A stacked and miniaturized radioisotope thermoelectric generator by screen printing

Accepted Manuscript Title: A stacked and miniaturized radioisotope thermoelectric generator by screen printing Authors: Zicheng Yuan, Xiaobin Tang, Yunpeng Liu, Zhiheng Xu, Kai Liu, Zhengrong Zhang, Wang Chen, Junqin Li PII: DOI: Reference:

S0924-4247(17)30386-2 https://doi.org/10.1016/j.sna.2017.10.055 SNA 10421

To appear in:

Sensors and Actuators A

Received date: Revised date: Accepted date:

22-3-2017 15-9-2017 22-10-2017

Please cite this article as: Zicheng Yuan, Xiaobin Tang, Yunpeng Liu, Zhiheng Xu, Kai Liu, Zhengrong Zhang, Wang Chen, Junqin Li, A stacked and miniaturized radioisotope thermoelectric generator by screen printing, Sensors and Actuators: A Physical https://doi.org/10.1016/j.sna.2017.10.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A stacked and miniaturized radioisotope thermoelectric generator by screen printing Zicheng Yuan1, Xiaobin Tang1,2,*, Yunpeng Liu1,2, Zhiheng Xu1, Kai Liu1, Zhengrong Zhang1, Wang Chen1, Junqin Li1 1 Department of Nuclear Science and Engineering, Nanjing University of Aeronautics and Astronautics, 29 General Road, Jiangning District, Nanjing, China 2 Jiangsu Key Laboratory of Material and Technology for Energy Conversion, Nanjing, China

Highlights  Proposed a miniaturized radioisotope thermoelectric generator (RTG) with a stacked and radial heat conduction structure.  Used electric heating as an equivalent radioactive isotope heat source.  Fabricated (Bi, Sb)2(Te, Se)3-based thermoelectric modules by screen printing with Isc of 3.5 uA, Voc of 15 mV, and Pmax of 12 nW.  Theoretically predicted the power density for 100-layer modules is 0.51 mW·cm−3. Abstract A radioisotope thermoelectric generator based on (Bi, Sb)2(Te, Se)3 thermoelectric material was designed as a miniature long-life power supply for low-power devices. In the finite element method simulation, the maximum hot-side temperature is approximately 400 K, and the voltage could reach 0.3 V for one single-layer module at 0.5 W heat source. Thermocouples films were prepared by screen printing. And one single-layer module, which was composed of five thermocouples, produces a power of 12 nW at 7.0 mV in the test. It is predicted that 100-layer modules would generate 2.31 mW at 2.34 V, and the maximum power density and maximum conversion efficiency are 0.51 mW·cm−3 and 0.46%, respectively. Keywords: miniaturized radioisotope thermoelectric generator, (Bi, Sb)2(Te, Se)3, thermocouples, stacked structure, screen printing 1. Introduction Radioactive isotope battery is a device that can convert radioactive decay particle energy into electrical energy. Since the 1950s, this battery has made great progress in various structures as well as in the development of different mechanisms. According to energy conversion mechanism, the radioactive isotope battery can be categorized into radiation heat conversion and radiation particle conversion. Radiation particle conversion, such as radiovoltaic [1–3] and radioluminescence [4,5], has been studied in recent years for the power supply of microelectromechanical system (MEMS). Radioisotope thermoelectric generator, called RTG, can provide high electric output power. RTG has no moving parts, as it uses thermoelectric couples to convert heat to electricity, and it can provide long-term stable power output for low-power sensing devices in remote areas [6]. Micro-radioisotope thermoelectric generators, for example, the energy supply for pacemakers [7] and the milliwatt radioisotope power supply [8] have hundreds of thermocouples array. A two-stage cascaded small radioisotope power source was designed for space applications and scientific missions [9]. Milliwatt-power RTG had 484 thermoelectric legs with large ratio of length to cross-sectional area [10]. Similar cases have a one-way heat flow structure, which is not sufficiently compact. We should change the thermoelectric legs and 1

