Possibility of NiCuZn Ferrites Composition for Stress Sensor Applications

Department of Physics, Changwon National University, Changwon 641773, Republic of Korea 2 School of Advanced Sciences, VIT University, Vellore 632014, India 3 School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen 518055, China Department of Materials Science and Nanotechnology, Yogi Vemana University, Kadapa 516227, India Department of Physics, Sri Krishnadevaraya University, Anantapur 515055, India


Introduction
It is well known that the effect of stress is very important in the magnetization process of ferrites [1].e magnetic properties of ferrites such as permeability, coercivity, and hysteresis loop change with the application of stress [2,3].e effect of external applied stress can be observed in ferrites as a change of inductance (Δ)% when compressive stresses are applied.ese variations of permeability with applied stress in these samples can be attributed to the magnetostrictive contributions of varied amounts of nickel and iron compositions.For small compressive stresses, the stress raises the initial permeability with negative magnetostriction and for large tensile stresses the permeability decreases [3].e magnetoelastic properties of ferrites are interesting subjects for investigations to determine the possibilities of their use in the construction of measuring sensors [4,5].Over the last decade, manganese substituted cobalt ferrites have been the dominant ferrite materials for stress sensor applications due to their large magnetomechanical effect and high sensitivity to stress [6][7][8][9][10][11]. Magnetic sensors play a signi�cant role in physical measurements used in all kinds of applications [12,13].e most oen used magnetic phenomena in today's magnetic sensor technology are the magnetoresistance [14,15], the magnetoimpedance [16,17], the magnetostriction [18,19], the electromagnetic induction [20], and the Hall effect [21].ere also exist other effects usable in sensing applications, both macroscopic and microscopic [22].Due to superior mechanical and magnetic properties in ferrites they seem to be suitable for construction of force and stress sensors [23,24].is led to systematic investigation of studies on the stress sensitivity of series of NiCuZn [25], MgCuZn, and NiMgCuZn [26] ferrites.Incidentally it has been noticed from these studies that certain NiCuZn ferrite compositions can be used for stress sensor applications.In view of this the details of synthesis of those NiCuZn ferrite compositions suitable for stress sensor applications and their stress dependence studies are reported in this paper.

Experimental Procedure
A series of six nonstoichiometric compositions of NiCuZn ferrite with compositions shown in Table 1 have been prepared by the conventional double sintering method using analytical grade NiO, CuO, ZnO, and Fe 2 O 3 in their respective proportions.ese oxides were weighed and intimately mixed in stoichimetric proportions.ese constituents were ball milled (RETSCH PM -200, Germany) in agate bowls with agate balls in acetone medium for 20 h.e slurry was dried and the dried powders were loosely packed in the form of cakes.ese cakes were presintered in closed alumina crucibles at 800 ∘ C for 2 h.e presintered cakes removed from the furnace were crushed and ball milled in an acetone medium in agate bowls with agate balls for another 24 hours to obtain �ne particle size.ese slurries a�er drying were sieved to obtain uniform particle size.
e presintered green powders were mixed with 2% polyvinyl alcohol as a binder and were compacted in the form of disks of diameter 10 mm and 2 mm height, toroids of 12 mm outer diameter (OD), 8 mm inner diameter (ID) and 4 mm height and cylinders of diameter 10 mm and length nearly 20 mm at 200 MPa with a suitable die.e compacted bodies were conventionally sintered in a programmable furnace (V.B.Ceramic Consultants, Chennai, India) at a temperature of 1250 ∘ C for 2 hours and were cooled to room temperature at 80 ∘ C/hr.e sintering schedule includes half an hour dwelling time at 110 ∘ C to remove moisture from the samples and one-hour binding burning time at 600 ∘ C. Proper care was taken to avoid zinc evaporation by providing ZnFe 2 O 4 .e density of the sintered specimens was measured by Archimedes's method.All the samples were structurally characterized using Philip high resolution X-ray diffraction system (PM 1730, Germany) with CuK  radiation.Microstructures of sintered samples were investigated using JOEL (JSM Model 6360, Japan) scanning electron microscope.e initial permeability (  )of these ferrite toroids was evaluated using the standard formulae from the inductance measurements carried out at a frequency of 10 kHz using computer controlled impedance analyzer (Hioki Model 3532-50 LCR Hi-Tester, Japan).ese measurements were carried out in the temperature range 30 to 300 ∘ C. In order to study the effect of external stress, uniaxial compressive stress parallel to the magnetizing direction was applied to the cylindrical shaped ferrite cores using uniaxial press system.e stress magnitudes were varied from 0 to 10 MPa.e change in inductance was measured using the abovementioned LCR Hi-Tester by employing 100-120 turns coil on each cylinder.e initial permeability was calculated using the relation where  is the series inductance;  is the number of turns of winding; ℎ is the height of the toroid in inches;   is the outer diameter of toroid; and   is the inner diameter of toroid, respectively.

