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

Abstract

By modifying the surface of silver powder, silver powder capable of reducing the line resistance in the case where the obtained silver powder is made into a paste to form an electrode and a method for producing the same are provided. When silver particles are precipitated from an aqueous solution containing silver ions, an aluminum compound is co-present in the aqueous solution, and a reducing agent having a pH-lowering effect on the aqueous solution or a reducing agent having a pH-lowering effect on the aqueous solution by addition of an oxidized product of the reducing agent is used as the reducing agent, thereby obtaining an aluminum oxide-coated silver powder that reduces the electrode resistance in the case where the precipitated silver powder is made into a paste to form an electrode.

Description

Technical Field

[0001] This invention relates to alumina-coated silver powder suitable for forming conductive pastes for electrodes and circuits in various electronic components, particularly for forming conductive pathways in solar cells, and a method for manufacturing the same, as well as the conductive paste itself. Background Technology

[0002] Previously, resin-based, sintered silver pastes were primarily used for forming electrodes and circuits in electronic components. Solar cells are a prime example of this application. For instance, in the case of amorphous silicon solar cells, these sintered conductive pastes can be used to form current collector electrodes such as finger electrodes or busbar electrodes. In recent years, there has been a growing demand for conductive pastes using silver powder, driven by miniaturization of electronic components and the need for finer wiring widths. For example, in the case of solar cells, increasing the light-receiving area to enhance power generation has created a pressing need for finer wiring. To achieve this finer wiring, it is necessary to reduce the line resistance of the conductive film formed using the conductive paste. Therefore, previous attempts have focused on improving the electrical properties of the final conductive film by modifying the surface of the silver powder.

[0003] For example, Patent Document 1 discloses a surface-modified silver powder used in sintering pastes for circuit formation, wherein an oxide or composite oxide containing at least one metallic element belonging to Groups 2 to 14 of the periodic table, such as alumina or silicon oxide, is fixed on the surface of metallic silver particles. The surface-modified silver powder disclosed in Patent Document 1 is mainly used in the formation of LTCC (low-temperature co-fired ceramic) circuit boards. The purpose of fixing alumina or the like on the surface of the metallic silver particles is to suppress thermal shrinkage during sintering of the conductive paste containing the silver powder.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2001-240901 Summary of the Invention

[0007] The technical problem to be solved by the invention

[0008] Patent Document 1 discloses silver powder obtained by modifying the surface of silver powder by fixing oxide onto its surface. However, the alumina-attached metal particles in Example 1 of Patent Document 1 are particles formed by adding 5 parts by mass of alumina to 100 parts by mass of silver powder and attaching them using a dry fixation method. While this can suppress thermal shrinkage during the firing of the conductive paste, due to the high volume resistivity of alumina, there is a problem that the effect of reducing the line resistance of the conductive film obtained by firing the conductive paste is small. When the line resistance of the conductive film cannot be reduced, fine wiring cannot be achieved.

[0009] The technical problem to be solved by the present invention is to provide silver powder and its manufacturing method that reduce line resistance when the silver powder is modified on the surface and then made into a paste to form electrodes with fine linewidth.

[0010] In addition, the technical problem to be solved in this invention is to provide a conductive paste that can reduce the line resistance of the conductive film.

[0011] means for solving technical problems

[0012] In order to achieve the above-mentioned problem, the inventors conducted repeated and in-depth research and found that by coating a trace amount of aluminum oxide on the surface of silver powder, the line resistance of the final electrode film with a narrow linewidth can be reduced, thereby completing the present invention as described below.

[0013] Specifically, to achieve the above-mentioned technical problems, the present invention provides:

[0014] (1) Alumina-coated silver powder, which is alumina-coated silver powder containing silver particles on the surface of alumina, wherein the amount of aluminum relative to the mass of the alumina-coated silver powder is more than 10 ppm by mass and less than 400 ppm by mass, and the cumulative 50% particle size D of the alumina-coated silver powder is measured using a laser diffraction scattering particle size distribution measuring device. 50 Its micrometer size ranges from 0.2 μm to 5.0 μm, and its true density is below 10.00 g / cm³.

[0015] (2) Preferably, the amount of aluminum in the alumina-coated silver powder mentioned in (1) is between 10 ppm and 100 ppm relative to the mass of the alumina-coated silver powder.

[0016] (3) Preferably, when observing the cross-section of the alumina-coated silver powder mentioned in (1) to (2), the number of silver particles with pores inside the particles is more than 70% of all silver particles.

[0017] (4) The alumina-coated silver powder mentioned in (1) to (3) above is preferably the D 50 With BET particle size D BET The ratio (D) 50 / D BET The value ranges from 1.00 to 2.50.

[0018] (5) The alumina-coated silver powder mentioned in (1) to (4) above preferably has a cumulative 90% particle size D based on the volume reference measured by a laser diffraction scattering particle size distribution measuring device. 90 Cumulative 10% particle size D based on volume 10 The difference (D) 90 -D 10) and the D 50 The ratio of (D) 90 -D 10 ) / D 50 The range is from 0.50 to below 2.00.

[0019] (6) Preferably, the thermal loss (Ig-loss) of the silver powder coated with alumina described in (1) to (5) above is 0.30% to less than 3.00%.

[0020] (7) Preferably, the alumina-coated silver powder described in (1) to (6) above satisfies the following condition: when the sample obtained by compression molding of the alumina-coated silver powder at 2.5 kg / mm² is subjected to a load of 5 mN / mm², the thermomechanical analysis (TMA) curve obtained by heating from room temperature to 900°C at a heating rate of 10°C / min is performed, and the temperature at which the expansion rate based on room temperature reaches 0.50% is above 150°C and below 350°C.

[0021] (8) Preferably, the silver powder coated with alumina described in (1) to (7) above satisfies that, in the aforementioned TMA curve, the temperature at which the shrinkage rate reaches 5.00% based on room temperature is above 450°C and below 850°C.

[0022] (9) The alumina-coated silver powder described in (1) to (8) above can be used as a sintering paste for forming electrodes of solar cell substrates.

[0023] In addition, the following invention is provided.

[0024] (10) A sintering paste for forming solar cell electrodes, comprising any one of (1) to (9) above, silver powder coated with alumina, an organic binder and an organic solvent.

[0025] As a method for manufacturing the above-mentioned alumina-coated silver powder, the following is provided:

[0026] (11) A method for manufacturing silver powder coated with alumina, comprising the following steps:

[0027] A process for preparing an aqueous solution of a silver-ammonia complex containing aluminum and having a pH of 10.0 or higher, comprising a "complex formation step" and a "pH adjustment step," wherein the complex formation step is a step in which a silver-ammonia complex is formed by adding ammonia to an aqueous solution containing silver ions to obtain an aqueous solution of the silver-ammonia complex; the pH adjustment step is a step in which a pH adjusting agent is added to the aqueous solution of the silver-ammonia complex to adjust the pH of the aqueous solution to 10.0 or higher; and the method used is selected from one or more of the following methods to prepare the aqueous solution of the silver-ammonia complex: a method of using the aqueous solution containing an aluminum compound in the complex formation step, a method of adding an aluminum compound to the aqueous solution in the period before the pH adjustment step is completed, and a method of adding an aluminum compound to the aqueous solution in the aqueous solution after the pH adjustment step is completed; and

[0028] The process involves adding a reducing agent to the aqueous solution of the silver-ammonia complex containing aluminum and having a pH of 10.0 or higher to reduce silver ions and precipitate silver particles, while simultaneously adjusting the pH of the aqueous solution to a level between 4.0 and 9.0 to precipitate aluminum oxide.

[0029] in,

[0030] The amount of aluminum supplied to the liquid via the aluminum compound is controlled such that the amount of aluminum relative to silver in the aqueous solution at the initial point of addition of the reducing agent is 10 ppm by mass or more but less than 400 ppm by mass.

[0031] The reducing agent is an organic compound containing one or more of the following groups: COOH, CHO, and OH.

[0032] (12) In the manufacturing method of alumina-coated silver powder described in (11) above, the reducing agent is preferably formaldehyde.

[0033] (13) The manufacturing methods described in (11) to (12) above may further include a step of adding a surface treatment agent to a slurry containing silver powder precipitated by adding the reducing agent.

[0034] (14) In the manufacturing method described in (13) above, it is preferred that the amount of the surface treatment agent added is more than 0.05% by mass and less than 0.50% by mass relative to the amount of silver in the silver-ammonia complex aqueous solution.

