Combustion Synthesis and Thermoelectric Properties of Beta-Gallia Rutile Intergrowths
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Beta-gallia rutile intergrowth materials (BGRs), a homologous series following Ga4Tin-4O2n-2 (15 ≤ n ≤ 25, odd), were investigated as potential n-type thermoelectric device candidates given their structural similarities to the strontium titanate Ruddlesden- Popper phases and misfit layer cobaltite materials. Compositions from n=15 through 25 (odd) were synthesized using solution combustion synthesis (C.S.). For comparison a sample of n=19 was prepared with solid-state synthesis (S.S.). The combustion synthesis yielded nanopowder samples of the anatase phase, which were transformed into the intergrowth phase after heating at 1400 °C in air. The X-ray diffraction analysis of the heat treated powder samples showed that the n=23 and 25 samples had rutile impurities and the n=15 had β-Ga2O3 impurities. Spark plasma sintering (SPS) produced reduced samples with highly dense microstructures, free of impurity rutile. β-Ga2O3, however, was still present in the n=15 sample. A systematic shifting the position of the diffraction peaks to higher angles was noted between the pre- and post-sintered samples, indicating a decrease in the unit cell size. Below 400 °C thermopower measured in argon was not temperature dependent, indicating the lack of thermally generated charge carriers. Combined with the increase in electrical conductivity with temperature, it was concluded that these BGR materials are polaron conductors, with mobility activation energies of 0.052 eV. Above 400 °C BGR begins to break down in to rutile and β-Ga2O3. Overall there is no discernable trend between electrical conductivity and composition, although in general thermopower increases with increasing n-value from 15 to 23. No conclusions could be drawn regarding thermal conductivity and composition due to the scatter in the thermal diffusivity data. The electrical conductivity of the C.S. samples were an order of magnitude lower than the S.S. sample, and the thermopower was higher. The lower carrier concentration in C.S. samples is hypothesized to be caused by nitrogen impurities – a byproduct of combustion synthesis –acting as acceptor-type substitutional dopants in the oxygen sublattice. Among the samples investigated, the n=19 S.S. sample showed the highest power factor (α2σ) and a higher ZT of 0.02 at 400 °C.
Advisory committee members: Scott Misture, David Lipke. Dissertation completed in partial fulfillment of the requirements for the degree of Masters of Science in Materials Science and Engineering at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University