(398ac) Evaluation of Environmental Materials As Thermal Witness Materials

Pyrotechnical events occur on very short time scales, generating temperatures of up to more than 2000 K and quench rapidly. These events are engineered to perform specifically for different applications. To guide the development of new materials used to generate these events, the temperature profile of the combustion clouds needs to be well described.

Measurement of temperature profiles is challenging in these high-stress environments. Electronic sensors may fail due to these stresses, and in addition may not offer the desired time resolution. Optical methods are limited by the optical thickness of the combustion environment, and may provide information of the periphery, or a line of sight average.

Thermal witness materials offer an alternative to real-time measurement methods.  Micron sized particles are inserted into the combustion environment, travel along with the expanding gases and undergo irreversible changes during the exposure. The extent of change is determined post exposure. A correlation is made between the extent of change and temperature-time profile the material has experienced.

While several engineered materials are under development as thermal witness materials, this study is aimed at evaluating environmental materials for the purpose of recovering temperature-time information after combustion events.  Examples of such environmental materials are minerals with volatile components which are partially lost during exposure.  Other materials with non-equilibrium structures, such as glasses, effectively remember thermal conditions when they are quenched-in from their corresponding high-temperature equilibrium state.  Both types of materials are sufficiently ubiquitous in both, remote and urban environments to be of interest for temperature forensics.

The examples evaluated include a synthetic jarosite (a complex sulfate hydrate), and silicate glasses.

Jarosites undergo decomposition losing (OH) and SO3. The decomposition process is complex and is governed by a set of serial and parallel reactions. The degree of decomposition depends on the experienced time-temperature profile, and can be determined using thermoanalytical methods. A correlation is made between the residual decomposition and the experienced environment.

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 both types of materials as thermosensors is limited by the resolution available for interrogation: themoanalysis for recovered decomposed minerals, and Raman spectroscopy for glasses.  These limitations will be quantified, and general conclusions regarding other related materials that show similar effects will be discussed.