Conductivity Of Thermoelectric Materials Research Articles

The Measurement of Thermal Conductivity with Comparative Methods For Thermolectric Materials

Research on the thermal conductivity of thermoelectric materials is still being carried out to control the flow of heat through the material so as to obtain a good thermoelectric material. The focus of this research was conducted to determine how to measure thermal conductivity using a comparative method, determine the value of the thermal conductivity of each sample, and determine the the effect of the porosity value on thermal conductivity. Measurements were made using variations of two types of materials, namely n-type Bi0,58Te1,42 and p-type Sb0,405Te0,595 and two sample sizes, namely (4×4×4) and (6×4×4) . The measurement of thermal conductivity using the comparative method, namely heater, thermocouple 1, aluminum metal, thermocouple 2, sempel, thermocouple 3 and heat sink are arranged in series to obtain data , , and for each increase in room temperature of 30 to 300. The importance of low thermal conductivity in thermoelectric materials is to be efficient when converting the material. The data obtained through measurement are then used to calculate the value of thermal conductivity. Based on the results obtained in this study, the lowest thermal conductivity value is Bi0,58Te1,42 (4×4×4) at 4,12 . The thermal conductivity of the thermoelectric materials will be smaller at the smaller the porosity for each sample variation. In the sample Bi0,58Te1,42 (4×4×4) , the smallest porosity value was 0,087 %. So in this research a good thermoelectric material is Bi0,58Te1,42 (4×4×4).

Assessing thermoelectric performance of quasi 0D carbon and polyaniline nanocomposites using machine learning

Thermoelectric materials have been widely recognized as a simple approach to harness green energy by converting thermal gradients into electrical energy. However, the intricate interplay between electrical conductivity, Seebeck coefficient, and thermal conductivity in thermoelectric materials presents a challenge to improving their efficiency. Traditional experimental methods and calculation methods have troublesome steps and long cycles for predicting new thermoelectric materials. In this work we present materials informatics-based approach, where statistical and machine learning models like correlation matrix, multiple linear regression, principal component analysis and artificial neural network were employed to find the relationship between features and thermoelectric performance. Furthermore, artificial neural network was used to analyze the roles of several compositional and microstructural features along with temperature on electrical conductivity, thermal conductivity, Seebeck coefficient and thermoelectric figure of merit (ZT) for PANI and quasi 0D carbon-based composites.

Enhanced thermoelectric properties of Bi0.5Sb1.5Te3/PbTe@C nanocomposites

It is widely acknowledged that the microstructure can be effectively modulated by a nano-secondary phase and further reduce lattice thermal conductivity in thermoelectric materials. However, considering their grain growth in the annealing and sintering process, it is crucial to discover appropriate nanoparticles, which can simultaneously inhibit the thermal conductivity and improve the power factor. Herein, we report the synthesis of PbTe@C core@shell nanocubes and their incorporation in commercial Bi0.5Sb1.5Te3(BST) powders. Since the carbon shell serves as a diffusion barrier to inhibit the growth of PbTe grains in high temperature process, several nano-sized PbTe@C particles can be preserved and observed in the BST bulk material. Thus, the lattice thermal conductivity (κlat) in the resulting composites was effectively reduced in the whole temperature range studied, which can be attributed to the scattering of high-density grain boundaries and large amounts of nano-incorporations. Meanwhile, the electrical properties are also improved upon the incorporation of PbTe@C and a large power factor of 29 μW cm−1 K−2 in the resulting composites was obtained. Furthermore, the average figure of merit value (ZTave) of the BST/0.3 wt%PbTe@C composite was 37% higher than that of the pure BST sample. This work provides a facile and novel strategy toward improving the thermoelectric performance of commercial BST.

Multiphysics Simulation of Seebeck Coefficient Measurement

Using the finite element method (FEM), we developed a simulation model based on our actual apparatus, which is used for measuring the Seebeck coefficient and the electrical conductivity of thermoelectric materials. In terms of geometry, materials, and physics, our simulation model corresponds to the essential details of the experimental device. This article describes the model in detail and highlights key technical characteristics. By comparing it with actual measurement results, the accuracy of the model has been verified. Based on this accurate model, a simulation comparison study on the changes of certain parameters was carried out. Finally, the results of these comparisons are discussed to gain a deeper understanding of the actual experimental process and to improve the accuracy of the measurement.

