Abstract:
An anode substrate constituted of a conductive film forming substrate and a reinforcing substrate having different thermal expansion coefficient and being bonded together by the arrangement of adhesive layers is disclosed. The substrate can prevent creation of cracks on the conductive film forming substrate when heating and cooling the anode substrate. The adhesive layers are arranged at an interval, each of the adhesive layers being formed into a shape selected from a group consisting of a rectangular strip shape and a curved strip shape. The adhesive layers are arranged in a pattern to be symmetry with respect to a center line of the arrangement of the adhesive layers extending perpendicular to a line connecting both longitudinal ends of the arrangement of the adhesive layer. Furthermore, the adhesive layers include an outer adhesive portion located outward among remaining adhesive layers, and the outer adhesive layers are arranged shorter than the remaining adhesive layers.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority of Japanese Patent Application No. 2010-082381 and the full contents of that application is incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a hermetically sealed vacuum container for a fluorescence emitting tube, such as a fluorescent display tube, more particularly, the present invention relates to a substrate such as an anode substrate constituting the hermetically sealed vacuum container. 
     BACKGROUND OF THE INVENTION 
       FIGS. 3A and 3B  show a conventional vacuum container of a fluorescent display tube (refer to Japanese Patent Application Publication No. 2003-68189).  FIG. 3A  is a perspective cross-sectional view of the vacuum container, and  FIG. 3B  is a cross-sectional view of the vacuum container taken along the line X 1 -X 1  shown in  FIG. 3A . The vacuum container is constituted of an anode substrate  11 , a front substrate  12  and side plates  13 . The anode substrate  11  is provided with a conductive film  14  formed thereon. The conductive film  14  includes an anode electrode made of a thin film forming a fluorescence emitting film and an anode wiring and the like. The vacuum container further includes a cathode electrode C which may be a thermal electron emitting filament. The anode substrate  11 , the front substrate  12  and the side plates  13  are made of glass and are integrally bonded together using frit-glass (not shown). Generally, the substrate and the side plate used for the fluorescent display tube are made of soda-lime glass. However, the use of the soda-lime glass to form the anode substrate  11  provided with the thin conductive film  14  may cause a migration problem which leads to a short-circuit between the electrodes and the wirings of the conductive film  14 . Therefore, the anode substrate  11  is generally made of high strain point glass in order to prevent the migration problem. 
     Furthermore, when forming the thin conductive film on the glass plate, it is desirable to use a thin glass plate to reduce the weight to facilitate the handling of the glass plate. Typically, the glass plate has thickness of about 1.8 mm. However, the glass plate having the thickness of about 1.8 mm is not strong enough for the use as the vacuum container of the fluorescent display tube. In view of this problem, Japanese Patent Application Publication No. H07-302559 proposes to provide a reinforcing glass plate bonded to the glass substrate provided with the thin conductive film.  FIG. 3C  is a cross-sectional view of an example of the conventional vacuum container provided with the anode substrate  11  provided with a reinforcing substrate  112 . More specifically, the anode substrate  11  is constituted of a substrate  111  on which the conductive film  14  is formed (hereinafter referred to as a conductive film forming substrate) and the reinforcing substrate  112  bonded to the conductive film forming substrate  111 . Using frit-glass  113  applied on the reverse surface of the conductive film forming substrate  111  (opposite to the surface of the substrate  111  on which the conductive film  14  in formed). In the anode substrate  11  fully covered with the frit glass  113  on the reverse surface of the conductive film forming substrate  111  shown in  FIG. 3C , air bubbles present between the conductive film forming substrate  111  and the reinforcing substrate  112  cannot be removed or released outside completely when the frit-glass  113  is heated and melted. In addition, the space between the conductive film forming substrate  111  and the reinforcing substrate  112  does not become uniform, because the frit-glass does not spread  113  between the conductive film forming substrate  111  and the reinforcing substrate  112  in an uniform thickness. 
     In view of the problems relating to the anode substrate  11  explained hereinabove, the inventors of the present invention have proposed an anode substrate  21  provided with strip-shaped frit-glass layers FG applied on a conductive film forming substrate  211  as shown in  FIG. 4 .  FIG. 4A  shows a cross-sectional view of the vacuum container having the anode substrate  21 ,  FIG. 4B  shows a cross-sectional view of the vacuum container taken along the line X 2 -X 2  shown in  FIG. 4A , and  FIG. 4C  shows the vacuum container of  FIG. 4A  seen from the direction of the arrow X 3  of  FIG. 4A  in which a reinforcing substrate  212  is eliminated for simplicity.  FIG. 4C  shows cracks created on the conductive film forming substrate  211 . 
