Download Synthesis of High Temperature Superconductors

Chemistry 185H
Laboratory #5: Synthesis and Characterization of High-Temperature Superconductors
Introduction
Superconductors are materials whose resistivity drops to zero at sufficiently low temperatures, in
contrast with metal conductors whose resistivity decreases as the temperature falls but remains
finite even at the lowest temperatures. The temperature below which the resistivity of a
superconductor goes to zero and the material becomes superconducting is known as the critical
temperature, Tc. Below Tc, a superconductor can sustain a current indefinitely with no loss due to
resistance, which is an electrical analog of friction. As current passes through a circuit containing
metal conductors and semiconductors, current is lost to resistive heating. A second property
possessed by all superconducting materials is an ability to exclude magnetic fields from their
interiors at temperatures below Tc, as illustrated in Figure 1. In the superconducting state, these
materials are perfectly diamagnetic. This phenomenon, called the Meissner effect, was discovered
in 1933 by Meissner and Ochsenfeld. The Meissner effect is responsible for the ability of
superconductors to levitate magnets.
Heike Kamerlingh Onnes discovered the
phenomenon of superconductivity in 1911, when he
observed that the resistance of mercury abruptly fell to
zero when cooled to 4 K by liquid helium.
Subsequently, other materials were discovered that
exhibited superconductivity; however, the
superconducting state could only be accessed through
cooling with liquid helium, which is expensive and
requires special handling. Consequently, the practical
applications of superconductors have been limited and Figure 1: Illustration of the Meissner effect.
highly specialized. A notable application of
(Taken from Teaching General Chemistry:
superconductors requiring liquid helium cooling is in
A Materials Science Companion, p. 305.)
the magnets that produce the high fields required for
nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI); the
coils of these magnets are constructed from Nb-alloy superconductors.
It has been a long-standing goal in the field of
superconductor research to develop materials that are
superconducting at temperatures which can be reached by
cooling with liquid nitrogen because liquid nitrogen is much
more readily available and less expensive than liquid helium.
A breakthrough came in 1987 when the research groups of
physicists Paul Chu and M. K. Wu, Jr. reported discovery of
the first material that was superconducting above the boiling
point of liquid nitrogen (77 K). You will synthesize this
material, YBa2Cu3Ox (x = 6.5–7), which is also known as a
Figure 2: Unit cell of YBa2Cu3O7.
1-2-3 high-temperature superconductor based on the Y:Ba:Cu
(Taken from Teaching General
ratio. Not only does this material exhibit superconductivity at Chemistry: A Materials Science
77 K, it is also relativity easy to synthesize. Subsequently,
Companion, p. 433.)
additional materials have been discovered that superconduct
at 77 K. The next steps required for widespread, practical application of high-temperature
superconductors include increasing the current-carrying capacity of these materials because
superconductivity is lost above a critical current density and developing the methods needed to
fabricate wires and films from these materials that can be incorporated in circuits.
The laboratory exercises which follow will be performed over two weeks. During the first
week, you will synthesize YBa2Cu3Ox high-temperature superconductors and examine their
structure using the Solid-State Model Kit. In the second week, you will physically and chemically
characterize the YBa2Cu3Ox material synthesized in week 1. Physical characterization will
include demonstrations of the Meissner effect and the loss of resistivity below the critical
temperature. The chemical characterization will use the results from an iodometric titration of
copper, which is present in both the +2 and +3 oxidation states, to determine the value of the
noninteger stoichiometric coefficient x for oxygen.
Background Reading
Read Section 3.14 and Box 5.2 (pp. 184-185) in Atkins/Jones. For additional information, the
interested student can consult the article “Superconductors: Better Levitation Through
Chemistry” by A. B. Ellis (Journal of Chemical Education, 1987, 64, 836).
Procedure-Week 1, Day 1
Synthesis of the High-Temperature Superconductor YBa2Cu3Ox
CAUTION: The chemicals used to synthesize YBa2Cu3Ox are toxic. Wear gloves while weighing
these compounds and grinding the reaction mixture. Grind the reaction mixture in the fume hood.
Wash your hands thoroughly after handing these compounds.
1. Measure out 0.500 g of Y2O3, which will be used as the limiting reagent in this synthesis, and
transfer it to an agate mortar. Do not use a porcelain mortar and pestle because contamination
of the reaction mixture with Al2O3 from the porcelain will reduce the superconducting
properties of the material synthesized. Also, avoid using metal spatulas which can introduce
other metal impurities. Instead, used one of the hard rubber spatulas provided.
