Nanoscale geometrical patterning for junctionless thermoelectrics

Chapter 8 - New paradigm for efficient thermoelectrics

It is demonstrated that spacer fabrics made of polyester can be coated with Al-doped ZnO (AZO) as thermoelectric (TE) material and conductive silver as contact material to enable smart textiles. An atomic layer deposition process was used for the AZO coating and the conductive silver paste was manually applied. A TE generator and cooling based on the Seebeck and Peltier effect can be observed if a temperature difference or direct current is applied, respectively. Both effects were proven to exist and evaluated. The Seebeck coefficient was determined of up to 50 µV · K−1 and a TE power of about 0.2 µW was generated. Without additional active heat dissipation or cooling, a temperature difference between both spacer fabric surfaces of up to 12 K was achieved. By changing the polarity of the electrical contacts it can significantly be shown that the cold-warm side is changed, as expected for a Peltier element. We observed experimentally the Peltier effect using a single element as well as electrically interconnected sample pairs, the latter could be important for the cooling application. Additionally, the heat transport through the spacer fabrics was tested and showed that there is no heat equalization between the surfaces via air and filaments at temperatures up to 50 °C. The Peltier effect as a fundamental TE effect, that enables site-specific and on-demand cooling applications, has not yet been comprehensively investigated using spacer fabrics and AZO as TE material. These investigations predict that AZO on spacer fabrics hold the special potential to enable flexible and textile solid-state cooling applications.

Micro or nano scale thermoelectric (TE) modules have received increasing attention because of their potential applications as energy supplyingand thermal managing components in microelectronic devices and micro-electro-mechanical systems (MEMS). In the present work, microscale thermoelectric modules are fabricated by combining mechanical cutting and photolithograph processes from nano-sized silicon carbide (SiC) particles reinforced Bi2Te3-based materials (Bi2Te3 for n type, and Bi0.5Sb1.5Te3 for p type) prepared by spark plasma sintering (SPS). The fabricated modules have 28 pairs of thermoelectric legs in an area of 3×3 mm2, and each of them is 200×400 µm2 in cross section and 600 µm in length, which is connected in series by Ni-Cu electrodes made with photolithograph patterning and magnetron sputtering. The thermoelectric performances of a p-n couple are simulated with the finite element method (FEM) under a thermal-electrical coupled multi-physics field for both electronic cooling (Peltier effect) and thermoelectric energy generation (Seebeck effect) working mold.

The Nernst and Seebeck effects in graphene with uniform Kekulé lattice distortion have been studied using the tight-binding model combined with the nonequilibrium Green's function method. Numerical results of this work showed that due to the electron–hole symmetry, the Nernst coefficient is an even function of the Fermi energy, while the Seebeck coefficient is an odd function regardless of the magnetic field. The Nernst and Seebeck coefficients show peaks when the Fermi energy crosses the Landau levels at high magnetic fields or crosses the transverse subbands at the zero magnetic fields. The peak height can be very large when the Fermi energy approaches the Dirac point, the Seebeck coefficient can reach about 0.78 mV/K, and the Nernst coefficient can reach about 0.95 mV/K at the corresponding hopping energy modification parameter δ=0.03 and T=0.009t/kB≈288 K. When δ=0.08 and T=0.024t/kB≈766 K, the Seebeck coefficient (or Nernst coefficient) is still up to about 0.78 mV/K (or 0.95 mV/K). This suggests that tunable Seebeck and Nernst coefficients can be achieved because the bandgap is a function of the corresponding hopping energy modification parameter δ. Experimentally, δ can be modulated by changing the type and amount of atoms adsorbed on graphene. In strong magnetic fields, the Nernst coefficient does not depend on the chirality of the nanoribbon.

