Ionisation associated with solid particles in flames—III. Probe studies

C O M B U S T I O N A N D F L A M E 26, 395-402 (1976)

395

lonisation Associated with Solid Particles in Flame IIl. Probe Studies E. R. MILLER, R.N. NEWMAN and F.M. PAGE Department of Chemistry, The University ~)f Aston in Birmingham, Gosta Green, Birmingham B4 7ET, England

Solid materials when added to flames in the form of fine particles produce ionisation, but the positive ion concentration, detected by an electrostatic probe, is random and appears to be associated with an interaction between the probe and an individual particle. The magnitude of the peaks currents observed and the results of auxiliary experiments show that the only tenable theory if the materials are pure is that a hot particle temporarily converts the cold probe to an emitting probe during the interaction, which is terminated either by the cooling or shedding of the particle. It is, however, not possible to exclude the ionisation of gaseous species arising from adherent volatile material present at less than I ppm level which could produce broadly similar effects.

Introduction Solid materials when added to flames as dust p r o d u c e m e a s u r a b l e ionisation, but the Langmuir-Williams Ill rotating probe shows that this ionisation is associated with the solid particle as such. Two theories which explain this are examined with inconclusive results.

Experimental The previous paper [2] in this series examined the electron number densities produced when solid particles were produced in flames from aqueous sprays, or vapours. It was found that the ionised particles were small and numerous (radius a = 10-Sm, number density, N = 10~ m -3) and often distinguished only with difficulty from background gaseous ionisation. The associated electrons were 10 to 100 times as numerous, and as it was believed that the

number of charges per particle could be a limiting factor in the ionisation, which could be relieved if the particles were larger, steps were taken to add fine powders to the flame. In addition, the number density of positive charges, rather than electrons, was measured in the hope that complementary information could be obtained, and because the height resolution of the method used was very much better than that of the resonant cavity. The results obtained were suprising but may be interpreted in terms of a particle-probe interaction, although the possiblilty that the interaction occurs through the mediary of a cloud of gaseous impurity cannot be ruled out.

The Addition of Dust Particles

The problem of introducing solid particles to a flame is central to all these studies. There is difficulty in achieving particle number densi-

Copyright O 1976 by The Combustion Institute Published by American Elsevier Publishing Company, Inc.

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E.R. MILLER, R. N. NEWMAN, AND F. M. PAGE

ties comparable to gaseous number densities without overloading the flame. A particle of l0 nm diameter contains 15000 sub units, 0.2 nm in diameter, and one of 1/xm will contain l0 s times as many. The accessible range of particle number densities is therefore limited, and the effect of even traces 0 0 -6) of gaseous impurities may mask the results sought. In the previous paper, Woolley [2] formed 20 nm particles by the pyrolysis of nickel carbonyl, following Egerton and Rudrakanchena [3]. Kelly and Padley [4] by a natural extension of the work of Belcher and Sugden [5] or Andrade [6] produced particles in the range 20 nm to 100 nm from aqueous salt sprays. These methods are of limited scope and produce only small particles, so it was decided to attempt to add solids directly to the flame. Gilbert [7] has shown that powder suspended in a liquid of moderate viscosity can be sprayed into a flame to produce a satisfactorily steady delivery, but since a different liquid mixture was required for each material, and since the evaporation of the last traces of liquid can cause premature disintegration of the particle [4 ], it was thought that the addition as a dry dust suspension would be preferable if adequate control could be achieved. Variations in particle number and particle size may both be expected. The former may be eliminated by careful control of the pick-up of dust and the latter either by use of a previously closely graded sample, or by passing the dustladen gas stream up a settling column or elutriator in the same way that the spray droplets of a solution are allowed to settle before entering the flame. After extensive trials, the most convenient and reproducible method of adding powders was found to be as follows: A portion of the nitrogen supply to the burner entered the dust cloud generator (Fig. l.) below a sintered glass disc. A few grams of the dust lay on top of the disc, and the whole was kept in oscillation by pressing against an eccentrically rotating rubber pad. Above the sintered glass disc, and connected to it by a B34 cone and socket joint was a length of 80 m m tubing. The coarser particles of dust settled out in this tube. In early experiments,