2. Carefully weigh out stoichiometric quantities of BaCO3 and CuO. Calculate the masses of
BaCO3 and CuO required before coming to lab based on the desired molar ratio Y:Ba:Cu =
1:2:3. Add the BaCO3 and CuO to the mortar. Careful weighing is essential because the
reagents must be present in nearly the exact 1:2:3 stoichiometric proportions to produce the
superconducting phase of yttrium barium copper oxide.
3. Grind the mixture of Y2O3, BaCO3, and CuO well using a pestle for approximately 20
minutes. Use a rubber spatula to scrape off any material that cakes on the side of the mortar.
The resulting powder should be uniform in color with no lumps and no black or white patches
present. Describe the appearance of the resulting reaction mixture. Why must you mix the
three powders?
4. Transfer the powder to a porcelain crucible. Load the crucible into the furnace rack, and write
down the position of your sample on the grid provided.
5. Your TA will complete the synthesis from here. The reaction mixture will be heated to 930 °C
for 12 hours and then will be cooled slowly in the furnace. Slow cooling is required to get an
oxygen content of x = 6.5-7. The crucible can be removed from the furnace when the furnace
temperature has fallen below 100 °C. The pellets and solid will be stored in a desiccator until
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your next lab period. If the solid is green, the material Y2BaCuO5, which is not a
superconductor, has formed. Additional heating at a slightly higher temperature (950 °C)
should convert this green material to the desired 1-2-3 high-temperature superconductor.
Examination of the Structure of YBa2Cu3O7 Using the Solid-State Model Kit
Consult page 45 of the Solid-State Model Kit Instruction Manual, 2nd ed. for instructions on
constructing a model for YBa2Cu3O7. Build the model, and then answer the following questions
about the structure of YBa2Cu3O7:
1. Sketch the unit cell for YBa2Cu3O7. How many Y, Ba, Cu, and O atoms are found in each unit
cell?
2. The copper in YBa2Cu3O7 is a mixture of +2 and +3 oxidation states. If yttrium has a +3
oxidation state while barium and oxygen have their normal oxidation states, what fraction of
the copper is in each of the two oxidation states?
3. In reality, YBa2Cu3Ox is a nonstoichiometric compound where x is less than 7 but greater
than 6.5. If x = 6.8, what percentage of the unit cells in YBa2Cu3Ox contain six oxygen atoms
instead of seven?
Include this material in the Discussion section of your lab report.
Procedure-Week 1, Day 2
Pressing of High-Temperature Superconductor Pellet
1. At this point, you will team up with another lab group. One group will provide the material
required to prepare a pellet for the physical characterization of the high-temperature
superconductor. The second group will provide the solid used in the chemical
characterization. The two groups will share samples.
2. Each group should grind up their solid high-temperature superconductor material using an
agate mortar and pestle. If a portion of the solid is green following heating in the furnace,
separate the green material from the black material. You should only grind up the black solid,
which is the superconducting phase.
3. The two groups should press one 2–3 mm thick pellet with the assistance of their TA. This
pellet may be fragile. Carefully transfer it to a crucible using plastic forceps. If the pellet
crumbles or shears when it is removed from the pellet press, grind it up in the mortar and
make a new pellet. Any remaining solid should be combined with that from the second lab
group in a crucible. Make sure that your name is marked on the crucibles before returning
them to the desiccator.
Procedure-Week 2
Physical Characterization of High-Temperature Superconductors
Below the critical temperature Tc, a superconductor excludes magnetic fields from its interior, a
property known as the Meissner effect. The Meissner effect can be demonstrated by cooling
YBa2Cu3Ox to 77 K with liquid nitrogen and then levitating a small, strong magnet. The magnet
induces a supercurrent on the surface of the superconductor. Within the perfectly diamagnetic
superconductor the induced magnetic field produced by the supercurrent exactly cancels the field
from the magnet with the net result that no magnetic field is created within the superconductor.
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The Meissner effect effectively prevents magnetic fields from penetrating the superconductor. At
temperatures below Tc there is no resistance to the flow of current in the superconductor creating a
second magnetic field outside the superconductor. The magnetic fields from the magnet and
superconductor repel each other resulting in levitation of the magnet as long as the temperature of
the superconductor is held below Tc.
Follow the procedure given below to demonstrate the Meissner effect using your YBa2Cu3Ox
pellet. Record your observations.