The problem of identifying patterns that are associated with the features of the structure and phase composition of new thermoelectric materials obtained by self-propagating high-temperature synthesis is considered. A measuring device has been developed to determine the Seebeck coefficient (thermoelectric motive force) of thermoelectric materials in the temperature range of 300–800 K in argon, air or vacuum. The design of the measuring device is described in detail, the capabilities of the device and the measurement error (less than 5 %) are discussed. The thermoelectromotive force of reference nickel samples in the temperature range of 300–800 K in an argon medium was measured by a differential method. Negative values of the Seebeck coefficient of the nickel sample were obtained throughout the studied temperature range, which indicates the predominance of electrons as the main charge carriers in the sample material. At room temperature, the measured value of the Seebeck coefficient is –19.05 mkV/K and decreases to a value of –25.71 mkV/K with an increase in temperature to 515 K. With a further increase in temperature to 640 K, the Seebeck coefficient monotonically increases to a value of –19.60 mkV/K. At temperatures above 640 K, the Seebeck coefficient continuously decreases and at 824 K reaches a value of –24.12 mkV/K. The Curie point is 644 K. The obtained values of the Seebeck coefficient for nickel in the temperature range 300–800 K are comparable with the data given in the literature. When calculating the Seebeck coefficient of the material, equations are used using the Seebeck coefficient values for the positive and negative thermocouple paths, which eliminates the need for additional measuring probes and contacts to measure the thermoelectric voltage on the sample. The set-up can also be used to make electrical resistance measurements using the standard 4-point method.

A new system to generate electric energy using thermoelectric generators (TEG’s) based on Peltier cells has been conceived, designed and built. The system consists of a Peltier cell assembly installed in a replica of an air conditioning circuit to benefit from hot and cold air flow to generate the appropriate temperature gradient. The new system has been characterized using a group of Peltier cells mounted on a dual thermostatic chamber where cold air from air conditioning equip and hot air coming from an industrial heater are flowing through independent half-chambers. The two half-chambers are separated by an insulation wall where Peltier cells have been inserted. Temperature difference between hot and cold air flow is used by the Peltier cells to generate a voltage and current using the Seebeck effect. Peltier cells are connected in series and parallel to increase voltage and current to obtain appropriate values compatibles with external applications. Experimental tests have been developed to characterize the new design obtaining electric current and voltage, thus power, from the Peltier cell assembly. The amount of power is proportional to the temperature difference between hot and cold side of the chamber through an exponential evolution with maximum performance for specific temperature difference. Power density of the TEG has been found of 2.5x10 4 W/m 2 for a temperature difference of 160º C. TEG assembly generates a current of 8 A and 5 VDC voltage at the peak power point. The system has been tested at temperature differences compatible with those created in air conditioning ducts to recreate a real situation. Power generation for set up conditions of 50º C at the hot side and 5º C at the cold one, like in conventional air conditioning ducts, has been found of very low value because of the low temperature difference. However, when using industrial conditions with hot temperature in the range 100º C to 130º C and cold temperature between -30º C and 0º C, the power generation has increased significantly, showing the critical influence of the temperature difference. The simulation analysis indicates that the new design is capable of generating enough power to cover energy demand in residential buildings.

Two-dimensional (2D) materials exhibit a number of improved mechanical, optical, and electronic properties compared to their bulk counterparts. The absence of dangling bonds in the cleaved surfaces of these materials allows combining different 2D materials into van der Waals heterostructures to fabricate p-n junctions, photodetectors, and 2D-2D ohmic contacts that show unexpected performances. These intriguing results are regularly summarized in comprehensive reviews. A strategy to tailor their properties even further and to observe novel quantum phenomena consists in the fabrication of superlattices whose unit cell is formed either by two dissimilar 2D materials or by a 2D material subjected to a periodic perturbation, each component contributing with different characteristics. Furthermore, in a 2D material-based superlattice, the interlayer interaction between the layers mediated by van der Waals forces constitutes a key parameter to tune the global properties of the superlattice. The above-mentioned factors reflect the potential to devise countless combinations of van der Waals 2D material-based superlattices. In the present feature article, we explain in detail the state-of-the-art of 2D material-based superlattices and describe the different methods to fabricate them, classified as vertical stacking, intercalation with atoms or molecules, moiré patterning, strain engineering and lithographic design. We also aim to highlight some of the specific applications of each type of superlattices.

