GB2236115A

GB2236115A - Process for preparing silver powder

Info

Publication number: GB2236115A
Application number: GB8921251A
Authority: GB
Inventors: Gerardus Hendricus Ma Scholten; Den Doel Allewijn Rudolf Van; Der Lee Gerrit Van
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1989-09-20
Filing date: 1989-09-20
Publication date: 1991-03-27
Also published as: GB8921251D0

Abstract

The invention is directed to a process for preparing silver powder, the powder obtained, the use of the powder for preparing silver containing layers and the products obtained by this use. The process comprises decomposing silver formate and/or silver citrate in glycerol in the presence of a complexing agent.

Description

Process for preparing silver powder The present invention is directed to a process for preparing silver powder. More in particular the invention is concerned with the production of submicron silver powder.

Submicron silver particles are generally applied in the form of thick-film pastes. There are two types of thick-film pastes: - high temperature thick-film pastes, and - polymer thick-film paste.

The majority of the silver powders used in high temperature thick-film pastes has a spherical morphology. The pastes is applied to a ceramic substrate by screen printing.

Subsequently the paste is dried and fired to remove all organic material and to obtain a sintered silver pattern.

In general there are four main routes for the production of submicron silver powder. These routes are -reduction of Ag+ ions with an organic reducing agent; -reduction of Ag+ ions with an inorganic reducing agent; -electrodeposition; and -plasma techniques.

The present invention is concerned with the firstmentioned main route, namely reduction of Ag+ ions with an inorganic reducing agent, in an inert organic medium (e.g.

glycerol) in which the silver compound is dissolved and/or suspended.

An important aspect in the preparation of silver powders is the final value of the specific surface area, or, in other words, the particle size of the silver powder.

In general the preparation of silver powders is carried out with one system that yields a powder having a specific surface area (mean particle size) within a relatively narrow range. Obtaining products outside the said range requires substantial modifications of the process, such as the use of other reducing agents.

It is an object of the present invention to provide a process for preparing silver powders, in which process powders within a very broad range of values for the specific surface area can be produced, by relatively small process modifications.

The present invention is thus directed at a process for preparing silver powder and comprises decomposing silver formate and/or silver citrate in glycerol in the presence of a complexing agent.

Surprisingly it has been found that it is possible to obtain silver powders having a different specific surface area, by the very easy method as described hereinabove, by changing temperature, silver formate or citrate concentration and/or concentration of complexing agent.

Particularly preferred is a method, whereby the silver component is decomposed in glycerol at a temperature of between 90 and 1500C, whereby silver powders having specific surface areas between 0.5 and 9 m2/g can be obtained.

The lower range of temperatures yields powders having a high specific surface area, whereas at higher temperature powders with a lower specific surface area are obtained.

Another way of influencing the particle size/specific surface area of the silver powder is the change in silver formate and/or whole concentration in the decomposition bath. At a constant silver formate citrate-complexing agent ratio the increase in amount of silver formate results in a decrease of specific surface area. The concentration of silver formate and/or citrate added to the decomposition bath ranges between 0,5 and 100 g/l, more preferably between 5 and 50 g/l.

A third possibility in influencing the specific surface area is a change in the amount of complexing agent.

The preferred complexing agents to be used in the process of the invention are organic acids or salts thereof, having reducing properties for silver ions. In particular the complexing agent is chosen from the group comprising formic acid, citric acid, salts thereof and mixtures of these. More in particular citric acid and/or salts thereof are used.

The amount of complexing agent is related to the silver-ion concentration and is preferably between 0.1 and 5, more in particular between 0.15 and 3 M/M-Ag+.

In the process of the present invention it is possible to use minor amounts of co-solvents, other than the glycerol, provided that these co-solvents do not interfere with the process.

The process of the present invention can be carried out by dissolving and/or suspending the suitable amount of silver formate in glycerol preheated to the reaction temperature. The complexing agent is added to the glycerol before the addition of silver formate or citrate. After addition of silver formate and/or citrate the reaction slurry is maintained at the reaction temperature for the required time.

The silver powder can then be filtered off,. and washed in a conventional manner.

During reaction the reactor is conventionally stirred.

The silver powder thus obtained can then be mechanically continued into smaller agglomerates. The powders thus obtained are build up of spherical primary particles with a narrow p.s.d. and are almost free of organic carbon residues.

