RU2206503C1 - Method of electrophoretic production of filaments from solid nanoparticls - Google Patents

Abstract

FIELD: materials technology. SUBSTANCE: proposed method is base on use of diamond nanoparticles preheated at temperature of 1200-1300 C under vacuum and then kept at temperature of 300-400 C in oxygen-containing atmosphere. Electrophoresis is performed in electric field formed by means of movable electrode in form of peaks. Proposed method makes it possible to produce long filaments from diamond nanoparticles and diamond fibrous material. EFFECT: enhanced efficiency. 3 cl, 1 tbl, 3 ex

Description

 The invention relates to the field of materials science, and in particular to methods for producing filamentary and fibrous materials. More specifically, it relates to methods for electrophoretic production of filamentary structures from solid particles, in particular diamond nanometer size.

 It is known that diamond-containing filaments of micron diameter are prepared by methods of chemical synthesis of microcrystalline diamond films (the so-called chemical vapor deposition methods) on the surface of solid filaments (wires) from another material [R. May et al. CVD diamond-coated fibers. Diam. Relat. Mater., V. 4, no. 5-6, pp. 794-797, 1995]. The method does not allow to obtain filaments with a diameter of less than 10 microns and is characterized by low efficiency and high cost associated with the use of diamond coating synthesis procedure. The yarns obtained in this way are chemically heterogeneous structures consisting of a diamond shell with a core of foreign material. In some cases (for example, a metal base), the core can be removed by chemical etching [P.G. Partridge et al. Chemical vapor deposited diamond fibers: manufacture and potential properties. Mater. Sci. Technolog., V. 10, no. 3, pp. 177-189, 1994], obtaining hollow chemically homogeneous diamond strands, however, this process is very slow and obtaining long diamond strands with a small diameter is difficult.

 A known method of electrophoretic production of threads from particles of solid material, based on the phenomenon of the growth of one-dimensional structures in an aqueous suspension of solid nanoparticles under the influence of an alternating electric field [K. D. Hermanson, S.O. Lumsdon, J.P. Williams, E.W. Kaler, O.D. Velev. Dielectrophoretic assembly of electrically functional microwires from nanoparticle suspension. Science, v. 294, pp. 1082-1086, 2001], selected as a prototype. In this method, using two electrodes located at a distance from fractions of a millimeter to 1 cm from each other, in an aqueous suspension (concentration 0.13-1.5 wt.%) Of gold particles (sizes 15-30 nm) create an alternating electric field (10-200 Hz, tension up to 250 V / cm), which stimulates fast (typical growth rate of 50 μm / s, can reach 0.5 mm / s) the growth of threads with a diameter of up to 1 μm, consisting of gold nanoparticles, in space between the electrodes. The method is characterized by high efficiency (productivity), allows you to get thin micron chemically uniform filaments of large length from solid particles of nanometer size. However, studies have shown that it is not applicable to dielectric materials and, in particular, to diamond nanoparticles.

 An object of the present invention is to provide a method for the electrophoretic production of filaments from solid nanoparticles, which makes it possible to use (utilize) diamond material, as well as the possibility of obtaining fibrous material from these filaments.

This task is achieved by preparing a suspension of diamond nanoparticles [RF Patent 2051093, BI, 1995, No. 36], preheated in a vacuum or inert atmosphere to temperatures of 1100-1300 ^o C, which provides the necessary surface properties of the nanoparticles for the formation of their one-dimensional associations in an electric field , as well as subsequent heating in oxygen at a temperature of 300-400 ^o C, which reduces the rate of spontaneous aggregation of diamond nanoparticles in an aqueous suspension and provides a high growth rate of threads, as well as the creation of an electric field with the help of electrodes, at least one of which is made in the form of a system of electrically connected metal parallel tips oriented in the direction of the electric field, and an increase in the distance between the electrodes during the growth of filaments, which ensures a high and uniform density of centers of nucleation of filaments and their increase lengths.

As a diamond-like substance, ultrafine diamond powder (UDD) was used [A.I. Lyamkin et al. Obtaining diamonds from explosives. DAN USSR, 1988, v. 302, N 3, p. 611-613], formed during the detonation of an explosive mixture of TNT with RDX in a 1: 1 ratio in an explosive chamber filled with carbon dioxide. UDD powders were isolated from condensed carbon products of the explosion by sequential chemical treatment in concentrated and dilute acids to remove non-diamond forms of carbon and metal impurities. The physicochemical properties of UDD were characterized using a set of analytical methods, including X-ray structural analysis, Raman spectroscopy, mass spectrometric and electron probe elemental analysis, and infrared spectroscopy. Samples are a dielectric carbon material with a diamond crystal structure. The crystallite size is 3-10 nm. The specific surface of the powder is 280 m ² / g. The composition of UDD includes: oxygen - up to 10%, nitrogen and hydrogen - up to 1%, other impurities - up to 2% in total.

 The invention is confirmed by the following.

