Plasma display panel

A plasma display panel is provided in which an operational quality is prevented from being deteriorated by resonance of a substrate. The plasma display panel includes a partition for dividing a discharge gas space defined by a pair of substrates and a sealing material in accordance with a cell arrangement of a display screen. There is a void space between the upper surface of the end portion of the partition and the surface of the opposed substrate, the surfaces contacting each other. The natural frequency of the portion from the inner edge of the void space to the inner edge of the sealing material is raised above audio frequency region of a human.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma display panel (PDP) and is useful for reducing acoustic noise in operation.

A display device utilizing a plasma display panel is becoming commonplace as a large screen television set. As such a display device is used widely at home, it has been requested to reduce a slight noise in operation.

2. Description of the Prior Art

A surface discharge type PDP for a color display includes a partition for preventing discharge interference between neighboring cells. There are arrangement patterns of the partition including a stripe pattern that divides a display area into columns of a matrix display and a mesh pattern that divides a display area into cells. When the stripe pattern is adopted, a plurality of partitions having a band-like shape in a plan view is arranged in the display area. When the mesh pattern is adopted, one partition (a so-called box rib) that defines each cell individually in a plan view is arranged in the display area. The partition has the height of 150-200 microns and defines a gap size between the substrates in the display area.

In general, a partition is made of a low melting point glass which is burned, and is formed in the following process. (A) Low melting point glass paste is applied to a glass substrate at a uniform thickness and is dried. (B) On the dried paste layer, a mask of a pattern corresponding to the partition is formed by a photolithography process. (C) Portions of the paste layer that are not masked are removed by a sand blast method in which a cutting material is blown. (D) After removing the mask, the patterned paste layer is burned.

In the process of forming a partition, some variation of height of the partition is inevitable. Especially, when the paste layer is patterned by the sand blast method and is burned as explained above, the sand blast causes a side cut, i.e., cutting under the mask in the sand blast process, so that the edge portion of the partition may be higher than other portions in a plan view in the subsequent burning process. More specifically, when a design value of the height of the partition is 140 microns, the edge portion becomes higher than other portions by approximately 30 microns. This phenomenon is called a “raise”, and the reason of the “raise” is considered to be uneven of thermal contraction stress. The “raise” phenomenon causes incomplete contact between the substrates in a PDP manufacturing process in which one substrate having a partition is placed on the other substrate. In the major portion of the partition forming area, the upper surface of the partition contacts intimately the surface of the opposed substrate. However, at the vicinity of the “raised” position of the partition forming area, only the raised edge portion of the partition contacts the surface of the opposed substrate. As a result, the substrate is curved microscopically, and a void space is generated between the upper surface of the partition and the surface of the opposed substrate. In this state of the PDP, the substrate is vibrated locally by periodical electrostatic attraction due to an application of a high frequency drive voltage for a display. Thus, minute acoustic noise is generated. This noise drops a quality of the display operation.

SUMMARY OF THE INVENTION

An object of the present invention is to prevent an operational quality from being deteriorated by resonance of a substrate.

According to one aspect of the present invention, the vibrating portion of a substrate that constitutes a plasma display panel is made to have a natural frequency higher than audio frequency region of a human, so that a user cannot hear the acoustic noise. Supposing that the audio frequency region of a human is 20-20000 hertz, it is the best to make the natural frequency higher than 20000 hertz. However, in the range above 16000 hertz, a sound is hard to hear unless its sound pressure is sufficiently large. Therefore, if the natural frequency is raised above 16000 hertz, the user cannot hear the acoustic noise substantially. Raising the natural frequency above 16000 hertz is useful as a practical method.

The natural frequency is determined by a length of the vibrating portion of the substrate, a thickness of the substrate, a density of the substrate and a Young's modulus of the substrate. The natural frequency can be raised by shortening the vibrating portion. In addition, the natural frequency can be raised by any method of enlarging the thickness of the substrate, using a substrate having a small density, or using a substrate having a large Young's modulus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be explained more in detail with reference to embodiments and drawings.