Two-dimensional (2D) materials have emerged as promising functional materials with many applications such as semiconductors and photovoltaics because of their unique optoelectronic properties. Although several thousand 2D materials have been screened in existing materials databases, discovering new 2D materials remains challenging. Herein, we propose a deep learning generative model for composition generation combined with a random forest-based 2D materials classifier to discover new hypothetical 2D materials. Furthermore, a template-based element substitution structure prediction approach is developed to predict the crystal structures of a subset of the newly predicted hypothetical formulas, which allows us to confirm their structure stability using DFT calculations. So far, we have discovered 267 489 new potential 2D materials compositions, where 1485 probability scores are more then 0.95. Among them, we have predicted 101 crystal structures and confirmed 92 2D/layered materials by DFT formation energy calculation. Our results show that generative machine learning models provide an effective way to explore the vast chemical design space for new 2D materials discovery.

Bismuth selenide (Bi2Se3) and bismuth telluride (Bi2Te3) are well-known compounds for thermoelectric (TE) cooling and generation applications near room-temperature. The performance of TE materials is quantified by a dimensionless figure of merit, ZT = α2σT/κ, in which α, σ, κ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and absolute temperature, respectively. Currently, enhancing the TE power factor (PF = α2σ) of Bi2Se3 and Bi2Te3 thin-films remains a challenge due to the coupling amongst TE material properties and the difficulty of growing stoichiometric films under elevated substrate temperatures (Ts), at which is beneficial for enhancing the σ. In this thesis study, n-type TE Bi2Se3 and Bi2Te3 thin films were grown on SiO2/Si substrates using pulsed laser deposition (PLD). The effects of the structure, composition, and morphology on the TE properties of Bi2Se3 and Bi2Te3 thin films were investigated by controlling background ambient pressures (P) and Ts in PLD depositions. We found that the deposition in relatively high P (≥ 40 Pa) could obtain stoichiometric films at extended Ts up to 300 °C for Bi2Se3 and 340 °C for Bi2Te3, which can reduce the carrier concentration (n) and significantly enhance the Seebeck coefficient (α), following the α~n-2/3 relation approximately. Furthermore, at high Ts- growths, the obtained structures of highly (00l)-oriented – layered of large crystallites led to the substantial increase in the carrier mobility µ and thus improve the σ (= nµe). For example, the stoichiometric Bi2Se3 films grown at grown at 300 °C and 40 Pa with highly (00l) oriented and layered-hexagonal platelets possessed the highest PF of 5.54 µWcm-1K-2, where ׀α׀ = 75.8 µV/K and σ = 963.8 S/cm. Similarly, the stoichiometric Bi2Te3 films grown at Ts = 220–340 °C and PAr = 80 Pa with highly (00l)-oriented and layered structures showed the best properties, with a carrier mobility µ of 83.9 – 122.3 cm2/Vs, an ׀α׀ of 172.8 – 189.7 µV/K, and a remarkably high PF of 18.2 – 24.3 µWcm-1K-2. In contrast, the Te-rich films deposited at Ts ≤ 120 °C with (015)-preferred orientations and columnar–small grain structures or the Te-deficient film deposited at 380 °C with Bi4Te5 polyhedron structure possessed poor properties, with µ < 10.0 cm2/Vs, ׀α׀ < 54 µV/K, and PFs ≤ 0.44 µWcm-1K-2. This study provides a comprehensive understanding the interrelationships between PLD processing conditions, microstructures, and TE properties of Bi2Te3-based thin films, promising for further improving the TE performance of materials and applications. In brief, the morphology of highly (00l) oriented–layered large crystallite structures and the stoichiometry predominantly contribute to the substantial enhancement of µ and ׀α׀, respectively, resulting in remarkable enhancement in PF.

