KR100708710B1 - Plasma display panel - Google Patents

Abstract

An object of the present invention is to provide a plasma display panel having a structure in which charge is prevented from cross talk to neighboring discharge cells and the exhaust of impurities is smoothly vented. Is a rear substrate disposed to form a discharge space between the front substrate and the front substrate, and is disposed between the front substrate and the rear substrate to partition the discharge cells together with the front substrate and the rear substrate. A partition wall formed with a double partition wall portion, discharge electrodes formed in the discharge cell, a phosphor layer coated in the discharge cell, and a discharge gas located in the discharge cell, wherein the double partition wall portion is disposed between the discharge cells adjacent in one direction. A discharge preventing wall portion which prevents discharge in a space between the first and second wall portions and the first and second wall portions separated from each other and arranged side by side A plasma display panel is provided.

Description

Plasma display panel {Plasma display panel}

1 is an exploded perspective view showing a conventional plasma display panel.

FIG. 2 is a cross-sectional view taken along the line II-II of FIG. 1.

3 is an exploded perspective view showing a plasma display panel according to a preferred embodiment of the present invention.

4 is an exploded perspective view illustrating an enlarged portion of FIG. 3.

FIG. 5 is a plan view schematically illustrating a state in which impurity gas is discharged between the partition walls of FIG. 3.

FIG. 6 is a cross-sectional view taken along line VI-VI of FIG. 3.

FIG. 7 is a plan view illustrating a relationship between a partition, a front dielectric layer, and discharge electrodes included in the plasma display panel of FIG. 3.

<Brief description of symbols for the main parts of the drawings>

100: plasma display panel 110: front substrate

112: sustain discharge electrode pair 113: X electrode

114: Y electrode 115: front dielectric layer

116: protective film 120: back substrate

130: partition 131: double partition

132: first wall portion 133: second wall portion

134: connection 135: discharge breaker

136: exhaust passage 138: vertical bulkhead portion

140: phosphor layer 150: partition wall

Ad: discharge area An: non-discharge area

The present invention relates to a plasma display panel, and more particularly, to a plasma display panel having a structure in which a partition wall is formed in double contact to facilitate the prevention of exhaust gas and cross talk of impurity gas, and to prevent misdischarge in a non-discharge region. It is about.

In general, a plasma display panel is a display device that discharges a sealed gas between two substrates on which electrodes are formed, and excites the phosphor layer by ultraviolet rays generated thereby to implement desired numbers, letters, or graphics.

The plasma display panel is classified into a direct current type and an alternating current type according to a type of driving voltage applied to a discharge cell, for example, a discharge type, and can be classified into a counter discharge type and a surface discharge type according to the configuration of the electrodes.

The DC plasma display panel has a structure in which all electrodes are exposed to a discharge space, and charges are directly transferred between corresponding electrodes. On the other hand, in the AC plasma display panel, at least one electrode is embedded in the dielectric layer, and instead of direct charge transfer between the corresponding electrodes, the ions and electrons generated by the discharge adhere to the surface of the dielectric layer to form a wall. A wall voltage is formed and discharge can be maintained by a sustaining voltage.

On the other hand, in the opposite discharge type plasma display panel, an address electrode and a Y electrode are provided to face each unit pixel, and address discharge and sustain discharge are generated between the two electrodes. On the other hand, in the surface discharge plasma display panel, an address electrode and corresponding X and Y electrodes are provided for each unit pixel to generate address discharge and sustain discharge.

The conventional surface discharge plasma display panel 10 includes a front substrate 22 and a back substrate 32 as shown in FIGS. 1 and 2.

The back substrate 32 includes address electrodes 35, a back dielectric layer 36 covering the address electrodes, a partition 40 partitioning discharge cells, both sides of the partition wall, and a lower substrate on which the partition walls are not formed. The phosphor layer 38 applied to is formed.

The front substrate 22 disposed to be spaced apart from and opposed to the rear substrate includes a sustain discharge electrode pair 23 for generating a sustain discharge, a front dielectric layer 26 covering the sustain discharge electrode pair 23, and a protective film. 28 is provided.

