RU2478141C2 - Modification method of material surface by plasma treatment - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: method involves loading of material to a chamber, vacuum pumping of the chamber, plasma treatment of material surface and its unloading. Plasma treatment is performed with cathode spots of vacuum arc discharge created in the chamber so that remelting of the material surface layer is provided. Pressure in the chamber is maintained at the level of not more than 1 Pa, voltage of vacuum arc discharge - not less than 10 V, and current of vacuum arc discharge - not less than 1 A. Excitation and maintenance of vacuum arc discharge is performed at application between cathode and anode of constant or impulse-periodic voltage, and localisation of cathode spots on the surface and control of their movement is performed with magnetic field.

EFFECT: improving efficiency and quality of modification of surface of materials and produced items.

6 cl, 2 dwg

Description

Technical field

The invention relates to the field of beam-plasma technologies for improving the operational properties of structural materials and products.

State of the art

In many cases, changing the physico-chemical characteristics of the surface layer of structural materials and products is a sufficient and cost-effective way to improve their operational properties. Currently, historically traditional similar technologies (galvanic coating, thermal hardening, cementing, polishing, etc.) are mainly replaced by environmentally friendly plasma-beam technologies. Although the production volume of functional coatings (including nanostructured coatings) using such technologies occupies the largest market segment, technologies of modifying the surface layer by processing with concentrated energy flows (laser radiation, electron beams, plasma flows) are of considerable interest. The use of various energy sources is determined by the specifics of specific tasks and has or may have its own niche in the use of modern surface modification technologies.

The results of processing structural materials and products with concentrated energy flows are the removal of surface contaminants (inclusions), surface polishing, however, the most important is the ability, under certain conditions, to change the microstructure and phase composition of the surface layer of materials and products and, thereby, improve their functional performance. In the first case, the grains are disaggregated (up to amorphization), in the second, the appearance of metastable phases and compounds, which cannot be formed under conventional heat treatment methods.

A necessary condition for (pseudo) amorphization of the microstructure is a high (≥ 10 ⁶ K / s) cooling rate of the molten layer (Luborsky F.E., Davis H.A., Liberman H.H. Amorphous metal alloys, M., Metallurgy, 1987, 582 p.). The use of pulsed sources of concentrated energy flows makes it possible under certain conditions to use the natural thermal conductivity outflow of heat into the material for rapid cooling of the surface layer. The requirements for the heating rate to the melting temperature can be estimated based on the need to use the adiabatic regime in which the energy absorbed in the surface layer remains within its limits for the duration of the pulse, i.e. not carried into the depths of the material.

Laser surface treatment of materials (hardening, amorphization, polishing, impact hardening, etc.) uses a pulsed (repetitively pulsed) radiation mode with a small beam diameter, so processing an area with centimeter or more dimensions requires the use of a beam scan. The high cost of equipment makes it possible to commercialize such technologies only for very specific operations, for example, manufacturing microoptical elements from glass ceramics (Veiko IP, Kieu OK Laser amorphization of glass ceramics: Basic properties and new possibilities for manufacturing microoptical elements // Quantum Electronics, 2007, v. 37, No. 1, pp. 92-98).

The use of electron beams for such operations is preferable to achieve relatively large (up to ~ 100 μm) thicknesses of the modified layers, however, the complexity, relatively high weight and size characteristics and the cost of such equipment significantly limit the use of this technology (Bakai AS, Borisenko AA Russel KC Amorphization kinetics under electron irradiation // Issues of Atomic Science and Technology, Ser. Physics of Radiation Damage and Radiation Materials Science, 2005, No. 4, pp. 108-113).

Modern sources (accelerators) of plasma flows allow the processing of surfaces with a relatively large area (e.g., ~ 300 × 300 mm ² , Litunovsky VN et al. Quasistationary plasma accelerators for experiments on thermonuclear fusion and technology // Plasma Devices and Operations, v. 2, Issue 2, 1992, pp. 111-123) in a single pulse to modify the topography, structure and composition of the surface layer of materials and products, including complex geometry. Such accelerators operate in a pulsed mode, limiting the performance of the process, and are also quite complex, which inhibits their commercial use.

