Induction Plasmatron Research Articles

Oxidation of Ceramic Materials Based on HfB2-SiC under the Influence of Supersonic CO2 Jets and Additional Laser Heating.

The features of oxidation of ultra-high-temperature ceramic material HfB2-30 vol.%SiC modified with 1 vol.% graphene as a result of supersonic flow of dissociated CO2 (generated with the use of high-frequency induction plasmatron), as well as under the influence of combined heating by high-speed CO2 jets and ytterbium laser radiation, were studied for the first time. It was found that the addition of laser radiation leads to local heating of the central region from ~1750 to ~2000-2200 °C; the observed temperature difference between the central region and the periphery of ~300-550 °C did not lead to cracking and destruction of the sample. Oxidized surfaces and cross sections of HfB2-SiC-CG ceramics with and without laser heating were investigated using X-ray phase analysis, Raman spectroscopy and scanning electron microscopy with local elemental analysis. During oxidation by supersonic flow of dissociated CO2, a multilayer near-surface region similar to that formed under the influence of high-speed dissociated air flows was formed. An increase in surface temperature with the addition of laser heating from 1750-1790 to 2000-2200 °C (short term, within 2 min) led to a two to threefold increase in the thickness of the degraded near-surface area of ceramics from 165 to 380 microns. The experimental results indicate promising applications of ceramic materials based on HfB2-SiC as part of high-speed flying vehicles in planetary atmospheres predominantly composed of CO2 (e.g., Venus and Mars).

Investigation of Heat Transfer Regimes in Subsonic Dissociated-Nitrogen Jets of a High-Frequency Induction Plasmatron under Additional Surface Heating by Laser Radiation

Investigation of Heat Transfer Regimes in Subsonic Dissociated-Nitrogen Jets of a High-Frequency Induction Plasmatron under Additional Surface Heating by Laser Radiation

Investigation of Heat Transfer Regimes in Subsonic Dissociated-Nitrogen Jets of a High-Frequency Induction Plasmatron under Additional Surface Heating by Laser Radiation

The heat transfer to a cylindrical water-cooled copper model was experimentally investigated in an induction VGU-4 high-frequency (HF) plasmatron of the Institute for Problems in Mechanics of the Russian Academy of Sciences. The model, 30 mm in diameter, equipped with a calorimetric transducer with a heat-adsorbing graphite surface, 13.8 mm in diameter, was exposed to the surface heating in the combined regime by nitrogen plasma and laser radiation and in the cases of the heating with only laser radiation or a nitrogen plasma jet. The experiments in the HF-plasmatron jets were performed at the pressure in the setup low-pressure chamber p = 1 × 104 Pa, nitrogen mass flow rate G = 2.4 g/s, and the plasmatron HF-generator anode power Na.p. = 22 kW. It is established that in the chosen experimental regimes the dissociated-nitrogen jet and the high-frequency induction discharge do not produce a considerable effect on the laser beam passing through them. The values of the heat flux density are obtained as functions of the laser radiation power delivered. The subsonic nitrogen plasma flow in the quartz discharge channel and in the low-pressure chamber of the VGU-4 setup is numerically modeled under the experimental conditions basing on the solution of the complete Navier–Stokes equations using the Patankar–Spalding method.

Thermal state of uncooled quartz discharge channel of powerful high-frequency induction plasmatron

Thermal state of uncooled quartz discharge channel of powerful high-frequency induction plasmatron

Разработка и исследование плазмохимической технологии получения смешанных оксидов урана и плутония из растворов

