Power Generation from Nanostructured Half-Heusler Thermoelectrics

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Power generation from nanostructured half-Heusler thermoelectrics for efficient and robust energy harvesting Daylon Black, Luke Schoensee, Joseph Richardson, Trenton Vleisides, Nicholas Kempf, Dezhi Wang, Zhifeng Ren, and Yanliang Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01042 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Applied Energy Materials

Power Generation from Nanostructured Half-Heusler Thermoelectrics for Efficient and Robust Energy Harvesting Daylon Black,1 Luke Schoensee,1 Joseph Richardson,2 Trenton Vleisides,1 Nicholas Kempf,3 Dezhi Wang,4 Zhifeng Ren,4 and Yanliang Zhang3* 1Department

of Mechanical and Biomedical Engineering, Boise State University, Boise, ID

83725 2Department 3

of Electrical and Computer Engineering, Boise State University, Boise, ID 83725

Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame

Indiana 46556 4Department

of Physics and the Texas Center for Superconductivity, University of Houston,

Houston, TX 77204, USA *[email protected] ABSTRACT We demonstrate a robust and high-power-density thermoelectric device fabricated using highperformance nanostructured bulk half-Heusler materials. A novel metal-ceramic composite was employed as electrodes to join the n-type and p-type half-Heusler elements in order to form a uncouple device. By matching the coefficient of thermal expansion of the nanostructured halfHeuslers, the composite electrodes reduce the interfacial shear stresses by 40% compared with the device using the conventional metal electrodes, which significantly improves the device reliability during operation under large temperature gradient. A three-dimensional finite element device model was built to calculate the device power output, efficiency and thermal stress during operation. The uncouple device delivers an ultrahigh power density of 8.6 W/cm2 and an efficiency of 6.2% under a temperature difference of 570 °C, which are within 2% and 10% of the finite element simulation respectively. Key Words: nanostructured half-Heusler, thermoelectric device, energy harvesting, high power density, coefficient of thermal expansion, thermal stress

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1 Introduction Waste heat is a largely untapped energy source with the potential to significantly improve energy efficiency and reduce emissions in broad fields. The waste heat from world’s electrical power generation translates to 15 terawatts of lost energy at the global scale.1 Statistical investigations indicate that over 50% of all fuel burned by industrial sources becomes waste heat. Broad implementations of waste heat recovery (WHR) systems would be highly beneficial to society because it would (1) utilize otherwise wasted thermal energy to produce electricity to either offset on-site consumption or to export to the grid, (2) represent a clean energy technology with zero new emissions from the use of waste heat instead of carbon-based fuels, and thus have indirect environmental and health benefits from reduced emissions, and (3) provide other benefits such as grid support and reliability, energy efficiency, and energy security. The thermoelectric generator (TEG) is a solid-state device that directly converts heat into electricity with no moving parts, which is a very attractive technology to recover waste heat from industrial processing, power plants, automobiles, solar thermal and geothermal energy. In addition, thermoelectric devices can enable self-powered systems by harvesting thermal energy to realize power-autonomy for broad applications where power delivery is challenging or expensive. Unlike other WHR technologies, the size of the TEG is highly scalable without significant cost changes due to the absence of moving parts and expensive machinery, making it especially advantageous for small-to-mid size energy harvesting systems. With an applied temperature gradient, TE devices convert thermal energy into electricity by means of the Seebeck effect. TE devices typically consist of doped n- and p-type semiconductors connected electrically in series and thermally in parallel.2 The maximum efficiency of a TEG can be estimated by

𝜂𝑚𝑎𝑥 =

Δ𝑇 1 + 𝑍𝑇𝑎𝑣𝑔 ― 1 , 𝑇ℎ 𝑇𝑐 1 + 𝑍𝑇𝑎𝑣𝑔 + 𝑇ℎ

(1)

where Th and Tc are the hot-side and cold-side temperatures of the TEG respectively, T is the temperature difference Th - Tc , ZTavg is the average thermoelectric figure of merit ZT between Th and Tc, Z = 2/ where , ,  are the Seebeck coefficient, electrical conductivity, and thermal 2


