CHENG Hui，ZHAO Mengxue，WU Wei

College of Chemical Engineering， Beijing University of Chemical Technology， Beijing 100029， China

Abstract

Objective Among the organic phase change materials， paraffin has high thermal and chemical stability， and small volume change during phase transition， non-corrosive and low cost. However， paraffin wax has problems such as poor thermal conductivity and photothermal conversion performance as well as easy leakage during practical use. Phase change microcapsules have good application prospects in thermal management and photothermal conversion. Thermal conductivity and solar absorption efficiency of their shell materials have a significant impact on their application performance. Titanium dioxide （TiO₂）， a semiconductor material， exhibits excellent thermal conductivity and high ultraviolet light absorption efficiency. However， it barely absorbs visible light. The addition of graphene can expand its light absorption range， improving the photothermal conversion efficiency of microcapsules. However， graphene lacks polar groups on its surface， making it difficult to combine with other materials. Although surface modification is commonly used to improve graphene’s surface energy， it often adversely affects the photothermal performance of the microcapsules. To address this， the study uses graphene oxide to exfoliate graphite and prepare hydrophilic graphene. This material retains excellent photothermal properties of graphene， while offering the polar functional groups of graphene oxide， allowing it to integrate effectively with TiO₂.

Methods Graphene oxide has both the excellent photothermal properties of graphene and the polar functional groups of graphene oxide， which are easy to combine well with TiO₂. In this paper， hydrophilic graphene was prepared by liquid-phase exfoliation of expandable graphite using graphene oxide as an additive. Preparation of microcapsules with paraffin as core and TiO₂ as shell using titanium butoxide （TBT） hydrolysis in N，N-Dimethylformamide（DMF） solution . Then hydrophilic graphene was added， and hydrophilic graphene was compounded with TiO₂shell through hydrogen bonding， and paraffin wax was used as the core to prepare to obtain hydrophilic graphene@TiO₂@paraffin composite phase change microcapsules. The properties of the resulting composite phase change microcapsules were systematically studied.

Results and Discussion In the graphene@TiO₂@paraffin composite phase change microcapsules， hydrophilic graphene was closely adsorbed onto the TiO₂ shell layer， resulting in spherical microcapsules with an average particle size of 50-150 μm and a rough surface for better dispersion. The optimal sealing performance was achieved when the mass ratio of paraffin wax to titanium butoxide （TBT） was 2∶1， the mass fraction of hydrophilic graphene was 1%， and the leakage was reduced to 6. 90% after 70 min. The microcapsules exhibited good thermal conductivity and latent heat storage. When the hydrophilic graphene mass fraction was 1%， the thermal conductivity of microcapsules was 0. 459 W/（m·K）， the paraffin wax encapsulation rate was 63. 33%， and the enthalpy of phase change was 175. 84 J/g. In addition， the core-shell structure improved the thermal stability of paraffin wax， delaying decomposition and reducing decomposition rate. Photothermal conversion experiments showed that the microcapsules had excellent photothermal conversion and storage ability. The addition of hydrophilic graphene significantly improved the photothermal conversion efficiency of the composite phase change materials （PCMs）， and the graphene@TiO₂@paraffin reached a maximum temperature of 64. 8 ℃ after 10 min of simulated solar irradiation， an increase of 31. 4 ℃ compared to pure paraffin wax.

Conclusion In this paper， hydrophilic graphene is successfully self-assembled onto the surface of TiO₂forming a shell layer for phase change microcapsules. Paraffin， TiO₂ and hydrophilic graphene are only physically bonded to each other. Composite microcapsules have a more uniform spherical structure with rough surface and good dispersibility. The addition of hydrophilic graphene can enhance the thermal conductivity and photothermal conversion performance of PCMs， so that the composite phase change microcapsules have good latent heat storage capacity. The core-shell structure of microcapsules can improve the thermal stability of paraffin waxes， and compared with pure paraffin waxes， the initial thermal decomposition temperature of microcapsules is increased， and the thermal decomposition rate is slower than that of paraffin waxes. The resulting PCMs demonstrate good thermal conductivity， thermal stability， and photothermal performance.

Keywords：solar energy； phase change material； microcapsule； graphene； paraffin wax