Johannes Georg Bednorz


Johannes Georg Bednorz

Johannes Georg Bednorz was born on May 16, 1950, in Nevenkirchen in what was then West Germany. He graduated from the University of Münster in 1976 and received his doctorate from the Swiss Federal Institute of Technology in Zurich in 1982. That year he went to work at the IBM Zurich Research Laboratory and met Müller, who invited him to participate in his study of superconductors--generally metals and alloys, whose electrical resistance becomes virtually nonexistent at temperatures close to absolute zero. Bednorz's own interest in the applications of superconductivity, especially in connection with the design of high-speed trains, made the project particularly appealing to him. Superconducting levitation would reduce power requirements in rail transportation: if a magnet is placed on a superconducting surface, it will float above it when the temperature of the surface is lowered.

Scientists have long sought materials that were superconductive at as high a temperature as possible to provide for the most effective electrical conduction for ordinary and practical use. After World War II, superconductors were put to many practical uses. Among these was the operation of large electromagnets, but they only worked at very low temperatures. Finding new materials would open up huge possibilities in reducing power requirements and preventing power losses in overhead power transmissions.

Many scientists were skeptical about the results until teams from the University of Tokyo, the University of Houston, and Bell Laboratories confirmed them. In 1987 researchers at the University of Houston, working with similar ceramics, announced their discovery of superconductors effective at 90° Kelvin, above the boiling point of liquid nitrogen. The same year President Ronald Reagan announced an initiative to develop superconductivity. Scientists around the world rushed to study ceramics and superconductivity in the liquid nitrogen temperature range.

The Nobel Prize committee took note of the excitement when they awarded Bednorz and Müller the physics prize, stating that the discovery had generated "an explosive development in which hundreds of laboratories the world over" were taking part. Still, the speed with which the award was given--only two years after the discovery was made--generated some surprise. Prize winners often wait much longer to be recognized.

Despite Bednorz's and Müller's discovery, no theoretical explanation for the behavior of the ceramic substances has yet been made. Nor did their ceramic materials immediately lend themselves to technological applications, since they are not easily made into wire. The Japanese have gone farthest in the field: the Nippon Steel Company developed a "melt-processing" technique to produce a superconducting wire out of the ceramics. Room-temperature superconductivity has yet to be achieved.

Supperconducivity applications

Some of the technological applications of superconductivity include the production of magnetometers based on SQUIDs, digital circuits (including those based on Josephson junctions and rapid single flux quantum technology), Magnetic Resonance Imaging (MRI) and Nuclear magnetic resonance (NMR), control magnets in particle accelerators and fusion reactors (tokamaks), power cables, and RF and microwave filters (e.g., for mobile phone base stations, as well as, military ultra-sensitive/selective receivers), and railgun and coilgun magnets.

The biggest application right now for superconductivity is in producing the large volume, stable magnetic fields required for MRI and NMR. This represents a multi-billion US$ market for companies such as Oxford Instruments, Siemens etc. The magnets typically use low temperature superconductors (LTS). These need to be cooled to liquid helium temperatures to superconduct. LTS is also used in high field scientific magnets because copper has a limit to the field strength it can produce.

The commercial applications so far for high-temperature superconductors (HTS) have been limited. HTS superconduct at temperatures up to that of liquid nitrogen which makes them cheaper to cool. The problem with HTS technology is that the currently known high-temperature superconductors are brittle ceramics which are expensive to manufacture and not easily turned into wires or other useful shapes.

Therefore the applications have been where HTS has some other intrinsic advantage i.e. in low thermal loss current leads for LTS devices (low thermal conductivity), RF and microwave filters (low resistance to RF), and increasingly in specialist scientific magnets, particularly where size and electricity consumption are critical (while HTS wire is much more expensive than LTS in these applications this can be offset by the relative cost and convenience of cooling).

1) Superconducting Magnets
Type II superconductors such as niobium-tin and niobium-titanium are used to make the coil windings for superconducting magnets. These two materials can be fabricated into wires and can withstand high magnetic fields. Typical construction of the coils is to embed a large number of fine filaments (20 micrometers diameter) in a copper matrix. The solid copper gives mechanical stability and provides a path for the large currents in case the superconducting state is lost. These superconducting magnets must be cooled with liquid helium. Superconducting magnets can use solenoid geometries as do ordinary electromagnets.

Most high energy accelerators now use superconducting magnets. The proton accelerator at Fermilab uses 774 superconducting magnets in a ring of circumference 6.2 kilometers. They have also found wide application in the construction of magnetic resonance imaging (MRI) apparatus for medical imaging.

2) Niobium-Titanium Superconductor

Niobium-titanium is a Type-II superconductor with a critical temperature of 10 K and a critical magnetic field of 15 Tesla. While both of these values are lower than those for niobium-tin, this material has become the material of choice for superconducting magnets because of its mechanical properties.
To make magnet wire, the niobium-titanium is formed into filaments finer than human hair and embedded in a matrix of solid copper. The fine filaments are advantageous because current flows only within a skin-depth of the surface of a superconductor. The solid copper forms a solid mechanical structure which will also carry the current if the superconducting phase is lost.

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