Method for producing copper powder

A method for stably producing a copper powder having a low degree of oxidation comprises generating copper hydroxide by adding an alkali to an aqueous solution containing copper ions, and reducing the copper hydroxide to obtain copper powder as a precipitate by adding hydrazine or a hydrazine compound to the aqueous solution until the aqueous solution yield a pH value in a range of from about 7 to 9.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for producing a copper powder, 
and more particularly to a method for producing a copper powder of a low 
oxidation state, which is useful as an electrically conductive paste for 
forming thick films. 
2. Prior Art 
Instead of using a noble metal such as gold, silver or palladium, copper 
powder is being employed more frequently these days in an electrically 
conductive paste material for forming thick films. This is because copper 
possesses excellent properties such as high conductivity and resistance 
against migration, and, in addition, it is available at a relatively low 
cost. 
Copper powder of the type for use as a thick-film conductive paste material 
has been produced by reducing a copper salt sparingly soluble in water, 
such as copper carbonate, using a reducing agent. However, this method 
results in a copper powder composed of coarse particles having a primary 
particle diameter of 1 .mu.m or larger, and the particles are often found 
agglomerated. 
Recently, copper powder produced from a water-soluble salt such as copper 
sulfate has become available. The water-soluble salt is converted into a 
hydroxide and then reduced to obtain the copper powder. The resulting 
copper powder consists of particles having a primary diameter of 1 .mu.m 
or smaller, and less agglomeration occurs with this powder. As a result, a 
dense conductive film can be obtained by using the copper powder as the 
conductive material. Moreover, when compared to a copper powder obtained 
by the conventional method using copper carbonate, the copper powder using 
a water-soluble copper salt provides, for instance, a capacitor electrode 
which is improved not only in electrical characteristics such as static 
capacitance, dielectric loss or insulation resistance, but also in 
mechanical properties. 
However, according to the above-explained conventional method, copper 
peroxide (CuO.sub.2) tends to be generated during the production of copper 
hydroxide, and cuprous oxide (Cu.sub.2 O) tends to form during the 
reduction of copper hydroxide to copper powder. This signifies that a 
copper powder having a high oxidation state may result from the process. 
When a copper powder of a high oxidation state is used as a conductive 
material, soldering failure can occur. Furthermore, capacitors with 
impaired electric properties are obtained when such a copper powder is 
utilized for the electrodes thereof, resulting in a low static 
capacitance, a high dielectric loss, etc. 
SUMMARY OF THE INVENTION 
An objective of the present invention is to provide a method for stably 
producing copper powder having a low oxidation state. Such a method for 
producing a copper powder comprises the steps of generating copper 
hydroxide by adding an alkali to an aqueous solution containing copper 
ions and reducing the copper hydroxide to obtain the copper powder as a 
precipitate by adding hydrazine or a hydrazine compound to the aqueous 
solution until the aqueous solution has a pH value in a range of from 
about 7 to 9. According to the present invention, a copper powder having 
low degree of oxidation and containing less residual copper oxides can be 
obtained by controlling the final pH to a range of from 7 to 9 after 
adding hydrazine or a compound thereof as a reducing agent. 
By forming a conductor by coating and firing a conductive paste prepared 
from the copper powder according to the present invention, a thick film 
copper conductor having excellent solderability can be obtained.

DETAILED DESCRIPTION OF THE INVENTION 
The method for producing a copper powder of the present invention is 
characterized in that it comprises generating copper hydroxide by adding 
an alkali to an aqueous solution containing copper ions, and reducing the 
copper hydroxide to obtain copper powder as a precipitate by adding 
hydrazine or a hydrazine compound to the aqueous solution until the 
aqueous solution has a pH value in a range of from about 7 to 9. 
The method of the present invention is also characterized in that hydrazine 
or the hydrazine compound is preferably added at a quantity corresponding 
to about 1 to 3 times the theoretical value necessary for reducing copper 
hydroxide to copper powder. 
