Experimental and Computational Study of the Microwave Absorption Properties of Recycled <i>α</i>-Fe2O3/OPEFB Fiber/PCL Multi-Layered Composites

The aim of this study was to fabricate multi-layered recycled α-Fe ₂O ₃/OPEFB fiber/PCL composites for microwave absorbing applications in the 1 - 4 GHz frequency range. The multi-layered composites were 6 mm thick and each consisted of a 2 mm thick layer of recycled α-Fe₂O₃/PCL composites at various loadings (5 wt% - 25 wt%) of 16.2 nm recycled α-Fe₂O₃ nanofiller, placed between two layers of 2 mm thick OPEFB fiber/PCL composites blended at a fixed ratio of 7:3. The real (ε') and imaginary (ε") components of the relative complex permittivity were measured using the open-ended coaxial probe technique and the values obtained were applied as inputs for the Finite Element Method to calculate the reflection coefficient magnitudes from which the reflection loss (RL) properties were determined. Both ε' and ε" increased linearly with recycled α-Fe₂O₃ nanofiller content and the values of ε' varied between 3.0 and 3.9 while the ε" values ranged between 0.26 and 0.64 within 1 - 4 GHz. The RL (dB) showed the most prominent values within the 1.38 - 1.46 GHz band with a minimum of -38 dB attained by the 25 wt% composite. Another batch of minimum values occurred in the 2.39 - 3.49 GHz range with the lowest of -25 dB at 2.8 GHz. The recycled α-Fe₂O₃/OPEFB fiber/PCL multi-layered composites are promising materials that can be engineered for solving noise problems in the 1 - 4 GHz range.

• Multi-Layered Composites
• Recycled <i>α</i>-Fe2O3
• Reflection Loss
• OPEFB Fiber
• Relative Complex Permittivity

1. Introduction

The advancements in contemporary wireless electronics at low RF/microwave frequencies have intensified the electromagnetic interference (EMI) problem for several electronic devices having civilian and military applications. The situation has given rise to the development of a wide range of absorber-based solutions aimed at offering electromagnetic compatibility to enhance the reliability and quality of these sophisticated electronic devices. Generally, the materials employed in EMI absorbing applications can be classified as dielectric, magnetic and magneto-dielectric (hybrid), with ferrites such as soft magnetic spinel ferrites [1] [2] [3], hard U and M-type ferrites [4] [5] being among the most widely used magnetic materials. Ferrite materials are known to be chemically stable and also have higher saturation magnetization and magnetic loss [6] as well as larger electrical resistivity but lack the appropriate balance of low density and high dielectric loss properties.

One approach which many researchers have exploited in recent years to reduce the effect of these limitations is to combine ferrite materials with carbon-based materials or conducting polymers to synthesize hybrid absorbers which have shown remarkable absorption characteristics over a wide range of microwave frequencies [4] [7] [8] [9] [10]. However, the replication of this approach using carbon bio-based materials and non-conducting polymers has received very little attention. For instance, carbon bio-based materials from agricultural residues such as oil-palm empty fruit bunch (OPEFB) fiber could serve as an inexpensive and green substitute to the frequently used carbon nanotubes and graphene oxides. A recent study by Abdalhadi et al. [11] reported that 100 µm grain-sized compacted OPEFB fiber had high dielectric loss values which ranged between 0.57 and 0.61 in the 1 - 4 GHz range. Additionally, OPEFB fiber is biodegradable, lighter and can be easily melt-blended with ferrites and non-conducting polymers such as polycaprolactone (PCL) [12].

Generally, the preparation of ferrite materials of different morphologies is often associated with a variety of expenses [13] [14] which ultimately make the materials relatively expensive. Recently, α-Fe₂O₃ (a corundum-type ferrite) was reportedly recovered from recyclable mill scale steel waste [15] [16] and the dielectric loss properties were enhanced [17] through processing techniques which were cheap and friendly to the environment. Recycled α-Fe₂O₃ could therefore be considered as a cheaper alternative to the most frequently used ferrites while at the same time saving the environment.

Moreover, when hybrid composites are employed as single-layered structures, they act as single barrier absorbers and are thus limited by the number of absorption assembly. Therefore multi-layered absorption structures [18] [19] [20] are designed to increase the absorption assembly with the view to improving the microwave absorption capacities of the composites. However, there is no information currently available on multi-layered absorbers based on recycled α-Fe₂O₃, OPEFB fiber and PCL composites.

