Superparamagnetic ZnFe2O4 Nanoparticles-Reduced Graphene Oxide-Polyurethane Resin Based Nanocomposites for Electromagnetic Interference Shielding Application

Superparamagnetic ZnFe2O4 spinel ferrite nanoparticles were prepared by the sonochemical synthesis method at different ultra-sonication times of 25 min (ZS25), 50 min (ZS50), and 100 min (ZS100). The structural properties of ZnFe2O4 spinel ferrite nanoparticles were controlled via sonochemical synthesis time. The average crystallite size increases from 3.0 nm to 4.0 nm with a rise of sonication time from 25 min to 100 min. The change of physical properties of ZnFe2O4 nanoparticles with the increase of sonication time was observed. The prepared ZnFe2O4 nanoparticles show superparamagnetic behavior. The prepared ZnFe2O4 nanoparticles (ZS25, ZS50, and ZS100) and reduced graphene oxide (RGO) were embedded in a polyurethane resin (PUR) matrix as a shield against electromagnetic pollution. The ultra-sonication method has been used for the preparation of nanocomposites. The total shielding effectiveness (SET) value for the prepared nanocomposites was studied at a thickness of 1 mm in the range of 8.2–12.4 GHz. The high attenuation constant (α) value of the prepared ZS100-RGO-PUR nanocomposite as compared with other samples recommended high absorption of electromagnetic waves. The existence of electric-magnetic nanofillers in the resin matrix delivered the inclusive acts of magnetic loss, dielectric loss, appropriate attenuation constant, and effective impedance matching. The synergistic effect of ZnFe2O4 and RGO in the PUR matrix led to high interfacial polarization and, consequently, significant absorption of the electromagnetic waves. The outcomes and methods also assure an inventive and competent approach to develop lightweight and flexible polyurethane resin matrix-based nanocomposites, consisting of superparamagnetic zinc ferrite nanoparticles and reduced graphene oxide as a shield against electromagnetic pollution.


Introduction
Recently, the rapid progress in electronic devices and information technology has endorsed the widespread utilization of high-power electromagnetic waves in scientific, commercial, civil, and military applications [1,2]. The abundant electromagnetic interference (EMI) in the environment has influenced the working of electronic devices [3]. Exposure to electromagnetic radiation has also influenced human health [4]. Electromagnetic shielding or absorption has been demonstrated to be one of the operative approaches to address electromagnetic pollution [5]. Therefore, electromagnetic shielding or absorption material is one of the best procedures of environment and health defense [6]. A lightweight, flexible,

Sonochemical Preparation of ZnFe 2 O 4 Nanoparticles
ZnFe 2 O 4 nanoparticles were synthesized by the sonochemical synthesis technique. First, 2.63 g Zn(NO 3 ) 2 ·6H 2 O and 7.56 g Fe(NO 3 ) 3 ·9H 2 O were mixed in 60 mL of deionized water. Additionally, aqueous 1.6 M sodium hydroxide (NaOH) was mixed in the above set solution with continuous stirring by a magnetic stirrer. Moreover, the attained mixed solution was placed to high-intensity ultrasonic waves for 25 min, 50 min, and 100 min with the use of the UZ SONOPULS HD 2070 Ultrasonic homogenizer (Berlin, Germany) at 70 W power and 20 kHz frequency. The achieved product was washed by utilizing deionized water and ethanol to eliminate unwanted chemical impurity. Finally, these washed nanoparticles were dried at 60 • C for 18 h. The prepared ZnFe 2 O 4 nanoparticles were designated as ZS25, ZS50, and ZS100 according to their sonochemical synthesis time of 25 min, 50 min, and 100 min, respectively.

Preparation of Reduced Graphene Oxide (RGO)
First, graphene oxide (GO) was prepared from natural graphite powders by utilizing the Hummers' method. Typically, the mixture comprised of graphite powder (1.5 g) and sodium nitrate (1.5 g) was added slowly to concentrated H 2 SO 4 (75 mL). The beaker, which contains the above reaction mixture, was placed on an ice bath, and potassium permanganate (9 g) was added gradually within 20 min, and this mixed solution was stirred further for 30 min under an ice bath (temperature in the range of 0-5 • C). This solution was stirred for an additional 48 h at room temperature. Afterward, 138 mL of deionized water was gradually added into the above mixture and stirred for 10 min. In this achieved mixture, 420 mL warm deionized water was then poured and kept under a continuous strong stirring. Moreover, H 2 O 2 (30 mL) was mixed into the above reaction mixture to eliminate the remaining KMnO 4 and stirred until the color of the product transformed into bright yellow, which signaled the formation of graphite oxide from graphite. Finally, the obtained bright yellow product suspension was centrifuged and washed with ethanol and deionized water until the pH~7 was reached. The obtained product was annealed at 60 • C in a vacuum oven for 24 h.
Vitamin C (10 g) was utilized as a reducing agent in the development of reduced graphene oxide (RGO) from 3 g of graphene oxide (GO). For this, the above-prepared GO was mixed in deionized water and additionally placed to high-intensity ultrasonic waves for 15 min with the use of UZ SONOPULS HD 2070 Ultrasonic homogenizer. Then, in this attained solution, vitamin C was added gradually, and then the obtained suspension was continuously stirred for the time of 3 h at temperature 90 • C. Additionally, the achieved product suspension was centrifuged and washed with ethanol and deionized water. Finally, the washed product was dried in a vacuum oven at 60 • C for 15 h.

Ultrasonic Preparation of Nanocomposites
Nanocomposites of polyurethane resin (PUR) (50 wt.%) with nanofillers (40 wt.% zinc ferrite nanoparticles and 10 wt.% RGO) were prepared. For the PUR matrix, component A and component B were used in the ratio of 100-50. For the preparation of nanocomposites, in a 25-mL beaker, isocyanate prepolymer (component A, Biresin U1404) were mixed with nanofillers (ZnFe 2 O 4 (90%) + RGO (10%)) by using a EURO-ST-D mechanical stirrer for 30 min and then sonicated by using a UP 400S ultra probe (Hielscher Ultrasonics GmbH, Teltow, Germany) (frequency: 24 kHz, power: 400 W) for 30 min in an ice bath. Further, amine (component B) as a curing agent was mixed to the above mixture and then sonicated at another 10 min by using a UP 400S ultra probe (frequency: 24 kHz, power: 400 W). Finally, the prepared sample was closed and retained in a drying oven, where the composite material was cured at 25 • C for 5 days. Three PUR-based nanocomposite using zinc ferrite nanoparticles (ZS25, ZS50, or ZS100) and RGO as nanofillers, namely, (i) ZS25-RGO-PUR, (ii) ZS50-RGO-PUR, and (iii) ZS100-RGO-PUR, were prepared. Additionally, rectangle-shaped samples 22.86 × 10.16 × 1 mm 3 were produced by cast molding.

