Parametric Study of the Ignition of Metal Powders by Electric Spark Graduate Mentor: Ervin Beloni Faculty Mentor: Prof. Edward Dreizin Bhavita Patel July 6 th 2008

Background: Metals as Fuel Additives Aluminum, other metals used as fuel additives –Example: Propellants, explosives, pyrotechnics Advantages of Metals: high energy density Shortcomings of Metals: –relatively low reaction rates

New Approaches New approaches to increase reaction rate: Synthesis can be done by: Mechanically alloyed powders Reactive nanocomposites By using new approaches the Reactivity increased But, Sensitivity increases as well Sensitivity needs to be decreased Sensitivity needs to be understood

Electro-Static Discharge (ESD) for Sensitivity Testing The ESD testing are based on US Bureau of Mines Report from 1940s. Part of current MIL-STD-1751A/NATO-AOP-7, this is the standard followed by many countries for such ESD testing. ESD offers a qualitative ranking of ESD sensitivity between different powders by comparing Minimum Ignition Energy (MIE) Skinner, D., Olson, D., Block-Bolton, A. “Electrostatic Discharge Ignition of Energetic Materials” Propellants, Explosives, Pyrotechnics 23, pp (1997) Magnesium (Mg) Powder Ignited Pin Electrode Sample Cup

ESD Ignition: Current Issues A most common ignition sensitivity test Many new reactive materials fail, not clear why –Production of new materials are not scaled up Test results can be affected by –Equipment model –Powder amount –Testing location –Testing personnel –Weather…such as Humidity Main problem: mechanism of ESD ignition is poorly understood for powders Possible processes causing ignition: –Thermal ignition as a result of direct heating of powder in the discharge –Thermal ignition as a result of Joule heating

Technical Approach Perform an experimental parametric study Determine how ignition is affected by the process parameters Establish a model that can adequately describe experiments –Different ignition mechanisms are expected to result in different effect of discharge parameters on ignition Challenge: design a parametric study to produce meaningful results

Basic Setup for ESD Testing Free powder placed in an electrode cup Capacitor charged to a specific voltage Capacitor discharges through pin electrode to powder bed Pulse parameters –Voltage, duration, current, overall energy Powder bed DC High Voltage Capacitor Voltage Switch 1Switch 2 Pin Electrode Distance (Gap)

Setting of a Parametric Study How is ignition affected by? –Material Magnesium & Aluminum –Particle size Spherical Mg 10.3 µm Spherical Al µm Spherical Al µm –Applied energy (capacitance & voltage) 2000 pF, 5000pF, 10000pF, etc. –Applied voltage 8kV, 10 kV, 12kV, 16kV, etc. –Pulse duration (Capacitance & Resistance) –Spark Configuration (gap) Output: optical trace (emission from ignited powder) –Measure ignition delay time ( the Delay time measured from the Spark to the increasing slope) –Other optical measurements to be considered in the future (spectral, intensity, etc.)

Experimental Setup with Diagnostics Cup diameter: 6 mm Cup depth: 0.45 mm Voltage Inductance Coil: 1 V = 1 A Current Inductance Coil: 1 V = 10 A

Outline of Experiments Conducted Powders –Spherical Mg 10.3 µm –Spherical Al µm –Spherical Al µm Test Al powders at 8 kV –Capacitances 2000pF (Does not Ignite) 5000 pF pF –Gap 0.2 mm 1.5 mm –Resistance 0 Ω Vary voltage at a given capacitance Repeat same experiments for Al powders and Mg powder with smaller weight. From each set of runs determine –Ignition delay –Spark energy Aluminum Powder

Size Distribution for Mg and Al

Processing of Current and Voltage

Emission Traces of AL Spark Shorter Ignition Delay Longer Ignition Delay

Processing of Delay Pulse

Ignition Delay v Energy for Mg powder

Ignition Delay v Energy for Al

Summary / Future Work SUMMARY: For Mg powder: ignition delay is a function of energy –Shorter delays at higher spark energies –Ignition delays do not decrease below about 0.5 ms For Al powders: ignition delay is a function of particle size –Shorter delay for finer particles –No detectable effect of energy –Larger error bars compared to Mg results: explained by a more difficult ignition FUTURE WORK: Reprocess data to attempt reducing the error bars. –Using another criterion to analyze previous data: by choosing some threshold value that is above the base signal noise. Additional experiments with new materials, varied settings