Abstract:
Implosion proof structure in a flat cathode ray tube having a panel to which the atmospheric pressure is exerted as the flat cathode ray tube is evacuated, including implosion proof means strapped or coated on an outer circumferential surface of a funnel in the vicinity of the panel, thereby enhancing an implosion proof strength of the flat cathode ray tube.

Description:
This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application Nos. P1999-56497 filed in Korea on Dec. 10, 1999, P2000-30319 filed in Korea on Jun. 2, 2000, and P2000-32775 filed in Korea on Jun. 14, 2000, which are herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flat cathode ray tube, and more particularly, to an implosion proof structure in a flat cathode ray tube for preventing implosion of the flat cathode ray tube. 
     2. Background of the Related Art 
     Referring to FIG. 1, a related art flat cathode ray tube is provided with a planar panel  1 , a funnel  3  smoothly curved from a sealing surface to the panel to a neck portion  3   a  having an electron gun sealed therein, welded to the panel  1  with Frit glass, and an electron gun  4  sealed in the neck portion for emitting red, green and blue electron beams toward the panel. In detail, there is a piece  2  of explosion proof glass attached to a front face of the panel for enhancing an explosion proof property of the panel  1 , and a fluorescent film  5  on an inside surface of the panel for emitting light as electron beams strike the fluorescent film  5 . There is a rectangular rail  6  on an inside surface of the panel, and a shadow mask  7  fitted to the rail  6  in an effective surface of the panel  1  having multiple slits for selecting a color from the electron beams. There is an inner shield  8  fixed at the rear of the rail for protecting the electron beams emitted from the electron gun and travelling toward the panel from geomagnetism, and a deflection yoke  9  on an outer circumferential surface of the neck portion of the funnel for deflecting the electron beams in horizontal and vertical directions. There is a band  11  strapped around the panel  1  for fastening a plurality of lugs  10  to an outer circumference of the panel  1 , for use in fastening the foregoing flat cathode ray tube to a sash of a monitor or a TV receiver. 
     Accordingly, when power is provided to the electron gun  4  sealed in the neck portion  3   a , to emit thermal electrons, the emitted electrons are accelerated and focused as they pass through a plurality of electrodes in succession, and are directed toward a screen side while being deflected in a vertical and a horizontal direction by the deflection yoke  9 . The electron beams emitted from the electron gun  4  and directed toward the screen side are involved in color selection as they pass through fine holes in the shadow mask  7 , and strike fluorescent material in the fluorescent film  5 . Eventually, a picture is reproduced as the fluorescent material emits lights resulting from an energy difference occurring when electrons in the fluorescent material is first excited and then dropped down to a base state. In order to enhance the electron emission, the cathode ray tube is passed through an evacuation process during its fabrication for keeping an inside of the cathode ray tube under a vacuum in a 10 −6 ˜10 −7  Torr range. 
     The evacuation process for the related art flat cathode ray tube will be explained, briefly. 
     Once the cathode ray tube having the funnel  3  fitted to the flat panel  1  is subjected to the evacuation process, and vacuumed down to a range of 10 −7 ˜10 −8  Torr, there is a pressure difference between an inside and outside of the cathode ray tube of at least 10 −6  Torr since outside of the cathode ray tube is at a 760 Torr atmospheric pressure. That is, the cathode ray tube is under a pressure of one atmosphere, i.e., 1.01325×10 5  N/m 2  pressure at all points thereof. Consequently, the panel and the funnel are deformed by the pressure until the outer and inner pressures come to an equilibrium, particularly, the panel  1  collapses in an inward direction of the cathode ray tube in a “c” direction in FIG.  2 . Moreover, as a provision for fixing the cathode ray tube that has been subject to the evacuation process to the sash of the monitor or the TV receiver, if the band assembly of the lugs  10  and the band  1  is strapped around the panel  1  under tension, the inward collapse of the panel becomes more serious. That is, as shown in FIG. 2, in the related art implosion