Comparative Study of Structural , Electrical , and Magnetic Behaviour of NiCu-Zn Nanoferrites Sintered by Microwave and Conventional Techniques

Ni 0.8−x Cu x Zn 0.2 Fe 2 O 4 spinel type ferrite nanoparticles have been synthesized by citrate precursor method. These nanoparticles were then given heat treatment using microwave and conventional sintering techniques. Various characterizations using X-ray powder diffraction (XRD), scanning electron microscope (SEM), LCR meter, and B-H loop tracer were carried out on the sintered specimens. The XRD spectra of these ferrites confirmed the formation of spinel structure. The average crystallite size calculated using Scherrer’s formula was found to be in the nanometer range, its value varying from 33 nm to 39 nm. Microwave sintered samples exhibited superior electrical and magnetic behaviour over their conventionally sintered counterparts. Feasibility of low temperature synthesis and promising properties will render these ferrites suitable for multilayer chip inductor applications.


Introduction
Spinel nanoferrites have generated enormous interest in the scientific community throughout the world due to their unique properties.Nanocrystalline spinel ferrites have excellent electrical and magnetic properties like high resistivity, low eddy current losses, high saturation magnetization, high Curie temperature, and large permeability at high frequency which makes them suitable for a wide range of applications such as in information storage systems, magnetic cores, microwave absorbers, medical diagnostics, radio frequency circuits, high quality filters, rod antennas, transformer cores, magnetic resonance imaging (MRI), and drug-delivery technology [1][2][3][4].A promising ferrite possessing the abovementioned characteristic features is the NiZn ferrite [5].
Considering the recent trend of technology, electronic devices are being processed in the form of microchips.Ferrites form a main component of such multilayer chip inductors (MLCIs) and are used with silver as the internal electrode.Since the melting point of silver is 961 ∘ C, the sintering temperature of the ferrite should be below 961 ∘ C to prevent silver diffusion into the ferrite.This temperature is quite low to cofire with NiZn ferrites whose sintering temperature is 1250 ∘ C. It has been reported that addition of CuO is capable of lowering the sintering temperature of NiZn ferrite with the conventional solid-state reaction method and coprecipitation method [6,7].Hence, for the present investigation, Cu substituted NiZn ferrites have been synthesized using the citrate precursor method.This synthesis method has been reported as a versatile method for obtaining nanocrystalline ferrite particles.It allows homogeneous distribution of ions at the molecular level, good control of stoichiometry, small particle size formation, low temperature synthesis, and high efficiency.Besides, it is simple, inexpensive, and less timeconsuming [3,8].
Lack of reports on a systematic investigation of the comparison of structural, electrical, and magnetic properties of NiCuZn ferrites sintered by microwave and conventional techniques prompted the present study.It has been reported that the microwave sintering technique tremendously (Merck, India).Stoichiometric amounts of nitrates and citric acid were dissolved separately in doubled distilled water.Metal nitrate solutions were mixed with citric acid solution in 1 : 1 molar ratio of nitrate to citric acid.The pH value of the solution was adjusted to 7 using ammonia solution.The resulting solution was heated at 100 ∘ C to form a viscous gel.Finally, the gel ignited in a self-propagating manner to form a fluffy loose ferrite powder.The as-burnt ferrite powders were ground using mortar and pestle.These powders were then mixed with 10% polyvinyl alcohol (PVA) as a binder and pressed to form pellets (10 mm diameter) and toroidal rings (30 mm outer diameter, 10 mm inner diameter, and 4 mm thickness) by applying a pressure of 10 ton cm −2 .These specimens were presintered at 600 ∘ C for 4 h in a programmable conventional furnace to expel the binder.The pellets and toroidal rings were then subjected to final sintering at 900 ∘ C (2 h for conventional sintering).Another set of samples was subjected to microwave sintering at a temperature of 900 ∘ C with a holding time of 30 min at a heating rate of 10 ∘ C/min.Finally, the samples were furnace-cooled.The pellets were coated with silver paste to provide electrical contact and the toroidal rings were wound with 70 turns of 30 SWG enamelled copper wire to form toroids.

