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The fast pyrotechnic reaction of silicon and red lead: heats of reaction and rates of burning
The fast pyrotechnic reaction of silicon and red lead: heats of reaction and rates of burning Sabeh S. Al-Kazraji
and Gwilym
J. Rees
Department of Chemical Engineering, The Polytechnic of Wales, Pontypridd, Mid-Glamorgan, UK (Received 12 May 1978)
Heats of reaction were measured with a bomb calorimeter and linear rates of burning by an electronic timer. The former reached a maximum at 10% Si for all sizes tested; the latter at 30% Si for coarse silicon and at 15% Si for fine silicon. An attempt was made to correlate both properties with rate of heat evolution. Reactions of mixtures containing <20% Si were violent owing to rapid rise of temperature to a high level, causing vaporization of products and rapid expansion of air above the delay element. Mixtures containing 20-50% Si were adaptable for practical use and reactions were mainly of the solid/solid type. The products are SiO2, Pb, and unreacted Si.
A previous paper’ has described thermal analysis studies of silicon/red lead mixtures, and the present work was concerned with measurements of the heats of reaction and linear rates of burning of compressed columns of compositons.
firing circuit the essential details of which are shown in Figure 2. PROCEDURE Safety measures
MATERIALS AND EQUIPMENT Silicon powders (designated A, B and C) having respectively the following average particle sizes and surface areas, 1.9pm and 6.258 m2/g, 3.9 pm and 2.543 m2/g, 5 pm and 1.448 m2/ g, were used in the experiments. They were 98% pure, the main impurity being metallic iron. The red lead (pb304) had an average particle size of 5 pm and surface area 0.442 m2/g. It was 95% pure, the main impurity being lead dioxide. Particle sizes of both materials were pre-determined, using a Coulter Counter Model TA particle size measuring equipment. Results were an average of measurements on five samples of each material. The Pb304/Si compositions were slurried by the addition of an aqueous solution of carboxymethyl cellulose which acts as a binder. The slurry was stirred thoroughly to ensure even distribution of the two species, and the water was evaporated by placing the slurried compositions on a steamheated copper tray. The binder prevents segregation of the Si and pb304 (owing to considerable density differences), and the final composition was in granular form which flowed easily and facilitated the pressing operation in delay tubes. A Gallenkamp Autobomb automatic adiabatic calorimeter Model No. CB-100 was used for measurement of heats of reaction. Die-cast metal delay tubes (Figure I) were used to contain the pressed compositions, and these were placed inside aluminium tubes fitted with electrical igniters crimped into position at one end of them. At the other end a plug of perspex was placed, to prevent ejection of slag and to protect the photo-electric cell. A Racal Universal Counter/Timer Model No. 9901 was used for measurement of linear burning times, coupled to a
0016-2361179/020139-05$2.00 0 1979 IPC Business Press
As described previously’, adequate precautions were necessary owing to the sensitivity of the compositions to impact, friction and static electricity.
Mixing procedure The Pb304 and Si powders were mixed by passing the previously weighed separate ingredients through 75 pm
Pyrokhnr
comporiiion
Delay element
Figure 1
Delay element and igniter assembly
FUEL, 1979, Vol 58, February
139
Pyrotechnic reaction of silicon and red lead: S. S. Al-Kazraji and G. J. Rees
sieve five times. A range of mixtures was prepared and each one was slurried in an aqueous solution of carboxymethyl cellulose, then dried ready for use.
Pb304/Si mixture. The temperature rise of the water was read by a Beckmann thermometer to O.OOl”C. Heat losses to the surroundings were eliminated by an outside jacket, filled with water and kept at the same temperature as the calorimeter throughout the test. This was done by a combination of very sensitive thermistors, one immersed in the calorimeter and the other in the water jacket, and an instantaneous-response electrode heater fitted in the jacket. The heat generated from the fuse-heads alone was measured by a separate experiment under identical conditions, by firing electrically ten fuse-heads alone in their assemblies. The amount of heat generated from ten fuse-heads was 594 J, and since five were used in the heat of reaction measurements, 297 J was deducted from the observed values to obtain the true heat of combustion of compositions.
