WO2012019819A1

WO2012019819A1 - Process to grow carbon nanotubes onto fibers

Info

Publication number: WO2012019819A1
Application number: PCT/EP2011/060611
Authority: WO
Inventors: Michel Calame; Matthias Graeber; Mickael Lucien Perrin; Christian Schoenenberger
Original assignee: Huntsman Advanced Materials (Switzerland) Gmbh
Priority date: 2010-08-13
Filing date: 2011-06-24
Publication date: 2012-02-16
Also published as: TW201215701A

Abstract

Process to grow carbon nanotubes onto carbon, glass or metal fibers comprising the following steps: a) Providing carbon, glass or metal fibers; b) Depositing onto the carbon, glass or metal fibers a layer of aluminum oxide with a thickness lower than 150 nm; c) Depositing onto the carbon, glass or metal fibers a layer comprising an iron catalyst; d) Growing carbon nanotubes onto the carbon, glass or metal fiber, preferably by chemical vapor deposition (CVD).

Description

Process to grow carbon nanotubes onto fibers

BACKGROUND OF THE INVENTION

Field of the invention

The present invention relates to a process to grow carbon nanotubes onto fibers, in particular carbon fibers, and to a method to produce composite materials with such fibers.

Related prior art

In our modern society one of the major drives of industry is the search for new materials, with a strong emphasis on strength, toughness and lightness. These advanced materials play a key role in our technological progress and help us to facilitate our daily life and to improve our living comfort. Such advanced materials are mostly composites, consisting of at least two parts: the so-called matrix and the reinforcement. Matrix materials are designed to surround and support reinforcement materials by maintaining their relative positions. Reinforcements are chosen for their special mechanical and physical properties and enhance the matrix properties.

Nowadays high-performance composite materials are mostly fiber-reinforced polymers with fibers such as fiberglass, carbon fiber, and Kevlar. For high-end applications, commonly used matrices are epoxy resins. These polymer matrix composites possess a variety of useful properties such as very high stiffness and strength, dimensional stability, good electrical properties, excellent corrosion resistance and a low density. Their implications are for example easy

transportability, high payload for vehicles and low stress for rotating parts, which make them highly attractive for both civil and defense implementations.

Composite materials however are in general not isotropic in nature, meaning that the

mechanical properties are dependent on the direction of the applied forces.

Although the in-plane loading and stresses have been handled by various configurations of fiber architectures such as unidirectional (1 D) fibers, woven (2D) fibers, and (3D) woven-fibre preforms, the intralaminar and interlaminar stresses have remained major issues. This relatively weak out-of plane behavior often leads to low interlaminar shear strength and to interlaminar failures such as delamination.

There are two ways to reinforce composites. The first option is to act on the resin. This

can be done by changing its chemical composition, or by adding fillers, like alumina, boron carbide, boron nitride, silicon carbide or magnesium oxide. Another good mechanical filler material can be carbon nanotubes (CNT). CNTs exhibit astonishing mechanical properties, like a Young modulus in the range of 0.4 to 4 TPa, depending on radius, chirality and type (single-walled or multi- walled). The widely used carbon steels or Cr-Mo steels are left far behind with moduli of 203 MPa and 213 MPa respectively, and even diamond with its 1.22 TPa has been superseded.

Another remarkable property of CNT's is their tensile strength. Values up to 150 GPa have been reported, which exceed by far the 2.8 GPa of diamond. Carbon nanotubes also exhibit an amazing behavior under compression. They are able to form kink-like ridges that can relax elastically when the compressive stress is released. Reversible deformations up to 50% of their length have been reported. And last but not least, CNT's have a density which has been measured to be around 1 .3-1.4 g/cm³. Combining this property with their amazing strength, one obtains a specific strength (strength to density ratio) up to 1 15 000 kN m / kg . This makes CNT the best of all known materials as far as mechanical properties are concerned. Even the strongest high-carbon steels only exhibit 580 kN m / kg. These mechanical properties make CNT's ideal to improve the overall stiffness, strength, impact resistance and performance in fatigue of high-end composite materials. However, the homogeneous dispersion of CNT's in the resin still remains a major issue.

The second option to improve composites is by acting on the fiber part. This can be

achieved by improving the quality of the carbon fibers, or by modifying their surface and maximizing the fiber/resin interface. Here, again, CNT may be good candidates due to their exceptional properties and enormous surface area to volume ratio.

