GB2355849A

GB2355849A - Light emitting cell comprising carbon nanotube structure

Info

Publication number: GB2355849A
Application number: GB0019710A
Authority: GB
Inventors: Lai-Cheng Chen; Chun-Hui Tsai
Original assignee: Delta Optoelectronics Inc
Current assignee: Delta Optoelectronics Inc
Priority date: 1999-08-10
Filing date: 2000-08-10
Publication date: 2001-05-02
Also published as: FR2797520B1; DE10039479A1; GB0019710D0; TW430857B; JP2001101987A; FR2797520A1; TW471238B

Abstract

A high brightness, power saving, and high accuracy light emitting cell is arranged to include a light emitting material 12 which is made of phosphor for emitting light in response to the collision of an electron beam, and an electron emitting unit having a carbon nanotube layer 18 for releasing an electron beam and emitting the electron beam to collide with said phosphor. Gate structure 221, 222, 223 is placed between the phosphor light emitting layer and the carbon nanotube electron emitting layer for the addressing of the electron beams.

Description

1 2355849 LIGHT EMITTING CELL AND METHOD FOR EMITTING LIGHT The present

invention relates to a light emitting cell, and more Z=1 particularly to a light emitting cell that utilizes carbon nanotubes for releasing an electron beam in order to ram against the phosphor.

1= A high-brightness, power-saving display nowadays is often emerged in many occasions for displaying significant messages, for example, the scoreboard in a large stadium, electronic board in a public place, road sign on a freeway, and so on. Typically, a display is a combination of a great deal of light emitting cells. Currently, the light emitting cell for constituting a display falls roughly into five categories: Incandenscent light bulb, small cathode ray tube, high voltage vacuum fluorescent display, small fluorescent lamp, and light-emitting diode.

The incandenscent light bulb utilizes the fundamentals of heating the filament to emit light. Because the temperature of the filament which is made of tungsten has to be kept around 900'C to 1500'C while the incandenscent light bulb is illuminating, the display that is constituted by incandenscent light bulbs is very power-consuming and thus the energy efficiency is very low. In addition, because the incandenscent light bulb can only emits yellowish white light, it will be quite difficult to be used to constitute a color display.

With respect to the cathode ray tube (CRT), the CRT utilizes electron beam to ram against the phosphor, therefore the luminescent efficiency of the CRT is very high. Theoretically, the energy efficiency of the CRT should be very high. Nevertheless, the electrons in a CRT are produced by heating a hot cathode formed by coating, an oxide that is I easy to release electrons, e.g. barium oxide, with the surface of metal.

I 2 While the hot cathode is heating, the oxide is capable of releasing hot electrons. Because the electron gun that is used to produce electrons is a point electron source, the temperature and current of the electron gun has to be boosted in order to obtain a higher electron density. Thus for a light emitting cell requiring to possess high-brightness, the life of the electron gun will inevitably be reduced, and the power consumption will be increased accordingly. On the other hand, because the size of CRT is quite huge, it is not suitable for constituting a high-accuracy display. Moreover,, the CRT display is very power-consuming. For example, the power consumption of a 25mx40rn CRT display is rated at 200OKW.

Though the power consumption of small CRT is only ten percents of that of the incandenscent light bulb, the point electron source will result in a low luminescent efficiency.

The high voltage vacuum fluorescent display (HVVFD) is similar to the CRT except that the point electron source is replaced with a line electron source. The line electron source is formed by coating an oxide that is easy to release electrons with a tungsten wire. Because the line electron source can emit numerous electrons to ram against the phosphor, the disadvantage of high power-consumption of the CRT display can be suppressed significantly. Besides, the HVVFD can integrate three original colors- red, green, and blue in a single cell, it is more suitable than CRT for constituting a color display with high resolution.

Nonetheless, though the HVVFD is much better than the CRT, the structure of HVVFD is quite complicated and it is uneasy to be manufactured. Moreover, it will consume a large quantity of power as heating the tungsten filament. For example, the power consumption of a 3 display that is constituted by HVVFD with the size of 25rnx4Orn is rated around 1000 M.

