Influence of Zirconium substitution on electrical properties of Nickel-Zinc Ferrites

The Zirconium substituted Nickel Zinc Ferrite nano particles with chemical composition of Ni(0.5-x)Zn(0.5-x)ZrxFe2O4, where the value of x varies from 0 to 0.2, have been synthesized by the sol-gel auto combustion method, using high purity nitrates and fuelling agent citric acid. The structural property was studies from X-ray diffraction spectra. The crystallite size decreases with increase in Zirconium substitution. The electrical property of the same was investigated by studying the dielectric constant, and AC conductivity. There are fluctuations in these properties with variation in Zirconium substitution in Ni-Zn Ferrite.


Introduction
In recent years nanoparticles have proved to be better from their bulk counter parts with respect to their extensive applications in the field of electronics, medical, agriculture, pharmacy and engineering fields [1].These nanoparticles have helped in the rapid progress in technology of 21 st Century because of their flexibility in tailoring required size, structure, and composition.The materials with diverse applications with specific electrical and magnetic properties have received considerable attention [1][2][3][4].Among them Ferrite is one which can be engineered so that they prove promising materials to be used in fields with varying electric as well as magnetic behaviour.
Among the Spinel ferrite group Nickel ferrite, Zinc ferrite and Nickel Zinc ferrite are one with wide range of applications.These are the ferrites that can be explored with different chemical compositions and substitutions, and can get varied properties.Nickel Zinc ferrite are used in applications like electronic devices [5], high frequency devices [5][6]., microwave devices [7][8], as sensors [9][10], for photo catalysis in waste water treatment [11][12], in biomedical applications such as magnetic carriers [13], drug delivery, anti microbial activity and many more.The method of synthesis also has an essential role in the determination of property of ferrites.The methods like Sol-gel method, Ceramic method, Hydrothermal method, Co-precipitation method, and many other are known for their specializations in deciding the nature of nanoparticles formed.

Method of synthesise
The nanoparticles of Zirconium substituted Nickel Zinc Ferrite were prepared by sol-gel auto combustion method.In this method the fuel/reducing agent used was Citric acid and metal nitrate precursor acts as oxidizer are dissolved in deionized water.The stoichiometric ratio of fuel to oxidizers was kept constant at 3:5.The precursor of metal nitrates in deionized water along with citric acid was continuously stirred for 30 minutes.Then the solution was then heated at 75 0 C so that the water in it evaporates forming a gelly solution.The gel is further heated to 150 0 C.As the temperature rises, foam starts to form from the reaction mixture, which latter forms a powder of desired ferrite on complete combustion.The powder is then grounded and calcinated at 500 0 C for 2 hours to remove the unreacted chemicals if any.These calcinated powders are the made into pellets and further sintered at 700 0 C for 2 hours.

Characterization techniques
The crystal structure of synthesized nano particles of Zirconium substituted Nickel-Zinc Ferrite is analyzed from X-ray diffraction measurements (XRD Model: SmartLab SE) recorded with X-rays of wavelength 1.5406Å.The AC Conductivity and impedance analysis was done using Wayne Kerr Precision Impedance analyzer 6500B.

Structural analysis
The Crystal structure of Zirconium substituted Nickel Zinc Ferrite was examined by X-ray diffraction studies.The XRD spectra of ferrite nanoparticles under investigation are as shown in Figure 1.These well defined sharp peaks in the spectra show the high crystallinity of synthesized ferrite.The X-ray diffraction spectra also validates the formation of Spinel Phase of Zirconium substituted Nickel-Zinc Ferrites as we can observe the intense peak at (311) plane and other peaks corresponding to (220), ( 222), (400), (422), (511), and (440) planes.The formation of crystalline phase of the ferrite under investigation was verified by referring to the particulars from the JCPDS card no.00-062-0437.
The size of crystallite of the nanoparticles was determined by employing Scherrer equation [14]: t=   (1) in Eq. (1) 't' is crystallite size, 'k' is Scherrer constant, 'λ' is wavelength of X-ray, 'β' is the full width at half maximum (FWHM) of the peak for (311) plane and 'θ' is Bragg angle.The lattice parameter of crystal system is obtained from the relation [14]: a =  ℎ √ℎ 2 +  2 +  2  (2) in Eq. ( 2) dhkl is the interplanar spacing of corresponding plane of Miller Indices (hkl).We can notice that the average size of crystallite of ferrite under study decreases from 29.98 nm to 14.73 nm as the Zirconium substitution increases.The size of crystallite and Lattice constant of nanoparticles under discussion are presented in Table 1.It has to be noted that both size of crystallite and Lattice constant decrease with increase in Zirconium substitution.

