Hollow silica sphere colloidal crystals: insights into calcination dependent thermal transport

Abstract

Colloidal crystals consisting of monodisperse hollow silica spheres represent a well-defined porous material class, which features a range of interesting optical, mechanical, and thermal properties. These hierarchically structured materials comprise micropores within the silica network, which are confined to a thin shell (tens of nanometers) of a hollow sphere (hundreds of nanometers). Using simple calcination steps, we markedly change the internal microstructure, which we investigate by a multitude of characterization techniques, while the meso- and macrostructure remains constant. Most importantly the rearrangement of the silica condensation network leads to a reduction in the total surface area and loss of micropores as demonstrated by N2 sorption and hyperpolarized (129)Xe NMR studies. Spin-lattice relaxation shows a drastic increase of the rigidity of the amorphous network. These microstructural changes significantly influence the thermal conductivity through such a porous silica material. We demonstrate a remarkably low thermal conductivity of only 71 mW m(-1) K(-1) for a material of a comparatively high density of 1.04 kg m(-3) at 500 °C calcination temperature. This thermal conductivity increases up to 141 mW m(-1) K(-1) at the highest calcination temperature of 950 °C. The great strength of hollow silica sphere colloidal crystals lies in their hierarchical structure control, which allows further investigation of how the internal microstructure and the interfacial contact points affect the transport of heat.

Thermal Transport in 3D Nanostructures

“…Altering the interparticle interactions will also influence the temperature dependence of the thermal conductivity. For instance, a change in the temperature dependence from to is observed for the silica hollow sphere colloidal crystal after 500 and 950 °C calcination, respectively …”

“…For instance, the hollow silica colloidal crystal shows a low thermal conductivity of 35 mW m −1 K −1 , which is comparable with polymer foam‐based thermal insulation materials . Studies show that the particle geometry, the packing density and symmetry, and the interparticle bonding strength play an important role on the thermal transport of the hollow particle colloidal crystals …”

Interfacial and volumetric sensitivity of the dry sintering process of polymer colloidal crystals: a thermal transport and photonic bandgap study

“…12 Recently, colloidal crystals have also drawn attention in the field of thermal transport in nanostructured materials. 13,14 Due to their highly-defined structure on the colloidal length scale, they can serve as a model system to get a deeper understanding of thermal transport in porous, particulate matter. Heat flow through a colloidal crystal is extremely sensitive to the size and bonding strength of the interfaces between neighbouring particles.…”

Synthesis of Hollow Silver Spheres using Spherical Vaterite‐type Calcium Carbonate as Template

“…Because hollow structured materials have low density, high surface area, high stability, and good permeation, and because the hollow part is able to encapsulate guest molecules, nano‐sized and micro‐sized hollow spheres are important types of these remarkable materials. Thus, these materials have potential applications as efficient adsorbents, catalysts, sensors, and coating materials, as well as in composites, energy materials, pigments, inks, lightweight fillers, cosmetics, lithium ion batteries, controlled release capsules, biomedical diagnosis and therapy, drug delivery, and so on . Various strategies have been developed for the fabrication of hollow structures, such as template‐assisted methods including soft and hard templates, the Kirkendall diffusion effect, and Ostwald ripening .…”

“…Thus, these materials have potential applications as efficient adsorbents, catalysts, sensors, and coating materials, as well as in composites, energy materials, pigments, inks, lightweight fillers, cosmetics, lithium ion batteries, controlled release capsules, biomedical diagnosis and therapy, drug delivery, and so on. [1][2][3][4][5][6][7][8][9][10][11] Various strategies have been developed for the fabrication of hollow structures, such as template-assisted methods including soft and hard templates, the Kirkendall diffusion effect, and Ostwald ripening. 12 The fabrication methods of hollow structures without templates involve ultrasonication, spray-drying, pyrolysis, laser pyrolysis, etc.…”