The sustain discharge electrode pair 23 is usually composed of an X electrode 24 and a Y electrode 25. The X electrode 24 generally includes a transparent X electrode 24a and a bus X electrode 24b disposed on one side of the transparent X electrode, and the Y electrode 25 is usually a transparent Y electrode 25a and the And a bus Y electrode 25b disposed on one side of the transparent Y electrode.

The space formed by the sustain discharge electrode pair 23 composed of a pair of the X electrodes 24 and the Y electrodes 25 and the address electrodes 35 intersecting the unit sub-pixels SP is disposed. To form.

In general, the subpixel SP is one of a red light emitting subpixel emitting red visible light, a green light emitting subpixel emitting green visible light, and a blue light emitting subpixel emitting blue visible light. The green light emitting subpixels and the blue light emitting subpixels are combined to form one pixel P. FIG.

On the other hand, the inner space of the front and rear substrates 22 and 32 are formed to be filled with an inert gas to have a discharge region.

At this time, the plasma display panel 10 includes a large amount of impurity gas in the protective layer 28 and the porous phosphor layer 38 having high moisture adsorption, and the impurity gas remains in the panel assembly and has many adverse effects on lifespan characteristics. This can cause problems such as permanent afterimage and discharge instability.

Therefore, it is necessary to discharge a large amount of impurity gas to the outside during the vacuum exhaust process, there is a problem that a separate exhaust passage is not formed in the conventional plasma display panel 10. In particular, when the closed partition 40 is formed, such as a matrix type, one discharge cell is blocked by the partitions, and the partitions have a thickness greater than or equal to a predetermined thickness to prevent crosstalk of charges. The ventilation resistance inside is increased, the smooth flow of the exhaust gas is blocked, so that the exhaust is extremely difficult, the time required for the exhaust is very long, and the exhaust is incomplete.

When the remaining gas is present in the discharge cell which is the discharge region due to incomplete exhaust gas, the residual gas acts as a factor causing mis-discharge by changing the driving conditions in the discharge cell.

In addition, when the conventional plasma display panel 10 is mounted in a low pressure state, for example, in an airplane, the front substrate 22 and the partition wall are instantaneously in a state where the air pressure inside the airplane is unexpectedly lowered compared to the normal atmospheric pressure. In this case, a breakage phenomenon occurs in which visible light Li leaks to the neighboring subpixel SP along the front substrate 22 and the partition 40. Occurs.

Such a halation phenomenon has a problem in that the performance of the panel is degraded, especially when occurring between adjacent pixels P. That is, the pixel P is one light emitting unit as is known, and one unit color is formed as a combination of colors of the subpixels SP in the pixel. Therefore, when visible light Li leaks between the pixels P, the image quality is not clearly seen in one pixel, so that the image quality is degraded, and the contrast contrast is reduced when the video is implemented.

An object of the present invention is to provide a plasma display panel having a structure for preventing cross talk of charges to neighboring discharge cells.

Another object of the present invention is to provide a plasma display panel having a structure in which exhaust of impurity gas is smoothly vented.

Still another object of the present invention is to provide a plasma display panel having a structure capable of preventing a halation phenomenon.

In order to achieve the above object, a plasma display panel according to a preferred embodiment of the present invention is:

Front substrate;

A rear substrate disposed to form a discharge space between the front substrate and the front substrate;

A partition wall disposed between the front substrate and the rear substrate to partition discharge cells together with the front substrate and the rear substrate, the partition wall having a double partition wall formed in one direction of the discharge cell;

Discharge electrodes formed in the discharge cell;

A phosphor layer coated in the discharge cell; And

And a discharge gas located in the discharge cell,

The double partition wall part includes a first and second wall part separated from each other by discharge cells adjacent in one direction and a discharge preventing wall part to prevent discharge in a space between the first and second wall parts.

In this case, the discharge electrodes may include sustain discharge electrode pairs in which X electrodes and Y electrodes are formed side by side in a pair of discharge cells on a lower surface of the front substrate, and the first wall portion and the second wall portion are the sustain discharge electrode pairs. It is preferable to be formed in parallel with the field.

Each of the X electrode and the Y electrode may include a bus electrode formed on the first wall portion or the second wall portion and a transparent electrode in contact with the bus electrode and partially protruding into the discharge cell.

In addition, the sustain discharge electrode pairs may be covered by a front dielectric layer, and the partition wall may be disposed under the front dielectric layer. In this case, at least one of the front dielectric layer and the partition wall is preferably colored to have a dark color when the respective colors overlap.

