Multi-strategy modulation towards negative dielectric properties in Ag nanoparticle-immobilized carbon fiber felt metacomposites

Abstract

In this work, Ag nanoparticle-immobilized carbon fiber felt metacomposites (CFF@PDA-Ag) were first fabricated by a chemical-plating method to achieve multi-strategy modulation of their negative dielectric properties. Three routes including Ag nanoparticle content regulation, heat treatment and compression were developed, and their effects on the negative dielectric performance were investigated. It was found that with the increase of deposited Ag nanoparticles, negative permittivity was achieved for the metacomposite with 15 wt% Ag nanoparticles (CFF@PDA-Ag15), resulting from the formation of three-dimensional conductive networks. After heating at 500 °C, the permittivity of CFF@PDA-Ag11 changed from a positive to negative value, which was attributed to its enhanced ac conductivity owing to the grain activation and interparticle bonding of melted Ag nanoparticles. In addition, the results indicated that the dynamic process also adjusted its dielectric properties. After compression, negative permittivity of CFF@PDA-Ag11 was observed and the corresponding absolute value increased with further compression. In addition, the Drude and parallel models composed of conductive carbon fibers and an air phase were used to explain the regulation mechanism of negative permittivity. This work developed a multi-strategy method for achieving adjustable negative permittivity based on Ag nanoparticles, and demonstrated its importance and significance for the development of novel metacomposites.

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1. Introduction

Compared with metamaterials with periodic array structures,^1–6 metacomposites realize negative electromagnetic parameters via traditional methods.^7–11 They have increasingly received widespread attention due to their many advantages including abundant raw materials and simple and controllable routes for synthesis. In order to meet the requirements of various electromagnetic applications, dielectric properties should exhibit good controllability.^12–20 Therefore, how to regulate the negative permittivity of metacomposites has gradually become a hot topic in this field.

It is well known that two-phase compounding with insulating and conductive phases is an effective method for preparation of metacomposites. Generally, ceramics and resin have been chosen for the insulating phase, while metal, carbon and conductive polymers have usually been used for the conductive phase.^21–36 The methods based on the composition and structure of the conductive phase have usually been used to regulate negative permittivity. Sun et al.²¹ reported flexible polyvinylidene fluoride composites with controlled negative permittivity using reduced graphene oxide (rGO) as the conductive phase. Zhou et al.³³ assembled Ag nanowires with high aspect ratios on thermoplastic polyurethane fibers to form three-dimensional conductive networks and observed adjustable negative permittivity. Hu et al.³⁷ also prepared a high entropy (Ti_0.25Zr_0.25Nb_0.25Ta_0.25)C ceramic with different sintering temperatures to realize controllable negative dielectric properties. Ma et al.³⁸ prepared an Al₂O₃ ceramic composite with carbon spheres, carbon black (CB), carbon fiber and graphene (GR) nanosheets and investigated the impact of different carbon fillers on its negative permittivity. Some core–sheath and hollow fibers have been designed and used as fillers to prepare metacomposites. For example, Qin et al.^39–43 investigated ferromagnetic microwires as the functional phase for preparation of polymer metacomposites.

Recently, some novel methods based on elemental doping, interface engineering and external field manipulation have also been developed to control negative dielectric properties. Guo et al.³¹ reported N and P co-doped GR and investigated the effect of induced defects on its dielectric properties. It was found that the dual-atom synergy adjusted the value of negative permittivity, and stable weak negative permittivity (from −30 to −40) was obtained. Modifying rGO/CNT heterogeneous interfaces with different polyelectrolytes also modulated their negative permittivity well.³² A multilevel heterogeneous interface including GR, CB and CaCu₃Ti₄O₁₂ was produced to accurately control the negative permittivity by controlling the carrier concentration.⁴⁴ The use of some external factors including temperature, pressure and magnetic fields has also been reported for metacomposites with a tunable dielectric response. A PVA/Ni@CNT film was fabricated to achieve broadband weak negative permittivity with good magnetic actuation properties.⁴⁵ Carbon fiber/Al₂O₃ composites produced with the hot-press method exhibited a negative response at different temperatures.⁴⁶ Although a large number of routes have been reported, there is a challenge to explore new technology for controllable negative permittivity.

Ag has the highest conductivity and is used as a conductive functional phase and effective filler for metacomposites. Xie et al.³⁴ fabricated Ag-SiO₂ metacomposites using SiO₂ microspheres as the templates and finally a weak negative permittivity of −100 was obtained. Qu et al.³⁵ fabricated a three-dimensional Ag network in a magnetic Bi₂₅FeO₄₀ matrix via a solid-state chemical reaction of Ag₂O, and broadband weak negative permittivity was observed. Pan et al.⁴⁷ constructed a Ag nanoparticle/melamine foam/polydimethylsiloxane system, achieving simultaneous negative permittivity and permeability across a 4.2 GHz bandwidth. Unfortunately, the influence of the structure and content of Ag nanoparticles on negative permittivity is still unclear, and hydrothermal or thermal decomposition methods are used to synthesize Ag nanoparticles.

Carbon fiber felt (CFF) with good electrical conductivity and flexibility and large specific surface area can be used as a matrix and electrode material in energy systems, environment applications, flexible electronics and intelligent sensing.^48–50 Here, Ag nanoparticle-immobilized CFF (CFF@PDA-Ag) metacomposites have been reported for the first time based on a facile chemical-plating method at room temperature. Considering that Ag nanopoarticles have a low melting point and their microstructures are easily controlled by heat treatment,⁵¹ modification of the content of Ag nanoparticles, heat treatment and compression were investigated to develop multi-strategy modulation of negative permittivity. The microstructure and composition were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The conductivity and dielectric properties were studied in detail. In addition, the Drude and parallel models of binary materials were used to fit and analyze the negative permittivity. The corresponding mechanisms of electron conduction and negative permittivity were also discussed and explained. The proposed method gave a new idea for modulation of negative permittivity based on metal nanoparticle-modified CFF metacomposites.

