Thermal properties of materials and their characterization by classic and transient methods (2017)

The problems of the measurement of thermophysical properties of materials are recently focused in many research and industrial branches. The problems of the last period are rising from the needs of increasing consumption of the energy. To solve the problems of how to get available of the renewable sources on one side and measures preventing energy losses on the other side are highly demanded. Thus the industry of new highly efficient and smart materials was established for a new production. This state invokes the need of development of the testing measurements and underlying physical methods and apparatuses. The other side of the problem is to understand of the thermophysical backgrounds. The question is how the materials structure is connected with the changes in measured thermophysical parameters. The paper gives the review of basic physical phenomena presented at the investigation of thermal properties of materials. One of the basic problems is to define the phenomena as we are observing them. Theoretically, all the observations in the literature that correspond to the phenomena are described in school books of the basic courses in physics and the underlying sciences. The book of Mr. Moore “Physical Chemistry”[1] seems to be a good source for the explanation of the basic problems in thermodynamics. The measure of the internal energy of materials is the temperature, and thus the thermal behavior reflects well any kind of the structure changes. Some illustrations are presented when the similar materials having nearly the same stoichiometry should possess different values of the specific heat, thermal diffusivity and thermal conductivity, commonly marked as the thermophysical parameters. The next problems in this research are the measurement methods. By single division, we can define some classes based on the type of generated temperature filed that is necessary for the material characterization. The classical methods are stationary techniques, steady state techniques and methods based on steady or balanced heat flow generation. In the last decades, the methods based on transient regime were developed and used. The basic problem when comparing the accuracy of the listed methods rises from so-called data consistency condition. This condition set the value of the thermal conductivity to be equal to the value of material volume density multiplied by thermal diffusivity and specific heat. Classical methods are measuring just one parameter. Usually when we have measured one of these parameters, and we will use the next one from literature, and put it into the data consistency equation to calculate the third one. Moreover, the calculated results obtained by variation of the couples of input parameters do not fit the data consistency condition at all. The problem is usually counted on the different origin of methods used for the measurements of particular parameters. In the comparison with the stationary methods, the transient methods are able to determine all three parameters from the single measurements and thus are able to fulfill this condition. The list of some transient methods developed and used at the Institute of Physics, SAS is given. In principle, there are two classes of techniques based on single probe sensors and double probe sensors. The most developed method is the pulse transient method that uses two probes for the measurement, the heat source and thermocouple. For this technique, more than ten models accounting different experimental arrangements and different types of disturbance effects are counted. The Hot Ball technique has also been used as the secondary type of humidity sensor. In this case, the sensor inserted into the porous media is calibrated on the water content in pores. The principle of dependence of the thermal conductivity on moisture content is used. The next class of the transient techniques uses single-probe sensors where the heat source and thermometer are unified. The new sensors for the hot circle and planar hot disc techniques were developed recently. The older single probe techniques are the hot wire, hot strip and hot disc. The mathematical background for the development of physical models uses a solution of the basic heat equation for the initial and boundary conditions stated according to the real experiment when possible. Some of the models developed are solving the particular problems with the disturbance effects that are increasing the uncertainty of the parameters estimation. In between the characteristic effects belong the heat capacity of the heat source-sensor itself, the heat contact resistance, the heat losses from the free sample surface and the temperature of the thermally stabilized heat exchangers in between which the experiment is usually realized.