• Energy Research

Thermoelectric devices are widely used as solid-state refrigerators and have potential energy generation appli-cations. Their characterization is key to develop more efficient devices and monitor their performance. Electrical impedance spectroscopy has been proved to be a useful method for the characterization of thermoelectric modules. However, deviations from current impedance models still exist in experimental results, especially in the high frequency part of the impedance spectrum, which limits its use. Here, we present a new comprehensive impedance model (equivalent circuit) which covers all the key phenomena that affects the module performance, and it is able to explain the observed deviations. The new equivalent circuit includes, as new additions, the thermal influence of the metallic strips (electrodes), combined with the thermal contact resistance between the metallic strips and the outer ceramic layer. Moreover, a new more accurate spreading-constriction impedance element, which considers the variation of the heat flow in the radial direction at the outer ceramic surfaces, is also developed. The comprehensive equivalent circuit was used to perform fittings to impedance spectroscopy measurements of modules fabricated by different manufacturers. From the fittings, it was possible to identify, among other key properties, the internal thermal contact resistances, whose direct determination is very chal-lenging. Thermal contact resistivities at the metallic strips/thermoelectric elements interface in the range 2.20 ×10-6-1.26 ×10-5 m2KW 1 were found. An excellent thermal contact was identified at the metallic strips/ceramic layers. This opens up the possibility of using impedance spectroscopy as a powerful tool to evaluate, monitor, and identify issues in thermoelectric devices. Funding for open access charge: CRUE-Universitat Jaume I

Thermoelectric (TE) devices can convert heat into electricity and have gained great interest as energy harvesters in many applications, such as self-powered sensors, solar TE generators and industrial waste heat recovery. Not many techniques are usually available for their characterization under operating conditions. Consequently, only current-voltage (power output) curves are typically reported to account for their performance, which provide limited information. Impedance spectroscopy, which has been mainly applied to the study of TE modules suspended in air or vacuum, has the advantage to be used under operating conditions. Here, we exploit this advantage and provide an in-depth analysis of the impedance response of standard TE devices when they operate under a small temperature difference. First, we derive the physical models (equivalent circuits) for the two most common operating conditions: (i) constant temperature difference and (ii) constant heat power input. Then, we use the obtained equivalent circuit to successfully fit experimental impedance measurements in both operating modes, which show significant differences. From the fittings, it is possible to obtain the thermal contact resistance between the TE device and the heat exchangers (heat source and heat sink) among other key parameters. In addition, we analyze the variations found in the impedance spectra for different current output levels. Moreover, we show how different electrical resistances, extracted from the impedance results, directly relate to the slope of the I-V curve (power output), and can provide very valuable information about the different processes that determine its value. All these results establish impedance spectroscopy as a very powerful tool to characterize TE devices under operating conditions and as a method that can provide highly valuable insights into their performance. Funding for open access charge: CRUE-Universitat Jaume I The authors acknowledge financial support from the Spanish Agencia Estatal de Investigación under the Ramón y Cajal Program (RYC-2013-13970), from the Generalitat Valenciana and the European Social Fund under the ACIF program (ACIF/2018/233) and BEFPI program (BEFPI/2019/030), from the Universitat Jaume I under the project UJI-A2016-08, and the technical support of Raquel Oliver Valls and José Ortega Herreros.

Thermoelectric materials can directly convert waste heat into electricity. Because of the vast amount of energy available as waste heat in our society, these materials could contribute to reduce our dependence on fossil fuels and their associated environmental problems. However, the heat to electricity conversion efficiency of thermoelectric materials is still a limiting factor, and extensive efforts are being undertaken to improve their performance. The search for more efficient materials is focused on the optimization of three properties (Seebeck coefficient, electrical resistivity, and thermal conductivity). Typically, these are determined as a function of temperature through independent measurements on two or more instruments, making thermoelectric characterization tedious and time consuming, which complicates the attainment of more efficient heat to electricity energy conversion. Here, it is demonstrated for the first time that complete thermoelectric characterization of a material may be achieved from single electrical measurement performed on one instrument only by employing the impedance spectroscopy method. A skutterudite sample is used for the demonstration, which is sandwiched between two stainless steel contacts in a four-probe arrangement and their properties are determined from 50 to 250 °C. This new approach shows good precision and agrees with characterization of the same sample performed with commercial equipment, illustrating the power of the technique to facilitate the rapid and efficient evaluation of thermoelectric materials.

