WO2019009136A1

WO2019009136A1 - Metal powder and method for producing same

Info

Publication number: WO2019009136A1
Application number: PCT/JP2018/024121
Authority: WO
Inventors: 広介六角; 友則宇野; 浅井　剛
Original assignee: 東邦チタニウム株式会社
Priority date: 2017-07-05
Filing date: 2018-06-26
Publication date: 2019-01-10
Also published as: CN110799285A; JP6553313B2; JP7080856B2; KR102278500B1; JP2019173179A; TW201906675A; CN110799285B; JPWO2019009136A1; TWI690381B; KR20200023442A

Abstract

Provided is a metal powder suitable for a conductive paste of an internal electrode, the metal powder being capable of improving reduction in the capacity of a capacitor associated with thinning of the internal electrode of a multi-layered ceramic capacitor. In the metal powder, the proportion, contained in the metal powder, of connected particles having an aspect ratio of 1.2 or more, a circularity of 0.675 or less, and a long diameter of three times or more the diameter of 50% of the metal powder among the connected particles formed by connecting metal particles is 500 ppm or less on a number basis.

Description

Metal powder and method for producing the same

One aspect of the present invention relates to a metal powder suitable for conductive paste applications used for electronic parts and the like, for example, conductive paste applications for internal electrodes of multilayer ceramic capacitors, and a method for producing the same.

The number of electronic components tends to increase as portable information terminals represented by smartphones and tablet terminals become multifunctional and sophisticated. For this reason, in order to mount on the main board of a limited area, the miniaturization and capacity increase of the ceramic capacitor mounted on the board are required.

With the miniaturization and increase in capacity of multilayer ceramic capacitors, thinning of internal electrodes of multilayer ceramic capacitors and reduction in resistance are required. For that purpose, it is required that the metal powder used for the internal electrode is an ultrafine powder having an average particle diameter of primary particles of 300 nm or less, 200 nm or less, and even 100 nm or less.

However, as the film thickness of the internal electrode becomes thinner, the problem that the capacitance of the capacitor is reduced becomes remarkable. This is because the dispersibility of the small particle size metal powder used for the thin layer electrode in the paste is poor, and a region in which the filling rate of the metal powder is low is generated in the electrode, and the shrinkage at the time of firing is large in that region It is considered that the volume of the electrode is reduced as a result of the generation of a large number of voids in the electrode layer.

As a means to cope with the above problems, for example, in Patent Document 1, sulfur is contained in nickel powder, and among sulfur present on the surface of nickel particles, sulfur present as sulfate ion and sulfur present as sulfide ion A nickel powder is disclosed that has improved sintering characteristics and dispersibility by defining its molar ratio.

Further, Patent Document 2 discloses a nickel powder in which the residual magnetization is reduced by adding a nonmagnetic metal element to nickel to make the a-axis length of the nickel crystal within a specific range, thereby suppressing aggregation.

International Publication Gazette "WO 2015/156080" (released on October 15, 2015) International Publication "WO2014 / 080600" (released on May 30, 2014)

However, there is a need for further solutions that improve the reduction in capacitance of the capacitor due to the thinning of the internal electrode.

Therefore, one aspect of the present invention aims to provide a metal powder suitable for the conductive paste of the inner electrode, which can improve the capacity reduction of the capacitor due to the thinning of the inner electrode of the laminated ceramic capacitor.

The present inventors have intensively studied to solve the above problems, and the proportion of particles of a specific shape in the metal powder is determined by the behavior of the metal powder, in particular, the dispersibility, the sintering start temperature, and the like. It has been found that the filling rate and the like are greatly affected, and one aspect of the present invention has been completed.

That is, in one aspect of the present invention, among connected particles formed by connecting metal particles, the aspect ratio is 1.2 or more, the degree of circularity is 0.675 or less, and the major diameter is 50% the number of metal powders The metal powder is characterized in that the proportion of connected particles that are three or more times in the metal powder is 500 ppm or less on a number basis.

According to one aspect of the present invention, the dispersibility of the metal particles in the electrode paste can be improved by setting the ratio of the connected particles having the above shape to 500 ppm or less, and the filling rate of the metal powder in the electrode Can be raised.

It is a figure which shows an example of the apparatus which manufactures a metal powder by a gaseous-phase reduction method. FIG. 2 is a view showing a metal powder production apparatus used in Example 1; It is a SEM image of the dry nickel powder obtained in Example 1.

