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331
EXPLOSIVE OXIDATION OF PENTABORANE
partial ordering undergone by the alloy either during the quench from the disordering temperature of 873" K., or during its dissolution period in the liquid tin. The reason for the latter possibility is that in Hultgren's earlier work, the quenched alloy a t 50" is dropped into liquid tin at 427', so that as the alloy is dissolved into the tin a t any one moment the as yet undissolved alloy is being heated through a range of temperatures a t which the rate of ordering is rapid.13 Although Hultgren's earlier data for the heat of formation of disordered Auo.&uo.6 are not correct, there is no reason to suspect his value of AHf for the ordered (face-centered tetragonal) Au0.6Cu0.6 of -2150 cal./g. atom. Hence, if we use his value for the ordered alloy and the present value for the disordered alloy, we find an energy of ordering of (-865 i 40) cal./g. atom. However, it is not known how this energy is apportioned between the two reactions that occur at this composition, namely, disordered alloy orthorhombic superlattice f.c. tetragonal superlattice. It is intended to examine this situation in detail. D. Solid Silver-Copper Alloys.-For the measurement of the enthalpy of formation of solid silver-copper alloys it was found necessary to operate the calorimeter a t a higher temperature than for the alloys already discussed, because the mutual solubility a t lower temperatures is quite limited. A temperature of 622" was chosen, even though this permits the investigation of only a small range of composition, in order to avoid whatever troubles may arise a t higher temperatures as yet unexplored by this calorimeter. The results of a series of differential determinations are listed in Table I, and are shown in Fig. 6. There do not exist any other data with which to compare these results except the point on Fig. 6 by Hultgren and Orr14 with which the present data are consistent.
700
-
600 -
500
-
-
-
(13) G . J. Dienes, Acta Met., 3, 549 (1955). (14) R. Hultgren and R. L. Om, private communication.
THE EXPLOSIVE OXIDATION OF PENTABORANE BY HARRY C. BADEN, WALTERH. BAUERAND STEPHEN E. WIBERLEY Walker Laboratory, Rensselaer Polytechnic Institute, Troy, New York Received October 17, 1967
T h e explosive reaction of pentaborane and oxygen has been investigated. The lower explosion limit for mixtures of pentaborane and oxygen on rapid mixing was studied as a function of mixture composition a t 15 and 21.5' and from 1 to 90 mm. pressure of pentaborane. Explosion pressures increased with decrease in temperature when clean vessels were used. Limits were found t o be higher in vessels coated with oxidation products than in clean vessels. I n coated vessels the explosion limit was lowered with increased vessel size, but no temperature dependence was found for such vessels. Addition of nitrogen lowered the pressures at which spontaneous explosion took place, while addition of diborane slightly inhibited the reaction. When carbon monoxide was added, erratic inhibition of the explosion occurred. Addition of 1% of iron carbonyl to pentaborane caused total inhibition of the oxidation a t 25'. When 0.1% of iron carbonyl was added to the pentaborane, both lower and upper temperaturepressure explosion limits were found for pentaborane and oxygen mixtures.
Because of the high energy release, the rapidity of reaction and the high flame speeds developed on oxidation, the boron hydrides offer a fruitful field for study. Since pentaborane, the most stable of t,he volatile boranes, is a liquid at room temperain hydrocarbons, it has advantages ture and for practical applications. However, there are
some properties of this compound which have limited its use. Pure pentaborane may ignite spontaneously on mixing with air under some conditi0ns.l A study of the kinetics and mechanism of the oxidation of pentaborane was undertaken in (1) D. T. Hurd, "An Introduction t o the Chemistry of the Hydrides." John Wiley and Sone. Inc.. New York. N. Y.,1952.
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H. C. BADEN,W. H. BAUERAND S.E. WIBERLEY
I I
I
0
I
' 6 4 c n ~ C o a k dBulb
I
10 20 OXYGEN PRESSURE,rnrn Hg.
