Silica aerogel composites with embedded fibres: a review on their preparation, properties and applications

Teresa Linhares ^ab, Maria T. Pessoa de Amorim ^b and Luisa Durães *^a
aCIEPQPF, Department of Chemical Engineering, University of Coimbra, 3030-790 Coimbra, Portugal. E-mail: luisa@eq.uc.pt; Fax: +351 239798703; Tel: +351 239798737
^b2C2T, Center of Science and Textile Technology of the University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal

First published on 19th September 2019

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1 Introduction

The aerogel was deemed as the ‘miracle material for the 21st century’¹ due to its highly porous structure, having a network of connected particles resembling a pearl necklace, with an air volume of 80–99.8%.² This unique layout imparts to aerogels the status of the lightest solid materials with the best insulation properties.^3–7 Other important attributes of this new-generation material are its high specific surface area (100–1600 m² g⁻¹) and a very low refractive index (1.007–1.240).^2,8

Adsorbents of harmful compounds,^9,10 sensors,¹¹ dielectric materials,¹² filtering media,¹³ Cherenkov detectors,¹⁴ kinetic energy absorbers,¹⁵ substrate for catalysts,¹⁶ carriers,¹⁷ extracting agents,^18,19 protective clothing,²⁰ and even art sculptures²¹ are among the reported end-uses of silica aerogels. Undoubtedly, thermal and acoustic insulation are the most significant applications of silica aerogels,^22,23 with profuse and detailed published studies.^24–29

In spite of the unique properties provided by the nanoscale porosity, the use of silica aerogels in common daily applications has been constrained, mainly due to their inherent brittleness that makes processing and handling difficult.^30–33 Other intrinsic drawbacks, such as dust release,^22,34,35 hydrophilicity,^9,33,35 volumetric shrinkage,^30,36,37 time for processing,^38–41 and ultimately, the prohibitive cost of silica aerogels^9,34,42,43 (8–20 fold more costly than polyurethane foams),⁵ need to be overcome, in order to broaden the applicability of such auspicious materials.

Strategies for reinforcement of silica aerogels are an effervescent topic of scientific research, either through the use of appropriate additives or by fine-tuning the synthesis conditions.^44,45 The manufacturing of silica aerogel composites with embedded fibres is one of the most effective techniques to widen their applications.^37,46–48 However, in terms of scientific reviews, there is a small number of publications on this topic, and generally each review covers a specific type of fibre or nanofibre.^49–53

This work intends to highlight the achievements of scientific research on silica-aerogel matrices with all types of fibres, presenting their properties and discussing strategies to encourage their common daily applications. The effect on aerogel composite properties of some parameters related to the fibres (morphology, orientation, amount, and thermal resistance) is analysed. The applications linked to the fibre-reinforced aerogel composites are also briefly addressed.

2 Brief description of silica aerogels: synthesis and properties

Silica aerogels are synthesised through sol–gel chemistry, defined by IUPAC⁵⁴ as the “Process through which a network is formed from solution by a progressive change of liquid precursor(s) into a sol, to a gel, and in most cases finally to a dry network”. The precursors can be either inorganic metal salts or metal alkoxides, but the relatively easy chemistry and versatility of silicon alkoxides make them the most used precursors in sol–gel chemistry of silica,⁵⁵ for example tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS). TMOS is more reactive than TEOS, but the higher price and the associated health hazards of TMOS turned TEOS the most common precursor among silanes.^6,56,57 The detailed mechanisms of silica aerogel synthesis are widely described in the available literature.^58–62

Table 1 summarises the typical properties of monolithic silica aerogels. However, each modified aerogel is unique,⁶³ with specific properties related to the synthesis conditions and used precursors.

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2.1 Hydrolysis and condensation/gelation

Sol–gel structures start from the hydrolysis of precursors and subsequent condensation into primary particles, evolving through the developing solution, and aggregating to form larger secondary particles, which link in a continuous network with liquid in the interstices.⁵¹

The sol–gel chemical reactions can be summarised according to Scheme 1, where hydrolysis and condensation occur in a parallel way.

The primary particles have diameters less than 2 nm and the secondary particles feature diameters around 10 nm or more. Gelation starts to occur when secondary particles link to each other, resulting in neck-regions between them, and a mesoporous network.^44,51 These structures are shown in Fig. 1.

The pH value is a decisive parameter for the relative rates of hydrolysis and condensation of alkoxysilanes, as outlined in Fig. 2, for precursors of the type Si(OR)₄. The condensation reaches the minimum rate around pH 1–3, since the isoelectric point of silica is achieved at pH 2, and thus the electrical mobility of silica particles is minimum.^59,64

Under acidic conditions, the hydrolysis reaction occurs at higher rate and the condensation/gelation is the rate-determining step, favouring the simultaneous formation of small oligomers with reactive Si–OH groups. This leads to a polymer-like gel, made from chains with few branches,^2,64 which are weakly cross-linked, and it easily loses its shape and tends to crush, as the chains impinge on one another due to internal movements. Hence, denser aerogels with small pores are obtained after acid catalysed synthesis.⁷¹ Under acidic conditions the hydrolytic reactions are favoured by proton donors, which readily attack the oxygen atoms due to their partial negative charge, while in a basic medium the slow hydrolysis reaction is the rate-determining step.^2,58 Under alkaline conditions condensation is faster, due to the slightly positive charge of Si atoms in the presence of proton acceptors,⁵⁸ and the hydrolysed species are readily consumed into larger and denser colloidal silica particles.^2,58 Such structures are more prone to prevent network dragging and crushing caused by internal pressure, yielding lighter aerogels with higher pore volume.⁷¹

In summary, the porosity of silica aerogels can be tailored through the balance between the hydrolysis and condensation rates, by adjusting the initial pH according to a specific precursor.⁷² As an example, silica aerogel density can be so different as 150 kg m⁻³ or 500 kg m⁻³, depending on synthesis conditions, respectively, in a basic or acidic medium.⁷³

The gelation or gel point is attained when a continuous network is formed, and the solution no longer flows under the effect of gravity.

2.2 Ageing

Chemical reactions of sol–gel chemistry still exist even after the gel point has been reached. The liquid inside the pores contains small particles, which are able to condense, as well as wandering monomers that will join the network.^2,74 That continuous process of cross-linking and coarsening due to the additional condensation is the so-called ageing.^75,76

Ostwald ripening refers to the phenomenon occurring at the micro level during ageing, in which the existing small particles with unreacted sites (OH moieties) tend to dissolve and reprecipitate into larger particles, or condense into more favourable regions of secondary particles, such as pores or crevices, particularly in the necks between them.^2,77 The micro-level condensation leads to additional stiffness of the gels, which causes contraction of the network and withdrawal of some solvent from the pores; this dimensional change is called syneresis.⁷⁸

The ageing, occurring at pH between 8 and 12, will determine the extent of shrinkage at the drying stage.⁷⁹ A silica matrix strengthened by adequate ageing will better resist the drying capillary pressures,⁷⁹ allowing an improvement of approximately 2-fold in the modulus of elasticity.⁴⁴

2.3 Drying

The ultimate goal of the sol–gel drying process relies on the solvent removal from the matrix by maintaining the gel network, yielding a porous solid with unchanged volume and shape.⁸⁰

During drying, there are two major factors affecting the shape of the solid porous structure of the gel. The first relates to the almost inevitable partial collapse of the network, as even the smallest shrinkage in the interior of the gel body causes a pressure gradient that results in cracks; second, the pore dimensions are different throughout the network, and hence neighbouring pores with different radii show different receding rates of menisci (faster on bigger pores). Thus, the walls between pores of different dimensions endure uneven levels of stress and therefore tend to crack due to the unbalanced forces.^75,81

There are several drying techniques, namely, supercritical drying (SCD), freeze-drying, and ambient pressure drying (APD). Along with the gel's composition, the drying of the gels dictates pore dimensions and their overall textural properties, and thus the chosen method is of high relevance.^80,82 While fibre-reinforced aerogels can be dried under ambient pressure with good results, native silica aerogels are prone to collapse and lose their monolithic shape.⁸³ In fact, the embedded fibres act as a supporting skeleton, preventing the aerogels' shrinkage caused by the drying capillary pressures.^37,47

Table 2 highlights the influence of drying conditions on the characteristics of silica aerogels: for a similar fraction of embedded fibres, SCD favours lighter materials (∼3 to 11%).^83,84

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APD can be tuned to be a suitable cost-effective alternative: for example, the volume shrinkage of silica aerogel composites reinforced with a micro-glass fibre mat was reduced from ≈62% to ≈22%, following a gradient multi-segment drying process after a surface treatment,⁸⁵ as described in Table 2.

All the composites listed in Table 2 exhibit characteristics of super-insulating materials, designated as such when the thermal conductivity is inferior to 20 mW m⁻¹ K⁻¹, measured at standard pressure and temperature.⁸⁶

2.4 Strategies to minimize APD shrinkage

APD is an advantageous drying technique when taking into account the involved costs, the low hazard levels and suitability for large-scale commercial applications.^46,103,104

In order to prevent irreversible densification or partial collapse due to stresses generated by capillary pressures, the preparation of aerogel-like APD materials requires specific adaptations, consisting of both chemical surface modification and skeleton strengthening.^37,80

The condensation of reactive groups in the network surface, mainly Si–OH, causes a gradual shrinkage that leads to the ensuing densification. Those reactions can be prevented after a surface treatment, carried out with reactive alkylsilane compounds, with non-hydrolysable Si–R alkyl groups,^{12,80,105,106} as is the case of HMDZ (hexamethyldisilazane) or HMDSO (hexamethyldisiloxane) (videTable 2).

