Ervin Beloni

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 µm and sponge powder with a volume mean diameter of 30 µm. 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 discharges Joule energy.

Model of heating and ignition of conductive polydisperse powder in electrostatic discharge

Heating of a conductive polydisperse powder by electrostatic discharge (ESD) is modelled numerically. Powder packing is described using a discrete element model; powder resistance is defined by geometry of particle contacts and properties of plasma produced by electrical breakdown between neighbour particles. A set of parametric calculations in combination with experimental data is used to determine necessary adjustable model parameters. The model predicts the temperature for each powder particle resulting from its heating by the ESD current. Location and packing of individual particles within the powder affects greatly their achieved temperatures and thus the likelihood of ignition. Consistently with experiments, a trend showing that smaller particles are generally heated to higher temperatures at a given ESD energy is detected for coarser powders; this trend becomes less clear for finer powders with particle sizes less than the breakdown distance given by the Paschen curve in air. Comparison of the experimental data and calculations suggests that the transition from single particle to cloud combustion occurs when the distance between the particles ignited by ESD becomes close to the flame size for the individual burning particle. This distance, inversely proportional to the number of ignited particles, is primarily determined by the ESD energy.