Latching Shape Memory Alloy Microactuator
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Transcript of Latching Shape Memory Alloy Microactuator
Latching Shape Memory Alloy Microactuator
ENMA490, Fall 2002S. Cabrera, N. Harrison, D. Lunking,
R. Tang, C. Ziegler, T. Valentine
Comments from GWR in yellow boxes.An excellent project and presentation overall.Presentation grade = A.- GWR
Outline
• Background• Problem• Project Development• Design• Evaluation• Applications• Summary/Future Research
ApplicationsDevice and Process Flow
Materials
Problem Statement• Assignment: Develop a design for a microdevice,
including materials choice and process sequence, that capitalizes on the properties of new materials.
• Survey: functional materials and MEMS
• Specific Device Goals:– Actuates– Uses Shape Memory Alloys – Uses power only to switch states
• Concept:– Latching shape-memory-alloy microactuator
NiTi SMA arm
Si island over valve
Project Stimulus
State of the Art: SMA microactuator – Lai et al. “The Characterization of TiNi Shape-Memory
Actuated Microvalves.” Mat. Res. Soc. Symp. Proc. 657, EE8.3.1-EE8.3.6, 2001.
– Uses SMA arms to raise and lower a Si island to seal the valve.
– Uses continuous Joule heating to keep valve open.
Joule heatingTOPVIEW:
SIDEVIEW:
• Martensite-Austenite Transformation
• Twinned domains (symmetric, inter-grown crystals)
Shape Memory Alloys
Austenite
Cooling
Polydomain Martensite
Applied Stress
Single-domain Martensite
Re-heating
Austenite
Applied Stress
Heat SMA2
valve opens
Heat SMA1
valve closes
SMA2 valve stayscools open
SMA1 magnet keepscools valve closed
INITIALDESIGN
Heat SMA1
valve close
s
Heat SMA2
valve open
s
SMA1 magnet keepscools valve closedSMA2 valve
stayscools open
FINALDESIGN
This slide would benefit from labeling the SMA1 and SMA2 for describing the actuation sequence
Cantilever Positions and Forces• Based on beam theory• Non-uniform shape change between SMA
and substrate causes cantilever bending– Thermal expansion causes bulk strain
(2-1)T– Martensite-austenite transformation
creates lattice strain =1-(aaust/amart)– Ω = [(2-1)T] or []
)232(2)()()(6
2221
21212121
22222
22111
21212121
ttttttEEbbtEbtEbttttEEbb
k
2
2kLd 3
3LEIdF
Dr. Wuttig commented that we should be more general in describing the lattice strain portion, not making the specific strains directly coupled to specific lattice constants.
Material PropertiesYoung’s Modulus (GPa)
Thermal Expansion Coefficient (*10-6/K)
Lattice Parameter (nm)
Si 190 2.33 N/A
GaAs 85.5 5.73 N/A
NiTi (martensite) 28-41 11 0.2889 (smallest axis)
NiTi (austenite) 83 6.6 0.3015
http://www.keele.ac.uk/depts/ch/resources/xtal/classes.html, http://cst-www.nrl.navy.mil/ lattice/struk/b2.html
Cantilever Positions and Forces
• Major assumptions:– Can calculate martensiteaustenite strain from
differing lattice constants– Properties change linearly with austenite-martensite
fraction during transformation
• Deflection– Large effect from SMA, negligible effect (orders of
magnitude less) from thermal expansion
Simulation
Simulation – Deflection Results
• 100μm long, 30μm wide, 2.5μm thick substrate, 0.5μm thick SMA• Tip deflection ≈ 39μm, Deflection < ≈ 21°, Tip force ≈ 0.23mN• Heat/cool cantilever 1: F(1) > F(magnet) > F(2)• Heat/cool cantilever 2: F(2) > F(magnet) > F(1)
0.1 0.5 1 5 10 15 20 25 30 35 40 45 50
1050100500100050001.E-06
1.E-05
1.E-04
1.E-03
1.E-02
Tip Deflection Scaling
SMA thickness (um)Length
(um)
Tip
def
lect
ion
(m)
L
0.3L
0.03L
Process Flow (Single Cantilever)-Silicon wafer (green) with silicon dioxide (purple) grown or deposited on front and back surfaces.
-Application of photoresist (orange), followed by exposure and development in UV (exposed areas indicated by green).