1. Prepare a cut-off Styrofoam cup approximately 1.5 cm tall to hold the YBa2Cu3Ox pellet and
liquid nitrogen.
2. Remove the YBa2Cu3Ox pellet from the crucible using plastic forceps. Remove any loose
material by gently scraping the pellet with a spatula. If the pellet has sheared, use the largest
piece. Note the color of the pellet. How does it compare to the color of the reactants?
3. Use plastic forceps to transfer the pellet to a Styrofoam cup, placing the flattest side up.
4. Place a small magnet on top of the YBa2Cu3Ox pellet using plastic forceps.
5. Obtain a Styrofoam cup full of liquid nitrogen from your TA. CAUTION: Liquid nitrogen is
extremely cold. Do not play around with the liquid nitrogen. Wear your safety goggles at all
times when working with liquid nitrogen. Do not allow your eyes, skin, or clothes to come
into contact with spilled liquid nitrogen. The skin is easily frozen or burned by liquid
nitrogen. If you spill liquid nitrogen on your clothing, quickly remove the clothing so that the
absorbed or trapped liquid nitrogen will boil away before freezing your skin.
6. Carefully pour liquid nitrogen into the cut-off Styrofoam cup until the pellet is covered. After
the pellet has cooled below the critical temperature, the magnet should levitate. Record your
observations. What happens if you tap the pellet gently with the plastic forceps?
7. When you are finished, let the liquid nitrogen evaporate. Leave the pellet and magnet in the
cut-off Styrofoam cup until they have warmed to room temperature.
A key attribute of superconductors is their loss of electrical resistance below some critical
temperature Tc. For the 1-2-3 high-temperature superconductors the critical temperature is above
the boiling point of liquid nitrogen (77 K). You will observe this property of superconductors for
yourself by first building a circuit to measure the resistance across your YBa2Cu3Ox pellet and
then making measurements as the pellet is cooled by liquid nitrogen.
1. Construct the circuit shown in Figure 3,
which consists of two 1.5 V AA batteries in
series, a 100 W resistor, and a round battery
holder. Make sure that the wires connecting
the round battery holder to the resistor and
D battery holder are sufficiently long to
permit placement of the round battery
holder in the cut-off Styrofoam cup without Figure 3. Circuit for measuring the change in the
resistance of YBa2Cu3Ox at temperatures below
having the resistor and batteries in contact
Tc. (Taken from Teaching General Chemistry: A
with the cup.
Materials Science Companion, p. 437.)
2. Carefully insert the YBa2Cu3Ox pellet into
the round battery holder. Use the plastic
forceps to manipulate the pellet.
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3. Measure the resistance across the pellet using a multimeter/voltmeter.
4. If necessary, obtain additional liquid nitrogen from your TA. Pour the liquid nitrogen into the
foam cup, being careful not to get liquid nitrogen on the resistor. When the superconductor
starts conducting, the resistor will become very hot. Do not touch the resistor. If liquid
nitrogen is splashed on the resistor, the thermal shock could cause it to break.
5. Measure the resistance as the pellet cools. Record your observations.
6. After completing your measurements, remove the batteries from the battery holder to prevent
further overheating of the resistor.
7. Allow the liquid nitrogen to evaporate.
8. After the pellet and round battery holder have reached room temperature, remove the pellet
from the battery holder using the plastic forceps.
Determination of the Copper Content in High-Temperature Superconductors by Iodometric
Titration
The YBa2Cu3Ox high-temperature superconductors are examples of nonstoichiometric
compounds in which the oxygen content is variable. The coefficient x in the formula has a
noninteger value in the range 6.5–7. You will determine the oxygen content indirectly through
iodometric titration of copper, which is present in both the +2 and +3 oxidation states in
YBa2Cu3Ox. In Titration A, the total copper content of the sample will be determined after
converting all of the Cu3+ to Cu2+ and allowing the Cu2+ to react with I- to form I3-. The triiodide
(I3-) produced is titrated with sodium thiosulfate (Na2S2O3), hence the classification of this redox
titration as an iodometric titration. The balanced chemical equation for the titration reaction is
I3- + 2 S2O32- → 3 I- + S4O62-
(1)
In Titration B, both Cu2+ and Cu3+ will be reacted with I- with each mole of Cu2+ producing half
a mole of I3- and each mole of Cu3+ producing one mole of I3-. The results of these titrations will
be used to determine the fraction of copper atoms in each oxidation state. The stoichiometric
coefficient for oxygen can then be calculated based on the distribution of copper between the two
oxidation states.
