Development of high performance microwave absorption modified epoxy coatings based on nano-ferrites

With the rapid spread of wireless technologies and increasing electromagnetic energy, electromagnetic waves (EMW) have become a severe threat to human health. Therefore, minimizing the harmful effects of electromagnetic wave radiation is possible through the development of high-efficiency EMW absorption coatings. The aim of this work was to generate microwave absorbance coatings containing synthesized nano-CuFe2O4 and nano-CaFe2O4. Firstly, nano-CuFe2O4 and nano-CaFe2O4 were synthesized using the sol–gel method. Then, their structure, electrical, dielectric, and magnetic properties were investigated to find out the possibility of using these materials in high-frequency applications (e.g., microwave absorbance coatings). After that, two dosages (2.5 wt% and 5 wt%) of nano-CuFe2O4 and nano-CaFe2O4 were incorporated into epoxy resin to prepare modified epoxy resin as microwave coatings. The dielectric studies show that the AC conductivity of the prepared samples is high at high frequencies. Additionally, the magnetic properties reveal a low coercivity value, making these samples suitable for high-frequency devices. The microwave results illustrate that adding nano-ferrites with high content enhances the absorption characteristics of the tested films. The results showed that the two films have two absorption bands with RL < –10 dB ranging from 10.61 to 10.97 GHz and from 10.25 to 11.2 GHz. The minimum return loss achieved for the two cases is −13 and −16 dB, respectively. Indicating that the film coated with CuFe has a better absorption value than the one coated with CaFe.


Preparation of the prepared nano-ferrites
As presented in Fig. 1, Fe(NO 3 ) 3 ⋅9H 2 O (0.2 M) and Ca(NO 3 ) 3 ⋅4H 2 O (0.1 M) were stirred at ambient temperature to prepare nano-CaFe 2 O 4 .Also, Fe(NO 3 ) 3 ⋅9H 2 O (0.2 M) and Cu(NO 3 ) 3 ⋅3H 2 O (0.1 M) were mixed to prepare nano-CaFe 2 O 4 .Then, 5% (w/v) CTAB was added to the two mixtures as a dispersing agent with continuous stirring at high speed.After that, 2 N NaOH was gradually added while being stirred frequently until the pH reached 8. Following thorough precipitation, the precipitates were cleaned with ethanol and deionized water before being set aside to dry at 80 °C.The dried dark red powders were then annealed at 550 °C in a muffle.

Characterization of the prepared nano-ferrites
The morphology and elemental composition of the synthesized nano-ferrites were investigated using scanning electron microscope connected to energy dispersive X-ray spectroscopy (SEM/EDX-JEOL JED 2300).Transmission electron microscope (TEM-JEOL JX 1230) was used to determine the particle shapes and sizes of the nano-ferrites.X-ray powder diffraction patterns (XRD) were investigated at ambient temperature using a Philip's diffractometer (Model PW1390), employing Ni-filtered Cu Kα radiation (λ = 1.5404Å).The diffraction angle, 2θ, was scanned at a rate of 2°/min.Particle Sizing Systems\ZPW388 obtained from Santa Barbara, Calif., USA was used.X-ray photoelectron microscopy (XPS) was performed by (KRATOS-AXIS) Ultra spectrometer.The magnetic properties were performed at room temperature using a vibrating sample magnetometer (VSM; Lake Shore, USA), and the maximum magnetic field is 20 KG.
The dielectric and electrical properties of the nano-ferrites were carried out over a frequency range (10 -1 to 10 7 ) and at ambient temperature.A high-resolution alpha analyzer (Novocontrol Technologies, GmbH & Co. KG) is employed to achieve these measurements.For measurements, a disc of the sample is sandwiched between two gold-plated brass electrodes of 10 mm in diameter in parallel plate geometry.The complex dielectric function was obtained by: where ε' is a real part and ε'' is an imaginary part or dielectric loss, i = √−1, and ν is the frequency.The complex conductivity σ* = σ' + iσ" and electric modulus M* = M' + iM" are determined and interpreted according to their relationships, as detailed in 23,24 .The frequency dependence of the three parameters (ε′, M″, and σ′) at room temperature will be considered here.These three parameters are interrelated to each other according to: which, Vol.:(0123456789)

Production of coatings formulations containing nano-ferrites
In this work, a ball mill was used to prepare epoxy resin composite coating according to the proportion of nanoferrites to epoxy resin mass fractions of 2.5% and 5%.Firstly, nano-CaFe 2 O 4 or nano-CuFe 2 O 4 was dispersed in a mixed solution of xylene and N-butanol (weight ratio of 7:3) using ultrasonic for 0.5 h.After that, the dispersed solution was mixed with the epoxy in a ball mill for 1 h.Finally, four paint formulations based on epoxy containing nano-CaFe 2 O 4 or nano-CuFe 2 O 4 at 2.5 and 5%, which were denoted as CaFe (2.5%), CaFe (5%), CuFe (2.5%), and CuFe (5%), were obtained.

