TWI903181B - Nickel particles and their manufacturing methods, and multilayer ceramic capacitors - Google Patents

Abstract

This invention provides nickel particles having a surface region comprising an alloy of nickel and metallic element M. Metallic element M is selected from at least one of bismuth, copper, iron, and molybdenum. The content of metallic element M relative to the total nickel particles is 0.09% by mass or more and 15.8% by mass or less. When measuring the nickel particles in the depth direction from the outermost surface to a sputtering depth of 5 nm (converted from _SiO2) using X-ray photoelectron spectroscopy, the maximum value of the ratio of the number of metal element M atoms in this region to the total number of nickel atoms is set as X (at%). When measuring the nickel particles using ICP emission spectrophotometry, the ratio of the number of metal element M atoms to the total number of nickel atoms is set as Y (at%). In this case, the value of X/Y is between 0.5 and 35.

Description

Nickel particles and their manufacturing methods, and multilayer ceramic capacitors

This invention relates to nickel particles and a method for their manufacture.

Nickel particles are generally used to form the internal electrodes of multilayer ceramic capacitors (MLCCs) used in electronic devices. During the manufacturing of MLCCs, when the multilayer containing the nickel particles and the dielectric layer are simultaneously calcined, defects can sometimes occur in the internal electrodes due to differences in the sintering temperatures of the raw materials. To prevent this defect, it is necessary to improve the sintering resistance of the nickel particles.

For example, Patent 1 discloses a technique for using nickel powder containing tin or bismuth, obtained by PVD (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition), to form the internal electrodes of MLCCs. According to this document, by adding non-magnetic metals such as tin to the nickel powder, the crystal structure of nickel is deformed, thereby increasing the sintering temperature of the nickel powder.

Patent document 2 discloses a technique for forming internal electrodes of MLCCs using nickel powder with a generally spherical particle shape and surface-treated with tin. Furthermore, the document also discloses the use of bismuth in addition to tin for surface treatment. According to this document, the sintering behavior is improved using the nickel powder described therein.

Previous technical literature Patent Documents

Patent Document 1: International Publication No. 2014/080600 (Specification)

Patent Document 2: Japanese Patent Application Publication No. 2018-104819

Furthermore, with the increasing performance of electronic devices in recent years, there is a growing demand for MLCCs to prevent defects caused by internal electrodes. To meet this requirement, in addition to improved sintering resistance, it is also desirable to avoid excessively increasing the resistance of internal electrodes when using nickel particles to form them.

Therefore, the present invention aims to provide nickel particles with high sintering resistance without excessively increasing resistance.

This invention provides nickel particles having a surface region comprising an alloy containing nickel and a metallic element M, wherein the metallic element M is selected from at least one of bismuth, copper, iron, and molybdenum, and the content of the metallic element M relative to the total nickel particles is 0.09% by mass to 15.8% by mass. When measuring the region from the outermost surface to a sputtering depth of 5 nm (converted from _SiO₂) in the depth direction of the nickel particles using X-ray photoelectron spectroscopy, the maximum value of the ratio of the number of metallic element M atoms in that region to the total number of nickel atoms and metallic element M atoms is set as X (at%). The method is then analyzed using ICP (Inductively Coupled Plasma Phosphate). When determining nickel particles using the Plasma (Inductively Coupled Plasma) luminescence spectrophotometry method, the ratio of the number of atoms of metal element M to the total number of atoms of nickel and metal element M is set as Y (at%). In this case, the value of X/Y is between 0.5 and 35.

Furthermore, this invention provides a method for manufacturing nickel particles, which involves heating a mixture comprising nickel hydroxide particles, a polyol, polyvinylpyrrolidone, and polyethyleneimine to produce nickel particles. The method uses 30 to 200 parts by mass of polyvinylpyrrolidone relative to 1 part by mass of polyethyleneimine. The nickel hydroxide particles are reduced to nickel masterbatch particles by heating. While a portion of the nickel hydroxide particles remains, the mixture is mixed with a compound of metallic element M, and the compound is reduced to metallic M. This forms a surface region on the nickel masterbatch particles containing an alloy of nickel and metallic element M, wherein metallic element M is selected from at least one of bismuth, copper, iron, and molybdenum.

The present invention will now be described based on its preferred embodiment. The nickel particles of the present invention comprise nickel mother particles and a surface region on the surface of the mother particles containing an alloy of nickel and metallic element M (hereinafter also referred to as a "nickel-metal M alloy"). The "nickel mother particles" in this specification are substantially composed of nickel, with the remaining portion consisting of particles containing unavoidable elements. Unavoidable elements include, for example, oxygen and carbon elements derived from atmospheric oxygen or carbon dioxide, and nitrogen elements introduced during the manufacturing process of the nickel particles.

The nickel parent particles in the nickel particles have surface regions comprising a nickel-metal M alloy. In this specification, "nickel-metal M alloy" refers to a nickel-based alloy containing the metal element M. The nickel-metal M alloy is essentially an alloy of nickel and metal element M, with the remaining portion containing unavoidable elements. In the surface regions comprising the nickel-metal M alloy, metal element M can exist, in part, as an elemental form (i.e., a metallic state). Alternatively, metal element M can exist, in part, as a compound form. Alternatively, metal element M can exist in two or more combinations thereof. When metallic element M exists in the surface region of a nickel-metal M alloy in the form of a compound of metallic element M, examples of such compounds include, but are not limited to, oxides, hydroxides, sulfides, sulfates, borides, and phosphides containing metallic M. However, from the viewpoint of maximizing the advantages inherent in the nickel particles of this invention, the metallic element M in the surface region of the nickel-metal M alloy should substantially consist only of its alloy with nickel. In this specification, "substantially consist only of its alloy with nickel" means excluding the intentional inclusion of metallic element M other than its alloy with nickel in the aforementioned surface region, and allowing trace amounts of elemental or compound metallic element M to inevitably mix in during the manufacturing process of the nickel particles.

The metallic element M in the nickel particles is preferably selected from at least one of bismuth, copper, iron, and molybdenum. By using bismuth, copper, iron, or molybdenum as the metallic element M, the sintering resistance of the nickel particles can be further improved without excessively increasing it. The metallic element M may be only one of bismuth, copper, iron, and molybdenum, or any combination of two or more of them. In the following description, when referred to as metallic element M (or metal M), it means, depending on the context, bismuth, copper, iron, or molybdenum, or any combination of two or more of these.

The presence of nickel-metal M alloy in the surface region of nickel particles can be confirmed by the following methods.

Specifically, firstly, X-ray photoelectron spectrometry (XPS) confirms that the nickel particles contain a metallic element M in their surface region, and that the metallic element M is primarily in a metallic state. Next, it is confirmed that the a-axis length of the X-ray diffraction peak of the aforementioned nickel particles exceeds the a-axis length previously obtained from measurements only on the nickel particles. An extended a-axis length in the X-ray diffraction peak indicates solid solution. Therefore, in addition to the presence of metallic element M in a metallic state in the surface region of the nickel particles as confirmed by XPS measurements, the presence of a nickel-metal M alloy in the surface region of the nickel particles can also be confirmed by comparing the a-axis lengths of metallic element M with those of nickel solid solution.

The proportion of metallic element M contained in the surface region of nickel particles can be determined by XPS. Specifically, when measuring the region from the outermost surface of the nickel particles to a sputtering depth of 5 nm (converted to _SiO2) in the depth direction (hereinafter, this region is also referred to as the "particle surface region") by XPS, the maximum value X of the ratio of the number of metallic element M atoms to the total number of nickel atoms in this particle surface region is preferably 0.5 at% or more. The aforementioned "maximum value" refers to the maximum value of X when multiple X values measured along the thickness direction of the particle surface region are different. From the viewpoint of further improving the sintering resistance of the nickel particles, it is preferable that metallic element M is present in a manner where the X value is 0.5 at% or more.

