The process of passing email. current through gas called gas discharge.
There are 2 types of discharges: independent and non-independent.
If the electrical conductivity of the gas is created. external ionizers, then el. the current in it is called. non-self gas discharge. V
Consider email diagram, comp. from a capacitor, galvanometer, voltmeter and current source.
Between the plates of a flat-plate capacitor there is air at atmospheric pressure and room temperature. If U equal to several hundred volts is applied to the capacitor, and the ionizer does not work, then the galvanometer does not register the current, however, as soon as the space between the plates begins to permeate. stream of UV rays, the galvanometer will begin to register. current. If the current source is turned off, the flow of current through the circuit will stop; this current represents a non-self-sustaining discharge.
j = γ*E – Ohm’s law for el. current in gases.
With a sufficiently strong electric field in the gas, the process of self-ionization begins, due to which the current can exist in the absence of an external ionizer. This kind of current is called a self-sustaining gas discharge. Self-ionization processes in general outline is as follows. In natural conventional There is always a small amount of free electrons and ions in a gas. They are created by such natures. ionizers, like cosmic ones. rays, radiation of radioactive substances, soda in soil and water. Quite strong electricity. the field can accelerate these particles to such speeds at which their kinetic energy exceeds the ionization energy when electrons and ions collide with neuters on the way to the electrodes. molecules will ionize these molecules. Arr. upon collision, new secondary electrons and ions are also dispersed. field and in turn ionize new neutrons. molecules. The described self-ionization of gases is called impact polarization. Free electrons cause impact ionization already at E = 10 3 V/m. Ions can cause impact ionization only at E = 10 5 V/m. This difference is due to a number of reasons, in particular the fact that the mean free path for electrons is much longer than for ions. Therefore, ions acquire the energy necessary for impact ionization at a lower field strength than ions. However, even at not too strong “+” fields, ions play an important role in self-ionization. The fact is that the energy of these ions is approx. sufficient to knock electrons out of metals. Therefore, the ions accelerated by the “+” field, hitting the metal cathode of the field source, knock out the electrons from the cathode. These knocked out electrons are decomposed. field and produce impact ionization of molecules. Ions and electrons, the energy of which is insufficient for impact ionization, can nevertheless, when colliding with molecules, cause them to become excited. state, that is, cause some energy changes in the electrical system. Neutral shells atoms and molecules. Exc. an atom or molecule after some time turns into normal condition, while it emits a photon. The emission of photons manifests itself in the glow of gases. In addition, photon, absorption. any of the gas molecules can ionize it, this kind of ionization is called photon ionization. Some photons hit the cathode, they can knock electrons out of it, which then cause impact ionization of neutrons. molecules.
As a result of impact and photon ionization and knocking out electrons from the “+” code by photons, the number of photons and electrons in the entire volume of the gas increases sharply (avalanche-like) and for the existence of a current in the gas an external ionizer is not needed, and the discharge becomes independent. The current-voltage characteristic of a gas discharge looks as follows.
The process of occurrence and formation of avalanches due to impact ionization, discussed above, does not lose the nature of a non-self-sustaining discharge, because If the external ionizer stops working, the discharge quickly disappears.
However, the occurrence and formation of a charge avalanche is not limited to the process of impact ionization. With a further, relatively small increase in voltage at the electrodes of the gas-discharge gap, positive ions acquire greater energy and, hitting the cathode, knock electrons out of it, what happens is secondary electron emission . The resulting free electrons on their way to the anode produce impact ionization of gas molecules. Positive ions on their way to the cathode in electric fields themselves ionize gas molecules.
If each electron knocked out from the cathode is capable of accelerating and producing impact ionization of gas molecules, then the discharge will be maintained even after the influence of the external ionizer ceases. The voltage at which a self-discharge develops is called circuit voltage.
Based on what has been said, independent discharge we will call such a gas discharge in which current carriers arise as a result of those processes in the gas that are caused by the voltage applied to the gas. Those. this discharge continues after the ionizer stops working.
When the interelectrode gap is covered by a fully conducting gas-discharge plasma, it begins breakdown . The voltage at which breakdown of the interelectrode gap occurs is called breakdown voltage. And the corresponding tension electric field is called punchy tension.
Let us consider the conditions for the occurrence and maintenance of independent discharge.
