When 2:00 PM - 4:00 PM Apr 29, 2015
Where 1005 H.H. Dow
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Thermoelectric Behavior of Quantum Dots Engineered Bulk Half-Heusler Nanocomposites

Yuanfeng Liu
Thesis Defense

Ferdinand Poudeu, advisor

Face to drastic reduction in natural resources of energy, thermoelectric materials research has received more and more attention in the past 20 years. In this thesis we focused our work on the development of half-Heusler based nanocomposite materials for application in thermoelectric technology. Half-Heusler alloys were chosen as our research focused due to its environmentally friendly, cheap elements and well as the mechanical stability the synthesized materials compared to other thermoelectric materials. Half-Heusler with 18 valence electrons is a narrow band gap semiconductor with large Seebeck coefficient. Howerver, the thermal conductivity can reach ~ 10 Wm-1K-1. In order to improve the thermoelectric properties of half-Heusler, two main strategies are applied. The first strategy is to increase mass fluctuation or point defects in the sample during solid-state reaction. The second strategy is to add a doping element to tune the carrier density or other phases to form additional phonon scattering centers.


This work focuses on the effects of  (1) the full-Heusler (FH) second phase, (2) heavily doping via Sb substitution at Sn sites, and (3) band gap engineering through Ge substitution at Sn sites on the electronic and thermal properties of half-Heusler (HH) matrices with composition Ti0.1Zr0.9NiSn and Zr0.25Hf0.75NiSn. In the first part of this work, we showed that the addition to excess elemental Ni in a HH matrix with composition Ti0.1Zr0.9NiSn via solid state reaction at high temperatures results in the formation of nanometer scale second phases with FH structure (Ti0.1Zr0.9Ni2Sn), coherently embedded within the HH matrix. We found that these embedded FH nanoinclusions enhance the carrier mobility and thermopower of the resulting HH/FH composites, when compared to the nanostructure-free HH matrix. It is suggested that the FH nanoparticles act as quantum dots with tunable band gap. The misalignment of the conduction band minima (CBM) at the HH/FH heterojunction creates a potential energy barrier, ∆E, which filters low energy electrons at the HH/FH interfaces while enabling transmission of high energy electrons across. In addition, the mass fluctuation at the HH/FH interfaces enhances phonon scattering leading to large reduction in the thermal conductivity. 


Building on results from Part-I, we explore in the second part the effect of FH nanoinclusions on electronic transports in Ti0.1Zr0.9Ni1+xSn matrix heavily doped via Sb substitution at Sn sites. Several samples of the Sb-doped nanocomposites, Ti0.1Zr0.9Ni1+xSn0.975Sb0.025 containing incremental volume fraction of FH second phases were synthesized through solid-state reaction of the elements. We found that, increasing the concentration of excess Ni, resulted in the agglomeration of small semiconducting FH particles into large FH aggregates with metallic conductivity. Metallic FH phase does not exhibit the energy filtering effects observed in Part I. Instead, large increase in the carrier density was observed in samples with increasing volume fraction of the FH second phase. This suggests additional doping of the Sb-doped HH matrix by the embedded metallic FH nanostructures. Therefore, large increase in the electrical conductivity and drastic drop in the thermopower were measured in most Sb-doped Ti0.1Zr0.9Ni1+xSn0.975Sb0.025 composites. However, large reduction in the thermal conductivity was observed in samples with increasing Ni concentration.


In the part-III, The effect of band gap engineering through isoelectronic substitution of Sn by Ge and mass fluctuation arising from the intermixing of Ti and Zr in the HH structure on the electronic and thermal transports of TixZr1-xNiSn0.975Ge0.025 HH matrix in the temperature range from 300 K to 775 K was investigated. A large reduction in the lattice thermal conductivity was observed with increasing Ti concentration. Surprisingly, the thermopower and electrical conductivity for the sample with x = 0.25 simultaneously increase with rising temperature. The combination of large reduction in lattice thermal conductivity via mass fluctuation at the Ti/Zr site with optimization of the thermopower and electrical conductivity through Ge substitution at the Sn site significantly improved the figure of merit from 0.05 to 0.48 at 775 K for the sample with x = 0.25.


In part IV, we apply the concept of energy filtering at HH/FH interfaces on Zr0.25Hf0.75Ni1+xSn1-ySby (x = 0~0.15 and y = 0.025, 0.01, 0.005, 0) composites with carrying doping levels. We observed that under heavily doped regime, the effect of energy filtering at the HH/FH interfaces on the overall carrier density of the Zr0.25Hf0.75Ni1+xSn1-ySby composites is marginal. We show that this behavior is due to the fact the extrinsic carriers originating from Sb substitutions at Sn sites occupy high-energy states in the conduction band of the HH matrix, leading to their effective transmission across the potential energy barrier at the HH/FH interfaces. Therefore, extrinsic carriers dominate electronic conduction in such system and the filtering of low energy intrinsic carriers at the HH/FH interfaces has negligible effect on the overall carrier density of the compounds.


In the last chapter, we explore the effect of the variation in the chemical composition of FH inclusions on the electronic and thermal properties of HH phases. This is achieved by reaction elemental Co with polycrystalline powder with composition Zr0.25Hf0.75NiSn. X-ray diffraction and transmission electron microscopy studies revealed the formation, through solid-state atomic diffusion, of nanoparticles of Co containing full-Heusler (FH) phases within the HH matrix. We found that the electrical conductivity the resulting Zr0.25Hf0.75NiCoxSn samples initially remains constant for samples with x ≤ 0.02, while the thermopower drastically drop for the sample with x = 0.02. For Co content x ≥ 0.05, the electrical conductivity gradually increases and the thermopower drops with increasing Co content. This electronic behavior suggests that at low Co content the electrical conduction is dominates by charge carrier’s compensation between the n-type HH matrix and isolated p-type Co-containing inclusions, whereas at high Co content, electronic percolation between Co-containing inclusions results in bipolar conduction within the Zr0.25Hf0.75NiCoxSn composites.