Note: Descriptions are shown in the official language in which they were submitted.
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SDK-D837/PCT
DESCRIPTION
Discharge Tube with Coating Material Applied on
Discharge Surface
TECHNICAL FIELD
The present invention relates to a discharge tube
suitably used as an arrester or a spark gap for supplying
a constant voltage to an ignition plug, a high-voltage
discharge lamp or the like, and more particularly relates
to a discharge tube with a coating material applied on
the discharge surfaces thereof.
BACKGROUND ART
Conventionally known discharge tube devices include
an arrester mounted on a commercial power line to
equipment for preventing intrusion of a spontaneous surge
voltage caused by thunder or a spark gap used with a
vehicle ignition plug or a high-voltage discharge lamp
for continuously supplying a constant high-voltage. It
is a common practice to apply various types of coating
material on the discharge surfaces of the electrodes of
the discharge tubes in order to minimize the initial
break-down voltage against a surge voltage for the
arrester or in order to produce a stable discharge
voltage for the spark gap. A spark gap, which has a
function of maintaining the discharge voltage below a
predetermined level, can, of course, be used as an
arrester.
The most generally known conventional coating
material is composed of water as a solvent, sodium
silicate, nitrocellulose or silicon rubber as a fixing
agent to which barium titanate or aluminum oxide is added
for improving the surge resistance.
However, it has been found that the discharge tube
with the above-mentioned additives applied on the
discharge surfaces thereof has a disadvantage in that,
when the rise time of the input surge voltage is
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shortened, the break-down voltage increases and the
function of an arrester or a spark gap cannot be fully
exhibited.
Especially for the spark gap, it has been found that
the breakdown voltage is reduced or otherwise stable
discharge characteristics are lost after continuous
discharge. Specifically, in order to supply a stable
constant voltage to a vehicle ignition plug or a
high-voltage discharge lamp, a high-voltage pulse of
several-msec duration must be applied or otherwise
special means must be taken, in which case it is
difficult to produce a stable discharge voltage.
DISCLOSURE OF THE INVENTION
The present invention has been developed to obviate
the above-mentioned problems, and a first object of the
invention is to provide a discharge tube capable of
producing a stable discharge voltage even with a surge
pulse having a short rise time. A second object of the
invention is to provide a discharge tube capable of
producing a stable discharge voltage even after
continuous discharge.
In order to achieve the above-mentioned objects,
according to the present invention, there is provided a
discharge tube configured as described below.
Specifically, a discharge tube according to the
invention comprises two electrodes arranged in opposed
relation to each other with an insulating member between
them for causing a discharge in the gap formed between
the two opposed discharge surfaces of the electrodes,
wherein a coating material containing 0.01 to 60 wgt % of
at least one alkali metal salt selected from potassium
bromide, potassium fluoride and sodium fluoride is
applied on the discharge surfaces of the two electrodes.
This configuration can produce a stable discharge
voltage even against a surge voltage having a short rise
time.
The alkali metal salt contained in the coating
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material is preferably 5 to 30 wgt %, or especially 10 to
20 wgt %.
Further, a stable discharge voltage is produced even
after continuous discharge by adding sodium silicate of
0.01 to 50 wgt % to the coating material.
Also, the sodium silicate contained in the coating
material is preferably 0.5 to 10 wgt %, or more
preferably 1 to 2.5 wgt %.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a sectional view of a discharge tube
according to the present invention.
Fig. 2 is a graph showing a voltage-time
characteristic curve of a discharge tube (with a coating
material containing potassium bromide) according to the
first embodiment of the invention.
Fig. 3 is a graph showing a continuous discharge
characteristic (indicating the upper and lower limits of
the wave) with high-voltage pulses continuously applied
to the discharge tube of Fig. 2.
Fig. 4 is a graph showing a voltage-time
characteristic curve of a discharge tube (using a coating
material containing sodium silicate and potassium
bromide) according to a second embodiment of the
invention.
Fig. 5 is a graph showing a continuous discharge
characteristic (indicating the upper and lower limits of
the wave) of the discharge tube of Fig. 4 continuously
supplied with high-voltage pulses.
Fig. 6 is a graph showing a continuous discharge
characteristic (indicating the upper and lower limits of
the wave) of the discharge tube of Fig. 4 supplied with a
continuous train of high-voltage pulses followed by
another continuous train of high-voltage pulses.
Fig. 7 is a graph showing a continuous discharge
characteristic (indicating the upper and lower limit of
the wave) of a discharge tube supplied with high-voltage
pulses according to a third embodiment of the invention.
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Fig. 8 is a graph showing a continuous discharge
characteristic (indicating the upper and lower limits of
the wave) of a discharge tube supplied with high-voltage
pulses according to a fourth embodiment of the invention.
