ESD sensitivity

One or more clip leads can be stmng in series to make the wrist strap wire long enough. Then the strap is fastened to the experimenter s arm. The circuit of Fig. 18.4 is then assembled, using an IGFET, quite analogous to the bipolar circuitry on 

Battery, nine volts Clip leads, 14 inch, one set. 

Lamp bulb, tungsten, 12 volt dc, green or blue Power MOSFET, N-channel Static control wrist strap Multimeter (or multitester ) 

Neon test light, 90 volts Power cord, lEC, with 3-prong plug 

Electrostatic discharges have been made responsible for many accidents in the manufacture ofmagnesium/Teflon -based compositions [19-21] (see also Chapter 18). Hubble et al. recently reviewed the complete manufacturing process of cylindrical 36 mm countermeasure flares and identified a number of possible operation steps prone to yield a large-surface electrostatic potential [22] (Table 18.7). 

Influence of sample composition and PTFE particle size on conductivity and breakdown conditions is listed in Table 19.6. The addition of conductive additives 

Composition Thickness (mm) Uc (kV) (MVm- ) Number samples before (Q- m- ) afler (a- m- ) 

Analysis of the samples that experienced breakdown shows the formation of conductive ducts. These ducts in the cases where ignition takes place undergo Joule heating and create hot spots that effect sample ignition. 

The dielectric breakdown voltage of MTV igniter compositions as a function of modifiers added has been studied in [28, 30). 

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A brief clarification needs to be made about ESD sensitivity. As ESD is the most difficult form of insult to guard against, great attention needs to be paid to materials that exhibit high sensitivity to it. ESD initiation sensitivity was measured for only the purest form of TATP. As in all other forms of insult, the pure TATP was only slightly more sensitive than PETN. A standard deviation was done for the measurements. Using simple statistical analysis, it can be shown that TATP will initiate approximately 1 to 2% of the time with energies as low as... 

HMTD displays some interesting characteristics. It was surprisingly fairly insensitive to impact compared to PETN and TATP. The impact sensitivity measured lay between the values for PETN and RDX. Even more surprising was the ESD sensitivity. HMTD is a very line powder when isolated. It has an appearance almost like that of talc. Fine powders are well known for being prone to static charge buildup. When transferring from one container to another, the static buildup in the HMTD was obvious by the way the powder stuck to the walls of the vessels involved. HMTD was more static sensitive than either of the standards tested, but it was less static sensitive than TATP. This was unexpected, and at the moment cannot be explained. The values obtained seem to indicate that HMTD has only moderate static sensitivity. 

Formulation Density (99%TMD)/g Detonation cm3 velocity/ms ERL impact sensitivity/cm BAM friction sensitivity/N ESD sensitivity/]... 

As is shown in Figure 6.4, the ESD values are greatly influenced by the particle size in these tests the sample must be carefully sieved prior to measuring. The finer the powder of a particular sample is, the higher the ESD sensitivity values are. Usually, compounds with ESD 0.1 J are classified as sensitive, those with ESD 0.1 J as insensitive. 

Fig. 6.4 Dependence of the ESD sensitivity on the particle size for hexogen (RDX) and octogen (HMX).

Tab. 13.6 Impact and ESD sensitivities of aero- and xerogel Fe203/Al thermite.

Naud et al. have preformed extensive work on the use of the Mo03/A1 nanothermite for application in electric matches. Electric matches prepared from this thermite mixture have been shown to have lower friction, impact, thermal and ESD sensitivities than the currently used matches, which contain toxic lead compounds making them both safer and more environmentally friendly to use. 

Sensitive components and assembhes must be enclosed in conductive, static-shielding bags, boxes, or wraps when not being worked on, unless otherwise protected. ESD-sensitive items must be removed from protective enclosures only at static dissipative or antistatic workstations. 

While increased sensitivity can make nano-thermites more dangerous to handle, the increased friction or impact sensitivity is also beneficial in some practical applications such as percussion primers. However some nano-thermites have increased ESD sensitivity, which has no current practical apphcation and is only a safety hazard. For example, a Bi203/Al nano-thermite has sufficient impact and friction sensitivity to be considered for use in ammunition primers, however it has an ESD sensitivity of 0.125 gj (40 nm Bi203, 41 nm Al). This is a static potential, which is easily achieved by the human body. This makes handling hazardous, especially since the Bi203/Al thermite has a combustion velocity of over 750 m/s which means that this thermite explodes rather than bums. The increased ESD sensitivity of nano vs. micron composites is believed to be the result of the increased ability of high surface areas to develop charges. 

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