The Role of Azides in Explosive Trains

A typical detonator has three sections, the first containing a (nondetonating) primer the initiator, lead or silver azide, is in the second section and finally, a secondary explosive forms the third section. Normally, each of the sections is filled or loaded by pressing the explosive into a metal sleeve or cup under pressures of several thousand pounds per square inch. The next component or element in an explosive train is an acceptor charge, containing a secondary explosive or lead the output Irom the secondary in the detonator is required to initiate the lead reliably (secondary explosive acceptor). 

In many detonators lead azide is found in both the priming mixture and the initiator and is followed by a secondary explosive such as PETN, RDX, or HMX. In those items that respond to mechanical stimuli, a firing pin is driven into the primer, igniting it. The flame (or blast) from the primer spreads to the azide, which builds up to a detonation and in turn initiates the secondary explosive in contact with it. A similar sequence of events occurs with explosive trains which respond to other mechanical, electrical, or thermal stimuli. The satisfactory functioning of the azide (or other energetic material) in any component or element, requires, therefore, a carefully planned sequence of response, energy release, and energy transfer within the limited dimensions of the explosive train. 

Because precise measurement techniques have not been available to determine the response and performance of detonator elements, it is not possible to state unequivocally which parameters are most important for their most effective or optimum design. Potentially important parameters can, however, be derived from well-known principles that govern the behavior of solid explosives. 

The parameters may be associated with the properties of the explosive as incorporated in the detonator, such as its composition, particle size, density (both in real terms and as a function of filling pressure), and with the volume. 

The need for data on the parameters is neither unique to the application of azides and solid explosives nor to the design of explosive-train elements, and numerous references to measurement techniques and relevant data are to be found throughout these two volumes and in standard handbooks [ 1 ]. However, the data on functional parameters, particularly within the constraints of element designs, represent a unique requirement, particularly because of the hazardous nature of the materials, the rapidity of the reactions involved, and the small quantities and dimensions available for measurement probes. In Section D of this chapter some recent techniques of measurement or observation are presented to illustrate current trends. One of the important parameters affecting the initiation and growth of reaction in the explosives is their vulnerability to shocks. Some recent techniques for quantifying these parameters are given in Section E below. 

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In considering the advanced diagnostics it should be remembered that the advances presented are concerned only with the azides, their properties and confinement, and the role of the substance earlier or later in the functioning sequence. Most of the work reported was done on production materials, and representative data that relate to the materials, rather than actual explosive-train components, is presented. 

It has been seen that lead azide plays both the role of donor and acceptor in explosive trains. In igniters and detonators lead azide acts as an acceptor by re-... 

Sensitivity plays a pivotal role in explosives technology, because on the one hand it is indicative of the hazards associated with handling a material, and on the other hand it is a key parameter determining the effectiveness of an explosive in an explosive train. In the former case occurrences of low probability are of interest, while in the latter case reliable functioning of an azide demands certainty that it will detonate in response to a given stimulus. 

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