Dissertations / Theses on the topic 'Thermopower'
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1
Koumoto, K., W. S. Seo, and S. Ozawa. "Huge thermopower of porous Y_2O_3." American Institute of Physics, 1997. http://hdl.handle.net/2237/6986.
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Oxley, John Paul. "Thermopower in two dimensional electron systems." Thesis, University of Nottingham, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293657.
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Abrahamson, Joel T. (Joel Theodore). "Energy storage and generation from thermopower waves." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/76474.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2012.Cataloged from PDF version of thesis.
Includes bibliographical references.
The nonlinear coupling between an exothermic chemical reaction and a nanowire or nanotube with large axial heat conduction guides a self-propagating thermal wave along the nano-conduit. The thermal conduit accelerates the wave by rapidly transporting energy to un-reacted fuel. The reaction wave induces what we term a thermopower wave, resulting in an electrical current in the same direction. At up to 7 W/g, peak power density is larger than that of many present micro-scale power sources (e.g. fuel cells, batteries) and even about seven times greater than commercial Li-ion batteries. Thermopower waves also tend to produce unipolar voltage pulses, although conventional thermoelectric theory predicts bipolar voltage. These waves also generate thermopower in excess of previous measurements in carbon nanotubes (CNTs) and therefore could increase figures of merit in a variety of thermoelectric materials. In this thesis, I have developed the theoretical framework to describe the thermal and chemical profiles of propagating reaction waves, and their electrical properties. My analysis yielded a new analytical solution for one-dimensional reaction and thermal diffusion systems with nth order kinetics that obviates many approximate or numerical approaches from the past 80 years. A generalized logistic. function describes the temperature and concentration profiles within the solid fuel and provides a solution for the wave velocity for a wide range of conditions. This approach offers new insight into such problems spanning several fields in science and engineering, including propulsion and self-propagating high temperature synthesis (SHS) of materials, as well as the dynamics of thermopower waves. Temperature and voltage measurements of thermopower waves on CNTs show that they can generate power as much as four times greater than predictions based on reference measurements of the Seebeck coefficient for static temperature gradients. We hypothesize that the excess thermopower stems from a chemical potential gradient across the CNTs. The fuel (e.g. picramide) adsorbs and dopes the CNTs ahead of the wave and desorbs and reacts behind the wave front. Furthermore, the excess thermopower depends on the mass of fuel added (relative to CNT mass), and the chemical potential difference matches the magnitude of the excess thermopower. Thus, a major conclusion of this thesis is that coupling to a chemical reaction can boost the performance of thermoelectric materials through differential doping. Thermopower waves can have well defined velocity oscillations for certain kinetic and thermal parameter values. Cyclotrimethylene-trinitramine (fuel) on multiwalled CNTs (conduit) system generates voltage oscillations of 400 to 5000 Hz. These frequencies agree with velocity oscillations predicted by my thermochemical model of the reaction wave, extended to include thermal transport within the conduits. Thermopower waves could thus find applications as new types of alternating current (AC) batteries and self-powered signal generators, which could easily be miniaturized. Microelectromechanical systems and sensors would benefit from thermopower wave generators to enable functions such as communications and acceleration that currently require large power packs. Additionally, the "self-discharge" rate of thermopower wave generators is extremely low in contrast to electrochemical storage, since their energy is stored in chemical bonds. Thermopower waves thus enable new energy storage devices and could exceed limitations of conventional thermoelectric devices.
by Joel T. Abrahamson.
Ph.D.
4
Isotalo, Heikki. "Thermopower in the characterization of electrically conducting polymers." [Hki] : Societas scientiarum Fennica, 1990. http://catalog.hathitrust.org/api/volumes/oclc/57960808.html.
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Anatska, Maryna Petrovna. "The transport coefficients in (R1.5Ce0.5)RuSr2Cu2O10-5 (R=Gd,Eu) rutheno-cuprates." Texas A&M University, 2006. http://hdl.handle.net/1969.1/5019.
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The thermal conductivity, thermopower, and electrical resistivity of
(R1.5Ce0.5)RuSr2Cu2O10-delta (R=Gd, Eu) polycrystalline samples with different oxygen
doping level are investigated in temperature range 1.8-300 K. Much attention is focused
on the dependence of the effect of the annealing in high oxygen pressures as well as the
effect of aging on transport coefficients in normal and superconducting states. It was
found that the process of deoxydation goes faster for Ru-1222(Eu) samples than for Ru-
1222(Gd) samples, which results in more pronounced granular effects in Ru-1222(Eu)
samples. The relative contribution to the thermal conductivity due to electrons and
phonons was estimated by using the Wiedemann-Franz relation and the resistivity data.
The calculation showed that the maximum electron contribution for Ru-1222(Eu) is
about 0.75% and that for Ru-1222(Gd) samples is around 4 %.
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Gibbings, C. J. "Thermoelectric properties of silicon inversion layers." Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.372871.
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Holland, Edward Robert. "Transport properties in electrically conductive polymeric materials." Thesis, Durham University, 1995. http://etheses.dur.ac.uk/5233/.
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Measurements on free standing films of the conductive polymer polyaniline (PANi) have revealed that charge transport within this material depends upon the level of intermolecular order. This factor is found to depend upon the method of sample preparation. PANi protonated by immersion of solid emeraldine base in aqueous methane sulphonic acid has low conductivity, 30-40 Scm(^-1). This can be enhanced, up to 250 Scm(^-1) if films are stretch oriented prior to protonation. Stretched samples have an electrical conductivity anisotropy factor of order 7 at 300 K, also revealed in their thermopower over the range 100 - 300 K. The behaviour of electrical conductivity with temperature is commensurate with charge transport in a disordered system. Protonation of PANi dissolved in meta cresol by addition of camphor sulphonic acid (CSA) yields material with conductivity of 250-300 Scm(^-1) Variation of the acid concentration has revealed a transition to a metallic response in conductivity (near 300 K) when 20-30% of polymer nitrogen sites are protonated. This character extends to progressively lower temperatures as protonation is increased to 60%. The metallic nature of this material is evident in the linear temperature dependence of thermopower and is ascribed to the presence of crystalhne regions within the polymer fihn, as revealed by an independent x-ray analysis The role of molecular order upon the properties of thin films of 3[2(S2-methylbutoxy)ethyl]-polythiophene has been investigated. Starting with polymer dissolved in 'good' solvent, quantities of nonsolvent lead to reorganisation of the sidechain groups when added. This promotes an increase in effective conjugation length which can be transferred to the solid state by the spin coating process as indicated by spectroscopic studies. With these films acting as the active layer in a field effect transistor the charge carrier mobility can be measured. It is found that as molecular order increases, mobility decreases from 10(^-5) cm(^2)V(^-1)s(^-1) to 710(^-8) cm(^2)V(^-1)s(^-1). This is ascribed to increased interchain separation and effects due to macroscopic aggregate grain boundaries.
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Kopp, Bastian [Verfasser]. "Thermopower of Atomic-Size Contacts at Low Temperature / Bastian Kopp." Konstanz : Bibliothek der Universität Konstanz, 2016. http://d-nb.info/1115727486/34.
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Srivastava, Gauri. "Low temperature measurement of thermopower in mesoscopic normal/superconducting nanostructures." Thesis, Royal Holloway, University of London, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.430893.
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Mahajan, Sayalee G. (Sayalee Girish). "Improving efficiency of ID thermopower wave devices and studying 2D reaction waves." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/101509.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2015.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 124-127).
With growing energy consumption, current research in the field is focused on improving and developing alternatives for energy storage and conversion. Factors such as efficiency of energy conversion, usability of this converted form of energy, power density, energy density etc. help us in determining the right energy source or conversion technology for any specific application. The main aim of this thesis was to study self-propagating reaction waves as a means of converting chemical energy into electrical energy. We carried out numerical simulations to study these self-propagating reaction wave systems and their heat transfer properties. Our analysis shows that for certain specific system heat transfer properties, self-propagating reaction waves can sometimes lead to superadiabatic temperatures, which are temperatures higher than the predicted adiabatic reaction temperature. Having energy available at higher temperature has advantages in heat harvesting applications such as thermoelectricity and thermophotovoltaics. We calculated the improvement in efficiency of a modified thermophotovoltaics setup, when the input is a reaction wave, operating under superadiabatic conditions. Experimentally, we studied these self-propagating reaction waves by launching I D thermopower waves. We demonstrated improved chemical-to-electrical conversion efficiency of these devices (from about 10-⁴ % to 10-² %) by operating with newer fuels such as sodium azide and sucrose with potassium nitrate on single-walled carbon nanotube-based thermal conduits. The net efficiency of operation of the device was also improved to up to 1% by using external thermoelectric harvesters to capture the heat energy lost via convection and radiation. We proposed a model combining the ID reaction heat and mass balance equations with the theory of excess thermopower to predict the output voltage profiles of thermopower wave devices and extract useful data from the voltage plots obtained experimentally. This model allows us to quantify the impact of the device-to-device variation of the fuel and thermal conduit properties, and can guide us to a better choice of fuel-thermal conduit pairs to improve the efficiency of operation. Finally, we experimentally studied 2D reaction waves. These waves were launched with a nitrocellulose fuel layer atop an aluminum foil thermal conduit. A wave front characteristic, the shape of these wave fronts, was studied as a function of heat loss. Energy released by these reactions was again harvested using external thermoelectrics to convert heat energy into electricity. We demonstrated that such a setup of 2D reaction waves can be used to illuminate a light-emitting diode (LED).
by Sayalee G. Mahajan.
