Download Superconductors: Better levitation through

Superconductors
Better Levitation through Chemistry
Arthur B. Ellis
University of Wisconsin-Madison, Madison. WI 53706
Nineteen eighty-seven will be remembered as an extraordinary year for the physical sciences. I t was a year in which
new superconductive materials created unprecedented excitement in the scientific community. And it was a year in
which the public became spellbound by new vistas in technology.
The title of this article reflects the notion that chemistry
can have a positive impact upon our lives-"better living
through chemistry". And it includes the phenomenon that
best captures the almost magical quality of superconductivity-levitation. First, I will outline the history of superconductivity, then describe the physical and chemical principles
upon which i t rests, and, finally, speculate on some of the
applications tbat may he forthcoming ( I ) .
Hlstory
Superconductivity owes its discovery to a Dutch physicist,
Heike Kamerlingh Onnes. In the early 1900's, Kamerlingh
Onnes accomplished the then remarkable feat of liquefying
helium. The ability to drop temperatures to a few degrees
Kelvin (helium boils at 4.2 K a t atmosoheric oressure) suddrnly o;,ent.d a iarger svindow for studies of thermal effects.
Kamerlineh Onnrs exvloited his new t d hs measurina the
electrical iesistance df metals a t these very low temperatures. In 1911, while measuring the resistance of mercury, he
found an astonishing result: at about 4 K, the resistivity
abruptly dropped below his ability to measure it. Nor was
mercury unique in displaying what appeared to be zeroresistance to a flow of electrical current. Several other metallic
elements exhibited the same effect a t temperatures of a few
degrees Kelvin. This remarkable electrical property was
treated as a diagnostic for a new state of matter, the superconducting state.
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About 20 years later, in 1933, another surprising characteristic of superconductivity was discovered by Meissner
and Ochsenfeld. They found that a superconducting material will not permit a magnetic field to penetrate its hulk, a
property that has come to be called the Meissner effect.
These two diagnostics of superconductivity, resistanceless
current flow and perfect diamagnetism, were appreciated
even 50 years ago as having tremendous technological implications. But two maior obstacles to imolementina- anv
- new
technology existed. First, extremely cold temperatures were
reauired to achieve the suoerconducting- state. Although
. liq.
uid helium could serve as a coolant, its scarcity and processing costs made it expensive; moreover, sophisticated equipment was required to handle it. The second problem was
tbat the superconducting state of these elemental metals
was easily destroyed by the application of modest external
magnetic fields or electrical transport currents, making their
use in electromagnetic applications impractical.
These problems prompted an intensive search for new
materials that would become superconducting a t higher
temperatures and retain their superconductivity in the presence of large magnetic fields and electrical currents. In particular, scientists who studied this phenomenon dreamed of
achieving superconductivity a t or above 77 K (-196 ' C ) , in
which case liquid nitrogen (boiling point, 77 K), which is
cheap and easily handled, could be used as a coolant.
Until recently, alloys of niobium, particularly Nb-Ti, had
been responsible for the greatest advances in superconductor technology. Their ability to remain in the superconducting state while supporting large electrical currents led to
limited but extremely important applications such as the
construction of powerful magnets. In 1973, superconductivity was observed at 23 K with NbaGe. Another breakthrough
10)
U d
Figure 1. The peridic table with elemems that exhibit superconductivity at ambient pressure indicated in boldface autilne; all d these elements requlre temperatures
of <10 K to main the superconducting state. Adapted from ref. 2.
838
Journal of Chemical Education
involved the observation of superconductivity with certain
organic salts, whose structur& feature one-dimensional
stacks of organic moieties that can be modified by chemical
substitution. These salts have required liquid-helium-range'
temperatures to become superconducting.
In spite of these advances, there was a feeling of despair
among some scientists that perhaps superconductivity a t 77
K was simply not meant to be. This gloomy picture reflected
the fact that, while more than 20 metallic elements exhibit
superconductivity (Fig. I),they do so only a t very low temperatures near that of liquid helium (2). Furthermore, most
bf the simple alloys or compounds based on combining two
or three elements also required disappointingly low temneratures for sunerconductivitv.