The field of the present invention relates actually to the modification of the surface of fibers that are used in fiber-reinforced polymer composites. Fiber- reinforced polymer composites are typically materials made from a resin matrix such as an epoxy resin and reinforcing material such as carbon or glass fiber woven mats, comprising individual cylindrical fiber filaments with diameters of the order few-tens of micrometers, typically from 5 to 100 micrometers. One of the distinct advantages of fiber-reinforced polymer composites compared to other materials is the combination of low weight and high strength. Fiber reinforced polymer composites are therefore commonly used in aerospace, automotive, or sports applications.

An important part of such composite material is the interface of resin matrix and reinforcing fiber, typically the weakest link in mechanical testing of the composite. To strengthen the interface often an organic sizing is applied to the surface of the fibers, ensuring a good wetting of the resin on the fiber. An approach to an even stronger resin-fiber interface is the modification of the fiber surface on the nanometer-scale, leading to a greatly increased resin-fiber interfacial area. In order to maximize the resin-fiber interfacial area, it is beneficial to modify the fiber surface with nanoparticles with a high aspect ratio such as nanorods, nanofibers, or nanotubes. These materials should cover a large fraction of the filament surface and extend perpendicular from the surface.

Carbon nanotubes with diameters in the nanometer range and lengths up to millimetres, typically from 20 to 200 micrometers, are an ideal candidate for the surface modification of fiber filaments, as they offer a large aspect ratio and, in addition, a very high stiffness.

The catalytic growth of carbon nanotubes at the surface of fiber filaments obtained by chemical carbon vapour deposition (CVD) has been shown to be beneficial for the resulting mechanical and electrical properties of a fiber- reinforced composite material. However, a desired geometry of dense arrays of long nanotubes extending perpendicular from the filament surface and fully covering is very difficult to achieve.

Thostenson and co-workers have demonstrated the catalytic growth of carbon nanotubes from a carbon vapor phase onto the surface of a carbon fiber filament. However, neither dense arrays nor growth perpendicular to the fiber were achieved. [Journal of Applied Physics, 91 (9), 6034-6037 (2002)].

The fabrication of highly-controlled carbon nanotube arrays on carbon fiber filaments is a major challenge. In particular, long perpendicular arrays (with respect to the filament surface) and a homogeneous surface coverage (with respect to the solid angle) can be hardly fabricated by conventional processes.

Examples of growth of carbon nanotubes on carbon fiber, which do not however result in arrays that are perpendicular to the fiber and which exhibit little control of the Carbon nanotube morphology can be found also in the International application WO2007 / 136613 A2.

International Publication WO2008/054409 discloses the growth of carbon nanotube forests on SiC-coated fibers. No homogeneous surface coverage with a perpendicular nanotube forests could be reached.

Garcia and co-workers have demonstrated the growth of vertical arrays of carbon nanotubes onto Aluminum fibers and highlighted beneficial effects on mechanical and electrical properties [Garcia et al., Composites Science and Technology 68 (2008), 2034-41].

The international application WO2008/054541 shows the catalytic growth of Carbon nanotubes forests on graphite fiber. However, the growth is not isotropic with respect to the fiber surface, in particular, only a part of the fiber filament surface is covered by carbon nanotubes.

The object of the present invention is to find a process to grow carbon nanotubes (CNT) onto fibers which does not exhibit the disadvantages of the conventional fabrication processes. An object of the present invention is, in particular, to find a process to grow carbon nanotubes (CNT) onto fibers, particularly carbon fibers, which is simple, not expensive and feasible on an industrial scale, which facilitates an homogeneous growth of dense arrays of vertical long carbon nanotubes on the whole surface of the fibers. Another object of the present invention is to produce composite materials by infiltrating fibers onto which carbon nanotubes have been grown, whereby the produced composite materials exhibit outstanding mechanical properties, in particular a high interlaminar shear strength.

The object of the present invention is solved according to the features of the following independent claims.

Summary of the invention

According to the invention, a process to grow carbon nanotubes onto carbon, glass or metal fibers comprises the following steps:

a) Providing carbon, glass or metal fibers;

b) Depositing onto the carbon, glass or metal fibers a layer of aluminum oxide with a thickness lower than 150 nm, for example between 1 and 100 nm; c) Depositing onto the carbon, glass or metal fibers a layer comprising an iron catalyst;

d) Growing carbon nanotubes onto the carbon, glass or metal fiber, preferably by chemical vapor deposition (CVD).

