Once the temperature distribution of the system components has been analyzed one can proceed to find the reasons that cause the existence of the thermal gradient in the sample. Next each of the factors that facilitates the existence of the temperature difference in the sample will be stated and explained.
Heat transfer between the sample and the support
The results of the thermal simulations developed with OpenFOAM® have demonstrated that the sample exchanges certain amount of heat by conduction with the support. These heat losses that take place in the sample base cause thermal asymmetry with respect to the horizontal centre plane of the sample, factor that increases the temperature gradient in the sample.
Therefore, the parameters to be taken into account with respect to this heat transfer are the thermal conductivity of the materials that comprise the support (boron nitride, alumina and steel) and its own geometry.
Temperature profile of the heating cylinder
The vanadium cylinder is the component that delivers thermal power to the system and, as a consequence, the hottest component. For this reason the temperature distribution given in its surface has a key role when it comes to study the temperature distribution of the sample.
As it has been seen, due to the component layout within the instrument D4, the temperature in the cylinder upper region tends to be higher than it is in the lower region. This fact makes the top of the sample see hotter points of the cylinder than the bottom of the sample, thus, the top of the sample tends to be hotter than the bottom.
Geometrical asymmetry of the instrument D4
As a summary, this is the main cause and, at the same time, the one that causes the two phenomena stated before. Thus, the horizontal geometrical asymmetry of the instrument components is the cause of the horizontal thermal asymmetry undergone by the sample, since the fact that the sample needs a support to stay elevated causes:
- that the sample releases heat by conduction to the support
- that the cylinder delivers radiative heat differently in the upper region and in the lower region, generating the already mentioned thermal areas, due to the presence of the support.