nottingham effect– release of heat at the cathode during autoelectronic emission and absorption of heat during thermionic field emission, due to the difference between the average energy of electrons approaching the cathode surface and leaving it. At low temperatures (with autoelectronic emission), the energy distribution of electrons practically does not differ from the Fermi distribution at absolute zero. Therefore, electrons with energies somewhat lower than the Fermi level go through the potential barrier into vacuum. In this case, the emitter is heated due to the energy of the electrons coming from the electrical circuit to the released levels. In the case of thermionic field emission (at a high temperature), electrons leave levels above the Fermi level. The filling of these levels with electrons coming from the circuit leads to cooling of the emitter. Discovered by W. B. Nottingham in 1941.

Multer effect– emission of electrons into vacuum from a thin dielectric layer on a conducting substrate in the presence of a strong electric field in the layer. Discovered by the American radio engineer L. Malter in 1936 in the A1 2 O 3 + Cs 2 O layer on A1. the emission current increases rapidly with an increase in the anode voltage. The Multer effect is due to the presence of a strong electric field in the layer, which leads to field emission from the substrate into the layer.

When bodies come into contact with vacuum or gases, electron emission is observed - the release of electrons by bodies under the influence of external influences: heating ( thermal emission) photon flux ( photoemission), electron flow ( secondary issue), ion flux, strong electric field ( autoelectronic or cold emission), mechanical or other "spoiling the structure" influences ( field emission).

In all types of emissions, except autoelectronic, the role of external influences is reduced to an increase in the energy of a part of the electrons or individual electrons of the body to a value that allows them to overcome the potential threshold at the boundary of the body, followed by exit and vacuum or another medium.

Multer effect applies:

A method for controlling the depth of a disturbed surface layer of semiconductor wafers, characterized in that, in order to enable automation and simplify the control process, the wafer is heated to a temperature corresponding to the maximum exoelectronic emission, which is controlled by one of the known methods, and the position of the emission peak determines the depth of the damaged layer;

An electronic turbine containing a cathode and an anode placed in a vacuum cylinder and a rotor with blades placed between them, characterized in that, in order to increase the torque on the turbine shaft, its rotor is made in the form of a set of coaxial cylinders with blades, fixed guide vanes are installed between the rotor cylinders a coating that provides secondary electron emission, for example, antimony-cesium. In the case of autoelectronic emission, an external electric field transforms the potential threshold at the body boundary into a barrier of finite width and reduces its height relative to the height of the initial threshold, as a result of which it becomes possible quantum mechanical tunneling electrons through the barrier. In this case, emission occurs without energy consumption by the electric field;

A method for measuring the volumetric concentration of hydrocarbons in vacuum systems by thermal decomposition of hydrocarbons on a heated pointed field cathode and recording the pyrolytic carbon accumulation time to one of the reference concentrations, characterized in that, in order to increase the measurement accuracy, the carbon accumulation time is recorded by changing the autoelectronic current value. The presence of thin dielectric films on the metal surface in strong fields does not interfere with the passage of electrons through the potential barrier. This phenomenon is called the Molter effect;

A cathode-beam storage tube with screen grids, characterized in that for the purpose of storing the recording for an indefinitely long time, one of the screen grids serving as a potential carrier is made of metals emitting secondary electron emission, covered with a dielectric film and having an effect.

Electron tunneling through potential barriers is widely used in special semiconductor devices - tunnel diodes. The height of the tunnel barrier can be influenced not only by the electric field, but also by other influences.

It is also used in a device that allows detecting magnetic domains with an inner diameter of not more than 1 micron, based on determining the change in the Fermi level of the investigated electrode by changing the height of the tunnel barrier and by its effect on the resistance value, the tunnel junction. The device is applicable in magnetic non-volatile and random access memory devices.

And also in the device for measuring the contact pressure of the tape on the magnetic head, containing elastic elements and sensors, characterized in that, in order to simultaneously carry out integral and discrete measurement of the indicated pressure, the measuring device is made in the form of a semi-cylinder, consisting of elastic elements forming on the body magnetic head, while the other edge of the half-cylinder is made free, and a sensor is installed under each strip of the comb, for example, with a tunnel effect.

tunnel effect– overcoming a potential barrier by a microparticle in the case when its total energy is less than the barrier height. The probability of passing through the barrier is the main factor that determines the physical characteristics of the tunnel effect. This probability is the greater, the smaller the mass of the particle, the narrower the potential barrier, and the less energy the particle lacks to reach the height of the barrier. In the case of a one-dimensional potential barrier, the characteristic is the transparency coefficient of the barrier, which is equal to the ratio of the flow of particles passing through it to the flow supplying the barrier. An analogue of the tunnel effect in wave optics: the penetration of a light wave into a reflective coating under conditions when, from the point of view of geometric optics, total internal reflection occurs.

