Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
1999-05-12
2001-11-20
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
C313S407000, C313S408000, C445S037000, C445S047000
Reexamination Certificate
active
06320306
ABSTRACT:
The invention relates to a shadow mask for color picture tubes.
BACKGROUND OF THE INVENTION
In a color picture tube having a shadow mask, the mask is arranged in direct proximity to the interior surface of the screen. Because the luminescent segments are produced on the interior surface of the screen, the geometry of the shadow mask is required to conform with the pattern of the luminescent segments when the color picture tube is in operation. Maximum impact accuracy of the electron on the luminescent segments is achieved when the aperture geometry of the shadow mask conforms with the distribution of the luminescent segments on the interior surface of the screen at the operating temperature. However, since only a small portion of the emitted electrons pass through holes in the mask and strike the luminescent segments and the majority of the electrons strike the mask directly, the mask is heated up to 80° C. as a result, giving rise to a change in mask geometry which results in doming of the mask (doming effect).
After doming, the aperture geometry of the shadow mask no longer conforms with the pattern of the luminescent segments, giving rise to imprecise electron impact. The color rendering quality of the screen is disturbed.
With high contrast pictures, different areas of the mask will be heated up to different levels, thus giving rise to partial doming of the mask (local doming) which also results in aberrations when the doming exceeds a tolerance.
A variety of attempts have been made to limit or prevent such disadvantageous thermal behavior of the shadow mask. Thus, various measures have been suggested to limit excessive heating of the mask.
U.S. Pat. No. 3,887,828 suggests applying to the metallic perforated mask a porous manganese dioxide layer and a thin layer of metallic aluminum on top thereof. The aluminum layer has contact with the shadow mask at the aperture edges only. It is electrically conducting and has an electron-absorbing property. Top of the aluminum layer is another layer of graphite, nickel oxide, or nickel iron.
The porosity of the manganese oxide layer is said to originate substantially from the individually arranged particles, the layer being sandwiched by the mask and thin aluminum layer. Due to the layer structure, heat generated by electron impact is intended to be kept away from the metallic perforated mask and emitted in the opposite direction.
This solution has various drawbacks. It has shown that keeping the generated heat away from the perforated mask is not feasible since the major part of the heat is not generated within the aluminum layer and the overlying graphite layer, but in the perforated mask. The electron-reflecting, electron-absorbing, and heat-emitting properties of the aluminum layer are too low. The heat-insulating sandwich structure arranged on top of the perforated mask now results in the opposite effect: The heat can be emitted only with difficulty.
DE 3,125,075 C2 describes an electron-reflecting layer directly coating the shadow mask. This layer contains heavy metals, particularly in the form of their carbides, sulfides or oxides. On electron impact, up to 30% of the electrons can be reflected, which means that the shadow mask is less heated. However, the major part of the electron beam still reaches the shadow mask, giving rise to undesirable heat generation therein and thus, general and local doming phenomena of the shadow mask.
U.S. Pat. No. 4,671,776 suggests coating the shadow mask with borate glass. The glass powder is sprayed onto the mask and is subsequently melted. The glass layer adheres very tightly to the backing. In operating conditions, the doming effect is diminished due to some heat-insulation but the major effect is from tensile forces within the mask resulting from the different expansion coefficients of the layer and the metal of the shadow mask. With such a coating, electron-reflecting effects can hardly be observed, so that a major part of the energy of the impacting electron beam still is transferred to the mask, giving rise to disadvantageous doming behavior.
Moreover, rigid fixation of the mask with a stable glass layer does not meet the higher requirements with respect to color picture quality in the multimedia age.
Another means of significantly limiting the undesirable doming phenomena is the use of high quality metal alloys, such as Invar, for the shadow mask, because this alloy has a particularly favorable thermal expansion coefficient. However, this material is highly expensive with respect to costs.
Moreover, since the cost percentage of the shadow mask with respect to the total cost of a color picture tube is already relatively high, the use of special metal alloys would result in a further increase of costs.
SUMMARY OF THE INVENTION
The object of the invention is now to largely avoid said doming of the shadow mask caused by the action of the electron rays, wherein low-cost steel is to be used as mask material.
According to the invention, the cathode-side surface of a perforated mask is provided with a heat-insulating layer, an electron-reflecting and electron-absorbing layer and a heat-emitting cover layer. Thereby, some of the electrons are reflected, while others are absorbed in the cover layer and transformed to heat, with the heat not acting directly on the perforated mask but being emitted into the interior of the tube due to the arrangement of a heat-insulating layer in accordance with the invention. Local temperature differences, which may give rise to partial doming of the perforated mask are also diminished. Such local temperature differences particularly occur with high contrast pictures.
The heat-insulating layer consists of temperature-resistant porous solids embedded in a binder. According to the invention, oxide, sulfide, silicon and/or aluminophosphate materials or material mixtures are provided. Among others, silicic acid, zirconium dioxide and titanium dioxide are suitable as porous oxides. In particular, the porous siliceous materials include the vast group of zeolites. Particularly suitable are the molecular sieves such as the natural molecular sieves chabazite, mordenite, erionite, faujasite, and clinoptilolite, as well as the synthetic zeolites A, X, Y, L, &bgr;, and/or those of the ZSM type. There is such a wide variety of zeolite structures that all the types cannot be mentioned here. Surprisingly, it was found that effective heat-insulation of the perforated mask can be achieved even with thin layers coating the mask. Likewise, advantageous effects result when using porous phosphate solids such as the so-called aluminophosphates, silicoaluminophosphates and metal aluminophosphates which can be produced by synthesis and are classified as small, medium, and large pore types.
Other suitable porous solids are intercalated clay minerals, layer phosphates, and silica gel as well as a variety of aluminosilicates.
More specifically, the electron-reflecting, electron-absorbing, and heat-emitting cover layer combined with the heat-insulating layer includes heavy metal compounds; here, particularly advantageous use can be made of bismuth oxide and bismuth sulfide as well as lead oxide and lead sulfide, and tantalum oxide, cerium oxide and barium titanate.
In particular, crystalline and glass-like silicates, phosphates, and borates are provided as a binder for the cover layer and the heat-insulating layer, whereby water glass, and low-melting glass such as solder glass, as well as metal phosphates were found useful. The binders mentioned are remarkable for their high adhesive properties both on the surface of the mask and between the layers, yielding a coating of extraordinary mechanical stability which results in additional dimensional stability of the perforated mask.
Coating of the layers is effected according to known coating procedures such as, for example, spraying the surface of the mask and therefore can be performed at favorable costs.
The heat-insulating layer has a layer thickness between 10 and 50&mgr;m with an average particle size between 1 and 10&mgr;m, while the hea
Heine Gunter
Neumann Peter
Schonert Bernhard
Schulke Ulrich
Day Michael H.
Gerike Matthew J.
Leydig , Voit & Mayer, Ltd.
Samsung Display Devices Co. Ltd.
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