Nickel powder

Abstract

To provide nickel powder capable of suppressing the generation of cracks and delamination upon multilayer ceramic capacitor production.SOLUTION: Nickel powder has an oxide film essentially consisting of NiO on the surface. In a componential composition of the surface of the nickel powder measured using an X-ray photoelectron spectroscopy (XPS), the substance amount of Ni to the total of Ni, NiO and Ni(OH)2 is above 15 mol% to 35 mol% or less, the substance amount of NiO is 50 mol% or more, the substance amount of Ni(OH)2 is above 0 mol% to 30 mol% or less, a sulfur content is 0.3 mass% or less when an oxygen content is 1.1 mass% or less, and a value obtained by multiplying the percentage of the sulfur content by the oxygen content is 0.15 or less when the oxygen content exceeds 1.1 mass%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a high-performance nickel powder used as an electrode material for multilayer ceramic parts. [Background technology]

[0002] Nickel powder is used as a material for capacitors in electronic circuits, particularly as a material for thick-film conductors that form internal electrodes of multilayer ceramic components such as multilayer ceramic capacitors (MLCCs) and multilayer ceramic substrates.

[0003] 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 multilayer ceramic capacitors has also increased significantly. As a result, inexpensive base metals such as nickel are being used as the metal powder for the internal electrode paste that constitutes the thick film conductor, instead of expensive precious metals.

[0004] In the process of manufacturing a multilayer ceramic capacitor, an internal electrode paste, which is 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 sheet on which the internal electrode paste has been printed and dried is then laminated so that the internal electrode paste-printed layers and the dielectric green sheet are alternately stacked, and then pressed together to obtain a laminate.

[0005] This laminate is cut to a specified size, and then the binder resin is removed by heat treatment (binder removal treatment).Furthermore, the laminate after binder removal treatment is fired at a high temperature of about 1300°C to obtain a ceramic molded body.

[0006] Then, external electrodes are attached to the obtained ceramic compact to obtain a multilayer ceramic capacitor. Because base metals such as nickel are used as the metal powder in the internal electrode paste that becomes the internal electrodes, the binder removal treatment of 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. In addition, firing after the binder removal treatment is carried out in a reducing atmosphere.

[0007] As multilayer ceramic capacitors become smaller and their capacitance increases, the internal electrodes and dielectric layers are becoming thinner. This has led to the particle size of the nickel powder used in the internal electrode paste becoming finer, with nickel powder with an average particle size of 0.4 μm or less being required, and nickel powder with an average particle size of 0.3 μm or less becoming the norm.

[0008] This type of 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, resulting in over-sintering of the nickel, causing spheroidization of the internal electrode film (reduced coverage), leading to problems such as reduced capacity and HALT degradation.

[0009] 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 of the nickel powder to the interior, which is said to result in a higher sintering temperature than conventional nickel powders and closer to the sintering temperature of the dielectric. Furthermore, because the surface composition is primarily NiO with a low proportion of Ni(OH)2, it is said to suppress the gas generated by the thermal decomposition of Ni(OH)2 during the binder removal process in the production of multilayer ceramic capacitors, thereby reducing the occurrence of cracks and delamination. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-223068 [Patent Document 2] International Publication No. WO2017 / 122689 Brochure [Patent Document 3] International Publication No. WO2017 / 069067 Brochure Summary of the Invention [Problem to be solved by the invention]

[0011] However, because the finer the nickel powder, the larger its specific surface area, nickel powder with an oxide film on its surface tends to contain a larger 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 causes cracks and delamination during firing.

[0012] Therefore, an object of the present invention is to provide nickel powder having an oxide film mainly composed of NiO on the surface thereof, in which the content of impurities such as oxygen is optimized. [Means for solving the problem]

[0013] As a result of extensive research, the 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 mass% or less and the oxygen content is 1.1 mass% or less, or when the sulfur content is 0.3 mass% or less and the oxygen content exceeds 1.1 mass%, the product of the sulfur content and the oxygen content as percentages is 0.15 or less.

