Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Discharge Lamp fitted with Cold Cathode
The invention relates to a discharge lamp having a
cold cathode, in particular a discharge lamp which is
operated by means of a dielectrically impeded discharge.
In dielectrically impeded discharges, at least one
electrode is separated 54from the discharge space by a
dielectric layer.
Here, the term "discharge lamp" signifies
radiators which emit light, that is to say visible
electromagnetic radiation, or else ultraviolet (W) and
vacuum ultra-violet (VUV) radiation.
The task of the electrodes of discharge lamps is,
inter alia, to supply the number of free electrons necessary
for maintaining a self-maintained discharge. These
electrons are essentially supplied by the cathode (in the
case of operation with a voltage of invariable polarity, for
example DC voltage or unipolar pulsed voltage) or by the
instantaneous cathode (in the case of operation with a
voltage of variable polarity, for example AC voltage or
bipolar pulsed voltage). Furthermore, free electrons are
generated in the cathode fall space positioned in front of
the (instantaneous) cathode.
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The cathode fall space is characterized by a field
. strength which is high by comparison with the remaining
space between the electrodes, that is to say the electric
potential with respect to the cathode drops relatively
strongly in this region. Consequently, a generation of
tree electrons in the cathode fall space is linked in
general to a correspondingly high power turnover. With
regard to as efficient as possible a generation of useful
radiation, that is to say of light or UV/VTJV radiation,
the aim is to reduce the cathode fall of discharge lamps.
Zn the final analysis, this requires increasing the
efficiency with which electrons~emerge from the cathode
surface .
One possibility consists in designing.the electrodes as
filaments, heating them and, if appropriate, additionally
coating them with an emitter paste, in order thereby to
improve the thermal electron emission. This relatively
complicated technique is applied, for example, in the
case of fluorescent lamps. Further disadvantages asso-
ciated with these so-called hot electrodes are the
limited service life of the lamp and the heating up of
the filling gas.
DE-U 295 O1 343 discloses a glow fluorescent lamp having
a cold cathodeT~electrodes are doped with manganese or
lanthanum in order to reduce the cathode fall.
US 5 418 424 describes a V'LTV radiation source having a
photocathode. Formed inside the radiation source, inter
alia, are Xe excimers which generate shortwave ViJV
radiation. The photocathode comprises a 50 nm thick
photoemitting layer, applied to a stainless steel sur-
face, and a grid electrode in parallel with said layer.
The V'UV radiation passes through the grid electrode onto
the photoemitting layer and releases electrons there by
means of the photo effect. These electrons then pass
through the grid electrode and thus maintain the'dis-
charge. A disadvantage is that the described mechanism
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for generating electrons is based on the VIJV radiation
generated by the discharge itself. Thus, during ignition of
the discharge, this electron source is not yet available, or
is so only to a limited extent. This can be seen in a
negative fashion in a loss of efficiency in the case of
discharges operated in pulses.
EP 675 520 A2 discloses a pot-shaped or beaker-
shaped electrode for miniature neon lamps for automobile
illumination. The inner wall of the electrode is uniformly
covered with an emitter layer.
Finally, US 5 159 238 describes a cathode for flat
discharge lamps. The cathode comprises electrically
conductive oxide particles which are embedded in a
low-melting glass, for example lead glass.
It is the object of the invention to remove or
mitigate the said disadvantages, and to provide a discharge
lamp having improved electron emission properties at a low
electrode temperature.
According to the invention there is provided a
discharge lamp comprising: at least one anode and a cold
cathode in an at least partially transparent discharge
vessel, the cold cathode having a ferroelectric between two
electrodes, a first one of the two electrodes having
openings for releasing electrons; means for releasing
electrons through the openings on the first electrode by
applying a first time-variant voltage signal with polarity
reversals to the two electrodes of the cold cathode; means
for accelerating the released electrons by applying a second
time-variant voltage signal to the at least one anode and to
the first electrode of the cold cathode; and means for
synchronizing the first and second time-variant voltage
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signals so that each polarity reversal of the first signal
time-variant voltage in a first direction is followed by an
increase in an absolute value of an amplitude of the second
time-variant voltage signal.
The basic idea of the invention consists in making
use of the capacity, demonstrated by R. Miller and
S. Savage, of
~
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ferroelectric materials to emit electrons (R. Miller and
S. Savage, Journal of Applied Physics 21, 1960, page
662ff), specifically for a cold cathode having improved
electron emission properties. The mechanism of electron
' 5 release is based, as is known, on a rapid reversal of
polarity inside the ferroelectric.
