Trident™ Thermal Conductivity Tech Tip: Measuring Anisotropic Samples Using Flex TPS Sensor

The following Tech Tip addresses the measurement of polymeric anisotropic using the Flex Transient Plane Source (TPS) sensor and the Anisotropic Utility available on Trident.  

Anisotropy is the property of a material that exhibits directionality with respect to the axis of observation (Figure 1). This property is becoming increasingly important, especially for thermal conductivity and effusivity of a material, with applications in aerospace, automotive, heat sinks and composite design leading the way. Anisotropy can be found in multiple different material’s types including polymers, engineered crystals, composite materials and geological samples. In the case of thermal conductivity of polymers and composite polymer materials, anisotropy arises from the directionality and alignment of polymer and polymer additives within the matrix. In-plane and out of plane thermal conductivity measurements can differ from 1 – 15 W/mK, especially those composite materials which contain carbon-based materials. Directionality within a crystalline material stems from a difference in symmetry within the crystal lattice. A classic example of an anisotropic crystal is sapphire (aluminium oxide) which possesses two different thermal conductivities with respect to orientation (36 W/mK along c-axis and 32 W/mK along a-axis).  

Figure 1 - (Left) A schematic representation of a material exhibiting anisotropic properties, and (right) examples of anisotropic materials. (Copyright Wikipedia Commons, 2020).

While these materials do exist, thermal conductivity measurement methodologies were heavily developed to explore isotropic or bulk samples, thus several methods have been explored to characterize these materials. These methods include modified transient plane source (MTPS), or transient plane source (TPS) methods, both possessing the tools to fully characterize these types of materials. The main difference between each of these methods is the number of measurements required to measure the anisotropic thermal conductivity of the material. With MTPS measurements, each axis in question needs to be measured, duplicating efforts for this measurement, whereas with TPS, the heat pulse is modelled and the measurement calculated from the 3D heat tensor which is recorded. Radial and in-plane thermal conductivity can be measured and separated.

TPS Anisotropic Utility Basics

Figure 2 - C-Therm FLEX Transient Plane Source Thermal Conductivity Sensor.

The transient plane source method employs a sensor which is made from a spiral of electrically conductive nickel and two layers of insulating Kapton (Figure 2). By applying a voltage to the sensor, a planar heat pulse maybe generated and measured by the sensor. By measuring the sample’s temperature vs time, thermal conductivity can be directly measured. Anisotropic bulk samples that are radial symmetric (a = b ≠ c) can be measured through this method, and the radial and in-plane thermal conductivity extrapolated from the linear heat pulse.

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