Thermoelectric Properties of Polycrystalline NiSi3P4

TL;DR

This study characterizes the thermoelectric properties of polycrystalline NiSi3P4, revealing its potential as a narrow-gap semiconductor with modest thermoelectric efficiency, and explores doping and alloying effects on its properties.

Contribution

It provides detailed measurements of NiSi3P4's thermoelectric properties and investigates doping and alloying strategies to enhance its performance.

Findings

Undoped NiSi3P4 behaves as a narrow gap semiconductor.

Boron doping introduces extrinsic holes and moderate doping levels.

The thermoelectric figure of merit zT reaches approximately 0.1 at 700K.

Abstract

The Hall and Seebeck coefficients, electrical resistivity and thermal conductivity of polycrystalline NiSi3P4 were characterized from 2 to 775K. Undoped NiSi3P4 behaves like a narrow gap semiconductor, with activated electrical resistivity \rho below room temperature and a large Seebeck coefficient of ~400uV/K at 300K. Attempts to substitute boron for silicon resulted in the production of extrinsic holes, yielding moderately-doped semiconductor behavior with \rho increasing with increasing temperature above ~150\,K. Hall carrier densities are limited to approximately 5x10^{19}/cm^3 at 200K, which would suggest the solubility limit of boron is reached if boron is indeed incorporated into the lattice. These extrinsic samples have a Hall mobility of ~12cm^2/V/s at 300K, and a parabolic band equivalent effective mass of ~3.5 times the free electron mass. At 700,K, the thermoelectric figure of merit zT reaches ~0.1. Further improvements in thermoelectric performance would require reaching higher carrier densities, as well as a mechanism to further reduce the lattice thermal conductivity, which is ~5W/m/K at 700K. Alloying in Ge results in a slight reduction of the thermal conductivity at low temperatures, with little influence observed at higher temperatures.