I
30
i
Pig. 1.-Composition explosion limits for pentaboraneoxygen mixtures in clean vessels and in vessels coated with reaction products, a t 16' except as noted otherwise.
the hope that conditions could be established for the most efficient use of this reaction. Simultaneously, a study of the explosive oxidation of diborane was undertaken2v4 in order to gain some knowledge of the mechanism of boron hydride combustion in general. A kinetic study of the pentaborane-oxygen aystern4 showed the existence of an explosion limit a t very low pressures and a t room temperature, implying the presence of a branching chain reaction. Investigations of flame spectra in the oxidation of pentaborane were made by Berl and co-workers,6 showing the presence of Bz03 in the explosion diffusion flame. Experimental Materials.-Pentaborane was prepared by the pyrolysis of diborane.6-8 After purification, various batches were assayed, by the cryoscopic m e t h ~ d to , ~ be 99.7 to 99.9% pure. The main pentaborane supply was maintained a t --20",except for short periods of time when small amounts were transferred to a vacuum line. These portions were kept frozen by Dry Ice slush, in a storage limb off the vacuum manifold. Infrared and mass spectrographic analysis of (2) W. Roth and W. H. Bauer, "Fifth Symposium on Combustion," Reinhold Publ. Gorp., New York. N. Y.,1955. (3) W. Roth and W. H. Bauer, THISJOURNAL, 60,639 (1956). (4) F. P. Price, J . A m . Chem. Soc., 73, 2141 (1951). (5) W.G. Berl, E. T Gayhart, H. L. Olsen, H. P. Broida and K. E. Shuler, J . Chem. P h y s . , 26, 797 (1956). ( 0 ) L. V. McCarty and P. A. DiGiorgio, J . A m . Chem. S O C . , ' 3138 ~~,
(1951). (7) A. Stock, "Hydrides of Boron and Silicon," Cornel1 University Press, Ithaca, N. Y..1933. (8) A. Burg and H. Schlesinger, Chem. Reus.,31, 13 (1942). (9) P. D.Zemany, Anal. Chsm.. 24, 348 (1952).
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this material failed to indicate the presence of any impurities. Matheson prepurified oxygen was used throughout the course of the study. The gas was admitted t o the vacuum line and was condensed in a trap at liquid nitrogen temperature. The middle third fraction of the liquid oxygen was allowed to vaporize and kept in a two-liter storage bulb for subsequent use in the oxidation experiments. Iron pentacarbonyl was obtained from the General Aniline and Film Corporation. A small amount of the liquid was introduced to the vacuum line, and alternately distilled and frozen three times before being frozen in a limb off the manifold. This limb was wrapped with black paper t o exclude all light. Apparatus.-A Pyrex glass vacuum system was employed for handling and storing materials. The vacuum manifold was fitted with gas storage bulbs, freeze-out limbs for holding pentaborane, and taps for the inlet of gases and for drying cleaned explosion bulbs. All joints and stopcocks were greased with Apiezon N, except those in the explosion apparatus where Dow Corning Silicon High Vacuum Grease was used. The reactants were mixed in spherical Pyrex explosion bulbs 2.0, 4.4or 6.4 cm. in diameter similar to those used by Price.* These bulbs were fitted with a capillary stopcock and standard taper joint to allow rapid exchange of used bulbs for clean bulbs. The explosion apparatus consisted of the explosion bulb, connected through a ground joint to a small calibrated volume, and a measuring pipet. Capillary lines led to the measuring manometer and manifold, the Bodenstein gauge, the measuring pipet, and through a capillary leak t o the vacuum manifold. The explosion apparatus could be completely submerged in a constant temperature-bath, which could be raised and lowered as desired. By means of this bath, the temperature of the reactants could be maintained to f 0 . 1 ' in the range -10 to 100'. Operating Procedure.-When an explosion test was made, the constant temperature-bath was raised around the explosion apparatus, and pentaborane was introduced to the explosion bulb and to the line to the manometer. The pressure was recorded, the stopcock to the bulb closed, and the excess entaborane re-collected in the storage freeze-out limb. $he measuring pipet and connecting lines were then evacuated and a pressure of oxygen sufficient to give the final desired pressure of this component in the explosion bulb was allowed to enter the pipet and the line to the measuring manometer. The oxygen pressure was recorded, the stopcock t o the pipet closed, and the apparatus again evacuated. The stopcock connecting to the manifold was then closed and the stopcock to the pipet opened. Manipulation of the air pressure above the mercury well compressed all the oxygen into the small calibrated volume above the explosion bulb. The stopcock on the explosion bulb was then opened to the calibrated volume, allowing oxygen to enter the bulb. As the pressure of oxygen was always much greater than the pressure of the pentaborane in the bulb, entry and mixing were very rapid. When explosion occurred, a bright flash, easily visible through the transparent-bath, was seen.
