Vacuum Baking
Foiled! NIST Unwraps What’s Baking Under Aluminum in Its Labs
April 5, 2023
By: Megan King
It’s National Bake Week here in the U.S. If you’re celebrating in your own kitchen, you may find yourself needing some aluminum foil. Whether you need a sheet of foil to keep that apple pie warm or you’re making something in an aluminum foil baking pan, foil has lots of uses in a baker’s kitchen.
Here at NIST, we do a different kind of baking, but we often use the same foil you use in your own kitchen. You won’t see our creations on The Great British Baking Show, and you definitely don’t want to eat them.
We often use foil in vacuum systems. Vacuums are spaces devoid of matter, in which air and other gases are almost completely removed. (Think about the phrase “done in a vacuum,” or to do something without any outside influences.) Vacuum chambers create pristine and highly controlled environments, so you can do things such as operating a world-class clock or studying the properties of a computer chip, without outside particles interfering.
NIST’s Flagship Clock Has to Bake Before It Can Tell Time
More than 20 years ago, the NIST-F1 cesium atomic clock became the instrument defining the second, and it helped contribute to the calibration of official U.S. civilian time and Coordinated Universal Time. These types of clocks are known as fountain clocks because they use a fountain-like movement of cesium atoms to measure frequency and time interval. Today, the NIST-F4 is slated to become the newest fountain clock at NIST. One day, it may become the nation’s reference standard for the SI second.
Before it can start ticking, the NIST-F4 must meet requirements for final pressure within its vacuum systems. This requires heat, or baking, as we call it. But — just like baking in your kitchen — everything has to heat up evenly across the clock. If one part doesn’t heat up uniformly, the temperature variations put stress on fragile components of the clock, and it could break.
The NIST-F4 is large, about 2 meters (or around 80 inches) tall. So, it would be difficult for us to build an oven to bake it in. Instead, our researchers wrap it with flexible electric heaters that deliver the heat. Aluminum foil keeps the heat in and allows it to remain uniform during the baking process.
Why foil? Well, aluminum foil is safe, easy to use, inexpensive and effective, said NIST researcher Vladislav Gerginov.
Gerginov used about 100 feet of foil to cover the clock for its weekslong baking process. He often uses standard kitchen foil. In some cases, he has to use a thicker foil than you’d use at home.
“It is used for the same reason as in quiche baking — to keep the whole system at similar temperature so that no part gets overheated while another stays cold,” Gerginov said. “It keeps the air flow minimal, conducts temperatures better, and is flexible, so we can wrap it around the complicated structure of the vacuum system.”
Enriching Silicon Needs Foil, Just Like Baking a Potato
Researchers in Josh Pomeroy’s laboratory are creating highly enriched silicon for quantum computing. Quantum computing uses the rules of quantum mechanics to solve complex problems.
NIST’s silicon enrichment apparatus has foil wrapped around its vacuum chambers. The chambers generate clean conditions, and to do this, they need to get really hot. To keep the vacuum system running optimally, the chamber must be heated (or baked) to 150 to 200 degrees Celsius (or about 400 degrees Fahrenheit).
Pomeroy and his team wrap heating tapes around the stainless-steel chambers, but stainless steel doesn’t spread heat out very well. Aluminum foil, however, does.
So, the chambers are wrapped in foil to keep the heat focused on the chamber and not in the rest of the room and to keep the heat even, without any cold spots or hot spots. It’s the same reason you wrap a potato in foil to bake it!
The foil stays on all the time unless researchers need to remove it to work on something underneath. If there are many repairs or developments, the lab may use many rolls of foil in a year. Otherwise, they leave it on as long as possible, and foil lasts for a very long time.
Baking Conduits for Tiny Neutrons Requires Aluminum to Manage the Heat
At NIST, we study and use subatomic particles known as neutrons. Neutrons have a mass similar to protons, but they do not have an electric charge. Neutron research is vital to helping advance our understanding of the universe. Neutrons are also used to study materials, leading to everyday applications, such as creating more fuel-efficient cars and learning more about biomolecules that could lead to new medicines.
