Computational Study of the Thermal Rectification Properties of a Graphene-Based Nanostructure.

Abstract

Thermally rectifying materials would have important implications for thermal management, thermal circuits, and the field of phononics in general. Graphene-based nanostructures have very high intrinsic thermal conductance, but they normally display no thermal rectification effects. The present study relates to a thermally rectifying material and, more particularly, to a graphene-based nanomaterial for controlling heat flux and the associated method determining the rectification coefficient. Thermal rectifiers using a graphene-based nanostructure as thermal conductors were designed. Vacancy defects were introduced into one end of the nanostructure to produce an axially non-uniform mass distribution. Modified Monte Carlo methods were used to investigate the effects of defect size and shape, vacancy concentration, and ribbon length on the thermal rectification properties. Anharmonic lattice dynamics calculations were carried out to obtain the frequency-dependent phonon properties. The results indicated that the nanoscale system conducts heat asymmetrically, with a maximum available rectification coefficient of about 70%. Thermal rectification has been achieved, and the difference in temperature dependence of thermal conductivity is responsible for the phenomenon. Defects can be tailored to modulate the temperature dependence of thermal conductivity. The power-law exponent can be negative or positive, depending upon the ribbon length and vacancy concentration. A computational method has been developed, whereby the numerous variables used to determine the rectification coefficient can be summarized by two parameters: the power-law exponent and the thermal resistance ratio. Accordingly, the rectification coefficient can be obtained by solving a simple algebraic expression. There are several structure factors that cause noticeable effects on the thermal rectification properties. Defect size, vacancy concentration, and ribbon length can affect the thermal conductance of the nanostructure symmetrically and significantly. Graphene-based nanostructure thermal rectifiers can be arranged in an array so as to provide thermal rectification on a macroscopic scale.