This paper presents the results of work on the application of plasma-chemical technology for producing uranium and plutonium oxides and their homogeneous mixtures from nitrate solutions of a given composition for the purpose of manufacturing fuel element cores. The technology is based on the process of high-temperature decomposition of aqueous solutions of salts by spraying them into a flow of the heat-transfer gas (air) heated to 5000 – 6000 K in a plasma torch. It is a highly productive short-stage process. No reagents are required for salt precipitation; there are no filtration and high-temperature treatment precipitation operations; the resulting powders are chemically active. It was found that the transformation time of salt solution drops to oxides is 0.05 – 0.10 s for the length of 0.5 – 0.7 m. The temperature in the reactor varies in the range of 1200 – 1800 K; no sintering and fusion of particles occurs. The apparatus-technological scheme of the plant has been developed. The technological equipment is made in a nuclear-safe design and installed in the exhaust boxes. The scheme includes a high-frequency induction plasmatron (plasma torch) with coaxial gas inlet into the discharge chamber. The high-frequency induction discharge is excited by inserting a grounded ignition electrode into the plasmatron discharge chamber. The plasmatron design provides the possibility of long continuous operation at high plasma temperatures with the creation of intensive plasma-jet processes for processing radioactive solutions. The reactor provides ways to prevent product accumulation on the walls (gas curtain method, electric pulse cleaning of cylindrical surfaces, vortex tangential gas supply at the level of the outlet holes of the injectors). Equipment for the separation of nanodispersed oxide powders from high-temperature steam- and powder-gas flows (vortex precipitators, filters with a metal-woven partition) has been developed and applied. The degree of oxide recovery in vortex precipitators is 88 – 92 % per stage, in filters with a metal-cloth partition the degree of purification reaches 105 – 108. Plasma-chemical oxides are nanocrystalline and nanoscale materials represented by crystalline and amorphous structures. Mixed powders are a homogeneous mixture of oxides in the form of solid solutions. The particle size does not exceed 3 μm; the crystallite size is about 10 nm. Fuel elements were manufactured from mixed powders at FGUP “Mayak”, which passed successful tests in the “Bor-60” reactor. Seven industrial fuel elements were manufactured for the BN-600 reactor, six of which were loaded into the reactor.

Behavior of Ultra-High Temperature Ceramic Material HfB2–SiC–Y3Al5O12 under the Influence of Supersonic Dissociated Air Flow

Ultra-high-temperature ceramic (UHTC) materials with a density of 94.5 ± 1.3% have been manufactured by hot pressing a (HfB2–30 vol % SiC)–5 vol % Y3Al5O12 composite powder, which has been prepared sol–gel method, at a moderate temperature of 1850°C (holding time, 30 min; pressure, 30 MPa). The oxidation resistance of this ceramic has been studied at elevated temperatures under the effect of a supersonic dissociated air flow (on a high-frequency induction plasmatron). The maximum surface temperature has been ~2550°C. An analysis of the kinetics of temperature changes depending on the heat load indicate that, when five percent of Y3Al5O12 by volume of HfB2–30 vol % SiC is introduced into the ceramics, the thermal conductivity of the material decreases. This is not critical from the point of view of the stability of the obtained sample to single temperature drops of ~700–1400°C in a few seconds with sharp heating and cooling of the sample. A decrease of ~35–40% in weight loss of the (HfB2–30 vol % SiC)–5 vol % Y3Al5O12) sample has been noted as the result of exposure as compared with unmodified analogs. Due to the presence of Y3Al5O12 in the initial ceramics, there is also a certain amount of the stabilized phase of cubic HfO2 in the oxidized layer in addition to the monoclinic.

Thermal Effect of Surface Catalysis in Subsonic Dissociated-Air Jets. Experiment on a High-Frequency Plasmatron and Numerical Modeling

Experiments on heat transfer in subsonic dissociated-air jets are performed on 100 kW VGU-4 induction plasmatron. The heat fluxes to the water-cooled surfaces of copper, silver, tantalum, beryllium, niobium, gold, molybdenum, and quartz are measured at the stagnation point on a cylindrical flat-nosed model, 50 mm in diameter, having rounded edges at 50 and 100 hPa pressures in the low-pressure chamber and the high-frequency (HF) generator power of 30 to 70 kW. At the same pressures and the HF generator powers of 45 and 64 kW the convective heating of a specimen of sintered silicon carbide is studied in the surface temperature range from 1720 to 1910 K. The predominance of the surface catalyticity effect on the heat flux with respect to nitrogen and oxygen atom recombination is demonstrated. Under the experimental conditions the air plasma flow in the discharge channel of the plasmatron, the subsonic jet flow past the cylindrical model, and the heat transfer to the stagnation point on the model are numerically simulated. Basing on the comparison of the experimental and calculated data on the heat fluxes to the surfaces of metals (Tw = 300 K), quartz (Tw = 572–722 K), and silicon carbide (Tw = 1720–1910 K) the quantitative catalyticity gradation of the materials considered with respect to heterogeneous recombination of nitrogen and oxygen atoms is established.

Modeling Thermal Gas Dynamic Processes of the Production of Silicon from Its Halides

A procedure was developed to model the gas-dynamic and thermal conditions of the production of powdered silicon from silicon tetrachloride and tetrafluoride in a high-frequency induction plasma chemical reactor. The model includes a description of the turbulent flow of a mixture of ideal viscous compressible gases while taking into account the induction heating of the gas by conduction, convection, and radiation, as well as taking into account the effect of the electromagnetic force on plasma motion. The powdered particles form according to the results of the thermodynamic calculations, and the particle distribution in the flow is described by the diffusion mechanism. The results of modeling the conversion of volatile silicon chloride and fluoride in a swirl-stabilized high-frequency induction plasmatron are presented.

