Determination of thermal diffusivity, conductivity, and energy release from the internal temperature profiles of energetic materials

A novel data processing technique has been developed to obtain thermal diffusivity, conductivity, and reaction heat release for energetic materials from Sandia Instrumented Thermal Ignition (SITI) experiments heated with a linear ramp temperature boundary condition. The method is based on the equivalence of the temperature responses of: (a) ramped temperature boundary condition with no internal heat generation and (b) uniform heat generation (that is, with a negative value) with constant temperature boundary conditions; which is true regardless of the spatial domain. For the specific case analyzed herein (the SITI apparatus), the midplane temperature profile is well represented by a quadratic expression in the radial coordinate for both ramped boundary temperature and uniform heat generation responses. Internal temperature data from temperature ramped SITI experiments with various pyrotechnics, propellants, and explosives were analyzed. Quadratic fits to the temperature profile data were made and the associated fitting coefficients were converted to yield thermal diffusivity directly. Thermal conductivity was then determined from thermal diffusivity, given knowledge of the material's specific heat capacity and density. Finally, because of the equivalence of the cases (a) and (b) above, their individual contributions to a combined temperature profile can be easily separated, thereby yielding internal heat generation as well. This technique allows for measurements of properties for pressed and powdered materials over a range of densities and temperatures. The technique is demonstrated using pyrotechnic materials (KClO4 and Ti/KClO4), a composite solid propellant (herein referred to as “Propellant A”, a class 1.3 AP-HTPB-aluminum propellant) and an explosive (PBX 9502).