Torsional Actuator Powered by Environmental Energy Harvesting from

Oct 17, 2016 - Synopsis. This work demonstrates a sustainable energy harvesting system directly converting naturally abundant low-grade thermal energy...
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Torsional Actuator Powered by Environmental Energy Harvesting from Diurnal Temperature Variation Dongseok Suh, Thuy Kieu Truong, Daniel G. Suh, and Seong Chu Lim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01502 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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Torsional Actuator Powered by Environmental Energy Harvesting from Diurnal Temperature Variation

Dongseok Suh,†,‡,* Thuy Kieu Truong,† Daniel G. Suh,‡,§ Seong Chu Lim,†,*





Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea

The Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, TX 75083, USA *

Corresponding authors: [email protected] (D. Suh); [email protected] (S. C. Lim)

Keywords: Environmental energy harvesting, diurnal temperature variation energy harvesting, torsional actuation, carbon nanotube yarn, phase-change material

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ABSTRACT

Inspired by a phase-change technology storing thermal energy in the form of latent heat, direct conversion of environmental temperature variation into a useful form of mechanical work is achieved using the drastic volume change of a phase-change material during solid–liquid phase transformation. A twisted carbon nanotube yarn in combination with a phase-change material functions as a backbone of torsional actuator, powered by the change of environmental temperature, because nanopores among carbon nanotubes or their bundles are completely filled with the phase-change material. By the proper selection of infiltrated material whose melting temperature lies within a diurnal temperature range, this hybrid yarn can be applied in an autonomous system using naturally abundant low-grade thermal energy flow.

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Introduction Day-and-night environmental temperature cycle occurs naturally every day. The energy density involved in this process is quite low spatially and temporally, but the absolute amount of energy governing this global phenomenon is enormous. Appropriate usage of this energy has been explored extensively through, for example, phase-change energy-storage technology, in which thermal energy is stored inside the phase-change material (PCM) in the form of latent heat.1-10 However, direct conversion of this low-grade thermal energy into another type of energy, such as mechanical work, has rarely been achieved, due mainly to the inherent low-density nature of input thermal energy and the low conversion efficiency. Therefore, an appropriate system design for the combination of thermal-to-mechanical energy conversion and energy-independent autonomous operation is required to facilitate environmental temperature energy harvesting. An example application is proposed in Figure 1a, in which temperature-dependent torsional actuation drives a vertical window-blind to maintain a comfortable environment under daily diurnal temperature variation.1-10

Many PCMs showing solid-liquid transition near room temperature have been used for thermal energy storage; e.g. organic materials such as alkanes and inorganic materials such as salthydrates.1-10 During the phase-transition process, substantial volume expansion or shrinkage (>10% change in specific volume) occurs, which is a characteristic of most PCMs.11 Because encapsulation is required for prevention of liquid-phase PCM leakage, such a volume change at the event of phase transition threatens the structural stability of the encapsulation layer and

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should be suppressed for long life-cycle operation.1-10 In other words, such a volume change in PCMs at the melting/freezing temperature is a drawback for thermal energy-storage applications.

However, this can promote direct conversion of thermal energy into mechanical work, for example, as illustrated in Figure 1b. Here we present a simple methodology for conversion of environmental energy flow into a useful form of work using carbon nanotube (CNT) yarn and a PCM. When the polymeric PCM is infiltrated inside the twisted CNT yarn, the volume expansion of PCM during the transition between solid and liquid phases drives torsional actuation of the CNT yarn due to its inherent chirally twisted structure, as introduced in Figure 1c.3, 12-18 Therefore, by selecting a PCM with a transition temperature within the range of diurnal environmental temperature variation, this torsional actuation can do mechanical work automatically, as shown schematically in Figure 1b. Because this process is driven by the natural low-grade thermal energy flow occurring every day, it does not require any extra power for the mechanical work, and so can be used for design of autonomous smart systems that scavenge energy from the environment.

