Pengcheng Zhai

Multi-Scale Microstructural Thermoelectric Materials: Transport Behavior, Non-Equilibrium Preparation, and Applications

Considering only about one third of the worlds energy consumption is effectively utilized for functional uses, and the remaining is dissipated as waste heat, thermoelectric (TE) materials, which offer a direct and clean thermal-to-electric conversion pathway, have generated a tremendous worldwide interest. The last two decades have witnessed a remarkable development in TE materials. This Review summarizes the efforts devoted to the study of non-equilibrium synthesis of TE materials with multi-scale structures, their transport behavior, and areas of applications. Studies that work towards the ultimate goal of developing highly efficient TE materials possessing multi-scale architectures are highlighted, encompassing the optimization of TE performance via engineering the structures with different dimensional aspects spanning from the atomic and molecular scales, to nanometer sizes, and to the mesoscale. In consideration of the practical applications of high-performance TE materials, the non-equilibrium approaches offer a fast and controllable fabrication of multi-scale microstructures, and their scale up to industrial-size manufacturing is emphasized here. Finally, the design of two integrated power generating TE systems are described-a solar thermoelectric-photovoltaic hybrid system and a vehicle waste heat harvesting system-that represent perhaps the most important applications of thermoelectricity in the energy conversion area.

Thermodynamic and mechanical properties of crystalline CoSb3: A molecular dynamics simulation study

Molecular dynamics simulations have been performed to study the fundamental thermodynamic and mechanical properties of single-crystalline skutterudite CoSb3 in the nanometric scale. The several interesting thermodynamic predictions, including linear thermal expansion coefficient, specific heat capacity, thermal conductivity, and temperature dependence of elastic constants, show excellent agreement with data available in the literature. The classic mechanical tests of uniaxial tension and compression are performed respectively at constant temperatures. The CoSb3 single-crystal exhibits nonlinear elastic response during the deformation process and the sustainable stress is very high, demonstrating its outstanding stability. An interesting phenomenon occurs at compression that the stress-strain curve undergoes a transition. The cause of the transition is an atomic reconstruction, which is observed and interpreted on the basis of interatomic interactions. Both of the failure patterns under tension and compres...

Electronic structure and transport properties of single and double filled CoSb3 with atoms Ba, Yb and In

We report an investigation of the electronic structures and electrical transport properties of the single-filled and double-filled CoSb3 skutterudites with Ba, Yb and In atoms by density functional calculations and Boltzmann transport theory. The band structure and the density of states of single and double filled CoSb3 are calculated and discussed. Based on the results of the band structure, the temperature dependence of Seebeck coefficients, the electrical conductivity, the power factor and the carrier concentration are computed, which are generally in good agreement with the experimental data. The results indicate that the combination of (Ba, In) could greatly improve the thermoelectric properties while the combination of (In, Yb) and (Ba, Yb) would have negative effect on the power factors, due to the fact that the interaction of Yb atoms with CoSb3 would result in a reduction of the electron mobility.

An Investigation on the Coupled Thermal–Mechanical–Electrical Response of Automobile Thermoelectric Materials and Devices

Thermoelectric (TE) materials, which can directly convert heat to electrical energy, possess wide application potential for power generation from waste heat. As TE devices in vehicle exhaust power generation systems work in the long term in a service environment with coupled thermal–mechanical–electrical conditions, the reliability of their mechanical strength and conversion efficiency is an important issue for their commercial application. Based on semiconductor TE devices wih multiple p–n couples and the working environment of a vehicle exhaust power generation system, the service conditions of the TE devices are simulated by using the finite-element method. The working temperature on the hot side is set according to experimental measurements, and two cooling methods, i.e., an independent and shared water tank, are adopted on the cold side. The conversion efficiency and thermal stresses of the TE devices are calculated and discussed. Numerical results are obtained, and the mechanism of the influence on the conversion efficiency and mechanical properties of the TE materials is revealed, aiming to provide theoretical guidance for optimization of the design and commercial application of vehicle TE devices.

Simulation and Design of Vehicle Exhaust Power Generation Systems: The Interaction Between the Heat Exchanger and the Thermoelectric Modules

Vehicle exhaust power generation systems (VEPGS), mainly consisting of a heat exchanger, cooling system, thermoelectric modules (TEMs), and clamping device, have attracted wide interest and attention for power generation from waste heat. In this work, systematical research was conducted to investigate the thermal performance, power output, and thermal stress of a VEPGS by using the multifield coupling method. Different from previous research, this work simulates a model that integrates the heat exchanger and TEMs, focusing on the effect of the TEMs on the thermal performance of the heat exchanger. It is found that the TEMs have a significant effect on the thermal performance of the heat exchanger. When not considering the effects of the TEMs, the hot-end temperature of the TEMs would be seriously underestimated, which would result in underestimation of the power output of the VEPGS and the level of thermal stress of the TEMs. Meanwhile, when considering the effect of the TEMs, the hot-end temperature distribution exhibits significant changes, and its temperature uniformity is significantly improved. The results suggest that, in VEPGS design and optimization, the interaction between the heat exchanger and TEMs should be considered. This study also contributes to a more accurate assessment method for VEPGS design and simulation.

