In thin handheld electronic devices, such as smartphone and tablet computers, high conductivity flexible graphite heat spreaders have become the passive thermal management solution of choice. Graphite is used in these applications because of its unique anisotropic properties of high in-plane thermal conductivity and low thru-thickness thermal conductivity. With the ratio of in-plane to thru-thickness thermal diffusivity and conductivity as high as 500:1, the anisotropic nature of graphite sheets defies isotropic thermal measurement techniques and complicates numerical analysis. Reliable measurement of in-plane thermal diffusivity is advantageous to the design and manufacture of these high performance graphite heat spreaders.
Angstrom’s method for determining thermal diffusivity is the fundamental technique for measuring this property in thin sheets. This technique involves applying a periodic heat load to one end of a sample and detecting the resulting temperature variations in the sample at different distances from the heat source. This paper compares two instruments based on Angstrom’s method. The first is a custom built laboratory device and the second is a commercially available instrument. The implementation of Angstrom’s method in each device is different. One unit uses a Peltier device to induce a sinusoidal heat load at one end of a long specimen and two thermocouples to determine the temperature profile. The other device steps a pulsed laser along a short specimen at incremental distances and uses a single fixed thermocouple to capture the temperature profile.
This paper compares thermal diffusivity measurements made with each device on several grades of graphite sheets, with nominal in-plane conductivity in the range of 300–1500 W/m·K. A Measurement System Analysis that uses statistical techniques to examine the reproducibility and repeatability of each instrument is performed and results are compared as a function of instrument, material and operator.