Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement.

Controlling thermomechanical anisotropy is important for emerging heat management applications such as thermal interface and electronic packaging materials. Whereas many studies report on thermal transport in anisotropic nanocomposite materials, a fundamental understanding of the interplay between mechanical and thermal properties is missing, due to the lack of measurements of direction-dependent mechanical properties. In this work, exceptionally coherent and transparent hybrid Bragg stacks made of strictly alternating mica-type nanosheets (synthetic hectorite) and polymer layers (polyvinylpyrrolidone) were fabricated at large scale. Distinct from ordinary nanocomposites, these stacks display long-range periodicity, which is tunable down to angstrom precision. A large thermal transport anisotropy (up to 38) is consequently observed, with the high in-plane thermal conductivity (up to 5.7 W m-1 K-1 ) exhibiting an effective medium behavior. The unique hybrid material combined with advanced characterization techniques allows correlating the full elastic tensors to the direction-dependent thermal conductivities. We, therefore, provide a first analysis on how the direction-dependent Young's and shear moduli influence the flow of heat.

Direct Measurement of the In-Plane Thermal Diffusivity of Semitransparent Thin Films by Lock-In Thermography: An Extension of the Slopes Method.

We present an extension of the well-known slopes method for characterization of the in-plane thermal diffusivity of semitransparent polymer films. We introduce a theoretical model which considers heat losses due to convection and radiation mechanisms, as well as semitransparency of the material to the exciting laser heat source (visible range) and multiple reflections at the film surfaces. Most importantly, a potential semitransparency of the material in the IR detection range is also considered. We prove by numerical simulations and by an asymptotic expansion of the surface temperature that the slopes method is also valid for any semitransparent film in the thermally thin regime. Measurements of the in-plane thermal diffusivity performed on semitransparent polymer films covering a wide range of absorption coefficients (to the exciting wavelength and in the IR detection range of our IR camera) validate our theoretical findings.

Anisotropic Thermal Transport in Spray-Coated Single-Phase Two-Dimensional Materials: Synthetic Clay Versus Graphene Oxide.

Directional control on material properties such as mechanical moduli or thermal conductivity are of paramount importance for the development of nanostructured next-generation devices. Two-dimensional materials are particularly interesting in this context owing to their inherent structural anisotropy. Here, we compare graphene oxide (GO) and synthetic clay sodium fluorohectorite (Hec) with respect to their thermal transport properties. The unique sheet structure of both allows preparation of highly ordered Bragg stacks of these pure materials. The thermal conductivity parallel to the platelets strongly exceeds that perpendicular to them. We find a significant difference in the performance between GO and synthetic clay. Our analysis of the textured structure, size of the platelets, and chemical composition shows that Hec is a superior two-dimensional component to GO. Consequently, synthetic clay is a promising material for thermal management applications in electronic devices where electrically insulating materials are prerequisites.