For an effective process it is important to have the combination of the Al- comprising and the Fe-comprising catalyst, homogeneously covering the surface of the fiber filament, without any shadowing effects caused by neighboring filaments. The homogeneous Al-coverage of the fiber filaments is reached, for example, through a deposition process called Atomic Layer Deposition (ALD), which is based on the sequential use of gas phase chemical processes.

Chemical vapor deposition (CVD) could be also used. An alternative deposition process of Al could be dipping the fibers in a solution containing the Al-catalyst. The homogeneous Fe-coverage of the filaments is reached, for example, through the dipping process of the fibers in a solution containing the Fe-catalyst.

According to a preferred embodiment of the invention, step a) is carried out by providing carbon fibers or graphite fibers, 0.005-0.080 mm in diameter, whereby several carbon fibers are twisted together to form a yarn, which is woven into a carbon fabric. Steps b) to d) are carried out on such a carbon fabric or even on a stack of carbon fabrics placed one over another.

According to a preferred embodiment of the invention, step a) comprises the step of placing carbon fabrics one over the other, so as to create a stack of fabrics comprising at least two fabrics, and the fabrics are sandwiched between two plates fixed at a fixed distance, so that no vertical expansion of the fabrics is possible during the growth of the carbon nanotubes. Steps b) to d) are carried out on such a "confined" carbon fabric or "confined" stack of carbon fabrics placed one over another.

According to a preferred embodiment of the invention, step b) is carried out with the process of Atomic Layer Deposition (ALD) or chemical vapour deposition (CVD) and the produced aluminum oxide layer exhibits over the whole surface of the fibers a homogeneous und uniform thickness up to 100 nm, for example between 1 and 50 nm.

According to a preferred embodiment of the invention, step b) comprises the step of depositing homogeneous single atomic layers of Al₂0₃ up to a thickness of 50 nm. The thickness of such a layer is for example between 1 and 50 nm, preferably between 1 and 20 nm.

According to a preferred embodiment of the invention, step c) is carried out by dipping the fibers or fabrics in an iron containing solution, for example an iron nitrate Fe(N0₃)₃ solution.

According to a preferred embodiment of the invention, step d) comprises the steps of:

• heating the fibers to a temperature between 500 and 1000 °C, preferably between 650 and 850 °C, to activate the deposited catalysts;

• placing the fibers in a carbon feedstock gas;

• optionally, adding hydrogen to the carbon feedstock gas, in order to improve the produced carbon nanotubes quality.

According to a preferred embodiment of the invention, the carbon feedstock gas is selected from the group consisting of acetylene, ethylene, methane, butane and propane.

According to a preferred embodiment of the invention, the deposited carbon nanotubes are perpendicular to the axis of the fibers onto which they have been deposited, without any preferential growth direction, and exhibit a growth length of at least 5 μηη, preferably at least 20 μηη, most preferably between 40 or 50 μηη and 2 mm.

Fibers or carbon fibers with carbon nanotubes grown onto their surfaces are produced preferably with the process according to the invention.

Also fabrics made with fibers with carbon nanotubes grown on the surfaces of the fibers are produced preferably with a process according to the invention, or fabrics are made preferably with fibers produced according to the invention. According to another aspect of the invention, a process to produce composite materials comprises the following steps:

• Providing fibers according to the invention or produced with the process

according to the invention;

• Infiltrating or infusing said fibers with curable liquid resins;

• Curing with heat or UV radiation or electron-beam radiation or microwave radiation or by electromagnetic induction the fibers infiltrated with resin, so as to produce a solid composite material.

Curing by heat is a preferred process on the industrial scale.

According to another aspect of the invention, a process to produce composite materials comprises the following steps:

e) Providing carbon fabrics with CNT according to the invention;

f) Disposing said carbon fabrics one over the other so as to create a stack of fabrics comprising at least two fabrics;

g) Infiltrating or infusing said carbon fabrics with curable liquid epoxy resins; h) Curing with heat or UV radiation or electron-beam radiation or microwave radiation or by electromagnetic induction the carbon fabrics infiltrated with epoxy resin, so as to produce a solid composite material.

