JP7735754B2 - Nickel powder - Google Patents

Description

The present invention relates to high-performance nickel powder used as an electrode material for multilayer ceramic components.

Nickel powder is used as a material for capacitors in electronic circuits, particularly as a thick-film conductor material for the internal electrodes of multilayer ceramic components such as multilayer ceramic capacitors (MLCCs) and multilayer ceramic substrates.

In recent years, the capacity of multilayer ceramic capacitors has increased, and the amount of internal electrode paste used to form the internal electrodes of these capacitors has also increased significantly. For this reason, inexpensive base metals such as nickel are now being used as the metal powder for the internal electrode paste that makes up the thick-film conductor, instead of expensive precious metals.

In the process of manufacturing multilayer ceramic capacitors, an internal electrode paste made from a mixture of nickel powder, a binder resin such as ethyl cellulose, and an organic solvent such as terpineol is screen-printed onto a dielectric green sheet. The dielectric green sheets on which the internal electrode paste has been printed and dried are then laminated so that the internal electrode paste-printed layers and the dielectric green sheets are alternately stacked, and then pressed together to obtain a laminate.

This laminate is cut to a specified size, the binder resin is then removed by heat treatment (binder removal process), and the laminate after binder removal is then fired at a high temperature of around 1300°C to obtain a ceramic molded body.

External electrodes are then attached to the resulting ceramic compact to produce a multilayer ceramic capacitor. Because base metals such as nickel are used as the metal powder in the internal electrode paste that forms the internal electrodes, the binder removal process for the laminate is carried out in an atmosphere with an extremely low oxygen concentration, such as an inert atmosphere, to prevent the oxidation of these base metals. Furthermore, firing after binder removal is carried out in a reducing atmosphere.

As multilayer ceramic capacitors become smaller and their capacitance increases, both the internal electrodes and dielectric layers are becoming thinner. This has led to the particle size of the nickel powder used in internal electrode paste becoming finer, with nickel powder with an average particle size of 0.4 μm or less being required, and the use of nickel powder with an average particle size of 0.3 μm or less becoming the norm.

Such fine nickel powder is highly active, making it difficult to control the decomposition behavior of the binder contained in the internal electrode paste and the nickel grain growth during firing. For example, the binder decomposes rapidly at around 600°C, causing structural defects such as cracks in the MLCC body, or abnormally promoting nickel grain growth, causing the nickel to over-sinter, leading to spheroidization of the internal electrode film (reduced coverage), and problems such as reduced capacity and HALT degradation.

To solve the above problems, Patent Document 1 discloses a nickel powder having a thin oxide layer substantially composed of nickel oxide on the surface. This surface oxide layer is extremely strong and is said to effectively reduce the activity of the powder and increase the sintering initiation temperature. Patent Document 2 also discloses a nickel powder having a dense, thick oxide film formed on the surface. Regarding the Ni2p3/2 spectrum peak in X-ray photoelectron spectroscopy (XPS), the area ratios of the peak attributable to metallic nickel (hereinafter sometimes referred to as the "Ni peak"), the peak attributable to the bonding state of nickel and oxygen (hereinafter sometimes referred to as the "NiO peak"), and the peak attributable to the bonding state of nickel _and hydroxyl groups (hereinafter sometimes referred to as the "Ni(OH) ₂ peak") are 5.0 to 15.0%, 55.0 to 80.0%, and 5.0 to 40.0%, respectively. This oxide film prevents oxidation from progressing from the surface to the interior of the nickel powder, resulting in a higher sintering temperature than conventional nickel powders, closer to the sintering temperature of the dielectric. Furthermore, the surface composition is primarily NiO with a low proportion of Ni(OH) ₂ , which suppresses the generation of gas due to the thermal decomposition of Ni(OH) ₂ during the binder removal process in the manufacture of multilayer ceramic capacitors, reducing the occurrence of cracks and delamination.

Japanese Patent Application Laid-Open No. 2008-223068 International Publication No. WO2017/122689 Pamphlet International Publication No. WO2017/069067 Pamphlet

However, because the finer the nickel powder, the greater its specific surface area, nickel powder with an oxide film on its surface inevitably tends to contain a higher amount of oxygen. When the oxygen content of nickel powder increases, gas generation and volume change due to reduction of nickel oxide increase during firing in a reducing atmosphere during the production of multilayer ceramic capacitors. This makes it difficult to obtain a dense electrode film and can lead to cracks and delamination during firing.

