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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Physics of air ionization. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur Air ionizers of various types, including Chizhevsky Chandeliers, are increasingly becoming part of our everyday life. Many radio amateurs make them themselves. However, not everyone understands what happens “at the tips of the needles” of the structure. What is the “fate” of the generated air ions and how to optimize the parameters and design of the air ionizer itself? These questions are considered by the author of the article. Without at all hoping to give an exhaustive answer to all the questions that arise, I will nevertheless try to talk about the physical processes that occur during ionization. We should probably start with a description of what the air around us physically represents. It consists of 78% molecular nitrogen N2 and 21% molecular oxygen 02 with a small admixture of carbon dioxide and inert gases. Gas molecules are very small, their diameter is about 2·10-10 m. A cubic meter of air under normal conditions (temperature 0°C and pressure 760 mm Hg) contains 2,5·1025 molecules. They are in continuous thermal motion, moving chaotically and continuously colliding with each other (Fig. 1). Actually, the pressure of air or other gases is explained by the impacts of molecules on the walls of the vessel.
Molecular physics teaches that the energy of thermal motion is proportional to the absolute temperature T and is equal to kT/2 for each degree of freedom of the molecule, where k = 1,38·10-23 J/K is Boltzmann's constant. Only at absolute zero temperature (T = 0 or -273,1°C) does thermal movement stop. It will be interesting for radio enthusiasts to note that the electrons in conductors, resistors, lamps and transistors are also subject to thermal motion, so a small, chaotically varying voltage appears at the terminals of these elements, called noise voltage. The noise power applied to the input of any amplifier or radio receiver is determined by the Nyquist formula: N = kTV, where B is the bandwidth. The velocities of molecules take on a variety of values, but in general they obey the Maxwell distribution. If we plot the speed v along the abscissa axis, and the number of molecules having a given speed, N(v), along the ordinate axis, we get a graph of the distribution of molecules by speed (Maxwell), shown in Fig. 2
The root mean square speed of molecules (it is slightly higher than the most probable one corresponding to the maximum of the curve) is under normal conditions about 500 m/s, which is 1,5 times higher than the speed of sound! It is absolutely clear that with such a high concentration of molecules and their enormous speeds, they often collide with each other, and the average free path does not exceed 0,25 microns (this is half the wavelength of light). One can only wonder how the ions “survive” in this nightmare crush! Let's look at them. Ions are the same atoms or molecules, but with a missing or attached “extra” electron. Recall that each atom contains a positively charged nucleus and an electron shell. The charge is quantized, and the minimum possible elementary charge is equal to the charge of the electron (e = 1,6-10-19 K). Any charge in nature is ne, where n is an integer, although it can be a very large number. The number of negatively charged electrons in an atom, equal to the number of positive charges in the nucleus, corresponds to the atomic number of the element in the periodic table. For example, a nitrogen atom has 7 electrons, an oxygen atom has 8. In general, the atom is electrically neutral and quite strong - energy must be expended to modify or destroy it. Particularly large amounts of energy are needed to split a nucleus; such energies are obtained only in special accelerators of charged particles or during nuclear reactions. The easiest way is to remove one outer electron from an atom. The work that must be done in this case is equal to the ionization energy. For double ionization of an atom (removal of two electrons) much more energy is needed. A light atomic or molecular ion very soon unites around itself a certain conglomerate of molecules and turns into a medium aeroion (I. Pollock), characterized by a much larger mass and lower mobility. Settling on microparticles, aerosols, dust particles, etc., these ions turn into heavy and superheavy aeroions (P. Langevin), which have even greater mass and even less mobility. These are no longer ions, but rather charged aerosols, the concentration of which depends entirely on the purity of the ionized air. The characteristics of air ions for fresh air outdoors are summarized in the table.
