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Thermal Diffusivity and Conductivity via Flash Method

PMIC recommends thermal diffusivity testing using the flash method for design applications, process control, quality assurance, and high sample throughput measurements.

 

The flash method is invaluable as a relatively inexpensive, fast test method, usually requiring only a small amount of material for testing.  PMIC introduced a TA Instruments Xenon Flash DXF-900 to its laboratory in 2018 and has since offered a variety of services for the nuclear, aerospace, and additive manufacturing industries.  A description of the method is available on the thermal conductivity testing services page.  The system can hold up to three specimens, allowing for batch measurements.  At room temperature, the measurement is almost instantaneous, while elevated temperatures require more time to allow for the system to heat up.  Thermal diffusivity testing using the flash method is a direct measurement method.  Unique to the flash method, diffusivity measurements increase precision with increase in temperature [1].

The flash method can also be used to calculate thermal conductivity at temperature using the ratio of maximum temperature rise, density, and thickness between the specimen and a reference material.  Unlike other thermal conductivity measurement methods such as ASTM E1225 or ASTM C518, specimens do not need to reach a steady state, significantly decreasing the time required for each measurement.

PMIC offers testing from 25 °C up to 900 °C, with sample geometries of Ø 12.7 mm and Ø 25.4 mm.  25.4 mm by 25.4 mm squares, liquids, pastes, and powders may also be tested in the system.  Although ideal for essentially fully dense, homogenous, and isotropic solid materials which are opaque, PMIC has also used the flash method to test transparent and semi-transparent materials.

For best results, PMIC recommends a specimen size of Ø 12.7 mm, as the smaller seize reduces lateral heat flow during testing.  Optimizing the thickness based on the material’s expected thermal diffusivity is emphasized in both ASTM E2585 and ASTM E1461 and PMIC recommends either providing multiple thicknesses over the range of 1 mm – 6 mm or reaching out to PMIC to determine an optimal thickness prior to sample development.  A significant factor for error of the flash method may be introduced by non-uniform thickness of solid specimens, as uniform heat-rise through the specimen is directly impacted by the specimen face flatness and parallelism.  Therefore, good specimen preparation plays an important role in obtaining best measurement results.  If the customer is interested in thermal conductivity over a temperature range, density as a function of temperature can significantly increase the accuracy of the test.

Measurements at PMIC are performed per ASTM E2585 Standard Practice for Thermal Diffusivity by the Flash Method and ASTM E1461 Standard Test Method for Thermal Diffusivity by the Flash Method.

[1] Fitzer, “Thermal Properties of Solid Materials, Project Section II – Cooperative Measurements of Heat Transport Phenomena of Solid Materials at High Temperatures,” Advisory Group for Aerospace Research and Development (AGARD), North Atlantic Treaty Organization, AGARD Report No. 606, 1973.