An increased concentration of hydrophobic moieties directly attached to the silicon atom, such as methyl groups, can lead to reversible shrinkage, the so-called ‘spring back’ effect.^107–109 It consists of a mechanical expansion of the gel network, which recovers almost to its original volume, as can be seen in Fig. 3. The phenomenon occurs during the last stage of the drying, when the porous structure is able to endure the drying loads, and the spring-back effect is driven by the repelling of hydrophobic functional groups.^43,109

The use of precursors with hydrophobic character, such as methyltrimethoxysilane (MTMS), with a non-hydrolysable methyl group, also prevents the collapse of gels during drying and imparts higher flexibility to the matrices.^35,107

Hybridization, through the incorporation of polymers in the silica backbone, is another technique of aerogel reinforcement that prevents shrinkage. It consists in the cross-linking of the network by adding monomers (e.g., acrylates and cyanates), leading to increased flexibility and/or mechanical strength of the hybrid material.^44,110–115 These aerogels are often termed X-aerogels.^116,117 However, as a consequence of the effective mass addition, the cross-linked gels tend to feature lower porosity, a detrimental outcome that leads to higher density materials, and a decrease of surface area,¹¹⁸ along with the inevitable penalization of the thermal conductivity.¹¹⁵

Another strategy consists in operating with drying control chemical additives, such as glycerol, formamide, dimethyl formamide, or tetramethyl ammonium hydroxide, added during the initial phase of silica gel synthesis. Their function is to promote the uniformity of pore size dimensions, thus minimizing stress asymmetries.⁸⁰

Finally, the embedment of fibres and nanofibres is recognized as an effective method of strengthening silica aerogels,^46,119–121 since the monolithic shape is preserved due to fibres, in spite of the inevitable fractures (both at micro and macro levels) occurring during evaporative drying.⁸³

The shrinkage and strengthening of wet gels have been, for a long time, a widely researched topic.^{30,44,51,59,122,123} For more details, the pioneering investigation of Phalippou et al.,¹²³ the NASA Tech Brief of Paik and colleagues³⁰ and the review work of Maleki and co-workers⁴⁴ are a convenient basis.

2.5 Insulation properties of silica aerogels

The extremely low thermal conductivity and the inorganic nature (non-combustible) of silica aerogels turned them into ideal substitutes for traditional insulating materials, such as polystyrene or polyurethane foams.^124–127 Currently, rock wool and glass wool are commonly used, and despite their inorganic nature, higher insulation performances are only achieved after increased thickness.^127,128Fig. 4 depicts the typical values of thermal conductivity of some insulating materials, along with those for the free air and an aerogel. Additionally, other important aspects have to be considered: while aerogel based thermal insulation materials require less than half of the thickness, when compared to other inorganic-based materials or polymeric foams, aerogels are much more expensive, around 8–9 times more expensive than conventional materials.¹¹⁵

According to a study sponsored by the European Commission, the use of aerogels as building insulation materials was estimated to reduce 30% of overall energy consumption and 25% of CO₂ emissions, yet providing the same comfort level.¹²⁹ Moreover, silica is the most abundant compound on the Earth¹³⁰ and the current environmental issues, such as limited energy resources,¹³¹ emphasize the aerogel application. These materials, being amorphous, have no hazardous effect either for human health or to the ecosystems.^124,132

Heat transfer in monolithic non-evacuated aerogels involves three components,^131,133,134 since the convection heat transfer can be neglected in porous materials where void spaces are smaller than 4 mm.^135,136 Thus, the total thermal conductivity can be described using eqn (1).

kT = ks + kg + kr	(1)

where k_T is the total thermal conductivity and k_s, k_g, and k_r refer, respectively, to the solid, gaseous and radiative transfer components of the first.

Aerogels, with a significant fraction of small pores, have also a tortuous silica structure, which minimises both the gaseous and solid conductivities. As an example, an aerogel with a density of 40 kg m⁻³, with circa 50% pores over 20 nm in diameter, typically exhibits a thermal conductivity of 18 mW m⁻¹ K⁻¹ at ambient pressure; a denser silica aerogel (circa 100 kg m⁻³), with 80% pores with a diameter below 20 nm, displays superior thermal insulation performance, namely 12 mW m⁻¹ K⁻¹ at ambient pressure.⁷⁹ Fricke and colleagues¹³⁷ first established relationships between silica aerogels' bulk density, ρ_b, and the three components of thermal conductivity (eqn (2)–(4)), by varying aerogel density between 70 and 230 kg m⁻³, as follows:

ks ∝ ρb1.5	(2)

kg ∝ ρb−0.6	(3)

kr ∝ (ρbe)−1	(4)

where e is the spectral mass attenuation coefficient, an optical property related to the efficiency of a material to attenuate infrared radiation (IR).¹⁴²

The specific surface area (SSA) of silica aerogels is also useful as an indicator of the thermal insulation performance of aerogels. For a given density, the smaller the size of the pores, the higher the SSA, and the better will be the insulation performance. This evidence was described by Wei et al.:¹³⁶ a silica aerogel with a density of 120 kg m⁻³ and a surface area of 400 m² g⁻¹ displayed a thermal conductivity of 20.1 mW m⁻¹ K⁻¹; by increasing the SSA to 1000 m² g⁻¹, the thermal conductivity was reduced to 13.7 mW m⁻¹ K⁻¹. Since the SSA is more easily assessed than the pore size distribution of the aerogels, SSA may be a valuable parameter in this context.

The solid thermal conductivity relates to the solid fraction of aerogels, i.e., it is influenced by the extent of crosslinking and network connectivity.^133,134 Heat conduction in the solid network occurs through the atomic lattice, due to the excitation of vibrational energy levels of interatomic bonds or even by free electron transport under thermal gradient.^143,144

Heat conduction in the gaseous state exists due to the collisions between molecules, since the faster ones transfer part of their kinetic energy to the slower molecules.¹⁴⁴ However, those movements are of minor relevance in the monolithic native silica aerogels, since the average pore dimensions are typically below 70 nm,¹³³ and the mean free path of air molecules at 1 atm and 23 °C is around 66 nm.¹⁴⁵ The Knudsen effect describes the reduced diffusivity directly linked to the pore dimensions, when each gas molecule collides more frequently with the pore walls than with neighbouring molecules,^146,147 leading to a reduced energy transfer through the gas molecules.¹⁴⁷

The radiative thermal conductivity relates mainly to the spectral mass attenuation coefficient, e. While the solid and gaseous thermal conduction requires a medium, the radiative thermal conductivity is based on electromagnetic waves, correlated to the photon's mean free path.¹⁴⁸ Therefore, this component is highly temperature-dependent, with a high factor-scaling exponent.¹⁴⁴ According to Fomitchev et al.,³ the radiative thermal conductivity of silica aerogels can be computed according to eqn (5):

kr = 16n2σT3ε−1	(5)

where n is the index of refraction, σ is the Stefan–Boltzmann constant, T is the temperature (K) and ε = ρ_be, the spectral attenuation coefficient.

Silica aerogels display poor absorptive properties in the near-infrared region, particularly below the wavelength 5–8 μm.^124,149 With a rise in temperature from 300 to 1000 K, the radiative thermal conductivity can lead to an increase from 8 to 92% of the total conductivity of native silica aerogels.¹⁵⁰

All the heat conductivity mechanisms within aerogels are sketched out in Fig. 5.

2.6 Mechanical properties of silica aerogels

The mechanical behaviour of native silica aerogels can be understood according to a proportionality that relates the Young's modulus, E, with material's bulk density, ρ_b,⁶⁵ described using eqn (6).

E ∝ ρbβ, β ≈ 3.2…3.8	(6)

The manufacture technique is somehow relevant, since aerogels synthesised under neutral or mild acidic conditions can be twice stiffer compared to similar density aerogels developed under basic conditions. Suitable ageing and heat treatments can also improve aerogel strength.⁶⁵ Nevertheless, even after optimizing synthesis conditions, silica aerogels are brittle materials, with a friable nature derived from their ionic-covalent bonds and high porosity.^80,151

Mechanical characterization of native aerogels is often a difficult task, either in terms of the specimen preparation or precisely measuring the small load forces applied.^63,117 Therefore, the evaluation is typically performed in compression and flexural modes.^80,117 Moreover, the tensile strength of aerogels can be deduced from the three-point bending flexural test.¹¹⁷

The compressive response behaviour of aerogels can be quite diverse: at higher densities, they tend to shatter after very little strain, exhibiting glass-like behaviour; at smaller densities, in the range of 80 to 150 kg m⁻³, silica aerogels can withstand compression up to 70% strain and recede back to the original volume.⁸⁰ A silica aerogel of 100 kg m⁻³ subjected to the three-point bending flexural test can sustain a maximum load of 0.02 MPa.¹¹⁷

The integration of fibres has been evidenced as an effective way to improve aerogels' mechanical performance, either during drying or in response to stress loads. As reported by Zhihua and co-workers,¹⁵² after the incorporation of 10 wt% ceramic fibres (Al₂O₃: ≈43–45%; SiO₂: ≈49–52%; ≈Fe₂O₃: 0.5–0.8%), the compressive strength of the reinforced aerogel increased six-fold in comparison with the pure silica aerogel, while the thermal conductivity (measured in air at room temperature) just increased from 23 to 29 mW m⁻¹ K⁻¹.

Regarding the effect of fibres on aerogel's properties, a detailed discussion is presented in the next section.

3 Silica-aerogels with embedded fibres

The embedment of fibres in silica aerogels can, to a large extent, prevent the shrinkage during their drying,^30,47,83 being as well an effective way to improve their mechanical properties.^{30,51,153,154} The challenge is, in spite of the reinforcement, to preserve their inherent insulation properties and high specific surface area.

3.1 Silica aerogel reinforcement strategies: a comparison

Among the current known techniques of silica aerogel reinforcement (previously summarised in Section 2.4), chemical cross-linking with reactive molecules or polymers leads to superior improvement in their mechanical performance.¹⁵⁵ For example, the compressive strength of amine-modified silica aerogels at the breaking point was ∼4.1 MPa, with a maximum strain of ∼5.7%; after being cross-linked with isocyanate, the mechanical strength at failure became improved by a factor of 45 (∼186 MPa, ∼77% strain). However, such an extraordinary improvement came at the cost of increased density (190 vs. 478 kg m⁻³).¹⁵⁶ The thermal conductivity of those composites was estimated from thermal diffusivity data as being 41 mW m⁻¹ K⁻¹,¹⁵⁶ which can yet be considered a good result taking into account the high density of isocyanate cross-linked composites. However, as detailed by Huang and colleagues,¹⁵⁷ when increasing the covalent bonds within a system, its thermal conductivity gets increased as well, since the direct contact between neighbouring particles is augmented.

