| Developing light weight,strong,and tough structural materials is of great importance for industrial and military applications.Many biological materials have long served as a source of inspiration for developing high-performance structural materials.With a very limited selection of relatively weak constituents,these biological materials realize outstanding mechanical properties by building sophisticated hierarchical architectures ranging from nano/micro to macroscopic levels.A prime example is nacre,which achieves both excellent strength and toughness by assembling 95 vol.%of brittle CaCO3 platelets and 5 vol.%of weak biopolymer into a ’brick-and-mortar’ architecture.During the past decade,various methods have been applied to develop high-performance nacre-mimetic materials.Specifically,freeze-casting holds promise to become a powerful technique with a balance between precise architectural control,easy scalability,versatility,and low cost,which are all important for the practical application of high-performance nacre-mimetic materials.Controlling ice nucleation and growth is crucial,as the architecture of the resulting porous material simply replicates the ice crystal morphology.In this context,parameters such as additives,cooling rate,temperature gradient direction,and external energized fields have been revealed to have a profound impact on the final architecture.However,it still lacks effective ways to precisely control the final architecture.Therefore,we have focused our study on controlling ice nucleation and growth as well as the structure,property,and application of the freeze-casted material.First,by introducing a linear wettability gradient onto the cold surface,a porous scaffold with long-range lamellar layers was fabricated.The wettability gradient yields successive ice nucleation and preferential ice growth,which act synergistically to control the orientation of the ice crystals and the architecture of the resulting porous material.Subsequent infiltration of this porous scaffold generates a high-performance bulk nacre-mimetic composites with excellent strength and toughness.In addition,cross-aligned and circular lamellar structures can be obtained by freeze-casting on surfaces modified with bilayer linear wettability gradient and radial wettability gradient,respectively.which are impossible to realize with conventional freeze-casting techniques.This study highlights the potential of harnessing the rich designability of surface wettability patterns to build bioinspired materials with complex architectures and high performance.Next,by covering the cold surface with a PDMS wedge(also referred to as "bidirectional freezing method"),a nacre-mimetic graphene/poly(vinyl alcohol)composite film was fabeicated,with both asperities and bridges introduced in addition to the lamellar layers to mimic the interfacial architectural interactions found in nacre.By carefully controlling the viscosity and component in the suspension,the nacre-mimetic composite film shows high strength,high toughness,and high stretchability.The fracture mechanism of the nacre-mimetic composite film was studied.With interfacial architectural design,the superstretchable composite film shows potential application in smart materials such as soft electronics.Finally,we extended the application of nacre-mimetic materials to the thermal insulation field.A silica foam with nacre-mimetic lamellar layers(albeit of polymer phase)was successfully fabricated by appling the bidirectional freezing method.Due to such anisotropic porous structure,the silica foam displays high compressive strength along the layers and low thermal conductivity across the layers,which is ideal for applications like building walls where these two properties can be fully utilized.Additionally,the silica foam is fire-resistant,which is crucial for its practical application.This study also highlights the possibility of combining intrinsically exclusive properties in engineering materials through constructing biomimetic structures. |
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