I
EDGAR
L. POLING' and H. P. SIMONS
West Virginia University Engineering Experiment Station, Morgantown,
W. Va.
Explosive Reaction of Diborane in Dry and Water-Saturated Air Mixtures of gases with varying concentrations of diborane and dry air gave higher maximum pressures than mixtures using wet air a t the exhaust end with needle CONSIDERABLE and valves and was equipped with a spark
interest has arisen in diborane and the other boron hydrides in recent years. Since the work on these hydrides was done by Stock (8), much research has been devoted to structure and reactions of the compounds. Price ( 5 ) has investigated the pressure limits of the explosion of diborane in oxygen. Several articles ( I , 2, 4) have been published concerning the pyrolysis of diborane and the pyrolytic production of higher boron hydrides. Weiss and Shapiro ( 9 ) have studied diborane hydrolysis. Roth (7) has investigated the explosive oxidation of diborane with dry oxygen a t low pressures, However, the physical properties of the explosive reaction of diborane in air and the effect of water vapor on these properties have not been reported. Hence, the rate of flame propagation and the pressure-time characteristics of the explosive reaction of diborane in dry and saturated air have been studied by the authors. Results of product gas analyses are given.
plug and a diaphragm-type strain gage constructed especially for ct. The pressure cell was one leg of a Wheatstone bridge, the output of which was fed to an Again, the trace was retographically. ct-gas samples from the reactor en in an evacuated bulb and analyzed for oxygen, hydrogen nitrogen in an Orsat gas analys
.
usual procedure was to transfer about 0.6 gram of diborane into the
vacuum system by successively admitting diborane to the system to a given pressure, and then freezing it out in a liquid nitrogen-cooled trap. The diborane sample was thawed over a carbon disulfide slush bath and refrozen twice with pumping between cycles, This gave diborane with an essentially constant vapor pressure. The diborane was admitted to the evacuated mixing reservoi! of the modified Toepler pump to a given partial pressure. The reservoir was then filled to atmospheric pressure with dry air or with air humidified in a system of bubbler bottles to a dew ,,point of about 28.5' C. The gases were '
L'
Apparatus and Procedure The diborane, obtained in a small cylinder from a commercial supplier, was handled during purification and transfer in a conventional glass vacuum system. Premixing of the diborane-air samples was carried out in a modified Toepler pump, one reservoir of which was fitted with a magnetically powered agitator. The rate of flame propagation runs were carried out in a stainless steel tube about 5 feet long and of 3/&nch inside diameter. The tube was equipped with a needle valve at the feed end, a gate valve a t the exit end, a spark plug for igniting the mixtures, and two glass windows approximately 4 feet apart through which the passage of the flame front was observed photoelectrically. The output from the photocells was fed to an oscilloscope, and the trace was recorded photographically. Pressure-time runs were carried out in a stainless steel reactor of about 500-ml. capacity which was fitted a t the feed end Present address, E. I. du Pont de Nemours & Go., Inc., Waynesboro, Va.
Figure 1 . The distance between the two breaks in the oscilloscope trace is the time required for the flame front to pass the two windows of the velocity tube A. B.
Typical rate of flame proqagation Typical sweep calibration
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speed of the oscilloscope was known a measure of the time required for the flame to pass between the two windows. Sweep speed was calibrated by an electrical-mechanical device which spaced pips a known time apart on a calibrating trace. When using the reactor, the pressure cell output plotted the relation of pressure in the reactor against time after ignition on the oscilloscope screen (Figure 2). Calibration of the oscilloscope deflection in terms of pressure was accomplished by a static calibration of the diaphragm cell against a precalibrated gage by using nitrogen under pressure fed to the reactor through the exhaust valve. A gas sample from the reactor was collected in an evacuated bulb, and then transferred to the buret of a n Orsat gas analysis apparatus where the oxygen was absorbed in an alkaline pyrogallate solution and the hydrogen was removed by passing the gases over copper oxide at 300' C. The remaining gas was taken as nitrogen. Results
Figure 2. Pressure-time traces on the oscilloscope screen for mixtures show the erratic dip of the curve ( A ) due to lower concentration of diborane and the leveling off (6) when the diborane concentration approaches stoichiometric
mixed for 3 minutes, and then transferred either to the reactor or to the velocity tube where they were ignited by closing and opening a switch admitting about 3 volts (direct current) to a n automobile spark coil whose output was fed to the spark plug. The spark ignited the mixture of gases and, at the
same instant, initiated the single sweep of the oscilloscope. When using the velocity tube, the photocell output caused two breaks in the oscilloscope trace as the flame front passed first one window and then the other (Figure 1). The distance between these breaks was, as the sweep
and Discussion
Results of the flame propagation study are presented in Figure 3. For mixtures using dry air, the best straight line through data points for mixtures containing less than a stoichiometric amount of diborane (6.53 mole %) follows the equation : RJ = 4984
+ 247.38Cd
where R , is the rate of flame propagation in feet per second and C d is the concentration of diborane in the original mixture in mole per cent. The equation for the best straight line through data points for mixtures containing more than a stoichiometric amount of diborane and using dry air is : RJ
=
6620 - 0.003Cd
where the nomenclature is as above. The second term on the right side of this equation is so small as to be inconsequential. For mixtures using wet air, the equation for the line through data points for mixtures containing less than 7.5 mole % diborane is:
0
0
3 L
u 0 e Q L
.-C
Rf
=
5029
+ 187.78Cd
and for mixtures containing more than 7.5 mole 70diborane is: RJ
L
0
Diborane
in original
mixture in mole percent
Figure 3. In mixtures using wet air, the water vapor decreases the rate of flame propagation due, probably, to hydrolysis of diborane
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In Figure 3, the effect of water vapor (3.9 mole 70water vapor based on the wet air) shifts the curve to the right about 1 mole % and decreases the rate of flame propagation for a given diborane concentration by about 120 feet per second. The shift to the right is probably due to hydrolysis of part of the diborane during premixing by water vapor in the wet air.
