CA1127283A - Carbon dioxide gas lasers - Google Patents

Abstract

ABSTRACT
A carbon dioxide laser comprises an envelope containing an electrode structure, and reflecting and partially reflecting mirrors. A tin (IV) oxide-supported palladium or platinum catalyst contained in porous structures of alumina or silica is mounted inside the envelope to provide a room temperature catalytic oxidation of CO during and following discharge of the laser in a pulsed or continuous wave mode of operation.

Description

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This invention relates to carbon dioxide (CO2) gas lasers in which laser emission is initiated by elec~rical discharge wikhin an envelope containing the gas.
A typical CO2 laser camprises an envelape or tube contain-ing a gas mixture e.g. of CO2, N2, and He, electrodes for praviding an electrical discharge within the gas, a reflecting c~nd a partiall~
reflecting mirror spaced apart and often parcillel to one another at opposite ends of the envelcpe.
Lasers can be divided into two groups, continuous wave (CW) and pulsed lasers; the former emits radiation continuously (when supplied with pawer) whilst the latter emits radia-tion in short bursts. Both groups may use a flcwing gas system or a sealed envelope ~ystem.
The electrical discharge in the gas causes a dissociation f C2 into CO and 2 In the flawing gas system the reaction gas products are swept out of the envela~e but in the sealed system these products remain within the envelape c~nd, unless recombined into C02, result in loss of output, for example by causing arcing between the electrodes with possible cansequential failure of the laser.
In general the difficulties caused by CO2 dissociation increase in severity as the pressure of the active gas is raised.
Thus CW lasers which operate at e.g. 20 Torr. do not have the pro-blem to the same ~' 1~ ~

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` ` : ' ' ~Z7'~3 extent as pulsed lasers operating at atmospheric pressure. At higher gas pres-sures transverse (of the laser axis) excitation of the discharge is normally used, these are termed TE lasers. Lasers operating a~ atmospheric gas pressure with transverse excitation are termed TeA (transverse exci-tatlon atmospheric pressure~
lasers. With these higher pressure gas lasers the presence of the dissociation products CO and 2 tends to cause the uniform electrical discharge to degrade into localised arcs with consequential dramatic loss of performance.
One form of TEA laser uses a trigger wire to produce subsidiary dis-charges to the anode causing ultra violet irradiation of the cathode. This produces the diffuse photo emission of electrons necessary for the creation of a uniEorm discharge and helps counter the arcing problem. Such a laser is des-; cribed in IEEE Journal of Quantum Electronics, ~ol. QEll, No. 9, September 1975, pages 774-778.
Another TEA laser is described in J. Phys. E. Sci. Instruments, Vol. 11, 1978, pages 311-315. rt uses two array~ of sliding arcs to provide ultra violet radiation which pre-ionlses the gas between the principal electrodes of the laser before high volts are applied to these, thus encouraging the ~ormation of a uniform discharge. Additionally a heated platinum wire is used to catalyse the recombination of C0 and 2 Use of a Pt wire ~n a C02 TEA laser is clescribed in J. Phys. E. Sci. rnstrum., ~ol. ll, 1978, pages 316-319. A disadvantage of such a heated wire is its power consumption, typically about 8 watt, which adds to the required batter~ weight ~n portable equipment and may result in thermal distortion of the laser envelope.
According to this inYent~on a carbon dioxide gas laser comprises ~
envelope containing a gaseous active medium including carbon dioxide, means for inducing an electric discl~arge in the gas within the envelope to provide a pumpsource, a reflecting mirror and a partially~reflecting mirror spaced apart at ~ 2 _ , ., opposite ends of the envelope to define a resonant cavity along the laser axis, and an energy abstracting means including the partially reflecting mirror, wherein ~he improvement comprises a structure including a cakalyst material mounted inside the envelope in intimatc conkact Wit]l the gas for khe catalyt:ic oxidation of carbon monoxide into carbon dioxide, said catalyst being selec-tcd from the group of tin (IV) oxlde-supported palladium, and tin (I~) oxide-supported platinum.

