A & M University, College Station, Texas, Supplements of 1960 to 1966. (1 1) L. M. Jackman and R. H. Wiley, J . Chem. SOC.,1960,2882 and 2887. (12) M. Martin, G. Martin, and P. Caubere, Bull. SOC. Chim. France, 1964, 3066. Unsaturated Aliphatic Acids and Esters (13) P. J. Collins and S . Sternhell, Australian J . Chem., 19, 317 (1966). (14) (a) J. A. Elvidge and P. D. Ralph,J. Chern. SOC.( B )243 (1966); (b) J . Clzern. SOC.(C) 387 (1966). (15) R. R. Frazer, Can. J . Chem., 38, 549 (1960). (16) R. R. Frazer and D. E. McGreer, ibid., 39, 505 (1961). (17) J. M. Purcell, S. G. Morris, and H. Susi, ANAL.CHEM., 38,588 (1966). (18) G. Slomp, Research Dept., The Upjohn Co., Kalamazoo, Mich., 49001, private communication, 1962. (19) M. van Gorkam and G. E. Hall, Spectrochim. Acta, 22, 990 (1966).
Unsaturated Aliphatic Aldehydes (20) A. W. Douglas and J. H. Goldstein, J. Mol. Specfry., 16, 1
(1965). (21) R. E. Klink and J. B. Stothers, Can. J . Chem., 44,45 (1966). (22) J. Wiemann, 0. Convert, H. Danechpejouh, and D. Lelandais, Bull. SOC.Chim. France, 1966, 1760. Unsaturated Aliphatic Ketones
(23) J. Kossanyi, Bull. SOC.Chim. France, 1965, 704. (24) C. Lumbrose and P. Maitte, ibid., p 315. Vinyl Ethers
(25) J. Feeney, A. Ledwith, and L. H. Sutcliffe,J . Chem. SOC.,1962, 2021.
RECEIVED for review January 25, 1968. Accepted April 25, 1968.
Simultaneous Differential Thermal Analysis-Thermogravimetric Analysis Technique to Characterize the Explosivity of Lead Azide V . R . P a i Verneker and J. N. M a y c o c k Research Institute f o r AdGanced Studies, Martin Marietta Corp., 1450 South Rolling Road, Baltimore, M d . 21227
A new thermogravimetric approach to distinguish between the fast decomposition and detonation of lead azide is discussed. This technique is based on the per cent weight loss of the sample and the heat evolved in a detonation. This study has also established both mass and heating rate criteria for detonations. The technique has then been applied to a study of the ageing kinetics of PbNo Cu, PbN6 H 9 0 CO? and PbN6 H 2 0 COz Cu at 0 O C , room temperature (25 "C) and 40 OC. The reactivity changes have been examined in three different ways, a thermogravimetric method, which can differentiate between a detonation or controlled decomposition, a differential thermal analysis technique of exotherm temperature shift, and a differential thermal analysis method whereby the sensitivity (in arbitrary units) is defined as the height of the exotherm over the half width of the peak.
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LEADAZIDE is widely used in military applications as a primary explosive because of its low cost and ease of efficient initiation. An increase in the sensitivity of explosive devices containing lead azide has been observed particularly after long periods of storage. This change in sensitivity has often been attributed to slow chemical reactions with metallic containers leading to the synthesis of highly unstable azides. According to Ubbelohde ( I ) , there are two main variants in routine tests for determining the thermal sensitivity of lead azide. In the first test, the explosive is heated in a suitable container, from room temperature to the ignition point with ignition occurring above a limiting temperature dependent o n the heating rate. Provided a standard rate of heating is used, the ignition temperature gives information on the sensitivity of PbN6. I n the second test, the container of PbN6
is suddenly plunged into a bath maintained at a constant temperature, and the time interval before ignition occurs is measured. Krien ( 2 ) has studied the application of differential thermal analysis (DTA) and thermogravimetry (TGA) in the examination of explosives. In this work, he had to choose low rates of heating (0.5-5 "C minute) for thermogravimetry (balancing problems) with the resultant flattening of the DTA peaks. With the Mettler simultaneous DTA-TGA analyzer used in the present work, it is possible to use any desired rate of heating, between 0.5 and 25' minute-', for accurate thermogravimetry. Because of the hazards involved in handling PbN6 small sizes are desirable. The Mettler micro stick can handle samples as low as 1 mg. Also, because the Mettler is equipped with a transparent quartz furnace it is possible to distinguish, visibly, between decomposition and detonation. This paper describes a new thermogravimetric technique to differentiat-e between detonation and decomposition of PbN6 and further describes the use of simultaneous DTA-TGA techniques to study the reactivity changes occurring in PbN6 stored under different environmental conditions, in particular the suspected increase in the sensitivity of lead azide exposed to copper with a n atmosphere of HzO and C 0 2present. EXPERIMENTAL Apparatus. The apparatus used for this study was a Mettler simultaneous DTA-TGA thermoanalyzer. The test sample of lead azide (pure or mixed with copper) and the reference A1203 were loaded into adjacent platinum cups of 3-mm diameter and 4-mm height. These same platinum cups act as one junction of the temperature sensing thermocouples. The positioning of the cups is shown in Figure 1, the circular shield being necessary for uniform temperature conditions at the measuring head, and also to function as
(1) J. L. Copp, S. E. Napier, T. Nash, W. J. Powell, H. Skelly, A. R. Ubbelohde, and P. Woodward, Proc. RGY.SOC.,Ser. A ,
241,25 (1949).
