The standard “ASTM D5470-12 Standard test method for thermal conductivity properties of thermally conductive electrical insulating materials” developed by the American Society for Testing and Materials (ASTM) describes a test method for measuring thermal impedance and calculating apparent thermal conductivity for thermally conductive electrical insulating materials ranging from liquid compounds to rigid solid materials.
The term thermal conductivity applies only to homogeneous materials. Thermally conductive electrical insulating materials are usually heterogeneous, and to avoid confusion this test method uses apparent thermal conductivity to determine the thermal conduction properties of both homogeneous and heterogeneous materials.
For the purposes of this standard, the following terms and definitions apply:
The test method described here is based on the idealized heat conduction between two parallel, isothermal surfaces separated by a test specimen of uniform thickness. The thermal gradient imposed on the specimen by the temperature difference between the two contacting surfaces causes heat flow through the specimen. This heat flow is perpendicular to the test surfaces and is uniform across the surfaces, with no lateral heat spread.
When two measuring rods are used, the measurements required by this standard are:
Based on the idealized test configuration, measurements are made to calculate the following parameters:
There is an interface thermal resistance between the sample and the test surfaces. These contact resistances are included in the calculation of the sample thermal impedance. The contact resistance varies greatly depending on the structure of the sample surface and the mechanical pressure applied to the sample by the test surfaces. Therefore, the compressive pressure applied to the sample should be measured and recorded as a secondary measurement required by the method, except for fluid samples where the applied pressure is negligible. The thermal impedance calculation consists of the sum of the sample thermal resistance and the interface thermal resistance.
The calculation of apparent thermal conductivity requires accurate determination of the thickness of the test specimen. Depending on the material type, different tools can be used to control, monitor, and measure the test specimen thickness. If the size of the specimen may change during testing, the test specimen thickness can be controlled with wedges or mechanical stops. If the size of the specimen may change during testing, the test specimen thickness can be monitored during testing by in situ thickness measurement. The thickness of the test specimen may be measured as manufactured at room temperature in accordance with Test Method C of ASTM D374 if it exhibits negligible pressure deflection.
This standard measures the steady-state thermal impedance of electrical insulating materials used to enhance heat transfer in electrical and electronic applications. This standard is particularly useful for measuring the thermal conduction properties of samples that are very thin or do not have sufficient mechanical stability to permit temperature sensors to be placed in the sample, as in ASTM E1225.
This standard applies an idealized heat flow pattern and specifies an average specimen test temperature. Thermal impedances measured in this manner are not directly applicable to most practical applications where these necessary uniform, parallel heat conduction conditions are not present.
This standard is useful for measuring the thermal impedance of the following types of materials.
The apparent thermal conductivity of a sample can be calculated from the measured thermal impedance and the measured sample thickness if the interface thermal resistance is negligibly small (nominally less than 1 percent) compared to the thermal resistance of the sample. The apparent thermal conductivity of a sample material can be accurately determined by excluding the interface thermal resistance. This is accomplished by measuring the thermal impedance of different thicknesses of the material under test and plotting the thermal impedance against the thickness. The inverse of the slope of the resulting straight line is the apparent thermal conductivity. The intersection at zero thickness is the sum of the contact resistances at the two surfaces. The contact resistance of hard test specimens (Type III) can be reduced by applying thermal grease or oil to the test surfaces.
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