ENG PHY-II UNIT 2: SUPERCONDUCTIVITY AND NANOMATERIALS

KEC GHAZIABAD ENGINEERING PHYSICS-II [Lecture wise-UNIT2] KEC GHAZIABAD 14 ENGINEERING PHYSICS-II 2014 Meissner Effect Lecture-1 Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. Figure: Meissner Effect The Meissner effect is an expulsion of a magnetic field from a superconductor during its transition to the superconducting state. If a magnetic field (B) is applied through a superconductor above transition temperature (Tc), magnetic field lines passes through it, however below Tc magnetic field lines expel out from the superconductor. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 8, 1911 in Hg, which has critical temperature of 4.2 K. Critical Temperature The temperature at which the transition from normal to superconducting state occurs is known as critical temperature. It is well defined temperature and specific to the particular element or material. It is not very sensitive to the small amount of impurities. It is denoted by Tc ExampleCritical temperature for Hg and MgB2 are 4.2 and 38 K respectively. Properties of superconductors 1. Current in these materials persists for long time. 2. These materials lie in the inner column of the periodic table. 3. These materials exhibit Meissner Effect. 4. Transition metals having odd number of valence electron are favorable for superconductivity. B0    B  o ( H  M )  H  M  0 M    1 H For superconductor, below Tc  M Magnetic susceptibility is defined as,    H Hence,   1 This shows that superconducting state is a state of perfect diamagnetism. [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 2 ENGINEERING PHYSICS-II 2014 Distinction between superconductor and perfect conductor Both superconductor and perfect conductor have zero resistivity, so distinction between them is an issue. The only method which is used to distinguish between these two is Meissner Effect. For superconductor When magnetic field is applied to a superconductor flux lines are ejected from the sample surface. i. e. B=0 For Perfect Conductor Maxwell’s IIIrd equation   B  E   t  T H c  H co 1     TC    2    Where, Tc is transition temperature. (i ) If a potential difference V is applied across a material of resistance R, then V  IR E  V IR  d d I 0 0 d For perfect conductor, R=0 E   B  F romeq (i ) 0 t B  cons tan t This shows that perfect conductor will allow to pass flux lines through it. Assignments: 1. Show the superconducting state is state of perfect diamagnetism. 2.How one can distinguish between a perfect conductor and superconductor? Explain 3.Define superconductivity and critical temperature. 4. Enlist the name of at least five superconductors and their critical temperature. Lecture 2 Critical Magnetic Field (Hc)The minimum applied field necessary to destroy superconductivity and further restore the normal resistivity is called the critical magnetic Hc. The value of crtical magnetic field changes with temperature of superconducting material. If Ho is the critical magnetic field at absolute zero, then Numerical: 1. The transition temperature for Pb is 7.2K. However at 5K it loses the superconducting property if subjected to a magnetic field of 3.3×104 A/M. Find the maxiumum value of H which will allow the metal to retain its superconductivity at 0K. Ans. Given, Tc =7.2 K, T=5K, Hc=3.3×104 A/M   T 2  H c  H co 1       TC   Hc 3.3  10 4  H co   2   T 2   5   1     1   7 . 2   T   C   3.3  10 4 3.3  10 4  H co    25  1  0.486 1    51.28  3.3  10 4  6.42  10 4 A/ M H co  0.514 2.Along thin superconducting wire of a metal produces a magnetic field 105×103 A/M on its surface due to the current through it at a certain temperature T. The critical field of the metal is 150×103 A/M at absolute zero. The critical temperature Tc of the metal is 9.2K. What is the value of T. [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 3 ENGINEERING PHYSICS-II 2014 Ans. Crtical magnetic field of superconductor is given as  T   H c  H co 1       TC   2 Hc   T        1  H co   TC     2 T    TC   H   1  c H co  (ii)Type –II superconductors- In these superconductors from superconducting state to normal state to normal state occurs in the range of magnetic field. The transition starts from Hc1 and ends at Hc2. Below Hc1. Below Hc1 sample is in superconducting and above Hc2, it is in normal state. Between Hc1 and Hc2, the state of material is known as Vortex state. 