Aluminate/cobalt composites developing for elevated-temperature and high-efficiency electromagnetic absorbing (EMA) materials

Summary

It is increasingly urgent to develop EMA materials that combine elevated-temperature and high-efficiency absorbing performances. Herein, aluminate/Co composites were prepared through mechanical mixing and hydration hardening. The microstructures of the specimens were controlled by adjusting the shape of Co particles, and cordierite particles were added to further regulate the distribution of Co particles. Flake-shaped Co particles exhibit superior electromagnetic parameters, while the excessively high conductivity of Co flakes filled specimens leads to a significant degradation in their impedance matching, resulting in a rapid attenuation of the electromagnetic performances. The temperature-dependent ionic conductivity plays an important role in controlling the elevated-temperature conductivity and can be suppressed by adding a second dielectric component. For a 2 mm specimen, the maximum reflection loss (RL_max) reaches −13.1 dB, and the effective absorption bandwidth (RL < −5 dB, ERL₅) covers the entire X band at 500°C, presenting great potential for developing into elevated-temperature and high-efficiency EMA materials.

Graphical abstract

Highlights

• •Cordierite particles were added to regulate the distribution of Co particles
• •Ionic conductivity plays an important role in controlling electromagnetic properties
• •Flaky Co particles exhibit superior electromagnetic parameters
• •Mm ACFC-10 specimen presents a wide ERL₅ covering t X-band at 500°C

Physics; Engineering; Materials science

Introduction

The rapid development of electronic information technology has led to a significant increase in stray electromagnetic waves in space, consequently resulting in increasingly severe electromagnetic interference problems for large electronic devices. Concrete structures, being the primary building material for such equipment, are endowed with electromagnetic absorption characteristics, which are beneficial to the normal operation of electronic devices. When confronted with elevated-temperature working environments, this poses enormous challenges to the temperature resistance performance of concrete-based absorbing materials.^1,2,3

To prepare elevated-temperature absorbing coatings, it is necessary to balance the physical and chemical properties of the coating substrate and absorber. Aluminum salt material possesses excellent temperature resistance and adjustable dielectric properties, making it a potential substrate material for elevated-temperature absorbing coatings. Ceramic materials, such as SiC, Ti₃SiC₂, Ti₃AlC₂,^4,5,6,7 are generally selected as high-temperature absorbers due to their temperature resistance and chemical stability. However, as for the elevated temperature below 500°C, their non-magnetic properties limit their ability to attenuate electromagnetic waves solely through dielectric loss characteristics. Mu et al. investigated the absorption properties of SiC_f/SiC–Al₂O₃ composites in the temperature range of 25°C–700°C at 8.2–12.4 GHz, and the composites show the ERL₁₀ of 3.4 GHz at 300°C, and 4.1 GHz at 400°C, respectively.⁸ Han et al. fabricated SiC-nanowire-reinforced SiC_f/SiC composites through an in situ growth of SiC nanowires on SiC fibers, and the specimens present an effective bandwidth of only 2.8 GHz at 600°C.⁹ Thus, the advantage of ceramic materials as the elevated-temperature absorbers is not prominent, especially at the temperature range below 500°C.

Co particles combine dielectric loss and ferromagnetic loss and still maintain high electromagnetic performance at high temperatures due to their high Curie temperature of 1131°C, thus presenting the potential to be used as a high-temperature absorber. Li et al.'s research has confirmed that Co-based absorbers exhibit excellent high-temperature stealth potential, but their maximum service temperature is limited to 673 K due to the constraints of the polyimide matrix.¹⁰ Zhou et al. prepared high-temperature stealth coatings using high-temperature ceramic materials as the substrate through spraying technology, which significantly improved their temperature resistance. However, the content and distribution of absorbers in the sprayed coating are uncontrollable, leading to poor high-temperature impedance matching characteristics and limiting the improvement of electromagnetic performance.¹¹

Numerous studies have shown that the microstructure of absorbing materials directly affects their electromagnetic properties, and that morphological characteristics have the most significant impact.^{12,13,14,15,16,17,18,19} Cao et al. found that flaky particles exhibited stronger dielectric loss characteristics than spherical particles, which is attributed to the enhanced interface polarization characteristics resulting from the larger aspect ratio of flaky particles. Furthermore, the flaky treatment of FeSi particles also mitigates the adverse effects caused by the skin effect, leading to an increase in the complex permeability of these particles.²⁰ Unlike the evolution law of electromagnetic properties at room temperature, the high-temperature electromagnetic loss mechanism is more complex. Some researchers have found that the temperature-dependent conductivity has a significant impact on the variation of high-temperature electromagnetic parameters. They have found that the main dielectric loss mechanism of the absorbing coatings transitions from polarization loss to conductive loss as the temperature increases, and the electron transport capacity is an important factor influencing the high-temperature dielectric properties.^21,22 Therefore, to explore the temperature-dependent conductivity of materials and keep it within a reasonable range is of great significance for optimizing electromagnetic properties.

Conventional high-temperature materials are typically fabricated through spraying and sintering processes. However, these preparation methods involve intense thermodynamic reactions, resulting in poor controllability over the internal microstructure of the materials. Consequently, there is an urgent need to develop a mild material preparation strategy to precisely regulate the high-temperature electromagnetic properties of composites via microstructure engineering. Aluminate-based materials exhibit outstanding thermal stability and tunable dielectric properties. Notably, they can be synthesized at room temperature while maintaining structural integrity and functionality under high-temperature conditions. These characteristics render them highly promising as matrix materials for high-temperature electromagnetic wave-absorbing coatings.

