ELECTRODES FOR GASEOUS DISCHARGE DEVICES

Abstract: An electrode structure suitable for use as an input or erase electrode (I or E) in a charge transfer type gas discharge display device (20) includes a conductive base (13 or 16), a layer consisting essentially of a low conductivity metal oxide layer (14 or 17) such as ruthenium oxide, and a thin coating of magnesium oxide dielectric (12) which improves performance of the device (20) but maintains the electrode essentially directly coupled to the ionizable gas in the device (20). The metal oxide layer (14 or 17) serves to protect the thin magnesium oxide dielectric coating (12) from contamination.

Description: ELECTRODES FOR GASEOUS DISCHARGE DEVICES Technical Field This invention relates to electrodes for gas¬ eous discharge devices of the kind including an envelope defining a channel containing an ionizable gas. Background Art Gaseous discharge devices of the kind specified are known from U.S. Patents No. 3,775,764; 3,781,600; and 4,027,197. These known gaseous discharge devices utilize the transfer of trapped charges resulting from the discharge. In the physical construction of a device using this principle, an ionizable gas is con¬ tained within an enclosure which has a plurality of dielectric-coated transfer electrodes arranged parallel but offset from one another on opposite side walls thereof. The transfer electrodes are σapacitively coupled to the ionizable gas by their dielectric coat¬ ing. As disclosed in the aforementioned U.S. Patent No. 4,027,197, a suitable thickness of this dielectric coating includes the range 13-25 microns. Typically, in¬ formation is entered into the device via an input electrode which does not have the capacitive-coupling dielectric coating and is, thus, direct-coupled to the ionizable gas. That is, the device is serially addressed by applying a voltage of predetermined magnitude between the direct-coupled input electrode and the first or nearest opposite dielectric-covered transfer electrodes. These two electrodes form the first cell within the device. By the proper application of a potential on the electrodes, the gas in the cells, formed by succes¬ sive pairs of nearest adjacent, opposite electrodes, is discharged and electric charge trapped on the coated walls of the electrode is used to transfer this gaseous discharge throughout the length of the device. Typi- cally, information is erased via an erase electrode, which, like the input electrode, does not have the capacitive dielectric coating previously mentioned, and is, thus, directly-coupled to the ionizable gas. As is disclosed in the aforementioned U.S. Patent No. 4,027,197, the interior walls of the tube adjacent the gas may be coated with a layer of magnesium oxide in order to improve the performance of the tube. All the electrodes including dielectric-coated transfer electrodes and the direct-coupled input and erase electrodes may be coated with the magnesium oxide layer for obtaining the characteristics of low operating volt¬ age and stable life of the device, as shown in Fig. 5. Typically, the magnesium oxide layer is made sufficiently thin in comparison with the dielectric coating of the transfer electrodes to preserve the direct coupled characteristics of the input and* erase electrodes. The magnesium oxide layer achieves a low operating voltage since magnesium oxide is an excellent secondary electron emitter. Furthermore, the magnesium oxide layer provides stable life since magnesium oxide is a refractory ater- ial which is immune to damage by ion bombardment and has a high melting point. It has been found that the operating charac¬ teristics of the known gaseous discharge device may alter, because contamination of the thin magnesium oxide coating over the input and erase electrodes fre¬ quently occurs due to sputtering of the electrode material during a heat treatment stage of the manufac¬ ture of the panel. Similar contamination occurs also as the result of gaseous discharge during use of the device. Gold or gold alloy input and erase electrodes have stable life characteristics. The use of gold, however, has the disadvantage of high cost, and, if the transfer electrodes are not also of gold, also requires the precise alignment of the separate input, erase and transfer electrode screenings during device manufacture. Disclosure of the Invention It is an object of the present invention to provide an electrode for a gaseous discharge device of the kind specified which is resistant to contamination and which is inexpensive. Therefore, according to the present invention, there is provided an electrode for a gaseous discharge device of the kind including an envelope defining a chan¬ nel containing an ionizable gas, characterized by a conductive base, a metal oxide layer formed on said base, and a dielectric layer formed on said metal oxide layer. It will be appreciated that in an electrode according to the invention the metal oxide layer serves to protect the dielectric layer from contamination and voltage- breakdown. Furthermore, the metal oxide layer is capable of permitting direct coupling of the electrode with the ionizable gas. A further advantage of an electrode according