Thermoelectric (TE) materials, or materials that can generate an electrical energy from temperature gradient, are promising for renewable energy technology. One fundamental aspect in the TE research is the demand to maximize the TE power-factor, PF = S2 σ, by having as large Seebeck coefficient (S) and electrical conductivity (σ) as possible. In the early 90s, Hicks and Dresselhaus proposed the PF enhancement by using low-dimensional materials, in which electrons are confined in certain directions and they move freely in the other directions. This quantum effect is known as the confinement length (L) effect, in which L is the thickness or diameter of the two-dimensional (2D) or one-dimensional materials, respectively. However, a key challenge is to understand the critical value of L, at which the PF can be significantly enhanced. Recently, we reevaluated the confinement theory of the low-dimensional materials to solve this issue. We showed that electrons are fully confined only when L is smaller than an intrinsic length Λ, the so-called thermal de Broglie wavelength, which depends on the materials and can be experimentally measured. Monolayer 2D materials naturally satisfy the condition of L < Λ since their confinement length is ∼ 1 nm, while their thermal de Broglie wavelength is ∼ 5-10 nm. Therefore, they could be a good candidate for TE materials. In this review article, we first review the TE materials with low dimensions. Then, we show the basic concept of the confinement effect and the consequence of such an effect. Finally, based on this effect, we turn our attention to the progress achieved recently in the TE properties of the 2D materials such as monolayer InSe, GaN electron gas, and SrTiO3 superlattices.

Bolometric arrays dominate in commercial terahertz and millimeter wave imagers. Bolometers tend to be slow and require an operating bias that increases the power budget. This paper presents first results on antenna-coupled thermo-electric junction detectors, a potentially attractive alternative to microbolometers. The nanoscale junctions have small thermal mass and can be much faster than bolometers. These junctions are heated by currents excited in spectrally-selective antennas and generate a voltage based on the Seebeck effect without any externally applied power. A pixel comprising a series array of antenna coupled junctions with peak response at 7.5 THz (40 μm wavelength) is designed and experimentally demonstrated. Responsivity of 5 MV/W, 500 Hz chopping frequency, and D* of 3.5 x 1010 Jones are demonstrated.

The search for nontoxic compositions in the thermoelectric field has motivated many researches to find alternatives to the toxic Pb-based systems, capable of reaching their high conversion efficiency. SnTe is gaining much attention during the past years due to its superior eco-friendly character, and its very similar crystal and band structure to that of PbTe. These makes SnTe a promising compound with great potential to answer the demand and use as a fair thermoelectric candidate. Most of the recently published studies mainly discuss the stoichiometric SnTe alloy. Only several focus on the effect of introducing excess tin/tellurium to the system. For that reason, this research aims to investigate in detail the nonstoichiometric SnxTe1-x co-doped by bismuth and indium/iodine, in an attempt to optimize the electronic properties.

As the global energy and environmental preservation needs continue to grow, the demand for renewable and clean energy conversion materials and devices continues to rise as well. Thermoelectric (TE) materials and devices can convert waste heat into electricity and therefore it can be a potential renewable and clean energy source. Organic and polymeric materials typically exhibit low electrical conductivities, high Seebeck coefficients, and orders of magnitude lower thermal conductivities as compared to their inorganic counterparts. However, the electrical conductivities of organic/polymeric materials are tunable via doping or molecular engineering. In this study, a series of carefully doped P3HT composites are systematically evaluated for heat as well as light modulated devices. Along with a high absorption coefficient, when the polymer film thickness is less than the penetration depth of the incoming photons, the photo effects are significant and could be very useful for light modulations of thermoelectric functions. With further systematic studies and a better understanding of the mechanisms behind the photo-Seebeck effect, the development of potential high-efficiency multi-function materials and devices appears feasible.

Changes in global energy trends and policies affect demands for scientific innovation. This article discusses the major emerging trends in the global energy sphere and the demands they will place on scientists for innovation in the next decade.

Changes in global energy trends and policies affect demands for scientific innovation. This article discusses the major emerging trends in the global energy sphere and the demands they will place on scientists for innovation in the next decade. With growing volumes and consumption of natural gas, innovation will be necessary for wider uses of natural gas. With this greater usage, scientists will be asked to find ways to monitor and reduce methane emissions from natural gas production, transportation, and consumption. Renewable energy also has an environmental impact, and with the increase in use of renewable energy, greener forms will need to be developed. Since electricity will be the largest sector in terms of energy demand growth, improvements will be necessary in cyber security of grids, in identifying less energy-intensive technologies for mining materials necessary for the electricity infrastructure, and in developing batteries and other forms of energy storage that do not require or require less rare minerals. Scientists need to think beyond the car and develop completely new transportation systems; great improvements also need to take place in maritime transportation, which is rapidly increasing.

