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A new thermoelectric concept using large area silicon PN junctions is experimentally demonstrated. In contrast to conventional thermoelectric generators where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we demonstrate a large area PN junction made from densified silicon nanoparticles that combines thermally induced charge generation and separation in a space charge region with the conventional Seebeck effect by applying a temperature gradient parallel to the PN junction. In the proposed concept, the electrical contacts are made at the cold side eliminating the need for contacts at the hot side allowing temperature gradients greater than 100K to be applied. The investigated PN junction devices are produced by stacking n-type and p-type nanopowder prior to a densification process. The nanoparticulate nature of the densified PN junction lowers thermal conductivity and increases the intraband traps density which we propose is beneficial for transport across the PN junction thus enhancing the thermoelectric properties. A fundamental working principle of the proposed concept is suggested, along with characterization of power output and output voltages per temperature difference that are close to those one would expect from a conventional thermoelectric generator.
Bi2Te2Se (BTS221) bulk crystals were recently discovered as an intrinsic 3D topological insulator. We have synthesized this material, and studied the transport properties of BTS221 from the thermoelectrics perspective. Temperature (T) dependent resistivity measurement indicates surface dominant transports in our sample at low T. We also report Seebeck measurement between 50K to room T.
Large electrocaloric (EC) effects in ferroelectric polymers and in ferroelectric ceramics have attracted great attention for new refrigeration development which is more environmental friendly and more efficient and thus could be an alternative to the existing vapor-compression refrigerators which consume large energy and release large amount of green house gas. However in the past, all EC effects investigations have been focused on solid state dielectrics. It is interesting to ask whether a large EC effect can also be realized in dielectric fluids. A dielectric fluid with large EC effect could lead to new design of cooling devices with simpler structures than these based on solid state EC materials, for example, they can be utilized as both the refrigerant and heat exchange fluid. Here we present that a large EC effect can be realized in the liquid crystal (LC) 5CB near it's nematic-isotropic (N-I) phase transition. The LC 5CB possesses a large dielectric anisotropy which can induce large polarization change from the isotropic phase to the nematic phase near the N-I transition. An isothermal entropy change of more than 23 Jkg-1K-1 was observed near 39 oC that is just above the N-I transition.
In this work, the thermoelectric properties of Se-doped Bi2Te3 are examined using first-principles density functional theory and semi-classical Boltzmann transport theory. Placing a single Se atom on the 3a Wyckoff position lowers the unit cell energy by approximately 3.6 eV, compared to the 6c Te position. The electronic structure of Bi2Te3 has minor changes upon Se doping. At carrier concentration of 1019 cm-3, the optimal thermopower, S, is obtained as 207 and 220 μV/K for n-type and p-type doping, respectively. Unlike the thermopower, the power factor, S2σ/τ, is highly anisotropic for the in-plane and cross-plane conduction. At carrier concentrations of 1019 cm-3, the best power factor is predicted to be around 1.05 and 1.4×1011 W/m·s·K2 for n-type and p-type doping, respectively.
In this work we report on opportunities for a colossal reduction in lattice thermal conductivity (LTC) of graded micro-nanoporous structures with inhomogeneous porosity which leads to the considerable improvement in thermoelectric figure of merit ZT. We employ the effective medium theory to calculate the LTC of a porous media with hole pores of variable radius and show that porous materials with inhomogeneous porosity are expected to have stronger reduction (about 30 times!) in thermal conductivity than those with pores of equal sizes. Such a reduction is caused by enhanced scattering of thermal phonons with the pore boundaries. We have studied the variations of the LTC as a function of porosity, pore sizes, geometry and the number of pore groups with different sizes. Our theoretical results show excellent agreement with experimental data.
The electrocaloric effect holds promise for possible application in refrigeration technologies. There is much interest in this subject and experimental studies have shown the possibility for creating materials with a modest sized electrocaloric response. However, theoretical studies lag behind the experimental effort due to the lack of computational methods to accurately study the finite temperature response. Here the freely distributed feram, an effective Hamiltonian molecular dynamics method, is demonstrated for predicting the electrocaloric response of BaTiO3.