The objective of this invention is to provide nickel powder with an oxide coating mainly composed of NiO on the surface, in which the content of impurities such as oxygen is optimized.

As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by controlling the component composition of Ni, NiO, and Ni(OH) ₂ on the surface of the nickel powder so that it falls within a specific range, and by achieving a compositional balance such that the sulfur content is 0.3% by mass or less and the oxygen content is 1.1% by mass or less, or when the sulfur content is 0.3% by mass or less and the oxygen content exceeds 1.1% by mass, the product of the sulfur content and the oxygen content as percentages is 0.15 or less.

In order to achieve the above object, the nickel powder of the present invention is a nickel powder having an oxide film mainly composed of NiO on its surface, and in the component composition of the surface of the nickel powder measured using X-ray photoelectron spectroscopy (XPS), the amount of Ni relative to the total of Ni, NiO, and Ni(OH) ₂ is 13 mol% or more and 35 mol% or less, the amount of NiO is 50 mol% or more, and the amount of Ni(OH) ₂ is more than 0 mol% and 30 mol% or less, and when the oxygen content is 1.1 mass% or less, the sulfur content is 0.17 mass% or less, and when the oxygen content exceeds 1.1 mass%, the value obtained by multiplying the percentage of the sulfur content and the percentage of the oxygen content is 0.15 or less.

The nickel powder of the present invention may have an average particle size of 0.03 μm to 0.4 μm.

The nickel powder of the present invention may have an H ₂ O adsorption amount per unit area of 0.10 mg/m ² or more and 0.70 mg/m ² or less at a relative pressure P/P ₀ =0.5.

The nickel powder of the present invention has a surface composition that is primarily composed of NiO, with the Ni ratio controlled to be relatively high. This results in a low oxygen content, which reduces the occurrence of cracks and delamination during the production of multilayer ceramic capacitors.

FIG. 1 is a diagram showing the results of thermogravimetric and differential thermal analysis of TG-DTA evaluation samples obtained from the nickel powders of Examples 1 to 4. FIG. 1 is a diagram showing the results of thermogravimetric and differential thermal analysis of a TG-DTA evaluation sample obtained from the nickel powder of Comparative Example 1. FIG. 10 is a diagram showing the results of thermogravimetric and differential thermal analysis of a TG-DTA evaluation sample obtained from the nickel powder of Example 5.

The nickel powder according to the present invention is described below. However, the present invention is not limited to this, and any modifications can be made without departing from the spirit of the present invention.

<1. Nickel powder>
The nickel powder according to the present invention is a nickel powder having an oxide coating mainly composed of NiO on its surface, and in the surface composition of the nickel powder measured using X-ray photoelectron spectroscopy (XPS), the amount of Ni relative to the total of Ni, NiO, and Ni(OH) ₂ is 13 mol% or more and 35 mol% or less, the amount of NiO is 50 mol% or more, and the amount of Ni(OH) ₂ is more than 0 mol% and 30 mol% or less. Because the surface composition is mainly composed of NiO and the oxygen content is kept low, the occurrence of cracks and delamination during the production of multilayer ceramic capacitors is suppressed, making the nickel powder suitable for use as internal electrodes of multilayer ceramic capacitors.

The surface composition will be explained in more detail. When the Ni2p spectrum of the nickel powder surface is analyzed by XPS, a Ni peak, a NiO peak, and a Ni(OH) ₂ peak are observed. Regarding the surface composition, the Ni peak corresponds to Ni, the NiO peak corresponds to NiO, and the Ni(OH) ₂ peak corresponds to Ni(OH) ₂ . The component composition ratios are determined from the area ratio of each peak to the total peak area of these three components. The preferred ranges for each component composition are: Ni is 13 mol% or more and 35 mol% or less, NiO is 50 mol% or more, and Ni(OH) ₂ is more than 0 mol% and 30 mol% or less (Ni + NiO + Ni(OH) ₂ = 100 mol%).

If the Ni ratio in the surface composition is less than 13 mol%, the oxygen content of the nickel powder becomes high, which may cause cracks or delamination during firing in a reducing atmosphere during the production of a multilayer ceramic capacitor. If the Ni ratio exceeds 35 mol%, the surface oxide film may become unstable, which may reduce oxidation resistance. A more preferable range of the Ni ratio is more than 20 mol% but not more than 30 mol%.

