JP7490528B2 - Copper Powder - Google Patents

Description

The present invention relates to copper powder.

Metal powders, which are aggregates of fine metal particles, and conductive pastes containing metal powders are widely used as raw materials for manufacturing various electronic components, such as wiring and terminals for low-temperature co-fired ceramic (LTCC) substrates and internal and external electrodes for multilayer ceramic capacitors (MLCC). Copper powder in particular is expected to be a replacement for nickel powder and silver powder, which have been widely used in the past, because the high conductivity of copper makes it possible to thin the internal electrodes of MLCCs and to miniaturize their external electrodes, as well as to significantly improve frequency characteristics.

For example, the following technology has been proposed for copper powder for conductive paste used to form electrodes for MLCCs:

Patent Document 1 discloses surface-treated copper powder with excellent sintering retardation. It describes how the sintering start temperature can be increased by subjecting the copper powder to a coupling treatment to provide a surface treatment layer made of elements such as Al and Si on the copper powder surface.

Patent Document 2 describes copper powder that has a volume cumulative particle size D50 measured by a laser diffraction scattering type particle size distribution measuring device of 0.20 μm to 0.70 μm, a ratio of crystallite size to D50 (crystallite size/D50) of 0.15 to 0.60 (μm/μm), and a ratio of oxygen content (O content) to specific surface area of 0.10 to 0.40 (wt% g/m2).

JP 2003-16832 A International Publication No. 2015/122251

The melting point of copper (about 1083 degrees) is lower than that of nickel (about 1455 degrees), and as the copper powder is refined, the specific surface area of the copper powder increases, so the melting point of the copper powder is further reduced. For this reason, when fine copper powder is used for the internal electrodes of a multilayer ceramic capacitor (MLCC), the copper powder starts to melt at a relatively low temperature, causing cracks in the electrode layer. In addition, the lower the melting point of the copper powder, the more likely it is that the electrode layer will shrink when the temperature is lowered, causing peeling between the dielectric layer and the electrode layer. Therefore, in order to solve such problems, it is important that the sintering start temperature of the electrode layer is close to the sintering start temperature of the dielectric layer. Specifically, it is necessary to increase the sintering start temperature of the copper powder that forms the electrode layer. In addition, when copper powder is used as a conductive paste, it is desirable to further improve the filling property of the copper powder.

However, although Patent Document 1 describes increasing the sintering start temperature, the only means for achieving this is to perform a surface treatment (coupling treatment) on the copper powder surface, and there is no mention of the issue of further improving the packing property.

In addition, in Patent Document 2, the objective is to provide a copper powder that can ensure low compression resistance and excellent conductivity even if it is fine copper powder, and there is no intention to further improve the packing property, as it is described that copper powder containing spherical or nearly spherical copper particles can obtain particularly excellent dispersibility.

Therefore, the present invention aims to provide copper powder that has a high sintering initiation temperature and high packing properties.

The copper powder according to one embodiment of the present invention has an average particle size ( _D50SEM ) of 0.1 μm or more and 0.5 μm or less, an average circularity of 0.80 or more, and a relationship between the average particle size ( _D50SEM ) and the BET specific surface area (S _BET ) satisfying the relationship (Formula 1).

The copper powder may have a ratio (D/D _50SEM ) of the average crystallite size (D) to the average particle size (D _50SEM ) of 0.1 or more and 0.5 or less.

The copper powder may also have a span S, expressed by formula 2, of 0.5 or more and 1.0 or less.

（ここで、（式２）のＤ_{９０ＳＥＭ}およびＤ_{１０ＳＥＭ}は、それぞれ電子顕微鏡観察の画像解析で得られた体積基準の粒子径ヒストグラムにおける累積頻度が９０％および１０％のときの粒子径を表す）

(Here, _D90SEM and _D10SEM in (Equation 2) represent particle sizes at which the cumulative frequencies in a volume-based particle size histogram obtained by image analysis of electron microscope observation are 90% and 10%, respectively.)

The copper powder may also have a thermal shrinkage rate of 17% or less at 1000°C.

According to the present invention, it is possible to provide copper powder with a high sintering start temperature and high packing properties. In addition, because the copper powder of the present invention has a high sintering start temperature, when used in the internal electrodes of an MLCC, it is possible to suppress the occurrence of cracks in the electrode layer and peeling between the dielectric layer and the electrode layer due to the shrinkage of the electrode layer when the temperature is lowered.

