Role of conducting filler on electromagnetic shielding performance of CaCu₃Ti₄O₁₂/CoFe₂O₄/Al/silicone composites over wide frequency range

Abstract

Electromagnetic interference (EMI) has become a critical issue in modern electronic devices, necessitating effective shielding materials for mitigating its adverse effects. This study investigates the electromagnetic shielding properties of CaCu₃Ti₄O₁₂(CCTO)/CoFe₂O₄ (CFO)/Aluminium(Al)/silicone composites over a broad frequency range, focusing on the influence of a conducting filler (Al) on their performance. The composites are fabricated through compression moulding involving the dispersion of CCTO/CFO/Al nanoparticles within a silicone matrix, with varying concentrations of conducting filler. The composites are formulated with varying ratios of Al to silicone with CCTO and CFO kept fixed by weight, denoted as CCAS-1, CCAS-2, CCAS-3, and CCAS-4 composites. The electromagnetic shielding effectiveness (SE) of the composites is evaluated using a vector network analyzer over the frequency range of 8 GHz to 22 GHz. Notably, the CCAS-4 composite exhibited a total shielding effectiveness (SE_T) of approximately 12 dB, corresponding to 94% microwave attenuation. This remarkable enhancement in shielding efficiency and SE is attributed to synergistic contributions of electrical conductivity, interfacial polarization, and magnetic property of the composite. These findings underscore the potential of CCAS composite can be an effective electromagnetic shielding material with promising applications in various technological domains.

Introduction

Divulge of confidential information due to electromagnetic interference (EMI) have emerged as serious problem with tremendous growth in electrical and electronics devices^1,2. Electromagnetic interference (EMI) has emerged as a significant concern due to the rapid proliferation of electronic and communication systems. EMI is generated by a wide range of sources, including wireless devices, power electronics, and high-frequency systems. Most common consumer electronics, such as mobile phones, Wi-Fi, and Bluetooth devices, typically operate in lower frequency ranges (kHz–GHz). In contrast, higher microwave frequency bands, such as the X-band (8–12 GHz) and Ku-band (12–18 GHz), are primarily utilized in radar, satellite communication, and advanced aerospace and defense technologies³. Uncontrolled electromagnetic emissions can adversely affect the performance of nearby electronic systems, leading to reduced accuracy and operational instability. Common manifestations of EMI include signal distortion in communication devices, image fluctuations in display systems, and interference in televisions, laptops, and mobile phones caused by surrounding electronic equipment or natural sources such as lightning⁴. Prolonged exposure to electromagnetic radiation may also pose potential health risks, particularly for individuals working in close proximity to high-intensity EM sources. Therefore, the development of efficient EMI shielding materials has become essential to mitigate electromagnetic pollution. Shielding materials function by reflecting and absorbing incident electromagnetic waves, thereby preventing their propagation and ensuring the reliable operation of electronic systems as well as environmental safety^5,6.

Flexible materials are now the preferred option for EMI shielding in electronics, replacing metal-based materials that are dense and prone to corrosion. Recently, significant attention has been devoted to advanced EMI shielding materials based on liquid metals, graphene hybrids, and polymer composites, owing to their excellent electrical conductivity, flexibility, and multifunctionality. For instance, graphene-encapsulated liquid metal composites and liquid metal/polymer systems have demonstrated promising EMI shielding performance along with stretchability and self-healing capabilities^7–10. Elastomers containing conductive, magnetic, and dielectric particles provide excellent shielding while covering the maximum surface area, making them suitable for mobile components, intricate shapes, and delicate circuits. Silicone rubber is a particularly promising elastomer due to its flexibility, resistance to corrosion and, good stretching strength¹¹. Adding various fillers (hybrid material) enhances the shielding effectiveness by increasing the number of multiple reflections inside the shield’s interior particles. Factors like high conductivity, high dielectric constant, high permeability, and low dielectric and magnetic loss affect EMI shielding capabilities¹². Interestingly, spinal ferrite-CoFe₂O₄ (CFO) has minimal magnetic loss, with absorption being mostly determined by their high permeability and large magnetic dipoles. The CaCu₃Ti₄O₁₂ (CCTO) has a massive dielectric constant and increased space charge polarization, reducing unwanted electromagnetic radiation in the surrounding area through electric dipoles. The CFO and CCTO are recently emerged as viable candidate for attenuation of EM wave in microwave region.