proof structure in the cathode ray tube, the strapping of the band  11  around the panel  1  with a tension, having an inward deformation along an axis of the tube of a bulb (the panel plus the funnel) having been through the evacuation process, makes the deformation more serious. Because stress is greater in the vicinity of a sealed surface of the panel  1  and the funnel  3 , breakage of the cathode ray tube may occur in the vicinity of the sealed surface due to permanent stresses coming from the one atmosphere pressure difference caused by the evacuation and the strapping force caused by the band around the panel  1 . Accordingly, the panel is susceptible to an implosion, in which the cathode ray tube may implode even by a small external impact, and may result in poor picture quality since a front face of the panel is not flat. 
     For preventing such an implosion of the panel, as an example, the panel in the related art flat cathode ray tube has a thickness at a central portion thereof set greater than a thickness the same region of a cathode ray tube with a conventional radius of curvature. However, the thicker panel causes the following problem. 
     In the evacuation process of the cathode ray tube during fabrication, the bulb is heated to a temperature in a range of approx. 340˜360° C. for extracting gas adsorbed in an inside surface of the bulb. A heat generated at a heater in a furnace heats an outer surface of the bulb by means of convection, and the heat at the outer surface of the bulb is transferred to the inside surface of the bulb by conduction. While glass has a thermal conductivity in a range of approximately 0.92×10−3(W/mm°K), the rail, a metal, has a thermal conductivity in a range of approximately 22.8×10−3(W/mm°K), i.e., the thermal conductivity of glass is relatively lower than metal. As heat conduction is inversely proportional to a thickness of the panel, the bulb may be broken by thermal stress resulting from a temperature difference between an inner surface and an outer surface of the bulb which becomes the greater as the thickness of the flat panel  1  increases. On the other hand, in a Frit sealing process in which the panel  1  and the funnel  3  are sealed with Frit glass carried out before the evacuation, when the Frit glass is crystallized to seal the panel  1  and the funnel  2 , the bulb is required to be heated up to approximately 440° C. according to a crystallization characteristic of the Frit glass. Therefore, when the thickness of the panel  1  is great, the bulb may be broken by a temperature difference between the inner surface and the outer surface of the bulb. In order to minimize such breakage, the heating process is required to be prolonged for heating the bulb slowly in an attempt to reduce the temperature difference between the inner surface and the outer surface of the bulb, which deteriorates yield, requires a greater time period for fabrication, and requires an increased amount of heat energy. In a case in which the panel  1  has a thickness equal to, or greater than 18.0 mm, a tint glass application with a light transmittivity of 75% at a thickness of 10.16 mm shows a light transmittivity below 40%, and a dark tint glass application with a light transmittivity of 46% at a thickness of 10.16 mm shows a light transmittivity below 28% (which is actually impossible to apply). Accordingly, there may be a limitation imposed on the design of the bulb in that only a clear glass application with a light transmittivity of 86% at a thickness of 10.16 mm and a semi-clear glass application with a light transmittivity of 82% at a thickness of 10.16 mm are possible. Because the bulb is liable to break when an external impact is applied if the permanent stress caused by the vacuum is excessive, an allowable vacuum stress is restricted to be below 85˜120 kgf/cm 2  (kilogram-force per square centimeter). 
