The explosion limits of five alkylsilanes were determined as a function of temperature and composition at a pressure of 1 atmosphere. Over a fuel concentration range of 2 to 10 percent, the lowest temperatures (zero C) below which explosion did not occur for the five fuels studied were: tetramethylsilane (CHsub3)sub4Si, 450 degrees; trimethlysilane (CHsub3)sub3SiH, 310 degrees;dimethylsilane (CHsub3)sub2SiHsub2, 220 degrees; methylsilane CHsub3SiHsub3, 130 degrees; and vinylsilane Hsub2C=CH-SiHsub3, 90 degrees. Explosion limits for hydrocarbons analogous to these silanes fall in a temperature range of 500 degrees to 600 degrees C. Since the explosion temperatures of the alkylsilanes are lower than those of the hydrocarbons and since they decrease as hydrogen atoms are substituted for methyl groups, it was concluded that the Si-H bond is more readily susceptible to oxidation than the C-H bond

Mcdonald, Glen E

Schalla, Rose L

English

NASA Technical Reports Server

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RESEARCH MEMORANDUM 
EXPLOSION AND COMBUSTION  PROPERTIES O F  ALKYLSILANES 
I - TEMPERATURE-COMPOSITION LIMITS O F  EXPLOSION FOR 
hKETHYL-, DIMETHYL-, TRIMETHYL-, TETRAMETHYL-, AND 
VINYLSILANE AT ATMOSHPERIC PRESSURE 
By Rose  L.  Schalla  and  Glen E. McDonald 
Lewis Flight Propulsion  Laboratory 
C leveland , Ohio 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS -- - . .  
WASHINGTON 
Februarg 5,1954 
https://ntrs.nasa.gov/search.jsp?R=19930087991 2020-06-17T11:37:18+00:00Z
M C A  RM E53IO1 
NATIONAL ADVISORY C m T I P Z  FOR AERONAUTICS 
€SEEARCH ~0~ 
'EXPLOSION AID COMBUSTION P R O ~ I E S  OF ALKyLsILfLNES 
r - TEMpERIITuIIE-ccrrroN LIMITS OF EXPLOSION FOR 
VTNYLSPIANE AT AlcMOSpHERIC PRESSURE 
By Rose L. Schalla and Glen E. McDonald 
SUMMARY 
The explosion limits of f ive  alkylsilanes were determined as a 
3 
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function of temperature and composition at a pressure of 1 atmosphere. 
Over a fuel concentration range of 2 t o  10 percent, the lowest temper- 
atures (OC) below which explosion did not occur f o r  the   f ive  fuels 
studied were : tetramethylsilane (CH3)4Si, 450°; trimethylsilane 
(CE3) 3SiH , 310° ; dimethylsilane (CH3) ~ S m g  , 220' ; methylsilane CH3SiH3, 
130'; and vinylsilane H2C=CH-SiH3, 90°. Explosion l imi t s  for  hydrocar- 
bons analogous t o  these sllanes fall i n  a temperature range of 500° t o  
600° C. Since the explosion temperatures of the alkylsilanes are lower 
are s&st i tuted  for  methyl groups, it was concluded tha t   the  S i - H  bond 
is  more readily  susceptible t o  oxidation  than  the C-E bond. 
? than  those of the hydrocarbons and since  they  decrease as hydrogen atoms 
4 
Combustion properties of hydrocarbon fue ls  such as flame speed, 
flammability limit, igni t ion energy, and quenching aistance have been 
extensively investigated. For cer tain conibustion processes, it i s  de- 
s i rable  t o  employ fue ls  which have more favorable co&ustion character- 
i s t ics   than  the hydrocarbons, but which possess similar physical 
properties. A class of compounds which m i g h t  meet these requirements 
i s  the  a lkyls i lanes .  Consequently, several alkylsilanes were prepared 
a t   t h e  NACA Lewis 1EtboratOrY; their   physical   properties (such as boiling 
point,  melting  point , heat of combustion, etc.  ) are  reported in refer- 
ence 1. A t  the time these allqylsilanes were synthesized, the conditions 
under which they could be safely handled and s tored,   par t icular ly   in  
contact w i t h  air, were not known. The present report descrfbes a study 
directed towad establishing the condftions of temperature and concen- 
t r a t ion  which w i l l  permit safe handling of t h i s   c l a s s  of fuels.  
