Gary Bulman

Superlattice-based thin-film thermoelectric modules with high cooling fluxes

In present-day high-performance electronic components, the generated heat loads result in unacceptably high junction temperatures and reduced component lifetimes. Thermoelectric modules can, in principle, enhance heat removal and reduce the temperatures of such electronic devices. However, state-of-the-art bulk thermoelectric modules have a maximum cooling flux qmax of only about 10 W cm−2, while state-of-the art commercial thin-film modules have a qmax <100 W cm−2. Such flux values are insufficient for thermal management of modern high-power devices. Here we show that cooling fluxes of 258 W cm−2 can be achieved in thin-film Bi2Te3-based superlattice thermoelectric modules. These devices utilize a p-type Sb2Te3/Bi2Te3 superlattice and n-type δ-doped Bi2Te3−xSex, both of which are grown heteroepitaxially using metalorganic chemical vapour deposition. We anticipate that the demonstration of these high-cooling-flux modules will have far-reaching impacts in diverse applications, such as advanced computer processors, radio-frequency power devices, quantum cascade lasers and DNA micro-arrays.

Development of low dark current SiGe-detector arrays for visible-NIR imaging sensor

SiGe based Focal Plane Arrays offer a low cost alternative for developing visible- NIR focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based IRFPAs will take advantage of Silicon based technology, that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance comparison for the SiGe based VIS-NIR Sensor with performance characteristics of InGaAs, InSb, and HgCdTe based IRFPAs. Various approaches including device designs are discussed for reducing the dark current in SiGe detector arrays; these include Superlattice, Quantum dot and Buried junction designs that have the potential of reducing the dark current by several orders of magnitude. The paper also discusses approaches to reduce the leakage current for small detector size and fabrication techniques. In addition several innovative approaches that have the potential of increasing the spectral response to 1.8 microns and beyond.

Controlled improvement in specific contact resistivity for thermoelectric materials by ion implantation

To obtain reduced specific contact resistivity, iodine donors and silver acceptors were ion-implanted into n-type and p-type (Bi,Sb)2(Se,Te)3 materials, respectively, to achieve >10 times higher doping at the surface. Implantation into n-type materials caused the specific contact resistivity to decrease from 1.7 × 10−6 Ω cm2 to 4.5 × 10−7 Ω cm2. Implantation into p-type materials caused specific contact resistivity to decrease from 7.7 × 10−7 Ω cm2 to 2.7 × 10−7 Ω cm2. For implanted thin-film superlattices, the non-implanted values of 1.4 × 10−7 Ω cm2 and 5.3 × 10−8 Ω cm2 precipitously dropped below the detection limit after implantation, ≤10−8 Ω cm2. These reductions in specific contact resistivity are consistent with an increase in tunneling across the contact.

Poisson Ratio of Epitaxial Germanium Films Grown on Silicon

An accurate knowledge of elastic constants of thin films is important in understanding the effect of strain on material properties. We have used residual thermal strain to measure the Poisson ratio of Ge films grown on Si ⟨001⟩ substrates, using the sin2ψ method and high-resolution x-ray diffraction. The Poisson ratio of the Ge films was measured to be 0.25, compared with the bulk value of 0.27. Our study indicates that use of Poisson ratio instead of bulk compliance values yields a more accurate description of the state of in-plane strain present in the film.

High-efficiency energy harvesting using TAGS-85/half-Heusler thermoelectric devices

To improve the thermal-to-electrical conversion efficiency of waste exhaust heat at temperatures in the vicinity of 750°C, RTI has combined two different high-performance materials to form a high ZT, hybrid thermoelectric (TE) device. Recently-developed enhanced “TAGS-85”, or e-TAGS, was employed as the p-leg, while the n leg was comprised of improved half-Heusler (HH) material. This hybrid material pair provides a high ZT, lead-free TE material solution for exhaust gas heat recovery for use in vehicle or industrial platforms. The improved HH material employs two novel techniques to reduce thermal conductivity: (1) high-energy milling, and (2) addition of coherent inclusions. Single n-/pcouples were produced that achieved a 9.2% efficiency with a power output of 205mW for Thot = 559°C and ΔT = 523K. This is a significant efficiency improvement at a lower hot side temperature with the hybrid e-TAGS/HH single couple over the performance of a conventional, all HH couple. By optimizing the cross sectional areas of the pellets for equal heat flow, the resulting asymmetric couple achieved 10.5% efficiency with a maximum power output of 317 mW at Thot = 537°C and ΔT = 497°C. A 49-couple hybrid module using other advanced HH materials paired with e-TAGS and operated with Thot up to 600°C reached a maximum efficiency of 10%. The improved module efficiency is believed to be due to both improved materials and optimized cross-sectional area ratios between the n- and p- elements.