We explore a model of heat transport between two heat reservoirs mediated by a quantum particle. The reservoirs are modeled as ensembles of harmonic modes linearly coupled to the mediator. The steady state heat current, as well as the thermal conductance are obtained for arbitrary coupling strength and will be analyzed for the cases of weak and strong coupling regimes. It is shown that the violation of the virial theorem – the imbalance between the average potential and kinetic energy of the mediator – can be considered as a measure of the coupling strength that takes into account all the relevant factors. The dependence of the thermal conductance on the coupling strength is non-monotonic and displays a maximum. Temperature dependence of the heat conductance may reach a plateau at intermediate temperatures, similar to the classical plateau at high temperatures. We will discuss the origin of Fourier’s law in a chain of macroscopically large, but finite subsystems coupled by the quantum mediators. We will also address the origin of the anomalously large heat current between the scanning tunneling microscope tip and the substrate in deep vacuum which was found in recent experiments.
High purity bulk graphite is applicable in many capacities in the nuclear industry. The thermal conductivity of graphite has been found to vary as a function of how its morphology changes on the nanoscale, and the type and number of defects present. We compute thermal conductivities at the nanolevel using large scale classical molecular dynamics simulations and by employing the Green-Kubo method in a set of in silico experiments geared towards understanding the impact of defects in the thermal conductivity of graphite. We present the results obtained for systems with 1– 3 vacancies, and compile a summary of some of the methods applied and difficulties encountered.
The thermal transport in amorphous/crystalline silicon superlattices with means of molecular dynamics is presented in the current study. The procedure used to build such structures is discussed. Then, thermal conductivity of various samples is studied as a function of the periodicity of regular superlattices and of the applied temperature. Preliminarily results show that for regular amorphous/crystalline superlattices, the amorphous regions control the heat transfer within the structures. Secondly, in the studied cases thermal conductivity weakly varies with the temperature. This, points out the presence of a majority of non-propagating vibrational modes in such systems.
As an approach to improve the thermoelectric properties of the polycrystalline Ca3Co4O9 misfit-layered oxide, substitutions of Co2+…4+ with the heavier cations Ru3+/4+ and In3+ were tested. Polycrystalline samples Ca3Co4-xRuxO9 and Ca3Co4-xInxO9 (0 < x < 0.21) were prepared via a solid-state-reaction route. For each sample the crystal structure was analyzed and a complete thermoelectric characterization was done within a temperature range of 300 K < T < 1125 K.
Both substitution strategies resulted in a significant decrease of the thermal conductivity (κ). For the In3+-substituted samples the decrease of the Seebeck coefficient (α) balanced the κ reduction so that no overall enhancement of the figure of merit (ZT) was found. The Ru3+/4+ substitution reduced the p-type carrier concentration and thus increases the electrical resistivity (ρel), while α became larger at low temperatures. Despite the reduction of the power factor, a small enhancement in ZT was observed in the case of x = 0.1 Ru substitution, due to the strong κ reduction. Considering the observed preferred orientation of the Ru-substituted crystallites, a maximum value of ZT = 0.14 perpendicular to the pressing direction is found at T = 1125 K, indicating that Ru substitution is a promising strategy for a further ZT improvement.
Grain boundaries are known to be able to impede phonon transport in the material. In the thermoelectric application, this phenomenon could help improve the figure-of-merit (ZT) and enhance the thermal to electrical conversion. Bi2Te3 based alloys are renowned for their high ZT around room temperature but still need improvements, in both n- and p-type materials, for the resulting power generation devices to be more competitive. To implement high density of grain boundaries into the bulk materials, a bottom-up approach is employed in this work: consolidations of nanocrystalline powders into bulk disks. Nanocrystalline powders are developed by high energy cryogenic mechanical alloying that produces Bi(Sb)Te(Se) alloy powders with grain size in the range of 7 to 14 nm, which is about 25% finer compared to room temperature mechanical alloying. High density of grain boundaries are preserved from the powders to the bulk materials through optimized high pressure hot pressing. The consolidated bulk materials have been characterized by X-ray diffraction and transmission electron microscope for their composition and microstructure. They mainly consist of grains in the scale of 100 nm with some distributions of finer grains in both types of materials. Preliminary transport property measurements show that the thermal conductivity is significantly reduced at and around room temperature: about 0.65 W/m-K for the n-type BiTe(Se) and 0.85 W/m-K for the p-type Bi(Sb)Te, which are attributed to increased phonon scattering provided by the nanostructure and therefore enhanced ZT about 1.35 for the n-type and 1.21 for the p-type are observed. Detailed transport properties, such as the electrical resistivity, Seebeck coefficient and power factor as well as the resulting ZT as a function of temperature will be described.
