WO2004039524A1

WO2004039524A1 - Method for manufacturing nano-scaled copper powder by wet reduction process

Info

Publication number: WO2004039524A1
Application number: PCT/KR2003/000918
Authority: WO
Inventors: In-Bum Jeong; Young-Sic Kim; Byoung-Yoon Lee; Yoon-Hyun Kim; Byong-Soek Jeong; Moon-Su Lee
Original assignee: Chang Sung Corporation
Priority date: 2002-10-30
Filing date: 2003-05-09
Publication date: 2004-05-13
Also published as: KR100486604B1; KR20040037824A; AU2003230317A1; US20040221685A1

Abstract

The present invention relates to a method for manufacturing a nano-scaled copper powder by a wet reduction process, comprising adding appropriate amounts of sodium hydroxide (NaOH) and hydrazine (N2H4) to an aqueous copper chloride (CuCl2) solution to finally obtain a copper powder having a particle size of 100 nm (0.1 nm) grade via an intermediate product such as a copper complex.

Description

METHOD FOR MANUFACTURING NANO-SCALED COPPER POWDER BY WET REDUCTION PROCESS

Technical Field

The present invention relates to a method for manufacturing a nano-scaled copper powder by a wet reduction process, and more particularly to a method for manufacturing a nano-scaled copper powder by a wet reduction process, comprising adding appropriate amounts of hydrazine (N₂H₄) and alkaline hydroxide to an aqueous copper salt(CuX, X^Cl^ Br₂, SO₄, (NO₃)₂ ) solution to finally obtain copper powders having lOOnm ~ l tm graded particle size via chelate.

Background Art

Copper (Cu) powder is employed in an electrically conductive paste material for multilayer passive devices, for example, a multilayer ceramic chip capacitor (MLCC). Recently, in order to produce the conductive material for inner electrode, a copper powder with a submicron scaled particle size ranging from 0.8/im

~ ljCtm has been used.

In this regard, development of a nano-scaled copper powder with good dispersibility may be considered. It is anticipated that such nano-scaled copper powder be applied to any miniaturized passive devices for which development is in progress in the pertinent art.

Meanwhile, in the fields of PDPs (Plasma Display Panels), FEDs (Field Emission Displays), automobile back light and the like using glass as a substrate, a metal conductive paste material is required to be sintered at a low temperature of 550 ^°C . Various application industries also tend to lower the sintering temperature. The use of a nano-scaled (100 nm) metal powder can keep pace with the trend of lowering the sintering temperature of a metal conductive paste material. Therefore, it is anticipated that the conductive paste material can be used for forming electrodes that have up till now been exclusively carried out by a plating method, due to a higher sintering temperature. Many different methods have been involved in the synthesis of a copper powder used in the conductive paste as described above, such as a gas phase method and a liquid phase method.

Conventional methods for manufacturing metal powders have various problems such as a low yield due to wide particle size distribution, large particle size, low sphericity, and difficulty in controlling a degree of oxidation. In order to overcome these problems, a wet method such as a liquid phase reduction method and a thermal decomposition method, as well as a gas phase method such as gas evaporation method and the like, have been developed.

Methods generally used for manufacturing metal powders are summarized, as follows.

With respect to a gas atomization method, a high-pressure inert gas is atomized to a molten metal flowing through a nozzle to obtain a metal powder. Although this method is suitable for mass production, it is difficult to prepare a nano-scaled powder, thereby powder yield being considerably lowered. Therefore, the gas atomization method is restrictively used.

With respect to a thermal decomposition method, a metal compound that has a wealc binding force between metal and anion is thermally decomposed using a gas reducing agent and milled to obtain a metal powder. This method provides a fine metal powder. However, because the metal powder may be burned during a heat treatment, the burned powder is required to be milled and classified.

Therefore, this method has a lower yield than a liquid phase reduction method when used in preparing a metal powder for forming a thick film conductive paste material.

In a gas evaporation method, an evaporation material is evaporated by heating its source under an inert gas such as He and Ar or an active gas such as CH and NH₄, and the evaporated gas is reduced and condensed in the seducing gas such as H₂ obtain a fine metal powder. This method is advantageous in preparing a metal powder having its particle size of 5nm ~ several tm. However, productivity is very low and thus the metal powder is very expensive.

