Non-self-sustained and independent charges. Ionization of gases

Gases at not too high temperatures and at pressures close to atmospheric are good insulators. If you place a charged electrometer in dry atmospheric air, then its charge remains unchanged for a long time. This is explained by the fact that gases under normal conditions consist of neutral atoms and molecules and do not contain free charges (electrons and ions). A gas becomes a conductor of electricity only when some of its molecules are ionized. For ionization, the gas must be exposed to some kind of ionizer: for example, an electric discharge, X-rays, radiation or UV radiation, a candle flame, etc. (in the latter case, the electrical conductivity of the gas is caused by heating).

When gases are ionized, one or more electrons are ejected from the outer electron shell of an atom or molecule, which leads to the formation of free electrons and positive ions. Electrons can attach to neutral molecules and atoms, turning them into negative ions. Therefore, in an ionized gas there are positively and negatively charged ions and free electrons. E electric current in gases is called a gas discharge. Thus, the current in gases is created by ions of both signs and electrons. A gas discharge with such a mechanism will be accompanied by the transfer of matter, i.e. ionized gases are conductors of the second kind.

In order to tear off one electron from a molecule or atom, it is necessary to perform a certain work A and, i.e. expend some energy. This energy is called ionization energy , whose values ​​for atoms of various substances lie within 4÷25 eV. Quantitatively, the ionization process is usually characterized by a quantity called ionization potential :

Simultaneously with the process of ionization in a gas, there is always a reverse process - the process of recombination: positive and negative ions or positive ions and electrons, meeting, recombine with each other to form neutral atoms and molecules. The more ions appear under the action of the ionizer, the more intense is the recombination process.

Strictly speaking, the electrical conductivity of a gas is never equal to zero, since it always contains free charges resulting from the action of radiation from radioactive substances present on the surface of the Earth, as well as from cosmic radiation. The intensity of ionization under the action of these factors is low. This slight electrical conductivity of the air is the cause of the leakage of charges of electrified bodies, even if they are well insulated.

The nature of the gas discharge is determined by the composition of the gas, its temperature and pressure, dimensions, configuration and material of the electrodes, as well as the applied voltage and current density.

Let us consider a circuit containing a gas gap (Fig.), subjected to continuous, constant in intensity action of an ionizer. As a result of the action of the ionizer, the gas acquires some electrical conductivity and current will flow in the circuit. Figure shows current-voltage characteristics (dependence of current on applied voltage) for two ionizers. Productivity (the number of pairs of ions produced by the ionizer in the gas gap in 1 second) of the second ionizer is greater than the first. We will assume that the performance of the ionizer is constant and equal to n 0 . At a not very low pressure, almost all the split off electrons are captured by neutral molecules, forming negatively charged ions. Taking recombination into account, we assume that the concentrations of ions of both signs are the same and equal to n. The average drift velocities of ions of different signs in an electric field are different: , . b - and b + are the mobility of gas ions. Now for region I, taking into account (5), we can write:

As can be seen, in region I, with increasing voltage, the current increases, since the drift velocity increases. The number of pairs of recombining ions will decrease as their speed increases.

Region II - saturation current region - all ions created by the ionizer reach the electrodes without having time to recombine. Saturation current density

j n = q n 0 d, (28)

where d is the width of the gas gap (the distance between the electrodes). As can be seen from (28), the saturation current is a measure of the ionizing effect of the ionizer.

At a voltage greater than U p p (region III), the speed of electrons reaches such a value that, when colliding with neutral molecules, they are able to cause impact ionization. As a result, additional An 0 pairs of ions are formed. The value A is called the gas amplification factor . In region III, this coefficient does not depend on n 0 , but depends on U. Thus. the charge reaching the electrodes at constant U is directly proportional to the performance of the ionizer - n 0 and voltage U. For this reason, region III is called the proportional region. U pr - proportionality threshold. The gas amplification factor A has values ​​from 1 to 10 4 .

In region IV, the region of partial proportionality, the gas gain begins to depend on n 0. This dependence increases with increasing U. The current increases sharply.

In the voltage range 0 ÷ U g, the current in the gas exists only when the ionizer is in operation. If the action of the ionizer is stopped, then the discharge also stops. Discharges that exist only under the action of external ionizers are called non-self-sustaining.

The voltage U g is the threshold of the region, the Geiger region, which corresponds to the state when the process in the gas gap does not disappear even after the ionizer is turned off, i.e. the discharge acquires the character of an independent discharge. Primary ions only give impetus to the occurrence of a gas discharge. In this region, I already acquire the ability to ionize massive ions of both signs. The magnitude of the current does not depend on n 0 .

In area VI, the voltage is so high that the discharge, once it has occurred, no longer stops - the area of ​​\u200b\u200bcontinuous discharge.

The process of current penetrating through a gas is called a gas discharge.