Characterization Techniques
The sonochemically prepared zinc ferrite nanoparticles X-ray Diffraction (XRD) was performed using an X-ray powder diffraction from Rigaku Corporation, Tokyo, Japan. The Raman spectroscopy of PUR-based nanocomposites was performed on a Raman spectrometer of Thermo Fisher Scientific, Waltham, MA, USA. XPS study of graphene oxide and reduced graphene oxide was performed on an X-ray photoelectron spectroscope of Kratos Analytical Ltd. (Manchester, UK). The FTIR spectroscopy of ZnFe 2 O 4 nanoparticles and PUR-based nanocomposites was performed on Nicolet 6700 (Thermo Scientific, Waltham, MA, USA). The high-resolution transmission electron microscope (JEOL JEM 2100) (JEOL, Peabody, MA, USA) was utilized to investigate the morphology and lattice fringes of ZnFe 2 O 4 nanoparticles. The surface morphology and structure of the PUR nanocomposites were investigated with an FE-SEM of FEI NanoSEM450 (The Netherland, FEI Company). Magnetic hysteresis curves of sonochemically prepared superparamagnetic ZnFe 2 O 4 nanoparticles were studied utilizing a VSM 7407, Lake Shore, Westerville, OH, USA. ZFC and FC temperature-dependent magnetization study of the sonochemically prepared zinc ferrite nanoparticles were investigated using a SQUID magnetometer of Quantum Design MPMS XL-7. The electromagnetic interference shielding effectiveness of the developed PUR-based nanocomposite with zinc ferrite nanoparticles and RGO as nanofillers was studied by using a vector network analyzer (Agilent N5230A, Agilent Technologies, Santa Clara, CA, USA) in 8. 2-12.4 GHz (X band).

X-ray Diffraction Study
The X-ray diffraction pattern of the sonochemically synthesized ZnFe 2 O 4 spinel ferrite nanoparticles at sonication times of 25 min, 50 min, and 100 min is displayed in Figure 1. The observed diffraction peaks correspond to the reflection of (220), (311), (222), (400), (331), (422), (511), (440), (531), and (442) planes of an Fd3m spinel crystal structure. Additionally, there is no presence of an impurity peak, which designates the high purity of spinel ferrite material. It is worth noting that as the sonication synthesis time increased, the intensity of diffraction peaks increased, and the width of the diffraction peak decreased, which suggests grain growth with an increase of sonication time. The average crystallite size of synthesized ZnFe 2 O 4 nanoparticles was studied by utilizing the Debye-Scherrer equation [25]: Additionally, structural parameters, such as ionic radii, hopping length for the octahedral and tetrahedral site, tetrahedral and octahedral bond length, tetrahedral edge, and the shared and unshared octahedral edge, for prepared ZnFe2O4 nanoparticles were as- Herein, λ, β, and θ are the wavelength of X-ray, the full-width at half maximum (FWHM), and the Bragg angle, respectively. The average crystallite size increases from 3.0 nm to 4.0 nm with an increase in sonication time, as shown in Table 1. The growth of spinel ferrite nanocrystals was associated with an increase in ultrasonic time [26]. The lattice parameter was determined by utilizing the following relation [25]: Herein, θ is the Bragg angle, and (hkl) are the Miller indices of the planes. The lattice parameter increases from 7.219 Å to 7.248 Å with an increase in sonication time from 25 min to 100 min, as shown in Table 1. The observed increase in the lattice constant with sonication time follows Vegard's law [27]. Generally, the lattice constant in the case of spinel ferrite correlates with microstructure, ordering/reordering of cations, valence states, and defects, etc. [28]. In the present work, the variation in the lattice constant can be attributed to changes in microstructure and ultrasonic-activated ordering/reordering of cations in ZnFe 2 O 4 spinel ferrite nanoparticles.
The X-ray density (d x ) of prepared spinel ferrite nanoparticles is evaluated by the following relation [25]: Herein, Z, M, N, and V are the number of the nearest neighbor, the molecular weight, the Avogadro number, and the volume of the unit cell (V = a 3 ), respectively. The evaluated value of the X-ray density was 8.51 g/cm 3 , 8.42 g/cm 3 , and 8.41 g/cm 3 for ZS25, ZS50, and ZS100 samples, respectively (Table 1). Thus, an increase of sonication time to 25 min, 50 min, and 100 min decreased the density of the prepared spinel ferrite nanoparticles.
Additionally, structural parameters, such as ionic radii, hopping length for the octahedral and tetrahedral site, tetrahedral and octahedral bond length, tetrahedral edge, and the shared and unshared octahedral edge, for prepared ZnFe 2 O 4 nanoparticles were assessed [29,30]. The variation in these parameters with sonication times of 25 min, 50 min, and 100 min was noticed, as mentioned in Tables 1 and 2. The increase in ionic radii, hopping length for the octahedral and tetrahedral site, tetrahedral and octahedral bond length, tetrahedral edge, and the shared and unshared octahedral edge for prepared ZnFe 2 O 4 nanoparticles with an increase of sonication time was noticed. Microstructure and ultrasonic-activated ordering/reordering of cations in ZnFe 2 O 4 nanoparticles was associated with an increase in sonication time, which can affect the physical properties of the material [31].

TEM Study
TEM measurements were carried out to investigate the structural features of prepared ZnFe 2 O 4 spinel ferrite nanoparticles. Figure 2 represents TEM and HRTEM images of prepared nanoparticles, namely ZS25, ZS50, and ZS100. The TEM image of ZS25 is depicted in Figure 2a, which shows particles in the range of 2-4.5 nm ( Figure S1 in supplementary material). The HRTEM image of ZS25 is shown in Figure 2b, which displays the lattice of (220) planes (d spacing 0.29 nm), (311) planes (d spacing 0.25 nm), and (400) planes (d spacing 0.21 nm) of ZnFe 2 O 4 spinel ferrite [32]. Further, Figure 2c depicts a lowresolution TEM image of the ZS50 sample, which illustrated that the product consisted of particles with sizes of 2.5-5 nm. Figure 2d shows lattice fringes with an interplanar spacing of 0.29 nm, which is consistent with (220) planes of spinel ferrite. Additionally, the TEM image of ZS100 is depicted in Figure 2e, which demonstrated that the prepared nanoparticles exhibited size 3-12 nm. Figure