PLessingof compositions in delay elements An air-operated hydraulic press was used to consolidate the experimental compositions in a 20-mm long die-cast delay element, by the following procedure. Granules of each experimental composition were introduced to the delay element in ten equal increments, each increment being compacted at 15.7 kPa pressure for a dwell time of 2 s. This incremental filling and compressing gives an evenly pelleted mixture; the total weight of composition in each delay element varies according to composition (0.44-l .OOg). On completion of pressing, each aluminium tube containing the filled delay element was fitted with an electrical cerium fuse-head igniter crimped into position. Each unit was then ready for use in the measurement of heat of reaction or burning time.
Measurement of burning time This was done by connecting Lhe aluminium tube, containing the die-cast delay element and fitted with the igniter, to a firing circuit and electronic timer. The operation of firing the igniter, which ignites the composition in the delay element, simultaneously started the timer. Termination of burning was recorded when the light from the end of the delay element fell on a photo-electric cell, which sent a signal to stop the timer. Ten measurements for each composition were performed, and mean values calculated.
Measurement of heat of reaction Five aluminium tubes, each containing delay elements filled with compressed experimental composition and fitted with fuse-heads, were used for each mixture studied. The fuse-head leads were connected to the firing circuit and inserted in the bomb, which was then filled with nitrogen at atmospheric pressure. The bomb was immersed in the calorimeter water. A 5 A current was then passed through the firing plug to fire the fuse-heads which, in turn, ignited the experimental
RESULTS These are shown in Table 1. Calculated heats of reaction If the combustion reaction is considered to proceed according to the equation Pb304 + 2 Si -+ 3 Pb + 2 SiO2 (685.6) (2 x 28.1) (3 x 207.2) (2 x 60.1)
A.C.
Firi”g
-
Mains
-
Figure 2
-
circull
(from National Bureau of Standards data*) the following values of the heats of formation apply:
Timer
MfPb304
Firing and timer circuit
Tab/e 7 Heat generated,
rate of burning and heat evolution
for PbaO&i
Silicon A Q
H
R.B.
R.H.
50150 55145 60140 65135 70130 75125 80120 85/l 5 90110 9515
788.4 904.4 983.1 1027.0 1086.4 i 108.2 1183.1 1269.3 1303.4 910.4
1126.1 1174.3 1170.2 I I 28.4 1 i 08.4 1056.0 1056.2 1066.5 1034.3 684.4
5.93 9.44 11.47 12.71 13.90 13.88 24.99 25.74 22.22 10.89
882.6 1740.1 2441.9 3068.9 3863.2 4277.5 9256.4 1151.8 11533.4 4335.2
Q = heat generated per gram of mixture (J/g) H = heat generated by mass containing 1 mole of Pb304 R.B. = rate of burning (cm/s) R. H. = rate of heat evolution (J/s)
FUEL,
1979, Vol 58,
February
kJ/mol
mixtures Silicon 6
Pb,O&i (wt /wt)
140
= -718.6
Silicon C
Q
H
R.B.
R.H.
Q
H
R.B.
605.6 720.2 793.1 921 .l 978.5 1093.1 1174.3 1251.3 1296.9 916.9
865.0 935.2 944.0 1012.0 998.3 1040.9 1048.3 1051.3 1029.1 689.3
3.88 6.92 8.94 11.66 16.30 13.43 10.87 10.06 9.44 4.61
517.3 1204.4 1783.6 2903.5 4708.5 4711.1 4462.5 4892.7 5259.7 3434.2
665.8 703.5 780.1 918.2 999.0 1076.8 1121.6 1189.8 1289.8 980.1
775.8 913.5 928.5 i 000.8 1019.2 1025.3 1000.1 999.7 1023.4 736.8
-
-
5.32 7.23 8.66 1I I .48 9.87 7.99 7.15 6.46 4.24
872.6 1416.2 2162.8 3394.9 3481 .i 3206.5 3263.9 3656.4 2049.0
(kJ)
R.H.
