JP3812359B2 - Method for producing metal powder - Google Patents

Description

【００３２】
【発明の効果】
本発明の方法では、球状で、結晶性が良く、かつ高分散性の金属粉末が容易に得られる。また原料化合物粉末を金属の融点以上の温度で加熱することにより、単結晶金属粉末を得ることが可能である。純度に影響を及ぼす添加剤や溶媒を使用しないので、不純物を含まない高純度の粉末が得られる。
【００３３】
更に本方法によれば、原料粉末の粒度コントロールにより粒径が揃った金属粉末を得ることができ、粒度の調整も容易である。従って分級工程の必要なく、粒度分布の狭い、極めて微細な、厚膜ペーストに適した粉末を得ることができる。
また原料を溶液、懸濁液状としないため、通常の噴霧熱分解法と比べて溶媒の蒸発によるエネルギーロスが少なく、ローコストで簡単に製造できる。しかも液滴の合着の問題がなく、比較的高濃度で気相中に分散させることができるため、効率が高い。
【００３４】
更に本法は、溶媒からの酸化性ガスの発生がないので、低酸素分圧下で合成する必要のある易酸化性の卑金属粉末の製造に適している。しかも化合物の選択により還元性ガスを外部から供給する必要なく、酸化を極力抑えることができるので、反応条件の設定が簡単である。しかも得られる金属粉末は活性が低く耐酸化性が良好であり、このため積層コンデンサ等の導体を形成するための導体ペーストに使用した場合、クラック等の構造欠陥のない、信頼性の高い部品を製造することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a metal powder suitable for electronics, and more particularly to a method for producing a highly crystalline metal powder useful as a conductive powder for a conductor paste.
[0002]
[Prior art]
The conductive metal powder used in the conductive paste for forming an electronic circuit has few impurities, is a fine powder having an average particle size of 0.1 μm or less to about 10 μm, and has a uniform particle shape and particle size. It is desirable that the particles are monodispersed particles that are free from aggregation. It is also required that the dispersibility in the paste is good and that the crystallinity is good so as not to cause non-uniform sintering. Especially when used for the formation of internal conductors and external conductors in multilayer ceramic electronic parts such as multilayer capacitors and multilayer inductors, in order to prevent structural defects such as delamination and cracks and to make the conductors thinner, it is finer and more granular. In addition to submicron particles with a uniform diameter and shape, there is a need for spherical, low-activity, highly crystalline or single-crystal metal powders that are resistant to expansion and contraction due to redox during firing and have a high sintering start temperature. Has been.
[0003]
That is, a multilayer ceramic electronic component generally includes an unfired ceramic green sheet such as a dielectric material or a magnetic material, and an inner conductor paste layer containing a noble metal powder such as palladium or silver-palladium or a base metal powder such as nickel or copper as a conductive component. However, when a base metal that easily oxidizes is used for the internal conductor, there are various problems. For example, when nickel powder is used as the conductive component of the internal conductor paste, the laminate is heated in an oxidizing atmosphere until the binder removal process, which is usually performed at a temperature of about 300 to 600 ° C. Completely burn and remove the organic vehicle inside. At this time, the nickel powder is slightly oxidized. Thereafter, firing is performed in an inert atmosphere or a reducing atmosphere, and reduction treatment is performed as necessary. However, it is difficult to completely reduce the nickel powder oxidized at the time of binder removal, which leads to a decrease in electrical characteristics such as an increase in resistance value. In addition, the expansion and contraction of the volume of the electrode occur along with this redox, but this volume change does not coincide with the sintering shrinkage behavior of the ceramic layer, so that structural defects such as delamination and cracks are likely to occur. In addition, nickel powder sinters quickly in a non-oxidizing atmosphere, and due to oversintering, the internal conductor becomes a discontinuous film, causing a resistance increase or disconnection, and there is a problem that the conductor thickness is increased. It is difficult to meet the demand for thinning of the inner conductor layer accompanying the increase in the number of laminated parts. Such oxidation or over-sintering is also a problem when an external conductor is formed by simultaneous firing using a nickel paste. Therefore, a highly crystalline nickel powder that is difficult to oxidize at least during binder removal and has a high sintering start temperature is desired.