(2) G. Krien, Explosicesfoffe,13, 205 (1965). VOL. 40, NO. 8, JULY 1968
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Table I. Detonation as a Function of Critical Mass of PbNs Heating rate 15 T/min, abnosphere: flowing He 10 Iitersbr Inference temp. of Mass Visual criterion Mass loss reaction 0.25 mg 1515 "C 3.0 mg Detonation 4 . 0 mg >I515 "C 4.0 mg Violent Deton.
the furnace control thermocouple. Both the test sample and reference material were loosely packed in the crucible to a sample weight of 2.0 mg. After the furnace was placed over the samples, the entire measuring head and balance unit were evacuated to approximately IO-' Torr after which they were filled with dry helium purified by passing through concentrated HSO1, KOH pellets, and P,Os towers. The addition of heated copper turnings, drying, and oxygen removal units, had no effect on any of the DTA traces and were therefore neglected. During a test run, a steady flow, 10 liters/hour, of helium was maintained around the samples. Materials. Lead azide was prepared by the conventional method of mixing aqueous solutions of lead acetate and sodium azide. The precipitated PbNs was thoroughly washed with distilled water and dried in a vacuum desiccator over P,O, and the sensitivity determined. This stock was then divided into different batches and stored at different temperatures (0 "C, 25 "C and 40 "C) in the desired atmospheres (inert gas or CO1 water vapor) either in the pure state or physically mixed with 10% by weight of copper powder. The water vapor conditions over the stored samples were adjusted to 75 Zrelative humidity and the CO, concentration to 300 to 450 ppm.
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RESULTS
Results o n the thermogravimetric approach to differentiate between detonation and decomposition of PbNs are shown in Tables I and 11. Table I shows that a critical mass is required to proceed from decomposition to detonation (keeping the heating rate and other experimental variables constant). Column 2 of this table gives a visual criterion for the differentiation, and column 3 reveals that during decomposition only 25% to 30Z of PhNs is lost whereas 100% loss is observed during detonation. When the heating rate is varied from a low to a high value, the reaction goes from decomposition to detonation (Table 11). As in Table I, column 2 shows a visual criterion for differentiating between decomposition and detonation, and column 3 indicates a 25-30% weight loss for decomposition and 100% weight loss for detonation. Column 4 gives a n analysis of the DTA exotherm associated
Figure 1. Head of the micro DTA-TGA stic:k used in the Mettler therrnoanalyzer This stick WBS used for all of the measuremenIS discussed in the text
with the weight loss. The sensitivity is calculated from the ratio of the height of the exotherm to its half width. As will be seen later, the sensitivity as calculated from the DTA exotherm can lead to the same conclusion as the thermogravimetric analysis. Results of the reactivity changes in PbNs stored under different atmospheres as a function of age and storage temperature of the material are summarized in Tables 111, IV, and V. The data presented in these tables is the average of multiple runs, with typical limits in the exotherm temperatures being +1 "C, and the limits in the sensitivity figures being f0.5 for values below 5 and f1 for higher values. In addition to the data presented in Tables 111, IV, and V a careful examination of the DTA plots reveals the presence o f a small exotherm at about 120 "C for all three systems but only up to the 4 week ageing period. This exotherm is reproducible but even up to
Table 111. Results of Reactivity Changes in PbNe System: PbN6
Age, weeks
Rate of Temperature of exotherm, "C heating Storage ("C/min) temperature: 0 "C 25 "C 40 "C 10
2 4 6 10 15
15 25 10 15 25 10 15 25 15 2.5 10
15 b
+ 10% Cu, He flow rate 10 liters/hr, and sample weight 2.0 mg.
