Electric lamp and discharge devices – Cathode ray tube – Envelope
Reexamination Certificate
1999-05-07
2002-06-11
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Envelope
C313S478000, C313S47700R
Reexamination Certificate
active
06404120
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a light transmissive substrate carrying a light transmissive low ohmic coating and in particular to a cathode ray tube comprising a display screen carrying a light transmissive low ohmic coating.
The invention further relates to a method of manufacturing an low ohmic coating on a substrate.
BACKGROUND AND SUMMARY OF THE INVENTION
Electroconductive coatings are inter alia used as antistatic layers on display screens of display devices, in particular cathode ray tubes (CRTs). Said layers have a sheet resistance, for example, of 10
6
to 10
10
&OHgr;/□ and are hence sufficiently electroconductive to ensure that a high electrostatic voltage present on the outside surface of the display screen is removed within a few seconds. Thus, the user does not experience an unpleasant shock if he touches the screen. Besides, the attraction of atmospheric dust is reduced.
Since electromagnetic radiation may be hazardous to health, shielding from electromagnetic radiation is becoming ever more important. Cathode ray tubes, such as display tubes for TVs and monitor tubes, comprise a number of radiation sources which may be hazardous to the user's health if he is exposed to said sources for a long period of time. A substantial part of the electromagnetic radiation generated can be screened off with metal in a simple manner via the housing of the cathode ray tube. However, radiation emitted via the display screen may substantially add to the amount of radiation to which the user is exposed.
This problem is solved by applying a well (electrically) conducting coating on the surface of the display screen. Said coating must also be sufficiently transparent in the wavelength range of from 400 to 700 nm, i.e. the transmission must be at least 60%. A well-known material which can be used for a transparent and well-conducting coating which meets said requirements is indium-doped tin oxide (ITO). Such a layer can be provided by means of vacuum evaporation or sputtering. Said method requires, however, expensive vacuum equipment. ITO layers can also be manufactured by firing spin-coated or sprayed layers of solutions of indium-tin salts. Said firing operation should be carried out at a temperature of at least 300° C. This temperature is much too high to be used with a complete display tube which, in order to preclude damage to parts of the display tube, can withstand temperatures of maximally 160° C.
In German Patent Application DE-A4229192, a description is given of the manufacture of an antistatic coating for, inter alia, a display screen, said coating being made from poly-3,4-ethylene dioxythiophene and a trialkoxysilane to improve the adhesion. By way of example, a coating is manufactured by providing a desalinated aqueous solution of poly-3,4-ethylene dioxythiophene, polystyrene sulphonic acid and 3-glycidoxypropyl trimethoxysilane on a glass plate, whereafter said glass plate is dried. Said poly-3,4-ethylene dioxythiophene is previously prepared by oxidatively polymerizing the monomer 3,4-ethylene dioxythiophene by means of an Fe(III) salt in water in the presence of polystyrene sulphonic acid to preclude precipitation. The antistatic layer thus obtained has a thickness of 0.6 &mgr;m (600 nm) and a sheet resistance of 50 k&OHgr;/□. This sheet resistance is sufficient to bring about an antistatic effect.
A disadvantage of said known layer is that the shielding against electromagnetic radiation is insufficient. Future standards require the electrical field intensity measured at a distance of 0.3 m from the display screen to be maximally 10 V/m in the frequency range 50 Hz-2 kHz and 1 V/m in the frequency range 2-400 kHz. Experiments have shown that in order to meet these requirements the sheet resistance must be below 3 k&OHgr;/□ and preferably maximally 1 k&OHgr;/□, taking into account that the sheet resistance may increase with time.
A property of the known antistatic layer is that it is of a blue colour, although it is transparent. Since the sheet resistance is inversely proportional to the layer thickness, a greater layer thickness will lead to a lower sheet resistance. However, as a result thereof the transmission of the layer in the orange-red wavelength range decreases substantially and the blue colour becomes even more intense.
It is an object of the invention to provide, inter alia, a substrate, like a display screen of a cathode ray tube, carrying a coating, said coating providing an effective shield against electromagnetic radiation and exhibiting good optical properties, such as a transmission of at least 60% in the wavelength range of from 400 to 600 nm. Preferably, the layer must be compatible with additional antireflective layers. A further object of the invention is to provide a simple method of manufacturing such light transmissive well-conducting coatings, and it must be possible, in particular, to carry out said method at relatively low temperatures (maximally 160° C.) at which no damage is caused to parts of a cathode ray tube.
These objects are achieved by a coated substrate as described in the opening paragraph, which is characterised according to the invention in that the coating is a mixed organic conductive polymer/transparent metal oxide coating having a layer thickness between 100 and 600 nm and a sheet resistance of less than 1 k&OHgr;/□. Depending on the thickness and/or the transparent metal oxide amount the sheet resistance can be between 100 and 600 &OHgr;/□. In accordance with the above-mentioned requirements, such a layer provides an excellent shield against electromagnetic fields. In addition, the composition of the coating is such that it can exhibit a transmission in excess of 60% in the wavelength range of from 400 to 700 nm. Metal oxides, like TiO
2
, and in particular SiO
2
are suited for use in the mixed conductive polymer/transparent metal oxide coating.
The much lower sheet resistance of the coating in accordance with the invention as compared to the known coating is obtainable by the method of preparing the coating as described hereinbelow.
An electroconductive coating in accordance with the invention, optionally with one or more additional scratch resistant layers can also suitably be used as a touch screen coating on a CRT or LCD display screen. By touching a certain part of the touch screen coating on the display screen, a local change in resistance is induced which is translated, via electronic controls, into a localisation and a subsequent action, such as opening a menu, turning pages etc. It is alternatively possible to write on the display screen with a pen, whereafter the writing is identified and processed.
For the additional layer use can possibly be made of a silicon dioxide layer having a thickness of from 50 to 250 nm. Using a tetraalkoxysilane, such as TEOS, as the precursor, such a layer can be provided in a simple manner by means of a sol-gel process, followed by curing at a relatively low temperature (≦160° C.).
The object of providing a simple method of manufacturing a transmissive electroconductive coating on a substrate (like a display screen of a cathode ray tube) is achieved in an embodiment in that the coating is manufactured by applying a layer of a solution of 3,4-ethylene dioxythiophene and an Fe(III) salt on the substrate, whereafter a treatment at an increased temperature is carried out, thereby forming a layer comprising poly-3,4-ethylene dioxythiophene and an Fe(III) salt, after which the layer is rinsed with an ethanolic metal oxide precursor, such as a SiO
2
precursor (e.g. a tetra alkoxy silane like TEOS) which is capable to extract Fe salts, thereby forming the electroconductive coating. Optionally an organic base can be added, a.o. to stabilise the system.
In general, polymers are slightly soluble in solvents such as alcohols. In order to obtain a processable polymeric solution, in the known method the polymerisation reaction is carried out in the presence of a large quantity of a stabilising polymer, such as polystyrene sulph
Aben Gerardus V. A.
Somers Michel J. M.
Guharay Karabi
Koninklijke Philips Electronics , N.V.
Patel Nimeshkumar D.
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