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Particle size dependence of the magnetic, dielectric and gas sensing properties of Co substituted NiFe2O4 nanoparticles
Accepted Manuscript Title: Particle size dependence of the magnetic, dielectric and gas sensing properties of Co substituted NiFe2 O4 nanoparticles Authors: E. Ranjith Kumar, Ch. Srinivas, M.S. Seehra, M. Deepty, I. Pradeep, A.S. Kamzin, M.V.K. Mehar, N. Krisha Mohan PII: DOI: Reference:
S0924-4247(18)30411-4 https://doi.org/10.1016/j.sna.2018.05.031 SNA 10792
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
7-3-2018 15-5-2018 17-5-2018
Please cite this article as: Kumar ER, Srinivas C, Seehra MS, Deepty M, Pradeep I, Kamzin AS, Mehar MVK, Mohan NK, Particle size dependence of the magnetic, dielectric and gas sensing properties of Co substituted NiFe2 O4 nanoparticles, Sensors and Actuators: A. Physical (2018), https://doi.org/10.1016/j.sna.2018.05.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Particle size dependence of the magnetic, dielectric and gas sensing properties of Co substituted NiFe2O4 nanoparticles
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E. Ranjith Kumara*, Ch. Srinivasb*, M. S.Seehrac, M. Deeptyb,d, I.Pradeepe, A.S. Kamzinf, M. V. K. Meharg, N. Krisha Mohanh a
Department of Physics, Dr. N.G. P. Institute of Technology, Coimbatore 643 048, India. Department of Physics, Sasi Institute of Technology & Engineering, Tadepalligudem 534101, India. c Department of Physics & Astronomy,West Virginia University,Morgantown, WV 26506, USA. d Department of Physics, Krishna University, Machilipatnam 521 001, India. e Department of Physics, Sri Krishna College of Engineering and Technology, Coimbatore 641008, India. f Ioffe Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg 194021, Russia. f Department of Physics, Government Degree College, Alamuru 533 233, India. g Department of Physics, Akkineni Nageswara Rao college, Gudiwada, 521 301, India.
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Corresponding author: [email protected] (E. Ranjith Kumar), [email protected] (Ch. Srinivas)
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Highlights
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Defect-free Ni0.8Co0.2Fe2O4 nanoparticles prepared using egg-white as bio-template;
Annealing from 600 C to 900 C controls particle size from 10 to 22 nm;
Increase in particle size increases saturation magnetization but lowers coercivity;
Core-shell model is used to explain size dependence of magnetic parameters;
Smaller particles perform better as gas-sensors for LPG, H2, NH3, and CO.
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Abstract Changes in the magnetic, dielectric and gas sensing properties of Ni0.8Co0.2Fe2O4 nanoparticles are reported. These polycrystalline nanoparticles (NPs) were prepared by evaporation method in the presence of egg-white as a bio-template. X-ray diffraction (XRD)studies of the samples showed single phase ferrite cubic structure without any secondary phases, with the crystallite size D = 10.5 nm, 16.4 nm and 21.9 nm for the
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samples heat-treated at 600°C, 750°C and 900°C respectively. TEM micrograph shows
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nearly spherical shaped particles with particle size consistent with the XRD results. The magnetic hysteresis loops measured at ambient show that with increase in D, saturation
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magnetization MS increases but coercivity HC decreases. This size dependence of MS and
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HC is interpreted in terms of the core-shell model with spins in the shell of thickness d = 0.4 nm not contributing to MS becauseof disorder resulting from the lower symmetry at
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the surface.The frequency dependence of the dielectric properties of the samples although
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typical of that reported in other ferrites show significant particle size dependence. The sample with the smallest D = 10.5 nm shows the best performance as a sensor for detecting different gases (LPG, H2, NH3, and CO) at the operating temperature of 250 oC using
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changes in the electrical resistivity on controlled exposure to gases as the criterion. Keywords: Nanoparticles; Chemical synthesis; Spinels; Magnetic properties; Dielectric
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constant; Gas sensors.
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1. Introduction The research area of magnetic spinels was brought into focus over 60 years ago and it has continued to be of great interest since then because of the many interesting properties and applications of the spinels [1-6]. The spinels have the general formula (A)[B2]X4with 2
the cubic unite cell containing 8 formula unites of AB2X4 with lattice constant of about 0.84 nm. In normal spinels such as ZnFe2O4 = (Zn12+)Fe23+]O4, the 8 tetrahedral A sites are occupied by divalent ions and the 16 octahedral B sites are occupied by trivalent ions. In
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inverse spinels such as Fe3O4 = (Fe3+) [Fe2+ Fe3+] O4, the A sites are occupied by trivalent ions whereas the B sites are occupied by both divalent and trivalent ions. The great diversity of magnetic spinels result from the fact that a variety of ions can be accommodated on the A and B sites sometimes resulting in mixed spinel structure. Spinels with anions other than O such as X= S, Se, Te etc. have also been investigated [4]. The
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inverse spinels like Fe3O4, CoFe2O4 and NiFe2O4 are ferrimagnets since Fe3+ moments on
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the A and B sites are aligned antiferromagnetically with resulting ferrimagnetism due to
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Fe2+, Co2+ and Ni2+ ions respectively with ordering temperature TC = 585°C, 520°C and This room temperature ferromagnetic-type
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585°C well above room temperature [5].
moment combined with their low electrical conductivity make magnetic spinels
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particularly useful in microwave communication industry and wherever low-eddy current
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with robust magnetism are desired [6].
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Bulk CoFe2O4 is a hard ferrimagnet with room temperature saturation magnetization MS ~ 87 emu/g and coercivity HC~ 1 kOe [7, 8]. On the other hand, NiFe2O4
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is a comparatively softer ferrimagnet with MS = 55 emu/g and HC = 400 Oe [8, 9]. Several
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studies have been reported in which divalent Co2+ and Ni2+ ions have been replaced by diamagnetic ions such as in MxCo1-X Fe2O4 and MxNi1-X Fe2O4with M= Zn2+, Mg2+[10-12]. The focus of the research reported here is on the nanoparticles of Ni0.8Co0.2Fe2O4in which the Co2+ ions are partially replaced by magnetic Ni2+ ions with the aim to have a material with controlled magnitudes of MS and HC at room temperature. Since in material like Fe3O4 3
strong dependence of MS and HC on the particle size has been reported and interpreted in terms of the core-shell model [13], it is desirable to investigate similar phenomenon in other system such as Ni0.8Co0.2Fe2O4. Here we report strong particle size dependence of the
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magnetic and dielectric properties of Ni0.8Co0.2Fe2O4system. These nanoparticles were also tested as gas sensors and it was observed that the sample with the smallest D = 10.5 nm shows the best performance for detecting different gases (LPG, H2, NH3, and CO) at the operating temperature of 250 oC using changes in the electrical resistivity on controlled exposure to the gases as the criterion. Details of the experimental procedures, results and
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their discussion are presented below.