A liquid phase reduction method is an exemplary chemical method for manufacturing a metal powder. This method can more easily control the shape of the powder and can prepare an ultrafine powder having a particle size of a submicron unit, compared with the aforementioned methods. The complete procedure of preparing a metal powder by reducing an initial precipitate is carried out in a liquid phase. In detail, a metal powder can be prepared by a procedure comprising a initial intermediate forming, producing an intermediate product and adding a reducing agent. The reducing agent comprises formalin, hydrazine, an organic compound and the like. Advantageously, the liquid phase reduction method provides easy control of the powder shape, high sphericity, and narrow particle size distribution. Furthermore, it is possible to prepare an ultrafine powder having a submicron-scaled particle size that is excellent in the surface property of the powder. Therefore, a powder that is high in tap density, one of the most important characteristics for a conductive paste material can be prepared. Despite these advantages, optimization of concentration, temperature, pH, and reaction rate is a prerequisite to prepare a metal powder.

A conventional wet method, such as the liquid phase reduction method, for preparing a copper powder controls the particle size of the powder through a multi- step reaction, as shown in Fig. 1.

In detail, in a first step, copper oxide (CuO) is precipitated by adding sodium hydroxide (NaOH) to an aqueous copper sulfate (CuSO₄) solution, and then filtered.

In a second step, a stable Cu₂O solution is obtained by reacting the obtained CuO with glucose (C₆Hι O₆), a representative aldohexose (a monosaccharide having

6 carbons and an aldehyde group).

In a third step, when the color of the resulting solution changed into a dark red due to the production of Cu₂O, glycine (NH₂-CH₂-COOH), a kind of amino acid, and arabic gum are added to the Cu O solution and uniformly dispersed. Then, hydrazine (N₂H₄) as a reducing agent is added to the mixture to thereby reduce

Cu₂O, to obtain a copper powder as a precipitate.

The glycine and arabic gum as the third additives are added to control the size and surface shape of the final copper powder.

By obtaining the copper oxide (CuO) as a precipitate, by adding sodium hydroxide (NaOH) to an aqueous copper sulfate (CuSO₄) solution, the effect of impurities that are left in the solution on the product can be minimized.

As described above, in the conventional wet method for preparing a copper powder, copper sulfate (CuSO₄) is used as a copper source. As a result, an anionic effect is reduced, whereby the particles of the copper powder become agglomerated. It is difficult to adjust the input condition by addition of the glycine and Arabic gum as an organic additive to control the size and surface shape of the copper powder, whereby a high degree of reproducibility cannot be afforded.

The particle size of the copper powder is different depending on the addition condition of the additives and thus it is difficult to control the particle size. The process is complicated due to many variables such as additives, reaction agents (NaOH, N₂H ), together with its quantity and method of addition and a solution temperature and requires a longer preparation time.

Relatively coarse copper powder, having a particle size of 0.5 to 4 m grade, is obtained and the particle size distribution of the powder is not uniform. In particular, because Cu O is a chemically stable intermediate product, the growth rate of the copper powder is slow. Therefore, it is difficult to maintain the sphericity of the powder surface.

For the forgoing reasons, it is difficult to prepare an ultrafine copper powder having a particle size of 0.1 /an (lOOnm) grade using the conventional wet method.

Disclosure of the Invention

Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for manufacturing an ultrafine copper powder having particle size of lOOnm grade ~ ljtffli grade by a wet reduction process, comprising the steps of adding sodium hydroxide (NaOH) to an aqueous copper chloride (CuCl₂) solution with high anionic effect, and reducing the resulting copper oxide (Cu_xO) by the addition of hydrazine (N H₄). The method is a relatively simple process and also affords a high degree of reproducibility. Furthermore, copper powder having particle size of lOOnm grade ~ lμm grade can be prepared which has good surface quality, narrow particle size distribution, and good powder sphericity.

In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a method for manufacturing a nano-scaled copper powder by a wet reduction process, comprising the steps of adding sodium hydroxide (NaOH) to an aqueous copper chloride (CuCl₂) solution to give an aqueous solution containing copper oxide and copper hydroxide; reducing the copper oxide and the copper hydroxide to obtain a nano-scaled copper powder as a precipitate by adding hydrazine (N₂H ) to the aqueous solution; and filtering and drying the precipitated nano-scaled copper powder.

