4015
THERMAL STABILITY OF NITRONIUM PERCHLORATE
Effects of Intentional Impurities on the Thermal Stability of Nitronium Perchlorate] by J. N. Maycock, V. R. Pai Verneker, and C. S. Gorzynski, Jr. Research Institute for Advanced Studies, M a r t i n Marietta Corporation, Baltimore, Maryland (Received M a y SI, 1968)
81287
The effects of adding impurities to nitronium perchlorate have been investigated by simultaneous differential thermal and thermogravimetric analyses and also by isothermal, constant-volume decompositions. The impurities have been chosen to (i) increase the number of cation vacancies, (ii) increase the number of anion vacancies, and (iii) produce steric hindrance effects. It has been shown that the thermal stability of nitronium perchlorate can be improved by the addition of an excess of cation vacancies and also ions which produce a cation-anion interaction in the lattice and vice versa that the thermal stability is decreased by an excess of anion vacancies.
Introduction The thermal decomposition and thermal stability of nitronium perchlorate have been studied in some detail and its decomposition kinetics and mechanisms are fairly well Possible changes in the thermal stability due to adding intentional impurities to the crystal lattice of nitronium perchlorate have not been reported although similar techniques applied to ammonium perchlorate have been used with some SUCcem5z6 As a result of the investigations of the thermal decomposition of pure nitronium perchlorate, it is probable that the rate-controlling step in the decomposition is the decomposition of the perchlorate ion Clod- +Clod.
+ e-
The rate of this reaction can be increased by removing the electrons and similarly it can be decreased if electron donors can be added to the system. Further, it is a well-established fact, that, at a given temperature for a uni-univalent ionic crystal, the product of the concentrations of anion and cation vacancies is a constant, the anion vacancy having a virtual positive charge and the cation vacancy a virtual negative charge, thus
[@][@I
= constant for any given
An attempt to stabilize nitronium perchlorate thermally has been made on the basis of the model just discussed. Using this model to achieve thermal stabilization requires that impurities be incorporated directly into the crystalline lattice of nitronium perchlorate. With many materials, e.g., potassium chloride,’ this can be achieved by adding the impurity to a melt of the host material. Solids which do not melt can therefore be “doped” only by recrystallization of the host and impurity from solution. A disadvantage of this method is that the degree of doping is not as efficient as growing from the melt. To dope nitronium perchlorate from solution, it was decided to use anhydrous (fuming) nitric acid as the solvent. Prior to any doping studies it was necessary to investigate the properties of pure nitronium perchlorate recrystallized from this solvent with respect to the properties of the “as received” Callery Chemical Co. nitronium perchlorate to determine if “ 0 3 is occluded within the lattice thereby increasing the reactivity.
Experimental Techniques Recrystallized nitronium perchlorate (NP) was prepared by dissolving 5 g of pure, dry Callery N P in 5 ml of “ 0 3 at approximately 70”. The resulting
T
Since the anion vacancy, @, has a virtual positive charge, it is capable of acting as an electron sink. Thus it should be possible to trap out electrons by increasing the concentration of anion vacancies and hence increase the rate of decomposition of nitronium perchlorate, and, vice versa, if the concentration of cation vacancies is increased, then the concentration of anion vacancies must decrease due to the law of mass action. The resulting doped material will contain fewer anion vacancies than the pure material and hence the rate of decomposition should decrease.