Each lab group will do either Titration A or Titration B and then share its results with a lab
group that has done the other titration. Make sure that you understand the titration procedure and
have everything assembled to carry out the titration before proceeding. It is important that these
titrations are carried out as rapidly as possible.
CAUTION: Handle all solutions containing perchloric acid (HClO4) with extreme care because
HClO4 is very corrosive. All work involving perchloric acid solutions should be carried out in a
fume hood.
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Titration A-Revised Procedure
1. Remove the solid made by heating the powder from the crucible, and transfer it to a clean
mortar using plastic forceps. Describe the appearance of the solid produced. Grind up a
portion of the solid using the pestle.
2. Accurately weigh out 100 to 150 mg of the powdered YBa2Cu3Ox prepared in week 1, and
transfer it to a clean beaker that can be used for the titration.
3. In a fume hood, dissolve the YBa2Cu3Ox powder in 10 mL of 1.0 M HClO4.
4. Boil the mixture gently for 10 minutes. Do not allow this solution to boil to dryness. While
the sample is boiling gently under the supervision of one member of your lab group, the
other member(s) can prepare the second and third titration samples following the procedures
given in Steps 1–4.
5. Allow these solutions to cool to room temperature before beginning the titration.
6. Dissolve 1.0–1.5 g of KI in 10 mL of distilled water, and immediately add the solution to the
beaker containing the dissolved YBa2Cu3Ox. Begin magnetic stirring. Do not carry out this
step until you are completely set up to do the titration.
7. Titrate with the standardized solution of Na2S2O3 provided. Be sure to record the
concentration of the standardized Na2S2O3 and starting volume of Na2S2O3 in the buret in
your laboratory notebook. Add 3 mL of starch solution just before the last trace of the I2 color
disappears. Adding starch prematurely can result in irreversible attachment of I2 to the starch,
making it difficult to see the endpoint. Note: The triiodide ion I3- exists in equilibrium with I2
and I- which is the reason the solution has the orange/brown color characteristic of I2(aq). The
solution should turn blue when the starch is added.
8. Titrate slowly from this point on. The blue color will disappear at the endpoint leaving a
cloudy white mixture. This solution containing suspended white solids should remain white
for 30–60 s. If the solution turns blue, you have just observed a false endpoint and you should
continue titrating slowly. Once you have reached the endpoint, record the volume of Na2S2O3
remaining in the buret
9. Repeat this procedure (Steps 6–8) two additional times.
Titration B-Revised Procedure
1. Remove the superconductor produced by heating the powder from the crucible, and transfer it
to a clean mortar using plastic forceps. Describe the appearance of the solid produced. Grind
up a portion of the solid using the pestle.
2. Accurately weigh out 100 to 150 mg of the powdered YBa2Cu3Ox prepared in week 1, and
place it in an Erlenmeyer flask.
3. In a fume hood, add 15 mL of 10% KI solution. The superconductor powder will not
dissolve.
4. Remove dissolved oxygen from the solution by vigorously bubbling nitrogen through the
solution for 10 minutes. Nitrogen can be delivered to the solution by attaching a Pasteur
pipette to the end of the nitrogen line. Make sure that the tip of the Pasteur pipette remains
submersed.
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5. Add 6 mL of 3.5 M HCl solution to the Erlenmeyer flask over a 5-minute period, while
stirring the solution magnetically. You should gently bubble nitrogen through the solution as
you complete the remainder of the titration procedure. The solution should turn brown as the
HCl is added indicating the formation of triiodide.
6. Continue to stir the solution for 10 minutes after the addition of the HCl solution is complete.
You will eventually see formation of a grayish white precipitate when the concentration of
CuI in the reaction mixture exceeds its solubility.
7. Titrate using the standardized solution of Na2S2O3 provided. Be sure to record the
concentration of the standardized Na2S2O3 and starting volume of Na2S2O3 in the buret in
your laboratory notebook. Add Na2S2O3 until the brown iodine color becomes pale. (A
0.100-g sample should require approximately 9–10 mL of Na2S2O3 to reach this point.) Add
3 mL of starch solution just before the last trace of I2 color disappears. Adding starch
prematurely can result in irreversible attachment of I2 to the starch, making it difficult to see
the endpoint. Note: The triiodide ion I3- exists in equilibrium with I2 and I- which is the
reason the solution has the orange/brown color characteristic of I2(aq). The solution should
turn blue when the starch is added.
