Foam Rubber-Based Three-Layer Flexible Composite for High-Efficiency Infrared Stealth and Joule Heating

Abstract

With the rapid development of infrared detection methods and military surveillance technologies, flexible and wearable infrared stealth materials (ISM) have attracted increasing attention. Inspired by the layered structure of penguins’ fat–feather–oil, this study prepared a three-layer MXene/waterborne polyurethane (WPU)-foam rubber-phase change microcapsule (PCM)/WPU composite material (M-F-P) via the solution blending and doctor-blading method. The outermost layer of the M-F-P composite is an MXene/WPU conductive film, which features a low infrared emissivity and Joule heating performance to adapt to suddenly cold environments. The porous foam rubber in the middle layer provides excellent thermal insulation performance, which effectively inhibits heat conduction and enhances infrared stealth efficiency. Meanwhile, as a four-directional elastic material, it exhibits deformation recovery capability in both the warp and weft directions as well as the 45° direction. The bottom layer of the PCM/WPU film has a phase change enthalpy of 154.3 J/g and possesses efficient thermal management capability. It achieves dynamic thermal regulation through the cycle of heat absorption at high temperatures and heat release at low temperatures.

1. Introduction

The rapid rise of infrared detection technology has been applied in various fields, with both its detection capability and precision reaching new heights. Based on the basic principle of thermal radiation, any object above absolute zero continuously radiates infrared rays [1], and infrared detection technology achieves target detection precisely by identifying the difference in infrared radiation between the target and the background environment [2]. In the military field, the infrared thermal signatures emitted by military equipment and personnel are highly susceptible to being captured by infrared detection systems, thereby exposing them to the threat of precise identification and strike. Particularly in individual soldier operations, significant thermal signals are continuously generated. Therefore, infrared stealth technology provides crucial support for the concealment of personnel and equipment on the battlefield [3]. Infrared stealth technology has emerged as a major research hotspot. To achieve infrared stealth, it is necessary to reduce the infrared radiation signal of the target. According to the Stefan–Boltzmann law (Equation (S1)) [4], there are two main approaches to reducing the target’s infrared radiation signal: one is to control the surface emissivity, and the other is to lower the surface temperature. Therefore, integrating these two strategies and developing a functional composite material that combines an internal thermal insulation structure, external low emissivity, and excellent flexible wearable properties has become the key to achieving efficient infrared stealth.

Three main approaches have been developed for controlling the surface emissivity of targets: coatings containing aluminum powder [5], fabrics doped with metal oxides [6,7], and composite metamaterials [8]. However, coatings formulated with aluminum powder are unsuitable for direct contact with human skin. They are prone to oxidative failure, have a relatively high weight, and the high gloss of aluminum powder makes them susceptible to light reflection. These drawbacks substantially restrict their usefulness in stealth applications [9]. For fabrics incorporating metal oxides, poor dispersion of oxide particles within polymer solutions often compromises fiber flexibility and elasticity. Composite metamaterials also face limitations, including demanding processing conditions, thermal instability of metal-based components, and a marked drop in performance when the incident angle exceeds 30°. Consequently, achieving effective infrared stealth requires tailoring material design to the specific operational demands and environmental conditions.

