Radiative cooling is an environmentally friendly, passive cooling technology that operates without energy consumption. Current research primarily focuses on optimizing the optical properties of radiative cooling films to enhance their cooling performance. In practical applications, thermal contact between the radiative cooling film and the object significantly influences the ultimate cooling performance. However, achieving optimal thermal contact has received limited attention. In this study, we propose and experimentally demonstrate a high-power, flexible, and magnetically attachable and detachable radiative cooling film. This film consists of polymer metasurface structures on a flexible magnetic layer. The monolithic design allows for convenient attachment to and detachment from steel or iron surfaces, ensuring optimal thermal contact with minimal thermal resistance and uniform temperature distribution. Our magnetic radiative cooling film exhibits superior cooling performance compared to non-magnetic alternatives. It can reduce the temperature of stainless steel plates under sunlight by 15.2 °C, which is 3.6 °C more than that achieved by non-magnetic radiative cooling films. The radiative cooling power can reach 259 W∙m−2 at a working temperature of 70 °C. Unlike other commonly used attachment methods, such as thermal grease or one-off tape, our approach allows for detachment and reusability of the cooling film according to practical needs. This method offers great simplicity, flexibility, and cost-effectiveness, making it promising for broad applications, particularly on non-horizontal irregular surfaces previously considered challenging.
Cooling constitutes a significant portion of global energy consumption, impacting the comfort, productivity, and well-being of individuals and society worldwide, and it contributes substantially to greenhouse gas emissions [1]. The growing demand for global cooling, driven by rapid urbanization, necessitates the development of sustainable and energy-efficient cooling solutions. Unlike conventional energy-intensive cooling strategies, radiative cooling technology, which operates without energy consumption, has garnered extensive attention recently. The basic principle of radiative cooling involves emitting thermal radiation to the extremely cold outer space through the atmospheric transparent window (8–13 μm) to cool objects, addressing critical environmental challenges posed by waste heat emission [2], [3]. In addition to high infrared emissivity in the atmospheric window, daytime radiative cooling materials must strongly reflect sunlight to achieve optimal cooling performance [4], [5], [6], [7], [8]. Recent research has focused on integrating radiative cooling with sustainable energy harvesting and developing wearable radiative cooling products [9], [10], [11], [12], [13]. In practical applications, improving the thermal conductivity and reducing the thermal contact resistance between the cooling film and the target objects are crucial for fully realizing the benefits of radiative cooling. However, this aspect has been largely overlooked [14].
Recent advancements in daytime radiative cooling have utilized photonic multilayer structures, metamaterials/metasurfaces, and polymer materials [15], [16], [17], [18]. Multilayer structures are typically thick (> 1 mm), with relatively high thermal resistance and comparatively low cooling performance [19], [20], [21], [22]. Additionally, their rigidity limits their application on non-flat surfaces. In contrast, metamaterial/metasurface structures can significantly reduce the thickness of radiative cooling films to tens of micrometers [23], [24], [25], [26]. However, their complex, high-cost, and time-consuming fabrication processes pose challenges for large-scale production. Polymer-based radiative cooling films [27], [28], [29], [30], [16], [31] are distinguished by their flexibility, ultrathin, and ultralightweight properties, as well as their compatibility with low-cost and large-scale manufacturing processes. These polymer materials can be flexibly applied to various shaped and textured surfaces [32], [33], significantly expanding the application scope of radiative cooling technology from conventional building applications to diverse objects and materials, particularly widely used iron or steel materials. Examples include vehicles, battery cabinets, large containers, boarding bridges, data centers, newspaper kiosks, distribution boxes, audio amplifiers, overpasses, base stations, warehouses, and storehouses (Note S1 in Appendix A). These materials possess high thermal conductivity, thermal capacity, and solar absorption, making them prone to heating under direct sunlight, thereby necessitating substantial cooling. However, no radiative cooling films have been specifically designed for these application scenarios.