Synergistic effect of grain boundaries and phonon engineering in Sb substituted Bi2Se3 nanostructures for thermoelectric applications

Phonon scattering by intrinsic defects and nanostructures has been the primary strategy for minimizing the thermal conductivity in thermoelectric materials. In this work, we present the effect of Isovalent substitution as a method to decouple the Seebeck coefficient and the thermal conductivity of antimony (Sb) substituted bismuth selenide (Bi2Se3). Transmission electron microscopy studies present the nanostructured Bi2-xSbxSe3 thermoelectric system represents the coexistence of hierarchical defect structure and dislocations. The observed giant reduction in thermal conductivity is due to the multi-scale phonon scattering caused by a combination of stacking faults, lattice dislocations and grain boundary scattering. This study reveals that a large number of dislocations about ∼1.09 × 1016 m−2 are particularly effective at lowering thermal conductivity. We achieved one of the ultra-low thermal conductivity values (∼0.26 W/m K) for the maximized dislocation concentration. Moreover, Isovalent substitution provides a new avenue for the reduction in thermal conductivity and significant enhancement in the Seebeck coefficient of thermoelectric materials.

The effect of ultrasmall grain sizes on the thermal conductivity of nanocrystalline silicon thin films

Nanocrystallization has been an important approach for reducing thermal conductivity in thermoelectric materials due to limits on phonon mean-free path imposed by the characteristic structural size. We report on thermal conductivity as low as 0.3 Wm−1K−1 of nanocrystalline silicon thin films prepared by plasma-enhanced chemical-vapor deposition as grain size is reduced to 2.8 nm by controlling hydrogen dilution of silane gas during growth. A multilayered film composed by alternating growth conditions, with layer thicknesses of 3.6 nm, is measured to have a thermal conductivity 30% and 15% lower than its two constituents. Our quantitative analysis attributes the strong reduction of thermal conductivity with decreasing grain size to the magnifying effect of porosity which occurs concomitantly due to increased mass density fluctuations. Our results demonstrate that ultrasmall grain sizes, multilayering, and porosity, all at a similar nanometer-size scale, may be a promising way to engineer thermoelectric materials.

Cumulative defect structures for experimentally attainable low thermal conductivity in thermoelectric (Bi,Sb)2Te3 alloys

Manipulation of thermal transport properties by defect engineering is an important but yet unresolved issue in thermoelectrics, since rational design strategies of multiple defect structures for minimizing lattice thermal conductivity are lacking. The key is to comprehend complex interaction between different frequency-dependent phonon scattering processes by multiple defect structures. Herein, individual contributions in lattice thermal conductivity reduction from each defect structure—point defects (0-dimensional, 0D), dislocations (1D), grain boundaries (2D), and nanoparticles (3D)—are characterized based on experimental results of commercial (Bi,Sb)2Te3-based alloys fitted to Debye-Callaway model. Then, the cumulative contributions by multiple defect structures are investigated interactively by estimating total phonon relaxation time. Therefore, the interactive role of multiple defect structures in reducing lattice thermal conductivity is elucidated. To extend the approach to other thermoelectric materials, PbTe-based alloys with multiple defect structures of point defects and grain boundaries were modeled as well. This approach enables to provide a thorough design of defect structures for realizing the experimentally attainable low lattice thermal conductivity in thermoelectric materials.

Investigation of structural, thermodynamic and energy state characteristics of the ZrNi1-xRhxSn solid solution

The peculiarities of crystal and electronic structures, thermodynamic and energy state characteristics of the ZrNi1-xRhxSn semiconductive solid solution were investigated. It has been shown that in the ZrNiSn compound simultaneously exist two types of structural defects of the donor nature which generate two donor bands with different ionization energy in the band gap: a) the donor band ɛD1, formed as a result of a partial, up to ~ 1%, occupation of 4a position of Zr atoms by Ni atoms (mechanism of “a priori doping”) and deep donor band ɛD2, formed as a result of partial occupation of the tetrahedral voids by Ni atoms (Vac). The substitution in 4c position of the Ni atoms by Rh ones in ZrNi1-xRhxSn generates structural defects of acceptor nature and creates an impurity acceptor band ɛA in the band gap, which, in addition to the existence of ɛD1 та ɛD2 donor bands, makes semiconductor highly doped and strongly compensated. The obtained results allow to understand the mechanisms of electrical conductivity of thermoelectric materials based on n-ZrNiSn and the ways of conscious optimization of their characteristics for obtaining the maximum efficiency of conversion of thermal energy into electric.

