WO2004078649A1

WO2004078649A1 - Synthesis of carbon nanotubes and / or nanofibres on a porous fibrous matrix

Info

Publication number: WO2004078649A1
Application number: PCT/GB2004/000866
Authority: WO
Inventors: Bojan Obrad Boskovic
Original assignee: The Morgan Crucible Company Plc
Priority date: 2003-03-03
Filing date: 2004-03-02
Publication date: 2004-09-16
Also published as: GB2399092B; GB0304826D0; GB2399092A

Abstract

A method of producing nanotubes and/or nanofibres by the catalytic decomposition of a gas feedstock on a catalyst is disclosed, in which the catalyst is impregnated and dispersed within a porous fibrous matrix.

Description

SYNTHESIS OF CARBON NANOTUBES AND/OR NANOFIBRES ON A POROUS FIBROUS MATRIX

This invention relates to the production of nanotubes and/or nano fibres by the decomposition of gases on a substrate. The invention further relates to the nanotubes and/or nanofibres produced, and to novel composite materials comprising the nanotubes and/or nanofibres with the substrate.

The carbon nanotubes (CNTs), originally reported by Iijima^[1] in 1991, were synthesized in a carbon arc-discharge. Since then, other authors have reported the growth of CNTs from an arc-discharge ^[2'³-¹ and other methods have been developed to synthesize nanotubes. CNTs have also been produced by vaporization processes using lasers'-⁴' ^5], electron beams¹⁶-¹ and solar energy ^[7]. Catalytic pyrolysis and chemical vapour deposition of hydrocarbons'-⁸' ⁹¹ are now widely used for carbon nanotube growth as simple and efficient methods. In addition to CNTs, similar methods have been used for the synthesis of carbon nanofibres (CNFs), also known as carbon filaments since the early 1950's^[10]. CNFs can be grown using catalytic decomposition of hydrocarbons over transition metal particles such as iron, cobalt, nickel, and their alloys at temperatures ranging from 500 to 1000 °C ^[I1]. A microwave plasma enhanced chemical vapour deposition (PECVD) process, used for the preparation of diamond and diamond-like carbon films, has been recently developed successfully for the growth of CNTs and CNFs^[12"17l Recently the first evidence of carbon nanofibres growth at room temperature using radio frequency PECVD (r.f. PECVD)¹¹⁸-¹ have been published. Nanotubes and nanofibres need not be of carbon alone and various other elements (e.g. boron) have been incorporated into nanotubes and nanofibres.

The most efficient of the known methods give growth of CNT and CNF using fine transition metal catalyst particles. Efficiency of the synthesis method is determined by the efficiency of the process of formation of small metal catalyst domains acting as nucleation seeds for CNT or CNF growth.

It is known that the metal catalysts can be deposited on a substrate. For example, WO01/85612 is directed to a method in which the catalyst is deposited on a porous carbon substrate, which is then electrically heated while feedstock gases are passed over the substrate. WO01/85612 discloses a number of processes for depositing the catalyst onto a substrate including:-

1) Producing a metal silicate gel, soaking carbon paper in the gel, and drying the paper to produce a carbon paper with a silicate containing iron catalyst particles deposited on the paper.

2) A suspension of fine metal particles is sprayed onto the carbon paper.

3) The fine catalyst particles are produced in a hollow cathode discharge apparatus 4) The method 3) used in conjunction with a plasma to prevent coalescence of the particles as they are discharged from the cathode. All of these methods result in the catalyst particles being distributed on the surface of the carbon paper.

The inventor has found that when the catalyst is impregnated and dispersed within a fibrous matrix, rather than being left on the surface, a more efficient deposition of nanofibres and/or nanotubes results. Dispersion of the catalyst throughout the fibrous . matrix appears to make the catalyst more active than when simply applied to the surface of a substrate. The inventors hypothesise, without wishing to be bound by this hypothesis, that dispersion within a fibrous matrix prevents agglomeration of the fine metal catalyst particles and so leads to a greater effective amount of catalyst being present. Alternatively, it may be that growth within a fibrous matrix present particularly good diffusion conditions for the feedstock gases.