In a typical radiative cooling system, heat conduction between the object and the cooling film is as vital as the thermal radiation performance of the cooling films. Poor heat conduction due to loose contact between the cooling film and the object leads to ineffective heat extraction and radiation by the cooling film, significantly compromising overall cooling performance [34], [35], [36]. Therefore, it is essential to develop methods that ensure firm attachment of radiative cooling films to object surfaces to improve thermal contact. For cost-effectiveness, the method should allow flexible attachment and detachment of radiative cooling films according to weather conditions and cooling requirements. Conventional methods using thermal interface materials, such as thermal conductive adhesives, improve thermal contacts [37], [38], [39]. However, uniformly applying thermal adhesives over large areas is technically challenging, and once installed, the cooling films cannot be removed and reused. Similarly, cool paints can achieve large-scale integration but are designed for permanent or long-term applications, making removal difficult when environmental temperatures are low. In comparison, thermal greases, as liquid thermal interface materials, can meet the demands of attaching and detaching radiative cooling films. However, their fluid nature makes them unsuitable for long-term use, especially on vertical surfaces. Furthermore, these strategies are not suitable for systems requiring frequent attachment and detachment of radiative cooling films.
In addition to using thermal interface materials to fill air gaps, increasing pressure between interfaces is an effective method to reduce thermal contact resistance. Li et al. [40] selected polyimide (PI) film as the substrate and used Maxwell pressure generated by high-voltage electricity to achieve tight thermal contact between the cooler and the object. This approach allows detachment and reuse of the radiative cooling film. However, electrostatic control requires a non-conductive substrate, severely limiting applicability, and the electrostatic field is not long-lasting, requiring additional energy input to maintain firm contact. Therefore, developing a radiative cooling film with low thermal contact resistance that can be easily attached to or detached from surfaces, enabling all-day cooling and stable long-term operation, remains challenging.
In this work, we demonstrate a flexible, magnetically attachable radiative cooling film that provides efficient all-day cooling and significantly reduces thermal contact resistance using stable magnetic attraction. The proposed magnetic radiative cooling film exhibits excellent cooling performance at both sub-ambient and above-ambient temperatures. Notably, at temperatures above ambient, it outperforms non-magnetic alternatives. When applied to an automobile engine cover, the magnetic cooling film achieved a daytime temperature drop of 25 °C. Additionally, it demonstrates outstanding mechanical properties for long-term stability and reusability. The robust magnetic radiative cooling film is suitable for practical applications in cooling objects with iron or steel surfaces mentioned above. Furthermore, it is advantageous for scenarios requiring frequent attachment or detachment based on weather conditions and cooling needs in Note S2 in Appendix A. The reusability of this film offers significant cost benefits for large-scale deployment of radiative cooling technology.
2. Methods
2.1. Sample characterization
The microstructure images were obtained by scanning electron microscope (RAITH150 Two, RAITH, Germany). The 3D image of the cooling film was taken by a 3D optical profiler (Bruker Contour GT-K1, Bruker, Germany). The reflectance (ρ) and transmittance (τ) were measured using an ultraviolet–visible–near infrared (UV–VIS–NIR) spectrometer (PerkinElmer Lambda 1050 UV/VIS/NIR Spectrometer, PerkinElmer, USA), a Fourier-transform infrared spectrometer (Nicolet IS50, Nicolet, USA), and a Fourier-transform infrared spectrometer (Bruker Hyperion 2000, Bruker, Germany). The emissivity (E) is obtained from the equation E = 1−ρ−τ.
2.2. Measurement of mechanical properties
The tensile strength of the radiative cooling films was measured using a universal testing instrument (SUNS UTM5015, SUNS, China). The dumbbell-shaped sample was 6 cm long and 2 cm wide, with a gauge length of 2 cm. The crosshead speed was set to 10 mm∙min−1.
2.3. Thermal measurement
The temperatures of the ambient air and the samples were measured using K-type thermocouples (KAIPUSEN TT-K-44, KAIPUSEN, China), connected to a programmable logic controller (PLC; Siemens 6ES7288-1ST30-0AA1, Siemens, Germany) with a resolution of ±0.1 °C. The thermocouples were calibrated before the measurements. Thermal images were captured using an infrared camera (FLIR A315, Teledyne FLIR, USA) with a working wavelength range of 7.5–13 µm.
