Superconductivity is a phenomenon observed in several metals and ceramic materials. When these materials are cooled to temperatures ranging from near absolute zero (-459 degrees Fahrenheit, 0 degrees Kelvin, -273 degrees Celsius) to liquid nitrogen temperatures (-321 F, 77 K, -196 C), they have no electrical resistance. The temperature at which electrical resistance is zero is called the critical temperature (Tc) and varies with the individual material. For practical purposes, critical temperatures are achieved by cooling materials with either liquid helium or liquid nitrogen. The following table shows the critical temperatures of various superconductors:
Material Type l Tc(K)
Zinc l metal l 0.88
Aluminum l metal l 1.19
Tin l metal l 3.72
Mercury l metal l 4.15
YBa2Cu3O7 l ceramic l 90
TlBaCaCuO l ceramic l 125
Because these materials have no electrical resistance, meaning electrons can travel through them freely, they can carry large amounts of electrical current for long periods of time without losing energy as heat. Superconducting loops of wire have been shown to carry electrical currents for several years with no measurable loss. This property has implications for electrical power transmission, if transmission lines can be made of superconducting ceramics, and for electrical-storage devices.
The classic demonstration of the Meissner Effect. A superconductive disk on the bottom, cooled by liquid nitrogen, causes the magnet above to levitate. The floating magnet induces a current, and therefore a magnetic field, in the superconductor, and the two magnetic fields repel to levitate the magnet.
Another property of a superconductor is that once the transition from the normal state to the superconducting state occurs, external magnetic fields can't penetrate it. This effect is called the Meissner effect and has implications for making high speed, magnetically-levitated trains (see How Maglev Trains Will Work for details). It also has implications for making powerful, small, superconducting magnets for magnetic resonance imaging (MRI).
How do electrons travel through superconductors with no resistance? Lets's look at this more closely.
The atomic structure of most metals is a lattice structure, much like a window screen in which the intersection of each set of perpendicular wires is an atom. Metals hold on to their electrons quite loosely, so these particles can move freely within the lattice -- this is why metals conduct heat and electricity very well. As electrons move through a typical metal in the normal state, they collide with atoms and lose energy in the form of heat. In a superconductor, the electrons travel in pairs and move quickly between the atoms with less energy loss.
As a negatively-charged electron moves through the space between two rows of positively-charged atoms (like the wires in a window screen), it pulls inward on the atoms. This distortion attracts a second electron to move in behind it. This second electron encounters less resistance, much like a passenger car following a truck on the freeway encounters less air resistance. The two electrons form a weak attraction, travel together in a pair and encounter less resistance overall. In a superconductor, electron pairs are constantly forming, breaking and reforming, but the overall effect is that electrons flow with little or no resistance. The low temperature makes it easier for the electrons to pair up (see A Teacher's Guide to Superconductivity for High School Students for details).
One final property of superconductors is that when two of them are joined by a thin, insulating layer, it is easier for the electron pairs to pass from one superconductor to another without resistance (dc Josephson effect). This effect has implications for superfast electrical switches that can be used to make small, high-speed computers.
The future of superconductivity research is to find materials that can become superconductors at room temperature. Once this happens, the whole world of electronics, power and transportation will be revolutionized.
Superconductors
Superconductivity was first noticed when liquid mercury was cooled to liquid Helium temperatures (4.2K) while its resistivity was being plotted. While approaching that temperature, the resistance was coming down linearly, when all of a sudden it dropped to zero Ohms! Dutch physicist Heike Kamerlingh Onnes was performing this experiment in 1911.
Since that time, other elements and combinations of elements have been shown to posses a superconducting state at various temperatures. This table shows the elements which become superconducting and the temperature at which it happens. Most research has been to find materials which are superconducting at higher temperatures. For example, the ceramics in kits you can buy become superconductors at about -186C. Using liquid nitrogen (LN2) which is at -196C, you can make that ceramic superconducting.
What is unique about a superconductor?
1. First, its resistance is really zero Ohms, nothing, nada, all gone! This means that if current were flowing in the material, it would produce no heat whatsoever.
2. Second, it will exclude any magnetic fields that come near it, like a magnetic mirror. If a north pole approaches the superconductor, the magnet will behave as though another magnet, just like itself, is approaching from the other side of the surface of the superconductor. At some distance, the magnet's north pole will start to repel the "other magnet's north pole", which is really a reflection of its own. It doesn't matter if it is a north or south pole, it will act the same way. This is the Meissner effect where a magnet will float, or levitate, above a ceramic of superconducting material.
