Processing and characterization of porous composites based on CaAl4O7/CaZrO3

According to previous research, CaAl4O7/CaZrO3 composites present good refractoriness, mechanical resistance and low thermal expansion coefficient (good thermal shock resistance), characteristics that insulating materials and lightweight structural components require. In this work, porous ceramics based on CaAl4O7/CaZrO3 were prepared from commercial powder combinations of m-ZrO2, Al(OH)3 and CaCO3. The powder mixture was processed by high-energy milling at different times (from 3 to 24 h). Different composites were obtained by solid state sintering at 1400 degrees C. The reaction was studied using thermal analysis techniques (TG-DTA), and the crystalline phases were analyzed by X-ray diffraction (XRD) and the Rietveld method. The resulting composites were also characterized. CaAl4O7 and CaZrO3 were the main phases present in these composites, with c-ZrO2 as secondary phase, and some scarce m-ZrO2. Porosity values varied from 54 to 59%, and flexural strengths ranged from 16.5 to 23.6 MPa. The increasing milling time produced an effective particle size reduction that accelerated the reactivity of the starting mixtures and reduced the grain size. The composite obtained by 24 h milling and sintered at 1400 degrees C (ACZ2414) presented the highest flexural strength and the lowest coefficient of linear thermal expansion. The mean thermal expansion coefficient of CaAl4O7/CaZrO3 composites was ca. 6 10-6 degrees C -1. This indicated that it is possible to manufacture ceramic bodies with expansion properties comparable to those of mullite, making it a suitable for insulation.