2  H  T  1  c  TC  H co  Given, Hc=105×103 A/m, Hco=150×103 A/m, Tc=9.2K Thus,   105  10 3  H  T   1  c   1  3  TC  150  10   H co   105  10 3  T  105    1   1   3  TC  150   150  10  T 3  45   0.55    TC 10  150  T  0.55Tc  0.55  9.2 T  5.06K Types of superconductors Superconductors are classified in two categories: (i)Type-I superconductorsIn these superconductors transition from superconducting state to normal state in presence of magnetic field occurs sharply at the critical value Hc. Meissner Effects in Type-I and type-II superconductors- Magnetic field through type-I superconductor is completely expelled out, however through type-II superconductor the behavior is different. Below Hc1 the magnetic field is completely expelled out and above Hc2 the magnetic field penetrates through material. In the range of Hc1 and Hc2, the magnetic field penetrates partially. The state between Hc1 and Hc2 is therefore known as mixed state or vortex state. Difference between type-I superconductors Effect/Parameter Type-I Meissner Effect Complete Transition Sharp at Hc Critical Field Other Name Example Applications and tye-II Type-II Partial Between Hc1 and Hc2 Low (~100- High (50T) 1000G) Soft Hard Hg, Nb Ceramic, Alloy Magneitc Permanent coils Magnets Assignments: 1.The transition temperature for Pb is 7.26K. The maximum critical filed for the material is 8 × 105A/m. Pb has to be used as superconductor subjected to a magnetic field of 4 × 104A/m. What [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 4 ENGINEERING PHYSICS-II 2014 precaution will have to be taken in terms of the temperature? 2.A superconducting material has a critical temperature of 3.7K in zero magnetic field of 0.306 Tesla at 0K. Find the critical field at 2K. 3.Calculate critical temperature of a superconductor when no magnetic field is present is Tc. Find the temperature at which the critical field becomes half of its value at 0K. 4. For a certain specimen, the critical fields are and for 14K and 13K respectively. Calculate the critical field at 5.0K. 5. Describe Type-I and type-II superconductors. Lecture 3 Critical Current The maximum current that can be passed in a superconductor without destroying its superconductivity is called critical current. This is denoted by Ic. If superconducting current is passing through a semiconductor wire of radius r, then I c  2rH c diameter 10-3 m at a temperature of 4.2K. Given the critical temperature for the sample is 7.18K and critical magnetic field is 6.5×104 A/m Solution:  T H c  H co 1     TC      4.2  2  H c  6.5  10 1       7.18   2  6.5  10 4 1  0.585 4  6.5  10 1  0.342  6.5  10 4  0.658  4.2  10 4 A/ m 4    10 3    4.28  10 4 I c  2rH c  2  3.14    2  I c  134 .3 A The critical current is given by The critical current density, Jc=Ic/area Jc  Ic  r 2 134.4  10 3   3.14    2  134.4 Jc  A/ m 2 6 0.785  10 J c  171.6  10 6 A/ m2 I c  2r H c  2 H  Numerical: 1. Determine the critical current and the critical current density for a superconducting rings of 2 Given, Tc=7.18 K, Hc(0)=6.5×104 A/m, T=4.2 K, diameter=10-3m Where, Hc is critical magnetic field for superconductor. Silsbee’s rule If a magnetic field is applied in transverse direction to the total magnetic field, then critical current Ic is given by This is known as Silsee’s rule. Persistent CurrentIf an electric current is set up in a perfect superconductor, it can persist for a very long time without any applied e.m.f. A current can be induced in a ring of superconducting material by cooling it in a magnetic field below a transition temperature and then switching off the field; when the field is off, the magnetic field outside the ring disappear but the flux inside the entire ring is trapped. Such a steady current which flows with undiminshing strength is known as persistent current.    