Investigating the performance and behaviour of porous silicon energetic materials.

Author: Andrew Plummer

Plummer, Andrew, 2017 Investigating the performance and behaviour of porous silicon energetic materials., Flinders University, School of Chemical and Physical Sciences

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Nanoporous structures etched into silicon wafers possess versatile and predictable morphology at nanometer scales. Loading these pore structures with an oxidising agent in a suitable carrier solvent forms an intimate fuel / oxidiser mixture able to react explosively with only minor initiating stimuli. Here, porous silicon (pSi) is formed through the electrochemical etching of mildly doped p-type wafers to form highly branched pore networks in the 3-5 nm pore size range, leaving the overall bulk dimensions of the wafer unchanged. The porosity, pore size and layer thickness are easily controlled through selection of etching parameters.

Burning rates of the system were found to be dependent on the choice of oxidising agent, porosity, layer thickness and degree of confinement. Sodium perchlorate demonstrated burning rates as high as ≈ 500 m.s-1 - other nitrate and perchlorate oxidising agents were shown to be suitable alternatives albeit with decreased burning rates. Perfluorinated compounds, notably perfluoropolyether (PFPE), were also considered as viable alternatives to sodium perchlorate.

Analysis of the flame profile by high-speed video is also presented, suggesting that the reaction type is a deflagration rather than a detonation. A strong plume of flame is emitted from the surface, indicating the potential for this material to perform useful work either as an initiator or as a propellant. Erratic burning rates were revealed to be due to material factors associated with the brittleness of the porous silicon, with fissures generated along which the advancing flame could jet.

pSi energetic materials were revealed to be as sensitive as comparable primary explosives. Loaded with sodium perchlorate, pSi was exceedingly sensitive, recording sensitiveness to initiation below the limit of detection for impact, friction and electrostatic discharge. However, the same system loaded with PFPE was insensitive to initiation by friction, yet remained extremely sensitive to impact or electrostatic discharge.

Infrared laser pulses could initiate pSi loaded with sodium perchlorate via either laser thermal ignition or laser-generated shock waves. Using Photon Doppler Velocimetry, it was determined that these waves are weak stress waves with a threshold intensity of 131 MPa in the silicon substrate. Shock generation was achieved through confinement of plasma, generated upon irradiation of an absorptive paint layer held against the substrate side of the wafer. These stress waves were below the threshold required for sample fracturing. Interestingly, the transparency of silicon in the infrared region could be exploited to increase the threshold of initiation by shining the laser through the backside of the supporting wafer, revealing a method of isolating the energetic material both from damaging environmental conditions (i.e. moisture) and from potentially risky electrical initiation trains.

The reaction between pSi and the loaded oxidising agent was studied using correlated differential scanning calorimetry (DSC) and FTIR spectroscopy for samples heated continuously between ambient and 500 oC. It was observed that the energetic reaction between pSi and sodium perchlorate depended on the presence of various hydride species on the surface of freshly etched pSi, and on formation of volatile free radical species released during either oxidation of the surface in the presence of air at about 200 oC or during desorption of the hydride above 270 oC in the absence of oxygen. However, energetic reactions between pSi and PFPE were delayed until pyrolysis of the PFPE above 390 oC in the absence of oxygen, suggesting PFPE’s suitability for pyrotechnics applications. Correlated thermal and spectroscopic methods of analysis gave new insights into the earliest stages of the reaction of these energetic materials.

Keywords: explosives, energetic materials, porous silicon, sensitiviteness

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2017
School: School of Chemical and Physical Sciences
Supervisor: Joe Shapter