Pyrotechnic reaction of silicon and red lead: S. S. Al-Kazraji and G. J. Rees 1500,
,
I
I
I
1300 F 2 : ..z :: E
1200 -
1100 -
” 6 cn G a D : z z
1000 -
900 -
BOO-
;; I” 700 -
\ ‘e
\LSilicon 600-
5001
0
I
I
I
I
10
20
30
10
Silicon Figure 3 calculated
kJ/mol
AHfPbO = -217.9
kJ/mol
m
Silicon C
I 50
60
1
I
70
I wt % 1
Heat evolved par gram of composition and experimental
AHfSiO2 = -903.7
B
versus % Si,
three different Si powders used, though the heat evolved from the fine grade (silicon A) is greater than that of the other two, particularly in the region 20-50% Si. The rate of burning (cm/s) when plotted against silicon content increases with decrease of % Si and reaches a maximum at 30% Si (silicon B and C) and at 15% for silicon A, and then decreases to 5% Si (Figures 4,5 and 6). The rate of heat evolved (R. H.) is calculated from the values of Q (J/g) and the weight burned per second (g/s). The weight of compressed composition burned was found to be proportional to time taken. For fine silicon (Figure 4) it is seen that heat evolved per second for various compositions reached a peak value at approximately the same percentage silicon as that at which rate of burning reached its peak, indicating that the two parameters are significantly related to each other. This pattern was true also for coarser grades (Figures 5 and 6) but it was observed to deviate at 10% Si, forming a second peak. However, at this point the time of burning is markedly influenced by the high pressure generated in the aluminium tube owing to the high temperature of reaction. Theoretically the maximum of rate of burning should coincide with maximum of heat of reaction. The observed discrepancy may be explained by the fact that the reactions take place at the surface and Si particles become coated with a layer of SiO2. This prevents complete reaction of the Si core even when in the molten state. Thus, in approaching the peak of the heat of reaction, the amount of Si is reduced and the layer of product, being thicker, causes the rate of reaction and hence the rate of burning to drop. This occurs at a point richer in Si than that for the heatgenerated maximum. The finer the Si, the better the agreement between rateof-burning and heat-of-reaction maxima, since larger surfaces are available giving better fuel/oxidizer contact. Also
For a 90/ 10 mixture of PbsO@i (1 g of composition), the Si is more than required by the equation, and 0.9 g of pb304 is therefore considered to have reacted completely. 60.1 x 2 SiO2 formed = --------x 685.6
Heat of reaction = 0.9 x 1000 = -1429
1300-
l Heat evolved 0 Rate of heat evolved I Rate of burning
0.9 g per 1 g mixture
60.1 x 2 --xx++ 685.6
903.7 60.1
718.6 685.6
1
J/g of mixture
=” 1200z .L llOO:: :
685.6 x __ 0.9
-30
-1L
-20
-13
-26
-12,
-24
-112
-22 2
-lo.
“u lOOO-
Heat evolved mole of Pb304 = -1429
-15
G
-9
0l ; 900a
-8 -7
= 1089 kJ -6 -5
DISCUSSION
z -18 .c” z = -16 2 2 2 -IL 6 z al (u -12 2 rY z m -10
-4
Pb304/Si compositins containing from 5 up to 50% Si have been examined; those containing more than 50% Si failed to maintain sustained reliable combustion. This was particularly true of the coarser grade (silicon C). From the graph of heat generated per g of composition against silicon content (Figure 3), this value increases with decrease of Si, reaching a maximum around 10% Si and then dropping sharply. This is of the same pattern for the
600 -
Silicon
-8
-3
-6
-2
-L
-1
-2
A twt%)
Heat evolved, rate of heat evolved and rate of burning Figure 4 versus amount of silicon A
FUEL,
1979,
Vol
58,
February
141
Pyrotechnic reaction of silicon and red lead: S. S. Al-Kazraji and G. J. Rees
l
Heat evolved 0 Rate of heat evolved l Rate of burning
-9
-18
-6
-16
700 -
-2
-4
600 -
-1
-2
0I ; a
900-
2 L P Q) 600‘;; I”
I 10
500’ 0
I I 30 LO Silicon B I wt%)
I 50
I 20
lOJ6