Oxidation of Porous HfB2–SiC Ultra-High-Temperature Ceramic Materials Rich in Silicon Carbide (65 vol %) by a Supersonic Air Flow

Porous HfB2–65 vol % SiC samples (porosity 34.5%) were produced by reactive hot pressing of HfB2–(SiO2–C) composite powder at 1800°C (heating rate 10 deg/min, holding duration 15 min) and 30 MPa. Using a high-temperature induction plasmatron, their resistance to oxidation by a supersonic dissociated air flow was studied (the heat fluxes in the course of the experiment were varied from 363 to 779 W/cm2). The observation of the temperature distribution over the surface of the sample during the experiment showed that a sharp increase in temperature from ~1770–1850 to ∼2600°C in the samples under investigation occurred at lower heat fluxes and shorter treatment times than that in denser HfB2–30 vol % SiC samples (porosity 9–11%). This indicated that increasing the density of the HfB2–SiC material and also increasing the silicon carbide content reduced the oxidation resistance. However, the fact that the studied sample withstood 37-min exposure to a high-enthalpy dissociated air flow (including 27 min at a surface temperature of 2560–2620°C) without destruction or complete oxidation makes it possible to assign it to ultra-high-temperature materials.

Numerical Simulation of the Evaporation Process of Monodispersed Quartz Particles in an Argon Plasma Flow of Induction Plasmatron

The processes of the heating and evaporation of quartz particles in an argon plasma flow of high-frequency induction plasmatron are numerically simulated within the model of a two-phase, collisionless, monodispersed mixture. The conditions for the implementation of a ring-type vortex flow pattern with full and partial particle penetration into the high-temperature zone are determined. The effect of particle feed rate on the gasdynamics of a dispersed flow is shown. The dependences of the evaporation efficiency on the main operating parameters of particle processing process, primarily, the particle feed rate, Joule power of the plasmatron, particle size and injection angle are established. Recommendations are given for the selection of the optimal operating parameters of the plasmatron and flow parameters of the processed quartz particles. It is shown that such a plasmatron with a power of about 5 kW provides a complete evaporation of quartz particle flow with diameter up to 50–70 μm and feed rate up to (8–10) × 10–5 kg/s at specific energy costs at a level of 50 MJ/kg.

Effect of the Surface Relief of HfB2-SiC Ceramic Materials on Their High-Temperature Oxidation

Samples of an ultra-high-temperature HfB2–30 vol % SiC ceramic material with an SiC crystallite size of ~50 nm have been fabricated by hot pressing of an HfB2–(SiO2–C) composite powder synthesized through a sol–gel route. It has been found that the selected hot-pressing conditions (1800°C, 15 min, 30 MPa) ensure complete conversion of silica to silicon carbide and afford a material with a relative density of 96.9%. The effect of a relief applied to the sample surface in the form of ring grooves with a depth of <0.5 mm on the oxidation of the material exposed to a subsonic dissociated air flow—temperature distribution over the surface, elemental and phase composition, and microstructure—has been studied using a high-frequency induction plasmatron.

The effects of subsonic and supersonic dissociated air flow on the surface of ultra-high-temperature HfB2-30 vol% SiC ceramics obtained using the sol-gel method

By hot pressing (1900 °C, 30 MPa, holding time 15 min) of HfB2-(SiO2-C) composite powder, an ultra-high-temperature ceramic material of HfB2-30 vol% SiC composition with nanocrystalline silicon carbide has been obtained. The effects of subsonic and supersonic dissociated air flow on the surface of produced materials have been studied on a high-frequency induction plasmatron in the geometry of a cylindrical sample with a flat face, fixed in a copper water-cooled holder with a 1 mm protrusion. It has been shown that a sudden rise in the average surface temperature of the samples to ∼2600 °C is characteristic for both modes, which is associated with the occurrence of local sites with a temperature >2000 °C and a subsequent increase in their area. It is a matter of evaporation of the borosilicate glass layer from the surface of the oxidized sample and formation of the ceramic layer of a highly catalytic and low thermal conductive porous HfO2 layer, which is confirmed by the emission spectroscopy data, XRD and elemental analysis of the material surface after the experiments. The features of heating the oxidized surface of the samples under the impact of subsonic and supersonic dissociated air flow have been noted: there are differences in the location of overheated sites, initiating a sharp temperature rise and the rate of growth of their area.

