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A new equipment for mass transport study in a free-burning d.c. arc in air
S~e~trochimica Acts,
Vol. SOB, pp. 31 to 44. Pergamon Press 1976.
Printed inNorthern Ireland
A new equipment for mass transport study in a free-burning d.c. arc in air V. M. VTJKANOVI& M. S. TODOROVI& P. S. TODOROVI~~ and T. A. MIHAILIDI Prirodno-matcmati&i hkultet, ElektrotehniEkifakultet, Tehnoloi3ko-metaluG.ki fakultet, Institut za C&u, Beograd (Received 16 April 1973. Revised 6 June 1974) Abstract-A method for injection of liquid substances into the arc plasma is proposed. This method, which is also applicable to other plasmas at atmospheric pressure, seems to be very convenient for the study of the transport properties of the injected elements. The results of time-resolved measurementsof the intensity of characteristicradiation emitted by the injected substances are presented in terms of residence times of particles in the plasma. The effect of the polarity of the arc was also studied. The cathode retains the substance in the vicinity of this electrode. 1. INTRODUCTION
THE PROCESSES of transportation of examined substance out of a d.c. arc, together
with their rates of entry, determine the particle densities in this plasma. It is well known [l, 21 that these densities together with temperature, electron density and atomic parameters, determines the intensity of light emitted by the substances. In order to increase the detection power in this source by increasing residence times of examined particles, the knowledge of processes governing the outwards transportation of substance from such plasma is required. These processes are [2] : (i) diffusion (due to concentration gradient, ambipolar and thermal diffusion); (ii) the influences of the axial and radial components of the electric field; (iii) convection of the gas. The technique to be described here was developed in order to enable the study of mass transport properties of substances involved in these processes with special respect to the residence times. It has been attempted to avoid other influences (such as those associated with continuous evaporation of substances or external gas streams) the arc used, for sake of simplicity of treatment and desired accuracy. 2. FUNDAMENTALS 2.1 Survey
OF MAIS TRANSPORT
STUDIES
of the state of investigations in literature
No survey of this kind can ever be complete. Therefore we shall only briefly quote some of the papers dealing with mass transport study, useful (in our opinion) for increasing of the power of detection in the d.c. arc. Convection is one of the processes, governing the mass transport, and rather independent of the examined substance. Such observations in vertical, free-burning d.c. arcs were done, e.g., by VAN STEKELENBTJRG [3]and HAGENAH [4],both using t At present at the Department of Electrical Engineering and Electronics, University of Liverpool, Great Britain. [l] P. W. J. M. BOUIUNS, Thory of Spectrochemical Excitation. Hilger and Watts, London (1966). [2] V. M. VUKANO~IO,Emtieionepektroskopie, p. 9. Akademie-Verlag, Berlin (1964). [3] L. H. M. VAN STEKELTCNB~~~, Phyeica 12,289 (1946). [4] W. HAUENAH,2. Phyeik 128,278 (1960). 31
32
V. M. VUEANO~I~, M. S. TODOBO&P. S. ToDo~~& and T. A. MIHAIL~DI
powder substances at the electrodes. Axial and radial distributions of the values of convection velooity were found by observing the motion of these substances (mostly carbon particles). In Ref. 4 the influence of convection at the energy balance was also thoroughly examined. BRIL [5] used the motion of the particles due to aonvection to examine the influence of the polarity of the sample electrode on the measured velocity. In order to study any of other processes of this kind, one needs to bring the substance of interest into the arc. A technique for this was proposed by RAIJUIBAUX and MAJXKH [S]. Designed especially for mass transport study, its main feature wss the needle moving transversally to the arc axis and bringing the substance into the arc. Together with proposing it, they used it for meesurement of residence times. The latter was achieved by observation of the time dependence of emitted line intensity, and these results were used for determination of the diffusion coefficients. The influence of different additives on the residence time was found [7] also with this technique. Further, UYKH and SERD applied such measurements to compare different spectrochemical sources [8]. KTJZNECOVA, R~BAUM and MALYEH [S] used them again, together with the theory of BOUMANSand DE GALAN [lo], to determine the influences of the arc current and dimensions of the gap on the detection power. This method for residence time determination in absorption and exit&ion zones was used also by TYKE, MEN’SHIKOV, MOROZOV and SHIPICIN[ 111, in their comparison of atomic absorption and emission methods. NAZAROV,GERA~IMOV and M~L’YUSexamined particular properties of atom transport through the zone of an a.~. arc [12] by the same method, applied at different values of the arc current. They also studied the effect of adding N&l and N&F. This method was also used by KAR’YAKIN, LAKTIONOV, PAVLENP;O and SKL’YARENKO [13].They explained the influence of lantanide oxide matrices on the determination of usual microimpurities by the velocities with which subs&mea leave the system, and the diffusion coefficients determined from the values of residence times. KRA~NOBAEVAand ZAD~OR~KAused the same technique for their investigations of the effect of additives at transport processes [14]. The increase of residence times in the presence of additives they ascribed both to the thermal parrtmeters and the decrease of the gradient of the eletric field in radial direction in such conditions. The influence of convection on the evaporation velocity from electrodes they found negligible. The technique of moving needle was also used by GOL’DFARBand IL’INA [ 161 [B] J. BRIL,C. R. Acad. Sci. Pa& 265B, 1277 (1967). [6] J. D. RAIK-AUMand V. D. MAIXKE,Opt.Spektroek. 9,426(1960). Kurdand V. D. M~~nrri,Opt. Spektrosk. 10,624(1901). [7] J. D. RAIKHB [s] V. D. MAIXKHand M. A. SERD, Opt. Spektrosk. 16,368(1964) [9] A. I. KTJZNE~OVA, J. D. R~mzm AUM and V. D. MALYKH, 2%. PrikZ. Spektfoek. 3,393 (1969). [lo] P. W. J. M. Boms and L. DE GALAN, Anal. Chem. 28,674 (1966). [ 1l] V. D. MALYKX& V. I. MEN’SHIKOV, V. N. MOROZOV and S. E. SHIPICIN,2%. PrikZ. Spektroek. 16,12 (1972). [12] T. V. NAZILBOV, K. 5. GERASMOV and A. V. MILYUFI Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim Nauk, 9,137 (1967). [13] A. V. KARYAKIN, N. V. LAKTIONOVA, I. V. PAWN-KO and Yu. S. SKLYARENKO, Izv. Sib., Otd. Akad. Nauk SSR, Ser. Khim. Nauk 9, 123 (1967). [ 143N. KUSNOB~EVA and Z. ZADQORSIEIL,Spectrochim. Acta 26B, 311 (197 1). [ 151 V. M. GOL’DFA~B and E. V. IL’INA. Opt. Spektroek. 11,445 (1961).