2. Experimental

2.1 Chemicals and reagents

CFF was obtained from Kunshan Longshengbao Electronic Materials Co., Ltd. Silver nitrate (AgNO₃, 99.8 wt%, AR) and glucose were obtained from Sinopharm Chemical Reagent Co. Ltd. Dopamine hydrochloride (DA, C₈H₁₁NO₂·HCl, 98 wt%), Tris–HCl buffer (1.0 M, pH 8.5) and polyvinylpyrrolidone K-30 (PVP, AR) were purchased from Shanghai Macklin Biochemical Co. Ltd.

2.2 Preparation of CFF@PDA-Ag

Fig. 1 shows the preparation process of CFF@PDA-Ag metacomposites. The CFF was cut into a rectangular shape of 30 mm × 10 mm, and then the CFF was cleaned with deionized water for 10 min to remove the surface contamination. Next, the cleaned CFF (0.6 g) was immersed in 300 ml of deionized water containing 0.3 g of DA. After stirring for 10 min, 3 mL of Tris–HCl buffer solution was added and it was continuously stirred for 24 h to prepare a CFF@polydopamine (CFF@PDA) composite. 0.06 g of PVP was dissolved in 30 mL of deionized water and 0.3 g of CFF@PDA was immersed in the solution. Then 0.1 g of AgNO₃ and 0.05 g of glucose were added, and the solution was stirred for 1 h to deposite Ag nanoparticles onto CFF@PDA via the chemical plating method.^52,53 After that, the CFF was taken out and dried at 60 °C in a vacuum oven for 12 h to obtain CFF@PDA-Ag. Using a similar process, 0.2, 0.4, 0.6, 0.8 and 1.0 g of AgNO₃ and 0.1, 0.2, 0.3, 0.4 and 0.5 g of glucose were used to modify the amount of Ag nanoparticles on the CFF. According to the loading mass ratio of Ag nanoparticles, the CFF@PDA-Ag composites were named CFF@PDA-Agx (where x is the mass ratio of Ag nanoparticles on the CFF, x = 7, 11, 15, 21, 28 and 40).

Fig. 1 Schematic diagram of the preparation process for CFF@PDA-Ag.

2.3 Heat treatment of CFF@PDA-Ag

The CFF@PDA-Ag11 was placed into an alumina ceramic boat and treated at 500 and 600 °C for 2 h in a N₂ tube furnace with a heating rate of 5 °C min⁻¹, respectively. After cooling to room temperature, the produced composites were marked as CFF@PDA-Ag11-500 °C and CFF@PDA-Ag11-600 °C.

2.4 Characterization

FESEM (Hitachi SU-70, Tokyo, Japan) was used to characterize the morphology and elemental composition of CFF@PDA-Ag. The phase composition was investigated by XRD (MiniFlex 600, Japan), and XPS (Thermo ESCALAB 250XI, USA) was used to study the chemical bonding states of CFF@PDA-Ag. The dc conductivity was measured with the four-probe method with a source measure unit (Keithley 2400, USA). The dielectric properties (capacitance, loss tangent and resistance) were characterized using an LCR meter (Keysight, E4980AL, USA) with a 16451B dielectric test fixture. The following formulae were used to obtain the permittivity (ε′), the imaginary part of the dielectric constant (ε″) and the dielectric loss angle (tan δ):

ε″ = D·ε′ (2)

where d is the sample thickness, A is the electrode area, C_p is the capacitance, D is the dielectric loss factor and ε₀ is the vacuum dielectric constant (8.85 × 10⁻¹² F m⁻¹).

3. Results and discussion

3.1 Microstructure and composition of CFF@PDA-Ag

FESEM was used to analyze the microstructures of the pure CFF, CFF@PDA and CFF@PDA-Ag, which are shown in Fig. 2. From Fig. 2a and b, it can be observed that the CFF was composed of many interlaced carbon fibers with high aspect ratios, exhibiting a unique three-dimensional network. The diameter of the carbon fiber was about 13 μm, and some grooves along the axial direction were distributed on the CFF surface. In addition, the fiber surface was rough and covered with a very villous structure. After the DA molecules were polymerized to form PDA on the CFF surface, CFF@PDA still exhibited a similar structure to the CFF (Fig. 2c). Fig. 2d shows the corresponding FESEM image of CFF@PDA-Ag28. Many Ag nanoparticles were uniformly modified on the fiber, forming a continuous Ag shell layer on the carbon fibers. Because the PDA had many nitrogen-containing groups, these groups provided many active sites for the Ag⁺, which were subsequently reduced to Ag nanoparticles by the reduction of glucose. It was found that the AgNO₃ concentration obviously controlled the mass of Ag nanoparticles on the fiber surface. FESEM images of CFF@PDA-Ag7 and CFF@PDA-Ag40 are shown in Fig. 2e and f. For the low AgNO₃ concentration, small Ag nanoparticles were formed and the whole carbon fiber was not completely covered. When a high AgNO₃ concentration was used, the content of Ag nanoparticles reached 40 wt%. Large amounts of Ag nanoparticles were produced and interacted with each other. Some large Ag nanoparticles with diameters of about 500 nm were also observed and the whole carbon fiber was encapsulated by a dense Ag shell. The EDS mapping confirmed the distribution of C, N, and Ag elements, as shown in Fig. 2g. The results indicated that these elements were uniformly distributed in the composite and CFF@PDA-Ag was successfully produced.

Fig. 2 FESEM images of (a and b) the CFF, (c) CFF@PDA, (d) CFF@PDA-Ag28, (e) CFF@PDA-Ag7 and (f) CFF@PDA-Ag40. (g) FESEM image and EDS maps of CFF@PDA-Ag40.

