Gallium oxide-deposited silver powder, method for producing same, and conductive paste

Abstract

[Problem] To provide a silver powder in which line resistance is reduced when the surface of silver powder is modified and the obtained silver powder is made into a past to be used for electrode formation, and a method for producing the same. [Solution] When metallic silver particles are deposited from an aqueous solution containing silver ions, by allowing a gallium compound to coexist in the aqueous solution, and by using a reducing agent which has an effect of lowering the pH of the aqueous solution upon addition or an oxidation product of which has an effect of lowering the pH of the aqueous solution, it is possible to obtain gallium oxide-deposited silver powder. When the deposited silver powder is made into a paste and used for electrode formation, the obtained electrode has low resistance.

Description

Gallium oxide coated silver powder, its manufacturing method and conductive paste

[0001] The present invention relates to a gallium oxide-coated silver powder suitable for use in a conductive paste for forming an electrical conduction path in elements such as electrodes and circuits of various electronic components, a method for producing the same, and a conductive paste. Note that, in this specification, gallium oxide is a concept that includes gallium oxide hydrate and gallium hydroxide.

[0002] Resin-type and baked-type silver pastes have traditionally been widely used to form electrodes and circuits for electronic components. In recent years, conductive pastes using silver powder have been required to achieve higher densities of conductor patterns and finer wiring lines due to the miniaturization of electronic components. To achieve finer wiring lines, it is necessary to reduce the line resistance of the conductive film formed using the conductive paste. To achieve this, attempts have been made to modify the surface of silver powder and improve the electrical properties of the resulting conductive film.

[0003] For example, Patent Document 1 discloses a surface-modified silver powder in which an oxide or composite oxide containing at least one metal element belonging to Groups 2 to 14 of the periodic table is fixed to the surface of metallic silver particles. The surface-modified silver powder disclosed in Patent Document 1 is intended to suppress thermal shrinkage when a conductive paste containing the silver powder is sintered.

[0004] Japanese Patent Application Laid-Open No. 2001-240901

[0005] The silver powder disclosed in Patent Document 1 has its surface modified by adhering an oxide to the surface. However, the oxides used in the examples disclosed in Patent Document 1 are only aluminum oxide and silicon oxide, and these oxides have high volume resistivities, so they are not very effective in reducing the line resistance of the conductive film.

[0006] The technical problem to be solved by the present invention is to provide a silver powder and a method for producing the same, which can be surface-modified to reduce line resistance when the resulting silver powder is made into a paste and used to form an electrode with a narrow line width.

[0007] As a result of intensive research to achieve the above-mentioned object, the inventors have found that by depositing a small amount of gallium oxide on the surface of silver powder, it is possible to reduce the line resistance of the finally formed electrode film having a narrow line width. Based on the above findings, the inventors have completed the present invention described below.

[0008] That is, in order to achieve the above-mentioned object, the present invention provides: (1) a silver powder consisting of silver particles having gallium oxide coated on the surface thereof, wherein the amount of gallium is 10 mass ppm or more and 900 mass ppm or less relative to the mass of the gallium oxide coated silver powder, and the volume-based cumulative 50% diameter D measured by a laser diffraction / scattering particle size distribution analyzer is ５０ (2) The gallium oxide-coated silver powder of the above item (1) preferably has a gallium content of 10 ppm by mass or more and 600 ppm by mass or less relative to the mass of the gallium oxide-coated silver powder.

[0009] The present invention also provides: (3) a method for producing gallium oxide-coated silver powder, comprising the steps of: adding ammonia to an aqueous solution containing silver ions and a gallium compound to form a silver-ammine complex; adding a pH adjuster to the aqueous solution containing the silver-ammine complex formed in the above step to adjust the pH of the aqueous solution to 10 or higher; and adding a reducing agent to the aqueous solution whose pH has been adjusted to 10 or higher to reduce the silver ions and precipitate silver particles; and simultaneously adding the reducing agent to adjust the pH of the aqueous solution to 3.3 or higher and 6.5 or lower to precipitate gallium. (4) In the method for producing gallium oxide-coated silver powder of (3), it is preferable that the reducing agent is an organic compound containing one or more of a COOH group, a CHO group, and an OH group in the molecule. (5) In the method for producing gallium oxide-coated silver powder of (3) or (4), it is more preferable that the reducing agent is formaldehyde.

