Thin film tungsten diselenide structures with ultra-low thermal conductivities
Materials scientists at the Universities of Oregon (UO) and Illinois at Urbana-Champaign (UI), USA, claim to have created a material with the lowest ever recorded thermal conductivity for a fully dense solid. This could lead to improved insulation in a variety of sectors.
The researchers synthesised thin films of tungsten diselenide (WSe2) structures with ultra-low thermal conductivities. Grown by a modulated elemental reactant method – controlling the order and disorder in two dimensional WSe2 sheets – the resulting textured nanocrystalline materials have a thermal conductivity of approximately 0.05 Wm-1K-1 under standard laboratory conditions.
According to project leader Professor David Cahill of UI, the physics behind this development is not clear. ‘Our working hypothesis is that a large fraction of the lattice vibrations are localised in the through thickness direction of the film.‘We all know that we can have nearly perfect electrical insulators – electrical conductivity is, for all practical purposes, zero for oxide glass or alumina. But, the physics of materials. This new material, WSe2, in the form that we describe, has a thermal conductivity that is a factor of six smaller than this “minimum” value.’
Researchers at Rensselaer Polytechnic Institute, USA, carried out molecular dynamics simulations of the novel material. The results demonstrated that the ultra-low thermal conductivity phenomenon in layered disordered structures is not limited to tungsten diselenide. Far from having an immediate application, the fabrication process offers an alternative to other methods where researchers have achieved low values by manipulating thin films of metals and oxides to adjust the interfaces of the materials by only a few nanometres.
‘We are [now] working to understand the details of the disorder in the stacking of WSe2 sheets and the ability to vary this systematically. Computational work is examining the nature [extended versus localised] of the vibration modes,’ says Cahill. ‘There is not a practical application because the synthesis method is slow – each layer of atoms is deposited one layer at a time – and therefore expensive. The important results is the demonstration of new physics – a thermal conductivity much smaller than what was previously thought [possible].’