If the ratio of NiO in the surface composition is less than 50 mol%, it means that the ratio of Ni(OH) ₂ is high, which may result in a decrease in the sintering start temperature during the production of multilayer ceramic capacitors, or in the occurrence of cracks or delamination during the binder removal process. A more preferable range for the ratio of NiO is 60 mol% or more.

If the amount of Ni(OH) ₂ in the surface composition exceeds 30 mol%, the sintering start temperature may decrease during the production of a multilayer ceramic capacitor, and cracks or delamination may occur during the binder removal process.

The oxygen content of the nickel powder according to the present invention is preferably 1.3% by mass or less based on the total nickel powder. The oxygen content must be considered together with the sulfur content, as described below. If the oxygen content exceeds 1.3% by mass, cracks and delamination may occur due to gas generation and volume changes caused by reduction during firing in a reducing atmosphere during the manufacture of multilayer ceramic capacitors. There are no particular restrictions on the lower limit of the oxygen content. As explained above in the surface composition, this nickel powder has an oxide film primarily composed of NiO on the surface, so the effective lower limit is approximately 0.5% by mass. The oxygen content can be measured, for example, using an oxygen analyzer using the inert gas fusion method.

The average particle size of the nickel powder according to the present invention is preferably 0.03 μm to 0.4 μm, in order to accommodate the recent trend toward thinner internal electrodes in multilayer ceramic capacitors. Of course, this does not preclude the use of nickel powder with an average particle size of more than 0.4 μm. The average particle size is determined by observing the nickel powder with a scanning electron microscope (SEM) and determining the diameter of the particles that can be seen in full detail in the photograph, and then calculating the number average particle size.

Nickel powders suitable for use in the internal electrodes of multilayer ceramic capacitors, such as the nickel powder of the present invention, may typically contain trace amounts of sulfur to suppress their catalytic activity. This is because the surfaces of nickel particles have high catalytic activity. If used as is without adding sulfur, for example, the nickel powder promotes the thermal decomposition of binder resins, such as ethyl cellulose resin, contained in the internal electrode paste during the binder removal process in the manufacture of multilayer ceramic capacitors. This causes the binder resin to decompose at low temperatures, significantly reducing the strength of the laminate. At the same time, large amounts of decomposition gas are generated, which can make the laminate more susceptible to cracking. On the other hand, even if sulfur is added to the nickel powder, cracks may still occur in the laminate depending on the oxygen content of the nickel powder. Therefore, as described below, it is necessary to consider the compositional balance between the sulfur content and oxygen content of the nickel powder.

As mentioned above, to incorporate sulfur into nickel powder, surface treatment is performed to adhere sulfur to the surfaces of the nickel particles, and the entire surface of the nickel particles is thinly and uniformly modified (coated) with sulfur. This is most preferable from the standpoint of demonstrating the binder resin decomposition inhibitory effect and reducing the impact of sulfur as an impurity on the properties of multilayer ceramic capacitors. However, as long as the binder resin decomposition inhibitory effect is achieved, the nickel particles may be partially modified (coated). In the present invention, the term "surface treatment" is used as a comprehensive concept to encompass both the entire modification (coating) of nickel particles and partial modification (coating) of nickel particles.

When the oxygen content in the nickel powder is 1.1% by mass or less, the sulfur content in the nickel powder is 0.3% by mass or less. If the sulfur content exceeds 0.3% by mass, internal electrode defects due to sulfur may occur, so it is preferably 0.2% by mass or less, and more preferably 0.17% by mass or less. There is no particular lower limit for the sulfur content.

Here, we consider the behavior of NiO and Ni(OH) ₂ on the surface of nickel powder contained in the internal electrode paste during the binder removal process in the manufacturing process of a multilayer ceramic capacitor and firing at a high temperature of about 1300°C. Ni(OH) ₂ on the surface of the nickel powder loses water at a temperature of around 200°C during the temperature rise process and becomes NiO, which exists on the surface of the nickel powder.

The nickel surface is coated with an oxide film (NiO). This oxide film (NiO) is reduced at 200-600°C (the temperature range in which residual carbon decomposes in a weakly reducing atmosphere) and releases oxygen (NiO → Ni + 1/2O ₂ ). The released oxygen burns the carbon remaining inside the electrode, generating gas, and the pressure of this gas causes structural defects such as cracks. If the amount of oxygen released (desorbed) from the oxide film is appropriate, it will stably burn the carbon (binder) remaining inside the electrode, but if excessive oxygen is released, it will rapidly burn the binder. On the other hand, the ease of oxygen release is related to the bond between oxygen and sulfur, and sulfur also plays a role in making it difficult for oxygen to be released. Therefore, the compositional balance of the sulfur and oxygen contents of the nickel powder can be said to be an important factor in determining the decomposition ability of residual carbon.

