Multifunctional Composite Material with Efficient Microwave Absorption\\ and Ultra-High Thermal Conductivity

Yun Wang(王云)$^{1,2}$, Tian-Cheng Han(韩天成)$^{1,2\hyperlink{s}{*}}$, Di-Fei Liang(梁迪飞)$^{1,2}$, and Long-Jiang Deng(邓龙江)$^{1,2}$

doi:10.1088/0256-307X/40/10/104101

⇦Chinese Physics Letters, 2023, Vol. 40, No. 10, Article code 104101⇨ Multifunctional Composite Material with Efficient Microwave Absorption and Ultra-High Thermal Conductivity Yun Wang (王云)1,2, Tian-Cheng Han (韩天成)1,2*, Di-Fei Liang (梁迪飞)1,2, and Long-Jiang Deng (邓龙江)1,2 Affiliations 1National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Chengdu 611731, China 2Key Laboratory of Multi-spectral Absorbing Materials and Structures of Ministry of Education, University of Electronic Science and Technology of China, Chengdu 611731, China Received 25 July 2023; accepted manuscript online 28 August 2023; published online 22 September 2023 ^*Corresponding author. Email: tchan@uestc.edu.cn Citation Text: Wang Y, Han T C, Liang D F et al. 2023 Chin. Phys. Lett. 40 104101 Abstract The increasing demands for electronic devices to achieve high miniaturization, functional integration, and wide bandwidth will exacerbate the heat generation and electromagnetic interference, which hinders the further development of electronic devices. Therefore, both the issues of microwave absorption and heat dissipation of materials need to be addressed simultaneously. Herein, a multifunctional composite material is proposed by periodic arrangement of copper pillars in a matrix, based on the wave-absorbing material. As a result, the equivalent thermal conductivity of the composite structure is nearly 35 times higher than the wave-absorbing matrix, with the area filling proportion of the thermal conductivity material being 3.14%. Meanwhile, the reflectivity of the composite structure merely changes from $-15.05$ dB to $-13.70$ dB. It is proved that the designed composite structure possesses both high thermal conduction and strong microwave absorption. The measured results accord well with the simulation results, which demonstrates that the thermal conductivity of the composite structure can reach more than 10 W$\cdot$m$^{-1}\cdot$K$^{-1}$ without significant deterioration of the absorption performance.

