Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2002-10-28
2004-03-30
Gupta, Yogendra N. (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C252S512000, C106S001190, C106S001210, C106S001280, C428S434000, C423S022000, C423S023000
Reexamination Certificate
active
06712998
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing a transparent conductive layer forming coating liquid (coating liquid for forming transparent conductive layers) which contains noble-metal-coated fine silver particles coated with gold or platinum alone or a composite of gold and platinum and are used for forming a transparent conductive layer on a transparent substrate. More particularly, it relates to a process for producing a transparent conductive layer forming coating liquid which can form a good transparent conductive layer providing a good reflection preventive effect and a good electric-field shielding effect and also having good transmitted-light profiles in the visible-light region and good weatherability when applied in front panels of display devices such as cathode ray tubes (CRT), plasma display panels (PDP), fluorescent display devices (VFD) and liquid-crystal display devices (LCD).
2. Description of the Related Art
With office automation (OA) made in recent years, a variety of OA instruments have been introduced into offices, and as office environment it has become no longer uncommon to do office works all day while facing display devices of OA instruments. Accordingly, in office works done sitting close to cathode ray tubes (CRTS; also called Braun tubes) of computers as an example of the OA instruments, it is required for display screens to be easy to watch and not to cause visual fatigue, as well as to be free from attraction of dust and electric shock which are due to the electrostatic charging on CRT surfaces.
Moreover, in addition to these, 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. Such electromagnetic waves are generated from deflecting coils and flyback transformers and a large quantity of electromagnetic waves increasingly tend to leak to surroundings as televisions become larger in size.
Now, the leakage of magnetic fields can be prevented in its greater part by designing, e.g., by the changing of deflecting coils in shape. As for the leakage of electric fields, it can be prevented by forming a transparent conductive layer on the front-glass surface of a CRT.
Measures to prevent such leakage of electric fields are theoretically the same as the countermeasures taken in recent years to prevent electrostatic charging. However, the transparent conductive layer is required to have a much higher conductivity than any conductive layers formed for preventing the electrostatic charging. More specifically, a layer with a surface resistance of about 10
8
&OHgr;/square is considered sufficient for the purpose of preventing electrostatic charging. However, in order to prevent the leakage of electric fields (i.e., electric-field shielding), it is necessary to form at least a transparent conductive layer with a low resistance of 10
6
&OHgr;/square or less, preferably 5×10
3
&OHgr;/square or less, and more preferably 10
3
&OHgr;/square or less.
Under such circumstances, as countermeasures for such a necessity, some proposals are made until now. In particular, as a method that can attain a low surface resistance at a low cost, a method is known in which a transparent conductive layer forming coating liquid prepared by dispersing conductive fine particles in a solvent together with an inorganic binder such as an alkyl-silicate is coated on a front glass for a CRT, followed by drying and then baking at a temperature of 200° C. or less.
This method making use of such a transparent conductive layer forming coating liquid is much simpler than any other transparent conductive layer forming methods employing vacuum evaporation (vacuum deposition), sputtering or the like, and can enjoy a low production cost. Thus, it is a method very advantageous as electric-field shielding that can be applied to CRTs.
As the transparent conductive layer forming coating liquid used in this method, a coating liquid is known in which indium tin oxide (ITO) is used as the conductive fine particles. Since, however, the resultant film has a surface resistance of as high as 10
4
to 10
6
&OHgr;/square, a corrective circuit for cancelling electric fields is required in order to sufficiently shield the leaking electric fields. Hence, there has been a problem of a production cost which is rather high correspondingly. Meanwhile, in the case of a transparent conductive layer forming coating liquid making use of a metal powder as the conductive fine particles, the resultant film may have a little lower transmittance than in the case of the coating liquid making use of ITO, but a low-resistance film of from 10
2
to 10
3
&OHgr;/square can be formed. Accordingly, such a coating liquid, which makes the corrective circuit unnecessary, is advantageous in cost and is considered to become prevailing in future.
Fine metal particles used in the above transparent conductive layer forming coating liquid are, as disclosed in Japanese Patent Applications Laid-open No. 8-77832 and No. 9-55175, limited to noble metals such as silver, gold, platinum, rhodium and palladium, which may hardly be oxidized in air. This is because, if fine particles of a metal other than noble metals as exemplified by iron, nickel or cobalt are used, oxide films are necessarily formed on the surfaces of such fine metal particles in the atmosphere, making it impossible to attain a good conductivity as the transparent conductive layer.
From another aspect, in order to make display screens easy to watch, anti-glare treatment is made to the surfaces of face panels so that the screens 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 layer thickness set at ¼ &lgr; and ¼ &lgr;, or ½ &lgr; and ¼ &lgr;, respectively (&lgr;: 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 constants n-ik (n: refractive index; i
2
=−1; k: extinction coefficient), the value of n is small but the value of k is extremely greater than that in ITO, 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).
Now, as stated above, fine metal particles used in the conventional transparent conductive layer forming coating liquid are limited to noble metals such as silver, gold, platinum, rhodium and palladium. To compare specific resistance of these, platinum, rhodium and palladium have a resistivity of 10.6, 5.1 and 10.8 &mgr;&OHgr;·cm, respectively, which are higher than 1.62 and 2.2 &mgr;&OHgr;·cm of silver and gold, respectively. Hence, it has been advantageous to use fine silver particles or fine gold particles in order to form a transparent conductive layer having a low surface resistance.
The use of fine silver particles, however, may cause a great deterioration due to sulfidation, oxidation or exposure to brine and ultraviolet rays to cause a problem on weatherability. On the other hand,
Gupta Yogendra N.
Sumitomo Metal & Mining Co., Ltd.
Vijayakumar Kallambella
Westerman Hattori Daniels & Adrian LLP
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