Superconductors and their applications Electrical resistance Using the

Superconductors and their applications

Electrical resistance Using the flow analogy, electrical resistance is similar to friction. For water flowing through a pipe, a long narrow pipe provides more resistance to the flow than does a short fat pipe. The same applies for flowing currents: long thin wires provide more resistance than do short thick wires. The resistance (R) of a material depends on its length, cross-sectional area, and the resistivity (the Greek letter rho), a number that depends on the material: The resistivity and conductivity are inversely related. The electrical resistance of a conductor is a measure of how difficult it is to push the charges along.

A semi-conductor will only conduct in one direction. After a certain amount of current is flowing, the voltage drop is almost constant. A condutor is like a simple wire. Current can flow in any direction. There is a fairly low resistance. A super condutor is a special material that at certain temperatures (usually very cold) has zero resistance. There a lot of uses for this, some haven't be realized on a large scale yet, and some have

SUPERCONDUCTORS • Superconductivity is a phenomenon in certain materials at extremely low temperatures , characterized by exactly zero electrical resistance and exclusion of the interior magnetic field (i. e. the Meissner effect) Will be Discussed later on • This phenomenon is nothing but losing the resistivity absolutely when cooled to sufficient low temperatures.

WHY WAS IT FORMED ? • Before the discovery of the superconductors it was thought that the electrical resistance of a conductor becomes zero only at absolute zero • But it was found that in some materials electrical resistance becomes zero when cooled to very low temperatures • These materials are nothing but the SUPER CONDUTORS. Examples: Lead, niobium nitride

WHO FOUND IT? • Superconductivity was discovered in 1911 by Heike Kammerlingh Onnes , who studied the resistance of solid mercury at cryogenic temperatures using the recently discovered liquid helium as ‘refrigerant’. • At the temperature of 4. 2 K , he observed that the resistance abruptly disappears. • For this discovery he got the NOBEL PRIZE in PHYSICS in 1913. • In 1913 lead was found to super conduct at 7 K. • In 1941 niobium nitride was found to super conduct at 16 K

SUPERCONDUCTING MATERIALS Superconductivity - The phenomenon of losing resistivity when sufficiently cooled to a very low temperature (below a certain critical temperature). Ø H. Kammerlingh Onnes – 1911 – Pure Mercury Resistance (Ω) 0. 15 0. 10 0. 0 Tc 4. 0 4. 1 4. 2 4. 3 4. 4 Temperature (K)

When cooled to sufficiently low temperatures, a large number of metals and alloys can conduct electric current without resistance. Obviously, these specific materials undergo a phase transition to a new superconducting state characterized by the complete loss of d. c. resistance below a well defined critical temperature, TC. Thus zero resistivity (ρ=0), i. e. infinite conductivity is observed in a superconductor at all temperatures below a critical temperature (ρ = 0 for all T < TC ).

Transition Temperature or Critical Temperature (TC) Temperature at which a normal conductor loses its resistivity and becomes a superconductor. • Definite for a material • Very good electrical conductors not superconductors eg. Cu, Ag, Au • Types 1. Low TC superconductors 2. High TC superconductors

Occurrence of Superconductivity Superconducting Elements TC (K) Sn (Tin) 3. 72 Hg (Mercury) 4. 15 Pb (Lead) 7. 19 Superconducting Compounds Nb. Ti (Niobium Titanium) Nb 3 Sn (Niobium Tin) 10 18. 1

Temperature Dependence of Resistance Electrical Resistivity ρ=ρo + ρ(T) • Impurities High Temperature Impure Metals ρ = ρo + ρ(T) Low Temperature Pure Metals ρ = ρ(T) Impure Metals ρ = ρo Pure Metals ρ=0 Superconductor

MEISSNER EFFECT • Material exhibits perfect diamagnetism or flux exclusion. • Reversible (flux lines penetrate when T ↑ from TC)

The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples Meissner effect When the superconducting material is placed in a magnetic field under the condition when T≤TC and H ≤ HC, the flux lines are excluded from the material. Transition temperature is the temperature at which a material changes from one crystal state (allotrope) to another. For example, when rhombic sulfur is heated above 96°C it changes form into monoclinic sulfur. When cooled below 96°C it reverts to rhombic sulfur.

The samples, in the presence of an applied magnetic field, were cooled below what is called their superconducting transition temperature. Below the transition temperature the samples canceled nearly all magnetic fields inside. They detected this effect only indirectly; because the magnetic flux is conserved by a superconductor, when the interior field decreased the exterior field increased. The experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconducting state.

In a weak applied field, a superconductor "ejects" nearly all magnetic flux. The magnetic field of these surface currents cancels the applied magnetic field inside the bulk of the superconductor. Therefore the conductivity can be thought of as infinite: a superconductor. It does this by setting up electric currents near its surface. Because the field expulsion, or cancellation, does not change with time, the currents producing this effect (called persistent currents) do not decay with time.