[0035] Invention Effects

[0036] By using the alumina-coated silver powder of the present invention, even when the silver powder is made into a paste for forming fine wire electrodes with a design linewidth of, for example, 20 μm or less, a conductive film with low line resistance can be obtained. Attached Figure Description

[0037] Figure 1 This is a cross-sectional SEM image of the silver powder obtained in Comparative Example 1 of the present invention.

[0038] Figure 2 XPS spectra (Al2p) of silver powder obtained in Examples 1 and 2 and Comparative Examples 2 and 3 of the present invention.

[0039] Figure 3 The TMA curves of silver powder obtained in the embodiments and comparative examples of the present invention are shown.

[0040] Figure 4 This is a cross-sectional SEM image of silver powder coated with alumina obtained in Example 1 of the present invention.

[0041] Figure 5 This is a cross-sectional SEM image of silver powder coated with alumina obtained in Example 2 of the present invention.

[0042] Figure 6 This is a cross-sectional SEM image of silver powder coated with alumina obtained in Example 3 of the present invention.

[0043] Figure 7 This is a cross-sectional SEM image of the silver powder obtained in Comparative Example 2 of the present invention.

[0044] Figure 8 This is a cross-sectional SEM image of the silver powder obtained in Comparative Example 3 of the present invention. Detailed Implementation

[0045] [Alumina-coated silver powder]

[0046] The alumina-coated silver powder of the present invention is composed of silver particles on which a trace amount of alumina is coated, and has a true density of 10.00 g / cm³ or less. These silver particles contain particles with pores that are closed from the outside. By coating the surface of the silver powder containing the porous silver particles with alumina, even when the alumina-coated silver powder is made into a paste to form a fine wire electrode with a designed linewidth of 20 μm or less, the line resistance can be reduced.

[0047] In this invention, silver particles coated with alumina refer to silver particles on which alumina is partially or completely coated. The distribution of aluminum coated on the surface of the silver particles can be confirmed using Auger electron spectroscopy (AES). Furthermore, whether the coated aluminum is an oxide can be confirmed by measuring the chemical bonding state of the aluminum using X-ray photoelectron spectroscopy (XPS). It should be noted that, within the scope of the effects of this invention, it is permissible for the portion of the alumina-coated silver powder particles to contain silver particles that are not coated with alumina on the surface. For example, the proportion of silver particles partially or completely coated with alumina on the surface can be 80% or more, preferably 90% or more, and up to a maximum of 100%, relative to all particles in the alumina-coated silver powder. If the proportion of silver particles partially or completely coated with alumina on the surface is within the above range, the effect of suppressing aluminum segregation (uniform distribution) in the conductive paste is excellent.

[0048] It should be noted that, in this invention, "coating" refers to the state in which aluminum oxide partially or completely coats the surface of silver particles. The aluminum oxide-coated silver powder obtained in this embodiment, as described below, is silver powder obtained by precipitating aluminum compounds in a solution as aluminum oxide coating the surface of the silver particles during the reduction precipitation of silver particles in the solution. Therefore, aluminum oxide can be uniformly coated on the surface of all the reduced and precipitated silver particles without segregation. Furthermore, the coating strength of the aluminum oxide on the silver particles can reach, for example, a strength that prevents the aluminum oxide from detaching from the surface of the silver particles even if the silver powder of this embodiment is pulverized. Thus, even when this aluminum oxide-coated silver powder is mixed into a conductive paste, the coating and the silver particles can remain homogeneous. That is, even in a conductive paste, the segregation of silver particles and aluminum oxide can be suppressed.

[0049] As mentioned earlier, alumina is an insulator, and it is foreseeable that the line resistance of the electrode film obtained by firing a paste containing silver powder coated with alumina would increase. However, the inventors have discovered that by controlling the amount of alumina coated on the surface of the silver powder, the line resistance of the final electrode film can be reduced, thus completing the present invention. The mechanism by which an appropriate amount of alumina is coated on the surface of the silver powder to reduce the line resistance of the final electrode film will be described below.

[0050] The term "aluminum content" in this specification refers to the amount of aluminum relative to the mass of the alumina-coated silver powder, as determined by inductively coupled plasma optical emission spectrometry (ICP-OES) after the alumina-coated silver powder has been completely dissolved in acid. It can be understood that in this invention, aluminum is primarily present on the surface of the silver particles, comprising both aluminum present within the silver particles and aluminum coated on the surface as alumina. The term "aluminum content" in this specification is the sum of the amount of aluminum present in the silver particles and the amount of aluminum coated on the surface.

[0051] It should be noted that since ICP-OES cannot perform state analysis, whether the aluminum present on the surface of the silver particles exists in the state of aluminum oxide, i.e., whether it is "silver particles with aluminum oxide coating the surface", can be determined by using X-ray photoelectron spectroscopy (XPS) to analyze the state of aluminum. In this invention, when the measured XPS spectrum shows a peak for aluminum in the binding energy range of 74.3 ± 1 eV, it is determined that the aluminum is not in the metallic state, but in the oxide state.

[0052] In this invention, the aluminum present on the surface of the silver particles is preferably aluminum oxide, but it may contain a small amount of elemental aluminum (metal) peaks. In the XPS measurements described later, the ratio of the height of the peak corresponding to elemental aluminum to the height of the peak corresponding to aluminum oxide is preferably less than 10%.

[0053] The aluminum content relative to the total mass of the silver powder coated with alumina is preferably 10 ppm by mass or more. If the aluminum content is less than 10 ppm by mass, the resistance reduction effect of the electrode film of the present invention may become insufficient. In addition, alumina is an insulator, so if the aluminum content is 400 ppm by mass or more, the line resistance reduction effect is weakened, which is not preferred. The aluminum content is more preferably 300 ppm by mass or less, more preferably 250 ppm by mass or less, and even more preferably 100 ppm by mass or less.

[0054] The true density of the alumina-coated silver powder of the present invention is 10.00 g / cm³ or less, preferably 9.90 g / cm³ or less, and more preferably 9.85 g / cm³ or less. In the present invention, the true density of the silver powder as a whole refers to the density of the silver powder taking into account the porosity (i.e., the "closed space inside the silver particles" mentioned above) that exists within the silver particles. If the true density of the silver powder is 10.00 g / cm³ or less, it is preferable because the state of the silver particles with internal porosity can be fully observed when observing the cross-section of the silver powder, and the sintering of the silver particles is sufficient during the firing of the conductive paste. On the other hand, the true density may also be, for example, 9.00 g / cm³ or more, 9.40 g / cm³ or more, or 9.70 g / cm³ or more.

[0055] When observing the cross-section of the silver particles constituting the alumina-coated silver powder of the present invention using a scanning electron microscope (SEM), it can be confirmed that there are micropores sealed from the outside inside the silver particles. In this alumina-coated silver powder, the proportion of silver particles having one or more pores inside the particles is preferably 70% or more of all silver particles, more preferably 75% or more, further preferably 80% or more, and most preferably 85% or more. If this value is 70% or more, after the sintering of the particle surface begins, the gas components in the pores inside the particles react rapidly, thereby easily promoting the overall sintering process of the silver particles and forming a dense conductive film, which is therefore preferred. It should be noted that the upper limit of the proportion is 100%.

[0056] It should be noted that in this specification, a pore with a diameter of 15 nm or more, observed using cross-sectional SEM images of silver particles taken at 10,000 to 40,000x magnification, is defined as a "pore". In this invention, "pore diameter (i.e., the diameter of the enclosed space inside the silver particle containing the pore)" refers to the outer diameter of the smallest circle that completely accommodates the pore when drawn in a scanning electron microscope image. The proportion of silver particles with pores inside the particles in this silver powder can be confirmed by the following ratio: the ratio of the number of particles with one or more pores inside to the number of silver particles observed by observing the aforementioned cross-sectional SEM images of silver particles taken at 10,000x magnification (containing 10 or more particles) in 5 or more fields of view.

[0057] The cumulative 50% particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on a volume reference. 50 Preferably, it is 0.2 μm or more, more preferably 1.0 μm or more, and even more preferably 1.5 μm or more. Furthermore, it is preferably 5.0 μm or less, more preferably 3.0 μm or less, and even more preferably 2.5 μm or less. If D 50 If the thickness is less than 0.2 μm, the viscosity of the conductive paste will increase excessively, which may cause line breaks when drawing wiring or similar tasks using the conductive paste, making it undesirable. Furthermore, when D... 50 When the diameter exceeds 5.0 μm, it becomes difficult to draw fine wiring when using conductive paste for wiring and other tasks, so it is not preferred.