heat conduction structure to improve power density. Whalen, et al in 2008 implemented the design of the wheel spoke thermoelectric module around the columnar heat source [11]. The author indicated the possibility of achieving a large output power and voltage by stacking with a thin thermoelectric material in a limited size, less than 25 μm [12]. Whalen used 11 pairs of 215 μm thick Bi2Te3 in the design of the wheel spoke thermocouple and optimized the thermopile by quartering. To obtain required output power, thin film device was made by magnetron sputtering [13]. Physical cutting methods are no longer applicable to thinner requirements in fabricating devices for radial structures; the molding methods of the thermoelectric material of thick films should be reconsidered. The current micron-scale thick film process of low-temperature thermoelectric devices has the following categories [14]: magnetron sputtering, inkjet printing, dispenser printing, screen printing, and stereolithography process and painting [14–18]. Screen printing has pattern accuracy control, low cost, and other advantages. Large number of thermocouples can be manufactured rapidly by screen printing, which is regarded as a developed and appropriate program. A miniaturized RTG with a multi-directional heat flow structure, that is a radial structure, was designed in this study. According to the binder and method of Cao, et al in 2016 [19,20], thermoelectric legs were produced by screen printing. 2. Model establishment 2.1. Model designing As shown in Fig. 1a, a stacked and miniaturized radioisotope thermoelectric generator was designed to a cylindrical box, which included thermocouples, polyimide film substrate, thermal insulator, radioisotope heat source, and a shell. The radioisotope heat source was also designed to be a cylinder in the center of the RTG. The stacked thermoelectric modules, which contains many single-layer modules, were located around the heat source in Fig. 1b. One single-layer module was made up of several thermocouples, and one thermocouple included a n-type leg and a p-type leg. This radial distribution structure makes thermal energy to flux in a radial pattern from the center of the heat source. Thermal flux flows through the thermocouples from the inside to the outside. With the design of the thermocouples and the heat source, the optimal controlling over the hot-side and cold-side temperatures can be realized. In addition, although the thin Bi–Sb–Te–Se alloy performs well for the RTG in the low-temperature case, it has the disadvantage of brittle property. To avoid this problem, we placed it on a flexible polyimide film. After the heat source becomes stable, it will provide electricity (Fig. 2). The red and blue represent the n-type and n-type thermoelectric semiconductors, respectively. The hole and electron carriers spread from the hot end to the cold end. Potential vector is present from the inside to the outside in the p-type, and that is opposite in the n-type. The device has several linked thermocouples, and it produces electrical potential difference when heat source was loading. To provide an acceptable performance, a large number of thermocouples are connected in series in Fig. 1b. Low potential n-type of one layer and the high potential p-type of another layer are linked together by the electrode in Fig. 1c. The top view on the left reflects the carrier-transporting, electric current, and electric potential with a central heat source; the upper right is a cross-section of junction P–N; and the lower right is a cross-section of the layer connection of the two modules. 2.2. The simulation of temperature distribution and electrical potential distribution of 2

thermocouples The temperature difference is a parameter that directly affects the performance of the thermoelectric generator. The heat transfer process and thermoelectric effect of a finite element analysis software COMSOL was used to simulate the temperature and potential distribution of the thermocouple [21] in order to roughly estimate the device size. The heat source power ranges from 0.1 W to 1 W under simulation boundary conditions. The substrate material is polyimide film and its thermal conductivity is 0.6 W·m−1·K−1 at room temperature. The one single-layer thermoelectric module has ten alloy legs, that mean five thermocouples, in a series connection. The length of thermoelectric leg is 13.5mm. The properties of thermoelectric materials are shown in Table 1. The value of heat transfer coefficient of the ambient air natural convection (h) ranges from 5 to 25 W m−2·K−1. The initial RTG temperature and ambient temperature were both set to 293 K. For the different values of h and heat power, simulation results show that the hot-end temperatures are approximately 310–410 K, and the voltages are approximately 0.03–0.3 V. We studied heat source power of 0.5 W and heat transfer coefficient of 10 W·m−2·K−1 as typical values for experimental size optimization.In Fig. 3, we observe that the temperature of the thermocouple in the leg length of 5 mm quickly reduces and then tends to room temperature. Different heat source power and air heat transfer coefficient have a similar distribution. The potential of an independent module increases in a clockwise direction with five thermocouples in series. This simulation results roughly indicate that the length of thermoelectric legs should be set to about 5 mm, and the output voltage of the thermocouples can be enhanced through series. 3. Optimization of thermocouple parameters 3.1. Equivalent circuit model The feasibility of the design is verified in the previous section. The load capacity of the series circuit is presented, and its equivalent circuit is provided in Fig. 4. We use equivalent circuit to discuss the number and size of thermocouples. Thus, a set of design parameters for optimizing power output is produced. where 𝑉L is the load voltage, 𝐸 is the Seebeck voltage of the RTG, 𝑅L is the load resistance, 𝑟 is the internal resistance, 𝛼np is the seebeck coefficient for the thermocouple, 𝛥𝑇 is the temperature difference, 𝑛 is the number of thermocouples, and 𝑃max is the maximum output power. 𝐸 L +𝑟

