Preparation and Electromagnetic Interference Shielding Performance of TPU/MWCNT/BiFeO₃ Composites

1. Introduction

Numerous studies have systematically investigated the effects of varying MWCNT loadings on the microstructure, mechanical properties and electromagnetic interference shielding effectiveness of binary TPU/MWCNT composites [1,2,3]. Relevant results demonstrate that as MWCNT loading increases, continuous and intact conductive networks are gradually constructed within the matrix, accompanied by a simultaneous improvement in the overall shielding effectiveness of the materials [4]. Once the MWCNT loading exceeds the percolation threshold, the composites deliver outstanding shielding performance across the Ku band (12.4–18.0 GHz), which fully verifies that conductive loss acts as the dominant mechanism responsible for electromagnetic wave attenuation in carbon nanotube-based composite systems [5]. Nevertheless, binary composites solely filled with MWCNTs possess obvious drawbacks. Their shielding performance heavily relies on conductive loss while contributing limited dielectric absorption loss to incident electromagnetic waves, leaving considerable room for the optimization of wave absorption capacity [6,7]. Meanwhile, high MWCNT loadings tend to induce severe filler agglomeration, drastically deteriorating mechanical indicators such as tensile strength and toughness of composites, elevating melt viscosity, and increasing the processing difficulty during molding [8].

Based on the above considerations, ternary TPU/MWCNT/BiFeO₃ composites were fabricated in this chapter by incorporating bismuth ferrite (BiFeO₃) particles with dielectric loss characteristics while fixing the MWCNT loading at 1 wt%. The selection of 1 wt% MWCNT as the fixed content is mainly based on the following reasons: at this loading, MWCNTs have constructed preliminary conductive networks within the TPU matrix, which can provide fundamental channels for conductive loss [9]. Meanwhile, sufficient interfacial space and room for synergistic effects are reserved for the incorporation of BiFeO₃, facilitating the investigation of the influence law of the second-phase filler on the overall performances of the composites.

As a room-temperature multiferroic material, bismuth ferrite (BiFeO₃) has attracted extensive attention in the field of electromagnetic functional materials in recent years [10,11]. Its unique perovskite crystal structure endows BiFeO₃ with simultaneous ferroelectric and antiferromagnetic ordering characteristics, delivering distinct dielectric relaxation behavior and desirable magnetic loss capacity in the microwave frequency band [12,13]. In terms of the loss mechanism, the dielectric loss of BiFeO₃ primarily originates from the relaxation processes induced by oxygen vacancies, ferroelectric domain wall movement, and dipole polarization, which enable its great potential as an electromagnetic wave absorbing material [14]. Previous studies have demonstrated that BiFeO₃ exhibits a high dielectric loss value within the frequency range of 12.4–18 GHz and possesses thermally stable dielectric properties, rendering it promising for high-temperature microwave absorption applications [15,16].

In this experiment, ternary TPU/MWCNT/BiFeO₃ composites with a fixed MWCNT loading of 1 wt% and varied BiFeO₃ contents were fabricated via a solution blending method. The effects of BiFeO₃ dosage on the microstructure, thermal stability, mechanical properties, and electromagnetic shielding effectiveness of the composites were systematically investigated. The synergistic mechanism between conductive fillers and dielectric fillers was analyzed, and the electromagnetic wave attenuation law under the dual conductive loss–dielectric loss mechanism was comprehensively explored [17,18].

2. Mechanism of Electromagnetic Interference (EMI) Shielding

Electromagnetic interference (EMI) shielding refers to a technical method that attenuates electromagnetic waves via conductive or magnetic materials, so as to block electromagnetic radiation from entering or escaping a specific area. According to Schelkunoff’s EMI shielding theory, three primary processes occur when electromagnetic waves impinge on the surface of shielding materials, namely reflection loss, absorption loss and multiple reflection loss.

Reflection loss (SER) represents the energy loss originating from the reflection of electromagnetic waves at the material surface due to impedance mismatch. When incident electromagnetic waves propagate from air (impedance Z0≈377 Ω) into the shielding material (impedance Zm), partial waves are reflected back into the air medium owing to the impedance difference between the two media. The magnitude of reflection loss depends on the electrical conductivity (σ) and magnetic permeability (μ) of materials, which can be expressed by the following equation.

$$SER=20{log}_{10}\left({\displaystyle \frac{Z_m}{{4Z}_0}}\right)=20{log}_{10}\left({\displaystyle \frac{1}{4}}\sqrt{{\displaystyle \frac{\sigma }{\pi f\mu }}}\right)$$

(1)

where f denotes the frequency of electromagnetic waves. It can be observed from the formula that higher electrical conductivity and magnetic permeability of the material lead to more remarkable reflection loss.