[0010] The present invention further provides: (6) a conductive paste using the gallium oxide-coated silver powder according to the above item (1) or (2).

[0011] By using the gallium oxide-coated silver powder of the present invention, a conductive film with low line resistance can be obtained even when the silver powder is made into a paste to form a thin line electrode with a designed line width of, for example, 15 μm or less.

[0012] The XPS spectra (Ga2p ３／２ ) is a depth profile analysis result of gallium for the gallium oxide-coated silver powder obtained in the examples and comparative examples of the present invention. A secondary electron (SEM) image of the gallium oxide-coated silver powder obtained in Example 2 of the present invention, and an Lα characteristic X-ray image of silver and gallium. A secondary electron (SEM) image of the gallium oxide-coated silver powder obtained in Comparative Example 2 of the present invention, and an Lα characteristic X-ray image of silver and gallium.

[0013] [Gallium oxide-coated silver powder] The gallium oxide-coated silver powder of the present invention is a silver powder having a small amount of gallium oxide coated on the surface thereof. By coating the surface of the silver powder with gallium oxide, the gallium oxide-coated silver powder is made into a paste, and even when a thin wire electrode having a design line width of 15 μm or less is formed, it is possible to reduce the line resistance.

[0014] The mechanism by which the line resistance of the electrode film is reduced by depositing gallium oxide is currently unknown, but the present inventors believe as follows. That is, it is presumed that the presence of gallium oxide on the surface of silver particles improves the sliding between silver particles in the paste. This improves the ejection properties of the paste from the printing plate and improves fine line printing properties, which is thought to lead to lower resistance of the electrode film finally obtained. From this perspective, gallium oxide may be present only near the surface of the silver particles.

[0015] In this specification, the "amount of gallium" in the silver powder refers to the amount of gallium relative to the mass of the gallium oxide-coated silver powder, which is measured by completely dissolving the gallium oxide-coated silver powder in acid and then using inductively coupled plasma optical emission spectroscopy (ICP-OES). In the present invention, gallium is present in large amounts on the surface side of the silver particles, and it is believed that there is gallium present in the silver particles and gallium coated on the surface as gallium oxide. In this specification, the "amount of gallium" refers to the sum of the amount of gallium present in the silver particles and the amount of gallium coated on the surface.

[0016] Since ICP-OES cannot perform state analysis, whether the gallium present on the surface of the silver particles is present in the form of gallium oxide, that is, whether the particles are "silver particles coated with gallium oxide on their surfaces," is determined by state analysis of gallium by X-ray photoelectron spectroscopy (XPS). In state analysis, gallium oxide is, for example, Ga ２ O ３ In the present invention, the gallium present on the surface of the silver particles is preferably gallium oxide, but may slightly contain, for example, a peak of gallium simple substance (metal). In the XPS measurement described below, the ratio of the height of the peak corresponding to gallium simple substance to the peak corresponding to gallium oxide is preferably 10% or less, more preferably 5% or less, and even more preferably 1% or less, and it is even more preferable that no peak corresponding to gallium simple substance is detected.

[0017] The amount of gallium is preferably 10 mass ppm or more relative to the total mass of the gallium oxide-coated silver powder. If the amount of deposition is less than 10 mass ppm, the effect of reducing the resistance of the electrode film of the present invention is insufficient. Furthermore, since gallium oxide is a type of insulator, if the amount of deposition exceeds 900 mass ppm, the effect of reducing line resistance is reduced, which is not preferable. It is more preferable that the amount of gallium is 10 mass ppm or more and 600 mass ppm or less.

[0018] The gallium oxide-coated silver powder of the present invention has a volume-based cumulative 50% diameter D measured by a laser diffraction scattering particle size distribution measuring device. ５０is preferably 0.2 μm or more and 5.0 μm or less. ５０ If the thickness is less than 0.2 μm, the viscosity of the conductive paste increases, making it difficult to increase the silver concentration in the conductive paste, and when the conductive paste is used to draw wiring or the like, it may break, which is not preferable. ５０ If the thickness exceeds 5.0 μm, it becomes difficult to draw fine wiring when drawing wiring or the like using the conductive paste, which is not preferable.