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2. Experimental methods
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2.1. Synthesis procedure
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To prepare nanoparticles (NPs) of Ni1-xCoxFe2O4(x = 0.2),analytical grade nickel nitrate [Ni(NO3)2.4H2O], cobalt nitrate [Co(NO3)2.6H2O], ferric nitrate [Fe(NO3)2.9H2O]
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were used in stoichiometric amounts along with egg-white solution. It has been found that
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the use of egg-white solution in the preparation avoids the formation of secondary phases
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such as α-Fe2O3 which have been reported to happen in other methods of synthesis [11,12]. In a typical experiment, 0.2 mol of nickel nitrate, 0.8 mol of cobalt nitrate and 2 mol of
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ferric nitrate solutions were slowly added to the egg white solution with rigorous stirring
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for 1 hour. Then the mixed solution was heat treatedat 100°C to obtain a dry precursor. The obtained precursor was ground into fine powder in an agate mortar and the powder was sintered at different temperatures 600°C, 750°C and 900°C for 2 h in air. As shown below, the heat-treated powder is shown to be single-phase Ni0.8Co0.2Fe2O4 spinel with particle size depending on the heat-treatment temperature. Based on the ratio of precursors used in 4
the synthesis, it is expected that Co substitutes at the A site although some site reversal is possible. These samples were structurally characterized followed by measurements of their magnetic, dielectric and gas-sensing properties.
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2.2. Characterization instruments Rigaku X-ray diffraction unit (Model ULTIMA III) was employed to record X-ray diffractograms of the samples using Cu-Kα radiation with wavelength λ = 0.15406 nm. The external morphology of the samples was examined by scanning electron microscopy (SEM with EDX) using JEOL 5600LV scanning electron microscope A Technai G20-
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stwinHRTEM working at an accelerating voltage of 200 kV was used to get TEM
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micrographs and selected-area electron diffraction (SAED). Hysteresis loops were
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recorded at room temperature employing a Vibrational Sample Magnetometer (Lakeshore
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model 7410).A digital LCR meter (Model TH2816A) was employed to measure the dielectric properties in the frequency range of 103 Hz to 106 Hz. The equipment and
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procedures for measurements of Ni0.8Co0.2Fe2O4 NPs as gas-sensors are described later in a
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separate section along with the results obtained from these measurements.
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3. Results and discussion 3.1 X-Ray Diffraction:
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The chemical phases of the obtained samples of nominal Ni0.8Co0.2Fe2O4were
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determined by X–ray diffraction (XRD) analysis. The X–diffraction spectra of Ni0.8Co0.2Fe2O4 ferrite NPs sintered at different temperatures 600°C, 750°C and 900°C are shown in Fig.2. The XRD patterns confirm the cubic spinel structure of the samples belonging to Fd3m space group. Within the accuracy of the XRD technique, any secondary phases besides the cubic spinel structure were not detected. The indexed diffraction peaks 5
with the Miller indices (220), (311), (222), (400), (422), (511), (440), (620) and (533) noted in Fig. 1 are consistent with the JCPDS card no. 01-077-0426 for the spinel structure with no extra phases. The sharpness of the diffraction peaks increases with the increase in
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heat treatment temperature, indicating the improvement of crystallinity of ferrite nanoparticles. The experimental lattice parameter a was determined using the well-known formula for the cubic structure given by [14] a d
h k 2
2
l
2
(1)
Where h, k, l are Miller indicesof the lines at different Bragg angles θandd is the d-
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spacings determined from the Bragg’s law: 2dsinθ =λ.The crystallite sizes D were
k
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D
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calculated from the most prominent (311) peak using the Debye-Scherrer formula[14]:
cos
(2)
D
where β is full width at half maxima (FWHM) in radians, and k is the shape factor taken as
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0.89.Using Eqs. (1) and (2) and the specifics of the (311) peak, the calculated values of
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lattice parameter a and crystallite size D for the three samples are as follows: a = 0.8401 nm, 0.8399 nm, 0.8398 nm and D = 10.5 nm, 16.4 nm and 21.9 nm for the samples heat-
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treated at 600°C, 750°C and 900°C respectively. These results show that increase in the
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heat-treatment temperature increases the crystallite size as expected from Oswald ripening and this is accompanied by a slight decrease in the lattice constant. The increase of the lattice constant with decrease in the crystallite size has also been reported in parent spinels of CoFe2O4[7] and NiFe2O4[8] as well as other oxides such as CuO [15] and CeO2 [16]. These increases in the lattice constant with decrease in D are interpreted to be due to the 6
increasing effect of the surface atoms which are weakly bound to their neighbors because of the breakdown of crystalline symmetry at the surface. Of course, the fraction of atoms on the surface increases inversely with the crystallite size D, resulting in the increasing
3.2. Results from SEM and TEM Investigations:
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effect of the surface atoms on the measured properties for the smaller particles.
All three samples were investigated using scanning electron microscopy (SEM), transmission electron microscopy(TEM) and selected area electron diffraction(SAED) and typical results are shown in Fig. 2 (a-c) for the sample heat-treated at 900°C. The
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morphology of the sample recorded by SEM and TEM shows that almost spherical
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particles.The size histograms determined by measuring the particle sizes for the three
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samples are shown Fig. 3. The average particle sizes for samples annealed at 600°C, 750°C
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and 900°C are 14.5 nm, 15.8 nm and 21.8 nm respectively with a wide distribution of particle sizes. These average sizes are in good agreement with the crystallite size D
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determined from the analysis of the XRD patterns except for the 600°C annealed sample
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for which the crystallite size D = 10.5 nm is somewhat smaller than the average particle of
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14.5 nm determined from TEM. This observation of the crystallite size (XRD) being less than the particle size (TEM) has been reported in other NP systems also such as Ni-Zn
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ferrite [11] and maghemite [17] and it is usually associated with the presence of grain
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boundaries in the particles.The SAED pattern of the ferrite sample recorded through a transmission electron microscope shows the superposition bright spot indicating the polycrystalline nature of nanoparticleswhich supports the particle distribution observed from XRD and TEM. 3.3 Magnetic Investigations: 7
The magnetic properties for the three Ni0.8Co0.2Fe2O4 samples heat-treated at 600°C, 750°C and 900°C were measured employing a vibrating sample magnetometer (VSM) and their respective hysteresis curves are shown in Fig.4. The magnetization curves
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indicate that all three samples have ferromagnetic-like behavior since coercivity is observed. However, the magnetization is not completely saturated even at 15 kOe, although for applied H > 10 kOe, the M vs. H curves are essentially linear with a comparatively negligible slope. The magnitude of the magnetization (Ms) measured at 15 kOe, remanent magnetization (Mr), coercivity (Hc), and squareness ratio (S=Mr/Ms) for the
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three samples heat-treated at 600°C, 750°C and 900°C are as follows: MS = 41.9, 45.4 and
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48.1 emu/g; Mr= 6.7, 10.5 and 16.9 emu/g; S = 0.16, 0.23 and 0.35; and HC = 1043, 793
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and 716 Oe respectively for the 600°C, 750°C and 900°C samples.