For industrial and public premises, the air environment of which is subjected to special treatment in air conditioning systems, the minimum required and maximum permissible standards for the concentration of light air ions of negative polarity are established - 600...50, positive - 000...400. The optimal concentration of light negative air ions is considered to be 50...000, positive - approximately half as much [3000]. In enclosed spaces, the concentration of useful light negative air ions usually does not exceed several tens. The concentration of harmful positive ones grows quickly, especially if there are people in the room and televisions, computer monitors and similar devices are working. Ionization mechanisms may be different. Photoionization occurs when a quantum of electromagnetic radiation (photon) collides with an atom or molecule. Impact ionization occurs when colliding with a rapidly moving, and therefore having high kinetic energy (mv2/2) particle. Thermal ionization caused by strong heating of the gas, such that the energy of thermal motion becomes comparable to the ionization energy. Finally, autoionization takes place under the influence of a strong electric field with a strength of 107...108 V/m, sufficient to “tear off” the outer electron of an atom by forces of electrostatic interaction [2]. Ionization energy can be measured, as expected, in joules (SI system of units), but it is much more convenient - in electron volts (1 eV = 1,6-10-19 J). In this case, it is numerically equal to the ionization potential P - the smallest accelerating potential difference that an electron must pass through in order to acquire energy eP sufficient to ionize an unexcited atom or molecule by electron impact. The ionization potentials of atomic nitrogen and oxygen are 14,5 and 13,6 V, respectively, but there are practically no atomic gases in the lower layers of the atmosphere. Nitrogen and oxygen molecules have different ionization potentials - 15,6 and 12,2 V. It is interesting to note that the ionization potential of molecular oxygen is noticeably lower, which already leads to an important practical conclusion: the ionizer must operate at the lowest possible voltage, at which light ions are still produced , - then healthy oxygen ions will predominate. Can gas molecules under normal conditions be ionized, or exchange charges during collisions caused by thermal motion? Obviously not, since the calculation of the average energy of translational motion of a molecule (3 degrees of freedom) gives the value ZkT/2 = 6·10-21 J, which is two and a half orders of magnitude less than the ionization energy. Under natural conditions, ultraviolet radiation from the Sun, radioactive elements in the earth's crust, thunderstorms and other electrical phenomena in the atmosphere ionize the air. Ions are also formed during the evaporation and spraying of water particles, as a result of the vital activity of plants and animals. For example, every human exhalation contains millions of positive ions [3], and cat hairs can create negative ions [4]. Ionization on high potential needles, as noted, occurs under the influence of an electric field with high intensity, and electrons are emitted from a negatively charged needle - after all, the metal has an abundance of “free” electrons that are not associated with the atoms of the crystal lattice, thanks to which the metal is a conductor. The work function of an electron in most metals is several electron volts, which is less than the ionization energy of the gas. Field electron emission [2] from a metal occurs at field strengths above 107 V/m and supplies primary electrons that serve only to initiate ionization processes. Along with it, a photoelectric effect can also occur - the knocking out of electrons by light quanta and ultraviolet radiation if the gas in the vicinity of the needle tip glows. The emitted electron does not remain free for long: after traveling a distance on the order of the mean free path, it will collide with a gas molecule and be attracted to it by electrical forces, forming a negative ion. The process of adding an electron to a neutral molecule no longer requires energy; moreover, this process even releases a small amount of energy. However, the “performance” of a needle operating in this way would be very low. It is interesting to accelerate an electron to such a speed that, colliding with a molecule, it knocks out another electron, which will also be accelerated by the field and knock out another one, etc. An electron avalanche is formed, flying from the tip of the needle. Positive ions are attracted to the negatively charged needle, accelerated by the field and bombard the metal, knocking out additional electrons. Electrons, combining with neutral molecules, form a stream of light negative air ions, scattering from the tip of the needle in the direction of the electric field lines. Ion bombardment probably supplies the bulk of the primary electrons. In order for electrons and ions to accelerate to energies sufficient for ionization, the field potential difference across the mean free path must be 12... 