[0019] In addition, when the cross section of the silver particles constituting the gallium oxide-coated silver powder of the present invention is observed with a scanning electron microscope (SEM), the presence of fine voids inside the silver particles can be confirmed in some cases. When there are closed voids inside, the mechanism is unclear, but the presence of the voids may contribute to reducing the line resistance of the thinned electrode, so it is preferable that there are voids.

[0020] [Production Method] In the method for producing the gallium oxide-coated silver powder of the present invention, a wet method is used, which is inexpensive and has excellent mass productivity.

[0021] [Starting material] In the method for producing gallium oxide-coated silver powder of the present invention, a mixed aqueous solution containing monovalent silver ions and a gallium compound is used as the starting material. As a source of silver ions, known inorganic silver salts such as silver nitrate (I), silver sulfate (I), silver carbonate (I), silver chloride (I), and silver oxide (I), which are industrially used, can be used. As a source of gallium compounds, inorganic gallium salts such as gallium nitrate, gallium acetate, and gallium oxalate can be used. Since gallium oxide is an amphoteric oxide, gallium reacts with gallium ions (Ga ) in the low pH range. ３＋ ), and in the high pH range, gallate ions (GaO ３ ３－ ) and dissolves in the form of gallium oxide (Ga ２ O ３ ) becomes the solid phase stability region.

[0022] Although not particularly specified in the present invention, the silver ion concentration in the mixed aqueous solution, which is the starting material, is preferably 1.0% by mass or more and 2.0% by mass or less at the preparation stage of the mixed solution. If the silver ion concentration is less than 1.0% by mass, the amount of silver powder that can be produced in one reaction will be reduced. If the silver ion concentration exceeds 2.0% by mass, the viscosity of the reaction solution after silver particle precipitation will increase, making it difficult to uniformly stir the reaction solution. Furthermore, the concentration of the gallium compound is preferably 0.5% by mass or more and 10% by mass or less in terms of gallium. If the gallium concentration is less than 0.5% by mass, the volume of the reaction solution will increase as the amount of gallium compound aqueous solution added to obtain the desired gallium oxide-coated silver powder increases, resulting in the use of a large amount of reagents, which is likely to be uneconomical. If the gallium concentration exceeds 10% by mass, the amount of gallium compound aqueous solution added to obtain the desired gallium oxide-coated silver powder will be small, and depending on the operating conditions, errors in the amount added may be large.

[0023] [Complex Formation Step] In the method for producing gallium oxide-coated silver powder of the present invention, silver ions are complexed with ammonium ions to form a silver-ammine complex. Ammonium salts such as aqueous ammonia, ammonium chloride, and ammonium carbonate can be used as the source of ammonium ions. When ammonium ions are used as the complexing agent, a silver-ammine complex is formed in an aqueous solution. In this case, since the coordination number of the ammine complex is 2, 2 moles or more of ammonium ions are added per mole of silver ions. The reaction temperature when forming the silver-ammine complex is preferably 20°C or higher and 40°C or lower. In consideration of the exothermic reaction that occurs when forming the silver-ammine complex, the reaction temperature when forming the silver-ammine complex is preferably within the above range in order to set the temperature of the reduction precipitation step described below to the desired temperature. If the reaction temperature is too low or too high, it takes time to adjust the temperature and increases energy costs, which is economically undesirable.

[0024] [pH Adjustment Step] Subsequently, a pH adjuster is added to the aqueous solution containing the silver-ammine complex and dissolved gallium compound obtained by the above step, and the pH of the aqueous solution is adjusted to 10 or more. The pH is adjusted to 10 or more in order to sufficiently strengthen the reducing power of the reducing agent. As the pH adjuster, hydroxides or carbonates of alkali metals or alkaline earth metals such as sodium hydroxide or calcium hydroxide can be used. In the method for producing gallium oxide-coated silver powder of the present invention, the upper limit of the pH of the aqueous solution is not particularly specified, but it is preferable to set the pH to 13 or less to avoid excessive use of the pH adjuster. The temperature of the pH adjustment step is 20°C or higher and 40°C or lower. If the reaction temperature is too low or too high, it takes time to adjust the temperature and increases energy costs, which is economically undesirable.