Ph. D.
11
Crump, Paul Andrew. "Classical and quantum electrical transport in two dimensional systems." Thesis, University of Nottingham, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319648.
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Sparks, Taylor David. "Oxide Thermoelectrics: The Role of Crystal Structure on Thermopower in Strongly Correlated Spinels." Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10243.
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This dissertation reports on the synthesis, structural and thermal characterization and electrical and thermal transport properties of a variety of strongly correlated spinels. General structure property relationships for electrical and thermal transport are discussed. However, the relationship between thermopower and features of the crystal structure such as spin, crystal field, anti-site disorder, and structural distortions are explored in depth. The experimental findings are reported in the context of improving existing oxide thermoelectric materials, screening for new materials or using thermopower as a unique characterization tool to determine the cation distribution in spinels. The need for improved n-type oxide thermoelectric materials has led researchers to consider mixed valence \((+3/+4)\) manganese oxides. Contrary to previous findings we report herein that the \(LiMn_2O_4\) compound reaches the relatively large n-type thermopower of \(-73 \mu V/K\) which is three times larger than the value observed in other manganese oxides, \(-25 \mu V/K\). The cause of this increase in thermopower is shown to be the absence of a Jahn-Teller distortion on the \(Mn^{3+}\) ions in \(LiMn_2O_4\). By avoiding this structural distortion the orbital degeneracy is doubled and the Koshibae et al.’s modified Heikes formula predicts a thermopower of \(-79 \mu V/K\) in good agreement with the experiment. Altering the \(Mn^{3+/4+}\) ratio via aliovalent doping did not affect the thermopower and is a second evidence of universal charge transport first reported by Kobayashi et al. The role of anti-site disorder was further examined in \(Fe_xMn_{1-x}NiCrO_4\) x=0, ½, ¾, 1 spinels but the effect on thermopower was inconclusive due to the presence of impurity phases. Next, the thermopower as a function of temperature in \(Co_3O_4\) was investigated as a means whereby the Wu and Mason’s 30 year old model for using thermopower to calculate cation distribution in spinels could be revisited. We report evidence that Wu and Mason’s original model using the standard Heikes formula and considering octahedral sites alone leads to a stoichiometrically inconsistent result at high temperatures. Alternate models are evaluated considering Koshibae et al.’s modified Heikes formula and accounting for tetrahedral site contributions. Furthermore, the effect of a possible spin state transition is considered.Engineering and Applied Sciences
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Möller, Thomas [Verfasser]. "Temperature Dependent Study of the Thermopower of Atomic and Molecular Scale Contacts / Thomas Möller." Konstanz : KOPS Universität Konstanz, 2020. http://d-nb.info/1213247276/34.
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Cole, Jody Richard. "The effect of doping on the anisotropic electrical resistivity and thermopower of YBa₂Cu₃Ox." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597825.
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This work reports the effect of oxygen doping on the anisotropic electrical resistivity and thermoelectric power of single crystal YBa2Cu3Ox and nominally 6%Ca-doped YBa2Cu3Ox. The a, b and c-axis thermopower of single crystal YBa2Cu3Ox has been measured over a wide doping range. The overdoped a-axis thermopower of single crystal YBa2Cu3Ox and nominally 6%Ca doped YBCO displays an unusual abrupt change in gradient at about 130K, which is independent of doping. This effect becomes more noticeable as the doping is increased. However it is not observed in any underdoped a-axis thermopower measurements. The a-axis thermopower an be scaled in terms of T*, in a similar way to polycrystalline YBCO. However the overdoped a-axis thermopower will not scale because of the abrupt gradient change and the negative value of the overdoped a-axis thermopower. At high dopings the b-axis single crystal thermopower of YBa2Cu3Ox shows markedly different behaviour to the a-axis thermopower, both in magnitude and temperature dependence. This is ascribed to a significant additional contribution from the chain conduction channel. However as the doping is progressively lowered the b-axis thermopower becomes more similar to the a-axis thermopower, reflecting the decreasing effect of the chain contribution on the b-axis thermopower. This can be observed in the in-plane thermopower anisotropy Sa(T)/Sb(T), which tends to unity as the doping decreases. The temperature dependence of the chain thermopower for several dopings has been determined from the measured in-plane thermopower and resistivity components, within the same model where the chains and planes conduct independently (ICPM). This temperature dependence is discussed within the usual Mott diffusion thermopower scenario. The c-axis thermopower has been measured for several dopings and is positive in magnitude and gradient, except at T*, where the gradient abruptly changes sign and the c-axis thermopower then becomes larger as the temperature is decreased.
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Ohta, Hiromichi, Yumi Masuoka, Ryoji Asahi, Takeharu Kato, Yuichi Ikuhara, Kenji Nomura, and Hideo Hosono. "Field-modulated thermopower in SrTiO3-based field-effect transistors with amorphous 12CaO・7Al2O3 glass gate insulator." American Institute of Physics, 2009. http://hdl.handle.net/2237/14153.
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Populoh, Sascha. "Investigation of the Mott transition in chromium doped V2 O3 by means of ultrasound and thermopower experiments." Paris 11, 2009. http://www.theses.fr/2009PA112044.
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Cette thèse présente une étude expérimentale de la transition de Mott dans V2O3 dopé au chrome par deux différentes techniques expérimentales entre la température ambiante à 500 K et sous pressions jusqu’à 6 kbar. La technique ultrasonore fournit une mesure directe de la compressibilité et a été utilisé pour estimer les effets de la réseau sur la transition de Mott. Nous avons été en mesure de mener la première étude systématique de l'influence des degrés de liberté du réseau sur la transition de Mott dans V2O3. On a vérifiée l'existence d'une température électronique critique Telc qui diffère de l'ordre de 3% ou 4% par rapport à la température de transition effectivement observée Tc. L'observation du comportement du mode transverse de la vitesse du son a permis de conclure que la transition de Mott à haute température a lieu sans brisure de symétrie et le paramètre d'ordre est un scalaire. Dans la deuxième partie de la thèse l'évolution de coefficient du Seebeck a été étudiée dans les mêmes conditions expérimentales. Les études autour du point critique nous ont permis d'établir un diagramme de phase expérimentale. Nous avons montré que le changement de S à la transition est dominé par le changement de la résistivité plutôt que par le changement de la symétrie particule-trou. En conséquence, nous avons pu relier les lois d’échelle pour les paramètres critiques relatifs à la conductivité à nos données. Dans le voisinage de la transition nos données sont assez bien reproduites par les exposants de champ moyen observés dans les mesures de conductivitéThis work presents an experimental study of the high temperature Mott transition in chromium doped V2O3 by means of two different experimental techniques at pressures between ambient pressure and 6 kbar and temperatures from room T to 500 K. Ultrasound measurements as a direct probe of the compressibility were used to estimate the effects of the lattice on the Mott transition. We were able to present the first systematic study of the influence of the lattice degrees of freedom on the high temperature Mott transition in V2O3 and could verify the existence of a critical electronic temperature Telc that differs in the order of 3% or 4% from the temperature of the transition Tc of the bulk sample. Furthermore, observation of the behaviour of the transversal mode of the speed of sound allowed to conclude that this transition occurs without symmetry breaking and the order parameter is a scalar. In the second part the evolution of the Seebeck coefficient was investigated under the same experimental conditions. The studies around the critical point allowed us to establish an experimental phase diagram. Furthermore, we were able to show that the change of the Seebeck coefficient at the Mott transition is driven by the change in the resistivity rather than by the change of the particle-hole symmetry. As a consequence of this we could relate the scaling laws for the critical parameters related to conductivity to our data. In the vicinity of the transition our data were quite well reproduced by the mean field exponents found in the conductivity experiments
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Böhnert, Tim [Verfasser], and Kornelius [Akademischer Betreuer] Nielsch. "Magneto-thermopower and Magnetoresistance of Co-Ni Alloy and Co-Ni/Cu Multilayered Nanowires. / Tim Böhnert. Betreuer: Kornelius Nielsch." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2014. http://d-nb.info/1052996698/34.
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Orovets, Christine M. "Search for Resonant Impurities in Bismuth and Bismuth-Antimony Alloys: Lithium, Magnesium, and Sodium." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1337715464.
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Burg, Tristan Kevin Materials Science & Engineering Faculty of Science UNSW. "Semiconducting properties of polycrystalline titanium dioxide." Publisher:University of New South Wales. Materials Science & Engineering, 2008. http://handle.unsw.edu.au/1959.4/41262.
