High frequency actuation of thin film NiTi

Sensors and Actuators A 111 (2004) 166–171

High frequency actuation of thin film NiTi Daniel D. Shin∗ , Kotekar P. Mohanchandra, Gregory P. Carman Mechanical and Aerospace Engineering Department, University of California at Los Angeles, 32-135 Engineering IV, 420 Westwood Plaza, Los Angeles, CA 90095-1597, USA Received 27 June 2003; received in revised form 15 September 2003; accepted 21 September 2003

Abstract The frequency response of thin film nickel–titanium (NiTi) shape memory alloy (SMA) membrane in three fluid mediums (air, silicon oil, and de-ionized water) was investigated in the present paper. Operation was investigated at different power input and duty cycles. Results showed an operating frequency of 40 Hz could be achieved in de-ionized water with appropriate power input and duty cycles. Larger operating frequencies could be achieved by applying power over a short heating time to minimize the temperature rise in the surrounding fluid. Results demonstrated the extreme sensitivity of frequency to operating medium, power input, and duty cycle. That is, changing one of these parameters, the frequency response could be changed by an order of magnitude. © 2003 Elsevier B.V. All rights reserved. Keywords: Frequency; Nickel–titanium; Thin film

1. Introduction Thin film shape memory alloys (SMA) possess unique characteristics that are useful in micro-actuators. A primary characteristic is the large strain (10% strain) for one actuation cycle or 2% strain for million cycles [1]. This, coupled with large actuation forces, produces energy densities exceeding 25 MJ/m3 . For comparison purposes, piezoelectric materials produce 0.1% strain and have energy densities on the order of 0.1 MJ/m3 [1]. While SMA have superior attributes, the frequency response is limited due to heat transfer characteristics. Thin film SMA with a large surface-to-volume ratio has recently been used to increase the frequency response of SMAs. Several thin film SMA devices had been manufactured with varying degree of success. For example, a 2 ␮m thin film of nickel–titanium (NiTi) was deposited and patterned over a polyimide layer and produced actuation frequencies greater than 20 Hz [2]. A bimorph SMA actuator was also fabricated by depositing a thin layer of NiTi over polyimide substrates [3]. The drive frequency of this device was limited to 0.1 Hz, because heat dissipation prevented the phase transformations back to martensite. Therefore, a wide range of operating frequencies had been achieved in air.

Thin film SMAs had also been used in devices with liquids such as micropumps. One notable example used two antagonistic NiTi membranes to create push–pull pumping motions by alternately heating the membranes [4–7]. A different design used a single NiTi diaphragm biased with pressurized nitrogen gas [8–11]. In all of these studies, the largest reported pumping frequency was 1 Hz due to heat dissipation issues. More recent micropumps fabricated with NiTi strips adhered to silicon membranes had shown superior performance [12–14]. These later designs produced drive frequency of 100 Hz, but insufficient cooling subsequently reduced diaphragm stroke over prolonged operations. Therefore, a detailed study needed to be conducted to determine the reason why such large variation in frequency response was reported. In this paper, the frequency response of thin film NiTi membrane is studied. The membrane uses an intrinsic two-way shape memory effect [15,16]. The membrane response in three fluid mediums is studied, air, water, and silicon oil. The power to actuate the film along with the duty cycle is varied. Results show that frequencies of 40 Hz can be achieved. The results also show that power supplied, duty cycle, and fluid mediums dramatically influence the reported results. 2. Experimental setup

∗

Corresponding author. Tel.: +1-310-825-9564; fax: +1-310-206-2302. E-mail addresses: [email protected] (D.D. Shin), [email protected] (G.P. Carman). 0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2003.09.026

The membrane actuator, shown in Fig. 1, is used to evaluate the frequency response of NiTi in various fluid

D.D. Shin et al. / Sensors and Actuators A 111 (2004) 166–171

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Fig. 1. NiTi membrane actuator.