𝑉L = 𝑅 𝑃=

𝑉L 2 𝑅L

× 𝑅L =

=

𝑛×𝛼np ×𝛥𝑇 1+𝑟/𝑅L

(𝑛×𝛼np ×𝛥𝑇)2 𝑅L 𝑅L2 +2𝑟𝑅L +𝑟 2

𝑅L = 𝑟 𝑃max =

(𝑛×𝛼np ×𝛥𝑇)2 4𝑟

(1) (2) (3) (4)

The load voltage VL and the power output can be solved using Eqs. (1) and (2). According to Eqs. (3) and (4), when the load resistance is equal to the thermocouple equivalent resistance, the maximum electric power 𝑃max dissipated in the load resistor is obtained. Keeping the internal resistance as low as possible in a limited size is necessary due to the negative correlation between 𝑃max and 𝑟. 3.2. Thermocouple model 3

The area proportion of the thermoelectric material remains the same (300/360). Then it was split into one to five pairs to change the number of the thermocouples as shown in Fig. 5. According to the results of the previous section, we chose a few lengths of thermoelectric legs in the vicinity of 5 mm. The five different radius sizes include 13.5, 10.5, 7.5, 4.5, and 1.5 mm, including 25 cases in total. All thermoelectric modules have the same thickness of 25 μm, and the diameter of the center of the heat source is always 3 mm. Other material properties are the same as those in the energy conversion simulation. To obtain resistance-related data, we applied the 1 A constant-current source at the end of the device, and we made the grounding on its other end. 𝜃=

𝑛2 𝑟

(5)

𝑃max ∝ 𝜃 (6) 𝜃 is defined as performance factor in this paper by ignoring 𝛥𝑇 and 𝛼np from Eq. (4). 𝑃max is proportional to the square of the number of thermocouples and is inversely proportional to the internal resistance of the device power supply. A positive correlation function relationship exists between n and 𝑅 .Fig. 6 describes the series resistance, average resistance, and performance factor, respectively; the color represents different thermoelectric leg length L. Eq. (6) indicates the number of thermocouples n and r is important to 𝑃max . For the uncertainty function relation between n and 𝑅, we made five cases, including 1–5 thermocouples. The increase in the number of thermocouples to average resistance in a large size (13.5 mm) is unfavorable; however, data show the opposite of this effect with a decrease in thermoelectric leg size. 𝛥𝑇 and 𝛼np are regarded as constants when the number of thermocouples is invariant. According to the change trend of 𝜃 in Fig. 6, an optimal solution of the size of thermocouples (thermoelectric leg length) exists. For the case list, the 4.5mm leg is the optimal design. The size of the electrodes in the model is set to be the same. When the length of the thermoelectric leg is close to the length of the electrode cover, the coverage area will result in a decrease in the effective length of the resistance. Because the size of 1.5mm is too small and the electrode cover is too large, the electrode coverage area has to be reduced, which makes the resistance effective length much larger. In practice, it is not convenient to connect the two electrodes when the size range is less than 4.5 mm leg, which cannot guarantee manufacturing accuracy. The appropriate size of thermoelectric leg length for 𝑃max ranges from 1.5 to 7.5 mm. Fig. 6c shows the trend that we should also add more thermocouples to improve performance. Because we consider the feasibility of the experimental process, the number of thermocouples has not increased. 4. Fabrication In this work, we referred to the optimization results of the previous section. The length of thermoelectric leg was 4.5 mm, and the diameter of center hole was 3 mm. Screen printing is used to shape the thick films for fabricating the thermoelectric conversion of the device. The screen mesh aperture is 150 mesh. Firstly, The p-type, n-type, and electrode are made into three kinds of screen printing plates. Subsequently, thermoelectric paste was prepared for printing on polyimide film substrate. Finally, the thermoelectric paste is annealed and fastened onto the substrate. 4.1. Thermoelectric paste synthesis The Bi0.5Sb1.5Te3 and Bi2Se0.3Te2.7 powders (Chengdu Alfa Metal Materials Co., Ltd) were 4