A new equipment for maw transport study in a free-burningd.c. arc in air
33
in their determinations of residence times and, values of the diffusion coefficient. They used photographic photometry of the high-speed camera photos of the arc, these being taken through interference filters for selection of characteristic light to obtain mean shift of the particles of the examined substances and maximum intensity variation with time (from frame to frame). It served, respectively, for evaluations of the diffusion coefficient and the residence time, the former being compared with calculations based upon method of HOWARD-ARNOLD [16]. The same technique was used to bring into the arc the substance, whose axial motion due to electric field was stopped by counter-directed adjustable forced convection, in experiments of GOL’DFARB,VAINBOIMand IL’INA [17]. The positions of halfmaximal intensity points, with the observed equalized velocities and the theoretical solution for the distribution of the substances in the arc gap, gave then the diffusion coefficient. In some of the works referred to so far [6-9, ll-151, the pulsed introduction of substances into the plasma was used to reduce the influence of the evaporation process on the study of transport phenomena. Another approach to the problem is the study of particle density distribution in the plasma in dynamic equilibrium, with continuous evaporation of the substance from the electrode. We would only mention the early work of GINSEL[18], and proceed to the first attempts of BOUMANS [19] and LAUWERIER[20] to apply this theory to the cathode layer arc. Following this line we would come to the theory of BOUMANS and DE GAUN [IO], studying the influence of axial transport at the line intensities in the steady and equilibrium state. As would be seen from this report, this model and the attached mathematical theory are widely used when such conditions are observed. GOL’DFARBand IL’INA [21] gave a theoretical treatment of the examined probe’s composition effect on the residence times, and thereby on the distribution of the examined substance in the arc gap, by considering mass transport properties of substances involved. Further theoretical efforts were made by KRINBER~[22] to determine the influence of the dimensions and mass of examined substances’ particles at the power of detection. This was based upon DE GUN’S experiments [23], and considerations of the influences of the electric field and diffusion at the transportation of particles out of system. An improved method for the calculation of the concentration field of microimpurity element in the interelectrode space was given by KRINBERQand SMIRNOVA [24]. They had to estimate the relative roles of different transport mechanisms,
[ 161 J. HOWARD ARNOLD, kg. Eng. Ohem. 22,lOQl (1930). [17] V. M. &L’DFABB, D. I. VAINBOIZX and E. V. IL’INA, vc’estn.Lenilagrad
247 (1963). [la] L. A. GINSEL,
Uni. Fiz.
Khim.
Thesis, University of Utrecht (1933). P. W. J. M. Boumms, Proc. 6th CoZZ. Spectr. Int., Amsterdam 1966; Spectrochim. Acta 11, 146 (1965). [20] H. A. LA~~EEIER, Appl. Soi. Rec. BA, 197 (1956). [21] V. M. GOL’DFARB and E. V. IL’INA, Uch. Zap. Leningrad. Gee. Pedagog. Inet. im. A. I. [lo]
Oertsena 339 (1966). [22] I. A. KBINBERO, Izv. Sib. Otd. Akud. Nauk SSSR, Ser. Rhim. Nauk 9, 4 (1967). [23] L. DE GALAN, J. Quant. Spectroec. Radiat. Transfer 5, 735 (1965). [24] I. A. KRINBER~ ctnd E. V. SMIRNOVA, Zh. Prikl. Spektroek. 10, 400 (1960). 3
34
V. M. VUXANO~IO,M. S. TODOBO&, P. S. TODOBOVIO and T. A. MIENIXDI
using diffusion coefficient values as could be calculated from the formulae of HIRSCHFELDER, CURTISS and BIRD [25], comparing them with experimental values from ref. 6, and the concentration KRINBERG)and S~NOVA
field with the experimental
data of ENQEL’SHT [26].
also provided the possibility to consider electrode geometry,
by different calculations of the concentration distribution for upper electrode being small or wide [27]. The volatilization rate of particles from the anode cavity into the discharge zone of d.c. arcs was studied by AVNY and GOLDBART [28]. These authors examined also the volatilization
rate, the total concentration and the axial A contribution
velocity of uranium particles using neutron activation analysis [29]. to the study
of concentration
distribution
was done by VUKANOVI~ [30], who Investigating
examined the effect of radial separation of masses in a d.c. arc plasma.
the influence of plasma composition on the distribution of substances in the arc gap, GOL’DFARB and IL’INA found an explicit relationship between the effect of “carriers” on spectral-line
intensities and the longitudinal
electric field [31].
They also found the tiuence
substances involved in the distribution studied.
transfer velocity
elements’
concentration
of
Using a high-speed camera ZOLO-
TUEHIN and ZIMINA [32] compared the experimentally the examined
of atoms in the
of other transport parameters
found axial dependence of
with the theoretical
one, the latter being
obtained by calculating the diffusion coefficient after HOWARD-ARNOLD [16] and the mobility after Einstein’s relation.
YAROSLAVSKA and ZOLOTUKHIN studied the veloc-
ity of evaporation
of substances from metal electrodes [33], calculating mobility and diffusion coefficients, and using the theory of BOUMANS [l] for Q. The influences of diffusion, convection
and electric field at this velocity were found. WIENECKE [34] determined the velocity field of high current arc by different methods. VUKANov10 [2] gave a method for calculation of the ratio of residence times with and without additives. VUKANO~I~ et al. [36, 361, as well as TODOROVI~ et al. [37] pointed out the role of the residences times on the increase of line intensities in an inhomogeneous
magnetic field.
A matter of more general interest is considered by
VUKANOVI~ [38]. [25] J. 0. HIRSCI~LDER, C. F. CUXTI~Sand R. B. BIRD, MO&~&W Theory of Gases and Liquids. John Wiley t Sons, New York (1964). [26] V. C. ENQEL’S~ md L. A. SPEKTOROV, Izv. Akad. Nauk SSSR, Ser. F&z. 26,887 (1962). [27] E. V. SMIRNOVA and I. A. KRINBERU, Zh. Prikl. Spektroek. 16, 17 (1972). [28] R. AVNI and Z. GOLDBART,Spectrochirn. Acta 2SB, 189 (1973). [29] R. AVNI, Z. GOLDBART,Spectrochim. Acta 28B, 241 (1973). [30] D. VUEANOV~~,Spectrochim Actu 22, 815 (1966). [31] V. M. GOL’DFARBand E. V. IL’INA, Zh. Prikl. Spektrosk. 5,381 (1966). and I. L. Z~NA, Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk 9, [32] G. E. ZOLO~ 106 (1967). [33] R. M. YAROSLAVSKAYAand G. E. ZOLOTUXJXIN, Zh. Prikl. Spektroek. 16, 592 (1969). [34] R. WIENEXKE,2. Phyeik 148,6. 128 (1955). [36] V. VWANOVIO, V. GEORQIJEVIO, N. KONJEVIOand D. VUKANOVI~, P~oG. 12th Coil. Spectr. Int. Ezeter 1965,p. 193.Adam Hilger, London (1966). [36] V. VWANOVIO, V. GEORQIJEVIO, D. VUXANOVIOand M. TODOROVIO, Spectrochim. Acta 24,B, 666 (1969). [37] TODOROVIO,V. M. VDXANOVI~, V. M. GEORQIJEVI~,T. A. MIHAILIDI and P. S. TODOROVIO,Proc. 9th Int. Conf. Phen. Ionized Gases, Bucharest 1969,p. 321. [38] V. M. VUKANOVI~,1Mag.Rem. Foly. 7l, 2 (1966).
A new equipment
for m8ss transport study in 8 free-burning d.c. 81% in air
35
The influence of diffusion at the heat conductivity, considered by BrmnORN[39], BROKAWand BUTLER [40] and BROKAW [41], indicates the importance of mass transport studies also for energy transport. The connection between the two, in the form of calculations of diffusion coefficients from the radial temperature distribution and heat conductivity data, is given by HERMANand MONTERDE GARCIA [42]. In the same line with these ones (not originating from mass transport studies in arc plasmas, but applicable there) is the work of HOWARD-ARNOLD [16], almost classical for the evaluation of diffusion coefficients. The results of WALKER and WESTENBERQ[43-461, extrapolated to arc temperatures, seem to show a good agreement, too, and possibilities for the determination of diffusion coefficients in flames, given by SNELLEMAN [47], can also be applied here, with the remark concerning necessary studies of thermal diffusion (after KEREKESand Aa [48], and discussions following Ref. 19). The conclusion of this survey could be, that much excellent work has been done in this field; nevertheless, the transport processes in the arc plasma are still insufliciently investigated. Development of new methods and techniques of direct investigations of the transport processes will contribute to better knowledge of quantitative relations in the plasmas of spectrochemical sources. 2.2 Basic de$nitions in our approach When the influence of continuous evaporation by pulsed injection is avoided and provided that there is no influence of external macroscopic streaming (easily seen to influence basic arc plasma equations of HOYAUX[49]), it is possible to adopt a simple model of mass transport in such experimental conditions. This model had to take into account influences of diffusion, electric field and convection (the statement based upon section 2.1 of this paper, especially ref. 2). By the term of usual (concentrational) diffusion we denote the motion of particles due to their number density’s gradient. This effect on the transportation of these particles outwards from the observed region can be described [50] by:
an=
z
DV%.