When lighter materials are sought, by encompassing a balance between moderate mechanical properties and superior insulation performance, this can be better accomplished through silica aerogel reinforcement with fibres, as disclosed in this literature review. Aerogel companies like Aspen Inc., Cabot Corporation, and Nano High-Tech Co., Ltd. grant further proof by producing aerogel blankets.¹⁵⁸

The appropriate fibre fraction along with their homogeneous dispersion will potentiate improved mechanical and/or insulation properties,⁵⁷ turning this into a versatile route to facilitate aerogel manufacture.^46,159–161 Additionally, the use of recycled or waste fibres^23,83 to strengthen silica aerogel composites is being studied at the laboratory scale, paving the way towards sustainable and lower cost materials,⁴⁰ with almost unlimited potential for further development.¹⁶²

There is a broad consensus (either among the scientific community or aerogel industry enterprises) about fibre embedment as being the most versatile and effective method of silica aerogel strengthening.^{30,46,119–121,155,158}

3.2 Techniques for preparation of silica aerogels with fibres

There are several techniques reported in the literature to reinforce silica aerogels with fibres. The pioneering investigation of Wang et al.¹³³ consisted of the incorporation of ceramic fibres, to further improve the radiative properties of doped aerogels due to their opacifying effect (mainly for high temperature applications), also providing extra strength to reduce their inherent fragility.¹³³ Such composites were (and still are) accomplished with dispersion of the fibres in the sol prior to the onset of gelation.^47,163,164 The mixture is then poured into a mould for gelation, and this procedure is typically used to manufacture strengthened and denser materials.^51,163

This procedure is briefly sketched out in Fig. 6a. The loading reinforcement limit is imposed by the uniformity of dispersion, since aggregates or bundles due to an excess of fibres might create weakened sites and decrease insulation performance of composites. This is more critical when using inorganic fibres. Some contextual examples are presented in Sections 3.5 and 3.6.

Flexible aerogel composites are generally obtained by pouring the sol over a fibre batting previously placed within a container, as illustrated in the scheme of Fig. 6b. A basic catalyst is added, just immediately before or after pouring the sol, to induce the gelation within a few minutes.^{22,33,165,166} The protocol in which the basic catalyst is added before pouring the sol is preferential, because the fibres may compromise the homogenization of the solution.

There are other versions for this route of composite preparation: (i) alternating the pouring of the sol with deposition of fibres, several times, according to the number of planned layers;¹²⁰ (ii) pouring all the alcosol into the mould and then immersing the fibres layer by layer^167,168 or (iii) pouring the sol into a container and soaking a fibre matting or entangled fibres at the onset of gelation.^169,170

To obtain silica aerogel composites in the form of thin films with extreme flexibility, an ideal technique of reinforcement involves the addition of electrospun nanofibres,^166,171 since there is an improved integration between the two phases.¹⁶⁶ The embedment procedures can be accomplished by impregnating a web previously formed¹⁷² or electrospinning the fibres in situ,¹⁶⁶ fully integrating the sol through precise control of gelation time.^166,173 A different procedure was described by Mazrouei-Sebdani and colleagues:¹⁷⁴ silica aerogel micro-granules were added to an electro-spinning polyethylene terephthalate (PET) solution, yielding a silica aerogel–PET nanofibre composite.

3.3 Inorganic and organic fibres as reinforcement materials of silica aerogels

The type of fibre, its intrinsic features, such as strength, density, length, diameter, length-to-diameter ratio, optical properties, curvature, and orientation angle, as well as the fraction of fibre content contribute to the ultimate features of silica aerogel composites, impacting their mechanical performance and insulation properties.^176–179 Hence, the accurate selection of the fibre type is essential, in order to create synergies in the conception of composites. For example, a thermal barrier for a high temperature environment must be manufactured with thermally resistant fibres that also have a large spectral complex refractive index throughout the infrared region;¹⁵⁰ if some degree of flexibility would be needed, a flexible fibre should be used in the appropriate fraction.¹²⁷

3.4 Influence of fibres on shrinkage and density of silica aerogel composites

Gel shrinkage is a key factor concerning the bulk density of aerogels. In fact, shrinkage or densification starts during crosslinking reactions when silica particles coalesce to form larger necks.⁷⁹ Shrinkage continues during ageing, when the silica matrix develops mechanical rigidity, with clusters restructuring due to the silica dissolution and re-deposition phenomena, which provides better resistance to the drying process,^79,83 as previously described.

The reported shrinkage values depend on the assessment method. One of the ways consists in the measurement of the initial volume of the mould,^208,209 or allowing the gelation to occur and measuring the volume of the wet gel,^42,83 in comparison to the final volume of the dried material; these methodologies will express higher shrinkage values, since the obtained values are cumulative values of ageing and drying shrinkage. The other way consists of carrying out the assessment after the ageing step, expressing solely the shrinkage due to drying.^57,210 Moreover, some authors report the linear shrinkage (usually in the diameter) and others report the volume shrinkage, the latter leading to significantly higher values. Often, the adopted method is not clearly disclosed.

As reported by Markevicius et al.,⁸³ during the ageing process of a pure silica gel, the volume shrinkage (occurring due to the ageing and drying processes) was around 20%, whereas with the addition of 25 wt% cellulosic fibres, a reduced shrinkage of ≈7% was achieved.

During drying, the embedded fibres also act as an effective skeleton frame that hinders shrinkage, since the casting volume is split into sub-volumes defined between two adjacent fibres. The internal movements of silica chains are inhibited, while the drying stresses are limited to small units, instead of one unit comprising the full body of the aerogel. Pure silica aerogels supercritically dried may undergo a linear shrinkage of around 5–10%, while the integration of fibres can reduce the shrinkage to neglectable levels.^30,83 As an example, the difference in the drying linear shrinkage (gel diameter) between a silica aerogel composite reinforced with silica fibre felt and the pristine counterpart (both supercritically dried in acetonitrile, at 295 °C and 5.5 MPa) was around 13%, and the reinforced aerogel had no apparent shrinkage.³⁰ Shrinkage in silica aerogel composites mainly occurs through thickness (z axis),^79,212 while monolithic non-reinforced aerogels shrink isotropically.⁷⁹

Paik and colleagues³⁰ refer to the composite density as being typically about 10% higher than the density of the corresponding neat aerogel. In general, the mere addition of fibres will lead to increased density, as listed in Tables 3 and 4. Several authors thoroughly addressed this issue, by studying the effect of different fractions of fibres embedded in silica aerogels, and dissimilar outcomes are described below.

Markevicius and co-workers⁸³ developed silica aerogel composites reinforced with Tencel® fibres (diameter of ca. 10 μm and length ≈ 2 mm). Tencel® is a commercial designation of a man-made cellulosic fibre, produced through the Lyocell process. The precursor was PEDS (pre-polymerized TEOS), and the gels were subjected to silylation in an HMDZ solution, for 72 h. According to their results, the higher the fibre content, the smaller the shrinkage (Fig. 10a), in contrast to density, which increased with increasing the fibre fraction (Fig. 10b). A similar trend was observed for aerogels obtained both by SCD and APD, but shrinkage also depends on the drying technique. The reported values are the cumulative shrinkage of the ageing and drying steps; while aerogels supercritically dried mostly shrunk during gel ageing, the evaporatively dried gels suffer additional densification due to the capillary pressures during drying (Fig. 10a). Thus, APD yielded generally higher densities than SCD, as presented in Fig. 10b.⁸³

Li and co-workers¹⁶⁴ reported another detailed observation of the range of low % of fibres. They synthesised silica aerogel composites from TEOS, with 0 to 1.5 volume% of sepiolite reinforcing fibres (2–10 μm length) dispersed in the sol, and dried under supercritical conditions. A small amount of fibres (0.5 vol%) prevented part of the shrinkage, resulting in slightly less dense materials (from 200 to 190 kg m⁻³), in spite of the mass addition; an increase of fibre content led to a denser material (up to 210 kg m⁻³), since the density of sepiolite fibres (2.0 g cm⁻³)¹⁶⁴ is much higher than the aerogel density.

A similar finding was published by Li et al.,³⁷ regarding silica aerogel composites reinforced with attapulgite fibres (a crystalline hydrated magnesium aluminum silicate). TEOS was the used precursor and a surface treatment was performed with trimethylchlorosilane (TMCS/ethanol/n-hexane solution), before APD of composites under a multistep temperature gradient. The mass of fibres varied from 0 to 20%, with linear shrinkage gradually diminishing from 32.3 to 10.2%, as depicted in Fig. 11a. Bulk density significantly decreased from 188 to 163 kg m⁻³ after adding 1 wt% fibres, and then gradually rose until 192 kg m⁻³. As claimed by the authors, fibres induced the aerogel matrix to withstand drying stresses, providing lighter materials with a precise addition of fibres.³⁷ It is worth highlighting the higher level of % shrinkage in APD compared to SCD, despite the effectiveness of hydrophobic surface treatments applied to the composites before APD.

In opposition to the monotonic decrease of shrinkage as a function of fibre content, Shao and colleagues⁵⁷ reported a different pattern, as depicted in Fig. 11b, regarding sodium silicate/MTES (1/0.3) composites reinforced with silica fibres (from 3.5 to 17.5 wt%), with a two-step MTES-TMCS modification. The APD dried composites achieved null volume shrinkage at 7 wt% fibre reinforcement. However, the optimal performance (a balance between insulation and mechanical behaviour) was achieved at 10.5 wt% (videTable 3). With further fibre addition, shrinkage increased and reached ∼24%, similar to the native, although two-step modified, silica aerogel. The authors attributed the fact to an incomplete surface modification caused by an excess of fibres that caused the composite densification.⁵⁷

Parmenter and Milstein⁴⁷ reported a different tendency when compared to those presented before. Both the density and shrinkage decreased along with the increase of fibre content.