Fig. 9 is a sectional view showing a configuration
of the electrode section of a discharge tube according to
another embodiment of the invention.
Fig. 10 is a graph showing a voltage-time
characteristic curve of a conventional discharge tube
supplied with high-voltage pulses.
Fig. 11 is a graph showing a continuous discharge
characteristic (indicating the upper and lower limits of
the wave) of an example of a conventional discharge tube
using a coating material containing sodium silicate of 25
wgt % and barium titanate of 5 wgt %.
Fig. 12 is a graph showing a continuous discharge
characteristic (indicating the upper and lower limits of
the wave) of an example of a conventional discharge tube
using a coating material containing sodium silicate of 6
wgt % and barium titanate of 3.5 wgt % applied thereto.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Prior to describing embodiments of the invention,
conventional discharge tubes with a coating material
applied to the discharge surfaces thereof will be
explained with reference to Figs. 10 to 12.
Fig. 10 is a graph showing a voltage-time
characteristic curve of a discharge tube with a coating
material containing sodium silicate and barium titanate
applied to the discharge surfaces thereof as an example.
In this graph, the ordinate represents the break-down
voltage with a surge pulse voltage applied, and the
abscissa represents the duration of a surge pulse
applied. The rise time (in voltage/time) of a surge
pulse is also shown. This graph indicates the change of
an actual break-down voltage with respect to the change
of the surge pulse duration and the rise time of a
discharge tube designed for a predetermined break-down
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time. The voltage-time characteristics of discharge
tubes having break-down voltages of 600 volts, 350 volts
and 200 volts are shown as an example of a specification.
As can be seen from this graph, a stable initial
discharge characteristic is exhibited up to about 400
V/msec. in rise time. With a shorter rise time, however,
the break-down voltage suddenly increases to such an
extent that the discharge tube fails to function as an
arrester.
Fig. 11 shows the initial discharge characteristic
(continuous discharge characteristic) of a discharge tube
using a coating material containing sodium silicate of 25
wgt % and barium titanate of 5 wgt % supplied with
high-voltage pulses of 5 msec. duration. Fig. 12 shows
an initial discharge characteristic (continuous discharge
characteristic) of a discharge tube using a coating
material containing sodium silicate of 6 wgt % and barium
titanate of 3.5 wgt % supplied with high-voltage pulses
of 5 msec. duration. Neither of the discharge tubes
could produce a stable continuous break-down voltage.
Now, preferred embodiments of the present invention
will be explained in detail with reference to Figs. 1 to
9.
Fig. 1 is a sectional view showing an embodiment of
a discharge tube 10.
Numeral 12 designates a cylindrical insulating
member composed of a ceramic such as alumina. Electrodes
14a, 14b are fixed in opposed relation to each other by
silver solder to the end openings of the cylindrical
3C insulating member 12, and a low-pressure inert gas, such
as argon or the like, is sealed in the hermetic
insulating member 12.
A carbon trigger line (not shown) is formed at
appropriate points on the inner wall of the insulating
member 12 for stabilizing the initial discharge.
The electrodes 14a, 14b composed of iron-nickel
alloy, iron-nickel-cobalt alloy or copper include
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discharge surfaces 17a, 17b, respectively, to discharge
in a gap 20 between the opposed discharge surfaces 17a,
17b. A coating material 16 is applied on the discharge
surfaces 17a, 17b.
(First embodiment)
According to the first embodiment of the invention,
the coating material 16 using water (pure water) as a
solvent is produced by adding a solvent containing 0.01
to 60 wgt % of potassium bromide constituting an alkali
metal salt to a well-known coating material using barium
titanate. The coating material 16 is coated and dried on
the discharge surface.
A specific example of the first embodiment will be
explained. Fig. 2 is a graph showing a voltage-time
characteristic curve of a specific example of the first
embodiment (a discharge tube using a coating material
containing barium titanate as a main component and 10 wgt
% of potassium bromide).
Comparison of this characteristic curve with the
voltage-time characteristic curve (Fig. 10) for a
conventional coating material containing barium titanate
not mixed with potassium bromide shows that the
conventional discharge tube has the disadvantage that
with a shorter rise time of the surge voltage, the
break-down voltage of even a 200-V discharge tube
increases and exceeds 2000 volts at 10 KV/~sec. This
indicates that even in the case where a discharge tube
having a specification of 200 volts is mounted as an
arrester on the equipment to be protected, an abnormal
voltage exceeding 2000 V will appear. The equipment,
therefore, cannot be positively protected and may be
broken. It can be seen from Fig. 10 that an abnormal
voltage exceeding 200 volts can be removed only from the
surge voltage of up to about 200 V/msec to 400 V/msec in
rise time.
In the discharge tube 10 according to the first
embodiment, by contrast, as shown in Fig. 2, the
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break-down voltage of a 200-V spark gap does not increase
as rapidly as that of the conventional discharge tube.
