Coating processes – Optical element produced – Transparent base
Reexamination Certificate
2002-04-30
2003-03-04
Barr, Michael (Department: 1762)
Coating processes
Optical element produced
Transparent base
C427S165000, C427S402000, C427S404000, C427S419100, C427S376100, C427S376600, C427S383100
Reexamination Certificate
active
06528112
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transparent conductive layered structure having a transparent substrate and a transparent conductive layer and transparent coating layer formed in succession on this substrate, which is used in front panels of display devices such as CRT, etc. The present invention particularly relates to a transparent conductive layered structure with excellent weather resistance, ultraviolet ray resistance, conductivity, etc., and with which a reduction in production cost is expected and a method of producing the same, and a coating liquid for forming a transparent conductive layer used in the production of a transparent conductive layered structure and a method of producing the same.
2. Description of the Prior Art
Many OA devices have been introduced to the office as a result of office automation (OA) in recent years and an environment in which the entire day work must be done facing the display of an OA device is no longer uncommon.
However, when a job is done next to a cathode ray tube (CRT) of a computer, etc., as an example of OA equipment, it must be easy to see the display screen 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 these requirements, etc., 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.
The above-mentioned electromagnetic waves are generated from deflecting coils and fly-back transformers and the development of larger televisions has led to a tendency toward leakage of increasingly larger amounts of electromagnetic waves around televisions.
For the most part, leakage of a magnetic field can be prevented by techniques such as changing the shape of the deflecting coil, etc. On the other hand, it is also possible to prevent leakage of an electric field by coating the front glass surface of a CRT with a transparent conductive layer.
Such methods for preventing leakage of an electric field are theoretically the same as measures that have been adopted in recent years to prevent electrostatic charging. However, the conductivity of the above-mentioned transparent conductive layer must be much higher than that of conductive layers that are formed to prevent electrostatic charging. That is, although surface resistance of 10
8
&OHgr;/□ (ohm per square) is enough to prevent electrostatic charging, a transparent conductive layer with at least as low a resistance as 10
6
&OHgr;/□ or less, preferably 10
3
&OHgr;/□ or less, is preferred in order for prevention of leakage of an electric field (electric field shielding).
Therefore, several suggestions have been made thus far for meeting the above-mentioned requirements, but of these, the method wherein a coating liquid for forming a transparent conductive layer of conductive microparticles dispersed with inorganic binder, such as alkyl silicate, etc., in a solvent is applied to the front glass of a CRT and dried and then baked at a temperature of 200° C. is known as a method with which low cost and low surface resistance can be realized.
In addition, this method that uses a coating liquid for forming a transparent conductive layer is very simple when compared to other methods of forming transparent conductive layers, such as vacuum evaporation and sputtering, has a low production cost, and is a very useful method in terms of electric field shielding treatment of CRTs.
It is known that the above-mentioned coating liquid that is used to form a transparent conductive layer used by this method employs indium tin oxide (ITO) as the conductive microparticles. However, because surface resistance of the film that is obtained is high at 10
4
to 10
6
&OHgr;/□, a corrective circuit for canceling the electric field is needed in order to sufficiently block leakage of an electric field. Therefore, there has been a problem in that production cost rises accordingly. 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 conductive layers that use metal powder for the above-mentioned conductive microparticles. Consequently, there is an advantage in terms of cost because the above-mentioned correcting circuit is not necessary, and this will probably be the prevailing 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 face panel surface to prevent reflection on the screen. 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; (&lgr; is wavelength), 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 parameters that make up 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 when compared to ITO 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 is obtained with the two-layered film structure.
However, as previously mentioned, the metal microparticles used in conventional coating liquid for forming a transparent conductive layer are limited to noble metals such as silver, gold, platinum, rhodium, palladium, etc. Nevertheless, when the specific resistance of these is compared, resistivity of platinum, rhodium, and palladium is 10.6, 5.1, and 10.8 &mgr;&OHgr;·cm, respectively, which is high when compared to the 1.62 and 2.2 &mgr;&OHgr;·cm of silver and gold. Therefore, it is more of an advantage to use silver microparticles and gold microparticles to form a transparent conductive layer with low surface resistance.
There was, however, a problem with weather resistance in that there was severe deterioration due to sulfurization, oxidation, and exposure to ultraviolet rays and brine, when silver microparticles were used, while when gold microparticles were used, there were none of the above-mentioned problems with weather resistance, but there were the same problem with cost as when platinum microparticles, rhodium microparticles, palladium microparticle
Kato Kenji
Yukinobu Masaya
Armstrong Westerman & Hattori, LLP
Barr Michael
Sumitomo Metal & Mining Co., Ltd.
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