Preparation and properties of the phase change materials for stable crop growth environment

Phase change material was developed for agricultural applications using microencapsulation. In this study. TiO₂@n-octadecane microcapsules (phase transition temperature at 25~33°C) were prepared, according to the suitable temperature for crop growth (around 30°C). The microcapsules were modified and optimized with the graphene oxide (GO), thus obtaining GO/TiO₂@n-octadecane microcapsules. The GO-modified TiO₂@n-octadecane-prepared microcapsules were dark gray powder particles. The color of the microcapsules was deepened, as the GO content increased. The modified TiO₂@n-octadecane microcapsules were detected by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and laser thermal conductivity meter for the morphology, chemical structure, and thermal conductivity of the sample microcapsules, respectively. The thermophysical properties of the samples were further tested using differential scanning calorimetry (DSC), and thermogravimetric analyzer (TGA). The experimental results indicated that: 1) The prepared microcapsules showed a compact spherical core-shell structure in the SEM images, with a uniform diameter ranging from 1-3μm on average and smooth shell surface, where the shell was 80nm thick with a thin sheet of aggregated TiO₂ attached to the surface. This structure was beneficial to prevent the leakage of n-octadecane during phase transition for the better thermal conductivity of phase change materials. Nanoscale GO sheets were also observed to attach to the surface of the microcapsules. 2) The characteristic peaks of n-octadecane and GO were observed in the FTIR characteristic curves of the modified microcapsules, respectively. Therefore, the synthesis process of modified microcapsules was a purely physical combination without any chemical reaction, indicating the successful preparation. 3) DSC results indicated that the melting phase change of microcapsules began at around 25°C, and then melted completely at around 33°C during heating. The enthalpy of melting microcapsules was about 120 J/g. The solidifying phase change began at about 25°C during cooling. The enthalpy of crystallization was about 116 J/g in the microcapsules. The encapsulation efficiency of TiO₂@n-octadecane microcapsules was 52.9%. The encapsulation efficiency of 1%, 2%, and 3%GO modified TiO₂@ n-octadecane microcapsules decreased to 43.3%, 41.2%, and 37.6%, respectively. 4) The thermal conductivity of the modified TiO₂@n-octadecane microcapsules increased by 58%, 86.3%, and 104.2%, respectively, compared with the unmodified ones. Therefore, the addition of GO significantly improved the thermal conductivity of microcapsules. But such an improving effect of GO was declined, as GO dosage increased. 5) The DSC test verified the microcapsules with multiple heating and cooling cycles. The prepared microcapsule phase change heat storage material shared better cycle stability and service life. TG analysis of the microcapsules revealed that both the weight loss rate and the encapsulation efficiency dropped, as the ratio of GO modification increased. Both the weight loss rate and the encapsulation efficiency rose, as the ratio of GO modification decreased. A sample with a high encapsulation efficiency presented the higher latent heat but lower thermal stability. In conclusion, the GO-modified microcapsules can be expected to serve as the broad application prospect, due to the high thermal conductivity, excellent stability, and more desirable latent heat of phase change. The finding can provide a strong reference for the secondary use of low-temperature energy.