Behavior of HfB2–30 vol% SiC UHTC obtained by sol–gel approach in the supersonic airflow

Using the sol–gel process, a composite powder of HfB2–(SiO2–C) composition was synthesized and then utilized for reactive sintering (hot pressing, 30 MPa; 1800 °C, 15 min) of ultra-high-temperature ceramic HfB2–30 vol% SiC composites. It was established that nanocrystalline silicon carbide (36 ± 2 nm) in a cubic modification is formed during hot pressing; HfO2 and HfC impurities were not found. The relative density of the obtained materials was 89.2 ± 2.3%. The long-term (40-min) oxidation resistance of the HfB2–30 vol% SiC sample was studied under the exposure to the supersonic dissociated airflow on a high-frequency induction plasmatron (heat flux changed from 232 to 779 W/cm2, pressure in the plasmatron chamber was 15 hPa), in a configuration that prevented significant heat discharge from the sample to a water-cooled holder (with 1-mm overhang). It was shown that, with a stepwise increase in the heat load in the initial stages, the temperature distribution over the front surface of the sample was relatively uniform. However, at the heat flux of 598 W/cm2, local overheated areas appeared in the central region, which spread over almost the entire surface of the sample within 1–2 min; the average temperature was ~2560 °C. Using emission spectroscopy data from the boundary layer above the sample surface, as well as XRD and SEM of the sample after exposure, it was shown that the sharp increase in temperatures from ~1500–1600 to 2500–2600 °C was associated with a change in the chemical nature of the surface, due to evaporation of borosilicate glass components and the appearance of a porous highly catalytic HfO2 with low thermal conductivity on the surface. It was noted that these processes under the exposure to the supersonic flow started at lower temperature than under the exposure to subsonic dissociated airflows.

Heat Transfer and Thermophysics of Subsonic Dissociated-Air Jets in Flow past a Cylindrical Model in a High-Frequency Induction Plasmatron

Heat transfer in the subsonic high-enthalpy air flow in the VGU-4 induction plasmatron is experimentally and numerically investigated for three versions of the sectional discharge channel with the exit section diameters of 30, 40, and 50 mm. The model was a flat-nosed cylinder, 20 mm in diameter. Using a Gamma code the enthalpy he and the velocity at the jet axis are numerically reconstructed from the experimental data. The method for approximately determine the enthalpy he on the basis of the experimental data and the Fay—Riddell formula for the heat flux is proposed. The effective dimensionless model radius reff is introduced, which makes it possible to take account for the dependence of the flow at the outer edge of the boundary layer on the model and discharge channel geometries and the plasmatron operation regime. The value of reff is calculated for the VGU-4 operation regimes with a simple cylindrical discharge channel and a sectional channel. The values of the enthalpy he calculated using the “exact” and approximate methods are compared: the greatest difference in the he values amounts to 6%.

Inconel 625/TiB2 Metal Matrix Composites by Direct Laser Deposition

This work presents results in the field of synthesis of new metal matrix composites with matrix NiTi and particles TiB2, and their use as additives to fabricate metal matrix composites based on the Inconel 625 alloy. NiTi-TB2 powders were obtained using self-propagating high-temperature synthesis. Composite NiTi-TiB2 particles were spheroidized on a high-frequency induction plasmatron. Composite NiTi-TB2 particles were mixed with metallic Inconel 625 powder with particle sizes of 50–150 µm. We used direct laser deposition by means of mixture of powders to grow samples with different contents of ceramics in the metal matrix. The process of direct laser deposition during the experiment was investigated. We have determined the peculiarities of the formation of the structure in metal matrix composites with different contents of titanium diboride. We have demonstrated the possibility of using Direct Laser Deposition (DLD) for fabricating items from ceramic metal materials. We have determined promising fields of further research for the purpose of obtaining efficient metal matrix composites using additive manufacturing technologies.

