Silica hollow nanosphere colloidal crystals feature a uniquely well-defined structure across multiple length scales. This contribution elucidates the intricate interplay between structure and atmosphere on the effective thermal diffusivity as well as the effective thermal conductivity. Using silica hollow sphere assemblies, one can independently alter the particle geometry, the density, the packing symmetry, and the interparticle bonding strength to fabricate materials with an ultralow thermal conductivity. Whereas the thermal diffusivity decreases with increasing shell thickness, the thermal conductivity behaves inversely. However, the geometry of the colloidal particles is not the only decisive parameter for thermal insulation. By a combination of reduced packing symmetry and interparticle bonding strength, the thermal conductivity is lowered by additionally 70% down to only 8 mW m −1 K −1 in vacuum. The contribution of gaseous transport, even in these tiny pores (<200 nm), leads to minimum thermal conductivities of ≈35 and ≈45 mW m −1 K −1 for air and helium atmosphere, respectively. The influence of the individual contributions of the solid and (open-and closed-pore) gaseous conductions is further clarified by using finite element modeling. Consequently, these particulate materials can be considered as a non-flammable and dispersionprocessable alternative to commercial polymer foams.

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.

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Polymer-shelled magnetic microbubbles have great potential as hybrid contrast agents for ultrasound and magnetic resonance imaging. In this work, we studied US/MRI contrast agents based on air-filled poly(vinyl alcohol)-shelled microbubbles combined with superparamagnetic iron oxide nanoparticles (SPIONs). The SPIONs are integrated either physically or chemically into the polymeric shell of the microbubbles (MBs). As a result, two different designs of a hybrid contrast agent are obtained. With the physical approach, SPIONs are embedded inside the polymeric shell and with the chemical approach SPIONs are covalently linked to the shell surface. The structural design of hybrid probes is important, because it strongly determines the contrast agent's response in the considered imaging methods. In particular, we were interested how structural differences affect the shell's mechanical properties, which play a key role for the MBs' US imaging performance. Therefore, we thoroughly characterized the MBs' geometric features and investigated low-frequency mechanics by using atomic force microscopy (AFM) and high-frequency mechanics by using acoustic tests. Thus, we were able to quantify the impact of the used SPIONs integration method on the shell's elastic modulus, shear modulus and shear viscosity. In summary, the suggested approach contributes to an improved understanding of structure-property relations in US-active hybrid contrast agents and thus provides the basis for their sustainable development and optimization.