**3.1 Design and fabrication of the ultrathin vapor chambers with wettability patterned surface on its evaporator**

**Figure 7(a)** presents the cross-section schematic of the ultrathin vapor chamber for case 1. Ultrathin copper (C1100P, 99.9%) is selected as the casing material and the #500 stainless-steel (SS 304) mesh covered with a layer of copper is adopted as the wick structure. A square array of micropillars integrated into the inner surface of the condenser is used for supporting the space of the vapor core. To investigate the effect of a multiscale micro/nanostructured surface, the only difference

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

#### **Figure 7.**

between case 1 and case 2 is the wick structure of case 2 is nanostructured to form a multiscale micro/nanostructured surface, as shown in **Figure 7(b)**. Case 3 which is shown in **Figure 7(c)** is developed based on the structure of case 2. A wettability pattern is fabricated on the inner surface of the evaporator casing material to examine its effect.

The square array of micropillars is fabricated through photolithography and electroplating, which is described below and shown in **Figure 8(a)**:

1.A 40 μm copper foil is cleaned with acetone, isopropanol, and DI water ultrasonic in sequence to remove organic contaminants. Then it is immersed into 20% sulfuric acid to dissolve the native oxide layer on its surface;

**Figure 8.**

*(a) the fabrication procedures of the micropillar array and (b) the picture of micropillar array [22].*

*The cross-section prototypes schematic of three ultrathin vapor chambers [22].*


The wick structure of case 1, copper-covered #500 stainless-steel (SS 304) mesh (wire diameter: 25 μm, wire spacing: 25 μm) is fabricated with the following method. First, the mesh is activated at a current of 2 A for 2 min in a stainlesssteel electroplating activation solution (From Beichen Limited Company, China) to enhance the bonding quality between the electroplated copper layer and the stainless-steel mesh. Then, the copper layer is formed by electroplating in a stationary solution with 0.8 M CuSO4 and 1.5 M H2SO4 at a current of 1 A for 5 min, as shown in **Figure 9(a)**. The nanostructured copper-covered stainless-steel mesh of case 2 is fabricated by oxidizing the copper layer through a chemical surface modification method [23]. The method is firstly immersing the copper-covered stainless-steel mesh into an aqueous solution with 0.065 M K2SO8 and 2.5 M KOH at 70°C for 30 min. Then the mesh is rinsed with water and dried in a 180°C oven for 1 h. A flower-like nanostructure is grown on the surface of the copper layer, as shown in **Figure 9(b)**.

**Figure 9.** *(a) The stainless-steel mesh with a layer of copper and (b) the mesh with a layer of flower-like nanostructure [22].*

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

**Figure 10.**

*(a) The fabrication procedures of a wettability pattern and (b) wettability check of a wettability patterned surface [22].*

The wettability patterned surface for case 3 is fabricated with the following method, as shown in **Figure 10(a)**:


**Figure 10(b)** shows a wettability check method using the condensation of water. The vapor condenses on the hydrophilic area, proving the success of the fabrication. The wettability pattern is composed of hydrophobic islands with a 45 μm side length and a 65 μm pitch distance between two nearby islands.

After the fabrication of all components, the mesh is sandwiched between the bottom and top casings, and then it is sealed by SnAg (97/3) solder. A tiny copper tube is connected to the inside of the chamber for the evacuating and feeding process. The dimensions of ultrathin vapor chambers are summarized in **Table 2**.


#### **Table 2.**

*Dimensions of ultrathin vapor chambers [22].*