At high voltages between the electrodes of the gas gap, the current increases greatly. This occurs due to the fact that electrons generated under the influence of an external ionizer, strongly accelerated by the electric field, collide with neutral gas molecules and ionize them. As a result of this, secondary electrons And positive ions(process 1, Fig. 8.4). Positive ions move towards the cathode and electrons move towards the anode. Secondary electrons re-ionize the gas molecules, and, therefore, total quantity electrons and ions will increase as the electrons move towards the anode like an avalanche. This is the reason for the increase in electric current. The described process is called impact ionization.
However, impact ionization under the influence of electrons is not sufficient to maintain the discharge when the external ionizer is removed. To do this, it is necessary that electronic avalanches be “reproduced”, i.e. so that new electrons appear in the gas under the influence of some processes. These are the following processes:
- positive ions accelerated by the electric field, hitting the cathode, knock electrons out of it (process 2);
- positive ions, colliding with gas molecules, transfer them to an excited state; the transition of such molecules to the ground state is accompanied by the emission of photons (process 3);
- a photon absorbed by a neutral molecule ionizes it, and the process of photon ionization of molecules occurs (process 4);
- knocking out electrons from the cathode under the influence of photons (process 5);
- finally, at significant voltages between the electrodes of the gas gap, a moment comes when positive ions, which have a shorter free path than electrons, acquire energy sufficient to ionize gas molecules (process 6), and ion avalanches rush towards the negative plate. When, in addition to electron avalanches, ion avalanches also occur, the current strength increases practically without an increase in voltage.
Gases are good insulators at temperatures that are not too high and at pressures close to atmospheric. This is explained by the fact that gases when normal conditions consist of neutral atoms and molecules and do not contain free charges (electrons and ions). A gas becomes a conductor of electricity when some of its molecules ionizes, To do this, the gas must be exposed to some kind of ionizer (for example, use a candle flame, ultraviolet and x-ray radiation, g-quanta, flows of electrons, protons, a-particles, etc.). The ionization energy of atoms of various gases lies in the range of 4 - 25 eV. In an ionized gas, charged particles appear that can move under the influence of an electric field - positive and negative ions and free electrons.
The passage of electric current through gases is called gas discharge.
Simultaneously with the process ionization gas, the reverse process always occurs - recombination process: positive and negative ions, positive ions and electrons, when meeting, combine with each other to form neutral atoms and molecules. The balance of their speeds determines the concentration of charged particles in the gas. The processes of ion recombination, as well as the excitation of ions, which does not lead to ionization, lead to glow gas, the color of which is determined by the properties of the gas.
The nature of the gas discharge is determined by the composition of the gas, its temperature and pressure, size, configuration and material of the electrodes, applied voltage, current density, etc.
Let us consider a circuit containing a gas gap subjected to continuous, constant-intensity action of an external ionizer.
As a result of ionization of the gas, a current will flow in the circuit, the dependence of which on the applied voltage is given in Fig.
On a section of the curve OA the current increases in proportion to the voltage, i.e. Ohm's law is satisfied. With a further increase in voltage, Ohm's law is violated: the increase in current strength slows down (section AB) and finally stops completely (section Sun). Those. we obtain a saturation current, the value of which is determined by the power of the ionizer. This is achieved when all the ions and electrons created by the external ionizer per unit time reach the electrodes during the same time. If in mode OS When the ionizer stops working, the discharge stops. Discharges that exist only under the influence of external ionizers are called dependent. With a further increase in the voltage between the electrodes, the current strength is initially slow (section CD), and then sharply (section DE) increases and the discharge becomes independent. A discharge in a gas that persists after the external ionizer stops working is called independent.
The mechanism for the occurrence of self-discharge is as follows. At high voltages, electrons arising under the influence of an external ionizer, strongly accelerated by the electric field, collide with gas molecules, ionize them, resulting in the formation of secondary electrons and positive ions. Positive ions move towards the cathode and electrons move towards the anode. The secondary electrons re-ionize the gas molecules, and therefore the total number of electrons and ions will increase as the electrons move toward the anode in an avalanche fashion. This causes an increase in electric current in the area CD. The described process is called impact ionization. Impact ionization under the influence of electrons alone is not sufficient to maintain the discharge when the external ionizer is removed. To maintain a discharge, it is necessary that electron avalanches be “reproduced,” that is, that new electrons arise in the gas under the influence of some processes. This occurs at significant voltages between the electrodes of the gas gap, when avalanches of positive ions rush to the cathode, knocking electrons out of it. At this moment, when in addition to electron avalanches, ion avalanches also arise, the current strength increases practically without an increase in voltage (section DE in Fig.), i.e. an independent discharge occurs. The voltage at which a self-sustained discharge occurs is called breakdown voltage.
