INTRODUCTION
The discovery of fullerenes and carbon nanotubes has opened a new research area in physics, chemistry and material science. Nanostructured materials may be defined as those materials whose structural element clusters, crystallites or molecules have dimensions in the 1 to 100 nm range. Clusters of atoms consisting of typically hundreds to thousands on nanometer (nm) scale are commonly called as nanoclusters. These small groups of atoms, in general, go by different names such as nanoparticles, nanocrystals, quantum dots and quantum boxes. Substantial work is being carried out in the domain of nanostructured materials and nanotubes in the past one decade. The explosion in both academic and industrial interest in these nanomaterials over the past decade arises from the remarkable variations in fundamental electrical, optical and magnetic properties that occur as one progresses from an ‘infinitely extended’ solid to a particle of material consisting of a countable number of atoms. Nanostructured materials had led to new basic science phenomena and several applications for the short, medium and long term.
Examples: nano-electronic devices, quantum wires, electron field emitters for ultra-thin TV screens, nanoprobes, high resolution tips for scanning and atomic force microscopes, sensors, ultrahigh strength composites, gas storage, nanodevices, and parts of nanomachines, among others. Carbon based nanomaterials and nanostructures including fullerenes and nanotubes play an increasingly pervasive role in nanoscale science and technology. Carbon nanotubes are currently being studied in an effort to understand their novel structural, electronic and mechanical properties to explore their immense potential for many applications in nanoelectronics, and as actuators and sensors.
Fullerenes and carbon nanotubes can be seen as curves pieces of graphite. Graphite is another polymorph of carbon.
GRAPHITE
Graphite has a crystal structure distinctly different from that of diamond and is also more stable than diamond at ambient temperature and pressure. Graphite is formed by flat hexagonal layers of carbon atoms separated 3.35 . The C-C distance is 1.42 and the structure belongs to P 63 mc space groups with lattice constants a = 2.46 and c = 6.71 . Graphite is highly anisotropic solid, structurally its interplanar spacing (3.35 ) is quite large compared to the in plane interatomic spacing (1.42 ). Physically its stiffness along the plane is quite large because of strong covalent bonds and in the perpendicular direction it is weak because of the Vander Waal’s force. The graphite structure is composed of layers of hexagonally arranged carbon atoms; within the layers. each carbon atom is
Figure 1: Structure of Graphite
bonded to three coplanar neighbor atoms by strong covalent bonds. Carbon when sp2 hybridized contains three strong π-bonds which lie in a triangular plane with bond angle of 120° between them and one weakly bonding π-bond perpendicular to the plane. Each carbon atom in the plane is covalently bonded to three others in plane with strong σ bonds of the sp2 hybridization, this network forms a continuous sheet of carbon. The fourth bonding electron participates in a weak Vander Waals type of bond between the layers. As a consequence of these weak interplanar bonds, interplanar cleavage is facile, which gives rise to the excellent lubricative properties of graphite. Also, the electrical conductivity is relatively high in crystallographic directions parallel to the hexagonal sheets. Graphite has metallic conductivity along the plane and semiconducting perpendicular to the plane. The anisotropy in conductivity is about 103.
Other desirable properties of graphite include: high strength and good chemical stability at elevated temperatures and in non oxidizing atmospheres, high thermal conductivity, low coefficient of thermal expansion and high resistance to thermal shock, high absorption of gases, and good machinability. Graphite is commonly used in heating elements for electric furnaces, as electrodes for ac welding, in metallurgical crucibles, in casting moulds for metal alloys and ceramics, for high temperature refractories and insulations, in rocket nozzles, in chemical reactor vessels, for electrical contacts, brushes and resistors, as electrodes in batteries, and in air purification devices.
FULLERENES
In the case of fullerenes, graphite acquires curvature due to the presence of pentagonal rings in the hexagonal graphite sheet. Fullerenes exists in discrete molecular form and consists of a hollow spherical cluster of sixty carbon atoms: a single molecule is denoted by C60. Each molecule is composed of groups of carbon atoms that are bonded to one another to form both hexagon (six-carbon atom) and pentagon (five carbon atom) geometrical configurations. One such molecule is found to consist of 20 hexagons and 12 pentagons which are arrayed such that no two pentagons share a common side; the molecular surface thus exhibits the symmetry of soccer ball. The material composed of C60 molecules that contains sixty carbon atoms in a network of sp2 bonding which forms a spherical structure, is known as buckminsterfullerene, from a number of perspectives, a molecule almost ‘custom built for nanoscience.
