How to Make a Yttrium-Based High-Temperature Superconductor (YBCO) Using University Laboratory Equipment | Observing Magnetic Levitation with Liquid Nitrogen

A representative yttrium-based high-temperature superconductor,
YBa₂Cu₃O_7-δ (YBCO, Y123)
can be prepared from raw material powders by solid-state reaction if university-level inorganic chemistry and materials science facilities are available.
Properly prepared YBCO has a superconducting transition temperature of around 90 K, so by cooling it with liquid nitrogen,
the levitation of a permanent magnet can be observed.

The author also has experience preparing small YBCO samples using laboratory equipment while attending university.
For an experiment on the scale of a student laboratory exercise, there is no need to prepare a large bulk sample,
and under favorable conditions, magnetic levitation can be confirmed in liquid nitrogen even with a small sintered body about the size of a grain of rice.

This article assumes that appropriate experimental facilities at a university, research institution, or similar facility are available,
and introduces the formulation, mixing, firing, grinding, shaping, and refiring of YBCO raw materials,
through to the observation of magnetic levitation using liquid nitrogen.

What Is YBCO?

YBCO is a cuprate high-temperature superconductor composed of yttrium (Y), barium (Ba), copper (Cu), and oxygen (O).
Its representative composition is
YBa₂Cu₃O_7-δ,
and it is also called “Y123” because the ratio of Y, Ba, and Cu is 1:2:3.

YBCO with an appropriate oxygen content and crystal structure has a superconducting transition temperature of around 90 K,
and becomes superconducting when cooled to approximately 77 K, the boiling point of liquid nitrogen.
Therefore, compared with low-temperature superconductors that require liquid helium,
it is a material that makes superconducting phenomena relatively easy to observe even in university student experiments.

What Kind of Superconductor Is Used in Maglev Trains?

When people hear the word superconductivity, many probably think of maglev trains.
However, the YBCO prepared in this experiment and the superconducting magnets installed on superconducting maglev vehicles
use different materials and cooling temperatures.

In Central Japan Railway Company’s superconducting maglev (SCMAGLEV),
the coils of the onboard superconducting magnets use
niobium-titanium alloy (Nb-Ti).
Nb-Ti is a representative metallic low-temperature superconducting material,
and common practical alloys have a critical temperature of approximately
9 K (approximately -264°C).
In an actual superconducting maglev system, liquid helium is used to cool it
to around 4 K (approximately -269°C),
allowing it to generate a powerful magnetic field in a stable superconducting state.

When a superconductor falls below its critical temperature, its electrical resistance disappears,
and the Meissner effect, which causes it to expel magnetic fields, appears.
However, it is not accurate to explain the mechanism by which a superconducting maglev levitates its vehicle
as “levitating solely because of the Meissner effect.”

In a superconducting maglev, the onboard superconducting magnets generate an extremely strong magnetic field,
and electromagnetic forces between this field and currents induced in the levitation and guidance coils on the ground
levitate and guide the vehicle.
In other words, the small-scale levitation experiment using YBCO and a permanent magnet in this article
is an experiment that makes the magnetic properties of superconductivity easy to observe,
whereas in an actual superconducting maglev,
extremely powerful superconducting electromagnets created using those properties play an important role.

Required Reagents and Equipment

Main Raw Materials

Main Instruments and Equipment

• Analytical balance
• Agate mortar and pestle
• Heat-resistant container such as an alumina crucible
• Electric furnace capable of operating at approximately 900–950°C or higher
• Heat-resistant tongs, heat-resistant gloves, etc.
• Local exhaust ventilation equipment for safely handling powders
• Die and hydraulic press for pellet formation
• Liquid nitrogen
• Appropriate insulated container such as a Dewar for liquid nitrogen
• Small, powerful permanent magnet (such as a neodymium magnet)
• Protective equipment such as safety glasses and, when necessary, a face shield

It Is Preferable to Use an Agate Mortar

Although an ordinary mortar can be used if you are simply mixing the raw materials,
for this experiment, the use of an agate mortar is recommended.

