LOCAL CHEMICAL BONDING AT GRAIN BOUNDARY OF Si_3N_4 CERAMICS

摘要

We perform a first principles molecular orbital calculation with special interest on the local chemical bonding at the grain boundary of Si_N_4. The computational result is twofold: 1) On the basis of our calculations using model clusters, the Si-L_(23) edge ELNES of the intergranular glassy film reported in literature is interpreted; The N/(O+N) ratio of the intergranular glassy film is found to be in the range of 30 to 40%. A model of atomic arrangement at the interface between the prismatic plane of β-Si_3N_4 and the glassy film is proposed which has no broken bond at the interface. 2) Magnitude of chemical bond-strength around rare-earth ions at an interstitial sites in bulk Si_3N_4 is examined by the calculation of bond overlap populations. The difference in solubility of rare-earth ions between α- and β-Si_3N_4 matrix is well explained. Finally the bond-strength around rare-earth ions at the interface between β-Si_3N_4 and intergranular glassy film is examined. The rare-earth ions are implied to be more stable at this interface than at the interstitial site of the bulk crystal. The grain-boundary bond-strength is suggested to be weakened when large rare-earth ions such as La~(3+) are present at this interface. This computational result explains why the interfacial bond strength varies with ionic radius of rare-earth ions in doped β-Si_3N_4 ceramics.