Figure 5 Heat evolved, rate of heat evolved and rate of burning versus amount of silicon B
I
1
I
I
I
Heat evolved Rate of heat evolved Rate of burning
-16
-16
with the finer Si there is closer agreement between experimental and calculated theoretical values for heat evolved. Reactions of mixtures containing 5, 10 and 15% Si were violent, and high pressures, inside the aluminium tubes, led to bursting. These compositions have no practical importance and their rates of reaction are very dependent on the pressures formed. Reaction will depend on the diffusion of vaporized oxides into the surface of the silicon and products. These reactions are different in nature from those where a large excess of Si (20-50%) is present. In the latter case solid/solid mechanisms prevail; the surface area of Si and the good fuel/oxidizer contact (controlled by the pressure in the pelleting process) are the controlling factors. Excess Si, which is unreacted, will act as a heat sink as it melts and hence keep the temperature at 1410°C until all the Si is molten. As the Si content approaches 15%, vaporization of lead at 1750°C and of unreacted lead oxides occurs. The heat generated from the reaction can be compared with the experimental results. Theoretically, stoichiometry occurs at 7.6% Si, and the experimental maximum heat generated was at 1% Si. X-ray diffraction and infrared analysis confirm the presence of elemental lead, fused silica and unreacted silicon. When the amount of heat liberated by a weight of mixture which contains one mole of Pb304 is plotted against % Si (Figure 7), it increases sharply as Si increases from 5 to 1% and reaches a maximum at 15%, staying constant up to about 25% Si. This, according to the treatment of Spice and Staveley3, indicates that there is a single reaction in this range (5-30% Si), and that the maximum at 15% Si is the point where the reducing agent uses up all the oxidizing agent. The reaction of Si with F’b304 can be considered to take place in two stages as indicated in a previous paper. First is the decomposition of Pb304 and then the reaction of Si with PbO and oxygen:
-3
Pb304
+ 3
PbO + ?402
(1)
Si + 02 + SiO2
(2)
2PbOtSi+SiO2+2Pb
(3)
From equation (I), oxygen gas forms 25% of the total amount of oxygen available for reaction.
-2
r
1200 -1
0: llOO-4
e” 2 lOOO-
600 I
-2
3 900-
5001 0
’ 10
I I I 20 30 LO Silicon C (wt%)
I 50
-0
-0
; r” 600.-5 z z! 700E 600
Figure 6 Heat evolved, rate of heat evolved and rate of burning versus amount of silicon C
142
FUEL, 1979, Vol 58, February
0
I 5
I
I
I
I
I
I
I
10
15
20
25
30
35
40
Silicon Figure
7
Heat evolved/mol
I wt
%)
Pbs04 versus % Si
45
Pyrotechnic reaction of silicon and red lead: S. S. Al- Kazraji and G. J. Rees
For compositions containing relatively large amounts of Si, thermogravimetric experiments have shown loss of some of the oxygen, indicating that the reaction is mainly of solid/solid nature. For compositions with a small amount of Si. the temperature reached was so high that some of the PbO was vaporized, and in this case the solid/solid reaction will not predominate. For a large excess of Si, above 30%, the reactions are incomplete owing to the presence of some Si particles not in contact with Pb304. Also it is very likely that the Si and 02 reaction is incomplete because of the high temperature needed, which may not be attained at large excesses of Si.
helpful discussions. We also thank the Science Research Council for the granting of a Cooperative Award in Science and Engineering (CASE) research studentship to one of us (S. S. Al-Kazraji), and R. Eather and J. Thompson, of the Royal Armament Research and Development Establishment, for advice on the firing circuit. Thanks are also due to D. Lea and associates of the Department of Electrical Engineering, Polytechnic of Wales for the construction of the firing circuit.