<Metal powder>
(Component metals)
In one aspect of the present invention, the metal powder is an aggregate of metal particles, and as a metal constituting the metal particles, silicon, copper, nickel, silver, molybdenum, iron, chromium, tungsten, tantalum, Cobalt, rhenium, platinum, palladium and the like, and alloys thereof can be mentioned. Among these, nickel, molybdenum, silver, tungsten, copper, platinum, palladium and their alloys are particularly preferable. In particular, nickel, copper, silver and their alloys are most preferred. These metal powders are suitably used as paste fillers, particularly fillers for conductive pastes.

(Number 50% diameter)
In one aspect of the present invention, the upper and lower limits of the number 50% diameter of the metal powder are not particularly limited, but for example, it is preferably 400 nm or less from the use as a filler of the conductive paste for internal electrodes of multilayer ceramic capacitors The thickness is more preferably 300 nm or less, still more preferably 200 nm or less, and most preferably 100 nm or less. Further, from the viewpoint of the production cost of the metal powder and the ignitability, it is preferably 10 nm or more, more preferably 20 nm or more, still more preferably 25 nm or more, and most preferably 50 nm or more.

In addition, "the number 50% diameter" means the diameter of the particle | grains corresponded to 50% of frequency (or accumulation) in the particle size distribution in the number-based basis of the metal particle which comprises metal powder. As for the number 50% diameter of metal powder, take a picture of metal powder with a scanning electron microscope, measure the particle size of about 1,000 metal particles from the picture using image analysis software, and obtain the metal The number 50% diameter can be calculated from the particle size distribution of the powder. In this case, the "particle size" is the diameter of the smallest circle circumscribed to the projected image obtained by the image analysis of the metal particles.

(Connected particles)
The metal powder may include secondary particles in which primary particles are aggregated, in addition to independent primary particles without aggregation. Among the secondary particles, “connected particles” are secondary particles remaining in the metal powder even after being crushed by a known crusher such as, for example, a jet mill, and typically, It means secondary particles in which primary particles are fused to each other. Among such "connected particles", particles having low sphericity (also referred to as sphericity), in particular, connected particles having an elongated shape exceeding a specific reference length in which a plurality of primary particles are connected in a line It has been found that the proportion of the metal powder significantly affects the behavior of the metal powder in the paste, such as the sintering start temperature and the filling rate.

In the present specification, unless otherwise specified, “connected particles” are, for convenience, “connected particles” in particles of a metal powder taken with a scanning electron microscope, in which “aspect ratio” is 1. 2 or more, refers to connected particles that are metal particles having a “roundness” of 0.675 or less and a “major axis” of three or more times the number 50% diameter of the metal powder.

Here, the “long diameter” is the length of the long side of the rectangle having the smallest area circumscribed to the projected image of the metal particle, and the “aspect ratio” is the length of the long side of the rectangle being the short side. Divided by

Moreover, "circularity" is a value calculated | required by following formula (1).
Circularity = (4π × [projected area of metal particles]) / [projected perimeter of metal particles] ² (1)
When the circularity is 1, the projected image of the particle is a true circle, and the three-dimensional shape of the particle can be expected to be close to a true sphere. In addition, as the degree of circularity approaches 0, the three-dimensional shape of the taken image has many irregularities, which can be expected to be a complicated shape.

The proportion of "connected particles" in the metal powder was determined by taking a picture of the metal powder with a scanning electron microscope and using image analysis software from about 40,000 pieces of metal particles taken in the picture. The number ratio obtained by measuring the number of metal particles having a circularity of 0.65 or less and a major diameter of 3 times or more of the number 50% diameter of the metal powder (hereinafter referred to as “connected particles” It is sometimes expressed as "rate". In addition, it is good to refer to the Example mentioned later for the conditions etc. which prepare the sample for imaging | photography of the image of metal powder.

In one aspect of the present invention, the proportion of connected particles contained in the metal powder is preferably 500 ppm or less, and more preferably 300 ppm or less, on a number basis. When the proportion of the connecting particles is in this range, it is possible to improve the dispersibility of the metal powder in the electrode paste and to obtain the effect of increasing the filling rate of the metal powder in the electrode. The above effects can be obtained even if the metal powder is ultrafine powder with a number 50% diameter of 400 nm or less, 300 nm or less, 200 nm or less, and 100 nm or less. Therefore, by using this metal powder as a filler of the conductive paste for internal electrodes, it is possible to prevent a decrease in capacity of the capacitor due to a defect of the electrode.

(Crystallite diameter)
In one embodiment of the present invention, the ratio (crystallite diameter / number 50% diameter) of the crystallite diameter of the metal powder to the number 50% diameter of the metal powder is preferably 0.50 or more, more preferably 0.55. It is more preferable that it is more than. In the metal powder having a connected particle ratio of 500 ppm or less, when the ratio of the crystallite diameter to the number 50% diameter is 0.50 or more, the sintering characteristics of the metal powder, particularly the smoothness of the sintered coating film is further improved can do.