8. Titrate slowly from this point on. The blue color will disappear at the endpoint leaving a
cloudy white mixture. This solution containing suspended white solids should remain white
for 30–60 s. If the solution turns blue, you have just observed a false endpoint and you should
continue titrating slowly. Once you have reached the endpoint, record the volume of Na2S2O3
remaining in the buret.
9. Repeat this procedure (Steps 1–8) two additional times.
Analysis of Titration Data
Lab groups should share their titration data (masses of YBa2Cu3Ox and volumes of Na2S2O3 used
in each titration), but the analysis of data should be carried out independently by each group. Lab
partners may discuss the analysis, but each student is responsible for performing his/her own
analysis of the data.
Titration A: As described above, the copper present in the 1-2-3 high-temperature
superconductors exists in the +2 and +3 oxidation states. This titration determines the total
amount of copper present in the sample. All of the Cu3+ present in the sample is converted to Cu2+
through oxidation-reduction reaction with the strong acid (HClO4)
YBa2Cu3O7 + 12 H+ → Y3+ + 2 Ba2+ + 3 Cu2+ + 6 H2O + 1/2 O2
(2)
Iodide I- is added to the sample and reacts with Cu2+ to produce triiodide I3-, the species titrated
with Na2S2O3.
2 Cu2+ + 5 I- → 2 CuI(s) + I3-
(3)
Equation 1 shows the reaction between I3- and S2O32- which occurs in the titration. Determine the
number of moles of I3- titrated, which is related to the copper content as follows
2 nA(I3-) = nA(Cutotal) = [nA(Cu2+) + nA(Cu3+)]
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(4)
where the quantities nA(X) represent the number of moles of species X present in sample A or
required to titrate sample A. To find the number of moles of each species present in a 1-g sample
or required to titrate a 1-g sample, divide equation 4 by the mass of sample A, mA.
2 nA(I3-)/mA = nA(Cutotal)/mA = [nA(Cu2+) + nA(Cu3+)]/mA
(5)
2 nA(I3-)/mA = n´(Cutotal) = [n´(Cu2+) + n´(Cu3+)]
(6)
where n´(X) represents the number of moles of species X in a 1-g sample. Division of equation 4
by the mass of the sample makes the number of moles independent of sample size and permits
comparison of the results from titrating samples of differing sizes. Find the quantity 2 nA(I3-)/mA
for each titration. Report the average and standard deviation of 2 nA(I3-)/mA for the three titrations.
Titration B: In this titration, any copper present in the +3 oxidation state is left in this
oxidation state and is allowed to react with iodide
Cu3+ + 4 I- → CuI(s) + I3-
(7)
The reaction of Cu2+ is given above in equation 3. Note the difference in the yield of I3- for the
two different reactions. One mole of I3- is produced for every mole of Cu3+ in the sample, while
only one half mole of I3- is produced for every mole of Cu2+. The relationship between the
number of moles of I3- determined in the titration and the moles of Cu2+ and Cu3+ is
nB(I3-) = 1/2 nB(Cu2+) + nB(Cu3+)
(8)
Divide equation 8 by mB, the mass of YBa2Cu3Ox used in each titration, to get an expression that
relates the quantity of I3- titrated to the moles of Cu2+ and Cu3+ in a 1-g sample of YBa2Cu3Ox
nB(I3-)/mB = 1/2 n´(Cu2+) + n´(Cu3+)
(9)
Find the quantity nB(I3-)/mB for each titration. Report its average and standard deviation from the
three titrations.
Analysis: Equations 6 and 9 can be solved simultaneously to determine the number of moles
of Cu2+ and Cu3+ in a 1-g sample of YBa2Cu3Ox. To determine the moles of Cu2+ in a 1-g sample
of YBa2Cu3Ox, subtract equation 9 from equation 6 and solve for n´(Cu2+), which yields the
expression
n´(Cu2+) = 2 [2 nA(I3-)/mA - nB(I3-)/mB]
(10)
Substitution of the resulting value for n´(Cu2+) into equation 6 or 9 will permit determination of
n´(Cu3+). Using the values for n´(Cu2+) and n´(Cu3+), calculate the percentage of copper atoms
that are in the +2 and +3 oxidation states. With this information, you could rewrite the formula for
the high-temperature superconductor as YBa2CuII3(0.YY)CuIII3(0.ZZ)Ox, where 0.YY and 0.ZZ
represent the percentages of Cu2+ and Cu3+, respectively. Use the normal oxidations states for
barium and oxygen and an oxidation state of +3 for yttrium to determine the stoichiometric
coefficient x for oxygen. You followed a similar approach in Week 1 as part of the model-building
exercise when you were asked to determine the relative amounts of Cu2+ and Cu3+ in
YBa2Cu3O7.
