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The transition to sustainable or green(er) cities requires the development and implementation of many innovative technologies. It is vital to ensure that these technologies are themselves as sustainable and green as possible. In this context, smart materials offer excellent prospects for application. They are capable of performing a number of tasks (e.g., repair, opening/closing, temperature measurement, storage and release of thermal energy) without embedded electronics or power supplies.
- smart materials
- sustainable cities
- green cities
- smart concrete
- SAW sensors
- shape-memory alloys
- phase-change materials
- meta-surfaces
1. Introduction
Over and above the environmental problems that the use of fossil fuels poses or may pose, the probable depletion of these same resources in the following decades is prompting us to rethink the infrastructure of our cities so as to drastically reduce their need for fossil fuels. The classic way to achieve this is to replace fossil fuels with renewable energy. However, it will be very difficult to completely replace fossil fuels with renewable energies in the short or even medium term [1]. It is therefore important to find solutions to reduce the overall energy consumption of our cities.
The most popular route to smarter, energy-efficient cities is the Smart City. Here, the latest Internet-of-Things technologies are used on a massive scale to acquire as much Data as possible and then use it to optimize, among other parameters, energy consumption [2,3,4]. However, this approach has a number of disadvantages, not least the need to manufacture and integrate millions of tiny electronic circuits into everyday objects and surrounding structures, then interrogate these communicating objects and store and process the data. These operations consume a lot of energy. The presence of electronics in everyday objects also poses obvious recycling problems. The disadvantages of using IoT-type technologies can therefore quickly outweigh the benefits when it comes to making cities greener [5].
2. Smart Materials for Green(er) Cities
2.1. Self-Healing Concrete
Introducing autonomous repairability and healability in cementitious materials represents an innovative strategy to extend the lifetime and reduce the CO2 footprint of buildings [6,7,8,9]. To date, concrete is the most widely applied material in the construction industry owing to its high mechanical properties paired with low production costs [10]. Concrete is formed by mixing cement, water, sand and coarse fillers. Subsequently, the cementitious phase is dried and cured, yielding calcium–silicate–hydrate (C–S–H) domains, which mainly contribute to the stiffness of the inorganic composite. While concrete benefits from a high compressive strength, its tensile properties (tensile strength and elongation) are limited [11]. Thus, concrete used for the construction of bridges, roads or buildings is prone to cracking during the lifetime of these structures [12]. Equipping concrete with a self-healing function is one way to ensure the structural integrity of buildings without costly and time-consuming maintenance and repair, as the material is able to close the damage zone without external intervention [13]. Here, it should be noted that cementitious materials—without a particular modification—can autonomously heal cracks, albeit at a limited crack size (<50 µm) [14]. This so-called autogenous healing can be based on various mechanisms: (i) a delayed hydration of residual cement reagents [15], (ii) the carbonation of dissolved calcium hydroxide [16], (iii) a swelling of the C–S–H phases [17] or (iv) the physical closure of cracks by small particles present in water [18]. However, in all cases, water is required to come in contact with the damage zone and to react with the concrete matrix or to transport the required reagents.
Another route to deposit materials and to physically seal small cracks in concrete is the precipitation of calcium carbonate with the help of selected bacteria [19]. Various types of bacteria have been reported that are able to survive the highly alkaline environment of cementitious materials and form calcium carbonate via different metabolisms (e.g., aerobic respiration of dissolved inorganic carbon, urea hydrolysis or nitrate reduction) [20]. While some bacterial spores can be directly mixed with cementitious materials, the majority of the reported concepts use carriers such as clay or activated carbon particles to extend the lifetime of the bacteria and protect them during the processing of concrete [21,22,23]. However, the limited lifetime of bacteria is only one issue when it comes to the industrial implementation of bacterial-based self-healable concrete. The bacteria also need optimum conditions to produce calcium carbonate via certain metabolic pathways. In particular, the availability of oxygen is vital for bacteria relying on aerobic respiration [24] and urea hydrolysis [25] to perform self-healing. In contrast, oxygen, which diffuses though microcapillaries in concrete, might become a problem for anaerobic bacteria [26].
The ability to withstand larger deformation and to reduce crack propagation in both autogenous and bacteria-based self-healing concretes can be significantly improved by adding selected fibers based on carbon [28], cellulose [29], thermoplastics [30] or glass [31]. In the presence of the fibers, crack propagation can be prevented by fiber bridging, which is a well-established way in fiber-reinforced composites to improve their fracture toughness [32].
In order to accelerate the healing time and slow down crack propagation, the addition of encapsulated repair agents has become a well-established self-healing strategy for concrete [34]. The liquid repair agents are embedded in microcapsules, which should break during a damage event and release the agent into the damage zone. At the same time, the microcapsules have to be sufficiently strong to withstand the mixing process and to be homogenously distributed through the concrete matrix. This makes the synthesis and design of the microcapsules quite challenging. Encapsulation materials are typically based on polystyrene methyl methacrylate [35], urea-formaldehyde [36], phenol–formaldehyde [37], glass [38] or ceramics [39]. Along with the type of materials, the mechanical properties, permeability and crack resistance of the capsules are also adjusted by wall thickness and surface chemistry [40]. Once broken, the physical and chemical repair of the crack in the concrete is induced by a chemical reaction of the healing agent (e.g., polymerization, curing), which solidifies the liquid at room temperature.
