RU2556938C2 - Method of obtaining colloidal solution of nano-sized carbon - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be used in obtaining coatings, reducing coefficient of secondary electronic emission, growing diamond films and glasses, elements, absorbing solar radiation. Colloidal solution of nano-sized carbon is obtained by supply of organic liquid - ethanol, into chamber with electrodes, injection of inert gas into inter-electrode space, formation of high-temperature plasma channel in gas bubbles, containing vapours of organic liquid. High-temperature plasma channel has the following parameters: temperature of heavy particles 4000-5000K, temperature of electrons 1.0-1.5 eV, concentration of charged particles (2-3)·10¹⁷ cm³, diameter of plasma channel hundreds of microns. After that, fast cooling within several microseconds is performed.

EFFECT: simplicity, possibility to obtain nanoparticles of different types.

3 cl, 1 dwg

Description

The proposed method for producing a stable nanoscale colloidal carbon solution relates to the field of nanotechnology.

Obtaining and studying nanostructured materials is of great interest from a scientific and applied point of view (unique electrical, magnetic, chemical, mechanical properties, catalytic activity, luminescent sv-va, etc.).

Fundamental interest is associated with structural features and physicochemical characteristics of the object (a large number of free carbon bonds, compact structure).

Of great interest are studies of such properties of nanofluids as thermal conductivity, density, viscosity, conductivity, and optical and magnetic characteristics.

The unusual properties of nanoparticles are the basis for many areas of applied nature:

- technology of new materials, pharmacology;

- a unique source of field electron emission;

- metal and semiconductor characteristics - the smallest electronic devices;

- the surface structure of the object allows you to use it as a container for liquid and gases, in particular hydrogen.

Recently, considerable interest has been attracted by work related to the production of thin films of nanostructured carbon to reduce the secondary emission factors of metals and dielectrics, growing diamond films and glasses, and obtaining stable colloidal solutions (solar energy absorber) (Robert Taylor, Sylvain Coulombe, Todd Otanicar, Patrick Phelan , Andrey Gunawan4, Wei Lv4, Gary Rosengarten, Ravi Prasher, and Himanshu Tyagi. Small particles, big impacts: A review of the diverse applications of nanofluids. J. Appl. Phys. 113, 011301 (2013)).

Various methods are known (physical, chemical, combined, etc.) for the formation of nanoparticles:

- electric arc,

- pulse-periodic arc and spark,

- laser ablation in gases and liquids,

- precipitation of chemical reaction products,

- pyrolysis in the presence of metal catalysts,

- electrical explosion of conductors,

- catalytic conversion of composite powders in flames, etc.

However, most of these methods are time-consuming and costly and complex, usually requiring separation of the useful product from the impurity. Carbon nanostructures are metastable states of condensed carbon; their preparation is possible only under conditions of deviation from thermodynamic equilibrium. Therefore, of great interest is the recent appearance of a number of works in which a pulsed electric discharge in liquids is used to synthesize carbon nanoparticles, metals, and various compositions. A short pulsed discharge promotes the creation of metastable phases of carbon as a result of carbon atomization in the high-temperature channel of the discharge and its subsequent rapid cooling (quenching).

The method is promising due to a number of features:

- simplicity and low cost of plants and raw materials;

- the ability to skeleton the synthesis process;

- the possibility of obtaining nanoparticles of various types;

- the presence of fluid around the plasma limits the possibility of its expansion and contributes to an increase in temperature and pressure, which favors the occurrence of exothermic chemical reactions.

A pulsed electrical discharge in a liquid can be realized in two ways. In one case, the pulse energy is ≥1 kJ, and in the second it does not exceed several J. The first case requires rather bulky and complex equipment, the reactor experiences significant shock loads. In addition, nanoparticles from nanoscale to micron are obtained, which requires additional efforts to separate them when used in various technologies. The carbon source in a liquid such as water is graphite electrodes. In the case of using organic liquid, the carbon supplier is the liquid itself.

The results of studies on the synthesis of carbon nanoparticles in organic liquids, in particular ethanol, are presented in (Journal of Physics D: Applied Physics, 43 (32). P.323001. Mariotti, D and Sankaran, RM (2010) Microplasmas for nanomaterials synthesis )

Closest to the proposed method is the method described in (Pulsed discharge production of nano- and microparticles in ethanol and their characterization. Parkansky N., Alterkop B., Boxman RL, Goldsmith S., Barkay Z., Lereah Y. Powder Technology 2005. T.150. No. 1. P.36-41), which uses a pulse-arc discharge in ethanol. Two electrodes are placed in ethanol (graphite, nickel, tungsten, etc.), pulse repetition rate f = 100 Hz, currents and voltage I = 100-200 A, U = 20 V, respectively, pulse duration τ = 30 μs, particles from nanoscale to micron.

The disadvantage of the described method is the instability of the colloidal solution (rather rapid precipitation), a wide particle size spectrum, as well as a rather complicated procedure of electrical breakdown in ethanol.

The technical result of the invention is simplicity and low cost, the possibility of obtaining nanoparticles of various types. In addition, it should be noted the following advantages of the proposed technical solution:

- a multi-electrode high-voltage pulse discharge with inert gas injection into the interelectrode space allows the formation of a stable nanostructured colloidal solution in ethanol. There is a certain threshold value of the specific energy input (J / cm ³ ) above which the colloidal solution is stable, the property of the solution does not change for more than a year. At lower specific energy depositions, precipitation and clarification of the liquid occur within 2-3 days;

- when the solution is heated to boiling point and subsequent cooling, the property of the colloid does not change;

- when current flows through a colloidal solution (electrophoresis), a rapid precipitation occurs and the liquid enlightens. At the same time, a nanostructured carbon film forms on the positive electrode;

- the size of the nanoparticles depends on the specific energy input. Near the threshold value of the specific energy input, their size is 5–10 nm, and they are a disordered carbon;

- nanopowder can be isolated from a colloidal solution by evaporation or as a result of electrophoresis.