Absorption loss (SEA) refers to the energy loss when electromagnetic waves penetrate the interior of materials and are attenuated and dissipated. As electromagnetic waves propagate inside the material, free electrons in conductive fillers generate ohmic loss under the action of electromagnetic fields; magnetic domains of magnetic fillers produce magnetic loss in alternating magnetic fields; dipoles of dielectric fillers undergo orientation polarization under alternating electric fields to induce dielectric loss. The magnitude of absorption loss is closely correlated with the electrical conductivity, magnetic permeability, dielectric constant and thickness of the material:

$$SEA=20{log}_{10}\left(e^{t/\delta }\right)=8.686{\displaystyle \frac{t}{\delta }}$$

(2)

where t represents the material thickness, and δ denotes the skin depth, which follows the equation δ=(πfμσ)−1/2. The absorption loss increases with rising frequency and elevated electrical conductivity.

Multiple reflection loss (SEM) corresponds to the energy loss generated by repeated reflection and transmission of electromagnetic waves inside materials. After electromagnetic waves penetrate the first interface, multiple reflections occur within the material or at interlayer interfaces, which extend the interaction duration between electromagnetic waves and the material and thus strengthen the attenuation effect. When the material thickness is much larger than the skin depth, the multiple reflection loss can be neglected.

Figure 1. Schematic illustration of EMI shielding mechanism.

Figure 1. Schematic illustration of EMI shielding mechanism.

3. Preparation and Process Route of TPU/MWCNT/BiFeO₃ Composites

3.1. Experimental Materials

The raw materials adopted in this work are listed in Table 1.

3.2. Experimental Equipment

The experimental instruments and equipment are listed in Table 2.

3.3. Component Ratios of TPU/MWCNT/BiFeO₃ Composite Materials

TPU/MWCNT/BiFeO₃ composites with a fixed MWCNT mass fraction of 1 wt%, and BiFeO₃ mass fractions of 0 wt%, 1 wt%, 3 wt%, and 5 wt%, respectively.

3.4. Fabrication Process of TPU/MWCNT/BiFeO₃ Composites

First, TPU pellets, MWCNT powder and BiFeO₃ powder were separately placed in a vacuum drying oven and dried at 100 °C for 6 h to remove moisture [19]. Glassware such as beakers and petri dishes used in the experiment were cleaned and dried for later use.

During sample fabrication, TPU pellets (45 g) were weighed into a dry beaker according to the designed component ratios. MWCNT powder (fixed at 1 wt% relative to the mass of TPU, 0.45 g) and BiFeO₃ powder with varied loadings (1 wt%, 3 wt% and 5 wt% relative to TPU mass, corresponding to 0.45 g, 1.35 g and 2.25 g, respectively) were weighed using weighing papers, followed by measuring 200 mL of DMF solvent. The MWCNT powder was first added into DMF and homogenously dispersed via a high-speed stirrer. Afterwards, BiFeO₃ powder with the corresponding mass fraction was introduced into the MWCNT dispersion, and stirring was continued to achieve uniform mixing. Finally, TPU pellets were incorporated, and stirring was maintained until TPU was completely dissolved to obtain a homogeneous TPU/MWCNT/BiFeO₃/DMF mixed solution.

The mixed solution was poured into petri dishes and vacuum-dried at 140 °C in a vacuum drying oven to evaporate the DMF solvent, yielding TPU/MWCNT/BiFeO₃ composites. The as-prepared materials were taken out and cut into small pieces. A thin layer of release agent was evenly sprayed onto the mold surface of a plate vulcanizer, followed by standing for a period until the release agent dried completely. Subsequently, the cut materials were loaded into the mold and placed on the plate vulcanizer for pre-melting at 185 °C for 15 min [20,21]. Afterwards, the temperature was kept constant, and a pressure of 8 MPa was applied and maintained for 20 min. After the pressure holding stage, the sample was cooled to room temperature under constant pressure before demolding, and TPU/MWCNT/BiFeO₃ composite specimens were finally obtained [22].

By fixing the MWCNT loading at 1 wt% and varying the addition amount of BiFeO₃, TPU/MWCNT/BiFeO₃ composites with BiFeO₃ mass fractions of 1 wt%, 3 wt% and 5 wt% were fabricated, respectively [23]. The fabrication procedure is illustrated in Figure 2.

4. Results and Discussion

4.1. X-Ray Diffraction (XRD) Analysis of TPU/MWCNT/BiFeO₃ Composites

The crystal structure of BiFeO₃ and its dispersion state in the matrix were confirmed by analyzing the X-ray diffraction (XRD) patterns of TPU/MWCNT/BiFeO₃ composites (Figure 3).

