Field emission is what happens when an external electric field pulls electrons out of a solid material. This phenomenon is at the heart of applications that range from advanced research to industry to defense. A few examples include:
- Electron microscopy: Electron microscope images are generated by a beam of free electrons produced by field emission. The images come from the electrons reflecting off of tiny structures, just as the images from a normal microscope or video camera come from reflected light.
- Displaying information: You can use field emission to light up a screen or video display in a controlled pattern. The display will light up in areas struck by electrons and remain dark in areas where the current is not flowing.
- Discharging unwanted voltages: Electronic charges can build up on the surfaces of spacecraft or satellites. If the structure does not safely disperse the charge, this can cause malfunction and damage. Field emission can neutralize this charge safely before it does damage.
- Ablating material: You can use field emission to purposely destroy material with a beam of high-energy electrons. The arc discharge method for making carbon nanotubes (CNTs) is an example of this. In arc discharge synthesis, electrons directed at a carbon source knock carbon atoms free. These free-flying carbon atoms re-assemble in the form of CNTs.
- Creating radiation of a controlled wavelength: You can use field emission to generate electromagnetic waves with a controlled frequency. This is a useful way to generate x-rays, microwaves or other types of radiation.
In this post we will give an overview of how field emission works. We will then get into why Galvorn CNT fibers are such an effective field emitter. If you're already familiar with the basics, click here to skip ahead to the "why Galvorn is so great" part. 🙂
What is field emission?
All material is made up of atoms. In each atom, negatively charged electrons orbit the positively charged protons in the atom's nucleus. The protons attract the electrons, binding the atom together. Field emission occurs when another source of positive charge creates an even stronger source of attraction.
Field emission can occur in any kind of material, even materials that are not good conductors of electricity. Insulating materials still have electrons, and a strong enough external electric field can strip them away.
What do we mean by "localized" electrons?
Atoms make up all solid material and they can hold themselves together in several different ways.
Non-conducting materials stay together in one of two ways:
- “Covalent” bonds, in which two neighboring atoms share one or more electrons
- “Ionic” bonds, in which an atom with a slight excess of positive charge steals one or two electrons from other nearby atoms. This leads to a positive ion and a negative ion which will tend to attract one another and remain bonded.
Normally, electrons stay closely bound to a single positively charged atomic nucleus. In a covalent bond, expand their range to orbit around two or three nuclei. This creates a more favorable (lower) energy state, which makes the bond hard to break.
Metals put a slight twist on this picture, and this twist is what makes them good conductors of electricity. The atoms of metallic elements have electrons with orbits that can expand to cover many other nearby atoms. When identical metallic atoms are together, these shared “conduction” electrons are able to hop back and forth between them. The electrons are, in a sense, free to roam around.
Now, zoom out and consider a larger chunk of material made from atoms that behave this way. Each atom adds a few more conduction electrons. The atoms wind up sharing these conduction electrons across the entire structure.
In a sense, this metallic material contains a continuous "ocean" of electrons. A few electrons stay tightly bound to each atomic nucleus, but the conduction electrons are able to move throughout the structure.
This phenomenon is what allows metal wires to have current flow through them. If you apply an electric field, you can create a net movement of the conduction electrons.
Now, in a manner of speaking, the electrons in this ocean hate each other, but they love the protons in the atomic nuclei. The conduction electrons, which are free to move around, will repel each other and try to get away from each other. The nuclei attract them, and the nuclei cannot move out of the material (at least, not easily). So, the electrons will spread out as far as they can, but they will stay within the bounds of the material… normally…
What is the "work function"?
Again, field emission is when an electric field pulls electrons away from (out of) a material. We call the energy required for an electron to escape the material the "work function" of that material.
In any conductor, there are:
- A group of electrons that are tightly bound to the protons in the atomic nuclei.
- A group of conduction electrons that are able to move around freely, not bound to any particular proton. These electrons are a bit higher up in energy.
- Some possible states that electrons could get into if you pour more energy into them. If you bombard those electrons with radiation, for example... These states have higher energy, but not enough energy to escape the overall pull of the nuclei.
All these electrons and electron states exist within an energy barrier that represents the edge of the material. None of the electrons can get too far away from the protons that attract them. At the edge of the material there is an energy "hill" that electrons simply cannot get over.
However, what if you apply an external electric field to the material? The barrier is no longer an infinitely high wall; instead, it takes the form of a triangular barrier.
Outside the material there is a drop in the electrical potential energy. If any electrons can get through the barrier, they will “roll downhill” down this potential energy slope. In other words, they will shoot away from the material surface and toward the source of that electric field. But first, the electrons must get through the barrier.
As the external electric field strengthens, the potential energy outside the material becomes steeper. It makes the triangular barrier both shorter and thinner.
The “work function” represents the difference in potential energy between two states:
- The potential energy in the highest energy state that electrons are currently occupying.
- The potential energy in the energy state where electrons can get to the top of the barrier.
In other words, the work function is the work required to pull electrons out of the material.
However, field emission is essentially a quantum mechanics phenomenon. The electrons aren’t going “over” this energy barrier; that would require them to gain energy from somewhere. The electric field isn’t giving them energy, it’s helping them convert potential energy into kinetic energy.
The electrons are able to go through the barrier because the strong external field has made that barrier "thin" enough. This is an example of quantum tunneling.
The Two Types of Field Emission
The two basic kinds of field emission are:
- Thermionic Emission: This involves heating the material to raise the energy level of the electrons. Exciting electrons to higher energy levels helps them move past the barrier more easily.
“Cold” field emission: This type of process uses electrons in their natural (i.e. room temperature) energy state. It means you need to use larger electric fields to get the electrons through the energy barrier. The advantage of this approach is that you have more control over the kinetic energy in the emitted electrons.