DOI:10.1088/0256-307X/40/10/104101 © 2023 Chinese Physics Society Article Text Flexibly manipulating electromagnetic waves and other information is undisputedly of central importance to our daily lives.[1] However, the heat and electromagnetic interference between modules seriously impede the further development of electronic devices. The heat generated by electronic devices significantly increases with the continuous enhancement of equipment integration and computational speed of system.[2] The electronic interference caused by higher frequencies and closer proximity of modules gets stronger, which further results in higher radiation of system. These issues can lead to a shortened lifespan or even complete malfunction of electronic equipment. However, single heat-conduction or wave-absorption materials could not address the issues of heat conduction and anti-interference simultaneously. Therefrom, developing materials with high thermal conductivity and excellent wave absorption performance is a potential solution to realize anti-interference and heat dissipation. Hence, materials with features of high thermal conductivity, strong attenuation, light weight, wide bandwidth, and adjustable frequency have gained considerable attention among researchers. The blending approach is a common design strategy used to realize absorption and heat conductivity performance, which typically involves adding wave absorption fillers[3] (such as iron carbonyl,[4-6] FeSiAl, ferrite, carbon black) and thermal conduction fillers (like Al$_{2}$O$_{3}$,[4,5] ZnO,[4,6] BN, AlN, and carbon-based materials) into the polymer matrix. The thermal conductivity of heat conductance-electromagnetic wave absorption materials (HC-EMWAMs) prepared by this method is usually below 3 W$\cdot$m$^{-1}\cdot$K$^{-1}$. The balance between wave absorption and thermal conduction performance can be effectively adjusted by modifying the ratio of multiple functional fillers. This method provides obvious benefits in the preparation process and is the most viable approach for achieving industrial-scale production currently. Laird, Fujipoly, and Kitagawa industries successfully introduced some HC-EMWAMs. However, the design contradictions exist in dispersion of the two functional fillers in the matrix. When the content of total filler is kept to be constant, a trade-off situation emerges, wherein enhancing one function comes at the cost of the others. Meanwhile, high filler content will affect the mechanical properties of materials. Then, some researchers prepared bifunctional fillers[7-12] by coating,[7-10] structural design,[11,12] and doping modification, to prepare HC-EMWAMs, which can greatly reduce the filling content. Although the thermal conductivity of the composite materials obtained by both the methods is low, usually not higher than 5 W$\cdot$m$^{-1}\cdot$K$^{-1}$, it is usually several times higher compared to those of the polymer matrices. Specifically, Zhang et al.[11] used carbon fibers to establish microscopic thermal conduction pathways, making the thermal conductivity of the material as high as 15.55 W$\cdot$m$^{-1}\cdot$K$^{-1}$, which is the polymer-based HC-EMWAMs with the highest thermal conductivity reported in the literature. Subsequently, some researchers replaced the polymer substrate by materials with high thermal conductivity[13-22] and then added lossy filler to prepare ceramics with high thermal conductivity and strong microwave absorption. These approaches have the potential to enhance mechanical properties, wave absorption, and superhigh thermal conductivity of materials, most of them are greater than 70 W$\cdot$m$^{-1}\cdot$K$^{-1}$. Among them, decreasing heat dissipation of phonons by enhancing the compatibility of the phase interface to obtain higher thermal conductivity was suggested to be the most important, while unfortunately leading to a narrower bandwidth and a bigger brittleness. Furthermore, Chen proposed the application of direct bonded copper technology[23] to connect the ferrite layer and copper layer, effectively reducing the thermal resistance at the interface and improving thermal conductivity. However, the copper/ferrite sandwich structure may face challenges in reducing their thickness and improving the material compound fastness on uneven surfaces. As for the polymer-based HC-EMWAMs, the low content of the wave absorption and thermal conduction fillers in matrix could not obviously improve the wave absorption and thermal conductivity. Instead, this will lead to a reduction of the equivalent electromagnetic parameters and the attenuation constant of the composite materials, resulting in a decrease in the wave absorption effect. The formation of the thermal conduction path will also be hampered, leading to a lower equivalent thermal conductivity. Moreover, microwave attenuating ceramics with superhigh thermal conductivity materials may deteriorate the wave absorption performance, being a mutual constraint between these two functions. Based on these problems, we hope to combine the high wave absorption capability of wave absorption materials (WAMs) and the high thermal conductivity of thermal conduction materials, with a simple preparation process and thin thickness of the composite material. Therefore, a multifunctional composite material is proposed in Fig. 1, $p$ represents the unit cell period, and $r$ denotes the radius of the filled high thermal conductivity material (HTCM). By modulating the periodic structure, it is possible to design a structure with high absorption and high heat conductivity. The thermal conductivity of the composite structure can reach more than 10 W$\cdot$m$^{-1}\cdot$K$^{-1}$ without significant deterioration of the absorption performance.

Fig. 1. The schematic diagram of the multifunctional composite material and periodic unit cell structure.

