1. Field of the Invention
This invention relates to a process for producing a noble-metal type fine-particle dispersion (liquid dispersion) in which noble-metal type fine particles have been dispersed in a solvent. More particularly, it relates to improvements in a process for producing a noble-metal type fine-particle dispersion in which the noble-metal type fine particles make up chainlike agglomerates, and also in a coating liquid for forming transparent conductive layers (hereinafter “transparent conductive layer forming coating liquid”) which is obtained by this process, a transparent conductive layered structure which is obtained by using the transparent conductive layer forming coating liquid, and a display device incorporated with such a transparent conductive layered structure.
2. Description of the Related Art
At present, in cathode ray tubes (CRTs; also called Braun tubes) used as computer displays and so forth, it is required for their display screens to be easy to watch and not to cause visual fatigue. Moreover, any ill influence on human bodies by low-frequency electromagnetic waves generated from CRTs is recently worried about, and it is desired for such electromagnetic waves not to leak outside. Against the leakage of such electromagnetic waves, it can be prevented by forming a transparent conductive layer on the front-panel surface of a display. For example, for preventing the leakage of electromagnetic waves (i.e., electric-field shielding), it is required to form at least a transparent conductive layer with a low resistance of 106 Ω/square or less, preferably 5×103 Ω/square or less, and more preferably 103 Ω/square or less.
Some proposals have been made on such low-resistance transparent conductive films. For example, proposed are methods such as a method in which a transparent conductive layer forming coating liquid in which fine conductive oxide particles of indium-tin oxide (ITO) or the like or fine metal particles have been dispersed in a solvent is coated on the front glass (front panel) of a CRT by spin coating or the like and the coating formed is dried, followed by baking at a temperature of about 200° C. to form the transparent conductive layer, a method in which a transparent conductive tin oxide film (nesa film) is formed on the front glass (front panel) by high-temperature chemical vapor deposition (CVD) of tin chloride, and a method in which a transparent conductive film is formed on the front glass (front panel) by sputtering of an indium-tin oxide, titanium nitride or the like.
The first-mentioned method making use of the transparent conductive layer forming coating liquid has been a very advantageous method because it is far simpler and can enjoy a lower production cost than the latter methods in which the transparent conductive film is formed by CVD or sputtering.
However, where, in the first-mentioned method making use of the transparent conductive layer forming coating liquid, the fine conductive oxide particles of indium-tin oxide (ITO) or the like are used as materials for the transparent conductive layer forming coating liquid, the transparent conductive layer formed has a surface resistance of as high as 104 to 106 Ω/square. Hence, this has not been adequate for shielding the leaking electric fields.
Meanwhile, in the case of the transparent conductive layer forming coating liquid having fine metal particles used therein, a transparent conductive layer having a low resistance of from 102 to 103 Ω/square can be formed although the film has a little lower transmittance than the coating liquid making use of ITO. Hence, this is considered to be a promising method in future.
As the fine metal particles used in the transparent conductive layer forming coating liquid, proposed are, as disclosed in Japanese Patent Applications Laid-open No. 8-77832 and No. 9-55175, particles of noble metals such as silver, gold, platinum, palladium, rhodium and ruthenium, which may hardly be oxidized in air. Incidentally, this publication also discloses that fine particles of a metal other than noble metals as exemplified by iron, nickel or cobalt may also be used. In practice, however, oxide films are necessarily formed on the surfaces of such fine metal particles in the atmosphere, and hence it is difficult to attain good conductivity as the transparent conductive layer.
In order to make display screens easy to watch, in CRTs, the surface of the front panel is subjected to, e.g., anti-glare treatment so that the screen can be restrained from reflecting light.
This anti-glare treatment can be made by a method in which a finely rough surface is provided to make diffused reflection on the surface greater. This method, however, can not be said to be preferable so much because its employment may bring about a low resolution, resulting in a low picture quality. Accordingly, it is preferable to make the anti-glare treatment by an interference method in which the refractive index and layer thickness of a transparent film is so controlled that the reflected light may rather interfere destructively with the incident light.
In order to attain the effect of low reflection by such an interference method, it is common to employ a film of double-layer structure formed of a high-refractive-index film and a low-refractive-index film each having an optical film thickness set at ¼ λ and ¼ λ, or ½ λ and ¼ λ, respectively (λ: wavelength). The film formed of fine particles of indium-tin oxide (ITO) as mentioned above is also used as a high-refractive-index film of this type.