317 321 324 320 322
320 324 326
317 322 328
325 330
328 332 299 322 318 323
327 327 331 338 319 324
323 330 321 320
System: PbNe
Age, weeks 2 4 6 10
15
b
40 "C
7.5D4 29 E5 39 E 13 D 15 E
1.7 D 12 D 66 E
5 12 18
18 52
13 D 76 E 0.8 D 3 D 5.2 D 18 E
11 D 20 E 1 D 6 D 6.5 D 16 E
25 D 29 E 3.3 D 24.0 E
D E
D D E
0.5 D 16 E 15 D 80 E
Results of Reactivity Changes in PbN6
Temperature of exotherm 0 OC Rate of heating Storage 25 "C 40 "C (OC/min) temperature: 0 "C 319 10 323 15 324 332 325 332 25 320 317 10 320 328 325 15 319 25 3 27 326 325 !5 330 329 328 25 327 291 329 15 332 306 338 25 318 10 15 318 317 319 25 325 324
Sensitivity height/half width Storage temperature:
0 OC
25 "C
40 "C
3 D 60 E 0.2 D 55 E
24
0.2 D 35 E 1.0 D 7.0 D
8.0 D 20.0 E 6.0 D 50 E 6.0 D 13 E
1.0 ~5 14
D E
1.0 D
70 E 6.5 D 15
E
0.8 D 1.2 D 18 25
D E
12 25
D E
Eb
1.01)
16
E
D represents a decomposition. E represents a detonation.
System: PbNB
Age, weeks 2 4 6
Results of Reactivity Changes in PbN6
+ H20 + COP + Cu, He flow rate l@iiters/hr, sample weight 2.0 mgs.
Rate of Temperature of exotherm ( " C ) heating Storage ("C min) temperature: 0 "C 25 "C 40 "C 15 25 15 25 10
331 332 324 326
321 326 324 332
15
326 327
323 323
329 329 317 317
326 328 317 318
25 10
10
15 25 10
15 a
25 "C
+ H2O + COS,He flow rate 10 liters/hr, sample weight 2 mg.
Table V.
b
331 335 319 327
0 "C
D represents a decomposition. E represents a detonation. Table IV.
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Sensitivity heightihalf width Storage temperature:
I5 25
323 324 326 327 314 322 316 320 320 322
Sensitivity heightihalf width Storage temperature:
0 "C
25 "C
40 "C
0 . 2 Da 7.0 D 4.0 D 18 E
16 D 60 Eb 13 D 20 E
4.0 D 22 E 10 D 22 E
4.0 D 14 E
17 20
2.0 D 25 E 2.0 D
5 D 30 E 3.0 D 40 E
8 23 5
10.0 E
D E
50
D E D E
7.0 D E
12
D represents a decomposition. E represents a detonation.