Alternatively, the composite materials can be produced in this way:

i) Providing fiber fabrics;

j) Disposing said fiber fabrics one over the other so as to create a stack of fabrics comprising at least two fabrics;

k) Optionally, sandwiching the fabrics between two plates fixed at a fixed

distance, so that no vertical expansion of the fabrics is possible during the growth of the carbon nanotubes;

I) Depositing onto the carbon, glass or metal fibers of the fabrics a layer of aluminum oxide with a thickness lower than 150 nm, for example between 1 and 100 nm;

m) Depositing onto the carbon, glass or metal fibers a layer comprising an iron catalyst;

n) Growing carbon nanotubes onto the carbon, glass or metal fiber, preferably by chemical vapor deposition (CVD);

o) Infiltrating or infusing said fiber fabrics with curable liquid epoxy resins;

p) Curing with heat or UV radiation or electron-beam radiation or microwave radiation or by electromagnetic induction the fiber fabrics infiltrated with epoxy resin, so as to produce a solid composite material.

Composite material produced with a process according to the invention are easy and inexpensive to produce on an industrial scale and exhibit outstanding mechanical properties, in particular a high interlaminar shear strength.

Surprisingly, they exhibit good mechanical properties, although less plies of fiber fabrics and less fibers are used than according to the conventional fabrication methods.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention are described in more detail with reference to the drawings. Therein:

FIG. 1 shows micrographs obtained with a Philipps XL-30 scanning electron microscope (SEM) of a carbon fiber cloth before and after carrying out the inventive process. After the inventive process highly aligned arrays of carbon nanotubes are attached to the fiber filament surface.

FIG. 2 shows on a SEM micrograph the controlled, homogeneous and nearly cylinder-symmetrical growth of carbon nanotube arrays (forests) at a carbon filament surface obtained thanks to the inventive process.

FIG. 3 a and b show typical carbon nanotube growth results obtained when skipping the Aluminum deposition (step b) and only dipping (5 x 10 min) the fiber cloth into the (Fe(N0₃)₃) / 2-propanol catalyst solution (c=50 mMol/L) before CVD growth (30 min at 725°C). The growth of carbon nanotubes is not homogeneous on the filament surface and vertical arrays are not formed.

FIG. 4 shows the CNT length in micrometer obtained with the inventive process as a function of the Al₂0₃ layer thickness measured in number of cycles of ALD (step b). The arrows in the legend point out which axis must be considered to read the correct scale. Pronounced maxima of the obtained nanotube array height exist. The maxima are at 50 cycles ALD for an lron(lll)Nitrate

concentration of 50 mMol/L and at 100 cycles for an lron(lll)Nitrate concentration of 15 and 30 mMol/L, respectively.

FIG. 5 shows the CNT length in micrometer obtained with the inventive process as a function of the temperature of CVD (step d) for 15 and 30 minutes. The arrows in the legend point out which axis must be considered to read the correct scale. A pronounced maximum occurs at 725°C.

FIG. 6 b) and c) show two scanning electron micrographs of carbon fiber cloths consisting of individual filaments with carbon nanotubes obtained with the inventive process. In Figure 6 b) the viewing direction is parallel to the fiber direction, in Figure 6 c) it is perpendicular. Stripes of parallel carbon nanotubes form inter-links between the carbon fiber filaments. Figure 6 a) shows untreated carbon fiber filaments before the inventive process as reference. DETAILED DESCRIPTION OF EMBODIMENTS

GROWTH OF CARBON NANOTUBES ON FIBERS EXPERIMENTAL SETUP

The experimental process steps and parameters used experimentally to grow the carbon nanotubes onto the fibers are described below.

Step a

HexForcer HexPrimeTMG0926 D 1304 TCT carbon fiber cloths available by Hexcel have been provided. Such carbon fibers (CF) contained 6k carbon fibers per yarn with a width of 2.17 mm per yarn.

Step b)

After an initial cleaning process in an UV ozone cleaner (model 42, by Jelight Company Inc) for ten minutes from each side the carbon fiber cloths were introduced in the deposition chamber of an Atomic Layer Deposition (ALD) apparatus (Savannah 100, by Cambridge NanoTech Inc.). The carbon fiber (CF) cloth is introduced in the ALD apparatus so that a thin layer of aluminum oxide (AIO_x) is deposited on the fiber surface.

An accurate description of the ALD process can be found in the User Manual of the used ALD apparatus Savannah 100, by Cambridge NanoTech Inc. or in [A. Rahtu B.S. Lim and R.G. Gordon. Atomic layer deposition of transition metals. Nature, 2:749-754, 2003].

