Ž .Sensors and Actuators 78 1999 163–167www.elsevier.nlrlocatersna

Dynamic thermo-mechanical properties of evaporated TiNi shapememory thin film

Eiji Makino ), Takayuki Shibata, Kazuhiro KatoElectronics and Information Engineering, Graduate School of Engineering, Hokkaido UniÕersity, Sapporo 060-8628, Japan

Received 10 December 1998; received in revised form 21 April 1999; accepted 23 April 1999

Abstract

Ž .The shape recovery and re-deflection responses of shape memory alloy SMA thin film to thermal cycles were investigated using thebulge method. It was deposited by flash evaporation and had a nominal composition of 50 at.% Ti–50 at.% Ni and a thickness of about 6mm. After being released from silicon substrates and undergoing vacuum-annealing to obtain memorisation of an initial flat shape, it wasdeformed into a cap shape of 5 mm in diameter by pressurisation at 400 kPa. Then, by applying 100 ms voltage pulses, its initial shapewas recovered by resistive heating at various energies. During these shape recovery and re-deflection cycles, change in displacement withtime was measured continuously using a laser displacement meter. The thin film exhibited shape recovery at energies for heating of morethan 1 J due to reverse martensitic transformation. Displacement due to shape recovery increased with increasing energy for heating,reaching saturation at around 100 mm at energies of more than 2 J. After heating was completed, the thin film deflected again due tomartensitic transformation under pressure. The period for each shape recovery and re-deflection cycle was about 600 ms at an energy of2.1 J. It exhibited stable shape recovery and re-deflection properties at up to 1000 cycles, which was the maximum number of thermalcycles tested. Finally, the pumping pressures and flow rates which might be expected with such an SMA micropump were also roughlyestimated. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Shape memory alloy; TiNi thin film; Flash evaporation; Shape recovery; Dynamic response; Bulge test

1. Introduction

Over the past decade, there has been increased interestw xin the development of microfluid handling devices 1–3 ,

which are used in micro systems such as chemical analysisand dosage systems. For certain types of microfluid han-dling device, such as micropumps, microvalves and microflow sensors, the micropump itself would constitute themost fundamental component.

A number of micropumps actuated by various means,such as piezoelectric, thermo-pneumatic, and electrostatic

w xmechanisms, have been developed 3–8 . These pumpshave proved capable of handling small volumes of fluid inthe order of micro litres. However, the pump pressure forsuch pumps is in the order of several tens of kPa, which isnot high enough for certain applications such as microanalysis systems which have complicated flow path struc-

) Corresponding author. Tel.: q81-11-706-6440; fax: q81-11-707-6581

tures and high flow resistance. For these applications, it isnecessary to develop a micropump with a high pumpingpressure of several hundreds of kPa.

Ž .Shape memory alloy SMA thin film is an importantw xmaterial for microactuators 9–13 , as it is capable of

exhibiting a large actuation force and a large displacement.We are attempting to develop a micropump with an SMAdiaphragm and two check valves, as shown schematicallyin Fig. 1. This pump will be actuated by the deflection andshape recovery movements of the SMA diaphragm duringcooling and heating. In order to realise such a micropump,we need SMA thin film with good thermo-mechanicalproperties.

We have already reported a fabrication method for TiNiSMA thin film and its steady-state deflection and shape

w xrecovery responses 14,15 . We demonstrated that it waspossible to deposit TiNi SMA thin film by flash evapora-tion, and that it exhibited stable shape recovery propertiesunder bias pressure.

The purpose of this paper is to describe the dynamicshape recovery and re-deflection properties of flash-

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 99 00222-8

( )E. Makino et al.rSensors and Actuators 78 1999 163–167164

Fig. 1. Schematic view of an SMA diaphragm micropump.

evaporated SMA thin film using the bulge test and toestimate the pumping pressure and flow rate of the SMAmicropump which we wish to develop.

2. Experimental

SMA thin film was prepared by flash evaporation usingsmall TiNi pellets, each of which had a weight of about 28mg. The details of the apparatus and the deposition condi-

w xtions have been already reported in the previous paper 14 .Over 20 flash evaporation cycles, SMA thin film with acomposition of 50 at.% Ti–50 at.% Ni and a thickness ofabout 6 mm was deposited on silicon substrates. Afterbeing released from the substrates, the free-standing filmwas then annealed at 5008C for 60 min in a vacuum toobtain memorisation of an initial flat shape.

