WO2006126808A1

WO2006126808A1 - A manufacturing method of metal-micro particles using pulse-type energy

Info

Publication number: WO2006126808A1
Application number: PCT/KR2006/001885
Authority: WO
Inventors: Sun Hee Park; Jin Yi
Original assignee: Ncmetal Inc.
Priority date: 2005-05-23
Filing date: 2006-05-19
Publication date: 2006-11-30
Also published as: KR20060120716A

Abstract

This invention relates to a method for preparing fine metal particles using pulse-type energy. Particularly, it relates to a method for preparing fine metal particles using the principle of electrical corrosion, the method comprising causing alkali metal ions to be present in water or organic solvent and, at the same time, placing at least two metal electrodes apart from each other in the solvent, and then applying to the metal electrodes an electric current, the size and direction of which change periodically with the passage of time in order to repeatedly change the polarities of the electrodes. According to the invention, it is possible to obtain fine metal particles by using electrical corrosion to induce the ionization of platinum group metals and metals having a strong oxide film formed thereon, which are not ionized by electrolysis, and to reduce the ionized metals. Also, it is possible to prepare fine particles from metals prone to oxidation, by the use of a solvent which is completely free of oxygen. In addition, it is possible to obtain non- aggregated, mono-dispersed fine particles, which can be applied in a powder injection molding process.

Description

A MANUFACTURING METHOD OF METAL-MICRO PARTICLES USING PULSE-TYPE ENERGY

Technical Field

[1] The present invention relates to a method for preparing fine metal particles using pulse-type energy. More particularly, the present invention aims to make it possible to prepare fine particles of all pure metals and metal alloys by using the principle of electrical corrosion while very rapidly changing the polarities of electrodes in a repeated manner.

Background Art [2] As a prior method for obtaining fine metal particles, Korean Patent Registration No.

0408647 discloses a method for preparing fine metal particles, comprising grinding metal oxide using ultrasonic ball milling, and then reducing the ground metal particles with hydrogen. However, this is a method for obtaining fine metal particles by finely grinding oxide without incorporation of impurities and then reducing the ground oxide particles with hydrogen. [3] For this reason, according to this method, particles of a metal which is difficult to obtain in an oxide form, cannot be prepared, and metal oxide which is not reduced by hydrogen cannot be prepared into fine metal particles. [4] For example, precious metals are very difficult to obtain in an oxide form, and even if an oxide thereof is obtained, it is highly expensive compared to the metal, which is not in the form of the oxide. [5] Also, aluminum oxide, chromium oxide, zinc oxide, titanium oxide and the like are not reduced by hydrogen. [6] Another problem is that, in the hydrogen reduction process, thermal treatment is essential, in which difficult-to-grind aggregates are formed together with the growth of reduced fine metal particles. [7] These aggregates are about a few microns in size and cause problems when applied to a powder injection molding process for manufacturing functional products having a complex shape. [8] Also, an organic binder does not penetrate inside the aggregates, but rather remains in spaces, resulting in the cracking of moldings due to the thermal expansion of gas during a binder removal process. [9] Also, it has been reported that, even if the organic binder well penetrates the aggregates, it is difficult to obtain completely compacted products in a sintering process, because very large pores are present between the aggregates. [10] As another method for obtaining fine metal particles, Korean Patent Application

No. 10-2003-0037065 discloses a method for preparing metal nanoparticles using an electrolysis process. In brief, the electrolysis process is a process for obtaining a given material by applying a direct electric current to two electrodes so as to produce cations in the anode and to supply electrons to the cations in the cathode. The technology described in said patent application is a method of preparing fine metal particles according to this electrolysis principle using pure metal electrodes without using metal salts and has an advantage in that it can prepare fine particles containing no impurities, unlike other mechanical methods for preparing particles.

[11] However, this method cannot avoid the growth and aggregation of particles due to contact between the particles, because it is a method wherein fine particles are obtained while metal particles ionized in the anode are subjected to phase transformation into a metal phase by receiving electrons in the cathode. This is because the generation of heat by electric current in the electrodes causes the growth of particles, but ultrasonic energy is insufficient to remove the electrical attractive force between the electrodes and the particles.

[12] Another important problem is that metals such as platinum group elements, which are chemically inert and not corroded by acid and alkali, and metals which have a strong oxide film formed on the surface thereof are not ionized, and thus metal particles cannot be obtained therefrom.

[13] Examples of such metals include platinum group metals such as platinum, palladium, ruthenium, rhodium, osmium and iridium, and metals such as aluminum, titanium and zinc.

[14] This is because the ionization of metals by direct current, used in electrolysis, simply depends on the principle that metal elements lose electrons due to the one- directional flow of electrons.