2 NACA RM E53L01 
Methyl-, dimethyl-, trfmethyl-, tetramethyl-, and vinylsilane were 
investigated in this study. Premixed fuel-air  mixtures, w i t h  the si lane 
concentration ranging from about 2 t o  10 percent by volume, were admitted 
t o  a heated, evacuated reaction vessel to a pressure of 1 atmosphere; 
and the lowest temperature a t  which explosion would occur was determined. 
The exploslon region of these five aUrylsilartes as a function of temper- 
ature and fuel-air composition was  therefore established. A comparison 
of the explosion region of the  alkylsilanes with the hydrocarbon com- 
pound which would resu l t  by replacing the s i l icon  atom with a cwbon 
atom was made wherever possible. 
Fuels. - The method of preparing the alkylsilanes used in t h i s  
study is described in reference 1. The tetrsmethylsilane had been puri- 
f i e d  from a commercially available product while the methyl-, dimethyl-, 
trimethyl-, and vinylsilmes had been  prepmed  by  reduction of the 
proper alkylchlorosilane wi th  lithium aluminwn hydride i n  anhydrous ethyl 
ether or dioxane. 
Procedure. - A diagram of the apparatus used f o r  determining the ex- 
plosion limits of the aUsylsilanes i s  shown i n  figure 1. Fuel and 
fi l tered air were admitted at the positions indicated and mixed in a 1- 
liter storage bulb by means of a magnetic stirrer. The pressure of the 
mixture i n  the  storage bulb was such that, when opened t o   t h e  heated, 
evacuated reaction vessel, the resulting pressure was I atmosphere. The 
temperature of the pyrex reaction vessel, 1- inches i n  diameter and 1 s  1 I 2 2 
inches i n  length, w a s  measured by a thermocouple attached to the  outside 
of the vessel. The relatively large diameter of the reaction vessel was 
selected t o  minwze  quenching effects.  
Admission t o  the  reaction vessel was usually made as rapidly a0 
possible although, 88 described later, the mixture was sometimes ad- 
mitted very slowly t o  determine the presence of cool flames. Occurrence 
of e,n explosion w a s  indicated by a strong surge of the mercury i n  the  
manometer, and occasionally a flme was seen t o  t r ave l  up the  tube lead- 
ing from the reaction vessel. Although such flames usually did not 
travel  very f a x ,  they did on several occasions flash back in to  the mixing 
bulb and cause a serious explosion i n  the line. Consequently, a roll of 
w i r e  gauze w a s  placed between the Mxhg bulb and the  reaction vessel t o  
serve as a flame trap. After this flame ar res te r  was installed, m 
further  trouble was encountered. 
The m i n i m  temperature for explosion w&s determined f o r  var ious  
f’uel-air mixtures of each given fuel. These minimum temperatures define 
the region of explosion and nonexplosion with respect t o  temperature and 
composltfon. 
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NACA RM E53L01 3 
Tetramethyl- and Trimethylsflanes 
Two distinctly  different  types of explosion  limits  were  encountered 
in  studying  the sllanes. The first  type,  typical of those  reported for 
hydrocarbons  at a pressure of 1 atmosphere, was found in this  investiga- 
tion  for  tetramethyl- and trimethylsilane.  For  both  tetramethyl- and 
trimethylsilane,  the  fuel-air  mixture  was  admitted  rapidly  to  the  evacu- 
ated  reaction  vessel  to a pressure of 1 atmosphere, and at a sufficiently 
high  temperature an explosion  occurred.  The explosion usually  occurred 
within 30 seconds  after  admission. If no rapid  reaction  took  place in 
this  length of time,  only a very slow increase  in  the  pressure was ob- 
served and no explosion  was  later  detected  even  after the intervals  of 
1 hour. It m s  observed  for  trimethylsilane  that  at  certain  temperatures 
below  the  explosion  region, a pressure  decrease would occur immediately 
after admission of the  fuel-air  mixture. 