The discharge operation can thus be started at about 430
v with the rise time of 10 KV/msec, thus exhibiting a
superior characteristic as an arrester. Consequently,
the equipment can be positively protected and is liable
to be broken at a lower rate.
Potassium bromide has a direct effect on the
stability of the discharge voltage, and is usable in the
range of 0.01 to 60 wgt %. The discharge characteristic,
however, becomes stable in the range of 5 to 30 wgt %, or
especially, the stability is improved in the range of 10
to 20 wgt % of potassium bromide. Other alkali metal
salts, such as potassium fluoride or sodium fluoride, may
be used in place of potassium bromide. At least one of
these three alkali metal salts may be contained in the
coating material with equal effect.
The discharge tube 10 according to the first
embodiment, as shown in Fig. 3, cannot maintain a stable,
constant discharge voltage in the case where high-voltage
pulses of 5 msec. or less in duration are continuously
applied as the initial discharge characteristic
(continued discharge characteristic). The discharge tube
10 therefore cannot be used as a spark gap.
(Second embodiment)
The discharge tube 10 has a similar mechanical
structure to the structure shown in Fig. l.
According to the second embodiment, the coating
material contains sodium silicate as a fixing agent.
Specifically, the coating material 16 uses water
(pure water) as a solvent. The solvent contains 0.01 wgt
% of sodium silicate and 0.01 to 60 wgt % of potassium
bromide. The coating material 16 is applied and dried on
the discharge surfaces.
The effect of the added sodium silicate begins to be
exhibited from about 0.01 wgt %, and the discharge
characteristic is most stable in the range of l to 2.5
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wgt %. The stability of the discharge characteristic is
gradually reduced for the content more than 2.5 wgt %.
When the content exceeds 40 wgt %, a dynamic current
discharge easily occurs. Therefore, the effect can be
sustained up to 50 wgt % of sodium silicate which is an
upper limit of the amount of sodium silicate that can be
dissolved in the solvent water. The content of sodium
silicate, however, should be maintained on the order of
several percent as far as possible
The second embodiment will be explained with
reference to a specific example. Fig. 4 is a graph
showing the voltage-time characteristic curve of a
specific example (sodium silicate of 2.5 wgt % and
potassium bromide of 10 wgt %) according to the second
embodiment.
As clearly understood from the graph of Fig. 4
showing the voltage-time characteristic curve of the
discharge tube 10 according to the second embodiment,
since the coating material contains potassium bromide,
the break-down voltage is prevented from increasing, as
in the first embodiment, even with a short rise time,
thereby making it possible to produce a superior
break-down time characteristic.
Fig. 5 shows the initial discharge characteristic of
a spark gap supplied with a high-voltage pulse not more
than 5 msec., having the discharge surfaces on which a
coating material containing 2.5 wgt % of sodium silicate
and 15 wgt % of potassium bromide is prescribed and
applied. As can be seen from Fig. 5, since the coating
material contains not only potassium bromide but also
sodium silicate, the initial discharge characteristic
(continuous discharge characteristic) as well as the
voltage-time characteristic shown in Fig. 4 can be
stabilized at 1000 V or thereabouts for the break-down
voltage.
Further, in the case where the coating material
contains sodium silicate, the initial discharge
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characteristic (continuous discharge characteristic) with
high-voltage pulses of 5 msec. or less continuously
applied is stabilized at about 1000 V as shown in Fig. 5.
The discharge tube thus can be used not only as an
arrester but also as a spark gap.
Fig. 6 shows the discharge characteristic after
applying a high-voltage pulse of 5 msec. or less 20000
times each for one second to the same discharge tube as
the one described above. As can be seen from Fig. 6, the
break-down voltage is maintained at 1000 V and a stable
discharge characteristic is exhibited even after
continuous long-time operation.
(Third embodiment)
The configuration of a discharge tube according to
the third embodiment is substantially the same as that cf
the discharge tube 10 described with reference to Fig. 1,
the only difference being the composition of the coating
material 16.
The coating material 16 uses water as a solvent
which contains 0.01 to 50 wgt % of sodium silicate and
0.01 to 60 wgt % of potassium fluoride. The coating
material 16 is applied and dried on the discharge
surfaces.
The amount of sodium silicate added is the same as
that explained with reference to the second embodiment
and will not be described.
Potassium fluoride has a direct effect on the
stability of the discharge voltage and can be used in the
range of 0.01 to 60 wgt %. The discharge characteristic
is stable with 5 to 30 wgt % of potassium fluoride, or
preferably about 10 to 20 wgt ~ at which high stability
is obtained.
Fig. 7 shows the initial discharge characteristic
(continuous discharge characteristic) obtained when a
high-voltage pulse of 5 msec. or less is impressed on a
spark gap using a coating material containing 2.5 wgt %
of sodium silicate and 15 wgt % of potassium fluoride
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prescribed and applied on the discharge surfaces thereof.