Preferably, the sodium hydroxide (NaOH) is added in an amount of 2 to

33 moles pe mole of the copper chloride (CuCl₂) when the aqueous copper chloride (CuCl₂) solution is kept within a temperature of 30 to 80 °C, and the hydrazine (N₂H₄) is added in an amount of 0.5 to 12 moles per mole of the copper chloride when the aqueous solution containing the copper oxide and the copper hydroxide is kept within a temperature of 40 to 80°C .

Preferably, in the step of producing the copper oxide and the copper hydroxide, before the addition of NaOH, silver nitrate (AgNO ) is added to the aqueous copper chloride (CuCl₂) solution in an amount of 1/1,000 to 1/10,000 moles per mole of the copper chloride.

In accordance with another aspect of the present invention, there is provided a method for manufacturing a nano-scaled copper powder by a wet reduction process, comprising the steps of adding hydrazine (N₂H ) to an aqueous copper chloride (CuCl₂) solution to give an aqueous solution containing a copper

.complex (Cu(N H₄)_mCl_n); adding sodium hydroxide (NaOH) to the aqueous copper complex solution to obtain a nano-scaled copper powder; and filtering and drying the nano-scaled copper powder.

Preferably, the hydrazine (N₂H₄) is added in an amount of 0.5 to 12 moles per mole of the copper chloride (CuCl₂) when the aqueous copper chloride (CuCl₂) solution is kept within a temperature of 20 to 70 ^°C , and the sodium hydroxide

(NaOH) is added in an amount of 2 to 33 moles per mole of the copper chloride when the aqueous solution containing the copper complex is kept within a temperature of 40 to 80^°C . Preferably, in the step of producing the copper complex (Cu(N₂H₄)_mCl_n), before the addition of hydrazine, silver nitrate (AgNO ) is added to the aqueous copper chloride (CuCl₂) solution in an amount of 1/1,000 to 1/10,000 moles per mole of the copper chloride.

Brief Description of the Drawings

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic flow diagram showing the conventional wet method for preparing a copper powder;

Fig. 2 is a schematic flow diagram showing the wet reduction method for preparing a nano-scaled copper powder according to the first embodiment of the present invention; F . 3 is a schematic flow diagram showing the wet reduction method for preparing a nano-scaled copper powder according to the second embodiment of the present invention;

Fig. 4 is a Scanning Electron Microscopy (SEM) photograph of the nano- scaled copper powder prepared according to the first embodiment of the present invention;

Fig. 5 is a SEM photograph of the nano-scaled copper powder prepared according, to the second embodiment of the present invention;

Fig. 6 is a SEM photograph of the nano-scaled copper powder prepared by adding a trace amount of silver nitrate for use with the first embodiment of the present invention; and

Fig. 7 is a SEM photograph of the nano-scaled copper powder prepared by adding a trace amount of silver nitrate for use with the second embodiment of the present invention.

Best Mode for Carrying Out the Invention

Hereinafter, the present invention will be described in more detail.

According to the present invention, copper chloride (CuCl₂) is used as a copper salt for preparing a copper powder, instead of copper sulfate (CuSO ) in a conventional wet method.

Copper chloride (CuCl₂) has an anionic group that is higher in terms of electronegativity, relative to copper sulfate (CuSO ), whereby the chlorine ion has a higher anionic effect than the sulfate ion in a solution. Therefore, agglomeration of the copper powder is more effectively prevented, thereby causing a much finer powder to be produced. Furthermore, copper chloride acts to effectively control the shape of the powder surface. For the forgoing reasons, according to the present invention, a nano-scaled copper powder is prepared by adding sodium hydroxide (NaOH) to an aqueous copper chloride solution (CuCl₂) to give copper oxide (CuO) and copper hydroxide (Cu(OH) ) as intermediate products, reducing the intermediate products using hydrazine (N₂H₄), followed by filtered and dried.

