## PREPARATION, STRUCTURE AND PROPERTIES OF A HIGH-TEMPERATURE SUPERCONDUCTOR

### Learning Experiences

• - Balancing a chemical reaction

• - Using an Analytical balance

• - Solid State Synthesis

• - Nonstoichiometry

• - Crystal structures

• - Levitation

• - Handling liquid nitrogen

### Pre-Lab Questions

1. Calculate the weights of BaO2 and CuO required to react stoichiometrically (1Y:2Ba:3Cu) with 0.60g of Y2O3 to produce YBa2Cu3O7.

2. What is a superconductor? Discuss two major physical properties usually associated with superconducting solids.

## Preparation, Structure and Properties of a High-Temperature Superconductor

### Laboratory Exercise

INTRODUCTION:

Onnes, a Dutch physicist, discovered in 1911 that mercury loses all resistance to electrical flow when cooled to about 4 K; thus, a current once started will flow continuously. Such a phenomenon is known as superconductivity. At ordinary temperatures, metals have some resistance to the flow of electrons, due to the vibration of the atoms which scatter the electrons. As the temperature is lowered, the atoms vibrate less and the resistance declines smoothly, until the material’s critical temperature, Tc, is reached. At this point, the resistance drops abruptly to zero (figure 1). If an electrical current is started in a superconducting ring, it will continue forever.

Figure 1. Electrical resistance of a superconductor.

Superconductors are also perfectly diamagnetic (i.e. they repel a magnetic field); this property was discovered in 1933 and named the Meissner effect. A magnetic field induces a current in a conductor; conversely, a current induces a magnetic field. When a magnet approaches a superconductor, it induces a current in the superconductor. Because there is no resistance to the current, it continues to flow, thus inducing its own magnetic field which then repels the magnet’s field. If the magnet is sufficiently small and strong, the repulsion will be enough to counterbalance the pull of gravity and the magnet will levitate above the surface of the superconductor.

Even though until recently the highest known critical temperature was only 23 K, observed in the intermetallic compound Nb3Ge, superconductors found a number of applications. The most common of these is for superconducting magnets for nuclear magnetic resonance machines used for medical imaging and in the research laboratory; you may find one of these in most chemistry departments. These instruments require liquid helium as the refrigerant, which is scarce and expensive. In a major breakthrough in 1986 (Nobel prize in 1987), Bednorz and Müller at IBM Zurich discovered superconductivity in copper containing oxides at over 30 K, instigating a massive worldwide research effort. This culminated in the discovery of a metallic oxide of yttrium, barium, and copper that was superconducting at about 92 K. This meant that liquid nitrogen (bp 77 K), which is cheaper and easily handled, for example in a Dewar flask (the laboratory version of a Thermos bottle), could be used as a coolant rather than helium. Related oxides now superconduct up to 125 K.

The compound, which has the formula YBa2Cu3O7-d, is readily prepared by heating an intimate mixture of yttrium oxide, barium peroxide, and cupric oxide at approximately 930oC for 10-12 hours. In this stage the crystalline structure is formed by the interdiffusion of ions, but it has a deficit of oxygen (dÅ 0.5). By cooling the material to 500°C and annealing at this temperature for 10-12 hours, allows it to react further with oxygen from the air, reducing d to less than 0.1. The overall reaction (with only the metal ions balanced) is given by:

Y2O3 + 2 BaO2 + 3 CuO ------> YBa2Cu3O7-d

This compound is often called the 1 2 3 material from the molar ratios of Y:Ba:Cu. The heating-cooling synthesis sequence is shown graphically in figure 2.

Figure 2. Heating-cooling sequence for synthesis of the 1-2-3 superconductor.

The unit cell structure of YBa2Cu3O7 is shown in Figure 3; although it appears complex, it’s basic building block is the simple perovskite structure, as you will see in the next few pages The main feature of the structure that is thought to be important in the superconductivity is the existence of the Cu-O 2-dimensional sheets extending infinitely through the material. Draw these sheets on this figure:

Figure 3. Unit cell of YBa2Cu3O7.