DIBORANE EXPLOSIVE REACTION thus reducing the effective concentration of diborane fed to the velocity tube. Reynolds and Gerstein (6) show the rate of flame propagation us. fuel concentration for various hydrocarbons. These curves show that the propagation rate increases with increasing fuel concentration below the stoichiometric point (maximum propagation rates for concentrations ranged from about 10 to 300/, higher than stoichiometric), and decreases sharply above this point. The increase was observed for diborane, but the decrease was not noted in the range studied (up to 20 mole % diborane in the original mixtures). Selected mixtures of hydrogen in oxygen and acetylene in oxygen, for which the flame propagation rates had been reported (3), were prepared and ignited in the tube in order to compare the results obtained with the present equipment with the earlier reported results. The average of the results was about 5% lower than the reported rates in hydrogen-oxygen mixtures, and checked to well within 1% of the reported values for acetylene-oxygen mixtures. Pressure-time trace photographs similar to those in Figure 2 were obtained for a number of runs using both wet and dry air in the reactant mixtures. Traces similar to that in Figure 2, B, were obtained for mixtures containing more than a stoichiometric amount of diborane. These show the pressure rising rapidly to a maximum, then falling off slowly. Traces similar to that in Figure 2, A , were obtained for mixtures containing less than a stoichiometric amount of diborane. The lower the diborane concentration, the lower and more pronounced the dip in the curve appeared; as the concentration of diborane approached stoichiometric, the dip became less pronounced and, at or about a stoichiometric concentration of diborane, disappeared to give traces similar to that of Figure 2, B. Analysis of the pressure-time trace photographs (Figure 2) showed that the maximum reactor pressure varied with the initial diborane concentration (Figure 4). The solid lines (Figure 4) are for mixtures using dry air, and the dotted lines for those using wet air. The higher dotted line and the higher solid line represent maximum pressures as read directly from the peaks of the pressure-time traces. However, in some cases these peaks were difficult to distinguish due to the vibration of the trace. Therefore, an extrapolation method, used to determine curves in Figure 4, was carried out by reading the pressure 3, 5 , IO, and 1 5 milliseconds (msec.) after ignition on each pressure-time trace. After a time lapse of 3 msec. and thereafter the vibration of the trace
240
r
I
I
I
I
,
, -
.$ 2 0 0 .-C
tI
160
?
Q
8 120
c
0
e 80
E
::
l0"l
I
s
s 40
0
2
I I I 1 4
6
Diborone
Figure 4.
-
8
10
12
~
I
I
--Dry
oir, -Dry ojr, A---A-Wet oir, e--*Wet air, 14 I6
I I
I
extrapoloted experjmental experimental exiropoloted 18 20
in origin01 mixture in mole percent
Maximum reactor pressure varies with initial diborane concentration
60
E c(
so
-
9
40
.a el
30 c 0 3
20
.-e
00
IO
e
=,
0
I
0 0
0
2
2
4
4
6 Diborane
6 Diborone
8 In original
10 12 14 16 mixture in mole percent
8 IO 12 14 16 in original mixture i n mole percent
i8
18
20
20
Figure 5. The actual amount of hydrogen in product gases, when compared with the theoretical amounts, shows that the reaction proceeded from oxidation to hydrolysis to both hydrolysis and pyrolysis-depending upon the concentration of diborane-in both dry (top) and wet (bottom) air mixtures VOL. 50, NO. 1 1
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60 c
0
50 a
0
.-
40
u)
M 0
30 c 0
0
g
eo
f
10
C
e
U >
I
0
2
0
6
4
Diborane
Figure 6.