The following describes in detail the embodiments of the present invention. However, the present invention can be implemented in various forms without departing from the spirit of the invention, and should not be construed as being limited to the description of the embodiments and examples exemplified below.

[1. Manufacturing method of copper powder]
The copper powder according to one embodiment of the present invention is a copper powder containing a plurality of copper particles. Hereinafter, a method for producing the copper powder according to this embodiment will be described.

[1-1. Generation of copper chloride gas]
In the method for producing copper powder according to the present embodiment, copper chloride gas is used. Copper chloride gas is produced by reacting metallic copper with chlorine gas using metallic copper as a raw material. The copper chloride gas produced by this production method uses metallic copper, which is less expensive than copper chloride, as a raw material instead of copper chloride, and can reduce costs. In addition, the amount of copper chloride gas produced can be controlled by using metallic copper, so the supply amount of copper chloride gas can be stabilized.

A specific method for producing copper chloride gas is to react metallic copper with chlorine gas at or below its melting point (e.g., 800°C to 1000°C) to produce copper chloride gas. The chlorine gas may contain substantially only chlorine, or it may be a mixed gas containing a diluting inert gas such as nitrogen or argon. By using a mixed gas, it is possible to easily and precisely control the amount of chlorine reacted with metallic copper.

[1-2. Reduction of copper chloride]
Next, the generated copper chloride gas is reacted with a reducing gas to reduce the copper chloride, thereby generating a primary powder of copper powder. As the reducing gas, for example, hydrogen, hydrazine, ammonia, methane, etc. can be used. The reducing gas can be used in an amount equal to or greater than the stoichiometric amount of the copper chloride gas. The generated primary powder of copper powder is quenched with an inert gas. In this embodiment, the cooling rate after the reduction reaction is related to the generation of linked particles, and appropriate conditions for these will be described later.

[1-3. Reduction of chlorine components]
In the reduction of copper chloride, hydrogen chloride is also produced along with copper powder. Hydrogen chloride is also produced by the reaction of unreacted chlorine with a reducing gas. These hydrogen chlorides contribute to a decrease in the purity of the copper powder. If chlorine derived from hydrogen chloride remains in the copper powder as copper chloride, it will accelerate the deterioration of electrodes and wiring made using the copper powder. Therefore, the copper powder obtained by the above manufacturing method may be treated to reduce the chlorine content of the copper powder.

Specifically, the chlorine components can be removed by treating the copper powder with an aqueous solution or suspension of a base. Examples of the aqueous solution of the base include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. The base concentration may be 0.1 mol/L or more, or 0.5 mol/L or more, and may be 1.5 mol/L or less, or 1.2 mol/L or less.

[1-4. Reduction of oxygen components]
Since copper is a metal that is relatively easily oxidized, the oxidation of copper powder is likely to progress not only to the surface of the copper particles but also to the inside. As the oxidation progresses, a layer of copper oxide is formed on the surface of the copper particles, and unevenness occurs. Such unevenness caused by oxidation causes a decrease in the conductivity of wiring and electrodes formed using the copper powder and a decrease in the flatness of the surface, which results in an increase in electrical resistance and poor contact in electronic components. In addition, the thermal shrinkage rate during sintering increases, making it easier for wiring and electrodes to peel off. Therefore, the copper powder obtained by the above manufacturing method may be treated to remove copper oxide or reduce the oxygen content in order to reduce the oxygen content of the copper powder.

Specifically, after the copper powder has been treated to reduce the chlorine content, the oxygen content is reduced by using a solution or suspension containing ascorbic acid, hydrazine, citric acid, etc. as a cleaning liquid. The copper powder is then washed with water, filtered, and dried.

In the following, solutions and suspensions that can be used in the process of reducing oxygen content will be collectively referred to as cleaning solutions. The solvents in cleaning solutions include water, alcohols such as ethanol and isopropyl alcohol, ketones such as acetone and methyl ethyl ketone, and mixtures of these solvents.

In this embodiment, the concentration of ascorbic acid is related to the occurrence of unevenness due to oxidation, so appropriate conditions for this will be described later.