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conductivity, respectively.3 Clearly, the maximum TE efficiency is governed by the Carnot efficiency, T/ Th, and a scaling factor which depends on the material’s ZT. The ZT of a TE material has been the guiding factor for engineering better materials and devices in the thermoelectric community for decades.4–6 The nanostructured half-Heusler alloys have gained immense interest in the last decade.6–9 The half-Heusler materials are known for their high Seebeck coefficient, high electrical conductivity, strong mechanical strength, but often have undesirable high thermal conductivity.10 Recent progresses on nanostructured half-Heusler materials have led to a significant reduction of lattice thermal conductivity and a peak ZT above 1 at moderately high temperatures (400-700 °C).11 While the majority of the literature to date focuses on the ZT improvement through nanostructuring and other strategy, the fabrication of high-temperature TEGs using nanostructured materials has been a challenge due to various reasons. Managing thermal stress at the interfaces is one of the greatest challenges in hightemperature TEG fabrication. The mismatch of the coefficient of thermal expansion between the metal electrode and the thermoelectric material typically results in device failure due to thermal stress at the interfaces. Despite several experimental studies on the fabrication of thermoelectric devices,11–16 there has been no comprehensive study on the nanostructured thermoelectric devices using sophisticated experimental characterization and theoretical modeling, and the thermal stress issue has been overlooked in previous reports. Herein, we report the fabrication, characterization, and multiphysics modeling of nanostructured bulk half-Heusler devices. The device electrical contact resistance, open circuit voltage, power output and efficiency are experimentally measured using custom designed testing system. The device thermal and electrical characteristics and thermal stress performances were simulated using a three-dimensional finite element device model. The device delivers an ultrahigh power density of 8.6 W/cm2 in excellent agreement with the theoretical predictions. Furthermore, the device using a novel metal-ceramic composite electrode demonstrates 40% thermal stress reduction compared with the device with pure metal electrodes.

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2 Experimental and modeling methods 2.1 Material Properties The n- and p- type half-Heusler material compositions used in the device fabrication are Hf0.25Zr0.75NiSn0.99Sb0.01 and Nb0.75Ti0.25FeSb, respectively. The half-Heusler material is prepared by arc melting elements of desired composition into an ingot. This ingot is ball milled into nanopowder, which is then pressed to form a disk using spark plasma sintering. The detailed material fabrication process for both n- and p- type materials is reported in literature.11, 17 The ntype material reaches a peak ZT of ~1 at about 500 °C and the p- type material reaches a peak ZT of ~1 in the vicinity of 700 °C. 2.2 Device Fabrication The bulk n- and p-type half-Heusler disks were polished and then diced into 1.5 x 1.5 x 2.4 mm3 legs. A unique copper-alumina composite structure consisting of an 0.64 mm thick alumina layer sandwiched between two 0.2 mm thick copper layers and bonded using a high temperature fusing technology was used as the metal electrode. The n- and p-type half-Heusler legs were centered and stacked between the metal electrodes. A 50 µm thick braze foil consisting of indium, copper, and silver was used to join the half-Heusler and copper in a vacuum furnace. A custom brazing fixture was applied to maintain certain pressure during vacuum brazing. Figure 1 shows an image of the polished half-Heusler disk (Figure 1a), diced legs (Figure 1b), and assembled device before brazing (Figure 1c) and after brazing (Figure 1d).

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Figure 1. TE device fabrication procedure (a) polished nanostructured half-Heusler disk (b) diced n-type and p-type half-Heusler legs (c) TE uncouple device before brazing (d) TE uncouple device after brazing. 2.3 Device Testing Methods A custom testing system was built to characterize the temperature, heat flow, power output and heat-to-power efficiency of the TE device. Figure 2 shows an image of the testing apparatus with the radiation shield and thermocouples removed for clarity. The device hot side consists of a custom machined copper heater block with a diameter of 10 mm. A cartridge heater was embedded within the block, and a radiation shield was attached surrounding the heater block to minimize radiation losses. The cold side of the device includes a nickel heat flow meter of 12 mm diameter, which is attached to an aluminum liquid cold plate. A series of thermocouples were embedded inside the heater block and the heat flow meter to measure the temperature and determine the heat flow. Graphite foils were used as thermal interface materials between all of the interfaces on the hot and cold side of the device, and ceramic spacers were used to thermally isolate the hot block from the rest of the apparatus. The entire apparatus is enclosed in a vacuum environment capable of reaching pressure level of 1 mTorr. During testing, the cold side of the TE device was maintained at 30°C while the hot side was increased from 200-600°C with 100°C increments. Once the system reached steady state for each hot-side temperature, the TEG peak power output was obtained by varying an external load resistance to match the internal TEG device resistance.