TEM Study
TEM measurements were carried out to investigate the structural features of prepared ZnFe2O4 spinel ferrite nanoparticles. Figure 2 represents TEM and HRTEM images of prepared nanoparticles, namely ZS25, ZS50, and ZS100. The TEM image of ZS25 is depicted in Figure 2a, which shows particles in the range of 2-4.5 nm ( Figure S1 in supplementary material). The HRTEM image of ZS25 is shown in Figure 2b, which displays the lattice of (220) planes (d spacing 0.29 nm), (311) planes (d spacing 0.25 nm), and (400) planes (d spacing 0.21 nm) of ZnFe2O4 spinel ferrite [32]. Further, Figure 2c depicts a lowresolution TEM image of the ZS50 sample, which illustrated that the product consisted of particles with sizes of 2.5-5 nm. Figure 2d shows lattice fringes with an interplanar spacing of 0.29 nm, which is consistent with (220) planes of spinel ferrite. Additionally, the TEM image of ZS100 is depicted in Figure 2e, which demonstrated that the prepared nanoparticles exhibited size 3-12 nm. Figure Figure 3 depicts the typical SEM image of RGO and prepared polyurethane resinbased nanocomposites. Wrinkled and curled graphene sheets can be noticed in Figure 3a. Further, the presence of RGO and prepared ZnFe2O4 nanoparticles in polyurethane resin can be noticed in SEM images of the surfaces of the PUR-based nanocomposites, as shown in Figure 3b-d. The increase in the thickness of RGO may be due to the agglomeration of RGO during the processing and formation of polymer nanocomposite [33].  Figure 3 depicts the typical SEM image of RGO and prepared polyurethane resinbased nanocomposites. Wrinkled and curled graphene sheets can be noticed in Figure 3a. Further, the presence of RGO and prepared ZnFe 2 O 4 nanoparticles in polyurethane resin can be noticed in SEM images of the surfaces of the PUR-based nanocomposites, as shown in Figure 3b-d. The increase in the thickness of RGO may be due to the agglomeration of RGO during the processing and formation of polymer nanocomposite [33].  Figure 3 depicts the typical SEM image of RGO and prepared polyurethane resinbased nanocomposites. Wrinkled and curled graphene sheets can be noticed in Figure 3a. Further, the presence of RGO and prepared ZnFe2O4 nanoparticles in polyurethane resin can be noticed in SEM images of the surfaces of the PUR-based nanocomposites, as shown in Figure 3b-d. The increase in the thickness of RGO may be due to the agglomeration of RGO during the processing and formation of polymer nanocomposite [33].

X-ray Photoelectron Spectroscopy
The prepared GO and RGO were examined by X-ray photoelectron spectroscopy (XPS). Figure 4 shows the XPS spectra of prepared graphene oxide (GO) and reduced graphene oxide (RGO). Figure 4a,c signifies the survey scan spectra of GO and RGO, which display the existence of carbon and oxygen. Figure 4b depicts the high-resolution XPS spectra of the C 1s region for GO. The deconvoluted C 1s peak displays the peak binding energy of 284.1 eV, 284.7 eV, 286.5 eV, 288.4 eV, and 290.0 eV, which resembles C=C (sp 2 carbon), C-C (sp 3 carbon), C-O, C=O, and O-C=O bonds, respectively [34]. Additionally, Figure 4d denotes the high-resolution XPS spectra of C 1s for RGO. It displays the peak binding energy of 284.4 eV, 285.9 eV, 287.7 eV, 289.1 eV, and 290.6 eV related to C=C, C-OH, C=O, O-C=O, and π-π* satellite bonds, respectively [35]. The XPS investigation demonstrated that after reduction treatment, the functional group of GO is reduced, and the sp 3 carbon is altered to sp 2 carbon.

X-ray Photoelectron Spectroscopy
The prepared GO and RGO were examined by X-ray photoelectron spectroscopy (XPS). Figure 4 shows the XPS spectra of prepared graphene oxide (GO) and reduced graphene oxide (RGO). Figure 4a,c signifies the survey scan spectra of GO and RGO, which display the existence of carbon and oxygen. Figure 4b depicts the high-resolution XPS spectra of the C 1s region for GO. The deconvoluted C 1s peak displays the peak binding energy of 284.1 eV, 284.7 eV, 286.5 eV, 288.4 eV, and 290.0 eV, which resembles C=C (sp 2 carbon), C-C (sp 3 carbon), C-O, C=O, and O-C=O bonds, respectively [34]. Additionally, Figure 4d denotes the high-resolution XPS spectra of C 1s for RGO. It displays the peak binding energy of 284.4 eV, 285.9 eV, 287.7 eV, 289.1 eV, and 290.6 eV related to C=C, C-OH, C=O, O-C=O, and π-π* satellite bonds, respectively [35]. The XPS investigation demonstrated that after reduction treatment, the functional group of GO is reduced, and the sp 3 carbon is altered to sp 2 carbon.

X-ray Photoelectron Spectroscopy
The prepared GO and RGO were examined by X-ray photoelectron spectroscopy (XPS). Figure 4 shows the XPS spectra of prepared graphene oxide (GO) and reduced graphene oxide (RGO). Figure 4a,c signifies the survey scan spectra of GO and RGO, which display the existence of carbon and oxygen. Figure 4b depicts the high-resolution XPS spectra of the C 1s region for GO. The deconvoluted C 1s peak displays the peak binding energy of 284.1 eV, 284.7 eV, 286.5 eV, 288.4 eV, and 290.0 eV, which resembles C=C (sp 2 carbon), C-C (sp 3 carbon), C-O, C=O, and O-C=O bonds, respectively [34]. Additionally, Figure 4d denotes the high-resolution XPS spectra of C 1s for RGO. It displays the peak binding energy of 284.4 eV, 285.9 eV, 287.7 eV, 289.1 eV, and 290.6 eV related to C=C, C-OH, C=O, O-C=O, and π-π* satellite bonds, respectively [35]. The XPS investigation demonstrated that after reduction treatment, the functional group of GO is reduced, and the sp 3 carbon is altered to sp 2 carbon.

