Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2001-07-24
2003-05-27
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C106S001180, C106S001190
Reexamination Certificate
active
06569359
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transparent conductive layer forming coating liquid for preparing a transparent conductive layer on a transparent substrate. The present invention particularly relates to a transparent conductive layer forming coating liquid that forms a transparent conductive layer with an excellent transmission profile in the range of visible light, weather resistance and provision of an excellent anti-reflection effect and electric field shielding affect in the case that a transparent conductive layered structure on which the above-mentioned transparent conductive layer is formed is used in front panels of display devices such as Braun tubes (CRTs), plasma display panels (PDPs), vacuum fluorescent displays (VFDs), liquid crystal displays (LCDs), and so on.
2. Description of the Related Art
Some conditions are required for a cathode ray tube (also called a Braun tube as mentioned above: CRT) now being used for computer displays, and so on. It must be easy to see the display screen
3
in order to prevent visual fatigue, as well as prevent deposition of dust and electric shock induced by the electrostatic charge on the CRT screen, etc. Furthermore, in addition to the requirements, there has recently been concern over the detrimental effects of low-frequency electromagnetic waves generated by CRTs on the human body and there is a demand for CRTs with which there is no leakage to the outside of such electromagnetic waves.
Further, in recent years, the problems of the above-stated electrostatic charge and leakage of electromagnetic waves are also pointed out in plasma display panels (PDP) used for wall-hung TVs, and so on, as in CRTs.
It is possible to prevent such leakage of electromagnetic waves, for example, by coating the front panel surface of a display with a transparent conductive layer.
The above-stated method for preventing the leakage of electromagnetic waves is theoretically the same as measures that have been adopted in recent years to prevent electrostatic charging. However, conductivity of the above-mentioned transparent conductive layer must be much higher than that of conductive layers that are formed to prevent electrostatic charging (surface resistance of approximately 10
8
to 10
10
&OHgr;/□, ohm per square).
That is, in CRTs, a transparent conductive layer with at least as low a resistance as 10
6
&OHgr;/□ or less, preferably 5×10
3
&OHgr;/□ or less, and more preferably 10
3
&OHgr;/□ or less is preferred for prevention of leakage of an electric field (electric field shielding). On the other hand, in PDPs, 10 &OHgr;/□ or less is demanded, for instance.
Moreover, several suggestions have been made thus far to take measures for the above-mentioned electric field shielding. For instance, in CRTS, there are proposals having been suggested, such as,
(1) a method wherein a coating liquid for forming a transparent conductive layer in which conductive oxide microparticles such as indium tin oxide(ITO) and so on, or metal microparticles dispersed in a solvent is applied to the front glass (a front panel) of a CRT and dried and then baked at a temperature of approximately 200° C. for forming the above-stated transparent conductive layer,
(2) a method for forming a transparent conductive tin oxide layer (a Nesa layer) on a front glass (a front panel) by a high temperature chemical vapor deposition (CVD) method of tin chloride, and
(3) a method for forming a transparent conductive layer on a front glass (a front panel) by sputtering indium tin oxide, titanium oxynitride and so on.
Also in PDPs, several methods have been proposed, such as,
(4) a method for forming a transparent conductive film on the above-stated front panel by sputtering metals such as silver and so on, and
(5) a method for forming a conductive film by setting an conductive mesh made by metal or metal-coated fibers on a front panel at the main device body side of the front panel in PDPs.
However, there are some problems in the method (5) in PDP, that is, although low surface resistance is obtained by using a conductive mesh, the transmittance gets lower and moire occurs. Furthermore, the manufacturing processes for forming a conductive layer are complicated and cost rises accordingly.
On the other hand, the method shown in (1) in CRTs is very simple when compared to other methods of forming a transparent conductive layer such as a CVD method or sputtering method shown in (2) to (4), and has a low production cost. As a result thereof, the method (1) that uses a coating liquid for forming a transparent conductive layer is a very useful method not only in the above-stated CRTs but also in PDPs.
However, in the method shown in (1) that employs conductive oxide microparticles such as indium tin oxide (ITO) and so on, as a coating liquid for forming a transparent conductive layer, surface resistance of the film that is obtained is high at 10
4
to 10
6
&OHgr;/□, which was not sufficient for blocking leakage of an electric field.
On the other hand, when compared to coating liquids that use ITO, a film with somewhat lower transmittance, but also low resistance of 10
2
to 10
3
&OHgr;/□, is obtained with coating liquids for forming transparent conducive layers that employ metal microparticles, and this will probably be the promising method of the future.
Moreover, the metal microparticles that are used in the above-mentioned coating liquid for forming the above-mentioned transparent conductive layer are limited to noble metals, such as silver, gold, platinum, rhodium, palladium, etc., that rarely oxidize in air, as shown in Japanese Patent Applications Laid-Open No. H 8-77832 and Laid-Open No. H 9-55175. This is because if microparticles of a metal other than a noble metal, such as iron, nickel, cobalt, etc., are used, an oxide film is invariably formed on the surface of such metal microparticles in an air atmosphere and good conductivity cannot be obtained as a transparent conductive layer.
Moreover, on the other hand, in order to make the display screen easy to see, anti-glare treatment is performed on the front panel surface to prevent reflection on the screen, for example, in CRTs.
This antiglare treatment is performed by the method whereby fine irregularities are made in the surface in order to increase diffused reflection at the surface, but it cannot be said that this method is a very desirable method because when used, resolution decreases and picture quality drops.
Consequently, it is preferred that antiglare treatment be performed by the interference method whereby the refractive index and film thickness of the transparent film be controlled so that there is destructive interference of the incident light by the reflected light.
A two-layered film structure wherein optical film thickness of film with a high refractive index and film with a low refractive index has been set at ¼&lgr; and ¼&lgr;, or ½&lgr; and ¼&lgr;, respectively, is usually used in order to obtain this type of low-reflection effect of the interference method, and film consisting of the above-mentioned indium tin oxide (ITO) microparticles is also used as this type of film with a high refractive index.
Furthermore, of the optical constant (n-ik, n: refractive index, i
2
=−1, k: extinction coefficient) of metals, the value of n is small, but the value of k is very large, and therefore, even if a transparent conductive layer consisting of metal microparticles is used, the same anti-reflection activity induced by interference of light as seen with ITO (film with a high refractive index) is obtained with the two-layered film structure.
Besides, as for the transparent conductive layered structure wherein the transparent conductive layer of this kind is formed on the transparent substrate, a specific feature is in recent years required to enhance contrast of pictures by controlling the transmittance to be set in the prescribed range (40 to 75%) less than 100% in order to ma
Fujita Kennichi
Kato Kenji
Tofuku Atsushi
Yukinobu Masaya
Kopec Mark
Oliff & Berridg,e PLC
Sumitomo Metal & Mining Co., Ltd.
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