Shape Memory Alloy Microactuator Research Articles

Three-dimensional thin-film shape memory alloy microactuator with two-way effect

A novel thin film (micrometer thickness) shape memory alloy (SMA) micro actuator is presented in this paper. The thin film SMA with composition of approximately 50:50 nickel titanium (NiTi) is sputter-deposited onto a silicon wafer in an ultra high vacuum system. Transformation temperatures of the NiTi film are determined by measuring the residual stress as a function of temperature. The transformation temperature is independent of the presence of chromium (Cr) used as an adhesion layer, or being exposed to air before annealing. A mixture of hydrofluoric acid (HF), nitric acid (HNO/sub 3/) and deionized (DI) water is used to etch the film. Different etch masks are evaluated to protect the NiTi film during the etching. Among the masks tested, a thick photoresist (AZ-4620) produces the best result. The NiTi membrane is hot-shaped into a three-dimensional (3-D) dome shape using a stainless-steel jig. Results indicate the membrane exhibits two-way effect. The performance of the SMA micro actuator is characterized with a laser measurement system for deflection versus input power and frequency response.

3D Simulation of a Shape Memory Microactuator

A three-dimensional (3D) model has been developed for simulation of the physical properties of an electrically driven shape memory alloy (SMA) microactuator used for control of a microvalve. The mechanical behavior is decribed by a two phase macromodel taking into account material and geometrical nonlinearity. The simulation makes use of mechanical, electrical and thermal finite element programs, which are coupled by a program for management of the simulation sequence as well as the exchange of data between the different finite element programs. Mechanical tests confirm the simulated forces and displacements. From the pressure-dependent gas flow in the valve an effective heat transfer coefficient is determined to simulate the convective heat exchange. The simulated heat transfer times are in quantitative agreement with experimentally determined time constants of the microvalve.

Modeling of an integrated shape memory alloy micro-actuator under bending sollicitation

In this paper, we give a model of shape memory alloys (SMA) thin blade working in bending. This work is based on the general study and the modeling of an integrated shape memory alloy micro-actuator controlled by thermoelectric effect called the « Ω module ». This micro-actuator is composed of a NiTi thin blade on which are placed two Bismuth Telluride ingots used to control the temperature of the SMA blade, by thermoelectric effect. The model of the bending behaviour is based on a general macroscopic description of the thermomechanical behaviour of shape memory alloys. The self-accommodating (pure thermal effect) and oriented (stress-induced) martensites are taken into account. The numerical resolution by finite differences leads to the comprehension of phase transformation for pseudo-elastic and thermal cycling bending tests

Effect of the composition and thermal annealing on the transformation temperatures of sputtered TiNi shape memory alloy thin films

After sputter-deposited amorphous TiNi films were subjected to heat treatment higher than 600 K, the shape memory alloys (SMAs) showed strong dependence of the transformation temperatures on heat treatment conditions and composition. Unlike Ni-rich films, the transformation temperatures of Ti-rich films are above the ambient temperature and largely depend on annealing temperatures from 600 to 1200 K. But they remain relatively constant on the various annealing temperature domains, 600–740 K, 740–1000 K, 1000 K +, delimited by exothermic peaks originating from the formation of precipitates. The measurements by differential scanning calorimetry, performed on TiNi thin films annealed below 740 K, showed transformation temperatures appropriate for medical applications and very small transformation temperature hysteresis of 3 K, resulting in a beneficial effect of SMA cyclic microactuators. Furthermore, using these lower annealing temperatures reduces both thermal and mechanical stresses and makes possible the development of SMA micro-actuators on substrates, that are unstable at elevated temperatures, and on electronic devices.

