Table of Contents

• Introduction
• Characteristics of Liquid Nitrogen
• Teacher's Guide to Handling and Storing Liquid Nitrogen
• Materials Needed for Experiments / Demonstrations
• Student Information Section
  • Pre test
  • Temperature Scales
  • Safety
  • Activity 1: Cryogenics and Common Materials
  • Worksheet #1
  • Activity 2: Resistance, Levitation, and Superconductivity
  • Worksheet #2
  • Post Test
• Appendix A: Sources
• Appendix B: Answers to Pre Test and Post Test
• Acknowledgments

Introduction

In 1911 Kamerlingh Onnes discovered superconductivity in mercury. Since then scientists have been trying to explain this strange phenomenon. Additional applications are being sought to take advantage of the remarkable power superconductivity holds.

The future of science lies in interesting young people in exploring the yet unexplained. As teachers we must compete with the "television" generation and present interesting new concepts in the classroom. Superconductivity and cryogenics may not be commonly explored in the middle school/junior high curriculum. This booklet presents you with some basic information to supplement the material presented in the accompanying video tape. The activities are designed for groups of one to four students. If small groups are difficult to organize in your situation, they can also be used for class presentation.

I hope that you find information that is useful in the classroom and the laboratory. Many of the questions proposed to the student are open-ended and may result in a variety of answers. If the information provided challenges the student to ask why, then superconductivity and cryogenics can open a new generation to one of the wonders of science.

Note: As with many materials used in science classes, using liquid nitrogen has some inherent dangers. You should read the section on handling and storing liquid nitrogen carefully and experiment with it before distributing it to students. You may also want to consider its dangers before organizing groups.

An excellent booklet "A Teacher's Guide to Superconductivity for High School Students" was written by Robert W. Dull and H. Richard Kerchner. It provides a more in-depth look at superconductivity and has an excellent reference section and glossary. A printed copy can be ordered by contacting the National Technical Information Service at the following address:

Be sure to specify order number DE95000551

Your comments and suggestions about this booklet would be most welcome. Please send them to:

Characteristics of Liquid Nitrogen

Liquid nitrogen is manufactured by cooling air to a temperature of 77 Kelvin (K). Nitrogen makes up about 79% of the air we breathe. Liquid nitrogen is very common and easy to obtain, if you know where to look. It is used by hospitals, doctors, universities, factories, welding shops, and cattle breeders.

To obtain liquid nitrogen, look in the yellow pages under Liquid Air or Gas- Industrial, Medical, etc. You may also be able to obtain it from a hospital, dermatologist, welding shop, or clinic. Liquid nitrogen usually costs less than $1 per liter, even in small quantities. A supplier may have a minimum charge of $10 or so for a small quantity because of the loss due to boil-off when filling a small container.

You may notice large trucks transporting liquid nitrogen on the highways. Industries that use large amounts of liquid nitrogen may have their own compressors, liquefiers, and dewars that hold several hundred liters of liquid nitrogen. A dewar is a specially insulated container for storing very cold liquids. It is similar to a vacuum-insulated glass thermos bottle. Large dewars may keep liquid nitrogen for several months. Doctors offices' commonly store liquid nitrogen in 10-20 liter dewars that hold liquid nitrogen for up to a month before it boils away.

You can use a common thermos bottle to store liquid nitrogen for 24-48 hours but only with a vented or partially-sealed lid. Insulated plastic containers may crack due to thermal shock. They can even explode into many small projectiles of frozen plastic.

When first observing liquid nitrogen, you may notice that it seems to be boiling. Since our common experiences tell us that boiling liquids must be hot, we may assume that it is hot also. The first observation is indeed true, the liquid nitrogen is boiling. However, it is not hot, it is extremely cold. Its boiling point is 77 K (- 321°F).

Because liquid nitrogen is boiling and is extremely cold, we must use special precautions when handling and storing liquid nitrogen.

Teacher's Guide to Handling and Storing Liquid Nitrogen

This section contains background information about safety and storage of liquid nitrogen. It contains more detailed information than the student section on safety. This is not intended as a comprehensive guide but will provide you with a reasonable amount of information about the dangers of liquid nitrogen.

Transporting Liquid Nitrogen

Safety Rule 1: Avoid transporting liquid nitrogen in the passenger compartment of the car, if possible.