The discharge electrodes may include a plurality of address electrodes formed on an upper surface of the rear substrate in a direction crossing the pair of sustain discharge electrodes, wherein the address electrodes are covered by a rear dielectric layer, and the partition wall is formed on an upper surface of the rear dielectric layer. It is preferably formed in.

Meanwhile, the discharge preventing wall part may include a connection part connecting the first wall part and the second wall part, and a discharge blocking part to reduce the space defined by the adjacent connection part and the first and second wall parts to be impossible to discharge. have. In this case, the discharge blocking unit may be integrally formed with the first wall portion or the second wall portion.

On the other hand, the plasma display panel according to another embodiment of the present invention:

Front substrate;

A back substrate forming a discharge space between the front substrate and the front substrate;

A partition wall partitioning discharge cells together with the front substrate and the rear substrate, the double partition walls being formed along a horizontal direction of the discharge cells, and colored partition walls;

Sustain discharge electrode pairs formed on a lower surface of the front substrate, the pair of X and Y electrodes extending in a horizontal direction for each discharge cell;

A colored front dielectric layer covering the sustain discharge electrode pairs;

A passivation layer covering the front dielectric layer;

Address electrodes formed in a vertical direction on an upper surface of the rear substrate;

A back dielectric layer covering the address electrode;

A phosphor layer coated in the discharge cell; And

And a discharge gas located in the discharge cell,

The double partition wall part may include a plurality of anti-discharge wall parts for preventing a discharge from occurring in a space between the first and second wall parts and the first and second wall parts arranged side by side in a horizontal direction between the discharge cells. Equipped.

Hereinafter, a plasma display panel according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings.

3 illustrates a plasma display panel 100 according to an exemplary embodiment of the present invention, FIG. 4 is an enlarged view of a portion of FIG. 3, FIG. 5 illustrates an arrangement of the partition wall of FIG. 3, and FIG. 6 is FIG. 3. This is taken along the VI-VI line of.

First, referring to FIGS. 3 and 4, the plasma display panel 100 according to the exemplary embodiment of the present invention may include a front substrate 110, a rear substrate 120, a partition wall 130, and discharge electrodes 112,. 122, a phosphor layer 140, and a discharge gas (not shown).

The front substrate 110 and the rear substrate 120 are disposed to face each other and form a discharge space therebetween. In this case, the front substrate 110 may be made of a material having good light transmittance such as glass, through which visible light (Li; see FIG. 5) is normally emitted to the outside. The back substrate 120 is also usually made of a material mainly containing glass.

The partition wall 130 is disposed between the front substrate 110 and the rear substrate 120 to define a discharge cell 150 together with the front substrate 110 and the rear substrate 120. In this case, an area in the discharge cell 150 partitioned by the partition wall 130 is called a discharge area Ad, and an area where no discharge space is formed by the partition wall 130 is called a non-discharge area An. do. The partition 130 will be described in detail later.

The discharge cells 150 are provided with a plurality of discharge electrodes 112 and 122 that cause discharge such as sustain discharge and address discharge. In this case, the discharge electrodes 112 and 122 may include sustain discharge electrode pairs 112. The sustain discharge electrode pairs 112 may be disposed in pairs with the X electrode 113 and the Y electrode 114 under the front substrate 110 for each discharge cell 150.

In particular, referring to FIG. 4, the X and Y electrodes 113 and 114 may include transparent electrodes 113b and 114b and bus electrodes 113a and 114a, respectively. That is, the X electrode 113 may be formed of the transparent X electrode 113b and the bus X electrode 113a, and the Y electrode 114 may be formed of the transparent Y electrode 114b and the bus Y electrode 114a. In this case, the transparent electrodes 113b and 114b are transparent electrodes made of indium tin oxide (ITO), and the bus electrodes 113a and 114a are discharge cells 150 to reduce the electrode line resistance of the transparent electrodes 113b and 114b. It is formed extending in one direction along the for example made of a metal material.

