Thermal breakdown modeling in polyimide films by solving the heat balance equation

The theoretical thermal breakdown in polyimide (PI) films was modeled at very high temperature between 320 °C and 400 °C using the heat balance equation. Its value was extracted from the internal temperature rise versus electric field of the sample up to its thermal degradation. The originality of this study appears in the consideration, for the first time, of the real temperature dependence of the heat capacity C
v
(T), compared to others studies where C
v
is fixed at a constant value. It has been possible by measuring it by DSC in a large temperature range up to 390 °C and by interpolating higher values using polynomial fitting. Moreover, the thermal conductivity has also been measured at room temperature only but its value was assumed to be constant and remaining low like in most of polymers. As an input, a simple thermal-activated current density has been used to solve the heat balance equation in a first approach. The results show that the use as an input of a constant value of the capacity C
v
(T
0
), where T
0
is the sample temperature, leads to large difference between both calculated and measured breakdown fields. In comparison, the implementation of the temperature dependence C
v
(T) during the solving of the model leads to closer quantitative results. Some future ways of enhancement of the model are proposed.