Fig. 3a presents optical photos of CFF@PDA, CFF@PDA-Ag7 and CFF@PDA-Ag40. CFF@PDA exhibited a black color, and CFF@PDA-Ag7 was dark gray. However, CFF@PDA-Ag40 exhibited a silver white color due to the high amount of Ag nanoparticles. XRD was used to characterize the phase composition of the composites, and Fig. 3b shows the XRD patterns of the CFF, CFF@PDA and CFF@PDA-Ag7. For the CFF and CFF@PDA, only two broad peaks appeared at 24° and 43°, attributed to the (002) and (100) crystal planes of carbon, which are typical characteristic peaks of amorphous carbon.⁵⁴ There were five strong diffraction peaks at 38°, 44°, 64°, 77°, and 82°, corresponding to the (111), (200), (220), (331), and (222) crystal planes of metallic Ag for CFF@PDA-Ag7.^55,56 The XRD results confirmed that Ag nanoparticles were immobilized onto the carbon fibers by chemical plating. The dc conductivity and density of CFF@PDA-Ag were also investigated, as shown in Fig. 3c. The densities of all samples ranged from 0.13 to 0.22 g cm⁻³, which were enhanced by the increase of immobilized Ag nanoparticles. With the increase of Ag nanoparticles, the dc conductivity of CFF@PDA-Ag also increased and reached a maximum of 390.8 (Ω m)⁻¹. The density of the produced CFF@PDA-Ag was still low and the composite could be placed on the petals, which still retained its original state without obvious deformation (Fig. 3d), indicating that CFF@PDA-Ag exhibited lightweight properties and showed potential for applications in the field of electromagnetic materials.

Fig. 3 (a) Optical images of CFF@PDA, CFF@PDA-Ag7 and CFF@PDA-Ag40. (b) XRD patterns of the CFF, CFF@PDA and CFF@PDA-Ag7. (c) Density and dc conductivity of CFF@PDA-Ag. (d) Optical image of CFF@PDA-Ag28 on petals.

To further investigate the chemical bonding structure of the composites, XPS was used to characterize the produced CFF@PDA-Ag. As shown in Fig. 4a, the XPS full spectrum indicated that CFF@PDA only had C, N and O elements. Besides these elements, the Ag element was also observed for CFF@PDA-Ag7. A high-resolution XPS C 1s fine spectrum is given in Fig. 4b; three peaks related to C–C (284.8 eV), C–O (286.0 eV), and C=O (287.9 eV) were observed, indicating the presence of many oxygen-containing functional groups. For the N 1s spectrum, the peaks were located at 399.8 eV, 400.3 eV, and 401.4 eV, resulting from C–N–C, C–N, and N–H groups, respectively, which confirmed the presence of PDA. As shown in Fig. 3d, the Ag 3d spectrum had two peaks – Ag 3d_3/2 (374.0 eV) and Ag 3d_5/2 (368.0 eV) respectively.^52,57 The results also further proved that Ag nanoparticles were successfully immobilized onto the CFF.

Fig. 4 (a) XPS survey spectra of CFF@PDA and CFF@PDA-Ag7. High-resolution XPS spectra of (b) C 1s, (c) N 1s and (d) Ag 3d of CFF@PDA-Ag7.

3.2 Dielectric properties of CFF@PDA-Ag

The ac conductivity of CFF@PDA-Ag was first studied to investigate the electron transmission process. It is well known that the ac conductivity consists of metal-like conductivity which is frequency independent and jump conductivity which is frequency dependent. The corresponding relationship is described by the Jonscher power law:

σ_ac = σ_dc + A(2πf)ⁿ. (4)

Here, σ_ac is the ac conductivity, σ_dc is the dc conductivity, A is a constant, and n is a power-law coefficient, which is usually between 0 and 1. Fig. 5 shows the ac conductivity of the produced CFF@PDA-Ag. It can be seen that the ac conductivity forms a straight line approximately parallel to the X-axis with low dispersion for CFF@PDA-Ag except for CFF@PDA-Ag40. This is attributable to the unique three-dimensional network of carbon fibers modified with Ag nanoparticles, which accelerates the electron transmission. The conductivity is not related to the frequency, indicating a metal-like conductivity mechanism (n ≈ 0). However, CFF@PDA-Ag40 showed different behavior and a decreasing trend at high frequencies due to the significant skin effect. The skin effect easily results in an uneven distribution of current across the cross-section of the conductor, and the surface has a higher current density and its interior has a lower current density. With the increase of frequency, the skin effect becomes more obvious and the resistivity is enhanced, which is ascribed to the reduction of the conductive area.^58–60 In addition, the ac conductivity increases with the amount of Ag nanoparticles, which is consistent with the dc conductivity results.

Fig. 5 (a) Ac conductivity, (b) dielectric constant, (c) dielectric loss and (d) tan δ values of CFF@PDA-Ag metacomposites.

The frequency dependences of ε′ in CFF@PDA-Ag with different amounts of Ag nanoparticles are presented in Fig. 5b. It was found that the ε′ of CFF@PDA-Ag increased with the amount of Ag nanoparticles from 0 to 11 wt%, which was attributed to the enhancement of the interface between Ag nanoparticles and CFF@PDA, resulting in an improvement of interface polarization. The ε′ of CFF@PDA-Ag11 reached about 2700 at 0.1 MHz; however, the ε′ of CFF@PDA was only 50. This indicated that the immobilization of Ag nanoparticles played important roles in improvement of the permittivity. When the Ag content was improved to 15 wt%, the ε′ suddenly changed from a positive to a negative value. Moreover, the |ε′| also increased with the amount of Ag nanoparticles. According to the Drude model, the conductive network in CFF@PDA-Ag15 was gradually constructed, while the frequency of delocalized plasma oscillation exceeded the electric field frequency at a certain frequency, resulting in the ε′ changing from a positive to a negative value.^61–64 The Drude model given below was used to fit the ε′ and the fitting results (solid blue line) were consistent with the experimental data (R² > 90%):

here is the real part of permittivity, ε_∞ is the permittivity for ω → ∞, τ_D is the damping term of delocalized electrons, ω represents the angular frequency of the electric field, and ω_p corresponds to the plasma frequency.

