WO2013136034A1

WO2013136034A1 - A reinforced material

Info

Publication number: WO2013136034A1
Application number: PCT/GB2013/000097
Authority: WO
Inventors: Hui QIAN; Alexander Bismarck; Emile Smith Greenhalgh; Milo Sebastan Peter SHAFFER
Original assignee: Imperial Innovations Limited
Priority date: 2012-03-12
Filing date: 2013-03-07
Publication date: 2013-09-19
Also published as: GB201204420D0

Abstract

There is provided an improved structural material comprising a structural arrangement of fibres, such as a carbon fabric or a weave of glass fibres, embedded within aerogel or xerogel, such as carbon or silica aerogel. Significantly improved mechanical properties are found. In embodiments, significantly improved electrical properties are also found. There is also provided a multifunctional supercapacitor comprising at least one aerogel-modified fabric electrode and/or aerogel-modified separator.

Description

A REINFORCED MATERIAL

Background

The present disclosure relates to the field of structural materials, such as structural

composites. The present disclosure also relates to the field of multifunctional materials, such as multifunctional electrodes for energy storage devices, such as supercapacitors.

Fibre filaments, such as carbon fibres, may be formed into cloths or fabrics by weaving, braiding or stitching, for example. Such fabric may possess desirable mechanical properties. For example, carbon fibre has high stiffness, high tensile strength and low weight. Structural fabrics may be used with other materials, such as polymers, to form reinforced composite materials.

A composite material (or "composite") comprises at least one matrix material and at least one reinforcing material. The matrix material surrounds and supports the reinforcing material. The matrix material is typically a polymer such as epoxy. The reinforcing material provides the desired physical properties, such as tensile stiffness and strength. The reinforcing material may be a woven fabric, such as a carbon fabric. Structural carbon fabrics are widely used as reinforcements in composite materials. The interaction between the matrix and reinforcing material provides a synergistic effect. The matrix may be used to bind the reinforcing fibres of a fabric together to prevent relative movement between the fibres and thereby permit the transfer of load between fibres. Such composites therefore provided a coherent, generally non-porous, structure that provides some mechanical and environmental stability.

Composites therefore provide material properties which cannot be achieved by individual materials.

One of the problems with conventional polymer-based fibre-reinforced composites is their relatively weak matrix-dominated properties compared to the fibre-dominated properties, such as in-plane shear strength, interlaminar shear strength and longitudinal compression strength, which limits the benefits and application of the composite materials. Extensive attempts have been made to improve the matrix-dominated properties by introducing nanoparticles, carbon nanotubes, and rubber phases into the matrix. Some improvements have been found at low loading densities but processing has made it difficult to create effective (higher) loading fractions of uniformly distributed, nanosized reinforcements. Essentially there is always a compromise between the stiffness and toughness of the matrix, which results in a compromise between the longitudinal compression strength and delamination fracture toughness.

The present disclosure relates to improving the mechanical properties of structural fabrics and fibre-reinforced composite material. There is disclosed a novel method for fabricating a nano-reinforced fibre structure for a composite.

The present disclosure also relates to providing an improved multifunctional material.

A multifunctional material is a material providing electrical and mechanical functionality, for example. Multifunctional structural power devices require structural and electrochemical energy storage/power functions to be combined in a single material. Multifunctional materials are being developed to improve system performance through a reduction in redundancy between subsystem com onents and functions. This unification of function has the potential to significantly reduce the weight and volume of mobile systems that currently rely on traditional, independent energy storage devices and load-carrying structural materials. It could provide crucial contributions towards future zero emission electrical vehicles, which require energy efficiency, sustainability and environmental friendliness. In addition, such useful and innovative materials are strong candidates for a wide range of energy-dependent applications, such as access panels for aircrafts, portable electronics, etc.

Summary of invention

In summary, there is provided a method to combine an aerogel or xerogel with a structural arrangement of fibres to form a mechanically, and electrically, improved structural material. The improved material may be referred to as an aerogel-modified, or xerogel-modifed, fabric. The modified fibre-based material in accordance with the present disclosure may be used itself as a structural material or used in a structural composite (e.g. infused with a secondary resin, such as a polymer resin) or used as a multifunctional material, for example. Improved structural composites in accordance with embodiments of the present disclosure may carry tensile stresses in the principle fibre direction in excess of 300 MPa, for example. The fibres in such structural fabrics in accordance with the present disclosure may carry tensile stresses in excess of 1500 MPa, for example. A structurally-improved material is formed because the aerogel, or xerogel, provides a nanoscale reinforcement about the fibres in the space normally occupied by conventional matrix material. This nanoscale reinforcement stiffens the environment around the fibres helping to resist shear deformations and fibre buckling effects that lead to matrix-dominated failures in conventional composites. Different fabrication methods may be incorporated to effect the structure.

A surprising advantage of aerogel- or xerogel-modification in terms of mechanical performance was discovered; the aerogel- or xerogel-coating around the fibres reinforces a surrounding polymer matrix, leading to dramatic improvements in the matrix-dominated composite performance. Increases in shear strength and modulus, up to 4.5-fold, is achieved. Such modified structural fabrics could therefore serve as a new generation of simple structural composites or multifunctional materials, for example, which is promising for applications in multifunctional structural power devices.

Aspects of the invention are defined in the appended independent claims.

The incorporation of aerogels or xerogels with the fibres significantly increased surface area, and hence electrochemical performance. In examples, aerogel modified-fabric was found to increase the electrochemical performance by 100-fold, resulting in aerogel-normalised specific capacitance of around 62 F/g (in an aqueous electrolyte). The associated increase in electrical conductivity may be a further benefit in multifunctional systems - but also in pure structural systems for, e.g. lightning strike protection which may be an issue for aircraft composites, for example,

A "structural" material or "structural arrangement" is, for example, a material which is designed to carry/resist mechanical loads well in excess (factor of over x l OO) of those induced by handling and self-weight. For example, a structural material, or the component elements of a structural material, are configured or arranged for significant load-bearing in comparison to their own weight.

"Aerogels" and "xerogels" are solids derived synthetically from gel in which the liquid component of the gel is at least partially removed. An "aerogel" or "xerogel", in the context of the present disclosure, may be formed by at least the following two steps. The first step is to gel a solution of precursor material. The gel may, for example, consist of interconnected chains of nanoparticles that form a bicontinuous rigid framework filled by the remaining solution mixture. The second step is to at least partially remove the remaining solution mixture. An "aerogel" or "xerogel" may be considered monolithic. A monolith is a single piece structure, although other structures may be embedded within it, for example. A monolith may also therefore be considered to be continuous.