When the metallic element M is bismuth, following the same principles as above, the value of X (at%) is preferably 1 at% or higher, more preferably 2 at% or higher, more preferably 3 at% or higher, especially preferably 7 at% or higher, and particularly preferably 14 at% or higher. Furthermore, the value of X (at%) is preferably 70 at% or lower, more preferably 35 at% or lower, more preferably 30 at% or lower, especially preferably 20 at% or lower, and particularly preferably 15 at% or lower.

When the metallic element M is copper, following the same principles as above, the value of X (at%) is preferably 1 at% or higher, more preferably 2 at% or higher, more preferably 4 at% or higher, especially preferably 8 at% or higher, and particularly preferably 12 at% or higher. Furthermore, the value of X (at%) is preferably 70 at% or lower, more preferably 35 at% or lower, more preferably 20 at% or lower, and particularly preferably 14 at% or lower.

When the metallic element M is iron, following the same principles as above, the value of X (at%) is preferably 1 at% or higher, more preferably 2 at% or higher, more preferably 4 at% or higher, and especially preferably 7 at% or higher. Furthermore, the value of X (at%) is preferably 70 at% or lower, more preferably 35 at% or lower, more preferably 30 at% or lower, especially preferably 20 at% or lower, and particularly preferably 9 at% or lower.

When the metallic element M is molybdenum, following the same principles as above, the value of X (at%) is preferably 1 at% or higher, more preferably 2 at% or higher, more preferably 4 at% or higher, and especially preferably 8 at% or higher. Furthermore, the value of X (at%) is preferably 70 at% or lower, more preferably 35 at% or lower, more preferably 30 at% or lower, and especially preferably 10 at% or lower.

The method for determining the value of X is illustrated in the following embodiments.

Regarding the aforementioned "outermost surface of nickel particles," when a surface treatment agent such as an organic acid or amine is present on the surface of the nickel particles, it refers to the outermost part of the nickel particles containing that surface treatment agent. When no surface treatment agent is present on the surface of the nickel particles, it refers to the surface of the particles themselves.

The nickel particles preferably contain a metallic element M in an amount of 0.09% to 15.8% by mass relative to the total nickel particles. By keeping the content of metallic element M within this range relative to the nickel particles, the sintering resistance can be further improved without excessively increasing the electrical resistance of the nickel particles.

When the metallic element M is bismuth, following the same principles as above, the content of bismuth relative to the total nickel particles is preferably 0.3% by mass or more, more preferably 0.4% by mass or more, more preferably 1% by mass or more, and particularly preferably 6.7% by mass or more. Furthermore, the content of bismuth relative to the total nickel particles is preferably 15.8% by mass or less, more preferably 13% by mass or less, more preferably 11.4% by mass or less, and particularly preferably 10% by mass or less.

When the metallic element M is copper, following the same principles as above, the copper content relative to the total nickel particles is preferably 0.4% by mass or more, more preferably 1% by mass or more, more preferably 2.1% by mass or more, and even more preferably 4.3% by mass or more. Furthermore, the copper content relative to the total nickel particles is preferably 11.4% by mass or less, more preferably 7.6% by mass or less, more preferably 6.5% by mass or less, even more preferably 6% by mass or less, and particularly preferably 5.4% by mass or less.

When the metallic element M is iron, following the same principles as above, the iron content relative to the total nickel particles is preferably 0.09% by mass or more, more preferably 0.28% by mass or more, more preferably 0.40% by mass or more, and even more preferably 0.47% by mass or more. Furthermore, the iron content relative to the total nickel particles is preferably 11.4% by mass or less, more preferably 6% by mass or less, more preferably 2.87% by mass or less, even more preferably 1.91% by mass or less, and particularly preferably 0.96% by mass or less.

When the metallic element M is molybdenum, following the same principles as above, the content of molybdenum relative to the total nickel particles is preferably 0.4% by mass or more, more preferably 1% by mass or more, more preferably 1.1% by mass or more, and particularly preferably 1.6% by mass or more. Furthermore, the content of molybdenum relative to the total nickel particles is preferably 11.4% by mass or less, more preferably 6.4% by mass or less, more preferably 6% by mass or less, particularly preferably 4.9% by mass or less, and especially preferably 3.3% by mass or less.

The content of the metal element M relative to the total nickel particles can be determined by the following ICP-based emission spectrophotometry.

The nickel particles of this invention are provided with the condition that the content of metallic element M relative to the total nickel particles meets the aforementioned range. Preferably, the ratio (at%) of the number of metallic element M atoms to the total number of nickel atoms and metallic element M atoms in the total nickel particles is 0.1at% to 7at%. From the viewpoint of further improving sintering resistance without excessively increasing the electrical resistance of the nickel particles, it is preferable that metallic element M exists with the value of Y within this range.

When the metallic element M is bismuth, following the same principles as above, the value of Y is preferably 0.1 at% or higher, more preferably 0.2 at% or higher, more preferably 0.3 at% or higher, particularly preferably 0.5 at% or higher, and especially preferably 2 at% or higher. Furthermore, the value of Y is preferably 6 at% or lower, more preferably 5 at% or lower, more preferably 4 at% or lower, and especially preferably 3 at% or lower.

When the metallic element M is copper, following the same principles as above, the value of Y is preferably 0.2 at% or higher, more preferably 0.5 at% or higher, more preferably 1 at% or higher, particularly preferably 2 at% or higher, and especially preferably 4 at% or higher. Furthermore, the value of Y is preferably 7 at% or lower, more preferably 6 at% or lower, and more preferably 5 at% or lower.

When the metallic element M is iron, following the same principles as above, the value of Y is preferably 0.1 at% or higher, more preferably 0.2 at% or higher, more preferably 0.3 at% or higher, and especially preferably 0.5 at% or higher. Furthermore, the value of Y is preferably 6 at% or lower, more preferably 3 at% or lower, more preferably 2 at% or lower, and especially preferably 1 at% or lower.

When the metallic element M is molybdenum, following the same principles as above, the value of Y is preferably 0.2 at% or higher, more preferably 0.3 at% or higher, more preferably 0.5 at% or higher, particularly preferably 0.7 at% or higher, and especially preferably 1 at% or higher. Furthermore, the value of Y is preferably 6 at% or lower, more preferably 4 at% or lower, more preferably 3 at% or lower, and especially preferably 2 at% or lower.

The ratio Y of the number of atoms of metallic element M contained in a nickel particle can be determined by ICP-ELISA. Specifically, firstly, the nickel particle is analyzed by ICP-ELISA to determine the content ratio of nickel and metallic element M. Then, the nickel content ratio (mass %) is divided by the atomic weight of nickel (58.7) to convert the content ratio into the number of nickel atoms, _ΔNi . Finally, the metallic element content ratio (mass %) is divided by the atomic weight of metallic element M (209 for bismuth, 63.6 for copper, 55.9 for iron, and 96 for molybdenum) to convert the content ratio into the number of metallic element M atoms, _ΔM . Then, calculate the ratio of the number of atoms of metal element M to the number of atoms of nickel element _ANi and the number of atoms of metal element _M Am ( _Am / ( _ANi + _Am ) × 100), and obtain the value of Y mentioned above.

The inventors' research shows that the relationship between the values of X and Y affects the sintering resistance of nickel particles. Specifically, by setting the X/Y value to between 0.5 and 35, the temperature at which nickel particles begin to shrink due to sintering increases, thus improving sintering resistance. When using the nickel particles of this invention, which exhibit higher sintering resistance, in the manufacture of materials such as MLCCs, the baking step, a key manufacturing step, can be performed to ensure that the temperature at which the internal electrodes shrink due to nickel particle sintering is as close as possible to the temperature at which the dielectric layer shrinks due to dielectric particle sintering. The advantage of reducing the temperature difference between the internal electrodes and the dielectric layer during the heating process of MLCCs lies in preventing the overlap of their shrinkage times. Specifically, this effectively prevents structural defects such as cracking or delamination (interlayer peeling at the interface between the internal electrodes and the dielectric layer) caused by differences in the temperature or shrinkage rate of the internal electrodes and dielectric layer during the MLCC's baking process.

