Synthesis and Characterizations of Novel Spinel Ferrites Nanocomposites Al0.5Cr0.5Zn0Fe2O4 and Zn0.5Cr0.5Al0Fe2O4

In this study, novel spinel ferrites nanocomposites containing aluminum chromium zinc nanoferrites, Al0.5Cr0.5Zn0Fe2O4 and Zn0.5Cr0.5Al0Fe2O4 have been fabricated and characterized to determine the properties of highly stable conduction materials. The nanocomposites have been synthesized through the sol–gel method. Zinc and aluminum-doped chromium ferrites were prepared with the stoichiometric composition ZnxAlx-0.5Cr0.5Fe2O4 with ammonium hydroxide solution (NH4OH) and polyethylene glycol (PEG) at different temperatures with consecutive steps. After sintering the final nanoferrites, characterizations for morphological, spectral properties, and crystallinity have been determined through scanning electron microscope (SEM), Fourier transformation infrared spectroscopy, and X-ray diffraction spectrometer, respectively. SEM micrographs presented that higher sample density and agglomeration of the nanocomposite outer surface with temperature increase. The investigation of the dielectric and conduction properties presented with varying sintering temperature and Al–Zn doping greatly influenced the dielectric properties of spinel nanoferrites dielectric properties: dielectric loss tangent and dielectric constant. The effects of various sintering temperatures provide synergistic effects on the morphology and dielectric conductivity features. The characterizations presented that the dopants (Al, Zn) enhanced the magnetic and electrical properties of both chromium nanoferrites which can be implemented in high frequency single-layered electromagnetic waves absorbing devices in electrical and medical appliances in future.


Introduction
Material scientists yearn to develop a material comprising extraordinary properties, controlled particle size, and structure that can be used in many industries. Although countless efforts have been progressed to achieve desired particles, struggles are still in continuation. It is a big challenge for a researcher to fabricate a nanomaterial with controlled structural features. Nowadays, 2D materials are emerging due to improved structural features that make them attractive for composite development. These materials have made progression in developing controlled composites at the atomic level [1]. Ferrites are one of the magnetic nanoparticles which are considered as most important due to atomic structures. Many scientists worked on the development of these magnetic particles to understand the behavior of material to be used in various technological industries. Nanoscale materials provide critical information related to magnetic behavior and its correlation with other metals. The main reason for enhanced magnetic properties is the size of ferrites in the nanometer range, making its properties like wall thickness and magnetic behavior dominant [2]. While talking of nanotechnology is defined as the understanding and control of matter at dimensions that are usually sandwiched between 1 and 100 nm. These unique phenomena enable novel applications of that field in many areas of daily life like medicine, sports, and automotive. Due to the rapid increase in industries, the development of nanoparticles also increases, although the Human race was exposed to nanomaterials a long time ago.
A well-known Scientist named Richard Zsigmondy introduced the term 'nano' when he first measured the particle size of gold colloids with the help of a microscope. He won the noble prize for his achievement in 1925 [3]. The firsttime study of structural, electrical, and magnetic characteristics was carried out by a group of researchers including Wille, Hilbert, and Forestier in 1930, followed by more advancement in the 1930-1935 era by Japanese scientists, who also worked on magnetic oxides. An essential property of magnets which was termed as loss-factor, and loss factor is used to measure loss tangent per permeability [4].
The unique properties of spinel ferrites like low dielectric loss and high saturation magnetization over a high range of frequencies play an essential role in identifying its technical applications [5][6][7]. The crystal structure of spinel ferrites has natural superlattice as well. AB 2 O 4 crystal structure has tetrahedral sites named A and octahedral sites named B. Spinel ferrites have crystal structure AB 2 O 4 having tetrahedral site marked as A and octahedral site marked as B. Spinel ferrites are further categorized as ferromagnetic, anti-ferrimagnetic, and paramagnetic due to vacant or available A and B sites. All of this shows different magnetic behavior [8][9][10]. The physicochemical, electrical, and magnetic properties of spinel ferrites are noticeably influenced by the incorporation of cations and their preferential distribution among the tetrahedral and octahedral interstitial sites, as well as particle size reduction (i.e., to produce a nanosized material) via manipulation of the preparation method to a great extent. These properties can be modified by changing the synthesis method, calcination temperature, compound composition, and type and concentration of the dopants [11,12]. It is noticed that due to the excellent properties of spinel ferrites (Nickel Ferrites, Zinc Ferrites) like having high stability, good optical features, easy to fabricate, and low-cost reactions, these are extensively used in various industrial applications. The main reason for outstanding properties is due to Fe 3+ ions [13].
Moreover, the interchange of trivalent metal ions in zinc ferrite considerably affects the physicochemical, magnetic, and electrical chattels [14]. Soft magnetic ferrites are commonly fabricated from salts of Zinc and chromium. The reason is the similar ions of chromium and iron with three valence electrons [15][16][17][18]. Different dielectric responses in ceramics can be deduced to probe the electrical inhomogeneities in separative regions, such as bulk, grain boundary, and electrode, regions [19]. In recent years, interest in nanometer catalysts of sulfides transition metals (Mo, Ni, W, etc.) for residue slurry-phase hydrocracking has been constantly increasing [20]. Currently, nanoparticles of spinel ferrite are synthesized by various techniques such as chemical coprecipitation [19], microwave combustion [14], hydrothermal [21], sol-gel [22], and ceramics [23] methods. In all these methods, the sol-gel auto-combustion synthesis method has more potential due to the synthetic approach and final yield percent of product [24]. This reported study explains the influence of sintering temperature on spectral and dielectric characteristics of Zinc Aluminum doped Chromium nanoferrites using a sol-gel synthesis approach.