PROCEDURE:

1. Preparation of the superconductor

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

Weigh out onto a piece of weighing paper 0.60 g of yttrium hydroxide, Y2O3, and transfer to a small beaker (make sure the beaker is dry). Weigh out stoichiometrically equivalent amounts (you calculated these as a pre-laboratory exercise) of barium peroxide, BaO2, and cupric oxide, CuO, transferring each in turn to the beaker.

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 material with a pestle for about 10 minutes; use a spatula to scrape the material off the sides of the mortar when it cakes there. Your final powder should be 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?

You will now make 2 pellets with this powder. Scrape the powdered mixture onto a creased piece of weighing paper. Divide the mixture in half using a second piece of weighing paper, one for each pellet. Your instructor will show you how to press each mixture into a pellet using the pellet press. Transfer the finished pellets into a small beaker labeled with your name and section number. [The pressed pellet is 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]. Place the labeled beaker containing your pellets on the front desk. The pellets will be placed in an alundum (a form of Al2O3) boat and heated in a furnace. The furnace will be heated to 930oC over a period of about 8-12 hours, held at 930oC for 12-16 hours, allowed to cool to 500oC and held there for 12-16 hours. Finally the furnace will be turned off and allowed to cool to room temperature. (See Figure 2). The cooled pellets, in their beakers, will be stored in a desiccator until the next laboratory period.

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

2. Crystal Packing - Structure of a Superconductor

The properties of a superconductor will obviously depend on the structure and packing of the atoms in its crystalline form. X-ray and neutron diffraction studies were most important in the elucidation of the 1-2-3 oxide structure, which belongs to a crystal family known as perovskites. Perovskites generally have a ratio of two metal atoms for three oxygen atoms, ABO3. Your TA will show you an x-ray diffraction pattern of one of the pellets made in Chemistry 111.

Model Building A. Perovskite Structure

The perovskite structure is named after the mineral CaTiO3. This structure is made up by corner sharing of TiO6 octahedra with Ca ions in the large cavities at the corners of the unit cell. Study the structure in Figure 4 and use the block models available to answer the following questions.

Figure 4. The perovskite structure of CaTiO3.

Questions - Perovskite Structure CaTiO3

1. Determine the oxidation state of the Ti if Ca and O have their normal oxidation states.

2. What is the basic unit cell structure for this type of compound?

3. What is the coordination number of:

Calcium ______________________

Titanium ______________________

Oxygen ______________________

4. Show that the unit cell of this compound corresponds to the formula CaTiO3. Remember that an ion shown as part of a unit cell does not contribute a whole ion to the cell unless it is wholly enclosed within the cell as the Ti ion is here. [Remember: How many unit cells is each atom in?].

5. How does the structure of WO3 differ from that of CaTiO3?

Model Building B. Structure of the Superconductor - YBa2Cu3O7

The 1-2-3 superconductor has a structure similar to perovskite. The resulting unit cell consists of three stacked cubic unit cells; it is considered to be orthorhombic rather than cubic, having an almost square base but rectangular sides (a = 3.817Å, b = 3.882Å, c = 11.671Å). Study the structures in Figure 5 and use your block models to answer the following questions.

Figure 5. Idealized structure of YBa2Cu3O7 obtained from X-ray, showing evolution from the perovskite structure shown in figure 4. (a) Stacking of 3 perovskite units; (b) shift of origin; and (c) removal of oxygen to give correct chemical composition.

Questions - Structure of Superconductor - YBa2Cu3O7

1. The copper in YBa2Cu3O7 may be considered to be a mixture of +2 and +3 oxidation states. If Y has a +3 oxidation state and Ba and O have their normal oxidation states, what fraction of the copper is in each of the two oxidation states?

2. In your laboratory notebook draw the idealized tetragonal unit cell if the yttrium and barium cubic structures were based exactly on the perovskite sub-structure.

3. Compare the coordination numbers of the barium and yttrium in both the ideal perovskite sub-structure and the structure actually obtained from X-ray diffraction analysis.

4. Comparing the two structures speculate as to why the 1-2-3 oxide structure may exhibit superconductivity.

5. Show that the orthorhombic unit cell obtained by X-ray diffraction analysis corresponds to the formula YBa2Cu3O7. Remember that an atom shown as part of a unit cell does not contribute a whole atom to the cell unless it is wholly enclosed within the cell.

