Study of the Thermoelectric Properties of RxCo4Sb12 (R=In, Ba) with InSb/GaSb Nano Inclusions

Abstract

Almost 80% of the world’s energy needs depend on oil, coal and natural gases. Use of natural gases, fossil fuels emit a large amount of CO2 to the earth’s atmosphere, which causes the greenhouse effect and results in an increase of the global temperature. The first step to reduce this problem is to use of renewable energy sources. One of the sustainable and challengeable energy sources is the use of thermoelectric (TE) materials, which can be used to convert a part of waste heat into electricity. The TE effect is the direct conversion of a temperature difference to electric voltage and vice versa through two mechanisms, the Seebeck effect and the Peltier effect. These days, Co4Sb12-based lead-free thermoelectric materials are among the most explored thermoelectric materials, efficient in the temperature range of 300 K – 800 K. They are a near-future potential candidate to be used in TE generator fabrication. But, the high lattice thermal conductivity of Co4Sb12 (~7.5 W/m-K at 300 K) due to strong Co-Sb covalent bonding, leads to a very low zT value (0.05 at 723 K) and low thermoelectric efficiency. Filling the voids at the 2a Wyckoff position in Co4Sb12 crystal structure by electropositive elements and homogeneous distribution of nanosized secondary phases in the bulk matrix are promising approaches to reduce the lattice part of thermal conductivity via enhanced phonon scattering. In this thesis, these two approaches were combined to enhance the thermoelectric efficiency of Co4Sb12-based materials. Chapter 1 of this thesis deals with the brief history of thermoelectricity and the different thermoelectric effects. The thermoelectric efficiency of the materials and devices are discussed with the thermoelectric transport properties. The available thermoelectric materials efficient at different temperature range is mentioned, followed by the motivation of choosing Co4Sb12-based materials. The thermoelectric properties and different approaches to enhance the thermoelectric efficiency of these materials are summarized. The motivation for this thesis is specified. The synthesis technique of materials, several characterization techniques, transport properties and mechanical properties measurement techniques are presented in chapter 2. In the first part of the thesis, the dispersion of InSb nano inclusions on the thermoelectric properties of RxCo4Sb12 (R=In, Ba) was investigated. In chapter 3, the thermoelectric properties of the nanocomposite of In0.5Co4Sb12 and InSb were investigated for the temperature range of 373 K to 723 K. The existence of partially oxidized InSb phase and In2O3 phase in the composite samples was detected. Submicrometer-sized InSb precipitates and uniform distribution of InSb nanoparticles (50-100 nm) inside the matrix grains were observed. The X-ray photoelectron spectroscopy showed an +1 oxidation state of In inside the voids, which resulted in reduced electrical resistivity (ρ) in In0.5Co4Sb12. The addition of InSb nanoparticles in the matrix increased ρ because of the scattering of charge carriers at the interfaces. An increase in the Seebeck coefficient (S) was observed in nanocomposites compared to In0.5Co4Sb12 because of the energy-filtering effect of the charge carriers. A significant reduction in lattice thermal conductivity (κL) of the composite samples was observed because of enhanced scattering of phonons of mid- to long wavelengths at the interfaces and rattling of In-filler in the voids, which was verified from the Raman analysis. These combined effects resulted in the maximum figure of merit of 1.1 at 723 K for (InSb)0.2+In0.5Co4Sb12, which is higher than the composite with in-situ formed InSb nano-inclusions. In chapter 4, the thermoelectric properties of nanocomposites of Ba0.3Co4Sb12 and InSb were investigated. The InSb nanoparticles of ~ 20 nm were dispersed in the matrix grains with few larger grains of about 10 μm due to the agglomeration of InSb nanoparticles. The +2 oxidation state of Ba in Co4Sb12 resulted in a low ρ value of the matrix. The increase in S and ρ of Ba0.3Co4Sb12 with the addition of InSb can be attributed to the energy filtering effect of low energy electrons at the interfaces. The power factor of the composites could not be improved compared to the matrix due to the very high ρ value. Minimum possible κL (0.45 W/m-K at 773 K) was achieved for the composite due to the combined effect of ratting of Ba filler atoms in the voids and enhanced scattering of phonons at the interfaces induced by InSb nanoparticles. As a result, the (InSb)0.15+Ba0.3Co4Sb12 composite exhibited improved thermoelectric performance with a maximum zT of 1.40 at 773 K and improved mechanical properties with higher hardness, Young’s modulus value and lower brittleness. In the second part, the influence of GaSb dispersion in the bulk matrix of RxCo4Sb12 was studied. In chapter 5, the combined effect of Ba-filling in the voids and GaSb nanophase incorporation in the matrix of Co4Sb12 have been studied for the thermoelectric properties. Nanocrystalline GaSb grains showed uniform distribution in the matrix along with few large grains of 1-3 μm. The +2 oxidation state of Ba in Co4Sb12 yielded the low ρ of the matrix. The GaSb addition in the matrix increased ρ slightly by enhancing the charge carrier scattering at the interfaces between matrix and GaSb. The GaSb grains (50-200 nm) could not filter the low energy charge carriers and resulted in invariant S values of the composites with that of the matrix. The reduction in κL for the sample with highest GaSb content can be attributed to the simultaneous effect of anharmonicity created by the Ba filler in the voids and enhanced scattering of phonons with mean free path of 10-100 nm at the interfaces. The reduced κL resulted in a zT of 1.0 at 773 K for (GaSb)0.4+Ba0.2Co4Sb12. In chapter 6, thermoelectric properties of nanocomposites consisting of In0.2Co4Sb12 and GaSb, were studied. (GaSb)x+In0.2Co4Sb12 nanocomposites were prepared from In0.2Co4Sb12 and GaSb master alloys, employing high energy ball milling and spark plasma sintering. CoSb2 and InSb secondary phases were found in the matrix. For the composites (x > 0.1) small amounts of the solid solution Ga1-xInxSb are found at the grain boundaries, originated from a reaction of GaSb with InSb. EBSD analysis revealed nanocrystalline grains of size 30-100 nm, which can be either of InSb, GaSb or Ga1-xInxSb dispersed in In0.2Co4Sb12. XPS revealed for In in the voids the +1 oxidation state, is responsible for the low ρ in In-filled Co4Sb12. The distribution of GaSb in the matrix increased ρ via scattering of the charge carriers at interfaces. S was significantly enhanced for the composite with lowest GaSb content likely due to the filtering of low energy charge carriers at the interfaces of GaSb and matrix phase. The composite with the lowest GaSb content (x=0.1) exhibits the lowest κL (1.56 W/m-K at 773 K) due to the simultaneous effect of In-filling the voids and incorporation of nanocrystalline GaSb in the matrix. The high S and low ρ for the composite x=0.1, together with the low κ resulted in the highest zT of 1.4 at 773 K with thermoelectric efficiency of 9.4 %. Chapter 7 discusses the summary of the results and the conclusions of the work done. The results obtained in the thesis show that the zT of thermoelectric materials can be improved using combined approaches of filling the void and dispersing nanosized second phase in the bulk matrix. The future research plan in skutterudite materials is discussed for further enhancing the thermoelectric efficiency.