RU2180160C1 - Method and device for producing fractal-like structure - Google Patents

Abstract

FIELD: plasma physics; electron-ion plasma processes and technologies. SUBSTANCE: proposed method and device may be used to obtain nanostructures and fractal-like aggregates when producing heterogeneous-phase working media of radiation sources, coatings possessing new physical properties, means and media for transmitting and transforming concentrated energy fluxes and electrical potential. Method involves generation of slightly ionized gas stream from source plasmaforming material, cooling of slightly ionized gas stream to condensation temperature, production of nanostructures from neutral and charged particles, aggregation of nanostructures to form clusters, and their growth to obtain fractal-like structures. Slightly ionized gas flow is obtained during pulsed diaphragm jet electrical discharge with flowing jets of high-temperature erosion products and with off-design parameter found experimentally for producing space-time structure of jet and for controlling aggregation process of fractal- like structures. Device implementing proposed method has pressurized discharge chamber incorporating power supply, evacuated gas system, ring electrodes whose inner holes are made in the from of truncated cones with their apexes facing each other, as well as diaphragm made of plasma-forming insulating material and mounted on ring electrode pin. Power supply is high-voltage adjustable-amplitude and adjustable-shape current pulse generator; diaphragm has round hole with appropriate radius r to length l ratio. Fractal-like structures can be produced from actually any material and aggregated to form structures of desired parameters: fractal dimension, linear size of aggregate and its components, anisotropic shape, etc. Fractal-like structures can be deposited on substrate surfaces. EFFECT: enhanced yield; enlarged functional; capabilities. 5 cl, 3 dwg

Description

The invention relates to plasma physics, mainly to the physics and technology of electron-ion plasma processes and technologies based on them, and can be used to obtain nanostructures (nanostructures (small clusters) - small macroscopic particles (number of atoms q≤10 ³ ), parameters ( values of the binding energy, ionization potential, electron affinity, etc.) which are monotonic functions of the number of atoms of their generators and have extremes for some atoms) and fractal-like aggregates when creating heterophasic workers media sources of radiation, coatings with new physical properties, for media transmission and transformation of concentrated energy flows and electric potential.

Known methods for producing fractal-like structures (fractal-like structures are understood to mean structures for which scaling relations that determine a fractal that differs from the topological dimension of a self-similar (in the sense of zooming) scale [Feder E. Fraktaly. M., Mir, 1991] are performed approximately a limited range of scales) and devices for their implementation, which use an electric explosion of wire, heating a tungsten spiral in a vacuum, laser irradiation of metals, explosion of material, burning SiH ₄ .

All currently known solutions for obtaining fractal-like structures of a substance have a number of disadvantages. First of all, this is a significant limitation on the choice of materials according to their chemical composition. Significant disadvantages are the inability to regulate the process of modifying the structure of the resulting aggregates — that is, to obtain fractal objects with given values of fractal dimension D in a wide range (for example, D = 1.3 - 2.5) and linear dimensions of the aggregate R _о . In addition, the specific content of fractal-like structures in the total volume of products obtained at the final stage of the aggregation process is usually small and does not exceed 5-10% for most known pulse methods.

A known method of producing fractal-like aggregates [Niklasson GA, Torebring A., Larsson C., Granqvist CG Phys. Rev. Lett. l988, V. 60, P. 1735] by passing an electric current through a wire containing the specified components, and then cooling the evaporated material during expansion into space, which leads to condensation and aggregation of the formed solid particles into clusters (a cluster is a finite and a large number of bound atoms or molecules (q≈10 ⁴ - 10 ⁶ ), similar to small particles). The method is limited in the choice of the material of the obtained fractal-like structures (Co, Ni, Al), the range of the obtained fractal dimension (1.7-1.8), the linear size of the obtained aggregates (about a micron).

 A device for implementing this method [Granqvist C.G., Burhman R. A. J. Appl. Phys. l976. V. 47. P. 2200] includes a chamber, a gas-vacuum system, a tungsten spiral heater with evaporated material (for example, cobalt) deposited on a wire, and a carbon grid coated with carbon. The evaporated metal was cooled during expansion in the chamber and deposited on a copper grid. The disadvantages of this device are the restriction on the choice of the evaporated material, the inability to control the process of aggregation of fractal-like structures using this device, as well as the low yield of the final product, which is no more than 1-2% of the total mass of the evaporated substance.