Thermoelectric technology has key benefits and strengths in many terrestrial energy recovery applications. Thermoelectric system cost is a key factor governing final decisions on the use of thermoelectric energy recovery systems in all terrestrial applications; thus cost being just as important as power density or efficiency for the adoption of waste energy recovery (WER) thermoelectric generators (TEG). New integrated cost analysis / thermoelectric analysis approaches have now shown key relationships and interdependencies between overall TEG system costs, including TE material costs, manufacturing costs, and specifically heat exchanger costs; and the TE performance design metrics such as TE material properties, TE device design parameters, heat exchanger performance metrics such as hot-side and cold-side conductances and UA values, and hot side heat flux in achieving optimal TEG WER designs. These new approaches have led to a new thermoelectric system economics paradigm that strongly influences TEG cost and performance decisions. While prior work provided foundations for the latest cost scaling analysis / TE performance analysis, this new work offers new insights and understandings and provides the basis for new thermoelectric system economics. Optimum TEG system cost conditions can now be tied directly to the TE materials, TEG design parameters, and heat exchanger design parameters through critical non-dimensional analysis. The non-dimensional analysis and metrics show the TEG system cost and performance interdependencies and intercouplings in one unifying and cohesive relationship. Prior work by T.J. Hendricks, S.K. Yee, and S. LeBlanc, J of Electronic Mater, 45, (3), 1751-1761, 2015 has shown that the system design that minimizes cost (e.g., the G [$/W] value) can be close to designs that maximize power, but these design regimes are not necessarily aligned with high system conversion efficiency or high specific power. Key sensitivities and interrelationships between critical cost metrics and critical TE performance and design metrics in the new thermoelectric system economics paradigm are explored. Quantitative data showing these sensitivities and their implications on TEG system design in terrestrial WER applications are presented. Critical non-dimensional parameter mapping has shown where heat exchanger cost- dominated conditions, TE material or manufacturing cost-dominated conditions, and combinations of cost conditions control and drive the overall TEG cost and performance. This new cost-performance paradigm shows the required pathways and challenges to achieving TEG system costs of $1 -$3/Welec.

The present work highlights the progress in the field of flexible thermoelectric generator (f-TEGs) fabricated by 3-D printing strategy on the typing paper substrate. In this study, printable thermoelectric paste was developed. The dimension of each planer thermoelectric element is 30mm*4mm with a thickness of 50 μm for P-type Bismuth Tellurium (Bi2Te3)-based/ poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) leg. A single thermoleg with this dimension can generate a voltage of 5.38 mV at a temperature difference of 70 K. The calculated Seebeck Coefficient of a single thermoleg is 76.86 μV/K. This work demonstrates that low-cost printing technology is promising for the fabrication of f-TEGs.

Recently, double perovskite-based oxide materials have been proposed for thermoelectric (TE) applications due to their environment-friendly nature, high-temperature stability, better oxidation resistance, and lower processing cost compared to conventional chalcogenides and intermetallics. In this review article, we have comprehensively summarized our recent research studies on Sr2B′B″O6-based double perovskites for high-temperature TE power generation. We have shown that decoupling of phonon-glass and electron-crystal behavior is possible in oxides by reducing thermal conductivity due to induced dipolar glassy state as a result of relaxor ferroelectricity. We have also introduced metal-like electrical conductivity (∼105 S/m) in these ceramics that are inherently insulator in nature. Moreover, we have observed interesting behavior of temperature-driven p–n type conduction switching assisted colossal change in thermopower in some of these oxides, hitherto, obtained only in chalcogenides. The charge transport mechanism in these complex oxides has been analyzed by small polaron hopping conduction model in conjugation with defect chemistry.

Cr-doped higher manganese silicides (HMSs) (Mn1-xCrx)Si1.75 (x = 0–0.35) have been prepared by repeated sintering from raw elemental powder using spark plasma sintering. The a- and cMn-axis length increases with increasing Cr content x. The results of powder X-ray diffraction and microstructural observation suggest that impurity phases, e.g. (Mn, Cr)Si and CrSi2, exist in the samples with x = 0.20 or above. The electrical resistivities and Seebeck coefficient decrease with increasing Cr content x. The Cr content x of 0.10 indicated the largest power factor at 850 K (1.39×10-3W/mK), followed in order by x of 0.25, 0, 0.05, 0.15, 0.20. To confirm the effect of Cr-doping on outputs of modules, two paired p-n modules consisting of n-type purchased Mg2Si and p-type Cr-doped HMS with x = 0, 0.05, 0.10, and 0.20 elements were prepared. The module consisting of (Mn0.9Cr0.1)Si1.75 showed the highest output, that is, 845 mW at 873 K on the hot side. There was approximately 8% improvement compared with that of the module consisting of Cr-free elements.

