Roadmap on thermoelectricity

The increasing energy demand and the ever more pressing need for clean technologies of energy conversion pose one of the most urgent and complicated issues of our age. Thermoelectricity, namely the direct conversion of waste heat into electricity, is a promising technique based on a long-standing physical phenomenon, which still has not fully developed its potential, mainly due to the low efficiency of the process. In order to improve the thermoelectric performance, a huge effort is being made by physicists, materials scientists and engineers, with the primary aims of better understanding the fundamental issues ruling the improvement of the thermoelectric figure of merit, and finally building the most efficient thermoelectric devices. In this Roadmap an overview is given about the most recent experimental and computational results obtained within the Italian research community on the optimization of composition and morphology of some thermoelectric materials, as well as on the design of thermoelectric and hybrid thermoelectric/photovoltaic devices

Chapter Managing Heat Transfer Issues in Thermoelectric Microgenerators

This chapter deals with heat transfer challenges in the microdomain. It focuses on practical issues regarding this matter when attempting the fabrication of small footprint thermoelectric generators (μTEGs). Thermoelectric devices are designed to bridge a heat source (e.g. hot surface) and a heat sink (e.g. ambient) assuring that a significant fraction of the available temperature difference is captured across the active thermoelectric materials. Coexistence of those contrasted temperatures in small devices is challenging. It requires careful decisions about the geometry and the intrinsic thermal properties of the materials involved. The geometrical challenges lead to micromachined architectures, which silicon technologies provide in a controlled way, but leading to fragile structures, too. In addition, extracting heat from small systems is problematic because of the high thermal resistance associated to heat exchanged by natural convection between the surrounding air and small bare surfaces. Forced convection or the application of a cold finger clearly shows the usefulness of assembling a heat exchanger in a way that is effective and compliant with the mechanical constraints of micromachined devices. Simulations and characterization of fabricated structures illustrate the effectiveness of this element integration and its impact on the trade-off between electrical and thermal behavior of the active materials in device performance

Managing Heat Transfer Issues in Thermoelectric Microgenerators

This chapter deals with heat transfer challenges in the microdomain. It focuses on practical issues regarding this matter when attempting the fabrication of small footprint thermoelectric generators (μTEGs). Thermoelectric devices are designed to bridge a heat source (e.g. hot surface) and a heat sink (e.g. ambient) assuring that a significant fraction of the available temperature difference is captured across the active thermoelectric materials. Coexistence of those contrasted temperatures in small devices is challenging. It requires careful decisions about the geometry and the intrinsic thermal properties of the materials involved. The geometrical challenges lead to micromachined architectures, which silicon technologies provide in a controlled way, but leading to fragile structures, too. In addition, extracting heat from small systems is problematic because of the high thermal resistance associated to heat exchanged by natural convection between the surrounding air and small bare surfaces. Forced convection or the application of a cold finger clearly shows the usefulness of assembling a heat exchanger in a way that is effective and compliant with the mechanical constraints of micromachined devices. Simulations and characterization of fabricated structures illustrate the effectiveness of this element integration and its impact on the trade-off between electrical and thermal behavior of the active materials in device performance

Roadmap on energy harvesting materials

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere

Synthesis, Transport, and Thermoelectric Studies of Topological Dirac Semimetal Cd3AS2 for Room Temperature Waste Heat Recovery and Energy Conversion