So finally we can say that the major Conditions for a material to be a superconductor i. Resistivity ρ = 0 i. Magnetic Induction B = 0 when in an uniform magnetic field

Applications of Meissner Effect • Standard test – proof for a superconductor • Repulsion of external magnets Magnet Superconductor

Superconducting Elements: Metallic elements are mostly superconductors. Their TC are typically of the order of a few Kelvin. Among metals, niobium exhibits the highest critical temperature of the pure elements, TC = 9. 2 K. Noble metals, copper, silver and gold, and alkaline metals, sodium and potassium, of which are excellent conductors of electricity at ambient temperatures, are not superconductors down to very low temperatures. Magnetic metallic elements do not exhibit superconductivity.

What’s about Semiconductor? The best known semiconductors, Si and Ge, become superconductors under a pressure of ~2 K bar with TC = 7 and 5. 3 K respectively. Other elements that become superconductors under pressure include P, As, Se, Y, Sb, Te, Ba, Bi, Ce and U.

Types of Superconductors • • • Type I Sudden loss of magnetisation Exhibit Meissner Effect No mixed state Soft superconductor Eg. s – Pb, Sn, Hg • • • Type II Gradual loss of magnetisation Does not exhibit complete Meissner Effect Mixed state present Hard superconductor Eg. s – Nb-Sn, Nb-Ti

Characteristic Properties of Superconductors

(i) Zero Resistivity, i. e. Infinite Conductivity Electrical Resistance ( ρ= 0 for all T < TC): The electrical resistance of a superconductor at all temperatures below a critical temperature TC is practically zero. If we assume the usual Ohm’s law (V = RI) describing the superconducting state

Effect of Magnetic Field (ii) Meissner-Ochsenfeld Effect (B = O inside the superconductor): The magnetic inductance becomes zero inside the superconductor when it is cooled in a weak external field. The effect is called the Meissner-Ochsenfeld effect. The superconducting metal always expels the field from its interior, and has The superconducting state of a metal exists only in a particular range of temperature and field strength. The condition for the superconducting state to exist in the metal is that some combination of temperature and field strength should be less than a critical value.

Its important to know that the Superconductivity of the metal will disappear if the temperature of the specimen is raised above its TC, or if a sufficiently strong magnetic is employed. There always exists some critical field Hc, above which superconductivity disappears. Element HC at 0 K (m. T) Critical magnetic field (HC) – Minimum magnetic field required to destroy the superconducting property at any temperature H 0 – Critical field at 0 K T - Temperature below TC TC - Transition Temperature Nb 198 Pb 80. 3 Sn 30. 9

Thermal Properties of Superconductors The thermal conductivity of superconductors undergoes a continuous change between the two phases and usually lower in a superconducting phase and at very low temperatures approaches zero. This suggests that the electronic contribution drops, the superconducting electrons possibly plays no part in heat transfer. The thermal conductivity of tin (TC = 3. 73 K) at 2 K is 16 W cm– 1 K– 1 for the superconducting phase and 34 W cm– 1 K– 1 for the normal phase.

Applications of Superconductors

Application— 1 Maglev (magnetic levitation) trains. These work because a superconductor repels a magnetic field so a magnet will float above a superconductor – this virtually eliminates the friction between the train and the track. However, there are safety concerns about the strong magnetic fields used as these could be a risk to human health. Levitation is the process by which an object is suspended by a force against gravity, in a stable position without solid physical contact. Yamanashi MLX 01 train in Japan

Application---2 Large hadron collider or particle accelerator. Superconductors are used to make extremely powerful electromagnets to accelerate charged particles very fast (to near the speed of light). Application---3 SQUIDs (Superconducting Quantum Interference Devices) are used to detect even the weakest magnetic field. They are used in mine detection equipment to help in the removal of land mines.

Application---4 “E-Bombs” The USA is developing “E-bombs”. These are devices that make use of strong, superconductor derived magnetic fields to create a fast, high-intensity electromagnetic pulse that can disable an enemy’s electronic equipment. These devices were first used in wartime in March 2003 when USA forces attacked an Iraqi broadcast facility. They can release two billion watts of energy at once.

Application---5 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.

Summary of Applications • Large distance power transmission (ρ = 0) • Switching device (easy destruction of superconductivity) • Sensitive electrical equipment (small V variation large constant current) • Memory / Storage element (persistent current) • Highly efficient small sized electrical generator and transformer • E bombs • SQUIDs (Superconducting Quantum Interference Devices) •

Medical Applications • NMR – Nuclear Magnetic Resonance – Scanning • Brain wave activity – brain tumour, defective cells • Separate damaged cells and healthy cells • Superconducting solenoids – magneto hydrodynamic power generation – plasma maintenance

SQUIDS (Super conducting Quantum Interference Devices)

Discovery: The DC SQUID was invented in 1964 by Robert Jaklevic, John Lambe, Arnold Silver, and James Mercereau. Principle : Small change in magnetic field, produces variation in the flux quantum. Construction: The superconducting quantum interference device (SQUID) consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions.

Types Two main types of SQUID: 1) RF SQUIDs have only one Josephson junction 2)DC SQUIDs have two or more junctions. Thereby, • more difficult and expensive to produce. • much more sensitive.

Construction : Consists of superconducting ring having magnetic fields of quantum values(1, 2, 3. . ) Placed in between the two josephson junctions

Explanation : • When the magnetic field is applied perpendicular to the ring current is induced at the two junctions • Induced current flows around the ring thereby magnetic flux in the ring has quantum value of field applied • Therefore used to detect the variation of very minute magnetic signals