Recycled toner-based Ni–Zn ferrites for microstrip patch antenna applications: structural, magnetic, and electromagnetic properties analysis

Waste printer toner powder was weighed and mixed with 50 mL of ethanol in a ratio of 1:10 in a beaker, whereby two NdFeB magnets were placed on the exterior sides of the beaker to facilitate the magnetic separation. The suspension was vigorously stirred to ensure homogeneous dispersion of the particles. After stirring, magnetic particles were attracted to the magnets, while non-magnetic particles remained suspended in the solution. The liquid above the sediment was carefully discarded, and the stirring and magnetic separation steps were repeated twice using fresh ethanol to enhance the purity of the magnetically separated particles. The collected magnetic powder, primarily magnetite (Fe₃O₄), was then dispersed in 15 mL of butyl acetate and stirred continuously for 60 min to remove impurities (non-magnetic powder) inside the lumps of toner. Following this purification step, the suspension was subjected to multiple centrifugation cycles to further eliminate residual contaminants. The collected magnetic powder was subsequently dried and left overnight in an oven at 90 °C. Later on, the dried powder was calcined at 500 °C for 2 h to decompose any remaining organic residues. Finally, the material was sintered at 1200 °C for 2 h to complete the extraction process.

To synthesize the nickel–zinc ferrites powder, hematite derived from the printer toner waste has been used and mixed thoroughly with nickel oxide and zinc oxide in a stoichiometric ratio corresponding to the nominal composition of Ni_0.3Zn_0.7Fe₂O₄. All precursors were initially homogenized with an agate mortar and later subjected to a high-energy ball milling process with a powder-to-ball weight ratio of 1:10 for 3 h at a rotational speed of 800 rpm to achieve further homogenization and particle size reduction. Following the milling process, the milled powders were sintered in a tube furnace at target temperatures ranging from 800 to 1200 °C with increments of 100 °C. A constant heating rate of 4 °C/min was applied, and the soaking time for each sintering temperature was maintained for 10 h.

In order to fabricate the microstrip patch antenna (MPA), FR4 was used as a substrate, while the sintered WNZF powders were made into a thick paste for a radiating patch component. To fabricate the WNZF paste, an organic binder was initially prepared by mixing 85 wt.% of linseed oil, 12.5 wt.% m-xylene, and 2.5 wt.% α-terpineol for 3 h [17]. Later on, the WNZF powder was mixed with the organic binder in a weight ratio of 60:40. The mixture was mixed using a vortex mixer for 30 min to achieve a homogeneous paste suitable for thick film printing. The viscosity of the prepared paste was measured using a Malvern Kinexus Rheometer equipped with a parallel plate geometry at room temperature under controlled shear rate mode using a fixed shear rate of 100 s⁻¹ [18]. The prepared paste was then printed onto an FR4 substrate via a screen-printing technique using a silk-screen frame with a measurement suitable for S-band frequencies. Following printing, the printed films were dried and cured at 150 °C for 40 min to promote solvent evaporation and ensure proper adhesion of the NZF thick film onto the substrate. Finally, the electrical connection to the MPA was carried out by attaching a SubMiniature version A (SMA) connector to the printed microstrip patch using silver paste and copper tape as a ground to complete the MPA assembly. The schematic diagram of the fabricated MPA and its specifications are given in Fig. 1 and Table 1, respectively. This integrated waste-to-functional-material workflow represents a novel and reproducible method for fabricating ferrite-based MPAs.

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To evaluate the viability of the oxidation–reduction process for hematite extraction and to investigate the structural evolution of waste-based nickel–zinc ferrites (WNZF) prepared using the extracted hematite, a Rigaku Miniflex II X-ray Diffractometer with Cu-Kα radiation was used and scanned from 20° to 80° with a scan step of 0.02°. A NETZSCH Differential Scanning Calorimeter (DSC) was used to investigate the thermal characteristics of the waste toner powder during the oxidation–reduction process, whereby the DSC scan was performed at 10 °C/min under an air atmosphere with baseline subtraction using an empty aluminium crucible measurement. The particle size confirmation of the extracted hematite was conducted using Talos L120C Transmission Electron Microscope. The sample morphology and grain size distribution of fabricated WNZF were examined using a Hitachi TM3030 Plus Scanning Electron Microscope, with the particle sizes were determined from the diameter measurement of 100 individual particles via an ImageJ software. The elemental composition was analysed via an Energy Dispersive X-ray (EDX) spectroscopy to confirm the chemical purity of the synthesized samples. The green density of the samples was assessed using Archimedes' principle using an ultrafiltered deionized water as a medium. The static and dynamic magnetic behaviours were both measured using a Lakeshore 704 Vibrating Sample Magnetometer and an ET4401 LCR-meter, respectively. Electromagnetic performance of MPA using WNZF as a radiating patch was evaluated using a Nano Vector Network Analyzer V2_2 with a frequency ranging from 2 to 4 GHz at 501 sweep points. The workflow of the complete procedure for the extraction process until MPA fabrications and characterization is illustrated in Fig. 2.