The current in the gas that occurs in the presence of an external ionizer is called dependent .

Let a pair of electrons and ions be let into the tube for some time, with an increase in voltage between the electrodes of the tube, the current strength will increase, positive ions begin to move towards the cathode, and electrons - towards the anode.

There comes a moment when all particles reach the electrodes and with a further increase in voltage, the current strength will not change, if the ionizer stops working, then the discharge will also stop, because. there are no other sources of ions, for this reason the discharge of ions is called non-self-sustaining.

The current reaches its saturation.

With a further increase in voltage, the current increases sharply, if you remove the external ionizer, the discharge will continue: the ions necessary to maintain the electrical conductivity of the gas are now created by the discharge itself. gas discharge that continues after the termination of the external ionizer is called independent .

The voltage at which self-discharge occurs is called breakdown voltage .

An independent gas discharge is maintained by electrons accelerated by an electric field, they have a kinetic energy that increases due to the electric field. fields.

Self discharge types:

1) smoldering

2) arc (electric arc) - for metal welding.

3) crown

4) spark (lightning)

Plasma. Plasma types.

Under plasma understand a strongly ionized gas in which the concentration of electrons is equal to the concentration of + ions.

The higher the gas temperature, the more ions and electrons in the plasma and the fewer neutral atoms.

Plasma types:

1) Partially ionized plasma

2) fully ionized plasma (all atoms decayed into ions and electrons).

3) High temperature plasma (T>100000 K)

4) low-temperature plasma (T<100000 К)

St-va plasma:

1) Plasma is electrically neutral

2) Plasma particles move easily under the action of the field

3) Have good electrical conductivity

4) Have good thermal conductivity

Practical use:

1) Conversion of thermal gas energy into electrical energy using a magnetohydrodynamic energy converter (MHD). Operating principle:

A jet of high-temperature plasma enters a strong magnetic field (the field is directed perpendicular to the drawing plane X), it is divided into + and - particles, which rush to different plates, creating some kind of potential difference.

2) They are used in plasmatrons (plasma generators), with their help they cut and weld metals.

3) All stars, including the Sun, stellar atmospheres, galactic nebulae are plasma.

Our Earth is surrounded by a plasma shell - ionosphere, outside of which there are radiation poles surrounding our Earth, in which there is also plasma.

The processes in the near-Earth plasma are caused by magnetic storms, auroras, and also in space there are plasma winds.

16. Electric current in semiconductors.

Semiconductors are ve-va, in which the resistance decreases with increasing t.

Semiconductors occupy 4 subgroups.

Example: Silicon is a 4-valence element - this means that in the outer shell of an atom, there are 4 electrons weakly bonded to the nucleus, each atom forms 4 bonds with its neighbors, when Si is heated, the velocity of valence e increases, and hence their kinematic energy (E k), the speed e becomes so great that the bonds do not withstand t break, e leave their paths and become free, in el. the field they move m-y nodes of the lattice, forming el. current. As t increases, the number of broken bonds increases, and hence the number of connected e increases, and this leads to a decrease in resistance: I \u003d U / R.

When the bond is broken, a vacancy is formed with the missing e; its crystal is not unchanged. The following process takes place continuously: one of the atoms providing the bond jumps to the place of the formed hole and the steam-electric bond is restored here, and where it jumped from, a new hole is formed. Thus, the hole can move throughout the crystal.

Conclusion: in semiconductors there are 2 types of charge carriers: e and holes (electron-hole conductivity)

Topic 7. Electrical conductivity of liquids and gases.

§one. Electric current in gases.

§2. Non-self-sustained and independent gas discharges.

§3. Types of non-self-sustained discharge and their technical use.

§four. The concept of plasma.

§5. Electric current in liquids.

§6. Laws of electrolysis.

§7. Technical applications of electrolysis (independently).

Electric current in gases.

Under normal conditions, gases are dielectrics and become conductors only when they are somehow ionized. Ionizers can be X-rays, cosmic rays, ultraviolet rays, radioactive radiation, intense heating, etc.

Ionization process gases is that under the action of an ionizer one or more electrons are split off from atoms. As a result, instead of a neutral atom, a positive ion and an electron arise.

Electrons and positive ions that have arisen during the action of the ionizer cannot exist separately for a long time and, recombining, form atoms or molecules again. This phenomenon is called recombination.

When an ionized gas is placed in an electric field, electric forces act on free charges and they drift parallel to the lines of tension - electrons and negative ions to anode(the 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 by donating or accepting electrons, thereby completing the circuit. An electric current is generated in the gas. Electric current in gases is called gas discharge. In this way, the conductivity of gases has an electron-ion character.

Non-self-sustained and independent gas discharges.

Let's assemble an 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, then an electric current will appear in the circuit, disappearing with the action of the ionizer.

