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Infrared Stealth and Multi-Band Compatible Stealth Materials

Lili Feng , Yiman Liu , Lin Yao , Rui Sun , Junhui He

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (6) : 1044 -1058.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (6) : 1044-1058 DOI: 10.7536/PC200947
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Infrared Stealth and Multi-Band Compatible Stealth Materials
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Abstract

With the rapid development of detection system and the improvement of detection accuracy, the demand for stealth technology is increasingly urgent. As the traditional infrared stealth materials are facing the severe challenge of multi-channel target detection, it is of great significance to develop new compatible stealth materials which can meet the requirements of infrared stealth, radar stealth, visible light stealth and laser stealth. The infrared stealth materials mainly aim at the infrared radiation characteristics of the target to design the material and structure, so as to reduce the prominence of the thermal infrared radiation signal in the background and the hit probability of the target by the thermal infrared guided weapon. This review summarizes the working principle, preparation methods and latest research progress of infrared stealth and compatible materials. Firstly, the structural characteristics, stealth mechanism and research results of the most promising infrared stealth materials, including photonic crystals, doped semiconductors, phase change materials and nano materials, are introduced. The materials to achieve infrared stealth and the specific stealth characteristics are focused. Secondly, infrared/radar compatible, infrared/visible light compatible, infrared/laser compatible and multi band compatible materials are discussed. In addition, the latest research progress is systematically summarized. Finally, the shortcomings and difficulties of infrared stealth materials and compatible materials are summarized, and the future research direction is prospected.

Contents

1 Introduction

2 Infrared stealth materials

2.1 Photonic crystals

2.2 Doped semiconductor

2.3 Phase change materials

2.4 Nano materials

2.5 Other materials

3 Compatible stealth

3.1 Infrared radar compatibility

3.2 Infrared and visible light compatibility

3.3 Infrared laser compatibility

3.4 Multi-band compatibility

4 Conclusion and outlook

Graphical abstract

Keywords

infrared stealth / coating / low emissivity / thermal radiation / multi-band compatible stealth

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Lili Feng, Yiman Liu, Lin Yao, Rui Sun, Junhui He. Infrared Stealth and Multi-Band Compatible Stealth Materials. Progress in Chemistry, 2021, 33(6): 1044-1058 DOI:10.7536/PC200947

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1 Introduction

With the development of modern military technology, infrared guidance with strong anti-jamming ability[3]. It is reported that in recent wars such as the Gulf War and the Afghan War, the rapid development of man-portable air defense system detection technology and the use of precision-guided weapons have gradually diversified the ways of reconnaissance and intelligence acquisition[1]. By means of air-ground observation, information processing and feature extraction, the infrared, radar, visible light and laser signals reflected or emitted by the target can be detected, and the target can be attacked quickly and accurately[2]. At present, one of the most widely used guidance methods to achieve precision strike is high precision guidance system, especially infrared guidance, which has caused serious damage to air combat weapons[4]. In order to improve the concealment, penetration and combat effectiveness of combat personnel and weapon systems on the battlefield, the research of advanced stealth technology has become a military focus of all countries in the world[5,6].
Stealth technology is a comprehensive technology to suppress the characteristics of radar, infrared, acoustic and optical signals by changing the external structure or surface treatment of the target, reduce its detectability, and make it difficult to be found, tracked, identified and attacked. It can be summarized as visible light camouflage, infrared camouflage, laser camouflage, radar camouflage and acoustic camouflage[1]. Among them, infrared stealth mainly refers to the structural design of materials, changing the infrared radiation characteristics of the target, reducing the infrared radiation signal of the target in the background, thus reducing the probability of target identification from the background, and realizing the stealth of the target[7].
Infrared refers to the electromagnetic wave whose wavelength is between 0.76 and 1000 μm. In the transmission process, the infrared ray will be attenuated by the interference of solid particles in the atmosphere and other substances, and the real infrared wave band reaching the ground surface is only 1 ~ 2.5 μm, 3 ~ 5 μm and 8 ~ 14 μm, as shown in Figure 1[8]. Infrared rays can be produced as long as the temperature of the substance is above absolute zero (-273.15 ℃). The radiation process of an object follows the Stefan-Boltzmann law:[9,10]
E = σε T 4
In the formula, E is the infrared radiation energy of the object; σ is the Stefan-Boltzmann constant; ε is the emissivity of the object; T is the absolute temperature of the object.
From the above formula, we can see that the amount of infrared radiation of an object depends on its absolute temperature and emissivity[11,12]. Therefore, the key to prepare infrared stealth materials is to control the infrared radiation of objects by reducing the temperature and emissivity, so as to reduce the detectability of the target[13].
Infrared detectors have been widely used in military reconnaissance, environmental monitoring, fire safety control and night vision due to their passive detection characteristics, accounting for about 30% of the detectors currently in use. The main working bands of infrared detectors are 1 ~ 2.5 μm, 3 ~ 5 μm and 8 ~ 14 μm atmospheric windows[14][15][16,17]. Therefore, it is of great significance for infrared applications to broaden the selection band of stealth materials to cover these three bands[18]. At the same time, the sensitivity of infrared detection equipment is also improving. The temperature resolution of thermal infrared reconnaissance technology has been improved from 0.1 ℃ to 0.01 ℃, and the spatial resolution has reached 0.03 mrad[19]. Therefore, infrared stealth materials play an important role in modern military and related industrial applications[20,21].
In the field of modern detection technology, the detection band is extended from visible light to infrared and radar bands, the traditional single-function camouflage coating is facing the severe challenge of multi-way target detection, and the multi-band detector has shown great potential in target acquisition and recognition, so a compatible stealth technology is needed. In recent years, with the rapid development of aerospace technology and the needs of modern warfare, aircraft components need to withstand more high temperature and thermal shock during flight, and at the same time, their flame retardancy, resilience and flexibility are also required in high temperature environments such as steelmaking, ironmaking, fire fighting, building decoration and so on[22,23]. Therefore, it is of great significance to develop infrared stealth materials with excellent thermal insulation properties. For other infrared applications, such as solar thermal devices or thermal camouflage, control of emissivity and absorptance is required over large surface areas ranging from a few square meters to even square kilometers. Therefore, it is a great challenge for new infrared stealth materials to prepare submicroscopic structures with large area, high fidelity and high efficiency[24]. In addition, under the action of dynamic environmental factors such as temperature, humidity and sunlight intensity, the difference of infrared radiation intensity between the object and the environment will also change dynamically, so it is necessary to develop infrared stealth materials whose emissivity changes with the change of external conditions. To sum up, multi-band, high temperature resistance, flame retardancy, lightweight, efficient preparation and dynamic change are the development trends of new infrared stealth materials[25].
In recent years, scientists have shown great interest in exploring new infrared stealth materials, mainly focusing on photonic crystals, doped semiconductors, phase change materials and nanomaterials. However, pure infrared stealth materials can not meet the needs of practical applications because of the narrow band. Therefore, multi-band stealth compatibility technology has become an important issue in the military field. In this review, the structural characteristics and stealth mechanisms of four types of infrared stealth materials are introduced, including photonic crystals, doped semiconductors, phase change materials and nanomaterials. The infrared stealth bands and the specific stealth characteristics are mainly introduced, and the latest research progress is systematically summarized. Then the compatible stealth conditions of infrared compatible radar, infrared compatible visible light, infrared compatible laser and multi-band compatible materials are discussed, and the outstanding achievements of compatible stealth materials at home and abroad in recent years are summarized in detail. Finally, the shortcomings of infrared stealth and compatible materials at present are analyzed, and the future development direction is prospected.