-Buffered oxide etch removes exposed oxide layer. Oxide underneath unexposed photoresist remains.
-Removal of photoresist in acetone/methanol is followed by KOH etch to remove exposed silicon until desired cantilever thickness is reached.
-Deposition of NiTi (yellow) via sputtering, followed by 500C anneal under stress to train SMA film.
-Deposition of magnetic material (blue) using a mask via sputtering on bottom of cantilever.
This schematic is more complete than that in the report. It would be helped if the wafer bonding/glueing step could be indicated somehow.
Process Flow (SMA Training)
• Small needles hold down cantilevers during post-deposition anneal
• Training process usually carried out at 500°C for 5 or more minutes
• Thin film will “remember” its trained shape when it transforms to austenite
• Degree of actuation determined by deflection of cantilever during training process
Small green circles indicate needle placement with respect to cantilever
wafer
Side view of needle apparatus
Non-Latching Power Cycle
• Energy use based on time spent in secondary state.– Energy = Power * Time
• Max energy used when 50% of time spent in secondary state.
• Above 50%, other type of actuator more efficient.
Non-latching Duty Cycle
0
20
40
60
80
100
0 10 20 30 40 50 60 70 80 90 100
Time Closed (%)
Cum
ulat
ive
Ener
gy C
onsu
med
(a
rb. u
nits
)
Normally open Normally Closed
Max energy usage
Latching Power Cycle
• Energy use based solely on number of switches.– Energy = Energy per cycle
* frequency of switching * time used
– Least energy used at low power to switch, low frequency of switching
• Low energy to switch, low frequency, latching is more energy efficient.
Latching Duty Cycles
0
5
10
15
20
25
30
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Switches (cycles * 2)
Cum
ulat
ive
Ene
rgy
cons
umed
(a
rb. u
nits
)
Low Power, Low Freq Low Power, High FreqHigh Power, Low Freq High Power, High Freq
The x-axis here is confusing, maybe misleading. If the x-axis is # switching actions, then the steps in energy expended should occur only at the switches. However, if the axis is time, then the faster switch frequency comes out right. More about this in the final report.
Power Considerations• Heat cantilevers to induce shape memory effect
– P = (m•c•T)/t = I2R• m - mass of cantilever, c - specific heat of cantilever, ΔT - difference
between Af and room temperature, t - desired response time
– Power differs slightly for martensite and austenite for constant I because of differing resistivity.
• From simulation:– Required current = 0.27 mA– Required power = 0.097 W
Applications and Requirements
• Electrical Contacts– Sensor– Circuit breaker
• Optical Switching– Telescope mirrors
• Gas/liquid Valves– Drug release system
device
outside world
TI thermal circuit breaker, http://www.ti.com/mc/docs/precprod/docs/tcb.htmSandia pop-up mirror and drive system, http://mems.sandia.gov/scripts/images.asp
Summary
• Final design: dual cantilever system with SMA and magnetic materials to provide latching action
• Power consumption lower than that of a non-latching design when switching occurs infrequently and uses little energy
• Future work:– Research magnetic material, packaging
– Specify application
– Continue analysis and optimization
– Build device
Backup
Shape Memory Effect
Free-energy versus temperature curves for the parent (Gp) and martensite (Gm) structures in a shape memory alloy. From Otsuka (1998), p.23, fig. 1.17.
Martensite-austenite phase transformation in shape memory alloys. From http://www.tiniaerospace.com/sma.html.
Material Choice: NiTi SMA
• Near-equiatomic NiTi most widely used SMA today
Property Value
Transformation temperature -200 to 110 C
Latent heat of transformation 5.78 cal/g
Melting point 1300 C
Specific heat 0.20 cal/g
Young’s modulus 83 GPa austenite; 28 to 41 GPa martensite
Yield strength 195 to 690 MPa austenite; 70 to 140 MPa martensite
Ultimate tensile strength 895 MPa annealed; 1900 MPa work-hardened
% Elongation at failure 25 to 50% annealed; 5 to 10% work-hardened
From http://www.sma-inc.com/NiTiProperties.html
Nickel-Titanium
B2 (cesium chloride) crystal structure. From http://cst-www.nrl.navy.mil/ lattice/struk/b2.html
B19’ crystal structure. From Tang et al., p.3460, fig.5.
Parent β (austenite) phase with B2 structure
Martensite phase with monoclinic B19’ structure