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ISSN: 2348-9812 Oxide thermoelectric performance has shown steady increase in the past 25 years. There has been wide interest in applications such as waste heat recovery and radioisotope thermoelectric generators (RTG’s) where potentially higher temperature use could lead to higher thermoelectric generator efficiency [1]. A higher use temperature will also yield a higher figure of merit (ZT). Zinc Oxide (ZnO) is a promising candidate as an n-type thermoelectric temperature due to its high temperature stability, non-toxic nature and low-cost [2]. ZnO is a wide direct gap semiconductor with high electron mobility and thermal conductivity [3]. ZnO has a high thermal conductivity which adversely effects thermoelectric performance. In one study, it was found that the room temperature value for Al-doped ZnO was 40 W/m K and 13 W/m K for dually doped (Al, Ga) ZnO [4]. The thermoelectric performance for Al-doped ZnO has been reported as a ZT=0.33 [5,6]. Nanostructuring has been used to improve thermoelectric efficiency in ZnO [7,8]. Using nanoparticles as the starting material, Al-doped ZnO had a thermal conductivity of ~2 W/m K at room temperature and a projected ZT ~ 0.44 at 1000K [7]. Introduction of nanovoids in the Al-doped ZnO samples resulted in a ZT ~0.6 at 1250 K [9,10]. However, there were difficulties in producing consistent homogeneity in the samples. Dually doping with aluminum and gallium resulted in a ZT=0.65 at 1247K [4]. Although the electrical conductivity decreased with the addition of gallium, the thermal conductivity of the sample was decreased to ~5 W/m K thus leading to the high value of ZT. It was also felt that nanostructuring also aided in the decrease in thermal conductivity. Spark plasma sintering (SPS) was used to densify duallydoped (Al, Ga) ZnO samples in another study [11]. Although, the thermal conductivity was decreased due to small grain sizes, the reported ZT of 0.14 was primarily due to a low power factor. It has been demonstrated that reprocessing thermoelectrics PbTe doped with 2 mol% Na and PbTe-SrTe (2 mol%) doped with 1 mol% Na in a SPS resulted in a higher ZT due to a reduced thermal conductivity [11]. This reduction in thermal conductivity was due to the formation of nanophases and mesophases in the materials which scattered a wider range of phonons. Introduction Nanometer sized ZnO powder was co-doped with gamma aluminum oxide and gallium oxide and sintered using a direct current sintering furnace. Sintered samples were reprocessed by crushing the samples in a Carver press, then milling for 4 hours. The reprocessed samples were then re-sintered in the direct current sintering furnace. Heat capacity, density and thermal diffusivity were measured in order to determine thermal conductivity as a function of temperature. Thermoelectric properties were measured. It was found that the thermal conductivity decreased from 7 W/m K to 3.5 W/m K at 805K by using the reprocessing technique. It is projected that the value will be less than 2 W/m K and the figure of merit greater than 0.65 at 1400K. In this study we used direct current sintering to densify co-doped Al, Ga ZnO samples and determine the thermal conductivity and thermoelectric figure of merit for singly processed and reprocessed material.

Reprocessing Leading to Lower Thermal Conductivity of ZnO Thermoelectrics