2 J c  1.72  108 A/ m 2 2.How much current can Pb wire, 1.0 mm in diameter, carry in its superconducting state at 4.2K? Given Bc= 0.0548T Ans. Critical current is given by I c  2rH c We know that Hc=Bc/µ o and r=diameter/2=1/2mm=0.5 mm I c  2r 0 Bc  2    0.5  10 3  I c  2  0.5  10 3  0.0548 4  10 7 0.0548 4  10 7 548 0.0548 Ic   10 4  4 4 [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 5 ENGINEERING PHYSICS-II 2014 I c  137 A Parameters affecting the superconducting property of materials Following parameters may destroy the superconductivity of any material (i) Temperature (ii) Magnetic field (iii) Current density For a superconductor to retain its superconducting state, the critical values of these parameters are required. A diagram for superconducting state can be obtained for these parameters. For x=λ, H ( x)  H o e 1 H  H ( x)  o e Hence, the penetration depth characterizes the distance to which a magnetic field penetrates into a superconductor and becomes equal to 1/e times that of the magnetic field at the surface of the superconductor. Typical values of λ range from 50 to 500 nm. The penetration depth is determined by the superfluid density, which is an important quantity that determines Tc in high-temperature superconductors. At a temperature T, penetration depth is given by Jc, Hc and Tc are the critical values of current density, temperature and magnetic field. It is clear from the phase diagram that superconducting region appear below the critical values of these parameters also a proper combination of these parameters is required. London Penetration depth Applied magnetic field through a superconductor does not suddenly drop zero at the surface but decays exponentially to zero according to equation H ( x)  H o e   x /   Where, Ho is the field at the surface, x is the distance from the surface, and λ is characteristic length and known as the London Penetration depth.  T  T    (0) 1     Tc    4    1 / 2 Where, λ(0) is penetration depth at 0K. It is given by  mo 0   2   0 ns e     1/ 2 Where, mo is the mass, and e is the charge on the electrons, and ns is the number of super electrons. Assignments 1.What is Silsbee’s rule? 2. Calculate the critical current which can flow through a long thin superconducting wire of aluminium of diameter 10-3m. Critical magnetic field for Aluminium is 7.9 10 3 A/ m . 3. Calculate the critical current density for 1mm diameter wire of lead at 4.2K. A parabolic dependence of critical filed Hc on T may be [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 6 ENGINEERING PHYSICS-II 2014 assumed. Given Tc(Pb) = 7.18K and Hc(Pb) = 6.5 × 104A/m. 4. State and explain London’s Equation of Superconductivity. Where, mo=9.1×10-31 Kg, µ o=12.56×10-7SI units, e=1.6×10-19 C, ns=1028 m-3  9.11031 0    12.56 107 10 28  1.6 10 19   Numerical: 1.The penetration depth λ of Hg at 3.5 K is about 750Ǻ. Find the penetration depth at 0K. Given Tc for Hg = 4.153K. Solution: The penetration depth at temperature T is given by  T  T    (0) 1     Tc  (0)   T     4  T 1     Tc  T  (0)   T 1     Tc  T  (0)   T 1     Tc                    4 1/ 2     (0)  750 1  0.854    3.5  4   7501       4.153   1/ 2  (0)  7501  0.521 / 2  7500.481 / 2   2    1/ 2 o 1 / 2 4 1/ 2  1/ 2 0  530 A 1 / 2 4   9.1  10 31 0    12.56  10 7  10 28  1.6  10 19  0.85  10 26 0  1.6  10 19 0  0.53  10 7 m Lecture 4   2   1/ 2 BCS theory This theory is given by Bardeen, Cooper and Scheffer in 1957. This theory propose attractive interaction between two electron through lattice vibration i. e.. phonon. The attractive interaction proceeds when one electron interacts with the lattice and deforms it. A second electron sees the deformed lattice and adjusts itself to take the advantage of deformation to lower its energy. Thus second electron interacts with first via the lattice deformation. The pair of these two electrons is known as copper pair. Since quanta of vibration is known as phonon. Hence, this interaction is also known as electron-phononelectron interaction.  (0)  7500.481 / 2  750  0.69  (0)  520 A 0 2. For a superconductor, the number of super electrons is 1028 m-3 and Tc=3K, find the penetration depth at OK. Ans. The penetration depth at OK is given as   o  0   2  n e   0 s  m 1/ 2 When an electron with wave vector K, destroyed the lattice, the lattice gain momentum as a result the momentum of electron decreases. So, a phonon of wave vector q is emitted. When another electron with wavevector K1 observes the energy from phonon its gets moementum. Therefore due to interaction occurs between two electrons with [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 7 ENGINEERING PHYSICS-II 2014 wave vector K-q and K+q. These electrons forms cooper pairs. generally contain fractional numbers to describe the doping required for superconductivity. Assignments 1.Write a short note on BCS theory. There are several families of cuprate superconductors and they can be categorized by the elements they contain and the number of adjacent copper-oxide layers in each superconducting block. For example, YBCO and BSCCO can alternatively be referred to as Y123 and Bi2201/Bi2212/Bi2223 depending on the number of layers in each superconducting block (n). The superconducting transition temperature has been found to peak at an optimal doping value (p =0.16) and an optimal number of layers in each superconducting block, typically n = 3. Below is the list of few cuprates. 