**Fei Kung and Ming-Chien Yang \***

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan; philip@allightec.com

**\*** Correspondence: myang@mail.ntust.edu.tw; Tel.: +886-2-2737-6528; Fax: +886-2-2737-6544

Received: 18 May 2020; Accepted: 8 June 2020; Published: 10 June 2020

**Abstract:** In this study, the epoxy powder was blended with graphene to improve its thermal conductivity and heat dissipation efficiency. The thermal conductivity of the graphene-loaded coating was increased by 167 folds. In addition, the emissivity of the graphene-loaded coating was 0.88. The epoxy powder was further coated on aluminum plate through powder coating process in order to study the effect on the performance of heat dissipation. In the case of natural convective heat transfer, the surface temperature of the graphene-loaded coated aluminum plate was 96.7 ◦C, which was 27.4 ◦C lower than that of bare aluminum plate (124.1 ◦C) at a heat flux of 16 W. In the case of forced convective heat transfer, the surface temperature decreased from 77.8 and 68.3 ◦C for a heat flux of 16 W. The decrease in temperature can be attributed to the thermal radiation. These results show that the addition of graphene nanoparticles in the coating can increase the emissivity of the aluminum plate and thus improving the heat dissipation.

**Keywords:** graphene; powder coating; thermal conductivity; heat dissipation; thermal radiation

### **1. Introduction**

Heat-dissipating coating is important for the stabilization and miniaturization of electronic components. As the aggregate density and power intensity of electronic components continue to increase, large amount of heat generated from these devices must be dissipated in a timely manner. However, the heat dissipation performance of today's electronic components cannot meet the requirements, thereby limiting the efficiency and service life of certain electronic components. To resolve this problem, heat-dissipating coating enhances the heat dissipation efficiency of the surface of a component [1]. It lowers the temperature of the heat-generating component in time and hence extends the service time and stability of components.

Literatures and patents on graphene heat-dissipating powder coating have been sparse; most of them confuse "heat dissipation" with "heat conduction" [2]. In general, the most important functions of a heat dissipation module in an electronic product include not only a rapid transfer of heat from the thermal source to the surface of the heat sink but also the ability to quickly disperse heat into the atmosphere through convection and radiation. A high thermal conductivity can only solve the problem of quick heat conduction. On the other hand, heat dissipation depends mainly on the heat dissipation area, profile, natural convection, and thermal radiation of the heat sink; it almost has nothing to do with the thermal conductivity of materials. Therefore, as long as the thermal conductivity is adequate, heat-dissipating coating can still be used as good heat dissipation modules for electronic products. Proper structural design of product or module can easily achieve a large heat dissipation surface area for convection. However, to achieve high heat dissipation efficiency through radiation, high thermal radiation coefficient is necessary [3].

Graphene is a nanomaterial with only one layer of carbon atoms. It features low density, low chemical activity, high thermal conductivity, large specific surface area, and high infrared emissivity. Graphene has superior heat conduction characteristics and its thermal radiation coefficient is greater than 0.95 [4]. Balandin et al. reported the thermal conductivity of suspended single-layer graphene measured near 5000 W m−<sup>1</sup> K−<sup>1</sup> , which is one of the highest thermal conductivity of the currently known materials [5]. Therefore, from the perspective of heat conduction, heat dissipation, or thermal management, graphene can effectively improve the heat dissipation performance of existing thermal dissipation products for electronic components, assemblies, and LEDs as long as graphene products can be configured to meet the design requirements. However, the stacking tendency of graphene led to poor dispersion and greater post-processing difficulties, thereby preventing graphene from exhibiting its superior characteristics [6,7].

Thermoset powder coating comprises thermoset resin, hardener, dye, filler, and additives. There are several types of thermoset powder coatings: epoxy resin, polyester, and acrylic resin. Table 1 compares the pros and cons of these three types of powder coatings. The constituents are first mixed according to a specific ratio, followed by hot extrusion and crushing and other preparation processes. The coating is then applied by an electrostatic spray or friction spray (a thermoset method) at ambient temperature. It is then baked, melted, and cured to form a shiny permanent coating for heat dissipation and corrosion prevention. [8,9]. Powder coating generally has a better thermal conductivity than solvent coating due to the better binding between the coating and the substrate. More thoroughly cured coating leads to more stable crosslinking and hence denser and tighter coating [10,11]. This favors the reduction of scattering in the "lattice vibration" of the thermal dissipation mechanism.


**Table 1.** Types and surface characteristics of resins.

The discussion of radiation and convection is rare. This study is aiming to investigate the enhancing effect of graphene-loading on the thermal dissipation performance of aluminum plate. The aluminum plate was attached to a heater as the heat source. The heat was transferred through the Al plate to the ambient atmosphere via convection and radiation. The plate was either bare or coated with a thin layer of polymer filled with graphene nanoflakes or boron nitride nanoparticles. The performance of the heat dissipation was evaluated by measuring the surface temperature on the plates with or without coating at a constant heat flux under forced convection or natural convection conditions. This study will demonstrate the significance of radiation heat transfer in the heat dissipation.

#### **2. Materials and Methods**

#### *2.1. Materials*

Graphene (AG05, grain size 5 µm, thickness 3.5 nm, aspect ratio 1429) was supplied by Allightec Co., Taichung, Taiwan. Aluminum plates (AL101001, Kuopont Chemical, Taoyuan, Taiwan) were used as the substrate for coating. The dimensions of the plate were 10 <sup>×</sup> <sup>10</sup> <sup>×</sup> 0.1 cm<sup>3</sup> . Epoxy resin (E12(604), Dow Chemical, Midland, MI, USA) and polyester (SJ4ET, Shenjian New Materials, Wuhu, China) were used as the matrix of the coating. Furthermore, hardener (HR0001, Kuopont Chemical, Taiwan) and additive (AD0001 Chemical, Kuopont, Taoyuan, Taiwan) were employed to give the coating (Table 2) both chemical resistance and weather resistance.


**Table 2.** Composition of powder coatings.