Results and Discussion
Figure 1 represents the typical X-ray diffractograms of nonstoichiometric NiCuZn ferrite samples, respectively.e Xray diffraction analysis of the ferrite samples shows the formation of single phase spinel structure.e X-ray lines show considerable broadening, indicating the �ne particle nature of the ferrite powder.e variation of the lattice parameter "" as a function of nickel ion content is depicted in Figure 2. It is noticed that the lattice parameter increases with the nickel ion content in the lattice.is variation can be explained on the basis of an ionic size difference of the component ions.It may be  pointed out here that the ionic radii of Ni 2+ ,Cu 2+ , and Zn 2+ are almost same with the experimental error that is, 0.72 Å, 0.74 Å, and 0.74 Å, respectively [27].However the lattice parameter is found to increase systematically with increase in nickel content.
e SEM photographs of the sintered samples were recorded to understand the microstructure of the NiCuZn ferrites.Scanning electron microscopy (SEM) of powder sintered at 1250 ∘ C/2 h shows an increase in particle size increasing the nickel concentration [28].Figures 3(a 1).It is well known that the initial permeability characteristics depend not only on chemical composition but also on the microstructure of the sintered body.Desired magnetic properties of ferrite can be achieved by the control of microstructures [29].Small and uniform grain size is favourable to obtain low power loss, but large grain size is favourable to get high permeability [30].
Figure 4 shows the variation of the initial permeability (  ) versus the nickel content for samples at room temperature (27 ∘ C).It can be noticed from the �gure that the initial permeability shows a maximum at 0.48 nickel content in these nonstoichiometric NiCuZn ferrospinels.is may be attributed to the fact that the magnetocrystalline anisotropy constant ( 1 ) becoming zero at this composition of nickel [31,32].
e initial permeability (  ) as the function of temperature (at constant frequency, 10 kHz) from room temperature to Curie point was also studied.e temperature dependence of magnetic anisotropy can be inferred from the temperature dependence of initial permeability as shown in Figure 5.It can be noted from Figure 5 that as the temperature increases the initial permeability (  ) remains constant up to a certain temperature and increases to a peak value and F 4: Variation of initial permeability with mole fraction of nickel at room temperature.
then abruptly falls to a minimum value.e temperature at which this abrupt fall takes place is the magnetic Curie transition temperature   .e magnetic initial permeability for the material is expected to strongly depend on the microstructure, as the initial permeability represents the mobility of magnetic domain wall in response to the small applied �eld [33].It is proved by Figures 3(a the magnetic permeability of the large-grain samples is larger than the �ne-grain ones.e initial permeability is an important magnetic property to study the quality of so ferrites.Generally, the initial permeability (  ) depends on two factors [34] namely, (1) contribution from spin rotation and (2) contribution from domain wall motion.But the contribution from spin rotation is smaller than domain wall motion [35].e permeability due to domain wall motion is given by [36] where   is saturation magnetization,  is mean grain size, and  is magnetic domain wall energy, which is proportional to magnetocrystalline anisotropy constant  1 [37,38].us where (  )  is the initial permeability of domain wall motion.e magnitudes of initial permeability (  ) increased with increasing content of nickel up to 0.48 aer decreasing with increasing nickel content and interestingly the Curie transition temperature remained fairly constant around 200 ∘ C. e �gure reveals that there is a �at temperature response of (  ) in all these samples.e Curie transition temperature and permeability behaviour as a function of nickel content in these ferrites were published in earlier communication [25].
e variation of ratio of inductance change (Δ)% as a function of applied compressive stress is shown in Figure 6.From an examination of Figure 6 it is clear that in samples a and b the (Δ)% values decrease with increasing external stress.In the case of samples d and f, there is an increase of (Δ)% up to certain applied stress and thereaer it decreases.At higher concentrations of nickel the (Δ)% increases up to a certain stress and decreases with further raise in external compressive stress showing a peak value.Exactly, similar behaviour was noticed by Kanada et al., [39] in NiMgCuZn ferrites at 4 MPa.For example these peak values are 3.5 MPa, 2 MPa, and 3.5 MPa for nickel contents 0.48, 0.50, and 0.52, respectively.In the present work, we have studied variation of inductance at low concentration of nickel.is type of behaviour dependency of (Δ)% on stress can be utilized for the development of a sensor for detection of stress variations.

Conclusions
e initial permeability measurements were carried out in the temperature range 30-300 ∘ C in NiCuZn ferrites.In the present study it has been noticed that there is a linear decrease of (Δ)% with increasing external applied stress in ferrite compositions with nickel 0.42, and 0.44 concentration.e compositions Ni 0.42 Cu 0.10 Zn 0.60 Fe 1.76 O 3.76 , and Ni 0.44 Cu 0.10 Zn 0.60 Fe 1.72 O 3.72 showed best magnetic properties among the �ve compositions and would be prominent materials for inductive stress sensor applications.

F 2 :
Lattice constant () as a function of nickel ion concentration of the NiCuZn system.
) and 3(b) show the microstructures of fracture surfaces of the sample b (Ni 0.44) and sample d (Ni 0.48) (Table