Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Plasma excitation
Reexamination Certificate
1998-05-01
2001-05-08
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Plasma excitation
C313S582000, C313S586000, C313S495000, C313S587000, C313S483000
Reexamination Certificate
active
06229582
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a display device comprising at least one compartment, which compartment contains an ionizable gas, walls of the compartment being provided with electrodes for selectively ionizing the ionizable gas during operation, and a voltage being applied across the electrodes during operation.
Display devices for displaying monochromatic or color images comprise, inter alia, direct-current plasma-display panels (dc PDPs) and direct-current plasma-addressed liquid-crystal display devices (dc PALC-displays), both types of display devices preferably being of the thin type.
A display device of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,596,431 (PHA 60 092). The thin-type display device described in said document comprises a display screen having a pattern of (identical) so-called data-storage or display elements and a plurality of compartments. Said compartments are filled with an ionizable gas and provided with electrodes for (selectively) ionizing the ionizable gas during operation. In the known display device, the compartments are mutually parallel, elongated channels (shaped in a so-called channel plate), which serve as selection means for the display device (the so-called 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) which ionize the ionizable gas, thus forming a plasma. If the voltage across the electrodes in one channel is switched off and the gas is de-ionized, the following channel is energized. On the display-screen side of the display device, the compartments are closed by a (thin) dielectric layer (“microsheet”) provided with a layer of an electro-optical material and further electrodes which serve as so-called data electrodes or column electrodes of the display device. The display device is formed by the assembly of the channel plate with the electrodes and the ionizable gas, the dielectric layer, the layer of the electro-optical material and the further electrodes.
In a plasma-display panel, a plasma-discharge is used, on the one hand, to directly excite phosphors of desirable display elements or, on the other hand, to generate light (for example UV light) which is used to excite phosphors of desirable display elements.
A disadvantage of the known display device is that such display devices exhibit a relatively high energy consumption.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a display device exhibiting a lower energy consumption.
To achieve this, the display device in accordance with the invention is characterized in that at least a part of the surface of the walls of the compartment is provided with a layer of a material for emitting secondary electrons, the electrodes being at least partly uncovered.
During the time that the gas is ionized, the plasma-discharge is maintained by applying a specific sustain current and a sustain voltage across the electrodes in the compartment. In a display device with a plasma-discharge in an atmosphere comprising an ionizable gas, this sustain current is much higher, on average, than in the case of a (pure) discharge between two electrodes under vacuum conditions. In the known display device, a (considerable) part of the electrons and ions in the plasma discharge are lost at the walls of the compartment. By covering, in accordance with the invention, at least a part of the surface of the walls of the compartment with a layer of a material having a (high) secondary electron-emission coefficient, the losses at the walls are reduced and the sustain current for maintaining the plasma in display devices in accordance with the invention is lower than in the known display devices. Said lower sustain current causes the energy consumption of the display device to be reduced. As the plasma-discharge is maintained by a direct current, the secondary electron-emitting material leaves the electrodes at least partly uncovered.
A material for emitting secondary electrons is taken to mean in this application, a material which emits one or more secondary electrons in response to a (primary) electron which is incident on (the surface of) the material. The yield &dgr; of secondary electrons is a function of the energy of the primary electrons E
p
, the maximum yield &dgr;
max
being achieved at a value of the primary electron energy of E
p
max
. The two primary-electron energy values corresponding to a yield of &dgr;=1 are commonly referred to as the first and the second crossover and are referenced E
I
and E
II
, respectively. Preferably, the secondary electron-emitting materials covering parts of the surface of the walls of the compartment of the display device have a relatively low first crossover energy value E
I
. As a result of the high density and a plurality of intercollisions, the primary energy values of the drifting electrons reaching the wall of the compartment are relatively low (in the range between 2 and 5 eV). The above-mentioned choice of E
I
enables a relatively high secondary-electron yield to be achieved.
In general, the display device comprises a number of compartments, each compartment comprising at least two electrodes for ionizing the gas.
An additional advantage of covering the walls of the compartment of the display device with the secondary emitting material is that the depth of the compartment can be reduced (for example by 10-20%). Such a reduction is possible because the walls contribute to the provision of (secondary) electrons to the plasma discharge, so that a compartment having a smaller depth is sufficient.
To bring about a uniform ignition of the plasma-discharge, it is desirable that the effective surface area of the electrodes should be as large as possible. To achieve this, the electrodes are preferably completely uncovered, that is, they are not provided with a layer of the secondary emitting material.
It is also desirable that the layer of the material provided on the walls of the compartment should not only have a high secondary-electron yield but also a high stability against ion and electron bombardment. To achieve this, an embodiment of the display device in accordance with the invention is characterized in that the material includes a material of the group formed by magnesium oxide (MgO), chromium trioxide (Cr
2
O
3
), silicon nitride (Si
3
N
4
) and yttrium trioxide (Y
2
O
3
). A material which can very suitably be used is magnesium oxide MgO (high stability against ion and electron bombardment) which has a measured secondary-electron emission coefficient &dgr; of approximately 5 to 11. Other suitable materials are TiO
2
, Ta
2
O
5
, AlN and Al
2
O
3
. Particularly Al
2
O
3
has a high secondary electron-emission coefficient (&dgr;≈10). The use of mixtures of said materials enables the desired properties of the layer of the secondary emitting material to be achieved.
A preferred embodiment of the display device in accordance with the invention is characterized in that the thickness of the secondary electron-emitting layer is above 20 nm. Too thin a layer (<5 nm) contributes only little to the secondary electron emission. Secondary electron-emission measurements carried out on said materials show that the yield of secondary electrons increases rapidly up to a layer thickness of approximately 20 nm, between 20 and 40 nm a further improvement of the yield is obtained and, at layer thicknesses≧40 nm, the yield of secondary electrons remains stable.
An embodiment of the display device in accordance with the invention is characterized in that the compartment comprises an elongated channel. If the display device is composed of a number of channel-shaped compartments, these channels are arranged so as to be mutually parallel. The secondary electron-emitting layer can be provided more readily in such elongated channels than in the plurality of compartments. In addition, in the case of channels, the ratio of the surface of the walls to the volum
Ngo Tulie
Sikes William L.
U.S. Philips Corporation
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