Study of Improved Thermoelectronic Properties of Inorganic Materials using Doping Technique

Alternative Title
Jinseo Kim
Alternative Author(s)
Jinseo Kim
일반대학원 에너지시스템학과
The Graduate School, Ajou University
Publication Year
Alternative Abstract
Thermoelectricity is one of the most attractive technologies in the field of renewable energy. A material useful for thermoelectric systems is evaluated through device efficiency and power factor. Because the material's properties determine these two factors, many attentions have been paid to improve the properties, especially the Seebeck coefficient with electrical and thermal conductivities. Recently, the development of nanotechnologies and doping technologies opened a lot of opportunities for that purpose. And thus, applying these technologies to enhance the conversion efficiency of thermoelectric materials became the hot trend. Two of the most attractive thermoelectric candidates are bismuth telluride (Bi2Te3, Bi-Te) and strontium titanate (SrTiO3, STO). Bi-Te exhibits excellent thermoelectric performance at low temperatures while STO works well at high temperatures. However, there are many challenges for them to be used in practical applications. Therefore, improvement of their quality, properties, and thermoelectric performance through processing and doping techniques is targeted in this dissertation. Regarding Bi-Te, the processing method is noticed because the effects of structural engineering on the thermoelectric properties have not fully be explored. I selected a combined approach of ballmilling and spark plasma sintering (SPS) that allow tuning the structural and electrical properties of Bi-Te at low cost. Here, two types of Bi-Te materials were processed, a mixture of Bi and Te powders (i.e., “SM-Bi-Te”) and an alloy of Bi-Te (i.e., “SA-Bi-Te”). They acted as a p-type semiconductor and a meta-metal, respectively. The difference between their thermoelectric properties was examined carefully. The ZT value of SA-Bi-Te (formed at 483 K) is 1.67 times higher than that of SM-Bi-Te when measured at 300 K. The ZT value of SM-Bi-Te (formed at 533 K) is 5.05 times higher than that of SA-Bi-Te when measured at 420 K. Interestingly, I found out the effect of pre-oxidation through this 2-step approach on the increment in the electrical conductivity and the figure of merit (ZT) of Bi-Te samples in a bulk form. These are discussed in detail. Next, doping of our Bi-Te materials is carried out. A comparison of their thermoelectric performance in bulk and film forms is conducted. The dopant here is fluorine (F), which serves as an electron donor and was introduced to Bi-Te through fluorine plasma. With an increase in plasma exposure time, nonlinear dependence of ZT of the F-doped bulk samples was observed. The peak improvement of its ZT at 300 K (0.986) or at 510 K (1.061) is recorded for the samples with 40-s exposure. That is because, after 40 sec of exposure, all surface areas were covered by F ions that hindered the enetration of coming F ions. Compared to the bulk samples, the F-doped film samples have a similar tendency of the ZT dependence but their ZT is smaller at the same exposure time. Some characterizations were carried out to find a reason for this phenomenon. In general, a significant improvement of ZT of Bi-Te can be achieved through processing and doping. Regarding STO, its thermoelectric performance is tuned by the use of Nb doping (Nb:STO). In this study, I used co-sputtering to introduce Nb dopants into STO at low temperatures. Measured at low temperatures, Nb:STO and STO films, particularly their electrical conductivities, have a different temperature dependence. The insulator-to-semiconductor transition temperature of STO film is over 450K while that of Nb:STO film is just above 350K. In other words, Nb doping helps STO have a higher electrical conductivity but a lower thermal conductivity. Thereby, Nb:STO would be applicable to thermoelectric systems working at low and elevated temperatures. In addition, Nb:STO films exhibit high thermal stability measured up to 973 K. It renders Nb:STO as a potential candidate for thermoelectric applications at elevated and high temperatures. In general, the optimization of thermoelectric materials is critical to expanding their potential for practical applications. My studies revealed the importance and advantages of the processing steps of thermoelectric materials regarding tuning their structural properties. Furthermore, our findings indicated the advantages and disadvantages of doping for Bi-Te and STO materials. Altogether, this dissertation presents the effects of processing and doping on the thermoelectric materials, and they are a vision for developing promising thermoelectric materials in the future.

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Graduate School of Ajou University > Department of Energy Systems > 4. Theses(Ph.D)
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