2 Infrared stealthy material

2.1 Photonic crystal

Photonic crystals (PCs) are dielectric systems whose dielectric constants vary periodically in different directions[26]. According to the direction of permittivity, it can be divided into one-dimensional PC, two-dimensional PC and three-dimensional PC[27]. There are two important concepts in photonic crystallography: photonic band gap and photonic localization[28]. When the light is irradiated by sunlight, the light beam with the frequency in the forbidden band range of the photonic crystal shows the characteristic of high reflection, and the light beam with the frequency in the passband range of the photonic crystal shows the characteristic of high transmission[29]. It is found that the spatial structure distribution, geometric structure and dielectric constant ratio of photonic crystals can be changed by introducing new dielectric materials such as superconductors and plasmas, so as to broaden the photonic band gap and improve the infrared stealth effect[30,31][32][33]. Photon localization is to introduce a defect with a certain frequency into the photonic crystal, and the photons matched with the defect will be confined in the defect, which will destroy the original symmetry of the photonic crystal and achieve low reflection at the wavelengths of radar, visible light and laser. This characteristic makes photonic crystals become a hot material for multi-band stealth applications and has attracted wide attention, and the research progress in this area will be described in detail later.
One-dimensional PC synthesis technology is mature and widely used, which has been reported in many papers. Kang et al. designed one-dimensional PC with good optical absorption capability by using dielectric cesium chloride and graphene respectively, and proved that the absorption rate of the PC was as high as 90% in the mid-infrared frequency range by combining theoretical calculation with experiments[34]. In order to realize the tunable photonic band gap, Ramanujam et al. selected gallium arsenide and silver nanoparticles to prepare one-dimensional PC, and the width of the photonic band gap can be effectively adjusted by controlling the pressure applied to the PC.When the applied pressure is 0 ~ 10 GPa, the photonic band gap width can be adjusted from 1. 953 μm to 1. 709 μm[35]. Li Kangwen et al. Selected Si and Y2O3 to construct a 24-layer one-dimensional PC[36]. The reflectivity of the structure is as high as 97.418% ~ 99.999% in the 3 ~ 5 μm infrared band. This report provides a new way and idea for the combination of photonic crystals and multilayer structures. Similar to Li Kangwen et al., Amri et al. Prepared Si/HMDSO composite coatings by alternately depositing 9 layers by sputtering and chemical vapor deposition[37]. The inorganic/organic hybrid one-dimensional photonic crystal has a low emissivity in the infrared range of 1. 08 ~ 1.99 μm, and the band gap is up to 0. 91 μm.
In order to save cost, Zhang et al. Prepared a one-dimensional heterostructure photonic crystal based on Te and ZnSe[38]. The photonic crystal has a high reflectivity of 86. 72% and 72. 91% at 3 ~ 5 μm and 8 ~ 14 μm, respectively, and the corresponding emissivity is 0. 072 and 0. 194, respectively. In the experiment, the radiation temperature of the photonic crystal is always lower than the background radiation temperature under different background temperatures, and the maximum radiation temperature difference is 31. 97 ℃. Due to the low emissivity of photonic crystals, this feature can effectively suppress the infrared radiation energy of potential targets. Because the raw materials are cheap and easy to obtain, the method can realize the large-scale production of the one-dimensional PC and lay a foundation for the commercial application of the product.
The three-dimensional PC can control the infrared light emissivity in three dimensions, and has more superior light control performance than the one-dimensional and two-dimensional PC, so it has a high application prospect in the field of infrared stealth. In view of this superiority, Qiu Lili et al[39].
They prepared three-dimensional PC by optimized vertical self-assembly method, and found that the center positions of the forbidden band of three-dimensional PC self-assembled by 1.00 and 1.20 μm monodisperse colloidal microspheres were 2.25 and 2.47 μm, respectively (Fig. 2a, B). The material can change the infrared radiation characteristics of the target, realize the stealth effect of the infrared band of the target, and has good potential competitiveness in the markets of photonic crystal templates, thermal barrier coating materials, infrared stealth technology and the like. Qiu et al., using the plane-wave expansion method, numerically simulated the model of cubic close-packed photonic crystals on the surface of polystyrene microspheres with a diameter of 2 μm, and determined that the final band gap was 3.24 ~ 3.44 μm (Fig. 2 C)[40]. At the same time, experiments were carried out to prepare 2 μm polystyrene photonic crystal films by self-assembly at the gas-liquid interface (Fig. 2D). The infrared absorption wavelengths are 3. 30 μm and 3. 42 μm, respectively, and the modulation of infrared radiation characteristics is realized in the 3 ~ 5 μm detection waveband, which provides a simple, economical, fast and efficient new method for the preparation of large diameter three-dimensional photonic crystal infrared stealth materials.