FTIR Spectroscopy
FTIR spectroscopy is an outstanding complementary characterization tool for Raman spectroscopy characterization of nanocomposites. Figure 6a represents the FTIR spectra of prepared ZnFe 2 O 4 nanoparticles. Two absorption bands at~545 cm −1 and~385 cm −1 were noted. The absorption band~545 cm −1 can be ascribed to the tetrahedral Zn 2+ (Zn-O) stretching vibration for the ZnFe 2 O 4 spinel ferrite crystal structure. The band 385 cm −1 can be assigned to the octahedral Fe 3+ (Fe-O) stretching vibration [38]. Additionally, Figure 6b displays the FTIR spectra of pure polyurethane resin (PUR) and its nanocomposites. The presence of an absorption band~545 cm −1 confirms the existence of ZnFe 2 O 4 in the developed nanocomposite. Furthermore, Figure 6b demonstrated other absorption bands related to main characteristics peaks for polyurethane, which was observed at~3310 cm −1 , 2966 cm −1 , 2868 cm −1 , 1728 cm −1 , 1640 cm −1 , 1534 cm −1 , 1223 cm −1 , and 1094 cm −1 . The absorption band~3310 cm −1 can be ascribed to the stretching vibration of the N-H group [39]. The bands at 2966 cm −1 and 2868 cm −1 can be attributed to the non-symmetric and symmetric stretching vibration of CH 2 . Further, the absorption bands at 1728 cm −1 , 1223 cm −1 , and 1094 cm −1 can be attributed to carbonyl (C=O), aromatic C-O stretching vibration, and C-O-C non-symmetric stretching vibration. Additionally, the band~1640 cm −1 can be ascribed as the abundance of amide I bands [40]. In addition, the absorption band at~1534 cm −1 can be associated with the N-H bonds of the urethane group [41]. In combination with Raman and FTIR spectroscopy results, the existence of spinel ferrite ZnFe 2 O 4 nanoparticles and reduced graphene oxide in the polyurethane matrix were verified.

FTIR Spectroscopy
FTIR spectroscopy is an outstanding complementary characterization tool for Raman spectroscopy characterization of nanocomposites. Figure 6a represents the FTIR spectra of prepared ZnFe2O4 nanoparticles. Two absorption bands at ~545 cm −1 and ~385 cm −1 were noted. The absorption band ~545 cm −1 can be ascribed to the tetrahedral Zn 2+ (Zn-O) stretching vibration for the ZnFe2O4 spinel ferrite crystal structure. The band ~385 cm −1 can be assigned to the octahedral Fe 3+ (Fe-O) stretching vibration [38]. Additionally, Figure 6b displays the FTIR spectra of pure polyurethane resin (PUR) and its nanocomposites. The presence of an absorption band ~545 cm −1 confirms the existence of ZnFe2O4 in the developed nanocomposite. Furthermore, Figure 6b demonstrated other absorption bands related to main characteristics peaks for polyurethane, which was observed at ~3310 cm −1 , 2966 cm −1 , 2868 cm −1 , 1728 cm −1 , 1640 cm −1 , 1534 cm −1 , 1223 cm −1 , and 1094 cm −1 . The absorption band ~3310 cm −1 can be ascribed to the stretching vibration of the N-H group [39]. The bands at 2966 cm −1 and 2868 cm −1 can be attributed to the non-symmetric and symmetric stretching vibration of CH2. Further, the absorption bands at 1728 cm −1 , 1223 cm −1 , and 1094 cm −1 can be attributed to carbonyl (C=O), aromatic C-O stretching vibration, and C-O-C non-symmetric stretching vibration. Additionally, the band ~1640 cm −1 can be ascribed as the abundance of amide I bands [40]. In addition, the absorption band at ~1534 cm −1 can be associated with the N-H bonds of the urethane group [41]. In combination with Raman and FTIR spectroscopy results, the existence of spinel ferrite ZnFe2O4 nanoparticles and reduced graphene oxide in the polyurethane matrix were verified.

Magnetic Properties
The magnetic characteristics of sonochemically synthesized spinel ferrite ZnFe2O4 nanoparticles were examined. Figure 7a depicts magnetic hysteresis curves of synthesized ZnFe2O4 nanoparticles at different sonication times of 25 min, 50 min, and 100 min. The prepared spinel ferrite nanoparticles exhibit zero remanent and zero coercivity, which is associated with superparamagnetic characteristics. The saturation magnetization value at the applied magnetic field of 800 kA/m was 0.78 Am 2 /kg, 1.05 Am 2 /kg, and 1.33 Am 2 /kg for ZS25, ZS50, and ZS100, respectively [42]. The reduced saturation magnetization of ZS25 is associated with the existence of a magnetically dead or anti-ferromagnetic layer on the border of the nanoparticle [43]. The observation of superparamagnetism or ferrimagnetism characteristics at room temperature in nanosized ZnFe2O4 spinel ferrite has been attributed to cation redistribution between Zn 2+ ions at the tetrahedral sites and Fe 3+