[0058] The BET specific surface area (SSA) of the alumina-coated silver powder of the present invention is preferably 0.10 m² / g or more, more preferably 0.20 m² / g or more, and even more preferably 0.30 m² / g or more; furthermore, it is preferably 1.20 m² / g or less, more preferably 1.00 m² / g or less, even more preferably 0.55 m² / g or less, and even more preferably 0.50 m² / g or less. If the BET specific surface area of ​​the alumina-coated silver powder is within the above range, it is easier to draw fine wiring when using conductive paste for wiring, etc., and therefore this is preferable.

[0059] The alumina-coated silver powder of the present invention D 50 The BET particle size (D) calculated from the aforementioned BET specific surface area (SSA) BET The ratio of (D) 50 / D BET The value is preferably between 1.00 and 2.50, more preferably below 2.00, and even more preferably below 1.50. It should be noted that D... BET The particle size (particle diameter) is the particle size assuming the particles are perfect spheres, calculated using equation (1) below. Furthermore, the method for determining true density will be described below. This ratio (D...) 50 / D BET In the absence of silver particle aggregation, the value is generally considered to be above 1.00. By controlling it below 2.50, it is easier to draw fine wiring when using conductive paste to draw wiring, and therefore it is preferred.

[0060] D BET (μm) = 6 / (BET specific surface area × true density) …(1)

[0061] Here, substitute the value of BET specific surface area in m² / g for “BET specific surface area” on the right side of equation (1), and substitute the value of true density in g / cm³ for “true density”.

[0062] The maximum particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on a volume reference. max Preferably, it is 15.0 μm or less, more preferably 12.0 μm or less, and even more preferably 10.0 μm or less. If D max For diameters below 15.0 μm, conductive paste can be used to draw fine wiring.

[0063] The cumulative 10% particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on the volume standard. 10Preferably, the particle size is 0.1 μm or more, more preferably 1.0 μm or more. Furthermore, preferably 3.0 μm or less, more preferably 2.0 μm or less, and even more preferably 1.5 μm or less. If the cumulative particle size D is 10%... 10 If the value is within the above range, the viscosity of the conductive paste can be prevented from becoming too high, and therefore it is preferred.

[0064] The cumulative 90% particle size D of the alumina-coated silver powder of the present invention, measured by a laser diffraction scattering particle size distribution measuring device, is based on a volume reference. 90 Preferably, the micrometer size is 1.5 μm or more, more preferably 2.0 μm or more, and even more preferably 2.5 μm or more. Furthermore, preferably 6.0 μm or less, more preferably 5.0 μm or less, and even more preferably 4.0 μm or less. If the cumulative D... 90 When the value is within the above range, it is easier to draw fine wiring when using conductive paste for wiring, etc., and therefore it is preferred.

[0065] The alumina-coated silver powder of the present invention achieves a cumulative particle size D of 90%. 90 Cumulative 10% particle size D based on volume 10 The difference (D) 90 -D 10 ) and cumulative 50% particle size D 50 The ratio of (D) 90 -D 10 ) / D 50 The value is preferably 0.50 to 2.00 or less, more preferably 1.50 or less, and even more preferably 1.30 or less. Wherein, (D 90 -D 10 ) / D 50 As an indicator of particle size distribution, the smaller this value, the sharper the particle size distribution of the silver powder coated with alumina, meaning the more uniform the particle size. In this invention, if it is within the above range, it is easier to draw fine wiring when using conductive paste to draw wiring, and therefore it is preferred.

[0066] The gravimetric loss (Ig-loss) of the alumina-coated silver powder of the present invention is preferably 0.30% or more, more preferably 0.40% or more, and even more preferably 0.50% or more, from the viewpoint of suppressing the agglomeration of the alumina-coated silver powder. Furthermore, from the viewpoint of suppressing the increase in the resistivity of the conductive film obtained by using a conductive paste containing alumina-coated silver powder, it is preferably 3.00% or less, more preferably 2.00% or less, and even more preferably 1.50% or less.

[0067] The alumina-coated silver powder of the present invention is compressed and molded under the conditions described below to obtain a pressed powder sample. When this sample is heated from room temperature (25°C ± 5°C) at a heating rate of 10°C / min, the temperature at which the expansion rate (hereinafter sometimes referred to as the "0.50% expansion temperature") reaches 0.50% based on room temperature is preferably between 150°C and 350°C. This expansion rate can be determined by using a thermomechanical analysis apparatus to obtain a TMA curve from room temperature to 900°C when the aforementioned sample is heated at a heating rate of 10°C / min. It should be noted that this expansion behavior is easily generated due to the presence of porosity within the silver powder particles; however, even with the same degree of porosity, the alumina-coated silver powder of the present invention tends to exhibit this expansion behavior significantly more strongly than silver powder without alumina coating. If the silver powder has an expansion rate of 0.50% or more at a temperature between 150°C and 350°C (i.e., silver powder with an expansion temperature of 0.50% between 150°C and 350°C), the sintering property between silver particles is improved due to the release of gas present in the aforementioned pores, which is effective in reducing the resistance of the electrode film. Preferably, the expansion rate in the temperature range of 200°C to 300°C on the TMA curve, based on room temperature, is 0.50% or more, more preferably 1.00% or more. Furthermore, it is preferable that the expansion rate in the temperature range of 200°C to 300°C, based on room temperature, is 5.00% or less, more preferably 3.00% or less. If it is 5.00% or less, a dense conductive film is easily obtained when forming the conductive film, which is preferred from the viewpoint of reducing resistance.

[0068] The alumina-coated silver powder of the present invention is compressed and molded according to the conditions described below to obtain a pressed powder sample. The thermomechanical analysis (TMA) curve of this sample, obtained by heating at a rate of 10°C / min from room temperature (25°C ± 5°C) to 900°C, shows a shrinkage rate of 5.00% based on room temperature (hereinafter sometimes referred to as the "5.00% expansion temperature"). This temperature is preferably 450°C or higher to 850°C or lower, more preferably 800°C or lower. If the 5.00% shrinkage temperature is within the above range, the organic components in the paste can easily escape, maintaining the strength of the electrode film after firing. The shrinkage rate based on room temperature in the TMA curve between 450°C and 850°C is preferably 5.00% or higher, more preferably 8.00% or higher. Furthermore, the shrinkage rate based on room temperature in the range between 450°C and 850°C is preferably 20.00% or lower, more preferably 15.00% or lower. If it is below 20.00%, the resistance of the electrode film after firing is reduced, which is therefore preferred.

[0069] The mechanism by which coating silver particles with alumina reduces the line resistance of the final electrode film is currently unclear, but the inventors have considered the following: It is hypothesized that the presence of trace amounts of alumina on the silver surface improves the sliding properties between silver particles within the paste. Therefore, it can be assumed that improved paste ejection from the printing plate and enhanced fine-line printability contribute to a reduction in the resistance of the final electrode film. From this perspective, alumina can exist only near the surface of the silver particles.

[0070] In addition to the mechanisms described above, the following mechanism also contributes to reducing the resistance of the electrode film when silver powder is coated with alumina: When the silver powder coated with alumina is heated, the gas inside the pores of the silver particles expands due to thermal expansion as it is released outwards, causing the silver particles to expand. Even with a very small amount of alumina coating, cracks may form in the alumina coating layer. This increases the exposed area of ​​metallic silver in the underlying silver particles, leading to metal / metal bonding between the silver powder particles coated with alumina. Therefore, improving the sintering performance of the silver powder coated with alumina results in a lower resistance of the final electrode film.

[0071] [Manufacturing Method]

[0072] The method for manufacturing alumina-coated silver powder of the present invention uses a wet process, which has excellent manufacturing cost and large-scale production efficiency.

[0073] [Starting material]

[0074] In the method for manufacturing alumina-coated silver powder of the present invention, an aqueous solution containing monovalent silver ions (aqueous solution containing silver ions) is used as the starting material. As the source of silver ions, industrially known inorganic silver salts such as silver nitrate (I), silver sulfate (I), silver carbonate (I), silver chloride (I), and silver oxide (I) can be used. It should be noted that an aluminum compound can also be added to the above aqueous solution to make it an aqueous solution containing both monovalent silver ions and aluminum ions. The aluminum compound can be added at any time, before the addition of ammonia, after the addition of ammonia, or after the addition of a pH adjuster, as long as it is before the addition of a reducing agent; there are no particular limitations.