The release of impurities such as oxygen from nickel powder at each temperature can be estimated by the rate of change in weight loss over time, ΔTG, measured by thermogravimetric differential thermal analysis (TG-DTA), since the impurities are present in trace amounts.

Figure 1 shows the results of thermogravimetric differential thermal analysis of TG-DTA evaluation samples obtained from the nickel powders of Examples 1 to 4. Figure 2 shows the results of thermogravimetric differential thermal analysis of TG-DTA evaluation samples obtained from the nickel powder of Comparative Example 1. Figure 3 shows the results of thermogravimetric differential thermal analysis of TG-DTA evaluation samples obtained from the nickel powder of Example 5.

The TG-DTA evaluation sample is composed of the same materials used in the internal electrode paste, such as nickel powder, binder resin, and barium titanate. This composition is nearly identical to the composition of the dried film of internal electrode paste obtained by printing the internal electrode paste on a dielectric green sheet and drying it, and is therefore nearly identical to the dried film of internal electrode paste obtained during firing. By evaluating a TG-DTA evaluation sample with this composition, we can determine the effect of nickel powder on the binder resin during the debindering process and predict defects such as cracks that may occur during firing of multilayer ceramic capacitors. The TG-DTA evaluation sample is obtained by preparing an internal electrode paste, applying the resulting internal electrode paste to a resin film, etc., and pulverizing the resulting dried product by evaporating and removing the organic solvents, such as terpineol, contained in the internal electrode paste. The internal electrode paste can also be prepared using a known kneading device, such as a three-roll mill.

For example, as shown in Figure 1, when the sulfur content of the nickel powder is 0.1% by mass or 0.11% by mass, even when the oxygen content is in the range of 0.91% by mass to 1.2% by mass, the ΔTG, which is the rate of change in weight loss over time in thermogravimetric differential thermal analysis (TG-DTA) of the nickel powder, shows peaks around 450°C, 500°C, and 650°C. On the other hand, as shown in Figure 2 (Comparative Example 1), when the oxygen content of the nickel powder exceeds 1.1% by mass and the sulfur content of the nickel powder is 0.13% by mass, the ΔTG, which is the rate of change in weight loss over time in thermogravimetric differential thermal analysis (TG-DTA) of the nickel powder, shows peaks around 350°C, 450°C, and 650°C.

Furthermore, as shown in Figure 3 (Example 5), even if the sulfur content of the nickel powder is 0.17% by mass, if the oxygen content is 1% by mass, the ΔTG peak near 370°C as shown in Figure 2 (Comparative Example 1) does not appear.

The difference between Figure 1 and Figure 2 is that the ΔTG peaks in Figure 1 appear around 450°C, 500°C, and 650°C, whereas in Figure 2, for nickel powder with an oxygen content of 1.2% by mass and a sulfur content of 0.13% by mass, the ΔTG peak appears around 370°C.

The nickel powder in Figure 3 (Example 5) has a ΔTG peak at 395°C, but there is no concern about cracking during firing of the MLCC. In other words, the thermal decomposition behavior of the binder resin in the internal electrode paste varies depending on the sulfur and oxygen content ranges of the nickel powder, and if debinding occurs at low temperatures, structural defects such as cracks caused by combustion gases are more likely to occur. Specifically, when the peak of the ΔTG decrease appears below 390°C, debinding occurs at low temperatures due to O generated from the nickel powder.

The sulfur content may be below the detection limit of analytical equipment used to analyze the content, such as a sulfur analyzer using the combustion method.

In other words, when the oxygen content in the nickel powder is 1.1% by mass or less, and the sulfur content in the nickel powder is 0.3% by mass or less, the peak of the decrease in ΔTG does not appear below 390°C, so debinding does not occur at low temperatures and structural defects such as cracks caused by combustion gases do not occur.

Furthermore, when the oxygen content in the nickel powder exceeds 1.1% by mass, the value obtained by multiplying the percentage of sulfur content in the nickel powder by the percentage of oxygen content is 0.15 or less, so the peak of the decrease in ΔTG does not appear below 390°C, debinding does not occur at low temperatures, and structural defects such as cracks caused by combustion gases do not occur.