As can be clearly seen from Fig. 7, the break-down
voltage is stable at about 1000 V.
The discharge characteristic of this discharge tube
obtained after high-voltage pulses of 5 msec. or less
are impressed 20000 times each time for one second is
found to be stable, though not shown. The break-down
voltage of 1000 V is maintained even after long-term
continuous discharge in the same manner as the discharge
tube of the second embodiment. The voltage-time
characteristic curve, though not shown, is also
substantially similar to that of the second embodiment.
(Fourth embodiment)
The configuration of a discharge tube according to
the fourth embodiment is substantially the same as that
of the discharge tube 10 described with reference to the
first embodiment, the only difference being the
composition of the coating material 16.
The coating material 16 uses water as a solvent and
contains 0.01 to 50 wgt % of sodium silicate and 0.01 to
60 wgt % of sodium fluoride. The coating material 16 is
applied and dried on the discharge surfaces.
The amount of the sodium silicate added is the same
as that explained with reference to the second embodiment
and will not be explained.
Sodium fluoride has a direct effect on the stability
of the discharge voltage, and is usable in the range of
0.01 to 60 wgt %. The content of 5 to 30 wgt %, however,
produces a stable discharge characteristic, and the
content of 10 to 20 wgt % is more preferable as the
stability is further improved.
Fig. 8 shows the initial discharge characteristic
obtained when high-voltage pulses of 5 msec. or less are
applied to a discharge tube using a coating material 16
containing 2.5 wgt % of sodium silicate and 15 wgt % of
sodium fluoride prepared and applied on the discharge
surfaces thereof. As can be seen from Fig. 8, the
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break-down voltage is stabilized at about 1000 V.
The discharge characteristic obtained a~ter applying
high-voltage pulses of 5 msec. or less in 20000 cycles
each one second in length, though not shown, indicates
that a break-down voltage of 1000 V and a stable
discharge characteristic are maintained even after
long-term continuous use in the same manner as the
discharge tube of the first embodiment. Also, the
voltage-time characteristic, though not shown, is
substantially identical to that of the second embodiment.
In the first to fourth embodiments described above,
potassium bromide, potassium fluoride or sodium fluoride
is used as an alkali metal salt contained together with
sodium silicate in the coating material 16.
Alternatively, the coating material 16 may contain a
mixture of a plurality of alkali metal salts with equal
effect, including a combination of potassium bromide and
potassium fluoride, a combination of potassium bromide
and sodium fluoride, a combination of potassium fluoride
and sodium fluoride or a combination of potassium
bromide, potassium fluoride and sodium fluoride.
In such a case, the content of each alkali metal
salt in the coating material 16 may be in the range of
0.01 to 60 wgt % as in the case where only one type of
alkali metal salt is used with sodium silicate.
Nevertheless, the discharge characteristic is stabilized
in the range of 5 to 30 wgt % or preferably in the range
of 10 to 20 wgt % where an especially high stability is
obtained.
As an example, assume that two alkali metal salts
(potassium bromide and potassium fluoride) are used. The
coating material 16 is prepared to contain 2.5 wgt % of
sodium silicate, 15 wgt % of potassium bromide and 15 wgt
~ of potassium fluoride. In the case of using all the
above-mentioned three alkali metal salts (potassium
bromide, potassium fluoride and sodium fluoride), on the
other hand, the coating material 16 preferably has the
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contents of 2.5 wgt % of sodium silicate, 15 wgt % of
potassium bromide, 15 wgt % of potassium fluoride and 15
wgt % of sodium fluoride. Also, the coating material
according to this invention can be applicable to a triode
discharge tube and the like.
Further, instead of a flat discharge surface of the
electrodes of the discharge tube 19 as shown in Fig. 1, a
recess 18 may be formed in the discharge surface of each
electrode as shown in Fig. 9. This configuration can
assure a large surface area contributing to a longer
service life.
Various preferred embodiments of the present
invention have been explained above. The inven~ion,
however, is not limited to these embodiments and can of
course be modified in various ways without departing from
the scope and spirit of the invention.
INDUSTRIAL APPLICABILITY
It will thus be understood from the foregoing
description that according to the present invention,
there is provided a discharge tube in which a stable
break-down voltage can be obtained even with a short rise
time of the surge voltage. The discharge tube according
to the invention can be used as an arrester, thereby
preventing the equipment involved from being damaged.
Further, in the case where the coating material
contains sodium silicate at the same time, a stable
break-down voltage is obtained even when hish-voltage,
high-frequency pulses are applied continuously. The
present invention, therefore, can exhibit a superior
characteristic even when used for a vehicle ignition plug
or a high-voltage discharge lamp.
