Aerogel-Based Multi-Functional Composite Structures—A Review

Zhuge Yan

(School of Civil Engineering and Surveying, University of Southern Queensland, Springfield, QLD 4300, Australia)



Aerogel-Based Multi-Functional Composite Structures—A Review

Zhuge Yan

(School of Civil Engineering and Surveying, University of Southern Queensland, Springfield, QLD 4300, Australia)

A review of the stat-of-the-art-of research on the applications of aerogels as part of the multi-functional composite structures is provided in this paper. The material characteristics and mechanical behaviour of polymer enhanced silica aerogels are reviewed first, followed by the applications to building structures. Aerogels have been in existence for many years; however, the engineering applications of aerogels have been limited due to their poor mechanical behaviour. Recently a new type of polymer enhanced silica aerogel, a nanostructured form of silica has been developed. The new material has low density, very low thermal conductivity, excellent acoustic insulation and high mechanical which makes it ideal for energy efficient building material. When thermal performance is important, insulation materials could be incorporated into composite structures. A few of commercial products have also been developed recently to promise the possibility of new development of composite structures using aerogels.

aerogels; nano materials; solid mechanics; multi-functional building material

1 Introduction

Global climate change and global warming resulting from carbon dioxide and greenhouse gases emissions are the most significant environmental challenges of the 21stcentury and the greatest problem facing humanity. As operational energy of building accounts for one third of world’s total energy use and over half of total greenhouse gas emissions[1-2], there is an urgent demand to improve the energy efficiency of buildings and to build”zero-energy buildings”. The design of energy efficient buildings is a complex task for architects and engineers and it can only be achieved if energy efficiency is combined with material efficiency[3]. The polymer enhanced silica aerogel, a nanostructured form of silica, is a low density nano-structured porous material with very low thermal conductivity, excellent acoustic insulation, high mechanical strength and energy absorption, which makes it to be ideal candidate for multi-functional use in wall panels.

Silica aerogels are attractive materials with low-density and high porosity assemblies of silica nanoparticles and possess desirable physical properties, such as lowest thermal conductivity among all solid materials (less than 20 mW/mK) and high acoustic impedance[4-5]. However, the engineering applications of aerogels have been limited due to their poor mechanical behaviour (fragility). More recently, it was discovered that by cross-linking the nano-particle building blocks of silica aerogels with polymeric tethers, the mechanical properties and ductility of aerogels could be significantly increased and there is a great potential for them to be used as structural materials, such as aerospace/space applications[5-8].

If aerogels are to be used as structural component, accurate estimation of inelastic deformation is essential. Poor performance in inelastic deformation was the cause for most structural failures due to earthquake. It is therefore critical to develop good constitutive models, which can precisely reproduce inelastic deformation of the material. A well-defined constitutive model will lead to the generation of accurate model-based predictions, such as finite element analysis.

Although the elastic properties of aerogels have been extensively studies, for processing and applications[9-11], the research on the stress-strain behaviour up to failure has been rare[6-7]and they are limited to the compressive behaviour of the material. No studies have been reported for shear behaviour of aerogels and no constitutive model has been developed.

Over the past 20 years or so, research efforts have been focused on understanding of mechanical behaviour and associated mechanics of cellular solids[9]. As a new type of cellular solids, the engineering applications of aerogels have been limited due to their fragility and hydrophilicity[7]. Recently, with the development of a new strong cross-linked silica aerogel by encapsulating the skeletal framework of amine-modified silica aerogels with polyurea (Fig.1)[5-8], there is a great potential of silica aerogels to be used as multi-functional building materials. However, most recent studies on aerogel applications in buildings are concentrated on thermal insulations where aerogels are treated as non-structural materials[12-14]. The thermal performance of aerogels are comprehensively reviewed by many researchers[12-14].As aerogels are highly porous, the possible application of aerogels as a multifunctional building material would be aerogel providing the thermal insulation and a sandwich panel containing the aerogel providing the structural support. Such sandwich panels could be used as external structural wall panels or roofs. As a least-cost energy strategy, thermal insulation of a building external wall can be seen as an investment in economic terms. Up to now, research published in this area is rare[15]. In this paper, a comprehensive review on the mechanical properties of aerogels and the possible applications as a multi-functional composite panels are presented.

