Improved Durability Nanotextured Aluminum Surfaces for Jumping Droplet Thermal Rectification

Jumping droplet thermal diodes (JDTDs) offer new capabilities for passive thermal rectification in thermal management of buildings, electronics, and batteries.^[1–3] However, the durability of JDTD devices is limited because the superhydrophobic (SHPB) surface degrades upon continuous exposure to condensing water vapor.^[4] In addition, most prior JDTDs documented in the literature have been constructed with copper surfaces, although aluminum surfaces are preferable for lightweight applications.^[2,3,5–8]<br/><br/>In this work, we quantify the durability of novel SHPB aluminum surfaces and JDTD devices. We manufacture our Al-based surfaces using scalable surface functionalization techniques based on hydrothermal nanostructuring, silanization, and a dip coat deposition of Teflon AF (amorphous fluoropolymer). We show that dip-coating Teflon AF on top of TFTS monolayer-coated nanostructured surfaces enhances surface durability compared to non-Teflon coated surfaces, thereby enabling prolonged operation and comprehensive thermal testing of the JDTD. Contact angle measurements, environmental scanning electron microscope (ESEM) images, and X-Ray photoelectron spectroscopy (XPS) measurements indicate slight changes in the surface topology and chemical composition after dip coating, confirming that the deposited coating is thin (&lt;20 nm).<br/><br/>Using our SHPB surfaces, we parametrically study steady-state thermal rectification as a function of gap height, fluid fill ratio, and heating orientation of the JDTD. We identify tradeoffs between the performance and the orientation dependence of operation by controlling the gap height and charge level. At larger gap sizes and lower charge levels, we observe enhanced thermal rectification only when the JDTD is heated from the bottom, while minimal thermal rectification is measured when the JDTD is heated from the top. At moderate charge levels and smaller gap sizes, operation is sustained in both orientations, but thermal rectification is reduced. We demonstrate a maximum thermal rectification ratio of 30 plus/minus 1.<br/><br/>To investigate potential implementation in thermal energy storage and harvesting applications, we show that the Al JDTD enables a half-wave thermal rectifier circuit that rectifies time-periodic temperature profiles, achieving thermal circuit effectiveness values up to 30 % of the ideal-diode limit at rectification ratios near 5. When coupled with thermal capacitances, such time-periodic thermal rectification could aid cold energy storage for passive cooling of components subjected to time-periodic thermal boundary conditions. Thus, our findings showcase the functionality of Al-based surfaces for thermal rectification and could guide future work aiming to apply Al JDTDs for improved thermal management.<br/><br/>References<br/>[1] H. Zhao, Y. Wu, H. Sun, B. Lin, M. Zhong, G. Jiang, S. Wu, <i>Renewable Energy</i> <b>2023</b>, 119278.<br/>[2] J. B. Boreyko, C.-H. Chen, <i>International Journal of Heat and Mass Transfer</i> <b>2013</b>, <i>61</i>, 409.<br/>[3] F. Zhou, Y. Liu, S. N. Joshi, E. M. Dede, X. Chen, J. A. Weibel, in <i>2017 16th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm)</i>, IEEE, Orlando, FL, <b>2017</b>, pp. 521–528.<br/>[4] M. J. Hoque, J. Ma, K. F. Rabbi, X. Yan, B. P. Singh, N. V. Upot, W. Fu, J. Kohler, T. S. Thukral, S. Dewanjee, N. Miljkovic, <i>Applied Physics Letters</i> <b>2023</b>, <i>123</i>, 110501.<br/>[5] J. B. Boreyko, Y. Zhao, C.-H. Chen, <i>Appl. Phys. Lett.</i> <b>2011</b>, <i>99</i>, 234105.<br/>[6] J.-X. Wang, P. Birbarah, D. Docimo, T. Yang, A. G. Alleyne, N. Miljkovic, <i>Phys. Rev. E</i> <b>2021</b>, <i>103</i>, 023110.<br/>[7] B. Traipattanakul, C. Y. Tso, C. Y. H. Chao, <i>International Journal of Heat and Mass Transfer</i> <b>2019</b>, <i>135</i>, 294.<br/>[8] M. Y. Wong, Y. Zhu, Y. Zeng, T. C. Ho, Y. Yang, H. Qiu, C. Y. Tso, <i>Advanced Engineering Materials</i> <b>2022</b>, <i>24</i>, 2100958.<br/><br/>*This work was supported by a NASA Space Technology Graduate Research Opportunity (80NSSC20K1220).