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

Abstract

To provide silver powder, a surface of which is modified so that, when the obtained silver powder is formed into a paste for electrode formation, the line resistance is reduced, and a method for producing the same.SOLUTION: When precipitating a metallic silver particle from an aqueous solution containing silver ions, a gallium compound is made to coexist in an aqueous solution. By using, as a reducing agent, a reducing agent that has an effect of lowering pH of the aqueous solution by addition thereof, or a reducing agent whose oxidation product has an effect of lowering pH of the aqueous solution, the precipitated silver powder is formed into a paste for electrode formation. By this electrode formation, gallium oxide-coated silver powder that forms a low-resistance electrode can be obtained.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[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. [Background technology]

[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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-240901 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[0007] As a result of intensive research to achieve the above-mentioned object, the present inventor has 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 electrode film having a narrow line width that is finally formed. Based on the above findings, the present inventors have completed the present invention described below.

[0008] That is, in order to achieve the above object, the present invention provides: (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 cumulative 50% diameter D on a volume basis measured by a laser diffraction scattering particle size distribution measuring device 50 The gallium oxide-coated silver powder has a particle size of 0.2 μm or more and 5.0 μm or less. (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] In the present invention, (3) A method for producing gallium oxide-coated silver powder is provided, 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 more; and adding a reducing agent to the aqueous solution whose pH has been adjusted to 10 or more 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 more and 6.5 or less, thereby precipitating gallium. (4) In the method for producing gallium oxide-coated silver powder according to the above item (3), the reducing agent is preferably 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 according to the above item (3) or (4), the reducing agent is more preferably formaldehyde.

[0010] In the present invention, (6) A conductive paste using the gallium oxide-coated silver powder according to the above item (1) or (2) is provided. [Effects of the Invention]

[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 fine line electrode with a designed line width of, for example, 15 μm or less. [Brief explanation of the drawings]

[0012] [Figure 1] 1 shows XPS spectra (Ga2p3 / 2) obtained for the gallium oxide-coated silver powders obtained in the examples and comparative examples of the present invention. [Figure 2] 1 shows the results of depth profile analysis of gallium for gallium oxide-coated silver powders obtained in the examples and comparative examples of the present invention. [Figure 3] 1 shows a secondary electron (SEM) image of the silver powder coated with gallium oxide obtained in Example 2 of the present invention, and Lα characteristic X-ray images of silver and gallium. [Figure 4] 1 shows a secondary electron (SEM) image of the silver powder coated with gallium oxide obtained in Comparative Example 2 of the present invention, and Lα characteristic X-ray images of silver and gallium. DETAILED DESCRIPTION OF THE INVENTION

[0013] [Gallium oxide coated silver powder] The gallium oxide-coated silver powder of the present invention is a silver powder having a minute 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, as determined by completely dissolving the gallium oxide-coated silver powder in acid and measuring it 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, i.e., whether the particles are "silver particles having gallium oxide deposited on their surfaces," is determined by state analysis of gallium using X-ray photoelectron spectroscopy (XPS). In state analysis, gallium oxide is, for example, Ga2O3. In the present invention, the gallium present on the surface of the silver particles is preferably gallium oxide, but may also contain a small amount of a peak corresponding to, for example, elemental gallium (metal). In the XPS measurement described below, the ratio of the height of the peak corresponding to elemental gallium 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 elemental gallium 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 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 exceeds 900 mass ppm, the effect of reducing line resistance is reduced, which is not preferable. The amount of gallium is more preferably 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 analyzer. 50 It is preferable that the thickness is 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] 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 may be confirmed. Although the mechanism is unclear when closed voids are present inside, the presence of such voids may contribute to reducing the line resistance of thinned electrodes, so it is preferable that voids exist.

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

[0021] [Starting materials] 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 a 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. Note that, since gallium oxide is an amphoteric oxide, gallium reacts with gallium ions (Ga 3+ ), and in the high pH range, gallate ions (GaO3 3- ) and becomes a stable solid phase of gallium oxide (Ga2O3) at near-neutral pH.

[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 amount of gallium compound aqueous solution added to obtain the desired gallium oxide-coated silver powder will increase, resulting in a large volume of the reaction solution and 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, the error in the amount added may be large.

[0023] [Complex formation process] 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 a 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 process] Subsequently, a pH adjuster is added to the aqueous solution containing the silver-ammine complex and dissolved gallium compound obtained by the above process to adjust the pH of the aqueous solution to 10 or higher. The pH is adjusted to 10 or higher to sufficiently strengthen the reducing power of the reducing agent. Examples of pH adjusters that can be used include hydroxides and carbonates of alkali metals or alkaline earth metals such as sodium hydroxide and calcium hydroxide. 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 lower to avoid excessive use of the pH adjuster. The temperature in 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 process] 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 and precipitate 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 stability 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 add a pH adjuster to lower the pH of the aqueous solution after the silver particles have precipitated, thereby coating gallium oxide on the surface of the silver particles. However, this method is not preferred because it increases the number of steps. It is also not preferred because it makes it difficult to uniformly coat gallium oxide on the particle surface. The reaction temperature when precipitating silver particles using the reducing agent is preferably 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.