3. Results and discussion
3.1. Principle of magnetic radiative cooling film
When the radiative cooling film (the cooler) is attached to an object, excess heat is first transmitted to the bottom interface between the object and the cooler. The heat is then conducted to the front surface of the cooler through its internal structures and finally radiated into cold outer space (Fig. 1(a)). Radiative cooling is achieved by emitting thermal radiation from the upper surface of the film. The cooling power depends on the temperature of the cooling film, which is strongly influenced by its thermal conductivity. To achieve significant temperature reduction, the thermal conductivity of the radiative cooling film should be maximized to transfer heat from the object to the cooling film efficiently. The thermal conductivity (K) of the cooling film can be expressed as K = , where d is the material thickness, and R is the thermal resistance. Thus, maximizing thermal conductivity involves either selecting a material with low thermal resistance or minimizing the film thickness. When heat is transferred between interfaces, resistance to heat transfer, known as thermal contact resistance (Rc), forms at the contact interface. This resistance is expressed as
$\mathrm{Rc}=\frac{\Delta T}{q}$
where ΔT is the temperature difference between the contact interfaces, and q is the heat flux through the contact interface. When the heat flow is constant, a larger thermal resistance leads to a larger temperature difference at the contact surfaces, impeding heat transmission. Here, we propose a low thermal resistance magnetic radiative cooling film composed of a polymer metasurface (PM) layer, a metal layer, and a flexible magnetic layer (Fig. 1(b)).
Metasurface and metamaterial structures [41], [42], [43] have been widely applied in thermal management due to their ability to manipulate spectral response with high precision [44], [45], [46], [47]. The PM structures on polyethylene terephthalate (PET) with silver (Ag) coating are promising for daytime radiative cooling due to their high reflection in the solar spectral regime and high thermal emissivity in the 8–13 µm atmospheric transparent window [29]. However, directly applying this to the object's surface without thermal interface materials introduces air gaps at the interface, increasing thermal contact resistance and compromising cooling performance. To address this, we introduce a thermally conductive flexible magnetic cooling film as the substrate to minimize thermal contact resistance through magnetic attraction. This approach enables the cooling film to be used in various applications involving steel or iron surfaces. The magnetic radiative cooling film, with its high thermal conductivity, can firmly attach to the target object, even with curved surfaces, eliminating air gaps and minimizing thermal contact resistance (Figs. 1(c) and (d)). Consequently, the temperature difference between the object and the cooling film can be reduced, achieving optimal cooling effects. Besides, the magnetic radiative cooling film exhibits excellent mechanical strength, ensuring long-term stability and reusability.
3.2. Characterization of the magnetic radiative cooling film
The designed magnetic radiative cooling films are manufactured through a low-cost, large-scale roll-to-roll process (Fig. 2(a)), which includes photo-imprinting, physical vapor deposition (PVD), and bonding. We use photo-imprinting technology to prepare PM structures on a PET film with a mask fabricated using laser nanoprinting technology [45], [48]. Next, we deposited a thin layer of silver (∼200 nm thick) on the back of the imprinted polymer film using the PVD method. Finally, the prepared film was bonded to the flexible magnetic cooling film using a thin layer of thermally conductive adhesive in Note S3 in Appendix A. The manufactured flexible radiative cooling film consists of three layers: a 50-μm-thick PET with a thermal conductivity of 0.24 W∙ (m∙K)−1 featuring a PM structure on top, a 200-nm-thick Ag back-reflector with a thermal conductivity of 427 W∙(m∙K)−1, and a 500-μm-thick flexible magnetic substrate with a thermal conductivity of 15 W∙(m∙K)−1. Photos of a roll of the magnetic radiative cooling film are shown in Figs. 2(b) and (c).