The small fluorescent lamp that utilizes ultraviolet rays to excite the phosphor can also be used to constitute a display. Unfortunately, the colors of the fluorescent lamp today are quite few, and its size is difficult to be dropped below I line/mm. Accordingly, it is somewhat difficult to be used to constitute an accurate display.

Light-emitting diode (LED) has been widely employed on a large display today. Though the red, green, and blue LED have been developed thus far, the high-brightness red and blue LEDs are uneasy to be manufactured, and the luminescent efficiency of LED is not comparable to that of the fluorescent lamp. In addition to the disadvantage of low luminescent efficiency, the LED has a serious view angle problem and thus it will not be suitable to be used to constitute a large display.

To conclude, the conventional light emitting cell has the following disadvantages:

(a) Low luminescent efficiency, (b) High energy consumption, and (c) Low resolution.

After analyzing the light emitting cells today, it can be found that the lurninescent efficiency by using the electrons to ram against the phosphor is superior than that by using other light emitting techniques.

Consequently, the small CRT has a better luminescent efficiency than incandenscent light bulb, light-ernitting diode, and so forth. However, the approach of producing electrons by heating is the major contribution to the power conSUmption in small CRT and HVVFD. If one is desired 3 4 to reduce the power consumption, a cold cathode will be the best choice for producing electrons in a light emitting cell.

In 1995, Rinzler first discovered that a carbon nanotube, which is composed of carbon material, can release electrons in "A simple and robust electron beam source from carbon nanotubes" by Philips G. Collins and A.

Zettl, Appl.f Phys. Lett, 69(13), pp. 1969-1971, 1996. In 1997, Wang et al. discovered that carbon nanotube can release numerous electrons at a low electric field, such as 0.8 Wpm, in "Field emission from nanotube bundle emitters at low fields" by Q.H. Wang, T.D. Corrigan, J.Y.

Dai, R.P.H. Chang, and A.R. Krauss, Appl., Phys. Lett, 70(24)f pp. 3308-3310, 1997. We have appreciated that a is high-brightness, power-saving, and high-accuracy light emitting cell can be brought out by combining a carbon nanotube at a low electric field and phosphor. The light emitting cell brought out thereby can be used to constitute a monochrome or a color display for displaying static texts and/or dynamic message picture, on an electronic board.

In a broad aspect, therefore the applicant has developed a light-emitting cell by using a carbon nanotube as an electron source for producing electrons to ram 2S against the phosphor for light emission.

An advantage of an embodiment of the present invention is to provide a high-brightness, low power consumption, and high luminescent efficiency light emitting cell.

3 01 A further aspect of the present invention provides a method for emitting light.

In particular, the invention provides a light emitting cell comprising a iight-emitting material which can emit light in response to the collision of an electron beam, and an e-lect-ron-emitting unit having a carbon nanotube layer as an electron source for releasing an electron beam and emitting the electron beam to ram against the light-emitting material.

Preferably, the light-emitting material is made of phosphor.

Preferably, the light-emitting cell further includes a panel for attaching the iight-emitting material thereto.

Preferably, the electron-emitting unit includes a substrate having a conductive line layer formed thereon for forming the carbon nanotube layer on the conductive line layer.

Preferably, the substrate is a glass substrate or a ceramic substrate.

Preferably, the light-emitting cell further includes a glass spacer for separating the panel from the substrate.

Preferably, a black matrix is further disposed adjacent to the light-emitting material for enhancing contrast.

Preferably, an aluminum layer is attached to the surface of the light-emitting material and the black matrix for enhancing reflection.

Preferably, the electron-emitting unit further comprises a gate formed above the carbon nanotube layer for controlling the electron beam to ram against the light-emitting material at a specific address.

Preferably, the electron beam is emitted by applying an electric field with an intensity of 0.8 V/pm to the electron beam to accelerate the electron beam.

A further aspect of the present invention provides a simplified fabrication process for the electron-emitting unit, including the steps of: (a) forming the carbon nanotube layer onto the substrate having the conductive line laver formed thereon, (b) forming a dielectric layer onto the carbon nanotube layer, (c) forming a conductor "I laver onto the dielectric layer, (d) forming a protec-or layer onto the conductor layer, (e) transferring a pa-:tern onto the protector layer, (f) removing a portion of t:-e protector layer according to the pattern, (g) removing the portion of the dielectric layer and the conductor layer not covered with the protector layer, and (h) stripping the residual protector layer.