[0075] Although not specifically specified in this invention, the concentration of silver ions in the aqueous solution used as the starting material is preferably between 1.0% by mass and 2.0% by mass during the solution preparation stage. When the silver ion concentration is less than 1.0% by mass, the amount of silver powder that can be produced in each reaction decreases, which is disadvantageous from the viewpoint of production efficiency. When the silver ion concentration exceeds 2.0% by mass, the viscosity of the reaction solution after the silver particles precipitate increases, and depending on the production equipment, it may be difficult to uniformly stir the reaction solution.

[0076] [Complex Formation Process]

[0077] In the method for manufacturing silver powder coated with alumina according to the present invention, silver ions are complexed with ammonium ions to form a silver-ammonia complex, thereby preparing an aqueous solution of the silver-ammonia complex. Ammonium salts such as ammonia, ammonium chloride, or ammonium carbonate can be used as the source of ammonium ions. When ammonium ions are used as the complexing agent, a silver-ammonia complex can be formed in the aqueous solution; however, in this case, since the coordination number of the ammonia complex is 2, more than 2 moles of ammonium ions are added per mole of silver ions. It should be noted that the reaction temperature for forming the silver-ammonia complex is preferably controlled between 10°C and 40°C. Since the formation of the silver-ammonia complex is an exothermic reaction, the reaction temperature for forming the silver-ammonia complex is preferably within the aforementioned range in order to set the temperature of the reduction precipitation process described later at the desired temperature. If the reaction temperature is too low or too high, temperature adjustment requires time, thus increasing energy costs.

[0078] [pH adjustment process]

[0079] Next, a pH adjuster is added to the aqueous solution of the silver-ammonia complex obtained from the aforementioned steps to adjust the pH of the aqueous solution to 10.0 or higher. Controlling the pH to 10.0 or higher is to sufficiently enhance the reducing power of the reducing agent. As the aforementioned pH adjuster, hydroxides or carbonates of alkali metals or alkaline earth metals such as sodium hydroxide or calcium hydroxide can be used. It should be noted that in the method for manufacturing alumina-coated silver powder of the present invention, the upper limit of the pH of the aqueous solution is not specifically specified, but to avoid excessive use of the pH adjuster, it is preferable to control the pH to 13.0 or lower. The temperature of the pH adjustment step is preferably controlled between 10°C and 40°C. If the reaction temperature is too low or too high, temperature adjustment requires time, thus increasing energy costs.

[0080] [Aluminum compounds]

[0081] In this invention, the aluminum compound is added at the time described above, either before or after the addition of ammonia, or after the addition of the pH adjuster. That is, the aluminum compound only needs to be added before the addition of the reducing agent in the silver particle precipitation process described later. This invention includes a step of adding the aluminum compound to the aforementioned aqueous solution containing silver ions, the aforementioned aqueous solution of the silver-ammonia complex, or the aforementioned aqueous solution with added pH adjuster.

[0082] As an aluminum compound, any compound that allows aluminum to be added to an aqueous solution in ionic form, depending on the pH range of the aqueous solution at the time of addition, is acceptable. For example, inorganic aluminum salts such as aluminum nitrate, aluminum acetate, and aluminum oxalate can be used. Furthermore, it is preferable to match the type of acid or base used; for example, using aluminum nitrate for silver nitrate aqueous solutions is also preferred. It should be noted that aluminum oxides are amphoteric oxides; therefore, aluminum is added in the low pH range in the form of aluminum ions (Al³⁺). + It dissolves in the form of aluminate ions (AlO2) in a high pH range. - It dissolves in the form of aluminum hydroxide (Al(OH)3 or Al2O3·3H2O) at a pH near neutral, forming a solid-phase stable region of aluminum oxide (Al2O3). It should be noted that here, aluminum oxide includes aluminum hydroxide (Al(OH)3 or Al2O3·3H2O).

[0083] The amount of aluminum supplied to the liquid by the aluminum compound (the total amount of aluminum supplied in the case of multiple supplies) is preferably such that, at the starting point of the addition of the reducing agent described later, the amount of aluminum in the aqueous solution is 10 ppm by mass or more and less than 400 ppm by mass relative to the amount of silver ions contained in the aqueous solution. By ensuring that the amount of aluminum relative to silver is 10 ppm by mass or more, it is easy to obtain the effect of reducing the resistance of the electrode film. In addition, since alumina is an insulator, by ensuring that the amount of aluminum relative to silver is less than 400 ppm by mass, it is effective in suppressing the effect of reducing the line resistance. More preferably, the amount of aluminum relative to silver is controlled to be 300 ppm by mass or less, more preferably 250 ppm by mass or less, and even more preferably 100 ppm by mass or less.

[0084] Furthermore, when adding aluminum compounds in an aqueous solution, the concentration is preferably between 0.5% by mass and 10.0% by mass (calculated as aluminum). When the aluminum concentration is less than 0.5% by mass, the volume of the reaction solution increases as the amount of aluminum compound aqueous solution added to obtain alumina-coated silver powder becomes more, requiring more reagents and thus becoming uneconomical. When the aluminum concentration exceeds 10.0% by mass, the amount of aluminum compound aqueous solution added to obtain alumina-coated silver powder becomes trace, and the error in the amount added can easily increase depending on the operating conditions.

[0085] [Silver particle precipitation process]

[0086] In the method for manufacturing silver powder coated with alumina according to the present invention, a reducing agent is added to the aqueous solution of a silver-ammonia complex containing aluminum and having a pH of 10.0 or higher obtained in the aforementioned steps to reduce the silver-ammonia complex, thereby causing silver particles to precipitate in the aqueous solution. At this time, a reducing agent that lowers the pH of the aqueous solution by adding the reducing agent is used. The reason for using a reducing agent that lowers the pH of the aqueous solution is that by adding this reducing agent, the pH of the aqueous solution can be lowered to the solid-phase stable region of the aforementioned alumina, thereby causing the alumina to coat the surface of the precipitated silver particles. If a reducing agent that does not lower the pH of the aqueous solution is used, adding a pH adjuster to lower the pH of the aqueous solution after the silver particles precipitate may allow the alumina to coat the surface of the silver particles, but this increases the number of steps in the manufacturing method, which is not preferable. Furthermore, it is difficult to uniformly coat the particle surface with alumina, which is also not preferable.

[0087] It should be noted that the reaction temperature for silver particle precipitation using the aforementioned reducing agent is preferably controlled between 10°C and 40°C. If the reaction temperature is too low or too high, temperature adjustment will require time, thus increasing energy costs.

[0088] [reducing agent]

[0089] The most significant technical feature of the method for manufacturing alumina-coated silver powder of the present invention is the use of a reducing agent that lowers the pH of an aqueous solution upon addition. The mechanism by which the pH of the aqueous solution is lowered by adding the reducing agent includes situations where the reducing agent itself has a pH-lowering effect, and situations where the oxidation product of the reducing agent has a pH-lowering effect.

[0090] As a reducing agent with such an effect, it is preferable to be an organic compound containing one or more of the following groups: COOH, CHO, and OH. Specifically, examples include aldehydes such as ascorbic acid, tartaric acid, formic acid, and formaldehyde.

[0091] When an organic compound with an intramolecular COOH group acts as a reducing agent, the pH of the aqueous solution decreases due to the addition of this reducing agent. In the case of a reducing agent with a CHO group that has a reducing effect, the CHO group reduces silver ions, and the CHO group is oxidized to form a COOH group, thus decreasing the pH of the aqueous solution. In the case of a reducing agent with an OH group that has a reducing effect, the OH group is oxidized and converted to a COOH group via the CHO group, thus decreasing the pH of the aqueous solution.

[0092] It should be noted that the presence of pores inside the silver particles constituting the aluminum oxide-coated silver powder of the present invention is presumed to be due to the gas generated by the decomposition of the aforementioned organic compounds being trapped inside the silver particles.

[0093] In the method for manufacturing alumina-coated silver powder of the present invention, the pH of the aforementioned aqueous solution is controlled to be between 4.0 and 9.0 by adding the aforementioned reducing agent. By setting the pH within the above-mentioned range, alumina can be coated onto silver particles from the mixed aqueous solution.