used for p-type and n-type thermoelectric materials, respectively, with a particle size determined by a 325 mesh (typically ≤ 10 μm) and a purity of 99.99%. The epoxy polypropylene glycol diglycidyl ether-based epoxy DER 732 (Dow Chemical) was used to adhere the thermoelectric legs to the polyimide film substrate. The paste consists of 85 wt% of alloy Bi–Sb–Te–Se powders and 15% of epoxy binder. The planetary mixer was used to mix the powder and solvent cement uniformly. The turbid liquid system was mixed at 1000 rpm for 3 min, paused for a minute, and then mixed at 1800 rpm for 3 min. 4.2. Formation of thermoelectric thick film using screen printing The p-type, n-type, and electrode were printed on a transparent medium plate successively. Then, the polyimide film substrate was fixed on the plate with preprinted pattern, which was applied to visual alignment before the thermoelectric legs were printed. Next, the thermoelectric legs were formed by drying the printed p-type, n-type, and electrode on the polyimide film substrate at 120 °C for 10 min successively. Samples were cut into round slices and a hole was punched at the center. Then, samples were annealed at 400 °C for 30 min in N2 atmosphere tube furnace to prevent oxidation. The size of samples shows in Fig. 7. 5. Electrical measurement and performance estimation The heat source rod (H = 15 mm, D = 3 mm, V = 0.106 cm3) was loaded vertically into the hole, which was at the center of samples. The lower part of the heat source bar was covered with a foamed plastic to reduce heat loss. A potential risk existed due to the large activity of radioisotopes in the current use of the resistive joule thermal simulation of radioactive fuel pellets for experimental research. 5.2. Measuring instruments and methods The output voltage was measured by a Keithley 4200 equipment. The temperatures of each point were measured by a multi-channel temperature inspection instrument with Pt-100 thermal resistor temperature sensor (Fig. 8). In the process of gradual stabilization of the temperature field, the output of the stable process was obtained. The temperature sensing probe measured the temperatures of air, hot end, and cold end in real time. The heat source rods at both ends of the DC power supply with a load power of 0.5 W was set as the value of equivalent thermal power. According to 238PuO2 (0.491 W·g−1, 5.62 W·cm−3) [22], 0.5 W was equivalent to 15 Ci purity of 85% 238PuO2 fuel pellets. 5.3. Measuring results In the first few minutes of the test, the temperature distribution of the device is a changing process. We measured nine sets of data from T1 to T9 over time (Fig. 9). The T1 to T6 upper layers were not covered with glass wool insulation material, whereas the T7 to T9 were all covered by aluminium silicate wool for thermal insulation (Fig. 9b). The hot-end temperature increased fast at the beginning. he measured open-circuit voltage was higher in the beginning, after 20 min voltage and temperature difference tend to be stable. The hot and cold heat flow at both ends had not yet stabilized; thus, the air convection heat transfer dominated the heat flow dispersion of the surface temperature of the device. The ambient temperature was 288 K. The cold end stabilized at 293 K. The hot end stabilized at 303 K. The temperature difference was maintained at 10 K. Fluctuation of the temperature difference is less than 4K. The device was coated with aluminum silicate wool, with cold and hot ends stabilizing at 300 K and 312 K, respectively. The device was less prone to temperature fluctuations; thus, it provided a more stable open-circuit voltage and maximum output power. The maximum output power is 12 nW 5