Figs. 2(d) and (e) show the cross-sectional and top-view scanning electron microscope (SEM; RAITH, Germany) images of the magnetic radiative cooling film. PET is a promising inexpensive material for daytime radiative cooling due to its transparency in the visible regime and strong thermal emissivity in the mid-infrared regime. Besides, PET has high mechanical strength and environmental stability, especially under intense UV illumination in Note S4 in Appendix A. To further enhance the emissivity of the PET layer in the atmospheric transparency window, we designed and fabricated a periodic trench-like PM structure composed of periodic square hole arrays (with a period of 8 μm and a hole width of 6.5 μm; Fig. S1 in Note S5 in Appendix A) on the surface of PET (Fig. 2(f)), allowing for the tuning and perfect matching of the structured PET emission spectrum with the atmospheric transparency window. The magnetic radiative cooling film exhibits omnidirectional absorption and emission capabilities in Note S6 and Fig. S2 in Appendix A, demonstrating a high infrared (IR) emissivity of 95% in the main atmospheric window of 8–13 µm and a high reflectivity of 90% in the solar spectrum of 0.3–2.5 µm (Fig. 2(g)). The flexible magnetic layer is a composite material with magnetic components arranged in an elastomer matrix. It is made from a combination of rubber polymer resin and Ba-ferrite powder, with a rubber polymer resin proportion of 35% (its magnetic parameters are presented in Table S1 in Appendix A). In practical applications, mechanical robustness is critical to ensure reusability and prolong the film's lifetime. To this end, we tested the mechanical strength of the cooling films with and without the flexible magnetic layer using an electronic tensile testing machine (Figs. 2(h) and (i)). The maximum tensile strain of the magnetic radiative cooling film is 60%, much larger than that of the non-magnetic radiative cooling film at 43%, indicating the flexible magnetic layer has significantly improved the overall tensile strength of the cooling film (Fig. 2(j)), beneficial for long-term stability, reusability, and cost-effectiveness.
To characterize the heat conduction and thermal contact resistance of the magnetic layer and compare it to non-magnetic ones, we placed three radiative cooling films (with and without the magnetic layer) of the same area on a constant temperature hot plate (Fig. 3(a)). The magnetic radiative cooling film (Sample 1) can be directly attached to the stainless steel surface of the hot plate. One of the non-magnetic radiative cooling film (Sample 2) is attached to the hot plate with thermal grease, and the other is directly placed on the hot plate without thermal grease (Sample 3) as a benchmark. The temperature mapping of all the samples was recorded using a thermal camera. As displayed in Fig. 3(b), the magnetic cooling film has the best thermal conduction from the object to the film, as its temperature approaches the preset temperature of 40 °C of the hot plate. Both the magnetic cooling film and the thermal grease film show much higher temperatures than the sample with an air gap, indicating that removing the air gap is critical to improving thermal conductivity. It is interesting to find that the temperature of Sample 1 is slightly higher than that of Sample 2, even with a larger film thickness. This is due to the much larger thermal conductivity of the magnetic layer in Note S7 and Fig. S3 in Appendix A. Additionally, the heat distribution from Sample 1 is much more uniform than that of Sample 2 in Fig. 3(b) due to the instability and uneven spread of thermal grease, compromising performance.
The excellent heat conduction from the object to Sample 1 is attributed to the high contact pressure provided by the magnetic attraction of the magnetic layer, which reduces the thermal contact resistance by minimizing gaps and increasing the contact area. We calculated the thermal contact resistance as a function of the applied pressure through simulation in Note S8 (Figs. S4 and S5) in Appendix A. The results show that the thermal contact resistance decreases with increased contact pressure (Fig. 3(c)). Generally, the pressure introduced by the magnetic layer increases with the thickness of the flexible magnetic layer, reducing thermal contact resistance. However, a thicker magnet layer introduces larger intrinsic thermal resistance. The overall cooling effect of the film requires a balance between decreased intrinsic thermal resistance (smaller thickness) and increased magnetic adhesion force (larger thickness). To optimize the thickness, we measured the magnetic adhesion force of cooling films with flexible magnetic layers of different thicknesses (300, 400, and 500 μm). The magnetic adhesion force for radiative cooling films with flexible magnetic layers of thicknesses 300, 400, and 500 μm are 0.6, 1.3, and 2.1 kPa, respectively. The results are used in the equation for calculating the thermal contact resistance. As shown in Fig. 3(c), the thermal contact resistance gradually decreases with the pressure increase. The contact resistance of the 500-μm-thick flexible magnetic layer is the lowest, at 6.6 K∙W−1. Meanwhile, its intrinsic thermal resistance is 3.3 × 10−5 K∙W−1, much less than the thermal contact resistance. Additionally, the cooling film will be less flexible if its thickness continues to increase. Therefore, we finally chose the 500-μm-thick flexible magnetic layer for our designed cooling film. The calculation results are experimentally verified by attaching magnetic radiative cooling films with flexible magnetic layers of thicknesses 300, 400, and 500 μm to the hot plate. As expected, the 500-μm-thick flexible magnetic layer cooling film shows the best thermal conduction, evidenced by the temperature closest to that of the preset hot plate (Fig. S5).
