Molecular Dynamics Study On Carbon Nanotubes-reinforced Nano-honeycombs

Nickel（Ni）,as an important material in electrochemical technology,is used widely as anode material in solid oxide fuel cells（SOFCs）.Nano-honeycombs（NNHCs）would be expected to an idea anode material for SOFCs because of its high surface area and highly ordered pore network.However,since SOFCs usually operate at a complicated situation,it is desired that the anode material have not only enough chemical and electrochemical stability,but also excellent mechanical properties to withstand stresses that arise during processing and service and so it is important to study the mechanical properties of NNHCs.Carbon nanotubes（CNTs）have been used in electrode materials of various cells and reinforcement fillers of different structural materials owing to their extraordinary properties.In this paper,the mechanical properties including elastic moduli E,ultimate strengths?_u and energy absorption per unit volume in deformation W_v of nickel nano-honeycombs（NNHCs）and CNT-reinforced NNHCs（CRNNHCs）under in-plane tension,in-plane compression,out-of-plane tension and out-of-plane compression are investigated by using the molecular dynamics（MD）.In addition,the in-plane compressive behaviors of aluminum（Al）nano-honeycombs（ANHCs）and CNT-reinforced ANHCs（CRANHCs）are investigated.The main research contents are organized as follows:（1）A representative volume element（RVE）of NNHCs is chosen for MD simulations to investigate the mechanical behaviors of the RVE in the in-plane and out-of-plane directions.In order to investigate the influence of relative density,7 classes of NNHCs with a center cylindrical pore of different radii are built.In order to investigate the influence of CNTs,we take one of the groups of NNHCs as the example by adding CNTs with different mass fractions to construct the model of CRNNHCs.All simulations are carried out at the temperature of 900 K and applying a constant strain rate of 5?10⁹ s^-1.The results show that the overall mechanical properties of the NNHCs,such as elastic moduli E,ultimate strengths?_u and energy absorption per unit volume in deformation W_v increase with the increase of the relative density and the out-of-plane mechanical properties are generally superior to those in the in-plane direction.As for the effects of CNTs to the mechanical properties of CRNNHCs,it strongly depends on loading directions.The existence of CNTs may result in a weakening in in-plane direction.The main reason is the interface between CNT and Ni is so weak that the load cannot be effectively transferred in the in-plane direction,resulting in defects at the interface.Compared to the in-plane loadings,the CNTs can bear more loads during the out-of-plane loadings resulting in a better reinforcement of CRNNHCs,and the reinforced effects increase with the increase of mass fractions of CNTsω_CNT.（2）Taking the CRNNHC with theω_CNTNT of 5.22‰and its corresponding NNHC as the example to investigate the influence of temperature on the mechanical behaviors of NNHCs and CRNNHCs under the four deformation situations.The samples are deformed at a temperature ranging from 300 K to 900 K under a constant strain rate of 5?10⁹ s^-1.The results show that the mechanical properties of both NNHCs and CRNNHCs decrease with the increase of temperature.Especially,elastic moduli E and ultimate strengths?_u decrease linearly.Under different temperature,the addition of CNTs may result in a weakening in the in-plane direction,but will result in a better reinforcement in the out-of-plane direction.（3）Taking the CRNNHC with theω_CNTNT of 5.22‰and its corresponding NNHC as the example to investigate the influence of strain rate on the mechanical behaviors of NNHCs and CRNNHCs under the four deformation situations.The samples are deformed at a strain rate ranging from 1?10⁸ s^-1 to 1?10¹00 s^-1 with a temperature of 900 K.The results show that elastic moduli E in both NNHCs and CRNNHCs are insensitive to the strain rate.?_u and W_v both in NNHCs and CRNNHCs increase with the increase of strain rate.Under different strain rate,the addition of CNTs may result in a weakening in the in-plane direction,but will result in a better reinforcement in the out-of-plane direction,which is similar to the situation of the temperature effect.（4）In addition,the in-plane compressive behaviors including elastic moduli and ultimate strengths of aluminum（Al）nano-honeycombs（ANHCs）and CNT-reinforced ANHCs（CRANHCs）are investigated by using the MD and analytical methods.And then the MD results are compared with the classical Gibson-Ashby expression.Elastic moduli and ultimate strengths increase with the increase in the relative density.The mechanical properties of CRANHCs are superior to ANHCs.As for elastic moduli,a simple model is proposed to predict radial elastic moduli E_r of CNTs so as to predict the elastic moduli of CRANHCs further.The result shows that the elastic moduli of CRANHCs can be predicted well by using the simple model.As for ultimate strengths,a modified Gibson-Ashby model is developed to estimate ultimate strengths of DCRANHCs,which shows a good agreement with the MD results except[110]direction.