§11. Electric current in gases

11.1 Ionization in recombination

Gases, unlike metals and electrolytes under normal conditions consist from electrically neutral atoms and molecules, and therefore they are not good electrical conductor. The experiments prove this. Charged electrometer on the isolated stand in a dry air does not lose its charge for a long time since it almost did not discharge through the air, because there no free charges in it. If two flat plates of air capacitor are or connected with power source and consistently turn galvanometer on, so that in the case of short circuit the galvanometer does not record a current, as an air space between the capacitor plates is an insulator. Having heated the air space of capacitor with a lit match, we will observe a noticeable current. Thus, to make the gas conductible, we must make or bring in it free charge carriers. This can be done, first of all, by way ionization of neutral atoms or molecules of gas. Ionization of gases is possible under the influence of space, x-ray or radioactive rays, due to the collision of atoms with fast electrons or other elementary or atomic particles at heating of gases and so on. In every case it happen the cleavage of one or more electrons from the shell of an atom or molecule. This process is called ionization. Consequently, at the ionization there are free electrons and positively charged ions. The free electrons, in turn, can be grabbed with neutral atoms or molecules, turning the last into the negative ions. Thus, in the gas under the influence of ionizer arise the negative ions and free electrons appear. The gas becomes a conductor of current.By the way, gases (e.g. air) under normal natural conditions have little conductivity caused by partial ionization of them by cosmic rays and radiation elements that are always in small numbers on the ground and in the air.

Atoms and molecules are stable systems of charged particles. For separation of the electron from the atom the energy must be spend expended that is called as energy or ionization operation. Ionization operation of atoms Ai of different gases is not the same. It depends on the chemical nature of gas and energy state of an electron in an atom or molecule. Ionization operation is expressed in electron volts.

Ionization energy is characterized with the ionization potential φi, that is considered as the potential difference that must be passed by an electron in the accelerating electric field to gain energy equal to the operation of ionization:

Ions and free electrons in the ionized gas, as well as neutral atoms and molecules are in constant random motion. In the case of approximationc of opposite charged particles they interconnect between each other and form neutral atoms and molecules. This process is called a recombination. During the recombination of positive ions and electron or two ions of opposite signs, energy is released that has been spent for their ionization. If the ionization of gas is long enough, under the condition of absence of an external electric field, the ionization and recombination processes are in the dynamic equilibrium. This means that the number of ion pairs generated by the ionizer for the time unit per volume unit of gas will equal to the number of neutral atoms and molecules which are formed in the same volume of gas for the same time.

The recombination of ions is accompanied by the release of energy mainly in the form of light radiation, that’s why the processes that take place at recombination are specific for luminosity of gas.

11.2. Non-self-maintained gas discharge in gases

Phenomena associated with the passage of electric current through the gas and are accompanied by changes in the state of gas (composition, pressure, energy states of molecules, etc.) is called an electric discharge in gases. Depending on the mechanism of ionization discharges in gases are divided into non-self-maintained discharge and self-maintained discharge. The electrical discharge is considered to be non-self-maintained if it occurs only under the influence of the ionizer, and without it the discharge disappears. Electrical conductivity of gases is researched with the help of gas discharge tube (Fig. 11.1) with two electrodes filled with the investigated gas. Voltage between the electrodes can be altered with the potency meters. Ionization is made engaged in random way, with the ultraviolet or X-rays. As it is seen from the current-voltage characteristics of electrical discharge (Fig. 11.2), in gas, with constant power of ionizer, initially with the change of voltage U the electric current varies linearly. With further increase in the voltage the dependence of I = f (U) becomes non-linear, and if U> U1, current strength is independent of voltage (Is = const). The current Is is called the saturation current. With increasing of voltage U> U2 there is a significant increase in current strength, which is accompanied by heat and light effects. Current in gases under non-self montained discharge is created with directional movement of ions and electrons under the influence of electric field.

