Parametric Study of the Ignition of Metal Powders by Electric Spark

Graduate Mentor: Ervin Beloni

Faculty Mentor: Prof. Edward Dreizin

Bhavita Patel

July 6th 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. 34-42 (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 bedDCHigh Voltage

CapacitorVoltageSwitch 1 Switch 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 3.0 - 4.5 µm

• Spherical Al 4.5 - 7.0 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

DC High Voltage Power Supply

Resistor/Capacitor Selector

Spectrometer

Chromel Wire

Oscilloscope

PMT 1

PMT 2

Pin Electrode

Fiber Optics

Sample Cup

Firing Test System

Interference Filters

Voltage Inductance

Coil Current Inductance

Coil

Switch

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 3.0 - 4.5 µm

– Spherical Al 4.5 - 7.0 µm

• Test Al powders at 8 kV– Capacitances

• 2000pF (Does not Ignite)• 5000 pF• 10000 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

Partical size (µm)0.1 1 10 100

Volu

me %

Mg 10.3 µm

Al (3.0-4.5) µm

Al (4.5-7.0) µm

Processing of Current and Voltage

Time, µs0 1 2 3 4 5 6

Cur

rent

, A; V

olta

ge, V

-400

-200

0

200

400

600 Current-Voltage-traces

Experimental currentFit currentExperimental voltage

2

22 2

2 1sin exp

4 24ACV R R

I t t tLC L LLC R C

E IV t

Emission Traces of ALS

hort

Sig

nal (

V)

0

1

2

3

4

5

6

-1

Time (s)0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07

-1

0

1

2

3

4

5

Spark

Spark

Shorter Ignition Delay

Longer Ignition Delay

Processing of Delay PulseE

mis

sion

sig

nal,

V

0

1

2

3

4

5

6 Raw Derivative

Time, ms

9.0 9.5 10.0 10.5 11.0 11.5 12.0

Der

ivat

ive,

V/s

0

500

1000

1500

Ignition delay

Zero-level signal

Spark

First peakof the signal derivative

Slope at the peak of the derivative

Ignition Delay v Energy for Mg powder

Measured Spark Energy (mJ)

0 20 40 60 80 100 120

Ign

itio

n D

ela

y (

ms

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0Recovered2

2000 pF - 0.2 mm - (6 - 16 kV, in 2 kV steps)5000 pF - 0.2 mm - 8 kV10000 pF - 0.2 mm - 8 kV2000 pF - 1.5 mm - 8 kV5000 pF - 1.5 mm - 8 kV10000 pF - 1.5 mm - 8 kV

Ignition Delay v Energy for Al

0 20 40 60 80 100 120 140

Ign

itio

n D

ela

y (

ms)

0

2

4

6

8

10

Measured Spark Energy (mJ)

Al (3.0-4.5) m 10000pF-0.2mm-8kVAl (3.0-4.5) m 5000pF-0.2mm-8kVAl (3.0-4.5) m 10000pF-1.5mm-8kVAl (3.0-4.5) m 5000pF-1.5mm-8kVAl (4.5-7.0) m 10000pF-0.2mm-8kVAl (4.5-7.0) m 5000pF-0.2mm-8kVAl (4.5-7.0) m 10000pF-1.5mm-8kVAl (4.5-7.0) m 5000pF-1.5mm-8kV

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