mediums. The membrane actuator is fabricated by sputter depositing 5 ␮m NiTi onto a 250 ␮m thick silicon substrate in an ultrahigh vacuum (UHV) chamber. The backside of the substrate (i.e. silicon) is patterned and etched to entirely remove the silicon in a square region having length of 14 mm. This yields a freestanding NiTi membrane attached along the square perimeter to silicon. Fabrication details are provided in Gill and Carman [15]. Once fabricated, the NiTi membrane is sandwiched between two molds forming a bubble shape shown in Fig. 2 and hot shaped. Hot shaping is a re-crystallization step, which involves heating the mold-membrane assembly up to 500 ◦ C for 20 min while pulling <6 × 10−6 Torr of vacuum in a vacuum chamber. Upon removal from the chamber and the mold, the membrane returns flat. When heated above the austenite transformation temperature, the dome shape forms in the NiTi film. Likewise, when cooled below the austenite transformation temperature, the membrane returns to initial flat shape (i.e. two-way effect). The diameter of the actuating dome is approximately 6 mm. Once fabricated, copper contact pads are placed along two opposing sides of the NiTi membrane as shown in Fig. 3. The pads reduce contact resistance and homogenize the current applied to the NiTi membrane for Joule heating. The silicon framework attached to the NiTi membrane is placed on an aluminum heat sink with silver conductive paste between the silicon and aluminum to improve heat conduction.

Fig. 2. Hot shaping jig with dome shaped plunger.

Fig. 3. Experimental setup for measuring membrane deflection.

If the experiment involves liquid, a plastic retaining wall is placed around the perimeter of the membrane so that liquid is contained on top of the NiTi membrane. For this study, de-ionized water or silicon oil was used as liquid mediums. The displacements of the NiTi membrane are measured from the lower side of the substrate-heat sink assembly using a Keyence LK-081 laser displacement meter. The displacement resolution of the laser is 3 ␮m with data sampling rate of 976 Hz, and the data is recorded with Tektronix TDS 460A oscilloscope. Current is applied to the copper contact pads and resistively heats the film. Fig. 4 illustrates the square-wave voltage signal used to actuate the NiTi membrane. This square-wave is produced from the VirtualBench Arbitrary Waveform Generator module working in conjunction with the National Instruments Data Acquisition System. The voltage output from the module is connected to the CRYDOM

Fig. 4. General input signal for heating the membrane.

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D.D. Shin et al. / Sensors and Actuators A 111 (2004) 166–171

Table 1 Experimental parameters used for various fluid media Duty cycle (%)

Power (W)

Operating frequency (Hz) 0.2

0.5

1.0

2.0

5.0

10.0

20.0

10

5.14 6.49

Water

Water

Water

Water Oil

Water Oil

Oil

Oil

30

0.94

Air

Air

Air

Air

Air

Air

50

0.94 0.99 5.14

Air Oil Water

Air Oil Water

Air Oil Water

Air Oil Water

Water

Water

Water

40.0

Water

CMX60D10 solid-state relay and acts as an on–off switch for the Keithley 228A voltage/current power supply. Rather than report voltage or current applied to the membrane we report the power applied to heat the membrane. The power required to heat the membrane is determined by power = U 2 R

(1) Fig. 5. One-dimensional heat conduction along the membrane length.

where U is the voltage and R is the resistance of the NiTi membrane which ranges from 0.164  at 22 ◦ C to 0.134  at 120 ◦ C. For presentation purposes, the power is calculated based on membrane resistance measured at 22 ◦ C. One complete cycle at a given frequency consists of applying the power to heat the film and removing the power to allow it to cool. Duty cycle is defined as the percentage of heating time applied during a given cycle. A detailed list of test parameters for the NiTi membrane is provided in Table 1. The membrane is tested in air, de-ionized water, and silicon. For air, 0.94 W is sufficient to heat the film beyond the transformation temperature and thus larger power inputs are not investigated. The duty cycle is changed from 30 to 50% at varying frequencies. For tests conducted in de-ionized water, a larger power (i.e. 5.14 W) is required to heat the film due to the dissipation of heat through the water. The duty cycle is changed from 10 to 50% while the frequencies tested are over a wide spectrum. Tests conducted in silicon oil are evaluated at two power inputs, namely 0.99 and 6.49 W. Smaller power is used with 50% duty cycle while at 10% duty cycle the power is increased to compensate for heat lost to surroundings.