Preferably, the carbon nanotube layer is formed by a printing process.

Preferably, the dielectric layer is made of an insulating material, such as a glass or a ceramic.

Preferably, the dielectric layer is formed by a printing process.

Preferably, the conductor layer is made of silver, nickel, or platinum.

Preferably, the conductor layer is formed by a printing process or an evaporation process.

Preferably, the protector layer is made of photoresist and be formed onto the conductor layer with a spin coating process.

Preferably, the pattern is transferred onto the protector layer by a lithography process, and alternatively, a printing process.

Preferably, the portion of the dielectric layer and the conductor layer not covered with the protector layer is removed by a sandblasting process.

In accordance with a further aspect of the present 2 invention, a variety of gate structures is provided. The gate can be a network conductor, a metal layer with an insulating layer formed thereon, an insulating layer with a metal layer formed thereon, an insulating layer formed between a first metal layer and a second metal layer, or a first insulating layer formed between a first metal layer and a second metal- layer and a second metal layer formed between the second metal layer and a third metal layer.

In accordance with a further aspect of the present invention, a variety of the addressing mechanism for the 3 light-emitting cell -1 also provided. One addressing mechanism is that the x-coordinate and the y-coordinate of the 30eCific address -for emitting light is determined by the carbon nanotube layer with a control circuit connected therewith. Another addressing mechanism is that the xcoordinate (y-coordinate) of the specific address for emitting light is determined by the metal layer of the gate and the y-coordinate (x-coordinate) of the specific address for emitting light is determined by the carbon nanotube layer. Otherwise, the x-coordinate of the specific address for emitting light is determined by the first metal layer of the gate and the y-coordinate of the specific address for emitting light is determined by the second metal layer of the gate.

Preferably, the whole light-emitting cell is enclosed in a vacuum environment.

Another aspect of the present invention is a method for emitting light, including the steps of: providing a light-emitting material which can emit light in response to the collision of an electron beam, providing an electron-emitting until having a carbon nanotube layer as an electron source for releasing an electron beam, and emitting the electron beam to ram against the lightemitting material.

Preferably, the light-emitting material is made of phosphor.

8 Now the foregoing and features of the present invention may best be understood through the following descriptions with reference to the accompanying drawings, in which:

Fig. I is a schematic diagram showing an embodiment of the light 5 emitting cell according to the present invention; Figs. 2 (a) - 2 (g) are schematic views showing the fabrication process of electron-emitting unit according to the present invention; Fig. 3 is a schematic diagram showing another embodiment of the light emitting cell according to the present invention; Fig. 4 is a schematic view depicting the implementation of the addressing mechanism of the light emitting cell according to the present invention; Figs. 5 (a) - 5 (e) are schematic views showing a variety of gate structures in the light emitting cell according to the present invention; and Figs. 6(a) - 6 (c) are schematic diagrams illustrating a variety of addressing mechanisms for the light emitting cell according to the present invention.

Please refer to Fig. I which shows an illustrative embodiment of the light emitting cell according to the present invention. Three piles of phosphors 12 each of red, green, and blue are attached to the panel 13.1 The block matrix (13M) I I is disposed adjacent to each pile of the phosphor for enhancing contrast, and an aluminum layer 14 is attached to the surface of the black matrix I I and phosphor 12 for enhancing reflection. The substrate 17 is a glass substrate or a ceramic Substrate with a conductive line layer 19 formed thereon. A carbon nanotube layer IS is formed on the conductive line layer 19, and a gate 16 which is Is 9 made of a network conductor is laid between the carbon nanotUbe layer 16 and the panel 13). The panel 13 and the substrate 17 is separate by the glass spacer 15. The whole light emitting cell is enclosed in a vaCLIUM environment.

In the light of the feature of carbon nanotube, as long as a low electric field, e.g. 0.8 V/pm, is applied between the gate 16 and the carbon nanotube layer 18, numerous electrons can be released from the carbon nanotube layer 18. While a high voltage, e.g. 5000 V, is applied to the panel 13 such that the panel 13 acts as an anode, the released electrons are accelerated to penetrate through the network gate 16 and rammed against the phosphor 12. The conductive line layer 19 can be taken as a control switch for controlling whether or not the carbon nanotubes 18 can emit electrons due to the applied voltage. Hence, a large color display can be brought out by combining a great deal of light emitting cells of the present invention.