An "aerogel" in the context of the present disclosure is, for example, a porous solid derived synthetically from gel in which the liquid component of the gel is replaced by gas without any substantial reduction in envelope volume. However, the skilled person will understand that some pore collapse may occur during drying. That is, an aerogel may partially shrink during drying. An aerogel may comprise a continuous nanostructured phase with continuous porosity; that is, an aerogel may be bicontinuous. An aerogel may be stiff. Carbon aerogel may be formed using a simple drying process, followed by a thermal carbonisation step. However, in some cases, particularly oxide systems, a distinction is more often made between "aerogels" and "xerogels", if simple drying processes lead to collapse.

A "xerogel" is, for example, a solid derived synthetically from gel in which the liquid component of the gel is removed by simple drying which causes collapse or shrinkage. That is, the surface area and envelope volume are decreased and the density is increased. Xerogels are typically formed of sol-gel oxide systems, such as silica. The collapse may be due to capilliary pressure/wetting effects. Special approaches - for example supercritical drying, solvent exchange or surface modification chemistry - may be employed to at least partially counteract the collapse. The skilled person will understand that a xerogel may only partially collapse. If the gel does not substantially collapse during drying, it may form an "aerogel" without any special measures.

"Embedding" is, for example, a process of fixing, wholly or partially within a surrounding material. Embedding is defined herein to include coating, substantially coating or partially coating with the surrounding material. Embedding may be achieved by infusing or impregnating, for example.

A "fabric" is, for example, a structure formed of an ordered arrangement of fibres or fibre filaments or substantially elongate elements. The component fibres or fibre filaments may be structurally arranged in ID, 2D or 3D - for example, woven, braided, knitted or stitched - to form a free-standing structure which is able to support load including significant external load once embedded in a matrix, for example. A fabric may also be referred to as a "cloth" or "weave". The word "fabric" includes unidirectional or bidirectional arrangements of fibres. The elements of the structural fabric may be activated, for example OH-activated. The fibres of the fabric may be continuous. "Continuous" fibres are ones which are so long that the ends of the fibres have a negligible effect on the mechanical performance. For carbon fibres, "continuous" would mean an aspect ratio of the individual fibres of at least 30, probably in excess of 100 (or even 1000 in some cases), to be truly considered as

"continuous". An ordered arrangement of continuous fibres for load-bearing may be referred to as a "structural fabric".

Brief description of the drawings

Embodiments of the present disclosure will now be described with reference to the accompanying drawings in which:

Figure 1 is SEM images of (a) as-received, and (b) - (d) CAG-modified carbon fabrics in accordance with the present disclosure;

Figure 2 shows mechanical characterisation of composite specimens fabricated using as- received and CAG-modified carbon fabrics including in-plane shear response of (a) DGEBA- based and (b) PEGDGE-based epoxy composites;

Figure 3 is (a), (b) photographs of failed PEGDGE-based composite specimens under in-plane shear testing, (c), (d) SEM images of the fracture surface, (a), (c) as-received and (b), (d) CAG-modified carbon fabrics;

Figure 4 shows measured adsorption and desorption isotherms of as-received and CAG- modified carbon fabrics;

Figure 5 is a schematic of an example structural energy storage device in accordance with embodiments of the present disclosure;

Figure 6 shows photographs of as-received (601) and silica aerogel-modified (603) glass fibre weaves;

Figure 7 shows SEM images of silica aerogel-modified glass fibre fabrics; and

Figure 8 shows adsorption and desorption isotherms of as-received and silica aerogel- modified glass fibre fabrics. In the figures, like reference numerals refer to like parts. Detailed description of the drawings

In overview, an improved fibre-based material is provided, having excellent electrochemical and mechanical properties, by modifying a structural arrangement of fibres, such as a structural fabric of continuous fibres, with aerogel or xerogel, such as carbon aerogel (CAG). Void space within the structural arrangement of fibres may be at least partially filled with the aerogel or xerogel. The aerogel- or xerogel-modification process can be scaled-up and fabrics with different surface properties fabricated.

Mention of "aerogel" in the following is by way of example only and the skilled person will understand that the disclosure is equally applicable to xerogels.

The aerogel may be a carbon although other materials are equally suitable, such as silica, chromia, tin oxide and modified aerogels containing different materials such as metal oxide, carbon nanofibres and carbon nanotubes. Organic aerogels may be equally applicable and may be more useful in multifunctional applications rather than pure mechanical or structural applications.

Mention of carbon aerogel, or CAG, in the following is by way of example only.

The base structural material may be a structural carbon fabric (CF) such as commerically available T300 or HTA from TISSA Glasweberei AG. T300 is a 5 harness satin weave having a weight of approximately 283 g/m² and HTA is a plain weave having a weight of

approximately 200 g/m². However, structural fabric comprising fibres, such as silica fibres, glass fibres, alumina fibres, ceramic fibres and aramid fibres are also suitable for aerogel modification in accordance with some embodiments of the present disclosure. Mention of structural carbon fabrics in the following is by way of example only.

Suitable fibres for carbon aerogel modification are able to withstand the high temperature step required for fabrication (see the Examples). For example, some aramid and glass fibres may not be compatible with the high temperature carbonisation step needed for carbon aerogel modification. For oxide aerogels, processing can be conducted at lower temperatures and therefore any structural fabric may be suitable from this point of view.

The fibre material may be chosen for chemical compatiblity with the resulting aerogel. In advantageous embodiments, the aerogel material is the same as the fibre material so a continuous well-adhered structure results after the high temperature step. For example, carbon aerogel modification of carbon fibres is particularly appealing. Similarly, silica aerogel is particularly compatible with silica glass fibres (or indeed oxide aerogels/fibres in general since the chemistry is quite interchangeable). In contrast, aramid fibres, though mechanically useful, are less compatible with water, so much sol-gel chemistry may be less suitable.

