Fiber-reinforced polymer composites (FRPC) have found wide demand in structural applications. Now, they are an integral part of many engineering solutions. In comparison with metals, polymer composites have superior mechanical performance, and reduced weight; they are less susceptible to fatigue and more corrosion resistant. The use of composites allows for manufacturing products with complex shapes, which reduces the number of parts, increases cost-effectiveness and reliability, and speeds up the assembly of products. As only one example, the implementation of FRPC technologies for blades of wind turbines has revolutionized the wind power industry, increasing the power output from the range of 2–3 MW to more than 12 MW due to the increase in blades’ size [1]. Another example is the high-end applications of the carbon/epoxy FRPC, the so-called “black aluminum”, in the aerospace industry [2]. Currently, the share of composite elements in the design of a modern aircraft reaches up to 50% ^[3][4][5][3,4,5]. The third example comes from marine vehicles where polymers find wide applicability [6]. The unique properties including thermal conductivity [7], electrical conductivity [8], and transparency to various types of radiation [9], are also highly demanded in various design applications.

Nowadays, new approaches in advanced structures and smart materials require material to conduct not only its primary mechanical or functional role, but to be multi-purpose, simultaneously addressing several demanded functionalities of the in-service support for a structure. One such task is to provide de/anti-icing of the working structural surfaces [10]. The formation of ice on hard surfaces can cause huge economic damage to society and poses a great danger [11]. The ice crust formation on the surface of an aircraft limits its performance [12], can significantly change the dynamic flight characteristics, and even lead to flight accidents ^[13][14][13,14]. Ice accumulation on ocean-going ships can change their balance and affect their safety [15]. The operation of wind turbines directly depends on the condition of the surface of the blades [16].

In general, ice formation can occur not only on FRPC, but on different surfaces, and cause a number of problems in everyday life, such as ice formation on roads and footpaths [17], on power line wires [18], and bridge cables [19], on systems of air recirculation [10] and roofs [20], etc. In all these cases, the formation of ice creates a negative impact on engineering structures or poses a danger to human life.

Thereby, it is important to develop effective heating methods that can protect working surfaces from ice formation. De/anti-icing functionality can be integrated into a composite part design. This problem is of special importance due to the necessity of operations in the Arctic and Antarctic regions.

A lot of anti/de-icing technologies are available on the modern market. Some of them can be widely used, whereas others are only for special applications. The most famous solutions are electrothermal [21], photothermal [22], ultrasonic [23], hydrophobic [24], and chemical [25].

2. Foreign-Object Heaters for FRPC

2.1. Metal Foils and Grids as FRPC Surface Heaters

The first steps in the implementation of the resistive heating of an FRPC part surface to prevent ice formation were carried out by placing an electrothermal material (heater) on the surface, with the heater being a type of functional electrical resistor that can convert electrical energy into heat. In one of the first industrial studies on anti/de-icing ^[26][27], several solutions were tested as a heater: etched metal foil grid, sprayed metal grid, knitted metal wire/glass fabric, a pierced expanded metal grid, and wires integrated into rubber. Different types of heaters were placed between the erosion shield material (Nickel alloy) and FRPC blade structure. Although this study demonstrated great future promise for the use of active ice removal systems, it also identified the common polymer burnout problem arising from excessive wire temperatures. Besides, the developed heaters, as external to the FRPC part, were likely to short-circuit against the conductive protective erosion shield material. From the current perspective, electrothermal anti/de-icing systems, which were used in the 70s in the form of thermal spacers, electrically heated foil, or electric heating elements made of metal or carbon fiber on the surface of FRPC products, were bulky, expensive, and often degraded the aerodynamic performance of the product. The placement of a heater on a surface of an FRPC structure is possible either on the internal side or on the external side. In [21], it was demonstrated that implementing an electrical heating element on the internal side of a composite structure led to increased power consumption due to the high temperature differences between the heat application surface and de-icing surface, separated by the FRPC laminate, which made this approach unprofitable. The metal foil heaters showed the best results when they were placed on the external surface of an FRPC structure ^[27][28].