The technical result is achieved by the fact that the method for producing a colloidal solution of nanosized carbon is carried out as follows, the organic liquid is fed into the chamber with electrodes, an inert gas is injected into the interelectrode space, a high-temperature plasma channel is formed in the gas bubbles, atomizing the carbon atoms followed by rapid cooling.

If the specific energy input into the liquid exceeds the threshold value, a stable colloidal solution is formed. Ethanol may be used as an organic liquid.

The drawing shows a device for producing a colloidal solution.

Our proposed method for producing a stable colloidal solution of nanosized carbon is based on the implementation of a pulsed high-voltage discharge in inert gas bubbles injected into an organic liquid (ethanol). As noted above, a characteristic feature of pulsed discharges in ethanol is the atomization of carbon in a high-temperature channel, followed by sharp cooling. The use of a high-voltage multi-electrode discharge device with gas injection into the interelectrode space, due to the specificity of the formation of the plasma channel and its cooling, opens up new possibilities for the formation of nanostructures and carbon nanofluids.

A dielectric chamber 1 is used, a multi-electrode discharge device 3 with gas injection into the interelectrode space located inside the chamber, placed in ethanol 2, which partially fills the chamber. The chamber 1 is equipped with a device for gas injection, a system for filling and pumping through it an organic liquid (ethanol). In this case, a high-voltage pulse generator 12 is connected to the discharge device. The device contains a pulse generator 5, Rogowski belt 6, a voltage divider 7, a spectrograph 8, an optical waveguide 9, nozzles for pumping liquid 10, and a nozzle for removing gas 13.

The device operates as follows.

An inert gas is injected into the discharge device 3 through the nozzle 4. To remove it from the reactor, a pipe 13 is used. After this, the reactor 1 is partially filled with liquid so that the discharge device 3 is completely in it. High voltage of a given value (U≤20 kV) and pulse repetition rate (f≤100 Hz) is supplied to the extreme electrodes of the discharge device. In the case of operation of the reactor in flow mode, the nozzles 10 provide the necessary flow rate. In gas bubbles 11 filled with alcohol vapor, interelectrode space through openings 5, a pulsed discharge occurs. In each of the interelectrode gaps, a high-temperature plasma channel is formed with a duration of several microseconds with the following parameters: temperature of heavy particles T = 4000-5000 K, electron temperature T _e = 1-1.5 eV, concentration of charged particles n = (2-3) · 10 ¹⁷ cm ³ , the diameter of the plasma channel is hundreds of microns. The energy invested in the discharge in one pulse, ≤2-3 J.

The atomization of carbon atoms occurs in the plasma channel. After the current pulse ceases, the plasma channel rapidly expands, which leads to its rapid cooling ("quenching") and the formation of carbon nonequilibrium nanostructures, thereby determining the characteristics and properties of the colloidal solution. The characteristic cooling time of the discharge channel is units, tens of microseconds. The dynamics of heating and cooling of the plasma channel significantly affects the parameters of the nanoparticles.

The specific energy input into the processed liquid is crucial for obtaining a colloidal solution. In the absence of a flow regime, the specific energy input γ is determined as follows:

, $$\gamma =\frac{W\cdot f\cdot t}{V}$$

,

W is the energy invested in the discharge in one pulse, f is the pulse repetition rate, V is the volume of the liquid, t is the time of processing the liquid.

In case of flowing mode:

$$\lambda =\frac{W\cdot f}{U}$$

,

U is the fluid flow rate per unit time (cm ³ / s). As the processing time of the liquid (specific energy input) increases, the liquid darkens, as a result of the formation of carbon nanoparticles and when a certain threshold value of the specific energy input is exceeded, a stable colloidal solution is formed (the precipitate does not precipitate for more than a year). At lower values of the specific energy input during a day or two, carbon falls to the bottom of the vessel, and the liquid clears.

The parameters of nanoparticles were studied by various methods: Raman scattering (Raman scattering), DLS (dynamic light scattering), X-ray diffractometry, electron microscopy, elemental composition, etc.

Note that when the colloidal solution is heated to a temperature close to the boiling point, and subsequent cooling, the solution remains stable. The threshold value of the specific energy input depends on the material of the electrodes.

The elemental composition of the powder of nanoparticles obtained by evaporation of a colloidal solution is as follows: C - 79.05%; O - 19.57%, the remaining detected elements are Si; K; Ti; Cr; Fe. The presence of oxygen is the result of its adsorption from air.

The results can be used to solve various applied problems, in particular, the production of metal coatings with a carbon film in order to reduce the secondary electron emission coefficient in the technology of growing diamond films and glasses, to create elements that absorb solar radiation, etc.

Claims (3)

1. A method of producing a colloidal solution of nanosized carbon, characterized in that the organic liquid is fed into the chamber with the electrodes, an inert gas is injected into the interelectrode space, a high-temperature plasma channel is formed in gas bubbles containing vapors of the organic liquid, with the following parameters: temperature of heavy particles T = 4000-5000K, the electron temperature T _e = 1-1,5eV, the charged particle density n = (2-3) · 10 ¹⁷ cm ^3, the diameter of the plasma channel hundreds microns, performing atomization carbon atoms, followed by conductive rapid cooling with a duration of several microseconds. 2. The method according to p. 1, characterized in that when exceeding the specific energy input into the liquid of a threshold value, a stable colloidal solution is formed. 3. The method according to p. 1 or 2, characterized in that ethanol is used as the organic liquid.

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