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Titanium dioxide, TiO2, has potential applications as a photoelectrode for photoelectrochemical generation of hydrogen by splitting water using solar energy and as a photocatalyst for water purification. This study is part of the UNSW research program to process TiO2-based oxide semiconductors as high-performance photoelectrodes and photocatalysts. This study investigates the effect of defect disorder on semiconducting properties of polycrystalline TiO2. This study involved the processing of high-purity polycrystalline TiO2 and determination of its semiconducting properties through measurement of electrical conductivity and thermoelectric power at elevated temperatures (1073-1323K) in controlled oxygen activities [1x10-13 Pa < p(O2) < 75 kPa]. The study included two types of experiments: Determination of electrical properties under conditions of gas/solid equilibrium. The data obtained was used to derive defect disorder and related semiconducting properties Monitoring of electrical properties during equilibration. This data was used to determine the chemical diffusion coefficient. The data obtained under equilibrium conditions indicates that oxygen may be used as a dopant to impose controlled semiconducting properties. In reduced conditions TiO2 is an n-type semiconductor and under oxidizing conditions TiO2 is a p-type semiconductor. The n-type behaviour is associated with oxygen vacancies as the predominant defects and titanium interstitials as the minority defects. The p-type behaviour is closely related to titanium vacancies that are formed during prolonged oxidation. Charge transport at elevated temperature was shown to involve substantial contribution from ions. Analysis of electrical properties enabled determination of several defect-related quantities including the activation enthalpy for oxygen vacancy formation, and the activation energy of the electrical conductivity components related to electrons, holes and ions. The kinetic data obtained during gas/solid equilibration enabled determination of the chemical diffusion coefficient which exhibited a complex dependence on nonstoichiometry. In addition, prolonged oxidation showed that equilibration occurred in two kinetic regimes. One for highly mobile oxygen vacancies and titanium interstitials which quickly reached an ??operational equilibrium?? within hours and another slow kinetic regime for equilibration of titanium vacancies over many thousand hours. The determined chemical diffusion coefficient data may be used to select the processing conditions required to impose uniform concentration of defects within a TiO2.
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Stalaučinskas, Tadas. "Klaipėdos šiluminės elektrinės konversija į šiuolaikinį meno centrą Danės gatvėje." Master's thesis, Lithuanian Academic Libraries Network (LABT), 2010. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2010~D_20100622_150120-55562.
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Magistro studijų baigiamojo darbo tikslas-- įvertinant esamą situacija bei atliekant reikalingus tyrimus, nustatyti Klaipėdos šiluminės elektrinės teritorijos konversijos galimybes, pateikti projektinius pasiūlymus. Atlikus architektūrinę ir urbanistinę analizę nuspręsta plėtoti visuomeninės ir komercinės infrastruktūros teritorijoją, integruojant ja į miesto planinę- erdvinę struktūrą. Planuojamą teritoriją siūloma išvalyti, paliekant architektūrinę ir konstrukcinę vertę turinčius pastatus. Šiluminės elektrinės pastatas tampa teritorijos ir miesto dalies akcentu, turintis glaudų ryšį su senamiesčiu. Trijų tūrių kompozicija racionaliai pritaikoma šiuolaikinio meno centro funkcijai, kuriame projektuojamos ekspozicijų erdvės, konferencijų salės, vestibiuliai, viešojo maitinimo, techninės patalpos ir kt. Rengiant projektą naudojamos netradicinės konstrukcinės sistemos. Darbą sudaro grafinė medžiaga ir tekstinė dalis: įvadas, analitinė dalis, duomenų apie pastatą apibendrinimas, pastato architektūrinis ir konstrukcijų techninės būklės įvertinimas, tyrimo medžiagos apdorojimas, architekturiniai ir konstrukciniai pastato renovacijos siulymai, literatūros sąrašas. Atskirai pridedami darbo priedai – grafinės medžiagos sumažintos kopijos, maketo nuotraukos.The aim of the final work of MA – identify the teritory conversion opportunities of Klaipeda thermopower station, evaluating the current situation and taking the relevant research methods . Develop comercial and public infrastructure area by using architectural and urbanistical analysis and integrating it into the target –spacial structure of the city. It is suggested to empty the target area, except the buildings that have architectural and structural value. The Thermopower station is going to be the accent of the teritory and the part of town, and have a close relation with the architecture of the oldtown. Three solids composition is adopted for the function of the modern art center, where exposition spaces, conference halls, lobbies, restaurants, technical rooms are being established. Untraditional constuctive systems were adjusted to prepare this project. The work contains graphic matereal and text part: introduction, analytical part, the summary of the building data, the evaluation of the building‘s architectural and constructive technical condition, processing of the research matereal, architectural and contructive renovation suggestions, bibliography. Appedixes: graphic matereal copies, photos of the model of the project.
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Köhler, Ulrike. "Thermoelectric transport in rare-earth compounds." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2008. http://nbn-resolving.de/urn:nbn:de:bsz:14-ds-1215004993567-14165.
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The focus of this thesis lies on the thermoelectric transport properties of rare-earth compounds containing Ce, Eu, and Yb. These systems have been investigated either to study fundamental problems or to evaluate their potential for thermoelectric applications.
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Cipiloglu, Mustafa Ali. "Thermoelectric Effects In Mesoscopic Physics." Phd thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/12604753/index.pdf.
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The electrical and thermal conductance and the Seebeck coefficient are calculated for one-dimensional systems, and their behavior as a function of temperature and chemical potential is investigated. It is shown that the conductances are proportional to an average of the transmission probability around the Fermi level with the average taken for the thermal conductance being over a wider range. This has the effect of creating less well-defined plateaus for thermal-conductance quantization experiments.
For weak non-linearities, the charge and entropy currents across a quantum point contact are expanded as a series in powers of the applied bias voltage and the temperature difference. After that, the expansions of the Seebeck voltage in temperature difference and the Peltier heat in current are obtained. Also, it is shown that the linear thermal conductance of a quantum point contact displays a half-plateau structure, almost flat regions appearing around half-integer multiples of the conductance quantum. This structure is investigated for the saddle-potential model.
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Köhler, Ulrike. "Thermoelectric transport in rare-earth compounds." Doctoral thesis, Technische Universität Dresden, 2007. https://tud.qucosa.de/id/qucosa%3A23707.
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The focus of this thesis lies on the thermoelectric transport properties of rare-earth compounds containing Ce, Eu, and Yb. These systems have been investigated either to study fundamental problems or to evaluate their potential for thermoelectric applications.
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Palacio, Morales Alexandra. "Thermoélectricité des composés fortement corrélés sous conditions extrêmes." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENY042/document.
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Cette thèse porte sous l'étude sous conditions extrêmes (basse température, fort champ magnétique et haute pression) des composés fortement corrélés du type fermions lourds. Trois composés ont été analysés UCoAl, UGe$_2$ et CeRh$_2$Si$_2$, en utilisant principalement de mesures thermoélectriques; une technique récente et très sensible dans le domaine des fermions lourds. À cette fin, de nouvelles dispositives de mesures de pouvoir thermoélectricité sous pression ont été développés au cours de cette thèse.Concernant le composé d'UCoAl, notre étude a permis d'analyser précisément la transition metamagnétique, induite par le champ magnétique, entre la phase paramagnétique (PM) et la phase ferromagnétique (FM) ainsi que, son évolution sous pression. Ainsi, nos mesures ont permis de compléter le diagramme de phase $(T,P,H)$ et notamment, de mettre en évidence la structure magnétique originale qui apparaît sous pression en forme de ```wings" pas des mesures thermoélectriques.Une fine analyse de la surface de Fermi de la phase FM$2$ d'UGe$_2$ a été réalisée grace à l'observation des oscillations quantiques du pouvoir thermoélectrique. Les résultats obtenues ont été comparés aux études conventionnelles des oscillations quantiques comme de ``de Haas-van Alphen" (dHvA) et de ``Suhbnikov-de Hass" (SdH) effets. Une très bonne accord entre les trois techniques a été constatée. % et montre les avantages d'utilisation des mesures du pouvoir thermoélectrique pour analyser les paramètres microscopiques des fermions lourds.% Les inconvénients de cette technique sont aussi présentés.Finalement, dans le système CeRh$_2$Si$_2$, la suppression du domaine antiferromagnétique (AF) sous champ magnétique $H_c sim 26$T et sous pression $Psim 1$GPa a été étudiée. Un très fort changement de la surface de Fermi à $H_c$ correspondant à la transition de l'ordre AF vers une phase paramagnétique polarisée (PPM), a été observé. Sous pression, des fluctuations magnétiques et une reconstruction de la surface de Fermi apparaissent autour de $P_c$. Ces fluctuations cachent la nature de la suppression de l'ordre AF vers un ordre paramagnétique (PM). L'étude du diagramme de phase $(T,H,P)$ révèle que les phases PM et PPM sont différentes, cependant des points en commun demeurentThermopower is a technique whose importance is related to the possibility of directly measuring electronic properties of the systems, as it is sensitive to the derivative of the density of states. In this work, the low temperature regime of strongly correlated electron systems has been studied using this technique. For that, a new pressure-field thermopower device was developed, and used, to determine $(T,P,H)$ phase diagrams of the itinerant ferromagnets UCoAl and UGe$_2$, and of the weak antiferromagnet CeRh$_2$Si$_2$.For example, in the case of UCoAl, this same technique was used to analyze the metamagnetic transition from paramagnetic (PM) to ferromagnetic (FM) phases and to study its evolution towards the quantum critical end point. The existence of exotic magnetic excitations in the ground state and around the critical end point were also evidenced.On the compound CeRh$_2$Si$_2$, the suppression of the antiferromagnetic (AF) order by magnetic fields and pressures was explored. A strong change of the Fermi surface at $H_c$, the field at which the suppression of the AF into the paramagnetic polarized (PPM) phase, was observed. We show that under pressure, the magnetic fluctuations around the critical pressure $P_c$ masked the Fermi surface reconstruction of the AF phase into the PM phase. The analysis of the $(T,P,H)$ phase diagram revealed that the non-ordered phases of this compound (PM and PPM) are different, therefore pressure and field behave as different suppressor mechanisms.In the UGe$_2$ compound, the analysis of its Fermi surface by thermopower quantum oscillations was performed as a last example of the utility and of the importance of this technique. To the best of the author knowledge, this is the first time that this technique was used in heavy fermion systems. A comparison to traditional probes such as de Haas-van Alphen and Shubnikov–de Haas effects was done. We observed a good agreement between them and we explain the advantages and the disadvantages of thermopower quantum oscillations technique over the traditional probes
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Zhang, Hongbin. "Relativistic Density Functional Treatment of Magnetic Anisotropy." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2009. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-25132.