Magnetic Properties
The magnetic characteristics of sonochemically synthesized spinel ferrite ZnFe 2 O 4 nanoparticles were examined. Figure 7a depicts magnetic hysteresis curves of synthesized ZnFe 2 O 4 nanoparticles at different sonication times of 25 min, 50 min, and 100 min. The prepared spinel ferrite nanoparticles exhibit zero remanent and zero coercivity, which is associated with superparamagnetic characteristics. The saturation magnetization value at the applied magnetic field of 800 kA/m was 0.78 Am 2 /kg, 1.05 Am 2 /kg, and 1.33 Am 2 /kg for ZS25, ZS50, and ZS100, respectively [42]. The reduced saturation magnetization of ZS25 is associated with the existence of a magnetically dead or anti-ferromagnetic layer on the border of the nanoparticle [43]. The observation of superparamagnetism or ferrimagnetism characteristics at room temperature in nanosized ZnFe 2 O 4 spinel ferrite has been attributed to cation redistribution between Zn 2+ ions at the tetrahedral sites and Fe 3+ ions at the octahedral sites [44,45]. Further, Figure 7b depicts magnetic hysteresis curves of prepared nanoparticle sample ZS25 at various temperatures 2 K, 77 K, and 300 K. At room temperature (300 K) and 77 K, the coercivity and remanent are zero for the ZS25 sample; however, it exhibits coercivity (67 kA/m) and remanent value (5 Am 2 /kg) at 2 K, which indicates superparamagnetic behavior of prepared ZS25 sample [46,47].
The temperature dependences of zero-field cooled (ZFC) and field-cooled (FC) of prepared nanoparticle sample ZS25 were also measured in wide temperature interval 2-300 K for magnetic field 7.96 kA/m. The ZFC and FC investigation is utilized to define the blocking temperature. Figure 7c displays the irreversibility of ZFC and FC curves and the occurrence of a maximum in ZFC curves. ZFC-FC curve display irreversibility characteristic due to the blocking/freezing and unblocking mechanism of the magnetic moment of magnetic nanoparticles [48]. The rise of ZFC magnetization with temperature is associated with an unblocking progression [49]. The ZFC and FC measurements for the ZS25 sample display the blocking temperature of 20 K. The prepared ZS25 sample displays ferromagnetic behavior below 20 K and superparamagnetic above 20 K. Above the blocking temperature, the magnetic moment of nanoparticles freely fluctuates in the applied magnetic field, which leads to superparamagnetism. Nevertheless, below the blocking temperature, the magnetic moment of each nanoparticle is blocked/freeze in the applied magnetic field direction, and the magnetic hysteresis with coercivity value 67 kA/m was noticed, as presented in Figure 7b. The blocking temperature depends on various factors such as effective anisotropy constant, magnetic coupling, particle size, applied magnetic field, etc. [50,51]. A research group, Qi Chen et al. [52], observed that the blocking temperature increases with the increase in particle size The temperature dependences of zero-field cooled (ZFC) and field-cooled (FC) of prepared nanoparticle sample ZS25 were also measured in wide temperature interval 2-300 K for magnetic field 7.96 kA/m. The ZFC and FC investigation is utilized to define the blocking temperature. Figure 7c displays the irreversibility of ZFC and FC curves and the occurrence of a maximum in ZFC curves. ZFC-FC curve display irreversibility characteristic due to the blocking/freezing and unblocking mechanism of the magnetic moment of magnetic nanoparticles [48]. The rise of ZFC magnetization with temperature is associated with an unblocking progression [49]. The ZFC and FC measurements for the ZS25 sample display the blocking temperature of 20 K. The prepared ZS25 sample displays ferromagnetic behavior below 20 K and superparamagnetic above 20 K. Above the blocking temperature, the magnetic moment of nanoparticles freely fluctuates in the applied magnetic field, which leads to superparamagnetism.
Nevertheless, below the blocking temperature, the magnetic moment of each nanoparticle is blocked/freeze in the applied magnetic field direction, and the magnetic hysteresis with coercivity value 67 kA/m was noticed, as presented in Figure 7b. The blocking temperature depends on various factors such as effective anisotropy constant, magnetic coupling, particle size, applied magnetic field, etc. [50,51]. A research group, Qi Chen et al. [52], observed that the blocking temperature increases with the increase in particle size of MgFe 2 O 4 nanoparticles. Further, Chao Liu et al. [53] reported the increase in the blocking temperature from 20 to 250 K with the increase in the size of the MnFe 2 O 4 nanoparticles from 4.4 to 13.5 nm.
It is well-known that the saturation magnetization (M s ) and coercivity (H c ) of the electromagnetic wave absorber material are the most important factor to influence the magnetic loss of the electromagnetic wave shielding material [54]. In general, for the application of electromagnetic interference shielding, an initial permeability (µ i ) signifies strong magnetic loss capacity of electromagnetic wave absorber material, which can be expressed as [55]: where a and b are constants, which have a dependence on the material. In the above relation, λ, ξ, and k are the magnetostriction constant, elastic strain parameter of the crystal, and proportionality coefficient, respectively [56]. The above equation signifies that the lower H c and higher M s are supportive of the value of µ i increasing and, consequently, the performance of electromagnetic wave absorption enhancing [57]. Superparamagnetic nanoparticles that exhibited zero coercivity and higher saturation magnetization could be high-performance electromagnetic interference shielding material. In the VSM study, it is noticed the ZS100 sample exhibited a high value of magnetization as compared to ZS25 and ZS100; therefore, the permeability of nanocomposites containing ZS100 would be higher. Further, this suggests that a nanocomposite based on ZS100 could have higher electromagnetic shielding performance as compared to ZS25 and ZS50.

Electromagnetic Interference Shielding Effectiveness (EMI SE) Study
The total electromagnetic interference (EMI) shielding effectiveness (SE T ) can be ascribed by the logarithmic ratio between the incoming power (P in ) and outgoing power (P out ) of electromagnetic radiation [58]: Herein, SE A , SE R , SE MR , and SE T are the absorption, reflection, multiple reflections, and total EMI shielding, respectively. In general, SE MR can be ignored when SE T is larger than 10 dB [58]. Hence, SE T can be expressed as: SE A in dB can be expressed as [4]: where f is the frequency of the electromagnetic wave; σ T is the total conductivity (S/cm) of shielding material; (µ r ) is the complex permeability of shielding material. It can be noticed that SE A is directly proportional to the conductivity (σ T ) and permeability (µ r ) of the shielding material. Further, SE R can be expressed as [4]: It can be noticed from the above expression that SE R is the function of the ratio of σ T and µ r . Furthermore, SE MR can be expressed as [4]: SE MR = 20 log 10 1 − 10 SE A 10 (9) SE MR can be neglected at SE A ≥ 10 dB. Figure 8 depicts the EMI-shielding performance of prepared nanocomposites based on the polyurethane matrix with sonochemically prepared superparamagnetic ZnFe 2 O 4 nanoparticles and RGO as nanofillers. The maximum total shielding effectiveness (SE T ) value for prepared nanocomposites of thickness 1 mm in the frequency range of 8.2-12.4 GHz was 12.7 dB, 13.8 dB, and 16.7 dB, for ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively. The EMI SE T value increases with an increase in the size of utilized superparamagnetic ZnFe 2 O 4 spinel ferrite nanoparticles in developed nanocomposites. The maximum SE A value was 3.6 dB, 5.9 dB, and 10.2 dB for prepared nanocomposites ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively. The higher SE A value of the ZS100-RGO-PUR sample indicates that this nanocomposite exhibits higher electrical conductivity and higher magnetic permeability. Additionally, the maximum SE R value was noticed to be 10.7 dB, 8.7 dB, and 6.7 dB for developed composite material ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively.

Electromagnetic Properties and Parameters
To elaborate more on the shielding characteristics of prepared superparamagnetic ZnFe2O4 and RGO-based PUR nanocomposites, complex permittivity and permeability of the nanocomposites were investigated. The real permittivity (ε′) implies the storage ability of electrical energy, while the imaginary permittivity (ε″) signifies energy dissipation. Figure 9a represents the frequency dependence real permittivity for developed nanocomposites in the 8.2-12.4 GHz frequency range. The value of ε′ is in the ranges of 6.8 to 7.3, 7.5 to 7.9, 8.4 to 9.1 for developed nanocomposites ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively. Figure 9b represents the frequency dependence imaginary permittivity for nanocomposites in the 8.2-12.4 GHz frequency range. The value of ε″ is in the range of 0.35 to 0.74, 0.55 to 0.88, 0.65 to 1.31 for ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively. The complex permittivity is the result of the polarizability of the nanocomposite material associated with the dipolar and electric polarization, initiated by an EM wave [5,67]. The input to the space charge polarization acts because of the heterogeneity of the nanocomposite material. In heterogeneous dielectric materials, there is an accumulation of virtual charges on the interfaces of two mediums with different dielectric constants and conductivities, which lead to interfacial polarization and is called Maxwell-Wagner polarization [68,69]. It can be observed that the values of ε′ and ε″ are increased with the increase of the size of superparamagnetic ZnFe2O4 nanoparticles in prepared nanocomposites.