 A known method of obtaining fractal-like aggregates selected by us as a prototype is [Lushnikov A. A., Pakhomov A.V., Chernyaev G.A. DAN USSR, 1987, 292, 86], which includes obtaining a stream of weakly ionized gas from an initial plasma-forming material, cooling a stream of weakly ionized gas to a condensation temperature, the formation of nanostructures from neutral and charged particles, the aggregation of nanostructures into clusters and their growth to fractal-like structures, where a weakly ionized gas get near the surface of the metal when irradiated with laser radiation. The main disadvantage of this method is the inability to control the process of structuring the resulting aggregates, as well as the low efficiency of the process of formation of nanostructures, which is due to the receipt of a small number of centers of formation of nanostructures.

 A device for producing products of high-temperature erosion of materials, we have chosen as a prototype [EV Kalashnikov // TVT. 1996. T. 34, 4. P. 501-505], including a sealed discharge chamber with a power source, gas vacuum system, ring electrodes, the inner holes of which are made in the form of truncated cones with their vertices facing each other, and made of a dielectric plasma-forming material and installed on the axis of the ring electrode with a diaphragm. Along with primary particles (in the form of monomers, clusters) and macroparticles - fragments of the destroyed diaphragm material in the form of droplets and fragments, fractal-like structures were formed at the post-discharge stage through the diaphragm opening as a result of cooling of the products of high-temperature erosion when filling the volume of the discharge chamber.

 However, this scheme for obtaining products of high-temperature erosion of materials does not allow to regulate the process of obtaining fractal-like structures and their structuring during the growth of aggregates. This leads to the fact that fractal-like aggregates are obtained with an uncontrolled spread of structural parameters in advance. In particular, the fractal dimension of aggregates D varies in the range from 1.6 to 1.9, and the specific content of fractal-like structures in the total volume of the obtained products does not exceed 5-10%.

 The claimed group of inventions allows us to stably and with high yield (more than 20% of the vaporized mass) to obtain both submicron and micron fractal-like aggregates of a given size, morphological and chemical structure, as well as to control the aggregation process in the early and late stages of coagulation of the above structures.

This technical effect is achieved by us when
- in a method for producing fractal-like structures, including obtaining a stream of weakly ionized gas from a plasma-forming material, cooling a stream of weakly ionized gas to a condensation temperature, forming centers of formation of nanostructures from neutral and charged particles, aggregation of nanostructures into clusters and their growth to fractal-like structures, weakly ionized gas is obtained when diaphragm pulsed electric discharge in the flow regime of jets of high-temperature erosion products with the parameter non-calculation n n, selected from the condition:
1≤n <250,
where n = P _crit / P _atm , P _crit is the pressure in the critical section of the jet when it expires from the orifice, P _atm is the pressure in the external environment with respect to the axial zone of the jet,
cooling the flow of weakly ionized gas to a condensation temperature is carried out during the decay of the discharge current t _decay , found from the condition
t _recession > t _cond,
where t _drop = 0.9 I _max K ^-1 , t _cond is the period of time during which the condensation of weakly ionized gas and the formation of charged nanostructures with a density of N ₊ and neutral nanostructures with a density of N _n , I _max is the value of the maximum discharge current, K is the steepness of the decay of the discharge current pulse,
the aggregation of nanostructures into clusters is carried out up to size r ₀ with a density of N _cl under the condition
k _rec (N ₊ ) ² >> k _coag (N _n ) ² ,
where k _rec is the constant of the recombination rate of positively charged nanostructures, k _coag is the constant of the coagulation rate of neutral nanostructures,
cluster growth is carried out to fractal-like structures of size R ₀ with fractal dimension D when the relation
n _cl ≈ (R ₀ / r ₀ ) ^D ,
where n _cl is the number of clusters in the aggregate;
- in a device for producing fractal-like structures, including a sealed discharge chamber with a power source, gas-vacuum system, ring electrodes, the inner holes of which are made in the form of truncated cones with their vertices facing each other, and a diaphragm mounted on the axis of the ring electrode, a high-voltage generator is selected as the power source current pulses of adjustable amplitude and shape, the diaphragm is made with a round hole, the surface of which is made of plasma-forming Container material with the ratio of radius r and of length 1 in the range of hole
0.5 <2r / l <2.0,
the radius R of the inner hole of the electrodes is selected from the condition
lnR / r> 1.6πLω / μμ ₀ (I _max ) ² t _n ,
where L is the distance of the diaphragm-ring electrode; ω is the average rate of ablation of the mass of the plasma-forming material of the diaphragm, μμ ₀ = 1.257 10 ^-6 GN / m; I _max - the value of the maximum discharge current; t _n is the development time of power magnetogasdynamic instabilities in the discharge plasma, and the value of I _{max is} chosen from the condition for l and r taken in cm
I _max > 2.5 10 ⁴ (l ^1.4 ) / r ^1.4 (1 + r / l), A.

