We report on our investigation into the use of III-V superlattice structures for thermoelectric (TE) applications. Preliminary review of III-V materials trends indicate that the GaSb/InAs superlattice system should offer one of the best potentials for high thermoelectric performance in the 500K-800K range. MOCVD growth of GaSb/InAs superlattice structures was carried out, and relevant structural, thermal, and electrical characterization has been performed. TEM and XRD results demonstrate a well-ordered superlattice structure. Thermal conductivity measurements reveal a reduction in the room-temperature thermal conductivity of GaSb/InAs superlattices (4.4-10.0 W/m-K), relative to either binary GaSb (32 W/m-K) or InAs (27 W/m-K). Additionally, we have worked to optimize the thermoelectric power factor (α2σ), studying both Se- and Te-doping of the superlattice structures, in an effort to demonstrate optimal thermoelectric performance. Our results demonstrate a maximum ZT of 0.36 at 400K for optimally doped n-type GaSb/InAs superlattice structures.

In this work, room temperature co-deposition of Mg and Si was used to successfully fabricate Mg2Si thin films on Si substrate by dual cathode magnetron sputtering (DCMS). Films were annealed at 380°C. Various Mg/Si sputtering power ratios have been examined. XRD, SEM and IR reflectivity measurements on grown and annealed films, reveal that annealing is enhancing the formation of crystalline Mg2Si.

Programs in the Defense Sciences Office (DSO) at the Defense Advanced Research Projects Agency (DARPA) bridge the gap from fundamental science to applications by identifying and pursuing the most promising ideas within the science and engineering research communities and transforming these ideas into new DoD capabilities. The Department of Defense (DoD) recognizes that our dependence on fossil fuel must be reduced and is actively supporting new technical solutions to this challenge. Moreover, advancements in alternative energy generation and storage technologies will render the warfighter more efficient, capable and mobile. Energy harvesting is a central pathway to solving this critical need. This paper will highlight two DARPA DSO programs which are making significant strides in the advancement in materials to improve energy harvesting technology with a focus on rapid transition to the end user. The Nanostructured Materials for Power (NMP) program is developing high performance thermoelectric materials and devices for energy harvesting with goals of energy conversion of 30%. The Low-Cost, Lightweight Portable Photovoltaics (PoP) program is developing integrated PV technologies with high power conversion efficiency (20%) in a form factor capable of being produced at low cost on flexible substrates. The paper will review the goals of the programs, discuss their implications, and highlight some of the research strategies underway.

A self resonating bimorph cantilever structure for fast temperature cycling in a pyroelectric energy harvester has been modeled using finite element method. Effect of constituting material properties and system parameters on the frequency and magnitude of temperature cycling and the efficiency of energy recycling using the proposed structure has been investigated. Results show that thermal contact conductance and heat source temperature play a key role in dominating the cycling frequency and efficiency of energy recycling. Studying the performance trend with various parameters such as thermal contact conductance, heat source temperature, device aspect ratio and constituent material of varying thermal conductivity and expansion coefficient, an optimal solution for most efficient energy scavenging process has been sought.

The work presents the model for the ionic thermoelectric phenomenons, which is based on the consideration of Grotthuss, hopping mechanism for the proton conductivity. The Seebeck coefficient and figure of merit are estimated in good agreement with experimental data.

The paper present a multimode optical fiber based solar lighting and photovoltaic multifunctional device. TiO2, PbS and conducting polymer poly(3-hexylthiophene) (P3HT) were coated on the optical core surface as active photovoltaic layers, with ITO and LiF/Al electrodes. The guided sunlight in the multi-moded fiber was totally internal reflected depending on the incident angle and the evanescent light mode from scattered radiation of light was absorbed by the active layer to generate the electric current. The optical-electrical behaviors such as the short circuit current, filling factor, and open circuit voltage were studied. Furthermore, optical loss and the evanescent field at the interface between the optical fiber core and ITO electrode thin layer was discussed in relation to the coupling light from the guiding medium to the devices. Comparison studies of chemical and physical thin film coating on optical fibers are discussed. In particular, we obtained in situ growth of quantum dot, composite electrolyte and plasmonic enhancement on TiO2. We have demonstrated waveguide like fiber device with photo current measurements and I-V characterizations. Furthermore, we have optimized the device transmission of visible light through total internal reflection, and PV conversion of evanescent light absorbed by solar active composite materials fabricated around optical fibers.

A recently developed generic model of a thermoelectric power generation system suggests a promising future for cost effective and scalable power generation. The model is based on co-optimizing the thermoelectric module together with the heat sink. Using this model, efficiency at maximum output power is calculated. It is shown that this approaches the Curzon-Ahlborn limit at very large Z values which is consistent with thermodynamic systems with irreversible heat engines. However, this happens only when the thermal resistances of the thermoelectric device with hot and cold heat sinks exactly match. For asymmetrical thermal resistances, the efficiency at maximum output power is different. This is consistent with the very recent results for the thermodynamic engines. Finally, we study the impact of lowering the thermal conductivity of the thermoelectric material or increasing its power factor and how these affect the performance of the thermoelectric power generation system.

