Laser discharge tube and electrode manufacturing method

A laser discharge tube having electrodes the peeling of which is reduced and a method of manufacturing electrodes of a laser discharge tube by which the peeling of the electrodes is reduced. The electrodes are adhered to the outside periphery of the tube wall of the laser discharge tube. The electrodes contain as components an electric conductive substance, an inorganic binder and boron nitride as a thermal expansion preventing substance for suppressing a coefficient of thermal expansion. When a silica glass tube is used as the laser discharge tube, the laser discharge tube has a coefficient of thermal expansion of +0.5.times.10.sup.-6. When the electrodes are mixed with boron nitride, the electrodes have a coefficient of thermal expansion of +2 to +7.times.10.sup.-6 and the coefficient of thermal expansion of the electrodes can be brought close to the that of the laser discharge tube by mixing boron nitride with the electrodes. Thus, the peeling of the electrodes caused by a difference of the coefficients of thermal expansion can be reduced.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a laser discharge tube used in a laser 
oscillator and a method of manufacturing electrodes of a laser discharge 
tube, and more specifically, to a laser discharge tube by which the damage 
of electrodes disposed on the outside periphery of the tube wall of the 
discharge tube is reduced and a method of manufacturing electrodes of a 
laser discharge tube. 
(2) Description of the Related Art 
A discharge tube used in a laser oscillator induces discharge in response 
to a high voltage applied thereto and outputs a laser beam produced by a 
laser gas excited by the discharge to the outside. Incidentally, the high 
voltage is applied to electrodes adhered to the outside periphery of the 
tube wall of the discharge tube and the discharge takes place between the 
electrodes confronting each other across the discharge tube. Therefore, 
discharge normally takes place within a region determined by the width of 
the electrodes. The discharge tube is composed of silica glass and the 
electrodes are composed of silver or the like which has good conductivity. 
The electrodes are formed by adhering silver to the outside wall of the 
discharge tube by metalizing or the like. 
Nevertheless, since a very high high-frequency voltage (e.g., 4000 V) is 
applied to the electrodes, the electrodes generates a considerable amount 
of power consumption. That is, since laser output from a laser oscillator 
is repeatedly turned on and off, the electrodes are repeatedly heated and 
cooled, by which strain stress is produced between the silica glass 
constituting the discharge tube and the metalized silver constituting the 
electrodes. Thus, the electrodes are deteriorated and peeled in a certain 
life time. A main factor for producing such strain stress is a difference 
between a coefficient of the silica glass tube and that of the metalized 
silver. More specifically, since the coefficients of thermal expansion are 
greatly different, the electrodes are lifted up by a heat cycle resulting 
from the turning on and off of a laser output so that the electrodes are 
deteriorated and peeled. 
Further, when a very high high-frequency voltage is applied to the 
electrodes, there is a possibility that dielectric breakdown is caused in 
regions other than the region between the electrodes and Corona discharge 
takes place. The Corona discharge is liable to take place at the portion 
of the discharge tube having a high temperature such as, for example, at 
the downstream of a laser gas flowing in the discharge tube. 
When a high voltage is applied to the electrodes, since the high voltage 
flows between the electrodes and the outside wall of the discharge tube, 
breakdown is caused at the edge portion of the electrodes and Corona 
discharge takes place. The Corona discharge travels once along the surface 
of the outside wall of the discharge tube and then gets into the discharge 
tube at a position apart from the electrodes by several millimeters. At 
the time, the silver is also melted and flown out and the flown-out silver 
grows to a tree-branch-shape along the outside wall of the discharge tube 
from the electrodes and covers the outside wall of the discharge tube in 
the vicinity of the electrodes with a width of several centimeters. When 
such a phenomenon occurs in which the silver serving as the electrode 
material migrates (so called an electro-migration), a dielectric strength 
is further lowered and Corona discharge is more liable to take place, and 
as a result, the flow-out of the electrode material and deterioration of 
the electrodes, and the like become more significant. 