In our example system that performs mechanical work using the environmental temperature change, twisted CNT yarn was employed as the backbone for torsional actuation driven by the volume change of infiltrated PCM.3, 15, 17, 19-22 Specifically, a neat CNT yarn has an inherently chiral structure of nanotube bundles because of the twisting of aligned CNT sheets, as shown in Figure 1d,12, 23-25 whose porous property enables filling of the yarn with various functional guest materials. In our approach, the materials examined in the phase-change energy storage

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technology were taken into consideration as an infiltration material firstly due to the marked volume change during the phase-transition process. When the specific volume varies drastically at the phase-transition temperature, torsional actuation is expected to occur for the opening or the closing of window blinds, as depicted in Figure 1c. The image of a neat yarn and that of PCM infiltrated yarn are shown in Figure 1e.

Experimental (Materials and Methods). Carbon nanotube (CNT) yarn. A free-standing CNT sheet was drawn mechanically from a vertically grown spinnable CNT forest manufactured at the NanoTech Institute at the University of Texas at Dallas, USA. The individual CNT sheets were stacked one by one to form a layer of 20 ~ 40 CNT sheets, (see Figure S1 in the SI) followed by post-twisting to convert the CNT sheets into a dual-Archimedean-type CNT yarn. (See Figure S2 in the SI for the information about the sample-dimension change during the twist-insertion process.) Infiltration of a phase-change material (PCM) into CNT yarn. After attachment of a rotor in the middle of the CNT yarn using a small amount of Pt paste, the lower part of the yarn was immersed into melted liquid icosane that was heated by the conventional hot plate for 10 min. Then, the whole sample was removed from the liquid icosane. (See Figure S3 in the SI for the experimental setup.) Before the start of actuation work, the sample was Joule-heated to evaporate excess icosane residues remaining at the surface of the CNT yarn. Rotation angle and temperature measurements. The rotation angle by the torsional actuation of PCM-infiltrated CNT yarn was estimated from a series of photographs taken by the USB microscope camera, using the fixed time-interval image capture function. The sample

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temperatures in Figures 2 and 3 were obtained using conventional thermocouples located near the sample.

Results and Discussion. As a proof-of-concept, we measured the torsional actuation of PCM-CNT hybrid yarn driven by the temperature variation, as shown in Figure 2a. PCM-CNT hybrid yarn was fabricated by infiltration of a liquid-phase guest material directly into the yarn. After preparation of the hybrid yarn in the form of half-infiltration, a rotor consisting of a small piece of Kapton tape was attached to facilitate monitoring of rotation. In the literature,3, 21, 26 it is confirmed well that half infiltration can promote reversible and maximum torsional actuation at the boundary of the two regions due to the difference in the torsional modulus of the materials (i.e. the neat CNT yarn and the PCM-CNT hybrid yarn). The sample was placed inside a glass jar and illuminated using an incandescent light to increase the ambient temperature near the yarn, as shown in Figure 2b. As the temperature changed inside the jar, rotor motion was recorded by the camera and decoded later to estimate the rotation angle.

Among many higher alkanes usable as volume-expandable materials in the solid-liquid phase transition, the icosane, C20H42, was used for infiltration into CNT yarn in this experiment due to its melting temperature of 36–38 °C, which is suitable for controlled measurement of this concept. (See Table S1 in the Supporting Information (SI).) In real applications, PCMs such as heptadecane (C17H36) or octadecane (C18H38) might be more appropriate because of their melting temperatures of 21 and 29 °C, respectively. However, no qualitative difference other than the

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melting temperature is expected in the operational characteristics of icosane compared with that of heptadecane or octadecane. Even in the morphologies of twisted CNT yarns infiltrated by different PCMs, they all show similar shapes as in Figure 1e and there were no noticeable differences.