Experimental Results First Explosion Limit at Room Temperature.When oxygen was added rapidly to pentaborane a t 21.5", explosion resulted above low critical pressures. Results obtained with an explosion bulb of 4.4 cm. diameter, show limits falling between those obtained for bulbs of 3.7 and 6.62 cm. diameter by Price,4 a t higher pressures of pentaborane. At lower pressures, below 2 mm. of pentaborane, the pressure limit curve obtained crossed that found by Price. Since the pressures involved in the explosion range studied by Price were SO low, below 5 mm., the temperature a t which explosion tests were made was lowered to 1.5'. Composition Limits at 15'.-When oxygen and pentaborane were mixed rapidly at 15", the explosion region was shifted t o higher pressures as shown in Fig. 2. Results of a typical set of explosion tests are plotted in Fig. 3 in detail. Al-
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EXPLOSIVE OXIDATIONOF PENTABORANE
333
though explosive and non-explosive regions were clearly defined, occasional erratic results were obtained. The shift of the explosion limit with increasing vessel size reported for room temperature in clean bulbs was also found a t 15'. However, the limit was found to be temperature sensitive. The limits were shifted to higher pressures with decreasing temperature, as shown in Fig. 1. Effect of Oxidation Product Coating.-The explosion of pentaborane-oxygen mixtures produces a deposit on the vessel surface. When repeated explosions were made in a single vessel, it was found that the pressure explosion limit curve was shifted to higher pressures. Reproducible limits were found which were shifted to much higher component pressures than those found in clean vessels of the same size, as is seen in Fig. 2. The limits obtained in coated bulbs also exhibited dependence on vessel diameter, but no temperature dependence was observed. Effect of Iron Pentacarbonyl on Explosive Reaction of Pentaborane and Oxygen.-It was concluded from the existence of a clearly defined lower explosion limit that the oxidation of pentaborane was initiated by a chain mechanism, as was found to be the case for diborane. Chain-breaking additives might therefore be expected to exist. It was found that addition of dibornne inhibited the explosion only slightly, and that carbon monoxide PRESSURE OXYGEN mmHg. produced an erratic inhibition of the explosion. Fig. 2.-Typical composition explosion limit data a t 15" Since it has been reported that iron pentacnrpentaborane-oxygen mixtures in 4.4 cin. Pyrex bulb bonyl, when added to liquid pentaborane, inhib- for ited the tendency of the borane to ignite spontane- coated with reaction products. ously on exposure to air, the effect of this carbonyl on the composition limits was investigated. The iron pentscarbonyl was added t o pentaborane iii a storage bulb, a t a concentration of 1% by volume, and samples of this mixture were transferred to the explosion bulb. Oxygen was then added and the result noted. These determinations were made at 24 f 1'. No explosions mere found in clean bulbs of either 4.4 or 6.4 cm. diameter with partial pressures of up to 140 mm. for pentaborane and 80 mm. for oxygen. When the concentration of iron pentacarbonyl was decreased to 0.01% of the pentaborane, explosions occurred at this concentration of additive when oxygen was added. Although the experiments with added carbonyl were made at room temperature, the pressures required for explosion were only slightly higher than those in the same bulbs a t 15' with no additive. A concentration of 0.1% added iron pentacarI I bony1 was selected for a more intensive survey of 25 45 65 85 the effect of this additive. It was discovered that TEMPERATURE OC. warming a bulb containing the unexploded mixture Fig. 3.-Temperature-pressure explosion limits for pentaof oxygen, pentnborane and iron pentacarbonyl was borane-oxygen, mixture ratio 1 to 3, with 0.1%iron carbonyl, sufficient to cause explosion. This phenomenon in clean Pyrex bulbs 4.4 cm. in diameter: x, explosion on hmting; 0, explosion on withdrawal; G, glow on mixing; E, suggested the possible existence of a set of pressure- explosion on mixing; and N, no reaction on mixing. temperature explosion limits; therefore, a constanttemperature hot oil-bath was introduced to the sys- occurred. Pressure was followed by means of a tem t o determine these limits. All tests in the Bodenstein quartz spiral gauge. The results of this study of this limit were made in clean bulbs 4.4 cm. study, given in Fig. 3, show the presence of an updiameter, and with a ratio of 302 to 1B5H9by vol- per and lower explosion limit for pentaborane-oxyume. Mixtures were made a t room temperature, gen mixtures in the presence of iron pentacarbonyl. and the temperature was increased until explosion The lower limit changes very little with tempera-
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W. P. SLEHTER AND ELAINE R. MANDELL
ture. The upper limit is more temperature sensitive, rising slowly a t first, but becoming almost vertical between 85 and 95’. At approximately 30’ and at mixture pressures near 10 mm., a glow reaction was noted, similar t o that described by Price4 for mixtures of pentaborane and oxygen without iron pentacarbonyl.