Furthermore, the nickel powder of this embodiment preferably has an H ₂ O adsorption amount per unit area at a relative pressure P/P ₀ = 0.5 of 0.10 mg/m ² or more and 0.70 mg/m ² or less, and more preferably 0.11 mg/m ² or more and 0.6 mg/m ^2. Here, the relative pressure refers to the ratio of the adsorption equilibrium pressure, which is the pressure at which the adsorption equilibrium state occurs at a constant adsorption temperature, to the saturated vapor pressure.

The wettability of the nickel powder with solvents and vehicles changes depending on the strength of the hydrophilicity/hydrophobicity of the surface, which significantly affects the disintegration and dispersibility of fine powder agglomerates, particularly as they become increasingly finer. If the _H2O adsorption amount of the nickel powder is less than 0.10 mg/( ^m2 /g), the hydrophobicity becomes too strong, which is undesirable, and the wettability with binders and solvents decreases. On the other hand, if the _H2O adsorption amount of the nickel powder exceeds 0.70 mg/( ^m2 /g), the hydrophilicity becomes too strong, which is undesirable, and the affinity with the binder decreases.

If it becomes difficult for the surfactant to adsorb to the nickel powder surface, wettability with the solvent or vehicle may deteriorate, resulting in insufficient disintegration of agglomerates and insufficient prevention of re-agglomeration, which can lead to reduced viscosity stability of the conductive paste and poor surface smoothness of the dried film, which is undesirable.

When using nickel powder as the conductive powder, it is preferable that the surface composition contain 50 mol% or more of NiO. If the amount of NiO is less than 50 mol%, the dispersant and binder resin will not be properly adsorbed, resulting in reduced viscosity stability, which is undesirable.

<2. Nickel powder manufacturing method>
The nickel powder according to the present invention is produced by filtering nickel particles in a slurry state, such as nickel particles crystallized by a reduction reaction in an aqueous solution using a wet method, and drying the filtered particles under a low oxygen partial pressure.

The nickel particles used here preferably have nickel hydroxide and the like on their surface, but are not exposed to the atmosphere after particle generation; nickel particles generated by a wet method before filtration are preferred. Of course, nickel particles generated by dry methods such as gas-phase reduction or plasma methods, or spray pyrolysis methods, can also be washed into a slurry state and the method described below can be applied, but nickel particles generated by these methods, such as dry methods, are exposed to the atmosphere and have a certain degree of surface oxidation. Therefore, it is preferable to remove the surface oxides by adding an acid or complexing agent capable of dissolving nickel oxides to a slurry containing nickel particles (hereinafter sometimes referred to as "nickel slurry").

The nickel slurry may be washed by adding a cleaning solution or solvent to remove impurities contained in the nickel particles or solvent. Any known cleaning method can be used, as long as the nickel particles in the slurry are not exposed to the atmosphere. For example, repeatedly adding and decanting the cleaning solution or solvent is preferable.

The nickel slurry can be filtered to produce a nickel cake, which can then be dried to obtain nickel powder. There are no particular restrictions on the filtration method, and any known method can be used, but it is preferable to subject the wet nickel cake to the drying process so that the nickel particles are not directly exposed to the atmosphere.

The drying process can be performed by known heat drying methods capable of adjusting the atmosphere or heat drying under reduced pressure, but it is preferable to heat dry in a low-oxygen atmosphere in which the oxygen partial pressure is controlled to at least 0.3 kPa to 5 kPa, preferably 0.5 kPa to 3 kPa. Specifically, the atmospheric pressure can be reduced. Heat drying in such a low-oxygen atmosphere slows down the progress of oxidation, and nickel powder can be obtained in which the amount of Ni in the surface composition is 13 mol % to 35 mol %.

The drying temperature for the drying treatment may be 60°C to 160°C. The drying temperature may be lower than 60°C, but this may result in a longer drying time and reduced productivity. A higher drying temperature is preferable because the Ni(OH) ₂ in the surface component composition is thermally decomposed and converted to NiO, and a drying temperature of 100°C or higher is particularly preferable. However, the higher the drying temperature, the more severe the drying agglomeration becomes, and the stronger the adhesion between nickel particles becomes. Even with the crushing treatment described below, the agglomerated nickel particles may not be disintegrated. Drying agglomeration is affected not only by the drying temperature but also by the drying time and atmospheric conditions, but a drying temperature of 160°C or lower is preferable, and 155°C or lower is even more preferable.

