This site is dedicated to an explanation of superconductivity and a closely related phenomenon, super fluidity. Briefly, superconductivity is a flow of electricity without resistance. This occurs very rarely, and only under very specific conditions.
This is only a primer in superconductivity; for more information you should check my annotated bibliography or my list of links. Any errors in accuracy are mine and mine alone. If you have any questions, concerns, or additional information, please contact me at: pcboutro@novice.uwaterloo.ca
This document is divided into several sections:
Superconductivity can be defined as the flow of electricity, absent of any resistance. What, though, does that really mean? And when can it happen?
Electricity itself is an astounding phenomenon. It arises from the basic structure of matter, and occurs almost instantaneously. Surprisingly, though, it is similar to a very familiar thing: the flow of water from streams and lakes into the ocean. Electricity flows through a circuit much like water flows through a stream. If there is a barrier in the stream, the water slows down. In the same way, if there is a gap in the circuit, electricity will not flow through it.
Superconductivity is a hot topic in physics circles these days. Depending
on where you turn, explanations of superconductivity range from incomprehensible
mathematical texts to shameless cheerleading for the marvels of technology.
An enormous amount of research is being focused on this single word, and
to most people the ultimate value remains unclear. This site is not an
authoritative text on superconductivity; far from it, in fact. As I am
progressing in my own understanding of superconductivity through my course
work at the University of Waterloo
I am adding details to this site. Perhaps the best quote about superconductivity
is Einstein's. As he was trying to explain his fascination with this seemingly
abstract and (then) academic phenomenon to a science reporter he said:
"It's charm is based on three words: zero, infinity,
and perfect." Roughly defined, superconductivity is the flow of electricity without
resistance. This occurs very rarely, and only under very specific conditions.
So rare was this phenomenon that it was not detected until 1908 by Kamerlingh
Onnes when he succeeded in liquefying helium, giving a means to cool other
substances to very low temperatures.
The story of superconductivity begins in 1908, when Kamerlingh Onnes succeeded
in liquefying helium. It had taken many years to achieve this, and the
techniques developed proved essential in later studies of other phemena,
including superconductivity. Onnes' success gave him a method of studying
other materials at low temperatures. In 1911, he was studying the electrical
conductivity of Mercury at very low temperatures when a startling observation
was made. While the electrical resistance had been steadily declining with
reduced temperature, at 4.2 degrees Kelvin it suddenly vanished. Onnes
called this phenomemon superconductivity. Within a few years Onnes was able to demonstrate that several other
substances were capable of exhibiting superconductivity. Tin was found
to superconduct at 3.8 degrees Kelvin; lead was found to superconduct at
6 degrees Kelvin. In a crucial experiment to demonstrate the efficacy of
this property, Onnes set a current flowing in a loop of lead cooled to
4 degrees Kelvin, removed the current source, and allowed the current to
run for one year. No measurable reduction in current was observed. The potential uses of these perfect conductors were many and varied
-- high intensity electromagnets and ecoonomical, long-distance power lines
were among the earliest to gain recognition. Two main problems existed,
however, to limit the immediate effectiveness and commercial viability
of superconducting materials. First, it was exceedingly difficult to maintain
the low temperatures required for superconductivity. Indeed, liquefying
helium was difficult enough in the first place, but to produce sufficient
quantities to keep a commercial superconductor in service was prohibitively
expensive. Second, for some unexplained reason, superconductivity was not
exhibited in the presence of either large magnetic fields or large current
densities. Current density, incidentally, is the current divided by the
area, or Amperes per metre squared. Both of these properties were required
for the industrial or commercial applications proposed for superconductivity.
Although the commercial viability of superconductors seemed nonexistent,
research continued into the mysterious origins of this phenomenon. No coherent
theory present at the time could explain this sudden elimination of resistance.
In 1933, Walthner Meissner was attempting to determine the origins of superconductivity,
and in the process discovered a second, crucial property of superconductors:
they are diamagnetic. That is, a superconductor will not allow a magnetic
field to penetrate into its interior. Instead, it sets of screening currents
that are equal in magnitude, but opposite in direction to those being applied,
thus leaving a net field of zero in its interior. When the screening currents
are set up, like poles are in proximity, and they will repel. Hence, the
stronger the initial applied magnetic field, the stronger the repulsion
created by the like poles created by screening currents. This phenomenon
of diamagneticism has been called the Meissner Effect. Although inexplicable
at the time, theories have been developed to explain the various properties
of superconductors. The next section covers the various
theories in some detail. In 1957 it was theorized that some materials could be found that would
be able to retain their superconducting properties in the face of high
current densities or strong magnetic fields. In 1960 theory was vindicated
with the discover of Nobium three Tin. Nobium three Tin is capable of superconducting
at 8.8 tesla, which is sufficient for most industrial applications. Thus,
a major obstacle to the commercial usage of superconductors had been removed.
The next two decades were characterized by a consolidation of the field
of superconductivity. That is, there were no discoveries of new properties
of the existing superconductors or of radical new classes of superconductors.