3. Properties of a superconductor

CAUTION! Liquid nitrogen is extremely cold: 77 K = -196oC =-321oF. 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.

a. Meissner Effect

Using the plastic forceps, remove the superconductor pellet from the beaker and place it in the cut-off Styrofoam cup. [If there is loose material on the pellet scrape it off gently with a spatula. If the pellet has sheared, use the thickest piece and place it flat side up.] Then place a small magnet on top of the pellet with plastic forceps.

Have your teaching assistant pour some liquid nitrogen from the Dewar flask into a second Styrofoam cup. Carefully pour some of the liquid nitrogen into the cut-off cup, covering the pellet. Some of it will boil away as the pellet, magnet, and cup cool; add more as necessary until the system is stable. When the pellet cools below the critical temperature, the magnet should “levitate” above the superconductor. Touch it gently with your forceps and it should spin. Remember to record all your observations, as you make them, in your laboratory notebook.

Allow the liquid nitrogen to evaporate and the pellet and magnet to warm to room temperature before handling them with your fingers.

b. Loss of Electrical Resistance by 4-probe technique (demonstration by TA)

The loss of resistance below the critical temperature can be measured by measuring the voltage drop across the pellet in a circuit. The 4-probe apparatus used is shown schematically in Figure 6. A battery generates a current , I, which passes through the pellet. The voltage drop, V=IR, along the pellet due to the pellet’s resistance, R, at room temperature is measured with the volt meter. When the material becomes superconducting, R=0, so V= 0 and no voltage drop should be measured. (The resistor in the circuit prevents the battery from being shorted when the superconductor loses its resistance. We use a 4-probe technique to eliminate contact resistances.)

Figure 6. The 4-probe conductance test circuit.

CAUTION! The resistor in the test circuit becomes quite hot. Do not touch the resistor or spill liquid nitrogen on it. Thermal shock could cause it to break, and you could burn your fingers.

In Chemistry 111 your TA will perform the following experiment in front of the class:

The pellet is carefully inserted into the 4-pronged holder; the outer leads are connected to the battery and the inner leads to the voltmeter. [Reverse the battery leads if the voltage is negative.]

After placing the holder in the Styrofoam cup, liquid nitrogen is slowly and carefully poured into the cup as in the Meissner experiment. When the pellet cools below Tc, it should become superconducting; note what happens to the voltmeter reading at this point?

The TA will then disassemble the experiment as follows, (1) disconnect the battery to prevent overheating the resistor, (2) let the nitrogen evaporate, (3) let the sample and holder return to room temperature, and (4) remove the sample from the holder.

OBSERVATIONS AND QUESTIONS:

Record you observations (as you make them) and answers in your lab notebook.

1. Discuss your observations. Does your product exhibit the characteristics of superconductivity? Does it appear to be fully or only partially superconducting? If not, what do you hypothesize went wrong? How would you identify the presence of second phases?

2. Why did you compress your reactants into pellets for your solid state reaction? Hint: what is the difference between reactions in solution and in the solid state?

3. The dream of the materials chemist is to discover a material that would be superconducting above room temperature, so that no refrigerant would be required. Suggest how the properties of such superconductors could be valuable in each of the following fields: computers/electronics, power generation/transmission, and rail transportation.

4. You have decided that the best way to secure your finacial future is to discover a room temperature superconductor. Using what you have learned from this experiment come up with some reasonable materials for synthesis. Use the basic perovskite strucure and consider the following in your choice of metal ions:

• a) oxidation state
• b) ratio of the cation/anion radius
• c) ionization potential of the cation
• d) type of hole it is fitting into
Model the system using CAChe software to see how your choice of metal ions has effected the crystal structure. For this structure the space group is Pmmm, a=3.820, b=3.886, c=11.683. The fractional coordinates are:
• atom x y z
• Y 0.5 0.5 0.5
• Ba 0.5 0.5 0.18393
• Cu(1) 0 0 0
• Cu(2) 0 0 0
• O(1) 0 0.5 0
• O(2) 0.5 0 0.37819
• O(3) 0 0.5 0.37693
• O(4) 0 0 0.15840