Figure 2. Custom built TE device measurement system.

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2.4 Modeling method The temperature distribution, electrical power output, and thermal deformation and stress of the TE device were simulated using a three-dimensional multi-physics model in ANSYS Finite Element Analysis (FEA) software.18,19 The temperature-dependent material properties for all the components in the device were used as input for the model.11, 17

Figure 3. ANSYS simulation setup (a) boundary conditions used (b) mesh used in simulation. Figure 3 shows the boundary conditions applied to the TE device (Figure 3a) and the mesh used in ANSYS (Figure 3b). Temperature conditions for the hot and cold side of the device were applied to the top and bottom of the uncouple respectively. The zero-voltage condition was applied to the end of the copper on the n- type leg. Electrical contact resistance of each joint in the device was measured and used as the input into the model. An external resistor with variable resistances was modeled using the ANSYS Parametric Design Language to vary the load and find the peak power and heat-to-power efficiency of the device. 3 Results and Discussion Figure 4 shows the electrical resistances scanned across the copper contacts and the n- and ptype materials using a custom probe station. The position variable shown in Figure 4c represents the resistance scan across the overall TE device height starting from the bottom copper layer (0 to 0.2 mm), to the TE leg (0.2 to 2.6 mm), and finally to the top copper layer (2.6 to 2.8 mm). 6


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The actual contact resistance value was calculated by taking the difference in resistance between the TE leg and the copper contact. The average contact resistivity for the n-leg and p-leg are 3.1 µΩcm² and 3.6 µΩcm² respectively, which yield a relatively low contact resistance compared with the resistances of the TE legs.

Figure 4. Electrical resistance scans across (a) p-type (b) n-type legs and ajacent electrodes, and (c) the schematic of measument diagram. The experimental and simulation device voltage and power output of the nanostructured halfHeusler device versus electrical current for each temperature step is shown in Figure 5. As shown in Figure 5b, a peak power is obtained when the external load resistance matches the uncouple electrical resistance. The experimental results match almost exactly with ANSYS simulation when the hot-side temperature is below 300°C. The experimental results drop slightly below simulation below 300°C because the contact resistances increase with temperature which is assumed to be a constant in simulation.

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Figure 5. Experimental and simulation (a) device voltage (b) device power output vs. electric current. The continious curve in the graph is the results from ANSYS simulation.

Figure 6. (a) Measured temperature profile in the heat flow meter at different locations (b) experimental and simulation heat flow vs. device hot-side temperature, and (c) schematic of heat flow measurement in the TE device measurement system. Figure 6 shows the temperature profile, and the heat flow measured using the heat flow meter located on the cold side of the TE device. The experimental heat flow is slightly higher than simulation and their differences increase with temperature due to the increasing radiation heat loss from the heater to the cold side of the device and heat flow meter which is not captured in the FEA model.

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Figure 7. Simulation and experimental results for the TE unicouple: (a) electrical resistance, (b) open circuit voltage, (c) peak power density and (d) peak efficicncy vs. hot side temperature.

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Figure 7 shows the experimental and simulation results for the electrical resistance, open circuit voltage, peak power density and peak efficiency of the TE device. As shown in Figure 7a, the experimentally measured electrical resistance is slightly higher than simulation value because the contact resistance Rc increases with temperature whereas the Rc was only measured at room temperature and used as a constant in simulation. The experimental open circuit voltage shown in Figure 7b increases from 39.7 mV at a ΔT of 170°C to 154.9 mV at a ΔT of 570 °C which is within 2% of the simulation. The peak power density is calculated by P/(An+Ap), where P is the peak output power of the device and An, Ap are the corresponding cross-sectional areas for the nand p- type legs respectively. The heat-to-power efficiency was calculated by η=P/(P+Q), where Q is the measured heat flow at the cold side of the device. The measured peak power density of the device reaches 8.6 W/cm2 at a ΔT of 570 °C, which is within 2% of the simulation result. The measured efficiency at a ΔT of 570 °C is 6.2%, about 10% lower than the simulation due to

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radiation heat loss from the heater to the heat flow meter which increases the measured heat flow Q.