Materials
For sample preparation, salts of nitrate including aluminum nitrate, chromium nitrate, zinc nitrate, iron nitrate, polyethylene glycol, ethanol, and ammonium hydroxide solution were purchased from Sigma-Aldrich, USA. These chemicals were analytical grade, and purity was 97-99% for all chemicals.

Sample Preparation
Zinc and Aluminum doped chromium ferrites were prepared with the stoichiometric composition Zn x Al x-0.5 Cr 0.5 Fe 2 O 4 where x ranging from 0 to 0.5 with the interval of 0.1 hexahydrated salts of zinc nitrate, aluminum nitrate, and chromium nitrate were dissolved in ethanol with continuous stirring with heating at 50 °C for 30 min. After that, 3 M NH 4 OH solution was added dropwise and increased temperature up to 100 °C followed by the addition of 1 ml PEG. The temperature was increased up to 220 °C following NH 4 OH addition to maintain pH 7 for next half hour. The whole reaction was completed in almost 3 h with 235 °C temperature. Then, samples were ground in mortar pestle to fine powders then placed in a furnace for sintering to remove all impurities. The resultant nanoferrites were ready for spectral, thermal, and dielectric properties studies.

Characterization
The morphology of the final products was studied by scanning electron microscopy (SEM, JSM 6940A, Jeol, Japan). The crystallinity of the final product has been characterized by XRD investigation using D/MAX2400 diffractometer with Cu Kα radiation (0.15418 nm). The wide-angle powder test employed a scanning speed of 6°/min, step width 0.04° with voltage 50 kV, current 150 mA. Absorption bands spectroscopic analysis was completed by FTIR (Perkin Elmer). 10 mm diameter pellets of designed nanostructure composites were used for calculating the complex permittivity (ε′ and ε″) and dielectric tangent loss (tan δ) by RF impedance/ Material analyzer (Agilent E4990A) in the frequency range of 25-7 MHz.