Let us consider self-non-maintained gas discharge quantified. For simplification we assume that gas discharge occurs between two flat electrodes, where the concentration of positive and negative ions same (n⁺ = n^- = n); charges of positive and negative ions are equal to the absolute value of the electron charge (q⁺ = q^-= | E |). In gas discharges uneven distribution ions between the electrodes often observed. In this case their concentration gradient dn / dx do not equal to zero and there is a noticeable diffusion flow of ions.

 

fig.11.1 fig. 11.2

In the case of a creation of potential difference between the electrodes, besides occurs flows of ions, directional movement of positive and negative ions, so the total current density is defined by the expression.

j=n⁺q⁺v⁺ +n^- q^-v^- - q⁺ D⁺

. (11.1)

where according speed directional movement and diffusion koe¬fitsiyenty positive and negative ions. If the concentration of ions around the same volume between the electrodes, the diffusion of ions poto¬kiv not. Then the expression (11.1) is simplified:

(11.2)

where mobility of gas ions, which are introduced similar to the ion mobility of electrolyte; E - electric field. Equation (11.2) is similar to Ohm's law. It would be the equivalent of Ohm's law provided that the factor ne (b⁺+b^-) is independent of E. This is true in the case of sufficiently small E, where there is linear current-voltage characteristics.

Let’s write the equation of ions balance in the gas by the presence of a constant electri current and influence the external ionizer. Let’s under the ionizer’s influence each second per unit volume formed ion pairs. Because every second recombination disappears per unit of volume n ion pairs. As a result of recombination every second per unit of volume a certain number of pairs of ions disappear which is proportional to both the concentration of negative ions n^-, and the concentration of positive ions n⁺ nthat is n=an² (α - coefficient of recombination; n + = n^-= n).

When electric current is present, the concentration of gas ions will be declining. If the electrode area S, and the distance between electrodes l, then at a current I (current density j), the reduction of ion number in volume unit is defined as follows:

11.3. Self maintained discharge in gases

With increasing voltage to values U> U2 current in the gas discharge increases sharply in the hundred or thousand of times. Experiment shows that under certain conditions stoppage of the ionizer does not affect the flow of the discharge. The electric current in the gas that passes without action external ionizer, called self maintained discharge. Self-discharge is maintained at very high voltage on the electrodes, under which a discharge that began independently creates a needed for further flow of electrons and ions. Replenishment of carriers c in self maintained discharge can various reasons, in particular various reasons, in particular through the mechanism of collision ionization of atoms (molecules) of gas. This is the process of knocking electrons out of atoms in a collision with a stream of fast electrons. Thus, the discharge becomes dependent on independent when new ions are formed as a result of internal processes occurring in the gas.

Primary electrons, causing an avalanche increase in the concentration of charge carriers in the gas space between the electrode may be due to tearing electrons strong electric field of individual gas molecules or cold cathode (field electron emissions), gas by space, γ- or x-rays more. Primary electrons to the free run are accelerated by strong external field and pick up speeds when their kinetic energy is equal to or greater and is of ionization of the gas. Thus there are positive ions and electrons new, which also provides energy required to ionization neutral gas atoms. This process of birth of new carriers repeatedly repeated quickly growing. So every electron, until it reaches the anode. This simplified picture of impact ionization. In fact, not every collision ionization, not all electrons to the free run acquire energy required to ionize neutral atoms. However, the process of ionization is just zbudzhen¬nya atoms when they move to higher energy levels and when returning to normal emit light and therefore gas in self maintained discharge illuminates.

Collision ionization takes place under the condition when kinetic energy of charged particles(electrons) W_к exceeds the ionization operation А_i of gases atoms, that is

W_к > А_i, , or ,

where — ionization potential. The electron acquires kinetic energy in the constant electric field along the nag of free run:

,

d — space between electrodes. The condition of collision ionization takes the form of:

.

From this condition it follows that to achieve collision ionization of gas atoms we have to increase voltage the electric field Е= U/ d or by increasing < l > (by dilution of gas).