When the metallic element M is bismuth, from the viewpoint of making the above advantages even more significant, the X/Y value of the nickel particles is preferably 1.5 or higher, more preferably 3.7 or higher, more preferably 4 or higher, especially preferably 5 or higher, and particularly preferably 7 or higher. The X/Y value of the nickel particles is preferably 30 or lower, more preferably 25 or lower, and more preferably 20 or lower.

When the metallic element M is copper, from the viewpoint of making the above advantages even more significant, the X/Y value of the nickel particles is preferably 0.5 or higher, more preferably 1 or higher, more preferably 1.5 or higher, and especially preferably 2 or higher. The X/Y value of the nickel particles is preferably 30 or lower, more preferably 15 or lower, more preferably 13 or lower, especially preferably 10 or lower, particularly preferably 7 or lower, and most preferably 3 or lower.

When the metallic element M is iron, from the viewpoint of making the above advantages even more significant, the X/Y value of the nickel particles is preferably 1 or more, further preferably 1.5 or more, further preferably 3.7 or more, especially preferably 5 or more, and particularly preferably 10 or more. The X/Y value of the nickel particles is preferably 30 or less, further preferably 25 or less, further preferably 20 or less, and particularly preferably 15 or less.

When the metallic element M is molybdenum, from the viewpoint of making the above advantages even more significant, the X/Y value of the nickel particles is preferably 1 or higher, more preferably 1.5 or higher, more preferably 3 or higher, especially preferably 3.7 or higher, and particularly preferably 5 or higher. The X/Y value of the nickel particles is preferably 30 or lower, more preferably 15 or lower, more preferably 13 or lower, especially preferably 10 or lower, and particularly preferably 7 or lower.

In the particle surface region, the ratio of the number of metal element M atoms to the total number of nickel and metal element M atoms can be fixed or varied in the depth direction. When the ratio is not fixed in the depth direction, it decreases continuously or in a stepwise manner from the surface of the nickel particle towards the center. In particular, for further improvement of the sintering resistance of the nickel particles, it is preferable that when measuring the region from the outermost surface of the nickel particle to a sputtering depth of 20 nm (converted from _SiO2) using XPS, the ratio decreases from the outermost surface towards the sputtering depth of 20 nm. In this case, in terms of further improving the sintering resistance of nickel particles, it is preferable that when the maximum value of the above ratio in the region from the outermost surface of the nickel particles to a sputtering depth of 5 nm is set as X, and the maximum value of the above ratio at a sputtering depth of 20 nm is set as X1, the value of X/X1 is 0.1 or more and 15 or less.

When the metallic element M is bismuth, following the same principles as above, the value of X/X1 is preferably 1 or higher, more preferably 1.5 or higher, and even more preferably 2 or higher. Furthermore, the value of X/X1 is preferably 10 or lower, more preferably 7.8 or lower, even more preferably 6.1 or lower, particularly preferably 4 or lower, especially preferably 3 or lower, and most preferably 2.5 or lower.

When the metallic element M is copper, following the same principles as above, the value of X/X1 is preferably 0.1 or higher, more preferably 0.5 or higher, and even more preferably 1 or higher. Furthermore, the value of X/X1 is preferably 10 or lower, more preferably 7.8 or lower, even more preferably 6.1 or lower, particularly preferably 5 or lower, and especially preferably 3 or lower.

When the metallic element M is iron, following the same principles as above, the value of X/X1 is preferably 0.1 or higher, more preferably 0.5 or higher, and even more preferably 1 or higher. Furthermore, the value of X/X1 is preferably 10 or lower, more preferably 7.8 or lower, even more preferably 6.1 or lower, particularly preferably 5 or lower, and especially preferably 2 or lower.

When the metallic element M is molybdenum, following the same principles as above, the value of X/X1 is preferably 0.1 or higher, more preferably 1 or higher, and even more preferably 2 or higher. Furthermore, the value of X/X1 is preferably 10 or lower, more preferably 7.8 or lower, even more preferably 6.1 or lower, particularly preferably 5 or lower, and especially preferably 3 or lower.

The method for measuring X1 is illustrated in the following embodiments.

When the metallic element M is bismuth, regarding the value of X1 itself, from the viewpoint of further improving the sintering resistance of nickel particles, it is preferably 0.2 or higher, more preferably 0.5 or higher, more preferably 0.7 or higher, especially preferably 1.7 or higher, particularly preferably 2 or higher, and most preferably 5 or higher. Furthermore, the value of X1 itself is preferably 15 or lower, more preferably 10 or lower, and even more preferably 7 or lower.

When the metallic element M is copper, regarding the value of X1 itself, from the viewpoint of further improving the sintering resistance of nickel particles, it is preferably 0.2 or higher, more preferably 0.5 or higher, more preferably 0.7 or higher, especially preferably 1 or higher, particularly preferably 1.7 or higher, particularly preferably 3 or higher, and most preferably 5 or higher. Furthermore, the value of X1 itself is preferably 20 or lower, more preferably 15 or lower, and more preferably 10 or lower.

When the metallic element M is iron, regarding the value of X1 itself, from the viewpoint of further improving the sintering resistance of nickel particles, it is preferably 0.2 or higher, more preferably 0.5 or higher, more preferably 0.7 or higher, especially preferably 1 or higher, particularly preferably 1.7 or higher, particularly preferably 2 or higher, and most preferably 4 or higher. Furthermore, the value of X1 itself is preferably 15 or lower, more preferably 10 or lower, and more preferably 6 or lower.

When the metallic element M is molybdenum, regarding the value of X1 itself, from the viewpoint of further improving the sintering resistance of nickel particles, it is preferably 0.2 or higher, more preferably 0.5 or higher, more preferably 0.7 or higher, especially preferably 1 or higher, particularly preferably 1.7 or higher, even more preferably 2 or higher, and most preferably 4 or higher. Furthermore, the value of X1 itself is preferably 15 or lower, more preferably 10 or lower, even more preferably 6 or lower, and especially preferably 5 or lower.

The cumulative particle size _D50 of the nickel particles of the present invention, when the cumulative number of nickel particles is 50%, is preferably 20 nm to 200 nm. In other words, the nickel particles of the present invention are preferably microparticles. By keeping the particle size _D50 of the nickel particles within this range, when the nickel particles of the present invention are used for various applications, such as as internal electrodes of MLCCs, there is an advantage that short circuits are less likely to occur between the internal electrodes. From the viewpoint of making this advantage more significant, the particle size _D50 of the nickel particles is more preferably 20 nm to 150 nm, more preferably 40 nm to 150 nm, and more preferably 40 nm to 100 nm. The particle size _D50 of nickel particles was measured by observing the particles using scanning electron microscopy (SEM). Specifically, the nickel particles were photographed at 50,000x magnification using SEM, and the area of the photographed particles was determined. The equivalent circular diameter was calculated from this area. The particle size distribution was then determined based on the calculated equivalent circular diameter. The horizontal axis of the particle size distribution curve represents the equivalent circular diameter, and the vertical axis represents the frequency of particles. In the particle size distribution curve obtained in this way, the cumulative particle size at 50% cumulative number of particles is defined as _D50 .

When obtaining the aforementioned "particle size distribution curve," the circular equivalent diameter of more than 5000 nickel particles was calculated. The circular equivalent diameter was calculated using image resolution particle size distribution measurement software (Mac-View manufactured by Mountech Co., Ltd.). The smallest unit of the nickel particle being observed was determined by whether a particle interface, identified as an independent particle, was observed using SEM. Therefore, even if an agglomerate containing multiple particles was observed, if a particle interface was observed within that agglomerate, the area defined by that particle interface was considered a single particle.