VOL. 40, NO. 6 , JULY 1966
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t 317
OC
350°C 332%
122%
Figure 2. A DTA-TGA plot for PbN6 aged 2 weeks a t room temperature; in H 2 0 and COz atmosphere, sample size 2.0 mg, heating rate lSO/min,and a flowing atmosphere of 10 liters/ hour of helium
4 weeks it was not always observed for all of the differently stored samples. With samples that did not detonate the decomposition exotherms can be divided into two classes, a symmetrical decomposition peak shown in Figure 2 and a broad decomposition exotherm shown in Figure 3. Figure 4 is a typical simultaneous DTA-TGA for a system undergoing a detonation. DISCUSSION
When lead azide is thermally decomposed, the reaction products are gaseous nitrogen and metallic lead. The nitrogen content of lead azide is about 28Z ; therefore a thermogravimetric analysis (weight loss us. temperature) would show a 28% weight loss. This is exactly what the data in Tables I and I1 show; however, when the thermal reaction leads to an explosive decomposition (detonation), the thermogravimetric analysis shows a 100% weight loss. Explanation for this is that detonation of PbN6 creates high temperatures on the order of 3300 "C such that the metallic lead formed during the detonation is vaporized. This simple thermogravimetric technique provides a very reliable method to differentiate between decomposition and detonation again eliminating the need for visual observations. Furthermore, very small sample sizes are sufficient for this type of test. With a constant heating rate, the sample mass becomes the critical factor to decide whether the reaction would lead to decomposition or detonation (Table I). I n order to differentiate between the sensitivities of different samples of PbNs, it is correct to say that the sample which requires the minimum weight to detonate is the most sensitive and vice versa. This is in complete agreement with earlier knowledge that a critical mass is required for detonation ( 2 ) . Results in Tables I and I1 show that this critical mass is very much dependent on the heating rate. Therefore, if the mass is maintained constant, the heating rate can be used to differentiate between lead azide samples of varying sensitivities. The lower the heating rate a t which a given mass of PbN6 detonates, the higher is its sensitivity and vice versa. When D T A is studied as a function of heating rate, the sensitivity, defined as the ratio of height of the exotherm to its half width, is seen to change from a low arbitrary value to a high one as the reaction goes from a decomposition process to detonation. A prior calibration against the thermo1328
ANALYTICAL CHEMISTRY
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Figure 3. DTA-TGA data for PbN6 aged 4 weeks in an H 2 0 COz atmosphere a t room temperature; sample size 2.0 mg, heating rate lO"/min, and a flowing atmosphere of 10 liters/hour of helium
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gravimetry is, however, necessary to give any meaning to this arbitrary sensitivity value. The D T A sensitivity method can be used, even without calibration, to study the reactivity changes in PbN6 stored under different atmospheres as a function of age and storage temperature. In addition, the D T A study can disclose other characteristics of PbN6 occurring at different temperatures prior to the detonation o r decomposition. A careful study of all the D T A data generated on this ageing study reveals that the small exotherm occurring around 120 "C is not always observed and that its appearance is also independent of the system in which PbN6 is stored and finally that after 4 weeks of storage, it is never observed. It is most probable, therefore, that it is a characteristic of the PbNe and not caused by the atmosphere or additives used in the ageing study. The exact phenomenon with which it is associated is hard to define; a detailed study is not possible because it does not always occur. We infer from the data (as we have never observed it after 4 weeks) that whatever causes the peak has annealed out after 4 weeks. It is not possible from our thermogravimetric data to say whether any weight loss is associated with this peak, although if there was any weight loss it must have been less than 0.05 mg--i.e., a 2.5z weight loss due to the sensitivity of the balance. Two tentative explanations are therefore possible: some P-PbNs is formed during the preparation of a-PbN6 and the peak in the DTA at 120 "C represents a crystal phase change. With the /3 form being less stable than the CY form it is conceivable that it is completely converted to CY after the 4 week storage. During the early stages of the thermal decomposition of CY-PbN6,the rate of the decomposition decelerates to a constant value which constitutes the major portion of an induction period. Pai Verneker and Forsyth (3) have observed this phenomenon and have shown that it depends on the particle size and the number of defect centers within the crystal. Although they have not studied the effect of ageing on the thermal decomposition of C Y - P ~ N their ~ , study of the photodecomposition of a-PbN6 ( 4 ) shows that these defect centers anneal out after a storage of 4 to 6 weeks. The exothermic peak which is seen in this work around 120 "C may well be associated with the initial decomposition. (3) V. R. Pai Verneker and A. C. Forsyth, Inorg. Nucl. Chem. Lett., 3, 257 (1967).
(4) V. R. Pai Verneker and A. C. Forsyth, J. Phys. Chem., 71, 3736 (1967).