at 7.0 mV when 𝛥𝑇 was 12 K at this time. The convection heat transfer of air and the conduction heat transfer of polyimide accounts for heat losses. We plan to print thermal insulation material (nano aerogel) around the thermal leg to prevent convection heat transfer, thereby effectively reducing the radial heat loss. At present, device is covered by aluminium silicate wool for thermal insulation in the experiment. At room temperature, a single thermoelectric leg was measured. The of resistance the single p-type was approximately 40–150 Ω, whereas that of the n-type was approximately 350– 850 Ω; the five pairs of thermocouple had a total resistance of 2–4 kΩ. The arc path produced additional resistance in terms of design. After loading the heat source, the temperature effect increased the total resistance by approximately 200 Ω. The average value of the seebeck coefficient reached 150 μV·K−1, which is comparable with that of the previous reports that applied the same method. This device generated load current was at the μA level, and the load power was at the nW level. 5.4. Performance evaluation of multi-layer modules RTG Based on the previous study, the performance of the multi-layer modules with the similar experimental conditions were theoretically predicted. The numerical prediction of multi-layer performance was calculated by finite element analysis software COMSOL. The seebeck coefficient, resistivity and thermal conductivity were obtained from Cao's works [20] in Table 1. The size of thermoelectric legs is 4.5 mm, heat source power is 0.5 W and heat transfer coefficient is 10 W·m−2·K−1. The specific parameters are calculated by formula (7) to (10). The multi-module series of numerical prediction was proposed in Table 3. 𝑉oc = 𝑁 × 𝐸 (7) 𝑃max =

𝜂=

𝑉oc 2 4𝑟

(8)

𝑑=

𝑃max 𝑉

(9)

𝑃ℎ𝑒𝑎𝑡 𝑃max

× 100%

(10)

A total of 50 or 100 layers was connected after the complete stacking of layers, and the overall volume (H = 25 mm, D = 15 mm) was V = 4.5 cm3. We calculated that the resistance was 23.6 Ω per layer with thermoelectric legs of 4.5 mm. The open circuit voltage of device is E = 46.7 mV with heat source power of 0.5 W and heat transfer coefficient of 10 W·m−2·K−1. As parasitic electrical losses stack up, thermal efficiency will somewhat improve, so knowing the relationship between number of discs and power output is not trivial. There is also the impedance matching when the power is in series. The internal resistance and load resistance values were equal to the size of the existence of the theoretical maximum output power. When the load changed within the reference range, the load capacity of the device could be measured from the graph of load resistance and load voltage–output power curve (Fig. 10). In experiments, it should be further decreased resistance by improving thermoelectric paste and annealing process to enhance output power. 6. Summary In this study, a stacked and miniaturized radioisotope thermoelectric generator was proposed. The stacked thick film thermocouples device was designed to provide large DC voltage without post-booster circuits while providing miniaturized features. If the stacked 6

thermocouples modules were implemented, the RTG could be applied to driving low-power sensing devices. We designed new RTG and manufactured a thick film module around the heat source. The simulation of the physical model by finite element analysis software COMSOL was conducted to verify the feasibility of the energy conversion process and roughly estimate the device size. This simulation results roughly indicate that the length of thermoelectric legs should be set to about 5 mm with 0.5 W heat source. The Pout values that were calculated omitted, because it is too far from the experimental data. The material is idealized, and the original purpose of modeling is to verify the feasibility and physical distribution of the scheme. The difference between the material in the fabrication process and the ideal setting value is mainly in electrical and thermal conductivity and seebeck coefficient. The influence factors of the biggest is the electrical conductivity. In order to optimize maximum output power, we used equivalent circuit to discuss the number and size of thermocouples. Thick film thermocouples around the center heat source were prepared by screen printing. A single-layer assembly contained 10 fan-shaped thermoelectric legs, that means 5 pairs of thermocouples, and a simulated heat source with 0.5 W. After the system becomes stable, one single-layer module generates open circuit voltage of 15 mV, short circuit current of 3.5 μA, and maximum output power of 12.6 nW at 7.0 mV. According to the experimental data in the literature, we calculated the ideal performances of the multi-layer modules RTG with the help of COMSOL in theory. One single-layer module obtains the maximum output power of 23.13 μW at 23.37 mV; 50layer coupled modules in series yield 1.16 mW at 1.17 V; and 100-layer coupled modules in series generate 2.31 mW at 2.34 V. For 100-layer, the maximum power density is 0.51 mW·cm−3 and the maximum energy conversion efficiency is 0.46%. The high thermocouple resistivity meanly contributes to the low measured performances. Much works will be done to improve the preparation technology in the future. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11675076, 11505096); the National Defense Basic Scientific Research Project (Grant No. JCKY2016605C006); the Jiangsu Planned Projects for Postdoctoral Research Funds (Grant No. 1601139B); the Funding of Jiangsu Innovation Program for Graduate Education (Grant No. KYLX16_0355); and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] Y. Chang, C. Chen, P. Liu, J. Zhang, A betavoltaic microcell based on Au/s-SWCNTs/Ti Schottky junction, Sensors Actuators, A Phys. 215 (2014) 17–21. doi:10.1016/j.sna.2013.08.015. [2] K. Zhang, G. Gui, P. Pathak, J.H. Seo, J.P. Blanchard, Z. Ma, Quantitative modeling of betavoltaic microbattery performance, Sensors Actuators, A Phys. 240 (2016) 131–137. doi:10.1016/j.sna.2016.01.028. [3] Y. peng Liu, X. bin Tang, Z. heng Xu, L. Hong, H. Wang, M. Liu, D. Chen, Influences of planar source thickness on betavoltaics with different semiconductors, J. Radioanal. Nucl. Chem. 304 (2015) 517–525. doi:10.1007/s10967-014-3879-2. [4] R. Walton, C. Anthony, M. Ward, N. Metje, D.N. Chapman, Radioisotopic battery and capacitor system for powering Wireless Sensor Networks, Sensors Actuators, A Phys. 203 (2013) 405–412. doi:10.1016/j.sna.2013.09.010. 7