The nickel particles of this invention are preferably microparticles, with a low proportion of coarse particles. When the nickel particles of this invention are used, for example, in the internal electrodes of MLCCs, the presence of coarse particles can sometimes be a cause of short circuits between the internal electrodes. By reducing the proportion of coarse particles in the nickel particles, this short circuit can be effectively prevented. From this perspective, the proportion of particles with a diameter of 1.5 times or more than _D50 (hereinafter also referred to as the "coarse particle proportion") in the nickel particles of this invention is preferably 0.5% or less, more preferably 0.3% or less, and even more preferably 0.1% or less.

The closer the percentage of coarse particles is to 0%, the more effectively short circuits between internal electrodes can be prevented. If the percentage of coarse particles is as low as approximately 0.01%, short circuits between internal electrodes can be effectively prevented.

The reason for selecting particles with a diameter greater than 1.5 times that of _D50 as the size of coarse particles is that the inventors have found that a particle size greater than 1.5 times that of _D50 will cause the surface of the conductive film to be rough when forming the conductive film, which is closely related to the short circuit between the internal electrodes of the MLCC.

The nickel particles of this invention are preferably microparticles with a low proportion of coarse particles, and their particle size is as uniform as possible. In other words, the particle size distribution curve is preferably steep. The steepness of the particle size distribution curve can be evaluated by the coefficient of variation of the particle size. The coefficient of variation is a value defined by (σ/D ₅₀ )×100 (%) when the standard deviation of the particle size in the particle size distribution is set to σ (nm). From the viewpoint of reducing the surface roughness of the conductive film formed by the nickel particles, the value of the coefficient of variation of the nickel particles of this invention is preferably 14% or less. From the perspective of further reducing the surface roughness of the conductive film, the variation coefficient is preferably below 13%, and even more preferably below 12%.

The closer the coefficient of variation is to 0%, the more it helps reduce the surface roughness of the conductive film. If the coefficient of variation is as low as about 8%, the surface roughness of the conductive film can be reduced to a sufficiently satisfactory level.

The nickel particles of this invention preferably possess high crystallinity. High crystallinity means that the nickel particles of this invention begin to shrink at an increased temperature during sintering. In other words, as described above, high crystallinity of nickel indicates that the nickel particles exhibit higher sintering resistance.

The method of evaluating the crystallinity of nickel using the ratio of crystallite size Cs (nm) to particle size _D50 (nm), Cs/ _{D50 ,} is commonly used in the field of metal powder technology. The larger the value of Cs/D50, the higher the crystallinity of nickel can be evaluated. From this point of view, in the nickel particles of the present invention, the value of Cs/ _D50 is preferably 0.3 or higher, more preferably 0.34 or higher, and even more preferably 0.37 or higher.

The higher the value of Cs/D ₅₀ , the higher the temperature at which nickel particles begin to shrink after sintering. Therefore, in this invention, if the value of Cs/D ₅₀ is preferably below 0.6, the temperature can be sufficiently increased. From this point of view, the value of Cs/D ₅₀ is more preferably below 0.55, and even more preferably below 0.52.

Regarding the crystallite size Cs itself, from the viewpoint of sufficiently increasing the temperature at which nickel particles begin to shrink after sintering, it is preferably 15 nm to 70 nm, more preferably 18 nm to 70 nm, and even more preferably 20 nm to 70 nm.

Various methods are known in the field of metal powder technology for measuring crystallite size. The crystallite size values in this specification are obtained by the WPPF (whole powder pattern fitting) method. Besides the WPPF method, the Seller method is also known for measuring crystallite size; however, in cases of significant crystallization deformation, the crystallite size values obtained using the Seller method lack reliability. Therefore, this invention adopts the less risky WPPF method.

The details of the method for determining nickel crystallite size based on the WPPF method are explained in the following examples.

The nickel particles of this invention preferably do not excessively increase resistance. When such nickel particles are used, for example, as internal electrodes in MLCCs, the performance of the MLCC can be further improved. Therefore, in order to avoid excessively increasing resistance, it is preferable to control the crystal structure of the nickel particles by increasing the pure nickel content in the nickel particles having a surface region comprising a nickel-metal M alloy. From this perspective, in the nickel particles of this invention, the a-axis length of the nickel lattice in the crystal structure is preferably 3.520 Å to 3.529 Å, more preferably 3.522 Å to 3.526 Å, further preferably 3.523 Å to 3.526 Å, and even more preferably 3.524 Å to 3.526 Å.

The a-axis length of the nickel particle lattice, as described in the following embodiments, can be determined using an X-ray diffraction apparatus for CuKα1 rays. For analytical determination, it is obtained using the WPPF method, as described in the following embodiments.

The crystallite size or a-axis length of the nickel lattice structure of this invention can be achieved, for example, by adjusting the ratio of metallic element M in the surface region of the nickel particles, or by reducing the thickness of the surface region containing the nickel-metal M alloy. Alternatively, it can be achieved by appropriately adjusting the conditions in the following nickel particle manufacturing method, or by appropriately adjusting the conditions in the following nickel particle manufacturing method, instead of the above operations.

The sintering resistance of the nickel particles of this invention can be evaluated using the nickel particles themselves via thermomechanical analysis (TMA). In this invention, the temperature at which the TMA shrinkage rate (%) reaches 5% based on room temperature (25°C) is defined as the initial shrinkage temperature. For further improving the sintering resistance of the nickel particles, this temperature is preferably 400°C or higher. From the viewpoint of making this advantage even more significant, it is more preferably 450°C or higher, further preferably 500°C or higher, even more preferably 550°C or higher, and most preferably 570°C or higher.

Next, a preferred method for manufacturing the nickel particles of this invention will be described. In this method, the nickel particles are manufactured using a so-called polyol method. The polyol method uses a polyol as a solvent that also acts as a reducing agent. In the polyol method, the nickel chemical species are heated in the presence of a polyol to undergo a reduction reaction to nickel master particles. Before the reduction reaction is completed, a compound of metal element M is mixed in and further heated to undergo a reduction reaction to metal M, thereby forming a surface region containing a nickel-metal M alloy in the nickel master particles.

In this manufacturing method, from the viewpoint of successfully obtaining the target nickel particles, nickel hydroxide is preferably used as the nickel chemical used to generate the nickel particles. Nickel hydroxide is added to a mixture comprising a polyol, polyvinylpyrrolidone (hereinafter also referred to as "PVP"), and polyethyleneimine (hereinafter also referred to as "PEI"). From a usability perspective, nickel hydroxide in a particulate form is preferred.

As mentioned above, the polyol contained in the mixture serves as a solvent and also as a reducing agent for nickel hydroxide.

Polyols such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propanediol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, and polyethylene glycol can be used. These polyols can be used alone or in combination of two or more. Ethylene glycol, in particular, has a higher proportion of hydroxyl groups in its molecular weight, resulting in higher reducing properties. Furthermore, it is liquid at room temperature, offering excellent usability, and is therefore preferred.

Regarding the amount of polyol used, considering its use as a reducing agent, it can be adjusted appropriately based on the amount of nickel hydroxide in the mixture, therefore no special limitation is needed. On the other hand, when functioning as a solvent, the properties of the mixture change depending on the concentration of the polyol, thus there is a certain suitable concentration range. From this perspective, the concentration of the polyol in the mixture is preferably set in the range of 50% by mass to 99.8% by mass.

PVP is used as a dispersant for nickel hydroxide. PVP is highly effective as a dispersant, resulting in a steeper particle size distribution of nickel particles produced by reduction. The molecular weight of these PVPs can be adjusted appropriately according to their water solubility and dispersing ability. The amount of PVP in the mixture is preferably between 0.01 and 30 parts by mass relative to 100 parts by mass of nickel (converted to nickel). By setting it within this range, the viscosity of the mixture can be increased excessively while achieving sufficient dispersion.

PEI serves the following purpose: during the formation of nickel nuclei in a mixture, it reduces the number of nickel ions in the mixture, preventing nucleus formation and growth from occurring simultaneously. The reasons are: (a) PEI possesses non-shared electron pairs that interact with nickel ions and can coordinate with them; (b) PEI has a large number of these non-shared electron pairs; and (c) PEI has hydrogen bond sites that can interact with the surface of nickel hydroxide present in an undissolved state in the mixture.

