Aluminum oxide-coated silver powder, method for manufacturing the same, and conductive paste

Abstract

This invention provides a silver powder and a method for producing the same, which reduce line resistance when the obtained silver powder is paste-formed and used to create electrodes, by modifying the surface of the silver powder. [Solution] When precipitating metallic silver particles from an aqueous solution containing silver ions, an aluminum compound is present in the aqueous solution, and a reducing agent that has the effect of lowering the pH of the aqueous solution when added, or a reducing agent whose oxidation products have the effect of lowering the pH of the aqueous solution, is used. This makes it possible to obtain aluminum oxide-coated silver powder that results in low electrode resistance when the precipitated silver powder is made into a paste and used to form electrodes.

Description

Technical Field

[0001] The present invention relates to aluminum oxide-coated silver powder suitable for use in conductive pastes for forming electrical conduction paths in elements such as electrodes and circuits of various electronic components, particularly solar cells, a method for producing the same, and a conductive paste.

Background Art

[0002] Conventionally, resin-type and fired silver pastes have been widely used for forming electrodes and circuits of electronic components. One of the typical applications is solar cells. For example, in the case of an amorphous silicon solar cell, the above-mentioned fired conductive paste is used for forming current collecting electrodes such as finger electrodes or bus bar electrodes. In recent years, conductive pastes using silver powder have been required to have a higher density of conductor patterns due to the miniaturization of electronic components and to be fine-lined with thinner wiring widths. For example, in the case of solar cells, it is necessary to increase the light receiving area in order to increase the power generation amount of the solar cell, and there has been a strong demand for fine-lining of the wiring. In order to fine-line the wiring, it is necessary to reduce the line resistance of the conductive film formed using the conductive paste. Therefore, conventionally, attempts have been made to modify the surface of silver powder to improve the electrical properties of the finally obtained conductive film.

[0003] For example, Patent Document 1 discloses surface-modified silver powder for a fired paste for forming a circuit in which at least one oxide or composite oxide containing a metal element belonging to Groups 2 to 14 of the periodic table such as aluminum oxide or silicon oxide is fixed to the surface of metal silver particles. The surface-modified silver powder disclosed in Patent Document 1 is mainly used for forming an LTCC (low-temperature co-fired ceramic) circuit board, and fixing aluminum oxide or the like to the surface of metal silver particles aims to suppress thermal shrinkage when the conductive paste containing the silver powder is sintered.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Japanese Patent Publication No. 2001-240901 [Overview of the project] [Problems that the invention aims to solve]

[0005] The silver powder disclosed in Patent Document 1 has its surface modified by fixing an oxide to its surface. However, the metal particles with aluminum oxide attached in Example 1 disclosed in Patent Document 1 were attached by a dry fixing method with 5 parts by mass of aluminum oxide added to 100 parts by mass of silver powder. While this suppresses thermal shrinkage when firing the conductive paste, the high volume resistivity of aluminum oxide results in a problem in that it does not effectively reduce the line resistance of the conductive film obtained by firing the conductive paste. If the line resistance of the conductive film cannot be reduced, fine wiring lines cannot be achieved.

[0006] The technical problem to be solved in this invention is to provide a silver powder and a method for producing the same, which reduces line resistance when the obtained silver powder is made into a paste and used to form electrodes with a narrow line width, by modifying the surface of the silver powder. Furthermore, a technical problem to be solved in this invention is to provide a conductive paste that can reduce the line resistance of a conductive film. [Means for solving the problem]

[0007] As a result of diligent research to achieve the above-mentioned objectives, the inventors discovered that by depositing a small amount of aluminum oxide on the surface of silver powder, it is possible to reduce the line resistance of the final electrode film with a narrow line width, and thus completed the present invention described below.

[0008] In other words, in order to achieve the above-mentioned objectives, the present invention provides: (1) Aluminum-coated silver powder composed of silver particles coated with aluminum oxide on the surface, wherein the amount of aluminum is 10 ppm by mass or more and less than 400 ppm by mass based on the mass of the aluminum-coated silver powder, and the volume-based cumulative 50% diameter D 50 measured by a laser diffraction scattering particle size distribution measuring device is 0.2 μm or more and 5.0 μm or less, and the true density is 10.00 g / cm 3 or less, and aluminum-coated silver powder is provided. (2) The aluminum-coated silver powder according to the above item (1) preferably has an aluminum amount of 10 ppm by mass or more and 100 ppm by mass or less based on the mass of the aluminum-coated silver powder. (3) When observing the cross-section of the particles, the aluminum-coated silver powder according to the above items (1) to (2) preferably has silver particles having voids inside the particles accounting for 70% or more of the total number of silver particles. (4) The aluminum-coated silver powder according to the above items (1) to (3) has a ratio (D 50 / D BET ) of 1.00 or more and 2.50 or less between the BET diameter D 50 and the BET diameter D BET is preferably. (5) The aluminum-coated silver powder according to the above items (1) to (4) has a difference (D 90 -D 10 ) between the volume-based cumulative 90% diameter D 90 and the volume-based cumulative 10% diameter D 10 , and a ratio ((D 50 -D 90 ) / D 10 ) of 0.50 or more and 2.00 or less between the difference (D 50 ) and the D 2 is preferably. [[ID=(6) The aluminum-coated silver powder according to the above items (1) to (5) preferably has a loss on ignition (Ig-loss) of 0.30% or more and 3.00% or less. (7) The aluminum-coated silver powder according to the above items (1) to (6) is applied to a sample obtained by compression molding the above aluminum-coated silver powder at 2.5 kg / mm 2 at 5 mN / mm 2In the TMA curve obtained by performing thermomechanical analysis (TMA) while applying a load and heating from room temperature to 900°C at a heating rate of 10°C / min, it is preferable that the temperature at which the expansion coefficient relative to room temperature reaches 0.50% is between 150°C and 350°C. (8) The aluminum oxide-coated silver powder described in items (1) to (7) above preferably has a temperature of 450°C or higher and 850°C or lower at which the shrinkage rate relative to room temperature reaches 5.00% in the TMA curve described above. (9) The aluminum oxide-coated silver powder described in items (1) to (8) above may also be for use as a baked paste for forming electrodes on a solar cell substrate.

[0009] Furthermore, the following inventions are provided. (10) A baked paste for forming electrodes of solar cells, comprising aluminum oxide-coated silver powder as described in any of items (1) to (9) above, an organic binder, and an organic solvent.

[0010] As a method for producing the above aluminum oxide-coated silver powder, (11) A process for producing a silver-ammine complex aqueous solution containing aluminum and having a pH of 10.0 or higher, comprising a "complex formation step" in which a silver-ammine complex is formed by adding ammonia to a silver ion-containing aqueous solution to obtain a silver-ammine complex aqueous solution, and a "pH adjustment step" in which a pH adjusting agent is added to the silver-ammine complex aqueous solution to adjust the pH of the aqueous solution to 10.0 or higher, wherein one or more methods selected from a method using the silver ion-containing aqueous solution containing an aluminum compound in the complex formation step, a method adding the aluminum compound to the aqueous solution during the period until the completion of the pH adjustment step, and a method adding the aluminum compound to the aqueous solution after the pH adjustment step has been completed, The process involves adding a reducing agent to the aforementioned aluminum-containing silver-ammine complex aqueous solution with a pH of 10.0 or higher to reduce silver ions and precipitate silver particles, while simultaneously lowering the pH of the aqueous solution to 4.0 or higher and 9.0 or lower, thereby precipitating aluminum oxide. A method for producing aluminum oxide-coated silver powder, The amount of aluminum supplied to the liquid by the above-mentioned aluminum compound is such that the amount of aluminum relative to the amount of silver in the aqueous solution at the start of the addition of the reducing agent is 10 ppm by mass or more and less than 400 ppm by mass. A method for producing aluminum oxide-coated silver powder is provided, wherein the reducing agent is an organic compound containing one or more of the COOH group, CHO group, and OH group in its molecule. (12) In the method for producing aluminum oxide-coated silver powder described in item (11) above, the reducing agent is preferably formaldehyde. (13) The manufacturing method described in items (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. (14) In the manufacturing method described in item (13), the amount of the surface treatment agent added is preferably 0.05% by mass or more and 0.50% by mass or less relative to the amount of silver in the silver-ammine complex aqueous solution. [Effects of the Invention]