The reason why the pH value during the reduction is limited to a range of 
from 7 to 9 is explained below. 
In general, a reductive reaction of copper hydroxide Cu(OH).sub.2 ! 
proceeds by first forming an intermediate product, cuprous oxide (Cu.sub.2 
O), and then forming the final product, copper (Cu), in a manner expressed 
below: 
EQU Cu(OH).sub.2 .fwdarw.Cu.sub.2 O.fwdarw.Cu 
Any intermediate product, i.e., cuprous oxide, which remains to the final 
stage increases the oxidation state of the copper powder. Thus, it can be 
seen that it is required to completely eliminate cuprous oxide by 
thoroughly reducing the starting copper oxide to copper powder. 
On the other hand, it is known that hydrazine (N.sub.2 H.sub.4) or a 
hydrazine compound, which is a reducing agent, decompose based on the 
following equation and exhibits reductive action: 
EQU 2N.sub.2 H.sub.4 .fwdarw.2NH.sub.3 +N.sub.2 +H.sub.2 
Hydrazine and hydrazine compounds are stable at a lower pH range, but 
decomposes more vigorously with increasing pH value. 
It can be seen from the foregoing that the pH value during the reductive 
reaction is adjusted to as high as possible, but not in excess of about 9, 
to prevent cuprous oxide from remaining after the reduction of copper 
hydroxide to copper powder. A preferred pH range is from about 7 to 9. It 
is more preferable to set a pH about 8. A pH value lower than 7 or higher 
than 9 is not preferred. If the pH value should be set lower than 7, the 
quantity of the intermediate product, i.e., cuprous oxide, which remains 
after the reduction reaction, increases. If the pH value is set at a value 
exceeding 9, it is more likely that copper hydroxide is oxidized into 
copper peroxide (CuO.sub.2). Eventually, non-reduced copper peroxide and 
half-reduced cuprous oxide are found to remain in large quantity because 
copper peroxide is extremely resistant to reduction. 
In an embodiment according to the present invention, the hydrazine or 
hydrazine compound is added in a quantity corresponding to about 1 to 3 
times the theoretical (stoichiometric) value necessary for reducing copper 
hydroxide to copper. If hydrazine or a compound thereof is added at a 
quantity less than the stoichiometric value for the reduction of copper 
hydroxide into copper, the reduction reaction may be incomplete, and 
non-reacted cuprous oxide would remain. If the amount of addition should 
exceed three times the stoichiometric value, the excess hydrazine or a 
compound thereof would be consumed and this not economic. 
Thus, as described in the foregoing, the method for producing copper powder 
according to the present invention comprises adding hydrazine or a 
hydrazine compound to an aqueous solution containing copper hydroxide 
until the pH value of the solution is adjusted in a range of from about 7 
to 9 to thereby effect a reducing reaction. In this manner, copper powder 
having a low degree of oxidation is obtained by preventing copper oxide 
such as cuprous oxide from remaining in the powder. 
Furthermore, the reduction reaction can be completed economically and 
efficiently by controlling the addition of the reducing agent, i.e., 
hydrazine or a compound thereof, to fall in a quantity corresponding to 
about 1 to 3 times the theoretical value necessary for reducing copper 
hydroxide to copper powder. Preferably the amount of reducing agent is 
greater than stoichiometric. 
The present invention is described in further detail below by making 
reference to some non-limiting examples. 
EXAMPLE 1 
An aqueous copper sulfate solution was obtained by dissolving 150 g of 
copper sulfate into 500 ml of pure water at 60.degree. C., and an aqueous 
alkaline solution prepared separately by dissolving sodium hydroxide into 
1,000 ml of pure water at a quantity equimolar to Cu.sup.2+ was added in 
the aqueous copper sulfate solution until the pH value of the solution 
became 4. A total of 200 ml of aqueous alkaline solution were added. Thus 
was obtained copper hydroxide. Then, hydrazine hydrate was added to the 
resulting aqueous solution containing copper hydroxide until the pH value 
of the solution became 9. Copper powder was obtained through the reduction 
of copper hydroxide in this manner by adding 200 ml of hydrazine hydrate 
to the solution. Subsequently, the copper powder was filtered off, and was 
dried after exchanging the adsorbed water by methanol. 