In this work, aluminate was selected as the matrix material, and spherical Co or flaky Co particles were used as additives to prepare aluminate/Co composite absorbing materials through mechanical mixing, coupled with a hydration hardening process. The microstructure characteristics of the composite coating were adjusted, and their influence on the elevated-temperature electromagnetic properties was clarified. Additionally, the temperature-dependent conductivity of the composite material was controlled by incorporating a second dielectric component in order to explore the influence of ionic conductivity on the elevated-temperature electromagnetic loss mechanism. The elevated-temperature absorption performances of these aluminate/Co composite materials were evaluated, resulting in an optimization strategy for high-temperature absorbing materials.

Results and discussions

Morphology and compositions

The representative morphology of raw particles was observed by SEM, as shown in Figures 1A–1D. The size of the raw Co microspheres ranges from 5 to 35 μm, presenting an average diameter of 20 μm. Co flakes exhibit a large aspect ratio with an average diameter of 40 μm and an average thickness of 800 nm. Both calcium aluminate and cordierite particles present irregular shapes, while the size of cordierite is larger than that of calcium aluminate.

Figure 1.

Characteristic morphology

The characteristic morphology of raw particles, (A) Co microspheres, (B) Co flakes, (C) calcium aluminate powders, and (D) cordierite powders. The representative morphologies of aluminate/Co composite materials, (E) AMC-10, (F) AFC-10, (G) ACFC-10. EDS images of ACFC-10, (g1) Co, (g2) Al, (g3) O, (g4) Ca, (g5) Si, and (g6) Mg. Scale bars: 50 μm (A, E–G), 20 μm (B–D).

High-temperature aluminate, as a typical substrate material, can be prepared at low temperatures and used at high temperatures. The hydration hardening process of aluminate at room temperature is as follows,^23,24

3(CaO·Al₂O₃)+21H₂O→CaO·Al₂O₃·10H₂O+2CaO·Al₂O₃·8H₂O+2Al(OH)₃ (Equation 1)

The metastable products, CaO·Al₂O₃·10H₂O and 2CaO·Al₂O₃·8H₂O, will be decomposed into 3CaO·Al₂O₃·6H₂O at high temperature as shown in the equations,^25,26

CaO·Al₂O₃·10H₂O+2CaO·Al₂O₃·8H₂O→3CaO·Al₂O₃·6H₂O+2Al(OH)₃+9H₂O (Equation 2)

Figures 1E–1G show the morphology of aluminate/Co composite materials. No obvious cracks can be observed in calcium aluminate, and Co particles can be distinguished clearly. The distribution of Co flakes is much more random due to their higher width-thickness ratio, which results in closer distances among Co flakes, even overlapping with each other. The introduction of cordierite powders also has an impact on the microstructure of AFC-10. It is believed that the differences in microstructure of composite materials will affect their conductivity, and then affect their electromagnetic properties. Moreover, the EDS patterns of ACFC-10 indicated that all components are uniformly distributed in the matrix, as shown in Figures 1G1–G6.

Figure 2Ashows the XRD pattern of calcium aluminate matrix, and the characteristic diffraction peaks corresponding to A1₂O₃, CaO, Al(OH)_3, and SiO₂ can be distinguished. Specifically, the diffraction peaks at 22.6°, 35.1°, 37.8°, 43.4°, 52.5°, 66.5°, and 68.2° are attributed to A1₂O₃ phase (PDF#10-0173), while those at 24.2°, 26.7°, 29.7°, 35.6°, 39.5°, 43.2°, 47.6°, and 48.4° correspond to CaO phase (PDF#28-0775). Furthermore, the diffraction peaks located at 18.8°, 20.4°, 27.8°, 40.6°, and 53.1° are associated with Al(OH)₃ (PDF#20-0011). A minor SiO₂ phase (PDF#35-0063) is also present in the matrix. This XRD phase analysis aligns with the hydration reaction equation of aluminates, providing evidence that the hydration hardening reaction of aluminates has been successfully accomplished. Upon the introduction of Co particles into the aluminate matrix, the characteristic diffraction peak at 2θ = 44.4° immediately emerges, corresponding to the (111) plane of cubic Co (PDF#15-0806). However, due to the low filling rate of Co particles in the composite samples, the intensity of the Co diffraction peak in the XRD spectra is relatively weak. It is noted that no diffraction peak of Co oxide was observed in the XRD pattern, which is attributed to the protection of Co microspheres by the aluminate matrix.

Figure 2.

Composition and microstructure

XRD patterns of composite specimen, (A) calcium aluminate matrix, (B) aluminate/Co composite materials.

Static magnetic properties and conductivity

The static magnetic properties of Co particles will affect the complex permeability of the composite specimens and demonstrate a pronounced temperature dependency. Figures 3A and 3B shows the hysteresis loops of Co microspheres and microflakes at different temperatures, and the static magnetic parameters are listed in Table 1. As the temperature increases, both M_s and H_c slowly decrease, but M_s still remains above 140 emu/g at 500°C, which is attributed to the higher Curie temperature of Co particles. Flaky Co particles possess a slightly decreased M_s while the higher H_c, which is attributed to their disrupted lattice integrity during ball milling. The defects inside the flake Co particles were reduced gradually as the temperature increased, and the coercivity of Co flakes decreased to 66.8 Oe after annealing at 500°C.^27,28,29

Figure 3.

Static magnetic properties and electrical conductivity

Magnetic loops of Co particles at different temperatures, (A) microsphere Co particles, (B) Flaky Co particles. Inset shows the detail of the magnetic hysteresis loop under a low applied field. (C) The temperature-dependent conductivity of composite specimens within the temperature range of 0°C–600°C.

Table 1.

