DENDRITIC SILVER POWDER

Abstract

Disclosed is a new dendritic silver powder which, when mixed with a synthetic resin, gives electroconductive films having sufficient electroconductivity. Even when the films produced from a mixture of the dendritic silver powder and a synthetic resin vary in thickness, the electroconductivity of the films can be maintained. The volume-cumulative particle diameter D50 (referred to as D50D) determined by adding the silver powder to water containing a dispersant, applying 300-watt ultrasonic waves to the resultant mixture for 3 minutes, and examining the dispersion with a laser diffraction/scattering type particle size analyzer is 1.0-15.0 m and that the ratio of the volume-cumulative particle diameter D50 (referred to as D50) determined by adding the silver powder to the water containing a dispersant and examining the mixture under the same conditions as for the D50D except that no ultrasonic waves are applied, to the D50D, D50N/D50D, is 1.0-10.0.

Claims

1. A dendritic silver powder comprising silver particles having a shape including a trunk and a plurality of branches that branch off perpendicularly or obliquely from the trunk and that have grown two- or three-dimensionally under observation of an electron microscope at a magnification of 3000 to 10000, the silver particles accounting for 50% by number or more based on all the silver particles being observed, wherein: D50D of the silver powder is 1.0-15.0 m, wherein D50D is defined by a volume cumulative particle diameter D50 of the silver powder, and wherein D50 is measured by a laser diffraction/scattering particle size distribution measurement apparatus with a pretreatment wherein a dispersion containing the silver powder, a dispersant and water is subjected to irradiation of 300 watts of ultrasonic waves for 3 minutes, and D50N/D50D of the silver powder is 1.0-10.0, wherein D50N is defined by a volume cumulative particle diameter D50 of the silver powder, and wherein D50 is measured by a laser diffraction/scattering particle size distribution measurement apparatus without the irradiation of ultrasonic waves, that is carried out in measuring D50D, to a dispersion containing the silver powder, a dispersant and water.

2. The dendritic silver powder according to claim 1, wherein a specific surface area measured according to a BET single-point method is 0.2-5.0 m.sup.2/g.

3. The dendritic silver powder according to claim 1, wherein the crystallite diameter is 500 -3000 .

4. The dendritic silver powder according to claim 2, wherein the crystallite diameter is 500 -3000 .

Description

EXAMPLES

[0073] Examples of the invention are described below. The invention, however, is not to be limited to the following Examples.

Observation of Particle Shape

[0074] For each of the silver powders (samples) obtained according to the Examples and Comparative Examples, the respective shapes of fifty random particles were observed with a scanning electron microscope at a magnification of 5000, and the form of silver powder particles accounting for 50% by number or more of all the silver powder particles is shown in Table 1.

[0075] Note that, in order to prevent the particles from overlapping one another during observation of the particle shapes, observation was performed by attaching a small amount of silver powder (sample) onto a carbon tape.

[0076] At this time, whether or not the shape was dendritic was determined by whether or not the particle had a shape including a trunk and a plurality of branches that branch off perpendicularly or obliquely from the trunk and that have grown two- or three-dimensionally.

Particle Size Measurement

[0077] A small amount, more specifically, 0.2 g of the silver powder (sample) obtained according to an Example or a Comparative Example was placed in a beaker, and 0.07 g of Triton X-100 (product of Kanto Chemical Co., Inc.) was added to and blended with the powder. Then, the powder was added to 40 mL of dispersant-containing water (dispersant: 0.3% SN-PW-43 solution (product of San Nopco)). This was then subjected to a dispersion treatment by applying 300 watts of ultrasonic waves for 3 minutes using an ultrasonic disperser US-300AT (product of Nihonseiki Kaisha Ltd.), to prepare a measurement sample. The volume cumulative particle diameter D50D of the measurement sample was measured using a laser diffraction/scattering particle size distribution measurement apparatus MT3300II (product of Nikkiso Co., Ltd.). In the measurement, the inside of the sample circulator and the flow path was washed with the dispersant-containing water (dispersant: 0.3% SN-PW-43 solution (product of San Nopco)); then, auto-zero calibration was performed while circulating the dispersant-containing water; thereafter, a measurement sample was added to a 200 mL cell in the circulator until the display showed that the concentration was within a measureable range; and then measurement was started after verifying that the concentration was stable within the measureable range.

[0078] On the other hand, the same silver powder was used to prepare a measurement sample in a similar manner as above except that no ultrasonic waves were applied, and the volume cumulative particle diameter D50N was measured according to the same conditions as above.

