IGNITION OF TITANIUM POWDER LAYERS BY ELECTROSTATIC DISCHARGE

Titanium powder heating and ignition by an electrostatic discharge (ESD) or spark was investigated. The effect of powder layer thickness and morphology was determined. Ti powder was chosen for these experiments because it is commonly found in energetic formulations, used for materials preparation by self-propagating high-temperature synthesis, and is extensively used in powder metallurgy. Two Ti powders were used: spherical powder with a volume mean diameter of 82 mu m and sponge powder with a volume mean diameter of 30 mu m. ESD current and voltage were measured in real time; powder ignition was characterized using an optical sensor and photographs of the produced burning particles. Different ignition modes were observed for powders with different morphologies and placed in layers with different thickness. For both spherical and sponge Ti powders prepared as monolayers, ESD initiation resulted in fragmentation of the initial particles. Produced particle fragments were ejected from the sample holder and burned as individual fine metal droplets. The burn times for such fragments were substantially shorter than expected for particles present in the starting Ti powders. Sponge powder placed in thicker layers ignited generating individual burning particles with combustion times close to those expected based on the particle size distribution. Spherical Ti powder placed in thicker layers was difficult to ignite and only a few short individual particle streaks were observed, which could be attributed to the finest particles present in the sample. When a titanium powder (either spherical and sponge) was placed in a layer with thickness greater than 0.1 mm, significant fusing of the particles was observed which reduced the powder heating by the discharge's Joule energy.