Methods of testing and evaluation of coating
Several ASTM standards, including hardness (ASTM D 6577), ductility (ASTM D 5638), impact resistance (ASTM D 2794), and pull-off strength (ASTM D 4541), were used to evaluate the coated films' elasticity, strength, and flexibility.

Microwave setup
tan δ = ε ′ ε ′′ The ability of materials to absorb microwaves is directly linked to their electromagnetic character- istics.To investigate the absorption properties of microwaves, the electromagnetic features of coatings containing nano-CaFe 2 O 4 and nano-CuFe 2 O 4 were measured using SPEAG-DAK 3.5 (200 MHz to 20 GHz).In this work, one thickness (1.5 mm) was used, according to the literature 25 , to examine only the effect of both concentrations on the absorption properties of microwaves.These measurements included the samples' relative complex permittivity, Eq. ( 1), which is made up of real and imaginary parts.The real part represents the storage capacity of dielectric energy, while the imaginary part indicates the dissipation of dielectric.The dielectric loss tangent is www.nature.com/scientificreports/used to describe the electromagnetic wave absorption properties of the absorbers.The reflection coefficients of rubber absorber sheets were measured using a transmission line technique in the 8-12 GHz frequency range at room temperature.A rectangular sample of rubber was placed in an aluminium specimen holder that connected the two waveguide sections, each 60 mm in length.The two waveguide sections were then connected to two ports of a Rhode and Schwartz model ZVA67 VNA vector network analyzer through two waveguide adaptors, as shown in Fig. 2. A full two-port transmission-reflection-line (TRL) calibration was performed to eliminate any loss due to the sample holder 9 .From the measured S 11 , the reflection loss (RL) was calculated as:

Results and discussion
X-ray diffraction (XRD)   where K = 0.89 is the Scherer constant, β is the width of the peak, θ is the Braggs diffraction angle, and λ is the wavelength of the X-ray 28 .The average crystal size of nano-CaFe 2 O 4 was calculated to be 42.4 nm, while nano-CuFe 2 O 4 was found to be 46.2 nm, clearly indicating that the synthesized nano-ferrites are of nano-scale.
To examine the morphology and particle size of both nano-CaFe 2 O 4 and nano-CuFe 2 O 4 , TEM and SEM were employed.TEM in Fig. 4a, b demonstrates that nano-CaFe 2 O 4 has a particle size range of 8.18 to 25.7 nm and nano-CuFe 2 O 4 has a particle size range of 13.37 to 33.70 nm.Additionally, the aggregation of nano-CaFe 2 O 4 and nano-CuFe 2 O 4 is made clear by TEM images, which may be triggered by the high surface energy and magnetic forces that occur between nanoparticles.On the other hand, SEM micrographs of synthesized nano-ferrites show their spinel structure, as seen in Fig. 4c, d.

Energy dispersive X-ray analysis (EDX)
EDX analysis was employed to figure out the constituent elements of nano-CaFe 2 O 4 and nano-CuFe 2 O 4 .According to an EDX investigation of the spectrum shown in Fig. 4e, f, the Fe, Ca, and O elements have been identified, which confirms the generation of nano-CaFe 2 O 4 .Additionally, EDX for nano-CuFe 2 O 4 precisely determines the presence of Cu, Fe, and O.

