Microscale shape memory alloy opens up application space for new smart devices
February 03, 2017
By Cory Knick, U.S. Army Research Laboratory/GTS
Green laser actuation of NiTi SMA microdevices
Bulk NiTi SMA transforms from tightly wound coil into paperclip shape when submerged into hot water
Scientists and engineers at the U.S. Army Research Laboratory are currently conducting research in processing smart materials for thermally activated thin-film shape memory alloys.
This novel class of smart materials and micromechanical systems, more commonly referred to as MEMS, processing technologies will enable devices for microsensing and actuation, event detection and device protection for a wide range of Army systems.
Smart materials, by design, have one or more properties, like stress or phase, which can be manipulated in a controlled fashion by external stimuli, such as temperature, moisture, pH, electric or magnetic fields.
One of these smart materials is a Nickel-Titanium alloy, often referred to as Nitinol, which exhibits advantageous properties such as shape memory and super elasticity.
Its unique properties are attributed to a reversible solid-solid phase transformation between two crystal phases, the austenite phase at high temperatures and the martensite phase at low temperatures.
The more compliant martensite phase undergoes a twinning process, which allows the material to sustain large deformation and transform back to its original austenite shape while heating the material above its phase transition temperature.
More intuitive examples include a NiTi paper clip that can be coiled completely out of shape, or a NiTi spring virtually straightened. Once dropped in hot water, they snap back to their original shape.
The NiTi material phase transformation is entirely reversible with the potential to exceed one billion reversals.
ARL's MEMS team is investigating the applicability of smart materials in microdevices such as shape memory alloy, or SMA, actuators in MEMS.
The team recently demonstrated devices capable of actuating up to 200 Hz, a 60% improvement over 125 Hz SMA devices reported by others, a feat they were able to achieve by demonstrating shape memory effect for extremely thin films of NiTi all the way down to 150 and 100 nm film thicknesses.
This study provides insights in to the deformation mechanisms and memory shape behavior at the microscale.
Originally discovered in 1959 by the Naval Ordinance Laboratory, the material has been used commercially in bulk form, but difficult to synthesize and demonstrate the shape memory effect in thin-films for microdevices until recently.
Subsequently, thermal stress is enhanced by depositing a platinum nano-film below the NiTi structure, creating significant deformation at the Pt-NiTi interface due to high deposition temperatures (600 °C) and mismatch in the thermal expansion between Pt and NiTi layers.
Once released from the substrate, residual stresses activate the material to curl at room temperature.
The MEMS researchers working in support of ARL's Materials Research Campaign have recently fabricated functional devices with a radius of curvature as low as 50 µm, a huge improvement from their first generation devices with 1.2 mm curl, and tighter than anything reported to date by other researchers.
These extremely tight folding devices fold up much tighter than other SMAs in the literature and represent a viable solution to meet specific program requirements.
The Pt/NiTi actuators will transform from the curled to the flat memory state when its temperature exceeds the phase transformation temperature of 70°C.
The NiTi phase changes rapidly, increasing stiffness and returns the material to its as-annealed shape, as quickly as three milliseconds as demonstrated by the ARL MEMS team.
Actuation temperatures can be controlled from -100 to 100 °C by manipulating the material's NiTi ratio, annealing temperatures, deposition and annealing temperatures. There is even a class of high temperature NiTi SMAs based on NiTiHf and NiTiPt for example, which allow actuation temperatures to be precisely controlled between 100 and 400 °C.
Actuation times of a few milliseconds can occur when the material is subjected to a heat source such as a pulsed laser beam or resistive heating.
Resolving the submicron-scaling of SMA actuators while maintaining the shape memory effect of Pt/NiTi nano-films is a major challenge. The ARL team has demonstrated reversible shape memory effects in 100 and 150 nanometer thick films, and plans to fabricate devices at these dimensions.
This is a noteworthy achievement because reliable shape memory effects in submicron films has been a major challenge and research thrust for those involved in this field.
Exploiting the thermal activation properties of the material will enable systems to omit external circuitry for active actuation by taking advantage of the passive response of these materials for microscale SMA actuators.
In addition, the new technology will instigate additional miniaturization of microdevices, on-chip diagnostics and thermal management applications.
When thinking about the potential application for microscale SMA in terms of the Army, ARL researchers envision many interesting applications that could one day benefit the Soldier.
Imagine a miniature device narrower than a strand of hair and only half a micron thick that would absorb incident laser light and heat up and phase change its way into actuation.
This would allow the Solider to remotely complete tasks such as control a switch or steer a beam of light.
For additional information, view the articles below or contact firstname.lastname@example.org:
 C. Knick, M. Srour, and C. Morris, "Characterization of sputtered nickel-titanium shape memory alloy and microfabricated thermal actuators," in Micro Electro Mechanical Systems, 2016. MEMS '16, Proceedings, IEEE., 29th Annual International Workshop on, pp. 256–261, Jan 2016. doi: 10.1109/MEMSYS.2016.7421677
 C. Knick, M. Srour, G. Smith and C. Morris, "Reduced Radius of Curvature Nitinol-on-Pt Bimorph Actuators Based on Reversible Shape Memory Effect," in ASME Conference on Smart Materials, Adaptive Structures, and Intelligent Systems (SMASIS 2016), September 2016.
 C. Knick and C. Morris, "Material and process development of thin film shape memory alloy MEMS actuator," in ASME Conference on Smart Materials, Adaptive Structures, and Intelligent Systems (SMASIS 2015), September 2015. doi: 10.1115/SMASIS2015-8801
 C. Knick, M. Srour, and C. Morris, "Reduced radius of curvature and optical actuation of nitinol shape memory alloy MEMS actuators," in Proceedings, 2016 Hilton Head Workshop on Solid-State Sensors, Actuators and Microsystems, 2016.
Contributions by Guest Author Cory Knick, U.S. Army Research Laboratory/GTS
The U.S. Army Research Laboratory is part of the U.S. Army Research, Development and Engineering Command, which has the mission to ensure decisive overmatch for unified land operations to empower the Army, the joint warfighter and our nation. RDECOM is a major subordinate command of the U.S. Army Materiel Command.