Impact of a Supersonic Dissociated Air Flow on the Surface of HfB2–30 vol % SiC UHTC Produced by the Sol–Gel Method

A new method to produce ultra-high-temperature ceramic composites under rather mild conditions (1700°C, 30 MPa, treatment time 15 min) was applied to synthesize a relatively dense (ρrel = 84.5%) HfB2–30 vol % SiC material containing nanocrystalline silicon carbide (average crystallite size ∼37 nm). The elemental and phase compositions, microstructure, and some mechanical properties of this material and also its thermal behavior in an air flow within the temperature range 20–1400°C were investigated. Using a high-frequency induction plasmatron, a study was made of the effect of a supersonic dissociated air flow on the surface of the produced ultra-high-temperature ceramic composite shaped as a flat-end cylindrical sample installed into a copper water-cooled holder. On 40-min exposure of the sample to the supersonic dissociated air flow, the sample did not fail, and the weight loss was 0.04%. Although the heat flux was high, the temperature on the surface did not exceed 1400–1590°C, which could be due to the heat transfer from the sample to the water-cooled model. The thickness of the oxidized layer under these conditions was 10–20 μm; no SiC-depleted region formed. Specific features of the microstructure of the oxidized surface layer of the sample were noted.

Experimental and Numerical Investigation of Heat Exchange between Underexpanded High-Enthalpy Air Jets and Cylindrical Models

Experiments on heat transfer in supersonic underexpanded high-enthalpy air jets are conducted on the VGU-4 induction plasmatron at the pressure in the compression chamber of 8.5 hPa. At the air flow rate of 3.6 g/s and the high-frequency generator powers of 45 kW(regime 1) and 64 kW (regime 2) the heat fluxes to the copper surface at the stagnation point of watercooled cylindrical models along the axes of dissociated air jets are measured. The models, 30 mm in diameter, could have a flat face or a hemispherical nose. In the same regimes, the stagnation pressures are measured using the Pitot tube in the shape of a cylinder, 30 mm in diameter, having either a flat face or a hemispherical bluntness with a receiving hole, 14 mm in diameter. For the experimental conditions calculations of flows in the plasmatron discharge channel and supersonic underexpanded jets issuing from the discharge channel are performed within the framework of the Navier–Stokes and Maxwell equations. The heat fluxes to the experimental models are computed and compared with the experimental data.

Heat Transfer in Subsonic Flows of Dissociated Nitrogen: HF Plasmatron Experiment and Numerical Simulation

Experiments on heat transfer in subsonic jets of dissociated nitrogen have been carried out on a IPG-4 induction plasmatron. The heat fluxes to copper, stainless steel, nickel, graphite, and quartz surfaces at the stagnation point of a water-cooled cylindrical flat-faced model 20 mm in diameter and dynamic pressures have been measured at a pressure of 50 hPa in the test chamber and a power of 35–65 kW of the HF generator. The experiments showed the influence of surface catalytic properties on the heat flux in relation to the nitrogen atom recombination. In the conditions of the experiments, a numerical simulation of nitrogen plasma flows in the discharge channel of plasmatron and the subsonic dissociated nitrogen jet flow around the cylindrical model has been carried out. The experimental and calculated data on heat fluxes to cooled copper, stainless steel, nickel, graphite, and quartz surfaces are compared. The quantitative catalyticity scale of the studied materials in relation to the heterogeneous recombination of nitrogen atoms is established.

Computation of inductively coupled air plasma flow in the torches

Software was developed to calculate electric field amplitude in the induction plasmatron discharge channel in framework of 2D model with far field boundary conditions. This code was integrated with the solver for Navier-Stokes equations for plasma flow in the plasmatron channel. Numerical simulation of inductively coupled plasma flow was carried out for the channels of IPG-4 and IPG-3 induction plasmatrons operating in IPMech RAS. Computations were performed twice: first, the electric field amplitude was calculated with the more accurate 2D model, next - with the simplified 1D model to provide detailed compare of the accuracy of results obtained with use of 1D model. The gas-dynamic parameters (flow velocity, temperature) at the exit section of the channels obtained with 1D model are within the error from 0.4% to 4% for IPG-4 plasmatron; the corresponding error for IPG-3 plasmatron is 16% to 26%. Numerical simulation results for IPG-3 plasmatron are presented for the first time.

Study of the Thermal Behavior of Wedge-Shaped Samples of HfB2–45 vol % SiC Ultra-High-Temperature Composite in a High-Enthalpy Air Flow

By spark plasma sintering, HfB2–45 vol % SiC ultra-high-temperature ceramic was prepared, from which wedge-shaped samples were cut. The behavior of the samples was examined in a flow of dissociated air produced by an induction plasmatron, where the surface temperature of the leading edges of the samples reached ~2700°C. The dependence of the temperature distribution gradient on the distance from the leading edges of the samples was experimentally investigated. For the samples after the experiments, the elemental and phase compositions were determined, and features of the surface microstructure in various regions of the sample and on its polished surface were studied.