Thermoelectric Power Conversion Research Articles

Nanocrystallization tuning of structural, thermoelectric and electrical properties of SrTiO3-doped vanadate glasses

Glass-ceramic nanocrystals (GCNs) were obtained by annealing parent SrTiO3-V2O5 (SV) glasses prepared using the melt-quenching technique at the crystallization temperature, Tc. The amorphous nature and glassy behavior of the quenched glasses were confirmed by XRD, DSC and FTIR spectroscopy. In the heat-treated samples, the crystal size was found to range from 40 to 80 nm for all studied samples. It was observed that as the SrTiO3 content in the GCNs increased, density (d) steadily rose. The change of vanadium ions concentration(C) has the predominant role for changing Seebeck coefficient in both glass and GCNs samples. The nanocrystallization process at temperatures close to the onset of Tc, lasting for one hour, notably increased the electronic conductivity of the initial glasses. Consequently, the modification in nanostructure resulted in enhanced conductivity. In comparison to the original glasses, the final materials demonstrated significantly improved electrical conductivity. The accumulation of V4+-V5+ pairs at the formed interlayer zones between nanocrystallites and the glassy phase is accountable for electron hopping in the current system, which is markedly higher than in the glassy matrix. The formed nanocrystallites play a crucial role in augmenting the conductivity of such nanomaterials. The maximum obtained value of the power factor (PF) is 0.9 × 10−4 mW/mk2 for the glass sample (x = 20) reflecting a very low thermoelectric power conversion efficiency while in GCNs, the PF values were well-enhanced to 0.6 mW/m.K2 at x = 20.

First-principles study on the lattice thermal conductivity of Janus In2Ge2Te3Se3 and In2Ge2Se6 bilayers：Candidate materials for thermoelectric devices in high-temperature conditions

Improvement of existing material properties is necessary for high-efficiency thermoelectric power conversion, but novel thermoelectric materials must also be predicted and synthesized. Here we solve the phonon Boltzmann transport equation using first-principles computations to examine the lattice thermal conductivity of Janus In2Ge2Te3Se3 and In2Ge2Se6 bilayers. Results show that the frequencies at which larger gaps appear in the intermediate and high-frequency optical branches of In2Ge2Te3Se3 are lower than those of In2Ge2Se6. As a result, the phonon dispersion curve of In2Ge2Te3Se3 shifts downward. Since Te atoms are heavier than Se atoms, compared to In2Ge2Se6, In2Ge2Te3Se3 has a lower overall phonon group velocity. Furthermore, the tight coupling of the in-plane acoustic modes and the soft bending of In2Ge2Te3Se3 in the finite layer thickness coupling result in an increase in the phonon-phonon scattering, a reduction in the phonon relaxation time, and a larger Grüneisen parameter, indicating that In2Ge2Te3Se3 is more anharmonic. At a temperature of 1000 K, the sum of all these parameters results in a minimum lattice thermal conductivity of 0.05 W/mK for In2Ge2Te3Se3 and 0.24 W/mK for In2Ge2Se6. This study sheds light on the Janus In2Ge2Te3Se3 and In2Ge2Se6 bilayer thermal transport capabilities, it may open the door for achieving thermal conductivity control in applications such as thermal management and thermoelectric devices.

Development of high-precision hardware and software tools for automated determination of the characteristics of thermoelectric devices

In this work, a high-accuracy setup was developed for the characterization of thermoelectric devices in the temperature range of 300-900 K. The output parameters of the thermoelectric devices, including the thermoelectric efficiency Z, Seebeck coefficient S, and internal resistance r, were measured. A technique, block diagram, and computer tools for automated measurement and preliminary processing of experimental data were developed for automated studies of the properties of semiconductor materials and thermoelectric power conversion modules. The developed tools were shown to have high efficiency. The complexity of the process of measuring the main electrical parameters of semiconductor materials was significantly reduced, and the accuracy of the obtained results was increased.

