Oxidation Behavior and Mechanical Strength of Mg₂Si Added with TiB₂

Abstract

Improving the oxidation resistance and mechanical strength of Mg 2 Si is an especially critical issue for practical use in thermoelectric devices. To solve this problem, a sintered sample was prepared by adding 1.1 mol% to 11.0 mol% of TiB 2 powder to Mg 2 Si (0.2 at% Sb) powder, and the oxidation resistance and mechanical strength were evaluated. The oxidation resistance was evaluated by raising the temperature from 300 K to 1023 K in a 200 Pa H 2 O atmosphere using an environmental scanning electron microscope and the sample surface condition was evaluated. In addition, the composition of the sample surface layer before and after oxidation was evaluated by energy dispersive X-ray spectroscopy (EDX). Mechanical strength was evaluated by a three-point bending test from 300 K to 823 K. Thermoelectric properties were measured from 300 K to 773 K.According to EDX analysis, the oxygen concentration in the sample without TiB 2 before and after thermal oxidation was 0.26 at% and 37.15 at%, respectively, while the concentration in the sample containing 3.3 mol% of TiB 2 was 0.02 at% and 6.59 at%, respectively. The bending strength of the sample without TiB 2 was 152.8 MPa at 773 K, and that of the sample with 3.3 mol% TiB 2 was 207.7 MPa at 773 K. The dimensionless figure of merit of Mg 2 Si (0.2 at% Sb) with 3.3 mol% TiB 2 was 0.49 at 773 K, which was the same as without addition of TiB 2 . It was found that the addition of up to 4.4 mol% of TiB 2 to Mg 2 Si can suppress oxidation and improve high temperature bending strength without affecting thermoelectric properties.