Rationale: There is no inherent danger in using the passenger compartment. However, if the liquid spills, several things could happen:

1. It could spill in the seat and freeze your skin through your clothes.
   
2. It could freeze and crack the seat, carpet, and other parts of the car (especially plastic parts).
   
3. It could reach your feet and freeze your shoes and/or skin.
   
4. The "fog" from a large spill could obscure your vision.
   
5. If the driver attempts to upright a spilled container, an accident could result from inattention to driving.
   
6. The pure nitrogen gas in a closed passenger compartment could result in a serious oxygen deficiency, even asphyxiation.

Solution: Transport the liquid in the trunk of your car away from materials it could damage if spilled.

Safety Rule 2: Use some method to keep the liquid nitrogen from spilling when transporting it.

Rationale: See Rule #1 for damage that can be caused.

Solution: Store the dewar or thermos in a box that cannot be easily tipped over. Pack around the dewar or thermos with paper or other material to keep it from tipping over. Be sure to use some material that will not be damaged if the liquid nitrogen should spill or slosh out.

Storing Liquid Nitrogen

Safety Rule #1: Never store liquid nitrogen in a closed container.

Rationale: When the liquid is boiling, it is giving off nitrogen gas and can develop extremely high pressures in a closed container. The container will develop enough pressure to explode!

Solution: Be sure the dewar or thermos bottle is properly vented. You can vent an ordinary thermos by drilling a 3/16" to 1/4" hold in the lid and cap. A stopper made from polyurethane foam can also be used as a temporary stopper. This foam is porous and will allow the gas to escape but will help prevent loss if tipped over. If the foam becomes saturated with liquid nitrogen, handle it with gloves until it has all drained or evaporated.

Safety Rule #2: Never store liquid nitrogen in an open container for a long period of time.

Rationale: Nitrogen has a lower boiling point than oxygen. If air can get into the container easily, the nitrogen can cool the air and liquid oxygen could accumulate in the bottom of the container. Before long you could be pouring oxygen from the container. Oxygen is a key component required for combustion, and pure oxygen can make a piece of wood burn explosively.

Solution: Use a container with only a small opening to allow the nitrogen gas to escape or one with a gas-penetrable polyurethane stopper. You may also use a rubber stopper with one hole in it.

Safety Rule #3: Never use liquid nitrogen in a tightly-closed room.

Rationale: The liquid nitrogen could cause oxygen deprivation if used for a long period of time in a tightly-closed room. A spill of 30 liters quickly expands (by a factor of 7/0) to 750 cubic feet.

Solution: Use only with adequate ventilation.

Handling Liquid Nitrogen

Safety Rule#1: Read and understand all safety rules before using liquid nitrogen.

Rationale: Frostbite will occur when liquid nitrogen contacts any part of the body. The longer the contact, the deeper the damage due to frostbite.

Safety Rule #2: Wear safety glasses or safety goggles at all times when using liquid nitrogen.

Rationale: Liquid nitrogen could quickly damage the eyes and eyelids.

Safety Rule #3: Wear appropriate clothing when using liquid nitrogen.

Rationale: Liquid nitrogen and the very cold objects in contact with it must be kept from prolonged contact with the skin. If spilled, it can accumulate in the folds of clothes, shirt tails, shoes, and even some gloves.

Solution: Wear a lab coat when using liquid nitrogen. Long pants can help control lap spills but standing up when using liquid nitrogen is an even better idea.

Safety Rule#4: Avoid all skin contact with liquid nitrogen.

Rationale: The liquid nitrogen and objects cooled in it can cause frostbite.

Solution: Never touch the liquid nitrogen or reach in it to retrieve objects. Gloves should be used when handling objects that have been removed from the liquid nitrogen. The gloves should:

1. Not absorb the liquid easily.
2. Be easy to remove quickly in case some liquid nitrogen is spilled into the glove.
3. Never be immersed in the liquid nitrogen-use plastic tweezers or forceps to retrieve objects from the liquid. Steel forceps are useful for handling heavier objects.

Safety Rule #5: Use care not to drop or cause sudden shock to materials cooled to liquid nitrogen temperatures.

Rationale: Objects cooled to liquid nitrogen temperatures can become brittle. They may shatter and send pieces of material flying or stick to your skin if touched. Avoid common glass and plastic. Remember that if a material shatters like glass, it may also cut like glass.

Solution: Allow any material removed from the liquid nitrogen to warm to room temperature before handling.

Safety Rule #6: Any material cooled in liquid nitrogen should be allowed to warm to room temperature before touching it.

Rationale: Materials, especially metal, will stay very cold for a long time when removed from the liquid nitrogen. If you touch the material with bare skin, you could easily get frostbite.