Experimental Details
Microwave sintering was carried out using a microwave furnace procured from VB Ceramic Consultants, Chennai, India.The microwave furnace uses a domestic microwave oven (VB Ceramic) with magnetrons operating at 2.45 GHz frequency at a power output of 4.4 kW.The temperature controller used was Eurotherm (Model 2416) microprocessor based PID programmer cum digital temperature indicator.The rate of heating and dwell with respect to time were preprogrammed and the entire operation was automatically controlled by the programmer.Temperature sensor output leads were directly connected to programmer as input.Raytek imported noncontact IR sensor was used to measure the temperature of sample directly.This sensor was fixed at the top of the furnace.SiC was selected as a microwave susceptor which reacted with the electromagnetic waves and created instant heat in the system.The specimen was placed in a crucible inside the cavity of the susceptor and was carefully placed in the furnace box.The sample centre and the centre of the sensor were aligned manually.The furnace was designed to work up to a maximum temperature of 1600 ∘ C. Insulation zone was constructed with Zircar (USA) SALI grade high alumina insulation.Such type of insulation helps the furnace for the fast rate of heating and serves for severe thermal shock heating cycles.The main structure was made with high quality stainless steel and outer shell made with mild steel angles and sheets with neat powder coat finish.

Characterizations.
The phase identification and structure analysis of the sintered samples were performed using Philips X'pert Pro X-ray diffractometer (microwave sintered samples) with CuK radiation ( = 1.5406Å).The crystallite size of each composition was determined using Debye-Scherrer relation [10] and lattice parameters were calculated from the (311) diffraction peak.X-ray densities were also determined.The microstructures were examined by using FEI QUANTA scanning electron microscope (SEM).Dielectric measurements as a function of frequency in the range 20 Hz-2 MHz were carried out at room temperature using a precision LCR meter E4980A.The electrical resistivity of the samples was determined.The real part of the dielectric constant was calculated using the formula where  is the capacitance of the pellet,  is the thickness of the pellet,  is the cross-sectional area of the flat surface of the pellet, and   is the permittivity of free space.Inductance () of the sintered toroids was measured in the frequency range of 20 Hz-2 MHz using a precision LCR meter E4980A.
The measured inductance was used to calculate the initial permeability   = /  , where  is the measured inductance and   is the air core inductance calculated using the relation where  is the number of turns of the coil ( = 70), ℎ is the thickness, OD is the outer diameter, and ID is the inner diameter of the toroidal specimen.Soft magnetic properties were measured on toroidal core samples at room temperature.The coercivity (  ) and saturation magnetization (  ) were determined from B-H hysteresis loops measured by B-H loop tracer (Ferrites India) working on 50 Hz mains frequency.2Θ (degree)

Results and Discussion
3.1.XRD and Structure Analysis.X-ray diffraction (XRD) patterns (Figure 1) of Cu substituted NiZn ferrites can be well indexed to JCPDS card number 01-071-3850, thereby establishing the formation of single phase cubic spinel structure.The patterns showed the absence of other impurity phases unlike the appearance of second phase of CuO or CuFe 2 O 4 for ferrite composition of  ≥ 0.3 prepared by coprecipitation method [11].The XRD parameters like lattice constant, X-ray density, and crystallite size for the microwave sintered (MW) Ni 0.8− Cu  Zn 0.2 Fe 2 O 4 ferrites are tabulated in Table 1.Lattice expansion occurs if the doping ion has larger radii than the displaced ion.In the present investigation, Cu 2+ ions of larger ionic radius 0.72 Å replace Ni 2+ ions of smaller ionic radius 0.69 Å and lattice constant is expected to increase.However, no significant increase is observed and this might be attributed to the fact that contribution to lattice constant does not solely depend on ionic radii but also on other interaction phenomena like the long range attractive coulomb force, bond length, and so forth which needs further investigation [12,13].Besides, it has also been reported that the presence of copper ions in different ionic states in the A and B sites is responsible for the nonlinear variation of lattice constant with increasing copper content [7].An increasing trend in density (theoretical and experimental) was observed with the increase in Cu 2+ ion concentration.This can be explained on the basis that atomic weight of Cu 2+ (63.55 amu) is greater than that of Ni 2+ (58.71 amu).The increase in experimental density with Cu 2+ substitution may be attributed to the acceleration of cation interdiffusion due to Cu 2+ ions and increase in reactivity of the fine ferrite grains which coalesce to form bigger grains resulting in pore reduction and volume shrinkage [14].Also, the high atomic mobility of Cu 2+ ions at relatively low temperature is an outstanding feature of Cu in increasing the density of the ferrite samples [15].For a given value of Cu 2+ concentration, the theoretical density is greater for the conventionally sintered samples (reported elsewhere) [16] than that of the microwave sintered ones.This could be due to longer sintering duration for the conventionally sintered samples.The same explanation will apply to the crystallite size as higher values of crystallite size are reported for the conventionally sintered samples [16] than the corresponding microwave sintered ones.The average crystallite sizes of all the samples are found to be in the range of 33 nm-39 nm clearly indicating that nanosized crystals can be effectively synthesized by the citrate precursor method.2).The fine grained microstructural feature of these samples is effective for MLCIs with thinner internal ferrite layers.It is observed that grain size increases with increasing Cu 2+ concentration up to  = 0.3.During sintering, Cu 2+ influences the microstructure by the formation of a liquid phase.The liquid phase induces higher atomic mobility of the Cu 2+ ions and this facilitates the grain growth [17].With further increase in Cu 2+ content, the average grain size decreases.This may be owing to the fact that Cu 2+ does not favour grain growth when present in excess amount.It has been reported that an intense microwave field concentrates around samples during microwave sintering resulting in ionization at the surface of ferrite particles.The kinetic energy of the ions at the grain boundaries increases due to the microwave field.This decreases the activation energy for a forward jump of the ions and increases the barrier height for a reverse jump.Hence, the forward diffusion of intergrain ions is enhanced, thereby accelerating the grain growth during sintering.As a result, grain size for the microwave sintered samples is found to be larger than the conventionally sintered ones [18].    is responsible for the conduction [20].During the process of electron hopping between Fe 2+ and Fe 3+ ions, localized electron accumulation takes place at the interface due to high resistance of the grain boundaries giving rise to space charge polarization.This results in a high value of dielectric constant at low frequencies.With further increase of frequency, the grains come into play, thereby reducing the value of dielectric constant rapidly.At still higher frequencies, the hopping of electrons cannot follow the frequency of the alternating field which leads to a decrease in contribution of space charge polarization.Hence, the dielectric constant decreases at high frequencies [21,22].Various other factors affecting polarization are structural homogeneity, density, grain size, and porosity of ferrite material.Addition of Cu 2+ in NiZn ferrite causes a decrease in structural homogeneity of the sample which may account for the increase in polarization [14].The hole exchange between Cu 2+ and Cu 1+ ions also plays a vital role in the conduction and polarization mechanism.The addition of Cu may enhance the concentration of Fe 2+ /Fe 3+ pairs in the B site, thereby resulting in an increase of dielectric constant [23,24].The increase in density may also be a major factor contributing to the increase in polarization with Cu 2+ incorporation.This is so because the increase in density decreases the porosity which will result in a higher number of polarizing species per unit volume [14].This may account for the higher values of dielectric constant for the conventionally sintered samples than the corresponding microwave sintered samples.