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To determine the saturation magnetization MS in the limit of H = ∞, we have plotted the high-H data of Fig. 4 as a plot of M vs. 1/H in Fig. 5 for the three samples. The
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extrapolated magnitudes of MS in the limit of 1/H =0 yields MS(∞) = 46, 49 and 52 emu/g
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for the samples annealed at 600°C, 750°C and 900°C respectively. These magnitudes of
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MS(∞) are about 8 % larger than those of MS (at 15 kOe) noted above. Since MS is not saturated at 15 kOe, it is important to use the magnitude of MS(∞) rather than MS(at 15
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kOe) in the calculation of magnetic moment µ = n B . This is done below.
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For ferromagnets/ferrimagnets, the measured saturation magnetization at T = 0 K is
given by MS = Nµ where N is the number of uncompensated moments each with magnetic moment of µ. This equation can be rewritten as MS = (NAn B )/MW, where MW is molar mass of ferrite sample, Ms is measured saturation magnetization per gram, NA is
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Avogadro’s number, and µ = n B with
B
as the Bohr magneton. This equation is strictly
valid at T = 0 K as MS usually decreases with increase in temperature due to thermal excitation of magnons. To get a theoretical estimate of n for Ni0.8Co0.2Fe2O4, we first use
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the measured MS for bulk CoFe2O4 and NiFe2O4 which equals 87 emu/g for CoFe2O4 [7] and 55 emu/g for NiFe2O4 [9].Since the observed MS in CoFe2O4 and NiFe2O4 is respectively due to Co2+ and Ni2+ moments only as noted in the introduction, the above magnitudes of MS yield µ = 3.7 B for Co2+ and µ = 2.3
B
for Ni2+. For bulk
Ni0.8Co0.2Fe2O4, using these magnitudes of µ of Co2+ and Ni2+ predicts µ = 2.58 B (n =
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2.58) and the corresponding MS = 61 emu/g for bulk sample of Ni0.8Co0.2Fe2O4 at T = 0 K,
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if the magnetic moments of Co2+ and Ni2+ in this system are parallel. Below, we compare
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MS in NPs of Ni0.8Co0.2Fe2O4.
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this results with the MS determined for bulk Ni0.8Co0.2Fe2O4 using the measured values of
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The plot of magnetization MS measured at 15 kOe and coercivity HC with respect to
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heat treatment temperature in Fig.6 shows that with increase in the heat treatment temperature which also increases the crystallite size, MS increases but HC decreases.
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Similar changes in MS and HCwith change is crystallite size D have been reported in Fe3O4
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nanoparticles [13]. In Fe3O4 NPs, the data of MS vs. D were fitted to the core-shell model in which the spins in shell of thickness d surrounding a core of diameter (D-2d) do not
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contribute to MS due to magnetic disordering of the surface spins. This model leads to the following variation of MS with D [13]: MS(∞) = MS(b)[1-(2d/D)] 3-------------- (3) where MS(b) is the saturation magnetization of the bulk sample of Ni0.8Co0.2Fe2O4. Using 9
the magnitudes of MS(∞) determined in Fig. 5 for the samples with D = 10.5 nm, 16.4 nm and 21.9 nm heat-treated at 600°C, 750°C and 900°C respectively, the fit of the data to Eq. (3) yields MS(b) = 58 emu/g and shell thickness d = 0.40 nm for the Ni0.8Co0.2Fe2O4
earlier assuming µ = 3.7 B for Co2+ and µ = 2.3
B
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system. This magnitude of MS(b)= 58 emu/g agrees well with MS(b) = 61 emu/g estimated for Ni2+ based on the reported MS
values for the bulk samples of the parent spinels of CoFe2O4 and NiFe2O4.
The observed variation of the coercivity HC with change in D shown in Fig. 6 is considered next. The fact that significant HC is observed implies that the particles of
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Ni0.8Co0.2Fe2O4. are not superparamagnetic at ambient. Also, HC decreases with increase in
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D implies that particles have multi-domain structure with domain walls. This issue of
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change in HC with change in crystallite size in Fe3O4 NPs has been discussed in detail
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recently by Lee et al. [18]. Another interpretation of this observation is likely related to
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increasing role of the surface spins whose fractional concentration increases with decrease
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in D. The surface spins are known to acquire additional anisotropy resulting in unusual enhancement of the anisotropy with decreasing particle size [19]. Since HC is directly
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proportional to anisotropy constant, the increase in HC with decrease in D follows. 3.4 Dielectric studies:
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The variation of dielectric constant and dielectric loss in the frequency range of
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103 Hz to 106 Hz for the three Ni0.8Co0.2Fe2O4 samples heat-treated at 600°C, 750°C and 900°C are shown in Fig.7 and Fig.8. As evident in Fig. 7, the dielectric constant for the sample heat-treated at 600oC with the smallest D = 10.5 nm is highest among the three samples at frequencies up to 105 Hz but it becomes the lowest at frequencies above 105 Hz. Similar trend of the frequency dependence of the dielectric constant and dielectric loss has 10
been reported in different spinel ferrites [7, 9, 20-23].Although quantitative interpretation of these observations is not yet available, a brief discussion is presented below on qualitative understanding of the particle size and frequency dependence of the dielectric
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properties based on arguments presented elsewhere [7,9,20-23]. It is noted that in general, contributions to dielectric properties arise from electronic, ionic, orientation and surface charge dielectric polarization.According to Koop’s theory, the ferrites are assumed to consist of conducting grains and insulating grain boundaries. As the ferrites are subjected to electric field of varying frequency, space charge
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polarization accumulates at the grain boundaries. The smaller particles have larger surface
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area per unit volume and so higher density of grain boundaries will tend to accumulate
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higher surface charge polarization. This will lead to higher dielectric constant for the
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smaller particles as observed in Fig. 7. The observed frequency dependence of the dielectric constant likely results from the dominance of surface charge polarization at
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lower frequencies compared to electronic and ionic polarizations. The dielectric constant
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and dielectric loss decrease rapidly with increase in frequency, essentially becoming
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independent at higher frequencies. Another interesting observation is that the size dependence of the dielectric constant at the higher frequencies is reversed in that the larger
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particles have higher dielectric constant. This is likely due to larger contributions from the
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ionic and orientation polarization because of the higher crystallinity of the larger particles.