The reason is that the sample becomes quite hard after firing.
When a fired YBCO-based sample is finely ground again, a large load is placed on the mortar,
and the surface of an ordinary mortar may become covered with scratches.
When the author actually prepared YBCO, the sample after firing was also quite hard,
and regrinding clearly required more force than mixing the original raw material powders.

In addition, abrasion of the mortar itself can cause impurities to contaminate the sample.
Because the properties of YBCO may deteriorate due to impurities or deviations in composition,
a hard, highly wear-resistant agate mortar is easier to use.

Care and Polishing When an Agate Mortar Becomes Scratched

Agate is a hard material, but repeatedly grinding hard sintered samples
may cause fine scratches on the inner surface of the mortar.
If the scratches are shallow, polishing may restore the surface to some extent.

Agate is a quartz-based material composed mainly of silicon dioxide (SiO₂),
so in stone and gemstone polishing, the surface is finished by progressively switching to finer abrasives.
For final polishing,
cerium oxide-based polishing compounds
are used for polishing quartz and agate.

1. Thoroughly wash the mortar and completely remove any sample powder adhering to it.
2. Inspect the scratched areas and, if necessary, polish them progressively using fine abrasives.
3. For the final finish, make a slurry of a fine abrasive such as cerium oxide with a small amount of water and polish the surface.
4. After polishing, rinse thoroughly with a large amount of water so that no abrasive remains.
5. Perform a final rinse with purified water or similar water, and allow the mortar to dry thoroughly before use.

However, if there are deep scratches that catch a fingernail, chips, or cracks,
it is safer not to attempt to grind and repair them yourself.
Because a mortar is subjected to strong forces during grinding,
if it is deeply damaged, consult the manufacturer or a scientific equipment supplier about repolishing or repair,
or consider replacing it.

In addition, when handling high-purity samples or samples for trace-element analysis,
the polishing compound itself may become a new source of contamination.
Rather than immediately returning a polished mortar to high-purity experiments,
wash it thoroughly and, if necessary, separate its uses.

Raw Material Formulation

Weigh the raw materials so that the atomic ratio of Y, Ba, and Cu is 1:2:3.
When Y₂O₃, BaCO₃, and CuO are used,
the basic stoichiometric ratio is as follows.

Y₂O₃ ＋
4BaCO₃ ＋
6CuO
→
2YBa₂Cu₃O_7-δ

For example, if the total amount of raw material powder before the reaction is approximately 2.00 g, an example of the amounts to weigh is as follows.

This does not mean that 2 g of YBCO will ultimately be obtained,
but is a guideline for when the total amount of raw material powder before the reaction is approximately 2 g.
Conditions vary depending on reagent purity and the experimental procedure used,
so if your laboratory or university has a specified procedure, give priority to that procedure.

How to Prepare YBCO

YBCO can be prepared by solid-state reaction.
Because solids are reacted with one another, simply mixing and firing them once may not allow the reaction to proceed uniformly.
Therefore,
mixing, firing, grinding, shaping, and refiring
are repeated to make the sample uniform.

1. Weigh the Raw Materials

Weigh Y₂O₃, BaCO₃, and CuO in the specified proportions.
Handle the powders gently to prevent them from dispersing,
and follow the SDS for the reagents being used and your laboratory’s rules for handling powders.

2. Thoroughly Mix and Grind in an Agate Mortar

Transfer the weighed powders to an agate mortar and mix them thoroughly.
In a solid-state reaction, the reaction proceeds from locations where different raw material particles are in contact,
so it is important to mix them uniformly while reducing the particle size.

As a guideline, carefully mix and grind the powder in the mortar for approximately 30 minutes.
Make the color and particle size of the powder as uniform as possible.

3. Perform the First Firing

Transfer the mixed powder to an alumina crucible and fire it in an electric furnace.
As one example,
raise the temperature to 920°C over approximately 5 hours, hold it at 920°C for approximately 10 hours,
and then cool it to around room temperature over approximately 10 hours
is one possible temperature profile.