The invention proposes a cold cathode having a sandwich-
type design. In this arrangement, a ferroelectric
material, for example a ceramic made from lead zirconium
l0 titanate (PZT) or lead lanthanum zirconium titanate
(PLZT) is arranged between two electrically conductive
surfaces, realized by plates, foils, films or similar,
for example. One or both surfaces have at least one or
more openings, for example in the~form of a perforated
l5 plate, wire grid or an appropriate applied layer. The
ferroelectric material is uncovered at the sites of the
openings, so that the free electrons generated by the
ferroelectric can pass through these openings in the
direction of the anode of the discharge device. The
20 sandwich-type cold cathode according to the invention
will also be termed a sandwich cathode below for the sake
of brevity.
During operation in a discharge lamp, a sequence of
rapidly alternating voltage pulses is applied to the two
25 electrically conductive surfaces. The sandwich cathode
therefore acts like an electric capacitor, the surfaces
corresponding to the capacitor electrodes. In this
scenario, the ferroelectric plays the role of the
capacitor dielectric. The rapid polarity reversal inside
30 the ferroelectric which was outlined at the beginning is
achieved in this way.
Tt is preferred for the sandwich cathode and, in parti-
cular, the ferroelectric layer to be of thin design in
order to keep as low as possible the level of the voltage
35 ~ pulses which is 'required for the polarity reversal.
Typical values for the thickness of the layer are less
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than 1 mm, values in the region of approximately 10 Fcm
and 0.2 mm being preferred.
The sandwich cathode can be used with various types of
cold cathode discharge lamps. The cold cathode according
to the invention develops its advantageous effect rode--
pendently of whether the anode has a dielectric
impediment or not. The only decisive factor in this
regard is that the sandwich cathode itself is located
inside the discharge vessel without a dielectric impedi-
went. The geometry of the sandwich cathode is adapted in
this case as optimally as possible to the geometry of the
discharge vessel.
The sandwich cathode is shaped as a hollow cylinder for
tubular discharge lamps, for example like the glow
fluorescent lamp, mentioned at the beginning, of
DE-U 295 01 343, or like the glow fluorescent lamp
disclosed in EP 700 074. The metal inner and outer walls
of the sandwich cathode each have a leading-in wire. The
two leading-in wires are led to the outside from the.
discharge vessel in a gastight fashion. It is thereby
possible to lead the rapidly alternating voltage pulses
to the sandwich cathode.
Advantageous use is likewise made of the sandwich cathode
in a discharge lamp which is operated by means of a
discharge impeded dielectrically on one aide. The sand-
wich cathode is arranged in the interior of the discharge
vessel, for example by being applied directly to the
inner wall. The anode is either fitted on the outer wall
or in the interior of the discharge vessel. In the latter
case, the anode is, however, separated from the actual
discharge by an additional dielectric, for example a thin
glass layer.
One advantage of the sandwich cathode consists a.n the
precise controllability of the instant of release of the
electrons. Specifically, the efficiency of the generation
. , ,.~-:,~___ . . .
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of useable radiation can be increased by the temporal
tuning of the voltage signals for the rapid polarity
reversal or the discharge. This holds, in particular, for
the pulsed discharge in accordance with WO 94/23442,
which is already distinguished in any case by improved
efficiency of the useable radiation. The increase in
efficiency is achieved by the specific provision of an
adequate number of electrons on the surface of the
sandwich cathode immediately before the ignition of the
l0 gas discharge. "'.
The increase in efficiency by the use of a sandwich
cathode according to the invention is, of course, limited
for the AC voltage excitation likewise used in the case
of dielectrically impeded discharges, since here the
l5 sandwich cathode acts as an anode a.n each second phase,
and consequently the electrons released there do not
contribute to the discharge.
The operating method according to the invention appears
as follows in the case of a dielectrically impeded
20 discharge operated in pulses. Firstly, a voltage signal
of rapidly reversing polarity is applied to the two
electric terminals of the sandwich cathode. As soon as a
sufficient number of electrons have been released on the
surface of the sandwich cathode owing to the rapid
25 polarity reversal of the ferroelectric thereby effected,
a voltage pulse is applied between the sandwich cathode
and the anode. The electrons are accelerated in the
direction of the anode as a result, and the discharge
therefore ignited.
30 The invention is explained in more detail below with the
aid of a few exemplary embodiments. In the drawing:
Figure 1a shows the top view of a strip-shaped cold
cathode according to the invention, having a
ferroelectric electron emitter/ of sandwich
35 design,
Figure lb shows the cross-section along the line AA of
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the sandwich cathode of Figure la,
Figure 2a shows the top view of an UV/V'U'V flat radiator
having the strip-shaped sandwich cathode of
Figure 1,
Figure 2b shows the cross-section along the line BB of
the W/VW flat radiator of Figure 2b,
Figure 3 shows a representation of the principle of the
electric circuit of the electrode arrangement
of Figure 2,
Figure 4a shows an excerpt from the time characteristic
of the voltage at the lamp electrodes in the
case of a pulsed mode of operation, and
Figure 4b shows the time characteristic, belonging to
Figure 4a, of the voltage at the sandwich
cathode in accordance with the method of oper-
ation according to the invention.