Measurement of Specific Surface Area

[0079] The specific surface area was measured according to the BET single-point method with a Monosorb (product of Yuasa Ionics).

Crystallite Diameter

[0080] The crystallite diameter was measured according to the Scherrer method (crystallite diameter measurement method by X-ray diffraction) using an Ultima IV X-ray diffractometer (product of Rigaku Co., Ltd.).

Evaluation of Sheet Resistance

[0081] 42.3 g of the silver powder (sample) obtained according to an Example or a Comparative Example, 99 g of a silicone resin (MRX-2269; product of Asahi Kagaku Kogyo Co., Ltd.) as a binder, and 1 g of an acrylic thickener (TT-615; product of the Dow Chemical Company) as a thickener were mixed, to prepare a paste.

[0082] Then, the paste was applied onto a silicone rubber sheet with a bar coater such that the width was 200 mm and the gap was 50 m. The paste was then dried in an atmospheric hot air drying oven at 90 C. for 60 minutes, to obtain a 40-m-thick coating film.

[0083] The sheet resistance value of the obtained coating film was measured by the four-point probe method using a resistivity meter (MCP-T600; product of Mitsubishi Chemical).

[0084] Note that, as for the silver powders obtained according to Examples 1 to 4, it was possible to measure a resistance value because the powder particles contacted one another sufficiently. As for the silver powder obtained according to Comparative Examples 1 and 2, the resistance value could not be measured as it was too high and exceeded the range (indicated as Unmeasurable in the Table).

Evaluation of Rate of Change in Electroconductivity upon Change in Film Thickness

[0085] 42.3 g of the silver powder (sample) obtained according to an Example or a Comparative Example, 99 g of a silicone resin (MRX-2269; product of Asahi Kagaku Kogyo Co., Ltd.) as a binder, and 1 g of an acrylic thickener (TT-615; product of the Dow Chemical Company) as a thickener were mixed, to prepare a paste.

[0086] Then, the paste was applied onto a silicone rubber sheet with a bar coater such that the width was 200 mm and the gap was 50 m. The paste was then dried in an atmospheric hot air drying oven at 90 C. for 60 minutes, to obtain a 40-m-thick coating film. The obtained coating film was cut into a 2-cm-wide, 15-cm-long strip, to obtain an evaluation film.

[0087] Then, one side of the film was fixed, and the other side was pulled to stretch the film from 15 cm to 19.5 cm and fixed in that state, and the sheet resistance value of the film for when the film prepared by mixing the dendritic silver powder and synthetic resin was stretched and the thickness of the film was changed was measured by the four-point probe method using a resistivity meter (MCP-T600; product of Mitsubishi Chemical).

Example 1

[0088] A DSE electrode was used for the anode, and a drum made of stainless steel (SUS 316) was used for the cathode, and the distance between the electrodes was set to 5 cm. As the electrolytic solution, a silver nitrate solution was electrolyzed while being circulated at 300 mL/min. At this time, the liquid temperature of the electrolytic solution was 25 C., the silver concentration was 20 g/L, the nitric acid concentration was 10 g/L, the citric acid concentration was 0.5 g/L, the amount of electrolytic solution was 30 L, the pH was 2.0, and the current density was adjusted to 750 A/m.sup.2, and electrolysis was performed for 60 minutes.

[0089] Silver deposited on the surface of the cathode was continuously scraped off with a scraper to collect silver powder. The collected silver powder was kept in pure water until the end of electrolysis.

[0090] After electrolysis, the powder was washed, surface treated, and filtered using a nutsche filter. First, the powder was washed with 5 L of pure water, then surface treated with 2.0 g of benzotriazole, and was then washed again with alcohol.

[0091] Then, the silver powder was transferred to a stainless-steel tray, and, using a fan-equipped shelf drier, was dried by being kept at room temperature for 15 hours in air atmosphere. After drying, the powder was sized using a sieve with 75-m openings, and particles that passed through the sieve were collected, to obtain a silver powder (sample).

Example 2

[0092] A silver powder (sample) was obtained in the same manner as Example 1, except that the silver concentration 20 g/L and the citric acid concentration 0.5 g/L were changed to a silver concentration of 10 g/L and a citric acid concentration of 0.1 g/L.

Example 3

[0093] A silver powder (sample) was obtained in the same manner as Example 1, except that the silver concentration 20 g/L, the nitric acid concentration 10 g/L, the citric acid concentration 0.5 g/L, the pH 2.0, and the current density 750 A/m.sup.2 were changed to a silver concentration of 30 g/L, a nitric acid concentration of 5 g/L, a pH of 2.5, and a current density of 1000 A/m.sup.2.

