# Smart Materials for Green(er) Cities, a Short Review
## 2 Smart Materials for Green(er) Cities
### Shape-Memory Alloys (NiTinol)
In the so-called martensite phase, at lower temperatures, the elementary cells of the crystal lattice can exist in two mirrored but energetically equivalent spatial configurations. When a mechanical load is applied to an SMA wire, it is the jumping of the elementary cells from one configuration to the other (and not elastic deformation) that makes it possible to strongly deform it. The deformation persists as long as the temperature of the wire  
Figure 3: SMA actuator (**a**) Test bench. A Hall sensor is used to measure the vertical displacement of the moving part (in red), to which the weight is attached. The white arrow points to the two cables used to heat the NiTinol wire using DC current; (**b**) observed vertical displacement over time using a heating current of 3 A and a weight of 0.5 kg. The power supply is automatically switched off immediately after actuation. The wire is then left to cool (passive air cooling). It returns to its initial position after about 4 s.  
does not exceed the martensite-austenite transition temperature. When this happens, the wire almost instantly returns to its memory shape. This is due to the fact that the crystal lattice can only exist in one configuration in the austenite phase. All elementary cells switch to this unique configuration when the temperature exceeds the martensite-austenite transition temperature, thus resetting the shape of the whole wire. The memory shape is fixed once and for all during the metallurgical manufacture of the wire in its solidification and cooling phase. One interesting property of SMA is that the austenite phase is more resistant to deformation than the martensite phase. This means that a weight lifted during the martensite-austenite transition and \"held up\" by the austenite phase can be heavy enough to deform back the material when it returns to the martensite phase. The weight can also be replaced by a spring, which is \"weaker\" than the austenite phase but \"stronger\" than the martensite phase. A system made of an SMA wire, and a spring can then operate cyclically between two states: 1. spring compressed above a threshold temperature and 2. spring relaxed below the threshold temperature. This property, which is used in the manufacture of actuators (see Figure 3: here, the attached weight resets the actuator), is also used in the manufacture of solar curtains, which can help to regulate the temperature within a building in a completely passive way [81]. Here, the SMA spring-wire mechanism is used to open or close small sunhades mounted side by side on an external panel. When the sun hits the curtain directly, the sunhades open. They then close again when direct light decreases. NiTimol has at least one other very interesting property for Green Tech/Green Cities applications. A NiTimol wire in the austenite phase (i.e., above its martensite-austenite transition temperature) can also be highly deformed. Once again, this is due to the ability of elementary cells to jump from one spatial configuration to another when subjected to mechanical loading. This time, however, when the applied load is released, the memory shape is immediately restored. In the austenite phase, a NiTimol wire therefore has super-elastic properties. It can be easily and very strongly deformed and immediately returns to its initial shape as soon as one stops actively deforming it. This is a highly useful property for many applications. Super-elastic stents placed in arteries that may be compressed by external forces in everyday life can easily undergo severe deformation (without damaging the artery) and then gently return to their memory shape as soon as compression ceases. NASA has also developed highly deformable super-elastic NiTimol wheels for its Mars exploration robots. These wheels are less susceptible than wheels made of thin aluminum foil to puncture by sharp stones [82]. In the field of Green Tech, it is the thermodynamics of the phenomenon that make it so interesting.