A Combinatorial Chemistry Study of YAG Nucleation
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A Combinatorial Chemistry Study of YAG Nucleation Randall S. Hay Air Force Research Laboratory Materials and Manufacturing Directorate WPAFB, OH 45433-6533, U.S.A. ABSTRACT A combinatorial method to screen large numbers of dopants that affect the nucleation of yttrium-aluminum garnet was developed. The method involves heat-treatment of electrontransparent sol-derived thin films on TEM grids, and counting the number and size of nuclei by TEM. Advantages and limitations of the technique are discussed. Preliminary results are presented. INTRODUCTION Polycrystalline yttrium-aluminum garnet (Y3Al5O12, YAG) is a promising material for ceramic fibers.(1-4) The creep resistance and microstructural stability of YAG are particularly attractive for high temperature structural ceramics. Thermal residual stresses are prevalent in common non-cubic structural ceramics such as alumina.(5) These stresses cause microcracking at large grain sizes and weaken the material,(6) but should be absent in YAG, which is cubic. Absence of such stress may account for the relatively high bend strengths of 1.7 GPa observed in developmental polycrystalline YAG fibers, with grain sizes of 0.7 to 3.2 µm.(3) Commercially available alumina and alumina-mullite fibers with higher strengths typically have grain sizes an order of magnitude smaller.(7, 8) Interest in YAG as an optical material is even more extensive. Nd-doped single-crystal YAG has long been used for lasers.(9) Recently there has been strong interest in polycrystalline YAG for highly efficient and economical lasers.(10-12) Densification of fine-grained polycrystalline YAG is difficult. The low diffusion coefficients of YAG inhibit densification.(1, 13-16) The high sintering temperatures (>1700ºC) used to make dense polycrystalline YAG for optical applications forms material with grain sizes >3 µm.(17-19) This grain size is too large for fine ceramic fibers, and furthermore, the dopants used to promote densification would be likely to have an adverse effect on the creep of polycrystalline fibers. Nucleation of YAG from sol-gel and solution precursors is typically sparse and site-saturated,(20, 21) and growth is spherulitic.(20) Typical nuclei densities are about 1/µm3.(20) This defines the smallest possible grain size, which at one micron is too large for densification without extensive grain growth.(22) To get the
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