The surface/interface energy theory based on three configurations proposed by Huang et al. is used to study the effective properties of thermoelastic nanocomposites. The particular emphasis is placed on the discussion of the influence of the residual interface stress on the thermal expansion coefficient of a thermoelastic composite filled with nanoparticles. First, the thermo-elastic interface constitutive relations expressed in terms of the first Piola-Kirchhoff interface stress and the Lagrangian description of the generalized Young-Laplace equation are presented. Second, the Hashin's composite sphere assemblage (CSA) is taken as the representative volume element (RVE), and the residual elastic field induced by the residual interface stress in this CSA at reference configuration is determined. Elastic deformations in the CSA from the reference configuration to the current configuration are calculated. From the above calculations, analytical expressions of the effective bulk modulus and the effective thermal expansion coefficient of thermoelastic composite are derived. It is shown that the residual interface stress has a significant effect on the thermal expansion properties of thermoelastic nanocomposites.
We report on the study of the self-focusing effects on the tunnel-ionization-induced injection in a laser wakefield accelerator. Targets composed of a gas mixture of 94% helium and 6% oxygen were used. The energy, energy spread, and charge of the generated electron beams can be adjusted by changing the input laser intensity and the plasma density, but the different aspects of the properties of the electron beams were not independent. It was inferred that the K-shell electrons of oxygen were ionized and injected into the plasma wake for acceleration when the laser intensity was increased beyond the threshold for generating O7+ by tunnel-ionization, due to the relativistic self-focusing in the propagation. Controlling the self-focusing of the laser beam by adjusting the input laser energy to shorten the distance over which the electrons were injected into the wake, quasi-monoenergetic electron beams were observed.