The performance of thermoelectric materials has been improved in the past few years by a significant reduction of the thermal conductivity, but this parameter has now reached its (amorphous) limit, which makes the enlargement of the power factor (PF) critical. Here we present a new concept, based on a hybrid system formed by a porous thermoelectric solid permeated by a liquid with an inert salt dissolved (electrolyte), which can be designed to significantly enhance the PF. The concept is demonstrated in an Sb-doped SnO2 porous film permeated with different inert salts (X+(BF4)−, X+ = Li+, Na+, K+) dissolved in 3-methoxypropionitrile (3-MPN). A 61.9% average decrease in the electrical resistivity without a significant variation of the Seebeck coefficient was found in the case of 1 M LiBF4 in 3-MPN. As a result, the PF was remarkably increased 3.4 times. On the other hand, 1-butyl-3-methylimidazolium (BMI+Y–, Y– = I–, BF4 –) ionic liquids were also employed as electrolytes. Using the BMII ionic liquid, the electrical resistivity showed a more significant average decrease of 82.5%; however, the absolute value of the Seebeck coefficient was reduced by 35%, finally resulting in an average enhancement of 2.4 times of the PF. The large enhancements achieved are attributed to the modification of the electrostatic environment of the porous solid by the ions in the electrolyte at the solid–liquid interface. These results establish a new strategy for the significant improvement of the PF which is not restricted to certain materials and can be potentially applied widely.
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 ability to produce nanoscale thermoelectric powders is particularly relevant to the development of nanostructured bulk materials. In this paper we report preparation of nanoscale thermoelectric powders by high-intensity ultrasonic milling (UM). The particles obtained have two unique features: the presence of a particle-size gap which distinguishes the newly created nanoparticles from the original particles, and significant changes of the surface morphology of the particles, suggesting potential manipulation of the surface states of micro or nano particles.
A proof of concept of the possibility to estimate thermal conductivity of bulk disc samples at room temperature by means of thermal decays is demonstrated. An experimental set-up was designed and fabricated, which is able to perform thermal transient measurements by using a specially designed multifunctional probe that has the ability to measure temperature at its tip. Initially, the probe is heated by a heater coil located in its interior until the tip temperature reaches a steady state. Then, the probe is contacted with a disc sample which produces a temperature decay until a new state is reached. The difference between the initial and final states temperatures shows a correlation with the thermal conductivity of the sample. Employing a calibration equation, obtained using reference materials, the thermal conductivity can be calculated. Reasonably good random and systematic errors (<13% and ~9% respectively) are obtained. Theoretical simulations performed using COMSOL show a good qualitative agreement with experimental results. This new method involves an inexpensive and simple set-up which can be especially useful for thermal conductivity screening and high-throughput measurements.
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