Solution: Allow materials to warm to room temperature or use gloves, plastic tweezers, or plastic forceps when handling them.

Safety Rule #7: Always have a plan for what the students can or should do should you leave the room.

Materials Needed for Experiments/Demonstrations

The experiments will be divided into two sections: "Cryogenics and Common Materials" and "Resistance, Levitation, and Superconductivity." The materials needed for these experiments are available from several sources (see Appendix A). Colorado Superconductor, Inc. and the other suppliers offer excellent kits for these purposes.

I. Cryogenics and Common Materials

II. Resistance, Levitation, and Superconductivity

OPTIONAL ACTIVITY 1

OPTIONAL ACTIVITY 2

Pre Test

Answer the following questions to the best of your ability. Your answers will not count as part of your grade.

1. Resistance is _____________________________.

2. The scale used to measure very cold temperatures without having to use negative numbers is the _____________________________.

3. The temperature at which atoms lose all of their energy is called _____________________________.

4. Cryogenics is the study of _____________________________.

5. Superconductivity was discovered by _____________________________.

6. When a material is in its superconducting state, _____________________________.

7. Superconductors are now commonly used for _____________________________.

8. Which of the following is coldest? _____________________________.

9. Objects cooled in liquid nitrogen : _____________________________.

10. When a magnet is placed on a superconductor which is in its superconducting state, it will ____________________________.

Temperature Scales

You may remember from one of your science classes that matter exists in three states—solid, liquid, and gas. A fourth state, plasma, has also been discovered and is being studied. With most materials the temperature determines the material's state. For example, if water is below 32°F., it is ice—a solid; above 212°F and it changes to steam—a gas; and of course at room temperature it's a liquid.

One thing we commonly do is heat water until it begins to boil which produces steam. Let's think of the opposite for a minute. How could we change steam to liquid? Cool it, of course. Air conditioners often drip water out the bottom-water that has been condensed (changed from a gas to a liquid) by the cold coils of the air conditioner. To do an experiment we can capture some of the steam in a closed container (being careful not to burn ourselves) and wait for it to cool to room temperature. Or we could put the container in contact with some ice cubes or into the refrigerator to speed the process.

If we put the water in the freezer, it would change to ice. One important thing to remember here is that at room temperature, the water will not burn or freeze us. However, if the water is very hot (steam) or very cold (ice) it can damage our skin. We are going to work with liquid nitrogen which is extremely cold. Even though it may look like water, it can damage our skin severely in a short period of time.

Before we experiment with liquid nitrogen and some very cold items, let's see how we represent these temperatures. Unfortunately, extremely cold temperatures cannot be measured with a common thermometer because it could easily freeze and even break! Instead, these very cold temperatures are measured with electronic devices.

The gases in the air we breathe—oxygen, nitrogen, carbon dioxide, etc. must be made very cold in order for them to change from gases to liquids. If only the Fahrenheit scale of temperature was available, scientists would have to use large negative numbers to talk about these temperatures. They would use -460°F. to represent absolute zero—the coldest temperature we can imagine. Absolute zero is the temperature where atoms lose all of their energy.

Instead of Fahrenheit, scientists use the Kelvin scale. 0 Kelvin is absolute zero and water freezes at 273 K. The room you are sitting in may be about 300 K. The drawing on the following page compares the three standard temperature scales—Fahrenheit, Celsius, and Kelvin.

No one has ever been able to cool a material to absolute zero to see what happens. However, liquid nitrogen (77 K.) and liquid helium (4.2 K.) have several common uses in medicine and science.

About Liquid Nitrogen

You are going to be using liquid nitrogen in your study of cryogenics and superconductivity. Nitrogen makes up about 79% of the air we breath. If ordinary air is cooled to 77 Kelvin, the nitrogen will turn to a liquid.

Liquid nitrogen is used by hospitals, doctors, welding shops, and many other industries. It is even used to do minor surgery on the skin.

It can be very dangerous if not handled properly. Be sure you understand all safety rules before using liquid nitrogen.

When you see liquid nitrogen, it is actually boiling the same way water boils. It just boils at a very cold temperature. Just like boiling water, it can quickly damage the skin. However, it freezes the skin instead of burning it.

Safety

Safety Rule #1: Read and understand all safety rules before using liquid nitrogen.

Safety Rule #2: Wear safety glasses or safety goggles at all times when using liquid nitrogen.

Reason: Liquid nitrogen could quickly and permanently damage the eyes and eyelids.

