Adiabatic-compression sensitivity

The major chemical components of emulsion explosives are fundamentally the same as those of slurry explosives, as shown in Table 9.4.[i l Instead of the sensitizers used for slurry explosives, a large number of hollow microspheres made of glass or plastics are incorporated to formulate emulsion explosives in order to obtain successive detonation propagation after the initiation of detonation. During detonation propagation into the interior of the explosives, an adiabatic compression results... 

The spots may be formed in two main ways 1) By friction on the confining surfaces of expls, on grit particles, or on crystals of the explosive itself and 2) By the adiabatic compression of small occluded gas bubbles the presence even of a small bubble can render most expls very sensitive to impact (Ref 19)... 

Stability. Lower explosive limit in air, 7.3% by volume sensitive to adiabatic compression (Angus Chemical Co., 1998) forms an explosive sodium salt which bursts into flame on contact with water (Budavari, 1998) Octanol/waterpartition coefficient (P) log P, -0.35 (Hansch et al., 1995) Conversion factor -, mg/m = 2.50 x ppm... 

Adiabatic Compression of Entrapped Gas or Vapor as a Cause of Initiation of Explosives. It has been shown by some Brit investigators that one of the important causes of initiation of sensitive expls by mech actionfsuch as impact) is the adia-... 

It is reported in Refs 42 67 that moderate quantities of waxy materials incorporated with such expl materials as HMX, RDX and PETN do not appreciably affect their initiation sensitivity, but do inhibit propagation to expin. Ref 67 further states that no relationship between the specific and latent heats of the desensitizers and its ability to desensitize could be found, and concludes that the desensitizer cannot be regarded merely as a thermal sink. This is somewhat in conflict with results of work in Ref 91, where it is reported that it has been established firmly that the sensitivity of the RDX compositions decreases with increasing specific heats of the additives, and a solid desensitizer functions primarily by absorbing heat from local regions of initiation including any hot spots which arise from adiabatic compression of occluded gas ... 

Ignition delays, investigated in adiabatic compression machines (89, 91, 92, 106, 182, 209, 212, 213), have been correlated with knock (90, 93-5, 111, 158, 160, 194, 203). Ignition occurs in two stages. Levedahl (106) examined the effects of temperature, density, and fuel type on the induction periods, ti and r2, corresponding to each ignition stage. A close correlation existed between total delay, r, and knock resistance. Sensitivity was explained in terms of the relative partial contributions of n and r2 to r. 

The bulk density of ANFO ranges from 0.80 to 0.87 g/cc. So, clearly about half of the ANFO is air or void space. All explosives require a certain amount of entrained void space in order to detonate properly. These void spaces also play a major role in the detonation reaction by creating hot spots under adiabatic compression in the detonation front.39 The amount of void space in any given explosive and the resultant change in density have a significant impact on the detonation properties like detonation velocity, sensitivity, and even energy release. 

This term denotes the increase of the detonation sensibility of explosives by finely dispersed air bubbles. The loss in sensitivity to detonation of gelatinous nitroglycerine explosives by long storage has been known since the time of Alfred Nobel it is due to the loss or coagulation of the air bubbles that may have been left in the explosive by the manufacturing process. This effect can be explained by the adiabatic compression and heating of the air inclusions as the detona-... 

Johansson and co-workers (7, 8, 9) have shown that heat transfer from a compressed spherical bubble does not increase the temperature of its liquid surface sufficiently to account for the impact sensitivity of liquid explosives the high sensitivity of nitroglycerin is postulated as arising from the fact that small droplets are readily formed by the impact and ignited by the compressed air. Bolkhovitinov (1) postulated crystallization of the liquid under the impact pressure, with the phase transition causing the temperature increase which causes explosions. Bowden (3) favors the adiabatic compression of gas bubbles combined with the dispersion of the explosive into fine particles as the mechanism for initiation by mechanical impact. 

Of the approximately 12 motion pictures we made of the impact initiation process, all show that the structure of the air bubble is broken down and replaced by a turbulence area. Ignition occurs at the former site of the bubble after an induction period. The compression ratio of the air bubble appears to be the major factor determining probability of initiation by impact. The mechanism for impact initiation of nitroglycerin therefore appears to be a quasi-adiabatic compression of the gas, with heat transfer accelerated by spray formation. Hot spots formed at the former site of the bubble undergo an accelerating exothermic reaction which proceeds to a deflagration. The possibility that liquid explosives under reduced pressure may be sensitized to weak impacts must be considered. 

Ultrasound propagation is adiabatic in homogeneous media at the frequencies typically used in US-based detection techniques. Therefore, although temperature fluctuations inevitably accompany pressure fluctuations in US, thermal dissipation is small and it is adiabatic compressibility which matters. As a second derivative of thermodynamic potentials, compressibility is extremely sensitive to structure and intermolecular interactions in liquids (e.g. the compressibility of water near charged ions or atomic groups of macromolecules differs from that of bulk water by 50-100%). 

Bowden and Yoffe 79. 80] pointed out the role of small trapped gas bubbles in the sensitivity of both liquid and solid explosives. The adiabatic compression of small trapped gas bubbles creates hot spots manifested by high temperature. Also friction occurs at the surface of the containing walls, on grit particles or between the crystals of explosives and forms an additional factor governing the sensitivity of explosives to impact. 

It was soon recognized that the presence of tiny air-bubbles in the slurry was a veiy good sensitizer. TIic bubbles adhere to TNT particles or Al powder and help detonation by creating hot spots by adiabatic compression. 

Some workers believe that adiabatic compression of interstitial gases is not important [38-40], because the shock sensitivity of some explosives is approximately the same for both vacuum and atmospheric pressure conditions. They consider that when an explosive is placed under vacuum it contains no interstitial gas. However, recent work showed that during the fracture of thermally unstable crystals decomposition products are produced [41,42]. The amount of products depended on the surface area created by fracture, the velocity of the fracture, and the crystal type. With lead azide the gases produced were equivalent... 

Using single crystals, it has been shown that thermal decomposition rates depend on the available areas of preferred surface and that induction periods decrease with increase in the surface area. However, virtually no other information is available on the sensitivities of different crystallographic faces of the azides. It is a matter of interest to determine whether the different thermal sensitivities are reflected in different sensitivities to adiabatic compression of the gas in the vicinity of the surfaces, or to electrostatic discharge, impact, or friction. Current- or field-initiation experiments on different azide surfaces (see the previous section) are also suggested by these observations. 

Studies of the sensitivity of EMs are mostly directed to impact and shock sensitivities (Sect. 3). A primary fragmentation of the given EM until adiabatic compression of a thin layer of its molecules by impact or shock wave is the unambiguously dominant view in this case. Physical Kinetics Model [97] was a typical representative of this view of mechanical spht-ting of the molecule at any dehberate bonds between its atoms by shock wave. One of the best developed models is Dlott s Model of the Multiphonon... 

Bowden [3] and others have attributed the ignition source to localized hot spots resulting from the adiabatic compression of trapped gas bubbles and of hard "grit" particles in the material. Other investigators, however, questioned whether the small actual amounts of heat thus liberated would be sufficient to cause ignition in the extremely short time of impact [6] and suggested that "trigger-actions" [2] other than hot spots could contribute to the sensitivity. 

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