     The vacuum container of  FIG. 4A  is constituted of the anode substrate  21 , the front substrate  22  and the side plates  23 . The anode substrate  21  includes the conductive film forming substrate  211 , the reinforcing substrate  212 , and the strip-shaped frit-glass layers FG constituted of the rectangular strip-shaped frit-glass layers FG 1  through FG 11 . The conductive film forming substrate  211  and the reinforcing substrate  212  are bonded together by means of the strip-shaped frit-glass layers FG 1  through FG 11 . The frit-glass layers FG 1  through FG 11  are equal in length and arranged at a predetermined interval. Furthermore, the frit-glass layers FG 1  through FG 11  are arranged so that the respective distance between both longitudinal ends of the frit-glass layers to both transverse ends, namely both upper and lower ends of the conductive film forming substrate  211  shown in  FIG. 4B  are equal. 
     The vacuum container of  FIGS. 4A and 4B  can solve the problems in the vacuum container of  FIGS. 3A through 3C  by forming the strip-shaped frit-glass layers on the conductive film forming substrate. However, there is still a problem in the vacuum container of  FIGS. 4A and 4B . That is, for the vacuum container of  FIGS. 4A and 4B , the conductive film forming substrate  211  is made of an expensive glass plate with a high strain point, while the reinforcing substrate  212  is made of the inexpensive soda-lime glass plate in order to reduce the manufacturing cost of the vacuum container. As a result, when the vacuum container is heated and cooled during a sealing process of the vacuum container, cracks are created at the conductive film forming substrate  211  as shown in  FIG. 4C . In  FIG. 4C , the cracks are created at four locations  211 C on the conductive film forming substrate  211  corresponding to the both longitudinal ends of the frit-glass layers FG 1  and FG 11 , namely outermost the terminating ends of the frit-glass layers FG 1  through FG  11  frit closest to the side plate  23 . 
     The formation of the cracks is caused by the difference in the thermal expansion coefficient between the conductive film forming substrate  211  and the reinforcing substrate  212  due to excessive stress applied locally at the location  211 C when the conductive film forming substrate  211  and the reinforcing substrate  212  having the different thermal expansion coefficient to each other are heated. Further to explanation regarding to the stress applied to the conductive film forming substrate  211  will be explained hereinafter. In this regard, the thermal expansion coefficient of the soda-lime glass is 93×10 −7 /degrees Celsius, the high strained point glass is 85×10 −7 /degrees Celsius and the frit-glass is 78×10 −7 /degrees Celsius. 
     SUMMARY OF THE INVENTION 
     In view of the above problems, an object of the present invention is to provide an anode substrate which can prevent cracks from forming on a conductive film forming substrate by bonding the conductive film forming substrate and a reinforcing substrate having different thermal expansion coefficient to each other by strip-shaped frit-glass layers. 
     In order to achieve the above object, the present invention provides a hermetically sealed vacuum container for a fluorescence emitting tube which comprises a substrate on which a conductive film is formed, a strip-shaped adhesive layers and a reinforcing substrate. The conductive film forming substrate and the reinforcing substrate having different thermal expansion coefficient are bonded together by a plurality of strip-shaped adhesive layers arranged at an interval. The adhesive layers are formed into a shape selected from a group consisting of a rectangular strip shape and a curved strip shape. The strip-shaped adhesive layers arranged in a pattern to be symmetry with respect to the center line of the strip-shaped adhesive layers extending perpendicular to the line connecting both longitudinal ends of the strip-shaped adhesive layers. Furthermore, the strip-shaped adhesive layers include outer adhesive layers located outward among remaining adhesive layers, and the outer adhesive layers are arranged to be shorter than the remaining adhesive layers. 
     Furthermore, the conductive film forming substrate may be made of high strain point glass, the reinforcing substrate may be made of soda-lime glass and the strip-shaped adhesive layers may be made of frit-glass. 
     The strip-shaped adhesive layers include at least two outer adhesive layers on both sides of an array of the adhesive layers, length of which becomes shorter in a stepwise toward the outside. 
     As described above, the substrate used for the vacuum container according to the present invention is constituted of the conductive film forming substrate and the reinforcing substrate having different thermal expansion coefficient and bonded together by the strip-shaped adhesive layers made of grit glass. Since the frit-glass layers arranged at an interval includes the outer frit-glass layers which are shorter than the remaining frit-grass layers, the stress to be applied to the conductive film forming substrate can disperse, thereby reducing the stress applied to one portion on the conductive film forming substrate. Consequently, even the conductive film forming substrate and the reinforcing substrate having different thermal expansion coefficient are bonded together using the strip-shaped frit-glass layers, the cracks on the conductive film forming substrate can be prevented. Furthermore, by arranging the outer adhesive layers to be shorter in a stepwise manner, the stress applied to one portion on the conductive film forming substrate can be significantly reduced. Thus, the cracks on the conductive film forming substrate can be prevented effectively. Furthermore, the curved frit-glass layers can significantly reduce the stress applied on the conductive film forming substrate. 
