Thermal Ground Plane (TGP) or Flat Heat Pipe
TGPs or flat heat pipes adopt a relatively much larger geometry in planar direction than traditional heat pipes to provide a potential for transporting or spreading a large amount of heat. They are finding their ways in the thermal cooling systems for applications such as computer chips, circuit boards, and space based radar systems which consume a high density of power. A flat heat pipe has wicking materials on the interior walls of its chamber to automatically pump the cooling fluids from the heat sink side to the evaporator side when transporting heat. The wicking materials enable heat pipes some unique functions that thermal siphons could not achieve. The flat geometry allows micro/nano-scale machining techniques to be applied for forming unique wicking materials such as carbon nanotubes, sintered copper, and micromachined structures. Besides wicking materials flat heat pipes have two other important components: a hermetically-sealed cavity and a working fluid. The cavity must be hermetically sealed to avoid input of any noncondensable gases by leaking. Ideally, only different phases of a working fluid are allowed in the chamber. The merit number is used to select a working fluid in Anderson's work for heat pipe systems. In this work water is adopted as the working fluid. For additional information, please contact Changsong Ding.
	Figure 1: Schematic of the titanium based flat heat pipe. Note that the Ti posts function as supporting structure for the sealing substrate Figure 2: SEM photograph of the 5μm/5μm(Dia./Gap) and 100μm/50μm (Dia/Gap) pillars. All pillars are ~50μm high. The nanostructured titania (NST, Fig. 2B) covers all the pillar surfaces. Figure 2D shows the SEM photograph of Figure 2C after gold plating.	Figure 2
	To investigate the thermal performance of the above proposed heat pipe device we packaged two separate substrates (sealing substrate and wicking substrate) as shown in Fig. 1 using a pulsed wave YAG laser welder (Neodymium-Doped Yttrium Aluminum Garnet, Nd: Y3Al5O12) with a wavelength of 1064nm. The bottom substrate contains the wicking material as shown in Fig. 2C with an area of 3cmx3cm. The top substrate has a cavity (200μmx3cmx3cm) with 16 Ti posts evenly located on it for supporting purposes. The net volume of cavity is 170μL. Τwo charging tubes were welded onto the device with laser welding techniques. The device is then purged with pressurized water vapor for 15minute to eliminate the air inside the device. Then it was charged with ~40μL water so that the water level is about ~10μm above the pillar top surface. The final packaged device is shown in Fig. 3. Figure 3A shows the side with wicking substrate and Figure 3B shows the front side which as supporting standoffs, charging tubes, and sealing caps. Figure 3 Figure 4
	A maximum of conductivity 500 W/m-K (with contact resistance included) was achieved with a steady state heat flow of 8.65W at 118^°C on the heat source side of the heat pipe. Contact area and thermal interface material thickness were measured for calculation of the thermal contact resistances of the interface material. Assuming that the thermal contact resistances at different temperatures are the same then we get the device thermal conductivity performance of ~700W/m-K at 118^°C with a heat flow of 8.65W. The titanium heat pipe architecture not only offers rigid, compatible and light weight building structure, but also provides the convenience of packaging at large scale thanks to the weldability of titanium substrates. The hermiticity of the laser welding and capping technique (Fig. 4) was conducted by helium leaking rate test method based on MIL-STD-883E standard. The helium leak detector was used in this test is ASM 142 with a sniffing unit, and its minimum detectable helium leak rate is 1x10^-11atm.cm^3/s. The test is conducted at room temperature while the pressure is maintained to be 1atm in the test chamber.