Figures la, 1b respectively show diagrammatically a
strip-shaped sandwich cathode 1 in top view and in a
cross-section along the line AA. The approximately 2 mm
thick sandwich cathode 1 comprises a bottom layer 2, a
top layer 3 and a ferroelectric layer 4 arranged between
the bottom and top layers. The ferroelectric layer 4
comprises an approximately 100 Ecm thick substrate made
from PZT (lead zirconium titanate). The bottom layer 2
and top layer 3 consist of platinum, which has been
applied to the top side and underside of the
ferroelectric substrate 4 with a layer thickness of
approximately Z E.cm in each case. At regular spacings of
approximately 200 E.tm, the top layer 3 has square openings
5 of dimensions 200 ~200 E,~.m'. The bottom layer 2 and top
layer 3 are each connected to a lead-in wire (not repre-
sented) in order to operate in a discharge lamp. In order
for the sandwich cathode 1 to emit electrons from its
openings 5, the two lead-in wiresfs~- are connected to
a voltage source (not represented) which supplies a
voltage signal with .tee-rapid polarity reversals.
Figures 2a, 2b respectively show in a diagrammatic
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representation a top view and the cross-section along the
line BB of an UV/VTJ'V flat radiator 6, that is to say a
flat discharge lamp, which is designed for the efficient
radiation of UV or VtJV radiation. The flat radiator 6
comprises a flat discharge vessel 7 having a rectangular
base area, a strip-shaped sandwich cathode 1 in accor-
dance with Figure 1, and two strip-shaped metal anodes 8.
The discharge vessel 7 comprises, for its part, a rec-
tangular baseplate 9 and a trough-type cover 10 (not
represented in Figure 2a), both made from glass. The
baseplate 9 and the cover 10 are connected to one another
in a gastight fashion in the region of their circumferen-
tial edges, and thus enclose the filling gas of the flat
radiator 6. The filling gas consists of xenon at a
filling pressure of 10 kPa. The anodes 8 are of the same
width and, like the sandwich cathode 1, are applied
parallel to one another on the inner wall of the
baseplate 9. By contrast with the, sandwich cathode 1, the
two anodes 8 are completely covered by a glass layer 11,
the thickness of which is approximately 150 ~,cm.
In one variant (not represented), the inner wall of the
cover 10 is coated with a fluorescent material or a
mixture of fluorescent materials, which converts into
visible light the UV/VUV radiation generated by the
discharge. This variant is a flat fluorescent lamp which
a.s suitable for general lighting or background lighting
of displays, for example LCD (Liquid Crystal Display).
In order to explain the method for operating discharge
lamps having a sandwich cathode, the arrangement of
Figure 2 and its electric circuit are represented
diagrammatically in Figure 3. In this case, the same
reference numerals denote the same features. The elec-
trodes of the sandwich cathode l, specifically the bottom
layer 2 and the top layer 3, are connected in each case
to an output pole of a~first voltage source 12 which
supplies a voltage signal U1. The anode 8 (only an anode
strip is shown in cross-section, for the sake of
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simplicity) is connected to a first pole of a second
voltage source 13, which supplies a voltage signal U=.
Furthermore, the top layer 3 of the sandwich cathode l is
additionally connected to the second pole of the second
voltage source 13.
Figures 4a and 4b likewise serve to explain the principle
of the method of operation, according to the invention
with reference to the example of a discharge operated a.n
pulses. These show in each case the time characteristic
of the two voltage signals Ul and U2 mentioned in Figure
3. Only one voltage pulse of a sequence of voltage pulses
which is unlimited in principle is shown a.n each case.
The temporal synchronization of the two voltage signals
Ul and Ua, inter alia, is essential. The first voltage
signal Ul is selected such that it rapidly reverses the
polarity at an instant t~, and thus starts the release of
electrons from the ferroelectric layer 4. Thereafter,
that is to say at a later instant t3>tz, a voltage pulse
of the voltage signal Uz is specifically started, which
accelerates the released electrons in the direction of
the anode 8 and thereby ignites or maintains the
discharge. The period for the polarity reversal of the
voltage signal Ul - calculated here as the temporal
spacing D = t4 - tl between the peaks of two voltage
pulses of different polarity which follow one another at
the instances tl and t,~ - is approximately 500 ns and
less. Typical values are of the order of magnitude of
100 ns or a few 100 ns.