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The viability of the oxidation–reduction process in extracting hematite has been investigated using an XRD analysis, as shown in Fig. 3. The XRD pattern reveals significant structural transformations as a function of the purification and heat treatment involved in the oxidation–reduction process. After being subjected to the chemical treatment and magnetic separation process, the XRD pattern shows the presence of a single-phase magnetite (Fe₃O₄; ref. code: 98–010-9827) with a cubic structure, a space group of Fd-3 m, and a dominant diffraction peak corresponding to the (113) plane. The main diffraction peaks for magnetite appear at 35.66° with broad peaks, which suggests that the material possesses small crystallite sizes and high lattice strains. After being subjected to the heat treatment at 500 °C, there is an existence of hematite peaks (Fe₂O₃; ref. code:98–001-2634), which indicates the phase transformation from magnetite to hematite. This indicates that during the heat treatment, magnetite is oxidized to hematite, whereby the reaction can be represented as 6Fe₃O₄ + O₂ → 4Fe₂O₃. During this process, oxygen from the air (air contains 21% oxygen) acts as an oxidizing agent, facilitating the conversion by accepting electrons from the iron atoms in magnetite. The heat treatment process promotes the rearrangement of iron and oxygen atoms to form a more stable hematite phase. In fact, the transformation of magnetite to hematite can occur more rapidly in smaller particles due to their higher surface area and greater reactivity. The oxidation process, which is diffusion-controlled, can proceed more quickly in smaller particles. Upon sintering at a higher temperature of 1200 °C, all the peaks indicate a single-phase hematite (ref. code: 98–001-2634) with a space group of R-3c and a dominant diffraction peak corresponding to the (104) plane. A notably narrower FWHM of 0.079° can be observed, reflecting an increase in crystallite size and improved crystallinity, as heat treatment promotes particle growth and reduces lattice defects.

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To further verify the efficiency of the hematite extraction from printer toner waste via the oxidation–reduction process, thermal analysis investigation has been carried out whereby the thermal behaviour of the sample was conducted using differential scanning calorimetry (DSC), as shown in Fig. 4. The DSC thermogram revealed two distinct glass transitions, with midpoints at 38.7 °C (ΔC_p = 0.670 J/g.K) and 222.8 °C (ΔC_p = 0.228 J/g.K), respectively. The presence of these two glass transition temperatures indicates the existence of amorphous phases, which may arise from residual additives and plasticizers in the printer toner waste. This behaviour is consistent with the complex composition of toner powders, which typically contain polymer resins, pigments, waxes, and various additives. The coexistence of multiple polymeric components or phases within the toner can therefore give rise to more than one distinct glass transition. Notably, a prominent endothermic peak was observed at 384.7 °C, with an enthalpy change of -377.2 J/g. This indicates a major phase transformation, corresponding to the conversion of magnetite (Fe₃O₄) to hematite (α-Fe₂O₃), where this phase transformation is endothermic. The observation of this transition is in agreement with previous reports [19], where the magnetite-to-hematite transformation typically occurs between 300 and 400 °C, depending on the sample purity, particle size, and atmospheric condition, which significantly affect the transformation kinetics [20]. This result is supported by the XRD analysis, which reveals that prior to the calcination process, all diffraction peaks correspond to the magnetite peaks, while after calcination at 500 °C, the XRD pattern shows characteristic peaks of both magnetite and hematite, indicating partial phase transformation. The formation of hematite from magnetite involves an initial oxidation, dissolution, and re-precipitation process. Prior to the phase transformation from magnetite (Fe₃O₄) to hematite (α-Fe₂O₃), the initial oxidation of magnetite occurs by the formation of maghemite (γ-Fe₂O₃) as an intermediate phase at the surface of the magnetite particles. This process involves the outward diffusion of Fe²⁺ ions from the magnetite, forming a magnetite/maghemite core/shell structure. Further heat treatment dissolves the maghemite in the surrounding solution and reprecipitates as hematite, whereby the transformation process can be written as 4Fe₃O₄ + O₂ → 6γ-Fe₂O₃ → 6α-Fe₂O₃. The presence of oxygen ensures that the transformation from maghemite to a more stable hematite can occur, as hematite is more stable at higher temperatures [21]. High crystallization and single-phase hematite are only achieved at higher temperatures, as confirmed by the XRD data after calcination at 1200 °C.