Thin sheets of monolithic CAGs with thickness around a few hundred microns can be fabricated. This structure has good electrical conductivity (for example, 20-200 S/cm) but poor mechanical stability. Embedding carbon paper into CAGs has been considered to help hold the CAG open, and increase the electrochemically active surface area. Carbon paper comprises a non-structural arrangement of short (not continuous) fibres in which the full mechanical benefit of the highly stiff and strong fibres is not achieved because the ends of the fibres dominate the mechanical stress distribution. In addition, the fibre volume fraction is low, as such carbon papers have high porosity. However, the inclusion of carbon paper also allows for some handling (for example, mechanical loading less than approximately 2 MPa) of the CAG. Electrochemical studies of carbon paper-CAG have been reported.

CAGs can be fabricated from a wide range of precursor gels, among which resorcinol- formaldehyde (RF) is the most widely used. In an embodiment, RF resin for example, from Indspec Chemical Co., containing oligomers formed by pre-mixing RF in a certain proportion, was used in order to shorten the gelation time. The formation of RF gels may be based on a sol-gel poly condensation mechanism and the organic gels may then be converted into CAGs through a carbonisation step under an inert environment, for example. In other embodiments, for example with oxide aerogels, the conversion process may be hydrolysis.

The aerogel precursors must wet the fibre used. Whilst fibre surface chemistry can usually be tailored to a matrix of choice, some systems will not work. For example, some solvents may degrade the fibres. The inventors have used two different methods to fabricate CAG-modified carbon fabrics. In an embodiment, RF sol-soaked carbon fabrics were pressed between two slides, such as glass slides.

In embodiments, the liquid component of the gel is replaced with gas without any reduction in envelope volume. In embodiments, a RF/formaldehyde gel is dried by a simple drying process and then carbonised at higher temperature to form carbon aerogel. A simple drying process may be used to form carbon aerogel without any special measures to avoid collapse because carbon systems do not tend to collapse owing to their lower porosity. However, in embodiments, some collapse of the pores does occur. Advantageously, the fibres of the fabric may prevent, or at least reduce, collapse.

In order to utilise the excellent mechanical properties of carbon fibres in a composite structure, it is desirable to obtain a high weight loading of carbon fibres embedded within an aerogel. Optionally, the primary fibre volume loadings is 30-90%, such as 50-70%, for example 60%. In a further embodiment, an infusion technique is utilised.

Vacuum-assisted infusion methods have been used for resin infusion in fibre-reinforced composite manufacturing (see, for example, C. Williams, J. Summerscales, S. Grove, Compos. Part A: Appl. Sei. Manuf. 1 96, 27). Advantageously, it was found that, compared to pressing, the pressure that could be applied on the carbon fabrics was higher in the vacuum- assisted infusion process. Lower CAG loading (15.9 wt.%) was observed for the infused specimens compared to that of the pressed specimens (22 wt.%), reflecting a higher volume fraction of primary reinforcing fibres.

The infusion method is found to be advantageous because it minimises the excess of aerogel on the surface of the fabric and/or maximises the amount of void space in the fabric filed with aerogel. By applying vacuum, air bubbles that lead to undesirable macroscopic voids in the final composite are minimised. Infusion provides a contoured bagged surface with uniform hydrostatic pressure thereby reducing voids between the fabric and a pressing surface (e.g. glass slide) that become resin rich regions. Advantageously, the infusion technique is found to better ensure that the system is not overloaded with aerogel; the process self-limits after the existing void space in the fabric is filled with precursor gel. Overloading can reduce the primary fibre loading and cause excessive swelling of the fabric.

Furthermore, the infusion technique is industrially scalable and can be applied to real components in generalised shapes. Infusion provides direct access to shaped components which is useful given that the aerogel is rigid once the precursor is converted.

The density of the aerogel should be sufficient to provide the required reinforcing effect. In an embodiment, the aerogel is 40-60vol%, optionally 50vol%, void space. In an embodiment, the aerogel-modified fabric may comprise by volume: 50-70%, optionally 60%, fibre; 10- 30%, optionally 20%, aerogel; and 10-30%, optionally 20%, void space (optionally, to be backfilled with polymer).

In multifunctional applications, the aerogel loading may advantageously be 5-15%, optionally 10% by volume, to provide sufficient surface area for electrochemistry.

The aerogel loses mass/volume during processing/carbonisation of the precursor and so filling the void space by infusion defines the resulting loading of aerogel depending on the yield of the precursor solution used.

In embodiments relating to multifunctional electrodes, aerogels are preferred to xerogels owing to their higher surface area. In other embodiments, collapsed xerogels may be preferred, as long as cracking is avoided. In embodiments in which thin laminates are formed, collapsed xerogels may be acceptable because the collapse could be accommodated by relaxation in the out of plane direction. In other embodiments, collapsed xerogels are preferred because they have higher density and are mechanically stronger.

Referring to Figure 1 , SEM characterisation shows a relative uniform coating of CAGs formed (Figs l b-Id) onto carbon fabrics (Fig l a). At least some of the fibres of the fabric are embedded within the aerogel. The gaps between carbon fibres are filled with CAGs (Fig. I c), which advantageously possess an interconnected pore structure with interstitial pores of a few tens of nanometres (Fig. 1 d). However, due to the low pressure applied on the carbon fabrics using the pressing route, CAG-rich regions were formed on both sides of the carbon fabrics, which may introduce resin-rich regions in the subsequent composite fabrication process and reduce the mechanical performance, particularly in bending or buckling. Advantageously, it was found that the fibres were not affected during the aerogel-modification process.

It is found that the aerogel-modified fabric has good stiffness and strength. In particular, the aerogel-modified fibres provide structural reinforcement and enhanced compression performance compared to conventional dry structural fibre configurations such as woven, braided, stitched or unidirectional fibres. The aerogel-modified fabric can therefore be used itself as a structural material, or used as a reinforcement for a composite.

In an embodiment, a number of layers of fabric are stacked and infused with aerogel precursor before the precursor is converted to aerogel. The lay-up may comprise multiple, pre-shaped pieces of fabric. In an embodiment, an aerogel precursor prepreg is formed and conventional lay-up or tape-winding used before conversion to aerogel. Once the aerogel is formed, the shape of the aerogel-modi ied fabric is set.

Such fabrication of the aerogel-modified fabric may encompass many conventional composite approaches. The aerogel-modified fabric may therefore itself be considered a type of "composite". The aerogel-modified fabric may also be considered a reinforced fabric which may optionally be combined with a matrix material, such as a dense polymer phase, using composite techniques. A distinction may therefore be drawn between the first resin which is the aerogel precursor and the optional second resin used to form a final phase - either by impregnation of a thermoplastic melt or a thermosetting resin system that is subsequently cured - such as in conventional composites. The thermosetting approach is preferable in the current context, in order to minimise viscosity during the second impregnation step. In embodiments, a final polymer phase fills the pores in the aerogel-modified fabric.