Full textAbstract:
Spin-orbit coupling (SOC) reduces the spatial symmetry of ferromagnetic solids. That is, the physical properties of ferromagnetic materials are anisotropic, depending on the magnetization direction. In this thesis, by means of numerical calculations with full-relativistic density functional theory, we studied two kinds of physical properties: surface magnetic anisotropy energy (MAE) and anisotropic thermoelectric power due to Lifshitz transitions. After a short introduction to the full-relativistic density functional theory in Chapter 2, the MAE of ferromagnetic thin films is studied in Chapter 3. For such systems, separation of different contributions, such as bulk magnetocrystalline anisotropy (MCA) energy, shape anisotropy energy, and surface/interface anisotropy energy, is crucial to gain better understanding of experiments. By fitting our calculating results for thick slabs to a phenomenological model, reliable surface MAE could be obtained. Following this idea, we have studied the MAE of Co slabs with different geometries, focusing on the effects of orbital polarization correction (OPC). We found that the surface anisotropy is mainly determined by the geometry. While OPC gives better results of orbital moments, it overestimates the MAE. In the second part of Chapter3, the effects of electric fields on the MAE of L10 ferromagnetic thin films are studied. Using a simple model to simulate the electric field, our calculations are in good agreement with previous experimental results. We predicted that for CoPt, even larger effects exist. Moreover, we found that it is the amount of screening charge that determines the magnetoelectric coupling effects. This gives us some clue about how to achieve electric field control of magnetization direction. In Chapter 4, Lifshitz transitions in L10 FePt caused by a canted magnetic field are studied. We found several Lifshitz transitions in ordered FePt with tiny features in DOS. Using a two-band model, it is demonstrated that at such transitions, the singular behaviour of kinetic properties is due to the interband scattering, and the singularity itself is proportional to the derivative of the singular DOS. For FePt, such singularity will be smeared into anomaly by chemical disorder. Using CPA, we studied the effects of energy level broadening for the critical bands in FePt. We found that for experimentally available FePt thin films, Lifshitz transitions would induce up to a 3% increase of thermopower as the magnetization is rotated from the easy axis to the hard axisSpin-Bahn-Kopplung reduziert die Symmetrie ferromagnetischer Festkörper. Das bedeutet, dass die physikalischen Eigenschaften ferromagnetischer Stoffe anisotrop bezüglich der Magnetisierungsrichtung sind. In dieser Dissertation werden mittels numerischer voll-relativistischer Dichtefunktional-Rechnungen zwei Arten physikalischer Eigenschaften untersucht: magnetische Oberflächen-Anisotropieenergie (MAE) und anisotrope Thermokraft durch Lifshitz-Übergänge. Nach einer kurzen Einführung in die relativistische Dichtefunktional-Theorie in Kapitel 2 wird in Kapitel 3 die MAE ferromagnetischer dünner Filme untersucht. In diesen Systemen ist es für ein Verständnis experimenteller Ergebnisse wichtig, verschiedene Beiträge zu separieren: Volumenanteil der magnetokristallinen Anisotropie (MCA), Formanistropie und Oberflächen bzw. Grenzflächenanisotropie. Durch Anpassen berechneter Daten für dicke Schichten an ein phänomenologisches Modell konnten verlässliche Oberflächen Anisotropien erhalten werden. In dieser Weise wurde die MAE von Co- Schichten mit unterschiedlichen Geometrien untersucht, wobei der Einfluss von Orbitalpolarisations-Korrekturen (OPC) im Vordergrund stand. Es wurde gefunden, dass die Oberflächenanisotropie hauptsächlich von der Geometrie bestimmt wird. Während OPC bessere Ergebnisse für die Orbitalmomente liefert, wird die MAE überschätzt. Im zweiten Teil von Kapitel 3 wird der Einfluss elektrischer Felder auf die MAE von dünnen ferromagnetischen Filmen mit L10-Struktur untersucht. Unter Verwendung eines einfachen Modells zur Simulation des elektrischen Feldes liefern die Rechnungen gute Übereinstimmung mit vorliegenden experimentellen Ergebnissen. Es wird vorhergesagt, dass für CoPt ein noch größerer Effekt existiert. Weiterhin wurde gefunden, dass die magnetoelektrische Kopplung von der Größe der Abschirmladung bestimmt wird. Dies ist eine wichtige Einsicht, um die Magnetisierungsrichtung durch ein elektrisches Feld kontrollieren zu können. In Kapitel 4 werden Lifshitz-Übergänge untersucht, die ein gekantetes Magnetfeld hervorruft. Es wurden mehrere Lifshitz-Übergänge in geordnetem FePt gefunden, welche kleine Anomalien in der Zustandsdichte hervorrufen. Mit Hilfe eines Zweiband-Modells wird gezeigt, dass an solchen Übergängen das singuläre Verhalten kinetischer Eigenschaften durch Interband- Streuung verursacht wird und dass die Singularität proportional zur Ableitung der singulären Zustandsdichte ist. In FePt wird durch chemische Unordnung diese Singularität zu einer Anomalie verschmiert. Der Einfluss einer Verbreiterung der Energieniveaus der kritischen Bänder in FePt wurde mittels CPA untersucht. Es wurde gefunden, dass in experimentell verfügbaren dünnen FePt-Filmen Lifshitz-Übergänge bis zu 3% Erhöhung der Thermokraft erzeugen, wenn die Magnetisierung von der leichten in die harte Richtung gedreht wird
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Momtaz, Zahra Sadre. "Transporte em nanoestruturas: fenômenos quânticos em poços duplos e triplos." Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-04012017-143327/.
Full textAbstract:
Nesta tese apresentamos os estudos de magnetotransporte em poços quânticos largos,\\\\ estreitos e triplos em campos magnéticos baixos. Dependendo dos estudos desejados, me-\\\\dimos a magnetoresistência em regime linear e não linear e sob a aplicação de corrente AC, irradiação de microondas e em gradiente de temperatura ao longo das amostras. Relatamos a observação de efeitos não lineares de corrente alternada em oscilações magneto-inter-sub-bandas de poços quânticos triplos. A oscilação MIS em sistemas de poços quânticos individuais e duplos e também os efeitos não lineares devido à corrente contínua foram estudados antes nestes sistemas. Nossos resultados são explicados de acordo com um modelo generalizado baseado na parte de não equilíbrio da função de distribuição de elétrons. A magnetorresistência não local sob irradiação de microondas é também estudada nesta tese. Os resultados obtidos proporcionam evidências para uma corrente de estado de borda estabilizada por irradiação de microondas, devido às ressonâncias não lineares e foram descritas por um modelo baseado em dinâmica não linear e mapa padrão de Chirikov. Finalmente, observamos uma correlação estreita entre as oscilações de resistência e oscilações de tensão de arraste do fônon induzidas por irradiação de microondas em um sistema bidimensional de eletrons sob campo magnético perpendicular. A influência da resistividade de dissipação modificada por microondas na tensão de arraste do fônon perpendicular ao fluxo de fônons pode explicar nossas observações. Além disso, características nítidas observadas na tensão de arraste do fônon sugerem que os domínios de corrente associados a estes estados podem existir na ausência de condução DC externa.In this thesis, we present the studies of magneto-transport in narrow , wide and triple quantum wells in low magnetic fields. Depending on the desired studies, we have measured the magneto-resistance both in linear and nonlinear regimes and under the application of AC current, microwave irradiation and temperature gradient along the samples. We have reported the observation of nonlinear effects of AC current on magneto-inter-sub-band oscillations (MIS) of triple quantum wells (TQWs). The MIS oscillations in single and double quantum well system and also nonlinear effects due to DC current have been studied before in these systems. Our results are explained according to a generalized model based on non-equilibrium part of electron distribution function. The nonlocal magneto-resistance under microwave irradiation is also studied within this thesis. The obtained results provide evidence for an edge-state current stabilized by microwave irradiation due to nonlinear resonances and have been described by a model based on the nonlinear dynamics and Chirikov standard map. Finally, we have observed the phonon-drag voltage oscillations correlating with the resistance oscillations under microwave irradiation in a two-dimensional electron gas in perpendicular magnetic field. The influence of dissipative resistivity modified by microwave on phonon-drag voltage perpendicular to the phonon flux can explain our observations. Moreover, sharp features observed in phonon drag voltage suggest the current domains associated with these states can exist in the absence of external DC driving.