A repeated healing of concrete, even if the damage occurs at the same site, can be realized by using hollow microchannels instead of capsules [46]. The channels mimicking vascular systems are able to store a larger amount of the healing agent and to transport/release it at the damage zone [47]. While the same healing mechanism and healing agents can be used as in the microcapsule approach, a low viscosity of the healing agent is crucial to filling it in an interconnected hollow fiber network.
2.2. Smart Concrete
Worldwide, the construction of concrete bridges, tunnels and buildings is extremely energy-intensive. There are three main options for reducing this energy consumption as much as possible. The first is, of course, to reduce construction activity altogether. However, this option is unrealistic. A second option is to improve cement production techniques and optimize the construction stages of buildings and major structures to make the construction industry less energy-intensive overall. A great deal of research and development work is being carried out worldwide to achieve this goal [61,62]. A third option is to extend the service life of reinforced concrete bridges, tunnels and buildings as far as possible.
In the case of bridges and tunnels, early detection of signs of deterioration, or Structural Health Monitoring (SHM), is nowadays mainly carried out using a battery of sensors mounted directly on the concrete surface [63]. These include deformation, temperature, vibration and humidity sensors. These sensors are usually connected to an electronic box for wireless data reading and transmission. These boxes are often equipped with photovoltaic panels to supply them with energy.
Less mature solutions already exist for measuring temperature or the state of deformation within concrete [64]. Most are based on embedded sensors connected to the exterior via electrical cables. These solutions work well, but the cables pose a number of problems. They require special care when the concrete is poured. They degrade over time in the concrete and can also facilitate water penetration, leading to corrosion of the reinforcing steel bars. Fully integrated solutions also exist to overcome these problems.
SAW sensors are a well-known technology and there is an abundance of literature on the subject [67,68]. SAW sensors essentially consist of a piezoelectric chip on which interdigital transducers (IDTs) and reflectors are deposited. IDTs and reflectors are both made of a series of very thin metal fingers. The piezoelectric chip is housed in a protective case. The IDT is connected to pins via very thin bonding wires. These pins pass through the housing, connecting the IDT to an external (electrical) signal source. IDTs can then convert an electrical signal into acoustic waves, and vice versa, thanks to the electromechanical coupling properties of the piezoelectric substrate. These acoustic waves, which propagate on the surface, can be reflected by the reflectors.
A very interesting SAW sensor configuration is where an IDT and several reflectors are deposited on the surface. Such SAW sensors are usually called “Delay Lines”. In the past, they have been used for numerous industrial applications, particularly in extreme environments [69,70]. The IDT is connected to a dipole antenna via the pins. An electromagnetic (EM) pulse sent by a reader to the sensor is converted into an acoustic wave by the IDT. This wave propagates on the surface and is reflected back to the IDT by the reflectors (each reflector reflects a fraction of the incident wave).
Measuring the temperature within concrete immediately after pouring and throughout the maturation phase can provide a wealth of useful information for civil engineering contractors. As concrete stiffens, it emits heat. The temperature rises significantly over hours or even days before slowly dropping and stabilizing at room temperature. The temperature rise is greatest at the center of the concrete element. The temperature profile, over several days, enables experts to verify that the concrete solidification phase proceeds smoothly and as expected.
Figure 1 shows the results of the tests in this field. An ID-Tag SAW temperature sensor is shown in Figure 1a. The sensor is marketed by the Austrian company sensideon (www.sensideon.com, accessed on 10 August 2023). It operates in the ISM Band at 2.45 GHz. The scholars manufactured (using 3D-printing) a protective casing in polymer material, into which scholars slipped the sensor (see Figure 1b). The aim was to avoid direct contact between the concrete and the antenna so as not to overly degrade the antenna’s electromagnetic properties. The housing also serves to protect the sensor and its antenna during casting. This is indeed a fairly brutal operation. Stones in concrete can hit the sensor quite hard. The sensor can also be damaged by the series of vibrations applied after casting for concrete compaction purposes (see Figure 1c). The scholars integrated the sensor into a concrete cube measuring 10 cm on each side (see Figure 1d).
Figure 1. Integration of a 2.45 GHz sensideon SAW ID-Tag sensor in concrete: (a) SAW ID-Tag with its antenna; (b) sensor and protecting housing, in formwork, prior to casting; (c) casting and shaking of concrete, to ensure proper concrete compaction; (d) concrete cube, with embedded sensor.
2.3. 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].
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).
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.
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 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.
To obtain the actuator displacement curve shown in Figure 3b, scholars used a current of 3 A for a voltage of 7 V. A consumption of only ~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].
NiTinol exists in different crystalline phases, depending on the temperature. 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 does not exceed the martensite–austenite transition temperature. When this happens, the wire almost instantly returns to its memory shape.
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.
NiTinol has at least one other very interesting property for Green Tech/Green Cities applications. A NiTinol 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 NiTinol 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.