The static magnetic properties of Co microspheres and microflakes at different temperatures

Properties	Co particles	25°C	100°C	200°C	300°C	400°C	500°C
Ms (emu/g)	Spherical Co	157.9	157.6	156.3	154.0	150.4	146.9
	Flaky Co	155.2	154.9	153.7	152.0	149.0	146.1
Hc (Oe)	Spherical Co	108.5	87.3	65.6	53.1	51.2	45.6
	Flaky Co	197.2	147.1	89.2	81.6	75.6	66.8

The temperature-dependent conductivity of composite specimens plays a crucial role in adjusting the conduction loss. Figure 3C shows the conductivity of aluminate matrix and aluminate/Co composite materials. Aluminate matrix materials present the low conductivity of 1.71 × 10⁻⁶ S/m at room temperature, while transferring from the insulator to semiconductor at 450°C and the conductivity increases to 1.16 × 10⁻⁵ S/m at 600°C. The variation of ion conductivity with temperature can be represented by the Arrhenius formula,^30,31,32

$$\sigma =Aexp\left(-\frac{E_a}{RT}\right)$$ (Equation 3)

where A is the Arrhenius constant, E_a is the ion activation energy, R is the ideal gas constant, and T is the absolute temperature. As for aluminate matrix, the ion mobility is low at room temperature, while the ion thermal disturbance and migration rate increase as the temperature rises, resulting in an increase in its conductivity.

Compared with various types of ions (H+, Al³⁺, Si⁴⁺, Ca²⁺, and O²⁻) in the matrix, the H⁺ ion, with the smallest radius, can migrate with the aid of crystal defects and ion gaps. The activation energy required for its migration under an electric field is lower, at only about 0.5 eV, enabling fast ion conduction. In contrast, the radii of other ions are relatively larger, and they can only migrate through internal defects in the crystal. The activation energy for their migration is generally between 1 and 2 eV. Therefore, H⁺ ion conduction predominates in the aluminate matrix.

When 10 vol. % Co particles were introduced into the aluminate matrix, the conductivity increases slightly, but both AMC-10 and AFC-10 are still insulators at room temperature and present a similar temperature dependence to the aluminate matrix. According to the microstructure of the AMC sample, Co microparticles are uniformly dispersed in the aluminate matrix; there is no contact between particles, and the electronic conductive path has not been formed, so the ionic conductivity is still the dominant factor affecting the conductivity of the composite sample. The impact of the introduction of Co particles on the conductivity of the sample may be related to changes in the conduction pathway; that is, the introduction of Co particles shortens the ion conduction pathway, shortens the ion migration distance during the conduction process, and improves the overall conductivity.

In a comparative analysis with AMC-10 specimens, flake-shaped Co particles exhibit a heightened aspect ratio and an intricate distribution in the aluminate matrix. Consequently, at an equivalent filling fraction, the interparticle distances among these flake-like Co particles are notably diminished, leading to a substantial reduction in the ion migration path length during the conduction process. This alteration profoundly impacts the conductivity of composite specimens, rendering it more sensitive to temperature variations. Specifically, the threshold temperature at which the AFC-10 specimen undergoes a transition from an insulating to a semiconducting state is correspondingly lowered (300°C).

The rapid ion H⁺ in the aluminate matrix has a significant impact on ionic conductivity at high temperature, while excessive conductivity may cause strong conduction loss, leading to impedance mismatch, and then attenuating the electromagnetic performances. Cordierite (Mg₂Al₄Si₅O₁₈), devoid of fast ion H⁺, exhibits relatively stable electromagnetic properties at high temperatures,^33,34,35 and the addition of cordierite may inhibit the surge in high-temperature conductivity by changing ion migration paths.³⁶ The incorporation of cordierite significantly reduces the conductivity of ACFC-10 specimens. Specifically, the ACFC-10 exhibits a conductivity of 4.9 × 10-6 S/m at 500°C, which is substantially lower than that of the AFC-10 specimen. The temperature at which the conductivity undergoes a rapid enhancement has been elevated from 300°C to 450°C, indicating the operating temperature range shifts toward higher temperatures. This notable reduction in conductivity is crucial for enhancing the high-temperature impedance matching and electromagnetic absorption characteristics.

Dielectric loss mechanism

Figure 4 shows the complex permittivity of the aluminate matrix in the temperature range of 25°C–500°C. At room temperature, the matrix exhibited low dielectric properties, characterized by a real permittivity (ε′) fluctuating around 6 and an imaginary permittivity (ε″) near 0.2. Upon heating to 400°C, ε′ slightly increased to approximately 7.5, while ε″ remained stable at around 0.2. Notably, at 500°C, a significant enhancement in complex permittivity was observed with increasing frequency, with ε′ exceeding 15 and ε″ surpassing 1 at 11 GHz.

Figure 4.

Complex permittivity of aluminate matrix

The complex permittivity of aluminate matrix measured at the temperature range of 25°C–500°C in X band, (A) the real part of permittivity, (B) the imaginary part of permittivity.

As for aluminate matrix, the dielectric loss mechanisms at lower temperatures are mainly caused by the dipole polarization, and the polarization relaxation enhances with increasing temperature according to Debye theory,^37,38,39

$${\epsilon }^{\prime }={\epsilon }_{\infty }+\frac{{\epsilon }_s-{\epsilon }_{\infty }}{1+{\omega }^2\tau {\left(T\right)}^2}$$ (Equation 4)

$${\epsilon }^{″}=\frac{\left({\epsilon }_s-{\epsilon }_{\infty }\right)}{1+{\omega }^2\tau {\left(T\right)}^2}·\omega \tau \left(T\right)+\frac{\sigma \left(T\right)}{{\epsilon }_0\omega }$$ (Equation 5)

where ω is the angular frequency, and τ(T) is relaxation time. ε_∞is the relative dielectric constant at high frequency limit, and ε_s is static permittivity. σ(T) is the temperature-dependent electrical conductivity, and ε₀ is the dielectric constant in vacuum.