A remarkable more than 3 times improvement in the power factor has been achieved in a porous thermoelectric oxide in the presence of an electrolyte containing a redox molecule.

In recent years, thermoelectric (TE) devices have been used in several refrigeration applications and have gained attention for energy generation. To continue the development of devices with higher efficiency, it is necessary not only to characterize their materials but also to optimize device parameters (e.g., thermal contacts). One attempt to increase the efficiency at the device level consists of the replacement of the typical ceramic layers in TE modules by metallic plates, which have higher thermal conductivity. However, this alternative device design requires the use of a very thin electrical insulating layer between the metallic strips that connect the TE legs and the outer external layers, which introduces an additional thermal resistance. Impedance spectroscopy has been proved to be useful to achieve a detailed characterization of TE modules, being even capable to determine the internal thermal contact resistances of the device. For this reason, we use here the impedance method to analyze the device physics of these TE modules with outer metallic plates. We show for the first time that the impedance technique is able to quantify the thermal contact resistances between the metallic strips and the outer layers, which is very challenging for other techniques. Finally, we discuss from our analysis the prospects of using TE modules with external metallic plates.

Skutterudite-based thermoelectric materials are promising candidates for waste heat recovery applications at intermediate temperatures (300–500 °C) owing to their high dimensionless figure of merit and power factor. Recently, several researchers have reported the high performance of skutterudite-based thermoelectric devices obtained by optimizing the crystal structure and microstructure of skutterudite materials and developing metallization layers for device fabrication. Despite extensive research efforts toward maximizing the power density and thermoelectric conversion efficiency of skutterudite-based devices, the thermoelectric properties of such devices after fabrication remain largely unknown. Here, we systematically investigated the factors that affect the thermoelectric properties of skutterudite-based devices within the range of practical operating temperatures (23–450 °C). We successfully prepared a two-couple skutterudite-based device with titanium metallization layers on both sides of the thermoelectric legs and characterized it using scanning and transmission electron microscopy and specific contact resistance measurements. Impedance spectroscopy measurements of the two-couple skutterudite-based device revealed the figure of merit of the device and enabled the extraction of three key thermoelectric parameters (Seebeck coefficient, thermal conductivity, and electrical conductivity). The impedance spectra and extracted parameters depended strongly on the measurement temperature and were mainly attributable to the thermoelectric properties of skutterudite materials. These observations demonstrate the interplay between the properties of thermoelectric materials and devices and can aid in directing future research on thermoelectric device fabrication.

Impedance spectroscopy has been shown as a promising method to characterize thermoelectric (TE) materials and devices. In particular, the possibility to determine the thermal conductivity λ, electrical conductivity σ, and the dimensionless figure of merit ZT of a TE element, if the Seebeck coefficient S is known, has been reported, although so far for a high-performance TE material (Bi2Te3) at room temperature. Here, we demonstrate the capability of this approach at temperatures up to 250 °C and for a material with modest TE properties. Moreover, we compare the results obtained with values from commercial equipment and quantify the precision and accuracy of the method. This is achieved by measuring the impedance response of a skutterudite material contacted by Cu contacts. The method shows excellent precision (random errors < 4.5% for all properties) and very good agreement with the results from commercial equipment (<4% for λ, between 4% and 6% for σ, and <8% for ZT), which proves its suitability to accurately characterize bulk TE materials. Especially, the capability to provide λ with good accuracy represents a useful alternative to the laser flash method, which typically exhibits higher errors and requires the measurement of additional properties (density and specific heat), which are not necessarily needed to obtain the ZT.