The Deflagration of Hydrazine Perchlorate - Advances in Chemistry

7 The Deflagration of Hydrazine Perchlorate

Downloaded by UNIV OF MASSACHUSETTS AMHERST on February 28, 2017 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0054.ch007

J.B.L E V Y , G. V O N ELBE, R. F R I E D M A N , T. W A L L I N , and S. J. A D A M S Atlantic Research Corp., Alexandria, Va. The deflagration of hydrazine perchlorate, both pure and with fuel and catalyst additives, has been investigated. Hydrazine perchlorate will deflagrate reproducibly if a few percent fuel is present. The deflagration process is catalyzed by copper chromite, potassium dichromate, and magnesium oxide. Deflagration rates have been measured photographically from 0.26 to 7.7 atm. A liquid layer was observed at the surface in these experiments. Vaporization rate measurements from 180°-235° C. have yielded the expression log P = 10.2 -6400/T for the vapor pressure of hydrazine perchlorate. Temperature profiles of the deflagration wave have been measured, and spectroscopic measurements of the flame temperature above a deflagrating strand have been made. The results are discussed in terms of the mechanism of deflagration of hydrazine perchlorate. 10

(mm.)

V I T T e are engaged i n a general program of research to understand the factors that govern the nature of the deflagration of composite solid propellants. O u r efforts have been devoted to studies of the oxidizer alone ever since early observations that ammonium perchlorate deflagrated as a monopropellant at rates comparable to those found for propellant formulations containing it ( J , 9 ) . Earlier work i n this laboratory dealt with the self-deflagration of ammonium perchlorate (16). W e report here o n studies with the related but more energetic material—hydrazine perchlorate. Hydrazine perchlorate is a white crystalline solid melting at 140°142° C . and having a density of 1.939 grams/cc. ( 5 ) . It forms a hemihydrate which can be dehydrated readily at 64.5° C . under vacuum. It has been reported (5) that dry hydrazine perchlorate can be detonated by shock or friction and that it has a shock sensitivity comparable to that of initiating explosives. W e have observed the usual precautions i n 5 5

Holzmann; Advanced Propellant Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


56

ADVANCED PROPELLANT CHEMISTRY

handling this material and have experienced explosions w i t h it only under extreme conditions—i.e., i n certain deflagration experiments. However, it is a very energetic material and must be handled with great care. The thermal decomposition of hydrazine perchlorate has been i n vestigated, and ammonium perchlorate was found to be a major product (13). W e know of no studies of the self-deflagration of hydrazine per chlorate. The results reported here are concerned w i t h studies of pure hydrazine perchlorate and hydrazine perchlorate œntaining small amounts of additives.

Experimental Preparation of Hydrazine Perchlorate. Hydrazine perchlorate was prepared b y titrating a solution of 8 5 % hydrazine hydrate to a p H of 3.2 with 4 8 % perchloric acid. This yielaed a stock solution which could be stored indefinitely. Hydrazine perchlorate was precipitated b y p o u r i n g a volume of this solution into 5 volumes of 2-propanol at 0° C . The hydra zine perchlorate was filtered, washed w i t h cold 2-propanol, and vacuum dried at 80° C . The material was analyzed iodometrically ( 3 ) . Purities > 9 9 % , as indicated b y the analysis, were obtained. T h e melting point was 1 4 2 ° 143° C . Processing Hydrazine Perchlorate. The hydrazine perchlorate used for the deflagration measurements was prepared i n the form of small spherical particles of fairly uniform size distribution b y means of a meltshot apparatus. I n this apparatus solid hydrazine perchlorate is fed into a spinning aluminum dish maintained at a temperature above the melting point of hydrazine perchlorate and fitted with a small lateral hole i n the side, which permitted the ejection of the molten spheres which cool as they fly through the air. It was found that 160° C . was a satisfactory temperature for the dish. W i t h the dish spinning at 2400 r.p.m. the par ticle sizes of the spheres obtained, as determined b y microscopic examina tion of a random selection, varied from 5Ο-θ00/χ. Analysis of material prepared i n this way indicated that no decomposition occurred during the shotting process. Strand Preparation. Strands were either tamped or pressed. Tamped strands were prepared b y pouring small increments of material Table I. Vaporization Rates

Temp.

