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UMD ENMA 490 - Latching Shape Memory Alloy Microactuator

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Latching Shape Memory Alloy Microactuator ENMA490 Fall 2002 S Cabrera N Harrison D Lunking R Tang C Ziegler T Valentine Outline Background Problem Project Development Design Evaluation Device and Process Flow Applications Summary Future Research Materials Applications 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 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 TOP VIEW SIDE VIEW Si island over valve NiTi SMA arm Joule heating Shape Memory Alloys Martensite Austenite Transformation Cooling Austenite Applied Stress Polydomain Martensite Applied Stress Re heating Austenite Single domain Martensite Twinned domains symmetric inter grown crystals Heat SMA2 valve opens SMA1 cools magnet keeps valve closed INITIAL DESIGN Heat SMA1 valve closes SMA2 stays cools valve open Heat SMA1 valve close s SMA2 stays cools valve open FINAL DESIGN Heat SMA2 valve open s SMA1 cools magnet keep valve closed 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 2 1 T or kL2 d 2 k F 3EId L3 6b1b2 E1 E 2 t1t 2 t1 t 2 b1 E1t12 2 b2 E 2 t 22 2 2b1b2 E1 E 2 t1t 2 2t12 3t1t 2 2t 22 Material Properties Young 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 6 6 0 3015 83 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 martensite austenite 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 100 m long 30 m wide 2 5 m thick substrate 0 5 m thick SMA Tip deflection 39 m 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 Tip Deflection Scaling 1 E 03 1 E 04 1 E 05 25 30 35 40 45 50 20 15 ss u m e n k c i h SMA t 10 5 1 0 5 u m 1 E 06 0 1 5000 1000 Le 500 ng 10050 10 th Tip deflection m 1 E 02 0 3L 0 03L L 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 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 Small green circles indicate needle placement with respect to cantilever wafer Thin film will remember its trained shape when it transforms to austenite Degree of actuation determined by deflection of cantilever during training process Side view of needle apparatus Non Latching Power Cycle Non latching Duty Cycle Energy use based on time spent in secondary state Cumulative Energy Consumed arb units 100 Max energy usage 80 Energy Power Time 60 Max energy used when 50 of time spent in secondary state 40 20 0 0 10 20 30 40 50 60 70 80 Time Close d Normally open Normally Closed 90 100 Above 50 other type of actuator more efficient Latching Power Cycle Latching Duty Cycles Cumulative Energy consumed arb units 30 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 25 20 15 10 5 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Switches cycles 2 Low Power Low Freq High Power Low Freq Low Power High Freq High Power High Freq Low energy to switch low frequency latching is more energy efficient Power Considerations Heat cantilevers to induce shape memory effect P m c T t I2R m mass of cantilever c specific heat of cantilever T 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 outside world Drug release system device TI thermal circuit breaker http www ti com mc docs precprod docs tcb htm Sandia 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


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