It is worth noting that CFF@PDA-Ag15, CFF@PDA-Ag21 and CFF@PDA-Ag28 all showed weak negative permittivity. Especially for CFF@PDA-Ag15, the ε′ at a frequency of 0.1 MHz was only −17.2, which was far lower than those of other reported metacomposites with Ag as the functional phase.^53,65–68

The imaginary part of permittivity, ε″, is used to evaluate the loss property of materials, presenting the dielectric loss. It consists of polarization and conductivity loss, which are expressed as follows:

Fig. 5c shows the relationship between the ε″ of the produced CFF@PDA-Ag and the frequency. It was found that the ε″ obviously reduced with frequency, due to the fact that the is inversely proportional to the frequency of an alternating electric field. Moreover, the interface polarization in the materials could not respond to the changes from high-frequency electric fields, resulting in a decrease of . The ε″ was also enhanced with the increase of the immobilized Ag nanoparticles because three-dimensional conductive networks and interface polarization were fabricated and enhanced. Fig. 5d shows the frequency-dependent tan δ values for the different CFF@PDA-Ag materials, and the CFF@PDA-Ag15 exhibited the maximum tan δ among the materials.

3.3 Dielectric properties of CFF@PDA-Ag after heat treatment

According to the above results, the negative dielectric properties were successfully modulated by controlling the amount of immobilized Ag nanoparticles. Considering that Ag nanoparticles have a low melting point and their microstructure is easily adjusted by temperature, the effect of heat treatment on the dielectric properties was also investigated. FESEM images of CFF@PDA-Ag11 after heat treatment at 500 and 600 °C are shown in Fig. 6. Ag nanoparticles were 10–600 nm in size and unevenly distributed in CFF@PDA-Ag11 (Fig. 6a). After heat treatment at 500 °C, Ag nanoparticles with an obvious spherical structure and sizes from 40 to 200 nm were observed (Fig. 6b). When the temperature was further increased to 600 °C, a lot of Ag nanoparticles melted into a complete and dense Ag shell on the carbon fiber (Fig. 6c). Fig. 7 shows the ac conductivity and dielectric properties of CFF@PDA-Ag11 before and after the heat treatment. As shown in Fig. 7a, the ac conductivity obviously increased after the heat treatment. After annealing at 600 °C, the ac conductivity was increased from 0.004 to 0.013 (Ω cm)⁻¹ at 0.1 MHz, which was about 3.25 times that of CFF@PDA-Ag11. This was attributed to the formation of good conductive networks from melted Ag nanoparticles after the heat treatment. CFF@PDA-Ag11-500 °C and CFF@PDA-Ag-600 °C both exhibited low frequency dispersion, indicating metal-like conductivity. As shown in Fig. 7b, CFF@PDA-Ag11 exhibited positive permittivity and after heat treatment, it was found that negative permittivity was realized (Fig. 7b). This was attributed to the increase of plasma oscillation frequency resulting from the high effective carrier density in the Ag conductive network formed after the heat treatment. This indicated that the heat treatment could reduce the content of Ag nanoparticles to achieve negative permittivity. The dielectric losses of CFF@PDA-Ag11-500 °C and CFF@PDA-Ag11-600 °C were both obviously improved compared with that of CFF@PDA-Ag11 (Fig. 7c), attributed to the fact that the conductivity loss and interface polarization were enhanced after the formation of a dense Ag shell. The frequency dependent tan δ values of CFF@PDA-Ag11 before and after heat treatment are shown in Fig. 7d. CFF@PDA-Ag11-500 °C showed the maximum tan δ value among these materials, which increased from 26.2 to 914.9 at 0.1 MHz.

Fig. 6 FESEM images of (a) CFF@PDA-Ag11, (b) CFF@PDA-Ag11-500 °C and (c) CFF@PDA-Ag11-600 °C.

Fig. 7 (a) Ac conductivity, (b) dielectric constant, (c) dielectric loss and (d) tan δ values of CFF@PDA-Ag11, CFF@PDA-Ag11-500 °C and CFF@PDA-Ag11-600 °C metacomposites.

3.4 Compression modulation

It was found that the CFF showed flexibility and could recover its original shape. So, the three-dimensional conductive network in CFF@PDA-Ag changed during the compression process, resulting in the corresponding changes in dielectric properties. In order to investigate the dynamic modulation effect of compression, the dielectric properties of the CFF@PDA-Ag materials with different thicknesses after compression were investigated. The ac conductivity, ε′ and ε″ of the compressed CFF@PDA-Ag are given in Fig. 8. It was observed that the ac conductivity subsequently increased with a decrease of the thickness due to the enhanced compression stress. The ac conductivity reached 0.009 (Ω cm)⁻¹ for CFF@PDA-Ag11 at 2.0 mm (Fig. 8a). This was attributed to the fact that the compression stress favoured the connection of carbon fibers with each other, as shown in Fig. 9. The variation of ε′ for CFF@PDA-Ag11 with different compression thicknesses is shown in Fig. 8b. After applying the compression, obvious changes in dielectric properties were observed. With increased compression, ε′ changed from positive to negative, and the absolute value was enhanced. Especially for CFF@PDA-Ag11 at 3.0 mm, the ε′ retained a weak negative value of about −130 throughout the whole testing frequency range. The maximum absolute value of ε′ could reach about 2400 at 0.1 MHz for a thickness of 2.0 mm.