The bus electrodes 113a and 114a may be formed in the non-discharge region An without being formed in the discharge region Ad. That is, the bus electrodes 113a and 114a having no transparent color may be formed outside the discharge area Ad, and thus, the visible light passing through the outside may not be absorbed by the bus electrode, thereby improving luminance of the panel. Can be. In this case, the transparent electrodes 113b and 114b may be formed such that a part of each of the discharge cells 150 contacts the bus electrodes 113a and 114a and another part of the transparent electrodes 113b and 114b protrudes into an internal space of the discharge cell 150. have.

The X and Y electrodes 113 and 114 may be formed on the bottom surface of the front substrate 110 and may be covered by the front dielectric layer 115. The front dielectric layer 115 is formed as a dielectric that can induce charge while preventing cations or electrons from colliding with the sustain discharge electrode pair 112 and damaging the electrodes during discharge.

The protective film 116 is preferably formed on the lower surface of the front dielectric layer 115. The protective film 116 serves to prevent damage to the dielectric layer due to the sputtering of plasma particles and to lower the discharge voltage and the sustain voltage due to secondary electron emission. In general, an MgO thin film by vapor deposition is used.

The discharge electrodes may include the address electrodes 122. The address electrodes 122 may extend in a direction crossing the sustain discharge electrode pairs 112 on an upper side of the rear substrate 120 facing the surface on which the sustain discharge electrode pairs 112 are disposed. Hereinafter, the direction in which the sustain discharge electrode pairs 112, in particular, the bus electrodes 113a and 114a extend (the horizontal direction in the drawing) is referred to as a first direction, and the direction in which the address electrodes 122 extend (the vertical direction in the drawing). Will be defined in the second direction.

Meanwhile, the address electrodes 122 may be covered by the back dielectric layer 125. Similar to the front dielectric layer 115, the back dielectric layer 125 is also formed as a dielectric capable of inducing charge while preventing cations or electrons from colliding with the address electrodes 122 upon discharge.

Meanwhile, the partition wall 130 includes a double partition wall part 131. In particular, the double partition wall part 131 includes the first wall part 132 and the second wall part 133 formed in the non-discharge area An between the discharge cells 150 adjacent in one direction, as shown in FIGS. 4 and 5. It is provided. The first wall part 132 and the second wall part 133 are disposed in parallel with each other so as to be formed separately along one direction of the discharge cell 150. In this case, the space between the first wall portion 132 and the second wall portion 133 forms the exhaust passage 136.

The exhaust passage 136 forms a passage through which the impurity gas G generated in the passivation layer 116 and the porous phosphor layer 140 is discharged to the outside during the vacuum exhaust process. That is, the impurity gas G from the discharge cell 150 flows through or passes through the first wall portion 132 or the second wall portion 133 and enters the exhaust passage 136 which is a non-discharge region An. Although not shown along the passage 136, it is guided to the exhaust hole and flows out. As a result, the ventilation resistance of the plasma display panel 100 is reduced, so that the exhaust gas flows smoothly, and the exhaust gas is easily exhausted, and the time required for the exhaust gas is shortened.

In addition, as shown in FIG. 6, visible light Li generated in one discharge cell 150 and passing through the front dielectric layer 115 is reflected by hitting the front substrate 110 and the first and second walls. It is inserted into the exhaust passage 136. In addition, visible light Li inserted between the front dielectric layer 115 and the front substrate 110 is also inserted into the exhaust passage 136 between the adjacent first wall portion 132 and the second wall portion 133. As a result, a halation phenomenon in which visible light Li leaks out to another discharge cell 150 is prevented from occurring, so that an image is clearly visible in one discharge cell 150, and the image quality of the image is improved, in particular, a video. In the real world, the contrast contrast is increased.

3 and 4, the bus X electrode 113a and the bus Y electrode 114a corresponding to one discharge cell 150 are moved to the non-discharge region An in order to improve the high definition and the brightness of the panel. Can be placed in the For example, a first wall 132 is formed under the bus X electrode 113a corresponding to one discharge cell 150, and a second wall 133 is formed in the second discharge cell 150 in a second direction. As a result, it may be formed under the bus Y electrode 114a corresponding to the adjacent discharge cell 150.

However, when the bus electrodes 113a and 114a are formed in the non-discharge region An, the distance between the bus electrodes 113a and 114a corresponding to the discharge cells 150 adjacent to each other becomes close to each other, and thus the The capacitance is increased between the bus electrodes 113a and 114a. Therefore, discharge may occur in the space in the exhaust passage 136 which is the non-discharge region An. In order to prevent this, the double partition wall part 131 further includes discharge preventing wall parts 134 and 135.