Let us consider the basic foundations of the theory of self maintained discharge. Note that for large values ​​of the field voltage E recombination of charge carriers can be neglected, because soon after the occurance of opposite charges they are diluted on the opposite sides by strong electric field.

Assume that the distance between the electrodes in a gas discharge tube d (fig. 11.3). Presume for the time t near cathode the primary electrons with the concentration n₀ was formed. Moving in the accelerating electric field, the electrons in their path's х will ionize gas atoms.

We consider that on the path х п of secondary electrons it was formed as a result of collision ionization in each unit volume. Let’s take the layer of gas dx >> <l>. Flowing through this layer, п electrones ionize atoms and their concentration on this path increases by dп. It’s

obviously, that dп is directly proportional to both the

number of electrons п, and the distance dх:

fig. 11.3

, (11.7)

α — collision of ionization coefficient, m ^-1. Integrating the last expression, we obtain:

п = Се^αx. (11.8)

When х = 0 С = п₀. Then

п = n₀е^αx. (11.9)

The number of electrons that reaches the anode unit area (х =d),

n_а = n₀е^αd. (11.10)

It follows from the formula (11.10)0 that for n₀ = 0 n_a is also equal to zero consequently collision ionization will not happen. To become a self maintained discharge, it is necessary that the electronic avalanche support themselves, ie that there was gas in another process in a gas by which new electrons will be formed. The secondary electron emission from the cathode during its bombardment by ions may be one of these processes. Denote asn_k the number of electrons released from per unit of surface for per unit of time under external ionizer and because the secondary electron emission. Then the flow density of electrons at the anode is

n_а = n_kе^αd. (11.11)

As a result of impact ionization the number of electrons that occurs in the avalanche is equal to the number of positive ions that were formed,

n_а – n_к = n_к (е^αd -1). (11.12)

During the bombarding of the cathode by ions due to the secondary electron emission the electrons from itwill be released where γ — proportionality factor (for metals γ <1). Then, taking into consideration, the effect of the external ionizer and secondary electron emission from the cathode, we can write down that:

. (11.13)

From it

. (11.14)

Flux density of electrons at the anode

. (11.15)

The anode current gas discharge completely determined by the motion of electrons, so the current density based on (11.15) be expressed as:

. (11.16)

At steady state discharge the current density must be the same throughout the period of gas discharge, so anywhere on the current consist of the sum of the densities of electronic j_е ionic j_i, current, so

j = j_e + j_i.

The analysis of the expression (11.16) shows that, when , current densityeven in thecase of termination of the external ionizer (n₀ = 0).Thus, the condition for self-discharge is equal

. (11.17)

Voltage at which the condition (11.17) can be carried out called the breakdown voltage of the gas or voltage ignition of the gas discharge.

The theory of self-discharge was first developed by the English physicist James. Townsend (1868-1957).

11.4.Smoldering discharge

A shape and a relative position of the electrodes, their mode (elevated power, type of cooling and other parameters options) determine the type of discharge.. Each type has a corresponding ionized gas, which is characterized by temperature, electrical conductivity, emission and absorption spectra and more.. Moreover, it appears that the state of some element of ionized gas for a specific type of discharge substantially depends on which area of the discharge gap contains the element and on its distance to the electrode.. n this regard, not only to distinguish the types of discharges, but also the region of the discharge gap that belongs to the same type of discharge.