2.2 Doped semiconductor

A doped semiconductor is composed of a semiconductor (metal oxide) and a dopant (carrier donor)[41]. Because the band gap of semiconductor is larger than the wavelength of infrared light, it can not absorb photons intrinsically, so the carrier donor plays a leading role in the absorption and reflection of photons[42]. With the increase of the doping carrier concentration, the metal ions are replaced and the electrons are transferred, so that the conductivity is increased, and the doped semiconductor material is transformed from a semiconductor to a conductor under the action of infrared light waves, so that the doped semiconductor material has low absorption and high reflection performance in the infrared wave band. By changing the carrier density, carrier mobility and carrier collision frequency, the absorptivity and emissivity of doped semiconductors can be changed, and then the infrared stealth effect can be enhanced[43]. Common infrared stealth doped semiconductors include: tin-doped indium oxide (ITO), aluminum-doped zinc oxide (ZAO), and antimony-doped tin oxide (ATO)[44⇓⇓~47][48,49][50].
ZAO has low infrared emissivity, good thermal insulation, electrical conductivity and optical properties, so it has received more attention in the field of infrared stealth[51]. Yu et al. designed a sandwich structure of carbon nanotube-doped aerogels (CNTAs) on polyimide (PI) fabric by hot-pressing method, and coated its outer surface with ZAO (Fig. 3A)[52]. By comparing the infrared thermal images of PI fabric, ZAO/PI fabric, CNTAs/PI fabric and ZAO/CNTAs/PI fabric (Figure 3B ~ e), it is found that PI fabric has a larger infrared radiation intensity under the heat source, and the infrared radiation intensity of ZAO/PI and CNTA/PI fabrics increases slowly under heating. However, the infrared radiation intensity of ZAO/CNTAs/PI fabric is very small, and the temperature difference between ZAO/CNTAs/PI fabric and the environment is also very small when it is heated under a heat source. Due to the presence of ZAO, the surface emissivity is less than 0.5, and the thermal conductivity of the middle layer aerogel is as low as 0.013 W/ (m · K). This experiment is the first time to realize the combination of aerogel and semiconductor, which not only lays the foundation for the development of doped semiconductor infrared stealth materials, but also expands the application field of doped aerogel.
ITO thin film is a kind of N-type degenerate semiconductor material with excellent infrared stealth performance[53]. Zhu et al. Prepared nano-columnar crystal ITO films by RF magnetron sputtering[54]. According to the analysis, the (400) preferred orientation of the nano-columnar crystal film is shown, and the increase of the number of carriers leads to the change of the energy band gap, thereby affecting the absorptivity and emissivity of the film in the infrared band. The film deposited at 200 W exhibits the best optical and electrical properties, with a resistivity of 1.44×10-4Ω·cm, an average transmittance of 88.49%, and an infrared reflectance of 89.18%. The nano columnar crystal ITO film has unique structure and excellent infrared stealth performance, and has good application prospect in the fields of flat panel displays, light-emitting diodes, solar cells and infrared stealth coatings. In a recent work, Zhu et al. deposited an ITO film with a sandwich structure consisting of an oxygen-poor layer and an oxygen-rich layer on a glass substrate (Fig. 4A)[55]. Figs. 4 B, C show the SEM top view and cross section of the sample, and it can be seen that the oxygen-poor layer at the bottom and the oxygen-rich layer in the middle are very different in morphology. Compared with the single-layer ITO film, the ITO film with the sandwich structure shows more excellent photoelectric properties (Fig. 4D), with a resistivity of 1.384×10-4Ω·cm and an infrared reflectance of 86.37%. The ITO films prepared by them provide a new idea for the preparation of transparent conductive films, and provide a unique opportunity for in-depth study of the effects of different sandwich structures on the photoelectric properties of films, which is of great significance.

2.3 Phase change material

According to the Stefan-Boltzmann law, reducing the surface temperature of the target object can reduce the infrared emission energy of the object[13]. There are two kinds of infrared stealth mechanisms of phase change materials: one is solid-liquid phase change, that is, in the process of phase change, a large amount of latent heat is absorbed or released by changing its physical properties, and the surface temperature of the target object is changed to achieve infrared stealth[56,57]; The second is the crystal phase transition, that is, the structure and optical properties change abruptly before and after the phase transition temperature, and the high transmission of infrared light changes into low emission to achieve infrared stealth[58]. Common phase change materials include microencapsulated phase change materials, vanadium dioxide and conductive polymer materials[59,60][61][62].