The aerogel-modified fabric may be formed into a composite using a suitable matrix material. A number of layers of aerogel-reinforced fabric may be stacked within the matrix of the composite. In embodiments, further infusion is used to load the polymer/organic phase. In embodiments, the second resin used to form the polymer/organic phase is cured.

The mechanical properties of the matrix of conventional composites are often relatively poor. Furthermore, many of the critical properties of a composite, particularly those that impose design limitations, are dominated by the matrix. Notably, the inventors have addressed this problem by using the continuous nanosized aerogels as a nanoreinforcement in the composite structure, leading to stiffening effects on the matrix-dominated properties. This unexpected benefit of aerogels to the mechanical performance of conventional polymer composites is novel and paves a way for significantly improved composite materials including improved multifunctional materials.

More specifically, it was found that when the aerogel-modified fabrics, in accordance with the present disclosure, were infused in a resin matrix, the aerogel-coating around the fibres extended into the matrix and enhanced the mechanical properties of the composite. In embodiments, it was also found that the electrical and/or electrochemical properties of the fabric were improved.

The inventors considered the effects of aerogel-coating on the in-plane shear properties of CAG-CF composites under ±45° tensile loading according to the standard ASTM D3518. By way of example only, three different matrix systems were compared, including conventional structural bisphenol-A (DGEBA)-based epoxy, polyethylene glycol diglycidyl ethers

(PEGDGE) based epoxy, and multifunctional polymer (PEGDGE with 10 wt.% ionic liquid electrolyte). At least the following resins are equally suitable: epoxy resin, acrylic resin (for example, polybutylacrylate), styrenic resin (polystyrene), polycarbonate resin, polyester resin, epoxidized phenolic resin, phenylenevinylene resin, fluorine resin, fluorenevinylene resin, phenylene resin and thiophene resin. The following thermoplastics may also be suitable: polyamides, polyether ether ketone (PEEK), polyetherimide (PEI), polyimide (PI), polyethersulfone (PES) and polyphenylene sulfide (PPS).

Typical shear stress-strain curves are shown in Figure 2a and 2b. It was found that higher load was achieved in the case of DGEBA-based composites leading to higher shear strength compared to the other two PEGDGE-based systems. The shear stress increased linearly up to around 0.3% strain, from which the shear modulus could be calculated. The failure of all specimens was found to involve damage accumulation in the matrix followed by delamination along the weave direction between the carbon and glass layers (see Fig. 3).

As-received 25.9 ± 2.2 4380 ± 60 45.0

DGEBA-

CAG- based epoxy 26.2 ± 0.5 5050 ± 210 40.7 modified

As-received 5.83 ± 0.14 201 ± 10 47.2

PEGDGE-

CAG- based epoxy 8.88 ± 0.12 911 ± 60 42.0 modified

As-received PEGDGE- 5.36 ± 0.07 276 ± 14 47.2

CAG- based epoxy

modified with 10 wt% IL 8.71 ± 0.1 1 895 ± 60 42.0

Table 1 : In-plane shear test results of composites containing as-rcceived and CAG-modified carbon fabrics with different polymer matrices (secondary resins). Two plies of glass fabrics were used in each sample.

As summarised in Table 1 , the inventors found that the shear strength of DGEBA-based composites remained fairly unchanged after C AG-modification of carbon fabrics. However, advantageously the shear modulus increased by around 15%, suggesting a reinforcing effect produced by the aerogels. As for the PEGDGE-based specimens, surprisingly, the inventors found that the presence of the CAGs improved the shear properties of the composite by an enormous amount, up to 1.6-fold improvement in shear strength and 4.5-fold in shear modulus, regardless of the addition of IL. The inventors achieved good infusion penetration of the epoxy matrix into the porous structure of aerogels as shown by similar fracture morphologies for the baseline and aerogel-modified specimens. The PEGDGE-based specimens exhibited features such as matrix plasticity and ductile fracture. In addition, the inventors found improved adhesion between the fibres of the fabric and the surrounding matrix. The in-plane shear test results demonstrated the unexpected stiffening effects of aerogels in the composite microstructure. Table 2a and 2b further shows the mechanical performance of a composite comprising a carbon fabric with and without aerogel-modification in accordance with the present disclosure. Notably, the inventors have found that the compressive strength and compressive modulus of composites comprising the aerogel-modified fabric is significantly enhanced. In particular, the matrix or interface dominated properties (delamination resistance, longitudinal compression strength, fatigue resistance etc) are improved.

Table 2a: Short beam shear test results

Table 2b: Compression test results

Once the aerogel-modified fabric is infused with resin, it is therefore shown that the presence of the aerogel significantly improves the mechanical performance of the composite. In embodiments, the process may be used to enhance the mechanical properties of conventional polymer composites. In other embodiments, the modified fabric may be used with a conventional structural resin to give enhanced polymer composite mechanical properties. In further embodiments, the modified fabric may be used with a tough/soft polymer to give both load-bearing and durability, such as to blast, impact or ballistics.

Moreover, the inventors identified that the electrical properties of the aerogel-modified fabric may be improved. In embodiments, the fabric may therefore be used with a multifunctional resin (such as a structural/ionic liquid blend) to give a multifunctional composite having electrical energy storage and load bearing capability. In further embodiments, the fabric may also be used with a purely electrical matrix (such as ionic liquid) to give electrical energy storage and modest load-bearing capability.

Electrochemical properties of carbon fabrics before and after CAG-modification, in accordance with embodiments, via different fabrication routes were studied using cyclic voltammetry (CV) measurements. A scan range (potential window) of -0.2 V to 0.2 V was used to provide representative capacitance results and allowed quick comparison between different specimens. Wider potential windows were tested up to ±1 V and consistent capacitance values were obtained. The CAG-modified carbon fabrics show good capacitive performance (see Table 3)

Increases in surface area were observed by BET for CAG-modified CPs (see Table 3). The increase of the surface area was consistent with the CAG loading leading to similar CAG- normalised specific surface area of approximately 740 m²/g.

The pressed aerogel-modified specimens exhibited higher capacitance than the infused one, which was consistent with the surface area results due to the higher aerogel-loading.

