Annotation of the results obtained in 2022
In the reporting year 2022, the following main tasks were planned to be completed. 1. Upgrade of the experimental model of the plasma generator and the complex of diagnostic equipment in accordance with the objectives of the project. 2. Experimental studies of the parameters and characteristics of a glow discharge at atmospheric pressure and generated plasma in a wide range of currents and durations, aimed at revealing the features of emission, ionization, and near-electrode processes. 3. Test experiments on obtaining and express analysis of the geometric parameters and elemental composition of particles of powder materials formed during the operation of an atmospheric pressure glow discharge. The work planned for the year has been completed in total. The result was the achievement of the following. 1. The upgrade of the plasma generator designed to explore the processes in a glow discharge at atmospheric pressure, leading to the appearance of atomic flows of particles of the cathode material, made it possible to directly observe and measure the parameters of optical emission from various areas of the discharge: cathode glow, positive column, and anode current spot. In conjunction with the modernization of the diagnostic complex, this makes it possible to qualitatively investigate the elemental composition of the discharge plasma, plasma parameters, as well as the temporal and spatial distributions of its optical emission with wavelength resolution. This approach is necessary to reveal the features of the generation of plasma flows with a high content of atomic metal particles in relation to the synthesis of ultrafine powders and coating deposition at atmospheric pressure. 2. The characteristics of a low-current discharge in an argon flow at atmospheric pressure have been studied. The conditions for the functioning of the discharge and the features of the optical radiation of its plasma are determined in relation to the implementation of the regime of stable generation of flows of metal atoms. The presence of excited atoms of cathode materials in the discharge plasma is unambiguously identified by the presence of the corresponding spectral lines. As a result of the analysis of current-voltage characteristics, a physical mechanism is proposed that explains the difference in discharge burning voltages for different materials of the cathode insert, based on the dependence on the surface tension force of the molten metal meniscus, which determines the conditions for the diffusion of atoms through its surface and their entry into the discharge as a plasma-forming medium. The study of the dynamics of the optical radiation of the plasma showed that the conditions for the self-sustaining of the discharge are determined by the radiation of argon atoms, which are excited during the decay of the discharge current as a result of three-particle recombination processes. The results of studying the emission spectra of atmospheric pressure discharge plasma by its various sections: cathode glow, positive column, anode spot, showed that the intensity of emission lines at the transitions of atoms and metal ions decreases with distance from the cathode surface. Based on the results of the study of the floating potential, a conclusion was made about an increase in the reduced electric field strength near the anode and, as a consequence, an increase in the energy of plasma electrons accelerated in this field. Therefore, the electrons in the anode part of the discharge interact more effectively with the atoms of the working inert gases than with the metal atoms, which have lower excitation and ionization potentials. 3. The processes of powder synthesis during the dispersion of cathode inserts made of magnesium, indium and zinc, due to their thermal erosion in a low-current (up to 1 A) discharge of atmospheric pressure in a flow of argon and helium, have been studied. As applied to the generation of metal-containing plasma flows at atmospheric pressure, this approach differs from more traditional ones in that the cathode is a source of metal atoms that enter the plasma by diffusion through the liquid metal meniscus surface. This method does not require the supply of raw materials in the form of metal powder, wire, or the injection of solutions of metal-chemical compounds into the discharge zone, since all processes occur due to the interaction of non-equilibrium low-temperature plasma with the cathode. The results of the study of the obtained powders by transmission electron microscopy and energy dispersive spectrometry showed that in the case of using a cathode insert made of magnesium and argon as a working gas, the particle size is 40 ± 7 nm. The shape of the individual particles is predominantly hexagonal. Simultaneously with these particles and their agglomerates, accumulations of shapeless particles several nanometers in size are observed. When dispersing a magnesium insert in a discharge plasma in a helium flow, the size of individual powder particles ranges from several units to 10 nm. The sizes of their agglomerates are up to 100 nm. The shape of individual particles, in this case, in the bulk, is not identifiable. However, there are hexagonal particles. The influence of the working type of gas on the shape of individual particles of powder material, in the case of using a cathode insert from indium, was not revealed. Predominantly cubic particles were observed. At the same time, the effect of the type of working gas on the size of nanoparticles is shown. In the case of argon, the particle size is approximately 16±7 nm. When discharged in a helium flow, the average particle size decreases to 13 ± 6 nm. A significantly greater effect of the working gas on the size and shape of the nanoparticles was revealed using zinc as the material of the eroded cathode insert. During the discharge in an argon flow, particles in the form of needles up to 200 nm long and up to 7 nm in diameter were synthesized. The use of helium as the working gas made it possible to synthesize particles smaller than 7 nm in size, the shape of which could not be identified. The results of the study of the elemental composition of the particles showed that, regardless of the type of working gas, the composition of the synthesized powders contained metal atoms used as eroded cathode inserts and oxygen atoms in proportions corresponding to the stoichiometric compositions characteristic of their oxides: MgO, ZnO, In2O3, and magnesium hydroxide Mg(OH)2 also. This experimental fact can be associated with the oxidation of metal particles as a result of interaction with oxygen already in the discharge plasma, since the working gases are not pure, where the volume fraction of oxygen is several ten-thousandths of a percent. Based on the results of the work, 2 papers were prepared. One paper was published in the journal Izvestiya vuzov. Physics (RSCI). Another article accepted for publication in the journal Applied Physics (RSCI, Scopus).

