Superconductivity | Properties of superconductivity |Types of superconductors | Application of superconductivity

In this article, we are going to learn about what is a superconductor, and what is superconductivity. types of superconductors, applications, and many things we are going to cover in this topic. So let’s start with the definition of superconductors and superconductivity.

what is a superconductor?

A state of the material in which it has zero resistivity is called superconductivity.

Superconductivity was discovered in 1911 by professor Heike Kamerlingh ones, who observed that the resistivity of mercury vanished completely below 4.2 K, the transition from normal conductivity occurring over a very narrow range of temperature of the order of 0.05 K.

Below figure 1. shows the resistance of mercury as a function of temperature.

In the superconducting state of a material, the electrical resistivity is zero and a persistent current can continue to flow without diminishing.

Superconductivity appears at low temperatures and in magnetic fields lower than a particular level.

Figure 2. shows an example of the variation in electrical resistivity with temperature.

The temperature at which superconductivity appears is called the critical temperature, T_c (or transition temperature).

Superconductors are materials that show superconductivity under certain conditions of temperature and magnetic field.

The state in which the superconductor does not show superconductivity is called the normal state.

A finite jump in the specific heat is observed at the critical temperature. Hence a change from a normal to the superconducting state, and vice versa is a second-order phase transition. It is shown experimentally that entropy is lower in the superconducting state than in the normal state.

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Also Read: Classification of Engineering Materials

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Properties of superconductors

1. The Meissner Effect

• In 1993 Meissner discovered that not only did superconductors exhibit zero resistance but also spontaneously expel all magnetic flux when cooled through the superconducting transition, that is, that is they are also perfect diamagnets. We call this the Meissner effect.
• The Meissner effect is not a consequence of zero resistance and Lenz law. The flux is expelled as the superconductor is cooled in a constant magnetic field.
• There is no time rate of change of the magnetic induction. Lenz’s law does not apply.
• Perfect diamagnetism is an independent property of superconductors and shows that superconductivity involves a change of thermodynamics state, not just a spectacular change in electrical resistance.
• This is a conceptually deepest property of superconductivity.

• It has been observed that when a long superconductor is cooled in a longitudinal magnetic field from above the transition temperature, the lines of induction are pushed out. Then inside the specimen, B = 0 .

• We know from the magnetic properties of materials that,

B = μo (H + M), for B = 0\; H =-M,

• consequently, since X_m=\frac{M}{H'}, we may state that magnetic susceptibility in a superconductor is negative this is referred to as perfect diamagnetism. This phenomenon is called the Meissner effect.
• Perfect diamagnetism and zero resistivity are two independent, essential properties of the superconducting state.

2. Effect of Magnetic Field

• Removal of the superconducting state does not only occur by raising the temperature but also by subjecting the material to a magnetic field.
• The critical value of the magnetic field for the destruction of superconductivity, H_C is a function of temperature, at T = T_C, H_C=0.
• With only small deviations, the critical field He varies with temperature according to the parabolic law

\boxed{H_C=H_0 \left [1- \left (\frac{T}{T_C} \right )^2 \right ]}

H₀ is the critical field at absolute zero and T_c is the transition temperature.

• The magnetic field that causes a superconductor to become normal from a superconducting state is not necessarily an external field it may arise as a result of electric current flow in the conductor.
• In a long superconductor wire of radius r, the superconductivity may be destroyed when a current I exceeds the critical current value Ic, which at the surface of the wire will produce a critical field Hc, given by,

\boxed{I_c=2\pi r H_c} called silsbee’s rule.

3. Frequency Effect

• Superconductivity is observed for d.c. and up to radio frequencies. It is not observed for higher frequencies.
• For a superconductor, the resistance is zero only when the current is steady or varies slowly.
• When the current fluctuates or alternates, a small absorption of energy roughly proportional to the rate of alternation occurs and is not yet fully understood.
• When the frequency of alteration rises above 10 MHz appreciable resistance arises, and at infrared frequencies (1013 Hz) the resistivity is the same in the normal and superconducting states and is independent of temperature.

