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Hot disk method (to determine thermal conductivity and capacity)

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Hot disk method (to determine thermal conductivity and capacity)

Thermophysical substance data constitute the basis for calculations and models and are indispensable for determining further safety-related data such as heat production, for example, or a specific heat quantity from measured temperatures. One method for determining some of these thermophysical data is the transient plane heat source (hot disk) method in accordance with ISO 22007-2.

With the help of this type of hot-disk measurement, the thermal conductivity λ, the thermal diffusivity a and the specific heat capacity per unit volume s can be obtained from a series of measurements. The specific heat capacity per unit volume is converted into the specific heat capacity cp using the previously established bulk density.

For a measurement, the test substance is placed in a special mould with variable volume and the mass of the sample is determined in order to calculate the bulk density of the sample using the actual volume. The hot disk sensor is then inserted horizontally into the center of the filled sample. The sensor serves as a source of heat while also acting as a dynamic temperature sensor. With a given electrical heating energy (mW) as well as a defined measuring time (s), a defined amount of energy is applied by the sensor into the center of the sample while the temperature change of the test object is measured at the same time.

Established measurement data such as heat capacity can be used, for example, for analysis of an adiabatic wire basket test or for analysis of an adiabatic heat-pressure accumulation test. Determining the exact heat capacity of a sample also makes it possible to determine more precise safety parameters.

A special, advanced application is offered by the parameter thermal diffusivity a, which can be used together with the established kinetic parameters (Arrhenius approach, zero-order kinetics) from an adiabatic heat-pressure accumulation test in order to calculate data for the self-accelerating decomposition temperature (SADT) as a function of various container sizes by applying the theory of heat explosion according to Frank-Kamenetskii.

 

[1]      ISO 22007-2 (2015-08-01): Plastics – Determination of thermal conductivity and thermal diffusivity, Part 2: Transient plane heat source (hot disk) method
[2] Kotoyori, Takashi: Diffusion and Heat Transfer in Chemical Kinetics. 1st edition.
Elsevier Science, 2005
[3] Frank-Kamenetskii, David A.; Appleton, John P. PubDiffusion and
Heat Transfer in Chemical Kinetics. 2nd Enlarged and Revised Edition. Plenum Press, 1969

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