A well-known environmentally friendly technology for cleaning the surface of metallurgical products using electric arc treatment, including the initiation of an electric arc discharge, the formation of cathode spots on the surface to be treated and cathode spots cleaning the surface to remove scale, oxide film and other contaminants ("Method of surface treatment of products by arc discharge in vacuum" , autosweet USSR No. 1695704, С23С 14/02, 1987; "Method for cathodic-vacuum surface treatment of metal parts", patent of the Russian Federation 2118399, 1996; autoswitch USSR 719,710, V08V 3/10, 1977; avt.svid USSR №935141, V08V 3/10, 1980;. Avt.svid USSR №1749279, S22V 9 / 20.1990)..

Processing hot metal billets in this way, for example, not only eliminates dross defect, but also is a guarantee against the formation of flocs, hairs and captures ("Method for surface treatment of products by arc discharge in a vacuum", RF patent No. 2144096, 1998).

There is also a method of processing materials with cathode spots of an arc discharge for pretreatment of metal surfaces (including in the presence of organic contaminants) for subsequent application of functional coatings (Yuya Kubo et al. Pre-treatment on metal surface for plasma spray with cathode spots of low pressure arc // Surface and Coating Technology, v. 200, Issue 1-4, 2005, pp.1168-1172; Takeda K. Dry cleaning of metal surfaces by a vacuum arc // Surface and Coating Technology, v.131, Issue 1- 3, 2000, pp. 234-238; Masaya Sugimoto, Koichi Takeda, Surface variation caused by vacuum arc cleaning of organic contaminant // Thin Solid Films, 506-507 (2006) p.337-341). Typically, such processing is performed at arc current values up to 120 A and a chamber pressure of 200-300 Pa. The technical result of the treatment is to increase the adhesion ability of the surface by removing the oxide layer and increasing the roughness.

It is known that in arc discharges with integrally cold electrodes (cathode and anode), the discharge current can concentrate on the electrodes in the so-called cathode or anode spots, from which the material of the corresponding electrodes evaporates. The formation of cathode and anode spots on the surface of the electrodes is largely determined by the pressure of the gas (s) at which the discharge occurs. At relatively high pressures (> 1 Pa), the arc discharge occurs in the form of simultaneously existing inactive (motionless) anode spots and mobile cathode spots, or in the form of only anode spots. The pressure range of 1-10 Pa is the transition pressure range from the mode of flow of the arc discharge with the simultaneous existence of both cathode and anode spots to the mode of flow of the arc discharge in the form of exclusively cathode spots (ed. USSR. No. 1152433, H01J 41/20, 1988 ) At pressures <1 Pa, an arc discharge occurs exclusively in the form of cathode spots, which are the only source of an electrically conductive medium (ionized vapors of the cathode material) in the interelectrode gap.

In the above technologies and known methods and devices for their implementation, the treatment with cathode spots is carried out at a process pressure substantially higher than 1 Pa. At such pressures, the medium for maintaining the arc discharge is created not only by ionized vapors of the cathode material, but also by ionization of the residual (or introduced working) gas and ionized vapors of the anode material. This significantly affects the microstructure and morphology of the treated surface, introducing them into the impurities of gases and anode material, which is a drawback that in some applications (for example, obtaining a fine-grained or amorphous surface layer of high purity) makes such methods unacceptable. In addition, with increasing gas pressure in the chamber, starting from 1 Pa and above, the cathode erosion rate by cathode spots decreases significantly (several times) compared to the erosion rate under vacuum conditions (p <1 Pa), and the mobility of the cathode spots decreases. This indicates a transition to a more stationary thermal process on the cathode surface with a significant reduction in the share of energy consumption for creating a conductive medium from the cathode material (i.e., for melting, evaporation and ionization of the cathode material), which reduces the efficiency of surface treatment of materials with cathode spots (Juttner B., Puchkarev V., Hantzsche E., Beilis I. "Cathode Spots", in the "Handbook of Vacuum Arc Science and Technology: fundamentals and applications", edited by RLBoxman, PJMartin and DM Sanders. Noyes Publications, Park Ridge, New Jersey, USA, 1995, pp. 145-149; Good V.M. Vacuum arc plasma in the presence of gas in the discharge gap // Physical surface engineering, 2005, v.3, No. 1-2, pp. 82-96; Logachev A.A., Chaly AM, Shkolnik S.M. Dynamics of spots on a copper cathode in a high-current vacuum arc // ZhTF, 1997, v.67, No. 4, p.133-136). Formally, this mode is classified as a low-pressure arc. Although at such pressures (p> 1 Pa) remelting of sections of the surface layer of the cathode (workpiece) is certainly possible, however, the efficiency of this process is noticeably lower, and the melt cooling rate is significantly reduced due to the much more stationary processes.