Electromagnetic Properties and Parameters
To elaborate more on the shielding characteristics of prepared superparamagnetic ZnFe 2 O 4 and RGO-based PUR nanocomposites, complex permittivity and permeability of the nanocomposites were investigated. The real permittivity (ε ) implies the storage ability of electrical energy, while the imaginary permittivity (ε ) signifies energy dissipation. Figure 9a represents the frequency dependence real permittivity for developed nanocomposites in the 8.2-12.4 GHz frequency range. The value of ε is in the ranges of 6.8 to 7.3, 7.5 to 7.9, 8.4 to 9.1 for developed nanocomposites ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively. Figure 9b represents the frequency dependence imaginary permittivity for nanocomposites in the 8.2-12.4 GHz frequency range. The value of ε is in the range of 0.35 to 0.74, 0.55 to 0.88, 0.65 to 1.31 for ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively. The complex permittivity is the result of the polarizability of the nanocomposite material associated with the dipolar and electric polarization, initiated by an EM wave [5,67]. The input to the space charge polarization acts because of the heterogeneity of the nanocomposite material. In heterogeneous dielectric materials, there is an accumulation of virtual charges on the interfaces of two mediums with different dielectric constants and conductivities, which lead to interfacial polarization and is called Maxwell-Wagner polarization [68,69]. It can be observed that the values of ε and ε are increased with the increase of the size of superparamagnetic ZnFe 2 O 4 nanoparticles in prepared nanocomposites.
Herein, εs and ε∞ are the static and infinite permittivity; ω = 2πf is the angular frequency; τ is the relaxation time; σ is the conductivity. It can be seen from the above relation that the ε′ and ε″ are the functions of ωτ. Hence, both the ε′ and ε″ are mutually dependent on one another. A relationship between ε′ and ε″ can be inferred after ignoring the contribution of σ and by eliminating ωτ [74]: From the above relation, it is easy to recognize that the curves of ε′ and ε″ would be a semi-circle, which is known as the Cole-Cole semicircle [75].  In general, the relaxation is associated with a delay in polarization concerning the change in the electrical field. Some obvious Cole-Cole semicircles can be noticed in Figure 9d, which signifies that the relaxation contributed to the dielectric loss. Additionally, one Cole-Cole semicircle represents a Debye dipolar relaxation, and the existence of more semicircles is attributed to multiple relaxation processes [76]. These other semicircles are associated with Maxwell-Wagner relaxation, electron/ion polarization, and interfacial polarization [77]. The multiple dielectric losses were responsible for the improvement of the absorption characteristics of PUR-based nanocomposites.
It is well-known that the real permeability (µ′) represents the storage ability of magnetic energy, and the imaginary permeability (µ″) signifies the magnetic loss. Figure 10a represents the frequency dependence of the real permeability (µ′) of PUR-based nanocomposites. The µ′ is in the range of 0.86 to 0.96, 0.91 to 0.99, and 0.90 to 1.09 for nanocomposites ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively. The value of real permeability (µ′) was increased with an increase of grain size of utilized superparamagnetic ZnFe2O4 spinel ferrite nanoparticles. Further, the value of µ″ is in the range of -0.06 to 0.03, −0.01 to 0.07, and 0.03 to 0.19 for the prepared composites ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively, as shown in Figure 10b. Remarkably, it is noticed that the µ″ exhibited negative value also for some PUR-based nanocomposites, which is associated with the motion of charges [78]. The relation between electrical conductivity (σ AC ) and imaginary permittivity (ε ) can be stated as [70]: Herein, ε o is the dielectric constant of free space; f is the frequency of the electromagnetic wave. The above relation signifies that the electrical conductivity will increase with an increase in the value of imaginary permittivity. Therefore, the enhanced value of the complex permittivity can be associated with the increase in the electrical conductivity of the prepared nanocomposites with an increase in the size of embedded superparamagnetic ferrite nanoparticles. Figure 9c represents the change in electrical conductivity with the frequency of prepared nanocomposites. The electrical conductivity is in the range of 1.9 × 10 −3 to 3.9 × 10 −3 S/cm, 2.5 × 10 −3 to 4.3 × 10 −3 S/cm, 2.9 × 10 −3 to 7.5 × 10 −3 S/cm for ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively.
Further, in reported literature by other researchers, the Debye theory is generally utilized to clarify the relaxation process of dipoles [71,72]. According to the Debye theory for dielectric loss characteristics, the real permittivity (ε ) and imaginary permittivity (ε ) can be written as [73]: Herein, ε s and ε ∞ are the static and infinite permittivity; ω = 2πf is the angular frequency; τ is the relaxation time; σ is the conductivity. It can be seen from the above relation that the ε and ε are the functions of ωτ. Hence, both the ε and ε are mutually dependent on one another. A relationship between ε and ε can be inferred after ignoring the contribution of σ and by eliminating ωτ [74]: From the above relation, it is easy to recognize that the curves of ε and ε would be a semi-circle, which is known as the Cole-Cole semicircle [75]. Figure 9d depicts the Cole-Cole plots for the developed PUR-based nanocomposites. In general, the relaxation is associated with a delay in polarization concerning the change in the electrical field. Some obvious Cole-Cole semicircles can be noticed in Figure 9d, which signifies that the relaxation contributed to the dielectric loss. Additionally, one Cole-Cole semicircle represents a Debye dipolar relaxation, and the existence of more semicircles is attributed to multiple relaxation processes [76]. These other semicircles are associated with Maxwell-Wagner relaxation, electron/ion polarization, and interfacial polarization [77]. The multiple dielectric losses were responsible for the improvement of the absorption characteristics of PUR-based nanocomposites.
It is well-known that the real permeability (µ ) represents the storage ability of magnetic energy, and the imaginary permeability (µ ) signifies the magnetic loss. Figure 10a represents the frequency dependence of the real permeability (µ ) of PUR-based nanocomposites. The µ is in the range of 0.86 to 0.96, 0.91 to 0.99, and 0.90 to 1.09 for nanocomposites ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively. The value of real permeability (µ ) was increased with an increase of grain size of utilized superparamagnetic ZnFe 2 O 4 spinel ferrite nanoparticles. Further, the value of µ is in the range of −0.06 to 0.03, −0.01 to 0.07, and 0.03 to 0.19 for the prepared composites ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively, as shown in Figure 10b. Remarkably, it is noticed that the µ exhibited negative value also for some PUR-based nanocomposites, which is associated with the motion of charges [78].  In general, the relaxation is associated with a delay in polarization concerning the change in the electrical field. Some obvious Cole-Cole semicircles can be noticed in Figure 9d, which signifies that the relaxation contributed to the dielectric loss. Additionally, one Cole-Cole semicircle represents a Debye dipolar relaxation, and the existence of more semicircles is attributed to multiple relaxation processes [76]. These other semicircles are associated with Maxwell-Wagner relaxation, electron/ion polarization, and interfacial polarization [77]. The multiple dielectric losses were responsible for the improvement of the absorption characteristics of PUR-based nanocomposites.
It is well-known that the real permeability (µ′) represents the storage ability of magnetic energy, and the imaginary permeability (µ″) signifies the magnetic loss. Figure 10a represents the frequency dependence of the real permeability (µ′) of PUR-based nanocomposites. The µ′ is in the range of 0.86 to 0.96, 0.91 to 0.99, and 0.90 to 1.09 for nanocomposites ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively. The value of real permeability (µ′) was increased with an increase of grain size of utilized superparamagnetic ZnFe2O4 spinel ferrite nanoparticles. Further, the value of µ″ is in the range of -0.06 to 0.03, −0.01 to 0.07, and 0.03 to 0.19 for the prepared composites ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively, as shown in Figure 10b. Remarkably, it is noticed that the µ″ exhibited negative value also for some PUR-based nanocomposites, which is associated with the motion of charges [78]. Additionally, the Globus equation is expressed as [79]: This equation signifies that to get a higher complex permeability, a higher saturation magnetization (MS), larger grain size (D), and smaller magnetocrystalline anisotropy constant (K1) are needed. The increased magnetization and larger grain size of the ZS100 sample may add to the larger permeability for prepared ZS100-RGO-PUR nanocomposites, as compared with ZS25-RGO-PUR and ZS50-RGO-PUR nanocomposites.