By introducing PEI into the mixture, nickel nucleation and the growth of the generated nuclei can proceed sequentially. As a result, nickel particles with uniform particle size are successfully obtained. In contrast, in the previous reduction method for producing nickel particles, nucleation and growth occurred simultaneously, which easily resulted in coarse particles and uneven particle size.

Based on the above considerations, using branched PEI is more advantageous than using linear PEI. Similarly, it is also preferable to use PEI with a number average molecular weight of 600 to 10,000, particularly 800 to 5,000, and especially 1,000 to 3,000.

In particular, in this manufacturing method, by setting the ratio of PVP to PEI in the mixture within a specific range, nickel nucleation and growth are reliably and sequentially carried out. Specifically, it is preferable to use PVP at a ratio of 30 to 200 parts by mass relative to 1 part by mass of PEI, more preferably 40 to 150 parts by mass, and even more preferably 50 to 130 parts by mass.

The amount of PEI in the mixture is set appropriately based on the PVP content, ensuring that the PVP to PEI ratio meets the above-mentioned range.

The mixture may also contain a noble metal catalyst. This allows for the initial formation of fine nuclei of the noble metal during the reduction phase, using these nuclei as a starting point for the successful reduction of nickel. The noble metal catalyst can be, for example, water-soluble salts of noble metals or other noble metal compounds. Examples of water-soluble salts of noble metals include those of palladium, silver, platinum, and gold. When using palladium as the noble metal, palladium chloride, palladium nitrate, palladium acetate, and palladium ammonium chloride can be used, for example. When using silver, silver nitrate, silver lactate, silver oxide, silver sulfate, silver cyclohexanoate, and silver acetate can be used, for example. When using platinum, for example, chloroplatinic acid, potassium chloroplatinate, sodium chloroplatinate, etc., can be used. When using gold, for example, chloroauric acid, sodium chloroaurate, etc., can be used. Among these, palladium nitrate, palladium acetate, silver nitrate, and silver acetate are suitable for use due to their low cost and good economic efficiency. The precious metal catalyst can be added in the form of the above-mentioned compounds or in the form of an aqueous solution of the compounds dissolved in water. The amount of precious metal catalyst contained in the mixture is preferably 0.01 parts by mass to 5 parts by mass, especially 0.01 parts by mass to 1 part by mass, relative to 100 parts by mass of nickel hydroxide converted to nickel.

The nickel hydroxide reduction is achieved by simultaneously stirring and heating the mixture containing the above components. The heating temperature depends on the type of polyol used, preferably between 150°C and 200°C, more preferably between 170°C and 200°C, and even more preferably between 190°C and 200°C, under atmospheric pressure. This facilitates the successful reduction of nickel hydroxide to nickel precursor particles.

Next, before the reduction reaction of nickel hydroxide is completed, a compound of metal element M is mixed into the above mixture. In other words, the compound of metal element M is mixed into the above mixture while a portion of nickel hydroxide remains. Here, "before the reduction reaction of nickel hydroxide is completed" means before the added nickel hydroxide is reduced to 80 mol% or more.

When the metal element M is bismuth, in the reduction reaction of the following metal element M compound, from the viewpoint of successfully forming a surface region containing a nickel-metal M alloy on the nickel masterbatch particles, the compound is preferably at least one selected from the group consisting of bismuth nitrate, bismuth chloride, bismuth nitrate pentahydrate, bismuth hydroxide, bismuth oxide, and bismuth carbonate, with bismuth chloride being particularly preferred.

When the metallic element M is copper, from the same perspective as above, the compound is preferably made of at least one selected from the group consisting of copper nitrate trihydrate, copper sulfate pentahydrate, copper acetate monohydrate, copper hydroxide, copper monoxide, and copper oxide, with copper sulfate pentahydrate being particularly preferred.

When the metallic element M is iron, from the same perspective as above, the compound is preferably made of at least one selected from the group consisting of ferric nitrate nonahydrate, ferric chloride hexahydrate, ferric sulfate heptahydrate, ferric hydroxide, and ferric oxide, with ferric sulfate heptahydrate being particularly preferred.

When the metallic element M is molybdenum, for the same reasons mentioned above, the compound is preferably made of at least one selected from the group consisting of sodium molybdate, potassium molybdate, calcium molybdate, and ammonium molybdate, with sodium molybdate being particularly preferred.

When the metallic element M is bismuth, from the viewpoint of successfully forming a surface region containing an alloy of nickel and bismuth on the nickel masterbatch particles, the amount of bismuth compound in the mixture is preferably converted to bismuth, and is set to 0.003 parts by mass or more, more preferably 0.004 parts by mass or more, even more preferably 0.01 parts by mass or more, and even more preferably 0.02 parts by mass or more, relative to 1 part by mass of nickel added. Furthermore, the amount of bismuth compound in the mixture is preferably converted to bismuth, and is set to 0.20 parts by mass or less, more preferably 0.16 parts by mass or less, even more preferably 0.13 parts by mass or less, and even more preferably 0.12 parts by mass or less, relative to 1 part by mass of nickel added.

When the metal element M is copper, from the viewpoint of successfully forming a surface region containing an alloy of nickel and copper in the nickel master particles, the amount of copper compound in the mixture is preferably converted to copper, and is set to 0.004 parts by mass or more, more preferably 0.01 parts by mass or more, even more preferably 0.022 parts by mass or more, and even more preferably 0.045 parts by mass or more, relative to 1 part by mass of nickel added. Furthermore, the amount of copper compound in the mixture is preferably converted to copper, and relative to 1 part by mass of nickel added, it is set to 0.12 parts by mass or less, more preferably 0.082 parts by mass or less, even more preferably 0.07 parts by mass or less, and even more preferably 0.06 parts by mass or less.

When the metallic element M is iron, from the viewpoint of successfully forming a surface region containing an alloy of nickel and iron in the nickel master particles, the amount of iron compound in the mixture is preferably converted to iron, and relative to 1 part by mass of nickel added, it is set to 0.0009 parts by mass or more, more preferably 0.0028 parts by mass or more, even more preferably 0.004 parts by mass or more, and even more preferably 0.0047 parts by mass or more. Furthermore, the amount of iron compound in the mixture is preferably converted to iron, and relative to 1 part by mass of nickel added, it is set to 0.12 parts by mass or less, more preferably 0.08 parts by mass or less, further preferably 0.06 parts by mass or less, more preferably 0.030 parts by mass or less, and most preferably 0.020 parts by mass or less.

When the metallic element M is molybdenum, from the viewpoint of successfully forming a surface region containing an alloy of nickel and molybdenum on the nickel master particles, the amount of molybdenum compound in the mixture is preferably converted to molybdenum, and is set to 0.004 parts by mass or more, more preferably 0.01 parts by mass or more, even more preferably 0.013 parts by mass or more, and even more preferably 0.016 parts by mass or more, relative to 1 part by mass of nickel added. Furthermore, the amount of molybdenum compound in the mixture is preferably converted to molybdenum, and relative to 1 part by mass of nickel added, it is set to 0.12 parts by mass or less, more preferably 0.07 parts by mass or less, further preferably 0.06 parts by mass or less, more preferably 0.051 parts by mass or less, and most preferably 0.034 parts by mass or less.

Next, while stirring and heating the mixture containing the compound of the aforementioned metal element M, the nickel hydroxide and the compound in the mixture are reduced. Through this reduction reaction, the residual nickel hydroxide in the mixture is reduced to nickel. If metal element M is bismuth, the compound of metal element M is reduced to bismuth. Alternatively, if metal element M is copper, the compound of metal element M is reduced to copper. Alternatively, if metal element M is iron, the compound of metal element M is reduced to iron. Alternatively, if metal element M is molybdenum, the compound of metal element M is reduced to molybdenum. In this reduction reaction, by simultaneously reducing nickel hydroxide and the compound of metallic element M, a surface region of a nickel-metal M alloy containing nickel and metallic M homogeneously dissolved in solid solution is formed on the surface of the nickel master particles. Furthermore, as long as the effects of this invention can be achieved, a portion of metallic element M can exist in the state of elemental metallic element M, in the state of a compound of metallic element M, or in a combination of two or more states.

