**4. An asymmetric vapor chamber with a nanostructured superhydrophobic condenser**

Compared with filmwise condensation occurring at a hydrophilic surface, dropwise condensation induced by a hydrophobic surface can largely increase the condensation heat transfer rate both under vacuum and atmosphere conditions [25]. Therefore, integrating a hydrophobic surface to the condenser of a vapor chamber is promising to enhance the thermal performance of the vapor chamber. In this section, the design and fabrication of a 70 mm × 70 mm × 3 mm asymmetric vapor chamber with a nanostructured superhydrophobic condenser are introduced firstly. Then the effect of the nanostructured superhydrophobic surface is discussed based on the experimental results.

### **4.1 Design, fabrication, and thermal performance test of the vapor chamber**

**Figure 13(a)** illustrates the design of the vapor chamber. To compare the thermal performance with a conventional vapor chamber, the major difference between the proposed one and the conventional one is the wick-laid condenser for the conventional one is replaced by a nanostructured superhydrophobic condenser.

#### **Figure 13.**

*(a) The schematic of the vapor chamber with a nanostructured superhydrophobic condenser and (b) the schematic of the evaporator base [26].*

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

As shown in **Figure 13(b)**, the evaporator base with an area of 70 × 70 mm2 is made of oxygen-free copper with a special configuration: (1) An array of studs is uniformly distributed on the copper substrate to prevent distortion caused by the pressure difference between the inner space of the vapor chamber and atmosphere; (2) A rectangular slot is set at the center of the substrate for sintering fine copper powders to enlarge capillary pressure in the center of the evaporator; and (3) A feeding tube is installed at the edge of the substrate to evacuate and feed working fluid. To fabricate the wick structure of the evaporator, a layer of size 57 μm copper powder is put into the center rectangular slot and then a layer of size 100 μm copper powder is covered the overall substrate. After that, a sintering process of the wick structure is performed in a 975°C hydrogen/nitrogen atmosphere for 2.5 h. This composite wick structure has an excellent backflow ability to prevent the occurrence of the partial dry-out as the smaller pores in the central wick provide higher capillary pressure.

The nanostructure of the condenser surface is fabricated with the method described in Section 3.1. Then, the nanostructured condenser is coated with a monolayer of FAS-17 using the method also described in Section 3.1. **Figure 14** shows the water contact angle of the condenser and the view of the flower-like nanostructure.

The thermal performance evaluation experimental setup is conducted on the LW-9510 platform which is described in Section 3.1. As the test power is much larger than that for the ultrathin vapor chamber, the cooling fan is replaced by a 170 × 80 mm<sup>2</sup> aluminum block attached to the outside surface of the condenser. The temperature of the aluminum block is set at 35°C and controlled by a water circulation system. Besides, the heater size is changed to 15 mm × 15 mm. In this study, the authors use the same parameter as that for the ultrathin vapor chamber, horizontal and vertical thermal resistance, to assess the thermal performance of this vapor chamber. The uncertainty of the heat load, temperature measurement, and the length measurement is ±0.1%, ±0.2%, and ± 0.4%, respectively [26].
