Solid Fuel Development for GasGenerating Microactuators

1
Solid Fuel Development for Gas-Generating
Microactuators

• Heather Hude
• April 7th, 2003

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Presentation Outline

• Project motivation
• Thesis objectives
• Background information
• Experimental procedures results
• Fuel preparation
• Thermal analysis
• Burn time testing
• Gas generation testing
• Conductive fuel testing
• Summary
• Conclusions
• Acknowledgments
• Questions???

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Project Motivation

• Development of a combustion-based microactuator
  to produce controlled gas jets
• Utilize solid fuel to ease fabrication
• Exhibit control over production of jets
• Applications
• Rapid jet firing for projectile guidance
• Primary focus
• Prolonged gas generation

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Thesis Objectives

• Identify MEMS processing compatible solid fuels
  capable of creating gas jets
• Develop handling and processing techniques
• Tailor the nature of the fuel so as to trade off
  between raw explosive power and chemical
  controllability
• Map out a wide range of fuels from slow, low
  energy gas production to high energy, rapid
  decomposition
• Develop test methods to characterize various
  solid fuels

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Literature Review of Previous Work

• Microrocket
• Maximize energy conversion to produce thrust
• Requires very fast decomposition reaction
• Applications
• Delivery of communication-equipped MEMS sensors
• Attitude control of microspacecrafts

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Previous Work vs. Proposed Actuator

• Microrockets
• Maximize energy conversion to create large thrust
• Rapid decomposition always required
• Igniters and combustion chambers fabricated
  separately
• Rely on high temperature to increase system
  pressure and create gas jet

• Proposed actuator
• Decomposition rate is controlled
• Decomposition rate is tailored for application
• Integrated igniter and combustion chamber
• Rely on generation of moles of gas to increase
  system pressure and create gas jet

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Types of Solid Fuels

• Heterogeneous or Composite
• Oxidizer and powdered fuel held together in a
  matrix with synthetic rubber
• Ammonium perchlorate, ammonium nitrate
• Al, Mg
• Organic binder

• Homogeneous or Double Base
• Oxidizer and Fuel are chemically linked
• Nitrocellulose
• Nitroglycerine
• Gas Generating
• Alkali azides (NaN3)
• Ammonium nitrate

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Chemicals Utilized

• Main fuel component
• Ammonium nitrate, ammonium perchlorate, sodium
  azide
• Rate modifying additives
• Magnesium, ammonium dichromate, aluminum,
  potassium nitrate
• Binders
• Hydroxyl terminated polybutadiene, glycidyl azide
  polymer

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Main Reaction Mechanisms

• Ammonium nitrate
• NH4NO3 ? HNO3 NH3
• NH4NO3 ? 2H2O N2O
• Ammonium perchlorate
• NH4ClO4 ? NH3 HClO4
• 2NH4ClO4 ? Cl2 4H2O 2NO O2
• Sodium azide
• NaN3 O2 ? Na2O 3N2

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Binder Chemistry

• Hydroxyl terminated polybutadiene (HTPB)
• Glycidyl azide polymer
• Curing agent
• Mondur MR methyl diphenyl isocyanate

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Solid Fuel Decomposition Rate

• Solid fuel rate equation
• r a P n Vielles Law
• Factors influencing decomposition
• Composition
• Particle size, particle size distribution
• Operating conditions - pressure, initial
  temperature, heat loss to surroundings
• Transient burning vs. steady state

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Overview of Experimental Procedures

• Fuel production and general processing
• Pressing fuels
• Demonstrate MEMS compatibility
• Fuel characterization
• Thermogravimetric analysis
• Burn time testing
• Extent of gas generation
• Conventional fuel modifications
• Conductive fuel

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Techniques for Fuel Preparation

• Prepared by hand in 30mL Pyrex beaker
• Binder added first other components added one at
  a time
• Thoroughly mixed
• Add curing agent
• Always 19 of binder
• 24 hours at room temperature to cure

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Fuel Formulations Prepared

• PSAN-based fuels
• 0-20 Mg to increase burn time
• 0-9 AD to increase burn time
• 6 formulations prepared
• AP-based fuels
• 0-10 Al to increase burn time
• 3 formulations prepared
• SA-based fuels
• 0-20 KNO3 to increase burn time
• 4 formulations prepared
• GAP-based fuels
• Replaced HTPB in some previously prepared fuels
• 10 formulations tested