[0025] [Silver Particle Precipitation Step] In the method for producing gallium oxide-coated silver powder of the present invention, a reducing agent is added to the alkaline aqueous solution containing the silver-ammine complex and dissolved gallium compound obtained in the above step to reduce the silver-ammine complex, thereby precipitating silver particles in the aqueous solution. In this case, a reducing agent that has the effect of lowering the pH of the aqueous solution by adding the reducing agent is used. The reason for using a reducing agent that has the effect of lowering the pH of the aqueous solution is that the addition of the reducing agent lowers the pH of the aqueous solution to the solid-phase stable region of gallium oxide, thereby coating gallium oxide on the surface of the precipitated silver particles. If a reducing agent that does not have the effect of lowering the pH of the aqueous solution is used, it is possible to coat gallium oxide on the surface of the silver particles by adding a pH adjuster after the silver particles have precipitated, but this method is not preferred because it increases the number of steps. Furthermore, it is not preferred because it becomes difficult to uniformly coat gallium oxide on the particle surface. It is preferable that the reaction temperature when precipitating silver particles using the reducing agent be 20°C or higher and 40°C or lower. If the reaction temperature is too low or too high, it takes time to adjust the temperature and the energy cost increases, which is economically undesirable.

[0026] [Reducing agent] The greatest technical feature of the method for producing gallium oxide-coated silver powder of the present invention is the use of a reducing agent that has the effect of lowering the pH of the aqueous solution by its addition. The mechanism by which the pH of the aqueous solution is lowered by the addition of a reducing agent is that the reducing agent itself has the effect of lowering the pH, and that the oxidation product of the reducing agent has the effect of lowering the pH.

[0027] As a reducing agent having such an effect, an organic compound containing one or more of a COOH group, a CHO group, and an OH group in the molecule is preferred.Specific examples include aldehydes such as ascorbic acid, tartaric acid, formic acid, and formaldehyde.In the case of an organic compound reducing agent having a COOH group in the molecule, the pH of the aqueous solution is lowered by adding the reducing agent.In the case of a reducing agent in which the CHO group has a reducing action, the CHO group reduces silver ions, and the CHO group is oxidized to become a COOH group, thereby lowering the pH of the aqueous solution.In the case of a reducing agent in which the OH group has a reducing action, the OH group is oxidized to become a COOH group via the CHO group, thereby lowering the pH of the aqueous solution.

[0028] In the method for producing gallium oxide-coated silver powder of the present invention, the pH of the aqueous solution is adjusted to 3.3 or more and 6.5 or less by adding the reducing agent. By setting the pH within the above range, gallium oxide can be coated onto the silver particles from the mixed aqueous solution.

[0029] The pH value lowered by adding the reducing agent can be the pH value after the precipitation of silver powder is completed, and in this case, the pH can be the measured pH value of the filtrate after the silver powder is filtered and recovered. This pH value can be controlled by the pH value in the pH adjustment step, the amount of pH adjuster added, the equivalent amount of ammonia in the complex formation step, or the amount of reducing agent. It is more preferable to set the pH at 6.0 or less. If the pH is below 3.3, gallium will react with GaOH. ２＋ , Ga ３＋ This is not preferable because it may result in the formation of ions.

[0030] By changing the amount of aqueous ammonia added in Example 1 described below, silver powder with a pH of the filtrate adjusted to 7.5 or higher had almost no gallium detected in the measurement of the amount of deposited gallium described below, silver powder with a pH adjusted to 6.4 had 40% of the gallium added to the mixed aqueous solution detected from the silver powder, and silver powder with a pH adjusted to 4.8 to 5.8 had almost all of the gallium added to the mixed aqueous solution detected from the silver powder. In other words, in the present invention, the pH is more preferably 4.8 or more and 5.8 or less.

[0031] From this, it is believed that when the pH of the mixed aqueous solution decreases from 10 or more to approach 6.5, gallium oxide begins to precipitate on the surface of the silver particles. Then, as the particles grow until the final pH is reached, a layer containing gallium (e.g., gallium oxide) is formed near the surface of the silver particles, and gallium oxide is deposited on the surface of the silver particles. Figure 2, described below, shows how gallium gradually decreases from the outermost surface toward the inside of the particles in a depth direction analysis of gallium from the silver particle surface by XPS measurement in combination with Ar ion sputtering, suggesting the presence of a layer containing gallium near the surface of the silver particles of the present invention. In other words, in the present invention, it is preferable that gallium gradually decreases from the outermost surface toward the inside of the particles in a depth direction analysis from the silver particle surface.