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Lab Report
Your laboratory report should include the following sections:
1. Introduction. Explain what superconductors are and the significance of the discovery of hightemperature superconductors. What special physical properties do superconductors have?
2. Experimental Methods. Summarize the procedures used without repeating all of the details
provided here. Emphasize any changes that you made in the procedure. Report the quantity,
manufacturer, and purity of each reagent used in the synthesis of YBa2Cu3Ox. Note the
concentration of the standardized Na2S2O3 solution used in the titration.
3. Results. This section should include descriptions of the YBa2Cu3Ox pellet and the solid
produced by heating the powdered reaction mixture. Report your observations from the
physical characterization of the superconductor pellets. Briefly present the reactions used in
Titrations A and B, and identify the species that are oxidized and reduced in these reactions.
Prepare Tables for Titration A and Titration B that report the mass of YBa2Cu3Ox used,
volume of Na2S2O3 required to reach the endpoint, nA(I3-) or nB(I3-), and 2 nA(I3-)/mA or
nB(I3-)/mB for each titration run. Indicate which lab group collected the data for each titration.
Provide the averages and standard deviations for 2 nA(I3-)/mA and nB(I3-)/mB in the text.
Calculate and report the percentages of the copper atoms that are in the +2 and +3 oxidation
states. Report the formula that you determined for YBa2Cu3Ox.
4. Discussion. Discuss the structure of YBa2Cu3Ox high-temperature superconductors based on
model building and the results of the iodometric titrations. Include a sketch of the
YBa2Cu3O7 unit cell. Compare the fraction of Cu atoms in the +2 and +3 oxidation states in
your sample with the values calculated for YBa2Cu3O7. What fraction of the unit cells in your
sample have only six oxygen atoms? How does this compare to the value calculated for
YBa2Cu3O6.8? An alternate method for synthesizing 1-2-3 high temperature superconductors
involves heating the reaction mixture in a tube furnace while flowing oxygen over the sample.
How would you expect the CuII:CuIII ratio of YBa2Cu3Ox synthesized using this method to
compare to the CuII:CuIII ratio in the material that you synthesized? Discuss the physical
properties of superconductors that you observed in this lab and how the physical properties of
superconductors differ from metal conductors. How do reactions in the solid state differ from
those in solution? To answer this question, think about why you had to grind the reactants
before pressing your pellet. If any part of this lab did not work, discuss what you think went
wrong and/or how you would improve the experiment.
5. Conclusions. Briefly summarize the key findings from your experimental work.
6. Appendix. Provide sample calculations from your analysis of the oxygen content of
YBa2Cu3Ox based on the iodometric titrations of copper. You should include the analysis of
the data for Titrations A and B.
The Results (and Appendix) will count for 45% of the lab report grade, and the Discussion will be
worth 30%.
Acknowledgments
The procedures for the synthesis of 1-2-3 high-temperature superconductors, the iodometric
titrations, and the physical characterization of YBa2Cu3Ox were adapted from those outlined in
Harris, D. C. Quantitative Chemical Analysis, 3rd ed.; W. H. Freeman and Co.: New York,
1991, pp. 416–417, 746–749.
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Ellis, A. B.; Geselbracht, M. J.; Johnson, B. J.; Lisensky, G. C.; Robinson, W. R. Teaching
General Chemistry: A Materials Science Companion; American Chemical Society:
Washington, DC, 1993, pp. 301–314, 429–438.
Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. Synthesis and Technique in Inorganic
Chemistry: A Laboratory Manual, 3rd ed.; University Science Books: Sausalito, CA, 1999,
pp. 17–26.
Instructions for construction of three-dimensional models of YBa2Cu3O7 were provided in
Lisensky, G. C.; Covert, J. C.; Mayer, L. A. Solid-State Model Kit Instruction Manual, 2nd
ed.; Institute for Chemical Education, University of Wisconsin-Madison: Madison, WI, 1994,
p. 45.
The original reference for the discovery of YBa2Cu3Ox high-temperature superconductors is
Wu, M. K.; Ashburn, J. R.; Torng, C. J.; Hor, P. H.; Meng, R. L.; Gao, L.; Huang, Z. J.; Wang,
Y. Q.; Chu, C. W. Phys. Rev. Lett. 1987, 58, 908.
Revised 8/05
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