The relaxation time exhibits a notable dependence on temperature and can be formulated using the Arrhenius equation,⁴⁰

$$\tau \left(T\right)={\tau }_0\exp \left(\frac{E_a}{RT}\right)$$ (Equation 6)

where τ₀ is a prefactor and E_a is the activation energy. As the temperature rises, the shortened relaxation time will contribute to enhanced permittivity, and the sharply enhanced conductivity at the elevated temperature also contributes to the increased permittivity. Notably, the permittivity of the aluminate matrix exhibits frequency-dependent enhancement at 500°C, which is different from the frequency dispersion characteristics at room temperature and contrary to that described by the Debye model.⁴¹ At elevated temperatures, the ions, particularly the weakly bound H⁺, are more prone to deviating from their equilibrium positions and migrating orderly under an external electric field, resulting in relaxation polarization. The motion equation of a single-bonded charge driven by an external electric field can be expressed as follows,⁴²

$$m\frac{{\partial }^{2}x}{\partial t^2}=q{E}_0{e}^{jwt}-\alpha x-2\eta \frac{\partial x}{\partial t}$$ (Equation 7)

where m, q, and x represent the mass, charge, and displacement of the bound charge, respectively, α is the coefficient of restoring force, while η is the damping coefficient.

Taking Lorentz correction into consideration, the real permittivity of the aluminate matrix containing N units of polarized bound charges can be obtained as follows,

$$\epsilon ={\epsilon }_s+\frac{N{q}^2}{{\epsilon }_{0}m}·\frac{{{\omega }_0}^2-{\omega }^2}{{\left({{\omega }_0}^2-{\omega }^2\right)}^2+{4\eta }^2{\omega }^2}$$ (Equation 8)

where ω₀ is the resonant frequency, and its value is considerably higher than the measured frequency.⁴³ Moreover, N and η present the temperature dependence as follows,

$$N\propto \exp \left(-\frac{E_a}{RT}\right)$$ (Equation 9)

$$\eta \propto \exp \left(-\frac{E_b}{RT}\right)$$ (Equation 10)

where E_a and E_b are the activation energies of electrons and the lattice, respectively, and their values decrease as the temperature increases, whereas according to Equations 2, 3, and 4, the permittivity increases with increasing frequency, which is consistent with the experimental results. Furthermore, the dipole polarization of the matrix material is dominant at lower temperatures, while the relaxation polarization of hot ions emerges and dominates dielectric losses at high temperatures. It is noted that both the thermionic relaxation polarization and ion conduction of the matrix material originate from the migration of ions and become apparent at higher temperatures; thus, the ion conductivity can be used as a macroscopic variable to assess the strength of the thermionic relaxation polarization.

Figures 5A and 5B show the complex permittivity of AMC-10 specimens within the frequency range of 8.2–12.4 GHz. After Co spherical particles are introduced into the aluminate matrix, the permittivity exhibits a similar temperature dependence to that of the aluminate matrix, but both ε′ and ε″ increase significantly. At room temperature, ε′ of AMC-10 increases from 6 to 7.5 after 10 vol. % Co microspheres are introduced. When the temperature rises to 500°C, ε′ first increases from 14.9 to 26.2 and then decreases sharply to 13.9, while ε″ continuously increases within the 8.2–12.4 GHz band. The incorporation of Co microspheres leads to the formation of a Co/aluminate heterojunction interface in the AMC-10 specimen, significantly enhancing the interface polarization of the composite specimen. Furthermore, the ionic conductivity of the AMC-10 coating undergoes a slight enhancement, and this enhanced conductivity of the specimens contributes to an increase in permittivity, particularly at elevated temperatures.

Figure 5.

Complex permittivity of aluminate/Co composites

The complex permittivity of composite specimens at different temperatures, (A and B) AMC-10, (C and D) AFC-10, (E and F) ACFC-10.

The complex permittivity of AFC-10 specimens was shown in Figures 5C and 5D. At room temperature, ε′ can reach 12, and ε″ is above 1, which is much higher than that of the AMC-10 specimen. As the temperature increases, both ε′ and ε″ increase significantly, exhibiting a similar temperature dependence to that of the aluminate matrix. When the temperature rises to 300°C, a distinct dielectric relaxation peak emerges at 11.7 GHz, indicating strong dielectric loss. Flaky Co particles with a larger aspect ratio possess a larger specific surface area, and the abundant ferromagnetic/dielectric heterogeneous interfaces contribute to enhanced interface polarization. Furthermore, the higher conductivity of the AFC-10 specimen also contributes to the enhanced conduction loss at elevated temperatures. However, the higher conductivity of the AFC-10 specimen may cause impedance mismatching, limiting the usability of the AFC-10 specimens at higher temperatures.

The conductivity test results show that the introduction of cordierite can significantly reduce the high-temperature conductivity of the composite specimen, which can greatly improve the impedance matching characteristics at elevated temperatures and increase the service temperature of the aluminate based composite materials. The complex permittivity of ACFC-10 decreases significantly after the introduction of cordierite, as shown in Figures 5E and 5F. When the temperature increases to 400°C, ε′ slowly increases from 11.7 to 14.9, and ε″ increases from 1.7 to 6.2. When the temperature further increases to 500°C, ε′ decreases from 19.1 to 1.8 and ε″ fluctuates around 15, and a significant relaxation peak appears near 10.5 GHz, which is attributed to the enhanced ionic conductivity. It is noted that the frequency-dependence permittivity at 500°C is much different, which is attributed to the enhanced ionic conductivity. Elevated temperature significantly improves ionic conductivity, which generates pronounced eddy currents on the surface that prevent electromagnetic wave penetration through the composite specimens.