[0014] In order to solve the above-mentioned problems, the nickel powder of the present invention is a nickel powder having an oxide film mainly composed of NiO on the surface, and in the component composition of the surface of the nickel powder measured using X-ray photoelectron spectroscopy (XPS), the amount of substance of Ni relative to the total of Ni, NiO, and Ni(OH)2 is 13 mol% or more 35 mol% or less, the amount of substance of NiO is 50 mol% or more, the amount of substance 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 % by mass or less, and when the oxygen content exceeds 1.1% by mass, the value obtained by multiplying the percentage of the sulfur content by the percentage of the oxygen content is 0.15 or less.

[0015] The nickel powder of the present invention may have an average particle size of 0.03 μm to 0.4 μm.

[0016] The nickel powder of the present invention has an HO adsorption amount per unit area of ​​0.10 mg / m at a relative pressure P / P = 0.5. 2 More than 0.70mg / m 2 It may be the following: [Effects of the Invention]

[0017] The nickel powder according to the present invention has a surface composition that is mainly composed of NiO, with the Ni ratio controlled to be relatively high, so that the oxygen content is low and the occurrence of cracks and delamination during the production of multilayer ceramic capacitors is suppressed. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 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. [Figure 2] FIG. 2 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. [Figure 3] 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. DETAILED DESCRIPTION OF THE INVENTION

[0019] The nickel powder according to the present invention will be described below, but the present invention is not limited to these and can be modified as desired without departing from the gist of the present invention.

[0020] <1. Nickel powder> The nickel powder according to the present invention is a nickel powder having an oxide film mainly composed of NiO on the surface thereof, 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 amount of Ni, NiO, and Ni(OH)2 is 13 mol% or moreThe amount of NiO is 50 mol % or more, and the amount of Ni(OH)2 is greater than 0 mol % and less than 30 mol %. As the surface composition is primarily 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 surface suitable for use as internal electrodes of multilayer ceramic capacitors.

[0021] 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. As for 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 ratio can be calculated from the area ratio of each peak to the total peak area of ​​these three components, and the preferred range of each component composition is as follows: Ni 13 mol% or more 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 %).

[0022] The ratio of Ni in the surface composition is 13 mole % less than If the Ni content exceeds 35 mol%, the oxide film on the surface becomes unstable, and oxidation resistance may decrease. A more preferable range for the Ni content is more than 20 mol% but not more than 30 mol%.

[0023] 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 cracks or delamination during the binder removal process. A more preferable range for the ratio of NiO is 60 mol% or more.

[0024] 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.

[0025] 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 change caused by reduction during firing in a reducing atmosphere during the production of a multilayer ceramic capacitor. There are no particular restrictions on the lower limit of the oxygen content. As explained above regarding the surface composition, this nickel powder has an oxide film mainly composed of NiO on the surface, so the substantial lower limit is approximately 0.5% by mass. The oxygen content can be measured, for example, using an oxygen analyzer based on the inert gas fusion method.

[0026] 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 of 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 particles that can be seen in the entire image, and then calculating the number average particle size.

[0027] 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 without 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 decomposes the binder resin at low temperatures, significantly reducing the strength of the laminate. At the same time, a large amount of decomposition gas is generated, which may lead to the formation of cracks in the laminate. On the other hand, even if sulfur is added to the nickel powder, cracks may 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.

[0028] As described above, to incorporate sulfur into nickel powder, surface treatment is performed to attach sulfur to the surfaces of nickel particles, and the entire surface of the nickel particles is thinly and uniformly modified (coated) with sulfur. This is most preferable from the viewpoint of exhibiting the binder resin decomposition suppression 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 suppression effect can be exhibited, a modification (coating) in which only a portion of the nickel particles is modified (coated) is also acceptable. In the present invention, the term "surface treatment" is used as a concept that encompasses both the entire modification (coating) of nickel particles and the partial modification (coating) of nickel particles.