If fractal precipitation is necessary, then in the discharge chamber an additional substrate is placed, previously purified by short-wave radiation (λ ≤ 400 nm) of the plasma of the discharge jet with the light flux density Ф ₀ = 10 ⁴ -10 ⁵ W / cm ² and duration 0.005 ms <τ <0 5 ms (see paragraph 2, claims). The substrate is placed between the ring electrode and the diaphragm at a distance A from the axis of the discharge gap, satisfying the ratio:
3R <A <5R,
where R is the radius of the hole in the ring electrode, m (see paragraph 4, the claims).

The substrate can also be placed in the discharge chamber orthogonal to the axis of the diaphragm behind the annular electrode at a distance P * satisfying the condition
A * <P * ≤P,
where P is the distance along the axis of the ring electrode from its surface facing the diaphragm to the chamber wall, A * = 24R, R is the radius of the hole in the ring electrode (see paragraph 5, the claims). In this case, the yield of fractal-like structures increases due to the aggregation of the erosion products of the diaphragm from the axial and axial zones of the jet passing through the hole of the ring electrode.

Figure 1 shows a diagram of a device for implementing the proposed method according to p. 3-5, where the power supply 1, annular anode 2, annular cathode 3, plasma-forming diaphragm 4, discharge vacuum chamber 5, switch 6, substrate 7, ignition device 8, synchronization circuit 9, substrate 10;
A is the distance from the axis of the discharge gap to the substrate 7, 1 is the thickness of the diaphragm, L is the interelectrode gap, P is the distance along the axis of the ring electrode from its surface facing the diaphragm to the chamber wall, 2R is the diameter of the hole in the ring electrode, 2r the diameter of the hole in the diaphragm, L _k is the inductance of the circuit, C ₀ is a high-voltage capacitive energy storage, R _k is the resistance of the circuit.

 In FIG. 2 shows a part of the discharge chamber with a photogram 11 of the discharge plasma jet at the current phase of the discharge, and also a substrate 10 is shown for deposition of aggregates at a distance P * from the surface of the ring electrode 3 to the substrate 10.

 Figure 3 presents an electron microscopic image of a fractal-like aggregate obtained under the gas-dynamic regime of the plasma flow of the jet and the off-set parameter n≈1-2.

A device that implements the claimed group of inventions works as follows: in the vacuum chamber 5 between the ring electrodes 2 and 3 through the plasma-forming diaphragm 4 form a jet diaphragm pulse discharge. To do this, to the ignition device 8 using the synchronization circuit 9, a start signal is supplied that opens the switch 6 (for example, an ignitron) of the power supply 1, which is a high-current discharge circuit with a high-voltage capacitive energy storage C ₀ . Then a high voltage is applied to the electrodes 2 and 3 and the capacitor bank C _{0 is} discharged at the load, the interelectrode gap (anode 2 - cathode 3).

The current impulse of the jet diaphragm discharge of the required shape, duration and amplitude is obtained by choosing the parameters of the discharge circuit and the discharge gap based on the electrical calculation - by choosing the interelectrode gap L, the thickness of the diaphragm l, the hole diameter 2r in it, the charging voltage U _{0 of the} batteries, the loop inductance L _to , the resistance of the circuit R _to and the composition of the plasma-forming material. In this case, a certain spatio-temporal structure of the flow of the erosion products of the plasma-forming material of the diaphragm with the zone of production of erosion products in the aperture opening is obtained; with an axial zone of blowing the discharge gap with erosion products; with the axial zone of the flow of erosion products, slightly compressed by the magnetic field of the discharge’s own current; the sheath zone and the braking zone behind the ring electrodes 2 and 3.