That is, the discharge preventing wall portions 134 and 135 limit the exhaust passage 136 to the extent that discharge is not possible, thereby preventing the discharge from occurring in the exhaust passage 136.

In this case, the discharge preventing wall parts 134 and 135 may include a connection part 134 and a discharge blocking part. The connection part 134 connects between the first wall part 132 and the second wall part 133, and the discharge blocking part is a space defined by the adjacent connection part 134 and the first and second wall parts 132 and 133. Reduce the space so that no discharge occurs at

In this case, the connection part 134 may be integrally formed with the first wall part 132 and the second wall part 133, and the discharge blocking part 135 may also be formed in the first wall part 132 or the second wall part. It can be formed integrally with one of the.

Meanwhile, the partition wall 130 may further include a vertical partition wall part 138 along with the double partition wall part 131. The double partition wall portion 131 is perpendicular to the address electrode 122. That is, the sustain discharge electrode pairs 112 are formed in the same first direction (horizontal direction) as the extending direction, and the vertical partition wall part 138 is formed in the direction (vertical direction) parallel to the address electrode 122. The vertical partition wall portion 138 extends from both sidewalls of the double partition wall portion 131, and may be integrally connected to each other to form a matrix or meander type.

Meanwhile, the phosphor layer 140 may be formed in the discharge cell 150. In this case, the phosphor layer 140 may be formed on the inner wall of the partition wall 130 and the upper surface of the back dielectric layer 125. The phosphor layer 140 includes a component that emits visible light by receiving ultraviolet rays emitted by the sustain discharge. In general, the red phosphor layer 140 formed on the red light emitting subpixel is Y (V, P) O ₄ : Eu. The green phosphor layer 140 formed of a green light emitting subpixel, including phosphors such as Zn ₂ SiO ₄ : Mn, YBO ₃ : Tb, etc., and the blue phosphor layer 140 formed of a blue light emitting subpixel. Include phosphors such as BAM: Eu and the like.

The discharge gas is disposed in the discharge cell 150.

Meanwhile, referring to FIG. 7, at least one of the front dielectric layer 115 and the partition wall 130 may be colored to have a dark color when the colors overlap each other. That is, the position where the front dielectric layer 115 and the partition wall 130 overlap each other is a non-discharge area An. When the non-discharge area An becomes dark, it is possible to increase the clear room contrast, which reduces the probability of the external light reflected back from the outside. That is, at least one of the front dielectric layer 115 and the partition wall 130, more preferably, the front dielectric layer 115 and the partition wall 130 may be colored to serve as a black stripe. Herein, dark means a color of 4 or less in the Munsell color system.

Hereinafter, an example of a process of patterning each functional layer of the back substrate 120 having the above structure will be described with reference to FIGS. 3 to 7 as follows.

First, a rear substrate 120 made of transparent glass is provided, and then a strip address electrode 122 is patterned on the rear substrate 120. The pattern for forming the partition wall 130 thereon after uniformly applying the raw material for the partition wall 130 having a predetermined thickness on the patterned back substrate 120 on the address electrode 122. Attaching a photosensitive film having a photomask, and exposing and developing the pattern of the partition wall 130 in a state in which the photo masks are aligned, and having a double partition wall portion 131 in one discharge cell 150 using a sand blasting apparatus. One partition 130 is completed. In this case, the vertical partition wall part 138 may be integrally formed together with the double partition wall part 131. After the partition wall 130 is completed, red, green, and blue phosphor layers 140 are applied to each discharge cell on the inner surface.

The completed rear substrate 120 includes a pair of sustain discharge electrode 112 having a bus electrode and a transparent electrode, a front dielectric layer 115 and a protective film 116 buried therein through a separate process. It is sealed with the front substrate 110.

After the front and rear substrates 120 are sealed to each other, the impurity gas inside the panel is exhausted, and then a gas mixed with an inert gas and Xe is sealed and the panel is separated from the exhaust device.

At this time, the impurity gas remaining inside the panel is easily discharged through the exhaust passage 136 of the double partition wall portion 131.