As a prerequisite to different material, it is crucial to investigate the EMI shielding properties of conducting filler (Aluminium-Al) based material which evolved a lightweight, low cost material. These materials primarily evolved as good shielding material due to high conductivity. In quest to attenuate EM radiations, conducting, dielectric and magnetic materials are explored for enhanced EMI shielding capability. Carbon-based and magnetic nanostructured materials have been widely explored for EMI shielding applications due to their combined dielectric and magnetic loss mechanisms. A comprehensive study is apprehended by Xu et al.¹³ on the EMI shielding properties of Al based foam. The Al foams exhibited the EMI SE of 75 dB in 130–1800 MHz frequency. Medjaouri et al.¹⁴ investigated the EMI shielding of surface modified Al₂O₃ particles incorporated in epoxy matrix. For instance, reduced graphene oxide reinforced iron oxide nanostructures and Fe₃S₄–carbon black nanohybrids have demonstrated effective EMI shielding performance in the microwave region, owing to synergistic interactions between conductive and magnetic components. These studies highlight the importance of hybrid systems in achieving enhanced attenuation through multiple loss mechanisms. Similar to previously reported carbon–magnetic hybrid systems, the present composites utilize combined dielectric and magnetic fillers to achieve EMI attenuation^15–18.

In present work, novel CCTO/CFO/Al/silicone flexible composites are fabricated by using compression molding method and their EMI shielding effectiveness is investigated for different frequency range. CCTO and CFO played a vital role in attenuating EM radiations, aluminium particles are seeped into the silicone matrix to form conductive network which facilitate the transfer of electrons of EM wave to the magnetic and dielectric particles. The agglomerated dielectric CCTO particles and magnetic CFO particles capture some of itinerary electrons of Al ions at the grain boundaries which hinder the travelling of EM wave out of shield i.e. low transmission of wave, thereby enhancing the EMI shielding capability. Here, influence of different content of Al on EMI shielding properties is investigated. Meanwhile, hardness of composites is also examined in detail.

Experimental

Copper nitrate trihydrate (Cu(NO₃)₂. 3H₂O), calcium nitrate tetrahydrate (Ca(NO₃)₂.4H₂O), are purchased from LOBA Chemie, titanium (IV) isopropoxide (C₁₂H₂₈O₄Ti) is purchased from Sigma Aldrich, ferric chloride hexahydrate (FeCl₃.6H₂O), sodium hydroxide (NaOH), cobalt chloride hexahydrate (CoCl₂.6H₂O) are purchased from CDH, silicone resin is purchased from Aahkels Premium, anhydrous citric acid, acetylacetone, Al particles are purchased from Sigma Aldrich of 0.6 mm of 99.9% purity distilled water (DI) prepared in laboratory used without further purification and modification.

Synthesis of CaCu₃Ti₄O₁₂ and CoFe₂O₄

The processes for synthesis of CCTO and CFO nanoparticles are reported in our previous literature^19,20. Where, acetylacetone (C₅H₈O₂, 99%+) and ethylene glycol is used as starting materials. Initially, 1.93 g (0.010 M) of citric acid is dissolved in distilled water with continuous stirring for 10 min. Separately, 3.09 mL (0.010 M) of titanium precursor is mixed with 0.90 mL (0.010 M) of acetylacetone under constant stirring for 5 min. Ethylene glycol and the citric acid solution are then added sequentially to the titanium precursor solution and stirred for 30 min to obtain solution (1) In parallel, 0.59 g (0.025 M) of calcium nitrate tetrahydrate and 1.77 g of copper nitrate trihydrate are dissolved in 20 mL distilled water with stirring for 30 min to form solution (2) This solution is added to solution 1 under continuous stirring, producing a translucent green solution. A few drops of aqueous ammonia are introduced, turning the mixture dark blue. The solution is heated at 80 °C to evaporate water, forming a blue gel, which further dried at 150 °C for 15 min. The resulting porous mass is ground into a fine black powder and finally calcined at 950 °C for 8 h to obtain CCTO.

