Superconnductivity

Topic: High Temperatures and Superconductors [EC01] A SUPER FUTURE AHEAD WITH SUPER CONDUCTIVITY ABSTRACT The discovery of superconductivity brings a tremendous change in the field of Science and Engineering from twentieth century onwards. The zero receptivity (or infinite conductivity) of a metal is known as superconductivity and this property of metal can be observed only at very low temperatures. Researchers are being carried out to develop superconductors for high temperatures. If it is made possible superconductors will bring  New revolutions in the field of miniaturization.  The traditional wheel will largely disappear with the introduction of magnetic levitation trains and vehicles.  The use of superconducting cables will almost eliminate electrical transmission losses.  The heat generation in microelectronic circuits will be almost eliminated and more powerful computers will come into existence. Finally development of superconductors may take the present world in to new dimensions. INTRODUCTION The discovery of superconductivity started from the findings of the Dutch physicist Heike Kammerlingh Onnes in 1911 that the resistance of mercury has an abrupt drop at a temperature of 4.1K and has practically a zero dc resistance value at temperature below 4.1K.This new phenomenon of zero resistance at low temperatures was soon found in many other metals and alloys. The temperature at which superconductivity first occurs in a material is termed as critical or transition temperature (Tc ). A new era in the study of superconductivity began in 1986 with the discovery of high critical temperature superconductors. This discovery has opened a new subject matter called the “High temperature superconductivity”. Efforts are also being made to develop “Organic Superconducting materials”. PROPERTIES OF SUPERCONDUCTORS CRITICAL FIELD: In superconductors their normal resistance may be restored if a magnetic field greater than the critical value Hc is applied to the specimen. Hc depends both on the material and on the temperature. It is zero Tc and as the temperature is reduced if increases, following approximately a parabolic law of the form.  T H C H O1  TC    2         It tends Ho as T approaches 00K.In a general the higher value of Tc the higher will be Hc Fig. Critical Magnetic field as a function of temperature. CRITICAL CURRENT DENSITY: An electric current is always associated with a magnetic field. Hence if a superconductor carries a current such that the field which it produces is equal to Hc, then the resistance of the sample will be restored. The current density at which it occurs is called the critical current density. TYPES OF SUPERCONDUCTORS: In order to study the types of superconductors we must have an idea about the BCS Theory. BCS THEORY: BCS Theory of Super Conductors: The fundamental idea underlying BCS theory is that electrons pair up with one another due to a special type of attraction (interaction).These pairs of electrons are called copper pairs. Normally two electrons repel each other. However, the electrons could attract each other via distortion of the lattice. The idea is if we consider an electron passing close to an ion, t here will be a momentary attraction between them which might slightly modify the vibrations of the ion. This in turn could interact with a second electron nearby which will also be attracted to the ion. But net effect of these two interactions is that there is an apparent attractive force between the two electrons and this would not have arisen if the ion had not been present. The BCS theory is able to explain all the properties shown by the superconductor. t Conductor Super Conductor Type 1: This type of substances discovered which are easily quenched by relatively week magnetic fields. The superconductivity in type 1 superconductors is modeled well by the BCS theory which relies upon electron pairs coupled by lattice vibration interactions. Sometimes these are also called as soft superconductors. Type 2: These are super conductors which are more robust than type 1. They vary for weak magnetic fields of low intensity and for high intensity magnetic fields. They allow sufficient current to flow which is generated by strong super conducting magnets. MEISSNER EFFECT OR DIAMAGNETISM: The expulsion of magnetic flux by the superconductors when they are placed in the magnetic field is known as Meissner’s Effect. From the Meissner’s effect the magnetic flux inside the superconducting specimen is zero but from the magnetic materials we know that B = µ 0 (H+M)  0 = µ 0 (H+M)  H = – M (or) χ = –1 Where χ is susceptibility of the material. From the above relation it can be said that the applied magnetic field get into utilize to magnetize the substance is an opposite direction. This is clearly, a diamagnetic behavior of the superconducting materials and its susceptibility In a superconductor the magnetization is in the direction opposite that of the external magnetic field. This is diamagnetism. If we take the coil to a region where there is a Magnetic Field the increase is the Magnetic flux through the coil will produce as induced emf and with it an induced current and an induced magnetic field. In according to Lenz’s law the induced field will be in the direction opposite the charge is the flux through the coil, in this case opposite the direction of the increasing external field through the superconducting coil. The induced emf disappears as soon as the coil comes to rest and the field through it stops increasing. In a coil made of normal wire the current and its magnetic field will then cease to exist. In a perfectly conducting coil, however, or on the surface of a cylinder, made of perfectly Flux Quantization Just as electric is quantized and occurs only is multiplies of electronic charge. So is the magnetic flux through a super conducting loop. The flux quantum is equal to h/2e (h-plank’s constant) equal to about 2*10-15T.m2. The remarkable fact is that the flux of the magnetic field through a non-super conducting area is restricted to nh/e, merely because it is surrounded by a superconductor. Josephson Effect: when an insulator is placed in between two superconductors, the super current flow across the insulator (which is of the order 10-50A) even in the absence of any voltage reference. This is known as υ. The Josephsons effect is of two types. 1. dc Josephson effect 2.ac Josephson effect 1.dc Josephson effect: Finding super currents across the Josephson’s junction without supplying in external energy to the Josephson arrangement is known as the dc Joseph son’s effect. 