Gabriele Karpinski

Macroscopic thermoelectric inhomogeneities in (AgSbTe 2) x (PbTe) 1-x

Exceptionally high thermoelectric figure of merit (zT>2), has been reported for (Ag1−ySbTe2)0.05(PbTe)0.95, which may involve the nanoscale microstructure. However, conflicting reports on the same materials claim only zT of 1 or less. Here we show that (Ag1−ySbTe2)0.05(PbTe)0.95 materials are multiphase on the scale of millimeters despite appearing homogeneous by x-ray diffraction and routine electron microscopy. Using a scanning Seebeck microprobe, we find significant variation of Seebeck coefficient (including both n-type and p-type behavior in the same sample) that can explain the discrepancy in reported zT. More homogeneous samples can be prepared with faster cooling rates.

Effect of ceramic dispersion on thermoelectric properties of nano-ZrO2∕CoSb3 composites

In the present work, nano-ZrO2∕CoSb3 composites were fabricated by milling ZrO2 and CoSb3 powders and hot pressing at different sintering temperatures. For the prepared compacts, the phase purity, microstructure, and temperature-dependent thermoelectric properties were characterized. The effect of nano-ZrO2 dispersion on composite electrical conductivity and thermal conductivity is strictly clarified by comparing the transport properties of the nondispersed and dispersed CoSb3 at identical porosity, so that the effect of porosity on thermoelectric parameters could be eliminated. The effect of the insulating inclusion itself on transport properties is also considered and eliminated using effective media theories. It is clearly verified that charge carrier scattering and phonon scattering occur simultaneously to lower the electrical conductivity and the thermal conductivity of CoSb3 due to the introduction of nano-ZrO2 inclusions. The investigated composites show higher electrical conductivity due to existe...

Macroscopic thermoelectric inhomogeneities in (AgSbTe2)x(PbTe)1−x

Exceptionally high thermoelectric figure of merit (zT>2), has been reported for (Ag1−ySbTe2)0.05(PbTe)0.95, which may involve the nanoscale microstructure. However, conflicting reports on the same materials claim only zT of 1 or less. Here we show that (Ag1−ySbTe2)0.05(PbTe)0.95 materials are multiphase on the scale of millimeters despite appearing homogeneous by x-ray diffraction and routine electron microscopy. Using a scanning Seebeck microprobe, we find significant variation of Seebeck coefficient (including both n-type and p-type behavior in the same sample) that can explain the discrepancy in reported zT. More homogeneous samples can be prepared with faster cooling rates.

Nano ZrO2/CoSb3 composites with improved thermoelectric figure of merit

Nano ZrO2/CoSb3 composites with different ZrO2 contents were prepared using hot pressing. The phase purity, the microstructure and the temperature-dependent transport parameters of the composites were investigated. The dimensionless figure of merit (ZT) of 0.18 of the non-dispersed CoSb3 preponderates the maximal value (0.17) of pure CoSb3 reported in the literature, which is attributed to the prepared sample having higher electrical conductivity due to the existence of a small amount of metallic Sb and lower thermal conductivity due to the fine-grained structure. Compared to non-dispersed CoSb3, a further improvement of 11% on ZT (0.20) was achieved in the composite with 0.05ZrO2 inclusions, which resulted from the enhanced ratio of electrical conductivity to thermal conductivity and the Seebeck coefficient. The nanodispersion method provides an effective approach to improving a material’s thermoelectric properties and performance.

Thermoelectric properties of hot-pressed skutterudite CoSb3

In the present work, skutterudite CoSb3 were fabricated by hot pressing at different sintering temperatures under vacuum and argon. For the prepared compacts, the phase, the microstructure, and the ...

Seebeck Scanning Microprobe for Thermoelectric FGM

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.

