Carbon nanotubes synthesis. Ing. Eva Košťáková KNT, FT, TUL

Transcription

1 Carbon nanotubes synthesis Ing. Eva Košťáková KNT, FT, TUL

2 Basic parameters: -Temperature (500, 1000 C ) -Pressure (normal, vacuum ) -Gas (ambient, inert atmosphere nitrogen, argon ) -Time (duration, time of delivery of carbon, the time maintaining stable conditions ) -Voltage, current (if it is a principle requiring such conditions) - Basic Input-carbon material (carbon source) - - The type and properties of the catalyst CNTs synthesis Not all manufacturing principles CNTs require all of these parameters. However, if the parameter in the technology used, it is fundamental to the achievement of certain structures and properties of nanotubes.

3 CNTs synthesis Carbon source basic material solid GRAPHITE AMORFOUS CARBON Amorphous carbon is a type ulíkového material in a non-crystalline, irregular form. It exists in the form of powder, and is a major constituent of substances such as charcoal, lamp black (carbon black) and charcoal. MOLECULAR PRECURSORS (gaseous or liquid hydrocarbons)

4 CNTs synthesis CATALYST For production of nanotubes is either added or not. Catalyst (Greek καταλύτης katalýtis) is a substance entering into a chemical reaction, it accelerates (or decelerates) while extending therefrom unchanged. Catalysts are either - permanently fixed in the solid substrate - "Floating" catalysts - fluidized, the melt - are impressed together with the gas phase within the production area

5 CNTs synthesis CATALYST For production of nanotubes is either added or not. Catalysts are most transition metals. It's actually the middle of the extended Mendeleyev's table - the rows from scandium to zinc, cadmium from the yttrium and lanthanum from the mercury - special group is then lanthanides and actinides. Such as Fe, Co, Ni, or alloys of metals such as Fe / Mo, Co / Mo, wherein one element acts as a catalyst and the second stabilizer having a support function. Metal acts as a dehydrogenation agent - hydrogen leaves, carbon is retained on the catalyst surface - metal + carbon-carbide formation. After raising concentration atoms on the surface of the catalyst occurs, and subsequently forming the cap tube.

6 CNTs synthesis CATALYST For production of nanotubes is either added or not. Size of the catalyst particles - best nanoparticles (average) indicates roughly the size (diameter) generated CNT, although in some instances may rise to several tubes from a single catalyst particle.

7 CNTs synthesis Type of catalyst - its morphology influences the structure of the emerging object.

8 The mechanism of nanotube growth Generally THAT, First is arranged (on the surface of the catalyst particles - is present in the production) cap, which acts as a seed (nucleus) of carbon nanotubes and then the tube is extended until the retained conditions for the growth of the tube. SWNT nucleation on the surface of the catalyst particle (iron)

9 The mechanism of nanotube growth There are two mechanisms for the growth of the nanotubes relative to the position of the catalyst particles: Tip-growth (catalyst inches with the top tube) Base-growth (catalyst remains firmly anchored in the substrate)

10 The mechanism of nanotube growth Mechanismus růstu base-growth MWNT growth from a FeCo crystal. The image sequence shows the growth of a multiwalled CNT from a FeCo crystal inside a larger host nanotube under electron irradiation at a specimen temperature of 600. base-growth

11 base-growth The mechanism of nanotube growth

12 Mechanismus růstu ze špičky Plovoucí proces Mechanismus růstu tip-growth The mechanism of nanotube growth tip-growth floating process

13 The mechanism of nanotube growth Pevný katalyzátor Tavenina Klastry seskupení molekul do nějakého celku s malými mezimolekulárními silami

14 The mechanism of nanotube growth After completion of growth of nanotubes occurs mostly to remove catalysts Metals other than the thermal expansion of the materials = C = cooling, separation of the catalyst from the tubes!

15 The main aim of CNTs production Produce large quantities of CNTs with high purity (purity), with uniform order (alignment), uniformity of properties of the produced nanotubes, and all at a low price == CNTs are then sold on the market. SEM sequence of nanotubes alignment obtained in plasma-cvd set-up for different growth time h/nanotubes/production.xml

16 PROBLEMS There are still a number of areas that are not yet clarified are still just the subject of research:?? How to ensure the growth of CNTs without surface defects on an industrial scale??? How to ensure the production of single CNTs = net??? How to ensure precise control of chirality in the production of CNTs?

