Fabrication And Mechanical Properties Of Metallic Quasi-BCC Beam Nanolattices

Mechanical metamaterials refer to a class of composite materials that have artificially designed structures and exhibit extraordinary mechanical properties that traditional materials do not have.Nanolattice is a new class of mechanical metamaterials with characteristic sizes on the order of hundreds of nanometers.Due to extremely special size effects,structural characteristics and material selection,the mechanical properties of this type of porous materials are very different from those of bulk materials,and it can even have better mechanical properties with lighter weight,which is expected to bring revolutionary applications in the field of high-performance functional materials in the future.Beam nanolattice is the main research object of nanolattice materials.However,to date,it is challenging to make breakthrough in the fabrication of metallic beam nanolattice with beam diameter of less than 200 nm,and thus their mechanical properties remain ambiguous.In the light of the problems mentioned above,a new type of quasi-body centered cubic(quasi-BCC)beam nanolattice mechanical metamaterial is proposed in this thesis,which is experimentally implemented with ion track technique.The characteristic size(beam diameter)of the quasi-BCC beam nanolattice can be as small as 30 nm,breaking through the size limit of the beam nanolattices(～200 nm).To prepare these metamaterials,based on the Heavy Ion Research Facility in Lanzhou(HIRFL),three-dimensional ion tracks were introduced into the polymer template by multi-rotation and multi-direction irradiation,taking full advantage of the parallel arrangement of ion tracks and nanoscale.After chemical etching,the three-dimensional ion tracks are transformed into highly interconnected three-dimensional nanopores.Using electrochemical deposition,we filled the nanopores with gold or copper and finally obtained the three-dimensional metallic quasi-BCC beam nanolattice materials.Furthermore,by controlling the ion irradiation rounds and irradiation directions,quasi-BCC beam nanolattices with different spatial configurations were obtained.By selecting the ion irradiation fluence,we achieved the relative density control of quasi-BCC beam nanolattices.By controlling the track etching conditions,the beam dimeter of the quasi-BCC beam nanolattices was precisely controlled in the nanometer scale(<100 nm),with the minimum size of 30 nm.By selecting the electrolyte composition,the quasi-BCC beam nanolattices of gold and copper are fabricated.Next,we used the conventional nanolattice mechanics testing method,processed cube samples by focused ion beam,and carried out quasi-static compression experiments by in situ nanomechanics test system with a scanning electron microscope to obtain the stress-strain curves,and studied the dependence of mechanical properties on beam diameter,three-dimensional structure,matrix materials and other major parameters.Finally,using Abaqus modeling,the mechanism behind the forementioned mechanical properties was discussed theoretically.The results have shown that the mechanical properties of Au quasi-BCC beam nanolattices exhibit an obvious size dependence and are related to the relative density.The strength and the stiffness of the Quasi-BCC-45°-Au-34 quasi-BCC beam nanolattice can be approximately doubled compared to the Quasi-BCC-45°-Au-117quasi-BCC beam nanolattice,and the compressive strength even exceeds that of bulk gold.The increase in strength can be ascribed to the size effects.The finite element simulation also proved the size effects play a key role in determining the mechanical properties.However,the increase in stiffness is not from the size effect.Our simulations show that there is no difference in stiffness between two samples.Comparing with Quasi-BCC-45°-Au-117,the increase in stiffness of Quasi-BCC-45°-Au-34substantially stems from smaller surface roughness.In addition,when the irradiation fluence is a constant,as the beam dimeter decreases,the relative density of the material decreases,and so does the compressive modulus and strength.Then,the metamaterial structure was further optimized through finite element simulation.The optimized Au quasi-BCC beam nanolattices have even better mechanical properties than those before optimization with the same relative density,which proves that the structural alignment of the beam nanolattices has a significant impact on the mechanical properties of the beam nanolattices.At the same time,the stress-strain curves of the Au quasi-BCC beam nanolattice under compression display three stages:elastic stage,plateau stage,and densification stage.Such a behavior is attributed to bending-dominated deformation mechanism,and it can withstand 80%strain without brittle collapse.Furthermore,the finite element simulation results show that both node offset and relative density reduction will reduce the stiffness and strength of the quasi-BCC beam nanolattices.The increase of surface roughness decreases the stiffness,but has no effect on the strength.The compression experiments show that the Cu quasi-BCC beam nanolattice have better mechanical properties than Au quasi-BCC beam nanolattice under certain conditions.The compressive strength of Cu quasi-BCC beam nanolattice with beam dimeters of 34 nm is also surprisingly higher than that of bulk Cu,and other mechanical properties are consistent with those of the Au quasi-BCC beam nanolattices.Finally,the high strength and deformation ability of the metallic quasi-BCC beam nanolattices give the material excellent energy absorption ability.Compared with natural porous materials and reported micro/nanolattice materials,the Cu quasi-BCC beam nanolattice has a record-high energy absorption capacity to date,up to 110±10 MJ/m~3.We believe this type of metallic quasi-BCC beam nanolattice has great potentials for multi-functional applications in fields such as heat transfer,electric conduction,catalysis applications,etc.In short,a new type of metallic quasi-BCC beam nanolattice with a minimum beam dimeter down to 30 nm has been realized using the ion track technique in this thesis.Their spatial configurations,beam dimeter,and material are well controlled.Our work establishes that Au and Cu quasi-BCC nanolattices have excellent compressive strength and energy absorption capacity,which substantially result from the relevant size reduction-induced mechanical enhancement,the quasi-BCC nanolattice architecture,and the synergy of the naturally high mechanical strength and plasticity of metals.The finite element simulations show that the deformation is governed by bending-dominated mechanism.Though this approach may be limited to materials that can be electrodeposited,it opens up the field to a fairly large number of metals.Thus,we expect our method can be extended to some other materials and/or spatial configurations where new physics and applications may be explored.