We present materials development in fabricating thin film devices for the conversion of wind energy as a sustainable energy source. We demonstrate the feasibility of piezoelectric polymer thin film devices to harvest wind energy in a miniature wind tunnel. Using an example of prototype device based on polyvinylidene fluoride (PVDF) thin film devices, we are able to obtain electrical power from the wind’s energy through the mechanical deformation of PVDF, such as that obtained from the films flapping in the wind. We have obtained a preliminary result of 1 mW power (at 15 mph wind) with a single layer of PVDF of 4 x 2 inches and 50 μm in thickness sandwiched between two thin gold electrode films. Additionally, the fracturing of metallic electrodes over time from the induced strain of this application lead to the significance of examining carbon nanotubes as compliant electrodes offering better mechanical properties while maintaining necessary electrical properties.

Experimental results are presented to demonstrate feasibility of small scale power generation using static reverse electrodialysis (RED) of CuSO4 solutions. In contrast to conventional macro scale reverse electrodialysis, the concentrated and diluate compartments were not refreshed, resulting in limited power delivery times. This is important in energy harvesting applications from limited supply of ionic concentrations. Maximum output power density of 0.17 μW·cm−2 was recorded using microfiltration membranes. The evolution of the open circuit output voltage with time is accurately modeled at various concentration ratios.

In this work we report on the infrared properties of the thermoelectric (1-x)PbTe-xPbSnS2 system with x=0.03, 0.06, 0.11 and 0.33. The results obtained by the analysis of the reflectivity spectra are discussed together with the structural and morphological characteristics obtained by XRD and SEM-EDS measurements. The system was found macroscopically homogeneous for x=0.03 and x=0.06 and phase separated for x=0.11 and x=0.33. The analyzed ~150cm-1 PbS impurity mode demonstrated a composition close to the PbTe0.98S0.02 for the major phase. The incorporation of PbSnS2 causes a reduction in plasma frequency (decrease in carrier frequency concentration) and an increase in carrier mobility.

In molecular solar energy harvesting systems, quantum mechanical features may be apparent in the physical processes involved in the acquisition and migration of photon energy. With a sharply declining distance-dependence in transfer efficiency, the excitation energy generally takes a large number of steps en route to the site of its utilization; quantum features are rapidly dissipated in an essentially stochastic process. In the case of engineered dendrimeric polymers, each such step usually takes the form of an inward hop between chromophores in neighboring generation shells. A physically intuitive, structure-determined adjacency matrix formulation of the energy flow affords insights into the key harvesting and inward funneling processes. A numerical method based on this analytic approach has now been developed and is able to deliver results on significantly larger dendrimeric polymers, with the help of large multi-processor computers. Central to this study is the interpretation of key features such as the relevance of a spectroscopic gradient and the presence of traps or irregularities due to conformational changes and folding. With the objective of fine-tune the funneling process, this model now allows the incorporation of parameters derived from quantum chemical calculations, affording new insights into the detailed operation of the harvesting process in a variety of dendrimer systems.

TAGS-85 is a well-known thermoelectric material based on germanium monotelluride that exhibits a second-order displacive transformation from a high-temperature cubic to a low-temperature rhombohedral polymorph. Recent efforts to improve the thermoelectric figure-of-merit through the addition of small amounts of the rare earth elements Ce and Yb have demonstrated a 25 percent increase in ZT at 700K in materials obtained by solidification from the melt. Preliminary analysis by x-ray diffraction of the chemically-modified alloy suggests a partial stabilization of the high-temperature cubic polymorph. 125Te NMR studies confirm the incorporation of rare earth cations into the GeTe-based lattice. Solid state synthesis has been successfully applied to the processing of rare-earth-doped TAGS-85 and has resulted in a further increase in ZT beyond the levels initially observed in melt-solidified materials. This is believed to be due to improved homogeneity in the distribution of the lanthanide.

Compaction of β-Zn4Sb3 was carried out by current-assisted short term sintering under pressure (STS) using different material and process parameters. Surface Seebeck mapping (PSM) of the compacted specimens (along the uniaxial pressing direction) shows a wide gradation ranging from ∼40–180 μV/K due to electro-migration of Zinc along the current direction. The wide distribution of S corresponds to a mixture of Zn-Sb phases [1] which arise depending on the extent of Zinc migration during the STS process. Variation in the material and process parameters (average particle size, heating rate, compaction time/temperature) results in different spatial distribution of S. Measurements of electrical (σ), thermal (κ) conductivities and Seebeck (S) coefficients between room temperature and 523 K were carried out on two specimens having different average S values and distributions as observed by the PSM. The results indicate an increase in the lattice thermal conductivity (κL) and subsequent lower ZT in the specimens compared to the reported values for β-Zn4Sb3.