     Furthermore, as another example of the related art implosion proof structure, since the flat cathode ray tube has a low implosion proof strength, an implosion proof glass is attached to a front face of the panel by using a resin for absorbing an external impact to the cathode ray tube. However, since a lamination process for attaching the implosion proof glass is required to be carried out in a separate clean room where a state of cleanness is maintained sufficient to prevent occurrence of foreign matter or blow holes, the fabrication process becomes complicated, and increases production costs. Further blow holes occur in the lamination process, defects increase in the cathode ray tube, and productivity is poor. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an implosion proof structure in a flat cathode ray tube that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an implosion proof structure in a flat cathode ray tube, which can moderate stress in the panel for enhancing an implosion proof strength of the cathode ray tube and preventing implosion of the cathode ray tube. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the implosion proof structure in a flat cathode ray tube having a panel the atmospheric pressure exerts thereto as the flat cathode ray tube is evacuated includes implosion proof means strapped or coated on an outer circumferential surface of a funnel in the vicinity of the panel. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: 
     In the drawings: 
     FIG. 1 illustrates a longitudinal section of a related art flat cathode ray tube; 
     FIG. 2 illustrates deformation of a panel in a related art flat cathode ray tube when evacuated, schematically; 
     FIG. 3 illustrates a side view with a partial cut away view of a flat cathode ray tube in accordance with a first preferred embodiment of the present invention; 
     FIG. 4 illustrates deformation of a panel in a flat cathode ray tube in accordance with a first preferred embodiment of the present invention when evacuated, schematically; 
     FIG. 5 illustrates a side view with a partial cut away view of a flat cathode ray tube in accordance with a second preferred embodiment of the present invention; and, 
     FIG. 6 illustrates a partial side sectional view of a flat cathode ray tube in accordance with a third preferred embodiment of the present invention, schematically. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. FIG. 3 illustrates a side view with a partial cut away view of a flat cathode ray tube in accordance with a first preferred embodiment of the present invention. 
     Referring to FIG. 3, the implosion proof structure in a flat cathode ray tube in accordance with a first preferred embodiment of the present invention includes a band  110  strapped around a flat portion of a funnel which is perpendicular to the panel for fastening lugs  100  thereto, to fasten the cathode ray tube to a sash of a monitor or TV receiver. In this instance, the band  110  is required to be strapped with a tension in a range of 600 kgf˜3000 kgf. If the tension is below 600 kgf, an amount of restoration (from a deformation caused by the evacuation to an original form by the tension of the band) is less than 10%, which is not a substantial improvement of the cathode ray tube deformation. On the other hand, since the improvement of the cathode ray tube deformation brought about when the tension is greater than 3000 kgf is almost the same when the tension is below 3000 kgf, no substantial improvement of the deformation is expected. That is, if the tension of the band  110  is below 600 kgf, an effect of the improvement is not enough in that the improvement is below 10%, while the tension greater than 3000 kgf provides little improvement of the deformation in comparison to a case of tension below 3000 kgf. An outer circumferential surface of the funnel  30  (the band  110  being fastened thereto) is a flat portion  120  perpendicular to the panel  101  larger than a width of the band  110 , for preventing the band  110  from slipping away from a proper position, while making strapping stable. 
     It is preferable that the width of the flat portion  120  of the funnel  30  to which the band  110  is strapped is larger than 16 mm for the following reasons. 
     As in the case of the related art, if the panel  101  and the funnel  30 , sealed together, are subjected to the evacuation process, contraction occurs, in which a central portion of the panel  101  collapses toward an inside of the cathode ray tube. As shown in FIG. 4, when the band  110  is strapped on the flat portion  120 , an outer circumferential surface of the funnel  30 , in the vicinity of the panel  101  under proper tension, strapping tension of the band is exerted in a direction shown as “a”. According to this, a rim of the panel displaces in a direction shown as “b”, and the central portion of the panel displaces in a direction shown as “c”. Those displacements restore the displacement of the panel  101  caused by the evacuation to a state that is close to an original state. The width of the band  110  versus the strapping tension can be expressed by the equation (1), below. 
     