Microstructural
The value of dielectric constant of these samples is low as compared to that of pure NiZn ferrites synthesized by the conventional solid-state process.Low sintering temperature of the samples must have resulted in lowering the possibility of ions to exist in different oxidation states.This in turn reduces the probability of hopping of charge carriers.Hence, a low value of dielectric constant is observed [14].
The variations of dissipation factor with frequency at room temperature for all the samples are shown in Figures 5 and 6.The dielectric loss decreased with increasing frequency, which is a normal behaviour of ferrites.This can be attributed to the fact that, in the low frequency region, which corresponds to a high resistivity (due to grain boundary), more energy is necessary for electron transfer between Fe 2+ and Fe 3+ ions.Hence, the loss is high.The high frequency region corresponds to a low resistivity (due to the grain), so a small amount of energy is necessary for electron hopping between Fe 2+ and Fe 3+ ions at the octahedral site.Hence,  the dielectric loss is low in this region.A peak at lower frequency region may be due to the existence of resonance between frequency of the externally applied electric field and the hopping frequency of the charge carriers.The domain wall resonance may also be a contributing factor [25].
Microwave sintered samples exhibited high loss factor values at lower frequencies, while the conventionally sintered samples possess relatively lower values.This can be explained by the fact that microwaves interact with high loss materials due to polarized species in the specimen.Exposure of highly absorbing species such as ferrites would result in the formation of polarized species within them [5].