Transition metal carbides and nitrides (MXenes), first reported in 2011, have attracted substantial interest for their low infrared emissivity and strong compatibility with stealth applications [10,11]. They can be conformally coated onto irregular surfaces and maintain excellent chemical stability and mechanical robustness [12,13]. Li et al. [14] pioneered the development of Ti₃C₂T_x thin films, demonstrating that the infrared emissivity of Ti₃C₂T_x thin films can be as low as 0.19 in the wavelength range of 7–14 μm. In addition, due to the increasingly urgent demand for infrared stealth in individual soldier operations in recent years, infrared stealth technology has gradually developed toward flexibility. Forming composite materials from MXene materials with low infrared emissivity and flexible substrates is an excellent design strategy. Among various flexible substrates, textiles are widely used due to their strong flexibility, light weight, and large usable area [15], but they exhibit poor thermal insulation performance. Another important means of infrared stealth is to control the target temperature, which is achieved by using thermal insulation materials to reduce the temperature difference between the target surface and the surrounding environment. Traditional flexible insulating materials such as wool, polystyrene foam, and cork typically exhibit thermal conductivities close to that of air and are essentially isotropic, making them less than ideal for effective thermal management [16]. Introducing porous structures is one of the effective strategies to enhance thermal insulation performance in the design of thermal insulation materials [17,18,19]. Porous structures can significantly inhibit thermal convection and achieve efficient thermal insulation by forming a solid–gas composite structure to block heat transfer paths [20,21]. The porous structure and low thermal conductivity of foam rubber, along with its unique three-dimensional porous network structure, can effectively store a large amount of static air and repeatedly reflect and scatter human body thermal radiation within the pores, thereby inhibiting gaseous thermal conduction [22,23,24] and providing a material basis for preparing ideal thermal insulation systems. Therefore, replacing textiles with foam rubber to form a composite material with MXene can synergistically achieve the dual functions of low infrared reflectance and thermal insulation.

Another important class of temperature-regulated infrared stealth materials is phase change materials (PCMs). As a key type of passive thermal management material, PCMs absorb or release latent heat during the phase transition and thereby maintain an approximately constant temperature. This characteristic narrows the temperature gap between a target and its surroundings, enabling thermal camouflage and infrared stealth [25,26,27]. For example, Xu et al. [28] prepared an infrared stealth suit by coating phase change microcapsules on the fabric, which can absorb a certain amount of heat and exhibit good infrared stealth capabilities. Zhang et al. [29] combined Kevlar fiber aerogel films with organic PCMs to achieve infrared camouflage functionality. By integrating the latent heat absorption and release effects of PCMs with the thermal insulation capability of porous materials, synergistic PTM performance can be realized [30,31]. Liu et al. [32] fabricated a double-layer film by superimposing porous silica aerogel and PCMs and demonstrated its dual-functional thermal regulation performance in harsh cold and hot environments. Notably, the thermal management mechanisms formed by organisms in nature through long-term evolution have provided abundant bionic inspiration for the development of advanced PTM materials. Many mammals (such as camels, polar bears, and penguins) have evolved remarkable strategies for maintaining stable body temperatures, and these mechanisms continue to inspire bio-inspired approaches to thermal management materials [33,34,35]. In particular, penguins are able to survive the extreme Antarctic climate through a three-tiered thermal insulation system consisting of a subcutaneous fat layer, a densely packed feather structure, and an oil secretion layer that enhances water repellency and heat retention [36]. Penguins rely on a substantial subcutaneous fat layer, which accumulates during periods of abundant food and serves as an energy reserve when resources become limited. Their specialized feather architecture traps air to form an insulating layer that suppresses heat loss. In addition, oil secreted from the uropygial glands imparts strong water repellency, preventing seawater from contacting the skin and further reducing thermal dissipation. This multi-layered and synergistic protective strategy also provides a highly valuable reference for the design of infrared stealth materials with efficient thermal insulation, thermal regulation capabilities, and infrared reflection properties.

Inspired by the multi-layered structure of penguin feathers, this study prepared a flexible three-layer M-F-P composite material via the scratch coating method, which simultaneously possesses favorable thermal management capability, excellent infrared stealth performance, and the Joule heating effect (Figure 1). The outer MXene/WPU layer provides low infrared emissivity and suppresses thermal imaging detectability. The middle foamed-rubber layer, characterized by low thermal conductivity and a highly porous structure, functions as both an insulating barrier and an air reservoir to limit heat loss. The innermost PCM/WPU layer undergoes reversible phase transitions that absorb excess heat at elevated temperatures and release stored heat under colder conditions, enabling active regulation of the temperature. By simultaneously regulating surface temperature and infrared emissivity, the M-F-P composite shows strong potential for infrared stealth applications. Its flexibility also offers a new avenue for developing advanced wearable thermal infrared stealth materials. More importantly, this work goes beyond the mere stacking of individual functions by proposing and validating a multilayer collaborative design based on the principle of “infrared reflection–thermal insulation–active thermal management.” This provides an innovative solution to the longstanding challenge faced by traditional stealth materials in balancing broad-spectrum infrared stealth, dynamic thermal response, and mechanical flexibility, thereby laying a scientific foundation for the design of next-generation adaptive infrared stealth composites.

