Simulation Of Using Process, Nano-improvement Of Corundum-Mullite Duplex Ceramic Kiln Furniture And Its Design And Preparation

With the rapid development of electric ceramics and components, due to such advantages of convenient operation, small temperature difference etc, pushing slab kiln gets widely attention and application in functional materials and components companies. Great progresses have been made in pushing slab production technologies for common ceramic sintering, but great differences of pushing slab for high temperature sintering of electric ceramics exist between us and foreign countries. With the continuously improvement of operation and equipments for functional ceramic materials and components sintering, the properties of higher reaction resistance, higher strength with lower weight, longer life etc become necessary, so that conventional pushing slabs such as clay, bauxite and Si₃N₄ bonded SiC can not satisfy the application requirements.Corundum-mullite duplex ceramic is a ceramic with corundum and mullite as raw materials, which combines the excellent characteristics of both materials, and it has excellent thermal shock resistance, better high temperature properties and outstanding chemical resistance. Therefore it has been applied as kiln furniture or fine ceramics in a lot of industries and becomes one of hotspots in high performance ceramics and refractories. However the present researches of the corundum-mullite duplex ceramic are mainly focused on composite proportion, granular match, moulding and firing, and few attentions are paid on the design and optimization of compositions, properties and micorstructure of duplex ceramics, resulting in the low properties and product stability, and the high temperature push slab long-termly rely on the importation.On the basis of the present state of corundum-mullite duplex ceramic, computational fluid dynamics (CFD) together with thermal conductivity inside solid was adopted to carry out mathematic simulation on the duplex ceramic in use, which obtains some characteristics on heating, distribution and variation of temperature field in duplex ceramic, and by means of which the compositions, microstructure and properties of duplex ceramic can be designed. Matrix of corundum-mullite duplex ceramic were prepared by total fine powders, the effect mechanism of micro-alumina and micro-silica addition and firing temperature on the matrix were discussed. The effect mechanisms of three factors (micro-alumina and micro-silica and firing temperature) on the properties of duplex ceramic were studied and optimized by orthogonal design method. Various solswere introduced into the duplex ceramic kiln furniture and the properties and microstructures of duplex ceramic by nano-improvement of sols and their mutual effect mechanisms were discussed. Corundum-mullite duplex ceramic push slab with high strength, low creep at elevated temperatures, good thermal shock resistance were produced, which achieves the design of the composition, microstructure and properties of duplex ceramic and satisfies the requirement in application. It is of important theoretical value and instructive meanings to the research and development of kiln furniture in high temperature industry and fine ceramic. Main results were as follows:1. Computational fluid dynamics (CFD) coupled with thermal conductivity inside solid was adopted to carry out mathematic simulation to the heating-up process of duplex ceramic push slab in kiln. The simulation indicated that obtained heat-flow density of push slab had silimar changes to the heat-up rate of klin wall, the heating-up rate at the surface of push slab presented a obvious delay compared to that of kiln wall, and the surface temperature of push slab was lower than that of sintering temperature of supporting product. When the temperature of kiln wall was 1680 ℃, the highest temperature of surface of push slab was 1350 ℃, the temperature difference between the surface and the interior (15 mm) of push slab was about 1150 ℃, which demands excellent thermal shock resistance on push slab, and it also indicates that thermal shock resistance is the most important index to push slab product. The temperature gradient of interia of push slab results in the tangential stress, once use of push slab indicates a thermal shock between air and high temperature, the thermal shock between air and high temperature of push slab must reach 120 times and the thermal shock between water and high temperature 60 times if the use times of push slab is required to reach 120 times. According to the above results, the composition (components, grains, matrix and additives), microstructure (micro crack, porosity, mullite skeleton and strengthen phase) and properties (thermal shock resistance, creep at elevated temperatures and physical properties were designed.2. The compositions, microstructure and properties of the corundum-mullite matrix were studied and the process parameters of corundum-mullite duplex ceramic klin furniture were optimized. The results showed that the density of the matrix decreased and the porosity increased with the increase of micro-silica addition, but the micro-silica addition had a little effect on the thermal shock resistance. The effect of micro-alumina powder on the properties of the matrix was related to the firing temperature and wasruleless. Firing temperature obviously increased the mechanical properties and decreased thermal shock resistance by affecting porosity and glass phase. Thermal shock resistance and other properties of duplex ceramic can be improved by controlling firing temperature and amount of micro-silica addition to adjust the formation of mullite and bonding state of matrix phases. On the basis of the studies on the matrix, the process parameters of the duplex ceramic kiln furniture were optimized by orthogonal design method, the effect mechanism of micro-alumina, micro-silica and firing temperature on the properties of duplex ceramic were analyzed and their interrelation were also discussed. The results showed that the effect orders of three factors were relate to the properties. For thermal shock resistance and creep, the effect order was: firing temperature> micro-alumina> micro-silica, and micro-alumina> micro-silica> firing temperature to mechanical properties. The optimization parameters of duplex ceramic were: micro-alumina of 11%, micro-silica of 3% and firing temperature of 1650 ℃. Under those conditions, the corundum-mullite duplex ceramic klin furniture was prepared with the density of 2.96 g/cm³, porosity of 18.5%, strength loss of 30% with water-colded at 1100 ℃ (64.7% with water-colded at 1200 ℃) and creep ratio of 0.99%. Micro crack, shape and size of pores, porosity, residual α-Al₂O₃, morphology of mullite and its bonding state with grains are the main effect characteristics of three factors on the microstructure and properties of duplex ceramic, by control of which thermal shock resistance, hot strength and creep of duplex ceramic can be improved.3. Various sols as substitution for polyvinyl alcohol were introduced into duplex ceramic kiln furniture as room temperature binder and their effects on the microstructure and properties were also discussed. Alumina sol, silica sol, aluminosilicate sol and zirconia sol can in-situ form nano or sub-micro meter alumina particles, mullite particles and zirconia particles, and improve the thermal shock resistance by toughening mechanisms of phase transition, nano-particles. The suitable introduction of sols promoted to the fining the grains of matrix and the formation of mullite phase, but too much addition increased the porosity, which affected the densification of duplex ceramic. With the addition of 3% alumina sol, the strength loss of duplex ceramic kiln furniture decreased from 64.7% to 49% (water-colded at 1200℃), and the thermal shock resistance increased by 25%.4. On the basis of above experimentals, corundum-mullite duplex ceramic push slab with excellent thermal shock resistance, low creep, and high wear resistance and strength, were produced. The push slab can be used at 1640 ℃ for more four months with a loadof eight kilogram in kiln, which is 1.5 times than the similar product prepared by Semens company in Germany. The duplex ceramic push slab has been produced more than 1000 tons. It is confirmed that the design of composition, structure and properties of corundum-mullite duplex ceramic has been realized to meet the requirement of high temperature kiln.