(1)
Here m represents particles’ number density of the observed substance, and D denotes its diffusion coefficient in arc conditions. The value of D is determined by all [39] F. BURHORN, 2. PhyaikUS,42 (1956). [40] J. N. BUTIJCRand R. S. BROKAW, J. Chem. [41] [42] [43] [44] [45] [46] [47]
Phye. 26, 1636 (1957). S. BROKAW, J. Chem. Phys. 22,1006 (1960). HERMAN and A. MONTER DE GARCIA, 2. Phyaik 205,313 (1962). E. WALKER and A. A. WESTENBERQ, J. Chem. Phya. 22, 1139 (1958). E. WALKER and A. A. WESTENBERO, J. Chem. Phys. 29, 1147 (1958). E. WALKER and A. A. WESTENBERO, J. Ghem. Phya. 21,579 (1959). E. WALKER and A. A. WESTEIDERGI, J. Chem. Phy.9.22,436 (1960). W. SNELLJCELN, The, University of Utrccht (1966). R. W. R. R. R. R.
[48] S. KEREKES and A. AQ, Proc. 14th Coil. Spectr. I&. De&recen 1967, p. 601. London (1968). [49] M. F. HOYAUX, Arc Phyeics. Springer-Verlag, Berlin (1968). [60] A. VON ENQEL, Ionized chases. aarcndon Press, Oxford (1955).
Adam Hilger,
36
V. M. VmovrB,
M. S. TODOROVI~,P. S. TODORO~I~and T. A. MIHAILIDI
the relevant interactions the particles of this substance are involved in (both as charged and neutral) under these circumstances, because of permanent charge exchange. Such treatment is inevitable, since the processes of ionization and recombination take place during time intervals definitively shorter than 1O-6s [l&j. The diffusion influences the particles’ motion velocity due to concentration gradient through the arc gap, regardless of the way in which D is determined or interaction potentials applied in its calculation, by [ 10, 291:
v, =
-igradn
The effect of ambipolar diffusion and thermal diffusion should also be taken into account in more detailed investigations of the problem [19, 48, 491. The axial electric field’s influence on the motion of the observed substance depends very much on its degree of ionization, yi, and this relation is [IjO]:
VE=
(3)
@Yi
where ,u is the mobility of ions of the observed substance, and E the axial electric field strength. As pointed out above (concerning the diffusion), the particles have to be considered as ions during ‘yt-th part of time unity, and as neutral atoms during the rest, and for the very same reason this influence is given in this form. The influence of convection in a vertical free-burning arc can be described by its velocity’s axial component, the direction of the latter being opposite to that of the gravitational field. Convection is caused by the difference of gas densities P,, and PI1 at ambient (Z’,,) and arc (T) temperatures, respectively, and the convection velocity is given 1261by: 1
vc =
J
2 po - P=gz.
(4
PT
The local gravitational acceleration is denoted here by g; z is assumed to be the axial distance from the top of the lower electrode. In our simplified model, the velocity of mass transport in axial direction is influenced by the algebraic sum of the velocities given by Equations 2, 3 and 4. The signs in the sum depend on the observed direction of motion with respect to the directions of axial electric field and convection. Some general form of this velocity can be assumed as dominant for axial mass transport and hence for the time during which the particles, leaving the arc by axial motion later on, stay in the arc. MANDEL’SHTAM [51] gave a simple relation for the residence time, T CEN -= at
--
N 7
(5)
where N represents the total number of observed substance’s particles in the arc. In the case of such an approach to the problems of transport phenomena, which is the basis of the considered model, local character of validity of Equations l-4 should be taken into account. [61] S. L. MANDEL’SIZTAM, Dokl. Akud.
Nauk SSSR,
8,669
(1938).
A new equipment for mass transport study in a free-burning d.o. arc in air
37
3. EXPERIMENTAL 3.1
Equipment
The former considerations led to the injection method described by TODOROVI~ The aqueous solutions of substances were injected into the arc in the form of “liquid bullets” of given velocity and length. The device we used for this purpose is shown in Fig. 1. Disks D, and D,, mounted on the same shaft, rotate at et al. [52].
n Arc
Fig. 1. Chopping and velocity selection system.
an angular speed w. The jet, coming from the capillary C, is chopped by the disk
The length L of the portion of the jet (i.e. “liquid bullet”) passing through the slit S, is therefore given by D,.
L0.L
ROJ
(6)
where v is the velocity of the jet, b the width of the slit, and R the radial distance of the slit to the axis of rotation. Velocity selection is done by disk D, which is provided with a slit S, (identical to S,, but shifted over an angle 8 with respect to it). The liquid bullet can pass through S, only if its transit time t, between the disks equals the delay of S, with respect to S,, i.e. d
8
t’=v=w
(7)
where d is the distance between the disks. The value of 8 is adjustable before the experiment in integer multiples of 15”. Equation 8 gives the velocity of bullets that pass through the device in the form v=-.
wa 8
[52] M. S. TODOROVI~,M. M. SIMI~, T. A. MIHAILIDI and P. S. TODOROVI~,Proc. 5th Yug. symp. Phya. Ion&. @%%, Herceg Novi 1970,p. 109.
38
V. pd. VUIUNOVI~,M. S. TODOROVI~, P. S. TODOROVIO and T. A. MIHAILIDI
Combining device
Equations
6 and 8 we obtain for the length of the bullet leaving the L =
bd
(9)
Fe
Thus, given a jet of velocity v, the angular speed must be chosen in compliance with Equation 8 in order that the fragments of the jet (“liquid bullets”), formed at D,, pass also through D,. Their length is then given by Equation 9. The whole device of Fig. 1 is placed in a box to avoid contamination of the surroundings by the spoilt solution, which is drained off from the box with a vacuum aspiration pump. A jet of velocity v is obtained with a device shown schematically in Fig. 2.
water
Solution -
Fig. 2. System for injection under constant pressure. A constant overpressure Ar, forces the solution to emerge from the bottle B, at the constant speed through the capillary C. It is provided that the direction of the jet can be changed mechanically, so that the direction of bullets’ motion through the vertical arc is always horizontal. Figure 3 shows the experimental relationship between jet velocity v and overpressure AI, in bottles B, and B,. The following values of the parameters of the system were found optimal: R = 10 cm, a=3cm, b = O-5 cm,
e=
300
Ar, = O-5 atm, v = 648 cm/a, 0 = 18 s-1 L = 0.29 cm. Accordingly, the time interval between the first touch of the bullet with the arc channel of O-6 cm in diameter and the point of leaving is 1.4 ms. Figure 4 represents photos of the solution bullet passing through arc plasma (without using an IF filter). It is obvious that the passing bullet causes only local
A new equipmentfor mass transport study in a free-burningkc. am in air
1
I
I
02
04
I
I
I
I
I
0.6
0.8
IO
12
14
39
Overpressure, otm Fig. 3. Experimental dependence of the liquid jet velocity on pressure.
perturbations of the burning arc. In Fig. 5 are represented schematically the bullet’s travel through the plasma and the development of a cloud of excited substance. Both these pictures and experimental observation of liquid bullets’ motion with, and without the arc on their way, indicate the fact that only a small fraction of the bullet evaporates, while its substantial part just passes through the arc. Though the exact mechanism leading to such partial evaporation is unknown to us, the
Fig. 5. Schematic representationof the bullet’s passage through the plrtsmaand the development of a cloud of exoited substance.
observed effect is in agreement with calculations of &ANZ [53] for the evaporation of water droplets traveling through the plasma. Circular region of light in Fig. 5, expanding while the main bullet leaves the arc, is caused by the emission of characteristic light from the cloud of the evaporated substance. The evidence for the last statement can be provided either by the use of colour film or placing the interference filter in front of the camera. In both cases the excited substance is clearly distingusihed from the background. [53] E.
