In this experiment you will

• synthesize a "high-temperature" 1-2-3-type superconductor,
• measure the critical temperature, T_c, of the superconductor, and
• determine magnetic properties of the superconductor

A Dutch physicist, Heike Kammerlingh Onnes, first detected the phenomenon of super-conductivity¹ in mercury metal in 1911. While measuring the resistance of mercury to electrical flow, Onnes made a profound discovery: Below 4.2 K, the resistivity of the metal dropped abruptly to zero. Since this time superconductors are known as materials that permit electrical current to flow with no loss of energy.¹ Mercury was not unique in displaying zero resistance to a flow of electrical current. Other metallic elements exhibited the same effect at temperatures of about 4 K. Thus, the remarkable electric property of zero resistance to a flow of electrical current at sufficiently low temperature was taken as a diagnostic for a new state of matter, the superconducting state. At ordinary temperature, metals have some resistance to the flow of electrons due to the vibrational movement of the atoms, which scatter the electrons. As the temperature is lowered, the atoms vibrate less causing less electron scattering, and the resistance declines smoothly. At a particular temperature, called the critical temperature, T_c, there is a sudden drop to zero resistivity which is characteristic of all superconducting materials.¹ Below T_c, a direct current can flow indefinitely in the material. To our present knowledge, once started, electrical current in a superconducting ring will continue forever, unless a force is applied to change the current or the temperature is changed. In 1933, a second important property of superconductors was discovered by Meissner and Ochsenfeld.¹ It was found that superconducting materials act as perfect diamagnetics in a magnetic field. The property that a superconducting material will not permit a magnetic field to penetrate its bulk is commonly referred to as the Meissner effect. The two properties of resistanceless current flow and perfect diamagnetism make superconductor technology important in a series of applications, ranging from supercomputers, superconducting magnets used in NMR spectroscopy, SQUIDS (superconducting quantum interference devices), Maglev (magnetic levitation) transportation, power storage and delivery, and communications, to name only a few.¹

Due to the tremendously important roles in these applications, extensive research has been devoted to the design of superconductors. Today, much interest is focused on a new class of materials, the so-called high temperature (> 30 K) cuprate superconductors due to the momentous discovery of superconductivity at 35 K in a ceramic material. In 1986, Georg Bednorz and K. Alex Müller² reported that the oxide of lanthanum, barium, and copper, La_2-xBa_xCuO₄, loses its resistance at 35 K. This discovery sparked a race among scientists worldwide to find compounds that are superconducting even at a higher temperature. In 1987, the year in which the Nobel prize in physics was awarded to Bednorz and Müller² reports, by Paul Chu and M. K. Wu³ indicated that the related material YBa₂Cu₃O_7-x, often referred to as the "1-2-3" superconductor, becomes superconducting at 93 K. The ease with which these compounds can be prepared, coupled with the vast commercial applications of high-temperature superconductors, has resulted in an exceptionally intense level of activity and interest in both the scientific and lay community.

The recent experimental results have necessitated a revision of the theories of superconductivity. At present, the extent to which the Bardeen, Cooper, and Schrieffer (BCS)⁴ theory, which had explained so successfully conventional low-temperature superconductors, can be applied to high-temperature superconductors remains unclear. Thus, a flood of theories has come forth to explain the phenomenon of high temperature superconductivity. In trying to understand what makes the 1-2-3 superconductor, researchers have devoted a lot of work to the study of the electronic, magnetic, thermal, and optical properties of these materials. W. A. Littel⁵ presents the structure and behavior of the charge carriers in these new materials. The chemistry of the high-temperature superconductors and the attempts of researchers in trying to tune the material chemically by substituting for Y, Ba, and Cu are reviewed by A. W. Sleight.⁶ In addition to the chemical and physical concepts presented in the articles, the interplay of theory and experiment can be examined, thereby exemplifying a chemist's contribution to such investigations.

In this experiment, you will synthesize a 1-2-3-type superconductor, measure its T_c, magnetic properties, and analyze the copper content to determine its composition. In addition, you will investigate the solid state structure of the high-temperature superconductor.

Safety: CAUTION! Many chemicals are toxic. Avoid creating or breathing dust when grinding. Avoid eye and skin contact. Wash hands thoroughly after handling.

Ia. Preparation of the 1-2-3 High Temperature Superconductor.

1. Place the following reagents in a 100 mL beaker:

1.10 g BaCO₃ (s), 0.316 g Y₂O₃ (s), and 0.520 g Cu (s)

2. In a fume hood, add 10 mL of concentrated nitric acid to the above mixture. Cover with a watch glass and boil the mixture in the hood on a hot plate set at the highest setting (about 500^oC). After 30 minutes of boiling, remove the watch glass and let the mixture boil to a nearly dry paste. Transfer the warm paste with a nickel spatula to a 3 cm square of gold or platinum foil on a porcelain dish.



3. In a well-ventilated area (preferably in a fume hood) drive off the NO₂ from the nitrate mixture by heating the paste in an oven at 960^oC for one hour.



4. Thoroughly grind the resulting black mass in a mortar to a homogeneous powder (about 20 min).



5. Repeat the synthesis again and after grinding the black mass combine with the black powder from step 4 and grind again. Divide the powder into two equal sized (approximately) samples.