(1) Supported by NASA, Jet Propulsion Laboratory, Contract No. NAS7-562, and U. S. Army Missile Command, Huntsville, Ala., Contract No. DA-01-021-AMC-l2596(Z). (2) (a) J. N. Maycook and V. R. Pai Verneker, J . Phgs. Chem., 71, 4077 (1967); (b) J. N. Maycock and V. R. Pai Verneker, ibzd., 72, 4004 (1968). (3) H. F. Cordes, ibid., 67, 1693 (1963). (4) M. D. Marshall and L. L. Lewis, Advances in Chemistry Series, No. 54, American Chemical Society, Washington, D. C., 1966, p 82. (6) V. R. Pai Verneker and J. N. Maycock, J . Inorg. Nucl. Chem., 29, 2723 (1967). Roy. Soc. (London), (6) J. N. Maycock and V. R. Pai Verneker, PTOC. in press. (7) P. W. M. Jacobs and J. N. Maycock, J . Phys. Chem. Solids, 24, 1693 (1963). Volume 78, Number 12 hTovember 1968
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J. N. MAYCOCK, V. R. PAIVERNEKER, AND C. S. GORZYNSKI, JR.
solution was allowed to cool to room temperature, with crystals first coming out of solution at about 40". The recrystallized nitronium perchlorate was then filtered, under reduced pressure, and dried by continuous pumping at lo-* torr for approximately 90 hr, at room temperature. With this drying procedure the recrystallized NP is in the form of a fine, dry powder. Doped specimens of nitronium perchlorate containing Ca2+, A13+, BFd-, s04'-, or Ca(BF& were prepared from the following salts: Ca(C10&, A1C13, N02BF4(NH4)&304, and NH4BFA. Specifically, the required mole per cent of impurity ion was mixed with the appropriate weight of nitronium perchlorate, dissolved in fuming H S 0 3 at 70", and allowed to cool to room temperature. The drying of the doped nitronium perchlorate was identical with that used for the pure material, The thermal stability of both the recrystallized pure nitronium perchlorate and the doped samples was characterized by simultaneous differential thermal analysis and thermogravimetric analysis (dta-tga) and also isothermal decompositions in a closed-volume, all-glass decomposition line. The experimental techniques for handling the samples and thermally characterizing them isothermally are described in earlier papers.2 The gaseous decomposition products from the doped samples mere found, mass spectrometrically, to be identical with those for pure nitronium perchlorate as discussed earlierS2a All pressure-time curves and dta-tga were reproducible with error ranges of 2% on the P-T curves and less than 1% on the t g plots.
Results and Discussion Four different sample preparations of N P were analyzed: (A) commercial N P as supplied by Callery Chemical Company; (B) NP recrystallized from fuming H N 0 3between 70 and 25", the crystals remaining in the acid solution 15 hr before filtering and drying; (C) ATP prepared as in (B) except that the crystals were filtered off and dried as soon as the crystallization appeared to be complete; (D) prepared as in (C) except that the crystallization temperature was lowered to only 55". The dta-tga analyses suggest that the general characteristics of samplks A, C, and D are similar. Adiabatic thermal decomposition (heating rate of 6" min-l in an He atmosphere) produced a broad endotherm with a peak at about 80" and two additional endotherms associated with weight loss processks centered at 120140 and 170-1 75 ", respectively, with decomposition being complete at about 200". Isothermal decompositions of these samples were performed at 110 and 150". These temperatures were chosen since the time for complete decomposition is of the order of 1 hr. The decompositions were carried out under 100 torr of dry He to suppress sublimation processes. It was found that samples A, C, and D showed the same decomposition characteristics, but The Journal of Phvsical Chemistry
"0
10
20
30
40
50
60
TIME ( M I N )
Figure 1. Comparison of the fractional decomposition as function of time for different N P samples being decomposed a t 110' under 100 torr of helium: 0, Callery from the interior of the batch; 0, N P from the surface of the batch; A, NP recrystallized from "03 and removed immediately; 0, N P recrystallized and left in solution for 15 hr; f , N P recrystallized and removed from the "03 solution at 55'.
