Electron Emission Introduction:
What is Electron Emission, Types of Electron Emission– The process of emission of electrons from a surface through provision of some specific external energy on the surface of some metal is called electron emission or electronic emission. Remember that energy required to emit electrons from the surfaces of various metals varies (that is electron emission takes place more easily in some metals as compared to some others). The phenomenon of escaping electrons from the metal surface into the surrounding by applying specific external energy is called electron emission. In other words, the process of emitting or receiving electrons through applying some specific procedure on a certain metal is called electron emission. Also, the method of excretion of independent electrons from a metal surface is called electron emission.
We know that every metal is composed of such atoms which are firmly bound in the shape of crystals and electrons of such atoms remain under influence of their respective atoms. (The structure of a metal material wherein metal’s atoms are found in allotted places at specific intervals with a specific arrangement, such a systematic structure of atoms is called crystal or lots). In other words, the existence of atoms of metal in their own specific places forming a pattern with a specific sequence and regular intervals is called crystal). However independent electrons in every metal do not always exist at particular or precise places. These independent electrons always keep moving on the metal in an irregular and more or less independent manner (The electrons which separate from the exterior/outer shell of an atom are called independent or free electrons of the atom). It has to be reminded that flow of current in wires, tubes, valves, and transistors becomes possible due to flow of these independent electrons. These independent electrons moving freely in the last shell of a metal conductor do not get excluded from the metal surface due to the force existing on its surface. Because this surface force prevents the expulsion of electrons from the metal surface at normal temperature (i.e. under normal circumstances independent electrons do not extricate from metal surface and mingle into the air or space surrounding the metal). However, if these independent electrons are provided some additional energy, they absorb it; resultantly their kinetic energy enhances. Thus, these independent electrons overcome the metal surface force and thus emitted from the metal surface. For example, if temperature of a metal conductor is increased too much, every independent electron rotating around outer shell of the metal atom’s surface gets increased energy. As a consequence, the kinetic energy of such independent electrons increases, and these electrons then emit from the metal surface after receiving extra energy. Therefore, the discharge or emission of electrons from the metal surface in such a manner is called electron emission.
The function of an electron tube (radio valve) depends on a stream of electrons, which acts as current carriers. (That’s to say the flow of current occurs due to a consistent flow of electrons). A metal electrode is being provided inside each of the tubes for producing this stream of electrons, which is called an emitter. An emitter is also normally called the cathode. Under normal room temperature, independent electrons in an emitter or conductor move freely on the surface of a metal. However, due to certain restraining forces or surface tension, which acts as a barrier, these independent electrons do not emit and enter into the surrounding atmosphere. (The forces which prevent electrons emission from the metal surface, is called energy barrier or the forces which restrict electrons inside the metal surface instead of letting them out, are called restraining force or energy barriers). In order to overcome the restraining force prevalent on the surface of a metal, electrons require definite energy, so that they can easily emit from an emitter’s surface. As kinetic energy generated by these electrons is insufficient, this specific energy is catered from a definite outside source. After absorption of exterior energy, the kinetic energy of these independent electrons increases to a reasonable limit, and as a result, they are able to breach the energy barrier on the metals’ surface and thus emit easily. The emission of electrons from a metal surface into the surrounding metal atmosphere in such a manner is called electron emission.
At absolute zero-degree temperature, the maximum quantity of explicit energy, which is absorbed by electrons and then emitted on the metal surface, is called the work function of that metal. In other words, work function is the quantity of work that is necessary for emission of electrons from a metal surface. Work function is reflected by Ew or ɸ and its unit is electron volt eV, which is equivalent to the quantity of work, resulting from movement produced through one volt-potential difference. Or energy receivable by an electron on a one-volt potential difference is called one electron volt. And its value is equivalent to 1.602×10-19joule. It is to be remembered that the work function of various materials is different.
Normally, Tungsten, Thoriated Tungsten, Oxide Tungsten and Oxide Coated materials are mostly used for electron emission. The efficiency of Oxide coated Tungsten is the highest and its life span is also the longest i.e. 20,000 hours. Moreover, this material is most suitable for low voltage tubes (cathode ray tubes). Tungsten is used in x-ray tubes, where operating temperature and operating voltage are too high (2500K⁰ and 50000 volts respectively). Thoriated Tungsten is a better emitter material, in which a small quantity (a few percent) of Thorium Oxide is combined with tungsten. It is used in high-powered tubes. An emitter with better efficiency and low work function is considered to be a good one. It should be mechanically and technically sound and have a high melting point.