It should be noted that during a discharge in gases, special condition substance called plasma. Plasma is a highly ionized gas in which the densities of positive and negative charges are almost equal. A distinction is made between high-temperature plasma, which occurs at ultra-high temperatures, and gas-discharge plasma, which occurs during a gas discharge. Plasma is characterized by the degree of ionization a - the ratio of the number of ionized particles to their total number per unit volume of plasma. Depending on the value of a, we speak of weakly (a is a fraction of a percent), moderately (several percent) and completely (close to 100%) ionized plasma.
There are four types of self-discharge: smoldering, spark, arc and corona.
1. Glow discharge occurs when low pressures. If to the electrodes soldered into glass tube 30 - 50 cm long, attach constant voltage at several hundred volts, gradually pumping air out of the tube, then at a pressure of ~ 5.3 - 6.7 kPa (several mmHg) a discharge occurs in the form of a luminous, winding reddish cord running from the cathode to the anode. With a further decrease in pressure (~13 Pa), the discharge has the following structure.
Directly adjacent to the cathode is a dark thin layer 1 – Aston's dark space, followed by a thin luminous layer 2 - first cathode glow or cathode film, followed by dark layer 3 - cathode (Crookes) dark space, which later turns into luminous layer 4 - smoldering glow, which has a sharp boundary on the cathode side, gradually disappearing on the anode side. It occurs due to the recombination of electrons with positive ions. Bordering the smoldering glow is the dark interval 5- Faraday dark space, followed by a column of ionized glowing gas 6 - positive column. The positive column does not have a significant role in maintaining the discharge. The applied voltage is distributed unevenly along the discharge. Almost all of the potential drop occurs in the first three layers and is called cathode potential drop.
The mechanism of layer formation is as follows. Positive ions near the cathode, accelerated by the cathode potential drop, bombard the cathode and knock electrons out of it. In the dark Aston space, electrons accelerate and excite molecules, which begin to emit light, forming cathode film 2. Electrons flying past film 2 without collisions ionize the gas molecules behind this film. Many positive and negative charges are formed. At the same time, the intensity of the glow decreases. This region is the cathode (Crookes) dark space 3. Electrons generated in the cathode dark space penetrate into the region 4 of the glow, which is caused by their recombination with positive ions. Next, the remaining electrons and ions (there are few of them) penetrate by diffusion into region 5 - Faraday dark space. It appears dark because the concentration of recombining charges is low. In area 5 there is electric field, which accelerates electrons and in the region of the positive column 6 they produce ionization, resulting in the formation of plasma. The glow of the positive column is mainly associated with transitions of excited molecules to the ground state. It has a color characteristic of each gas. In a glow discharge, only three of its parts are of particular importance for its maintenance - up to the glow. In the cathode dark space, electrons and positive ions are strongly accelerated, knocking electrons out of the cathode (secondary emission). In the region of smoldering glow, impact ionization of gas molecules by electrons occurs. The positive ions formed during impact ionization rush to the cathode and knock out new electrons from it, which, in turn, again ionize the gas, etc. In this way, the glow discharge is continuously maintained.
Application in technology. The glow of the positive column, which has a color characteristic of each gas, is used in gas-discharge tubes to create advertisements (neon gas-discharge tubes give a red glow, argon - bluish-green) and in fluorescent lamps.
2. Spark discharge occurs at high electric field strengths (~3 10 b V/m) in a gas under pressure of the order of atmospheric pressure. The explanation of spark discharge is given on the basis streamer theory according to which the appearance of a brightly glowing spark channel is preceded by the appearance of faintly glowing accumulations of ionized gas - streamers. Streamers arise both as a result of the formation of electron avalanches through impact ionization and as a result of photon ionization of the gas. Avalanches, catching up with each other, form conductive bridges from streamers, along which following points time and powerful streams of electrons rush, forming spark discharge channels. Due to the release of a large amount of energy during the processes considered, the gas in the spark gap is heated to a very high temperature (about 10 4 o C), which leads to its glow. Rapid heating of the gas leads to an increase in pressure and the appearance of shock waves, which explains sound effects during a spark discharge. For example, crackling in weak discharges and powerful rumbles of thunder in the case of lightning.