sample B decomposed much faster than the others. The early portion, fractional decomposition a 0 + 0.2, of these tests is shown in Figure 1. A tentative explanation for this observed difference in reactivity is based on the possibility of trace quantities of perchloric acid monohydrate diffusing into the sample. This could be possible owing to the small amount of HzO present in the HN03. Assuming this as a possibility, then H,O+ClOd-, being very reactive, could act as centers for the initiation of the decomposition of the nitronium perchlorate. A similar effect has been observed with ammonium perchlorate.* This study of the growth parameters of nitronium perchlorate implies that doped crystals grown from nitric acid will be free of solvent inclusions provided they are removed from the solution immediately upon formation or if the recrystallization is done above 55". The Ca2+-dopedNP samples were thermally analyzed by constant-volume isothermal decompositions, under vacuum, at 150,110, and 90". Upon completion of the decompositions there was no residue at 150" and only a small amount at 110 and 90". The residue is probably nitrosonium perchlorate as discussed earlier.2b Figure 2 is representative of the decomposition pressure curves at 110" for pure and for and 10-1 mol % Caz+doped NP. As is clearly seen, the thermal stability is in the order 10-l > 10-4 > 0 mol %. Analysis of other (8) J. N. Maycook, V. R. Pai Verneker, and L. Rouoh, Jr., Inorg. Nucl. Chenz. Letters, 4, 119 (1968).
4017
THERMAL STABILITY OF NITRONIUM PERCHLORATE
under 100 torr of helium gave uniform final pressures for both pure and Ca2+-dopedNP. Similar effects have been observed with ammonium perchlorate.6 Samples of Ca2+-doped N P were also analyzed by simultaneous dta-tga techniques. The only marked difference in the dta traces is that the endotherm centered about 80" is drastically reduced. From the mechanism of decomposition as proposed in an earlier paper2bthis implies that the reaction
260-
uz W
340-
waK W
-
20
-
NOzC104 --t NOClOd
20
30 40 TIME (MINUTES)
50
__
60
Figure 2. Comparison between the isothermal decompositions of pure and Ca2+-doped N P a t 110' under vacuum mol % of Ca2+-doped NP; conditions: 0, pure NP; 0 , 0, 10-1 mol yo of Ca2+-doped N P (2.2-1. closed volume).
+ 0.502
has been inhibited. This is further illustrated from the tga data as pure Callery "fluff" loses 10% by weight before 100" with a heating rate of 6" min-' whereas the 10+ mol % Ca2+-doped material only loses 3.5% by weight. Samples (25 mg) of the A13+-dopedN P were analyzed by simultaneous dta-tga in a flowing dry He atmosphere (10 l./hr) and a heating rate of 6' min-l. There was no distinctive difference in the dta traces for the doped sample with respect to the Callery "fluff ." A comparison of the weight loss among pure NP, A13+-doped samples, and mol % ' Ca2+-doped N P is given in Table I. Table I : Sample Weight Loss between Room Temperature and Specified Temperature (6' min-1 Heating Rate) in He Atmosphere c -
MOLE
o/o
Ca++ DOPANT
Figure 3. Relationship between the rate of thermal decomposition and concentration of Ca2+ dopant, for isothermal decompositions, under vacuum, a t 110'.