2.3.1 Microcapsule

Microcapsule is a kind of micro-container which is made of polymer or inorganic wall (wall material) to completely wrap up the trace of solid, gas and liquid (core material)[63]. The microcapsule phase change material is made into infrared phase change camouflage material by means of coating, integration and doping, and the infrared camouflage can be realized by controlling the temperature difference between the target and the background by absorbing or releasing heat[64⇓~66]. The selection of core material and wall material is the key to realize the infrared stealth performance of microcapsules. The core material is required to have low thermal conductivity and heat storage density, low degree of supercooling and phase separation, and stable performance, while the wall material of microcapsules is required to have good mechanical properties, good compactness, good film-forming and other characteristics[67⇓~69][70]. For example, Yu et al prepared phase change microcapsules with paraffin wax as the core material and urea-formaldehyde resin as the shell material by in-situ polymerization, with an encapsulation rate of 60.1% and an average particle size of 425.7 nm (Figures 5A ~ C)[71]. The results show that the fabric coated with microcapsules can block the external heat, slow down the heating rate, effectively reduce the temperature by 5 ~ 10 ℃, and reduce the infrared emissivity accordingly (Figure 5d), which has a certain infrared camouflage ability. This work provides a new idea for the application of phase change microcapsules in the preparation of military protective clothing.

2.3.2 Vanadium dioxide

Vanadium dioxide (VO2) is a new type of infrared stealth material with reversible phase transition properties. The phase transition heat of monoclinic VO2 is 51.84 J/G[72]. When The phase transformation temperature reaches 68 ℃, the monoclinic semiconducting phase (M1 phase, space group P21/c, unit cell parameters :am1=5.75Å, bm1=4.52Å, cm1=5.38Å, β = 122.6 °) with high resistivity at low temperature transforms into the rutile metallic phase (R phase) with low resistivity at high temperature.The space group is P42/mnm, the unit cell parameters are :at=bt=4.55Å and ct=2.86Å), and the high transmission of infrared light is changed into high reflection[73][74,75]. At present, the known thin film synthesis methods can be used to prepare VO2 thin films, which can be roughly divided into physical and chemical methods. Physical methods mainly include sputtering deposition and pulsed laser deposition, while chemical methods include sol-gel, polymer-assisted deposition and chemical vapor deposition.
The phase transition temperature is the key factor affecting the application of VO2. The main methods to control the phase transition temperature are doping, interfacial stress, particle size and so on. Among them, the elements Mo, Ti and Ni in the doping method have been widely studied[76⇓~78][79⇓~81][82]. Chen et al. carefully studied the combination of W and vanadium dioxide, and prepared W-doped VO2 films with different doping rates within a certain power range by using a double-target co-sputtering method (Figure 6)[83]. The phase transition temperature of the undoped pure VO2 film is 64.3 ℃, and the phase transition temperature is reduced to 54.8 ℃ when the tungsten doping ratio is 0.5%. When the doping rate is 1%, the temperature drops to 46.2 ℃; When the doping ratio is 1.5%, the temperature drops to 41.3 ℃; When the doping ratio is 2%, the phase transition temperature is further reduced to 36.5 ℃. By controlling the W doping rate, large-scale production can be achieved, and this report opens up a new way to prepare phase change materials with large area, high fidelity and high efficiency.
In addition, Liu et al also prepared spherical VO2 nanopowders with pure phase and good crystallization[84]. The average particle size is 91.5 nm, and the infrared radiation intensity can be automatically adjusted to reduce the radiation temperature. Below the phase transition temperature of 67.2 ℃, the VO2 presents a continuum state with high emissivity and high radiation intensity, and when the temperature increases, the VO2 will transit to the metallic state with low emissivity and low radiation intensity. It is worth mentioning that they have successfully prepared snowflake-like VO2(M) using similar methods. Interestingly, after 10 cycles of heating and cooling in air atmosphere, the phase change properties of VO2 powder did not decrease, which greatly improved the practical application value of VO2 phase change materials[85]. In this regard, He Junhui et al., based on the results of finite-difference time-domain (FDTD) calculations, proposed a controllable method for the preparation of HSNs/VO2 bilayers of multifunctional SiO2 hollow nanospheres[86]. In the experiment, it was observed that the temperature difference between the blank glass and the coated glass increased with the increase of the irradiation time. After 5 minutes of thermal radiation, the temperature difference reaches 7 ℃, which fully proves that the prepared HSNs/VO2 double-layer coating has excellent anti-infrared radiation effect, and also shows that the film has great application potential in transparent energy-saving intelligent windows of high-rise buildings.