Figure 1. Schematic illustration of a composite inspired by the skin–fat structure of a penguin for infrared stealth at night.

2. Experimental Section

2.1. Materials

All chemical reagents were obtained commercially and used without any further purification. Titanium Aluminum Carbide (Ti₃AlC₂, 400 mesh, CAS No. 196506-01-1) powders were supplied by Jilin 11 Technology (Jiling, China). Lithium fluoride (LiF, CAS No. 7789-24-4) was provided by Aladdin (Shanghai, China). Hydrochloric acid (HCl, CAS No. 7647-01-0) was brought from Beijing Chemical Reagents (Beijing, China). 35 °C phase change microcapsules (PCM) were purchased from Jiuosen Co., Ltd. (Taiwan, China), and waterborne polyurethane (WPU, CAS No. 9009-54-5, 35%) was obtained from Jitian Chemical (Shenzhen, China). The foam rubber was purchased from Zhongke Haishi Company (Beijing, China).

2.2. Preparation of MXene/WPU Film

The synthesis process of the MXene solution is based on the previous report [10]. Initially, 1.5 g of LiF was dissolved in 20 mL of 9 M HCl, and the mixture was heated to 40 °C under magnetic stirring for 15 min. Subsequently, 1.0 g of MAX (Ti₃AlC₂) was gradually added to the solution, which was then continuously stirred at 40 °C for 48 h to etch the aluminum layer, yielding a Ti₃C₂T_x slurry. The resulting Ti₃C₂T_x solution was washed with 2 M HCl, shaken manually until the solution was uniform, and then centrifuged at 3000 rpm for 2 min. The precipitate was washed with deionized water until the pH of the precipitate was around 6. The final precipitate was continuously sonicated with deionized water in an ice water bath for 90 min, and the solution containing the exfoliated MXene nanosheets was centrifuged at 3500 rpm for 20 min. The resulting upper solution was the MXene solution (Figure 2a). The MXene solution was added to WPU and continuously stirred to obtain a 30 wt.% MXene/WPU solution. The solution was scraped onto a glass plate using a coating machine, with a thickness of 400 μm. The film was dried at 40 °C for 4 h to obtain the MXene/WPU film.

Figure 2. Schematic illustration of the fabrication of the M-F-P composite. (a) The synthesis process of MXene/WPU film; (b) The synthesis process of PCM/WPU film.

2.3. Preparation of M-F-P Composite Materials

The PCM powder was added into the WPU solution and continuously stirred to obtain a 10 wt.% PCM/WPU solution (Figure 2b). The resulting solution was scraped and coated on foam of 1 mm, 2 mm and 3 mm with a coating machine, with a thickness of 300 microns, and dried at room temperature for 5 h. The MXene/WPU film (with the contact surface with the glass plate facing upward) was bonded to the foam rubber PCM/WPU using WPU and dried at 40 °C for 2 h to obtain the sample. They are designated as M-F1-P, M-F2-P and M-F3-P based on the thickness of the foam rubber.