ACKNOWLEDGEMENTS
1 Al-Kazraji, S. S. and Rees, G. J. Combust.Flame 1978,31, 105 2 National Bureau of Standards, Technical Notes 270-3, Jan. 1968 3 Spice, J. E. and Staveley, L. A. K. J. Sot. Chem. Ind. 1949,68, 348
The authors thank Nobel’s Explosives Co. Ltd for financial support and materi::ls, and J. Morman and T. McLaughlin for
REFERENCES
FUEL, 1979, Vol 58, February
143
and Gwilym
J. Rees
Department of Chemical Engineering, The Polytechnic of Wales, Pontypridd, Mid-Glamorgan, UK (Received 12 May 1978)
Heats of reaction were measured with a bomb calorimeter and linear rates of burning by an electronic timer. The former reached a maximum at 10% Si for all sizes tested; the latter at 30% Si for coarse silicon and at 15% Si for fine silicon. An attempt was made to correlate both properties with rate of heat evolution. Reactions of mixtures containing <20% Si were violent owing to rapid rise of temperature to a high level, causing vaporization of products and rapid expansion of air above the delay element. Mixtures containing 20-50% Si were adaptable for practical use and reactions were mainly of the solid/solid type. The products are SiO2, Pb, and unreacted Si.
A previous paper’ has described thermal analysis studies of silicon/red lead mixtures, and the present work was concerned with measurements of the heats of reaction and linear rates of burning of compressed columns of compositons.
firing circuit the essential details of which are shown in Figure 2. PROCEDURE Safety measures
MATERIALS AND EQUIPMENT Silicon powders (designated A, B and C) having respectively the following average particle sizes and surface areas, 1.9pm and 6.258 m2/g, 3.9 pm and 2.543 m2/g, 5 pm and 1.448 m2/ g, were used in the experiments. They were 98% pure, the main impurity being metallic iron. The red lead (pb304) had an average particle size of 5 pm and surface area 0.442 m2/g. It was 95% pure, the main impurity being lead dioxide. Particle sizes of both materials were pre-determined, using a Coulter Counter Model TA particle size measuring equipment. Results were an average of measurements on five samples of each material. The Pb304/Si compositions were slurried by the addition of an aqueous solution of carboxymethyl cellulose which acts as a binder. The slurry was stirred thoroughly to ensure even distribution of the two species, and the water was evaporated by placing the slurried compositions on a steamheated copper tray. The binder prevents segregation of the Si and pb304 (owing to considerable density differences), and the final composition was in granular form which flowed easily and facilitated the pressing operation in delay tubes. A Gallenkamp Autobomb automatic adiabatic calorimeter Model No. CB-100 was used for measurement of heats of reaction. Die-cast metal delay tubes (Figure I) were used to contain the pressed compositions, and these were placed inside aluminium tubes fitted with electrical igniters crimped into position at one end of them. At the other end a plug of perspex was placed, to prevent ejection of slag and to protect the photo-electric cell. A Racal Universal Counter/Timer Model No. 9901 was used for measurement of linear burning times, coupled to a
0016-2361179/020139-05$2.00 0 1979 IPC Business Press
As described previously’, adequate precautions were necessary owing to the sensitivity of the compositions to impact, friction and static electricity.
Mixing procedure The Pb304 and Si powders were mixed by passing the previously weighed separate ingredients through 75 pm
Pyrokhnr
comporiiion
Delay element
Figure 1
Delay element and igniter assembly
FUEL, 1979, Vol 58, February
139
Pyrotechnic reaction of silicon and red lead: S. S. Al-Kazraji and G. J. Rees
sieve five times. A range of mixtures was prepared and each one was slurried in an aqueous solution of carboxymethyl cellulose, then dried ready for use.
Pb304/Si mixture. The temperature rise of the water was read by a Beckmann thermometer to O.OOl”C. Heat losses to the surroundings were eliminated by an outside jacket, filled with water and kept at the same temperature as the calorimeter throughout the test. This was done by a combination of very sensitive thermistors, one immersed in the calorimeter and the other in the water jacket, and an instantaneous-response electrode heater fitted in the jacket. The heat generated from the fuse-heads alone was measured by a separate experiment under identical conditions, by firing electrically ten fuse-heads alone in their assemblies. The amount of heat generated from ten fuse-heads was 594 J, and since five were used in the heat of reaction measurements, 297 J was deducted from the observed values to obtain the true heat of combustion of compositions.
