Controllable Design And Thermal Performances Of Montmorillonite Nanosheets Based Thermal Energy Storage Materials

Montmorillonite is a typical clay mineral with abundant reserves and low prices.It can be easily exfoliated into two-dimensional nanosheets(MtNS)just by physical ultrasonic and shearingtreatment due to the hydration and swellingproperties.However,this unique property of montmorillonite has not been well exploited to date.On the other hand,high-tech thermal storage technology can improve energy utilization efficiency as well as promote energy conservation and emission reduction.The development of low-cost and high-performance thermal energy storage technology is a major demand on account of the energy crisis and environment pollution,and achievinga sustainable clean energy strategy is China's basic energy national policy.Therefore,it is essential to develop MtNS based heat storage materials,which has great economic significance and social benefits in the fields of clay minerals materials and sustainable energy sources.In this work,natural montmorillonite has been exfoliated into MtNS and used as the supportingmaterials for stearic acid(SA)phase change materials.MtNS based sensible and latent heat storage materials were designed and constructed,includingMtNS/SA microcapsule phase change materials,three dimensional MtNS framework based shape-stabilized phase change materials(3D-MtNS/SA),and MtNS/H2O nanofluids.The morphological characteristics,synthesis mechanism and thermal properties of these thermal storage materials were studied,and the main conclusions obtained in the paper are as follows:(1)MtNS/H₂O nanofluids have been developed for the collection and storage of solar thermal energy.The electrostatic repulsion and hydration repulsion between MtNS led to the superb stable suspension of MtNS/H₂O nanofluids.The MtNS/H₂O nanofluids showed enhanced thermal transfer ability,which enables the efficient solar heat collection and sensible heat storage.(2)MtNS and SA are self-assembled into microcapsule composite phase change materials with MtNS shell and SA core under the electrostatic interactions.The MtNS/SA composite phase change materials have an ultra-high weight fraction of SA(88.88 wt%)due to the very thin MtNS shell.The meltingand freezinglatent heats of the MtNS/SA composite phase change materials are as high as 181.04 J/gand 184.88J/g,respectively,which are much higher than that of previous reported clay-based composite phase change materials.(3)The MtNS shell in the MtNS/SA microcapsule phase change material provides effective protection to the SA core,leadingto the prevention of leakage of the phase change material duringthe phase change process,and the improvement of the structural stability,thermal stability and cyclingperformances of the phase change material.(4)LoadingAgNP into MtNS/SA microcapsule composite phase change materials can significantly improve the thermal conductivity of the composite phase change materials,while ensuringa high latent heat capacity.The thermal conductivity of MtNS/AgNP@SA is 0.82 W/m K,which is 310.0%higher than that of SA.(5)The constructed 3D-MtNS/SA shaped-stabilized phase change materials simultaneously showed high thermal energy storage capacity and good thermal conductivity.The 3D-MtNS framework with massive porous structure encapsulated95.2 wt%SA,resulted in a huge meltinglatent heat(195.75 J/g)and freezinglatent heat(198.78 J/g).The inner-connected MtNS inside the composite phase change materials provided fast heat transfer path,which effectively improved the thermal conductivity of phase change materials The porous structure provided strongcapillary force to effectively prevent leakage of phase change materials duringphase transition.(6)AgNW and MtNS are assembled into a three-dimensional framework together,which can still encapsulate very high content of SA phase change materials and achieve a huge thermal storage capacity.Besides,only a small amount of AgNW can greatly improve the thermal conductivity of the shape-stabilized phase change materials,the thermal conductivity of AgNW@3D-MtNS(0.613 W/m K)has increased 199.02%compared to SA.The AgNW conductive filler is fixed thus to solve the defect that heat conductive filler is easily agglomerated duringthe solid-liquid phase transition.