Investigation On Infrared-radiative Composite Ceramics Of Mullite

High efficient infrared radiative composite ceramics of mullite-ferrite were studied and obtained.Firstly, we investigated the sintering condition of the mullite, in which the synthesis temperature should be above 1500℃. The relationship between the mullite sintering temperature and its production was explored. It was found that the composite of mullite and SiO2 can be easily obtained, and the doped Y2O3 evidently could reduce the sintering temperature and change the crystal lattice constant a little. Furthermore, the resistence to thermal shock was improved by doped Y2O3.Wo also investigated the sintering conditions of ferrite with spinelle structure, The sintering raw materials were composed of CuO andα- Fe2O3 with different ratios. It was found the sintering temperature was dependent on this ratio. The structure for the obtained composite were revealed XRD. The results showed that the ferrite with a spinel structure was obtained by using CuO andα-Fe2O3 , whichwas affected by the ratio CuO/α-Fe2O3. As the amount of Fe2O3 is too larger, the ferrite with inverse-spinel structure was easily obtained, whereas that of CuO is too larger, the ferrite with spinel structure was aquired.Infrared-radiative ceramic materials for grain-drying were prepared based on the characteristic infrared absorbance of grains. The sintering raw materials were mainly composed of Al2O3 and SiO2, while Fe2O3, CuO and Y2O3 were used as doping materials. The infrared radiation for the ceramics(mullite and doped mullite)was in a wave band range from 2.5 to 15μm, and its efficiency varied from 0.845 to 0.933. Optimal compositions, which was determined by using an orthogonal experimental design method, were Al2O3 71.8%,SiO2 28.2 %, Fe2O3 2.5%, CuO 2.5%, and Y2O3 2.5%.The structures for obtained samples and their dependence on the dopants and doped amount were characterized by XRD, The percentages of mullite in the specimens ranged from 70% to 90%. It was found that while the amount of the doped CuO increased , the growth grate for mullite dropped dramatically, but the doped Fe2O3 and Y2O3 had no significant influence on the growth rate. The reduction in growth rate by doped CuO is caused by a ralatively high degree of lattice distortion for mullite, This doping thus increased the Gibbs energy, and decreased the difference of Gibbs energy between Al2O3-SiO2 and mullite, resulting in a lower "driving force" in the reaction. On the other hand, the doped Fe2O3 and Y2O3 caused an insignificant lattice distortion.The infra-red radiation for the ceramics was measured, which showed that the doping of Fe2O3, CuO, Y2O3 increased the efficiency for the infra-red radiation ofmullite significantly. Fe2O3, CuO and Fe2O3 influenced the infrared radiation for mullite differently. The optimal dopping ratio of Fe2O3, CuO, Fe2O3are 2.5%,5% and 2.5%, respectively. The efficiency for the infra-red radiation of the specimen reached 0.938 in the wave band from 5 to15μm which was due to that the ferrite with a spinel structure was favored by doping Fe2O3 and CuO. The lattice distortion of mullite caused by the doping Cu2+ also contributed to the increase in the efficiency for the infrar-ed radiation.In our experiment, the increase in the efficiency for the infra-red radiation with increasing the amount of doped CuO could be ascribed to the formation of ferrite and lattice distortion.At first, the doped Fe2O3 involved in the formation of ferrite,leading to the enhancement in the efficiency for the infra-red radiation. However, overdose doping had a negtive effect in enhancing the efficiency for the infra-red radiation, which was due to that overdose doping Fe2O3consumed more CuO, leading to the reduction in the lattice distortion caused by doping Cu2+. Furthermore, the overdose doping Fe2O3 might prevent CuO from entering the crystal lattice of mullite, reducing the contribution from Cu2+ to the efficiency for the infra-red radiation.The reason why inclusion of Y2O3 was not obviously related to the efficiency for the infra-red radiation could be that the radius of Y3+ ion was too large to enter the crystal lattice of mullite, and a significant lattice distortion could not appear in mullite by doping Y2O3. Thus, doping Y2O3 had not a dramatic impact on theefficiency for the infra-red radiation of sample. However, a small amount of inclusion of Y2O3 notably decreased the synthesis temperature and improved the resistance to thermal shock as well.