(764d) Measuring Thermal Histories Using Fragile Glass Crystallization

Thermal witness materials generally exploit irreversible processes to record the temperature history of their environment.  The general challenge is to decouple time from temperature, i.e. to distinguish the extent of the irreversible process caused by a short exposure at higher temperature from a relatively longer exposure at a lower temperature.  This requires that multiple processes occur during exposure of the witness materials and can be usefully quantified after exposure.  The approach presented here proposes to use the crystallization of particles of a fragile glass as the irreversible process, and the melting of the glass as a boundary condition.  Specifically, bio-active glass 45S5 crystallizes with known kinetics above 900 K, and melts above 1420 K.  Glass particles with a size distribution of 10 – 50 µm and initially at room temperature are inserted in a hot environment and exposed for a period of time on the order of 0 – 100 ms, after which they are allowed to cool naturally.  The heat transfer from the environment to the particles depends on the particle size, and therefore at any given environment temperature large particles will heat up more slowly and may not crystallize substantially.  Smaller particles may heat up for long enough to crystallize, and even smaller particles may reach the melting point.  After exposure, a representative sample of the glass particles are collected and examined by polarized light microscopy.  The smallest and largest particle sizes observed to have crystallized provide two observable parameters that can be related to the environment temperature and the duration of the exposure.  Quantification of these relations will be presented, and time and temperature ranges will be identified that can usefully be characterized using this method.  The feasibility of microscopic examination will be demonstrated.