CFO nanoparticles are synthesized using the co-precipitation method. Stoichiometric amounts of FeCl₃·6 H₂O (2 mol), CoCl₂·6 H₂O (1 mol), and NaOH (8 mol) are separately dissolved in 100 mL of deionized water under continuous stirring. The mixed chloride precursor solution is then added dropwise to the NaOH solution, resulting in the formation of a brown precipitate. The mixture is further stirred for 30 min to ensure homogeneity. The precipitate is washed and centrifuged multiple times to remove residual salts, followed by drying and calcination at 200 °C for 2 h to eliminate impurities. Finally, fine cobalt ferrite nanoparticles are obtained.

Briefly, CCTO and CFO particles exhibit average sizes in the nanometer range (∼200 nm), CFO- (50 nm) respectively as determined from TEM analysis. The aluminium (Al) powder used in this study is of micron-scale particle size, as specified by the supplier. The particle size and associated surface area play a crucial role in governing interfacial polarization and conductive network formation within the composite matrix.

Fabrication of CCTO/CFO/Al/ silicone composite

The CCTO/CFO/Al/silicone flexible, stretchable and thin composites are fabricated by using compression moulding at room temperature. The appropriate proportion of CCTO, CFO, and Al are weight and mixed properly via mortar pestle and added into silicone resin to form slurry. The slurry is then mechanically mixed to get uniform dispersability and bubble free casting. After addition of curing agent, the slurry is poured into a circular shape compression moulder and compressed by hydraulic press so that silicone is forced to flow within cavity. The final product obtained as 0.5 mm thick circular film. The composite of CCTO/CFO/Al/silicone is abbreviated as CCAS composites. Different composites of silicone elastomers are obtained by varying compositions of aluminium powder as shown in Table 1.

Table 1.

Composition of CCAS composites.

Sample	CCTO (%)	CFO(%)	Al (%)	Silicone(%)
CCAS-1	25	25	0	50
CCAS-2	25	25	5	45
CCAS-3	25	25	10	40
CCAS-4	25	25	15	35

Characterizations

Prepared powders are analysed by D8 Advance Bruker X-ray diffractometer over the 2θ range of 20◦-80◦. Morphology of CCAS composites is obtained by Nova NanoSEM 450 field emission scanning electron microscope (FESEM). HIOKI IM 3536 LCR meter is used to investigate the dielectric and electrical properties of composites. The EMI shielding performance of CCTO composites are measured by using agilent make PNA N5222A using waveguide method. The network analyzer is properly calibrated for elimination of instrumental errors. The samples are enclosed between two waveguide 90 flanges. Shielding effectiveness due to absorption (SE_A) and reflection (SE_R) of composites are stimulated by scattering parameters S11 and S21 obtained from PNA as follows^21,22

1

2

3

4

The total shielding effectiveness of a material is generally expressed as the sum of contributions from reflection absorption and multiple internal reflections:

5

In many cases, the contribution of multiple reflections (SE_Mr) is neglected when the total shielding effectiveness exceeds ~ 15 dB, as the internally reflected waves undergo significant attenuation within the material. Under such conditions, the simplified relation,

6

is considered valid. However, in the present study, the observed SE_T values (~ 12 dB) are below this threshold. Therefore, neglecting the multiple-reflection term may not be strictly appropriate, and the calculated values of SE_R and SE_A should be considered approximate. It is expected that multiple internal reflections within the heterogeneous CCTO/CFO/Al/silicone matrix contribute to the overall attenuation, particularly due to interfacial polarization and impedance mismatch at filler–matrix boundaries. Despite this limitation, the simplified approach provides a reasonable qualitative understanding of the shielding mechanisms in the composites.

The EMI shielding efficiency (%) is obtained by using the given relation.