In metals, among parameters constituting an optical constant n−ik (n: refractive index; i2=−1; k: extinction coefficient), the value of n is small but the value of k is great, and hence, also when the transparent conductive layer formed of fine metal particles is used, the effect of low reflection that is attributable to the interference of light can be attained by the double-layer structure as in the case of ITO (a high-refractive-index film).
In recent years, in addition to the above characteristics such as good conductivity and low reflectance, as CRT screens are made flatter, transparent conductive layered structures in which the transparent conductive layer of this type has been formed are further demanded to have characteristics by which their transmittance can be adjusted within a stated range lower than 100% (stated specifically from 40% to 95%, and commonly from 40% to 75%) to improve the contrast of images. To meet such a demand, it is also common to mix fine color-pigment particles or the like in the transparent conductive layer forming coating liquid.
Here, the reason why the transparent conductive layer having a low transmittance is formed in flat-screen CRTs is as follows: Face panels (front panels) of the flat-screen CRTs have a structure that the outer surface of the panel is flat and the inner surface thereof has a curvature. Hence, the face panel differs in thickness between the screen center and its periphery. This causes in-plane non-uniformity of brightness when conventional color glass (e.g., semi-tinted glass; transmittance: about 53%) is used in panel glass. Accordingly, a high-transmittance panel glass and a low-transmittance transparent conductive layer are combined so as to achieve both the in-plane uniformity of brightness and the improvement in contrast (the contrast is improved as the transmittance is lowered).
However, there has also been a problem that the addition of fine color-pigment particles or the like tends to make the transparent conductive layer have a little low conductivity.
Now, for a conductive layer having fine metal particles used therein, it is desirable that, since metals are originally not transparent to visible light rays, fine metal particles in a quantity as small as possible form conducting paths in the transparent conductive layer in a good efficiency in order to achieve both the high transmittance and the low resistance in the above transparent conductive layer. That is to say, as structure of a conductive layer formed by coating on a substrate a commonly available transparent conductive layer forming coating liquid composed chiefly of a solvent and fine metal particles, and drying the coating formed, it is necessary for the layer to have a structure in which microscopic openings (spaces) have been introduced into a layer of fine metal particles, i.e., a network structure.
Formation of such a network structure can provide a transparent conductive layer having low resistance and high transmittance. This is because the network part comprised of fine metal particles functions as conducting paths on the one hand and the part of openings formed in the network structure has the function to improve light ray transmittance, as so presumed.
As methods of forming the network structure of fine metal particles, they may include, in rough classification, the following methods.
(1) Methods of forming the network structure by causing fine metal particles to agglomerate with one another in the course that the transparent conductive layer forming coating liquid is coated and the coating formed is dried to form a film.
More specifically, a method in which, since the fine metal particles tend to agglomerate compared with fine oxide particles, the solvent composition and so forth of the transparent conductive layer forming coating liquid is appropriately selected so that the fine metal particles may necessarily agglomerate with one another to a certain extent in the course of coating and drying for film formation to obtain the network structure (see Japanese Patent Applications Laid-open No. 9-115438, No. 10-1777, No. 10-142401, No. 10-182191 and so forth); and
a method in which an agglomeration-inducing agent, an agglomeration-accelerating high-boiling solvent or the like is intentionally further added to the transparent conductive layer forming coating liquid so as to actively accelerate the agglomeration between fine metal particles in the course of coating and drying to obtain a network structure (see Japanese Patent Applications Laid-open No. 10-110123, No. 2002-38053 and so forth).
(2) Methods of forming the network structure by coating a transparent conductive layer forming coating liquid in which agglomerates of fine metal particles have been dispersed, and drying the coating formed.
More specifically, a method in which a dispersion of fine metal particles having been made to gather in the form they have minute holes (i.e., in the form of rings), without bringing primary particles of the fine metal particles into a uniformly monodisperse state, is used (see Kogyo Zairyo (Industrial Materials), Vol. 44, No. 9, 1996, pp. 68–71); and
a method in which a transparent conductive layer forming coating liquid in which chainlike agglomerates comprised of fine metal particles having agglomerated in the form of chains have been dispersed in advance is used (see Japanese Patent Application Laid-open No. 2000-124662).