ALD was then used to depose aluminum oxide on the CF with Trimethylaluminium (TMA) as precursor and water (H₂0). The thickness of the aluminum oxide layer can be controlled by the number of ALD cycles that are run. Each cycle deposits a fraction of a monolayer onto the fiber cloth. Typically a growth rate of 0.1 nm of deposited aluminum oxide per cycle is achieved and a number of cycles ranging from 10 to 200 have been run, corresponding to obtained thicknesses of the AI2O3 layers ranging from 1 nm to 20 nm.

Such a film is thin, homogeneous and covering the whole surface of the fibers. A particular advantage of this technique is the homogeneity of the deposited layer in terms of low surface roughness and spatial coverage of the substrate. In particular, there are no shadowi ng effects on the fi ber filaments due to neighboring filaments , as wo u l d be the case for conventional deposition techniques such as thermal evaporation or sputtering.

An homogeneous AI2O3 layer covering the whole surface of the fibers can be produced not only by ALD, but also with other methods. It could also be possible to deposit the aluminum oxide layer from a solution containing Al, by dipping the fibers in such a solution. Similar surface coverage and the absence of shadowing effects are expected as well. Another alternative process to produce the Al layer would be chemical vapor deposition (CVD). The same furnace used for the following growth of CNT could be used in this case, with evident cost advantages for the industrial production scale.

Step c)

The fiber cloth is then dipped in Ferric nitrate (Fe(N0₃)₃) / 2-propanol solution (Iron(lll) nitrate deposition). By dipping, a thin layer of Iron is deposited on top of the aluminum oxide. Such a film is homogeneous and covering the whole surface of the fibers.

The Fe catalyst application on the aluminum oxide -coated fiber cloth is performed as follows: Ferric nitrate (Fe(N0₃)₃) is dissolved in 2-propanol with concentrations ranging from 10 to 60 mMol/l and sonicated for about 10 minutes to ensure a well-suspended solution. The carbon fiber yarns are cut into 3 cm long samples and submerged n times for t minutes in the catalytic solution (with 1 < n < 25 and 1/6 < t < 50 min). Between the dips the samples were dried at room temperature.

Step d)

The samples were then ready for CNT growth and put on a quartz boat and loaded in the chemical carbon vapor deposition (CVD) furnace. The fiber cloth is introduced into a CVD chamber with feedstock gas flow and Carbon nanotubes grow at the surface of the fiber.

The furnace used for CVD (MTF 12/38/250 tube furnace from Carbolite) consists of a quartz tube with length of 1 m, an inner diameter of 30 mm and an outer diameter of 35 mm. Samples are loaded at room temperature and heated up under a protective flow of argon with a flow rate of 1000 seem (standard cubic centimeters per minute). Once the desired synthesis temperature has been reached, the Hydrogen flow (flow rate 500 seem) and Ethylene flow (flow rate 85 seem) are turned on and the argon flow is stopped. The synthesis temperature could be varied between 650 and 850 °C. The growth time in the furnace could be varied between 1 and 120 minutes. After growth completion the ethylene flow is turned off and the samples are cooled down under a protective flow of

Hydrogen and Argon. At 500 °C the Hydrogen flow was stopped. The samples are unloaded when the temperature has dropped below 350 °C.

In order to characterize the obtained CNT growth, sample inspection was done using a scanning electron microscope (Philips XL-30). EXAMPLES AND CHARACTERIZATION Example 1

Carbon nanotubes have been grown on carbon fibers with the specific process parameters reported in Table 1 .

Table 1

FIG. 1 shows micrographs obtained with a Philipps XL-30 scanning electron microscope (SEM) of a carbon fiber cloth before and after the inventive process carried out with the process parameters according to Table 1. After the inventive process, highly aligned arrays of carbon nanotubes are attached to the fiber filament surface.

FIG. 2 shows on a SEM micrograph the controlled, homogeneous and nearly cylinder-symmetrical growth of carbon nanotube arrays (forests) at a carbon filament surface after the inventive process has been carried out with the process parameters according to Table 1. Thanks to the inventive process, dense arrays of long carbon nanotubes essentially perpendicular to the carbon fiber axis have been grown, which cover homogeneously the whole surface of the fiber and exhibit a length of approximately 10 micrometer.

FIG. 6 b) and c) show two scanning electron micrographs of carbon fiber cloths consisting of individual filaments with carbon nanotubes after the inventive process has been carried out with the process parameters according to table 1 . Figure 6 a) shows untreated carbon fiber filaments before the inventive process as reference.

