Thermal Conductivity Measurements: Importance of Measuring Under Representative Application Conditions

By Jacob Pierce, Market Analyst

Thermal conductivity is a property that describes a material’s ability to conduct heat. Measuring thermal conductivity under representative conditions is important because it gives a better understanding of the thermal properties of materials as they are applied in the application. By knowing the conductivity value of a material under different conditions, you can determine its suitability for specific applications. Below are a select few of the important testing conditions that should be considered in obtaining representative thermal conductivity for your intended purpose.

Figure 1 – Trident Thermal Conductivity Platform with MTPS sensor, capable of operating in a wide range of environmental conditions for truly representative testing  

Temperature

When measuring the thermal conductivity of a material, considerations must be made as to what temperature the material will be operating in. Measuring the thermal conductivity of a material at room temperature will not provide sufficient insight into how that material will at varying temperatures. For example, a thermal interface material used in an electronic assembly will often be subject to various temperatures inside an enclosure. It is, therefore, beneficial to measure the conductivity of the TIM under representative temperatures in understanding its performance for the entire operating temperature range, as oftentimes in TIM applications, a weak temperature dependence of thermal conductivity is desirable. Pictured below in Figure 2 is the relationship between thermal conductivity and varying temperatures of three epoxy resin composites [1], measured using the Modified Transient Plane Source (MTPS) sensor from C-Therm Technologies. The relationship shows a notable increase in conductivity as the temperature increases from room temperature to approximately 60 °C.

Figure 2 – Epoxy resin-based TIMs filled with Ag nanoparticle-decorated graphene nanosheets – School of Environmental and Materials Engineering, College of Engineering, Shanghai

Humidity

Moisture content is an additional environmental condition that can significantly affect the thermal conductivity of hygroscopic materials. This is mainly prevalent in materials such as foams, insulations, and porous adsorbents. A paper published by the Interdisciplinary Graduate School of Engineering Studies in Japan [2] studied the effects of relative humidity on the effective thermal conductivity for zeolite-based absorbents. The MTPS sensor was placed inside of a control cell with the humidity level managed by an evaporator. Figure 3 below shows a photo of the experimental setup, with the resulting data shown in Figure 4.

Figure 3 – Experimental setup for measuring thermal conductivity under varying relative humidity levels – Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Japan

Figure 4 – Results of effective thermal conductivity measurements at varying levels of relative humidity

Compression

For non-rigid materials susceptible to changes in density, the thermal conductivity is known to be impacted by the densification of the medium. This is important in measuring textiles, powders, foams, and insulations. One of the increasingly important application spaces dealing with compression is the electronics industry with gap pads and other thermal interface materials. Gap pads typically experience compression to some extent while in use, densifying under the compressed load. Some compression is likely appropriate for measurements, but it should be representative of the application. An ideal option for these measurements would be to test under a range of applied forces for representative results. This can be done by pairing the MTPS sensor with C-Therm’s CTA Compression Test Accessory (CTA), allowing for not only testing under representative conditions but also in a repeatable manner. Figure 5 below shows the relationship between thermal conductivity and the applied force of a gap pad-style thermal interface material, as measured with the MTPS sensor and the CTA.

Figure 5 – Relationship between thermal conductivity and the applied force of a gap pad-style thermal interface material, as measured with the MTPS sensor and the CTA.

Note: TIM Testers

TIM Testers are often used for performing impedance measurements of TIMs (Thermal Impedance vs Thermal Conductivity), which are then used to calculate ‘apparent’ thermal conductivity according to the ASTM D5470 standard. Issues can start to arise, however, as certain types of TIMs will require higher forces for measurements while using a TIM Tester, up to 500 PSI in some cases. These high levels of force result may result in excessive densification, which will impact the measurement and the calculated apparent conductivity. This can produce results of higher thermal conductivities, which may seem beneficial at first glance, but is not necessarily a representative result. It should also be noted that the standard has no stated value of precision or bias. It is stated that measurements were taken by laboratories in a round-robin study, in which the calculation of apparent thermal conductivities was found to be +/- 18% of the mean values from all of the labs involved.

Further Reading

To read more about thermal conductivity measurements under representative testing conditions, please see the blog post here: Why is the thermal conductivity so low? The effect of interstitial air on the effective k of powders.

References

[1] Chen, L., Zhao, P., Xie, H., & Yu, W. (2016). Thermal properties of epoxy resin based thermal interfacial materials by filling ag nanoparticle-decorated graphene nanosheets. Composites Science and Technology, 125, 17–21. https://doi.org/10.1016/j.compscitech.2016.01.011