Superconductors

Crisanta U. Ganado
MST Physics
 Superconductors

 Superconductors are the material having almost

zero resistivity and behave as diamagnetic below
the superconducting transiting temperature.
 Superconductivity is the flow of the electric current
without resistance in certain metals, alloys, and
ceramics at temperatures near absolute value.
 The Discovery

Superconductivity was first discovered in 1911

by the Dutch physicist, Heike Kammerlingh
Onnes.
 The Discovery

Onnes felt that a cold wire's resistance would

dissipitate. This suggested that there would be a
steady decrease in electrical resistance, allowing
for better conduction of electricity.
Resistance in a Conductor
Temperature being dropped?
 The Discovery

Onnes passed a current through a very pure

mercury wire and measured its resistance as he
steadily lowered the temperature. Much to his
surprise there was NO RESISTANCE at 4.2K.
Temperature Review!
 The Discovery

At some very low temperature point, scientists felt that

there would be a leveling off the resistance reached
some ill-defined minimum value allowing the current
with little or no resistance.
Superconductivity

At 4.2 K, the electrical

resistance (opposition
of a material to the flow
of electrical current
through it) vanished.
Meaning, extremely
good conduction of
Electricity -
Conductivity.
How will electrons flow in
superconductors?
 Important Factors to Define a
Superconducting State

The superconducting state is defined by the three

very important factors:
1. Critical Temperature (Tc)
2. Critical Magnetic Field (Hc)
3. Critical Current Density (Jc)
 The Critical Temperature (TC)

This is the critical temperature at which the resistivity of a

superconductor goes to zero. Above this temperature the
material is non-superconducting, while below it, the
material becomes superconducting.
 The Critical Temperature (TC)
The temperature which a material Metal Tc (K)
electrical resistivity drops to
absolute zero is called the Critical Aluminum 1.2
Temperature or Transition
Temperature. Tin 3.7
Below Critical temperature, Mercury 4.2
material is said to be
superconducting and above this it is Niobium 9.3
said to be in normal state. Below Niobium-
this temperature, the 17.9
Tin
superconductors also exhibits a
variety of several astonishing Tl-Ba-Cu-
125
magnetic and electrical properties. oxide
The Critical Magnetic Field (HC)

  T 2 
H C  H O 1    
  TC  

Above this value of an externally applied magnetic field a

superconductor becomes non-superconducting. This
minimum magnetic field required to destroy the
superconducting state is called the critical magnetic field.
The Critical Current Density (JC)

The scientific notation representing the "critical current

density" or maximum current that a superconductor can
carry without becoming non-superconductive.
For superconductors, maximum critical current density is
when the current density is strong enough to quench the
superconducting state.
 General Properties of
Superconductors
 Electrical Resistance: Virtually zero electrical
resistance.
 Effect of Impurities: When impurities are added to
superconducting elements, the superconductivity is
not loss but the Tc is lowered.
 Effects of Pressures and Stress: Certain materials
exhibits superconductivity on increasing the pressure
in superconductors, the increase in stress results in
increase of Tc value.
 General Properties of
Superconductors
 Isotope Effect: The critical or transition temperature
T c value of a superconductors is found to vary with
isotopic mass. The transition temperature is inversely
proportional to the square root of isotopic mass of
single superconductors.
 Magnetic Field Effect: If strong magnetic field
applied to a superconductors below its Tc , the
superconductors undergoes a transition from
superconducting state to normal state.
Types of Superconductors

Type I Type II
T < TC T > TC
Type I Superconductors
Type I: Low TC
Superconductors
(Superconducting State)
T < TC
 Superconductors that
require liquid helium
coolant are called low
temperature
superconductors.
 Liquid helium
temperature is 4.2 K
above absolute zero.
 The Meissner Effect
“The complete expulsion of all magnetic field by a
superconducting material.”

Exhibiting diamagnetic properties to the total exclusion of

all magnetic fields. (Named for Walter Meissner.) This is a
classic hallmark of superconductivity and can actually be
used to levitate a strong rare-earth magnet.
 The Magnetic Levitation

Magnetic Levitation, maglev, or magnetic suspension is a

method by which an object is suspended with no support
other than magnetic fields. Magnetic force is used to
counteract the effects of the gravitational and any other
accelerations.
The two primary issues involved in magnetic levitation are
lifting force: providing an upward force sufficient to
counteract gravity, and stability: insuring that the system
does not spontaneously slide or flip into a configuration
where the lift is neutralized.
 The Magnetic Levitation

The levitation of a magnet above a cooled superconductor,

the Meissner Effect.
 Type I Superconductors
 Soft Superconductors are those which can tolerate
impurities without affecting the superconducting
properties.
 Also called as Soft Superconductors.
 Only one critical field exists for these superconductors.
 Critical field value is very low.
 Exhibits perfect and complete Meissner Effect.
 The current flow through the surface only.
 These materials have limited technical applications
because of very low field strength value.
 Examples: Pb, Hg, Zn, etc.
Type II Superconductors
Type II: HighTC
Superconductors
(Normal State)
T > TC
 Superconductors
having their TC values
above the temperature
of liquid nitrogen
(77K) are called the
high temperature
conductors.
 Type II Superconductors
 Hard superconductors are those which cannot tolerate
impurities, the impurity affects the superconducting
material.
 Also called as Hard Superconductors.
 Two critical fields: HC1 (lower) and HC2 (upper).
 Critical field value is very high.
 Don't exhibit perfect and complete Meissner Effect.
 It is found that current flows throughout the material.
 These materials have wider technology of very high
field strength value.
 Example: Nb3Ge, Nb3Si.
 Applications
Magnetically Levitated Vehicles (MagLev Vehicles)
 Based on two techniques:
Electromagnetic Suspension and
Electrodynamic Suspension
 In EMS, the electromagnets installed
on the train bogies attract the iron
nails. The magnets wrap around the
iron and attractive upward force is lift
the train.
 In EDS levitation is achieved by
creating a repulsive force between the
train and guide ways.
 The basic idea of this is to levitate it
with magnetic fields so that there is no
physical contact between the trains
and guideways. Consequently, the
maglev train can travel at high speed
of 500 km/h.
 Applications
Military: Sea Minesweeping Applications

The Advanced Lightweight

Influence Sweep System
(ALISS) contains a 5 ft
diameter superconducting
magnet built by General
Atomics, and is shown
below mounted on a high
speed Navy Test craft. The
system has successfully
demonstrated effective and
reliable magnetic mine
countermeasures.
 Applications
Particle Accelerators
Superconducting
LHC Cryodipole: A
computer generated
electromagnets
diagram of a dipole have many
magnet section of the exciting uses;
beam-line at the LHC, application in
indicating the use of particle
superconductors. accelerators.
Particle
accelerators where
Magnetic Field: The the charged
magneti field particle travels in a
produced by the LHC circular path
dipole's require a strong
superconducting coils. magnetic field to
bend the particle.
 Applications
MRI: Magnetic Resonance Imaging

MRI systems use a

superconducting magnet, which
consists of many coils or
windings of wire through which a
current of electricity is passed,
creating a magnetic field of up to
2.0 tesla. Maintaining such a
large magnetic field requires a
good deal of energy, which is
accomplished by
superconductivity, or reducing
the resistance in the wires to
almost zero.
 Applications
And many more...

 Particle Accelerators
 Electric Motors
 Military
 Computing
 Medical
 Transportation
 Generators
 Power Transmission
 B-Field Detection (SQUIDS)
Superconductor Elements
Superconductors

Crisanta U. Ganado
MST Physics