JP2009013490A - Nickel powder and method for producing the same - Google Patents

Abstract

A nickel powder having a small average particle diameter of nickel powder, a uniform particle diameter of nickel powder, a high sintering start temperature, excellent dispersibility in a conductive paste and flatness when used as an internal electrode. To provide.
A nickel powder containing chromium oxide and / or chromium hydroxide, which is screen-printed on an alumina substrate with a nickel coating weight of 0.45 mg / cm ² and fired. When the nickel coating film is formed, the nickel powder has a density of 50% or more. This nickel powder is prepared by preparing a colloidal solution in which composite colloidal particles composed of palladium and silver are dispersed, mixing the solution, a reducing agent, and an alkaline substance to prepare an alkaline colloidal solution. It is obtained by adding an aqueous salt solution to produce nickel particles.
[Selection] Figure 1

Description

  The present invention relates to nickel powder used as a material for a conductive paste and a method for producing the same. The conductive paste is used as a thick film conductor for forming internal electrodes in multilayer components such as multilayer ceramic capacitors (Multi-Layer Ceramic Capacitors; “MLCC”), multilayer ceramic substrates, and other electronic circuits. .

  The multilayer ceramic capacitor is used in an electronic circuit, and the multilayer ceramic capacitor has a structure in which internal electrode layers and dielectric layers are alternately stacked and external electrodes are provided at both ends. Conventionally, noble metals such as silver and palladium have been used as the material of the internal electrode layer of the multilayer ceramic capacitor, but at present, conversion to low-cost nickel is in progress.

  In this multilayer ceramic capacitor, generally, a conductive paste containing fine nickel powder is screen-printed on a dielectric layer made of an alumina substrate or a green sheet, laminated, and then fired in a reducing atmosphere. However, it is manufactured by cutting to a predetermined size and providing external electrodes. The conductive paste is manufactured by kneading fine nickel powder, a resin such as ethyl cellulose, terpineol, and the like in an organic solvent or the like.

  By the way, in electronic devices, high performance, downsizing, high capacity, and high frequency are progressing. For this reason, in the design of electronic circuits, multilayering and thinning have progressed, and higher stacking with different materials has also progressed. In the multilayer ceramic capacitor, thinning corresponding to this has also been progressing. . Therefore, it is necessary to reduce the thickness of the dielectric layer and the internal electrode layer constituting this. Specifically, as the dielectric layer of the multilayer ceramic capacitor is made thinner, the thickness of the internal electrode of the fired multilayer ceramic capacitor is currently reduced to about 1 to 3 μm, and further 1 μm or less. Even thick ones are starting to be offered.

  In particular, in order to reduce the thickness of the internal electrode screen-printed on the dielectric, it is necessary to reduce the particle size of the nickel powder contained in the conductive paste for printing the nickel coating film that will be the internal electrode. For this reason, spherical nickel powder having an average particle diameter of 0.2 to 0.4 μm is used as the nickel powder used for the internal electrode conductive paste of the multilayer ceramic capacitor.

  However, the use of nickel powder having such a small particle size causes the following problems.

  When producing a thin internal electrode, a firing step in a reducing atmosphere is indispensable as described above, but the sintering start temperature of nickel powder having an average particle size smaller than 0.4 μm is about 500 ° C. . This is a very low temperature with respect to the sintering start temperature of the dielectric material of about 1000 ° C. Therefore, when heating is performed for sintering the dielectric layer, the sintering of the nickel powder proceeds more than necessary. As a result, there arises a problem that the nickel coating is divided into islands. This tendency becomes more prominent as the nickel coating thickness of the electrode becomes thinner and as the particle size of the powder used becomes smaller. Thus, when an electrode interrupts in the shape of an island, it does not function as an internal electrode.

  Therefore, the following proposal has been made as a method for solving such a problem.

  For example, in Patent Document 1, a solution containing a thermally decomposable compound capable of forming a composite oxide and a nickel raw material is sprayed into a ceramic tube heated at a high temperature, and thermally decomposed so that the surface is a composite oxide. By obtaining the coated nickel powder, the sintering start temperature of the nickel powder is brought as close as possible to the sintering temperature of the dielectric layer, and the shrinkage behavior can be approximated to the ceramic of the dielectric layer. However, with this method, it is difficult to make the coating state of the composite oxide uniform, and the composite oxide layer on the surface peels off during the pasting process, and the sintering suppression effect cannot be obtained satisfactorily. There are many.

  In Patent Document 2, at least one selected from the group consisting of oxides and composite oxides containing at least one metal element belonging to Group 2-14 of the periodic table within the range of atomic numbers 12-56, 82 is provided. A composite nickel fine powder adhering to the surface and a method for producing the same are disclosed. According to this, it is said that island-like sintering of nickel powder can be suppressed by shifting the rapid thermal shrinkage start temperature of nickel fine powder to a higher temperature side. However, it is very difficult to form an oxide layer uniformly on the surface of the nickel powder, and the oxide or composite oxide on the surface is peeled off during the pasting process, and the oxide layer is destroyed during firing. Since the sintering starts from the spot, the sintering temperature cannot be shifted to a sufficiently high temperature side.

