Pawan Gogna

Bio: Pawan Gogna is an academic researcher from California Institute of Technology. The author has contributed to research in topics: Thermoelectric effect & Thermoelectric materials. The author has an hindex of 10, co-authored 27 publications receiving 4325 citations. Previous affiliations of Pawan Gogna include Massachusetts Institute of Technology & Jet Propulsion Laboratory.

New Directions for Low-Dimensional Thermoelectric Materials**

Abstract: Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof-of-principle studies on quantum-well superlattice and quantum-wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.

Nanostructured Bulk Silicon as an Effective Thermoelectric Material

Abstract: Thermoelectric power sources have consistently demonstrated their extraordinary reliability and longevity for deep space missions and small unattended terrestrial systems. However, more efficient bulk materials and practical devices are required to improve existing technology and expand into large-scale waste heat recovery applications. Research has long focused on complex compounds that best combine the electrical properties of degenerate semiconductors with the low thermal conductivity of glassy materials. Recently it has been found that nanostructuring is an effective method to decouple electrical and thermal transport parameters. Dramatic reductions in the lattice thermal conductivity are achieved by nanostructuring bulk silicon with limited degradation in its electron mobility, leading to an unprecedented increase by a factor of 3.5 in its performance over that of the parent single-crystal material. This makes nanostructured bulk (nano-bulk) Si an effective high temperature thermoelectric material that performs at about 70% the level of state-of-the-art Si0.8Ge0.2 but without the need for expensive and rare Ge.

Low-Threshold Photonic Crystal Laser

Abstract: We have fabricated photonic crystal nanocavity lasers, based on a high-quality factor design that incorporates fractional edge dislocations. Lasers with InGaAsP quantum well active material emitting at 1550 nm were optically pumped with 10 ns pulses, and lased at threshold pumping powers below 220 μW, the lowest reported for quantum-well based photonic crystal lasers, to our knowledge. Polarization characteristics and lithographic tuning properties were found to be in excellent agreement with theoretical predictions.

High-performance InAs quantum-dot lasers near 1.3 μm

Abstract: High-performance quantum dot (QD) lasers near 1.3 μm were fabricated using four stacks of InAs QDs embedded within strained InGaAs quantum wells as an active region and a reactive-ion-etched 5-μm-ridge waveguide design. For a 1.5-mm-long cavity QD laser, ground-state continuous-wave (cw) lasing has been achieved with a single facet output power of 15 mW at temperatures as high as 100 °C, while at room temperature having a differential quantum efficiency of 55% and a single facet output power of 50 mW. The characteristic temperature T0 for ground-state cw lasing is 78 K up to our temperature measurement limit of 100 °C.

High Temperature Thermoelectric Properties of Yb 14 MnSb 11 Prepared from Reaction of MnSb with the Elements

Abstract: Compounds of the Yb14MnSb11 structure type are the highest efficiency bulk p-type materials for high temperature thermoelectric applications, with reported figures of merit (ZTs) as high as ∼1.3 at 1275 K. Further optimization of ZT for this structure type is possible with the development of a simple synthetic route. However, this has been difficult to achieve because of the small amount of Mn required compared with Yb and Sb. A simple synthetic route for Yb14MnSb11 has been developed utilizing a combination of ball milling and annealing to produce phase-pure material followed by spark plasma sintering for consolidation. The materials have been characterized by powder X-ray diffraction before and after spark plasma sintering. The stoichiometric reaction of Yb, Sb, and MnSb provides phase-pure powder by X-ray diffraction. Upon cycling to temperatures greater than 1272 K, Yb14MnSb11 shows the presence of Yb11Sb10. Additional samples with 5% and 10% excess Mn were also investigated. Adding 5–10% excess Mn do...

Complex thermoelectric materials.

Abstract: Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.

Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials

Abstract: If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. We show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films. We show that WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.

High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys

Abstract: The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100°C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250°C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86°, 106°, and 119°C with hot-side temperatures set at 50°, 100°, and 150°C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials.