Inorganic Chemistry Lab
Experiment 1 The Synthesis and Characterization of a High Temperature
Superconductor YBa2Cu3Ox
(1) Objective
In this experiment, you will synthesize the high temperature superconductor YBa2Cu3Ox,
and demonstrate its superconductivity by showing the Meissner effect.
(2) Background
The discovery of high temperature copper oxide superconductors (specifically the
compound YBa2Cu3O7, often referred to as 1-2-3 superconductors) created an extensive burst
in the study of materials science and superconductivity theory. The resulting interest in
chemical aspects of superconductivity and the ready availability of a refrigerant, liquid N2
(b.p. = 77 K), necessary to demonstrate superconducting phenomena created a secondary
increase in the incorporation of these topics in chemical education.
However, preparation of the homogeneous, well-ordered superconductor material
necessary for technological applications or for the experimental studies of properties has
proven to be a great synthetic challenge. YBa2Cu3Ox tends to phase separate after formation,
producing a mixture of superconducting, metallic, and insulating phases. In addition, it is
difficult to prevent the formation of unwanted impurity phases, including nonsuperconducting copper oxides, hydroxides, and carbonates due to reaction with air. Much of
the work in the field has been focusing on these synthetic difficulties.
Synthetic routes used have included methods typically employed for ceramic processing.
Homogeneous starting materials of extremely small particle size are important for the
preparation of well-ordered single-phase materials. The traditional method of ceramic
synthesis involves the grinding of metal powder oxides in the proper stoichiometry, followed
by heating in a furnace. Hand-grinding stoichiometric mixtures of Y2O3, BaCO3, and CuO
results in powdered material of average particle size ~1 µm. However, these "shake and
bake" methods have several disadvantages. The repeated mixing and grinding of the oxide
powders needs to be conducted in a fume hood to minimize exposure to dust; the mixing
itself might be incomplete due to the relatively large grain sizes of commercially available
oxides, resulting in failure to achieve the intimate mixing necessary to create a homogeneous
composition. More elegant co-precipitations of mixtures of complex metal salts, such as the
carbonates, nitrates, or oxalates, also have been used in attempts to obtain finer (≤1 µm)
particle dispersions and better homogeneity. The homogeneous co-precipitation synthetic
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Inorganic Chemistry Lab
method produces a reproducibly consistent ceramic precursor powder which can be
processed into a reliable superconductor.
(3) Equipment and Materials
1. Equipment
High Temperature Tube Furnace, alumina crucible, heat-proof gloves, 250 mL
Erlenmeyer flask, condenser, 250 mL beaker, 100 mL beaker, Buchner funnel, Buchner
flask, pH paper, crystallizing dish, thermometer, feeder funnel
2. Materials
Urea, Oxalic acid, Y(NO3)3‧ 6H2O, Cu(NO3)2‧ 3H2O, Ba(NO3)2, 95% C2H5OH, Y2O3,
BaCO3, CuO
(4) Synthesis of YBa2Cu3Ox
4.1 Co-precipitation approach
YBa2Cu3 oxide pre-ceramic powder is prepared by the method of homogeneous coprecipitation. An aqueous urea/oxalic acid solution of the metal salts in the proper 1-2-3
stoichiometry is prepared and heated. At a temperature between 80 and 100˚C hydrolysis of
urea takes place with the simultaneous evolution of CO2 and NH3 according to the following
equation:
CO(NH2)2 + H2O  2 NH3 + CO2
As the urea hydrolyzes, the pH of the solution gradually rises. Increasing the pH causes
the metal ions to precipitate out as their hydroxide, oxalate or carbonate salts. The final
stoichiometry of the powder is determined by the final pH of the solution which in turn is
determined by the initial molarity of the urea solution. An initially high urea concentration
favors the proper 1-2-3 stoichiometry in the pre-ceramic powder. The precipitate is separated,
washed and dried. It is then heated at 900˚C for at least 16 hours in air to burn out all the
residual carbon. The powder obtained at this stage is pressed into a pellet and sintered at
900˚C for 4 hours, followed by annealing in oxygen at 500˚C for another 16 hours. This
results in a material with a composition of YBa2Cu3Ox (6.5 < x < 7.0).
1. Weigh 40 g of urea to a 100 mL beaker. Weigh 3.15 g of oxalic acid on the weighing
paper. Add the above urea and oxalic acid to a 250 ml Erlenmeyer flask via a feeder
funnel. Add 18 mL of deionized (DI) water. It will prepare a solution that is 12 to 14 M
in urea and 0.5 M in oxalic acid (C2O4H2.2H2O).
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Inorganic Chemistry Lab
2.
3.
4.
5.
6.
7.
NOTE: 18 mL DI water will be added to the Erlenmeyer flask in batches. Firstly, add
about 15 mL DI water to the mixture of urea and oxalic acid. The remaining 3 mL of DI
water will be added to the Erlenmeyer flask after the addition of the nitrates in step 3 to
rinse the used glassware in the aim of accurate transfer of materials.
Place a crystallizing dish (for water bath) onto a hot plate and clamp the flask inside the
crystallizing dish. Add enough water to the dish so as to cover the urea solution. Heat the
solution at 70 ˚C under stirring to dissolve. Use an additional kerosene thermometer to
monitor the temperature of the water bath.
In a 100 ml beaker, weigh 1.900 g of Ba(NO3)2, 2.635 g of Cu(NO3)2‧ 3H2O and 1.393 g
of Y(NO3)3‧ 6H2O and stir the mixture with a glass rod. Weigh Ba(NO3)2 firstly, then
Cu(NO3)2‧ 3H2O and lastly Y(NO3)3‧ 6H2O. Carefully add this mixture to the urea
solution via a feeder funnel and stir to dissolve. Use the remaining 3 mL of DI water to
rinse the used the glassware in the aim of accurate transfer of materials.
WARNING - both the yttrium and copper nitrates are extremely hydroscopic (i.e., they
absorb moisture from the air and become nonstoichiometric).
Fit the flask with condenser. Heat the solution until it is at 90-100 ˚C and then continue
heating for 1 hour. Evolution of CO2 and NH3 should be observed during heating.
After 1 hour of heating, turn off the magnetic stirring. Draw a little clear solution with a
pipette, cool the solution to room temperature and measure its pH value with pH paper. If
the pH is 7, allow the solution to cool to room temperature, otherwise, continue heating
until the pH reaches a value of 7.
Add ~50 ml of DI water to the solution and stir it for 5 minutes before filtering. This will
help dissolve any unreacted urea from the copper oxide precipitate. Filter the pale blue
precipitate in a Buchner funnel using a suction apparatus. If the mixture filters slowly, stir
the precipitate with 50 ml more DI water and re-filter. Wash the precipitate with DI water
followed with ethanol. Leave the suction on for several minutes to dry the powder then
place the mixture in a petri dish and let it dry in an oven at 140 ˚C for six hours.
Grind the powder in the mortar and pestle. Place the powder in an alumina crucible and
calcine it in air by heating at 900 ˚C for 16 hours in a tube furnace, using the heating
curve shown in Figure 1.
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Inorganic Chemistry Lab
Figure 1. Heat treatment profile for step 7 and 8
8. Examine the powder after it has been removed from the tube furnace and cooled.
YBa2Cu3Ox is black. A green coloration in the powder indicates the formation of
Y2BaCuO5 and CuO. The presence of a small portion of green phase usually does not
interfere with the superconductivity. But if the powder is dominated by Y2BaCuO5 then
grind the powder in the mortar and pestle again. And then place the powder in an alumina
crucible and calcine it in air by heating at 900˚C for 16 hours in a tube furnace again
using the heating curve shown in Figure 1.
9. If the powder is black, grind the powder in the mortar and pestle. At this stage weigh 1.6
g of the powder, and mold it into a pellet (3 mm thick, 12 mm in diameter) through the
use of a compressing and tableting machine. The pellet must be removed with extreme
care to avoid crumbling it.
10. The pellets should be sintered under nitrogen to give them structural integrity. Before
turning on the furnace adjust the nitrogen flow to 1-2 bubbles per second. Place the
pellets in an alumina crucible and heat the material under nitrogen at 900 ˚C for 4 hours.
Then annealing at 500 ˚C for 24 hours under oxygen keeping the oxygen flow to 2
bubbles per second. Turn off the furnace, and allow it to cool to room temperature while
maintaining the oxygen flow. The material obtained after this procedure should be black.
The heating curve is shown in Figure 2.
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Inorganic Chemistry Lab
Figure 2. Heat treatment profile for step 10
4.2 Hand-grinding approach
Weigh out 225.8 mg (1 mmol) of Y2O3, 789. 4 mg (4 mmol) of BaCO3, and 477.4 mg(6
mmol) of CuO. Note that these weights correspond to a 1: 2: 3 molar ratio of metals, try to
make sure that the actual masses are within a few tenths of a milligram of the masses given
here. In a hood, combine the powders in a clean agate mortar and grind the mixture with an
agate pestle until the powders are thoroughly mixed (about 15 min). The final powder should
be of a uniform gray color with no lumps. Use a compressing and tableting machine (14 Mpa,
2.5 min) to make a small pellet (3 mm thick, 12 mm in diameter) out of your gray powder.
