(1 of 10)Dielectric materials are greatly desired for electromagnetic absorption applications. Lots of research shows that conduction loss and polarization are two of the most important factors determining complex permittivity. However, the detailed dissipation mechanisms for the improved microwave absorption performance are often based on semiempirical rules, lacking practical data relationships between conduction loss/polarization and dielectric behaviors. Here, a strategy of introducing point defects is used to understand such underlying relationships, where polarizability and conductivity are adjustable by manipulating oxygen deficiency or heteroatoms. Based on first principles calculations and the applied oxygendeficient strategy, dielectric polarization is shown to be dominant in determining the permittivity behaviors in semiconductors. Meanwhile, the presented nitrogen doping strategy shows that conduction loss is dominant in determining the permittivity behavior in graphitized carbon materials. The validity of the methods for using point defects to explore the underlying relations between conduction loss/polarization and dielectric behaviors in semiconductor and graphitized carbon are demonstrated for the first time, which are of great importance in optimizing the microwave absorption performance by defect engineering and electronic structure tailoring.

An exceptionally large axial electrostrictive deformation is demonstrated in single walled carbon nanotubes using Hartree-Fock and density functional quantum mechanics simulations. Armchair and zigzag open-ended tubes and capped tubes are studied and in all of them the external electric field induced axial strains can be greater than 10% for a field strength within 1 V/A. The corresponding volumetric and gravimetric work capacities are predicted to be three and six orders higher than those of the best known ferroelectric, electrostrictive, magnetostrictive materials and elastomers, respectively.

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Recently discovered ultralow friction ͑superlubricity͒ between incommensurate graphitic layers has raised great interest in understanding the interlayer interaction between graphene sheets under various physical conditions. In this work, we have studied the effects of interlayer distance change and in-sheet defects in modifying the interlayer friction in graphene sheets by extensive molecular-force-field statics calculations. The interlayer friction between graphene sheets with commensurate or incommensurate interlayer stacking increases with decreasing interlayer distance, but in the case of incommensurate stacking, ultralow friction can exist in a significantly expanded range of interlayer distance. The ultralow interlayer friction in the incommensurate stacking sheets is insensitive to the in-sheet defect of vacancy at a certain orientation. These results provide knowledge for possibly controlling friction between graphene sheets and offer insight into their applications.