The high thermal performance of the magnetic radiative cooling films can also be seen from the histograms and highly uniform contour plots of temperature distribution (Figs. 3(d) and (e) and Note S9 in Appendix A). As shown in Fig. 3(d), due to the magnetic attraction and high thermal conductivity, the average temperature of Sample 1 is 39.59 °C, with a standard deviation (Std) of 0.11. The temperature difference (ΔT) between the hot plate and the surface of Sample 1 is less than 0.5 °C, indicating the heat transfer from the object to the cooling film is 99% efficient. More importantly, the low Std confirms a highly uniform temperature distribution, which is essential for high-performance radiative cooling. The average temperature and Std of Sample 2 are 39.46 °C and 0.15, respectively, indicating a higher temperature difference and less uniform temperature distribution. This shows that the application of thermal grease highly depends on the properties of the grease (including thermal conductivity and viscosity) and the operator's skill in applying the grease, which causes uncertainty in achieving consistent performance (for details in Video S1 in Appendix A). In contrast, using the magnetic radiative cooling film is significantly simpler and provides more consistent results. Finally, the average temperature and Std of Sample 3 are 37.25 °C and 1.69, respectively, showing the worst performance in all cases due to the large air gaps and high thermal contact resistance. Therefore, simply attaching cooling films to objects is ineffective in practical applications.
3.3. Cooling performance of the magnetic radiative cooling film
We evaluated the cooling performance of magnetic radiative cooling films by measuring the temperature difference and cooling power (Fig. 4). Figs. 4(a) and (b) show the schematic diagram and photo of the experimental devices in Note S10 (Figs. S6 and S7) in Appendix A. To accurately quantify the film’s performance, it is essential to minimize heat exchange between the operating environment (inside the chamber) and the ambient environment. A foamed nitrile butadiene rubber (NBR) thermal insulating layer and foam fixture were designed to ensure maximum thermal insulation. The chamber exterior is coated with a reflective aluminum layer to avoid heating from sunlight during the day and to minimize heat loss due to unwanted thermal radiation (Fig. 4(b)). Additionally, a 10-μm-thick polyethylene (PE) film is used as the chamber window layer to reduce conduction and convection heat exchange between the radiative cooling film and the ambient environment. The PE film has high transmission in both solar and infrared spectra, reaching up to 95% and 90%, respectively, ensuring the PE window does not affect the film's performance. Moreover, a weather station was built to accurately measure the ambient temperature without wind interference.
Our experiment was conducted in Weihai City (China). The magnetic cooling film, placed in the measurement chamber in Fig. 4(b) without attachment to any cooling-required object, reached 6.1 °C below ambient temperature under a solar irradiance of 721 W∙m−2 (Fig. S6(a)) at noontime when the ambient temperature was 26.5 °C, and the average humidity was 28.8% (Figs. 4(c) and S5(a)). On a clear night, even with humidity reaching 55.1% (Fig. S6(b)), the magnetic radiative cooling film achieved up to 8.6 °C lower than ambient (Fig. 4(d)). Furthermore, the magnetic cooling film still achieved a maximum temperature drop of 3.5 °C (Fig. S6(c)) on a cloudy day with an average solar irradiance of 89 W∙m−2 and a humidity of 44.6% (Fig. S6(d)). These results demonstrate the magnetic cooling films' ability to reach temperatures lower than the ambient temperature.
