Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
2002-05-24
2003-07-29
Vu, David (Department: 2821)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
C148S336000, C420S094000
Reexamination Certificate
active
06600259
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a semi-tension mask (also called semi-stretched-tension mask or SST mask) formed of a Fe—Ni alloy for use in a cathode-ray tube or picture tube (formerly Braun tube) and, more particularly, to a Fe—Ni alloy which has excellent magnetic properties and is outstandingly capable of controlling beam drift under the influence of geomagnetism of a mask made of it, a semi-tension mask made of the alloy, and a color picture tube using the semi-tension mask.
PRIOR ART
A picture tube is provided with a perforated mask as a mechanism through which electron beams from electron guns precisely strike the intended color phosphor dot on a phosphor screen to give a desired color tone. Masks for picture tubes are roughly divided into two types; shadow mask type or a mask workpiece formed with dots or slots for the passage of electron beams by etching and then formed to the mask shape by pressing, and aperture grille type or a mask workpiece formed with elongated slits for the passage of electron beams by etching and then pulled vertically and tentered or stretched on a frame.
For the shadow mask type, Fe-36% Ni alloy (invar) is commonly used as it controls the doming phenomenon that results from thermal expansion. For the aperture grille type whose structural features rarely allow doming due to thermal expansion, mild steel that is higher in the coefficient of thermal expansion but is less expensive is employed.
With both advantages and disadvantages the two types have dominated the market. More recently, what is known as semi-tension mask has come on the scene as a new type combining the merits of the former two.
The semi-tension mask is made by vertically stretching a mask workpiece that has been formed with dots or slots for the passage of electron beams by etching and supporting it on a frame (tentering) rather than pressing the sheet, as in the aperture grille type. In the early stage of its development, the mask needed its workpiece be stretched not only vertically but also horizontally, i.e., in a total of four directions. The four-direction stretching sometimes broke the mask workpieces. To preclude this danger, experiments were made on stretching the workpieces only in two vertical directions and favorable results were obtained. The mask made by the improved method of stretching the workpiece with weak forces in two directions, rather than in four directions, is designated semi-stretched tension mask, or briefly semi-tension mask.
FIGS. 1 and 2
are schematic views illustrating, respectively, a semi-tension type mask and an aperture grille type mask. Both masks are stretched vertically, or upwardly and downwardly. In a semi-tension mask a multiplicity of columns of vertical slots are formed from edge to edge. Each column of slots comprises a number of slots separated by bridges formed between the ends of adjacent slots. An aperture grille mask, by contrast, has a multiplicity of long vertical slits extending from nearly top to nearly bottom and arranged sidewise from edge to edge. It requires damper wires to control the vibration of the mask that is caused by sound sources such as speakers. The bridges of a semi-tension mask are the portions left unetched when the slots have been formed by etching. They serve to prevent the columns of vertical slots from being twisted. Because of the presence of bridges in the individual columns of slots, the mask is also called a bridged tension mask.
Compared with the shadow mask type formed by press work, the semi-tension mask type permits the picture tube to be more flattened with greater brightness and higher resolution. Moreover, the presence of bridges enables the semi-tension type to be superior to the aperture grille type in vibration resistance, with no necessity of damper wire. Further, the former requires relatively low loads for vertical stretching, which helps reduce the cost.
On the other hand, the semi-tension mask type has the possibility of doming with thermal expansion due to the low stretching force and bridging, as compared with the aperture grille type. To prevent the phenomenon, the use of Fe—Ni alloys of low thermal expansion, especially invar, is under study. It has, however, been found that the use of invar that has hitherto been used for superfine color display in the manufacture of a semi-tension mask causes a problem of serious beam drift along edges of the mask. Ordinarily a semi-tension mask is made substantially flat for flat picture tubes (center height of the curved screen/diagonal length of the screen <0.1%). Thus at the edges of the mask the angle that a beam makes with the mask is small at the edges, with the consequence that a slight beam drift of an electron beam from an electron gun results in an increased amount of mislanding on the phosphor. One of the intended features of a semi-tension mask is to obtain high luminance by increasing the aperture ratio. This sacrifices the magnetic shield characteristic of the mask itself to such an extent that beam drift is likely to take place.
Another problem is wrinkling of the mask. In the manufacture of a semi-tension mask a workpiece formed with dots or slots by etching is blackened, welded to a frame, and stretched under predetermined loads. Here the term “blacken” is used to mean a treatment in which a mask workpiece is heated in a steam or combustion gas atmosphere to form a black film such as an iron oxide film on the surface so that the mask looks black. The blackened work is stretched under given loads, welded to a frame, and baked to be freed of strains that have resulted from welding and other operation. It has recently been found that, during the baking, the invar sheet under tension by the frame undergoes plastic deformation at elevated temperature, or creeping. Once it occurs, creeping causes elongation of the mask and the “tension down” or relief of loads which, in turn, lead to wrinkling, deterioration of vibration characteristics, and various other problems. Thus the drawbacks of the semi-tension mask type include the tension down upon baking in the course of the fabrication, leading to wrinkling and other major troubles.
PROBLEMS THAT THE INVENTION IS TO SOLVE
From the viewpoint of thermal expansion, mild steel and the like are not suited as materials for semi-tension mask, and it has been necessary to use a low-thermal-expansion alloy such as invar, with improved magnetic shield characteristic. When used in the manufacture of a semi-tension mask, the invar that is used for a shadow mask of the pressed type with no magnetic shield problem, deteriorates the magnetic shield characteristic of the semi-tension mask. Careful investigation of the manufacturing steps has revealed that this difference is caused by a significant change in the magnetic characteristic of the material with stretching, i.e., before and after the stretching.
To be more concrete, it is necessary with the pressed type shadow mask to allow its bridges (the portions between the apertures in the form of dots or slots through which electron beams pass) to have sufficient (self-shape-retaining) strength to keep its shape (curvature). In order to attain high luminance, the etching factor (amount of etching in the depth direction/amount of side etching) should be high so as to form a multiplicity of dots or slots by etching while securing the bridges, and for that purpose it is customary to treat the workpiece in such a manner as to avoid the collection in the rolled surface of the closest packed (111) plane where the corrosion rate is low. We tentered the material on a frame, stretching it in the rolling direction (the <100>direction) and in the directions (<110> directions) at angles of 90° and 45° to the rolling direction, and these magnetic properties were determined. With a geomagnetic shield after AC demagnetization as in a picture tube, the higher the residual magnetic flux density (Br) and the smaller the coercive force (Hc), the better the magnetic shield properties. Hence the magnetic shield properties are improved as the
Mori Masazumi
Ono Toshiyuki
Drinker Biddle & Reath LLP
Nippon Mining & Metals Co., Ltd.
Vu David
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