Aiming to improve thermoelectric properties of half-Heusler compounds MNiSn (M = Ti, Zr, Hf), phase equilibria in the (Ti,M)NiSn systems have been investigated focusing on the half-Heusler phase separation between TiNiSn and ZrNiSn, TiNiSn and HfNiSn. Solubility limit of an element M in each half-Heusler phase was determined by XRD and EPMA. Nearly single phase alloys were fabricated by the directional solidification using optical floating zone melting method to evaluate intrinsic thermoelectric properties of the alloys. Lattice thermal conductivity can be reduced in (Ti,M)NiSn alloys by phonon scattering due to the solid solution effect of M site substitution. Moreover, electrical properties can be enhanced by the Ti addition in (Ti,Zr)NiSn alloys. Among present MNiSn alloys, (Ti0.15,Zr0.85)NiSn alloy has the highest power factor of 5.3 mWm-1K-2 at around 745 K.

The U.S. military uses large amounts of fuel during deployments and battlefield operations. Consequently, the U.S. military has a strong need to develop technologies that increase fuel efficiency and minimize fuel requirements all along the logistics trail and in all battlefield operations. There are additional requirements to reduce and minimize the environmental footprint of various military equipment and operations and reduce the need for batteries (non-rechargeable) in battlefield operations. The tri-agency SERDP (Strategic Environmental Research and Development Program) office is sponsoring a challenging, high-payoff project to develop a lightweight, small form-factor, soldier-portable advanced thermoelectric generator (TEG) system prototype to recover and convert waste heat from a variety of deployed equipment with the ultimate purpose of obtaining additional power for soldier battery charging, advanced capacitor charging, and other battlefield power applications. The project seeks to achieve power conversion efficiencies of 10% (double current commercial TE conversion efficiencies) in a system with ˜1.6-kW power output for a spectrum of battlefield power applications. In order to meet this objective, the project is taking on the multi-faceted challenges of tailoring LAST/LASTT-based thermoelectric (TE) materials for the proper temperature ranges (300 K – 700 K), fabricating these materials with cost-effective hot-pressed and sintered processes while maintaining their TE properties, measuring and characterizing their thermal fatigue and structural properties, developing the proper manufacturing processes for the TE materials and modules, designing and fabricating the necessary microtechnology heat exchangers, and fabricating and testing the final TEG system. The ultimate goal is to provide an opportunity to deploy these TEG systems in a wide variety of current military equipment. This would help the Army in achieving one of the Office of Secretary of Defense’s major strategic objectives to maintain and enhance operational effectiveness while reducing total force energy demands. The presentation will review the progress made on 1) the performance of LAST / LASTT TE materials and tailoring their temperature dependency; 2) evaluating the structural (Elastic modulus, Poisson’s ratio and mechanical strength) properties of these materials, 3) development of the necessary LAST/LASTT-based TE modules, 4) development of the required hot- and cold-side microtechnology heat exchangers, and 5) the overall system designs for 30 kW and 60 kW TQG applications and potential performance pathways/differences for these two TQG cases. This work leverages critical fundamental research performed by the Office of Naval Research in developing LAST/LASTT materials.

Directionally solidified alloys in the Ru-Mn-Si system exhibit a particular microstructure including columnar compositional variation due to the formation of many different chimney-ladder phases along the growth direction. Despite the existence of the compositional variation, the crystal orientations of the neighboring chimney-ladder phases are preserved. Over the compositional interfaces, the metal sublattice is considered to be continuous while the Si sublattice is not. Heat treatment of the directionally solidified alloy with the nominal composition of Ru0.10Mn0.90Si1.732 at 1100°C coarsens the compositional domains so as to reduce the density of the compositional interfaces. The values of the thermal conductivity increase with the decrease in the density of the compositional interfaces whereas those of the Seebeck coefficient and electrical resistivity are almost unchanged after the heat treatment. It is considered that the thermoelectric properties of the chimney-ladder compounds in the Ru-Mn-Si system can be enhanced by introducing a high density of the compositional interfaces.

In order to perturb global warming and realize a sustainable global energy system, enhancements in the energy efficiency are required. One of the reliable technologies to reduce the greenhouse gas emissions and the consumption of fossil fuel that is attracting attention is thermoelectric technology, which can directly convert heat into electricity and consequently increase the energy conversion efficiency of power generation by combustion. Magnesium silicide (Mg2Si) has been identified as a promising advanced thermoelectric material operating in the temperature range from 500 to 800 K. Compared with other thermoelectric materials that operate in the same conversion temperature range, such as PbTe, TAGS (Ge-Te-Ag-Sb) and CoSb3, Mg2Si shows promising aspects, such as the abundance of its constituent elements in the earth’s crust and the non-toxicity of its processing by-products, resulting in freedom from care regarding prospective extended restriction on hazardous substances.

