Gaosheng Fu

Fabrication of Thermoelectric Devices Using Thermal Spray: Application to Vehicle Exhaust Systems

Thermoelectric devices produce electricity directly from heat; they are small, have no moving parts, and are quiet. Commercially available thermoelectric devices, however, are expensive and labor intensive to produce, and come in very limited form factors. This article presents initial results for the use of thermal spray to directly fabricate thermoelectric devices. The target application is automotive exhaust systems and other high-volume heat sources. In this work, FeSi2 and Mg2Si metal silicides were sprayed. Characterization of the Mg2Si deposits indicates that both the thermal conductivity and the Seebeck coefficient are roughly one half the values of bulk Mg2Si. The electrical conductivity, however, is several orders of magnitude lower than bulk measurements in the literature, with likely reasons including impurities in the starting powder, oxidation during spraying, and using an undoped material. FexCo4−xSb12 skutterudite material has also been sprayed; however, not enough powder was available to fabricate samples large enough for characterization. The steps required to fabricate a thermoelectric device are presented, including the formation of the bottom and top metallic layers and the thermoelectric legs using thermal spray and laser micromachining. A technique for bridging the air gap between adjacent thermoelectric elements for the top layer based on a sacrificial filler material has also been demonstrated.

Thermoelectric properties of magnesium silicide fabricated using vacuum plasma thermal spray

The thermoelectric properties of magnesium silicide samples prepared by Vacuum Plasma Spray (VPS) are compared with those made from the conventional hot press method using the same feedstock powder. Thermal conductivity, electrical conductivity, Seebeck coefficient, and figure of merit are characterized from room temperature to 700 K. X-ray diffraction and scanning electron microscopy of the samples are obtained to assess how phase and microstructure influence the thermoelectric properties. Carrier concentration and Hall mobility are obtained from Hall Effect measurements, which provide further insight into the electrical conductivity and Seebeck coefficient mechanisms. Low-temperature electrical conductivity measurements suggest a 3D variable range hopping effect in the samples. VPS samples achieved a maximum ZT = 0.16 at 700 K, which is around 30% of the hot press sample ZT = 0.55 at 700 K using the same raw powder. The results suggest that thermal spray is a potential deposition technique for thermoelectric materials.

Sensors for Small Modular Reactors Powered by Thermoelectric Generators

The aim of this work is to develop thermoelectrically powered sensing and actuating devices for normal and off-normal conditions in Small Modular Reactors (SMRs). This is realized using thermoelectric generators (TEGs) placed on key reactor components such as pipes, pump housings, heat exchangers or reactor vessels to capture waste heat and produce usable electric power. The electrical power generated is used to drive sensors, signal conditioning, and wireless communication to relay critical plant information to operators and emergency crew. Surplus power can be stored in batteries and/or super capacitors for actuation and other high-current, short-duration power needs. A review of key SMR reactor types and candidate locations for installing these devices is presented. The design and manufacturing process of an integrated TEG assembly that attaches to a schedule 40 steam pipe and preliminary test data of the system from an installation performed on a conventionally-fueled cogeneration power plant located on site are presented. The nominal pipe temperature is 350°C, and the assembly is designed to reduce the temperature to 230°C on the hot side of the TEG. A cold side temperature of 70°C was obtained using a large heat sink cooled by natural convection to the ambient. Designs for an enclosure system are also presented to protect the electrical components from fire and radiation.Copyright

Thermoelectric Properties of Magnesium Silicide Depositedby Use of an Atmospheric Plasma Thermal Spray

The thermoelectric properties of magnesium silicide (Mg2Si) samples prepared by use of an atmospheric plasma spray (APS) were compared with those of samples prepared from the same feedstock powder by use of the conventional hot-pressing method. The characterization performed included measurement of thermal conductivity, electrical conductivity, Seebeck coefficient, and figure of merit, ZT. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDX) were used to assess how phase and microstructure affected the thermoelectric properties of the samples. Hall effect measurements furnished carrier concentration, and measurement of Hall mobility provided further insight into electrical conductivity and Seebeck coefficient. Low temperature and high velocity APS using an internal-powder distribution system achieved a phase of composition similar to that of the feedstock powder. Thermal spraying was demonstrated in this work to be an effective means of reducing the thermal conductivity of Mg2Si; this may be because of pores and cracks in the sprayed sample. Vacuum-annealed APS samples were found to have very high Seebeck coefficients. To further improve the figure of merit, carrier concentration must be adjusted and carrier mobility must be enhanced.

