Discrete space-time model for heat conduction: Application to size-dependent thermal conductivity in nano-films

- Heat conduction model with discrete variables is introduced.
- The model describes space and time non-Fourier effects.
- Cross-plane heat conduction in nano film is considered.
- The size-dependant thermal conductivities are obtained.
- The model describes transition from diffusion to ballistic regimes.

An analytical discrete-variable model has been developed to describe heat conduction in nano-sized systems. The model assumes that the system consists of a homogeneous array of cells with characteristic size h; each cell interacts with the nearest neighbors in discrete time step τ and all the cells compute their new state simultaneously. In the continuum limit h→0 and τ→0, the model reduces to classical heat diffusion equation of parabolic type or heat conduction equation of hyperbolic type, depending on the choice of scaling invariant. The model is applied to heat conduction in nano-films with emphasis on the transition from the diffusive to ballistic heat transport, which occurs with decreasing film thickness. This model provides a simple method for predicting in a self consistent manner the effective cross-plane thermal conductivities, the temperature jump at the boundaries, the heat flux across the film, and the temperature gradient within the film as functions of the film thickness. The results are in good agreement with molecular dynamic and Monte Carlo simulations.