Presentation on theme: "Identify that metals possess a crystal lattice structure"— Presentation transcript:
1 Identify that metals possess a crystal lattice structure Caroline Chisholm CollegePhysicsMetals are made of atoms arranged in a regular three-dimensional pattern or lattice. The classical model describes valence electrons as the common property of all atoms in the lattice, forming an ‘electron cloud’.Describe conduction in metals as a free movement of electrons unimpeded by the latticeRemember:In conductors, valence electrons are free to move like a cloudThe presence of an electric field causes the derandomisation of this ‘cloud’ motionE fieldThe electrons ‘drift’ with a velocity vdMuch of their energy is given up through collisions with the lattice.Identify that resistance in metals is increased by the presence of impurities and scattering of electrons by lattice vibrationsThe thicker the wire, the more electrons & the greater the current that canflow for a given voltage. The atoms that form the lattice vibrate more as their temp. increases. As the electrons begin to move, they collide with impurities and tiny imperfections in the lattice. As a result, the resistance increases. Superconductors have different electron movement - passing through the lattice unimpeded.
3 Caroline Chisholm College Physics Superconductors are materials thatlose all electrical resistancebelow certain temperaturesDescribe the occurrence in superconductors below their critical temperature of a population of electron pairs unaffected by electrical resistanceCaroline Chisholm CollegePhysicsWhen a current is magnetically induced in a superconducting ring, it continues without measurable decrease for many months!The temperature at which a material becomes superconducting is called the critical temperature.Below this temperature, once electrons are excited to energy levels (current), there is an energy gap - electrons cannot decay by collisions with ions in the lattice because the lattice has insufficient thermal energy. So they are free to move through the lattice without collisions.Above the critical temperature, the energy gap is zero so the lattice does have enough energy to scatter the electrons
4 Caroline Chisholm College PhysicsDiscuss the BCS theoryIn a superconductor, the electrons travel in pairs and move quickly between the atoms with less energy loss.As a negatively-charged electron moves through the space between two rows of positively-charged atoms (like the wires in a window screen), it’s charge pulls inward on the atoms (a bit like rolling a heavy bowling ball along a mattress) . This distortion (and local concentration of positive charge caused by phonons) attracts a second electron to move in behind it. This second electron absorbs the phonon and encounters less resistance, much like a passenger car following a truck on the freeway encounters less air resistance.The two electrons form a weak attraction caused by phonon exchange, travel together in a pair (Cooper pairs) and encounter less resistance overall. In a superconductor, electron pairs are constantly forming, breaking and reforming, but the overall effect is that electrons flow with little or no resistance. The low temperature makes it easier for the electrons to pair up.
5 Caroline Chisholm College Process information to identify some of the metals, metal alloys and compounds that have been identified as exhibiting the property of superconductivity and their critical temperaturesCaroline Chisholm CollegePhysicsType 1 SuperconductorsThe Type 1 category of superconductors is mainly comprised of metals and metalloids that show some conductivity at room temperature. They require incredible cold to slow down molecular vibrations sufficiently to facilitate unimpeded electron flow in accordance with what is known as BCS theory. BCS theory suggests that electrons team up in "Cooper pairs" in order to help each other overcome molecular obstacles - much like race cars on a track drafting each other in order to go faster. Scientists call this process phonon-mediated coupling.Type 1 superconductors - characterized as the "soft" superconductors - were discovered first and require the coldest temperatures to become superconductive. They exhibit a very sharp transition to a superconducting state (see above graph) and "perfect" diamagnetism - the ability to repel a magnetic field. Below is a list of known Type 1 superconductors along with the critical transition temperature - known as Tc - below which each superconducts. (www.superconductors.org)plus othersLead (Pb) Lanthanum (La) Tantalum (Ta) Mercury (Hg) Tin (Sn) Indium (In) Thallium (Tl) Rhenium (Re)7.2 K K K K K K K KProtactinium (Pa) Thorium (Th) Aluminum (Al) Gallium (Ga) Gadolinium (Gd) Molybdenum (Mo) Zinc (Zn)1.40 K K K K K K K
6 Caroline Chisholm College Process information to identify some of the metals, metal alloys and compounds that have been identified as exhibiting the property of superconductivity and their critical temperaturesCaroline Chisholm CollegePhysicsType 2 SuperconductorsExcept for the elements vanadium, technetium and niobium, the Type 2 category of superconductors is comprised of metallic compounds and alloys. The recently-discovered superconducting "perovskites" (metal-oxide ceramics) belong to this Type 2 group.They achieve higher Tc's than Type 1 superconductors by a mechanism that is still not completely understood. Conventional wisdom holds that it relates to the planar layering within the crystalline structure (see above graphic). Although, other recent research suggests the holes of hypocharged oxygen in the charge reservoirs are responsible. (Holes are positively-charged vacancies within the lattice.)The superconducting cuprates (copper-oxides) have achieved astonishingly high Tc's when you consider that by 1985 known Tc's had only reached 23 K. To date, the highest Tc attained at ambient pressure has been 138 K. One theory predicts an upper limit of about 200 K for the layered cuprates. Others assert there is no limit. Either way, it is almost certain that other, more-synergistic compounds still await discovery among the high-temperature superconductors.