Facile composite engineering to boost thermoelectric power conversion in ZnSb device

Zinc antimonide (ZnSb) is one of the alternatives for commercial thermoelectric materials due to its non-toxic, low-cost, and earth-abundant nature. However, its simple crystal structure causes strong phonon vibrations, which enhance lattice thermal conductivity. In this work, we systematically studied the effect of γ-Al2O3 nano-inclusions on ZnSb. Our results show that composite engineering imparts lattice phonon scattering for reduced thermal conductivity and low-energy carrier filtering for enhanced Seebeck coefficient. The obtained figure of merit in the ZnSb+5% γ-Al2O3 sample at 673 K is nearly two-fold higher than the pristine sample. Our fabricated 2-leg ZnSb+5% γ-Al2O3 device displayed a power generation of 0.11 μW at ΔT of 200 °C. Furthermore, adding γ-Al2O3 nano-inclusions improve the mechanical and thermal stabilities due to grain boundary hardening and dispersion strengthening. Overall, the addition of γ-Al2O3 nano-inclusions to ZnSb enhancing the Seebeck coefficient, reducing thethermal conductivity, and improving mechanical and thermal stability significantly.

Thermal design of standalone miniature thermo-electric power conversion system using variable area liquid fuel combustor

The novel standalone miniature thermo-electric power conversion system using variable area liquid fuel combustor is proposed in the present work. There is a need to provide an alternative energy source for portable electronic devices to cater to its need for high energy density requirements, and the proposed system is a feasible option for the same. The various critical parameters required for designing a thermoelectric power conversion system are identified as per the requirement of standalone nature and flame stabilization. The 15 W thermo-electric power conversion system is designed based on these parameters. The variable area combustor and the concentric fuel tube helps for the natural circulation of fuel and air inside the combustor and flame stability. The temperature of liquid fuel film over the fuel orifice exit is calculated by equating mass diffusion transfer number with thermal diffusion transfer number at steady state condition assuming the Lewis number unity. The Thin metallic porous media of ∼ 10 mm diameter is suggested based on the thermal analysis over the fuel orifice exit to enhance the fuel evaporation rate.

Anomalous Nernst Effect in an Antiperovskite Antiferromagnet

The anomalous Nernst effect (ANE), the generation of a transverse electric voltage by a longitudinal temperature gradient, has attracted increasing interest from researchers recently, due to its potential in thermoelectric power conversion and close relevance to the Berry curvature of the band structure. Avoiding the stray field of ferromagnets, the ANE in antiferromagnets (AFMs) has the advantage of realizing highly efficient and densely integrated thermopiles. Here, we report the observation of the ANE in an antiperovskite noncollinear AFM, ${\mathrm{Mn}}_{3}\mathrm{Sn}\mathrm{N},$ experimentally, which is triggered by the enhanced Berry curvature from Weyl points located close to the Fermi level. Considering that antiperovskite ${\mathrm{Mn}}_{3}\mathrm{Sn}\mathrm{N}$ has a rich magnetic phase transition, we modulate the noncollinear AFM configurations by the biaxial strain, which enables us to control its ANE. Our findings provide a potential class of materials to explore the Weyl physics of noncollinear AFMs as well as realizing antiferromagnetic spin caloritronics that exhibits promising prospects for energy conversion and information processing.

Review: Surface orientation effects on Pool-boiling with plain and enhanced surfaces

A Pool Boiling (PB) is crucial to safe and efficient operations in various applications such as thermoelectric power conversion, refrigeration, electronics, transportation, microfluidics, biomedical engineering, metallurgical industry, and space exploration. However, the poor counter-current flows near the heated surface limit the maximum PB cooling efficiency via a premature surface dryout, i.e., poor Critical Heat Flux (CHF) and Heat Transfer Coefficient (HTC). The two-phase flow substantially changes as the surface orientation deviates from the upward facing, leading to the CHF and HTC changes. This paper reviews the effects of surface orientation on the two-phase flows and their PB performance using various coolants for plain and engineered surfaces. This review includes both the experimental and theoretical approaches to understand HTC and CHF, including tailored two-phase flow for enhanced HTC and CHF. This review also discusses future research directions for engineered surface under different surface orientation.