In addition, the crystallite diameter of a metal powder calculates | requires the half value width of a diffraction peak with X-ray-diffraction apparatus, and is computed by the formula of Scherrer shown below.

(1)
Crystallite diameter = (0.9 × [X-ray wavelength]) / ([peak half width] × cos [diffraction angle])

For example, the crystallite diameter of the Ni powder is determined from the half value width of the diffraction peak of the (111) plane, the (200) plane, and the (220) plane.

(Coarse particles)
In one aspect of the invention, the metal powder can include coarse particles. Here, the coarse particles are spherical or substantially spherical particles having an aspect ratio of less than 1.2 or a circularity of more than 0.675, and a metal whose major axis is at least three times the number 50% diameter of the metal powder. It means particles. That is, the coarse particles are primary particles or secondary particles having a large major diameter and a near spherical shape like the connected particles although the aspect ratio or the degree of circularity does not satisfy the requirements of the connected particles. The proportion of coarse particles contained in the metal powder is preferably 15 ppm or less on a number basis, and more preferably 5 ppm or less. In a metal powder having a connected particle ratio of 500 ppm or less, when the ratio of coarse particles is in this range, the electrode layer can be smoothed when used as a conductive paste filler of the internal electrode of the multilayer ceramic capacitor, It is possible to prevent a defect such as a short circuit between them.

The ratio of "coarse particles" in the metal powder was determined by taking a picture of the metal powder with a scanning electron microscope and using image analysis software from about 60,000 pieces of metal particles taken in the picture. Number ratio obtained by measuring the number of metal particles less than 1.2 or having a circularity of 0.675 or more and having a major axis of at least three times the number 50% diameter of metal powder (hereinafter referred to as “coarse particle percentage It may be written as ".

<Method of producing metal powder>
The metal powder according to an aspect of the present invention can be produced, for example, by a known method such as a gas phase method or a liquid phase method. In particular, gas phase reduction methods such as vapor phase reduction method in which metal powder is produced by contacting metal halide gas with reducing gas, or spray pyrolysis method in which pyrolyzable metal compounds are sprayed and pyrolyzed are It is easy to control the particle size of the produced metal fine powder, and spherical particles can be produced efficiently. For this reason, it is easy to control the number of 50% diameter of the metal powder, the connected particle ratio and the coarse particle ratio to be in the suitable range. Hereinafter, a vapor phase reduction method will be described as one embodiment of a particularly preferable method for producing a metal powder.

(Gas phase reduction method)
In the gas phase reduction method, the gas of vaporized metal halide is reacted with a reducing gas such as hydrogen. In particular, the gas phase reduction method is a more preferable method of producing a metal powder from the viewpoint of being able to precisely control the particle size of the metal powder to be produced, and further to prevent the generation of coarse particles.

A known method can be used as a method of obtaining a metal halide gas in the vapor phase reduction method. For example, it is possible to adopt a method in which a solid metal halide such as anhydrous cobalt chloride is heated, sublimated and transported to a reduction part by an inert gas. Alternatively, a method can be adopted in which a halogen gas is brought into contact with a solid metal as a raw material to continuously generate a metal halide gas. In particular, from the point of stability of quality such as particle size distribution and prevention of contamination to the formed metal powder, halogen gas is brought into contact with solid metal as raw material to continuously generate metal halide gas, and this metal It is preferable to introduce the halide gas directly to the reduction unit.

An example of the apparatus which manufactures a metal powder by a gaseous-phase reduction method is shown in FIG. In the apparatus shown in FIG. 1, the reactor containing the reduction reaction zone c has a bottomed cylindrical shape, and a metal halide gas nozzle a is attached to one end of the reactor, whereby metal halide gas and A mixed gas with an inert gas is supplied. Moreover, the reducing gas nozzle b is attached to the same end in the said reaction apparatus. The metal halide is reduced in the reduction reaction area c by the reducing gas supplied from the reducing gas nozzle b into the reactor, whereby the metal powder d is generated (reduction reaction step). A cooling gas nozzle e is attached to the other end of the reactor, and the metal powder d generated by the cooling gas supplied from the cooling gas nozzle e into the reactor is rapidly cooled to prevent aggregation of the metal particles. Do. The recovery pipe f is attached to the reactor, and the metal powder d is sent to the recovery device through the recovery pipe f.