As discussed above, the carbon nanotube can release numerous electrons at a low electric field, the difficulties encountered by the CRT and HVVFD can be significantly overcome. In the mean time, the well known three original colors- red, green, blue, can be integrated in a single light emitting cell. Consequently, the light emitting cell can be applied to constitute a power-saving, full-color electronic board.

The electron-emitting unit in this embodiment comprises the gate 16 and the carbon nanotube layer 18 formed on the conductive line layer 19.

Instead of laying an independently- imp I e m ente d gate 16 above the Z:) C, carbon nanOtUbe layer 18, a simplified fabrication process for the electron-emitting unit can be developed for dissolving the difficulty of multilayer alignment. The procedure of the fabrication process for the 9 io electron-emittinc, unit is illustrated in Figs. 2 (a) - 2 (g) and described C-) step by step in the following:

1. Printing a carbon nanotube layer 62 onto a glass substrate or a ceramic substrate 61 with a conductive line layer formed thereon.

2. Printing an insulating material or a dielectric material, such as glass, onto the carbon nanotube layer 62 to act as the dielectric layer 63.

3. Printing a conductor layer 64, such as silver, nickel, or platinum onto the dielectric layer 63 by a printing process or an evaporation process tp act as a gate.

4. Forming a protector layer 65, such as photoresist, onto the conductor layer 64 by a spin coating process.

5. Transferring a pattern onto the protector layer by a lithography process and a printing process.

6. Removing a portion of the protector layer 65 according to the 15 transferred pattern.

7. Removing the portions of the uncovered conductor layer and dielectric layer 651 by a sandblasting process.

8. Eventually, stripping the residual protector layer 65.

The finished electron-emitting unit is shown in Fig. 2(g). The carbon nanotube layer 62 is separate with the gate 64 by a dielectric layer 63. Thus, the gate can be implemented together with the carbon nanotube layer in a simple fabrication process, and the difficulty of multilayer alignment can be overcome with ease.

Please refer to Fig. which shows another embodiment of the light 1 emitting cell according to the present invention. It is worthy to note that the gate structure is shown in an amplified diagram on the left side of Fig. 3. The gate structure of this embodiment comprises a first metal layer I () I t 221, an insulating layer 223, and a second metal layer 222. When the electron beam is accelerated by the applied electric field and is about to penetrate through the gate 16, only the electrons at a specific address can penetrate the through hole 224 of the gate 16 and ram against the phosphor 12. The implementation of the addressing mechanism is depicted in Fig. 4.

Please refer to Fig. 4. When a positive voltage is applied to the conductive lines 21 of the conductive line layer on the substrate, the whole conductive line can emit electrons. However, only when the second metal layer 31 is also inputted with a positive voltage, the emitted electrons can penetrate through the gate and ram against the phosphor to emit light. It is evident that the x-coordinate of the address for the electron beam to penetrate through the gate is determined by the metal line layer on the substrate, and the y-coordinate is determined by the second metal layer of the gate.

Figs. 5 (a) - 5 (e) illustrates a variety of gate structures of the light emitting cell according to the present invention. Fig. 5 (a) is a prototype of the gate structure, which is made of a network conductor 40. Such gate is applicable on condition there is no need to setup the address for emitting light or the address for emitting light is determined by the cathode. The gate shown in Fig. 5 (b) includes a metal layer 41 and an insulating layer 42, and the address for emitting light is controlled b the C) y metal layer 41. The gate structure of Fig. 5 (c) is the same as the gate structure of Fig. 5 (b), except that the relative position of the metal layer 25 41 and the insulating layer 42 are exchanged. The gate structure of Fig. 5 (d) is an alternative design of the gate structure of Fig. 5 (b) and Fig. 5 C> (c), which includes a first metal layer 411, an insulating layer 42, and a i I second metal layer 412. The x-coordinate of the address for emitting liaht is determined by the first metal layer 411 and the y-coordinate of the address for emitting light is determined by the second metal layer 412. The gate structure of Fig. 5 (e) is a modified design of the gate structure of Fig. 5 (d), which includes a first metal layer 411, a first insulating 421, a second metal layer 412, a second insulating layer 422, and a third metal layer 413. The addressing mechanism of Fig. 5 (e) is the same as that of Fig. 5 (d). As for the function of the third metal layer 4131 it is used to focus the emitted light on a spot.