The twerature-camposition  curves  for  these two fuels  are shown i n  
figure 2. Determimtions  were d e  of both the tqerature at which ex- 
plosion  occurred in 15 seconds  or less and the  temperature  below  whfch 
no ealosion occurred. As shown in figure 2, these  two  temperatures do
not  differ by more  than 15O C; and, consequently,  the minimum explosion 
temperature  must  lie  somewhere  witbin  this 150 C temperature  range.  Rich 
mlxtures of tetramethylsilane and air ignlted  at a temperature as low as 
4500 C, while rich trimethyuihne-air mixtures required a temperature of 
only 3100 C. 
In  figure 3 are  presented  the  temperature-composition eqlosion 
curves  for  several  hydrocaxbon  compounds  as  given i n  ref rences 2 to 5. 
The boundary between  explosion  and  nonexplosion  for  the hydrocarbons is 
between 5000 and 600° C. The  temperature-composition  explosion  curves 
for  tetramethyl-  and  trimethylsilane  are  very similar to  those of the 
hydrocarbons, except  that  the  tetramethylsilane  temperatures  are  about 
1000 C lower asd the  trimethylsilane  temperatuxes are abaut ZOOo C 
lower  than  those of the  hydrocarbons. 
The  temperature-composition  explosion  curves  for  representative 
hydrocarbons as s h m  in figure .3 lie in a narrow temperature range, 
while a marked  separation  is s h m  in figure 2 to  exist  between 
tetramethyl- and trimethylsilane.  The  apparent higher reactivity of 
trimethylsilane  indicates  that  structural  changes  within  the  alkyl- 
silanes may make =ked  differences in their  conibustion  properties. 
One  reaction  bulb  was  used  throughout  the  determinations  of  the 
tetramethyl-  and  trimethylsilane  explosion  limits.  During  the  course of 
these  explosions small mounts of Si02 precipitated  aut on the  walls of 
I 
4 NACA RM E53L01 
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the reaction chamber. The reaction vessel was therefore replaced by a 
clean chamber, and a point on the trimethylsilane curve was redetermined. c 
The explosion temperature obtained with the  clean  vessel was ident ical  
with that previously determined; and, consequently, the possibility that  
small amounts of Si02 were inhibiting  or promoting the explosion was 
eliminated. .. 
Dimethyl-, Methyl-, and Vinylsilanes 
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M A second type of explosion limit was encountered with the dimethyl-, 
methyl-, and vinylsilanes; and the temperature-composition curves for  
these  three compounds could not be obtained by the technique used far 
tetramethyl- and trlmethylsilane. ?he temperatCFes below which no ex- 
plosion occurred (reported in fig. 4 )  6erve to  indicate  the temperature 
region for safe handling of a given fuel concentration. However, once 
temperatmes above the  nonexplosaon region were reached, an explosion 
always took place before the mixtures reached a pressure of 1 atmos- 
phere i n  the reaction vessel. Mlxtures were therefare admitted very 
slowly t o  observe at what pressure $he first indfcation of reaction a d  
occur. By this procedure it was observed tha t  a ser ies  of minor explo- 
sions, probably cool  flames, occurred over a pressure range of 5 t o  10 
cen the te r s  of mercury. After this pressure range was exceeded, no 
further reaction  took  place when the mixture was slowly admitted t o  
a pressure of 1 atmosphere a t  temperatures Just  above the nonexplosion 
region. " 
" 
Since a very vigorous reaction took place when the mhture was rap- ? 
idly admitted, it is  believed that the cool-flame reaction occurring a t  a 
pressure of 5 to 10 centimeters set off an errplosive reaction when addi- 6 
t iona l  material was being rapidly supplied. The temperature-composition 
explosion curves for  dimethyl-, methyl-, and vinylsilane actually rep- 
resent the appearance of cool flames. The cool flames are capable of 
causing a true explosion if additional fuel-air mixture is lmedlately 
available to react. If cool fhmes did not occur, the explosion temper- 
ature  for  an air mixture of these fuels at an init ial  pressure of 1 at- 
mosphere would be expected t o  be higher thas the  temperatures  recorded 
here for  these compounds. . . .  
