl3 system at 900°C, using one ultrasonic vibrator. In DN-USPT, the composite powder was prepared by simultaneous spray pyrolyses of a water–ethanol solution in an Al(NO3)3–Si(OC2H5)4 system and one in a ZrOCl2–YCl3 system at 900°C, using two ultrasonic vibrators. When these composite compacts were fired at a temperature between 1400 and 1700°C for 5 h, the relative densities attained maxima, i.e., 87.2% (SN-USPT) and 95.5% (DN-USPT), at the firing temperature of 1500°C. Densification of the DN-USPT-derived powder compact proceeded more markedly than that of the SN-USPT-derived powder compact.

Microstructures and mechanical properties of mullite-(yttria, magnesia- and ceria-stabilized) zirconia composites

The microstructures and some mechanical properties of composites containing mullite and each of three different zirconias stabilized with low concentrations of yttria, magnesia, and ceria, have been studied. A sol-gel derived, high-purity, mullite was used as a matrix phase. In the present study, composites were prepared by conventional sintering of mullite and zirconia milled powder mixture. In all the composite materials, large fractions of the tetragonal zirconia (ZrO2) transformed into monoclinic form during cooling from the fabrication temperature. In the use of ceria-stabilized ZrO2, large internal macroscopic voids appeared in the sintered body. The thermal expansion hysteresis associated with the tetragonal/monoclinic transformation was evident only in the mullite/yttria-stabilized ZrO2 composite from which the Ms temperature could be determined.

Sintering of Mullite Prepared through Two Routes

In the present investigation, two different types of mullite, namely, electrofused and sol-gel derived materials, were selected to have a comparative study of their properties. The mullites were compacted at a pressure ranging from 300 to 350 MPa followed by sintering at 1650° and 1700°C for one hour. It was found that the sol-gel mullite showed better mechanical properties. Dielectric constant of the sample showed a direct relation with the sintered density. Higher A12O3 containing mullite showed higher thermal shock resistance.

Effect of rigid inclusions on the sintering of mullite synthesized by sol-gel processing

The effect of crystalline particulate inclusions of mullite or zirconia on the sintering and crystallization of a mullite powder matrix was investigated as a function of the inclusion volume fraction and size. The mullite powder was synthesized by sol-gel processing and, within the limits of X-ray diffraction, was amorphous. Composites containing up to 22.5 vol % zirconia reached almost full density after sintering at 1500 °C for 1 h. Under identical conditions, the sintered density of the composites containing crystalline mullite inclusions was considerably lower. The zirconia inclusions were inert but the mullite inclusions enhanced the independent nucleation and growth rate of the mullite crystals in the matrix. The lower sintering rate of the matrix reinforced with crystalline mullite is attributed to the enhanced matrix crystallization.

Effects of Rare Earth Oxide Addition on the Sintering of Mullite

Effects of various kinds of rare earth oxides (Y2O3, Er2O3, Sm2O3, La2O3 and CeO2) addition on the sintering of mullite and the precipitation behavior of secondary phases in the mullite sintered bodies were investigated. The sintering of the mullite compacts, to which a small amount of these rare earth oxides was added, was performed at 1500°-1600°C. The morphologies of the rare earth silicates precipitated and remained on the grain boundaries of the sintered bodies could be classified into three categories, i.e., crystalline, glassy and decomposed substances, depending upon the kinds of the rare earth oxides added. The microstructures of the mullite sintered bodies changed as the amount of the additives varied. The three point bending strength of the sintered mullite increased with the addition of 0.5wt% of Y2O3 or CeO2. On the other hand, excessive amount of these additives would result in the decrease of the three point bending strength. It was found that the maximum value of three point bending strength was evidently related to the melting point of the secondary phases existed in the sintered bodies of muilite.

Characteristics of solid-phase synthesis of mullite from oxides

The authors have investigated solid-phase synthesis of mullite and developed a procedure for quantitative x-ray analysis of mullite in two-phase mixtures of mullite and corundum. They have shown the advantage of joint vibrogrinding of the initial components as opposed to separate grinding during synthesis and sintering of mullite from quartz sand and commercial alumina. The onset of mullitization was noted at 1200°C, and complete mullitization at 1750°C (residence 16 h).

The gaseous medium as a factor in the formation and service of mullite and mullite-corundum refractories

In an investigation of the sintering of mullite and mullite-corundum chamottes in oxidizing and reducing media it was established that kaolin conversion and mullite formation set in a hydrogen atmosphere about 100°C sooner than in an oxidizing medium. At 1300–1500°C in a reducing medium the corundum crystallizes rapidly as a result of the break-up of the mullite so that the density and strength increase and sintering is delayed to some extent. High-density mullite and mullite-corundum chamottes can be obtained by firing up to 1700°C in an oxidizing medium. When tested in the lining of carbon black reactors and in the checker of a generator mullite-corundum refractories proved to be more durable than other industrial grade refractories. The saving accruing from their introduction is considerable. The maximum permitted service temperature in the linings and checkers of heat-using equipment should be governed by the proportion of reducing agent (H2, CO, C) in the gaseous medium.