LITHIUM PERCHLORATE OXYGEN CANDLE Pyrochemical Source of Pure Oxygen M E Y E R M . M A R K O W I T Z , D A N I E L A. A N D H A R V E Y S T E W A R T , J R .
B O R Y T A ,
Research and Engineering Center, Foote Mineral Co., Exton, Pa. The interaction of the oxidant lithium perchlorate with minor additions of the elemental fuels aluminum, boron, iron, magnesium, manganese, silicon, and titanium was studied in an attempt to formulate a thermochemically self-propagating condensed phase source of pure oxygen. Ignition and combustion characteristics were investigated b y the use of differential thermal analysis, thermogravimetry, chlorine evolution, and burning rate measurements, and b y chemical and x-ray analyses. The final oxygen-generating composition was a compact (in weight per cent) of 84.82% LiC104, 10.94% Mn, and
4.24% LizO2.
At a pressed
density of 2 . 3 2 grams per cc., the available oxygen content of this mixture is equivalent to that of an equal volume of liquid oxygen at its boiling point ( -
183" C.).
s
M A N is subjected to environments isolated from a continuous supply of respiratory oxygen, he is forced to carry with him his own breathing atmosphere. These restricted environments encompass such varied sites as submarines ( Z ) , space capsules (24).and mines (8:2 5 ) . Under average levels of human activity and diet, the need for oxygen corresponds to a consumption of about 2 pounds per man-day with the concomitant production of 2.3 pounds of carbon dioxide ( 7 7 ) . This physiological stoichiometry results in a respiratory quotient or RQ (volume of COn produced per volume of 0 2 used) of 0.83. I n nonregenerative applications, oxygen may be carried along as the elemental material in the form of liquid oxygen or as the compressed gas (,?7),or as some chemically combined form capable of release through chemical reaction (gas-solid interaction, pyrolysis). The type of oxygen supply system used will depend to a great degree on the length of the desired mission and the weight- and power-carrying facilities available (34). For emergency situations and for needs of relatively short duration, the chemically combined oxygen sources are of considerable utility. They are characterized by prolonged storage life with consequent ease of logistics, the need for relatively simple regulatory equipment, and the ability to function with little or no auxiliary power.
A
lithium ozonide (LiOB) (70) are included in Table I for the sake of completeness, but as yet they have not been synthesized in states of high purity. A prototype development in the use of halates as oxygen sources is incorporated in the so-called .'chlorate candle" (26, 32, 33), where the oxidation of a small amount of iron by sodium chlorate provides sufficient heat to decompose a considerable excess of sodium chlorate and yield substantially pure oxygen gas. Early attempts a t the exploitation of this concept led to disastrous explosions and cast serious doubts on the inherent safety of these pyrochemically self-substaining oxygen sources. However, the most recent developments of this type of oxygen system have resulted in a linearly burning composite of 92% NaC103, 4% steel wool, and 4% Ba02. which yields about 40% available oxygen (26). T h e present report relates to major improvements in the oxygen candle concept with the use of lithium perchlorate as the oxygen source and a more energetic reducing agent as the fuel component. Some data pertaining to readily available fuel elements are presented in Table 11. O n the basis of heat release, boron appears to be the most efficient fuel. However, the fuel ulti-
Properties of Various Oxygen Sources Required f o r Available Oxygen I Man-Day Compound Denszty Wt. 7 0 G./cc. Lb." Cc. 626 60.1 1.45 3.35 2.43 LiCIOl 3.84 689 52.0 1.31 2.53 NaCIO, 4.33 777 46.2 1.16 2.52 KClOa 3.77 639 53 0 1 42 2.63 LiClO? 808 1.13 4.43 NaCldJ 2.49 45.1 998 0 91 5.10 KC103 2.32 39.2 4 . 4 3 1373 0 66 1 43 46.1 98 3 ' 2 ,H202 Liquid02at -183°C. 1.14 100.0 1 . 1 4 2 00 797 LiO2 , , 61.6 .. . 3.25 .. . LiOa ., 72 8 ... 2 75 . . Table 1.