In Figure 6 b) the viewing direction is parallel to the fiber direction, in Figure 6 c) it is perpendicular. Stripes of parallel carbon nanotubes form inter-links between the carbon fiber filaments. The crosslinking of filaments inside the cloth is evident.

Increasing the fiber interface through the growth of carbon nanotubes can improve the mechanical properties of a composite material obtained, for example, by infiltrating with epoxy resins such fibers. The mechanical properties of such a composite can, however, be also improved by the crosslinking and interlinking between the fibers. Fig. 6 shows that the fibers produced according to the invention could be certainly advantageously used for the production of composite material exhibiting outstanding mechanical properties.

Looking at these morphologies from a mechanical point of view, it is evident that such interlinked fibers could exhibit the capability to improve the mechanical properties of composite materials produced with such fibers. The CNT's will lead to an increase of the fiber/epoxy interface surface area. The fiber/epoxy interface is considered to be the interface between the reinforcement (fibers + CNT's) and the matrix (epoxy resin for example). Since CNT's have a enormous surface to volume ratio, growing CNT's on the

fibers will increases this interface area. Several interfiber links may be formed.

Concerning interplay links, it will also occur when the CNT length is comparable or larger than the interplay spacing which is typically a few hundred micrometers.

The CNT's may lead to a major improvement of the interface area and interfiber links may be formed. By using long CNT's, interply links may occur.

The drawback of this kind of reinforcement is that the CNT's will push apart the carbon

fibers, which will drastically increase the volume of the fabrics. This leads to a reduction of the fiber volume fraction (FVF, fiber volume/total volume) of the cured composite. The FVF mainly determines the mechanical properties of a composite. For carbon/epoxy composites, the ideal FVF is 60%. By increasing the volume of the fabric, on one hand the FVF of the cured composite will decrease, deteriorating its properties, but on the other hand the interface area will increase and interply links may be formed.

Example 2 (comparative)

Carbon nanotubes have been grown on carbon fibers with the specific process parameters reported in Table 2.

Table 2 FIG. 3 a and b show typical carbon nanotube growth results obtained with the conventional process represented by the process parameters of Table 2, where the Aluminum deposition (step b) is skipped and only dipping (5 x 10 min) the fiber cloth into the (Fe(N0₃)₃) / 2-propanol catalyst solution (c=50 mMol/L) is performed before CVD growth (30 min at 725°C). The growth of carbon nanotubes is not homogeneous on the filament surface and vertical CNT arrays are not formed.

Example 3

Carbon nanotubes have been grown on carbon fibers with all the combinations of specific process parameters reported in Table 3.

Table 3

A control of the carbon nanotube length can be achieved by controlling the Aluminum oxide layer thickness.

The catalytic efficiency of the combination of the Aluminum and the Iron catalyst depends sensitively on the thickness of the aluminum oxide coating and the concentration of Iron(lll) Nitrate in the dipping solution.

FIG. 4 shows the CNT length in micrometer obtained with the inventive process carried out with all the combinations of the specific process parameters reported in Table 3.

Fig. 4 shows that pronounced maxima of the obtained nanotube array height exist. The maxima are at 50 cycles of aluminum oxide deposition for an lron(lll)Nitrate concentration of 50 mMol/L and at 100 cycles for an lron(lll)Nitrate concentration of 15 and 30 mMol/L, respectively. No vertical nanotube arrays have been observed below 50 cycles run in the aluminum oxide deposition, which corresponds to an estimated obtained aluminum oxide thickness of about 5 nm. Above 50 cycles run in the aluminum oxide deposition, carbon nanotubes lengths between 10 and 50 micrometers have been measured.

Example 4

Carbon nanotubes have been grown on carbon fibers with all the combinations of specific process parameters reported in Table 4.

Table 4

FIG. 5 shows the CNT length in micrometer obtained with the inventive process carried out with all the combinations of specific process parameters reported in Table 4.

A Control of carbon nanotube length can be achieved by controlling the growth temperature of the CVD process.

