Topics in..
. feature
Edited by S. 2. LEWIN, New York University, New York 3, N.Y.
and technological applicstions of excited gases. "One can perceive from the literature that up until now the applications have involved oxygen, nitrogen, ea~bon mono- and dioxide, helium, and argon primarily." There is still a.vmt amount of untried reactions, processes and applications involving excited earhonyls, halides, hydrides, gases containing sdfur, nitrogen. phmphonn, silicon, ete.
These articles, most of which are to be contributed by guest authors, are intended to serve the readers of this JOURNAL by calling attention to new developments i n the theory, design, or availability of chemical laboratory instrumentation, or by presenting useful insights and explanations of topics that are of practical importance to those who use, or teach the use of, modern instrumentation and instrumental techniques.
Discharge Experimental Requirements
XXVIII. Research With Electrodelessly
Radiofrequency Generation and Gas Excitation
Discharged Gases John R. Hollohan. Trocerlab, A Division of Loborofory for Elecfronics, lnc., Richmond, California This paper will describe some rather straightforward uses of excited gases for resehreh or technological purposes, and some general laboratory, vacunm, and R F engi~leeringrequirements for assembling and operating dinchsrge apparatmes. There are m y itumber of pnhlished work? on some of thc deeper kinetic and sppectrcscopie aspects of gaq phase processes occurring in the discharge. l\fannella (2), for ousmple, hm disewsed the nat,ore and recent interoretations of active nitroeen wluvh I II. I w I)IOIIIICPII i l , elrl.troddt.-. ~ l i c I : . 'I'lw ila.c>t-.ion .sf lht, t . 8 1 d 1 y .S~mrty,So. :K, I - C , I I P V O ~of , ~ ~. . r l i . w reactions and processes some of which were induced hy ddischarge met,hods and simulate those occurring in the upper sir mosphere (3). The atom-molecule and atom recombination reactions, and chamcte~istic spectra due to various de-excitation modes are discussed. Recently, Young (4) presented current theories and experimenbd results from st.udies on the airglow, most of which were performed iu a microwave discharge. The plasma t,ype encotmtered in the n~icrowaveor rediofrquenry electrodel&q discharge is a "low temperature," low p r e s sure plamm, not, too unlike the kind of cold discharge set up het.ween a cathode and anode of the internal electrode glow discharge apparatus. Thin "cold plasma" (temperatnre usually i ~ pto 500°K) di* charge a t about 100 microns ta several ndlimetea prewnre is in eontrant t,o the more commonly taken description of a plamm, that is, a much higher temperature (50110 to 50,OOO'K) and density plasma, surh ai that of aplasma jet. I n the cold plasma one has, however, a non-isolhrmd plnxns, in t,hat the temperature of atoms and molecules, ionic or neutral, is much less than the temperature of the elerbrons, which have a concentrsr tion of lO'0-lO's ern-' under the u s u d d i s charge conditions. The temperatures of
the electrons may be on the order of ten3 of thomands of degrees while the atomic and molecular species have moderat,e gas temperatures of a few hundred degrees. The temperatures one measure-, however, will always be that composite temperature of the plasma due to the neutral and ionic species. The electronic temperature can he defined if we assume a Maxwellian velocity distribution and calculates corresponding temperature. The excited ges plasma, once produced in the flow diseharee manner.. ~rovidesone wit,h a oowerfd means to induce chemical
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kinetic experiments. The general motivation behind using excited gas reactive atomic, ionic, and energetic species ir that one can carry out chemical reactions or processes a t temperatures mueh lower t,han those usually required, or a t a. mueh faster rate than is observed a t a given bemperature. The electrodeless method freeri one from problems of contamination by consumable electrodes and provides a uniform volume of excited, reactive ga?. It hss been amply demonstrated that free radicals, ions, and excited molecules generated in a closely-controlled, stahle electrodeless diachxrze can he utilized to nrodnce useful non-excited gase?. Systems of interest can he quite diverse: homogeneous gas reaction a t discharge conditions; gas-solid reactions; reactions involving solids of different physical diiponition, such as ultrafine sub-micron powders, organic or inorganic phases; reactions producing products in the gas p h a ~ ewhich are pumped out for suhseqnent analysis; reactions for effecting syntheses, or reducing organic media to residual inorganic ash. There exist9 a. wide variety of research
I n Figure 1 of the previous article (1) the critical components of a flow di~charge configuration were schematically shown. The experiment,d section here r i l l assume familiarity with the layout of a typical discharge set-up. The crit,ical components of a discharge system sre:
1. the radiofreqmncy generator or microwave magnetron, the region of excitation, i.e., coil or microwave cavity, and the plasma reactor, 2. tLvrteuum pump and means of monitoring the pressure, gas handling regulator, flowrstor, and bleed valves. Aside from these very essential components, one may he involved with the appropriate temperature measuring device, atom concentration probe?, gas titration assembly, traps, etc. Since the rwearch application areas of excited gases are so diverse, i t is impossible to generalize in the description of apparatus and discharge requirements. Moreover, one can work with either the microwave or radiofrequency method of diccharge. The microwave nece~sitattes the use of terminal wave-guide cavities (see Figure 2 of the previous article) from which the microwave radiation pours and the excitation takes place. Sinee in general the microwave discharge apparatus is more diklicult in an engiweering sense to set, up and, is less versatile, the R F discharge technique will be the one emphz-ized here. The radiofrequency method depends npun delivery of the energy from the R F generator to a "plasma activator" hy means of coaxial cable. From there the energy is delivered to the region of excitation. There are two primary ways gas can be excited using KF energy. These are the ring type or inductive discharge and the parallel plate or capacitive diichnrge methods. These two configuration.; are shown in Figures 1 and 2. I t is into either of these types of discharge regions that. energy comes from the generator. The electrodeless nature of the discharges is evident. The glassware of the vacuum (Continued on page A498)
Volume 43, Number 6, June 1966
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A497
Chemical instrumentation
Figure 1. A ring or inductive type of dirchorge coil.
