Download - Laser induced non - stationary processes in gaseous mixtures through porous media

Specovrhimica Acta, Vol. 46A. No. 4, pp. 659 - 663, 1990. 0584 - 8539190 $3.00 + 0.00

Printed in Great Britain. 0 1990 Rrgamon Press plc

LASER INDUCED NON - STATIONARY PROCESSES IN GASEOUS MIXTURES THROUGH POROUS MEDIA

1.ursu* , R.Alexandrescu*, I.N.Mihailescu*.I.Morjan*,M.Stoica*, ** M.Zoran*

A.M.Prokhorov , V.G.Bordo**, I.Ershov**, Yu.N. Petrov**

V.A.Kravchenko**,

*)CentraZ Inatitute of Physics, Bucharest, **) Institute of General Physics, Moscow,

'1. I nfaoduat.ion

POE MC-6, Romania U.S.S.R.

The investigations concerning the action of laser radiation on gaseous molecular flows in transit through a porous membrane pointed to a diffusion process with many specific features /1,2/ which were analysed in connection with the po- larisation interaction of molecules in the electromagnetic field of resonant laser radiation /3/.

In this work we present a.new and more complete experimental study on the behaviour of gas - mixture flows in a laser field when transiting through a metalized fine - porous glass membrane. One component of the gas mixture resonantly absorbed the CO2 laser radiation. Long irradiation times were chosen in order to analyse the various featuress of the diffusion process in a non-stationary regime, but also from a practical point of view, so to evaluate the possibilities of gas separation in such systems.

An analysis of the diffusional process in the given experimental conditions takes account of the interference of several interrelated factors such as surface diffusion, temperature of the surface and in the gas phase, thermal desorption and relaxation of excited molecules, etc.

2. Expchimental

We have investigated the transit of a mixture of difluorethane and argon through a glass fine-pored membrane under the action of frequency stabilized C.W. CO2 laser radiation, which could be tuned to different vibration - rotation lines in the 10.4 nm and 9.4 ym bands. C2H4F

-P was the resonant gas in the mixture,with

an absorption band centered at 948 cm The membrane was made of borosilicate glass, having a diameter of 10 mm, a thickness of 7 mm and a pore diameter of 80 R. The specific surface of pores was 100 m2/g while the pore volume was 0.18 cm3/g. On one side of the membrane a 1000 ?? thick gold layer was deposited. The membrane separated the cell in two compartments (Fig.l), into one of which gases were admitted while the other one was connected to a MI-1201 V mass spectrometer. The mixture of gases entered the cell at pressure between 0.5 and 4 torr, while at the exit of the membrane the pressure was *lo-8 torr. In these conditions the flows through the membrane were of the order of several mg/hour.

metallised porous membrane yus midwe (4% turf)

Fig. I. Experimental set-up

When tuning the laser from one vibration-rotation line to another, the inten- sity of laser radiation was maintained constant and equal to 60W/cm2 by means of KBr plate attenuators. The laser perpendicularly irradiated the metalized surface of the membrane, in the same direction as the gas mixture penetration into the membrane.

The mixture concentration was mass spectrometrically monitored by measuring the ionic peaks at m/e=40 (Ar) and 51 (C2H4F2) with a 15s time delay. The output signal of the mass spectrometer for the chosen molecuie was recorded, in the ab- sence (qo) and under the action of laser radiation (q ), respectively.

659

666 I. URSU et al.

The experiments were carried out during several weeks, in which period of the time the membrane might have suffered some unexpected physical transforma - tions which could affect the gas diffusion process. One such factor is the re- flectivity of the .input metal surface, which decreases in time by the deposition of impurities, the membrane acting as a gas sucction filter. Also possible, but difficult to control is the modification of pore transparency, which might be changed at elevated temperature 131.

3. R eb u.ttb

The experimental results we present below refer to different stages in the membrane utilization which will be quoted as (I) for the first stage (new membrane), (II) for a transition stage (membrane after %30 hours of operation) and (III) for a final stage (membrane after %60 hours of operation).

Fig.2 represents the time dependence of the relative flow variation q*/q, for the two components Ar and C2HgF2 in a mixture Ar:C2HgF2 = l:l, diffusing through membrane(I a total input pressure p = 1 torr. lOP(20) laser line (v = 944 cm-l

At the switch-on of the ) there is an increase of C2H4F2 flow during the

first 4 to 5 minutes followed by a gradual decrease till the flow reaches smal- ler values than the stationary q. value

Fig.2. The dependence of the relative flow variation q*/q, for the mlxture components C2H4F2 and Ar diffusing through membrane (11, on laser irradiation time t.