  In Patent Document 3, a metal salt such as chromium is used to solve problems such as a large sintering temperature difference between the nickel electrode and the dielectric, cracking or peeling of the electrode in the firing process, or firing failure of the dielectric. It is disclosed that a conductive paste is produced by obtaining a spherical nickel alloy powder having a uniform particle size by volatilizing with nickel chloride and reducing in air. In this manufacturing method, the sintering start temperature of the nickel powder itself can be shifted to a high temperature side, but the temperature at which shrinkage starts is still about 540 ° C., and nickel that is not alloyed in a temperature range higher than that. The shrinkage rate is equivalent to that of powder.

  Moreover, when manufacturing a thinner internal electrode, there are the following problems.

  As conditions for obtaining a good thin internal electrode, not only the sintering start temperature of the nickel powder but also the dispersibility of the nickel powder and the flatness of the surface of the internal electrode are important factors. Flatness is an important factor because multilayer ceramic capacitors with a thin internal electrode layer often have a very thin dielectric layer. In the crimping process, the convex portions on the surface of the internal electrode penetrate through the dielectric layer laminated thereon, so that a short circuit failure of the internal electrode is likely to occur.

  The flatness of the internal electrode surface is greatly influenced by the dispersibility and the uniformity of the particle diameter of the powder used for electrode production. If a powder having poor dispersibility or a powder having a non-uniform particle size is used, unevenness is likely to be formed on the surface of the internal electrode due to agglomeration of the powder, and the flatness may be deteriorated.

  Therefore, even if nickel powder having a small average particle diameter as described above is used, if the dispersibility of the nickel powder is poor or the particle diameter is not uniform, the powder will aggregate and the nickel coating film In some cases, the thickness of the film becomes uneven and a convex portion is formed on the surface of the coating film, which may hinder the production of a thin multilayer ceramic capacitor.

  Incidentally, the nickel powder having a smaller particle diameter than before has a tendency to deteriorate in dispersibility due to dry agglomeration or the like or to have a nonuniform particle diameter. Due to these reasons, if irregularities are formed on the surface of the internal electrode or if coarse particles are present on the internal electrode, the protrusions and coarse particles penetrate into the dielectric layer during lamination, and in the worst case, penetrate the dielectric layer. Such a phenomenon may cause a short circuit failure.

  Therefore, when such nickel powder is used, consideration for the flatness of the internal electrode is important.

In order to solve such a problem, Patent Document 4 discloses a method for producing a nickel powder having a small particle size and good dispersibility by adding a predetermined amount of hydrazine as a reducing agent to a nickel chloride aqueous solution having a predetermined concentration and reacting them. Is disclosed. However, in this method, in order to obtain nickel powder with good dispersibility, it is necessary to adjust the reducing conditions such as lowering the nickel concentration in the liquid or significantly increasing the hydrazine concentration, and the average particle size is 0. It is difficult to stably manufacture a product smaller than 3 μm even if the reducing conditions are optimized.
JP-A-11-124602 JP 2000-282102 A JP 2002-60877 A Japanese Patent Laid-Open No. 06-336601

  As described above, nickel powder can be used for a conductive paste by shifting the sintering start temperature to the high temperature side to prevent electrode breakage during firing, and having excellent dispersibility in the conductive paste and flatness when used as an internal electrode. Has not yet been proposed.

  In view of the above-mentioned present situation, the present invention aims to provide a conductive paste containing nickel powder that enables the multilayer ceramic capacitor to be thinned. Specifically, the average particle diameter of the nickel powder is small, and An object of the present invention is to provide a nickel powder having a uniform particle size of nickel powder, a high sintering start temperature, and excellent dispersibility in a conductive paste and flatness when used as an internal electrode.

The conductive paste of the present invention is nickel powder containing chromium oxide and / or chromium hydroxide, and is screen-printed with a nickel coating weight of 0.45 mg / cm ² on an alumina substrate using the nickel powder. When the nickel coating film is formed by firing, the density of the nickel coating film is 50% or more.

  The average particle diameter of the nickel powder is preferably 0.05 to 0.4 μm.

  In the method for producing a conductive paste of the present invention, a colloidal solution in which composite colloidal particles composed of palladium and silver are dispersed is prepared, and the colloidal solution, a reducing agent, and an alkaline substance are mixed to prepare an alkaline colloidal solution. Then, it is preferable that nickel particles are generated by simultaneously adding a chromium salt and a nickel salt aqueous solution to the alkaline colloidal solution or by adding a nickel salt aqueous solution containing a chromium salt.

  The amount of the chromium salt is preferably 0.01 to 5% by mass with respect to the amount of nickel added as the nickel salt aqueous solution.

  Moreover, it is preferable that the quantity of the said palladium shall be 1-5000 mass ppm with respect to the quantity of the nickel added as said nickel salt aqueous solution.

  Furthermore, it is preferable that the amount of silver is 0.01 to 50 mass ppm with respect to the amount of nickel added as the aqueous nickel salt solution.

  In preparing the alkaline colloidal solution, it is preferable to add a protective colloid agent to disperse the composite colloidal particles.

  It is preferable that the addition amount of this protective colloid agent shall be 1-5000 mass ppm with respect to the quantity of nickel added as said nickel salt aqueous solution.

  As the protective colloid agent, gelatin can be preferably used.

  Although the nickel powder of the present invention has a high sintering start temperature despite the small average particle diameter, the sintering of the internal electrode film during firing can be suppressed. Moreover, it has a uniform particle size and has good dispersibility in the conductive paste.