syslem passes either axially dc,w~,1Iw indwlive coil or hetweeu the parallel, capacitive disrharge plates. Each method h w its ndvnntnpe; dependingon theproject at hand. It shmdd he menlioned at this point that the glow chararterislies of the m i c n w a v ~and I1F ave quite different. The RF glow can he made to extend virtually thmughout the entire system, while ihe microwave has its greatest glow intensity at the rnvify then diminishm rat,her rapid1.v until o w ran find nrtive species from the disrhxrge still persiding into a region free of the glow of the plasmn. I t ir for this reason, pl.irnarily, thal, gas renclions are sltldicd spectroscapirally wilh the mirrownve method, since thenpertranrenot cornplir;rled hy the confusing spertra of the glowing plasma. Any glow or light lhnt is seen in ihi-; ]region
Figure 2. A system orrembled far rapocilive dixharge5. A manifold of four tuber is shown entering a large reaction chomber containing o sample o f polymer foam to be treated with excited oxygen.
(Continued mz page ~1500)
A498
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Journal o f Chemical Educofion
Chemical instrumentation
P L A S M A ACTiVATOR
MC L E O D
BLEi
downstream from the microwave cavity raises from chemilnmineseent reactions or emissionsfrom de-excitingspecies. The ILF exciter region, inductive or cspacit,ive, can be seen in relation to the rest of the components in Figures 3 and 4. I n either method, the optimiaztion of coupling the R F energy into the disch~rging gas is done by matching t,he gas load impedance to t,he impedance of the amplifier plate ontput circuit oi the generator. The impedance matching is achieved by a tuning process in the plasma activator and, once tuned, the matched circuit is quite stable so long as the dischasge parameters-gas, pressure, and flow rnte--do not change. The power is coupled into the plasma with the plasma. acting essentially ass, core. Some of the power is lost due to heating efiects, while the remainder goes into ionization of the gas. This is the sum total of the fornard power, approximately, from the generator. There is s n amount of Dower which is reflect,ed back to the
the gas, one can measure the net power being supplied to the h d , i.e., excited gas, as the difterende between the forward and reflected power. This is measured conveniently with an R F wattmeter inserted in the circuit as shown schematically in Figures 3 and 4. I n a. commercial version, the power meter can be m integral part of the circuitry of the R F generator (the KFG-BOO, Tracedab, Inc.). One "tunes"
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Journal o f Chemical Fducafion
V4 LV
TO DIFFUSION
and MECH4NiCAL PUMPS
COAX C A B L E
GENERATOR
Figure 3. Schmotic of o dixharge orrembly utilizing on inductive discharge coil. case, the inductive exciter coil ir an integrol part of the plasma activator.
to minimum reflected pawer by means of adjusting variable condensers in the plasma activator. Thus, for example, of 250 watts being generated, as little as 0.5 watts can be found reflected after optimizing the line impedance t,o the load impedance by means of tuning with the plasma. activator. A commercially avdilable plasma. activatar which inchtdes the discharge (induetive) coil, associated tuning circuitry, and flawmeter is shown in Figme 5. This low temperature plasma discharge unit (the PAI-BOO, Tracerlab, Inc.) is to be tmed in conjunction with the RFG-600 R F generator, Figure 6. This generator has a maximum discharge power of 300 watts. Coaxid cable (RG/8p 50 ohm) provides
In this
energy transmission to the plasma activatar and discharge coil from the generator. As mentioned earlier the use of the eapscitive discharge configwation may be mare desirable. This allows discharges to be set up under identical conditions in several inlet tubes simultitneonsly. The front cover of this issue shows such a manifold system under discharge conditions. The advantage of this disch:trge method is that t,he plasma reactor, hence thesystem under study, can be made much more isothermal than a reactor that eonsists of a single inlet of discharged gas which passes thrangh the reactor and out a downstream exit. In this case tempera(Conlinued on page A608)
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