Figure 6 shows the ε′-ε″ curves of ACFC-10 specimens, in which the relaxation loss can be evaluated using the Cole−Cole semicircle according to Debye theory,⁴⁴

$${\left({\epsilon }^{\prime }-\frac{{\epsilon }_s+{\epsilon }_{\infty }}{2}\right)}^2+{\left({\epsilon }^{″}\right)}^2={\left(\frac{{\epsilon }_s-{\epsilon }_{\infty }}{2}\right)}^2$$ (Equation 11)

where ε_s is the static dielectric constant, and ε_∞ is the dielectric constant at infinite frequency. At room temperature, the ACFC-10 specimen exhibits two semicircles in the ε′-ε″ plots, which may be originated from dipole polarization and interface polarization within the measured frequency range. The ε′-ε″ plot changes as the temperature increases, but a similar profile can be maintained, and two Cole-Cole semicircles can still be distinguished. However, the profile of the ε′-ε″ plots changes significantly when the temperature rises to 400°C, and the ε′-ε″ plots consist of lines, which indicates that conduction losses gradually dominate the dielectric losses. It is noted that the ε′-ε″ plots changes sharply, which is attributed the abnormal electromagnetic losses caused by impedance mismatch.

Figure 6.

Dielectric loss mechanism

Plots of ε′ versus ε″ for ACFC-10 specimens at different temperatures, (A) 25°C, (B) 100°C, (C) 200°C, (D) 300°C, (E) 400°C, (F) 500°C.

Ferromagnetic loss mechanism

Figure 7 shows the complex permeability of AMC-10, AFC-10, and ACFC-10 specimens at different temperatures. The AMC-10 specimen exhibits poor ferromagnetic loss, with a μ″ value of only 0.04 at room temperature, while that of the AFC-10 specimen significantly increases to 0.10. The shape anisotropy of Co particles directly influences their permeability as follows,^45,46

$${\mu }^{\prime }=1+\frac{{\left(2.8\right)}^{2}4\pi M_s\left[H_k+4\pi M_s\left(D_{\perp }{-D}_e\right)\right]}{f_0^2}$$ (Equation 12)

$${\int }_0^{\infty }{\mu }^{″}\left(f\right)fdf=k_A\frac{\pi }{2}{\left(\overline{\gamma }4\pi M_s\right)}^2$$ (Equation 13)

where H_k is the magnetocrystalline anisotropic field, f₀ is the natural resonant frequency, k_A is the dimensionless factor, $\overline{\gamma }$ is the gyromagnetic ratio, D_⊥ is the perpendicular demagnetization factor, and D_e and D_h are the parallel demagnetization factors along the hard and easy axis, respectively. For spherical particles, D_⊥ = D_h = D_e = 1/3, but for flaky particles, D_⊥≈1, and D_h = D_e ≈ 0. According to Equation 10, the increased D_⊥ together with the decreased D_e contributes to the enhanced permeability of the AFC-10 specimen. Furthermore, the flake treatment of Co particles can significantly suppress the eddy current effect due to the skin effect, thereby improving the permeability of particles.

Figure 7.

Complex permeability of aluminate/Co composites

The complex permeability of composite specimens at different temperatures, (A and B) AMC-10, (C and D) AFC-10, and (E and F) ACFC-10.

As the measured temperature increases, the real part of the permeability of each specimen rises, whereas the imaginary part of the permeability gradually decreases and eventually falls below zero. As is known, the magnetic crystal anisotropy constant progressively diminishes as the temperature increases, and ferromagnetic particles attain complete magnetic crystal isotropy at the Curie temperature. Given that the Curie temperature of Co particles (∼1150°C) significantly exceeds the measurement temperature of 500°C, the ferromagnetic properties are sustained. In the composite specimen, Co particles are randomly dispersed in the aluminate matrix, and the easy-magnetization axes are similarly oriented randomly in all directions, thereby macroscopically manifesting the composite specimen as magnetocrystalline isotropic. As the temperature elevates, the magnetocrystalline anisotropy constant diminishes, enabling the magnetic moment in the hard axis to rotate toward the external magnetic field, leading to an augmentation in the magnetization vector and, consequently, an enhancement in the permeability of the composite specimen. Nevertheless, the decreasing M_s with increasing temperature will contribute to a decrease in permeability according to Equations 10 and 11. Additionally, the reduced magnetocrystalline anisotropy also results in a decrease in the intrinsic coercive force of the Co particles, thereby mitigating the hysteresis loss during magnetic moment rotation. Furthermore, the diminished defects within the Co particles at high temperatures also attenuate the ferromagnetic loss. Consequently, the permeability initially increases and then decreases as the temperature rises, exhibiting its peak at 400°C.

The permeability exhibits a weak frequency dependence at room temperature, while μ′ decreases rapidly with increasing frequency, and μ″ assumes a significant negative value, indicating a notable permeability decay at elevated temperatures. It is noteworthy that this permeability decay is concomitant with a surge in permittivity, which can be attributed to the enhanced conductivity. The pronounced variations in electromagnetic parameters will attenuate the absorbing performance of composite specimens, hence necessitating the regulation of the conductivity of the composite specimens. In the case of the AMC-10 specimen, the temperature at which permeability decay occurs is 500°C. Conversely, for the AFC-10 specimen, owing to its high conductivity, the temperature at which permeability decay manifests is reduced to 300°C. Following the incorporation of cordierite, the temperature threshold for permeability decay in the ACFC-10 specimen is restored to 500°C, suggesting that the ACFC-10 specimen possesses superior high-temperature electromagnetic properties that effectively inhibit conductivity at elevated temperatures.

