12.1 Design of Monodisperse Ceramic Powders
Many of the current limitations to the performance of ceramics in high -technology areas are a direct consequence of non -optimum physico -chemical properties of the powders used in their fabrication. The average particle size, particle size distribution, degree of agglomeration, and dispersion characteristics of the precursor powders combine to limit the macroscopic homogeneity of the ceramics in the green and fi nal sintered states. The microstructural homogeneity of the ceramics can be optimized by processing powders with tightly controlled characteristics. Monodisperse metal oxide powders derived from sol -gel processes are prime examples of these materials. Such powders offer many processing advantages over conventional ceramic powders, since their spherical shape and narrow particle size distribution allow for a tight control over the packing density of the powder particles in the green ceramic. Since there are very few agglomerates, the particles pack very uniformly, with residual pore sizes in the green ceramic often being on the order of just two particle diameters -that is, less than 1 ยต m. While dispersions of those powders settle only slowly, very high green densities are attainable due to statistical ordering of the particles with residual pore size that often are on the order of one particle diameter. This is a crucial advantage, since residual porosity or agglomerates in a fi red structural ceramic will act as crack initiation sites or create defects in the material, hence lowering its overall performance and strength. In processing conventional ceramic powders, it is very diffi cult to eliminate any residual porosity and fl aws that might result from poor powder packing and the presence of agglomerates. Densifi cation is typically accomplished by applying pressure and/or adding sintering aids to control grain growth (see Chapter 2 ). Processing by hot isostatic pressing ( " HIPing " ) is extremely costly and therefore undesirable, and is also limited to simple geometric shapes. The addition of sintering aids can adversely affect the hightemperature properties of the material by lowering the melting point, and therefore is also undesirable. With monosized powders, such processing steps are not necessary to achieve good densities, microstructural uniformity, and superior sinterability.