[en] Highlights: • An ultra-thin flat heat pipe was developed. • The wick was treated to be superhydrophilic. • The flat heat pipe can manage heat flux up to 490 W/cm². • An extremely low thermal resistance was obtained. - Abstract: Miniaturization of high-performance electronics demands heat dissipation components with small size while high performance. This paper presents an ultra-thin flat heat pipe (UTFHP) with a total thickness of 0.95 mm and an inner height of 0.55 mm. Novel wick structure made of superhydrophilic sintered copper mesh screen was employed to provide strong capillary force as well as low flow resistance for the working fluid. Two high-heat-flux heaters were soldered discretely on the UTFHP to evaluate its heat transfer characteristics both under natural air convection and forced water cooling conditions. It is found that the UTFHP had much lower evaporator temperature and much smaller thermal resistance compared with copper sheet, no matter for one heater or two heaters situation, demonstrating excellent heat transfer capability. Moreover, the UTFHP could tolerate 490 W/cm² without dryout. The transition heat flux was 302.5 W/cm² in term of thermal resistance, at which the minimum thermal resistance was 0.039 °C cm²/W, indicating the proposed ultra-thin flat heat pipe can be a promising solution for cooling high power density electronics.

[en] Highlights: • Mechanism of thermal spreading enhancement for a microscale flat plate heat pipe was explored. • Nanoscale grooves formed on smooth metal wires contribute to achieving superhydrophilic wick. • Quasi-loop-type two phase circulation was verified in the microscale flat plate heat pipe. • The microscale flat plate heat pipe has a highest thermal conductivity of 2.88 × 10⁴ W/(m⋅K). • The microscale flat plate heat pipe can further reduce the temperature of smartphone chips by 10 °C. -- Abstract: For flat plate heat pipes, under the premise of holding high thermal spreading performance meanwhile being as thin as possible for cooling smartphones, the flow resistance should be minimized and the capillary force be maximized. Firstly, the mechanism of superhydrophilicity in a specially treated wick was explored. Secondly, a visual study on a microscale flat plate heat pipe which can regulate two-phase flow was conducted. The typical two-phase flow patterns were identified in the heating and cooling zones under different heat loads and working orientations. Under the joint action of the above functions, a 500 µm thick flat plate heat pipe was developed and tested under natural air convection and compared with graphite and copper flakes. The results demonstrate that the microscale flat plate heat pipe has a maximum equivalent thermal conductivity up to 2.88 × 10⁴ W/(m⋅K), more than 80 times the value of copper and 36 times of graphite , which is superior to any reported thin film heat spreader so far. The proposed microscale flat plate heat pipe is an ideal solution to cool high-end smartphone chips.

[en] Highlights: • A 1.2 mm thick miniature loop heat pipe was developed. • The mLHP can manage a wide range of heat loads at natural convection. • A minimum mLHP thermal resistance of 0.111 °C/W was achieved at 11 W. • The proposed mLHP is a promising solution for cooling mobile electronics. - Abstract: In this paper, we present a miniature loop heat pipe (mLHP) employing a 1.2 mm thick flat evaporator and a vapor line, liquid line and condenser with a 1.0 mm thickness. The mLHP employs an internal wick structure fabricated of sintered fine copper mesh, comprised of a primary wick structure in the evaporator to provide the driving force for circulating the working fluid, and a secondary wick inside the liquid line to promote the flow of condensed working fluid back to the evaporator. All tests were conducted under air natural convection at an ambient temperature of 24 ± 1 °C. The proposed mLHP demonstrated stable start-up behavior at a low heat load of 2 W in the horizontal orientation with an evaporator temperature of 43.9 °C and efficiently dissipates a maximum heat load of 12 W without dry-out occurring. A minimum mLHP thermal resistance of 0.111 °C/W was achieved at a heat load of 11 W in a gravity favorable operation mode, at which the evaporator temperature was about 97.2 °C. In addition, an analytical analysis was conducted, and the devised equation could be used to evaluate the performance of the mLHP.