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

Zuyuan Wang,Markus Retsch,Patrick Hummel,Konrad Rolle,Bernd A F Kopera,Anna M Lechner,George Fytas,Theresa Schilling,Josef Breu,Alexandra Philipp

doi:10.1002/anie.201911546

Abstract

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.

Highlights

Heat management is crucial in many applications important for fueling the growth of our technology-driven society
Whereas we find a clear correlation between the moduli and the thermal conductivity, we cannot deduce which change in mechanical modulus causes which effect to the thermal transport
Fully delaminated hectorite platelets can be processed into hybrid Bragg stacks with unique properties, with the polymer polyvinylpyrrolidone being the intercalated second component

Summary

Introduction

Heat management is crucial in many applications important for fueling the growth of our technology-driven society. Whereas heat transport represents an effective, far-field phenomenon, it is decisively governed by the material structure[1] and chemistry[2] on the microscale Extreme phenomena of both heat dissipation and thermal insulation have been demonstrated in nanostructured and hybrid materials. The expected effective material properties, such as mechanical reinforcement, optical transparency, and electrical or thermal conductivity, have been often found inferior to the high expectations The reason for such shortcomings is that the nanocomposite structure, the soft-hard interface, is poorly controlled. The combination of unique hybrid materials and advanced characterization techniques provides an unprecedented insight into the physics of direction-dependent nanomechanical and thermal transport properties in strongly anisotropic materials with polymer confinement

Results and Discussion

Conclusion

Conflict of interest