2.3.3 Conductive polymer

Conductive polymer is a kind of polymer material that can be transformed from insulator to conductor by chemical or electrochemical doping, usually with conjugated π bonds[87,88]. At room temperature, the intrinsic conducting polymer is not conductive, but with the redox process, ions will be inserted into the polymer backbone to neutralize the charge, after this "doping", the polymer has conductivity, and the conductivity is between 10-6~105S/cm[89][90]. Therefore, conductive polymer materials not only have the unique characteristics of polymer materials, such as processability, light specific gravity, simple process, green and environmental protection, but also have the high conductivity of conductors[91]. Such materials can undergo a reversible transition from an insulator with low electrical conductivity to a metal with high electrical conductivity at room temperature, and at the same time, their high reflection of infrared light changes to high transmission[92]. By combining the reversible doping and dedoping properties of conductive polymers with their ability to absorb infrared light of a certain wavelength band, conductive polymers can play a huge potential in the field of infrared stealth[93,94]. Among them, conducting polymers such as polyaniline, polypyrrole, polyacetylene and polythiophene have become the focus of research because of their structural diversity, simple composite processing, strong molecular design and easy control of electromagnetic parameters[95⇓~97].
Polyaniline is one of the most promising conductive polymers because of its high conductivity, good charge storage capacity, easy synthesis and low cost of raw materials[98⇓~100]. He et al. Synthesized a spiral-shell helical polyacetene (SHPA) by copolymerization of L-serine grafted propargylamine (LSA) and propargylamine (PA) (fig. 7A)[101]. In the range of 8 ~ 14 μm, the infrared emissivity is 0.451. At the same time, they explored the infrared absorption mechanism and concluded that, first of all, the self-assembly of hydrogen bonds into snail-like helical structures reduces the vibration of — NHCO-, thus reducing the absorption of energy. Secondly, the introduction of amino can enrich the composition of intramolecular hydrogen bonds, reduce the vibration of unsaturated bonds, and reduce the potential energy of the system. Finally, the bond length of the main conjugated chain is more even in the solvent, which makes the conjugation effect enhanced and the system more stable (Fig. 7 B). This strategy undoubtedly provides a new direction for the realization of controllable infrared stealth. As another example, Ju et al. prepared a composite fiber using polyurethane-antimony tin oxide (PU-ATO) (fig. 7C)[102]. The PU-ATO not only has infrared stealth and thermal radiation stealth properties, but also has hydrophobicity to prevent a decrease in stealth properties due to wetting of the PU-ATO composite fiber by absorbed water (fig. 7 d, e). Because of its wearability and hydrophobicity, PU-ATO composite fibers are expected to open up a new market in infrared stealth products.

2.4 Nanomaterial

Nanomaterials have become one of the most potential functional stealth materials because of their small particle size, excellent wave absorption, broadband, good compatibility, small mass and thin thickness[103]. The stealth mechanism of nanomaterials mainly includes three aspects: [104~107]:(1) small size effect. When electromagnetic waves such as infrared and radar, which are much larger than their own size, pass through, the transmittance of waves is high and the reflectivity is low, so that the received signal of the detector is weakened. (2) Surface effect. Compared with conventional materials, the specific surface area of nanoparticles is larger, and when electromagnetic waves pass through, multiple scattering will occur. At the same time, with the increase of the number of surface atoms, the surface activity of particles is enhanced, resulting in magnetization, and electromagnetic energy is converted into heat energy, which is conducive to the absorption of electromagnetic waves. (3) quantum size effect. The energy level of the particle electron changes, forming a new wave absorption effect. At the same time, the quantum size affects the shift of the absorption edge of the material, thus changing the absorption bandwidth.
For the first time, Liu et al. Dispersed carbon black and nanoporous silicon into anodic aluminum oxide (AAO) as a template to prepare an excellent infrared stealth material (Fig. 8 a, B)[108]. This structure broadens the light absorption wavelength of silicon to the mid-infrared region, and the AAO containing carbon black can easily transfer the thermal characteristics of objects to evade the tracking of infrared detectors. Experiments show that the nanomaterial can effectively absorb infrared light in the range of 2.5 ~ 15.3 μm, and the absorption rate is as high as 97.5% (Fig. 8 C, d). In the same year, Jia et al. Used magnetron sputtering technology to prepare nano-copper films on aramid fabrics (Fig. 8e)[109]. The results show that the infrared reflectivity of the deposited layer increases with the increase of the thickness of the copper film. When the sputtering time is 10 min, the infrared reflectivity is the highest, reaching more than 90%. Figure 8 f ~ H shows the thermal images recorded by infrared thermal imaging of the researcher's arm under various conditions. It can be seen that the radiation temperature of the aramid fabric sprayed with copper for 10 minutes is the lowest. Based on its excellent thermal camouflage performance, this nano-copper film creates new opportunities for the application of infrared camouflage in the field of military security and surveillance.

2.5 Other materials

2.5.1 Hydrotalcite-like material

Hydrotalcite-like compounds (LDHs) are [110~113] composed of positively charged layers of divalent metal ions and trivalent metal ions and interlayer anions through the interaction between the charges and the hydrogen bonding between interlayer water molecules and the layers. The general formula can be expressed as [ M 1 - x 2 + M x 3 + (OH)2]x+(An-)x/n·mH2O,M2+ represents a divalent metal ion, M3+ represents a trivalent metal ion, An- represents an interlayer anion, X represents the molar ratio of M2+/M2++M3+, and m represents the number of interlayer water molecules[114]. In which that infrared absorption range can be Chan by adjusting the composition. The common preparation methods include co-precipitation method, hydrothermal synthesis method, crystallization isolation method, ion exchange method and calcination recovery method. In fact, the infrared radiation characteristics of the camouflage target are similar to those of the background environment, which can also achieve the effect of camouflage and stealth. Green vegetation is the basic background of military camouflage simulation. For this reason, Yang et al successfully prepared three kinds of LDH by co-precipitation method, which have similar emissivity to green vegetation to achieve infrared stealth effect[115]. The results show that the emissivity of Mg/Al-LDH, Zn/Al-LDH and Zn/Mg/Al-LDH are 0. 968, 0. 939 and 0. 911, respectively, in the 8 ~ 14 μm waveband, and the emissivity of Mg/Al LDH is the closest to that of plant leaves.This kind of hydrotalcite material has great research value in the future infrared stealth materials and military clothing design.