CARBON NANOTUBES
Another molecular form of carbon has recently been discovered that has some unique and technologically promising properties. ljiyama of NEC Japan in 1991 first announced the synthesis of carbon nanotubes. In his experiments on the arc discharge between graphite electrodes, along with the collected shoot particles there were some fine tube like structures. This on careful analysis revealed that they are carbon nanotubes (CNTs). Its structure consists of a single sheet of graphite, rolled into a tube, both ends of which are capped with C60 fullerene hemispheres. The tubes can be either open at their ends or capped at one or both ends with half a spheroidal fullerene.
NANOMATERIAL ADVANTAGE
Nanocrystalline materials are three dimensional solids composed of nanometre-sized grains, or crystallites. Because of their unique structure, which is characterized by ultrafine grains and a rather high density of crystal lattice defects, these materials have extra ordinary fundamental properties that could be exploited to make ‘next-generation’ super-strong materials.
High strength, or hardness has already been observed in many nanomaterials. But in most cases such nanomaterials have very low ductility–they fail when their shape is changed. Some even become brittle when a force or deforming load is applied. Strength and ductility are usually opposing characteristics: the higher the strength, the lower the ductility, and vice versa. This correlation is associated with the nature of plasticity: the more difficult it is for dislocations to appear and to move, the stronger but brittle and less ductile is any crystalline material.
Nanomaterials are also set to introduce several advantages in the electronics and computing industry. Their use will permit an increase in the accuracy of the construction of electronic circuits on an atomic level, assisting in the development of numerous electronic products.
The very large surface-to-volume ratio of nanomaterials is especially useful in their use in the medical field, which permits the bonding of cells and active ingredients. This results in the obvious advantage of an increase in the likelihood of successfully combatting various diseases.
DISADVANTAGES OF NANOMATERIALS
Currently, one of the main disadvantages associated with nanomaterials is inhalation exposure. This concern arises from animal studies, the results of which suggested that nanomaterials such as carbon nanotubes and nanofibers may cause detrimental pulmonary effects, such as pulmonary fibrosis. Further possible health risks are ingestion exposure and dust explosion hazards.
Additionally, there are still knowledge gaps regarding nanomaterials, meaning the manufacturing process can often be complex and difficult. The overall process is also expensive, requiring optimum results - especially regarding their use in consumer goods - to avoid financial losses.
SUPERCONDUCTORS
Superconductivity is the set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor.
Unlike an ordinary metallic conductor, whose resistance decreases gradually as its temperature is lowered even down to near absolute zero, a superconductor has a characteristic critical temperature below which the resistance drops abruptly to zero. An electric current through a loop of superconducting wire can persist indefinitely with no power source.
Figure 1: Temperature comparison
The superconductivity phenomenon was discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes. Like ferromagnetism and atomic spectral lines, superconductivity is a phenomenon which can only be explained by quantum mechanics. It is characterized by the Meissner effect.
The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature.
A superconductor with little or no magnetic field within it is said to be in the Meissner state. The Meissner state breaks down when the applied magnetic field is too strong.
Important points of superconductors:
(i) Some elements offer zero resistance property while carrying current when worked at below certain temperature.
(ii) Direct current (DC) flowing in the coil produces strong sustained magnetic field even the current becomes zero it is maintained.
(iii) Magnetic field is utilized as stored energy.
Above the critical temperature, a superconductor has no notable effect when a magnetic field is applied, as the magnetic field is able to pass through the superconductor unhindered. If the superconductor is below its critical temperature, the applied magnetic field is expelled from inside of the superconductor and bent around it, called as Meissner effect, shown in figure. Because surface current that flows without resistance create magnetization within superconductor is equal and opposite to the applied magnetic field, resulting in cancelling out the magnetic field everywhere within the superconductor. This results in superconductor having a magnetic susceptibility of -1, that means it exhibits perfect diamagnetism.