These authors, who first studied the qualitative correlation between fibre reinforcement and the shrinkage behaviour of silica aerogel composites, used TEOS as the precursor and a mixture of ceramic fibres (68% silica with 3 μm diameter, 20% alumina with 2–4 μm diameter and 12% aluminoborosilicate with 8 μm diameter, all with 1.27 cm length), and the mass percentage of the reinforcement material varied from 0 to 25%. The fibres were added to the silica-based solution, and the composites were supercritically dried. They generally concluded that the higher the fibre fraction, the lower the density of aerogel composites. This was attributed to the support of the matrix provided by the fibres, which leads to a decrease of total shrinkage.⁴⁷

The observations reported by Gao et al.²¹³ in a subsequent study are in line with those of Parmenter and Milstein.⁴⁷ The aerogel composites were TEOS-based, reinforced with ceramic fibres (mainly composed of SiO₂ and Na₂O) and supercritically dried in ethanol. The fraction of fibres varied from 0 to 11.5 volume%, leading, respectively, to a variation of the composite bulk density between 202 and 142 kg m⁻³, accompanied by a decrease in shrinkage in the z direction, achieving a value of only 0.6%.²¹³

In spite of very different findings, there are some notes that can be outlined:

(1) The bulk density of silica aerogel composites is related to the type and fraction of fibres used for reinforcement;

(2) As a rule of thumb, the volume shrinkage of silica aerogel composites generally decreases with the increase of fibre content.

3.5 Effect of fibre content on the mechanical performance of silica aerogel composites

The porous nanostructure of silica aerogels can lead to a very low compressive strength, a property that might be critical in the case of load-bearing applications.^186,214

Mechanical compressive failures in silica aerogel composites can occur through different mechanisms, such as buckling, delamination, shear deformation and fibre rupture. Buckling is the main cause of fibre break-up, when the stress load is transmitted to the reinforcing fibres, leading to their fracture.²¹⁵

According to the work of Parmenter and Milstein,⁴⁷ both the fibre content and the material density determine the compressive strength behaviour of reinforced composites. These findings are depicted in Fig. 12, in which part (a) refers to the lower densities and part (b) refers to the higher densities. A small amount of ceramic fibre addition, such as 5 wt%, increased the elastic modulus relative to that of the pure aerogel, and this was particularly noticeable in structures with higher density (Fig. 12b), resulting thus in stiffer aerogels. With further increase of the fraction of fibres, the Young's modulus decreased, and hence the composites exhibited a higher degree of flexibility (Fig. 12a, at 10 wt%), which can be advantageous to overcome the inherent brittleness and difficult handling of silica aerogels.

Gao and co-workers²¹³ extended the study of mechanical performance by measuring tensile, bending and compressive strengths as a function of fibre content and density of silica aerogel composites reinforced with ceramic fibres (synthesis conditions and variables previously described). The mechanical strength increased up to 1.2–1.4 MPa with increasing mass of fibres up to 9.4 volume%, and then decreased, in accordance with the results described by Parmenter and Milstein⁴⁷ for the aerogel composites with the highest fraction of reinforcing fibres.

Regarding the results of the different mechanical strength tests as a function of density, Gao et al.²¹³ reported an almost linear relationship between tensile/bending/compressive strengths and density.

Boday and colleagues⁶⁸ reported a similar trend for silica aerogel composites reinforced with organic polyaniline nanofibres, using a three-point flexural compression test. The mechanical strength of composites linearly grew up to 0.06 MPa (the higher strength was recorded at a density of 74 kg m⁻³, with 3 wt% nanofibres), and then decreased monotonically due to a higher fraction of polyaniline, corresponding to a higher density of composites. The authors conjectured that after the maximum strength, the subsequent decrease was caused by the numerous fibres interfering with the interconnections between silica particles in the network. The best mechanical performance of a polyaniline–silica composite aerogel was 200% higher than that of the pure silica aerogel of the same density. They also reported that the polyaniline–silica aerogel composites supported 8500 times their own weight, in opposition to the 5000–6500 times of the pure silica aerogel counterpart.⁶⁸

Apart from the fraction of reinforcement material, the drying technique also determines the mechanical performance of aerogel composites, since it influences the level of shrinkage and the related density, as previously described. This was evidenced by Jaxel and co-workers,⁴² using the 3 point-bending test performed on aerogel composites of silica–Tencel® (fibres ≈ 14 μm diameter and 8 mm length). The fibre volume fraction varied from 0 to ≈2%; the samples were both ambient pressure and supercritically dried, giving densities of 103 and ≈124 kg m⁻³, respectively, for the neat silica aerogel and the composite with the higher content of Tencel® fibres. The flexural results are presented in Fig. 13a (there are no results for ambient pressure drying with 0% fibre content, due to the loss of the monolithic shape during drying⁴²). The supercritically dried composites displayed higher stiffness, expressed by the higher values of moduli and maximum stress, compared to the ambient pressure dried composites, which showed higher flexibility (Fig. 13a and b). The authors attributed this fact to the gel’s partial fragmentation during APD. Under stress loading, the fragmented parts are sustained by the fibres that smoothly move into new rearrangements, while pure silica aerogels tend to break completely.⁴²

Liao and colleagues¹¹⁹ and Hou and co-workers¹⁸⁶ reported similar outcomes, summarised in Fig. 13c and d for: (c) a TEOS-based silica aerogel, reinforced with aligned glass fibres (added to sol in layers perpendicularly oriented), modified by a TMCS/n-hexane solution and subjected to APD;¹¹⁹ (d) a composite developed from a mixture of precursors, TEOS and zirconium oxychloride, reinforced with dispersed ZrO₂ fibres added to the ZrO₂–SiO₂ solution, and subjected to SCD.¹⁸⁶ As depicted in Fig. 13c, the increase of reinforcement material fraction, from 0 to 9 wt% glass fibres, resulted in an increase of compressive strength of the composites, from 0.72 MPa to 21.03 MPa (almost 30-fold), while the bulk density increased from 115 to 227 kg m⁻³ (less than 2-fold).¹¹⁹ The silica–zirconia composites reinforced with zirconia fibres exhibited a positive and almost linear correlation of bulk density and compressive strength with fibre wt%, between 0 and ≈10 wt% (Fig. 13d). Beyond that point, the mechanical strength decreases with the addition of fibre mass. Hou et al.¹⁸⁶ explained these results through an excess of fibres that led to clusters (non-homogeneous dispersion), which favoured fibre slip off under stress load and the composite failure.¹⁸⁶

The mechanical response of silica aerogel composites engineered for insulation at high/extreme temperature must fulfill two important requisites: (i) the ability to maintain thermal performance when subjected to a load; (ii) displaying the necessary degree of elasticity under that load.²¹⁶

The loss of integrity of silica aerogel composites under stress load is often the main factor of degradation of their insulation performance due to induced cracks.^216,217

3.6 Effect of fibre content on the thermal conductivity of silica aerogel composites

The fraction of fibres reinforcing silica aerogel composites dictates their thermal conductivity, which can also be influenced by the type of fibre.^85,177 The trend observed for density in the silica–sepiolite and silica–attapulgite composites (Section 3.4) is similar to the trend of thermal conductivity: a small fraction of fibres decreased the thermal conductivity relatively to that of the neat aerogel, while for a higher amount of fibres the thermal conductivity increases. In fact, higher porosity prevailed over a small mass addition of the inorganic fibres, and thermal conductivity decreases due to the increase of voids; but, by increasing the amount of fibres, thermal conductivity increases almost linearly,^37,164 as shown in the two examples of Fig. 14.

Other systems of silica aerogels with alumina, silica or silicon carbide fibres were reviewed and explained by Lee & Cunnington.¹⁷⁷ According to their study, a low content of fibres, with residual contact between them, has a negligible effect on the solid heat conduction of the composites.¹⁷⁷

Liao and co-workers¹¹⁹ studied the effect of the fraction content of aligned glass fibres (5–20 μm diameter) on the thermal conductivity of silica aerogel composites. Four layers of fibres were alternately deposited in perpendicular directions and permeated with silica sol. The content of glass fibres was 1, 4.2, 7 and 9 wt%. The increasing mass of fibres led to different thermal performance behaviour: from 1 to 4.2 wt%, there was almost no increase in thermal conductivity, attributed to a low contact between fibres, separated by the aerogel in-between; a further increase in the fibre content, from 4.2 to 7 wt%, increases the mutual contact between fibres due to the nearest adjacent layers of glass fibres, and thermal conductivity rises from 26 to 32 mW m⁻¹ K⁻¹, until full contact of fibres; above 7 wt% fibres, the thermal conductivity increases only slightly.¹¹⁹

Lu and colleagues¹³⁵ evaluated the thermal conductivity using different patterns between adjacent SiC fibres embedded in a silica aerogel. They assessed three possibilities: (i) strong contact, for effective contact between fibres; (ii) non-ideal contact, with air and cracks between fibres; and (iii) ideal contact, when the space around each fibre was filled by the aerogel. The results are shown in Fig. 15a. The ideal contact leads to the lowest thermal conductivity, suggesting that with lower fibre fraction (with reduced probability of mutual contact) better insulation properties can be achieved.¹³⁵

Dissimilar achievements are also reported in the literature. Jiang and colleagues⁸⁵ developed a silica aerogel composite, reinforced with a microglass fibre mat (2–4 μm diameter), with a volume content of 4.5, 6.8, or 9.1%. The results are depicted in Fig. 15b: the higher the volume of fibres, the lower the thermal conductivity of the composite. According to the authors, the results can be understood in light of the small diameter of the glass fibres, and the consequent high specific surface area. Thus, the higher the fibre content, the higher the ability to suppress the radiative heat conduction. This is supported by the slope of thermal conductivity curves with increasing temperature: the composite with higher fibre content exhibited the lower slope.⁸⁸

In Fig. 15c, corresponding to the work of Yuan et al.,²¹⁸ the trend is the opposite: the higher the content of glass fibres, the lower the insulation performance of the silica aerogel composite, which was attributed to an eventual higher connection between fibres, enabling the creation of new paths for heat leaking,²¹⁸ but it can also be related to the higher diameter of glass fibres, which was 12 μm, while Jiang and co-workers⁸⁵ used fibres with a smaller diameter (2–4 μm).