The effect of temperature variation on the rotation angle change is shown in Figure 2c. When the incandescent light was switched on, the temperature increased and torsional actuation of the halfinfiltrated hybrid yarn occurred. When the light was switched off, the rotated yarn returned to almost its original position as the temperature decreased. One interesting feature (Figure 2c) was the similarity between variations of temperature and rotation angle and the deviation occurring when the sample was cooled. This can be ascribed to the delay of rotation at around 33 °C, which was related to the PCM phase transition from liquid to solid. In the region of increasing temperature, rotation was delayed due to the solid to liquid phase transition, but the signal is unclear due to the rapid increase in temperature. This initial experiment demonstrates the feasibility of torsional actuation of a hybrid CNT yarn due to environmental temperature change.

To evaluate the rotation of hybrid CNT yarn according to temperature variation, we established an experimental setup inside an environmental chamber (Incufridge RS-IF-203, Revolutionary Science) to simulate the temperature variation occurring naturally every day. Figure 3a shows the experimental setup for the environmental temperature control. A small metallic bar was attached at the junction between the neat CNT yarn and hybrid CNT yarn, and the angle was measured using photographs taken successively during temperature variation at 1 °C per 3 min. The

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temperature inside the environmental chamber changed from around 20 to 50 °C, as measured by the thermocouple thermometer near the yarn. Figure 3b shows series of photographs taken every 50 s during repeated heating and cooling cycles. These show rotation of the metallic bar due to the variation in temperature encompassing the PCM melting temperature.

The rotation angle as a function of temperature is analysed in Figure 4 using the photographs in Figure 3. When the temperature was controlled as a function of time in Figure 4a, torsional actuation driven by repeated environmental temperature variation (Figure 4b) did not give coincident initial and final angle positions. This might originate from incomplete and nonuniform infiltration of PCM, which is not desirable for reversible mechanical work. However, when the initial and the final positions were normalised (Figure 4c), a reversible feature in two repetitive cycles was evident. Specifically, as the temperature increased from 20 to 50 °C, onestep sharp rotation occurred at 31 to 37 °C, followed by saturation of the angle change at higher temperatures. In contrast, as temperature decreased from 50 °C, there was a significant delay of rotation angle at around 35 °C, which decreased further to the saturation region below 27 °C. This significant delay in rotation angle was identical to the results in Figure 2.

The origin of such a delay is likely related to the phase-change of the PCM infiltrated in the CNT yarn. The most interesting feature in Figure 4c is that rotation occurs in a sharp single step when the temperature increased, but a two-step angle change appears when the temperature is decreased. From basic knowledge of thermodynamics, the easiest way to explain this is the super-cooling phenomenon. When the sample is cooled, the initial rotation occurs sharply from

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36 to 35 °C. Thereafter, the solidification of PCM in a super-cooling state occurs slowly below the melting temperature of the PCM, which induces the second rotation at 34 to 28 °C. During the heating process there is no such anomaly, but one-step rotation occurs from solid to liquid. Because two successive cycles yielded the same results, it is clear that such phenomena involving heating, cooling and a super-cooling processes drive the torsional actuation powered by environmental temperature variation. Alternatively, the delayed crystallization process also can be qualitatively explained in terms of the confinement effect of liquid PCM inside nanopores among CNT fibrils.27 It might be plausible to consider those two effects simultaneously for the understanding of pronounced hysteretic behavior in Figure 4c.

For application as a smart window, a further experiment was performed. (See Figure S4 in the SI). A 5 cm × 5 cm piece of aluminium foil was cut into pieces and connected to the top neat yarn and the bottom PCM-infiltrated yarn to mimic the function of a heat or light chopper, as in typical vertical window blinds. In a simple test inside a glass jar heated by infrared light, which induced temperature variation of 28 to 40 °C, this system yielded a roughly repetitive 150 ° angle rotation.