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Figure 5 shows the XRD pattern for nickel–zinc ferrites prepared using hematite extracted from the toner waste (WNZF). The XRD pattern reveals a clear evolution of phases before and after the sintering process. Before milling, the dominant zinc oxide phase (ref. code: 98–010-5281) was observed with a sharp peak at 36.7° and a narrow FWHM of 0.079°, indicating good crystallinity. After milling, the most prominent peak shifts to hematite (ref. code: 98–001-2634) as shown in position 33.24° with a broader FWHM of 0.276°. It suggests a reduction in crystallite size and an increase in the structural disorder due to the mechanical milling process. During this stage, fresh interfaces with defective regions are created, thus reducing the crystallization in the particles [22]. As subjected to the sintering at 800 °C, the formation of zinc ferrite (ZnFe₂O₄; ref. code: 98–001-5740) with a cubic structure and space group of

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Fd-3 m is observed, and the FWHM decreases to 0.256°, which indicates the onset of the crystallite growth. Upon sintering, ZnO reacts with hematite and forms zinc ferrites as an intermediate phase, which then further reacts with NiO to yield the single phase of NZF (ref. code: 98–006-8765) at sintering temperatures of 900 °C. The formation of zinc ferrites allows for a more uniform distribution of Zn²⁺ and Fe³⁺ ions before the incorporation of Ni²⁺, leading to better control over the final cation rearrangement within the NZF spinel structure [23]. The FWHM values systematically decrease from 0.197° at 900 °C to 0.079° after sintering at 1200 °C, which signifies a consistent improvement in crystallinity and crystallite size with increasing sintering temperature. This enhancement is due to higher thermal energy facilitating atomic diffusion and grain growth. The consistent peak positions in these varied sintering temperatures suggest that the spinel NZF phase remains stable without decomposition or secondary phase formation. The detailed structural parameter changes of all the samples are given in Table 2.

To further evaluate the reactivity and the morphological uniformity of the waste-derived hematite particles before being used as a raw material for NZF fabrication, the particle size characterization of the particles after heat treatment at 1200 °C has been carried out using TEM, as presented in Fig. 6. Since particle size strongly influences surface reactivity during sintering, the particle size was analysed to confirm its dimensions, with the corresponding histogram confirming the particle size distribution. The TEM image shows particulate agglomeration with a size distribution of the submicrometer range, with sizes predominantly between 150 and 300 nm and an average particle size of 0.213 μm. The observed distribution provides a crucial baseline for understanding the behaviour of the powder particles during subsequent thermal transformation. In fact, finer size particles are desired as high surface area from finer size particles results in high driving force and free energy, thereby triggering reaction between particles during the sintering process.

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The microstructural evolution of WNZF after sintering is analysed based on the SEM micrographs in Fig. 7. These micrographs reveal a notable transformation in grain size and morphology, which is quantitatively supported by the data given in Table 3. At the sintering temperature of 800 °C, fine and closed-packed grains with an average size of 0.78 μm can be observed with evident porosity. The relative density values are also low, indicating that the sintering process at this stage is dominated by initial particle bonding and rearrangement with limited atomic diffusion. As the sintering temperature increases to 900 °C and 1000 °C, a noticeable increase in average grain size to 1.29 μm and 1.42 μm can be observed, accompanied by corresponding increases in the relative density. These changes reflect the activation of intermediate sintering mechanisms such as grain boundary diffusion and pore shrinkage, resulting in more effective mass transport and the elimination of smaller pores. These can be observed in the SEM micrographs of the samples which display a uniform grain structure with visibly reduced porosity compared to the lower temperature sample. Further sintering at 1100 °C and 1200 °C, the microstructural changes become more pronounced, and the average grain size increases further to 1.55 μm at 1100 °C and increases significantly to 2.48 μm at 1200 °C. These observations are consistent with the dominance of final sintering mechanisms, such as lattice diffusion and grain boundary migration, which facilitate rapid grain growth and the closure of remaining pores [24]. However, the substantial grain growth observed at 1200 °C suggests the onset of abnormal grain growth, which can lead to a less uniform microstructure and potentially affect the materials’ functional properties.