Morphological Study of Designed Nanoferrites
Scanning electron microscope (SEM) micrographs are provided sample porosity, surface roughness, size of grain, distribution of particle size, material homogeneousness, and intermetallic distribution and diffusion. Figure 1 is depicted the micrographs of synthesized zinc aluminum ferrites and chromium-doped zinc-aluminum ferrites (Zn x Al x-0.5 Cr 0.5 Fe 2 O 4 ) by varying x = 0 and x = 0.5 with the difference of 0.5 nanopowders with chelating compound polyethylene glycol (PEG300) at three different sintering temperatures 500, 600 and 700 °C. It is examined in SEM pictures that considered ferrites possess an angular assembly having critical directions neighboring surfaces. SEM micrographs of resultant samples in which composition x = 0.0 and x = 0.5 for Zn and Al with elevated temperature show that gain size and sample density increase, as well as agglomeration, occur with increasing temperature. On the other side, substitutions of Zn and Al elements reveal more uneven forms and huge masses because of accumulation as shown in Fig. 1 (1a,  1b, 2a, 2b, 3a, 3b) due to increasing temperature [25,26].

Functional Group Study of Designed Nanoferrites
IR spectrum of prepared ferrites with substitution of Aluminum and Zinc is represented in Fig. 2a and b.
At 800-900 cm −1 , captivation's highest peak is associated with vibrating metals spreading at a tetrahedral place between 500 and 700 cm −1 . Moreover, the absorption peak is associated with metal vibration spreading at the octahedral B site. It has been observed from the FTIR spectrum that captivation bands of dehydrated gel at 3300 cm −1 are significant marks of the O-H band (hydroxyl group). At 2400 cm −1 , the peak shows that adsorption of CO 2 from the environment is achieved. Existence of carboxyl group C=O and traces of NO 3− ions at 1630 and 1395 cm −1 in the position of captivation bands match up to extending vibrating, correspondingly. But crews or bands at 1070, 830, and 650 cm −1 are generally connected with distortion of the C-H group due to stretching tremor crew in obtained results [19,27].

Crystalline Study of Designed Nanoferrites
XRD is used to analyze the nanostructure of considered nanocomposite illustrations (Zn x Al x-0.5 Cr 0.5 Fe 2 O 4 ), as displayed in Fig. 3a and b with the value of x = 0 and x = 0.5. The structure of single-phase cubic nanoferrites is proven International Center for Diffraction Data (ICDD) with card #01-083-1730 [28]. Solid evidence for accumulation of spinel assembly of nanoferrites is resembled with deflection heights of correlated planes at (222), (400), (311), (511), (440), (422), and (220). It is evident from the broadest range in the XRD outline that, synthesized ferrite has nano-sized particles. To measure the average size of crystallite of nanoferrites, Debye Scherer's formula is used (D) by considering of widen peak with the high strength (311) [29]. Figure 3a shows different peaks at (311), (220), and (200) of Al 0.5 Cr 0.5 Fe 2 O 4 in which Aluminum is shown with 0.5 compositions at 500, 600, and 700 °C. It observed there are peaks of Aluminum at (311), (220), and (200) getting sharp with increased temperature from 500 to 700 °C, because high strength of the main deflection peak of spinel aluminum ferrite at (311) even was depended as a quantity of its crystalline point. It is evident from patterns that all peaks become sharp and high intensities, which means strengthened crystallization when sintering temperature increases. Figure 3b  getting sharp with elevated temperature from 500 to 700 °C because high strength of highest deflection peak of spinel Zinc ferrite at (311) plane was depended as a degree of its crystalline notch. It is evident from patterns that all peaks become sharp and have high intensities, strengthening crystallization when sintering temperature increases. Zn 2+ ions usually exist in a B octahedral place. Cation dispersal is established from the crystal lattice constant of theoretical valuation. This displays good arrangement with investigational and ICDD cards of corresponding spinel ferrites [30].