4. Entropy

• Entropy increases on going from a superconducting state to a normal state.

5. Thermal Conductivity

• In an ideal superconductor, there is a marked drop in thermal conductivity when superconductivity sets in.
• In non-ideal superconductors, an increase in thermal conductivity on becoming superconducting has been observed in a few specimens.

6. Isotope Effect

• It has been observed that the critical temperature of superconductors varies with isotopic mass.
• The relation is.

M^{1/2}=constant

T_c \propto \frac{1}{\sqrt{M}}

Where M is the mass of the isotope.

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Types of Superconductor

From their magnetic properties, superconductors are classified into two groups:

1. Type-I Superconductor ( Soft Superconductor)
2. Type-II Superconductor (Hard Superconductor)

1. Type-I Superconductor:

• Those superconductors that show perfect diamagnetism up to critical magnetic field Hc and go to the normal state are called type-I superconductors.
• Soft metals such as lead or indium belong to this group.
• The critical field of these superconductors is low.
• They have a low melting point.
• These materials obey Silsbee’s rule and show the Meissner effect.
• The transition from normal to superconducting state is sharp.
• They have low values of H_c and T_c.

2. Type-II Superconductor:

• Hard metals and alloys that have different magnetization characteristics are called type-II superconductors.
• They are characterized by high transition temperatures,
• high critical field,
• incomplete Meissener effect,
• break down of Silsbee’s rule and or
• broad transition region.

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Difference between type 1 and type 2 Superconductors

Type 1 Superconductor	Type 2 Superconductor
These Superconductors are called soft superconductors.	These superconductors are called hard superconductors.
Only one critical field exists for these superconductors	Two critical fields Hc1 (lower critical field) and Hc2 ( upper critical field) exist for these superconductors.
The critical field value is very low.	The critical field is very high.
These superconductors exhibit a perfect and complete Meissner effect.	These do not exhibit a perfect and complete Meissner effect.
These materials have limited technical applications because of their very low field strength value.	These materials have wider technological applications because of their very high field strength value.
Example: Pb, Hg, Zn, etc.	Example: Nb3, Ge, Nb3Si. Y1Ba2Cu3O7, etc.

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Superconducting Materials

Since 1911 superconductivity has been found to occur in some metallic elements and hundreds or thousands of alloys and compounds.

Some of these materials are listed in the table below along with the transition temperatures.

Element	Tc (K)	Alloy or Compound	Tc (K)
Al	1.19	Nb-Ti	8
In	3.41	Nb-Zr	11
La	5.9	Nb3Sn	18.3
Nb	9.2	Nb3Ge	22.5
Pb	7.18	V3Si	17
Re	1.70	Nb N	17.3
Sn	3.72	Y Ba2 Cu3 O7	93
Ta	4.48	Bi Sr Ca CuO	107
Tc	8.22	Tl Ba Ca Cu O	120
V	5.13	Th2 Ca Ba2 Cu2 O10	125

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Application of superconductors

Superconducting materials are of interest because they promise low joule heating losses in electric power generation and transmission systems.

Today, superconducting magnet technology is in the mature stage: many large magnets for nuclear fusion experiments have been constructed, the superconducting magnetic resonance imaging (MRI) market is going steadily, and an experimental magnetically levitated train is under development as a commercial transportation system.

Superconductivity is also entering the field of electronics.

Superconducting technology has the inherent ability to provide devices, components, and systems that can greatly reduce overall system weight, volume, and input power.

Superconducting technology can provide breakthroughs in spacecraft design and performance (many devices).

Superconducting magnets find applications in the following areas:

• Magnets for nuclear fusion
• Magnets for high-energy physics
• Generators and motors
• Magnetically Levitated Transportation
• Superconducting magnets for energy storage
• Magnetic resonance imaging and other applications (in medicine).

Superconducting materials are used as electronic switching devices called cryotrons (based on the destruction of a superconducting state in a strong magnetic field).

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