In detail, the first step of adding sodium hydroxide (NaOH) to an aqueous copper chloride solution (CuCl ) to give an aqueous solution containing copper oxide (CuO, Cu₂O) and copper hydroxide (Cu(OH)₂) as intermediate products is shown as the following Scheme 1 :

Scheme 1

4CuCl₂ + 6NaOH ^■» CuO + Cu₂O + Cu(OH) ₂ + 8C1^" + 6Na⁺ + 2H₂O In the above reaction, NaOH is added to produce copper oxide and copper hydroxide. The amount of the added NaOH ranges from 2 to 33 moles per mole of CuCl₂. If the amount of NaOH exceeds 33 moles, the obtained aqueous solution is changed into a strong basic solution. Therefore, a reduction reaction does not completely occur in the subsequent step of adding N₂H₄. Furthermore, such addition is uneconomical and a large amount of ions are left in the aqueous solution, resulting in an increase of impurities. On the other hand, if the amount of NaOH is less than 2 moles, the desired intermediate product, copper oxide (Cu_xO) is not obtained. As a result, subsequent reaction cannot be accomplished.

It is preferable to limit the temperature of the aqueous CuCl₂ solution to a range of 30 to 80 ^°C upon the addition of NaOH. If the temperature of the aqueous CuCl₂ solution is less than 30 ^°C, it is difficult to prepare the intermediate products. On the other hand, if the temperature of the aqueous CuCl₂ solution exceeds 80 ^°C, the intermediate products are quickly prepared, thus causing severe agglomeration. At the same time, because the reduction reaction is carried out at too high a temperature, 100 ^°C or more, the thermal stability of the intermediate products is lowered.

The second step of reducing the copper oxide (CuO) and copper hydroxide (Cu(OH)₂) to obtain a copper powder as a precipitate using hydrazine (N₂H₄) is shown as the following Scheme 2: Scheme 2 CuO + Cu₂O + Cu(OH)₂ + 8C1^" + 6Na⁺ + 2N₂H

^■» 4Cu_(S) + 6Na⁺ + 8CT + 2N₂ + H₂t + 4H₂O

In the above reaction, the amount of the added N H₄ ranges from 0.5 to 12 moles per mole of CuCl₂. If the amount of N₂H is less than 0.5 moles, the reduction reaction may be incomplete. On the other hand, if it exceeds 12 moles, although the reaction rate is increased, the product is severely agglomerated and the surface quality of the copper powder is lowered.

It is preferable to add hydrazine to an aqueous solution containing the copper oxide (CuO) and copper hydroxide (Cu(OH)₂) when the temperature of the aqueous solution is kept within a range of 40 to 80°C . If the temperature is less than 40 ^°C , the reduction reaction is not easily carried out, resulting in an incomplete reduction reaction. On the other hand, if it exceeds 80^°C, the reduction reaction is easily carried out but it is carried out at too high a temperature, thereby causing agglomeration of the product.

The precipitated copper powder is filtered to eliminate NaCl salt and then dried under a non-oxidizing atmosphere, to thereby finally produce a nano-scaled copper powder.

Meanwhile, before the addition of sodium hydroxide (NaOH) to the aqueous copper chloride (CuCl₂) solution, silver nitrate (AgNO₃) may be added in a trace amount of 1/1,000 to 1/10,000 moles per mole of the copper chloride. Because silver is reduced faster than copper, the addition of silver nitrate enables an increase in the reduction rate of the copper.

That is, silver acts as a catalyst to promote the nucleation of copper, thereby increasing the reduction rate of copper. As a result, the total reduction rate of copper is increased. In the presence of silver nitrate (AgNO₃), the first step is carried out as the following Scheme 3: Scheme 3 4CuCl₂ + (6+χ)NaOH + χAgNO₃ (χ - 0.001 ~ 0.0001)

- CuO + Cu₂O + Cu(OH)₂ + 2H₂O + χAgO + 8C1^" + 6Na⁺ + χNO₃ ^" + (2+χ)H₂O

Hydrazine is added to the aqueous solution obtained according to Scheme 3 to thereby give a copper powder, as the following Scheme 4: Scheme 4

CuO + Cu₂O + Cu(OH)₂ + 2H₂O + χAgO + 8C1^" + 6Na⁺ + χNO₃ ^" + N₂H₄ ^■» 4Cu₍s) + χAg₍s₎ + 8C1^" + 6Na⁺ + χNO₃ ^" + (2+χ)N₂t + (l+χ)H₂t

+ (4+χ)H₂O In the above reaction, the obtained copper powder is filtered to eliminate NaCl salt and a nitrate ion (NO₃ ^") and then dried under a non-oxidizing atmosphere to thereby finally produce a nano-scaled copper powder. Meanwhile, according to the present invention, a nano-scaled copper powder can also be prepared, even though the addition sequence of NaOH and hydrazine (N₂H₄) is changed.