 

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Annotation of the results obtained in 2023
In the reporting year 2023, the following main tasks were planned. 1. Research on the characteristics of thermal erosion of cathode insert materials in relation to the generation of flows of metal atoms. 2. Experimental study of near-electrode and volumetric processes in the plasma of an atmospheric pressure glow discharge under conditions of generating flows of metal atoms with high spatial and temporal resolution. 3. Study of the structure of ultrafine powders and coatings obtained during the functioning of an atmospheric pressure glow discharge. The work planned for the year has been completed in full. The result was the achievement of the following results. 1. The processes leading to erosion of the cathode material in a pulsed glow discharge at atmospheric pressure in the mode of generating flows of metal atoms, at a voltage of 150 - 300 V, a current of 500 - 600 mA, a pulse duration of 9 - 12 μs, a pulse repetition rate of 60 - 100 kHz, in flows of inert gases argon and helium, at a flow rate of 1 l/min have been studied. When the discharge operates with the specified parameters near the cathode, a power density of 15 – 20 W/mm2 is ensured. The cathode of the discharge system is a cylindrical rod made of a refractory material - molybdenum with a cavity into which an insert of a low-melting metal (Mg, Zn, In) is placed, and is a crucible for this insert. It is shown that with the diffusion of heat from the near-cathode region of the discharge, the cathode - crucible is heated to 700 degrees Celsius, which is sufficient to melt the specified materials of the cathode insert. In the molten state, metals (Mg, Zn, In) expand and are partially displaced from the cavity, forming a convex meniscus. Metal atoms are captured by the gas jet from the vapor layer near the surface of the liquid metal meniscus and transported through the discharge plasma, participating in collisions with electrons, resulting in excitation and ionization. In the case of heating the crucible with a heat source not related to the nature of the gas discharge, but with identical power and gas-dynamic characteristics, uniform heating of the molybdenum crucible is ensured, but noticeable erosion of the low-melting insert cannot be identified. It is shown that to generate flows of metal atoms as a result of thermal erosion of the cathode insert, the most optimal way is to transfer heat from the near-cathode region of the glow discharge, with the localization of the cathode glow directly at the location of the cathode insert. As a result of the implementation of this condition, erosion rates of 5.5E-5 g/s (grams per second) for magnesium and 33E-5 g/s for zinc are ensured. 2. Spatial and temporal distributions of radiation intensity in the plasma of a glow discharge at atmospheric pressure in a flow of inert gas were studied at minimum values of the discharge current of 50 – 60 mA, below which it goes out, in the mode of generating a flow of metal atoms at a discharge current of up to 600 – 800 mA , as well as when switching to a high-current discharge mode with cathode spots, with a current of up to 500 A. It is shown when a discharge operates with a minimum current, the intensity distribution in the visible range of plasma radiation from a positive column in the space between the cathode and anode can take on a structure in the form of a sequence of alternating luminous and darker sections in comparison. A study of the dynamics of this distribution showed the highest intensity is observed after the current decays and is determined primarily by the emission of excited argon atoms, which can occur as a result of photorecombination, electron deceleration on ions and atoms, as well as a result of three-particle interactions with the intermediate formation of a molecular argon ion. In the mode of generating fluxes of metal atoms, it is shown that the increase and decrease in radiation intensity, in the spectral ranges corresponding to the lines of singly charged metal ions, have a time course similar to the waveforms of the voltage and current of the discharge. The radiation corresponding to the lines of excited metal atoms, in addition to the main pulse, the dynamics of which corresponds to the discharge current, has a surge in the intervals between current pulses and decreases to the background level at the time of the front of the next current pulse, which is one of the conditions for self-sustaining discharge. Because of studying the breakdown dynamics during the transition of a low-current glow discharge to a high-current mode with cathode spots, it was demonstrated that in the interval, over subnanosecond times, a diffuse streamer with a diameter of 3 ± 1 mm develops, which is comparable to the length of the discharge gap. This is due to the radial component of the electric field, as well as the generation of runaway electrons near the cathode. 3. Under the thermal influence of a low-current glow discharge on a section of the cathode containing inserts made of low-melting metals, with a power density of 15 – 20 W/mm^2, in combination with the gas-dynamic effect of an inert gas jet, the generation of flows of metal atoms injected from the discharge system through a hole in the anode into a container on the walls of which they cooled and stuck together into nano-sized agglomerates. As a result of these processes, powders of the oxides of these metals are obtained. When producing powder coatings in an inert gas atmosphere using the method of measuring ohmic resistance, it was shown that their material is electrically conductive particles. Access of oxygen to coatings leads to an increase in their resistance at a rate of approximately 11 kOhm/s. Since the size of the powder particles did not exceed several tens of nanometers, oxidation occurred throughout the entire volume of the sprayed material. This is confirmed by the results of transmission electron microscopy and X-ray diffraction analysis, which did not reveal the content of metal particles. Studying the structure of individual nanosized powder particles using transmission electron microscopy, as well as aggregations of powders in the form of coatings using X-ray diffractometry, showed identical results. In the case of using an erodible cathode insert made of indium, the particles have a structure based on a body-centered cubic lattice (bcc In2O3). In the case of using magnesium, diffraction analysis showed that particles of two types are observed - magnesium oxide, which is characterized by a face-centered cubic lattice (fcc MgO), and magnesium hydroxide Mg(OH)2, which has a hexagonal structure, with a predominant content of the oxide phase. When the zinc insert is eroded, the phase composition of the particles is represented by a single phase based on zinc oxide with a hexagonal close-packed lattice (hcp ZnO). Based on the results of the work, 4 articles were prepared. Three articles were published in the journal Ceramics (WoS, Scopus). Another article has been accepted for publication in the journal Ceramics International (Scopus Q1).

 

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