A similar mode of arc discharge was also used in the method of modifying the properties of structural steel with a cathode spot of a low pressure arc (Demidenko VV, Potemkin GV, Remnev G.E. et al. Modification of properties of structural steel with a cathode spot of a low pressure arc // Physics and chemistry of material processing, 2010, No. 5, p. 43-49), which is selected as a prototype.

The processing process is described by the following sequence. After evacuation of the chamber to a pressure of 1 Pa, nitrogen was admitted into it to a pressure of -1000 Pa. The product was used as the cathode — a steel sheet St.3 measuring 0.5 × 0.8 m ² . An anode assembly with an annular graphite anode is located above the cathode. Processing was carried out by a long arc in the mode of a single cathode spot with an arc voltage of 25-35 ° V and an arc current of 100-150 A.

After the discharge was initiated, the surface temperature of the graphite anode rose to 2000 ° С, which caused its evaporation and the formation of an intense flux of carbon atoms to the cathode (product). Arc burning occurred in a gas-vapor mixture of atomic carbon, nitrogen, iron and oxygen. The last two streams emanate from the cathode and are significantly (50-100%) ionized. The expansion of the charged component of the cathode torch and the movement of the cathode spot were controlled by applying a tangential magnetic field of B = 10-20 mT to the cathode region of the voltage drop.

Such a processing process allows you to effectively clean the surface of oxide films and grease, as well as to modify the structure and phase composition of the surface layer, not only by treating the surface with a cathode spot, but also due to the thermodynamically nonequilibrium process of saturation of the surface melt with atoms and ions of the working gas and anode. In particular, supersaturation of the surface layer with a thickness of 30–40 μm with carbon made it possible to reduce the grain size in the layer, which led to an increase in corrosion resistance. In this case, however, the microhardness after processing did not change, or significantly decreased (up to 3 times).

However, this method can be used only for a limited range of materials (in which both gas and carbon impurities or compounds can have a positive effect) and is unsuitable for obtaining pure modified surface layers.

The purpose of the invention is to increase the efficiency and quality of surface modification of the material by plasma treatment, to obtain clean and highly pure modified surface layers of the material.

This goal is achieved by the fact that the plasma treatment of the surface of the material is carried out by cathode spots of a vacuum arc discharge to ensure remelting of the surface layer of the material, while the pressure in the chamber is maintained no more than 1 Pa, the voltage of the vacuum arc discharge is not less than 10 V, the current of the vacuum arc discharge is not less than 1 A.

In special cases, the implementation of the proposed method of plasma surface treatment to excite and maintain a vacuum arc discharge using a constant or pulse-periodic voltage between the cathode and the anode with an arbitrary ratio of pulse duration and pause, as well as the inert and / or working gas inlet into the vacuum chamber.

