JP2015158001A - Nickel powder production process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a nickel powder containing no coarse particles, and production process thereof, which is suitable for an internal electrode of thin layered multilayer ceramic capacitor; and specifically a process for producing a nickel powder in which the average particle size is 0.1 to 0.3 μm and in which the ratio of coarse particles having a particle size of 1.0 μm or more is 50 ppm or less of the total number of particles and in which the oxygen content is reduced.SOLUTION: The nickel powder production process is characterized in comprising: a step for forming a water slurry in which the raw material nickel powder is at high dispersion state in water, and wet classification treating the water slurry to form a nickel slurry; and a step for drying the nickel slurry obtained from the wet classification treatment step to form a nickel powder, and heat the nickel powder under a reduction atmosphere.

Description

  The present invention relates to a method for producing nickel powder that can be suitably used as an internal electrode of a multilayer ceramic capacitor (MLCC).

  Conventionally, nickel powder has been used as a material for a conductive paste for producing a thick film conductor. This thick film conductor is used for the formation of electric circuits, the electrodes of multilayer ceramic components such as multilayer ceramic capacitors and multilayer ceramic substrates, etc. Especially in multilayer ceramic capacitors, the increase in the number of layers is progressing due to the demand for smaller size and higher capacity. The amount of conductive paste used for that purpose has also increased significantly. For this reason, as the metal powder used for the conductive paste, the use of expensive noble metals is avoided, and inexpensive base metals such as nickel are mainly used.

This multilayer ceramic capacitor is manufactured, for example, by the following method.
First, a conductive paste obtained by kneading a nickel powder, a resin such as ethyl cellulose, and an organic solvent such as terpineol is screen-printed on a dielectric green sheet to produce an internal electrode. Next, dielectric green sheets are laminated and pressure bonded so that the printed internal electrodes are alternately overlapped. Thereafter, the laminate is cut into a predetermined size, subjected to a binder removal treatment for removing the resin such as ethyl cellulose used as an organic binder, and then fired at a high temperature to 1300 ° C. to obtain a ceramic body. And an external electrode is attached to this ceramic body, and a multilayer ceramic capacitor is malignant.

  By the way, as described above, the base powder such as nickel is more prevalent than the precious metal in the metal powder in the conductive paste to be the internal electrode. Therefore, the nickel powder or the like is not oxidized in the binder removal treatment of the laminate. Thus, it is performed in an atmosphere in which the oxygen content is extremely small.

  Further, in recent years, in multilayer ceramic capacitors that have been required to be downsized and increased in capacity, in order to achieve the reduction in size and capacity, both internal electrodes and dielectrics constituting the multilayer ceramic capacitor have been made thinner. It has been. In particular, the particle diameter of nickel powder used for the internal electrode is mainly 0.5 μm or less.

  The nickel powder used for the internal electrode is required to have various characteristics, but it is important that one of them does not contain coarse particles. The reason is that if coarse particles are included, the coarse particles protrude from the internal electrode layer, and contact with another internal electrode layer causes a short circuit.

  Furthermore, nickel powder containing a large amount of oxygen, that is, if the oxygen content is high, the volumetric shrinkage of the nickel powder increases during firing in a reducing atmosphere, and the continuity of the electrode cannot be maintained. In order to generate cracks and delamination in the capacitor due to the occurrence, resulting in a decrease in the capacity of the capacitor, a low oxygen content is required.

In the nickel powder that requires such characteristics, first, as a method for removing coarse particles, the nickel powder is slurried using water as a medium, and the slurry is classified with a hydrocyclone, whereby 1 μm of coarse particles of 2 μm or more are obtained Has been proposed (Patent Document 1).
This method is effective for removing coarse particles of 2 μm or more, but is insufficient for removing coarse particles of the order of 1 μm to 2 μm. Further, since the treatment is performed in water, there is a problem in that the amount of hydroxide and the like on the nickel surface increases, and as a result, the oxygen content of nickel increases.

  Therefore, as a method for removing hydroxide or carbonate on the nickel surface, a method of forming a fatty acid metal salt on the surface of each particle of nickel powder under specific conditions and then heat-treating has been proposed (Patent Document 2). However, this method is effective in suppressing the progress of sintering between particles during heat treatment and suppressing the increase in coarse particles, but does not mention the coarse particles originally present.