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Romanovsky, Igor Alexandrovich. "Novel properties of interacting particles in small low-dimensional systems." Diss., Available online, Georgia Institute of Technology, 2006, 2006. http://etd.gatech.edu/theses/available/etd-07102006-041659/.
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Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2007.Landman, Uzi, Committee Member ; Yannouleas, Constantine, Committee Member ; Bunimovich, Leonid, Committee Member ; Chou, Mei-Yin, Committee Member ; Pustilnik, Michael, Committee Member.
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Pawula, Florent. "Particularités des oxydes de ruthénium sondées par l'effet Seebeck." Thesis, Normandie, 2018. http://www.theses.fr/2018NORMC225/document.
Full textAbstract:
Dans son ensemble, cette thèse porte sur la synthèse, l’étude structurale et l’étude des propriétés magnétiques et de transport de différentes familles d’oxydes de ruthénium, présentant des comportements électroniques et magnétiques variés, de structure rutile, hexaferrite de type R et hollandite. Le but de ce travail était l’étude des particularités des oxydes de ruthénium sondées par l’effet Seebeck dans les matériaux suivants : RuO2 de structure rutile (chaînes d’octaèdres de RuO6 liés par leurs arêtes, interconnectées par leurs sommets) à transport de type Boltzmann dominé par les interactions électron-phonon, les hexaferrites de type R BaCo2Ru4O11 et BaMn2Ru4O11 (octaèdres de RuO6 liés par les arêtes, formant des plans kagomé, et octaèdres de RuO6 liés par une face) ferromagnétiques doux et mauvais métaux, et deux nouvelles hollandites Sr1.5Ru6.1Cr1.9O16 et Ba1.5Ru6.1Cr1.9O16 (doubles chaînes de RuO6 liés par les arêtes, interconnectées par les sommets) avec agglomérats de spins localisés. La synthèse de ces deux nouvelles hollandites par réaction à l’état solide a permis de mettre en évidence l’existence de magnétorésistance négative dans cette famille de composés. Cette thèse montre que le comportement du coefficient Seebeck S d'oxydes de ruthénium à structures constituées d'octaèdres RuO6 majoritairement liés par leurs arêtes présente deux comportements différents. À basse T, S dépend fortement de la structure cristallographique et de la structure électronique associée. Par contre, dans la limite haute T, S tend vers une valeur commune indépendamment de la structure comme rapporté ici pour les hexaferrites de type R et les hollandites, et comme observé précédemment dans la pérovskite SrRuO3 (octaèdres RuO6 liés par les sommets) ferromagnétique métallique et dans la quadruple pérovskite LaCu3Ru4O12 (octaèdres RuO6 liés par les sommets) métallique présentant un magnétisme de type Pauli. Dans ces hexaferrites de type R BaCo2Ru4O11 et BaMn2Ru4O11 et dans ces deux nouvelles hollandites Sr1.5Ru6.1Cr1.9O16 et Ba1.5Ru6.1Cr1.9O16, le coefficient Seebeck à haute température atteint une valeur dominée par l’entropie de spin du ruthéniumThis thesis deals with the synthesis, the structural study and the magnetic properties and electronic transport studies of different ruthenium oxide families, presenting various magnetic and electronic behaviors, with rutile, R-type hexaferrite and hollandite structures. The goal of this thesis was the study of the ruthenium oxide peculiarities probed by the Seebeck effect in the following materials: RuO2 rutile (edge-shared RuO6 chain interconnected by their vertices) with Boltzmann type transport dominated by electron-phonon interactions, BaCo2Ru4O11 et BaMn2Ru4O11 R-type hexaferrites (edge-shared RuO6 octahedra, forming kagome planes, and face-shared RuO6 octahedra) soft ferromagnetic bad metals, and two new hollandites Sr1.5Ru6.1Cr1.9O16 et Ba1.5Ru6.1Cr1.9O16 (double chains of edge-shared RuO6 octahedra, interconnected by their vertices) with localized transport and cluster-glass behavior. The synthesis of both new hollandites by solid state reaction allowed us to show the existence of negative magnetoresistance in this compound family. This thesis shows that the behavior of the Seebeck coefficient of ruthenium oxides with structures mainly consisting of edge-shared RuO6 octahedra presents two different behaviors. At low T, S strongly depends on the crystallographic structure and on the associated electronic structure. On the other hand, in the high T limit, S tends a common value independently of the structure as reported here for the R-type hexaferrites and the hollandites and as previously observed in the ferromagnetic metal SrRuO3 perovskite (apex-shared RuO6 octahedra) and in the metallic with Pauli-type magnetism quadruple perovskite LaCu3Ru4O12 (apex-shared RuO6 octahedra). In these R-type hexaferrites BaCo2Ru4O11 and BaMn2Ru4O11 and these two new hollandites Sr1.5Ru6.1Cr1.9O16 and Ba1.5Ru6.1Cr1.9O16, the high temperature Seebeck coefficient reaches a value dominated by the Ru spin entropy term
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Lipp, Dieter. "Thermische Tieftemperatureigenschaften von Seltenerd-Übergangsmetall-Borkarbiden." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2002. http://nbn-resolving.de/urn:nbn:de:swb:14-1021367425218-47062.
Full textAbstract:
The present work reports on thermal low-temperature properties of rare earth transition metal borocarbides such as specific heat, thermal conductivity and thermopower. The influence of structural disorder, caused by stoichiometric variations and substitutions of the rare earth element or the transition metal, on the thermal and superconducting low-temperature properties is investigated. The structural disorder results in the reduction of the superconducting transition temperature Tc, of the Sommerfeld value gamma, of the upper critical magnetic field Hc2(0), of the negative curvature of the H-dependence of the T-linear specific heat contribution gamma(H), and in the reduction of the positive curvature of Hc2(T) near Tc. But isoelectronic rare earth substitutions do not result in the transition from clean to dirty limit. Due to Pt-substitutions similar reductions of thermal and superconducting properties are observed. The behaviour of Hc2(0) and the concentration dependence of the positive curvature of Hc2(T) near Tc point to the transition from clean to quasi-dirty limit in the case of Pt-substitutionsIn der vorliegenden Arbeit werden Untersuchungen zu thermischen Tieftemperatureigenschaften, wie der spezifischen Wärmekapazität, der Wärmeleitfähigkeit und der Thermokraft, an supraleitenden Seltenerd-Übergangsmetall-Borkarbiden vorgestellt. Es wurde der Einfluß von gezielt hervorgerufener Unordnung im kristallographischen Aufbau, die durch isoelektronische Substitutionen des Seltenerd-Elements und des Übergangsmetalls sowie durch Söchiometrievariationen erzeugt wurde, auf die thermischen und supraleitenden Tieftemperatureigenschaften untersucht. Folge der strukturellen Unordnung ist eine Reduzierung der charakteristischen Eigenschaften, wie der Sprungtemperatur der Supraleitung Tc, der Sommerfeldkonstanten gamma, des oberen kritischen Magnetfelds Hc2(0), der negativen Krümmung in der Feldabhängigkeit des T-linearen Beitrags zur spezifischen Wärme gamma(H) sowie eine Reduzierung der positiven Krümmung in der Temperaturabhängigkeit von Hc2(T). Isoelektronische Substitutionen auf dem Seltenerd-Platz führen aber nicht zum Erreichen des dirty limit. Eine Reduzierung der relevanten supraleitenden und thermischen Eigenschaften durch Pt-Beimengungen wird ähnlich wie im Falle der Lu-Substitution festgestellt. Die Konzentrationsabhängigkeit von Hc2(0) sowie die Krümmung von Hc2(T) weisen hier auf einen Übergang vom clean limit zum quasi-dirty limit durch die Pt-Substitution hin
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Isotta, Eleonora. "Nanostructured thermoelectric kesterite Cu2ZnSnS4." Doctoral thesis, Università degli studi di Trento, 2021. http://hdl.handle.net/11572/315174.