Disclosure of invention

It is known that at a pressure p≤1 Pa, a vacuum arc discharge mode is realized, which occurs exclusively in the vapors of the cathode material, and the generation of these vapors is carried out by cathode spots 1, 2 or 3 of the type formed on the cathode surface. The type of cathode spot formed on the cathode surface is determined by the state of the surface (its purity), the current of the vacuum arc discharge and the time of its existence (Zykova N.M., Kantsel V.V., Rakhovsky V.I., ZhETF, 1965, vol. 48, No. 5, p.677; Kesaev I.G. Cathodic processes of an electric arc, Publishing House "Science", M., 1968; Juttner V., Puchkarev VF, Hantzsche E., Beilis I. "Cathode Spots", in the "Handbook of Vacuum Arc Science and Technology: fundamentals and applications", edited by RL Boxman, PJ Martin and DM Sanders, Noyes Publications, Park Ridge, New Jersey, USA, 1995, pp. 73-281). A characteristic feature of the development of a vacuum arc discharge at such pressures is the concentration of the discharge current in microscopic unsteady channels (cathode spots), where the current density and power can reach 100 MA / cm ² and 10 ⁹ W / cm ^2, respectively (due to the small size of the channels - 10 microns and the duration of their existence ~ 10 ^-7 s), determines the blasting mechanism of electron emission from microscopic inhomogeneities (geometric and structural) of the surface of the cathode by a strong (about 10 MV / m), the local electric field in the thin skin prick todnoy region (Anders A. Metal plasma immersion ion implantation and deposition:. a review // Science & Coatings Technol, v 93, 1997, pp.158-167.). The pressure in such a plasma channel (cathode spot) can reach 10 ¹⁰ Pa, which determines its very rapid (explosive) expansion and limits its duration (Mesyats GA et al. Pulsed Electrical Discharge in Vacuum, Springer Verlag, Berlin, 1989). In this case, the cooling rate of the plasma of the cathode spots is determined not only by its free expansion into the interelectrode gap, but also by the high radiation losses of the erosion plasma, which ultimately limits the lifetime of the plasma of a single cathode spot to 10 ^-7 -10 ^-8 s (Anders A. Metal plasma immersion ion implantation and deposition: a review. Science & Coatings Technol., v. 93, 1997, pp. 158-167). When the microdischarge is completed in one cathode spot, the voltage at the cathode-anode gap is restored, and the microdischarge occurs on another part of the cathode surface (another cathode spot). Thus, the movement of the cathode spot along the cathode surface is not a material movement, but a process associated with the generation and decay of the discharge current channels at the active emission points of the cathode surface (cathode spots).

The high power density of the microdischarge at a pressure of p≤1 Pa provides intensive heating of a cathode surface area of about 10 μm in size to the phase transition temperatures of the cathode material, and the high temperature region is much larger than the size of the cathode (emission center) (Valuev V.P., Valueva T.V. Vacuum-arc discharge at an integrated cold cathode // Instrument and Technology, 2010, No. 29, v. 3, p. 36-49). Since, based on the idea of the diffusion nature of heat removal from the molten layer deep into the material, it is shown that the melt cooling rate is related to the heating power Р _h as ∂Т / ∂t ~ Р ² _h (Alekseev V.A., Konkashbaev I.K., Kiselev E.A. et al. // Letters in ZhTF, 1983, 9, issue 1, pp. 42-45), the melt cooling rate necessary for amorphization is 10 ⁷ -10 ⁹ K / s, which is obviously achievable for integrally cold cathodes (materials and products) at the above level of power values in the cathode spot P _h ~ 10 ⁹ W / cm ² .

, где χ, с, ρ - коэффициент теплопроводности, удельная теплоемкость и плотность материала соответственно, а Т=T_исп - температура поверхности, принимаемая равной температуре испарения, имеет максимальные значения в диапазоне 0,5 - 2 МВт/см² для основных (Сu, W, Al, Ti и др.) конструкционных материалов (Литуновский В.Н. Исследование взаимодействия плазмы с поверхностью в режимах импульсной плазменной обработки материалов // Препринт НИИЭФА, П-0992, Федеральное Агентство по атомной энергии, ФГУП "НИИЭФА им. Д.В. Ефремова" 2005, 10 с.), что существенно меньше уровня мощности нагрева (~10⁹ Вт/см²). Таким образом, выполняется критерий эффективности процесса аморфизации - адиабатический режим нагрева, при котором энергия, поглощаемая поверхностным слоем материала в процессе нагрева до температуры фазовых переходов, остается в его пределах вплоть до окончания импульса нагрева.Indeed, the heat transfer capacity in the depth of the material

, where χ, s, ρ is the coefficient of thermal conductivity, specific heat and density of the material, respectively, and T = T _isp - surface temperature, taken equal to the temperature of evaporation, has maximum values in the range of 0.5 - 2 MW / cm ² for the main (Сu , W, Al, Ti, etc.) of structural materials (V. Litunovsky. Investigation of the interaction of plasma with the surface in pulsed plasma treatment of materials // Preprint NIIEFA, P-0992, Federal Agency for Atomic Energy, Federal State Unitary Enterprise NIIEFA named after D. .V. Efremova "2005, 10 pp.), Which is significantly less nya heating power (~ 10 ⁹ W / cm ^2). Thus, the criterion for the efficiency of the amorphization process is fulfilled - the adiabatic heating mode, in which the energy absorbed by the surface layer of the material during heating to the temperature of phase transitions remains in its range until the end of the heating pulse.