(Metal halide gas)
As metal halide gas, silicon chloride (III) gas, silicon (IV) chloride gas, copper (I) chloride gas, copper (II) chloride gas, nickel chloride gas, silver chloride gas, molybdenum chloride gas (III) gas Molybdenum (V) gas, iron (II) gas, iron (III) gas, chromium (III) gas, chromium (VI) gas, tungsten (II) gas, tungsten (III) gas, Tungsten (IV) gas, tungsten (V) gas, tungsten (VI) gas, tantalum (III) gas, tantalum (V) gas, cobalt chloride gas, rhenium (III) gas, rhenium chloride (IV ) Gas, and rhenium chloride (V) gas, platinum fluoride (VI) gas, palladium fluoride (II) gas and mixtures thereof Gas and the like. Most preferably, they are nickel chloride gas, copper (I) chloride gas, copper (II) chloride gas, silver chloride gas, and a mixed gas thereof.

The metal halide gas can be generated by reacting a solid metal charged in a chlorination furnace (not shown) with a halogen gas. The temperature in the chlorination furnace is a temperature at which the raw material metal is halogenated, and may be equal to or lower than the melting point of the raw material metal. For example, when producing nickel chloride gas from metallic nickel, the temperature should be 800 ° C. or higher to sufficiently proceed the reaction, and be 1483 ° C. or lower, which is the melting point of nickel, considering the reaction rate and the durability of the chlorination furnace. A range of 900 ° C. to 1200 ° C. is preferred.

Further, it is more preferable to control the partial pressure of the metal halide gas by appropriately diluting the generated metal halide gas with an inert gas such as helium, argon, neon and nitrogen. Specifically, the amount of metal halide gas generated is adjusted by adjusting the amount of halogen gas supplied in the halogenation furnace, and the amount of mixing is adjusted by adjusting the amount of inert gas supplied to the generated metal halide gas. The partial pressure of the metal halide gas in the gas (in other words, the mol% concentration of the metal halide gas in the mixed gas) is adjusted. Here, when the partial pressure of the metal halide gas is high, the particle size of the metal powder formed is large, and as the partial pressure is reduced, the particle size decreases, the metal powder formed by the partial pressure of the metal halide gas Particle size distribution can be controlled. While being able to set arbitrarily the quality of the metal powder produced | generated by this, quality can be stabilized. When passing through the metal halide gas nozzle a shown in FIG. 1, the partial pressure of the metal halide gas in the mixed gas of the metal halide gas and the inert gas is set to 1.0 when the total pressure of the mixed gas is 1.0. 0.01 to 0.95 (Pa / Pa), more preferably 0.01 to 0.7, still more preferably 0.01 to 0.6, and most preferably 0.01 to 0.5. Such partial pressure range maintains the production efficiency of the metal powder high, while the particle size, particle size distribution, particle shape, its crystallinity, and the quality of the sinterability and the like of the target metal ultrafine powder Is a preferred embodiment for producing

In the following description, for convenience, the term "metal halide gas" may also include the meaning of "metal halide gas containing an inert gas (that is, mixed gas)".

(Reducing gas)
The reducing gas for reducing the metal halide gas includes hydrogen gas, hydrogen sulfide gas, ammonia gas, carbon monoxide gas, methane gas, and a mixed gas thereof. Particularly preferred are hydrogen gas, hydrogen sulfide gas, ammonia gas, and mixed gas thereof. When hydrogen sulfide gas is contained in the reducing gas, metal particles in the obtained metal powder may contain sulfur as a component.

In addition, it is preferable to introduce the theoretical amount (chemical equivalent) or more necessary for reduction of the metal halide gas, and the supply amount of the reducing gas supplied from the reducing gas nozzle b into the reactor is limited, and is limited Although not preferred, it is preferable to supply a reducing gas equivalent to 300 to 10000 mol%, more preferably 1000 to 6000 mol% of the theoretical amount.

(Reduction reaction area)
The “reduction reaction area” is an area that occupies a part in the reactor, and is located in the vicinity of the tip of the metal halide gas nozzle a, and the generation of metal particles by the reaction of the metal halide gas and the reducing gas is performed It is an area. The “reduction reaction area” is also an area including at least a point at which the metal halide gas supplied into the reactor starts to contact with the reducing gas, and a point at which metal particles begin to be generated, In such a case, a bright flame similar to a combustion flame emitted by gaseous fuel such as hydrocarbon is generated by black body radiation. In addition, the metal generated in the reduction reaction region forms a nucleus, and passes through the reduction reaction region while growing the nucleus.

The average temperature in the reduction reaction zone is set to a temperature at which the supplied metal halide gas can be rapidly reduced. As an example, when nickel chloride gas is used as the metal halide gas, the average temperature of the reduction reaction zone c shown in FIG. 1 is usually 900 to 2,000 ° C., preferably 1,000 to 1,800 ° C., more preferably Is 1,200-1,600.degree.