The XRD results show that the TPU/MWCNT/BiFeO₃ composite with 1 wt% BiFeO₃ exhibits characteristic diffraction peaks of BiFeO₃ at 2θ of 22.5°, 32.2°, 33.2°, 45.9°, and 57°. For the TPU/MWCNT/BiFeO₃ composites with 3 wt% and 5 wt% BiFeO₃, additional characteristic peaks of BiFeO₃ appear at 2θ of 39.7°, 51.6°, and 52.6°. Specifically, the diffraction peak at 22.5° corresponds to the (012) crystal plane, the peak at 32.2° corresponds to the (104) crystal plane, and the peak at 33.2° corresponds to the (110) crystal plane of BiFeO₃. The diffraction peak at 39.7° is assigned to the (006) or (202) crystal plane, the peak at 45.9° to the (024) crystal plane, the peak at 51.6° to the (116) crystal plane, the peak at 52.6° to the (018) crystal plane, and the peak at 57° to the (122) or (018) crystal plane of BiFeO₃. The appearance of these typical BiFeO₃ diffraction peaks verifies the successful fabrication of target composites. With the increase of BiFeO₃ content from 1 wt% to 5 wt%, the intensity of BiFeO₃ characteristic peaks gradually increases, indicating the simultaneous improvement in crystallinity and loading of BiFeO₃. Notably, the main diffraction peak at 32.2° shows the most significant intensity enhancement, suggesting the possible preferred orientation of the (104)/(110) crystal planes.

The coexistence of shoulder peaks at 2θ = 24° and 2θ = 25.5° in the 1 wt% BiFeO₃ composite demonstrates the uniform dispersion of MWCNTs in the TPU matrix. As the BiFeO₃ content increases, the intensity of the peak at 24° decreases, while the shoulder peak at 25.5° disappears and shifts toward a higher angle, approaching the intrinsic peak position of pure MWCNTs. This phenomenon indicates that high-content BiFeO₃ induces the agglomeration of MWCNTs. The stress of MWCNTs inside the agglomerates is released, and the interplanar spacing recovers to the original state of pristine MWCNTs [24,25].

In addition, the position of the amorphous diffraction peaks of TPU in all composite samples remains unchanged with the variation of BiFeO₃ content, revealing that the incorporation of BiFeO₃ does not alter the average chain spacing of TPU molecular chains, and the amorphous structure of the TPU matrix maintains stable [26,27]. Moreover, the intensity of TPU amorphous diffraction peaks decreases monotonously with the increasing BiFeO₃ loading, which is mainly attributed to the dilution effect caused by the increased inorganic filler content [28,29,30].

4.2. Survey X-Ray Photoelectron Spectroscopy (XPS) Analysis of TPU/MWCNT/BiFeO₃ Composites

The full X-ray photoelectron spectroscopy (XPS) survey spectra of TPU/MWCNT/BiFeO₃ composites (Figure 4) illustrate the surface elemental composition of composites with different BiFeO₃ contents. Characteristic peaks of C1s, O1s, N1s, Si2p, and Si2s were detected on the surface of all samples [31]. After the introduction of BiFeO₃, weak Fe2p peaks appeared in the spectra, whereas the Bi4f signals were barely observable. For the TPU/MWCNT1% composite, the C1s, O1s, N1s, Si2s, and Si2p peaks are located at 284.8 eV, 532.3 eV, 400 eV, 153.2 eV, and 102 eV, respectively. The Fe2p peak of the TPU/MWCNT1%/BiFeO₃1% composite is centered at 710.8 eV, while the Bi4f peak of the TPU/MWCNT1%/BiFeO₃3% composite is located at 163.2 eV.

By comparing the XPS survey spectra of samples with different BiFeO₃ contents, it can be observed that the height of the C1s peak shows no obvious variation with the increasing BiFeO₃ loading, while the O1s peak height exhibits an overall upward trend. The intensities of the Si2p and Si2s peaks are also significantly enhanced, which is most prominent for the TPU/MWCNT1%/BiFeO₃5% sample. The signals of N1s, Bi4f and Fe2p peaks are weak and hard to distinguish in the survey spectra.

Quantitative analysis based on XPS survey spectra (Table 3) further reveals the evolution of surface elemental composition. The Bi4f signals of all BiFeO₃-containing samples are extremely weak (<0.02 at.%), and the atomic percentage of Fe2p ranges from 0.72% to 0.96%, without a monotonic increase with the rising BiFeO₃ loading [32]. The total Si content (Si2s + Si2p) rises markedly from 16.59% for TPU/MWCNT1% to 32.89% for TPU/MWCNT1%/BiFeO₃5% (16.7% for Si2s and 16.19% for Si2p) [33]. The oxygen content fluctuates between 20.86% and 23.72%, while the carbon content varies from 41.38% to 61.33%.

This observation is strongly associated with the intrinsic characteristics of BiFeO₃ raw powder, residual mold release agent, and residual DMF solvent [34].

(1) Analysis of Bi/Fe signals: The as-used BiFeO₃ powder is a micron-sized product with a mesh size of −200 mesh (particle size < 74 μm), which is far larger than the detection depth of XPS (~10 nm). Besides, BiFeO₃ particles are tightly encapsulated by the TPU matrix, making it difficult for XPS to detect Bi signals inside the particles. The trace Fe signal (<1 at.%) detected on the sample surface may originate from an ultrathin Fe-rich layer (e.g., Fe₂O₃) on the surface of BiFeO₃ particles, or the leaching and surface migration of trace iron ions during sample preparation. The Fe content does not rise with the increased BiFeO₃ loading, demonstrating that the “effective Fe source” exposed on the surface is limited, and the Fe signal may be further attenuated by the covering layer of residual release agent [35].