Sn clathrates are promising phonon glass, electron crystal materials (PGEC), in which the phonon free paths are short and the electron free paths are long. We analysed the relaxed structure of Sn clathrates using four different Density Funtional Exchange-Correlation functionals. The phonon structures were investigated as a first step in order to determine the phonon contribution to the thermal conductivity. We determined the Seebeck coefficient and electrical conductivity of the clathrate compound and the thermoelectric figure of merit. A glimpse into the dynamics of the system for the evaluation of the thermoelectric and electronic properties is presented.

Motivated by low cost, low toxicity, mechanical flexibility, and conformability over complex shapes, organic semiconductors are currently being actively investigated as thermoelectric (TE) materials to replace the costly, brittle, and non-eco-friendly inorganic TEs for near-ambient-temperature applications. Metal–organic frameworks (MOFs) share many of the attractive features of organic polymers, including solution processability and low thermal conductivity. A potential advantage of MOFs and MOFs with guest molecules (Guest@MOFs) is their synthetic and structural versatility, which allows both the electronic and geometric structure to be tuned through the choice of metal, ligand, and guest molecules. This could solve the long-standing challenge of finding stable, high-TE-performance n-type organic semiconductors, as well as promote high charge mobility via the long-range crystalline order inherent in these materials. In this article, we review recent advances in the synthesis of MOF and Guest@MOF TEs and discuss how the Seebeck coefficient, electrical conductivity, and thermal conductivity could be tuned to further optimize TE performance.

The characterization of nanostructured samples with at least one restricted dimension like thin films or nanowires is challenging but important to understand their structure and transport mechanism and to improve current industrial products and production processes. We report on the development of a chip-based platform to simultaneously measure the in-plane electrical and thermal conductivity, the Seebeck coefficient as well as the Hall constant of a thin film in the temperature range from 120 K up to 450 K and in a magnetic field of up to 1 T. Due to the design of the setup, time consuming preparation steps can be omitted and a nearly simultaneous measurement of the sample properties is achieved. Typical errors caused by different sample compositions, varying sample geometries, and different heat profiles are avoided with the presented measurement method. As a showcase study displaying the validity and accuracy of our system, we present measurements of the thermoelectric properties of a 110 nm Bi87Sb13 thin film in the temperature range from 120 K up to 450 K.

Phosphorene is a new-emerging two-dimensional material with many fascinating electronic and thermal properties. Using nonequilibrium Green's function technique, we investigate the thermoelectric transport properties of phosphorene in the ballistic transport regime. We find that while the electronic conductance and thermal conductance of phosphorene are highly anisotropic, the Seebeck coefficient is isotropic. The maximum predicted thermopower reaches 2500 μV/K. We also find that the Wiedemann–Franz law is valid only when the chemical potential is inside valence band or conduction band. When the chemical potential is near the valence band maximum or conduction band minimum; however, the Wiedemann–Franz law becomes invalid, and interestingly, the figure of merit ZT reaches its maximum value. We also find that figure of merit ZT increases with the increase of temperature, and ZT in the armchair direction is much higher than that in the zigzag direction. By analyzing the various effects on ZT, we discuss the possible routines to enhance figure of merit ZT.

Bi2Sr2Co2Ox (BSC-222) bulk materials have been prepared in air by single hot pressing or partial melting followed by hot pressing. Thermal transport properties of as-prepared samples were compared with BSC-222 samples prepared by single partial melting. Samples prepared through hot pressing have a high bulk density and improved preferential grain orientation. Bulk density and preferential grain orientation are significantly better in samples processed by partial melting followed by hot pressing. In samples prepared by partial melting, the presence of sub-micrometer-sized secondary phases Bi0.75Sr0.25Ox, Sr-based oxide, and CoO, is observed. The lattice contribution to the total thermal conductivity decreases significantly in partially melted BSC-222 samples. By using a classical phonon transport model, this result demonstrates that the decrease in lattice thermal conductivity in partially melted BSC-222 samples may be linked not only to porosity but also to the presence of defects, induced by partial melting process, which may play an important role in phonon scattering.

Thermoelectric properties of segmented nanowires and nanobelts are studied by means of the Kubo-Greenwood formula and a real-space renormalization plus convolution method. The tight-binding and Born models are respectively used for the calculation of electronic and lattice thermal conductivities. In particular, we investigate the thermoelectric figure of merit (ZT) of periodic and quasiperiodically segmented nanowires with two different cross sections, where the segments of the quasiperiodic one are ordered following the Fibonacci sequence. The results show an increase of ZT when the cross section area of nanowires diminishes. In addition, we present results of ZT in segmented nanobelts with an inhomogeneous cross section. For both nanowires and nanobelts, the quasiperiodicity seems to be an important enhancing factor of ZT.

