# Smart Materials for Green(er) Cities, a Short Review
## 2 Smart Materials for Green(er) Cities
### Shape-Memory Alloys (NiTinol)  
Shape memory alloys, especially those based on NiTinol, have a range of properties that make them very interesting for applications in many fields [73], from biomedical [74] to aerospace [75], and not forgetting construction and civil engineering [76] (see also Section 2.1). SMAs certainly have the property of returning to a pre-programmed shape when the temperature exceeds a certain threshold. Above all, they have the ability to do so very quickly and forcefully. Figure 3a shows a test bench equipped with an FG-ONE SMA actuator, produced and marketed by the German company Hoffmann [77]. The actuator is essentially made up of a thin SMA wire attached to either side of its housing. In its initial position, the wire is deformed into a \"V\" shape by a weight attached to its center. The wire is electrically connected to a power source. The current can then be switched on to heat up the wire quickly above its threshold temperature. The wire almost instantly resumes its memory shape, stretching horizontally between its two supports. As it does so, it lifts the weight attached to it by 4.5 mm (see Figure 3a). The exerted force is sufficient to lift a weight of almost 1.5 kg in just a few milliseconds (see Figure 3b). This property alone means that we can already envisage a number of applications for green(er) cities. One possible application is in the field of electrical grid safety. The copper busbars used to conduct current in electrical power cabinets are connected to each other by screws, which can loosen over time (due to vibrations in particular). When the contact between the two busbars deteriorates, electrical resistance increases at the contact point. This generates Joule effect losses, causing the contact to heat up rapidly. This overheating can have catastrophic consequences, leading to the outright destruction of  
Figure 2: In situ temperature measurement in a small concrete block: (**a**) Maximum interrogation distance, at the end of the solidification (maturation) phase; (**b**) temperature data, over time, during solidification phase (data were acquired between 28 February and 2 March 2022). the cabinet and more. Regular maintenance is required to avoid these problems. SMA material clamps, U-shaped, for example, could be used to re-press the busbars together in the event of abnormal heating [78]. The resistivity of NiTinol also changes when it switches from its deformed state below the temperature threshold to the memory-shape, above this threshold. This would make it possible to detect actuation and send an alarm signal, enabling maintenance personnel to intervene quickly. SMA actuators could also find applications as safety components in the energy production sector, e.g., in nuclear power stations. The increasing complexity of computer systems used to control nuclear power plants can make them vulnerable to malicious acts by hackers, for example [79]. What would happen if one of these hackers managed to take control of a safety valve, which must open or close when the temperature of the reactor core cooling circuit exceeds a critical value? Here, SMA actuators could take over and automatically actuate the valve without any possible external interaction. It is not possible to hack a passive system without on-board electronics. To obtain the actuator displacement curve shown in Figure 3b, we used a current of 3 A for a voltage of 7 V. A consumption of only \(\sim\)3 J was therefore required to lift a 0.5 kg mass by 4.5 mm, in a bit less than 150 ms. This is another advantage of SMA actuators: they are energy efficient [80]. For certain applications, they could advantageously replace other solutions, such as solenoid actuators. However, only if the application does not require a quick resetting of the actuator. Indeed, resetting the actuator requires it to cool below its transition temperature, which can take several seconds in the case of centimeter-sized actuators (see Figure 3b). NiTinol exists in different crystalline phases, depending on the temperature.