The disclosed infusion method was found to be an efficient way to introduce aerogel-coating onto the structural fibres, leading to remarkable improvements in surface area.

ΓΑ Π.

BET . CAG-

CAG Pore Pore Specific normalised ,. ,

, ,. surface , . ,_{iL c} normalised loading volume width capacita ce surface

, . ° area , 3 , _λ . ,JT. _λ capacitance

(wt.%) . 2 , _Λ (cm /g) nm F/g_c+CAG) area !L

(m /_gc+c^Ac) (m gcAG) ^{(F gCAc)}

As-

0.209 ±

received n a 2.9E-4 n/a 0.06 ± 0.01 n/a n/a

0.003

CAG- modified

22.0 163.1 ± 1.8 0.188 4.6 14.3 ± 0.2 741.4 65.2 (press)

CAG- modified

15.9 1 18.0 -t 1 .6 0.092 3.1 8.7 ± 0.3 742.0 54.8 (infusion)

CAG- modified

9.5 80.7 ± 0.7 0.138 6.8 5.9 ± 0.4 849.5 62.1 (infusion

scaled-up)

Table 3: Surface properties of as-received and CAG-modified carbon fabrics and specific capacitances derived from cyclic voltammetry .at a scan rate of 5 mV/s in 3M KC1 using a three-electrode cell.

Considerable increases in adsorption quantity were observed after the CAG modification (see Fig. 4). Both of the CAG-modificd specimens, shown in Fig. 4, demonstrate type IV curves according to the IUPAC classification, exhibiting hysteresis loops between the adsorption and desoiption isoiherms. The formation of such hysteresis is consistent with the mesoporosity feature of the CAG-modified specimens, since the mechanism of filling by capillary condensation in mesopores differs from that of the mesopore emptying.

The infusion process may be scaled-up and, in an embodiment, CAG-modified carbon fabrics with dimensions of 22 1 7 cm were fabricated. The size of the scaled-up specimens was determined by the active area of the chamber furnace used in the carbonisation process. The process disclosed herein was found to be suitable for producing fabrics of a size in the range. As shown in Table 3, typical CAG-loading around 9 wt.% was obtained, which was slightly lower compared to the small specimens fabricated using the same method. This is because less viscous RF sol (i.e. shorter stirring period after preparation of RF sol) was infused for scale-up process to prevent gelation occurring during the infusion process. Similar specific capacitance normalised to the CAG-Ioading (around 62 F/g), were achieved for the scaled-up specimens. The fabrication process may be applied on other types of carbon fabrics with different surface chemistries and morphologies, such as chemically-activated HTA carbon fabrics with rough surface structure and T300 carbon fabrics with different sizing and weaving type.

Improvements in surface area and electrochemical performance arc observed regardless of the fibre type and properties, demonstrating the universality of the fabrication process of CAG- modified structural carbon fabrics.

In embodiments, the aerogel-modified fabric is used as an electrode in a multifunctional structural energy storage device such as a supercapacitor or battery. Advantageously, in embodiments, the aerogel-modified fabric was coated with metal oxide optionally, a transition-metal oxide such as Mn0₂. It is found that the metal oxide coating notably increases the (pseudo)capacitance of the fabric thereby further improving its performance as an electrode, see Table 4. The metal oxide may increase the energy capacity of the battery electrode.

The selection of metal oxide is determined by the potential window of interest such as energy storage capacity, electrochemical stability etc. In an embodiment, Mn or Fe oxide is used because it is inexpensive and based on a relatively light metal. In other embodiments, Zn and Ni oxides are highly suitable.

Table 4: Specific capacitance tests in aqueous solution of 3M KC1 solution from 0V to 0.8V at 5 mV/s In embodiments, a metal oxide coated aerogel-modified fabric is used as a hybrid electrode. An energy storage device may be formed using aerogel-modified fabric electrodes wherein the first electrode is coated with a first metal oxide, for example Mn0₂, and the second electrode is coated with a second metal oxide, for example FeO_x. Such a device may be considered an asymmetric hybrid.

A distinction is drawn between coating an aerogel-modified fabric with oxide and using an insulating oxide aerogel such as silica, titatnia, alumina and other sol-gel aerogels. Insulating oxide aerogels are not relevant to multifunctional electrode applications. However, in embodiments, an insulating structural fabric (such as a structural arrangement of glass fibres) is modified using an insulating oxide aerogel, in accordance with the present disclosure, to form a multifunctional structural electrolyte/separator. In embodiments, such aerogel- modified fabrics are used in a multifunctional structural electrochemical energy storage device.

Owing to the improved mechanical and electrical properties explained above, the coated or uncoated aerogel-modified structural fabric may be used as a component, such as an electrode, in an energy storage device, such as a supercapacitator or battery.

As an energy storage device, supercapacitors, i.e. electrical double layer capacitors (EDLCs), have attracted increasing interest due to their high power density, long cycle life, and good reversibility compared to batteries. The energy storage capability of supercapacitors strongly depends on the specific surface area of the electrodes.

To date, two main strategies have been pursued for fabrication of multifunctional structural power devices. A straight-forward approach is a multifunctional structure: physically embedding energy storage devices into conventional fibre-reinforced composites (see, for example, T. Pereira Z. Guo, S. Nieh, J. Arias, H. T. Hahn, Compos. Sci. Technol. 2008, 68, 1935) or using structural composite laminates as packaging to protect the devices (see, for example, J. Thomas, M. Qidwai, J. in. Metals Mater. Soc. 2005, 57, 18). These devices were reported to operate normally when the composites were modestly mechanically loaded. However, this approach is limited in mass/volume savings and suffers from problems such as delamination at the device/composite interface. Alternatively, a potentially more beneficial way is to produce truly multi functional materials, consisting of truly multifunctional device/composite components, which simultaneously and synergistically provide structural and electrochemical energy storage functions (see, for example, M. Shaffer, E. Greenhalgh, A. Bismarck, P. Curtis, 2007; J. F. Snyder, D. J. O'Brien, D. . Bacchle, D.E. Mattson, E. D. Wetzel, in smasis2008: Proceedings of the Asme Conference on Smart Materials, Adaptive Structures and Intelligent Systems - 2008. Vol 1 , 2009, 1).

WO2007/125282 discloses a supercapacitor having first and second electrodes wherein each electrode comprises a composite of a mat of conducting fibres bound by an electrolytic resin, and separated by a porous insulator.