[0029] 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 the sulfur content 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.

[0030] 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 process of manufacturing multilayer ceramic capacitors, from the binder removal process to firing at high temperatures of around 1300°C. Ni(OH)2 on the surface of the nickel powder loses water at around 200°C during the heating process and becomes NiO, which remains on the surface of the nickel powder.

[0031] The nickel surface is covered 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 / 2O2). 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 stably burns 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 O and S, and S also plays a role in making O release more difficult. Therefore, the compositional balance of the sulfur and oxygen contents of the nickel powder is an important factor in determining the decomposition ability of residual carbon.

[0032] The release of impurities such as oxygen from nickel powder at each temperature can be estimated by ΔTG, the rate of change in weight loss over time in thermogravimetric differential thermal analysis (TG-DTA), because the amount of impurities is very small.

[0033] Fig. 1 shows the results of thermogravimetric differential thermal analysis of TG-DTA evaluation samples obtained from the nickel powders of Examples 1 to 4. Fig. 2 shows the results of thermogravimetric differential thermal analysis of TG-DTA evaluation samples obtained from the nickel powder of Comparative Example 1. Fig. 3 shows the results of thermogravimetric differential thermal analysis of TG-DTA evaluation samples obtained from the nickel powder of Example 5.

[0034] The TG-DTA evaluation sample is composed of the common 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 internal electrode paste film obtained by printing the internal electrode paste on a dielectric green sheet and drying it, and thus to the dried internal electrode paste film obtained during firing. Evaluation using a TG-DTA evaluation sample with this composition allows us to understand 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, and pulverizing the resulting dried product after evaporating and removing the organic solvent, 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.

[0035] For example, as shown in Figure 1, when the sulfur content of the nickel powder is 0.1 mass% or 0.11 mass%, even if the oxygen content is in the range of 0.91 mass% to 1.2 mass%, the rate of change in weight loss over time, ΔTG, measured by 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 mass% and the sulfur content of the nickel powder is 0.13 mass%, the rate of change in weight loss over time, ΔTG, measured by thermogravimetric differential thermal analysis (TG-DTA) of the nickel powder shows peaks around 350°C, 450°C, and 650°C.

[0036] Furthermore, as shown in Figure 3 (Example 5), even if the sulfur content of the nickel powder is 0.17 mass%, if the oxygen content is 1 mass%, a peak in ΔTG near 370°C as shown in Figure 2 (Comparative Example 1) does not appear.

[0037] 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 the ΔTG peak in Figure 2 for the nickel powder with an oxygen content of 1.2 mass% and a sulfur content of 0.13 mass% appears around 370°C.

[0038] The nickel powder in Figure 3 (Example 5) shows 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 range of sulfur and oxygen contents in 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 decrease in ΔTG appears at 390°C or below, debinding occurs at low temperatures due to O generated from the nickel powder.

[0039] The sulfur content may be below the detection limit of an analytical instrument used for content analysis, such as a sulfur analyzer using a combustion method.

[0040] In other words, when the oxygen content in the nickel powder is 1.1% by mass or less, if 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.

[0041] Furthermore, when the oxygen content in the nickel powder exceeds 1.1% by mass, the value obtained by multiplying the percentage of the sulfur content in the nickel powder by the percentage of the oxygen content is 0.15 or less, and therefore the peak of the decrease in ΔTG does not appear below 390°C. As a result, debinding does not occur at low temperatures, and structural defects such as cracks caused by combustion gases do not occur.

[0042] In addition, the nickel powder of this embodiment has an HO adsorption amount per unit area of ​​0.10 mg / m at a relative pressure P / P = 0.5. 2 More than 0.70mg / m 2 It is desirable that the concentration is less than 0.11 mg / m 2 More than 0.6mg / m2 Here, the relative pressure refers to the ratio of the adsorption equilibrium pressure, which is the pressure at adsorption equilibrium at a certain adsorption temperature, to the saturated vapor pressure.

