Strength of Lead Azide

Until recently, little attention has been given to the dielectric breakdown strength of lead azide values reported in the literature vary from 20 to 300 kV/cm. Past studies were primarily directed to the dependence of the dielectric strength on such factors as sample thickness, density, temperature, etc. Little attention was given to the type or shape of electrode, the electrode material, or the degree of contact between sample and electrodes. Thus the measured dielectric strength was determined by the conditions of the measurement, which were usually different for each investigation. 

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In another study, Sukhushin and coworkers [35] studied the impulse breakdown strength of lead azide pellets of different thicknesses and densities. The effect of changing electrode polarity was also investigated. The specimen was a wafer pressed on a polished, hardened steel cylinder into a polyvinyl chloride sheath. The outer spherical electrode was of 10 mm diam and positioned against the outer surface of the specimen. Square-wave pulses, 1.5-40 //sec, were used to determine the breakdown voltages. 

Figure 25. Dielectric strength of lead azide as a function of density and thickness [35].

F ure 21. Dependence of dielectric strength on volume fraction of lead azide [34],... 

A. Zimmerschied, F. Davies, Investigation of Lead Azide Dielectric Strength, The Boeing Company Report No. T2-4096-1, Seattle, Washington, 1974. 

A plot of P vs. Up has the slope poDs and passes through the origin. If the Hugoniot curve for the detonation products of lead azide at the C-J point were known (the curve for unreacted RDX is known [40]), it would be possible to determine the strength of the shock wave generated in RDX by the detonation wave from lead azide. It is to be recalled that the Chapman-Jouget model as modified calls for a reaction zone at the end of which the reactants have been completely converted to products in equilibrium and travel at the local sonic velocity. The end of the reaction zone is often called the C-J plane, and the associated pressure and temperature called the C-J pressure and temperature. 

Stengach, The Dielectric Strength of Pressed Lead Azide , ZhPriklMekhiTekhFiziki 1 (1972),... 

Properties Bright-yellow crystals or amorphous powder or granules. Sp. gr. 1.63. Explodes when heated to a temperature of 180°. Less sensitive to impact than either lead azide or mercury fulminate, much less sensitive to friction than the latter, and does not get dead-pressed. Cannot be detonated under water with a No. 8 electric detonator. The strength (sand-bomb test) is about, twice that of mercury fulminate or lead azide. The initiating power is greater than... 

Pressed lead azide is a two-phase dielectric system consisting of azide crystals of various dimensions and air in the pores at ambient pressure. Its dielectric strength varies with the density, crystal dimensions, thickness, etc. 

The effect of density on the dielectric strength of dextrinated lead azide, normal lead styphnate, and silver azide was also investigated (Figure 16). The dielectric strengths of dextrinated lead azide and normal lead styphnate increased with density. The breakdown level of silver azide was relahvely unaf-... 

Pitts [31] reported a dielectric strength of 36 kV/cm for a 0.5-mm-thick lead azide pellet of 3.7 g/ml density. The explosive was confined in a block between two 4.2-mm-diam flat-bottom drill rods. A 500-pF capacitor discharge was used to determine the dielectric strength. No description was noted in the report of the type of azide investigated. 

Figure 19. Average electric field strength for detonation of dextrinated lead azide as a function of thickness [11].

Table VII. Lead Azide Dielectric Strength as a Function of Electrode Curvature [34]...

Brish and coworkers considered the following mechanisms for initiation (1) The impact pressure of light. Calculation showed, however, that this only amounted to 10" bar and could be disregarded. (2) Electrical breakdown. The breakdown strength of compressed lead azide is 10 V/cm. Since at the critical intensity for initiation the laser light produces an average electric field of 0.7 X 10 V/cm, and self-focusing effects could increase this a further 3-5 times, this is clearly a possible mechanism. (3) Photochemical decomposition... 

For lead azide of density 3 X 10 kg/m Sukhushin et al. believe that microdischarges in the pores of the pellet are important. The pores reduce the dielectric strength of the pellet. When the microdischarges are of critical size, the substance initiates. 

It was originally assumed that the deterioration of LA in a detonator, because of its decomposition by carbon dioxide and the formation of basic lead azide or basic lead carbonate, would decrease its initiation efficiency and hence decrease overall ability of the detonator to perform with the desired strength. Danilov et al., however, published that the lead carbonate that forms during the reaction of LA with carbrni dioxide creates a surface layer which protects LA from further decomposition [3]. The mechanism of deterioration of LA depends upon hydrolysis cmiditions. According to Blay and Rapley, if the hydrolysis is not accelerated by abnormal conditions, the deterioration does not proceed beyond an acceptable level in service detonators [36]. The same thinking was reported in 1975 by Lamnevik who did not notice any loss of function in an LA detonator due to LA degradation [3, 31]. 

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