[0004]
On the other hand, palladium, a noble metal, oxidizes at a relatively low temperature during firing and is reduced when heated to a higher temperature, which causes structural defects due to mismatch in sintering shrinkage behavior between the electrode layer and the ceramic layer. . Accordingly, it is desired that palladium and palladium alloy are not easily oxidized, but spherical high crystalline powder, particularly single crystal powder is very excellent in terms of oxidation resistance.
[0005]
Conventionally, a spray pyrolysis method is known as a method for producing such highly crystalline metal powder. As described in Japanese Patent Publication No. 63-31522, the spray pyrolysis method is a fine liquid obtained by spraying a solution containing one or more metal compounds or a suspension in which these are dispersed. A method of depositing a metal or alloy powder by heating the droplet at a temperature higher than the decomposition temperature of the metal compound, preferably at a temperature close to or higher than the melting point of the metal, and thermally decomposing the metal compound It is. According to this method, a highly crystalline or single crystal, high density, highly dispersible true spherical metal powder or alloy powder can be obtained. Unlike the wet reduction method, solid-liquid separation is not necessary, and manufacturing is easy, and since no additives or solvents that affect the purity are used, there is an advantage that a high-purity powder containing no impurities can be obtained. is there. In addition, the particle size can be easily controlled, and the composition of the produced particles basically matches the composition of the starting metal compound in the solution, so that there is an advantage that the composition can be easily controlled.
[0006]
However, this method uses water or an organic solvent such as alcohol, acetone or ether as a solvent or dispersion medium to form the metal compound of the raw material as droplets, so that energy loss during pyrolysis is large and cost is high. There is a problem. That is, in this process, the metal compound is thermally decomposed simultaneously with the evaporation of the solvent by heating, or after the evaporation of the solvent, the metal compound is thermally decomposed. A lot of energy is required. In addition, the particle size distribution of the generated powder may be increased due to the coalescence or breakage of droplets in the reaction vessel. For this reason, it is difficult to set reaction conditions such as the spray rate, the droplet concentration in the carrier gas, and the residence time in the reactor. In particular, in the case of base metal powders such as nickel, iron, cobalt, and copper, it is necessary to perform thermal decomposition in a strictly controlled reducing or weakly reducing atmosphere in order to prevent oxidation. Further, when water is used as a solvent, it is easily oxidized at a high temperature because of the oxidizing gas generated by the decomposition of moisture, and it is difficult to obtain a powder having good crystallinity.
[0007]
A method for producing ultrafine metal particles by a vapor phase method is also well known. For example, to obtain nickel powder, nickel chloride is evaporated and reduced with a reducing gas at a high temperature. However, the powder obtained by the precipitation reaction from the gas phase is likely to aggregate and it is difficult to control the particle size. Also, it is impossible to make alloys of metals with different vapor pressures with a precisely controlled composition.
[0008]
Japanese Patent Publication No. 11-503205 describes a method of reducing a metal compound powder such as tungsten oxide by a solid-gas reaction using a reducing agent. In this method, the raw metal compound powder is chemically reduced by passing it through a heated reaction chamber together with a gaseous reducing agent and optionally a carrier gas in a predetermined track. This reaction occurs when the reducing gas comes into contact with the raw material to be reduced. Therefore, if the raw material is a solid powder, the area in contact with the gas is small compared to the above-described gas phase method, so that it can be completely completed in a short time. It is difficult to reduce to metal. Even when using a cyclone to lengthen the track or rupture the particles, unreacted products and incompletely reduced products are likely to remain, so processes such as track and reducing gas supply methods Setting parameters is difficult. Therefore, it is considered difficult to obtain a spherical highly crystalline powder having a uniform particle size suitable for electronics.