ABSTRACT SYNTHESIS, TRANSPORT, AND THERMOELECTRIC STUDIES OF TOPOLOGICAL DIRAC SEMIMETAL CD3AS2 FOR ROOM TEMPERATURE WASTE HEAT RECOVERY AND ENERGY CONVERSION by The University of Wisconsin-Milwaukee, 2017 Under the Supervision of Professor Nikolai Kouklin Rising rates of the energy consumption and growing concerns over the climate change worldwide have made energy efficiency an urgent problem to address. Nowadays, almost two-thirds of the energy produced by burning fossil fuels to generate electrical power is lost in the form of the heat. On this front, increasing electrical power generation through a waste heat recovery remains one of the highly promising venues of the energy research. Thermo-electric generators (TEGs) directly convert thermal energy into electrical and are the prime candidates for application in low-grade thermal energy/ waste heat recovery. The key commercial TE materials, e.g. PbTe and Bi2Te3, have room temperature ZT of less than 1, whereas ZT exceeding 3 is required for a TEG to be economically viable. With the thermoelectric efficiency typically within a few percent range and a low efficiency-to-cost ratio of TEGs, there has been a resurgence in the search for new class of thermo-electric materials for developing high efficiency thermo-to-electric energy conversion systems, with phonon-glass electron-crystal materials holding the most promise. Herein, we focus on synthesis, characterization and investigation of electrical, thermo-electrical and thermal characteristics of crystalline Cd3As2, a high performance 3D topological Dirac semimetal with Dirac fermions dispersing linearly in k3-space and possessing one of the largest electron mobilities known for crystalline materials, i.e. ~104-105cm2V-1s-1. Suppression of carrier backscattering, ultra-high charge carrier mobility, and inherently low thermal conductivity make this semimetal a key candidate for demonstrating high, device-favorable S and in turn ZT. In this work, a low-temperature vapor-based crystallization pathway was developed and optimized to produce free standing 2D cm-size crystals in Cd3As2. Compared to the bulk crystals produced in previous studies, e.g. Piper-Polich, Bridgman, or flux method, Cd3As2 samples were synthesized over a considerably shorter time ( only a few hours), were single crystals and highly stochiometric. A high thermopower of up to 613 μV K−1 and the electrical conductivity of ~ 105 S/m were registered within the temperature range of 300–400 K. A 1ω-method based on the transfer function was applied to probe a thermal conductivity, k of Cd3As2 platelets. The results yield k of ~2.4 W/m.K in the confirmation that the thermal conductivity of Cd3As2 crystals is to approach the amorphous limit at the room temperature. With its peak thermopower attained at the low temperature range of ~300-400 K, high electrical conductivity and amorphous limit thermal conductivity, crystalline Cd3As2 grown via a low-T vapor based method demonstrates ZT \u3e3; the results confirm that as-produced Cd3As2 platelets hold a high promise and is another phonon-glass electron-crystal TE material for the development of next generation, high efficiency thermo-electric generators and refrigerators operating under normal conditions

FINITE ELEMENT AND IMAGING APPROACHES TO ANALYZE MULTISCALE ELECTROTHERMAL PHENOMENA

Electrothermal effects are crucial in the design and optimization of electronic devices. Thermoreflectance (TR) imaging enables transient thermal characterization at submicron to centimeter scales. Typically, finite element methods (FEM) are used to calculate the temperature profile in devices and ICs with complex geometry. By comparing theory and experiment, important material parameters and device characteristics are extracted. In this work we combine TR and FEM with image blurring/reconstruction techniques to improve electrothermal characterization of micron and nanoscale devices