In this current work, recycled α-Fe₂O₃/OPEFB fiber/PCL multi-layered composites were fabricated using recycled α-Fe₂O₃/PCL and OPEFB fiber/PCL composites. The recycled α-Fe₂O₃/PCL composites consisted of various loadings (5 wt% - 25 wt%) of 16.2 nm recycled α-Fe₂O₃ nanofiller while the OPEFB fiber/PCL composites were blended at fiber to PCL ratio fixed at 7:3. The open-ended coaxial (OEC) probe method was then used to determine the relative complex permittivity properties which were subsequently used as inputs in the Finite Element Method to calculate the reflection coefficient magnitude in the L-S band from which the reflection loss performances of the composites were evaluated. A simulation of the x-component (V/m) of the electric field distribution within the composites was subsequently carried out to further assess their absorbing properties.

2. Materials and Methods

2.1. Preparation of Recycled α-Fe 2 O 3 /PCL Composites

5 wt% of 16.2 nm recycled α-Fe₂O₃ nanopowder, prepared from industrial mill scale waste using methods described by Mensah et al. [17], was mixed with 95 wt% PCL (Sigma-Aldrich, St. Louis, MO, USA) in a Brabender Extruder (Brabender GmbH & Co. KG, Duisburg, Germany) for 15 minutes at 65˚C to synthesize the composite of total mass 25 g. The procedure was repeated using 10 wt%, 15 wt%, 20 wt% and 25 wt% of the nanofiller and 90 wt%, 85 wt%, 80 wt% and 75 wt% of PCL respectively. 8.33 g of each of the composites were then placed in 6 cm × 3.6 cm × 0.20 cm molds and hot-pressed at a pressure of 110 kg/cm² into flat blocks.

2.2. Preparation of OPEFB Fiber/PCL Composite

Using the procedure outlined in Abdalhadi et al. [11], purified OPEFB (Ulu Langat Oil Palm Mill, Dengkil, Selangor, Malaysia) was milled into 100 µm grain size and 17.5 g of the fiber was melt-blended with 7.5 g PCL using the Brabender Extruder (Brabender GmbH & Co. KG, Duisburg, Germany) for 15 minutes at 65˚C to prepare the composite of total mass 25 g. By applying a pressure of 110 kg/cm², 8.33 g portions of the blended composite were then hot-pressed into 6 cm × 3.6 cm × 0.20 cm flat blocks. The choice of OPEFB fiber to PCL ratio of 7:3 was to ensure that more fiber was used while the ability of the extruder to mix the materials uniformly was not compromised during the melt-blending process.

2.3. Preparation of the Multi-Layered Composites

As shown in Figure 1, the multi-layered composites were prepared by inserting each of the 6 cm × 3.6 cm × 0.20 cm recycled α-Fe₂O₃/PCL composites between two layers of the 6 cm × 3.6 cm × 0.20 cm OPEFB fiber/PCL composites before

placing in a 6 cm × 3.6 cm × 0.70 cm mold and hot-pressed at a pressure of 110 kg/cm² to a final thickness of 0.60 cm and mass 25 g.

3. Characterization

3.1. Morphology

The distribution of the OPEFB fiber and recycled α-Fe₂O₃ nanoparticles within the PCL matrix was examined using the Field Emission Scanning Electron Microscope (JEOL JSM-7600 FESEM, JEOL, Tokyo, Japan).

3.2. Relative Complex Permittivity

The relative complex permittivity values (real, ε'; imaginary, ε") of all the samples were determined at room temperature by means of the OEC probe connected to a Vector Network Analyzer (Agilent N5230A PNA-L, Agilent Technologies, Santa Clara, USA) and measurements were taken in the microwave frequency range of 1 - 4 GHz. The measurements were obtained by pressing the coaxial probe firmly onto the broader surfaces of the samples while ensuring that there were no air spaces or gaps between the surfaces in contact.