JP 2001-62332 A JP 2003-129105 A

The present invention has been made in view of such problems, and it is an object of the present invention to provide a suitable nickel powder and a method for manufacturing the same in order to produce a thin-layered multilayer ceramic capacitor internal electrode. .
Specifically, production of nickel powder having an average particle size of 0.1 to 0.3 μm, the number of coarse particles of 1.0 μm or more being 50 ppm or less of the total number of particles, and a reduced oxygen content It aims to provide a method.

  The inventors of the present invention have made it possible to form an internal electrode of a thin-film multilayer ceramic capacitor in which nickel powder forms a highly dispersed state in a water slurry, is subjected to wet classification from that state, and then is heated through a reducing atmosphere. The present inventors have found that a suitable nickel powder that forms can be obtained, and have completed the present invention.

  That is, the first invention of the present invention is obtained by forming a water slurry in which the raw material nickel powder is highly dispersed in water, subjecting the water slurry to a wet classification process to form a nickel slurry, and a wet classification process. The nickel slurry is dried to form nickel powder, and the nickel powder is heated in a reducing atmosphere.

  According to a second aspect of the present invention, there is provided a nickel powder manufacturing method, wherein the raw material nickel powder in the first aspect is nickel powder produced by a wet reduction method.

  The third invention of the present invention uses a wet reduction method in which the raw material nickel powder in the second invention crystallizes nickel powder from an aqueous nickel salt solution, a reducing agent, and an alkaline colloidal solution containing palladium and silver. A nickel powder production method, characterized in that the nickel powder is produced.

  According to a fourth aspect of the present invention, there is provided a nickel powder manufacturing method characterized in that the reducing atmosphere in the first to third aspects is an inert gas atmosphere containing 1 to 50% by volume of hydrogen.

  According to a fifth aspect of the present invention, there is provided the nickel powder manufacturing method, wherein the heating temperature in the step of heating the nickel powder in the reducing atmosphere according to the first to fourth aspects is 150 to 350 ° C. .

  In a sixth aspect of the present invention, the amount of coarse particles having a particle diameter of 1.0 μm or more in the nickel powder produced by the step of heating the nickel powder under the reducing atmosphere in the first to fifth aspects is The nickel powder production method is characterized in that the amount of particles is 50 ppm or less.

  A seventh invention of the present invention is the nickel powder manufacturing method according to the sixth invention, wherein the oxygen content of the nickel powder is 1.4% by mass or less.

  An eighth invention of the present invention is the nickel powder manufacturing method according to the sixth or seventh invention, wherein the average particle size is 0.1 to 0.3 μm.

According to the method for producing nickel powder according to the present invention, the number of coarse particles having an average particle size of 0.1 to 0.3 μm, 1.0 μm or more is 50 ppm or less of the total number of particles, and the oxygen content is 1.4. Nickel particles having a mass% or less can be produced.
Therefore, if it is used for the internal electrode of a thin-film multilayer ceramic capacitor, the capacity of the capacitor is not reduced due to coarse particles or oxygen content, and an industrially significant effect is achieved.

  In the production method of the present invention, the nickel powder is made into a water slurry to obtain a highly dispersed state, and then wet classification is performed, and the classified water slurry is dried and then heated in a reducing atmosphere.

  As a processing step for removing coarse particles other than the present invention, a method of obtaining a nickel powder by once dry-classifying the obtained nickel powder can be considered. However, in the method of obtaining the nickel powder by once dry-classifying the obtained nickel powder, the coarse particles are often not removed to a sufficient level, and the reliability of removing the coarse particles is low.

<Manufacturing method of nickel powder>
Hereinafter, the present invention will be described in detail.
In the present invention, nickel powder obtained by various methods such as a wet reduction method, a gas phase method such as a CVD method or a plasma method, or a spray pyrolysis method can be used as a raw material nickel powder. Since it is made into a slurry, it is suitable to apply to the nickel powder obtained by the wet reduction method which can be easily made into a water slurry. The nickel powder obtained by the wet reduction method will be described below as a specific example of the raw material nickel powder, but is not limited to this.

The step of obtaining the nickel powder by the wet reduction method may use a known method, and the nickel powder is crystallized by adding a reducing agent, an aqueous nickel salt solution and, if necessary, a complexing agent or a dispersing agent.
In the present invention, it is desirable to further add an alkaline colloidal solution containing palladium and silver described in Japanese Patent No. 4957172 in order to obtain raw material nickel powder having an average particle size of 0.1 to 0.3 μm.