FRANZ,
Chenz.Araalyticzna14,1207
(1969).
40
V. M. VUKANO~I~, M. S. TODOROVI~, P. S. TODOROVIO and T. A. MIEAILIDI
Upon considerations in connection with Fig. 4, and evaporation calculations in ref. 53, we base our belief that the risks of arc plasma perturbation are rather small with this technique. Our earlier investigations performed by introducing the substance into the plasma on a movable needle show that in this case the perturbation of plasma is considerably greater. The same reasons are valid in connection with applicability of our simple model of mass transport. Residence times were measured with the equipment shown schematically in Fig. 6. The arc is projected with a magnification l/0 on the window of the photomultiplier (PM), The output signal of the PM is followed with an oscilloscope Arc
PM
c 0 CR0
Fig.
6.
Schematic representation of the equipmentfor the measurementsof the residencetimes. PM = photomultiplier; C.R.O. = cathode ray oscilloscope.
(Tektronix 545A). Characteristic radiation is selected with Zeiss interference filters: IF589, IF675, IF625 and IF650 (for Na 5889-5892 A, Li 6707 A, orange CaO bands and red SrO bands, respectively), in front of the PM. 3.2 Measurements of the residence times The signal at the oscilloscope screen, which corresponds to the intensity of characteristic radiation emitted from the observed region by the examined substance, was recorded with tine-camera. The velocity of the film was synchronized with the sweep speed of the oscilloscope. For every recorded pulse, T was determined as the time of its decrease from maximum to l/e of the maximum value. The value T,,, represents the time, during which the pulse decreases from its maximum to the half of it. Simple blanking of parts of the window of the PM enabled observations of the arc excluding or including the electrode regions. The mean values of T and T,,, were found as arithmetic means of these quantities over all the pulses recorded under the same experimental conditions (i.e. the examined substance, its concentration in the solution, the observed region, and in some cases the upper electrode’s polarity). All the results were obtained at an arc current of 9A, and with graphite electrodes of O-6 cm in diameter. Table 1 (for Li and Na) and Table 2 (for Na, CaO and SrO) are for observations of the arc without and with the electrode regions, respectively. The
Fig. 4. An example of high-speed camera film demonstrating the passage of a bullet through t,hc plasma. Time interval bctwcm photos 0.2.5 ms.
A new equipment
for mass transport study in a free-burning d.c. arc in air
41
Table 1. Dependence of residence time and T,,, on the direction of the electric field (electrode regions not observed Test substance Concentration in aqueous solution Upper electrode Residence time r (ms) Tl12 0-1
Li 12 mol/l. + 7.3 5.1
46 3.2
N& 6 mol/l. + 3.5 2.4
2.0 I.4
Table 2. Dependence of residence time and Tin on the solution concentration and the direction of the electric field (electrode regions included in the observation) Test substance Upper electrode Residence a time 7 (ms) TI12 (ms) Residence b
time 7 (ms) Tl/2 ew
c
Residence timer (ms) T1/2 bw
a b c
(solution)
Na
CaO
SrO
+
-
5.0 3.4
5.1 3.5
4.3 3.0
4.4 3.1
4.3 3.0
5.1 3.5
4.2 3.0
3.8 26
4.0 2.8
6.4 45
4.0 2.8
3.3 2.3
+I-
+/-
6 mol/l. of Na and 15 mol/l. of both Ca and Sr; 3 mol/l. of Na and 0.75 mol/l. of both Ca and Sr; 2 mol/l. of Na and 05 mol/l. of both Ca and Sr.
results in Tables 2a, 2b and 2c are for different values of the concentration of the examined substances in the solution. In the case of injection of Na when the arc including the electrode regions was observed, another method of averaging the pulses was also applied. The time axis of each pulse was divided into 0.3 ma intervals, starting from the beginning of the pulse in one case, and from its maximum in the other. The arithmetical means of the pulse heights at these points were determined over all the pulses recorded under the same experimental conditions. The results are shown in Figs 7a and 7b for the whole pulses and their decreasing parts (starting from maxima), respectively. Since the maxima did not appear at the same moment in all the pulses, thus varying their form, there is essential difference between these two hinds of mean pulses. Because the evaporation processes are more likely to influence the increasing parts of the pulses, the decreasing parts could be more interesting for studies of particles’ motion outwards from the arc. Clearly, although essentially determined by transport processes, the time variation of the whole pulse is also affected by the injection method. Therefore the quantities t,(total pulse duration time) and r9 (integral pulse intensity), obtained from the mean pulses of Fig. 7. a and listed in Table 3, characterize not only the transport properties, but also the pulses one could expect in our method.
42
V. M. VCCKANOVI~,M. S. TODOBOVI~, P. S. TODORO~I~ and T. A. MIHAILIDI 3ms
Fig. 7a. Mean pulses obtained on the oscilloscope screen when pulses were averaged from the beginning.
3ms
Fig.
7b. Mean pulses obtained on the oscilloscope screen when averaging was started from the pulse maxima.
A new equipment for mam transport study in a free-burningd.c. arc in air
43
Table 3. Dependence of averaged pulse for Na on
concentrationof solution (t&in&ion of the pulse from 10% of XXI8Xim81heighth at the beginning till the same value at the end of pulse, S-integral of the pulse) Solution concentration (mol/l.) Pulse duration td (ms) Integral of the pulse,S
(arbitrav units)
6
3
2
14.6
13.1
12.3
126
7.7
6.4
4. DISCUSSION Table 4 represents a comparative survey of the results of different authors. Comparatively good agreement can be seen, though differences in method existed. One of the possible explanations of the discrepancy for Li could be perhaps its concentration in the solution. It has to be noticed, however, that the exponential form of the intensity dependence on time does not completely correspond to the experimental pulses we obtained. A further interpretation of T, in the sense of a measure of time variation of intensity of light emitted by the injected substance, is an open question. The comparison of the influence of the polarity of the electrodes on the residence times between observations of the arc exclusive and inclusive of electrode regions indicate extraordinary importance of the electrode effects, especially the cathode retaining. Namely, while higher values of the residence times were found for anode up than for cathode up in the case when only the gap was observed, including of electrode regions in observation of transport led even to the inverse effect of polarity. The measuring error, expressed in the form of mean square deviation, did not exceed 30x, and normally is rather less. It seems that the substance, travelling very quickly through the arc gap and causing short pulses from that region with cathode up, stays unproportionally long in the vicinity of this electrode having reached it. With anode up, the retaining in the gap is longer, due to smaller velocity of travel for Table 4. Comparison of some of our results with the literature data Li
Examined substance Results from: cathode up
Na
Residence time, (ma) 2.0 4.6
Present work anode up GOL’DBARB, IL’INA [I51 RAIKE~BAUX, MALYKH [7] MALYRH, SERD [8]
7.3 2(14-2-7) l(1.4) 1.2
3.5 3(2*2-3.9)
l-7
44
V. M. VUKANOVIO,M. S. TODOROVIC?,P. S. TODOROVIO and T. A. MIHAITJDI
Fig. 8. An example of idealized development of excitation and its dependence on the electrode polarity ,fl < t, < ta < t, < 1,
most of the substance there, but it seems as if there is no electrode retaining now. Schematic representation of these effects is given in Fig. 8. These interpretations completely correspond to the observations by high-speed camera method. Therefore we consider, that these effects essentially determine the application possibility of the cathode region for spectrochemical analysis with high power of detection.
