Markus Retsch1
University of Bayreuth1
Thermal transport can be significantly influenced by the presence of nano- and mesostructures and the interfaces that exist in such materials. This structuring can be employed to realize super-insulation properties as well as efficient heat spreading. Consequently, nanoscale thermal transport has attracted a lot of research interest for the past 20 years.[1]<br/>Colloidal materials represent an ideal platform to access well-defined devices structured on hierarchical length scales. Colloidal superstructures, which can be amorphous or crystalline, cover size ranges from a few nanometers up to micrometers and beyond. Additionally, one can quickly implement different kinds of materials and fabricate heterostructured composites and hybrid materials.<br/>In this contribution, I will focus on two extreme cases of thermal transport derived from colloidally self-assembled materials. The first example will cover extremely anisotropic thermal conductors based on 1D crystalline Bragg stacks. Spray-coating of nematic dispersions of 1 nm thick clay dispersions with a distinct amount of water-soluble polymers leads to strictly hard-soft alternating layers. The ratio between in-plane and cross-plane thermal conductivity reaches up to 38 in such fully dielectric materials.[2] Based on a complete mechanical tensor analysis, we can also assess the influence of the nanostructure on the phonon mean free path in such systems.<br/>The second example comprises an isotropic, thermally insulating material based on colloidal ensembles of hollow silica nanoparticles. The combination of low density, disorder, and interparticle constriction results in ultralow thermal conductivities at room temperature.[3] When using such particle packings at high temperatures, the thermal insulation is compromised by an increasing amount of thermal radiation. Laser flash analysis allowed us to directly measure the temperature-dependent transition from purely conducting to radiative transport mechanisms. Combined with a detailed optical and theoretical characterization of the colloidal assembly structures, we work out the relevance of their mid-infrared optical properties.[4]<br/>1. Cahill, D. G.; Braun, P. V.; Chen, G.; Clarke, D. R.; Fan, S.; Goodson, K. E.; Keblinski, P.; King, W. P.; Mahan, G. D.; Majumdar, A.; Maris, H. J.; Phillpot, S. R.; Pop, E.; Shi, L., Nanoscale thermal transport. II. 2003–2012. Appl. Phys. Rev. 2014, 1 (1), 011305.<br/>2. Wang, Z.; Rolle, K.; Schilling, T.; Hummel, P.; Philipp, A.; Kopera, B. A. F.; Lechner, A. M.; Retsch, M.; Breu, J.; Fytas, G., Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement. Angew Chem Int Ed 2020, 59 (3), 1286-1294.<br/>3. Ruckdeschel, P.; Philipp, A.; Retsch, M., Understanding Thermal Insulation in Porous, Particulate Materials. Adv. Funct. Mater. 2017, 27 (38), 1702256.<br/>4. Neuhöfer, A. M.; Herrmann, K.; Lebeda, F.; Lauster, T.; Kathmann, C.; Biehs, S.-A.; Retsch, M., High Temperature Thermal Transport in Porous Silica Materials: Direct Observation of a Switch from Conduction to Radiation. Adv. Funct. Mater. 2021, doi: 10.1002/adfm.202108370.