I t was against this setting that Georg Hednorz and K. Alex
Mi~llcr.two scientist3 working at the IBM facility in Zurich,
reported a startling result in early 1986: an oxide of lanthanum, barium, and copper lost its resistance a t about 30 K (3).
That a metallic oxide, generally regarded as a mineable ore
(or worse, as a form of dirt), should not only exhibit superconductivity but do so a t such a high temperature struck
many in the scientific community as incredible. Suddenly, a
whole
new familvof materials. ceramics. was r i ~ for
e investi----~
-~~
gation, and the pace of researih accelerated markedly.
In early 1987, groups a t the University of Houston and
University of Alabama, headed by physicists Paul Chu and
M. K. Wu, Jr., respectively, announced (4) the discovery of a
related oxide that was superconducting above 77 K! Superconductivitv at the temperature of liquid nitrogen (in fact,
above 90 K)became a ;eality, to the delight of many researchers who never expected to see it. In fact, we cannot
rule out the possibility that room temperature superconductivity is in the offing. At this writing, several research groups
have seen tantalizing evidence for such a possibility.
Let usnow turn to the physics and chemistry that underlie
these momentous advances.
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vacancy
interstitial
impurity
Physical Properties of Superconductors
Loss of Resistance
A material's oassaee into the suoerconductine state is
characterized b; an &rupt change k its resistivzy as i t is
cooled. In ordinarv conductine
- materials the flowing- electrons that represent current always encounter some resistance, which can he likened to friction. Ohm's law relates the
current, I, to the voltage, V, (the difference in electrical
potential between points in a circuit that causes electrons to
flow) and resistance, R, thus: V = ZR.
The source of the resistance is the scattering of electrons
from the atoms that make up the conducting material. Scattering occurs if the atoms are vibrating andlor if lattice
defects are nresent. Defects. which are illustrated in Fieure
2, occur when a normally periodic arrangement of atoms, a
crvstal lattice. is interrunted hv imnuritv atoms. hv dis~lacemerit of atoms to posiiions that they "would not nokwlly
occupy (interstitials), and by the absence of atoms (vacan~ i e ~ )
Figure 3 graphs the resistivity of a superconducting material (the resistivity is proportional to resistance) as a function of temperature. As the sample is cooled, there is initially
a smooth decline in resistivity with temperature; with less
thermal energy available, the atoms vibrate less, causing less
scattering. In a normal metal, this decline ceases a t very low
temperatures when scattering becomes limited by fixed defects: a limitine or residual resistivitv exists. What characterizes the superconducting state, however, is that there is a
sudden drop to zero resistivity a t what is called the critical
temperature, T,.Below T,, a direct current can flow indefinitely in the material, so far as anyone has been able to
determine. This is a form of perpetual motion that is one of
the amazing-.
~ r o.~ e r t iof
e sthe su~erconductinestate.
The transition from a normaimetal to a s~perconductor
can be regarded as a phase change. But unlike the phase
change associated with, say, conversion of a liquid to a gas,
the enthalpy and entropy of the phase change are zero (if no
magnetic field is present); there is a change in the specific
heat of the material, however.
-
The Melssner Effect and Levitation
Besides resistanceless current flow, the other characteristic of a superconducting material is that it is perfectly diamagnetic. The expulsion of magnetic field lines from the
interior of a material as it passes from the normal to the
superconducting state, the Meissner effect, is illustrated in
Figure 4.
Figure 2. Types of lattice defects
Temperature
Figure 3. Resistivity as a function of temperature for a superconductor
Figure 4. The Meissner effect. A superconductar (shaded sphere) expels
magnetic field lines from its interior.
Volume 64
Number 10 October 1987
837
In order to appreciate how the Meissner effect operates to
produce levitation, we need t o review some of the principles
of electromagnetism. As recently as the beginning of the
19th century, electricity and magnetism were believed to be
two separate phenomena. In 1820, however, the Danish
physicist Oersted observed a connection between them. As
illustrated in Figure 5, Oersted found that when he passed
an electric current through a wire, a deflection was observed
in the magnetic pointer of a compass; in other words, the
current had induced a magnetic field. Superconducting
magnets are based on this principle: a magnetic field induced by passing current through continuous superconducting coils of wire will persist indefinitely, since there is no
resistance to the flow of current.
