Sensitivity of Azides

It is thus not surprising that, even neglecting differences in apparatus, data that relate the impact sensitivities of explosives are generally not consistent, a comment which applies even to the sensitivity of lead azide relative to that of secondary explosives. For example, there exist data by Kondrikov [30] showing that in a comparatively well-instrumented apparatus lead azide appeared less sensitive to impact than TNT, PETN, and TNB. Tables II and III illustrate the degrees of consistency obtainable with present apparatus. 

Another parameter which may be assessed in comparing data is the time delay for the initiation following impact. Cook [32] found the delay in primary explosives to be virtually independent of the potential energy of the drop weight (Table IV). 

Impact energy (J) Average delay (psee) Delay range (psec) 

90% Service lead azide/10% beta lead azide 11.91 0.081 

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Impurities, such as grit, shreds of cotton, even in small quantities, sensitize an expl to frictional impact. That is why utmost cleanliness must be exercised in the preparation of expls. There are differences in the sensitivity of azides to mechanical and thermal influences. They have been correlated with the structure of the outer electronic orbits, the electrochemical potential, the ionization energy and the arrangement of atoms within the crystal. Functions of the polarizability of the cation are the plastic deformability of the crystals, and their surface properties. The nature of cation in an azide, such as Pb(Nj)2, has little effect on the energy released by the decomposition, which is vested in the N ion. The high heat of formation of the N2 molecule accounts... 

The sensitivity of azides to the impact of the falling hammer or weight, or to collisions with other massive objects, is among the properties commonly determined when a compound or artifact is fust prepared [5,8]. A crude impact test is easy to devise, but it is only one of several tests that assess the effect of comparatively weak mechanical action on explosives (the effect of strong shocks... 

Table V. Ball and Disk Impact Sensitivities of Azides [41,42]...

The influence of impurities on sensitivity has practical significance from a safety viewpoint, and the ability to reduce the sensitivity of azides with certain impurities appears possible. Increasing the detonation velocity of the azides by adding impurities could produce more powerful detonations for critical applications. 

Available information on the impact sensitivity of azides, predominately lead azide, shows that the data are affected by the test conditions and the condition of the material itself. The energy and the rate of energy transmitted by impact to the test sample are critical for initiation. 

Because of the extensive discussion in Volume 1, the sensitivity of azides to heat is only briefly touched upon in this chapter. The effects of high-energy (UV) radiation of low intensity are also discussed at length in Chapter 7 of Volume 1. 

The sensitivity of azides to heat is one of their properties which can be most precisely determined. The more practically useful substances, such as lead and silver azides, do not detonate until temperatures close to or at their melting points are attained. Among technologically important sohd explosives such as TNT, tetryl, and RDX, the relatively high melting points of lead and silver azides (<300°C) and the good vacuum stability in standard tests are perhaps not representative of their overall sensitivity. Once a threshold temperature has been attained in the azides, the transition from slow decomposition to detonation is... 

Sensitization by electron transfer can occur in resist systems. A good example is the sensitization of azide photolysis by aromatic hydrocarbons, which proceeds by the coupled reactions, ... 

In literature, there are several attempts described to predict and calculate the impact sensitivity of energetic materials. Most of these papers deal only with nitro- and nitrate ester compounds, whereas the impact sensitivity of azides has not been the subject of detailed calculations so far, apart from recently published structure-sensitivity correlations on inorganic azides. ... 

The vertical (Franck-Condon) excitation energy ( V) itself is an important photochemical characteristic of azide. This calculated parameter corresponds to the experimentally measured maximum of the long-wavelength absorption band and characterizes the region of spectral sensitivity of azide. From Table 12 it is seen that monotonically decreases with an increase in the size of the azide molecule for both neutral and protonated azides. The decrease in is symbatic to the above-discussed decrease in the HOMO-LUMO gap, which is an essential part of E, and corresponds to the bathochromic shift of the absorption band observed in the spectra of aromatic compounds with an increase in the number of annelated aromatic rings [55]. 

Hence, it follows that the requirement of long-wavelength light sensitivity of azides is contradictory. On the one hand, to exhibit bands in the visible region, an azide should be a dye, and most of dyes have an extended system of conjugated 7l bonds and are cations. On the other hand, upon an increase in the size of the 7l system and positive charging, the azide loses photoactivity and the quantum yield of photodissociation of the azido group sharply decreases to < 0.01. This problem is considered below. 

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