Preparation, lodom tric Analysis, and Classroom Demonstration of uperconductivity in YBa2C~308-x
2
Daniel C. Harris, Marian E. Hills, and Tenell A. Hewston Chemistry Division, Research Department, Naval Weapons Center. China Lake, CA 93555 After the discovery in 1911that mercury loses itselectrical resistivity when cooled below 4 K, superconductivity was not seen above 23 K in other materials until 1986. In that year the floodgates of research were opened by a report that an oxide of barium, lanthanum, and copper was superconducting up to 35 K ( I , 2). The transition temperature of this compound, Ba,La2-,CuOa (z a 0.15) (3-9, was exceeded in 1987 by that of another oxide, YBa2Cu30s-, ( x I),which is superconducting up to 100 K (6, 7). This article describes a student preparation of YBanCusOs-,, a classroom demonstration of its superconductivity, and an analytical chemistry experiment dealing with the oxidation state of copper in the material. Superconductors As the name implies, one outstanding property of a superconductor is zero electrical resistivity when it is cooled to a sufficiently low temperature (Fig. 1).Once started, electrical current in a superconducting ring will continue forever unless a force is applied to change the current. How long is forever? An experiment (8) with the superconductor Nh3Zr showed that the current decayed less than one part per billion per hour, implying a resistivity of less than ohmm. A second outstanding feature of superconduetors is that the magnetic field, B, inside a bulk specimen is zero. This is called the Meissner effect. When a field is applied to a specimen, currents flow in the outer skin of the material such thatthe applied field is exactly opposed by the induced field, and the net field inside is zero.' A magnetic field that decays exponentially as i t enters the bulk is in the skin of the supercouductor. The depth at which the internal field decreases to l/e times the external value is called the penetration depth and is of the order of 10-100 nm at temperatures well below the superconducting transition temperature. A paramagnetic substance is attracted into a magnetic field and a diamagnetic substance is repelled by a magnetic field. Figure 2 shows the magnetic susceptibility2 of a sample of YBa~Cu30~.65 as a function of temperature. The temperature near 100 K at which the susceptibility begins to curve downward is considered to be the onset superconducting transition temperature, T,. Above T, this sample is paramagnetic, but it becomes strongly diamagnetic as the temperature is lowered. If the applied magnetic field exceeds a critical value, superconductivity is lost. In a TypeImaterial (Fig. 3), the field
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measured n SI units of tesla (T).The field inside a solid is equal to the app1:ed feld B. plus the inducedfield,mM: B = B.- 7 ."OM, - where no is the permeabili~vof free ~ ~ aand c e~ itheimaanetization induced in the solid. The value of B is zero inside a suoe&onductor. so "AM = -6, Magner~zaton is eqdai to the magnebc doole moment & unlt vo .me an0 nas the unlrs amperes per meter For a current t tlowmg around a loop of area a, the magnetic dipole moment has the magnitude 1 . a, with units of amperes. meters2. Magnetic susceptibility is measured by observing the force exerted on a solid by a magnetic field gradient. its measurement and units are described in references Sand 10.
200
Figure 1.Direct current electrical resistance of a pellet of YBaPu.0.~.
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2 -0.025 ' The magnetic field B is also called the magnetic induction and is
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TEMPERATURE. K ss Cer Figure 2. Magnetic susce~tibilityof YBa.Cu.0. .. mole of Cu) corrected far the diamagnetic contributions of Me elements. The molar magnetic s u s ceptibllity in the SI units of this figureis equal to 4r X lo@ times the molar susceptibility in cgs units. ~
Volume 64 Number 10 October 1987
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I Bc
APPLIED FIELD
SUPER-
( C O N O U C T I N G FVORTEX ~ STATE
TNoRMAL-
TYPE II
Figure 3. Behavior of superconductorsin a magnetic field.The direction of the induced field opposes the applied field.