DC Resistivity.
The DC electrical resistivity is an important parameter of low temperature sintered ferrite for MLCI application.The variations of DC resistivity versus Cu 2+ content for microwave (MW) and conventionally sintered (Con) pellets are shown in Table 1.Thakur and Singh [22] have reported that the variation of resistivity with Cu 2+ content can be attributed to the grain size of the samples.Samples having small grains possess a higher number of grain boundaries.These boundaries are regions of mismatch between energy states of adjacent grains and hence act as a barrier to the flow of electrons.Additional advantage of small grain size is that it aids in reducing Fe 2+ ions as oxygen moves faster in smaller grains and hence retains the iron in the Fe 3+ state.This decreases the probability of electron hopping in smaller grains resulting in higher resistivity.However, resistivity may also be influenced by other factors like hopping of charge carriers, lattice site distribution, and so forth.The resistivity of the samples under study does not exactly follow the abovementioned trend, which needs further investigation.by microwave and conventional sintering techniques are shown in Figures 7 and 8.It is observed that    remains almost constant up to a frequency of 10 kHz and thereafter it shows a gradual decline for the compositions with Cu 2+ content of 0.2, 0.3, 0.4, and 0.5, while for the samples with Cu 2+ content of 0.0 and 0.1, the values of    remain almost unchanged over the entire frequency range studied.It is reported that    versus frequency curve is characterized by a critical frequency beyond which    decreases rapidly from its static value.Initially, when there was great disparity between the drive frequency and the precision frequency, the value of    remained static.The decrease in    at a particular frequency is due to the onset of ferrimagnetic resonance.As the drive frequency increases and matches the precision frequency, the precessing spins absorb power from the drive field and as a result    decreases.It is therefore expected that    will begin to decrease at a lower frequency if    is large.This is in agreement with the experimentally observed values.This means that the only loss mechanism in ferrites is the spin resonance [26].It has also been reported that the decreasing trend in permeability with increasing frequency is due to nonmagnetic impurities between grains and intragranular pores which act as pinning points.This considerably reduces the contribution of the motion of spin and domain walls to the permeability [12].Two-Magnetizing Phenomenon.Spin rotation and domain wall motion accounting for the permeability of polycrystalline ferrite can be represented as   = 1 +   +  spin , where   = 3  2 /4, and  spin = 2  2 /  , where   is susceptibility due to domain wall motion,  spin is susceptibility due to spin rotation,   is saturation magnetization,  is average grain diameter,  is domain wall energy, and   is total anisotropy constant [27].Accordingly,    varies linearly with grain size.Globus et al. [28] also assumed that permeability arising due to wall motion is likely to vary linearly with grain size while permeability contribution to spin rotation was assumed to be independent of grain size.Globus [29] found linear dependence of permeability on grain size for several NiZn ferrites.These reasons account for the abrupt increase in initial permeability with increasing Cu 2+ content, which attains a maximum value at 0.3 Cu 2+ concentration (as shown in Table 2).The higher value of grain size possibly explains the higher value of initial permeability for the microwave sintered specimens than the conventionally sintered ones.2. These values are low as compared to those reported by Dimri et al. [14].The exchange interaction between the ions at the tetrahedral (A) sites and octahedral (B) sites in the crystallographic lattice is responsible for the variation of saturation magnetization   with concentration of Cu 2+ ions.In spinel ferrites, Zn 2+ ions occupy the A site while Ni 2+ and Cu 2+ ions have strong preference for the B site and Fe 3+ ions partially occupy the A and B sites.Fe 3+ (d 5 ), Ni 2+ (d 8 ), Cu 2+ (d 9 ), and Zn 2+ (d 10 ) cations of the ferrite have 5, 2, 1, and 0 unpaired electrons, respectively, and some magnetic moment in  B [1].The resultant magnetization, , is the difference of the magnetizations  B and  A of the B and A sublattices, respectively, whereby the former generally has a higher value.As the magnetic moment of Cu 2+ (1.3 B ) is less than that of Ni 2+ (2.3 B ), the substitution of Ni 2+ with Cu 2+ will decrease the net magnetic moment and exchange interaction between A and B sublattices [30].Consequently, 4  values will decrease.But the experimental values of 4  are found to increase with Cu 2+ ion concentration up to  = 0.3.This is indicative of the presence of Cu 2+ ions on the A sites.Rahman and Ahmed [7] have also reported the presence of Cu 2+ ions on the A sites.Hence, Cu 2+ ions may have a tendency to occupy both sublattices [31].This can alter the cation distribution to a greater extent which is a deciding factor in determining the overall magnetic properties.The incorporation of Cu 2+ ions will force some of the Fe 3+ ions to migrate from the A sites to the B sites.With the magnetic moment of Fe 3+ ions (5 B ) being greater than that of Cu 2+ ions, the migration causes an increase in magnetic moment of B sites and a decrease in magnetic moment of A sites.This results in an increase in net magnetization  =  B −  A .With further addition of Cu ions, deviations are observed.More Fe 3+ ions migrate to the B sites thereby increasing the magnetic moment of the B site.The magnetic moments of the remaining few Fe 3+ ions on the A sites are no longer able to align all the moments of the B ions antiparallel to themselves,
Characterization.The SEM micrographs for Ni 0.8− Cu  Zn 0.2 Fe 2 O 4 nanoparticles are shown in Figure 2. Microstructural analysis allows determination of the average grain size of the samples (Table
0.8− Cu  Zn 0.2 Fe 2 O 4 ferrites are depicted in Figures 9 and 10.The variation of 4  values with Cu 2+ ion content is shown in Table