6. Place one of the powder samples in a KBr–type pellet press and apply enough pressure (about 4 tons) to make a pellet. Place the pellet and the other powder sample into porcelain dishes.



7. Heat the pellet and the powder in a tube furnace under flowing (about 50 mL/min) oxygen. The program for heating the pellet is as follows: heat rapidly to 960^oC, soak in the O₂ atmosphere at 960^oC for ten hours, cool to 400^oC over ten hours (ramp cooling), and then turn off the oven, allowing the sample to cool to room temperature. The flow of O₂ can be stopped during the latter part of the ramp cooling.



8. Store each in a sealed vial for protection from moisture.

Ib. Alternative Procedure for the Synthesis of the High Temperature Superconductor.

1. Weigh 0.60 g of Y₂O₃ and transfer it to a small beaker (make sure the beaker is dry). Weigh out stoichiometrically equivalent amounts of BaO₂ and CuO, transferring each to the beaker.



2. These three materials must now be thoroughly mixed to obtain good results. In a hood, place the powdered materials in a mortar. Then mix and grind the materials with a pestle for about 10 minutes; use a nickel spatula to scrape the material off the sides of the mortar when it cakes there. Your final powder should be of a uniform color with no lumps and no black or white spots or patches visible. What color is your powder, and why must you mix the starting powders?



3. You will now use half of the powder to make a pellet as described above.



4. Place the pellets in an alundum (a form of Al₂O₃) boat and heat in a furnace and proceed as described above. The pressed pellets are quite fragile and may shear crosswise or crumble when ejected from the die. If it shears or crumbles, crush it in your mortar and repress.

The finished pellets should be dark gray to black. A dark green material is a second phase of composition Y₂BaCuO₅, which does not superconduct.

II. Determination of the Magnetic Properties of the 1-2-3 Superconductor.

CAUTION! Liquid nitrogen is extremely cold. Do not spill it on your skin or clothing. Severe frostbite or freezing of the flesh can occur. Remove clothing that becomes saturated with liquid nitrogen, because the liquid may be held within the spaces in the fabric, freezing the skin underneath. A drop or two spilled on the skin is not dangerous because the outer layer of the drop will vaporize, forming an insulating layer of gas.

Demonstration of the material's superconductivity and the Meissner Effect.

1. How high does the pellet lift a small, strong magnet when it is cooled in a bath of liquid nitrogen? Carefully place a magnet on top of the superconducting material in a low-cut polystyrene cup. Slowly pour liquid nitrogen into the cup. The superconductor begins to cool. At a particular temperature, the magnet rises and remains suspended over the superconducting material. Compare the distance of this rise to the maximum possible distance the magnet can be lifted. This maximum distance is found by gradually raising the magnet toward a horizontal iron plate until the magnet jumps to the plate.



2. Can the pellet be magnetized? Hang the pellet on a thread under the surface of some undisturbed liquid nitrogen. Note that it swings either way when approached by both poles of a magnet. Remove the pellet from the liquid nitrogen. Now, try to magnetize the pellet by sandwiching it between two attracting magnets and lowering the sandwich into the liquid nitrogen. Pull off the magnets while the pellet is still in the liquid nitrogen so that it stays in the superconducting state. If it has been magnetized it will rotate in a favored direction when approached sideways by a pellet magnet or approached directly by a horseshoe magnet.



3. How will the superconducting powder behave in a magnetic field? Sprinkle some of the superconducting powder into a small Dewar flask containing liquid nitrogen and a permanent magnet. The superconducting powder will avoid high field regions leaving a bare spot close to the magnet (see R. Baker, J. C. Thompson, J. Chem. Educ. 1987, 64, 853).

III. Determination of T_c

What is the critical temperature, T_c? T_c is found by placing a thermocouple sensor under the pellet and lowering it into a small dewar flask. The small, strong magnet is suspended on a thread just over the pellet. The temperature at which the magnet just moves from or toward the pellet is the T_c. Adding or removing liquid nitrogen varies the temperature.

IV. Solid State Structure of the 1-2-3 Superconductor.

Using all information technologies available to you, search for the solid state structure of the superconductor, describe and discuss it, and, if time permits, use the software packages available to you to model the structure of the superconducting material. Is there a correlation between structure and superconductivity?

(1) For reviews see:(a) M. H. Whangbo; C. C. Torardi; Acc. Chem. Res. 1991, 24, 127. (b) C. N. R. Rao; B. Raveau; ibid. 1989, 22, 106. (c) R. Simon; A. Smith; Superconductors, Conquering Technology's New Frontier; Plenum Press: New York, 1988. (d) R. J. Cava; Science 1990, 247, 656.

(2) Bednorz, J. G.; Müller, K. A. Z. Phys. B. 1986, 64, 189.

(3) Wu, M. K.; Ashburn, J. R.; Torng, C. J.; Hor, P. H.; Meng, R. L.; Gao, L.; Huang, Z. J.; Wang,

(4) Y. Q.; Chu, C. W. Phys. Rev. Lett. 1987, 58, 908.

(5) Bardeen, J.; Cooper, L.N.; Schrieffer, J. R. Phys. Rev. 1957, 108, 1175. Littel, W. A. Science 1988, 242, 1390.

(6) Sleight, A. W. Science 1988, 242, 1519.