Types of Electron Emission
The supplementary energy required for the emission of electrons from a metal surface can be obtained through any of the following methods. In other words, the following different methods can be resorted to for superfluous energy required for electron emission.
- Thermionic or Primary Emission
- Secondary Emission
- Photo Electric Emission
- Field Emission
All these methods of electron emission are applied in different electronic appliances. However, the Thermal or Thermionic method of emission is the most important one of all these methods. The application of this method is more common in electronic tubes.
The details of electron emission mentioned above are as below:
Thermionic (Primary) Emission
This is most widely used for electron emission. In this method, emitter metal or electron emitting object is heated, due to which the thermal energy or kinetic energy of free electrons present on the surface of metal magnifies. Thus, due to an increase in the energy of these electrons, the emission rate from the emitter surface also increases substantially. The quantity of electrons emission from the metal surface depends on metal temperature.
This method of electron emission from a blistering metal resembles a great extent to the method of liquid evaporation in which vapors emit from the surface of the liquid. When a liquid is warmed, its molecules convert into vapors by getting sufficient energy in order to prevail over the restraining force of the liquid. The quantity of molecules turning into vapors increases as the temperature of the liquid is raised. Similarly, when a piece of metal or metal body is warmed to a specific temperature (2500⁰ C) by placing it in tubes with an air vacuum or filled with inert gases (this metal piece is called cathode or emitter), free electrons on the metal surface get enough energy so as to emit from the surface through overpowering restraining force. It is to be remembered that this task is performed in a vacuum tube so that air molecules do not interfere with the process. In order to enhance electron emission, sometimes some gas is filled in the tube. Heat energy is provided via passing current from emitters or cathodes made with high resistance wire. The number of electrons emitted from the metal surface depends on temperature. Electron emission increases as temperature increases. The thermionic emitter is made from low work function material in order to achieve higher efficiency; so that material could be operated on low temperature as well (i.e. secondary emission could be initiated)
A moving particle striking with a hard metal surface provides enough energy to electrons making it possible to emit through breaking energy barrier on the metal surface. Such electrons are called secondary electrons. In secondary emission, electrons on a metal surface emit through ion or mechanical collision of electrons. The bombardment of high-velocity ions on a metal surface resulting in the emission of electrons from the metal surface is called secondary emission. Or the process of striking fast-moving electrons on a metal surface resulting in the emission of electrons from the surface is called secondary emission. In other words, high-velocity bombardment is done on some metal surface (cathode), resulting in the emission of electrons from the cathode surface, which is called secondary emission. When high-speed electrons suddenly bombard the metal surfaces, they transfer their high-speed kinetic energy to atoms and electrons found on the surface, striking it. Some of the high speed or bombardment electrons strike the metal surface in a direct collision with free electrons emitting it from the metal surface. In this method, electrons that bombard the metal surface are called primary electrons while the electrons which get free as a result of this collision with the metal surface, are called secondary emission electrons. However, it is important to remember for this end that the availability of primary electrons from some other emitter is very essential so that the objective of secondary emission could be achieved through the bombardment of the same with the secondary emitting surface.
The velocity of electrons emitted from a metal surface as a result of secondary emission is normally low. And such electrons are unnecessary. Further, the metal surface is also influenced due to the bombardment of primary electrons. A carbon layer is put on the metal surface to protect it from unnecessary secondary emissions. (clear graphite is used for this purpose). Thus, very few secondary electrons can emit from this carbon shrouded layer surface). It must be remembered that secondary emission does not depend on temperature.
The metal surface condition and low work function are very crucial for secondary emission, which must necessarily be inculcated. Besides, the secondary emission also depends on the following:
- Energy from Primary Electrons
- Emitting Material
- Mass of Bombarded Molecules
- Type of Primary Molecules
- Surface Condition of Target
- Bombardment Angle on Target
Although secondary emission is an accidental condition, and it is usually considered unsuitable for electric appliances, however, it is at the same time considered important and beneficial in several electronic gadgets such as cathode ray tubes, electron multiplier tubes, radar storage tubes & electronic computers, etc. Undoubtedly, secondary emission does not influence the process of diodes, however vacuum tubes wherein two or more electrodes are found, it has an important bearing on its mechanism. In fact, secondary emission has been responsible for cathode active resistance characteristics.