[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 can be either the reducing agent itself having the effect of lowering the pH or the oxidation product of the reducing agent having 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 preferable. Specific examples include aldehydes such as ascorbic acid, tartaric acid, formic acid, and formaldehyde. In the case of reducing agents that are organic compounds containing COOH groups in their molecules, the addition of the reducing agent lowers the pH of the aqueous solution. In the case of reducing agents in which the CHO group has a reducing effect, the CHO group reduces silver ions and is oxidized to COOH groups, lowering the pH of the aqueous solution. In the case of reducing agents in which the OH group has a reducing effect, the OH group is oxidized to COOH groups via the CHO group, 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, the pH of the filtrate was adjusted to 7.5 or higher, and in the measurement of the amount of deposited gallium described below, almost no gallium was detected in the silver powder. In the silver powder adjusted to a pH of 6.4, 40% of the gallium added to the mixed aqueous solution was detected in the silver powder. In the silver powder adjusted to a pH of 4.8 to 5.8, almost all of the gallium added to the mixed aqueous solution was detected in the silver powder. In other words, in the present invention, the pH is more preferably 4.8 or higher and 5.8 or lower.

[0031] From this, it is believed that when the pH of the mixed aqueous solution decreases from 10 or higher to approach pH 6.5, gallium oxide begins to precipitate on the surface of the silver particles. It is then believed that a layer containing gallium (e.g., gallium oxide) forms near the surface of the silver particles as the particles grow until the final pH is reached, 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 interior of the particles in a depth profile 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 interior of the particles in a depth profile 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 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 to be uniformly deposited on the silver particle surfaces. Therefore, the amount of gallium compound added to the mixed solution in advance can be reduced, and silver powder can be obtained with a low amount of gallium in the silver powder measured in the measurement of the amount of deposited gallium, as described below. 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 at 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 deposited only on the surface of the precipitated silver particles, which is preferable. As the aldehyde, formaldehyde is more preferably used from the viewpoints of availability and reducing power. The amount of the reducing agent added is preferably 1 equivalent or more relative to silver in order to increase the silver yield, and may be 2 equivalents or more relative to silver, for example, 10 equivalents or more and 20 equivalents or less.

[0034] [Separation and recovery process] 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 means such as decantation or a filter press can be used. 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 falls below a predetermined value. After washing, the silver particles may be subjected to a drying step in an aggregated state such as a cake.

[0035] The drying process can be carried out using a vacuum dryer or an airflow dryer. In the drying process, 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 mixer with a stirring rotor and stirred to apply 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, it is possible to effectively prevent the silver particles in the silver powder from sintering with each other. [Example]

[0036] [Measurement of Gallium Amount] The amount of gallium deposited on the gallium oxide-coated silver powder was measured using inductively coupled plasma optical emission spectroscopy (ICP-OES). One gram 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 cooling, the heated sample was diluted to 100 mL with pure water. 5 mL of the supernatant was then removed and diluted again to 100 mL with pure water to prepare the sample for ICP analysis. Measurements were performed using the standard addition method. A calibration curve was created by adding gallium to a standard solution containing 5N silver and adjusted to the same silver concentration as the sample. Quantitative analysis was performed using an Agilent 5800 ICP-OES (Agilent Technologies).

[0037] [State analysis of gallium] The state of the gallium deposited on the silver particles was analyzed by X-ray photoelectron spectroscopy (XPS). XPS measurements were performed using a scanning X-ray photoelectron spectrometer, PHI5000 Versa Probe III, manufactured by ULVAC-PHI, Inc. The X-ray source was monochromated AlKα radiation, with an accelerating 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. Ga2p for gallium chemical state analysis 3 / 2 Using the spectrum, the pass energy was 69 eV, the integration time was 80 ms, the measurement energy interval was 0.125 eV / step, and the number of accumulations was 25. In addition, charge correction was 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 500. Depth profiling of the gallium oxide-coated silver particles was performed using Ar ion sputtering, with the ion sputtering rate being 10 nm / min in SiO2 equivalent.

[0038] [Particle size distribution measurement] The cumulative 10% particle size (D) of the gallium oxide-coated silver powder 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 Corporation). 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 placed in the above-mentioned device, and the particle size distribution was determined using the attached analysis software. During the measurement, an SDC device was used as the circulator of the laser diffraction / scattering particle size distribution analyzer, and the setting value of the "flow rate (%)" of the circulator was set to 60.

[0039] [BET specific surface area measurement] The BET specific surface area of ​​the gallium oxide-coated silver powder was measured by the 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, a N2 / He (30 / 70) mixed gas at a gas flow rate of 25 mL / min, and degassing conditions before measurement at 60°C for 10 minutes.