fig. 11.4

The simplest and most studied type of discharge, where the gas is in a non-stable state is smoldering discharge. It is observed in gases at low pressures (about 10³ pascal and lower). Smoldering discharge is called a self-discharge, which release electrons from the cathode is due to the bombardment of its positive ions and photons generated in the gas. To monitor the discharge of this type we can take glass tube 30-50 cm long, in which two electrodes are soldered (fig 11.4, а). When the voltage between the electrodes is several hundred volts and gas pressure in the tube is about 7∙10³ pascals, the self-discharge in the form of bent thin filaments appears, on the pressure of 133∙10^-1 —133∙10^-2 pascals charge looks like that shown schematically in fig. 11.4, а. The discharge gap between the cathode and the anode is divided into several areas. Narrow, so-called Aston dark space directly adjacent to the cathode 1. In this layer, electrons released from the cathode, do not have sufficient energy to excite the atoms and molecules of gas. Its width is a few tenths of a millimeter . Thin light layer 2, which is called the cathode glow adjacent to this layer. In this layer there is excitation of atoms and molecules without electron ionization. In the transition to the normal state the excited atoms emit light. For the cathode layer is a layer 3 Dark who called cathode dark space or сrookes dark space. In this space there is ionization of atoms and molecules and the growth of electronic avalanches. The area of cathode dark space is essential for the maintenance of self-discharge, because it produced positive ions, which determines the secondary electron emission from the cathode. A light layer 4 is called smoldering glow. It is caused by the recombination of electrons with ions and atoms transitions from the excited state to the normal. This layer has a sharp boundary from the cathode. Its brightness gradually decreases and becomes a so-called Faraday dark space 5. Area 1 - 5 are called the cathode discharge parts. All the processes by which the discharge becomes self-discharge happen in these areas. The luminous region 6, which reaches the anode lies behind the Faraday space. This area is called the positive pole illuminating. Sometimes this pole is divided into several layers or strata. The positive pole - is an ionized gas, and its glow is caused mainly by recombination of electrons with positive ions. Positive pole does not affect the maintenance of self-discharge. However, it is the most interesting region in terms of the application smoldering discharge.

The elucidation of the nature of the physical processes that occur in each region of the smoldering discharge, helps the potential distribution curves φ and the voltage of electric field E along the discharge tube l (fig. 11.4, b, c). Because of the positive pole of the course depends on the capacity of line l, and the voltage is constant, then it follows that the concentration of positive and negative charges are equal. If the gas discharge tube anode make moving, then its movement to the cathode cathode discharge region 1 - 5 remain unchanged, and positive pole will decline to extinction it. With further movement of the anode layer into smoldering sphere glow disappears. Discharge stops when the anode is on the verge spaces l and 2. Experiments show that when the power of the current during the discharge is small, the cathode potential decline is independent of the current strength. Change amperage just makes resizing luminous surface of the cathode. This decline is called the normal cathode potential decline. When the current strength reaches a certain value, the entire surface of the cathode is covered with fluorescent tape and cathodic potential recession starts to increase with increasing current strengthIn this case it is called anomalous cathode decline, and discharge - abnormal smoldering discharge. It turns out that the normal decline cathodic potential depends on the cathode material and the kind of gas Cathode potential decline thus proportional to the work function of electrons from the cathode. This dependence makes it possible to produce gas-discharge tube with low voltage lighting category. Neon lamps, iron electrodes are covered with a layer of barium discharge ignition voltage of about 70 V. These lamps are widely used as indicator.

Since the substance of the cathode in a glow discharge gradually transformed into a vapor state, such a discharge is widely used for cathodic sputtering of metals. If the front of the cathode glow discharge to place different objects, they will be covered with a thin layer of metal cathode. Since metal mirrors made ​​of high quality

Smoldering discharge is also widely used as a light source (fluorescent lamps, discharge tubes advertising, etc.).

Unlike gas cathode gas discharge of the positive column of a high degree of spatial uniformity parameters along the column. A characteristic feature of the positive column is the ratio of electric field to gas density and the average electron energy is set regardless of the strength of the current flowing in the discharge, and the applied voltage to the electrodes.

. The discharge in the atomic gas is significantly different from the level of molecules-polarized gas. This is due to the presence of their vibrational and rotational degrees of freedom. The presence of these additional degrees of freedom determines the basic properties of the discharge in molecular gases and enables the widespread use of this type of discharge in various laser, plasma-chemical and other plants. The creation of molecular gas with a gas which the help of is not in equilibrium state discharge makes it possible to use it to speed up chemical reactions.