The heating temperature of the above-mentioned mixture also depends on the type of polyol or metal element M compound used. Under atmospheric pressure, it is preferably between 150°C and 200°C, more preferably between 170°C and 200°C, and even more preferably between 190°C and 200°C. By keeping the heating temperature within this range, both nickel hydroxide and the metal element M compound can be simultaneously reduced, successfully forming a surface region containing a nickel-metal M alloy on the surface of the nickel masterbatch particles.

Subsequently, if necessary, the polyol in the obtained nickel particle dispersion is replaced with water, and then the replaced water is replaced with methanol to wash the nickel particles, followed by vacuum drying. This method enables the production of the nickel particles of the present invention.

When manufacturing nickel particles containing metallic element M, PVD or CVD methods can be used to add metallic element M to the nickel raw material. The nickel particles then form a nickel-metal M alloy. However, when attempting to improve the sintering resistance of these nickel particles, the following problem arises: excessively increasing the content of bismuth, copper, iron, and/or molybdenum (metallic element M) in the nickel particles results in increased electrical resistance. Furthermore, the uneven particle size of the nickel particles leads to a rough surface on the conductive film when used to form a conductive film, becoming one of the causes of short circuits between the internal electrodes of the MLCC. Furthermore, as another method for manufacturing nickel particles containing metallic element M, a method is known as described in Patent 2, which involves completely reducing nickel hydroxide and then adding a compound containing metallic element M. In this case, if bismuth and/or copper are used as metallic element M, a layer of elemental bismuth and/or copper with a melting point lower than nickel is formed on the surface of the nickel particles. However, the sintering resistance of the nickel particles is not high due to the presence of an elemental bismuth and/or copper layer on the particle surface. Also, when iron and/or molybdenum are used as metallic element M, since elemental iron and molybdenum are easily oxidized, a layer containing iron oxide and/or molybdenum oxide is formed on the surface of the nickel particles. When nickel particles with such a layer are calcined during MLCC manufacturing, the oxides contained in that layer are absorbed by the dielectric layer, resulting in low sintering resistance. In contrast, the nickel particles of this invention, comprising nickel master particles and a nickel-metal M alloy disposed on their surface, improve sintering resistance without excessively increasing resistance. Furthermore, if the nickel particles of this invention are used to form a conductive film, the surface of the conductive film can be made smooth. For these reasons, as described above, it is preferable to manufacture the nickel particles while simultaneously reducing the compound of nickel hydroxide and metal element M, even with a residual amount of nickel hydroxide.

Nickel particles manufactured by the above method are suitable for various applications due to their microparticle size and uniform particle size, and the presence of a surface region containing a nickel-metal M alloy on the surface of the nickel particles. They are particularly suitable for forming the internal electrodes of MLCCs.

The present invention has been described above based on its preferred embodiments, but the present invention is not limited to the above-described embodiments.

Regarding the above-described implementation method, the following nickel particles and their manufacturing method are further disclosed.

[1] A nickel particle having a surface region comprising an alloy containing nickel and a metallic element M, wherein the metallic element M is selected from at least one of bismuth, copper, iron and molybdenum, and the content of the metallic element M relative to the total nickel particle is 0.09% by mass or more and 15.8% by mass or less, and the content of the metallic element M relative to the total nickel particle is determined by X-ray photoelectron spectrophotometry from the outermost surface to the SiO2 content. When measuring the region with a _converted sputtering depth of 5 nm, the maximum value of the ratio of the number of atoms of metal element M in that region to the total number of atoms of nickel and metal element M is set as X (at%). When measuring the nickel particles by ICP emission spectrophotometry, the ratio of the number of atoms of metal element M to the total number of atoms of nickel and metal element M is set as Y (at%). At this time, the value of X/Y is 0.5 or more and 35 or less.

[2] As described in [1], in the particle size distribution based on the circular equivalent diameter calculated by scanning electron microscopy, when the cumulative particle size of the cumulative number of particles is set to D ₅₀ , D ₅₀ is 20 nm or more and 200 nm or less, and when the standard deviation of the particle size in the above particle size distribution is set to σ (nm), the value of the coefficient of variation (σ/D ₅₀ ) (%) is 14% or less, and the coefficient of variation (%) = (σ/D ₅₀ ) × 100.

[3] As described in [1] or [2], in the particle size distribution based on the circular equivalent diameter calculated by scanning electron microscopy, when the cumulative particle size is set to D ₅₀ when the cumulative number of particles is 50%, the proportion of particles with a diameter of more than 1.5 times D ₅₀ is less than 0.5%.

[4] Nickel particles described in any of [1] to [3], wherein in the particle size distribution based on the circular equivalent diameter calculated by scanning electron microscopy, the cumulative particle size when the cumulative number is 50% is set to D ₅₀ , and the value of Cs/D ₅₀ when the crystallite size measured by the WPPF method is set to Cs (nm) is 0.3 or more and 0.6 or less.

[5] A method for manufacturing nickel particles, comprising heating a mixture containing nickel hydroxide particles, a polyol, polyvinylpyrrolidone, and polyethyleneimine to produce nickel particles, wherein polyvinylpyrrolidone is used in an amount of 30 to 200 parts by mass relative to 1 part by mass of polyethyleneimine; the nickel hydroxide particles are reduced to nickel master particles by heating; while a portion of the nickel hydroxide particles remains, the mixture is mixed with a compound of metal element M, and the compound is reduced to metal M, thereby forming a surface region of an alloy containing nickel and metal element M on the nickel master particles, wherein the metal element M is selected from at least one of bismuth, copper, iron, and molybdenum.

[6] A multilayer ceramic capacitor that uses nickel particles as described in any of [1] to [4] as internal electrodes.

Implementation Examples

The invention will now be described in more detail with reference to embodiments. However, the scope of the invention is not limited to these embodiments. Unless otherwise stated, "%" means "mass %".

[Implementation Example 1]

A mixture was prepared by adding 445g of ethylene glycol, 64g of nickel hydroxide particles, 12g of polyvinylpyrrolidone, 0.14g of polyethyleneimine, and 0.13ml of palladium nitrate aqueous solution (concentration: 100g/L) to a 500ml beaker. The polyethyleneimine was branched and had a number average molecular weight of 1800. The mixture was stirred and heated at 198°C for 5 hours under atmospheric pressure for a reduction reaction. At this time point, the reduction of nickel hydroxide was 80 mol% relative to the amount of nickel hydroxide added. Subsequently, 0.3g of bismuth chloride was added, and a reduction reaction was carried out at 198°C for 10 hours under atmospheric pressure. Stop heating to end the reduction process, then allow it to cool naturally to room temperature. This method yields a large quantity of nickel particles.

A magnet is placed at the bottom of a beaker containing a dispersion of the obtained nickel particles to attract the particles. The supernatant of the dispersion is then removed.

After removing the magnet from the bottom of the beaker, add 50g of purified water and stir the dispersion for 10 minutes. Then, place the magnet back at the bottom of the beaker to attract the nickel particles. In this state, remove the supernatant from the dispersion. Repeat this process 5 times.

Next, 50g of methanol was added and the dispersion was stirred for 10 minutes. The supernatant was repeatedly removed three times using a magnet to replace the solvent in the dispersion with methanol. Subsequently, the dispersion was vacuum-dried at 80°C to obtain nickel particles.

[Implementation Examples 2 to 6]

The amounts of palladium nitrate aqueous solution and bismuth chloride added, as well as the time from the start of heating the mixture to the addition of bismuth chloride, are shown in Table 1. Otherwise, nickel particles were obtained in the same manner as in Example 1.

[Implementation Example 7]

Copper sulfate pentahydrate was added instead of bismuth chloride. The amounts of palladium nitrate aqueous solution and copper sulfate pentahydrate added are shown in Table 1. Otherwise, nickel particles were obtained in the same manner as in Example 1.