2 Microstructure and Mechanical Properties of Aerogels

2.1 Microstructure and Material Characterisation

The microstructure performance of aerogels has been extensively researched since the development of new processing schemes and synthetic routes in later 1980s[6-8,11,16-18]. The microstructure, comprising nano-sized pores and linked primary particles, with a “pearl-necklace”-like fractal network, as well as the elemental composition, can be tailored by solution chemistry via a process known as the base-catalysed sol-gel method (Fig.1)[8,17,18]. The large void space between those chains of particles is responsible for the low bulk density of aerogels, which is about half of that of dense silica, and the failure normally begins at the inter-particle necks.

A good microstructure model of aerogels requires sound characterisation techniques. There are a number of tools that have been applied to determine the microstructural characteristics of silica aerogels. Scattering techniques have been used to investigate aerogel structures[4,17]. Using small angle neutron scattering (SANS) and nuclear magnetic resonance (NMR), it has been observed that aerogels are fractal structures characterised by a fractal dimension varying from 3 to 1.8[4]. Scanning electron microscopy (SEM) is a powerful method for analysing porous materials and it has also been used to study the changes in the morphology of polymer enhanced aerogels under inelastic compression[6]. More recently, X-ray tomography has been used to generate a 3D microstructure of aerogels[5].

The elasto-mechanical behaviour of aerogels has been studied experimentally by many researchers through compression test, three-point bending test or ultrasonic techniques[9,17,19]. They found that the elastic modulus is proportional to the density by the scaling exponent between 2-4, depending upon solution chemistry and processing conditions. Gibson and Ashby[9]proposed “2” for the open-cell foam model where bending is the dominant mode of deformation. Initially, a higher than predicted scaling exponent for aerogels was found[20]. Ma et al[19]attempted a systematic study of the dependence of the scaling relationship on the microstructure of the gel network. They found out a power-law relationship with an exponent of 3.6 which agreed well with an ultrasonic test result (Fig.2a). They also concluded that many previous derivations with the higher exponents were due to the presence of dangling mass in the network which raised the density without contributing to the stiffness. A lower result (exponent=3.11) was found recently through a computer simulation where the range of densities was broader[4](Fig.2b).

Based on these testing results, researchers argue that the open-cell foam model which was well documented by Gibson and Ashby[9]was not appropriate to model aerogels unless the density changes of the network are taken into account by varying the network thickness without altering the connectivity[13, 15]. The mechanical property of aerogel does not simply depend on the density (porosity) alone, but is affected by the pore size (or pore volume) distribution and connectivity. However, the mechanical (structural) property and pore connectivity relationship of aerogels with high porosity is still not well understood.

2.2 Mechanical Properties of Polymer Enhanced Aerogels

Recent studies have been conducted on enhancing mechanical properties via polymer cross-linking[5-8]. Aerogels were prepared by cross-linking an amine-modified silica framework produced by congelation of Aminopropyltriethoxysilane (APTES) and Tetramethylorthosilicate (TOMS) with polyurea formed by isocyanate, followed by heat-treatment, washing and drying with supercritical CO2.

Since the development of new strong cross-linked silica aerogel with the potential application as load-carrying components, the stress-strain relationship and associated compressive strength have been investigated by a limited number of researchers[6-7]. It was observed that in compression, cross-linked aerogels were linearly elastic under small compressive strains (<4%) and then exhibit yielding (until40% strain), followed by densification and inelastic hardening; they ultimately fail at77% strain[6](Fig.3). The failure was due to lateral tensile stresses instead of compressive buckling which was common for open-cell foam[16]. The ultimate compressive strength is around 45 times higher than native (non-cross-linked) aerogels while the density only increased around 2.5 times and the thermal conductivity remained low[6]. The relatively high compressive stress at ultimate failure in combination with its low thermal conductivity would render the polymer enhanced aerogels as a potential multifunctional material for applications where catastrophic failure could not be tolerated. The energy absorption is also very high, making the material suitable for energy absorption in structures subjected to impact loadings.

The effect of strain rate on the ultimate compressive strength was studied by [7] using a split Hopkinson pressure bar (SHPB). Although the Young’s modulus increases with an increase in strain rate, the overall stress-strain curves did not change significantly for strain rate up to 0.35 s-1. However, under higher strain rates (114-4 386 s-1), the stress-strain curves are higher than those at lower rates, indicting a stiffening behaviour. They also found that the dynamic failure was similar to the failure behaviour under quasi-static compression which was due to lateral tensile stresses.

In general, aerogels exhibit complex mechanical behaviour: linear in tension and plastic in compression. Compared with compression testing, there have been very limited tests being conducted related to tensile behaviour of polymer enhanced aerogels through three-point bending test[6]. The testing results indicated the material exhibits a bi-model behaviour, where the elastic modulus in compression is considerably lower than the modulus from bending tests[6]. Therefore, additional tensile testing must be performed to obtain the actual Young’s modulus in tension.

