Electrode activating compound for gas discharge tube

An electrode activating compound for a gas discharge tube such as a surge voltage arrester, triggerable gas discharge tubes and flash tubes in which a titanium oxide is used in place of the radioactive thorium oxide. Preferably, the titanium oxide is provided by a reduction of a titanium dioxide with a reducing agent such as BaAl.sub.4 or metallic titanium or by oxidation of a titanium hydride in either an oxidizing atmosphere or with an oxidizing compound.

BACKGROUND OF THE INVENTION 
The present invention is concerned with an electrode activating compound 
for a gas discharge tube containing a metal oxide selected from a metal of 
the fourth subgroup of the periodic system. 
An electrode activating compound, which utilizes a metal oxide from the 
fourth subgroup of the periodic table, for the electrodes of a voltage 
overload arrester or a surge voltage arrester is disclosed in U.S. Pat. 
No. 3,691,428 which was based on German O.S. No. 19 35 734. In this U.S. 
patent, an electrode activating compound included thorium oxide. 
Electrode activating compounds are generally used in the case of gas 
discharge tubes and according to the purpose of the application are 
composed to significantly influence the important electric characteristic 
quantities in each case. Where the gas discharge tube is utilized as a 
surge voltage arrester or is used as a controllable switching tube, which 
is known as a cold cathode thyratron, or is used as a flash tube, an 
important requirement is directed towards a low glow-arc transition. This 
is defined as a momentary current value in the case of which the fired gas 
discharge crosses over from a glow discharge into an arc discharge. Low 
values produce a good firing behavior, above all, in the case of firing 
via a third electrode with low firing currents, and by means of good 
current carrying capacity, they produce a long service life. Beyond this, 
the minimum operating voltage is advantageously of a small value. 
A low glow-arc transition can be attained with an electrode activating 
compound which, among other things, contains a radioactive thorium oxide 
as an effective constituent. The security measures, which are necessary 
because of the radioactivity and therefore the danger to the environment, 
as well as the considerable waste disposal costs, provide a serious 
disadvantage to the use of thorium oxide as an electrode activating 
compound. 
SUMMARY OF THE INVENTION 
The present invention is based upon the problem of replacing the 
radioactive thorium oxide by means of a material, which is not 
radioactive. In addition, the material of the invention is not expensive 
and in particular it does not combine a low glow-arc transition with other 
disadvantages such as it could be too easily atomized, vaporized so that 
conducting deposits would occur upon the interior wall of the gas 
discharge container and therefore create short circuits. 
For the solution of this problem, the invention is directed in providing an 
electrode activating compound containing a metal oxide of a metal selected 
from the fourth subgroup of the periodic table, said metal oxide being 
titanium oxide. 
With a "titanium oxide", it is to be expressed that depending upon the 
purpose of the application and depending upon the desired electrical 
characteristic qualities in connection with other factors such as 
electrode surface, electrode spacing, gas pressure, internal firing and 
self firing, the titanium in the oxide is either quadrivalent or has a 
lower valence and it can also be composed non-stoichiometrically in its 
valence. It is essential that titanium oxide is a non-toxic and is an 
inexpensive substance. It is also essential that the titanium oxide in the 
electrode activating compound, even in the case of extremely small 
currents and low voltages, enables an unwavering arc discharge to be 
maintained. Titanium oxide combines a good electron emission ability with 
a relatively poor thermal conductivity. 
In order to obtain the desired titanium oxide in the electrode activating 
compound, one has essentially two possibilities. The compound is generally 
applied as a mixture in the form of a paste upon the electrode or upon the 
electrodes and is formed in a forming process with the compound which is 
active in the operation. One possibility consists in beginning with 
quadrivalent titanium dioxide and during the forming process producing a 
lower valent titanium oxide. The other possibility consists in beginning 
from either a group consisting of titanium or titanium hydrides in the 
forming process and producing a higher valent titanium oxide. 
In accordance with this, and according to one design of the invention, it 
is proposed that the electrode activating compound be formed from a 
material, which has a constituent selected from a group consisting of 
titanium and titanium hydride, includes an oxidation agent and is applied 
to the electrode. During a forming process of the electrodes, titanium 
oxide is formed at least partially due to oxidation. In another proposed 
method, a material mixture containing a constituent selected from a group 
consisting of titanium and titanium hydride is applied on the electrode 
and during the forming process is partially oxidized in an oxygen 
containing gas atmosphere. 
According to the other proposed process, an electrode activating compound 
is produced by applying on the electrodes a material which contains a 
titanium dioxide and a reducing agent. In the presence of the reducing 
agent or means during the forming process of the electrodes, the titanium 
dioxide is reduced at least partially into a lower valent titanium oxide. 