[0009]
In Japanese Patent Publication No. 36-9163, silver, nickel, copper organic polybasic acid salt is thermally decomposed in air or inert gas at a relatively low temperature, ie, 100 to 500 ° C. A method for producing the mixture is described, and it is also stated that fine metal powder with a particle size of several microns or less is obtained by grinding. However, in this method, not only the particle size cannot be strictly controlled, but also the particle shape cannot be maintained when heated to a temperature above or near the melting point of the metal in order to increase crystallinity. It can no longer be reduced to several microns or less.
[0010]
[Problems to be solved by the invention]
The object of the present invention is a spherical, uniform particle size, high purity, high density, high dispersibility and fine high crystallinity suitable for thick film pastes, especially for conductive pastes for producing ceramic multilayer electronic components. It is to obtain metal powder. Another object is to provide a novel method for producing such metal powder in a low cost and simple manner.
[0011]
[Means for Solving the Problems]
That is, in the present invention, one or more kinds of thermally decomposable metal compound powders are supplied to a reaction vessel using a carrier gas, and the metal compound powders are dispersed in the gas phase at a concentration of 10 g / l or less. A highly crystalline metal powder, characterized by being heated at a temperature higher than its decomposition temperature and having a melting point of the metal Tm ° C. (Tm−200) ° C. or higher. The manufacturing method is the gist.
[0012]
The present invention also provides a method for producing the highly crystalline metal powder, wherein the metal compound powder is a homogeneous mixed powder or composite powder of metal compounds containing two or more metal elements, and the metal powder is an alloy powder. To do.
Furthermore, the present invention provides a highly crystalline metal powder produced by the above method, a conductor paste containing the highly crystalline metal powder, and a ceramic laminated electron comprising a conductor layer formed using the conductor paste. The gist of the parts.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The metal powder produced by this method is not particularly limited, but is particularly suitable for the production of base metal powders such as copper, nickel, cobalt and iron and noble metal powders such as silver, palladium, gold and platinum. By selecting the raw metal compound powder, a mixed powder or alloy powder of a plurality of metals can be obtained. The “metal powder” of the present invention includes such mixed powders and alloy powders.
[0014]
Thermally decomposable metal compound powders used as raw materials for metal powders include inorganic substances such as hydroxides, nitrates, sulfates, carbonates, oxynitrates, oxysulfates, chlorides, oxides, ammonium complexes, and phosphates. Compounds and organic compounds such as carboxylates, resinates, sulfonates, acetylacetonates, and monovalent or polyvalent alcoholates of metals are used. In particular, hydroxides, carbonates, carboxylates, resinates, acetylacetonates, alcoholates and the like are preferred because they do not produce harmful by-products after thermal decomposition.
[0015]
Two or more kinds of metal compound powders can be mixed and used as the raw material powder.
When producing an alloy powder, the raw material powder of the alloy component metal may be simply mixed and supplied at a predetermined composition ratio. However, a plurality of metal components are contained in one particle of the raw material powder at a constant composition ratio. It is desirable to use a composite powder that has been pre-composited to be included. As a method for compounding, there are known methods such as a method in which a metal compound powder as a raw material is mixed in advance, heat-treated until the composition becomes uniform and then pulverized, a sol-gel method, and a coprecipitation method. Double salts, complex salts, metal double oxides, and the like may also be used.
[0016]
In this method, it is considered that one metal or alloy particle is generated per one pyrolytic metal compound particle of the raw material. For this reason, the ratio of the particle size of the generated metal particles varies depending on the type of the metal compound, but is approximately proportional to the particle size of the raw material powder. Therefore, in order to obtain a metal powder having a uniform particle size, metal powder having a uniform particle size is used. When the particle size distribution of the raw material powder is wide, it is desirable to adjust the particle size by pulverizing, crushing or classifying in advance with a pulverizer or classifier before dispersing in the gas phase.