DISCUSS I O N  
A comparison of the explosion temperatures of the allrylsilmes 
with hydrocarbon fuels  (see f ig .  3) shows the marked decreaee of  the non- 
explosive region for the silanes as compared with t ha t  of typical  hydro- 
carbons which would be f o r m e d  if the s i l icon   a tom were .replaced by 
carbon. As already pointed out, the explosion curves of the hydrocar- 
bons show l i t t l e  variation with changes in fuel structure.  The eqlo- 
sion curves of tetramethyl-, trimethyl-, dimethyl-, and methylsilane are 
each separated from the next member of the ser ies  by about 100° C. The 
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NACA RM E53LOl 5 
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reac t iv i ty  is obviously increased by s d s t i t u t i n g  hydrogen atoms f o r  the 
silanes than f o r  the hydrocarrbons and since it decreases as hydrogen 
atoms are  substituted f o r  methyl groups, it appears very probable t h a t  
the Si-H bond i s  more readily  sribject  to  oxidation  than  the C-H bond. 
This conclusion might Ellso have been reached by  considering  the  theo- 
r e t i c a l  bond-strength calculations of Pauling (ref. 6) which show the 
strength of the SI-H bond t o  be appreciably w e a k e r  than that of the C-H 
d methyl groups. Since the  explosion  temperature is lower f o r  the alkyl- 
w bond.  Bond length measurements by B r o c b y  and Beach (ref.  7) also show 
41 the  Si-H bond to be considerably  longer and therefore weaker  than the 
C-H bond. 
Walsh (ref .  8) has proposed that oxidation reactions of the hydro- 
carbons proceed by free radicals.  To permit the -formation of these 
radicals the first step in the oxidation must involve the vulnerability 
of the molecule t o  bond-breaking reactions. Walsh proposes that radi- 
ca ls  are f irst  formed by removal of a hydrogen atom from 
w h i c h  is then srib3ect to attack  by  addition of 02 t o  the 
valence. A general over-all scheme stding wlth a f r e e  
therefore be presented as foUaKs (ref. 9) : 
The f a c t  that the  alkylsilanes  oxidize more readily  than 
and that hydrogen atoms can be removed more readi ly  from 
the molecule, 
f r ee  carbon 
radical  R can 
the  hydrocarbons , 
the silanes,  
would lend support t o  t he   i n i t i a l   s t ep  proposed f o r  the hydrocarbon oxi- 
dation mechanism, namely, the removal of a hydrogen atom. If the silane 
oxidation then follows the mechanism of the hydrocarbon oxidation,  the 
II 
4 scheme presented i n  equation (1) might also be  adapted. 
Since the explosion curve for vinylsilane i s  about 1300 C lower 
than f o r  dimethylsflane, the HzC=CH warp appears to introduce appreci- 
ably more reac t iv i ty   in to  the molecule than does the CzHs radical. Ear- 
ever, even mixtures of vinylsilane vapor and a i r  are nonexplosive below 
900 C; thus, the most reactive  fuel  studied can be seSely handled at 
room temperature without danger of a purely spontaneous reaction. Spon- 
taneous combustion which might resu l t  when a large volume of a l iqu id  
si lane fuel is exposed t o  air f o r  long time periods was not studied in 
this reseazch. 
I n  an investigation of the nonexplosive region of five  alkylsilanes 
as a function of temperature and concentration at a pressure of 1 a tms-  
phere, the follawing results were obtained: 
6 NACA RM E53L01 .. . 
I 
1. Over a fuel concentration range of 2 t o  10 percent, the 10W6t 
temperatures (OC) below which exp1osion"did not 0'6CZir T o r  the f i v e  fuels 
studied were tetrmnethylsilane (CH3)4SI, 45Q0; trimethyleilane (CR3)3SiH, 
310'; dimethylsilane (CI-13)2SiHz, 220'; methyls-- CH3SiH3, 130°; and 
vinylsilane H2C=CH-SiH3, 90'. 
t 
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peratures between 
1. The large 
2. Explosion temperatures of' alkylsilanes were found t o  be lower 
than  those of analogous hydrocarbon compounds which have explosion tern- 
500° and 600° C. b " 
CONCLUDING. REMARKS 
variation in eqlosion temperatures among the silanes 
indicates that substituting hydrogen atoms f o r  the methyi groups de- 
cidedly  increased  their  reactivity and may cause marked changes i n  the 
combustion properties among  members of . .  t h i s  . . .. class  of fuels.  