That is, when the coefficients of thermal expansion has different values, 
the electrodes are lifted up by the heat cycle and deteriorated and 
peeled. Corona discharge is more liable to take place in the deteriorated 
portions and the peeled portions of the electrodes. Therefore, the 
deterioration of electrodes which start once is accelerated by Corona 
discharge. 
As described above, thermal expansion and Corona discharge caused by power 
consumption is made more significant when a temperature increases, by 
which the deterioration and peeling of electrodes are accelerated. 
Although the deterioration and the like of the electrodes can be prevented 
by lowering a voltage applied thereto, power to be supplied is lowered 
accordingly. Thus, a laser beam machining requiring a large amount of 
power cannot be executed. Further, although there is a method of lowering 
the temperature of the electrodes by mounting heat radiation plates on the 
electrodes, thermal expansion arising on the surface where the electrodes 
are adhered to the discharge tube cannot be avoided by this method. Thus, 
Corona discharge caused at the downstream of a laser gas where the 
temperature of the discharge tube is raised to high cannot be also 
prevented. 
SUMMARY OF THE INVENTION 
Taking the above into consideration, an object of the present invention is 
to provide a laser discharge tube by which the peeling of electrodes is 
reduced. 
Further, another object of the present invention is to provide a method of 
manufacturing a laser discharge tube by which the peeling of electrodes 
can be reduced. 
To solve the above object, according to the present invention, there is 
provided a laser discharge tube for inducing a discharge in response to an 
applied high-frequency voltage and exciting a laser gas, which comprises 
electrodes adhered to the outside periphery of the tube wall of the laser 
discharge tube, and further contain an electric conductive substance 
having a positive coefficient of thermal expansion, a thermal expansion 
preventing substance having a negative coefficient of thermal expansion 
and an inorganic binder for adhering the electric conductive substance and 
the thermal expansion preventing substance to the outside periphery of the 
tube wall. 
Further, there is provided a method of manufacturing electrodes of a laser 
discharge tube, which comprises the steps of making paste by mixing in a 
solvent an electric conductive substance having a positive coefficient of 
thermal expansion, a thermal expansion preventing substance having a 
negative coefficient of thermal expansion, an inorganic binder for 
adhering the electric conductive substance and the thermal expansion 
preventing substance to the outside periphery of the tube wall of the 
laser discharge tube and an organic binder for making the electric 
conductive substance, the thermal expansion preventing substance and the 
inorganic binder to a paste state, coating the paste to the laser 
discharge tube according to the configuration of the electrodes, heating 
and drying the paste to volatilize the solvent and, thermally decomposing 
the organic binder as well as dissolving the inorganic binder by baking 
the paste and adhering the electric conductive substance and the thermal 
expansion preventing substance to the outside periphery of the tube wall. 
Since the electrodes adhered to the outside periphery of the tube wall 
contain as constituting substances the electric conductive substance 
having a positive coefficient of thermal expansion, the thermal expansion 
preventing substance having a negative coefficient of thermal expansion 
and the inorganic binder for adhering the electric conductive substance 
and the thermal expansion preventing substance to the outside periphery of 
the tube wall, a difference between the coefficient of thermal expansion 
of the electrodes and that of the laser discharge tube can be reduced. 
Further, according to the method of manufacturing the electrodes, first, 
the electric conductive substance, the thermal expansion restricting 
substance, the inorganic binder and the organic binder for making the 
electric conductive substance, the thermal expansion restricting substance 
and the inorganic binder to a paste state are mixed in the solvent to make 
paste. The paste is coated to the laser discharge tube according to the 
configuration of the electrodes. Next, the paste is heated and dried to 
volatilize the solvent. Further, the paste is baked to thermally decompose 
the organic binder as well as dissolve the inorganic binder so that the 
electric conductive substance and the thermal expansion restricting 
substance are adhered to the outside periphery of the tube wall to thereby 
make the electrodes. In such a manner, the laser discharge tube having the 
electrodes whose coefficient of thermal expansion is close to that of the 
discharge tube can be made. 