Application: in radio elements based on the tunnel effect - tunnel diodes.

Thermionic Emission- emission of electrons by heated bodies in vacuum or other media. Only those electrons can leave the body, the energy of which is greater than the energy of an electron at rest outside the body. The number of such electrons at T-300 K is very small and increases exponentially with temperature. Therefore, the thermionic emission current is noticeable only for heated bodies. In the absence of a "suction" electric field, the emitted electrons form a negative space charge near the emitter surface, which limits the thermionic emission current.

Thermionic emission underlies the operation of thermoelectric cathodes used in many electrovacuum and gas-discharge devices.

Thermionic energy converter is a device for converting thermal energy into electrical energy based on the phenomenon described above. Its action is based on the following process: electrons "evaporate" from the cathode (hot metal surface with a high work function), which, having flown through the interelectrode gap, "condense" on the anode (cold metal); current flows in the external circuit; its efficiency exceeds 20%.

Ion-electron emission– emission of electrons by the surface of a solid body into vacuum during the bombardment of the surface with ions; The coefficient of ion-electron emission y is equal to the ratio of the number of emitted electrons n i to the number of ions incident on the surface n j . For slow ions, y practically does not depend on the energy and mass mj, but depends on their charge (for singly charged ions, y ≈ 0.2, for multiply charged ions, y can exceed unity).

Ion-electron emission also depends on the energy of ionization and excitation of ions on the work function of the target substance. When the speed of the ions reaches 6-7-10 6 cm / s, its character changes dramatically.

At first, y grows in proportion to ej, then as (si)" 2 , at Vj = 10 8 - 10 9 cm/s, a maximum is reached, then there is a decrease.

If a slow ion approaches the surface of a solid body, then the electron of the solid body can go to the ion and neutralize it. Such a transition is accompanied by the release of energy and some of the electrons that have received it can leave the body. When bombarded by fast ions, an intense electrical exchange occurs, in which the electron flies into vacuum.

Vacuum is understood as a gas or air in a state of the highest rarefaction (pressure of the order of ). Vacuum is a non-conductive medium, since it contains an insignificant amount of electrically neutral particles of matter.

To obtain an electric current in vacuum, a source of charged particles - electrons is needed, and the movement of electrons in vacuum occurs practically without collisions with gas particles.

The source of electrons is usually a metal electrode - the cathode. In this case, the phenomenon of the release of electrons from the cathode surface into environment called electron emission.

Free electrons in a metal in the absence of an external electric field randomly move between the ions of the crystal lattice.

Rice. 13-6. Double electrical layer on the metal surface.

At room temperature, no electrons escape from the metal due to the insufficient value of their kinetic energy. Part of the electrons with the highest kinetic energy, during their movement, goes beyond the surface of the metal, forming an electron layer, which, together with the layer of positive ions of the crystal lattice located under it in the metal, forms a double electric layer (Fig. 13-6). The electric field of this double layer counteracts the electrons tending to leave the conductor, i.e., it is inhibitory for them.

For an electron to go beyond the metal surface, it is necessary for the electron to impart energy equal to the work that it must do to overcome the retarding effect of the double layer field. This work is called the work function. The ratio of the output energy to the electron charge is called the output potential, i.e. .

The work (potential) of the output depends on the chemical nature of the metal.

The values ​​of the output potential for some metals are given in Table. 13-1.

Table 13-1

Depending on the way in which the additional energy necessary to exit the metal is imparted to the electrons, the types of emission are distinguished: thermionic, electrostatic, photoelectronic, secondary, and under the impact of heavy particles.

Thermionic emission is the phenomenon of the release of electrons from the cathode, due solely to the heating of the cathode. When a metal is heated, the speeds of electrons and their kinetic energy increase and the number of electrons leaving the metal increases. All electrons emerging from the cathode per unit time, if they are removed from the cathode by an external field, form electricity emissions. As the cathode temperature rises, the emission current increases slowly at first, and then faster and faster. On fig. 13-7 curves of the emission current density, i.e., the emission current per unit cathode surface, expressed in A/cm2, are given depending on the temperature T for various cathodes.

Rice. 13-7. Curves of emission current density depending on temperature for various cathodes: a - oxide; b - tungsten, covered with thorium; c - uncoated tungsten.