°Z

Area sq. cm.

Duration sec.

Weighty grams subcharged limed

453 463 473 492 508

4.90 0.50 4.90 0.50 4.90

18,900 21,240 18,900 2,220 2,400

1.56 0.244 1.70 0.239 1.65


0.22 0.073 0.54 0.036 08.3


7.

LEVY ET AL.

Hydrazine Perchlorate

57

into a tube and tamping each increment gently with a Teflon rod. Pressed strands were prepared i n a steel mold by means of a hydraulic press. Pressures of ~40,000 p.s.i.g. gave strands of 9 5 - 9 8 % of crystal density, which was considered adequate. Pressing operations were performed remotely. The mixtures of hydrazine perchlorate and the fuels or catalysts were prepared by mixing the hydrazine perchlorate shot with the finely ground other ingredients i n an ordinary vee mixer for several hours. The uniform deflagration rates observed with the various mixtures attest to the homo geneity of strands prepared i n this way. Sublimation Experiments. The sublimation experiments were per formed with a conventional cold-finger vacuum sulblimation apparatus with a removable cold finger. The apparatus was evacuated by an oil pump to about 5 microns, lowered into a thermostat, and the timer started. T w o sublimation apparatuses were used. A t first a fairly small one with a cross-sectional area of 0.5 sq. cm. was used to keep the amount of hydrazine perchlorate required down to about 0.5 grams. Subsequently a larger apparatus having a cross-sectional area of 4.90 sq. cm. was used with amounts of hydrazine perchlorate of the order of 1.5-2.0 grams. A t the conclusion of the experiment the sublimate was carefully re moved from the cold finger and weighed. The weight of the residue was found by weighing the outer tube, washing out the residue, and reweighing the tube. The analyses were performed by iodometry. Flame Temperature Measurements. A tungsten ribbon filament lamp, calibrated oy the National Bureau of Standards for the temperature range 1100°-2300° C , was used for these measurements which were per formed i n the conventional manner (17).

Results The experiments performed i n this program are grouped into: (a) experiments i n which vaporization rates of pure hydrazine perchlorate were measured; (b) deflagration rate measurements; (c) temperature profile measurements; ( d ) flame temperature measurements. Vaporization Rate Measurements. These experiments were per formed i n the glass sublimation apparatuses described under Experi mental. The surface area of the liquid was quite undisturbed by bubbles of Hydrazine Perchlorate 70* X % Hydrazine R f Perchlorate in Vaporation „ % SubRest- grams/sq. residue recov. limate due cm.-sec. 1.32 99 2.38 99.8 99.4 97 0.163 6.85 99 1.14 5.84 95.0 99.0 99 0.200 32.4 0.79 99.0 98 70.5 100.0 ate

0


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ADVANCED

PROPELLANT

CHEMISTRY


during these experiments, and its magnitude was constant during an ex periment The temperature of the liquid was assumed to be that of the bath i n which the apparatus was immersed. The results are given in Table I. The relation between vaporization rate and vapor pressure is given by ( 7 ) :

g = vaporization rate in grams c m . " ^ec."" a = evaporation coefficent Ρ = vapor pressure in dynes em.""

M = molecular weight ο vaporizing species Τ = absolute temperature R = gas constant in ergs mole"" deg.~

1

2

1

l

It can be seen that, once an assumption is made for the value of M, the only quantity still unknown i n the above equation is a, the evapora tion coefficient, which must have a finite value equal to or less than 1. If it is assumed that a is constant but unknown, then the vapor pressure at any given temperature is proportional to the vaporization rate, and the enthalpy of vaporization may be found from the Clausius-Clapeyron type treatment. If a value is assigned to a, then vapor pressure values and the entropy of vaporization can be calculated as well. If the entropy of vaporization found i n this way is a reasonable value, then the assumed value of a receives support. The latter procedure has been adopted here, and a value of unity has been taken for a. The reasons for choosing this value are: (a) Values for a for a wide variety of substances have been reported (18), and for the majority of cases values close to unity (i.e., within a factor of 2 or 3) have been reported; (b) The cases (15, 21) where α « 1 are restricted to solids, and the explanation offered has been that an adsorbed layer at the surface interferes with the vaporization process. Since hydrazine perchlorate i n this experiment was molten, this condition does not apply. (Values of 0.02 to > 0.25 have been reported for liquid water (18).) The vapor pressure values obtained from the data of Table I by letting a = 1 and Μ η 66 (the average for hydrazine and perchloric acid-(vide infra) are tabulated i n Table II and plotted as the ClausiusClapeyron expression in Figure 1. The data i n Figure 1 are fairly linear, and the line which has been drawn visually through them yields the equation:

log,oP(nu»).