[0032] In this production method, gallium ions are added to the mixed solution in advance, and gallium oxide is deposited on the silver particle surfaces by a pH transition during reduction precipitation. This suppresses the formation of gallium oxide particles or gallium particles isolated from the silver particles, as compared to Patent Document 1, and allows gallium oxide fixed to the surface of the silver particles to be uniformly deposited. Therefore, the amount of gallium compound added in advance to the mixed solution can be reduced, and silver powder can be obtained in which the amount of gallium in the silver powder measured in the measurement of the amount of deposited gallium described below is kept low. In other words, in the present invention, it is preferable that gallium oxide near the surface of the silver particles is uniformly deposited on the entire silver particle within the aforementioned concentration range.

[0033] When an aldehyde is used as a reducing agent, silver particles are precipitated to a certain extent, and the pH of the aqueous solution becomes within the above-mentioned pH range after the concentration of the carboxylic acid, which is the oxidation product, increases, so that gallium oxide is preferably deposited only on the surface of the precipitated silver particles.As the aldehyde, from the viewpoint of ease of availability and reducing power, it is more preferable to use formaldehyde.The amount of the reducing agent added is preferably 1 equivalent or more relative to silver, and may be 2 equivalents or more relative to silver, for example, 10 equivalents or more and 20 equivalents or less, in order to increase the silver yield.

[0034] [Separation and Recovery Step] The silver powder coated with gallium oxide obtained by the above series of steps is separated and recovered using a known solid-liquid separation means, and then washed with water as needed and dried. Known solid-liquid separation methods can be used, for example, decantation or a filter press. The end point of washing may be determined using the electrical conductivity of the washing water. Specifically, the end of washing is determined when the electrical conductivity of the washing water becomes a predetermined value or less. The silver particles after washing may be subjected to a drying step in a cake-like aggregated state.

[0035] The drying step can be carried out using a vacuum dryer or an airflow dryer. In the drying step, a high-pressure air flow can be blown onto the aggregates of silver particles, or the cake or spherical silver powder in the drying process can be placed in a stirrer with a stirring rotor and stirred to impart a dispersing force to the cake or silver powder in the drying process, thereby facilitating dispersion and drying. The drying temperature for the silver powder is 100°C or lower. If the temperature of the silver powder is 100°C or lower, sintering of the silver particles in the silver powder can be effectively suppressed.

[0036] [Measurement of Gallium Amount] Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to measure the amount of gallium deposited on the gallium oxide-coated silver powder. 1 g of sample (silver powder) was precisely weighed, 15 mL of pure water, and 10 mL of nitric acid (for precision analysis) were added, and the mixture was heated at 200 ° C. for 30 minutes. After allowing the heated sample to cool, the volume was adjusted to 100 mL using pure water, and 5 mL of the supernatant was taken from the sample and adjusted to 100 mL again using pure water to prepare a sample for ICP analysis. The standard addition method was used for measurement. A calibration curve was created by adding gallium to a standard solution with a silver concentration similar to that of the above sample using 5N silver. An Agilent 5800 ICP-OES from Agilent Technologies was used for quantitative analysis.

[0037] [State Analysis of Gallium] The state analysis of the gallium coated on the silver particles was carried out by X-ray photoelectron spectroscopy (XPS). For the XPS measurement, a scanning X-ray photoelectron spectrometer, PHI5000 Versa Probe III manufactured by ULVAC-PHI, Inc., was used. The X-ray source used was monochromated AlKα rays, with an acceleration voltage of 15 kV, an output of 25 W, an X-ray incident angle of 90 degrees, and a photoelectron take-off angle of 45 degrees. For the chemical state analysis of gallium, Ga2p ３／２ The spectrum was used, with pass energy of 69 eV, integration time of 80 ms, measurement energy interval of 0.125 eV / step, and number of accumulations of 25. Charge correction was also performed with the C-C bond energy set to 284.8 eV. Note that, only in Example 1 described later, the number of accumulations for the chemical state analysis of gallium was set to 500. Ar ion sputtering was also used for depth profiling of the silver particles coated with gallium oxide. The ion sputtering rate was set to SiO ２ This is equivalent to 10 nm / min.