induced in the superconductor is equal in magnitude (hut o ~ ~ o s iin t edirection) to the aDDlied field.. UD .. . to the critical firid. B,. At this point, the material luars its superconducri\.its.The valucoftherritical field isa functionoftem~erature and decreases as the temperature increases, hecoking zero a t T,.YBa2Cu30s-, is a Type I1 material in which the applied magnetic field is exactly cancelled by the induced field up to a lower critical field, BC1(Fig. 3). Between BC1and Bc2 the applied field is not completely balanced by the induced field, and the material is said to be in a vortex state. The sample loses superconductivity above Bc2.A cylinder of material in the vortex state contains filaments in the superconducting state and filaments in the normal state. As lone as superrunducting filaments are present, there is a superronductinr uath from one end to the other. and the resistivltv is zero. ~ i i h applied e field is increased, the fraction of n o r i a l material increases and the fraction of su~erconductinr!material decreases. Structure of YBaZCunOa-, Figure 4 shows the unit cell of YBazCun07,which is derived from a defect perovskite structure. Defect structure means that vacancies (unoccupied atomic sites) are present. There are ~ositionsfor nine atoms of 0 in the unit cell. hut only seven are occupied.3 Why are there not nine 0 atoms in the unit cell? The normal oxidation states of Y and Ba are +3 and +2, respectively. If all Cu were +2, the formula of the compound would he Y B a z C ~ 3 0with ~ ~ 6.5 02-needed to balance the cation charges. If all Cu were Cu3+,which is an unusual oxidation When counting atoms In the unit cell in Figure 4, remember that most atoms in the diagram are shared by more than one unit cell. Atoms on a face of the cell are shared by two cells, so the number of these atoms must be divided by two. Atoms on edges are shared by four unit cells, and those at vertices are shared by eight unit cells. 848
Journal of Chemical Education
0 COPPER
0 OXYGEN
(2OXYGEN VACANCY
state for Cu, the formulawould he YBa2Cu30s.The observed composition is variable, hut hovers near YBa2Cu307,which implies that one third of the Cu is Cu3+and the remainder is C o z - .This assignment of oxidation states is purelv formal. populaThe material is metallic ahore 7;. so it must have a . . tion of mobile electrons. The structure in Figure 4 is an idealization emphasizing the location of oxygen vacancies and the approximately
LIQUID NITROGEN LEVITATING MAGNET
SOLID SUPERCONDUCTOR STYROFOAM
\
OVERHEAD
PROJECTOR
Figure 5. Demonstrating magnetic levitation with an overhead projector. square planar coordination of Cu by 0. All 0 positions in the horizontal plane containing Y are vacant. The other vacancies are located in the top and bottom Cu-0 planes. Each Cu atom is surrounded by four 0 atoms with a Cu-0 distance of approximately 194 pm. The coordination of the eight Cu atoms in the planes ahove and below Y could he considered as square pyramidal if the next-nearest 0 atoms at 238 pm are included. The eight 0 atoms surrounding the Y atom are not coplanar with the Cu atoms as drawn hut are puckered out of the Cu planes toward the Y atom. An idealized description of the Cu-0 network is that there are horizontal sheets of vertex-sharing Cu04 squares. These alternate with perpendicular CuO4 squares sharing two opposites vertices to form infinite strings.
Figure 6. Projected image produced by apparatus in Figure 5.
net pruduce a magnetic firld that repels the magnet. Tc, show to an entire clash, the lwitarion experiment is set upon thelitayeof an overhead projector, asshown in Figures 5 and 6. PIXI: the magnet on top of the solid superconductor in a low-cut expanded polystyrene cup. TWO-mirrorssupported at 45' angles direct the projector light across the superconductor. T o bring the image into focus, move the cup along the horizontal line between the two mirrors. Liquid nitroaen is ooured into the shallow cun. . . and the sunerconducrmr hegins I O cool. When i r cools sufficiently, themagnet ~lmrnaricallsrises and remains susoended to the delieht of all observers. lodometric Analysis of Copper Oxldation States In YBa2Cu308-,
In Experiment A Y B ~ ~ C U is~ dissolved O ~ - ~ in dilute HCI, in which Cu3+is rapidly reduced to Cu2+(11)
Preparation of the Superconductor
Place in a mortar 0.750 g of Yz03, 2.622 g of BaC03, and 1.581 g of CuO (atomic ratio Y:Ba:Cu = 1 2 3 ) . (Ordinary reagent-grade chemicals are suitable and the scale can he varied over a wide range.) Grind the mixture well with a pestle for 20 min, and transfer the powder to a porcelain crucihle or boat. Heat in the air in a furnace at 920-930 "C for 12 h or longer. Turn off the furnace, and allow the sample to cool slowly in the furnace. This slow coolingallows atmosphericoxygen to he taken up by the sample and produces the desired oxygen content. The crucihle may he removed when the temperature is below 100 'C. The hlack solid mass can he gently dislodged from the crucihle for the demonstration described below. Alternatively, the solid mass may he ground for 20 min and reheated to produce higher quality material. If the powder is green instead of hlack, raise the temperature of the furauce by 20 "C and reheat.