13 V. This means that the field strength E = dU/dl must be 12 V/0,25 μm = 50 MV/m (megavolts per meter!). This huge value of the field strength should not be confusing - it is actually obtained in real ionizers. The described avalanche ionization is accompanied by other interesting phenomena. Some atoms receive energy from collisions with electrons and ions that is insufficient for ionization, but transfers the atom to an excited state (electrons of excited atoms move to higher orbits). Everything in the world strives for balance, and very soon the excited atom, passing into the ground (equilibrium) state, releases excess energy in the form of a quantum of electromagnetic radiation. The energy of quanta of infrared (thermal) radiation is less than about 2 eV, visible (light) - 2...4 eV, quanta with higher energy belong to the ultraviolet range. All these radiations of low intensity are present during the ionization of gases. Visible radiation quanta (photons) create a glow at the tips of the needles, which can be observed in absolute darkness, preferably with a microscope, in the form of a very beautiful bluish star. It is generally accepted that a good ionizer should not have a glow from the needles, but, apparently, there is always a weak glow, and the size of the star is very small. The movement of ions in the air due to several reasons. Diffusion is caused by the same thermal movement of molecules. Thanks to diffusion, different gases in one volume are mixed, odors spread quite quickly, and the temperature is equalized. The rate of diffusion of any gas, particles, molecules or ions is proportional to the concentration gradient, or the degree to which their number changes with distance. This leads to equalization of concentration throughout the volume over time. In air, the diffusion rate is usually very small and is measured in centimeters per second. Light ions move much faster under the influence of an electric field. The speed of an ion in an electric field is determined by its mobility: v = u·E. For example, a light negative ion of molecular oxygen, having a mobility of 1,83 cm2/Vs, acquires a speed of about 2 m/s at a field strength slightly above 10 kV/m. Ions move strictly along the field lines, and by drawing a picture of the field lines in the room, we also get a picture of the ion flows. If there is an ordered movement of all molecules (wind, draft, fan jet), then the ions, of course, are carried away by this flow and move with it. This movement is superimposed on the movement under the influence of the field according to the usual rules of vector addition of velocities. At the same time, due to frequent collisions, the ions recombine - when a negative and a positive ion collide, an electron passes from one to the other and two neutral atoms or molecules are formed. By attracting neutral molecules, light ions become “heavier” and turn into medium ions. As a result, their concentration decreases over time. The average lifetime of a light negative ion is estimated at tens of seconds [3]. It follows that it is impossible to store ions in a closed room “for future use,” and those who believe that by turning on the ionizer for half an hour before going to bed, they will breathe ionized air all night are wrong. It is better if the ionizer works constantly, but with low output, so as to create a not too high, optimal ion concentration. Field concentration on needles. To create or at least evaluate the field pattern near the ionizer and in the surrounding space, it is convenient to divide the problem into two: calculate the “microfield” at the tip of the needle, and then, considering the entire structure of the ionizer as a single electrode, get an idea of the “macrofield” in the entire volume of the room. This technique is often used in electrodynamics, “matching” (equating) the fields at the boundary of the regions under consideration. Let's start with the needle. Since the time of M. Faraday, it has been known that the electric field lines are always perpendicular to the conducting surface (as well as to any equipotential surfaces), and are not interrupted anywhere, starting at positive charges and ending at negative ones. They can leave or come from infinity, which is impossible for closed spaces. The field strength is directly proportional to the density of the field lines, and at the surface - to the surface charge density. Using these rules, we will draw a picture of the lines of force at the tip of the needle with a radius of curvature r (Fig. 3).
Conventionally, it is shown that each line of force ends at a charge (-). It can be seen that both field lines and charges are concentrated at the tip of the needle, where the structure of the field is the same as that of a ball of radius r. Let us use the formulas known from the general physics course for the field strength and potential of a sphere with charge q: E = q/4πεε0r2, U = q/4πεε0r. Excluding the charge q and dielectric constants εε0, we obtain E = U/r, which coincides with the result of a more rigorous derivation [5]. It turns out that not only the potential on the needle, but also its sharpness is involved in creating a field sufficient for ionization. Thus, at the tip of a needle with a radius of curvature of 10 μm = 10-5 m, even at a voltage of U = 1 kV, a very strong field with a strength of 108 V/m appears. This is quite consistent with the experimental results [6], when a noticeable ion current was observed at fairly low voltages and large distances between the electrodes. The microstructure of the metal probably also helps the flow of charges. In Fig. Figure 4 shows an image of a copper single crystal pre-polished and then subjected to ion bombardment, taken with a scanning electron microscope with a magnification of 3000 [2]. It is likely that at the edges of these impressive “peaks” and “craters” the microfield intensity should increase greatly.