0 IO 12 14 in original mixture in mole percent
16
18
20
Comparison of experimental curves for hydrogen in product gases The wet oir curve shows o shift i o w o r d higher diborane concentration
.
c
f
20
Q 0
I
01 0
2
4
6 Diborane
I 1 I I 8 10 12 14 16 in original mlxtura in mole percent
I
18
20
Figure 7. More than the theoretical amount of oxygen remained in product gases, indicating the reaction was incomplete The wet oir curve a g o i n shows a shift t o w a r d higher diborone concentration
was much less and the pressure could be read much more exactly. The pressure at these points was plotted against time after ignition and was extrapolated to zero time to obtain a maximum pressure. These extrapolated curves lie slightly below the experimental curves in Figure 4, but the maximum points fall a t about the same diborane concentrations for both the extrapolated and experimental curves. The highest reactor pressure (Figure 4) attained by mixtures using dry air occurred a t a concentration of about 14 mole 7’diborane; the maximum for mixtures using wet air occurred a t about 10 mole % diborane. It is possible that the higher pressure obtained in mixtures using dry air was due to the slightly higher rate of flame propagation seen for these mixtures. The higher rate of reaction would tend to produce the product more quickly, thereby causing the pressure to rise to a higher value before condensation of water vapor and cooling of the hot gases could offset the pressure rise.
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Product-gas analyses were carried out for oxygen, hydrogen, and nitrogen. Figure 5 shows the actual amount of hydrogen found (data points and solid line) in relation to theoretical amounts (dotted lines) which have been present if the reaction proceeded either as an oxidation B2Hs 4- 3 0 2 -+ B z 0 3 3H20 (oxid.), as pyrolysis BzHs + 2B 3H2 (pyr.), as hydrolysis BpHs 6H20 2H3B03 6H2 (HOH), or as hydrolysis until all water from the oxidation was consumed, then by pyrolysis (HOH) (pyr.). I n Figure 5, the reaction apparently proceeds as in oxidation up to a stoichiometric amount of diborane, then proceeds about as a hydrolysis up to about 14 mole yo diborane, above which point both hydrolysis and pyrolysis appear to be involved. Figure 5 shows similar curves for mixtures using wet air. The break in the theoretical curve below 1 mole % diborane is caused by producing hydrogen by hydrolysis in the mixer as part of the diborane reacts with water vapor present in the wet air. The experimental curve does not show that
INDUSTRIAL AND ENGINEERING CHEMISTRY
+
+
+ +
+
this hydrogen was produced, but it is believed that it was produced and was burned during the explosive reaction. If this theoretical amount of hydrogen produced in the mixer were added to the experimental curve, this curve would fall essentially at the same point on the theoretical curves as for dry air in Figure 5. The relation of the experimental curves for hydrogen in product gases from mixtures of wet and dry air are shown in Figure 6. The wet air curve shows the same shift toward a higher diborane concentration as u a s observed in the flame propagation study. Figure 7 shows the amount of oxygen remaining in product gases for mixtures using wet and dry air. In all cases. there was more than the theoretical amount of oxygen remaining, indicating that the reaction did not go entirely to completion. The remaining oxygen was about 1 mole % more than theoretical in mixtures using dry air and about 2 mole % more than theoretical in mixtures using wet air. The shift toward a higher diborane concentration of the break point between oxidation and hydrolysis is seen again. A spot check (made by impinging the gases on a filter paper moistened with a 5% solution of silver nitrate in n-pentylamine) of product gases gave a positive test for traces of volatile boron hydrides, but a specific test for decaborane was negative. Decaborane specifically produces a deep red color in a solution of 5 ml. of C.P. quinoline in 100 ml. of xylene. The hydrides detected were more probably pyrolysis products than residual diborane. Acknowledgment The authors wish to acknowledge gratefully the assistance of the West Viiginia University Engineering Experiment Station, through which this project was administered.
Literature Cited (1) Bragg, J. K., McCarty, L. V., Korton, F. J.. J , Am. Chem. Sac. 73. 2134 (1951). (2) Clarke, R. P., Pease, R.’W., Zbzd.,73, 21 52- (1951). (3)Di xon, H:B., Trans. Roy. SOC. (London) A184, 97 (1893). (4) McCarty, L. V., DiGiorgio, P. A, J . Am. Chem. Sac. 73, 3138 (1951). (5) Price, F. P., Zbid.,7 2 , 5361 (1950). (6) Reynolds, T. W., Gerstein, M., “Third Symposium on Combustion, Flame, and Explosion Phenomena,” pp. 190-4, Williams and Wilkins, Baltimore, Md., 1949. (7) Roth, W., “A Study of the Explosive Oxidation of Diborane,” Dissertation, Rensselaer Polytechnic Inst., 1954. (8) Stock, A., “Hydrides of Boron and Silicon,” Cornel1 Univ. Press, Ithaca, New York, 1933. (9) Weiss, H. G., Shapiro. I., J . Am. Chtm. SOC. 75, 1221 (1953). RECEIVED for review September 11, 1956 ACCEPTED February 15, 1957 Work supported by Callery Chemical Co. under contract from Bureau of Aeronautics, Department of Navy. I--
