Enhancement of Thermal Stability of Filled Skutterudite Thermoelectric Materials through Nano-SiO_2 Coating

Single-phase Ba0.3In0.2Ni0.05Co3.95Sb12 bulk materials were synthesized by melting and spark plasma sintering(SPS) methods.Ba0.3In0.2Ni0.05Co3.95Sb12/SiO2 nanocomposite material with nano-SiO2 coating was prepared by wet chemical coating processing.Emphases were given on the evolution of microstructure,chemical composition,and thermoelectric properties of two kinds of(Ba,In) double-atom filled skutterudite based bulk materials during periodic thermal cycling 2000 times in the range of 300-723-300 K.It was found that the thermal stability of(Ba,In) double-atom filled skutterudite was remarkably improved after nano-SiO2 coated.The microstructure and chemical composition along the grain boundaries of single-phase Ba0.3In0.2Ni0.05Co3.95Sb12 bulk material obviously changed,while the thermal cycling had no effect on Ba0.3In0.2Ni0.05Co3.95Sb12/SiO2 nanocomposite.Thus the nano-SiO2 stabilized the grain boundary and prevented the elements diffusing and volatilizing from intra-grain.The ZT values of two kinds of materials were very close and kept unchanged at 300 K and 500 K during the thermal cycling.The ZT values of single-phase bulk materials gradually decreased at 800 K.However,the ZT values of nanocomposite mainly depended on the complex changes in thermal conductivity and increased with prolonging of quenching time,then reached 1.12 at 800 K after quenching for 1800 times,which was even higher than those of unquenched Ba0.3In0.2Co3.95Ni0.05Sb12/SiO2 nanocomposite(ZT=1.08) and single-phase Ba0.3In0.2Co3.95Ni0.05Sb12 bulk material(ZT=1.10) quenched for 1800 times.