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The characterization "thermal diode" has been used to portray systems that spread heat very efficiently in a specific direction but obstruct it from flowing in the opposite direction. In this study, a planar vapor chamber with a wickless, wettability-patterned condenser is fabricated and tested as a thermal diode. When the chamber operates in the forward mode, heat is naturally driven away from the heat source; in the reverse mode, the system blocks heat backflow, thus working as a thermal diode. The low-profile assembly takes advantage of the phase-changing properties of water inside a closed loop comprised of a classical thin-wick evaporator opposing a wickless wettability-patterned condenser, when the chamber operates in the forward (heat-transporting) mode. The wettability patterned plate -when on the cooled side- enables spatial controlled dropwise and filmwise condensation and offers an efficient transport mechanism of the condensed medium on superhydrophilic wedge tracks by way of capillary forces. The same chamber acts as a thermal blocker when the opposing wick-covered plate is on the cool side, trapping the liquid in the pores and blocking heat flow. With this system, thermal diodicities exceeding 20 have been achieved, and are tunable by altering the wettability pattern as needed for different purposes. The present vapor chamber - thermal diode design could be well-suited for an extensive range of thermal-management applications, ranging from aerospace, spacecraft, and construction building materials, to electronics protection, electronics packaging, refrigeration, thermal control during energy harvesting, thermal isolation, etc.

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