January 20, 2025
Thermal contact resistance (TCR) is an important concept for engineers and scientists working on thermal management systems as it affects efficiency and performance of many devices such as electronic cooling systems and industrial heat exchangers. A lower TCR improves heat transfer between two objects.
When two bodies come in contact, heat flows from the hotter to the colder body. But the temperature drop is observed at the interface of the two surfaces due to the surface roughness and air gaps present between the two surfaces. This is due to the concept called thermal contact resistance.
The temperature drop observed on the interface due to microscopic level surface roughness of the objects.
The formula of TCR is:
R=ΔT/Q
where:
R = Thermal contact resistance (m²K/W)
ΔT = Temperature difference across the interface (K)
Q = Heat flow through contact surface (W/m²)
The unit of TCR, m²K/W, represents the degree of thermal resistance per unit area. The lower value of ‘R’ indicates more efficient heat transfer, which is vital for high-performance thermal systems.
There are many factors that determine its magnitude:
Surface roughness: Thermal resistance stems from the inherent gaps on the surface due to the roughness of it. Smoother surfaces reduce the air gaps, thus will have a lower TCR. Hence, to enhance thermal conductivity material surfaces are made to be as smooth and planar as possible to improve heat conduction at the interface.
Materials properties: Metals like copper, silver and aluminum have high thermal conductivity and thus reduces TCR. On the other hand, low conductivity materials like ceramics and titanium increase it.
Contact pressure: Force applied per unit area on two surfaces that are in contact with each other can be defined as contact pressure. High contact pressure compresses the air gaps between the interfaces, as a result increasing the effective contact area, and thus lowering the TCR.
Interfacial materials: Thermal interface resistance can be reduced by employing interface materials like grease that can establish intimate contact between the two surfaces. These materials are highly conformable and displace as much air as possible when coming in contact.
Oxidation of metallic surfaces: At high temperatures, metallic surfaces may oxidize, forming an oxide layer on the interface which has a lower thermal conductivity than the base metal. This additional layer becomes another barrier to heat flow, thus increasing TCR.
Temperature: Elevated temperatures can initiate changes in material properties, causing deformation at the interface. While this does increase the contact area, it also degrades the material properties, which reduces the effectiveness of thermal conduction.
To accurately measure this involves techniques and instruments to capture heat flux and temperature differences across the interface.
To delve deeper into the differences between transient and steady-state techniques for thermal conductivity measurement, read our comprehensive guide.
Thermal contact resistance is a pivotal factor in designing efficient thermal systems. By understanding its principles and reducing its effects, engineers can improve thermal management, leading to enhanced performance and sustainability. Whether you’re optimizing electronic devices or industrial equipment, mastering TCR is essential for maintaining a competitive edge.
Thermal contact resistance is the resistance to heat flow at the junction where two materials touch. The tiny air gaps due to surface roughness makes the heat transfer less efficient.
It is calculated using the formula: R= ΔT/Q, where ΔT is the temperature difference and Q is the heat flow through the surface.
There are a few ways to reduce TCR such as using smoother surfaces, increasing contact pressure and applying thermal interface materials like thermal grease to the surfaces.
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M. Grujicic, C. Z. (2005). The effect of thermal contact resistance on heat management in the electronic packaging. Applied Surface Science, 290-302.
Yaoqi Xian, P. Z. (2018). Experimental characterization methods for thermal contact resistance: A review. Applied Thermal Engineering, 1530-1548.