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The observed trends can be attributed to the sequential activation of different sintering mechanisms. At lower temperatures, surface diffusion and initial neck formation between particles predominate, resulting in limited densification and grain growth. As the temperature increases, grain boundary diffusion becomes more prominent, promoting densification and moderate grain growth [25]. At the highest sintering temperatures, lattice diffusion and grain boundary migration dominate, leading to rapid grain coarsening and nearly complete densification [26]. The enlargement of grain size and the reduction of porosity contribute to enhanced magnetic ordering by facilitating the displacement of magnetic domain walls, thereby increasing the magnetization of the samples [27]. While higher temperatures generally improve density and reduce porosity, excessive grain growth can negatively impact the materials’ properties due to the development of inhomogeneous microstructures. Therefore, optimal sintering conditions should be carefully selected to balance densification and grain growth, ensuring the desired combination of microstructural features and functional properties in the final samples.

To determine the elemental composition of the synthesized WNZF, energy-dispersive X-ray (EDX) spectroscopy was conducted. As shown in Fig. 8, the EDX spectra revealed the presence of only Ni, Zn, Fe, and O elements, regardless of their sintering temperature, which confirms the chemical purity of the fabricated samples. The detection of C is attributed to the sample holder used during the EDX analysis. The quantitative elemental composition is presented in Table 4. As the sintering temperature increases to 1000 °C, a notable decrease in Zn content is observed. Although the boiling point of bulk Zn is 907 °C, the submicron-scale dimensions of the particles result in a significantly larger surface-to-volume ratio, which increases the surface energy and accelerates atomic diffusion. These conditions promote the earlier onset of Zn volatilization, effectively causing Zn to evaporate at lower temperatures than its standard boiling point. Additionally, sintering at relatively lower temperatures, for instance at 800 °C, results in incomplete mixing and non-uniformities between the raw materials, which potentially causes an abnormal expansion of ZnO [28]. These factors contribute to the pronounced Zn evaporation observed at 1000 °C, thereby adversely affecting the magnetic properties of the WNZF samples. At higher sintering temperatures of 1100 °C and 1200 °C, the elemental composition of the samples is quite stable. This suggests that Zn evaporation predominantly occurs around 1000 °C. At elevated temperatures, enhanced atomic mobility and improved solid-state diffusion facilitate more complete reactions, leading to the effective incorporation of Zn into the NZF lattice, thereby suppressing further Zn volatilization [29].

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In determining the suitability of the extracted hematite to be used as a raw material for NZF for technological applications, understanding the magnetic behaviour of the WNZF is essential for evaluating the functional performance of the material. As the magnetic performance of ferrites is inherently influenced by their microstructural characteristics [30], it is necessary to examine the magnetic properties of the synthesized WNZF samples to establish a structure–property relationship. In soft ferrites, hysteresis parameters such as saturation magnetization, coercivity, and remanent magnetization influence the materials’ magnetic response and overall efficiency of MPA. Details of the measured magnetic and electromagnetic parameters are given in Table 5 for clarity.

To evaluate the effectiveness of hematite extraction from printer toner waste, the magnetic properties of the samples before and after heat treatment were analysed using VSM, as shown in magnetization (M) as a function of magnetic field (H) curve in Fig. 9. Prior to calcination, the extracted sample exhibited a saturation magnetization (M_s) of 42.52 emu/g, consistent with the presence of magnetite phase, which has a good ferromagnetic behaviour. This value is significantly lower than that of commercial magnetite with M_s of 67.89 emu/g, likely due to impurities or partial oxidation in the waste-derived sample. Upon heat treatment at 500 °C, a phase transformation to hematite from magnetite reduced the M_s to 3.49 emu/g, while further calcination at 1200 °C yielded single-phase hematite with an M_s of 0.44 emu/g. This value is comparable to that of commercial hematite, as reported in previous studies [31], suggesting the viability of this waste-derived material for practical use. These findings demonstrate that magnetite recovered from toner waste can be effectively converted into hematite with magnetic characteristics close to commercial standards.

