Electric lamp and discharge devices: systems – With cathode or cathode heater supply circuit
Reexamination Certificate
2000-02-25
2001-11-20
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
With cathode or cathode heater supply circuit
C315S112000, C315S118000, C313S582000, C313S584000
Reexamination Certificate
active
06320318
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a display device having a display panel comprising at least one compartment which contains an ionizable gas mixture including at least a basic gas and an additional gas, the compartment being provided with electrodes for selectively ionizing the ionizable gas mixture during operation, and the display panel comprising means for supplying the additional gas to the ionizable gas mixture.
Display devices for displaying monochromatic images or color images comprise plasma-addressed liquid crystal display devices, referred to as PALC displays, preferably of the flat-panel type. PALC displays are used, for example, as displays for television and computer applications.
A display device of the type described in the opening paragraph is known from, for example published European patent application EP 0 816 898. The flat-panel type display device described in this application has a display screen with a pattern of (identical) data storage or display elements and a plurality of compartments. The compartments are filled with an ionizable gas mixture and are provided with electrodes for selectively ionizing the ionizable gas mixture during operation. In the known display device, the compartments have the shape of parallel, elongated channels in the form of a channel plate functioning as selection means for the display device. These are the plasma-addressed row electrodes. By applying a voltage difference across the electrodes in one of the channels of the channel plate, electrons are emitted from the cathode. The electrons will ionize the ionizable gas so that a plasma is formed. When the voltage across the electrodes in one channel is switched off and the gas mixture is deionized, a subsequent channel is switched on. At the display screen side of the display device, the compartments are closed by a relatively thin dielectric layer referred to as the microsheet. This layer is provided with electro-optical material, and further electrodes which function as the data electrodes or column electrodes of the display device. The further electrodes are provided on a substrate. The display panel is constituted by the assembly of the channel plate with the electrodes and the ionizable gas mixture, the dielectric layer, the layer of the electro-optical material and the further electrodes.
In a PALC display panel, the panel is addressed row by row. The resolution is determined by the number of rows which can be written per second. The minimally required time for writing a row is determined by the time required to ignite the plasma, the time required to charge the microsheet and the time required to switch off the plasma, referred to as the afterglow decay time. The shorter the afterglow decay time, the larger the number of rows which can be written per second and the higher the maximally achievable resolution.
A gas mixture which is known to be suitable for a relatively short afterglow decay time is a He-H
2
mixture. However, a problem with such a mixture is the stability with respect to time. The quantity of H
2
is relatively small (typically less than 3%) and the probability that the equilibrium pressure of H
2
in the mixture remains constant is very small. This pressure variation is caused by the loss of hydrogen, for example, due to implantation in the electrodes during operation of the panel, or because there may be hydride formation. At a too low hydrogen pressure, the afterglow decay time will be too long, while the plasma will no longer function optimally at a too high hydrogen pressure. Said European patent application proposes providing the panel with a material which supplies and absorbs hydrogen so as to control the hydrogen pressure in the compartments.
A drawback of this solution is that, for the material used as a hydrogen source, the equilibrium pressure of hydrogen in the material is dependent on the temperature. Since the operating temperature of a PALC display panel may vary between 0° C. and about 80° C., a variation of more than a factor of 100 may occur in the hydrogen pressure, which is not acceptable.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a display device in which the hydrogen pressure in the display panel is maintained substantially constant.
To this end, the display device according to the invention is characterized in that the display device is provided with temperature-stabilizing means for stabilizing the temperature of the means for supplying the additional gas.
Since there is a loss of the additional gas, resulting in a reduction of the concentration of the additional gas, and since the afterglow decay time preferably remains below a given value, it is desirable to take measures so as to maintain the partial pressure of the additional gas in the ionizable gas mixture substantially constant. The partial pressure of the additional gas can be controlled by providing the display device with means which supply additional gas. The materials used for this purpose yield a constant partial pressure of the additional gas within a given concentration range of the additional gas. For this given concentration range, this pressure is thus substantially independent of the concentration of the gas in the material, but is dependent on the temperature of the material. By stabilizing the temperature of the material supplying additional gas, said temperature dependence and, consequently, the variation of the partial pressure of the additional gas in the ionizable gas mixture is compensated. In this way, the pressure of the additional gas in the compartments can be maintained at the desired level.
A preferred embodiment of the display device according to the invention is characterized in that the additional gas is a gas of the group constituted by hydrogen, deuterium, or hydrogen deuterium.
Important parameters of the plasma discharge cycle of the display device are the electric conductivity of the plasma discharge and the decrease of conductivity in the afterglow decay time. When the decrease of the conductivity of the plasma discharge progresses too slowly, the discharge may continue while a subsequent data row is already being written, which is undesirable. A conductivity which decreases too rapidly also has detrimental effects.
Helium (He) is the most frequently used basic gas in display devices of the type described in the opening paragraph. The ignition voltage of the plasma discharge may be decreased by adding small quantities of a gas (typically of the order of several percents) to the helium. Such gases generally have an ionization potential which is lower than that of helium. The gas mixtures formed are referred to as Penning mixtures. A known additional gas is hydrogen (H
2
). By using such gas mixtures, not only the ignition characteristic of the plasma discharge is influenced but also, for example, the current which is necessary to maintain the discharge, and the afterglow decay characteristic of the discharge. The properties of the plasma discharge cycle of the display device can be influenced by making a suitable choice of the additional gas. The additional gas may be alternatively deuterium (D
2
) or hydrogen deuterium (HD), so that, in comparison with the addition of hydrogen, the plasma discharge will have a lower ignition and sustain voltage and a longer afterglow decay time.
A further embodiment of the display device according to the invention is characterized in that the temperature-stabilizing means comprise a Peltier element accommodating the means for supplying the additional gas.
A Peltier element is a very suitable element with which the means for supplying the additional gas can be both cooled and heated.
A further embodiment of the display device according to the invention, comprising an exhaust tube, is characterized in that the Peltier element is accommodated in the exhaust tube, and in that at least a part of the heat exchanger of the Peltier element forms part of the wall of the exhaust tube.
The exhaust tube has an exhaust connection for the display devic
Bisschops T.
Schreuders Herman
Van Slooten Udo
Philogene Haissa
U.S. Philips Corporation
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