Fig. 8 (a) Ac conductivity, (b) dielectric constant and (c) dielectric loss values of CFF@PDA-Ag11 at different thicknesses after compression.

Fig. 9 Schematic diagram of the structural changes in CFF@PDA-Ag after compression and recovery.

The change in dielectric constant can be explained using the capacitor model theory:^69,70

εⁿ = v₁ε₁ⁿ + v₂ε₂ⁿ (7)

ε = v₁ε₁ + v₂ε₂. (9)

Here, ε is the dielectric constant of a mixture of two phases, v₁ and ε₁ are the volume fraction and relative dielectric constant of one phase, and v₂ and ε₂ are the volume fraction and relative dielectric constant of the other phase. n represents different connection modes: when n is −1, it corresponds to the series mode (eqn (8)), and when n is 1, it represents the parallel mode (eqn (9)).

Considering that the produced CFF@PDA-Ag had high porosity, it could be regarded as a binary material composed of conductive fibers and an air phase, which fitted the parallel model. Due to the high effective carrier concentration and good network structure of conductive carbon fibers, the dielectric constant ε₁ was considered as a negative value. During the compression process, the fiber structure deformed and became dense, which effectively expelled the air phase from the materials and resulted in a significant increase of the volume fraction v₁ and the corresponding decrease of v₂. After the compression, the effective carrier concentration increased and the negative ε₁ became small (the absolute value became big). However, for the air phase, the ε₂ was 1.0 and the v₂ was reduced after the compression. So, according to eqn (9), when the compression reached the threshold, the absolute value of the negative v₁ε₁ was equal to the positive v₂ε₂ and the ε reached 0. Further compression caused CFF@PDA-Ag to exhibit negative dielectric performance.

4. Conclusions

In summary, CFF@PDA-Ag metacomposites were successfully fabricated by chemical plating of Ag nanoparticles onto the CFF to develop a multi-strategy for achieving controllable negative dielectric properties. It was found that modification of the amount of Ag nanoparticles, heat treatment and compression all effectively regulated the negative dielectric properties. With the increase of deposited Ag nanoparticles, CFF@PDA-Ag exhibited adjustable negative permittivity resulting from the formation of three-dimensional conductive networks. After heat treatment at 500 °C, a permittivity change from positive to negative was observed even for the 11 wt% Ag nanoparticles, which was attributed to the enhanced conductivity from the melted Ag shell on the carbon fibers. During the compression, CFF@PDA-Ag11 also showed a change from positive to negative permittivity, and the absolute value of negative permittivity subsequently increased to 2400 with further compression. The parallel model of a binary material composed of conductive fibers and an air phase was also used to analyze the dielectric properties. The permittivity transformation was mostly attributed to the fact that the absolute value of the negative permittivity of carbon fibers increased and the volume fraction of the air phase reduced after the compression. The proposed method in this work provides novel modulation routes based on metal content, heat treatment and compression and shows potential applications for fabrication of multifunctional metacomposites.

Author contributions

Xuan Yang: writing – original draft, methodology, investigation, data curation. Lixin Xuan: conceptualization, methodology. Weiwei Men: resources, investigation. Muwen Niu: methodology, investigation. Xiao Wu: supervision, software. Jingyu Bi: methodology, data curation. Lei Qian: supervision, writing – review & editing.

Conflicts of interest

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.

Data availability

The data supporting this article is not available because it is related with our ongoing research work and the legal confidentiality.

Acknowledgements

This work was supported by the Natural Science Foundation of Shandong Province (No. ZR2024QE129) and the Project of Joint Laboratory of Electromagnetic Structure Technology between AVIC Special Institute and Shandong University (No. 2025-16-XMMJ-0248).