Dielectric Elastomer Actuators (DEAs) consist of an elastomeric layer sandwiched between two compliant electrodes. An electric field applied between the two electrodes will lead to a compression of the elastomer due to the Maxwell’s pressure. DEA can be used for many active applications such as pumps, muscles and so on, where the voltage drives the motion, but they can also operate inversely for energy harvesting or for sensor applications, when the displacement of charges due to a change in thickness is stored or detected. Energy harvesting systems like buoys using wave energy or shoe soles extracting energy from walking have been demonstrated. In this contribution we investigate polychloroprene (CR) as a new material for DEA and describe its potential for use in energy harvesting. To this end, a full characterization of the material properties was undertaken. We find that the very high permittivity combined with good mechanical properties makes this material a promising novel candidate for the energy harvesting application.

Mechanical energy harvesting from ambient vibrations is an attractive renewable source of energy for various applications. Prior research was solely based on lead-containing materials which are detrimental to the environment and health. Therefore, lead-free materials are becoming more attractive for harvesting applications. The present work is focused on the development of lead-free piezoelectric materials based on solid solution having composition (KNa)NbO3-xABO3, (where A = Li, and B = Nb; x = 0, 5, 5.5, 6, and 6.5 wt%). The solid solutions of the above ceramics were prepared by using solid-state reaction method. The X-ray diffraction spectra exhibited single phase formation and good crystallinity with LiNbO3 addition up to x = 6.5 wt%. Dielectric studies reveal that the composition with LiNbO3 = 6.5 wt% exhibits superior properties suitable for piezoelectric energy harvesting applications. The nanoscale piezoelectric data obtained with piezoresponse force microscopy provide a direct evidence of strong piezoelectricity with LN doping. The best piezoelectric properties are obtained for the composition K0.5Na0.5NbO3 – 6.5%LiNbO3.

The dependency of Young’s and shear modulus on relative density and temperature was determined for three series of skutterudites: p-type MmyFe3CoSb12 (y = 0.70, 0.80), p-type DDyFe3CoSb12 (y = 0.65, 0.68) and n-type Sr0.07Ba0.07Yb0.07Co4Sb12. For all three groups of materials the dependency of Young’s modulus, E, on the density and temperature is linear: E(d) = 4.1(1)×d – 268(3) GPa (d is the relative density in %), ΔE/ΔT = 0.20(1) GPa/K, thus allowing extrapolation of E as a function of temperature and towards 100% density.

A polarization-independent, broadband, antireflecting compound aperture array is designed, fabricated and characterized. The structure is composed of an aluminum film with a periodic array of perforations (apertures) configured in a square lattice with a unit cell consisting of two apertures with different diameters, both filled with silicon oxynitride. Light-channeling waveguide cavity modes of different energies are excited within the two different apertures within each unit cell thereby transmitting different parts of the solar spectrum into the substrate. Experimental characterization shows low reflection (<10%) and low diffuse backscatter. Numerous applications of the films include light detecting and emitting structures and devices.

Thermoelectric (TE) properties of erbium-silicon co-doped InxGa1-xN alloys (InxGa1-xN: Er + Si, 0≤x≤0.14), grown by metal organic chemical vapor deposition, have been investigated. It was found that doping of InGaN alloys with Er atoms of concentration, N[Er] larger than 5x1019 cm-3, has substantially reduced the thermal conductivity, κ, in low In content InGaN alloys. It was observed that κ decreases as N[Er] increases in Si co-doped In0.10Ga0.90N alloys. A room temperature ZT value of ~0.05 was obtained in In0.14Ga0.86N: Er + Si, which is much higher than that obtained in un-doped InGaN with similar In content. Since low In content InGaN is stable at high temperatures, these Er+Si co-doped InGaN alloys could be promising TE materials for high temperature applications.

Thermoelectric generators are actively being pursued to recover waste heat from the auto exhaust gas to improve vehicle fuel economy. Efficiency of a thermoelectric generator is defined as the ratio of electrical power output to the heat input. In a typical thermoelectric generator, a heat exchanger captures the heat from the medium (ex: hot exhaust gas heat) and this heat needs to be transferred to the hot end of the thermoelectric elements with minimum losses. It is important to understand and minimize these thermal losses to improve the efficiency of a thermoelectric generator. Accurate measurement of the thermal interface resistance parameters is also important because they are used in a comprehensive thermoelectric system model to predict the performance of the generator under actual use conditions. To understand the factors influencing the thermal interface resistance, and to determine the effective thermal interface resistance between the heat exchanger and the thermoelectric hot shunts in a prototype generator that is currently being developed for auto exhaust heat recovery application, we have designed and built a test setup to characterize the thermal interface resistance under high heat flux conditions. Measured temperature profiles in the test sample, heat input into the test device and its geometry are fed into a thermal model to extract the thermal conductance parameters. Factors affecting the thermal interface resistance and the influence of different interface materials were evaluated. Suitable solutions with minimum thermal loss were selected for building the prototype thermoelectric generator for waste heat recovery application and validating the system model.