Here we have tried to introduce reusing of industrial waste of Si sludge as a source material for Mg2Si, because the current product inversion rate of Si for semiconductor LSI devices remains at 25 to 30 %, while most of the remainder is disposed of as a waste; this is mainly discharged as sludge from grinding and polishing processes. It is possible that the reuse of this waste Si could be effective in both reducing the cost of source Si and in the reduction of industrial waste. On the other hand, recycled materials of standard lightweight magnesium alloys based on the Mg-Al-Zn-Mn system, such as AZ91 or AM50, were also introduced as a Mg source for Mg2Si synthesis. The concept of this work is a production of wasted heat recovery device using environmentally-benign Mg2Si by means of industrial waste or less pure recycled sources.

The efficiency of a thermoelectric device is characterized by the dimensionless figure of merit, ZT=S2σT/κ, of its constituent thermoelectric material, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. As a target for practical use, ZT value exceed unity, which gives about 10 % conversion efficiencies, is expected. So far, we succeeded to obtain the Mg2Si with ZT=1.08 using rather pure Si (99.999% : solar grade) and Mg (99.95%) sources. In this article, we report multifarious fabrication processes in order to realize ZT value as high as unity and the detailed thermoelectric properties concerning Mg2Si initiated from reused Si sludge and the recycled Mg-alloy sources. In conjunction, we will also discuss the practical output-power characteristics of the samples with the formation of Ni electrodes by monobloc sintering. A tentative generated power density from the wasted heat at 773 K was ˜2 KW/m2.

Recent experimental and theoretical studies have shown that the thermal to electrical power conversion efficiency (as measured by the thermoelectric figure of merit) can be enhanced in nanocomposite materials. Primarily, these efforts to improve the thermoelectric efficiency rely on reducing the lattice thermal conductivity through nanostructuring of the materials or the introduction of a second nanometer-scale phase into the composite material. Here, we show that the inclusion of semimetal nanoparticles into bismuth telluride (Bi2Te3) can result in both an increase in the electronic transport properties (so called "power factor") as well as a decrease in lattice thermal conductivity. The effect of different volume fractions of Bi nanoinclusions (3% and 5%) on the thermal and electrical properties of the composite are reported. A marginal increase in the thermoelectric figure of merit is achieved for 3% metal nanoinclusion, whereas a significant improvement in the figure of merit could be achieved for 5% nanoinclusions in the Bi2Te3 thermoelectric matrix.

Copper aluminum oxide (CuAlO2) with delafossite structure is a promising candidate for high temperature thermoelectric applications because of its modest band gap, high stability and low cost. We investigate magnesium doping on the aluminum site as a means of producing higher electrical conductivity and optimized Seebeck coefficient. Powder samples were synthesized using solid-state reaction and bulk samples were prepared using both cold-pressing and hot-pressing techniques. Composition analysis, microstructural examination and transport property measurements were performed, and the results suggest that while hot-pressing can achieve high density samples, secondary phases tend to form and lower the performance of the materials.

The standardization of thermoelectric generator (TEG) performance measurement is of a major interest for a usage of them in industrial energy harvesting application. The effects influencing the TEG measurement are regarded and are addressed in this article. The dramatic influences of different measurement parameters are pointed out. The output power dependence on the contact pressure is investigated as a major attempt. The influence of contact pressure on the temperature of the TEG-substrate at the hot side is observed with a high resolution IR camera.

Different approaches are discussed in order to obtain the accurate efficiency of a TEG. The heat flux method is influenced by many effects that can falsify the results. The Harman method is used as another approach to determine the figure of merit to calculate the efficiency.

In this paper, we present a systematic study of the transient cooling in different Si/SiGe superlattices as well as bulk silicon microrefrigerators. Transient thermoreflectance imaging is used to obtain the temperature map of the device with sub micrometer spatial, 100ns temporal and 0.1C temperature resolution. It is shown that Peltier cooling dominates in the first 10-30 microseconds before Joule heating in the active and buffer layers reach the top surface. The transient characterization shows that at the optimum current for maximum steady-state cooling, the response of bulk silicon cooler is 25% faster than the 3 microns thick superlattice device and that of the 6 microns thick superlattice is 25% slower. However, it is possible to increase the cooling speed by a factor of two or three, down to 3.6 microseconds, by overdriving the current at the expense of the reduced steady-state cooling.