(2) Analysis of Si, O and C signals: The JD-909A mold release agent adopted during compression molding leaves substantial residues on the sample surface, which act as the primary source of Si and a major contributor to O and C signals [36]. The residual amount of release agent rises with increasing BiFeO₃ loading (the Si atomic percentage increases from 16.59% to 32.89%), revealing that high BiFeO₃ dosage elevates the viscosity of the composite system and facilitates the transfer of more release agent onto the material surface. In addition, residual DMF solvent (boiling point = 153 °C; drying temperature = 140 °C) contributes partial O and N signals [37]. The fluctuation of O1s content ranging from 20.86 at.% to 23.72 at.% originates from the combined contributions of intrinsic oxygen in TPU, oxygen from residual DMF, and oxygen contained in Si–O bonds of the release agent.

(3) Analysis of N1s signal: The atomic percentage of N1s fluctuates between 0.49 at.% and 1.22 at.% without an obvious regular trend, which verifies the existence of trace nitrogen-containing residues (DMF) on the sample surface, yet their contribution to elemental signals is far lower than that of release agent residues.

Table 3. Relative atomic percentages (at.%) of surface elements for TPU/MWCNT/BiFeO₃ composites.

Samples	C1s (at.%)	O1s (at.%)	N1s (at.%)	Si2s (at.%)	Si2p (at.%)	Bi4f (at.%)	Fe2p (at.%)
TPU/MWCNT1%	61.33	20.86	1.22	8.71	7.88	0	0
TPU/MWCNT1%/BiFeO₃1%	50.01	22.84	0.49	12.83	12.87	0	0.96
TPU/MWCNT1%/BiFeO₃3%	57.82	20.99	0.53	10.08	9.85	0.01	0.72
TPU/MWCNT1%/BiFeO₃5%	41.38	23.72	1.13	16.7	16.19	0.01	0.86

Table 3. Relative atomic percentages (at.%) of surface elements for TPU/MWCNT/BiFeO₃ composites.

Samples	C1s (at.%)	O1s (at.%)	N1s (at.%)	Si2s (at.%)	Si2p (at.%)	Bi4f (at.%)	Fe2p (at.%)
TPU/MWCNT1%	61.33	20.86	1.22	8.71	7.88	0	0
TPU/MWCNT1%/BiFeO₃1%	50.01	22.84	0.49	12.83	12.87	0	0.96
TPU/MWCNT1%/BiFeO₃3%	57.82	20.99	0.53	10.08	9.85	0.01	0.72
TPU/MWCNT1%/BiFeO₃5%	41.38	23.72	1.13	16.7	16.19	0.01	0.86

4.3. FT-IR Spectroscopic Analysis of TPU/MWCNT/BiFeO₃ Composites

To investigate the effect of BiFeO₃ incorporation on the microstructure of TPU/MWCNT/BiFeO₃ composites, Fourier-transform infrared (FT-IR) spectroscopy analysis was performed on the TPU/MWCNT/BiFeO₃ composites (Figure 5).

By comparing the Fourier-transform infrared spectra of TPU/MWCNT1% and TPU/MWCNT/BiFeO₃ composites, no characteristic absorption peaks assigned to BiFeO₃ (such as the Fe–O stretching vibration peak at approximately 550 cm⁻¹) were detected in the spectra of all composites. This phenomenon can be attributed to the low loading content of BiFeO₃ (≤5 wt%) and its micron-sized particles (-200 mesh), whose infrared signals were shielded by the strong absorption of the TPU matrix [38].

When the BiFeO₃ content was 1 wt% and 5 wt%, the peak intensities of the carbonyl peaks of hard segments (1727.5 cm⁻¹, 1700.6 cm⁻¹), amide II band (1532.2 cm⁻¹), and C–O–C ether bond peaks of soft segments (1230.4 cm⁻¹, 1165.3 cm⁻¹, 1136.9 cm⁻¹, 1078.5 cm⁻¹) all exhibited an upward trend. This is ascribed to two factors: the uniform dispersion of low-dose BiFeO₃ strengthens interfacial interactions, and the overall confinement effect on molecular chains induced by the network structure formed by BiFeO₃ at high loadings. Nevertheless, when the BiFeO₃ loading reached 3 wt%, the intensities of all the above-mentioned peaks declined back to levels close to those of the TPU/MWCNT1% reference sample. This observation implies that 3 wt% may be the critical concentration at which BiFeO₃ undergoes a transition from uniform dispersion to network formation within the matrix. Near this critical point, the original interfacial interactions are partially disrupted, while a complete new filler network has not yet been established, resulting in a temporary weakening of the confinement effect on the molecular chains of hard and soft segments.

