Selection and Evaluation of Materials for tehrmoelectric Applications

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MEMS the state of the art and future
challengesPaul RonneyDept. of Aerospace
Mechanical EngineeringUniv. of Southern
California, Los Angeles, USAYiguang Ju
Department of Engineering MechanicsTsinghua
University, Beijing, China
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Outline

• Part 1 Introduction to MEMS
• What is MEMS?
• Fabrication techniques
• Applications
• The market for MEMS
• Opportunities for the future
• What can the government do to help?
• Part 2 Power MEMS as an example of MEMS
  development

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What is MEMS?

• Micro-Electro-Mechanical Systems (MEMS) is a
  technology that
• Leverages Integrated Circuit fabrication
  technology by adding additional functions, for
  example
• Mechanical
• Chemical
• Biological
• Optical
• Mass-produces ultra-miniaturized components at
  low cost
• Enables radical new micro-system applications,
  for example
• Pressure / acceleration sensors
• Power production
• Medical devices
• Optical switches

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Advantages of MEMS
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Microscale fabrication techniques

• Bulk Micromachining
• Deep reactive ion etching
• Surface Micromachining
• LIGA
• Others
• EFAB
• Micro EDM
• 3-D Lithography
• Laser Micromachining

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Anisotropic Wet Etching of Silicon
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Deep Reactive Ion Etching
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Surface Micromachining
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LIGA process
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EFAB (Electrochemical FABrication) (NEW)

• Analogous to macroscale rapid prototyping,
  solid freeform fabrication - enables
  fabrication of arbitrarily complex 3D structures
• Selective electroplating of structural and
  sacrificial metals
• Developed at University of Southern California
• Electrochemistry can also be used to deposit
  other types of materials, e. g.
• Thermoelectric
• Magnetic
• Electrically insulating
• Catalytic
• Can use existing mechanical design software
  modeling tools
• No clean room required for device fabrication -
  much less expensive than silicon-based techniques
• Commercialization by MEMGen Inc., Torrance, CA,
  USA

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EFAB key technology Instant Masking

• Pre-fabricated masks serve as reusable printing
  plates
• Polymer mask patterned on anode using
  conventional photolithography
• Lithography for all layers done in parallel,
  prior to, separate from device fabrication,
  allowing
• Low-cost, self-contained automated machine
• Mask outsourcing - possible collaboration with
  Chongqing

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EFAB process flow
Selectively deposited material (usually
sacrificial)
Blanket deposited 2nd material (usually
structural)

(
b
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(
c
)
(
a
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(
d
)
(
e
)
(
f
)
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EFAB results

• 12-layer chain, 290 ?m wide (worlds
  narrowest?)
• Minimum feature size 20 µm
• First-generation microcombustor built

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Applications for MEMS

• Pressure transducers
• Accelerometers
• Gyroscopes
• New areas
• Optical switches
• Gas turbines
• Nano-satellite systems
• Drug delivery
• Power MEMS

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Switches for fiber-optic networks

• Many possible approaches, MEMS and non-MEMS
• 3D much higher density of switches than 2D, MEMS
  fabrication required

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Space applications
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Advanced aircraft applications

• Smart skin - senses reduces air drag
• Micro-mixing enhancement in engines
• Sensing in Gas turbine engine Environment
• Flow
• Vibration
• Temperature
• Strain
• Pressure Sensors for Stall/Surge Control
• Fuel Valve Position Sensors
• Chemical Sensors for Emissions Monitoring

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Microfluidic system for bio-chemical sensing
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Drug delivery systems

• Micromachined needles connected to individual
  microvalves and supply reservoirs
• Each reservoir may contain different
  type/concentration of drug
• May be combined with on-chip biosensor

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Drug delivery systems (2)

• (a) Drug delivery chamber
• (b) Two electrodes (AgCl/Ag electrode and IsOx
  electrode) for monitoring pH
• (c) Metal valves

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MEMS Market (U. S. estimate)
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Conclusions (MEMS)

• Many potential MEMS applications - has been
  demonstrated in USA
• China can become a significant force in MEMS
  development because of its existing
  infrastructure and its large yet highly educated
  workforce
• What can the government do to help?
• Difference between Japan and USA USA time from
  research to market is much shorter - why?
• Support and stimulate joint collaborative
  research between universities and companies
• Attract different sources of funding to sustain
  research - government, workshop registration
  fees, company staff training
• Government provides funds to university for
  facilities that companies can rent to test new
  ideas before buying their own facilities
• DARPA funds applied research on MEMS but allow
  universities and companies to retain intellectual
  property rights for non-government applications

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Conclusions (MEMS)

• Expect 95 of 100 projects to fail (success of
  other 5 will more than pay for 95 failures)
• Balance between traditional MEMS areas and
  radical new areas
• Traditional Chinese successes in international
  markets based on production cost advantages,
  especially lower labor costs
• High technology successes in high value-added
  markets depend on making use of skilled,
  educated, motivated Chinese workforce
• How to judge the future of MEMS technologies?
• High value added - unit cost of complete system
  is high
• Enabling technology - cant work without MEMS
  devices
• Collaboration between industry and universities
  essential
• Inter-disciplinary activity essential

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Microscale power generation (Power MEMS)

• USC effort supported by U. S. Defense Advanced
  Research Projects Administration (DARPA)