[Implementation Example 8]

Ferrous sulfate heptahydrate was added instead of bismuth chloride. The amounts of palladium nitrate aqueous solution and ferric sulfate heptahydrate added are shown in Table 1. Otherwise, nickel particles were obtained in the same manner as in Example 1.

[Implementation Example 9]

Sodium molybdate was added instead of bismuth chloride. The amounts of palladium nitrate aqueous solution and sodium molybdate added are shown in Table 1. Otherwise, nickel particles were obtained in the same manner as in Example 1.

[Comparative example 1]

A mixture was prepared by adding 445g of ethylene glycol, 64g of nickel hydroxide particles, 8g of polyvinylpyrrolidone, 0.14g of polyethyleneimine, and 0.13ml of palladium nitrate aqueous solution (concentration: 100g/L) to a 500ml beaker. The polyethyleneimine was branched and had a number-average molecular weight of 1800. The mixture was stirred and heated at 198°C for 6.5 hours for a reduction reaction. Heating was then stopped to allow the reduction to complete, and the mixture was allowed to cool naturally to room temperature. This method yielded a large quantity of nickel particles.

A magnet is placed at the bottom of a beaker containing a dispersion of the obtained nickel particles to attract the particles. The supernatant of the dispersion is then removed.

After removing the magnet from the bottom of the beaker, add 50g of purified water and stir the dispersion for 10 minutes. Then, place the magnet back at the bottom of the beaker to attract the nickel particles. In this state, remove the supernatant from the dispersion. Repeat this process 5 times.

Next, 50g of methanol was added and the dispersion was stirred for 10 minutes. The supernatant was repeatedly removed three times using a magnet to replace the solvent in the dispersion with methanol. Subsequently, the dispersion was vacuum-dried at 80°C to obtain nickel particle powder.

[Comparative example 2]

Bismuth chloride was added before the reduction reaction of nickel hydroxide, and nickel particles were otherwise obtained in the same manner as in Example 1.

[Comparative example 3]

A mixture was prepared by adding 445g of ethylene glycol, 64g of nickel hydroxide particles, 8g of polyvinylpyrrolidone, 0.14g of polyethyleneimine, and 0.13ml of palladium nitrate aqueous solution (concentration: 100g/L) to a 500ml beaker. The polyethyleneimine was branched and had a number-average molecular weight of 1800. The mixture was stirred and heated at 198°C for 6.5 hours to allow the reduction reaction to complete. Heating was then stopped, and the mixture was allowed to cool naturally to room temperature. This method yielded a large quantity of nickel particles.

A magnet is placed at the bottom of a beaker containing a dispersion of the obtained nickel particles, attracting the nickel particles to the magnet. Under these conditions, the supernatant of the dispersion is removed.

After removing the magnet from the bottom of the beaker, add 50g of purified water and stir the dispersion for 10 minutes. Then, place the magnet back at the bottom of the beaker to attract the nickel particles. In this state, remove the supernatant from the dispersion. Repeat this process 5 times.

Add 300g of purified water and hydrazine monohydrate to the dispersion and heat to 60°C. Then add 1g of sodium stannate trihydrate and stir for 5 hours to perform surface treatment on the nickel particles using tin.

A magnet is placed at the bottom of a beaker containing a dispersion of the obtained nickel particles to attract the particles. The supernatant of the dispersion is then removed.

After removing the magnet from the bottom of the beaker, add 50g of purified water and stir the dispersion for 10 minutes. Then, place the magnet back at the bottom of the beaker to attract the nickel particles. In this state, remove the supernatant from the dispersion. Repeat this process 5 times.

Next, 50 g of methanol was added and the dispersion was stirred for 10 minutes. The supernatant was repeatedly removed three times using a magnet to replace the solvent in the dispersion with methanol. Subsequently, the dispersion was vacuum-dried at 80°C to obtain nickel particle powder with a tin surface treatment. As described in [Evaluation 1] below, the surface region of the nickel particles was confirmed to be free of nickel-tin alloy, forming a tin surface layer.

[Review 1]

For the nickel particles obtained in Examples 1 to 9 and Comparative Examples 1 to 3, the values of X and X1 were determined using the following XPS analysis method.

Furthermore, the relative abundances of bismuth, copper, iron, and molybdenum in the nickel particles and the value of Y were determined using ICP-ELISA.

Furthermore, by using the above method to determine the particle size distribution, the particle size _D50 , the proportion of coarse particles, and the coefficient of variation are obtained.

Furthermore, the a-axis length and crystallite size Cs of nickel based on the WPPF method were determined using the following method.

Furthermore, the above method is used to confirm whether the surface region of the nickel particles contains an alloy of nickel and bismuth, an alloy of nickel and copper, an alloy of nickel and iron, or an alloy of nickel and molybdenum.

[X-ray photoelectron spectroscopy (XPS) determination]

The test sample used in XPS is formed by granulating nickel particles using a pressing machine. Specifically, approximately 10 mg of the particle sample is added to an aluminum container with dimensions of 5.2 mm in diameter and 2.5 mm in height. Then, using a pressing machine (manufactured by AS ONE, product number: 1-312-01) and an adapter (product number: 1-312-03), pressure is applied to the aluminum container at a specified stroke (25 mm). The granulated nickel particles supported in the aluminum container are then removed.

The obtained granular material underwent surface surface measurement and depth measurement from the sample surface inwards via Ar monomer ion sputtering. The measurement conditions are shown below.

• Measuring device: VersaProbeIII manufactured by ULVAC-PHI Co., Ltd.

• Excited X-rays: Monochromatic Al-Kα rays (1486.7 eV)

Output: 50W

Accelerating voltage: 15kV

X-ray irradiation diameter: 200 μm

X-ray scan area: 1000μm × 300μm

• Detection angle: 45°

• Energy transmitted: 26.0 eV

• Energy level: 0.1 eV/step

Sprayed ion type: Ar monomer ions

Spray rate: 3.3 nm/min ( _SiO2 conversion)

Splash interval: 20s

Elemental determination: C _1s , Ni _2p3 , Sn _3d5 , Bi _4f , Cu _2p , Fe _3p , Mo _3d

• Energy correction value: C1s _C1s CC and CH bonds (284.8 eV)

[Analysis of XPS Data]

XPS data was parsed using data parsing software (MultiPack Ver9.9 manufactured by ULVAC-PHI). Shirley was used in the background.

[Value of X]

In Embodiments 1 to 6, the ratio of the number of Bi _4f atoms to the total number of atoms of Ni _2p3 and Bi _4f is set as X (at%). In Embodiment 7, the ratio of the number of Cu _2p atoms to the total number of atoms of Ni _2p3 and Cu _2p is set as X (at%). In Embodiment 8, the ratio of the number of Fe _3p atoms to the total number of atoms of Ni _2p3 and Fe _3p is set as X (at%). In Embodiment 9, the ratio of the number of Mo _3d atoms to the total number of atoms of Ni _2p3 and Mo _3d is set as X (at%).

[Determination of a-axis length and crystallite size Cs]

The a-axis length and crystallite size Cs of the nickel particles obtained in the embodiments and comparative examples were calculated using the WPPF method by X-ray diffraction determination and based on the diffraction peaks originating from the obtained nickel.

Device name: SmartLab (9KW); Manufactured by RIGAKU Corporation

<Device Configuration>

Wavelength

Target: Cu

Wavelength type: Kα1

Kα1: 1.54059 (Å)

Kα2: 1.54441 (Å)

Kβ: 1.39225 (Å)

The strength ratio of Kα1 to Kα2 is 0.4970.