2.4. Phase-Change Materials
Almost 25% of the energy produced worldwide is used to heat and cool homes and commercial buildings [86]. Therefore, passive cooling techniques are a highly promising family of solutions to help reduce the energy consumption of homes and buildings. Passive cooling techniques usually use large heat capacity materials (heat sinks), such as building materials or water, to mitigate temperature rises due to heat sources, such as hot ambient air, direct solar heat gain and internal heat gain [87]. Another way to massively expand the heat capacity of a building (in a pre-defined temperature range) is to incorporate phase-change materials (PCM) into building materials [88].
PCMs can store or release especially large amounts of thermal energy during the solid–liquid or liquid–solid phase transition. In Figure 4, scholars present the operating principle of an ideal PCM material (the curves do not correspond to real PCM materials) [89].
Figure 4. Stored heat around Tm (typical curves). Red: PCM with a solid–liquid transition temperature Tm; Blue: standard material, with no phase change, in the considered temperature range.
The ideal PCM for passive air conditioning applications must have a small volume change, be non-toxic and non-corrosive, have high thermal conductivity and specific heat capacity, and not supercool or decompose. Only PCMs that have a phase transition close to human comfort temperature can be used. This temperature is located between 18 and 24 °C [91].
PCMs are particularly well suited to passive air-conditioning applications in hot countries, such as sub-Saharan Africa, where the days are hot and the nights cold. However, they can also find applications in other parts of the world.
2.5. Acoustic Metasurfaces
Metamaterials form a new class of materials whose physical properties derive essentially from their micro- or meso-structure, and not from their nature [93]. In particular, it is possible to manufacture metamaterials with properties unknown in nature, such as materials with a negative optical index [94]. In addition, the properties do not depend on the selected material (e.g., polymer, metal, ceramics).
Acoustic metasurfaces form a sub-class of metamaterials. Their ability to manipulate acoustic waves (passively) by means of structured plates that are very thin in relation to the wavelength gives them very interesting properties for “green acoustic” applications. They could indeed replace standard acoustic panels, often made of a wooden panel with holes masking a thick layer of absorbent porous material. Metasurface panels can attenuate sound (even at low frequency) without the need for this absorbent layer, often made from materials that are difficult to recycle, such as glass wool or polyurethane [95].
Acoustic metasurfaces can be used to manipulate the angles of reflection and transmission of incident waves, making it possible, for example, to generate large sound-free areas [97,98,99,100]. These surfaces are made up of geometric patterns designed to interact with incident waves. However, the patterns must have a characteristic size close to a quarter of a wavelength, which limits the application to low frequencies only. Other metasurfaces modify wave reflection and transmission coefficients at their interface [99] or directly absorb incident waves. Some of these absorbing metasurfaces are based on soft and flexible resonating membranes that can damp low-frequency acoustic waves [100,101]. However, these membranes are often fragile, which limits their range of application. Another category of absorbing metasurfaces is based on the Helmholtz resonator principle [102,103,104,105].
2.6. Combination of Smart Materials
Combining several of the solutions discussed above is also a promising approach to developing high-added-value applications. One example is the possible combination of SMA-actuated solar curtains and PCM panels (e.g., inside the rooms) for passive air-conditioning in buildings. PCM materials could also be integrated into concrete walls. SAW sensors could then be used to measure (wirelessly and without embedded electronics) the temperature at the heart of the wall. Initially, this would help in the development of PCM-based solutions for different types of building, and/or different regions of the world. It could also be used to optimize the operation of an active air-conditioning system installed as a complement to the passive air-conditioning system.
Figure 11 shows the results of a feasibility study carried out in this field. A sensideon SAW ID-Tag sensor and its 3D-printed holder (see Figure 11a) were placed directly into a hollow, 15 cm × 15 cm × 15 cm concrete cube (Figure 11b), subsequently filled with PCM material in granulated form (Figure 11c). The walls are 2.5 cm thick. The phase transition temperature of the used PCM lies between 45 and 50 °C. The cube was then placed in a climatic chamber with the temperature set at 65 °C (the air within the chamber reaches this temperature within a few minutes). The heating was then turned off after 20 h. The experimental curve in Figure 11d shows the evolution of the temperature at the heart of the cube, obtained wirelessly and without embedded electronics, using the SAW sensor. The plateaus observed in the temperature rise and fall phases are due to the phase change of the PCM material. The PCM stabilizes the temperature in both directions. These results demonstrate the technical feasibility of the concept discussed in this paragraph and offer interesting prospects for applications.
Figure 11. Integration of a sensideon SAW ID-Tag sensor in a hollow concrete cube, filled with PCM granulates: (a) SAW ID-Tag with its 3D-printed support; (b) sensor and support within the cube; (c) cube filled with PCM granulates; (d) temperature evolution within the PCM material, measured with the SAW sensor. The cube was placed in a climatic chamber with a set temperature of 65 °C. The reader’s antenna was also placed in the climate chamber approximately ten centimeters away from the cube’s surface. After 20 h, the temperature inside the climate chamber was set to 20 °C.
This entry is adapted from the peer-reviewed paper 10.3390/app13169289
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