Lead azide activation energy

Many investigations are reported on azides of barium, calcium, strontium, lead, copper, and silver in the range 100 to 200°C (212 to 392°F). Time exponents were 6 to 8 and activation energies of 30 to 50 kcal/g mol (54,000 to 90,000 Btu/lb mol) or so. Some difficulties with reproducibility were encountered with these hazardous materials. 

According to Garner and Gomm [37] the activation energy of the thermal decomposition of lead azide is 38.0 kcal/mole, assuming that the reaction can be expressed by an equation of the form p=kt. 

Fig 18 Effect of Ultraviolet Irradiation on Induction Time, 50% Point, and Activation Energy of Colloidal Lead Azide (Ref 153)... 

The problem of the action of radiation on azides, particularly lead and barium azide has been review cd [116]. Irradiation prior to thermal decom-Jposition often effects a reduction or elimination of the induction period, a de-ICrcasc in activation energy and an increase in the rate of decomposition. Zak-rov and co-workers [117] have found that the application of a moderate electric field can affect the rate of thermal decomposition of azides. 

Copp et al. believed that percussion sensitiveness (with the ball and disk apparatus) was very complex and involved, in addition to the formation of hot spots through friction, a tribochemical reaction in which there was a more direct transfer of mechanical energy to activation energy than was the case when the mechanical energy is first converted to heat. An example of this is shown with experiments using nickel and tin disks to confine Service lead azide in which the surfaces of the low melting point lessened the grit sensitivity but enhanced the impact sensitivity. 

Figure 19. Effect of ultraviolet irradiation on induction time, 50% point, and activation energy of colloidal lead azide [117].

There is, however, no direct experimental evidence for this hypothesis on the contrary, single-crystal photoconductivity measurements by Dedman and Lewis [64] suggest that thermal exciton formation is the first step in decomposition. Formation of the exciton for lead azide occurs with a thermal activation energy of 122 kJ/mole, which is very close to the experimental values of 123 kJ/mol and 125 kJ/mol for thermal decomposition, implying that exciton formation is the rate-determining step in decomposition. 

Singh observed that doping lead azide with small amounts of (BiNa) (0.24 wt %) results in increased rate and decreased activation energy for thermal decomposition [101]. On this basis it could be theorized that the rate-controlling step is the transfer of an electron from the azide ion to the conduction band and that if this energy barrier is reduced, for example by adding an impurity which introduces new energy levels in the band gap, decomposition rate is enhanced. 

In spite of the observed effects of (FeNs) " and Fe " on the decomposition kinetics and absorption energy levels of lead azide, neither impurity significantly altered the activation energy for thermal decomposition. Thus, it is likely that these impurities affect the thermal decomposition rate of lead azide by altering the equilibrium concentration of reacting species (possibly N3) or sites, rather... 

Both a- and -forms of lead(II) azide are very sensitive to shock and to thermal decomposition, but the activation energy for decomposition of the / -form is considerably lower than that of the a-form, and at about 260°C the violence of detonation of the)8-form is about twenty times greater than that of the a-form227. 

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