The silver powders can be used for preparing pastes, such as conductive pastes, which are used for preparing multilayer ceramics, ceramic capacitors, but also for conductive patterns on ceramic or polymeric substrates. In view thereof the present invention also covers the use of silver powders for preparing silver layers on various substrates, such as ceramic and polymeric materials and the products thus obtained.

Depending on the type of application, the applied film of silver powder containing paste is dried, and optionally sintered. This last embodiment is of course only possible when sinter resistant substrates have been used. During sintering the silver powder particles form a conductive sinter layer.

Suitable high temperature thick film paste compositions contain ethyl-cellulose, a glass-forming material, terpineol or comparable material and the silver powder, optionally in combination with other metal powders such as palladium.

The composition, which may contain other additives is prepared by mixing and/or milling components to a paste. The paste is brought onto the surface of a substrate by suitable means, such as a sieve printing apparatus.

The printed material can subsequently be dried and, depending on the use, be fired to burn the organic material of the ink. Suitable firing temperatures are between 900 and 11000C.

The present invention is now elucidated on the basis of some examples, which are not intended to limit the invention.

EXAMPLES 1-13 Silver formate, added in an amount of 10 g/l was decomposed during 15 min.in a 250 ml stirred vessel (1250 rpm, axial stirrer) filled with a solution of citric acid in glycerol, preheated to the reaction temperature (1150C).

Spherical powders having various specific surface areas were obtained by variation of the ratio of citric acid to silverformate. The specific surface area was determined using the method of Brunauer-Emmet-Teller, as described in Metals Handbook, 9th ed. Vol. 7, Powder Metallurgy, pp. 262-267.

In table 1 the results of Examples 1-5 have been given.

TABLE 1

Analytical results ; CA/Ag+ SSA CA/AQ+ SSA C-org Oxygen Example No. m2/ % (m/m) % (m/m) 1 0 0.7 2 0.25 2.3 0.11 3 0.5 3.4 0.15 0.35 4 1 3.0 0.13 0.34 5 2 3.6 0.25 0.83 CA = citric acid In a second series of experiments (Nos. 6-9) the temperature of the decomposition bath was varied at a constant citric acid/silverformate ratio of 0.5. The results are given in table 2.

TABLE 2

1 Analytical results Temperature SSA C-org Example No. OC m2/q % (m/m) 6 100 8.2 0.32 7 115 3.9 0.11 8 130 0.9 0.05 9 150 0.7 0.40 CA = citric acid In a third series of experiments (Nos. 10-13) the silver formate concentration was varied at a reaction temperature of 1150C and a citric acid/silver formate ratio of 0.5. The results are given in table 3.

TABLE 3

Analytical results HCOOAg CA/Ag+ SSA C-org Example No. q/l m/m. m2/q % (m/m) 1 10 0 0.7 - 10 10 0.5 3.4 0.15 11 20 0.5 2.8 0.12 12 30 0.5 2.2 0.23 13 40 0.5 - 1.7 CA r citric acid

Claims

Claims 1. Process for preparing silver powder, comprising decomposing silver formate and/or silver citrate in glycerol in the presence of a complexing agent.
2. Process according to claim 1, wherein as complexing agent an organic acid having reducing properties for silver ions is used.
3. Process according to claim 2, wherein the complexing agent has been chosen from the group consisting of formic acid, salts thereof, citric acid, salts thereof, as well as mixtures of two or more of these.
4. Process according to anyone of claims 1-3, wherein powders are prepared having a specific surface area of 0.5 to 9 m2/g.
5. Process according to anyone of claims 1-4, wherein the temperature is between 90 and 1500C.
6. Process according to anyone of claims 1-5, wherein the total silver formate and/or silver citrate concentration is between 0,5 and 100 g/l, preferably between 0.5 and 100 g/l, preferably between 5 and 50 g/l.
7. Process according to claim 6, wherein citric acid and salts thereof are used.
8. Process according to anyone of claims 1-7, wherein the concentration of complexing agent is between 0.1 and 5, preferably between 0.15 and 3 mol/mol Ag.
9. Process for preparing silver powder as claimed in claim 1, substantially as described hereinbefore, especially with reference to the examples.
10. Silver powder prepared in accordance with anyone of the claims 1-9.
11. Use of the silver powder according to claim 10, for the preparation of silver containing layers.
12. Products obtained by the use of silver powder according to claim 11.