2.4. Characterization

The X-ray diffraction patterns were conducted by the SmartLab SE spectrometer (Rigaku, Japan) with a scattering angle ranging from 3° to 80°. The microscopic morphologies were observed by a field emission scanning electron microscope (COXEM, 30AX plus, Republic of Korea) at an acceleration voltage of 15 kV. The samples needed to be sprayed with Au on the surface before testing. The chemical structures of samples were researched by the Fourier-transform infrared spectroscopy (Thermo Fisher Nicolet iS10m, America) using the ATR method. The wavenumber range used for analysis was 500–4000 cm⁻¹. The mechanical properties of the samples were measured using an Instron 5969 universal testing machine. Tensile tests were performed in accordance with the ASTM D412-16 [37] standard, and compression tests were conducted following the ASTM D575 [38] standard. The strain rate for cyclic tensile and compression tests was set at 10 mm/min, and all tests were carried out at room temperature (25 °C). The thermal conductivity of foam rubber was measured using the guarded hot plate heat flow meter method. The test instrument was the DRPL-3B thermal conductivity tester, following the ASTM D5470-17 [39] standard. The phase change behavior of PCM was measured with the differential scanning calorimeter (DSC 204F1, Netzsch, Germany). The samples were conducted at a heating/cooling rate of 10 °C/min under a nitrogen atmosphere. The AVIO R550-Pro thermal imager was applied to monitor surface radiative temperatures and capture infrared images. Thermogravimetric (TG) curves were obtained using a German thermogravimetric analyzer (TGA209F1, NETZSCH, Germany). The mechanical properties were measured with a materials testing machine (Instron 5969, America).

3. Results and Discussion

3.1. Microstructure and Composition

After diluting the MXene solution, the MXene dispersion exhibits a light green color characteristic of Ti₃AlC₂ and a Tyndall effect (Figure 3a) [14,40]. Figure 3b shows the XRD patterns of the Ti₃AlC₂ and MXene. The successful synthesis of MXene is evidenced by the characteristic peaks of (004) and (006) as well as the left shift of (002) from 9.5° to 5.8° [41]. Fourier infrared spectroscopy in Figure 3d shows that a series of FTIR absorption peaks of WPU are detected in the MXene/WPU sample. The representative peaks at 3310 cm⁻¹, 2900 cm⁻¹, 1700 cm⁻¹, 1110 cm⁻¹ and 1525 cm⁻¹ can be considered as N-H, C-H, C=O, C-O-C stretching vibration peaks and N-H group bending vibration peaks, respectively. The peak of the N-H stretching vibration in MXene/WPU is much broader than that in pure WPU, and the peak of the bending vibration of the amino group in the urethane moiety also shifts from 1540 cm⁻¹ to 1525 cm⁻¹. These phenomena collectively confirm the successful miscibility of MXene with WPU. Compared with pure WPU, the MXene/WPU composite exhibits no obvious chemical damage, as evidenced by the retention of the characteristic peaks of N-H, C=O, and C-O-C functional groups. The incorporation of MXene does not destroy the backbone structure of WPU, and WPU endows MXene with flexibility [42]. As shown in Figure 3d,d₁ and Figure S1, the cross-sectional SEM images of the WPU film and the MXene/WPU film illustrate the differences in their microstructures. It can be seen that after the introduction of MXene nanosheets, the cross-section of the MXene/WPU film displays a distinct rough structure, indicating that the MXene nanosheets have been successfully incorporated into the WPU matrix and formed a layered structure with a specific orientation, which is consistent with previous literature reports [43,44,45]. As shown in Figure 3e,e₁, a large number of PCMs can be observed from the cross-sectional SEM images of the PCM/WPU film. Phase change enthalpy is usually used to evaluate the material’s ability of isothermal heat absorption and release; the higher the phase change enthalpy, the more excellent the performance. DSC is an effective method for measuring the phase change enthalpy of materials. Figure 3f examines the phase change behavior of the PCM through DSC analysis. The PCM has excellent heat storage capacity (ΔHm = 154.3 J/g), enabling effective temperature regulation and facilitating infrared stealth in various scenarios. Thermal stability tests were conducted on the PCM to verify their reliability in practical applications. The thermal decomposition process of the PCM was obtained via thermogravimetric analysis (TG) as shown in Figure 3g. There are two main thermal decomposition stages for the PCM, corresponding to the PMMA polymer as the shell layer (200 °C to 295 °C) and n-eicosane as the core layer (300 °C to 400 °C), respectively. The PCMs do not decompose below 100 °C, ensuring their thermal stability within the phase change temperature range.