The main exotherm i s followed by one or two additional small exotherms. During decomposition, the residue turns yellow a t these temperatures and indicates the formation of small amounts of oxides of P b (the oxygen is a residual impurity in the helium gas). When the temperature was raised further, the formation and final decomposition of higher oxides is seen. The main exotherm which is symmetrical during the first two weeks becomes broad except in the case of P b N 6 Cu. The other two systems had water and CO?; this, therefore, suggests that some basic lead azide is formed. The reactivity changes (changes in the sensitivity) as a function of the age of the material have been investigated in three different ways : thermogravimetric method, which can differentiate between decomposition and detonation, as a function of heating rate; differential thermal analysis method of shift in the exotherm temperature; and differential thermal analysis method whereby the sensitivity in arbitrary units is defined as the height of the exotherm over the half width. Thermogravimetric Method. Careful qualitative analysis of the data in the tables for (PbN6 H 2 0 C o t ) and (PbNs H,O COz Cu) reveal that during the first 4 weeks copper desensitizes P b N 6 ; however, from 4 weeks to 15 weeks a sensitization by copper is seen. The change from desensitization to sensitization, qualitatively, coincides with the formation of basic lead azide as shown by the broadening of the main exotherm. Whether this has any meaning or not, it is not possible to say at this stage. One can, however, speculate that formation of basic lead azide, being a chemical reaction, facilitates the incorporation of copper into the basic lead azide; copper can enter the basic lead azide crystal lattice either as Cu+ o r as Cu2+or in the form of some complex. A sensitization would be observed in either case because the anion vacancy concentration has increased or because of the copper ion being colored (5, 6). The desensitization effect during the first 4 weeks may also then be explained by saying that diffusion of copper into the crystal lattice has not yet taken place. Pai Verneker and Avrami (7) have shown in the case of barium azide that small concentrations of metal, less than l.O%, increase the reactivity, whereas metal in concentrations of more than 1.0% causes a desensitization. By virtue of its high thermal conductivity the metal will tend to inhibit hot spot formation and thus inhibit the explosive decomposition. The present system had 10% C u and by analogy with barium azide would desensitize lead azide. The observation of desensitization followed by sensitization caused by copper is true at all the storage temperatures. DTA Method of Shift in the Exotherm Temperature. An analysis based on this method does not permit the assignment of the role of copper on the ageing characteristics of PbNs. I n most cases, irrespective of whether C u is there o r not, an initial desensitization followed by a sensitization is seen. This generalization is in agreement with the one drawn from the approach of thermogravimetry. DTA Method of Sensitivity of Arbitrary Units. The results of this method are very similar t o those of the thermogravimetric approach and it is apparent that C u desensitizes in the initial 4 weeks after which a sensitization is observed, This work has shown a new thermogravimetric approach to
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Figure 4. DTA-TGA data for PbN6 aged 2 weeks at room temperature in an H 2 0 CO, atmosphere
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With the 25 '/rnin heating rate, 2.0 mg of sample, the detonation characteristics are very obvious
differentiate between decomposition and detonation. This method in conjunction with D T A has enabled us to study the ageing characteristics of a-PbN6. The data obtained leads to the following conclusions: A small exotherm occurring around 120 " C in a D T A trace appears a t times during the first 4 weeks of ageing after which it is never observed. This, we speculate, is associated either with a crystal phase change, or more likely, with the initial gas evolution peak as found in the rate of gas evolution us. time plots for the thermal decomposition of a-PbN6 (3). The main exotherm which is symmetrical during the first two weeks becomes broadened and unsymmetrical except in the case of PbNG Cu system. This implies that water and COz react with PbN6 forming a basic salt. The sensitivity of the system PbN6 f H 2 0 C02 goes through a cyclic process of an increase, a decrease and a n increase in sensitivity during the 15 week period of ageing. By comparing these results with those on the system PbN6 H 2 0 Con Cu, it appears that copper desensitizes in the first 4 weeks after which a sensitization is observed. The desensitization is tentatively attributed to a relatively large concentration of metal which acts as a heat sink, and the sensitization is attributed to colored ions of C u diffusing into the basic lead salt.
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ACKNOWLEDGMENT
The authors thank L. L. Rouch, Jr., for his valuable assistance in this experimental study. Maycock, V. R. Pai Verneker, and C . S. Gorzynski, Jr. Spectrochim. Acta, 23A, 2849 (1967). (6) V. R. Pai Verneker and A. C . Forsyth, J . Phys. Cliern., 72, 111 (1968). (7) V. R. Pai Verneker and L. Avrami, ibid., 72,778 (1968). ( 5 ) J. N.
RECEIVED for review March 19, 1968. Accepted April 30, 1968. Work supported by Picatinny Arsenal, U. S. Army Munitions Command under Contract DAAA21-67-C-1074. VOL. 40, NO. 8, JULY 1968
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