7

Results and discussion

Structural properties

It is evident from Fig. 1a that the CCTO sample exhibits sharp and well-defined diffraction peaks, indicating good crystallinity and phase formation. The prominent peaks corresponding to (220), (400), and (422) planes confirm the formation of cubic perovskite CCTO structure. The high intensity and narrow width of the peaks suggest improved crystallinity due to calcination at 950 °C. No additional impurity peaks corresponding to secondary phases such as CaTiO₃ or CuO are observed, indicating that the stoichiometric ratio is well maintained during synthesis. The overall diffraction pattern is in good agreement with standard data ( JCPDS file no.75-2188) confirming the formation of phase-pure CCTO. The absence of peak broadening further suggests relatively larger crystallite size compared to typical nanoscale materials. Thus, the XRD results confirm the successful synthesis of highly crystalline CCTO suitable for further composite fabrication.

Fig. 1.

XRD pattern of CCTO and CFO.

The XRD pattern of CFO (Fig. 1b) confirms the formation of a cubic spinel structure. The prominent peak at 2θ ≈ 35.4° corresponds to the (311) plane, while additional peaks at ~ 30°, 43°, 57°, and 62–64° are indexed to (220), (400), (511), and (440) planes, respectively, matching standard data. The absence of impurity peaks indicates phase-pure CFO. The relatively broad diffraction peaks suggest the nanocrystalline nature of the material, attributed to the co-precipitation method. This nanoscale feature enhances surface area and interfacial polarization, which is beneficial for magnetic loss mechanisms. Consequently, CFO contributes significantly to electromagnetic wave attenuation in the composite system.

Microstructural properties

Figure 2 shows the surface morphology of CCAS composites. As evident from graphs, CFO, CCTO, and Al exhibited good dispersion in silicone matrix however some agglomeration of particles is also observed. For CCAS-1 composite, large CCTO granular can be seen. As the Al loading increase, the surface of sample become rough with bumping of large granules. Distribution of Al particles in CCAS composite increases the formation of conductive network with reduced gap between the particles which inhibiting the attenuation of EM wave²³.

Fig. 2.

FESEM images of CCAS composites.

In addition, roughness of composite’s surface increases with decrease in content of silicone and with increase in Al content to 15wt% as clearly visible in Fig. 1d for CCAS-4 composite.

Figure 3 demonstrates the TEM graph of CCTO and CFO separately. The CCTO particles exhibit irregular morphology with noticeable agglomeration and an average particle size of ~ 200 nm. The agglomeration is attributed to high surface energy and particle coalescence during the high-temperature calcination process²⁴. Distinct grain boundaries indicate the crystalline nature of the particles.

Fig. 3.

Transmission electron microscope (TEM) image of (a) CCTO (b) CFO particles at 200 nm and 50 nm scale respectively.

The significant size disparity among these fillers leads to a hierarchical microstructure, which is advantageous for electromagnetic attenuation. The CFO nanoparticles show quasi-spherical morphology with a relatively uniform size distribution and an average particle size of ~ 50 nm. The smaller size is associated with the co-precipitation synthesis route, which enables controlled nucleation²⁵. CFO nanoparticles provide a high specific surface area, resulting in an increased number of interfaces that enhance interfacial polarization. In contrast, the relatively larger CCTO particles contribute to bulk dielectric polarization due to their high dielectric constant. Furthermore, the presence of micron-sized Al particles facilitates the formation of conductive pathways at lower percolation thresholds by bridging the gaps between ceramic particles. This multi-scale particle distribution improves filler packing efficiency, reduces interparticle distance, and promotes the formation of an interconnected conductive network. Consequently, the synergistic interaction between particle size, surface area, and spatial distribution significantly enhances charge accumulation at interfaces, electrical conductivity, and overall EMI shielding performance of the composites.

Dielectric, and electrical properties

The electrical, and dielectric properties of the composites were evaluated to gain further insight into the EMI shielding mechanism. As shown in Fig. 4a, the electrical conductivity increases systematically from CCAS-1 to CCAS-4 with increasing filler content. This behavior indicates the formation of improved conductive pathways within the composite due to the incorporation of Al particles, which enhances charge transport and contributes to conduction loss and reflection of electromagnetic waves. Figure 4b presents the frequency-dependent dielectric constant of the composites. The relatively high dielectric constant, (~ 350) particularly at higher filler loading, can be attributed to the presence of CCTO and the associated interfacial polarization effects. The decrease in dielectric constant with increasing frequency is characteristic of polarization relaxation, indicating reduced dipolar response at higher frequencies. These dielectric properties contribute to energy dissipation through polarization loss mechanisms may leads to good EMI shielding performance. Similar synergistic effects between conductive and magnetic fillers have been reported by Govindasamy^26–29 where, iron oxide nanoparticle and MWCNT-based composites, leading to enhanced EMI shielding performance.

