Role of vacancy defects on the lattice thermal conductivity in In2O3 thermoelectric nanocrystals: a positron annihilation study

High purity $$\hbox {In}_2\hbox {O}_3$$ nanopowders were subjected to different thermal treatments to investigate the role of defects on the lattice thermal conductivity. The powders were first treated by spark plasma sintering (SPS) at $$700\,^\circ\hbox {C}$$ and annealed in air between 700 and $$1300\,^\circ\hbox {C}$$ . X-ray diffraction measurements show that the samples are single phase, and the diffraction peaks become sharper with increasing of annealing temperature, indicating improvement in crystallinity and increase in grain size. The $$\hbox {In}_2\hbox {O}_3$$ nanopowders were also treated by SPS sintering at different temperatures without subsequent annealing. On the contrary, the average grain size of $$\hbox {In}_2\hbox {O}_3$$ treated by SPS has no obvious change with the increase in sintering temperatures. Positron annihilation measurements reveal large amounts of monovacancies and vacancy clusters in the $$\hbox {In}_2\hbox {O}_3$$ nanocrystals. The monovacancies gradually recover and the vacancy clusters transform into smaller vacancies with increasing annealing or sintering temperatures. The lattice thermal conductivity increases with the increase in annealing or sintering temperature, which shows close correlation with the recovery of vacancy defects after heat treatment. This gives us strong evidence that vacancy defects play an important role on the suppression of lattice thermal conductivity in nanostructured thermoelectric materials.