Green Route for Fabrication of Water-Treatable Thermoelectric Generators

Since future energy harvesting technologies require stable supply and high-efficiency energy conversion, there is an increasing demand for high-performance organic thermoelectric generators (TEGs) based on waterproof thermoelectric materials. The poor stability of n-type organic semiconductors in air and water has proved a roadblock in the development of reliable thermoelectric power generators. We developed a simple green route for preparing n-type carbon nanotubes (CNTs) by doping with cationic surfactants and fabricated films of the doped CNTs using only aqueous media. The thermoelectric properties of the CNT films were investigated in detail. The nanotubes doped using a cationic surfactant (cetyltrimethylammonium chloride (CTAC)) retained an n-doped state for at least 28 days when exposed to water and air, indicating higher stability than that for contemporary CNT-based thermoelectric materials. The wrapping of the surfactant molecules around the CNTs is responsible for blocking oxygen and water from attacking the CNT walls, thus, extending the lifetime of the n-doped state of the CNTs. We also fabricated thermoelectric power conversion modules comprising CTAC-doped (n-type) and sodium dodecylbenzenesulfonate- (SDBS-) doped (p-type) CNTs and tested their stabilities in water. The modules retained 80 ± 2.4 % of their initial maximum output power (at a temperature difference of 75°C) after being submerged in water for 30 days, even without any sealing fills to prevent device degradation. The remarkable stability of our CNT-based modules can enable the development of reliable soft electronics for underwater applications.

Low lattice thermal conductivity and its role in the remarkable thermoelectric performance of newly predicted SiS2 and SiSe2 monolayers

For high-efficiency thermoelectric power conversion, not only improvement of existing materials properties but also prediction and synthesis of new thermoelectric materials are needed. Here, we have carried out a systematic investigation on the thermoelectric performance of newly predicted two dimensional (2D) semiconducting SiS2 and SiSe2 monolayers using density functional theory (DFT) and solving the Boltzmann transport equations (BTEs) for electrons and phonons. Our computed value of lattice thermal conductivity (kph) in SiSe2 monolayer is ultralow, which results in a high thermoelectric figure of merit (zT) value of 0.86 (0.83) for p-type (n-type) at 900 K in SiSe2 monolayer. While in SiS2 monolayer, zT value are 0.77 (p-type) and 0.71 (n-type) at 900 K. The values of kph are attributed to low group velocity, strong anharmonicity and phonon–phonon coupling of acoustic and low-frequency optical branches, leading to larger scattering, smaller mean free path, and shorter lifetime of phonons. It is also found that p-type doping is more effective than n-type doping to get optimal power factor (PF) and zT. Our findings suggest that newly predicted semiconducting SiSe2 and SiS2 monolayers can be very promising thermoelectric materials for the fabrication of high-efficiency thermoelectric power generators to convert waste heat into electricity.

Thermoelectric Generation with Impinging Nano-Jets

In this study, thermoelectric generation with impinging hot and cold nanofluid jets is considered with computational fluid dynamics by using the finite element method. Highly conductive CNT particles are used in the water jets. Impacts of the Reynolds number of nanojet stream combinations (between (Re1, Re2) = (250, 250) to (1000, 1000)), horizontal distance of the jet inlet from the thermoelectric device (between (r1, r2) = (−0.25, −0.25) to (1.5, 1.5)), impinging jet inlet to target surfaces (between w2 and 4w2) and solid nanoparticle volume fraction (between 0 and 2%) on the interface temperature variations, thermoelectric output power generation and conversion efficiencies are numerically assessed. Higher powers and efficiencies are achieved when the jet stream Reynolds numbers and nanoparticle volume fractions are increased. Generated power and efficiency enhancements 81.5% and 23.8% when lowest and highest Reynolds number combinations are compared. However, the power enhancement with nanojets using highly conductive CNT particles is 14% at the highest solid volume fractions as compared to pure water jet. Impacts of horizontal location of jet inlets affect the power generation and conversion efficiency and 43% variation in the generated power is achieved. Lower values of distances between the jet inlets to the target surface resulted in higher power generation while an optimum value for the highest efficiency is obtained at location zh = 2.5ws. There is 18% enhancement in the conversion efficiency when distances at zh = ws and zh = 2.5ws are compared. Finally, polynomial type regression models are obtained for estimation of generated power and conversion efficiencies for water-jets and nanojets considering various values of jet Reynolds numbers. Accurate predictions are obtained with this modeling approach and it is helpful in assisting the high fidelity computational fluid dynamics simulations results.

Significant Phonon Drag Enables High Power Factor in the AlGaN/GaN Two-Dimensional Electron Gas.