This is feasible because CuCl reacts with hydrazine (N H₄) to form a copper complex (Cu(N₂H₄)_mCl_n). In detail, hydrazine (N₂H₄) is added to an aqueous copper chloride (CuCl₂) solution in an amount of 1 to 12 moles per mole of the copper chloride at a temperature of 20 to 70 ^°C to produce a copper complex (Cu(N₂H )_mCl_n) as an intermediate product. This reaction can be simplified as the following Scheme 5:

Scheme 5 CuCl₂ + N₂H_4excess ^■» Cu(N₂H₄)_mCl_n + (2-n)CT (n=l , 2)

In the above reaction, if the temperature of the aqueous copper chloride solution is less than 20 ^°C , the desired intermediate product is not obtained. Rather, an undesirable intermediate product may be obtained or such an undesirable reaction may occur. On the other hand, if it exceeds 70 ^°C , the desired intermediate product is obtained and at the same time, a partial reduction reaction thereof may occur.

If the amount of the added hydrazine (N₂H ) is less than 1 mole, the desired intermediate product is not obtained; while, if it exceeds 12 moles, a large amount of ions are left in the aqueous copper chloride solution, thereby increasing impurities. Furthermore, a partial reduction reaction may occur. Then, sodium hydroxide (NaOH) is added to an aqueous solution containing the copper complex in an amount of 2 to 33 moles per mole of the copper chloride (CuCl ) at a temperature of 40 to 80^°C to separate a nano-scaled copper powder from the copper complex. This reaction is shown as the following Scheme 6:

Scheme 6 Cu( ₂H₄)_mCl_n + χ NaOH(χ =2 ~ 33) + (2-n)CT (n=l , 2)

- CuO + mN₂H₄ + 2C1^" + χ Na⁺ + (x -l)OH^" + * H₂

^ Cu_(s) + 2CT + x Na⁺ + Qc -l)OH^" + £ mN₂ + £ (m-l)H₂ + mNH₃ + H₂O In the above reaction, if the temperature of the aqueous solution containing the copper complex is less than 40 ^°C, a reduction reaction is not easily carried out and

, even then has a slow reaction rate. On the other hand, if the temperature exceeds 80 ^°C, the reduction reaction is increased but the copper powder is easily agglomerated due to a too high temperature.

If the amount of the added sodium hydroxide (NaOH) is less than 2 moles, the reduction reaction is not easily earned out. On the other hand, if it exceeds 33 moles, the reduction reaction is increased but a large amount of ions are left in the aqueous solution, thereby increasing impurities. Furthermore, excess NaOH is wasteful from an economical point of view. Then, the obtained nano-scaled copper powder is filtered and dried, to thereby finally give an ultrafine copper powder having a particle size of 100 nm grade.

Meanwhile, in the step of producing the copper complex (Cu(N₂H₄)_mCl_n), before the addition of hydrazine (N H₄), silver nitrate (AgNO₃) is added to the aqueous copper chloride (CuCl₂) solution in an amount of 1/1,000 to 1/10,000 moles per mole of the copper chloride in order to promote the reduction reaction rate of copper complex. This reaction is summarized as the following Scheme 7:

Scheme 7

CuCl₂ + NzHtexcess + yAgNO₃ (y=0.001 ~ 0.0001)

^ Cu(N₂H₄)_mCl_n + yAg⁺ + yNO₃ ^" + (2-n)Cf (n=l , 2) Subsequent to producing the copper complex (Cu(N₂H₄)_mCl_n) by the addition of a trace amount of silver nitrate and hydrazine, sodium hydroxide (NaOH) is added to separate a copper powder from the aqueous solution containing the copper complex (Cu(N₂H )_mCl_n). This reaction is summarized as the following Scheme 8: Scheme 8