2.3 Analytical and Numerical Simulation of the Mechanical Behaviour of Aerogels

The finite element method (FEM) has been employed to model the mechanical behaviour of aerogels at the microscopic scale[19]. As the sol-gel process involves the assembly of small particles to form clusters and network, a diffusion-limited cluster-cluster aggregation (DLCA) algorithm has been used by a many investigations to generate mesoporous aerogel structure[19]. Although there are many other aggregation algorithms available, since gels with different structural features show a similar scaling relationship, the choice of any particular aggregation algorithm should be less important[19]. The network connectivity information was then transferred to the FEM. By representing every inter-particle bond as a stiff beam element and every particle as a rigid node, the mechanical properties of the model were evaluated as a framework of beams, and the modulus was calculated from the energy stored during deformation[19].

More recently, different modelling methods, such as Molecular Dynamics (MD) simulations and Material Point Method (MPM) have been used to model the mechanical behaviour aerogels[4,5]. The interatomic potential is the key component of MD simulation which accounts for the potential energy due to the distance between atoms and change of orientation and bond angle of atoms[4]. The MD simulation successfully predicted the elastic modulus of aerogels, but failed to accurately predict the compressive strength of the material. In general, MPM gave the same results as FEM, but it can simulate complex problems involving large nonlinear deformation and complex surface contacts. MPM has been applied to model the microstructural behaviour of aerogels in 3D[5]. However, the size of the aerogel microstructure simulated was limited by the available computational power and the model simulated was smaller than the actual testing specimen.

3 Aerogel Applications in Multi-Functional Composite Structures

A very interesting aspect with aerogels is that they can be produced as either opaque, translucent or transparent materials, thus enabling a wide range of possible building applications[13]. Although the higher cost of aerogels prevents its widely application in cost-sensitive building industries, more researches were carried out in the past few years and resulted in several commercial products, specially designed for building application. Currently, a few different groups of building applications can be noticed for aerogel insulation such as granular aerogel-based translucent insulation materials and transparent monolithic aerogels[21].

3.1 Opaque granules aerogels products

There are several commercial products using granules in the form of particles or blankets.Whereas monolith silica aerogels are very fragile, granular aerogels has a higher thermal conductivity than monolithic aerogels, due to the higher conductivity air (25 mW/(mK)) between the granules. The thermal conductivity of compacted granular silica aerogels can be reduced to 13 mW/(mK), nearly half the value of the uncompacted granules[15]. Cabot ThermalWrapTM®blanket products consists of aerogel granules within non-woven fibres that produce an extremely compressible and very thin (up to 8 mm) insulation material. Another insulation material developed by Cabot is called NanogelTM®(fumed silica aggregate) which is a hydrophobic aerogel produced as particles[22]. As shown in Figure 4, a solution of silica (waterglass) is precipitated to form silica particles (the sol), flocculated into open network (the hydrogel), and dried (supercritical drying) to remove the liquid from the sol-gel. Particle sizes range from 0.01 to 4 mm, with a density between 120 and 180 kg/m3to suit a variety of bulk infill applications.

Other similar aerogels based materials are developed by Aspen, such as Pyrogel®and Cryogel®[23]. The products are textile-like blankets with thickness up to 10 mm and a thermal conductivity of 13.8 mW/mK at 0°. The aerogel composite may be prepared by adding fibres or a fibrous matrix to a pre-gel mixture to increase the tensile strength of the material.

3.2 Application ofopaque granules aerogels to multifunctional sandwich panels

Due to the high cost of the material and the new polymer enhanced aerogels which were discussed in Section 2.2 are not commercially available yet, the engineering application of aerogels in multi-functional panel is limited to aerospace/mechanical engineering, such as car panels. For building structures, the research was rare. Only two publications in the literature, one from Massachusetts Institute of Technology (MIT)[15]and University of Delaware[23]. There is also a commercial product from Kalwall®, where translucent sandwich wall panels were developed[24](will be reviewed in section 3.3). A truss-core sandwich panel filled with compacted aerogel granules, designed to provide both mechanical support and thermal insulation was developed by MIT recently [15]. Solid polystyrene was chosen as panel material. Aerogels used were Silica-based P100 series granular aerogels purchased from Cabot, with a density between 120 and 180 kg/m3.

