Metal nanotubes. TUBALL - revolutionary carbon nanotubes for the tire industry

Fullerenes and carbon nanotubes. Properties and application

In 1985 Robert Curl, Harold Croteau And Richard Smalley completely unexpectedly discovered a fundamentally new carbon compound - fullerene , the unique properties of which have caused a flurry of research. In 1996, the discoverers of fullerenes were awarded the Nobel Prize.

The basis of the fullerene molecule is carbon- this unique chemical element, distinguished by its ability to combine with most elements and form molecules of the most varied composition and structure. From your school chemistry course, you, of course, know that carbon has two main allotropic states-graphite and diamond. So, with the discovery of fullerene, we can say that carbon acquired another allotropic state.

First, let's look at the structures of the molecules of graphite, diamond and fullerene.

Graphitehas layered structure (Fig.8) . Each layer consists of carbon atoms covalently bonded to each other in regular hexagons.

Rice. 8. Graphite structure

Adjacent layers are held together by weak van der Waals forces. Therefore, they slide easily over each other. An example of this would be a simple pencil - when you drag a graphite rod over paper, the layers gradually “peel off” from each other, leaving a mark on it.

Diamondhas three dimensions tetrahedral structure (Fig. 9). Each carbon atom is covalently bonded to four others. All atoms in the crystal lattice are located at the same distance (154 nm) from each other. Each of them is connected to the others by a direct covalent bond and forms in the crystal, no matter what size it is, one giant macromolecule

Rice. 9. Diamond structure

Due to the high energy of C-C covalent bonds, diamond has the highest strength and is used not only as a precious stone, but also as a raw material for the manufacture of metal-cutting and grinding tools (readers may have heard about diamond processing of various metals)

Fullerenesgot their name in honor of the architect Buckminster Fuller, who invented similar structures for use in architectural construction (therefore they are also called buckyballs). Fullerene has a frame structure that is very reminiscent of a soccer ball, consisting of “patches” of 5- and 6-gonal shapes. If we imagine that there are carbon atoms at the vertices of this polyhedron, then we get the most stable fullerene C60. (Fig. 10)

Rice. 10. Structure of fullerene C 60

In the C60 molecule, which is the best known and also the most symmetrical representative of the fullerene family, the number of hexagons is 20. Moreover, each pentagon borders only on hexagons, and each hexagon has three common sides with hexagons and three with pentagons.

The structure of the fullerene molecule is interesting in that inside such a carbon “ball” a cavity is formed, into which, thanks to capillary properties atoms and molecules of other substances can be introduced, which makes it possible, for example, to transport them safely.

As fullerenes were studied, their molecules were synthesized and studied, containing different numbers of carbon atoms - from 36 to 540. (Fig. 11)

a B C)

Rice. 11. Structure of fullerenes a) 36, b) 96, c) 540

However, the diversity of carbon frame structures does not end there. In 1991, a Japanese professor Sumio Iijima discovered long carbon cylinders called nanotubes .

Nanotube is a molecule of more than a million carbon atoms, which is a tube with a diameter of about a nanometer and a length of several tens of microns . In the walls of the tube, carbon atoms are located at the vertices of regular hexagons.

Rice. 13 Structure of a carbon nanotube.

a) general view of the nanotube

b) nanotube torn at one end

The structure of nanotubes can be imagined this way: we take a graphite plane, cut a strip out of it and “glue” it into a cylinder (in reality, of course, nanotubes grow in a completely different way). It would seem that it could be simpler - you take a graphite plane and roll it into a cylinder! – however, before the experimental discovery of nanotubes, none of the theorists predicted them. So scientists could only study them and be surprised.

And there was something to be surprised about - after all, these amazing nanotubes weigh 100 thousand.

times thinner than a human hair turned out to be an extremely durable material. Nanotubes are 50-100 times stronger than steel and have six times less density! Young's modulus – The level of resistance of the material to deformation is twice as high for nanotubes as for conventional carbon fibers. That is, the tubes are not only strong, but also flexible, and their behavior resembles not brittle straws, but hard rubber tubes. Under the influence of mechanical stresses exceeding critical ones, nanotubes behave quite extravagantly: they do not “tear”, do not “break”, but simply rearrange themselves!

