Multiphase thermoelectrics : effect of second phase in ex-SITU Cu7Te4-Bi0.4Sb1.6Te3 and in-SITU SnTe-SnSe nanocomposites
Tan, Li Ping
Date of Issue2014
School of Materials Science and Engineering
Thermoelectric (TE) materials are a potential source of alternative energy, and have been the subject of research since the 1950s, with their figure of merit (ZT) fluctuating at about 1. Only recently were breakthroughs made in research, based on approaches like nanostructuring, presence of multiple phases or presence of these multiple phases at various length scales in the material, leading to improved ZT values of more than 2. This thesis work focused on the use of multiphase ex-situ and in-situ nanocomposites for TE properties enhancement over the single phases. The material system for the ex-situ nanocomposites, Cu7Te4-Bi0.4Sb1.6Te3 (BST), was chosen based on earlier reports on the feasibility of synthesis via a solid state synthesis method. Here, further improvements were made: surfactant-free nanorods of Cu7Te4 were fabricated via a facile synthesis method, where Te nanorods were used as templates, and then added to BST. The material system for in-situ nanocomposites, SnTe-SnSe, was chosen based on its phase diagram, which has two single phase regions (a SnTe-rich and a SnSe-rich solid solution) and a two phase region. The fabrication method via melt spinning allowed for rapid solidification and microstructural optimization through subsequent processing by ball milling and heat treatment. In the ex-situ nanocomposite, the BST matrix alone yielded a maximum power factor of 2.46x10-3 W/m.K2 at 300 K, while the addition of 5 wt % of Cu7Te4 (5wtNC) nanorods increased and shifted the maximum power factor to 3.30x10-3 W/m.K2 at 345 K. The maximum ZT obtained for the 5wtNC was 1.14 at 444 K, which was nearly 23% higher than the peak ZT achieved for the BST matrix. However, issues like agglomeration of nanorods and cracking of samples were faced. To mitigate them, the focus was shifted to the use of in-situ nanocomposites instead. For the sample made up of a nominal composition 25 mol % SnSe and 75 mol % SnTe (25SnSe), a maximum power factor of 1.31x10-3 W/m.K2 was obtained at 494 K, which was 1.5 times that of SnTe. The peak ZT of 25SnSe was 0.19 at 548 K, which was 3 times higher than the peak ZT of SnTe. Further characterization to elucidate the reasons behind the enhancement of TE properties in 25SnSe led to the discovery of metastable behaviour in the orthorhombic second phase: the cubic SnTe-based ternary phase that formed after melt spinning transformed into a mixture of cubic and orthorhombic ternary phases (referred to as second phase) after ball milling. The mass fraction of this second phase increased with low applied pressure, and TE properties measurements carried out indicated that the second phase was important in the reduction of electrical resistivity and maintaining high Seebeck coefficient at higher temperatures. Transmission electron microscopy showed the presence of second phase nanoprecipitates in the cubic matrix in both the ball milled and hot pressed samples, and high temperature X-ray diffraction measurements showed that the phase transformation pathway in 25SnSe was due to the movement of Te and Se atoms. Calculations using the Miedema model for the enthalpy of formation of the various phases showed that the less stable phases transformed with application of heat or pressure, to more stable compositions and/or mass fraction. These results give an insight into how the phases evolve, and how processing variables can play a role to optimize the constituent phases and further improve TE properties. The results above indicate the effectiveness of using the multiphase concept in our work. It is concluded that (1) depending on the type of matrix, the presence of an appropriate second phase can effectively mitigate negative effects in the matrix, leading to enhanced electrical properties; (2) there is an optimum mass fraction of nano-sized second phase that will be useful for TE properties enhancement; (3) the presence of these precipitates at the micron and nano scale will be useful in scattering a larger range of phonons.