Strongly Anisotropic Thermal and Electrical Conductivities of Self-assembled Silver Nanowire Network

TL;DR

This paper presents a self-assembled silver nanowire network with highly anisotropic thermal and electrical conductivities, offering a potential solution for thermal management in flexible electronics by efficiently spreading heat in-plane while shielding from heat cross-plane.

Contribution

It introduces a highly anisotropic silver nanowire network with aligned nanowires, demonstrating significant in-plane thermal and electrical conductivity differences and analyzing their temperature dependence.

Findings

In-plane thermal conductivity is 37 W/m-K; cross-plane is 0.36 W/m-K.

Anisotropy ratio of 3 for both thermal and electrical conductivities.

Anisotropy ratio remains constant down to 50 K, indicating contact effects dominate transport.

Abstract

Heat dissipation issues are the emerging challenges in the field of flexible electronics. Thermal management of flexible electronics creates a demand for flexible materials with highly anisotropic thermal conductivity, which work as heat spreaders to remove excess heat in the in-plane direction and as heat shields to protect human skin or device components under them from heating. This study proposes a self-assembled silver nanowire network with high thermal and electrical anisotropy with the potential to solve these challenges. The in-plane thermal conductivity of the network along the axial direction of silver nanowires is measured as 37 W/m-K while the cross-plane thermal conductivity is only 0.36 W/m-K. The results of measurements of electrical and thermal conductivities suggest that abundant wire-wire contacts strongly impede thermal transport. The excellent alignment of nanowires results in the same anisotropy ratio of 3 for both thermal and electrical conduction in the two in-plane directions. The ratio remains unchanged as the temperature decrease to 50 K, which indicates that wire-wire contacts lower the thermal and electrical conduction in the two directions to the same extent and their effect is independent of temperature. In addition, phonon softening markedly reduces the Debye temperatures of the network, which are fitted from the electrical resistivity data. As a result of phonon thermal conduction, the Lorenz numbers of the film in the two directions, which are approximately the same, are larger than the Sommerfeld value at room temperature and decrease as temperature decreases because of small angle scattering and the reduced phonon contribution. This nanowire network provides a solution to the emerging challenges of thermal management of flexible electronics.