Project to be submitted for the partial fulfillment of CBSE, Class XII, Practical Examination

2006-07

Recipe Of A SuperconductorSubmitted by: Kartik Gupta

Submitted to: Mrs. Geetha Nair

Curriculum Vitae

Name: Kartik Gupta Father’s Name: Lt Col Jayant Gupta Name of the School: The Study Senior Secondary

School, Udaipur

CBSE Roll Number: CBSE Registration Number: A/05/03732/059693 Address: 12-B, Pologround, Saheli Marg, Udaipur Mode of Project: Single

CertificateThis is to certify that Mr. Kartik Gupta of Class

XII has satisfactorily completed the course of experiments and the project report in practical Chemistry prescribed by the Central Board of Secondary Education in the laboratory of The Study Senior Secondary School, Udaipur in the year 2006-07.

Date :

Signature of the Teacher-in-charge

Signature of the Principal

Acknowledgement

I dedicate this project to Mrs. Geetha Nair. Her able guidance has worked wonders and provided us with a new dimension for learning chemistry.

Preface

Have you ever imagined making a superconductor in your own school lab! Making one now is as easy as cooking a packet of Maggi-2 minute noodles (although it wouldn’t take 2 minutes to make and you will have to prepare the ‘Tastemaker’)!!

But first let us know what exactly a superconductor is. We will subsequently be dealing with its chemical and physical theory and then go on to make one…

Recipe Of A Recipe Of A SuperconductorSuperconductor

Above: Crystal structure of YBa2Cu3O7 - the so-called "1-2-3" superconductor

Contents What is Superconductivity? A Brief History of Superconductivity Superconductivity Theory – Background Recipe Of A Superconductor Applications of Superconductors

What is Superconductivity?Superconductivity is a phenomenon displayed by some materials when they are cooled below a certain temperature, known as the superconducting critical temperature, Tc.Below Tc, superconducting materials exhibit two characteristic properties:

•Zero electrical resistance•Perfect diamagnetism (the Meissner effect)

Zero electrical resistance means that no energy is lost as heat as the material conducts electricity - this has many applications.The second of these properties, perfect diamagnetism, means that the superconducting material will exclude a magnetic field - this is known as the Meissner effect (after its discoverer), and can be used to display extraordinary physical effects.

Superconducting materials can be categorised into one of two types:

Type I Superconductors - which totally exclude

all applied magnetic fields. Most elemental superconductors are Type I.

Type II Superconductors - which totally exclude low applied magnetic fields, but only partially exclude high applied magnetic fields; their diamagnetism is not perfect but mixed in the presence of high fields. Niobium is an example of an elemental Type II superconductor.

Both types exhibit perfect electrical conductivity, and can be restored to 'normal' conductors in the presence of a sufficiently strong magnetic field.

A Brief History of Superconductivity

The Discovery of Superconduction The Meissner Effect Theory of Superconduction High Temperature Superconduction

The Discovery of Superconduction

•Before the discovery of superconduction, it was already known that cooling a metal increased its conductivity - due to decreased electron-phonon interactions).

•After the 'discovery' of liquefied helium, allowing objects to be cooled to within 4K of absolute zero, it was discovered (by Onnes, 1911) that when mercury was cooled to 4.15K, its resistance suddenly (and unexpectedly) dropped to zero (i.e. it went superconducting).

•In 1913, it was discovered that lead went superconducting at 7.2K. It was then 17 years until niobium was found to superconduct at a higher temperature of 9.2K. Onnes also observed that normal conduction characteristics could be restored in the presence of a strong magnetic field.

Graph Showing sudden drop in resistivity when the temperature of mercury is lowered to 4.5 K.

The Meissner Effect

It was not until 1933 that physicists became aware of the other property of superconductors - perfect diamagnetism. This was when Meissner and Oschenfeld discovered that a superconducting material cooled below its critical temperature in a magnetic field excluded the magnetic flux. This effect has now become known as the Meissner effect.

The limit of external magnetic field strength at which a superconductor can exclude the field is known as the critical field strength, Bc.Type II superconductors have two critical field strengths; Bc1, above which the field penetrates into the superconductor, and Bc2, above which superconductivity is destroyed, as per Bc for Type I superconductors.

Above: The Meissner effect - a superconducting sphere in a constant applied magnetic field excludes the magnetic flux.

                                                                   

Theory of Superconduction

Fritz and Heinz London proposed equations to explain the Meissner effect and predict how far a magnetic field could penetrate into a superconductor, but it was not until 1950 that any great theoretical progression was made, with Ginzburg-Landau theory, which explained superconductivity and provided derivation for the London equations.

