Optimizing Thermoelectric Properties of In Situ Plasma-Spray-Synthesized Sub-stoichiometric TiO_2−x Deposits

In this article, an attempt has been made to relate the thermoelectric properties of thermal spray deposits of sub-stoichiometric titania to process-induced phase and microstructural variances. The TiO_2−x deposits were formed through the in situ reaction of the TiO_1.9 or TiO_1.7 feedstock within the high-temperature plasma flame and manipulated via varying the amounts of hydrogen fed into in the thermal plasma. Changes in the flow rates of H₂ in the plasma plume greatly affected the in-flight particle behavior and composition of the deposits. For reference, a high-velocity oxy-fuel spray torch was also used to deposit the two varieties of feedstocks. Refinements to the representation of the in-flight particle characteristics derived via single particle and ensemble diagnostic methods are proposed using the group parameters (melting index and kinetic energy). The results show that depending on the value of the melting index, there is an inverse proportional relationship between electrical conductivity and Seebeck coefficient, whereas thermal conductivity has a directly proportional relationship with the electrical conductivity. Retention of the original phase and reduced decomposition is beneficial to retain the high Seebeck coefficient or the high electrical conductivity in the TiO₂ system.