To summarize, Co flakes exhibit higher permittivity and permeability, whereas the high conductivity of the AFC-10 specimen leads to the generation of severe eddy currents, thereby attenuating its electromagnetic properties. The incorporation of cordierite can effectively mitigate the conductivity of the composite specimen, which in turn enhances its high-temperature absorption performance.

Meanwhile, micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation (LLG) are conducted to evaluate the ferromagnetic loss mechanisms at varying temperatures.^47,48,49

$$\frac{dM}{dt}=-{\gamma }_{G}M\times H_{eff}-\frac{{\alpha }_G}{M_s}\left[M\times \left(\frac{dM}{dt}\right)\right]$$ (Equation 14)

$$H_{eff}=H_{ani}+H_{ex}+H_d+H_{zee}$$ (Equation 15)

$$E_a=K_{\alpha }\left({\alpha }_1^2{\alpha }_2^2+{\alpha }_2^2{\alpha }_3^2+{\alpha }_3^2{\alpha }_1^2\right)$$ (Equation 16)

$$E_{ex}=-\sum _{i<j}A\left(R_L{R}_m\right)\overrightarrow{S}\left(R_L\right)·\overrightarrow{S}\left(R_m\right)$$ (Equation 17)

where M represents the magnetization vector, γ_G denotes the gyromagnetic ratio of the electron, α_G is the damping constant (α_G = 0.05). H_eff serves as the effective field, comprising contributions from the anisotropy field (H_ani), the exchange field (H_ex), the demagnetic field (H_d), and the Zeeman field (H_zee). During the simulation, the crystal structure of Co particles was set as an fcc structure, the M_s of Co particles was set as 14 × 10⁵ A/m, the magnetocrystalline anisotropy constant K_α was 4.5 × 10³ J/m, and the exchange constant A was 9 × 10⁻¹² J/m. To ensure the precision of the calculations, a mesh size of 2 nm × 2 nm×2 nm was utilized to discretize the particle geometry. A 10 mW RF field was selected as the field source, and its direction is incident along the horizontal direction. Moreover, the initial magnetization direction was regulated to be randomly distributed. The simulation time step was 10⁻⁹ s, and the simulation process consisted of 20 cycles.

The exchange coefficient A(T) and the magnetocrystalline anisotropy constant K(T) of Co particles exhibit temperature dependence, and they are correlated with that of M_s through a power-law scaling relationship,⁵⁰

$$\frac{A\left(T\right)}{A\left(T^{\ast }\right)}={\left[\frac{M_s\left(T\right)}{M_s\left(T^{\ast }\right)}\right]}^2$$ (Equation 18)

$$\frac{K\left(T\right)}{K\left(T^{\ast }\right)}={\left[\frac{M_s\left(T\right)}{M_s\left(T^{\ast }\right)}\right]}^{\sim 2}$$ (Equation 19)

where the value T∗ is chosen as the room temperature. A(T) and K(T) are calculated based on the measured M_s at various temperatures and listed in Table 2. As the temperature increases, the decrease in M_s is bound to contribute to the decreased A(T) and K(T), which subsequently leads to the dampening of the anisotropy energy and exchange energy. However, for Co, A(T) and K(T) can still be maintained at a relatively high level at 500°C, due to the high Curie temperature of Co (1131°C).

Table 2.

The temperature-dependent A(T) and K(T) calculated based on the power-law scaling relationship

Parameters	25°C	100°C	200°C	300°C	400°C	500°C
Ms (×105A/m)	14	13.9	13.8	13.6	13.3	12.5
A (×10−12J/m)	9	8.874	8.739	8.496	8.122	7.173
K1 (×103J/m)	4.5	4.437	4.370	4.248	4.061	3.586

Figures 8A and 8B illustrate the precession of magnetic moments in spherical and flake Co particles, respectively, under an alternating magnetic field, with magnetic domains differentiated by color. The relaxation dynamics of these magnetic moments significantly contribute to ferromagnetic loss. During precession under the alternating field, exchange energy, demagnetization energy, anisotropy energy, and Zeeman energy exhibit periodic fluctuations, as depicted in Figure S2, indicating the pronounced ferromagnetic relaxation. In comparison with these four energy components, the anisotropy energy contributes minimally to energy losses, whereas the exchange energy assumes a preeminent role in energy dissipation within Co nanoparticles. Furthermore, the hysteretic movement of magnetic domains under an alternating field gives rise to significant demagnetization energy and Zeeman energy, both of which subsequently contribute to ferromagnetic losses.

Figure 8.

Ferromagnetic loss mechanism

The precession of magnetic moments in Co particles under an alternating magnetic field during a sinusoidal period, (A) spherical Co particles and (B) flaky Co particles. The frequency of the alternating magnetic field was set to 1 GHz. The magnetic domains were differentiated by color.

(C) The total energy loss of Co particles in the temperature range of 0°C–500°C obtained from micromagnetic simulation based on the LLG equation.