[0094] The pH value lowered by adding the aforementioned reducing agent can be used as the pH value after silver powder precipitation is complete. In this case, this pH value can be set as the measured pH value of the filtrate after filtering and recovering the silver powder. This pH value can be controlled by the pH value in the pH adjustment process described above, the amount of pH adjusting agent added, the equivalent of ammonia in the complex formation process, or the amount of reducing agent. The pH value is more preferably set to 6.0 or higher. When the pH value is lower than 4.0, aluminum may transform into Al. 3+ The ions cause the aluminum oxide on the surface to redissolve, which is undesirable. Furthermore, when the pH is greater than 9.0, almost no aluminum is detected in the determination of the aluminum coated with the silver powder, which is also undesirable.

[0095] It should be noted that when the reducing agent in Example 1 described below is changed from formalin to hydrazine, the pH of the filtrate changes from 10.0 to 11.0, and almost no aluminum is detected in the determination of the amount of aluminum coated on the obtained silver powder described below.

[0096] Therefore, it is believed that when the pH of the mixed aqueous solution decreases from above 10.0 to near 9.0, aluminum oxide begins to precipitate on the surface of the silver particles. Thus, as the particles grow until the final pH is reached, an aluminum-containing layer (e.g., aluminum oxide) forms near the surface of the silver particles, thereby gradually coating the surface of the silver particles with aluminum oxide.

[0097] Therefore, in the manufacturing method of the present invention, aluminum ions are pre-contained in the aforementioned mixed solution, and the pH change during the precipitation process of the reducing agent causes alumina to coat the surface of the silver particles. Thus, compared to Patent Document 1, the generation of alumina particles or aluminum particles separated from the silver particles can be suppressed, and the fixed alumina can be uniformly coated on the surface of the silver particles. Therefore, the amount of aluminum compound pre-contained in the mixed solution can be suppressed, resulting in silver powder with a lower aluminum content, which will be measured in the aluminum content determination described later. That is, in the present invention, the alumina near the surface of the silver particles is preferably uniformly attached to the entire silver particle within the concentration range described above.

[0098] When aldehydes are used as reducing agents, silver particles precipitate to a certain extent. As the concentration of the oxidation product, carboxylic acid, increases, the pH of the aqueous solution reaches the aforementioned pH range. Therefore, from the viewpoint of facilitating the coating of alumina onto the surface of the precipitated silver particles, this is preferable. As for the aldehyde, formaldehyde is more preferred from the viewpoint of ease of acquisition and reducing power.

[0099] To improve the silver yield, the amount of the aforementioned reducing agent added is preferably 1 equivalent or more relative to silver, or 2 equivalents or more relative to silver, for example, 10 equivalents or more to 20 equivalents or less.

[0100] [Surface treatment agent addition process]

[0101] In the method for manufacturing alumina-coated silver powder of the present invention, a surface treatment agent can be added to the solution containing alumina-coated silver particles obtained in the aforementioned silver particle precipitation step. This yields alumina-coated silver particles coated with the surface treatment agent.

[0102] Examples of surface treatment agents added to the solution containing silver particles coated with alumina include fatty acids, compounds with azo structures, fatty acid salts, surfactants, organometallic chelate forming agents, and protective colloids. From the viewpoint of easy and uniform adhesion to the surface of silver powder, the surface treatment agent is preferably one or more selected from the group consisting of fatty acids, compounds with azo structures, and fatty acid salts.

[0103] [Surface treatment agent]

[0104] Examples of fatty acids include benzyl acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, and linolenic acid. They can be used alone or in combination of two or more.

[0105] Examples of fatty acid salts include the salts of the fatty acids listed above. Examples of salts include, for example, sodium salts and potassium salts.

[0106] Examples of compounds with azole structures include benzotriazole, sodium benzotriazole, and potassium benzotriazole. They can be used alone or in combination of two or more.

[0107] The amount of surface treatment agent added to the solution containing silver particles coated with alumina is preferably 0.05% by mass or more, more preferably 0.10% by mass or more, and even more preferably 0.15% by mass or more, relative to the amount of silver in the silver-ammonia complex aqueous solution. If it is 0.05% by mass or more, it is effective in maintaining the dispersibility of the obtained silver powder coated with alumina well. On the other hand, the amount of the aforementioned surface treatment agent is preferably 1.00% by mass or less, more preferably 0.30% by mass or less, and even more preferably 0.20% by mass or less. When the amount of the aforementioned surface treatment agent is 1.00% by mass or less, it is easier to suppress the excessive organic matter content in the obtained silver powder coated with alumina, and when using a conductive paste containing silver powder coated with alumina to obtain a conductive film, it is beneficial to suppress the increase in resistance value.

[0108] A surface treatment agent is added to a solution containing silver particles coated with alumina, preferably after adding a reducing agent to an aqueous solution of a silver-ammonia complex and allowing a predetermined time for addition. In this invention, the addition of the surface treatment agent inhibits the growth of silver particles, thus causing silver particles to precipitate and promoting the precipitation of alumina due to pH changes. The timing of addition, which allows silver in the solution to precipitate as silver particles, can be arbitrarily determined.

[0109] [Separation and recycling process]

[0110] After separating and recovering the alumina-coated silver powder obtained from the aforementioned series of processes using known solid-liquid separation methods, it is washed and dried as needed. Known solid-liquid separation methods include, for example, decantation or a filter press. The endpoint of the washing process can be determined by the conductivity of the washing water. Specifically, the washing process is considered complete when the conductivity of the washing water reaches a predetermined value or below. The washed silver particles, etc., can be supplied to the drying process in a cake-like or other agglomerated state.

[0111] [Drying Engineering]

[0112] The drying process can be carried out using vacuum drying or airflow dryers. During the drying process, high-pressure air jets can be sprayed onto the silver particles or other aggregates, or the cake-shaped and spherical silver powder from the drying process can be added to a mixer equipped with a stirring rotor for stirring. This disperses the silver powder, allowing for operations based on dispersion or drying. The drying temperature of the silver powder is controlled below 100°C. Maintaining the silver powder temperature below 100°C effectively inhibits the sintering between silver particles within the powder.

[0113]

Example

[0114] [Determination of Aluminum Content]

[0115] The amount of aluminum coated in alumina-coated silver powder was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). 1 g of the sample (silver powder) was accurately weighed and added to 15 mL of pure water and 10 mL of nitric acid (for precision analysis). The mixture was heated at 200°C for 30 minutes. After the heated sample cooled naturally, it was brought to a final volume of 100 mL with pure water. 5 mL of the supernatant was collected and brought to a final volume of 100 mL with pure water to prepare the sample for ICP analysis. A standard addition method was used for the determination. Aluminum was added to a standard solution containing 5N silver to form a silver concentration equivalent to that of the sample, thus establishing a calibration line. Quantitative analysis was performed using an Agilent 5800 ICP-OES instrument manufactured by Agilent Technologies.

[0116] [Aluminum Phase Analysis]

[0117] The state of silver-coated aluminum was analyzed using X-ray photoelectron spectroscopy (XPS). XPS measurements were performed using a PHI 5000 VersaProve III scanning X-ray photoelectron spectroscopy system manufactured by ULVAC Fiennes GmbH. The X-ray source was monochromatic Al Kα line, with an accelerating voltage of 15 kV, an output power of 25 W, an X-ray incident angle of 90 degrees, and a photoelectron extraction angle of 45 degrees.

[0118] Chemical state analysis of aluminum was performed using Al 2p spectroscopy, with a pulse energy of 69 eV, an integration time of 80 ms, an energy interval of 0.125 eV / step, and a cumulative count of 500. Charge correction was performed using the C / C bond binding energy of 284.8 eV.

[0119] [Scanning Electron Microscopy (SEM) Observation]

[0120] The cross-section of silver particles was observed using a scanning electron microscope (SEM) according to the following method to confirm the existence of internal pores in the silver particles.

[0121] First, silver powder is added to resin and a curing agent for curing, and then the cured resin is cut. Next, the cut surface is ground using a cross-section polishing machine to expose the cross-section of the silver particles. A scanning electron microscope is then used to observe each silver particle to confirm whether there are any pores inside the silver particles.