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The challenge of microcombustion

• Hydrocarbon fuels have numerous advantages over
  batteries
• 100 X higher energy density
• Much higher power / weight power / volume of
  engine
• Inexpensive
• Nearly infinite shelf life
• More constant voltage, no memory effect, instant
  recharge
• Environmentally superior to disposable batteries
• but converting fuel energy to electricity with
  a small device has not yet proved practical
  despite numerous applications
• Foot soldiers
• Portable electronics - laptop computers, cell
  phones,
• Micro air and space vehicles

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The challenge of microcombustion

• Most approaches use scaled-down macroscopic
  combustion engines, but may have problems with
• Heat losses - flame quenching, unburned fuel CO
  emissions
• Heat gains before/during compression
• Limited fuel choices need knock-resistant
  fuels, etc.
• Friction losses
• Sealing, tolerances, manufacturing, assembly

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Cox Tee Dee .010 Weight 0.49 oz.Bore
0.237 6.02 mmStroke 0.226 5.74
mmDisplacement 0.00997 cu in (0.163 cm3)RPM
30,000Power 3 watts
Smallest existing combustion engine
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Some power MEMS concepts
Wankel rotary engine
Free-piston engine
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Some power MEMS concepts

• Issues
• Friction, heat losses
• Very tight manufacturing tolerances
• High production cost
• Very high rotational speed needed to achieve
  compression (speed of sound doesnt scale!)
• Fuel may always need to run on hydrogen

Micro gas turbine engine (MIT)
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Some power MEMS concepts

• Non-IC engine concepts possible enabling
  technologies, but dont address complete system

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Our approach - microFIRE

• Integrated microscale power generation system
• Combustion
• Heat transfer
• Electrical power generation
• Fabrication assembly
• Swiss-roll heat recirculating burner with
  toroidal 3-D geometry
• Direct thermoelectric conversion of heat to
  electricity
• Monolithic fabrication of the entire device with
  EFAB
• Being developed by MEMGen, Inc.
• gt 10 million venture capital funding in first
  year of existence

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microFIRE approach (1) Combustion

• Swiss roll heat recirculating burner -
• minimizes heat losses
• Toroidal 3-D geometry - further
• reduces losses - minimizes
• external temperature on all surfaces

One-dimensional counterflow combustor / heat
exchanger
Two-dimensional Swiss-roll burner

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microFIRE approach (2) - Power generation

• Thermoelectric (TE) power generation elements
  embedded in wall between hot (outgoing product)
  and cold (incoming reactant) streams

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microFIRE approach (3) - Fabrication

• EFAB (Electrochemical Fabrication)
• Enables fabrication of arbitrarily complex 3D
  structures
• NASA Jet Propulsion Laboratory proprietary
  process for electrochemical deposition of Bi2Te3
  thermoelectric elements - Process-compatible with
  EFAB, enabling monolithic fabrication of entire
  device!
• Targets
• Weight 500 mg
• Volume 0.04 cc
• Power 100 mW
• Efficiency gt 10

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microFIRE advantages

• Integrated combustor / heat exchanger / power
  generation
• Heat losses / flame quenching problems minimized
• External T (IR signature, touch-temperature
  hazards) minimized
• Direct conversion, no moving parts!
• No friction losses
• No tight manufacturing tolerances
• Rugged, reliable, low maintenance
• Quiet, stealthy, no vibration
• Long life (no wear or fatigue-induced breakage)
• Compact
• Can use wide variety of conventional hydrocarbon
  fuels without pre-processing

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Fabrication of macroscale test devices

• Development approach build macroscale models,
  test, develop numerical simulation capability,
  design microscale device
• Soligen rapid prototyping process for 2-D and
  3-D designs in Al2O3 - SiO2 ceramic

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Mesoscale experiments

• Wire-EDM fabrication
• Pt igniter wire / catalyst

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Combustion modes

• Combustion usually in flameless mode - no
  visible flame!

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Quenching limits

• Area-averaged V can be 30x stoichiometric burning
  velocity, even with mixture 33 leaner than
  conventional lean limit no insulation
• Lower limit can be reduced dramatically with
  catalytic Pt strips
• but it can also be increased dramatically

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Numerical modeling

• FLUENT software package, 2D 3D simulations

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Numerical modeling

• High fuel
• Low fuel
• Reaction rates Temperatures

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Conclusions (microFIRE)

• Combustion in microscale devices feasible even at
  low temperatures compatible with thermoelectric
  elements, but will probably require heat
  recirculation catalytic assistance
• Combustion behavior under such conditions quite
  different from conventional flames
• Expect similar findings in most other microscale
  systems - performance cannot be predicted based
  only on macroscale results

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Challenges for Power MEMS

• microFIRE-specific
• Developing calibrating gas-phase surface
  chemistry sub-models
• Modelling electrochemical processes - rely less
  on empirical testing
• Catalyst preparation, degradation restoration
• Challenges for all micro-chemical/thermal/fluid
  systems
• Auxiliary components - valves, pumps, fuel tanks
• System integration and packaging

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Thanks to

• Chongqing Science Technology Commission
• Chongqing University
• and especially U. S. Defense Advanced Research
  Projects Administration (DARPA) Microsystems
  Technology Office !!!