Polymers with carbon nanotubes
July 6, 2009
Sarrià Chemical Institute (IQS) has since 1994 of the laboratory of materials science, where Dr. Carles Colominas works with nanocomposites, synthesized layers by means of steam and technology Sol-Gel, via wet chemical depositionfor ultra hard materials with hardness from 10 to 40 times higher than the steel. The most senior Center on nanomaterials is AIN (Navarre Industry Association) in Cordovila. Add the Institute of science and technology of polymers (CSIC), Madrid, Aimplas (Institute of polymer technology), in Paterna (Valencia), and finally, included Nanospain (www.nanospain.org). There are 275 research groups and 1,200 researchers.
But at the industrial and commercial level, do find a company to do business with nanotechnology? There is some exception that proves the rule. Pere Castell, Nanozar, says that the industry sees nanotechnology as a market with high risk, not acceptable for it, and that Spain suffers from a lack of highly qualified technical personnel. The 'miracle' properties of nanomaterials are a reality, but turn them into an industrial success is harder still. Progress will be continued, but slow.
There are many kinds of nanomaterial, for example, the nanoarcillas (an easy subject for reinforcing polymers) and carbon, one or many layers of nanotubes. The latter are very difficult to manipulate. They are networks exagonales of trivalent carbon curved and closed, with a length of several microns. Their diameters range between 1 and 20 nanometers. They were discovered by S. Iijima, in Japan in 1991. There are two types: simple wall, or single and multiple wall, up to 100 layers.
To understand the problem, simply cite the NIMS (National Institute for Materials Science), of Tsukuba, Japan. Research teams available to those cannot afford any company, nor those of international level. Only the State: high-voltage microscopy and synchrotron of electrons, among others.
Chemists, through strategies below have above, synthesized Molecular materials that present physical properties of extraordinary interest. The nanochemistry is a tool of inestimable value to the development of artificial molecular machines. So believes Tomás Torres barley, from its chemistry Chair of the UAM. The chemical may offer electronic engineers systems car assembled, as bricks at the molecular level to the construction of underwater electronic and electro-optic, far below the current in use size.
The nanochemistry manipulates materials at the nanometer scale, between 1-100 nm. It involves an impact of several orders of magnitude on the submicrométrica technology, which is the basis of the current electronics. His interest is obvious. It is to build pieces of matter such as atoms and molecules, supramolecular entities in the scale Nano with specific properties. He tries to manipulate structures at molecular level and provide them with new properties, found in conventional materials.
Molecular materials are synthesized in isolation and are organized in some sort of phase condensed, able to present non-conventional physical properties. The properties of nanostructured materials depend on the effects of size, the dimensionality of the system changes. Nanocrystalline materials consist of usually crystalline blocks, and there is a marked difference with glasses and gels, which are microstructurely homogeneous.
The best energy to do work to a machine or molecular switch are photons and electrons. A very repeated phrase, which gave Richard Feynman on December 29, 1959: There's plenty of room in the bottom.
The strength or hardness of crystalline material, according to the Hall-Petch effect, dependent on the size of the grain. Any pressure, as the grain size decreases, the metal hardens. Young module characterized the behavior of elastic material, according to the direction in which a force is applied, and for values of voltage in the range of complete reversibility of deformations. The value of Young's modulus is defined by the ratio of the voltage applied to the material and caused deformation. Smaller grain, greater is the Young's modulus, the force to be applied, so that the metal in the area of plasticity is greater.
With the module of shear so: smaller is the grain, the greater the force that needs to be done to achieve a given distortion.
The Hall-Petch effect is true for grains of a diameter of 12 nm. For minor grain hardness decreases. There are dislocations within a grain, which propagate to the nearby grain, and weaken the metal. Minor size, more difficult is the spread of the dislocation and the appearance of the metal plasticity. But the practical results are not encouraging, do not confirm the theory. Thermodynamics suggests that the nanostructures synthesized probably will never achieve the uniformity and perfection of the electronic circuits made with silicon crystals, obtained with conventional lithography. The simple reduction in size, by itself, can not bring the expected increase in features.
The National Center of microelectronics (CNM) nanotubes
The CNM works the British doctor Adrian Bachtold, who has made statements which we will try to summarize here. It tries to shed light on the quantum properties of the NTC and its behavior in applications. The Centre has the largest clean-Spain room and professionals with broad international experience. They are interested in the mechanical and electrical properties of the NTC. For example: how one electron circulates through the NTC, which is its vibration, or see if there is what type and quantum phenomena.
After having made the first logical circuit from nanotubes, it seems that there has been little progress. For more than two decades that refers to the possibility of replacing the Silicon by NTC, to improve its performance. Unless our work, appeared in Science, in 2001, he failed to prove, that a logical circuit with a few molecules could be constructed. This did not mean that NTC-based computers appear in a short time. Today day get NTC is something simple and cheap, but its handling remains very complex. On the other hand the NTC can behave like metals, semiconductors and superconductors. Let us recall here that the diamond pure carbon is insulating.
We do not have techniques to manipulate the NTC with efficiency. In addition, they have randomly on a surface, which forces to locate them, and it is not easy. The NTC should be at a precise position on the substrate (polymer), which will strengthen.
The NTC can be a good choice to replace silicon. The properties of the NTC are clearly superior to the latter. For example, phenomena such as the conductivity or the transductancia. This is important, so that the circuits to operate at higher speeds, and therefore have better transistors. It should dominate the management of the atoms of carbon, which is what defines, whether it is going to be superconducting or not.