Effect of Planar Rattling on Suppression of Thermal Conductivity in Thermoelectric Materials

We studied the thermoelectric properties, crystal structure, and phonon dynamics of LaOBiS2−xSex. We found that LaOBiS2−xSex has an extremely low lattice thermal conductivity of 1 W/mK around room ...

Enhanced Phonon Boundary Scattering at High Temperatures in Hierarchically Disordered Nanostructures

Boundary scattering in hierarchically disordered nanomaterials is an effective way to reduce the thermal conductivity of thermoelectric materials and increase their performance. In this work, we investigate thermal transport in silicon-based nanostructured materials in the presence of nanocrystallinity and nanopores at the range of 300–900 K using a Monte Carlo simulation approach. The thermal conductivity in the presence of nanocrystallinity follows the same reduction trend as in the pristine material. We show, however, that the relative reduction is stronger with temperature in the presence of nanocrystallinity, a consequence of the wavevector-dependent (q-dependent) nature of phonon scattering on the domain boundaries. In particular, as the temperature is raised, the proportion of large wavevector phonons increases. Since these phonons are more susceptible to boundary scattering, we show that this q-dependent surface scattering could account for as much as a ∼ 40% reduction in the thermal conductivity of nanocrystalline Si. The introduction of nanopores with randomized positions magnifies this effect, which suggests that hierarchical nanostructuring is actually more effective at high temperatures than previously thought.

Hopping Time Scales and the Phonon-Liquid Electron-Crystal Picture in Thermoelectric Copper Selenide.

The suppression of transverse phonons by liquidlike diffusion in superionic conductors has been proposed as a means to dramatically reduce thermal conductivity in thermoelectric materials [H. Lui etal. Nat. Mater. 11, 422 (2012)NMAACR1476-112210.1038/nmat3273]. We have measured the ion transport and lattice dynamics in the original phonon-liquid electron-crystal Cu_{2}Se using neutron spectroscopy. We show that hopping time scales are too slow to significantly affect lattice vibrations and that the transverse phonons persist at all temperatures. Substantial changes to the phonon spectrum occur well below the transition to the superionic phase, and the ultralow thermal conductivity is instead attributed to anharmonicity.

Experimental and modeling evidence for the reduction of thermal conductivity in Mg2Si by fine tuning the nano & micro-structural features

In this paper, experimental and modeling evidence for reduced thermal conductivity in Bi-doped Mg2Si is presented. Upon doping, lattice thermal conductivity was reduced monotonically and, at the same time, MgO content was increased while grain size was decreased. A model has been developed, which takes into account various phonon-scattering mechanisms, the 3-phonon (Umklapp) process, electron-phonon interaction and phonon-scattering by the grain-boundaries or nano-scale MgO inclusions. The model was successfully applied and excellently describes the temperature dependence of lattice thermal conductivity as well as its evolution vs. doping. Application of the model suggests that lattice thermal conductivity is mainly affected by both nano-MgO precipitates and Umklapp processes. Exploring the effect of the grain size and nano-precipitates, our model predicts that the two phonon-scattering mechanisms are interconnected; for a given grain size there is an optimal value for nano-precipitate concentration, and vice-versa. This conclusion may lead to a general approach in the optimization of lattice thermal conductivity in thermoelectric materials, by fine tuning both the nano- as well as the micro-structural features.

Electrically tunable thermal conductivity in thermoelectric materials: Active and passive control

Applications involving the use of thermoelectric materials can be found in many different areas ranging from thermocouple sensors, portable coolers, to solar power generators. Generally, they can be subdivided by the direction of energy conversion. While the Peltier effect is used in solid-state refrigeration, the Seebeck effect is responsible for the conversion of temperature gradients into electrical voltage in energy harvesting systems. However, this paper proposes a novel approach to the use of thermoelectric couples, treating them as variable insulators in thermal systems. Here, we demonstrate that thermal conductivity in thermoelectric materials can be externally controlled by electrical parameters such as electrical load or DC voltage in passive and active systems, respectively. Active mode is a good solution when a complete insulation or a high control of thermal conductivity is needed. Passive mode permits a thermal conductivity increment of 1 + ZT times with respect to semiconductor initial thermal conductivity. Results open new doors and new opportunities for 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 (Δ T). 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 Δ T lower than 59 K and rises to 12.6% with Δ T up to 251 K, while the error between the discrete numerical model-calculated output power and experiment remains lower than 2% even when Δ T is above 251 K.