The emission of electrons from a metal surface due to the impact of light is referred to as Photo-Electric Emission and it is used in phototubes. When light rays fall on sensitive metals or specific metal materials (e.g. Potassium, Radium, Sodium, Lithium, Lepidus, etc.), some of its free electrons absorb light energy for getting light, due to which their kinetic energy increases. As a consequence, the emission of electrons begins from such surfaces. The emission of electrons in this way is called photoelectric emission. In other words, when light falls on a particular piece of metal under the process of photoelectric emission, the energy available in light gets transferred to free electrons found in the metal, due to which their kinetic energy or velocity increases quite significantly. Consequently, these electrons start emitting from the metal surface. This method of emission of electrons through reflecting light rays on a metal surface is called photoelectric emission. Photoelectric emission depends on the following:
- Electrons that are equivalent directly to the proportion of light intensity falling on a particular piece of metal surface or emitter surface are emitted.
- Radiation frequency
- Emission starts when light falls on a metal surface and stops when reflection of light blocks
- A definite light frequency is required for metals with specific surfaces. Electron emission does not occur with a frequency less than this particular frequency.
- The maximum quantity of kinetic energy released by electrons for light purposes from sensitive metal surfaces is independent of light flex.
It is to be bear in mind that a semi-cylindrical shape is designed for cathode or emitter for photoelectric emission so that more and more light could fall on it. A particular frequency, on which the process of electronic emission kicks off when light falls on a particular metal surface irrespective of light intensity, is called threshold frequency of that surface. As threshold frequency changes from the ultraviolet region to the infra-red region for different metal surfaces, therefore a light frequency less than threshold frequency cannot initiate the process of photoelectric emission.
Photoelectric emission was first discovered by Hertz. In 1965, Ian Steen presented the theory of light energy which is traditionally known as quantum theory. According to this theory, light in a photoemissive cell provide work function energy to free electrons from the surface. Per bundle or per quantum energy depends on frequency of light, which is reflected by the following formula.
W= hf, Quantum Energy, Joules
Here h denotes plank constant and its value is 6.624×10-34joules seconds, while f mean frequency. It has been proved through experiments that when an electron absorbs some frequency “f” rays, quantum energy is produced within it which is shown by “hf”. It is necessary to remember that the quantity of kinetic energy absorbed in electrons increases with an increase in the quantity of “hf”.
Field emission is also called cold cathode emission. As we know, electrons are negatively charged particles, therefore when a high positive field is applied to free electrons of a metal surface, these electrons start emitting from the surface of the metal due to appropriate energy or adequate gravitational force. The mechanism, by which the effect of a high voltage magnetic field (i.e. high positive voltage) is administered on some metal surface for emission of electrons, is called Field Emission. Another electric field is brought near the metal surface or cathode in this method of electronic emission due to which electrons start to omit from the cathode. This process of emission of electrons from metal surfaces due to the influence of an external field is called field emission. The stronger the field, the greater is the quantity of free electrons being emitted from the surface of the cold cathode or cold emitter.
In other words, if a high positive electric field is exerted on metal surface, emission of electrons from the surface of the material is not directly possible. If field intensity is increased to 106volts per second, there will be a rapid increase in electron emission. This is called high field emission. Various precautions are to be kept in mind while designing high voltage x-ray tubes, so that risk of producing high voltage intensity in electrodes could be averted. Also, likely damage from a high voltage current thus produced from through such emission can be thwarted. This method is not so vastly used in modern circuits. The tubes working on this method are called cold cathode tubes as the cathodes in these tubes are not warmed up to emit electrons. A positively charged body attracts electrons towards itself (there is negative charge on electrons and two opposite charges pull each other) therefore, the anode present in the tube, pulls electrons from the metal cathode towards itself, provided that anode has been exerted adequate positive high potential. Some gas is inserted by devoiding the tube of air. The voltage, on which field emission starts, is called threshold voltage. Its value for different materials varies. It is be noted that field emission does not rely on temperature (that’s this kind of emission is almost free from the effect of temperature). It depends on the intensity of electric field and the work function of the relevant metal. This kind of emission is possible on normal room temperature.
Note: Apart from the afore-mentioned 4 types of electronic emission, there is another form of electronic emission as well which is referred to as radioactive emission. This procedure is not so popular and its application is limited to certain specific places e.g. x-rays and some other medical equipments and tools.
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