First-Hand:Serendipity and Superconducting Magnets
Contributed by: Morris Tanenbaum, IEEE Life Fellow
After my work on silicon transistors, I was offered the opportunity to transfer to the Metallurgical Research Department (later known as the Materials Science Department) and head a new Subdepartment devoted to basic research. At Bell Labs, Subdepartments typically consisted of 10 or so Members of Technical Staff (MTS) with a comparable number of technicians and other support staff. A Department usually consisted of four Subdepartments. The Metallurgical Research Department was responsible for research and development of metals, ceramics, semiconductors and other non-organic materials. During the age of electro-mechanical switching, specialty metals for springs, electromagnets, electrical contacts, etc were critical for the reliability of the network. The Department was populated historically and primarily by expert metallurgical engineers who developed new materials and process technology and also served as consultants to the manufacturing engineers in Western Electric, the manufacturing arm of the Bell System.
A few years earlier the management of the Research Division of Bell Labs decided that it was time to increase the research depth of the Metallurgical Research Department. A few Ph.D.’s were hired and they formed the core of my new Subdepartment. Among them was Gene Kunzler, a Ph.D. from the University of California, Berkeley where he had studied the physical properties of solids at ultra low temperatures. At Bell Labs, he was currently measuring the electrical and magnetic properties of single crystals of copper at liquid helium temperatures and working with the theoretical physicists in the Physics Department to understand the electronic structure that gave rise to the high electrical conductivity of copper.
Masers and Magnets
One day as I was sitting in my cubicle, Rudi Kompfner came in, sat down and said “I have a problem and wonder if you could help me”. Rudi was in the Systems Research area of the Research Division and was currently studying masers (the microwave predecessor of the laser) and how they might be used in telecommunications. He was the inventor of the traveling wave tube and had a very interesting background himself. He had studied architecture in Austria, his home country, migrated to England to escape the Nazis, practiced architecture there but then became involved with electronics as World War II began. Later, he came to the U.S. and Bell Labs.
The maser was attractive because of the ability to achieve very low noise amplification by operating at liquid helium temperatures. Rudi and his colleagues also wanted to operate at high frequencies which required high magnetic fields, 10,000 gauss or higher. Since they were operating at liquid helium temperatures, their problem was how to obtain high magnetic fields at those ultra low temperatures. The active maser crystals had to be mounted in high vacuum Dewar vessels but the electromagnets had to be external to the Dewar vessel because of the heat generated by the magnets. This created a large gap between the magnets and the maser crystal which, in turn, required large magnets and used large amounts of electric power, much of which was dissipated as heat. Rudi wondered if the magnet coils could be made of superconductors which could be immersed in the liquid helium with the maser crystal and operated without generating any heat. He was aware that superconductors, in general, abhorred magnetic fields and their superconductivity disappeared at fields of a few thousand gauss or lower.
Rudi was also aware that new superconductors had recently been discovered by Berndt Matthias at Bell Labs which had higher transition temperatures (the temperature at which superconductivity appeared) and he wondered if perhaps they could also tolerate higher magnetic fields. I suggested that we talk to Gene Kunzler whose apparatus would be well suited to test superconductivity at high magnetic fields. Gene was very interested.
Gene started out with alloys of bismuth and lead. The metallurgical engineers in the Department prepared wires of these alloys and, while they sustained higher magnetic field than the pure metals, they did not reach the 10,000 gauss level that was needed. Kunzler then moved on to other alloys where Mathias had found higher critical temperatures. Now they started getting some very interesting results and with molybdenum-rhenium alloys reached magnetic fields of 15, 000 gauss and Komfner’s problem was solved. However, Kunzler’s appetite was not satisfied. First, he suggested that I give him an “incentive”. I agreed that for the demonstration of any magnet that produced a sustained magnetic field above the 15,000 gauss already achieved, I would reward him with a bottle of scotch for each additional 2,000 gauss.
In 1954, Mathias and his colleagues had discovered that the compound Nb3Sn (niobium-tin) had the highest critical temperature (18K) of any known material at that time. (Since that time, other superconductors have been discovered with much higher transition temperatures.) Unfortunately, unlike the molybdenum-rhenium alloys, niobium-tin was a hard, brittle ceramic and could not be drawn into wire. Working with Ernie Buehler (again!), Frank Hsu and Jack Wernick, they conceived of a process of making a tube of Niobium covered with Monel (a commercial nickel alloy), filling the tube with the proper mixture of niobium and tin metal powders, drawing the resulting tube into a wire, winding a coil of the resulting wire and finally heating the coil to a high temperature where the niobium and tin powders reacted to form the niobium-tin compound in the desired coil shape. (Since the Monel was not a superconductor at any temperature, it served as an insulator in the coil.) The resulting magnet produced a magnetic field of 88,000 gauss–and I owed Kunzler 36 bottles of scotch! He settled for 2 cases and we had a grand party. Later fields above 100,000 gauss were achieved.
Applications and the Future
The superconducting magnet technology that was produced has application far beyond masers (which have since been generally superseded by other solid state and optical devices). Today’s MRI machines that are widely used in medical diagnosis use superconducting magnets and would be impractical without them. Superconducting coils are used as electrical energy storage devices by utilities. They are widely used in high energy physics studies of the basic structure of matter and the electrical and optical properties of materials at high magnetic fields. They have promise in the development of magnetic levitation for mass transportation systems, large electrical motors and dynamos and in replacing ultra high voltage, AC electrical transmission systems with high amperage DC systems,. There are likely other applications not yet recognized.
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