[0043] The wettability of the nickel powder with the solvent or vehicle changes depending on the hydrophilicity or hydrophobicity of the surface, which has a significant effect on the disintegration and dispersibility of agglomerates of fine powder, especially as it becomes increasingly finer. The H2O adsorption capacity of nickel powder is 0.10 mg / (m 2 / g), the hydrophobicity is too strong and the wetting with the binder and solvent is reduced, which is not preferable. 2 / g), the hydrophilicity becomes too strong and the affinity with the binder decreases, which is not preferable.

[0044] If it becomes difficult for the surfactant to adsorb to the nickel powder surface, the wettability with the solvent or vehicle may deteriorate, which may result in insufficient disintegration of aggregates and insufficient suppression of re-agglomeration, resulting in a decrease in the viscosity stability of the conductive paste and poor surface smoothness of the dried film, which is undesirable.

[0045] When nickel powder is used as the conductive powder, the surface composition preferably contains 50 mol% or more of NiO. If the amount of NiO is less than 50 mol%, the dispersant and binder resin are not properly adsorbed, which results in a decrease in viscosity stability, which is undesirable.

[0046] <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.

[0047] The nickel particles used here preferably have nickel hydroxide and the like on their surfaces but are not exposed to the atmosphere after particle generation, and nickel particles generated by a wet method before filtration are preferred. Of course, nickel particles generated by a dry method such as a gas-phase reduction method or a plasma method or a spray pyrolysis method can be washed to form a slurry, and the method described below can be applied. However, nickel particles generated by these methods, such as a dry method, 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").

[0048] The nickel slurry may be washed by adding a washing liquid or solvent to remove impurities contained in the nickel particles or solvent. Any known washing method can be used as the washing method, as long as the nickel particles in the slurry are not exposed to the atmosphere. For example, repeated addition and decantation of the washing liquid or solvent is preferable.

[0049] The nickel slurry is filtered to form a nickel cake, which is then dried to obtain nickel powder. The filtration method is not particularly limited and any known method can be used, but it is preferable that the wet nickel cake is subjected to the drying treatment so that the nickel particles are not directly exposed to the air.

[0050] As the drying treatment, known heat drying in which the atmosphere can be adjusted or heat drying under reduced pressure can be used, 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. By heat drying in such a low-oxygen atmosphere, the progress of oxidation slows down, and the amount of substance of Ni in the surface composition decreases. 13 mol% or more Nickel powder containing 35 mol % or less can be obtained.

[0051] 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 Ni(OH)2, one of the surface components, 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.

[0052] The drying time for the drying treatment is not particularly limited, but is preferably set to 20 minutes to 24 hours. By adjusting these drying treatment conditions, the surface composition measured using XPS can be adjusted to 13 mol% or more An oxide film is formed on the nickel powder, in which the amount of NiO is 35 mol % or less, the amount of NiO is 50 mol % or more, and the amount of Ni(OH)2 is greater than 0 mol % and less than 30 mol %.

[0053] Since the nickel powder that has undergone the drying treatment may have particles bonded to each other due to dry agglomeration, it is preferable to perform a crushing treatment to loosen the agglomerated nickel particles. In the crushing treatment, 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 applied.

[0054] 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.

[0055] 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, an 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.

[0056] The nickel salt is not particularly limited as long as it is a nickel salt that is readily soluble in water, and one or more types selected from nickel chloride, nickel sulfate, and nickel nitrate can be used.

[0057] By adding a metal more noble than nickel to a nickel salt solution, the metal more noble than nickel acts as a nucleating agent, being reduced first to form initial nuclei during reduction crystallization of nickel. Particle growth from these initial nuclei results in the production of fine nickel particles. Examples of metal salts of metals 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 ppm] (moles of palladium salt / moles of nickel × 106) is preferably in the range of 0.2 mol ppm to 100 mol ppm.

