Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2000-04-27
2001-05-08
Jenkins, Daniel (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C075S374000
Reexamination Certificate
active
06228141
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to nickel fine powder comprising nickel and a trace amount of phosphorus dispersed in the nickel as well as a method for preparing the same and more particularly to nickel fine powder which comprises nickel and a trace amount of phosphorus dispersed in the nickel, whose particle size is uniform and which is excellent in resistance to heat shrinkage as well as a method for preparing the same.
(b) Description of the Prior Art
Heretofore, there have been proposed a variety of methods for the preparation of nickel fine powder. These methods can roughly divided into two categories, one of which is a dry method comprising the steps of using a nickel salt such as nickel chloride as a starting material, subjecting the nickel salt to vapor-phase reduction with hydrogen and then cooling the resulting nickel vapor to give nickel fine powder; and the other of which is a wet method comprising the steps of mixing an aqueous solution containing a nickel salt such as nickel chloride with an aqueous solution of an alkali hydroxide to form nickel hydroxide and then reducing the resulting nickel hydroxide to give nickel fine powder.
Moreover, regarding methods for controlling the particle size of nickel fine powder, there have been known, for instance, those comprising adjusting the nickel salt vapor concentration and/or the amount of hydrogen gas to be introduced during the reduction step for the dry preparation method; and those comprising controlling, for instance, the concentration of each reaction component, the reaction temperature and the kinds and concentrations of additives, for the wet preparation method. However, there can be prepared simply nickel fine powder whose average particle size falls within the range of from 0.2 to 0.5 &mgr;m, even if nickel fine powder having a small particle size is prepared according to either of the foregoing preparation methods.
It is difficult to prepare nickel fine powder having a average particle size smaller than that defined above according to the presently existing technology. In particular, there has never been proposed any technique for the steady preparation of nickel fine powder whose particle size is uniform and is less than 0.2 &mgr;m.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide nickel fine powder which comprises nickel and a trace amount of phosphorus dispersed in the nickel, whose particle size is uniform and is not more than 0.5 &mgr;m, in particular less than 0.2 &mgr;m and which is excellent in the resistance to heat shrinkage as well as a method for easily and steadily preparing such nickel fine powder.
The inventors of this invention have conducted various studies to achieve the foregoing object, have found that if nickel fine powder is prepared by reducing nickel hydroxide in the presence of a specific amount of phosphorus ions, the particle size of the resulting nickel fine powder is reduced in proportion to the amount of the coexisting phosphorous ions, that this would permit the quite easy control of the particle size and the steady preparation of nickel fine powder whose particle size is quite uniform, that the resulting nickel fine powder comprises nickel and a trace amount of phosphorus dispersed in the nickel and that the phosphorus-containing nickel fine powder is quite excellent in its resistance to heat shrinkage, and thus have completed the present invention on the basis of these findings.
According to an aspect of the present invention, there is provided nickel fine powder which comprises nickel and phosphorus dispersed in the nickel in an amount ranging from 0.01 to 2% by weight based on the weight of the nickel, and whose particle size is not more than 0.5 &mgr;m.
The nickel fine powder of the present invention is further characterized by the fact that it has a rate of linear heat shrinkage, as determined at 1000° C. of not higher than 8%.
The rate of linear heat shrinkage as determined at 1000° C. is herein defined by the following expression:
R
=
L
-
l
L
×
100
wherein,
R=rate of linear heat shrinkage as determined at 1000° C.
L=length of specimen determined at room temperature,
l=length of specimen determined at 1000° C.
According to another aspect of the present invention, there is provided a method for preparing nickel fine powder which comprises nickel and phosphorus dispersed in the nickel in an amount ranging from 0.01 to 2% by weight based on the weight of the nickel, and whose particle size is not more than 0.5 &mgr;m. The method comprises the step of reducing nickel hydroxide in the presence of phosphorus ions in such an amount that the molar ratio thereof to nickel ions ranges from 0.01 to 2 to give the nickel fine powder.
According to a further aspect of the present invention, there is also provided another method for preparing nickel fine powder which comprises nickel and phosphorus dispersed in the nickel in an amount ranging from 0.01 to 2% by weight based on the weight of the nickel, and whose particle size is not more than 0.5 &mgr;m. The method comprises the steps of mixing an aqueous solution of a nickel salt with an aqueous solution of an alkali hydroxide to form nickel hydroxide and reducing the resulting nickel hydroxide to give nickel fine powder, wherein a phosphorus ion source is added to the reaction system at any stage of the production method to thus reduce the nickel hydroxide in the presence of phosphorus ions in such an amount that the molar ratio thereof to nickel ions ranges from 0.01 to 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The nickel fine powder of the present invention comprises nickel and phosphorus dispersed in the nickel in an amount ranging from 0.01 to 2% by weight based on the weight of the nickel. The presence of phosphorus in the nickel ensures the improvement of the resistance to heat shrinkage of the powder. When nickel fine powder is heated and sintered in a nitrogen gas atmosphere, the conventional nickel fine powder free of any phosphorus shows not less than 10% of linear heat shrinkage as determined at 1000° C., while the phosphorus-containing nickel fine powder according to the present invention has a rate of linear heat shrinkage as determined at 1000° C. smaller than that observed for the conventional nickel fine powder, although the phosphorus-containing nickel fine powder is finer than the conventional nickel fine powder. For instance, the nickel fine powder of the present invention has a rate of linear heat shrinkage, as determined at 1000° C., of not more than 8% even if the particle size thereof is 0.01 &mgr;m. In other words, the phosphorus-containing nickel fine powder of the invention has such characteristic properties that the linear heat shrinkage is very low as compared with the conventional nickel fine powder free of any phosphorus.
The advantage due to the foregoing characteristic properties would be exhibited in particular when the nickel fine powder of the invention is used, for instance, as a material for internal electrodes of laminated ceramic condensers. Such a laminated ceramic condenser is manufactured by alternatively putting ceramic dielectric material and internal electrodes in layers, then attaching them together under press and unifying them through firing. When using the conventional nickel fine powder free of any phosphorus, nickel undergoes shrinkage at a low temperature during the firing step because of the difference in the rate of linear heat shrinkage between a ceramic dielectric material (e.g., BaTiO
3
) and the nickel electrode. Contrary to this, the linear heat shrinkage is inhibited due to the presence of phosphorus dispersed in the nickel in case of the phosphorus-containing nickel fine powder of the invention and this accordingly results in the reduction of the difference in the rate of linear heat shrinkage between the ceramic dielectric material and the nickel electrode, although the nickel fine powder of the invention is finer than the conventional one. The nickel
Araki Takayuki
Hayashi Takao
Toshima Yoshiharu
Yamaguchi Yasuhide
Jenkins Daniel
Mitsui Mining & Smelting Co. Ltd.
Young & Thompson
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