Thermodynamics of Thermoelectric Phenomena and Applications

Fifty years ago, the optimization of thermoelectric devices was analyzed by considering the relation between optimal performances and local entropy production. Entropy is produced by the irreversible processes in thermoelectric devices. If these processes could be eliminated, entropy production would be reduced to zero, and the limiting Carnot efficiency or coefficient of performance would be obtained. In the present review, we start with some fundamental thermodynamic considerations relevant for thermoelectrics. Based on a historical overview, we reconsider the interrelation between optimal performances and local entropy production by using the compatibility approach together with the thermodynamic arguments. Using the relative current density and the thermoelectric potential, we show that minimum entropy production can be obtained when the thermoelectric potential is a specific, optimal value

Counterintuitive example on relation between ZT and thermoelectric efficiency

The thermoelectric figure of merit ZT, which is defined using electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature T, has been widely used as a simple estimator of the conversion efficiency of a thermoelectric heat engine. When material properties are constant or slowly varying with T, a higher ZT ensures a higher maximum conversion efficiency of thermoelectric materials. However, as material properties can vary strongly with T, efficiency predictions based on ZT can be inaccurate, especially for wide-temperature applications. Moreover, although ZT values continue to increase, there has been no investigation of the relationship between ZT and the efficiency in the higher ZT regime. In this paper, we report a counterintuitive situation by comparing two materials: although one material has a higher ZT value over the whole operational temperature range, its maximum conversion efficiency is smaller than that of the other. This indicates that, for material comparisons, the evaluation of exact efficiencies as opposed to a simple comparison of the ZTs is necessary in certain cases.Comment: 12 pages, 2 tables, 2 figure

Organic Micropollutants in Small River Systems – Occurrence, Fate and Effects

The number of anthropogenic organic chemicals is continuously increasing and with it their substantial use in, for instance, industrial and domestic applications, agriculture and medical use. Many of these compounds are intentionally synthesized to be persistent (e.g., flame retardants, coatings) or bioactive (e.g., pesticides, pharmaceuticals), with some exhibiting the unintended characteristic of being bioaccumulative (e.g., polycyclic aromatic hydrocarbons, per- and polyfluoroalkyl substances). Thus, they pose an imminent risk to the environment with potentially far-reaching implications. Organic micropollutants are released into the aquatic environment through various input sources such as surface runoff, wastewater treatment plant effluents, combined sewer overflows, industrial sites or are, in the case of pesticides, intentionally dispersed into the environment. Inputs can be point or diffuse sources that are often not easy to identify and characterize. Rivers act as a conduit for micropollutant transport, integrating diffuse and point sources within their entire catchment and hosting important transformation processes. Therefore, the present work aimed to gain a detailed understanding of the occurrence and (toxic) effects of organic micropollutants in rivers that is required to predict their fate in the environment. In this doctoral thesis, I investigated the chemical and toxicological profile of the Ammer River, in Southwest Germany, under varying hydrological conditions. The Ammer River is representative of small rivers in karstic systems and in densely populated temperate climate regions. I characterized input sources and the fate of organic micropollutants at the scale of the Ammer catchment via a combination of chemical analysis (liquid chromatography–mass spectrometry) and environmentally relevant in-vitro bioassays, during a series of field sampling campaigns. Organic indicator chemicals were selected based on their application, environmental relevance, occurrence in previous studies and degradability in order to indicate different input sources and in-stream processes. The bioanalytical test battery was further improved and complemented by the development of a novel in vitro bioassay (Oxygen Consumption Rate assay) that can account for two different modes of action of mitochondrial toxicity in environmental samples. Under dry weather conditions and using chemical analysis and in-vitro bioassays, I identified a wastewater treatment plant as the major input source of organic micropollutants governing the chemical and toxicological profile of the Ammer River. Organic micropollutants were uncovered to be discharged from different input sources, with the wastewater treatment plant as the dominant input source of pharmaceuticals, industrial and household chemicals and biocides. In an 8 km long section of the Ammer, downstream of the wastewater treatment plant, compound concentrations and biological effects decreased and dilution and loss processes were uncovered. The tributaries in that river section contributed little to the overall load of compounds and mixture effects in the Ammer due to their relatively low discharge, but showed a different chemical and toxicological profile. During a storm event, the chemical and toxicological profile of the Ammer significantly changed. The numbers, concentrations and fluxes of organic micropollutants and associated effects were generally higher and suspended particulate matter turned out to be an important transport vector for effects and for hydrophobic target compounds. Organic micropollutants discharged from agricultural and urban areas, combined sewer overflow and the wastewater treatment plant. Thus, changing hydrological conditions may trigger the occurrence and the increase of organic micropollutant concentrations, mass fluxes, associated effects and the importance of particle-facilitated transport in rivers, which can pose a risk to the aquatic environment and are, at present, not considered in risk assessment and management options. Although both approaches, chemical analysis and in-vitro bioassays, are complementary as they cover different compounds and compound classes, they showed a similar pollution profile of the Ammer. In this thesis I showed that the chemical and toxicological profile of a river is highly variable in time and space, specifically driven by varying hydrological conditions that act to partition input sources of organic micropollutants, and thus their individual mass-flux contribution. It is therefore important for the investigation of the fate of organic micropollutants in rivers, of factors driving in-stream processes (e.g., degradation, sorption) and for future regulatory monitoring efforts to consider and properly capture these spatiotemporal variations

An investigation of the water supply at Charleston, Illinois

Thesis (BS)--University of Illinois, 1915Typescrip

Impact of the Dopant Species on the Thermomechanical Material Properties of Thermoelectric Mg2Si0.3Sn0.7

