Fundamental knowledge of the processes occurring in the reaction between selenium and barium peroxide should lead to ability to produce more closely timed delay fuses. These fuses can be used more effectively and safely in mining where successive detonation of charges is necessary
I
LEWIS B. JOHNSON, Jr. Research Laboratories for the Engineering Sciences, University o f Virginia, Charlottesville, Va.
Burning Selenium and Barium Peroxide Powders Apparatus and Procedure The reaction between gray selenium and barium peroxide powders takes place b y three processes. The effects o f the first can be measured, after the powdered mixture i s heated to temperaiures as low as 50" C. for as little as 32 hours, as a slight lowering of the heat output and activation energy and an increase in burning rate. They are believed to be caused b y an increased area of contact between the reactants brought
FOR
exothermic reactions, velocities often must be controlled within narrow limits and, therefore, a basic knowledge of mechanisms and kinetics is important,
about by the preheating. The second process appears to be a slow, exothermic, product-forming reaction between the materials, which is not self-sustaining. Its effects are a considerable decrease in the eventual heat output and burning rate and an increase in the activation energy. The third i s the ignition reaction, which begins at about 265" C., is strongly exothermic, and i s self-propagating.
For the reaction between selenium and barium peroxide powders, this report presents evidence for the presence of three distinTuishable processes.
All calorimetric determinations were made with a Parr Model 1401 peroxide bomb calorimeter. The general instructions for its use were followed, except that sodium peroxide was not added, because the heat powder sample itself supplied the oxygen necessary for combustion. The samples consisted of 15.00 grams of heat powder (selenium and barium peroxide) and were fired with an electric hot wire fuse. The apparatus used for burning-rate determination consisted of t w u thick circular metal plates, the lower section of which contained a 26-cm. groove, 0.5 cm. wide, machined on the arc of a circle 10 cm. in diameter. The upper section contained a protrusion which fitted snugly into the groove in the lower wction. A radial hole at one end of the 25-gram groove permitted iqniticrn sample of heat powder was placed in the groove, the upper plare was fitted in A\
VOL. 52, NO. 3
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241
Along with the samples of mixed heat powders, separate samples of the metal powder and the oxidant were given identical treatments, to determine whether the effects noted were due to evaporation, atmospheric oxidation, or a reaction between the metal powder and the oxidant. Samples were mixed and tested after preheating of individual components. All samples were prepared from the same batches of barium peroxide and selenium. The barium peroxide had a purity of about 90y0 ; the major impurities were barium carbonate and hydroxide. The selenium was the gray, metallic powder and had a purity of 98%. Although the impurities may have had some effect upon the individual burning properties of the samples, it is believed that this effect was canceled by using the same materials throughout the investigation.
Figure 1. Calculated and experimental values o f heat expected from formation of end products agreed very well
place, and the apparatus was compressed to 10,000 pounds total force in a hydraulic press. I t was then removed from the press and one end of the train was ignited with a hand torch. Flashes of light from two holes drilled 25 cm. apart along the length of the groove were used to start and stop a timer to obtain the burning time for the 25-cm. length of the powder train. A modification of the apparatus described by Henkin and McGill ( 7 ) was used to measure the activation energies of the heat powders. A 1-gram sample of the powder to be tested was placed in a thin-walled copper tube 8 cm. long and 7 mm. in diameter. The tube was then plunged about 4 cm. deep into a molten lead bath whose temperature was measured by a sheathed, Chromel-Alumel thermojunction. .4 stopwatch was used to measure the time required to ignite samples at various temperatures. A plot was made of the logarithm of the ignition time, t , against the reciprocal of the absolute temperature, T. From the relationship : Loget
=
E/RT
Results Thermodynamics and stoichiometry of the heat powder system were studied using calorimetric values determined as a function of composition. Then, using data from thermodynamic tables (4, heat expected from the formation of various end products was calculated (Figure 1). For kinetic studies, samples containing 15 and 85 weight of selenium and barium peroxide, respectively, were used, which had been preheated for 2,8, 16, 32, 64, and 128 hours at 50°, loo', l j O o , and 200' C. However, results for all heating times agreed qualitatively; therefore, only data for 32 hours are presented here. The weight loss of barium peroxide was constant at about O.lyc,while the loss of selenium increased with tempera-