Vol. SOB, pp. 31 to 44. Pergamon Press 1976.
Printed inNorthern Ireland
A new equipment for mass transport study in a free-burning d.c. arc in air V. M. VTJKANOVI& M. S. TODOROVI& P. S. TODOROVI~~ and T. A. MIHAILIDI Prirodno-matcmati&i hkultet, ElektrotehniEkifakultet, Tehnoloi3ko-metaluG.ki fakultet, Institut za C&u, Beograd (Received 16 April 1973. Revised 6 June 1974) Abstract-A method for injection of liquid substances into the arc plasma is proposed. This method, which is also applicable to other plasmas at atmospheric pressure, seems to be very convenient for the study of the transport properties of the injected elements. The results of time-resolved measurementsof the intensity of characteristicradiation emitted by the injected substances are presented in terms of residence times of particles in the plasma. The effect of the polarity of the arc was also studied. The cathode retains the substance in the vicinity of this electrode. 1. INTRODUCTION
THE PROCESSES of transportation of examined substance out of a d.c. arc, together
with their rates of entry, determine the particle densities in this plasma. It is well known [l, 21 that these densities together with temperature, electron density and atomic parameters, determines the intensity of light emitted by the substances. In order to increase the detection power in this source by increasing residence times of examined particles, the knowledge of processes governing the outwards transportation of substance from such plasma is required. These processes are [2] : (i) diffusion (due to concentration gradient, ambipolar and thermal diffusion); (ii) the influences of the axial and radial components of the electric field; (iii) convection of the gas. The technique to be described here was developed in order to enable the study of mass transport properties of substances involved in these processes with special respect to the residence times. It has been attempted to avoid other influences (such as those associated with continuous evaporation of substances or external gas streams) the arc used, for sake of simplicity of treatment and desired accuracy. 2. FUNDAMENTALS 2.1 Survey
OF MAIS TRANSPORT
STUDIES
of the state of investigations in literature
No survey of this kind can ever be complete. Therefore we shall only briefly quote some of the papers dealing with mass transport study, useful (in our opinion) for increasing of the power of detection in the d.c. arc. Convection is one of the processes, governing the mass transport, and rather independent of the examined substance. Such observations in vertical, free-burning d.c. arcs were done, e.g., by VAN STEKELENBTJRG [3]and HAGENAH [4],both using t At present at the Department of Electrical Engineering and Electronics, University of Liverpool, Great Britain. [l] P. W. J. M. BOUIUNS, Thory of Spectrochemical Excitation. Hilger and Watts, London (1966). [2] V. M. VUKANO~IO,Emtieionepektroskopie, p. 9. Akademie-Verlag, Berlin (1964). [3] L. H. M. VAN STEKELTCNB~~~, Phyeica 12,289 (1946). [4] W. HAUENAH,2. Phyeik 128,278 (1960). 31
32
V. M. VUEANO~I~, M. S. TODOBO&P. S. ToDo~~& and T. A. MIHAIL~DI
powder substances at the electrodes. Axial and radial distributions of the values of convection velooity were found by observing the motion of these substances (mostly carbon particles). In Ref. 4 the influence of convection at the energy balance was also thoroughly examined. BRIL [5] used the motion of the particles due to aonvection to examine the influence of the polarity of the sample electrode on the measured velocity. In order to study any of other processes of this kind, one needs to bring the substance of interest into the arc. A technique for this was proposed by RAIJUIBAUX and MAJXKH [S]. Designed especially for mass transport study, its main feature wss the needle moving transversally to the arc axis and bringing the substance into the arc. Together with proposing it, they used it for meesurement of residence times. The latter was achieved by observation of the time dependence of emitted line intensity, and these results were used for determination of the diffusion coefficients. The influence of different additives on the residence time was found [7] also with this technique. Further, UYKH and SERD applied such measurements to compare different spectrochemical sources [8]. KTJZNECOVA, R~BAUM and MALYEH [S] used them again, together with the theory of BOUMANSand DE GALAN [lo], to determine the influences of the arc current and dimensions of the gap on the detection power. This method for residence time determination in absorption and exit&ion zones was used also by TYKE, MEN’SHIKOV, MOROZOV and SHIPICIN[ 111, in their comparison of atomic absorption and emission methods. NAZAROV,GERA~IMOV and M~L’YUSexamined particular properties of atom transport through the zone of an a.~. arc [12] by the same method, applied at different values of the arc current. They also studied the effect of adding N&l and N&F. This method was also used by KAR’YAKIN, LAKTIONOV, PAVLENP;O and SKL’YARENKO [13].They explained the influence of lantanide oxide matrices on the determination of usual microimpurities by the velocities with which subs&mea leave the system, and the diffusion coefficients determined from the values of residence times. KRA~NOBAEVAand ZAD~OR~KAused the same technique for their investigations of the effect of additives at transport processes [14]. The increase of residence times in the presence of additives they ascribed both to the thermal parrtmeters and the decrease of the gradient of the eletric field in radial direction in such conditions. The influence of convection on the evaporation velocity from electrodes they found negligible. The technique of moving needle was also used by GOL’DFARBand IL’INA [ 161 [B] J. BRIL,C. R. Acad. Sci. Pa& 265B, 1277 (1967). [6] J. D. RAIK-AUMand V. D. MAIXKE,Opt.Spektroek. 9,426(1960). Kurdand V. D. M~~nrri,Opt. Spektrosk. 10,624(1901). [7] J. D. RAIKHB [s] V. D. MAIXKHand M. A. SERD, Opt. Spektrosk. 16,368(1964) [9] A. I. KTJZNE~OVA, J. D. R~mzm AUM and V. D. MALYKH, 2%. PrikZ. Spektfoek. 3,393 (1969). [lo] P. W. J. M. Boms and L. DE GALAN, Anal. Chem. 28,674 (1966). [ 1l] V. D. MALYKX& V. I. MEN’SHIKOV, V. N. MOROZOV and S. E. SHIPICIN,2%. PrikZ. Spektroek. 16,12 (1972). [12] T. V. NAZILBOV, K. 5. GERASMOV and A. V. MILYUFI Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim Nauk, 9,137 (1967). [13] A. V. KARYAKIN, N. V. LAKTIONOVA, I. V. PAWN-KO and Yu. S. SKLYARENKO, Izv. Sib., Otd. Akad. Nauk SSR, Ser. Khim. Nauk 9, 123 (1967). [ 143N. KUSNOB~EVA and Z. ZADQORSIEIL,Spectrochim. Acta 26B, 311 (197 1). [ 151 V. M. GOL’DFA~B and E. V. IL’INA. Opt. Spektroek. 11,445 (1961).