The reducing agent or means which is included in the material for one thing 
can be metallic titanium, a barium-aluminum alloy, or an alkali compound 
which is selected from a group consisting of potassium azide and potassium 
boranate. The use of titanium, as a reducing compound is disclosed in 
German Pat. No. 1,951,601. It is also noted that U.S. Pat. No. 3,676,743, 
which is based on German Pat. No. 1,950,090, disclosed utilizing a 
barium-aluminum alloy as a coating on electrodes in a voltage overload 
arrester to produce an essentially lower electron work function and a 
stabilizing effect. 
With the help of the electrode activating compound in accordance with the 
present invention, both a high response direct voltage and a high maximum 
operating voltage of the gas discharge tube are obtained. In addition, a 
low minimum operating voltage and low arc drop voltage will be obtained. 
By means of a low energy conversion, the service life is very high because 
the glow range can be kept low with voltage and current. 
The large ratio which is attained with the "maximum operating voltage 
without spontaneous firing" to the "minimum operating voltage with 50% 
probability of firing in the case of admission with a specific triggering 
pulse" in the case of a triggerable gas discharge tubes can be utilized 
technically in an advantageous manner. The maximum operating voltage may 
be very high, or the minimum response direct voltage, which is still 
triggerable, can be very low. However, also the demand can be raised for 
example to this extent that data sheet specifications are fulfilled not 
only in one polarity, but rather in both polarities. 
Further, low firing currents are advantageous especially in the case of 
triggerable gas discharge tubes. Discharge currents under 10 mA can be 
generated simply without glass-to-metal sealed lead-through for firing the 
main discharge. A conducting cover which is applied on the exterior of the 
container, will suffice with an alternating voltage of at least 2 kV at 
0.1 MHz to overcome the capacity resistance of the container wall. 
Prerequisite for the firing of the arc is then only a significant high gas 
pressure of approximately 400 m bar to 500 m bar in the discharge 
container. The firing begins at the cathode upon a very small spot with a 
glow discharge of a high power density for example, several kW per 
cm.sup.2, and after approximately 10.sup.-5 seconds, an arc low end, which 
glows white and emits electrons, occurs. As soon as this subdischarge path 
has fired to the container wall, the gas discharge tube can transfer into 
a main discharge path or respectively in the case of a flash tube can 
short circuit a flash capacitor if its charging voltage lies noticeably 
over the arc drop voltage of the subdischarge path. A filling gas, an 
inert gas like argon, can be utilized, or xenon can be used if light 
efficiency or color plays a role. 
If the electron activating compound contains in addition to the titanium 
oxide, an alkali halide in particular potassium iodide, potassium bromide 
or potassium chloride, additional advantages will occur. The use of alkali 
halide in over voltage arresters is disclosed in the above mentioned 
German Pat. No. 1,951,061. 
In practice, a mixture of titanium dioxide, barium-aluminum alloy and 
potassium halide are to be recommended. The composition of this mixture is 
to be accomodated to the pressure of the gas atmosphere. The potassium 
halide is a compound selected from a group consisting of potassium iodide, 
potassium bromide and potassium chloride. 
Various ranges are possible for example, TiO.sub.2 from 2% to 60%, 
BaAl.sub.4 from 5% to 50% and in the case of KX from 0% to 80% where 
X.dbd.Cl, Br, or I. The firing voltage is determined by the ratio of 
TiO.sub.2 and the reducing agent. In the case of ratio of the TiO.sub.2 
:BaAl.sub.4 &lt;1, the firing voltage drops and a brownish or violet wall 
deposit will occur. 
As optimum examples, with a gas pressure of 450 m bar of argon, a 
composition of 40% TiO.sub.2, 40% BaAl.sub.4 and 20% KX can be used. When 
the gas pressure is 90 m bar then a mixture of 10% TiO.sub.2, 20% 
BaAl.sub.4 and 70% KX is utilized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The principles of the present invention can be utilized in a gas discharge 
tube such as a surge voltage arrester having a so called button arrester 
shape which has truncated cone-shaped electrodes 2 and 3 which are facing 
towards each other and spaced apart by a tubular shape insulating body 1 
which are secured together in a gas tight arrangement. The material for 
the insulating body preferably is glass or ceramic. The material for the 
electrodes 2 and 3 consists of a Ni-Fe alloy, or Ni-Fe-Co alloy. As 
illustrated each of the electrodes 2 and 3 on a surface which lies 
opposite one another, has a layer 4 applied. This layer 4 contains an 
electrode activating compound according to the present invention which 
contains a titanium oxide. 
Although various minor modifications may be suggested by those versed in 
the art, it should be understood that I wish to embody within the scope of 
the patent granted hereon, all such modifications as reasonably and 
properly come within the scope of my contribution to the art.