Study of the Stability of Na _0.7 CoO ₂ Thermoelectric Materials under Shock Dynamic Loading in a Shock Tube

Introduction

The thermoelectric material is a material that shows large thermo power, low resistivity and low thermal conductivity. Recently, layered cobalt oxides have been extensively investigated as a promising candidate as thermoelectric material. Important feature of sodium cobalt oxides is that, the sodium ions (Na ⁺) randomly occupy the regular site by 50% and the sodium content can go up to 70% changes[1]. In this sense layered cobalt oxides, NaCo ₂ O ₄ should be written as Na _x CoO ₂ (x = 0.5) and the compound is quite promising for thermoelectric power generation. Terasaki et al. found that a single crystal NaCo ₂ O ₄ exhibits good thermoelectric property[2]. These layered oxides consist of two layers: CoO ₂ layer and Na ion layer. CoO ₂ layers acting as an electron reservoir which are responsible for the electrical conductivity and large thermoelectric power. Na ions layer sandwiched between two neighboring CoO ₂ layers adjust the concentration of electron in CoO ₂ layers and decrease the thermal conductivity along the stacking direction c[3]. Since the discovery of moderately large thermoelectric power (Seebeck coefficient) together with high electrical conductivity in Na _x CoO ₂(x is 0.70), experiments were done to find new phases for thermoelectric conversion applications[4]. A single-crystal X-ray diffraction study confirmed that Na _x CoO ₂ (x = 0.74) adopted the hexagonal P6₃/mmc space group[5]. Neutron diffraction and electron diffraction study shows that the crystal structure of the oxides is strongly dependent on sodium content. The crystal structure of Na _0.75 CoO ₂ was studied at ambient and low temperatures down to 10 K at pressures up to 40 GPa using a diamond cell shows an increase in Co-O bond length and decrease in Na-O bond length[6]. Here we present the experimental results on the interaction of shock heated test gases with Na _x CoO ₂ at high temperature and moderate reflected shock pressure.