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

The aim of this study was to fabricate multi-layered recycled α-Fe2O3/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.


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 carbonbased 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-Fe 2 O 3 (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-Fe 2 O 3 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]

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/cm 2 , 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.

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-Fe 2 O 3 /PCL composites between two layers of the 6 cm × 3.6 cm × 0.20 cm OPEFB fiber/PCL composites before

Morphology
The distribution of the OPEFB fiber and recycled α-Fe 2 O 3 nanoparticles within the PCL matrix was examined using the Field Emission Scanning Electron Microscope (JEOL JSM-7600 FESEM, JEOL, Tokyo, Japan).

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.

Microwave Absorption
The reflection coefficient magnitudes (|S 11

Morphology
The respective distributions of OPEFB fiber and recycled α-Fe 2 O 3 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)

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-Fe 2 O 3 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-Fe 2 O 3 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   [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-Fe 2 O 3 /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-Fe 2 O 3 /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-Fe 2 O 3 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-Fe 2 O 3 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-Fe 2 O 3 nanofiller ( Figure 6) was observed, indicating that higher absorption could be attained by increasing the nanofiller content beyond 25 wt%.

Microwave Absorption
The  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-Fe 2 O 3 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.

Conclusion
In this study, 6 mm thick recycled α-Fe 2 O 3 /OPEFB fiber/PCL multi-layered composites were successfully prepared and investigated for their complex per-