Project Narratives Project title: Team Members: Statement of the project goals: The project's role in support of the strategic plan:

Transcription

1 Project Narratives Project title: Thermoelectric Energy Generators Based on High Efficiency Nanocomposite Materials Team Members: Project Leader: Daryoosh Vashaee Graduate Student: Amin Nozariasbmarz Statement of the project goals: This project targets novel thermoelectric materials designed to produce high performance thermoelectric generators (TEG) optimized to operate at body temperature. In order to achieve this goal, thermoelectric nanocomposite materials based on bismuth antimony telluride with large figures of merit, ZT and small thermal conductivities are being developed. This effort is coupled with materials and system modeling efforts in ASSIST to develop materials that match the system s external thermal impedances while maintaining a high ZT and proper mechanical properties for device fabrication. The project's role in support of the strategic plan: Thermoelectric energy harvesting is one of the key harvesting modalities for strategic research in the center. This program has the responsibility of producing the high performance TEG materials for both rigid and flexible TEGs for ASSIST platforms. Discussion of fundamental research, educational, or technology advancement barriers and the methodologies used to address them: The research was an integrated effort from material to a working system for body heat energy harvesting, which had not been actively pursued prior to this work. Therefore, there were new science, fundamental, and engineering challenges that we had not foreseen at each stage. We identified such barriers as described below and developed the pathway to address them in order to build the system. The existing expertise and the facilities in the ERC were especially helpful. In particular: (1) Currently, for room temperature applications, the state-of-the-art thermoelectric materials are based on alloys of Bi-Sb-Se-Te. For example, (BixSb1-x)2Te3 nanocomposites have shown nearly 50% improvement in figure of merit, ZT, compared to the conventional bismuth antimony telluride alloys [1]. However, for wearable TEGs, in addition to high ZT, the material has to have much lower thermal conductivity. This requirement is mainly due to the skin thermal resistance and the constraints for using a large heat sink in a wearable platform. It is possible to reduce the thermal conductivity by increasing the porosity of the nanocomposite structure. However, the porosity would reduce the mechanical properties resulting in fragile legs, which are difficult to cut or package. Even more challenging is the task of making electrical contacts to porous materials due to the horizontal micro-cracks that disrupt the electrical current. (2) In order to address the requirement for very low thermal conductivity and high ZT, a novel nanocomposite structure composed of the Bi-Sb-Te alloy with glass inclusion was developed. The new structure allowed reducing the thermal conductivity without deteriorating the density or the ZT. The nanocomposite structure has nearly 50% smaller thermal conductivity compared to the 9

2 conventional bismuth antimony telluride alloys. Such a small thermal conductivity was achieved with only less than 5% porosity and with a high ZT of 1.2 at room temperature. (3) Maintaining a high voltage for the boost converter unit to work efficiently under different thermal conditions is indeed challenging. The efficiency of the boost converter is 20% at 10mV input voltage and 50% at 20 mv input voltage most of the time. Therefore, the TEG must be designed to deliver voltage >20mV to avoid losses in the boost converter. In order to increase the output voltage and the power, the TE legs were cut to have a small cross sectional area of 0.6mm 0.6mm and heights of 2mm and 4mm. This was a challenging task due their very high aspect ratio. We devised a method for cutting these legs with nearly 100% yield. The long and thin legs enabled to increase the fill factor while maintaining a large temperature gradient across the TEG for higher output voltage and power. (4) It is worth noting that, due to the low power mode of operation, the joule heating in wearable TEG devices is often negligible compared to the other factors allowing to increase the aspect ratio in favor of the thermal impedance requirements. (5) Making good electrical contacts to bismuth telluride based nanocomposites is known to be challenging compared to their single or poly crystalline form. In order to make good electrical contacts, a proper solder and soldering procedure was identified to bond the TE legs directly to the metal pads on the ceramic substrate without the need to metalize them prior to bonding. This approach simplified the packaging of the TEG device and the relieved the challenges associated with the metallization step. (6) Maintaining a uniform heat distribution across the TEG devices is often difficult especially for application on a non-planar surface like the wrist or the arm. Non-uniform heat flow through a large area can reduce the efficiency as the dark TE legs will act as a load than a generator wasting the power. Such non-uniformity can happen due to the non-uniform heat transfer through the skin and/or the convection around the arm or the arm. To address this issue, a thermal spreader was devised under the TEG to distribute the heat uniformly across the TE legs. In addition, the spreader increased the effective surface area resulting in higher output power from the TEG. Figure 1: (a) High-resolution transmission electron micrograph of bulk nanocomposite BiTe-SbTe. Energy dispersive spectroscopy (EDS) elemental maps are also shown for the corresponding red box (right). The inset shows a magnified view of the interface of the Bi rich grain and surrounding Sb rich grains; (b) SEM micrograph of the cross-section of layered structure of BiTe ingot prepared via induction melting. The inset shows a magnified view of the structure exhibiting the highly crystalline nature of the ingot; (c) SEM micrograph of BiTe-SbTe powder after 1 hour milling. The inset shows a magnified view of a particle with smaller crystallites. 10