Example 4

[0094] A silver powder (sample) was obtained in the same manner as Example 1, except that the silver concentration 20 g/L, the citric acid concentration 0.5 g/L, and the current density 750 A/m.sup.2 were changed to a silver concentration of 30 g/L and a current density of 1500 A/m.sup.2.

Comparative Example 1

[0095] To 0.8 L of pure water was dissolved 12.6 g of silver nitrate, and 24 mL of 25% ammonia water and 40 g of ammonium sulfate were further added, to prepare a silver ammine complex salt aqueous solution (silver concentration: 10 g/L; molar ratio NH.sub.3/Ag.sup.+: 12; 20 C.; pH 9.4).

[0096] Using DSE electrode plates for both the anode and cathode, this silver ammine complex salt aqueous solution, as an electrolytic solution, was electrolyzed at a current density of 200 A/m.sup.2 at a solution temperature of 20 C., and electrolysis was performed for 1 hour while scraping off the electrodeposited silver powder particles from the electrode plate with a scraper at suitable intervals.

[0097] Then, a slurry including the scraped-off silver powder particles was filtered with a nutsche filter, and the particles were washed with pure water and further with alcohol, and were dried at 70 C. for 12 hours in an air atmosphere, to obtain a silver powder (sample).

Comparative Example 2

[0098] A DSE electrode was used for the anode, and a plate made of stainless steel (SUS 316) was used for the cathode, and the distance between the electrodes was set to 5 cm. A silver nitrate solution was used as the electrolytic solution. The liquid temperature of the electrolytic solution was 25 C., the silver concentration was 20 g/L, the nitric acid concentration was 10 g/L, the citric acid concentration was 0.5 g/L, the amount of electrolytic solution was 3.0 L, the pH was 2.0, and the current density was adjusted to 750 A/m.sup.2, and electrolysis was performed for 60 minutes, while scraping off the silver powder particles electrodeposited on the surface of the cathode with a scraper at suitable intervals.

[0099] After electrolysis, the powder was washed, surface treated, and filtered using a nutsche filter. First, the powder was washed with 5 L of pure water, then surface treated with 2.0 g of benzotriazole, and was then washed again with alcohol. Then, using a fan-equipped shelf drier, the silver powder was dried at 60 C. for 8 hours in air atmosphere, to obtain a silver powder (sample).

TABLE-US-00001 TABLE 1 Comparative Comparative Example 1 Example 2 Example 3 Example 4 Example 1 Example 2 D50D (m) With ultrasonic 11.0 3.0 13.9 8.4 3.2 8.8 wave dispersion D50N (m) Without 22.0 24.4 34.6 52.2 59.0 213.9 ultrasonic wave dispersion D50N/D50D 2.0 8.1 2.5 6.2 18.4 24.3 SSA (m.sup.2/g) 0.52 0.95 1.31 1.60 3.42 3.47 Sheet resistance 8.9E04 2.4E03 1.0E03 3.8E03 Unmeasurable Unmeasurable ( .Math. cm); (high (high Amount of filler resistance) resistance) filled: 70% Sheet resistance 9.8E04 3.2E03 1.5E03 4.2E03 Unmeasurable Unmeasurable after change in (high (high film thickness resistance) resistance) ( .Math. cm); Amount of filler filled: 70% Crystallite 1758 1067 887 780 366 750 diameter () Shape Dendritic Dendritic Dendritic Dendritic Dendritic Dendritic

Consideration

[0100] When observed with an electron microscope at a magnification of 5000, all of the silver powders (samples) obtained according to Examples 1 to 4 and Comparative Examples 1 and 2 were dendritic silver powders wherein silver powder particles, each having a shape including a trunk and a plurality of branches that branch off perpendicularly or obliquely from the trunk and that have grown two- or three-dimensionally, accounted for 50% by number or more of all the silver powder particles being observed.

[0101] Taking into consideration the results for the Examples and the results of tests heretofore conducted by Inventors, it was found that, by setting the aforementioned ratio D50N/D50D within a predetermined range, a dendritic silver powder at least having a D50D of 1.0-15.0 m can offer sufficient electroconductivity even when mixed with a synthetic resin and made into an electroconductive film, and the electroconductivity of the film can be maintained even when the film prepared by mixing the dendritic silver powder and synthetic resin is stretched and the film thickness is changed.