Fig. 4.

(a) Electrical conductivity and (b) dielectric constant of CCAS composites.

Electromagnetic shielding performance

Figure 5 depicted the effect of different contents of Al on EMI shielding performance of CCAS composites in X-band. The CCAS composites depicted frequency dependant nature. Shielding performance of CCAS composites are measured for 0.5 mm thick samples. Additionally, the silicone rubber is significant parameter for considering CCAS composites for potential applications as it provides high flexibility, and strength. It can be said that significant increase in electrical conductivity with increase in Al content, may render an improvement in shielding performance. Interestingly, for the EMI shielding application of the CCAS composites, the Al³⁺ must retain its superior electrical conductivity^30,31. The composites without Al content exhibited SE_R ~ 0.5 dB and SE_A ~ 0.3 dB at 8 GHz frequency. However, a hump for CCAS-1 is observed from 9.5 GHz to 10.5 GHz frequency where SE_A ~ 0.9 dB. This uneven behaviour of CCAS-1 composite is due to heterogeneous distribution of CCTO and CFO particles that leads to impedance matching and absorption of impinged wave by particles inside the shield. Addition of 15wt% Al content increases the SE from 0.8 dB to 6.5 dB that is corresponding to 76% shielding efficiency as shown in Fig. 5d. However, the interaction of electromagnetic waves with the CCTO/CFO/Al/silicone composites involves reflection at the surface, transmission into the material, and subsequent attenuation through absorption and internal reflections. Upon incidence, a portion of the wave is reflected due to impedance mismatch, primarily influenced by the conductive Al network, while the remaining wave penetrates the material and is attenuated via dielectric polarization (CCTO), magnetic losses (CFO), and conductive losses.

Fig. 5.

EMI shielding (a) SE_R, (b) SE_A, (c) SE_T, and (d) shielding efficiency of CCAS composites in 8–12 GHz.

The frequency-dependent shielding behavior is governed by impedance matching and skin-depth effects, which control the balance between reflection and absorption. In addition, the periodic oscillations observed in the shielding effectiveness curves arise from interference effects caused by multiple internal reflections within the finite-thickness sample. This behavior is further influenced by the high dielectric nature of CCTO, which reduces the wavelength inside the material (λ = λ₀/√ε_r), leading to enhanced phase differences and more pronounced constructive and destructive interference at specific frequencies^32,33.

Figure 6 demonstrate the shielding effectiveness in frequency 12–15 GHz where SE_R shows linear behaviour i.e. no discernable change in SE of composites. It is clear from the graphs as the loading of conductive filler increase with ceramics in the matrix, the SE_T shows a noticeable improvement from 2 dB to 6.5 dB (Fig. 6c). The combine interactions of Al particles with CCTO and CFO particles are responsible for higher EMI SE³⁴. For the CCAS composite, the shielding value are ~ 8 dB at 15 GHz which responsible for ~ 80% of attenuation efficiency of EM wave. In these composites, SE due to reflection is larger as compare to SE due to absorption; It should be noted that since SET values are below 15 dB, multiple internal reflections may also contribute to the overall shielding effectiveness.

Fig. 6.

EMI shielding performance of CCAS composites in frequency 12–15 GHz.

Figure 7 depicted the EMI shielding of CCAS composites at frequency 15–18 GHz. It is observed from the graphs, EMI shielding performance increase with increase in Al loading which corroborate a non-linear trend of SE. The EMI SE is significantly ascribed by reflection and absorption. High silicone content and low Al content depicted lower shielding. This behavior is result of non-electrically conductive silicone, which decreases the contact area of Al filler, leading to barrier effect between CCTO, CFO, and Al particles within silicone matrix. CCAS composites shows increasing trend of EMI SE (~ 11 dB) with increase in Al content to 15wt%.

Fig. 7.

EMI shield performance of CCAS composites in 15–18 GHz.