  Therefore, if a conductive paste containing such nickel powder is used, the nickel coating film has a high density that does not break into islands during the production of the multilayer ceramic capacitor, has flatness, and is due to defective production of internal electrodes. Since no obstacles such as short circuit occur, a high-quality internal electrode can be manufactured with a high yield.

The nickel powder according to the present invention contains chromium oxide and / or chromium hydroxide, and the nickel powder is screen-printed on an alumina substrate with a coating weight of 0.45 mg / cm ² to obtain a nickel coating film. Is formed, the density of the nickel coating film is 50% or more.

  Here, the density ratio is calculated as follows.

  First, ethyl cellulose (20% by mass) is added to terpineol (80% by mass) and heated to 80 ° C. with stirring to prepare a terpineol solution in which ethyl cellulose is dissolved. Subsequently, 45% by mass of this solution, 40% by mass of the nickel powder of the present invention, and 15% by mass of terpineol are kneaded in a three-roll mill to produce a conductive paste.

A nickel coating is screen printed onto a 2.54 cm (1 inch) square alumina substrate with a nickel coating weight of 0.45 mg / cm ² and dried at 120 ° C. for 1 hour. The alumina substrate is fired at a temperature rising rate of 10 ° C./min up to 1250 ° C. in a weakly reducing (N ₂ / H ₂ ) atmosphere. The alumina substrate on which the nickel coating film after firing is formed is irradiated with light illumination light from the back side of the printing surface, and the light transmission state of the printing surface is 400 times magnification, 160 μm in length and width in an optical microscope. Photographed at 230 μm (see FIG. 2), the occupied area of transmitted light was measured for each photographed image, and the density was calculated according to the following formula.

  Dense rate (%) = (total imaging area−transmitted light area) / (total imaging area) × 100

  The nickel coating weight is calculated from the difference in weight of the alumina substrate before and after acid dissolution of the nickel coating after the nickel coating on the alumina substrate is acid-dissolved after measuring the density.

  In addition, as described above, the density is calculated based on the measured transmitted light area, and the transmitted light area is to confirm the location of the nickel powder on the alumina substrate by the presence or absence of the transmitted light. Is obtained. That is, the portion without transmitted light is in a state where the light illumination light is blocked by the nickel powder and cannot be transmitted, and indicates that the nickel powder is present in the portion, and conversely, the portion with the transmitted light is In this situation, the light illumination light is transmitted through the nickel powder without being obstructed, and it is indicated that the nickel powder is not present at the location, so that the location where the nickel powder is present can be confirmed.

  Hereinafter, the nickel powder used for the electrically conductive paste of this invention is demonstrated in detail.

  The nickel powder of the present invention is produced by simultaneously adding a chromium salt and a nickel salt aqueous solution to the alkaline colloid solution, or by adding a nickel salt aqueous solution containing a chromium salt to the alkaline colloid solution. However, it is thought that chromium precipitates as chromium hydroxide or chromium oxide and is taken into the nickel particles when they are formed.

  As described above, chromium hydroxide and chromium oxide are taken into the nickel powder and remain in the powder as it is. Therefore, in the pasting process, chromium hydroxide and chromium oxide are peeled off or missing from the nickel powder. There is nothing to do. Further, even during sintering, these chromium hydroxide and chromium oxide are present inside the nickel powder, so that they hardly concentrate at the interface of the nickel powder as the nickel is sintered. Continue to stay in the membrane.

  By continuing to exist in the nickel coating film in this way, chromium hydroxide and chromium oxide reduce the number of contact points between the nickel fine particles in the nickel powder, while preventing the movement of nickel atoms and physically burning the nickel. Sintering is inhibited and the sintering start temperature is shifted to the high temperature side. As a result, it is considered that the sintering of the internal electrode film made of nickel powder is suppressed until the temperature becomes high, and the phenomenon that the internal electrode film is isolated (disconnected) in an island shape is suppressed.

  However, if the addition amount of the chromium salt is less than 0.01% by mass with respect to the amount of nickel added as the nickel salt aqueous solution, the effect of suppressing the electrode from being island-shaped hardly appears. Further, when the addition amount exceeds 5% by mass, the effect reaches its peak, and a large effect due to the increase in the content cannot be obtained. Therefore, the upper limit of the addition amount is set to 5% by mass from the viewpoint of cost effectiveness. Therefore, the addition amount of the chromium salt is preferably 0.01 to 5% by mass, and more preferably 0.1 to 0.5% by mass.

  Note that the amount of chromium contained in the powder is proportional to the amount of chromium salt added during the reaction, and the chromium content after powder production is about 40 to 60% of the amount of chromium salt added. Therefore, in order to increase the amount of chromium in the powder, the amount of chromium salt added during the reaction may be increased.

In order to verify the above view, when a nickel coating film having a coating weight of 0.45 mg / cm ² was formed using a conventionally used 0.4 μm nickel powder obtained without adding a chromium salt, The density of the nickel coating film was about 30%. This 0.45 mg / cm ² is the coating weight corresponding to 0.5 μm in thickness of the nickel coating film. As described above, this coating weight causes island breaks in the nickel coating film, and the internal electrodes Since it becomes non-conductive, the multilayer ceramic capacitor could not obtain a capacitance.