In the production method of the present invention, first, the nickel powder obtained by the above method is used as the raw material nickel powder, and after the raw nickel powder is made into a water slurry and highly dispersed, a wet classification step is performed.
In this step, the nickel powder (raw material nickel powder) obtained by the wet reduction method and the reaction solution are solid-liquid separated by a known method, pure water is added to the solid phase component, and the nickel powder is removed from the pure water. It is desirable to form a water slurry that has been highly dispersed in and then classify the water slurry with a wet classifier. In the case of nickel powder obtained by a method other than the wet reduction method, it is directly purified without solid-liquid separation. Water may be added to form a water slurry. In addition, washing may be added to remove the impurity component before forming the water slurry.

As a high dispersion method, a wet counter jet mill, a wet pulverizer that grinds the surface of the powder particles in the slurry by rotating a stirring blade having the same diameter as the inner wall of the stirring vessel at high speed, for example, an optimizer (Sugino) When processed using Machine Co., Ltd., TK Fillmix (made by Tokushu Kika Kogyo Co., Ltd.) or the like, a dispersion state that can be classified in a wet-classification process in a subsequent process can be easily obtained. In addition, as another means of the wet pulverizer, an apparatus for accelerating the slurry pressurized by a pump through a narrowed flow path and colliding with a hard diamond plate, for example, a microfluidizer (Mizuho Industry Co., Ltd.), Nanomizer (Yoshida Machine Industry Co., Ltd.), etc. can be used.
For example, when an optimizer (manufactured by Sugino Machine Co., Ltd.) is used, a highly dispersed slurry can be obtained under the condition that a water slurry having a solid content concentration of 30% by weight is pressurized at 2000 atm and collided 15 times. Moreover, dispersibility can be further improved by repeating this operation a plurality of times.

  For the wet classification, an apparatus using a cyclone system or a centrifugal system, for example, a hydrocyclone (manufactured by Nippon Chemical Machinery Co., Ltd.), LCSS (manufactured by CMS Co., Ltd.), nano cut / micro cut (manufactured by Krettek) or the like is used. be able to.

Next, the nickel powder produced by drying the solid phase component obtained by solid-liquid separation of the classified water slurry is heated in a reducing atmosphere.
In this step, excess oxides and hydroxides on the surface of the nickel powder formed from the formation of the highly dispersed water slurry to the wet classification are removed.

The drying after the wet classification may be performed using a known method, and the heating conditions of the nickel powder obtained after the drying in a reducing atmosphere are as follows: the amount of hydrogen gas in which the hydrogen concentration is 1 to 50% by volume as the reducing atmosphere to be used It is desirable to use a mixed gas consisting of an inert gas and supply it to the heating furnace.
When the hydrogen gas concentration is less than 1% by volume, removal of nickel surface oxide, surface hydroxide, etc. does not proceed sufficiently, and the effect does not appear clearly. On the other hand, even if the hydrogen gas concentration is higher than 50% by volume, the effect is not changed.
The inert gas is not particularly limited, and nitrogen gas, argon gas, or the like can be used.

Furthermore, the heating temperature is desirably 150 to 350 ° C. When the heating temperature is less than 150 ° C., the removal of hydroxide and the like on the surface of the nickel powder does not proceed sufficiently, and the effect does not appear clearly. When the heating temperature is higher than 350 ° C., coarse particles are generated due to necking and sintering of nickel powders, which is not desirable.
The furnace used for heating is not particularly limited as long as it can be used in a reducing atmosphere, and a batch furnace, a roller hearth furnace, a pusher furnace, or the like can be used.

<Characteristics of nickel powder according to the present invention>
[Number of coarse particles]
A feature of the nickel powder according to the present invention is that the number of coarse particles having a particle size of 1.0 μm or more is 50 ppm or less of the total number of particles. When the number of coarse particles exceeds 50 ppm, when used for the internal electrode of a laminated ceramic capacitor having a thin layer, coarse particles may protrude from the internal electrode layer, and may contact and short-circuit with another internal electrode layer. The particle diameter is a diameter measured from a scanning electron microscope (SEM) image.

[Average particle size]
Further, the nickel powder preferably has an average particle size of 0.1 to 0.3 μm.
Nickel powder having an average particle size of less than 0.1 μm is liable to form secondary particles due to aggregation, and the classification efficiency is lowered. On the other hand, when the average particle size exceeds 0.3 μm, the number of nickel powders contained in each layer of the internal electrode is reduced and the continuity of the electrode layer is maintained when used for the internal electrode of the thin-layered multilayer ceramic capacitor. May disappear.