The total Cu content can then he measured by treatment with iodide Cu2+(aq)+ 21-(aq)
-
CuI(s) + '&(aq)
(2)
and titration of the liberated iodine with standard thiosulfate Each mole of Cu in YBa2Cu308-, is equivalent to one mole of S ~ 0 3 in ~ -Experiment A. In Experiment B YBa2Cu308-, is dissolved in HCI solution containing I-. In this case, the C I P selectively oxidizes two moles of I' (and precipitates with a third mole)
Classroom Levitation Demonstration
The solid hlack mass of YBazCuaOs-, taken directly from the crucihle is used for this demonstration. Alternatively, the hlack powder obtained by grinding the mass can he gently pressed into a crucihle (or pressed in a pellet press) and reheated. To demonstrate superconductivity to a small group, cool the solid product in liquid nitrogen and place a small magnet such as a 1.5- X 8-mm Teflon-coated stirring bar over the superconductor. If positioned carefully with a plastic tweezer, the magnet will levitate in the air ahove the superconductor. Alternatively, place the magnet on the solid before cooling, and the magnet will rise when the solid is cooled. Currents induced in the superconductor by the mag-
The moles of S ~ 0 3 required ~to titrate the liberated I2 are equivalent to Cu2+ 2Cu3+in Experiment B. Experiment A gives the total Cu content of Y B ~ ~ C U ~ O ~ - ~ , and the difference hetween the results of Experiments A and B gives the Cu3+ content. With these two pieces of information, it is possible to calculate the value of x in the formula Y B ~ & U ~ ODifferent ~ - ~ preparations give values of x that hover about the value x = 1.It is instructive to carry out each titration three times. Using the standard deviations as a measure of the uncertainty of each result, carry out a propagation of uncertainty analysis to find the uncertainty in the value of x .
+
Volume 64
Number 10
October 1987
849
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T h e results of iodomktric & l y s i s are in good agreement with those calculated from the mass lost b y YBazCusOs-, when i t is h e a t e d to 1000 OC u n d e r Hz (11)
0.03MNa2Sp03 Solution. ~issol've3.7 g of NazS203.5H20 plus 0.05 g of Na2C08 in 500 mL of freshly boiled distilled water. Add 3 drops of chloroform and store in a capped amber bottle. The NaZCO3and chloroform act as preservatives. The solution is stable for a period of days or weeks but should be restandardized after several weeks. Starch Indicator. A slurry containing 1gof soluble starch plus 1 mg of HgIp (a preservative) in 10 mL of distilled water is poured into 90 mL of boiling distilled water to give a nearlv clear solution that should he stablefor several weeks in a closed bottle. Standard Cu. Weigh accurately 0.5-0.6 g of reagent Cu wire into a 100-mL volumetric flask. In a fume hood, add 6 mL of distilled water and 3 mL of 70%nitric acid and hoil gently on a hot plate until the solid has dissolved. Add 10 mL of distilled water and boil gently. Add 1.0 g of urea or 0.5 g of sulfamic acid and hoil for 1 min to destroy HNOz and oxides of nitrogen that interfere with the iodometric titration. Cool to room temperature and dilute to 100 mL with 1.0 M HC1. Standardization of Nn2S203With Cu. The ease of I- oxidation by O2 in acid requires that the titration he done as rapidly as possible under a brisk flow of N2or AI. The titration vessel is a 180mL tall-form beaker (or a 150-mL standard beaker) with a loosely fitting two-hole cork a t the top. One hole serves as the inert gas inlet and the other is for the buret. Pipet 10.00 mL of standard Cu into the beaker and flush with inert gas. Remove the cork just long enough to pour in 10 mL of distilled water containing 1.0-1.5 g of KI (freshlv dissolved) and heein mametie stirrine. Titrate with ~ a. A~0. .solution , in a 50-mi huret.addine 2 droos of starch iust bri.,rr the last trace of l2 cdor disappears. If starch is added rolr soon,there ran be irrwersible attarhment uf 12mthestarchand the
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Journal of Chemical Education
endpcnt is ha'ider to distinguisli. The Na2S203should he standardized three or four times and the average molarity computed.
Experiment A Accurately weigh 150-200 mg of YBazCuaOs-, and dissolve it in 10 mL of 1.0 M HCI in a titration beaker in a fume hood. Boil gently far 10 min to ensure destruction of Cu3'. Cool to room temperature and place the cork with gas inlet and buret on the beaker and begin gas flow. Quickly add 10 mL of water containing 1.0-1.5 g KI and titrate as above with magnetic stirring. Emeriment B Arcurattdy weigh 1.3-20U rng of YRnrCu O,., into the trtratbn vesscl, and hegin mert ya.i flou. Add 10 mL uf 1.0 11 IICI O 7 M KI and rrir mncnrtically fur 1 min. Add 10 ml. of distilled water and complete the titretion as above Acknowledgment W e a r e grateful t o K. T. Higa a n d W. A. Weimer for helping devise t h e levitation demonstration. D a t a for Figures 1a n d 2 were kindly provided b y B. Chamberland, University of Connecticut. T h i s work was supported b y t h e Office of Naval Research. Literature Cited
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8. Pile, J.: Mil1s.R. G.Phya. R e ~ L e f f 1363.10.93. . 9 . Lindoy, L. F.; KataviC: Buseh, D. H. J . Chsm. Educ. 1972,19,117. 10. Pars, G.;Suteliffe, H. J.Chem.Educ. l971,48,180. 11. Harris, D. C.; Hewt0n.T. A. il Solid State Chem. 1987,in prwd