  The average particle diameter of the nickel powder of the present invention is preferably in the range of 0.05 to 0.4 μm. When the average particle diameter exceeds 0.4 μm, it becomes impossible to reduce the thickness of the internal electrode produced using such powder to about 1 μm. In order to achieve a further reduction in the thickness of the internal electrode, it is preferable to reduce the average particle diameter. However, if the average particle diameter is less than 0.05 μm, the cohesive force of the powder becomes too large, and it is very difficult to form a paste. The problem that it becomes difficult arises and is not preferable. The average particle diameter can be determined by measuring all the particle diameters of nickel particles within a predetermined range in observation with a scanning electron microscope (SEM).

  As will be described later, the average particle diameter is achieved by forming nuclei in each particle of the nickel powder with palladium instead of nickel, and nickel is deposited on the palladium nuclei to form particles.

  In addition, the variation (uniformity) of the particle size of the nickel powder in the present invention is obtained by measuring the particle size of each particle by image processing, and counting the frequency for all the particle sizes based on the measurement result. Each particle size measured was determined by creating a particle size distribution graph with the particle size on the X axis and the frequency on the Y axis. Between the minimum particle size and the maximum particle size of this graph, the particle size (d10) at the frequency of 10% and the particle size (d90) at the frequency of 90% are calculated from the minimum particle size, and the ratio ( By calculating (d10 / d90), the particle size variation (uniformity) can be obtained. The ratio obtained by such calculation is desirably 0.3 or more. When the ratio (d10 / d90) is 0.3 or more, it means that the powder has a uniform particle size with a small variation in particle size in the particle size frequency distribution.

  The uniformity of the particle size is achieved by reducing the particle size of the nickel powder and improving the dispersibility by the production method described later, thereby suppressing the generation of coarse particles and connected particles.

  Next, the manufacturing method of the nickel powder which concerns on the electrically conductive paste of this invention is demonstrated in detail.

  In the present invention, first, a colloidal solution in which composite colloidal particles composed of palladium and silver are dispersed is prepared. The colloidal solution is prepared by mixing a predetermined amount of an aqueous palladium salt solution and an aqueous silver salt solution.

  The amount of palladium is preferably 1 to 5000 ppm by mass with respect to the amount of nickel to be added later as an aqueous nickel salt solution. When the amount of palladium is less than 1 ppm by mass, the number of colloidal particles serving as nuclei decreases, and the particle size of the resulting nickel particles may increase. On the other hand, even if the amount of palladium is more than 5000 mass ppm, there is almost no further effect on the refinement of the obtained nickel particles, and a large amount of expensive noble metal is used, resulting in poor cost performance. . Also, if the average particle size becomes too small, the cohesive force increases, making it difficult to disperse in the paste. As a result, a multilayer ceramic capacitor in an agglomerated state is manufactured, and the dielectric layer This may cause a short circuit failure or the like.

  Moreover, it is preferable that silver amount shall be 0.01-50 mass ppm with respect to the nickel amount added later. When palladium and silver are combined to form colloidal particles, a small amount of silver exerts an effect of suppressing the formation of coarse nickel particles and connected particles. This is thought to be due to the fact that palladium is refined by the addition of silver and the number of colloidal particles acting as nuclei increases. If the amount of silver is less than 0.01 ppm by mass, the above effect may be hardly obtained, and if it is more than 50 ppm by mass, there will be hardly any further effect on the refinement of the nickel particles obtained.

  Here, the palladium salt aqueous solution is not particularly limited, and for example, an aqueous solution containing at least one selected from palladium chloride, palladium nitrate, palladium sulfate and the like may be used as the palladium salt aqueous solution. It is most preferable to use an aqueous solution containing palladium chloride that can be easily adjusted.

  On the other hand, as the silver salt aqueous solution, for example, a silver nitrate aqueous solution can be used.

  Next, the colloidal solution, the reducing agent, and the alkaline substance are mixed to prepare an alkaline colloidal solution. The method for preparing the alkaline colloid solution, that is, the method for dispersing the composite colloidal particles of palladium and silver in the alkaline reducing agent solution is not particularly limited. For example, after preparing a colloidal solution in which composite colloidal particles of palladium and silver are dispersed, a reducing agent and an alkaline substance are added to the colloidal solution, or a reducing agent solution in which a colloidal solution and an alkaline substance are added individually. And a method of mixing these solutions. In general, the colloidal solution is dropped into an alkaline reducing agent solution.

  In addition, when mixing the colloidal solution with the reducing agent solution, for example, a protective colloid agent is preferably added to the reducing agent solution in order to further disperse the composite colloidal particles composed of palladium and silver.

  As the protective colloid agent, any material may be used as long as it surrounds composite colloidal particles composed of palladium and silver and contributes to the formation of the protective colloid, and gelatin is most preferable. In addition, polyvinylpyrrolidone, gum arabic, sodium hexametaphosphate, polyvinyl Alcohol or the like can also be used as a protective colloid agent.

  In addition, it is preferable that the addition amount of the protective colloid agents such as gelatin is 1 to 5000 ppm by mass with respect to the nickel amount added as the nickel salt aqueous solution. If the added amount of the protective colloid agent is less than 1 ppm by mass, the formation of the protective colloid may be insufficient, and the colloidal particles may be agglomerated, and coarse particles and connected particles may be generated in the reduced nickel powder. There is. Moreover, when this addition amount exceeds 5000 mass ppm, there is a possibility that the protective colloid becomes too much and unreduced nickel remains.