Dielectric Constant
All ferrite dielectric constants measured from frequency series 25 Hz to 7 MHz as in Fig. 4a and b. The graphs show the change of dielectric constant with incidence for the same absorptions of Zn 2+ and Al +3 at elevated temperatures (500, 600, 700 °C). Perfectly, ferrites display dielectric dispersal were decreased in dielectric constant takings with rising of frequency from 25 Hz to 5 MHz. An instant decline in dielectric constant is happened in the low rate of recurrence division and lessens in the high incidence portion, and almost all methods are self-leading of frequency. Dielectric dispersion method in nano-ferrite materials described based on Koop's spectacle concept of dielectrics and Maxwell-Wagner model. Accordingly, the Maxwell-Wagner theory dielectric standard comprises improved conducting units alienated by less conducting particle borders. Consequently, particles have higher permittivity and conductivity, although particle margins are less conductive and show lesser permittivity values. At lower incidences, unit margins are more operational than particles in electrical transmission. The value of the dielectric constant is higher because of the thinner particle boundary. Upper values of dielectric constant (ε/) explained at minor incidences are similarly described on the basis of interfacial polarization because of irregular dielectric arrangement. In Fig. 4a and b it shows with the increase in frequency the dielectric constant decrease with elevated temperature (500, 600, 700 °C) of the same composition of two different (Zn and Al) samples [31].

Dielectric Loss
Dielectric loss calculates a dielectric material's characteristic degeneracy of electromagnetic energy. It can be parameterized in terms of either the loss angle δ or equivalent loss tangent tan δ. Dielectric loss disperses electrical energy by movement of charges in an alternating electromagnetic field as polarization changes direction.
When the material is intense within temperature series, regular arrangement of dipoles with field declines, thus divergence and dielectric constant decreases. At low temperatures, alignment manner cannot contribute to polarization. As a consequence of particle structure and porosity, inconsistencies in the arrangement may be created. Dielectric loss tangent (tan delta) is signified in Fig. 5a and b. It has been deceptive that the occurrence of relaxation peak monitors the rise in incidence loss tangent declines. Debye relaxation model is helpful to describe the occurrence of easing peak. Current work displays, relaxation peak at incidence 470 kHz and transfers to concerning the lesser frequency with the rise in Zn 2+ and Al +3 content at increasing temperature 500, 600, and 700 °C. In Zn nanoferrites and Al nanoferrites with the increase in temperature flowing of relaxation peak from more excellent to lesser frequency side exposes establishment of dipole-dipole interactions which reason interruption to the rotation of dipoles. Due to interruption of dipoles rotation, resonance among dipoles and applied field happens at a lesser frequency. The results expression explained that decrease in dielectric constant and loss tangent with the increase in temperature (500, 600, and 700 °C) respectively with same composition Zn 2+ and Al +3 in the sample [32].

Conclusion
There were two diverse compositions Al 0 . 5 Cr 0.5 Zn 0 Fe 2 O 4 and Zn 0.5 Cr 0.5 Al 0 Fe 2 O 4 , of spinel nanoferrites synthesized by the auto combustion sol gel method. The structure, and dielectric properties of these spinal ferrites have been investigated in current research. All samples which were prepared exhibit FCC spinal structure. Scanning electronic microscopy (SEM) confirmed the spherical shaped structure of fabricated nanoferrites that were also in the nano range. Clusters in these materials were also premeditated by SEM; it's due to the magnetic nature of these ferrites. XRD explained the spinal structure of nano ferrites by showing the peak of the spinel cubic structure. FTIR characterization explained two prominent absorption bands due to Tetrahedral on A-site and Octahedral on B-site in spinel ferrites. Temperature variations and Al-Zn doping greatly inclined the dielectric properties of spinel nanoferrites dialectic properties like dielectric loss tangent, and dielectric constant work examines that these properties reduce with increase of frequency which is typical dielectric dispersion manners these nanoferrites can be used for multipurpose like in high frequency single-layered electromagnetic waves absorbing devices, in medical fields as well as in electrical devices. The addition of dopants (Al, Zn) enhanced the magnetic and electrical properties of chromium nanoferrites. To have better and deeper understanding, detailed magnetic study of these ferrites will be proceeded in further study. The synthesis methods and use of various sintering temperature is also greatly affected the critical properties of ferrites, including structural and magnetic properties of the present systems of nanoferrites for many electronic applications.
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