2.Define cooper pairs. 3.The penetration depth  (T ) of Hg at 3.5K is about 750 A . Find the penetration depth at 0K. Given Tc for Hg = 4.153K 4.Determine the penetration depth in mercury at 0K, if the critical temperature of mercury is 4.2K and penetration depth is 57nm at 2.9K. 5. Calculate the London penetration depth or Tc = 3.7K, superconducting electron density (ns) = 7.3 × 103kg/m3, Atomic Weight = 118.7 and effective mass = 1.9m0 0 Lecture 5 High-temperature superconductors (HTS) Materials that behave as superconductors at unusually high temperatures are known as High Tc superconductors. HTS have been observed with transition temperatures as high as 138 K (−135 °C). There are two categories of these superconductors (i) Compounds of copper and oxygen (socalled "cuprates") (ii) iron-based compounds (the iron pnictides) The cuprate superconductors adopt a perovskite structure. The copper-oxide planes are checkerboard lattices with squares of O2− ions with a Cu2+ ion at the centre of each square. The unit cell is rotated by 45° from these squares. Chemical formulae of superconducting materials Formula No. of Cu-O Tc Notation planes (K) in unit cell YBa2Cu3O7 123 92 2 Bi2Sr2CuO6 Bi-2201 20 1 Bi2Sr2CaCu2O8 Bi-2212 85 2 Bi2Sr2Ca2Cu3O6 Bi-2223 110 3 Tl2Ba2CuO6 Tl-2201 80 Tl2Ba2CaCu2O8 Tl-2212 108 2 [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] 1 Page 8 ENGINEERING PHYSICS-II 2014 Tl2Ba2Ca2Cu3O10 Tl-2223 125 3 TlBa2Ca3Cu4O11 Tl-1234 122 4 HgBa2CuO4 Hg-1201 94 1 HgBa2CaCu2O6 Hg-1212 128 2 HgBa2Ca2Cu3O8 Hg-1223 134 3 much smaller because no space for heat would be required. Computers of today need a great deal of space for cooling.Computers are being developed today that use Josephson junctions. The Josepson effect states that electrons are able to flow across an insulating barrier placed between two superconducting materials. Josephson junctions have a thin layer of insulating materials squeezed between superconductive material. Josephson junctions require little power to operate, thus creating less heat. 5.Josephson Devices Applications of Superconductors 1.Superconducting Transmission Lines Since 10% to 15% of generated electricity is dissipated in resistive losses in transmission lines, the prospect of zero loss superconducting transmission lines is appealing. High Current densities above 10,000 amperes per square centimeter are considered necessary for practical power applications, and this threshold has been exceeded in several configurations. 2.Superconducting Motors and Generators Superconducting motors and generators could be made with a weight of about one tenth that of conventional devices for the same output. This is the appeal of making such devices for specialized applications. Motors and generators are already very efficient, so there is not the power savings associated with superconducting magnets. It may be possible to build very large capacity generators for power plants where structural strength considerations place limits on conventional generators. 3.Superconducting Magnetic Energy Storage Superconducting magnetic energy storage (SMES) stores electricity for long periods of time in superconductive coils. SMES will be used by electrical utilities some day. 4.Computers If computers used superconducting parts they would be much more faster than the computers today. They would Devices based upon the characteristics of a Josephson junction are valuable in high speed circuits. Josephson junctions can be designed to switch in times of a few picoseconds. Their low power dissipation makes them useful in highdensity computer circuits where resistive heating limits the applicability of conventional switches. 6.SQUID Magnetometer The superconducting quantum interference device (SQUID) consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions. The device may be configured as a magnetometer to detect incredibly small magnetic fields -- small enough to measure the magnetic fields in living organisms. Squids have been used to measure the magnetic fields in mouse brains to test whether there might be enough magnetism to attribute their navigational ability to an internal compass. 