1268
result of the transference number experiments above 280’ with Pyrex or quartz fritted disk$’ must be viewed with suspicion. 6. The conventional shear viscosities of AgX03 and havc been determined over the same temperature range as the conductivities. The viscosity of A g S 0 3 was found to be somewhat higher than that previously determined,4 while the temperature range for the viscosity of T K O 3 has been e ~ t e n d e d . ~In the case of TIKOs, the viscosity was also determined using a modified Ostwald type viscosimeter in which the flow rate was determined by a Pyrex medium fritted disk located a t the end of the conventional long capillary tube. The viscosities determined by these two methods gave the same results.
COMMUNICATIONS TO THE EDITOR
7. A calculation of the self-diffusion cocfficients using the Stokes-Einstein relation and the viscosity values just determined leads to a calculated I ) N o ~ much larger in T K O 3than in AgN03 and almost similar D T I +and DAa+ values. A combination of the selfdiffusion values thus calculated with the KernstEinstein relation gives a calculated equivalent conductance larger for TINO, as compared with AgN0,. Experimentally, however, we find the equivalent conductance of TlN03to be about 60% of that of AgNO,.
(6) F. R. Duke and G. Victor, in press. (7) I. G. Murgulescu and D. Topor, 2. Physik. Chem., 219, 134 (1962).
COMMUNICATIONS T O T H E EDITOR
The Use of Flash Heating to Study the Combustion of Liquid Metal Droplets’
Sir: Investigations of the combustion of liquid metal droplets are important in order to understand basic oxidation to disperse toxic or radioactive metals safely during high-speed ablative entry into the earth’s a t m ~ s p h e r eto , ~control metal fires, or conversely to use metal flames for achieving high temperatures5 or large thrusts in propulsion devices.6 In earlier studies, metal droplets have been produced in plasma t o r ~ h e s ,or~ burning propellant strands.’ These experiments have had limitations imposed on the oxidizing atmospheres by the source of heat used to prepare the droplets. Clearly, it would be desirable to use a means of heating that does not itself modify the atmosphere in which the oxidation is to be carried out. Furnaces operating above the melting point of the metal satisfy this requirement to some extent,lOall although it may be difficult to obtain sufficiently large energy transfer rates to melt the more refractory metals. P’lash h e a t i n g l * seems ~ ~ ~ to offer considerable promise as an atmosphere-independent method of preparing droplets of the higher melting metals. In this technique, an intense pulse of light from a capacitor discharge lamp is used to heat solids with high specific areas rapidly to very high temperatures. A number of refractory metals in the form of thin wires, foils, and fine powders have been melted or vaporized this The Journal of Physical Chemistry
way.12-14 The pulses of light normally have millisecond durations, with radiant energies of 10-30 joules/cm.2 flash. l5 These previous experiments indicate that droplets of any of the metals may be prepared by flash heating in atmospheres that may vary widely (1) Work was performed under the auspices of the U. S. Atomic Energy Commission. (2) E. M. Mouradian and L. Baker, Jr., Nucl. Sci. Eng., 15, 388 (1963). (3) I. L. Branch and J. A. Connor, Jr., Nucleonics, 19, No. 4, 64 (1961). (4) L. Liebowits, L. Baker, Jr., J. G. Schnizlein, L. W. Mishler, and J. D. Bingle, Nucl. Sci. Eng., 15, 395 (1963). (5) A. V. Grosse and J. B. Conway, Ind. Eng. Chrm.. 50, 663 (1958). (8) D. A. Gordon, “Solid Propellant Rocket Research,” Academic Press, New York, N. Y., 1960, p. 271; W. M. Frrssell, C. A. Papp, D. L. Hildenbrand, and R. P. Sernka, ibid., p. 259; W. A. Wood, ibid., p. 287. (7) R. Friedman and A. Macek, “Ninth Symposium (International) on Combustion.” Academic Press, New York, N. Y., 1963, p. 703. (8) C. M. Drew, A. 8. Gordon, and R. H. Knipe, A.I.A.A. Conference on Heterogeneous Combustion, Palm Beach, Fla., Dec , 1963. (9) Private communication, A. E. Levy-Pascal, Astropower, Inc., Newport Beach, Calif. (10) H . M. Cassell and I. Liebman, Combust. Flame, 3 , 467 (1959). (1 1) Private communication, J. L. Beal. Cornell Aeronautical Laboratory, Buffalo. N. Y. (12) J. Eggert. Physik. Bl., 10,549 (1954); J . Phys. Chem., 6 3 , 11 (1958). (13) L. S. Nelson and J. L. Lundberg. ihid., 6 3 , 433 (1959). (14) L. S. Nelson and N. A. Kueblcr, Rev. Sci. Instr., 34,806 (1963). (15) N. A. Kuebler and L. S. Nelson, J . Opt. Soc. Am., 51, 1411 (1961).