A new equipment for maw transport study in a free-burningd.c. arc in air
33
in their determinations of residence times and, values of the diffusion coefficient. They used photographic photometry of the high-speed camera photos of the arc, these being taken through interference filters for selection of characteristic light to obtain mean shift of the particles of the examined substances and maximum intensity variation with time (from frame to frame). It served, respectively, for evaluations of the diffusion coefficient and the residence time, the former being compared with calculations based upon method of HOWARD-ARNOLD [16]. The same technique was used to bring into the arc the substance, whose axial motion due to electric field was stopped by counter-directed adjustable forced convection, in experiments of GOL’DFARB,VAINBOIMand IL’INA [17]. The positions of halfmaximal intensity points, with the observed equalized velocities and the theoretical solution for the distribution of the substances in the arc gap, gave then the diffusion coefficient. In some of the works referred to so far [6-9, ll-151, the pulsed introduction of substances into the plasma was used to reduce the influence of the evaporation process on the study of transport phenomena. Another approach to the problem is the study of particle density distribution in the plasma in dynamic equilibrium, with continuous evaporation of the substance from the electrode. We would only mention the early work of GINSEL[18], and proceed to the first attempts of BOUMANS [19] and LAUWERIER[20] to apply this theory to the cathode layer arc. Following this line we would come to the theory of BOUMANS and DE GAUN [IO], studying the influence of axial transport at the line intensities in the steady and equilibrium state. As would be seen from this report, this model and the attached mathematical theory are widely used when such conditions are observed. GOL’DFARBand IL’INA [21] gave a theoretical treatment of the examined probe’s composition effect on the residence times, and thereby on the distribution of the examined substance in the arc gap, by considering mass transport properties of substances involved. Further theoretical efforts were made by KRINBER~[22] to determine the influence of the dimensions and mass of examined substances’ particles at the power of detection. This was based upon DE GUN’S experiments [23], and considerations of the influences of the electric field and diffusion at the transportation of particles out of system. An improved method for the calculation of the concentration field of microimpurity element in the interelectrode space was given by KRINBERQand SMIRNOVA [24]. They had to estimate the relative roles of different transport mechanisms,
[ 161 J. HOWARD ARNOLD, kg. Eng. Ohem. 22,lOQl (1930). [17] V. M. &L’DFABB, D. I. VAINBOIZX and E. V. IL’INA, vc’estn.Lenilagrad
247 (1963). [la] L. A. GINSEL,
Uni. Fiz.
Khim.
Thesis, University of Utrecht (1933). P. W. J. M. Boumms, Proc. 6th CoZZ. Spectr. Int., Amsterdam 1966; Spectrochim. Acta 11, 146 (1965). [20] H. A. LA~~EEIER, Appl. Soi. Rec. BA, 197 (1956). [21] V. M. GOL’DFARB and E. V. IL’INA, Uch. Zap. Leningrad. Gee. Pedagog. Inet. im. A. I. [lo]
Oertsena 339 (1966). [22] I. A. KBINBERO, Izv. Sib. Otd. Akud. Nauk SSSR, Ser. Rhim. Nauk 9, 4 (1967). [23] L. DE GALAN, J. Quant. Spectroec. Radiat. Transfer 5, 735 (1965). [24] I. A. KRINBER~ ctnd E. V. SMIRNOVA, Zh. Prikl. Spektroek. 10, 400 (1960). 3
34
V. M. VUXANO~IO,M. S. TODOBO&, P. S. TODOBOVIO and T. A. MIENIXDI
using diffusion coefficient values as could be calculated from the formulae of HIRSCHFELDER, CURTISS and BIRD [25], comparing them with experimental values from ref. 6, and the concentration KRINBERG)and S~NOVA
field with the experimental
data of ENQEL’SHT [26].
also provided the possibility to consider electrode geometry,
by different calculations of the concentration distribution for upper electrode being small or wide [27]. The volatilization rate of particles from the anode cavity into the discharge zone of d.c. arcs was studied by AVNY and GOLDBART [28]. These authors examined also the volatilization
rate, the total concentration and the axial A contribution
velocity of uranium particles using neutron activation analysis [29]. to the study
of concentration
distribution
was done by VUKANOVI~ [30], who Investigating
examined the effect of radial separation of masses in a d.c. arc plasma.
the influence of plasma composition on the distribution of substances in the arc gap, GOL’DFARB and IL’INA found an explicit relationship between the effect of “carriers” on spectral-line
intensities and the longitudinal
electric field [31].
They also found the tiuence
substances involved in the distribution studied.
transfer velocity
elements’
concentration
of
Using a high-speed camera ZOLO-
TUEHIN and ZIMINA [32] compared the experimentally the examined
of atoms in the
of other transport parameters
found axial dependence of
with the theoretical
one, the latter being
obtained by calculating the diffusion coefficient after HOWARD-ARNOLD [16] and the mobility after Einstein’s relation.
YAROSLAVSKA and ZOLOTUKHIN studied the veloc-
ity of evaporation
of substances from metal electrodes [33], calculating mobility and diffusion coefficients, and using the theory of BOUMANS [l] for Q. The influences of diffusion, convection
and electric field at this velocity were found. WIENECKE [34] determined the velocity field of high current arc by different methods. VUKANov10 [2] gave a method for calculation of the ratio of residence times with and without additives. VUKANO~I~ et al. [36, 361, as well as TODOROVI~ et al. [37] pointed out the role of the residences times on the increase of line intensities in an inhomogeneous
magnetic field.
A matter of more general interest is considered by
VUKANOVI~ [38]. [25] J. 0. HIRSCI~LDER, C. F. CUXTI~Sand R. B. BIRD, MO&~&W Theory of Gases and Liquids. John Wiley t Sons, New York (1964). [26] V. C. ENQEL’S~ md L. A. SPEKTOROV, Izv. Akad. Nauk SSSR, Ser. F&z. 26,887 (1962). [27] E. V. SMIRNOVA and I. A. KRINBERU, Zh. Prikl. Spektroek. 16, 17 (1972). [28] R. AVNI and Z. GOLDBART,Spectrochirn. Acta 2SB, 189 (1973). [29] R. AVNI, Z. GOLDBART,Spectrochim. Acta 28B, 241 (1973). [30] D. VUEANOV~~,Spectrochim Actu 22, 815 (1966). [31] V. M. GOL’DFARBand E. V. IL’INA, Zh. Prikl. Spektrosk. 5,381 (1966). and I. L. Z~NA, Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk 9, [32] G. E. ZOLO~ 106 (1967). [33] R. M. YAROSLAVSKAYAand G. E. ZOLOTUXJXIN, Zh. Prikl. Spektroek. 16, 592 (1969). [34] R. WIENEXKE,2. Phyeik 148,6. 128 (1955). [36] V. VWANOVIO, V. GEORQIJEVIO, N. KONJEVIOand D. VUKANOVI~, P~oG. 12th Coil. Spectr. Int. Ezeter 1965,p. 193.Adam Hilger, London (1966). [36] V. VWANOVIO, V. GEORQIJEVIO, D. VUXANOVIOand M. TODOROVIO, Spectrochim. Acta 24,B, 666 (1969). [37] TODOROVIO,V. M. VDXANOVI~, V. M. GEORQIJEVI~,T. A. MIHAILIDI and P. S. TODOROVIO,Proc. 9th Int. Conf. Phen. Ionized Gases, Bucharest 1969,p. 321. [38] V. M. VUKANOVI~,1Mag.Rem. Foly. 7l, 2 (1966).
A new equipment
for m8ss transport study in 8 free-burning d.c. 81% in air
35
The influence of diffusion at the heat conductivity, considered by BrmnORN[39], BROKAWand BUTLER [40] and BROKAW [41], indicates the importance of mass transport studies also for energy transport. The connection between the two, in the form of calculations of diffusion coefficients from the radial temperature distribution and heat conductivity data, is given by HERMANand MONTERDE GARCIA [42]. In the same line with these ones (not originating from mass transport studies in arc plasmas, but applicable there) is the work of HOWARD-ARNOLD [16], almost classical for the evaluation of diffusion coefficients. The results of WALKER and WESTENBERQ[43-461, extrapolated to arc temperatures, seem to show a good agreement, too, and possibilities for the determination of diffusion coefficients in flames, given by SNELLEMAN [47], can also be applied here, with the remark concerning necessary studies of thermal diffusion (after KEREKESand Aa [48], and discussions following Ref. 19). The conclusion of this survey could be, that much excellent work has been done in this field; nevertheless, the transport processes in the arc plasma are still insufliciently investigated. Development of new methods and techniques of direct investigations of the transport processes will contribute to better knowledge of quantitative relations in the plasmas of spectrochemical sources. 2.2 Basic de$nitions in our approach When the influence of continuous evaporation by pulsed injection is avoided and provided that there is no influence of external macroscopic streaming (easily seen to influence basic arc plasma equations of HOYAUX[49]), it is possible to adopt a simple model of mass transport in such experimental conditions. This model had to take into account influences of diffusion, electric field and convection (the statement based upon section 2.1 of this paper, especially ref. 2). By the term of usual (concentrational) diffusion we denote the motion of particles due to their number density’s gradient. This effect on the transportation of these particles outwards from the observed region can be described [50] by:
an=
z
DV%.