Internal differentiation occurred for the C–H stretching vibration peaks attributed to soft segments at a BiFeO₃ loading of 3 wt%: the intensity of the asymmetric C–H stretching peak (2959.7 cm⁻¹) slightly decreased, while the intensity of the symmetric C–H stretching peak (2915.1 cm⁻¹) marginally increased. Such differentiation further verifies the unique state at 3 wt% — the overall orderliness of soft segments declines (reflected by the weakened asymmetric stretching peak), yet the local mobility of molecular chains rises (reflected by the intensified symmetric stretching peak). This serves as a typical characteristic of the structural transition point [39,40].

The N–H stretching vibration peak of hard segments (3323.8 cm⁻¹) slightly increased at all BiFeO₃ loadings, demonstrating that interfacial hydrogen bonding interactions between BiFeO₃ and hard segments persistently exist, yet such interactions are "masked" by the variation in filler dispersion state at the loading of 3 wt%.

4.4. Effect of BiFeO₃ Content on Thermal Stability of TPU/MWCNT/BiFeO₃ Composites

Figure 6 display the thermogravimetric analysis (TGA) and corresponding derivative thermogravimetry (DTG) curves of TPU/MWCNT/BiFeO₃ composites. Combined with the data listed in Table 4, it can be found that the incorporation of low loading BiFeO₃ slightly improves the thermal stability of the composites: the 5% weight loss temperature (T5%) of TPU/MWCNT1%/BiFeO₃1% and TPU/MWCNT1%/BiFeO₃3% increases marginally. Nevertheless, when the BiFeO₃ loading rises to 5%, the T5% value becomes slightly lower than that of TPU/MWCNT1% [41]. This reveals that excessive BiFeO₃ tends to agglomerate within the matrix or trigger localized catalytic degradation, thereby inducing slight premature decomposition [42]. In contrast, the char residue yield exhibits a continuous upward trend with increasing BiFeO₃ content, gradually rising from 10.42% to 16.01%. This phenomenon is mainly attributed to the outstanding high-temperature thermal stability of BiFeO₃ itself and its char-promoting capacity, which facilitates the formation of a denser protective carbon layer [43,44].

With the increase in BiFeO₃ loading, the temperature at the maximum decomposition rate continuously decreases from 441.90 °C to 419.17 °C, while the absolute value of the maximum mass loss rate rises from 1.054 %/min to 1.372 %/min. This indicates that the incorporation of BiFeO₃ accelerates the reaction process in the main decomposition stage, allowing the material to reach its decomposition peak at a lower temperature. This observation is highly consistent with the morphological evolution of DTG curves. The DTG curve of TPU/MWCNT1% exhibits a typical two-stage decomposition feature with a shoulder peak on the left side of the main peak. After the addition of BiFeO₃, the shoulder peak almost disappears, and the curve transforms into a nearly single peak. It can be inferred that BiFeO₃ preferentially catalyzes or interferes with the decomposition of TPU hard segments, narrowing or advancing their decomposition temperature range to overlap with that of soft segments, thereby concentrating the thermal decomposition reaction [45,46].

TGA-FTIR coupled characterization was carried out for further investigation. As displayed in Figure 7, all BiFeO₃-containing samples present stronger peak intensities in their three-dimensional FTIR spectra compared with TPU/MWCNT1%, demonstrating that the incorporation of BiFeO₃ accelerates the release of pyrolysis products or alters the composition of gaseous degradation products. Although the TPU/MWCNT1%/BiFeO₃3% sample exhibits higher peak intensity than TPU/MWCNT1%, its absorbance values in the high-temperature region are lower instead. This phenomenon can be explained as follows: BiFeO₃ with a loading of 3% catalyzes the rapid premature emission of partial pyrolysis products at the initial decomposition stage, which reduces the amount of gas available for escape at later temperatures. Alternatively, BiFeO₃ may facilitate more pyrolytic fragments to participate in char-forming reactions and retain them in the condensed phase, thus leading to weaker gas emission signals under high-temperature conditions.

4.5. Fracture Morphology of TPU/MWCNT/BiFeO₃ Composites

To preliminarily observe the dispersion state of BiFeO₃ particles within the TPU/MWCNT matrix, the fracture surfaces of TPU/MWCNT/BiFeO₃ composites were characterized using a super-depth-of-field microscope. Figure 8 presents the fracture morphologies of samples with various BiFeO₃ loadings at a magnification of 500×.

As can be seen from Figure 9, unlike the uniform dark background observed for the TPU/MWCNT sample (Figure 8), numerous scattered bright spots or light-colored particles emerge in the micrographs after the incorporation of BiFeO₃. These bright spots correspond to pristine BiFeO₃ particles and their agglomerates. As the BiFeO₃ loading increases, the distribution density of bright spots within the field of view rises remarkably, indicating a greater number of observable BiFeO₃ particles per unit area. Nevertheless, it can also be found that more large-sized, high-brightness agglomerates appear when the BiFeO₃ loading reaches 3% and 5%. In particular, obvious particle agglomeration occurs in local regions of the 5% BiFeO₃ sample. This reveals that BiFeO₃ at high loadings tends to form agglomerates in the TPU/MWCNT matrix, leading to deteriorated dispersion uniformity.

