•Graphene film wrapped on the NiTi alloy exhibits good anticorrosion and biocompatibility.•The supervisory of graphene is its flexibility, which keeps excellent mechanical stability upon deformation.•The behavior of graphene for inhibiting poisonous Ni ions releasing was explored through theoretical calculation.Due to their unique shape memory effect and superelasticity, NiTi shape memory alloys have been considered for a wide range of biomedical applications. However, they are still controversial because of the potential toxic, carcinogenic and allergic effects caused by Ni2 + release for a long term use. Wrapping protective layers with good flexibility and biocompatibility is significant for inhibiting the poisonous Ni2 + releasing from NiTi. Here, we report a novel method to protect the NiTi and enhance its biocompatibility by using graphene fabricated via a modified chemical vapour deposition (CVD) technique. The graphene layer not only prevents effectively the leak of Ni2 + but also improves the biocompatibility of NiTi upon deformation. The detailed mechanism for enhancing the anti-corrosion and biocompatibility of NiTi alloy by using graphene is also explored. Compared with traditional surface modification layer, graphene obtained by CVD is chemically inert and highly flexible, possesses both good anti-corrosion and biocompatibility properties, which may improve the surface coatings for NiTi alloys and promote more application of graphene in biomedical materials.A large sheet of graphene was fabricated by a CVD method, and transferred onto the NiTi shape memory alloy substrate. It is calculated that the significant inhabitation of Ni2 + releasing is kinetically hindered caused by the high energy barrier built upon graphene wrapping even under stain or with Cl−. The graphene film obtained by CVD is chemically inert and highly flexible, which demonstrates a significant reduction of Ni2 + releasing from NiTi and greatly enhances its anti-corrosion ability. Additionally, both blood and tissue cells display a better attachment on the NiTi with graphene manifests the improvement of its biocompatibility.Download high-res image (175KB)Download full-size image

With the aim to optimize alkaline treatment of zeolites to obtain hierarchical zeolites, dissolution and absorption mechanisms relevant to mesopore formation were investigated at an atomistic scale by density functional calculations. In the dissolution processes, dealumination is energetically more favorable than desilication, though both processes can occur. The dissolved Al species prefer to be absorbed back onto zeolite surfaces whereas the dissolved Si species tend to aggregate in solution. The dissolution process promotes but the absorption process hampers the mesopore formation, laying foundation for understanding the mesoporosity influenced by the variations of zeolite framework and solution.

To gain an atomistic-level understanding of the thermal and chemical responses of condensed energetic materials under thermal shock, we developed a thermal shock reactive dynamics (TS-RD) computational protocol using molecular dynamics simulation coupled with ReaxFF force field. β-Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) was selected as a a target explosive due to its wide usage in the military and industry. The results show that a thermal shock initiated by a large temperature gradient between the “hot” region and the “cold” region results in thermal expansion of the particles and induces a thermal–mechanical wave propagating back and forth in the system with an averaged velocity of 3.32 km s−1. Heat propagating along the direction of thermal shock leads to a temperature increment of the system and thus chemical reaction initiation. Applying a continuum reactive heat conduction model combined with the temperature distribution obtained from the RD simulation, a heat conduction coefficient is derived as 0.80 W m−1 K−1. The chemical reaction mechanisms during thermal shock were analyzed, showing that the reaction is triggered by N–NO2 bond breaking followed by HONO elimination and ring fission. The propagation rates of the reaction front and reaction center are obtained to be 0.069 and 0.038 km s−1, based on the time and spatial distribution of NO2. The pressure effect on the thermal shock was also investigated by employing uniaxial compression before the thermal shock. We find that compression significantly accelerates thermal–mechanical wave propagation and heat conduction, resulting in higher temperature and more excited molecules and thus earlier initiation and faster propagation of chemical reactions.

To gain an atomistic-level understanding on physical and chemical processes occurring at the interfaces of hypergolic propellants, we carried out the first reactive dynamic (ReaxFF) simulations to study the reactive hypergolic mixture of monomethylhydrazine (MMH) and dinitrogen tetroxide (NTO), in comparison with the ethanol (EtOH) and NTO mixture that is reactive but nonhypergolic. Our studies show that the MMH–NTO mixture releases energy more rapidly than the EtOH–NTO mixture upon mixing the fuels and oxidizers. We found that the major early chemical reactions between MMH and NTO are hydrogen abstractions and N–N bond scissions. The MMH–NTO mixture has more reaction channels than EtOH–NTO based on statistical analyses of chemical reaction events and channels at early stages of the dynamics. Analyzing the evolution of product distribution over reaction time shows that the oxidizer (NO2) diffuses into the fuels (MMH or EtOH) for the occurrence of reactions, demonstrating the influence of physical mixing on chemical reactions. Our simulations suggest that effective hypergolic systems require fuels with low energy barriers of H abstractions and/or bond scissions and oxidizers with large diffusion mobility for efficient physical mixing.