The drying time for the drying treatment is not particularly limited, but is preferably 20 minutes to 24 hours. By adjusting these drying treatment conditions, an oxide coating is formed on the nickel powder, whose surface composition, as measured by XPS, is such that the amount of Ni is 13 mol % to 35 mol %, the amount of NiO is 50 mol % or more, and the amount of Ni(OH) ₂ is more than 0 mol % to 30 mol %.

Since nickel powder that has undergone drying may have particles bonded together due to dry agglomeration, it is preferable to carry out a crushing process to loosen the agglomerated nickel particles. For crushing, dry crushing methods such as spiral jet crushing and counter jet mill crushing, wet crushing methods such as high-pressure fluid collision crushing, and other general-purpose crushing methods can be used.

As an example of nickel particles crystallized by a reduction reaction in an aqueous solution using a wet method, which is suitable for producing the nickel powder described above, the crystallization method for nickel particles described in Patent Document 3 will be described below.

In this wet method for precipitating nickel particles, an amine compound is added to a reaction solution containing at least a water-soluble nickel salt, a salt of a metal nobler than nickel, hydrazine as a reducing agent, and a mixture of alkali hydroxide and water, and the nickel salt is reduced with hydrazine to crystallize the nickel particles. A sulfide compound may also be added to the reaction solution.

The nickel salt is not particularly limited as long as it is readily soluble in water, and one or more selected from nickel chloride, nickel sulfate, and nickel nitrate can be used.

By including a metal more noble than nickel in the nickel salt solution, the metal more noble than nickel is reduced first during reduction crystallization of nickel, acting as a nucleating agent to form initial nuclei. Particle growth from these initial nuclei produces fine nickel particles. Examples of metal salts more noble than nickel include water-soluble copper salts, gold salts, silver salts, platinum salts, palladium salts, rhodium salts, and iridium salts, with palladium salts being particularly preferred. When using palladium salts, the ratio of palladium salt to nickel [moles per million] (moles of palladium salt / moles of nickel × 106) is preferably in the range of 0.2 mol ppm to 100 mol ppm.

Hydrazine is preferably used as the reducing agent, and in addition to anhydrous hydrazine, hydrazine hydrate, which is a hydrazine hydrate, may also be used.

Since the reducing power of hydrazine increases as the pH of the reaction solution increases, alkali hydroxide is used as a pH adjuster to increase alkalinity. The alkali hydroxide is preferably an alkali metal hydroxide, and more preferably one or more selected from sodium hydroxide and potassium hydroxide. The amount of alkali hydroxide added should be such that the pH of the reaction solution is 9.5 or higher, preferably 10 or higher.

Amine compounds are well known for their ability to accelerate reduction reactions as complexing agents. However, specific amine compounds are preferably incorporated into the reaction solution because they have the effect of inhibiting the self-decomposition of hydrazine and inhibiting the bonding of nickel particles together. Amine compounds that inhibit the self-decomposition of hydrazine are compounds that contain two or more primary amino groups (-NH ₂ ) or secondary amino groups (-NH-) in their molecules. Specifically, ethylenediamine, diethylenetriamine, propylenediamine, etc. are preferred. The amount of amine compound incorporated is in the range of 0.01 mol % to 5 mol %, more preferably 0.03 mol % to 2 mol %, in terms of the ratio of moles of amine compound [mol %] (moles of amine compound / moles of nickel × 100).

Sulfide compounds are compounds containing one or more sulfide groups (-S-) within their molecules. When used in combination with the specific amine compounds mentioned above, they significantly enhance the ability to inhibit the self-decomposition of hydrazine and also inhibit the bonding of nickel particles together. Among sulfide compounds, compounds that exhibit these effects most prominently are those that also contain at least one carboxyl group (-COOH) or hydroxyl group (-OH) within their molecules. Specifically, L-methionine, L-ethionine, thiodiglycolic acid, etc. are preferred. The amount of sulfide compound blended should be in the range of 0.01 mol% to 5 mol%, more preferably 0.03 mol% to 2 mol%, as the molar ratio of sulfide compound [mol %] (moles of sulfide compound / moles of nickel x 100).