Neutrons show you things at subatomic sizes that you would not otherwise be able to see. Neutrons can show you hydrogen within materials such as fuel cells, similar to the way an X-ray can show a doctor your bones.
At the NIST Center for Neutron Research, neutrons often travel through a 60-meter-long guide tube. This tube has the air removed from it to prevent it from impeding the flow of neutrons.
At the end of the guide tube is a scientific instrument we use for experiments. That apparatus is periodically wrapped in aluminum foil and electric heaters to allow its metallic walls to “bake” efficiently, removing residual gas molecules. When it cools down, the vacuum inside the apparatus is improved, allowing the instruments to work better. This is important because otherwise neutrons can interact with any residual gas that lingers inside the apparatus, and that could lead to the loss of neutrons we need for research, said NIST researcher Scott Dewey.
But that’s not the only place where aluminum plays an important supporting role in this research.
Special windows have to be placed at the beginning and end of the apparatus to keep the air out and allow neutrons in. If air got into the instrument, the neutrons would run into the air. This would scatter or absorb the neutrons, making them unavailable to work with.
The most effective material to seal the scientific instrument during this process is an aluminum can, the kind that holds soda (or pop, depending on where you are reading this).
Soda cans are a perfect material for this work because they’re very lightweight and yet also extremely strong. Soda companies have had a strong incentive to keep the can strong but also lightweight for transporting it. Dewey calls the cans “miracle material” in the lab.
Dewey and his fellow researchers use empty soda cans, which are made up almost entirely of aluminum, to serve as a protective “window” around the neutron apparatus. Dewey confirmed he does not drink the soda beforehand!
First, researchers flatten the can. Then, they buff it to remove all the paint and then shape it into a disk. Then they have to add holes to the disk in the appropriate spots for attaching to the apparatus. It’s tricky work because the cans cannot be wrinkled, or they won’t stand up to having atmospheric pressure on one side and the vacuum on the other.
Once affixed, it’s the soda cans’ job to keep air out of the apparatus. That’s something that properly crushed aluminum cans are perfectly suited to do.
Foil Keeps Everything Clean in a Room Where Everything Has to Be Really Clean
In a lab at NIST where we research very tiny things (billionths of a meter), everything has to be perfectly clean. So much so that researchers don head-to-toe “bunny suits” to work in the room, so no dirt or dust from the outside world can get in. When working with nanotechnology, very small tech that is used in computer chips and other uses in our everyday lives, we use some specialized equipment that also has to stay clean. Enter: foil.
At the very beginning of many nanofabrication processes, researchers put aluminum foil inside a circular piece of equipment known as a spin coater. The spin coaters coat a synthetic substance sensitive to light, called a photosensitive polymer, on a base to form a uniform film. Because it’s spinning like an old-fashioned record, researchers poke a hole in the middle of the foil to allow the spin coater’s interior to fully function, while the rest of it is covered in foil.
The foil captures the polymer residue that flies off the edge of the base as it spins rapidly. The spin coaters’ users can simply take it out and dispose of it. It minimizes the amount of time the equipment is down for cleaning. They use foil for the same reason you put it on a baking sheet — to reduce time and effort spent cleaning!
An additional piece of equipment, the electron beam evaporator in the lab, helps to create a metal or oxide thin film for various experiments. During the evaporation, electrons generated from tungsten filaments at high voltage melt a sample of solid metal or oxide inside the evaporator. Vapor from the melted materials condenses on the base to form the thin film.
However, the vapor also condenses on the walls of the evaporator. To minimize the cleaning time, shields wrapped with aluminum foil are installed inside. The foil catches materials researchers use for their experiments such as tungsten, platinum, gold, chrome, and nickel.
Each month or so, maintenance staff change out the foil, keeping the inside of the equipment clean.
ABOUT THE AUTHOR
Megan King
Megan King is a writer-editor at NIST and edits the Taking Measure blog. After graduating from John Carroll University, she began her career as a newspaper journalist, covering county fairs and school...
https://www.nist.gov/blogs/taking-measure/foiled-nist-unwraps-whats-baking-under-aluminum-its-labs