The reducing agent used in the present invention is not particularly limited, and was prepared using a water-soluble hydrazine compound containing at least one selected from hydrazine, a hydrazine compound, sodium borohydride, and the like, as described above. It is preferable to use a hydrazine aqueous solution or the like. Among these reducing agents, hydrazine (N ₂ H ₄ ) is most preferable, particularly in that the component has few impurities.

  The alkaline substance to be mixed with the colloidal solution is not particularly limited, and may be any water-soluble alkaline substance such as sodium hydroxide, potassium hydroxide, and ammonia. In the present invention, these water-soluble alkaline substances and water-soluble hydrazine compounds such as hydrazine and hydrazine hydrate are mixed in pure water to prepare an alkaline hydrazine aqueous solution, which can be used as an alkaline reducing solution. it can.

  As the alkaline reducing agent solution, a mixed solution of sodium hydroxide and hydrazine hydrate is preferable. However, if the pH of the mixed aqueous solution is less than 10, the reaction rate becomes slow, and reduction precipitation of nickel hardly occurs. Therefore, it is particularly preferable to use the mixed aqueous solution whose pH is adjusted to 10 or more.

  The present invention is a nickel powder produced by simultaneously adding a chromium salt and an aqueous nickel salt solution to an alkaline colloidal solution as described above, or adding an aqueous nickel salt solution containing a chromium salt to an alkaline colloidal solution, The feature of the production is that the composite colloidal particles composed of palladium and silver are dispersed in the alkaline colloid solution in advance before adding the nickel salt aqueous solution. By using the alkaline colloid solution, The details of the mechanism by which the particle size of the nickel particles produced by reduction is uniformized and refined are unknown. However, this mechanism is presumed as follows.

  Palladium and silver have a higher oxidation-reduction potential than nickel, so they become nuclei when nickel particles precipitate, and it is thought that nickel precipitates and grows into these nuclei, so that nickel nuclei do not form. In addition, it is presumed that nickel particles are generated.

  In addition, since the composite colloidal particles composed of palladium and silver are present in the alkaline colloidal solution in a uniformly monodispersed state, when a nickel salt aqueous solution is added, Therefore, nickel is considered to cause uniform nuclear growth.

  Furthermore, by adding not only palladium but also silver, the aggregation of palladium is suppressed, so the formation of coarse particles and connected particles is suppressed, and the addition amount of palladium and silver is controlled within a predetermined range. Thus, composite colloidal particles composed of palladium and silver in a monodispersed state with a more uniform particle size are produced.

  In addition, by adding a protective colloid agent in an amount within a predetermined range to form a protective colloid, aggregation of the composite colloidal particles is further suppressed, and a monodispersed state is promoted.

  For this reason, it is thought that the produced nickel particles are in a monodispersed state with a uniform particle size, and it is difficult to form nickel particles that become coarse particles or connected particles.

  In addition, since the composite colloidal particles composed of palladium and silver become the nuclei of the nickel particles during nickel precipitation as described above, the number of nuclei of the nickel particles changes with the number of composite colloidal particles. . Therefore, when many composite colloidal particles exist in the alkaline colloidal solution, nickel is dispersed and dispersed into a large number of nuclei to form nickel particles. The particle size of the nickel particles is easily miniaturized. Conversely, when the number of composite colloidal particles present in the alkaline colloidal solution is small, the number of nuclei is also small, so that the amount of nickel that precipitates in each nuclei increases, and nucleation grows, causing individual nickel to grow. It is considered that the particle size of the particles tends to be coarsened.

  Thus, when reducing and precipitating nickel from an aqueous nickel salt solution, the composite colloidal particles composed of palladium and silver are used as a reducing aid that serves as a nucleus for the reduction and precipitation of nickel and promotes the nucleus growth of the nickel particles. Furthermore, by controlling the number of the composite colloidal particles, it is considered that the particle diameter of the produced nickel particles can be made uniform and fine.

  The nickel salt aqueous solution used in the present invention is not particularly limited, and for example, an aqueous solution containing at least one selected from nickel chloride, nickel nitrate, nickel sulfate and the like can be used. However, among these aqueous solutions, a nickel chloride aqueous solution that can simplify waste liquid treatment is particularly preferable.

  The chromium salt used in the present invention is desirably water-soluble from the reaction step, and chromium nitrate, chromium sulfate, chromium sulfide and chromium oxide can be used, but chromium chloride is particularly preferred.

  The chromium salt may be added to the alkaline colloid solution simultaneously with the nickel salt aqueous solution during the nickel particle generation reaction, or may be added in advance to the nickel salt aqueous solution added to the alkaline colloid solution.

  Since the nickel powder of the present invention has the above characteristics, the sintering start temperature is high although the average particle diameter is small, so that the sintering of the internal electrode film during firing can be suppressed. Moreover, it has a uniform particle size and has good dispersibility in the conductive paste.

  Therefore, when the conductive paste containing such nickel powder is used, the nickel coating film is not interrupted in the form of islands during the production of the multilayer ceramic capacitor, and there is no trouble such as a short circuit due to defective production of the internal electrode. A high-quality internal electrode with a high density can be manufactured with high yield. Moreover, the flatness of the nickel coating film is also good.

  Specific examples will be described below.

  The examples of the present invention and the comparative examples were evaluated as follows.