to the invention when utilized as an input or erase electrode is that the same conductive base material can be used for the input, erase and trans¬ fer electrodes, thereby avoiding both the use of expensive gold or gold alloy input and erase electrodes and any need for an additional processing step of align¬ ing separately formed input and erase electrodes with the transfer electrodes during manufacture of the gas¬ eous discharge device. According to a further feature of the inven¬ tion, the metal oxide layer has a sheet resistivity of less than 5.5 x 10 . ohms per square. . Brief Description of the Drawings One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic cross-sectional view of a conventional direct-coupled input electrode in a plasma charge transfer display device; Fig. 2 is a schematic cross-sectional view of a conventional direct-coupled erase electrode in a plasma charge transfer display device; Fig. 3 is a schematic cross-sectional view of an input electrode incorporating the principles of the present invention; Fig. 4 is a schematic cross-sectional view of an erase electrode incorporating the principles of the present invention; and Fig. 5 is a schematic cross-sectional view of a plasma charge transfer display device embodying the principles of the present invention. Description of the Preferred Embodiment Referring to Fig. 5, a plasma display panel 20 comprises enclosure or substrate 7 of any suitable dielectric material, such as a relatively thin, flat clear glass plate, defining a channel 8 containing an ionizable gas, such as neon and hydrogen, at a pre¬ determined pressure. The display panel is provided with a pair 6-6 of keep-alive electrodes on the inner opposite walls 10-10 of the substrate, opposite one another, an input electrode I and an erase electrode E. A plurality of electrodes 9 identified as 1, , 2*. , 3-, , 4, etc. to 1„n, 2n__, 3n__, 4„n are located on the inner walls 10-10 of the substrate, opposite one another, in parallel alignment but laterally offset to subject the ionizable gas to an electric field when a suitable potential is applied between two nearest-adjacent opposite electrodes. Each succeeding pair of opposite nearest-adjacent electrodes ( e.g., I- , ^_y 2 ι r 2 i"" 3 l' 3 l~ 4 l' 4 1~ 1 2' etc * ) forms a cell. In the embodiment shown, all the elec¬ trodes except the input electrode I and the erase electrode Ξ are capacitively coupled to the encapsulated ionizable gas (coated with a dielectric layer 11). All of the electrodes, including the input and erase elec¬ trodes, are covered in addition with a transparent dielectric 12, such as the aforementioned magnesium oxide. The magnesium oxide layer 12 improves, the operational performance of the device 20, as discussed hereinabove. The input electrode I and erase electrode E, provided with the magnesium oxide layer are directly coupled to the ionizable gas. Alternating electrodes on one side of the display panel are connected electri¬ cally in separate groups. That is, electrodes numbered l- j _, 1 2 , I3, etc., are connected together to a terminal 1 while those numbered 3,, 3 2/ 3 , etc. are connected to a terminal 3. In like manner, on the opposite side, all electrodes numbered 2,, 2 2 , 2-,, etc. are connected to terminal 2 and those numbered 4, , ' 4~, 4,, etc. are connected to terminal 4. Terminals 1, 2, 3, 4 can be connected through a suitable switch (not shown) to a suitable voltage source (not shown) for sequentially applying voltage pulses to each of the transfer elec¬ trodes. Typically, the input electrode is made of the same conductive material as the transfer electrodes and is directly coupled to the encapsulated ionizable gas. Although metal or carbon may be used for forming the conductive base of the input and transfer electrodes, one conventional material that has been found to work very well is a paste consisting of* 50-95% by weight silver and 50-5% by weight glass frit. This material, when fired, provides a conductive metallic electrode due to the -silver particles dispersed in the glass matrix with the glass serving to adhere the silver to the substrate. In prior art devices, the input and erase electrodes shown respectively in Figs. 1 and 2 were bare except for the thin magnesium oxide coating 12 applied over them. In a preferred embodiment of the present invention, shown in Figs. 3 and 4, a coating 14 is applied between the conductive base 13 of the input electrode I and the magnesium oxide dielectric layer 12 and coating 17 is applied between the conductive base 16 of the erase electrode E and magnesium oxide layer 12. This coating 14 (and 17) is a slightly conductive metal oxide having a high electrical resistivity. To determine the upper limit of the desirable 5 surface electrical resistivity of the coating 14, (and 17) consider a typical plasma display panel for which the time (t) between voltage pulses applied to the input electrode is 80 us and which has an input electrode of width, w, of 6 mils (150 microns) and length, h, of 30 10 rails (750 microns) whose capacitance, C, is typically 0.55 pf. Assume that it is desired that the partially conductive coating on the input be such that the time between pulses