7.Magnetically Levitated Trains Perhaps the most famous and fascinating superconducting invention is magnetically levitated trains, or "maglev" trains. Maglev trains have no wheels and friction. The trains float silently on a magnetic field due to diamagnetic behaviour. Assignments 1.What are High Temperature superconductors? 2. Name few High Temperature superconductors and list out the characteristics of such type of materials. 3. Discuss the characteristics of superconductors in superconducting state. [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 9 ENGINEERING PHYSICS-II 2014 4. Discuss the application of superconductors. Lecture 6 Nanomaterials The materials whose particle size lie in the range 1-100 nm , known as nanomaterials. ExamplesNanoferrite, Fulleren, Carbon nanotube These properties have properties very much different from bulk materials (particle size >100 nm). For example 1. Paramagnetic materials becomes super paramagnetic when approaches to nanoregime. 2. Gold nanoparticles appear deep red to black in solution. Nanoparticles exhibit anomalous behavior due following reasons 1. Large surface (S) to volume (V) ratio These materials has large S/V ratio, which leads to significant changes in chemical or surface related properties. 4. Exchange length lax is comparable to the particle size which leads to modification in magnetic properties of these materials. Exchange length lax is defined as the distance between two neighboring spin. Production of nanomaterialsGenerally two approaches are used for producing nanoparticles (i) Bottom-up approaches-The nanoparticles are synthesizing by staring from molecular/atomic level. Chemical reaction methods are falls in this category. Follwing methods comes in the bottom-up approach (a)Hydrothermal method (b)citrate precursor method (c) micro-emulsion method (d)Nitrate method (ii) Top-down approaches-This methodology involves production of nanomaterials starting from bulk precursor. Bulk precursor is crushed into nanopartcles using various methods. Few of them are (a)Mechanical Milling (b)Electron-beam evaporation (c)rf sputtering (d)Pulsed Laser Deposition Buckyballs or Fullerene 2. Optical Confinement- Due to optical confinement the energy levels of nanoparticles are discrete. This gives rise to change in optical properties of nanomaterials. 3. In these materials generally mean free path is comparable to the particle size, which leads to change in electrical properties. A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, tube, and many other shapes. Spherical fullerenes are also called buckyballs, and they resemble the balls used in football (soccer). The first fullerene molecule to be discovered, and the family's namesake, buckminsterfullerene (C60), was prepared in 1985 by Richard Smalley, Robert Curl, James Heath, Sean O'Brien, and Harold Kroto at Rice University. In this molecule each carbon atom is bonded to three adjacent carbon atoms are arranged in sphere about a nanometer in diameter. In this structure carbon atoms are situated at 60 chemically equivalent vertices that are connected by 32 faces. Out of which 12 are pentagonal and 20 are hexagonal. In this structure [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 10 ENGINEERING PHYSICS-II 2014 Figure: Arrangement of carbon atom in a C60 molecule. The ball like molecules binds each other in solid state to form a crystal having face centered cubic. Lecture 7 Creation of buckyballs Buckyballs are created by following methods: (1) Carbon Arc Buckyballs are created in a inert gas, He, medium. This helium atmosphere is created by bell-jar apparatus which is filled with 100 Torr of helium. Inside the bell-jar two carbon rods should be placed slightly apart from each other. With these two carbon rods one can create a carbon arc between the two rods using the energy apparatus. This energy apparatus needs to provide some where between 100 and 200 amps, closer to the 200, of current at around 220 voltage, a portable arc welder can fill these requirements. This arc should be left on for approximately 10-15 seconds. After this a 5-10 minuet cool down period is recommended. This should have produced a good amount of soot on the inside of the bell-jar. This soot contains not only Buckministerfullerenes but other fullerenes and carbon molecules as well. The next step is to purify the carbon 60 and extract the molecules from the soot. (2) Solar Production of Fullerene (3) Pyrolytic Production of Fullerenes Figure: Arrangement of C60 in a crystal The buckyballs has two categories: (a) Larger molecules: C70, C82, C60 (b) Smaller molecules: C22 Assignments 1.What do you mean by Nanotechnology? 