3.5. Ni0.8Co0.2Fe2O4 Particles as Gas Sensors: In this section, we present experimental procedures and results obtained from using the three samples of Ni0.8Co0.2Fe2O4. as sensors for four different gases viz. liquefied
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petroleum gas (LPG), H2, NH3, and CO. The schematic diagram of the apparatus used for these measurements is shown in Fig. 9. To construct the sensor, the samples of Ni0.8Co0.2Fe2O4. were ground in an agate mortar
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and pestle. These ground powders were mixed with deionized water to obtain a paste, which was then applied to the a 10- mm-long alumina tube (outer diameter = 5mm; inner diameter = 3 mm) followed by drying and calcination at 400°C for 2 h. Two silver (or platinum) electrodes were installed onto the tube at a distance of 6 mm from each other. For varying the temperature of the sensor, a heater made up of a Ni–Cr coil was mounted
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in the tube. The working temperature of the sensor was measured with a chromel-alumel
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thermocouple placed on the heater. Performance and sensibility of the sensors were tested
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by measuring changes in the electrical resistivity on exposure to LPG, the gas for which
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maximum changes in the resistivity were observed among the four gases of LPG, H2, NH3, and CO(Fig.9). To determine the maximum sensor response threshold, both the operating
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temperature (OT) and the LPG concentration were varied. As is seen from the schematics
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shown in Fig.9, the load resistor RL is connected with a sensor element A. The sensor
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response is defined as: S = (Ra – Rg)/Rg , where Ra and Rg are the resistances of the sensor in air and in the studied gas, respectively. The results from these measurements are shown in
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Fig. 10(a, b, c, d). The response time is defined as the time necessary to achieve 90% of the
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conductivity of the equilibrium value after applying the test gas. The recovery time is the time needed for the initial changes in the resistivity in air to be established in the sensor after the gas is turned off. From the results shown in Fig. 10(d) carried out at the optimum OT = 250°C, the response time is about 40s and the recovery time is about 60s for all three sensors. 12
To determine the optimum OT, Fig. 10(a) shows the gas sensor response parameters S = (Ra – Rg)/Rg of the three samples (sample A-600C, Sample B-750C and Sample C-900C) of Ni0.8Co0.2Fe2O4 on exposure to 1000 ppm of LPG plotted as a function
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of the operating temperature. The measurements were done at the temperatures of 50, 100, 150, 200, 250, and 300°C. As seen in Fig. 10(a), the sensor response strongly depends on the operating temperature of the sensor becoming maximum at OT = 250°C and decreasing at the higher temperatures.This shows that the working temperature optimization is the important parameter to control sensor response for detecting LPG using Ni0.8Co0.2Fe2O4
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[24-28].
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The sensor response to different LPG concentrations at OT = 250°C is shown in
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Fig. 10(b) for the three sensors based on the three samples of Ni0.8Co0.2Fe2O4 noted above.
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Similarly, Fig. 10(c) shows the comparative response of the three sensors to four different gases (LPG, NH3, H2, and CO) at concentrations of 1000 ppm, again at OT = 250 oC.
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Finally, Fig.10(d) compares the change in S for the three sensors to LPG under OT = 250 o
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C as the exposure time of the sensors to LPG is varied. After reaching the maximum value
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of S after about 60 s, the gas is turned off and magnitude of S then decreases, eventually reaching almost zero. As noted earlier and evident in Fig. 10 (d), for all three sensors, the
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response time to achieve 90 % of the change in S is about 40 seconds, and the recovery
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time after the gas is turned off is about 60 seconds. However, the changes in S are the largest for the sensor made of sample A of Ni0.8Co0.2Fe2O4 consisting of the smallest crystallites of D = 10.5 nm obtained by annealing at 600oC as compared to the other two sensors consisting of larger crystallites of Ni0.8Co0.2Fe2O4. This higher sensor response of the sensor made of smaller crystallites of sample A is likely due to higher surface area per 13
unit mass of the crystallites in sample A since the total surface area per unit mass of a sample increases inversely with the crystallite size D of the particles present in the sample. Form Fig. 10(c), it is also evident that the response is the highest for LPG as compared to
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the response to H2, NH3, and CO gases. The response time and the recovery time are also essential parameters for gas sensors, From these data it can be concluded that Ni0.8Co0.2Fe2O4 based sensors are promising sensors in gas analysis, the sensor made of the smallest crystallites being the best. 4. Conclusions
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Synthesis of pure phase of Ni0.8Co0.2Fe2O4 nanoparticles of different sizes is
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reported here using the simple evaporation method assisted with egg white as a bio-
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template. The crystallite size D is controlled with the calcination temperature of the
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obtained product, the higher crystallite size yielding larger crystallites. In this method, egg white may control the shape of nanoparticles acting as a binder-cum-gel for its gelling,
D = 10.5 nm, 16.4 nm and 21.9 nm show hysteresis loops with saturation
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samples with
D
foaming and emulsifying characters. Magnetic measurements at ambient on the three
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magnetization MS increasing but coercivity HC decreasing with increase in D. These results are interpreted in terms of the core-shell model with a shell of thickness d =0.4 nm
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containing disordered spins not contributing to MS. Results from the frequency dependence
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of the dielectric constant and dielectric loss on the three samples are presented and discussed in terms of contributions of space charge polarization accumulated at the insulating grain boundaries. Experiments on the use of the three samples of Ni0.8Co0.2Fe2O4 nanoparticles as sensors for detecting different gases (LPG, H2, NH3, and CO) show the best performance for the sample with the smallest crystallite size D = 10.5 nm, likely due 14
to its higher surface area per unit mass of the sample. The synthesis procedure described here with egg white as a bio-template is well suited to prepare ferrite samples without secondary phases.
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14. B. D. Cullity,“Elements of X-Ray Diffraction” (Addison-Wesley Publishing Co, 1956).
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15. A. Punnoose, H. Magnone, M. S. Seehra and J. Bonevich, Phys. Rev. B 64 (2001), 174420 (8 pages). 16. S. Deshpande, S. Patil, S. Kuchibhatla, and S. Seal, Appl. Phys. Letters. 87, 133113(2005). 17. K. Pisane, E. Despeaux, and M. S. Seehra: J. Magn. Magn. Mater. 384, 148-154 (2015). 18. J.S. Lee, J. M. Cha, H. Y. Yoon, J-K Lee, and Y. K. Kim, Sci. Rept. 5, 12135, (2015). 19. K. L. Pisane, Sobhit Singh, and M. S. Seehra, Appl. Phys. Letters. 110, 222409, (2017). 20. M. Penchal Reddy, G. Balakrishnaiah, W. Madhuri, M. Venkata Ramana, N. Ramamanohar Reddy, K. V. Siva Kumar, V. R. K. Murthy, R. Ramakrishna Reddy, J. Phy. Chem. Sol. 71(2010) 1373 – 1380.
