In-operando crystallization study of zinc nitrate hexahydrate using zinc oxide nucleators
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abstract
AbstractSupercooling (TSup) is one of the crucial problems in utilizing a phase change material (PCM), which has been attempted to overcome using nucleators possessing small lattice disregistry without comprehensive understanding of the nucleation phenomenon. Here, this work studies the interactions between nucleators and PCM via inoperando direct visualization for the first time, to the best of our knowledge, to better understand the crystallization process during freezing of a PCM using zinc nitrate hexahydrate (ZNH) and a zinc oxide (ZnO) nucleator. According to our inoperando study, freezing was randomly initiated by only a fraction of the nucleator particles during each thermal cycling. However, previously unresponsive nucleators also suddenly crystalized PCM when the propagating crystal encountered them, improving the crystal initiation and thereby TSup. Consequently, for equal wt% of ZnO, better nucleation behavior was obtained using uniformly distributed small nucleator particles throughout the PCM, as compared to either large or poorly distributed aggregated nucleators. DSC results using ZnOneedle as nucleator confirmed a 38% and 33.3% lower TSup upon employing 5wt% small (individual) particles (~4m on average), as compared to using equal wt% of small (aggregated) particle clusters (~200m) and large particles (~46m), respectively. Crystallization of ZNH caused additional hairlike ZnO growth preferentially decorated along lateral faces of the original nucleator particles. The inoperando studies are valuable tools to correlate inherent crystallization phenomena to the practical thermal energy storage properties of the system.