3 Achievements in Year 3 and Previous Years: (1) Development of p type (Bi,Sb)2Te3 nanocomposites Two types of nanocomposites were developed: (a) The first material composed of nano bulk (Bi2Te3)-(Sb2Te3) composite mixture with enhanced thermoelectric figure-of-merit resulted from simultaneous enhancement of power factor and reduction of thermal conductivity. The composite material is composed of nanocrystallites of (Bi1-xSbx)2Te3 (Figure 1) with x changing from zero to one with mean value of 0.8. The chemical analysis performed by energy dispersive spectroscopy confirmed the composite nature of the bulk samples as shown in Figure 1 (a). Figure 1 (b) also shows the scanning electron microscope (SEM) image of the layered structure of the ingot. It is observed that the ingot is consisted of nanocrystallites with thickness in the range of sub-200 nm (inset). SEM micrograph of the composite powder prepared from the two binary ingots after 1 hour of ball milling is also depicted in Figure 1 (c). It is observed that relatively large particles (~ 500 nm) consist of smaller crystallites. Such crystallites play a key role in affecting the thermoelectric properties of the composite sample. This nanocomposite mixture possesses the large thermoelectric power factor of W/m- K 2 and small thermal conductivity of 1.1 W/m-K at 105 C compared to the conventional Bi-Sb- Te thermoelectric alloys. Simultaneous enhancement of the power factor and reduction of the thermal conductivity in this structure led to the enhanced figure-of-merit with maximum ZT of ~1.5 at 105 C, showing 50% improvement over that of the conventional Bi0.4Sb1.6Te3 ingot. Our theoretical modeling showed that the crystallite sizes are comparable to the charge carrier energy relaxation length, which is a condition to observe such enhancement in power factor. As Figure 2(a) depicts the nanocomposite sample possesses higher electrical conductivity than the crystalline sample due to its higher defect concentration. Figure 2(b) shows the temperature dependence of the Seebeck coefficient. Large Seebeck coefficient values were maintained at a high carrier concentration through energy filtering. Nanocomposite sample possesses the maximum PFT of Figure 2: Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, (c) thermal conductivity and (d) figure of merit for bulk nanocomposite of (Bi,Sb)2Te3) with glass inclusions (red symbols). The corresponding properties for conventional p type (Bi,Sb)2Te3 is also shown for comparison. All properties of the nanocomposite material were measured along the z axis of the ingot. 11

