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Thermoelectric devices could play an important role in making efficient use of our energy resources but their efficiency would need to be increased for their wide scale application. There is a multidisciplinary search for materials with an enhanced thermoelectric responses for use in such devices. This volume covers the latest ideas and developments in this research field, covering topics ranging from the fabrication and characterization of new materials, particularly those with strong electron correlation, use of nanostructured, layered materials and composites, through to theoretical work to gain a deeper understanding of thermoelectric behavior. It should be a useful guide and stimulus to all working in this very topical field.
For the efficient utilization of energy resources and the minimization of environmental damage, thermoelectric materials can play an important role by converting waste heat into electricity directly. Nanostructured thermoelectric materials have received much attention recently due to the potential for enhanced properties associated with size effects and quantum confinement. Nanoscale Thermoelectrics describes the theory underlying these phenomena, as well as various thermoelectric materials and nanostructures such as carbon nanotubes, SiGe nanowires, and graphene nanoribbons. Chapters written by leading scientists throughout the world are intended to create a fundamental bridge between thermoelectrics and nanotechnology, and to stimulate readers' interest in developing new types of thermoelectric materials and devices for power generation and other applications. Nanoscale Thermoelectrics is both a comprehensive introduction to the field and a guide to further research, and can be recommended for Physics, Electrical Engineering, and Materials Science departments.
This work presents the development and application of analytical, numerical, and experimental methods for the study of thermal and electrical transport in nanoscale systems, with special emphasis on those materials and phenomena which can be important in thermoelectric and semiconductor device applications. Analytical solutions to the Boltzmann transport equation (BTE) using the relaxation time approximation (RTA) are presented and used to study the thermal and electrical transport properties of indium antimonide (InSb), indium arsenide (InAs), bismuth telluride (Bi2Te3), and chromium disilicide (CrSi2) nanowires. Experimental results for the thermal conductivity of single layer graphene supported by SiO2 were analyzed using an RTA-based model and compared to a full quantum mechanical numerical BTE solution which does not rely on the RTA. The ability of these models to explain the measurement results as well as differences between the two approaches are discussed. Alternatively, numerical solutions to the BTE may be obtained statistically through Monte Carlo simulation for complex geometries which may prove intractable for analytical methods. Following this approach, phonon transport in silicon (Si) sawtooth nanowires was studied, revealing that thermal conductivity suppression below the diffuse surface limit is possible. The experimental investigation of energy transport in nanostructures typically involved the use of microfabricated devices or non-contact optical methods. In this work, two such approaches were analyzed to ascertain their thermal behavior and overall accuracy as well as areas for possible improvement. A Raman spectroscopy-based measurement design for investigating the thermal properties of suspended and supported graphene was examined analytically. The resulting analysis provided a means of determining from measurement results the thermal interface conductance, thermal contact resistance, and thermal conductivity of the suspended and supported graphene regions. Previously, microfabricated devices of several different designs have been used to experimentally measure the thermal transport characteristics of nanostructures such as carbon nanotubes, nanowires, and thin films. To ascertain the accuracy and limitations of various microdevice designs and their associated conduction analyses, finite element models were constructed using ANSYS and measurements of samples of known thermal conductance were simulated. It was found that designs with the sample suspended were generally more accurate than those for which the sample is supported on a bridge whose conductance is measured separately. The effects of radiation loss to the environment of certain device designs were also studied, demonstrating the need for radiation shielding to be at temperatures close to that of the device substrate in order to accurately calibrate the resistance thermometers. Using a suspended microdevice like those analyzed using finite element analysis, the thermal conductivities of individual bismuth (Bi) nanowires were measured. The results were correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk Bi in the same crystal direction, the thermal conductivity of a single-crystal Bi nanowires of 232 nm diameter was found to be 3 - 6 times smaller than bulk between 100 K and 300 K. For polycrystalline Bi nanowires of 74 nm to 255 nm diameter the thermal conductivity was reduced by a factor of 18 - 78 over the same temperature range. Comparable thermal conductivity values were measured for polycrystalline nanowires of varying diameters, suggesting a grain boundary scattering mean free path for all heat carriers in the range of 15 - 40 nm which is smaller than the nanowire diameters. An RTA-based transport model for both charge carriers and phonons was developed which explains the thermal conductivity suppression in the single-crystal nanowire by considering diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 1019 cm−3. Using a similar experimental setup, the thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of higher manganese silicide (HMS) nanostructures were investigated. Bulk HMS is a passable high temperature thermoelectric material which possesses a complex crystal structure that could lead to very interesting and useful nanoscale transport properties. The thermal conductivities of HMS nanowires and nanoribbons were found to be reduced by 50 - 60 % compared to bulk values in the same crystal direction for both nanoribbons and nanowires. The measured Seebeck coefficient data was comparable or below that of bulk, suggesting unintentional doping of the samples either during growth or sample preparation. Difficulty in determining the amorphous oxide layer thickness for nanoribbons samples necessitated using the total, oxide-included cross section in the thermal and electrical conductivity calculation. This in turn led to the determined electrical conductivity values representing the lower bound on the actual electrical conductivity of the HMS core. From this approach, the measured electrical conductivity values were comparable or slightly below the lower end of bulk electrical conductivity values. This oxide thickness issue affects the determination of the HMS nanostructure thermoelectric figure of merit ZT as well, though the lower bound values obtained here were found to still be comparable to or slightly smaller than the expected bulk values in the same crystal direction. Analytical modeling also indicates higher doping than in bulk. Overall, HMS nanostructures appear to have the potential to demonstrate measurable size-induced ZT enhancement, especially if optimal doping and control over the crystallographic growth direction can be achieved. However, experimental methods to achieve reliable electrical contact to quality four-probe samples needs to be improved in order to fully investigate the thermoelectric potential of HMS nanostructures.
