Scientists at Osaka University, Panasonic Corporation, and Waseda University used scanning electron microscopy (SEM) and X-ray absorption spectroscopy to determine which additives cause crystallization in supercooled aqueous solutions. This work may help the development of new energy storage materials based on latent heat.

If you put a bottle of water in the refrigerator, you will take out a solid icicle after a few hours. However, if the water has few impurities and is undisturbed, it may not freeze, but will remain as a supercooled liquid. Be careful, because this state is very unstable, if shaking or adding impurities, water will crystallize quickly. As evidenced by many YouTube videos. Supercooling is a phenomenon in which an aqueous solution remains liquid without freezing, even if its temperature is below the freezing point. Although many studies have been conducted on additives that cause freezing of supercooled liquids, the details of the mechanism are still unclear. One potential application may be latent heat storage materials, which rely on freezing and melting to capture and then release heat.

a) HAADF-STEM images of replicated membranes prepared from samples #1, #2, #11 and #13 of tetra-n-butyl ammonium 3-methylvalerate (TBA-3MP) aqueous solution. In samples #11 and #13, 5-10 nm silver nanoparticles were observed. In sample #13 containing silver nanoparticles and F-anions, there are clusters of 10-30 nm even at 281 K. As the temperature decreases further, the number density of clusters increases, and then the TBA-3MP semi-clathrate hydrate crystallizes with a low degree of supercooling. b) SE-STEM images of different areas of the same film as "13-a" in panel a). A cluster of 10-30 nm in size is wrapped with a silver nanoparticle (the black dot indicated by the red arrow).

Now, a team of researchers led by Osaka University has shown that silver nanoparticles are very effective in inducing the crystallization of clathrate hydrates. The clathrate hydrate physically looks like ice and consists of a hydrogen-bonded water cage with guest molecules inside.

Using SEM and freeze-fracture replication methods, we captured the moment when the new clusters encapsulated the silver nanoparticles in the aqueous solution of the latent heat storage material. Corresponding author Professor Takeshi Sugahara explained that this is because the nanoparticles act as "seeds" or nucleation sites. , Used to form tiny clusters. Once started, the remaining solute and water molecules will quickly form additional clusters, and then the clusters will be densified leading to crystallization.

The researchers found that although silver nanoparticles tend to accelerate the formation of these clusters, other metal nanoparticles, such as palladium, gold, and iridium, do not promote crystallization. The supercooling suppression effect obtained in this study will help realize the practical application of clathrate hydrate as a latent heat storage material, Professor Sugawara said. As described in this study, material design guidelines for enhanced subcooling control may lead to the application of latent heat storage materials in solar and heat recovery technologies to improve efficiency.