Dielectric and electrical investigations
The real part of complex permittivity, ε', is illustrated graphically as a function of frequency for CaFe 2 O 4 and CuFe 2 O 4 at room temperature in Fig. 6a.Three different trends are clearly shown in the figure.The lower range of frequencies (0.1-10 Hz) shows a clearly linear increase with decreasing frequency.This originated from the contribution of charge carriers' transport, which causes ac conductivity.Further increases in frequency show shoulder-like behavior in the intermediate range of frequencies.This behavior may be attributed to the wellknown interfacial polarization due to the accumulation of some ions at the interfaces at the borderers between different components and their heterogeneous structures 36 .
Similar behavior was also shown on the frequency dependence of dielectric loss ε′′(f), as illustrated in Fig. 6b.This confirms that both parameters are not independent.At the higher limit of the frequency window (≥ 10 MHz), the permittivity values of both compositions collapse together and become independent of frequency.This can be explained according to the fact that the alteration of all kinds of polarizations lags behind the frequency of the externally applied electric field.This phenomenon became well known in many composites recently 37 .
The representation of the imaginary part of the electric modulus M''(f) is usually used in dielectric characterization because it suppresses undesirable capacitance effects due to electrode contacts and provides a clear view of the DC conduction and dipole relaxation [36][37][38] .Figure 7 illustrates graphically the imaginary part of the electric modulus as a function of frequency.The two investigated processes are clear peaks in this representation.Both processes seem to be faster in the case of the CuFe 2 O 4 composite.This may indicate that the degree of freedom of the Cu ions is higher than that of the calcium ions.
Generally, this investigated behavior can be explained on the basis of the Maxwell-Wagner bi-layered model, which is in agreement with Koop's dispersion phenomenological theory 39,40 .In this model, the dielectric material is composed of two layers.One of them is the grain boundary, which is a conductive layer, and the other is the grain boundary, which is a poorly conductive layer.The grain boundary is more active at lower frequencies, while its activity becomes less and the grain's activity dominates at higher frequencies.This is due to the movement of charge carriers (electrons) in the material that can move in the direction of the electric field by hopping mechanisms to the grain boundary at lower frequencies.The charge carriers accumulate at the grain boundary (a poorly conductive layer), forming space-charge polarization and causing the high value of the dielectric constant.As the frequency increases, the probability of reaching the grain boundary by the charge carriers is weak because they cannot follow the direction of the applied electric field, which thus causes the dielectric constant to decrease until it reaches a constant value [40][41][42] .
In the presented study, ε' has a high value at lower frequencies because the electron hopping between cations (Fe 3+ ↔ Fe 2+ in both the CaFe 2 O 4 and CuFe 2 O 4 samples besides Cu 2+ ↔ Cu 1+ in the CuFe 2 O 4 sample as confirmed from XPS) takes place in the direction of the applied field, whereas it has a low value at higher frequencies due to the electron hopping between cations not following the alternating field 41,43 .Additionally, CuFe 2 O 4 displayed improved dielectric properties compared to CaFe 2 O 4 over the frequency range.This refers to the increase in charge carrier number that decreases the resistance of the sample, and consequently, the polarization increases, resulting in a high value of ε' 40,44,45 .
The dependence of AC electrical conductivity on frequency for CaFe 2 O 4 and CuFe 2 O 4 at room temperature is shown in Fig. 8.It is observed that the conductivity increases as the frequency increases for the investigated samples due to the increased field applied to the charge carriers, which causes an increase in their hopping or tunnelling 46 .In ferrites, the electrical conductivity is due to the exchange of electrons between ions of the same element that have different valence states 41,47 .At low frequencies, the conductivity is low because of the grain boundary effect that reduces hopping electrons between Fe 3+ and Fe 2+ .In contrast, the effect of the grain boundary becomes less at high frequencies, and the grain effect is dominant, and leading to hopping electrons 41 .Also, the AC conductivity of CuFe 2 O 4 is higher than that of CaFe 2 O 4 owing to the cationic distribution in the samples and the flow of charge carriers because of the grain effect, which is more active than the grain boundary effect 44,47 .
From the obtained results, it can be concluded that the real part of the complex permittivity and dielectric loss of the prepared nano-ferrites showed the dielectric behavior of these samples.Therefore, they can be used as

Magnetic properties
Figure 9 shows the magnetic hysteresis plots of nano-CaFe 2 O 4 and nano-CuFe 2 O 4 at room temperature.The inset figure illustrates the magnified vision of the low-field magnetization region.The S shape in the hysteresis loop for the examined samples indicates the ferromagnetic behavior of the samples, which is somewhat strong for nano-CuFe 2 O 4 while weak for nano-CaFe 2 O 4 .The values of magnetic parameters as coercivity, H c , remanent magnetization, retentivity, M r , saturation magnetization, M s , and squarence ratio, M r /M s , are calculated from hysteresis plots and tabulated in Table 1.
It is noticed that the value of M s for nano-CuFe 2 O 4 is higher than that of nano-CaFe 2 O 4 .The coercivity, Hc, of the samples is very low, which means that these samples can be considered soft magnets 49,50 .Moreover, the low value of coercivity indicates the negligible hysteresis loss of microwave energy which makes these samples candidates for high-frequency devices 2,51 .