The copper powder thus obtained was subjected to X-ray diffraction analysis 
to calculate Cu.sub.2 O/Cu ratio by weight from the intensity ratio of the 
peaks observed in the diffractogram. The result is shown in Table 1. 
A conductive paste was prepared from the copper powder. The resulting paste 
was applied to a ceramic substrate and was fired to evaluate the 
solderability of the conductor. 
More specifically, a conductive paste was prepared by kneading 80% by 
weight of the copper powder, 5% by weight of a Pb--B--Zn based glass 
powder, and 15% by weight of an organic vehicle obtained by dissolving an 
acrylic resin into .alpha.-terpineol solvent. The conductive paste thus 
obtained was applied to an alumina substrate by screen printing, and was 
fired at 600.degree. C. under an atmosphere containing 1,000 ppm or less 
of oxygen. 
Subsequently, a flux obtained by dissolving rosin into methanol was applied 
to the conductive portion, and the resulting structure was immersed into a 
Sn--Pb eutectic solder melt at 230.degree. C. for a duration of 5 seconds. 
The solderability was evaluated by the wet area ratio of the conductor 
immersed into the solder melt. The result is given in Table 1. 
EXAMPLE 2 
An aqueous solution was obtained by dissolving 1,000 g of copper chloride 
into 3,000 ml of pure water at 70.degree. C., and an aqueous alkaline 
solution prepared separately by dissolving sodium hydroxide into 1,000 ml 
of pure water at a quantity equimolar to Cu.sup.2+ was added in the 
aqueous solution until the pH value of the solution became 3.5. A total of 
900 ml of aqueous alkaline solution were added. Thus was obtained copper 
hydroxide. Then, hydrazine hydrate was added to the resulting aqueous 
solution containing copper hydroxide until the pH value of the solution 
became 7. In this manner, copper powder was obtained through the reduction 
of copper hydroxide by adding 900 ml of hydrazine hydrate to the solution. 
Subsequently, copper powder was filtered off, and was dried after 
exchanging the adsorbed water by methanol. 
The copper powder thus obtained was subjected to X-ray diffraction analysis 
to calculate Cu.sub.2 O/Cu ratio by weight from the intensity ratio of the 
peaks observed in the diffractogram. The result is shown in Table 1. 
A conductive paste was prepared in the same manner as in Example 1 except 
for using the copper powder obtained above, and the resulting conductive 
paste was applied and fired for the evaluation of the solderability of the 
resulting conductor. The result is given in Table 1. 
COMATIVE EXAMPLE 1 
An aqueous solution was obtained by dissolving 400 g of copper sulfate into 
1,000 ml of pure water at 60.degree. C., and an aqueous alkaline solution 
prepared separately by dissolving sodium hydroxide into 1,000 ml of pure 
water at a quantity equimolar to Cu.sup.2+ was added in the aqueous 
solution until the pH value of the solution became 4. A total of 500 ml of 
aqueous alkaline solution were added. Thus was obtained copper hydroxide. 
Then, hydrazine hydrate was added to the resulting aqueous solution 
containing copper hydroxide until the pH value of the solution became 6. A 
copper powder was obtained in this manner through the reduction of copper 
hydroxide. The total amount of hydrazine hydrate added into the solution 
was 300 ml. Subsequently, the copper powder was filtered off, and was 
dried after exchanging the adsorbed water by methanol. 