4 ~1.7 W/m-K at 104 C, which is 20% higher than that of the crystalline sample (i.e. 1.4 W/m-K). This value is, to the best of our knowledge, the highest reported power factor for (BixSb1-x)2Te3 structures. Among recent reports on (BixSb1-x)2Te3 structures with large ZT values, maximum PFT of 1.43 W/m-K was reported by Poudel et al, 1.29 W/m-K by Fan et al.[2], and 1.08 W/m-K by Xie et al.[3]. Figure 2(c) shows the thermal conductivity of the nanocomposite sample as a function of temperature. Total thermal conductivity of the sample decreased mainly through the reduction of its lattice (κl) and bipolar (κl) components as a result of nanostructuring. Minimum thermal conductivity of ~1.1 W/m-K was achieved at 104 C. To the best of our knowledge, this is the only re- port of simultaneous enhancement of power factor and reduction of thermal conductivity with respect to the crystalline structure, which happens for the entire temperature range of C. Figure 2(d) depicts the temperature dependence of the figure of merit (ZT) of the nanocomposite (Bi2Te3)0.2(Sb2Te3)0.8 sample with respect to that of the crystalline alloy. Significant increase in power factor alongside the reduction in thermal conductivity led to the maximum ZT of ~1.5 at 104 C, showing 50% improvement compared with that of the crystalline structure. Note that sample possesses ZT values above 1 from room temperature to 200 C. The theoretical calculations considering the multiband structure of the material is in good agreement with the experimental data over the whole temperature range. (b) The second type based on p-type (Bi,Sb)2Te3 also showed enhancement of thermoelectric figure-of-merit but more reduction of thermal conductivity compared to the first nanocomposite material. Nanocomposites with different ZT values were obtained by adjusting the electrical conductivity of the material while keeping the Seebeck coefficient constant at 200 µv/k. Our model calculations showed that for a given thermal conductivity, improving the ZT improved the output power as expected; however, more interestingly, the output power increased more rapidly with decreasing the thermal conductivity than increasing the ZT. The need for small thermal conductivity is due to the large parasitic thermal resistances that exist at the skin/teg and TEG/air interfaces. These resistances make it extremely challenging to achieve a large temperature gradient across the device. We developed nanocomposites of (Bi,Sb)2(Se,Te)3 with the key property of small thermal conductivity [4,5]. The reduction of thermal conductivity is mainly due to the phonon scattering at the grain boundaries. Figure 3: Transmission Electron Microscopy image of the grain boundaries in (Bi,Sb) 2Te 3 based nanocomposites being explored in ASSIST for high performance TEG operation on human body. Figure 3 shows the Transmission Electron Microscopy (TEM) image of the interface of several grains in the nanocomposite bulk sample. These grains have usually sizes smaller than the mean free path of the majority of phonons but larger than the mean free path of the charge carriers. Therefore, the reduction in thermal conductivity often happens without significant deterioration of the electrical conductivity [6]. Electrons and phonons in nanocomposites experience several transport processes that do not occur in other thermoelectric or electronic materials. Phonon scattering at grain boundaries, dopant diffusion and precipitation, reduced bipolar thermal diffusion of electron-hole pairs, electron transitions through the same and different valleys at grain boundaries, thermionic emission at grain boundaries, and partial relaxation of the carrier energy at nanoscale grains are some of the main 12

5 mechanisms that can significantly affect the thermoelectric properties of nanocomposites [7]. Therefore, with further engineering of the grain size, alloy composition ratios, carrier concentration, grain boundary potential, etc. we can further optimize the thermoelectric properties according to the application requirements. The temperature dependency of the thermoelectric properties of the developed nanocomposite material is shown in Figure 4. This nanocomposite material is composed of nanocrystallites of (Bi1-xSbx)2Te3 embedded with glass inclusions. It possesses the small thermal conductivity of 0.77 W/m-K and ZT 1.2 at room temperature. The electrical conductivity and Seebeck coefficient at room temperature are 720 S/cm and 210 µv/k, respectively. Compared to conventional p type thermoelectric materials, with thermal conductivity of 1.4 W/mK and ZT 0.9 at room temperature, this nanocomposite structure is much more suitable for body heat energy harvesting. (2) Fabrication of TEG devices with rigid substrates Two different sizes of TE legs with sizes of 0.6mm 0.6mm 4mm and 0.6mm 0.6mm 2mm were cut and packaged into a TEG device (Figure 4). The TEG devices were measured in terms of the output power and voltage and compared with the results from commercial TE devices. Both devices were tested under similar conditions sitting on a 2mm thick PDMS on a hot plate at 37C. Commercial device: 3.3mV/cm 2 and 0.24 μw/cm 2 NCSU device: 11.4 mv/cm 2 and 1.0 μw/cm 2 Our device produced 3.45 times more voltage and 4.2 times more power compared to one of the best commercial TE devices. We did not use a heat spreader or a heat sink for this test. It is expected that the TEG will generate higher power on the testbed with a heat spreader and a heat sink. Figure 4: Thermoelectric legs cut and packaged into a TEG device. Plans for next year: (1) The primary focus will be on the development of the n type nanocomposites appropriate for body heat energy harvesting. Unfortunately, the n type bismuth telluride alloys have smaller ZT than their p type counterparts mainly due to their low Seebeck coefficient. This is mainly due to lower degeneracy of the conduction band compared with the valence band. Therefore, our focus, in addition to reducing the thermal conductivity by nanostructuring, will be on the techniques for enhancing the Seebeck coefficient such as tuning the composition of the grains, their size, the interface potentials, and the grain alignment. The last one is especially important due to the 13