I
I 7 0 0 4 0 I0OL
I
+ constant
the activation energy, E, was obtained by multiplying the slope of the line obtained by the constant, R. The differential thermal apparatus consisted of a vertical-tube furnace equipped with a temperature equalizer and two temperature recorders. The first recorder measured the sample temperature; the second, the differential temperature. The vertical tube was 7 cm. in diameter. -46.5-cm. cylindrical nickel rod, 15 cm. long, was centered in the tube and served as a temperature equalizer. T w o holes of I-cm. diameter and 7-cm. length were drilled in the top of the nickel block equidistant from the center. These holes contained the sample and reference material. The reference material was barium peroxide. The sample temperature was recorded on a 0- to 100-mv. Brown strip chart re-
242
corder, Model 153X 12-V-II-III-30-A3. The differential temperature was recorded on an adjustable-zero, adjustable-span, Brown strip chart recorder, Model S Y 153X18-(VA)-II-I11-(6). A 0- to 1-mv. span was used. The 35-ohm furnace coil was heated by 110-volt, 60cycle current which gave a linear heating rate of 5' C. per minute over the temperature range used. A standard procedure was developed for handling and testing the heat powder samples. The metal powders and oxidants were stored in a desiccator over calcium chloride. Samples were weighed and mixed in a dry room where the relative humidity was 7 to 10% at 75' F., placed in aluminum pans, and heated for different lengths of time at different temperatures in a drying oven. They were then cooled in the dry room and portions Xvere weighed for the tests.
I
I
:j
0 -heal
2 0 020 5 0 u ,
:
0
,
,
,
outpit
,
g-burning rote
A-activation
25
50
75 Preheat
0C
125
Temperature
150
,
,
energy
175
200
1°C)
Figure 2. Preheating lowered heat output values for a mixture of selenium and barium peroxide but had little effect on activation energy
INDUSTRIAL AND ENGINEERING CHEMISTRY
SELENIUM A N D B A R I U M P E R O X I D E ture. M'ithin experimental error, the weight loss of the heat powder mixture was the sum of the losses of the constituents. In no instance was the weight loss of either constituent or the mixture greater than lY0. h-o deficit in heat output was caused by preheating the separate components, but there was a change caused by prcheating the mixture. The value for the unheated mixture of 1570 selenium and 85% barium peroxide was 89.5 kcal. per mole; preheating caused the values to be lower. The changes are sholsn in Figure 2. The burning rate of the unheated mixture was about 0.32 cm. per second. I t increased with preheating temperature up to about 100' C., reached a maximum value of about 0.40 cm. per second for samples heated to 100' C., then gradually decreased for samples heated to h gher temperatures. Preheating had little effect on the activation energies of the mixtures. However, a slight decrease in activation energy was found for samples heated to about 100' C., an increase, for ones heated to higher temperatures. Each point obtained was the average of five runs having a mean deviation of + l o % . Each line was drawn using eight points which covered a temperature range from 340' to 460' C. Results of thr differential thermal analyses are summarized in Figure 3, which shows a composite of the curves for all mixtures shown in the table. .4 measurable heat output by the sample began at about 190' C. and increased steadily until the sample fired at about 265' C. A small endotherm at about 215' C. was due to the melting of selenium in the sample. There was a slight variation of firing temperature, depending on the amount of preheating the sample had undergone. This is not shown on the graph, but approximate firing temperatures are listed in the table. In general, the more the samples were preheated, the higher the eventual firing temperature.
Discussion If the samples of heat powder were fired in an inert atmosphere, two reactions occurred :
+ 3BaOz = Base04 + 2 B a 0 2Se + 2Ba02 = BaSe04 + Base Se
(1) (2)
The stoichiometry of Reaction 1 is satisfied a t a 1 to 3 mole ratio of the reactants and has a calculated heat output of 95.3 kcal. per mole, or 162 calories per gram. The stoichiometry of Reaction 2 is satisfied at a 1 to 1 mole ratio of reactants and has a calculated heat output of 121 kcal. per mole, or 244 cal. per gram. The weight ratios for the above reactions
'O0I
I
/
I
/
t " 0
/
/
-05
- 0-3
a
16
-0.1 Di f f e re n t i a I
24 Time
01 (m v.)