[0017]
In the present invention, it is important to thermally decompose the solid metal compound powder in a state dispersed in the gas phase. Moreover, in the reaction vessel, it is necessary to perform pyrolysis in a state where the raw material powder is dispersed at a low concentration so that the raw material powder and the generated powder do not collide with each other. For this reason, the concentration in the gas phase must be 10 g / l or less. If the concentration is higher than this, a metal powder having a uniform particle size cannot be obtained due to collision between the powders. The dispersion concentration can be appropriately determined according to the dispersion apparatus and the thermal decomposition apparatus to be used. If it is 10 g / l or less, there is no particular limitation. However, if the concentration is too low, production efficiency deteriorates, so 0.01 g / l or more is desirable.
[0018]
The means for dispersing the raw material powder in the gas phase is not particularly limited, and an ordinary disperser is used. In order to perform thermal decomposition while maintaining a low concentration dispersion state, for example, a tubular reaction vessel heated from the outside is used, and the raw material powder is supplied together with a carrier gas from one opening at a constant flow rate. The metal powder produced by being pyrolyzed through the inside is recovered from the other opening. The flow rate and transit time of the powder and carrier gas mixture are set according to the apparatus used so that the powder is sufficiently heated to a predetermined temperature. Heating may be performed from the outside of the reaction vessel in an electric furnace, a gas furnace, or the like, or fuel gas may be supplied to the reaction vessel and the combustion flame may be used.
[0019]
The carrier gas is not particularly limited in the case of a noble metal, and an oxidizing gas such as air, oxygen or water vapor, an inert gas such as nitrogen or argon, a mixed gas thereof or the like is used. In the case of base metals such as nickel and copper that are easily oxidized, an inert gas is used. However, in order to enhance the antioxidant effect by making the atmosphere during thermal decomposition weakly reducible, a reducing gas such as hydrogen, carbon monoxide, or methane is used in combination. May be.
[0020]
One of the features of the present invention is that no strict atmosphere adjustment is required during heating. In particular, in the case of base metals, by using as a raw material a metal compound that can generate carbon monoxide, hydrogen, methane, etc. when pyrolyzed in an inert gas to create a reducing atmosphere, it can be reduced to the reaction system from the outside. There is no need to supply gas and a metal powder with almost no oxidation is obtained. For example, when nickel powder is produced by a spray pyrolysis method using a conventional aqueous solution, it is usually necessary to introduce a strictly controlled amount of reducing gas in order to suppress nickel oxidation. However, in the method of the present invention, for example, when a carboxylate powder such as nickel acetate is used as a raw material and pyrolysis is performed in a nitrogen atmosphere, carbon monoxide and hydrogen are generated by decomposition of the carboxylic acid radical, and the inside of the reactor is a reducing atmosphere. Therefore, nickel powder with almost no oxidation can be obtained.
[0021]
The metal powder obtained by the method of the present invention is spherical primary particles having a uniform particle size and no aggregation. Also, the crystallinity is good, there are few defects inside the particles, and there are almost no grain boundaries. For this reason, the activity is low despite being a fine powder. In particular, base metals such as nickel, iron, cobalt, copper, and easily oxidizable metals such as palladium are not easily oxidized, and can be stored stably in the air, while maintaining oxidation resistance up to high temperatures. Therefore, when used as a conductor paste for inner conductors and outer conductors of multilayer capacitors, there is an increase in resistance due to oxidation of conductive metals, and occurrence of structural defects such as delamination and cracks due to oxidation and reduction during firing. Therefore, a capacitor having excellent characteristics can be manufactured.
[0022]
When the heating temperature is lower than Tm ° C. when the melting point of the target metal or alloy is Tm ° C. (Tm−200), spherical high crystalline metal powder cannot be obtained. In particular, in order to obtain a true spherical single crystal metal powder with a smooth surface, it is desirable to carry out at a temperature higher than the melting point or higher of the metal or alloy for the heat treatment. In addition, when the metal produces oxides, nitrides, carbides, or the like during or after thermal decomposition, it is necessary to perform heating under conditions that decompose these.
[0023]
【Example】
Next, the present invention will be specifically described with reference to Examples and Comparative Examples.