Composite Structure. The main objective of the structural design is to address the issues of low thermal conductivity and narrow bandwidth found in the methods mentioned above, and the disadvantages of double-layer materials in thickness and surface adhesion. At the same time, we hope to maximize the inherent advantages of both materials. Therefore, we adopt periodic filling of the wave-absorbing materials with uniform high-heat-conducting materials, enabling the formation of a macroscopic heat conduction path throughout the entire structure in the thickness direction. The heat is transferred to the surface through the filling materials, thereby increasing the equivalent thermal conductivity of the overall structure. However, this structure sacrifices the wave-absorbing performance to a certain extent. Therefore, it is necessary to design the size of the periodic heat conduction materials, so as to ensure the wave absorbing characteristics of the structure. In short, it is equally necessary to comprehensively consider the thermal conductivity and absorption performance before determining the size parameters of composite structures. Design of Thermal Conductivity Characteristics. Since the length and width are much larger than the thickness of the composite structure, the problem can be simplified to a one-dimensional heat transfer problem in the thickness direction only. In the entire heat transfer process, the issue of heat conduction will inevitably arise. According to the Fourier law, the equivalent thermal conductivity of composite structure can be calculated based on the equation \begin{align} \kappa_{\rm eff} =\kappa_{\rm h} g+\kappa_{\rm a} (1-g), \tag {1} \end{align} where $\kappa_{\rm eff}$ is the equivalent thermal conductivity of the composite structure, $\kappa_{\rm a}$ and $\kappa_{\rm h}$ are the thermal conductivities of wave absorption material and the heat conduction material respectively, and $g$ is the area ratio of the heat conduction material. The influences of $\kappa_{\rm a}$ and $g$ on the equivalent thermal conductivity $\kappa_{\rm eff}$ of the composite structure are shown in Fig. 2(b). It is clear that the larger the values of $\kappa_{\rm a}$ and $g$, the better the equivalent thermal conductivity, suggesting that the composite structure is equipped with better dissipation performance. Design of Wave-Absorbing Characteristics. We take the vertical reflectance as the reference standard to evaluate the wave absorbing performance of the composite structure. Adding periodic high thermal conductivity material indeed leads to a decrease in wave absorbing performance. Moreover, when the electromagnetic wave is incident on the interface of the two materials with different electromagnetic parameters, the electromagnetic wave entering the structure will be changed due to the difference of impedance matching. On the other hand, the change in the equivalent dielectric parameter of the composite structure also results in a variation in the attenuation constant, which in turn affects the absorption performance. In general, high thermal conductivity materials have low dielectric constants. Therefore, in order to maintain the absorbing performance of the composite structure, $g$ should be kept below 15% as much as possible. Considering the combined influence of the two material's area ratio on both thermal conductivity and absorption performance, selecting copper with high thermal conductivity can greatly improve the equivalent thermal conductivity of the composite structure with a low ratio. However, cross-shaped metal structures should be avoided. When the electromagnetic wave is vertically incident on the metal surface, some of them will be reflected. At the same time, the periodic cross-like structure will exhibit capacitance and inductance characteristics on the surface, thereby suppressing absorption. Therefore, choosing structures such as circle, squares, and hexagons are preferred as they have little change on absorbing and thermal conductivity properties with the same filling ratio. Here, we selected the circle as research object for simulation and experiment. Electromagnetic Simulation. Taking the single periodic element structure as the simulation object, the impact of the structural parameters on the absorbing performance is simulated. The dielectric and permeability parameters of the absorption material are depicted in Fig. 2(a). Figure 2(c) displays the simulation results for varying radii $r$, while keeping the periodic size $p$ fixed at 2 mm.

Fig. 2. Electromagnetic parameters of the absorbing material and the influence of the ratio of thermal conductive materials: (a) electromagnetic parameters of the absorbing material, (b) effect of the ratio of thermal conductive material and its thermal conductivity on the equivalent thermal conductivity $\kappa_{\rm eff}$, and (c) effect of varying $r$ on reflectivity when $p=2$ mm.

