Carbon nanotubes are a nanoscopic configuration of carbon atoms arranged in the form of hexagonal meshes that form a cylindrical structure, similar to a tube.

The characteristics of these rolled graphene layer structures vary substantially depending on the orientation of the hexagons that form them about their axis, standing out because they surpass the most known materials in their physical properties. Due to their unique properties, they have a lot of important uses in different fields. This article aims to discuss the methods used to synthesize carbon nanotubes in detail.

Introduction

In the field of nanomaterials, in the specific case of polymeric nanocomposites, new applications are opened by the use of carbon nanotubes as reinforcing agents as reinforcing agents dispersed in the continuous phase. These "nanotubes" can be considered both as reinforcing agents (relative to the mechanical properties) but also electrical and mechanical, which they can provide to the polymer matrix.

In fact, about their chemical-physical characteristics, carbon nanotubes show highly attractive properties. From the moment of their discovery 1991 as a waste product of the synthesis process of the parent molecule [fullerene] and the consequent in-depth study of their characteristics and potential applications, the interest on the part of researchers around the world has been gradually increasing.

One of the problems that has previously blocked a more invasive use of nanotubes in the most diverse fields is that of the inherent difficulties in synthesis processes at a sustainable price. In fact, currently on the market, it is only available in limited and very expensive quantities.

Another very important problem is the control of the size and morphology of nanotubes, ultimately making real nanotechnology. However, all this has led many research centers of various levels to undertake very in-depth studies and research on the subject. The potential applications are almost unlimited and range from polymeric nanocomposites with improved mechanical properties to all those applications where very special and peculiar electrical, thermal and optical properties are required.

Structure of Carbon Nanotubes

Carbon nanotubes were discovered accidentally by researcher Sumio Iijima, in the waste products in the fullerene production process. In fact, carbon nanotubes have a structure strongly linked to that of fullerene (third crystalline form of C60 carbon, the famous Buckyball very similar to a soccer ball). Carbon nanotubes are made up of flat graphite (graphene) sheets of carbon atoms to form a hexagonal structure rolled up to delineate a cylindrical structure.

Two forms of nanotubes are usually considered

1. Single walled nanotubes (SWNT)
2. Multi walled nanotubes (MWNT)

Single-walled carbon nanotubes have a diameter ranging from 1 to 10 nm, but generally the diameter is around 2 nm. Nanotubes are characterized by a high length / diameter ratio (10e4 10e5) and it is precisely for these and other characteristics that carbon nanotubes assume such captivating behaviors.

Between the walls of the multi-walled carbon nanotubes, weak interactions are established which seem to arise to stabilize the structure upon growth during the synthesis phase. The diameter of multi-walled nanotubes is generally much larger than single walled ones up to a few tens of nanometers.

There are also major imperfections of single-walled carbon nanotubes. For these reasons, single-walled nanotubes are much more valuable and very expensive commodities.

Production Methods of Carbon Nanotubes

There are two main methods to produce carbon nanotubes, high-temperature methods, and the Chemical Vapor Deposition method.

1. High-Temperature Methods
2. Chemical Vapor Deposition Method

1. High Temperature Method

This method used by Sumio Iijima consists of inserting two graphite rods, the anode and the cathode, in an electric arc production chamber, filled with non-reactive gases. When they are brought together and an electric current flows through them, an arc is produced, it is consumed to form a plasma whose temperature can reach 6000 °C, which instantly vaporizes the tip of one of the two rods. Carbon gas produced during this vaporization forms nanotubes which generally have 20 to 30 walls. If metal catalysts are added inside the anode, it is possible to obtain single-walled carbon nanotubes.

The synthesized nanotubes are almost perfect. However, the process is so complex that in the end, we have little control over the outcome. One of the main problems is the formation of other forms of carbon which are very difficult to separate from nanotubes, moreover, the high temperature necessary for the synthesis of nanotubes is restrictive for them, they tend to partially melt and adhere.

To get more information about the synthesis of carbon nanotubes,

you can read our blog post here.

1.1 Laser ablation

Technique carried out by the group of R. Smalley, at the University of Houston (USA), which consists of bombarding a graphite rod with continuous laser radiation (with the continuous flow) or pulsed (with short pulses, it can be more powerful than the continuous laser). This process allowed the reaction temperature to drop to 1200 °C. Graphite is either vaporized or expelled in small fragments of a few atoms, the process is more expensive, but has a higher synthesis yield than the electric arc production method, it also allows easier control, which allows the study of the synthesis conditions and their modeling to obtain only the desired products.

1.2 Solar Reactor

This method consists of concentrating solar energy on a graphite target to reach the vaporization temperature. This process makes it possible to synthesize an average of 0.1 g to 1 g of nanotube per "experiment", the production and operating mode is close to that by continuous laser, it is, therefore, suitable, like the latter, for in situ studies (behavioral study, carried out by observation in the situation).

Advantages and disadvantages of High temperature methods

Advantages

1. Synthesis of single-walled nanotubes (whereas with the other methods one obtains only multi-walled nanotubes or an inseparable mixture).
2. Formation of very pure products.