References

1. W. Ma, H. Yang, J. Zhao, W. Li, H. Ding, S. Qu, Y. Liu, X. Hu, R. Li, Q. Tao, M. Mo, W. Zhai and X. Song, Multi-physical lattice metamaterials enabled by additive manufacturing: Design principles, interaction mechanisms, and multifunctional applications, Adv. Sci., 2025, 12, 2405835.
2. X. Wang, F. Qin, Y. Duan, G. Yang, W. Huang and Z. Huang, Dielectric-based metamaterials for near-perfect light absorption, Adv. Funct. Mater., 2024, 34, 2402068.
3. H. Wang, Y. Lyu, S. Bosiakov, H. Zhu and Y. Ren, A review on the mechanical metamaterials and their applications in the field of biomedical engineering, Front. Mater., 2023, 10, 1273961.
4. X. Ni, S. Yves, A. Krasnok and A. Alu, Topological metamaterials, Chem. Rev., 2023, 123, 7585–7654.
5. C. Qian, I. Kaminer and H. Chen, A guidance to intelligent metamaterials and metamaterials intelligence, Nat. Commun., 2025, 16, 1154.
6. Y. Zhou, S. Wang, J. Yin, J. Wang, F. Manshaii, X. Xiao, T. Zhang, H. Bao, S. Jiang and J. Chen, Flexible metasurfaces for multifunctional interfaces, ACS Nano, 2024, 18, 2685–2707.
7. Y. Zhou, Y. Wang, S. Qiu, W. Zhao, S. Wang, H. Bao, Y. Qu and Z. Wen, Microscopic response mechanism of epsilon-negative and epsilon-near-zero metacomposites, Research, 2025, 8, 556.
8. A. Le Duigou, M. Grabow, F. Scarpa, J. Deschamps, C. Combescure, K. Labstie, J. Dirrenberger, M. Castro and U. Lafont, Bioinspired 4D printed tubular/helicoidal shape changing metacomposites for programmable structural morphing, Adv. Mater. Technol., 2025, 10, 2570009.
9. G. Li, Z. Guo, Z. Sun, P. Wang, J. Bi, J. Wang, Y. Sha and L. Qian, Strategies for the effective design and regulation of carbon-based metacomposites: a review, J. Mater. Chem. C, 2024, 12, 7146–7161.
10. Z. Leng, Z. Yang, X. Tang, M. Helal, Y. Qu, P. Xie, Z. El-Bahy, S. Meng, M. Ibrahim, C. Yu, H. Algadi, C. Liu and Y. Liu, Progress in percolative composites with negative permittivity for applications in electromagnetic interference shielding and capacitors, Adv. Compos. Hybrid Mater., 2023, 6, 195.
11. F. Qin, D. Estevez and H. Peng, Design philosophy of metacomposite materials based on functional fibers, Mater. China, 2019, 38, 319–340.
12. J. Xiao, B. Zhan, M. He, X. Qi, X. Gong, J. Yang, Y. Qu, J. Ding, W. Zhong and J. Gu, Interfacial polarization loss improvement induced by the hollow engineering of necklace-like PAN/carbon nanofibers for boosted microwave absorption, Adv. Funct. Mater., 2024, 35, 2316722.
13. Y. Qu, Y. Zhou, Y. Luo, Y. Liu, J. Ding, Y. Chen, X. Gong, J. Yang, Q. Peng and X. Qi, Universal paradigm of ternary metacomposites with tunable epsilon-negative and epsilon-near-zero response for perfect electromagnetic shielding, Rare Met., 2024, 43, 796–809.
14. J. Xiao, B. Zhan, M. He, X. Qi, Y. Zhang, H. Guo, Y. Qu, W. Zhong and J. Gu, Mechanically robust and thermal insulating nanofiber elastomer for hydrophobic, corrosion-resistant, and flexible multifunctional electromagnetic wave absorbers, Adv. Funct. Mater., 2024, 35, 419266.
15. Q. Liang, M. He, B. Zhan, H. Guo, X. Qi, Y. Qu, Y. Zhang, W. Zhong and J. Gu, Yolk–shell CoNi@N-doped carbon-CoNi@CNTs for enhanced microwave absorption, photothermal, anti-corrosion, and antimicrobial properties, Nano-Micro Lett., 2025, 17, 167.
16. Y. Qu, Y. Zhou, Q. Yang, J. Cao, Y. Liu, X. Qi and S. Jiang, Lignin-derived lightweight carbon aerogels for tunable epsilon-negative response, Adv. Sci., 2024, 11, 2401767.
17. Z. Wang, X. Li, L. Wang, Y. Li, J. Qin, P. Xie, Y. Qu, K. Sun and R. Fan, Flexible multi-walled carbon nanotubes/polydimethylsiloxane membranous composites toward high-permittivity performance, Adv. Compos. Hybrid Mater., 2020, 3, 1–7.
18. Z. Wang, K. Sun, Y. Yuan, X. Yao, Q. Hou, C. Song and R. Fan, Silver nanowires epsilon-negative metacomposites in constructing laminated structure meta-capacitors, Small, 2025, 2501848.
19. J. Zhou, X. Li, J. Xu, H. Ye, Y. Wang, M. Zhao and B. Yang, Direct high-throughput fabrication of meta-fiber composites with high strength and malleable toughness via bioinspired assemble-spinning strategy, ACS Appl. Polym. Mater., 2024, 6, 12854–12865.
20. G. Li, Y. Chen and G. Wei, Continuous fiber reinforced meta-composites with tailorable Poisson's ratio and effective elastic modulus: Design and experiment, Compos. Struct., 2024, 329, 117768.
21. K. Sun, M. Zhao, P. Yang, M. Chen, Q. Hou, W. Duan and R. Fan, Negative permittivity of reduced graphene oxide/polyvinylidene fluoride membranous composites adjusted by heat treatment, Rare Met., 2024, 43, 5964–5967.
22. R. Ma, C. Cheng, Y. Liu, J. Wang, J. Zhou, Z. Hu, H. Cui, J. Li and R. Fan, Temperature dependence of negative permittivity behavior in graphene/alumina ceramic metacomposites, J. Eur. Ceram. Soc., 2024, 44, 3012–3019.
23. J. Dai, H. Jiang, Z. Guo and J. Qiu, Tunable epsilon-and-Mu-near-zero metacomposites, Adv. Funct. Mater., 2024, 34, 2308338.
