# Hydrates for Cold Storage: Formation Characteristics, Stability, and Promoters
## 3 Results and Discussion
### Crystal Morphology of Hydrates  
Information on the characteristics of hydrate formation and the related promoting factors is essential for the effective use of a hydrate as the working substance for cold storage. Microscopic crystallization is of great significance in the identification and analysis of samples and their respective formation mechanisms. [PERSON] and his coworkers had conducted comprehensive studies of hydrate crystal morphology in different experiment conditions [50; 51; 52; 53]. They observed the formation and growth of crystal growth of ionic semi-clathrate hydrate formed in CO\({}_{2}\) gas + TBAB aqueous solution system, and they found that the hydrate crystals initially grew in the liquid phase, instead of growing at the gas/liquid interface [52; 53]. [PERSON] et al. [54] synthesized the single crystals from THF hydrate in the laboratory, and their microscopic analysis showed that inhibitors can change the growth characteristics of such crystals. [PERSON] et al. [55] studied the particle distribution in the hydrate slurry of THF and ice at the microscopic level and concluded that this slurry can be used as a refrigerant. The microscopic evaluation of the morphological characteristics of the hydrate crystals was performed herein to obtain the characteristics of formation of R141b, TME, and TBAB/TBAC hydrates. As shown in Figure 3a, the R141b hydrate crystals were mainly granular or dendritic, and microporous structures were distributed on the hydrate surface. The overall crystal distribution was uneven with local clusters present. The formation rate increased greatly after the ice cubes were added, possibly because supercooling is conducive to the formation of the hydrate crystals. However, once the crystals were stable, the solution was isolated into small chambers, and the non-diffused R141b clusters and liquid water were separated. The diffusion of R141b molecules was suspended, and there was no further formation of the hydrate crystals [19]. These results shows that the micro-environment for the formation of the R141b hydrate may be affected by two factors that restrict each other, namely diffusion of the molecules and the rate of crystal nucleation. As shown in Figure 3b, the TME hydrate presented an ellipsoid shape when viewed on the 200 \(\upmu\)m scale. At 100 \(\upmu\)m resolution, vein-like structures were observed on the surface of the ellipsoid where the vein distribution was uniform with a certain regularity. At 50 \(\upmu\)m resolution, crystal grains were observed at the junction of the veins and were exposed to the outer surface (air). In addition, the crystals were pentagonal, hexagonal, and partly irregular. The nucleation and growth rates may determine the hydrate morphology. The nucleation rate increased at higher grain growth rates, and the diminished space may restrict growth of the crystal nucleus, which could be considered the cause of the formation of irregular hydrate crystals. This phenomenon is similar to that reported by other researchers [56]. The single crystal X-ray structure analysis indicated that TME hydrate crystals were lamellar, with alternating TME and water layers [27]. The formed hydrate may constitute a barrier, which easily hindered diffusion of the guest molecules and heat exchange, which in turn decreased crystal growth and increased grain sizes. As shown in Figure 3c, the crystal grains at the edge of the TBAB/TBAC hydrate were large with an irregular polygon morphology, which is similar to previous studies [52; 53]. The hydrate surface was uneven, with small and raised structures distributed throughout. These projecting structures were interconnected in a vein-like shape. Densely distributed crystal grains with uneven heights and a gill-like morphology were present on the surface of the hydrate crystal, and the crystals were very dense.