Preparation Of Bioinspired Multiscale Structural Materials Through Surface-Groove-guided Directional Freezing

The structure design and function exploration of new materials are the eternal themes of the development of materials science.In the fields of aerospace,lightweight equipment,energysaving buildings,and tissue engineering,lightweight,strong,and tough materials are of vital importance as the strategic demands.After the long period of evolution,natural materials have possessed these excellent mechanical properties through constructing complex hierarchical architecture.Learning from nature and exploring the "structure-function" relationship will provide a powerful guidance for improving the properties of the existing materials and developing new counterparts.Mollusk shell is a typical representative of lightweight and strong materials.Among them,mother-of-pearl,one of the most widely-explored natural materials in recent years,improves the fracture toughness of materials with high inorganic content through constructing a layered "brick-and-mortar" structure.Compared with nacre,conch shells have an extra macroscopic three-layer cross-lamellar complex architecture,which endows them with an order of magnitude higher fracture toughness than natural nacre.Inspired by the formation process of natural materials,researchers have successfully developed various bottom-up technologies to realize the construction of bioinspired hierarchical architectures,such as layer-by-layer assembly,3D-printing,and ice-templating technique.However,restricted by the material system and resolution,chemical composition,hierarchical architecture,and mechanical properties of bioinspired materials obtained through these methods are not as good as those of natural materials,and the size limitation of the final materials also hinders their applications in practical life.Ice-templating technique achieves the ordered assembly of various building blocks and the construction of hierarchical architecture through controlling ice formation.Specifically,ice nucleation and growth directly affect ice morphology,and the latter determines the architecture of the final materials.Based on the above principles,many attempts have been made to achieve the microstructure control of ice-templated materials through regulating ice nucleation and growth.For example,the construction of long-range lamellar architecture has been successfully realized through changing the temperature field of the cold surface with dual temperature gradient or tailoring surface property with wettability gradient to control ice formation.However,the scaleup fabrication and complex assembly are still hindered by the gradient length and designability,respectively.Therefore,this research aims to explore ice nucleation and growth on structured surface with scalability and designability.We demonstrate the orientation mechanism of ice crystals on grooved surfaces,investigate the relationship between the microstructure and properties of bioinspired materials with the designability of grooved surface,such as Poisson’s ratio,fracture toughness,and impact resistance,and realize the fabrication of large-sized bioinspired structural materials with the scalability of grooved surfaces,which expands the practical applications of biomimetic multifunctional structural materials.The main contents of this research are divided into the following three parts:First,the growth mechanism of ice crystals on the grooved surface was investigated and a new control strategy of ice-templated architecture was proposed.Oriented grooves with various width and distance were introduced on the cold surface by photolithography.Freezing processes of various suspensions with different thermal conductivities on grooved surfaces were investigated through in situ observation.It was found that surface grooves that can induce the preferential orientation of ice crystals have a size effect.Specifically,surface grooves with size in a certain range can regulate the ice orientation through the merging behavior of ice crystals in a single groove or localized temperature gradient.Utilizing the designability of surface grooves,the graphene aerogel with negative Poisson’s ratio was prepared through ice-templated assembly.This sufficiently confirms the possibility of constructing complex hierarchical architecture through surface-groove-induced freezing process.Next,nanomaterials were orderly assembled into biomimetic hierarchical architecture with the designability of surface grooves.Inspired by the architecture of conch shell,surface grooves with three-layers cross-orientation were introduced on the cold finger with the photolithography and the suspension of alumina nanoplatelets was directionally frozen on this surface.On the basis of the orientation mechanism of ice crystals,the width and distance of surface grooves are both 10μm.After the infiltration of the freeze-dried scaffold with epoxy resin,the final composite perfectly replicates the composition and multiscale architecture of natural conch shell.The strength and toughness of this conch-shell-inspired cross-lamellar composite are 2.5 and 20 times higher than those of natural nacre,2.5 and 2 times higher than those of natural conch shell.This composite holds great application potentials in personal protective equipment,such as helmet and body armor.Finally,a continuous freezing device was built using the cold surface with large-area oriented grooves to realize the scale-up fabrication of bioinspired materials with hierarchical architecture.Based on the research of size-effect grooves,the large-area oriented grooves with specific sizes were produced through sanding on the cold surface.The suspension of the alumina nanoplatelets was directionally frozen on such a cold surface,a porous scaffold with nacre-mimetic lamellar architecture was obtained,and the orientation mechanism of ice crystals on the sanded surface was also demonstrated.Different from traditional freezing process,the continuous freezing was realized under the constant temperature gradient by transferring the suspension through the fixed cold source.As a result,the scale-up fabrication strategy can promote the practical applications of bioinspired structural materials.