Figure 3. (a) The Tyndall effect of the MXene aqueous solution. (b) The XRD patterns of Max and MXene. (c) FTIR spectra of the WPU film and MXene/WPU film. (d,d₁) SEM images of the MXene/WPU film. (e,e₁) SEM images of the PCM/WPU film. (f) DSC curves of PCM. (g) TG curve of PCM.

3.2. Thermal Insulation and Heat Retention Performance

As shown in Figure 4a, the foam rubber exhibits a three-dimensional porous network with a uniformly distributed closed-cell structure, and the pore size primarily falls within the range of 120–150 μm. The foam rubber maintains a low thermal conductivity of 0.034 W/(m·K) across different thicknesses (Table S1). An infrared camera was used to compare foam rubber and cotton fabric of identical size and thickness (≈3 mm). When both samples were placed in a refrigerator, the IR images (Figure 4c) show that foam rubber remained yellow even after 60 min, whereas cotton cooled rapidly, with its color shifting quickly from yellow to blue–green. When placed on a 50 °C heating plate, the cotton fabric again exhibited a faster color transition—from yellow–green to red—during heating, as shown in Figure 4d. During both cooling and heating, the surface covered by foam rubber consistently shows a pronounced color contrast relative to the environment, indicating a significant temperature difference and confirming its thermal insulation capability. To more accurately assess its insulation performance, thermocouples were placed between the textile samples and the skin to monitor microclimate temperature changes. As shown in Figure 4e,f, after 30 min in a −13 °C environment, the temperature beneath the foam rubber stabilized at 35 °C, whereas the cotton fabric dropped more noticeably, reaching a minimum of 31.3 °C. Under a 50 °C environment, the foam rubber maintained a temperature of approximately 40 °C, while the cotton fabric increased to 47 °C. These results further demonstrate the superior thermal insulation and heat retention properties of foam rubber. For foam rubber, heat transfer occurs first through solid-phase conduction along the cell walls. The presence of numerous closed cells greatly extends the solid conduction pathway, thus reducing overall heat-transfer efficiency [19]. Secondly, it possesses a unique three-dimensional porous network structure with closed-cell pores that store substantial volumes of static air. Heat undergoes multiple reflections and scattering within these pores, further inhibiting thermal conduction (Figure 4g).

Figure 4. (a) SEM image of foam rubber. (b) Comparison of thermal conductivity between foam rubber and other materials [22,36,40,46,47,48,49,50]. (c) Infrared images of foam rubber and cotton fabric at cold temperatures (0 °C) and (d) high temperatures (50 °C). (e) Temperature variation plots for foam rubber and cotton fabric in environments of −13 °C and (f) 50 °C. (g) Heat transfer mechanism of foam rubber.

