Effect of Pre-Oxidation on Thermoelectric Properties of Bismuth Telluride Compacted By Spark Plasma Sintering
Authors: Kim Jinseo , Duy Le Thai , Lee Sang Yeon , Seo Hyungtak 
Published: February 2020

Research on harvesting alternative energy sources has got a lot of attention to adapt human demands for energy while reducing the environmental pollution by the extensive use of fossil resources. Thermoelectricity is a technology based on the different temperature on two terminals of couples of p-n semiconductors to convert heat into electricity. By this way, waste heat can be recycled into usable and storable energy at low cost. Thus, it is esteemed as the most noteworthy technology in the field of renewable energy. However, for widespread usage, there are many challenges relating to the choice of thermoelectric materials as well as platform design for mass-production, and thus improvement of energy conversion efficiency.
Among thermoelectric materials, the binary system of Bismuth and Telluride (Bi-Te) is widely used due to the high thermoelectric performance that can be obtained at a low temperature range of 300 - 500 K. However, controlling the ratio and crystallization of Bi-Te is critical. Therefore, many methods have been developed to construct thermoelectric systems based on Bi-Te with a high crystallization and in a large scale, such as chemical vapor deposition (CVD), sputtering, atomic layer deposition (ALD), spark plasma sintering (SPS), and so on. Among them, SPS is the fastest approach to produce high quality of Bi-Te systems with a low materials consumption. Therefore, we carried out the study with this approach. In other hands, the oxidation of raw Bi-Te powder can occur under ambient conditions. However, effects of the oxidation on the thermoelectric property of this system have not fully explored yet. Therefore, our work carried out to investigate the quality of the Bi-Te system compacted from their nanorized raw powders and to understand effects of pre-oxidation on the thermoelectric performance of this system.
In details, the Bi-Te system has the n-type (Bi-Te alloy, Sigma Aldrich) and p-type components (Bi-Te mixed powder). The powder was prepared by mixing the raw Bi and Te powders (Sigma Aldrich) and then ground through a ball-milling process for 15 hr, resulting in a nanorized powder. After that, coin-like samples (diameter of 15 mm) were compacted through SPS by the use of the nanorized powder (p-type) and alloy (n-type). X-ray diffraction (XRD), energy dispersive spectrometry in transmission electron microscopy (TEM-EDS) and X-ray photoelectron spectroscopy (XPS) were utilized in order to analyze the structural properties of the nanorized powder, alloy, and compacted samples. Here, in XRD measurement, an insignificant change in the positions and intensities of peaks was observed in the samples with different mixing ratios of Bi-Te. Furthermore, TEM-EDS measurements showed information of the atomic arrangement and fractions of Bi-Te components that were not uniformly mixed. These hint that the pre-oxidation of these powders could occur and effect on the bonds between Bi-Te. Through the use of XPS, the intermolecular chemical bonds of the materials were revealed. As a result, the compacted samples contained a small portion of oxygen (either Bi-O or Te-O). These oxide bonds hindered Bi-Te interactions, leading to the low mixing-uniformity and small change in XRD patterns.
To evaluate the thermoelectric performance of the system, its electrical conductivity depending on temperature was measured by a semiconductor parameter analyzer while its thermal conductivity was measured using the laser flash analysis (LFA). Then, Seebeck coefficient and figure of merit of the system were calculated. As a result, it is found that the increase of temperature (300 - 420 K) at one sample terminal went along with a slightly improved Seebeck coefficient but a small decrease of the figure of merit. That was because the conductivity of the n-type component increased while that of the p-type component decreased under heat. As well known, the resistance of semiconductors decreases at higher temperatures due to the increase of charge concentrations. However, our nanorized powder contained oxides that can not only hinder charge transferring pathways but also be charge trapping sites. Besides, oxygen atoms/molecules inside the powder and from the air can diffuse into the low packing crystal lattice structure of the p-type component, leading to an increase of oxide defects and charge scatterings (i.e., lower conductance) here. Generally, from the experiment observation, the pre-oxidation has a negative effect on the thermoelectric property of the Bi-Te system, especially the p-type component. Therefore, a passivation is necessary to gain better the efficiency.