Staff of the Department of Thermophysics is involved in the development of new innovative class of the testing methods – transient methods for measuring of the thermophysical properties of materials. Principle of this class of methods is based on generation of a small heat disturbance inside a specimen and the measuring of the following temperature response. Methods are included into this class where measuring probes are embedded into the material thus surface interfering effects are suppressed. Table 1 gives an overview of the basic experimental arrangement of the transient methods.
Table 1.: Transient methods and the corresponding parameters that can be determined by particular method.
Theory and the methodology of the following transient methods were worked out in our department:
Principle: A heat pulse is generated inside the specimen and the following temperature response is measured. The heat pulse is produced due to Joule heating from the electrical resistance of the planar source. The thermocouple measures the temperature response to the heat pulse.
The thermophysical parameters are determined according used model. Ideal model is used that assumes:
Such model is characterized by formulas that describe the temperature response
where r is density and and R is electrical resistance of the planar heat source.
Two procedures can be used for calculation of the thermophysical parameters:
Ideal time window is limited by sensitivity coefficients and its linear dependency. Experimental realization of the method causes deviations from the ideal model that limits the time window. Time window is indicated in figures by box. Several interfering effects distort the ideal temperature response:
Heat source and the heat loss effects.
The heat source and the heat loss effects deform the temperature response at the short times and at the long times. Instead of ideal function (1) or (2), the function (6) is used for fitting (case of heat loss effects).
where J0(x) and J1(x) are Bessel functions s=H / λ and an are the roots of the equation:
The real heat pulse width deforms the temperature response. Instead of ideal function (1) or (2), the function (7) is used for fitting.
where Q*(x) is error function
The real heat source capacity effect shifts in a way shown in upper figure the temperature response. Instead of ideal function (1) or (2), the function (8) is used for fitting.
and B = c2ρ2/2bc1 ρ1