11.5. Spark discharge

With the gradual increase in voltage between two electrodes placed in a gas at normal pressure, there is a separate category, called spark. Thus the air gap between the electrodes is penetrated by brightly illuminating a thin channel zigzag shape with branching (Fig. 11.5). Spark discharges in gases is a breakdown of the dielectric gas that occurs in the case of a certain electric field strength Ec (critical field strength or breakdown voltage). This strenght depends on the type of gas and its condition on the shape of the electrodes. For air at normal conditions Ek ≈ 3∙10⁶ v/m. The critical value of the strenght is almost linearly dependent on the gas pressure in a wide range of its

. (11.18)

However, after discharge gap "break" spark channel resistance of the gap is small and the channel passes through a brief pulse of current importance, which leads to release of large amounts of heat. The temperature of the gas in the discharge gap reachesK. Instantly heated gas expands to form a cylindrical shock waves. This is accompanied by sound effects (or thunder crackling)

fig.11.5 fig.11.6

Experiments show that the channel spark discharge begin to grow sometimes from the positive electrode sometimes from the negative one, and sometimes even from some point in between.

The field strength at the electrodes depends on the curvature of the electrode surface. Therefore, the minimum voltage at which for a given distance between the electrodes spark discharge begins it is vary for different electrode shapes. Thus, at a voltage of V in air,under normal, spark discharge occurs between edges in the range of 220 mm, and between the flat electrodes - in a span of 36.7 mm. If you reduce the distance between the electrodes at a constant voltage, the electric field in the gas gap will grow and when you reach critical strength Ek spark discharge occurs. The greater the distance between the electrodes, the greater the voltage required between them, in which the first spark slip. This dependence on the size of the spark gap voltage as the basis of the high-voltage spark voltmeter.

Consider the mechanism of spark discharge. Initially it was thought that the spark discharge is caused by the same process as the glow discharge, ie bulk ionization of molecules elektrons and secondary electron emission of positive ions. However, as shown by research, these processes can not explain a number of peculiarities of the spark discharge. Thus, according to the mechanism given time to develop discharge sparks should be about 10^-4—10^-5 s. Oscillographic studies show that the time of spark discharge is much smaller and is 10^-7—10^-8 s. The mechanism of spark discharge theory explains the streamer discharge, which was developed in 1940 by T. Mick D. Loeb. This theory qualitatively explains the main features of the discharge. According to this theory the occurence of brightly glowing sparks channel is preceded by the appearance of luminous areas of high conductivity - streamers, their origin is explained by the occurrence of electron avalanches near the cathode. Under the influence of streamers ionization and excitation of atoms and molecules of gas are take place. Light photons emitted by excited atoms and molecules in the path of the anode, ionize the gas and create new beginnings of new streamers. Thus, the cause of streamers is not only the formation of electron avalanches due to impact ionization, but the ionization of the gas emission that occurs in the category of discharge (photoionization). After the electronic avalanche that occurred near the cathode due to photoionization, the emergence of new avalanches is take place. These avalanches in their development produce other avalanches. This process is quite fast. While the first avalanche size reaches the AB region of high ionization (streamer) will have dimensions of SP (Figure 11.6: Avalanche symbolically shaded cones and wavy lines show the direction of propagation of radiation from regions of high ionization).

In solid and liquid dielectrics spark discharge is the main form of self conduction associated with the destruction of the dielectric. In solid dielectrics spark leaves the hole, so the word "breakdown" corresponds to its contents.

Along with streamers extending from the cathode to the anode (negative streamers) there are also streamers propagating from the anode to the cathode (positive streamers).

11.6. Corona discharge

Corona discharge is observed at relatively high gas pressure (atmospheric pressure), which is in rather inhomogeneous electric field. Such a field can be created between two electrodes, one surface of which has a large curvature (wire, tip). The presence of the second electrode is not required, because its role can perform the grounded body. When the electric field near the electrode with greater curvature reaches 3∙10⁶ V/m, there is a glow around it. It looks like a shell, or corona surrounding this electrode. Crown, which occurs around the negative electrode, called the negative and positive electrode around - positive. The mechanism of discharge in both cases is different.