The truss segments were curt from a 1.6 mm thick sheet of polystyrene then assembled into the 3D truss core. The voids between the trusses in the panel were filled with granular aerogels. The uniaxial compressive testing was conducted for the truss-core sandwich panels, it was indicated that the core failed by plastic buckling followed by debonding. The compressive strength was around 0.5 MPa (Fig.5). The bending test was also performed and it was found that failure was observed to initiate by debonding followed by buckling of the struts. The average peak failure load was 2.65±0.15 kN, corresponding to a shear stress in the core of 0.378±0.018 MPa. If the mechanical properties of the sandwich panels could be improved to carry the design load, this kind of multifunctional sandwich panel could dramatically reduce energy consumption in the buildings and also serve as a structural element.

Hybrid composite panels consisting of flexible silica aerogelcore and glass fibre reinforced epoxy face-sheets were developed at the University of Delaware using Aspen®aerogel blankets[23]. Special methods were developed for sealing the aerogel blanket from resin infusion during processing. The results indicated that the thermal resistance was largely improved using 4 layer aerogels, but the flexural strength reduced by 34% and the stiffness reduced by 46%.

3.3 Aerogels applications in translucent walls and roofs

In recent years, aerogels applications in building for daylighting goals become widespread[12]. This is due the fact that aerogels have a combination of low thermal conductivity and a high transmittance of daylight[21]. Recently a lightweightsandwich panel of fiberglass-reinforced polyester with a filling of filled with NanogelTM®(Cabot) aerogel was developed by Kalwall®[24]. The details of the wall design is shown in Figure 6. The wall panel can provide U.05/R20 insulating performance while offering exceptional light transmitting qualities. A pre-engineered Skyroofs®was also developed (Fig.7) which offering the energy-efficient, diffuse daylighting. The roofs were designed to withstand hurricane-force winds and high snow loads.

3.4 Aerogels incorporated concrete

Lightweight concrete usually possesses superior thermal insulation properties compared to conventional concrete due to the large amount of air void in the concrete matrix and in the lightweight aggregate[25]. However, presence of voids will reduce the mechanical strength of concrete. In order to overcome this problem, experimental research has been carried out recently using aerogel particles as aggregate for lightweight and thermal insulating concrete[25-27].

A series of laboratory testing was conducted by adding 10 to 60% of aerogel particles by volume[25]. The effect of aerogels on the compressive and flexural tensile strength is shown in Figure 8. It is clearly indicated that with the increase of aerogel content, the compressive/flexural strength decreased. However, the failure mode is more ductile with aerogels.

Similar research was carried out by Ng et al.[26]on ultra-high performance concrete (UHPC) where up to 80% by volume of aerogel particles was used in the mix design. They found that 50% of aerogel contents would produce an optimum results, with a compressive strength of 20 MPa and thermal conductivity of 0.55 W/(mk) (Fig.9).

4 Conclusions

By combining thethermal insulation properties into construction material would be ideal to create a multi-functional building material. Silica aerogels are attractive materials with low-density and high porosity assemblies of silica nanoparticles and possess desirable physical properties, such as lowest thermal conductivity among all solid materials, high acoustic impedance and high transmittance. In this paper, the possibility of applications of aerogels as part of a multi-functional structural elements is discussed. Because of their many environmental benefits there has been an increase in the research over the past few years. However, the research on the inelastic behaviour and the constitutive model of areogles has been rare. As aerogels are highly porous, possible application of aerogels as a multi-functional building material would be in the form of a sandwich panel. However, research in this area is rare. Therefore, systematic investigations is required for the performance of polymer enhanced silica aerogels as multifunctional building materials.

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[责任编辑：俞启灏]

1004-6011(2016)03-0065-08

2016-07-25

诸葛燕(1963—)，女，教授，博士生导师，博士，研究方向：复合材料、结构加固、绿色混凝土材料.

基于气凝胶的多功能复合建筑结构

诸葛燕

(南昆士兰大学 土木工程与测绘学院， 斯普林菲尔德 澳大利亚 4300)

对气凝胶在多功能复合结构中的应用进行了回顾. 首先介绍了材料的性能和强化二氧化硅聚合物的特性，然后是其在建筑结构中的应用. 由于气凝胶力学特性差，即使问世以来，它的工程应用也是有限的. 近年来一种新型的强化二氧化硅气凝胶聚合物，二氧化硅钠诺形式开发了出来. 由于其具有低密度，很低的热传导性，良好的隔绝效果和高的力学性能，这种新材料成为理想的能效建筑材料. 当处于热性能是非常重要的场合，隔绝材料就可以加装在复合结构中. 目前市场已经开发出了一些产品，它们可以保证使用气凝胶的复合结构有进一步发展的可能.

气凝胶； 纳诺材料； 固体力学； 多功能建筑材料

TB34

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