[0011] By using the aluminum oxide-coated silver powder of the present invention, a conductive film with low line resistance can be obtained even when the silver powder is pasteurized to form fine wire electrodes with a design line width of, for example, 20 μm or less. [Brief explanation of the drawing]

[0012] [Figure 1] This is a cross-sectional SEM image of the silver powder obtained in Comparative Example 1 of the present invention. [Figure 2] These are the XPS spectra (Al2p) of the silver powder obtained in Examples 1 and 2 and Comparative Examples 2 and 3 of the present invention. [Figure 3] These are the TMA curves of the silver powder obtained in the examples and comparative examples of the present invention. [Figure 4] This is a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 1 of the present invention. [Figure 5] This is a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 2 of the present invention. [Figure 6]This is a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 3 of the present invention. [Figure 7] This is a cross-sectional SEM image of the silver powder obtained in Comparative Example 2 of the present invention. [Figure 8] This is a cross-sectional SEM image of the silver powder obtained in Comparative Example 3 of the present invention. [Modes for carrying out the invention]

[0013] [Silver powder coated with aluminum oxide] The aluminum oxide-coated silver powder of the present invention is composed of silver particles with a small amount of aluminum oxide coated on their surface, and has a true density of 10.00 g / cm³. 3 The silver powder is as follows. The silver particles include those that have voids inside that are closed off from the outside. By coating the surface of the silver powder containing silver particles with internal voids with aluminum oxide, it becomes possible to reduce line resistance even when the aluminum oxide-coated silver powder is made into a paste and a fine wire electrode with a design line width of 20 μm or less is formed.

[0014] In this invention, silver particles coated with aluminum oxide refer to silver particles on which aluminum oxide is coated on part or all of the surface. In this invention, the distribution of aluminum coated on the surface of silver particles can be confirmed using Auger electron spectroscopy (AES), and whether or not the coated aluminum is an oxide can be confirmed by measuring the chemical bonding state of aluminum using X-ray photoelectron spectroscopy (XPS). It is acceptable for some of the particles in the aluminum oxide-coated silver powder to be silver particles on which aluminum oxide is not coated on the surface, as long as the effects of this invention are achieved. For example, the ratio of silver particles on which aluminum oxide is coated on part or all of the surface to the total particles in the aluminum oxide-coated silver powder should be 80% or more, preferably 90% or more, and the upper limit should be 100% or more. If the ratio of silver particles on which aluminum oxide is coated on part or all of the surface is within the above range, the effect of suppressing the uneven distribution of aluminum in the conductive paste is excellent.

[0015] In this invention, "adhesion" refers to a state in which aluminum oxide is adhered to silver particles so as to cover part or all of their surface. The aluminum oxide-adhered silver powder obtained in this embodiment is produced by reducing and precipitating silver particles in a solution, causing aluminum compounds in the solution to precipitate as aluminum oxide and adhere to the surface of the silver particles, as will be described later. Therefore, aluminum oxide can be adhered to the surface of all the reduced-precipitated silver particles without segregation, and the strength of the adhesion of aluminum oxide to the silver particles can be set so that, for example, the aluminum oxide does not detach from the surface of the silver particles even when the silver powder of this embodiment is crushed. As a result, even when the aluminum oxide-adhered silver powder is mixed into a conductive paste, the adhered material can be kept uniform together with the silver particles. In other words, the segregation of silver particles and aluminum oxide can be suppressed even in a conductive paste.

[0016] As described above, aluminum oxide is a type of insulator, and it is expected that the line resistance of the electrode film obtained by firing a paste using silver powder coated with aluminum oxide will increase. However, the inventors have discovered that the line resistance of the final electrode film can be reduced by controlling the amount of aluminum oxide coated on the surface of the silver powder, and have completed the present invention. The mechanism by which the line resistance of the final electrode film is reduced by coating the surface of the silver powder with an appropriate amount of aluminum oxide will be described later.

[0017] In this specification, the "aluminum content" in silver powder refers to the amount of aluminum relative to the mass of aluminum oxide-coated silver powder, measured by inductively coupled plasma atomic emission spectroscopy (ICP-OES) after completely dissolving the aluminum oxide-coated silver powder with acid. In this invention, aluminum is thought to be abundant on the surface side of the silver particles, with some aluminum present in the silver particles and others coated on the surface as aluminum oxide. Therefore, the "aluminum content" in this specification refers to the sum of the amount of aluminum present in the silver particles and the amount of aluminum coated on the surface.

[0018] Since ICP-OES cannot perform state analysis, whether the aluminum present on the surface of the silver particles exists in the form of aluminum oxide, that is, whether the particles are "silver particles coated with aluminum oxide," is determined by state analysis of aluminum using X-ray photoelectron spectroscopy (XPS). In this invention, if the aluminum spectrum in the XPS spectrum obtained by measurement has a peak in the binding energy range of 74.3 ± 1 eV, it is determined that the aluminum is in an oxidized state and not in a metallic state.

[0019] In this invention, the aluminum present on the surface of the silver particles is preferably aluminum oxide, but it may also contain a small amount of peaks corresponding to elemental aluminum (metal). In the XPS measurement described later, the ratio of the height of the peak corresponding to elemental aluminum to the peak corresponding to aluminum oxide is preferably 10% or less.

[0020] The amount of aluminum is preferably 10 ppm by mass or more relative to the total mass of the aluminum oxide-coated silver powder. If the amount of aluminum is less than 10 ppm by mass, the effect of reducing the resistance of the electrode film of the present invention may be insufficient. Furthermore, since aluminum oxide is a type of insulator, an amount of aluminum of 400 ppm by mass or more is undesirable because it reduces the effect of lowering the line resistance. The amount of aluminum is more preferably 300 ppm by mass or less, even more preferably 250 ppm by mass or less, and even more preferably 100 ppm by mass or less.

[0021] The aluminum oxide-coated silver powder of the present invention has a true density of 10.00 g / cm³. 3 The following is the value: 9.90 g / cm³ 3 Preferably, it is 9.85 g / cm³. 3 The following is more preferable. In this invention, the true density value in the silver powder refers to the density of the silver powder taking into account the voids present inside the silver particles (i.e., the "closed spaces inside the silver particles" mentioned above). The true density of the silver powder is 10.00 g / cm³.3 The following conditions are preferable because, when observing the cross-section of the silver powder, sufficient silver particles with internal voids can be observed, and the sintering of the silver particles proceeds sufficiently when the conductive paste is fired. On the other hand, the true density is, for example, 9.00 g / cm³. 3 The above is 9.40 g / cm³. 3 The above, or 9.70 g / cm³ 3 That's fine too.

[0022] In the aluminum oxide-coated silver powder of the present invention, when the cross-section of the silver particles constituting the silver powder is observed with a scanning electron microscope (SEM), it can be confirmed that there are fine voids inside the silver particles that are closed from the outside. In the aluminum oxide-coated silver powder, the proportion of silver particles having one or more voids inside the particle is preferably 70% or more of the total silver particles, more preferably 75% or more, even more preferably 80% or more, and most preferably 85% or more. This value of 70% or more is preferable because, after the sintering of the particle surface begins, the reaction of gas components in the voids inside the particles facilitates the sintering of the entire silver particle, and a dense conductive film is formed. The upper limit of the proportion is 100%.

[0023] In this specification, voids are defined as particles with a diameter of 15 nm or more, as observed using cross-sectional SEM images of silver particles taken at 10,000 to 40,000 times magnification. In this invention, "void diameter (i.e., diameter of the closed space inside the void-containing silver particle)" means the diameter of the outer circle when drawing the smallest circle that contains all the voids in a scanning electron microscope image. The proportion of silver particles with voids inside in the silver powder can be confirmed by observing cross-sectional SEM images of silver particles (containing 10 or more particles) taken at 10,000 times magnification in 5 or more fields of view, and calculating the ratio of the number of particles with one or more voids inside to the number of silver particles observed.