. "- 
.. . . , ". -.- 
2. Since the explosion temperature i s  lower f o r  the alkylsilanes 
than f o r  the hydrocarbons and since it decrease6 as hydrogen atoms are 
substituted f o r  methyl groups, the  Si-E bond is probably more euscept- 
ible   to   oxidat ion than the C-H bond. 
" 
Lewis Flight Propulsion Laboratory 
National Advisory Committee f o r  Aeronautics 
Cleveland, Ohio, November 25, 1953 
1. Tannenbaum, Stanley, Kaye, Sanruel,  and  Lewenz,  George F.: Synthesis 
and Properties of Some Alkylsilanes. Jour. Am. Chem. SOC., vol. 
75,  no. 15, Aug. 5, 1953, pp. 3753-3757. 
2. Townend, Donald T. A,,  and Mandlekar, M. R.  : The Influence of Pres- 
sure on the Spontaneous Ignition of .InfJmr&ble Gas-Air Mixtures. 
I1 - Pentane-Air Mixtures. Proc. Roy. SOC. (London), ser . A, vol. 
143, no. A 848, Dec. 4, 1934, pp.  168-176. 
3. Townend, D. T. A., Cohen, L. L., and Mandlekar, M. R .  : The In- 
fluence of Pressure on the Spontaneoys Ignition of Inflammable 
Gas-Air Mixtures. I11 - Hexane- and Isobutane-Air Mixtures. 
Proc. Roy. SOC. (London), ser. A, vol. 146, no. A 856, A%. 1, 
1934, pg. 113-129, 
" - . . . . . . . - " .. 
b 
NACA RM E53L01 7 
r 
4. Townend, D. T. A . ,  and CJmniberlain, E. A. C.: The Influence of Pres- 
sure on the Spontaneous Ignition of Inflammable G a s - A i r  Mixtures. 
IV - Methane-, Ethane-, and Propane A i r  Mixtures. Proc. Roy. SOC. 
(London), ser. A, vol. 154, no. A 881, Mar. 3, 1936, pp. 95-ll2. 
5. &ne, G. P., and Tamend, D. T. A.: The Influence of Pressure on the 
Spontaneous Ignition of Inflammable G a s - A i r  Mixtures - The Simpler 
Olef ines. Proc. Roy. SOC.  (London) , ser. A, vol. 160, no. A 901, 
w May 18, 1937, pp. 174-187. 
E 
4 6. Pauling, Linus: The Nature of the Chemical Bond. Second ed., 
Cornell Univ. Press, 1940, p. 53. 
7. Brockway, L. 0. , and Beach, J. Y. : The Electron Diffraction Investi- 
gation of the Molecular Structures of (I) Phosphorus Oxytrichlor  ide , 
Oxy-dichlorofluoride, Oxychlorodifluoride, Oxytrifluoride, Fluoro- 
dichloride, Pentafluoride, and Trifluorodichloride, and of ( 2 )  
Disilane,  Trichlorosilane, and Eexachlorodisilane.  Jour. Am. Chem. 
SOC., V O ~ .  LX, YIO. 8, A u ~ .  5, 1938, p ~ .  1836-1846. 
8. Walsh, A. D. : Processes in the Oxidation of Hydrocarbon Fuels. 1 - 
The Point of Oxygen Attack. Trans. Faraday SOC. , vol. 42, no. 286, 
Mar.-Apr. 1946, pp. 269-283. (See also  vol. 43, 1947, pp. 297-313.) 
9. Lewis , Bernard, and von Elbe , Guenther : Corfbustion, Fla~ws and 
Explosions of Gases. Academic Press , Inc. (New York) , 1951 
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Explosion and combustion properties of alkylsilanes I : temperature-composition limits of explosion for methyl-,dimethyl-,trimethyl-,tetramethyl-,and vinylsilane at atmospheric pressure

Explosion and combustion properties of alkylsilanes I : temperature-composition limits of explosion for methyl-,dimethyl-,trimethyl-,tetramethyl-,and vinylsilane at atmospheric pressure

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