Depending on the rate of ablation of the mass ω of the plasma-forming material, an erosion plasma with high specific parameters (temperature from 2000 to 70,000 K at pressures from 0.1 to 100 MPa) is obtained in the aperture opening depending on the current density from 1 kA / cm ² to 1 MA / cm ² .

Aggregation of fractal-like structures from erosion products is carried out in the zone of the jet shell (substrate 7) and the zone of deceleration of the jet behind the ring electrode (substrate 10). In this case, the flow regime of products of high-temperature erosion in the axial zone of the jet with an off-set parameter n is selected from the condition:
1T≤n <250,
where n = P _crit / P _atm , P _crit is the pressure in the critical section of the jet when the diaphragm _flows out of the hole, P _atm is the pressure in the external environment with respect to the axial zone of the jet.

 From the neutral and charged particles of the formed stream of weakly ionized gas in the zone of the jet shell and the zone of deceleration of the jet behind electrodes 2 and 3, the necessary and stable number of centers of formation of nanostructures of a given composition is obtained.

The flow of weakly ionized gas to the condensation temperature is cooled by reducing the discharge current from the maximum value I _max with a selected slope K, providing a sharp expansion of the axial zone of the jet, slightly compressed by the azimuthal magnetic field of the discharge current. In this case, the current decay time t _{decay is} significantly greater than the time interval t _cond , necessary for the condensation of all erosion products in the flow of expanding weakly ionized vapor, i.e.

t _recession >> t _cond .

The time interval t _Kond , during which the condensation of weakly ionized gas is found from the expression
t _cond ≈3g ^1/3 (k ₀ N _c ) ^-1 ,
where q is the average number of atoms in the cluster, k _o ≈ 1.9T ^1/2 m ^1/6 ρ ^-2/3 , T = T _s is the condensation temperature, m is the mass of the atom, ρ is the density of the substance of the nanostructures, N _c is atomic density before condensation of a weakly ionized gas flow.

The fulfillment of this condition is necessary so that condensation processes in the expanding ionized gas proceed at relatively high gas pressures, when its pressure is compared with the saturated vapor pressure at a given temperature. This is necessary to increase the centers of formation of nanostructures, as well as for stable and efficient aggregation of nanostructures into clusters with their subsequent growth to fractal-like structures. To achieve this, the inner surface of the aperture opening is made of a plasma-forming material with a ratio of radius r and length l of the hole in the range
0.5 <2r / l <2.0,
the radius R of the inner hole of the electrodes is selected from the condition
lnR / r> 1.6πLω / μμ ₀ (I _max ) ² t _n ,
where L is the distance of the diaphragm - the ring electrode, ω is the average velocity of the mass transfer of the plasma-forming material of the diaphragm, μμ ₀ = 1.257 10 ^-6 GN / m, I _max is the value of the maximum of the discharge current, t _n is the development time of the power magneto-gas-dynamic instabilities in the discharge plasma, the value of I _max must correspond to the following condition for l and r taken in cm
I _max > 2.5 10 ⁴ (l ^1.4 ) / r ^1.4 (l + r / l), A.

This makes it possible, when the current drops from I _max to 0.1I _max per t _{drop, to} cool the flow of weakly ionized gas to the condensation temperature and to carry out a multi-stage process of aggregation of fractal-like structures in the zone of the shell of the discharge jet and behind the cutoff of the ring electrode.

It was also experimentally established that for the full use of the evaporated material in the formation of nanostructures and then clusters, the condition (N _n + 2N ₊ ) q = N _a must be fulfilled. For this, expansion and cooling of a weakly ionized gas is realized during the condensation time, found from the condition
t _cond = 3g ^1/3 (k ₀ N _c ) ^-1 ,
where t _cond is the period of time during which condensation of the weakly ionized vapor occurs, q is the average number of atoms in the cluster, N _c is the density of atoms during cooling of the flow of weakly ionized gas, k _o ≈ 1.9 T ^1/2 m ^1/6 ρ ^{- 2/3} , T = T _cond is the condensation temperature, m is the mass of the atom, ρ is the density of the substance of the nanostructures.