The quantum restriction effect of charge carriers in two-dimensional materials can significantly improve their power factors. MXene, as a new type of two-dimensional double transition metal material, has attracted extensive attention due to thermoelectric properties, and higher controllability than single transition metal MXene, which has potential applications in thermoelectric devices. In this work, new two-dimensional monolayer double transition metal MXene, i.e. TiZrCO&lt;sub&gt;2&lt;/sub&gt; and VYCO&lt;sub&gt;2&lt;/sub&gt;, are designed and their stabilities, electronic and thermoelectric properties are studied by the first principles and Boltzmann transport theory. It has been shown that both are indirect bandgap semiconductors with mechanical, thermodynamic and kinetic stability, and their thermoelectric properties (Seebeck coefficients, electrical and electronic thermal conductivities and lattice thermal conductivities) in a temperature range from 300 K to 900 K are studied. For the optimal carrier concentration at 300 K, the p-type TiZrCO&lt;sub&gt;2&lt;/sub&gt; power factor is 11.40 mW/(m·K&lt;sup&gt;2&lt;/sup&gt;), much higher than that of n-type one, and the VYCO&lt;sub&gt;2&lt;/sub&gt; power factor of p-type (2.80 mW/(m·K&lt;sup&gt;2&lt;/sup&gt;)) and n-type (2.20 mW/(m·K&lt;sup&gt;2&lt;/sup&gt;)) are similar to each other. At 300 K, TiZrCO&lt;sub&gt;2&lt;/sub&gt; and VYCO&lt;sub&gt;2&lt;/sub&gt; have low lattice thermal conductivities of 5.08 W/(m·K) and 3.22 W/(m·K), respectively, and the contributions of optical phonon to the lattice thermal conductivity are both about 30%, i.e. 2.14 W/(m·K) and 1.09 W/(m·K) at 900 K, respectively. At the same time, it is found that at 300 K, when the material sizes of TiZrCO&lt;sub&gt;2&lt;/sub&gt; and VYCO&lt;sub&gt;2&lt;/sub&gt; are within 12.84 nm and 5.47 nm respectively, their lattice thermal conductivities are almost unchanged, and can be adjusted by adjusting the compositions. At 900 K, the thermoelectric value of p-type TiZrCO&lt;sub&gt;2&lt;/sub&gt; and VYCO&lt;sub&gt;2&lt;/sub&gt; reach 1.83 and 0.93, respectively, which are better than those of n-type, 0.23 and 0.84. The double transition metals MXene TiZrCO&lt;sub&gt;2&lt;/sub&gt; and VYCO&lt;sub&gt;2&lt;/sub&gt; have better thermoelectric properties than the single transition metal MXene (such as Sc&lt;sub&gt;2&lt;/sub&gt;C(OH)&lt;sub&gt;2&lt;/sub&gt;, &lt;i&gt;ZT&lt;/i&gt; = 0.5), and have the potential applications in new thermoelectric materials with excellent comprehensive properties. A set of calculation methods used in this paper can also provide some reference for exploring the thermoelectric properties of a new double transition metal element MXene.

The thermoelectric effect is attracting a renewed interest as a concept for energy harvesting technologies. Nanomaterials have been considered a key to realize efficient thermoelectric conversions owing to the low dimensional charge and phonon transports. In this regard, recently emerging two-dimensional materials could be promising candidates with novel thermoelectric functionalities. Here we report that FeSe ultrathin films, a high-Tc superconductor (Tc; superconducting transition temperature), exhibit superior thermoelectric responses. With decreasing thickness d, the electrical conductivity increases accompanying the emergence of high-Tc superconductivity; unexpectedly, the Seebeck coefficient α shows a concomitant increase as a result of the appearance of two-dimensional natures. When d is reduced down to ~1 nm, the thermoelectric power factor at 50 K and room temperature reach unprecedented values as high as 13,000 and 260 μW cm−1 K−2, respectively. The large thermoelectric effect in high Tc superconductors indicates the high potential of two-dimensional layered materials towards multi-functionalization.

Thermoelectric (TE) energy conversion, which directly transforms waste heat into usable electricity, presents a crucial technology for sustainable power generation and energy efficiency enhancement. The emergence of two-dimensional (2D) materials has profoundly impacted this field by providing an atomically thin platform for unprecedented control over electronic and thermal transport properties. This comprehensive review critically analyzes the latest progress, persistent challenges, and future opportunities of 2D materials for advanced TE applications. We begin by systematically evaluating the synthesis and processing toolkit, correlating techniques from top-down exfoliation to bottom-up chemical vapor deposition with their specific impacts on microstructure and final device performance. Subsequently, we provide a critical assessment of the fundamental TE performance of key 2D families, including graphene, transition metal dichalcogenides (TMDs), MXenes, black phosphorus (BP), and hexagonal boron nitride (h-BN). The discussion details how advanced engineering strategies, such as strain modulation, layer number control, chemical doping, and heterostructure (HS) design, can dramatically enhance the power factor (PF) while simultaneously suppressing lattice thermal conductivity (κL). Finally, we showcase the successful translation of these materials into practical applications, encompassing flexible and wearable TE generators (WTEGs), self-powered sensors, and integrated energy harvesting systems.