0.3
0.5
32
40
48
(rnin.)
Figure 3. Measurable heat output began at 190" C. and increased steadily until the sample fired at 265" C.
are 13.45 Se to 86.55 BaO? and 31.80 Se to 68.20 BaOn, respectively, for Reactions 1 and 2. The 10% difference between the experimental and calculated curves over the first 2070 composition can probably be attributed to impurities in the selenium and barium peroxide. This is not true of the increasing discrepancy past the Z0y0 composition value. Rough measurements of peak temperatures reached during the reaction showed that mixtures containing up to 15y0 selenium attained temperatures of 500" to 600" C., while those containing more than 15% selenium reached 600' to 1000' C. U p to about 15 or 20y0 selenium, the fired mixtures probably do not attain a temperature high enough to vaporize the selenium (boiling point 688' C.), For these mixtures, the reaction goes essentially to completion. The higher temperatures attained during the firing of samples containing more
than about 20cG selenium vaporize part of the selenium. Some of the vapor evidently distills away from the barium peroxide and the reaction does not proceed to completion. Experimental evidence verifying this is a coating of red amorphous selenium found on the top and sides of the calorimeter cup after firing the latter samples. I n an abundance of oxygen, the barium selenide formed in Reaction 2 would proceed to barium selenate. Although the atmosphere in the calorimeter was air in all instances, the free space in the calorimeter was only about 15 cc. or about 3 cc. of oxygen. For mixtures capable of using oxygen, this would yield an additional 9 cal., or, for a 15gram sample, about 0.5 cal. per gram. Because of the closer agreement between calculated and experimental values for the mixtures near the 1 to 3 mole ratio mixture, it was believed advisable to use this mixture for kinetic studies. VOL. 52, NO. 3
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Effect of Preheating Heat Powder Samples upon Weights, Calorimetric Values Burning Rates, Activation Energies, and Firing Temperatures
(Preheat time, 32 hours) Preheat Temp.,
c.
Sample Unheated mixture Se4 BaOlb BaOZa Se6 15% Se 85% Ba0ZC Sea BaOzb BaOz Seb 15% Se BaOzC Se5 BaOzb BaOP Seb 15% Se 85% BaOf Sea BaOlb BaOP Se6 15% Se 85% B a 0 f
+
+
+ + + + + + + + + +
Preheated.
Unheated.
... 50 50 50 100 100 100 150 150 150 200 200 200
Weight Heat Burning Change, Output, Rate, % Kcal./Mole Cm./Sec.
Energy,
Firing Temp.,
89.5 89.5 89.5 88.9 89.5 89.5 88.4 89.5 89.3 84.8 88.9 89.5 77.5
5.3
265
...
0.00 -0.10 -0.10 0.00 -0.10 -0.10 -0.10 -0.10 -0.18 -0.80 -0.11 -0.98
0.32 0.33 0.32 0.34 0.32 0.32 0.40 0.31 0.33 0.35 0.32 0.32 0.27
Kcal./ Mole
... ... 5.0 ... ... 4.6 ... ... 4.9 ... ... 5.9
"C.
... ...
265 265
... ... 270 ... ... 285
Preheated together.