The different trends of thermal conductivity as a function of fibre fraction, by comparing the composites developed by Jiang et al.,⁸⁵ Liao et al.¹¹⁹ and Yuan et al.,²¹⁸ are in accordance with the degree of contact between fibres in the structures; when disposed in layers, the stronger contact between fibres potentiates an increase of conductivity.¹³⁵ Nonetheless, all the composites provided better insulation features than commercial fibre glass insulators, whose typical values of thermal conductivity are around 40 mW m⁻¹ K⁻¹,⁸³ as shown in Fig. 4.

In the aforementioned development of Markevicius and colleagues,⁸³ cellulosic fibres (Tencel®, 10 μm diameter and 2 mm length) were added to a silica sol at concentrations of 1, 1.5, 2 and 3 wt% (corresponding to 10, 15, 18 and 25 wt% of the dried composite). The influence of APD vs. CO₂ SCD on the insulation performance of composites was also studied. The thermal conductivity of cellulosic fibres is higher than that of the silica aerogel structure, ranging from ∼0.72–5.7 W m⁻¹ K⁻¹ in cellulose nanocrystals, to lower values of ca. 0.22–0.53 W m⁻¹ K⁻¹ in more organized man-made materials, such as cellulosic films.²¹⁹ Therefore, it is expected that the addition of cellulosic fibres, while providing reinforcement of silica aerogels, would have a detrimental effect on their insulation features. In fact, the thermal conductivity of composites slightly increased with the increase in the fraction of fibres, as depicted in Fig. 16.

The SCD composites showed ≈7% better performance in terms of insulation, attributed to the reduced shrinkage, thus lower density, compared to the APD composites.⁸³

This group of researchers also assessed the insulation properties of other cellulosic fibres compared to those of Tencel®, namely flax and recycled paper fibres, all with 25 wt% embedded fibres in the silica matrix. According to their results the silica–Tencel® composites performed around 13–14% better.⁸³ Yet, all the composites still exhibit values below the thermal conductivity of free air under standard conditions (25 mW m⁻¹ K⁻¹), with insulation performance similar to that of super-insulating materials. This was attributed to the mesoporous structure of the gel, not significantly damaged during drying.

The incorporation of cotton fibres also increases the conductivity of silica aerogels, in accordance with the work of Rezaei and Moghaddas.¹⁹² Even though the structural strength of composites increased, in order to maintain their thermal conductivity below the thermal conductivity of free air, the cotton fibre addition was limited to a maximum content of 60 wt% of the dried aerogel, according to the authors. These composites were obtained from a water glass precursor and APD, after surface modification with TMCS in hexane.¹⁹²

In the development of pectin–silica composite aerogels, by dissolving pectin in water-glass-derived silicic acid solution, the as-prepared high methoxy content pectin nanofibres remained interpenetrated within the silica matrix.¹⁵⁵ The aerogels synthesised at pH 1.5, 3 and 5 and reinforced with 5, 10 and 20 wt% pectin nanofibres exhibited improved insulation properties when compared to the pristine aerogel counterpart. While the pectin–silica composites displayed thermal conductivity values between 14 and 17 mW m⁻¹ K⁻¹, the thermal conductivity of the native silica aerogel was 17.4 mW m⁻¹ K⁻¹.¹⁵⁵ One possible reason for these results may be the smaller dimensions of pores in the hybrid aerogels,³¹ since the diameter of pores in the composites was ≈11 nm, while it was around 14 nm in the native silica aerogel, on average.¹⁵⁵ As revealed by Zhao et al.,¹⁵⁵ the composite synthesised at the smaller pH and with a small load of pectin fibres exhibited the lowest thermal conductivity: 14.2 mW m⁻¹ K⁻¹.

Additionally to the size of pores, their relative volume is also a key factor of insulation performance, as demonstrated by Shao et al.:⁵⁷ while the pristine silica aerogel (ρ_b = 125 kg m⁻³, APD), with a mesopore volume of 96.6% displayed a thermal conductivity of ∼20 mW m⁻¹ K⁻¹, after the addition of 17.5 wt% silica fibres (ρ_b = 146 kg m⁻³, APD) the mesopore volume lowered to 77.7% and the thermal conductivity increased to ∼30 mW m⁻¹ K⁻¹ (even so, a very good result was achieved with composites).

3.7 Influence of fibre morphology/size

The physical adhesion between the silica phase and the reinforcing fibrous matrices tends to be improved with the roughness of fibres' surface.²³² Other relevant parameters of fibres for the final properties of aerogel composites are mentioned below.

3.8 Influence of fibre orientation

The absorption and scattering characteristics of each fibre are related to the angle of incidence, defined between the incident radiation and the plane normal to the longitudinal axis of the fibre.^177,237

Apart from the optical properties and volume fraction, fibre orientation is an important parameter for the thermal conductivity of aerogel composites. According to the studies of Lee & Cunnington,¹⁷⁷ 25 wt% SiC fibres with 15 μm diameter randomly oriented in a plane normal to the heat flux imparted the lowest conductivity to a silica aerogel composite (ρ_b = 105 kg m⁻³), while the fibres randomly oriented in space imparted the highest value. In order to improve the mechanical properties, mainly the shear strength, the incorporation of fibres randomly oriented in space, along with other fibres in the plane normal to the heat flux, might be an effective solution, despite the increase of the thermal conductivity, especially for high temperature applications.¹⁷⁷

In fact, the total thermal conductivity of silica aerogel composites can be adjusted accordingly to the inclination angle of embedded fibres, either in low or high temperature environments. Zhao and co-workers¹⁷⁸ reported the influence of the orientation of glass fibres, by varying the heat flux incident angle, φ, between 0° and 40° (φ = 0° when fibres are perpendicular relative to the heat direction): the thermal conductivity of aerogel composites increased from 20 to 55 mW m⁻¹ K⁻¹, at 250 K, and from 80 to 195 mW m⁻¹ K⁻¹, at 1250 K, as illustrated in Fig. 22a. Lu and co-workers¹³⁵ corroborated and complemented those findings by evaluating the effect of increasing the volume fraction of SiC fibres, oriented in the planes perpendicular and parallel to the heat flux, on the thermal conductivity of aerogel composites, with the perpendicular array being the best option for thermal insulation (Fig. 22b). Moreover, the difference in the plotted curves of the total thermal conductivity between the planes (the perpendicular and the parallel array of fibres) gradually increased along with the volume of fibres, due to the worst insulating properties of SiC fibres compared to the pristine silica aerogel matrix.¹³⁵

Fibre orientation also influences the mechanical properties of silica aerogel composites. The multilayer effect on silica aerogel composites was studied by Wu et al.,¹²⁰ using different combinations of glass fibre alignment (5–20 μm diameter and 3% volume). They were deposited longitudinally (L) and transversally (T), in six different designs (LLLL, LTLL, LLTT, LTLT, LTTL, and LTTT), added sequentially and permeated with a silica sol, as depicted in Fig. 23. All the fibre layers in the same direction (LLLL design) imparted to the composite the highest compressive strength, 3.70 MPa at 50% strain, more than two-fold relatively to the composite with alternate directions of fibre layers (LTLT), which withstood a load of 1.71 MPa. According to the authors, the perpendicular direction alignment between two adjacent layers might be prone to create weakened stress points.¹²⁰

As a conclusion of this topic, it can be stated that the thermal and mechanical behaviour of fibre-reinforced silica aerogels can be tailored to the intended application by design of fibre alignment, either to achieve optimal insulation features¹⁷⁷ or improved response in mechanical performance.^119,120

4 Applications of fibre–silica aerogel composites

The most important applications of fibre-reinforced silica aerogel composites are presented here.

4.1 Thermal insulation

According to our survey, thermal insulation is the preferred application of silica aerogel composites. Heat barriers must be highly porous materials, in order to display low thermal conductivity. Silica aerogel composites fulfill such a requisite.

Different fibrous matrices were/are used for aerogel reinforcement, which might enhance their applicability as thermal insulation barriers: glass fibres; ceramic fibres, such as xonotlite,¹³⁶Cerachrome®, Saffil® or Q-Fiber® Felt;³ aramid fibres, such as Nomex®,³ or Kevlar®;^121,167 zirconium dioxide;¹⁸⁶ polypropylene;¹²⁷ cellulosic nanofibres;²³⁸ cotton;¹⁹² and even flax.⁸³

Table 6 describes the relevant features of silica aerogel composites, when they have been envisioned for insulation purposes in room temperature environments. Some of the reported values therein were obtained from the graphical results reported in the cited literature. The presentation of the studies is ordered according to the drying technique used (SCD, freeze-drying (FD) or APD). Regarding the thermal conductivity results, it is generally perceived that a better insulation performance is accomplished by SCD, mainly related to the residual shrinkage during drying, thus yielding composites with higher porosity. Good results were as well achieved after freeze-drying. Nevertheless, very good results are disclosed with APD drying, revealing the beneficial effect of fibre embedment on inhibiting the loss of porosity during drying.

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There is also a correlation between thermal conductivity and bulk density, as mentioned before: the insulation feature tends to deteriorate with increasing density, which might be attributed to a higher fraction of fibres.