Further, based on the following relations [80]: and utilizing electromagnetic parameters for ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR nanocomposites, the dielectric loss tangent (tanδε) and magnetic loss tangent (tanδµ) were evaluated. Figure 10c represents dielectric loss tangent vs. frequency curves for prepared PUR-based nanocomposites. The dielectric loss tangent (tanδε) of samples ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, fluctuated with an increase of frequency of electromagnetic wave between 0.05 to 0.10, 0.06 to 0.11, and 0.07 to 0.15, respectively. Additionally, the dielectric loss is related to dipole polarization and interfacial polarization at higher frequencies [81]. It can be also noticed that the dielectric loss (ε″) value of the ZS100-RGO-PUR sample is much higher than the other two samples (i.e., ZS25-RGO-PUR, and ZS50-RGO-PUR). The higher dielectric loss in the ZS100-RGO-PUR sample is associated with enhanced electrical conductivity and dielectric constant induced by micro-currents and polarization in nanocomposites [82]. The magnetic loss tangent variation with the frequency of an electromagnetic wave of prepared PUR-based nanocomposites is presented in Figure 10d. It can be perceived that the magnetic loss tangent fluctuated between −0.06 to 0.03, −0.01 to 0.07, and 0.03 to 0.19 for samples ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively.
It is well-known that natural resonance, exchange resonance, and eddy current are the main contributors to the magnetic loss of nanoparticles [83]. The eddy current loss can be stated by the following relation when the size of magnetic nanoparticle (D) is smaller than the skin depth (δ) [84]: Additionally, the Globus equation is expressed as [79]: This equation signifies that to get a higher complex permeability, a higher saturation magnetization (M S ), larger grain size (D), and smaller magnetocrystalline anisotropy constant (K 1 ) are needed. The increased magnetization and larger grain size of the ZS100 sample may add to the larger permeability for prepared ZS100-RGO-PUR nanocomposites, as compared with ZS25-RGO-PUR and ZS50-RGO-PUR nanocomposites.
Further, based on the following relations [80]: and utilizing electromagnetic parameters for ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR nanocomposites, the dielectric loss tangent (tanδ ε ) and magnetic loss tangent (tanδ µ ) were evaluated. Figure 10c represents dielectric loss tangent vs. frequency curves for prepared PUR-based nanocomposites. The dielectric loss tangent (tanδ ε ) of samples ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, fluctuated with an increase of frequency of electromagnetic wave between 0.05 to 0.10, 0.06 to 0.11, and 0.07 to 0.15, respectively. Additionally, the dielectric loss is related to dipole polarization and interfacial polarization at higher frequencies [81]. It can be also noticed that the dielectric loss (ε ) value of the ZS100-RGO-PUR sample is much higher than the other two samples (i.e., ZS25-RGO-PUR, and ZS50-RGO-PUR). The higher dielectric loss in the ZS100-RGO-PUR sample is associated with enhanced electrical conductivity and dielectric constant induced by micro-currents and polarization in nanocomposites [82]. The magnetic loss tangent variation with the frequency of an electromagnetic wave of prepared PUR-based nanocomposites is presented in Figure 10d. It can be perceived that the magnetic loss tangent fluctuated between −0.06 to 0.03, −0.01 to 0.07, and 0.03 to 0.19 for samples ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR, respectively.
It is well-known that natural resonance, exchange resonance, and eddy current are the main contributors to the magnetic loss of nanoparticles [83]. The eddy current loss can be stated by the following relation when the size of magnetic nanoparticle (D) is smaller than the skin depth (δ) [84]: where f is the electromagnetic wave frequency; ρ is the electric resistivity of the nanoparticles. Based on this above relation, C o = f −1 (µ ) −2 µ should be constant, if the magnetic loss is mainly contributed from the eddy current loss. It can be seen in Figure 11a that the value C o is not constant for all the prepared PUR-based nanocomposites. It signifies that the eddy current loss would not be a dominant contributor to magnetic loss. where f is the electromagnetic wave frequency; ⍴ is the electric resistivity of the nanoparticles. Based on this above relation, Co = f −1 (µ′) −2 µ″ should be constant, if the magnetic loss is mainly contributed from the eddy current loss. It can be seen in Figure 11a that the value Co is not constant for all the prepared PUR-based nanocomposites. It signifies that the eddy current loss would not be a dominant contributor to magnetic loss. Besides dielectric and magnetic losses, skin depth (δ) is another important factor that stimulates the absorption of electromagnetic waves. Skin depth states the distance at which the field drops to 1/e of the incident value and stated as [85]: Herein, f is the frequency; σ is the electrical conductivity; µ is the permeability. This relation signifies that skin depth reduces with an increase in frequency, permeability, and conductivity. Figure 11b depicts the frequency dependence variation of skin depth of the prepared PUR-based nanocomposites. A smaller skin depth states a stronger absorption capacity [86]. The skin depth of samples ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR fluctuated with an increase in the frequency of electromagnetic waves between 0.08 to 0.14 mm, 0.08 to 0.12 mm, and 0.06 to 0.11 mm, respectively. The prepared ZS100-RGO-PUR nanocomposite exhibits smaller skin depth and, therefore, stronger absorption. Besides dielectric and magnetic losses, skin depth (δ) is another important factor that stimulates the absorption of electromagnetic waves. Skin depth states the distance at which the field drops to 1/e of the incident value and stated as [85]: Herein, f is the frequency; σ is the electrical conductivity; µ is the permeability. This relation signifies that skin depth reduces with an increase in frequency, permeability, and conductivity. Figure 11b depicts the frequency dependence variation of skin depth of the prepared PUR-based nanocomposites. A smaller skin depth states a stronger absorption capacity [86]. The skin depth of samples ZS25-RGO-PUR, ZS50-RGO-PUR, and ZS100-RGO-PUR fluctuated with an increase in the frequency of electromagnetic waves between 0.08 to 0.14 mm, 0.08 to 0.12 mm, and 0.06 to 0.11 mm, respectively. The prepared ZS100-RGO-PUR nanocomposite exhibits smaller skin depth and, therefore, stronger absorption.
The attenuation constant (α) is an important factor that governs the electromagnetic wave absorption capabilities of shielding nanocomposites. It can be assessed by the following relation [87]: Figure 11c displays the frequency dependence variation of the attenuation constant (α) of prepared PUR-based nanocomposites. The high attenuation constant (α) value of the prepared ZS100-RGO-PUR nanocomposite compared with other samples demonstrated high absorption of electromagnetic waves. Another key factor that governs electromagnetic wave absorption is impedance matching. It is stated by the modulus of the normalized characteristic impedance (Z), which can be calculated by utilizing the following relation [88]: where Z 1 = Z o µ r /ε r ; Zo is the impedance in free space; ε r is the value of complex permittivity; µ r is the value of complex permeability. Figure 11d shows the variation of the impedance matching coefficient for prepared PUR-based nanocomposites with frequency. The values of Z are below one, and the ZS100-RGO-PUR nanocomposite has a lower Z value compared to other samples. A high attenuation constant and moderate Z value of ZS100-RGO-PUR nanocomposite provided the high value of EMI-shielding effectiveness [89]. The schematic illustration of the electromagnetic interference shielding mechanism in the prepared nanocomposite is shown in Figure 12. When electromagnetic waves interact at the surface of the prepared nanocomposite, a part of it is reflected, another part is absorbed, and the remaining part has multiple reflections and scattering [90]. The reflection is associated with moving charge carriers interacted with electromagnetic waves [91]. The absorption signifies the dissipation of energy of the electromagnetic waves due to the interaction of electromagnetic waves with the electric and magnetic dipoles [92]. The multiple reflections are reflections at different surfaces or interfaces present due to inhomogeneity within the prepared nanocomposite. The prepared nanocomposites consisted of conductive RGO sheets and ZnFe 2 O 4 magnetic nanoparticles not only improve impedance matching but also creates a micro-current network and attains interfacial polarization [93]. The appropriate conductivity of RGO sheets gifted the prepared nanocomposites exhibited a moderate conductivity loss. Residual functional groups and defects in RGO sheets and ZnFe 2 O 4 originated dipole polarization and defect polarization [94]. The interaction of electromagnetic waves also creates hopping and migrating electrons across the defects of RGO sheets.