Figs. 6 (a) - 6 (c) shows three addressing mechanism for the light emitting cell according to the present invention. Fig. 6 (a) illustrates a gate addressing mechanism. The first metal layer 521 of the gate determines the x-coordinate of the address for emitting light, while the second metal layer 523 of the gate determines the y-coordinate of the address for emitting light. The emitted electron beam can selectively penetrate through the gate at a specific address which is determined by the first metal layer 521 and the second metal layer 522 and then ram against the phosphor.

Fc, iz-V 6 (b) shows another gate addressing mechanism for the light emitting cell according to the present invention. The metal layer 525 of C the gate determines the x-coordinate (y-coordinate) of the address for emitting light, and the carbon nanotube layer 511 determines the ycoordinate (x-coordinate) for emitting light. Fig. 6 (c) shows a cathode addressing mechanism for the light emitting cell according to the present Z:) invention. The address for emitting light is determined by the carbon nanotube layer 5 1 with a control circuit 56 connected therewith. The control circuit 56 encodes the control signal S and then controls the z::12 specific carbon nanotubes 511 to release electrons. The released electrons will be accelerated by the applied electric field to penetrate the gate 52 and ram against the phosphor 53 to emit light.

In summary, the light emitting cell of the present invention utilizes 5 the carbon nanotube to release electrons at a low electric field, and emits the electrons to ram against the phosphor, therefore it can possess the characteristics of high luminescent efficiency, low power consumption, and high resolution. Accordingly, it is suitable to be used to constitute a large display.

While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the present invention which is defined by the appended claims.

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Claims

CLAIMS: 1. A light emitting cell comprising:

a light-emitting material which can emit light in response to the collision of an electron beam; and an e I ectron- emitting unit having a carbon nanotube layer as an electron source for releasing an electron beam and emitting the electron beam to ram against the light-emitting material.
2. The light emitting cell of claim 1, wherein the light-emitting material is made of phosphor.
3. The light emitting cell of claims I to 2, further comprising a panel for attaching the light-emitting material thereto.
4. The light emitting cell of claims I to 3, wherein the electronemitting unit includes a substrate having a conductive line layer formed thereon for forming the carbon nanotube layer on the conductive line layer.
5. The light emitting cell of claim 4, wherein the substrate is a glass substrate.
6. The light emitting cell of claims 4 to 5, wherein the substrate is a ceramic substrate.
7. The light emitting cell of claims I to 6, further including a spacer for 20 separating the panel from the substrate.
8. The light emitting cell of claims 7, wherein the spacer is made of glass i
9. The liaht emitting cell of claims I to 8, further comprising a black 4:) CD matrix disposed adjacent to the light-emitting material for enhancing contrast.
10. The light emitting cell of claims I to 9, further comprisinc, a metal z::, ID layer attached to the surface of the light-emitting material and the black matrix for enhancing reflection.
11. The li ht emittina, cell of claims I to 10, wherein the metal layer is 9 ZD made of aluminum.
12. The light emitting cell of claims I to 10, wherein the electron- t') Z:) emitting unit further comprises a gate formed above the carbon nanotube layer for controlling the electron beam to ram against the light-emitting material at a specific address.
13. The light emitting cell of claims 1 to 12, wherein the electron beam is emitted by applying an electric field to the electron beam to accelerate the electron beam.
14. The light emitting cell of claim 13, wherein the electric field has an intensity of 0.8 V/gm.
15. The light emitting cell of claim 1, wherein the electron-emitting unit is formed by the steps of: forming a carbon nanotube layer onto a substrate having a conductive line layer formed thereon; forming a dielectric layer onto the carbon nanotube layer; forming a conductor layer.onto the dielectric layer; fon- ning a protector layer onto the conductor layer; transferring a pattern onto the protector layer; removing a portion of the protector layer according to the pattern; removing the portion of the dielectric layer and the conductor layer not covered with the protector layer; and stripping the residual protector layer.
16. The light emitting cell of claim 15, wherein the carbon nanotube layer is formed by a printing process.
17. The light emitting cell of claims 15 to 16, wherein the dielectric layer is made of an insulating material.