Following Oersted's obsenrations, two other scientists,
Michael Faraday and Joseph Henry, investigated the related
question of whether a magnetic field could induce a current
in an electrical conductor. They found that a magnet could
induce a current if the magnet was constantly in motion (or
more fundamentally, if the magnetic flux linking the coil is
continuously changing), as shown in Figure 6. Our present
large-scale electricity needs are now met by generators that
work on this ~ r i n c i ~ l e .
The coupling of electrical and magnetic phenomena is
eleeantlv demonstrated in a levitation experiment. We begin
hycoolihg a pellet so as to make it su~rconducting.If an
appropriate light-weight, strong magnet is now placed ahove
the pellet, it will hover, as if by magic, over the pellet. The
magnet retains this position so long as the pellet remains
superconducting; once the pellet warms to its normal state,
the magnet will no longer remain suspended in air.
Why does levitation occur? Electromagnetism tells us that
as the magnet first approaches the pellet, it will induce a
current in the surface of the superconductor. Because the
superconductor has no resistance, this current remains even
after the magnet stoos movine. I t is a su~ercurrent.The
supercurrent in turn induces a magnetic field. The induced
magnetic field has just the right strength and geometry to
cancel completely the effects of the magnetic field, arising
from the maenet.
. . . in the bulk of the suoerconductor. The
interior d the supercondurtor is thuv perfectly diamagnetic.
as demanded bv the Meiisner effect. Ourside of the superconductor, however, the field caused by the magnet a n d t h e
field induced by the superconductor repel one another, just
as two north or two south poles of conventional magnets
would. The result is that the magnet is suspended in limbo,
its position dictated by the equilibrium between the downward gravitational force and the upward magnetic repulsive
f0rce.h its ability to respond electromagnetically to movements of the magnet, the superconductor can be regarded as
a kind of magnetic mirror.
Levitation is easily demonstrated with the new superconductors havine T, ahove 77 K. A pellet of the material is
cooled with &id nitrogen, u s i n i a Styrofoam stand (an
inverted coffee CUD,for example). When placed above the
pellet, a standard Iefrigeratormagnet, if small enough, can
become airborne. However, the lift is more dramatic with
strong, rare-earth-based magnetic compounds like SmCoS
and FelaNdlB.
.. - We have cleveloped a technique, using an
overhead projector, placed on its back, and mirror, for
enabling a large audience to view alevitation experiment (5).
This is truly a demonstration accomplished with magnets
and mirrors!
Compass
The Mechanism of Superconductivity
Superconductivity baffled theorists for many years after
its discovery. I t was not until the late 1950's, some 40-odd
years after Kamerlingh Onnes' experiments, that a satisfactory theory was found. Called the BCS theory after its authors. Bardeen. Cooper, and Schrieffer, i t satisfactorily accounts for virtually h l of the properties of the traditional,
low-T, superconductors (2).The cornerstone of the theory is
a not& that a t first seems counterintuitive: electrons are
attracted to each other to form what are called Cooper pairs.
That two electrons, both negatively charged, would feel an
attraction seems to violate what we know about simple electrostatic interactions. The key to the attraction, though, is
that i t is mediated by the lattice. Positive ions, whose positions are fixed in the lattice, result in any metal because the
valence electrons of atoms composing the metal have been
removed from their individual atoms and move about freely.
Figure 7 shows that, as these electrons responsible for conductivity travel past the positive ions, the ions are drawn
toward the path of the electron by electrostatic attraction.
n
Dry C e l l
Figwe 5. Oersted's experiment. The magnetic needle of a compass placed
beneath a conductor carrying current will be deflected by the Induced magnetic field
Figure 6. Moving a magnet through a coiled wire induces a current (mare
a voltage is induced. which drives the cunent). detected with a
a~c~rately,
galvanometer.
