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Many phase change materials (PCMs) are found to crystallize without exhibiting a glass transition endotherm upon reheating. In this paper, we review experimental evidence revealing that these PCMs and likely other hyperquenched molecular and metallic systems can crystallize from the glassy state when reheated at a standard rate. Among these evidences, PCMs annealed below the glass transition temperature T g exhibit slower crystallization kinetics despite an increase in the number of sub-critical nuclei that should promote the crystallization speed. Flash calorimetry uncovers the glass transition endotherm hidden by crystallization and reveals a distinct change in kinetics when crystallization switches from the glassy to the supercooled liquid state. The resulting T g value also rationalizes the presence of the pre- T g relaxation exotherm ubiquitous of hyperquenched systems. Finally, the shift in crystallization temperature during annealing exhibits a non-exponential decay that is characteristic of structural relaxation in the glass. Modeling using a modified Turnbull equation for nucleation rate supports the existence of sub- T g fast crystallization and emphasizes the benefit of a fragile-to-strong transition for PCM applications due to a reduction in crystallization at low temperature (improved data retention) and increasing its speed at high temperature (faster computing).
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Switching between Crystallization from the Glassy and the Undercooled Liquid Phase in Phase Change Material Ge 2 Sb 2 Te 5
Abstract Controlling crystallization kinetics is key to overcome the temperature–time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3Tmto compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge2Sb2Te5with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperatureTg. Moreover, highly unusual for glass‐forming systems, crystallization at conventional heating rates is observed more than 50 °C belowTg, where the atomic mobility should be vanishingly small.
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- PAR ID:
- 10460175
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Materials
- Volume:
- 31
- Issue:
- 39
- ISSN:
- 0935-9648
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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