PLessingof compositions in delay elements An air-operated hydraulic press was used to consolidate the experimental compositions in a 20-mm long die-cast delay element, by the following procedure. Granules of each experimental composition were introduced to the delay element in ten equal increments, each increment being compacted at 15.7 kPa pressure for a dwell time of 2 s. This incremental filling and compressing gives an evenly pelleted mixture; the total weight of composition in each delay element varies according to composition (0.44-l .OOg). On completion of pressing, each aluminium tube containing the filled delay element was fitted with an electrical cerium fuse-head igniter crimped into position. Each unit was then ready for use in the measurement of heat of reaction or burning time.
Measurement of burning time This was done by connecting Lhe aluminium tube, containing the die-cast delay element and fitted with the igniter, to a firing circuit and electronic timer. The operation of firing the igniter, which ignites the composition in the delay element, simultaneously started the timer. Termination of burning was recorded when the light from the end of the delay element fell on a photo-electric cell, which sent a signal to stop the timer. Ten measurements for each composition were performed, and mean values calculated.
Measurement of heat of reaction Five aluminium tubes, each containing delay elements filled with compressed experimental composition and fitted with fuse-heads, were used for each mixture studied. The fuse-head leads were connected to the firing circuit and inserted in the bomb, which was then filled with nitrogen at atmospheric pressure. The bomb was immersed in the calorimeter water. A 5 A current was then passed through the firing plug to fire the fuse-heads which, in turn, ignited the experimental
RESULTS These are shown in Table 1. Calculated heats of reaction If the combustion reaction is considered to proceed according to the equation Pb304 + 2 Si -+ 3 Pb + 2 SiO2 (685.6) (2 x 28.1) (3 x 207.2) (2 x 60.1)
A.C.
Firi”g
-
Mains
-
Figure 2
-
circull
(from National Bureau of Standards data*) the following values of the heats of formation apply:
Timer
MfPb304
Firing and timer circuit
Tab/e 7 Heat generated,
rate of burning and heat evolution
for PbaO&i
Silicon A Q
H
R.B.
R.H.
50150 55145 60140 65135 70130 75125 80120 85/l 5 90110 9515
788.4 904.4 983.1 1027.0 1086.4 i 108.2 1183.1 1269.3 1303.4 910.4
1126.1 1174.3 1170.2 I I 28.4 1 i 08.4 1056.0 1056.2 1066.5 1034.3 684.4
5.93 9.44 11.47 12.71 13.90 13.88 24.99 25.74 22.22 10.89
882.6 1740.1 2441.9 3068.9 3863.2 4277.5 9256.4 1151.8 11533.4 4335.2
Q = heat generated per gram of mixture (J/g) H = heat generated by mass containing 1 mole of Pb304 R.B. = rate of burning (cm/s) R. H. = rate of heat evolution (J/s)
FUEL,
1979, Vol 58,
February
kJ/mol
mixtures Silicon 6
Pb,O&i (wt /wt)
140
= -718.6
Silicon C
Q
H
R.B.
R.H.
Q
H
R.B.
605.6 720.2 793.1 921 .l 978.5 1093.1 1174.3 1251.3 1296.9 916.9
865.0 935.2 944.0 1012.0 998.3 1040.9 1048.3 1051.3 1029.1 689.3
3.88 6.92 8.94 11.66 16.30 13.43 10.87 10.06 9.44 4.61
517.3 1204.4 1783.6 2903.5 4708.5 4711.1 4462.5 4892.7 5259.7 3434.2
665.8 703.5 780.1 918.2 999.0 1076.8 1121.6 1189.8 1289.8 980.1
775.8 913.5 928.5 i 000.8 1019.2 1025.3 1000.1 999.7 1023.4 736.8
-
-
5.32 7.23 8.66 1I I .48 9.87 7.99 7.15 6.46 4.24
872.6 1416.2 2162.8 3394.9 3481 .i 3206.5 3263.9 3656.4 2049.0
(kJ)
R.H.