2.5.2 Biomimetic material

Biomimetic infrared stealth materials are new types of infrared stealth materials synthesized and prepared based on the microstructure characteristics, color change principle or electromagnetic wave reflection characteristics of organisms. The skin of cephalopods such as squid and octopus is an excellent color-changing system. The muscle tissue in the skin can control the expansion or contraction of vesicles in pigment cells, and then regulate infrared light[116]. Gorodetsky et al. imitated the above biological functions and adjusted the wrinkle structure on the surface of the film material by mechanical stretching, thus changing the reflectivity and diffuse reflection, thus realizing the dynamic regulation of broadband and narrowband infrared reflection[117]. In addition to reducing the infrared reflectivity, it is also an effective way to achieve infrared stealth by reducing the infrared radiation ability of the target through thermal insulation. The hollow hair of polar bears and other mammals living in the cold environment is an excellent thermal insulation material, which can effectively block the infrared radiation to the external environment, reduce the loss of heat, and achieve the effect of infrared stealth[118]. Gui et al. Prepared a wearable ordered porous fiber material by using self-developed freeze-spinning technology through bionic hollow hair structure[119]. The material not only has efficient heat insulation and heat preservation functions, but also has good processability, air permeability and comfort, so that a wearer can realize thermal stealth in the observation of an infrared imaging device, and the material is of great significance to the fields of developing novel heat insulation and heat preservation combat uniforms used in alpine regions, infrared stealth military camouflage materials and the like.

3 Compatible Stealth

The application of military target detection system and precision guidance system poses a great challenge to the survival of military targets, so the research of advanced stealth technology has become a military focus of the world[5]. Visible light stealth is used to defend the enemy's naked eye and visible light detection system against the reconnaissance of military targets. Infrared stealth uses stealth technology to reduce the infrared radiation energy of the target, so that the radiation characteristics of the target and the background are consistent. Laser stealth reduces the reflectivity of the target surface in the working band of the laser, which makes the laser unable to receive the reflected echo and reduces the detectability of the target[120]. Single stealth technology is not enough to protect targets in all directions, and multi-band stealth compatible technology has become an important issue in the military field[121,122]. Multi-band compatible camouflage materials can reduce the probability of military targets being found and effectively improve the survivability of weapons and equipment in the battlefield[123].

3.1 Radar and infrared compatibility

Among many detection technologies, radar detection (2 ~ 18 GHz) and infrared detection are the two most effective and common detection technologies[124]. The radar detector detects the target by receiving the reflected wave generated after the electromagnetic wave transmitted by the detection equipment is blocked[125,126]; The infrared detector receives the radiation energy of the object itself and finds the target. Infrared/radar compatible stealth materials have become a hot research topic in the field of stealth[127]. However, different detection methods make radar wave and infrared stealth mechanism different, and they restrict each other. Radar absorbing materials are required to have the characteristics of high absorption and low reflection, while infrared stealth materials are required to have the characteristics of high reflection and low emission[128,129]. In addition, radar absorbing materials will attenuate the incident electromagnetic wave and then convert it into heat, resulting in the increase of target temperature, which in turn restricts infrared stealth[130]. From Kirchhoff's law, it follows that for opaque material[131]
ε + R = 1
Where ε is the emissivity; R is the reflectivity.
Both ε and R are functions of wavelength, and the wavelength difference between infrared wave and radar wave is large, so the compatible material must meet the condition that ε is small and R is large in the infrared band, and the reflectivity R is small in the radar band.
The materials with radar camouflage and infrared camouflage characteristics can be made into coated particle structure or composite film structure to achieve compatible stealth. For example, Zhong et al. prepared a multifunctional metamaterial composite structure, as shown in Fig. 9a, B, which consists of an infrared layer (IRSL), a radar absorbing layer 1 (RAL1), a radar absorbing layer 2 (RAL2) and a frequency selective transport layer (FSTL) from top to bottom[132]. The metamaterial has a reflectivity of less than 10% in the 1.5 to 9 GHz range and a transmission peak of 50% near 3.8 GHz (Figure 9c), while also maintaining a low emissivity (0.52) in the 8 to 14 μm range. This work not only realizes the compatible stealth of infrared and radar bands, but also lays a foundation for the further expansion of the subsequent band range. Yang et al. Successfully coated the surface of metamaterial electromagnetic wave absorber with polyurethane/aluminum composite coating with low infrared emissivity, and systematically studied the effect of the floating rate of aluminum particles on the infrared emissivity[133]. The results show that the infrared emissivity of the coating decreases with increasing buoyancy (Fig. 9d). When the content of Al powder is 20% and the suspension rate is 90%, the infrared emission ability of the infrared coating is only 0.204, and the infrared coating has excellent electromagnetic wave absorption performance, which can achieve good infrared radar compatibility conditions.
In addition, Xu et al. Designed and fabricated a subsurface composed of a radar absorbing layer and an infrared shielding layer[134]. The infrared emissivity can be flexibly adjusted by adjusting the filling ratio (metal area/total area) of the top metal portion. Both the simulation and experimental results show that the surface has strong absorption peaks at 6. 35,8.38,12.10,15.37 and 18. 05 GHz, and the emissivity is as low as 0. 32 in the infrared band of 3 ~ 14 μm. Unlike the subsurface prepared by Xu et al., Qu et al. Designed a subsurface compatible with radar and infrared stealth using ITO film (Fig. 10 a, B)[135]. When the sheet resistance of ITO is 60 Ω/sq, the absorptivity is more than 90% in the frequency range of 5. 8-8. 3 GHz. At the same time, the infrared emissivity is about 0.52 in the range of 3 ~ 14 μm. It should be noted that when the sheet resistance is 150 Ω/sq, the subsurface has a radar absorptance of more than 90% and an infrared emissivity of about 0.75 in the 6.8 ~ 13.4 GHz range (Figure 10 C ~ f). The structure has the remarkable advantages of small thickness and light weight, and is of great significance in stealth engineering.