Silica aerogel composites with inorganic fibres, developed under appropriate synthesis conditions (such as low pH, longer ageing times or surface modification), can inhibit extreme temperature heat transfer. These materials can be applied, for example, for protecting building steel frames during fires, thus preventing the structure from softening and collapse, as described by Motahari & Abolghasemi.²⁰⁸

Along with the effective thermal insulation at high temperature, high-tech applications also require efficient mechanical performance,^135,179 which is affected by the skeletal rearrangement of aerogels induced by increasing temperature (the aforementioned aerogel's sintering). This phenomenon may result, for example, in a partial reduction of the Young's modulus.²¹²

Table 7 highlights some of the properties of aerogel composites developed for high temperature environments.

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As previously mentioned, the compressive strength of unmodified silica aerogels (with a density of 96 kg m⁻³) is approximately 0.04 MPa.¹⁸⁸ The mechanical performance of silica aerogel composites, even when subjected to high temperatures, remained significantly higher, this being a fundamental requisite for such application fields. Also, the bending strength of native silica aerogels with a density of ≈100 kg m⁻³ is around 1 MPa,²¹¹ which is below that observed by Jiang et al.⁸⁵ and Liu et al.²³⁰

As reported by Jiang et al.,⁸⁵ after reinforcement with glass fibres, and in spite of the temperature (650 °C), silica composites can endure high mechanical strength, still being a superior insulating material, with a thermal conductivity of 22 mW m⁻¹ K⁻¹, which is below the thermal conductivity of free air under standard conditions.

4.2 Adsorption of harmful compounds

Farmer²³⁹ first reported the use of aerogels (carbon-based) in a pioneering technique for continuous removal of ionic pollutants from aqueous streams, or deionization and electrochemical purification of brackish water (≤5000 ppm). After polarization of the operating cell, through which the salty solution flows, the imposed field deflects anions and cations, which are then electro-sorbed by carbon aerogel electrodes, with surface areas increased up to 2600 m² g⁻¹.²⁴⁰

Since then, the application of aerogels in environmental remediation has been an increasing practice,^10,74 and the trend is also valid for silica aerogels with embedded fibres.^244,245 The chemical versatility of silica aerogels (along with their rough surface and huge surface area) allows them to be tailored for specific adsorption purposes. For example, the grafting of non-polar groups decreases their surface energy, enhancing the interactions of aerogels with other non-polar species, such as polluting oils,^246–248 as seen in Fig. 24.

That induced hydrophobicity enhances the buoyancy of aerogels in polluted waters, unlike several organic and natural sorbents, which are often used due to their known efficiency (such as low grade cotton, kapok or rice straw).²⁴⁹

In addition, the incorporation of small amounts of fibres in aerogels improves their mechanical performance, allowing the repeatability of adsorption and desorption processes.²⁴⁵

Table 8 summarises some of the available results on removal of organic pollutants from the environment, performed by silica aerogel composites with quite different fibrous materials.

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Superhydrophobic materials were successfully developed, after a surface modification with TMCS in n-hexane: (i) a polyacrylonitrile–silica aerogel displayed a contact angle of 169° (6 h silylation treatment, at 40 °C);²⁴⁵ (ii) an attapulgite–silica aerogel composite exhibited a contact angle of 153° (soaked for 36 h for silylation treatment, at 60 °C).²⁴⁴ Additionally, Wei et al.²⁵⁰ evaluated the comparative performance of different fibres in terms of their chemical structure, concluding that the adsorption capacity for organic pollutants is around 6 to 8 times the composite weight.²⁵⁰ Slightly better performance was achieved with composites reinforced with hydrophobic fibres, as expected for non-polar pollutants.

In light of the results, fibre-reinforced silica aerogels are remarkable adsorbent materials. Moreover, Shi et al.²⁴⁵ stated that diesel oil adsorption/desorption processes could be repeated at least 100 times.

4.3 Acoustic insulation

Noise pollution has become a critical matter in people's daily life, a problem demanding more effective solutions in the prevention of health issues.²⁵¹ Aerogel-based sound attenuation barriers are among the related cutting edge technologies.²⁵²

Noise mitigation is dependent on material's thickness,²⁵³ interrelated with the wavelength: in order to attenuate a sound wave of wavelength l, the thickness of each discrete insulation layer must be at least l/2.¹⁶⁹

Sound wavelength increases with decreasing frequency, and hence, a low frequency range (100–2000 Hz) requires thicker insulation barriers,²⁵⁴ diminishing the available and useful space, which can be particularly detrimental in confined spaces. Consequently, aerogel composites are the best-suited materials for the insulation of low frequencies, due to their efficiency at reduced thickness.¹⁶⁹

The porous nature of silica aerogels, which hinders heat transfer, is also an ideal feature to achieve effective acoustic insulation.^4,5 The sound attenuation relies on the fraction of energy loss, since the acoustic waves are successively transferred through the gas and solid phases, gradually reducing their amplitude.²⁵⁵

The acoustic insulation provided by aerogels is a function of several factors, related to the preparation procedures: material's density, interstitial gas pressure and their overall texture.²⁵⁵ For example, surface roughness and larger pores are advantageous features.^256,257 This was demonstrated by Martin and colleagues,²⁵⁶ in their work on the development of silica aerogels by using polyethylene glycol (PEG) to control pore dimensions. Polymers are known to be used as porogen additives, allowing controlled design of hierarchically porous materials.⁷² For example, by increasing the concentration of high molecular weight PEG in the sol, aerogels with higher pore dimensions can be produced.^72,256 The sound velocity across native aerogels was reported as 241.0 m s⁻¹, being reduced to 103.2 m s⁻¹ after PEG addition at a concentration of 10.2 mg L⁻¹.²⁵⁶

Similar to the heat transport, there is a relationship between sound propagation and aerogel's density,⁶⁵ according to eqn (8):

Vsound ∝ ρbα, α ≈ 1.0…1.4	(8)

The propagation speed of longitudinal sound waves through native silica aerogels is typically 100 m s⁻¹, for densities around 100 kg m⁻³.^258–260 In the case of extremely light aerogels, for instance with 5 kg m⁻³, a sound velocity of 20 m s⁻¹ was reported by Groß and co-workers,⁷⁰ although this value had been achieved after aerogel's evacuation.

In spite of the improvement of acoustic insulation with the decrease of aerogels' density, their inherent fragility poses difficulties for most end applications.²⁵⁹ The incorporation of fibres makes possible easy handling, allowing the applicability of lighter aerogels to be extended, while endowing the materials with a higher degree of flexibility. By using polyethylene terephthalate (PET) nonwoven fabric with 5 mm thickness and a density of 37 kg m⁻³, flexible PET–silica aerogel insulation blankets were manufactured with two different methods: (1) by pouring the silica sol over the nonwoven fibre mat before gelation and (2) by dipping the fibrous mat into a dispersion of ethanol and hydrogel granules that were already aged.²⁶¹ In the last method, the sol was allowed to gel and, after gelation, it was subsequently broken into particles by ultrasonication, which were then aged in an ethanol bath.²⁶¹ The purpose of Oh et al.'s²⁶¹ work was to simplify aerogel production processes, mainly in terms of costs, since aerogel granulates can be used to build effective insulator systems,^115,262 being less expensive than monolithic aerogels.^262–264 The composites developed with the first method were actually the most effective in acoustic insulation, especially at frequencies between 1000 and 6000 Hz. The analyses were performed in terms of sound absorption coefficient, described as the fraction of sound energy absorbed by a material, with values ranging between 0 and 1 (1 – perfect absorber). At frequencies of 2000 and 5000 Hz, the samples developed with method 2 (ρ_b around 158 kg m⁻³) displayed a value of ≈0.2, while in method 1 (composites at ρ_b around 65 kg m⁻³) it was >0.4.²⁶¹

An aerogel composite based on polyacrylonitrile and silica electrospun nanofibres (with apparent density = 9.6 kg m⁻³), developed under freeze-drying and cross-linked with heat treatment, featured better insulation performance compared to a highly commercial insulating fabric made from polypropylene fibres, adequate to coat the passenger compartment of transport vehicles. This study of Si and colleagues¹⁶⁰ was carried out with aerogel composites of 5 mm thickness and an areal density of 48 g m⁻²; the commercial insulating material was twice thicker, with an areal density of 210 g m⁻².

Motahari and colleagues¹⁶⁹ used a cotton fibre nonwoven mat (with an areal density of 800 g m⁻² and 1 cm thickness) as the support matrix to synthesise in situ silica aerogels, and assessed the influence of the molar ratio of MeOH/TEOS and ageing time on the composites' sound absorption. The mass of the aerogel in the samples was around 45%. The performance of a composite with a density of 88 kg m⁻³ (molar ratio of 11 and aged for 24 h) was significantly better than that of the original fibrous matting (more than 1-fold, on average), mainly at frequencies below 2000 Hz.¹⁶⁹

Feng and co-workers²³ manufactured silica aerogel composites (1 cm thickness) with cellulose fibres recycled from paper waste. The best performance in sound attenuation was achieved at 2 wt% fibre concentration and 146 kg m⁻³ density. According to the authors, the coefficient of sound absorption reached 0.5, a higher value compared to that of the commercial polystyrene foams. Moreover, researchers claimed that the cellulose–silica composites have the potential to be scaled up for industrial dimensions, with the interest in them relying on cost-effectiveness and higher degrees of flexibility.