A thermal gradient not only generates a heat current but could also result in a usable voltage difference if applied to a suitable material. Conversely, it is possible to use voltage differences to drive heat currents. The effects related to the mutual conversion of temperature and voltage differences are known as the thermoelectric (TE) effects. TE materials have been used for energy harvesting and solid-state cooling applications for decades. Nevertheless, their performances need significant enhancements to overcome the growing demands for applications. The challenge is that a good TE material needs to have a high electrical conductivity like in a metal, a high Seebeck coefficient like in an insulator, and a low thermal conductivity like in a glass. A number of strategies have been proposed to meet these diverse demands. Dimension reduction is a promising route to enhance the efficiencies of conventional TE materials, and it was proposed long before the innovation of 2D materials. The emergence of 2D materials has stimulated intensive research into their TE properties, with numerous approaches proposed to enhance their performance. Here, we review the recent literature on the TE properties of 2D materials by focusing on the employed enhancement strategies. In addition to the effects of quantum confinement, strategies such as nanostructuring, strain engineering, edge/surface functionalization, creating defects/vacancies, and doping are discussed thoroughly. Approaches to incorporate the intrinsic electronic properties, to utilize the materials with intrinsically low lattice thermal conductivity, and proposed techniques to further lower thermal conductivity are also reviewed. Lastly, the properties of emerging 2D materials with potentially high TE efficiencies are summarized.

New and more efficient thermoelectric (TE) materials that make use of nanotechnology have been developed. These new materials, called quantum wells (QW), are composed of alternating layers of 10 nm thick silicon and SiGe films. They can be deposited by various techniques and magnetron sputtering was used to obtain uniform layered structures that exhibited no degradation of the TE properties or microstructure after thermal aging. For QW thin films, the heat and current flow are “in plane” and in this orientation all of the thermoelectric properties (the Seebeck coefficient, electrical resistivty, and the thermal conductivity) are improved to increase the TE Figure of Merit, ZT (see equation 3, p. 4, for the definition of Z). From the most recent QW test data, ZTs greater than 3 at room temperature have been obtained which constitutes a significant improvement over the state-of-the-art (SOTA) bulk thermoelectrics which have ZTs less than 1. QW materials have the best measured TE power factor (Seebeck coefficient squared divided by electrical resistivity) and, combined with low thermal conductivity substrates, should provide very high efficiency TE modules. The QW TE materials with ZTs greater than 3 lead to conversion efficiencies greater than 20 percent, which allows for much wider commercial applications, particularly in the applications such as the waste-heat recovery from truck engines, refrigeration, and air conditioning, where the SOTA bulk TE modules were shown to be technically feasible but economically unjustified due to low conversion efficiencies. With higher efficiency QW materials, these applications become economically attractive. For the refrigeration and air conditioning applications, the QW TE materials are predicted to have higher coefficients of performance (COP) than the SOTA vapor compression systems, with the additional advantages of having no compressors, no moving parts, no refrigerants, no vibrations, no noise, and practically no maintenance. With such significant advantages, it is very important to have independent confirmation of the QW TE properties that lead to such improved performance. Three independent researchers have confirmed the previously measured QW TE properties using conventional test techniques, and a totally new test technique was developed to measure the TE properties and performance and the results provided yet another confirmation of the superior TE performance of the QW materials versus the SOTA bulk thermoelectrics. The temperature range for the applications is anticipated to be as low as −150C to the upper temperature of 1000C, with the power generation capacity ranging from milliwatts to kilowatts and cooling capacity ranging from watts to several tons of refrigeration.

Crystal structure, microstructure, and electronic transport properties of β-Zn4Sb3 thermoelectrics: effects of Zn intercalation and deintercalation