Title:
METHOD AND MEANS FOR OPERATING A PLASMA DISPLAY PANEL
United States Patent 3614526


Abstract:
A thermally triggered plasma display panel of the gas-discharge type comprising a plurality of discrete electrically isolated gas-containing cells. One side of each cell is coupled to a common transparent electrode. The other side of each cell is coupled to a corresponding highly resistive thermal electrode. All of the electrodes in the plasma display panel are separated from the gas medium by a thin transparent dielectric coating of glass. The method by which ionization is established is to apply a continuous alternating field across the common electrode and all of the thermal electrodes, thus placing each cell under the influence of a continuous alternating electric field. The magnitude of the applied electric field is insufficient to establish ionization, at normal cell pressures, within the respective cells, but is sufficient to sustain ionization upon the initiation of ionization within the respective gas cells. Ionization is initiated by the momentary application of current, which produces a quick heat. An increase in cell pressure, caused by the quick heat, results in a lower ionizing voltage. Therefore, the current pulse, in cooperation with the alternating electric field, initiates ionization within the cells corresponding to the current-pulsed thermal electrodes. The magnitude of the alternating field is sufficient to sustain ionization once ionization is initiated, thus providing the illumination necessary for the display of the desired information.



Inventors:
JANNING JOHN L
Application Number:
05/041260
Publication Date:
10/19/1971
Filing Date:
05/28/1970
Assignee:
NATIONAL CASH REGISTER CO.:THE
Primary Class:
Other Classes:
313/15, 313/582, 315/98, 315/115, 330/169
International Classes:
H01J17/49; H04N5/66; (IPC1-7): H01J11/00
Field of Search:
313/15,188 315
View Patent Images:



Primary Examiner:
Kominski, John
Claims:
What is claimed is

1. A plasma display device, comprising:

2. A device in accordance with claim 1 wherein said common electrode is transparent, said common electrode further having a thin transparent dielectric coating thereon.

3. A device in accordance with claim 1 wherein said thermal electrodes have a thin dielectric coating thereon.

4. A plasma display device, comprising:

5. A device in accordance with claim 1 wherein said thermal electrodes and said transparent electrode are separated from said gas by a thin dielectric coating.

6. A plasma display device, comprising:

7. A device in accordance with claim 3 wherein said first substrate and said film electrode deposits are transparent.

8. A method of generating a display in a plasma display panel formed of a plurality of gas-containing cells, each cell coupled to a common electrode and each cell further coupled to corresponding thermal electrodes, comprising the steps of:

9. The method as specified in claim 8 wherein said directed heat is generated by momentarily applying current pulses to selected ones of said thermal electrodes.

Description:
BACKGROUND OF THE INVENTION

The present invention relates to a plasma display panel for the visual display of information and the method in which the display is initiated into a plasma state by the momentary pulsing of any electric current through selected portions of the display in cooperation with an alternating electric field.

The prior art is replete with many different kinds and types of display devices which have been developed for the visual display of information. Devices of this type are exemplified in U.S. Pat. No. 3,127,535, issued Mar. 31, 1964, to Harold T. Westerheim, U.S. Pat. No. 3,376,454, issued Apr. 2, 1968, on the application of Elmer O. Stone, U.S. Pat. No. 3,499,167, issued Mar. 3, 1970, on the application of Theodore C. Baker et al., and British Pat. Nos. 1,161,832 and 1,161,833, issued Aug. 20, 1969, to University of Illinois Foundation.

Illumination conforming to the type of information to be displayed is achieved by the ionization of selected plasma cells which conform to the particular information to be displayed. In order for discharge or ionization to occur within a cell, it is necessary to have free electrons which, when under the influence of an electric field, create large numbers of secondary electrons, thus creating an avalanche of electrons i.e., ionization.

Gases in an ionized state are often referred to as a plasma, and the cells containing such gases are known as plasma cells.