In a practical way, silica aerogels embedded with fibres can be tailored to be used as efficient sound barriers with reduced thickness compared to the commonly applied materials, such as rock wool panels or polystyrene foam, broadening the applicability of aerogels where the space for insulation is limited.^23,265

4.4 Sensors

The low refractive index of silica aerogels can be harnessed for the development of sensing devices, for example, to be used in waveguides and for supporting the performance of optical fibres.²⁶⁶ Due to the total reflection phenomenon, a light beam can travel long distances within optical fibres, which are known as well for their immunity against electronic interference and ionizing radiation, large bandwidth, signal accuracy and for being light weight and flexible.^267–269

Conventional optical fibres have thick walls, the cladding, which has a smaller refractive index in comparison to the core. So, light is successively reflected inside the core, providing that the beam refraction angle exceeds the critical angle (θ_c), as sketched out in Fig. 25a. When the light is totally confined to the waveguide region, the optical fibres are the blueprint adequate to the communication field,²⁶⁶ but such a design hinders their application in other sensing mechanisms. On the other hand, if the light would be able to surpass the boundaries of the travelling area, even in a small portion, it could interact with the surrounding environment, for example, by means of scattering, absorption or fluorescence.^268,270 This is the case of evanescent field sensing performed by tapered fibres, or tapers,²⁷⁰ of which an example is drafted in Fig. 25b. This type of optical fibre is used as a sensor of temperature, external refractive index,^271,272 optical properties of materials,²⁷⁰ in extended environmental monitoring^268,272 or clinical diagnosis.²⁶⁸

Tapered fibres are optical fibres stretched over a defined length into thinner diameters, as displayed in Fig. 25b, in which the sensing mechanism occurs though the so-called evanescent field: around the fibre's waist, an oscillating field of released energy is generated, remaining spatially concentrated in the vicinity of the source.²⁶⁶

After the high temperature stretching processes to narrow the waveguide of glass fibres, the inherent fragility of those materials is further increased, as the radial dimensions can be diminished from 125 μm to a few hundreds of nanometres.²⁶⁶ The evanescent region can be as thin as less than 1 μm, and therefore is quite prone to contamination or to be damaged.²⁷⁰ Therefore, a suitable shield is necessary in order to protect the sensing properties of the evanescent field, for example, from the influence of dust or environment water.^266,270 Silica aerogels provide the ideal protection: their high porosity and absorptive properties, combined with hydrophobic functionalization, are the sought features. Moreover, a hydrophobic aerogel can be used in the evanescent field for detection of dissolved gases in water.²⁶⁶ But the more important related property is the low refractive index of silica aerogels, and hence they do not interfere significantly with light emissions in the evanescent field.^266,273

Additionally to the embedment of optical fibres in aerogels,^266,273 either by sol–gel coating²⁷⁰ or by laying the fibres on aerogel's surface,²⁷⁴ another version of the applicability of aerogels in this technical field is by filling their hollow-core with the sol, allowing in situ aerogel synthesis.²⁶⁶ Despite the unusual manufacture of such silica aerogel composites, according to Grogan²⁶⁶ this conception of waveguide allows broader tailoring of the optical sensor properties, such as the control of the flow of light.

The earlier mentioned polyacrylonitrile–silica aerogel composites, developed by Si and co-workers,¹⁶⁰ revealed elasticity-responsive electrical conduction. The electrically conductive carbon fibres were obtained after carbonization of composites, by using the polyacrylonitrile electrospun nanofibres as the precursor for the graphitised carbon, with the electrical conductivity value reported as 0.25 S cm⁻¹. The flexible composite was connected to a flash light-emitting diode in a 3 V circuit; when subjecting the composites to a stress load up to 50% of compressive strain, the normalized electrical resistance decreased by 70%. The researchers attributed this decrease in electric resistance to temporary contact established between fibres during compression, adding new electrical conduction paths. In spite of the broad availability of pressure sensors, silica aerogel composites may also have a role in this application, as described here.

Chemiresistor gas/vapour sensors were prepared by Boday and co-workers,⁶⁸ by imparting electrical conductivity to a silica aerogel through the addition of polyaniline nanofibres. The work aimed at the application of these silica aerogel composites in the detection of acidic (HCl) and basic (NH₃) gaseous molecules in the environment. The time response accomplished by composites with 9.6 wt% polyaniline nanofibres was similar to that of thin sensor films cast on electrodes, in which the concentration of polyaniline needed to be significantly higher (several orders of magnitude). Moreover, while the time response of thin films decreased with thickness, the performance of polyaniline–silica aerogels was independent of the layer thickness, allowing this way thicker materials, hence easier handling of detection materials. The researchers hypothesized that the speed of response and sensitivity of the polyaniline nanofiber–silica aerogel composites could be attributed to more open mesoporosity and increased surface areas, around two orders of magnitude higher than those of the film of polyaniline fibers. Boday et al.⁶⁸ also mentioned that polyaniline–silica aerogel composites, with 16.5 wt% polyaniline nanofibres, displayed higher electrical conductivity values than a polyvinyl alcohol aerogel loaded with 12 mg L⁻¹ of carbon nanotubes.

4.5 Technical and protective apparel

The concept of conventional insulation clothing is based on airy textile structures (non-woven fabrics), flexible foams (polyurethane layers) or fibres with a hollow core. Higher thermal insulation performance requires higher thickness (successive layers of fabric or foam), which might restrain the user's movements.¹³⁹

Aerogels are indeed a major breakthrough when applied to apparel, due to their proved efficiency at reduced thickness. Fabrics with integrated aerogels can provide up to six times more insulation performance, with the same fabric thickness, than conventional insulating clothing.¹³⁹ Aerogel based materials encompass the needed features to be used for protection from thermal and electric hazards, to protect workers exposed to molten substances or from flames or radiant heat.³³

Aspen Aerogels Inc. led the manufacturing of aerogel blankets, whose industrial production started around 2003,⁸⁰ with the insulation products resembling textile-like materials.¹²³ The production techniques involve alkoxysilane precursors and CO₂ supercritical drying,⁸³ with the product being around 10 times more expensive than conventional materials with the same level of insulation.¹³² Notwithstanding, when it comes to people operating in an extreme temperature environment, it is quite important neither to compromise the functional performance of the protective apparel nor hamper the action of users.⁵ Two examples are presented below.

A cold-water diving suit with incorporated aerogel exhibited ≈76% improved thermal insulation, compared to a market standard material with a similar thickness, generally used for cold weather clothing (a micro-fibrous polypropylene batting). During the same study, another comparison was made with neoprene foam, which is generally used in wetsuits, revealing a 5-fold improvement for the silica aerogel composite. The solution developed by Aspen Aerogels consists in the embedment of the aerogel in a nonwoven fabric in the inner layer of the diving suit, providing a thermal resistance of 12 to 14 clo(†), depending on the measurement under different pressure conditions. Additionally, the thermal performance of this aerogel-based garment did not exhibit a significant loss with a soft laundering maintenance.²⁷⁵

The extreme weather conditions experienced by mountain climbers were also assessed with an Aspen Aerogels product, known as Pyrogel®. As stated by Ananthan et al.,⁵ after three Mont Blanc routes, two climbers reported the unexpected no cold sensation, at severe temperatures of −20 to −25 °C and a wind speed of 70–80 km h⁻¹.

Other studies aimed at better protection of fire fighters during their exigent duties. Aerogel blankets, with applicability in fire fighters’ protective clothing, were developed by formation of a silica network within a meta-aramid fibrous batting. The composites, dried at ambient pressure, were subjected to silylation treatment with TMCS, imparting hydrophobicity, decreased density and consequently better results in thermal insulation. After a comparative evaluation with conventional non-aerogel blankets, the silica aerogel blankets revealed an increased protection of 58% regarding burn injury hazard.³³

Aerogel based materials prevent heat flux, which is advantageous for inhibiting the incoming heat, but may hinder the necessary heat exchange balance of the human body.^276,277 Shaid and co-workers²⁷⁸ evaluated the heat stress of fire fighters, when the body temperature rises beyond the safe limits. In their approach, they combined the use of aerogel particles along with phase change materials (PCMs). PCMs have the ability to store and release heat according to the changing environment temperature. A mixture of a dispersion of nanoporous aerogel particles, melted eicosane (an organic PCM with a phase transition temperature around 37 °C) and a coating textile binder was applied to the inner layer of the Nomex® fabric(‡), by an impregnation process. The aerogel particles and the binder paste were applied to the outer layer of the Nomex® fabric. PCMs absorbed metabolic heat, circumventing the reduction of heat exchange that results from the almost null permeability of the aerogel treated fabric. According to the authors, the aerogel/PCM mixture proved to be efficient for fire fighters’ equipment, since the time until the pain threshold (when the human body temperature rises up to 44 °C) was delayed by more than 2-fold.²⁷⁸ The advantages of silica aerogel porosity for the infiltration of melt PCMs for textile thermoregulatory applications are thoroughly detailed in other studies.^279,280

Another benefit of the aerogel-based fabrics is their relatively reduced thickness, hence less bulky aesthetic appearance, which allows adding design to protective clothing, for example, in ski suits.¹³⁹

4.6 Other applications

According to several authors,^{16,153,281–283} silica aerogels reinforced with ceramic quartz fibre felt can provide a structural frame for the encapsulation of enzymes, allowing their use for repeated cycles.^153,281 Enzymes were added to the silica sol before gelation and the gel was supercritically dried with CO₂. The aerogel encapsulation technique potentiates a homogeneous dispersion of enzymes,¹⁶ enabling higher concentrations, as well as the enhancement of the catalytic activity per mass of enzyme.¹⁵³ The enzymes studied were lipases, used for the esterification of lauric acid with 1-octanol,^153,281 transesterification of sunflower seed oil with methyl acetate¹⁶ and methanol,²⁸² and partial transesterification of sunflower oil with ethanol.²⁸³

Karout and co-workers¹⁵³ also tried to use carbon fibre felt in this context, which they claimed to provide extra reinforcement to silica aerogels. In terms of catalytic activity, the results were as good as those obtained by enzyme encapsulation in aerogels reinforced with ceramic fibre felt. However, the authors reported that hydrophobic carbon felt performed better with silica precursors with high ratio of hydrophobic groups, while ceramic felt should be preferentially used to strengthen aerogels synthesised with low proportion of hydrophobic groups.¹⁵³

Other applications of silica aerogels reinforced with fibres are mentioned in the literature as filtering media,^13,160 corrosion resistant insulators,²⁸⁴ separators of electric charged materials,^160,285 shock absorbers²⁸⁶ or even tissue engineering scaffolds.^160,287

Also, the non-toxic nature of silica aerogels and sufficient biocompatibility with mammals allow them to be used in the pharmaceutical industry, for example as supports of controlled drug delivery.^287–289

5 New trends and future perspectives

Apart from the well-established method of fibre embedment, another emerging way to develop flexible and reinforced aerogel composites is through the scaffold concept. It consists of a two-step manufacturing process, starting with the assembly of a macro-porous solid template (either a rigid or flexible structure), which in turn will be dipped in the silica sol that undergoes gelation, filling with a nanostructured matrix the macropores of the template. As an example, Mahadik et al.²⁸⁴ used 2-ethylhexyl acrylate for the scaffold synthesis, and MTMS as the silica precursor. Both networks remained closely adhered, surrounded by each other without additional chemical bonds. Flexible composites were developed, encompassing super-hydrophobicity (165°), low density (123 kg m⁻³) and low thermal conductivity (≈45 mW m⁻¹ K⁻¹).