Full textAbstract:
To support the growing global demand for energy, new sustainable solutions are needed both economically and environmentally. Thermoelectric waste heat recovery and energy harvesting could contribute by increasing industrial process efficiency, as well as powering stand-alone devices, microgenerators, and small body appliances.The structural complexity of quaternary chalcogenide materials provides an opportunity for engineering defects and disorder, to modify and possibly improve specific properties. Cu2ZnSnS4 (CZTS, often kesterite), valued for the abundance and non-toxicity of the raw materials, seems particularly suited to explore these possibilities, as it presents several structural defects and polymorphic phase transformations. The aim of this doctoral work is to systematically investigate the effects of structural polymorphism, disorder, and defects on the thermoelectric properties of CZTS, with particular emphasis to their physical origin. A remarkable case is the order-disorder transition of tetragonal CZTS, which is found responsible for a sharp enhancement in the Seebeck coefficient due to a flattening and degeneracy of the electronic energy bands. This effect, involving a randomization of Cu and Zn cations in certain crystallographic planes, is verified in bulk and thin film samples, and applications are proposed to exploit the reversible dependence of electronic properties on disorder. Low-temperature mechanical alloying is instead discovered stabilizing a novel polymorph of CZTS, which disordered cubic structure is studied in detail, and proposed deriving from sphalerite-ZnS. The total cation disorder in this compound provides an uncommon occurrence in thermoelectricity: a concurrent optimization of Seebeck coefficient, electrical and thermal conductivity. These findings, besides providing new and general understanding of CZTS, can cast light on profitable mechanisms to enhance the thermoelectric performance of semiconducting chalcogenides, as well as delineate alternative and fruitful applications.
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Isotta, Eleonora. "Nanostructured thermoelectric kesterite Cu2ZnSnS4." Doctoral thesis, Università degli studi di Trento, 2021. http://hdl.handle.net/11572/315174.
Full textAbstract:
To support the growing global demand for energy, new sustainable solutions are
needed both economically and environmentally. Thermoelectric waste heat recovery
and energy harvesting could contribute by increasing industrial process efficiency, as
well as powering stand-alone devices, microgenerators, and small body appliances.
The structural complexity of quaternary chalcogenide materials provides an
opportunity for engineering defects and disorder, to modify and possibly improve
specific properties. Cu2ZnSnS4 (CZTS, often kesterite), valued for the abundance and
non-toxicity of the raw materials, seems particularly suited to explore these
possibilities, as it presents several structural defects and polymorphic phase
transformations.
The aim of this doctoral work is to systematically investigate the effects of structural
polymorphism, disorder, and defects on the thermoelectric properties of CZTS, with
particular emphasis to their physical origin.
A remarkable case is the order-disorder transition of tetragonal CZTS, which is found
responsible for a sharp enhancement in the Seebeck coefficient due to a flattening and
degeneracy of the electronic energy bands. This effect, involving a randomization of
Cu and Zn cations in certain crystallographic planes, is verified in bulk and thin film
samples, and applications are proposed to exploit the reversible dependence of
electronic properties on disorder. Low-temperature mechanical alloying is instead
discovered stabilizing a novel polymorph of CZTS, which disordered cubic structure
is studied in detail, and proposed deriving from sphalerite-ZnS. The total cation
disorder in this compound provides an uncommon occurrence in thermoelectricity: a
concurrent optimization of Seebeck coefficient, electrical and thermal conductivity.
These findings, besides providing new and general understanding of CZTS, can cast
light on profitable mechanisms to enhance the thermoelectric performance of
semiconducting chalcogenides, as well as delineate alternative and fruitful applications.
32
Lipp, Dieter. "Thermische Tieftemperatureigenschaften von Seltenerd-Übergangsmetall-Borkarbiden." Doctoral thesis, Technische Universität Dresden, 2001. https://tud.qucosa.de/id/qucosa%3A24137.
Full textAbstract:
The present work reports on thermal low-temperature properties of rare earth transition metal borocarbides such as specific heat, thermal conductivity and thermopower. The influence of structural disorder, caused by stoichiometric variations and substitutions of the rare earth element or the transition metal, on the thermal and superconducting low-temperature properties is investigated. The structural disorder results in the reduction of the superconducting transition temperature Tc, of the Sommerfeld value gamma, of the upper critical magnetic field Hc2(0), of the negative curvature of the H-dependence of the T-linear specific heat contribution gamma(H), and in the reduction of the positive curvature of Hc2(T) near Tc. But isoelectronic rare earth substitutions do not result in the transition from clean to dirty limit. Due to Pt-substitutions similar reductions of thermal and superconducting properties are observed. The behaviour of Hc2(0) and the concentration dependence of the positive curvature of Hc2(T) near Tc point to the transition from clean to quasi-dirty limit in the case of Pt-substitutions.In der vorliegenden Arbeit werden Untersuchungen zu thermischen Tieftemperatureigenschaften, wie der spezifischen Wärmekapazität, der Wärmeleitfähigkeit und der Thermokraft, an supraleitenden Seltenerd-Übergangsmetall-Borkarbiden vorgestellt. Es wurde der Einfluß von gezielt hervorgerufener Unordnung im kristallographischen Aufbau, die durch isoelektronische Substitutionen des Seltenerd-Elements und des Übergangsmetalls sowie durch Söchiometrievariationen erzeugt wurde, auf die thermischen und supraleitenden Tieftemperatureigenschaften untersucht. Folge der strukturellen Unordnung ist eine Reduzierung der charakteristischen Eigenschaften, wie der Sprungtemperatur der Supraleitung Tc, der Sommerfeldkonstanten gamma, des oberen kritischen Magnetfelds Hc2(0), der negativen Krümmung in der Feldabhängigkeit des T-linearen Beitrags zur spezifischen Wärme gamma(H) sowie eine Reduzierung der positiven Krümmung in der Temperaturabhängigkeit von Hc2(T). Isoelektronische Substitutionen auf dem Seltenerd-Platz führen aber nicht zum Erreichen des dirty limit. Eine Reduzierung der relevanten supraleitenden und thermischen Eigenschaften durch Pt-Beimengungen wird ähnlich wie im Falle der Lu-Substitution festgestellt. Die Konzentrationsabhängigkeit von Hc2(0) sowie die Krümmung von Hc2(T) weisen hier auf einen Übergang vom clean limit zum quasi-dirty limit durch die Pt-Substitution hin.
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Chickering, William Elbridge. "Thermopower in Two-Dimensional Electron Systems." Thesis, 2016. https://thesis.library.caltech.edu/9320/2/thesis.pdf.
Full textAbstract:
The subject of this thesis is the measurement and interpretation of thermopower in high-mobility two-dimensional electron systems (2DESs). These 2DESs are realized within state-of-the-art GaAs/AlGaAs heterostructures that are cooled to temperatures as low as T = 20 mK. Much of this work takes place within strong magnetic fields where the single-particle density of states quantizes into discrete Landau levels (LLs), a regime best known for the quantum Hall effect (QHE). In addition, we review a novel hot-electron technique for measuring thermopower of 2DESs that dramatically reduces the influence of phonon drag.
Early chapters concentrate on experimental materials and methods. A brief overview of GaAs/AlGaAs heterostructures and device fabrication is followed by details of our cryogenic setup. Next, we provide a primer on thermopower that focuses on 2DESs at low temperatures. We then review our experimental devices, temperature calibration methods, as well as measurement circuits and protocols.
Latter chapters focus on the physics and thermopower results in the QHE regime. After reviewing the basic phenomena associated with the QHE, we discuss thermopower in this regime. Emphasis is given to the relationship between diffusion thermopower and entropy. Experimental results demonstrate this relationship persists well into the fractional quantum Hall (FQH) regime.
Several experimental results are reviewed. Unprecedented observations of the diffusion thermopower of a high-mobility 2DES at temperatures as high as T = 2 K are achieved using our hot-electron technique. The composite fermion (CF) effective mass is extracted from measurements of thermopower at LL filling factor ν = 3/2. The thermopower versus magnetic field in the FQH regime is shown to be qualitatively consistent with a simple entropic model of CFs. The thermopower at ν = 5/2 is shown to be quantitatively consistent with the presence of non-Abelian anyons. An abrupt collapse of thermopower is observed at the onset of the reentrant integer quantum Hall effect (RIQHE). And the thermopower at temperatures just above the RIQHE transition suggests the existence of an unconventional conducting phase.
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Lou, Hou-Lun, and 羅晧倫. "Dynamic Thermopower of Ni-DNA System." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/259qhg.
Full text
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Lee, Jui-Kan, and 李瑞康. "Transport and Thermopower Studies of Indium Tin Oxide Films." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/41260849119722087583.
Full textAbstract:
碩士輔仁大學
物理學系
98
We have measured the resistance, thermopower and magnetoresistance of a series of annealed indium tin oxide (ITO) thin films (thickness 21nm) and compared with Bloch-Grüneisen law, Seebeck effect and two-dimensional weak-localization effect, respectively. We extracted Debye temperature , electron-phonon coupling constant , Fermi energy , carrier concentration , phase breaking time . This observation provided a direct experimental proof of the free-electron-like characteristic of the energy band structure of ITO. Besides, our results also revealed the electron-phonon interaction was very weak in ITO films. Our extracted values of ~1050 K and cm-3 are very close to previous studies, and are larger one order of magnitude than metals.