3.3. Mechanical Properties

Figure 5a presents the stress–strain curves of 1-mm-thick foam rubber stretched along the warp, weft, and 45° directions. All three curves show pronounced elastic behavior: even at large strains of 225–275%, the material exhibits neither a clear yield plateau nor noticeable plastic flow. This response suggests that the foam rubber can recover its original shape once the load is removed, a defining characteristic of four-way stretch materials. The differences in curve shape and strength among the three directions indicate that the foam rubber displays anisotropic mechanical properties. To investigate the macroscopic mechanical properties of the foam rubber, we conducted a uniaxial tensile test under a fixed elongation rate (100%) and obtained the stress–strain curve as shown in Figure 5b. This curve clearly reveals the typical mechanical characteristics of the foam rubber as a porous polymer material. Firstly, throughout the stretching process, the stress level exhibited by the material was extremely low (<0.15 MPa), indicating that the foam rubber has an extremely low elastic modulus and excellent flexibility. Secondly, the stress–strain relationship of the material showed a significant nonlinearity. In the initial stage of strain (approximately 0–25%), the stress increased gradually, corresponding mainly to the bending of the foam pore walls and the elastic buckling of the pore edges. As the strain further increased (>50%), the curve slope gradually increased, and the stress rise rate accelerated. This is attributed to the gradual compaction and collapse of the pores within the foam, the onset of the densification effect, and the enhancement of the material’s resistance to deformation. The entire deformation process reflects the gradual extension and orientation of the polymer chain segments and the irreversible damage accumulation of the bubble structure, which is a comprehensive manifestation of the material’s viscoelastic and hyperelastic behaviors (Table S2). Figure 5c shows the cyclic strain curves of the 3 mm-thick foam rubber in the 1st, 25th, and 50th cycles after being stretched to a fixed elongation of 100% (Figures S2 and S3 show the cyclic strain curves of the 1 mm-thick and 2 mm-thick foam rubber). From the first cycle to subsequent cycles, it is observed that the area of the hysteresis loop significantly decreases, indicating a clear stress softening. Moreover, with each cycle group of unloading, the residual strain gradually increases, directly proving the accumulation of permanent deformation (Table S3), indicating that during the initial cycle, the elastic recovery rate gradually decreases, and then the performance tends to stabilize.

Figure 5. (a) Stress–strain curves of foam rubber-1 under warp, weft, and 45° direction tensile tests. (b) Stress–strain curves of foam rubbers under tensile test at a fixed elongation (100%). (c) Stress–strain curves of foam rubber (3 mm) under cyclic tensile tests at 100% elongation. (d) Stress–strain curves of M-F-P composites under tensile tests. (e) Stress–strain curves of M-F-P composites under composites tests. (f) Stress–strain curves of M-F3-P composites under cyclic composites tests. (g–i) Digital Photographs of M-F-P composites.

Figure 5d shows the tensile stress–strain curves of the M-F-P composites. All samples exhibit typical behavior of flexible materials, characterized by low yield strength and large strain in the plastic deformation region, with fracture elongation exceeding 150%, indicating a certain degree of ductility. From M-F1-P to M-F3-P, increasing the foam rubber thickness effectively enhances the load-bearing capacity of the composites without compromising their ductility. Figure 5e presents the compressive stress–strain behavior of the M-F-P composites. At small strains, the stress increases linearly with strain, representing the elastic region dominated by the elastic bending of the cell walls. The curve then enters a broad and relatively flat plateau region, where stress rises slowly with increasing strain. This stage reflects the material’s ability to absorb energy during continuous deformation as the cellular structure begins to buckle, yield, and gradually collapse. When the strain exceeds approximately 50–60%, the curve rises steeply, marking the onset of the densification region. At this point, most of the cells have collapsed, the solid frameworks come into contact, and the material’s resistance to further deformation increases sharply. Figure 5f presents the stress–strain curve of M-F3-P after 50 cycles of compression. (The stress–strain curve of M-F1-P and M-F2-P after 50 compression cycles are shown in Figures S4 and S5). Compared with the first cycle, the loading curves of the 25th and 50th cycles have significantly shifted downward. This indicates that the stress level required to achieve the same strain gradually decreases with each cycle, suggesting that the effective stiffness of the material decreases due to fatigue. As the cycles progress, the area of the hysteresis loop gradually decreases and stabilizes. To investigate the effect of heating and cooling cycles on the mechanical properties of the material, we conducted 10 heating and cooling cycle tests on the M-F3-P samples (one cycle consists of holding at −10 °C for 1 h → holding at 25 °C for 1 h → holding at 50 °C for 1 h). After the cyclic tests, there was no significant change in the tensile and compressive properties of the samples (Figure S6), indicating that the material can still maintain excellent mechanical properties after multiple heating and cooling cycles. This indicates that the material’s ability to dissipate energy initially declines and then gradually reaches a stable state, suggesting that the material has excellent buffering effects. M-F-P material possesses the advantage of being lightweight, allowing it to be placed upon flowers without causing them to bend. M-F1-P material exhibits excellent flexibility and foldability during arbitrary bending, curling, and folding processes (Figure 5g–i).