Conclusions
In summary, superparamagnetic ZnFe2O4 spinel ferrite nanoparticles were prepared successfully by the sonochemical synthesis approach at various ultra-sonication times of 25 min (ZS25), 50 min (ZS50), and 100 min (ZS100). The average crystallite size increased from 3.0 nm to 4.0 nm with an increase in sonication time. The lattice parameter increased from 7.219 Å to 7.248 Å with an increase in sonication time from 25 min to 100 min. The increase in ionic radii, hopping length for the octahedral and tetrahedral site, tetrahedral and octahedral bond length, tetrahedral edge, and shared and unshared octahedral edge for prepared ZnFe2O4 nanoparticles with an increase in sonication time is associated with The magnetic characteristics of the superparamagnetic ZnFe 2 O 4 nanoparticles component in the developed nanocomposites provided a degree of magnetic loss such as natural resonance and eddy current loss. It improves the impedance matching between complex permittivity and permeability, which provides well absorption condition for elec-tromagnetic waves [95]. The improved electromagnetic wave shielding characteristics of the ZS100-RGO-PUR nanocomposite can be primarily attributed to the inclusive acts of magnetic loss, dielectric loss, and appropriate attenuation constant derived from various nanofillers in the matrix.

Conclusions
In summary, superparamagnetic ZnFe 2 O 4 spinel ferrite nanoparticles were prepared successfully by the sonochemical synthesis approach at various ultra-sonication times of 25 min (ZS25), 50 min (ZS50), and 100 min (ZS100). The average crystallite size increased from 3.0 nm to 4.0 nm with an increase in sonication time. The lattice parameter increased from 7.219 Å to 7.248 Å with an increase in sonication time from 25 min to 100 min. The increase in ionic radii, hopping length for the octahedral and tetrahedral site, tetrahedral and octahedral bond length, tetrahedral edge, and shared and unshared octahedral edge for prepared ZnFe 2 O 4 nanoparticles with an increase in sonication time is associated with cation redistribution in ZnFe 2 O 4 nanoparticles with an increase in sonication time. The prepared spinel ferrite nanoparticles exhibited zero remanent and zero coercivity, which is associated with superparamagnetic characteristics. The prepared magnetic ZnFe 2 O 4 nanoparticles (ZS25, ZS50, and ZS100) and electrically conductive reduced graphene oxide (RGO) were embedded in a polyurethane resin (PUR) matrix to develop lightweight and flexible nanocomposites for electromagnetic interference shielding application. The maximum total shielding effectiveness (SE T ) value for developed nanocomposites of thickness 1 mm in the range of 8.

Funding:
We thank the financial support of the Czech Science Foundation (GA19-23647S) project at the Centre of Polymer Systems, Tomas Bata University in Zlin, Czech Republic. One author, Anju, also acknowledges the financial support by the internal grant no. IGA/CPS/2020/003 for specific research from Tomas Bata University in Zlín.