[15] In other words, it is very difficult, only by this electrical energy, to ionize the chemically inert metal and the surface of the metals having an oxide film formed thereon.

[16] Still another problem is that, because a solvent is based on water, metals which tend to be oxidized by dissolved oxygen and oxygen generated in the electrolysis of water react with oxygen just after reduction in the cathode so as to be converted into oxides. Disclosure of Invention Technical Problem

[17] The present invention has been made to solve the above-described problems occurring in the art, and it is an object of the present invention to make it possible to obtain fine metal particles by using electrical corrosion to induce the ionization of platinum group metals and metals having a strong oxide film formed thereon, which are not ionized by electrolysis, and to reduce the ionized metals. Another object of the present invention is to make it possible to prepare fine particles from metals prone to oxidation, by the use of a solvent which is compeltely free of oxygen. Still another object of the present invention is to make it possible to obtain non- aggregated, mono- dispersed fine particles, which can be applied in a powder injection molding process. Technical Solution

[18] The present invention provides a method for preparing fine metal particles using the principle of electrical corrosion, the method comprising the steps of: causing alkali metal ions to be present in water or organic solvent and, at the same time, placing at least two metal electrodes apart from each other in the solvent, and then applying to the metal electrodes an electric current, the size and direction of which change periodically with the passage of time in order to repeatedly change the polarities of the electrodes.

[19] To change the polarities of the electrodes, an electric current, the size and direction of which change periodically with the passage of time, is used.

[20] Also, said electric current has a waveform selected from among sign wave, square wave, triangular wave and saw-tooth wave.

[21] The electric current having such a waveform has the effect of additionally applying pulse-type energy to the electrodes, unlike the case of the prior electrolysis process (direct current) depending on the flow of a simple electric current.

[22] In other words, in the present invention, electrical energy and kinetic energy are applied to the electrodes, and thus it is easy to ionize metal material which is difficult to ionize by the prior electrolysis process. Also, because the polarities of the electrodes repeatedly change, there are not metal particles attached to the electrodes.

[23] In the present invention, the reduction of the ionized metal does not depend on electrons emitted from the cathode, but rather is achieved by dissolving an alkali metal having strong ionization tendency in an electrolyte so as to induce the alkali metal to transfer electrons to target electrons.

[24] In other words, alkali metals such as sodium (Na), lithium (Li) and potassium (K) have a very strong ionization tendency, and thus are present as cations in water or organic solvent such as alcohol.

[25] When hydroxides, oxides or compounds of said alkali metals are dissolved in water or organic solvent, the alkali metals will be present as cations.

[26] Alkali metals, which can be used in the present invention, include metal citrate, metal ascorbate and metal acetate, and are not limited to any particular substance, because these are present as cations when dissolved.

[27] Accordingly, the alkali metal cations instantaneously become neutral by receiving electrons from the electrodes and transfer the electrons to metal cations. [28] In other words, the alkali metal becomes cationic again, and the metal ions are reduced by receiving the electrons from the alkali metal to produce metal particles. [29] Thus, because the metal ions are not reduced by receiving electrons from the electrodes, it is possible to avoid that the phenomenon that the metal ions are attached to the electrodes. [30] Meanwhile, when water is used as the solvent, dissolved oxygen is present in the water, and oxygen gas is generated due to electrolysis. [31] Thus, metals having strong oxidizing power, when reduced after ionization, bind to oxygen to form metal oxides. [32] To avoid this phenomenon, in the present invention, a solvent in which electrolysis does not occur is used, and the concentration of hydrogen ions is increased. [33] The solvents in which electrolysis does not occur include alcohol, acetone, methyl ethyl ketone (MEK), benzene and toluene. [34] The hydrogen ion concentration can be increased by adding an organic acid, which does not react with metals. [35] Examples of the organic acid includes 3-oxo-L-gulofuranolactone, and

2-hydroxy-l,2,3-propanetricarboxylic acid, but are not limited thereto in the present invention. [36] As described above, when the hydrogen ion concentration of the solvent in which electrolysis does not occur is increased, and the electrodes are applied with electric current having a waveform selected from among sign wave, square wave, triangular wave and saw-tooth wave, the prepared fine metal particles do not undergo any oxidation. [37] Thus, metals having a very high oxidizing power, for example, aluminum, titanium, zinc and tantalum, can also be prepared into fine particles. [38] According to the present invention as described above, metals, including precious metals such as platinum, gold, silver and palladium, but excluding alkali metals on the

Periodic Table of Elements, can be prepared into fine particles having a size of less than 100 nm. [39] The fine metal particles thus prepared are obtained as a dispersion in solvent, mixed with an organic protective coating or an organic binder for powder injection molding, and stored and maintained in a state in which they do not react with oxygen even after drying.