The dependence of the emission current density on temperature and work function is expressed by the Richardson-Dashman equation:

where A is the emission constant; for metals it is equal to; T is the absolute temperature of the cathode, K; - base of natural logarithms; - work function, eV; is the Boltzmann constant.

Thus, the emission current density increases proportionally and so that a cathode made of a material with a low work function and a high operating temperature is needed to obtain a large emission current.

If the electrons that have flown out of the cathode (the emitted electrons) are not removed from it by an external accelerating field, then they accumulate around the cathode, forming a volume negative charge (electron cloud), which creates a decelerating electric field near the cathode, which prevents the further escape of electrons from the cathode.

Electrostatic electron emission is the phenomenon of the release of electrons from the cathode surface, due solely to the presence of a strong electric field near the cathode surface.

The force acting on an electron in an electric field is proportional to the charge of the electron and the field strength F - ee. At a sufficiently high strength of the accelerating field, the forces acting on an electron located near the cathode surface become large enough to overcome the potential barrier and eject electrons from the cold cathode.

Electrostatic emission finds use in mercury valves and some other appliances.

Photoelectronic emission is the phenomenon of the release of electrons, due solely to the action of radiation absorbed by the cathode, and not associated with its heating. In this case, the cathode electrons receive additional energy from light particles - photons.

Radiant energy is emitted and absorbed by certain portions - quanta. If the quantum energy, determined by the product of the Planck constant of the radiation frequency v, i.e., more work exit for the material of this cathode, then the electron can leave the cathode, i.e., photoelectron emission will take place.

Photoelectronic emission is used in solar cells.

Secondary electron emission is the phenomenon of the exit of secondary electrons, due solely to the impact of primary electrons on the surface of a body (conductor, semiconductor). Flying electrons, called primary, meeting a conductor on their way, hit it, penetrate into its surface layer and give part of their energy to the electrons of the conductor. If the additional energy received by the electrons upon impact is greater than the work function, then these electrons can go beyond the conductor.

Secondary electron emission is used, for example, in photomultipliers to amplify the current.

Secondary emission can be observed in vacuum tubes in which the anode is exposed to electrons flying from the cathode. In this case, secondary electrons can create a flow that is opposite to the "working" one, which worsens the operation of the lamp.

Electron emission under the impact of heavy particles is the phenomenon of the release of electrons, due solely to the impact of ions or excited atoms (molecules) on the surface of the body - the electrode. This type of emission is similar to the secondary electron emission considered above.

INDUSTRIAL ELECTRONICS

Chapter XII

EV DEVICES

With regard to automation production processes In all branches of industry, industrial electronics, the science of the technical use of electronic, ionic, and semiconductor devices, has become of great importance.
main feature electronic appliances(electronic tubes) consists in the fact that the passage of electric current in them is associated with the movement of electrons in a vacuum, and the control of moving electrons is carried out by an electric field.
Ionic devices called devices in which the electric current is a flow of electrons and charged particles - ions in a highly rarefied gaseous medium under the action of electric field forces.
semiconductor devices are such devices in which an electric current is created by electrons and holes moving under the influence of an electric field (the processes associated with hole conduction are described in § 146 of Chapter XIV) in a semiconductor medium.

§ 126 Electronic emission

The operation of electronic and ionic devices is based on the use of electron emission. The latter consists in the release of electrons from the surface of metals into a vacuum or a rarefied gas. The movement of these electrically charged particles creates a current in electronic and ionic devices. The main types of electron emission used in electronics are thermionic, secondary electronic and photoelectronic.
Thermionic emission. In metals, around each atom there are electrons that are loosely bound to it. Some of these electrons, torn off from their nuclei, are in random motion. The speed of the chaotic movement of these free electrons depends on the temperature of the metal: the higher the temperature, the faster the electrons move.
At certain temperatures (900 - 1000 ° C and above), the speed of movement of a part of the electrons becomes so significant that, overcoming the forces of attraction of the nuclei of atoms, they break out of the metal and fly out of it. This phenomenon is called thermionic emission.
Different metals have different numbers of electrons emitted at the same temperature. Sodium, potassium, cesium, barium and some other metals have the highest thermionic emission.
At very high temperatures, the heated metal begins to evaporate and this limits the possibility of increasing thermionic emission by increasing the temperature.
Secondary electron emission. If a metal plate is placed in a vacuum at a certain distance from the electrode from which the electrons are emitted and a positive potential is applied to it, then the electrons emitted from the surface of the electrode, carrying a negative electric charge, will be attracted to the plate and hit it at high speed. Under the impact of fast-flying electrons, other electrons will be knocked out from the surface of this plate, called secondary emission electrons.
One of the varieties of secondary emission is the emission of electrons under the influence of the bombardment of material by electrically charged particles - ions, the mass of which is much greater than the mass of electrons. The emission of electrons from the surface of materials under the action of ion bombardment is used in the operation of ion devices.
Photoelectronic emission. Photoelectronic emission occurs under the influence of light, ultraviolet and other rays falling on the surface of materials.
The light flux can be considered as a stream of tiny particles called photons.
The speed of photons (the speed of light) is about 300,000 km/s. Photons, hitting the surface of the material, knock out electrons from it.
The phenomenon in which electrons are ejected from a material under the influence of light energy is called photoelectric effect. This phenomenon is used in photocells.

