Fast Scanning Calorimetry Studies of Superheated Polycrystalline Films: Insights into the Glass Transition in the Limit of High Heating Rates

The behavior of rapidly heated thin-film materials is relevant for a broad range of existing and emerging technologies. Experimental investigations of superheated crystalline phases are important because they may speed the development of generalized theoretical approaches to studying a variety of other non-equilibrium processes, such as glass softening. Using a novel Fast Scanning Calorimetry technique (FSC), capable of heating rates in excess of 100,000 K/s, we investigated the non-equilibrium melting mechanism for micrometer-scale polycrystalline films of several molecular compounds. In all cases, the phase transition occurs heterogeneously, i.e. melting originates on a sample’s surfaces and progress into the sample’s bulk via a transformation front interface. Remarkably, every compound’s melting transformation during rapid heating is characterized by an “anomalously” high activation energy barrier. These barriers indicate that, when confined by a propagating melting front, constituent diffusion occurs in a high defect density, glass-like amorphous phase. We will discuss the fundamental significance of this recent discovery on the glass transition phenomena and the potential of exploiting the interfacial effect of FSC to probe the properties of glassy films.