For perpetual operation of this system under repeated environmental temperature cycles, several issues must be considered, such as the stability of the PCM inside the CNT yarn and drift of the final rotation angle. The first thing that can be immediately applicable to solve those issues empirically is the addition of pre-heat-and-cool cycles for a sufficient amount of time. When the structure of the PCM-CNT yarn is considered, although encapsulation to prevent leakage of the

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liquid phase PCM was not applied in the above experiments, it could be fairly stable due to the considerable amount of tiny pores inside the CNT yarn formed among the nanotube bundles. The stability of the liquid-paraffin-filled nano-pores of CNT yarn was demonstrated during operation of the electrolyte-free torsional and tensile actuator, as high-viscosity liquid PCM was retained without leakage due to the surface tension even up to one million cycles.3, 14, 28-30 In contrast, the issue of the offset-angle drift during repeated phase transitions of the PCM must be solved before application of this thermal-to-mechanical conversion device. It is expected that other types of CNT yarn, such as two-plied CNT yarns twisted in the reverse direction, can prevent drift of the final angle position during repeated cycles by balancing the tendency to show untwisting behaviour.3, 19

As a final discussion point, it is worthy to consider the energy and/or power density as well as the conversion efficiency of the proposed system. In the structural point of view, the PCM-CNT hybrid yarn is comparable to the formerly reported hybrid carbon nanotube yarn artificial muscle,3 although they are operated in a completely different way. Because the latter, driven by the Joule-heating, showed the energy density of the order of one kilojoule per kilogram and the power density of a few tens kilowatt per kilogram,3 we could estimate the equivalent or the moderately less energy/power densities for the PCM-CNT hybrid yarn at the best case. However, when the different operation method is taken into account, the energy/power density might be much lower than that. Additionally, in terms of the efficiency, the former system reports the electrical-to-mechanical conversion efficiency of less than 1 %.3 In spite of these unfavourable conditions, our PCM-CNT hybrid yarn system is worthwhile to study because it uses the

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enormous low-grade thermal energy that is available in nature, free-of-charge, which overcomes the disadvantage of such a low conversion efficiency.

In summary, we presented an energy conversion device that directly transforms the environmental temperature cycle into torsional mechanical work without any additional external power. The thermal expansion of a PCM at the solid-liquid phase transition within the range of environmental temperature variation drives torsional actuation of the twisted CNT yarn infiltrated by the PCM. The simplicity of the system design, the abundance of materials and the free-of-charge energy will facilitate use of this system for autonomous energy generation or direct mechanical work, such as smart window applications.3, 31-32

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Figure 1. Schematic illustrations of torsional actuation powered by environmental diurnal temperature variation: a) The concept of a smart window autonomously closing (opening) when temperature increases (decreases). b) Reversible torsional actuation of an imaginary vertical window blind attached to the PCM-half-infiltrated CNT yarn. (c) Expected correlation between the specific volume change of the infiltrated PCM material and the rotation angle of the CNTyarn torsional actuator as a function of temperature. (d) Stacked CNT sheets before the twistinsertion (top) and twisted CNT yarn after 200 turns (bottom). (e) Scanning electron microscope images of a neat CNT yarn (left) and an PCM (icosane)-infiltrated CNT yarn (right).

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Figure 2. Torsional actuation of PCM-CNT hybrid yarn driven by light and heat: a) Detailed configuration of the torsional actuator consisting of neat CNT yarn, Kapton tape rotor and PCMinfiltrated yarn. All items such as the thermocouples (for measurement of sample temperature) and the USB camera (for measurement of rotor angle) were installed inside the jar. b) Photograph and schematic illustration of the overall experimental setup, including the jar containing the hybrid yarn and showing operation of the light bulb outside the jar. c) Sample temperature and rotation angle over time. The initial sudden increase in temperature corresponds to the switching-on of the incandescent lamp and the temperature decreases markedly when the incandescent lamp is switched off.