Pyrotechnic reaction of silicon and red lead: S. S. Al-Kazraji and G. J. Rees 1500,
,
I
I
I
1300 F 2 : ..z :: E
1200 -
1100 -
” 6 cn G a D : z z
1000 -
900 -
BOO-
;; I” 700 -
\ ‘e
\LSilicon 600-
5001
0
I
I
I
I
10
20
30
10
Silicon Figure 3 calculated
kJ/mol
AHfPbO = -217.9
kJ/mol
m
Silicon C
I 50
60
1
I
70
I wt % 1
Heat evolved par gram of composition and experimental
AHfSiO2 = -903.7
B
versus % Si,
three different Si powders used, though the heat evolved from the fine grade (silicon A) is greater than that of the other two, particularly in the region 20-50% Si. The rate of burning (cm/s) when plotted against silicon content increases with decrease of % Si and reaches a maximum at 30% Si (silicon B and C) and at 15% for silicon A, and then decreases to 5% Si (Figures 4,5 and 6). The rate of heat evolved (R. H.) is calculated from the values of Q (J/g) and the weight burned per second (g/s). The weight of compressed composition burned was found to be proportional to time taken. For fine silicon (Figure 4) it is seen that heat evolved per second for various compositions reached a peak value at approximately the same percentage silicon as that at which rate of burning reached its peak, indicating that the two parameters are significantly related to each other. This pattern was true also for coarser grades (Figures 5 and 6) but it was observed to deviate at 10% Si, forming a second peak. However, at this point the time of burning is markedly influenced by the high pressure generated in the aluminium tube owing to the high temperature of reaction. Theoretically the maximum of rate of burning should coincide with maximum of heat of reaction. The observed discrepancy may be explained by the fact that the reactions take place at the surface and Si particles become coated with a layer of SiO2. This prevents complete reaction of the Si core even when in the molten state. Thus, in approaching the peak of the heat of reaction, the amount of Si is reduced and the layer of product, being thicker, causes the rate of reaction and hence the rate of burning to drop. This occurs at a point richer in Si than that for the heatgenerated maximum. The finer the Si, the better the agreement between rateof-burning and heat-of-reaction maxima, since larger surfaces are available giving better fuel/oxidizer contact. Also
For a 90/ 10 mixture of PbsO@i (1 g of composition), the Si is more than required by the equation, and 0.9 g of pb304 is therefore considered to have reacted completely. 60.1 x 2 SiO2 formed = --------x 685.6
Heat of reaction = 0.9 x 1000 = -1429
1300-
l Heat evolved 0 Rate of heat evolved I Rate of burning
0.9 g per 1 g mixture
60.1 x 2 --xx++ 685.6
903.7 60.1
718.6 685.6
1
J/g of mixture
=” 1200z .L llOO:: :
685.6 x __ 0.9
-30
-1L
-20
-13
-26
-12,
-24
-112
-22 2
-lo.
“u lOOO-
Heat evolved mole of Pb304 = -1429
-15
G
-9
0l ; 900a
-8 -7
= 1089 kJ -6 -5
DISCUSSION
z -18 .c” z = -16 2 2 2 -IL 6 z al (u -12 2 rY z m -10
-4
Pb304/Si compositins containing from 5 up to 50% Si have been examined; those containing more than 50% Si failed to maintain sustained reliable combustion. This was particularly true of the coarser grade (silicon C). From the graph of heat generated per g of composition against silicon content (Figure 3), this value increases with decrease of Si, reaching a maximum around 10% Si and then dropping sharply. This is of the same pattern for the
600 -
Silicon
-8
-3
-6
-2
-L
-1
-2
A twt%)
Heat evolved, rate of heat evolved and rate of burning Figure 4 versus amount of silicon A
FUEL,
1979,
Vol
58,
February
141
Pyrotechnic reaction of silicon and red lead: S. S. Al-Kazraji and G. J. Rees
l
Heat evolved 0 Rate of heat evolved l Rate of burning
-9
-18
-6
-16
700 -
-2
-4
600 -
-1
-2
0I ; a
900-
2 L P Q) 600‘;; I”
I 10
500’ 0
I I 30 LO Silicon B I wt%)
I 50
I 20
lOJ6
Figure 5 Heat evolved, rate of heat evolved and rate of burning versus amount of silicon B
I
1
I
I
I
Heat evolved Rate of heat evolved Rate of burning
-16
-16