To confirm the cooling capacity of the magnetic cooling film when attached to a cooling-required object, we measured its performance on stainless steel under sunlight, as shown in Fig. 4(f). Three identical 0.2-mm-thick stainless steel plates of the same size were placed on thermal insulation fixtures in the chamber to measure cooling performance (photo shown in Fig. 4(e)). One stainless steel surface was covered by a non-magnetic radiative cooling film using thermal conductive grease, another was covered by the magnetic radiative cooling film based on magnetic suction, and the last one was uncovered as a benchmark. High-accuracy thermocouples measured the temperature of the stainless steel's lower surface and the ambient temperature. Most setups in published works [49], [50], [51] measure the temperature directly underneath the cooling film, assessing the film's capability to cool itself. In contrast, our setup measures the temperature drop on the stainless steel, showing the effectiveness of our magnetic radiative cooling film in cooling objects. High cooling performance can only be achieved when heat is effectively extracted from the object due to low thermal resistance.
As shown in Fig. S6(e), the peak solar irradiance was near 600 W∙m−2, and the humidity was 30.7%. Under these conditions, the temperature of the stainless steel plate under the magnetic radiative cooling film was 15.2 °C lower than that of the bare stainless steel. In comparison, the temperature of the stainless steel plate under the non-magnetic radiative cooling film was only 11.6 °C lower than the bare one. Under strong sunlight, the stainless steel's temperature became much higher than the ambient temperature. When covered with the magnetic cooling film, the temperature was effectively reduced, approaching ambient temperature and significantly lower than the non-magnetic one in Fig. 4(g). At night, both the magnetic and non-magnetic radiative cooling films could cool the stainless steel plate to below ambient temperature in Fig. 4(h) at a humidity of 39.4% (Fig. S6(f)), with the magnetic cooling film performing better (0.5 °C lower) due to better thermal contact. Due to the large thermal capacity of stainless steel, achieving sub-ambient temperature cooling performance under strong solar irradiation is challenging. When cooling films are applied to objects, the system's temperature, including the radiative cooling film and objects, varies upon several parameters, such as the temperature, specific heat, and volume of the object to be cooled. We demonstrated the cooling performance by applying our cooling films to a high-temperature stainless steel plate and measuring the plate's temperature. The cooling capacity of the magnetic radiative cooling film is used to cool both the film and the stainless steel plate, which has a much larger thermal capacity. Therefore, a significantly larger amount of heat needs to be removed from the system. As displayed in Fig. 4(g), the temperature of the stainless steel plate under the magnetic radiative cooling film was slightly higher than the ambient temperature (1.7 °C).
We further measured the cooling power of the magnetic radiative cooling film. The principle involves using a heater to actively compensate for the cooling power, as shown in Figs. S7(a) and (b). In this way, instant cooling power can be calculated from the heater's power input in Note S11 in Appendix A. We first measured the cooling power at ambient temperature. The experimental results are shown in Fig. S7. On a sunny day, when the average solar irradiance, relative humidity, and ambient temperature were 582 W∙m−2, 30.7%, and 22.6 °C (Figs. S7(c) and (d)), respectively, the cooling power of the magnetic cooling film reached 89 W∙m−2 (Fig. S7(c)). On a clear night, with an ambient temperature of 16.1 °C and relative humidity of 39.4% (Figs. S7(e) and (f)), the cooling power of the film was 65 W∙m−2 (Fig. S7(e)). Even on a cloudy day, with solar irradiance, ambient temperature, and humidity of 398 W∙m−2, 24 °C, and 36.9%, respectively (Figs. S7(g) and (h)), the average cooling power maintained at 43.7 W∙m−2 (Fig. S7(g)). This experiment demonstrates that the magnetic radiative cooling film exhibits superior all-day cooling capability.