In search for non-toxic thermoelectric materials that are stable in air at elevated temperatures, zinc oxide has been shown to be one of only few efficient n-type oxidic materials. Our bottom-up approach starts with very small (<10 nm) Al-doped ZnO nanoparticles prepared from organometallic precursors by chemical vapor synthesis using nominal doping concentrations of 2 at% and 8 at%. In order to obtain bulk nanostructured solids, the powders were compacted in a current-activated pressure-assisted densification process. Rapid thermal annealing was studied systematically as a means of further dopant activation. The thermoelectric properties are evaluated with regard to charge carrier concentration and mobility. A Jonker-type analysis reveals the potential of our approach to achieve high power factors. In the present study, power factors larger than 4×10-4 Wm-1K-2 were measured at temperatures higher than 600 °C.
Latest nanotechnology concepts applied in thermoelectric (TE) research have opened many new avenues to improve the ZT value. Low dimensional structures can improve the ZT value as compared to bulk materials by substantial reduction in the lattice thermal conductivity, κL. However, the materials were not feasible for the industrial scale production of macroscopic devices because of complicated and costly manufacturing processes involved. Bulk nanostructured (NS) TEs are normally fabricated using a bulk process rather than a nano-fabrication process, which has the important advantage of producing in large quantities and in a form that is compatible with commercially available TE devices.
We developed fabrication strategies for bulk nanostructured skutterudite materials based on FexCo1-xSb3. The process is based on precipitation of a precursor material with the desired metal atom composition, which is then exposed to thermochemical processing of calcination followed by reduction. The resultant material thus formed maintains nanostructured particles which are then compacted using Spark Plasma Sintering (SPS) by utilizing previously optimized process parameters. Microstructure, crystallinity, phase composition, thermal stability and temperature dependent transport property evaluation has been performed for compacted NS FexCo1-xSb3. Evaluation results are presented in detail, suggesting the feasibility of devised strategy for bulk quantities of doped TE nanopowder fabrication.
Rare-earth telluride compounds are characterized by their high performance thermoelectric properties that have been applied to the development of functional materials . Recently, May and co-workers reported that nanostructured bulk lanthanum telluride (La3-xTe4, 0 ≤ x ≤ 1/3) by mechanical ball-milling exceeded the figure of merit (ZT) of 1 at high temperatures near 1300K [2-3]. Since the increased thermoelectric efficiency of nanostructured materials is due to the enhancement of phonon scattering introduced by quantum confinement, thin films have also generated significant scientific and technological interest [4-6]. Here, we report on the electrodepostion of lanthanum telluride and lanthanum thin films in ionic liquids in ambient conditions. Surface morphologies varied from needle-like to granular structures and depend on deposition conditions. This novel electrochemical synthesis approach is a simple, inexpensive and laboratory-environment friendly method of synthesizing nanostructured thermoelectric materials.
A complete dry processing route is developed for the fabrication of thermally-conductive carbon nanotube (CNT)-copper oxide (CuOx) heterostructures. This was achieved by the deposition of copper (Cu) onto CNTs and subsequent annealing in Ar and air environment to convert the coated Cu into CuOx nanoparticles. The survivability and diameters of CNTs were studied to ensure their integrity after the multiple processing steps and annealing temperatures (400 °C). The as-produced CNTs, air/Ar-annealed CNTs, Cu-coated CNTs, and CNT-CuOx heterostructures were characterized to study their structure, phase, and morphology using microscopy, elemental analysis, X-ray diffraction, and sheet resistance. It was observed that CNTs could survive the processing conditions and became coated with CuOx nanoparticles. The sheet resistance of CNTs coated with CuOx nanoparticles was ∼4 times greater than the as-produced CNTs. The Raman spectroscopy-based estimation of thermal conductivity of CNTs and CNT-CuOx heterostructures showed 2-7 times enhancement for the latter as compared to pure CuOx. In conclusion, such hybrid CNT-based heterostructures are promising for applications in thermal management.
The thermopower properties of GaN nanowires with transitional metal impurities are investigated in the framework of constrained spin density functional theory (DFT) calculations. The nanowires are connected to nanoscopic Al electrodes, which ensure a natural coupling to the wurtzite structure of the nanowires. We investigate the thermoelectric properties comparatively for the pristine GaN nanowire and the system with one Mn adatom. Our study points out the predicted qualitative behavior for systems with a peak in the total transmission, as well as the sign change in the thermopower. For the system with the magnetic impurity we find an enhanced conductance, thermopower and figure of merit. The detectable spin current polarization suggests the device structure may be also used in low temperature sensing applications.