Therefore, including heating, holding, and cooling, one firing process
takes approximately 25 hours.
Because appropriate conditions vary depending on the characteristics of the furnace, the amount of sample, the crucible, the atmosphere, and other factors,
consider this only as a guideline for a university-level experiment.

4. Grind Again After Cooling

After confirming that the sample has cooled sufficiently, remove it,
and finely grind it again in an agate mortar.

A sample that has been fired once may become extremely hard, unlike the original raw material powder.
Do not try to forcibly crush a large lump all at once,
but instead break it down little by little and return it to a uniform powder.

5. Form the Powder into a Pellet

Place the ground sample into a die and use a hydraulic press to form it into a pellet.
If the only purpose is to observe magnetic levitation, a large sample is unnecessary,
and even a small sample only a few millimeters in size can be observed.

Always follow the rated specifications of the die and press for the applied pressure.
Because damage to the die can lead to a serious accident,
use the method and load specified for the particular equipment.

6. Fire the Shaped Sample Again

Place the shaped sample back into the electric furnace.
As a guideline, the same conditions as the first firing can be used:
raise the temperature to 920°C over approximately 5 hours → hold at 920°C for approximately 10 hours → cool over approximately 10 hours.

When the author actually prepared the sample,
magnetic behavior could be confirmed even after approximately two cycles of processing that included shaping, firing, and grinding.
However, the required number of cycles varies depending on factors such as how thoroughly the raw materials are mixed, particle size, and firing conditions.

If the reaction is insufficient after two cycles,
repeat
regrinding → shaping → refiring
additional times.
The number of repetitions itself is not the objective;
what is important is reducing unreacted and nonuniform regions and forming the desired YBCO phase.

7. Adjust the Oxygen Content

For YBCO, simply achieving Y:Ba:Cu = 1:2:3 is not sufficient.
As indicated by the chemical formula
YBa₂Cu₃O_7-δ,
the amount of oxygen in the crystal has a major effect on its superconducting properties.

Adjust the oxygen state through cooling after high-temperature treatment and, when necessary, annealing in an oxygen atmosphere.
Because this process varies considerably depending on the furnace and laboratory procedures used,
give priority to the conditions specified by your institution.

Precautions When Using an Electric Furnace

Do Not Carelessly Look Inside the Furnace

Do not unnecessarily look directly into an electric furnace while it is firing or at a high temperature.
If a sample or container bursts or cracks,
or if some sudden impact causes hot fragments of the sample to fly out,
they may strike the face or eyes directly.

Hot fragments entering the eyes in particular can cause serious eye injury,
and in the worst case, there is a risk of loss of vision.
When opening or closing the furnace, do not bring your face directly in front of the furnace opening,
and use safety glasses, a face shield, or other protective equipment specified by the facility.

Beware of Burns

Crucibles and samples heated to 900°C or higher are extremely dangerous.
It may not be possible to determine from appearance alone whether they are still hot.
Use heat-resistant tongs and appropriate heat-resistant protective equipment,
and handle them only after confirming that they have cooled sufficiently.

Confirming Magnetic Levitation with Liquid Nitrogen

Completed YBCO looks like a black ceramic at room temperature.
Because its appearance alone cannot determine whether it has become superconducting,
cool it to approximately 77 K with liquid nitrogen and observe its magnetic behavior.

Magnet to Use

To observe magnetic levitation,
use a small, powerful permanent magnet.
Among readily available options, a neodymium magnet is suitable.

A magnet that is too large will also be heavier,
so when the sample is small, a small, lightweight magnet with strong magnetic force is easier to observe.
Because neodymium magnets can crack or pinch fingers if they snap together forcefully,
take care when handling the magnets themselves.

Observation Method

The simplest method is first to cool the YBCO sufficiently with liquid nitrogen,
and then bring a neodymium magnet close to it to check for repulsion or levitation.

1. Place the YBCO sample in an appropriate insulated container.
2. Add liquid nitrogen and cool the sample sufficiently.
3. Wait until the vigorous boiling of the liquid nitrogen subsides.
4. Carefully bring a small neodymium magnet toward the sample using tweezers or a similar tool.
5. Check whether the magnet is repelled or levitates and stabilizes above the sample.