(Average particle size)
The average particle diameter is obtained by measuring the particle diameter based on observation with a scanning electron microscope (SEM). Specifically, it is as follows. A diagonal line is drawn on a SEM photograph (4.5 μm long × 6.5 μm wide) of 20,000 times. Two lines are drawn on both sides of the diagonal line in parallel with the diagonal line at a position 0.5 μm away from the diagonal line. The distance between the two drawn lines is 1 μm. And all the particle size of the nickel powder particle which the whole was contained between the drawn two lines was measured, and the average particle size was calculated | required based on them.

(Particle size variation (uniformity))
Measurement of particle size variation was performed by image processing. First, the particle size of each particle was measured by image processing on an SEM image observed at a magnification of 10,000 with an SEM. Next, for each measured particle size, in the graph of particle size frequency distribution, the particle size (d10) having a frequency corresponding to a position of 10% and the particle size having a frequency corresponding to a position corresponding to 90% (from the smallest particle size) d90) and the ratio d10 / d90 was calculated.

(Dense rate)
Calculation of the density was performed as follows.

  First, ethyl cellulose (20% by mass) was added to terpineol (80% by mass) and heated to 80 ° C. with stirring to prepare a terpineol solution in which ethyl cellulose was dissolved. Subsequently, 45% by mass of this solution, 40% by mass of the obtained nickel powder, and 15% by mass of terpineol were kneaded with a three-roll mill to produce a conductive paste.

Using the conductive paste, a nickel coating film was screen-printed on a 2.54 cm (1 inch) square alumina substrate with a nickel coating weight of 0.45 mg / cm ² and dried at 120 ° C. for 1 hour. The alumina substrate was baked by raising the temperature to 1250 ° C. at a rate of 10 ° C./min in a weakly reducing (N ₂ / H ₂ ) atmosphere. The alumina substrate on which the nickel coating film after firing is formed is irradiated with light illumination light from the back side of the printing surface, and the light transmission state of the printing surface is 400 times magnification, 160 μm in length and width in an optical microscope. Images were taken at 230 μm (see FIG. 2), and the occupied area of transmitted light in each photographed image was measured, and the density was calculated according to the following equation.

  Dense rate (%) = (total imaging area−transmitted light area) / (total imaging area) × 100

  Further, the nickel coating weight was calculated from the difference in weight of the alumina substrate before and after the nickel coating was dissolved by acid, after the nickel coating film on the alumina substrate was acid-dissolved after the density ratio was measured.

(Arithmetic mean surface roughness)
A nickel coating film was screen-printed on a 2.54 cm (1 inch) square alumina substrate using the nickel ink, and dried at 120 ° C. for 1 hour to prepare a dry film having a 10 mm square and a film thickness of 1 μm. The arithmetic average surface roughness (Ra) was measured for the dried film.

  The arithmetic average surface roughness (Ra) is measured based on the standard of JIS B0601-1994. Judgment of pass / fail is made on the basis of the arithmetic average surface roughness (Ra) of 0.1 μm, which is a commonly used 0.4 μm nickel powder, and the measurement result of 0.12 μm or less is regarded as acceptable. did.

(Examples 1 to 11)
Gelatin, which is a protective colloid agent, an alkaline substance, and hydrazine were mixed with a composite colloidal solution composed of palladium and a small amount of silver to prepare an alkaline colloidal solution for reducing nickel.

  The production of the alkaline colloid solution for reducing nickel was specifically performed as follows. First, gelatin was dissolved in 6 L of pure water, and then hydrazine was mixed so that the concentration of hydrazine was 0.02 g / L. Next, a mixed solution of palladium and a small amount of silver was prepared, and dropped into the solution containing gelatin and hydrazine as a reducing agent to obtain a colloidal solution. After adding sodium hydroxide, an alkaline substance, to this colloid solution to adjust the pH to 10 or higher, hydrazine was further added until the concentration of hydrazine reached 26 g / L, and a colloid composed of palladium and a small amount of silver was mixed. A colloidal solution was prepared and used as a solution for reducing nickel.

  The content of palladium, silver, and gelatin in the alkaline colloidal solution is palladium: 1 to 5000 mass ppm, silver: 0.01 to 50 mass ppm, gelatin: 1 to 1 with respect to the total mass of nickel in the nickel salt aqueous solution. Each was changed within the range of 5000 ppm by mass. The contents of palladium and silver in the solution were measured by ICP emission spectroscopy. These addition amounts are shown in Table 1, respectively.

  Further, a solution prepared by dissolving nickel chloride in advance and dissolving chromium chloride in a nickel chloride aqueous solution having a nickel concentration of 100 g / L is dropped into the alkaline colloid solution by 0.5 L to reduce the nickel powder. Obtained. In addition, content of chromium chloride was changed in the range of chromium: 0.01-5 mass% with respect to the total mass of nickel in nickel chloride aqueous solution.

  Subsequently, the nickel powder obtained by the reduction treatment was further vacuum-dried at 150 ° C. for 12 hours to obtain a dried nickel powder.

  Table 1 shows the average particle diameter, arithmetic average surface roughness (Ra), d10 / d90, and density of the obtained nickel powder.

  1 and 2 are scanning electron microscope (SEM) observation photographs (magnification 20000 times, length 4.5 μm × width 6.5 μm) of the nickel powder obtained as Example 2 and Example 7 of the present invention. Each is shown. 4 and 5 were screen-printed on an alumina substrate using nickel ink based on the nickel powder obtained as Example 5 (dense rate 51%) and Example 6 (dense rate 72%) of the present invention. An optical microscope backlight observation image photograph (magnification 400 times, length 160 μm × width 230 μm) in the measurement of the density of the nickel coating film is shown.