Figure 6. Simulation results with the hot-side and cold-side temperatures of the device at 600°C and 30°C (a) temperature gradient (b) load voltage (c) current density (d) joule heating (e) heat flux (f) device figure of merit ZT. Figure 6 shows the solved FEA contours for the TE device when the hot and cold sides were maintained at 600°C and 30°C respectively. Figure 6a show the temperature gradient along the entire uncouple. From the simulation it is clear only a ~520°C temperature difference occurs across the TE leg when temperature difference across the entire device is 570°C, and near 50°C temperature drop occurs across the top and bottom alumina layers. Figure 6b shows the voltage gradient across the device when the load resistance matches the internal resistance of the device. Figure 6c and Figure 6d show the current density and joule heating effects across the TE device. The ZT distribution as a function of position for both p- and n- legs is shown in Figure 6f, indicating a significantly lower average ZT than the peak ZT during device operation.

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Figure 7. Equivalent von-Mises stress (top 3 figures) and shear stress (bottom 3 figures) at the TE leg hot-side interface for three electrode configurations. avg and max are the average and maximum nodal stresses on the leg hot side. Finite element simulations were performed in ANSYS to study the thermal stress performance of the uncouple with different electrode configurations. The three configurations studied include the use of double copper on alumina (DCA), single copper on alumina (SCA), and copper only. The hot-side and cold-side temperatures on the device were set to 600°C and 30°C, respectively. Temperature-dependent stress-strain curves were used for the copper to accurately capture plastic deformation and stresses. The TE legs and ceramic were modeled as linear elastic components. All contacts between components were set to bond with adjacent layers. For this study both the equivalent von-Mises stress and the shear stress are used for comparison since most TE devices failures occur when these stresses exceed the critical bonding strength at the metal-TE interfaces.20–26 The hot side of the TE leg typically experiences the largest thermal strains and the bonding is most susceptible to failure due to the weakened bonding strength at high temperatures. Figure 7 shows the equivalent von-Mises stress and the shear stress at the TE leg hot-side surface for the three electrode configurations. The copper only electrode has the highest thermal stress due to a considerable mismatch of the coefficient of thermal expansion 12


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(CTE) between the copper (16.710-6 /K) and the half-Heusler TE materials (10-1210-6 /K). The CTE of alumina is 5.410-6 /K, and thus the copper and alumina combination yields an effective CTE at the copper interface that closely matches the CTE of the TE materials, resulting in significantly reduced thermal stresses. The DCA configuration shows the minimum average and max stress at the TE leg hot side for both stress criteria. By having copper on both sides of the alumina, the entire DBC package can expand more uniformly with minimum bending due to balanced forces on the two sides of the alumina. Our novel electrode design using metal-ceramic composite to manage thermal stresses can be applied in TE device fabrication based on a wide range of materials besides the half-Heusler materials. The concept of effective CTE match can be applied to design a composite electrode to match the CTE of any TE material as long as the TE material’s CTE is in between the chosen metal and ceramic combination. Furthermore, the composite electrode design approach can be adapted for broad applications beyond thermoelectric devices where thermal stress management is of great importance. 4 Conclusions A comprehensive experimental and modeling study was performed to investigate all the critical performance criteria of a thermoelectric generator device. Using a novel metal and ceramic composite structure, a reliable metal and TE materials contact is realized through a hightemperature brazing process, which not only yield a sufficiently low electrical contact resistance but also a significant reduction in the thermal stresses compared with conventional electrode design. The nanostructured half-Heusler materials produces an ultrahigh power density of 8.6 W/cm2 and a moderate efficiency of 6.2% due to the high power factor and moderate ZT. The finite element modeling results show an excellent agreement with the experiments by considering the electrical contact resistances. Future work will focus on improving the efficiency of these devices by improving the average ZT values across the entire device working temperature region, and demonstrating these high-performance devices for broad waste heat recovery and energy harvesting applications.

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5 ACKNOWLEDGMENTS This work is funded by the US Department of Energy, Office of Nuclear Energy, under Award number DE-NE0008255. 6 REFERENCES (1)

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Power Generation from Nanostructured Half-Heusler Thermoelectrics

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