7 Caroline Chisholm College PhysicsType 2 Superconductors (cont.)The first superconducting Type 2 compound, an alloy of lead and bismuth, was fabricated in 1930 by W. de Haas and J. Voogd. But, was not recognized as such until much later, after the Meissner effect had been discovered. The first of the oxide superconductors was created in 1973 by DuPont researcher Art Sleight when Ba(Pb,Bi)O3 was found to have a Tc of 13K. The superconducting oxocuprates followed in 1986.Type 2 superconductors - also known as the "hard" superconductors - differ from Type 1 in that their transition from a normal to a superconducting state is gradual across a region of "mixed state" behavior. A Type 2 will also allow some penetration by an external magnetic field into its surface. While there are far too many to list in totality, some of the more interesting Type 2 superconductors are listed below by similarity and with descending Tc's. (www.superconductors.org)Hg0.8Tl0.2Ba2Ca2Cu3O K (record-holder) Tl2Ba2Ca2Cu3O K Bi1.6Pb0.6Sr2Ca2Sb0.1Cu3Oy 115 K (thick film on MgO substrate)
9 Perform an investigation to demonstrate magnetic levitation Caroline Chisholm CollegePhysicsAnalyse information to explain why a magnet is able to hover above a superconducting material that has reached the temperature at which it is superconductingLevitation of a magnet above a cooled superconductor, the Meissner Effect,has been well known for many years. If a superconductor is cooled below its critical temperature while in a magnetic field, the magnetic field surrounds but does not penetrate the superconductor. The magnet induces current in the superconductor which creates a counter-magnetic force that causes the two materials to repel. This can be seen as the magnet is levitated above the superconductor. Keep in mind that this will occur if the strength of the applied magnetic field does not exceed the value of the critical magnetic field (H) of the superconductor. If the magnetic field becomes too strong, it can penetrate the interior of the material and lose its superconductivity. In addition the force of repulsion must exceed the weight of the magnet.(www.ornl.gov)This happens because the appliedmagnetic field causes currents toflow in the superconductor whichcreate a magnetic field to cancelthe internal magnetic fieldThe magnetic field created by thecurrents also serves to repel theoriginal magnetic fieldTHIS IS DIAMAGNETISM
10 Caroline Chisholm College Physics Process information to discuss possible applications of superconductivity and the effects of those applications on computers, generators and motors and transmission of electricity through power gridsDiscuss the advantages of using superconductors and identify limitations to their useCaroline Chisholm CollegePhysicsThe major limitation is the need for low tempsMagnetic-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.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.Magnetic Resonance Imaging (MRI) was actually discovered in the mid 1940's. But, has only recently become an indispensable medical tool with the development of powerful computers to quickly process the large volume of data that is generated.
11 Caroline Chisholm College Physics Process information to discuss possible applications of superconductivity and the effects of those applications on computers, generators and motors and transmission of electricity through power gridsDiscuss the advantages of using superconductors and identify limitations to their useCaroline Chisholm CollegePhysicsHigh-energy particle research hinges on being able to accelerate sub-atomic particles to nearly the speed of light. Superconductor magnets make this possible.SUPERCONDUCTING ELECTRONS WILL ALSO TUNNEL THROUGHINSULATORS AND CREATE SUPERFAST SWITCHESElectric generators made with superconducting wire 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. Computer applications also (speed, size).Because no electrical energy is lost, relatively thin wires can carry huge currents, and no loss means thatelectrical energy can be stored -Superconducting magnetic energy storage (possibly)
12 Caroline Chisholm College Physics Gather and process information to describe how superconductors and the effects of magnetic fields have been applied to develop a maglev trainCaroline Chisholm CollegePhysicsanimationThe EMS system useselectromagnets of likepolarity in the guiderail and the trackThe passing superconductor inducesa current in the coils, making themelectromagnetsIn the EDS system, the train is levitated using the Meissner effectwith superconducting magnet on the train andelectrically conductive strips on the trackanimationIn both systems the train is accelerated alongthe track by rapidly alternating electromagnets(produced by alternating current)which create a series of attractions and repulsions
14 SOLID LIQUID GAS ASTROPHYSICS! Caroline Chisholm College Physics SUPERCONDUCTIVITY - change in state?A change in state is usually a reversible changein the physical properties (including appearance)brought about by a temperature changeThe physica l properties of superconductors do change at the critical temperature - they exhibitdiamagnetism and zero resistance.However, there is not a change in appearanceDEBATABLE!ASTROPHYSICS!