3.3. Microwave Absorption

The reflection coefficient magnitudes (ǀS₁₁ǀ) of the composites were determined theoretically from the Finite Element Method performed on COMSOL Multiphysics software version 3.5 (COMSOL AB, Stockholm, Sweden) based on the microstrip model geometry. The model consisted of a tetrahedral element/mesh of an R/T Duroid 5880 dielectric substrate (length = 6.0 cm, width = 5.0 cm, thickness = 0.15 cm) having a signal line (6.0 cm × 0.15 cm) imprinted along its broader side. The measured ε' and ε" values of the composites were fed into the software as inputs for the calculation of the ǀS₁₁ǀ in the 1 - 4 GHz range. To visualize the electric field distribution within the composites, arrow representations of the simulated x-component (V/m) of the field were subsequently obtained after performing parametric analysis at 4 GHz.

4. Results and Discussion

4.1. Morphology

The respective distributions of OPEFB fiber and recycled α-Fe₂O₃ nanoparticles within the PCL matrix was examined from the FESEM micrographs of the surface morphologies of the composites depicted in Figure 2. The micrograph presented in Figure 2(a) for the OPEFB fiber/PCL blend shows no agglomeration, implying that there was an even distribution of the fiber throughout the PCL matrix. The micrographs in Figures 2(b)-(d) represent the surface morphologies of selected recycled α-Fe₂O₃/PCL composites with 5 wt%, 15 wt% and 25 wt% of the recycled α-Fe₂O₃ nanoparticles respectively.

A careful examination of the micrographs shows an even dispersal of bright spots which can be attributed to recycled α-Fe₂O₃ due to its higher atomic number contrast as compared to the PCL matrix. The intensity of the dispersal,

which increased with the recycled α-Fe₂O₃ nanofiller content, was without agglomeration for all the composites. The homogeneous distribution of the OPEFB fiber and recycled α-Fe₂O₃ nanoparticles in the PCL matrix indicates that they were fully integrated in the composites to provide the interfacial bonding which enhanced the permittivity properties of the composites.

4.2. Relative Complex Permittivity

The relative complex permittivity properties of the composites were determined from the ε' and ε" values using the OEC technique in the frequency range of 1 - 4 GHz. As presented in Figure 3 and Figure 4 respectively, the variation in ε' and ε" clearly depict increasing values with recycled α-Fe₂O₃ nanofiller content for all the composites. The ε' values obtained were between 3.0 and 3.9 while the ε" values ranged between 0.26 and 0.64. For instance, at 2.4 GHz, the ε' values were 3.2, 3.3, 3.4, 3.5 and 3.7 while the ε" values were 0.29, 0.37, 0.40, 0.46 and 0.54 for the 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt% composites respectively.

Since ε' and ε" values largely depend on contributions from orientation, interfacial, electronic and atomic and polarizations [21], the combined utilization of OPEFB fiber and recycled α-Fe₂O₃ nanofiller in the PCL matrix enabled orientation polarization [22] and interfacial polarization [23] respectively to dominate causing the observed increase in the permittivity values. These results therefore suggest that the relative complex permittivity properties of the composites could

be enhanced by increasing the recycled α-Fe₂O₃ nanofiller content, in view of the constant composition of OPEFB fiber. This is consistent with previous studies where the complex permittivity of polymer-based composites increased when fillers having high ε' and ε" values were utilized [1] [10] [24] [25].

A comparison of the measured ε' and ε" values with those of some composites synthesized from ferrites, epoxy and carbon nanotubes is presented in Table 1. At the stated frequencies, the ε' of the 25 wt% recycled α-Fe₂O₃/OPEFB fiber/PCL multi-layered composite appears to be comparable to that of 0.01 wt% CNT + epoxy + CoO (3 mm thick) composite but lower than the remaining composites.

However, the ε" values are much higher than the other composites, suggesting that the 25 wt% recycled α-Fe₂O₃/OPEFB fiber/PCL multi-layered composite could serve as an appropriate substitute since materials with high ε" values tend to possess higher absorption properties making them capable of attenuating electromagnetic waves.

Additionally, the effect of the recycled α-Fe₂O₃ nanofiller content on the microwave absorption tendencies of the composites was preliminarily evaluated from the variation of loss tangent (Tan δ) with frequency. As shown in Figure 5, the profiles depict a clear linear correlation between absorption properties and increase in recycled α-Fe₂O₃ nanofiller composition for all the composites. For instance, at 2.4 GHz, a strong correlation between loss tangent and fractional composition (x) of the recycled α-Fe₂O₃ nanofiller (Figure 6) was observed, indicating that higher absorption could be attained by increasing the nanofiller content beyond 25 wt%.