In typical thermoelectric energy harvesters and sensors, the Seebeck effect is caused by diffusion of electrons or holes in a temperature gradient. However, the Seebeck effect can also have a phonon drag component, due to momentum exchange between charge carriers and lattice phonons, which is more difficult to quantify. Here, we present the first study of phonon drag in the AlGaN/GaN two-dimensional electron gas (2DEG). We find that phonon drag does not contribute significantly to the thermoelectric behavior of devices with ∼100 nm GaN thickness, which suppresses the phonon mean free path. However, when the thickness is increased to ∼1.2 μm, up to 32% (88%) of the Seebeck coefficient at 300 K (50 K) can be attributed to the drag component. In turn, the phonon drag enables state-of-the-art thermoelectric power factor in the thicker GaN film, up to ∼40 mW m-1 K-2 at 50 K. By measuring the thermal conductivity of these AlGaN/GaN films, we show that the magnitude of the phonon drag can increase even when the thermal conductivity decreases. Decoupling of thermal conductivity and Seebeck coefficient could enable important advancements in thermoelectric power conversion with devices based on 2DEGs.

Enhanced thermoelectric properties of graphene-based ferromagnetic-superconductor junctions, Andreev reflection effect

We have theoretically studied ballistic graphene-based Ferromagnetic/Superconductor (FG/SG) junction operated as thermoelectric generators. Ferromagnetism and superconductivity in graphene are assumed to be induced by superconducting and ferromagnetic electrodes placed on the top of the graphene sheet. We calculate the normal and Andreev reflection probabilities using the Blonder-Tinkham-Klapwijk (BTK) formalism to solve a four-dimensional version of the Dirac-Bogoliubov-de Gennes equation. By a special choice of system parameters it is possible to achieve a condition in which thermal conductance declines significantly while electrical conductance increases. We also calculate output power and thermoelectric power-conversion efficiency at low temperature based on such a structure and show that the power generation efficiency is close to the Carnot limit.

Influence of Quantum Interference on the Thermoelectric Properties of Molecular Junctions.

Molecular junctions offer unique opportunities for controlling charge transport on the atomic scale and for studying energy conversion. For example, quantum interference effects in molecular junctions have been proposed as an avenue for highly efficient thermoelectric power conversion at room temperature. Toward this goal, we investigated the effect of quantum interference on the thermoelectric properties of molecular junctions. Specifically, we employed oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring ( para-OPE3) and meta-connected central ring ( meta-OPE3), which both covalently bind to gold via sulfur anchoring atoms located at their ends. In agreement with predictions from ab initio modeling, our experiments on both single molecules and monolayers show that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ∼20 μV/K) compared with para-OPE3 (with ∼10 μV/K). Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions.

A quantum-dot heat engine operating close to the thermodynamic efficiency limits.

Cyclical heat engines are a paradigm of classical thermodynamics, but are impractical for miniaturization because they rely on moving parts. A more recent concept is particle-exchange (PE) heat engines, which uses energy filtering to control a thermally driven particle flow between two heat reservoirs1,2. As they do not require moving parts and can be realized in solid-state materials, they are suitable for low-power applications and miniaturization. It was predicted that PE engines could reach the same thermodynamically ideal efficiency limits as those accessible to cyclical engines3-6, but this prediction has not been verified experimentally. Here, we demonstrate a PE heat engine based on a quantum dot (QD) embedded into a semiconductor nanowire. We directly measure the engine's steady-state electric power output and combine it with the calculated electronic heat flow to determine the electronic efficiency η. We find that at the maximum power conditions, η is in agreement with the Curzon-Ahlborn efficiency6-9 and that the overall maximum η is in excess of 70% of the Carnot efficiency while maintaining a finite power output. Our results demonstrate that thermoelectric power conversion can, in principle, be achieved close to the thermodynamic limits, with direct relevance for future hot-carrier photovoltaics10, on-chip coolers or energy harvesters for quantum technologies.

Study of different heat exchange technologies influence on the performance of thermoelectric generators

A key challenge in thermoelectric power conversion is to create a significant temperature difference (ΔT) across the thermoelectric generator (TEG). And one approach to create a larger temperature difference is to enhance heat transfer of the hot side of TEG. Thus, when a thermoelectric generator has been designed and manufactured, it is found that the heat exchangers of the thermoelectric device determines its performance. This paper investigated the differences as well as the advantages and disadvantages of three typical heat exchangers in a thermoelectric setup, and the power consumed by the auxiliary equipment to improve the thermoelectric performance were taken into account. For this purpose, a mathematical model has been developed. The accuracy of this model was verified via the experiments, by constructing and testing the prototypes. The results illustrate the net power output performance of thermoelectric devices with three different type of heat exchangers. The air cooling exchangers are shown a minimum auxiliary consumption whereas the heat pipe cooling exchanger are shown to be the most effective. At last, this article gets an economic analysis, and a new evaluation index associated with the power consumption of the auxiliary equipment is proposed. The results provided some practical guidelines for the design and application of practical thermoelectric power generations.