Our nation discards more than 50% of the total input energy as waste heat in various industrial processes such as metal refining, heat engines, and cooling. If we could harness a small fraction of the waste heat through the use of thermoelectric (TE) devices while satisfying the economic demands of cost versus performance, then TE power generation could bring substantial positive impacts to our society in the forms of reduced carbon emissions and additional energy. To increase the unit-less figure of merit, ZT, single-crystal semiconductor nanowires have been extensively studied as a building block for advanced TE devices because of their predicted large reduction in thermal conductivity and large increase in power factor. In contrast, polycrystalline bulk semiconductors also indicate their potential in improving overall efficiency of thermal-to-electric conversion despite their large number of grain boundaries. To further our goal of developing practical and economical TE devices, we designed a material platform that combines nanowires and polycrystalline semiconductors which are integrated on a metallic surface. We will assess the potential of polycrystalline group III-V compound semiconductor nanowires grown on low-cost copper sheets that have ideal electrical/thermal properties for TE devices. We chose indium phosphide (InP) from group III-V compound semiconductors because of its inherent characteristics of having low surface states density in comparison to others, which is expected to be important for polycrystalline nanowires that contain numerous grain boundaries. Using metal organic chemical vapor deposition (MOCVD) polycrystalline InP nanowires were grown in three-dimensional networks in which electrical charges and heat travel under the influence of their characteristic scattering mechanisms over a distance much longer than the mean length of the constituent nanowires. We studied the growth mechanisms of polycrystalline InP nanowires on copper surfaces by analyzing their chemical, optical, and structural properties in comparison to those of single-crystal InP nanowires formed on single-crystal surfaces. We also assessed the potential of polycrystalline InP nanowires on copper surfaces as a TE material by modeling based on finite-element analysis to obtain physical insights of three-dimensional networks made of polycrystalline InP nanowires. Our discussion will focus on the synthesis of polycrystalline InP nanowires on copper surfaces and structural properties of the nanowires analyzed by transmission electron microscopy that provides insight into possible nucleation mechanisms, growth mechanisms, and the nature of grain boundaries of the nanowires.
In this paper, synthesis of Cu2ZnSnSe4 (CZTSe) materials by using simple and cost-effective solid state reaction method from the elemental Cu, ZnO, SnO and elemental Se powders are carried out. The SEM images show spherical, non-uniform size with aggregation of nanopowders. The phase separation and thermal analysis of the milled powders suggested that most of the starting powders reacted because of a mechanical alloying effect during milling process. After the solid state reaction at above 500 °C, the nanopowders crystallized into stannite single phase, which are confirm by XRD spectra. The thermoelectric properties of synthesized powder are under study.
The present work investigates the fabrication, thermal conductivity (TC) and rheological properties of water based carbon nanotubes (CNTs) nanofluids (NFs) prepared using a two-step method. As-received (AR) CNTs heated and the effect of heat treatment was studied using X-ray diffraction and thermogravimetric analysis. The AR-CNTs and heat-treated CNTs (HT-CNTs) were dispersed with varying concentration of surface modifiers Gum Arabic (GA) and TritonX-100 (TX) respectively. It was found that heat treatment of CNTs effectively improved the TC and influenced rheological properties of NFs. Scanning electron microscopy analysis revealed TX modified NFs showed better dispersion ability compared to GA. Surface modification of the CNTs was confirmed by Fourier Transformation Infrared (FTIR) analysis. Zeta potential measurement showed the stability region for GA modified NFs in the pH range of 5-11, whereas pH was between 9.5-10 for TX NFs. The concentration of surface modifier plays an extensive role on both TC and rheological behavior of NFs. A maximum TC enhancement of 10% with increases in viscosity around 2% for TX based HT-CNTs NFs was measured. Finally comparison of experimental TC results with the predicted values obtained from a model demonstrated inadequacy of the predictive model for CNT NFs system.
The thermal conductance of a gold/water interface has been found to change as a function of the surrounding’s adhesion energy. We measure the thermal conductance of a lithographically prepared gold nanowire with a thin film nanoscale thermal sensor composed of AlGaN:Er3+. The temperature of the nanowire is measured as a function of incident laser intensity. The slope of this plot is inversely proportional to the thermal conductance of the nanoparticle/surrounding’s interface. We show that the conductance of the nanoparticle/water interface increases with the molality of the solution. This was tested with multiple solutes including NaCl, and D-Glucose. The interfacial conductance of pure water is reported to be 44 MW/m2K and the conductance saturates to 100 MW/m2K at a molality of 0.21 m.