     According to the present invention, the conductive film forming substrate and the reinforcing substrate are bonded together by the strip-shaped frit-glass layers. Thus, air bubbles present between the conductive film forming substrate and the reinforcing substrate can be completely released outside during the heating and melting process of the frit-glass. In addition, the thickness of the frit-glass layer between the film conductive substrate and the reinforcing substrate can be uniform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic plan views of an anode substrate of a vacuum container according to the present invention, in which  FIG. 1A  illustrates an embodiment strip-shaped frit-glass layers formed on a conductive film forming substrate and  FIG. 1B  illustrates another embodiment of strip-shaped frit-glass layers; 
         FIGS. 2A through 2D  are schematic plan views of an anode substrate of a vacuum container of the present invention illustrating the stress distribution at the conductive film forming substrate of the anode substrate, in which  FIGS. 2A and 2B  illustrate a conventional anode substrate; 
         FIGS. 3A ,  3 B, and  3 C are a perspective, a cross-section, and a partially broken cross-section views of a conventional vacuum container of a fluorescent display tube respectively; and 
         FIGS. 4A through 4C  are a vertical cross-section, a horizontal cross-section, and a plan view of another conventional vacuum container provided with an anode substrate having a conductive film forming substrate and a reinforcing substrate bonded together by mean of strip-shaped frit-glass layers. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will be described below in reference with  FIGS. 1A ,  1 B and  FIGS. 2A through 2D . 
       FIG. 1A  shows an embodiment of a substrate  211  provided with a conductive film (not shown) constituting a vacuum container of the present invention. The basic structure of the vacuum container of the present invention shown in  FIG. 1  is similar to a conventional vacuum container shown in  FIG. 4A . Thus, the same reference numerals are used to indicate the similar components. As shown in  FIG. 1 , the vacuum container of the present invention is constituted of a front substrate (not shown), side plates  23  and an anode substrate on which a conductive film are formed, a reinforcing substrate and a strip-shaped frit-glass layers FG 1  through FG 11 . The conductive film forming substrate  211  and the reinforcing substrate are bonded together by the strip-shaped frit-glass layers FG 1  through FG 11 . The conductive film forming substrate  211  may be made of high strain point glass while the reinforcing substrate may be made of other glass such as soda-lime glass. 
     As shown in  FIG. 1A , the frit-glass layers FG 1  through FG 11  are is formed into a rectangular strip shape. The frit-glass layers FG 1  through FG 11  are arranged on the reverse surface of the conductive film forming substrate  211  at a predetermined interval corresponding to the opposite surface of the substrate  211  on which the conductive films are formed. Among the frit-glass layers FG  1  through FG 11 , the frit-glass layers FG 1  and FG 11  located outermost adjacent to the side plate  23  have shortest longitudinal length, and FG 2  and FG  10  next to the frit glass layers FG 1  and FG 11  are longer than the frit-glass layers FG 1  and FG 11  but shorter than the remaining frit-glass layers FG 3  through FG  9  located inward. Thus, the frit-glass layers FG 1  through FG  11  are arranged so as to satisfy the equation of FG 1 =FG 11 &lt;FG 2 =FG 10 &lt;FG 3 =FG 4 =FG 5 =FG 6 =FG 7 =FG 8 =FG 9 . 
     Each of the frit-glass layers FG 1  through FG  11  extends along the vertical central line SL 1 . As shown in  FIG. 1A , the vertical central line SL 1  passes through the frit-glass layer FG 6  located at the center of the grit glass layers FG 1  through FG 11 , however, if the number of the frit layers is even, then the vertical central line passes through between two centrally located frit-glass layers. Furthermore, the center of each frit-glass layers FG 1  through FG  11  is arranged side by side along the horizontal central line SL 2 . Accordingly, the strip-shaped frit-glass layers are arranged to be symmetric with respect to the vertical central line SL 1 . In other words, an arrangement of the frit-glass layers FG 1  to FG  5  and an arrangement of the frit-glass layers FG 7  to FG  11  are symmetric with respect to the vertical central line SL 1 . The vertical and the horizontal central lines SL 1  and SL 2  are arranged to intersect orthogonally at the center of the conductive film forming substrate  211 . 