When metal particles are formed in the reduction reaction zone, the temperature of the metal particles is about 100 to 600 ° C. higher than the atmospheric temperature (average temperature) in the reduction reaction zone by the heat of reaction of the metal halide gas. To reach This "maximum temperature" may cause variations in the metal particles formed in the reduction reaction zone. Here, when the atmospheric temperature in the reduction reaction region is different depending on the place, the “highest reachable temperature” reached by the metal particles is different depending on the position where the metal particles are generated. If the variation in the maximum temperature reached is large, coupled particles and coarse particles are likely to be generated. For this reason, it is preferable that the range of the dispersion | variation with the highest achieved temperature by the heat of reaction which a metal particle emits is 80 degrees C or less, and it is more preferable that it is 50 degrees C or less. If the variation in the maximum temperature reached at the time of particle formation is large, coarse particles are likely to be generated in places where the temperature is higher than the ambient temperature, and fine particles that cause connected particles in locations where the temperature is lower than the ambient temperature Is easy to generate. Here, the connected particle rate is reduced to 500 ppm or less by setting the width of the highest attainable temperature at which particles are generated, that is, the difference between the highest value and the lowest value of the highest reachable temperature, to less than 80 ° C. And the coarse particle rate can be reduced to 15 ppm or less. Each of the metal particles having reached the “maximum reached temperature” is then discharged from the reduction reaction zone and cooled.

The temperature change at the time of particle generation in the reduction reaction region of the reactor can be determined by calculation by fluid simulation.

For fluid simulation, fluid simulation software (manufactured by ANSYS, Inc., trade name ANSYS CFX) is used to create a simulation model including a reducing portion, divided into hexahedral meshes about 2 mm apart, gas flow rate and temperature, And calculation is performed by giving the wall surface temperature of the device as the boundary condition. The k-ε model is used for the turbulent flow model and the vortex dissipation model is used for the reaction model.

In addition, as a method of reducing the temperature variation due to the difference in the place in the reaction apparatus which the reduction reaction area occupies, for example, the whole wall surface surrounding the reduction reaction area is known heating means, for example, microwave heating device, electric heater The method of heating uniformly by laser heating, a gas burner, or these combination etc. is mentioned. Moreover, it is more preferable to use a microwave heating device among these from the viewpoint of contamination prevention of an impurity, and energy efficiency.

The gas containing the metal powder generated in the above reduction reaction zone flows out of the reduction reaction zone, contacts with the cooling gas, and mixes. Thereby, the metal powder is rapidly cooled to a temperature of 400 ° C. or less (cooling step). It is possible to further suppress that the particles of the metal powder are joined together to be connected particles by rapid cooling.

The above-mentioned cooling gas is an inert gas, and nitrogen gas, helium gas, argon gas, neon gas, hydrogen gas and their mixed gas may be mentioned. The temperature of the cooling gas is usually 0 to 100 ° C., preferably 0 to 50 ° C., more preferably 0 to 30 ° C. The flow rate of the cooling gas is 50 to 300 times the flow rate of the metal powder per hour. Thereby, the cooling rate for cooling the metal powder can be made faster than 10,000 ° C./sec, and the connected particle rate can be reduced.

In the method of producing metal powder by the above vapor phase reduction method, it is desirable to adjust the concentration (partial pressure) of metal halide gas, flow rate, temperature distribution of reduction reaction area, cooling rate of generated metal powder, etc. A metal powder having a 50% diameter, a connected particle rate, and a coarse particle rate can be obtained.

The metal powder obtained by the above method preferably removes residual metal halide (washing step). The method of removing the metal halide is not particularly limited. For example, a method of removing the easily soluble metal halide by suspending the metal powder in a liquid satisfying specific conditions of controlled pH and temperature. Alternatively, the volatile metal halide can be vaporized and removed by maintaining the temperature at a high temperature below the sintering temperature of the metal powder in a reduced pressure environment. As one example, the metal powder is preferably washed by using an aqueous carbonic acid solution adjusted to a pH range of 4.0 to 6.5 as a washing solution. Unreacted metal halide gas can be suitably removed. In addition, the metal powder can be deaggregated by replacing the cleaning solution containing the metal powder with pure water or removing the carbonic acid by heating, and the metal powder can be redispersed. Therefore, the content of connecting particles can be further suitably reduced.

If the treatment is performed in the liquid phase after removing the metal halide, the metal powder slurry is dried (drying step). The drying method is not particularly limited, and known methods can be used. Specific examples thereof include flash drying, heating and drying, vacuum drying, etc., in which the catalyst is brought into contact with a high temperature gas for drying. Among these, air flow drying is preferable because it can suppress aggregation of particles.