dopant levels showed, as expected, that the thermal stability order is 1 > > > > 0 mol % for for each of the temperatures investigated-150, 110, and 90". A comparison between the rates of decomposition at 110" for pure and doped N P is shown in Figure 3. Kinetic analyses of these isothermal data reveal that the activation energy for the doped samples between 100 and 150" is 15 kcal mol-'. This value is in agreement with the value found for pure NP2b and is therefore in agreement with the proposed model for stabilization as this depends only on changing the rate constant and not on the activation energy. Figure 2 also clearly shows that the final decomposition pressure for the Ca2+-doped samples is less than that of pure NP. Visual observation also revealed a greater degree of sublimation for the Ca2+ samples under vacuum conditions. Decompositions performed
Sample
72'
Pure N P 10-4 mol yo AlS+ 10-3 mol yo Ala+ mol % A13+ IO-' mol % A13+ mol % Cat+ mol yo Ca2+BFa-
5.5 2.8 1.4 2.0 1.0 1.7 0.75
Wt loss, %-----1000
10.0 6.5 4.0 2.8 2.5 5.4 2.4
132'
20.2 8.5 7.0
7.4 10.0 12.2 3.2
A check on the validity of the tga data was made by isothermally decomposing 100 mg of A13+-doped N P at 110" and comparing it with pure and Ca2+- (same dopant level) doped NP. To make this test more meaningful, these samples were also selected so that their "age" was the same. As is seen in Figure 4, where the fractional decomposition is plotted as a function of time, it is apparent that the A13+-doped N P exhibits better thermal stability than the pure or Ca2+-doped NP. Again the A13+-doped N P showed more sublimation than pure N P when the decompositions were run under vacuum conditions. The choice of the Sod2- ion dopant was made on the basis of checking the validity of this thermal stabilization technique. With this ion incorporated in the lattice of NP, the net result is an increase in anion vacancies which, based on the model, should decrease mol yo Sod2-the t,hermal stability of NP. A Volume 7.9, Number 1.9 November 1968
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J. N. MAYCOCK, V. R. PAIVERNEKER, AND C. S. GORZYNSKI, JR. Table I1 : Isothermal Decomposition Analyses
Sample
Pure N P N P doped, 5 mol % NOsBFd N P doped, 5 mol % NHdBF4 N P doped, IO-' mol % ' CaZ+
I
0
I
I
10
20
I
I
30 40 TIME ( M I N )
I
I
50
60
I
70
Figure 4. Relationship between the fractional decomposition ( 0 1 ) and time for isothermal, constant-volume decompositions, a t l l O o , of 100-mg samples of: x, pure, mol yo Ca2+-doped NP; recrystallized NP; 0, mol % Ala+-doped NP; 0, 10-2 mol % 0, 5 x SOa2--doped NP.
doped N P sample was isothermally decomposed at temperatures in the range 110-150" under vacuum. The fractional decomposition of this material at 110" is compared with pure N P and Ca2+- and A13+-doped N P in Figure 4. In addition to the observed increase in reactivity, the S02--doped samples also exhibited more sublimation than the pure NP. Kinetic analysis of the data in the temperature range 110-150" also shows an activation energy of 15 kcal mol-1 for the S042--doped samples. The infrared and Raman studies of some nitronium salts by Nebgen, et U Z . , ~ showed a possible correlation between stability and anion-cation interaction. This can be seen for the series N02BF4, which is ionic having little anion-cation interaction and is stable, NO2C1O4, which is partially covalent with more anion-cation interaction and is unstable, and finally N02NOa,which is covalent with the most anion-cation interaction and is very unstable. Using this hypothesis it seems possible that incorporating BF4- ions into the lattice of N P may increase its stability. BF4- ions were substituted in the lattice of N P by making H N 0 3 solutions of N P and N02BF4 at about 70°, cooling to room temperature, and then filtering and drying the crystals. The N02BF4 was dried a t 140" for 6 hr before use. Since the desired effect is expected to work on a macroscopic rather than a microscopic scale, the dopant level used was 5 mol %. NH4BF4was also tried as an alternative to N02BF4 since i t does not require any pretreatment before being used as a dopant. After drying, these samples were analyzed by isothermal, constant-volume decompositions at 110 and 150'. A comparison of the data for this type of analysis for pure NP and Ca2+- and BF4-doped N P is given in Table 11. The data of Table I1 clearly show that the BF4- ion did not make any improvement over the Ca2+dopant The Journal of Physical Chemistry