[0122] For example, "EpoFix resin" and "EpoFix curing agent" manufactured by Struers Co., Ltd. can be used as the resin and curing agent mentioned above. Alternatively, "ArBlade 5000" manufactured by Nippon High Technology Co., Ltd. can be used as the cross-section polishing machine mentioned above. Furthermore, "JSM-IT800SHL" manufactured by Nippon Egis Corporation can be used as the scanning electron microscope mentioned above.

[0123] It should be noted that in this specification, "pores inside the silver particle" does not refer to the uneven space on the surface of the silver particle, but rather to the enclosed space inside the silver particle.

[0124] [Particle size distribution determination]

[0125] The cumulative 10% particle size (D) of alumina-coated silver powder was determined using a laser diffraction scattering particle size distribution analyzer (MicroTrack MT-3300 EXII, manufactured by Microtrac BEL Corp.) as a volume reference. 10 ), cumulative 50% particle size (D 50 ), cumulative 90% particle size (D 90 ), cumulative 100% particle size (D maxDuring the determination, 0.1 g of sample was added to 40 mL of isopropanol (IPA) for dispersion. An ultrasonic homogenizer (US-150T, 19.5 kHz, probe tip diameter 18 mm, manufactured by Nippon Seiki Co., Ltd.) was used for dispersion. The dispersion time was set to 2 minutes. The dispersed sample was then fed into the apparatus, and the particle size distribution was determined using the accompanying analytical software.

[0126] It should be noted that the circulator of the aforementioned laser diffraction scattering particle size distribution measuring device used an SDC (sample delivery controller) device during the measurement, and the "flow rate (%)" setting of the circulator was 60.

[0127] [BET specific surface area]

[0128] The BET specific surface area of ​​silver powder coated with alumina was determined using a Macsorb HM-model 1210 instrument manufactured by MOUNTTECH, according to the BET single-point method utilizing nitrogen adsorption. It should be noted that in the determination of the BET specific surface area, the sample weight was set to 3.0 g, a N2:He mixture of 30:70 was used, the gas flow rate was set to 25 mL / min, and the degassing conditions before the determination were 60℃ for 10 minutes.

[0129] [True density determination]

[0130] True density is determined using the constant volume expansion method (the "gas specific gravity bottle method" in the Japanese Pharmacopoeia) and a dry automatic densitometer using helium gas (manufactured by Microtrac, device name: AccuPyc II1340). Specifically, helium gas is introduced into a container until a certain pressure is reached, and the volume of the silver powder is measured from the volume of the gas at that point. The true density is calculated by dividing the mass of the silver powder by this volume. It should be noted that the helium gas cannot reach the closed spaces (i.e., pores) on the particle surface at this pressure. Therefore, the true density is measured in a way that includes the closed pores inside the silver particles that are not connected to the outside. Therefore, generally, the higher the proportion of pores inside the silver powder, the lower the true density of the silver powder.

[0131] [Thermomechanical Analysis]

[0132] Thermomechanical analysis (TMA) of silver powder coated with alumina was performed using the Thermo plus EVO2 series TMA8311 manufactured by Rigaku Corporation. First, 0.3 g of silver powder coated with alumina was weighed and placed into a specified mold with a diameter of 5 mmφ. The mold was then pressed with a 50 kg load for 1 minute to form a circular plate-shaped test sample. The test sample was placed in the sample holder of the aforementioned thermomechanical analysis (TMA) apparatus (Thermo plus EVO 2 series TMA8311). A measuring load of 98 mN was applied through the measuring probe, and the temperature was increased from room temperature (25℃±5℃) to 900℃ at a heating rate of 10℃ / min. Thermomechanical analysis (TMA) was performed, and the expansion rate and contraction rate of the test sample were determined using the following equations (2) and (3) for the length L (mm) distribution of the circular plate-shaped test sample at each temperature.

[0133] Expansion rate (%) in thermomechanical analysis = 100 × (LL) RT ) / L RT …(2)

[0134] Shrinkage rate (%) in thermomechanical analysis = 100 × (L) RT -L) / L RT …(3)

[0135] Here, L RT The length (mm) of a circular plate-shaped sample is measured at room temperature (25℃±5℃).

[0136] [Determination of Ig-loss value]

[0137] In this specification, "Ig-loss" refers to the change in mass when heated from room temperature (25℃±5℃) to 800℃. Specifically, it is an indicator of the amount of non-silver components in the silver powder, and an indicator of the amount of residual components in the silver powder, such as additives or surface treatment agents used in the silver powder manufacturing process. The Ig-loss of alumina-coated silver powder is calculated as follows: a silver powder sample is accurately weighed (weighing value: w1), placed in a magnetic crucible, heated to 800℃, and then held at 800℃ for 30 minutes as sufficient time to reach constant weight. After cooling, it is weighed again (weighing value: w2). The Ig-loss is calculated from w1 and w2 according to the following formula (4).

[0138] Loss on heating (Ig - loss) (mass%) = 100 × (w1 - w2) / w1 ... (4)

[0139] [Manufacturing method of conductive paste]

[0140] A mixture was prepared by mixing alumina-coated silver powder, aluminum powder (metallic aluminum: 99.87% by mass, iron: 0.09% by mass, silicon: 0.04% by mass, SEM average particle size: 2.0 μm), glass frit (glass powder: containing PbO as the main component, and containing B2O3, SiO2 and other oxides), ethyl cellulose, TEXANOL, butyl carbitol acetate, tributyl citrate, oleic acid, triacetin, methylphenyl polysiloxane, hydrogenated castor oil, and fatty acid amide. The composition of the mixture is shown in Table 1. The obtained mixture was fed into a paddleless rotary stirring degassing device (V-mini300 manufactured by EME Corporation), premixed for 30 seconds at a rotation speed of 1000 rpm, and then kneaded using a three-roll mill (80S manufactured by EXAKT Corporation) through a roller gap of 100 μm to 20 μm to obtain a conductive paste.

[0141] Table 1

[0142]

[0143] [Determination of line resistance]

[0144] Using the conductive paste obtained in the above steps, straight lines were printed by screen printing. The designed linewidth was 20 μm, and the line length was 150 mm. Printing was performed using a Micro-tec printer at a squeegee speed of 350 mm / s. A silicon substrate with a thickness of approximately 170 μm (for solar cell applications, texturing and SiN) was used for printing. x (Film formation completed). After printing, the wafer was dried in a dryer at a temperature set at 200°C for 5 minutes, and then fired in a solar cell firing furnace (manufactured by NGK) at a peak temperature of 740°C on the surface of the silicon wafer to obtain a sample for measuring line resistance.

[0145] The resistance value of the electrode after firing (the line resistance value of the conductive film) was measured using a digital multimeter (manufactured by ADC) by contacting the two ends of the printed electrode with the test terminals.

[0146] [Comparative Example 1]

[0147] Commercially available silver powder (manufactured by Tonghe Electronics Technology Co., Ltd., 4-8FD) was used as the test powder. The line resistance was measured according to the above steps, and the result was 16.9Ω. In this invention, when the line resistance is lower than this value, it is determined that the invention has the desired effect.

[0148] Figure 3The TMA curve of the silver powder obtained from Comparative Example 1 is shown in Figure 1. The silver powder obtained from Comparative Example 1 began to expand at around 220°C, with a maximum expansion rate of 0.47%, which was less than 0.50%. Afterwards, it recovered to its original sample length at around 320°C and began to shrink, reaching a shrinkage of over 5.00% at 417°C. Additionally, Figure 1 shows a cross-sectional SEM image of the silver powder obtained from Comparative Example 1. It should be noted that the white scale bar in the lower right corner of this SEM image is 1 μm long. Figure 4 , Figures 5-7 (The same applies to the silver powder obtained in Comparative Example 1). The silver powder obtained in Comparative Example 1 has pores inside, and the proportion of silver particles with pores is 90%.

[0149] Table 2 shows the aluminum content and the cumulative 10% particle size (D) of the silver powder used in the test. 10 ), cumulative 50% particle size (D 50 ), cumulative 90% particle size (D 90 ), cumulative 100% particle size (D max ), BET specific surface area, BET particle size (D) BET D 50 With D BET (D) BET The ratio of (D) 50 / D BET The cumulative 90% particle size (D) of the volume standard was measured using a laser diffraction scattering particle size distribution measuring device. 90 ) and the cumulative 10% particle size on a volume basis (D 10 The difference (D) 90 -D 10 ) and D 50 The ratio of [(D)] 90 -D 10 ) / D 50 The following parameters are used: Ig-loss value, proportion of porous silver particles, true density, 0.50% expansion temperature, 5.00% contraction temperature, and the aforementioned line resistance (the same applies to the following examples).