A requirement for efficient structural composite supercapacitors is the development of multifunctional electrode materials that possess high energy storage capability and improved mechanical performance. Composites in accordance with the present disclosure show great potential as multifunctional electrode structures. However, as can be understood from the foregoing, the fabrics and composites disclosed herein are equally applicable in other fields, in particular as structural composites.

Figure 5 shows a structural energy storage device, in accordance with embodiments of the present disclosure, comprising two aerogel-modified structural fabric electrodes each comprising carbon fabric (507) embedded within carbon aerogel (505). There is shown a separator (503), such as a glass fabric separator, sandwiched between the two aerogel- modified electrodes. The structure is immersed in an electrolyte matrix (501 ), such as a polymer electrolyte matrix. The energy storage device may be fabricated with any suitable electrolyte, for example, an ionic liquid.

In embodiments, the separator (503) is also aerogel-modified in accordance with the present disclosure, for example, using a non-conducting oxide aerogel to improve mechanical and/or electrochemical performance, of the insulating structural separator.

Ionic liquids (IL) are very interesting in energy storage applications due to their unique physicochemical properties such as high thermal and hydrolytically stability, negligible vapour pressure, relatively high ionic conductivity, and large electrochemical windows (up to 7 V). This can boost both the energy and power density, which are both proportional to the square of the applied voltage. In an embodiment, supercapacitors with different separator configurations were fabricated using IL and CAG-modified carbon fabric electrodes in accordance with the present disclosure. Impedance spectroscopy (IS) and the capacitance, calculated in the low frequency range, show similar values to those obtained from CV measurements.

The inventors have considered the effects of separator thickness on the electrochemical properties. Polypropylene (PP) membranes are widely used in battery industry and have low thickness (25 μηι) and high porosity (-55%). Glass fabrics (GF) may be used as separators in multifunctional structural supercapacitors. The inventors have found that GF of a thickness of 0.05 to 0.5 mm, for example around 0.16 mm, was advantageous because thinner fabrics are not dense enough to prevent the carbon fibres poking through and causing short-circuiting. The inventors found that ESR increased with increasing separator thickness, leading to reduced power density that is inversely proportional to the resistance

Supercapacitors based on CAG-modified carbon fabric electrodes and ionic liquid (IL) electrolyte showed up to 180-fold increases of specific capacitance compared to the one with unmodified carbon fabrics, leading to remarkable improvements in energy density, i.e. from 0.02 to 2.6 Wh/kg. In addition, the electrochemical performance increased as the thickness of the separator decreased. The inventors found good accessibility of IL into the porous CAG structure to achieve high EDL capacitance. The energy density calculated based on a working voltage of 4 V increased from about 0.02 to 2.6 Wh/kg after CAG-modification of the carbon fabric electrodes.

However, in order to achieve multifunctionality of supercapacitor devices, a solid-state electrolyte may be advantageous. In general, polymer electrolytes are attractive because they can lead to flexible, compact, laminated solid-state structures free from leaks and available in different geometries. In an embodiment, IL-modified epoxy matrix based on polyethylene glycol diglycidyl ether (PEGDGE), cross-linked with amine-based cross linker, was used as a solid-state polymer electrolyte for fabrication of a composite electrode for a structural supercapacitor.

PEGDGE-based epoxy matrix has modest mechanical properties compared to traditional structural epoxy matrix (such as bisphenol-A based epoxy), but has high ionic conductivity due to their beneficial structure in supporting efficient ion transport. The inventors found that specific capacitances measured using CV showed improvements in the CAG-modified composite, but the increases were considerably lower than the ones obtained in aqueous electrolyte (3M KCI solution) or pure IL. The inventors attribute this to the reduced ionic conductivity of the electrolyte, from around 9> 10^"3 S/cm for pure IL to 2.8x 10^'5 S/cm for PEGDGE epoxy mixed with 10 \vt.% IL. A decrease in ionic conductivity of the electrolyte could lead to reduced ion mobility and transport rate. Meanwhile, the electrolyte/electrode interface could also affect the accessibility of IL and ion storage on the electrode surface.

C_g_CV C_a_CV Cv_CV CgJS Ca_IS

Electrode Separator Electrolyte

mF/g mF/cm² mF/cm³ mF/g mF/cm²

As-received 9.80 0.39 7.68 7.31 0.29

PP CAG-mbdified 1 180 52.3 858 1330 58.7

As-received 7.56 0.30 4.96 7.68 0.31

GF IL CAG-modified 1110 48.9 644 1320 58.5

As-received 7.13 0.29 3.75 6.90 0.28

2GF

CAG-modified 990 43.6 479 1250 55.1

As-received 7.75 0.31 6.08 5.63 0.23

PP CAG-modified 28.6 1.27 20.8 976 43.1

PEGDGE/

As-received 6.57 0.26 4.31 6.45 0.26

GF 10 wt.%

CAG-modified 8.20 0.36 4.83 237 10.5

IL

As-received 9.52 0.38 5.01 10.3 0.41

2GF

CAG-modified 12.5 0.55 6.09 65.4 2.89

C_V_IS ESR E CV E IS P

Electrode Separator Electrolyte

mF/cm³ mQ/cm² Wh/kg Wh/kg W/kg

As-received 5.73 6.6 0.022 0.016 7365

PP CAG-modified 960 8.6 2.63 2.95 5392

As-received 5.03 9.9 0.017 0.017 4910

GF IL CAG-modified 770 12.7 2.46 2.94 3659

As-received 3.63 14.1 0.016 0.015 3452

2GF

CAG-modified 610 18.3 2.19 2.77 2530

As-received 4.41 338.3 0.017 0.013 76.8

PP CAG-modified 710 283.5 0.064 2.17 90.2

PEGDGE/

As-received 4.23 1673.2 0.015 0.014 15.5

GF 10 wt.%

CAG-modified 140 1549.8 0.018 0.53 16.5

IL

As-received 5.42 2889.9 0.021 0.023 16.8

2GF

CAG-modified 31.8 2954.2 0.028 0.15 15.3

Table 5: Electrochemical properties of supercapacitors fabricated using as-received and CAG- modified carbon fabrics with different separator materials (PP and GF). Gravimetric (electrode mass-normalised) (Cg), area-normalised (Ca) and volumetric (Cv) capacitance values were derived from cyclic voltammetry (CV) and impedance spectroscopy (IS). Equivalent series resistance (ESR) was obtained from impedance spectroscopic measurements. The energy (E) and power densities (P) were calculated based on a voltage of 4 V. The coefficients of variation were all around 5%.

