Exceptional ceramic parts having laser optical transparency have been fabricated and are commercially available. These parts are typically made starting with a very pure co-precipitated powder which is then slip cast in the presence of a gelling agent to form the green structure or preform prior to sintering. A uniform slurry of high purity powder is poured into a plaster mold which sucks the water out of the slurry by capillary forces and produces the green structure after drying. Using fluid flow and surface tension to consolidate the ceramic powder allows parts to be made with a uniform powder packing. However, because the mold removes the water, slip casting can only be used for relatively thin parts. The need for a very porous surface on the mold also introduces another variable in the green structure fabrication. The porous mold usually made of commercial gypsum may also be a source of contamination. Moreover, the presence of the gelling agent, or its by products, in the final structure is an impurity that adversely affects the optical properties of the ceramic. Cold uniaxial pressing and cold isostatic pressing have also been used to make transparent parts. However, inter-particle friction during the pressing process tends to prevent densification in the center of the part so that size of the part must be kept small enough that this does not cause porosity.
In order to increase the driving force for sintering, a finer nano-sized powder than that produced by precipitation may be used. This can be especially important for achieving high transparency needed for lasers. Finer particles because of their increased surface area sinter more easily. Very small trapped pores are also less effective in scattering light.
However, smaller nano-sized particles behave differently than larger (such as micrometer) sized particles during green structure consolidation. For instance, smaller particles experience more friction as they move past one another in a die making it more difficult to produce a uniform structure through cold pressing, especially where larger parts are desired. The higher surface area of finer particles also requires more water for wetting making it difficult to get the solids loading high for slipcast slurries. As a result, after slip casting there is significant shrinkage on drying often leading to cracking and other problems. Finer particles are more susceptible to surface-area-dependent chemical reactions, as may occur between a porous mold and certain ceramic powders. Therefore, a method of preparing transparent ceramics using nano-sized particles without the problems associated with traditional methods would be very beneficial to the advancement of this field.