[0024] The aluminum 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. 50It is preferable that the particle size is 0.2 μm or larger, more preferably 1.0 μm or larger, and even more preferably 1.5 μm or larger. Furthermore, it is preferable that the particle size is 5.0 μm or smaller, more preferably 3.0 μm or smaller, and even more preferably 2.5 μm or smaller. D 50 If the thickness is less than 0.2 μm, the viscosity of the conductive paste becomes too high, which is undesirable because it may cause disconnections when using the conductive paste to draw wiring or other designs. Also, D 50 If the thickness exceeds 5.0 μm, it becomes difficult to draw fine wiring when using conductive paste, which is undesirable.

[0025] The aluminum oxide-coated silver powder of the present invention has a BET specific surface area (SSA) of 0.10 m². 2 It is preferable that it be 0.20m or more. 2 It is more preferable that it be 0.30m or more. 2 It is even more preferable that it be 1.20m or more. 2 It is preferable that it be less than or equal to 1.00 m 2 It is more preferable that it be less than or equal to / g, and 0.55m 2 It is even more preferable that it be less than or equal to / g, and 0.50m 2 It is even more preferable that the value be less than or equal to / g. If the BET specific surface area value of the aluminum oxide-coated silver powder is within the above range, it is preferable because it makes it easier to draw fine wiring when using conductive paste to draw wiring and the like.

[0026] The aluminum oxide-coated silver powder of the present invention is D 50 And the BET diameter (D) calculated from the BET specific surface area (SSA) mentioned above. BET ) ratio (D 50 / D BET ) is preferably 1.00 or more and 2.50 or less, more preferably 2.00 or less, and even more preferably 1.50 or less. BET This is the particle size (particle diameter) assuming the measured particle is a perfect sphere, and is calculated by the following equation (1). The method for measuring true density will be described later. This ratio (D 50 / DBET The coefficient of silver particles is generally considered to be 1.00 or higher when there is no aggregation of silver particles, and setting it to 2.50 or lower makes it easier to draw fine wiring when using conductive paste to draw wiring, which is preferable. D BET (μm)=6 / (BET specific surface area x true density) …(1) Here, the "BET specific surface area" on the right-hand side of equation (1) has the unit m. 2 The BET specific surface area value, expressed in g / g, is substituted into the "True density" field, with the unit g / cm² being used. 3 The value of the true density represented by is substituted.

[0027] The aluminum oxide-coated silver powder of the present invention has a maximum volume-based diameter D measured by a laser diffraction scattering particle size distribution analyzer. max It is preferable that the particle size is 15.0 μm or less, more preferably 12.0 μm or less, and even more preferably 10.0 μm or less. max If the thickness is 15.0 μm or less, it is possible to draw fine wiring using conductive paste.

[0028] The cumulative 10% diameter D of the aluminum oxide-coated silver powder measured by the laser diffraction scattering particle size distribution analyzer of the present invention is measured by the present invention. 10 The diameter is preferably 0.1 μm or larger, more preferably 1.0 μm or larger. It is also preferably 3.0 μm or smaller, more preferably 2.0 μm or smaller, and even more preferably 1.5 μm or smaller. Cumulative 10% diameter D 10 If the value is within the above range, it is preferable because it prevents the viscosity of the conductive paste from becoming too high.

[0029] The cumulative 90% diameter D of the aluminum oxide-coated silver powder measured by the laser diffraction scattering particle size distribution analyzer of the present invention is determined by volume. 90The particle size is preferably 1.5 μm or larger, more preferably 2.0 μm or larger, and even more preferably 2.5 μm or larger. Furthermore, it is preferably 6.0 μm or smaller, more preferably 5.0 μm or smaller, and even more preferably 4.0 μm or smaller. Cumulative 90%D 90 If the value is within the above range, it is preferable because it makes it easier to draw fine wiring when using conductive paste to draw wiring and other patterns.

[0030] The aluminum oxide-coated silver powder of the present invention has a cumulative diameter of 90% D 90 and volume-based cumulative 10% diameter D 10 The difference (D 90 -D 10 ) and cumulative 50% diameter D 50 The ratio ((D 90 -D 10 ) / D 50 (D 90 -D 10 ) / D 50 This value is an indicator of particle size distribution, and the smaller this value, the sharper the particle size distribution of the aluminum oxide-coated silver powder, meaning that the particle size is uniform. In the present invention, if the value is within the above range, it is preferable because it makes it easier to draw fine wiring when using conductive paste to draw wiring.

[0031] The aluminum oxide-coated silver powder of the present invention preferably has an ignition loss (Ig-loss) value of 0.30% or more from the viewpoint of suppressing aggregation of the aluminum oxide-coated silver powder, more preferably 0.40% or more, and even more preferably 0.50% or more. Furthermore, it is preferable that the Ig-loss value be 3.00% or less from the viewpoint of suppressing the increase in the resistance value of the conductive film obtained using a conductive paste containing aluminum oxide-coated silver powder, more preferably 2.00% or less, and even more preferably 1.50% or less.

[0032] In the aluminum oxide-coated silver powder of the present invention, it is preferable that when a sample of the compacted powder obtained by compression molding the aluminum oxide-coated silver powder under the conditions described later is heated from room temperature (25°C ± 5°C) at a heating rate of 10°C / min, the temperature at which the expansion rate relative to room temperature reaches 0.50% (hereinafter sometimes referred to as the "0.50% expansion temperature") is between 150°C and 350°C. This expansion rate can be determined from the TMA curve obtained from room temperature to 900°C when the above sample is heated at a heating rate of 10°C / min using a thermomechanical analyzer. Note that this expansion behavior is more likely to occur when there are voids inside the silver powder particles, but even with a similar void ratio, the aluminum oxide-coated silver powder of the present invention tends to exhibit greater expansion behavior compared to silver powder that is not coated with aluminum oxide. If the expansion rate reaches 0.50% or more between 150°C and 350°C (i.e., the 0.50% expansion temperature is between 150°C and 350°C), the sinterability between silver particles increases due to the release of gas present in the voids, which is effective in reducing the resistance of the electrode film. Preferably, the expansion rate relative to room temperature is 0.50% or more in the temperature range of 200°C to 300°C of the TMA curve, and more preferably 1.00% or more. Furthermore, preferably, the expansion rate relative to room temperature is 5.00% or less in the temperature range of 200°C to 300°C, and more preferably 3.00% or less. If it is 5.00% or less, a dense conductive film is more easily obtained when a conductive film is formed, which is preferable from the viewpoint of reducing resistance.

[0033] The aluminum oxide-coated silver powder of the present invention preferably has a TMA curve obtained by thermomechanical analysis of a compacted powder sample obtained by compression molding the aluminum oxide-coated silver powder under the conditions described later, where the temperature at which the shrinkage rate relative to room temperature reaches 5.00% (hereinafter sometimes referred to as the "5.00% shrinkage temperature") is between 450°C and 850°C, and more preferably 800°C or lower. If the 5.00% shrinkage temperature is within the above range, organic components in the paste will be more easily released, and the strength of the electrode film after firing can be maintained. Preferably, the shrinkage rate relative to room temperature reaches 5.00% or higher, and more preferably 8.00% or higher, in the TMA curve between 450°C and 850°C. Furthermore, preferably, the shrinkage rate relative to room temperature reaches 20.00% or lower, and more preferably 15.00% or lower, in the TMA curve between 450°C and 850°C. A value of 20.00% or less is preferable because it provides the effect of reducing the resistance of the electrode film after firing.

[0034] The mechanism by which the line resistance of the final electrode film decreases when aluminum oxide is deposited on the surface of silver particles is currently unknown, but the inventors believe the following: They hypothesize that the presence of a small amount of aluminum oxide on the surface of the silver particles improves the sliding properties of the silver particles within the paste. This improves the discharge of the paste from the printing plate, thereby improving fine line printing performance, and ultimately leading to lower resistance in the final electrode film. From this perspective, aluminum oxide may only be present near the surface of the silver particles.