Ginzburg-Landau theory has been largely superseded by BCS theory, which deals with superconduction in a more microscopic manner.BCS theory was proposed by J. Bardeen, L. Cooper and J. R. Schrieffer in 1957 - it is dealt with in the Theory section. BCS suggests the formation of so-called 'Cooper pairs', and correlates Ginzburg-Landau and London predictions well.However, BCS theory does not account well for high temperature superconduction, which is still not fully understood.

High Temperature Superconduction

The highest known temperature at which a material went superconducting increased slowly as scientists found new materials with higher values of Tc, but it was in 1986 that a Ba-La-Cu-O system was found to superconduct at 35K - by far the highest then found. This was interesting as BCS theory had predicted a theoretical limit of about 30-40K to Tc (due to thermal vibrations).Soon, materials were found that would superconduct above 77K - the melting point of liquid nitrogen, which is far safer and much less expensive than liquid helium as a refrigerant.

Although high temperature superconductors are more useful above 77K, the term technically refers to those materials that superconduct above 30-40K.In 1994, the record for Tc was 164K, under 30GPa of pressure, for HgBa2Ca2Cu3O8+x.

Superconductivity Theory - Background

Electron Conduction Transport

Metals conduct electricity via delocalised electrons within the metal lattice in a metal, the atoms lose valence electrons to form a lattice of positively-charged cations. The valence electrons are then delocalised throughout the lattice, and are free to move between the cations - these electrons are the current carriers.

The simplest way of explaining conductivity is by using the Drude model. The Drude model makes the assumption the conducting electrons

do not interact with the cations (the "free electron approximation"), except for collisions, where the velocity of the electron abruptly and randomly changes direction as a result of collision ("relaxation time approximation");

maintain thermal equilibrium throughout collisions ("classical statistics approximation");

do not interact with each other ("independent electron approximation").

Above: The Drude model: approximates the metal to a lattice of cations through which delocalised electrons flow.

For our purposes, it is necessary to adopt a modified instance of the Drude model, whereby the electrons are assumed to have zero electrical potential between the cations, but near the cations the potential is negative - that is, the free electron approximation outlined above is not adopted.

Above: Abandoning the free electron approximation: the potential is negative near the cations and zero in the region between ions.

Electron-Phonon Interaction

The cations within the lattice are oscillating about their equilibrium positions due to thermal energy. The resulting propagating lattice vibrations are called phonons, as they are essentially sound waves.The electrons then interact with the cations as they move through the lattice - causing charge distortions that propagate along the lattice structure, in turn causing distortions in the periodic potential. These distortions can affect the motion of another electron at some distance that is also interacting with the lattice in a similar way - this is thus called an electron-phonon interaction, and is an integral part of Cooper pair formation.

From this, it is simple to see why conductivity decreases with temperature - increased thermal energy will cause the cations to oscillate more violently; the electron-phonon interactions are greater and so impede the flow of electrons through the lattice. Conceptually, it is simplest to visualise this as the cations physically 'knocking' the electrons off their paths:

Above: Lattice distortion: around an electron causes an increase in positive charge density that will propagate along the lattice with the cation vibrations.

Recipe Of A Superconductor

Making your own Superconductors

Making your own Superconductors

With the advent of high temperature superconduction, it is relatively simple to prepare and use a ceramic high temperature superconductor. What follows are brief instructions for making an yttrium-barium-copper-oxide superconductor - these are taken from the instructions provided with a superconductor fabrication kit that was marketed by Colorado Futurescience. The method is typical of ceramic processes in scientific research…

Contents of the Recipe

Equipment Method

1. Mixing the chemicals2. Calcination3. Intermediate firing(s)4. The final oxygen annealing

Testing the superconductor

Equipment

To make an yttrium-barium-copper-oxide superconductor, you will need:

Yttrium Oxide Barium Carbonate (TOXIC) Cupric Oxide A Laboratory Furnace or a converted pottery

kiln. Labware made of alumina. An Oxygen Source Liquid Nitrogen and a rare-earth magnet for

testing and demonstrating the superconductors

Method

There are a number of methods of producing ceramic superconductors like this, but the simplest is the so-called "shake and bake" method, which involves a four step process:

Mixing the chemicals; Calcination (the initial firing); The intermediate firing(s) (oxygen annealings); The final oxygen annealing. The number of intermediate firings and the length of the

firings are largely up to the user. In general, the more intermediate firings, and the longer the duration of the firings under oxygen flow, the better the superconductor. But definite signs of superconductivity can usually be obtained without any intermediate firing at all. In fact, if the initial mixing of the chemicals is sufficiently thorough, the intermediate firing is not necessary at all.