In most prior art devices, ionization has been accomplished by photon bombardment of the cathode surface with ultraviolet light for providing free electrons, or by the use of very high voltage pulses across the plasma cell electrodes. Some of the prior art even goes so far as to provide a live cell (that is, a cell which is continually kept in a plasma state i.e., in an ionized state), thus insuring that a supply of free electrons is available to any adjacent cells that may have need for them. The applicant has eliminated the need for the aforementioned means necessary in initiating ionization in a gas cell by simply utilizing the heat-generating characteristics of highly resistant thermal electrodes in cooperation with an electric field.

Applicant's structure is novel in that it provides simple means for initiating ionization of gas in selected cells, thereby decreasing the cost of this type of display device.

SUMMARY OF THE INVENTION

The invention relates to a thermally triggered plasma display panel in which there are formed a plurality of discrete, gas-containing cells. On one side of the cells is positioned a transparent conductive coating, which functions as a common electrode for each and every cell which the conductive coating overlies. The second side of each cell is individually connected to a thermal electrode which, in turn, is individually connected to a current source. The transparent conductor and all of the thermal electrodes are separated from the gaseous medium by a thin layer of glass.

The illumination necessary for viewing is provided by applying pressures alternating electric field across every cell via the transparent electrode and the thermal electrodes. The magnitude of this alternating electric field is insufficient to cause ionization in any of the cells, thereby requiring the application of additional energy in order to establish ionization in the gas cells. The additional energy is realized from the heat produced by the application of a current pulse through selected thermal electrodes, which raises the internal pressure of the associated gas cells. Since the internal pressure of a contained gas is dependent on temperature, the increased pressure lowers the ionizing voltage required by the gas cells. Therefore, at the higher pressures the alternating electric field is sufficient to initiate ionization within the associated cells, thus providing an illuminated display. All pressure returns to normal after cooling; however, ionization is sustained for so long as the alternating electric field is applied.

In some of the prior art devices, it is necessary to utilize what are commonly referred to as wall charges for the operation of displays utilizing gas cells. The present invention obviates this requirement, since ionization can be carefully and accurately controlled by the application of current pulses to the thermal electrodes in cooperation with the electric field.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an isometric view of a visual display device embodying the principles of the instant invention.

FIG. 2 is a curve illustrating the firing voltage characteristics versus pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the plasma display panel is shown generally at 20. The panel consists of plates 22 and 26, each having first and second sides, which may also be designated as outside and inside surfaces, respectively. At least one of the plates is transparent, preferably the plate 22, and may be fabricated from a dielectric material such as glass. The second side of the plate 22 has a transparent electrode 28, approximately 4,000-Angstroms thick, attached thereto. This transparent electrode 28 may be formed from tin oxide, cadmium oxide, or a thin gold-film deposit. The transparent electrode may be deposited on the plate 22 by conventional methods, such as thin metal or metal oxide deposition techniques. The transparent conductor 28 is covered by a thin dielectric coating 38, approximately 0.001-inch thick, such as glass. The plate 26 has deposited thereon an array of thermal electrodes 30, which are on the order of 0.020 inch wide and on the order of 4×10-6 -inch thick. The thermal electrodes 30 may be fabricated from electrically resistant materials such as nichrome, rhenium, tantalum, or other such materials. These thermal electrodes are deposited by well-known conventional techniques. Each individual thermal electrode 30 is connected to an input line 32 and a grounded output line 34. These thermal electrodes 30 are further covered by a light dielectric deposit 36, such as glass, thus preventing the electrodes 30 from being in physical contact with the gas. The glass dielectric coatings 36 and 38 also eliminate deterioration of the electrodes 28 and 30.