A variety of new superconductors of the Nobium three Tin variety were discovered,
and which were capable of bearing high current densities and strong magnetic
fields. More importantly, perhaps, were the efforts of engineers at companies
such as Supercon Inc., Intermagnetics General Corporation, and Oxford Instruments
in the 1960s and 1970s. These companies focused on applying superconductors
to a wide range of instrumentation and other devices; they refined manufacturing
techniques to create commercially reliable and economically affordable
superconductors. Key successes in this field include: While the discovery of Nobium three Tin -type superconductors removed
a key obstacle to commercial application of superconductivity and the micro-electronics
revolution spurred a host of new innovations and commercial applications,
a problem yet remained. Many of the most useful applications remained tantilizingly
out of reach because of the critical temperatures involved. All superconductors
found before 1980 had a critical temperature below 20 degrees Kelvin. To
achieve such low temperatures required elaborate preparations and safeguards
which were simply not feasible for large-scale projects. The sheer volume
of liquid hydrogen required to cool a 100 km powerline to its critical
temperature would quickly overwhelm the savings generated by the efficient
transport and minimal resistance. Similarly, although superconductors had
potential to revolution the personal computer (which was just then emerging
as a mainstream research tool that could replace the reliance on mainframes)
the cost of cooling a computer to 20 degrees Kelvin erased the potential
productivity gains. Other problems included repairing superconducting components
(it could not be done on the fly -- a total system shutdown would be required)
and the space restriction imposed by the massive refrigeration systems
needed. The malleability of superconductors also became an issue, as many
were found to be highly brittle and unsuitable for use in wires or delicate
components. In the 1980s, the focus shifted imperceptibly from utilizing
the existing superconductors to finding new ones. The crucial properties
required were high critical temperatures and improved malleability. The next major advance in superconductor research came in 1986. Two
scientists at the IBM Zuric Laboratory -- Alex Mueller and Georg Bednorz
-- spent most of 1984 and 1985 searchingg for superconductors with a high
critical temperature in a group of metal oxide ceramic metals called the
perovskites. In 1986 they believed that they had procured evidence to show
barium lanthium copper oxide to superconduct with a critical temperature
of 30 kelvins. They released a tentative report to allow for peer review.
At the annual meeting of the Materials Research Society in Boston that
fall, two researchers from the University of Tokyo were not only able to
confirm Mueller and Bednorz's results, but also to show that barium lantium
copper oxide was diamagnetic (or exhibited the Meissner Effect), as were
all other superconductors. A few months later, the exact critical temperature
was established as 57 K, a new record. Researchers began a frenetic search
for new superconductors chemically similar to barium lanthium copper oxide,
and excitement began to build. On February 16th, 1987, Paul Chu and his team of researchers from the
University of Houston reported finding a critical temperature of 93 K in
barrium yttrium copper oxide. Other researchers substituted various rare
earth elements for the yttrium, and this prompted the discovery of ten
new superconductors. In 1988, two new copper oxide ceramics using bismuth and thallium were
found to have critical temperatures of 110 K and 125 K respectively. This
temperature represented a major landmark in superconductivity research,
for it allows a switch away from expensive helium-based refrigeration and
to nitrogen-based refrigeration. Liquid nitrogen is about as expensive
as beer or milk, so large-scale commercial applications began instantly
feasible.
When Kammerlingh Onnes first observed superconductivity he believed that
some sort of radical change had occured on the microscopic level; perhaps
he was observing something akin to a change of state, he thought. This
idea has proved to be a cornerstone of superconductivity theories. Although
it is no longer considered to be a change of state, the concept of a fundamental
change on the atomic level has fuelled subsequent research. Onnes soon realized that he was not dealing with a straight-forward
chemical change or reaction. There was no observable equilibrium state
between the semi-conducting and superconducting states of matter, he noted.
Instead, Onnes likened it to the change from sand to glass, as opposed
to a chemical reaction or change, such as water expanding when it freezes.
Glass is formed when the liquid form is cooled too quickly, without allowing
sufficient time for the molecules to order themselves. The change is permanent:
once formed glass cannot be melted and allowed to solidify into anything
other than glass. A key observation was made by Fritz London. He remarked that, in one
way, the superconductor acted like a single particle of vast size, and
not a collection of atoms. Although he could not recommend a mechanism,
London was certain that this was the explanation for the Meissner effect
and the diamagnetism displayed by superconductors. The first theory to combine the ideas of both Onnes and London was called
the "Two Fluids Model." It first became popular in the 1920s,
although it still has some modern day adherents. Elements of this theory
were incorporated into some of the other modern day theories. The key idea
is that there are two types of electrons in metals -- normal and superconducting.
For some reason, low temperatures allow the superconducting electrons to
increase in number, until they are in sufficient number to predominate
the material's physical characteristics. Several faults existed with this
theory, including the dubious mechanism for the transference between 'normal'
and 'superconducting' electrons. In 1957 Barden, Cooper, and Schrieffer (BCS) theorized that all electrons
in a superconductor develop a mutual attraction and flow together as a
frictionless fluid. They suggested that electrons with opposite spins were
paired, and that these 'Cooper Pairs' would condense into an electrical
superfluid at low temperatures, where the energy levels would be a little
below normal electron energy levels. The electrons forming the 'Cooper
Pairs' were those with similar values for the primary, secondary, and magnetic
quantum numbers, but opposite values for their spin quantum number.
Superconductivity for non-scientists
This section is an attempt to make the broad topic of superconductivity available to everyone, regardless of their scientific or mathematical backgrounds. My belief is that scientific principles, when well explained, are of broad appeal. I would greatly appreciate any suggestions on how to make this section accomplish that goal more effectively.
Superconductivity occurs in some substances when they are cooled below
a certain critical temperature. Scientists call this temperature the "critical
temperature".
Historical Overview
Theoretical Explanations
Similarly, Onnes argued, superconductivity is not a half-way occurence.
Once the temperature is reduced to the critical point, any current, regardless
of its magnitude, will be superconducted; if the temperature is raised
beyond the critical marker, then the current will peter out, regardless
of other factors. Incidents that lack an equilibrium state, like the formation
of glass or the occurence of superconductivity, are the exception in nature.
For that reason it tends to be more difficult to theorize about them and
to posit valid explanations for their occurence. Technological Applications