As described above, according to the plasma display panel of the present invention, an exhaust passage is formed in at least one of the partition walls disposed in one discharge cell, so that the exhaust gas remaining in the panel can be easily discharged, and the exhaust performance is improved. As a result, the electrical and optical characteristics of the panel can be greatly improved.

In addition, charges flowing into the neighboring discharge cells across the partition wall between adjacent discharge cells are trapped in the exhaust passage, thereby preventing cross talk and halation of the charges.

In addition, the non-discharge region becomes dark, and the bright room contrast ratio is improved.

In addition, since the bus electrode is positioned in the non-discharge region, the luminance is improved, and the double barrier rib is provided with the prevention prevention wall portion, thereby preventing the occurrence of erroneous discharge in the non-discharge region.

Although the present invention has been described with reference to one embodiment shown in the drawings, this is merely exemplary, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

Claims (13)

Front substrate; A rear substrate disposed to form a discharge space between the front substrate and the front substrate; A partition wall disposed between the front substrate and the rear substrate to partition discharge cells together with the front substrate and the rear substrate, the partition wall having a double partition wall formed in one direction of the discharge cell; Discharge electrodes formed in the discharge cell; A phosphor layer coated in the discharge cell; And And a discharge gas located in the discharge cell, The double partition wall part may include a first and second wall parts separated from each other by adjacent discharge cells in one direction and a discharge preventing wall part to prevent discharge in a space between the first and second wall parts. Plasma display panel. The method of claim 1, The discharge electrodes may include sustain discharge electrode pairs in which X electrodes and Y electrodes are formed side by side in a pair of discharge cells on a lower surface of the front substrate, And the first wall portion and the second wall portion are formed in parallel with the sustain discharge electrode pairs. The method of claim 2, Each of the X electrode and the Y electrode includes a bus electrode formed on the first wall portion or the second wall portion and a transparent electrode in contact with the bus electrode and partially protruding into the discharge cell. . The method of claim 1, The discharge electrodes are covered by a front dielectric layer, And the barrier rib is disposed under the front dielectric layer. The method of claim 4, wherein And the front dielectric layer and the partition wall are colored to have a dark color when the respective colors overlap each other. The method of claim 2, The discharge electrodes may include a plurality of address electrodes formed on an upper surface of the rear substrate in a direction intersecting the sustain discharge electrode pairs. The address electrodes are covered by a back dielectric layer, And the partition wall is formed on an upper surface of the back dielectric layer. The method of claim 1, The discharge preventing wall portion: A connection portion connecting the first wall portion and the second wall portion; And And a discharge blocking portion for reducing the space defined by the adjacent connecting portion and the first and second wall portions to be impossible to discharge. The method of claim 7, wherein And the discharge blocking unit is integrally formed with the first wall unit or the second wall unit. Front substrate; A back substrate forming a discharge space between the front substrate and the front substrate; Partitioning the discharge cells together with the front substrate and the back substrate, the partition walls having a double partition wall formed along the horizontal direction of the discharge cells and colored; Sustain discharge electrode pairs formed on a lower surface of the front substrate, the pair of X and Y electrodes extending in a horizontal direction for each discharge cell; A colored front dielectric layer covering the sustain discharge electrode pairs; A passivation layer covering the front dielectric layer; Address electrodes formed in a vertical direction on an upper surface of the rear substrate; A back dielectric layer covering the address electrode; A phosphor layer coated in the discharge cell; And And a discharge gas located in the discharge cell, The double partition wall part may include a plurality of anti-discharge wall parts for preventing a discharge from occurring in a space between the first and second wall parts and the first and second wall parts arranged side by side in a horizontal direction between the discharge cells. Plasma display panel characterized in that it comprises. The method of claim 9, Each of the X electrode and the Y electrode includes a bus electrode formed on the first wall portion or the second wall portion and a transparent electrode in contact with the bus electrode and partially protruding into the discharge cell. . The method of claim 9, And the front dielectric layer and the partition wall are colored to have a dark color when the respective colors overlap each other. The method of claim 9, The discharge preventing wall portion: A connection portion connecting the first wall portion and the second wall portion; And And a discharge blocking portion for reducing the space defined by the adjacent connecting portion and the first and second wall portions to be impossible to discharge. The method of claim 12, And the discharge blocking unit is integrally formed with the first wall unit or the second wall unit.

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