Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
2002-01-02
2004-09-07
Beck, Shrive P. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C427S573000, C427S575000, C427S577000, C427S249100, C423S44500R, C423S447300, C118S7230MA
Reexamination Certificate
active
06787200
ABSTRACT:
DISCUSSION OF THE BACKGROUND
FIELD OF THE INVENTION
The present invention concerns a process and a device for depositing, by electron cyclotron resonance plasma, films of carbon nanofibre webs.
In addition, the invention concerns the films of web obtained in this manner.
The technical field of the application may be defined, in a general manner, as that of depositing films of carbon on a substrate.
Such films are, in particular, films of carbon that emit electrons, but we have also sought to develop processes whose purpose is to synthesise films of diamond, and profitably employ the mechanical, optical and electrical properties of diamond at temperatures generally between 400° C. and 1,000° C., or in order to make DLC (“Diamond Like Carbon”) type carbon films, generally at low temperature (20° C. to 400° C.) and with a high level of C—C sp3 bonding, in particular for their mechanical properties.
Such films are principally amorphous.
More precisely, the present application is particularly concerned with the preparation of carbon films formed of nanotubes or nanofibres.
BACKGROUND OF THE INVENTION
Table
1
, at the end of the description, shows different devices and processes for depositing, under vacuum, carbon films, used mainly for depositing emissive carbon.
This table highlights two different categories of deposition processes.
The first category involves CVD processes (“Chemical Vapour Deposition”) in which a gas of organic molecules (often methane) is introduced as a mixture, with or without hydrogen, into a device that enables the C—C, C-H and H—H bonds to be broken by electron impact with, for example, the use of a hot filament, the introduction of microwave power, the use of a radio frequency (RF) polarisation or the use of an electron cyclotron resonance (ECR).
Depending on the device used, the operating pressure is either high (filament, microwave, radio frequency) or low (ECR, RF). The result is a dissociation and an ionisation of the particles, which increases as the pressure decreases. The energy that needs to be supplied for the reaction that transforms the gas into a solid is considerably decreased by the breaking of the covalent bonds (for example CH
4
) of organic molecules.
It is thus possible to obtain graphite or diamond type crystallised structures at lower substrate temperatures (for example, 400° C. instead of 800° C.). The polarisation of the substrate also makes it possible to favour crystallisation at lower temperature, thus enabling the use of a wider variety of substrates.
The second category of deposition processes groups together the processes, called PVD processes (Physical Vapour Deposition), which involve the direct deposition of carbon atoms or ions, which may be achieved by spraying a graphite target by arc, by laser ablation, by a beam of ions or by evaporation.
The quality and the structure of the films, at a given temperature, mainly depend on the energy of the incident carbon ions or atoms.
In the case of the preparation of carbon nanotube or nanofibre films, which is of particular interest to us, within the scope of the present application, the PVD and CVD processes described above are also used.
Document (18) “High yield of single-wall carbon nanotubes by arc discharge using Rh-Pt mixed catalysts”, Y, Saito, Y. Tani, N. Miyagawa, K. Mitsushima, A. Kabuya, Y. Nishina (Chemical Physics Letters 294 (1998), Pages 593-598) and Document (19) “Helical microtubes of graphitic carbon” S. Iijima (Nature, vol. 354,6 Nov. 1991) pages 56 and 58) concern processes for producing carbon nanotubes by PVD processes, with a direct supply of C° carbon atoms by laser ablation or electric arc. Document (18) describes, more precisely, a process for preparing carbon nanotubes using evaporation by arc between two graphite electrodes in helium, at high pressure (50-1 520 torrs). Binary mixtures of metals from the platinum group, such as rhodium and platinum, are used as catalysts.
The object of document (19) is the synthesis of tube type carbon structures in the form of pins by evaporation by arc discharging with a carbon electrode, within an enclosure filled with argon at 100 torrs.
The nanotubes or nanofibres can also be prepared by CVD processes, via catalytic de-hydrogenation of organic molecules such as acetylene or methane.
The device used may be make use of hot filaments, a radio frequency system or the injection of microwaves at high pressure, which generates atomic hydrogen and radicals or ions, such as CH
3
+
, CH
3
0
, CH
0
, etc.
However, it should be pointed out that forming truly organised architectures of carbon nanofibres or nanotubes and not random and unorganised deposits has been explored up to now.
Document (15) “Large-Scale Synthesis of Aligned Carbon Nanotubes”, W. Z. Li, S. S. Xie, L. X. Qian, B. H. Chang, B. S. Zou, W. Y. Zhou, R. A. Zhao, G. Wang (Science, vol. 274-6. December 1996, pages 1 701-1703) describes the synthesis of aligned carbon nanotubes using a process based on PECVD (Plasma Enhanced Chemical Vapour Deposition) of carbon from the decomposition of acetylene from a gaseous mixture of acetylene and nitrogen, with the deposition being catalysed by microparticles of iron imprisoned within the porous silica that forms the substrate.
Images obtained by scanning electron microscope show that the nanotubes are markedly perpendicular to the surface of the silica and form rows of tubes separated from each other by around 50 micrometers length and spaces of around 100 manometers.
Document (16) “Growth of Highly oriented Carbon nanotubes by plasma-enhanced hot filament chemical vapour deposition, Z. P. Huang, J. W. Xu, Z. F. Ren, J. H. Wang, M. P. Siegal, P. N. Provencio (Applied Physics Letters, vol. 73, number 26,28 December 1998 pages 3845-3847) also describes the growth of orientated carbon nanotubes on monocrystalline and polycrystalline nickel substrates by the PECVD process, by using a hot filament. The carbon nanotubes have diameters of 10 to 500 nm and a length of 0.1 to 50 micrometers. Acetylene is used as the carbon source and ammonia is used as the diluting gas and for the catalysis.
“Electron Field emission from phase pure nanotube films grown in a methane/hydroen plasma, O. M. Kuttel, O. Groening, Ch. Emmenegger, L. Schlapbuch (Applied Physics Letters, vol. 73, number 15,12 October 1998, pages 2 113-2 115) concerns the growth of films of carbon nanotubes on silicon substrates by CVD, from a mixture of methane and hydrogen, using a microwave plasma at a substrate temperature of 900° C. to 1 000° C. Iron or nickel is deposited, beforehand, on the substrate in order to act as a catalytic seed for growing the nanotubes.
None of the processes described above allow organised architectures of carbon nanofibres or nanotubes to be made with strong bonds between the tubes in order to form a spider's web (2D structure).
SUMMARY OF THE INVENTION
We have seen that the alignment of the nanofibres or nanotubes could certainly be obtained (15) (16), but that, unless particular precautions are taken, the carbon nanotubes often develop (17) in a random unorganised manner, in the form of a jumble of filaments or spike structures without C—C bonds between the tubes (1D structure).
Although attempts, aiming to develop interconnections, have been carried out (18) by adding nanograins of catalyst, one again obtains, in this case, a disordered and random structure without strong C—C bonds between the tubes.
In addition, none of the processes described above allow films of nanotubes to be prepared and, moreover, organised architectures of carbon nanofibres or nanotubes, such as webs of nanofibres or nanotubes directly from organic molecules and without a catalyst.
Finally none of the processes allows the deposition of nanofibres or nanotubes over a large surface, in other words generally greater than or equal to 1 m
2
.
There is therefore a need for a process for depositing webs of carbon nanofibres or nanotubes, not requiring a catalyst, which allows the deposition of such nano-architectures over large areas at a relatively low temper
Delaunay Marc
Semeria Marie-Noëlle
Beck Shrive P.
Commissariat a l'Energie Atomique
Markham Wesley D
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