3.4. Infrared Stealth Performance

The infrared stealth performance of M-F1-P, M-F2-P, and M-F3-P was evaluated using an infrared thermal imager. All samples were first placed on a temperature-controlled heating plate set at 37 °C (simulating skin-surface temperature). During heating, the infrared images of both the background and the targets were recorded, as shown in Figure 6a, and the actual temperature evolution is presented in Figure 6b. Among the samples, M-F3-P exhibited the lowest apparent surface temperature in the infrared images. This result indicates that increasing the foam rubber thickness from 1 mm to 3 mm significantly enhances the infrared stealth capability by effectively suppressing the infrared radiation signature of the target, demonstrating the tunability of the M-F-P composites. After 3 h of heating at 37 °C, the apparent infrared radiation temperatures of M-F3-P, M-F2-P, and M-F1-P were only 20.6 °C, 22.1 °C, and 23.1 °C, respectively—approximately 10 °C lower than the actual temperature. These results confirm that the MXene layer imparts low infrared emissivity to the composites, enabling excellent infrared stealth performance. To further verify the role of the MXene layer’s low emissivity in infrared stealth, white adhesive tape was applied to the surface of M-F3-P as a blackbody reference. As shown in Figure S7, the taped region indeed exhibited a higher radiation temperature than the MXene-coated surface. Comparison of Figure 6c and Figure S8 shows that after heating at 75 °C for 3 h, all three M-F-P samples maintained low apparent infrared radiation temperatures (28.8 °C, 30.4 °C, and 33.6 °C), which were substantially lower than their actual temperatures (47.5 °C, 51.0 °C, and 56.9 °C, respectively). The thermal plates covered with M-F-P composites displayed infrared images that blended almost seamlessly with the background, demonstrating a pronounced mid-temperature infrared stealth effect. Figure S9 presents the infrared comparison images of the M-F-P composites after heating at 100 °C for 80 min. Even under these elevated conditions, M-F3-P maintained a relatively low apparent temperature (35.9 °C), indicating a degree of infrared stealth. Infrared imaging of the composite cross-section revealed a bottom-to-top heat-transfer process, with substantial heat blocked in the middle and lower layers during heating (Figure 6d). To evaluate the durability of the infrared stealth performance, M-F3-P was subjected to ten consecutive cycles of ultrasonic treatment in water (20 min per cycle), followed by drying. Figure 6e shows the infrared images of the samples after the 10th ultrasonic cycle, following 3 h of heating at 37 °C. The composites retained their infrared stealth performance, with no noticeable degradation in thermal camouflage capability, confirming their robustness and practical application potential. Figure 6f presents the infrared image of M-F1-P placed on a human palm, where the covered skin blends into the background, demonstrating excellent infrared concealment. The M-F-P materials also exhibit good flexibility and wearability and can be fabricated into a face mask worn by the user (Figure 6g). When placed on a fighter jet model heated to 100 °C, the material continued to provide effective infrared thermal camouflage (Figure 6h). Figure 6i illustrates the infrared stealth mechanism of M-F-P: when an object is at a higher temperature than its surroundings, it radiates substantial infrared energy to the environment. Therefore, to achieve infrared stealth, the infrared emission of high-temperature objects must be effectively concealed. In this work, the infrared stealth mechanism of M-P-F arises from the synergistic contribution of three components: (1) The innermost PCM layer regulates temperature by rapidly absorbing and storing heat during the phase transition at elevated temperatures and releasing heat under cold conditions. (2) The middle porous foam–rubber layer, with its high porosity, provides excellent thermal insulation and effectively blocks heat transfer from high-temperature objects. (3) The outer MXene layer, characterized by its low infrared emissivity, substantially reduces the detectability of the target in thermal imaging. By simultaneously controlling the surface emissivity and temperature, the M-P-F composite exhibits extremely low infrared radiation intensity, enabling effective infrared camouflage for personnel and equipment during nighttime operations.