To obtain fractal-like aggregates with a given fractal dimension D and size R _0, the growth process is carried out in the cluster-cluster mode of aggregation by maintaining for a time t a _{decrease in the} temperature T * of the medium in the coagulation region within 0.8 T _cond ≤T * <T _cond . Moreover, the following relations are valid
N _cl ≈6k _rec (N ₊ ) ² q ^1/6 (k ₀ N _n ) ^-1 ,
n _cl ≈ (R ₀ / r ₀ ) ^D ,
where N _cl is the density of fractal-like clusters, k _o ≈ 1.9 T ^{* 1/2} m ^1/6 ρ ^-2/3 , n _cl is the number of clusters in a fractal-like aggregate; R ₀ is the size of the fractal-like aggregate, D is the fractal dimension of the aggregate, r ₀ is the cluster size. The change in temperature T * in the above range is provided by the selected value of the slope of the current K.

 Thus, creating and changing the spatio-temporal structure of the jet and the discharge mode by selecting the parameters of the discharge gap and the size of the aperture, control the process of structuring the resulting fractal-like aggregates at the stage of their growth.

The obtained fractal-like structures, if necessary, are additionally deposited on the surface of the jet located in the zone of the shell at a distance A of the substrate 7 and placed in the jet braking zone at a distance P * from the ring electrode 3 of the substrate 10, cleaned immediately before the short-wave plasma radiation is deposited from the axial zone of the jet at the current phase of the discharge with a light flux density Ф ₀ = 10 ⁴ -10 ⁵ W / cm ² and a duration of 0.005 ms <τ <0.5 ms.

Concrete example
As a specific example of the execution of the claimed group of inventions, we describe the preparation of fractal aggregates of size R ₀ = 2 ± 0.3 μm with a fractal dimension D = 1.6 ± 0.2 when processing ≈45% of the vaporized diaphragm material into fractal-like aggregates; see figure 3.

The discharge was formed in a vacuum chamber 5, in which the pressure P _nach = 10 Pa, through an opening in the plasma-forming diaphragm 4 between the ring electrodes 2 and 3 with power from a capacitor bank 1 with circuit parameters U ₀ = 5 kV, C ₀ = 2.8 mF , L _k = 5.6 μG, R _k = l, 9 mOhm. Spectrally pure carbon with a cylindrical hole diameter of 2r in the range of 4.0 mm and a thickness of 1 from 4.0 mm was used as the plasma-forming material of the diaphragm 4.

The diameter of the hole in the 2R ring electrodes of graphite was 40 mm. The distance from the surface of the diaphragm 4 to the ring electrodes 2 and 3 was 50 mm with an interelectrode gap L of 104 mm. At a multichannel diagnostic complex, not shown in Fig. 1, a single current pulse of 400 μs duration with a current amplitude of 75 kA was recorded to realize a gas-dynamic plasma flow regime with an off-set parameter n = 1-2 at a carbon mass ablation rate ω = 63.9 g / from.
The shape of the pulse of the discharge current and the derivative of the discharge current were recorded using Rogowski belts, and the voltage drop across the discharge gap was measured using two capacitive voltage dividers.

The steepness of the current drop K was chosen equal to 9 * 10 ⁶ A / s, while for t _drop = 7.5 10 ^-3 s >> t _cond = (1-5) 10 ^-7 s, charged nanostructures with a density of N ₊ = ( 2-0.6) 10 ¹⁴ cm ^-3 , neutral nanostructures with a density of N _n = (1-2) 10 ¹³ cm ^-3 and clusters of size r ₀ ≤4-5 nm with a constant recombination rate of positively charged nanostructures k _rec = 1 * 10 ^-8 cm ³ s ^-1 and the coagulation rate constant of neutral nanostructures k _coag = (4-5) * 10 ^-10 cm ³ s ^-1 .

 The structural parameters of fractal-like structures obtained as a result of aggregation by the method described above were determined by electron microscopy (TEM) using cell decomposition of a two-dimensional image of the studied structure (Covering Set Method).

The fulfillment of the above conditions allowed to obtain a qualitatively new technical effect, namely:
- receive fractal-like structures from almost any source material;
- aggregate structures with the necessary structural parameters: fractal dimension, linear size of the aggregate and its constituent elements, shape anisotropy, etc .;
- to carry out a significant yield of the obtained fractal-like structures from the initial plasma-forming material;
- precipitate fractal-like structures on the surface of the substrate.