Safety Rule #3: Wear appropriate clothing when using liquid nitrogen.

Reason: Liquid nitrogen and the very cold objects it produces must be kept from prolonged contact with the skin. If spilled, it can accumulate in the folds of clothes, shirt tails, shoes, and even some gloves.

Solution: Wear a lab coat when using liquid nitrogen. Stand when doing experiments.

Safety Rule #4: Avoid all skin contact with liquid nitrogen.

Reason: The liquid nitrogen and objects cooled in it can cause frostbite.

Solution: Never touch the liquid nitrogen or reach in it to retrieve objects. Gloves should be used when handling objects that have been removed from the liquid nitrogen.

The gloves should:

1. Not absorb the liquid easily.
2. Be easy to remove quickly in case some liquid nitrogen is spilled into the glove.
3. Never be immersed in the liquid nitrogen—use plastic tweezers or plastic forceps to retrieve objects from the liquid. Steel forceps are useful for handling heavier objects.

Safety Rule #5: Use care not to drop objects still cold from the liquid nitrogen.

Reason: Objects cooled to liquid nitrogen temperatures can become brittle. They may shatter and send pieces of material flying.

Safety Rule #6: Any material cooled in liquid nitrogen should be allowed to warm to room temperature before handling.

Reason: Materials, especially metal, will stay very cold for a long time when removed from the liquid nitrogen. If you touch the material with bare skin, they could stick to your skin or freeze your skin.

Solution: Allow materials to warm to room temperature or use gloves, plastic tweezers, or plastic forceps when handling them.

Safety Rule Summary

1. Read and understand all safety rules before using liquid nitrogen.
2. Wear safety glasses or safety goggles at all times when using liquid nitrogen.
3. Wear appropriate clothing when using liquid nitrogen.
4. Avoid all skin contact with liquid nitrogen.
5. Use care not to drop objects still cold from the liquid nitrogen.
6. Any material cooled in liquid nitrogen should be allowed to warm to room temperature before handling.

Activity 1: Cryogenics and Common Materials

Cryogenics is a branch of science that explores production and effects of very cold materials. We know that many plastics and rubber will crack and break at very cold temperatures. The first few activities we will do are designed to help you see what happens when common materials are cooled to liquid nitrogen temperature, 77 K.

Materials needed:

**SAFETY**

You are going to be using liquid nitrogen and handling some very cold materials. Read and understand all safety procedures carefully and get your teachers permission before beginning.

Procedure 1

Many materials, including rubber and most plastics, become very brittle when cooled to liquid nitrogen temperature. You can easily break them while they are still cold. You may want to try similar experiments with other materials. If you use a material that can absorb the liquid nitrogen, handle it in such a way that the liquid can drain onto a safe surface, not on you or your clothes.

Procedure 2

Procedure 3

Worksheet #1

Procedure 1

1. What happened to the cold rubber band when you tried to bend or stretch it?

2. What happened to the flower?

Procedure 2

3. Why do you think the balloon deflated?

4. Where did the air inside the balloon go?

5. When the balloon was deflated you may have noticed some liquid inside the balloon. What do you think the liquid was?

6. What happened to the helium-filled balloon? Why?

Procedure 3

7. Describe how the solder changed when cooled.

Activity 2: Resistance, Levitation, and Superconductivity

Procedure 1

Watch the videotape about superconductivity and answer the questions on Worksheet # 2.

Procedure 2: Resistance Measurements

This procedure is to familiarize you with how to use an ohm meter and what happens to some materials when they are cooled.

Materials needed:

Procedure 3: Current flow in a wire (OPTIONAL)

This procedure demonstrates two things that happen when current flows through a wire: (1) a magnetic field is created and (2) resistance causes heat. It's also valuable to understand that if current flowing through a wire can produce heat, it can also create a fire if a wire's capacity is exceeded.

Materials needed:

Procedure 4: Levitation

The videotape and Procedure 3 demonstrated that current flowing through a wire produces a magnetic field. We are now going to show that a superconducting material is capable of repelling a magnetic field. The reduced supercurrent in a superconductor creates an internal magnetic field exactly opposite of the field of a small magnet placed close to it.

You already know that if you place the North end of a magnet close to the North end of another magnet, they will repel. The same thing happens in a superconducting material. If the magnet is placed on the superconductor, it will levitate above the material. This phenomenon is being explored as a way to produce bearings for motors that have almost no drag (resistance to moving).