The magnetic radiative cooling films can be applied to any steel or iron surfaces to reduce cooling costs, such as on vehicles or base stations. In these scenarios, the temperatures of the objects are typically higher than the ambient environment. Therefore, it is essential to characterize the cooling performance at different high working temperatures to gain a comprehensive understanding in Note S11. For cooling above ambient temperature, convection and conduction between objects and the environment are significant components of the cooling power that cannot be ignored. To quantify the net contribution from radiative cooling, we measured both the radiative cooling power and the cooling power from convection and conduction in Note S12 in Appendix A.
One example is the cooling power at a working temperature of 30 °C, as shown in Fig. 5(a). The cooling power from convection and conduction is 23 W∙m−2, while the radiative cooling power is 104 W∙m−2, accounting for 82% of the total cooling power of the system, indicating that radiative cooling dominates passive cooling above ambient temperature. Under similar weather conditions (specific parameters are shown in Note S12), the cooling power of the magnetic radiative cooling film at working temperatures of 30, 40, 50, 60, and 70 °C averaged 104, 133, 173, 217, and 259 W∙m−2, respectively (Fig. 5(b)). As expected, the cooling power of the magnetic radiative cooling film gradually increases with the rise in working temperature, consistent with theoretical predictions of radiative cooling power.
Fig. 5(c) compares the experimentally measured and theoretically calculated radiative cooling powers based on atmospheric transmittance according to local climate conditions in Note S13 (Figs. S8 and S9) in Appendix A. There is acceptable agreement between the experimental and theoretical results in Fig. 5(c). The slight difference may be due to the calculation using a fixed atmospheric transmittance, which varies in real-time during the measurement. To investigate this, we calculated the cooling power at different atmospheric transmittance levels (Fig. S8). The measured results fall within the theoretically calculated range between the highest and lowest transmittance (Fig. S8). The calculation also suggests that cooling power can be much higher when atmospheric transmittance is high. Finally, Fig. 5(d) shows that as the working temperature increases, the contributions from convection and conduction also increase, but radiative cooling remains the dominant factor. The measured cooling powers of the magnetic cooling film at various operating temperatures, as shown in Fig. 5, demonstrate its high-power characteristics.
3.4. Practical applications of magnetic radiative cooling film
The magnetic radiative cooling film exhibits excellent flexibility, making it suitable for covering curved surfaces. We attached both magnetic and non-magnetic radiative cooling films of identical size (20 cm × 30 cm) to the same corrugated steel tile, as shown in Fig. 6(a). Notably, there were air gaps wider than 1 cm between the concave surface of the tile and the non-magnetic film (Fig. 6(a), right). In contrast, the magnetic radiative cooling film adhered tightly and conformally to the tile without additional fixing mechanisms (Fig. 6(a), left). When the tiles were placed vertically, the non-magnetic cooling film required additional adhesive tape to stay in place, and the air gap between the film and the tile increased due to reduced contact pressure and wind. Meanwhile, the magnetic radiative cooling film remained firmly attached. The thermal images in Fig. 6(b) demonstrate that the temperature of the magnetic radiative cooling film was approximately 4 °C lower than that of the non-magnetic film under sunlight. This result highlights the advantages of applying magnetic radiative cooling films on non-horizontal and curved surfaces.
For outdoor applications, the durability of the cooling film for long-term use is critical [52], [53]. The average peeling strength of the magnetic substrate is 82 N∙m−1, which fully meets usage requirements and prevents easy detachment in Fig. 6(c)[54]. As shown in Fig. 6(d), the tensile strain curves of the magnetic cooling film showed no significant change after 1000 cycles of repeated attachment and detachment. The measured reflectance spectrum remained almost unchanged after 1000 cycles of attachment and detachment in Fig. 6(e), indicating high reusability. In contrast, the cooling film with thermal grease exhibited significant damage to the silver reflective layer after undergoing the same process (Figs. S9(a) and (b), and Video S2 in Appendix A). In addition, to test long-term stability, we measured the reflection spectra after three days of solar simulator irradiation or seven days of constant temperature heating at 50 °C in Fig. S9(b). Corrosion and abrasion resistance tests were also conducted in Note S14 (Fig. S10) in Appendix A. The reflection spectra of the magnetic cooling film remained almost unchanged after these tests.