Manufacturing issues of thin film NiTi microwrapper

Manufacturing issues related to a thin film NiTi shape memory alloy (SMA) microactuator (i.e. microwrapper) have been investigated using both wet and dry etching techniques. Results show that wet etching the amorphous film produces a cleaner pattern than the crystallized film. Transformation temperatures are not affected by the pre-exposure of the NiTi film to air before crystallization. However, this process produces breakage in the NiTi film at sacrificial layer steps. This is believed to be due to residual stresses developed between the film and substrate during sputtering. The film breakage is overcome by dry etching the film with an ion-milling technique. Curvature in the microwrapper arms is induced using either a bi-layer material (i.e. polyimide and NiTi) or a functionally gradated NiTi film. Results show that when heated the microwrapper arms flatten due to shape memory effect and curl up to form a cage-like structure when cooled.

An SMA actuator controlled by Peltier effect.

Microtechnologies and microsystems engineering use new active materials for microactuators and microsensors. In this field, Shape Memory Alloys (SMA) are good candidates for microactuation. They can actuate microrobots and are able to provide very important forces, but have low dynamic response, especially for cooling. Joule effect is an easy and classical way to heat the SMA actuators, but cooling is not so easy. Dynamic response of the actuator depends on cooling capabilities. This paper describes a reversible way for heating and cooling SMA microactuators, based on the Peltier effect. Using Peltier effect, a positive or a negative electrical current is able to absorb or produce heat in the SMA actuator. The integrated SMA microactuator presented here in the millimeter size. It consists of a thin blade of SMA working in flexion and placed in junction with a material with high thermoelectric features. This material is Bi2 Te3, i.e. bismuth telluride, N an P doped. As far as we know, it is one of the first investigations with a real measurement of the motion of the SMA actuator using integrated thermoelectric junctions. Using this system, we are able to control the temperature of the SMA blade in a range of -10°C to 90°C. Some experimental results are given.

Fabrication of TiNi shape memory alloy microactuators by ion beam sputter deposition

We report on the production of thin films of TiNi shape memory alloy, grown by ion beam sputter deposition (IBSD) using a Kaufman-type source, for microactuator applications. IBSD is a vacuum-coating process in which a target is bombarded by accelerated ions from a showered ion beam source and sputtered atoms of the target material are deposited onto a nearby substrate. In this work, argon ions at energies up to 1500 eV and current densities of are used to bombard sectored targets of titanium and nickel in order to deposit TiNi films onto unheated substrates. The films were characterized by electrical resistivity measurements and x-ray reflectometry. R-phase and martensitic transformations are seen without high-temperature annealing and the shape memory properties are compared with those of films prepared by DC and RF magnetron sputtering.

Thin film shape memory alloy microactuators

Thin film shape memory alloys (SMAs) have the potential to become a primary actuating mechanism for mechanical devices with dimensions in the micron-to-millimeter range requiring large forces over long displacements. The work output per volume of thin film SMA microactuators exceeds that of other microactuation mechanisms such as electrostatic, magnetic, thermal bimorph, piezoelectric, and thermopneumatic, and it is possible to achieve cycling frequencies on the order of 100 Hz due to the rapid heat transfer rates associated with thin film devices. In this paper, a quantitative comparison of several microactuation schemes is made, techniques for depositing and characterizing Ni-Ti-based shape memory films are evaluated, and micromachining and design issues for SMA microactuators are discussed. The substrate curvature method is used to investigate the thermo-mechanical properties of Ni-Ti-Cu SMA films, revealing recoverable stresses up to 510 MPa, transformation temperatures above 32/spl deg/C, and hysteresis widths between 5 and 13/spl deg/C. Fatigue data shows that for small strains, applied loads up to 350 MPa can be sustained for thousands of cycles. Two micromachined shape memory-actuated devices-a microgripper and microvalve-also are presented.

Fabrication of Silicon-Based Shape Memory Alloy Micro-Actuators

ABSTRACTThin film shape memory alloy has been integrated with silicon in a new actuation mechanism for micro-electro-mechanical systems. This paper compares nickel-titanium film with other actuators, describes recent results of chemical milling processes developed to fabricate shape memory alloy micro-actuators in silicon, and describes simple actuation mechanisms which have been fabricated and tested.