However, to avoid occasional failures of the 1 to 3 mixture to fire, it was necessary to fire a mixture containing a slightly higher percentage of selenium. The 15 weight yo mixture was used, therefore, throughout the kinetic studies. The fact that the weight loss of barium peroxide was the same at all preheat temperatures indicates that it was due io the evaporation of a small amount of water from the material. The weight loss of selenium, on the other hand, increased with temperature and must have been caused by an actual loss of metal powder, due to evaporation of selenium in absence of oxygen. I n the presence of oxygen, it occurs by surface oxidation of the metal, followed by evaporation of the oxide. The heat output of the unheated powder was 89.5 kcal. per mole. Values for the preheated materials were lower than this only when the constituents were heated together. The maximum deficit in heat output was about 15Yc, while the maximum weight loss was only about 1%. Therefore, the heat output deficit was not caused by evaporation of seleniumfrom the sample. Furthermore, the differential thermal analyses showed a slow evolution of heat beginning at about 190' C. and gradually increasing with temperature up to the firing temperaturr of about 265' C. These facts show that there is a preignition, heat-producing reaction between the selenium and barium peroxide. This reaction can be stopped by removing the heat source at any time prior to reaching the firing temperature. I n other words, the preignition reaction is not self-sustaining. However, because the heat output is a thermodynamic quantity and depends only upon the initial and final states of the constituents, it follows that the p-eignition reaction is a product-forming one. 'The formation of a reaction product by
244
Activation
the preignition reaction would, in effect, dilute the original heat powder mixture with an inert material. The addition of an inert diluent to a heat powder mixture invariably decreases the burning rate of the mixture. However, preheating to about 100' C. for 32 hours actually increased the burning rates of the mixtures. Here again, the effect was noted only when the constituents were heated together. Therefore, a mechanism must have been present which increased the burning rate while it decreased the heat output slightly. Numerous variables affect the burning rate of a heat powder mixture. Among these are area of contact between the constituents, active impurities, inert impurities, compaction of the sample, pressure exerted by gases formed or present during the reactions, and quenching by the carrier. By using the same apparatus and techniques throughout, and by testing samples of the individual components along with samples of the mixture, it appears that all effects have been canceled except an increase in the area of contact between the constituents. Huttig, Worl, and Weirzer ( 3 ) studied the solid-state reaction between the oxides of zinc and aluminum to form ZnAlz04. They showed that thr first step in the rraction was the formation of a coating of zinc oxide on the surface of the aluminum oxide. Such a mechanism would bring larger surface areas of the reacting species in conrace and thereby cause the burning reaction to take place more rapidly. Selenium atoms may migrate to form a coating on the barium peroxide at temperatures below the ignition point. Further work is necessary to determine whether this is a solid-state migration or occurs in the vapor phase. There is, of course, a limit to the effect brought about by increasing the area of contact between the reacting species.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Once all exposed surfaces of barium peroxide are covered with selenium, the burning rate decreases rapidly with increasing amounts of reaction products. There is still some question as to whether the slow, product-forming, exothermic reaction below 265' C. is the same reaction as the self-propagating reaction which occurs above this temperature. Attempts were made to analyze the products of both reactions by x-ray diffraction. Two ashes were prepared, one by ignition of a mixture and the other by heating a powder mixture to 200' C. for 1000 hours. (The latter sample could not then be ignited by any means.) X-ray diffraction patterns for the two ashes appeared identical, but the lines were not distinct enough to be absolutely conclusive. If the products of the slow reaction are the same as those of the rapid reaction, there must be a reason why some atoms and molecules react at the lower temperatures. Work by Hill and Wallace ( 2 ) and by Spice and Stavely ( 5 ) has shown that at least some reactions of this general type, regardless of their rates, occur in two distinct steps. Hill and Wallace, using the temperature profile method, found that the reaction between molybdenum and potassium permanganate, going to completion in about 1 second, had a first step at about 120' C., with the rest of the reaction occurring above this temperature. The peak temperature was about 850' C. The authors considered that molybdenum was mobile because of its small size and suggested that the first step was diffusion of the molybdenum into The permanganate along grain boundaries and the second step was penetration of the permanganate lattice by the molybdenum. Spice found similar steps in slower reactions, the ones studied being iron plus potassium dichromate and iron plus barium peroxide. Such a step as that found by the above authors may exist for the system composed of selenium and barium peroxide, the temperature at which lattice penetration occurs being about 265' C., which is the temperature at which the reaction becomes self-propagating. Further work is necessary to show whether this is true. literature Cited (1) Henkin, H., McGill, R., IND.ENG.
CHEM.44, 1391-5 (1952). (2) Hill, R. A. W., Wallace, A. A., Nature 178, 692-3 (1956). (3) Huttig, G. F., Worl, H., Weitzer, H. H., Z. anorg. u . allgem. Chem. 283, 207-16 (1956). (4) Rossini, F. D., others, "Selected Values of Chemical Thermodynamic Properties,'' National Bureau of Standards, 1947-50. (5) Spice, J. E., Stavely, L. A . K., J . Cirem. Sac. Znd. 68, 313 (1944).
RECEIVED for review July 17, 1959 ACCEPTEDOctober 29, 1959