Currently, the maximum length of nanotubes is tens and hundreds of microns - which, of course, is very large on an atomic scale, but too short for everyday use. However, the length of the resulting nanotubes is gradually increasing - now scientists have already come close to the centimeter mark. Multiwalled nanotubes 4 mm long were obtained.

Nanotubes come in a variety of shapes: single-walled and multi-walled, straight and spiral. In addition, they demonstrate a whole range of the most unexpected electrical, magnetic, and optical properties.

For example, depending on the specific folding pattern of the graphite plane ( chirality), nanotubes can be both conductors and semiconductors of electricity. The electronic properties of nanotubes can be purposefully changed by introducing atoms of other substances inside the tubes.

The voids inside fullerenes and nanotubes have long attracted attention.

scientists. Experiments have shown that if an atom of some substance is introduced inside a fullerene (this process is called “intercalation”, i.e. “incorporation”), this can change its electrical properties and even turn an insulator into a superconductor!

Is it possible to change the properties of nanotubes in the same way? It turns out yes. Scientists were able to place inside a nanotube a whole chain of fullerenes with gadolinium atoms already embedded in them. The electrical properties of such an unusual structure were very different from both the properties of a simple, hollow nanotube and the properties of a nanotube with empty fullerenes inside. It is interesting to note that special chemical symbols have been developed for such compounds. The structure described above is written as Gd@C60@SWNT, which means "Gd inside C60 inside a Single Wall NanoTube."

Wires for macrodevices based on nanotubes can pass current practically without generating heat, and the current can reach a huge value - 10 7 A/cm 2 . A classical conductor at such values would instantly evaporate.

Several applications of nanotubes in the computer industry have also been developed. Already in 2006, emission monitors with flat screens operating on a matrix of nanotubes will appear. Under the influence of a voltage applied to one end of the nanotube, the other end begins to emit electrons, which hit the phosphorescent screen and cause the pixel to glow. The resulting image grain will be fantastically small: on the order of a micron!(These monitors are studied in the peripheral devices course).

Another example is the use of a nanotube as a scanning microscope tip. Usually such an edge is a sharpened tungsten needle, but by atomic standards such sharpening is still quite rough. A nanotube is an ideal needle with a diameter of the order of several atoms. By applying a certain voltage, it is possible to pick up atoms and entire molecules located on the substrate directly under the needle and transfer them from place to place.

The unusual electrical properties of nanotubes will make them one of the main materials for nanoelectronics. Based on them, prototypes of new elements for computers were made. These elements make devices smaller by several orders of magnitude compared to silicon ones. The question of which direction the development of electronics will go after the possibilities for further miniaturization of electronic circuits based on traditional semiconductors are completely exhausted is now being actively discussed (this may happen in the next 5-6 years). And nanotubes have an undeniably leading position among promising candidates for the place of silicon.

Another application of nanotubes in nanoelectronics is the creation of semiconductor heterostructures, i.e. structures of the "metal/semiconductor" type or the junction of two different semiconductors (nanotransistors).

Now, to produce such a structure, it will not be necessary to grow two materials separately and then “weld” them together. All that is required is to create a structural defect in it during the growth of the nanotube (namely, to replace one of the carbon hexagons with a pentagon) simply by breaking it in the middle in a special way. Then one part of the nanotube will have metallic properties, and the other will have semiconductor properties!

And other similar structures that can be called by the general term carbon frame structures. What is it?