If the Magnet Is Repelled and Does Not Stabilize, Cool It While Applying a Magnetic Field

If the sample is cooled first and the magnet is then brought close,
the repulsive force may cause the magnet or sample to be pushed sideways,
making the position unstable when using a small sample.
In that case,
determine the relative positions of the magnet and superconductor first, and then cool them with liquid nitrogen while maintaining that arrangement
may make stable levitation easier to observe.

For example, place the YBCO sample on top of a small neodymium magnet,
carefully position them so that they do not shift relative to each other, and then add liquid nitrogen to cool them.
When the sample falls below its critical temperature, part of the magnetic field present during cooling
becomes pinned to defects and other features inside the sample through flux pinning,
making the relative positions of the magnet and sample easier to stabilize.

After sufficient cooling, slightly lifting the sample with tweezers or a similar tool,
or carefully adjusting its position relative to the magnet,
may create a more stable levitation state than when the sample is simply strongly repelled.

With a good sample, levitation may be observed even with a sample about the size of a grain of rice,
depending on the combination with the magnet.
As the sample returns to room temperature, the superconducting state is lost, and magnetic levitation can no longer be maintained.

Precautions When Using Liquid Nitrogen

The temperature of liquid nitrogen is approximately -196°C.
Although it is a relatively commonly used cryogen in laboratories,
improper handling can lead to severe frostbite, eye injuries, oxygen-deficiency accidents, and other hazards.

Do Not Put Bare Hands into Liquid Nitrogen

Do not put your hands or fingers into liquid nitrogen as a prank or demonstration.
When liquid nitrogen contacts the skin, it rapidly freezes tissue,
creating a risk of severe frostbite and tissue damage.
If the damage is deep, tissue necrosis may also occur.

With extremely brief contact, a layer of nitrogen gas may form between the liquid and the skin surface,
but this does not guarantee safety.
If the skin is wet, if liquid enters clothing or gloves,
or if the contact time becomes longer, severe frostbite may result,
so demonstrations claiming that “it is safe to put your hand into liquid nitrogen for just an instant” should not be imitated as a safe procedure.

Protect Your Eyes and Face

Splashing may occur when liquid nitrogen is poured.
If it enters the eyes, it can cause severe frostbite and eye injury,
so wear safety glasses and, depending on facility rules, use a face shield or similar protection as well.

Do Not Place It in a Sealed Container

Liquid nitrogen greatly increases in volume when it vaporizes.
If placed in a sealed container, the internal pressure can rise and cause the container to rupture.
Always handle liquid nitrogen in a dedicated Dewar or another container
that allows vaporized nitrogen to escape safely.

Do Not Use Large Amounts in Poorly Ventilated Areas

Nitrogen gas itself does not have a strong odor or similar warning property.
When a large amount of liquid nitrogen vaporizes, it displaces the surrounding oxygen,
creating a risk of oxygen deficiency.
Do not use large quantities in a small, poorly ventilated room,
and follow the facility’s requirements for ventilation systems, oxygen concentration management, and related safety measures.

Do Not Touch Equipment Cooled by Liquid Nitrogen Either

The liquid itself is not the only hazard.
Metal tweezers, containers, samples, and other objects immersed in liquid nitrogen also become extremely cold.
Touching them with bare hands may cause the skin to freeze to the object and become injured,
so do not touch them directly until they have warmed sufficiently.

Possible Reasons Why the Magnet Does Not Levitate Properly

Even if a black sintered body has been produced, the magnet will not necessarily levitate cleanly.
If it does not work properly, possible causes include the following.

• The weighing ratio of the raw materials is incorrect
• Insufficient mixing or grinding
• Inappropriate firing temperature or holding time
• Unreacted raw materials or secondary phases remain
• Insufficient sintering
• Inappropriate oxygen content
• The sample has not been sufficiently cooled with liquid nitrogen
• The magnet is too heavy for the sample, or its magnetic force is too weak
• Impurities were introduced during preparation

If Impurities Are Introduced, the Sample May Instead Stick to the Magnet

When the author actually prepared YBCO,
instead of repelling the magnet as a superconductor,
the sample was attracted to and stuck to the magnet
in one case.