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'112'~,~2B3 The structure for inducing an electric fdischarge may be spaced elec-trodes, or radio frequency induction.
The catalyst may be incorporated into porous structures such as activated alumina, silica etc., and be formed into strips or shee-t~. ','he amount of Pd or Pt within or on the SnO2 may vary, a typical value is around 2~
by weight. The catalyst oF,erates unheated at ambient tem~,eratures even in very cold climates, but it may include a small power consumption heater for initial use until the laser envelope reaches its operating temperature.
'~he laser may ke a aw or a pulsed laser with a sealed envelope or an unsealed enveloE~e. The laser may inclwde a device such as a fan to circulate the laser gas.
The use of tin (IV) oxide-supported palladium has previously been suggested for use in automobile exhaust systems, G. Croft and M. J. Fuller, Nature, Vol. 269, 13 October 1977, page 585/6. ~Iowever auto~obile exhaust systems operate at elevated temperatures and it has previously been considered necessary to use such high temperatures for catalytic oxidation of CO by 2 and N2. In addition catalytic exhaust systems operate with considerable excess air rat~er than stoichiometric amounts of CO-and 2 The invention will now be described, by way of example only, with reference to the acccmpanying drawings of which:-Fiyures 1, 2 are plan and side sectional views of a transverselyexcited atmospheric pressure CO2 laser;
Figure 3 is a circuit diagram for the laser of Fiyure 1.
As shown in the drawings a laser comprises an envelope 1 made of glass and expansion matched Ni-Fe-Co alloy. m e envelope 1 is filled with yas a-t 760 Torr. pressure and a composition by vDlume of 40% CO2, 20~ N2, 40~ ~. Iwo Rogowski-profiled nickel electrodes 2, 3 are spaced apart by alumina spacers 4 ~v~

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2~7%~3 parallel to the laser axis 5. These electrodes 2, 3 are supported by alum m a blocks 6, 7 which also carry electrical connections 8, 9 to the exterior oE -theenvelope 1. At one end of the envelope 1 is a gold plated fully reflecting mirror 10 arranged normal to the laser axls 5 whils-t at-the other end of the envelope 1 is a multilayer-dielectric-coated germanium 85% reflecti~rl mirror llalso arranged normal to the laser axis 5. At both sides of the envelope are rows of tee shaped tungsten rods 12, 13 which pass into the envelope 1 in a gas tight manner. The pointed inner ends of the rods are spaced apart e.g. by 2mm arc gaps. Each row of rods 12, 13 is connected to earth via capacitors 14, 15 e.g. 4 pF capacitors, and the rcws connected in parallel as sho~n in Figure 3.
The energy for the arcs is provided by two 900 pF 16, 17 capacitors charged to 30 kV by an HT circuit 18 and connected via a triggered spark gap SGl. The energy for the electrodes 2, 3 is provided by a 10 nF capacitor 19 also charged to 30 kV by a HT circuit 20 and connected via a spark gap SG2. A delay 21, e.g.
2 ~s, allows sequential firing of SGl and SG2.
Strips 22 of porous alumina incorpora~ing 1.8% Pd-~nO2 (or 1.3%
Pt-SnO2) are mounted inside the envelope. Alternatively catalyst coated strips ox rods may be mounted in the envelope. m e (Pd-SnO2) catalyst may be prepared by standard processes e.g. by impregnating SnO2 gel with H2PdC14 solution or cation-exchanging with Pd(NH3)4(0H)2, or by co-precipitating Pd(OH)2 and hydrated SnO2 from chloride solution with KOH followed by washing, drying, and re-washing. This results in SnO2 particles supporting Pd in and/or on the sur-face. Such processes have been developed by the Tin Research Institute and des-cribed by G. C. Bond, L. R. Molloy and M. J. Fuller, J. Chem. Soc. (Chem.
Communications) pages 796/7, 1975. These processes resul-t in a tin (IV) oxide-supported palladium which is a more effective catalyst ~or the oxidation of CO
than either SnO2 or Pd is individually, particularly a-t low temperatures.

