Surface Effects On The Mechanical Properties Of A Nanoporous Material

Nanoporous materials are widely used in various industries due to the accuracy in nanoscale and their excellent properties.With the development of science and the recognition to sustainable development,the requirements could not be fully satisfied by traditional materials.Especially for some precision equipment and instruments,the materials with small scale but remarkable properties should be used,such as the field of aerospace,the weight of materials should be reduced as much as possible,and the small scale materials which are larger surface area and stronger adsorption capacity should be used in chemistry and medicine.However,these demands can be resolved by nanoporous materials to a certain extent,and the application of nanoporous materials have been promoted by the progress of manufacturing technology in recent years.It is noted that the properties of nanoporous materials are influenced by some conditions,including surface effect and volume effect,and the mechanical properties are mainly affected by the surface effect.Based on surface elasticity theory and yield strength theory,Young’s modulus and yield strength of regular polygonal prism nanoporous materials were derived.Then,the effects of surface,porosity and microstructure on Young’s modulus and yield strength of nanoporous materials were analyzed,and Young’s modulus of hierarchical nanoporous materials was studied further.On the basis of surface elasticity theory,Young’s modulus prediction model of regular polygonal prism nanoporous materials was established,and the effects of surface elastic modulus,residual surface stress and porosity on Young’s modulus of different regular polygonal prisms were studied in detail.The results reveal that Young’s modulus is mainly controlled by the surface elastic modulus,but it is not sensitive to the residual surface stress.This trend is in good consistency with the existing work.Moreover,for different regular polygonal prism structures,the difference of Young’s modulus tends to decrease with the increasing porosity,and the maximum Young’s modulus always relies on a critical side number of six,regardless of the surface effect and porosity.Based on the yield strength theory and combined the surface elasticity theory,the strength prediction model of four-sides regular polygonal prism was established.Considering the surface energy effect,the effects of surface bulk modulus,surface shear modulus and porosity on the yield strength of nanoporous materials were analyzed respectively.The results indicate that the change trend of yield strength is controlled by the two surface modulus,and the yield strength increases with the two surface modulus increasing,but it is not sensitive to the surface bulk modulus.Then,the yield strength significantly improves with the porosity reducing.Furthermore,for three different regular polygonal prism structures,the maximum yield strength depends upon the side number three at all times,regardless of the effects of surface energy and porosity.For the hierarchical nanoporous materials,based on the theory of unit cell nanoporous materials,Young’s modulus prediction model of six-side regular polygonal prism hierarchical structure was established.Without considering surface effect,Young’s modulus of regular polygonal prism hierarchical structure was obtained.The results of theoretical prediction show that the stiffness of nanoporous materials can be effectively reduced by the hierarchical structure,and Young’s modulus of the hierarchical structure decreases about 80% ～ 90%,compared to the first level structure.In regular polygonal prism hierarchical structures,the maximum Young’s modulus always relies on a critical side six,but the difference between regular polygonal prism hierarchical structures is smaller than that between unit cell structures.