[Oxygen content]
The oxygen content of the nickel powder is preferably 1.4% by mass or less. If the oxygen content exceeds 1.4% by mass, the volumetric shrinkage of the nickel powder increases during firing of the multilayer ceramic capacitor in a reducing atmosphere, and the continuity of the electrode cannot be maintained, and the gas due to oxide reduction This is because generation of cracks and delamination in the capacitor causes a decrease in the capacitance of the capacitor.

  Hereinafter, the present invention will be described in detail using examples, but the present invention is not limited to these examples. The nickel powder was evaluated as follows.

[Average particle size]
A scanning electron microscope (SEM, JSM-5510, manufactured by JEOL Ltd.) was used to obtain a photograph of an SEM image (field of view: vertical 9.6 μm × horizontal 12.8 μm) at a magnification of 10,000. This SEM image is measured using image analysis software (Mac-View, manufactured by Mountec Co., Ltd.) to measure the area and number of particles in which the entire shape of the particle shape can be seen, and from these, the diameter of each particle is obtained and averaged. Calculated.

[Number of coarse particles]
Using a scanning electron microscope, 20 views of a SEM image (field of view: length 19.2 μm × width 25.6 μm) at a magnification of 5000 times are obtained. This SEM image of 20 fields of view is measured using the image analysis software to measure the area and number of particles in which the entire shape of the particles can be seen, and from these, the diameter of each particle is obtained. Were counted as coarse particles.

[Oxygen content of nickel powder]
The oxygen content of nickel powder was measured with an analyzer (manufactured by LECO, TC436AR).

[Production of raw material nickel powder]
Below, the detail of raw material nickel powder preparation using the wet reduction method is shown.
An alkaline hydrazine solution was mixed with an alkaline colloidal solution composed of palladium, a small amount of silver and gelatin to prepare an alkaline colloidal solution for reducing nickel.
The content of palladium, silver, and gelatin in the prepared alkaline colloidal solution is palladium: 10 mass ppm, silver: 0.1 mass ppm, gelatin: 1 with respect to the total mass of nickel in the nickel salt aqueous solution as the starting solution. It was set as mass%. The contents of palladium and silver in the solution were analyzed by ICP emission spectroscopic analysis.

The production of the alkaline colloid solution for reducing the nickel was specifically performed as follows.
First, after a predetermined amount of gelatin was dissolved in 300 L of pure water, hydrazine was mixed so that the concentration of hydrazine was 0.02 g / L to prepare a solution containing gelatin and hydrazine.
Next, a 10 L mixed solution of pure water, a predetermined amount of palladium salt and silver salt was prepared, and dropped into the previously prepared solution containing gelatin and hydrazine to obtain a colloidal solution.

  A sodium hydroxide aqueous solution is added to this colloidal solution to adjust the pH to 10 or more, and hydrazine is further added until the nickel weight: hydrazine weight becomes 1: 3.75, so that the composite composed of palladium and a small amount of silver is added. An alkaline hydrazine solution mixed with colloidal particles was prepared and used as an alkaline colloid solution for reducing nickel. At this time, pure water was further added so that the total solution amount was 400 L.

  And 25 L of nickel chloride aqueous solution whose nickel concentration is 100 g / L was dripped at this alkaline colloid solution as nickel salt aqueous solution, nickel was reduced, and raw material nickel powder was obtained.

The process of making the obtained raw material nickel powder highly dispersed in water as a water slurry and performing wet classification is as follows.
The obtained nickel powder was separated from the solution after the wet reduction reaction, 6 L pure water was added to the nickel powder cake, and suspended with a blade stirring type stirrer. Next, the process of opposing collision at 2000 atmospheric pressure by an optimizer (manufactured by Sugino Machine Co., Ltd.), the obtained water slurry is allowed to stand, and the supernatant liquid is removed by decantation is repeated 10 times to obtain a nickel powder dispersed water slurry. It was.

  Next, the operating pressure of hydrocyclone (NHC-1 type (manufactured by Nippon Chemical Machinery Co., Ltd.)) was adjusted to 1.2 MPa, the treatment volume was 200 L / hr with pure fluid, and nickel powder dispersed water slurry Processed. Thereafter, solid-liquid separation and vacuum drying were performed to obtain nickel powder.