The degree of control on the Carbon nanotube growth that can be reached in the process is well illustrated in Figure 5. The length of the carbon nanotubes grown on the fiber surface exhibits a pronounced maximum at 725°C. The position of this maximum is independent of growth time (squares denote 15 min CVD, stars 30 m i n CVD). The sharp peak of carbon nanotube length points out the homogeneous catalyst distribution obtained on the fiber surface. Carbon nanotubes lengths between 20 and 70 micrometers have been measured. PRODUCTION OF COMPOSITE MATERIALS

EXPERIMENTAL SETUP

In order to test the mechanical properties of composites produced with carbon fibers with carbon nanotubes, test samples needed to be made.

Step e

Carbon fibers mats HexForcer HexPrimeTMG0926 D 1304 TCT were provided by Hexcel and cut into pieces of approximately 10cm x 10cm.

Step f

2, 4 or 6 plies were placed one over the other. Such stacks of plies have been subjected to the process steps a) to d) described above, in order to grow carbon nanotubes onto the fibers. The applied process parameters correspond to the ones in Table 1 used for Example 1.

CNT growth on carbon fibers has been realized using two different strategies: ^'free growth' and ^'confined growth'.

In ^'free growth' no mechanical constraints are imposed on the volume of the carbon fiber fabrics during the CNT growth process In ^'confined growth' Molybdenum plates are used to confine the sample and limit its volume expansion during CNT growth, with a fixed interplate distance. The stack of fabrics is sandwiched between the two molybdenum plates, which are screwed together at a fixed distance.

Step g

After CNT growth the samples were ready to be infused with an epoxy system (resin and hardener).

The selected epoxy system was Araldite LY 8615 (resin) with the Aradur 8615 (hardener), obtainable in the market by Huntsman Advanced Material.

Infusion was realized using two different techniques.

The first one is called "vacuum bagging". The fiber mats are placed on a rigid Teflon plate. With sealant tape and transparent nylon foil a vacuum bag is made and an inlet and outlet tube are added. The outlet tube is used to connect a vacuum pump.

The purpose of the inlet tube is to guide the epoxy system into the bag. Both tubes are reinforced to prevent collapse caused by vacuum. The epoxy system is prepared. 2:1 mass fraction of the resin and the hardener. The two components are mixed by stirring for at least 2 minutes to ensure a homogeneous mixture. This mixture is then placed in a vacuum chamber to degas the air bubbles, which might be present in the epoxy. The vacuum pump is then switched on and the inlet closed to create vacuum inside the bag.

The inlet is then slowly opened to allow the epoxy system to enter the bag and infuse the fabrics.

When a sufficient amount of resin has entered the bag the inlet is closed while letting the outlet open. This causes the epoxy system to be sucked through the sample and the air bubbles trapped inside the sample to be evacuated. When the epoxy system has infused the entire stack of fabrics and the air bubbles have left the bag, the composite sample is ready for curing.

This occurred for 8 hours at 100 °C , while remaining under vacuum.

The second technique used to infuse the fabrics is called "pressurized infusion".

The setup is similar as the one for "vacuum bagging", but instead of applying pressure by vacuum pumping, pressure was now applied by applying a weight on top of the fabrics. The weight was distributed over the sample using a Teflon plate. The pressure on the samples is estimated to be around 0.4 bar. This is less than for vacuum bagging, but allowed to infuse more than 10 composite samples simultaneously. This is not possible using vacuum bagging, since the pump is not powerful enough to infuse several composite samples at the same time.

The produced composite samples were cured for 3 hours at approximately 80 °C. Step h

All produced composite samples received a final curing of 3 hours at 180 °C to ensure a full curing and complete the curing process. The samples were then cut into suitable dimensions for the mechanical tests. The produced composite plates exhibited a thickness ranging from 2 to 6 mm.

EXAMPLES AND CHARACTERIZATION

When the composite samples were ready, they could be submitted to mechanical tests to investigate the influence of CNT growth on the mechanical properties on the composites. 3-point bending tests were performed on the composite samples to measure the interlaminar shear strength (ILSS) and the flexural modulus, according to ISO standards (ISO norm 14130/97). For reference composite specimens, whereby the CNT have been grown conventionally according to the process parameters of Table 2 (no Al₂0₃ layer) and the "free growth" conditions were used, 6 plies of carbon fiber fabrics were necessary to reach the required values of ILSS between 20 and 25 MPa and the required values of the flexural modulus between 1 .8 and 3 GPa.

For inventive composite specimens, whereby the CNT have been grown according to the process parameters of Table 1 (with Al₂0₃ layer) and the "free growth" conditions were used, only 4 plies of carbon fiber fabrics were necessary to reach the required values of ILSS between 20 and 25 MPa.