[0038] [Particle size distribution measurement] The cumulative 10% particle diameter (D １０ ), cumulative 50% particle diameter (D ５０ ), cumulative 90% particle diameter (D ９０) was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300 EXII, manufactured by Microtrac Bell Co., Ltd.). For the measurement, 0.1 g of sample was added to 40 mL of isopropyl alcohol (IPA) and dispersed. An ultrasonic homogenizer (Nippon Seiki Seisakusho, US-150T: 19.5 kHz, tip diameter 18 mm) was used for dispersion. The dispersion time was 2 minutes. The dispersed sample was subjected to the above-mentioned device, and the particle size distribution was determined using the attached analysis software. Note that an SDC device was used as the circulator of the laser diffraction / scattering particle size distribution analyzer during measurement, and the setting value of the "flow rate (%)" of the circulator was 60.

[0039] [BET Specific Surface Area Measurement] The BET specific surface area of ​​the gallium oxide-coated silver powder was measured by a BET single-point method using nitrogen adsorption using a Macsorb HM-model 1210 manufactured by MOUNTECH Co., Ltd. The BET specific surface area was measured using a sample weight of 3.0 g and N ２ A gas mixture of HCl / He (30 / 70) was used at a gas flow rate of 25 mL / min, and degassing conditions before measurement were 60° C. for 10 minutes.

[0040] [Scanning Electron Microscope Observation] For scanning electron microscope (SEM) observation of the gallium oxide-coated silver powder, a JSM-IT800SHL manufactured by JEOL Ltd. was used. Furthermore, characteristic X-ray (Lα ray) images of silver and gallium were taken using an energy dispersive X-ray fluorescence analyzer (EDX) attached to the same device. In the present invention, whether gallium is uniformly present on the silver particle surface was confirmed using ImageJ, an image processing software capable of binarizing the obtained characteristic X-ray (Lα ray) image of gallium. Specifically, using the software, the peripheries of 10 arbitrary particles were manually set from the gallium distribution image, and the distribution area of ​​gallium per particle was determined by binarization. The average value of the distribution area ratio of gallium per particle was calculated by dividing by the area of ​​the entire particle. In addition, in the present invention, the threshold value for binarization was set to a value 1 lower than the maximum value (254 in the present invention), and the white area was designated as the distribution area of ​​gallium. In the present invention, the average distribution area ratio of gallium per particle is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more.

[0041] [Method of manufacturing conductive paste] A conductive paste containing gallium oxide-containing silver powder, aluminum powder (metallic aluminum: 99.87 mass %, iron: 0.09 mass %, silicon: 0.04 mass %, SEM average diameter: 2.0 μm), glass frit (glass powder: containing PbO as the main component, B ２ O ３ , SiO ２ A mixture was obtained by mixing ethyl cellulose, texanol, butyl carbitol acetate, tributyl citrate, oleic acid, triacetin, methylphenylpolysiloxane, hydrogenated castor oil, and fatty acid amide. The composition of the mixture is shown in Table 1. The mixture was placed in a propellerless rotary and revolving mixer deaerator (V-mini300 manufactured by EME Corporation) and premixed for 30 seconds at a revolution of 1000 rpm. After that, the mixture was kneaded using a three-roll mill (80S manufactured by EXAKT) with the roll gap changed from 100 μm to 20 μm, to obtain a conductive paste.

[0042]

[0043] [Measurement of Line Resistance Value] Using the conductive paste obtained by the above procedure, a linear pattern was printed by screen printing. The line had a design line width of 12 μm and a length of 150 mm. A Microtec printer was used for printing at a squeegee speed of 350 mm / s. A silicon substrate (for solar cell applications, textured and coated with SiNx) approximately 170 μm thick was used for printing. After printing, the substrate was dried for 5 minutes in a dryer set at 200°C and then baked in a solar cell baking furnace (manufactured by NGK) at a peak temperature of 720°C on the wafer's top surface to prepare a sample for line resistance value measurement. The resistance value of the electrode after baking (the line resistance value of the conductive film) was measured using a digital multimeter (manufactured by ADC Corporation) by placing measurement terminals on both ends of the printed electrode.