Fig. 2 shows the experimental apparatus used in thebulge test for measuring the dynamic responses of shaperecovery and re-deflection in the deposited thin film. Itwas clamped onto a specimen holder with an O-ring, andthen nitrogen gas was applied at a pressure of 400 kPathrough a hole of 5 mm in diameter to obtain deformation.Gas pressure was measured with a semiconductor pressure

Ž .transducer Copal Electronics, PAH2000-203G .A DC voltage pulse of 100 ms in width was applied to

the thin film to heat it up resistively. During heating,electric currents were measured and energy was calculated.After heating for 100 ms, the voltage pulse was switchedoff, and the thin film was air-cooled.

Fig. 2. Experimental apparatus for bulge measurement.

When it bulged through pressurisation at room tempera-ture, it formed a spherical cap shape. In this state, wecontinuously measured change in height at the centre ofthe spherical cap as shape recovery during heating and asre-deflection during cooling using a laser displacement

Ž .meter Keyence, LC2400 .To investigate the effect of number of thermal cycles on

shape recovery properties, displacement-time diagramswere measured at thermal cycles ranging from 1 to 1000cycles at a pressure of 400 kPa. For these measurements,DC voltage pulses of a 100 ms in width were applied tothe thin film for periods of 5 s for heating.

3. Results and discussion

3.1. Shape recoÕery and re-deflection

Fig. 3 shows changes in displacement due to shaperecovery and re-deflection with time during heating andcooling for various energies at a pressure of 400 kPa. Atthe beginning of the measurements, the thin film wasunder a state of deflection due to gas pressure. Under thiscondition, the thin film was heated for 100 ms and thenair-cooled. During this heating and cooling process, dis-placements from the initial deflection state were measuredcontinuously.

At energies of less than 1 J, no shape recovery wasobserved, because the temperature of the thin film was nothigh enough to cause reverse martensitic transformationŽ .As: 688C . At 1 J, shape recovery began to occur, anddisplacements for recovery increased with increasing en-ergy. After the heating voltage was switched off and thetemperature decreased, the thin film became soft and de-flected again due to the occurrence of martensitic transfor-

Fig. 3. Changes in displacement during shape recovery and re-deflectionwith time at various energies for heating at 400 kPa.

( )E. Makino et al.rSensors and Actuators 78 1999 163–167 165

Fig. 4. Shape recovery vs. energy for heating.

Ž .mation Ms: 658C . It then recovered its initial cap shapeafter a certain period, depending on energy for heating.

Displacement due to shape recovery was measured onFig. 3 and plotted in Fig. 4 as a function of energy forheating at a pressure of 400 kPa. At low heating energiesof below 1 J, no displacement was observed. With anincrease in energy from 1–2 J, displacement increaseddrastically, indicating the occurrence of reverse martensitictransformation. At energies of more than 2 J, displacementbecame stable.

Fig. 5 shows time duration from initiation of heating tocommencement of shape recovery and to termination ofre-deflection as a function of energy at a pressure of 400kPa. These data were taken from the displacement-timediagrams shown in Fig. 3. Termination of re-deflectionmeans 95% recovery of the initial cap shape during cool-ing.

Time taken for commencement of shape recovery wasless than 100 ms. This decreased with increasing energyfor heating, as the temperature of the thin film rose fasterat higher energies.

On the other hand, time taken for the termination ofre-deflection increased with increasing energy, as a longertime was necessary for the thin film to cool down afterheating at higher energies. In particular, at energies of

Fig. 5. Time duration from initiation of heating to commencement ofshape recovery and to termination of re-deflection vs. energy for heating.

Fig. 6. Changes in displacement during shape recovery and re-deflectionwith time for various thermal cycles at 400 kPa. Shape recovery wasconducted at 2.1 J heating.

more than 4 J, a longer duration of more than 1 s wasnecessary to recover the initial cap shape during the re-de-flection process. This means that, in order to obtain ashorter cycle time, it is counter-productive to apply toohigh an energy to the thin film, although the time for shaperecovery commencement is shorter at higher energies.Considering the displacements of shape recovery shown inFig. 4 together with cycle time, an energy of about 2 Jwould appear to be appropriate.

3.2. Effect of number of thermal cycles on shape recoÕeryand re-deflection properties

Fig. 6 shows change in displacement with time duringthe shape recovery and re-deflection process for variousthermal cycles. These measurements were conducted at anenergy of 2.1 J.