A third plate 24, approximately 0.015-inch thick, is illustrated in FIGURE 1 as having a plurality of holes 40 etched therethrough. These holes 40 are positioned on the plate 24 in such a manner that, when the plate 24 is positioned on the plate 26, each hole 40 is in alignment with a corresponding thermal electrode 30. The completed assembly 20 is achieved when the plate 24 is sandwiched by the plates 22 and 26. The plates 22, 24, and 26 are sealed together by cement or glass frit, thereby resulting in an airtight structure. However, at least one surface of the plate 24 is roughed up, so that gas is able to transfer from one cell to another, given enough time and pressure differential existing between cells. Evacuation means (not shown) are utilized for evacuating the gas cells formed by the holes 40 of any air which they may contain and for introducing a gas mixture, such as neon, argon, and nitrogen, into those cells.

The input lines 32 are connected to a current source 42, and the output lines 34 are connected to a current source 42, and the output lines 34 are connected to ground. A selective thermal control 44, which may be a digital control on a computer, is utilized for selectively pulsing current through the desired electrodes. The transparent electrode 28 on the plate 22 is connected to a voltage source 46.

Prior to a discussion of the operation of the plasma display assembly 20, it is in order to discuss some of the fundamental rules which apply to this type of display cell. Ionization in this type of cell is initiated by a firing voltage Vf. The firing voltage Vf is, in turn, dependent upon the pressure of the particular gas mixture contained in the plasma cell and is also dependent upon the distance between the electrodes to which Vf is applied. The curve illustrated in FIG. 2 is typical of firing voltage Vf versus gas pressure of gas contained within a rigid structure. It can be seen that, as the pressure increases, the firing voltage Vf decreases to a minimum and begins to increase. The internal pressure of a gas mixture within a contained cell is dependent on temperature, since the total number of gas molecules, the gas volume, and the distance between the electrodes to which Vf is applied are all constant. The pressure thus is proportional to the absolute temperature and inversely proportional to the volume. Since the volume in a rigid cell cannot change, the pressure is dependent entirely upon the temperature. Therefore, by changing the pressure within the cells by decreasing or increasing the cell temperature, the firing voltage Vf can be increased or decreased. It is desirable to utilize a firing voltage which is low in magnitude, in that this lowers the cost of fabrication and the cost of the electronic drivers necessary in the operation thereof.

Once ionization is initiated, it can be sustained with a lower voltage Vs; i.e., the sustaining voltage. Thus in most prior art devices two voltage levels are necessary to operate this type of gas cell. The normal cell pressure (NCP)-- i.e., the cell pressure at room temperature-- is shown in FIG. 2. It can be seen that increasing the pressure of a cell between NCP and A lowers the firing voltage requirements for the gas cell. A lower Vf can be used within the range of A and B. This results in the ionization (i.e., creation of a plasma) within that cell, sustained by a sustaining voltage Vs. The instant invention obviates the need for two voltage levels, in that the sustaining voltage Vs can now be utilized as the firing voltage simply by increasing the cell pressure.

The following description of operation contemplates a plasma display panel containing gas which is in a nonionized state. Upon the application of an alternating voltage Vs by the voltage source 46, which can be any suitable voltage transformer, an alternating electric field is established across all the cells which are a part of the panel 20. The strength of the electric field is insufficient to ionize the contained gases. Ionization is achieved by passing a current pulse through selected ones of the thermal electrodes. The thermal electrodes may be selected by the selective thermal control 44, which may be digital computer control or a conventional switching device. The current pulse produces a quick heat, in the range of 300° Centigrade, for periods ranging from 3 to 10 milliseconds, depending upon the particular cell parameters, which causes a pressure rise in those gas cells corresponding to the selected thermal electrodes. The higher pressure, in cooperation with the pulsating electric field, supplied by Vs, causes ionization to occur in the selected gas cells, thereby producing illumination therein. In order to erase a display, it is merely necessary to interrupt the sustaining voltage for a period of time, thereby allowing the ionized gases to revert to their normal molecular state.

In order to aid those skilled in the art, in the practice of this invention, the aforementioned dimensions for construction of the plasma display device are given by way of exemplification. The electrical characteristics are not given, since these are a matter of choice, depending upon the supply voltage, the initial gas pressure, the gas mixture, the ambient temperature, and the size of the cavity 40.