[5] X. Tang, Z. Xu, Y. Liu, M. Liu, H. Wang, D. Chen, Physical Parameters of Phosphor Layers and their Effects on the Device Properties of Beta-radioluminescent Nuclear Batteries, Energy Technol. 3 (2015) 1121–1129. doi:10.1002/ente.201500268. [6] D. Woerner, A Progress Report on the eMMRTG, J. Electron. Mater. 45 (2016) 1278–1283. doi:10.1007/s11664-015-3998-8. [7] A.R. Kahn, J.D. Hixson, J.E. Puffer, E.E. Bakken, Three-years’ clinical experience with radioisotope powered cardiac pacemakers., IEEE Trans. Biomed. Eng. 20 (1973) 326–31. doi:10.1109/TBME.1973.324283. [8] D.T. Allen, N.D. Hiller, J.C. Bass, N.B. Elsner, Fabrication and Testing of Thermoelectric Modules and Milliwatt Power Supplies, Sp. Technol. (2004) 521–528. doi:10.1063/1.1649613. [9] A. Lieberman, A. Leanna, M. McAlonan, B. Heshmatpour, Small thermoelectric radioisotope power sources, AIP Conf. Proc. 880 (2007) 347–354. doi:10.1063/1.2437473. [10] V. V. Gusev, A.A. Pustovalov, N.N. Rybkin, L.I. Anatychuk, B.N. Demchuk, I.Y. Ludchak, Milliwatt-power radioisotope thermoelectric generator (RTG) based on plutonium-238, J. Electron. Mater. 40 (2011) 807–811. doi:10.1007/s11664-011-1579-z. [11] S.A. Whalen, C.A. Apblett, T.L. Aselage, Improving power density and efficiency of miniature radioisotopic thermoelectric generators, J. Power Sources. 180 (2008) 657–663. doi:10.1016/j.jpowsour.2008.01.080. [12] V. Raag, Proceedings of the 11th Intersociety Energy Conversion Engineering Conference, vol. 2, State Line, NV, September 1976, pp. 1586–1590 [13] C. a Apblett, S. a Whalen, M.W. Moorman, T.L. Aselage, M.P. Siegal, S.K. Frederick, A Miniaturized mW Thermoelectric Generator For NW Objectives : Continuous , Autonomous , Reliable Power For Decades, Online. (2006). doi:10.2172/899375 [14] M. Orrill, S. LeBlanc, Printed thermoelectric materials and devices: Fabrication techniques, advantages, and challenges, J. Appl. Polym. Sci. 134 (2017). doi:10.1002/app.44256. [15] S.H. Park, S. Jo, B. Kwon, F. Kim, H.W. Ban, J.E. Lee, D.H. Gu, S.H. Lee, Y. Hwang, J.-S. Kim, D.-B. Hyun, S. Lee, K.J. Choi, W. Jo, J.S. Son, High-performance shape-engineerable thermoelectric painting., Nat. Commun. 7 (2016) 13403. doi:10.1038/ncomms13403. [16] S.J. Kim, J.H. We, B.J. Cho, A wearable thermoelectric generator fabricated on a glass fabric, Energy Environ. Sci. 7 (2014) 1959. doi:10.1039/c4ee00242c. [17] J. Weber, K. Potje-Kamloth, F. Haase, P. Detemple, F. Völklein, T. Doll, Coin-size coiled-up polymer foil thermoelectric power generator for wearable electronics, Sensors Actuators, A Phys. 132 (2006) 325–330. doi:10.1016/j.sna.2006.04.054. [18] C.-C. Hsiao, Y.-S. Wu, Fabrication of flexible thin-film thermoelectric generators, J. Chinese Inst. Eng. 34 (2011) 809–816. doi:10.1080/02533839.2011.591552. [19] Z. Cao, E. Koukharenko, M.J. Tudor, R.N. Torah, S.P. Beeby, Flexible screen printed thermoelectric generator with enhanced processes and materials, Sensors Actuators, A Phys. 238 (2016) 196–206. doi:10.1016/j.sna.2015.12.016. [20] Z. Cao, M.J. Tudor, R.N. Torah, S.P. Beeby, Screen Printable Flexible BiTe-SbTe-Based Composite Thermoelectric Materials on Textiles for Wearable Applications, IEEE Trans. Electron Devices. PP (2016). doi:10.1109/TED.2016.2603071.

