Applications of impedance spectroscopy in thermoelectricity

t is widely used in a lot of different fields (solar cells, fuel cells, corrosion, supercapacitors, batteries, etc.). • Powerful and very reliable equipment are available in the market. • It allows the separation of the physical processes occurring in a device

Desarrollo de pigmentos anticorrosivos mediante técnicas electroquímicas

Treball Fi de Grau en Enginyeria en Tecnologies Industrials. Codi: ET1040. Curs acadèmic: 2014-201

Nuevo método de medida de la conductividad y difusividad térmica usando un módulo termoeléctrico

Treball Final de Màster Universitari en Enginyeria Industrial. Codi: SJA020. Curs acadèmic: 2016/201

A novel vacuum pressure sensor using a thermoelectric device

We present the proof-of-concept of a new vacuum pressure sensor based on a new operating principle. The new sensor is formed by the simple combination of a thermoelectric (TE) module that is contacted at both sides by a bent copper plate. The vacuum pressure is related to the change in the thermal contact resistance that exists between the outer ceramic surfaces of the TE module and the copper plate, since heat transfer through the ceramic/copper interface is found to be influenced by the amount of air present in the interface gaps (vacuum pressure). The variations of the thermal contact resistance produce a change of the TE module voltage when a fixed current is applied to it. By monitoring this voltage simultaneously to the response of a commercial pressure sensor at different vacuum pressures inside a vacuum chamber, a calibration equation was identified, which enables obtaining the vacuum pressure from the voltage signal. Random errors lower than 10% were found in the 0.1 to 250 mbar range, which is the pressure range that the sensor can properly sense. This new device is inexpensive, simple to fabricate and integrate, and benefits from the high stability of TE modules

Impedance Spectroscopy Analysis of Thermoelectric Modules Fabricated with Metallic Outer External Layers

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.Funding for open access charge: CRUE-Universitat Jaume

Comprehensive impedance spectroscopy equivalent circuit of a thermoelectric device which includes the internal thermal contact resistances

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

Characterization of thermal contacts between heat exchangers and a thermoelectric module by impedance spectroscopy

Heat to electricity energy conversion efficiency of a thermoelectric (TE) device is not only influenced by the TE materials properties, but it also depends on the temperature difference between both sides of the TE legs. Keeping this temperature difference as close as possible to the temperature difference between the heat sink and the heat source is crucial to maximize the TE device performance. However, achieving this is quite difficult, mainly due to the thermal contact resistance at the interfaces between the TE module and the heat sink/source. In this study, it is analyzed the effect of this thermal contact resistance on the impedance spectroscopy response of a TE module that is thermally contacted by two aluminum blocks, which act as heat exchangers. A new theoretical model (equivalent circuit) that takes into account the thermal contact resistance is developed, which includes two new elements that depend on this parameter. The equivalent circuit is tested with experimental impedance measurements where the thermal contact is varied. It is demonstrated that using this equivalent circuit the thermal contact resistivity can be easily determined, which opens up the possibility of using impedance spectroscopy as a tool to quantify and monitor this crucial property for the TE device performance

Large power factor improvement in a thermoelectric oxide using liquid electrolytes

Comunicació presentada a Conference on Modern Concepts and New Materials for Thermoelectricity (11-15 Mar 2019, Trieste (Italy))• A new hybrid system formed by a nanostructured mesoporous solid permeated by a liquid electrolyte has been conceived to improve the thermoelectric power factor. • The concept has been demonstrated employing Sb:SnO2 and different electrolytes. • More than 3 times improvement in the power factor has been achieved by a 61.9 % reduction of the electric resistance of the system without modifying the Seebeck coefficient using LiBF4 1 M in 3-methoxipropionitrile. • An imidazolium iodide ionic liquid produces an 82.5 % drop in the electric resistance although with a reduction in the Seebeck coefficient, leading to 2.4 times improvement in the PF

Measurement of thermal conductivity and thermal diffusivity using a thermoelectric module

A proof of concept of using a thermoelectric module to measure both thermal conductivity and thermal diffusivity of bulk disc samples at room temperature is demonstrated. The method involves the calculation of the integral area from an impedance spectrum, which empirically correlates with the thermal properties of the sample through an exponential relationship. This relationship was obtained employing different reference materials. The impedance spectroscopy measurements are performed in a very simple setup, comprising a thermoelectric module, which is soldered at its bottom side to a Cu block (heat sink) and thermally connected with the sample at its top side employing thermal grease. Random and systematic errors of the method were calculated for the thermal conductivity (18.6 % and 10.9 %, respectively) and thermal diffusivity (14.2 % for both errors) employing a BCR724 standard reference material. Although errors are somewhat high, the technique could be useful for screening purposes or high-throughput measurements at its current state. This new method establishes a new application for thermoelectric modules as thermal properties sensors. It involves the use of a very simple setup in conjunction with a frequency response analyzer, which provides a low cost alternative to most of currently available apparatus in the market. In addition, impedance analyzers are reliable and widely spread equipment, which facilities the sometimes difficult access to thermal conductivity facilities.The authors wish to acknowledge financial support from the Ramón y Cajal programme (RYC-2013-13970) and the Accelerated Metallurgy Project, which is co-funded by the European Commission in the 7th Framework Programme (contract NMP4-LA-2011-263206), by the European Space Agency and by the individual partner organizations

Experimental conditions required for accurate measurements of electrical resistivity, thermal conductivity, and figure of merit (ZT) using Harman and impedance spectroscopy methods

The Harman method is used extensively for the characterization of the dimensionless figure of merit ZT of thermoelectric (TE) materials and devices. However, its accuracy has often been questioned, since in many cases there are relatively high errors associated with the method. The impedance spectroscopy technique, which has recently been shown as a suitable tool to also characterize TE materials and devices, has some similarities with the Harman method, and can also directly provide ZT. In order to obtain reliable measurements in both methods, there are some common critical points that must be taken into account, such as for example the requirement of fully adiabatic conditions, and a negligible Joule effect. In this study, we have evaluated the effect of different experimental conditions in the accuracy of both methods using a sample with known TE properties. Our analysis has led to the identification of different sources of errors and other issues that have not been clearly identified to date that can lead to inaccurate results. Namely, the need of a homogeneous Peltier effect at the junctions, problems arising from the use of Ag paint, and the selection of the right value for the current perturbation applied to the system. These problems and sources of error need to be identified and carefully considered if accurate results are to be obtained