[0058] The reducing agent is preferably hydrazine, and in addition to anhydrous hydrazine, hydrazine hydrate, which is a hydrazine hydrate, may also be used.

[0059] The alkali hydroxide is used as a pH adjuster to increase alkalinity, since the reducing power of hydrazine increases as the pH of the reaction solution increases. The alkali hydroxide is preferably an alkali metal hydroxide, 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.

[0060] Amine compounds are well known for their ability to accelerate reduction reactions as complexing agents. However, certain amine compounds are also effective in inhibiting the self-decomposition of hydrazine and inhibiting the bonding of nickel particles together, making them preferable for incorporation into the reaction solution. Amine compounds with the effect of inhibiting the self-decomposition of hydrazine are compounds containing two or more primary amino groups (-NH2) or secondary amino groups (-NH-) in the molecule. Specifically, ethylenediamine, diethylenetriamine, propylenediamine, etc. are preferable. The amount of the amine compound is in the range of 0.01 mol% to 5 mol%, more preferably 0.03 mol% to 2 mol%, in terms of the molar ratio of the amine compound [mol %] (moles of amine compound / moles of nickel × 100).

[0061] Sulfide compounds are compounds containing one or more sulfide groups (-S-) in the molecule. When used in combination with the specific amine compounds, they significantly enhance the inhibitory effect on 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) in the molecule. Specifically, L-methionine, L-ethionine, thiodiglycolic acid, etc. are preferred. The amount of sulfide compound blended is 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 × 100).

[0062] In the crystallization procedure, the reduction reaction is initiated when the aqueous solutions of a water-soluble nickel salt, a salt of a metal nobler than nickel, hydrazine, and an alkali hydroxide are all mixed together to prepare a reaction solution. The order in which the aqueous solutions are mixed until the reaction solution is prepared is not particularly limited, and they may be mixed in any order. The amine compound and sulfide compound may be added to at least one of the aqueous solutions before the reduction reaction is initiated, that is, during the preparation, or to the reaction solution after the reduction reaction has begun.

[0063] The temperature during crystallization is set to a range of 40°C to 90°C, more preferably 50°C to 80°C, when the reaction solution is prepared, that is, when the reduction reaction starts. [Example]

[0064] 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 HO adsorption, were evaluated as follows.

[0065] (Average particle size) The nickel powder was observed using a scanning electron microscope (SEM, manufactured by JEOL Ltd., JSM-7100F) to obtain an image (SEM image), and the diameter of particles whose entire shape could be confirmed was determined from the photograph by image analysis. The number average particle size was taken as the average particle size.

[0066] (Surface composition) The surface composition of the nickel powder was determined using an XPS (Versa Probe II, manufactured by ULVAC-PHI, Inc.) by performing waveform separation using peak fitting from the Ni2p spectrum of nickel 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 ratio of these areas.

[0067] (Oxygen content) The measurement was carried out using an oxygen analyzer (TC436, manufactured by LECO Corporation) using an inert gas fusion method.

[0068] (Sulfur content) Measurement was carried out using a sulfur analyzer (CS600, manufactured by LECO Corporation) using a combustion method.

[0069] (H2O adsorption amount) After vacuum degassing the nickel powder for 8 hours at 25°C, the specific surface area of ​​the nickel powder was measured using a specific surface area and pore size distribution analyzer (Microtrackbell, BELSORP-miniX) with nitrogen as the adsorbate. Furthermore, after vacuum degassing the nickel powder for 8 hours at 25°C, the adsorption / desorption isotherm of the nickel powder was measured using a high-precision vapor adsorption analyzer (Microtrackbell, BELSORP-AQUA 3). The amount of H2O adsorption at a relative pressure (P / P0) of 0.5 was determined from the nickel powder adsorption / desorption isotherm. The obtained amount of H2O adsorption at a relative pressure (P / P0) of 0.5 was divided by the specific surface area to calculate the amount of H2O adsorption per unit area.