FIGS. 1A and 1Bshow a general structure of a PDP according to the present invention.FIG. 1Ais a plan view, andFIG. 1Bis a cross section ofFIG. 1Aalong1B—1B line. A PDP1comprises a pair of substrate structural bodies10and20. The substrate structural body means a plate-like structural body including a substrate having a size larger than a display screen60and at least one other element constituting a panel. The substrate structural bodies10and20are made independently of each other and are placed so as to oppose and overlap each other. The peripheral portions of the opposed area are sealed with a sealing material35to be a unit. The gap between the opposed substrate structural bodies10and20sealed with the sealing material35makes a discharge gas space. The substrate structural body10has a dimension protruding from both sides of the substrate structural body20in the horizontal direction, while the substrate structural body20has a dimension protruding from both sides of the substrate structural body10in the vertical direction. On these protruding portions, electrode terminals drawn out of the display screen60are arranged for being connected to a driving circuit. The display screen60has a dimension that the peripheral portion thereof is apart from the sealing material35by approximately 15 millimeters.

FIG. 2is a diagram showing an example of a cell structure of a PDP. InFIG. 2, the portion including three cells of the PDP1corresponding to one pixel display is shown apart from a pair of substrate structural bodies so that the inner structure can be seen easily.

In each of the cells constituting the display screen, display electrodes X and Y and address electrodes A cross each other. The display electrodes X and Y are arranged on the inner surface of the front glass substrate (the front substrate)11. Each of the display electrodes X and Y includes a transparent conductive film41that forms a surface discharge gap and a metal film (a bus electrode)42that extends over the entire length of the row. The display electrode pairs are covered with a dielectric layer17having the thickness of approximately 30-50 microns, and the surface of the dielectric layer17is coated with a protection film18that is made of magnesia (MgO). The address electrodes A are arranged on the inner surface of the back glass substrate21and are covered with a dielectric layer24. On the dielectric layer24, band-like partitions29having the height of approximately 140 microns and being made of a low melting point glass are arranged so that one partition29is positioned between address electrodes A. These partitions29divide the discharge gas space into columns in the direction along the row of the matrix display and define the size of the discharge gas space in the front and back direction. Each of column spaces31of the discharge gas space corresponds to each column and is continuous over all rows. The inner surface of the back side including upper surfaces of the address electrodes A and side faces of the partitions29is covered with fluorescent material layers28R,28G and28B of red, green and blue colors for a color display. Italic letters R, G and B inFIG. 2represent light emission colors of the fluorescent materials. The fluorescent material layers28R,28G and28B are excited locally by ultraviolet rays emitted by the discharge gas and emit light.

FIG. 3is a schematic diagram of a structure of a main portion of the PDP. InFIG. 3, elements of the front substrate structural body except the glass substrate11are omitted, and elements of the back substrate structural body except the glass substrate21and the partition29are omitted. Actually, the thickness of the glass substrates11and21is 2-3 millimeters, while the thickness of the dielectric layer is approximately 30 microns that is sufficiently small. In addition, the electrodes and the protection film are thinner than the dielectric layer.

In the PDP1, the partitions29are formed on the back glass substrate21as mentioned above, and the end portion thereof is raised to be higher than other portions. The height ΔH of the raised portion295at the end portion of the partition is approximately 30 microns. The sealing material35is made of a low melting point glass that has a softening point lower than the partition material. Therefore, in the sealing process for glass-fusing the glass substrate11and the glass substrate21, the partition29is not softened. As a result, the end portion of the glass substrate11is deformed to curve slightly in the sealing process, so that a void space33having the length L2is formed between the glass substrate11(strictly the dielectric layer17) and the upper surface of the partition29. A so-called floating structure in which the glass substrate11is supported unstably (this portion of the structure is called a “beam”) is formed over the range of the length L from the inner edge of the void space33to the inner edge of the sealing material35that is the fixed edge. In the PDP1having the above-mentioned beam, a buzz sound95is generated during a display operation. Namely, when applying a high frequency drive voltage to cells, a periodical electrostatic attraction force works between the display electrode X or Y and the address electrode A that are opposed to each other via the discharge gas space. Thus, the beam portion of the glass substrate11is vibrated uniquely by absorbing a vibration energy corresponding to the resonance frequency thereof. According to the present invention, the natural frequency of the beam is higher than the audio frequency region of a human, so that a user of the PDP1cannot hear the buzz sound. In other words, the buzz sound95is eliminated in an artificial manner.