For most materials (Ti, Al, Cu, Zr, steel, etc.), maintaining a discharge in intrinsic vapor at pressures p≤1 Pa does not cause problems, however, for some materials (Mo, W, etc.) it can be difficult to transition to ultra low pressures. In this case, after evacuation, a stream of inert and / or working gas stabilizing the discharge can be introduced into the chamber. However, the condition p≤1 Pa must be observed in this case as well in order to realize the discharge flow regime exclusively in pairs of the cathode material (pairs of the processed material).

The total processing time is a statistical superposition of pulsed processes in individual high-current microdischarges spontaneously (or under the influence of a magnetic field) moving along the surface of the cathode (product). Although the process of existence of a separate microdischarge is purely pulsed (τ _p ~ 10 ^-7 s), in general, the operation mode of the device for implementing the inventive processing method can be characterized as a high-performance stationary process. The performance of this process can easily be controlled by the amount of discharge current. An increase in the discharge current leads to a “division” of the cathode spot, i.e. an increase in the number of simultaneously active emission centers, each of which has approximately the same parameters (Anders A. Cathodic arc plasma deposition, Vac. Technol. & Coating, v.3, No.7, 2002, pp. 27-35). The use of pulse-periodic instead of constant voltage provides an additional opportunity to vary the productivity of the processing process by changing the ratio of the duration of the voltage pulse and pause.

The invention allows to carry out a qualitatively different (than in analogues and prototype) surface modification of materials and products, namely, changing and homogenizing the microstructure and phase composition of the surface layer, including its amorphization, while ensuring the cleanliness of the modified layer from gas and / or other inclusions and / or their compounds with the processed material, which ultimately leads to improved operational properties of materials (corrosion resistance, wear resistance, increased hardness, etc.).

The invention also allows the formation of unique pure materials and alloys on the surface of products, alloying the surface of structural steels (for example, nickel, or aluminum). Due to the high cooling rate of the molten layer, it is possible to form compounds that do not form under ordinary (equilibrium) conditions (Fe / Pb, Mo / Cu, etc.). It is known that highly pure materials and alloys can be obtained by electron beam melting in vacuum, during which re-melting of all material in vacuum occurs at a pressure of (1-10 ^-3 ) Pa and removal of gas and other impurities dissolved in the material (Bruckmann G. and Scholz H. "General Metallurgical Aspects of Vacuum Treatment", in the "Handbook of Vacuum Arc Science and Technology: fundamentals and applications", edited by RL Boxman, PJ Martin and DM Sanders, Noyes Publications, Park Ridge, New Jersey, USA 1995, pp. 544-559). In many applications, however, the requirements for the purity of materials and alloys are imposed only on their surface. In this case, there is no need for a vacuum remelting of the entire material, a surface remelting at the same pressures is sufficient, which can be easily implemented using the proposed method. This greatly simplifies and reduces the cost of the process.

Brief Description of the Drawings

The inventive method of processing materials and products is illustrated by sketches of possible structural devices for its implementation. Figure 1 presents a diagram of one of the possible variants of a device for processing products of cylindrical shape.

The device consists of a vacuum chamber 1 with a quick-detachable loading hatch 2, which serves as the anode of a vacuum arc discharge; a water-cooled hollow cylindrical cathode holder 3 of the workpiece to be connected to the vacuum chamber using a flange 4 with connectors 5 for supplying current and cooling water; the workpiece 6, placed on the cathode-holder with ensuring good electrical and thermal contacts; the upper 7 and lower 8 electromagnetic coils located in the cavity of the cathode-holder in such a way that their middle sections are in the plane of the upper and lower bases of the workpiece; insulator 9, used for electrical isolation of the anode and cathode. The vacuum pumping of the chamber is carried out through the flange 10 on the vacuum chamber, and the gas inlet through the valve 11.

Figure 2 presents a diagram of one of the possible variants of a device for processing products of a flat shape. The device consists of a vacuum chamber 1 with a quick-detachable loading hatch 2, which serves as the anode of a vacuum arc discharge; a water-cooled cathode-holder of the processed products 3, connected to the vacuum chamber using a flange 4 with connectors 5 for supplying current and cooling water; the workpiece 6, placed on the cathode-holder with ensuring good electrical and thermal contacts; the upper 7 and lower 8 electromagnetic coils located on the vacuum chamber above and below the plane of the workpiece, respectively; insulator 9, used for electrical isolation of the anode and cathode. The vacuum pumping of the chamber is carried out through the flange 10 on the vacuum chamber, and the gas inlet through the valve 11.