The NTC define a rigid, robust and with a very light body structure, but we know very little about another mechanical property: the vibration. Understanding would open the door to many applications, optimize energy use in mobile telephony, to the identification of weak forces (very sensitive sensors), or detect the mass of a single atom. Also expected much of quantum computing.
Before this, what really matters is the basic science behind the NTC. For example, to see to what extent the NTC behave according to the principles of quantum mechanics. Also interested in its behavior depending on the temperature, or the fluctuation of the atoms at that level.
The reality is that we cannot still understand well what happens on the nano scale. So far Dr. Adrián Bachtold, NJC. If it cannot be measured on the nanoscale, you can either move in nanoscience.
It is disappointing, after much research in molecular engineering of Nanosystems, the lack of success in technological and commercial exploitation. The manipulation of molecular structures is a very difficult issue. And it is not worth remembering that we changed the structures of polymers, which today we find everywhere. It's molecules that can be considered large.
Production of NTC
The best known method for the production of NTC is the sublimation of the carbon in an inert atmosphere. Also download by electric arc. It is generation of plasma through a current between two electrodes of coal, at a distance of 1 mm and in an inert atmosphere. The electrodes reach 3500 ° C. The alignment, performance and quality of the NTC depend on the conditions of the arch and the stability of plasma. Other methods include laser ablation and catalytic decomposition of hydrocarbons.
The chirality is a property that must be counting on the NTC. A chiral object has no axis of rotation. Chiral molecules have the property of diverting (rotate) the plane of polarized light that passes through them. The plane is diverted certain angle. If the light rotates right, this molecule is dextrógena, swi is on the left, is "left-handed".
In the NTC the chirality is equivalent to the direction in which a sheet of graphite of one atom thick rolled up to form the nanotube. The graphite sheet comes from the Ribbon, coiled form the NTC. It is easy to imagine that the tape can be cut from the sheet forming different angles from 0 up to 90 °. The chirality and, therefore, the properties of the NTC, dependent on the cutting angle. It is not possible to achieve the same chirality NTC, because some NTC are metal (drivers) and other semiconductors. All tubes are formed to assembling hexagons, but the chirality varies. It is a mixture.
The problem remains, as in the monolayer, than in the multilayer tubes. Separate the NTC by chirality is not possible, because, due to the attractive Van der Waals forces, the NTC tend to autoensamblarse, to form fibers or bundles.
The news of last January, comes from Duke University. Jie Liu and Patricia Crawford claim that they have managed to grow exclusively on semiconductor type NTC. Its merit is having achieved an exact combination of two alcohols with argon and hydrogen, with copper as a catalyst. It will have to confirm their method.
There are zero chirality NTC: the nanotube seen laterally shows rows of hexagons vertical, parallel to the axis of the nanotube. Nanotubes with chirality have rows of hexagons that form an angle with the axis of the NTC. The intermingling of the NTC, to mix them with thermoplastic resins, greatly increases the viscosity.
The most practical method of selecting NTC is the chemical vapor deposition which is carried out in the IQS, Barcelona. It is based on the decomposition of gaseous hydrocarbons (benzene and acetylene) called precursors, on a substrate graphite, covered by transition metals (cobalt, iron, nickel) that act as catalyst. Temperatures ranging from 700 to 1500 ° C. The NTC of this method are not straight but curved, forming a conglomerate contaminated with catalyst. The diameter of the NTC is quite uniform.
When the pyrolysis is carried out in the presence of organo-metallic compounds, are aligned NTC. The growth of the NTC initially is fast, but much decreases when the catalyst is encapsulated by the NTC. The density is very low compared to graphite, which is a great advantage for certain applications.
The theoretical mechanical properties of the NTC are fascinating, but in practice such high values are not available. Their characteristics depend on much of the high radio/length relationship and of the surface/volume ratio, which in turn means a drastic increase of the interfacial area.
Moreover, they are highly elastic, i.e. ideal candidates for reinforcement in polymers, compared with the traditional reforzantes. For its ability to withstand high deformations offer a considerable advantage over carbon fibers. It is assumed that he has been an ideal dispersion in the polymer matrix. As the NTC have agglomerates, is a challenge to achieve effective dispersion.
In fact, the dispersion is achieved based on a mixed, which acts as a shear and that breaks the NTC. If the mixing is energy, reduces the length of the NTC, which is not serious. In fact, the radio/length of the NTC remains high, is reduced from 250 to 1,000.
This is the price to be paid to achieving an acceptable dispersion. The final properties of the nanopolímero depend on the degree of dispersion and alignment of the NTC. The orientation and the degree of alignment of the NTC identifies with images of TEM and x-ray diffraction.
The NTC give the polymer not only a great improvement of the mechanical, but also electrical properties: conductivity, increased by 10 orders of magnitude, an advantage over the conductive electrical charges.
Acrilonitrito-butadiene-styrene (ABS), polystyrene and polypropylene are used to form multi-component, where the dispersion of the NTC is carried out using a mixer. The successful scattering analyses by SEM and TEM techniques.
The transfer of burden of the polymer matrix to the NTC is conditioned by the interfacial Union, the molecular structure of the polymer and its ability to form helices arranged around the single nanotube.
The uniform distribution is achieved with a concentration of NTC low, less than 1% by weight, at least in the case of the PS and PMMA. Increase the concentration to the 2.5 or 10% changes the properties of cargo, but the disadvantages outweigh the advantages.
Epoxy matrices can not be strengthened with NTC, due to the weak Interfacial unions between the two phases. NTC-polymer chemical bonds give rise to an effective transfer.
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