Thermoelectric Performance of n-type Layered Perovskites

Open Access
Author:
Magagnosc, Daniel
Area of Honors:
Materials Science and Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Dr Clive A Randall, Thesis Supervisor
  • Clive A Randall, Thesis Supervisor
  • Suzanne E Mohney, Honors Advisor
Keywords:
  • thermoelectric
  • layered perovskite
  • ferroelectric
  • Seebeck coefficient
  • electrical conductivity
Abstract:
In this work, single phase perovskite layer structure materials consisting of Sr2Nb2O7, tungsten doped Sr2Nb2O7, Sr2Ta2O7 and solid solutions of Sr2Nb2O7 and Sr2Ta2O7 were investigated as possible thermoelectric materials. X-ray diffraction was used to determine phase development and to measure the lattice parameter for all compositions. Samples were reduced under oxygen partial pressures of 10-6, 10-14, and 10-16 atm to enhance the n-type electronic conductivity. The resultant materials were analyzed via x-ray diffraction, and key properties such as electrical conductivity and Seebeck coefficient were measured. At 10-16 atm oxygen, Sr2Nb2O7 converted to Sr5Nb5O17. Sr2Ta2O7 and solutions of Sr2Nb2O7 and Sr2Ta2O7 were observed to resist reduction and no new phases formed. Sr2Ta2O7 was too resistive to measure the conductivity or Seebeck coefficient; this material is relatively stable to reduction processes, and therefore maintains high resistivities that are difficult to measure with the MMR Technologies SB-100 Programmable Seebeck Controller used here. Electrical conductivity was measured for solutions of Sr2Nb2O7 and Sr2Ta2O7. The conductivity and Seebeck coefficient of doped Sr2Nb2O7 was found to depend strongly on tungsten concentration and reduction conditions. Electrical conductivity increased with decreasing oxygen pressure and ideal tungsten concentrations were found at each oxygen pressure. Anomalies were observed in the Seebeck coefficients at oxygen pressures of 10-14 atm and greater. At 10-16 atm oxygen, the Seebeck coefficients were found to vary systematically. The thermoelectric power factors were low for all compositions indicating that polycrystalline Sr2Nb2O7 was a poor thermoelectric material in the temperature range studied. However, the power factor showed an increasing trend indicating that doped Sr2Nb2O7 may be a suitable thermoelectric material at higher temperatures of ~1000 °C and with crystallographic texture.