[1-5. Surface treatment]
As described above, copper is a metal that is relatively easily oxidized. Therefore, in order to suppress the oxidation of the surfaces of the copper particles, the copper powder obtained by the above-mentioned production method may be subjected to a surface treatment.

Specifically, the copper powder is treated with a solution or suspension containing a surface treatment agent. Examples of the surface treatment agent that can be used include nitrogen-containing heteroaromatic compounds such as benzotriazole and its derivatives, triazole and its derivatives, thiazole and its derivatives, benzothiazole and its derivatives, imidazole and its derivatives, and benzimidazole and its derivatives.

[1-6. Other steps]
As an optional step, the obtained copper powder may be subjected to a step of drying, classification, crushing, sieving, etc. Classification may be dry classification or wet classification, and in the case of dry classification, any method such as air classification, gravity field classification, inertial field classification, centrifugal field classification, etc. may be adopted. Crushing may be performed, for example, using a jet mill. Sieving may be performed by vibrating a sieve having a desired mesh size and passing the copper powder through it. By performing classification, crushing, and sieving treatments, it is possible to further narrow the particle size distribution of the copper powder.

2. Characteristics of copper powder
The characteristics of the copper powder according to one embodiment of the present invention are as follows:

[2-1. Average particle size]
The average particle size of the copper powder refers to the particle size at which the cumulative frequency in the volumetric particle size histogram of the copper powder is 50%. This is the particle size weighted by the volume of each particle contained in the powder. As expressed by the following formula, the particle size d _i (i is a natural number from 1 to k, i≦k) of particles The frequency F of particles having a particle diameter d _i is obtained by dividing the total volume by the total volume of all particles contained in the powder. The particle diameter at which this frequency F is accumulated and reaches 50% is the median. Here, the average particle diameter obtained by image analysis of observation with an electron microscope is expressed as D _{50 SEM} , and the average particle diameter obtained by laser diffraction method is expressed _{as D 50 .}

Here, V _i is the volume of a copper particle having a particle diameter d _i , and n _i is the number of copper particles having a particle diameter d _i .

The following describes how to calculate the volume V _i and particle diameter d _i in the average particle diameter (D _50SEM ) obtained by image analysis of an electron microscope. In a micrograph of copper powder observed with an optical microscope or an electron microscope, copper particles (e.g., 100 to 10,000, typically 500) whose outlines have been confirmed are visually observed. Next, the particle diameter d _i is calculated from the surface area S _i of the visually observed copper particles according to the following formula.

Next, the volume V _i of the copper particles is calculated from the calculated particle diameter d _i by the following formula.

Specifically, for example, a scanning electron microscope (SEM: SU5000 manufactured by Hitachi High-Technologies Corporation) can be used to analyze approximately 500 copper particles present in one field of view of an SEM image at a magnification of 15,000 times using image analysis software (Macview 4.0 manufactured by Mountec Co., Ltd.).

Moreover, the average particle size (D ₅₀ ) obtained by the laser diffraction method refers to the volume-based average particle size, as described above.

Specifically, for example, a particle size distribution measuring device (LA-960 laser diffraction/scattering particle size distribution device manufactured by Horiba, Ltd.) is used to create a particle size distribution on a volume basis, and the median value is used as the average particle size.

The average particle size ( _D50SEM ) of the copper powder according to this embodiment is preferably 0.1 μm or more and 0.5 μm or less. By sintering the copper powder satisfying this range, a metal film with a small thickness can be obtained, which can contribute to, for example, thinning of electrodes of MLCC, and finer wiring and terminals of electronic components such as LTCC substrates. In addition, the aggregation of copper particles can be suppressed, which makes the copper powder easier to handle.

In order to meet the challenge of further thinning the electrode layer, the upper limit of the average particle size (D _50SEM ) must be 0.5 μm or less, preferably 0.4 μm or less, and more preferably 0.35 μm or less. The lower limit of the average particle size (D _50SEM ) must be 0.1 μm or more. Furthermore, copper powder with a small lower limit of the average particle size (D _50SEM ) is difficult to manufacture, and if the average particle size (D _50SEM ) is too small, the copper particles may easily aggregate with each other, making handling difficult. Copper powder with an average particle size (D _50SEM ) of 0.1 μm or more and 0.5 μm or less can be obtained by optimizing the conditions of the vapor phase growth method.