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21. N. Thomas, P.V. Jithin, V.D. Sudheesh, V.Sebastian, Ceram. Int.43 (9) (2017)
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7305 – 7310.
22. E. Ranjit Kumar, R. Jayaprakash, J. Magn. Magn. Mater. 366(2014) 33 – 39.
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23. R. K. Panda, R. Muduli, S. K. Kar, D. Behera, J. Alloy. Compd. 615 (2014) 899–
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905.
(2013).
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24. A. Stanoiu, C.E. Simion, S. Somacescu, Sens. Actuators B: Chem. 186, 687–694
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25. Qinqin Zhao, Dianxing Ju , Xiufeng Song, Xiaolong Deng, Meng Ding, XijinXu, Haibo Zeng, Sens. Actuators B: Chem 229 (2016) 627-634. 26. Qinqin Zhao, Dianxing Ju, Xiaolong Deng, Jinzhao Huang, Bingqiang Cao,
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Xijin Xu, Sci. Rep. 5 (2015) 7874.
27. Qinqin Zhao, Xiaolong Deng, Meng Ding, Lin Gan, Tianyou Zhai and Xijin Xu ,
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Cryst EngComm, 17 (2015) 4394-4401.
28. Xiao Wang, Shouwei Zhang, MinghuiShao, Jinzhao Huang, Xiaolong Deng, Peiyu
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Hou, Xijin Xu, Sens. Actuators B: Chem 251 (2017) 27–33.
BiographiesE. 16
Dr. Ranjith Kumar, Assistant Professor of Physics. At present time he is at Department of Physics in Dr. N.G.P. Institute of Technology, Coimbatore. Hisresearch activity concerns with the preparation, characterization and developmentof magnetic and semiconducting nanoparticles for sensor applications.
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Figures with captions:
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Fig. 1 XRD spectra of Ni0.8Co0.2Fe2O4 nanoparticles heat-treated at different temperatures:
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(A) 600°C, (B) 750°C, and (C) 900°C.
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Fig.2.For the Ni0.8Co0.2Fe2O4 nanoparticles heat-treated at 900°C, micrographs of SEM is
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shown in (a), TEM in (b) and SAED pattern in (c).
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Fig. 3. Particle size histograms and TEM micrographs the three samples of Ni0.8Co0.2Fe2O4 heat-treated at 600°C, 750°C, and 900°C.
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Fig. 4 Room temperature hysteresis curves of Ni0.8Co0.2Fe2O4 nanoparticles heat- treated
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at (A) 600°C, (B) 750°C and (C) 900°C.
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Fig. 5. Plot of the magnetization (M) vs inverse magnetic field (1/H) for the three samples annealed at (A) 600°C, (B) 750°C, and (C) 900°C. The extrapolation of the linear fits to
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the data for lower 1/H is used to determine the saturation magnetization in the limit of 1/H
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= 0 (H = ∞).
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Fig.6.Plots of saturation magnetization measured at 15 kOe and coercivity against heat-
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treatment temperatures of the samples. The lines joining the points are visual aids.
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Fig. 7. Frequency dependence of dielectric constant of Ni0.8Co0.2Fe2O4 nanoparticles heattreated at (A) 600°C, (B) 750°C, and (C) 900°C. The lines joining the points are visual
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guides.
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Fig.8. Frequency dependence of dielectric loss of Ni0.8Co0.2Fe2O4 nanoparticles heat-
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treated at (A) 600°C, (B) 750°C, and (C) 900°C. The lines joining the points are visual
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guides.
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Fig. 9. Schematics of the gas sensor response measuring setup. A—gas sensor, B—electron gauge block. 1—gas supply, 2—gas output, 3—sample, 4— electrodes, 5—heating wire, 6— alumina tube, 7—substrate, 8—temperature controller.
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Fig. 10. Response of the three sensors made of Ni0.8Co0.2Fe2O4 nanoparticles annealed at 600 oC, 750 oC and 900 oC. In (a), changes in the response parameter S are plotted as the operating temperature (OT) is varied to determine the optimum OT = 250 oC; In (b), the concentration of LPG is varied at OT = 250 oC; In (c), the sensor response is compared for the four gases at OT = 250 oC; and in (d) changes in S are plotted as a function of the
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exposure time using OT = 250 oC and 1000 ppm of LPG. The line connecting the data
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points are visual guides.
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S0924-4247(18)30411-4 https://doi.org/10.1016/j.sna.2018.05.031 SNA 10792
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
7-3-2018 15-5-2018 17-5-2018
Please cite this article as: Kumar ER, Srinivas C, Seehra MS, Deepty M, Pradeep I, Kamzin AS, Mehar MVK, Mohan NK, Particle size dependence of the magnetic, dielectric and gas sensing properties of Co substituted NiFe2 O4 nanoparticles, Sensors and Actuators: A. Physical (2018), https://doi.org/10.1016/j.sna.2018.05.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Particle size dependence of the magnetic, dielectric and gas sensing properties of Co substituted NiFe2O4 nanoparticles
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E. Ranjith Kumara*, Ch. Srinivasb*, M. S.Seehrac, M. Deeptyb,d, I.Pradeepe, A.S. Kamzinf, M. V. K. Meharg, N. Krisha Mohanh a
Department of Physics, Dr. N.G. P. Institute of Technology, Coimbatore 643 048, India. Department of Physics, Sasi Institute of Technology & Engineering, Tadepalligudem 534101, India. c Department of Physics & Astronomy,West Virginia University,Morgantown, WV 26506, USA. d Department of Physics, Krishna University, Machilipatnam 521 001, India. e Department of Physics, Sri Krishna College of Engineering and Technology, Coimbatore 641008, India. f Ioffe Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg 194021, Russia. f Department of Physics, Government Degree College, Alamuru 533 233, India. g Department of Physics, Akkineni Nageswara Rao college, Gudiwada, 521 301, India.
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Corresponding author: [email protected] (E. Ranjith Kumar), [email protected] (Ch. Srinivas)
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*
Highlights
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Defect-free Ni0.8Co0.2Fe2O4 nanoparticles prepared using egg-white as bio-template;
Annealing from 600 C to 900 C controls particle size from 10 to 22 nm;
Increase in particle size increases saturation magnetization but lowers coercivity;
Core-shell model is used to explain size dependence of magnetic parameters;
Smaller particles perform better as gas-sensors for LPG, H2, NH3, and CO.
