Thermoelectric Investigations of Si/β-FeSi2 composite

Abstract

This thesis focuses on thermoelectric properties of environmentally friendly, cheap and non-toxic Si/β-FeSi2 composite materials. Although thermoelectric power generation is reliable and environmentally friendly technology to convert waste heat into a useful form of energy, the cost and toxicity of the state-of-art materials limits the practical application. Practical application of this technology needs the exploration and development of the thermoelectric efficiency of cheap and non-toxic materials. The efficiency of the thermoelectric generator depends on the operational temperature and the dimensionless figure of merit (ZT) of the involved thermoelectric material. Dimensionless figure of merit, ZT = S2σT/(κe + κL), where S is the Seebeck coefficient, σ is the electrical conductivity, κe and κL are the respective electronic and lattice thermal conductivities and T is the absolute temperature. Thus, a material should be electrically conducting and thermally insulating in nature to have better thermoelectric performance. Composite materials are preferred because they offer different designing options and they can scatter different wavelength of phonons and reduce the thermal conductivity. Chapter I introduces the basic principles of thermoelectric effects and the parameters involved in the thermoelectric performance and the ways adopted to improve the thermoelectric performance. Chapter II explains the materials and methods used to synthesize Si/β-FeSi2 composite and processing steps to obtain highly dense pellets for the study. Description and working principle of the instruments and methods used to study temperature-dependent thermoelectric properties have been mentioned.

Chapter III deals with the chemical doping on microstructurally engineered Si/β-FeSi2 composite. Al-doping on Si/β-FeSi2 composite has been carried out to synthesize p-type composite and the effect of doping on thermoelectric properties have been discussed. The experimental thermal conductivity of the samples was fitted to Debye-Callaway model to understand the contribution from various scattering processes for thermal conductivity reduction. In Chapter IV, an attempt was made to tune the particle size of Si on Si/Al doped β-FeSi2 composite. The microstructural engineering coupled with chemical doping found to increase the thermoelectric performance of the composite. Further, three different microstructures have been developed by varying the processing steps and the effect of processing steps on thermoelectric properties of the composite has been studied in chapter V. In chapter VI, n-type Si/β-FeSi2 has been synthesized by phosphorus doping and the effect of variation in silicon particle size on thermoelectric properties of the composite have been studied. Chapter VII deals with incorporation of Fe in phosphorous doped Si, to make Si/silicide interfaces with different concentrations. Effect of silicide concentration on the thermoelectric properties of the composite have been studied. In Si-rich conditions, Si/α-Fe2Si5 composite have been stabilized. Transport properties of the composites with α- and β-FeSi2 interfaces have been compared. Si/α-Fe2Si5 composite shows better thermoelectric performance, due to the presence of ‘Fe’ vacancy in α-phase. This Fe vacancies act as point defect scattering centres for phonons to lower thermal conductivity. This α- phase forms the coherent interface with Si in ‘c’ direction and lead to higher electrical conduction in the composite. Chapter VIII describes the synthesis of Sb dispersed p-, n-type Si/β-FeSi2. Si/β-FeSi2, Sb/β-FeSi2 and Sb/Si interfaces lead to enhanced phonon scattering and lower the thermal conductivity of the composite. Chapter IX explains the effect of dispersing V2O5 in thermoelectric properties of β-FeSi2. Dispersing 10 wt. % of V2O5 forms SiO2 precipitates by internal oxidation and lead to drastic decrease in thermal conductivity. Chapter X summarizes the work carried out in the thesis and provides insights into the strategies that could lead to a higher thermoelectric performance in the composites.