The average particle size ( _D50SEM ) and average particle size ( _D50 ) of the copper powder can be controlled within the above-mentioned ranges by appropriately setting conditions such as the temperature during chlorination of metallic copper, the flow rate of chlorine gas, the ratio of chlorine gas to diluent gas, the temperature during reduction of copper chloride, and the flow rate ratio of copper chloride gas to reducing gas.

[2-2. BET specific surface area (S _BET )]
The specific surface area is the surface area per unit amount (usually per gram). The specific surface area is an important physical property value related to the structure of a surface or the structure of a particle.

The BET method is a method for measuring the surface area of a powder by a gas-phase adsorption method. In the BET method, the surface area of 1 g of a sample, i.e., the specific surface area, is calculated from the adsorption isotherm. That is, the specific surface area can be calculated by calculating the amount of adsorption based on the BET formula and multiplying it by the area of the surface occupied by one adsorbed molecule. For example, nitrogen gas, argon gas, krypton gas, carbon monoxide gas, or carbon dioxide gas can be used as the adsorbed gas, and the amount of adsorption can be measured from the change in pressure or volume of the adsorbed gas. In this specification, the specific surface area measured by the BET method using nitrogen gas as the adsorbed gas is described as the BET specific surface area (S _BET ).

Specific measurements using the BET method involve vacuum degassing at a temperature of 120 degrees as a pretreatment, adsorbing nitrogen gas as the adsorption gas, and then calculating the BET specific surface area using the BET formula. Specifically, the BET specific surface area can be calculated using, for example, a fully automatic specific surface area measuring device, a Macsorb (registered trademark) measuring device (manufactured by Mountec Co., Ltd.).

[2-3. Relationship between _D50SEM and S _BET ]
Moreover, the BET specific surface area (S _BET ) and the average particle diameter (D _{50 SEM} ) of the copper powder according to this embodiment satisfy the relationship expressed by (Formula 1).

The left side of Equation 1 can be expanded as follows: Here, it is assumed that the copper particles are true spheres with a diameter D _{50 SEM} , and the density of the copper particles is ρ.

Therefore, the left side of (Equation 1) can be said to be a parameter that is correlated with density.

When the density of the copper particles is high, it can be considered that the bonds between the copper atoms are strong. Conversely, when the density of the copper particles is low, it can be considered that the bonds between the copper atoms are weak. Therefore, the left side of (Equation 1) can be considered to represent the strength of the bonds between the copper atoms. Copper powder that satisfies (Equation 1) has strong bonds between the copper atoms of the copper particles, a high sintering start temperature, and can suppress the occurrence of cracks in the electrode layer and peeling between the dielectric layer and the electrode layer due to shrinkage of the electrode layer when the temperature is lowered.

Furthermore, _D50SEM and S _BET are inversely proportional when the particle shape is uniform. If the particle shape is elliptical or if there are irregularities on the particle surface, S _BET increases, and the value of 6/( _D50SEM ·S _BET ) decreases. Therefore, the inventors of the present invention considered that focusing on 6/( _D50SEM ·S _BET ) would lead to an index of the proportion of spherical particles, and used it as one of the evaluation indexes for packing property.

[2-4. Average circularity]
The copper powder of the present invention has a shape close to a perfect sphere. More specifically, the average circularity of the copper powder is, for example, preferably 0.80 or more, preferably 0.88 or more, and more preferably 0.91 or more. The average circularity is one of the parameters that represent the shape of the copper powder, and is a value obtained by analyzing an image obtained by observing the copper powder under a microscope, determining the circularity C of a plurality of particles (for example, 500 particles), and averaging the results. The circularity C of each individual copper particle is expressed by the following formula.

Here, A is the perimeter of the projected surface of each particle in the microscope image, and B is the perimeter of a circle with an area equal to the area of this projected surface. The copper powder of the present invention has a high average circularity, which results in good packing properties.