[0035] In addition to the mechanism described above, the following mechanism is also thought to contribute to the low resistance of the electrode film in the case of aluminum oxide-coated silver powder. When aluminum oxide-coated silver powder is heated, the gas present in the voids within the silver particles that make up the silver powder is released to the outside due to heating, causing thermal expansion and thus expanding the silver particles themselves. In this case, if the amount of coated aluminum oxide is small, it is thought that cracks may form in the aluminum oxide-coated layer, increasing the exposed area of ​​metallic silver of the underlying silver particles, and resulting in metal / metal bonding between the aluminum oxide-coated silver powders. As a result, the sinterability of the aluminum oxide-coated silver is improved, and as a result, the final electrode film has low resistance.

[0036] [Manufacturing method] The present invention provides a method for producing aluminum oxide-coated silver powder, which utilizes a wet process that offers advantages in terms of manufacturing cost and mass productivity.

[0037] [Starting materials] In the method for producing aluminum oxide-coated silver powder of the present invention, an aqueous solution containing monovalent silver ions (silver ion-containing aqueous solution) is used as a starting material. As a source of silver ions, known inorganic silver salts used industrially, such as silver(I) nitrate, silver(I) sulfate, silver(I) carbonate, silver(I) chloride, and silver(I) oxide, can be used. Alternatively, an aqueous solution containing monovalent silver ions and aluminum ions may be obtained by adding an aluminum compound to the above aqueous solution. The timing of adding the aluminum compound may be before the addition of ammonia, after the addition of ammonia, or after the addition of the pH adjuster; there are no particular restrictions as long as it is before the addition of the reducing agent.

[0038] Although not specifically defined in this invention, the silver ion concentration in the starting aqueous solution is preferably 1.0% by mass or more and 2.0% by mass or less at the solution preparation stage. 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 small, which may be disadvantageous in terms of productivity. If the silver ion concentration exceeds 2.0% by mass, the viscosity of the reaction solution after silver particle deposition will increase, and depending on the manufacturing equipment, it may be difficult to uniformly stir the reaction solution.

[0039] [Complex formation process] In the present invention's method for producing aluminum oxide-coated silver powder, silver ions are complexed with ammonium ions to form a silver-ammine complex, which is then converted into an aqueous solution of the silver-ammine complex. As a source of ammonium ions, ammonia water, ammonium chloride, ammonium carbonate, or other ammonium salts can be used. When ammonium ions are used as a complexing agent, a silver-ammine complex is formed in the aqueous solution. In this case, since the coordination number of the ammine complex is 2, at least 2 moles of ammonium ions are added per mole of silver ions. The reaction temperature for forming the silver-ammine complex is preferably between 10°C and 40°C. The reaction temperature for forming the silver-ammine complex is preferably within the above range, considering that an exothermic reaction occurs during the formation of the silver-ammine complex, in order to set the temperature of the reduction deposition step described later to a desired temperature. If the reaction temperature is too low or too high, temperature adjustment will take time, leading to increased energy costs.

[0040] [pH adjustment process] Subsequently, a pH adjusting agent is added to the silver-ammine complex aqueous solution obtained in the above step to adjust the pH of the aqueous solution to 10.0 or higher. The pH is raised to 10.0 or higher in order to sufficiently increase the reducing power of the reducing agent. As the pH adjusting agent, alkali metals such as sodium hydroxide and calcium hydroxide, or alkali earth metal hydroxides and carbonates can be used. In the method for producing aluminum oxide-coated silver powder of the present invention, there is no particular upper limit to the pH of the aqueous solution, but it is preferable to keep the pH at 13.0 or lower to avoid excessive use of the pH adjusting agent. The temperature of the pH adjustment step is preferably between 10°C and 40°C. If the reaction temperature is too low or too high, it will take time to adjust the temperature, which will increase energy costs.

[0041] [Aluminum compounds] In the present invention, the timing of adding the aluminum compound may be before the addition of ammonia, after the addition of ammonia, or after the addition of the pH adjuster, as described above. In other words, the aluminum compound may be added before the addition of the reducing agent in the silver particle precipitation step described later. The present invention includes a step of adding the aluminum compound to the silver ion-containing aqueous solution, the silver-ammine complex aqueous solution, or the aqueous solution to which the pH adjuster has been added.

[0042] As for the aluminum compound, any compound that can be added to the aqueous solution in an ionic state depending on the pH range of the aqueous solution at the timing of addition is acceptable. For example, inorganic aluminum salts such as aluminum nitrate, aluminum acetate, and aluminum oxalate can be used. It is also preferable to match the type of acid or base used. For example, it is preferable to use aluminum nitrate in an aqueous silver nitrate solution. Note that since aluminum oxides are amphoteric oxides, aluminum ions (Al) are present in the low pH range. 3+ ), in the high pH range, aluminate ions (AlO2 - It dissolves in the state shown above, and at a pH near neutral, it enters the solid phase stable region of aluminum oxide (Al2O3). Here, aluminum oxide is a concept that includes aluminum hydroxide (Al(OH)3 or Al2O3·3H2O).

[0043] The amount of aluminum supplied into the solution by the aluminum compound (or the total amount of aluminum supplied if supplied in multiple stages) is preferably such that, at the start of the addition of the reducing agent described later, the amount of aluminum relative to the amount of silver ions in the aqueous solution is 10 ppm by mass or more and less than 400 ppm by mass. Setting the amount of aluminum relative to silver to 10 ppm by mass or more makes it easier to obtain the effect of lowering the resistance of the electrode film. Furthermore, since aluminum oxide is a type of insulator, setting the amount of aluminum relative to silver to less than 400 ppm by mass is effective in suppressing the effect of lowering the line resistance. It is more preferable to set the amount of aluminum relative to silver to 300 ppm by mass or less, even more preferable to 250 ppm by mass or less, and even more preferable to 100 ppm by mass or less.

[0044] Furthermore, when adding an aluminum compound as an aqueous solution, the concentration is preferably 0.5% by mass or more and 10.0% by mass or less as aluminum. If the aluminum concentration is less than 0.5% by mass, the amount of aqueous aluminum compound solution added to obtain the desired aluminum oxide-coated silver powder increases, which in turn increases the volume of the reaction solution and the amount of reagents used, making it uneconomical. If the aluminum concentration exceeds 10.0% by mass, the amount of aqueous aluminum compound solution added to obtain the desired aluminum oxide-coated silver powder becomes very small, and depending on the operating conditions, the error in the amount added may become large.

[0045] [Silver particle deposition process] In the present invention's method for producing aluminum oxide-coated silver powder, a reducing agent is added to the aluminum-containing silver-ammine complex aqueous solution obtained in the above step, which has a pH of 10.0 or higher, to reduce the silver-ammine complex and precipitate silver particles in the aqueous solution. At this time, the reducing agent used is one that has the effect of lowering the pH of the aqueous solution when added. The reason for using a reducing agent that has the effect of lowering the pH of the aqueous solution is that by adding the reducing agent, the pH of the aqueous solution is lowered to the solid-phase stable range of the aluminum oxide, and aluminum oxide is deposited 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 lower the pH of the aqueous solution by adding a pH adjusting agent after the silver particles have precipitated and deposit aluminum oxide on the surface of the silver particles, but this method is undesirable because it increases the number of steps. Furthermore, it is undesirable because it becomes difficult to uniformly deposit aluminum oxide on the particle surface. Furthermore, it is preferable that the reaction temperature when precipitating silver particles using the aforementioned reducing agent be between 10°C and 40°C. If the reaction temperature is too low or too high, it will take time to adjust the temperature, which can lead to increased energy costs.

[0046] [Reducing agent] The greatest technical feature of the present invention's method for producing aluminum oxide-coated silver powder is the use of a reducing agent that, upon addition, lowers the pH of an aqueous solution. The mechanism by which the pH of an aqueous solution decreases upon addition of a reducing agent includes cases where the reducing agent itself has a pH-lowering effect, and cases where the oxidation products of the reducing agent have a pH-lowering effect.