24. J. Chen, G. Qin, Y. Shi, K. Pan, J. Du and J. Qiu, Negative permittivity and negative magnetic susceptibility of polypyrrole nanorings/carbon nanotubes multi-dimensional metacomposites in the radiowave frequency range, Org. Electron., 2022, 104, 106470.
25. Y. Shi, K. Pan, M. Moloney and J. Qiu, Strain sensing metacomposites of polyaniline/silver nanoparticles/carbon foam, Composites, Part A, 2021, 144, 106351.
26. A. Uddin, D. Estevez and F. Qin, From functional units to material design: A review on recent advancement of programmable microwire metacomposites, Composites, Part A, 2022, 153, 106734.
27. Q. Jiang, Y. Qiao, C. Xiang, A. Uddin, L. Wu and F. Qin, Metacomposite based on three-dimensional ferromagnetic microwire architecture for electromagnetic response, Adv. Compos. Hybrid Mater., 2022, 5, 3190–3200.
28. H. Wu, R. Yin, Y. Zhang, Z. Wang, P. Xie and L. Qian, Synergistic effects of carbon nanotubes on negative dielectric properties of graphene–phenolic resin composites, J. Phys. Chem. C, 2017, 121, 12037–12045.
29. K. Sun, C. Wang, J. Tian, Z. Zhang, N. Zeng, R. Yin, W. Duan, Q. Hou, Y. Zhao, H. Wu and R. Fan, Magnetic-driven broadband epsilon-near-zero materials at radio frequency, Adv. Funct. Mater., 2024, 34, 202306747.
30. R. Yin, W. Zhao and L. Qian, Nitrogen doping effect of graphene nanosheets towards metacomposites with tunable negative permittivity, Compos. Commun., 2020, 19, 16–19.
31. Z. Guo, Z. Sun, P. Wang, B. Du and L. Qian, Dual heteroatoms-induced defect engineering in graphene towards metacomposites with epsilon-near-zero response, Appl. Surf. Sci., 2023, 638, 158073.
32. Z. Guo, A. Li, Z. Sun, Z. Yan, H. Liu and L. Qian, rGO/CNTs heterogeneous interface modified by polyelectrolyte towards modulating negative permittivity of meta-composites, Appl. Surf. Sci., 2023, 613, 156074.
33. Y. Zhou, Y. Qu, L. Yin, W. Cheng, Y. Huang and R. Fan, Coassembly of elastomeric microfibers and silver nanowires for fabricating ultra-stretchable microtextiles with weakly and tunable negative permittivity, Compos. Sci. Technol., 2022, 223, 109415.
34. P. Xie, Z. Zhang, Z. Wang, K. Sun and R. Fan, Targeted double negative properties in silver/silica random metamaterials by precise control of microstructures, Research, 2019, 11, 1021368.
35. Y. Qu, Y. Chen, J. Ding, Q. Peng and X. Qi, Epsilon-negative response induced by magnetic/dielectric dual percolation in Ag/Bi₂₅FeO₄₀ metacomposites, Appl. Phys. Lett., 2025, 127, 041703.
36. J. Tian, R. Fan, P. Yang, H. Wu, S. Jiang, Y. Zhou and A. Sarychev, Flexible silver nanorods/carbon fiber felt metacomposites with epsilon-near-zero property adjusted by compressive deformation, Rare Met., 2023, 42, 3318–3325.
37. Z. Hu, C. Chen, H. Cui, M. Liu, M. Liu, X. Wang, Y. Liu, C. Li, Y. Li, Y. Zhao, Z. Wang and R. Fan, Adjustable negative permittivity behavior of high entropy (Ti_0.25Zr_0.25Nb_0.25Ta_0.25)C ceramic with different sintering temperatures, J. Eur. Ceram. Soc., 2025, 45, 117178.
38. R. Ma, C. Cheng, J. Wang, X. Hu and R. Fan, Tunable negative permittivity behavior in alumina ceramic composites with different carbon fillers, Ceram. Int., 2025, 51, 2043–2051.
39. A. Uddin, F. Qin, D. Estevez, S. Jiang, L. Panina and H. Peng, Microwave programmable response of Co-based microwire polymer composites through wire microstructure and arrangement optimization, Composites, Part B, 2019, 176, 107190.
40. D. Estevez, F. Qin, Y. Luo, L. Quan, Y. Mai, L. Panina and H. Peng, Tunable negative permittivity in nano-carbon coated magnetic microwire polymer metacomposites, Compos. Sci. Technol., 2019, 171, 206–217.
41. Y. Luo, D. Estevez, F. Scarpa, L. Panina, H. Wang, F. Qin and H. Peng, Microwave properties of metacomposites containing carbon fibres and ferromagnetic microwires, Research, 2019, 3239879.
42. Y. Luo, F. Qin, F. Scarpa, J. Carbonell, M. Ipatov, V. Zhukova, A. Zhukov, J. Gonzalez, L. Panina and H. Peng, Left-handed metacomposites containing carbon fibers and ferromagnetic microwires, AIP Adv., 2017, 7, 056110.
43. Y. Luo, F. Qin, J. Liu, H. Wang, F. Scarpa, P. Adohi, C. Brosseau and H. Peng, Dielectric properties of composites containing melt-extracted Co-based microwires, Compos. Commun., 2016, 1, 20–24.
44. X. Zhu, Y. Qu, X. Qi and Y. Liu, Negative permittivity and ε‘-near-zero responses tuned by multi-level heterogeneous interface in ternary metacomposites, Ceram. Int., 2025, 51, 35046–35055.
45. X. Tang, Z. Zhang, P. Xie, Q. Hou, G. Liang, Z. Ni, Q. Huang and H. Wu, Theoretically designed epsilon-negative materials: Ni nanoparticles encapsulated CNTs and their magnetic driven properties, Chem. Eng. J., 2024, 488, 150899.
46. Y. Liu, C. Cheng, J. Zou, J. Fu, J. Wang, J. Zhou, R. Ma, H. Cui, Z. Hu, T. Wang, Y. Du and R. Fan, Highly tunable negative permittivity of carbon nanofiber/alumina metacomposites at different external temperatures, Composites, Part A, 2023, 173, 107660.
47. K. Pan, S. Yu, J. Du, Y. Li and J. Qiu, Neuromorphic double-negative metacomposites with fano resonance and photonic skin characteristics, Adv. Funct. Mater., 2025, e13091.
