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High-T/sub c/ superconductor and its use in superconducting magnets
[report]

M.A. Green

1988
unpublished

The discovery of the high-critical-temperature superconductor is a very important solid-state-physics event. 1 The popular press, and to a lesser extent some of the scientific journals, has claimed wonderful things will unfold because of the discovery of high-critical-temperature superconductors (high-T >. Many of the proposed uses for the high-T superconductor involve the creation of a magnetic field using superconducting coils. This report will assess what is known about the high-T
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... e high-T superconductors and take a realistic look at their potential use in various kinds of superconducting magnets. Based on what is known about high-T superconductors, one can make a "wish list" of things that will make such materials useful for magnets. Then, the following question 1s asked. If one had a high-T superconductor with the same properties as modern niobium-titanium superconductor, how would the superconductor work 1n a magnet environment? Finally, this report will show the potential Impact of the Ideal high-T superconductor on: 1) accelerator dlpole and quadrupole magnets, 2) superconducting magnets for use in space, and 3) superconducting solenoids for magnetic resonance Imaging. What Is Known about the High-T Oxide Superconductor? The h1gh-T oxide superconductors are an extension of a group of superconductors known as the perovskite class of superconductors 2 that Includes materials such as WOjReOj, SrTIOj, 3 and Ba(PbBi)0 3 . The last material has been known for some years, but it has a critical temperature of only 10 K. Perovskltes have been studied for some years since many people thought that they might have some unusual superconducting or magnetic properties.* The discovery of a perovsklte-type superconductor with a critical temperature above 35 K is considered to be a very important advance in metallurgy and solid-state physics. The high-T oxide superconductors, which are not true perovskite structures, are more complex than perovskltes, yet they exhibit similar properties. The high-T superconductors, at this tine, appear to be divided into two types-the lanthanum-strontium type and the barium-yttrium type. 5 It might be argued that the two types are really subsets of a single type of metal/copper-oxide superconductor with oxygen as the most important element. The superconductor that has received the greatest attention has been the REa 2 Cu,0 7 , where R is yttrium or one of the Lanthanide rare earths. (There have been unverified reports from China that superconductivity can be obtained without the Lanthanide or yttrium.) 6 person considering the use of the h1gh-T oxide superconductor for generating a magnetic field. There have been few, 1f any, direct measurements of H -at low temperatures (e.g., 4.2 K) for these materials. The claims of high values of H « are based on extrapolations based on a measurement of dH 2 /dT at or near T and on the experimental observations in other materials that high T implies that H -Is high also. If one extrapolates dH ,/dT for the onset of the drop in resistivity, one might get y H -at T -0 K as high as 360 tesla, based on a linear extrapolation model. (If one uses an extrapolation model that many conventional superconductors follow, 10 one gets a value of F 0 H C 2 " 260 tesla at T -0.) On the other hand, if one extrapolates dH c2 /dT based on measurements of zero resistance near T , the value of y-H.g at T -0 K would be in the 14-to 20-tasla range, depending on the extrapolation model. The rather low predicted value of zero-resistance y 0 H.. 2 at T " 0 K probably can be explained using a granular model, where regions of high-", superconductor are surrounded by regions of low-H " superconductor. B>11 There is evidence that the zero-resistance dH "/dT slope for one type (Y-Ba-Cu-0) makes a sharp change at about 60 K; i.e., below 60 K the value of H -increases more rapidly than it does above 60 K. If this were true for all types of Y-Ba-Cu-0, the estimated value for the zero-resistance H c2 would be much higher. (The material that K. Noto et al. examined might have a value of H " in the 55-to 80-tesla range depending on the model used for the extrapolation. 12 ) It is expected that the value of the superconductor H c2 is a function of crystal -5c. H , Measurements Measurements of n 0 H c] by the National Bureau of Standards on Y-Ba-Cu-0 superconductor show two (or more) phases of the superconductor. 14 (H . is the lower critical field.) One phase has a value of y 0 H c , of 0.016 tesla to 0.035 tesla at T -0 K, whereas the other phase has a zero-temperature ji H . of 0.0014 'tesla to 0.0030 tesla. (The variation Is based on the model used to extrapolate the value of n 0 H c i at T -0 K from measurements of dH ,/dT at or near T .) The measured value of H . should be compared to ii.H i -0.014 tesla for Nb-TI, n 0 H cl . 0.018 tesla for NbgSn, and M o H cl " °-037 tes1a for V 3 G ad. Critical-Current Density The potential for high critical-current density (J ) has been demonstrated by IBM in samples of oriented multicrystal Y-Ba-Cu-0 on a strontium tltanate substrate. 15 The IBM measured values were J (4.2 K, 0 T) -5 x 10 10 A m~2 and J c (77 K, 0 T) -10 s A m" 2 . Bulk samples of Y-Ba-Cu-0 have yielded much lower measured values for the critical current. Measurements of critical current by direct voltage measurements of a sample yield measured critical-current densities that are an order of magnitude or more lower than those obtained by magnetization. 1 " 17 (The IBM orlented-film measurements were made using magnetization methods.) Measurements of critical current at fields above zero using voltage-drop measurements show a sharp reduction 1n the critical-current density at 77 K

doi:10.2172/7150598
fatcat:kunh5sueijddzpwjghxpfs7ct4