[2-5. Average crystallite size]
The crystallite size is an index that indicates the length of the region that can be considered as a single crystal. Each copper particle has a single or multiple crystallites. The average crystallite size is the average of the individual copper particles. The average crystallite size is the average value of the crystallite size. The average crystallite size is determined by measuring various parameters (wavelength λ of the X-ray used, half width of the diffracted X-ray) obtained by X-ray diffraction measurement of copper powder. The value is defined as a value obtained by substituting the above-mentioned parameters (angle β, Bragg angle θ) into the Scherrer equation shown below, where K is the Scherrer constant.

Specific measurement conditions for the average crystallite size are an acceleration voltage of 45 kV and a discharge current of 40 mA. For example, an X-ray diffractometer (Spectris X'PertPro) is used to determine the half-width of the diffraction peak of the (111) plane of the copper crystal with CuKα radiation, and the average crystallite size can be calculated using the Scherrer formula shown above. In this specification, the average crystallite size is referred to as D.

[2-6. Ratio (D/D _50SEM )]
The ratio (D/D _50SEM ) of the average crystallite diameter (D) to the average particle diameter (D _50SEM ) of the copper powder according to this embodiment is preferably 0.10 or more and 0.50 or less. The copper powder contained in the above can be obtained by vapor phase growth under appropriate growth conditions. The vapor phase growth method can produce copper powder with a large average crystallite size due to the high temperature at which it is grown. If the ratio is 0.10 or more, the average crystallite size becomes large and the sintering start temperature becomes high, which is preferable. If the ratio is more than 0.50, it is preferable that the crystallite size is so large. Since copper powder is difficult to obtain, the upper limit of the average crystallite size is preferably 0.50.

[2-7. Ratio ( _D50 / _D50SEM )]
The ratio of the average particle diameter (D ₅₀ ) obtained by a laser diffraction method to the average particle diameter (D 50 _SEM ) obtained by image analysis of the copper powder observed by an electron microscope according to this embodiment (average particle diameter (D ₅₀ )/average The particle size (D _{50 SEM} ) is not more than 1.8, preferably not more than 1.7.

When the average particle size (D ₅₀ )/average particle size (D _{50 SEM} ) is within the above range, aggregation between copper powder particles is reduced, and therefore the dispersibility of the copper powder can be increased.

[2-8. Span S]
The particle size of the copper powder according to the present embodiment has a small variation and a narrow particle size distribution, and therefore has high packing properties. More specifically, the span S represented by (Equation 2) is preferably 0.5 to 1.0, and more preferably 0.5 to 0.7.

Here, D _90SEM and D _10SEM are particle sizes at which the cumulative frequencies in a volume-based particle size histogram obtained by image analysis of electron microscope observation are 90% and 10%, respectively, and are indexes representing particle size distribution.

[2-9. Heat shrinkage rate]
The thermal shrinkage rate of the copper powder according to this embodiment at 1000° C. is 17% or less, and preferably 15% or less.

Generally, a copper powder coating formed from a conductive paste containing copper particles has small gaps between the copper particles, and by eliminating these gaps and increasing the filling ability, a copper coating with lower resistance can be obtained.

The copper powder according to this embodiment is within the above range, so that the gaps between the copper particles in the coating film formed from the conductive paste obtained using this powder are easily filled, making it possible to further reduce the electrical resistance value.

The thermal shrinkage was determined by the following method. Copper powder was filled into a cylindrical cylinder with a diameter of 5 mm and a height of about 40 mm. A punch was pressed into the top of the housing and pressurized at 0.87 ton/ ^cm2 for 10 seconds to mold the copper powder into a cylindrical shape with a height of about 5 mm. The molded body was placed so that the long axis was in the vertical direction, and heated in a heating furnace while applying a load of 98.0 mN in the long axis direction. Heating was performed from room temperature (20 ° C) to 1000 ° C at a heating rate of 5 ° C / min under a nitrogen gas flow containing 2 vol % hydrogen gas (flow rate 300 mL / min). During heating, the height change (expansion / contraction) of the molded body was monitored, and the thermal shrinkage at 1000 ° C was calculated according to the following (Equation 3).

Here, _h20 is the height of the compact at 20°C and _h1000 is the height of the compact at 1000°C.