[0047] As reducing agents having such effects, organic compounds containing one or more of the COOH group, CHO group, and OH group in their molecule are preferred. Specifically, examples include aldehydes such as ascorbic acid, tartaric acid, formic acid, and formaldehyde. In the case of an organic compound reducing agent having a COOH group in its molecule, the pH of the aqueous solution decreases upon addition of the reducing agent. In the case of a reducing agent where the CHO group has reducing properties, the CHO group reduces silver ions, and the CHO group is oxidized to a COOH group, thus decreasing the pH of the aqueous solution. In the case of a reducing agent where the OH group has reducing properties, the OH group is oxidized to a COOH group via the CHO group, thus decreasing the pH of the aqueous solution. Furthermore, it is presumed that the presence of voids within the silver particles constituting the aluminum oxide-coated silver powder of the present invention is due to the gas generated by the decomposition of the aforementioned organic compound being trapped inside the silver particles.

[0048] In the method for producing aluminum oxide-coated silver powder according to the present invention, the pH of the aqueous solution is adjusted to 4.0 or higher and 9.0 or lower by adding the reducing agent. By setting the pH within the above range, aluminum oxide can be coated onto silver particles from the mixed aqueous solution.

[0049] The pH value that decreases by adding the aforementioned reducing agent can be the pH value after the precipitation of silver powder is complete, in which case the pH can be the pH measurement of the filtrate after the silver powder has been filtered and recovered. This pH value can be controlled by the pH value in the pH adjustment step described above, the amount of pH adjusting agent added, the equivalent amount of ammonia in the complex formation step, or the amount of reducing agent. It is more preferable that the above pH be 6.0 or higher. If the above pH falls below 4.0, aluminum will be Al 3+ This is undesirable because it may cause the aluminum oxide on the surface to redissolve as ions. Furthermore, if the pH is greater than 9.0, it is undesirable because almost no aluminum will be detected when measuring the aluminum coated with the resulting silver powder. Furthermore, when the reducing agent in Example 1, described later, was changed from formalin to hydrazine, the pH of the filtrate changed from 10.0 to 11.0, and almost no aluminum was detected in the amount of aluminum deposited on the resulting silver powder, as described later.

[0050] From this, we believe that when the pH of the mixed aqueous solution decreases from above 10.0 to approach 9.0, aluminum oxide begins to precipitate on the surface of the silver particles. Then, as the particles grow until the final pH is reached, a layer containing aluminum (e.g., aluminum oxide) is formed near the surface of the silver particles, and the aluminum oxide adheres to the surface of the silver particles.

[0051] In this manufacturing method, aluminum ions are pre-included in the mixed solution, and aluminum oxide is deposited on the surface of the silver particles by pH transition during reduction deposition. Compared to Patent Document 1, this method suppresses the formation of isolated aluminum oxide particles and aluminum particles from the silver particles, and allows for uniform deposition of aluminum oxide fixed to the surface of the silver particles. Therefore, the amount of aluminum compound to be pre-included in the mixed solution can be reduced, and silver powder with a low amount of aluminum in the silver powder, as measured in the later-described measurement of the amount of deposited aluminum, can be obtained. In other words, in the present invention, it is preferable that the aluminum oxide near the surface of the silver particles is within the aforementioned concentration range and uniformly attached to the entire silver particle.

[0052] When aldehydes are used as reducing agents, silver particles precipitate to some extent, and after the concentration of carboxylic acid, which is an oxidation product, increases, the pH of the aqueous solution reaches the aforementioned pH range. This is preferable from the viewpoint of making it easier for aluminum oxide to adhere to the surface of the precipitated silver particles. As for the aldehyde, formaldehyde is more preferable from the viewpoint of ease of availability and reducing power. The amount of reducing agent added is preferably 1 equivalent or more relative to silver in order to increase the yield of silver, and may also be 2 equivalents or more relative to silver, for example, 10 equivalents or more and 20 equivalents or less.

[0053] [Surface treatment agent addition process] In the method for producing aluminum oxide-coated silver powder of the present invention, a surface treatment agent may be added to the solution containing the aluminum oxide-coated silver particles obtained in the silver particle precipitation step. This yields aluminum oxide-coated silver particles coated with the surface treatment agent. Examples of surface treatment agents to be added to the solution containing the aluminum oxide-coated silver particles include fatty acids, compounds having an azole structure, fatty acid salts, surfactants, organometallic chelating agents, and protective colloids. Here, from the viewpoint of being easily uniformly attached to the silver powder surface, it is preferable that the surface treatment agent be one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acid salts.

[0054] [Surface treatment agent] Examples of fatty acids include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, and linolenic acid. These may be used individually or in combination of two or more. Examples of fatty acid salts include salts of the fatty acids listed above. Examples of salts include sodium salts and potassium salts. Examples of compounds having an azole structure include benzotriazole, benzotriazole sodium salt, and benzotriazole potassium salt. These may be used individually or in combination of two or more.

[0055] The amount of surface treatment agent added to the solution containing aluminum oxide-coated silver particles 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-ammine complex aqueous solution. An amount of 0.05% by mass or more is effective in maintaining good dispersibility of the resulting aluminum oxide-coated silver powder. On the other hand, the amount of surface treatment agent added 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 surface treatment agent added is 1.00% by mass or less, it is easier to suppress the amount of organic matter in the resulting aluminum oxide-coated silver powder from becoming too high, which is advantageous in suppressing the increase in resistance when a conductive film is obtained using a conductive paste containing aluminum oxide-coated silver powder.

[0056] It is preferable to add the surface treatment agent to the solution containing aluminum oxide-coated silver particles after a predetermined time has elapsed since adding the reducing agent to the silver-ammine complex aqueous solution. In this invention, since the growth of silver particles stops when the surface treatment agent is added, the silver particles precipitate, the precipitation of aluminum oxide due to the change in pH progresses, and the timing of addition can be arbitrarily determined so that the silver in the solution precipitates as silver particles.

[0057] [Separation and Recovery Process] The silver powder coated with aluminum oxide obtained through the above-described series of steps is separated and recovered using known solid-liquid separation methods, then washed with water as necessary and dried. Known solid-liquid separation methods include, for example, decantation or a filter press. The end of the washing process may be determined using the electrical conductivity of the washing water. Specifically, washing is determined to be complete when the electrical conductivity of the washing water falls below a predetermined value. The silver particles after washing may be subjected to the drying process in an aggregated state such as a cake.

[0058] [Drying process] The drying process can be carried out using vacuum drying or an airflow dryer. In the drying process, operations may be performed to promote dispersion and drying by blowing a high-pressure airflow onto the aggregate of silver particles, or by introducing the cake or the spherical silver powder in the drying process into a stirrer with a stirring rotor and stirring it. The drying temperature of the silver powder should be 100°C or lower. If the temperature of the silver powder is 100°C or lower, sintering of the silver particles in the silver powder can be effectively suppressed. [Examples]

[0059] [Measurement of aluminum content] Inductively coupled plasma atomic emission spectrometry (ICP-OES) was used to measure the amount of aluminum deposited on aluminum oxide-coated silver powder. 1 g of the sample (silver powder) was accurately 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 the heated sample cooled, it was diluted to 100 mL using pure water, 5 mL of the supernatant was taken, and the volume was again diluted to 100 mL using pure water to prepare the sample for ICP analysis. The standard addition method was used for measurement. A calibration curve was created by adding aluminum to a standard solution using 5N silver, with a silver concentration similar to that of the sample. For quantitative analysis, an Agilent 5800 ICP-OES from Agilent Technologies was used.

[0060] [Analysis of the state of aluminum] The state of aluminum deposited on silver particles was analyzed by X-ray photoelectron spectroscopy (XPS). A scanning X-ray photoelectron spectrometer, PHI5000 Versa Probe III, manufactured by ULVAC-PHI, Inc., was used for the XPS measurements. Monochromatic AlKα rays were used as the X-ray source, with an acceleration voltage of 15kV, an output of 25W, an X-ray incidence angle of 90 degrees, and a photoelectron extraction angle of 45 degrees. For the chemical state analysis of aluminum, Al2p spectroscopy was used, with a pass energy of 69 eV, integration time of 80 ms, measurement energy interval of 0.125 eV / step, and 500 integration cycles. Charge correction was also performed using a CC bond energy of 284.8 eV.