Fig. 1. Dust cloud generation.

an elutriator was constructed from a graded series of such tubes, but problems of turbulence generated at the entry point could not be overcome, and the multitube set did not produce a significantly sharper particle size distribution than the single settling tube. The dust-laden gas was mixed with the remainder of the flame gases and passed into the burner through a narrow tube, in which the gas velocity was too high to allow any dust to settle out. The burner itself was of conventional design as described by Padley & Sugden [8] except that the capillary tubes (0.032 in. i.d.) were soldered into individual holes in the brass partitions, spaced in a hexagonal pattern at 0.068 in. centres. The total area of the flame was then

IONISATION WITH SOLID PARTICLES IN FLAMES--Ill just large enough to allow for expansion of the gases on combustion at 2100 K, and the resulting flame was parallel sided. The burner layout is shown in Fig. 2.

397

cemented into a boron nitride insulator which could be spring loaded so as to be in the flame for a predetermined time before being hit and knocked aside by the electrostatic probe. The probe used in the majority of these studies was 1.75 mm in diameter and was formed on a length of 33 s.w.g platinum wire which was supported by a boron nitride insulator. The probe was held 100 volt negative with respect to the grounded burner top. Care was taken to screen all leads to minimise interference, and to clean the probe frequently. The probe was rotated at 60 rpm for the powder studies, but for other experiments was replaced by a 3 mm diameter probe on a 1 mm wire which at 20 rpm could withstand the shock of hitting a free-falling ~/s in. diameter ball bearing or at 5 rpm, of pushing aside a spring loaded insulated cylinder. The probe current was displayed on an oscilloscope triggered by a reed switch operated by a magnet attached to the probe. The Response of the Probe

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Fig. 2. The burner. Certain experiments were carried out in which Y8 in. diameter ball bearings were dropped axially through an inverted flame. The burner used in these experiments was identical with that of Fig. 2 in all respects save the single axial tube which extended from the burner face through the base (now the top) which guided the ball bearings into the flame. The flames studied were all rather hot and fast-burning, and the unusual operating attitude had little effect on the flame shape. In certain other experiments, a short length of 1 mm rod, either of tungsten or iridium was

Oscillograms showing the probe response to gaseous ions (CsC l sprayed into the flame) and to solid particles (aluminium powder, 53 micron to dust) are presented in Fig. 3. The " s p i k y " appearance in the presence of solid particles is clear, and similar oscillograms were obtained with a variety of particles: aluminum, alumina, c a r b o n , v a n a d i u m p e n t o x i d e , t u n g s t e n carbide, iron and l a n t h a n u m hexaboride--all nominally l-l0/~ in diameter, though no spikes were seen when solutions of uranium, chromium or vanadium salts which are known to give rise to particles of smaller size were sprayed into the flame. The appearance of spikes was always associated with the presence of solid particles but could be obscured by low sensitivity or high particle number density. In order to establish a quantitative scale, the probe was calibrated by determining the relative response to different concentrations of caesium in a solution sprayed into a hot flame. The resulting measurements were fitted to a theoretical curve to produce a simultaneous

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E.R. MILLER, R. N. NEWMAN, AND F. M. PAGE the probe potential may emit electrons by thermionic emission. (4) The particles may be electrically inert, but associated chemically with a region of ionised gas.