=

10.2

-

-p

The heat of vaporization, ΔΗ„, from the above slope, is 29.2 kcal./mole.


7.

Hydrazine Perchlorate

LEVY E T AL.

Table II.

Vapor Pressue of Hydrazine Ptorchiorate

Temp., ° K.

Vapor Pressure, mm.


453 463 473 492 508

Figure 1.

1.16 3.08 2.66 15.1 33.4

X X Χ X X

10-* 10~ 10" 10~ 10 ~ 4

4

4

4

Rate of vaporization of hydrazine perchlorate

It is of interest to consider this value i n terms of an assumed vaporization process where vaporization occurs with dissociation as is believed to be the case for ammonium perchlorate. The equilibrium is N H C10 (1) τ± N H ( g ) + HC10 (g) 2

6

4

2

4

4

The heat of formation of crystalline hydrazine perchlorate is —42.5 k c a l . / mole. This was estimated from the heats of formation of the ions i n solu tion, the heat of hydration of anhydrous hydrazine perchlorate to the hemihydrate, and the heat of solution of the hemihydrate (11). A value of 3.84 kcal./mole has been reported for -the heat of fusion ( 19) yielding —38.7 kcal./mole for the heat of formation of the liquid. T h e heats of formation of gaseous perchloric acid and gaseous hydrazine are —1.1 (6)



60

ADVANCED PROPELLANT CHEMISTRY

and 22.75 (22) kcal./mole, respectively. These values lead to an enthalpy change of 60.4 kcal./mole for the above equihbrium. The heat of vapori zation of 29.2 kcal./mole would correspond to an enthalpy change for the above equihbrium of 58.4 kcal./mole. The agreement supports the belief that the vaporization rates are proportional to the pressures, and that the vaporization process is dissociative. The standard entropy of vaporization for l i q u i d hydrazine perchlorate calculated from the vapor pressure expression is 64 cal./deg. mole. A l though values for the standard entropies of gaseous hydrazine and per chloric acid are known, the value for hydrazine perchlorate is not, and hence it is not possible to compare the above experimental figure with a calculated figure. It is possible to compare it with another case where the entropy of vaporization is known and would be expected to have a comparable value. Ammonium perchlorate has been selected for this purpose. The standard entropy of vaporization of crystalline ammonium per chlorate can be calculated from the standard entropies of crystalline a m monium perchlorate and gaseous ammonia and perchloric acid. The values, i n cal./deg. mole are 44.02, 45.967 (14), and 70.7 (10), respec tively, yielding a value of 72.7 cal./deg. mole for the entropy change for: NH C10 (c) 4

NH,(g) + HC10 (g)

4

4

(The experimental value (12) determined from vapor pressure measure ments is 71.0 cal./deg. mole. ) The entropy of fusion of hydrazine per chlorate may be calculated from the cited heat of fusion and the melting point and is found to be 9 cal./deg. mole. Thus the entropy change for the process N H a0 (c) 2

6

N H ( g ) + HC10 (g)

4

2

4

4

is found to be 73 cal./deg. mole. This is i n reasonable agreement w i t h the value for ammonium perchlorate and supports the belief that a = 1 for hydrazine perchlorate is a reasonable assumption. ( H a d it been as sumed that a for hydrazine perchlorate were of the same order as ammo nium chloride,—i.e., ( I S ) , the data would have yielded an entropy of vaporization of 100 cal./deg./mole. ) A final point is worth noting i n comparing ammonium perchlorate and hydrazine perchlorate. The experimentally determined vapor pres sure expression for the former (12) is

The Deflagration of Hydrazine Perchlorate - Advances in Chemistry

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