Cu(N₂H₄) _mCl_n + yAg⁺(y=0.001 -0.0001) + yNO₃- + (2-n)CT (n=l, 2)

+ x NaOH (x -2 ~33)

CuO + mN₂H₄ + 2C1^" + χ Na⁺ + (χ -l-y)OH^" + yAgO

+ yNO₃ ^" + £ (l + y)H₂ Cu₍s) + Ag_(S) + 2CF + x Na⁺ + (x -l-y)OH^" + yNO₃ ^_ + * mN₂

+ £ (m-y-l)H₂ + mNH₃ + (l+y)H₂O The copper powder obtained is filtered and dried under a non-oxidizing atmosphere to thereby finally produce a nano-scaled copper powder.

The present invention will hereinafter be described more specifically by non- limiting preferred examples.

Example 1 According to a conventional wet method, first, a sodium hydroxide (NaOH) with varying concentrations was added to IOOM. of an aqueous copper sulfate (CuSO₄) solution to produce an aqueous solution containing copper oxide (Cu_xO) as a precipitate. Then, the copper oxide was filtered and recovered.

Distilled water and glucose (C₆Hι₂O₆) were added to the obtained copper oxide and agitated until the color of the solution changed into dark red. As a result, an aqueous solution containing a stable Cu₂O was obtained.

Then, glycine (NH₂-CH -COOH) and arabic gum were added to the aqueous solution containing Cu O and then dispersed uniformly.

Then, Cu O was reduced to a copper powder as a precipitate by mixing hydrazine (N H₄), as a reducing agent, into the dispersion and then dried.

The result of the conventional wet method is given in Table 1.

Table 1

Sample No. CuS0₄ NaOH N₂H₄ : Glycine Average particle size (μm)

11 1 3 : 3 : 0.1 0.4

12 1 3 : 4 : 0.25 0.5

13 1 : 2 : 7 : 0.30 1

- Numbers in respective components denote M ratios based on 2M CuS0₄

- Glucose and arabic gum as additives were added in an amount of 0.1M, respectively, based on 2M CuS0₄.

As shown in Table 1, the average particle size of each copper powder

(samples 11 to 13) prepared by the conventional wet method varied, depending on the amounts of added reaction agents and additives. Specifically, the particle size distribution ranged from about 0.4 to lμm.

Example 2 According to the present invention, first, lOOmβ of 2M aqueous CuCl₂ solution was heated to a temperature of 30 to 80 ^°C and vigorously agitated at that temperature.

Sodium hydroxide (NaOH) was at a time added to the aqueous copper chloride solution at the above temperature. Because the particle size of the final product, copper powder, depends on the concentration of the sodium hydroxide, the amount of the sodium hydroxide can be adjusted according to the desired particle size.

After the addition of sodium hydroxide (NaOH), hydrazine (N₂H₄) was added to the resulting aqueous solution at a temperature of 40 to 80 ^°C, to obtain a copper powder. In this case, hydrazine (N₂H ) was added at a time.

The copper powder obtained according to the above procedure was washed with secondary distilled water and filtered. The filtered copper powder was dried at an appropriate temperature under a non-oxidizing atmosphere to thereby finally obtain a nano-scaled copper powder.

That is, the conventional wet method for preparing a copper powder comprises various processes such as filtering, recovering and addition of distilled water. However, the wet reduction method according to the present invention is carried out in one reaction vessel and the process for recovering a copper powder is carried out only once.

The particle size distribution of the copper powder obtained according to the present invention is given in Table 2.

Table 2 Particle size distribution of copper powder according to the concentration of NaOH

As shown in Table 2, in case of samples 22 to 26, NaOH was added with varying concentrations of 2 to 33 moles per mole of the copper chloride (CuCl₂). It can be seen that when the concentration of hydrazine (N₂H₄) is constant, as the [NaOH]/[CuCl₂] ratio increases, the particle size distribution of the copper powder increases.

This is because the ratio of the obtained copper oxides, Cu₂O and CuO, varies according to the concentration of sodium hydroxide (NaOH).

As the amount of sodium hydroxide (NaOH) increases, a stable intermediate product, Cu O, is produced in a large amount. As a result, a reduction reaction is not easily carried out.