(a)

In order to assess and eliminate these processes it will be assumed that the particle diameter is 20 × 10-6 m and at a temperature of 2300 K. The thermionic emission of electrons by particles has been treated to varying degrees of sophistication by Einbinder [9], F. T. Smith [10] or Sop and Dimick [11] and this work is discussed in the earlier paper [12]. For more than 10 electrons per particle the approximations used by Sugden and Thrush [12] are valid, and their equation will be used:

Ne=

(b) Fig. 3. Probe response: (a) to gaseousions, (b) to dust particles.

calibration of the sprayer and of the probe. It was observed that the aluminium powder produced spikes of an area equivalent to 5 x 10-12 coulombs or 3 × 107 charges. If each spike does correspond to the interaction of the probe with a single particle, four processes may be considered: (1) The particle, which is at the plasma potential, may transfer charge to the highly negative probe. (2) The particle may lose electrons by the thermionic emission and be raised above the plasma potential so that additional charge must be transferred to bring it to probe potential. (3) The particle in contact with the probe and at

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where np is the number density of particles of mean radius r. The delivery of weighed amounts of powder suggested that np was about 50 cm -3 so that, taking (b for aluminium to be 3.57 eV, Ne is l.l × 106 and the number of charges per particle 2.2 × l04. This is far below the measured value, and no slight adjustment will raise it, so that no mechanism based on simple thermal ionisation of the particle can account for the observed spikes. At the temperature of experiment, the particle is likely to be liquid, and the maximum charge that it can carry without disintegration may be estimated from Rayleigh's theory of instability [13] to be 4.7 x 10-~ e.s.u or 9 x 106 charges which again underwrites the improbability of the observed spike being due to a single charged particle hitting the probe. The second and third possible explanations are closely related in the sense that current flows from the probe as a result of collision with a particle. In the second possibility this current flows simply because probe and particle are at differing potentials and the quantity flowing, Q = CV, may be calculated from the capacity of the particle as a sphere from the probe potential to be 7 x 10~ charges per particle. The third possibility, in which the hot par-

I O N I S A T I O N WITH SOLID P A R T I C L E S IN F L A M E S - - I l l

ticle can emit thermionically while in contact with the charged probe is more difficult to assess. The maximum current which could be drawn is many times that observed, but the duration of this current, limited either by the cessation of probe-particle contact or by the cooling of the particle, is not easy to calculate. Some generalisation can be made since both mechanisms would be expected to depend on particle radius, the former linearly and the latter as the square. The capacitative mechanism would not be expected to depend on the thermionic functions of the material nor upon the temperature, while the emissive mechanism should depend upon both. The powders added to the flame were coarsely sized and aggregated so that distinction b e t w e e n the mechanisms was not easy, and attention was given to collisions which would certainly be single particles. To this end, stainless steel ball bearings were dropped through the centre tube of an inverted burner. It was found that a fastburning flame with temperature 2507 K was stable in this configuration. In this flame, occasional interactions were observed, but the need for millisecond precision in dropping the ball bearings made observation difficult. Nevertheless two observers were able to distinguish that spikes occurred when the probe narrowly missed the balls. The ball bearings were therefore subjected to a rigorous cleaning and degreasing process to remove any traces of contamination which might give rise to an ionisation blanket. After this, no spikes were observed under any conditions, although ball-probe contact was achieved, and it was concluded that no significant capacitative charge transfer was occuring. No c o n t r i b u t i o n from an emissive mechanism could be expected, as the temperature of the massive ball bearings was unaffected by their short stay in the flame. Therefore a variant of this experiment was tried, in which a short cylindrical wire mounted in a boron nitride insulator was struck by the probe. The insulated wire was spring loaded to avoid damage and could be introduced into the flame shortly before the collision, so that it was effectively cold, or sufficiently in advance of the probe for it to have reached a high tempera-

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ture before collision. There was generally a temperature drop of 400 K along the wire, and the hotter end was found, by pyrometry, to be about 700-800 K below the temperature of the flame, but there was a correlation between the wire temperature and the flame temperature and the use of the latter to plot Fig. 4 is meaningful. In this diagram it is seen that the response of the cold probe alone in passing through the flame is barely distinguished from the cold probe striking the cold wire, but that when the wire had got hot an increased current was observed, and that this current became exponentially larger as the flame temperature increases. Similar results were observed with an iridium wire, but the exponential increase was time dependent probably because of the alteration in the work function of the surface by adsorption [14], and for the same reason the

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E.R. MILLER, R. N. NEWMAN, AND F. M. PAGE

divergence between hot and cold responses became more marked at higher flame temperatures. Having regard to the limitation on the temperature of the wire, it is evident that the emissive charge transfer process is adequate to explain the observed spikes but that it is not possible to exclude an explanation based on contamination.

