Development of Highly Anisotropic Porosity in YAG Ceramic by Unidirectional Freeze Casting Method

Abstract

In recent years, porous ceramics have been widely used in many applications. Among the fabrication techniques to produce porous ceramics, freeze casting has recently shown much attention due to its unique ability to form highly anisotropic porosities throughout the structure. Yttrium aluminum garnet (YAG) has been chosen as for the fabrication of anisotropic porous ceramics since it is an important advanced structural and functional materials. The present study is broadly divided into two parts. The first part of the work involves the synthesis of phase-pure YAG by using wet-chemistry (co-precipitation) and taking yttrium and aluminum nitrates as precursors. The co-precipitated powder was subsequently calcined at 1100oC for 1 hour, subjected to high-energy milling to reduce the particle size to below 5 μm. The second part of the thesis comprises of unidirectional freeze casting of the YAG suspensions in a custom-built freeze-casting set up at different freezing temperatures, viz., -10 to -50oC at an interval of ten degrees. The resultant microstructures of the horizontal surfaces (i.e. at a direction perpendicular to ice-growth direction) at all freezing temperatures exhibited lamellar structures with long range structural ordering. The structural wavelength (one of the critical parameter to define the microstructure) and the lamella thicknesses followed the negative power law with the exponents of 0.71 and 0.33 respectively, with the freezing front velocity. Further investigation into the details of the microstructure revealed that, it gradually undergoes from planar (unsteady state) to lamellar (steady-state) structures through a transition state. Unidirectional freeze-casting was also performed at different concentration of YAG suspensions, viz., 10, 20, and 30 Vol% and the corresponding microstructures were examined. Compressive stress of the porous samples exhibited an increasing trend with increase in solid loading. While at constant solid loading higher strengths were obtained by decreasing the structural wavelengths. These highly anisotropic porous YAG ceramics may find their suitable applications in high temperatures filters, high temperature catalysts.