Papers by Author: Eckhard Müller

Abstract: For more than a decade, skutterudites such as cobalt antimonides have been widely studied as a promising thermoelectric (TE) material for high-temperature applications. High thermoelectric figure of merit (ZT) in this material system can be achieved by suitable doping or by filling the interstitial voids with guest atoms. One of the best improvements in ZT is reported when indium (In) is used as additive to cobalt-antimonide skutterudites, as has been done in this study. Compaction of the grinded powders was carried out by a current-assisted short-term sintering device, which significantly reduces the process time in comparison to conventional hot pressing. Phase homogeneity of the bulk material has been probed by X-ray powder diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX). TE properties (i.e. electrical conductivity, Seebeck coefficient and thermal conductivity) have been analyzed in the temperature range from 300 K to 700 K. The functional homogeneity of the samples was screened by the Potential & Seebeck Microprobe (PSM). Adapted from these results the effect of indium addition to short-term sintered cobalt-antimonide based skutterudites with absence of impurity phases will be discussed.

Abstract: The FGM principle plays an important role in enhancing the efficiency of thermoelectric devices. While a thermoelectric generator (TEG) is typically operating in a large temperature difference, attractive conversion efficiency of a particular semiconductor is restricted to a small temperature range. Hence, when employing a semiconductor with its highest possible efficiency at the respective temperature in the internal temperature field along a stacked TEG, the overall conversion efficiency of the device may be considerably enhanced. Similarly, the FGM principle can be employed for linearization of thermal sensors. The output voltage (response) of the sensor is proportional to the Seebeck coefficient of the material the sensor is made of. Since the Seebeck coefficient is strongly temperature-dependent, the sensor response is not linear with temperature. However, combining in a stack two or more semiconductors which temperature dependence of the Seebeck coefficient are complementary to each other, results in a sensor with linear response (i.e. its output is proportional to the temperature difference, or heat flux, respectively.) Stacking of several materials to each other or grading a semiconducting sample requires a technique which can scan the Seebeck coefficient profiles S(x) along the stack. Accordingly a Seebeck micro-probe technique has been developed for scanning the surface of a sample monitoring S with a resolution of down to 10 µm within the temperature range from -15°C to 60°C. An additional option of such a device is the scanning of the electrical potential along the stack under current flow [1]. Thus, related experimental data on the local profiles of the electrical conductivity and Seebeck coefficient along the stack (or continuously graded FGM) will be available. The apparatus has been automated so that extended areas may be scanned providing two-dimensional images. Additionally, several samples can be scanned in one automatic run.

Abstract: Commercialization of Peltier coolers has progressed during last years and special efforts have been undertaken to enhance the efficiency of thermoelectric (TE) devices. Along with the continued search for advanced TE materials, the concept of FGM offers a strategy of gradual improvement of device performance. In reality a functional gradient in a TE material means a related spatial variation of all TE properties – Seebeck coefficient, electrical, and thermal conductivity – whereas the most relevant effect is linked to the gradient of the Seebeck coefficient. Due to the spatial dependence of the Seebeck coefficient, Peltier heat is absorbed or released inside the TE element under current flow (distributed Peltier effect) which can be exploited to shape the internal temperature profile in a desired manner. Starting from the first principles of thermoelectricity, a differential equation governing the coupling of thermal and electrical transport is derived within the frame of a one-dimensional model. It is shown that this approach can be also used to model multi-segment Peltier cooling devices. Temperature profiles T(x) have been calculated for a segmented TE element within the framework of a constant parameters theory. The work presents an analytical model for performance evaluation of multiply-segmented Peltier elements. The problem is treated in a one-dimensional approach for a p-type stack containing N segments of different properties. Assuming constant TE material properties in each of the segments, the differential equation of TE transports has been solved to obtain the temperature profile T(x) in each segment. With the material properties values in each segment representing volume average values this model gives an excellent approximation also for continuously graded elements. The boundary conditions of the TE problem set-up, as conservation of heat at any intermediate junction between the segments, and fixed temperature at the cold and hot end of the element, lead to a linear equation system, which can be easily solved by means of standard methods. From the solution, all desired performance parameters can be deduced. Based on realistic material data exemplary calculations are presented for stacked and continuously graded elements. To demonstrate the developed numerical algorithm, gradients of the Seebeck coefficient are mainly considered. Calculations have been performed for N = 2, 5, 10, and continuous gradients. As target parameters, the C.O.P. and the cooling power have been calculated as functions of the electric current. As well, the minimum temperature of the cold side has been determined for various shape of the Seebeck gradient. It is shown that the TE FGM effect can be almost completely utilized already by a stack of two to five homogeneous segments. The results allow for giving an estimation on the order of magnitude of performance improvement of both discontinuously and continuously graded Peltier cooling devices. The model calculation was implemented with the software tool MATHEMATICA. The code provides an easy to handle convenient instrument for performance estimation of non-homogeneous Peltier pellets. Technological studies for controlled fabrication of those pellets are underway.

Abstract: Recently, there is a strong interest in developing superior thermoelectric materials with the aim to improve the performance of a thermoelectric device. However, the performance of a thermoelectric generator (TEG) can be considerably improved also by applying a graded composition along the temperature gradient inside the thermoelectric device so that at each position the respective material achieves its maximum thermoelectric performance (TE FGM principle). Combining the high efficiency of Bi2Te3 (used at low temperatures) and general durability of FeSi2 (applied up to high temperatures) will result in a thermoelectric device with enhanced efficiency operating in air at a wide temperature range. The challenge is to contact these dissimilar materials without any negative impact on the performance of TEG. Besides providing a good electrical and thermal contact between Bi2Te3 and FeSi2 the junctions have to remain mechanically and chemically stable over long term. A Bi2Te3-SiO2 composite interlayer was used to adjust the different coefficient of thermal expansion (CTE) of FeSi2 (≈ 7·10-6 K-1) and Bi2Te3 (≈ 19·10-6 K-1). Due to low chemical stability of the Bi2Te3/FeSi2 contact at elevated temperatures (1000 h @ 300°C) a contacting material (diffusion barrier) based on Ni, Zn, Ti, and ZnTe was tested. Some contacts show excellent chemical and mechanical stability, though the electrical properties of the contacts do not meet the requirements (e.g. ZnTe is a wide gap semiconductor with high electrical resistivity). According to very recent studies at the Zn-based diffusion barriers a very thin layer of undoped ZnTe growing at the Bi2Te3/Zn interface causes the deterioration of the contact resistance. Ideas solving this problem are outlined and discussed.