8

[21] A. Khajepour, F. Rahmani, An approach to design a 90Sr radioisotope thermoelectric generator using analytical and Monte Carlo methods with ANSYS, COMSOL, and MCNP, Appl. Radiat. Isot. 119 (2017) 51–59. doi:10.1016/j.apradiso.2016.11.001. [22] R.C. O’Brien, R.M. Ambrosi, N.P. Bannister, S.D. Howe, H. V. Atkinson, Safe radioisotope thermoelectric generators and heat sources for space applications, J. Nucl. Mater. 377 (2008) 506–521. doi:10.1016/j.jnucmat.2008.04.009.

Biographies Zicheng Yuan received the BEng degree in radiation protection and environmental engineering from Chengdu University of Technology, in 2015. He is researching his PhD in nuclear technology and materials engineering at Department of Nuclear Science and Engineering at Nanjing University of Aeronautics and Astronautics. His research interests are currently focused on radiation thermoelectric conversion device design, thermophotovoltaic converter design for radioisotope power systems.

Xiaobin Tang received his BEng degree in materials science and engineering from Nanjing University of Aeronautics and Astronautics, in 2000. He received his PhD degree in measuring and testing technologies and instruments from the same university, in 2009. He is now a professor in Department of Nuclear Science and Engineering at Nanjing University of Aeronautics and Astronautics. He is China Nuclear Society Radiation Physics Branch; Jiangsu Provincial Biomedical Engineering Society of medical physics professional committee deputy secretary-general, standing committee. He presided over the national level, provincial and ministerial level and other research projects more than 30 items. Tang has published over 60 journal papers and 20 patents (11 authorized) in nuclear technology and other disciplines in the field of research work, such as nuclear materials, nuclear medicine, nuclear instrument, etc. Tang’ research interests include radiation energy conversion mechanism and nuclear battery technology, material irradiation effect and new materials for nuclear use, new technique and dose effect of radiation therapy, radiation detection imaging and nuclear instrument development, and space radiation physics and nuclear technology application. Yunpeng Liu received his BSc degree in applied physics from Nanjing University of Aeronautics and Astronautics, in 2009. He received his PhD degree in nuclear technology and materials engineering from the same university, in 2014. He is currently working at Department of Nuclear Science and Engineering at Nanjing University of Aeronautics and Astronautics as a lecturer. He presided research projects supported by the National Natural Science Foundation of China, the Natural Science Foundation of Jiangsu Province, China Postdoctoral Science Foundation. He has published 10 papers 9

related to his research. He holds and has licensed 7 patents. His current research interests are application of Monte Carlo method in particle transport, design and preparation of new isotope battery, and space laser/X-ray coupled communication technology. Zhiheng Xu received the BEng degree in nuclear engineering and nuclear technology from East China University of Technology, in 2012. Since then, Zhiheng is an MD-PhD candidate majored in nuclear technology and materials engineering at Nanjing University of Aeronautics and Astronautics. His research interests are radiation energy conversion mechanism and nuclear battery technology. As a PhD student, he is focused on the development and fabrication of radioluminescent nuclear batteries as well as theory and potential applications of new semiconductor and nano-sized fluorescent material.