t 5 16
18. The light emitting cell of claims 15 to 17, wherein the insulating material is a alass.

t)
19. The light emitting cell of claims 15 to 18, wherein the insulating material is a ceramic.
20. The light emitting cell of claim 15, wherein the dielectric layer is formed by a printing process.
2 1. The light emitting cell of claim 15, wherein the conductor layer is made of one selected from a group consisting of a silver, a nickel, and a platinum.
22. The light emitting cell of claim 15, wherein the conductor layer is formed by a printing process.
23. The light emitting cell of claim 15, wherein the conductor layer is formed by an evaporation process.
24. The light emitting cell of claim 15, wherein the protector layer is 15 made of photoresist.
25. The light emitting cell of claim 15, wherein the protector layer is formed by a spin coating process.
26. The light emitting cell of claim 15, wherein the pattern is transferred onto the protector layer by a lithography process.
27. The light emitting cell of claim 15, wherein the pattern is transferred onto the protector layer by a printing process.
28. The light emitting cell of claim 15, wherein the portion of the dielectric layer and the conductor layer not covered with the protector layer is removed by a sandblasting process.
29. The light emitting cell of claim 12, wherein the gate comprises a network conductor.

16 1 17 3
30. The light emitting cell of claim 29, wherein the x-coordinate and the y-coordinate of the specific address for emitting light is determined by the carbon nanotube layer with a control circuit connected therewith.

3
3 1. The light emitting cell of claim 12, wherein the gate comprises an 5 insulating layer and a metal layer formed thereon.
32. The light emitting cell of claim 3 1, wherein the x-coordinate of the specific address for emitting light is determined by the metal layer and the y-coordinate of the specific address for emitting light is determined by the carbon nanotube layer.
33. The light emitting cell of claims 31 to 32, wherein the y-coordinate of the specific address for emitting light is determined by the metal layer and the x-coordinate of the specific address for emitting light is determined by the carbon nanotube layer.
34. The light emitting cell of claim 12, wherein the gate comprises a 15 metal layer and an insulating layer formed thereon.
35. The light emitting cell of claim 34, wherein the x-coordinate of the specific address for emitting light is determined by the metal layer and the y-coordinate of the specific address for emitting light is determined by the carbon nanotube layer. 20
36. The light emitting cell of claims 34 to 35, wherein the y-coordinate of the specific address for emitting light is determined by the metal layer! and the x-coordinate of the specific address for emitting light is determined by the carbon nanotube layer.
37. The light emitting cell of claim 12, wherein the gate comprises an 25 insulating layer formed between a first metal layer and a second metal layer.

i's
38. The light emitting, cell of claim 37, wherein the x-coordinate of the specific address for emitting light is deten-nined by the first metal layer and the y-coordinate of the specific address for emitting light is determined by the second metal layer.
3) 9. The light emitting cell of claim 12, wherein the gate comprises a first insulating layer formed between a first metal layer and a second metal layer, and a second insulating layer formed between the second metal layer and a third metal layer.
40. The light emitting cell of claim 39, wherein the x-coordinate of the 10 specific address for emitting light is determined by the first metal layer and the y-coordinate of the specific address for emitting light is determined by the second metal layer.
41. The light emitting cell of claim 1, wherein the light emitting cell is enclosed in a vacuum environment.
42. A method for emitting light, comprising steps of providing a lightemitting material which can emit light in response to the collision of an electron beam; providing an electron-emitting unit having a carbon nanotube layer as an electron source for releasing an electron beam; and emitting the electron beam to ram against the light-emitting material.
43. The method of claim 42, wherein the light-emitting material is made of phosphor.
44. The method of claim 42, wherein the electron beam is emitted by applying an electric field between to the electron-beam to accelerate the electron beam.
45. The method of clairns 4') to 44, wherein the electric field has an intensity of 0.8 V/Lm.

18 19
46. The device substantially as hereinbefore described with reference to the accompanying Figs. 1, 3, and 4.

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