17 Výroba CNTs ENERGY needed for the growth of carbon nanotubes is typically supplied by heating the precursor or catalyst. The best known manufacturing principles are the following three: ARC DISCHARGE (in an inert gas atmosphere, in water) Laser ablation - lapped LASER CVD (Chemical Vapor Deposition)

18 Electric discharge electric arc The strong electric field causes electrons uprooting of atoms and molecules of gas (gas ionization). Electric current for this condition is known as electrical discharge and consists of a mixture of free electrons and positive, eventually. negative ions in the gas. Electric discharge lasts a short time - until the discharge of the external electric field. Electric current of gas at high temperature is called the arc The oldest technique of CNTs Iijima discovered CNTs coating on the cathode used in the electric arc.

19 Electric discharge electric arc The arc is generated between two electrodes under the following conditions: voltage: 20-30V Current: A Pure or doped graphite electrode The distance between the surfaces of the electrodes 1-3 mm The inert atmosphere (He, Ar) - pressure in the processing chamber is controlled during the process of manufacturing a vacuum to prevent oxidation of the materials produced Discharge Time: 10-60s

20 Electric discharge electric arc The arrangement of electrodes: homo-electrode (cathode and anode are made of carbon) hetero-electrode (carbon is the cathode and the anode is a metal (e.g., molybdenum) Ukládání trubic Carbon electrodes can be pure or doped catalysts (cobalt, nickel, etc). CATALYSTS increases the quality and quantity MADE nanotubes.

21 J. Applied Physics, Vol.92, No.5, september 2002

22 LASER ABLATION Process: Inert gas laser beam "shooting" on the carbon target Carbon vapor is carried produkovýny and flow of inert gas to the water cooled collector metal (typically Al or Cu) Nanostructures are placed on the surface of the collector This technology is actually enhancement technology using an electric arc. A typical setup: The quartz tube furnace. The tube is sealed and connected to a vacuum system and inert gas reservoir. The laser beam enters into the quartz tube through a special window. Carbon target (target) is located in the center of quartz tube and is rotated in the direction of the laser beam. At the other end of the tube is a water cooled metal collector.

23 LASER ABLATION

24 CVD Chemical vapor deposition The process of growth of CNTs involves heating the catalyst (usually located on the substrate) at high temperature in a tubular furnace and blowing a hydrocarbon gas through the tube after a certain period. Key parameters for CVD nanotube production are: hydrocarbons (type and flow rate) Catalyst and substrate The temperature in the furnace

25 CVD Chemical vapor deposition substrate It must withstand the reaction temperatures. Typically, these are metal oxides (Al2O3, SiO2, TiO2... or quartz.... But also metals, minerals, carbon fibers. The substrate is supplied with a catalyst or catalyst particles are no longer part of the substrate! The catalyst is: - Firmly fixed in solid - Floating (molten, fluidized) - Gaseous (allowed to enter the furnace together with the gas atmosphere or a source of carbon)

26 CVD Chemical vapor deposition

27 CVD Chemical vapor deposition The catalyst particles can remain rooted to the substrate during the growth of CNT (base-growth) or can be lifted from the substrate and stop at the growing tip of the tube (tip-growth). In both cases, the carbon is added to the side of the catalyst. Type growth tubes is given by the surface properties of the catalyst-substrate and the forces acting on the catalyst surface.

28 CVD Chemical vapor deposition This is an image of a carbon nanotube structure (or "architechure") grown by chemical vapor deposition on a silicon substrate, by John Hart, a post-doctoral associate at MIT. Architectures are formed by selforganization of carbon nanotubes as they grow upward from a silicon substrate and a catalyst layer. If the catalyst is uniformly distributed, nanotubes grow everywhere on the substrate. How the nanotubes organize is defined by how they "push" and "pull" each other to produce the architectures. If the catalyst is only located in certain areas (patterned), then nanotubes grow only in those areas. In this image, the catalyst is patterned by photolithography, where a light-sensitive polymer is used to specify where the catalyst is placed. Each structure consists of thousands to millions of parallel nanotubes (the density of nanotubes growing from a substrate is about 20 billion per square centimeter). The larger towers in "metropolis" are 200 micrometers wide, which is approximately the width of two human hairs. The image was taken using a scanning electron microscope.

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