Almost similar value to SE_A and SE_R are obtained which demonstrate that both the mechanism is considerably prevalent at frequency range 18–22 GHz as shown in Fig. 8 Meanwhile, value of SE elevated gradually with increase in Al content up to 15 wt %. By adjusting Al content in CCAS composites, about 94% of wave can be blocked. There is increase in SE_T at higher frequency for CCAS composites. Enhancement in EM shielding up to 12 dB is due to increased % of Al. It is worth noting that SE value of CCAS-3 and CCAS-4 is almost comparable. This may be because addition of larger % of Al with CCTO and CFO particles in silicone matrix leads to formation of conductive network of charge carriers. These free charges are drifted towards absorbing material and captured into the junction of matrix where absorbing particles are significantly accumulated³⁵. Therefore, this is plausibly specifying the requirement of minimum electrical conductivity to obtained desirable absorption dominant EMI shielding. Multiple internal reflections are formed due to multiple conductive loops formed by Al network inside the matrix which help increasing the shielding effectiveness.

Fig. 8.

EMI shielding performance of CCAS composite in 18–22 GHz.

The EM shielding properties of CCAS composites are studied at different frequency bands. To understand the shielding mechanism of CCAS composites, EM wave is impinged on the surface of CCAS composites some part of wave is reflected due to large accumulation of charge particles of Al filler at surface of CCAS composites and residual part is enter inside the shield and attenuated by interfacial loss derived from CCTO and CFO particles. It is clear that in band (8–18 GHz), SE_R found to be larger as compare to SE_A, with larger content of Al. However, SE_A is continuously increasing from 8 GHz to 22 GHz. At higher frequency band, the SE_A found to be higher than as SE_R. It is well known, reflection is mainly dominated due to higher electrical conductivity of shield’ surface and absorption are primarily arisen due to large nomadic charges (arise due to absorption of photon energy by bound electrons) and interfacial polarization in the shield³⁶. However, at frequency (18–22 GHz), incremental loading of Al will result in expand in conductive paths. Therefore, interface area and electric dipoles in CCAS composites rapidly decayed transforming incident EM wave energy to heat energy. In addition, conversion of EM energy into heat is due to no-zero resistance of eddy current. Consequently, it is harder for EM waves (not reflected wave) to escape from CCAS composites with higher Al content, resulting in larger SE_A value. In addition, heterogeneous junction of CCTO/CFO/Al is appearing inside shield, where large accumulation of particles facilitates the attenuation of wave via internal multiple reflections. The observed shielding values ascribed to impedance matching between targeted EM wave and shield. Larger Al content at higher frequency increases the SE and % shielding efficiency from 76% to 94%. This demonstrates how higher frequency increases the EMI SE due to prominent electrical conductivity of Al, CCTO’s dielectric characteristics and CFO’s magnetic characteristics^37,38. It is important to note that the observed improvement in EMI shielding effectiveness with increasing Al content is accompanied by a simultaneous reduction in the silicone matrix fraction. As a result, the enhancement in shielding performance cannot be attributed solely to the increased Al loading. Instead, it arises from the combined effects of higher conductive filler concentration, improved percolation network formation, and a reduced proportion of the insulating polymer matrix, all of which contribute to enhanced electrical conductivity and electromagnetic attenuation. With increasing frequency, reduced skin depth enhances attenuation, while improved impedance matching allows greater wave penetration into the material, increasing internal absorption.

Figure 9 presents a conceptual phase diagram describing the interplay between electrical conductivity, dielectric constant, and magnetic permeability in determining the EMI shielding behavior of the composites. The axes represent relative variations (low to high) of these parameters rather than absolute quantitative values, as the exact thresholds depend on material composition, frequency, and microstructural characteristics. In general, materials with high electrical conductivity tend to exhibit reflection-dominated shielding due to impedance mismatch at the surface, whereas materials with balanced dielectric and magnetic properties promote absorption-dominated shielding through improved impedance matching and energy dissipation. The present CCTO/CFO/Al/silicone composites lie in an intermediate regime, where the combined contributions of conductive, dielectric, and magnetic components facilitate moderate shielding effectiveness. This fig. serves as a qualitative framework to guide the design of EMI shielding materials, emphasizing that optimal performance can be achieved by tuning conductivity to exceed the percolation threshold while maintaining sufficient dielectric and magnetic losses for effective absorption.