2.What is naoscience? 3.What is nano particle ?Explain 4.Write down the bottom up and top down approach? 5.Why the properties of naoparticle differ from its bulk materials? 6.How to visualize nanoscale? Carbon Nanotube Carbon tubes were discovered by S. Iijima. It is a tube-shaped material, made of carbon, having a diameter measuring on the nanometer scale. It is sheet of graphite called graphene, rolled into a cylindrical structures. In these structures each carbon atom is covalently bonded to three other carbon atom. At face of each carbon nanotubes carbon atoms are arranged in hexagonal type geometry. If the ends of tubes are closed, then carbon atoms are arranged in pentagonal structures at the ends. Carbon nanotubes can be of few nm in diameter and upto several nm in length. Hence they have large length to diameter ratio (~106). [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 11 ENGINEERING PHYSICS-II 2014 (ii)Multiwalled carbon nanotubesMulti-wall nanotubes can appear either in the form of a coaxial assembly of SWNT similar to a coaxial cable, or as a single sheet of graphite rolled into the shape of a scroll. The diameters of MWNT are typically in the range of 5 nm to 50 nm. The interlayer distance in MWNT is close to the distance between graphene layers in graphite. Carbon Nanotubes have many structures, differing in length, thickness, and in the type of helicity and number of layers Types of carbon nanotubes on the basis of number of layers Carbon nanotubes may be of two types. Single walled carbon nanotubes(i)Single-wall nanotubes (SWNT) are tubes of graphite that are normally capped at the ends. They have a single cylindrical wall.Most SWNT typically have a diameter of close to 1 nm. The tube length, however, can be many thousands of times longer. SWNT have unique electronic and mechanical properties which can be used in numerous applications, such as field-emission displays, nanocomposite materials, nanosensors, and logic elements. Assignments 1) Write down the bottom up and top down approach? 2) Why the properties of naoparticle differ from its bulk materials? 3) How to visualize nanoscale? Lecture 8 Types of Nanotubes on the basis of helicity Nanotubes on the basis of helicity are categorized by defining chrial vector. Chrial vector is defined as the unit vector in an infinite graphene sheet that describes how to to roll up graphene sheet to make nanotubes. The way the graphene sheet is wrapped is represented by a pair of indices (n,m). If a1 and a2 are the two unit vectors in graphene sheet then chiral vector is defined as    Ch  na1  ma 2 The diameter of an ideal nanotube can be calculated from its (n,m) indices as follows d  a n 2  nm  m2  where a = 0.246 nm. [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 12 ENGINEERING PHYSICS-II 2014 (3) Chiral Nanotube This type of nanotube exhibits twist or spiral around the axis of nanotubes. For these nanotubes, n#m i.e. Ch  na1  ma 2  On the basis of chiral vector (orientation) nanotubes are defined as   (1) Arm chair In this nanotube there is a line of hexagonal parallel to the axis of tube. i.e. Ch  na1  na 2 For this nanotube, n=m       Ch  na1  a 2  (2) Zig-Zag Nanotubes In this nanotubes there is a line of carbon bonds down to the centre of hexagonal. For this nanotube, m = 0, Assignments 1) How many category of carbon nanotube are there? 2) What is carbon nanotube ? 3) What type of hybridization is present in carbon nanotube? 4) A arm chair carbon nano tube have diameter 1.35 nm, calculate the chiral vector for arm chair CNT.( Hint :- d = (n2 + m2 + nm)1/2 0.0783nm) i.e. Ch  na1   [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 13 ENGINEERING PHYSICS-II 2014 Lecure 9 Production of Nanotubes Following methods are used for producing carbon nanotubes. (1)Arc discharge Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a current of 100 amps that was intended to produce fullerenes. Laser ablation In laser ablation, a pulsed laser vaporizes a graphite target in a high-temperature reactor while an inert gas is bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes. The laser ablation method yields around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature. However, it is more expensive than either arc discharge or chemical vapor deposition.