with the finer Si there is closer agreement between experimental and calculated theoretical values for heat evolved. Reactions of mixtures containing 5, 10 and 15% Si were violent, and high pressures, inside the aluminium tubes, led to bursting. These compositions have no practical importance and their rates of reaction are very dependent on the pressures formed. Reaction will depend on the diffusion of vaporized oxides into the surface of the silicon and products. These reactions are different in nature from those where a large excess of Si (20-50%) is present. In the latter case solid/solid mechanisms prevail; the surface area of Si and the good fuel/oxidizer contact (controlled by the pressure in the pelleting process) are the controlling factors. Excess Si, which is unreacted, will act as a heat sink as it melts and hence keep the temperature at 1410°C until all the Si is molten. As the Si content approaches 15%, vaporization of lead at 1750°C and of unreacted lead oxides occurs. The heat generated from the reaction can be compared with the experimental results. Theoretically, stoichiometry occurs at 7.6% Si, and the experimental maximum heat generated was at 1% Si. X-ray diffraction and infrared analysis confirm the presence of elemental lead, fused silica and unreacted silicon. When the amount of heat liberated by a weight of mixture which contains one mole of Pb304 is plotted against % Si (Figure 7), it increases sharply as Si increases from 5 to 1% and reaches a maximum at 15%, staying constant up to about 25% Si. This, according to the treatment of Spice and Staveley3, indicates that there is a single reaction in this range (5-30% Si), and that the maximum at 15% Si is the point where the reducing agent uses up all the oxidizing agent. The reaction of Si with F’b304 can be considered to take place in two stages as indicated in a previous paper. First is the decomposition of Pb304 and then the reaction of Si with PbO and oxygen:
-3
Pb304
+ 3
PbO + ?402
(1)
Si + 02 + SiO2
(2)
2PbOtSi+SiO2+2Pb
(3)
From equation (I), oxygen gas forms 25% of the total amount of oxygen available for reaction.
-2
r
1200 -1
0: llOO-4
e” 2 lOOO-
600 I
-2
3 900-
5001 0
’ 10
I I I 20 30 LO Silicon C (wt%)
I 50
-0
-0
; r” 600.-5 z z! 700E 600
Figure 6 Heat evolved, rate of heat evolved and rate of burning versus amount of silicon C
142
FUEL, 1979, Vol 58, February
0
I 5
I
I
I
I
I
I
I
10
15
20
25
30
35
40
Silicon Figure
7
Heat evolved/mol
I wt
%)
Pbs04 versus % Si
45
Pyrotechnic reaction of silicon and red lead: S. S. Al- Kazraji and G. J. Rees
For compositions containing relatively large amounts of Si, thermogravimetric experiments have shown loss of some of the oxygen, indicating that the reaction is mainly of solid/solid nature. For compositions with a small amount of Si. the temperature reached was so high that some of the PbO was vaporized, and in this case the solid/solid reaction will not predominate. For a large excess of Si, above 30%, the reactions are incomplete owing to the presence of some Si particles not in contact with Pb304. Also it is very likely that the Si and 02 reaction is incomplete because of the high temperature needed, which may not be attained at large excesses of Si.
helpful discussions. We also thank the Science Research Council for the granting of a Cooperative Award in Science and Engineering (CASE) research studentship to one of us (S. S. Al-Kazraji), and R. Eather and J. Thompson, of the Royal Armament Research and Development Establishment, for advice on the firing circuit. Thanks are also due to D. Lea and associates of the Department of Electrical Engineering, Polytechnic of Wales for the construction of the firing circuit.
ACKNOWLEDGEMENTS
1 Al-Kazraji, S. S. and Rees, G. J. Combust.Flame 1978,31, 105 2 National Bureau of Standards, Technical Notes 270-3, Jan. 1968 3 Spice, J. E. and Staveley, L. A. K. J. Sot. Chem. Ind. 1949,68, 348
The authors thank Nobel’s Explosives Co. Ltd for financial support and materi::ls, and J. Morman and T. McLaughlin for
REFERENCES
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