Discussion The results of this investigation support the conclusion of Price4 that the reaction of pentaborane and oxygen is of the branched chain type. No chain carrying intermediate has been isolated but the inhibiting effect of small amounts of iron carbonyl clearly indicates that oxygen atoms may be involved in the chain mechanism. The well defined lower composition explosion limit is sensitive to both vessel diameter and coating with reaction products. As the limit is found at very low pressures in clean vessels, the glass must have a comparatively low chain-breaking efficiency. In coated vessels, the limit occurs at considerably higher component pressures, indicating a greater capacity for chain breaking. Although the addition of nitrogen gas had little effect on the limit observed in d e a n bulbs, in coated vessels the addition of nitrogen lowered the explosion limit pressures. This would be expected if the nitrogen molecules hindered diffusion of chain carriers to the surface, thus decreasing the rate of destruction. The solid coating formed on explosion varied from a brown color when pentaborane rich mixtures were exploded to a white color when oxygen rich mixtures were used. All white solids were soluble in methanol or ethanol. While the brown solid was not completely soluble in these solvents, it did dissolve in either dilute or concentrated nitric acid. Infrared analysis showed only the presence of boric acid. Some boron hydride
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polymers are yellow solids or oils, which may account for the yellow coloration observed in some experiments. The niajor portion of the brown solid is believed to consist of elemental boron and boric acid. The total inhibition of the spontaneous explosion of pentaborane a t room temperature is notable. Iron carbonyl is presumed to inhibit oxidation chain reactions by the destruction of active oxygen species. lo Whether these are oxygen atoms or oxygen-containing intermediates was not determined. The upper temperature-pressure explosion limit exhibited when only 0.1% of iron carbonyl is present is thought to be due to increase of chain breaking in the gaseous phase at higher pressures, sufficient to lead to inhibition of the explosion when combined with the action of iron pentacarbonyl. The failure to thus far locate an upper temperaturepressure explosion limit for pure pentaborane-oxygen mixtures need not eliminate the possibility of existence of such a limit. The method of test used by Price, and in this investigation, involves high flow gradients and high concentration gradients during the sudden mixing of oxygen with pentaborane. An upper limit may well be located under different experimental conditions. The partial oxidation of pentaborane on slow addition of oxygen” indicates that speed of mixing may be an important factor in the initiation of explosive condit,ions. Acknowledgment.-Acknowledgment is made to Dr. C. C. Clark for helpful advice and suggestions. This work was carried out under the sponsorship of the Olin Mathieson Chemical Corporation. (10) B. Lewis and G. von Elbe, “Combustion, Flames and Explosions of Gases,” Academic Press, Inc., New York, N. Y., 1951. (11) H. C. Baden, S. E. Wiberley and W. H. Bailer, THIS JOURNAL, 69, 287 (1955).
MOLECULAR STRUCTURE AND MOTION I N IRRADIATED POLYETHYLENE1 BY W. P. SLICHTER AND ELAINE R. MANDELL Bell Telephone Laboratories, Inc., Murray Hill, New Jersey Received October 8% 1967
Molecular structure’and motion have been studied in polyethylene subjected t o high-energy irradiation from an atomic pile and from electrons. Measurements involved X-ray diffraction and proton magnetic resonance spectroscopy. Contrary to earlier findings, it is shown that high-energy irradiation inflicts change concurrently in the crystalline and the amorphous regions. Lattice defects are introduced into crystallites. The characteristic separations between chains .are altered in both the crystalline and the amorphous regions, and the two regions merge at higher dosages. With increaslng dosage, the irradiation causes progressively greater constraint to even small-scale motions.
1. Introduction It is well known that the physical and chemical properties of polyethylene are markedly changed by high energy irradiation. These changes occur from bombardment in nuclear reactors, from high energy electrons, and from y-rays. Evidently the changes involve several processes, including crosslinking, chain scission and double bond formation, (1) Presented at the 132nd Meeting of the American Chemical .Society, New York, September 9, 1957.
for which mechanisms have been proposed by a number of authors. Earlier studies dealt with highly branched polyethylenes. This paper reports on additional studies of the morphology of irradiated polyethylene, both branched and linear, and of the extent of chain motion in irradiated polyethylene. Two ranges of dosage were used: comparatively light dosage, of the sort which is of interest in polymer technology; and very extensive irradiation, which so thoroughly changes the mate-
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