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Figure 3. Torsional actuation of a PCM-CNT hybrid yarn driven by environmental temperature variation: a) Photograph and schematic illustration of the experimental setup, including a temperature change chamber in which the environmental temperature for the PCM infiltrated CNT yarn was controlled. Torsional actuation was monitored using the metallic bar attached at the junction between the neat CNT yarn and the PCM-infiltrated CNT yarn (bottom). b) Photographs of metal bars rotated by torsional actuation. Photographs were taken every 50 s while the environmental temperature inside the chamber was changed by 1 °C /minute. Temperature was measured using a thermocouple located near the rotor (not shown).

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Temperature o ( C)

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Figure 4. Plot of temperature-dependent torsional actuation of a C20H42-infiltrated CNT yarn introduced in Figure 3: a-b) Temperature variation (a) and the corresponding rotation angle (b) according to repeated environmental heating and cooling. c) Normalised rotation angle of the hybrid CNT yarn. The angles at 25 °C and 50 °C at each temperature cycle were normalised as values of 0 and 1.

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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. “The physical properties of various higher alkanes. Sheet drawing and collection process with the U-shaped sample jig rotated by the stepper motor. Sample dimension change during the CNT yarn-twisting process. Experimental setup for half-infiltration of PCM into the CNT yarn. Experimental setup for rotation test of a large-size window.”

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (D.S.) * Email: [email protected] (S.C.L.)

Present Addresses § Department of Applied Mathematics, Brown University, Providence, RI 02912, USA

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF2013R1A1A1076063 (D.S.) and NRF-2015R1D1A1A02062402 (S.C.L.)), funded by the Ministry of Science, ICT & Future Planning, Republic of Korea.