Application in technology. To ignite the combustible mixture in engines internal combustion and protection of electrical transmission lines from overvoltages (spark gaps).
3. Arc discharge. If, after igniting a spark discharge from a powerful source, the distance between the electrodes is gradually reduced, then the discharge becomes continuous, i.e. an arc discharge occurs. In this case, the current increases sharply, reaching hundreds of amperes, and the voltage across the discharge gap drops to several tens of volts. The arc discharge can be obtained from a source low voltage, bypassing the spark stage. To do this, electrodes (for example, carbon) are brought together until they touch, they become very hot with electric current, then they are separated and obtained electric arc. At atmospheric pressure, the arc discharge has a temperature of ~3500 o C. As the arc burns, a depression is formed on the anode - a crater, which is the hottest point of the arc. the arc discharge is maintained due to intense thermionic emission from the cathode, as well as thermal ionization of molecules caused by high temperature gas
Application - for welding and cutting metals, producing high-quality steels (arc furnace) and lighting (spotlights, projection equipment).
4. Corona discharge- high-voltage electrical discharge at high (for example, atmospheric) pressure in a sharply inhomogeneous field near electrodes with a large surface curvature (for example, a tip). When the field strength near the tip reaches 30 kV/m, a glow appears around it, looking like a crown, which gives rise to the name of this type of discharge. This phenomenon was called in ancient times the fires of St. Elmo. Depending on the sign of the corona electrode, negative or positive coronas are distinguished.
Application - in electric precipitators used to purify industrial gases from impurities, when applying powder and paint coatings.
Electrical self-sustained and non-self-sustained discharge occurs in various gas environments under certain conditions. As a rule, a person uses an independent discharge. The article characterizes these phenomena.
What is in gases?
Before considering a gas discharge, independent and non-self-sufficient, let us define this phenomenon. A discharge is understood as the occurrence of an electric current in a gas. Since gaseous media are insulators by their nature, this means that the current is due to the presence of free carriers in them electric charge. In addition to them, an electric field must also exist so that the charges acquire directional movement.
An electric field can be created by applying an external potential difference to a volume of gas (presence of electrodes: negative cathode and positive anode).
The following processes can be sources of charge carriers:
- Thermal ionization. It arises due to the mechanical collision of high-energy gas particles (atoms, molecules) and knocking out electrons from them. This process is activated when the temperature increases.
- Photoionization. Its essence lies in the absorption of a high-energy photon by an electron and its separation from the atom.
- Cold electron emission. It occurs due to bombardment of the cathode surface by ions.
- Thermionic emission. This process is due to the evaporation of high-energy electrons from the cathode and their participation in the subsequent ionization of the plasma.
The named processes underlie the classification of discharge types (independent and non-independent).
The concept of discharge independence
Let's consider the case with a cathode tube. It is a sealed container in which there is some gas under a certain pressure. At the ends of this tube there are electrodes. If a small potential difference is applied to them, then practically no current will arise. This is due to the lack sufficient quantity charge carriers.
If you heat the gas or expose it to ultraviolet radiation, the voltmeter will immediately detect the appearance of a current. This is a clear example of a non-self-sustaining discharge. It is called that because for its existence an external source of ionization (radiation, temperature) is necessary. As soon as this source is removed, the voltmeter readings will again become zero.
If, in the absence of external sources of ionization, the voltage between the electrodes of the tube is increased, a current will begin to appear, which will go through several stages (saturation, increase, decrease). In this case, they talk about an independent electrical discharge. It no longer requires external sources; the necessary charge carriers are generated within the system itself. The processes of their formation remain the same as for a non-self-sustaining discharge. At high voltages and high current densities, thermal emission of cathode electrons is also added.
Current-voltage characteristics of the discharge
It is convenient to study gas self-sustaining and non-self-sustaining discharges if we use the dependence of voltage on current (or vice versa), which is usually called the current-voltage characteristic. It allows you to judge not only the magnitude of voltage and current in the system, but also the electrical processes occurring in it.
Below is the current-voltage characteristic, which reflects all the main phases of discharge development.
As you can see, there are three of them: dark, smoldering and arc. Later in the article we will describe these phases in more detail.
Dark discharge
It is described by the interval AC. As the voltage U increases, the current I increases due to an increase in the speed of ion movement. However, these speeds are low, so a non-self-sustained discharge occurs. In the BC region it reaches saturation and becomes independent, since the speed of the ions becomes sufficient to knock electrons out of it when bombarding the cathode. These electrons lead to additional ionization of the gas.