Fig. 9.

Design guidance for absorbing and shielding material by a relationship between conductivity, dielectric and magnetic properties with impedance match.

Hardness

The mechanical behaviour of material after curing is clearly understood by hardness test. The composites are cured at room temperature. Figure 10 shows the effect of filler content on hardness of material. The result clearly indicates that hardness of material varied with filler content in silicone matrix. The silicone matrix is much softer with reduced elastic modulus which substantially tailored by varying filler content. The hardness achieved by CCTO and CFO sample (CCAS-1) is about 78^o whereas reinforcement of Al increase the hardness by 7˚ as that of CCAS-1 i.e. a discernible change can be seen. Moreover, hardness of material tends to enhanced by addition of Al filler. About 78^o hardness is achieved by composite without Al content however greater hardness of composite is witnessed with reinforcement of filler. Inclusion of aluminium with increased wt% from 5 to 15 increases the hardness of composites linearly that means filler led to stiffening and reinforcing effect³⁹. Sample ‘CCAS-4’ turned out to be formulation of highest filler content shows maximum hardness in which 15wt% Al played vital role for improving mechanical tenacity. Moreover, higher hardness value arises due to increase in cross linking density of silicone (((CH₃)₂ Si-O)_n) and filler⁴⁰. The hardness is one of important property of materials as it breaks the deposited film of water/moisture on the material surface. Furthermore, large filler content attributed to better filler-matrix interaction which synergistically affected the hardness. Good degree of hardness is accentuated by addition of filler which decreases the micro surface defects. These composites can provide good resistance to penetration and deformation. The higher loading of filler renders diversion of applied force. The pattern obtained for hardness is in agreement with Kumar et al. hardness of silicone rubber composites depend on amount of inclusion of filler content (graphene oxide and iron oxide)⁴¹.

Fig. 10.

Shore A hardness of CCAS composites.

Conclusion

In this study, flexible CCTO/CFO/Al/silicone composites were successfully fabricated and evaluated for EMI shielding performance over the frequency range of 8–22 GHz. The incorporation of Al as a conductive filler resulted in a measurable enhancement in shielding effectiveness, with a maximum SET of ~ 12 dB (~ 94% attenuation) observed for the highest filler loading. The improvement in shielding behavior is attributed to combined effects of electrical conductivity, interfacial polarization, and dielectric–magnetic loss mechanisms. Despite this improvement, the overall shielding effectiveness remains moderate compared to typical application benchmarks (~ 20 dB). The present composites may therefore be more suitable for applications requiring lightweight, flexible materials with moderate EMI shielding, such as wearable electronics or low-intensity EMI environments. these composites can function effectively as lightweight materials or as secondary layers in multi-layer shielding systems. Further enhancement in shielding performance is expected through optimization of filler loading, improved dispersion, and development of more efficient conductive networks within the polymer matrix.

Author contributions

Neelam Kumari: Methodology, Validation, Formal analysis, Writing – original draft.  Amena Salim: Collecting resources.  Rahul Singhal: Validation, Supervision. Vishant Gahlaut: Validation, Data curation.  Jigar Limbachiya: Funding acquisition. Ravi Hegde: Funding acquisition. Bhuwaneshwar Semwal: Funding acquisition. Pushpendra Kumar: Funding Acquisition, Validation, Umesh Kumar Dwivedi: Conceptualization, Validation, Formal analysis, Writing – review & editing, Supervision.

Funding

Open access funding provided by Manipal University Jaipur.

Data availability

The datasets generated during current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors affirm the following financial interest/personal relationship which may be considered as potential competing interests: Dr. Umesh Kumar Dwivedi, Amity University Rajasthan confirms a relationship with Indian Space Research Organization and its Regional Academic Centre for Space (RAC-S), and Amity University Rajasthan.

Ethical approval

The authors declare that the submitted manuscript is original. They acknowledge the current research has been conducted ethically, and the final shape of the research has been agreed upon by all authors.

Consent to participate

The authors provide consent to participate in this study.