The crystallization procedure involves mixing aqueous solutions of a water-soluble nickel salt, a salt of a metal nobler than nickel, hydrazine, and an alkali hydroxide, and the reduction reaction begins when the reaction solution is prepared. The order in which the aqueous solutions are mixed before the reaction solution is prepared is not particularly limited, and they can be mixed in any order. The amine compound or sulfide compound can be added to at least one of the aqueous solutions before the reduction reaction begins, or during the preparation process, or added to the reaction solution after the reduction reaction has begun.

The temperature during crystallization, at the time the reaction solution is prepared, i.e., at the time the reduction reaction begins, should be in the range of 40°C to 90°C, more preferably 50°C to 80°C.

The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples. The properties of the nickel powder, such as average particle size, surface composition, oxygen content, sulfur content, and H ₂ O adsorption, were evaluated as follows.

(Average particle size)
The nickel powder was observed using a scanning electron microscope (SEM, manufactured by JEOL Ltd., JSM-7100F) to take an image (SEM image), and the diameter of particles whose entire shape could be confirmed was determined from the photographic image by image analysis, and the number average particle size was taken as the average particle size.

(Surface composition)
The surface composition of the nickel powder was determined by using XPS (Versa Probe II, manufactured by ULVAC-PHI, Inc.) to perform waveform separation from the Ni2p spectrum of nickel by peak fitting to calculate the areas of the Ni peak, NiO peak, and Ni(OH) ₂ , and then determining the mole percentages of Ni, NiO, and Ni(OH) ₂ from the area ratios of these peaks.

(Oxygen content)
The measurement was carried out using an oxygen analyzer (TC436, manufactured by LECO Corporation) using an inert gas fusion method.

(Sulfur content)
The measurement was carried out using a sulfur analyzer (CS600, manufactured by LECO Corporation) based on a combustion method.

( _H2O adsorption amount)
The nickel powder was degassed under vacuum at 25°C for 8 hours, and then the specific surface area of the nickel powder was measured using nitrogen as the adsorbate with a specific surface area/pore distribution measuring device (Microtrackbell, BELSORP-miniX). Furthermore, the nickel powder was degassed under vacuum at 25°C for 8 hours, and then the adsorption/desorption isotherm of the _H2O adsorption amount of the nickel powder at 25°C was measured using a high-precision vapor adsorption measuring device (Microtrackbell, BELSORP-AQUA ₃ ). The _H2O adsorption amount at a relative pressure (P/ _P0 ) of 0.5 in the adsorption/desorption isotherm of the nickel powder was determined. The obtained _H2O adsorption amount at a relative pressure (P/P0) of 0.5 was divided by the specific surface area to calculate the _H2O adsorption amount per unit area.

(Example)
[Crystallization process]
Using the method described in Patent Document 3, nickel particles were crystallized by a reduction reaction. Specifically, 5 kg of nickel chloride hexahydrate (based on nickel mass) was dissolved in 100 L of pure water to prepare a nickel salt solution. A 0.5 mol % L-methionine aqueous solution (based on nickel), a 0.5 ppm palladium (II) chloride ammonium aqueous solution (based on nickel mass), 60% hydrazine hydrate, and a 25% sodium hydroxide aqueous solution were added in this order to prepare a reaction solution, and crystallization by reduction reaction was initiated. Subsequently, a 1.0 mol % ethylenediamine aqueous solution (based on nickel mass) was added to the reaction solution, and nickel particles were crystallized. It was confirmed that all of the nickel components in the solution had been reduced to metallic nickel.

[Cleaning and filtration process]
The nickel slurry obtained by the crystallization step was first left to settle, the supernatant was removed by decantation, and the same amount of the supernatant as that from which the pure water and sulfuric acid aqueous solution had been removed was added to obtain a nickel slurry with a pH of 8.3. Thioglycolic acid was added to this nickel slurry to perform surface treatment of the nickel particles, and then the nickel slurry was left to settle, and the supernatant was removed by decantation. During decantation, enough liquid was left to prevent the nickel particles from being exposed to the atmosphere.

The nickel slurry was then transferred to a Tanabe Willtec centrifuge (model CO-24), where it was filtered and washed with purified water while adjusting the flow rate to ensure that the nickel particles were not exposed to the air at all times, and solid-liquid separation was carried out to obtain a wet nickel cake.