Starting with the first published works on the electrochemical deposition of thermoelectric (TE) V–VI compounds in the early nineties, these past two decades have seen a steady increase in scientific interest and publications on this topic. This is hardly surprising, as TE devices offer unique opportunities for power generation in virtually any environment (“energy harvesting”) or demanding cooling applications through the Peltier effect. This review first provides an overview of the advances in the electrodeposition of n- and p-type thin films based on Bi2(Te, Se)3 and (Bi, Sb)2Te3, the currently best-known TE materials for room temperature applications. The overview includes information about the electrolyte and the deposition conditions as well as the achieved composition, thickness, morphology, and TE properties of the deposited films. Additionally, we present the state-of-the-art and recent developments in electroplating-based fabrication processes for microscale TE devices.

The peculiarities of the Seebeck coefficient and power factor are studied in porous thermoelectric materials with spherical hollow pores of varying diameter from nanometer to micrometer length scales. The pores are assumed to be randomly dispersed throughout the matrix material. The influence of trap centers situated at pore interfaces on the power factor is investigated. Using the model based on gamma distribution of the pore sizes, the analytical expression is obtained for the power factor at the arbitrary level of the Fermi energy. Limiting cases of nondegenerate and degenerately doped porous semiconductors are examined as well. The results are compared with calculations for a multilayer composite in which each layer contains pores of a single length-scale. It is shown that the presence of hollow pores with multiscale hierarchical disorder leads to more considerable enhancement in the thermopower over its value in the bulk. Necessary conditions for the enhancement of the power factor are found.

Thermoelectric generation is one of the strongest candidates for recovering the waste heat from industry and transportation. Some of oxides and silicides are considered to be promising thermoelectric materials because of their high oxidation resistance. Several types of modules using p-type Ca3Co4O9/n-type CaMnO3 and p-type MnSi1.75/n-type Mn3Si4Al2 have been prepared and shown around 4 kW/m2 of maximum power density. The present study described the challenging enhancement of the thermoelectric figure of merit ZT of both oxide and silicide compounds. Introduction of secondary phases and low bulk density using a partial melting method is found to be effective for reducing phonon thermal conductivity in the promising Bi2Sr2Co2Ox. The grain size and distribution of the secondary phases can be controlled by optimizing the parameters of the partial melting method. On the other hand, detailed crystallographic structure of a new n-type Mn3Si4Al2 is clarified and leads to the enhancement of the ZT values by elemental substitution.

The (3 + 1)-dimensional crystal structure of the Nowotny chimney–ladder compound Rh17Ge22 was revealed using powder x-ray diffraction technique. As in the case of the higher manganese silicide MnSiγ (known as Mn11Si19, etc.), this germanide consists of two tetragonal subsystems of [Rh] and [Ge] with an irrational c-axis ratio γ = cRh/cGe, and hence the structural formula can be represented as RhGeγ. As expected from first-principles calculations of the approximate structure and the valence electron count, n-type (negative Seebeck coefficient) conduction was experimentally confirmed over the whole temperature range of 333–984 K. Although the absolute value of the Seebeck coefficient was limited to |S| ≤ 53 μV/K, a high electrical conductivity (σ = 4.8 × 103 S/cm) yielded a reasonable power factor of S2σ ∼ 1 mW/K2 m above 600 K. A maximum dimensionless figure-of-merit of ∼0.1 at 900 K was expected using thermal conductivity data of a sintered pellet.

Thermoelectric (TE) generators have very important applications, such as emerging automotive waste heat recovery and cooling applications. However, reliable transport properties characterization techniques are needed in order to scale-up module production and thermoelectric generator design. DOE round-robin testing found that literature values for figure of merit (ZT) are sometimes not reproducible in part for the lack of standardization of transport properties measurements. In Sandia National Laboratories (SNL), we have been optimizing transport properties measurements techniques of TE materials and modules. We have been using commercial and custom-built instruments to analyze the performance of TE materials and modules. We developed a reliable procedure to measure thermal conductivity, seebeck coefficient and resistivity of TE materials to calculate the ZT as function of temperature. We use NIST standards to validate our procedures and measure multiple samples of each specific material to establish consistency. Using these developed thermoelectric capabilities, we studied transport properties of Bi2Te3 based alloys thermal aged up to 2 years. Parallel with analytical and microscopy studies, we correlated transport properties changes with chemical changes. Also, we have developed a resistance mapping setup to measure the contact resistance of Au contacts on TE materials and TE modules as a whole in a non-destructive way. The development of novel but reliable characterization techniques has been fundamental to better understand TE materials as function of aging time, temperature and environmental conditions.