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Basic Fuel Processing

• Pressing fuels
• Manual lamination press
• Thin, flat sheets of fuel
• Use
• Preparing TGA samples
• Packing fuels for MEMS compatibility
• Done by hand
• Into micro-scale devices
• Use
• Combustion testing

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Fuel Pressing Results

• Successfully produced 0.4 0.5mm thick cured
  fuel sheets using a mold (750-1000psi)
• Fairly reproducible
• Samples were cut with 3.5mm hole punch for use in
  TGA
• Successfully produced sheets as thin as 0.15mm
  without mold (3750psi)
• Little reproducibility
• No control over fuel spreading variable
  composition consistency

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MEMS Compatibility Results

• Done by hand to prevent destruction of substrate
• Successfully packed fuels into prototype devices
  of various sizes
• Volume ranges - 23mm3 to 0.02mm3
• Cross-sectional area ranges - 46mm2 to 0.03mm2
• Aspect ratio ranges 0.07 to 2.5

0.5mm
0.25mm
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Thermal Analysis Techniques

• Thermogravimetric analysis (TGA)
• Temp. range 30-550C
• Heating rate 100C/min
• Generates plots of weight as a function of time
  and temperature
• Determine decomposition temps.
• Qualitatively compare each fuel formulation

Balance Arms
Sample Pan
Seiko Instruments, Inc. DG/DTA 320
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TGA Results

• Decomposition temperatures
• Fairly good agreement with values reported in
  literature
• Results can be affected by experimental
  conditions
• Each sample was in powder form
• Subjected to a heating rate of 100C/min

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TGA Results Effect of Sample Wt

• Each sample cut from
• the same sheet of fuel
• Larger sample, faster
• decomposition rate
• Due to additional heat
• supplied from
• exothermic reaction
• Peak DTA measurement for larger sample is over 6
  times larger than smaller sample
• Max temperature reached for larger sample was
  383C, while smaller sample was 276C
• Requires extremely reproducible method of sample
  preparation

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TGA Results Effect of Heating Rate

• Each sample cut from
• same sheet of fuel
• 100C/min 5.025mg
• 10C/min 5.040mg
• Faster heating rate,
• faster decomposition rate
• Conclusion TGA not reliable method to
  characterize solid fuels
• Heating rate is not fast enough to provide
  combustion conditions
• Difficult to prepare reproducible samples

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Burn Time Testing

• Develop test structure to reproduce combustion
  conditions
• Relatively small, reproducible volumes
• Rapid heating rates
• Three sets of experiments
• Burn time comparisons
• Burn time prediction
• Nozzle effects

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Sample Preparation

• Mix batch of fuel smear unset fuel into
  pre-weighed, alumina test structure
• Press fuel with fingertips remove excess
• Press pre-weighed Ti igniter onto fuel
• Allow to cure undisturbed for 24 hrs

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Burn Time Testing Procedure

• Position sample under low power microscope
• Connect sample to DC power supply (3.5V)
• Initiate video capture system and stopwatch
• Simultaneously start timer, power, and video
  recording
• Analyze video with Quicktime

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Sample Video

• Power to sample is initiated

• Start of combustion reaction

• Middle of combustion reaction

• End of combustion reaction

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Effect of Rate Modifying Additives

• Additives to PSAN-based fuel
• Increasing Mg content decreased effective burn
  time
• Increasing AD content had diminishing effect on
  burn time 3 AD had max. effect
• Adding Al to AP-based fuel
• Increasing Al content decreased effective burn
  time

Fuel composition 20 HTPB, 0-20Mg balance PSAN
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Effect of Rate Modifying Additives (Cont)

• Adding KNO3 to SA-based fuels
• KNO3 oxidizes unreacted Na metal
• Exothermic reaction supplies addition heat to
  accelerate reaction
• Increasing KNO3 content had diminishing effect
• 10 KNO3 produces max effect
• Greater than 15 by weight creates excess oxidizer

Fuel Composition 20 HTPB, 0-20KNO3, balance SA
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Effect of Binder

• GAP successfully decreases burn time in fuels
  without metal fuel
• Increase thermal energy supplied to burning
  surface
• Previously reported that metal fuels have
  detrimental effect on burn time and gas
  production of GAP-based fuels

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Burn Time Prediction and Nozzle Effect Procedures

• Burn Time Prediction
• Add Al spacers to bottom of combustion chamber
• Decrease characteristic length from 0.5mm to
  0.2mm
• Test as previously described
• Nozzle Effect
• Cut circular nozzles from
• alumina tile
• Clamp onto prepared sample
• Test as previously described
• Also tested with microphone