The above and other objects, features and advantages of the present 
invention will become apparent from the following description when taken 
in conjunction with the accompanying drawings which illustrate preferred 
embodiments of the present invention by way of example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described below with 
reference to the drawings. 
FIG. 2 is a view schematically showing the arrangement of a laser 
oscillator using a laser discharge tube according to the present 
invention. The laser discharge tube 1 is a pipe with a circular cross 
section comprising a dielectric material (e.g., silica glass). Two 
electrode portions 2, 3 are helically disposed on the outside periphery of 
the laser discharge tube 1 at the same pitch. A laser gas 10 flows in the 
laser discharge tube 1 in the direction of a tube axis indicated by an 
arrow, and when a high-frequency voltage is applied between the electrodes 
2 and 3 from a high-frequency power supply (not shown), a discharge takes 
place in the laser discharge tube 1 between the electrodes confronting 
each other across the discharge tube 1 and the laser gas 10 is excited. A 
totally reflecting mirror 4 and an output coupling mirror 5 are disposed 
at the opposite ends of the laser discharge tube 1 to constitute a 
Fabry-Perot reflector and a beam discharged from the molecules of the 
excited laser gas is amplified and a part thereof is output from the 
output coupling mirror 5 as a laser beam 11. The laser beam 11 is 
irradiated to a workpiece for the laser machining thereof. 
FIG. 1 shows the arrangement of the laser discharge tube according to the 
present invention, which is a cross sectional view taken along the line 
1--1 of FIG. 2. The cross section of the laser discharge tube 1 comprises 
the laser discharge tube 1 and the electrodes 2 and 3 adhered to the 
outside periphery la of the tube wall of the laser discharge tube 1. A 
silica glass tube is used as the laser discharge tube 1. The electrodes 2 
and 3 contain as components an electric conductive substance, a thermal 
expansion restricting substance, and an inorganic binder for adhering the 
electric conductive substance and the thermal expansion restricting 
substance to the outside periphery of the wall tube and are formed by 
being vapor deposited on the outside periphery la of the tube wall by 
metalization. Silver is used as the electric conductive substance, boron 
nitride (BN) is used as the thermal expansion restricting substance, and a 
lead type substance (PbO.B.sub.2 O.sub.3) is used as the inorganic binder. 
The silica glass tube has a coefficient of thermal expansion of 
+0.5.times.10.sup.-6 or higher. Silver has a positive coefficient of 
thermal expansion of +10.times.10.sup.-6 or higher. Boron nitride has a 
negative coefficient of -0.48.times.10.sup.-6. Consequently, electrodes 
containing boron nitride in a weight ratio of 1-10% have a coefficient of 
thermal expansion of +2 to +7.times.10.sup.-6. That is, the coefficient of 
thermal expansion of the electrodes can be brought considerably close to 
that of the silica glass tube. 
Next, a method of manufacturing the laser discharge tube described above 
will be described. In this example, a silica glass tube is used as the 
laser discharge tube, powdered silver is used as the electric conductive 
material of the electrodes, boron nitride is used as the thermal expansion 
restricting substance, and a lead type substance (PbO.B.sub.2 O.sub.3) is 
used as the inorganic binder. An organic binder for making the silver, 
boron nitride and the lead type substance (PbO.B.sub.2 O.sub.3) to a paste 
state and a solvent for dissolving them are used as other necessary 
substances. An acrylic resin is used as the organic binder and terpineol 
is used as the solvent. 
First, the silver, boron nitride, inorganic binder (PbO.B.sub.2 O.sub.3) 
and acrylic resin are mixed in the terpineol and made to a paste state. At 
this time, the boron nitride is contained in a weight ratio of 1 to 10% to 
the total weight of the silver, boron nitride and inorganic binder 
(PbO.B.sub.2 O.sub.3). The thus prepared paste is referred to as electric 
conductive paste. The electric conductive paste has a good dispersing 
property into mesh as well as suitable viscosity and fluidity. Further, 
since the electric conductive paste has a good transferring property when 
it is coated onto the surface of the discharge tube by screen printing or 
offset printing, a smooth configurational accuracy and uniform electric 
characteristics of an electrode pattern can be obtained. 