838
Journal of Chemical Education
-
a
Flgure 7. The coupling (vertical lines) of two electrons (large blocks) Into
Cooper pairs is mediated by lattice atoms (dots),whose positions are affected
by electronic motion.
And because the ions are much more massive than the electron and move more sluggishly, this "wake" of displaced
nositive charee
lone enough to attract a second elec. . nersists
.
trun. f$.rmingtht, Couper pair.'l'hr pair is thus bound by the
mutual artniction of earh elrctron fi,r the ~ositiveions in the
lattice. Important characteristics of the Cooper-pair electrons are that their spins are paired and the combined momentum of the pair is not affected by electron scattering.
Consequently, scattering does not provide the energy transfer between the electrons and lattice that produces resistance in a normal conductor. In the superconducting state
most of the conduction electrons are bound in Cooper pairs.
A crude way to think of these pairs is as diatomic molecules. And like such molecules, they can be dissociated if
sufficient energy is present to disrupt the bonding. The
value of T,is a reflection of this energy: as a superconductor
is warmed, the number of Cooper pairs drops as T, is apnroached.
Tempernture is not rhr only experimental parameter that
call affect the formntion of Cooper pairs. We have seen that.
when magnetic fields are applied to a superconductor, currents will he induced in the surface of the superconductor. If
these fields and corresponding currents are sufficiently
large, they can impart so much energy to the superconductor
that the Cooper pairs will be dissociated and the superconductivity destroyed. Many potential applications of superconductors are thus ~recludedbv the critical field H, (and
corresponding critical current) a t which this conversion
from suoerconductine to normal behavior occurs; recall that
small magnetic field; and electrical currents destroyed superconductivity in the early superconductors. Figure 8
shows how T,varies with H, for a typical superconductor.
The points on the curve represent the critical temperature at
u,hich the phase change fntm normal cunductor t o s u p r r c ~ n ductor takes d a r e in the presence of different magnetic field
strengths. AS-themagnet& field increases, lower values of T,
result.
The BCS theory makes no restriction on how large T,can
be. But at present we do not know the extent to which the
theory can be applied to the new superconductors.
Chemical Properties of a Hlgh-T, Superconductor
Svnthesis and Oxidation States
The superconducting oxide that has created such a maelstrom of research activitv has the deceptively simple formula, YBa&~307-~(x 5 0.1). I t is often referred-to as the
"1-2-3" compound because of the Y.Ba:Cu stoichiometry.
Making a pellet of the compound is straightforward, although several steps are involved ( 5 , 6 ) .In a typical procedure, the starting materials, Y203, BaC03, and CuO, are
ground together and heated to about 950 OC. After cooling,
nellets can be messed and then sintered at 950 "C. Sinterina
involves heating just below the melting point; this process
~ r o m o t e bondine
s
between the erains comnosina the pellet.
;hereby increasing the densityUand strength o r t h e pellet:
v
6
Superconductor
Normal
conductor
Temperature
Figure 8. Relationship of H, and T, for a typical supercanductor.
Once the pellet has been sintered, it is heated in 0 2 a t 500600 "C and slowly cooled to room temperature.
The oxygen content of YBa2Cua07-, has been estimated
from standard chemical methods, which indicate that the
material is a nonstoichiometric compound: instead of having
an integral number of oxygen atoms, the best supercondncting materials appear to have values of x of about a tenth or
less. As will he discussed in more detail below, nonstoichiometric compounds are common in the solid state and reflect
chemistrv associated with lattice defects.
Formai oxidation states can be assigned to the elements of
the 1-2-3 compound. Based upon the normal oxidation
states of -2 for the oxide oxygen atom, +3 for the yttrium
atom, and +2 for each barium atom, the resulting average
oxidation state for each conoer
. . atom is 713 (for x = 0). We
can intrrpret this nonintegral oxidation state for Cu to mean
thnt. on aieraee. two-thinlsof the C'u is wesent in Cu2' sites
in the lattice &d one-third of the Cu isin Cw3+sites.