Thermoelectric generators are an excellent option for waste heat reuse. Materials for such devices have seen their thermoelectric properties improving constantly. The functioning of a generator, however, does not only depend on thermoelectric properties. Thermal and mechanical properties play a decisive role in the feasibility of any thermoelectric generator. To shed light on the properties exhibited by thermoelectric materials, we present the temperature dependent characterization of Young’s modulus and coefficient of thermal expansion for Mg(2)Si(0.3)Sn(0.7). Comparing undoped to Bi-doped n-type and Li-doped p-type material, we investigated the influence of doping in the relevant temperature regime and found the influences to be minor, proving similar properties for n- and p-type. We found a Young’s modulus of 84 GPa for the p-type and 83 GPa for the n-type, similar to that of the undoped compound with 85 GPa. The thermal expansion coefficients of undoped, as well as n- and p-type were equally similar with values ranging from 16.5 to 17.5 × 10(−6) 1/K. A phase analysis was performed to further compare the two materials, finding a similar phase distribution and microstructure. Finally, using the gathered data, estimations on the possible thermally induced stresses under a temperature difference are provided to evaluate the relevance of knowing temperature dependent thermal and mechanical properties

Upscaled Synthesis of n‐and p‐Type Thermoelectric Skutterudite Single Legs by Gas Atomization and Current‐Assisted Sintering

CoSb3‐based Skutterudites are among the best materials for thermoelectric generator (TEG) applications in the intermediate temperature range up to 500 °C. Synthesis of these materials is usually performed on a laboratory scale in materials research. In order to be suitable for an industrial low cost production of TEG technologies capable of delivering large amounts of thermoelectric (TE) materials are needed. A process mastering this challenge is gas atomization, which has been adapted to the requirements of TE materials, in particular CoSb3‐based Skutterudites .It is found that despite rapid solidification taking place in the atomization process the produced powder material contains only traces of the target Skutterudite phase. Microstructure investigation shows a very fine dispersion on the micrometer scale of CoSb, CoSb2, and Sb phases in the atomized particles, making diffusion paths for the formation of the Skutterudite phase short. This allows the use of short‐term heat treatment to achieve almost single phase material of high functional homogeneity. Different thermal post‐treatments are evaluated leading to a content of >98% of the Skutterudite phase in large ingots. Doping and filling by varying the starting composition is applied to tune the materials to n‐ and p‐type conduction, respectively, and led to an increase of their thermoelectric figure of merit ZT up to values of 0.9 and 0.72 for n‐ and p‐type material, respectively. Gas atomization can be used in combination with current‐assisted hot pressing to produce p‐ and n‐type Skutterudite single legs in large quantities in comparably short time. The results on ZT shown in the present work are measured on still not optimized materials and prove the possibility of Skutterudites fabrication by gas atomization with moderate ZT values

Native point defects and low pp-doping efficiency in Mg2(Si,Sn)Mg_2 (Si,Sn) solid solutions: A hybrid-density functional study

We perform hybrid-density functional calculations to investigate the charged defect formation energy of native point defects in $Mg_2 Si$, $Mg_2 Sn$, and their solid solutions. The band gap correction by hybrid-density functional is found to be critical to determine the charged defect density in these materials. For $Mg_2 Si$, $Mg$ interstitials are dominant and provide unintentional $n$-type conductivity. Additionally, as the $Mg$ vacancies can dominate in $Mg$-poor $Mg_2 Sn$, $p$-type conductivity is possible for $Mg_2 Sn$. However, the existence of low formation energy defects such as $Mg_{Sn}^{1+}$ and $I_{Mg}^{2+}$ in $Mg_2 Sn$ and their diffusion can cause severe charge compensation of hole carriers resulting in low $p$-type doping efficiency and thermal degradation. Our results indicate that, in addition to the extrinsic doping strategy, alloying of $Mg_2 Si$ with $Mg_2 Sn$ under $Mg$-poor conditions would be necessary to enhance the $p$-type conductivity with less charge compensation.Comment: Main: 17 pages (including title, abstract, main, references, figure captions. 4 figures). This manuscript is accepted for publication in JALCOM. The article will be published as Gold Open Acces

Phase Noise of SAW Delay Line Magnetic Field Sensors

Surface acoustic wave (SAW) sensors for the detection of magnetic fields are currently being studied scientifically in many ways, especially since both their sensitivity as well as their detectivity could be significantly improved by the utilization of shear horizontal surface acoustic waves, i.e., Love waves, instead of Rayleigh waves. By now, low-frequency limits of detection (LOD) below 100 pT/Hz can be achieved. However, the LOD can only be further improved by gaining a deep understanding of the existing sensor-intrinsic noise sources and their impact on the sensor's overall performance. This paper reports on a comprehensive study of the inherent noise of SAW delay line magnetic field sensors. In addition to the noise, however, the sensitivity is of importance, since both quantities are equally important for the LOD. Following the necessary explanations of the electrical and magnetic sensor properties, a further focus is on the losses within the sensor, since these are closely linked to the noise. The considered parameters are in particular the ambient magnetic bias field and the input power of the sensor. Depending on the sensor's operating point, various noise mechanisms contribute to f0 white phase noise, f-1 flicker phase noise, and f-2 random walk of phase. Flicker phase noise due to magnetic hysteresis losses, i.e. random fluctuations of the magnetization, is usually dominant under typical operating conditions. Noise characteristics are related to the overall magnetic and magnetic domain behavior. Both calculations and measurements show that the LOD cannot be further improved by increasing the sensitivity. Instead, the losses occurring in the magnetic material need to be decreased