Exhibits Reversible Cycling Between Distinct Mechanical and Light-Reflecting States Controlled by Temperature
This thin film material combines the characteristic shape-memory behavior of heat-responsive alloys with tunable optical properties. Shape memory alloys are materials that can be contorted at low temperatures yet inevitably return to their original shape after a single heating/cooling cycle. Such predictable, heat-activated shape changes make them attractive for actuators and other devices that convert energy into mechanical motion. The global market for smart materials with motor or actuator applications was over $10 billion in 2010 . While shape memory alloys have traditionally been studied for their mechanical performance, interest in their optical properties has grown due to emerging opportunities in plasmonics, sensing, and nano-optics. Accordingly, demonstrations of alloys whose shape memory controls useful optical effects are needed.
Researchers at the University of Florida have developed a nickel-titanium thin-film shape-memory alloy that exhibits iridescence and surface plasmon resonance when nanostructured. These useful light-matter interactions can then be turned off simply by heating the material, thereby triggering a shape-memory-effect transition that restores the material to its original geometry. This cycle between different optical behaviors can be repeated indefinitely.
Application
Smart materials and devices that undergo reversible changes in both shape and optical properties in response to heat
Advantages
- Recovers its original form when heated, allowing devices with heat-responsive shape transformation and actuation
- Dramatically alters its interaction with light during shape recovery, enabling opportunities in plasmonic sensing and biosensing
- Demonstrates shape memory effect in a thin-film material, achieving compatibility with lower frequency devices and smaller scale applications, such as micro-electro-mechanical systems (MEMS) implanted in the body for medical purposes
Technology
The atoms that form certain materials, such as metals and metal alloys, arrange themselves in repeating patterns known as crystals. In most cases, the same material can form different crystals under different conditions, such as temperature and pressure. Shape memory alloys intelligently navigate these transitions, returning to their original form after deformation. This nickel-titanium alloy forms a smooth, shiny sheet in its original form, martensite. Applying several thousand pounds of pressure to give the alloy a nanostructured shape induces a phase transition known as detwinning, which renders it colorfully iridescent. Strikingly, the alloy readily reverts to its original shiny sheet form when heated just above room temperature (60 °C) due to a heat-driven transition from detwinned martensite to austenite. This linking of distinct reflective properties to the two crystals exhibiting the shape memory effect yields improved nano-optical devices, such as light-emitting diodes and plasmonic biosensors. Plasmonic biosensors rely on a light-matter interaction known as surface plasmon resonance that can detect viruses infecting the body. Notably, surface plasmon resonance is enabled in the nanostructured state but absent in the recovered smooth state. This capability makes it attractive for applications in plasmonic biosensors, nano-optical devices, and advanced sensing platforms.
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