MicroActuator | High Temp Thin Film SMA | MicroRelay
This actuator consists of a rectangle of silicon, 5 mm wide and 8 mm long, that has been partially removed to leave a poppet supported by 8 TiNi thin film 'microribbons'. The lower drawing shows these mmicroribbons stretched. When electrical current runs through these microribbons, they are warmed by Joule heating and the shape memory property causes them to contract. The upper view shows these 'microribbons' in their contracted state. The poppet displacement is more than 100 micrometers, and it can exert a force of as much as 0.5 Newtons.
High Temperature Thin Film Shape Memory Alloy
Funded in part by DARPA, TiNi Alloy Company has developed high temperature thin film shape memory alloy composed of titanium, nickel, and hafnium (TiNiHF). This film has desirable thermomechanical characteristics with an austenite finish temperature of as high as 170 C. It has high ductility, great strength, and shape recovery of up to 4 percent.
The photograph above shows TiNiHf valve actuators microfabricated on a silicon substrate. Each actuator consists of a rectangular frame of silicon, 8 mm long and 5 mm wide, a central poppet etched from the same single crystal silicon wafer as the frame, and four microribbons of TiNiHf that connect the frame and the poppet.
When a force is exerted perpendicular to the plane of the frame, the poppet moves and the TiNiHf microribbons are stretched longitudinally. When an electric current passes through the microribbons it generates Joule heat, the TiNiHf transforms to austenite, contracts and moves the poppet back into the plane of the frame. A force of up to 0.5 Newton is produced, and displacement as much as 100 micrometers. Flow rates as high as one liter per minute were recorded with current of 150 milliamperes applied to the actuator.
HOW IT WORKS
The latching mechanism, invented by Maj. John Comtois, USAF, consists of two beams disposed end-to-end: one is forked, the other pointed. In one stable position the two ends do not touch. In the second stable position, the pointed end is caught in the fork: both beams are slightly bent so that the ends are pressed together. This produces a low-ohmic contact. The beams are displaced into contact by a first TiNi actuator, and out of contact by a second TiNi actuator.
TiNi MicroRelays are fabricated on silicon substrates by MEMS (micro-electro-mechanical) processes derived from microelectronics production. Sacrificial layers of aluminum are used to release structures from the substrate. TiNi SMA film is sputter-deposited. Nickel beams are electroplated. Patterning uses photoresists, including SU-8. Electrical connection is by evaporated gold.
The latching microrelay is suited for use wherever current of up to one ampere is to be controlled and where small size is necessary. The devices are non-volatile: when power is turned off they do not change state. Power is consumed only during change of state.
Relays can be made in a variety of sizes to meet differing requirements. Linear arrays may be placed with individual devices as close as 100 microns, and two dimensional arrays with up to one thousand components in one square centimeter. Thus TiNi MicroRelays may be used for applications as diverse as protection of individual IC components and cross-bar switching networks.
Prototype microrelays were made in an SBIR program funded by the Air Force, but re not available at this time. The work required to revive the program is high, so there are no plans at this time to market microrelays.