Properties Of Ge-Te Based Amorphous/nano-crystal In-situ Thermoelectric Nanocomposites

Thermoelectric materials are a kind of semiconductor great materials which can directly convert electrical power to heat, or conversely. They are interesting for applications in thermoelectric cooling devices and power generators. The dimensionless figures of merit ZT of traditional thermoelectric materials have long been about 1. The figure of merit of thermoelectric materials can be significantly improved by nanotechnology, since the thermal conductivity can be decreased more than the electric conductivity of the materials.In the present work, a new kind of amorphous/nano-crystal in-situ composites was developed. Nano-crystal was in-situ precipitated from the Ge-Te based amorphous matrix. The structure, morphologies and the mechanical of the precipitation of the nano-composites were investigated by XRD, SEM, HRTEM and Raman scattering. Kinetics of reaction, nucleation and crystal growth mechanism during the synthesis of nano-composite have been experimentally investigated and discussed in detail. The electrical properties of GeTe-based amorphous, amorphous/nano-crystal composites and crystal composite have been measured. Some important results of the present work are listed as fellows.1. Amorphous Ge₁₅Te₈₅, Ge₁₅Sb₃Te₈₂, Ge₂₀Te₈₀ and Ge₃₀Te₇₀ bulks have been successfully prepared by quenching the melt to liquid nitrogen. The short-range order were calculated by amorphous Bragg equation, and the range is about 4-5 atoms. The short-range order was also investigated by HRTEM, which is consistent with the calculated result.2. The amorphous transition behavior and crystallization kinetics of GeTe-based amorphous have been analysized by differential scanning calorimetry (DSC). The activation energy of the glass transition, the activation energy of crystallization, and the dimensionality of growth have been determined use Kissinger and its modified formula, JMA and its modified formula. The crystallization was identified as 3 dimensional processes. The glass transformation ability have been researched and Ge₂₀Te₈₀ has been chosen as the best materials as the amorphous matrix.3. The nanocrystals were precipitated in the amorphous matrix under the temperature field outside the samples. Te, GeTe and amorphous phase were identified using XRD. There are four periods in the transformation investigated by Raman scattering: (1) a - Te→ c - Te , Ge(a) + 4Te(c)→GeTe₄(c) ; (2) GeTe₄ (c) + Ge(a)→ 2GeTe₂ (c) , a-Te→c-Te ; (3)GeTe₂+Ge(c)→2GeTe(c), a-Te→c-Te; (4)a-Te→c-Te.4. The mechanism of the nanocrystal in-situ precipitation, the nuclear, the growth mechanism and the morphologies of the nanocomposites and the interface between amorphous phase and crystal phase has been investigated using HRTEM.5. The melt crystal and the annealed Ge₂₀Te₈₀ have been prepared, two kinds of GeSbTe alloys were prepared, one is melt spinning method and the other one is quenched into water. The morphologies of the GeTe-based crystal were investigated by SEM. The phase of rapidly solidified GeSbTe based layer alloys is cubic and the hot pressed one is hexagonal. The phase transformation from the facc-centered cubic to hexagonal structure makes the grain finer.6. The electrical properties of amorphous, amorphous/nano-crystal, crystal Ge₂₀Te₈₀ and GeSbTe alloys have been measured. For the amorphous Ge-Te based semiconductor, the electrical conductivity is very low at the room temperature, and high Seebeck coefficient. For the amorphous/nanocrystal nanocomposite, the electrical conductivity increase and the Seebeck coefficient keep a high level. The power factor of the nanocomposite anneal at 170 °C for 2 hours is 10.8×10^-3 Wm^-1K². The electrical property is improved largely.