The dark charge received this name because its glow is practically zero: low plasma concentration, low currents (10 -8 A), lack of recombination of ions and electrons.
Glow discharge
On the current-voltage characteristic it corresponds to the zone between points C and F. From the figure it can be seen that the voltage changes (falls and rises), and the current is constantly increasing. Two subzones are of interest:
- OE points - normal glow discharge. The reason for the current increase here is associated with an increase in the plasma area in the gas. That is, at first these are narrow small channels, then due to the cold emission of electrons they expand until they reach the entire volume of the tube. From this moment there is a transition to the next subzone.
- EF points - abnormal discharge. The current of this self-discharge in the gas begins to increase due to hot electron emission. The temperature of the cathode gradually increases and it begins to emit negatively charged particles.
All neon and fluorescent lamps operate in the normal glow discharge region.
Spark and arc discharges
These types independent categories cover the FG zone in the figure. The most complex processes take place here.
When the voltage between the electrodes increases to the maximum value (point F), and the thermal emission of electrons from the cathode is activated, then favorable conditions to form an unstable spark discharge. It represents short-term breakdowns (microseconds), which have a characteristic zigzag shape. A striking example in nature is lightning in the atmosphere.
The discharge occurs through narrow channels called streamers. They are narrow broken lines of highly ionized plasma that connect the cathode surface to the anode. The current strength in them reaches tens of thousands of amperes.
Stabilization of the spark charge leads to the formation of a stable arc (point G region). In this case, the entire volume of gas in the tube is highly ionized plasma. The surface of the cathode is heated to 5000-6000 K, and the anode - to 3000 K. Such strong heating of the cathode leads to the formation of so-called “hot spots” on it, which become a powerful source of thermionic electrons and cause erosive wear of this electrode. The voltage during an arc discharge is not high (several tens of volts), but the current can reach 100 A or more. The welding arc is a prime example of this type of discharge.
Thus, the existence of self-sustaining and non-self-sustaining discharges in gases is due to the mechanisms of its ionization and plasma formation with increasing voltage and current in the system.
Topic 7. Electrical conductivity of liquids and gases.
§1. Electric current in gases.
§2. Non-self-sustaining and independent gas discharges.
§3. Types of non-self-sustaining discharge and their technical use.
§4. The concept of plasma.
§5. Electric current in liquids.
§6. Laws of electrolysis.
§7. Technical applications of electrolysis (do it yourself).
Electric current in gases.
Under normal conditions, gases are dielectrics and only become conductors when they are ionized in some way. Ionizers can serve x-rays, cosmic rays, ultraviolet rays, radioactive radiation, intense heating, etc.
Ionization process gases is that under the influence of an ionizer one or more electrons are split off from atoms. As a result, a positive ion and electron appear instead of a neutral atom.
Electrons and positive ions generated during the action of the ionizer cannot exist separately for a long time and, when reunited, again form atoms or molecules. This phenomenon is called recombination.
When an ionized gas is placed in an electric field, electric forces act on the free charges and they drift parallel to the tension lines - electrons and negative ions to anode(an electrode of some device connected to the positive pole of the power source), positive ions - to cathode(an electrode of some device connected to the negative pole of a current source). At the electrodes, ions turn into neutral atoms, giving or accepting electrons, thereby completing the circuit. An electric current arises in the gas. Electric current in gases is called gas discharge. Thus, the conductivity of gases is electron-ionic in nature.
Non-self-sustaining and independent gas discharges.
Let's collect electrical circuit, containing a current source, a voltmeter, an ammeter and two metal plates separated by an air gap.
If you place an ionizer near the air gap, an electric current will appear in the circuit, disappearing with the action of the ionizer.
Electric current in a gas with non-self-conducting is called non-self-sustaining gas discharge. Graph of the dependence of the discharge current on the potential difference between the electrodes - current-voltage characteristic of the gas discharge:
OA is a section where Ohm’s law is observed. Only some of the charged particles reach the electrodes, some recombine;
AB - the proportionality of Ohm's law is violated and, starting from the current, does not change. The highest current possible with a given ionizer is called saturation current ;
Sun – independent gas discharge, in this case, the gas discharge continues even after the termination of the external ionizer due to ions and electrons resulting from impact ionization(ionization of electric shock); occurs when the potential difference between the electrodes increases (occurs electron avalanche).