Thermoelectric energy harvesting for a solid waste processing toilet

Over 2.5 billion people do not have access to safe and effective sanitation. Without a sanitary sewer infrastructure, self-contained modular systems can provide solutions for these people in the developing world and remote areas. Our team is building a better toilet that processes human waste into burnable fuel and disinfects the liquid waste. The toilet employs energy harvesting to produce electricity and does not require external electrical power or consumable materials. RTI has partnered with Colorado State University, Duke University, and Roca Sanitario under a Bill and Melinda Gates Foundation Reinvent the Toilet Challenge (RTTC) grant to develop an advanced stand-alone, self-sufficient toilet to effectively process solid and liquid waste. The system operates through the following steps: 1) Solid-liquid separation, 2) Solid waste drying and sizing, 3) Solid waste combustion, and 4) Liquid waste disinfection. Thermoelectric energy harvesting is a key component to the system and provides the electric power for autonomous operation. A portion of the exhaust heat is captured through finned heat-sinks and converted to electricity by thermoelectric (TE) devices to provide power for the electrochemical treatment of the liquid waste, pumps, blowers, combustion ignition, and controls.

Combining previously unpaired materials enables better energy harvesting

The transportation sector accounts for a significant fraction of domestic petroleum usage, with consumption over the next 10 years estimated to range from 14 to 15 million barrels per day.1 The US Department of Energy estimates a continued increase in petroleum consumption in future decades despite significant efforts to improve fuel economy and develop alternative fuels. In the typical internal combustion engine, only about 25% of the energy produced by burning fuel is used to move the vehicle, with most of the remaining energy ( 2/3) leaving the engine in the form of heat.2 Approximately 40% of this waste heat leaves through the engine exhaust, and the rest is ejected to the atmosphere through the engine cooling system. Conversion of this waste heat—which has a high temperature (i.e., high quality)—to electricity using thermoelectric (TE) technology would enable an improvement in automobile fuel economy of up to 5%. However, achieving this goal requires significant advancements in TE technology. Specifically, those improvements involve using high-efficiency TE materials with a high-TE figure of merit (ZT) that are able to operate reliably at high temperatures. In addition, the TE modules and heat exchangers must be inexpensive enough to meet extremely challenging cost targets. In our work, we have combined two high-performance TE materials that have not previously been paired to form a high-ZT hybrid TE device. We accomplished this with the novel approach of using an enhanced alloy called ‘e-TAGS’3–5—tellurium, silver, germanium, and antimony (Te50Ag6:52Ge36:96Sb6:52, or TAGS-85) containing 1% of the rare earth element ytterbium (Yb)—as the p-leg material, and combining it with improved half-Heusler (HH) materials containing titanium, hafnium, zirconium, nickel, tin, Figure 1. Comparison of efficiency between a symmetric hybrid e-TAGS/HH single couple, an asymmetric hybrid e-TAGS/HH couple, and an all-HH single couple as a function of hot-side temperature (Thot). The asymmetric performance reached a maximum efficiency of 9.7%. e-TAGS: Advanced TAGS (tellurium, silver, germanium, antimony) alloy. HH: Half-Heusler.

Progress in Bi2Te3-based superlattice thermoelectric materials

Thin film superlattice (SL) based thermoelectric (TE) devices offer the potential for improved efficiency and high heat flux cooling over conventional bulk materials. We have demonstrated external cooling of 55K and heat pumping capacity of 128 W/cm2 in single couples and temperature differences as high as 102K in three stage cascade structures. The high heat flux pumping capacity of these in thin film devices are also attractive for hot-spot cooling in electronics. These same materials have also been successfully employed in power generation and energy harvesting applications. In this presentation, we will discuss recent RTI advances in Bi2Te3-based thin-film SL (TFSL) devices for cooling and energy harvesting applications.