Carbon framework structures are large (and sometimes gigantic!) molecules made entirely of carbon atoms. One can even say that carbon frame structures are a new allotropic form of carbon (in addition to the long-known ones: diamond and graphite). The main feature of these molecules is their skeleton shape: they look like closed, empty “shells” inside. The most famous of the carbon framework structures is the C 60 fullerene, the completely unexpected discovery of which in 1985 caused a boom in research in this area (the Nobel Prize in Chemistry for 1996 was awarded to the discoverers of fullerenes Robert Curle, Harold Kroteau and Richard Smalley). In the late 80s and early 90s, after a technique for producing fullerenes in macroscopic quantities was developed, many other, both lighter and heavier fullerenes were discovered: starting from C 20 (the minimum possible fullerene) and up to C 70, C 82, C 96, and higher.

However, the diversity of carbon frame structures does not end there. In 1991, again completely unexpectedly, long, cylindrical carbon formations called nanotubes were discovered. Visually, the structure of such nanotubes can be imagined as follows: we take a graphite plane, cut a strip out of it and “glue” it into a cylinder (caution: such folding of a graphite plane is only a way to imagine the structure of a nanotube; in reality, nanotubes grow in a completely different way). It would seem that it is simpler - you take a graphite plane and roll it into a cylinder! - however, before the experimental discovery of nanotubes, none of the theorists predicted them! So scientists could only study them - and be surprised!

And there were many surprising things. Firstly, the variety of shapes: nanotubes could be large and small, single-walled and multi-layered, straight and spiral. Secondly, despite their apparent fragility and even delicacy, nanotubes turned out to be an extremely strong material, both in tension and bending. Moreover, under the influence of mechanical stresses exceeding critical ones, nanotubes also behave extravagantly: they do not “tear” or “break”, but simply rearrange themselves! Further, nanotubes demonstrate a whole range of the most unexpected electrical, magnetic, and optical properties. For example, depending on the specific folding pattern of the graphite plane, nanotubes can be both conductors and semiconductors! Can any other material with such a simple chemical composition boast at least part of the properties that nanotubes have?!

Finally, the variety of applications that have already been invented for nanotubes is striking. The first thing that suggests itself is the use of nanotubes as very strong microscopic rods and threads. As the results of experiments and numerical modeling show, the Young's modulus of a single-walled nanotube reaches values of the order of 1-5 TPa, which is an order of magnitude greater than that of steel! True, currently the maximum length of nanotubes is tens and hundreds of microns - which, of course, is very large on an atomic scale, but too short for everyday use. However, the length of nanotubes produced in the laboratory is gradually increasing - now scientists have already come close to the millimeter mark: see the work [Z. Pan et al, 1998], which describes the synthesis of a 2 mm long multiwalled nanotube. Therefore, there is every reason to hope that in the near future scientists will learn to grow nanotubes centimeters and even meters long! Of course, this will greatly influence future technologies: after all, a “cable” as thick as a human hair, capable of holding a load of hundreds of kilograms, will find countless applications.

Another example where a nanotube is part of a physical device is when it is “mounted” on the tip of a scanning tunneling or atomic force microscope. Usually such an edge is a sharpened tungsten needle, but by atomic standards such sharpening is still quite rough. A nanotube is an ideal needle with a diameter of the order of several atoms. By applying a certain voltage, it is possible to pick up atoms and entire molecules located on the substrate directly under the needle and transfer them from place to place.

The unusual electrical properties of nanotubes will make them one of the main materials for nanoelectronics. Prototypes of field-effect transistors based on a single nanotube have already been created: by applying a blocking voltage of several volts, scientists have learned to change the conductivity of single-walled nanotubes by 5 orders of magnitude!

Another application in nanoelectronics is the creation of semiconductor heterostructures, i.e. metal/semiconductor structures or the junction of two different semiconductors. Now, to produce such a heterostructure, it will not be necessary to grow two materials separately and then “weld” them together. All that is required is to create a structural defect in it during the growth of the nanotube (namely, to replace one of the carbon hexagons with a pentagon). Then one part of the nanotube will be metal, and the other will be a semiconductor!

Several applications of nanotubes in the computer industry have already been developed. For example, prototypes of thin flat displays operating on a matrix of nanotubes have been created and tested. Under the influence of a voltage applied to one end of the nanotube, electrons begin to be emitted from the other end, which fall on the phosphorescent screen and cause the pixel to glow. The resulting image grain will be fantastically small: on the order of a micron!