2.2. Metal Coatings as FRPC Surface Heaters

The early external heaters of FRPC structures were difficult to use and not effective. Later, engineers found ways to increase their capabilities. Nowadays, one of the top ways to create a cheap and repairable heating coating on an FRPC surface is by thermal spraying technology ^[28][29]. In ^[29][30], the authors applied a flame-sprayed nickel-chromium (NiCr) coating on the FRPC surface for use as a heating element. Application of the coating with high-temperature thermal technology was shown not to destroy the integrity and mechanical properties of the FRPC laminate due to the implementation of a protective sand-epoxy layer. The resulting coating was found to provide uniform heating. Testing showed that when cooled to −25 °C, the FRPC surface temperature maintained above 0 °C. The technology of thermal spraying of metal films also allows for the application of coatings to FRPC surfaces of complex geometries and the repair of damaged coatings ^[30][31]. The current system used on Boeing 787, requires a steady state temperature of 6 °C for effective anti-icing under −18 °C operational ambient conditions, expending 11.8 kW/m², not taking into account the energy absorbed by the composite structure itself ^[31][32]. Moreover, the deposition of metal layers on the polymer can be conducted by other technologies including physical vapor deposition ^[32][33], chemical vapor deposition ^[33][34], and plasma-enhanced chemical vapor deposition ^[34][35]. These methods are relatively expensive and not suitable for manufacturing thick metal coatings (over 100 μm) at high deposition rates ^[35][36]. Nevertheless, they allow for obtaining layers from non-traditional materials, such as transparent and electroconductive indium tin oxide or extra-thin metal films. However, for transparent applications, it is more interesting to use systems based on thin layers of single-wall carbon nanotubes (CNT) ^[36][37].

2.3. Metal-Based Heaters Imbedded into FRPC

A large area of research in anti/de-icing was devoted to the placement of heating elements embedded into an FRPC product. However, heater implantation into an FRPC product may lead to the degradation of its functional or mechanical properties, especially interlaminar. In ^[37][38], the authors demonstrated that the implantation of a foil as a heater led to the development of delamination in the FRPC part under high loads. However, in ^[38][39] this drawback was not observed for perforated metal foils as contact pads supplying electrical current to other types of heating elements inside the FRPC. Authors in ^[39][40] presented a numerical and experimental development of the concept of a thermoelement based on NiCr wires to be embedded into FRPC profiles of wind turbine blades as an active anti-icing system. It was experimentally shown that the edge region of the profile was the most susceptible to icing due to the maximal convective heat transfer over this region and the fluid load. For anti-icing in cold and dry conditions, the temperature at the leading edge was kept at 60 ± 3 °C for low wind speed. The minimum surface temperature of the rest of the FRPC profile was maintained at 26 °C. The power consumption of the system was 8.3 kW/m², which is lower than 9.2 kW/m²—the power consumption for a similar aluminum profile with outside heaters put to the same icing conditions. Another interesting case is to combine embedded heating elements with electrically conductive fibers in a fiber metal laminate (FML) as an FRPC structure ^[40][41]. The FML systems are widely used, and their production technologies are well studied. The use of such a combined anti-icing system can lead to a decrease in product weight, especially for outdoor structures. In ^[41][42], heated glass laminate aluminum-reinforced epoxy composite structure (GLARE) was studied as one of the most widely utilized FMLs. Since in ^[37][38] it was shown that metal foil embedded into an FRPC can cause delamination, for the GLARE as a serial product it was necessary to demonstrate that an embedded heater does not worsen mechanical properties; in particular, the absence of linear viscoelastic creep. Authors of ^[41][42] showed that the metal layers and glass fibers in GLARE offset the effect of interlaminar creep in the heated state. Continuous physical aging slows down this process in long-term temperature and stress loading. The overall creep effect is thereby limited, which leads to applications of heated GLARE in FRPC structures ^[42][43][43,44]. In studies ^{[39][40][41][42][43]}[40,41,42,43,44] discussed above, the process of embedding the heating elements into the FRPC structures was time- and effort-consuming. It can suffer from manufacturing inconsistencies and human errors. ^[44][45] proposed to introduce 3D printing to automate the manufacturing process. The authors used continuous NiCr wire and thermoplastic as 3D printing material to create a heater embedded into an FRPC plate. The NiCr-heaters in thermoplastic volume were printed by a meander pattern without a gap (i.e., as a continuous filament). A meander pattern was chosen as providing an evenly distributed heat flux on the surface of the FRPC plate. Then, heating plates were covered with a layer of Kapton film for electrical insulation.  The application of metal-based heaters integrated into FRPC looks promising. Moreover, optimization of the manufacturing process, thanks to the possibility of 3D printing to lay the wire inside the product, allows for reducing the risks of malfunction. Nevertheless, embedded metal heaters have a big disadvantage—they are nearly impossible to be repaired.