This phenomenon can be attributed to the limited compatibility between inorganic ceramic BiFeO₃ particles and the organic TPU matrix. Meanwhile, the van der Waals forces among particles strengthen with the rising particle concentration, which intensifies the tendency of agglomeration.

4.6. Effect of BiFeO₃ Content on Tensile Properties of TPU/MWCNT/BiFeO₃ Composites

Tensile tests were carried out on TPU/MWCNT/BiFeO₃ composites with different BiFeO₃ loadings (Figure 10). It can be observed from the stress-strain curves in Figure 10(a) that all samples exhibit the characteristics of filled elastomers with strain softening behavior. The slope of each curve continuously decreases from the onset of stretching until fracture, without an obvious strain hardening stage.

With a fixed MWCNT loading of 1 wt%, BiFeO₃ was incorporated into the matrix, and the tensile properties of the composites continuously deteriorated as the BiFeO₃ content increased. As listed in Table 5, the tensile strength of the reference TPU/MWCNT1% sample is 12.80 MPa. After adding 1% BiFeO₃, the tensile strength drops to 8.01 MPa with a reduction of 37.4%, followed by 5.60 MPa at 3% BiFeO₃ (a 56.3% decrease), and only 4.49 MPa at 5% BiFeO₃ (a 64.9% reduction).The elongation at break decreases from 349.10% for the reference sample to 140.79% (1% BiFeO₃), 66.20% (3% BiFeO₃) and 33.27% (5% BiFeO₃), and the elongation of the 5% BiFeO₃ sample merely accounts for 9.53% of the reference value.Tensile toughness suffers an even sharper decline from 32.40 MJ/m³ to 7.75 MJ/m³ (1% BiFeO₃), 2.09 MJ/m³ (3% BiFeO₃) and 0.67 MJ/m³ (5% BiFeO₃). The toughness of the 5% BiFeO₃ sample is almost completely lost, reaching only 2.1% of the reference sample.

The continuous deterioration of mechanical performance with increasing BiFeO₃ loading can be attributed to the following reasons:(1) BiFeO₃ particles embed into the interstices of the MWCNT network and exert a dilution effect, disrupting the continuity of the original MWCNT conductive network.(2) The weak interfacial adhesion between inorganic particles and the TPU matrix induces premature debonding during tensile loading, which acts as stress concentration sites and crack initiation sources.(3) Severe agglomeration of BiFeO₃ particles at high loadings generates large-scale defective structures.

The synergistic effect of these three factors comprehensively degrades the strength, ductility and toughness of the composites. Notably, the sample filled with 5 wt% BiFeO₃ nearly loses typical elastomeric characteristics and evolves toward brittle material behavior.

(Note: In subgraphs (b)–(d), 0% BiFeO₃ corresponds to TPU/MWCNT1%, 1% BiFeO₃ corresponds to TPU/MWCNT1%/BiFeO₃1%, 3% BiFeO₃ corresponds to TPU/MWCNT1%/BiFeO₃3%, and 5% BiFeO₃ corresponds to TPU/MWCNT1%/BiFeO₃5%.)

4.7. Effect of BiFeO₃ Content on Electromagnetic Interference Shielding Performance of TPU/MWCNT/BiFeO₃ Composites

Figure 11 presents the electromagnetic interference shielding effectiveness (SE) of TPU/MWCNT/BiFeO₃ composites with different BiFeO₃ loadings in the frequency range of 12.4–18 GHz. As shown in Figure 11(a), the maximum SE value of the composites rises first and then declines with increasing BiFeO₃ content. Without BiFeO₃ (i.e., TPU/MWCNT1%), the maximum SE is 9.20 dB. After incorporating 1 wt% BiFeO₃, the maximum SE increases to 10.69 dB. When the BiFeO₃ loading rises to 3 wt%, the maximum SE reaches a peak value of 11.47 dB, approximately 24.7% higher than that of the TPU/MWCNT1% sample. Nevertheless, further increasing the BiFeO₃ loading to 5 wt% causes the maximum SE to drop to 10.11 dB.

Figure 11(b) compares the values of absorption shielding efficiency (SEA ) and reflection shielding efficiency (SER ) for all samples at 18 GHz. According to the histogram data, both SEA and SER show an upward-then-downward trend with increasing BiFeO₃ loading, and SEA exhibits a more prominent variation range.For the sample without BiFeO₃ filler, SEA is 5.35 dB and SER is 3.84 dB. At a BiFeO₃ loading of 1 wt%, SEA rises to 6.61 dB and SER increases to 4.08 dB. When the loading reaches 3 wt%, SEA and SER reach their maximum values of 7.16 dB and 4.32 dB, respectively. Further raising the BiFeO₃ loading to 5 wt% causes SEA and SER to decline to 6.16 dB and 3.95 dB, respectively.

This reveals that the incorporation of BiFeO₃ primarily improves the wave absorption loss capacity of the composites. Such improvement originates from interfacial polarization, dipole polarization and multiple scattering effects induced by ferroelectric BiFeO₃, which accelerate the dissipation of electromagnetic waves inside the material. Nevertheless, excessive BiFeO₃ particles tend to agglomerate in the matrix, breaking the conductive network constructed by MWCNTs and worsening impedance matching, which ultimately reduces the electromagnetic interference shielding effectiveness [47].