As temperature increases, the saturation magnetization, exchange coefficient, and anisotropy constant all decrease, leading to a reduction in these four energy losses, as shown in Figure S3. Notably, this attenuation of energy loss of flaky particles is more pronounced at higher temperatures, as evidenced in Figure 8C. Specifically, spherical Co particles exhibit inferior performance, with an energy loss of 3.65 × 10–17 J/V at room temperature, which significantly decreases to 2.45 × 10–17 J/V at 500 °C, representing a 32.9% reduction in high-temperature energy loss. In contrast, flaky Co particles display an energy loss of 3.73 × 10–17 J/V at room temperature, which diminishes to 3.14 × 10–17 J/V at 500°C, resulting in a more modest 15.8% attenuation in energy loss characteristics. Therefore, it can be inferred that the flaking treatment of ferromagnetic particles holds potential for enhancing their ferromagnetic loss characteristics, suggesting a promising avenue for improving the performance of such materials in relevant applications.

In summary, excellent impedance matching characteristics are the prerequisite for achieving superior electromagnetic performance in composite materials, as shown in Figure 9. The aluminate/Co composites simultaneously possess ferromagnetic loss, dielectric loss, and conduction loss, indicating their potential to be developed as an efficient wave-absorbing material. However, the increase in conductivity with rising temperature not only enhances conduction loss but also leads to impedance mismatch, causing incident electromagnetic waves to reflect off the material surface. Therefore, to achieve the excellent electromagnetic performance of high-temperature wave-absorbing materials, their impedance matching characteristics must be properly controlled. In this study, the conductivity of aluminate/Co composites was effectively regulated through the introduction of a secondary dielectric component, providing an effective approach for optimizing the performance of elevated-temperature electromagnetic absorbing materials.

Figure 9.

Schematic of the electromagnetic loss of the aluminate/Co composites

Electromagnetic absorbing performances

EMA performances of AMC-10, AFC-10, and ACFC-10 specimens were calculated according to the transmission line theory.^51,52 Figures S4–S6 show the reflection loss for AMC-10, AFC-10, and ACFC-10 specimens at the 8.2–12.4 GHz band with the typical thicknesses of 1.5 mm, 2 mm, 2.5 mm, and 3 mm. As the thickness increases, RL_max peaks shift toward the lower frequency, which corresponds to the quarter-wavelength cancellation theory.⁴⁶

For comparison, the absorbing performances of specimens with the thickness of 2 mm are shown in Figure 10 and listed at Table 3. The AMC-10 specimen exhibits poor EMA performance in X band. As the temperature increases, its wave-absorbing performance improves slightly, but the reflection loss (RL) does not exceed −5 dB during the measurement frequency band. In contrast, the AFC-10 sample demonstrates a significant enhancement in EMA performance, with an RLmax of −12.5 dB and an ERL of 5 dB covering the entire X-band at 300°C. Moreover, the absorbing peak shifts to the lower frequencies, which is caused by the increasing electromagnetic parameters according to the quarter-wavelength cancellation theory. However, due to the high conductivity of the AFC-10 specimen, abnormal electromagnetic parameters occur at 400°C, limiting further improvement in its wave-absorbing performance. By incorporating cordierite, the high-temperature conductivity of the ACFC-10 sample is significantly reduced, contributing to a slight decrease in wave-absorbing performance at lower temperatures. Nevertheless, its high-temperature EMA performance is notably improved. For a 2 mm ACFC-10 specimen, when the temperature reaches 400°C, the RLmax is −9.1 dB, and the ERL5 can reach 3.4 GHz. As the temperature continues to rise to 500°C, the RLmax reaches −13.1 dB, with an ERL10 of 0.4 GHz and an ERL5 covering the entire X-band.

Figure 10.

Reflection loss

EMA performances of specimens with the typical thickness of 2 mm, at the frequency range of 8.2–12.4 GHz, (A) AMC-10 specimen, (B) AFC-10 specimen, and (C) ACFC-10 specimen.

Table 3.

EMA performances of aluminate/Co composite specimens at the typical thickness of 2 mm

Specimens	RLmax (dB)	ERL10 (GHz)	ERL5 (GHz)	Tempetature (°C)
AMC-10	−3.7 (11.5 GHz)	–	–	500
AFC-10	−12.5 (8.2 GHz)	1.1 (8.2–8.8, 9.5–10.0)	4.2 (8.2–12.4)	300
ACFC-10	−9.1 (11.3 GHz)	–	3.1 (9.1–12.4)	400
	−13.1 (8.2 GHz)	0.4 (8.2–8.6)	4.2 (8.2–12.4)	500

In summary, AFC-10 demonstrates superior wave-absorbing performance compared to AMC-10. However, the high high-temperature conductivity of AFC-10 results in impedance mismatch in high-temperature environments. The introduction of cordierite to restrict high-temperature ionic conductivity enables ACFC-10 to maintain excellent electromagnetic absorption characteristics even in high-temperature environments, offering a feasible approach for the preparation and optimization of high-temperature wave-absorbing coatings.

Conclusions

In this work, aluminate/Co composite absorbing materials were prepared to satisfy the design requirements for structural/functional integrated components. The microstructure characteristics of the composite coating were controlled by adjusting the shape of Co particles, and cordierite particles were added to further regulate the distribution of Co particles. The temperature-dependence ionic conductivity plays an important role in controlling the elevated-temperature electromagnetic properties and can be suppressed by adding a second dielectric component. AFC-10 displays the higher electromagnetic properties than that of AMC-10, while its impedance matching rapidly decays with increasing temperature. Micromagnetic simulation results demonstrate that flake-shaped Co particles exhibit higher ferromagnetic loss than spherical Co particles, both at room temperature and under elevated temperature conditions. The elevated-temperature absorption performances of these aluminate/Co composite materials were evaluated, and an optimization strategy for elevated-temperature absorbing materials was obtained. For a 2 mm ACFC-10 specimen, when the temperature reaches 400°C, the RL_max is −9.1 dB, and the ERL₅ can reach 3.4 GHz. As the temperature continues to rise to 500°C, the RL_max reaches −13.1 dB, with an ERL₁₀ of 0.4 GHz and an ERL₅ covering the entire X-band.