The natural frequency F of the beam illustrated inFIG. 3is expressed in the following equality.F=an2⁢π⁢⁢L2·Eh212⁢⁢ρ(1)

Here, anis a constant (=22) in the case of the fixed edge, L is the distance from the inner edge of the void space to the sealing material, E is a Young's modulus of the front substrate, h is the thickness of the front substrate, and ρ is a density of the front substrate.

Since the natural frequency F is inversely proportional to the square of L as shown in the equality (1), the natural frequency F becomes higher as the length L of the beam becomes shorter. Furthermore, as shown inFIG. 4, the amplitude of the natural vibration (i.e., the sound pressure of the buzz sound) becomes smaller as the length L of the beam becomes shorter. Therefore, the problem of the buzz sound is solved by shortening the length L of the beam. However, the length L2of the void space33shown inFIG. 3is dependent on the raise quantity of the partition29and the pressure of the filled discharge gas, so it is not easy to shorten the length L2. On the other hand, the length L1from the end of the partition29(i.e., the raised portion295) to the sealing material35can be shortened relatively easily by redesigning the dimension, which is a realistic method for shortening the beam.

FIRST EXAMPLE

In the PDP having the front substrate11made of a high distortion point glass having E=78 GPa and ρ=2770 kg/m3, the relationship between the length L of the beam and the natural frequency F is as shown inFIGS. 5 and 6. As shown inFIG. 6, the measured value of the natural frequency F when h=0.0028 meters is substantially identical to the calculated value.

In the case where the length L2of the void space33is 0.01 meters, in order to raise the natural frequency F above the upper limit value 20000 Hz of the audio frequency region, the length L1is set to the value that satisfies the conditions below.

When a substrate having h=0.0028 meters is used, L1is less than 0.017 meters.

When a substrate having h=0.0020 meters is used, L1is less than 0.013 meters.

When a substrate having h=0.0010 meters is used, L1is less than 0.006 meters.

SECOND EXAMPLE

In the PDP having the front substrate11made of a soda glass having E=73 GPa and ρ=2500 kg/M3, the relationship between the length L of the beam and the natural frequency F is as shown in FIG.7. In the case where the length L2of the void space33is 0.01 meters, in order to raise the natural frequency F above the upper limit value 20000 Hz of the audio frequency region, the length L1is set to the value that satisfies the conditions below.

When a substrate having h=0.0028 meters is used, L1is less than 0.018 meters.

When a substrate having h=0.0020 meters is used, L1is less than 0.013 meters.

When a substrate having h=0.0010 meters is used, L1is less than 0.007 meters.

As explained above, by shortening the length L of the beam, the natural frequency F of the beam is raised above the audio frequency region. However, without being limited to this method, any other method such as thickening the substrate, using a substrate having a small density, or using a substrate having a large Young's modulus can be adopted so as to raise the natural frequency F. In other words, it is sufficient that the thickness h of the front substrate11satisfies the inequality (2) or that the density ρ satisfies the inequality (3) or that the Young's modulus E satisfies the inequality (4).h>2⁢π⁢⁢L2⁢fmaxan·12⁢⁢ρE(2)

Here, h is the thickness of the front substrate, L is the distance from the inner edge of the void space to the sealing material, an is a constant (=22), fmaxis the upper limit value of the audio frequency region of a human, ρ is a density of the front substrate, and E is a Young's modulus of the front substrate.ρ<E12·(an⁢h2⁢π⁢⁢L2⁢fmax)2(3)

Here, ρ is a density of the front substrate, E is a Young's modulus of the front substrate, anis a constant (=22), h is the thickness of the front substrate, L is the distance from the inner edge of the void space to the sealing material, and fmaxis the upper limit value of the audio frequency region of a human.E>12⁢⁢ρ·(2⁢⁢π⁢⁢L2⁢fmaxan⁢h)2(4)

Here, E is a Young's modulus of the front substrate, ρ is a density of the front substrate, L is the distance from the inner edge of the void space to the sealing material, fmaxis the upper limit value of the audio frequency region of a human, anis a constant (=22), and h is the thickness of the front substrate.