Thermoelectric generators are able to directly convert heat into electrical energy. They could have a great potential in terms of energy harvesting, but unfortunately, the best thermoelectric materials are rare and pollutant.Silicon and Germanium would be attractive materials if their thermoelectric efficiency were improved. For this purpose, nanostructuring is a possible route, for instance via the introduction of rough boundaries or interfaces between materials.Recently, polytype nanowires (composed of a sequence of cubic and hexagonal phases of Si and Ge) have been fabricated, but the experimental characterization of such complex nanostructures with exotic materials is challenging.In this thesis, we study the details of thermal transport in nanostructures with numerical simulations. An original Monte Carlo method is developed, with a full band emph{ab initio} description of materials. It includes models for the rough boundaries and the solid-solid interfaces. Molecular Dynamics simulations are also performed to characterize the properties of interfaces.We confirm that the hexagonal phases of Si and Ge have lower thermal conductivity than their cubic counterparts. The full band model shows a strong anisotropy in the heat flux.Usual semi-analytical models failed to reproduce the thermal conductivity of simulated nanostructures with rough boundaries. Besides, those boundaries tend to focus the heat flux in the main direction of the nanostructure. Finally, some polytype interfaces can have an interfacial conductance almost as low as Si/Ge interfaces, and thus could improve significantly the thermoelectric efficiency of polytype nanowires. The presented Monte Carlo method could easily be used with a wide range of materials,and it can model arbitrarily complex nanostructures. In the future, the results from Molecular Dynamics simulation will be used to parametrize a more realistic model of solid-solid interfaces.
Edited by the initiators of a priority research program funded by the German Science Foundation and written by an international team of key players, this is the first book to provide an overview of nanostructured thermoelectric materials -- putting the new developments into perspective alongside conventional thermoelectrics. As such, it reviews the current state of research on thermoelectric Bi2Te3 nanomaterials, covering advanced methods of materials synthesis, characterization of materials structures and thermoelectric properties, as well as advances in the theory and modeling of transport properties. Nanomaterials-based thermoelectric devices are also discussed with respect to their properties, their suitability for different energy generation applications, and in light of their commercialization potential. An outlook on the chances, challenges and future directions of research rounds off the book, giving a straightforward account of the fundamental and technical problems - plus ways to overcome them.
The first book of its kind?providing comprehensive information on oxide thermoelectrics This timely book explores the latest research results on the physics and materials science of oxide thermoelectrics at all scales. It covers the theory, design and properties of thermoelectric materials as well as fabrication technologies for devices and their applications. Written by three distinguished materials scientists, Oxide Thermoelectric Materials reviews: the fundamentals of electron and phonon transport; modeling of thermoelectric modules and their optimization; synthetic processes, structures, and properties of thermoelectric materials such as Bi2Te3- and skutterudite-based materials and Si-Ge alloys. In addition, the book provides a detailed description of the construction of thermoelectric devices and their applications. -Contains fundamentals and applications of thermoelectric materials and devices, and discusses their near-future perspectives -Introduces new, promising materials and technologies, such as nanostructured materials, perovskites, and composites -Paves the way for increased conversion efficiencies of oxides -Authored by well-known experts in the field of thermoelectrics Oxide Thermoelectric Materials is a well-organized guidebook for graduate students involved in physics, chemistry, or materials science. It is also helpful for researchers who are getting involved in thermoelectric research and development.
Ten years ago, D.M. Rowe introduced the bestselling CRC Handbook of Thermoelectrics to wide acclaim. Since then, increasing environmental concerns, desire for long-life electrical power sources, and continued progress in miniaturization of electronics has led to a substantial increase in research activity involving thermoelectrics. Reflecting the latest trends and developments, the Thermoelectrics Handbook: Macro to Nano is an extension of the earlier work and covers the entire range of thermoelectrics disciplines. Serving as a convenient reference as well as a thorough introduction to thermoelectrics, this book includes contributions from 99 leading authorities from around the world. Its coverage spans from general principles and theoretical concepts to material preparation and measurements; thermoelectric materials; thermoelements, modules, and devices; and thermoelectric systems and applications. Reflecting the enormous impact of nanotechnology on the field-as the thermoelectric properties of nanostructured materials far surpass the performance of conventional materials-each section progresses systematically from macro-scale to micro/nano-scale topics. In addition, the book contains an appendix listing major manufacturers and suppliers of thermoelectric modules. There is no longer any need to spend hours plodding through the journal literature for information. The Thermoelectrics Handbook: Macro to Nano offers a timely, comprehensive treatment of all areas of thermoelectrics in a single, unified reference.