In producing copper powder by the vapor phase growth method according to this embodiment, the inventors focused in particular on the cooling rate of the copper powder obtained by the reduction reaction between copper chloride and a reducing gas, and the concentration of ascorbic acid in the cleaning solution. This is because by adjusting the cooling rate of the copper powder obtained by the reduction reaction between copper chloride and a reducing gas, it is possible to reduce the number of connected particles, thereby obtaining copper particles with improved packing properties. In addition, by adjusting the concentration of ascorbic acid in the cleaning solution, it is possible to remove copper oxide from the surfaces of the copper particles, thereby reducing unevenness and obtaining copper particles with improved packing properties.

Therefore, the cooling rate of the copper powder obtained by the reduction reaction of copper chloride with a reducing gas is 500°C/sec or more, preferably 750°C/sec or more, and more preferably 1000°C/sec or more.

The concentration of ascorbic acid in the cleaning solution may be 5% by mass or more and 25% by mass or less, or 10% by mass or more and 20% by mass or less.

The cooling rate of copper powder obtained by the reduction reaction of copper chloride with a reducing gas refers to the temperature difference between the temperature of the copper powder produced by the reduction reaction and the temperature of the copper powder produced by the reduction reaction when the copper powder is brought into contact with a cooling inert gas to lower the temperature of the copper powder, divided by the time required to obtain the temperature difference.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1
Spherical metallic copper with an average diameter of 1.5 cm was placed in the chlorination furnace as a raw material, and the temperature of the chlorination furnace was heated to 900°C. The volume ratio of chlorine gas and nitrogen gas in the mixed gas introduced from the chlorine introduction tube at the top of the chlorination furnace was 29:61. The volume ratio of chlorine gas and nitrogen gas in the mixed gas introduced from the chlorine introduction tube at the bottom of the chlorination furnace was 2:98. As a result, the volume ratio of chlorine gas and nitrogen gas in the mixed gas introduced from the chlorine introduction tube at the top and the chlorine introduction tube at the bottom of the chlorination furnace was 1:0.17. Under these conditions, metallic copper and chlorine gas were reacted to generate copper chloride gas.

The generated copper chloride gas was introduced into a reduction furnace heated to 1150°C. In addition, 4600 mol% hydrogen gas relative to the copper chloride gas and 24600 mol% nitrogen gas relative to the copper chloride gas were introduced into the reduction furnace. The copper chloride was reduced to generate copper. The generated copper was cooled using nitrogen gas at a cooling rate of 750°C/sec to form individual copper particles, and copper powder was obtained as an aggregate of copper particles.

The copper powder obtained was then subjected to a process for reducing the chlorine and oxygen components. Specifically, a 40% by weight aqueous solution of sodium hydroxide (approximately 300 mL) and a 10% by weight aqueous solution of ascorbic acid (approximately 300 mL) were added to the copper powder obtained, and the resulting mixture was stirred at room temperature (25°C) for 30 minutes. After stirring was completed, the mixture was filtered, washed with water, and dried to obtain copper powder with reduced chlorine and oxygen components.

Then, a surface stabilization treatment was performed. Specifically, an aqueous solution (approximately 300 mL) containing 0.30 wt % benzotriazole as a surface treatment agent was added to the copper powder treated with the ascorbic acid aqueous solution at room temperature (25°C), and the resulting mixture was stirred for 30 minutes. After stirring was completed, the mixture was left to stand, the supernatant was removed, and the mixture was dried to obtain the copper powder of Example 1.

Example 2
Copper powder of Example 2 was obtained under the same conditions as in Example 1, except that the cooling rate was 500° C./sec.

Example 3
Copper powder of Example 3 was obtained under the same conditions as in Example 1, except that the cooling rate was 500° C./sec and the ascorbic acid concentration was 20 mass %.

Example 4
The copper powder of Example 4 was obtained under the same conditions as in Example 1, except that the cooling rate was 1000° C./sec and the ascorbic acid concentration was 20 mass %.

Example 5
Copper powder of Example 5 was obtained under the same conditions as in Example 1, except that the cooling rate was 1000° C./sec.

Example 6
The copper powder of Example 6 was obtained under the same conditions as in Example 1, except that the cooling rate was 750° C./sec and the ascorbic acid concentration was 20 mass %.

(Comparative Example 1)
The copper powder of Comparative Example 1 was obtained by utilizing the wet method disclosed in JP 2007-254846 A.

(Comparative Example 2)
The copper powder of Comparative Example 2 was obtained by utilizing the wet method disclosed in Patent Document 1.