This sample was considered to have been unsuccessfully prepared,
and there is a possibility that magnetic impurities such as iron were introduced during the process.
However, because elemental analysis of the impurities in that sample was not performed at the time,
it cannot be concluded that iron was the cause.

This experience also shows the importance of avoiding contamination not only from reagent purity,
but also from mortars, pestles, spatulas, dies, crucibles, and other equipment as much as possible.
Particular care should be taken during the grinding process to prevent contamination caused by wear of the equipment.

If an X-ray diffractometer (XRD) is available,
it can be used to determine whether the desired YBCO phase has formed in the completed sample
and whether secondary phases or other phases are present.
Furthermore, by measuring electrical resistance as a function of temperature using the four-probe method,
the superconducting transition itself can be confirmed in addition to magnetic levitation.

Meissner Effect and Flux Pinning

Magnetic levitation of superconductors is often explained in terms of the “Meissner effect,”
but flux pinning is also important for understanding the levitation of YBCO.

YBCO is a type-II superconductor.
In a type-II superconductor, depending on the conditions, magnetic flux enters the material in a quantized state,
and
flux pinning
occurs when the magnetic flux becomes pinned to defects and other features in the material.

As a result, rather than the magnet simply being repelled,
its positional relationship with the superconductor can become fixed to some extent,
and a state in which the magnet appears to levitate stably may be observed.

Summary

With university-level equipment, YBCO can be prepared
from Y₂O₃, BaCO₃, and CuO by solid-state reaction.

During preparation,
accurate weighing, thorough mixing, firing, regrinding, shaping, refiring, and adjustment of the oxygen state
are important.
When the author actually prepared YBCO, results could be confirmed after approximately two cycles of processing,
but depending on the conditions, grinding and firing may need to be repeated additional times.

In addition, because the sample becomes extremely hard after firing, an agate mortar is easier to use,
and it is also important to prevent impurities from being introduced from the equipment.
The fact that the author actually experienced a failed sample that was instead attracted to a magnet
also demonstrates that sample purity cannot be ignored.

Even if the completed sample is only about the size of a grain of rice,
by cooling it sufficiently with liquid nitrogen and using a small, powerful neodymium magnet,
the characteristic magnetic behavior of a superconductor can be observed visually.
On the other hand, because both high-temperature furnaces and liquid nitrogen can cause serious accidents,
the experiment must be performed under the safety management of a university or research institution.

References

• R. Jack Hanni, Crystal Synthesis of Selected Quantum Materials,
  Brigham Young University–Idaho, 2021.
  For YBCO, it describes a method in which the material is ground in an agate mortar for 30 minutes,
  heated to 920°C over 5 hours, held for 10 hours, cooled over approximately 10 hours,
  then reground and pelletized before being refired using the same temperature profile.
• University of Wisconsin–Madison Environment, Health & Safety,
  Cryogenic Liquids.
  It explains the hazards of cryogenic liquids, including liquid nitrogen, such as frostbite, cold burns, and oxygen deficiency.
• University of California, Berkeley, Department of Chemistry,
  Safe Handling of Cryogenic Liquids.
  It explains that even brief contact with cryogenic liquids can cause tissue damage,
  as well as the serious injuries that can result from prolonged contact.
• Central Japan Railway Company (JR Central),
  SUPERCONDUCTING MAGLEV (SCMAGLEV).
  It explains that niobium-titanium alloy is used in the coils of the onboard superconducting magnets,
  and that they are cooled to extremely low temperatures using liquid helium.
• U.S. Department of Energy,
  DOE Explains…Superconductivity.
  It explains the basics of zero electrical resistance and magnetic-field exclusion (the Meissner effect) below the critical temperature.
• Funakoshi Co., Ltd.,
  Mortar Made of Agate and Quartz Glass.
  It introduces agate mortars as having a polished finish and being used to improve sample recovery.