To compare the methods (1) with the methods (2), the methods (2) have an advantage that a developed network structure can be formed with ease because the agglomerates of fine metal particles have been completed in advance in the transparent conductive layer forming coating liquid.
On the other hand, there may be other problem that filters tend to clog at the time of filtering treatment of the transparent conductive layer forming coating liquid, or that coating film defects may occur if the agglomeration of fine metal particles has proceeded in excess.
However, the above can be said to be preferable methods from the viewpoint that a transparent conductive layer having good conductivity can be formed as long as the agglomerates of fine metal particles that have been formed in advance in the transparent conductive layer forming coating liquid have sufficiently high dispersion stability and the size of the agglomerates has been controlled to be hundreds of micron or less.
Here, in the methods (2), as methods of forming the agglomerates of fine metal particles in advance in the transparent conductive layer forming coating liquid (or a fine-metal-particle dispersion used in producing the transparent conductive layer forming coating liquid), the following methods (a) to (e) are known as disclosed in, e.g., Japanese Patent Applications Laid-open No. 2000-124662, No. 11-329071 and No. 2000-196287.
(a) A method in which a water-soluble salt such as sodium salt, potassium salt, calcium salt or ammonium salt, an acid such as hydrochloric acid, nitric acid, phosphoric acid or acetic acid or an alkali such as sodium hydroxide or ammonia is added to a dispersion of fine metal particles to make the dispersibility of fine metal particles unstable, to form the agglomerates of fine metal particles.
(b) A method in which, at the stage where fine metal particles dispersed in the transparent conductive layer forming coating liquid are prepared from an aqueous solution of a metal salt, the pH and so forth of the aqueous solution are controlled within stated ranges to form the agglomerates of fine metal particles.
(c) A method in which a dispersion of fine metal particles is kept at tens of degree of temperature which is not higher than the boiling point of a dispersion solvent, for several hours to tens of hours to form the agglomerates of fine metal particles.
(d) A method in which an organic compound such as an alcohol is added to a dispersion of fine metal particles to control the polarity of a dispersion solvent, to form the agglomerates of fine metal particles.
(e) A method in which a dispersion of fine metal particles is subjected to mechanical dispersion treatment such as sand mill treatment or impact dispersion treatment to form the agglomerates of fine metal particles.
Now, in the above methods (a) to (d), the methods (a) and (d) are not practical because they are methods in which the dispersion stability of fine metal particles is made to lower (the zeta potential of the system lowers and the stability lowers) to form the agglomerates and hence, if left as it is, the agglomeration may gradually proceed as the fine metal particles are kept unstable. Accordingly, in order to make the stability of the system higher, it is necessary to remove any destabilization factor(s) [in the method (a), the water soluble salt, the acid or the alkali; in the method (d), the organic compound such as an alcohol]. However, this step is so complicated that these methods have not been preferable methods.
The method (c) is a simple method because the dispersion of fine metal particles may only be kept heated. However, such a transparent conductive layer forming coating liquid of the kind that originally the agglomerates are formed by heating at tens of degree of temperature can not be said to ensure high dispersion stability of the fine metal particles themselves contained therein. Hence, there has been a problem that the agglomerates formed have also a low dispersion stability. If on the other hand the fine metal particles themselves have a high dispersion stability, it takes a long time to form the agglomerates by heating at tens of degree of temperature. Thus, this method can not still be said to be practical.
The method (b) is a method in which the agglomerates of fine metal particles are formed at the stage where the fine metal particles are prepared from an aqueous metal salt solution. Hence, there is a problem that the agglomerates further agglomerate one another and settle in, e.g., a concentrating step taken thereafter for preparing the transparent conductive layer forming coating liquid, and further it is necessary to determine the state of agglomeration of fine metal particles in advance. Thus, this method has been inconvenient in that the state of agglomeration of the fine metal particles can not be changed at will in the subsequent stage.
In addition, the method (e) is a method in which mechanical dispersion treatment is carried out to form the agglomerates of fine metal particles, and hence it has had a problem that it requires an expensive treatment equipment and also the step of treatment can not be said to be simple.