The scope of the invention is as set out in the claims in the light of the following illustrative description with reference to the figures in which:-

Figs. la and lb are micrographs of a carbon cloth substrate Fig. 2 is a micrograph of a ceramic paper substrate Fig. 3 is a micrograph showing nano fibre and nano tube growth within a carbon substrate

Figs. 4a and 4b are micrographs showing nanofibre and nanotube growth within the carbon cloth substrate of Figs, la and lb. Fig 5. is a micrograph showing nano fibre and nanotube growth within the carbon cloth substrate of Figs, la and lb.

Figs. 6a and 6b. are micrographs showing nanofibre and/or nanotube growth inside the ceramic paper substrate of Fig. 2

In the following description reference is made throughout to carbon nanofibres and carbon nanotubes. The same principles can be applied to doped nanofibres and nanotubes (e.g. nanofibres or nanotubes containing boron).

Carbon and ceramic cloth and paper matrix were used to demonstrate the efficient CNT growth using a thermal CVD method.

Catalyst and substrate preparation

Fine iron powder catalyst (6-8 μm in diameter) obtained from Goodfellow Ltd, Cambridge, UK was firstly dispersed in isopropanol (IP A) using an ultrasonic bath for 20-30 min.

Then 2.5 mm thick VCL N carbon cloth, obtained from Morgan Speciality Graphite, Fostoria, OH, USA (Fig. 1) with pore size greater than approximately 50 x 50 μ ; or 3 mm thick ceramic paper with pore sizes greater then approximately 10 x 10 μm, obtained from Isofrax (Fig. 2) were soaked in the suspension and left in the ultrasonic bath for 30 min.

The samples were then dried producing a fibrous matrix with an impregnated finely dispersed metal powder.

Nanofibre/nanotube production

A tube furnace with controlled atmosphere was used.

In order to evacuate the air Ar was introduced into the furnace with 300 seem flow rate for 25 min. To reduce the impregnated metal powder catalyst hydrogen was next introduced into the furnace with 200 seem flow for 120 min at 450 °C. Carbon nanotubes and nanofibres were grown using ethylene (800 seem) and hydrogen (200sccm) mixture at 650 °C for 2 hours. After that the furnace was cooled down in Ar flow (200 seem).

Synthesised carbon nanotubes and nanofibres have variation of diameters from approximately 10 rtm to 150 nm and length of few microns (Fig. 3). According to the SEM examination it was observed that almost all iron powder was transformed into the seeds for carbon nanotubes and nanofibres growth. The nanotubes/nano fibres are produced in clumps originating from the surface of the catalyst particles. Catalyst particles were observed on the tip of CNF indicating the tip growth model'-¹¹-'. Examples of carbon nanotubes successfully grown inside carbon cloth are illustrated in Figs. 4 and 5, and inside ceramic paper in Fig. 6.

The amount of fibre produced could be controlled using variation of catalyst loading. Very high density of CNT and CNF growth inside carbon cloth was illustrated in Fig. 4, which used O.Olg of Fe powder 6-8 microns in diameter in a 2 x 2cm sample of carbon cloth. (2.5mg/cm²).

When 0.1 g of fine iron powder was dispersed in 20 x 20 cm carbon cloth (0.25mg/cm²), 2.1 g of carbon nanotubes/nanofibres were produced (Fig.5) a yield of -2000%.

(Carbon nanotube yield was calculated using the formula CNT yield

where mem is the mass of carbon nanotubes in the final product, and ITI_CAT is the mass of the catalyst inside the fibre matrix after impregnation. Similar yield calculations are used in the literature'-²⁰-'). This corresponded to about 14% by weight nanotubes/nanofibres in the cloth based upon the weight of cloth plus nanotoubes/nanofibre. The best yield the inventor has heard rumour of is -600%, although the best he is aware of in the written literature is -120% ^[19].

Efficient CNT growth was achieved, although density of CNT growth for a small catalyst loading (Fig. 5) was not very high comparing with the example illustrated in Fig. 4. Even with the smaller loading (Fig. 5) tensile strength of the cloth with CNT was increased by approximately 10%, thermal conductivity by 18% and thermal diffusivity by 11%. It is expected that improvement of mechanical, electrical and thermal properties would be higher with higher CNT growth density.