Limitations of the study

Although the aluminate/Co composite demonstrates promising high-temperature EMA performance, several limitations should be noted:

Narrow Effective Absorption Bandwidth: The optimized specimen achieves effective absorption (RL < −5 dB) across the X-band but can’t cover broader frequency ranges, limiting multi-band applications.

Potential Oxidation of Co Particles: At elevated temperatures (>500°C), metallic Co flakes may undergo oxidation, degrading electromagnetic properties and long-term stability.

Temperature-Dependent Conductivity Trade-off: Although ionic conductivity suppression improves high-temperature performance, further optimization is needed to balance conductivity and impedance matching at extreme temperatures (>500°C).

Scalability Challenges: The hydration hardening process, while effective, may face reproducibility issues in large-scale production due to particle dispersion heterogeneity.

Resource availability

Lead contact

Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Dr Zhenjie Guan (guanzj@hit.edu.cn)

Materials availability

All the materials of this study are available from the lead contact without restriction upon request.

Data and code availability

• •All data reported in this article will be shared by the lead contact upon request.
• •No original code was generated for this study.
• •Additional information on the data reported in this article and their analysis is available from the lead contact on request.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China(Grant No. 52301188), the 2022 Postdoctoral Funding of Heilongjiang Province, and the Key Laboratory Fund for Precision Thermal Processing of Metals.

Author contributions

Zhen-Jie Guan: Conceptualization, methodology, investigation, and writing. Bo-An Yang: Validation and calculation. Yong Yuan: Supervision and validation. Jian-Tang Jiang: Supervision, review, and editing. Yang Li: Investigation and software. Bo Song: Formal analysis and validation. Xue-Yin Sun: Methodology and investigation. Yuan-Xun Gong: Funding acquisition and supervision. Shao-Jiu Yan: Resources and validation. Wen-Zhu Shao: Supervision. Liang Zhen: Supervision and review.

Declaration of interests

The authors declare no competing interests.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Aluminate (CaO-Al2O3)	Hebei Hongyao Mineral Products Processing Co., LTD	CAS: 12042-68-1
Cordierite (Mg2Al4Si5O18)	Sinopharm Chemical Reagents Co., Ltd	CAS: 1302-88-1
Co particles	Changsha TIJO Metal Powders Co., Ltd	CAS: 7440-48-4
Deposited data
All data reported in this paper will be shared by the lead contact upon request.
Software and algorithms
The Object Oriented MicroMagnetic Framework (OOMMF)

Method details

Materials

Commercially available aluminate (CaO-Al₂O₃) was used as the matrix materials and cordierite (Mg₂Al₄Si₅O₁₈) was uesd as the second dielectric component without further treatment. Microsphere Co particles (below 400 mesh) were provided by Changsha TIJO Metal Powders Co., Ltd. The flaky Co particles were obtained through an typical ball-milling process referring to our previous work.²² The as-milled Co particles were sieved to obtain the uniform flaky Co particles in the range of 300–400 mesh.

Fabrication of aluminate/Co composite materials

Aluminate together with 10 vol. % of Co particles were mixed, deionized water was added to make mixed mortar followed by mechanical agitation for 20 min to ensure that the absorbent is evenly dispersed in the aluminate matrix. The mixed mortar was transferred to a rectangular mold with the size of 22.86×10.16 mm, formed at a pressure of 10 MPa, and the resulted rectangular specimens was hardened in a closed water vapor environment for 72 h at room temperature. The aluminate/microsphere Co and aluminate/flaky Co composite specimens, named as AMC-10 and AFC-10, were obtained after annealed at 500 °C for 2 h to remove excess free water in an air atmosphere. In addition, cordierite with the same mass fraction as aluminate was added into and Co flake content still maintains at 10 vol.%, and then the aluminate/cordierite/flaky Co composite specimens were obtained and named as ACFC-10.

Characterization

The morphology of particles and the cross-sectional view of absorbing specimens was observed on a scanning electron microscope (SEM, Zeiss Supra 55). The cross section of absorbing specimens was polished before observation. X-ray diffractometer (XRD, PANalytical X’Pert PRO) with a Cu Ka radiation was utilized to analyze the phase compositions of aluminate/Co composite materials. The high-temperature conductivity of aluminate/Co composite materials was examined using a four-probe resistance tester, and the high temperature environment with the temperature range of 25∼600°C was provided by the tube furnace in an air atmosphere. To reduce the test error, the conductivity was measured 5 times at each temperature and the average value was taken. The static magnetic properties of pure Co particles were measured by a physical property measurement system (PPMS, Quantum Design Dynacool-14T) with a vibrating sample magnetometer (VSM) option at 25°C, 100°C, 200°C, 300°C, 400°C and 500°C, respectively. The elevated-temperature electromagnetic properties of these absorbing specimens were performed on a vector network analyzer (VNA, Agilent N5230A) equipped with electromagnetic heating furnace, and the measured electromagnetic filed frequency range was 8.2–12.4 GHz and the temperatures were set as at 25°C, 100°C, 200°C, 300°C, 400°C and 500°C, respectively. Waveguide specimens were made into rectangular blocks with the size of 10.16×22.68×2.0 mm.

Micromagnetic simulation

The micromagnetic simulation of OOMMF software based on Landau-Lifshitz-Gilbert equation (LLG) was carried out to reveal the ferromagnetic loss at elevated temperature, and the calculation grid is set as 2×2×2 nm.

Quantification and statistical analysis

There are no quantification or statistical analyses to include in this study.

Published: September 23, 2025