Mechanical properties
The mechanical features of CaFe (2.5%), CaFe (5%), CuFe (2.5%) and CuFe (5%) are shown in Fig. 10a.The figure shows that the hardness of CaFe (5%) and CuFe (5%) is higher than those containing 2.5%, which are approximately 240.56 and 243, respectively.Coatings containing 5% provided the best hardness values due to their high content in the film, resulting in the formation of a uniform film that had acceptable hardness.Meanwhile, the coatings with 5% had low impact resistance and ductility due to the presence of a significant quantity of epoxy rings that formed an inflexible and dense macromolecular framework.These characteristics, which result in a stiff and brittle structure, are a direct consequence of the hardener's depletion of all reactive sites.

Pull-off strength results
The pull-off test was used to determine the level of adhesion for coated panels that were both dry and wet.An equation has been used to estimate the adhesion loss (ψ) values as follows: where α D denotes the dry adhesion and α w represents the wet adhesion.
Figure 10b demonstrates that the films coated with CaFe (5%) and CuFe (5%) delivered the best dry and wet adhesion strengths with low adhesion loss, which are approximately 4 and 4.8%, respectively, while the adhesion loss of CaFe (2.5%) and CuFe (2.5%) is in the range of 31.57and 29.59%, respectively.The presence of nanoferrites with high content has improved both adhesion strength and reduced adhesion loss.This is attributed to   www.nature.com/scientificreports/ the good arrangement of the nano-particles, which can block the whole voids in the matrix and form a tight film that could restrict the diffusion of aqueous solution through the film, so the adhesion is not affected 52 .

Microwave measurements
The variation of the return loss of the four films coated with CaFe (5% and 2.5%) and CuFe (5% and 2.5%) is shown in Fig. 11.From this result, it can be observed that adding nano-ferrites with high content enhances the absorption characteristics of the tested films, as the two films coated with 2.5% of nano-ferrites have a return loss of around −5 dB.However, the two films coated with (5% wt) of nano-ferrites have two absorption bands with RL < -10 dB ranged from 10.61 to 10.97 GHz for CuFe and from 10.25 to 11.2 GHz for CaFe, with centre frequencies of 10.75 GHz and 10.8 GHz for CuFe and CaFe, respectively, as shown in Fig. 13.The minimum return loss achieved for CaFe is −13 dB and for CuFe is −16 dB, respectively.Indicating that the film coated with CuFe has a better absorption value than that coated with CaFe, and the coated films containing a high ratio of both nano-ferrites (e.g., 5%) offer the best absorption value.Therefore, the dielectric measurements in the range 8-12 GHz have been performed for 5% of both CaFe and CuFe. Figure 12 represents the measured ε ′ , ε ′′ and tanδ ǫ of the two films coated with CaFe and CuFe at a high concentration of 5%.From this figure, it can be observed that the values of ε ′ have a clear resonance at 10.75 GHz.Also, tanδ ǫ is an effective parameter for indicating the dielectric loss capability of the absorber.A moderate value  a combination of dielectric loss, magnetic loss, multiple reflections, scattering, and interface effects.Figure 14 illustrates a schematic diagram of the mechanism of microwave absorption.The dielectric loss occurs because of the polarization of the electric dipoles within the material in response to the applied electric field of the electromagnetic wave.Both nano-CuFe 2 O 4 and nano-CaFe 2 O 4 , exhibit dielectric properties that cause them to absorb and dissipate energy from the incident electromagnetic waves.Another absorption mechanism is magnetic loss.Nano-ferrites possess a high magnetic permeability, which allows them to interact strongly with the magnetic component of the electromagnetic wave.This interaction leads to the conversion of the magnetic energy into heat, resulting in energy absorption and attenuation of the electromagnetic wave.Also, the synthesized nanoferrites contribute to multiple reflections and scattering of the incident electromagnetic waves.Due to their small size and unique morphology, the nano-ferrite particles can scatter the incoming waves in multiple directions.This scattering effect enhances the path length of the waves within the coatings, increasing the opportunity for absorption and dissipation of energy.The interfaces between the nano-ferrite particles and the surrounding matrix (such as epoxy resin) also play a role in the absorption mechanism.These interfaces can lead to additional absorption due to interfacial polarization and frictional losses.All these mechanisms collectively contribute to the effective absorption of microwave energy within the coatings, reducing the reflection of electromagnetic waves.It's important to note that the specific absorption characteristics of nano-CuFe 2 O 4 and nano-CaFe 2 O 4 can depend on factors such as their composition, particle size, morphology, and the surrounding matrix material 9 .