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Figure 10 shows the hysteresis loops of the WNZF samples sintered at temperatures ranging from 800 to 1200 °C, illustrating a clear influence of sintering temperature on the magnetic properties of the samples. The narrow hysteresis loops observed in the synthesized samples demonstrate their soft magnetic behaviour, suggesting low energy loss and suitability for magnetic device applications. The M_s exhibits an increase with increasing sintering temperature, from 23.11 emu/g at 800 °C to 36.17 emu/g at 1200 °C. This enhancement in M_s is closely correlated with the microstructural evolution of the WNZF. As sintering temperature increases, grain growth occurs, leading to larger average grain sizes and a reduction in internal stresses and lattice defects. Larger grains facilitate more coherent magnetic domains and reduce the spin disorder at the grain boundary, thereby increasing the net magnetic moment and resulting in higher M_s values [32]. In contrast, both remanent magnetization (M_r) and coercivity (H_c) demonstrate a decreasing trend with increasing sintering temperature. M_r decreases from 1.24 emu/g at 800 °C to 0.10 emu/g at 1200 °C, while the H_c values decline markedly from 33.18 Oe to 0.99 Oe over the same temperature range. These reductions can be attributed to the microstructural changes induced by sintering. During the milling process preceding sintering, the WNZF particles undergo repeated deformation and fracturing, resulting in a high density of defects and internal stresses that act as pinning centres for magnetic domain wall movement. These imperfections impede the domain wall movement, thereby increasing the H_c. As sintering temperature increases, the grain growth occurs, these defects are released out, and the grain boundary becomes less obstructive to the domain wall movement. Larger grains caused by higher sintering temperature typically contain multiple magnetic domains separated by domain walls, which can move more freely under an applied magnetic field, reducing the energy barrier for magnetization reversal and thus lowering the H_c.

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Furthermore, the decrease in H_c with increasing grain size is consistent with the inverse relationship between H_c and grain size in ferrimagnetic materials [33]. Larger grains reduce the magnetocrystalline anisotropy energy, which is a main contributor to the H_c, facilitating easier magnetization reversal; hence in the present investigation, it has been used to achieve lower magnetic loss. This grain size effect is significant in optimizing the magnetic properties of ferrites for high-frequency applications. Enhanced M_s coupled with reduced H_c at higher sintering temperatures indicated improved magnetic performance, which is essential for achieving efficient magnetic tuning and low-loss operation in MPA applications. Furthermore, according to Neel’s theory of ferromagnetism, the strength of superexchange interactions between magnetic moments in Fe³⁺–Fe³⁺ ions and Fe³⁺–Ni²⁺ ions in NZF sublattices would be more intense in samples with higher phase purity. A higher sintering temperature would be an important factor in enhancing the magnetic properties of the samples.