(1)
Here m represents particles’ number density of the observed substance, and D denotes its diffusion coefficient in arc conditions. The value of D is determined by all [39] F. BURHORN, 2. PhyaikUS,42 (1956). [40] J. N. BUTIJCRand R. S. BROKAW, J. Chem. [41] [42] [43] [44] [45] [46] [47]
Phye. 26, 1636 (1957). S. BROKAW, J. Chem. Phys. 22,1006 (1960). HERMAN and A. MONTER DE GARCIA, 2. Phyaik 205,313 (1962). E. WALKER and A. A. WESTENBERQ, J. Chem. Phya. 22, 1139 (1958). E. WALKER and A. A. WESTENBERO, J. Chem. Phys. 29, 1147 (1958). E. WALKER and A. A. WESTENBERO, J. Ghem. Phya. 21,579 (1959). E. WALKER and A. A. WESTEIDERGI, J. Chem. Phy.9.22,436 (1960). W. SNELLJCELN, The, University of Utrccht (1966). R. W. R. R. R. R.
[48] S. KEREKES and A. AQ, Proc. 14th Coil. Spectr. I&. De&recen 1967, p. 601. London (1968). [49] M. F. HOYAUX, Arc Phyeics. Springer-Verlag, Berlin (1968). [60] A. VON ENQEL, Ionized chases. aarcndon Press, Oxford (1955).
Adam Hilger,
36
V. M. VmovrB,
M. S. TODOROVI~,P. S. TODORO~I~and T. A. MIHAILIDI
the relevant interactions the particles of this substance are involved in (both as charged and neutral) under these circumstances, because of permanent charge exchange. Such treatment is inevitable, since the processes of ionization and recombination take place during time intervals definitively shorter than 1O-6s [l&j. The diffusion influences the particles’ motion velocity due to concentration gradient through the arc gap, regardless of the way in which D is determined or interaction potentials applied in its calculation, by [ 10, 291:
v, =
-igradn
The effect of ambipolar diffusion and thermal diffusion should also be taken into account in more detailed investigations of the problem [19, 48, 491. The axial electric field’s influence on the motion of the observed substance depends very much on its degree of ionization, yi, and this relation is [IjO]:
VE=
(3)
@Yi
where ,u is the mobility of ions of the observed substance, and E the axial electric field strength. As pointed out above (concerning the diffusion), the particles have to be considered as ions during ‘yt-th part of time unity, and as neutral atoms during the rest, and for the very same reason this influence is given in this form. The influence of convection in a vertical free-burning arc can be described by its velocity’s axial component, the direction of the latter being opposite to that of the gravitational field. Convection is caused by the difference of gas densities P,, and PI1 at ambient (Z’,,) and arc (T) temperatures, respectively, and the convection velocity is given 1261by: 1
vc =
J
2 po - P=gz.
(4
PT
The local gravitational acceleration is denoted here by g; z is assumed to be the axial distance from the top of the lower electrode. In our simplified model, the velocity of mass transport in axial direction is influenced by the algebraic sum of the velocities given by Equations 2, 3 and 4. The signs in the sum depend on the observed direction of motion with respect to the directions of axial electric field and convection. Some general form of this velocity can be assumed as dominant for axial mass transport and hence for the time during which the particles, leaving the arc by axial motion later on, stay in the arc. MANDEL’SHTAM [51] gave a simple relation for the residence time, T CEN -= at
--
N 7
(5)
where N represents the total number of observed substance’s particles in the arc. In the case of such an approach to the problems of transport phenomena, which is the basis of the considered model, local character of validity of Equations l-4 should be taken into account. [61] S. L. MANDEL’SIZTAM, Dokl. Akud.
Nauk SSSR,
8,669
(1938).
A new equipment for mass transport study in a free-burning d.o. arc in air
37
3. EXPERIMENTAL 3.1
Equipment
The former considerations led to the injection method described by TODOROVI~ The aqueous solutions of substances were injected into the arc in the form of “liquid bullets” of given velocity and length. The device we used for this purpose is shown in Fig. 1. Disks D, and D,, mounted on the same shaft, rotate at et al. [52].
n Arc
Fig. 1. Chopping and velocity selection system.
an angular speed w. The jet, coming from the capillary C, is chopped by the disk
The length L of the portion of the jet (i.e. “liquid bullet”) passing through the slit S, is therefore given by D,.
L0.L
ROJ
(6)
where v is the velocity of the jet, b the width of the slit, and R the radial distance of the slit to the axis of rotation. Velocity selection is done by disk D, which is provided with a slit S, (identical to S,, but shifted over an angle 8 with respect to it). The liquid bullet can pass through S, only if its transit time t, between the disks equals the delay of S, with respect to S,, i.e. d
8
t’=v=w
(7)
where d is the distance between the disks. The value of 8 is adjustable before the experiment in integer multiples of 15”. Equation 8 gives the velocity of bullets that pass through the device in the form v=-.
wa 8
[52] M. S. TODOROVI~,M. M. SIMI~, T. A. MIHAILIDI and P. S. TODOROVI~,Proc. 5th Yug. symp. Phya. Ion&. @%%, Herceg Novi 1970,p. 109.
38
V. pd. VUIUNOVI~,M. S. TODOROVI~, P. S. TODOROVIO and T. A. MIHAILIDI
Combining device
Equations
6 and 8 we obtain for the length of the bullet leaving the L =
bd
(9)
Fe
Thus, given a jet of velocity v, the angular speed must be chosen in compliance with Equation 8 in order that the fragments of the jet (“liquid bullets”), formed at D,, pass also through D,. Their length is then given by Equation 9. The whole device of Fig. 1 is placed in a box to avoid contamination of the surroundings by the spoilt solution, which is drained off from the box with a vacuum aspiration pump. A jet of velocity v is obtained with a device shown schematically in Fig. 2.
water
Solution -
Fig. 2. System for injection under constant pressure. A constant overpressure Ar, forces the solution to emerge from the bottle B, at the constant speed through the capillary C. It is provided that the direction of the jet can be changed mechanically, so that the direction of bullets’ motion through the vertical arc is always horizontal. Figure 3 shows the experimental relationship between jet velocity v and overpressure AI, in bottles B, and B,. The following values of the parameters of the system were found optimal: R = 10 cm, a=3cm, b = O-5 cm,
e=
300
Ar, = O-5 atm, v = 648 cm/a, 0 = 18 s-1 L = 0.29 cm. Accordingly, the time interval between the first touch of the bullet with the arc channel of O-6 cm in diameter and the point of leaving is 1.4 ms. Figure 4 represents photos of the solution bullet passing through arc plasma (without using an IF filter). It is obvious that the passing bullet causes only local
A new equipmentfor mass transport study in a free-burningkc. am in air
1
I
I
02
04
I
I
I
I
I
0.6
0.8
IO
12
14
39
Overpressure, otm Fig. 3. Experimental dependence of the liquid jet velocity on pressure.
perturbations of the burning arc. In Fig. 5 are represented schematically the bullet’s travel through the plasma and the development of a cloud of excited substance. Both these pictures and experimental observation of liquid bullets’ motion with, and without the arc on their way, indicate the fact that only a small fraction of the bullet evaporates, while its substantial part just passes through the arc. Though the exact mechanism leading to such partial evaporation is unknown to us, the
Fig. 5. Schematic representationof the bullet’s passage through the plrtsmaand the development of a cloud of exoited substance.
observed effect is in agreement with calculations of &ANZ [53] for the evaporation of water droplets traveling through the plasma. Circular region of light in Fig. 5, expanding while the main bullet leaves the arc, is caused by the emission of characteristic light from the cloud of the evaporated substance. The evidence for the last statement can be provided either by the use of colour film or placing the interference filter in front of the camera. In both cases the excited substance is clearly distingusihed from the background. [53] E.