It is natural to ask why the 1-2-3 oxide had not been
prepared before, given the tremendous amount of research
that has been conducted with oxides. The answer lies in the
large number of elements that form oxides and the difficulty
in predicting which of these combinations will yield new
phases. Most of the elements in the periodic table form solid
oxides, so that the number of ways we can combine three
oxides is staggeringly large. Furthermore, we cannot always
predict which combinations will lead to solid solutions. For
example, AI2O3and Cr203 are miscible in any proportion to
yield compounds of the type (A12-,Cr,)03, where x can take
anv value from 0 to 2: in contrast. LiCl and KC1 are mutuallv
in'oluble (7). whether compounds form solid solutions or
simnlv retain their identities in a ~hvsicalmixture is often
determined by X-ray and neutron diffraction: the interaction of X-rays and neutrons with a periodic crystal lattice
yields a diffraction pattern that serves as a fingerprint for
identifying the formation of a new phase. Both X-ray and
neutron diffraction patterns of the 1-2-3 oxide were used to
demonstrate that it was a new material.
In trying to understand what makes the 1-2-3 oxide so
special, researchers have tried to tune the material chemically by substituting for Y, Ba, and Cu. A variety of rare earth
elements have been substituted for Y without greatly disturbing the superconducting characteristics of the material.
However, substitution for Ba or Cu adversely affects superconductivity (8).
~
~
~
Structure
Because the structure of a solid is intimately related to its
physical properties, X-ray and neutron diffraction have assumed a prominent role in the characterization of the 1-2-3
oxide. The structure of this superconductor belongs to the
perovskite family. Perovskites characteristically have aratio
of two metal atoms for each three oxygen atoms. Representative comnounds with the nerovskite structure are CaTiOp
(ca2+;~ i ~ ; )NaNbos
,
( ~ a + l N b ~ and
+ ) , ~ a ~ 1 (0~3a ~AI"~);
+;
in these simnle
the metal oxidation states must
. ~erovskites
.
sum I,, tfiin order to yield nnelectricnlly neutral compound.
Shown in Fieurc 1 is the solid-state structure for the mineral perovskite, CaTi03. Note that the larger Ca2+cation is
a t the center of the cubic unit cell, the smaller Ti4+ions are
located a t each corner of the cube, and the 02-ions bisect the
edges of the cube.
If the 1-2-3 compound had an idealized perovskite structure, it would possess nine oxygen atoms in its formula (metal-to-oxygen atom ratio of 23) and would have the tripledecker structure sketched in Figure 10a. The resulting idealized unit cell, consisting of three stacked cubic unit cells, is
now tetragonal rather than cubic, having a square base but
rectangular sides. Note that the top and bottom compartments of the tetragonal unit cell contain Ba2+ions and that
the middle compartment contains a Y"+ ion. The Cu ions
Volume 64
Number 10 October 1967
839
occupy the corners of the cubes composing the unit cell,
while the oxide ions again hisect the cube edges.
Why is the ideal structure not adopted by the 1-2-3 oxide
compound? The answer presumably lies in the excessively
high oxidation state required of Cu. A formula of YBa2Cu309
leads to an average copper oxidation state of 1113, which
implies contrihutions from both Cu3+ and Cu4+ oxidation
states. The fact that tetravalent Cu compounds are extremely rare strongly suggests that the average Cu oxidation state
must be +3 or less. One avenue for achieving a lower oxidation state is to expel oxygen atoms from the lattice: as we saw
above, the observed stoichiometry of the 1-2-3 oxide with
annroximatelv seven oxveeu atoms vields an average copper
oxidation statk of 713, whLch impliescontrihutions from bbth
Cu2+and Cu3+ions (but not from Cu4+ions).
How the structure is tuned by varying the oxygen content
is illustrated in Figure lob. Although it is difficult (due to
crystal disorder problems) to locate all of the oxygen atoms
in the structure, the general geometrical pattern appears to
be that four of the 12 02-ions surrounding the Y3+ion have
been lost as well as four other 02-ions, two from the top face
of the unit cell and two from the bottom.