If the magnet were sitting on the superconducting ceramic when it wasn't a superconductor, and then you started to cool the ceramic, when it becomes superconducting the magnet will start to lift up off of the ceramic and begin to levitate.
One problem, though, is that if the magnetic field of the current flowing within the superconductor becomes large enough, the ceramic will drop out of superconductivity, even if it is cold. Large magnetic fields will destroy the superconducting state. So, there is always a balance between the temperature, the magnitude of the magnetic field due to the current, and the molecular structure in determining the suitability of the superconductor for a particular application. There are always new things on the horizon, though.
An excellent site which describes research in superconductors is at:
http://www.physics.ubc.ca/~supercon/supercon.html
The above photos are the kit I purchased from Arbor Scientific. They have a complete kit, P8-9702, an economy kit, P8-9701, and a high density kit, P8-9715. The above is the high density kit. Edmund Scientific also has a kit, 38-169. Arbor includes some great notes and instructions with their kits. The kit is actually manufactured by Superconductive Components, Inc. at http://www.superconductivecomp.com/.
Another quality source for kits and information is at http://www.futurescience.com/welcome.html. They have an excellent site describing the history and explanation of this phenomenon, and a description on how to make your own ceramic superconductors!
The above kit comes with a petri dish, the two ceramics shown, and plastic (non-magnetic) tweezers. The lower density ceramic comes with a smaller NIB magnet, the higher density ceramic on the right comes with a larger NIB. The ceramics are about 1" diameter.
To carry the LN2, I have a dewar from International Cryogenics (part number IC-5D), that holds 5 liters of LN2. When full, the LN2 can last for up to 21 days in the dewar. I also have a dewar flask made of HDPE that can hold 2 liters (from Electron Microscopy Sciences as part number 62038-02, also available from Sargent Welch as part number WLS-34755-B. A 90mm x 280mm glass dewar is available from Labglass as part number LG-7681-102), and has a large mouth, making it easy to submerse objects. Remember, LN2 is very, very, very cold! Always, always wear gloves and goggles when pouring it. Here is a table of various events and the temperatures at which they happen, in Fahrenheit, Celsius, and Kelvin.
Temperatures F C K
water boils 212.0 100.0 373.2
body temp 98.6 37.0 310.2
room temp 77.0 25.0 298.2
water freezes 32.0 0.0 273.2
mercury freezes -37.8 -38.8 234.4
dry ice -108.4 -78.0 195.2
liquid Oxygen -297.4 -183.0 90.2
liquid Nitrogen -320.8 -196.0 77.2
liquid Helium -452.1 -269.0 4.2
absolute zero -459.7 -273. 20.0
For this, K = C + 273.2
and F = (9/5) * C + 32
Experiment
Here are some photos of the magnets floating above the ceramics. Very cool!
This shows the lower density ceramic with a couple of different magnets floating above it. I placed a copper disk, about 5/16" by 3" diameter, in the petri dish and placed the ceramic on top of that. Another demonstration is to drop the magnet onto the copper disk, watching it float downward at a slower than usual rate, then rest on the surface of the copper disk. This is an example of eddy currents.
These are with the higher density ceramic, which will be suspended in the air if you lift the magnet as shown. The ceramic would also suspend the magnet under it if you flip them over. To use this ceramic, you need to have the magnet sitting on the ceramic as it cools to its superconducting state. Then pick up the magnet (the ceramic will come with it), hold it in the air for a few seconds, allowing the ceramic to warm up. As it does so, it will start to fall away from the magnet. You can then float the magnet above the ceramic, and it will be horizontal, not angled as with the other ceramic. The first photo in the next row shows that the magnet is floating about 10mm above the ceramic! This effect is called pinning, where there is some penetration of the magnetic field into the superconducting ceramic, different from the other ceramic which does not allow any penetration of the field.
These three photos show how the fields of the magnets affect each other, even from a distance of about 15"! One magnet is floating above the high density ceramic, the other is in my hand which I rotate, causing the floating magnet to move around.
A very interesting effect I noticed was that when a small magnet was floating above the high density ceramic, it would stay in place even when the ceramic was tilted onto its edge! The next two photos show three and four steel ball bearings stuck to the magnet, floating above the high density ceramic, slowly rotating. Great effect! (I removed the copper disk for this one.)
