Electronic Structure and Bonding of β‐SiAlON

Abstract

The ␣-and ␤-SiAlONs are ceramic solid solutions with charge-neutral substitutions in ␣-and ␤-Si 3 N 4 . They have high potential for applications as structural materials. We have calculated the electronic structure and bonding of ␤-Si 6 -z Al z O z N 8 -z for z ‫؍‬ 0, 1, 2, 3, 4 using a first-principles method. Total energy calculations show that the bulk modulus of ␤-Si 6 -zAl z O z N 8 -z decreases as z increases, in general agreement with experimental trends. Simultaneous substitution of the (Si,N) pair by (Al,O) results in impurity-like states in the upper portion of the bandgap of ␤-Si 3 N 4 . As z increases, more and more states are introduced into the gap, forming a new conduction band (CB) edge for SiAlON. At z ‫؍‬ 4, the calculated bandgap is ϳ1.3 eV. Density of states (DOS) calculations show the top of the valence band remains steep for all z, and the bottom of the CB is formed predominately by Si-O antibonding states. Orbitally resolved partial DOS calculations in the CB region are used to predict the trends of the electron-energy-loss near-edge spectra (ELNES) of Si-L 2,3 , Al-L 2,3 , Si-K, Al-K, O-K, and N-K edges in ␤-SiAlON. The impurity-like states near the CB edge result in pre-edge structures in all ELNES spectra. Effective charge and bond order calculations show that the overall bond strength in ␤-SiAlON decreases only slightly as z increases. Although the stronger Si-N bonds are replaced by weaker Al-O bonds, the remaining Si-N and Al-O bonds actually strengthen as z increases because of the effective charge redistribution after substitution. This is a very interesting finding that may partly explain the superior mechanical properties of the SiAlON system that render them suitable for structural applications.

FP-LMTO Studies of Ordered and Disordered ?-SiAlONs

“…The calculated dielectric gap values at x ¼ 1:5 are 4.1, 3.7 and 2.2 eV for Si 3 N 4 , d-and o-SiAlONs respectively. No new impurity-like isolated states near the CB edge reported earlier [5] were found in either o-or d-SiAlONs.…”

“…The calculated dielectric gap values at x ¼ 1:5 are 4.1, 3.7 and 2.2 eV for Si 3 N 4 , d-and o-SiAlONs respectively. No new impurity-like isolated states near the CB edge reported earlier [5] were found in either o-or d-SiAlONs.The atomic ordering effect in SiAlONs (Al, O atoms form extended 1D-like structures along the z-axis of the crystal as predicted in our earlier paper [6]) brings about an essential splitting of O2s,2p states along the G-K-M directions of the BZ. For the considered supercell, the cohesion energy of the o-Si 6Àx Al x O x N 8Àx (x ¼ 1:5) is 2.33 eV higher than that of the disordered state.…”

<i>Ab initio</i> Modeling of the Stress-Strain Response of SiAlON (Si<SUB>6-<i>z</i></SUB>Al<SUB><i>z</i></SUB>O<SUB><i>z</i></SUB>N<SUB>8-<i>z</i></SUB>, <i>z</i> = 0.5 and 1)

“…It is generally accepted that between the P6 3 =m and P6 3 configurations, both of which have been used to describe the -Si 3 N 4 unit cell structure, the difference is insignificant and results in similar data of the structural properties of the single crystal. 3,34) In this study the relaxed supercell structure was found to resemble the P6 3 =m symmetry configuration. As mentioned, the SiAlON structure is obtained by a substitution of Si and N pairs with Al and O, respectively.…”

“…Therefore, for the SiAlON polymorphs investigated, it is reasonable to accept, in all cases, a decrease in the 'ideal' strength of the lattice. 2,3,17) The decrease in the 'ideal' strength of the SiAlON polymorphs is, to a degree, expected and reasonable. What is even more interesting is the characteristic behavior, in the case of -SiAlON, of the stress-strain curve below the maximum induced stress limit.…”

“…2) It is well-known that the mechanical properties and fracture behavior of sintered silicon nitride composite materials, at elevated temperatures above 1200 C, are directly related to the intergranular glassy phase, which is mostly made up of oxides, at triple junctions or between the single crystal Si 3 N 4 grains (generally, -Si 3 N 4 particles). 3) Additionally, it is known that the thickness and properties of the intergranular phases (including other properties 4) ) can be altered, using various sintering additives, where in most cases rare-earth elements are used. The addition of the elements aluminium and oxygen into sintered materials effects the growth of grains and the strength of the crystalline-glass interface.…”