<Evaluation>
The average particle diameters D _50SEM , D _10SEM , D _90SEM , average particle diameter D ₅₀ , average circularity, specific surface area S _BET , average crystallite diameter D, sintering initiation temperature and thermal shrinkage of the copper powders of Examples 1 to 6 and Comparative Examples 1 and 2 were measured.

<Average particle size _D50SEM , _D10SEM , _D90SEM >
The average particle diameters _D50SEM , _D10SEM , and _D90SEM were measured as follows. Using a scanning electron microscope (SEM: SU5000 manufactured by Hitachi High-Technologies Corporation, the same applies below), images of 500 copper particles present in one field of view of an SEM image at a magnification of 15,000 times were analyzed using image analysis software (Macview 4.0 manufactured by Mountec Co., Ltd.). The average particle diameters _D50SEM , _D10SEM , and _D90SEM were calculated from the particle diameters of the individual copper particles obtained by this analysis.

<Average particle diameter _D50 >
The average particle diameter _D50 was measured as follows: Copper powder was added to an alcohol solvent so that the laser light transmittance was 80 to 90%, and the mixture was dispersed ultrasonically for 5 minutes. The average particle diameter _D50 was then measured using a particle size distribution analyzer (LA-960 laser diffraction/scattering particle size distribution analyzer, manufactured by Horiba, Ltd.).

<Average circularity>
The average circularity was measured as follows: Using an SEM, about 500 copper particles present in one field of view of an SEM image at a magnification of 15,000 times were analyzed using image analysis software (Macview 4.0 manufactured by Mountech Co., Ltd.), and the circularity C of each of the about 500 copper particles was determined, and the average circularity was calculated from the average.

<Specific surface area S _BET >
The specific surface area S _BET was measured as follows: In accordance with "6.2 Fluidized method (3.5) One-point method" of JIS R 1626-1996 (Method for measuring specific surface area of fine ceramic powder by gas adsorption BET method), vacuum degassing was performed at a pretreatment temperature of 120°C, nitrogen gas was adsorbed, and then the specific surface area S _BET was measured using a fully automatic specific surface area measuring device Macsorb (registered trademark) measuring device (manufactured by Mountec Co., Ltd.).

<Average crystallite diameter D>
The average crystallite diameter D was calculated using the Scherrer formula and the half-width of the diffraction peak of the (111) plane of the copper crystal obtained using an X-ray diffractometer (X'PertPro manufactured by Spectris Co., Ltd.) with CuKα rays generated under conditions of an acceleration voltage of 45 kV and a discharge current of 40 mA.

<Sintering start temperature>
The sintering start temperature was determined by the following method. Copper powder was filled into a cylindrical cylinder with a diameter of 5 mm and a height of about 40 mm. A punch was pressed into the top of the housing and pressed at 0.87 ton/ ^cm2 for 10 seconds to mold the copper powder into a cylindrical shape with a height of about 5 mm. The molded body was placed so that the long axis was in the vertical direction, and heated in a heating furnace while applying a load of 98.0 mN in the long axis direction. Heating was performed from room temperature (25 ° C) to 1000 ° C at a heating rate of 5 ° C / min under a nitrogen gas flow containing 2 vol % hydrogen gas (flow rate 300 mL / min). During heating, the height change (expansion / contraction) of the molded body was monitored, and a thermomechanical analysis curve was obtained. Based on this thermomechanical analysis curve, the temperature at which the height change (contraction) of the molded body started and the contraction rate reached 5% was adopted as the sintering start temperature. The sintering start temperature was judged as "good" when it was 680° C. or higher, "passable" when it was 660° C. or higher and less than 680° C., and "unacceptable" when it was less than 660° C.

<Heat shrinkage rate>
The thermal shrinkage was determined by the following method. Copper powder was filled into a cylindrical cylinder with a diameter of 5 mm and a height of about 40 mm. A punch was pressed into the top of the housing and pressurized at 0.87 ton/ ^cm2 for 10 seconds to mold the copper powder into a cylindrical shape with a height of about 5 mm. The molded body was placed so that the long axis was vertical, and heated in a heating furnace while applying a load of 98.0 mN in the long axis direction. Heating was performed from room temperature (20 ° C) to 1000 ° C at a heating rate of 5 ° C / min under a stream of nitrogen gas containing 2 vol % hydrogen gas (flow rate 300 mL / min). During heating, the height change (expansion / contraction) of the molded body was monitored, and the thermal shrinkage at 1000 ° C was calculated according to the above-mentioned (Equation 3).