As for the capacitance measurements using impedance spectroscopy it was found that, only the local ions were affected and the ion transport was efficient to achieve the electrochemical charging of the double layer surface on the electrodes. Thus improvements in electrochemical performance by aerogel-modification were evident compared to the ones observed in the CV study regardless of the separator configurations. Furthermore, the structural supercapacitors based on IL-modified PEGDGE epoxy and CAG-modified carbon fabric electrodes, in accordance with an embodiment exhibited much higher ESR than the devices using pure IL^! electrolyte, which the inventors attribute to the decreased ionic conductivity of the electrolyte. In addition, the inventors found that separator effects became more apparent, resulting in around 10-fold increase in ESR when the thickness increased from 25 to around 300 μπι.

Relatively soft/weak polymer electrolyte formations (usually preferred for good ionic conductivity) may therefore be compensated by the aerogel microstructure of the modified fabrics, leading to both good structural and electrochemical performance. The inventors have demonstrated that aerogel-modification not only improves the specific capacitance of fabric electrodes, but also reinforces the electrolyte matrix.

In summary, the introduction of the aerogel nanoarchitecture significantly increases the surface area of conventional structural fabrics and hence the electrochemical performance by 100-fold with less than 10 wt.% aerogel-loading, resulting in aerogel-normalised specific capacitance of around 62 F/g in aqueous electrolyte. Supercapacitors based on CAG-modified carbon fabric electrodes and ionic liquid electrolyte exhibited up to 180-fold increases of specific capacitance compared to that of unmodified structural carbon fabrics, leading to remarkable improvements in energy density, e.g. from 0.02 to 2.6 Wh/kg with a polymer membrane separator, for example. In addition, the electrochemical performance of the supercapacitors, increased as the thickness of the separator decreased.

Multifunctional structural supercapacitors with the disclosed CAG-modified carbon fabric with a polymer solid electrolyte matrix (PEGDGE-based epoxy with 10 wt.% IL) demonstrate the multifuiictionality of the composite. The aerogel-coating around the fibres reinforces the surrounding polymer electrolyte matrix, leading to dramatic enhancement of the matrix- dominated composite performance.

Enhancements in shear modulus ranged from 1.2-4.5 fold increases are found, with more noticeable effects whilst using non-structural matrix materials, such as PEGDGE-based epoxy. As less rigid structure is needed for development of polymer solid electrolyte to obtain high ionic conductivity, the strategy of using aerogels in multifunctional structural supercapacitors could compensate the modest mechanical performance. Furthermore, the stiffening effects of aerogels around the fibres on the surrounding matrix, similar to the usage of carbon nanotubes grafted onto carbon fibres, may also support the fibres against microbuckling, which is the critical composite failure mode associated with fibres under longitudinal compression. Examples

All chemical reagents were purchased from Si ma-Aldrich, and were used as-received. Example 1 - carbon aerogel modification of a carbon fabric

Fabric: HTA 3k plain weave carbon fabric (200 g/m², TISSA Glasweberei AG) was used as the electrode materials. T300 3k 5 -satin-harness weave carbon fabric (283 g/m², ACG) and chemically-activated HTA carbon fabrics were used.

Fabrication of CAG-modified carbon fabrics: Resorcinol-formaldehyde (RF) polymer was prepared using the commercially-available RF resin (AX2000, iNDSPEC Chemical Corporation). The AX-2000 resin contains about 73.1 wt.% resorcinol, at a R:F molar ratio of 2: 1. Sodium hydroxide ( OH) was used as the catalyst (C) and the R:C molar ratio was 50: 1 . Extra formaldehyde (37 wt.% solution) was added to the mixture to keep the R:F ratio at 1 :2. The weight percentage of the RF in mixture was controlled to be 40% by adjusting the quantity of the diluents distilled water. The mixture was tightly sealed and stirred for 2 hours. The formation of RF gels was based on the sol-gel polycondensation mechanism and the organic gels were then converted into CAGs through a carbonisation step under an inert environment. Two different methods were applied to coat carbon fabrics with RF: (1 ) Pressing route: carbon fabrics were soaked in RF solution for 2 hours and then pressed between two clean glass microscope slides (102x152 mm, 1.2-1.5 mm, Logitech Ltd.), which were clamped on both ends to form a thin film of RF coating onto the carbon fabrics. Then the pressed sample was taped and wrapped in aluminium foil and placed in air-tight box to retard evaporation. (2) Infusion route: RF solution was infused into the carbon fabrics based on resin infusion under flexible tooling (RIFT) method that is designed for fibre-reinforced polymer composite fabrication (see below ' Fabrication of structural supercapacitors' for details). Specimens prepared using different methods were both cured at room temperature, 50°C and 90°C with a period of 24 h at each temperature. The dried specimens were then carbonised at 800°C for 30 min in a furnace (Lenton ECF 12/30) under N₂, with a flow rate of 0.5 L/min. Example 2a - silica aerogel modification of a continuous glass weave

Fabric: continuous glass fibre weaves such as plain weave, 200 gm^'2 from TISSA Glasweberei AG, 842.0200.01.

Example recipe for silica sol: Tetramethyloxysilane (TMOS) mixed with distilled water, acidified with HCl to a pH of 2-3. H₂G7Si molar ratios in the range of 3 to 4. Preferably choose a sol composition (medium density) that avoids the need for supercritical or other careful drying, The Sols are prepared in an air-tight round bottom flask by mixing acidified water with the silicate precursor until the solution becomes clear. The sol is then ready for impregnation via pressing or preferably (vacuum assisted) resin transfer moulding (as in Example 1). After impregnation, the fabrics should be sealed and left at room temperature for 48 h to allow gelation, then aged in their sealed moulds at 50°C for one week to allow condensation reactions to take place. Drying of the gels can subsequently be carried out in air raising the temperature to 210°C in a standard oven, either slowly over a period of a week, or rapidly over two hours. The aero/xerogels may optionally be stabilized in a furnace at 450°C in air for 2hs, before optional further impregnation of the secondary resin.

Example 2b - silica aerogel modification of a continuous glass weave

Fabric: continuous glass fibre weaves such as plain weave, 200 gm-2 from TISSA Glasweverei AG, 842.0200.01.

