Directional Thermal Conductivity of a NIST Certified Reference Material using the Transient Plane Source Anisotropic Test Method

The Transient Plane Source Thermal Conductivity System, your powerful ally in thermal analysis, is currently the only instrument in the world capable of measuring both bulk thermal conductivity and directional thermal conductivity of materials.

Recently the bulk thermal conductivity (W/m∙K) and anisotropic or directional thermal conductivity (W/m∙K) was measured for a NIST Certified Reference Material using the TPS System. The results were compared to certificate thermal conductivity values supplied by NIST, generated from steady state test methods.

When measuring anisotropic thermal properties with the TPS System, it is assumed:

• The main directions in the material are orthogonal and can be described in terms of x-, y- and z- axes.
• The properties along the radial direction (y- and z- axes) are identical and the properties along the axial direction (x- axis) are different.

When making measurements it is vital to orient the sample and TPS sensor correctly according to the main directions in the material. The TPS sensor is positioned so that it would be perpendicular to the axial direction (through plane) and parallel with the radial direction (in plane).

As only one measuring position with the TPS sensor is possible, volumetric heat capacity (MJ/m³·K) of the sample is entered in order to determine directional thermal conductivity.

Where bulk thermal conductivity is an average representation of the thermal properties of a sample and given that it was expected the NIST Certified Reference Material would be anisotropic due to fiber orientation in the radial direction of the sample, we expected a lower thermal conductivity in the axial direction than the radial direction and the bulk thermal conductivity would be some weighted average of the thermal properties of the sample in two planes.

This was confirmed in testing. See Table 1 below.

Table 1. Anisotropic and Bulk Thermal Conductivity of NIST Certified Reference Material

Material	Stated λ (W/m·K)	Axial λ (W/m·K)	Radial λ (W/m·K)	Bulk λ (W/m·K)
NIST Certified Reference Material	0.0209	0.0198	0.0400	0.0285

Notes: Stated λ is certificate thermal conductivity; Axial λ is through plane thermal conductivity measured using the TPS Anisotropic Module; Radial λ is in plane thermal conductivity measured using the TPS Anisotropic Module; Bulk λ is bulk thermal conductivity measured using the TPS Standard Analysis Module. All measurements were made at ambient temperature and pressure.

The NIST Certified Reference material (Cp: 872 J/kg·K) was expected to be anisotropic and this was confirmed. The radial thermal conductivity was 2.0 times higher than the axial thermal conductivity. The bulk thermal conductivity, a weighted average of the directional thermal conductivity results, was not in the range of the stated thermal conductivity. BUT the axial thermal conductivity was with a relative difference of 5.4% between axial thermal conductivity and the stated thermal conductivity values.