Due to differences in the degree of reduction, the particle size distribution of the obtained copper powder becomes less uniform as the concentration of NaOH increases.

In case of sample 21, in spite of good characteristics of the copper powder, reaction rate was slow and thus productivity was lowered. Sample 27 had a fast reaction rate but the average particle size distribution of the copper powder exceeded 0.5 m. Therefore, samples 21 and 27 are not preferable to prepare a nano-scaled copper powder.

In particular, if the amount of added hydrazine (N₂H₄) exceeds 12 moles per mole of the copper chloride, a reaction rate is increased but the copper powder is easily agglomerated. As a result, the surface quality of the copper powder is lowered. Therefore, it is preferable to limit the amount of hydrazine to up to 12 moles.

As shown in Table 3, copper powder having a particle size of 100 nm or less grade was easily obtained when the molar ratio between CuCl and NaOH was 1 :2. In addition, the physical properties of the copper powder, such as particle size distribution and particle shape, were excellent.

Table 3 shows chemical components of the copper powder obtained according to Example 2 under the condition of CuCl₂ : NaOH : N₂H = 1 :2:12.

Table 3

Example 3

First, lOOm. of 2M aqueous CuCl₂ solution was heated to a temperature of 20 to 70 ^°C and vigorously agitated at that temperature.

Hydrazine (N₂H ) was mixed into the resulting aqueous copper chloride solution at the above temperature in an amount of 1 to 12 moles per mole of the copper chloride and vigorously agitated for about 5 minutes.

Subsequently, when the aqueous solution containing hydrazine was kept at a temperature of 40 to 80 ^°C, sodium hydroxide (NaOH) was added thereto.

The sodium hydroxide (NaOH) was added with varying concentrations of 2 to 33 moles per mole of the copper chloride (CuCl₂).

Then, the copper powder obtained according to the above procedure was washed with secondary distilled water and filtered. The filtered copper powder was dried to obtain a nano-scaled copper powder.

The particle size distribution of the finally obtained copper powder is given in Table 4.

Table 4 Particle size distribution of copper powder according to the concentration of NaOH

33 1 12 8 0.20 0.31 0.40 0.31

34 1 12 16 0.21 0.51 0.75 0.51

- Numbers in N₂H₄ and NaOH denote molar ratio based on 2M CuCl₂

Table 4 shows the particle size distribution according to the amount of NaOH per mole of the copper chloride (CuCl₂). As shown in Table 4, as the amount of NaOH increased, the particle size of the obtained powder increased and a wide particle size distribution was obtained.

As shown in Table 4, when the amount of added NaOH was 2 to 4 moles per mole of CuCl₂, a copper powder having a particle size of 100 nm grade was obtained. Furthermore, dispersibility and shape of the powder surface were excellent.

Although the intermediate product of the present example was different than that of Example 2, the final result was almost similar. It can be seen from the forgoing that the method of Example 3 is suitable for preparing a copper powder having a particle size of 100 nm grade, similar to Example 2.

Example 4

According to Example 4, a trace amount of silver nitrate (AgNO₃) is further added to the aqueous copper chloride solution in Examples 2 and 3. Because silver is reduced faster than copper, the addition of silver nitrate enables the promotion of a heterogeneous nucleation of copper, thereby increasing the reduction rate of copper.

That is, a trace amount of silver nitrate was added to the aqueous copper chloride solution in Example 2. Then, sodium hydroxide and hydrazine were added in sequence to obtain a copper powder.

A trace amount of silver nitrate was added to the aqueous copper chloride solution in Example 3. Then, hydrazine and sodium hydroxide were added in sequence to obtain a copper powder.

The copper powder according to Example 4 was compared with those according to Examples 2 and 3 in terms of the particle size distribution of a copper powder. The results are given in Table 5.

Table 5 Particle size distribution of copper powder according to presence or absence of

AgNO₃

Table 5 shows a particle size distribution according to the presence or absence of the optional additive, AgNO₃ under the condition of CuCl : NaOH : N₂H₄ = 1 :2:12.

In sample 41, NaOH and N₂H₄ were in sequence added to an aqueous (CuCl +AgNO₃) solution, and in sample 42, N₂H₄ and NaOH were added in sequence to an aqueous (CuCl₂+AgNO₃) solution, for the purpose of obtaining a copper powder.