Carbon nanotubes growth could however also be achieved inside any thermally sensitive fibre matrix (e.g polymer or organic fibre matrix) if r.f. PECVD method is used for CNF growth as described in ref. 18. Carbon nanotubes or nanofibres grown inside the fibre matrix could be extracted using high gas flow, or if the substrate fibre is suitable, by. dissolution, reaction, melting, vaporisation, or otherwise removal of the fibre matrix.

Composite materials with carbon nanotubes and/or nanofibres grown inside a fibre matrix could be used as filter materials, where filter pore size is controlled by the density of the carbon nanotubes/nanofibres grown inside the fibre matrix. Very dense material, similar to that shown in Fig. 4a, could be used for bio-hazard filters where small pore size is of extreme importance. Very dense materials with a lot of carbon nanotubes are expected to also have extremely good mechanical properties which can be used as a reinforcement and even bullet-proof materials.

The high thermal conductivity of these materials may be of use in automotive and aerospace applications and for heat distribution or hot spot control. The high electrical conductivity of these materials could be used for example in electronic components packaging, as gas diffusion layers in fuel cells, as electromagnetic shielding, as oxygen sensors (the resistivity of carbon nanotubes has been shown to vary with oxygen concentration'-²¹-'). This method could be further developed for production of carbon nanotube based sensing devices inside the fabric. This fabric can then be used as a layer inside smart, bio-hazard and bullet-proof uniforms.

A composite cloth comprising a porous or fibrous matrix filled with nanofibres and/or nanotubes can be used in the production of rigid composite articles, e.g. by layering and impregnating with polymers to form a continuous polymeric phase filling the porosity of the porous material. Methods of forming such polymer-cloth composites are well known in relation to carbon fibre and glass fibre composite materials.

References

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Claims

1. A method of producing nanotubes and/or nanofibres by the catalytic decomposition of a gas feedstock on a catalyst, in which the catalyst is

5 impregnated and dispersed within a porous fibrous matrix.

2. A method, as claimed in Claim 1, in which the catalyst is formed within the porous fibrous matrix by decomposition of a catalyst precursor.

10 3. A method, as claimed in Claim 1 or Claim 2, in which:- a) the catalyst or catalyst precursor is prepared as a suspension or solution b) the porous fibrous matrix is placed in the suspension or solution c) the porous fibrous matrix and suspension are agitated ultrasonically to impregnate and disperse the catalyst or catalyst precursor within the

15 fibrous matrix.

4. A method, as claimed in any one of Claims 1 to 3, in which the porous fibrous matrix is a carbon based material.

20 5. A method, as claimed in any one of Claims 1 to 3, in which the porous fibrous matrix is a ceramic based material.

6. A method, as claimed in any one of Claims 1 to 3, in which the porous fibrous matrix is a polymeric material.

25.

7. A method, as claimed in any one of Claims 1 to 6, in which the nanofibres and/or nanotubes are removed from the porous fibrous matrix.

8. A method, as claimed in Claim 7, in which the porous fibrous matrix is 30 dissolved, reacted, melted, vaporised, or otherwise removed to leave the nanotubes or nanofibres.

9. A composite material comprising a porous fibrous matrix which is a fibrous matrix and nanotubes/nanofibres grown within the porosity of the porous fibrous matrix.

10. A composite material, as claimed in Claim 9, comprising a porous fibrous matrix and >10% by weight nanofibres and/or nanotubes based on the weight of porous fibrous matrix, nanofibres, and nanotubes.

11. A composite material, as claimed in Claim 9 or Claim 10, in which the porous fibrous matrix is a carbon based material.

12. A composite material, as claimed in Claim 9 or Claim 10, in which the porous fibrous matrix is a ceramic based material.

13. A composite material, as claimed in Claim 9 or Claim 10, in which the porous fibrous matrix is a polymeric material.

14. A composite material, as claimed in any one of Claims 9 to 13, additionally comprising a continuous polymeric phase.

15. A cloth comprising the composite material of any of Claims 9 to 14.

16. A filter comprising the composite material of any of Claims 9 to 14.

17. A fabric reinforced with the composite material of any of Claims 9 to 14.

18. A heat spreader comprising the composite material of any of Claims 9 to 14.

19. Packaging for electrical components comprising the composite material of any of Claims 9 to 14.

20. A gas diffusion layer for a fuel cell, comprising the composite material of any of Claims 9 to 14.

1. An electromagnetic shield comprising the composite material of any of Claims 9 to 14.