For inventive composite specimens, whereby the CNT have been grown according to the process parameters of Table 1 (with Al₂0₃ layer) and the "confined growth" conditions were used, only 2 plies of carbon fiber fabrics were necessary to reach the required values of ILSS between 20 and 25 MPa.

Thanks to the process to produce composites according to the invention, composite materials with satisfactory mechanical properties can be surprisingly obtained using less plies of carbon fiber fabrics, less fiber volume fraction (FVF) and less carbon fibers.

These advantages become unexpectedly more dramatic, if the CNT are grown using the method of the "confined growth", whereby the plies of fiber fabrics are sandwiched between two plates fixed at a predetermined distance, so that no vertical expansion of the fabrics is possible during the growth of the carbon nanotubes.

The best results have been obtained using "confined growth". Similar ILSS values and flexural moduli were obtained as for the reference samples using only 33% of the fabrics. This allows to save up to 67% of the carbon fabrics, while preserving the composite's properties. Confining the fabrics between two metal plates prevented the swelling of the composite and promoted the creation of interfiber and interply links.

Claims

1. A process to grow carbon nanotubes onto carbon, glass or metal fibers comprising steps in the following sequence: a) Providing carbon, glass or metal fibers;

b) Depositing onto the carbon, glass or metal fibers a layer of aluminum

oxide with a thickness lower than 150 nm, for example between 1 and 100 nm;

c) Depositing onto the carbon, glass or metal fibers a layer comprising an iron catalyst; and

d) Growing carbon nanotubes onto the carbon, glass or metal fiber,

preferably by chemical vapor deposition (CVD).

2. Process according to claim 1 , whereby step a) is carried out by providing carbon fibers, 0.005-0.080 mm in diameter, whereby carbon fibers are twisted together to form a yarn, which is woven into a carbon fabric.

3. Process according to claim 2, whereby step a) comprises the step of placing carbon fabrics one over the other, so as to create a stack of fabrics comprising at least two fabrics, and the fabrics are sandwiched between two plates fixed at a fixed distance, so that no vertical expansion of the fabrics is possible during the growth of the carbon nanotubes.

4. Process according to any of the preceding claims,

whereby step b) is carried out with the process of Atomic Layer Deposition (ALD) or chemical vapour deposition (CVD) and the produced aluminum oxide layer exhibits over the whole surface of the fibers an homogeneous und uniform thickness up to 100 nm, for example between 1 and 80 nm.

5. Process according to any of the preceding claims,

whereby step b) comprises the step of depositing homogeneous single atomic layers of Al₂0₃ up to a thickness of 50 nm, for example between 1 and 20 nm. Process according to any of the preceding claims,

whereby step c) is carried out by dipping the fibers in an iron containing solution, for example an iron nitrate Fe(N0₃)₃ solution.

Process according to any of the preceding claims,

whereby step d) comprises the steps of: heating the fibers to a temperature between 500 and 1000 °C, preferably between 650 and 850 °C, to activate the deposited catalysts;

placing the fibers in a carbon feedstock gas;

optionally, adding hydrogen to the carbon feedstock gas, in order to improve the produced carbon nanotubes quality.

Process according to claim 7,

whereby the carbon feedstock gas is selected from the group consisting of acetylene, ethylene, methane, butane and propane.

9. Process according to any of the preceding claims,

whereby the deposited carbon nanotubes are perpendicular to the axis of the fibers onto which they have been deposited, without any preferential growth direction, and exhibit a growth length of at least 5 μηη, preferably between 10 and 100 μηη.

Fibers or carbon fibers with carbon nanotubes grown onto their surfaces, produced with a process according to any of the claims 1 to 9.

Fabrics made with fibers with carbon nanotubes grown on the surfaces of the fibers produced with a process according to any of the claims 1 to 9, or fabrics made with fibers according to claim 10.

Process to produce composite materials comprising the following steps:

• Providing fibers according to claim 10 or produced with the process according to any of the claims 1 to 9;

• Infiltrating or infusing the fibers with curable liquid resins;

• Curing with heat or UV radiation or electron-beam radiation or

microwave radiation or by electromagnetic induction the fibers infiltrated with resin, so as to produce a solid composite material.

13. Process to produce composite materials comprising the following steps: e) Providing carbon fabrics according to claim 1 1 ;

14. Composite material produced with a process according to any of the claims 12 to 13.