[0040] [Scanning electron microscope observation] The gallium oxide-coated silver powder was observed using a scanning electron microscope (SEM) manufactured by JEOL Ltd., JSM-IT800SHL. Furthermore, characteristic X-ray (Lα) images of silver and gallium were taken using an energy dispersive X-ray fluorescence analyzer (EDX) attached to the same instrument. In the present invention, whether gallium was uniformly present on the silver particle surface was confirmed using ImageJ, an image processing software capable of binarizing the obtained characteristic X-ray (Lα) image of gallium. Specifically, using the software, the perimeters of 10 arbitrary particles were manually set from the gallium distribution image, and the gallium distribution area per particle was determined by binarization. The average value of the gallium distribution area ratio per particle was calculated by dividing the gallium distribution area ratio by the total area of ​​the particle. 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 gallium distribution area. 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] [Conductive paste manufacturing method] A mixture was prepared by mixing gallium oxide-containing silver powder, aluminum powder (metallic aluminum: 99.87% by mass, iron: 0.09% by mass, silicon: 0.04% by mass, SEM mean diameter: 2.0 μm), glass frit (glass powder: PbO as the main component, containing BO, SiO, and other oxides), 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 planetary mixer / deaerator (EME Corporation, V-mini300) and premixed for 30 seconds at 1000 rpm. It was then kneaded using a three-roll mill (EXAKT 80S) with the roll gap reduced from 100 μm to 20 μm, producing a conductive paste.

[0042] [Table 1]

[0043] [Measurement of line resistance] Using the conductive paste obtained using the above procedure, a linear shape 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, with a squeegee speed of 350 mm / s. A silicon substrate (for solar cell applications, textured and with SiNx film already formed) with a thickness of approximately 170 μm 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 top surface of the wafer to prepare a sample for measuring line resistance. The resistance value of the electrode after firing (line resistance value of the conductive film) was measured with a digital multimeter (manufactured by ADC Corporation) by applying measuring terminals to both ends of the printed electrode.

[0044] [Comparative Example 1] When the line resistance was measured using commercially available silver powder (4-8FD manufactured by DOWA High-Tech Co., Ltd.) using the procedure described above, it was 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 measured. Table 2 shows the cumulative 10% particle diameter (D 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 wt% 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 wt% to obtain a mixed aqueous solution containing silver ions and gallium ions. Next, 90.8 g of 28.0 wt% aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added while stirring the mixed solution, and the 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 wt% aqueous sodium hydroxide solution was added to the mixed aqueous solution to adjust the pH to 12.11. 3 minutes after the addition of the aqueous ammonia solution, 251 g of a 26 wt% 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 a 1.55% by mass stearic acid emulsion was added. Stirring was then stopped, and the solid matter was filtered using a Nutsche filter. The pH of the filtrate was 5.35. A 5 L beaker was used for the reaction, equipped with a baffle and a two-stage turbine blade. The obtained solid matter was washed with water until the electrical conductivity of the water-passing liquid 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 placed in a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) and crushed twice for 30 seconds using a dial scale of 100 to obtain the gallium oxide-coated silver powder according to Example 1.

[0047] The amount of gallium in the gallium oxide-coated silver powder was measured and found to be 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 in this example. A small peak was observed in the spectrum near a binding energy of 1187 eV. This peak was attributed to gallium oxide, indicating the presence of gallium oxide on the surface of the silver powder obtained in this example. Note that no peaks of metallic gallium were observed in the 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 even when the gallium amount was 90 ppm by mass, it was found that 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 amount of gallium and the cumulative 10% particle diameter (D 10 ), cumulative 50% particle diameter (D 50 ), cumulative 90% particle diameter (D 90 ), the BET specific surface area and the line resistance are also shown.

[0049] [Example 2] Gallium oxide-coated silver powder according to this example was obtained in the same manner 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 oxide (Figure 1). Figure 2 shows the results of a depth profile analysis of gallium by XPS measurement in combination with Ar ion sputtering. The vertical axis in 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] Figure 3 shows a secondary electron (SEM) image of the gallium oxide-coated silver powder obtained in Example 2 and Lα characteristic X-ray images of silver and gallium. As shown in Figure 3, the average distribution area ratio of gallium per particle in Example 2 was 92%. This indicates 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 quantification, so the gallium peak could not be detected. However, in view of 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 even when the gallium amount was 460 ppm by mass, it was found that 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 amount of gallium in the gallium oxide-coated silver powder obtained in this example, and the cumulative 10% particle diameter (D 10 ), cumulative 50% particle diameter (D 50 ), cumulative 90% particle diameter (D 90 ), the BET specific surface area and the 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. The gallium XPS spectrum of the gallium-coated silver powder is shown in Figure 1, and the results of the depth profile analysis of gallium are shown in Figure 2. It can be seen that gallium oxide is coated on the surface of the silver powder in this comparative example as well. Furthermore, Figure 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 in 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 the silver powder, if the gallium content expressed as a mass ratio to the mass of the gallium oxide-coated silver powder exceeds 900 ppm by mass, it was found that the line resistance increases when it is pasted to form an electrode. Table 2 shows the amount of gallium and the cumulative 10% particle diameter (D 10 ), cumulative 50% particle diameter (D 50 ), cumulative 90% particle diameter (D 90 ), the BET specific surface area and the line resistance are also shown.

[0054] [Table 2]

Claims

1. A silver powder consisting of silver particles having gallium oxide coated 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 measuring device 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 with respect to the mass of the gallium oxide-coated silver powder.

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

4. The 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.

Images