Example 1
Nickel acetate tetrahydrate powder was supplied to an air-flow type pulverizer at a supply rate of 5 kg / hr, and pulverized and dispersed with nitrogen gas at a flow rate of 200 l / min. The obtained nickel acetate tetrahydrate powder had an average particle size of about 1.0 μm and a maximum particle size of about 3.0 μm. The concentration of nickel acetate tetrahydrate powder in the gas phase is 0.4 g / l. While maintaining this powder concentration, this gas-solid mixture was introduced into a reaction tube in an electric furnace heated to 1550 ° C., heated and decomposed, and the powder produced by the bag filter was collected.
[0024]
When the obtained powder was analyzed with an X-ray diffractometer, it was confirmed to be a metal nickel single crystal powder. Further, when observed with a scanning electron microscope (SEM), it was a true spherical particle having no aggregation, an average particle diameter of 0.5 μm, and a maximum particle diameter of 2.0 μm. When thermogravimetric analysis was performed in air, no oxidation occurred up to 350 ° C. Since the oxidation temperature of the polycrystalline nickel powder having an average particle size of 0.5 μm obtained by the wet method is 250 ° C., it can be seen that the nickel powder of the present invention is a stable powder.
[0025]
Examples 2 and 3
Nickel powder was produced in the same manner as in Example 1 except that the temperature of the electric furnace was 1300 ° C. and 1650 ° C., respectively. The characteristics of the obtained powder are shown in Table 1.
Example 4
Nickel acetate tetrahydrate powder was pulverized and dispersed with nitrogen gas in the same manner as in Example 1 except that the supply rate to the pulverizer was 62.5 kg / hr. This gas-solid mixture was introduced into a reaction tube heated to 1550 ° C. in the same manner as in Example 1 and pyrolyzed to produce nickel powder. When introduced into the reaction tube, the nickel acetate tetrahydrate powder had an average particle size of about 2.5 μm, a maximum particle size of about 6.0 μm, and a powder concentration in the gas phase of 5.0 g / l. The characteristics of the obtained powder are shown in Table 1.
[0026]
Examples 5 and 6
Nickel powder was produced in the same manner as in Example 1 except that nickel formate dihydrate powder and nickel oxalate dihydrate powder were used instead of nickel acetate tetrahydrate powder, respectively. The characteristics of the obtained powder are shown in Table 1.
Comparative Example 1
Nickel acetate tetrahydrate powder was pulverized and dispersed with nitrogen gas in the same manner as in Example 1 except that the supply rate to the pulverizer was 150 kg / hr. This gas-solid mixture was introduced into a reaction tube heated to 1550 ° C. in the same manner as in Example 1 and pyrolyzed to produce nickel powder. The average particle diameter of the nickel acetate tetrahydrate powder when introduced into the reaction tube was about 5.0 μm, and the powder concentration in the gas phase was 12.0 g / l. When the obtained powder was observed with an SEM, a large number of highly crystalline particles were fused to form huge amorphous particles, and the particles had a wide particle size distribution.
[0027]
Comparative Example 2
Nickel powder was produced in the same manner as in Example 1 except that the temperature of the electric furnace was 1100 ° C. As shown in Table 1, the obtained powder was amorphous and had a wide particle size distribution, was an aggregate of fine crystals, and had low crystallinity. The oxidation resistance was also low.
Example 7
Nickel acetate tetrahydrate powder and copper acetate powder were mixed in advance so that the weight ratio of metal components was Ni: Cu = 7: 3, and powder was produced in the same manner as in Example 1.
[0028]
The obtained powder was confirmed to be a single crystal nickel-copper alloy by X-ray diffraction. The characteristics are shown in Table 1.
Example 8
Palladium chloride powder is used as the raw material, the dispersion concentration in the gas phase when introduced into the reaction tube is 1.0 g / l, air is used as the grinding gas and carrier gas, and the temperature of the electric furnace is 1600 ° C. A powder was produced in the same manner as in Example 1 except that
[0029]
The obtained powder was confirmed to be a metal palladium single crystal powder by X-ray diffraction. The characteristics are shown in Table 1.