Fabrication of silica aerogel-modified glass fibres: Silica sols were prepared from tetraethylorthosilicate (TEOS), H20, C2H50H, HCl, and NH40H in the molar ratio of 1 :3.5:3.9:7.8x 10-4:5.7x10-3 using a two-step acid-base-catalysed procedure at room temperature. In the first step, TEOS was partially hydrolysed and condensed by mixing with H20, C2H50H, and HCl catalyst. The second step consisted of mixing with NH40H catalyst and additional amount of H20. The amount of H20 was divided into the HCl fraction and the NH40H fraction in a weight ratio of 4: 1. The sols were then ready for impregnation via pressing or preferably (vacuum assisted) resin transfer moulding (as in Example 1). After impregnation, the fabrics should be sealed and left at room temperature for 48 h to allow gelation, and then aged in C2H50H for 48 h. Ambient pressure drying was performed in air at room temperature for 72 h and then in oven at 80°C, 100°C and 120°C with a period of 2 h at each temperature.

Characterisation of silica aerogel-modified glass fibres: Silica aerogel modification was successfully performed on a continuous glass fibre weave (Figure 6). SEM characterisation clearly showed that silica aerogel was uniformly coated onto glass fibres (Figure 7), leading to a hierarchical structure that is promising for structural composite applications. Similar to the carbon aerogel-carbon fibre system, the introduction of silica aerogel resulted in significant increase in adsorption quantity (Figure 8) and specific surface area (Table 6).

Table 6: Characterisation of as-received and silica aerogel-modified glass fibre weaves.

Example 3 - fabrication of a structural supercapacitor

Two different secondary resins were used: polyethylene glycol diglycidyl ether (PEGDGE, Mn~526) and diglycidylether of bisphenol-A (DGEBA). Tri-ethylene-tetramine (TETA, molar ratio of PEGDGE.TETA-3.T) and 4,4'-diamino dicyclohexyl methane (PACM, molar ratio of DGEBA:PACM~2.3: l) were used as the cross-linker for PEGDGE and DGEBA, respectively. To produce a multifunctional electrolyte matrix, 10 wt.% ionic liquid, l-ethyI-3- methylimidazolium bis(trifluoromethylsulfonyl) imide (EMITFSI, purity 98%), was mixed in PEGDGE (82.6 wt.%) and the mixture was stirred until a homogeneous solution was obtained. TETA (7.4 wt.%) was then added to the solution. All resin mixtures were degassed before infusion using the RIFT method. RIFT can be considered as a variant on resin transfer moulding (RTM) in which one tool face is replaced by a flexible film allowing for more flexibility in the layup of the laminate. The layup for producing structural supercapacitors consisted of two carbon-aerogel modified carbon fabric electrodes (optionally, as Example 1), which were sandwiched two aerogel-modified or unmodified fabrics (e.g. based on glass fabric, plain weave, 200 g/m2, 842.0200.01, TISSA Glasweberei AG (optionally, as Example 2) or polypropylene membrane, PP monolayer 3500, Celgard) as the separator (mirrored at the mid-plane to ensure the laminates were symmetrical). Copper tape (with conductive adhesive, 542-5511 , RS components), as the current collector, was applied to the carbon fabrics for electrochemical characterisation. The resin was then infused into the vacuum bag containing the fabrics under 1 bar. The curing of PEGDGE-based and DGEBA-based specimens was performed at 80 °C for 24 hours and at room temperature for 48 hours, respectively.

Claims

1 . A structural material comprising a structural arrangement o f fibres embedded within an aerogel or xerogel.

2. A structural material as claimed in claim 1 wherein the structural arrangement of fibres is a fabric.

3. A structural material as claimed in any preceding claim wherein the fibres are woven, unidirectional, braided, stitched and/or structurally-aligned.

4. A structural material as claimed in any preceding claim wherein the fibres are continuous.

5. A structural material as claimed in any preceding claim in which the fibres are carbon fibres.

6. A structural material as claimed in any preceding claim wherein the aerogel or xerogel component is continuous or monolithic.

7. A structural material as claimed in any preceding claim wherein the aerogel or xerogel component has continuous porosity, optionally, mesoporous and/or microporous.

8. A structural material as claimed in any preceding claim wherein the aerogel or xerogel is carbon aerogel.

9. A structural material as claimed in any preceding claim wherein the aerogel or xerogel is an oxide aerogel or xerogel.

] 0. A structural material as claimed in any preceding claim wherein the structural material is further infused with a secondary resin. .

1 1. A structural material as claimed in claim 10 wherein the secondary resin is a structural polymer resin.

12. A structural material as claimed in claim 10 wherein the secondary resin is an ionic liquid or multifunctional polymeric electrolyte.

13. A structural material as claimed in claim 12 wherein the multifunctional polymeric electrolyte is an ionic liquid-modified epoxy matrix.

14. A structural material as claimed in any preceding claim coated with metal oxide.

15. A multifunctional electrode for an energy storage device comprising a structural material as claimed in any preceding claim.

16. A multifunctional separator for an energy storage device comprising a structural material as claimed in any preceding claim.

17. A composite comprising the structural material as claimed in any of claims 1 to 14.

18. A method of improving the mechanical performance of a material comprising a structural arrangement of fibres, the method comprising: embedding the fibres in an aerogel or xerogel.

19. A method as claimed in claim 18 further comprising the step of forming the aerogel or xerogel in-situ from a precursor gel.

20. A method as claimed in any of claim 19 wherein the step of forming the aerogel or xerogel comprises at least partially immersing the structural arrangement of fibres with the precursor gel and applying pressure, for example by pressing.

21. A method as claimed in any of claims 19 wherein the step of forming the aerogel or xerogel comprises vacuum-infusing the fabric with the precursor gel.

22. A method as claimed in any of claims 18 to 21 wherein the precursor gel is resorcino!- formaldehyde "RF" and, optionally, further comprises at least one oligomer.

23. A method as claimed in claim 22 further comprising heating the precursor gel in an inert atmosphere.

24. A method as claimed in any of claims 18 to 23 further comprising coating the aerogel or xerogel with metal oxide.

25. A method as claimed in any of claims 18 to 24 wherein the improved mechanical performance is an increase in at least one mechanical property selected from the group comprising: shear modulus, shear strength, compressive strength, compressive modulus and tensile strength.

26. A structural material, multifunctional electrode, multifunctional separator, composite or method of improving the mechanical performance of a structural material substantialiy as hereinbefore described with reference to the accompanying drawings.