N-type Organic Materials for Thermoelectric Applications

Harvesting waste heat as a renewable energy source could allow us to power small devices in everyday life, from medical devices to wireless networks from heat sources all around us. In particular, the use of organic materials as the active thermoelectric component opens up the possibility of flexible, printed electronics and ease of cheaper mass reproducibility. In this work, 3 topics are explored: (i) the use of graphene-based materials for thermoelectric applications, (ii) understanding how heat can move through polymer thin films with topographical features, in particular P3HT, and (iii) the effect ladderisation has on the polymer BBB and the resulting thermal and structural properties of the laddered structure, BBL. Graphene is a versatile material with intrinsically high carrier mobility. However, having impressive electronic properties is not wholly advantageous for thermoelectric energy harvesting as it usually leads to a high electronic thermal conductivity which reduces a material’s zT. In an attempt to remedy this, many researchers have successfully merged graphene with polymer. One approach is to covalently bond functional groups on to the graphene surface, then polymerise a layer of monomers on top. The functional groups interrupt the repetitive structure of graphene, inducing more phonon scattering events which reduces the lattice thermal conductivity. This process also reduces carrier mobility, hence reducing electronic thermal conductivity and carrier conductivity which is an unwanted but inevitable effect. A layer of polymer is hence grown on top to restore some electronic pathways. This was the approach deployed in this work, the polymer of choice was PEDOT, thus a sulfonate group was chosen to be the functional group. While the thermoelectric properties of pure graphene in this work yield values agreeing with literature, it was clear that the substrate choice played a noticeable part. It was found that graphene on a silicon nitride (Si3N4) terminated substrate, a higher Seebeck coefficient of ~25 μV/K was measured in comparison to ~17 μV/K for graphene on aluminium oxide (Al2O3) terminated substrates. This lead to zT reaching a maximum value of ~3x10-3. This may be explained by a possible band gap opening of 0.22 eV, observed with UPS, which was not observed for graphene on Al2O3. Raman spectroscopy showed that the D-band associated with disorder and defects within the graphene lattice was present for graphene on Al2O3, and not for graphene on Si3N4 which could also explain the lower Seebeck coefficient as this parameter is also dependent on carrier mobility. For functionalised graphene/PEDOT films, again, samples on Si3N4 performed better than films on Al2O3. Using XPS, it was found that a larger concentration of functional groups were bonded to the graphene surface for films on Al2O3 which could be due to the different fermi levels of the graphene on their respective substrate materials, and also due to the presence of more graphene edges on graphene/ Al2O3 films (as shown by the D-band in the Raman spectrum). The surface functionalisation was successful in reducing the thermal conductivity, however, the electrical conductivity was heavily damaged, in particular for films on Al2O3 where the electrical conductivity is almost an order of magnitude less and thermal conductivity was approximately half that values seen with films on Si3N4. The lower concentration of functional groups seen in the films on Si3N4 were hence beneficial to the system. Raman spectroscopy also revealed a different morphology between the two sample types where a higher degree of crystallinity due to shorter chains is seen in the films on Si3N4 and can also contribute to the higher electronic properties. Overall, it is shown that graphene works as a good base material for thermoelectric materials, and it is possible to exploit parameters such as substrate choice and functionalisation efficiency to tune the thermoelectric parameters. All in-plane thermal conductivity measurements throughout my work rely on the simple assumption that heat flux is homogeneous and one dimensional. However, for thin films, where topographical roughness is inevitable, heat flux will begin to deviate from the ideal scenario and the measured values will begin to deviate from true intrinsic values of the material. My second project focuses on understanding how measured values are affected by a simple rectangular dip/trough on the surface of a thin film and whether modelled scenarios can be used to represent realistic scenarios. Finite element modelling was used to represent a segment of a doped P3HT thin film, of thermal conductivity 0.4 W/mK, and a thickness and width of 300 nm and 1500 nm respectively, and a single surface feature. The film is modelled with a membrane layer underneath, of thickness 144 nm, representing a substrate with a thermal conductivity of 2.6 W/mK. Fourier’s law was then used to extract a thermal conductivity value that represents a real measured value. Fourier’s law states that the local heat flux is proportional to the area in which it travels through – therefore we’d only expect a measured thermal conductivity value to deviate from the material’s intrinsic value if the area of the film changes perpendicular to the heat flux. By comparing the extracted thermal conductivity to the intrinsic thermal conductivity of the material, defined in the model, it is shown that as the feature for deeper (at constant width), the extracted value deviated super linearly from the intrinsic value. However, with a full 500 nm wide crack in the film, the extracted value is only 40% lower than the intrinsic thermal conductivity which is only possible due to the presence of a more thermally conductivity membrane (membrane is 10 times more conductive than film). Colour scaled images show that heat is redirected into the membrane to allow continuous heat flow to the other side of the crack. In the case the membrane is less thermally conductive than the film, the membrane is not able to redirect the heat effectively enough and the extracted thermal conductivity drops by almost 100% (film is 20 times more conductive than the membrane). A much less significant effect is seen when keeping the depth of the feature constant whilst varying the width. This is because the change is now parallel to the heat flow. For this section, the feature depth was kept constant at 210 nm with varying widths. The peak deviation from intrinsic thermal conductivity is seen when the width of the feature is ~66% of the full simulated length, beyond this point the extracted thermal conductivity begins to converge back to the material’s intrinsic value. At this point, the maximum deviation was ~25% lower than the intrinsic thermal conductivity. When the width of the surface feature is 50% of the segment width, the original film height gets treated as peaks. Thus, the asymmetric nature of this curve tells us that heat is redirect much more efficiently into the constricted areas, as opposed to the peaks. This is, again, due to the aid of a more thermally conductive membrane. When the film is more thermally conductive than the membrane, no aid is given and the maximum deviation of the extracted thermal conductivity peaks symmetrically when the feature is at 50% of the simulated segment. In this case, the maximum deviation is much higher, at ~35%. The same is therefore seen for a suspended film. My final project explores the affect that ladderising a polymer from a single strand to a double strand has on thermal conductivity. The two polymers studied were the single stranded polymer, BBB and the double stranded ladder polymer, BBL, both of which are very similar in structure. The thermal conductivity of BBB was 0.28 W/mK which is typical of an amorphous polymer. The thermal conductivity of BBL was significantly higher at ~1 W/mK. The high thermal conductivity can be attributed to the fact that ladder polymers are much stiffer due to the double bonds between units and may also exhibit a higher order of crystallinity in comparison to an amorphous polymer. The amorphous and semi-crystalline nature of BBB and BBL respectively are confirmed by GIWAX data. This is interesting due to the significant percent increase in thermal conductivity in doped BBB, where it a small degree of crystallinity might be expected. BBB had an activation energy of 0.25 eV whereas the activation energy of BBL was lower, at 0.10eV. This suggests that the BBL structure has shallower traps associated with disorder which promotes carrier mobility and phonon propagation, agreeing with the thermal conductivity data. The structure of these polymers were analysed further using FTIR where it is clear that doping affected the two structures differently. Doped BBL is seen to have a slight peak shift associated with the neutral C=O bond which can suggest a stronger electron-phonon coupling. The spectra also suggests the dopant is affecting the C=N, C-N, C=O and C-C units within BBL, however, the dopants are residing locally near the C=O units within BBB. The more delocalised nature of polarons within doped BBL may explain the wider polaron band seen in the UV-VIS spectrum