Using the same atomic microscope, it is possible to record and read information from a matrix consisting of titanium atoms lying on an -Al 2 O 3 substrate. This idea has also already been implemented experimentally: the achieved information recording density was 250 Gbit/cm 2 . However, in both of these examples, mass application is still far away - such high-tech innovations are too expensive. Therefore, one of the most important tasks here is to develop a cheap method for implementing these ideas.

The voids inside nanotubes (and carbon framework structures in general) have also attracted the attention of scientists. In fact, what will happen if an atom of some substance is placed inside a fullerene? Experiments have shown that intercalation (i.e. introduction) of atoms of various metals changes the electrical properties of fullerenes and can even turn an insulator into a superconductor! Is it possible to change the properties of nanotubes in the same way? It turns out yes. In [K.Hirahara et al, 2000], scientists were able to place inside a nanotube a whole chain of fullerenes with gadolinium atoms already embedded in them! The electrical properties of such an unusual structure were very different from both the properties of a simple, hollow nanotube and the properties of a nanotube with empty fullerenes inside. How, it turns out, the valence electron, given by the metal atom at everyone’s disposal, means a lot! By the way, it is interesting to note that special chemical designations have been developed for such compounds. The structure described above is written as Gd@C 60 @SWNT, which means "Gd inside a C 60 inside a Single Wall NanoTube."

It is possible not only to “drive” atoms and molecules individually into nanotubes, but also to literally “pour” matter. As experiments have shown, an open nanotube has capillary properties, that is, it seems to draw a substance into itself. Thus, nanotubes can be used as microscopic containers for transporting chemically or biologically active substances: proteins, poisonous gases, fuel components and even molten metals. Once inside the nanotube, atoms or molecules can no longer get out: the ends of the nanotubes are securely “sealed”, and the aromatic carbon ring is too narrow for most atoms. In this form, active atoms or molecules can be safely transported. Once at their destination, the nanotubes open at one end (and the operations of “soldering” and “unsoldering” the ends of the nanotubes are quite possible with modern technology) and release their contents in strictly defined doses. This is not science fiction; experiments of this kind are already being carried out in many laboratories around the world. And it is possible that in 10-20 years, diseases will be treated on the basis of this technology: say, pre-prepared nanotubes with very active enzymes are injected into the patient’s blood, these nanotubes are collected in a certain place in the body by some microscopic mechanisms and are “opened” at a certain moment time. Modern technology is almost ready for implementation...

Carbon nanotubes are a material that many scientists dream of. High strength coefficient, excellent thermal and electrical conductivity, fire resistance and weight coefficient are an order of magnitude higher than most known materials. Carbon nanotubes are a sheet of graphene rolled into a tube. Russian scientists Konstantin Novoselov and Andrei Geim received the Nobel Prize in 2010 for its discovery.

For the first time, Soviet scientists were able to observe carbon tubes on the surface of an iron catalyst back in 1952. However, it took fifty years for scientists to see nanotubes as a promising and useful material. One of the striking properties of these nanotubes is that their properties are determined by geometry. Thus, their electrical properties depend on the angle of twisting - nanotubes can demonstrate semiconductor and metallic conductivity.

What is this

Many promising areas in nanotechnology today are associated with carbon nanotubes. Simply put, carbon nanotubes are giant molecules or framework structures that consist only of carbon atoms. It is easy to imagine such a nanotube if you imagine that graphene is folded into a tube - this is one of the molecular layers of graphite. The method of folding nanotubes largely determines the final properties of this material.

Naturally, no one creates nanotubes by specially rolling them from a sheet of graphite. Nanotubes form themselves, for example, on the surface of carbon electrodes or between them during an arc discharge. During discharge, carbon atoms evaporate from the surface and connect with each other. As a result, nanotubes of various types are formed - multi-walled, single-walled and with different twist angles.