Dielectric Studies
The Dielectric studies are helpful in the investigation of electrical properties of materials.These dielectric properties are dependent on composition of the material, method involved in synthesis, temperature under which the study is carried out and also electric field frequency.These investigations help in knowing the nature and transport of electric charge carriers.Under this discussion of dielectric properties we study the frequency dependency of dielectric constant, dielectric loss and AC conductivity of the materials.The relative permittivity of material is studied as Dielectric constant (εr).Dielectric constant of a material gives us the measure of its ability to store electrical energy.The relative permittivity or dielectric constant is a complex parameter which consists of two parts; viz.real part of εr represented by ε` and imaginary part of εr represented by ε``.
The calculation of real part (ε`) of dielectric constant is done by utilizing the relation that follows ε` = In Eq. ( 3) 'Cp' represents parallel capacitance (in farad),'t' represents pellet's thickness, 'A' denotes pellet's cross section area and 'ε0' denotes absolute permittivity.The variance of dielectric constant's real part with frequency is represented in figure 2.An increase in frequency brings down the value of ε`.This value of 'ε`' is high at lower frequency, however we can observe that these values decrease as frequency goes up and almost saturates at higher frequency.We can obtain the dielectric constant's imaginary part (ε``) by employing the relation given below ε``= ε` tan δ (4) According to Eq. ( 4), 'tan δ' is dielectric loss tangent.The frequency dependent change of dielectric constant's imaginary part is shown in figure 3.As observed for ε`, the ε`` also exhibits same behavior.The values of ε`` also goes down as the frequency rises up.'ε`' is high for lower frequency and the values decrease with increase in frequency.This values of ε`` almost saturates at higher frequency.The dielectric loss (tan δ) is the energy lost due to varying electric field.The figure 4 shows the frequency dependency of tan δ of Zirconium substituted Nickel-Zinc Ferrite at room temperature.The plots show that lower frequencies result in a high dielectric loss, which decreases as frequency increases.

AC conductivity
The AC conductivity (σac) of Zirconium substituted Nickel-Zinc Ferrites are measured at different frequencies at room temperature.The relation between the dielectric constant and dielectric losss of the nanoparticles is utilized to study AC conductivity (σac), σac = ωε0ε`tanδ (5) In Eq. ( 5) 'ω' represents the angular frequency (ω=2πf), 'ε0' represents permittivity of free space (8.869 x 10 -12 F/m), 'ε`' represents real part of dielectric constant and 'tan δ' represents dielectric loss.This relation (5) was helpful in representing the deviations in AC conductivity as frequency changes and is depicted in Figure (5).We can observe that at lower frequency the dependency of conductivity of frequency is less significant, but as frequency increases the conductivity also increases at higher frequencies.Maxwell-Wagner Theory and Koop Theory explain this change in electrical conductivity, according to which we can observe the increase in AC conductivity as frequency increases.The theories say that the grain boundaries are more active at lower frequencies which results in lowering of hopping frequency of the electrons between Fe 3+ and Fe 2+ .Whereas conductive grains at higher frequency are more active, resulting in increased hopping of Fe 3+ and Fe 2+ with frequency [15][16].As we can also observe that at lower frequency the Zirconium substitution has increased conductivity, which shows that Zirconium substituted Nickel-Zinc Ferrite, can be useful in applications that need high AC conductivity at low frequency.

Conclusion
Our focus in this work has been on the synthesis of Zirconium substituted Nickel-Zinc Ferrite employed by Sol gel Auto combustion method for the investigation of its structural and electrical properties.The structure of synthesized ferrite nanoparticles was analyzed from XRD studies, which confirms spinel structure of ferrite, where the increase in Zirconium concentration raises the crystallite size and lattice parameter.The dielectric studies show that the dielectric constant decreases and AC conductivity increases with increase in frequency.We can also observe a remarkable impact of Zirconium substitution on the dielectric properties of Nickel-Zinc ferrite.These properties are some indications that could the material is suitable for the applications in sensors and in microwave devices.