Quantitative Studies of the Spikes Having been successful in excluding two of the possible mechanisms for spike production it was decided to make a close study of the spikes themselves. Typical oscillograms are shown in Fig. (5) superimposed on a 1 kHz square wave time marker. The spikes show evidence of skewness due to wave shaping, but since the oscilloscope rise time was only 23 ns, the pulse shaping of the trailing edge of the spikes, having a time constant of the order of a millisecond, must arise from another cause, for example from a coaxial lead capacity of the order of 100 pf and an interelectrode flame resistance of the order of 35 MfL The leading edge of the spikes had a rise time of only 200 /zsec and a slope which was lineally dependent on the peak height. Such behaviour is found when a transient event of short duration is observed by a circuit of long time constant. Under these circumstances the peak height is a reasonably constant fraction of the hypothetical final value provided that the duration is equally constant. The random nature of the spikes, due to the distribution of sizes, directness of collision and other variability led to an arbitrary method of measurement. A series of photographs was taken, each oscilloscope trace showing a number of spikes, and the series was projected at constant magnification on to a grid, and the peak height of the largest 10% were taken as representing the interaction. Some judgment was exercised, and occasional" rogue" points were omitted. The first experiments were carried out with low work function materials, lanthanum hexaboride (~b = 2.66 ev; m.p 2500 K; dia 3.4 /xm) and barium oxide (~b = 1.66 ev; m.p 2198 K; dia 2.6/zm) and the spike height related to the spike width at half height. The spike height

Fig. 5. Expanded oscillograrn of a single spike.

is a measure of the final value only if the duration of the event is constant and small compared to the shaping time. Two series of measurements were made, one with the normal (cold) probe, and one where the probe was preheated by passing through 10 cm of a (horizontal) secondary flame before entering the powder laden flame. It was expected that preheating the probe would slow the cooling of the particle and extend the duration of the pulse, but as Fig. (6) shows, there is no distinction between hot and cold probes. In addition, when no powder was added, the oscilloscope base line was steady, whether with hot or cold probe. But in powder laden flames, the base line for the hot probe drifted, though not at every pass through the flame. It was concluded that occasionally a particle struck the probe and adhered to it, thus emitting when the probe was hot, but that the majority of collisions beLanthanum hcxaborid¢ o Cold "probe • Hot probe

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IONISATION WITH SOLID PARTICLES IN FLAMES--III tween particles and the probe were glancing, short duration, contacts during which no significant cooling occurred. Exactly similar observations were made with barium oxide, except that the spike heights observed were considerably greater then with l a n t h a n u m hexaboride.

The Temperature Dependence of the Spike Height The mean spike height was very low just above the burner, but increased to reach a constant value at 3 cm height in the flame which was unchanged up to 7 cm from the burner. This profile is similar to the measured temperature profile of the undoped flame and the temperature dependance of the spike height was therefore examined, working in the plateau at 4 cm above the burner surface. The height showed an exponential dependance on temperature, and the results for the three materials, barium oxide, lanthanum hexaboride and tungsten carbide, which had similar particle sizes are shown in Fig. (7). The refractories LaB~ and WC may be represented by lines with slopes corresponding to 2.20 ev (literature [15] values

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× Borium oxide

401

2.07-2.66) and 2.50 ev (literature value 3.6-3.9) respectively. The LaB6 was synthesised from pure materials [16] but the WC was contaminated with sodium, whose adsorption on the surface may profoundly lower the work function. Barium oxide was best represented by two lines, the low temperature slope corresponding to 0.6 eV and the slope above 2280 K corresponding to 1.94 eV. It is tempting to associate the break in the curve with the melting point of BaO (2200 K) but though little is known about work function changes of liquids, it is not thought likely that the change on melting would exceed 1 eV. The work function of barium oxide is, h o w e v e r , susceptible to nonstoichiometry, and traces of free metallic barium in the oxide lattice can lower the work function of the surface markedly, so that the figure of 0.6 eV is not unreasonable. On melting, the free metal atoms may diffuse to the surface and become oxidised much more readily and the work function would then be expected to be close to that of the stoichiometric oxide (1.66 eV) as is found. Aluminium powder was also examined, and was found to give very small spikes, of nearly constant amplitude over the range 2250-2500 K but not outside these limits. The low amplitude and high temperature required is consistent with the higher work function of the metal, but since self-sustaining combustion occurs above 2300 K in the vapour phase, no conclusion can be drawn.