Composite phase change materials (PCM) were prepared by mixing exfoliated graphite nanoplatelets (xGnP) into paraffin wax. The two types of graphite nanoplatelets that were investigated were xGnP-1 having thickness of 10 nm and a diameter of 1 um and xGnP-15 having the same thickness with a platelet diameter of 15 um. Direct casting and two roll milling were used to prepare samples. Scanning electron microscopy images show that the nanofillers disperse very well in paraffin matrix without noticeable agglomeration. Paraffin/xGnP-15 PCM consistently exhibited higher thermal conductivity than xGnP-1 PCM. The improvement in thermal conductivity was as high as 5 fold for xGnP-15 composites and 2 fold for xGnP-1 composites at 4 vol%. The aspect ratio, particle orientation, and interface density between the conductive particles and the polymer matrix were found to be the critical parameters in determining the conductivities of the resulting nanocomposites. The thermal physical properties of the nanocomposites were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). It was found that the latent heat of nanocomposites was not negatively affected in the presence of xGnP particles and the thermal stability improved.

Mechanical energy can be converted to electrical energy using a dielectric elastomer generator (DEG). The maximum amount of energy that can be harvested from a DEG is constrained by various modes of failure and operational limits. Known limiting mechanisms include electrical breakdown, electromechanical instability, loss of tension and rupture by stretch. These limits define a cycle where maximum energy can be harvested. The cycle was represented on work-conjugate planes, which can be used as a guide for the design of practical cycles. The amount of energy harvested is larger when a DEG is subject to equal-biaxial stretching.

An intermediate-band material based on thiospinel semiconductor MgIn2S4 is presented. This material is proposed as high efficiency photovoltaic material for intermediate-band solar cells. We analyze V substitution for In in the parent compound MgIn2S4 and the formation of the V d-states intermediate band. For the proper characterization of the width and position of this band inside the band gap, the standard one-shot GW method within the plasmon-pole approximation is applied. The electronic properties thus obtained are discussed and compared to those studied with Density Functional Theory (DFT), and the advantages and the limitations of the two methods are discussed. In addition, DFT electronic-charge density analysis is shown.

In order to apply P(VDF-TeFE) piezoelectric polymer to micro-generator as a membrane, the polymer is deposited on a substrate by spin-coating method. In this process, some changes of properties and quality of the film are caused by a solvent which was used to dissolve the polymer granule. Since a solvent affects the film properties and surface stability, we have carried out the thermal process at a temperature higher than melting point. However, this thermal process due to a temperature higher than Curie point of the polymer causes a falling-off in electrical properties. In this paper, we have studied a method to prevent a falling-off in electric properties of P(VDF-TeFE) film even though the film is annealed at a temperature higher than Curie point.

Solar thermal power generation is fast becoming cost competitive for utility scale electricity. Parabolic trough concentrators have proven economical and reliable but their efficiency is limited by the maximum temperature of the heated fluid. This work will explore the possibility of adding a thermoelectric power generator (TEG) as a topping cycle at high temperature to increase the overall efficiency of the system. In this design the perimeter of the receiver tube is covered with thermoelectrics so that the absorber temperature is raised and the energy rejected from the TEG is used to heat the fluid at its originally specified temperature. A heat transfer analysis was carried out on the design to determine the overall system efficiency. Our findings show the state of the art solar thermal trough collector is not a good application for a ZT=1 thermoelectric material. To increase the overall power output of the system by approximately 10% would require a ZT=3 material.

For the material (Pb0.95Sn0.05Te)1-x(PbS)x nanostructuring from nucleation and growth and spinodal decomposition were reported to enhance the thermoelectric figure of merit over bulk PbTe, producing ZT of 1.1 - 1.4 at 650 K for x = 0.08[1]. Thermoelectric modules made from (Pb0.95Sn0.05Te)1-x(PbS)x materials with various hot-side metal electrodes were fabricated and tested. Short circuit current was measured on unicouples of Pb0.95Sn0.05Te – PbS 8% (n-type) legs and Ag0.9Pb9Sn9Sb0.6Te20 (p-type) legs over 10 (A) for a hot side temperature of 870K, and a cold side of 300K. Hot pressed (Pb0.95Sn0.05Te)1-x(PbS)x materials were also investigated for module fabrication. Investigations of the electrical properties of hot-pressed (Pb0.95Sn0.05Te)1-x(PbS)x materials are presented along with the latest advancements in the fabrication and characteristics of modules based on the processing of these materials.

Further development in the area of vibration energy harvesting is limited by the lack of efficient methods to adapt the harvester to its surroundings. To this end, we propose an innovative passive way of automatic passive resonance frequency tracking.

