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Hirt 1 Impact of Additives on Thermionic Cathodes a Thesis

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Hirt 1 Impact of Additives on Thermionic Cathodes a Thesis Hirt 1 Impact of Additives on Thermionic Cathodes A Thesis Presented to the Honors Tutorial College Ohio University In Partial Fulfillment Of the requirements for Graduation From the Honors Tutorial College with the degree of Bachelor of Science in Physics By Benjamin Hirt May 2018 Hirt 2 This thesis has been approved by The Honors Tutorial College and the Department of Physics and Astronomy Dr. Martin Kordesch Professor, Department of Physics and Astronomy Thesis Adviser Dr. David Drabold Director of Studies, Honors Tutorial College Physics Dr. Cary Frith Dean, Honors Tutorial College Hirt 3 Table of Contents Abstract.......................................................................................................................4 1.0 Introduction ...................................................................................................5 1.1 Electrons in Solids.............................................................................................5 1.2 Free Electron Model...........................................................................................6 1.3 Tunneling...........................................................................................................8 1.4 Vacuum equipment.............................................................................................9 1.5 History of Cathodes.........................................................................................13 1.6 Emission Regions.............................................................................................15 1.7 Mean Free Path................................................................................................17 1.8 Thermionic Cathodes.......................................................................................19 1.9 Plan for thesis...................................................................................................24 2.0 Materials and Methods.......................................................................25 2.1 Setup.................................................................................................................25 2.2 Measurements..................................................................................................29 2.3 Materials..........................................................................................................32 3.0 Results................................................................................................................33 3.1 Oxide Heater....................................................................................................33 3.2 Fowler-Nordheim.............................................................................................36 3.3 Additives..........................................................................................................40 3.4 Discussion........................................................................................................43 4.0 Conclusion- ...................................................................................................43 5.0 Afterwards, Imaging the Surface...............................................44 5.1 Magnetic Lensing Theory................................................................................44 5.2 Methods............................................................................................................45 5.3 Results..............................................................................................................54 5.4 Discussion........................................................................................................57 5.5 Conclusion.......................................................................................................57 Acknowledgments.............................................................................................59 References-..............................................................................................................60 Hirt 4 Abstract High frequency electromagnetic waves are essential for communications through earth's atmosphere for things such as missiles and satellites. While semi-conductor materials have replaced many uses of thermionic cathodes, semi-conductors cannot be used to create high frequency communications which are essential for national security. The current best thermionic cathodes, scandate cathodes, have reproducibility issues and are not well understood with regards to how the cathodes achieve low work functions that make them good electron emitters. I attempted to learn more about how these thermionic cathodes establish low work function surfaces by looking at the electron emission of cathodes with and without additives by measuring current vs voltage curves. I found that the cathodes with additives had better electron emission at similar temperatures which means that they have lower work functions. Hirt 5 1.0 Introduction Satellite communications are essential for modern day civilization. From GPS to long distance communications, society uses satellites in a multitude of ways. High frequency communications are essential for satellites and missiles because high frequency electromagnetic radiation penetrates the earth's atmosphere. Vacuum cathodes can be used to create high frequency communications.1 Thermionic cathodes are essential for satellite operation and therefore there is an interest in making the cathodes last longer and work more efficiently. 1.1 Electrons in Solids How the electrons behave in a material determines many different properties of that material. Generally what determines good conductivity is how easily electrons can change energy levels, where energy levels being closer together (in energy) allows electrons to move more easily between levels. The easier the movement between energy levels is, the better the conductivity. The most likely electrons to move are the lowest binding energy ones, the ones in the valence and conduction bands. The valance band is the highest level electrons will fill at 0K. The conduction band is the energy level right above that. The position of these two bands in energy level depends on what type of material is under consideration. In metals the conduction and valence bands overlap, in semi-conductors they are close, and in insulators they are far apart.2 The placement of the valence band and the conduction band determine many other properties that are related to electrons, such as how much energy is required to knock an electron out of a solid into vacuum, and to continue doing that with more electrons, which is the process of electron Hirt 6 emission. Figure 1: Electron energy levels in different materials 1.2 Free Electron Model In metals, we can treat the outermost valence electrons to be relatively shielded from the positively charged ions in the metal by the other electrons closer in. But the electrons are still trapped in a potential at the surface, which is known as the work function, and they cannot escape from the positive ions. The outermost electrons are still bound to the metal, but the electrons can move about freely, which accounts for many properties of metals, such as their electrical, thermal, and optical properties. The allowed energies in this region are closely spaced. The outermost electrons in a metal are best to be thought as not belonging to an atom, but instead the whole crystal.2 Hirt 7 If we were to think of atoms in a solid, the nuclei would be stationary and positive. If we were to solve the Schrodinger equation for the electrons in these solids, we would find that the solutions are only solvable in some regions. This leaves some electron energy levels with no solutions (so no electrons are at those energy levels), which are known as gaps, and other regions where solutions are possible, and they are known as bands3. The Pauli exclusion principle states that no two electrons in the same system can have the same energy state as another electron. Electrons fill the lowest energy levels first in an atom, and then because the electrons can't share the lower energies, they have to move up to the higher levels that are less tightly bound to the solid (low binding energy). If we were to fill an atom full of electrons at 0K, the highest energy to which the electrons are filled is called the Fermi energy (Ef). Pauli's exclusion principle means that two free electrons in the crystal cannot be at the same energy state. Energy bands result from the overlap of the levels of each individual atom due to the large number of atoms in a solid (on the order of 1023). This is why we think of energy bands instead of discrete energy levels.2 The Fermi Distribution function for free electrons gives the probability of finding an electron at a certain energy E: 1 f (E)= ((e((E−Ef )/ kT ))+1) Where f(E) is the probability of that energy or higher, E is the energy, Ef is the Fermi energy, k is the Boltzmann constant, and T is temperature.2 Using the Fermi Distribution, we can find out how likely we are to find an electron at a certain energy level given a temperature. Hirt 8 1.3 Tunneling Electrons exists as waves and particles, and act strangely compared to what we normally observe how the world works. In quantum mechanics, there is a chance that a particle can “tunnel” through a barrier if the particle has enough energy to be in the next region over past the barrier. Tunneling means that you don't actually need all the energy required to get over a barrier in quantum mechanics, just enough to exists past the barrier. You
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