Disadvantages

1. Formation of clusters that must be dissociated to be able to apply them.
2. No control over the length of the nanotubes.

2. Chemical Vapor Deposition (CVD) Method 

Chemical Vapor Deposition (CVD) Method for Carbon Nanotube Synthesis

The CVD method is carried out using a liquid or gaseous carbon source to which a metallic precursor is added. Ferrocene (organometallic compound) is frequently used (sometimes nickelocene). The aerosol solution transformed into fine droplets is then transported by an inert gas (generally argon) to a furnace at a temperature between 750 °C and 900 °C.

Nanotubes "grow", either on the glass wall of the tube or on a silicon plate, placed here to facilitate the recovery of the nanotubes, the nanotubes are recovered after reaction on the plate where they are aligned (the continuous supply of reagents will force the nascent nanotubes to take up as little space as possible, therefore all aligning in one direction, vertically above the place where they "grow"). Multi-leaf, aligned nanotubes with a length of approximately 200m are obtained. After the reaction, the nanotubes still contain some impurities (mainly the starting metal, iron, or nickel), which must be removed. The nanotubes are, therefore “baked” again, under an inert gas atmosphere (the presence of dioxygen would destroy the nanotubes), which has the effect of opening the half-fullerenes at the ends, allowing impurities to come out. This “re-baking” also has the advantage of making the nanotubes even more rectilinear, by eliminating any defects (part of a “broken” graphene layer).

Detail

CVD (Chemical Vapor Deposition) from aerosols is one of the methods of catalytic CVD. This method is based on the catalytic decomposition of a liquid or gaseous hydrocarbons, such as toluene or acetylene, in the presence of a metallocene such as a ferrocene. The specificity of the process lies in the fact that it is carried out in a single step: the reactor is supplied simultaneously with a catalytic source and a carbon source. There are different equipment depending on the direction of injection of the precursors and the size of the samples synthesized:

• equipment in vertical configuration making it possible to obtain samples with a surface area of the order of several hundred cm2
• equipment in horizontal configuration making it possible to obtain samples with a surface area of the order of cm2 to a few tens, or even hundreds of cm2;

All this equipment is made up of 3 parts: an aerosol generator making it possible to produce a set of droplets most often accompanied by an evaporator, a furnace in which a reactor is placed, and a set of cooling and evacuation systems.

To get more information about carbon nanotube sponges,

you can read our blog post here.

Two types of aerosol generators are used: an ultrasonic aerosol generator or an injection system of the automotive injector type. The ultrasonic aerosol generator is composed of a piezoelectric ceramic which vibrates and creates, by micro-cavitation, an aerosol on the surface of the liquid, which is then transported by the gas flow. The injector system consists of a needle valve whose opening is controlled in duration and frequency and it operates in pulsed mode.

CVD for the Synthesis of Vertically Aligned Carbon Nanotubes and Graphene

The decomposition of the catalytic and carbon precursors occurs in two stages. Initially, the metallocene will decompose thermally giving rise to the formation of catalytic particles in the gas phase which will, in a second step, generate the catalytic decomposition of the carbon precursor. This results in the formation of a deposit on the walls of the reactor or various substrates (quartz, silicon, stainless steel, aluminum, carbon and carbon fiber fabrics, etc.). This deposit consists of carbon nanotubes aligned like a brush mat. Small equipment in the horizontal configuration is also used for the growth of graphene on metals.

Advantages and disadvantages

Advantages

• Formation of very pure products, practically flawless.
• Synthesis of multi-walled nanotubes.
• formation of easily dissociable aligned nanotubes.

Disadvantages

• high deposition temperatures generate residual stresses (material fatigue).

Properties of Carbon Nanotubes

The unique properties of Carbon Nanotubes are listed below

• High tensile and compressive strength
• High Young's modulus
• Flexibility

• High current density per transverse area
• Superconductivity under certain conditions
• High thermal conductivity
• High reactive surface

Applications

Depending on the previous properties, the following potential or real applications arise

• Storage devices for fuel cells
• Reinforced Composite Polymers
• Conductive polymers
• Flame retardants
• Nanoelectronics
• Battery electrodes
• fuel Membranes
• Elements for interaction with light
• Solar cells
• Pharmaceutical transport and release systems
• X-ray devices
• Data storage (pc memories)
• Field emission display FED
• Sensors
• Artificial muscles
• Separation membranes.
• Chemical catalysis

Conclusion

Carbon nanotubes are similar to tiny rolled sheets of graphite with nanometric diameters and lengths on the order of microns. They are unique materials with exceptional mechanical, electrical, optical, thermal, and chemical properties that make them suitable for improving many existing products and even for generating new ones. It is manufactured by high temperature methods and chemical vapor deposition methods, each having its own advantages and disadvantages, as described above.

There are many applications that can obtain important benefits when incorporating carbon nanotubes. For example Composite materials reinforced with nanotubes, flat screens that use nanotubes as field emitters, biological and chemical sensors to detect pollutants, drug delivery, or fuel cells. In general, sectors such as electronics, materials, sensors, biotechnology, chemistry, energy, mechanics, scientific instrumentation, and photonics could be favored by the introduction of carbon nanotubes in many of their products.

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