Field indoors. As you move away from the tip of the needle, the field strength drops rapidly (inversely proportional to the square of the distance, while the field can still be considered spherical), and at a distance of 1 cm in our example (U = 1 kV, r = 10 μm) it would be only 100 V/ m. Obviously, this is not so, and here we already find ourselves in the macrofield region, so we must be guided by other considerations. Let, for example, the “classical” “Chizhevsky chandelier” hang at a height h above, although poorly conducting, a large-sized table (Fig. 5).
With some stretch, we consider the field between the chandelier and the table to be uniform (the field lines are parallel). Then E = U/h, and putting U = 30 kV and h = 1,5 m, we obtain E = 20 kV/m. Here it’s time to turn to the “Sanitary Rules and Norms” of the State Committee for Sanitary and Epidemiological Supervision [7]! They allow electrical substation personnel to work at this field strength for no more than 5 hours, and during the entire working day, a field strength of less than 15 kV/m and an ion current density of no more than 20 nA/m2 are allowed. The latter can be measured by connecting a microammeter between a conductive plate placed on the upper surface of the table and the positive terminal of the chandelier’s power source, then dividing the “current from the sheet” (in the words of A.L. Chizhevsky) by its area. According to the above estimates, the chandelier operates at the limit of what is permissible and in its original form is more suitable for large halls than for living rooms. This is also evidenced by the data on ion concentrations obtained experimentally by the author during the operation of the Elion-135 ionizer (Diode plant, manufactured in 1995). The assessment was made based on the charge and discharge rate of the electroscope and gave a concentration value of the order of 300 ions/cm000 at a distance of about 3 m from the ionizer. The “current from a sheet” with an area of 2 m0,5, lying at a distance of 2 m under the “chandelier,” was about 1,7 nA, which gives a current density six times greater than the permissible one. Apparently, given such high productivity, the device has a pulse mode of operation. Of course, Ohm’s law has not been canceled, and the ion current must return to the positive pole of the power source. The conductivity of the walls, floor and ceiling is quite sufficient for the passage of microscopic ionic current. We find the equivalent resistance by dividing the voltage on the “chandelier” by its current. Let us assume that in the example under consideration the “chandelier” current is 1 μA, then the equivalent resistance will be 30 kV/1 μA = 30 GOhm. The “return wire” is the reinforcement of reinforced concrete walls, hidden wiring and, in general, any volumetric, albeit isolated object that has sufficient capacity to “absorb” a weak ionic current. In this case, the object will be charged negatively. An attempt to depict a picture of the lines of force around a “chandelier” in an empty room is made in Fig. 6.
The power lines are thicker where the distance to the walls or ceiling is shorter. There the field strength is higher and ions rush there. Their “travel time” is a few seconds at most, and they are mostly useless to you. What to do? Lower the “chandelier” lower so that it is closer to the floor than the ceiling, and as far away from surrounding objects as possible, then stand, sit or lie under it. Then the flow of ions will rush predominantly towards you. Dust and aerosols. Small, well-insulated objects - particles of dust, smoke, water droplets, etc. - are quite quickly electrified in the ionizer field. The process goes like this: the neutral particle is first polarized, that is, positive charges accumulate on the side facing the ionizer, and negative charges on the opposite side (see Fig. 3). The former are attracted stronger (they are closer) than the latter are repelled, so the particle will fly to the ionizer, remaining neutral. But a flow of ions moves towards them, which will soon compensate for the positive charge, as a result the entire particle will become negatively charged. Now it will fly along the power line from the ionizer and settle where the line ends. We must expect that over time, stains from settled dust will remain on the ceiling and wallpaper, and repairs will be needed. Sometimes the pattern of internal reinforcement appears very clearly on the walls and ceiling. Such undesirable phenomena indicate, firstly, that the ionizer was installed incorrectly, and secondly, that it was not turned on in clean air. In conclusion, I would like to wish good luck to the experimenters, health to the patients, and both to the readers who have mastered this article, expressing the hope that they will also express their wishes and thoughts on the issues raised. Literature
Author: V.Polyakov, Moscow
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