126!)
~ O M M U N I C A T I O N STO T H E I h I T O R
in conipositiori and pressure, and may be neutral, ionized, or dissociated. We report here on the use of flash heating to form single droplets of niolten zirconium (melting point 1855’) with highly reproducible diameters between 100 and 500 I.(. The droplets were prepared from barefully cut squares of foil dimensions: 0.2 to 2.0 mm. square, 10 p thick). Oxidations were performed in air at pressiires between 50 arid 629 torr. The apparatus in which the oxidations were performed is shown in Fig. 1. It corisisted of a vertical glass oxidation tube, the upper end of which was surrounded with a helical capacitor discharge lamp. A t the top of the tube, a mechanism operated by a rotary solenoid released single squares of zirconium foil which fell downward toward the flash heating region. ,Just as the foils passed through, the lamp mas triggered; the resulting thermal emission melted the zirconium and formed brilliantly incandescent droplets. The behavior of the glowing droplets as they fell was recorded by high speed photography. The flash lamp was powered by a Trion 1’s-100 lasw
1\
TIME FOR EXPLOSION OF ZIRCONIUM DROPLETS IN QUIESCENT AIR.
\
265 MICRONS
L 234 MICRONS
4
ROTARY SOLENOID (45.ROTATION)
Jf---i~pp
LUCITE INSERT CEMENTED INTO GROUND JOINT
TUBING VACUUM
S H W FROM WHICH SQUARE OF FOIL IS RELEASED WHEN SOLENOID ROTATES
GROUND JOINT
FLASH TUBE
TO CAPACITOR BANK
-
TO CAPACITOR
TO GASSUPPLY
u(PL0SIoN OF
TO VACUUM PUMP -. ..
-
4 1
Figure 1. Flash heating apparatus for studying the oxidation of freely-falling metal droplets. To load square of foil o n t o shelf, rotary release mechanism is raised above ground joint.
3 A I R PRESSURE (TORR) Figure 2. Time required for zirroniurn droplets t o euplode during free fall in quiespent air, measured from start of heating flash. Points with vertical bars are averages of from three to five separate determinations; average deviation is indicated by distance frani point to end of bar.
stimulator which operated a t 4 kv. and delivered 3600 joules. The fused quartz flash lamp was surrounded by an externally sllvered cylindrical glass reflector. A cylindrical volume approximately 30 mm. in diameter and 65 mm. long was illumiliatcd. Several results obtained with zirconium droplets will be presented here to illustrate the capability arid accuracy of the method. A more complete report n i l 1 be presented later. The most impressive feature of the combustion of a zirconium droplet falling freely in air is the explosion or sparking that occurs a short time after the drop1c.t is formed. This is easily seen in a darkened room with the unaided eye and appears plainly on motion picture frames recorded a t 5000 frames/src. This c4Tect has been observed by others in zirconium and other both directly and by inference from the presence of shattered hollow spheres after the con?t)ustion.
COMMUNICATIONS TO THE EDITOR
1270
We have found that, the induction period before explosion is quite sensitive both to air pressure and to the diameter of the droplet. A plot of the time interval between the start of the heating flash and the explosion observed on our films is shown in Fig. 2. Each point is the average of three to five separate determinations. The average deviation between points is rrbout * 3 msec., or about +2%. I t will be noted that the induction period increases with increasing particle diameter and with decreasing air pressure. Collection of particles in glass dishes inserted a t various heights in the oxidation tube enabled us to follow the oxidation process as the droplets fell. This is based upon the observation that the incandescent droplets apparently quench rapidly when they strike a glass surface. Table I summarizes the appearance of residues collected a t various levels in the oxidation tube when 265-p diameter droplets fell through air a t 625 torr. The normal distance for explosion of droplets with this diameter was 32 f 2 cm., measured downward from the midpoint of the flash lamp's helix. The cause of the explosions is not known a t present, but the films show a rapidly expanding luminous burst in the first frame or two (0.2-0.4 mscc.) a t the start of the explosion. I t is thus likely that the driving force is the expansion of a gas a t high pressure. The explosions may also be related to the change from the
Table I : Appearance of Residues Collected at Various Distances when a 265-p Zirconium Droplet Falls in Air a t 62.5 Torr (Explosion Distance = 32 f 2 cm.) Distance below flash lamp, cm.