1. Q I 1OP (20)

2. m

3. A 1 9P (22)

4. V

The separation coefficient n may be defined as the ratio between the flow variation of C2H4F2 and Ar respectively, n = (q*/qo)C,HgF2/(q*/qo)Ar. For the

flow parameters presented in Fig.2 and during resonant laser irradiation a maximum value n= 1.13 was found which diminishes in time.

If the nonresonant 9P(22) line (v = 1045 cm- 1) is employed then for both components in the mixture the flows are slowly decreasing but with a different slope (curves (3) and (4) in Fig.2).

By using a mixture with excess Ar (Fig.3), the time dependence of C2HgF2 flow variation through membrane (II) under resonant 1OP (20) laser line shows a small increase which lasts a short time (less than 1 minute) and then suddenly decreases and reaches rather high values e.g., q*/qo = 0.52 after 26 minutes of irradiation. In contrasts, there is a small flow decrease which keeps almost constant ((q*/qo)C2HgF2 = 0.95) if the nonresonant 9P(22) laser line is used (curves (3), (4) in Fig. 3). This flow behaviour with a nonresonant laser line was quite general for all membrane types and flow parameters.

4" Yu

Fig.3. The tempora 1 dependence of the relative flow variation q*-/q, for C2H4F2

and Ar diffusing through membrane (II).

Fig. 3 i switch-on of ture). After ry q, value

For the

Laserinducednon-stationaryprocessesingaseousmixtures 661

llustrates the nonstationary character of the flow not only at the laser radiation but also at the switch-off (right-side of the pic- a slight variation the flows begin to tend to the initial stationa-

same mixture concentration as above, the evolution of the separation of irradiation is shown in Fig.4. The results refer to stage (II) of the membrane. A maximum value IJ= 1.21 is reached in the time interval from 10 to 12 minutes of irradiation and an input pressure of 2 torr. As the pressure becomes lower, the separation coefficient also decreases.

coefficient during the first 15 minutes

7 t

membrane (17) Ar:C,H./+4:1

12

I?

I” ” ” ” ’ c 2 4 6 8 IU 12 14 16&.7iR)

Fig.4. The temporal variation of the separation coefficient for different pressures at the membrane (II) input.

I3 2 torr A I torr 0 0,5 torr

The following experimental results presented in Figs. 5 to 8 refer to membrane (III).

Fig, 5 shows the relative flow variation for C2H4F2 and Ar, after the switch-on of the lOP(20) laser line.The flow of the resonant and nonresonant component share almost similar temporal behaviour.

The main features of flow variation in Fig. 5 i.e. a decrease of the flow in the first few minutes till a minimum value is reached, followed by a slow in-

crease for long lasting irradiation times (between 10 and 30 minutes), charac- terize all the results obtained with membrane III for different input pressures (p= 1 to 4 torr) and two opposite mixture concentr;tions (Ar:C2H4F2=4 and 1:4). The slope of the upward part of these curves (depicting C2H4F2 relative flow increase) i.e. the ratio between flow variation interval A(q*/qo) and time interval At, has a marked dependence on pressure, as visible from Fig.6.

Fig.5. The dependence of the relative flow Fig.6. The dependence of

variation q*/qo on laser irradiation A (q*/q,) /At for C2H4F2 time for C2H4F2 and Ar diffusing on mixture total pressure

through membrane (III).

membrane (gl) (11 C, 14 ,‘z : Ar= I:4 czIC, H4 /2 : Ar= 4:?

(II

I I I e 2 4 6 8 f (min)

0 J-5 turr . 1 fUff 0 2 forr A 3 forr

Fig. 7. The dependence of the minimum value of C2H4F2 flow variation on laser irradlatlon time.

662 I. URSU et al.

We note also that the minimum value of the C2H4F2 flow decrease in Fig.5 moves towards shorter irradiation times with increasing input pressures. In Fig.7 curve (1) the minimum occurs at ~7 minutes for p=O.5 torr but at only %2 minu-

tes for p=3 torr. This dependence seems to be evenmore pronounced for a mixture with C2H4F2 in excess (curve (2) in the mixture Ar:C2H4F2 = 1 : 4).

'I

t

, , ( , ) , , , c

Fig.8. 2 4 6 8 Ill 12 14 16 f8 20 22 24 26C(m/i)

The temporal variation of the separation coefficient for two opposite mixture concentrations : Ar : C,HqF2 = 4 : I and 1 : 4.

