Thermoelectric Properties of Iron-Silicide with various baoudaries

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There are problems such as the global warming and fossil fuel depletion crisis. Alternative energy to current energy is desired to solve these problems. One of prospective energies is generated from thermoelectric conversion. The conversion efficiency and generating power become higher with higher figure of merit ZT and higher power factor P. The ZT and P are given byZT=σα^2/κ=σα^2/(κ_<el>+κ_<ph>),P=σα^2,where T is temperature, σ electrical conductivity, α Seebeck coefficient, κ_<el> and κ_<ph> being thermal conductivity due to electron and phonon transportations.The β-FeSi_2 attracts attention for the thermoelectric conversion, because it is abundant and free of toxicity, being high heat-resistant and oxidation-resistant. The polarity of β-FeSi_2 is convertible between p- and n-type with selecting the dopant of third element. The figure of merit and power factor of β-FeSi_2 are enhanced by mixing substances such as Ag, or SiC ceramics etc. It is interesting how the thermoelectric prop … More erties depend on microstructures, or fabrication processes of β-FeSi_2 which is mixed with such elements or compounds.The objective of the present research is to clarify the relation between the thermoelectric properties and microstructures or fabrication processes in the Co doped β-FeSi_2 which is mixed with Ag,Cu or Sb ete. In order to enhance the thermoelectric performance, crystal grain boundaries and phase interfaces were controlled on the basis of phase transformation, mechanical alloying, spin-casting and diffusion. The microstructures were investigated by using a transmission electron microscope with an energy dispersion X-ray microanalysis equipment. The Seebeck coefficient, and electrical conductivity were measured and related to the microstructures. The drawn conclusions are given as follows.The Ag,Cu,or Sb-rich phase is observed at the grain boundaries and in the matrix of the β phase.The Seebeck coefficient decreases nonlinearly with increasing the element content. The Seebeck coefficient is large in the ribbon mixed with Ag. The Seebeck coefficient is independent of timing of mixing with Ag. The electrical conductivity decreases nonlinearly with increasing Ag content. The electrical conductivity is large in the discs which are formed by pressing powder. The power factor reaches to maximum near 2-3 at% Ag for the discs.Carrier mobility, effective electron mass, or scattering factor of the discs differs from that of ingot, or ribbon, because of different microstructures.The powder becomes fine and round with ball milling. Sintering reduces the amount of the α phase and increases the amount of ε phase, resulting in decreasing of the β phase and increasing in the ε phase after annealing. The electrical conductivity and the power factor are enhanced by ball-milling, sintering and annaling. Less