Evaluation of environmental materials as thermal witness materials

New and complex energetic materials are under development for achieving tunable pyrotechnical events for applications such as neutralization of biological weapons, bunker busters and many others. To guide the development of hybrid materials, the pyrotechnical environment they produce requires higher degree of characterization i.e. good description of spatial and temporal temperature distribution. Temperature measurements in pyrotechnical events are especially challenging, where the temperature of the environment rises more than 2000 K on microseconds to few milliseconds time scale. These environments produce high thermal stress where traditional sensors like thermocouples, optical pyrometers struggle to describe the dynamic changes in the environment. The presented research focuses on the development of thermal witness materials that are injected into the combustion environment, where they travel with the expanding gases and undergo a quantifiable physical change/transformation. The extent of change is determined post exposure and a correlation is made between the extent of observed change and the exposed time-temperature profile. The thermosensors under investigation, Jarosite and silicate glass are materials that are ubiquitous in both urban and rural environments. Jarosites undergo decomposition losing (OH) and SO3 upon thermal exposure. The decomposition mechanism is complex and is governed by a set of serial and parallel reactions. The degree of decomposition depends on the exposed thermal profile, and can be easily determined using thermogravimetric analysis. A correlation is made between the residual decomposition and the experienced environment. The limitations and sensitivity for these sensors are presented. The structures of silicate and borate glasses are strongly dependent on the quench rate during glass formation. As an indicator, the glass transition temperature varies linearly with the logarithm of the quench rate. Structural aspects, specifically the degree of connectedness of network forming units [SiO4] and [BO3] can be investigated using Raman spectroscopy. This allows one to recognize variations in the temperature, and cooling rate experienced by the material as it cooled from the high-temperature environment. The use of these sensors to reconstruct the environment temperature profile is under development. The fundamental idea defining the methodology to recover thermal history is discussed