To more intuitively demonstrate the durability of our film, we compared the cooling performance before and after 1000 cycles of attachment and detachment through experimental measurements in Note S15 (Fig. S11) in Appendix A. Two identical magnetic radiative cooling films with the same area were prepared: One served as the reference, while the other underwent 1000 cycles of attachment and detachment. The ratio of the cooling power of the two was used as a parameter to characterize durability. The comparison results are shown in Fig. 6(f). Before the 1000 cycles test, the cooling power of the tested film was 98.4% of the reference, and after the test, the cooling power was 97.8% of the reference, with only a negligible difference of 0.6% in Fig. S11. These results confirm the reusability and durability of our magnetic cooling film.
To simulate practical applications, we attached the magnetic cooling film to the hood of a car under the sun and recorded the temperature using a thermal camera in Figs. 7(a) and (b). The magnetic cooling film adhered firmly to the surface (Fig. 7(a)) and could be repeatedly attached and detached (Video S3 in Appendix A). As shown in Fig. 7(b), the temperature of the magnetic film was significantly lower than the area without a film, with a maximum temperature difference of 25 °C. Furthermore, the magnetic film can be firmly attached to vertical surfaces, including sidewalls of base stations, containers, or temporary structures (e.g., kiosks, toll booths, and pump rooms). We attached the magnetic cooling film to the sidewall of a pump room in Fig. 7(c), achieving a temperature drop of over 10 °C compared to the surrounding area in Fig. 7(d), significantly improving comfort inside the structure during hot summer conditions. Therefore, the magnetic radiative cooling film can be readily applied to various metallic surfaces, providing immediate passive cooling and energy savings.
4. Conclusion
We have developed a high-power, flexible, and magnetically attachable radiative cooling film, experimentally confirming its capability for all-day radiative cooling. The proposed PM-structured PET/Ag/flexible magnetic structure monolithic architecture enhances infrared emissivity through the periodic structure on the PET surface and reduces thermal contact resistance with objects. Both experimental and theoretical results demonstrate that the magnetic radiative cooling film significantly reduces thermal contact resistance. The stronger the magnetic attraction, the smaller the thermal contact resistance at a given magnetic layer thickness. Whether the cooling application is sub-ambient or above-ambient, the magnetic radiative cooling film exhibits superior cooling performance compared to the non-magnetic variant, with a more pronounced effect at above-ambient working temperatures. Under identical conditions, the magnetic cooling film can achieve a temperature drop of 15.2 °C on the surface of stainless steel, which is 3.6 °C more than that achieved by the non-magnetic cooling film. As the working temperature increases, the cooling power of the magnetic radiative cooling film also increases continuously. Even at low atmospheric transmittance, the radiative cooling power can reach 259 W∙m−2 at a working temperature of 70 °C.
The magnetic radiative cooling film we propose is specifically designed for applications on steel and iron surfaces, which have extensive outdoor applications in vehicles, facilities, and building infrastructures that store substantial heat in hot environments. To demonstrate the effectiveness of the magnetic cooling films in various application scenarios, we have quantified the cooling performance on different stainless steel surfaces, as well as on vehicles, corrugated steel tiles, and the sidewalls of a shed. Additionally, we verified the mechanical robustness, reusability, and stability of the magnetic cooling film. This high-power magnetic radiative cooling film not only addresses the heat extraction problem between the cooling film and the object but also adapts to various applications, significantly expanding the practical application prospects of radiative cooling.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Australia Research Council through the Discovery Project scheme (DP190103186 and DP220100603), the Industrial Transformation Training Centres scheme (IC180100005), the Future Fellowship scheme (FT210100806), the Future Fellowship scheme (FT220100559), the Discovery Early Career Researcher Award scheme (DE230100383), the Shenzhen Science and Technology Program (GJHZ20240218113407015), the Natural Science Foundation of Shandong Province (ZR2021ME162), and the Key Research and Development Program of Shandong Province, China (2022SFGC0501).
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