Google Search
A5: Nanoscale and low dimensional effects
Reduction of the phononic contribution to thermal conductivity has been one of the most active fields in thermal engineering due to its effect on important aspects related to energy production like thermoelectric efficiency. In the last decades, several alternatives have been exploited in this direction. The most explored has been nanoscaling in structures like thin films, wires or dots. A complementary method has been combining two materials using different strategies, like for instance alloys, superlattices and, in the last years, compositionally graded materials. The development of all these devices is based on the known role that size reduction and composition modulation plays on relaxation times, group velocities and specific heats. In particular, results from composition graded materials have yielded experimental results suggesting the need of new scattering terms in order to make good predictions.
At the same time, in the last years the role of normal scattering has been extensively studied as it is widely accepted that despite its non resistive nature, it has a profound effect on thermal conductivity. In a recently published work a Kinetic Collective Model (KCM), able to predict the thermal conductivity from the splitting of the heat flux in a collective (normal dominated) and a kinetic (resistive dominated) term has been presented. Like in the Callaway Model, KCM can be combined with ab-initio relaxation times in order to eliminate any parameter from the expressions but due to its more natural separation of relaxation times can give a deeper insight on thermal transport behavior.
We present a generalization of the KCM able to obtain a single expression for the thermal conductivity of inhomogeneous materials. On this basis, the phenomenology associated to non-homogeneous relaxation times is explained, and it is demonstrated that a direct consequence is the theoretical prediction of thermal rectification in some graded systems. This prediction is compared with recent experimental results where this behaviour has been observed.