Kai Liu received the BEng degree in nuclear reactor engineering from University of South China, in 2015. He is a MD candidate in nuclear technology and applications engineering at Nanjing University of Aeronautics and Astronautics. His current research interests are mechanism and practical design of nuclear batteries and exploring more energy conversion mechanisms to convert radioactive decay energy into electrical energy.

Zhengrong Zhang received the BEng degree in nuclear engineering and nuclear technology from the University of South China, in 2012. He is a MDPhD candidate in nuclear technology and materials engineering at Nanjing University of Aeronautics and Astronautics. His research interests are currently focused on Radiation effects on materials and new nuclear application materials. As a PHD student, he is focused on the development and fabrication of nuclear material as well as theory and potential applications of new semiconductor and shielding material.

Wang Chen received the BEng degree in nuclear engineering and nuclear technology from China Three Gorges University, in 2012. He is master candidate in nuclear technology and application at Nanjing University of Aeronautics and Astronautics. His research interests are currently focused on radioactive isotope particle energy conversion, radioluminescent nuclear battery of quantum dot.

10

Junqin Li received the BEng degree in materials science and engineering from the Shenyang Aerospace University, in 2015. She is master candidate studied in materials science and technology at Nanjing University of Aeronautics and Astronautics. Her research interests are currently focused on the synthesis and potential applications of new thermoelectric materials.

Figures

Fig. 1. Layout of stacked and miniaturized RTG: (a) the schematic cross section of this design; (b) the stacked thermocouples modules with heat source.

Fig. 2. Operating principle of the RTG: (a) one single-layer module and its electrical potential distribution; (b) the way to connect the p- and n- types; (c) the way to connect the up- and down- layers modules.

11

Fig. 3. Calculated (a) temperature distribution and (b) electrical potential distribution of a single-layer module with 0.5 W heat source h is 10 W·m−2·K−1. XOY plane is the size scale in (a, b).

Fig. 4. Equivalent circuit model.

12

Fig. 5. Number of different thermocouples; red is p-type, and blue is n-type. Each case is assumed to have same values 𝛥𝑇 and 𝛼np; thus, Eqs. (5) and (6) can be derived.

13

Fig. 6. Calculated values of internal resistance. (a) Device resistance; (b) Average resistance of the thermocouple; (c) Performance factor .

Fig. 7. Prepared samples after annealing.

14

Fig. 8. Measuring methods. (a) Measurement parameter interface; (b) static performance measurement; (c) output performance measurement.

15

Fig. 9. Performance measurement curve. (a) I-V/P output curve; (b) T-V/P fluctuation curve.

Fig. 10. Load curve of theoretical estimation with 0.5 W heat source.

Table 1 The properties of thermoelectric materials in experiment [20]. pn-type type −1 Seebeck coefficient (μV·K ) 108.5 −138.4 −1 Conductivity (S·m ) 27000 10030 Thermal conductivity 1.036 0.426 −1 −1 (W·m ·K ) Table 2 The measured ambient, cold end, hot end temperature and the temperature difference between cold and hot end. ambient cold hot temper 16

temperature T1 T2 T3 T4 T5 T6 T7 T8 T9

287.8 K 288.3 K 288.5 K 288.7 K 288.3 K 287.5 K 287.7 K 288.3 K 288.6 K

end end temperature temperature 292 K 300 K 292 K 304 K 293 K 304 K 293 K 303 K 294 K 303 K 289 K 302 K 296 K 310 K 299 K 311 K 300 K 312 K

Table 3 The performance estimation of multi-layer device. mo open maximum power dule circuit output number voltage 𝑁 𝑉𝑜𝑐 𝑃𝑚𝑎𝑥 1 46.73 23.13 μW at 23.37 mV mV 50 2.34 V 1.16 mW at 1.17 V 100

4.67 V

2.31 mW at 2.34 V

17

ature difference 8K 12 K 11 K 10 K 9K 13 K 14 K 12 K 12 K

device power density 𝑑

conversi on efficiency 𝜂

0.26 mW·cm−3 0.51 mW·cm−3

0.23% 0.46%