The oxygen vacancies are defects in the lattice that are
governed by chemical equilibria. I t is helpful, in fact, to
regard the solid as a solvent wherein vacancies, impurities,
and interstitial atoms act as solutes that are subiect to the
same kinds of mass action expressions that are used t o describe aaueous acid-base chemistry. In this case, the concentration df oxygen vacancies is coupled to the oxidation states
of the Cu ions composing the lattice: as the oxygen content
declines, so too does the average Cu oxidation state. Specifically, since the solid must remain electrically neutral, each
oxygen
titanium
calcium
Figure 9. The perovskite structure of CaTiO, possesses a cubic unit cell (the
fundamental building block of a threedimansianal lattice). The conelation of
the unit cell with the chemical formula is established by determining lhe enent
to which each atom in lhe unit ceil contributes to this particular unit cell: The
single Ca" ion Is fully contained in and belongs exclusively tothis ceil; each of
the eight Ti4+ ions is shared by eight unit cells (8 X 118 = 1 Ti4+ ion); and each
01 the twelve 02-ions is shared by four unit cells (12 X 114 = 3 0'- ions). The
ions Sires are not drawn to scale.
b
a
-~
Fiaure 10. la1
YBa,CulOq,
. . Idealized unit cell of the hvoolhetical
..
. . . which is
essumed to oe oared an a perovsrlte-lire wb-sw~cture.ib) Idealwed structure
of the 1-2-3 Oxioe compamd. YBarC~,O,., ablaned hom X-ray d'nraction
m a y e s . The ion szes are no1 drawn to scale. Aoapled from ref 8.
840
Journal of Chemical Education
02-ion that is removed reauires comnensatorv removal of
two units of positive charge; this can he accomplished if, for
example, two Cu3+ centers are converted to C U ~centers.
+
An
equation describing the equilibrium that couples oxygen
vacancies, V, with Cu oxidation states is,
Are there any clues to the superconductivity of the 1-2-3
oxide from its structure? Speculation has focused on the
hondine and coordination reouirements of Cu. The oxveen
vacauci& in the 1-2-3 structu>e create sheets and chaihs of
Cu atoms, linked through the remaining oxygen atoms. One
consequence of such a bonding arrangement is that the
physical properties of the material, including superconductivity, will show anisotropy, that is, they are dependent on
the lattice direction along which they are measured. Much
more information is needed, however, before the remarkable
properties of this particular 1-2-3 oxide can be explained.
Applications
Will the hieh-T- oxide sunerconductors transform the
technological Lndsiape of ou; world? Applications, a t least
in nrincinle.
. . increase with critical temnerature: liauid-nitrogen-hased superconductivity pcrmits geographically broader anulicntiuns
thahdoes technolorn
..
-- based on liquid helium;
and the discovery of a room-temperature superconductor
has the potential to bring superconductive devices into every
household. Whether these applications come to fruition will
depend on whether some formidable problems in the realm
of materials science can be overcome.
If we think hack to the notion that the high-T, materials
are really a form of dirt, the difficulty in fabricating them
into wires, ribbons, and films is readily appreciated. One of
the major technical challenges associated with the ceramics
is to find ways to mold them into useful shapes. If this can be
done, applications await in such diverse areas as the transmission of electrical power, transportation, recreation, magnet-intensive technoloeies,
- . and education.
The ability of a superconductor to support a resistanceless
direct current can be exploited in the lossless transmission of
electrical power. At present, a substantial fraction of electricity is lost as heat through the resistance associated with
traditional conductors. Whether conventional transmission
of electricitv will he affected by the ceramic is difficult to
assess. ~ h e i eis some loss of energy when alternating current, the form of current provided by utilities, is being transferred. A laree-scale shift to sunerconductine technoloevwill
-.
also hingeon whether wires can be prepared from the ceramics that retain their superconductivity at 77 K while supporting large current densities.