26.07.14 11:28

Electronic emission is the departure of an electron from a metal (or any other body) and its transition to another phase (gas or vacuum) under the action of additional energy imparted to the electron from the outside. Although in the atoms of the surface layer of any body, electrons are at different energy levels and, therefore, have different energies, without energy supply from the outside, only a very small fraction of them (those electrons that have increased energy) can pass through the potential barrier and enter another phase; the remaining electrons always remain within their atoms and molecules.

Several types of electron emission are observed (photo, thermal, etc.), including autoelectronic (cold) emission - the ejection of electrons under the action of strong electric fields. The latter kind of emission requires a very high field gradient. N. A. Krotova, V. V. Karasev, Yu.

The values ​​of the discharge potentials, the gaps between the film and the substrate, and the electrification density of the surfaces were obtained by the authors on the basis of calculations using an auxiliary graph (the Paschen curve), and the energy of the electric double layer was assumed to be equal to the maximum work of separation.

Experiments on the establishment of electron emission during the detachment of films from substrates were carried out using a vacuum adhesion meter at a pressure of about 10-5 mm Hg. Art. A fluorescent screen was placed on a glass plate against the separation boundary at a distance of 1 cm. The screen flashed bluish-green light when various polymer films were removed from the glass substrate in vacuum; in the dark, the glow was clearly visible even at a distance of several meters.

It was noticed that the glass plate also glows with a greenish light in the places where the film is torn off.
Photographic plate blackening

Consequently, radiation appears at the separation boundary, which causes fluorescence of the screen and glass. Further experiments showed that this radiation also causes a blackening of the photographic plate if it is fixed in place of the fluorescent screen. The photographic plate blackened when detached from the substrate of the most diverse high polymers in their structure: natural and synthetic rubbers, gutta-percha, polyisobutylene, various cellulose ethers, vinyl polymers, etc.

Radiation was observed when these polymers were detached from both glass and metal. The authors concluded that this radiation could not be either X-rays or visible light: it was a stream of electrons. They are also convinced of this by their experience in studying the influence of a magnetic field on radiation. As is known, photons, i.e. visible and x-rays in a magnetic field do not deviate from their rectilinear path: the magnetic field does not act on them.

Positively or negatively charged particles behave differently: the former deviate towards the negative pole, the latter towards the positive. At a magnetic field strength of about 25–30 oersted, the authors managed to obtain a fairly clear image of an undeflected and deflected electron beam.

IN Lately(1965-1966), the author of the book, together with Yu. M. Evdokimov, investigated electrical phenomena at the substrate-adhesive contact boundary using a new method. The adhesion of some polymers to silicate glass and metals (grade X and 1Kh18N9T steel) was studied. The polymers chosen were cellulose ethers (acetobutyrate (ABC), acetopropionate and cellulose tripropionate), chlorosulfopolyethylene, and adhesive patch.

Gluing these polymers to the substrates was prepared by pouring 10% polymer solutions in appropriate solvents (acetone, carbon tetrachloride) onto the degreased surfaces of the substrates, except for the sticky patch, which was duplicated with the substrate by rolling with a rubber roller.
Electronic emission

The electrical phenomena of the selected systems (substrate + film) were studied on a vacuum adhesion meter and according to the method proposed by B. V. Deryagin and N. A. Krotova.

After detachment, the disconnected surfaces of all systems had electrical charges of opposite sign, which were monitored on a string electrometer (see its description below). Glass and metals were positively charged in all cases, and the detached polymer surfaces were negatively charged. Photographs of the electron emission arising from the separation of polymer films from glass and metals were obtained.

