of some P-Si3N4 and some iron, along with a-Si,N4, it appears that liquid iron silicide penetrated into the a-Si3N4 region and that part of the (Y phase dissolved in the iron silicide and reprecipitated as P-Si3N4. A similar process, involving solution in liquid Si, was discussed by Morgan.I6 This, then, represents another mechanism involving iron silicide by which @-Si3N4 might be formed in reactionsintered silicon nitride.
Characterization of Grain-Boundary Segregation in MgO 385
IV. ConclusionsA TEM study of reaction-sintered silicon nitride suggests that large @-Si3N, grains grow in liquid iron silicide, whereas the muchsmaller a-Si,N4 grains, which occur at some distance from the liquid, apparently grow from the vapor. Between the large @and small a-Si3N4 grains there is a zone where a-and @-Si3N4 coexist with iron silicide, suggesting that, in this region, a-Si3N4 is converted to the @ form by solution in, and reprecipitation from, liquid iron silicide. References ID. R. Messier and P. Wong, "Kinetics of Nitridation of Si Powder Compacts," 'S. S. Lin, "Mass Spectrometric Studies of the Nitridation of Silicon," ibid.. 'S. S. Lin. "Comoarative Studies of Metal Additives on the Nitridation of J . Am. Ceram. Sac., 56 [9] 480-85 (1973).58 [7-81 271-73 (1975).Silicon," ibid., 60 [1'2] 78-81 (1977). , "The a/P-Si,N4 Question," J . Mafer. Sci., 15 [3] 791-93 (I 980).Using Auger electron spectroscopy, segregation of Ca and Si corresponding to the adsorption of a partial monolayer adjacent the boundary in polycrystalline MgO was observed. Segregation of Sc is different in nature; the distribution consists of a 3.0-nm wide region corresponding to a positive space charge layer balancing a negative boundary charge. Various segregation models are applied to the observed behavior.