After several minutes, you will notice a liquid substance forming on the surface of the copper disk, and on the ceramic. You will also notice that when a magnet is close to that liquid, it will jump up to the magnet, then quickly evaporate! This liquid is oxygen which condenses out of the air onto the surface (same reason water condenses out of the air and onto a colder surface - like a cold glass of lemonade in the summer). Check the table above and see that oxygen becomes a liquid at a warmer temperature than liquid nitrogen. Interesting phenomenon, because liquid oxygen is paramagnetic, meaning that it is slightly attracted to stronger magnetic fields! In this photo, you can see the drop of liquid oxygen starting to form under the magnet on the ceramic, just before it jumps up to the magnet.
What good is this property of superconductivity? Here are some areas where this technology will be a help. (This information is from a display at the Franklin Institute of Science.)
1. Transmission of electrical power. Today, about 9% the power generated at the electrical power stations is wasted as heat in the wires which carry it from the station to the end user. If these wires were superconductors, there would be a tremendous savings. In addition, if the generators themselves used superconducting wires, that would be an even greater amount of savings.
2. In the area of medicine, MRI (magnetic resonance imaging) machines use powerful magnets to create the fields necessary to help physicians see into the body without having to perform surgery. Using superconductors would allow stronger magnets to be built, providing clearer pictures of various types of cells and tissues.
Another area is the production of SQUIDS which are magnetically sensitive sensors, sensitive enough to be able to detect electrical activity within the brain! They would be used to help diagnose and track changes in brain activity to determine if medicines or treatments are helping.
3. Future trains will use superconductors to provide a method to levitate the train above the tracks, reducing friction with the wheels, allowing the train to travel faster with less energy.
www.calpoly.edu/~cm/studpage/clottich/fund.html
4. If computers used superconductors, then they could be made smaller, with smaller wires which would not heat up due to resistance, which would operate faster because the computer chips are closer to each other.
The Type 1 category of superconductors is mainly comprised of metals and metalloids that show some conductivity at room temperature. They require incredible cold to slow down molecular vibrations sufficiently to facilitate unimpeded electron flow in accordance with what is known as BCS theory. BCS theory suggests that electrons team up in "Cooper pairs" in order to help each other overcome molecular obstacles - much like race cars on a track drafting each other in order to go faster. Scientists call this process phonon-mediated coupling because of the sound packets generated by the flexing of the crystal lattice.
Type 1 superconductors - characterized as the "soft" superconductors - were discovered first and require the coldest temperatures to become superconductive. They exhibit a very sharp transition to a superconducting state (see above graph) and "perfect" diamagnetism - the ability to repel a magnetic field completely. Below is a list of known Type 1 superconductors along with the critical transition temperature (known as Tc) below which each superconducts. The 3rd column gives the lattice structure of the solid that produced the noted Tc. Surprisingly, copper, silver and gold, three of the best metallic conductors, do not rank among the superconductive elements. Why is this ?
| Lanthanum (La) Tantalum (Ta) Mercury (Hg) Tin (Sn) Indium (In) Palladium (Pd)* Chromium (Cr)* Thallium (Tl) Rhenium (Re) Protactinium (Pa) Thorium (Th) Aluminum (Al) Gallium (Ga) Molybdenum (Mo) Zinc (Zn) Osmium (Os) Zirconium (Zr) Americium (Am) Cadmium (Cd) Ruthenium (Ru) Titanium (Ti) Uranium (U) Hafnium (Hf) Iridium (Ir) Beryllium (Be) Tungsten (W) Platinum (Pt)* Lithium (Li) Rhodium (Rh) | 7.196 K 4.88 K 4.47 K 4.15 K 3.72 K 3.41 K 3.3 K 3 K 2.38 K 1.697 K 1.40 K 1.38 K 1.175 K 1.083 K 0.915 K 0.85 K 0.66 K 0.61 K 0.60 K 0.517 K 0.49 K 0.40 K 0.20 K 0.128 K 0.1125 K 0.023 K (SRM 768) 0.0154 K 0.0019 K 0.0004 K 0.000325 K | FCC HEX BCC RHL TET TET (see note 1) (see note 1) HEX HEX TET FCC FCC ORC BCC HEX HEX HEX HEX HEX HEX HEX ORC HEX FCC HEX BCC (see note 1) BCC FCC |
*Note 1: Tc's given are for bulk (alpha form), except for Palladium, which has been irradiated with
He+ ions, Chromium as a thin film, and Platinum as a compacted powder.