With the increase of $r$, the absorption peak weakens and shifts towards lower frequencies. This behavior can be attributed to the decrease in the equivalent electromagnetic parameters, which in turn affects the attenuation constant. The attenuation constant determines the rate of attenuation of electromagnetic waves in the materials. The absorbing material, serving as the base medium of the composite structure, will be designated as the reference group for comparison with the composite structure. The absorbing performance of the composite structure increases from $-15.05$ dB to $-13.70$ dB when $r=0.2$ mm. This is due to the equivalent electromagnetic parameter[24] of the metal periodic array, which was reported in Ref. [25] as follows, when $p/r\ge 10$:[26] \begin{align} \varepsilon = \varepsilon_{0} \left({{ \begin{array}{*{20}c} {1} & {0} & {0} \\ {0} & {1} & {0} \\ {0} & {0} & 1-[k_{\rm p}^{2}/k^{2}] \\ \end{array}}}\right). \tag {2} \end{align} If the base medium for the standard line coal is replaced with another homogeneous medium, Eq. (2) will remain valid by replacing $\varepsilon_{0}$ with the corresponding dielectric.[27] The relative dielectric constant of the metal array in the in-plane direction within the plane is approximately 1, indicating that it has minimal impact on the electromagnetic properties when $p/r\ge 10$. Therefore, the composite structure can efficiently absorb electromagnetic waves without significant energy dissipation. When using a high thermal conductivity material such as metal as a periodic array, the ratio of $p$ to $r$ must be greater than or equal to 10. Based on the above discussion for the equivalent thermal conductivity and absorbing effect of the composite structure, it is appropriate to set the parameters $p$ and $r$ as 2 mm and 0.2 mm, respectively, to meet our goal. In this case, the equivalent thermal conductivity of the composite structure is 12.946 W$\cdot$m$^{-1}\cdot$K$^{-1}$, which is dozens of times higher than reference absorbing material $\kappa_{\rm a}=0.36$ W$\cdot$m$^{-1}\cdot$K$^{-1}$. At the same time, the absorption peak of the composite structure shifts from $-15.05$ dB to $-13.70$ dB, the intensity of the absorption peak is only reduced by 1.35 dB. Experimental Verification. Increasing $p$ and $r$ in proportion has almost negligible effect on thermal conductivity and absorption performance. Therefore, taking the processing difficulty and precision into account, the parameters $p$ and $r$ are chosen as 5 mm and 0.5 mm, respectively, for the fabrication of a sample with dimensions measuring $25\,{\rm mm} \times 25\,{\rm mm} \times 1.1$ mm intended for temperature measurement. Similarly, we confirmed the values of $p$ as 10 mm and $r$ as 1 mm, and a sample with the size of $200\,{\rm mm} \times 200\,{\rm mm} \times 1.1$ mm was fabricated for the reflectivity test. Figure 3 shows the reflectivity experimental results. The experimental setup and test specimens as shown in Fig. 3(a). Figures 3(b) and 3(c) illustrate the simulation results and measured results of the reflectivity property of the composite structure, which are then compared with the reference absorbing material. The measured results exhibit a similar trend to the simulation results with slight deviations, which can be attributed to environmental uncertainties during the measurement of electromagnetic parameters.

Fig. 3. Result of reflectivity when $r=1$ mm and $r=10$ mm: (a) reflectivity test device and experimental sample, (b) simulation results of reference and composite structure, and (c) experimental results of reference and composite structure.

Fig. 4. Temperature test platform and test samples.

To evaluate the heat dissipation capability of the composite structure, the temperature of the heating plate is set as a constant of 50 ℃, which serves as a thermal source providing continuous heat input from the bottom of the specimen. Insulated cotton is used to surround the sample to simulate an adiabatic environment. An infrared camera is used to capture the highest temperature on the material surface. The testing instrument and samples are depicted in Fig. 4. The absorbing material of the same size is used as the reference group similarly. A layer of PDMS film is applied to both specimens. This helps eliminate the influence of low metal reflectivity on the infrared image. The temperature rise curve throughout the entire process is depicted in Fig. 5, in which surface temperature first sharply elevates and then rises slowly over time. The temperature difference between the two samples is greater than 3 ℃ in the first 5 s, and reaches the maximum temperature difference of 6.1 ℃ at the second seconds. The composite structure reaches thermal equilibrium after heating for 23 s, while the maximum temperature of the reference specimen is 49.0 ℃, taking approximately 45 s to reach thermal equilibrium. This finding verifies the higher thermal conductivity of the composite structure, enabling faster heat transfer to the surface. In addition, the steady-state temperature of the composite structure specimen ranges from between 50.5 ℃ and 50.7 ℃, while the steady-state temperature of the reference absorbing material specimen ranges in 49.8–50.0 ℃, indicating that the absorbing material exhibits a high thermal resistance, consequently implying a lower thermal conductivity. Finally, based on the measurement results of reflectance and thermal conductivity on the samples, it can be concluded that the proposed periodic composite structure effectively enhances the thermal conductivity while maintaining the effective absorption performance.