Fig: state of metal in normal conducting state and superconducting state
Superconductors can be divided into two classes according to how this breakdown occurs.
1.1. Type-I Superconductors: There are thirty pure metals which exhibit zero resistivity at low temperatures and have the property of excluding magnetic fields from the interior of the superconductor (Meissner effect). They are called Type I superconductors.
The superconductivity exists only below their critical temperatures and below a critical magnetic field strength.
Type I superconductors are well described by the BCS theory, which relies upon electron pairs coupled by lattice vibration interactions.
Remarkably, the best conductors at room temperature (gold, silver, and copper) do not become superconducting at all. They have the smallest lattice vibrations, so their behavior correlates well with the BCS Theory.
A type I superconductor consists of basic conductive elements that are used in everything from electrical wiring to computer microchips. At present, type I superconductors have Tc between 0.000325 °K and 7.8 °K at standard pressure. Some type I superconductors require incredible amounts of pressure in order to reach the superconductive state. One such material is sulphur which requires a pressure of 9.3 million atmospheres (9.4 x 1011 N/m2) and a temperature of 17 °K to reach superconductivity. Some other examples of type I superconductors include Mercury - 4.15 °K, Lead - 7.2 °K, Aluminum - 1.175 °K and Zinc - 0.85 °K.
The Type I superconductors have been of limited practical usefulness because the critical magnetic fields are so small and the superconducting state disappears suddenly at that temperature.
1.2. Type-II Superconductors: Starting in 1930 with lead-bismuth alloys, a number of alloys were found which exhibited superconductivity; they are called Type II superconductors. They reach a superconductive state at much higher temperatures when compared to Type I superconductors.
They were found to have much higher critical fields and therefore could carry much higher current densities while remaining in the superconducting state.
Type I superconductors are sometimes called "soft" superconductors while the Type II are "hard", maintaining the superconducting state to higher temperatures and magnetic fields. Type II superconductors can also be penetrated by a magnetic field whereas a type I cannot.
Figure 2: Magnetic field variation of Type Ӏ
(A) and Type ӀӀ (B) superconductor
The superconducting state cannot exist in the presence of a magnetic field greater than a critical value, even at absolute zero. This critical magnetic field is strongly correlated with the critical temperature for the superconductor, which is in turn correlated with the band gap. Type II superconductors show two critical magnetic field values, one at the onset of a mixed superconducting and normal state and one where superconductivity ceases.
It is the nature of superconductors to exclude magnetic fields (Meissner effect) so long as the applied field does not exceed their critical magnetic field. This critical magnetic field is tabulated for 0K and decreases from that magnitude with increasing temperature, reaching zero at the critical temperature for superconductivity. The critical magnetic field at any temperature below the critical temperature is given by the relationship:-
PROPERTIES OF SUPERCONDUCTORS
- The superconducting state is perfectly diamagnetic in nature.
- They have zero resistivity.
- They have infinite conductivity.
APPLICATIONS OF SUPERCONDUCTOR
- Magnetic-levitation is an application where superconductors perform extremely well. Transport vehicles such as trains can be made to "float" on strong superconducting magnets, virtually eliminating friction between the train and its tracks. Not only would conventional electromagnets waste much of the electrical energy as heat, they would have to be physically much larger than superconducting magnets.
- Niobium – Titanium (Nb-Ti) alloy (microfilaments) embedded in a copper core is used as superconducting material. To utilize the superconducting property of alloy it is kept below critical temperature about 4.2 K (-269 ̊C) where the resistance offer to the current is zero. To maintain the temperature at critical value require electrical power that why it is important to operate the system with minimum heat loss. This critical temperature is maintained in helium filled tank by using some cryogenic refrigerator. Helium is used as working fluid because only helium is liquid at this critical temperature. Nb-Ti was the first material for the practical manufacture of superconducting wire.
- Recently, power utilities have also begun to use superconductor-based transformers and "fault limiters".
- Superconductors also used as energy storage devices which store the energy in the form of magnetic field and utilize it, to supply the load in micro grid.
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