Further it is worthwhile to compare the temporal evolution of the separation coefficient in case of two different mixture input pressure of 2 torr. In the mixture with C2H4F2 in excess 0 keeps at low values, while in both cases it decreases at greater pressures.

4. Vi6 CuddiOtl

al Temporal behaviour of relative flow variation i) Corresponding to the three stages of membrane utilization, there are dif-

ferent types of behaviour of gas flows exposed to laser radiation which is in resonance with one of the mixture components (Figs. 2,3 and 5). On the other hand, in spite of differences in the temporal evolution one may observe similarities between corresponding flows. So if we imagine a compression in time of curves (1) and (2) in Fig.2, these are very similar to the beginning of curves (1) and (2) in Fig.3, while the latter reproduce themselves quite well in the first 2 to 3 minutes of flow evolution in Fig.5. We believe that the reason for the dis- tinct behaviour of flows at different membrane stages is that a clean, reflect- ing membrane surface (Fig.2) inhibits heating effects for a rather long time (4-5 minutes). On the contrary a damaged surface which has caught impurities and adsorbed molecules heats very rapidly under laser radiation (less than 1 minute in Fig.3).

ii) If we further compare the gas flows under the action of a resonant and a nonresonant laser line (Figs.2 and 3) one may note quite different temporal behaviour especially for short irradiation times. More exactly, just after the switch-on of laser radiation there is an increase of the flows (more pronounced for C2H4F2) if the resonant lOP(20) laser line irradiates the system. The decrease which follows is similar in form for both C2H4F2 and Ar flows but different in characteristic time of decay.

In contrast, if a nonresonant laser line was used then the starting flow increase was never experimentally found.

We interpret the flow decrease in all three membrane stages as an effect of the thermal action of laser radiation. At the same time the flow increase is most likely due to a nonthermal, resonant effect, which was formerly observed in other different heterogeneous systems and irradiation geometries /1 i 51.

We must observe that the rather complex features of gas flow diffusion in porous membranes, are somehow simplified in our specific experimental conditions.In a laser field, acting on the internal metal deposited surface of the membrane the radiation does not practically penetrate into the pores'volume. The induced modi- fications in the flow transit are attributed to surface processes, i.e. adsorption and/or desorption of molecules on the input surface of the membrane, followed by diffusion on surface towards pore entries.

These simplifying premises predict a proportionality between gas flow at the membrane exit q and the product of surface diffusion coefficient D and surface concentration of molecules N at the input face of the membrane /3/. At thermal equilibrium, N % exp(U/kT) and D % exp(-Q/kT), where U is the adsorption potential which accounts for the dynamical coupling of the adsorbate molecules to the substrate and Q is the potential barrier the molecule must overcome to migrate between adsorption sites, Q < U /g/.

A theory which was developed in /2 to 5/ demonstrates that the resonant mechanism which govern adsorption and diffusion of gas flows in porous media (or capillaries) is the deepening of the adsorption potential for resonant molecules.

New experimental evidence obtained especially in diffusion processes occuring in metal capillaries /3f pointed to the possibility of another resonant mechanism which has its origin in the vibrational-relaxation processes of adsorbed molecules on surface /7/.

Laserinducednon-stationaryprccessesingaaaeusmixhlres 663

In gas phase, vibrational energy redistribution of excited molecules has been thoroughly studied (see e.g. /a/) and is now quite well understood.

The system adsorbate-adsorbent on surface implies a mixture of internal molecular vibrations and external modes /9/ in which, by a weak interaction (physical adsorption) the internal modes are hardly changed. We may suppose that the a- symmetric top C2H4F2 molecule is raised to upper levels of its resonant mode (the fundamental vibration A" Ill/) by absorption of a IR quantum from the Co2 laser radiation. For excited adsorbed molecules the potential barriers between different surface sites may be substantially reduced thus allowing increased surface migration. Then, the surface diffusion coefficient increases and so does the gas flow in transit through porous membrane. It is worthwhile to note that for 10.6 pm radiation the quantum energy is 2.7 Kcal/mole which is a characteristic value for energy barriers in physical adsorption /lo/.

The decay of molecular modes on the surface may involve many channels of deactivation /7/. In our opinion the conversion of intramolecular vibration energy into translational motion (surface V-T relaxation) may be favoured as compa- red to other deactivation mechanisms, e.g. generation of substrate phonons or non- adiabatic decay into electron-pair excitation in metal conduction band 1121.