In the same conditions, the use of AgNO₃ resulted in small average particle size and small particle size deviation. Furthermore, AgNO₃ acted to increase the reaction rate. Samples 41 and 42 exhibited almost same characteristics of copper powders as corresponding sample 22 of Example 2 and sample 31 of Example 3.

As shown in Figs. 6 and 7, the copper powders prepared using AgNO₃ exhibited smaller average particle size and smaller particle size deviation than those in corresponding Examples 2 and 3.

The copper powders prepared according to the conventional wet method (samples 11 to 13) had relatively coarse average particle size of 0.4 to ljMii. On the other hand, the copper powders prepared according to the present invention had ultrafine average particle size of lOOnm grade ~ l m grade.

Industrial Applicability

As apparent from the above description, the present invention provides two mechanisms for preparing an ultrafine copper powder having a particle size of (lOOnm grade ~ l m grade). The two mechanisms are very suitable for preparing an ultrafine copper powder having a particle size of 100 nm grade. The nano-scaled copper powder prepared according to the present invention is excellent in particle size distribution and dispersibility. The particle size of the copper powder varies according to the amount of sodium hydroxide (NaOH), thereby making it possible to control the particle size and distribution thereof. At the same time, due to heterogeneous nucleation of copper by the addition of the optional additive, silver nitrate (AgNO₃), the reduction reaction rate of copper increases and the particle size of the copper powder becomes finer.

Accordingly, the method according to the present invention is a relatively simple process and also affords a high degree of reproducibility. In addition, copper powder having a particle size of 100 nm can be prepared which has a good surface quality, narrow particle size distribution, and good powder sphericity.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

Claims:

1. A method for manufacturing a nano-scaled copper powder by a wet reduction process, comprising the steps of: adding sodium hydroxide (NaOH) to an aqueous copper chloride (CuCl₂) solution to give an aqueous solution containing copper oxide and copper hydroxide; reducing the copper oxide and the copper hydroxide to obtain a nano-scaled copper powder as a precipitate by adding hydrazine (N₂H₄) to the aqueous solution; and filtering and drying the precipitated nano-scaled copper powder.

2. The method as set forth in claim 1, wherein the sodium hydroxide (NaOH) is added in an amount of 2 to 33 moles per mole of the copper chloride (CuCl ) when the aqueous copper chloride (CuCl₂) solution is kept within a temperature of 30 to 80 ^°C, and the hydrazine (N₂H₄) is added in an amount of 0.5 to 12 moles per mole of the copper chloride when the aqueous solution containing the copper oxide and the copper hydroxide is kept within a temperature of 40 to 80 ^°C .

3. The method as set forth in claim 1 or claim 2, wherein in the step of producing the copper oxide and the copper hydroxide, before the addition of NaOH, silver nitrate (AgNO₃) is added to the aqueous copper chloride (CuCl₂) solution in an amount of 1/1,000 to 1/10,000 moles per mole of the copper chloride.

4. A method for manufacturing a nano-scaled copper powder by a wet reduction process, comprising the steps of: adding hydrazine (N H₄) to an aqueous copper chloride (CuCl₂) solution to give an aqueous solution containing a copper complex (Cu(N H₄)_mCl_n); adding sodium hydroxide (NaOH) to the aqueous copper complex solution to obtain a nano-scaled copper powder; and filtering and drying the nano-scaled copper powder.

5. The method as set forth in claim 4, wherein the hydrazine (N₂H₄) is added in an amount of 0.5 to 12 moles per mole of the copper chloride (CuCl₂) when the aqueous copper chloride (CuCl₂) solution is kept within a temperature of 20 to 70 °C, and the sodium hydroxide (NaOH) is added in an amount of 2 to 33 moles per mole of the copper chloride when the aqueous solution containing the copper complex is kept within a temperature of 40 to 80°C .

6. The method as set forth in claim 4 or claim 5, wherein in the step of producing the copper complex (Cu(N₂H₄)_mCl_n), before the addition of hydrazine, silver nitrate (AgNO₃) is added to the aqueous copper chloride (CuCl₂) solution in an amount of 1/1,000 to 1/10,000 moles per mole of the copper chloride.