A composite is a material that is made up of two or more constituents that remain distinct and identifiable after mixing. Composites commonly consist of a matrix and either one or multiple fillers. Matrices used to form composites include metals, polymers and ceramics. Through the mixing of multiple components, scientists and engineers seek to take advantage of certain properties of each individual component.

Depending on the desired application of a composite, it may be important to consider if that material is anisotropic. An excellent tool for the characterization of anisotropic materials is the Hot Disk Transient Plane Source (TPS) technique. Due to the alignment of fillers in composites, they will sometimes be anisotropic, and thus require special treatment. The Hot Disk TPS instrument has a special module designed specifically to measure directionally dependent thermal conductivity and thermal diffusivity. Thus thermal conductivity values for both directions can be obtained with input of volumetric heat capacity (which can be obtained using the heat capacity module).

A common outcome for researchers is to create composites that have either a higher or lower thermal conductivity than the pure matrix. These composites can then be used as building materials, thermal interface materials and phase change materials, among others. Research performed by Miller et al.1 at the Michigan Technological University demonstrates the ability of the Hot Disk TPS technique to measure anisotropic properties of composite materials accurately. A variety of composites were prepared by the addition of different amounts of synthetic graphite particles to a liquid crystal polymer. An extruder was used to prepare the various composites with synthetic graphite particles ranging from 0 to 70 wt. %

Using the TPS anisotropic module the directionally dependent thermal conductivity of each composite was determined. Directionally dependent thermal diffusivity can also be obtained in this way. The Hot Disk TPS technique provides results with a better than 5% accuracy and 2% reproducibility.

Not only were the composites tested by the TPS technique, but also by the steady-state guarded heat flow meter method (ASTM F433) in order to compare the data obtained from each method.

Table 1. Thermal conductivity values obtained from the guarded heat flow meter and TPS techniques at varying amounts of synthetic graphite filler. k represents thermal conductivity.

Wt.% of Synthetic Graphite Particles	Through-Plane k Guarded Heat Flow Meter (W/mK)	Through-Plane k Hot Disk TPS (W/mK)	In-Plane k Hot Disk TPS (W/mK)
0	0.2169	0.22	0.22
10	0.2935	0.284	1.426
20	0.3869	0.385	2.029
30	0.5464	0.545	2.938
40	0.7064	0.702	5.881
50	1.1081	1.101	9.020
60	1.5586	1.545	16.03
70	2.6251	2.624	32.55

The guarded heat flow meter method measures through-plane thermal conductivity, which when compared to the through-plane thermal conductivity obtained from TPS testing shows excellent agreement. This data shows that the Hot Disk Transient Plane Source (TPS) technique is an excellent method for determination of anisotropic properties of composite materials.