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Wang, Yu-Hao, and 王宇豪. "Thermopower,specific heat and magnetic properties in FeSi2 nanoparticles." Thesis, 2003. http://ndltd.ncl.edu.tw/handle/02994040106672007475.
Full textAbstract:
碩士輔仁大學
物理學系
91
The high-temperature α-phase of FeSi2 is metallic while low-temperature β-phase is a semiconductor . In this thesis we present the study of thermopower S, thermal conductivityκand electrical conductivityσ of α and β-phases of FeSi2 . Furthermore, the values of Figure of Merit Z = S2σ/κwere calculated and the results shows β- FeSi2 had higher thermpower as well as higher efficiency Z compared with theα- FeSi2. The nanoparticles of β- FeSi2 were fabricated by different methods such as flash evaporation, EDM and ball milling. We discuss how the size effects on S,κandσ inβ- FeSi2. The result show that the values of S, κ and σ in β- FeSi2 nanoparticles were smaller than the bulk. The specific heat of β- FeSi2 nanoparticles was higher than the bulk at 0.3 to 30 K . The magnetic susceptibility of β- FeSi2 nanoparticles and bulks were measured from 2 to 300 K, and the results reveals thatβ- FeSi2 nanoparticles have superparamagnetism and Blocking Temperature at 70 K.
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Duarte, Nicolas Benjamin Tadigadapa Srinivas A. "Thermopower measurement of gold nanowire systems using a micromachined workbench." 2008. http://etda.libraries.psu.edu/theses/approved/WorldWideIndex/ETD-3314/index.html.
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Chih-Ting, Chen, and 陳志挺. "Thermopower , thermal conductivity and specific heat in Bi2Te3 bulk and nanoparticles." Thesis, 2003. http://ndltd.ncl.edu.tw/handle/38081379361892512692.
Full textAbstract:
碩士淡江大學
物理學系
91
This experiment is aim to improve on TE material quality as Seebeck coefficient and thermal conductivity by analyzing, and hopefully increasing, the TE figure of merit, Z, which is an important factor related to efficient energy conversion. Fortunately, theory and literature have revealed that by decreasing the dimensions of our samples, we can increase Seebeck coefficient, and therefore, improve on the Z.So,we reduce our specimen Bi2Te3 size by milling and EDM methods and try to study the relationship between sample size and it’s thermopower, thermal conductivity and specific heat.
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Su, C. H., and 蘇家弘. "Set-Up of Simultaneous Real-Time Measurements of High-Temperature Resistivity and Thermopower." Thesis, 2004. http://ndltd.ncl.edu.tw/handle/41013154974946927011.
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碩士國立彰化師範大學
光電工程技術研究所
92
Abract This thesis describes the set up of simultaneous real-time measurements of high-temperature resistivity and thermopower. This system is used to study the thermoelectric properties of hot-pressed Bi2Te3 and Na0.65Co1-xNixO2. We use current sourcemeter Keithley 2400, digit multimeter Keithley 2000 and high-temperature vacuum furnace, which were interface by the IEEE488 to measure the high-temperature resistivity and thermopower simultaneously. Data acquisition is automatically controlled by a user-writer program usingVB6.0 language. Bi2Te3 was prepared via metal-organo complex method and hot pressing process at various combination of temperature and pressure. The resistivity and thermopower of Bi2Te3 were measured from 77K to 573K. From XRD patterns, on the preferential orientation (0,0,15) reflection plane is observed for all the hot-pressed Bi2Te3 samples. These results show that the electrical resistivity decreases and thermopower exhibits a optimum values with increasing temperature. Based on the combination of resistivity and thermopower data, we can conclude that the mobility increases with increasing temperature. The carrier concentration shows minimum value probably due to the lattice deformation and the composition deviation of tellurium from stoichiometric Bi2Te3. The power factor of the hot-pressed sample prepared at the pressure of 220 MPa and at 350℃ is 8.8μW/cmK2. Polycrystalline samples of Na0.65Co1-xNixO2 were prepared by rapid heat-up method and high-temperature resistivity and thermopower were measured up to 573K. We find that the phase stability of Na0.65Co1-xNixO2 is poor in vacuum and the measurement is not reliable because of phase formation of Co3O4. With temperature increasing, the resistivity of Na0.65Co1-xNixO2 decreases and the thermopower increases. Therefore, Na0.65Co1-xNixO2 has better thermoelectric properties at high temperature as compared to low temperatures and shows promising application at high temperatures.
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Jayaraman, Aditya. "Tuning the electrical and thermoelectric properties of bilayer graphene by band engineering." Thesis, 2020. https://etd.iisc.ac.in/handle/2005/5188.
Full textAbstract:
Graphene and its two dimensional (2D) analogues have sustained the interests of the
researchers for quite some time because of the rich physics they offer including many
quantum phenomena and non-trivial topological properties. Bilayer graphene specifically
offers far more tunability in its electronic properties because of the ability to break the
inversion symmetry and open a band gap by an application of a transverse electric field.
In this thesis. we study the electrical and thermoelectric properties of bilayer graphene
under the influence of different types of external parameters namely transverse electric
field, uniaxial strain and the superlattice potential due to the moiré lattice formation.
While the opening of a band gap is a promising property for device applications, the
presence of disorder and localized states within the gap implies that the electron transport
still takes place via other mechanisms such as variable range hopping. In addition, bilayer
graphene can still host robust edge states when the gap is open in the bulk because of
its non-trivial marginal topology. The first part of the thesis attempts to establish the
nature of transport in strongly localized bilayer graphene and bridge the two conflicting
transport mechanisms that are predicted to occur namely the edge transport and the
2D hopping transport. Our results show possible evidence of one dimensional hopping
transport occuring via the edges, possibly assisted by electron-electron interaction.
Bilayer graphene has another interesting anomaly in its band structure near the band
edge, i.e Lifshitz transition. While the energy scale associated with the trigonal warping
induced Lifshitz transition is very small resulting in the masking of its effects on the
transport properties, it can be enhanced with the help of external parameters such as
strain and the electric field (D). We observe anamolous plateau like features in the
thermopower measurements, close to the charge neutrality point in a ultrahigh mobility
bilayer graphene. We have explained these unique features with the presence of van-Hove
singularity near the Lifshitz transition which has been enhanced due to uniaxial strain and
D. The observation of these effects in thermopower measurements with no corresponding
features in the conductance measurements shows the extreme sensitivity of thermopower
to the low energy features in density of states.
Finally, we study the electrical and the thermoelectric properties of bilayer graphene
aligned with two boron nitride layers resulting in the formation of a supermoiré lattice
resulting in the modification of energy dispersion of BLG due to zone folding. The overlay
of the two two-layer moiré superlattices results in a third superlattice, whose period can
be larger than the maximum period (> 14 nm) in a graphene/hBN system. We observe
multiple resistance ridges very close to the charge neutrality point which could possibly
emerge due to higher order minibands in graphene-hBN moiré superlattices. The resistance
at the secondary maxima seems to saturate to a quantized value of the resistance
upon the application of an electric field possibly indicating a transport through one dimensional
conducting modes. We also observe multiple sign changes in the thermopower
measurements corresponding to the resistance ridges confirming these peaks as arising
due to the miniband formation. This demonstration can pave way for constructing higher
order moiré systems possibly resulting in the formation of ultra flat bands in graphene
moiré superlattices.
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Tseng, Kai-Sheng, and 曾凱笙. "Negative Capacitance Effect and Thermopower in C6H2X2(NH2)2-Single Molecule Junctions (X=H, F, Cl, Br)." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/d5v33h.
Full textAbstract:
碩士國立交通大學
電子物理系所
106
We study the effect of functional substitution in negative capacitance and thermopower in single-molecule junctions from first-principles, The single-molecule junction is formed by a C6H2X2(NH2)2 molecule (x=H,F,Cl,Br) sandwiched between two Au electrodes connected to a battery. The electric dipole moment is induces by the asymmetric molecule due to functional substitution, and thus forming a ferroelectric single-molecule junction. When a voltage is applied, negative capacitance may occur owing to the inversion of the electric dipole moment. Firstly we relax the structure of C6H2X2(NH2)2 by total energy calculation using VASP. It’s charge density of C6H2X2(NH2)2 is applied as the initial guess density for current-carrying wave functions obtained self-consistently in the framework of density-functional theory combined to Lippmann-Schwinger equation under various voltages. The non-equilibrium wave functions are applied to calculate the charge distribution, electric multipole moments current and Seebeck coefficient, we also investigate the correlation between electric multipole moments and the current and Seebeck coefficient.
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Mitra, Richa. "Coulomb drag to thermopower response in dimensionally mismatched InAs nanowire-Graphene heterostructures and anisotropic noise in ReS2." Thesis, 2020. https://etd.iisc.ac.in/handle/2005/4770.