The copper powder thus obtained was subjected to X-ray diffraction analysis 
to calculate Cu.sub.2 O/Cu ratio by weight from the intensity ratio of the 
peaks observed in the diffractogram. The result is shown in Table 1. 
A conductive paste was prepared in the same manner as in Example 1 except 
for using the copper powder obtained above, and the resulting conductive 
paste was applied and fired for the evaluation of the solderability of the 
resulting conductor. The result is given in Table 1. 
COMATIVE EXAMPLE 2 
An aqueous solution was obtained by dissolving 1,000 g of copper sulfate 
into 2,000 ml of pure water at 65.degree. C., and an aqueous alkaline 
solution prepared separately by dissolving sodium hydroxide into 2,000 ml 
of pure water at a quantity twice the molar content of Cu.sup.2+ was 
added in the aqueous solution until the pH value of the solution became 4. 
A total of 1,100 ml of aqueous alkaline solution were added. Thus was 
obtained copper hydroxide. Then, hydrazine hydrate was added into the 
resulting aqueous solution containing copper hydroxide until the pH value 
of the solution became 10. A copper powder was obtained in this manner 
through the reduction of copper hydroxide. The total amount of hydrazine 
hydrate added into the solution was 1,000 ml. Subsequently, the copper 
powder was filtered off, and was dried after exchanging the adsorbed water 
by methanol. 
The copper powder thus obtained was subjected to X-ray diffraction analysis 
to calculate Cu.sub.2 O/Cu ratio by weight from the intensity ratio of the 
peaks observed in the diffractogram. The result is shown in Table 1. 
A conductive paste was prepared in the same manner as in Example 1 except 
for using the copper powder obtained above, and the resulting conductive 
paste was applied and fired for the evaluation of the solderability of the 
resulting conductor. The result is given in Table 1. 
TABLE 1 
______________________________________ 
Final pH in Solder- 
reductive Cu.sub.2 O/Cu 
wetted 
reaction (wt ratio) 
area ratio (%) 
______________________________________ 
Ex 1 9 0.019 .gtoreq.80 
Ex 2 7 0.017 .gtoreq.80 
Comp. Ex. 1 
6 0.049 .ltoreq.50 
Comp. Ex. 2 
10 0.033 .ltoreq.50 
______________________________________ 
From the results shown in Table 1, it can be seen that the copper powders 
obtained by the method according to the present invention yield a low 
degree of oxidation with a Cu.sub.2 O/Cu ratio (by weight) in a low range 
of from 0.017 to 0,019. Furthermore, as is evaluated by the solder-wetted 
area ratio of 80% or higher, it can be seen that the solderability of the 
conductor using the copper powder according to the present invention is 
favorable. 
In contrast to the results above for the copper powder of the present 
invention, the powder obtained with a final pH value of lower than 7 at 
the completion of the reduction reaction (Comparative Example 1) yields a 
Cu.sub.2 O/Cu ratio as high as 0.049, and the conductor which results 
therefrom by coating and baking yields an unfavorable solder-wetted area 
ratio of 50% or lower. Furthermore, the powder obtained with a final pH 
value exceeding 9 at the completion of the reduction reaction (Comparative 
Example 2) also yields a Cu.sub.2 O/Cu ratio as high as 0.033, and the 
conductor which results therefrom by coating and baking yields an 
unfavorable solder-wetted area ratio of 50% or lower. 
Although the aqueous solution containing copper ions is obtained by 
dissolving copper sulfate or copper chloride into pure water in the 
Examples above, the present invention is not only limited thereto. Any 
water-soluble copper compound such as copper nitrate can be used in the 
place of copper sulfate or copper chloride. 
Furthermore, in the production of copper hydroxide, any other alkali such 
as ammonia, potassium hydroxide, etc., can be used in the place of sodium 
hydroxide. 
The reducing agent employed above is hydrazine hydrate. However, various 
other hydrazine compounds such as hydrazinium chloride or hydrazinium 
sulfate can be used as well. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.