2.4. Carbon-Based Heaters Imbedded into FRPC

As an alternative to embedding foreign materials as heating elements into FRPC, the possibility is often present to imbed FRPC-related materials, for example, FRPC reinforcing elements, as heat sources. Historically, this approach was developed in parallel with external heaters. One of the pioneering works in this area confirmed the possibility of using carbon fibers as heaters. In ^[38][39], the authors manufactured an FRPC heater based on industrial carbon fibers and resin; in this case, the mesh from nickel was used as electrical contact pads. However, the developed heater possessed low efficiency and high heterogeneity of the generated heat field on the surface of the FRPC. On a 10 cm segment, the temperature drop was 15 °C. Moreover, the authors had to overcome the difficulty of making electrical contacts with carbon fibers. Currently, these problems are solved by functionalizing the surface of carbon fibers using the electroconductive sizing of Ni, Cu, Zn, Pt, Ag, or their alloys ^{[45][46][47][48][49]}[46,47,48,49,50]. A modern study of carbon fibers showed their excellent properties as heaters ^[50][51]. 

2.5. Comparative Analysis of the Foreign-Object Heater Technologies

The comparison of different heaters technologies is presented in Figure 1 0 and in Table 1. The analysis of six main criteria shows that in the case of heaters on FRPC surfaces, Figure 10a, the thermal sprayed technology allows for obtaining the best solution. For the case of embedded heaters, Figure 10b shows that carbon textiles, CNT, and graphene buckypapers allow for obtaining heaters with the best properties and low cost. The carbonaceous nanofiller mat heaters have several advantages: high thermal- and electro-conductivities, sustainability, simplicity of implementation, and improved mechanical properties of FRPC. However, heaters based on these materials have low maintainability: the impossibility of easy repairs due to embedment. From this point of view, nowadays thermal sprayed technology is considered the most applicable option for mass production since these heaters for FRPC have high maintainability due to accessible surface coating, are easily manufactured, and allow for obtaining good thermal characteristics at low cost. The foreign-object heaters have been around for a while and come in various types. However, existing solutions became widely applied in FRPC heaters production only recently. Early solutions had low efficiency, were difficult to manufacture, and had a high cost. Modern technologies, efficient and commercially viable, to produce metal alloy surface heaters for FRPC have been developed with the help of advanced manufacturing technologies based on robotic systems. Currently, the main disadvantages of metal alloy surface heaters are that they create an additional load on the FRPC and are prone to the risk of electric shock.  The main disadvantage of embedded heaters for FRPC, unlike surface ones, has always been the complexity or even impossibility of their repair. This fact greatly hindered their development. However, a large amount of research and development of technologies to produce nanoscale materials has opened new opportunities in this approach. The main advantage of using heating elements manufactured from nanoscale materials is the extremely high reliability, since even with significant damage they continue to remain operational. Moreover, embedded nanoscale heaters are integrated into FRPC and do not add extra weight or load, instead, they enhance the material’s mechanical properties, unlike metal alloy surface heaters. However, new technologies obtaining heating elements from nanoscale materials with given characteristics have just started developing. Manufacturing heaters based on nanoscale materials for large-area FRPC is expensive and not scalable so far. This area of research is actively developing and has great prospects, especially in improving the physical properties of heating elements from nanomaterials.