Non-uniform thermal properties of an alumina granule/epoxy potting compound

In harsh environments, including high shock and vibration, magnetic devices such as transformer coils are potted to enhance thermal performance and provide mechanical protection. One potting compound frequently used is epoxy containing alumina particles. A nominally isotropic and uniform potting compound consisting of about 70 to 80% by volume 14–28 mesh (0.6 to 1.2 mm across) alumina granules in low viscosity epoxy was tested to determine its thermal properties. Examination by optical microscopy revealed that there was significant variation in volume fraction of alumina particles by location. The specific heat and thermal conductivity of the compound were measured using a Differential Scanning Calorimeter and guarded heater method based on the ASTM D5470-06. The thermal properties were found to vary with time, location, and temperature; with the specific heat ranging from 1.00 J/g°C ± 14% at 25°C to 1.22 J/g°C ± 12% at 125°C and an apparent thermal conductivity of 2.56 W/m·K ± 23%. Users of such compounds should be aware that the thermal properties are not necessarily constant in time or uniform, and assuming that they are could lead to significant errors when modeling their performance.