6 stronger dependency of the ZT on the crystal orientation in the n type material compared to the p type. (2) We will also work on a new type of TEG devices based on a quasi-vertical thin film structures. The material will be grown using a dual beam Pulsed Laser Deposition and the device will be processed via micro-fabrication steps. This enables of TE legs per cm 2 to be connected in series (and/or parallel) circuits that would increase the output voltage to several volts. (3) We start a new thrust for developing implantable TEG devices for medical applications. We will re-engineer our TEG devices for this purpose. The bio-compatible packaging of the TEG device is of most importance. Several FDA approved materials will be investigated for packaging. In particular, parylene has proven to provide ultimate conformal coating for device protection. It has ultra-low thermal conductivity (good for TEG) and is FDA approved (comply with USP class VI Plastics requirements and are MIL-I-46058C / IPC-cc-830B listed as class XY). (4) Hybrid solar-thermoelectric generators will be further pursued for wearable application. For this purpose, solar cells will be grown on the heat sink on top of the TEG devices. The power generated by the solar cell will be combined with the TEG power through the boost convertor. The solar cell will consist of hydrogenated amorphous silicon (a-si:h) films grown by plasmaenhanced chemical vapor deposition (PECVD). Expected milestones and deliverables for the project: - Synthesis, characterization, and optimization of the n type nanocomposite material - Fabrication of TEGs with p-type and n-type nanocomposite legs - PLD grown thermoelectric films and their properties optimization - a-si:h thin film solar cell growth and integration with TEG devices - Bio-compatible packaging of the TEG devices for implantable applications Member company benefits: We anticipate any thermoelectric company relying on pick-and-place tooling and conventional rigid packaging can benefit from the advances made in thermoelectric materials in this program. Course implementation information: A new course was developed in the area of energy harvesting. ECE 492/592, Solid State Solar and Thermal Energy Harvesting was offered in the spring semester 2015 at NCSU for the first time. It is in line with the ASSIST educational plan and studies the fundamental and recent advances of energy harvesting from solar and thermal energies. The course was offered to our senior undergraduate and graduate students in the area of Nanoelectronics and Photonics, as well as anyone interested to learn about solar cells and thermoelectrics science and technology. This course will be available online starting Spring 2015 and it will be one of the candidates for oneon-one course sharing efforts with FIU. 1. Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, High Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys, Z. Science 2008, 320,

7 2. Fan, S.; Zhao, J.; Guo, J.; Yan, Q.; Ma, J.; Hng, H. H. p-type Bi0.4Sb1.6Te3 nanocomposites with enhanced figure of merit, Appl. Phys. Lett. 2010, 96, Xie, W.; Tang, X.; Yan, Y.; Zhang, Q.; Tritt, T. M. Unique nanostructures and enhanced thermoelectric performance of melt-spun BiSbTe alloys, Appl. Phys. Lett. 2009, 94, Nikhil Satyala, Armin Tahmasbi Rad, Zahra Zamanipour, Payam Norouzzadeh, Jerzy S. Krasinski, Lobat Tayebi, Daryoosh Vashaee, Reduction of Thermal Conductivity of Bulk Nanostructured Bismuth Telluride (Bi2Te3) Composites Embedded with Silicon Nanoinclusions, Journal of Applied Physics, DOI / (2014) 5. N. Satyala, A. Tahmasbi Rad, Z. Zamanipour, P. Norouzzadeh, Jerzi. S. Krasinski, L.Tayebi, D. Vashaee, Influence of germanium nano-inclusions on the thermoelectric power factor of bulk Bismuth Telluride (Bi2Te3) alloy, J. Appl. Phys. 115, (2014) 6. Arash Mehdizadeh Dehkordi, Daryoosh Vashaee, Enhancement in thermoelectric power factor of polycrystalline Bi0.5Sb1.5Te3 by crystallite alignment, Physica Status Solidi (a), DOI: /pssa (2012) 7. Nikhil Satyala, Payam Norouzzadeh, and Daryoosh Vashaee, Nano Bulk Thermoelectrics: Concepts, Techniques, and Modeling, Book chapter in Thermoelectrics at Nanoscale, Springer, (2014). 15