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Abstract Changes in the magnetic, dielectric and gas sensing properties of Ni0.8Co0.2Fe2O4 nanoparticles are reported. These polycrystalline nanoparticles (NPs) were prepared by evaporation method in the presence of egg-white as a bio-template. X-ray diffraction (XRD)studies of the samples showed single phase ferrite cubic structure without any secondary phases, with the crystallite size D = 10.5 nm, 16.4 nm and 21.9 nm for the
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samples heat-treated at 600°C, 750°C and 900°C respectively. TEM micrograph shows
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nearly spherical shaped particles with particle size consistent with the XRD results. The magnetic hysteresis loops measured at ambient show that with increase in D, saturation
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magnetization MS increases but coercivity HC decreases. This size dependence of MS and
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HC is interpreted in terms of the core-shell model with spins in the shell of thickness d = 0.4 nm not contributing to MS becauseof disorder resulting from the lower symmetry at
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the surface.The frequency dependence of the dielectric properties of the samples although
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typical of that reported in other ferrites show significant particle size dependence. The sample with the smallest D = 10.5 nm shows the best performance as a sensor for detecting different gases (LPG, H2, NH3, and CO) at the operating temperature of 250 oC using
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changes in the electrical resistivity on controlled exposure to gases as the criterion. Keywords: Nanoparticles; Chemical synthesis; Spinels; Magnetic properties; Dielectric
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constant; Gas sensors.
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1. Introduction The research area of magnetic spinels was brought into focus over 60 years ago and it has continued to be of great interest since then because of the many interesting properties and applications of the spinels [1-6]. The spinels have the general formula (A)[B2]X4with 2
the cubic unite cell containing 8 formula unites of AB2X4 with lattice constant of about 0.84 nm. In normal spinels such as ZnFe2O4 = (Zn12+)Fe23+]O4, the 8 tetrahedral A sites are occupied by divalent ions and the 16 octahedral B sites are occupied by trivalent ions. In
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inverse spinels such as Fe3O4 = (Fe3+) [Fe2+ Fe3+] O4, the A sites are occupied by trivalent ions whereas the B sites are occupied by both divalent and trivalent ions. The great diversity of magnetic spinels result from the fact that a variety of ions can be accommodated on the A and B sites sometimes resulting in mixed spinel structure. Spinels with anions other than O such as X= S, Se, Te etc. have also been investigated [4]. The
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inverse spinels like Fe3O4, CoFe2O4 and NiFe2O4 are ferrimagnets since Fe3+ moments on
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the A and B sites are aligned antiferromagnetically with resulting ferrimagnetism due to
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Fe2+, Co2+ and Ni2+ ions respectively with ordering temperature TC = 585°C, 520°C and This room temperature ferromagnetic-type
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585°C well above room temperature [5].
moment combined with their low electrical conductivity make magnetic spinels
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particularly useful in microwave communication industry and wherever low-eddy current
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with robust magnetism are desired [6].
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Bulk CoFe2O4 is a hard ferrimagnet with room temperature saturation magnetization MS ~ 87 emu/g and coercivity HC~ 1 kOe [7, 8]. On the other hand, NiFe2O4
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is a comparatively softer ferrimagnet with MS = 55 emu/g and HC = 400 Oe [8, 9]. Several
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studies have been reported in which divalent Co2+ and Ni2+ ions have been replaced by diamagnetic ions such as in MxCo1-X Fe2O4 and MxNi1-X Fe2O4with M= Zn2+, Mg2+[10-12]. The focus of the research reported here is on the nanoparticles of Ni0.8Co0.2Fe2O4in which the Co2+ ions are partially replaced by magnetic Ni2+ ions with the aim to have a material with controlled magnitudes of MS and HC at room temperature. Since in material like Fe3O4 3
strong dependence of MS and HC on the particle size has been reported and interpreted in terms of the core-shell model [13], it is desirable to investigate similar phenomenon in other system such as Ni0.8Co0.2Fe2O4. Here we report strong particle size dependence of the
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magnetic and dielectric properties of Ni0.8Co0.2Fe2O4system. These nanoparticles were also tested as gas sensors and it was observed that the sample with the smallest D = 10.5 nm shows the best performance for detecting different gases (LPG, H2, NH3, and CO) at the operating temperature of 250 oC using changes in the electrical resistivity on controlled exposure to the gases as the criterion. Details of the experimental procedures, results and
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their discussion are presented below.
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2. Experimental methods
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2.1. Synthesis procedure
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To prepare nanoparticles (NPs) of Ni1-xCoxFe2O4(x = 0.2),analytical grade nickel nitrate [Ni(NO3)2.4H2O], cobalt nitrate [Co(NO3)2.6H2O], ferric nitrate [Fe(NO3)2.9H2O]
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were used in stoichiometric amounts along with egg-white solution. It has been found that
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the use of egg-white solution in the preparation avoids the formation of secondary phases
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such as α-Fe2O3 which have been reported to happen in other methods of synthesis [11,12]. In a typical experiment, 0.2 mol of nickel nitrate, 0.8 mol of cobalt nitrate and 2 mol of
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ferric nitrate solutions were slowly added to the egg white solution with rigorous stirring
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for 1 hour. Then the mixed solution was heat treatedat 100°C to obtain a dry precursor. The obtained precursor was ground into fine powder in an agate mortar and the powder was sintered at different temperatures 600°C, 750°C and 900°C for 2 h in air. As shown below, the heat-treated powder is shown to be single-phase Ni0.8Co0.2Fe2O4 spinel with particle size depending on the heat-treatment temperature. Based on the ratio of precursors used in 4
the synthesis, it is expected that Co substitutes at the A site although some site reversal is possible. These samples were structurally characterized followed by measurements of their magnetic, dielectric and gas-sensing properties.
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2.2. Characterization instruments Rigaku X-ray diffraction unit (Model ULTIMA III) was employed to record X-ray diffractograms of the samples using Cu-Kα radiation with wavelength λ = 0.15406 nm. The external morphology of the samples was examined by scanning electron microscopy (SEM with EDX) using JEOL 5600LV scanning electron microscope A Technai G20-
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stwinHRTEM working at an accelerating voltage of 200 kV was used to get TEM
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micrographs and selected-area electron diffraction (SAED). Hysteresis loops were
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recorded at room temperature employing a Vibrational Sample Magnetometer (Lakeshore
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model 7410).A digital LCR meter (Model TH2816A) was employed to measure the dielectric properties in the frequency range of 103 Hz to 106 Hz. The equipment and
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procedures for measurements of Ni0.8Co0.2Fe2O4 NPs as gas-sensors are described later in a
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separate section along with the results obtained from these measurements.