In addition to the static magnetic properties characterized by hysteresis measurement, the dynamic magnetic response of the WNZF samples can be further elucidated through complex permeability (μ* = μ’– jμ”) analysis. Complex permeability is a fundamental parameter in evaluating the frequency-dependent magnetic behaviour of ferrites, especially for high-frequency device applications such as MPAs. The complex permeability of WNZF, which represents the magnetic characteristics of the samples, consists of two components: the real part of permeability (µ’) and the imaginary part or loss factor (µ”). The real part of permeability (µ’) corresponds to the material’s capacity to store the magnetic energy, while the imaginary part (µ”) corresponds to the magnetic loss factor of the samples, associated with lagging of domain wall motion and spin rotation under an alternating magnetic field [34]. Figure 11 shows the frequency dependence of the real permeability (μ’) for WNZF samples sintered at various temperatures. For all samples, µ’ exhibits an increase with frequency, which can be attributed to the enhanced mobility of domain walls and the reduction of demagnetizing fields as the microstructure evolves with sintering. Notably, samples sintered at higher temperatures display higher values, which correlate closely with increased density and grain size, as confirmed by the XRD and microstructural analysis. Larger grains reduce the volume fraction of grain boundaries, which act as a barrier to the domain wall movement, thereby facilitating the formation of highly mobile domain walls. Furthermore, the removal of pores during grain growth minimizes stress concentrations that could otherwise hinder the easy direction of magnetization, thus promoting higher permeability [35]. These findings are consistent with previous research [36] that related improved magnetic permeability to reduced microstructural defects and increased densification in ferrite materials. Besides that, a scrutiny observation on the μ’ values also revealed that the negative or near-zero μ’ values for samples sintered at 800 °C to 1000 °C. This can be explained by the high resistivity from porous and fine-grained microstructures formed at these lower sintering temperatures. Low-temperature sintering leads to a high-volume fraction of intergranular pores and weakly connected grains, which in turn produce high resistivity and disrupt the magnetic circuit. This porous microstructure behaves similarly to a dilute magnetic composite, where the presence of non-magnetic gaps from porosity interrupts the magnetic pathways, thus reducing the effective μ’ to near-zero or even negative values. Furthermore, the magnetic properties of the samples are significantly suppressed by the demagnetizing field originating from the amorphous phase that remains in the samples at these lower temperatures. Once the sintering temperature is raised above 1000 °C, the samples develop higher density and better structural ordering. Consequently, the μ’ values increase sharply, rising from 1.30 at 800 °C to 25.97 at 1100 °C, and further to 181.72 at 1200 °C sintering temperature.

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Figure 12 presents the variation of the imaginary permeability (μ”) with frequency, revealing a similar increasing trend. The increase in μ” at higher frequencies indicates an increase in magnetic losses, which arise from the phase lag between the applied alternating magnetic field and the response of the magnetic domains. These losses are predominantly attributed to two mechanisms: domain wall relaxation and spin rotation. At lower frequencies, domain wall motion contributes primarily to the magnetic losses; as frequency increases, spin rotation becomes more significant, leading to a rise in μ”. The observed dispersion in μ” is typically associated with the resonance phenomenon, where the energy dissipation due to lagging of domain wall displacement and spin rotation is maximized just before the resonant frequency is reached. All in all, the removal of porosity, reduction in grain boundary fractions, and elimination of the structural defects collectively enhance the complex permeability, thereby improving the complex permeability and the material’s suitability for high-frequency EM applications.

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In order to evaluate the efficiency of EM power transfer from the feed to the WNZF radiating element, the return loss measurement has been carried out. However, only WNZF samples sintered at temperatures between 800 and 1000 °C were suitable for the fabrication of MPA using the screen-printing technique. This is attributed to the fine particle size observed in these samples, which enables uniform dispersion and smooth transfer through the silk mesh during the screen-printing process. In contrast, samples sintered at higher temperatures of 1100 and 1200 °C exhibited significantly larger and more agglomerated particles, resulting in poor flowability and blockage of the silk mesh, thereby rendering them unsuitable for screen printing and subsequent antenna MPA fabrication. As the thick film paste for screen printing is made using a constant powder-to-binder ratio of 60:40, the rheological behaviour of the fabricated paste has been impacted. As the grain size increases with sintering temperature, the viscosity of the paste also increases, as shown in Fig. 13. This occurs because larger particles create a more rigid particle network, which restricts the movement of the binder and increases resistance to flow. The increased grain size also reduces the surface area for binder–particle interaction, thus resulting in a mechanical hindrance from the larger and less flowable particles. When the viscosity is too high due to a larger grain size, the paste becomes difficult to transfer and spread, resulting in uneven film formation on the substrate. This is consistent with the expectation that higher grain size leads to increased viscosity and poor printability, even when the powder-to-binder ratio is held constant. As a consequence, electromagnetic measurements were only conducted on the MPA fabricated using WNZF samples sintered at 800 to 1000 °C, as only these samples could be processed and effectively printed into an even thick film required for MPA.