The pellet must be removed with extreme care to avoid crumbling it. Transfer the gray pellet
into a preweighed alumina boat and record the total weight of the starting reagents. Place the
boat near the center of the tube in the furnace. Position the tube in the furnace next to a
thermocouple. Because the furnace temperature is controlled by the thermocouple, it is
important to position the thermocouple probe near the reaction boat (but on the outside of the
tube). Fit the ends of the tube with gas adapters, connecting one adapter to an oxygen
cylinder and the other end to an oil bubbler so that the gas flow can be monitored. Before
turning on the furnace, adjust the oxygen flow to 1-2 bubbles per second. Then heat the
sample to 970 °C for 5 h.
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Inorganic Chemistry Lab
Figure 3. Heat treatment profile
Turn off the power to the furnace (the temperature controller should be left on), leaving
the oxygen flowing. Allow the temperature to reach 700 °C before opening the furnace to
accelerate the cooling. Once the sample has cooled to 200 °C, turn off the oxygen, and
disassemble the apparatus. When the tube is cool enough to handle, remove it from the
furnace. The sample should be black, and the particles should be sintered together. Weigh the
boat with the sample still in it, and regrind the sample to a fine powder; the particles should
be black and lustrous.
(5) Test of Meissner Effect
The determination as to whether or not the YBa2Cu3Ox made is a high- temperature
superconductor is most easily done by observing the Meissner effect. When a material
makes the transition from the normal to superconducting state, it actively excludes magnetic
fields from its interior; this is called the Meissner effect. The superconducting transition
temperature for YBa2Cu3Ox is ~92 K. Cooling a pellet of the material in liquid nitrogen (77
K) and levitating a ferromagnet over its surface is a positive test for superconductivity in
these materials. It is best to use samarium-cobalt or neodymium-iron-boron ferromagnets
(which should be about one-third or less the size of the pellet) as they possess stronger
magnetic fields which allow the magnet to levitate higher above the superconductor. The cut
off bottom of a styrofoam coffee cup makes a suitable reservoir for the liquid nitrogen and
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Inorganic Chemistry Lab
pellet. A pair of plastic tweezers is useful for handling the magnets as they often drift off the
superconductor and into the liquid nitrogen. Care must be observed when handling the liquid
nitrogen as its extremely low temperature can cause frostbite.
Figure 3. The Meissner effect occurring when a superconductor cooled by liquid nitrogen
levitates a magnet.
(6) Laboratory report
Table 1: Molecular weight (MW), mass, amounts and stoichiometric coefficient for starting
materials in the co-precipitation approach
Starting materials
MW/(g·mol-1) Mass/g Amount/mmol
Y(NO3)3 ﹒6H2O
Ba(NO3)2
Cu(NO3)2 ﹒3H2O
Urea
(H2N)2CO
Oxalic acid
C2O4H2﹒2H2O
Stoichiometric
coefficient
___
___
Table 2: Molecular weight (MW), mass, amounts and stoichiometric coefficient for starting
materials in the hand-grinding approach
Starting materials MW/(g·mol-1) Mass/g Amount/mmol
Y2O3
BaCO3
CuO
Stoichiometric
coefficient
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Inorganic Chemistry Lab
Table 3: Properties of your product YBa2Cu3Ox
Product
YBa2Cu3Ox
Color
Meissner effect (Y/N)
(7) Safety precautions
In addition to the usual precautions, conduct all grinding and transfer operations in a hood
wearing gloves. Barium is quite toxic and inhalation of it poses the most significant chemical
hazard in this experiment.
(8) Questions
1.
2.
3.
4.
Why YBa2Cu3Ox is called high temperature superconductor?
What is the function of urea in the preparation of YBa2Cu3Ox?
What was the purpose of each of the procedures (i.e. calcination, sintering and annealing)
in the furnace?
Did your sample display the Meissner effect? Discuss the reason.
(9) Additional literature references
1.
2.
3.
4.
5.
Several articles in J. Chem. Ed. 1987, 64, 836-853.
Liu, R.S.; Chang, C. T.; Wu, P. T. Inorg. Chem. 1989, 28, 154.
Holland, G. F; Stacy, A. M. Acc. Chem. Res. 1988, 21, 8.
Müller, K. A.; Bednorz, J. G. Science 1987, 237, 1133.
Pool, R. Science 1988, 241, 655.
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