Many additional elements can be coaxed into a superconductive state with the application of high pressure. For example, phosphorus appears to be the Type 1 element with the highest Tc. But, it requires compression pressures of 2.5 Mbar to reach a Tc of 14-22 K. The above list is for elements at normal (ambient) atmospheric pressure. See the periodic table below for all known elemental superconductors (including Niobium, Technetium and Vanadium which are technically Type 2).
**Note 2: Normally bulk carbon (amorphous, diamond, graphite, white) will not superconduct at any temperature. However, a Tc of 15K has been reported for elemental carbon when the atoms are configured as highly-aligned, single-walled nanotubes. And non-aligned, multi-walled nanotubes have shown superconductivity near 12K. Since the penetration depth is much larger than the coherence length, nanotubes would be characterized as "Type 2" superconductors.
***Note 3: For a list of elements that are naturally diamagnetic, click HERE.
Author's Comment: The information posted on this page was obtained from a variety of sources including, but not limited to, the CRC Handbook of Chemistry and Physics, the Technische Universität München, Reade Metals and Minerals Corp., industry news sources, and various private researchers. A special thanks to Professor Bertil Sundqvist, Department of Experimental Physics, Umea University, Sweden, also to Dr. Jeffery Tallon, Industrial Research Ltd., New Zealand, and to Dr. James S. Schilling, Department of Physics, Washington University.
7.19600 kelvin = -446.7172 degrees Fahrenheit
Superconductors, Orbits, Magnetic Field
Question - I recently learned about Superconductor and Meissener
Effect.
When superconductor meets low temperature, its electricity resistance
becomes zero, thus when the Superconductor receives magnetic energy, it
reflects back same amount energy that it can levitate a magnet on the
superconductor. We know that Earth itself is big magnet and contains
magnetosphere. Then superconductor itself should be levitated from the
ground because it reflects back the same amount of magnetic energy
received, but it does not happen you know.
How about in outer atmosphere, like low orbit 200km altitude?
Would superconductor move away from the earth?
-----------------
Interesting question. The superconductor will generate a magnetic field
through the Meisner Effect even in the relatively low magnetic field of the
earth. Earth's magnetic field is around one gauss or even less, away from
the magnetic poles of the earth. The magnetic field generated by the
superconductor would not exceed that of the magnetic field of the earth and
this is just not enough energy density to levitate. If the superconductor
were in orbit then the magnetic field from the earth would be smaller since
one is moving away from the source of the field. This would in turn lead to
less energy than on the earth. Just to add one more point, not exactly
directly related to your question. Magnetic fields are used to propel
projectiles. This type of propulsion device was devised more than 40 years
ago at MIT.
Magnesium diboride is occasionally referred to as a high-temperature superconductor because its Tc value of 39 K is above that historically expected for BCS superconductors. However, it is more generally regarded as the highest Tc conventional superconductor, the increased Tc resulting from two separate bands being present at the Fermi energy.
Fulleride superconductors[30] where alkali-metal atoms are intercalated into C60 molecules show superconductivity at temperatures of up to 38 K for Cs3C60.[31]
Some organic superconductors and heavy fermion compounds are considered to be high-temperature superconductors because of their high Tc values relative to their Fermi energy, despite the Tc values being lower than for many conventional superconductors. This description may relate better to common aspects of the superconducting mechanism than the superconducting properties.
Theoretical work by Neil Ashcroft predicted that liquid metallic hydrogen at extremely high pressure should become superconducting at approximately room-temperature because of its extremely high speed of sound and expected strong coupling between the conduction electrons and the lattice vibrations.[32] This prediction is yet to be experimentally verified.
All known high-Tc superconductors are Type-II superconductors. In contrast to Type-I superconductors, which expel all magnetic fields due to the Meissner Effect, Type-II superconductors allow magnetic fields to penetrate their interior in quantized units of flux, creating "holes" or "tubes" of normal metallic regions in the superconducting bulk. Consequently, high-Tc superconductors can sustain much higher magnetic fields.