Table 1

Comparison of ε' and ε" at some specified frequencies

Material	2 GHz		4 GHz		Reference
	ε'	ε"	ε'	ε"
0.01 wt% CNT + epoxy + CoO (3.5 mm thick)	3.5	0.30	3.5	0.25	[26]
0.01 wt% CNT + epoxy + CuO (3.5 mm thick)	4.2	0.40	4.1	0.41	[26]
U-type barium hexaferrite-epoxy composite	4.5 - 5.2	<0.30	4.7 - 5.0	<0.30	[4]
α-Fe2O3 + OPEFB fiber + PCL multilayered composite (25 wt%, 6 mm thick)	3.7	0.54	3.5	0.64	This study

4.3. Microwave Absorption

The microwave absorbing properties of the composites were examined from the reflection loss (RL) values calculated from the expression [27];

R L = 20 log | S 11 |

|S₁₁| represents the magnitude of the reflection coefficient of the composites theoretically calculated using the Finite Element Method. Figure 7 shows a positive correlation between the RL (dB) values and increases in recycled α-Fe₂O₃ nanofiller content within the stated frequency range. Generally, the values were all lower than −10 dB, which suggests that all the composites can be predicted to possess 90% microwave absorption capabilities in the 1 - 4 GHz frequency range. The most prominent RL (dB) values were located in the 1.38 - 1.46 GHz band with a minimum value of −38 dB attained by the 25 wt% composite. A secondary set of minimum values were also obtained for the 15 wt% - 25 wt% composites in the 2.39 - 3.49 GHz with the lowest of −25 dB at 2.8 GHz.

The predicted high absorption properties of the composites could be attributed to their highε" values resulting from the enhanced interfacial polarization

due to the combination of the OPEFB fiber and recycled α-Fe₂O₃ nanofiller. These simulated results demonstrate that the recycled α-Fe₂O₃/OPEFB fiber/PCL multi-layered composites can significantly absorb microwaves and would also be environmentally friendly and cheaper alternatives for applications in the 1 - 4 GHz.

In order to visualize the distribution of the electric field within the composites to assess their absorbing properties, the x-component (V/m) of the field was simulated and its direction away from the conducting strip analyzed at 4 GHz using arrows. As depicted in Figure 8(a), the arrows for the unloaded microstrip shows a scattered electric field distribution array particularly in the immediate vicinity of the conducting strip located at the center. However, each of the sample-loaded microstrip shown in Figures 8(b)-(f) demonstrates symmetrically and equally dispersed electric field directions alongside the conducting strip line that projects upwards and then back to ground. By increasing the recycled α–Fe₂O₃ nanofiller content, the size of the electric field returning to ground becomes smaller due to absorption. This result is very much consistent with Figure 4 in the sense that, the higher the ε" values of the composites the fewer the electric field that is distributed to ground as a result of absorption. The simulation of the electric field distribution therefore indicates that the composites are capable of absorbing microwaves in the 1 - 4 GHz region.

5. Conclusion

In this study, 6 mm thick recycled α-Fe₂O₃/OPEFB fiber/PCL multi-layered composites were successfully prepared and investigated for their complex permittivity and microwave absorption properties in 1 - 4 GHz. The results showed that the relative complex permittivity properties of the composites increased linearly with recycled α-Fe₂O₃ nanofiller content and the ε' values obtained varied between 3.0 and 3.9 while the ε" values ranged between 0.26 and 0.64 within 1 - 4 GHz. Additionally, a strong correlation between loss tangent and fractional composition of the recycled α-Fe₂O₃ nanofiller was established at 2.4 GHz which indicated that higher absorption could be attained by increasing the nanofiller content beyond 25 wt% The Reflection loss (dB) of the composites showed the most prominent values within the 1.38 - 1.46 GHz band with a minimum of −38 dB attained by the 25 wt% composite. Another batch of minimum values was also obtained in the 2.39 - 3.49 GHz with the lowest of −25 dB at 2.8 GHz. The recycled α-Fe₂O₃/OPEFB fiber/PCL multi-layered composite are promising materials that can be engineered for solving low frequency (below 4 GHz) noise problems while inherently providing low density, low-cost and environmental benefits.

Acknowledgements

The authors wish to thank the Institute of Advanced Materials (ITMA), UPM for the use of the material synthesis and characterization laboratory and the Department of Physics, UPM for providing access to the RF/microwave laboratory.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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