Advantageous Effects

[40] As described above, according to the present invention, it is possible to prepare fine metal particles having a size of less than 100 nm, which contain no impurity. Also, it is possible to produce fine particles of the platinum group metals which are chemically inert. Furthermore, it is possible to prepare fine particles of all pure metals and metal alloys by completely inhibiting the oxidation of the fine particles. Thus, the present

W_{h l(t}e_igoss invention can be used in an unlimited range of applications and is expected to be highly useful. Best Mode for Carrying Out the Invention

[41] Hereinafter, the present invention will be described in further detail by examples and comparative examples in conjunction with reference figures.

[42] 1. Example (1) [43] Sodium hydroxide is dissolved in water to prepare a solvent. In the solvent, two platinum (Pt) electrodes are placed apart from each other, and then a "sinusoidal electric current" is applied to the electrodes, thus preparing fine platinum particles.

[44] 2. Comparative Example (1-1) [45] Sodium hydroxide is dissolved in water, and two platinum (Pt) electrodes are then immersed therein. Then, a "direct electric current" is applied to the electrodes.

[46] 3. Results [47] As shown in Reference Figure 1, it can be seen that, in the case of Example (1), the platinum electrodes had a weight loss of about 12% after 12 hours, whereas, in the case of Comparative Example (1-1) comprising applying the direct electric current, the platinum electrodes had no weight loss.

[48] This suggests that platinum is not ionized by electrolysis, but can be ionized by the sinusoidal electric current.

[49] [Reference Figure 1] [50]

T| - a-;

. O example 1

b i -,

2 i ^■ comparative examplE 1 -1

0 . ϋ 2 4 b !> 10

Tim&jhouir}

[51] [Reference Figure 2]

[53] Reference Figure 1 shows measurement results for a change in platinum electrode weight as a function of current application time in Example 1 and Comparative

Example 1-1. [54] Reference Figure 2 is a transmission electron microscopy photograph of the fine platinum particles prepared according to Example 1. As shown in Figure 2, the size of the fine platinum particles is about 4 nm. [55] 4. Example (2)

[56] Sodium acetate is dissolved in alcohol. To increase the hydrogen ion concentration of the alcohol, 3-oxo-L-gulofuranolactone is then dissolved in the alcohol, thus preparing a solvent. [57] In the solvent thus prepared, two aluminum (Al) electrodes are placed apart from each other, and then a sinusoidal electric current is applied to the electrodes. [58] 5. Results

[59] As can be seen in Reference Figure 3, the aluminum was not oxidized, but rather was present in the form of pure aluminum. [60] [Reference Figure 3]

[61]

2n{degree} [62] [Reference Figure 4] [63]

A ^ φ

.» - *

$

«< φ

#

10 nm

[64] Reference Figure 3 shows analysis results for the X-ray diffraction pattern of the fine aluminum particles prepared according to Example (2). [65] Reference Figure 4 is a transmission electron microscope photograph of the fine aluminum particles prepared according to Example (2). As shown in Reference Figure 4, the size of the fine aluminum particles is about 5 nm.

Claims

[1] A method for preparing fine metal particles using pulse-type energy according to the principle of electrical corrosion, the method comprising the steps of: causing alkali metal ions to be present in water or organic solvent and, at the same time, placing at least two metal electrodes apart from each other in the solvent, and then applying to the metal electrodes an electric current, the size and direction of which change periodically with the passage of time.

[2] The method of Claim 1, wherein the electric current to the metal electrodes electric current has a waveform selected from among sign wave, square wave, triangular wave and saw-tooth wave and has a voltage of more than 2 V.

[3] The method of Claim 1, wherein the step of causing alkali metal ions to be present in water or organic solvent is performed by dissolving hydroxides, oxides or compounds, which contain Na, Li and/or K.

[4] The method of Claim 1, wherein metal cations ionized in the electrodes are reduced by receiving electrons from the alkali metal instantaneously reduced by the applied electric current, so as to form the fine metal particles.

[5] The method of Claim 1, wherein the organic solvent is any one selected from the group consisting of alcohol, MEK, benzene and toluene, which do not generate oxygen gas, because electrolysis does not occur therein.

[6] The method of Claim 1, which further comprises adding an organic acid for increasing hydrogen ion concentration to the organic solvent in order to inhibit the oxidation of the metal.

[7] The method of Claim 6, wherein the organic acid is any one selected from among

3-oxo-l-gulofuranolactone, and 2-hydroxy-l,2,3-propanetricarboxylic acid.