3. Result and discussion For comparison purposes, an analytical heat transfer model is presented to estimate the power required to heat the NiTi membrane in a given time. The model assumes conductive heat transfer from the center to the silicon substrate as shown in Fig. 5. Heat transfer by convection is assumed negligible. The one-dimensional model is given by
 ∂ ∂T ∂T k + q˙ = ρCp (2) ∂x ∂x ∂t

where T is temperature, x the length along the NiTi membrane, t the time. The constants k is thermal conductivity, ρ the density, and Cp is specific heat. The term q˙ is rate of heat generated per unit volume defined as ratio of power required to heat the membrane to its volume. power q˙ = (3) volume The material parameters used for the model are listed in Table 2. The model is evaluated using an explicit finite-difference method. The boundary conditions are T(x, 0) = 300 K
 L T , t = 300 K 2

(4)

The silicon substrate at the edge of the membrane behaves as a heat sink at an ambient temperature. The temperature difference to transform the membrane from martensite to an austenite phase is T = 100 K. The heating time t is varied from 0.005 to 2.5 s depending on frequency and duty cycle. Using the thermal model in Eq. (2) with the boundary conditions in Eq. (4), the power required to raise the membrane temperature in a given heating time can be obtained from Eq. (3). The theoretical power required Table 2 Input parameters for one-dimensional heat transfer analysis NiTi properties Thermal conductivity, k (W/m K) Density, ρ (kg/m3 ) Specific heat, Cp (J/kg K) Volume (m3 )

18 6450 837 1.76 × 10−9

D.D. Shin et al. / Sensors and Actuators A 111 (2004) 166–171

Fig. 6. Baseline measured in stagnant air at 0.2 Hz using 50% duty cycle.

to heat the membrane within each heating time ranges from 159.6 to 0.43 W for 0.005 to 2.5 s, respectively. An experimental displacement versus time plot for the membrane is presented in Fig. 6. Here the membrane is actuated in stagnant air at an operating frequency of 0.2 Hz. The duty cycle in these tests is 50%, with a heating time of 2.5 s. The thermal model suggests 0.43 W of power completely heats the membrane but 0.94 W is required during the tests. The difference between theoretical and experimental results provides an indication of the amount of Joule heat lost to the surroundings, e.g. air and substrate-heat sink assembly. Fig. 6 shows the curves associated with cooling and heating in each cycle. The negative displacements indicate that the membrane deflects downward when actuated. The membrane reaches a maximum displacement of −2200 ␮m when heated. As for the cooling curve, the membrane returns to its initial flat shape at zero displacement implying that it completely cools during the cycle. These results suggest that both complete forward (i.e. austenite) and reverse phase transformation is occurring. The important points on this graph are the maximum displacement on heating and the return location on cooling. Fig. 7 plots membrane displacements achievable in air operated at different frequencies. For each frequency tested,

Fig. 7. Displacement vs. frequencies in stagnant air using 50% duty cycle.