Conclusion I'01

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Fig. 7. Temperaturedependenceof spike height (Barium oxide, Lanthanium hexaboride, Tungsten carbide).

The transient probe response ("spike") observed when a Langmuir-Williams rotating probe is used to study flames containing micron sized powders is associated with an interaction between the probe and the particle of the powder. There is evidence that for materials of low thermionic work function the hot particle collides with the cold probe and converts it into an emitting probe. This collision is usually a transient event, and the particle leaves the probe while still hot, after an interaction lasting about 200/zsec but occasionally a particle may adhere p e r m a n e n t l y . The

402

E. R. M I L L E R , R. N. N E W M A N , A N D F. M. P A G E

maximum current (4 x 10-7 amp for LaB6 at 2487 K) was less than that expected (2.4 × 10-3 amp for a 15 /x particle with work function 2.66 eV) because of this brief interaction, because of pulse shaping by the detection circuit and because of the uncertainty in the temperature. Nevertheless the maximum currents observed showed a dependence on the flame temperature which was consistent with the known surface properties of the material and with the chemistry of their interaction with the flames. It was not possible to predict the currents exactly because of the chemical interactions and caution must be observed in using thermionic data derived from readily defined experimental conditions to assess behaviour in flames. It has not been possible to exclude the possibility that surface contamination by low ionisation potential impurities could affect the observations. Indeed it has been shown [17] that pure sodium chloride can produce the phenomena described here, through slow evaporation and the diffusion of the vapour away from the solid. Some of the powders used showed traces of alkali metal impurities and the low work function of the tungsten carbide powder may well have been due to adsorption of sodium. Nevertheless, the general description of the phenomenon as due to the transient interaction

of particle and probe producing an emitting probe seems to be correct for many of the powders studied. References 1. Soundy, R. G. and Williams, H., AGARD, P & E Panel 26th meeting. Pisa, 1965, p. 165. 2. Page, F. M. and Woolley, D. E., Combust. Flame, in press. 3. Egerton, A. C. and Rudrakanchana, V., Proc. Roy. Soc. A225 427 (1954). 4. Kelly, R. and Padley, P. J., Nature 216, (1967). 5. Belcher, H. and Sugden,T,M., Proc. Roy. Soc. A202, 17 (1950). 6. Andrade, E. N. da C., Phil. Mag. 24, 15 (1912). 7. Gilbert, P. T., Anal, Chem. 34, 1025 (1962). 8. Padley, P. J. and Sugden, T. M., 8th Symposium on Construction, Williams and Wilkins, Baltimore, 1962, p. 162. 9. Einbinder, H., J. Chem. Phys. 26, 948 (1957). 10. Smith, F. T., Proc. 3rd Conference on Carbon 419, (Pergamon 1959). 11. Soo, S. L. and Dimick, R. E., lOth Symposium on Combustion, Butterworth, Cambridge, 1964, p. 699. 12, Sudgen T. M. and Thrush B. A., Nature 168, 703 (1951). 13, Rayleigh, Lord, Phil. Mag. 14, 184 (1882). 14. Farragher A. L., Ph.D Thesis, University of Aston (1966). 15. Fomenko V. S. and Samsonov G,, Handbook o f Thermionic Properties, Plenum Press, 1966. 16. Borax Consolidated Limited, private communication to ERM. 17. Newman R. N. and Page F. M., Combust. Flame 20, 171 (1973). Received 22 August 1974, revised 6 January 1976