The electric conductive paste is spirally coated onto the surface of the 
silica glass tube according to the electrode configuration of the silica 
glass tube by screen printing. Next, the discharge tube is heated and 
dried. At this time, the terpineol as the solvent in the paste is 
volatilized. Further, the discharge tube is baked. At the time, the 
acrylic resin as the organic binder is thermally decomposed and the 
inorganic binder (PbO.B.sub.2 O.sub.3) is dissolved. The dissolved 
inorganic binder (PbO.B.sub.2 O.sub.3) causes the powdered silver to 
adhere each other as well as acts to adhere the silver and boron nitride 
to the surface of the silica glass tube. As a result, the silver and boron 
nitride are vapor deposited on the laser discharge tube as the electrodes. 
Then, the silver, inorganic binder (PbO.B.sub.2 O.sub.3) and boron nitride 
finally remain as the components of the electrodes. The laser discharge 
tube provided with the electrodes containing boron nitride is made as 
described above. 
Physical properties of the electrodes made as described above will be 
described with reference to the graphs. 
FIG. 3 is a graph showing the relationship between a weight ratio of boron 
nitride to electrodes and a coefficient of thermal expansion. Electrodes 
which do not contain boron nitride has a coefficient of thermal expansion 
of about +10.times.10.sup.-6. When boron nitride having a coefficient of 
thermal expansion of -0.48.times.10.sup.-6 is contained in the electrodes 
and an amount of the boron nitride is increased, the coefficient of 
thermal expansion of the electrodes is lowered. Then, when the amount of 
boron nitride is increased to 10%, the coefficient of thermal expansion of 
the electrodes becomes +2.times.10.sup.-6. 
FIG. 4 is a graph showing the relationship between a weight ratio of boron 
nitride to electrodes and a volume intrinsic resistivity. In the graph, a 
value of the volume intrinsic resistivity of the electrodes is increased 
by increasing an amount of boron nitride to be contained in the 
electrodes. Then, when the amount of boron nitride is increased to 10%, 
the volume intrinsic resistivity of the electrodes becomes 
12.times.10.sup.-4 .OMEGA..cm. 
The followings can be considered from FIGS. 3 and 4. When an amount of 
boron nitride is too little, although a resistance value is suppressed to 
a low level, a coefficient of thermal expansion is almost unchanged. If 
the coefficient of thermal expansion is almost unchanged, an effect for 
suppressing strain stress caused by the repetition of heating and cooling 
is lost. If so, a meaning for containing boron nitride is lost. 
Consequently, boron nitride is preferably contained in a weight ratio of 
1% or more. 
On the other hand, when an amount of the boron nitride is too much, 
although a coefficient of thermal expansion is suppressed to a low level, 
a resistance value is increased. When the volume intrinsic resistivity 
becomes 12.times.10.sup.-4 .OMEGA..cm or higher, the capability of the 
electrodes is lowered and an amount of power consumption is also 
increased. When the amount of power consumption is increased, an amount of 
thermal expansion is increased accordingly. Thus, the electrodes are 
liable to be deteriorated and peeled by the heat cycle caused by the 
turning on and off of a laser output, which results in an adverse effect 
to an output of a strong laser beam. Thus, a suitable weight ratio of 
boron nitride is 10% or less. Because of the reasons as mentioned above, 
an excellent effect can be obtained when boron nitride is contained in a 
weight ratio of 1 to 10%. 
When the amount of boron nitride is continuously increased, the coefficient 
of thermal expansion is rapidly reduced up to the weight ratio of the 
boron nitride of 5%. When the amount of the boron nitride is further 
increased and the weight ratio thereof exceeds 7%, the amount of reduction 
of the coefficient of thermal expansion becomes considerably small, from 
which it can be found that the optimum weight ratio of the boron nitride 
is 5-7% at which the coefficient of thermal expansion is balanced best 
with the volume intrinsic resistivity. 
