Study On Synthesis And Preparation Of Corundum/Mullite/Gahnite-based Multiphase Materials

Gahnite is characterized by its prominent performances such as high melting temperature(1950℃), low thermal expansion coefficient(5.5 × 10-6/℃, 25-1000℃) as well as high thermal stability and good resistance to acid and basicity. In early 1993, Dr. Peter put forward an idea that it is possible to substitute gahnite for mullite used as the raw material for ceramic cup of blast furnace. It is well known that there exists a composition triangle in phase diagram for ternary system of Al2O3-SiO2-ZnO, responding to three high temperature phases of corundum, mullite and gahnite. Through control of chemical composition and technical process, it is possible to synthesize corundum-mullite-gahnite multiphase materials theoretically.This paper firstly reviewed the research literatures about the physical and chemical basis of the ternary system of Al2O3-SiO2-ZnO and correlated materials synthesis. Based on the review, the main objectives of this work had been determined.The thermodynamic analysis indicated that corundum-mullite-gahnite multiphase materials can be synthesized. The investigation of x-ray diffraction (XRD) and scan electron microscope (SEM) proved the validity of thermodynamics research. Investigation by thermogravimetry (TG), differential scanning calorimetry (DSC) and x-ray diffraction (XRD) indicated that using Al(OH)3 and ZnO as raw materials, the endothermic curve caused by dehydration reaction of Al(OH)3 and subsequently exothermic curve of Al2O3-ZnO synthesis reaction were overlapped. Non-isothermal kinetics study showed that the mechanism of ZnO-Al2O3 solid-phase reaction was well fitted with Cater's Equation. Active energy of solid-phase reaction derived from the experiments by the methods of Ozawa Equation avoiding the function of reaction mechanism was about 63.48KJ/mol, which was almost the same value of active energy deduced from Cater's Equation.Study on the synthesis of corundum-mullite-gahnite multiphase materials by one-step sintering process using high purity raw materials showed that the more the content of gahnite in multiphase materials, the high the volume expansion would be introduced in the system, which resulted in the sintering difficulty of the multiphase materials. While the content of gahnite was controlled about 30wt% and the content of mullite was about 70wt%, the densified multiphase materials can be obtained after fired at 1600℃ or 1700 ℃. The formation of new phase such as mullite and gahnite by solid-phase reaction in multiphase materials can benefit the sintering of materials at 1700℃, decreased apparent porosity to less than 5%. Reducing atmosphere inhibited the synthesis of mullite andgahnite, played a negative role in the densification of multiphase materials. Based on orthogonal designed experiment, the synthesis of corundum-mullite-gahnite multiphase materials by two-step sintering process showed that the light fired temperature and holding time mostly influenced the apparent porosity of the multiphase materials. Further study on the light firing time indicated that at the light firing temperature of 1300℃ and died fire temperature of 1600℃ for 3h, the optimal light fired time was about 1h. Study on the synthesis of corundum-mullite-gahnite multiphase materials by two-step sintering process using kaolinite as raw material showed that impurities introduced by kaolinite mainly increased the liquid amount at high temperature, which can assist sintering of specimen. The experiment results indicated that the samples with lower gahnite content had better sinterability than others. However, if the gahnite content was relative high, the promoting effect on sinterability caused by kaolinite was not significant. Study on the synthesis of corundum-mullite-gahnite multiphase materials by one-step sintering process using kaolinite showed that corundum-mullite-gahnite multiphase materials with the apparent porosity less than 5% can be obtained through sintering at 1700℃ for 3h. SEM observation showed that the phases in corundum-mullite-gahnite multiphase ma...