TECHNICAL FIELD

The present invention relates to mixed silver powder that is suitable for forming a conductive film on a surface of a member having stretchability, a method for producing the same, and a conductive paste using the mixed silver powder.

BACKGROUND ART

In recent years, an electronic equipment that is referred to as a wearable device capable of being mounted on the human body has been actively developed as shown, for example, in PTL 1. The wearable device is frequently provided on cloth having a stretchability, such as the device described in PTL 2, and therefore the electrode constituting the circuit of the wearable device is demanded to have a capability of forming at a low temperature and a stretchability.

Examples of a material that is capable of forming an electrode with a conductive film on a member having a flexibility at a low temperature include the silver pastes described in PTL 3 and PTL 4. The silver pastes contain, as a major component, a low temperature curing resin as a binder having mixed therewith silver powder formed of metallic silver fine particles as a conductive filler, and the silver powder used is mainly a flaky material. The silver pastes can form a circuit of a conductive film on a surface of a stretchable member, such as cloth, in such a manner that a circuit pattern is formed by such a measure as screen printing or the like, and then the resin as the binder is cured at a low temperature, but have a problem that the volume resistivity of the conductive film is significantly increased when the member is stretched and contracted.

As a technique for improving the characteristics of the silver paste for forming a conductive film, for example, PTL 5 and PTL 6 describe mixed silver powder having plural kinds of silver powder having different shapes, but these related techniques do not describe mixed silver powder that is suitable for forming a conductive film on a stretchable member. For example, PTL 5 describes a silver powder mixture obtained by mixing filamentous silver powder with one kind or two kinds of granular silver powder and flaky silver powder, but the silver powder mixture intends to improve the thixotropy of the silver paste, and the stretchability of the baked film of the silver paste is insufficient for applying to a stretchable member.

CITATION LIST

Patent Literatures

PTL 1: JP-A-2015-083045PTL 2: JP-A-2014-203815PTL 3: JP-A-2016-014111PTL 4: JP-A-2015-073105PTL 5: JP-A-2015-065098PTL 6: JP-A-2013-105525

SUMMARY OF INVENTION

Technical Problem

In the techniques described in PTL 1 and PTL 2, the flaky silver powder is used as a conductive filler for providing a good conductivity through the overlap of the flaky silver powder in the conductive layer. In the techniques, however, the silver paste is cured at a low temperature, and the flaky silver powder is only in contact with each other simply physically in the cured resin. Therefore, in the case where the conductive film formed by curing the silver paste is stretched and contracted, it is considered that the contact among the silver powder is lost to result in the increase of the volume resistivity of the conductive film.

As a result of the earnest investigations by the present inventors, it has been found that in the case where a circuit of a conductive film is formed on a stretchable member with a conductive paste using, as a conductive filler, a silver powder mixture obtained by mixing filamentous silver powder having a particular shape and good low temperature sintering capability with silver powder having another shape, the extent of the decrease of the volume resistivity of the conductive film after stretching and contracting the member is decreased, and thus the present invention has been completed.

The silver powder mixture of plural kinds of silver powder different in shape as described above can be produced by mixing flaky silver powder and filamentous silver powder having the prescribed shapes, and can be efficiently produced at low cost by a wet reduction method of silver ion.

Accordingly, an object of the present invention is to provide a silver powder mixture that is suitable for forming a conductive film on a surface of a member having a stretchability, a method for producing the same, and a conductive paste using the silver powder mixture.

Solution to Problem

The object of the present invention can be achieved by one embodiment of the present invention, i.e., a silver powder mixture containing filamentous silver powder including a spherical part and a filamentous part, the filamentous part extending from the spherical part and having a short axis length that is smaller than a particle diameter of the spherical part.

It is possible that the silver powder mixture contains the filamentous silver powder and flaky silver powder, and has a projected area ratio of the filamentous silver powder of 20% or more. In this case, it is preferred that the filamentous part of the filamentous silver powder has an average long axis length of 2 μm or more and 20 μm or less and an average short axis length of 50 nm or more and 900 nm or less, and the flaky silver powder has an average particle diameter of 1 μm or more and 50 μm or less and an aspect ratio, which is defined by a ratio of an average long diameter and an average thickness, of 1.5 or more, the silver powder mixture is preferred, and it is more preferred that the silver powder mixture has a projected area ratio of the filamentous silver powder of 20% or more in the mixed silver powder.

It is more preferred that the silver powder mixture has a content of granular silver powder of less than 20%.

The present invention provides filamentous silver powder having a shape including a spherical part and a filamentous part extending from the spherical part and having a short axis length that is smaller than a particle diameter thereof, and also provides a silver powder mixture containing filamentous silver powder having that shape and flaky silver powder.

As a method for producing the silver powder mixture, there is provided a production method including: adding one kind of a salt of copper or aluminum and ethylenediaminetetraacetic acid to a silver nitrate aqueous solution to provide a silver complex solution; after adding ethylenediaminetetraacetic acid, retaining the silver complex solution under stirring for 90 seconds or more (preferably 110 seconds or more); and adding a reducing agent containing one kind or two or more kinds of L-ascorbic acid, erythorbic acid, and salts thereof to the retained silver complex solution.

In the production method, it is possible that the silver complex solution before adding the reducing agent has a silver concentration of from 0.05 mol/L to 1 mol/L, and an amount of the reducing agent added is from 0.05 mol/L to 1 mol/L based on the liquid amount of the silver complex solution before adding the reducing agent. It is also possible that in the silver complex solution, an amount of the one kind or two or more kinds of a salt of copper and aluminum added is from 0.001 to 0.02 in terms of molar ratio with respect to silver, and an amount of the ethylenediaminetetraacetic acid added is from 0.0005 to 0.015 in terms of molar ratio with respect to silver.

The present invention provides a conductive paste including the silver powder mixture.

Advantageous Effects of Invention

According to the present invention, filamentous silver powder and a silver powder mixture that are suitable for forming a conductive film on a surface of a member having a stretchability can be produced by an inexpensive wet reduction method.