Let us recall some experimental features obtained with resonant irradiation. We interpret the small amount of Ar flew increase at beginning of irradiation interval (Figs. 2,3) as collisional transfer of kinetic energy from "fast" resonant

C2H4F2 molecules to Ar atoms. The flow decrease of both Ar and C2H4F2 flows may be attributed to an enhanced heating of the internal membrane surface, with sub- sequent induced thermal desorption. The heating is accelerated by resonant absorption in the adsorbate (the rate of flow decrease is greater with increased C2H4F2 partial pressure (Fig.7)).

The slow flow increase at long irradiation times and a rather imperfect re- flecting surface (Fig.5) points to a competition between two processes: first, there is a cooling of the internal surface dut to energy exchange between the surface and the gas above it and second, there might be a desorption process from the external surface of the membrane. The cooling rate grows faster with increasing pressure (Fig.6).

b.l Gas mixture separation coefficients. The nonstationary behaviour of the separation coefficients as depicted in

Figs.4 and 8 reveal a rather complicated interdependence between resonant action of laser radiation and heating and cooling processes at surface, with specific temporal development. For a given substrate, these processes are determined by the total pressure of the gas mixture as well as by the ratio of the components. An optimum value of n is obtained for instance, at a gas pressure around 2 torr (FPg.4) which means a balance between a rather important resonant effect for C2H2F2 flow and the thermal heating of the surface.

Most relevant for the energy transfer processes occuring at surface is the lowering of the separation coefficient with increasing content of the resonant component in mixture (Fig.8). We believe that the transfer of kinetic energy from "fast" resonant molecules to Ar atoms becomes more effective, stimulating surface diffusion of Ar atoms also and thus drastically decreasing the values of the separation coefficient.

Wu may conclude that the resonant mechanism observed at the transit of gas mixtures flows through porous membranes in a laser radiation field may be inter- preted in terms of a surface diffusion stimulation by the process of vibrational excitation and subseqllent V-T surface relaxation. Although complicated with heating and cooling effects, this process may lead to rather important values of the separation coefficient between resonant and nonresonant components.

Redc&?nct?h

1.

2.

3.

4.

5.

6. 7.

8.

9.

10.

$fr:

I.Ursu,R.Alexandrescu,N.Comaniciu,C.Grigoriu,I.N.MIhailescu,I.Morjan,A.M.Prok- horov, N.V.Karlov,V.A.Kravchenko, N.A.Orlov,Yu.N.Petrov,J.Mol.Structure,ll4, 317 (1984). I.Ursu, R.Alexandrescu,V.Draganescu,I.N.Mihailescu,I.Morjan,A.M.Prokhorov,N.V. Karlov, V.A.Kravchenko,A.N.Orlov, Yu.N.Petrov, J. Physique 46, 883 (1985). V.A.Kravchenko,A.N.Orlov, Yu.N.Petrov,A.M.Prokhorov, "Resonance Heterogeneous Processes in a Laser Field" (in Russian),Trudi IOFAN,v.ll, ed.Nauka,Moscow1988 V.A.Kravchenko,Yu.N.Petrov,S.p.Surov, V.A.Sikchougov, Visokochistie Vestchest- va 3, 94 (1937). I.Ursu,R.Alexandrescu,I.Morjan,I.N.Mihailescu,A.M.Prokhorov,A.S.Lagutchev,A.N. Orlov, Yu.N.Petrov, J.Appl. Phys. 65, 4173 (1989). J.R.Dacey, Ind. and Eng. Chemistry 6, 27 (1965). B.J.Lundquist in "Surface Studies wTth Lasers" ed.F.R.Ausseneg,A.Leitner,M.E. Lipitsch, p.14, Springer, Berlin, Heidelberg, New York Tokyo, 1983. J.I.Steinfeld in "Energy Storage and Redistribution in'Molecules", ed.J.Hinze v:gQi;;+;;rn I$.$;eiorE. New York,, 1983.

Gilles, Riehl in Vibration on Surfaces", ed. R.Caudano, J.M.

A.A.Lucas, i.l?, Plenum Press New York, London, 1982. S.Ross,J.P.Olivier,"On Physical Adsorption".John Wiley, N.York,London,Sydney, 1964. N.Solimeno B.P.Dailey, J. Chem. V.P.Zhdano~o~T,$nbH~~~~n~~~~.

Bh s., Physica P

22, 2042 (1954)

Russian), and Chemical Processes on Surface" (in

Download - Laser induced non - stationary processes in gaseous mixtures through porous media

Top Related