Grain Size Effects on Energetic Material Properties: Is nano the solution

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Grain Size Effects on Energetic Material
Properties Is nano the solution?

• J. Addiss, H. Czerski, D.M. Williamson, J.E.
  Field and W.G. Proud
• Fracture and Shock Physics Group, Cavendish
  Laboratory, Cambridge,
• CB3 0HE, United Kingdom

University of Cambridge
Cavendish Laboratory
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Thank you

• Organising Committee Conference
• Acad. V. Fortov
• Russian Colleagues

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Why?

• Several reasons
• Fundamental science
• Safety less sensitive
• Functional more reproducible output or effect
• Application can we miniaturize the energetic
  system
• Use existing chemicals to keep ageing under
  control

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Contents

• Shock to Detonation sensitivity
• Deflagration to Detonation burn characteristics
• Temperature results
• Channel Results
• Internal and Surface Structure
• Future Challenge

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Conventional PETN

• Conventional PETN
• Seive 85 has been used as control
• Irregular angular grains
• Approximately 180?m diameter
• Columns pressed to different densities

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Ultrafine PETN

• Sub-micron primary particles
• Large secondary agglomerations
• Fluffy to handle

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Conventional RDX

• Grade 1 fine RDX
• Supplied dry
• Angular grains
• Different shape to the PETN

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Sensitivity testing small scale gap tests
Confinement height 25mm
Charge column width 5mm
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Results for 3.64mm Barrier
NOTE - Coarse Time Resolution
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Results for 5.59mm Barrier
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3.63 mm Streak
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3.67 mm Streak
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3.71 mm Streak
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Results for Conventional PETN
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Results for ultrafine PETN
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Results for RDX
Gap Thickness / mm
Density
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Type I DDT

• Slow conductive burn
• Convective burning stage
• Plug formation
• Accelerating compressive burning
• Plug reaches critical velocity and pressure
• SDT event
• Detonation

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Experimental Technique
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Experimental Arrangement
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Type I DDT

• Ultrafine PETN
• Medium density - 70 TMD
• All conventional systems - same

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Type II DDT

• Ultrafine PETN
• Low density column (29 TMD)

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Type II DDT ultrafine PETN 30 TMD
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Type II DDT - ultrafine RDX
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Conclusions 1

• Flame propagation in convective burning is
  heavily influenced by nature of granular bed
• Plug formation is influenced by nature of bed
  compaction
• Compressive burning controlled by hot-spot
  mechanisms in gas space collapse

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Thermocouples

• Type II DDT
• Thin type K thermocouples
• Equally spaced

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Thermocouples
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Enhanced detonation velocity

• Experiments designed to probe the phenomena of
  enhanced detonation velocity
• Reaction ignited thermally
• Once reaction reaches optical fibre, EBW
  detonator is fired

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Enhanced detonation velocity

• c - EBW detonator
• d - Steel
• confinement

e - Optical fibre f - Ultrafine PETN g - Thermal
ignition
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Channel investigation

• Possible causes of enhanced detonation
• Increased temperature
• Channel effects
• Earlier compaction
• Channel effects were investigated

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Geometry effects

• Charges pressed to 40 TMD
• Charge II with 1.5 mm channel
• Detonation velocity doubles

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Witness plates

• (a) - Without channel
• (b) - With channel

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Conclusions 2

• Both RDX and PETN have been shown to be less
  sensitive in at nanometric length scales to long
  duration shocks
• Sensitivity to this type of shock has been shown
  to increase with porosity
• Type of reaction changes with particle size and
  porosity of the sample
• Previously observed by a Russian group in picric
  acid and an American group in tetryl and some
  high energy propellants.

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But what about links between the morphology of
RDX crystals and their sensitivity?
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Characterisation of morphology

• Anything which will lead to more, or
  faster-growing hotspots during the shock to
  detonation transition.
• This study - We consider the raw material only
• Polymer-bonded systems French (SNPE)
• Look for structures that affect hot-spot (energy
  concentration)

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Type I
Critical gap 7.8mm Mean particle size
27mm Mean no. of voids 4.1 Mode no. of voids 2
Critical gap 10.3mm Mean particle size
16mm Mean no. of voids 1.5 Mode no. of voids 0
50mm
Class 5 Type - 10-30mm
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Type II
Critical gap 7.5mm Average crystal dia.
29mm Average number of voids 3.1 Mode no. of
voids 3
Critical gap 8.1mm Average crystal dia
12mm Average number of voids 0.1 Mode no. of
voids 0
50mm
Class 5 Type - 10-30mm
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Mercury Porosimetry
Class 5
Red lines represent the more sensitive sample.
Class 1
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Conclusions 3

• Presence of more voids does not increase
  sensitivity (seem to be the reverse!)
• No correlation is seen between type of surface
  features and sensitivity
• BUT ESEM data - sensitivity was shown to link
  to surface defect density! (Both in this and SNPE
  research)

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Future work is nano a solution?

• Following parameters (nano v conventional)
• Shock sensitivity lower Good
• Larger range of density Good
• Deflagration to Detonation type I or type II
  possibly useful
• Very Sensitive to short high level shocks (not
  reported here)
• NEXT CHALLENGE - CONTROL OF SURFACE DEFECTS

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Acknowledgements

• Engineering and Physical Sciences Research
  Council
• MoD (UK)
• dstl
• QinetiQ
• SNPE
• Dr. Michael Gifford for excellent research in
  establishing the basis of this study

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