[0150] It should be noted that in this comparative example, aluminum compounds were not used to supply aluminum to the liquid, and commercially available silver powder (mentioned above) was used as the test powder.

[0151] [Example 1]

[0152] To 3482.7 g of a silver nitrate aqueous solution containing 53.7 g of silver, 1.1 g of a 5% by mass aluminum nitrate nonahydrate (manufactured by High Purity Chemical Research Institute Co., Ltd., specification purity 98.0+%) aqueous solution was added. While stirring at 332 rpm, 113.2 g of a 28% by mass ammonia aqueous solution (manufactured by Pure Science Co., Ltd.) was added, generating an aluminum-containing silver ammonia complex aqueous solution. The aluminum content in the added aluminum nitrate nonahydrate aqueous solution was 70 ppm by mass relative to the silver content. One minute after adding the ammonia aqueous solution, 10.0 g of a 20% by mass sodium hydroxide aqueous solution was added, and the solution temperature was adjusted to 26.5°C. At this point, the pH of the solution was 12.1. Next, after adding the above ammonia aqueous solution to the aluminum-containing silver nitrate aqueous solution, 3 minutes later, 251.4 g of 25.9% by mass formalin (manufactured by Mitsubishi Chemical Corporation) was added all at once as a reducing agent. Fifteen seconds after adding the reducing agent, 6.1 g of an emulsified aqueous solution containing 1.55% stearic acid was added to the slurry containing the precipitated silver powder. Stirring was then stopped, and the slurry containing silver particles was filtered and washed with water until the conductivity dropped below 0.5 mS / m. The slurry was then vacuum-dried at 73°C for 10 hours to obtain silver powder. At this point, the pH of the filtrate was 6.3. The reaction was conducted in a 5-liter beaker using baffles and two turbine blades. 50 g of the silver powder obtained in the above steps was loaded into a sample mill (manufactured by Kyoritsu Rikko Co., Ltd., SK-M10) and milled twice for 30 seconds at a dial value of 100 to obtain alumina-coated silver powder (experimental powder) according to Example 1.

[0153] The aluminum content of the silver powder coated with alumina was determined, and the result showed that the aluminum content relative to the mass of the silver powder coated with alumina was 70 ppm, and the loss on heating (Ig-loss) was 0.95%. Figure 2 The XPS spectrum of the alumina-coated silver powder obtained in this embodiment is shown. A small peak is observed near the binding energy of 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating the presence of alumina on the surface of the silver powder obtained in this embodiment. It should be noted that no peak of metallic aluminum (binding energy near 72.6 eV) was observed by XPS measurement.

[0154] Figure 3 The TMA curve of the alumina-coated silver powder obtained in Example 1 is shown. The alumina-coated silver powder obtained in Example 1 began to expand at around 220°C, reaching an expansion rate of 0.50% or more at 227°C, with a maximum expansion rate of 1.90%. Afterward, it recovered to its original sample length at around 420°C and began to shrink, reaching a shrinkage rate of 5.00% or more at 559°C. Furthermore, Figure 4A cross-sectional SEM image of the silver powder coated with alumina obtained in Example 1 is shown. The silver powder coated with alumina obtained in Example 1 has pores, with a porosity of 86% silver particles and a true density of 9.88 g / cm³.

[0155] Using the silver powder coated with alumina obtained in this embodiment, the line resistance was measured according to the above steps, and the result was 16.5 Ω. This value is lower than that of Comparative Example 1, indicating that even with an aluminum content of 70 ppm by mass, the line resistance is reduced when alumina is coated on the surface of the silver powder, made into a paste, and formed into an electrode.

[0156] [Example 2]

[0157] Except for the addition of 3.2 g of aluminum nitrate nonhydrate aqueous solution, alumina-coated silver powder (experimental powder) according to this embodiment was obtained using the same procedures as in Example 1. The amount of aluminum in the added aluminum nitrate nonhydrate aqueous solution was 210 ppm by mass relative to the amount of silver. At this time, the pH of the solution before reduction addition was 12.1, and the pH of the filtrate was 6.4.

[0158] The aluminum content of the silver powder coated with alumina was 210 ppm by mass, and the loss on heating (Ig-loss) was 0.92%. XPS analysis confirmed that the coated aluminum was an oxide. Figure 2 Furthermore, the alumina-coated silver powder obtained in Example 2 began to expand at around 220°C, reaching an expansion rate of 0.50% or more at 226°C, with a maximum expansion rate of 1.80%. Afterward, it returned to its original sample length at around 540°C and began to shrink, reaching a shrinkage rate of 5.00% or more at 759°C. Figure 3 ).in addition, Figure 5 The image shows a cross-sectional SEM image of the silver powder coated with alumina obtained in Example 2. The silver powder coated with alumina obtained in Example 2 is porous, with a porosity of 84% silver particles and a true density of 9.79 g / cm³.

[0159] Using the silver powder coated with alumina obtained in this embodiment, the line resistance was measured according to the above steps, and the result was 16.7 Ω. This value is lower than that of Comparative Example 1, indicating that even with an aluminum content of 210 ppm by mass, the line resistance is reduced when alumina is coated on the surface of the silver powder, made into a paste, and formed into an electrode.

[0160] [Example 3]

[0161] Except that the amount of aluminum nitrate nonahydrate aqueous solution added was 0.8 g and the amount of 20% sodium hydroxide aqueous solution added was 15.0 g, the alumina-coated silver powder (experimental powder) according to this embodiment was obtained by following the same steps as in Example 1. The amount of aluminum in the added aluminum nitrate nonahydrate aqueous solution was 50 ppm by mass relative to the amount of silver. At this time, the pH of the solution before the addition of the reducing agent was 12.5, and the pH of the filtrate was 6.8.

[0162] The aluminum content of the silver powder coated with alumina is 50 ppm by mass, and the loss on heating (Ig-loss) is 0.72%. XPS analysis confirmed that the coated aluminum is an oxide. Figure 2 Furthermore, the alumina-coated silver powder obtained in Example 3 reached an expansion temperature of 232°C exceeding 0.50%, with a maximum expansion rate of 0.55%. Afterward, the sample length recovered to its pre-measurement length around 277°C, and then began to shrink, reaching a shrinkage temperature of 483°C exceeding 5.00%. Figure 3 ).in addition, Figure 6 The image shows a cross-sectional SEM image of the silver powder coated with alumina obtained in Example 3. The silver powder coated with alumina obtained in Example 3 is porous, with a porosity of 87% silver particles and a true density of 9.48 g / cm³.

[0163] Using the silver powder coated with alumina obtained in this embodiment, the line resistance was measured according to the above steps, and the result was 16.7 Ω. This value is lower than that of Comparative Example 1, indicating that even with an aluminum content of 50 ppm by mass, the line resistance is reduced when alumina is coated on the surface of the silver powder, made into a paste, and formed into an electrode.

[0164] [Comparative Example 2]

[0165] Except that the amount of aluminum nitrate nonhydrate aqueous solution added was 7.3 g, the alumina-coated silver powder (test powder) according to this comparative example was obtained by following the same procedures as in Example 1. The amount of aluminum in the added aluminum nitrate nonhydrate aqueous solution was 480 ppm by mass relative to the amount of silver. At this time, the pH of the solution before the addition of the reducing agent was 12.1, and the pH of the filtrate was 6.4.

[0166] The aluminum content of the alumina-coated silver powder obtained in this comparative example was 480 ppm by mass, and the loss on heating (Ig-loss) was 0.93%. The XPS spectrum of aluminum for this alumina-coated silver powder is shown in [image / image / description]. Figure 2In this comparative example, aluminum oxide was also coated on the surface of the silver powder. Furthermore, the aluminum oxide-coated silver powder obtained in Comparative Example 2 began to expand around 220°C, reaching an expansion rate of 0.50% or more at 225°C, with a maximum expansion rate of 1.70%. Afterward, it returned to its original sample length around 600°C and began to shrink, but the maximum shrinkage rate of the silver powder obtained in Comparative Example 2 was 0.43%, and no shrinkage exceeding 5.00% was observed. Figure 3 Additionally, in Figure 7 The image shows a cross-sectional SEM image of the silver powder coated with alumina obtained in Comparative Example 2. The silver powder coated with alumina obtained in Comparative Example 2 is porous, with a porosity of 91% silver particles and a true density of 9.81 g / cm³.

