Report on Malcolm F. Nicol Graduate Scholar Research Activities
As global energy requirements become increasingly demanding, it is paramount to pursue methods and techniques with potential to increase energy efficiency through reduction or conversion of waste heat energy, to produce clean, reliable, and renewable fuel, and to reduce the overall energy consumption at a global scale. Specifically, thermoelectric (TE) materials can convert waste heat energy into useful electrical energy via a thermal gradient and have a vast array of applications including commercial TE refrigeration, energy-efficient engines in the automotive industry, and radioisotope TE generators for NASA satellites. The efficiency of TE materials is governed by the dimensionless figure of merit ZT with a larger value of ZT implying a more efficient TE material. ZT is defined as, ZT = (α2 σ / κ) T, where α, σ, κ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the absolute temperature of the TE material, respectively. Enhancement of TE efficiency thus requires optimization of the electrical and thermal properties of the materials by increasing the Seebeck coefficient and electrical conductivity while decreasing the thermal conductivity at a particular temperature. Methods to optimize these properties have included modifications to the electronic structure near the Fermi level through chemical doping and alloying of TE materials which can enhance the Seebeck coefficient and electrical conductivity as well as nano-structuring and synthesis of cage-like structures to decrease the thermal conductivity. Furthermore, synthesis of new materials and accessing new phases of existing materials through use of high-pressure and high-temperature techniques is a powerful tool to modify electrical and thermal properties and aids in identifying more efficient TE materials. This project has focused on developing a technique to measure thermal and electrical properties at high-pressure and temperature conditions alongside using synchrotron radiation to probe structural properties simultaneously.
We have designed and developed a specialized cell assembly for high-pressure/high-temperature electrical and thermal measurements for use with the Paris-Edinburgh (PE) press. This project has been a collaborative effort between HiPSEC at UNLV (Jason Baker, Ravhi Kumar (PI)), the High-Pressure Collaborative Access Team (HPCAT) (Changyong Park, Curtis Kenney-Benson) at Argonne National Laboratory (ANL), and Los Alamos National Laboratory (Nenad Velisavljevic). Specifically, the HPCAT, Sector 16 BM-B beamline which can be equipped with a PE press is the primary location for this technique. A photograph of the beamline hutch is depicted in Figure 1 (left) indicating the X-ray side and the collimator and detector side as well as labeling a few important features such as the X-ray camera for radiography imaging, the PE press containing the sample cell assembly at the center, and the high-pressure oil line which provides ability to compress the cell assembly within the PE press. Figure 1 (right) is a zoomed in photograph of the sample cell assembly which is situated at the center between two tungsten carbide anvils which are connected to electrodes to provide power for a heater inside the cell assembly.
Figure 1. Photograph of HPCAT Sector 16 BM-B beamline with PE press installed
A schematic of the cell assembly designed for this project is depicted in the Figure 2. The sample is located at the center and is sandwiched between two K-type thermocouples, which act as electrical and thermal probes, and two diamonds discs which allow for ideal electrical isolation and thermal conductance. Heat is applied to the sample by a graphite heater located at the top of the cell assembly which causes a temperature difference across the sample. By measuring the electrical resistivity, Seebeck coefficient, and thermal conductivity alongside X-ray radiography imaging and X-ray diffraction measurements, a full characterization of the electrical, thermal, and structural properties of a material can be simultaneously performed as a function of pressure and temperature.
Figure 2. Schematic of specialized cell assembly for thermoelectric measurements in PE press at HPHT conditions.
Figure 3 displays a radiography image of the entire sample cell assembly as it sits in the PE press before compression has occurred. This figure is a combined image of approximately 20 separate radiography images each covering approximately a 0.8 mm by 0.8 mm square. As labeled in the figure, the individual parts of the cell assembly are clearly visible due to differing absorption of the incoming X-rays. The radiography images centered on the sample can be used to determine the thickness of the sample and thus the separation of the thermocouples for use in the calculation of the electrical resistivity and thermal conductivity by calibrating the number of pixels in the image to a specific value.
Figure 3. Combined radiography images to produce view of entire sample cell assembly situated in PE press.
A more detailed description of this cell assembly and application to Bi and PbTe samples has been published in the Journal of Synchrotron Radiation [Baker et al. 2016]. Other materials have been explored utilizing this specialized cell assembly and are in preparation for publication. Additionally, this technique is available for general users at the Sector 16 BM-B beamline at Argonne National Laboratory.
Baker et al. J. Synchrotron Rad. (2016). 23, 1368-1378