With a beam of electrons that have similar energies, you can more easily predict the following:
- What speeds the electrons will have.
- The directions in which the electrons will move.
- How they will interact with other materials.
How do we make field emission happen?
Field emission requires a voltage source (energy input).
In a standard electric circuit, the voltage source pushes the electrons over an energy barrier. Electrons move in a loop around the circuit, similar to a water pump pushing water up and then letting it flow back down. The electrons pushed into the portion of the circuit near the voltage source all want to get away from each other, so they move back around the circuit and create a flow of electric current.
Step 1: Introduce a break in the circuit
The first step in creating a field emitter is to introduce a break in the circuit while keeping the voltage source active. The voltage source will continue to move electrons to one side of the circuit, causing negative charge to build up on that side. Meanwhile, because electrons are leaving the far side of the circuit, that side becomes positively charged.
Those charges will build up until there is sufficient pushback from the built-up charges. Eventually the difficulty of moving more electrons will balance out the power of the voltage source.
Step 2: Create a strong electric field
Next, we need to create a strong electric field between the positive and negative sides of the circuit. If we introduce two parallel plates near the break in the circuit, charge will accumulate within those plates. This will create a strong, uniform electric field between the two sides.
The positively charged plate is the anode, and the negatively charged plate is the cathode.
The electric field between the two plates puts an attractive force on the electrons that are gathering in the cathode. This will lead to field emission of those electrons, if that electric field becomes strong enough.
The interesting wrinkle: surface geometry matters!
Here's when field emission design gets interesting (and challenging!): The strength of the electric field felt by electrons on the cathode's surface depends greatly on the shape of that surface.
Pointed or raised structures will experience a much stronger electric field at their tips. This is not just because they are physically closer to the other plate; the sharper the tip, the stronger the electric field will be. This phenomenon described by a parameter called the "field enhancement factor." The geometry of the surface determines how strong the field enhancement will be.
You can achieve field emission with lower voltage sources if the field enhancement factor is strong. Generally, the best field emission comes from a cathode surface with spikes or tips that are as thin as possible.
Sharp geometries are hard to maintain with high current (it gets HOT!)
The thin, sharp shapes that we like for field emission create a problem, however.
In applications with high field emission current, a LOT of electrons are going to flow out through the surface. They will all flow through those thin tips. This creates a high density of electric current, which in turn creates heat.
Keeping these sharp tips in good condition can get difficult if the field emission current is high. If the cathode is metal it might melt. If it’s a carbon source, or some other structure that does not melt easily, it could burst. Maintaining this kind of geometry with high current applications is challenging.
See below, a Silicon Carbide (SiC) Field Emitter made by NIST in 2013.
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By National Institute of Standards and Technology - Silicon Carbide Field Emitter, Public Domain, Link
Galvorn CNT fibers are delivering on the field emission promise of CNTs
People have been interested in using carbon nanotubes in cathodes for field emission for some time. The durability and stiffness of CNT molecules make them suitable for thin field emission structures. Because of their electrical and thermal conductivity, CNTs have a suitably low work function (comparable to metals). They have good thermal stability compared to metals, and won't melt in high heat.
Historically, researchers have used CNTs as individual emitters or assembled them into arrays. Atomic Force Microscopy (AFM) can arrange individual CNTs on a surface. However, this is a time consuming and complex process.
Producing Galvorn is an efficient, scalable way to obtain large surface areas of well-aligned CNTs. At first, the structure of Galvorn might seem like a drawback for field emission. The CNTs in Galvorn align with each other in the plane of the material, and the surface of Galvorn fiber and films is smooth. However, it turns out the field emission current itself actually creates the critical tall thin geometric features.
DexMat and AFRL are discovering key benefits with Galvorn CNT cathodes
DexMat has been working with Air Force Research Laboratory (AFRL) on field emission applications for some years now. We have discovered some interesting benefits using a high performance material like Galvorn.
Galvorn has pretty incredible mechanical properties, including conductivity and high tensile strength. As a form of carbon, it does not burn or melt and can therefore withstand high temperatures. Also, the CNTs in Galvorn fabric or film, have an aligned structure that can be beneficial for field emission.
When you lay the material down initially, there are no tall, pointy structures coming off the surface. Within the material, the CNTs run parallel to the surface.
However, once you charge up the device with voltages sufficient for field emission, it creates bursts in the material. Electric charge builds up in the skin of the fibers; negatively charged CNT bundles repel away from one another. Eventually these bundles will peel away from the surface, leading to an exfoliated, fibrillated structure.
Image Source: Advanced space charge limited field emission cathodes
"(a) (Top left): Overview of a site at the top layer, representing the level of structural degradation at ∼3000 shots. (b) (Top right): View of the surface layer where Coulomb explosions have collected, leaving a roughened surface. (c) (Bottom left): View of the second layer, where ruptures are typically individual, although some small collections are present. (d) (Bottom right): Close view of an individual rupture site showing its scale and jagged geometry."
This fibrillated structure, with many thin CNT bundle tips is ultimately what enables great field emission from Galvorn.
Typically, you create this field emission in a vacuum. Otherwise things would get in the way of the electrons. Because there’s no oxygen around, the current can go even higher. But if the current reaches a certain point, theoretically the thermal energy will eventually degrade.
If these fibrils burn out, a lot of charge simply builds up in a different spot, which then fibrillates. As a result, we're seeing that Galvorn carbon nanotube fibers make for a pretty durable cathode source.
Try Galvorn in Your Own Field Emission Application
If you are doing field emission work and would like to try Galvorn, you can purchase samples from our online store. Our team of experts is available to discuss your application goals. I and the rest of our team of experts will be happy to advise on what Galvorn product will best suit your needs.