[0044] [Comparative Example 1] When a commercially available silver powder (4-8FD manufactured by DOWA Hi-Tech Co., Ltd.) was used and the line resistance was measured using the procedure described above, it was found to be 66.7 Ω. In the present invention, it is determined that the effect of the invention has been achieved when the line resistance is lower than the line resistance value. Table 2 shows the cumulative 10% particle diameter (D) on a volume basis of the commercially available silver powder used in Comparative Example 1. １０ ), cumulative 50% particle diameter (D ５０ ), cumulative 90% particle diameter (D ９０ ), BET specific surface area and line resistance as above.

[0045] [Example 1] 0.66 g of a 5% by mass aqueous solution of gallium nitrate (manufactured by Kojundo Chemical Laboratory Co., Ltd.) was added to 3,506 g of a silver nitrate aqueous solution with a silver ion concentration of 1.53% by mass to obtain a mixed aqueous solution containing silver ions and gallium ions. Next, while stirring the mixed solution, 90.8 g of 28.0% by mass aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added, and the liquid temperature of the mixed aqueous solution was adjusted to 28.0 ° C. Next, while continuing to stir, 1 minute after the addition of the aqueous ammonia solution, 9.96 g of a 20% by mass aqueous sodium hydroxide solution was added to the mixed aqueous solution to adjust the pH to 12.11, and then 3 minutes after the addition of the aqueous ammonia solution, 251 g of a 26% by mass aqueous formaldehyde solution (manufactured by Mitsubishi Chemical Corporation) was added all at once as a reducing agent to precipitate silver particles.

[0046] Fifteen seconds after adding the reducing agent to the mixed aqueous solution, 6.13 g of 1.55% by mass stearic acid emulsion was added. Stirring was then stopped, and the solid matter was filtered using a Nutsche filter. At that time, the pH of the filtrate was 5.35. A 5 L beaker was used for the reaction, and a baffle and a two-stage turbine blade were used. The obtained solid matter was washed with water until the electrical conductivity of the liquid after passing water was 0.5 mS / m or less, and then vacuum dried at 73 ° C. for 10 hours. 50 g of the silver powder obtained in the above process was charged into a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) and crushed twice for 30 seconds using a dial memory 100 to obtain the gallium oxide-coated silver powder according to Example 1.

[0047] When the amount of gallium in the gallium oxide-coated silver powder was measured, it was 90 ppm by mass relative to the mass of the gallium oxide-coated silver powder. Figure 1 shows the XPS spectrum of the gallium oxide-coated silver powder obtained by this example. A small peak was observed in the spectrum near a binding energy of 1187 eV. This peak was attributed to gallium oxide, and it was found that gallium oxide was present on the surface of the silver powder obtained by this example. Note that no peak of metallic gallium was observed by XPS measurement.

[0048] When the line resistance was measured using the gallium oxide-coated silver powder obtained in this example using the procedure described above, it was 21.3 Ω. This value was lower than that of Comparative Example 1, and it was found that even if the gallium amount was 90 ppm by mass, by coating the surface of the silver powder with gallium oxide, the line resistance when it was pasted to form an electrode was reduced. Table 2 shows the gallium amount of the gallium oxide-coated silver powder obtained in this example, the cumulative 10% particle diameter (D １０ ), cumulative 50% particle diameter (D ５０ ), cumulative 90% particle diameter (D ９０ ), BET specific surface area and the above-mentioned line resistance are also shown.

[0049] [Example 2] Gallium oxide-coated silver powder according to this example was obtained using the same procedure as in Example 1, except that the amount of gallium nitrate aqueous solution added was 3.28 g. The pH of the filtrate was 5.51. The amount of gallium in the gallium oxide-coated silver powder was 460 ppm by mass, and XPS measurement confirmed that the coated gallium was an oxide (Figure 1). Figure 2 shows the results of depth profile analysis of gallium using XPS measurement in combination with Ar ion sputtering. The vertical axis of Figure 2 represents the atomic percentage of gallium among all elements, including C. The gallium peak gradually decreased after the start of measurement, indicating that gallium was present in large amounts near the surface of the silver powder.

[0050] FIG. 3 shows a secondary electron (SEM) image of the gallium oxide-coated silver powder obtained in Example 2 and an Lα characteristic X-ray image of silver and gallium. From FIG. 3, the average value of the distribution area ratio of gallium per particle in Example 2 was 92%. This shows that gallium oxide is uniformly distributed on the silver powder surface. In Example 1, an attempt was made to obtain an Lα characteristic X-ray image of gallium, but the measurement device used in the present invention was below the lower limit of quantitation, so the gallium peak could not be detected. However, considering the manufacturing method of the present invention, it is presumed that the gallium oxide-coated silver powder obtained in Example 1 also has gallium oxide uniformly distributed on the silver powder surface, as in Example 2.