Fractional --decomposition, br-20 min 6 min a t 150° a t llOo
0.44 0.05 0.22 0.06
0.58 0.22 0.17 0.25
which in turn is not quite as good as the Ala+ dopant. However, since the BFI- ion apparently does improve the stability of pure NP, an attempt to incorporate both Ca2+ and BF4- into the N P lattice was attempted. As Ca(BF& was unavailable, it was decided to use both Ca(C104)2 and N02BF4 as the dopants. Crystals containing both Ca2+ and BF,- ions were grown and dried as for the other doped samples. Initially a doped sample was grown from a solution containing 5 X mol % Ca(C104)~and 5 X mol % N02BF4. Simultaneous dta-tga data showed no improvement over the 5 X mol % Ca2+-doped NP. A further sample was grown from an HNOI solution containing 9 X mol % N02BF4 and mol % Ca(C104)2. Samples (25 mg) of this material were analyzed by simultaneous dta-tga. A comparison between pure N P and differently doped samples (doping level held constant) is given in Table I, where it is clear that the (Ca2+ BF,-)-doped XP is the most thermally stable of the series. Samples of pure NP, recrystallized from HNOa, lo-' mol % Ca2+-doped NP, and 5 X mol % Ala+doped N P have been stored, under vacuum, at room temperature. All samples, after recrystallization, were dried by pumping at torr for 90 hr. Isothermal, constant-volume decompositions were then carried out at 110" on the appropriate days. The final decomposition gas pressures are given in Table 111. A possible interpretation of this data is based on the postulated reaction mechanism for the temperature range below l l O o , i.e.
+
NO2C104 +NOCIO,
+ 0.502
Assuming this reaction to be correct implies that both the Ca2+ and Ala+ dopants inhibit the conversion of nitronium perchlorate into nitrosonium perchlorate. If this is the case, then it is to be expected that the pure N P will give lower final gas pressures as a function of aging, as is seen in Table 111. The lower final gas pressure of the aged, pure NP is due to testing 100-mg samples of material that inevitably contains an increasing per cent of nitrosonium perchlorate as the aging continues. For the Ca2+- and Ala+-doped samples (9) J. W. Nebgen, A. D. MoElroy, and H. F. Klodowski, Inorg. Chem., 4, 1796 (1965).
THERMAL STABILITY OF NITRONIUM PERCHLORATE Table 111: Total Gas Pressures
( p ) for
4019
Decomposition of “Aged” Samples at l l O o a Day-
Pure N P 10-l mol % Ca2+-doped N P mol % AP+-doped N P 5 X a
1
16
2000 2000 2000
1230 1900 2000
30
7
46
60
860 1900
400
2000
2000
300 1600 2000
*..
90
300 1600 2000
90-cmS reaction chamber.
this conversion to nitrosonium perchlorate is inhibited; thus the final gas pressures are indicative of still nominally pure NP. This explanation is substantiated from dta analysis as the endotherm for the above reaction at 80” is not present on the dta trace for the material which has been aged. Thermogravimetric analyses also show a reduction in the first weight loss process for the aged material. Aging, then, appears to favor the conversion of N P into nitrosonium perchlorate, although the dopants Ca2+and A13+tend to inhibit this conversion. The nitronium perchlorate-nitrosonium conversion is also, obviously, temperature dependent, such that storage at low temperatures will decrease this conversion, as also will an overpressure of oxygen.2b
Conclusions The observed increase and decrease in the thermal
stability of N P caused by the addition of M2+, M3+, and X2- can be explained by the change in cation and anion vacancies, respectively, as discussed in the Introduction. A model of this type is expected to show a larger stabilization effect for M3+than for M2+; this is observed for the cases of A13+and Ca2+. The observed increase in sublimation for both cationand anion-doped samples is unexpected. It is possible that this can be attributed to an increase in “effective” surface area by the increase in vacancy concentration which would be expected t o facilitate the sublimation processes. A similar effect has been noticed with ammonium Although ammonium and nitronium perchlorate appear to be dissimilar, it is possible, based on these experiments, to see a similarity between the two in their dependence on the point-defect structure for some of their thermal characteristics.
Volume 79, Number 18 November 1088