FRANZ,
Chenz.Araalyticzna14,1207
(1969).
40
V. M. VUKANO~I~, M. S. TODOROVI~, P. S. TODOROVIO and T. A. MIEAILIDI
Upon considerations in connection with Fig. 4, and evaporation calculations in ref. 53, we base our belief that the risks of arc plasma perturbation are rather small with this technique. Our earlier investigations performed by introducing the substance into the plasma on a movable needle show that in this case the perturbation of plasma is considerably greater. The same reasons are valid in connection with applicability of our simple model of mass transport. Residence times were measured with the equipment shown schematically in Fig. 6. The arc is projected with a magnification l/0 on the window of the photomultiplier (PM), The output signal of the PM is followed with an oscilloscope Arc
PM
c 0 CR0
Fig.
6.
Schematic representation of the equipmentfor the measurementsof the residencetimes. PM = photomultiplier; C.R.O. = cathode ray oscilloscope.
(Tektronix 545A). Characteristic radiation is selected with Zeiss interference filters: IF589, IF675, IF625 and IF650 (for Na 5889-5892 A, Li 6707 A, orange CaO bands and red SrO bands, respectively), in front of the PM. 3.2 Measurements of the residence times The signal at the oscilloscope screen, which corresponds to the intensity of characteristic radiation emitted from the observed region by the examined substance, was recorded with tine-camera. The velocity of the film was synchronized with the sweep speed of the oscilloscope. For every recorded pulse, T was determined as the time of its decrease from maximum to l/e of the maximum value. The value T,,, represents the time, during which the pulse decreases from its maximum to the half of it. Simple blanking of parts of the window of the PM enabled observations of the arc excluding or including the electrode regions. The mean values of T and T,,, were found as arithmetic means of these quantities over all the pulses recorded under the same experimental conditions (i.e. the examined substance, its concentration in the solution, the observed region, and in some cases the upper electrode’s polarity). All the results were obtained at an arc current of 9A, and with graphite electrodes of O-6 cm in diameter. Table 1 (for Li and Na) and Table 2 (for Na, CaO and SrO) are for observations of the arc without and with the electrode regions, respectively. The
Fig. 4. An example of high-speed camera film demonstrating the passage of a bullet through t,hc plasma. Time interval bctwcm photos 0.2.5 ms.
A new equipment
for mass transport study in a free-burning d.c. arc in air
41
Table 1. Dependence of residence time and T,,, on the direction of the electric field (electrode regions not observed Test substance Concentration in aqueous solution Upper electrode Residence time r (ms) Tl12 0-1
Li 12 mol/l. + 7.3 5.1
46 3.2
N& 6 mol/l. + 3.5 2.4
2.0 I.4
Table 2. Dependence of residence time and Tin on the solution concentration and the direction of the electric field (electrode regions included in the observation) Test substance Upper electrode Residence a time 7 (ms) TI12 (ms) Residence b
time 7 (ms) Tl/2 ew
c
Residence timer (ms) T1/2 bw
a b c
(solution)
Na
CaO
SrO
+
-
5.0 3.4
5.1 3.5
4.3 3.0
4.4 3.1
4.3 3.0
5.1 3.5
4.2 3.0
3.8 26
4.0 2.8
6.4 45
4.0 2.8
3.3 2.3
+I-
+/-
6 mol/l. of Na and 15 mol/l. of both Ca and Sr; 3 mol/l. of Na and 0.75 mol/l. of both Ca and Sr; 2 mol/l. of Na and 05 mol/l. of both Ca and Sr.
results in Tables 2a, 2b and 2c are for different values of the concentration of the examined substances in the solution. In the case of injection of Na when the arc including the electrode regions was observed, another method of averaging the pulses was also applied. The time axis of each pulse was divided into 0.3 ma intervals, starting from the beginning of the pulse in one case, and from its maximum in the other. The arithmetical means of the pulse heights at these points were determined over all the pulses recorded under the same experimental conditions. The results are shown in Figs 7a and 7b for the whole pulses and their decreasing parts (starting from maxima), respectively. Since the maxima did not appear at the same moment in all the pulses, thus varying their form, there is essential difference between these two hinds of mean pulses. Because the evaporation processes are more likely to influence the increasing parts of the pulses, the decreasing parts could be more interesting for studies of particles’ motion outwards from the arc. Clearly, although essentially determined by transport processes, the time variation of the whole pulse is also affected by the injection method. Therefore the quantities t,(total pulse duration time) and r9 (integral pulse intensity), obtained from the mean pulses of Fig. 7. a and listed in Table 3, characterize not only the transport properties, but also the pulses one could expect in our method.
42
V. M. VCCKANOVI~,M. S. TODOBOVI~, P. S. TODORO~I~ and T. A. MIHAILIDI 3ms
Fig. 7a. Mean pulses obtained on the oscilloscope screen when pulses were averaged from the beginning.
3ms
Fig.
7b. Mean pulses obtained on the oscilloscope screen when averaging was started from the pulse maxima.
A new equipment for mam transport study in a free-burningd.c. arc in air
43
Table 3. Dependence of averaged pulse for Na on
concentrationof solution (t&in&ion of the pulse from 10% of XXI8Xim81heighth at the beginning till the same value at the end of pulse, S-integral of the pulse) Solution concentration (mol/l.) Pulse duration td (ms) Integral of the pulse,S
(arbitrav units)
6
3
2
14.6
13.1
12.3
126
7.7
6.4
4. DISCUSSION Table 4 represents a comparative survey of the results of different authors. Comparatively good agreement can be seen, though differences in method existed. One of the possible explanations of the discrepancy for Li could be perhaps its concentration in the solution. It has to be noticed, however, that the exponential form of the intensity dependence on time does not completely correspond to the experimental pulses we obtained. A further interpretation of T, in the sense of a measure of time variation of intensity of light emitted by the injected substance, is an open question. The comparison of the influence of the polarity of the electrodes on the residence times between observations of the arc exclusive and inclusive of electrode regions indicate extraordinary importance of the electrode effects, especially the cathode retaining. Namely, while higher values of the residence times were found for anode up than for cathode up in the case when only the gap was observed, including of electrode regions in observation of transport led even to the inverse effect of polarity. The measuring error, expressed in the form of mean square deviation, did not exceed 30x, and normally is rather less. It seems that the substance, travelling very quickly through the arc gap and causing short pulses from that region with cathode up, stays unproportionally long in the vicinity of this electrode having reached it. With anode up, the retaining in the gap is longer, due to smaller velocity of travel for Table 4. Comparison of some of our results with the literature data Li
Examined substance Results from: cathode up
Na
Residence time, (ma) 2.0 4.6
Present work anode up GOL’DBARB, IL’INA [I51 RAIKE~BAUX, MALYKH [7] MALYRH, SERD [8]
7.3 2(14-2-7) l(1.4) 1.2
3.5 3(2*2-3.9)
l-7
44
V. M. VUKANOVIO,M. S. TODOROVIC?,P. S. TODOROVIO and T. A. MIHAITJDI
Fig. 8. An example of idealized development of excitation and its dependence on the electrode polarity ,fl < t, < ta < t, < 1,
most of the substance there, but it seems as if there is no electrode retaining now. Schematic representation of these effects is given in Fig. 8. These interpretations completely correspond to the observations by high-speed camera method. Therefore we consider, that these effects essentially determine the application possibility of the cathode region for spectrochemical analysis with high power of detection.