After drying the metal powder, the metal powder may break up the aggregation of particles generated by the drying (grinding process). The method for breaking up the aggregation of the metal powder is not particularly limited, and known methods can be used. As a specific example, a jet mill, a bead mill, etc. which make particles collide with each other by a high pressure gas flow can be mentioned. When removal of aggregation is not sufficient in one pass of crushing, multiple passes of crushing may be performed, but the diameter of the crystallite may be reduced by excessive crushing and the sintering characteristics may be deteriorated. Adjustment is necessary. That is, it is preferable to adjust the number of times the metal powder is passed through the crusher so that the ratio of the crystallite diameter to the number 50% diameter of the metal powder is maintained at 0.5 or more.

The metal powder according to one aspect can be suitably produced by the production method described above. However, the invention of this application does not exclude the aspect which reduces the ratio of the connection particle | grains and the coarse particle which are contained in the said metal powder by classifying metal powder.

The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention.

Examples will be shown below, and the embodiment of the present invention will be described in more detail. Of course, the present invention is not limited to the following examples, and it is needless to say that various aspects are possible as to details.

Example 1
The apparatus shown in FIG. 2 was heated to an atmosphere temperature of 1,100 ° C. by an electric heater, and a mixed gas of nickel chloride gas and nitrogen gas was introduced from the metal halide gas nozzle a. Here, the partial pressure of the nickel chloride gas was 0.037, assuming that the total pressure of the mixed gas was 1.0. At the same time, hydrogen gas was introduced into the reactor from the reducing gas nozzle b, and nickel chloride gas was reduced in the reactor to obtain a metal powder (nickel powder) d.

During the nickel formation reaction, the nickel powder produced by the heat of reaction is heated to about 1,400 ° C., and the gas flow containing the produced nickel powder is gaseous fuel such as hydrocarbon by black body radiation of the nickel powder. It was observed as a luminous flame resembling a burning flame.

In addition, the wall surface around the region where this luminous flame is generated (that is, the reduction reaction region c) is heated with a microwave heating device g at a frequency of 2.45 GHz and an output of 4.9 kW to bring nickel chloride gas and hydrogen gas into contact. Temperature variations in the area were reduced. At this time, when the dispersion of the highest attainable temperature of nickel particles at the time of particle formation is determined using fluid simulation software (ANSYS, Inc., product name ANSYS CFX), the range of the highest reachable temperature at the time of particle generation is the largest It was 40 ° C.

In the fluid simulation, a simulation model including a reducing portion was prepared, divided into hexahedral meshes of about 2 mm intervals, and calculation was performed with the flow rate of the gas, the temperature of the gas, and the wall temperature of the apparatus as boundary conditions. The k-ε model was used for the turbulent flow model, and the vortex dissipation model was used for the reaction model.

The produced metal powder (nickel powder) d is mixed with nitrogen gas of 25 ° C. introduced from the two cooling gas nozzles e, cooled to 400 ° C. or less, and led to a bag filter (not shown) by a recovery pipe f The nickel powder was separated and collected.

The recovered nickel powder is dispersed and precipitated five times in water (in the washing solution) whose pH and temperature are properly controlled. After removing the remaining nickel chloride, the water content is 0.5 with a flash dryer. The drying process was performed so that it became% or less. Furthermore, disintegration by a jet mill was performed for one pass. The obtained nickel powder is apply | coated and photographed on a glass plate so that it may become thickness about 1 micrometer, and the SEM image of dry nickel powder is shown in FIG.

Example 2
In the reactor in the same manner as in Example 1, except that the partial pressure of nickel chloride gas in the mixed gas of nickel chloride gas and nitrogen gas is 0.15, and the output of the microwave heating device is 2.8 kW. The nickel chloride gas was reduced to produce a nickel powder. The temperature difference in the region where the nickel particles are formed was 45 ° C. at maximum.

[Example 3]
The reaction was performed in the same manner as in Example 1, except that the partial pressure of nickel chloride gas in the mixed gas of nickel chloride gas and nitrogen gas was 0.29, and the output of the microwave heating device was 3.2 kW. The nickel chloride gas was reduced to produce a nickel powder. The maximum temperature range of the nickel particles at particle formation was at most 65 ° C.

Comparative Example 1
A nickel powder was produced in the same manner as in Example 1 except that the output of the microwave heating device g was changed to 0. The maximum temperature range of the nickel particles at the time of particle formation was 84 ° C. at the maximum.