 The advantages of the described methods for producing fractal-like structures and devices based on them allow using them also for modeling the properties of long-lived and energy-intensive plasma formations (fractal plasmoids), when an understanding of the nature of objects such as ball lightning and other phenomena of fundamental scientific importance is required.

Claims (5)

1. A method of producing fractal-like structures, including obtaining a stream of weakly ionized gas from the initial plasma-forming material, cooling the stream of weakly ionized gas to a condensation temperature, the formation of nanostructures from neutral and charged particles, the aggregation of nanostructures into clusters and their growth to fractal-like structures, characterized in that the weakly ionized gas receive at a jet diaphragm pulsed electric discharge in the flow regime of jets of high-temperature erosion products with the parameter eraschetnosti n, chosen from the condition
1≤n <250,
where n = P _crit / P _atm ;
P _crit - pressure in the critical section at the expiration of the jet from the opening of the diaphragm, Pa;
P _ATM - pressure in the external environment with respect to the axial zone of the jet, Pa,
cooling the flow of weakly ionized gas to a condensation temperature is carried out at a decay time of the discharge current t _decay , found from the condition
t _recession >> t _cond ,
where t _recession = 0.9l _max K ^-1 , s;
t _cond is the period of time during which condensation of a weakly ionized gas occurs and the formation of charged nanostructures with a density of N ₊ and neutral nanostructures with a density of N _n , s;
l _max - the value of the maximum discharge current, A;
K is the steepness of the decay of the discharge current pulse, A / s,
the aggregation of nanostructures into clusters is carried out up to size r ₀ under the condition
k _rec (N ₊ ) ² >> k _coag (N _n ) ² ,
where k _rec is the constant of the recombination rate of positively charged nanostructures, cm ³ s ^-1 ;
k _coag is the coagulation rate constant of neutral nanostructures, cm ³ s ^-1 ,
cluster growth is carried out to fractal-like structures of size R ₀ with fractal dimension D when the relation
n _cl ≈ (R ₀ / r ₀ ) ^D ,
where n _cl is the number of clusters in the aggregate. 2. The method according to p. 1, characterized in that the obtained fractal-like structures are additionally deposited on the surface of the substrate, cleaned immediately before the deposition of the jet plasma by short-wave radiation at the current phase of the discharge with the light flux density Ф ₀ = 10 ⁴ -10 ⁵ W / cm ² and Duration 0.005 <τ <0.5 ms. 3. A device for producing fractal-like structures, including a sealed discharge chamber with a power source, gas-vacuum system, ring electrodes, the inner holes of which are made in the form of truncated cones with their vertices facing each other, and a diaphragm mounted on the axis of the ring electrode, characterized in that as a source a high-voltage current pulse generator of adjustable amplitude and shape is selected, the diaphragm is made with a round hole, the surface of which is made and plasma-forming material, with the ratio of radius r and of length l openings selected from the range of
0.5 <2r / l <2.0,
the radius R of the inner hole of the electrodes is selected from the condition
ln R / r> 1.6πLω / μμ ₀ (I _max ) ² t _n ,
where L is the distance of the diaphragm-ring electrode, m;
ω is the ablation rate of the mass of the plasma-forming material of the diaphragm, kg / s;
μμ ₀ = 1.257 × 10 ^-6 GN / m;
I _max - the value of the maximum discharge current, A;
t _n - time of development of power magnetogasdynamic instabilities in the discharge plasma, s,
the value of I _max corresponds to the condition for l and r taken in cm
I _max > 2.5 • 10 ⁴ (l ^1.4 ) / r ^1.4 (1 + r / l), A,
and the distance P along the axis of the ring electrode from its surface facing the diaphragm to the chamber wall is selected from the conditions
P> A *,
where A * = 24R, m;
R is the radius of the hole in the ring electrode, m 4. The device according to p. 3, characterized in that between the one of the ring electrodes (for example, the cathode) and the diaphragm an additional substrate is placed parallel to the axis of the discharge gap and at a distance A from it, satisfying the relations:
3R <A <5R. 5. The device according to p. 3, characterized in that orthogonal to the axis of the aperture opening behind the ring electrode, an additional substrate is placed at a distance P * satisfying the condition
A * <P * ≤P.

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