Vers des métamatériaux thermoélectriques à base de super-réseaux verticaux : principes et verrous technologiques

Metamaterials offer the benefit of obtaining improved physical properties over natural materials. In this work, we explore a new variety of thermoelectric metamaterials based on silicon micro- and nano- structuration, in the form of vertical superlattices for use in energy-related applications. Additionally, we focus on a route towards fabricating these materials using simple and low-cost means compared to prior attempts. The first part of this thesis serves as an introduction to the thermal phenomena which form the basis for electrical conduction and heat dissipation by thermionic emission and phonon scattering at the nanoscale. These principles forms the crux of the device. This section also details the characterization principles and results using the 3ω and 2ω methods for thermal measurement. The second part of this thesis describes both top-down and bottom-up approaches towards fabricating nanoscale superlattices from single-crystalline silicon. The novel proposed vertical architecture raised technological challenges that were tackled through the exploration of original experimental techniques for producing high aspect ratio (HAR) structures in an effective manner and over large surface areas. These techniques include the use of traditional lithography patterning and subsequent extrusion of volumic structures. Additionally, the use of nanofibers and diblock copolymers as templates for further etching of HAR silicon nanostructures are also presented to bring us closer to the ultimate goal of the projectLes méta-matériaux offrent la possibilité d'obtenir des propriétés physiques nettement améliorées en comparaison avec celles des matériaux naturels. Dans ce travail, nous explorons une nouvelle variété de métamatériaux thermoélectriques à base de micro-et nano-structuration du silicium, sous la forme de super-réseaux verticaux, avec comme visée applicative la récupération d'énergie thermique ainsi que le refroidissement. En outre, nous focalisons nos efforts sur une méthodologie expérimentale permettant la réalisation de ces matériaux par des moyens simples et peu coûteux. La première partie de cette thèse sert d'introduction aux phénomènes thermiques qui constituent la base de la conduction électrique et de la dissipation de chaleur dans les nanostructures, respectivement par émission thermo-ionique et par la diffusion de phonons. Cette partie détaille également les principes et résultats de caractérisation thermique à l'aide des méthodes 3ω et 2ω. La deuxième partie de cette thèse décrit les approches de micro- nanostructuration descendante « top-down » et ascendante « bottom-up », en vue de la fabrication de super-réseaux nanométriques sur du silicium mono-cristallin. La nouvelle architecture verticale proposée soulève des défis technologiques qui sont traités à travers l'exploration de techniques expérimentales originales pour produire, d'une manière efficace et sur de grandes surfaces, des structures submicroniques à fort facteur de forme. Ces techniques comprennent l'utilisation de motifs résultant de lithographie traditionnelle combinée à l'extrusion pour en produire des structures volumiques. En outre, l'utilisation de nanofibres et de diblocs copolymères comme nano-motifs géométriques sont également présentés pour nous rapprocher davantage de l'objectif ultime du proje