Conclusions
In summary, new and cost-effective electromagnetic absorbing coatings that may effectively minimize electromagnetic radiation were successfully developed by the introduction of nano-CuFe 2 O 4 and nano-CaFe 2 O 4 modified epoxy resin.The study examined the mechanical characteristics of coatings that absorb electromagnetic waves, as well as the impact of coating impermeability and EMW absorption qualities.From this investigation, the following results are drawn: • The addition of proper concentration of nano-CuFe 2 O 4 and nano-CaFe 2 O 4 can effectively improve the adhe- sion and hardness of epoxy resin coating.Among them, the optimized modified epoxy coating with nanoferrites content of 5% exhibits the best enhancement effect, which can be attributed to the good arrangement of the nano-particles, which can block the whole voids in the matrix and form a tight film that could restrict the diffusion of aqueous solution through the film.• The microwave results illustrate that adding nano-ferrites with high content (e.g., 5%) enhances the absorp- tion characteristics of the tested films.• From this result, it can be observed that adding nano-ferrites with high content enhances the absorption characteristics of the tested films, as the two films coated with 2.5% of nano-ferrites have a return loss of around −5 dB.However, the two films coated with (5% wt) of nano-ferrites have two absorption bands with RL < -10 dB ranged from 10.61 to 10.97 GHz for CuFe and from 10.25 to 11.2 GHz for CaFe, with centre frequencies of 10.75 GHz and 10.8 GHz for CuFe and CaFe, respectively.Indicating that the film coated with CuFe has a better absorption value than that coated with CaFe.• Thus, the novel electromagnetic-absorbing coatings synthesized in this work can be widely applied for the purpose of protecting the environment from building electromagnetic pollution and reducing electromag- www.nature.com/scientificreports/netic radiation caused by health hazards to people.They can also be used in television stations, airports, docks, and navigation beacons to eliminate reflection interference and reduce electromagnetic radiation.

Figure 5
Figure5demonstrates the zeta potential of nano-CaFe 2 O 4 and nano-CuFe 2 O 4 .Zeta potential is widely recognized for providing insight on the stability of nanoparticles in the media in which they are dispersed.The particle is considered stable if the potential is within the range of + 30 to −30 mV.The figure illustrates that both nano-ferrites fall within the indicated range, demonstrating that they are stable and do not aggregate under the intended usage conditions29 .

Figure 9 .
Figure 9.The magnetic hysteresis plots of nano-CaFe 2 O 4 and nano-CuFe 2 O 4 at room temperature.

Figure 10 .
Figure 10.(a) Mechanical properties and (b) pull-off test results of the prepared coatings.

Figure 11 .Figure 12 .Figure 13 .
Figure 11.Frequency response of the reflection loss of the prepared coatings.
possible valence states of each cation in nano-CuFe 2 O 4 and nano-CaFe 2 O 4 have been determined by per- forming XPS analysis.Figure5bis the full XPS spectra of nano-CuFe 2 O 4 and nano-CaFe 2 O 4 .The XPS spectra exhibit characteristic peaks at binding energies representing C 1s, O 1s, Fe 2p, Ca 2p, and Cu 2p.The Fe 2p 3/2 and Cu 2p 3/2 spectra can be well-fitted by two synthetic curves, indicating two possible valences for each ion, as shown in Fig.5c, d.In Fig.5c, two peaks are shown at binding energies of 710.02 and 709.9 eV for the Fe 2p3/2 line in nano-CuFe 2 O 4 and nano-CaFe 2 O 4 , respectively.These peaks are attributed to Fe 2+ .On the other hand, the peaks at binding energies of 712.63 and 711.78 eV in both nano-CuFe 2 O 4 and nano-CaFe 2 O 4 , respectively, are attributed to Fe3+30,31.Figure5ddisplays two peaks at binding energies of 933.2 and 936.06 eV for the Cu 2p 3/2