  In the step of heating the dried nickel powder in a reducing atmosphere, the obtained nickel powder is treated in a hydrogen-nitrogen mixed gas atmosphere having a hydrogen concentration of 1.4% by volume at a heating temperature of 180 ° C. and a heating time of 60 minutes. went.

Table 1 shows the results of evaluating the average particle size, the number of coarse particles, and the oxygen content of the obtained sample.
The average particle diameter of the obtained sample was 0.18 μm, and the number of coarse particles of 1.0 μm or more was 31 ppm of the total number of particles. The oxygen content of the nickel powder was 1.2% by mass.

(Comparative Example 1)
Nickel under the same conditions as in Example 1 except that the raw nickel powder obtained by the wet reduction method in Example 1 was placed in a highly dispersed state in water and the wet classification step and the heating step in a reducing atmosphere were replaced. A powder was prepared.
In other words, nickel powder is generated, solid-liquid separated, vacuum dried, heated in a hydrogen-containing gas atmosphere, made highly dispersed in water, wet-classified, and solid-phase components vacuum dried after solid-liquid separation. It was. Table 1 shows the results of evaluating the average particle size, the number of coarse particles, and the oxygen content of this sample.

(Comparative Example 2)
A nickel powder was produced under the same conditions as in Example 1, except that the step of wet-classifying the nickel powder obtained by the wet reduction method in Example 1 into a highly dispersed state in water was omitted.
That is, a nickel powder was produced, solid-liquid separated, and vacuum dried, and heated in a hydrogen-containing gas atmosphere. Table 1 shows the results of evaluating the average particle size, the number of coarse particles, and the oxygen content of this sample.

(Comparative Example 3)
A nickel powder was produced under the same conditions as in Example 1 except that the step of heating the nickel powder in a reducing atmosphere by the wet reduction method in Example 1 was omitted.
That is, nickel powder was produced, made highly dispersed in water, wet-classified, solid-liquid separated, and vacuum-dried. Table 1 shows the results of evaluating the average particle size, the number of coarse particles, and the oxygen content of this sample.

  From Table 1, in Example 1, since the number of coarse particles of 1.0 μm or more is 50 ppm or less of the total number of particles and the oxygen content is 1.4 mass% or less, the coarse particles and the oxygen content are small. It can be seen that nickel powder is obtained.

On the other hand, in Comparative Example 1, the number of coarse particles is suppressed to 50 ppm or less of the total number of particles. However, since wet classification is performed after heating in a reducing atmosphere, the oxygen content increases and the oxygen content increases. It is not less than 1.4% by mass.
In Comparative Example 2, the oxygen content is suppressed to 1.4% by mass or less, but since the wet classification is not performed, the number of coarse particles is larger than 50 ppm of the total number of particles. It turns out that it is not able to reduce.

  Furthermore, in Comparative Example 3, the number of coarse particles is suppressed to 50 ppm or less of the total number of particles, but since the heat treatment is not performed in a reducing atmosphere after wet classification, the oxygen content is 2.1% by mass, It turns out that the oxygen content of nickel powder has not been reduced.

Claims (8)

Forming a water slurry in which the raw material nickel powder is highly dispersed in water, and wet-classifying the water slurry to form a nickel slurry;
Drying the nickel slurry obtained by the wet classification process to form nickel powder, and heating the nickel powder in a reducing atmosphere.   The method for producing nickel powder according to claim 1, wherein the raw material nickel powder is nickel powder produced by a wet reduction method.   The raw material nickel powder is a nickel powder produced by a wet reduction method in which nickel powder is crystallized from an aqueous nickel salt solution, a reducing agent, and an alkaline colloidal solution containing palladium and silver. The manufacturing method of the nickel powder of Claim 2.   The said reducing atmosphere is an inert gas atmosphere containing 1-50 volume% hydrogen, The manufacturing method of the nickel powder of any one of Claims 1-3 characterized by the above-mentioned.   The method for producing nickel powder according to any one of claims 1 to 4, wherein a heating temperature in the step of heating the nickel powder in the reducing atmosphere is 150 to 350 ° C.   The number of coarse particles having a particle diameter of 1.0 μm or more in the nickel powder produced by the step of heating the nickel powder in the reducing atmosphere is 50 ppm or less of the total number of particles. 6. The method for producing nickel powder according to any one of 5 above.   The method for producing nickel powder according to claim 6, wherein the oxygen content of the nickel powder is 1.4 mass% or less.   The nickel powder production method according to claim 6, wherein an average particle diameter of the nickel powder is 0.1 to 0.3 μm.