The main classification of nanotubes is based on the number of layers that make them up:

Single-walled nanotubes are the simplest type of nanotubes. Most of them have a diameter of the order of 1 nm with a length that can be thousands of times greater;
Multilayer nanotubes, consisting of several layers of graphene, they fold into the shape of a tube. A distance of 0.34 nm is formed between the layers, that is, identical to the distance between the layers in a graphite crystal.

Device

Nanotubes are extended cylindrical carbon structures that can have a length of up to several centimeters and a diameter of one to several tens of nanometers. At the same time, today there are technologies that make it possible to weave them into threads of unlimited length. They can consist of one or more graphene planes rolled into a tube, which usually end in a hemispherical head.

The diameter of nanotubes is several nanometers, that is, several billionths of a meter. The walls of carbon nanotubes are made of hexagons, at the vertices of which there are carbon atoms. Tubes can have different types of structure, which affects their mechanical, electronic and chemical properties. Single-layer tubes have fewer defects; at the same time, after annealing at high temperatures in an inert atmosphere, it is possible to obtain defect-free tubes. Multiwalled nanotubes differ from standard single-walled nanotubes in a significantly wider variety of configurations and shapes.

Carbon nanotubes can be synthesized in different ways, but the most common are:

Arc discharge. The method ensures the production of nanotubes in technological installations for the production of fullerenes in the plasma of an arc discharge, which burns in a helium atmosphere. But here other arc combustion modes are used: higher helium pressure and low current densities, as well as larger diameter cathodes. The cathode deposit contains nanotubes up to 40 microns in length; they grow perpendicularly from the cathode and are combined into cylindrical bundles.
Laser ablation method . The method is based on the evaporation of a graphite target in a special high-temperature reactor. Nanotubes are formed on the cooled surface of the reactor in the form of graphite evaporation condensate. This method allows one to predominantly obtain single-walled nanotubes with control of the required diameter by temperature. But this method is significantly more expensive than others.
Chemical vapor deposition . This method involves preparing a substrate with a layer of catalyst - these can be particles of iron, cobalt, nickel or combinations thereof. The diameter of nanotubes grown using this method will depend on the size of the particles used. The substrate is heated to 700 degrees. To initiate the growth of nanotubes, carbon-containing gas and process gas (hydrogen, nitrogen or ammonia) are introduced into the reactor. Nanotubes grow on areas of metal catalysts.

Applications and Features

Applications in photonics and optics . By selecting the diameter of nanotubes, it is possible to ensure optical absorption in a wide spectral range. Single-walled carbon nanotubes exhibit strong saturable absorption nonlinearity, meaning they become transparent under sufficiently intense light. Therefore, they can be used for various applications in the field of photonics, for example, in routers and switches, for creating ultrashort laser pulses and regenerating optical signals.
Application in electronics . At the moment, many methods have been announced for using nanotubes in electronics, but only a small part of them can be realized. The greatest interest is in the use of nanotubes in transparent conductors as a heat-resistant interfacial material.

The relevance of attempts to introduce nanotubes in electronics is caused by the need to replace indium in heat sinks, which are used in high-power transistors, graphics processors and central processing units, because the reserves of this material are decreasing and its price is rising.

Creation of sensors . Carbon nanotubes for sensors are one of the most interesting solutions. Ultrathin films of single-walled nanotubes may currently become the best basis for electronic sensors. They can be produced using different methods.
Creation of biochips, biosensors , control of targeted delivery and action of drugs in the biotechnology industry. Work in this direction is currently underway. High-throughput analysis performed using nanotechnology will significantly reduce the time it takes to bring a technology to market.
Today it is growing sharply production of nanocomposites , mostly polymer. When even a small amount of carbon nanotubes is introduced into them, a significant change in the properties of polymers is ensured. This increases their thermal and chemical stability, thermal conductivity, electrical conductivity, and improves their mechanical characteristics. Dozens of materials have been improved by adding carbon nanotubes;

Composite fibers based on polymers with nanotubes;
ceramic composites with additives. The crack resistance of ceramics increases, protection of electromagnetic radiation appears, electrical and thermal conductivity increases;
concrete with nanotubes – increases grade, strength, crack resistance, reduces shrinkage;
metal composites. Especially copper composites, whose mechanical properties are several times higher than those of ordinary copper;
hybrid composites, which contain three components at once: inorganic or polymer fibers (fabrics), a binder and nanotubes.