4.8. Analysis of Electromagnetic Shielding Mechanism of TPU/MWCNT/BiFeO₃ Composite Materials

To deeply investigate the effect of BiFeO₃ loading on the electromagnetic shielding mechanism of TPU/MWCNT/BiFeO₃ composites, Figure 12 present the transmission coefficient (T), reflection coefficient (R) and absorption coefficient (A) of TPU/MWCNT/BiFeO₃ composites at 18 GHz. It can be observed from the plotted data that the transmission coefficient T of the composites decreases continuously with the rising BiFeO₃ content. The transmission coefficient T reaches 0.41 for the sample without BiFeO₃ (namely TPU/MWCNT with 1 wt% MWCNT). When the BiFeO₃ loading is 1 wt%, 3 wt% and 5 wt%, the transmission coefficient T declines to 0.30, 0.23 and 0.18, respectively. This reveals that the incorporation of BiFeO₃ effectively blocks the penetration of electromagnetic waves and progressively improves the shielding performance.

The reflection coefficient R shows a monotonically increasing trend with the increase of BiFeO₃ content, rising from 0.43 at 0 wt% BiFeO₃ to 0.57 at 5 wt% BiFeO₃. This is attributed to the introduction of BiFeO₃ ferroelectric filler, which further aggravates the impedance mismatch between the composite and air, leading to more electromagnetic waves being reflected at the material surface. In contrast, the absorption coefficient A first rises and then slightly declines as BiFeO₃ loading increases. The absorption coefficient A increases from 0.16 to 0.22 at 1 wt% BiFeO₃, and reaches a maximum value of 0.25 at a BiFeO₃ loading of 3 wt% [48].

The remarkable enhancement of absorption coefficient A demonstrates that the incorporation of BiFeO₃ mainly strengthens the electromagnetic wave absorption and dissipation capacity of the composites. The underlying mechanisms are summarized as follows:(1) As a ferroelectric material, BiFeO₃ possesses a high dielectric constant. Its addition multiplies internal interfaces within the composite and generates abundant interfacial polarization centers.(2) Interfacial polarization effect emerges at the interfaces between BiFeO₃ and MWCNT, as well as between BiFeO₃ and the TPU matrix, which accelerates the attenuation of incident electromagnetic waves [49]. (3) BiFeO₃ particles introduce abundant lattice defects and electric dipoles. Dipole polarization is triggered under external electromagnetic fields, which further dissipates electromagnetic wave energy.

In summary, the electromagnetic shielding performance of TPU/MWCNT/BiFeO₃ composites originates from the synergistic effect of reflection loss and absorption loss. Notably, absorption loss is greatly boosted after BiFeO₃ doping and achieves the optimal shielding effect at a BiFeO₃ loading of 3 wt%.Bismuth ferrite nanoparticles act as multiferroic dielectric fillers, which convert the energy of incident electromagnetic waves (with similar interaction rules to solar light) into thermal loss through dipole polarization and interfacial polarization. In the composite matrix, interconnected multi-walled carbon nanotubes (MWCNTs) construct a continuous conductive network. The migration of free electrons in this network triggers ohmic loss, endowing the material with primary electromagnetic interference (EMI) shielding capacity. The dielectric thermal loss and conductive shielding effect produce superposition and synergistic enhancement, thereby establishing a dual attenuation system of "conductive loss + dielectric thermal loss" to realize multidimensional and composite EMI shielding performance.

5. Conclusions

In this experiment, with the MWCNT loading fixed at 1 wt%, BiFeO₃ particles with varying loadings (0 wt%, 1 wt%, 3 wt%, 5 wt%) were incorporated. Ternary TPU/MWCNT/BiFeO₃ composites were successfully fabricated via solution blending followed by hot-pressing molding. The effects of BiFeO₃ loading on the microstructure, thermal stability, mechanical properties and electromagnetic interference shielding effectiveness (EMI SE) of the composites were systematically investigated. Particular emphasis was placed on discussing the synergistic effect and electromagnetic wave attenuation law under the dual mechanisms of conductive loss and dielectric loss. The main conclusions are summarized as follows:

(1) Characterizations including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and survey X-ray photoelectron spectroscopy (XPS) were performed. Characteristic diffraction peaks assigned to BiFeO₃ were observed in the XRD patterns, and the peak intensity increased with rising BiFeO₃ loading, which verified the successful fabrication of target composites. The incorporation of high BiFeO₃ loadings (3 wt% and 5 wt%) induced localized agglomeration of MWCNTs, recovering their interlayer spacing to the intrinsic state of pristine nanotubes. FT-IR results indicated that 3 wt% BiFeO₃ served as a critical concentration for the transition from uniform dispersion to interconnected network structure within the matrix, at which point the confinement effect on hard and soft segment molecular chains of TPU was temporarily weakened. Survey XPS spectra revealed that BiFeO₃ particles were tightly encapsulated by the TPU matrix, and only trace Fe signals could be detected on the sample surface. The surface Si atomic content of specimens increased remarkably with the increase of BiFeO₃ loading (rising from 16.59 at.% to 32.89 at.%), which was ascribed to the elevated system viscosity induced by high filler loading, facilitating the migration of more release agent onto the material surface [50].