2. ac Josephson effect: Finding super currents of ac frequency flowing across Josephson junction with the supply of dc current to the ac Josephson arrangement is known as ac Josephson Effect. The ac frequency across the Josephson junction can be given as, υ= (2eV)/h. Where (2e) =charge of copper pair electrons, V=potential difference across the junction, h= plank’s constant. APPLICATIONS OF JOSEPHSON EFFECT: SQUID-Superconducting Quantum Interface Device: A loop that contains or more Josephson junctions for flux detection (or) measurement is called SQUID. SQUIDS can be used for the measurement of small magnetic field and their small changes. Military Applications: Squids can be used to detect Mines and submarines. APPLICATIONS OF SUPERCONDUCTORS IN REAL LIFE: Magnetic Resonance Imaging (MRI) of a human Skull: An area where superconductors can perform a life saving function is in the field of biomagnetism. Doctors need a non- invasive means of determining what’s going on inside the human body. By impinging a strong superconductor-derived magnetic field into the body, hydrogen atoms that exist in the body’s water and fat molecules are forced to accept energy from the magnetic field. They then release this energy at a frequency that can be detected and displayed graphically by a computer. Squids are capable of sensing a change in a magnetic field up to 100 billion times that the force that moves the needle on a compass. With this technology, the body can associated with MRI’s. Electrical applications: The normal and superconducting states in a super conducting material are reversible. Further the resistance suddenly drops to zero, conducting. These characters help us to use the super conducting materials as electrical switches. The temperature or magnetic field. The superconductor switches operated by magnetic field are called “cryotrons”. Superconductors are used as fuses; these can be used in windings of electric motors, generators, transformers etc. Electric Generators: Electric generators made with superconducting wise are far more efficient than conventional generators wound with copper wire. In fact their efficiency is above 99% and their size about half that of conventional generators. These facts make them very lucrative ventures for power utilizes. In one instance 250 pounds of superconducting wire replaced 18,000 pounds of vintage copper cable, making it over 7000% more space efficient. Power transmission: persistent currents should make the superconductors more preferable than normal cables. In fact, superconducting wires have been fabricated and used for power transmission. However this method is not at cost – effective the cables are being used only in experiments. With the development of HTS’s and design technology we can hope to employ superconductors for long-distance power transmission economically in near future. Superconducting Micro Chip: This will lead to smaller and much more powerful supercomputers. Silicon chips may be packed more densely to store information. Computers may be as fast as the nerve cells of the brain. PetaFlop Computers: Petaflop computers are a thousand trillion floating point operati o n s p e r second. Today’s fastest computing operations have only reached “teraflop” s peeds – trillions of operations p e r second. It has been conjectured that devices on the order of 50 manometers in size along with unconventional switching mechanisms, such as the Josephson junctions associated with superconductors, will be necessary to achieve these blistering speeds. Satellite Communications: Among emerging technologies is a stabilizing for earth- orbiting satellites that employs the properties of imperfect superconductors to reduce friction to near zero. Ultra-sensitive, Ultra- fast are being adapted to telescopes due to their ability to detect a single photon of light. Internet Communications: Superconductors may even play a role in Internet Communications soon. And since Internet data traffic is doubling every 100 days, superconductors’ technology is being called upon to meet this super need. MAGNETIC APPLICATIONS: Magnetic Leviation: A magnet stays leviating above super conductor because flux pinning. Flux pinning occurs in tiny defects in the crystalline of the superconducting material. Image the magnetic field around the magnet like lines of force. The superconductor repels the majority of the magnetic force lines, which support and leviate the magnet .A smaller portion of the lines of force become trapped in the defects in the superconductor matrix, and are held in place. These trapped lines of force (flux pinning) are reason the magnet doesn’t slide fall off the superconductor. As in the type2 superconductors have the upper critical field has more than 500 Kilo Gauss (>50 Tesla), they are used in manufacture of commercial solenoids (super magnets). Superconductors are also used as frictionless bearings. Type1 superconductors can be used as magnetic shielding and flux trapping devices. Storing Electrical Power: Once the current is induced in the superconducting materials, its lack of resistance allows the induced current to flow forever. These permanent currents in a super conductor also produce a magnetic field around the super conductor, creating a powerful Electro Magnet. Which has great applications in wide range of fields' Tunneling Effect: As dc current flows across the Josephson junction without any voltage source and further a small voltage (1 micro volt) produces high frequency (484 MHz) oscillations across such junction. Thus superconductors are use full for R.F generators. MEDICAL APPLICATIONS: Superconducting devices like, SQUIDS, Superconductors are also employed in magnetic resonance imaging (MRI) diagnostic techniques which rare widely used in these days. Growing Importance: These amazing applications which will give a new dimension to our world are being taken seriously, the flowing details gives its range of importance. 1 The National Science foundation along with NASA and various Universities are currently trying to develop PetaFlop Computers. 2. General electric has estimated the potential worldwide market for super conducting generators in the next decade at around 20-30 billion dollar. GE is currently developing a 100-megavolt-amphere prototype to confirm viability. 3. Both US & Japan have places to replace under ground copper power cables with super conducting cables cooled with liquid nitrogen. 4. According to various estimates, the worldwide market for super conductor products is projected to grow near $90 billions by near 2010 and $200 billion by 2020. CONCLUSION: With the development of superconductors at high temperatures, the growth in the field of science & technology has been tremendously improved. I n future ‘superconductors’ with their amazing properties will rule the world hence ‘There is no susceptibility of life, without superconductor’. Reference: 1. Applied physics by Dr.M.Chandra Sekhar and Dr.P.Appalanaidu. 2. Solid- state physics by S.O. Pillai. 3. Material Science and Engineering V. Raghavan. 4. Material Science by M. Arumugam. 5. Solid state physics by Belluvvi