**5.2 Effect of the multiscale micro/nanostructured wick structure on the thermal performance of two-phase heat spreaders**

The thermal performance of two types of the asymmetric vapor chamber with different wick structures on the evaporator is compared with each other, as shown in **Figure 17**. The vertical thermal resistance for both vapor chambers decreases from around 0.63°C/W to 0.32°C/W when the heat flux increases from 5 W/cm2 to the maximum test heat flux, which is shown in **Figure 17(a)**. However, the micro/ nanostructured wick structure gives a quicker decrease of the vapor chamber at the low heat flux range compared with the bare sintered wick. For example, the vertical thermal resistance of the vapor chamber with micro/nanostructured wick shows a 0.25°C/W decrement with the increase of the heat flux from 5 W/cm2 to 35 W/cm2 . While with the bare sintered wick, it only decreases 0.18°C/W. Besides, the micro/ nanostructured wick structure enhances the vertical thermal performance for all the test heat flux compared with the bare sintered wick. These can be explained as follows: In the low heat flux range, thin-film evaporation is the dominant heat transfer mode for the evaporator. The thin-fin array nanostructure grown on the microstructure surface increase the superficial area of the wick structure, leading to an enlargement of the thin-film evaporation area. Thus, the heat transfer rate of the evaporator is improved. In the high heat flux range, nucleate boiling replaces the thin-film evaporation to become the dominant heat transfer mode in the evaporator. As shown in **Figure 16(b)**, the nanostructure roughens the wick structure when compared with the bare sintered wick shown in **Figure 16(a)**, resulting in more nucleation sites in the wick structure. Thus, the extent of boiling in the evaporator is enhanced and the heat transfer rate is increased. Moreover, the backflow ability of the multiscale micro/nanostructured wick structure is enhanced owing to its super hydrophilicity. Thus, the vapor chamber can work well at high heat flux conditions.

#### **Figure 17.**

*Thermal performance comparison of the vapor chambers with different wick structures: (a) vertical thermal resistance and (b) horizontal thermal resistance [29].*

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

The enhancement of the horizontal thermal performance, presented in **Figure 17(b)**, can be also explained with the advantages provided by the nanostructure. The improved thin-film evaporation at the low heat flux on the micro/nanostructured wick structure increases the amount of the vaporized liquid spreading to the whole space of the vapor chamber, resulting in a reduction of the horizontal thermal resistance by 30–50% within the heat flux ranging from 5 W/cm<sup>2</sup> to 35 W/cm<sup>2</sup> . Besides, the micro/nanostructured wick structure gives a more stable horizontal thermal performance from low heat flux region to high heat flux region. Generally, the vapor chamber with a multiscale micro/ nanostructured wick structure has a better horizontal thermal performance than that with a bare sintered wick structure.

The thermal performance of the ultrathin vapor chamber can be also enhanced by nanostructuring the wick structure to form a micro/nanostructured wick structure. As shown in **Figure 12(a)** and **(c)**, the difference of the horizontal thermal resistance between case 1 and case 2 under the optimum charge ratio (62.8% for case 1 and 60.4% for case 2) can be neglected within the range of heat flux from 8.59 W/cm<sup>2</sup> to 19.84 W/cm<sup>2</sup> . However, it becomes obvious when the heat flux increases to 23.91 W/cm<sup>2</sup> owing to the occurrence of serious partial dry-out on the evaporator of case 2. A large increment of the vertical thermal resistance for case 1 is also found at the heat flux of 23.91 W/cm<sup>2</sup> while it for case 2 just slightly increases, which are shown in **Figure 12(b)** and **(d)**. These also prove the enhancement of the backflow ability induced by the multiscale wick. Besides, the vertical thermal resistance under the optimum charge ratio for case 2 is smaller than that for case 1 due to the enhancement of the thin-film evaporation that happened on the evaporator.

The authors also adopted the micro/nanostructured wick structure for another type of two-phase heat spreaders, the thermal ground plane, which works in a one-dimensional way [30]. By combining a two-layer structure with the multiscale wick, the thermal performance of the thermal ground plane is enhanced. Therefore, growing nanostructure on the microstructure surface to form a multiscale wick is an efficient way to improve the thermal performance of various kinds of two-phase heat spreaders.