Cellulose derived scaffolds, often involving the freeze-drying technique in their manufacture,^46,241,290 are also being used as silica aerogel reinforcement frameworks. The broad availability of a renewable source might suggest a more sustainable approach of the concept, but among the current drying technologies, freeze-drying is considered too expensive due to its inefficiency in energy consumption.²⁹¹ Nonetheless, in the scientific research domain, good results can be found for silica aerogels reinforced with cellulose templates, both in terms of robustness and thermal insulation (in spite of the unmodified freeze-dried nanofibrous scaffolds typically encompassing high values of thermal conductivity²⁹²). Zhao and co-workers²⁴¹ developed a silylated framework, by adding MTMS to an aqueous suspension of nanofibrillated cellulose (with such a modification, the authors intended as well to further improve the adhesion between the scaffold and the aerogel particles); after that, the silica sol at the onset of gelation was cast into a mould containing the previously prepared scaffold. Additionally, the gels were subjected to a hydrophobic treatment with HMDZ and supercritically dried. The composites displayed super-insulating properties (less than 20 mW m⁻¹ K⁻¹), and the compressive modulus increased by 55% and maximum tensile strength increased by 126%, compared to the 100% silica aerogel reference sample.²⁴¹

Other researchers were inspired by nature, by mimicking birds' nests.^243,293 Birds developed the expeditious technique of filling the tree branch structures with highly insulating materials, such as feathers or animal wool, decreasing the lap-branch holes to below the mean free path of air. The aerogel mimic was achieved with mullite fibres, as the macrostructure, and silica–boron²⁹³ or silica–zirconia²⁴³ sols to fill the interstices. High performance materials were then synthesised, able to endure load stress and extreme temperatures. For example, in terms of compressive strength, an increase up to 1.05 MPa was reported, around ten-fold compared to the reference pristine aerogel;²⁴³ or improved flexibility, with the limit of elastic region defined at 2.2 MPa (more than two fold in comparison with silica aerogels), and rebound resilience at 2.0 MPa, still performing at ≈86%, even when subjected to an environment of 1000 °C.²⁹³ Moreover, it has been recently accepted that ceramic nanocrystalline nanofibres combine the high aspect ratio with a nanometer-sized grain, encompassing the needed flexibility to be used for 3D elastic assemblies.¹⁹⁰

After this exhaustive literature survey, there is absolutely no doubt over the huge potential of silica aerogels, mainly for insulation purposes.²⁹⁴ According to Koebel et al.,¹⁴⁰ “buildings are a tremendous market opportunity”, since transport enterprises have been investing in energy efficient systems, mainly for economic motivations. Now, as the climate change protocols demand for diminished CO₂ emissions²⁹⁵ along with EU Directives on building energy efficiency,²⁹⁶ heating, ventilation and air conditioning of buildings remain as the tackling issues,¹⁴⁰ and so insulating materials with improved efficiency are effervescently sought.^284,297

Vacuum insulation panels (VIPs)^140,298,299 and aerogels^140,294,298 were accepted as being the cutting edge technology in this context, whose typical values of thermal conductivity are around 4 and 13 mW m⁻¹ K⁻¹, respectively. Both materials are very expensive, but VIPs seem to be more cost-effective,²⁹⁸ with their use being suggested in the retrofit of public historic buildings (even if it was with some restrictions).²⁹⁹ However, the thermal conductivity of VIPs tends to deteriorate with ageing, due to moisture and air penetration by diffusion,^140,298 while aerogels can be made hydrophobic, with no loss of performance due to ageing.²⁹⁸

Currently, the aerogel market provides high value-added products, in areas that justify their cost,²⁹⁸ mainly in the form of aerogel blankets, as mentioned above (Section 3.1).

It is predicted that, in near future, the decreased costs of aerogels will propel their use as the main insulation resource; in 2008, the cost of an aerogel was ≈£ 3000 m⁻³, and it is estimated that, by 2050, its price will fall below £500 m⁻³.³⁰⁰

Several strategies have been already implemented to improve aerogels’ cost-effectiveness:

(1) Cost-effective precursors, as is the case of sodium silicate,^4,46 or those retrieved from agro-industrial wastes, such as rice hull ash;⁴

(2) Diversification of aerogel reinforcement materials, taking into account their availability and footprint,¹⁶² favouring those obtained from wastes^23,83 or other industrial by-products;¹⁵⁹

(3) Simplified processes that decrease the manufacturing time and reduce wastes;^41,46,301

(4) APD;^40,300

(5) For specific needs only accomplished with SCD, the CO₂ fluid might be a sustainable option.^89,302 This is not, however, a consensual conclusion. According to a life cycle assessment (on the laboratory scale) for silica aerogels synthesized under high and low temperature SCD, the greater environmental burdens (both in energy consumption and in CO₂ emissions) were due to the CO₂ SCD aerogels.³⁰³

A recent successful case of aerogel technology transfer from the laboratory to market, due to a cost reduction of ≈70% (although in a granular form; brand-named Quartzene®), is a material synthesised with affordable precursors and APD. Also, according to the authors, their manufacture is environmentally friendly.³⁰⁴

5.1 Final considerations

Aerogels are viewed as a new class of materials in terms of their chemical architecture,⁵⁸ being some of the most promising nanomaterials for effective insulation barriers, also taking advantage of non-flammable properties of silica.²⁶⁵ However, aerogels' inherent friability derived from the porous structure, and current prohibitive manufacturing costs, impairs their up-take by the market.^115,294

Fibre embedment in silica aerogels imparts additional strength to the composites, while acting as starting spots for the development of colloidal networks during the gelation event.⁶⁸ Thus, fibres tend to act as nucleation agents,⁶⁸ diminishing manufacturing time and ultimately the price of aerogels. However, further research is needed to fully assess the effect of the presence of fibres during gelation and correlate the porous nanostructure features with the type/properties of fibres and the possible chemical interaction between the two phases. This can be performed by using high-resolution microscopy techniques and molecular modelling studies. In fact, the outstanding thermal insulation performance of silica aerogels relies on their nanoporous structure. Thus, any change of the silica network that causes the enlargement of pores may have a detrimental effect on the insulation performance of aerogels, and must be prevented. Their high thermal efficiency at reduced thickness makes them appropriate in cases of space limitation, as can be the case of retrofitting of important old building facades,^305,306 where the requirements of historical preservation include the maintenance of their appearance.

The production of large-scale silica aerogel monolithic shapes can be better accomplished with extra reinforcement, by following the composite approach, involving the embedment of fibrous materials, mainly in the matt form, but also after fibre dispersion in the initial sol.^83,307 Besides the already implemented industrial processes (for example, Aspen Inc., Cabot Corporation, and Nano High-Tech Co., Ltd), novel methodologies are being researched at the laboratory domain to scale-up aerogel manufacture.^115,241,308

As already exposed, aerogel syntheses have numerous constraints and are difficult to control, which often cause the lack of a standardised product.²⁹⁴ But there are promising scenarios: a patented process of the French company SEPAREX S.A., involving SCD, allows greater uniformity of the internal structure and better results in terms of thermal conductivity.²⁹⁴ Another solution may be the evacuation and sealing of silica aerogels, which can reduce their thermal conductivity by 50%,¹³⁹ a possible way of circumventing the loss of insulation performance due to fibre addition, considering the composite approach.

The traditional insulation materials, either in the form of mineral fibre panels or polymeric foams, currently account for the largest market shares, due to their relatively accessible costs and high moisture resistance.²³ In line with an European report on aerogels,²⁹⁴ the focus of public-funded research should be geared towards increasing the market attractiveness, capitalising on aerogels’ unique features. As referred by Fricke²⁵⁸ (going back to 1992), there is a need for effective environmentally friendly, non-hazardous, non-inflammable thermal insulation materials to replace polymeric foams. The frenetic scientific research in silica aerogels with embedded fibres is in pursuit of those goals.

Also, the successful development of such auspicious materials may provide unexpected insights for new advanced applications, for example, as novel drug carriers, either for agriculture or health applications, tackling the two most important sectors of growing worldwide population.

Disclaimer

The mention to enterprises or market products does not stand for recommendation or endorsement by the authors nor by the Royal Society of Chemistry.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge Professors Maria Helena Gil, António Portugal and Jorge Coelho for their support in the improvement of the text. Funding: this work was funded by Fundação para a Ciência e Tecnologia, I.P., through a doctoral degree grant [SFRH/BD/131819/2017]; this work was also supported by the European Regional Development Fund (ERDF), through COMPETE 2020 – Operational Programme for Competitiveness and Internationalization, combined with Portuguese National Funds, through Fundação para a Ciência e Tecnologia, I.P. [POCI-01-0145-FEDER-006910 (UID/EQU/00102/2013); POCI-01-0145-FEDER-007136 (UID/CTM/00264/2013)].

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Footnotes

† Clothing insulation can be expressed in clo units. 1 clo is the thermal resistance assigned as 0.155 m2 K W−1, where 1 clo represents the insulation of a suit provided to a resting man to maintain the comfort at 21.1 °C, with a relative humidity of less than 50% and an air velocity of 0.102 m s−1.309

‡ Nomex® is the commercial brand of an aramid fibre with meta-oriented linkages, stable at a temperature as high as 500 °C for small periods of time; for long term exposure, it can withstand until 220 °C.185

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