Materials needed:

NOTE: A disk or ring of superconducting material may be used for this activity, either should work.

Procedure 5: Superconductor switch (OPTIONAL)

This procedure is designed to show how resistance is greatly reduced when a coil of superconducting material is cooled to liquid nitrogen temperature.

Materials needed:

NOTE: This demonstration requires low-resistance contacts on the superconducting coil. If the switch fails to operate, it may be due to poor contact between the coil and its leads.

Worksheet #2

Procedure 1

1. What is the most common use of superconductors today?

2. How will superconductors be used in transportation?

3. What is electrical resistance?

4. How are magnetic fields and electrical current flow related?

5. What is absolute zero? How many degrees Kelvin? Fahrenheit?

6. Why wear safety glasses?

7. Why can we use only non-metallic tools in the liquid nitrogen?

8. What happens to your skin if liquid nitrogen stays on it too long?

9. What is a superconductor?

10. What happens if you place a magnet on a superconductor (a) at room temperature? (b) when it is in its superconducting state?

Procedure 2

11. In step 10, what was the reading on the ohm meter?

12. In step 12, what was the reading?

13. What were the readings for one carbon resistor?

Warm:
Cold:

14. Why do you think the carbon resistor differed from the copper coil?

Procedure 3 (optional)

15. (a) What happens to the flat wires on the back of the tester when it is touched to the battery?
(b) Why?

16. How does the tester work?

17. What would you expect to happen if you move the compass near an appliance cord with the appliance on?

Procedure 4

18. Why do you think the magnet levitates?

19. Why does it levitate higher when the disk is cooled first without the magnet in place?

Procedure 5

20. What causes the bulb to light in this experiment?

21. Can you think of a practical application for this?

Post Test

Answer the following questions to the best of your ability. Your answers will count as part of your grade.

1. Superconductors are now commonly used for _____________________________.

2. Resistance is _____________________________.

3. Which of the following is coldest? _____________________________.

4. When a material is in its superconducting state, ____________________________.

5. Cryogenics is the study of _____________________________.

6. The scale used to measure very cold temperatures without having to use negative numbers is the _____________________________.

7. Superconductivity was discovered by _____________________________.

8. The temperature at which atoms lose all of their energy is called _____________________________.

9. When a magnet is placed on a superconductor which is in its superconducting state, it will _____________________________.

10. Objects cooled in liquid nitrogen: ____________________________.

Appendix A

Superconductor kits and supplies can be obtained from the following sources:

CeraNova Corporation produces helical coils of YBCO. These coils are useful in laboratory superconductivity demonstrations, especially where high resistances above liquid nitrogen temperatures are needed. Contact CeraNova Corporation, 14 Menfi Way, Hopedale, MA 01747, Telephone or fax: (508) 473-3200

Colorado Superconductor, Inc. sells several superconducting kits which demonstrate the Meissner effect, as well as measurement of T_c, H_c and current density. Contact Colorado Superconductor, Inc., Department F4, P. O. Box 8223, Fort Collins, CO 80526, Telephone: (303) 491-9106

Edmund Scientific sells superconducting ceramic disks for educational laboratory demonstrations. The kit includes a disk of YBa₂Cu₃0₇, holder, instructions, and bibliography. Contact Edmund Scientific, 101 East Gloucester Pike, Barrington, NJ 08007, Telephone: (609) 573-6250

Futurescience, Inc. sells a variety of superconducting kits for classroom demonstration and student use. Kits fit nicely on your bookcase and hold the necessary items. One kit has a videotape with extensive safety content, simple cryogenic demonstrations, and examples of activities that can be performed with other kits. Contact Futurescience, P. O. Box 17179, Colorado Springs, CO 80935, Telephone: (303) 797-2933 or (719) 634-0185, Fax: (719) 633-3438

Sargent-Welch Scientific sells a superconductivity demonstration kit, which includes experiments demonstrating the Meissner effect, zero-resistance and quantum mechanical effects, and the variables of T_c, J_c, and H_c. Contact Sargent-Welch Scientific Company, 7300 North Linden Avenue, Skokie, IL 67007, Telephone: (800) SARGENT

Appendix B

Answers to Pre Test

Answers to Post Test

Acknowledgments

This handbook was written during the summer of 1994 at the Oak Ridge National Laboratory (ORNL) operated by Martin Marietta Energy Systems.

Partial support for this effort at ORNL was provided by the Department of Energy (DOE) Teacher Research Associates Program and by the DOE Office of Energy Management—Superconducting Technology Program.