Advantages and disadvantages

Among the advantages of carbon nanotubes are:

Many unique and truly useful properties that can be used in the implementation of energy-efficient solutions, photonics, electronics, and other applications.
This is a nanomaterial that has a high strength coefficient, excellent thermal and electrical conductivity, and fire resistance.
Improving the properties of other materials by introducing a small amount of carbon nanotubes into them.
Open-ended carbon nanotubes exhibit a capillary effect, meaning they can draw in molten metals and other liquid substances;
Nanotubes combine the properties of solids and molecules, which opens up significant prospects.

Among the disadvantages of carbon nanotubes are:

Carbon nanotubes are not currently produced on an industrial scale, so their serial use is limited.
The cost of producing carbon nanotubes is high, which also limits their application. However, scientists are working hard to reduce the cost of their production.
The need to improve production technologies to create carbon nanotubes with precisely defined properties.

Prospects

In the near future, carbon nanotubes will be used everywhere; they will be used to create:

Nanoscales, composite materials, ultra-strong threads.
Fuel cells, transparent conducting surfaces, nanowires, transistors.
The latest neurocomputer developments.
Displays, LEDs.
Devices for storing metals and gases, capsules for active molecules, nanopipettes.
Medical nanorobots for drug delivery and operations.
Miniature sensors with ultra-high sensitivity. Such nanosensors could find use in biotechnological, medical and military applications.
Space elevator cable.
Flat transparent loudspeakers.
Artificial muscles. In the future, there will be cyborgs, robots, and people with disabilities will return to a full life.
Engines and power generators.
Smart, light and comfortable clothing that will protect you from any adversity.
Safe supercapacitors with fast charging.

All this is in the future, because industrial technologies for the creation and use of carbon nanotubes are at the initial stage of development, and their price is extremely expensive. But Russian scientists have already announced that they have found a way to reduce the cost of creating this material by two hundred times. This unique technology for producing carbon nanotubes is currently kept secret, but it is set to revolutionize industry and many other areas.

Introduction:

Nanotubes can act not only as a material under study, but also as a research tool. Based on nanotubes, for example, it is possible to create microscopic scales. We take a nanotube, determine (by spectroscopic methods) the frequency of its natural vibrations, then attach the sample under study to it and determine the frequency of vibrations of the loaded nanotube. This frequency will be less than the oscillation frequency of a free nanotube: after all, the mass of the system has increased, but the rigidity has remained the same (remember the formula for the oscillation frequency of a weight on a spring). For example, in the work it was discovered that the load reduces the oscillation frequency from 3.28 MHz to 968 kHz, from where the weight of the load was obtained 22 +- 8 fg (femtograms, i.e. 10-15 grams!)

Another example where a nanotube is part of a physical device is to “mount” it on the tip of a scanning tunneling or atomic force microscope. Usually such an edge is a sharpened tungsten needle, but by atomic standards such sharpening is still quite rough. A nanotube is an ideal needle with a diameter of the order of several atoms. By applying a certain voltage, it is possible to pick up atoms and entire molecules located on the substrate directly under the needle and transfer them from place to place.

Carbon nanotubes (tubulenes) are extended cylindrical structures with a diameter from one to several tens of nanometers and a length of up to several centimeters, consisting of one or several hexagonal graphite planes rolled into a tube and usually ending in a hemispherical head, which can be considered as half a fullerene molecule

Nanotube structure:

To obtain a nanotube (n, m), the graphite plane must be cut along the directions of the dotted lines and rolled along the direction of the vector R .