Comparative Example 2
A nickel powder was produced in the same manner as in Example 2 except that the output of the microwave heating device g was changed to 0 and crushing by a jet mill was performed in three passes. The maximum temperature range of the nickel particles at particle formation was 95 ° C. at the maximum.

Comparative Example 3
A nickel powder was produced in the same manner as in Example 3 except that the output of the microwave heating device g was changed to 0 and the partial pressure of nickel chloride gas was changed to 0.33. The maximum temperature range of the nickel particles at the time of particle formation was 90 ° C. at the maximum.

[Evaluation]
With respect to the dried nickel powders obtained in Examples 1 and 2 and Comparative Examples 1 and 2, the number 50% diameter, the connected particle ratio, the crystallite diameter, and the coarse particle ratio were measured by the following methods.

Moreover, the dispersibility to a solvent, the sintering characteristic, the filling rate, and the smoothness of the coating film were evaluated by the following methods.

a. Take a picture of metal nickel powder with a 50% diameter scanning electron microscope (product name: JSM-7800F, manufactured by Nippon Denshi Co., Ltd.), and use the image analysis software (product name: MacView 4.0, product name made by Mountech Co., Ltd.) from the photograph Then, the particle diameter of about 1,000 particles was measured to calculate the number 50% number of particles. The particle diameter is the diameter of the smallest circle circumscribed to the metal particle projection image.

b. Connection Particle Rate Photograph a picture of metallic nickel powder with a scanning electron microscope (product name JSM-7800F, manufactured by Nippon Denshi Co., Ltd.), and use the image analysis software (product name: MacView 4.0, product name, manufactured by Mountech Co., Ltd.) from the photograph. The number of connected particles with an aspect ratio of 1.2 or more and a circularity of 0.675 or less among about 40,000 particles, the major axis of which is at least three times the number 50% of the number determined above Was calculated. In this case, the length of the long side of the rectangle having the smallest area circumscribed to the metal particle projection image is taken as the long diameter, and the value of (long side length × short side length) is taken as the aspect ratio. Further, the value of (4π × [projected area of metal particle]) / [projected circumferential length of metal particle] ² is taken as the degree of circularity.

c. Crystallite diameter The crystallite diameter is the diffraction peak of the (111), (200), and (220) planes of the nickel crystal measured with an X-ray diffractometer (Spectroly Co., Ltd. Panalical Division, trade name X'Pert PRO). The half width is determined, and the value of the crystallite diameter calculated by Scherrer's formula crystallite diameter = (0.9 × X-ray wavelength) / (peak half width × cos [diffraction angle)] Average value. The measurement conditions were such that the accelerating voltage of the X-ray tube was 45 kV and the current value was 40 mA, and the wavelength of the X-ray was Cu-Kα. The incident side of the X-ray was 0.04 radian for the solar slit, 15 mm for the mask, 1/2 ° for the divergence slit, and 1 ° for the antiscatter slit. The detector side had a solar slit of 0.04 radian and a scattering prevention slit of 5.5 mm. The scan rate was 0.04 ° / s.

d. Coarse particle rate Photograph a picture of metallic nickel powder with a scanning electron microscope (Nihon Denshi Co., Ltd., trade name JSM-7800F), and use the image analysis software (Mauntech Co., Ltd. trade name MacView 4.0) from the photograph Spherical particles or spherical particles having an aspect ratio of less than 1.2 or a circularity of more than 0.675 among about 600,000 particles, and having a major diameter equal to or more than three times the 50% diameter number. Calculated.

e. Dispersibility 100ml of 5wt% aqueous solution of polycarboxylic acid dispersant was added to 0.5g of nickel powder and dispersed for 60 seconds with an output of 600W and an amplitude width of 30μm using an ultrasonic dispersator (brand name GSD600AT, manufactured by Ginsen Co., Ltd.) did. After dispersion, suction filtration is performed at a suction pressure of 0.1 MPa using a membrane filter (pore diameter: 1 μm, filter diameter: 25 mm) (manufactured by GE Healthcare Bio-Sciences Co., Ltd., trade name Nucripore membrane). The aggregation behavior of the nickel powder was evaluated as shown in Table 1.

f. Sintering properties and packing ratio 1 g of nickel powder, 3% by weight of camphor and 3% by weight of acetone are mixed, this mixture is filled into a cylindrical metal container with an inner diameter of 5 mm and a length of 10 mm and compressed at 500 MPa to give test pellets Was produced. The thermal contraction behavior of this test pellet is measured using a thermomechanical analyzer (trade name TMA 8310, manufactured by Rigaku Corporation) under a 1.5% by volume hydrogen-nitrogen reducing gas atmosphere at atmospheric pressure, a temperature rise rate of 5 ° C. It measured on the conditions of / min.