Improved thermoelectric power factor and conversion efficiency of perovskite barium stannate

The highly dispersive conduction band and high temperature stability contribute to the excellent electrical properties when BaSnO3 is n-type doped.

Experimental investigation of flame stability limits of a mesoscale combustor with thermally orthotropic walls

Flame stability limits were experimentally investigated for a parallel plate mesoscale combustor with orthotropic walls. The combustor walls were made of pyrolytic graphite, which has orthotropic thermal conductivity of 350 W/m-K in ab-plane and 3.5 W/m-K in the c-plane at room temperature. Infrared images of the combustor walls showed that the pyrolytic graphite plates had uniform temperature distributions at all operating conditions tested. Compared to an isotropic material (stainless steel), pyrolytic graphite showed a greater high velocity limit (HVL) and thereby wider flame stability limit. Thermal performance of the combustor was also evaluated via an energy balance, wherein, heat losses were found to dominate the thermal fluxes. The uniform temperature profile and the lack of a distinctive “hot spot” in the thermal images of the pyrolytic graphite indicate that it is a suitable combustor material for thermoelectric and thermophotovoltaic power conversion applications. Future work should focus on the thermal management so as to avoid excessive heat losses leading to an improvement in the thermal performance of the combustor.

Heat collection and supply of interconnected netlike graphene/polyethyleneglycol composites for thermoelectric devices.

The key challenges in thermoelectric power conversion are creating a significant temperature difference and obtaining more heat energy through a thermoelectric device. Herein, graphene/polyethyleneglycol composites (G-PEGs) were proposed as a heat supply for thermoelectric devices. The G-PEGs not only afford a lot of conductive pathways for heat transfer but also act as highly thermally conductive reservoirs to hold phase-change materials for thermal energy collection, storage and release. The concept described in this study holds great promise in designing multifunctional composites for heat collection, transport, and supply in thermoelectric power conversion.

Nonlinear thermovoltage and thermocurrent in quantum dots

Quantum dots are model systems for quantum thermoelectric behavior because of their ability to control and measure the effects of electron-energy filtering and quantum confinement on thermoelectric properties. Interestingly, nonlinear thermoelectric properties of such small systems can modify the efficiency of thermoelectric power conversion. Using quantum dots embedded in semiconductor nanowires, we measure thermovoltage and thermocurrent that are strongly nonlinear in the applied thermal bias. We show that most of the observed nonlinear effects can be understood in terms of a renormalization of the quantum-dot energy levels as a function of applied thermal bias and provide a theoretical model of the nonlinear thermovoltage taking renormalization into account. Furthermore, we propose a theory that explains a possible source of the observed, pronounced renormalization effect by the melting of Kondo correlations in the mixed-valence regime. The ability to control nonlinear thermoelectric behavior expands the range in which quantum thermoelectric effects may be used for efficient energy conversion.

Super-adiabatic combustion in Al2O3 and SiC coated porous media for thermoelectric power conversion

The combustion of ultra-lean fuel/air mixtures provides an efficient way to convert the chemical energy of hydrocarbons and low-calorific fuels into useful power. Matrix-stabilized porous medium combustion is an advanced technique in which a solid porous medium within the combustion chamber conducts heat from the hot gaseous products in the upstream direction to preheat incoming reactants. This heat recirculation extends the standard flammability limits, allowing the burning of ultra-lean and low-calorific fuel mixtures and resulting a combustion temperature higher than the thermodynamic equilibrium temperature of the mixture (i.e., super-adiabatic combustion). The heat generated by this combustion process can be converted into electricity with thermoelectric generators, which is the goal of this study.The design of a porous media burner coupled with a thermoelectric generator and its testing are presented. The combustion zone media was a highly-porous alumina matrix interposed between upstream and downstream honeycomb structures with pore sizes smaller than the flame quenching distance, preventing the flame from propagating outside of the central section. Experimental results include temperature distributions inside the combustion chamber and across a thermoelectric generator; along with associated current, voltage and power output values. Measurements were obtained for a catalytically inert Al2O3 medium and a SiC coated medium, which was tested for the ability to catalyze the super-adiabatic combustion. The combustion efficiency was obtained for stoichiometric and ultra-lean (near the lean flammability limit) mixtures of CH4 and air.