Shape-Memory Micropumps

Abstract

Motivated by many experimental efforts to develop suitable shape-memory micropumps, we propose a multiscale framework to study the behavior of pressurized films. We use recoverable deflection as a measure to design large stroke micropumps and develop a model to estimate it. We show that the recoverable deflection of a polycrystalline shape-memory film depends on the transformation strain of the underlying martensitic transformation, the texture and especially on the size effects. We find that flat grains are preferable to long grains in columnar films concerning the purpose of large recoverable strain. We also show that common sputtering texture is not ideal for recoverable deflection in both Ti-Ni and Cu-based shape-memory films. It turns out that{100} Cu-based films may have better behavior than Ti-Ni films. We conclude with comparison with experiment. The interest in microelectromechanical system (MEMS) applications has recently motivated many experimental efforts to develop suitable microactuators and micropumps. These devices have a wide range of applications in fields such as drug delivery, inkjet printing and cooling systems of elec- tronic circuits. However, common MEMS-integrated actua- tion scheme has enjoyed limited success in delivering a rea- sonable work output from the extremely small size of devices, and therefore both high stroke and force are the key require- ment for selecting actuating materials. Shape-memory alloys show great promise in this aspect since their work density is significantly higher than that of other types of materials. 26) These alloys are able to recover large strain and are capable of high force, which in turn directly transmit large stoke and high pressure in micropumps. In addition, the disadvantage of low response rate caused by cooling and heating bulk shape- memory alloys can be greatly improved at small scales be- cause of the increase in the surface area to volume ratio. This makes these alloys in the form of thin films ideal for use in MEMS-integrated actuation scheme, and we seek to develop a micromechanical model to understand the behavior of film at an extremely small thickness. In this paper, we propose a framework to study the behavior of pressurized shape-memory thin films with intended appli- cation to large stoke micropumps. Our work was motivated by the recent experiment on the fabrication of SMA actuated micropumps. 16, 17, 26) They have reported that a Ti-Ni microp- ump exhibits the largest work out per cycle per unit volume amongst various common actuator systems. 26) However, the ratio of the deflection to the half-edge length of diaphragm has been observed around several values from 0.04 to 0.12 which only correspond to 0.2%-1.4% small strain. Our the- oretical prediction is around 0.15 which is larger than these experimental observations, and the discrepancy is not com- pletely understood here. We believe there should be a plenty of room to improve this critical ratio to design large stroke mi- cropumps. In particular, we show that {100} Cu-based shape-