1. Mixing the chemicals

The starting mix is a grey powder made by thoroughly mixing yttrium oxide, barium carbonate and cupric oxide in the ratios 1:2:3 (This superconductor is often referred to as "1-2-3" as a result) -

Yttrium Oxide, Y2O3 - 11.29 grams Barium Carbonate, BaCO3 - 39.47 grams Cupric Oxide, CuO - 23.86 grams

2. Calcination

For the initial heat treatment, called calcination, the mix is heated at 925-950 degrees Celsius for about 18-24 hours. This first treatment may be done in a crucible or evaporating dish made of alumina or of a good grade of laboratory porcelain. This forms the basic crystal structure of YBa2Cu3O6.5, and gets rid of the carbon dioxide from the barium carbonate. (Barium carbonate is used instead of barium oxide because barium oxide of any reasonable purity is difficult to obtain.

Also, exposing barium oxide to air tends to quickly convert much of it to barium carbonate and barium hydroxide.) The result of this first firing is a porous black or very dark gray clump. The coloration should be fairly even. An uneven green coloration is an indication that the powders are not as thoroughly mixed as they should have been, and that extra time and care should be taken to insure thorough grinding and mixing on subsequent steps. The material will seem to shrink rather dramatically during the initial firing as it loses its carbon dioxide and becomes much denser than the original powder mix.

3. Intermediate firing(s)

The porous black clump is ground into a fine powder and placed in the furnace in an alumina dish. After the furnace temperature reaches about 500 degrees Celsius, begin a slow flow of oxygen into the furnace. This heat treatment under oxygen flow is called oxygen annealing. A final furnace temperature of 925 to 975 degrees Celsius is recommended for the intermediate firings. A temperature much higher than this will result in a material that is much harder to re-grind. Temperatures above 1030 degrees Celsius may destroy the crystal structure.

After the mix has heated in the furnace for at least 18 hours at 925-975 degrees Celsius, reduce the temperature slowly. If you plan to test the sample for superconductivity after this firing, the cooling rate must be no more than 100 degrees per hour until 400 degrees Celsius is reached. The rate of cooling from 400 degrees down to room temperature can be increased to about 200 degrees per hour. If you do not plan to test for superconductivity after this firing, a cooling rate in excess of 100 degrees per hour may be used; however a cooling rate in excess of 250 degrees per hour is not recommended. Do not remove the oxygen flow until the indicated furnace temperature has fallen below 400 degrees Celsius.

The material should be thoroughly re-ground in a mortar and pestle (or similar device) between each firing. (If, after an intermediate firing, there is some green coloration in the resultant disk, it is important to take extra time and care in re-grinding and mixing the material before the next firing.) Problems that occur in the mixing and grinding process in any of these steps are often due to hard, coarse particles being mixed in with the finely powder material. An ordinary kitchen tea strainer can come in handy at this point to separate the coarser particles or lumps so they may be ground separately.

IMPORTANT: If you an ordinary tea strainer, make sure it is made of a non-magnetic material, or make sure you are satisfied that none of the material in the sifter or strainer will contaminate the chemicals. Even very small quantities of magnetic materials in the chemical mix can diminish or destroy the potential superconductivity. (It is also for this reason that "ceramic grade" chemicals, which tend to have iron impurities, are not often usable for making superconductors.) Shortcuts in grinding the materials, such as using an electric coffee grinder, often contaminate the compound with elements that destroy the superconducting properties. Some contaminates will destroy superconductivity in very tiny amounts. To keep your chances of success high, grinding with a good-quality mortar and pestle is the best method. This manual grinding can be an arduous process, but the results are worth the trouble.

4. The final oxygen annealing

The sample should be thoroughly reground, and the resultant black powder placed back in the alumina dish. The thickness of the layer of loose powder in the dish should match the desired thickness of the final superconducting disk. For this final firing, the powder should be as finely-ground and as densely-packed as possible. Do NOT pack the powder into the dish by pressing on it from the top (as this can makes the superconductor tend to stick to the alumina dish). Better results can usually be obtained by tapping the alumina dish with a pestle or a similar object so that the particles of the mix settle together in an evenly packed disk.