[Drying process]
The wet nickel cake was dried using an EYELA stationary vacuum dryer (model VOS-451SD). The drying conditions were as shown in Table 1, and the pressure was reduced from the start of temperature increase until the drying temperature was reached and the temperature was then lowered to 30°C, with air being introduced to a pressure of 6 kPa. The oxygen partial pressure in the drying atmosphere was 1.2 kPa. The drying temperature was maintained at 100°C or higher for 45 minutes or longer. Once the drying temperature had lowered to 30°C, the amount of air introduced was gradually increased to return to atmospheric pressure, and drying was completed, yielding nickel powder.

[Paste formation and preparation of TG-DTA evaluation sample]
An internal electrode paste was prepared using the obtained nickel powder. The internal electrode paste was formulated so that 100 parts by mass of nickel powder contained 4 parts by mass of ethyl cellulose, 2 parts by mass of polyvinyl butyral resin, and 10 parts by mass of barium titanate powder. Furthermore, the ethyl cellulose and polyvinyl butyral resin were dissolved in terpineol in advance to prepare a vehicle, which was then added during the preparation of the internal electrode paste. The obtained internal electrode paste was applied to a PET film, and the terpineol was evaporated and removed in a drying oven at 120°C to obtain a dried film. The obtained dried film was crushed in a mortar to prepare a TG-DTA evaluation sample. The TG-DTA sample contained nickel powder, ethyl cellulose, polyvinyl butyral resin, and barium titanate powder. This composition was approximately the same as the composition of a dried film obtained by printing the internal electrode paste on a dielectric green sheet and then drying it, i.e., the dried film of the internal electrode paste when fired.

[evaluation]
The average particle size, surface composition, and sulfur and oxygen contents of the obtained nickel powders of Examples 1 to 5 and Comparative Example 1 were measured. The results are summarized in Table 1.

In addition, TG-DTA evaluation samples obtained from the nickel powders of Examples 1 to 5 and Comparative Example 1 were subjected to thermogravimetric differential thermal analysis (TG-DTA) to determine the rate of change in weight loss over time, ΔTG. Table 1 also shows the low-temperature peak temperatures of ΔTG. The TG-DTA heating conditions were a first debinding process in which the temperature was increased from room temperature to 310°C at 5°C/min, held at 310°C for 3 hours, and then cooled to room temperature. The second debinding process was a temperature profile in which the temperature was increased from room temperature to 800°C at 5°C/min and then cooled to room temperature. The atmosphere in the first debinding process was _N2 gas, and the atmosphere in the second debinding process was a mixed gas of 2% _H2 and _N2 . In Table 1, the surface composition is expressed in mol%, and the S and O contents are expressed in mass%.

Furthermore, the amount of H ₂ O adsorption per unit area was calculated for the obtained nickel powders of Examples 1 to 5 and Comparative Example 1. The results are shown in Table 2. In Table 2, the unit of the amount of H ₂ O adsorption is mg/m ² .

As shown in Table 1, Examples 1 to 5, which have specified sulfur and oxygen contents, show ΔTG peaks at temperatures higher than approximately 390°C. On the other hand, Comparative Example 1, which has sulfur and oxygen contents outside the range of this embodiment, shows ΔTG peaks at temperatures lower than approximately 390°C. When a multilayer ceramic capacitor is manufactured using an internal electrode paste containing the nickel powder of Comparative Example 1, defects such as cracks may occur.

As described above, the present invention can provide a nickel powder that can suppress the occurrence of cracks and delamination during the production of multilayer ceramic capacitors.
It was also found that the amount of H ₂ O adsorbed per unit area was the lowest in Comparative Example 1.

Claims (3)

A nickel powder having an oxide coating mainly composed of NiO on its surface,
In the component composition of the surface of the nickel powder measured using X-ray photoelectron spectroscopy (XPS), the amount of Ni relative to the total of Ni, NiO, and Ni(OH) ₂ is 13 mol% or more and 35 mol% or less, the amount of NiO is 50 mol% or more, and the amount of Ni(OH) ₂ is more than 0 mol% and 30 mol% or less;
When the oxygen content is 1.1% by mass or less, the sulfur content is 0.17 % by mass or less,
A nickel powder in which, when the oxygen content exceeds 1.1 mass %, the value obtained by multiplying the percentage of the sulfur content by the percentage of the oxygen content is 0.15 or less. The nickel powder according to claim 1, having an average particle size of 0.03 μm to 0.4 μm. 3. The nickel powder according to claim 1, wherein the amount of H ₂ O adsorbed per unit area at a relative pressure P/P ₀ =0.5 is 0.10 mg/m ² or more and 0.70 mg/m ² or less.