[0061] [Scanning electron microscope (SEM) observation] The presence of voids within the silver particles was confirmed by observing the cross-section of the silver particles using a scanning electron microscope (SEM) in the following manner. First, silver powder was placed in resin and a hardener and allowed to solidify, and the solidified resin was then cut. Next, the cut surface was polished with a cross-section polisher to expose the cross-section of the silver particles, and each silver particle was observed with a scanning electron microscope to confirm the presence or absence of voids inside the silver particles. For example, the resin and curing agent mentioned above can be "Epofix resin" and "Epofix curing agent" manufactured by Storuas. For example, the cross-section polisher can be "ArBlade5000" manufactured by Nippon High Technologies. For example, the scanning electron microscope can be "JSM-IT800SHL" manufactured by JEOL Ltd. In this specification, "voids within silver particles" refers to enclosed spaces within silver particles, not spaces in the surface irregularities of the silver particles.

[0062] [Particle size distribution measurement] Volume-based cumulative 10% particle size (D) of aluminum oxide-coated silver powder 10 ), cumulative 50% particle diameter (D 50 ), cumulative 90% particle diameter (D 90 ), cumulative 100% particle diameter (D max The particle size distribution was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300 EXII, manufactured by Microtrac Bell Co., Ltd.). For the measurement, 0.1 g of the sample was dispersed in 40 mL of isopropyl alcohol (IPA). An ultrasonic homogenizer (US-150T: 19.5 kHz, tip diameter 18 mm, manufactured by Nippon Seiki Seisakusho Co., Ltd.) was used for dispersion. The dispersion time was 2 minutes. The dispersed sample was subjected to the above apparatus, and the particle size distribution was determined using the accompanying analysis software. Furthermore, during measurement, an SDC (Sample Delivery Controller) device was used in the circulation chamber of the laser diffraction / scattering particle size distribution analyzer, and the "flow velocity (%)" setting of the circulation chamber was set to 60.

[0063] [BET specific surface area measurement] The specific surface area (BET) of aluminum oxide-coated silver powder was measured using a single-point BET method with nitrogen adsorption, employing a Macsorb HM-model 1210 measuring instrument from MOUNTECH. For the BET specific surface area measurement, a sample weight of 3.0 g was used, with a gas mixture of N2:He=30:70, a gas flow rate of 25 mL / min, and degassing conditions of 60°C for 10 minutes before measurement.

[0064] [Measurement of true density] The true density was measured using a dry automatic helium gas densimeter (Micromeritix, instrument name: AccuPyc II 1340) by the constant volume expansion method (the "gas pycnometer method" in the Japanese Pharmacopoeia). Specifically, the volume of silver powder was measured from the gas volume when helium gas was filled into the container until a constant pressure was reached, and the true density was calculated by dividing the mass of the silver powder by that volume. Note that the helium gas at this time cannot reach the closed spaces (i.e., voids) on the particle surface. Therefore, the measurement of true density includes the density of closed voids inside the silver particles that are not connected to the outside. Consequently, the true density of the silver powder tends to decrease as the proportion of voids inside the silver powder increases.

[0065] [Thermomechanical Analysis Measurement] Thermomechanical analysis (TMA) of aluminum oxide-coated silver powder was performed using a Rigaku Thermo plus EVO 2 series TMA8311. First, 0.3 g of aluminum oxide-coated silver powder was weighed and placed into a predetermined mold with a diameter of 5 mmφ. A disc-shaped sample was then prepared by pressing it with a load of 50 kg for 1 minute using a press machine. This sample was placed in the sample holder of the thermomechanical analysis (TMA) apparatus, and a load of 98 mN was applied using a measuring probe. The temperature was increased from room temperature (25°C ± 5°C) to 900°C at a heating rate of 10°C / min to perform thermomechanical analysis (TMA). The expansion and contraction rates of the sample were measured for the length L (mm) profile of the disc-shaped sample at each temperature using equations (2) and (3) below. In thermomechanical analysis, the coefficient of thermal expansion (%) = 100 × (LL RT ) / L RT …(2) Shrinkage rate in thermomechanical analysis (%) = 100 × (L RT -L) / L RT …(3) Here, L RT This is the length (mm) of the disc-shaped sample at room temperature (25℃ ± 5℃).

[0066] [Measurement of loss on ignition (Ig-loss)] In this specification, "Ignition Loss (Ig-loss) value" refers to the change in mass when heated from room temperature (25°C ± 5°C) to 800°C. Specifically, it is an indicator of the amount of non-silver components present in the silver powder, and indicates the amount of residual components in the silver powder, such as additives and surface treatment agents used in the manufacturing process of the silver powder. The Ignition Loss (Ig-loss) value of aluminum oxide-coated silver powder was calculated by precisely weighing the silver powder sample (weighed value: w1), placing it in a magnetic crucible, heating it to 800°C, holding it at 800°C for 30 minutes (a sufficient time to reach a constant weight), then cooling and weighing again (weighed value: w2), and calculating the Ig-loss value from w1 and w2 using the following formula (4). Ignition loss (Ig-loss) value (mass%)=100×(w1-w2) / w1 …(4)

[0067] [Method for manufacturing conductive paste] A mixture was obtained by mixing aluminum oxide-coated silver powder, aluminum powder (metallic aluminum: 99.87% by mass, iron: 0.09% by mass, silicon: 0.04% by mass, SEM average diameter: 2.0 μm), glass frit (glass powder: mainly containing PbO, and containing B2O3, SiO2 and other oxides), ethylcellulose, 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 obtained mixture was placed in a propellerless self-rotating agitator defoaming device (EME Co., Ltd., V-mini300) and pre-mixed for 30 seconds under conditions of rotation at 1000 rpm. Then, using a three-roll roller (EXAKT 80S), the mixture was kneaded by passing it through a roll gap from 100 μm to 20 μm to obtain a conductive paste.

[0068] [Table 1]

[0069] [Measuring line resistance] Using the conductive paste obtained by the above procedure, a linear shape was printed by screen printing. The linear shape had a design line width of 20 μm and a length of 150 mm. A Microtec printing press was used for printing, and the squeegee speed was 350 mm / s. A silicon substrate with a thickness of approximately 170 μm (for solar cell applications, with texture formation and SiNx film deposition) was used for printing. After printing, the samples were dried for 5 minutes in a dryer set to a temperature of 200°C, and then fired in a solar cell firing furnace (NGK) under conditions where the peak temperature of the wafer surface reached 740°C to prepare samples for line resistance measurement. The resistance value of the electrode after firing (line resistance value of the conductive film) was measured using a digital multimeter (manufactured by ADC Corporation) by placing measuring terminals on both ends of the printed electrode.

[0070] [Comparative Example 1] Using commercially available silver powder (4-8FD, manufactured by DOWA High-Tech Co., Ltd.) as the test powder, the line resistance was measured using the procedure described above and was found to be 16.9 Ω. In this invention, the effect of the invention is judged to have occurred when the line resistance decreases below this value.

[0071] Figure 3 shows the TMA curve of the silver powder obtained in Comparative Example 1. The silver powder obtained in Comparative Example 1 began to expand at around 220°C, with a maximum expansion rate of 0.47% and an expansion rate of less than 0.50%. Subsequently, it returned to the sample length before measurement at around 320°C and began to contract, reaching a temperature of 417°C at which it achieved a contraction of 5.00% or more. Figure 1 shows a cross-sectional SEM image of the silver powder obtained in Comparative Example 1. The length of the white bar in the lower right of this SEM image is 1 μm (similar in Figures 4, 5-7). The silver powder obtained in Comparative Example 1 had voids inside, and the proportion of silver particles with voids was around 90 percent.

[0072] Table 2 shows the aluminum content of the silver powder used as the test powder, and the cumulative 10% particle size (D) based on volume. 10 ), cumulative 50% particle diameter (D 50 ), cumulative 90% particle diameter (D 90 ), cumulative 100% particle diameter (D max), BET specific surface area, BET diameter (D BET )、D 50 and BET diameter (D BET ) ratio (D 50 / D BET ), the volume-based cumulative 90% diameter D 90 measured by a laser diffraction scattering particle size distribution measuring device and the volume-based cumulative 10% diameter D 10 difference (D 90 -D 10 ) and D 50 ratio ((D 90 -D 10 ) / D 50 ), loss on ignition (Ig-loss) value, the proportion of silver particles having voids, true density, 0.50% expansion temperature, 5.00% shrinkage temperature, and the line resistance described above (the same applies in each of the following examples). In addition, in this comparative example, there is no supply of aluminum into the liquid by an aluminum compound, and commercially available silver powder (described above) is used as the test powder.