       
           W=T /( t×σ )  (1), 
       
     
     where, ‘W’ denotes a width of the band, ‘t’ denotes a thickness of the band, ‘T’ denotes the strapping tension, and ‘σ’ denotes a yielding strength of the band. In general, a material used as the band in the cathode ray tube has the yielding strength of approx. 32 kgf/cm 2 , and ‘t’ in a range of 1.2 mm. Therefore, according to the equation (1), it can be known that the width of the band  110  is required to be at least 16 mm in order to have the strapping tension of the band greater than 600 kgf. According to this, it can also be known that a width of the flat portion  120  of the outer circumferential surface of the funnel formed perpendicular to the panel  101  is required to be at least 16 mm for stable fastening of the band  110  around the outer circumferential surface of the funnel  3 . 
     FIG. 5 illustrates a side view with a partial cut away view of a flat cathode ray tube in accordance with a second preferred embodiment of the present invention, wherein the band  110  in the first embodiment is replaced with a wire  130  in the second embodiment. Since a strapping tension of the wire  130  is also required to be in a range of 600 kgf˜3000 kgf in strapping the wire  130 , conditions for the wire  130  can be derived from the equation (1), as follows. 
     
       
           W=T /( t×σ )  (1), 
       
     
     where, ‘T’ denotes the strapping tension, and ‘σ’ denotes a yielding strength of the wire  130 , and a sectional area of the band W×t may be substituted with π×R 2 , to express the radius &gt;R= of the wire as an equation (2) below. 
     
       
           R=√T /(π×σ) 
       
     
     where, T denotes the strapping tension, σ denotes a yielding strength of the wire  130 , and R is a radius of the wire  130 . For example, if a wire  130  of a chrome steel with a yield strength 41.8 kgf/mm 2  is used, a required radius of the wire  130  is 2.5 mm or greater from the equation (2), when the strapping tension is greater than 600 kgf. 
     A deformation behaviour of the flat cathode ray tube with the aforementioned implosion proof structure in accordance with a second preferred embodiment of the present invention will be explained. 
     In the evacuation process after the funnel is welded to the panel, and the electron gun is sealed in the funnel, as shown in FIG. 2, the cathode ray tube is involved in collapse of the central portion of the panel toward an inside of the cathode ray tube, and the rim extended outward. In this instance, when the band and the wire  130  have the same sectional areas, the wire  130  has a contact area smaller than the band, to require a smaller width of the flat portion than the band. As shown in FIG. 4, when the band or wire  130  is strapped around the outer circumferential surface  120  of the funnel in the vicinity of a welded region of the panel and the funnel of the foregoing flat cathode ray tube with a tension, the strapping tension is applied in an “a” direction, so that the rim of the panel displaces in the “b” direction, and the central portion of the panel displaces in a “c” direction, offsetting the deformation caused by the evacuation, and restoring the cathode ray tube to a form close to a form before the evacuation. Since the offsetting of the deformation reduces the permanent stress in the flat cathode ray tube, the flat cathode ray tube is made to have an anti-implosion strength which can withstand an external impact. 
     FIG. 6 illustrates a partial side sectional view of a flat cathode ray tube in accordance with a third preferred embodiment of the present invention schematically, wherein a hardening adhesive  140  is applied to an outer circumferential surface of a front portion of the funnel in the vicinity of a welded region of the panel  101  and the funnel  30 . The hardening adhesive  140  is of a material hardened by oxygen, heat, or water to have a certain tensile strength (such as a ceramic adhesive). In the evacuation, the deformation (and a consequential tensile stress) occurs at the welded region of the panel and the funnel mostly, i.e., a force {circle around ( 2 )} applied to the funnel from the atmospheric pressure causes a maximum vacuum stress at the welded region in a short axis direction of the panel and the funnel, which in turn causes a deformation of the cathode ray tube as shown in dashed lines in FIG.  6 . However, the hardening adhesive  140  coated on the outer circumferential surface of the funnel forms a force {circle around ( 1 )} opposing the force {circle around ( 2 )} from the atmospheric pressure which exerts in a direction the panel collapses toward the inside of the cathode ray tube, and makes a balance against the force {circle around ( 2 )} from the atmospheric pressure, restoring the flat cathode ray tube to form before the evacuation as shown in solid line in FIG. 6, which may be described in detail as follows. 
     The force from the hardening adhesive  14  to the panel of the cathode ray tube can be defined similar to the equation (1) as shown below. 
     
       
           W=Ta /( t×σ )  (1), 
       
     
     where, Ta denotes the force applied to the cathode ray tube from atmospheric pressure, and, since it is required to apply a strapping tension at least equal to the Ta, the hardening adhesive is required to have a yield strength ‘σ’, a thickness ‘t’ and a width ‘W’. And, the strapping force from the hardening adhesive  140  to the outer circumferential surface of the funnel can be expressed as an equation (3), below. 
     
       
           T=p×R×W   (3), 
       
     
     where, ‘T’ denotes the strapping force from the hardening adhesive  140  to the funnel, ‘p’ denotes a pressure from a unit area of the hardening adhesive, and R denotes an outer circumference of the funnel, and ‘W’ denotes a width of the hardening adhesive. Because the force from the hardening adhesive  140  to the outer circumference of the funnel  3  is required to be equal to, or greater than the force from the atmospheric pressure to the panel, for prevention of the deformation of the panel, a relation of the equations (1) and (2) can be expressed as inequalities shown below. 
     