3mm, 2mm, 1mm, 0.5mm nozzles
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Burn Time Prediction Results

• Based on Vielles Law
• Reaction rate should be constant at constant
  pressure
• Burn time should be a linear function of sample
  thickness
• One possible trendline presents non-linearity
• Suggests reaction may be diffusion limited
• Agrees with previously reported reaction
  mechanism
• Large variation in data

Fuel Composition 20 HTPB, 20 Mg, 60 PSAN
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Burn Time Prediction Results (Cont)

• In this case, burn time shows linear trend
• Slight variation in composition
• Theoretically, reaction mechanism is same as
  previous slide
• Large variation in data visible
• Error in mixing fuels
• Error inherent to data capture system

Fuel Composition 20 GAP, 20 Mg, 3 AD, 57
PSAN
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Burn Time Prediction Results

• Results from AP-based fuels were same as those
  encountered with PSAN-based fuels
• Cannot be concluded that decomposition reaction
  becomes diffusion limited on small length scales
  or that the rate remains constant
• More work must be done, particularly with smaller
  characteristic lengths
• Results from SA-based fuels exhibit linear trend
  as expected
• Decreasing characteristic length successfully
  decreases burn time

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Sample Video with 2mm diameter Nozzle

• Power to sample is initiated

• Start of combustion reaction

• Middle of combustion reaction

• End of combustion reaction

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Effect of Varying Nozzle Diameter

• Decreasing nozzle diameter, increases linear burn
  rate
• Assume pressure is inversely proportional to
  nozzle diameter
• Each fuel tested exhibited this trend

Fuel Composition 20 GAP, 20 Mg, 3 AD, 57 PSAN
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Gas Production Testing

• Collect non-water soluble combustion gases
• Prove fuel effectiveness for use in
  gas-generating microactuators

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Gas Production Testing Procedure

• Prepare sample as previously described
• Connect sample to power supply slide into
  vessel
• Seal vessel
• Apply 3.5V to sample
• Collect combustion gases

Water Column
Pressure Vessel
Electrical Connection
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Gas Generation Results

• GAP-based fuels produce more gas than HTPB-based
  fuels, as expected
• In theoretical calculations, complete combustion
  of organic binders was assumed

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Gas Generation Results

• Possible sources of error in theoretical
  calculations
• One decomposition reaction was assumed for each
  main fuel component
• NH4NO3 ? 2H2O N2O
• Complete combustion of binder monomer was assumed
• CH2CHCHCH2 11/2O2 ? 4CO2 3H2O
• Possible sources of error in gas collection
  technique
• Comparatively large volume of apparatus
• Volume of vessel 164mL
• Average volume of gas collected 14.5mL
• Solubility of some production gases in water

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Conductive Solid Fuels Testing

• Add graphite to solid fuel mixtures
• Further ease device fabrication

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Conductive Solid Fuel Testing Procedures

• Prepare fuel as previously described
• Add 20 by volume graphite to mixture
• Test for burn time
• Attach Ti igniter and cure for 24 hrs
• Record burn time as previously described
• Test for conductivity
• Fill alumina combustion chamber with fuel
• Insert electrodes on two opposite sides of
  chamber
• Pass current through sample
• Calculate resistance and resistivity

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Conductive Fuel Burn Time Testing Results
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Conductive Fuel Conductivity Results
Fuel Composition 20 HTPB, 10 Al, 25 Graphite,
45 AP
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Summary

• Identified gas-generating solid fuels suitable
  for MEMS processing
• Conventional and conductive
• Demonstrated ability to modify the burn time for
  many possible applications
• Addition of various amounts of rate modifying
  chemicals
• Adjusting characteristic burn length
• Modifying system pressure

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Conclusions

• In general, AP-based fuels burn the fastest and
  PSAN and SA-based fuels are comparable
• GAP-based fuels burn faster than HTPB-based fuels
  when no metal fuel is present
• AP-based fuels still burn fastest followed by
  SA-based fuels, and then PSAN-based fuels
• GAP-based fuels produce more gas than fuels
  prepared with HTPB
• AP-based fuels produce the most, followed by
  SA-based fuels, and then PSAN-based fuels
• Addition of 20 graphite by weight enables fuel
  to be conductive

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Acknowledgements

• Dr. Mark Allen advisor
• DARPA funding
• Combustion Sub-group
• Brian English, Ed Birdsell, Yanzhu Zhao
• Richard Shafer lab manager
• Hollie Reed TGA
• Family friends

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QUESTIONS???