Characterisation of silica aerogels for vibro-acoustic applications

This article-based thesis comprises of a collection of four journal papers, three of which are published and one of which is currently accepted for publication in the Journal of Acoustical Society of America pending revisions. Each article constitutes a chapter written and formatted in manuscript form. The overall aim of these papers is to understand the acoustical properties of silica aerogels used for vibro-acoustic applications. There has been a lack of knowledge on the relationship between the acoustical and non-acoustical properties of aerogels which usually come in highly porous, granular (e.g., millimetre-size particles) and powder (e.g., micron-size particles) aerogels. For this purpose, silica aerogels have been provided in the powder and powder-embedded fibreglass mat form by our industry partner – Armacell. Alongside this, aerogels have also been synthesised in monolithic and granular form in the laboratory. These materials have been carefully characterised acoustically and non-acoustically to provide new insights into the relations between their chemical, microstructural and acoustical characteristics.
This thesis uses an extensive materials characterisation process to support the experimental and analytical modelling procedure used to allow us to predict the in-situ performance of these materials in industry after carefully understanding the physical morphology such as pore size and porosity which underpins their acoustic phenomena. Advanced theoretical models have been used to explain the observed acoustical behaviour and has linked this to the material microstructure. It has been found that most of the current work only focuses on absorption, transmission loss, layer thickness and morphology like pore and particle size. The work carried out in this thesis focuses on mathematical models that use fundamental acoustical quantities such as frequency-dependent bulk modulus, complex dynamic density, and complex dynamic compressibility. These properties have been linked to the intrinsic material parameters and parameters of the air filling the porous structure of aerogels.
In the case of powder aerogels, it has been found that the dynamic loss factor is a key parameter required to predict the measured acoustic absorption coefficient. It has also been found that the sound absorption depends non-linearly on the sound pressure excitation. In the case of powder silica aerogels embedded into fiberglass mats it has been found that the classical model for the visco-thermal and inertia effects in the pores can predict unrealistic pore size and porosity value to achieve good fit with the data. In the case of granular silica aerogels, it has been found that the rarefied gas flow and heat transfer in the inner-particle transport macropores, inter-scale (voids to/from inner-grain pores) pressure diffusion, and inter-scale (transport- to/from meso pores) mass diffusion can be a dominant phenomenon. This research suggests that a refined model is required to predict the acoustical properties of aerogels to take into account their complex hierarchical pore structure and properties of air in sub-micron pores.