Other potential applications of the new superconductors
appear in the field of electronics. For example, miniaturization and increased s ~ e e dof computer chius are limited bv
the generation of heat and the charging time of capacitors
arising from the resistance of the interconnectinn metal
tilrns.'i'he use of the new ceramics may rrwlt in more denseIs packed rhips that could transmit information more rapid& by orders i f magnitude.
The use of superconductors for transpottation has already
been established using liquid helium as a coolant. A prototype levitated train system has been constructed in Japan by
equipping the train with superconducting magnets and placing magnets on the track. Since the Meissner effect permits
levitation of a magnet with a superconductor or of a superconductor with a magnet, a variety of engineering strategies
for transportation are possible: Magnetic vehicles riding
above a superconducting freeway are shown in Figure 11.
Once the vehicle is propelled, be i t by gasoline, a magnet, or
blast of air, little energy should be required for sustained
motion; on the other hand, means for starting, stopping, and
changing direction will need to be developed. It should be
-
-
Figure 12. Moving can be e breeze with levitation
Figure 11. Levitation
gives new meaning to "free"-ways.
noted, too, that although the idea of three-dimensional personal transportation is appealing, a new set of traffic-control
problems will await these magic-carpet rides.
Recreation is an area that is ideally suited for superconductiveflights of fantasy. I t is easy to imaginelevitating toys
and facilities. For example, a su~erconductingrink would
literally to walk on air. A
enable a magnetically shbd
superconducting yki slope could create new forms of slalom
and downhill competition. Furniture whose height can he
adjusted by superconducting c4ectromagnet.i would provide
new perspectives on interim and exterior decorating. And, as
illustrated in Figure 12, moving cumbersome personal helongings could become cheaper and easier.
Superconducting magnets are crucial components of seveta1 technologies. Magnetic resonance imaging (MRI) is
playing an increasingly prominent role in medicine. The
intense magnetic fields that underlie this technique provide
a unique diagnostic tool for physiological studies. Similarly,
the particle accelerators that serve the high-energy physics
community are dependent on high-field superconducting
magnets. The recent controversy surrounding the Superconducting Super Collider (SSC) also illustrates the political
ramifications of new technologies. At issue is whether the
multibillion-dollar project should be constructed now, using
existing liquid-helium-based superconductor technology, or
whether construction should be postponed to embrace the
developing liquid-nitrogen-hased technology. Proponents of
waiting argue that a substantial savings in cooling costs
would result, assuming that the oxides can be fabricated into
useable forms. Advocates of immediate construction believe
that the cooling costs are a relatively small portion of the
SSC costs and argue that developing the oxides to the point
where they can be used in the SSC may take too long and
offer little benefit.
While the aforementioned applications are a t varying
stages of development, the educational landscape is already
being reshaped by the new superconductors. The 1-2-3 oxide
is readily prepared, permitting demonstrations of levitation
throughout the country. As I hope this article has illustrated,
levitation can be used as a starting point to discuss physics,
chemistrv. materials science. and even nolitics. Most oeoole
are absokely entranced b; a levitate'd magnet. ~ n once
d
thev have said. "Gee whiz".. thev
" ask whv it works.. soeculate
.
on what i t can be used for, and say, "Gee whiz", again. What
more can you ask for in science education?
Acknowledgment
I am grateful to my research group and to J. E. Nordman,
D. C. Larhalestier, J. C. Wright, L. F. Dahl, and J. Stewart
for helpful comments. I thank Cheryl Agulnick for drawing
the levitation cartoons. The University of Wisconsin Graduate School is acknowledged for generous support of our studies of superconductors.
Llterature Cited
1. General references for this ertido include the following, Is) R e - l n n e s , A. C.: Rhoderick. E. H. lntrnduelion to Suparconducliuify; Pergamon: Oxford, 1876. lb) Grd, R.
2.
2.
4.
6.
6.
7.
8.
New Ymk. 1973. IcI Beehgasrd, K.:
Safeand Simpl~E1ectricalErprrimanfs;Douer:
Jerom~,D.Sei.Am.1982,247.52.Id) Little, W.A.Sci.Am. 1965,212,21. le)Kunzler,
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Volume 64
Number 10
October 1987
841
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