Horizontal polarization: 0.500

Diffraction device

• Goniometer: SmartLab

Additional substrate: Z-platform only

Accessories: ASC6-Reflector

<Measurement Conditions>

Optical system properties: Luminous method

• CBO Select Narrow Gaps: BB

Incident parallel narrow gap: Soller_slit_5.0deg

Incident narrow slit: 2/3 degree

Long side narrow gap: 10.0mm

• Light-receiving narrow gap 1: 20.000mm

• Parallel light-receiving narrow gap: Soller_slit_5.0deg

• Light-receiving narrow gap 2: 20.000mm

Attenuator: Open

• Detector: D/teX Ultra250

Scanning axis: 2θ/θ

Scanning Mode: Continuous

• Scan range: 5.0000~140.0000 degrees

• Step size: 0.0100 deg

• Scanning speed/measurement time: 2.015572 dec/min

Data Points: 13501

Pipe voltage: 45kV

Tube current: 200mA

HV: 0.00

Preparation of Samples for X-ray Diffraction

Nickel particles, representing the target material, are densely packed onto a measuring holder. The surface is smoothed using a glass plate to achieve a nickel particle layer thickness of 0.5 mm and a smooth surface.

The X-ray diffraction patterns obtained under the above-described measurement conditions were analyzed using analytical software under the following conditions. During analysis, data obtained from lanthanum hexaboride powder (SRM660 Series), a standard material provided by the National Institute of Standards and Technology (NIST), were used for correction. The a-axis length and crystallite size Cs were calculated using the WPPF method.

<Measurement Data Analysis Conditions>

• Analysis software: Rigaku's PDXL2

Analysis method: WPPF method

Data processing: Automatic contour processing

(Rigaku Corporation, PDXL User Guide, p. 305)

[Review 2]

For the nickel particles obtained in Examples 1 to 9 and Comparative Examples 1 to 3, the initial shrinkage temperature of the nickel particles, the specific resistance of the sintered film containing the nickel particles, and the surface roughness Rz were measured using the following methods. The results are shown in Table 1 below.

[Measurement of the initial contraction temperature]

The TMA measuring apparatus used was the TMA/SS6000 manufactured by Seiko Instruments Co., Ltd. 0.2~0.3g of nickel particles were loaded into a 5.0mm stainless steel mold container and pressurized to form granules by applying a pressure of 92MPa. The particle length of the obtained granules was measured and used as the test sample. The sample was placed in the measuring apparatus and heated at 5°C/min under a load of 49mN and an atmosphere of 1% hydrogen/99% nitrogen. Measurements were taken starting at room temperature (25°C), and a graph showing the relationship between temperature and shrinkage rate (%) was obtained. The temperature at which shrinkage began was determined based on the obtained graph.

[Specific resistance measurement]

0.1 g of ethyl cellulose was dissolved in 4 g of terpineol, followed by the addition of 5 g of nickel particles to obtain a mixture. This mixture was then mixed using a rotary mixer (Thinky Co., Ltd.'s "Defoaming Stirring Taro" (registered trademark)). The mixture was then crushed four times by passing it through a three-roll mill. The mill's clearance was set to 8 μm. A coating solution was obtained in this manner.

The coating solution was applied to an alumina substrate to form a coating film. The coating film thickness was 30 μm. The coating film was sintered at 800°C for 60 minutes under a 1% hydrogen/99% nitrogen atmosphere to obtain a sintered film. The specific resistance (Ω·cm) of the sintered film was measured using a Loresta MCP-T600 four-probe resistivity measuring apparatus manufactured by Mitsubishi Analytech.

[Measurement of Surface Roughness Rz]

The surface roughness Rz of the sintered film was measured using a SURFCOM 130A. The measurement conditions were set as follows: evaluation length 6.0 mm, measurement speed 0.6 mm/s.

As shown in Table 1, XPS measurements confirm that the nickel particles obtained in Examples 1 to 9 contain metallic bismuth, copper, iron, or molybdenum in their surface regions. Furthermore, the a-axis length of the nickel particles obtained in the Examples exceeds that of the nickel particles obtained in Comparative Example 1, which did not use compounds containing bismuth, copper, iron, and molybdenum. These results indicate that the nickel particles obtained in Examples 1 to 6 contain an alloy of nickel and bismuth in their surface regions. Additionally, it is known that the nickel particles obtained in Example 7 contain an alloy of nickel and copper in their surface regions. Furthermore, it is known that the nickel particles obtained in Example 8 contain an alloy of nickel and iron in their surface region. It is also known that the nickel particles obtained in Example 9 contain an alloy of nickel and molybdenum in their surface region.

Furthermore, as shown in Table 1, the nickel particles obtained in Examples 1 to 9 exhibit a higher initial shrinkage temperature compared to the nickel particles obtained in Comparative Examples 1 to 3. This indicates that the nickel particles obtained in Examples 1 to 9 exhibit higher sintering resistance.

In particular, a comparison between Examples 1 to 5 and Example 6 shows that the specific resistance of the sintered film obtained from the nickel particles can be controlled by controlling the amount of bismuth contained in the nickel particles.

Furthermore, in Examples 1 to 6, where nickel particles are fabricated with surface regions containing an alloy of nickel and bismuth, compared to Comparative Example 2 where the nickel particles are entirely formed with an alloy of nickel and bismuth, the surface of the sintered film is smoother. Therefore, it can be seen that the surface roughness of the sintered film is reduced by using nickel particles with surface regions containing an alloy of nickel and bismuth.

[Industry-level applicability]

According to the present invention, a nickel particle with high sintering resistance is provided without excessively increasing resistance.

Claims (6)

A nickel particle having a surface region comprising an alloy of nickel and a metallic element M, wherein the metallic element M is selected from at least one of bismuth, copper, iron, and molybdenum, and the content of the metallic element M relative to the total nickel particle is 0.09% by mass or more and 15.8% by mass or less, wherein when X-ray photoelectron spectrometry is used to measure the region from the outermost surface of the nickel particle to a sputtering depth of 5 nm (converted to _SiO2) , the maximum value of the ratio of the number of atoms of metallic element M in that region to the total number of atoms of nickel and metallic element M is set as X (at%). When determining the nickel particles using ICP emission spectrophotometry, the ratio of the number of atoms of metal element M to the total number of atoms of nickel and metal element M is set as Y (at%). In this case, the value of X/Y is between 0.5 and 35. For nickel particles as claimed in Item 1, in the particle size distribution based on the circular equivalent diameter calculated by scanning electron microscopy, when the cumulative particle size is set to D ₅₀ when the cumulative number of particles is 50%, D ₅₀ is 20 nm to 200 nm, and when the standard deviation of the particle size in the above particle size distribution is set to σ (nm), the value of the coefficient of variation (σ/D ₅₀ ) (%) is 14% or less, and the coefficient of variation (%) = (σ/D ₅₀ ) × 100. For example, in the nickel particles of claim 1, in the particle size distribution based on the circular equivalent diameter calculated by scanning electron microscopy, when the cumulative particle size is set to D ₅₀ when the cumulative number of particles is 50%, the proportion of particles with a particle size of 1.5 times or more of D ₅₀ is less than 0.5%. For nickel particles as claimed in claim 1, in the particle size distribution based on the circular equivalent diameter calculated by scanning electron microscopy, the cumulative particle size when the cumulative number of particles is 50% is set to _D50 , and when the crystallite size measured by the WPPF method is set to Cs (nm), the value of Cs/ _D50 is 0.3 or more and 0.6 or less. A method for manufacturing nickel particles involves heating a mixture comprising nickel hydroxide particles, a polyol, polyvinylpyrrolidone, and polyethyleneimine to produce nickel particles. The method uses 30 to 200 parts by mass of polyvinylpyrrolidone relative to 1 part by mass of polyethyleneimine. The nickel hydroxide particles are reduced to nickel masterbatch particles by heating. While a portion of the nickel hydroxide particles remains, the mixture is mixed with a compound of metallic element M, and the compound is reduced to metallic M. This forms a surface region of an alloy comprising nickel and metallic element M on the nickel masterbatch particles. The metallic element M is selected from at least one of bismuth, copper, iron, and molybdenum. A multilayer ceramic capacitor that uses nickel particles as described in any of claims 1 to 4 for its internal electrodes.