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(16) Kwon, C. H.; Lee, S.-H.; Choi, Y.-B.; Lee, J. A.; Kim, S. H.; Kim, H.-H.; Spinks, G. M.; Wallace, G. G.; Lima, M. D.; Kozlov, M. E.; Baughman, R. H.; Kim, S. J., High-power biofuel cell textiles from woven biscrolled carbon nanotube yarns. Nat Commun 2014, 5, 3928. (17) Lima, M. D.; Hussain, M. W.; Spinks, G. M.; Naficy, S.; Hagenasr, D.; Bykova, J. S.; Tolly, D.; Baughman, R. H., Efficient, Absorption-Powered Artificial Muscles Based on Carbon Nanotube Hybrid Yarns. Small 2015, 11 (26), 3113-3118. (18) Kosidlo, U.; Omastová, M.; Micusík, M.; Ćirić-Marjanović, G.; Randriamahazaka, H.; Wallmersperger, T.; Aabloo, A.; Kolaric, I.; Bauernhansl, T., Nanocarbon based ionic actuators—a review. Smart Mater Struct 2013, 22 (10), 104022. (19) Chun, K.-Y.; Hyeong Kim, S.; Kyoon Shin, M.; Hoon Kwon, C.; Park, J.; Tae Kim, Y.; Spinks, G. M.; Lima, M. D.; Haines, C. S.; Baughman, R. H.; Jeong Kim, S., Hybrid carbon nanotube yarn artificial muscle inspired by spider dragline silk. Nat Commun 2014, 5, 3322. (20) Lee, S.-H.; Kim, T. H.; Lima, M. D.; Baughman, R. H.; Kim, S. J., Biothermal sensing of a torsional artificial muscle. Nanoscale 2016, 8 (6), 3248-3253. (21) Mirvakili, S. M.; Pazukha, A.; Sikkema, W.; Sinclair, C. W.; Spinks, G. M.; Baughman, R. H.; Madden, J. D. W., Niobium Nanowire Yarns and their Application as Artificial Muscles. Adv Funct Mater 2013, 23 (35), 4311-4316. (22) Kwon, C. H.; Chun, K.-Y.; Kim, S. H.; Lee, J.-H.; Kim, J.-H.; Lima, M. D.; Baughman, R. H.; Kim, S. J., Torsional behaviors of polymer-infiltrated carbon nanotube yarn muscles studied with atomic force microscopy. Nanoscale 2015, 7 (6), 2489-2496. (23) Zhang, M.; Atkinson, K. R.; Baughman, R. H., Multifunctional Carbon Nanotube Yarns by Downsizing an Ancient Technology. Science 2004, 306 (5700), 1358-1361. (24) Zhang, X.; Jiang, K.; Feng, C.; Liu, P.; Zhang, L.; Kong, J.; Zhang, T.; Li, Q.; Fan, S., Spinning and Processing Continuous Yarns from 4-Inch Wafer Scale Super-Aligned Carbon Nanotube Arrays. Adv. Mater. 2006, 18 (12), 1505-1510. (25) Ghemes, A.; Minami, Y.; Muramatsu, J.; Okada, M.; Mimura, H.; Inoue, Y., Fabrication and mechanical properties of carbon nanotube yarns spun from ultra-long multi-walled carbon nanotube arrays. Carbon 2012, 50 (12), 4579-4587. (26) Foroughi, J.; Spinks, G. M.; Wallace, G. G.; Oh, J.; Kozlov, M. E.; Fang, S.; Mirfakhrai, T.; Madden, J. D. W.; Shin, M. K.; Kim, S. J.; Baughman, R. H., Torsional Carbon Nanotube Artificial Muscles. Science 2011, 334 (6055), 494-497. (27) Michell, R. M.; Blaszczyk-Lezak, I.; Mijangos, C.; Müller, A. J., Confinement effects on polymer crystallizatoin: From droplets to alumina nanopores. Polymer 2013, 54, 4059-4077. (28) Chen, R.; Cui, Y.; Tian, H.; Yao, R.; Liu, Z.; Shu, Y.; Li, C.; Yang, Y.; Ren, T.; Zhang, G.; Zou, R., Controllable Thermal Rectification Realized in Binary Phase Change Composites. Sci. Rep. 2015, 5, 8884. (29) Huang, X.; Xia, W.; Zou, R., Nanoconfinement of phase change materials within carbon aerogels: phase transition behaviours and photo-to-thermal energy storage. J. Mater. Chem. A 2014, 2 (47), 19963-19968. (30) Zhang, S.; Tao, Q.; Wang, Z.; Zhang, Z., Controlled heat release of new thermal storage materials: the case of polyethylene glycol intercalated into graphene oxide paper. J. Mater. Chem. 2012, 22 (38), 20166-20169. (31) He, S.; Chen, P.; Qiu, L.; Wang, B.; Sun, X.; Xu, Y.; Peng, H., A Mechanically Actuating Carbon-Nanotube Fiber in Response to Water and Moisture. Angew. Chem. Int. Ed. Engl. 2015, 54 (49), 14880-14884.

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(32) Haines, C. S.; Lima, M. D.; Li, N.; Spinks, G. M.; Foroughi, J.; Madden, J. D. W.; Kim, S. H.; Fang, S.; Jung de Andrade, M.; Göktepe, F.; Göktepe, Ö.; Mirvakili, S. M.; Naficy, S.; Lepró, X.; Oh, J.; Kozlov, M. E.; Kim, S. J.; Xu, X.; Swedlove, B. J.; Wallace, G. G.; Baughman, R. H., Artificial Muscles from Fishing Line and Sewing Thread. Science 2014, 343 (6173), 868872.

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Table of Contents (TOC) Graphic

The manuscript title Torsional Actuator Powered by Environmental Energy Harvesting from Diurnal Temperature Variation

The names of all authors Dongseok Suh, Thuy Kieu Truong, Daniel G. Suh, and Seong Chu Lim

Brief synopsis This work demonstrates sustainable energy harvesting system directly converting naturally abundant low-grade thermal energy flow (such as diurnal temperature variation) into mechanical work.

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