Understanding Thermal Insulation in Porous, Particulate Materials

Significance 

Recent technological advances contributed to the evolution of thermal insulation materials. Currently, the need for efficient thermal insulation cannot be overstated owing to our limited energy resources. Furthermore, established materials have proven inadequate for their prescribed role pushing the need further for excellent insulators. Aerogels emerged as promising thermal insulators. However, aerogels suffer from poor mechanical properties and elaborate, energy-intensive fabrication. Silica hollow spheres, on the other hand, offer excellent control over their hierarchical assembly structure. In their case, the geometry of the colloidal particles is not the only decisive parameter that determines the thermal insulation. This, therefore, calls for further research to elucidate other critical factors.

Researchers led by Professor Markus Retsch at the University of Bayreuth in Germany conducted a series of studies to improve our understanding of thermal insulation mechanisms in porous, particulate materials. The goal of the researchers was to shade light on the fundamental limit of thermal conductivity in hollow silica sphere ensembles. They discovered this limit through critical structure-property relationships. Furthermore, they investigated several parameters including: the aspect ratio of the hollow particles, the degree of order in the colloidal assembly, and the bonding strength between adjacent particles, among others. Their work was recently published in Advanced Functional Materials.

The researchers began their studies by synthesizing a variety of hollow silica spheres. The team then obtained colloidal crystals by slow evaporation of a concentrated particle dispersion under ambient conditions. Subsequently, they characterized their material using transmission electron microscopy, scanning electron microscopy, and nitrogen sorption measurements. Laser flash analysis determined the effective thermal diffusivity of the colloidal assemblies. Combining the thermal diffusivity measurement with the density of the colloidal crystals and their corresponding specific heat capacity allowed to calculate the thermal conductivity.

The authors observed a decrease in thermal conductivity when the particle size was increased and the shell thickness reduced. Besides the particle geometry, they investigated the number and strength of the inter-particle contact points. The researchers reduced both the packaging density of the spheres in the assembly and the bonding strength between adjacent particles. These alterations resulted in a substantial reduction of the thermal conductivity and diffusivity in a vacuum.

Markus Retsch and his research team successfully presented a holistic picture of the thermal transport properties of dense silica hollow nanoparticle packings. Their work concentrated on the influence of the hollow sphere size and shell thickness. The thermal diffusivity is mainly influenced by the shell thickness. Altogether, their comprehensive study on the interplay of vacuum- and gas-dependent thermal diffusivity and thermal conductivity in nanostructured materials will help in developing new and innovative insulation materials.

Understanding Thermal Insulation in Porous, Particulate Materials

Left:Transmission electron microscopy: Hollow silica nanoparticles can be fabricated with high structural precision based on a templating strategy. Particle diameter and shell thickness can be controlled with nanometer precision.

Middle: Scanning electron microscopy: Self-assembly of such monodisperse colloidal particles readily yields highly periodic superstructures. The particles crystallize in a face-centered cubic symmetry. 

Right:Optical microscopy: Slow drying of colloidal dispersions leads to macroscopically free-standing films with a thickness of several hundred micrometers. The highly crystalline order can be inferred from the bright opalescent color.

About the author

Markus Retsch studied Polymer- and Colloid Chemistry at the University of Bayreuth (Germany) from 2001 – 2006. He received his Ph.D. degree from the Max Planck Institute for Polymer Research/University of Mainz (Germany) in 2009. Following a postdoctoral stay at the Massachusetts Institute of Technology (MIT, USA) from 2009 – 2011, he was appointed Junior-Professor at the University of Bayreuth (Germany). In 2013 he received a Lichtenberg professorship provided by the Volkswagen foundation, and in 2016 he was awarded an ERC starting grant.

The Retsch group focuses on functional colloidal materials with a particular interest in energy conversion and energy conservation technologies. Topics of interest are colloidal self-assembly processes, thermal transport in nano- and mesostructured materials, broadband nanophotonic engineering and plasmonic superstructures and their relevance for photovoltaic applications.

About the author

Pia Ruckdeschel studied Polymer and Colloid Chemistry as an undergraduate from 2008 to 2011 and received a Master’s degree in Polymer Science in 2014 from the University of Bayreuth. Following this, she received her Ph.D. degree in 2018. Her research focused on colloidal self-assembly and thermal transport properties of nano- and mesostructured materials, in particular hollow silica nanospheres.

About the author

Alexandra Philipp is currently a Ph.D. student in the Physical Chemistry I department of the University of Bayreuth. She studied Chemistry as an undergraduate at the University of Bayreuth. Following this, she obtained a Master’s degree in Polymer Science in 2014 from the same university.

Her current research interests lie in the field of thermal transport in nano- and microstructured materials, including the modeling of heat transfer through colloidal assemblies and the anisotropic thermal conductivity in polymeric nonwovens and layered materials.

Reference

Pia Ruckdeschel, Alexandra Philipp, and Markus Retsch. Understanding Thermal Insulation in Porous, Particulate Materials. Advanced Functional Materials. 2017, volume 27.

 

Go To Advanced Functional Materials

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