If the crown is negative, the positive ions generated by electronic avalanches, accelerated inhomogeneous electric field near the cathode and cause secondary electron emission. These electrons give rise to new electronic avalanche. Because of the distance from the wire field strength decreases, at some distance from the electronic avalanches are break down. The distance covered by the electronic avalanche is the thick ness of the crown. It is always less than the distance between the electrodes. When corona discharge electronic avalanche is not fully penetrate the layer of gas that is a partial breakdown of the gas gap. Electrons that fall into the dark zone of corona discharge, adhered to the neutral gas atoms, causing negative ions. In the case of pure electropositive gases, negative ions are not formed thus the the charge carriers in this area will be electrons. Research found that adding a small amount of electronegative gas to electropositive on it significantly reduces the power of the discharge current under the same conditions. In the dark zone the discharge is dependent.

If the crown is positive, the electronic avalanches occur not as a result of secondary electron emission, but by photoionization radiation through the voluminous crown layer near the anode. Positive ions in a dark area of the crown move to the cathode. For the occurrence of corona discharge in air is necessary that the electric field strength on the electrode surface was not the less than the initial field strenght E of the crown, which is determined by the empirical formula PeakkV / cm,

δ — density of the gas within its density at atmospheric pressure and temperature 25 °С; r_{0 -} radius of the cylindrical wire, where occurs crown.

Along with fixed current flow in the corona discharge the interrupted phenomenon may also occured. In the negative corona these effects depend on the nature of the gas, humidity, the presence of dust in it, the state of the surfaces of the electrodesIn the discharge current pulses are observed regularly. They represent a series of avalanches, accompanied by photoionization in the surrounding gas volume. In the positive corona interrupted phenomena caused by the presence of pulses of two types: avalancheing and streamering. The presence of discontinuous phenomena in the corona discharge is causing significant radio interference.

With increasing voltage between the electrodes the so-called dark zone corona disappears and there is a spark discharge. Corona discharge is intermediate between the glow and spark discharges.

Corona discharge should be considered in high voltage engineering, as this occurring loss of electricity. In order to prevent corona on high voltage power lines wires must not have defects in the form of sharp protrusions.

Corona discharge is used in electric filters, intended for the treatment of industrial gases from solid and liquid contaminants (smoke in the manufacture of sulfuric acid fumes shops foundry of ferrous and non-ferrous metals, etc.)

11.7. Arc discharge

If you get a spark discharge from a powerful power source, and then gradually decrease the distance between the electrodes, the intermittent discharge is continuous, ie, there is a new type of discharge, which is called the arc. In this current strenght increases dramatically, and the voltage on the discharge gap drops to a few tens of volts.

Arc discharge can be formed, bypassing the spark discharge. To do this a large the electrodes pull together to contact them occurs electrical resistance in the places of contact. As a result, a significant amount of heat is released and electrodes heated. If electrodes separate to a certain distance there is an arc discharge between them. In this way at 1802 V. Petrov invented the arc discharge.

Electric arc in a gas at atmospheric pressure is formed through special form of carbon electrodes, which are made by pressing powdered graphite and bindering substances. During the arcing the distance between the electrodes is about 5 mm, current - 10 - 20, and the voltage between them - 40 - 50 V. In the process of arcing carbon cathode progressed the concavity formed at the anode, which is called the arc crater. His temperature at atmospheric pressure is up to 4000 ° C and at a pressure of 20 atm is more than 7000 °C. Cathode arc has a lower temperature than the anode, so at the atmospheric pressure its temperature reaches 3500 ° C. In an electric arc with metal electrodes rapid evaporation of metal, is occured a significant amount of heat is spent. Therefore, the temperature of this arc is lower than the arc of carbon electrodes, and is 2000 - 2500 ° C.

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