Enhanced Phonon Scattering And Related Mechanisms In Si Based Thermoelectric Materials

The increased demand for sustainable energies has prompted the imperative searches for new energy conversion technologies in the past decades. Thermoelectric (TE) materials can realize the direct conversion between heat and electrical energy with the merits of compactness, low noise, no pollutants and high reliability. The Si-based thermoelectric materials with the abundant constituent elements, non-toxicity, environmental friendliness and good thermoelectric properties, have attracted much attention. However, high lattice thermal conductivity of Si-based TE material limits its TE performance.Boundary scattering and point defect scattering are two important phonon scattering mechanisms in TE materials. In bulk Si TE materials, smaller grain sizes can enhance boundary scattering by limiting the mean free path of phonons, and thus lattice thermal conductivity can be greatly reduced. In Mg2Si TE material, Mg vacancy can scatter phonons effectively and result in low lattice thermal conductivity. In the present work, Si-based material and MgaSi1-Sbx TE material were studied. The main results are listed below:(1) Grain-refined heavily doped Si1-xPx were synthesized by ball-milling and spark plasma sintering methods. Microstructural observation reflects that pervasive nanograins with average grain size of ～800nm have been achieved. A small amount of P doping results in an increase in the carrier concentration and reduced lattice thermal conductivity κL. Thermal conductivity of Si0.94P0.06 is merely 13.8 Wm-1K-1 at 300K, almost 90% decrease compared with the single crystal Si. Further analysis shows that, apart from boundary scattering and point defect scattering, electron phonon (EP) scattering plays an extremely important role in the decrease of κ. Heat carrying phonons scattered by electrons accounts for ～36% of all scattered phonons for Si0.94P0.06 at room temperature. Benefiting from the sharp decline of the lattice thermal conductivity, the zT value of the Si0.94P0.06 reaches ～0.6, a factor of ～3.0 compared with the single-crystal material, experimentally indicating EP scattering may play a more important role in the reduction of thermal conductivity to improve thermoelectric figure of merit in grain-refined silicon.(2) P-type B doped SiGe alloys were also prepared by ball milling. B doping effectively increases the carrier concentration and optimizes the electrical properties. Lattice thermal conductivity of the material is reduced by the enhanced boundary scattering, which is caused by the reduced grain sizes due to ball milling.In addition, lattice thermal conductivity of the material is further reduced due to the point defect scattering and carrier-phonon scattering caused by doping. At room temperature, the thermal conductivity of Si0.8Ge0.2B0.04 is ～4 Wm-1K-1. Due to the improvement of the electric transport properties and the decreased thermal conductivity, the figure of merit zT is improved. The maximum figure of merit zT reaches 0.42 for the sample Si0.8Ge0.2B0.04 at 850K,2.5 times higher than that of Si0.8Ge0.2B0.002.(3) Phase-pure Mg2Si1-xSbx was achieved by solid state reaction for 24h at 1073K. Sb doping can significantly reduce lattice thermal conductivity of the samples. Point defect scattering help reduce lattice thermal conductivity when the content of Sb is small (x≤0.1). As Sb content increases (x≥0.2), the concentration of Mg vacancy increases. The increased Mg vacancy act as phonon scattering centers, reducing the lattice thermal conductivity. The lattice thermal conductivity of x= 0.6 sample at room temperature was-1.1 Wm-1K-1, decreasing ～86% when compared with x= 0.02 sample. Benefiting from the sharp decline of the lattice thermal conductivity, the zT value of Mg2Si0.4Sb0.6 reaches-0.5 at 750K.