(Comparative Examples 1 and 2)
Except for the addition amount of silver deviating from the preferable range (0.01 to 50 mass ppm) in the present invention (Comparative Example 1: 0.005 mass ppm, Comparative Example 2: 60 mass ppm), Examples 1 to In the same manner as in No. 3, nickel powder was obtained.

  Table 2 shows the average particle diameter, arithmetic average surface roughness (Ra), d10 / d90, and density of the obtained nickel powder.

(Comparative Examples 3 and 4)
Except for the addition amount of palladium deviating from the preferred range (1 to 5000 mass ppm) in the present invention (Comparative Example 3: 0.05 mass ppm, Comparative Example 4: 6000 mass ppm), Examples 4 and 5 Similarly, nickel powder was obtained.

  Table 2 shows the average particle diameter, arithmetic average surface roughness (Ra), d10 / d90, and density of the obtained nickel powder.

(Comparative Examples 5 and 6)
Except for the addition amount of gelatin which is a protective colloid agent is outside the preferable range (1 to 5000 mass ppm) in the present invention (Comparative Example 5: 0.05 mass ppm, Comparative Example 6: 7500 mass ppm). In the same manner as in Examples 6 and 7, nickel powder was obtained.

  Table 2 shows the average particle diameter, arithmetic average surface roughness (Ra), d10 / d90, and density of the obtained nickel powder.

  3 shows a scanning electron microscope (SEM) observation photograph (magnification 20000 times, length 4.5 μm × width 6.5 μm) of the nickel powder obtained as Comparative Example 6.

(Comparative Examples 7 and 8)
Examples 8 to 11 except that the addition amount of chromium deviates from the preferred range (0.01 to 5% by mass) in the present invention (Comparative Example 7: 0.005% by mass, Comparative Example 8: no addition). In the same manner, nickel powder was obtained.

  Table 2 shows the average particle diameter, arithmetic average surface roughness (Ra), d10 / d90, and density of the obtained nickel powder.

  FIG. 6 shows an optical microscope backlight observation in the measurement of the density of a nickel coating film screen-printed on an alumina substrate using a nickel ink based on the nickel powder obtained as Comparative Example 8 (density 32%). An image photograph (magnification 400 times, length 160 μm × width 230 μm) is shown.

(Comparative Example 9)
YH-6 manufactured by Sumitomo Metal Mining Co., Ltd., which is a conventionally used 0.4 μm nickel powder, was adopted as Comparative Example 9, and the average particle diameter, density and arithmetic average surface roughness of the nickel powder were Evaluation was performed in the same manner. The evaluation results are shown in Table 2.

  Examples 1 to 11 are examples in which the addition amounts of palladium, silver, gelatin and chromium are within a preferable range in the production of nickel powder according to the conductive paste of the present invention.

  As is clear from Table 1, the average particle diameters of the nickel powders according to Examples 1 to 11 are in the range of 0.05 to 0.4 μm. Moreover, the ratio (d10 / d90) of the particle size frequency of nickel powder is 0.8 or less, and the particle size is uniform.

  Moreover, the value of arithmetic average surface roughness (Ra) is also lower than the reference value of 0.12 μm, and all show a good dispersion state.

  Furthermore, since the density is 50% or more, it is possible to prevent island breaks in the nickel coating film that occur during sintering.

  In Comparative Example 1, the addition amount of silver deviates from the preferred range in the production of nickel powder according to the conductive paste of the present invention, and therefore the average particle size (0.41 μm) and the density (40%) values are the same. It does not meet the requirements of the invention (50% or more).

  Moreover, the ratio (d10 / d90) of the particle size frequency of the nickel powder is 0.85, which exceeds the preferable value of 0.8.

  On the other hand, in Comparative Example 2, the addition amount of silver exceeds the upper limit of the preferable range in the production of nickel powder according to the conductive paste of the present invention, but its average particle size (0.15 μm), arithmetic average surface roughness (Ra) (0.04 μm) and the density (65%) are the same values as in Example 2. From this, in the present invention, it can be seen that even if the addition amount of silver exceeds 50 mass ppm, it is not possible to expect improvement of the above evaluation values.

  Further, in Comparative Example 3, since the addition amount of palladium deviates from the preferable range (1 to 5000% by mass) in the production of the nickel powder-containing conductive paste of the present invention, the average particle size value (0 .44 μm) is outside the preferred range of the present invention (0.05-0.40 μm).

  Moreover, the ratio (d10 / d90) of the particle size frequency of the nickel powder is 0.85, which exceeds the preferable value of 0.8.

  That the ratio (d10 / d90) is 0.8 or less means that in the particle size frequency distribution, there are many particles on the larger particle size side and fewer particles on the smaller particle size side, That is, it means that the nickel particles are mainly composed of particles having a relatively large particle size, and the variation in the particle size is small.

  In Comparative Example 4, 6000 mass ppm of palladium is added, and is not much different from the case of adding 5000 mass ppm in terms of the effect on arithmetic average surface roughness (Ra) and density characteristics (Example 5). There are difficulties in using more expensive noble metals, and there are also problems in terms of effects on average particle size characteristics.