Halate Compounds a s Oxygen Sources
Classically. the alkali metal halates (chlorates and perchlorates) have been used to prepare oxygen gas by thermal decomposition reactions. These materials are known to pyrolyze for the most part according to the over-all reactions : M C 1 0 3 = MC1 3 I2O2and M C 1 0 4 = MC1 -I- 2 0 2 (72). T h e oxygen availability for some of these compounds is presented in Table I. Of the materials tabulated, lithium perchlorate sh0x.s the highest potential oxygen content on both a weight (60.17,) and volume basis ( 1 . 4 5 grams of 0 2 per cc.) and actually contains about 277, more oxygen per cc. than liquid oxygen itself. Lithium superoxide (LiOz) (37) and
+
, ,
a
7 man-day oxygen requirement taken as 2 lb.
VOL. 3
NO. 4
DECEMBER 1964
321
Table II.
Fuel
.41 B Fe Fe Fe Mg Mn Mn Mn Mn Si Ti
Density 2.70 2.45 7 86 7 86 7.86 174 7 20 7.20 7.20 7.20 2.40 4 . 50
f ' " " " " " " '
l A
Properties of Various Fuels
Com6ustzon Product AlaO:i, corundum
BnOa FeO. wustite Fe304 Fe2Oa MgO MnO MnsOi Mn2Os
MnO? .%On, m-quartz TiOa, rutile
Heat of Combustion Kcal./gram Kcnl./cc. fuel fuel 7.41 20 01 14.11 34.57 1 17 9 20 1 63 12 81 1 80 14 14 5 91 10 28 1 67 12 02 2.01 14.47 2.08 14.98 16.27 2.26 7.47 10.46 4.71 21.20
mately to be used in conjunction with lithium perchlorate must be capable of producing linearly propagating, smooth combustion \\ith no serious side reactions interfering with the release of substantially pure oxygen. O n these accounts the use of boron as a fuel in this application is inappropriate and manganese metal powder, despite its lower heat of combustion, appears to provide the best compromise fuel component.
0 X
W
f
20-
1
0
n
PURE
2
LiCLO4
W
I
wl 80.2
[ A R G O N ATM.)
Experimental Procedures
Thermoanalytical Techniques. Changes occasioned by the heating of lithium perchlorate and its admixtures with various fuels \vere followed by the techniques of differential thermal analysis (DTA4)( 3 6 ) and thermogravimetric analysis (TGA4) (5). I n the D T A method, thermal effects under a steadily rising temperature (8.5' C. per minute) are determined by the simultaneous recording of sample temperature, T , against the temperature difference between the sample and a n inert reference material, S T : ignited alumina. T h e furnace design ( 7 7 ) , heating rate controls ( 2 0 ) , and specific application of D T A to perchlorate salts (27) have been described. For experiments carried out in inert argon atmospheres, a D T A sample holder ( 7 6 ) was used. T h e corresponding weight changes were resolved by continuous automatic weighing (TGA) ( 7 4 ) using the heating chamber and temperature corrections (75) found appropriate for this technique. Chemicals Used. Some properties of the chemicals used in these studies are given in Table 111. T h e specific surface areas (square meters per gram) were determined by a dynamic nitrogen adsorption method (28, 30).
0
100
200
300
400
500
600
700
TEMP. , O C. Figure 1. rate
Thermal behavior of pure lithium perchlo-
A
PEAK OFF SCALE
B.
Results and Discussion
Lithium Perchlorate-Fuel Interactions. THERMAL DEPURELITHIUM PERCHLORATE.T h e pyrolysis
COMPOSITION O F
of this material has been repeatedly studied by the D T A (73. 77. 27, 22) and T G A (74, 77-79) techniques. As indicated by- the DTA curve of Figure l A , the sequence of events on heating the compound is: the dissociation reaction, H?O (small endotherm a t 150' C.), LiClOd. H,O = LiC10, occasioned by the presence of a small amount of moisture in the oxidant; fusion of anhydrous lithium perchlorate, LiClOa (solid) = LiC104 (liquid) (large endotherm a t 247' C.); thermal decomposition and crystallization sequence, LiC104 2 0 1 (gas), LiC104 (liquid) = (liquid) = LiCl (liquid) LiCl (solid) 2 0 2 (gas)