We present a new approach employing mechanical non-linear behaviour of the system to track the vibration frequency peak. An analytical model representing these nonlinear harvesting systems has been developed and analysed. Experimental results obtained with custom fabricated MEMS devices show an experimentally verified frequency adaptability of over 36% for a clamped-clamped beam device at 2g (1g=9.81m.s-2) input acceleration. We believe that the proposed solution is perfectly suited for autonomous industrial machinery surveillance systems, where vibrations with high accelerations that are necessary for enabling this solution are abundant.

We propose a solid state heat pump based on photon assisted heat transfer between two large-area light emitting diodes coupled by the electromagnetic field and enclosed in a semiconductor structure with a nearly homogeneous refractive index. Ideally the thermophotonic heat pump (THP) allows heat transfer and electricity generation at the Carnot efficiency, but in reality there are several factors that limit the efficiency. We present a numerical model that accounts for the most important losses of the thermophotonic heat pump to study the operating regimes and the fundamental limitations of the THP structure. The results show that the thermophotonic heat pump has potential to outperform heat pumps based on the thermoelectric effect especially for heat transfer across large temperature differences. In energy harvesting applications the performance of the THP is good for small power densities, but drops at high power densities unless the losses in the structure can be efficiently minimized.

We present three approaches to harvesting ambient energy, based on complementary physical effects in order to exploit three kinds of energy available for harvesting:

Mechanical vibrations

Mechanical vibration energy can be harvested with resonant mechanical structures excited by the vibrations; this scavenged energy is then transformed into electricity by piezoelectric materials. We propose to use thin piezoelectric layers deposited on the resonant structure.

An on-chip integrated prototype, using a seismic mass at the end of a Si cantilever beam coated with a piezoelectric AlN film (overall volume 5 mm3), produces 30 nW of rectified power at 3 V under 0.4 g acceleration, which is enough to power a wireless sensor. Power conditioning electronics adapted to ultra-low voltage input is also proposed, which can efficiently charge a storage capacitor under very low accelerations. The fully micro-fabricated System on a Package combines a MEMS generator and ASIC power management circuit.

Evolutions of ambient temperature

A permanent magnet is attached to a PZT/brass bimorph, and placed near a "thermo-magnetic" material (FeNi) which Curie temperature can be tuned around the ambient. During slow cooling or heating by the ambient air, the FeNi magnetic properties vary slowly and thus the attraction force between the FeNi and the NdFeB magnet varies slowly. Despite the slow global evolutions, the opposition between the linearity of the bimorph bending and the strong non-linearity of the magnetic attraction over distance leads to brutal clamping or release of the magnet around Tc: the piezo can then efficiently convert these fast deformations into electricity. Operational temperature range can be set by tuning the FeNi composition to obtain a predefined Curie point; hysteresis between the cooling/heating thresholds can be tuned by modifying the gap between the magnet and the FeNi.

The cm-scale prototype generates a peak voltage of 35 V on release (T > Tc). For a discharge time of 0.1 s through a 1 MΩ load, the total energy harvested is 13.5 μJ per stroke, i.e. 1.35 mW of average power (respectively -14 V, 2.2 μJ and 0.22 mW on clamping when T < Tc). Output power is surprisingly constant for various load values tested. Scaling effects must now be addressed to assess the feasibility of an integrated MEMS generator and its potential efficiency.

Low-frequency, large amplitude bending from human motion:

A very light, flexible electro-active polymer membrane (50 x 30 mm, 31 μm thick) operates in a large frequency spectrum from quasi-static to dynamic range. It scavenges 0.1 mJ per cycle at 1 Hz, under 170 V and constant charge Q: 100 μW is enough to supply a low-power system.

Improvements under investigation include achieving poling voltage with piezoelectric polymer or electrets polymer, using multi-stack scavengers to miniaturize the structure and to decrease the poling voltage V, and integration into textile.

Thermoelectric properties of a homologous series of Magnéli phase titanium oxides TinO2n-1 (n = 2, 3..) have been investigated. Dense polycrystalline specimens with nominal composition of TiO2-x (x = 0.10, 0.20) have been prepared by conventional hot-pressing. X-ray diffraction analysis has revealed that prepared specimens are slightly reduced during hot-pressing. Electrical conduction is of n-type for all prepared titanium oxides and electrical resistivity and absolute values of Seebeck coefficient decrease with increasing oxygen deficiency. The carrier concentration of Magnéli phase titanium oxide increases with increasing oxygen deficiency. Lattice thermal conductivity decreases with increasing oxygen deficiency by more than 60% at room temperature and 40% at 773K compared to TiO2, which can be due to the presence of dense planar defects. The largest thermoelectric figure of merit Z, 1.6×10-4 K-1 at 773K, was obtained in TiO1.90 hot pressed specimen.