An ideal nanotube is a graphite plane rolled into a cylinder, that is, a surface lined with regular hexagons with carbon atoms at the vertices. The result of such an operation depends on the angle of orientation of the graphite plane relative to the axis of the nanotube. The orientation angle, in turn, determines the chirality of the nanotube, which determines, in particular, its electrical characteristics

The chirality of nanotubes is indicated by a set of symbols (m, n) indicating the coordinates of a hexagon, which, as a result of folding the plane, must coincide with the hexagon located at the origin.

Another way to indicate chirality is to indicate the angle α between the direction of folding of the nanotube and the direction in which adjacent hexagons share a common side. However, in this case, to fully describe the geometry of the nanotube, it is necessary to indicate its diameter. The chirality indices of a single-walled nanotube (m, n) uniquely determine its diameter D. The indicated relationship has the following form:

Where d 0 = 0.142 nm - the distance between neighboring carbon atoms in the graphite plane. The relationship between chirality indices (m, n) and angle α is given by the relation:

Among the various possible directions of folding of nanotubes, those for which alignment of the hexagon (m, n) with the origin of coordinates does not require distortion of its structure are distinguished. These directions correspond, in particular, to the angles α = 0 (armchair configuration) and α = 30° (zigzag configuration). The indicated configurations correspond to chiralities (m, 0) and (2n, n), respectively.

(types of nanotubes)

Single-walled nanotubes:

The structure of single-walled nanotubes observed experimentally differs in many respects from the idealized picture presented above. First of all, this concerns the vertices of the nanotube, the shape of which, as follows from observations, is far from an ideal hemisphere.

A special place among single-walled nanotubes is occupied by the so-called armchair nanotubes or nanotubes with chirality (10, 10). In nanotubes of this type, two of the C-C bonds that make up each six-membered ring are oriented parallel to the longitudinal axis of the tube. Nanotubes with a similar structure should have a purely metallic structure.

Multi-walled nanotubes:

Multi-walled nanotubes differ from single-walled nanotubes in a much wider variety of shapes and configurations. The variety of structures is manifested in both longitudinal and transverse directions.

The “Russian dolls” type structure (Fig. a) is a collection of cylindrical tubes coaxially nested into each other. Another type of this structure (Fig. b) is a collection of coaxial prisms nested within each other. Finally, the last of the above structures (Fig. c) resembles a scroll. For all structures in Fig. The characteristic value of the distance between adjacent graphite layers is close to the value of 0.34 nm, inherent in the distance between adjacent planes of crystalline graphite.

The implementation of a particular structure of multi-walled nanotubes in a specific experimental situation depends on the synthesis conditions. An analysis of the available experimental data indicates that the most typical structure of multi-walled nanotubes is a structure with sections of the “Russian nesting doll” and “papier-mâché” type alternately located along the length. In this case, smaller “tubes” are sequentially inserted into larger tubes. This model is supported, for example, by facts on the intercalation of potassium or ferric chloride into the “intertubular” space and the formation of “bead”-type structures.

Discovery history:

As is known, fullerene (C 60) was discovered by the group of Smalley, Kroto and Curl in 1985, for which these researchers were awarded the Nobel Prize in Chemistry in 1996. As for carbon nanotubes, it is impossible to give an exact date for their discovery. Although Iijima's observation of the structure of multi-walled nanotubes in 1991 is well known, there is earlier evidence for the discovery of carbon nanotubes. So, for example, in 1974-1975. Endo et al. have published a number of papers describing thin tubes with a diameter less than 100 Å prepared by vapor condensation, but a more detailed structural study has not been carried out. In 1977, a group of scientists from the Institute of Catalysis of the Siberian Branch of the USSR Academy of Sciences, while studying the carbonization of iron-chromium dehydrogenation catalysts under a microscope, recorded the formation of “hollow carbon dendrites”; a mechanism of formation was proposed and the structure of the walls was described. In 1992, an article was published in Nature, which stated that nanotubes were observed in 1953. A year earlier, in 1952, an article by Soviet scientists Radushkevich and Lukyanovich reported electron microscopic observation of fibers with a diameter of about 100 nm, obtained from the thermal decomposition of oxide carbon on an iron catalyst. These studies were also not continued.