The implementation of the invention

The inventive method is as follows. After vacuum evacuation of the device to a residual pressure of not more than 1 Pa between the cathode (workpiece 6) and the anode (vacuum chamber 1), a vacuum arc of constant or pulsed voltage of at least 10 V with a discharge current of at least 1 A is excited. To facilitate initiation and maintaining a vacuum arc discharge, it is possible to supply an inert gas to the vacuum chamber at a flow value providing an integral pressure in the chamber of not higher than 1 Pa. The entire discharge current of the vacuum arc is concentrated on the cathode in one (or several) cathode spots, which randomly move along the cathode surface. For uniform processing by cathode spots of the entire surface of the product, the movement of the cathode spots is controlled by a magnetic field.

In the case of a cylindrical product (Fig. 1), the magnetic field of the arched configuration on the surface of the product is created by two electromagnetic coils located in its cavity. The cathode spots in such a magnetic field are localized under the apex of the magnetic arch and rotate in the azimuth direction at high speed (up to 100 m / s). The position of the top of the magnetic arch is determined by currents in the electromagnetic coils and can vary from the plane of the upper base of the cylinder (when only the upper electromagnetic coil is turned on) to the plane of its lower base (when only the lower electromagnetic coil is turned on). Uniform surface treatment of the product by cathode spots is carried out when controlling currents in electromagnetic coils (manually or according to the program).

In the case of a flat product (figure 2), a solenoidal magnetic field on the surface of the product is created by two electromagnetic coils located on the vacuum chamber. The cathode spots in such a magnetic field, rotating in the azimuthal direction, also move in the direction of the acute angle formed by the intersection of the magnetic field lines with the surface of the product. When the upper coil is turned on, this movement corresponds to the direction to the center of the product, when the lower coil is turned on, to its periphery. Uniform surface treatment of the product by cathode spots is carried out when controlling currents in electromagnetic coils (manually or according to the program).

Processing performance may vary by the magnitude of the discharge current and / or the ratio of the duration of the current and pause during pulsed power supply.

Energy costs for processing cathode spots according to the proposed method do not exceed 0.5 kWh / m ² . This is significantly less than the corresponding values for surface modification using laser, electron-beam, and other plasma technologies. In addition, the simplicity of the equipment and, accordingly, its cost also compares favorably with the proposed method from the above technologies.

Thus, the proposed method in comparison with the prototype and other methods of similar purpose ensures the achievement of the goal is to increase the efficiency and quality of surface modification of the material by plasma treatment.

Claims (6)

1. A method of modifying the surface of a material by plasma treatment in vacuum, including loading the material into the chamber, vacuum pumping the chamber, plasma treating the surface of the material and unloading it, characterized in that the plasma treatment is carried out by cathode spots of a vacuum arc discharge excited in the chamber to ensure remelting of the surface layer of the material while the pressure in the chamber is maintained no more than 1 Pa, the voltage of the vacuum arc discharge is at least 10 V, the current of the vacuum arc discharge is at least 1 . 2. The method according to claim 1, characterized in that the excitation and maintenance of a vacuum arc discharge is carried out when a constant voltage is applied between the cathode and anode, and the plasma processing performance is controlled by changing the value of the discharge current. 3. The method according to claim 1, characterized in that the excitation and maintenance of a vacuum arc discharge is carried out when a pulse-periodic voltage is applied between the cathode and anode, and the plasma processing performance is controlled by changing the magnitude of the discharge current and / or the ratio of the duration of the current pulse and pause. 4. The method according to claim 1, or 2, or 3, characterized in that after evacuation the chamber is filled with an inert gas and / or working gas, in the medium of which the excitation and maintenance of the vacuum-arc discharge are carried out. 5. The method according to claim 1, or 2, or 3, characterized in that the localization of the cathode spots on the treated surface and control their movement on the surface is carried out by a magnetic field. 6. The method according to claim 4, characterized in that the localization of the cathode spots on the treated surface and control their movement on the surface is carried out by a magnetic field.

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