Layered Oxides as Potential Thermoelectrics

Abstract

Thermoelectrics are devices that can convert waste heat into usable electrical energy. There is a longstanding challenge to synthesize materials with large conversion efficiencies and lower Cost per Watt ratios so that thermoelectricity can be a commercially viable source of energy. The chief candidates for thermoelectric application are intermetallic semiconductors but due to reasons like toxicity, stability and cost viability they have found very little success. But in recent years transition metal oxides have emerged as a possible alternative. Oxides as possible thermoelectric materials came into consideration after the discovery of large thermopower and low resistivity in the layered oxide NaCo2O4. Following this discovery layered oxide compounds like the Ruddlesden-Popper series have been of great interest in this area of research. In this project the La-Ni-O family of the Ruddlesden-Popper series are studied for their structural and thermoelectric properties. The general formula of this series is given as Lan+1NinO3n+1 and in this project the compounds La2NiO4 (n=1) and La3Ni2O7 (n=2) along with a few compositions each with a Ca2+ dopant at the La3+ site was synthesized via solid state synthesis and studied for its thermoelectric properties. La2NiO4 has a tetragonal crystal structure with I4/mmm space group symmetry whereas La3Ni2O7 has an orthorhombic crystal structure with Fmmm space group symmetry. Rietveld refinement was done using these space groups for the corresponding compounds to calculate the lattice parameters. High temperature X-Ray Diffraction analysis was also performed on some of the samples to look for any structural phase transition. Thermal conductivity measurements showed a decrease in thermal conductivity with increasing temperature for all samples and a general increase in thermal conductivity with increasing dopant concentration. Electrical resistivity is also observed to be decreasing with increasing temperature.