Electric current in a gas with non-self-conductivity is called non-self-sustained 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 - a section on which Ohm's law is observed. Only a part of the charged particles reaches the electrodes, a part recombines;

AB - the proportionality of Ohm's law is violated and, starting from, the current does not change. The highest current that is 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 action of the external ionizer due to the ions and electrons that have arisen as a result of impact ionization(ionization of electric shock); occurs with an increase in the potential difference between the electrodes (occurs electronic avalanche).

The process of passing email. current through the 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. nesamost. gas discharge. V

Consider. email scheme, comp. from a capacitor, a galvanometer, a voltmeter and a current source.

Between the plates of a flat capacitor is air at atmospheric pressure and room t. If a U equal to several hundred volts is applied to the capacitor, and the ionizer does not work, then the current galvanometer does not register, however, as soon as the space between the plates begins to penetrate. flow of UV rays, the galvanometer will start registering. current. If the current source is turned off, the flow of current through the circuit will stop, this current is a non-self-sustained discharge.

j = γ*E - Ohm's law for el. current in gases.

With a sufficiently strong e. field in the gas begins the process of self-ionization, due to which the current can exist in the absence of an external ionizer. This kind of current is called an independent gas discharge. The processes of self-ionization in general terms are as follows. In nature. conv. A gas always contains a small amount of free electrons and ions. They are created by such natures. ionizers, like space. rays, radiation of radioactive substances, soda in soil and water. Fairly strong email. the field can accelerate these particles to such speeds at which their kinetic energy exceeds the ionization energy when electrons and ions collide on the way to the electrodes with neutrons. molecules will ionize those molecules. arr. upon collision, new secondary electrons and ions also accelerate. field and in turn ionize new neutrons. molecules. The described self-ionization of gases is called impact polishing. Free electrons cause impact ionization already at E=10 3 V/m. Ions, on the other hand, 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 for electrons the mean free path 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. enough to knock electrons out of metals. Therefore, the ions dispersed by the “+” field, hitting the metal cathode of the field source, knock out the electrons from the cathode. These knocked-out electrons field and produce impact ionization of molecules. Ions and electrons, the energy of which is insufficient for impact ionization, can nevertheless lead them into excitation when colliding with molecules. state, that is, to cause some energy changes in the email. shells of neutral atoms and molecules. Excit. an atom or molecule after some time goes into a normal state, while it emits a photon. The emission of photons is manifested in the glow of gases. In addition, a photon, absorb. any of the gas molecules can ionize it, this kind of ionization is called photonionization. Some of the photons hit the cathode, they can knock electrons out of it, which then cause impact ionization of the neutron. molecules.

As a result of impact and photon ionization and knocking out electrons from the “+” code by ions by photons, the number of photons and electrons in the entire volume of the gas increases sharply (avalanche-like) and an external ionizer is not needed for the existence of a current in the gas, and the discharge becomes independent. CVC of the gas discharge is as follows.

The process of occurrence and formation of avalanches considered above due to impact ionization does not lose the character of a non-self-sustained discharge, since in the event of termination of the external ionizer, the discharge quickly disappears.

However, the emergence and formation of an avalanche of charges is not limited to the process of impact ionization. With a further, relatively small increase in voltage, on the electrodes of the gas-discharge gap, positive ions acquire more energy and, hitting the cathode, knock electrons out of it, occurs secondary electron emission . The resulting free electrons on the 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 ejected from the cathode is capable of being accelerated and producing impact ionization of gas molecules, then the discharge will be maintained even after the action of the external ionizer ceases. The voltage at which an independent discharge develops is called closing 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 due to the voltage applied to the gas. Those. this discharge continues even after the termination of the ionizer.

When the interelectrode gap is covered by a completely conducting gas-discharge plasma, it breakdown . The voltage at which the breakdown of the interelectrode gap occurs is called breakdown voltage. And the corresponding electric field strength is called breakdown tension.

Let us consider the conditions for the emergence and maintenance of an independent discharge.

At high voltages between the electrodes of the gas gap, the current increases greatly. This is due to the fact that the electrons arising under the action of an external ionizer, strongly accelerated by an electric field, collide with neutral gas molecules and ionize them. As a result of this, secondary electrons and positive ions(process 1, figure 8.4). Positive ions move towards the cathode and electrons move towards the anode. Secondary electrons again ionize the gas molecules, and, consequently, the total number of 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 action of electrons is not sufficient to maintain the discharge when the external ionizer is removed. For this, it is necessary that the electron avalanches be “reproducible”, 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 an electric field, hitting the cathode, knock out electrons from 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, the process of photon ionization of molecules occurs (process 4);
• knocking out electrons from the cathode under the action 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 mean free path than electrons, acquire energy sufficient to ionize gas molecules (process 6), and ion avalanches rush to the negative plate. When, in addition to electron avalanches, there are also ion avalanches, the current increases almost without increasing the voltage.