Further, the deterioration and peeling of the electrodes can be more 
prevented by the provision of a ceramic coating layer on the outside 
periphery of the electrodes of the laser discharge tube. The ceramic 
coating layer is made as described below. 
A case will be described here in which a ceramic type paint mainly composed 
of titanium composite oxide (TiO, Al.sub.2 TiO.sub.5, MgTiO.sub.3, etc.) 
is used as the ceramic material of a dielectric layer. When the ceramic 
type paint is used, the paint mixed with a binder is coated onto the 
electrodes 2 and 3 and baked in a thermostat at several hundred degrees of 
temperature to make the electrodes 2, 3. Silicon or Tyranno-polymer 
(Si--Ti--C--O) is used as the binder. The electrodes 2, 3 are completely 
covered with the thus made ceramic coating layer (dielectric layer). 
Incidentally, when the ceramic type paint is used, the electrodes can be 
adjusted to exhibit a low coefficient of thermal expansion and a high 
coefficient of heat transfer by the mixture of titanium composite oxide. 
As described above, a coefficient of thermal expansion can be reduced to a 
value of about one fifth by the mixture of boron nitride with the 
electrodes. Then, a difference between the coefficient of thermal 
expansion of the electrodes and that of the silica glass tube is 
considerably reduced, by which strain stress caused when the laser 
discharge tube is repeatedly heated and cooled can be suppressed. As a 
result, the deterioration and peeling of the electrodes caused by a change 
of temperature can be suppressed. Then, the occurrence of Corona discharge 
can be also prevented by the suppression of the deterioration and peeling 
of the electrodes. 
Further, since the deterioration and peeling of the electrodes are 
difficult to be caused, the life of the laser discharge tube is greatly 
prolonged. Further, a power to be supplied which has been suppressed to a 
low level to prevent the harmful influence of Corona discharge and the 
like can be sufficiently increased. The power to be supplied can be 
increased to about two times a conventional level, which leads to the 
reduction of size and the increase of output of a laser apparatus. 
Although silver is used as the electric conductive substance in the above 
description, an electric conductive material such as silver palladium and 
the like may be used. 
Although terpineol is used as the solvent, butylcarbitol acetate may be 
used as the solvent. 
Although the lead type substance (PbO.B.sub.2 O.sub.3) is used as the 
inorganic binder, a zinc type substance (PbO.B.sub.2 O.sub.3.ZnO) may be 
used. 
Although silica glass is used as the laser discharge tube, other dielectric 
material which is highly resistant to breakdown (e.g., alumina, aluminum 
titanate) may be used. 
Although the electric conductive paste is coated by screen printing, it may 
be coated by offset printing. 
Although spiral electrodes are metalized to the discharge tube, 
plate-shaped electrodes may be metalized. 
As described above, according to the present invention, since the 
electrodes of the laser discharge tube are mixed with the thermal 
expansion restricting substance, the coefficient of thermal expansion of 
the electrodes can be brought close to that of the silica glass tube and 
the like constituting the discharge tube, so that the peeling of the 
electrodes caused by a difference of the coefficients of thermal expansion 
can be prevented. 
Further, since the electrodes are made in such a manner that the thermal 
expansion preventing substance is mixed with the electrodes of the laser 
discharge tube and the paste is made by the mixture of the organic binder 
and the inorganic binder, the electrodes of the laser discharge tube can 
be made in which the peeling of the electrodes caused by a difference of 
the coefficients of thermal expansion can be prevented. 
Thus, the life of the laser discharge tube can be greatly prolonged by the 
electrodes whose peeling can be prevented. 
Further, the power to be supplied of the discharge tube can be increased by 
the electrodes whose peeling can be prevented, whereby an output from a 
laser oscillator can be increased. 
The forgoing is considered as illustrative only of the principles of the 
present invention. Further, since numerous modifications and changes will 
readily occur to those skilled in the art, it is not desired to limit the 
invention to the exact construction and applications shown and desired, 
and accordingly, all suitable modifications and equivalents may be 
regarded as falling within the scope of the invention in the appended 
claims and their equivalents.