During the first thermal cycle, after the thin film hadshown a small displacement due to shape recovery duringheating, it deflected and returned to a point far exceeding

Fig. 7. Shape recovery vs. number of cycles at 2.1 J heating.

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Fig. 8. Time duration from initiation of heating to commencement andtermination of shape recovery, and to termination of re-deflection vs.number of cycles at 2.1 J heating.

that of the initial displacement during cooling. This couldnot have been caused by the thermo-mechanical propertiesof the thin film. However, it may have been caused by anextra displacement of the thin film due to thermal soften-ing and expansion of the O-ring used to clamp it in orderto prevent leakage of the nitrogen gas used for pressurisa-tion. It would seem that, although the thin film exhibited anormal displacement due to shape recovery, an excessivedisplacement took place in the reverse direction. Thistendency, however, reduced markedly during the secondcycle, eventually disappearing within several cycles. Dur-ing the tenth cycle, the thin film returned to almost itsoriginal position after cooling.

After 10 cycles, displacement change due to shaperecovery and re-deflection became stable, and proved ca-pable of maintaining this stability for up to at least the1000 thermal cycles tested.

Fig. 7 shows the relationship between displacement ofshape recovery and number of thermal cycles. Apart fromthe first cycle, displacement was about 135 mm during thefirst few cycles, then decreased with increasing number ofthermal cycles. This decrease in displacement was due towork hardening of the thin film, as slip deformation isintroduced into thin film under high bias pressures such as

w x400 kPa, as mentioned in the previous paper 15 . Afterseveral hundreds of cycles, displacement due to shaperecovery was almost constant at about 100 mm.

Fig. 8 shows time duration from initiation of heating tocommencement of shape recovery and termination of re-deflection as a function of the number of thermal cycles.The time taken to reach termination of deflection is impor-tant in estimating the cycle time of an SMA actuator.

Shape recovery commenced early within a short periodof about 60 ms after initiation of heating in every thermalcycle, and terminated quickly within about 140 ms.

However, it took longer for re-deflection to terminate,as cooling had to take place via thermal conduction throughthe specimen holder and air cooling, without resort to any

special cooling device. Time taken for termination ofre-deflection was about 600 ms, independent of the num-ber of thermal cycles. This means that, if it were subjectedto the same cooling conditions, SMA thin film could beactuated at a fastest cycle time of 600 ms.

4. Conclusions

The dynamic thermo-mechanical properties of TiNi thinfilm deposited by flash evaporation were investigated us-ing the bulge method. At a bias pressure of 400 kPa, acircular shape thin film of 5 mm in diameter exhibited ashape recovery of about 100 mm during heating at 2.1 Jfor 100 ms, and a thermo-mechanical cycle period of about600 ms.

If such SMA thin film was applied to a micro pump asan actuation diaphragm with a diameter of 5 mm, adisplacement of 100 mm in the cap shape would give a

Ž .pumping volume of 1 ml micro litre . Moreover, a periodof 600 ms for one thermo-mechanical cycle would exhibitan actuation frequency of 1.7 Hz. Based on these values,an SMA diaphragm micropump would exhibit a pumpingflow rate of about 100 mlrmin at a pumping pressure of400 kPa.

In the present study, it was not possible to ascertain thetemperature of the thin film in real time during the heatingand cooling process. In order to describe this relationshipbetween temperature and displacement due to deflectionand shape recovery, we are going to fabricate a microthermocouple on evaporated SMA thin film for the nextstep of this work. When we have completed work on thedynamic thermo-mechanical properties of this SMA thinfilm, we will apply it to a micropump.

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Eiji Makino received the BS, MS and PhD degrees in precision engineer-ing from Hokkaido University, Sapporo, Japan in 1969, 1971 and 1985,respectively. From 1971 to 1976, he worked at Toshiba. He joined thefaculty of engineering, Hokkaido University in 1976. His research inter-ests include microfabrication technology and microelectromechanical sys-tems.

Takayuki Shibata received the BS and MS degrees in precision engineer-ing from Hokkaido University, Sapporo, Japan in 1987 and 1989. Afterworking for Sumitomo Electric Industries for two years, he joined thefaculty of engineering, Hokkaido University in 1991. He is workingtoward the PhD degree in ultra precision machining of silicon and quartzsubstrates. His research interests include surface micromachining ofdiamond thin film and microelectromechanical systems.

Kazuhiro Kato received the BS and MS degrees in precision engineeringfrom Hokkaido University, Sapporo, Japan in 1995 and 1997. He iscurrently with Riso Kagaku.