[0051] When the line resistance was measured using the gallium oxide-coated silver powder obtained in this example using the procedure described above, it was 49.6 Ω. This value was lower than that of Comparative Example 1, and it was found that even if the gallium amount was 460 mass ppm, by coating the surface of the silver powder with gallium oxide, the line resistance when it was pasted to form an electrode was reduced. Table 2 shows the gallium amount of the gallium oxide-coated silver powder obtained in this example, the cumulative 10% particle diameter (D １０ ), cumulative 50% particle diameter (D ５０ ), cumulative 90% particle diameter (D ９０ ), BET specific surface area and the above-mentioned line resistance are also shown.

[0052] Comparative Example 2 Gallium oxide-coated silver powder according to this comparative example was obtained using the same procedure as in Example 1, except that the amount of gallium nitrate aqueous solution added was 6.55 g. The pH of the filtrate was 5.44. The amount of gallium in the gallium oxide-coated silver powder obtained in this comparative example was 950 ppm by mass. For the gallium oxide-coated silver powder, the XPS spectrum of gallium is shown in FIG. 1, and the results of the depth profile analysis of gallium are shown in FIG. 2. It can be seen that gallium oxide is coated on the surface of the silver powder in this comparative example as well. Furthermore, FIG. 4 shows a secondary electron (SEM) image of the gallium oxide-coated silver powder obtained in Comparative Example 2, as well as Lα characteristic X-ray images of silver and gallium. In Comparative Example 2, the average distribution area ratio of gallium per particle was 94%, indicating that gallium oxide was uniformly distributed on the silver powder surface.

[0053] When the line resistance was measured using the gallium oxide-coated silver powder obtained by this comparative example using the procedure described above, it was 75.1 Ω, which was higher than that of Comparative Example 1. Therefore, when gallium oxide is coated on the surface of silver powder, if the gallium content expressed as a mass ratio to the mass of the gallium oxide-coated silver powder exceeds 900 mass ppm, it was found that the line resistance increases when it is pasted to form an electrode. Table 2 shows the gallium amount of the gallium oxide-coated silver powder obtained by this comparative example, the cumulative 10% particle diameter (D １０ ), cumulative 50% particle diameter (D ５０ ), cumulative 90% particle diameter (D ９０ ), BET specific surface area and the above-mentioned line resistance are also shown.

[0054]

Claims

1. A silver powder consisting of silver particles coated with gallium oxide on the surface, wherein the amount of gallium is 10 mass ppm or more and 900 mass ppm or less relative to the mass of the gallium oxide-coated silver powder, and the volume-based cumulative 50% diameter D measured by a laser diffraction scattering particle size distribution analyzer ５０ Gallium oxide-coated silver powder having a particle size of 0.2 μm or more and 5.0 μm or less.

2. The gallium oxide-coated silver powder according to claim 1, wherein the amount of gallium is 10 ppm by mass or more and 600 ppm by mass or less relative to the mass of the gallium oxide-coated silver powder.

3. A method for producing gallium oxide-coated silver powder, comprising the steps of: adding ammonia to an aqueous solution containing silver ions and a gallium compound to form a silver-ammine complex; adding a pH adjuster to the aqueous solution containing the silver-ammine complex formed in the above step to adjust the pH of the aqueous solution to 10 or higher; and adding a reducing agent to the aqueous solution whose pH has been adjusted to 10 or higher to reduce the silver ions and precipitate silver particles, while simultaneously adding the reducing agent to adjust the pH of the aqueous solution to 3.3 or higher and 6.5 or lower to precipitate gallium.

4. A method for producing gallium oxide-coated silver powder according to claim 3, wherein the reducing agent is an organic compound containing one or more of a COOH group, a CHO group, and an OH group in the molecule.

5. The method for producing gallium oxide coated silver powder according to claim 3, wherein the reducing agent is formaldehyde.

6. A conductive paste using the gallium oxide-coated silver powder according to claim 1 or 2.

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