Potential-Seebeck-microprobe (PSM): measuring the spatial resolution of the Seebeck coefficient and the electric potential

Thermoelectric power generators are typically operating in a large temperature difference; indeed the properties of thermoelectric semiconductors vary with temperature. Thus the overall conversion efficiency is strongly dependent on spatial variations of the material properties according to the temperature profile along the entire thermoelectric generator element. Similarly, a functionally graded module is capable of accomplishing thermal sensors with linearised characteristics over a wide temperature range. The Seebeck-coefficient S is a measure of the electrically active components in a material. Different components in a single unit become visible by measuring the local S with a scanning thermoprobe. This applies accordingly for the electrical conductivity and therefore the behaviour of the material in a certain temperature gradient becomes predictable. A scanning Seebeck microprobe has been combined with the measurement of the electric potential along the surface of semiconducting or metallic material. A heated probe tip is placed onto the surface of the sample under investigation, measuring the Seebeck coefficient. Using a specially designed sample holder, an AC current can be applied to the specimen, allowing for the detection of the voltage drop between one current contact and the travelling probe tip. This voltage is proportional to the electrical conductivity at the tip position. With this technique a spatially resolved imaging of the Seebeck coefficient as well as the electrical conductivity can be performed. Furthermore the electrical contact resistance between different materials becomes visible, e.g., in segmented thermoelectric or other devices.

Numerical performance estimation of segmented thermoelectric elements

Functionally graded and segmented thermoelements have been considered for long, aiming at improving the performance of thermogenerators (TEG) which are exposed to a large temperature difference. A numerical algorithm has been previously developed using the software MATHEMATICA and has been applied for modelling homogeneous and segmented Peltier elements. It is capable to calculate the exact temperature profile along a segmented element in a one-dimensional model. The algorithm is based on the constant properties assumption (CPA) in each of the segments and is also providing the opportunity of treating quasi-continuous gradients. Integral quantities like the voltage drop over the element and performance parameters like cooling power and C.O.P. (for a Peltier cooler) or output power and efficiency (for a TEG) are deduced taking into account the real temperature dependence of the materials properties. This algorithm was inserted in a loop (varying the current density) to determine optimum operation parameters at given temperature difference. Numerical parameter studies based on quasi-continuously grade elements, CPA in each of the segments, and preassuming constant volume average of the figure of merit over the whole element (ZT) provide guidelines for advantageous TE gradients in Peltier coolers and TEG. A practical example is illustrating the quantitative improvement of performance achievable by segmentation of a Peltier cooler.

Application Overview of the Potential Seebeck Microscope

The scanning Potential Seebeck Microscope (PSM) turned out to be a suitable tool to investigate material properties not only for thermoelectrics. Numerous cooperation and projects which were successfully accomplished by DLR and Panco and their national and international partners have shown the wide spectrum of application for this measurement instrument. The continuing extension of applications and further developments on this instrument were documented within several publications [Platzek, et al., 2005, Platzek, et al., 2005, Chen, et al., 2005, Ziolkowski, et al., 2006, Platzek, et al., 2003] showing the scientific output achieved by applying the PSM. With regard to the further developments which have been made and the results obtained so far, this work will give an overview of the possible applications of the PSM. This multiplexed informations will mark the present status of development and will give an outlook for further goals to reach

Processing and Characterization of Nano-structured ZrO2/CoSb3 Thermoelectric Composites

The addition of ceramic inclusion to a thermoelectric matrix could reduce the thermal conductivity of the composite, which is attributed to phonon scattering on the generated interfaces. The electrical conductivity of the composite, however, could also be reduced due to additional charge carrier scattering. The performance of the thermoelectric composite, therefore, depends on the resulting ratio of electrical conductivity to thermal conductivity, which results from the entire scattering effects on phonons and charge carriers. In the present work, nano-sized ZrO2 powders of different contents, which were expected to minimize their scattering impact on charge carriers, were dispersed into sub-micron-sized CoSb3 powders via ball milling. The as-milled powders were consolidated into dense compacts by hot pressing. The phase, the microstructure, and the thermoelectric properties of the prepared compacts were characterized. The correlation of phase purity, microstructure, and thermoelectric properties (electrical conductivity, thermal conductivity, and ratio of electrical conductivity to thermal conductivity), with the ceramic content and sintering temperature is presented. The results show how the performance of the investigated thermoelectric composites can be affected by the dispersion of nano-sized ceramic inclusions. It is noted that the selection of appropriate inclusion content is crucial to maintaining or improving the ratio of electrical conductivity to thermal conductivity