Polarization field engineering of GaN/AlN/AlGaN superlattices for enhanced thermoelectric properties

A novel polarization field engineering based strategy to simultaneously achieve high electrical conductivity and low thermal conductivity in thermoelectric materials is demonstrated. Polarization based electric fields are used to confine electrons into two-dimensional electron gases in GaN/AlN/Al0.2Ga0.8N superlattices, resulting in improved electron mobilities as high as 1176 cm2/Vs and in-plane thermal conductivity as low as 8.9 W/mK. The resulting room temperature ZT values reach 0.08, a factor of four higher than InGaN and twelve higher than GaN, demonstrating the potential benefits of this polarization based engineering strategy for improving the ZT and efficiencies of thermoelectric materials.

Solvothermal synthesis of nano-sized skutterudite Co1−xNixSb3 powders

Nanostructuring is one of the effective approaches to lower the thermal conductivity of thermoelectric materials for improving its figure of merit. The nano-sized uniform skutterudite Co1−xNixSb3 (x = 0, 0.05, 0.075, 0.125, 0.15, and 0.25) thermoelectric powders were synthesized in triethylene glycol solution by using CoCl2, NiCl2, and SbCl3 as precursors and NaBH4 as the reductant. Different synthesis conditions were studied to pursue pure and uniform skutterudite CoSb3 powders. The powders were characterized by X-ray diffraction, field emission scanning electron microscope, and energy-dispersive X-ray analysis. Experimental results show that a Ni-doped skutterudite Co1−xNixSb3 single phase was obtained at 290 °C for 12 h. The powders are spherical, small, and uniform. As x increases from 0 to 0.25, the unit-cell parameter a increases from 0.9044 to 0.9065 nm and the particle size increases from 10 to 30 nm.

Ultralow Thermal Conductivity in Polycrystalline CdSe Thin Films with Controlled Grain Size

Polycrystallinity leads to increased phonon scattering at grain boundaries and is known to be an effective method to reduce thermal conductivity in thermoelectric materials. However, the fundamental limits of this approach are not fully understood, as it is difficult to form uniform sub-20 nm grain structures. We use colloidal nanocrystals treated with functional inorganic ligands to obtain nanograined films of CdSe with controlled characteristic grain size between 3 and 6 nm. Experimental measurements demonstrate that thermal conductivity in these composites can fall beneath the prediction of the so-called minimum thermal conductivity for disordered crystals. The measurements are consistent, however, with diffuse boundary scattering of acoustic phonons. This apparent paradox can be explained by an overattribution of transport to high-energy phonons in the minimum thermal conductivity model where, in compound semiconductors, optical and zone edge phonons have low group velocity and high scattering rates.

Thermoelectric Properties of Hot-Pressed Ultra-Fine Particulate Sige Powder Alloys with Inert Additions

ABSTRACTThe objective of the work reported here is to reduce the thermal conductivity of thermoelectric materials in order to improve their figureof- merit and conversion efficiency. Theory predicts that the addition of ultra-fine, inert, phonon-scattering centers to thermoelectric materials will reduce their thermal conductivity [1]. To investigate this prediction, ultra-fine particulates (20Å to 120Å) of silicon nitride have been added to boron doped, p-type, 80/20 SiGe. All of the SiGe samples produced from ultra-fine powder have lower thermal conductivities, than that for standard SiGe, but high temperature heat treatment increases the thermal conductivity back to the value for standard SiGe. However, the SiGe samples with silicon nitride, inert, phonon-scattering centers, retained the lower thermal conductivity after several heat treatments. A reduction of approximately 25% in thermal conductivity has been achieved in these samples.

Peculiarities of thermophysical measurements of thermoelectric materials

This paper examines a method for direct measurement of the heat losses in determining the thermal conductivity of thermoelectric materials.

Measurement of thermal conductivity using the Peltier effect

A modification of the Harman `Z-meter' technique for the measurement of the thermal conductivity of thermoelectric materials is described. Heat losses are taken into account in the thermal conductivity equation, and an experimental technique derived which enables the losses to be eliminated. The theory is tested by measurements on a specimen of FeSi2-4% CoSi2 alloy.