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0.94 W of power is applied at a 50% duty cycle. Two data points are presented (i.e. square and circle) in the figure. The square data points show that the maximum displacement reached a steady state during heating of the membrane. The circle data points provide the membrane’s return location steady state when cooled during the cycle. The figure in the insert provides a description on the data used to produce the 0.2 Hz data points. Note that the returning location is not always back to zero displacement. For example at 1 Hz, the return location is approximately 300 ␮m while the maximum displacement is near 1700 ␮m. When reviewing the entire frequency spectrum tested, the maximum displacement remains relatively stable at 1700 ␮m, which appears to suggest that the membrane is sufficiently heated to induce full forward transformation (i.e. austenite). On the other hand, the return location (circle) displacement shifts further away from the initial zero displacement or flat configuration as frequency increases. This increases up to 1100 ␮m at 2 Hz, which implies that duration of cooling is insufficient at higher operating frequencies. These higher frequencies are not presented. In separate tests conducted on the membrane in air, the duty cycle is reduced to 30% while applying an identical power of 0.94 W. Reducing the duty cycle increases the cooling time and reduces the heating time. Fig. 8, similar in context to Fig. 7, shows membrane displacements at each frequency. Despite reducing the heating time, the membrane appears to have been heated sufficiently at low frequencies (e.g. 0.2 Hz) because 1900 ␮m displacements were achieved. As the operating frequency increased, the maximum displacement reduces sharply to 900 ␮m. This indicates that the duration of heating time was insufficient at elevated frequencies. In contrast, the cooling appears to have improved when compared to Fig. 7. In Fig. 8, the membrane’s return location converges to 600 ␮m as the operating frequency approaches 10 Hz, whereas in the previous results it shifted continuously to maximum of 1200 ␮m at 2 Hz. These results suggest that at these operating conditions, 10 Hz is achieved in air, however, full transformation is not occurring. They also serve to demonstrate the sensitivity of frequency response to duty cycle.

Fig. 8. Displacement vs. frequencies in stagnant air using 30% duty cycle.

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Fig. 9. Displacement vs. frequencies in de-ionized water using 10% duty cycle.

3.1. De-ionized water The displacements of the membrane in de-ionized water at each frequency were plotted in Fig. 9. The results were obtained by applying 5.14 W of power at 10% duty cycle. The power was larger than used in stagnant air because of the larger thermal conductivity of water. The larger thermal conductivity resulted in additional heat lost to the surroundings. This was evident because the theoretical power required to heat the membrane ranged from 1.88 to 32.05 W for the frequencies studied in Fig. 9. Even with 5.14 W of power, the maximum displacement decreased from 180 to 80 ␮m when the frequency increased from 0.2 to 5 Hz, respectively. At frequencies beyond 5 Hz, the membrane did not displace. These results, i.e. value of the maximum displacements, suggested insufficient heating and thus limited frequency response. The membrane was also tested in de-ionized water at a duty cycle of 50%. The increase in duty cycle compensated for the insufficient heating time discussed in the preceding paragraph. Applied power remained at 5.14 W. The membrane displacement versus frequency plot was shown in Fig. 10. In these tests, operation of the membrane up

Fig. 10. Displacement vs. frequencies in de-ionized water using 50% duty cycle.

Fig. 11. Displacement vs. frequencies in silicon oil using 50% duty cycle.

to 40 Hz was achievable. The maximum displacement was larger when compared to previous tests in water because of the longer duty cycle. The maximum ranged from 410 ␮m at 0.2 Hz to 260 ␮m at 40 Hz. The maximum displacement decreased as frequency increased due to insufficient heating. Furthermore, the membrane did not return to the flat configuration because it was not completely cooled. The reason for incomplete cooling was the temperature of the water immediately adjacent to the membrane also increased. This increase prevented the membrane from cooling. Nonetheless for these specific operating conditions, frequencies of up to 40 Hz were achievable. Once again, the results demonstrate the difficult balance between applied power and duty cycle to achieve maximum frequency. 3.2. Silicon oil Fig. 11 provided displacement versus frequency plots for a membrane in silicon oil. The silicon oil had lower thermal conductivity than de-ionized water, i.e. 0.131 W/m K for oil compared to 0.613 W/m K for water. The power applied to the membrane operated in silicon oil was initially 0.99 W with a duty cycle of 50%. The frequency obtainable with these inputs reached a maximum of 2 Hz, much smaller than water at 40 Hz with an identical duty cycle but higher power. The displacement of the membrane in oil decreased from 680 ␮m at 0.2 Hz to 320 ␮m at 2 Hz. The maximum displacement decreased due to the decreased heating time as the frequency increased. The return location of the membrane ranged from 450 ␮m (0.2 Hz) to 210 ␮m (2 Hz). Although it decreased to 210 ␮m at 2 Hz, it did not return completely to zero displacement because the temperature of the adjacent oil increased preventing complete cooling. Fig. 12 showed the displacement versus frequency plot for a membrane in silicon oil with 6.49 W applied at a 10% duty cycle. The return region of the membrane at 2 Hz was comparable to the previous results (i.e. 2 Hz using 0.99 W at 50% duty cycle), near 250 ␮m. This verified that the temperature of the membrane was comparable to the previous test data. The maximum displacement ranged from 670 to