[0167] Using the silver powder coated with alumina obtained in this comparative example, the line resistance was measured according to the above steps, and the result was 19.2 Ω, which is an increase compared to the resistance of Comparative Example 1. Therefore, it can be seen that when alumina is coated on the surface of silver powder, if the aluminum content, expressed as a mass ratio relative to the mass of the silver powder coated with alumina, is 400 ppm or more, the line resistance will increase when it is made into a paste and formed into an electrode.

[0168] [Comparative Example 3]

[0169] While stirring 3231.0g of a silver nitrate aqueous solution containing 50.8g of silver at 174rpm, 103.2g of a 28% by mass ammonia solution (manufactured by Junko Chemical Co., Ltd.) was added. After 30 seconds of ammonia addition, 0.5g of a 5% by mass sodium carbonate aqueous solution was added, followed by another 30 seconds, and then 0.5g of a 5% by mass PEI600 aqueous solution was added. The temperature was adjusted to 33.5°C. At this point, the pH of the solution was 12.1. Next, 3 minutes after the addition of ammonia, 302.8g of a 1.86% by mass hydrazine hydrate aqueous solution (manufactured by Mitsubishi Gas Chemical Co., Ltd.) was added as a reducing agent. 10 seconds after the addition of the reducing agent, 1.7g of a 5.0% by mass aluminum nitrate nonhydrate aqueous solution was added. 20 seconds after the addition of the reducing agent, 110g of a 67.5% by mass nitric acid aqueous solution was added. The aluminum content in the added aluminum nitrate nonhydrate aqueous solution was 120 ppm by mass relative to the silver content.

[0170] Furthermore, after adding the reducing agent, 5.1 g of an aqueous emulsion containing 1.55% stearic acid was added to the slurry containing precipitated silver powder after 30 seconds. Stirring was then stopped, the slurry containing silver particles was filtered, washed with water until the conductivity became below 0.5 mS / m, and then vacuum dried at 73°C for 10 hours to obtain silver powder. At this point, the pH of the filtrate was 8.8. The reaction was performed using a 5L beaker with baffles and a three-stage turbine blade. 50 g of the silver powder obtained in the above steps was fed into a sample mill (manufactured by Kyoritsu Rikko Co., Ltd., SK-M10), and pulverized twice for 30 seconds at a dial value of 100 to obtain alumina-coated silver powder (experimental powder) according to Comparative Example 3.

[0171] The aluminum content of the alumina-coated silver powder obtained in this comparative example was 115 ppm by mass, and the loss on heating (Ig-loss) was 0.20%. The XPS spectrum of aluminum for this alumina-coated silver powder is shown in [reference needed]. Figure 2 As can be seen, in this comparative example, alumina also coats the surface of the silver powder. Furthermore, the alumina-coated silver powder obtained in Comparative Example 3 did not exhibit expansion of more than 0.50%, and began to shrink from around 440°C, reaching a shrinkage of more than 5.00% at 553°C. Additionally, Figure 8 A cross-sectional SEM image of the silver powder coated with alumina obtained in Comparative Example 3 is shown. In the silver powder coated with alumina obtained in Comparative Example 3, the proportion of porous silver particles was 0%, there were no pores, and the true density was 10.42 g / cm³.

[0172] Using the alumina-coated silver powder obtained in this comparative example, the line resistance was measured according to the above steps, and the result was 17.1 Ω, which is an increase compared to the line resistance of Comparative Example 1. Therefore, it can be seen that when the surface of the silver powder is coated with alumina, and there are no pores in the alumina-coated silver powder, the line resistance increases when it is made into a paste and formed into an electrode.

[0173]

Claims

1. Alumina-coated silver powder, which is alumina-coated silver powder containing silver particles that coat the surface of alumina, wherein, The aluminum content relative to the mass of the silver-coated alumina powder is 10 ppm by mass or more but less than 400 ppm by mass; the cumulative 50% particle size D of the silver-coated alumina powder, measured using a laser diffraction scattering particle size distribution measuring device, is... 50 Its micrometer size ranges from 0.2 μm to 5.0 μm; its true density is below 10.00 g / cm³.

2. The alumina-coated silver powder according to claim 1, wherein, The amount of aluminum relative to the mass of the silver powder coated with alumina is between 10 ppm and 100 ppm.

3. The alumina-coated silver powder according to claim 1, wherein, When observing the cross-section of the particles, more than 70% of all silver particles have pores inside.

4. The alumina-coated silver powder according to claim 1, wherein, The D 50 With BET particle size D BET The ratio (D) 50 / D BET The value ranges from 1.00 to 2.

50.

5. The alumina-coated silver powder according to claim 1, wherein, The cumulative 90% particle size D of the volume standard, measured by a laser diffraction scattering particle size distribution measuring device. 90 Cumulative 10% particle size D based on volume 10 The difference (D) 90 -D 10 ) and the D 50 The ratio of (D) 90 -D 10 ) / D 50 The range is from 0.50 to below 2.

00.

6. The silver powder coated with alumina according to claim 1 has a heat loss (Ig-loss) of more than 0.30% to less than 3.00%.

7. The alumina-coated silver powder according to claim 1, wherein, The thermomechanical analysis (TMA) curves obtained by compressing the silver powder coated alumina at 2.5 kg / mm² and applying a load of 5 mN / mm² to the sample, and heating at a rate of 10℃ / min from room temperature to 900℃, show that the temperature at which the expansion rate reaches 0.50% based on room temperature is above 150℃ and below 350℃.

8. The alumina-coated silver powder according to claim 1, wherein, The thermomechanical analysis (TMA) curves obtained by compressing the silver powder coated alumina at 2.5 kg / mm² and applying a load of 5 mN / mm² to the sample, and heating at a rate of 10℃ / min from room temperature to 900℃, show that the temperature at which the shrinkage rate reaches 5.00% based on room temperature is above 450℃ and below 850℃.

9. The alumina-coated silver powder according to claim 1, used as a sintering paste for forming solar cell electrodes.

10. A sintering paste for forming solar cell electrodes, comprising the alumina-coated silver powder of claim 1, an organic binder, and an organic solvent.

11. A method for manufacturing silver powder coated with alumina, comprising the following steps: A process for preparing an aqueous solution of a silver-ammonia complex containing aluminum and having a pH of 10.0 or higher, comprising a "complex formation step" and a "pH adjustment step," wherein the complex formation step is a step in which a silver-ammonia complex is formed by adding ammonia to an aqueous solution containing silver ions to obtain an aqueous solution of the silver-ammonia complex; the pH adjustment step is a step in which a pH adjusting agent is added to the aqueous solution of the silver-ammonia complex to adjust the pH of the aqueous solution to 10.0 or higher; and the method used is selected from one or more of the following methods to prepare the aqueous solution of the silver-ammonia complex: using the aqueous solution containing an aluminum compound in the complex formation step, adding an aluminum compound to the aqueous solution in the period before the pH adjustment step is completed, and adding an aluminum compound to the aqueous solution after the pH adjustment step is completed; and The process involves adding a reducing agent to the aqueous solution of the silver-ammonia complex containing aluminum and having a pH of 10.0 or higher to reduce silver ions and precipitate silver particles, while simultaneously adjusting the pH of the aqueous solution to a level between 4.0 and 9.0 to precipitate aluminum oxide. in, The amount of aluminum supplied to the liquid via the aluminum compound is controlled such that the amount of aluminum relative to silver in the aqueous solution at the initial point of addition of the reducing agent is 10 ppm by mass or more but less than 400 ppm by mass. The reducing agent is an organic compound containing one or more of the following groups: COOH, CHO, and OH.

12. The method for manufacturing alumina-coated silver powder according to claim 11, wherein, The reducing agent is formaldehyde.

13. The method for manufacturing alumina-coated silver powder according to claim 11, further comprising the step of adding a surface treatment agent to a slurry containing silver powder precipitated by adding the reducing agent.

14. The method for manufacturing alumina-coated silver powder according to claim 13, wherein, The amount of the surface treatment agent added is more than 0.05% by mass and less than 0.50% by mass relative to the amount of silver in the silver-ammonia complex aqueous solution.

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