Thermoelectric materials are solid-state energy converters whose combination of thermal, electrical, and semiconducting properties allows them to be used to convert waste heat into electricity or electrical power directly into cooling and heating. These materials can be competitive with fluid-based systems, such as two-phase air-conditioning compressors or heat pumps, or used in smaller-scale applications such as in automobile seats, night-vision systems, and electrical-enclosure cooling. More widespread use of thermoelectrics requires not only improving the intrinsic energy-conversion efficiency of the materials but also implementing recent advancements in system architecture. These principles are illustrated with several proven and potential applications of thermoelectrics.

http://engin1000.pbworks.com/f/TE_rev.pdf

Cooling, heating, generating power, and recovering waste heat with thermoelectric systems.

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.

/pdf/high-thermoelectric-performance-of-nanostructured-bismuth-4evuadx0tz.pdf

High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys

http://absimage.aps.org/image/MAR08/MWS_MAR08-2007-001627.pdf

High Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys.

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

/pdf/prospects-of-colloidal-nanocrystals-for-electronic-and-3dxpyhl3r7.pdf

Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Approximately 90 per cent of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30-40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2-4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20-300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.

Enhanced thermoelectric performance of rough silicon nanowires

Soft x-rays for deep sub-100 nm lithography, with and without masks

We demonstrate Purcell enhancement of single NV centers in L3 photonic crystal cavities made from high-purity single-crystal diamond. Furthermore, NV centers were created using an implantation mask in the cavity high field region.

Targeted creation and Purcell enhancement of NV centers within photonic crystal cavities in single-crystal diamond

The simultaneous imaging of magnetic fields and temperature (MT) is important in a range of applications, including studies of carrier transport, solid-state material dynamics, and semiconductor device characterization. Techniques exist for separately measuring temperature (e.g., infrared (IR) microscopy, micro-Raman spectroscopy, and thermo-reflectance microscopy) and magnetic fields (e.g., scanning probe magnetic force microscopy and superconducting quantum interference devices). However, these techniques cannot measure magnetic fields and temperature simultaneously. Here, we use the exceptional temperature and magnetic field sensitivity of nitrogen vacancy (NV) spins in conformally-coated nanodiamonds to realize simultaneous wide-field MT imaging. Our "quantum conformally-attached thermo-magnetic" (Q-CAT) imaging enables (i) wide-field, high-frame-rate imaging (100 - 1000 Hz); (ii) high sensitivity; and (iii) compatibility with standard microscopes. We apply this technique to study the industrially important problem of characterizing multifinger gallium nitride high-electron-mobility transistors (GaN HEMTs). We spatially and temporally resolve the electric current distribution and resulting temperature rise, elucidating functional device behavior at the microscopic level. The general applicability of Q-CAT imaging serves as an important tool for understanding complex MT phenomena in material science, device physics, and related fields.

Wide-field Magnetic Field and Temperature Imaging using Nanoscale Quantum Sensors

Electrical power densities of up to 33 W/cm2 and up to 12 W/cm2 were obtained for n-type and p-type PbTeSe-based stand-alone thermoelectric devices, respectively, at modest temperature gradients of ∼200 °C (Tcold = 25 °C). These large power densities were enabled by greatly improving electrical contact resistivities in the thermoelectric devices. Electrical contacts with contact resistivities as low as 3.9 × 10−6 Ω cm2 and 4.0 × 10−6 Ω cm2 for n- and p-type telluride-based- materials, respectively, were developed by investigating several metallization schemes and contact layer doping/alloy combinations, in conjunction with a novel contact application process. This process exposes heated semiconductor surfaces to an atomic hydrogen flux under high vacuum for surface cleaning (oxide and carbon removal), followed immediately by an in-situ electron-beam evaporation of the metal layers.

High-output-power densities from molecular beam epitaxy grown n- and p-type PbTeSe-based thermoelectrics via improved contact metallization

Solid-state quantum emitters that couple coherent optical transitions to long-lived spin states are essential for quantum networks. Here we report on the spin and optical properties of single tin-vacancy (SnV) centers in diamond nanostructures. Through magneto-optical spectroscopy at 4 K, we verify the inversion-symmetric electronic structure of the SnV, identify spin-conserving and spin-flipping transitions, characterize transition linewidths, and measure electron spin lifetimes. We find that the optical transitions are consistent with the radiative lifetime limit and that the spin lifetimes are longer than for other inversion-symmetric color centers under similar conditions. These properties indicate that the SnV is a promising candidate for quantum optics and quantum networking applications.

Michael Walsh

Papers

Soft x-rays for deep sub-100 nm lithography, with and without masks

Targeted creation and Purcell enhancement of NV centers within photonic crystal cavities in single-crystal diamond

Wide-field Magnetic Field and Temperature Imaging using Nanoscale Quantum Sensors

High-output-power densities from molecular beam epitaxy grown n- and p-type PbTeSe-based thermoelectrics via improved contact metallization

Transform-limited photons from a tin-vacancy spin in diamond