Influence of Nanopores on the Tensile/Compressive Mechanical Behavior of Crystalline CoSb3: A Molecular Dynamics Study

Recently, many experimental studies have reported that inserting nanopores into thermoelectric materials can both remarkably reduce the thermal conductivity and significantly improve the thermoelectric performance of the target material. Research on nanoporous materials has thus been attracting much attention worldwide. However, most of the studies mainly focus on the preparation of nanoporous material and the effect of different geometrical sizes of nanopores on thermal conductivity and thermoelectric properties of the nanoporous material. In this paper, the mechanical behavior of crystalline CoSb3 with nanopores under uniaxial tensile/compression is studied by means of the molecular dynamics method. The emphasis is on the influence of porosity, temperature and strain rate on the tensile/compressive mechanical behavior of nanoporous CoSb3. The simulation results show that both failure patterns under tension/compression are typical brittle fractures. The elastic modulus decreases with the growing porosity, and the porosity and the elastic modulus are inversely proportional to each other. The increase of temperature results in a linear degradation of the elastic modulus and the ultimate strength. The elastic modulus and the ultimate strength under uniaxial compression are greater than those under uniaxial tension. The present study sheds light on the future application of nanoporous CoSb3 thermoelectric materials.

Validation of discrete numerical model for thermoelectric generator used in a concentration solar system

In a concentration solar system, large temperature differences occur between the hot- and cold-junction of thermoelectric module due to concentration technology. Therefore, the nonlinear temperature dependence of the Seebeck coefficient, the electric conductivity, and the thermal conductivity of thermoelectric materials should not be neglected in performance evaluation of thermoelectric generator. Herein, a discrete numerical model, with advantage of low-computational expense, high accuracy, and broad application scope, is built to design the thermoelectric generator in a concentration solar system. Temperature-dependent material properties and contact resistance are both incorporated in the discrete numerical model. Besides, an experiment is carried out to measure the performance of Bi2Te3 thermoelectric generator operating under various temperature differences (ΔT). Finally, the discrete numerical model-calculated results are compared to the experimental data and the theoretical results computed by the commonly used constant properties model. Results show that the variation between the constant properties model-calculated output power and the measured one is less than 4% with ΔT lower than 59 K and rises to 12.6% with ΔT up to 251 K, while the error between the discrete numerical model-calculated output power and experiment remains lower than 2% even when ΔT is above 251 K.

The influence of Zn vacancy on thermal conductivity of β-Zn4Sb3: A molecular dynamics study

In our present work, the effect of the Zn-vacancy concentration on the lattice thermal conductivity of β-Zn4Sb3 at room temperature is studied by using the nonequilibrium molecular dynamics approach. Along both the x- and z- axes, the heat flux and the temperature gradient exponentially decay and increase respectively. The lattice thermal conductivity of the single crystal bulk β-Zn4Sb3 rapidly decreases when there exists Zn atom vacancy, and then the lattice thermal conductivity slowly falls further with the growing Zn atom vacancy proportion, which suggests that the Zn atom vacancy (nv) to the lattice thermal conductivity (kvac) leads to a scaling law of kv ∼ nv−α This phenomenon is attributed to the fact that the existence of vacancy scattering can significantly decrease the mean free path. When the vacancy proportion of Zn atom reaches 10%, that is the vacancy model of β-Zn4Sb3, the lattice thermal conductivity of β-Zn4Sb3 is 1.32 W/mk and 1.62 W/mk along the x- and z- axes respectively, which drops b...

Numerical simulation on the interface debonding in solid propellant under large deformation by a cohesive zone model

A new algorithm based on the artificial interaction force between neighbouring particles are introduced to build microstructure models of solid propellant, and the numerical simulation on the non-linear properties of solid propellants under large deformation are performed by using the remeshing technique based on the finite element code ANSYS. The binder/particle interface debonding is modelled through cohesive zone models. The results show that the interface debonding of large particles precedes that of small particles. The overall strains at which interface debonding takes place and the predicted effective stresses of propellants decrease with the decreasing of interface strength. At large strains, the load is mainly carried by the binder network, which is formed due to severe interface debonding. The simulated microscopic deformation modes of solid propellants are well consistent with those of experiments.

Ti3SiC2–(Ti3SiC2–SiC) functionally graded materials by spark plasma sintering reactive synthesis method Part 2 – fabrication and characterization

Abstract Ti3SiC2–(Ti3SiC2–SiC) functionally graded materials were in situ fabricated successfully by the method of spark plasma sintering reactive synthesis which was found to be applicable for the fabrication of functionally graded materials with good graded composition, microstructure as well as hardness distribution. It was also shown from the characterisation results that, with the increase in intermediate grade layers, the cracks due to residual thermal stress tend to be reduced, which is consistent with former finite element analysis results.