Figure 6. (a) IR images of M-F-P composites on a thermal stage of 37 °C. (b) Top surface temperature–time curves of M-F-P composites on a hot target of 37 °C. (c) IR images of the M-F-P composite on a thermal stage of 37 °C. (d) Thermal infrared images of M-F3-P for cross-section on a hot stage of 38 °C. (e) IR image of M-F1-P after 10 repeated heating cycles. (f) IR image of a volunteer holding the M-F2-P composite on hand for the IR stealth demonstration. (g) IR image of volunteers wearing masks made of the M-F2-P composite. (h) IR image of 100 °C fighter model. (i) The infrared stealth mechanism of M-F-P composites.

3.5. Electrical Heating and Visual Stealth Performance

Joule heating is observed and implemented according to the connection method shown in Figure 7a [51]. As shown in Figure 7b–e, when current flows through the material, electrical energy is converted into thermal energy due to resistance. Under infrared thermal imaging, the material exhibits Joule heating behavior at low voltages. The equilibrium temperature increases with rising load voltage when 2.5, 3.0, 3.5, and 4.0 V are applied, respectively. Figure 7f demonstrates the material’s capacity for Joule heating when subjected to varying voltages within a 15 °C cold environment. At 4.0 V, the material stabilizes at approximately 40 °C, effectively countering the cold to provide warmth for the human body. This low-voltage heating ensures operational safety, positioning the material as a viable candidate for secure heating applications. Figure 7g demonstrates the temperature stability of M-F1-P during repeated heating and cooling cycles. The temperature curves remain consistent between the first and fifth cycles, indicating that M-F1-P exhibits excellent Joule heating stability. The material achieves tunable infrared stealth through the synergistic effect of MXene’s low emissivity and foam rubber’s thermal insulation, while also demonstrating safe low-voltage Joule heating capability and excellent cyclic stability over more than 5 heating/cooling cycles. Its core strength lies in the effective integration of these two key functionalities.

Figure 7. (a) The schematic illustration of Joule heating tests. (b–e) IR image of M-F1-P composite under different applied voltages. (f) Real-time temperature variation at different applied voltages. (g) Temperature stability of M-F1-P composite in different heating and cooling cycles at an applied voltage of 4.0 V. (h) Comparison of the properties of M-F-P composite with other materials [22,52,53,54].

4. Conclusions

Inspired by the feather–oil–fat structure of penguins, this paper successfully fabricated an M-F-P composite material featuring a three-layer structure comprising MXene/WPU foam rubber-PCM/WPU. Through synergistic interactions between its layers, the M-F-P composite achieves outstanding infrared stealth and Joule heating capabilities. Maintained for 3 h under simulated human body temperature conditions, the material’s surface temperature remains approximately 23 °C under infrared thermal imaging, blending seamlessly with the ambient background and demonstrating excellent infrared concealment. Concurrently, the material exhibits outstanding flexible wearability. Mechanical testing indicates the M-F-P composite possesses low yield strength, demonstrates substantial strain within the plastic deformation zone, and achieves a fracture elongation exceeding 150%, exhibiting considerable ductility. When subjected to a 4.0 V voltage, the M-F-P composite maintains a stable material temperature of approximately 40 °C, effectively countering cold conditions to provide warmth to the human body. This M-F-P composite, integrating adaptive thermal management, efficient infrared stealth, Joule heating, outstanding wearability, design flexibility, and mechanical properties, demonstrates broad application prospects in the field of flexible smart camouflage.