From the measurement results, 5% shrinkage temperature was determined as the sintering start temperature, and the sintering characteristics of the nickel powder were evaluated as shown in Table 2. The higher the sintering start temperature, the better the heat resistance.

Further, the filling rate of the fired body was determined from the shrinkage rate when heated to 700 ° C. by the formula (density of fired body 焼成 bulk density of nickel), and was evaluated as shown in Table 3. As the filling rate is higher, shrinkage due to firing when applied to the electrode is less likely to occur.

g. Smoothness of the coating 100 ml of a 5% by weight aqueous solution of polycarboxylic acid dispersant is added to 0.5 g of nickel powder, and an output of 600 W and an amplitude of 30 μm are obtained using an ultrasonic dispersion machine (manufactured by Ginsen Co., Ltd., trade name GSD 600 AT). It dispersed for 60 seconds. After the dispersed slurry was allowed to settle for 10 minutes to sediment, the supernatant was discarded, and about 100 mg of the sedimented slurry was applied on a quartz plate with a 5 μm applicator. A nickel coating on a quartz plate is heated in an atmosphere of 1.5 volume% hydrogen-nitrogen reducing gas at a heating rate of 5 ° C. using an electric furnace (trade name SLT-2035 D, manufactured by Motoyama Co., Ltd.) The temperature was raised under the conditions of 1 / minute, and firing was performed at 1,000 ° C. for 1 hour. The surface roughness (Sz: maximum height; height between the highest peak and the deepest valley) of the fired coating is measured with a digital microscope (trade name VHX-1000, manufactured by Keyence Corporation), The smoothness was evaluated as (Sz value / number of 50% diameter of nickel powder) as shown in Table 4.

Table 5 shows the 50% diameter, linked particle ratio, crystallite diameter, and coarse particle ratio of nickel powder of Examples and Comparative Examples in Table 5, and evaluation of dispersibility, sintering characteristics, filling ratio, and smoothness of coating film. The results are shown in Table 6.

In Example 1, no coarse particle was found in the photographed particles, so the evaluation result was set to be less than the detection limit.

As is apparent from Tables 5 and 6, although the nickel powder of Example 1 has the same number of 50% diameter as that of Comparative Example 1, the percentage of connected particles and the ratio of coarse particles are low. It can be seen that the dispersibility, the sintering characteristics, the filling rate and the smoothness of the fired coating film are excellent. In addition, although the nickel powder of Example 2 has the same number of 50% diameter as that of Comparative Example 2, the crystallite diameter is large and the number of connected particles is small, so the dispersibility, sintering characteristics and filling rate Is excellent.

From the above results, the metal powder of the present invention has excellent dispersibility, sintering characteristics, filling rate and smoothness of the fired coating film in the manufacturing process of the multilayer ceramic capacitor, and as a result, the generation of voids in the electrode layer is suppressed. It has been proved to be effective for preventing the decrease in capacity of the capacitor.

The present invention can be suitably used as a metal powder for the conductive paste of the internal electrode of a laminated ceramic capacitor.

a metal halide gas nozzle b reducing gas nozzle c reduction reaction area d metal powder e cooling gas nozzle f collection pipe g microwave heating device

Claims

Among connected particles formed by connecting metal particles, connected particles having an aspect ratio of 1.2 or more, a circularity of 0.675 or less, and a major axis of 3 times or more of the number 50% diameter of metal powder A metal powder, wherein the proportion contained in the metal powder is 500 ppm or less on a number basis.
The number of coarse particles having an aspect ratio of less than 1.2 or a circularity of more than 0.675 and a major axis of at least three times the 50% diameter of the metal powder is contained in the metal powder in a ratio of the number The metal powder according to claim 1, which is 15 ppm or less on the basis of the standard.
The metal powder according to claim 1 or 2, wherein the number 50% diameter is in the range of 10 nm or more and 400 nm or less.
The metal powder according to any one of claims 1 to 3, wherein the ratio of the crystallite diameter to the number 50% diameter of the metal powder is 0.50 or more.
A method for producing a metal powder, comprising a reduction reaction step of reacting a metal halide gas with a reducing gas,
In the reduction reaction step, the average temperature of the reduction reaction zone through which the metal halide gas passes is a temperature not higher than the melting point of the metal powder, and the width of the maximum temperature reached when forming metal particles is 80 ° C. or less A method of producing a metal powder, characterized in that
The method for producing a metal powder according to claim 5, comprising a cooling step of cooling the metal powder to a temperature of 400 ° C or less after the reduction reaction step.