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Figure 14 shows the return loss (RL) of WNZF sintered at various temperatures measured using a vector network analyzer in the 2 to 4 GHz frequency range. The negative RL and corresponding resonant frequencies for each sample are summarized in Table 5. Increasing the sintering temperature from 800 to 1000 °C significantly enhances the microwave absorption properties of the samples. The sample sintered at 800 °C exhibits a negative return loss of -2.808 dB at 3.75 GHz, indicating weak microwave absorption. The 900 °C sample shows moderate improvement, with a return loss of -5.910 dB at 3.69 GHz. Notably, the sample sintered at 1000 °C demonstrates a substantial increase in absorption, achieving a negative return loss of -15.154 dB at a lower resonant frequency of 2.73 GHz. The observed trend can be attributed to microstructural changes induced by higher sintering temperatures. Elevated temperatures promote grain growth and densification, reducing porosity and enhancing the magnetic properties of the samples. This improvement facilitates better impedance matching and stronger microwave absorption, as evidenced by the more negative return loss values. Lower (more negative) return loss values indicate improved impedance matching and reduced signal reflection, which are critical for optimal MPA performance. For efficient MPA operation, a return loss below -10 dB is considered acceptable, which corresponds to less than 10% power being reflected. In practical MPA design, a return loss of at least -9.5 dB is typically required to ensure minimum signal reflection and efficient energy transfer [37]. -3 dB is an indicator where EM waves can be transmitted as much as 50%, and -10 dB is an indicator where EM waves can be absorbed by 90%. As shown in Fig. 14(a), the sample sintered at 800 °C exhibits no discernible peak reaching the -3 dB threshold. This is attributed to the small grain size and the predominance of an amorphous phase, which results in poor EM response with low negative return loss. In contrast, Fig. 14(b) shows that the sample sintered at 900 °C exhibits a distinct resonance peak surpassing the -3 dB threshold, indicating 50% of microwave absorption. Further enhancement is observed in Fig. 14(c), where the sample sintered at 1000 °C shows a peak reaching -10 dB, signifying approximately 90% microwave absorption and improved electromagnetic performance. Furthermore, the shift of the resonant frequency towards lower values with increasing sintering temperature suggests an increase in the effective permeability of the material, which is consistent with previous reports by Ahsan et al. [38]. At a smaller grain size after sintering at a lower temperature, the samples exhibit a single domain state, leading to higher domain wall resonant frequencies of 3.75 GHz. As grain size increases with increasing sintering temperature up to 1000 °C, the sample transitions from single domain to multi-domain state, which enhances initial permeability but reduces domain wall resonant frequency at 2.73 GHz.

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Furthermore, a clear relationship between the microstructural evolution and the EM performance can be described as follows: The microstructural evolution induced by the increasing sintering temperature strongly influences the interaction of EM waves with the WNZF-based MPA. At lower sintering temperatures (800 °C to 900 °C), fine grains and high porosity introduce numerous grain boundaries that act as scattering centres and pinning sites for domain wall motion. This limits domain wall movement and reduces permeability, also acting as scattering centres for incident EM waves. Consequently, the return loss values remain relatively low (-2.808 dB at 800 °C).

As sintering temperature increases to 1000 °C, grain growth and densification reduce porosity and the grain boundary fraction. This microstructural refinement facilitates easier magnetic domain movement, leading to enhanced permeability (μ’ = 7.96 at 100 kHz) and improved impedance matching. As a result, the return loss reaches -15.154 dB at 2.73 GHz, indicating excellent microwave absorption and MPA performance.

At excessively high sintering temperatures (> 1100 °C), abnormal grain coarsening and Zn volatilization degrade microstructural uniformity. Although the permeability continues to increase (μ’ = 181.72 at 1200 °C sintering temperature), the associated agglomeration reduces processability for screen printing and may not translate into practical MPA improvements. Hence, the observed trends in permeability and return loss clearly demonstrate that optimal EM performance is achieved at intermediate sintering temperatures, where a balance between grain growth, densification, and homogeneity is established. These findings demonstrate that optimizing the sintering temperature of the samples is a viable approach for developing better performance of MPA materials for wireless applications. Unlike conventional ferrite studies that rely on commercial precursors, the present work demonstrates that toner-derived ferrites can achieve competitive microwave absorption properties, highlighting the novelty of waste-based ferrite application in MPAs. For instance, as shown in Table 6, the M_s achieved in the present study (36.17 emu/g) is comparable to values reported for conventional NZF synthesized from commercial precursors. Similarly, the return loss of -15.154 dB at 2.73 GHz is competitive with ferrite-based MPAs in the previous literature. These findings confirm that recycled toner-derived ferrites can match, or even surpass, the conventional materials, thus strengthening the case for sustainable material substitution.