**4.2 The effect of the nanostructure condenser on the thermal performance of the vapor chamber**

The thermal performance of the asymmetric vapor chamber is compared with that of a conventional vapor chamber and a copper with the same dimensions, as shown in **Figure 15**. The charge amount of the asymmetric vapor chamber is set at

#### **Figure 14.**

*Features of the condenser: (a) front view of the condenser; (b) water contact angle of the condenser; and (c) amplified view of the nanostructure [26].*

**Figure 15.**

*Thermal performance comparison of the vapor chamber with a conventional vapor chamber and copper plate: (a) horizontal thermal resistance and (b) vertical thermal resistance [26].*

1.71 g and 1.26 g. The pressure inside the vapor chamber is evacuated to 3.6 kPa. The conventional vapor chamber refers to a vapor chamber with symmetrical structure, which consists of two layers of copper mesh with 40 μm and 80 μm wire diameter sintered on both the evaporator and the condenser as a wick structure. Therefore, the evaporator and the condenser have the same hydrophilicity feature. The copper plate is a solid pure T2 copper solid heat spreader. The conventional vapor chamber and the copper plate have the same dimension as the asymmetric vapor chamber.

The asymmetric vapor chamber shows a much lower horizontal thermal resistance compared with the conventional vapor chamber and copper plate, as shown in **Figure 15(a)**. The horizontal thermal resistance of the asymmetric one is kept at around 0.02°C/W for all the testing heat flux. Especially, the horizontal thermal resistance at 15 W/cm2 heat flux for the asymmetric one is only 1/5 of that for the conventional one which has a vertical thermal resistance even higher than that for the copper plate. The difference of the horizontal decreases with the increase of the heat flux and is eliminated at the heat flux of 82 W/cm2 . This proves that a nanostructured superhydrophobic condenser can enhance the temperature uniformity of the vapor chamber.

**Figure 15(b)** presents the vertical thermal resistance of different heat spreaders. Both types of the vapor chamber have a lower thermal resistance compared with the copper plate. Besides, the thermal resistance of the asymmetric one when the heat flux is less than 70 W/cm2 is lower than that of the conventional one. However, when the heat flux is larger than 70 W/cm<sup>2</sup> , the asymmetric one with 1.26 g water has a larger thermal resistance than that of the conventional one. This can be explained by insufficient water flowing back to the evaporator. Thus, increasing the amount of water may lower the vertical thermal resistance at the high heat flux for the asymmetric one, which is proved by the asymmetric one with 1.71 g shows a best vertical thermal performance at the heat flux of 82 W/cm<sup>2</sup> .

In summary, the asymmetric vapor chamber with a nanostructured superhydrophobic condenser has a better thermal performance both in horizontal and vertical aspects when compared with the conventional vapor chamber. This can be explained as follows: the mode of the condensation on the condenser of the conventional vapor chamber is filmwise condensation which occurs at the hydrophilic surfaces while dropwise condensation is the mode for the condensation on the condenser of the asymmetric vapor chamber. Dropwise condensation has at least 3 times the heat transfer rate of filmwise condensation [25]. Thus, the thermal resistance of the condenser for the asymmetric vapor chamber is lower than that for the conventional one. Besides, droplets formed during dropwise condensation

### *Multiscale Micro/Nanostructured Heat Spreaders for Thermal Management of Power Electronics DOI: http://dx.doi.org/10.5772/intechopen.100852*

can return to the evaporator by directly contacting or falling into the evaporator surface, while liquid film formed during filmwise condensation only can return the evaporator through the wick structure. Therefore, dropwise condensation induced by the nanostructured superhydrophobic surface also improves the backflow ability of the wick structure. As a result, a larger amount of heat is removed and spread to the whole inner space in the asymmetric vapor chamber.

The authors presented wettability modification and patterning can enhance the thermal performance of vapor chambers in this and the last sections. In the next section, another surface modification method, which is growing nanostructured on the microstructure to form a multiscale micro/nanostructured wick structure, also can improve the thermal performance of vapor chambers.