3.2 Visible-infrared compatible

Visible light and infrared compatible camouflage is the need of strategy and battle in the field of military target camouflage, and it is also the deepening and expansion of camouflage material technology research[136]. The performance of visible light and infrared compatible camouflage materials should meet the following two requirements: First, in the visible light band (0.38 ~ 0.76 μm), its spectral characteristics and background spectral characteristics should be as consistent as possible, so that the camouflage materials should be able to blend into the natural background[137][138]. Secondly, in the infrared band (1 ~ 2.5 μm, 3 ~ 5 μm and 8 ~ 14 μm), camouflage materials should have appropriate infrared emissivity to reduce the difference of infrared radiation between the target and the background, thereby reducing the probability of being detected and identified by infrared detectors. At present, there are two main methods to realize the visible light and infrared compatible camouflage of materials. First, the materials with visible light camouflage characteristics and low infrared emissivity characteristics are made into coated particle structures or composite film structures, such as coatings and composite film [139~141]. Secondly, a single special material is prepared, which can meet the requirements of visible and infrared compatible camouflage, such as special doped materials and rare earth materials[142].
Ye et al. Reported a new method to achieve visible and infrared compatibility by designing dielectric/metal/dielectric structures. In this study, ZnS/Ag/ZnS thin films were prepared by particle swarm optimization method[143]. The spectral characteristics of the film are shown in Figure 11A. The samples with sand green and yellow green coatings have high visible light reflectivity and excellent visible light stealth performance. It is found that the average visible light transmittance of the film is 89% and the infrared reflectance is 95% (Fig. 11B), and the visible light transmittance and infrared reflectance of the sample remain above 80% at an oblique incidence angle of 60 °. Unlike the particle swarm optimization method, Qi et al. Successfully prepared a one-dimensional photonic crystal with specific surface colors (khaki, brown, navy and cyan) and ultra-low infrared emissivity by depositing Ge and ZnS on quartz glass substrates by magnetron sputtering.The macroscopic picture and SEM image of the prepared Ge/ZnS one-dimensional photonic crystal are shown in Figure 11c and Figure 11d. The average reflectivity of the Ge/ZnS one-dimensional photonic crystal reaches 95.1% in the 3 ~ 5 μm infrared band, which meets the requirements of visible-infrared compatible stealth[144].
Materials with variable emissivity can self-adjust their infrared radiation characteristics, and have important applications in infrared compatible stealth technology[145]. Liu et al synthesized BCG/CaCl2/PEG-g-CDA composites with different mass ratios by liquid phase reaction method[146]. The color of the composite changed significantly from green to yellow when the temperature was increased from 30 ° C to 55 ° C, and reversibly changed back to green when the temperature was decreased to room temperature (fig. 12A). At the same time, the infrared emissivity of the composite increases rapidly in the temperature range of 50 to 62 ° C, with a variation of 0.23 from 0.7 to 0.93 (Fig. 12 B). In addition, they explained the mechanism of thermochromism and tunable infrared emissivity. This reversible thermochromism is due to the structural difference caused by the molecular structural transition of BCG between the open-ring quinone structure and the closed-loop lactone structure (Fig. 12c), while the tunable emissivity is attributed to the change in resistivity under temperature fluctuations. The preparation method is simple and low in cost, and has important reference value for preparing visible light and infrared compatible materials.

3.3 Laser-infrared compatible

Laser and infrared detection equipment play a very important role in the military. In recent years, both of them are developing towards the direction of composite detection, which greatly increases the probability of finding important targets[147,148]. Therefore, it is necessary to study the infrared and laser compatible stealth materials. Laser is usually based on the active detection mode, based on the high reflection characteristics of the target surface, through the detection of echo signals to determine the location of the target. Common lasers used for ranging and guidance include 1.06 μm Nd ∶ YAG laser, 1.54 μm erbium-doped laser and 10.60 μm CO2 laser[149,150]. Therefore, laser stealth is aimed at laser stealth at wavelengths of 0.93, 1.06, 1.54 and 10.6 μm. Laser stealth requires materials to have low reflectivity and high absorptivity for incident laser, so as to greatly reduce its echo power. Infrared stealth requires materials to have high reflectivity and low emissivity to infrared radiation, so as to effectively suppress infrared radiation. In order to achieve compatible stealth, the infrared emissivity and the reflectivity at the laser wavelength of the material must be reduced at the same time, which are contradictory and difficult to achieve with traditional coatings[151].
Photonic crystals provide a new way to solve the problem of infrared laser compatible stealth because of its excellent optical properties. Miao et al. Used the characteristic matrix method in thin film optical theory to deposit Te, ZnSe and Si alternately on silicon wafers, and obtained inhomogeneously doped one-dimensional photonic crystals[152]. The average emissivity of the photonic crystal is 0. 0845 at 3 ~ 5 μm, and 0. 281 at 8 ~ 14 μm, and the reflectivity is 28% at 10. 6 μm. Because of the low cost and the unique properties of reducing infrared emissivity of flaky aluminum powder, Huang et Al. Prepared Al/ATO composites by co-precipitation method[153]. The results show that the absorptivity of Al/ATO composites increases significantly with the increase of Sn/Sb molar ratio. When the Al content is 20% and the Sn/Sb ratio is 10 ∶ 1, the Al/ATO composite has the best infrared laser stealth performance, in which the infrared emissivity is 0. 708 and the reflectivity is 43. 4%.
In practical applications, not only the normal incidence but also the oblique incidence must be considered. Chen et al designed a selective metamaterial absorber for 1.54 μm laser and mid-infrared atmospheric window[154]. The simulation results show that the absorptivity reaches 99% at 1.54 μm, and the emissivity is less than 0.1 and 0.06 at 3 ~ 5 μm and 8 ~ 14 μm, respectively (Figure 13 a). The analysis of thermal emission characteristics shows that the structure can suppress 90% and 94.9% of the blackbody radiation in the 3 ~ 5 μm and 8 ~ 14 μm bands, respectively (Figure 13 B). At the same time, they studied the influence of the dielectric layer on the absorption performance, calculated the influence of the thickness of the polyimide layer and the loss angle tangent on the absorption coefficient, and found that the structure still has good absorption performance at a large incident angle (Fig. 13C, d), which is undoubtedly a great progress in infrared laser compatible stealth.