[0073] [Example 1] To 3482.7g of an aqueous silver nitrate solution containing 53.7g of silver, 1.1g of a 5% by mass aqueous solution of aluminum nitrate notahydrate (manufactured by Kojun Chemical Laboratory Co., Ltd., standard content 98.0+%) was added. While stirring at 332 rpm, 113.2g of a 28% by mass aqueous ammonia solution (manufactured by Junsei Chemical Co., Ltd.) was added to produce an aqueous silver ammine complex solution containing aluminum. The amount of aluminum in the added aluminum nitrate notahydrate solution was 70 ppm by mass relative to the amount of silver. One minute after adding the ammonia solution, 10.0g of a 20% by mass aqueous solution of sodium hydroxide was added, and the solution temperature was adjusted to 26.5°C. At this time, the pH of the solution was 12.1. Next, three minutes after adding the ammonia solution to the aluminum-containing silver nitrate solution, 251.4g 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 emulsion aqueous solution containing 1.55% by mass of stearic acid was added to the slurry containing the precipitated silver powder. The stirring was then stopped, the silver particle slurry was filtered, and washed with water until the electrical conductivity was 0.5 mS / m or less. The mixture was then vacuum-dried at 73°C for 10 hours to obtain silver powder. The pH of the filtrate was 6.3. A 5 L beaker was used for the reaction, along with baffles and a two-stage turbine blade. 50 g of the silver powder obtained in the above process was placed in a sample mill (Kyōritsu Riko Co., Ltd., SK-M10) and crushed twice for 30 seconds using the dial setting of 100 to obtain aluminum oxide-coated silver powder (test powder) according to Example 1.

[0074] When the aluminum content of the aluminum oxide-coated silver powder was measured, it was found to be 70 ppm relative to the mass of the aluminum oxide-coated silver powder, and the ignition loss (Ig-loss) was 0.95%. Figure 2 shows the XPS spectrum of the aluminum oxide-coated silver powder obtained in this example. A small peak was observed in the spectrum around a bond energy of 74.3 ± 1 eV. This peak is attributed to aluminum oxide, indicating that aluminum oxide is present on the surface of the silver powder obtained in this example. Notably, no peak for metallic aluminum (around a bond energy of 72.6 eV) was observed in the XPS measurement.

[0075] Figure 3 shows the TMA curve of the aluminum oxide-coated silver powder obtained in Example 1. The aluminum oxide-coated silver powder obtained in Example 1 began to expand at around 220°C, reaching an expansion of 0.50% or more at 227°C, with a maximum expansion rate of 1.90%. Subsequently, it returned to the sample length before measurement at around 420°C and began to contract, reaching a contraction of 5.00% or more at 559°C. Figure 4 shows a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 1. The aluminum oxide-coated silver powder obtained in Example 1 contained voids, with the proportion of voided silver particles being 86%, and the true density being 9.88 g / cm³. 3 That was the case.

[0076] Using the aluminum oxide-coated silver powder obtained in this embodiment, the line resistance was measured using the procedure described above and was found to be 16.5 Ω. This value was lower than that of Comparative Example 1, indicating that even with an aluminum content of 70 ppm by mass, coating the surface of the silver powder with aluminum oxide reduces the line resistance when it is paste-formed to create electrodes.

[0077] [Example 2] The aluminum oxide-coated silver powder (test powder) according to this example was obtained using the same procedure as in Example 1, except that 3.2 g of aluminum nitrate notahydrate aqueous solution was added. The amount of aluminum in the added aluminum nitrate notahydrate aqueous solution was 210 ppm by mass relative to the amount of silver. At that 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. The aluminum content of the aluminum oxide-coated silver powder was 210 ppm by mass, and the ignition loss (Ig-loss) value was 0.92%, confirming by XPS measurement that the coated aluminum was an oxide (Figure 2). The aluminum oxide-coated silver powder obtained in Example 2 began to expand at around 220°C, reaching an expansion of 0.50% or more at 226°C, with a maximum expansion rate of 1.80%. Subsequently, it returned to the sample length before measurement at around 540°C, began to contract, and reached a contraction of 5.00% or more at 759°C (Figure 3). Figure 5 shows a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 2. The aluminum oxide-coated silver powder obtained in Example 2 contained voids, with a void-containing silver particle ratio of 84% and a true density of 9.79 g / cm³. 3 That was the case.

[0078] Using the aluminum oxide-coated silver powder obtained in this embodiment, the line resistance was measured using the procedure described above and was found to be 16.7 Ω. This value was lower than that of Comparative Example 1, indicating that even with an aluminum content of 210 ppm by mass, coating the surface of the silver powder with aluminum oxide reduces the line resistance when it is paste-formed to create electrodes.

[0079] [Example 3] The aluminum oxide-coated silver powder (test powder) according to this example was obtained using the same procedure as in Example 1, except that 0.8 g of aluminum nitrate notahydrate aqueous solution was added and 15.0 g of 20% by mass sodium hydroxide aqueous solution was added. The amount of aluminum in the added aluminum nitrate notahydrate aqueous solution was 50 ppm by mass relative to the amount of silver. At that 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. The aluminum content of the aluminum oxide-coated silver powder was 50 ppm by mass, and the ignition loss (Ig-loss) value was 0.72%, confirming by XPS measurement that the coated aluminum was an oxide (Figure 2). Furthermore, the temperature at which the aluminum oxide-coated silver powder obtained in Example 3 reached an expansion of 0.50% or more was 232°C, and the maximum expansion rate during expansion was 0.55%. Subsequently, it returned to the sample length before measurement at around 277°C, began to contract, and reached a temperature of 483°C at which a contraction of 5.00% or more was reached (Figure 3). Figure 6 shows a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Example 3. The aluminum oxide-coated silver powder obtained in Example 3 contained voids, with a void-containing silver particle ratio of 87%, and a true density of 9.48 g / cm³. 3 That was the case.

[0080] Using the aluminum oxide-coated silver powder obtained in this embodiment, the line resistance was measured using the procedure described above and was found to be 16.7 Ω. This value was lower than that of Comparative Example 1, indicating that even with an aluminum content of 50 ppm by mass, coating the surface of the silver powder with aluminum oxide reduces the line resistance when it is paste-formed to create electrodes.

[0081] [Comparative Example 2] The aluminum oxide-coated silver powder (test powder) for this comparative example was obtained using the same procedure as in Example 1, except that 7.3 g of aluminum nitrate notahydrate aqueous solution was added. The amount of aluminum in the added aluminum nitrate notahydrate aqueous solution was 480 ppm by mass relative to the amount of silver. At that 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. The aluminum content of the aluminum oxide-coated silver powder obtained in this comparative example was 480 ppm by mass, and the ignition loss (Ig-loss) value was 0.93%. The XPS spectrum of aluminum for this aluminum oxide-coated silver powder is shown in Figure 2. In this comparative example as well, it can be seen that aluminum oxide is coated on the surface of the silver powder. Furthermore, the aluminum oxide-coated silver powder obtained in Comparative Example 2 began to expand at around 220°C, reaching an expansion of 0.50% or more at 225°C, with a maximum expansion rate of 1.70%. Subsequently, it returned to the sample length before measurement at 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 of 5.00% or more was observed (Figure 3). Figure 7 shows a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Comparative Example 2. The aluminum oxide-coated silver powder obtained in Comparative Example 2 had voids, and the proportion of silver particles with voids was 91%, with a true density of 9.81 g / cm³. 3 That was the case.