     By providing the strip-shaped frit-glass layers of shorter length on the both the frit-glass layers FG  1  through FG 11 , the stress applied to the conductive film forming substrate  211  disperses so that the stress is applied to the locations on the conductive film forming substrate  211  corresponding to the both ends of the frit-glass layers FG 1 , FG 2  and FG 3 , and FG 9 , FG 10 , and FG 11 , and the stress becomes relatively small at that locations. As the result, production of cracks can be prevented. The number of the frit-glass layers is not limited to that disclosed herein. Also, the number of the shorter frit-glass layer may be selected arbitrarily, but it should be at least 1. The greater the number of the shorter frit-glass layers, the smaller the chance of the cracks being created, since the stress applied to the conductive film forming substrate can be dispersed according to the number of the shorter frit-glass layers. 
     In this embodiment shown in  FIG. 1A , the conductive film forming substrate  211  is 91×44 mm in size and 1.8 mm in thickness. The reinforcing substrate (not shown) is 91×44 mm in size which is the same as the conductive film forming substrate  211  and 1.3 mm in thickness. The side plate  23  is 2.35 mm in thickness and 3.5 mm in height. The width of each of the frit-glass layers FG 1  through FG 11  is 2 mm. A distance S 1 , S 2  and S 3  (shown in  FIG. 1A ) corresponding to the distance from the transverse end of the conductive film forming substrate  211  to the longitudinal end of each of the respective frit-glass layers FG 3 , FG 2  and FG 1  is S 1 =8.35 mm, S 2 =11.35 mm and S 3 =15.35 mm. The distance S 1  is the same for the frit-glass layers FG 3  through FG 9 . A distance S 4  (shown in  FIG. 1A ) from the longitudinal end of the conductive film forming substrate  211  to the transverse end of the frit-glass portion FG 1  is S 4 =8.35 mm. A space between each of the frit-glass layers S 5  is S 5 =7.18. However, these sizes and distances are only examples and may be chosen arbitrarily. Although in this embodiment shown in  FIG. 1A , the frit-glass layers FG 1  through FG 11  are arranged so that the longitudinal direction thereof extends along the vertical central line SL 1 , the longitudinal direction of the frit-glass layers FG 1  through FG 11  may extend along the horizontal central line SL 2 . 
     Another embodiment of the present invention will be explained with reference to  FIG. 1B . In this embodiment, the same reference numerals are used for the components similar to those of the embodiment shown in  FIG. 1A .  FIG. 1B  shows another embodiment of an arrangement of the frit-glass layers FG 1  through FG 9 . The frit-glass layers FG 1  through FG  9  are arranged at a predetermined interval with respect to each other. Each of the frit-glass layers FG 1  through FG  4  and FG  6  through FG 9  is arranged into a curved rectangular strip-shaped and concaved toward the side plate  23 . More specifically, a pair of the frit-glass layers FG 1  and FG 9 , FG 2  and FG 8 , FG 3  and FG 7  as well as FG 4  and FG 6  are arranged in an arc of an ellipsoid or a circle fashion on both sides of the center of the conductive film forming substrate  211 . There is also provided a frit-glass layer FG 5  arranged into a circular shape and located at the center of the conductive film forming substrate  211 . The shape of the frit-glass layer FG 5  may be formed into other shapes such as an ellipsoidal shape and rectangular shape. The frit-glass layers FG 2  and FG 8  are arranged shorter than the frit-glass layers FG 3  and FG 7 , as well as the frit-glass layers FG 1  and FG 9  are arranged shorter than the frit-glass layers FG 2  and FG 8 . The strip-shaped frit-glass layers are arranged in a pattern to be symmetric with respect to the vertical central line SL 1 . In other words, the arrangement of the frit-glass layers FG 1  to FG  4  is symmetric to the arrangement of the frit-glass layers FG 6  to FG  9 . 
     According to this embodiment, since the frit-glass layers FG 1  through FG 4  and FG 6  through FG 9  are formed into the curved shape, the stress applied to the conductive film forming substrate  211  becomes smaller than that of the embodiment having the frit-glass layers FG 1  through FG 11  shown in  FIG. 1A  which are not curved. As a result, the creation of a crack can be prevented more effectively. 
     In the above embodiments, the frit-glass layers FG 1  through  11  of  FIG. 1A  and the frit-glass layers FG 1  through FG 9  of  FIG. 1B  are located on the reverse surface of the conductive film forming substrate  211  within a range defined by the side plates  23 . Furthermore, the high strain point glass used to form the conductive film forming substrate  211  may be alkali-free glass or low-alkali-free glass. Furthermore, the conductive film forming substrate and the reinforcing substrate do not need to be made of glass and may be made of insulating material. Furthermore, the frit-glass used to form the frit-glass layers may be replaced with an adhesive including insulating material other than glass. 