Fig. 5. Infrared temperature test result: the variation of surface temperature with heating time for absorption material and composite structure.

In addition, this structure offers high design flexibility. Apart from the flexible adjustment of both structural parameters and filling shapes, it is also possible to select suitable absorption materials with desired absorption bandwidth and absorption frequency to meet specific application requirements. The thermal conductive material used for filling is not limited to copper. In some scenarios where electrical conductivity is limited, insulated thermal conductive composites, such as BN and Al$_{2}$O$_{3}$, can be chosen. In summary, we report a multifunctional composite material that incorporates high thermal conductivity material periodically into the microwave absorption material. The study focuses on investigating the influence of structural parameters on the microwave absorption effect and thermal conductivity effect of the composite structure. Indeed, increasing the proportion of high thermal conductivity materials results in a higher equivalent thermal conductivity for the composite structure. When considering the influence on microwave absorption performance, it is important to ensure that $p/r\ge 10$, otherwise absorption capacity will deteriorate significantly. In this study, with a copper ratio of 3.14%, the equivalent thermal conductivity increases up to approximately 35 times, while the absorption performance only increases to 1.35 dB. In the thermal test, the composite structure reaches a steady state faster than the reference absorbing material, and it also achieves a higher steady-state temperature. This confirm that a higher thermal conductivity of the composite structure is comparable to the absorption material. The reflectivity test results are consistent with the simulation and theory. Both the simulation and experimental results confirm that the composite structure possesses high thermal conductivity and high wave absorption characteristics simultaneously, providing a flexible strategy for heat conduction-electromagnetic wave absorption materials application. References Intelligent metasurfaces: control, communication and computingBubble-templated rGO-graphene nanoplatelet foams encapsulated in silicon rubber for electromagnetic interference shielding and high thermal conductivityA review and analysis of microwave absorption in polymer composites filled with carbonaceous particlesEnhanced Thermally Conductive and Microwave Absorbing Properties of Polymethyl Methacrylate/Ni@GNP NanocompositesSynchronously enhanced electromagnetic wave absorption and heat conductance capabilities of flower-like porous γ-Al2O3@Ni@C compositesSynergistic enhancement of thermal conduction and microwave absorption of silica films based on graphene/chiral PPy/Al2O3 ternary aerogelsSynergistically enhanced heat conductivity-microwave absorption capabilities of g-C3N4@Fe@C hollow micro-polyhedra via interface and composition modulationA Through‐Thickness Arrayed Carbon Fibers Elastomer with Horizontal Segregated Magnetic Network for Highly Efficient Thermal Management and Electromagnetic Wave AbsorptionStudy on Aluminum Nitride Microwave Attenuation Ceramics with High Thermal ConductivityLossy AlN–SiC composites fabricated by spark plasma sinteringSpherical glassy carbon/AlN microwave attenuating composite ceramics with high thermal conductivity and strong attenuationHigh-thermally conductive AlN-based microwave attenuating composite ceramics with spherical graphite as attenuating agentDense AlN/FeSiAl composite ceramics with high thermal conductivity and strong microwave absorptionStructural, thermal and dielectric properties of AlN–SiC composites fabricated by plasma activated sinteringMethod for fabricating microwave absorption ceramics with high thermal conductivityThermal Conductivity and High-Frequency Dielectric Properties of Pressureless Sintered SiC-AlN Multiphase CeramicsEffect of Laser Ablation on Microwave Attenuation Properties of Diamond FilmsBonding microwave absorbing ferrites to thermal conducting copperHyperbolic metamaterials: fusing artificial structures to natural 2D materialsStrong spatial dispersion in wire media in the very large wavelength limitDispersion and Reflection Properties of Artificial Media Formed By Regular Lattices of Ideally Conducting Wires

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