3.4 Multi-band compatible

The development of omni-directional and full-band reconnaissance technology requires the in-depth study of composite stealth[155]. In addition to the response of infrared light wave, stealth coatings also require materials to meet the compatibility of different bands such as visible light, laser, infrared, radar wave, sound wave and so on[156].
In 2017, Zhang et al. Prepared a one-dimensional photonic crystal with infrared-laser-radar compatible stealth by periodically stacking Ge/Se/Si and introducing defects into its structure, and then innovatively combining radar absorbing materials (RAM) with it[157]. The reflectivity is as high as 89. 6% in the infrared band of 8 ~ 14 μm, and as low as 39. 8% in the laser reflection band of 10. 6 μm.In the radar high frequency band of 7. 8 ~ 18 GHz, the transmitted energy is attenuated by 10 dB compared with that without RAM. Similar to the Ge/Se/Si multilayer structure, Chen et al. Proposed a composite grating structure composed of Ag and ZnS[158]. The surface phonon polaron was excited by SiC grating, and the absorption peak was observed at 10. 6 μm. Ag and ZnS are selected to design the multilayer film with high absorption at 1.06 μm, high visible light transmittance and high reflection at infrared band. The simulation results show that the spectral reflectance is 95% in the wavelength range of 1 ~ 14 μm, the absorptivity is 94% and 89% in the wavelength range of 1. 06 μm and 10. 6 μm, respectively, and the average transmittance is more than 75% in visible light.
Recently, Li et al. designed a photonic structure with thermal management and multiband compatible stealth (Fig. 14 a-c)[159]. The results show that the structure has an emissivity of 0.25 and 0.33 at 3 to 5 μm and 8 to 14 μm, respectively, and an absorptance of up to 90% at 10.6 μm (Figure 14 d). At the same time, it is found that the visible spectrum of different colors can be obtained by using the anti-reflection resonance of the top silicon film, and the reflectivity in the visible spectrum is red-shifted with the increase of silicon thickness (Fig. 14e), so the selected color can be used for visual camouflage. For example, samples with a silicon thickness of 20 nm can be used for visible light stealth in deserts, and samples with a silicon thickness of 30 and 40 nm have the potential for visible light stealth in forests. The photonic structure can realize the effective compatibility of three bands of infrared, laser and radar, and promote the multifunctional development of multi-band compatible stealth materials.

4 Conclusion and prospect

Due to the long-term development of detection means and the existence of unknown detection forms, weapons and military equipment are facing more severe threats, and the demand for strengthening stealth technology is increasingly urgent. A single infrared stealth material can not meet the needs of practical applications because of its narrow band. Therefore, multi-band stealth compatibility technology has become an important issue in the military field. In this paper, photonic crystals, doped semiconductors, phase change materials, nanomaterials and other emerging stealth materials are systematically discussed, and the structural characteristics and stealth mechanisms of various materials are classified and discussed.The infrared stealth band and specific stealth characteristics are focused on, and the infrared compatible visible light, infrared compatible radar, infrared compatible laser and multi-band compatible materials are systematically summarized. The new infrared stealth materials are mainly developed in the direction of "light, thin, strong and wide" multi-spectrum stealth.
Although a large number of scientific research work has made it widely developed, there are still many serious challenges in infrared stealth in China. First of all, the related research of infrared stealth in China is not perfect, the theory is not deep enough, and there is a lack of new materials. With the military game of various countries, single stealth technology should have a more sophisticated level, while multiple stealth compatible technologies, such as infrared-radar or infrared-visible compatible stealth technologies, should also be paid attention to and developed. Thirdly, the traditional camouflage protection technology is static and has limitations, which can not adapt to environmental changes and lose camouflage effect. Therefore, in order to further improve the survival and penetration ability of military targets in complex battlefield environment, to avoid the search and attack of photoelectric detection system and infrared precision guidance weapons, the development of intelligent infrared stealth materials which can actively adapt to the environment is an important development trend of camouflage and stealth technology. In addition, in the laboratory, the preparation of infrared stealth metamaterials is mostly based on flat surfaces. However, the actual infrared stealth targets, such as aircraft, tanks, ships and missiles, usually have complex and changeable surface curvature, and the coating may also be subjected to repeated high and low temperature thermal shock, high temperature airflow erosion, solid particles erosion, combustion impurities corrosion and other environmental tests. Therefore, the key of engineering application is to improve the high temperature service performance and flexibility of the coating. It is hoped that the efforts focused on the above aspects will further promote the development of infrared stealth at home and abroad.

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Funding

National Key Research and Development Program of China(2017YFA0207102)

National Natural Science Foundation of China(21271177)

National Natural Science Foundation of China(21571182)

Fundamental Research Funds for the Central Universities(2021YJSHH23)

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, CAS

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