D.D. Shin et al. / Sensors and Actuators A 111 (2004) 166–171

[4]

[5]

[6]

[7]

Fig. 12. Displacement vs. frequencies in silicon oil using 10% duty cycle.

[8] [9]

110 ␮m at 2 to 20 Hz, respectively. The maximum displacement reached 670 ␮m because larger power provided additional heat to the membrane. The maximum displacements eventually decreased to 110 ␮m at 20 Hz because of insufficient heating due to reduced heating time. Once again, depending upon fluid medium a delicate balance between applied power and duty cycle existed to maximize frequency.

[10] [11]

[12]

[13]

4. Conclusions High frequency actuation of thin film NiTi membrane in stagnant air, de-ionized water, and silicon oil was investigated. Results had show that applied power to actuate the NiTi membrane significantly influence the operating frequency. For maximum frequency, the applied power should be as large as possible with duty cycle as small as possible. This reduced heating of the surrounding and allowed the film to cool more quickly. For the tests conducted here, de-ionized water with the largest heat conduction coefficient produced the largest frequency, i.e. 40 Hz. This was not believed to be an upper bound but was dependent upon operating conditions and desired response.

[14]

[15]

[16]

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Biographies Acknowledgements This project was made possible by the Defense Advanced Research Projects Agency (DARPA), Ephrahim Garcia’s Compact Hybrid Actuator Program managed by AFRL under the guidance of Kyle Henderson. References [1] P. Krulevitch, A.P. Lee, P.B. Ramsey, J.C. Trevino, J. Hamilton, M.A. Northrup, Thin film shape memory alloy actuators, J. Microelectromech. Syst. 5 (1996) 270–282. [2] J. Walker, K. Gabriel, Thin-film processing of TiNi shape memory alloy, Sens. Actuators 21 (1990) 243–246. [3] J.L. Seguin, M. Bendahan, A. Isalgue, V. Esteve-Cano, H. Carchano, V. Torra, Low temperature crystallised Ti-rich NiTi shape memory

Daniel D. Shin graduated from the Mechanical and Aerospace Engineering Department at University of California, Los Angeles, in 2000 with a PhD. He is currently a postdoctoral research fellow in the Mechanical and Aerospace Engineering Department at UCLA. His research interest includes studies of thin film shape memory alloys, MEMS, magnetostrictive materials, and composites. Kotekar P. Mohanchandra received MS in physics and PhD in materials science from Mangalore University at India in 1994. He is currently a research associate in the Mechanical and Aerospace Engineering Department at University of California, Los Angeles. His current area of research includes preparation and characterization of smart materials. Gregory P. Carman graduated from the Engineering Science Mechanics Department at VPI in 1991 with a PhD. He is currently a full professor in the Mechanical and Aerospace Engineering Department at UCLA. Professor Carman is interested in fundamental studies on ferromagnetic shape memory alloys, thin film shape memory alloys, magnetostrictive materials, piezoelectric, and fiber optic sensors.