(2) Cross-sectional morphology observations were carried out using a super-depth-of-field microscope. Bright spots corresponding to BiFeO₃ particles dispersed in the matrix could be clearly observed under the microscope. As the BiFeO₃ loading increased, the distribution density of bright spots rose. Nevertheless, larger agglomerates emerged in the samples filled with 3 wt% and 5 wt% BiFeO₃, demonstrating that the dispersion uniformity of BiFeO₃ deteriorated and the agglomeration tendency intensified at high filler loadings.

(3) Thermogravimetric analysis (TGA) was adopted to investigate the thermal stability of TPU/MWCNT/BiFeO₃ composites. Low BiFeO₃ loadings (1 wt% and 3 wt%) slightly increased the initial decomposition temperature (T₅%) of the composites, while T₅% decreased at a loading of 5 wt%, revealing that excessive BiFeO₃ might trigger slight early-stage decomposition. The char residue rate continuously increased from 10.42% to 16.01% with rising BiFeO₃ loading, which originated from the char-promoting effect of BiFeO₃. Nevertheless, the temperature corresponding to the maximum decomposition rate declined steadily from 441.90 °C to 419.17 °C, and the DTG curves transformed from two-stage decomposition profiles into single peaks. This indicates that BiFeO₃ accelerates the reaction rate of the main decomposition stage and preferentially catalyzes the decomposition of hard segments in TPU. TGA-FTIR characterization manifests that the release intensity of pyrolysis products from BiFeO₃-containing samples is generally higher than that of the BiFeO₃-free system. Notably, the absorbance of the sample with 3 wt% BiFeO₃ decreases significantly in the high-temperature region, demonstrating that BiFeO₃ at this loading level can facilitate the participation of pyrolysis products in char-forming reactions and thereby suppress gas emission at high temperatures [51].

(4) The tensile mechanical properties of TPU/MWCNT/BiFeO₃ composites were systematically studied. With a fixed MWCNT loading of 1 wt%, the mechanical performances continuously deteriorated as the BiFeO₃ content increased. Compared with the reference sample TPU/MWCNT-1 wt%, the tensile strength of TPU/MWCNT-1 wt%/BiFeO₃-5 wt% decreased from 12.80 MPa to 4.49 MPa, representing a 64.9% reduction. The elongation at break plummeted from 349.10% to 33.27%, with a loss of 90.5%. The tensile toughness dropped sharply from 32.40 MJ/m³ to 0.67 MJ/m³, merely 2.1% of the reference value. Such degradation can be attributed to three factors: the dilution effect of BiFeO₃ on the MWCNT conductive network, stress concentration induced by weak interfacial adhesion between fillers and matrix, and severe particle agglomeration at high filler loadings. The specimen filled with 5 wt% BiFeO₃ almost lost the intrinsic elastomeric characteristics of TPU.

(5) The influence of BiFeO₃ loading on the electromagnetic interference shielding performance of TPU/MWCNT/BiFeO₃ composites was investigated, and the corresponding electromagnetic shielding mechanism was analyzed:① The total shielding effectiveness (SEₜ) of the composites exhibited an upward-then-downward trend with the increase of BiFeO₃ loading. At the frequency of 18 GHz, the maximum SEₜ of TPU/MWCNT-1 wt%/BiFeO₃-3 wt% reached 11.47 dB, approximately 24.7% higher than that of TPU/MWCNT-1 wt% (9.20 dB). Nevertheless, when BiFeO₃ loading rose to 5 wt%, the maximum SEₜ decreased to 10.11 dB. Both absorption loss (SEₐ) and reflection loss (SEᵣ) followed the identical variation tendency, and the fluctuation range of SEₐ was much more remarkable.② As BiFeO₃ loading increased, the transmission coefficient (T) decreased continuously from 0.41 to 0.18, while the reflection coefficient (R) increased monotonically from 0.43 to 0.57. By contrast, the absorption coefficient (A) first increased and then slightly declined, peaking at 0.25 at 3 wt% BiFeO₃. This reveals that the incorporation of BiFeO₃ mainly strengthens the absorption loss capacity of the composites. The underlying mechanisms are as follows: BiFeO₃ acts as a ferroelectric filler to generate interfacial polarization and dipole polarization effects; numerous multi-interfacial polarization centers are constructed among BiFeO₃, MWCNT and TPU matrix, which facilitate the dissipation of electromagnetic waves inside the composite. However, severe agglomeration of excessive BiFeO₃ (5 wt%) breaks the continuity of the MWCNT conductive network and deteriorates impedance matching, thereby reducing the shielding effectiveness.