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3. Results and discussion 3.1 X-Ray Diffraction:
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The chemical phases of the obtained samples of nominal Ni0.8Co0.2Fe2O4were
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determined by X–ray diffraction (XRD) analysis. The X–diffraction spectra of Ni0.8Co0.2Fe2O4 ferrite NPs sintered at different temperatures 600°C, 750°C and 900°C are shown in Fig.2. The XRD patterns confirm the cubic spinel structure of the samples belonging to Fd3m space group. Within the accuracy of the XRD technique, any secondary phases besides the cubic spinel structure were not detected. The indexed diffraction peaks 5
with the Miller indices (220), (311), (222), (400), (422), (511), (440), (620) and (533) noted in Fig. 1 are consistent with the JCPDS card no. 01-077-0426 for the spinel structure with no extra phases. The sharpness of the diffraction peaks increases with the increase in
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heat treatment temperature, indicating the improvement of crystallinity of ferrite nanoparticles. The experimental lattice parameter a was determined using the well-known formula for the cubic structure given by [14] a d
h k 2
2
l
2
(1)
Where h, k, l are Miller indicesof the lines at different Bragg angles θandd is the d-
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spacings determined from the Bragg’s law: 2dsinθ =λ.The crystallite sizes D were
k
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D
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calculated from the most prominent (311) peak using the Debye-Scherrer formula[14]:
cos
(2)
D
where β is full width at half maxima (FWHM) in radians, and k is the shape factor taken as
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0.89.Using Eqs. (1) and (2) and the specifics of the (311) peak, the calculated values of
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lattice parameter a and crystallite size D for the three samples are as follows: a = 0.8401 nm, 0.8399 nm, 0.8398 nm and D = 10.5 nm, 16.4 nm and 21.9 nm for the samples heat-
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treated at 600°C, 750°C and 900°C respectively. These results show that increase in the
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heat-treatment temperature increases the crystallite size as expected from Oswald ripening and this is accompanied by a slight decrease in the lattice constant. The increase of the lattice constant with decrease in the crystallite size has also been reported in parent spinels of CoFe2O4[7] and NiFe2O4[8] as well as other oxides such as CuO [15] and CeO2 [16]. These increases in the lattice constant with decrease in D are interpreted to be due to the 6
increasing effect of the surface atoms which are weakly bound to their neighbors because of the breakdown of crystalline symmetry at the surface. Of course, the fraction of atoms on the surface increases inversely with the crystallite size D, resulting in the increasing
3.2. Results from SEM and TEM Investigations:
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effect of the surface atoms on the measured properties for the smaller particles.
All three samples were investigated using scanning electron microscopy (SEM), transmission electron microscopy(TEM) and selected area electron diffraction(SAED) and typical results are shown in Fig. 2 (a-c) for the sample heat-treated at 900°C. The
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morphology of the sample recorded by SEM and TEM shows that almost spherical
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particles.The size histograms determined by measuring the particle sizes for the three
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samples are shown Fig. 3. The average particle sizes for samples annealed at 600°C, 750°C
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and 900°C are 14.5 nm, 15.8 nm and 21.8 nm respectively with a wide distribution of particle sizes. These average sizes are in good agreement with the crystallite size D
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determined from the analysis of the XRD patterns except for the 600°C annealed sample
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for which the crystallite size D = 10.5 nm is somewhat smaller than the average particle of
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14.5 nm determined from TEM. This observation of the crystallite size (XRD) being less than the particle size (TEM) has been reported in other NP systems also such as Ni-Zn
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ferrite [11] and maghemite [17] and it is usually associated with the presence of grain
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boundaries in the particles.The SAED pattern of the ferrite sample recorded through a transmission electron microscope shows the superposition bright spot indicating the polycrystalline nature of nanoparticleswhich supports the particle distribution observed from XRD and TEM. 3.3 Magnetic Investigations: 7
The magnetic properties for the three Ni0.8Co0.2Fe2O4 samples heat-treated at 600°C, 750°C and 900°C were measured employing a vibrating sample magnetometer (VSM) and their respective hysteresis curves are shown in Fig.4. The magnetization curves
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indicate that all three samples have ferromagnetic-like behavior since coercivity is observed. However, the magnetization is not completely saturated even at 15 kOe, although for applied H > 10 kOe, the M vs. H curves are essentially linear with a comparatively negligible slope. The magnitude of the magnetization (Ms) measured at 15 kOe, remanent magnetization (Mr), coercivity (Hc), and squareness ratio (S=Mr/Ms) for the
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three samples heat-treated at 600°C, 750°C and 900°C are as follows: MS = 41.9, 45.4 and
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48.1 emu/g; Mr= 6.7, 10.5 and 16.9 emu/g; S = 0.16, 0.23 and 0.35; and HC = 1043, 793
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and 716 Oe respectively for the 600°C, 750°C and 900°C samples.
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To determine the saturation magnetization MS in the limit of H = ∞, we have plotted the high-H data of Fig. 4 as a plot of M vs. 1/H in Fig. 5 for the three samples. The
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extrapolated magnitudes of MS in the limit of 1/H =0 yields MS(∞) = 46, 49 and 52 emu/g
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for the samples annealed at 600°C, 750°C and 900°C respectively. These magnitudes of
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MS(∞) are about 8 % larger than those of MS (at 15 kOe) noted above. Since MS is not saturated at 15 kOe, it is important to use the magnitude of MS(∞) rather than MS(at 15
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kOe) in the calculation of magnetic moment µ = n B . This is done below.
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For ferromagnets/ferrimagnets, the measured saturation magnetization at T = 0 K is
given by MS = Nµ where N is the number of uncompensated moments each with magnetic moment of µ. This equation can be rewritten as MS = (NAn B )/MW, where MW is molar mass of ferrite sample, Ms is measured saturation magnetization per gram, NA is
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Avogadro’s number, and µ = n B with
B
as the Bohr magneton. This equation is strictly
valid at T = 0 K as MS usually decreases with increase in temperature due to thermal excitation of magnons. To get a theoretical estimate of n for Ni0.8Co0.2Fe2O4, we first use
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the measured MS for bulk CoFe2O4 and NiFe2O4 which equals 87 emu/g for CoFe2O4 [7] and 55 emu/g for NiFe2O4 [9].Since the observed MS in CoFe2O4 and NiFe2O4 is respectively due to Co2+ and Ni2+ moments only as noted in the introduction, the above magnitudes of MS yield µ = 3.7 B for Co2+ and µ = 2.3
B
for Ni2+. For bulk
Ni0.8Co0.2Fe2O4, using these magnitudes of µ of Co2+ and Ni2+ predicts µ = 2.58 B (n =
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2.58) and the corresponding MS = 61 emu/g for bulk sample of Ni0.8Co0.2Fe2O4 at T = 0 K,
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if the magnetic moments of Co2+ and Ni2+ in this system are parallel. Below, we compare
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MS in NPs of Ni0.8Co0.2Fe2O4.
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this results with the MS determined for bulk Ni0.8Co0.2Fe2O4 using the measured values of
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The plot of magnetization MS measured at 15 kOe and coercivity HC with respect to
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heat treatment temperature in Fig.6 shows that with increase in the heat treatment temperature which also increases the crystallite size, MS increases but HC decreases.