[40] Plasma torch catalyst particles is fed into the plasma, and then cooled down to form single-walled carbon nanotubes. Different single-wall carbon nanotube diameter distributions can be synthesized. Chemical vapor deposition (CVD) During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt,[84] iron, or a combination.[85] The metal nanoparticles can also be produced by other ways, including reduction of oxides or oxides solid solutions. The diameters of the nanotubes that are to be grown are related to the size of the metal particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700°C. To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen or hydrogen) and a carbon-containing gas (such as acetylene, ethylene, ethanol or methane). Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. Assignments 1) Write the typical properties and uses of carbon Nanotube 2) Explain the detailed method of production of carbon-nanotube by any one method with diagram? 3) What is carbon Buckuball and write their properties and uses Lecture 10 Properties of buckyballs The method is similar to arc-discharge in that both use ionized gas to reach the high temperature necessary to vaporize carbon-containing substances and the metal catalysts necessary for the ensuing nanotube growth. The thermal plasma is induced by high frequency oscillating currents in a coil, and is maintained in flowing inert gas. Typically, a feedstock of carbon black and metal [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Properties of Carbon Bucky balls: 1. Because of spherical shape , Bucky ball have extremely stable configuration which is resilient to impact and deformation 2. The C60 buck balls can withstand high Page 14 ENGINEERING PHYSICS-II 2014 temperature and pressure. 3. Bucky ball have aromatic nature as electron are free to move among other bonds in hexagon carbon rings. 4. Bucky ball are infect the only known carbon allotropes which are soluble, have limited solubility , in most of the solvents. 5. By doping buck ball they can be electrically insulating , conducting , semiconducting or even superconducting Uses of Bucky balls: 1. Hydrogen storage as almost every carbon atom in C60 absorb a hydrogen atom without disrupting the buckyball structure , making it more effective than metal hydrides .This could lead to application in fuel cell. 2. Bucky are now being considered for uses in the field of medicine, both for diagnostic and drug delivery purpose. 3. The scanning Tunneling microscope in one of the tools in microscopy which uses needle OF the BUCKYBALLS. 4. Doping metals onto the surface of Bucky ball offers the possibility for become catalysts. Properties of Carbon Nanotube: 1. CTNs have high electrical conductivity 2. CTNs have Very high tensile strength. 3. CNTs have highly flexible can be bent considerably without damage. 4. CNTs have high thermal conductivity in the axial direction. 5. CNTs have low thermal expansion coefficient. 6. CNTs are good electron field emitters. 7. CNTs have a hiogh aspect ratio (length=1000x diameter) ranging from every life items like clothes and sports gear to combat jackets. 2. Nanotube based transistors have been made that operate at room temperature. 3. Carbon Nanotube has also been proposed as a possible gene delivery vehicle and for use in combination with radiofrequency fields for destroys cancers cells. Application of nanotubes Carbon Nanotube Technology can be used for a wide range of new and existing applications:              Conductive plastics Structural composite materials Flat-panel displays Gas storage Antifouling paint Micro- and nano-electronics Radar-absorbing coating Technical textiles Ultra-capacitors Atomic Force Microscope (AFM) tips Batteries with improved lifetime Biosensors for harmful gases Extra strong fibers Applications of nanotechnology Assignments 1) Explain how a Buckyball can be used as anti-oxidant, medical imaging and drug delivery system? 2) Write the advantage of nanotechnology over the conventional technology Uses of Carbon Nanotube: 1. Because of the great mechanical properties of the carbon nanotube , a variety of structure have been proposed [UNIT-II:SUPERCONDUCTIVITY & NANOMATERIALS] Page 15