represents three time constants. In other words, t=3T or R=t/3C where T is the time constant 15 and R is the resistance of the coating. Since, by definition, R=KL/wh where K " is the bulk resistivity of the coating material and L is the thickness of the coating over the input electrode, this yields a bulk • resistivity =wht/3CL=2.2 x 10 10 ohm-mil (5.6 x 10 11 ■ 20 ohm-microns) for a 0.4 mil (10 microns) thick layer, or a sheet resistivity of 5.5 x 10 ohms per square. This represents the maximum possible sheet resistivity of the coating 14 (and 17) in order to remove the charge on the input electrode before the next voltage pulse is applied 25 to the input electrode. Different values for t, T, etc. would, of course, change the value of the maximum ac¬ ceptable surface resistivity. A preferred example of a suitable material for coatings 14 and 17 is a ruthenium oxide-based resistor 30 paste. Formula No. 600-105 sold by Thick Film Systems Inc., Santa Barbara, California. This paste, having an unfired viscosity at 25°C of 750 + 150 poise, when fired at a temperature of about 600°C is believed to become essentially glass but nevertheless has a finite small 35 conductivity due to the ruthenium oxide material dis¬ persed within the glass. The ruthenium oxide conductive coating eliminates contamination of the magnesium oxide by the input electrode. . A suitable pre-fired thickness of the ruthenium oxide-based material 14, 17 is in the range of 10-30 microns. While the preferred thickness of the coating is believed not limited to the above mentioned range, D.C. operation was preserved and the stability and other operational characteristics of the display d-evice were found to be the same when ruthenium oxide coating of the above thickness range was used. The above-described ruthenium oxide coating is rated as having a sheet resistivity of approximately 1 megohm per square when fired at a temperature of 600°C. Although no precise values of resistivity are available, it is estimated that the ruthenium oxide coating re¬ sistivity resulting from the exemplary 575°C firing temperature is about 10 megohms per square. Both values are well below the established upper limit of about 5.5 x 10 ohms per square. Other metal oxides that may be used for the coating 14 are resistor pastes made from oxides of thallium, palladium, iridium, indium, tungsten, tan- tallum, rhodium, copper, bismuth, and lead. ~ ~ The erase electrode E (Fig. 4) at the other end of the display panel is used to clear the display. The erase electrode in the present embodiment, like the input electrode I, is directly coupled to the encap¬ sulated gas by means of the metallic oxide coating 17 applied between the bare erase electrode conductive base 16 and the magnesium oxide film 12. The above dis¬ cussion of the metallic oxide coating for the input electrode applies to the erase electrode as well and the preferred metal oxide is -also the ruthenium oxide-based material. Typically, a display panel has several par¬ allel channels 8 running in a horizontal direction along the panel length. Each channel has its own input and erase electrodes; the erase electrodes are connected together since no unique selection is required at the erase electrode. The several channels 8 generally share the same 1, 2 , 3, 4 pattern of transfer electrodes. In construction of such a plasma charge trans¬ fer device, all of the electrodes 1,, 2,, 3-,, 4,, etc. including the input electrode I (conductive base 13 thereof)" and the erase electrode E (conductive base 16 thereof) can be formed on the substrate walls 10-10, by using a silk screening technique to pattern the elec¬ trodes and then firing the "green" electrode material. Another conventional method of forming the electrodes is by a photoresist technique in which the conductive pattern is achieved by etching away a conductive coating applied on the inner surface of each of the substrate walls 10-10. Next, all of the electrodes except the input and erase electrodes are covered with the di¬ electric layer 11. The input and erase electrodes are then separately covered with the metallic oxide layer which is fired preferably at _575°C to form layers 14, 17 for the input and erase electrodes. Over all of the electrodes and their respective coating materials, the dielectric layer 12 is formed to ultimately form an enclosure to contain the ionizable medium. The two substrates 10-10 are then aligned and joined to the substrate 7 by heat treatment. An exhaust port in one of the substrates is utilized to evacuate -the cavities, i.e., channels 8, and thereafter an ionizable gas is introduced therein, and the device sealed. Operation of plasma panels 20 is well-known in the art, see for example U.S. Patent No. 3,781,600 issued to William E. Coleman and Clarence W. Kessler, and will be but briefly described here. In actual operation of the device, a keep-alive cell is formed by the pair of keep-alive electrodes 6-6 which are capa- citively coupled to the gas. The electrodes 6-6 are connected to a source 18 of alternating pulse voltage of suitable magnitude to ionize the gas within the keep- alive cell for facilitating "firing" or discharge of the first cell