[0082] When the line resistance was measured using the aluminum oxide-coated silver powder obtained in this comparative example, following the procedure described above, it was found to be 19.2 Ω, which was higher than that of Comparative Example 1. Therefore, it was found that when aluminum oxide is coated onto the surface of silver powder, if the aluminum content, expressed as a mass ratio to the mass of the aluminum oxide-coated silver powder, is 400 ppm by mass or more, the line resistance increases when it is paste-formed to create electrodes.

[0083] [Comparative Example 3] 3231.0 g of a silver nitrate aqueous solution containing 50.8 g of silver was stirred at 174 rpm, and 103.2 g of a 28% by mass ammonia aqueous solution (manufactured by Junsei Kagaku Co., Ltd.) was added. 30 seconds after adding the ammonia aqueous solution, 0.5 g of a 5% by mass sodium carbonate aqueous solution was added, and another 30 seconds later, 0.5 g of a 5% by mass PEI600 aqueous solution was added to adjust the solution temperature to 33.5°C. At this time, the pH of the solution was 12.1. Next, 3 minutes after adding the ammonia aqueous solution, 302.8 g of a 1.86% by mass hydrazine aqueous solution (manufactured by Mitsubishi Gas Chemical Co., Ltd.) was added all at once as a reducing agent. 10 seconds after adding the reducing agent, 1.7 g of a 5.0% by mass aluminum nitrate nonahydrate aqueous solution was added, and 20 seconds after adding the reducing agent, 110 g of a 67.5% by mass nitric acid aqueous solution was added. The amount of aluminum in the added aluminum nitrate nonahydrate aqueous solution was 120 ppm by mass relative to the amount of silver. Furthermore, 30 seconds after adding the reducing agent, 5.1 g of an emulsion aqueous solution containing 1.55% by mass of stearic acid was added to the slurry containing the precipitated silver powder. The stirring was then stopped, the silver particle slurry was filtered, and washed with water until the electrical conductivity was 0.5 mS / m or less. The mixture was then vacuum-dried at 73°C for 10 hours to obtain silver powder. The pH of the filtrate was 8.8. A 5 L beaker was used for the reaction, along with baffles and a three-stage turbine blade. 50 g of the silver powder obtained in the above process was placed in a sample mill (Kyōritsu Riko Co., Ltd., SK-M10) and crushed twice for 30 seconds using the dial setting of 100 to obtain aluminum oxide-coated silver powder (test powder) related to Comparative Example 3.

[0084] The aluminum content of the aluminum oxide-coated silver powder obtained in this comparative example was 115 ppm by mass, and the ignition loss (Ig-loss) value was 0.20%. The XPS spectrum of aluminum for this aluminum oxide-coated silver powder is shown in Figure 2. In this comparative example as well, it can be seen that aluminum oxide is coated on the surface of the silver powder. Furthermore, the aluminum oxide-coated silver powder obtained in Comparative Example 3 did not exhibit expansion of more than 0.50%, began to contract from around 440°C, and reached a temperature of 553°C at which it reached a contraction of more than 5.00%. Figure 8 shows a cross-sectional SEM image of the aluminum oxide-coated silver powder obtained in Comparative Example 3. The aluminum oxide-coated silver powder obtained in Comparative Example 3 had zero percent of silver particles with voids, was void-free, and had a true density of 10.42 g / cm³. 3 That was the case.

[0085] When the line resistance was measured using the aluminum oxide-coated silver powder obtained in this comparative example, following the procedure described above, it was found to be 17.1 Ω, which was higher than that of Comparative Example 1. Therefore, it was found that when aluminum oxide is coated on the surface of silver powder, and the aluminum oxide-coated silver powder does not have voids, the line resistance increases when it is pasteurized to form electrodes.

[0086] [Table 2]

Claims

1. Aluminum oxide-coated silver powder consists of silver particles coated with aluminum oxide on their surface, wherein the amount of aluminum is 10 ppm by mass or more and less than 400 ppm by mass relative to the mass of the aluminum oxide-coated silver powder, and the volume-based cumulative 50% diameter D is measured by a laser diffraction scattering particle size distribution analyzer. ５０ The particle size is between 0.2 μm and 5.0 μm, and the true density is 10.00 g / cm³. ３ The following is silver powder coated with aluminum oxide.

2. The aluminum oxide-coated silver powder according to claim 1, wherein the amount of aluminum is 10 ppm by mass or more and 100 ppm by mass or less with respect to the mass of the aluminum oxide-coated silver powder.

3. The aluminum oxide-coated silver powder according to claim 1, wherein, when the particle cross-section is observed, the number of silver particles having voids inside the particle is 70% or more of the total silver particles.

4. The aforementioned D ５０ and BET diameter D ＢＥＴ The ratio (D ５０ / D ＢＥＴ The aluminum oxide-coated silver powder according to claim 1, wherein the ratio is 1.00 or more and 2.50 or less.

5. The cumulative 90% diameter D on a volume basis measured by a laser diffraction scattering type particle size distribution measuring device ９０ and the cumulative 10% diameter D on a volume basis １０ the difference between (D ９０ − D １０ ), and the ratio of the above D ５０ ((D ９０ − D １０ ) / D ５０ ) is 0.50 or more and 2.00 or less, the aluminum oxide-coated silver powder according to claim 1.

6. The aluminum oxide-coated silver powder according to claim 1, wherein the ignition loss (Ig-loss) is 0.30% or more and 3.00% or less.

7. The aluminum oxide-coated silver powder according to claim 1, wherein the aluminum oxide-coated silver powder is distributed at 2.5 kg / mm³ ２ The sample obtained by compression molding was given 5 mN / mm² ２ Aluminum oxide-coated silver powder, wherein, in a thermomechanical analysis (TMA) performed by heating from room temperature to 900°C at a heating rate of 10°C / min under the applied load, the temperature at which the expansion coefficient relative to room temperature reaches 0.50% is between 150°C and 350°C.

8. The aluminum oxide-coated silver powder according to claim 1, wherein the aluminum oxide-coated silver powder is distributed at 2.5 kg / mm³ ２ The sample obtained by compression molding was given 5 mN / mm² ２ Aluminum oxide-coated silver powder, wherein, in a thermomechanical analysis (TMA) performed by heating the material from room temperature to 900°C at a heating rate of 10°C / min under the applied load, the temperature at which the shrinkage rate relative to room temperature reaches 5.00% is between 450°C and 850°C.

9. The aluminum oxide-coated silver powder according to claim 1, for use in a firing paste for forming electrodes of solar cells.

10. A baked paste for forming electrodes of solar cells, comprising the aluminum oxide-coated silver powder described in claim 1, an organic binder, and an organic solvent.

11. The process includes a "complex formation step" to obtain a silver-ammine complex aqueous solution by adding ammonia to a silver ion-containing aqueous solution to form a silver-ammine complex, and a "pH adjustment step" to adjust the pH of the aqueous solution to 10.0 or higher by adding a pH adjusting agent to the silver-ammine complex aqueous solution, wherein one or more methods selected from the following are used: using the silver ion-containing aqueous solution containing an aluminum compound in the complex formation step, adding the aluminum compound to the aqueous solution during the period until the pH adjustment step is completed, and adding the aluminum compound to the aqueous solution after the pH adjustment step is completed, to produce a silver-ammine complex aqueous solution containing aluminum and having a pH of 10.0 or higher. The process involves adding a reducing agent to the aforementioned aluminum-containing silver-ammine complex aqueous solution with a pH of 10.0 or higher to reduce silver ions and precipitate silver particles, while simultaneously lowering the pH of the aqueous solution to 4.0 or higher and 9.0 or lower, thereby precipitating aluminum oxide. A method for producing aluminum oxide-coated silver powder, The amount of aluminum supplied to the liquid by the above aluminum compound is such that the amount of aluminum relative to the amount of silver in the aqueous solution at the start of the addition of the reducing agent is 10 ppm by mass or more and less than 400 ppm by mass. A method for producing aluminum oxide-coated silver powder, wherein the reducing agent is an organic compound containing one or more of the COOH group, CHO group, and OH group in its molecule.

12. The method for producing aluminum oxide-coated silver powder according to claim 11, wherein the reducing agent is formaldehyde.

13. A method for producing aluminum oxide-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 aforementioned reducing agent.

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

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