Sample Materials for CSAT Paper -1 (G.S.) Pre 2013: "Science & Technology: Superconductivity"

Subject: Science & Technology:
Topic: Superconductivity

Ques. 1 : What is superconductivity?

Ans. Superconductivity is a phenomenon occurring in certain materials at low temperatures characterised by complete absence of electrical resistance and the exclusion of the interior magnetic field (the Meissner effect).

Superconductivity occurs in a wide variety of materials including simple elements like tin and aluminium, various metallic alloys, some heavy-doped semiconductors and certain ceramic compounds containing planes of copper and oxygen atoms. The latter class of compounds, known as cuprates are also known as high temperature superconductors (HTs).

Superconductivity does not occur in noble metals like gold and silver nor in most ferromagnetic metals, although a number of materials displaying both superconductivity and ferromagnetism have been discovered in recent years.

In 1911, Kammerlingh Ones discovered that a certain temperature, and often within a narrow temperature range, the electrical resistivity of many metals and alloys drops suddenly to zero. Ones observed that resistance of mercury vanishes suddenly at 4.2 k. Conventional superconductors exhibit superconductivity at relatively lower temperatures. Unconventional superconductors in particular the high-temperature superconductors (HTs) superconductor at much higher temperatures (though still far below room temperature) .

Ques. 2 : Discuss in brief the properties of superconductors?

Ans. Most of the physical properties of super-conductors vary from material to material, such as the heat capacity and the critical temperature at which - superconductivity is destroyed. On the other hand, there is a class of properties that are independent of the underlying material.

1. Zero Electrical Resistance: All superconductors have zero resistivity to low applied currents when there is no magnetic field present.

2. Critical Temperature T: In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature To. The value of this critical temperature varies from material to material.
Conventional superconductors usually have critical temperatures ranging from less than 1K to around 20 K. Unconventional superconductors such as cuprate superconductors (e.g. 4Ba2 Cu307) have much higher critical temperature. 4Ba2Cu307 has a critical temperature of 92K. Explanation on how these materials exhibit superconductivity at higher temperatures is unsatisfactory.

3. Meissner Effect: When a superconductor is placed in a ‘weak’ external magnetic field H, the field penetrates the superconductor for only a short distance \ called the penetration depth, after which it decays rapidly to zero. This is called the Meissner effect, and is a defining characteristic of superconductivity. For most superconductors, the penetration depth is of the order of 100 nm.

The Meissner effect breaks down when the applied magnetic field is too large. Superconductors can be divided into two classes according to how this breakdown occurs:

1. Type I Superconductor: In them, superconductivity is abruptly destroyed when the strength of the applied field rises above critical value Hc. For example, Most pure elemental superconductors (except vanadium, niobium, technetium and carbon nanotubes) are type I.

2. Type II superconductor: In Type II superconductors, when the field applied is beyond a critical value HC1 it leads to a mixed state, in which increasing amount of magnetic flux penetrates the material, but there remains no resistance to flow of electric current. But at second critical field strength, HC2, superconductivity is destroyed.

Applications and uses of Superconductors

I. Medical:

Magnetic resonance imaging
Biotechnical Engineering

II. Electronics:

SQUIDs
Josephson Junction Devices
Particle Accelerators
Sensors
Transistors
Circuitry connections

III Industry

Seperation
Magnets
Sensors and Transducers
Magnetic shielding

IV Transportation

Magnetically levitated vehicles
Marine Propulsion

V Power Generation

Motors
Generators
Energy Storage
Transmission
Fusion
Transformers and Inductors

Ques. 3 : Give an account of the technological innovations which is based on supercon-ductivity?

Ans. There have been many technological innovations based on superconductivity. Superconductors are used to make the most powerful electro magnets known to man. Super conducting magnets are essential components of several technologies. Magnetic Resonance Imaging (MR1) is playing an increasingly important role in diagnostic medicine. The intense magnetic fields that are needed for these instruments are a perfect application of superconductors. Similarly, particle accelerators used in high- energy physics study are very dependent on high field super conducting magnets.

The field of electronics holds great promise for practical applications of superconductors. The miniaturisation and increased speed of computer chips are limited by the generation of heat and the charging time of capacitors due to the resistance of the interconnecting metal films. The use of new super-conductive films may result in more densely packed chips which could transmit information more rapidly by several orders of magnitude. Super conducting electronics have achieved impressive accomplishments in the field of digital electronics. Superconductors are used to build Josephson junctions which are the building blocks of SQUIDs, (Superconducting Quantum Interference Devices) the most sensitive magnetometers known.

Magnetic-Levitation is an application where superconductors perform extremely well. Transrport vehicles such as trains can be made to ‘float’ on strong superconducting magnets, virtually eliminating friction between the train and its tracks. The conventional electromagnets when compared to superconductors would waste, much of electrical energy as heat and would be physically much larger than superconducting magnets. Magnetic Levitation Trains (MLT) currently exist in Japan and West Germany. MLTs offer an alternative to air transportation between cities up to a few hundred miles apart. Japanese MLTs generally run at speeds up to 500 kmph.

The ability of superconductors to conduct electricity with zero resistance can be exploited in the use of electrical transmission lines. Currently, a substantial fraction of electricity is lost as heat through resistance, associated with traditional conductors such as copper or aluminium. A large-scale shift to superconductivity technology depends on whether wires can be prepared from brittle ceramics that retain their superconductivity at higher temperatures (TTK) while supporting large current densities.

New applications of superconductors will increase with critical temperature The liquid nitrogen-based superconductors that exhibit superconductivity at higher temperature has provided industry more flexibility to utilise superconductivity as compared to ‘conventional’ liquid helium superconductors. The possible discovery of room temperature superconductors has the potential to bring superconducting devices into our every day lives.
Promising future industrial and commercial applications include transformers, power storage devices, motors, electric power transmission, etc. Most applications today employ the well-understood conventional superconductors, but it is expected that high- temperature superconductors will soon become more cost-effective in many cases. The rapid progress in the field of superconductivity leads one to believe that applications of superconductors is limited by only one’s imagination and time.

Ques. 4 : Briefly discuss the research developments in the field of superconduc-tivity?

Ans.
Recognising the importance of promoting superconductivity R&D and applications in the country, an apex body with Prime Ministers as chairman and a Programme Management Board (PMB) was constituted in 1987. In February 1991, the National Super conductivity Science and Technology Board (NSTB) replaced the apex body and the PMB.

The National Superconductivity Programme (NSP) was launched in 1988. In Phase-I (88-91) 65 projects were started in several institutions including the leading laboratories in DAE, CSIR, IITs in phase-N (91-95) 6 new projects were started. The following are the some of the achievements in superconductivity R&D.

1. High Temperature Liquids: Scientists at National Physical Laboratory (NPL), New Delhi have developed a new SQUID at Liquid Nitrogen Temperature (UK). This is considered to be on the high temperature side so far as superconductors are concerned.

2. Superconducting Magnetic Ore separators: A Superconducting high gradient magnetic separator (SCHGS) has been developed at BHEL, Hyderabad with the help of NPL, BARC, Mumbai and National Mineral Development Corporation.

3. Super Conducting Compound: Monophasic compounds with a critical transition temperature (Tc of 110 K, 90 K and 80 K have been obtained by the Department of Nuclear Physics of Madras University.

4. Super Conducting Generator: Engineers at BHEL research centre at Hyderabad have built arid tested India’s first superconducting generator, and synchronised it with Andhra Pradesh Power Grid. The achievement has ushered India into a new era of power generation at ultra low temperature.

Answer of Question 1: A

Answer of Question 2: B