[0070] (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 the 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 the nickel mass), a 0.5 ppm palladium (II) chloride ammonium aqueous solution (based on the 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 the nickel mass) was added to the reaction solution to crystallize nickel particles. It was confirmed that all of the nickel components in the solution had been reduced to metallic nickel.

[0071] [Cleaning and filtration process] The nickel slurry obtained by the crystallization process 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 treat the surface of the nickel particles, and then the slurry was left to settle, and the supernatant was removed by decantation. During decantation, enough liquid was left behind to prevent the nickel particles from being exposed to the atmosphere.

[0072] The nickel slurry was then transferred to a Tanabe Willtec centrifuge (model CO-24), where it was filtered and washed while adjusting the flow rate of pure water so that the nickel particles were not exposed to the air at all times, and a wet nickel cake was obtained by solid-liquid separation.

[0073] [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 the temperature rise until the drying temperature was reached and then the temperature was 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 the pressure to atmospheric pressure, completing the drying and obtaining nickel powder.

[0074] [Paste making and preparation of TG-DTA evaluation samples] The resulting nickel powder was used to prepare an internal electrode paste. The internal electrode paste was formulated with 100 parts by weight of nickel powder, 4 parts by weight of ethyl cellulose, 2 parts by weight of polyvinyl butyral resin, and 10 parts by weight of barium titanate powder. The ethyl cellulose and polyvinyl butyral resin were dissolved in terpineol to prepare a vehicle, which was then added during the preparation of the internal electrode paste. The resulting 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 resulting dried film was crushed in a mortar and used as a TG-DTA evaluation sample. The TG-DTA sample consisted of nickel powder, ethyl cellulose, polyvinyl butyral resin, and barium titanate powder. This composition was nearly identical to the composition of the dried film obtained when the internal electrode paste was printed on a dielectric green sheet and then dried, i.e., the dried film of the internal electrode paste when fired.

[0075] [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.

[0076] 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 a rate of 5°C / min, held at 310°C for 3 hours, and then cooled to room temperature. The temperature profiles for the second debinding process were also set: a first debinding process in which the temperature was increased from room temperature to 800°C at a rate of 5°C / min, and then cooled to room temperature. The atmosphere for the first debinding process was N2 gas, and the atmosphere for 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%.

[0077] [Table 1]

[0078] Furthermore, the amount of H2O 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 H2O adsorption is mg / m 2 is.

[0079] [Table 2]

[0080] As shown in Table 1, in Examples 1 to 5, which have predetermined sulfur and oxygen contents, a peak of ΔT G is observed at a temperature higher than about 390° C. On the other hand, in Comparative Example 1, which has sulfur and oxygen contents outside the ranges of this embodiment, a peak of ΔT G is observed at a temperature lower than about 390° C. When a multilayer ceramic capacitor is manufactured using an internal electrode paste using the nickel powder according to Comparative Example 1, defects such as cracks may occur.

[0081] 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. Moreover, it was found that the amount of H2O adsorption per unit area was the lowest in Comparative Example 1.

Claims

1. A nickel powder having an oxide coating mainly composed of NiO on its surface, The surface composition of the nickel powder measured using X-ray photoelectron spectroscopy (XPS) was Ni, NiO, and Ni(OH). ２ The amount of Ni is 13 mol % or more and 35 mol % or less, the amount of NiO is 50 mol % or more, and Ni(OH) ２ is more than 0 mol % and not more than 30 mol %, 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.

2. The nickel powder according to claim 1, wherein the average particle size is 0.03 μm to 0.4 μm.

3. Relative pressure P / P ０ H per unit area at =0.5 ２ O adsorption amount 0.10 mg / m ２ 0.70mg / m or more ２ The nickel powder according to claim 1 or claim 2, wherein:

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