The criteria for the thermal shrinkage rate were: a thermal shrinkage rate of 15% or less was "good", a thermal shrinkage rate of 17% or less was "passable", and a thermal shrinkage rate of over 17% was "unacceptable". If the thermal shrinkage rate at 1000°C is 17% or less, the filling property is improved, and the gaps between the copper particles in the coating film formed from the conductive paste obtained using this are easily filled, making it possible to further reduce the electrical resistance value.

The above evaluation results are shown in Table 1 along with various parameters.

The copper powders of Examples 1 to 6 were produced by a vapor phase growth method, and therefore had a small average particle size _D50SEM or average particle size _D50 , and a large average crystallite size D. As a result, D/ _D50SEM was large, and the sintering start temperature was high, at 660°C or higher. This result is in contrast to the result of Comparative Example 1, which was produced by a wet method. Since the copper powders of Examples 1 to 6 have a high sintering start temperature of 660°C or higher, if used for the internal electrodes of an MLCC, it is possible to suppress the occurrence of cracks in the electrode layer and peeling that occurs when the copper powder and dielectric in the MLCC are sintered, and it is possible to provide an MLCC with a high yield.

The copper powder of Comparative Example 2 has a high sintering start temperature of 720°C, even though the D/D _50SEM is small. Comparative Example 2 was produced by a wet method, and it is highly likely that multiple substances (Si, Al, Zn, etc.) are attached to the copper powder surface due to the effect of the coupling treatment. Therefore, it is presumed that the sintering start temperature is high due to the effect of impurities on the copper powder surface.

In contrast to the copper powders of Comparative Examples 1 and 2, the copper powders of Examples 1 to 6 had a 6/(D _{50 SEM} ·S _BET ) of 5.5 or more, and therefore had a good thermal shrinkage rate at 1000° C. of 17% or less.

In contrast to the copper powders of Comparative Examples 1 and 2, the copper powders of Examples 1 to 6 had high average circularity of 0.88 or more. In particular, Examples 3, 4, and 6 had very good values of 0.91 or more.

In contrast to the copper powders of Comparative Examples 1 and 2, the copper powders of Examples 1 to 6 had D ₅₀ /D _50SEM of 1.8 or less, which was a good value.

One embodiment of the present invention can be implemented by combining appropriate components as long as they are not mutually inconsistent. In addition, a product in which a person skilled in the art has appropriately added or removed components or modified the design based on one embodiment of the present invention, or in which a process has been added or omitted or conditions have been changed, is also included in the scope of the present invention as long as it contains the gist of the present invention.

In addition, even if there are other effects and advantages different from those brought about by one embodiment of the present invention, if they are clear from the description in this specification or can be easily predicted by a person skilled in the art, they are naturally understood to be brought about by the present invention.

The copper powder according to the present invention has the characteristics of a high sintering temperature and good packing property, and therefore can be suitably used for electrodes of MLCC.

Claims (3)

A copper powder containing a plurality of copper particles having a sintering start temperature of 660° C. or more ,
the plurality of copper particles does not contain silver;
the average particle size ( _D50SEM ) obtained by image analysis of observation with an electron microscope is 0.1 μm or more and 0.35 μm or less, the average circularity is 0.80 or more, and the average particle size ( _D50SEM ) and the BET specific surface area (S _BET ) satisfy the relationship formula (Formula 1);
The copper powder has a ratio (D/D _50SEM ) of the average crystallite diameter (D) to the average particle diameter (D _50SEM ) of 0.1 or more and 0.5 or less .

The copper powder according to claim 1 , wherein the span S represented by (Equation 2) is 0.5 or more and 1.0 or less.

(Here, _D90SEM and _D10SEM in (Equation 2) represent particle sizes at which the cumulative frequencies in a volume-based particle size histogram obtained by image analysis of electron microscope observation are 90% and 10%, respectively.) 3. The copper powder according to claim 1 , which has a thermal shrinkage rate of 17% or less at 1000°C.