 Example 1

A portion of UDD powder (0.2 g) with an average diamond particle size of 4 nm was loaded into a crucible of a high-temperature vacuum furnace and heated at a temperature of 10 ^o C / min to temperatures of 900-1600 ^o C with continuous pumping of a vacuum chamber providing a pressure of no higher than 3 • 10 ¹⁵ Torr. After the sample was cooled to room temperature and removed from the vacuum chamber, the diamond phase was analyzed by X-ray diffraction or Raman spectroscopy. A suspension of UDD with a concentration of 0.8 wt.% In distilled water was prepared by dispersion in an ultrasonic bath. After dispersion, the suspension was poured into an electrophoretic cell with two metal electrodes located at a distance of 0.5 cm from each other. An alternating voltage with a frequency of 100 Hz and an amplitude of 150 V was applied to the electrodes. The formation of filaments between the electrodes and their growth rate were monitored using an optical microscope. After completion of the formation of filaments, they were removed from the electrophoretic cell, dried, and their thickness was measured using a scanning electron microscope.

 The table shows the parameters of the process of growth of threads and their properties for various modes of preparation (annealing) of UDD.

When using UDD annealed in vacuum at temperatures less than 1100 ^o C, the filaments between the electrodes are not formed. At such temperatures, the formation of a surface layer of nanoparticles with certain electrophysical properties that ensure continuous growth of the filament does not occur. The growth of threads in this case stops immediately after its inception. For UDD, heated in vacuum at temperatures above 1300 ^o C, structural transformations affect the volume of diamond nanoparticles and the resulting material mainly consists of carbon with a non-diamond structure. It should be noted that due to the small particle size (4 nm) and large surface / volume ratio, the 39% content of the non-diamond phase after annealing at 1300 ^° C is equivalent to only 2-3 atomic layers on the surface of a spherical particle.

 Example 2

The operations are performed as in example 1, however, after vacuum annealing of the UDD and before preparing the suspension, the UDD sample is heated in air at temperatures of 300-400 ^o С. The growth rate of the UDD threads oxidized at 350 ^o С was two times higher than the rate for unoxidized samples ceteris paribus. Investigations of the composition of surface functional groups by methods of thermal desorption mass spectrometry and IR spectroscopy showed the presence of carbonyl groups of a certain structure after such oxidation. These groups ensure the stability of the suspension (reduction of aggregation with subsequent sedimentation of aggregates) and thereby prevent a decrease in the concentration of UDD in the suspension over time. For oxidation temperatures of 250 ^o C and below, a positive effect was practically not observed (a weak degree of surface oxidation). At oxidation temperatures in excess of 400 ^° C, the growth rate of the filaments sharply decreased due to the “burn-out” of the surface layer formed on diamond nanoparticles during vacuum annealing and necessary for the implementation of dielectrophoretic growth of filaments. This is confirmed by mass spectrometric studies of the process of UDD oxidation by atmospheric oxygen.

 Example 3

The operations are carried out as in example 1, using UDD annealed in vacuum at 1300 ^o C, but to form an electric field, an electrode is used in the form of a system of tips located at a distance of 0.5-1.0 mm from each other and directed to the opposite electrode. This provides a high and uniform density of centers of nucleation of filaments (local areas with a substantially inhomogeneous field) over the entire surface of the electrode, as well as a high degree of branching and splicing. This electrode is arranged to move relative to the second electrode along the direction of the electric field. Previously, in an independent experiment, the growth rate of the filaments is determined under the used conditions (field parameters, suspension concentration) with stationary electrodes. After applying the field and the beginning of the growth of filaments, the electrodes are moved apart at a constant speed not exceeding the growth rate of filaments (5.2 μm / s). At higher speeds, growth ceases at a certain distance between the electrodes. When the electrodes are placed one above the other, the exit of the needle electrode with a bunch ("beard") of growing threads from the suspension is allowed (a kind of pulling of the threads from the suspension). The constant concentration of particles in the suspension, necessary to ensure the operability of the method, was maintained either by using a cell with a large amount of suspension (the mass of particles in the suspension is much higher than the mass of the formed filaments) or by slowly pumping a suspension with a constant concentration through the cell . In this case, tangles of up to 10 cm or more are formed. In contrast to the use of ordinary flat fixed electrodes, when the filaments grow randomly and their density does not exceed several units per 1 cm ² , in this case it is possible to form a fibrous "tangled" material such as felt or felt fabric.

Claims (3)

1. The method of producing strands from solid nanoparticles, which consists in preparing an aqueous suspension of nanoparticles and creating an alternating electric field in the suspension using two electrodes, characterized in that the aqueous suspension is prepared from diamond nanoparticles preheated in vacuum at 1200-1300 ^o C. 2. The method according to claim 1, characterized in that after heating in vacuum, the diamond nanoparticles are heated at 300-400 ^o With in an oxygen-containing atmosphere.  3. The method according to claim 1, characterized in that to create an electric field as one of the electrodes use a system of electrically connected metal parallel tips oriented in the direction of the electric field, and after creating an electric field, the distance between the electrodes is gradually increased with a speed not exceeding thread growth rate.

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