39 kelvin = -389.47 degrees Fahrenheit
The strongest type-I superconductor, pure lead has a critical field of about 800 gauss
The earths magnetic field is approximately 0.5 gauss
Below Hc1 the superconductor excludes all magnetic field lines. At field strengths between Hc1 and Hc2 the field begins to intrude into the material. When this occurs the material is said to be in the mixed state, with some of the material in the normal state and part still superconducting. Type I superconductors have Hc too low to be very useful. However, Type II superconductors have much larger Hc2 values. YBCO superconductors have upper critical field values as high as 100 tesla.
Once set in motion, electrical current will flow “forever” in a closed loop of superconducting material. This makes it the closest thing we know to a perpetual motion machine we have today.
Another instrument relying on superconductors is called a superconducting quantum interference device or SQUID. It is capable of detecting a change in a magnetic field one billion times weaker than the force that moves the needle of a compass. The device is used to probe the body without the need for the strong magnetic fields of the MRI. One such use is to look at the brain.
An international research team has discovered that a magnetic field can interact with the electrons in a superconductor in ways never before observed. Andrea D. Bianchi, the lead researcher from the Université de Montréal, explains in the January 11 edition of the journal Science what he discovered in an exceptional compound of metals -- a combination of cobalt, indium and a rare earth -- that loses its resistance when cooled to just a couple of degrees above absolute zero.
Superconductivity is a phenomenon which occurs in certain materials and which manifests itself by a complete loss of electrical resistance. An important area in the study of superconductors is how they respond to magnetic fields. Besides their obvious relevance to practical applications, such studies are an ideal way to obtain a deeper understanding of the fundamental aspects of superconductivity.
When subjected to a magnetic field, most superconductors will generate vortices (electric tornadoes) which confine them in tubes of magnetic flux. Such vortices have been described by a model for which Alexei Abrikosov and Vitaly Ginsburg received a Nobel Prize in 2003. However, the results obtained by Eskildsen and his colleagues reveal a radical departure from the usual behavior.
“Even in materials such as the high-temperature or heavy-fermion superconductors, where we do not presently understand the microscopic nature of the superconducting state, the Abrikosov-Ginzburg-Landau picture has, for more than 50 years, provided us with a phenomenological description of the vortices,” Eskildsen said. “But in CeCoIn5, as our measurements demonstrate, this paradigm breaks down, forcing us to rethink our understanding of superconductivity.”
The discovery of superconductivity in certain ceramic materials at temperatures as high as 140K (-208 degrees Fahrenheit), well above the boiling point of liquid nitrogen, opened up new possibilities for applications. Superconductors are currently being used or developed for a wide range of applications including electric power transmission, ship propulsion motors, magnetically levitated trains, magnets for medical imaging, and digital filters for high-speed communications.
"This discovery sharpens our understanding of what, literally, holds the world together and brings physicists one step closer to getting a grip on superconductivity at high temperatures. Until now, physicists were going around in circles, so this discovery will help to drive new understanding," said Prof. Bianchi, who was recruited to UdeM as a Canada Research Chair in Novel Materials for Spintronics last fall and performed his experiments at the Paul Scherrer Institute in Switzerland, in collaboration with scientists from ETH Zurich, the University of Notre Dame, the University of Birmingham, U.K., the Los Alamos National Laboratory and the Brookhaven National Laboratory.
Magnetic tornado that grows stronger
Using the Swiss Spallation Neutron Source (SINQ), Prof. Bianchi and his team cooled a single-crystal sample of CeCoIn5 down to 50mK above absolute zero and applied a magnetic field nearly high enough to entirely suppress superconductivity. They found that the core of the vortices feature electronic spins that are partly aligned with the magnetic field.
This is the first experimental evidence that a theory that describes the properties of superconducting vortices and, for which Abrikosov and Ginzburg received the Nobel Prize in 2003, which does not generally apply in magnetically-induced superconductors.
"When subjected to intense magnetic fields, these materials produce a completely new type of magnetic tornado that grows stronger with increasing fields rather than weakening," said Prof. Bianchi. "The beauty of this compound is how we can experiment without breaking it."
Superconductors hold great promise for technological applications that will change how modern civilization can store and transmit energy - arguably some of the most pressing challenges today. Other notable applications include superconducting digital filters for high-speed communications, more efficient and reliable generators and motors, and superconducting device applications in medical magnetic resonance imaging machines.
The first superconductor was discovered nearly a hundred years ago, and in most materials this curious state with no resistance was shown to arise from the interaction of the electrons with the crystal; however, in this new material, superconductivity is thought to arise from magnetic interactions between electrons.
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