Thermal Stability And Radiation Tolerance Of Nanostructured Ni-Carbon Nanotube Hybrid

The development of future fusion nuclear energy systems calls for materials that can withstand higher temperature,ultra-high neutron irradiation dosage,and bombardment of high-flux plasmas.The key to the commercial application of fusion energy is the advancement of materials with exceptional radiation tolerance.Extensive researches in the past few decades have shown that despite the details of microstructural damage vary drastically in different materials,the essential cause of radiation damage is the continuous generation and clustering of irradiation-induced point defects.The formation of complex defects such as voids,dislocation loops,phase decomposition,segregation,and bubbles,led to serious hardening,swelling,and embrittlement of the material.There are two strategies to improve the radiation tolerance of materials.One is by introducing high-density defect sinks,such as grain boundaries(GBs),interfaces,surfaces,etc.,to absorb irradiation-induced point defects and inhibit their clustering to form complex defects.Representatives of this strategy include nanocrystalline,nanolayer,and nanoporous materials.The other is by complex alloying composition,which is also known as high-entropy alloys.The unique local structural and chemical variation altered the intrinsic mass transportation properties of the material such as the migration energy and formation energy of interstitial atoms and vacancies,which will improve the recombination probability of interstitial atoms and vacancies,thereby reducing the density of irradiation point defects in materials.Combined with all the existing theories of radiation damage,we designed and developed a hybrid film with Ni nanocrystals anchored on a single wall carbon nanotubes(SWCNTs)bundled networks,and systematically investigated its thermal stability and radiation tolerance performance.The mechanisms of interactions between irradiation-induced point defects and the hybrid were also studied using molecular dynamics simulations.The main results and conclusions are as follows:(1)We designed and fabricated a nano-network structural Ni-SWCNT hybrid by Ni sputtering onto a SWCNTs scaffold using magnetron sputtering technology.The influence of deposition time and temperature on Ni-SWCNT diameter and Ni nanograin size of the hybrid was systematically investigated.It was found that the NiSWCNT diameter increases linearly with deposition time,and the Ni grain size increases with increasing deposition temperature.The Ni-SWCNT hybrid transformed into a highly stable bamboo-like structure at elevated temperature.The bamboostructural Ni-SWCNT composite showed no grain growth during annealing until the Ni nanowires fractured into separated nano-particles due to Rayleigh instability at 873 K.On one hand,GB migration is driven by GB curvature,so that the flat Ni GBs in the hybrids are highly stable.On the other hand,the formation of GB grooves reduces the free energy of the Ni-SWCNT system and has a pinning effect on Ni GBs,which inhibits the further growth of Ni nano-grains.(2)The Ni-SWCNT hybrid showed excellent radiation tolerance performance during He ion irradiation experiments.No obvious irradiation-induced defects were detected by transmission electron microscope in Ni-SWCNT hybrids with diameter smaller than 30 nm,regardless of the initial Ni grain size.The excellent radiation tolerance of the hybrid can be ascribed to the following aspects:First,the highly stable network structure of SWCNTs bundles help preserved the network morphology of the hybrid.Second,the sink density in Ni-SWCNT hybrid is about 1.5～2 times higher than that of traditional bulk nanocrystalline materials,which effectively eliminate the irradiation-induced defects;Third,the high-energy GBs and the Ni-SWCNT interfaces in the Ni-SWCNT hybrids are effective defect sinks,which further improved the radiation tolerance of the hybrid.(3)To further improve the structural stability of the Ni-SWCNT hybrids against annealing and irradiation,an amorphous carbon(AC)layer was coated onto the surface of Ni-SWCNT,and labeled as AC-Ni-SWCNT.After annealing at 1073 K for 5 h,the grain size of AC-Ni-SWCNTs is about 42.6 nm,exhibiting a further improved stability compared with Ni-SWCNT hybrids.No obvious irradiation-induced defects were detected in AC-Ni-SWCNT hybrids with diameter smaller than 70 nm after 5 dpa irradiation at 673 K.At similar irradiation dose level,the radiation damage of AC-NiSWCNT is much higher than the Ni-SWCNT samples and other reported nanostructured materials and coarse-grained materials.The AC layer with ultra-low surface energy greatly reduced the total interfacial energy of the Ni-SWCNT hybrid.It also exerts a compressive hoop stress on the inner Ni grains,which inhibit the migration of GBs and the Rayleigh instability induced fragmentation of Ni nanowires.The AC coating greatly improved the thermal stability of the Ni-SWCNT hybrids through both kinetic and thermodynamic aspects.The high density of Ni GBs and Ni-C interfaces and the compressive hoop stress on Ni nano-grains effectively enhanced the annihilation of irradiation-induced defects and enabled the excellent radiation tolerance of AC-Ni-SWCNT hybrid.(4)The AC-Ni-SWCNT hybrids with excellent bending flexibility can be readily fabricated in meter-scale and paved on to arbitrary surfaces as irradiation shielding material in nuclear reactors.Moreover,the AC coating can also be replaced by metal oxide materials with high thermal stability,such as alumina.This shell-nanocrystallinecarbon nanotube hybrid network structure is also applicable to other metals and alloys,which provides an important approach to the practical use of nanocrystalline materials in extreme environments.