Radiation heat transfer can be as important as convection heat transfer in electronics cooling, especially in natural convection and low-airflow applications. In part 1 of this 2 part article, we’ll talk a bit about heat sink anodization, and how it affects radiation cooling in thermal management. In part 2 we’ll cover the heat sink anodization process and different types of anodization.
Depending on the type of surface treatment used, radiation heat transfer is enhanced in two distinct ways: by increasing the emissivity of the surface or by increasing the surface area. The effect of both these parameters is shown in the Stephen-Boltzmann Equation:
Where, q is the amount of radiation energy that is exchanged between two bodies whose temperatures are TH and TC. FHC is the surface shape factor, σ is the Stephen-Boltzmann coefficient, and ε and A are the emissivity and area of the surface, respectively.
The Stephen-Boltzmann Equation indicates that the temperature difference between the hot and cold bodies is the driver for radiation heat transfer. However, in almost all electronics applications, neither of these temperatures is a controlled parameter. Therefore, radiation heat transfer can only be enhanced by increasing the emissivity and surface area.
The most common method of surface treatment in electronics cooling is surface anodizing. Anodizing can be an environmentally safe electrochemical process that thickens and toughens the naturally occurring protective oxide layer on the surface of metal parts. Surface anodizing changes the microscopic texture of a metal, making the surface durable, corrosion and weather resistant. In the process, the metal forms the anode (the positive electrode) of an electrolytic circuit. Through an acidic electrolytic solution, the electrical current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the metal anode (the positive electrode), building up a deposit of metal oxide. The acid action is balanced with the oxidation rate to form a coating with microscopic pores, 10-150 nm in diameter. The thickness of the metal oxide depends on the anodizing method and is anywhere from 0.5 to 115 microns thick. It is controlled by electrolyte concentration, acidity, solution temperature, and the process electrical current and voltage .
A typical anodizing process will form an oxidized layer on a metal surface with a thickness ranging anywhere from 1.8 to 25 microns. However, some industrial applications require parts with exceptionally high wear resistance. The process used on such surfaces is termed hard anodizing and produces surfaces that are thick compared to standard anodizing. The coating of a hard anodized surface can be greater than 25 microns thick and is made under special conditions of high current density, very low temperature and the use of special electrolytes .
Various protective benefits and aesthetically pleasing colors have extended the use of anodization to many industrial and commercial applications. For electronics cooling, however, the advantages of surface anodizing are the dielectric isolation of the cooling components from their electronics environment, and the significant increase in their surface emissivity. The increase in the emissivity coefficient on the anodized surfaces of heat exchangers, electronics cabinets and enclosures, heat sinks, etc. is typically on the order of 0.83 to 0.86 . When compared to the emissivity coefficient of bare aluminum, 0.04 to 0.06 , the importance and significance of enhancement of radiation heat transfer would become evident.
The pores created by acidic anodizing on aluminum can easily absorb dyes. Colored dyes are often used on heat sinks for cosmetic and marketing purposes. The color of anodization has no impact on radiation heat transfer. A clear anodized surface has the same emissive characteristics as a black anodized surface. To further protect the surface of dyed anodized heat sinks from corrosion, they are usually sealed by immersion in boiling hot de-ionized water or steam .
In part 2 we’ll cover the heat sinks anodization process and different types of anodization.
- Radiation Heat Transfer and Surface Area Treatment, Qpedia Thermal eMagazine, June 2008.
- Edwards, J., Coating and Surface Treatment Systems for Metals, Finishing Publications Ltd. and ASM International, 1997.
- Aluminum Anodizer Council Web Forum.
- Gustavsen, A. and Berdahl, P. Spectral Emissivity Of Anodized Aluminum And Thermal Transmittance Of Aluminum Windows Frames, Nordic Jounnal Of Building Physics, Vol.3, 2003.
- Ozisik, N.,Heat Transfer: A Basic Approach, McGraw-Hill, 1984.
- Highly Emissive Ion Beam Textured Surfaces for Improved Cooling of Electronic Devices, Electronics Cooling Magazine, September 1997.
- Chi, T. , Ballinger, R., Olds, R., and Zecchino, M., Surface Texture Analysis Using Dektak Stylus Profilers, Veeco Instruments, Inc.