48. Q. Men, S. Wang, Z. Yan, B. Zhao, L. Guan, G. Chen, X. Guo, R. Zhang and R. Che, Iron-encapsulated CNTs on carbon fiber with high-performance EMI shielding and electrocatalytic activity, Adv. Compos. Hybrid Mater., 2022, 5, 2429–2439.
49. X. Ye, Y. Chen, J. Yang, H. Yang, D. Wang, B. Xu, J. Ren, D. Sridhar, Z. Guo and Z. Shi, Sustainable wearable infrared shielding bamboo fiber fabrics loaded with antimony doped tin oxide/silver binary nanoparticles, Adv. Compos. Hybrid Mater., 2023, 6, 106.
50. J. Bi, Z. Sun, Z. Guo, S. Tian, G. Li and L. Qian, Negative permittivity enhanced reflection and adsorption of electromagnetic waves from carbon fiber felt/carbon nanotubes, J. Mater. Chem. C, 2024, 12, 13974–13984.
51. H. Granbohm, J. Larismaa, S. Ali, L. Johansson and S. Hannula, Control of the size of silver nanoparticles and release of silver in heat treated SiO₂-Ag composite powders, Materials, 2018, 11, 80.
52. W. Wang, Y. Jiang, S. Wen, L. Liu and L. Zhang, Preparation and characterization of polystyrene/Ag core–shell microspheres – A bio-inspired poly(dopamine) approach, J. Colloid Interface Sci., 2012, 368, 241–249.
53. G. Li, Z. Guo, Z. Sun, J. Bi, J. Wang, Y. Sha and L. Qian, Honeycomb network structure constructed by silver nanoparticles achieving negative permittivity at low percolation threshold, Mater. Today Phys., 2024, 46, 101521.
54. X. Tang, F. Xie, Y. Lu, Z. Chen, X. Li, H. Li, X. Huang, L. Chen, Y. Pan and Y. Hu, Intrinsic effects of precursor functional groups on the Na storage performance in carbon anodes, Nano Res., 2023, 16, 12579–12586.
55. C. Cheng, Y. Jiang, X. Sun, J. Shen, T. Wang, G. Fan and R. Fan, Tunable negative permittivity behavior and electromagnetic shielding performance of silver/silicon nitride metacomposites, Composites, Part A, 2020, 130, 105753.
56. H. Gu, X. Xu, M. Dong, P. Xie, Q. Shao, R. Fan, C. Liu, S. Wu, R. Wei and Z. Guo, Carbon nanospheres induced high negative permittivity in nanosilver-polydopamine metacomposites, Carbon, 2019, 147, 550–558.
57. M. Zhou, J. Wang, G. Wang, Y. Zhao, J. Tang, J. Pan and G. Ji, Lotus leaf-inspired and multifunctional Janus carbon felt@Ag composites enabled by in situ asymmetric modification for electromagnetic protection and low-voltage joule heating, Composites, Part B, 2022, 242, 110110.
58. Q. Chávez, F. Godínez, F. Méndez and A. Aguilar, Prediction of temperature profiles and ampacity for a monometallic conductor considering the skin effect and temperature-dependent resistivity, Appl. Therm. Eng., 2016, 109, 401–412.
59. X. Wang, S. Liao, Y. Wan, P. Zhu, Y. Hu, T. Zhao, R. Sun and C. Wong, Electromagnetic interference shielding materials: Recent progress, structure design, and future perspective, J. Mater. Chem. C, 2022, 10, 44–72.
60. M. Chen, M. Gao, F. Dang, N. Wang, B. Zhang and S. Pan, Tunable negative permittivity and permeability in FeNiMo/Al₂O₃ composites prepared by hot-pressing sintering, Ceram. Int., 2016, 42, 6444–6449.
61. L. Zhu, Exploring strategies for high dielectric constant and low loss polymer dielectrics, J. Phys. Chem. Lett., 2014, 5, 3677–3687.
62. Y. Thakur, T. Zhang, C. Iacob, T. Yang, J. Bernholc, L. Chen, J. Runt and Q. Zhang, Enhancement of the dielectric response in polymer nanocomposites with low dielectric constant fillers, Nanoscale, 2017, 9, 10992–10997.
63. M. Bordag, Drude model and Lifshitz formula, Eur. Phys. J. C, 2011, 71, 1788.
64. Q. Zhao, L. Kang, B. Du, H. Zhao, Q. Xie, X. Huang, B. Li, J. Zhou and L. Li, Experimental demonstration of isotropic negative permeability in a three-dimensional dielectric composite, Phys. Rev. Lett., 2008, 101, 027402.
65. Y. Zhou, Y. Qu, L. Yin, W. Cheng, Y. Huang and R. Fan, Coassembly of elastomeric microfibers and silver nanowires for fabricating ultra-stretchable microtextiles with weakly and tunable negative permittivity, Compos. Sci. Technol., 2022, 223, 109415.
66. K. Sun, L. Wang, Z. Wang, X. Wu, G. Fan, Z. Wang, C. Cheng, R. Fan, M. Dong and Z. Guo, Flexible silver nanowire/carbon fiber felt metacomposites with weakly negative permittivity behavior, Phys. Chem. Chem. Phys., 2020, 22, 5114–5122.
67. Z. Shi, R. Fan, Z. Zhang, K. Yan, X. Zhang, K. Sun, X. Liu and C. Wang, Experimental realization of simultaneous negative permittivity and permeability in Ag/Y₃Fe₅O₁₂ random composites, J. Mater. Chem. C, 2013, 1, 1633.
68. M. Han, Z. Shi, W. Zhang, K. Zhang, H. Wang, D. Dastan and R. Fan, Significantly enhanced high permittivity and negative permittivity in Ag/Al₂O₃/3D-BaTiO₃/epoxy metacomposites with unique hierarchical heterogeneous microstructures, Composites, Part A, 2021, 149, 106559.
69. G. Li, Z. Sun, Z. Guo, P. Wang, B. Du, S. Tian, H. Ding, Y. Qiu, J. Bi and L. Qian, Dynamic modulation of permittivity properties via compression of carbon nanotube-impregnated cotton for wide epsilon-near-zero bandwidth, Adv. Compos. Hybrid Mater., 2024, 7, 83.
70. R. Newnham, D. Skinner and L. Cross, Connectivity and piezoelectric–pyroelectric composites, Mater. Res. Bull., 1978, 13, 525–536.

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