       
           T≧Ta , and  p×R≧σ×t   (4). 
       
     
     That is, since the force Ta from the atmospheric pressure to the cathode ray tube is constant, after the yield strength of the hardening adhesive is fixed, the thickness &gt;t= and the width ‘W’ are fixed according to equations (1) and (4), i.e., t≧Ta/(σ×W), and W≧Ta(p×R). And, in order to make the hardening adhesive to compress the flat cathode ray tube effectively, it is required to set a difference of thermal expansion/contraction coefficients between the hardening adhesive (after hardening) and the funnel to be approx. 5×10 −7 /° C., for maintaining constant compression as the hardening adhesive and the funnel  140  expand/contract in similar ratios when heat is generated by the electron beams upon operation of the flat cathode ray tube. If the hardening adhesive  140  has a small thermal expansion coefficient, the hardening adhesive  140  expands less than the funnel when the flat cathode ray tube is in operation, compressing the funnel excessively, and bulges the panel forward. If the hardening adhesive  140  has a great thermal expansion coefficient, the hardening adhesive  140  expands larger than the funnel, failing to compress the funnel effectively (collapse of the panel is occurs). 
     As an example of such a coating of hardening adhesive, the width and the thickness of a ceramic adhesive coated on a 17″ cathode ray tube will be calculated. In this instance, as the atmospheric pressure is 0.01034 kg/mm 2  and the 17″ flat cathode ray tube has a panel area of 97900 mm 2 , the force T from the atmospheric pressure to the front face of the panel is 1012 kgf. As the ceramic adhesive has a yield strength of 25 kg/mm 2 , and a length of the outer circumference of the funnel is approximately 1260 mm, the thickness ‘t’ of the hardening adhesive  140  is set to be 0.5 mm since t≧Ta/(σ×W) according to the equation (1). Then, a pressure per unit area of the funnel from the ceramic adhesive is 0.0099 kg/mm 2  according to the equation (4). As the width ‘W’ of the ceramic adhesive is W≧Ta/(p×R), the width ‘W’ is greater than 81 mm. 
     Thus, since the displacements that occurred in the evacuation of the flat cathode ray tube are restored by a strapping force of the band, wire, or the hardening of adhesive on and around the funnel, a thickness of the panel can be reduced substantially as the implosion proof strength of the panel is enhanced. That, in turn, facilitates reducing a temperature difference between the inner and the outer circumferential surface of the panel  101  in the Frit sealing, and evacuation processes when the panel  101  and the funnel  30  are welded. That is, tint glass with a reflectivity of 0.045 and a light absorptivity of 0.04626 or clear glass with a reflectivity of 0.045 (the same as the tint glass) and a light absorptivity of 0.00578, are used. If the panel is formed of tint glass, the panel has a thickness of 18.0 mm and a light transmittivity of 40% or below. Eventually, since the present invention permits reduction of the panel thickness, the limitation on the design of the flat cathode ray tube is reduced in that not only the clear glass, but also tint glass can be used. As the panel has a sufficient implosion proof strength, no implosion proof glass is required. In this instance, it is apparent to a person skilled in this field of art that there may be a variety of applications, such as an application wherein the band is not necessarily used for fastening the lugs, but, a band for fastening the lugs may be strapped around the panel and/or another band may be strapped around the funnel. 
     As has been explained, the implosion proof structure in a flat cathode ray tube of the present invention can restore a cathode ray tube to an original form, for preventing an implosion of the cathode ray tube, by strapping or coating a band, wire, or hardening adhesive around the funnel to moderate a permanent stress occurring in the cathode ray tube due to a pressure difference between an inside and outside of the cathode ray tube. The enhancement of the implosion proof strength of the cathode ray tube caused by the strapping or coating eases a limitation of the panel design as even a thin panel can meet an allowable vacuum stress. Since no implosion proof glass is required on the front face of the panel, the fabrication process is simplified, productivity is improved, and production costs are reduced. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the implosion proof structure in a flat cathode ray tube of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come with the scope of the appended claims and their equivalents.