  Comparative Example 5 added 0.05 mass ppm, and Comparative Example 6 added 7500 mass ppm gelatin, and the amount added deviated from the preferred range (1 to 5000 mass%) in the production of nickel powder according to the conductive paste of the present invention. Therefore, in both cases, the reaction does not proceed properly, and the average particle size exceeds 0.4 μm, which is the upper limit of the preferred range (0.05 to 0.4 μm) of the nickel powder according to the conductive paste of the present invention. In particular, in Comparative Example 6, since the amount of gelatin added was large, the average particle size was 1.12 μm, and the ratio of particle size frequency (d10 / d90) was 1.01, and the variation in particle size was The nickel powder was large and had large particles, and the arithmetic average surface roughness (Ra) value was 0.18 μm, which exceeded the reference value of 0.12 μm.

  In Comparative Examples 5 and 6, the ratio (d10 / d90) of the particle size frequency of the nickel powder is 0.85 and 1.01, respectively, which exceeds the preferable value of 0.8.

  Comparative Example 7 has a chromium addition amount of 0.005% by mass and less than the preferable range (0.01 to 5% by mass) in the production of nickel powder according to the conductive paste of the present invention. Since chromium is not added at all, the dense ratios are 34% and 32%, respectively, which are lower than the range of 50% in the nickel powder according to the conductive paste of the present invention.

  In addition, as described above, Comparative Example 9 was evaluated by using YH-6 nickel powder manufactured by Sumitomo Metal Mining Co., Ltd., which is a conventional product, as a comparative example for the present invention, in the same manner as the other examples. Therefore, no chromium is added at the time of powder production, and the density is as low as 30%.

  As described above, Comparative Examples 7 to 9 are examples in which a small amount of chromium is added or no addition, and the density of these is as low as 30 to 34%. On the other hand, Examples 1, 2, and 10 having an average particle diameter equal to that of the comparative example, to which an appropriate amount of chromium was added, showed a high density ratio of 75 to 64%. From these examples, it is clear that high density can be obtained by adding chromium.

  As described above, the conductive paste of the present invention has good dispersibility of the nickel powder and can effectively suppress the island-like phenomenon of the nickel sintered film. In addition, even in the conventionally used 0.4 μm nickel powder, the effect of improving the density is sufficiently exhibited in the present invention, and a nickel coating thinner than the conventional multilayer ceramic capacitor is used. It is possible to realize the thinning of the internal electrode layer of the multilayer ceramic capacitor.

It is a scanning electron microscope (SEM) observation comparative photograph (magnification 20000 times, length 4.5 micrometers x width 6.5 micrometers) which shows each state of nickel powder of Example 2. It is a scanning electron microscope (SEM) observation comparative photograph (magnification 20000 times, length 4.5 micrometers x width 6.5 micrometers) which shows each state of nickel powder of Example 7. It is a scanning electron microscope (SEM) observation comparison photograph (magnification 20000 times, length 4.5 micrometers x width 6.5 micrometers) which shows each state of nickel powder of comparative example 6. It is an optical microscope backlight observation image comparative photograph (magnification 400 times, length 160 micrometers x width 230 micrometers) which shows each dense state of the nickel coating film screen-printed on the alumina substrate using the nickel powder of Example 5. It is an optical microscope backlight observation image comparative photograph (magnification 400 times, length 160 micrometers x width 230 micrometers) which shows each dense state of the nickel coating film screen-printed on the alumina substrate using the nickel powder of Example 6. It is an optical microscope backlight observation image comparison photograph (magnification 400 times, length 160 micrometers x width 230 micrometers) which shows each dense state of the nickel coating film screen-printed on the alumina substrate using the nickel powder of comparative example 8.

Claims (9)

Nickel powder containing chromium oxide and / or chromium hydroxide,
When the nickel powder is used to screen-print on an alumina substrate with a nickel coating weight of 0.45 mg / cm ² and baked to form a nickel coating, the density of the nickel coating is 50% or more. Nickel powder, characterized in that   The nickel powder according to claim 1, wherein the average particle size is 0.05 to 0.4 μm.   A colloidal solution in which composite colloidal particles composed of palladium and silver are dispersed is prepared, and the colloidal solution, a reducing agent, and an alkaline substance are mixed to prepare an alkaline colloidal solution, and a chromium salt is added to the alkaline colloidal solution. And nickel salt aqueous solution is added simultaneously, or nickel salt aqueous solution containing chromium salt is added to produce nickel particles.   The method for producing nickel powder according to claim 3, wherein the amount of the chromium salt is 0.01 to 5% by mass with respect to the amount of nickel added as the aqueous nickel salt solution.   5. The method for producing nickel powder according to claim 3, wherein the amount of palladium is 1 to 5000 ppm by mass with respect to the amount of nickel added as the nickel salt aqueous solution.   The method for producing nickel powder according to any one of claims 3 to 5, wherein the amount of silver is 0.01 to 50 ppm by mass with respect to the amount of nickel added as the aqueous nickel salt solution. .   The method for producing nickel powder according to any one of claims 3 to 6, wherein a protective colloid agent is added to disperse the composite colloidal particles when preparing the alkaline colloidal solution.   The method for producing nickel powder according to claim 7, wherein the amount of the protective colloid agent added is 1 to 5000 ppm by mass with respect to the amount of nickel added as the aqueous nickel salt solution.   The method for producing nickel powder according to claim 7 or 8, wherein gelatin is used as the protective colloid agent.