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Coulomb drag technique is a notable tool for probing electron-electron interaction in double-layer systems, where the momentum and energy transfer from one layer to the other layer as a result of Coulomb interaction. In such a scenario, allowing current through a layer induces an open-circuit voltage in the nearby layer known as the 'Drag voltage'. In two-dimensional bilayer heterostructures, correlated charge inhomogeneity breaks the electron-hole symmetry and gives rise to a nonzero drag signal at the charge neutrality point. Many theories have been proposed to explain the drag signal with no consensus among them. In the first part of the thesis, we address this question by studying the Coulomb drag in novel drag systems consisting of a two-dimensional (2D) graphene and a one-dimensional (1D) InAs nanowire (NW) heterostructure exhibiting distinct results from 2D-2D heterostructures. For monolayer graphene (MLG)-NW heterostructures, we observe an unconventional drag resistance peak near the Dirac point due to the correlated interlayer charge puddles. The drag signal decreases monotonically with temperature T^(-2) and with the carrier density of NW n^(-4), but increases rapidly with magnetic field B^2. These anomalous responses, together with the mismatched thermal conductivities of graphene and NWs, establish the energy drag as the responsible mechanism of Coulomb drag in MLG-NW devices. In contrast, for bilayer graphene (BLG)-NW devices the drag resistance reverses sign across the Dirac point, and the magnitude of the drag signal decreases with the carrier density of the NW n^(-1.5), consistent with the momentum drag but remains almost constant with magnetic field and temperature. This deviation from the expected T^2 arises due to the shift of the drag maximum on graphene carrier density.
In the second part of the thesis, we present the thermoelectric properties of graphene in the NW-graphene heterostructure using the nanowire as a local nano-heater. Contrary to conventional thermoelectric measurement, where a heater is placed at one side, here we have used the InAs NW with a diameter of ~ 50 nm as a local heater at the middle of the heterostructure and measure the thermoelectric response across the graphene as a function of temperature (1.5K - 50K) and carrier concentration. The thermoelectric voltage in the NW-bilayer graphene (BLG) heterostructure follows Mott's response with a sign change around the Dirac point. In contrast, the thermoelectric voltage in NW-monolayer graphene (MLG) heterostructure show anomalous oscillations around the Dirac point, which is completely missing in the Mott's response extracted from the resistance data. This discrepancy is understood by the modification of the local density of states (DOS) in MLG due to a cavity formed by the 1D electrostatic potential of the NW. Moreover, using Fourier's law, we theoretically estimate the temperature profile for the heterostructure, which shows that the gradient of temperature is dominant around the graphene part underneath of the NW, and thus sensitive to detect the local DOS.
In the last part of the thesis, we demonstrate the 1/f noise study in ReS2 transition metal dichalcogenide to characterize its in-plane anisotropy. The intrinsic structural deformity in ReS2 gives rise the in-plane anisotropy which is reflected in conductance and other measurement data. Our study shows that in a few-layer ReS2, 1/f noise is also anisotropic. By comparing the anisotropies, we found noise spectroscopy to be more sensitive than the conductance measurement.
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Ryu, Yeontack. "Energy Carrier Transport In Surface-Modified Carbon Nanotubes." Thesis, 2012. http://hdl.handle.net/1969.1/148118.
Full textAbstract:
Carbon nanotubes are made into films or bulks, their surface or junction morphology in the networks can be modified to obtain desired electrical transport properties by various surface modification methods. The methods include incorporation of organic molecules or inorganic nanoparticles, debundling of nanotubes by dispersing agents, and microwave irradiation. Because carbon nanotubes have unique carrier transport characteristics along a sheet of graphite in a cylindrical shape, the properties can be dramatically changed by the modification. This is ideal for developing high-performance materials for thermoelectric and photovoltaic energy conversion applications. In this research, decoration of various organic/inorganic nanomaterials on carbon nanotubes was employed to enhance their electrical conductivity, to improve thermoelectric power factor by modulating their electrical conductance and thermopower, or to obtain n-type converted carbon nanotube. The electrical conductivity of double-wall nanotubes (DWNTs) decorated with tetrafluoro-tetracyanoquinodimethane (F4TCNQ) was increased up to 5.9 × 10^5 S/m. The sheet resistances were measured to be 42 Ω/sq at 75% of transmittance for HNO3/SOCl2-treated DWNT films, making their electrical conductivities 200~300% better than those of the pristine DWNT films. A series of experiments at different ion concentrations and reaction time periods were systematically performed in order to find optimum nanomaterial formation conditions and corresponding electronic transport changes for better thermoelectric power factor. For example, the thermoelectric power factors were improved by ~180% with F4TCNQ on DWNTs, ~200% with Cu on SWNTs, and ~140% with Fe on single-walled nanotubes (SWNTs). Also SWNTs was converted from p-type to n-type with a large thermopower (58 μV/K) by using polyethyleneimine (PEI) without vacuum or controlled environment. This transport behavior is believed to be from charge interactions resulted from the difference between the work functions/reduction potentials of nanotubes and nanomaterials. In addition, different dispersing agents were utilized with DWNT and SWNTs to see a debundling effect in a film network. The highest electrical conductivity of ~1.72×10^6 S/m was obtained from DWNT film which was fabricated with a nanotube solution dispersed by chlorosulfonic acid. Debundling of nanotubes in the film network has been demonstrated to be a critical parameter in order to get such high electrical property. In the last experiment, Au nanoparticle decoration on carbon nanotube bundle was performed and a measurement of themophysical properties has done before and after modifying carbon nanotube surface. Carbon nanotube bundle, herein, was bridged on microdevice to enable the measurement work. This study demonstrates a first step toward a breakthrough in order to extract the potential of carbon nanotubes regarding electron transport properties.
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"Synthesis, Structures and Properties of Thermoelectric Materials in the Zn-Sb-In System." Doctoral diss., 2011. http://hdl.handle.net/2286/R.I.9179.
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abstract: The challenging search for clean, reliable and environmentally friendly energy sources has fueled increased research in thermoelectric materials, which are capable of recovering waste heat. Among the state-of-the-art thermoelectric materials β-Zn4Sb3 is outstanding because of its ultra-low glass-like thermal conductivity. Attempts to explore ternary phases in the Zn-Sb-In system resulted in the discovery of the new intermetallic compounds, stable Zn5Sb4In2-δ (δ=0.15) and metastable Zn9Sb6In2. Millimeter-sized crystals were grown from molten metal fluxes, where indium metal was employed as a reactive flux medium.Zn5Sb4In2-δ and Zn9Sb6In2 crystallize in new structure types featuring complex framework and the presence of structural disorder (defects and split atomic positions). The structure and phase relations between ternary Zn5Sb4In2-δ, Zn9Sb6In2 and binary Zn4Sb3 are discussed. To establish and understand structure-property relationships, thermoelectric properties measurements were carried out. The measurements suggested that Zn5Sb4In2-δ and Zn9Sb6In2 are narrow band gap semiconductors, similar to β-Zn4Sb3. Also, the peculiar low thermal conductivity of Zn4Sb3 (1 W/mK) is preserved. In the investigated temperature range 10 to 350 K Zn5Sb4In2-δ displays higher thermoelectric figure of merits than Zn4Sb3, indicating a potential significance in thermoelectric applications. Finally, the glass-like thermal conductivities of binary and ternary antimonides with complex structures are compared and the mechanism behind their low thermal conductivities is briefly discussed.Dissertation/Thesis
Ph.D. Chemistry 2011
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Pandey, Tribhuwan. "Addressing the Challenges in Designing of Efficient Thermoelectric Materials." Thesis, 2015. https://etd.iisc.ac.in/handle/2005/4806.
Full textAbstract:
Thermoelectric materials offer applications in conversion of waste heat to useful electrical energy
and are promising sources of renewable energy. To use a material efficiently in thermoelectric
application it is necessary to have high figure of merit (ZT). The task of increasing ZT is
challenging because of the competing relation between electrical conductivity, thermopower,
and thermal conductivity. For enhancing ZT of a thermoelectric material, it is necessary to
have a good understanding of its electronic structure as well as transport properties. The goal
of the present work is to develop an understanding of the thermoelectric properties of selected
materials and address some of the fundamental challenges for achieving enhanced performance
in these materials. In this thesis, I have employed density functional theory based calculations
combined with Boltzmann transport theory, to study electronic structures, and electronic and
thermal transport properties of several promising class of thermoelectric materials, including
the transition metal silicides (FeSi2, CrSi2), Zintl compounds, sulphide (Bi2S3), intermetallics
and transition metal dichacogenides (MX2 (M = Zr, Hf and X = S, Se), and MoS2). Based on
the comprehensive study of electronic and thermoelectric properties we conclude that different
strategies are required to improve the ZT of different class of materials. Our findings provide a
better understanding of materials properties and can be generalized to other materials as well
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Hamad, Nagat El-Sabaey Farag. "Magnetische und Elektrische Eigenschaften von Nd0.66(Sr1-yLiy)0.34MnO3 Manganiten." Doctoral thesis, 2003. http://hdl.handle.net/11858/00-1735-0000-0006-B0B6-E.
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Walter, Marvin. "The tunnel magneto-Seebeck effect in magnetic tunnel junctions." Doctoral thesis, 2013. http://hdl.handle.net/11858/00-1735-0000-0022-5E09-E.
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