formed by the input electrode I and the first electrode 1,. The device is serially addressed by applying suitable voltage pulses to the input electrode and/or the electrode 1,. When the potential between the input electrode I and the first transfer electrode 1, is above a threshold (or firing) voltage, V. > V- (where V. is the potential difference between the input and the first electrode, and V f is the cell firing voltage), a gaseous discharge occurs. This discharge between the input and the first electrode is quickly extinguished, however, because the trapped charge (or wall charge, as is conventionally known) on the electrode 1- gives rise to a voltage Vwc opposing the initially applied voltage. Next, a voltage V 5 is applied between the first and second transfer electrodes, 1, and 2, respectively. When V of suitable polarity is applied to the first and second transfer electrodes, Vwc adds algebraically such that the total voltage between the two is greater than the firing voltage V_ and a gaseous plasma discharge occurs. It should be noted that if no discharge had occurred in the first cell I-l-i, no trapped charge would be present on the electrode 1 I- . Then, when Vs is ap- plied between the first and "second electrodes, no gas¬ eous plasma discharge would occur in the cell 1,-2, . By sequentially applying V to successive cells and. thus utilizing the trapped charge on the electrode wall of the previously-discharged cell, this charge and the trapped charge initiated by the input pulse can be transferred to any cell along the length of the plasma charge transfer device. Focusing now on the novel aspects of this invention, the slightly conductive metal oxide coat¬ ing 14 on the input electrode enables direct electrical connection.of the input electrode I to the encapsulated gas without the previous contamination problems. This is possible because the metal oxide coating functions both as a highly resistive protective cover, and also. in a limited sense, as a conductor. That is, when a potential suitable for inputting charge is applied to the input electrode such that V. > V f and therefore a plasma discharge is initiated in cell I- , the high resistivity coating 14 dissipates any undesirable charge buildup on the input electrode in time for the next voltage pulse to be. applied to the input. However, the coating 14 has sufficient conductivity to give rise to only a slight current through the coating and thereby reduce the possibility of breakdown of the overlying protective magnesium oxide coating on the input elec¬ trode. The conductive coating 17 on the erase elec¬ trode enables direct electrical connection of the erase electrode E to the encapsulated gas, like the input electrode I, without the previous contamination prob¬ lems. Upon discharge of the next to the last cell in the device (the cell formed by the elect •rodes 3n-4_n_ adjacent the erase electrode E), wall charge is formed on the wall of the electrode 4 . Then, upon discharge of the last cell, 4 -E, the wall charge is transferred to the erase electrode which, being maintained at the relatively low or ground voltage, extinguishes the discharge. The high resistivity coating 17 dissipates undesirable charge build up on the erase electrode and protects the overlying magnesium oxide coating in the same manner as explained in connection with the input electrode coating 14.

Claims: CLAIMS : 1. An electrode for a gaseous discharge device (20) of the kind including an envelope (7) defining a channel (8) containing an ionizable gas, characterized by a conductive base (13 or 16), a metal oxide layer (14 or 17) formed on said base, and a die¬ lectric layer (12) formed on said metal oxide layer. 2. An electrode according to claim 1, characterized in that said metal oxide layer (14 or 17) has a sheet resistivity of less than 5.5 x 10 ohms per square. 3. An electrode according to claim 2, characterized in that said metal oxide layer (14 or 17) consists essentially of ruthenium oxide. 4. An electrode according to claim 1, characterized in that said dielectric layer (12) con¬ sists of magnesium oxide. 5. An electrode according to claim 1, characterized in that said conductive base (13 or 16) consists essentially of 50% to 95% by weight silver and 50% to 5% by weight glass. 6. An electrode according to claim 1, characterized in that said electrode (I) is an input electrode. 7. An electrode according to claim 1, char¬ acterized in that said electrode (Ξ) is an erase elec¬ trode. 8. A gaseous discharge device (20) including an envelope (7) defining a channel containing an ioniza¬ ble gas, and a plurality of transfer electrodes (1-, 2, , 8. ( concluded ) 3,, etc.) characterized by an input electrode (I) accord- ing to any one of the preceding claims, said transfer electrodes ( , 2-,, 3-, etc.) being insulated from said gas by a further dielectric layer which capacitively couples said transfer electrodes to said gas. 9. A gaseous discharge device (20) according to claim 8, characterized by an erase electrode (E) according to any one of claims 1 to 7.

Inventors: HALL S
Applicants: NCR CO, NCR CORP
Application year: 1981
Main classification: H01J17
IPC codes: H01J11, H01J17, H01J61