Electronic emission was found in all systems under study. The photographs show uneven blackening of the plate. Apparently, electron emission centers arise as a result of detachment of individual regions of the polymer from the substrate. It is clearly seen that the radiation proceeds parallel to the film surface: the greatest radiation is noticeable at the separation boundary, and the least radiation is observed for the emitting polymer film.

When chlorosulfopolyethylene is detached from glass, separate flashes are obtained, which, apparently, occur when separate sections of the polymer are detached from glass; With the removal of the separation boundary from the plate, blackening of the photographic plate is not observed, most likely due to the low radiation intensity.

Electron emission is also confirmed by a screen shot made of paper when the photographic plate is exposed to radiation observed when the polymer film is detached from the substrate. The photograph clearly shows a dark area - a consequence of the shielding of rays by a paper screen, and light areas - the results of the action on the plate of electrons that did not meet an opaque screen on their way. These pictures serve as direct evidence of electron emission, which indicates a certain role of electrical forces that carry out the adhesion of bodies upon contact.

Lecture 2

Formation of negative ions

It has been established that halogens during ionization are able to add electrons to form negative ions (halogens: F, Cl, Br, J). F has the highest electron affinity, which is often introduced into the arc in the form of salts (CaF2) in order to suppress porosity in the weld metal. The attachment of negative ions by F atoms leads to a decrease in the concentration of free electrons in the arc plasma, although the total number of charged particles remains constant. The electrons carry the bulk of the current. Negative F ions are heavy, slow-moving particles that carry out current transfer much worse. Therefore, when substances containing F are introduced into the welding zone, the stability of the arc burning deteriorates sharply, especially when welding with alternating current. Therefore, electrodes of the UONI 13/45 type, containing a significant amount of CaF2 in their composition, are used at direct current. If welding must be done on alternating current, then ionizing substances are introduced into the composition of such coatings, or arc stabilization is used using oscillators or pulse generators.

Emission of electrons from the cathode surface

To pull out an electron from the cathode, it is necessary to overcome the forces of attraction of the electron by the positive charges of the cathode. To do this, you need to spend a certain amount of work, which is called the work function. The value of the work function depends on the material of the cathode and the state of its surface (the presence of oxide and other films). For the process in the welding arc, two types of electron emission are of primary importance: thermionic and autoelectronic.

Thermionic emission occurs when the cathode surface is heated. In this case, individual electrons can receive energy sufficient to perform the work function and leave the cathode surface. In the absence of an electric field, an electron cloud is formed above the cathode surface, and the further process of electron emission stops.

Over time, individual electrons from the space charge return to the charge body and are drawn into the metal. Electrons are simultaneously emitted and drawn back into the metal. During prolonged heating of the metal with constant temperature an equilibrium emission density is established (the number of emitted electrons is equal to the number of drawn in).

The electron current density can be calculated using the formula:

j = AT 2 exp(-j/kt)

where j is the work function.

As the temperature increases, the thermionic emission current density increases. At the temperature of the welding arc, such a density of thermionic emission is established that is sufficient to maintain a stable arc discharge.

Field emission. In order to facilitate the emission of electrons from the metal, the heated metal - the cathode is placed in an electric alternating field. The field poles are arranged as follows: ²-² on the metal, ²+² on the opposite electrode - the anode.

The electric field completely or partially destroys the spatial electric charge. This facilitates the emission of electrons from the cathode and increases the equilibrium emission density, which is calculated from the same dependence.

The equation for the current of thermo- and field emission takes the form:

In an electric field, the work function of an electron decreases by the amount

Δj= 0 3/2 E 1/2,

where E is the field strength.

Emission under the influence of an electric field is called autoelectronic. Welding is characterized by both types of emission.

Reducing the work function from the electrode surface can serve as one of the ways to stabilize the arc discharge.

Table - Work function from the cathode surface for various materials

In the presence of oxide films on the surface of the electrode, the work function is significantly reduced, especially strongly reduce j films of oxides of alkali and alkaline earth metals. In order to improve the stability of the arc during welding W electrodes, oxides are introduced into the composition of the electrodes La, such electrodes are called lanthanated. Previously used electrodes contained 1.5-2.5% thorium dioxide. VT-15 and VT-25 (1.5-2.5% thorium dioxide). The arc does not wander over the metal surface.

Abroad and in our country, attempts were made to increase stability by reducing j electron from the surface of the consumable electrode. For this, an activated wire was used; covered with a thin layer of salt. The best effect is given by cesium salts (provides a low ionization potential). In this case, droplets of molten metal are crushed.