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1~L27Z~3 Similarly, the Pt-SnO2 catalyst may be prepared by standard processes e.g. by ion-exchanging a platinum chloride-ammonia complex (H2PtC16 ~ NH3) with SnO2 gel, follcwed by reduction. This resu~ts in SnO2 particles suppor~ing Pt in and/or near the surface. Such processes have been developed by the Tin ~esearch Institute.
m ese processes result in a tin (IV) oxide-supported platinum which is a more effective catalyst or the oxidation of C0 than either SnO2 or Pt is individually, particul æ ly at low temperature.
The catalyst may be incorporated into porous structures by standard processes as described for example in R. L. Ivloss, Experimental Methods in Catalytic Research, Vol. 2, pages 43-94, Academic Press Inc. 1976.
In operation the spark gap SGl is closed ~hich results in a series of arcs being formed in sequence along the two rows of rods 12, 13. m is provides pre-ionising ultra-violet radiation into the gas between the main electrodes 2, 3. After about 2 ~s ~depending on the gas composition) the spark gap SG2 is closed resulting in the main electrical discharge and a consequential burst of laser radiation through the mirror 11. As a result some CO2 dissociates into CO
and 2 m e catalyst assists in the recombination of CO and 2 into 2 The amount of catalyst needed depends on the laser pulse repetition frequency (PR~) (and hence generation of 2) the acceptable degradation in laser performance (increasing 2 concentration reduces perfonmance), the temperature of the catalyst and its availability to the CO and 2 For a given pulse repetition frequency and permitted 2 concentration t~le amount of catalyst must be incxeased until recc~bination matches dissociation. The rate of re-co~bination varies with temperature. It has been found for example, that 2 and CO are renDved by SnO2Pd from an initial mLxture of 34% C02, 20~ N2, 40% ~Ie, 4% CO and 2~ 2~ at a rate of about 0.005 cm3/s/g per unit mass of catalyst at -30C and 0.3 cm3/s/g - --at 40C with a loyarithmic relation at immediate tempera-tures. In similar condi-tionsl it has been found that 2 and CO are rem~ved b~ SnO2.Pt at a rate of about 0.06 cm3/s/y at 17C. This shows that the catalysts can operate at a~bient temper.atures even in very cold climates.
Use of SnO2-Pd or SnO2-Pt as catalysts is not limited to TBA lasers sin oe they can be used with other C02 gas lasers to increase useful life -times.
For example they can ke used with low pressure 20 Torr. CW lasers and CW wave-guide lasers (gas pressure about 100 Torr.).

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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A carbon dioxide gas laser comprising a sealed envelope containing a gaseous active medium including carbon dioxide, means for inducing an electric discharge in the gas within the envelope to provide a pump source, a reflecting mirror and a partially reflecting mirror spaced apart at opposite ends of the envelope to define a resonant cavity along the laser axis, and an energy abstracting means including the partially reflecting mirror, wherein the improvement comprises a structure including a catalyst material mounted inside the envelope in intimate contact with the gas for the catalytic oxidation of carbon monoxide into carbon dioxide, said catalyst being selected from the group of tin (IV) oxide-supported palladium, and tin (IV) oxide-supported platinum.

2. A laser according -to claim 1 wherein the structure is porous and incorporates the catalyst within its pores to increase the surface area of catalyst exposed to the gas.

3. A laser according to claim 2 wherein the porous structure is porous alumina.

4. A laser according to claim 2 wherein the porous structure is porous silicon.

5. A laser according to claim 1 wherein the structure is in the form of strips having a surface coating of the catalyst.

6. A laser according to claim 1 wherein the means for induc-ing an electric discharge comprise shaped electrode structures arranged either side the laser axis for transverse excitation of the gas.

7. A laser according to claim 6 wherein a further electrode structure with spark gaps is provided between the shaped electrode structures to one side of the laser axis for providing pre-ionisation of the gas.