For this final heat treatment, heat the sample to between 950 degrees and 1000 degrees Celsius for about 18 hours. The higher temperature is better, but be sure of the accuracy of your temperature indicator before getting too close to 1000 degrees. Temperatures above 1020 degrees risk decomposition of the crystal structure and the possibility of the material sticking to the alumina dish. On the other hand, a final oxygen annealing at only 950 degrees Celsius will yield a superconductor that will crack easily, but will otherwise be satisfactory.

Testing the superconductor

The most foolproof test for superconductivity is the simplest. This is the test for diamagnetism using small rare earth magnets made of samarium-cobalt or neodymium-iron-boron. Use a very small rare-earth magnet at first. Start with a rare-earth disk magnet about 6 mm. in diameter. If you have made a good-quality superconductor, the magnet will levitate at least 3 mm. above the surface of the superconducting disk. A superconductor with poor levitation can usually be improved by re-grinding it and giving it an additional oxygen annealing.

When a superconductor levitates a magnet, a magnetic mirror image is formed in the superconductor of the levitating magnet due to the exclusion of the magnetic field (the Meissner effect). The magnetic mirror image insures that there is always a north pole induced in the superconductor directly below the north pole of the levitating magnet. There is a south pole induced in the superconductor directly below the south pole of the levitating magnet. This mirror image moves with the magnet as the magnet is moves, so that the disk magnet can be given a rapid spin without affecting its levitation. In fact the magnet may continue to spin for quite a long time because its spinning encounters no friction other than the friction of air resistance.

Applications of Superconductors

Efficient Electricity Transportation Magnetic Levitation Magnetic Resonance Imaging (MRI) Synchrotrons and Cyclotrons (Particle

Colliders) Fast Electronic Switches

Efficient Electricity Transportation

Superconductors have many uses - the most obvious being as very efficient conductors; if the national grid were made of superconductors rather than aluminium, then the savings would be enormous - there would be no need to transform the electricity to a higher voltage (this lowers the current, which reduces energy loss to heat) and then back down again.Superconducting magnets are also more efficient in generating electricity than conventional copper wire generators - in fact, a superconducting generator about half the size of a copper wire generator is about 99% efficient; typical generators are around 50% efficient.

The US Department of Energy are actively encourages the use of superconductors as energy efficient devices.At the moment, the problem lies with the critical temperature - unless a material is found that can superconduct above 300K, some sort of cooling system needs to be employed, which would be expensive, although companies are developing prototypes - in December 1998, Pirelli Wire built a test 150ft cable that transmitted electricity using high temperature superconducting materials.

Magnetic Levitation

So-called 'MagLev' trains such as the Yamanashi MLX01 train show above have been under development in Japan for the past two decades - the train floats above the track using superconducting magnets; this eliminates friction and energy loss as heat, allowing the train to reach such high speeds.

Magnetic Resonance Imaging (MRI)

MRI is a technique developed in the 1940s that allows doctors to see what is happening inside the body without directly performing surgery. The development of superconductors has improved the field of MRI as the superconducting magnet can be smaller and more efficient than an equivalent conventional magnet.

Synchrotrons and Cyclotrons (Particle

Colliders)Particle Colliders like CERN's Large Hadron

Collider (LHC) are like very large running tracks that are used to accelerate particles (i.e. electrons, positrons, hadrons and more) to speeds approaching the speed of light before they are collided with one another or other atoms, usually to split them (this was how many sub-nuclear particles such as taus and neutrinos were discovered).

They do this by cycling the particle using magnetic fields, continually increasing the speed of the particle.

The first project to use superconducting magnets was the proton-antiproton collider at Fermilab.

Fast Electronic Switches

Type II superconductors can be used to as very fast electronic switches (as they have no moving parts), due to the way in which a magnetic field can penetrate into the superconductor - this has allowed Japanese researchers to build a 4-bit computer microchip (compared to today's 32-bit and 64-bit processors) operating at about 500 times the speed of current processors, where heat output is currently a major problem with typical speeds approaching the 1GHz mark.

An article in Superconductor Week focuses upon the efforts of NASA, DARPA and others to build a 'petaflop' (a thousand-trillion floating point operations per second - compared to today's 'teraflop' (1 trillion Flops per sec) computers) computer using superconductor technology.

Conclusion

Although superconductivity remains an area of high interest in both Chemistry and Physics, yet its applications are nevertheless in the dormant phase. The basic problem lies with its multiple operational barriers along with the financial aspect of the technology. It costs trillions of dollars to setup an infrastructure involving superconductors but sooner or later cheaper ways to device them shall be developed and their utility will exceed their price.

Bibliography And References

This project has been made with references made from:

www.wikipedia.com www.w3.org