Thermoelectric properties of DC-sputtered filled skutterudite thin film

The Yb filled CoSb3 skutterudite thermoelectric thin films were prepared by DC magnetron sputtering. The electrical conductivity, Seebeck coefficient, thermal conductivity, and figure of merit ZT of the samples are characterized in a temperature range of 300 K to 700 K. X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy are obtained to assess the phase composition and crystallinity of thin film samples at different heat treatment temperatures. Carrier concentrations and Hall mobilities are obtained from Hall Effect measurements, which provide further insight into the electrical conductivity and Seebeck coefficient mechanisms. The thermal conductivity of thin film filled skutterudite was found to be much less compared with bulk Yb filled CoSb3 skutterudite. In this work, the 1020 K heat treatment was adopted for thin film post process due to the high degree of crystallinity as well as avoiding reverse heating effect. Thin film samples of different thicknesses were pre...

Review of Waste Energy Resource in Vehicle Engine Exhaust

Energy recovery from vehicle engine exhaust has attracted considerable interest recently. Key parameters associated with the engine exhaust, including temperature, mass flow rate, maximum extractable energy, and optimum location for energy extraction all factor strongly into the materials research and device design for waste heat recovery. This review paper compiles available data in literature on the vehicle engine exhaust resources for several different vehicles, and under various operational conditions. Three vehicles types, namely, mid-size sedans, light duty trucks, and heavy duty trucks, have been considered, and the driving cycles including Federal Test Procedure (FTP) series, Highway Fuel Economy Test (HFET) and New European Driving cycle (NEDC) are considered in this review. The results show the average temperatures at highway driving cycle and city driving cycle remain in the ∼500–650°C and ∼200–400 °C range, respectively. The mass flow rate varies significantly with vehicle size. The available thermal power calculated based on the collected data is 3–10 kW.© 2012 ASME

Thermoelectrically Powered Sensing for Small Modular Reactors

This project aims to develop thermoelectric generator (TEG)-based devices for sensing during normal and off-normal conditions in Small Modular Reactors (SMRs). TEGs will be placed on key reactor components including pipes, pump housings, heat exchangers and reactor vessels. The heat is conducted by heat pipes to the TEGs and removed by a heat sink in natural convection. The electrical power generated by is then used to drive sensors and wireless communications. The estimated power generated by one TEG is 19 W, and sensors with related circuit only require less than 7 W. Extra power can be stored into batteries and used for actuation and similar high-current, short duration power needs. Initial enclosure designs are also presented to protect the electrical devices from fire, force, water, and radiation. Preliminary experiments have been set up for testing TEGs. An experimental test stand design has been simulated, and is now being built. Radiation dosage at different locations and its effects on electrical devices and TEGs are also investigated. A fin analysis of the cooling side of the TEG is also presented. According to the results, the annular finned-tube with an inner pipe diameter of 58 cm can provide a maximum heat dissipation of 1,700 W.Copyright

Heat Transfer Modeling and Geometry Optimization of TEG for Automobile Applications

Thermoelectric Generators (TEG) can be used on automobiles to harvest energy from exhaust waste heat. Besides the improvement on material side, the optimization of geometry of the module is also important to maximize the output power density and will be addressed in this paper. A thermal resistance network is established based on heat conduction and radiation from heat source to heat sink. Although the heat transfer model is based on cylindrical exhaust pipe geometry, the thermo-element is approximated as plane geometry because the ceramic layer and thermoelectric layer are much smaller compared with the exhaust pipe diameter. The TE material we proposed to recover waste heat energy is magnesium silicide (Mg2Si), which has a reasonable figure of merit in the automobile exhaust temperature range, and the process is thermal spray compatible, which is a mass productive method currently under investigation. Another material that used for comparison is titanium oxide. Based on the Seebeck coefficient, thermal and electrical conductivity of our thermal sprayed samples, the thermoelectric leg length and the area ratio between thermoelectric element and total module area are optimized for maximum power density output. The optimal leg length is around 0.85mm, and the air gap is as small as possible. A parameter sensitivity analysis is conducted to investigate the influence of ceramic layer thickness, exhaust pipe radius, electrical contact resistance, hot and cold side temperature, Seebeck and electrical conductivity on the optimal leg length.© 2012 ASME

Thermoelectric Properties of Magnesium Silicide Prepared by Thermal Spraying

The paper presents the thermoelectric properties of Mg2Si coatings by Atmospheric Plasma Spray (APS), High Velocity Oxy-Fuel Spray (HVOF) and Vacuum Plasma Spray (VPS). Thermal spraying, a flexible, industry-scalable and cost-effective manufacturing process, was first applied to prepare Mg2Si as thermoelectric materials. The characterization of Mg2Si coatings including thermal conductivity, electric conductivity, and Seebeck coefficient are reported. XRD and SEM analysis of the coatings are presented as well.Copyright