Construction And Performance Study Of Biomimetic Skin-Flesh Structure High-Temperature Composite Thermal Energy Storage Materials

Thermal energy storage is a crucial technology for addressing the temporal and spatial mismatch between energy production and consumption.Phase change thermal storage is considered the most promising thermal storage technology due to its nearly constant-temperature storage process and high energy density.In industrial processes and the field of solar thermal power generation,widely available and cost-effective molten salts are primarily used as high-temperature thermal storage materials.However,molten salts face significant technical challenges such as low thermal conductivity,strong corrosiveness,and leakage that need to be overcome.Encapsulating molten salt as shape-stabilized phase change material（ss-PCM）or units is currently a vital approach to address the aforementioned issues.Presently,corrosion-resistant materials like stainless steel or ceramic shells are used to encapsulate molten salt.While this method solves the problem of leakage,it has not fully resolved issues related to thermal conductivity and corrosion.The use of porous materials（metal,ceramic,or carbon）to adsorb molten salt or the mixing and compression of molten salt with ceramic or carbon powders,leveraging the flexible pores formed between particles,significantly enhances the thermal conductivity of the thermal storage material.These approaches have also made initial progress in addressing molten salt leakage issues.However,they still face challenges in preventing molten salt leakage and volatilization during hundreds or even thousands of thermal cycles,making it difficult for current composite phase change materials to support the long-term stable and efficient operation of molten salt thermal storage systems.To address the aforementioned issues,this study draws inspiration from the stable flesh tissue and moisture prevention in the pulp of a loofah to propose a biomimetic design strategy for high-temperature composite thermal storage materials known as the"Skin Flesh"structure（SF）.The strategy involves the utilization of a rigid or flexible porous ceramic framework combined with molten salt to achieve a highly thermally conductive core while simultaneously creating a low porosity or dense outer skin on the surface of the core to prevent molten salt leakage.This approach effectively addresses issues related to thermal conductivity disparities,high corrosion resistance,and leakage.Based on the biomimetic design strategy outlined above,research was conducted on the construction and performance of composite thermal storage materials featuring a biomimetic"Skin Flesh"structure.A porous silicon carbide material with a pore-silk structure was prepared using an organic foam impregnation method.This material was then combined with molten salt to form the core part of the biomimetic"Skin Flesh"structure.High-temperature wettability tests showed excellent wettability between the silicon carbide ceramic surface and the molten salt,allowing for molten salt to be pressure-infiltrated into over 98%of the porous ceramic pores.Corrosion and mechanical performance studies demonstrated that the porous silicon carbide with a pore-silk structure could withstand molten salt corrosion and solid-phase thermal expansion.Using the transient plane source method,the thermal conductivity of the composite material was found to be five times that of pure molten salt.Heat transfer studies indicated that the heat storage capacity of the composite material,constructed using porous silicon carbide with a porosity of 80%and a pore density of 20 PPI（Pores per linear inch）,was 1.7 times that of pure molten salt.On the porous ceramic preform prepared using the impregnation method mentioned above,a ceramic slurry was applied to the surface and subsequently sintered to obtain SF porous ceramics（SFPC）.Molten salt was directly infused into the SFPC to create the"Skin Flesh"structure composite phase change material（SF-CPCM）.Calculations and leakage tests showed that an SFPC with a skin thickness of 2 mm,a particle size of 6μm,and an equivalent diameter of 60 mm achieved excellent anti-leakage performance and molten salt loading capacity.After undergoing 500 cycles of thermal cycling tests,the SF-CPCM did not exhibit cracking or significant leakage.Numerical simulations indicated that SFPC,with both porous ceramic and skin sintered simultaneously,had a continuous core-skin thermal conductive network.This structure provided significant advantages in heat transfer and heat storage compared to three common molten salt encapsulation forms:those with skin but no core,those with core but no skin,and those with heterogeneous core-skin.However,it’s worth noting that the batch production consistency of SFPC based on the impregnation method was not high.Additionally,when the SF-CPCM continued cycling up to 850 times,cracks formed due to alternating thermal stresses.Therefore,it is essential to develop a more controllable process with higher cyclic durability for SF-CPCM.Inspired by the phenomenon of coconut’s middle husk buffering external impact stress,we propose the incorporation of a porous carbon layer between the phase change material and the ceramic shell.This layer serves to absorb the expansion of the molten salt,preventing the rupture of the shell during thermal cycling and thereby enhancing the cyclic durability of the SF-CPCM.The specific construction method involved using a hybrid cold pressing process to prepare a solid-solid phase change material（ss-PCM）consisting of MgO,expanded graphite（EG）,and ternary chloride（TC）.A layer of phenolic resin was applied to its surface,and then a dense ceramic body was wrapped around the resin-coated surface using a dry pressing process.Finally,a co-sintering process at 700°C was employed to obtain a co-sintered SF-CPCM with a dense SiC shell and a porous carbon layer.Electron microscope images showed that the SiC ceramic shell was dense,and after soaking for one hour,the water absorption rate of the ceramic shell with a SiC mass fraction of 65%was only0.25%.The porous carbon layer exhibited porous elasticity,which not only absorbed the thermal expansion of the molten salt but also maintained close contact with the ss-PCM and ceramic shell,reducing interfacial thermal resistance.Thermal conductivity tests indicated that when the ceramic shell had a SiC mass fraction of65%and a thickness of 3 mm,the thermal conductivity of the co-sintered SF-CPCM could reach 7.73 W/m·K.Compression tests demonstrated that the compressive strength of SF-CPCM could reach 42.1 MPa,and after continuous heating for 50hours,the mass loss rate of SF-CPCM was only 1%.The introduction of rice husk carbon（RHC）,which possesses an in-situ three-dimensional interconnected porous SiO₂-C structure,to replace a portion of the original"flesh"section that contained EG and MgO was undertaken.This replacement resulted in a strong connection between the thermal conductivity enhancer and structural support material,limiting the migration of molten salt during thermal cycling and slowing down the performance degradation throughout the cycling process.Initially,various RHC-based ss-PCMs with different molten salt mass fractions were prepared.The results showed that the introduction of RHC allowed ss-PCM to load up to 65%of molten salt.Even at a 60%molten salt mass fraction,the thermal conductivity remained as high as 6.91 W/m·K.Even after 1000cycles of thermal cycling testing,the ss-PCM maintained its shape stability.When compared to ss-PCM without RHC,the mass loss rate and thermal conductivity degradation were minimal.Both the co-sintered SF-CPCM and RHC-based co-sintered SF-CPCM underwent 2000 cycles of thermal cycling testing.The results indicated that after 2000 cycles,neither material deformed or cracked.However,the latter exhibited lower mass loss and less thermal conductivity degradation compared to the former.The cyclic performance of co-sintered SF-CPCM was significantly improved over SF-CPCM prepared using slurry-based methods,and RHC notably enhanced the cyclic stability of SF-CPCM.The biomimetic skin-flesh structured thermal storage materials developed in this article are suitable for large-scale high-temperature thermal storage applications,including heating,industrial waste heat utilization,and solar thermal power generation.This research provides theoretical and material support for the development of revolutionary,high-efficiency,and highly reliable thermal storage technologies.