Example 9
Palladium chloride powder and silver acetate powder are mixed in advance so that the weight ratio of metal components is Pd: Ag = 2: 8, so that the dispersion concentration in the gas phase is 0.4 g / l, and the electric furnace A palladium-silver alloy single crystal powder was obtained in the same manner as in Example 8 except that the temperature was 1400 ° C. The characteristics are shown in Table 1.
[0030]
Comparative Example 3
A powder was produced in the same manner as in Example 9 except that the temperature of the electric furnace was 900 ° C. As shown in Table 1, the obtained powder was a mixture of palladium-silver alloy powder and palladium oxide powder having low crystallinity.
Further, from the results of Example 9 and Comparative Example 3, it can be seen that the palladium-silver alloy powder obtained in the present invention is extremely excellent in oxidation resistance.
[0031]
[Table 1]

[0032]
【The invention's effect】
According to the method of the present invention, a spherical metal powder having good crystallinity and high dispersibility can be easily obtained. Moreover, it is possible to obtain a single crystal metal powder by heating the raw material compound powder at a temperature equal to or higher than the melting point of the metal. Since no additive or solvent affecting the purity is used, a high-purity powder containing no impurities can be obtained.
[0033]
Furthermore, according to this method, a metal powder having a uniform particle size can be obtained by controlling the particle size of the raw material powder, and the particle size can be easily adjusted. Therefore, it is possible to obtain a very fine powder suitable for a thick film paste with a narrow particle size distribution without the need for a classification step.
In addition, since the raw material is not in the form of a solution or suspension, energy loss due to evaporation of the solvent is less than that in a normal spray pyrolysis method, and the production can be easily performed at low cost. In addition, there is no problem of droplet coalescence, and since it can be dispersed in the gas phase at a relatively high concentration, the efficiency is high.
[0034]
Furthermore, this method is suitable for the production of an easily oxidizable base metal powder that needs to be synthesized under a low oxygen partial pressure because no oxidizing gas is generated from the solvent. Moreover, since it is not necessary to supply a reducing gas from the outside by selecting a compound, the oxidation can be suppressed as much as possible, so that the reaction conditions can be easily set. Moreover, the obtained metal powder has low activity and good oxidation resistance, and therefore, when used as a conductor paste for forming a conductor such as a multilayer capacitor, a highly reliable part free from structural defects such as cracks. Can be manufactured.

Claims (9)

One or more kinds of thermally decomposable metal compound powders are supplied to a reaction vessel using a carrier gas, and the metal compound powders are dispersed in the gas phase at a concentration of 10 g / l or less. A method for producing a highly crystalline metal powder, wherein the metal powder is produced by heating at a temperature higher than the decomposition temperature and the melting point of the metal is Tm ° C (Tm-200) ° C. The method for producing a highly crystalline metal powder according to claim 1, wherein the metal compound powder is a homogeneous mixed powder or composite powder of metal compounds containing two or more kinds of metal elements, and the metal powder is an alloy powder. The method for producing a highly crystalline metal powder according to claim 1 or 2, wherein the carrier gas is an inert gas, and the metal powder is nickel powder, copper powder or alloy powder containing nickel and / or copper. The highly crystalline metal powder according to any one of claims 1 to 3, wherein the metal compound powder is a metal compound powder that can be pyrolyzed in an inert gas so that the atmosphere during pyrolysis can be reduced. Manufacturing method. The method for producing a highly crystalline metal powder according to claim 4, wherein the metal compound powder is a metal carboxylate powder. The method for producing a highly crystalline metal powder according to claim 1 or 2, wherein the metal powder is a palladium powder or an alloy powder containing palladium. A highly crystalline metal powder produced by the method according to claim 1. A conductor paste comprising the highly crystalline metal powder according to claim 7. A ceramic multilayer electronic component, wherein a conductor layer is formed using the conductor paste according to claim 8.