     The results obtained through a simulation of stress distribution at the conductive film forming substrate will be explained with reference to  FIGS. 2A through 2D .  FIG. 2A  shows the conventional film substrate having a conventional arrangement of frit-glass layer shown in  FIG. 4C .  FIG. 2B  shows the stress distribution resulted from the strip-shaped frit-glass layer of  FIG. 2A .  FIG. 2C  shows the conductive film forming substrate  211  according to the present invention having the arrangement of frit-glass layer shown in  FIG. 1A .  FIG. 2D  shows the stress distribution resulted from the arrangement of frit-glass of  FIG. 2D . 
     The simulation was performed according to a finite element method. The following describes conditions for the simulation. The conductive film forming substrate  211  and the reinforcing substrate  212  are bonded together by means of the frit-glass layers FG 1  through FG 11  to from the anode substrate  21 , and the side plates  23  are bonded to the anode substrate. The simulation was performed for a  114  portion of the anode substrate  21  (indicated by the solid line in  FIGS. 2A and 2C ). The size of the conductive film forming substrate  211 , the reinforcing substrate  212  and the frit-glass layers FG 1  through FG 11  are the same as the embodiment shown in  FIG. 1A , except the height of the side plate  23  is set to 1.75 mm. Furthermore, the conductive film forming substrate  211  and the reinforcing substrate  212  were bonded together by heating the anode substrate  21  to melt the frit-glass followed by cooling the anode substrate  21  down to a room temperature (25 degrees C.). The melted frit-glass solidifies at a temperature of 380 degrees C. 
     In the arrangement of frit-glass layers shown in  FIG. 2A , a stress (a tensile stress) applied to the conductive film forming substrate  211  becomes greatest at the location  211 C 11  which is adjacent to the longitudinal end of the frit-glass layers FG 11  as shown in  FIGS. 2A and 2B , and the maximum value of the stress is about 3.801 kgf/mm 2 (37.3 MPa). In the arrangement of frit-glass layers shown in  FIG. 2C , the stress applied to the conductive film forming substrate  211  shows peaks at the locations  211 C 9 ,  211 C 10  and  211 C 11  corresponding to the longitudinal end of each of the frit-glass layers FG 9 , FG 10  and FG 11  as shown in  FIGS. 2C and 2D , while the stress induced at the location  211 C 11  being the greatest. The maximum value of the stress at the location  211 C 11  is about 1.876 kgf/mm 2 (18.4 MPa). The stress at the respective peaks described above becomes smaller in order of the stress at the location  211 C 11 ,  211 C 10  and  211 C 9 , the stress at the locations  211 C 9  being the smallest. 
     From the results obtained through the foregoing simulation, it is observed that, by employing the arrangement of frit-glass layers FG 1  through FG 11  with the outer frit-glass layers which are shorter than the other frit-glass layers, the peak of the stress on the conductive film forming substrate  211  disperses to several locations on the conductive film forming substrate  211 , with the stress at each peak being relatively small. Consequently, creation of a crack on the conductive film forming substrate  211  can be prevented. 
     In the embodiments explained hereinabove, the anode substrate is provided with the conductive film including the anode electrode and the anode wiring, however, the conductive film may be provided to both of the anode substrate and the front substrate. Although the rectangular conductive film forming substrate is shown, the conductive film forming substrate may be formed into various shapes but the shape need to be rectangular, e.g. square, rhombus, trapezoid or parallelogram. In addition, the conductive film forming substrate does not need to be the same in size with the to reinforcing substrate. Furthermore, in the embodiments described herein, the vacuum container includes at least four rectangular side plates. However, the four side plates may be formed in one, or in case of not forming the conductive film on to the front substrate, the side plates and the front substrate may be formed in one to form a cap shape. 
     According to the embodiments of the present invention, each of the frit-glass layers FG 1  through FG 11  is formed continuously, however the rectangular frit-glass layers may be formed with a plurality of dots. In addition, the fluorescent display tube described herein may be provided with a field emission cathode instead of the thermal-electron emitting filament. In addition, the present invention may be applied to other fluorescence emitting tube or device such as, an image display device or a light source having a vacuum container. 
     The embodiments described herein are only representative embodiments and are not intended to limit the present invention. It will be understood that various modifications to the embodiments may be made without departing the frame of the present invention.