Tuesday, December 6, 2011

Magnetic Susceptibility (Establishment of a New Theory)

The initial set-back as discussed earlier, however didn't demoralize the researchers and many of them were struggling to solve this puzzle. However, one of the remarkable achievements towards understanding the superconducting state of matter was the discovery of Meissner Effect after about twenty-two years from the discovery of superconductors. In 1933, Meissner and Ochsenfeld experimentally verified that the internal field flux (B$_0$) is essentially zero in case of pure metals. They did an experiment in which he cooled down a superconductor with a background field. Also, a test coil was wrapped over the superconducting specimen. The specimen was then cooled down with a field surrounding it. As the normal to superconducting transition occurred in the specimen, all the flux was expelled out of the specimen. Hence, from the susceptibility equation,
H + 4\pi \chi H = 0
Thus, the susceptibility is strongly negative at - 1/4$\pi$ showing a perfectly dia-magnetic behavior of the specimen in its superconducting state. The appearance of magnetic flux in the specimen core can also conclude the destruction of superconducting state of the matter. The transition curves are also reversible for a pure sample of lead. However, this is not universal. The reversibility in the transition curve can only be obtained in a well constrained experiment. One of the most important factors is that the sample is metallurgically pure and topologically single. Also, if the core of the ring is vacuum or non-superconducting, some hysteresis is seen. This is because of the induced field in the ring which opposes the decay. In practice, the inherent impurity in the specimen also causes flux trapping and hysteresis.

Magnetic Susceptibility (Another Surprise!!)

Another consequence with the superconducting magnets is that they offer very low magnetic susceptibility. Whenever a metal is exposed to external magnetic field, Eddy current is set up in the material, which then decays depending on the resistance offered by the material. However, in case of superconductors, Eddy current does not decay and hence expels the magnetic field from its vicinity.
Mathematically, from the first Maxwell equation
\nabla V = -{\dot B}/c
But the voltage inside a superconductor is zero in steady D.C. state and hence, B = B$_0$. Thus, the flux in a material in its superconducting state becomes constant at a level which was inherent to it when the resistance of the material was lost. This phenomena gave a back up support to the previously experienced problem of critical field in superconductors. 

With all such obstacles, the technological implementation of the superconductor could not be done for more than two decades. It is not that the researches in this area were stopped. In fact, a number of other researchers and young scientists were motivated towards making a no-loss magnet. However, practical uses remain a dream for people for quite a long time. 


Monday, December 5, 2011

Why Mercury was chosen by Onnes as the test substrate where as there were other metals available ?

The answer is a simple one. 

Anyone doing an experiment would try to make the results as accurate as possible. In those days, mercury was available easily with a very high purity. Thus, the residual resistance measurement could have produced highly accurate results. 

Generation of Magnetic Field using Superconductors : An Initial Set-back

The disappearance of electrical conductivity in many metals at certain critical temperature gave a boost to the hope of people thinking about powerful electromagnets and other electrical technologies. They thought, if a specimen can take current without any Ohmic (resistive) losses then the dream of constructing high-field electromagnets can be fulfilled. In 1913, Onnes fabricated a small coil of Lead keeping this in mind and cooled it down to its superconducting state, i.e. below 4.2 K. What was driven him to do so was -- he wanted to flow high current through it without significant power loss, and generate high magnetic field. However, he along with the entire science community were greatly disappointed when it was found that the magnet was unable to produce more than few hundred Gauss even when it is operated below its superconducting temperature. Above a certain field, it was behaving as if a normal conductor is being charged with current. This superconducting to normal transition was termed as quench. With the help of further investigations, it was found that the superconducting behavior of the metals was also disappearing at a certain field generated either internally or externally (few hundred Gausses). Thus, to the disappointment of all, it was found that the superconductors can be operated within the critical temperature and critical field (which was too low to make an electromagnet).
For straight superconductors carrying high current, the quenching occurs when the field generated at the surface of the wire crosses the critical field. 

Sunday, December 4, 2011

Invention of Superconductivity

The electrical resistivity of pure metals was known to drop rapidly with temperature. Several attempts were going on to find the residual resistivity of the metals which would replicate the inherent residual impurities. On Eighth April 1911, Dutch physicist Kamerlingh Onnes found that the electrical resistivity of mercury tended to become zero suddenly at about 4.25 K (about 269 degrees below the temperature of ICE. On earth the minimum temperature that exists in the pole is ~ 240 K). Later on, this reversible and reproducible phenomena was seen in Lead at 7.2 K, Tin at 3.7 K and few other metals.

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During those days, due to the limitation in instrumentation, the question of resistance dropping to immeasurable level at temperature below the critical temperature arose. In order to validate this, Onnes made a ring of Tin and cooled it with a background field. As the ring went pass its critical temperature, the field was removed which induced a circular current in the ring. It was pointed that even if small resistance is present in Tin, then the induced current would decay in finite time.

But Onnes could not predict any measurable change in the current after several days (In present days, with high purity of metals and increased measurement accuracy, the time has been extended to years). It was concluded that the resistance is far below the practicable limits and hence it can be considered as `zero'.

The state of metals showing this phenomena below a critical temperature was termed as "Superconducting state."
Now the question for you:  

Saturday, December 3, 2011

What is Superconductivity ??

Well, as you all might be knowing, all the metals are also known as conductor as they allow current to pass through them easily.  For example:  Iron, Copper, Aluminum  etc. You might have seen wires made of these metals. And when current is passed thorough them, it gets heated. This heating is because of the resistance it offers to the passing current and is known as Joule's heating.  Some of the metals can carry more currents without getting heated compared to other. This is because of their inherent configuration (I will discuss some other time). Again, when the metals are cooled, the conduct even more current, which means, their resistivity towards current (electrical resistivity) decreases.
In 1911, while cooling down mercury (one of the metals) to ultra-low temperature, Kamerlingh Onnes found an entirely new behavior of it. He found that the electrical resistivity of mercury suddenly fell down to zero at a certain temperature near to 4 K. This process was reversible and was found on some more elements. This indicated a complete loss of resistivity in some of the metals at a very low temperature. This behavior of a conductor was termed as Superconductivity

Friday, December 2, 2011

Now a days, superconductors are the front line candidate to replace many of our commonly used technologies. Starting from a small motor up to the biggest and toughest reactors, Superconductors are now dominating all over. While I would intend to keep the content of this sites in a very common language, let me just list out some of the ongoing usages and researches on the technologies including superconductors.
  8. MRI
AND many more...

I would go through each of them in a very detailed manner. However, don't expect me to go through them in a systematic way from top to bottom.

The question to all of you out there is : -