Pack cementation to prevent the oxidation of CoSb3 in air at 800 K

Abstract The skutterudite CoSb3 is a well-known thermoelectric material widely used to produce electricity from heat. This material is commonly employed under vacuum but in a near future it is expected to be used at high temperatures under oxidative atmospheres, e.g. in air. The lifetime of the material may be affected by the oxidative environment, considerably limiting the use of the thermoelectric equipment. The present research describes the degradation mechanisms of CoSb3 from oxidation experiments carried out under a flow of synthetic air at 800 K. The skutterudite material is progressively oxidized after 15 h, 50 h, 100 h and 1000 h of treatment, producing three oxides on the CoSb3 surface (CoSb2O4/CoO·Sb2O3, Sb2O4 and CoSb2O6). These three oxides have different growth kinetics and they are produced in various amounts as a function of the oxidation time. Next, as a solution for an appropriate oxidation protection, this work explores the chemical vapor deposition (CVD) process named pack cementation to synthesize a protective coating on CoSb3. This process produces a surface layer made of aluminum antimonide (AlSb) and cobalt aluminide (Al9Co2). The oxidation experiments carried out on the coated CoSb3 highlight the protective properties of this innovative surface layer. The coating is a protective barrier against oxygen that keeps the CoSb3 substrate unaffected by the flow of air at 800 K for 1000 h. Consequently, pack cementation is an efficient process to synthesize a protective surface layer that makes CoSb3 usable under oxidative environments.