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Similar changes in MS and HCwith change is crystallite size D have been reported in Fe3O4
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nanoparticles [13]. In Fe3O4 NPs, the data of MS vs. D were fitted to the core-shell model in which the spins in shell of thickness d surrounding a core of diameter (D-2d) do not
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contribute to MS due to magnetic disordering of the surface spins. This model leads to the following variation of MS with D [13]: MS(∞) = MS(b)[1-(2d/D)] 3-------------- (3) where MS(b) is the saturation magnetization of the bulk sample of Ni0.8Co0.2Fe2O4. Using 9
the magnitudes of MS(∞) determined in Fig. 5 for the samples with D = 10.5 nm, 16.4 nm and 21.9 nm heat-treated at 600°C, 750°C and 900°C respectively, the fit of the data to Eq. (3) yields MS(b) = 58 emu/g and shell thickness d = 0.40 nm for the Ni0.8Co0.2Fe2O4
earlier assuming µ = 3.7 B for Co2+ and µ = 2.3
B
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system. This magnitude of MS(b)= 58 emu/g agrees well with MS(b) = 61 emu/g estimated for Ni2+ based on the reported MS
values for the bulk samples of the parent spinels of CoFe2O4 and NiFe2O4.
The observed variation of the coercivity HC with change in D shown in Fig. 6 is considered next. The fact that significant HC is observed implies that the particles of
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Ni0.8Co0.2Fe2O4. are not superparamagnetic at ambient. Also, HC decreases with increase in
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D implies that particles have multi-domain structure with domain walls. This issue of
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change in HC with change in crystallite size in Fe3O4 NPs has been discussed in detail
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recently by Lee et al. [18]. Another interpretation of this observation is likely related to
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increasing role of the surface spins whose fractional concentration increases with decrease
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in D. The surface spins are known to acquire additional anisotropy resulting in unusual enhancement of the anisotropy with decreasing particle size [19]. Since HC is directly
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proportional to anisotropy constant, the increase in HC with decrease in D follows. 3.4 Dielectric studies:
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The variation of dielectric constant and dielectric loss in the frequency range of
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103 Hz to 106 Hz for the three Ni0.8Co0.2Fe2O4 samples heat-treated at 600°C, 750°C and 900°C are shown in Fig.7 and Fig.8. As evident in Fig. 7, the dielectric constant for the sample heat-treated at 600oC with the smallest D = 10.5 nm is highest among the three samples at frequencies up to 105 Hz but it becomes the lowest at frequencies above 105 Hz. Similar trend of the frequency dependence of the dielectric constant and dielectric loss has 10
been reported in different spinel ferrites [7, 9, 20-23].Although quantitative interpretation of these observations is not yet available, a brief discussion is presented below on qualitative understanding of the particle size and frequency dependence of the dielectric
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properties based on arguments presented elsewhere [7,9,20-23]. It is noted that in general, contributions to dielectric properties arise from electronic, ionic, orientation and surface charge dielectric polarization.According to Koop’s theory, the ferrites are assumed to consist of conducting grains and insulating grain boundaries. As the ferrites are subjected to electric field of varying frequency, space charge
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polarization accumulates at the grain boundaries. The smaller particles have larger surface
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area per unit volume and so higher density of grain boundaries will tend to accumulate
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higher surface charge polarization. This will lead to higher dielectric constant for the
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smaller particles as observed in Fig. 7. The observed frequency dependence of the dielectric constant likely results from the dominance of surface charge polarization at
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lower frequencies compared to electronic and ionic polarizations. The dielectric constant
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and dielectric loss decrease rapidly with increase in frequency, essentially becoming
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independent at higher frequencies. Another interesting observation is that the size dependence of the dielectric constant at the higher frequencies is reversed in that the larger
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particles have higher dielectric constant. This is likely due to larger contributions from the
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ionic and orientation polarization because of the higher crystallinity of the larger particles.
3.5. Ni0.8Co0.2Fe2O4 Particles as Gas Sensors: In this section, we present experimental procedures and results obtained from using the three samples of Ni0.8Co0.2Fe2O4. as sensors for four different gases viz. liquefied
11
petroleum gas (LPG), H2, NH3, and CO. The schematic diagram of the apparatus used for these measurements is shown in Fig. 9. To construct the sensor, the samples of Ni0.8Co0.2Fe2O4. were ground in an agate mortar
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and pestle. These ground powders were mixed with deionized water to obtain a paste, which was then applied to the a 10- mm-long alumina tube (outer diameter = 5mm; inner diameter = 3 mm) followed by drying and calcination at 400°C for 2 h. Two silver (or platinum) electrodes were installed onto the tube at a distance of 6 mm from each other. For varying the temperature of the sensor, a heater made up of a Ni–Cr coil was mounted
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in the tube. The working temperature of the sensor was measured with a chromel-alumel
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thermocouple placed on the heater. Performance and sensibility of the sensors were tested
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by measuring changes in the electrical resistivity on exposure to LPG, the gas for which
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maximum changes in the resistivity were observed among the four gases of LPG, H2, NH3, and CO(Fig.9). To determine the maximum sensor response threshold, both the operating
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temperature (OT) and the LPG concentration were varied. As is seen from the schematics
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shown in Fig.9, the load resistor RL is connected with a sensor element A. The sensor
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response is defined as: S = (Ra – Rg)/Rg , where Ra and Rg are the resistances of the sensor in air and in the studied gas, respectively. The results from these measurements are shown in
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Fig. 10(a, b, c, d). The response time is defined as the time necessary to achieve 90% of the
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conductivity of the equilibrium value after applying the test gas. The recovery time is the time needed for the initial changes in the resistivity in air to be established in the sensor after the gas is turned off. From the results shown in Fig. 10(d) carried out at the optimum OT = 250°C, the response time is about 40s and the recovery time is about 60s for all three sensors. 12
To determine the optimum OT, Fig. 10(a) shows the gas sensor response parameters S = (Ra – Rg)/Rg of the three samples (sample A-600C, Sample B-750C and Sample C-900C) of Ni0.8Co0.2Fe2O4 on exposure to 1000 ppm of LPG plotted as a function
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of the operating temperature. The measurements were done at the temperatures of 50, 100, 150, 200, 250, and 300°C. As seen in Fig. 10(a), the sensor response strongly depends on the operating temperature of the sensor becoming maximum at OT = 250°C and decreasing at the higher temperatures.This shows that the working temperature optimization is the important parameter to control sensor response for detecting LPG using Ni0.8Co0.2Fe2O4
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[24-28].
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The sensor response to different LPG concentrations at OT = 250°C is shown in
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Fig. 10(b) for the three sensors based on the three samples of Ni0.8Co0.2Fe2O4 noted above.
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Similarly, Fig. 10(c) shows the comparative response of the three sensors to four different gases (LPG, NH3, H2, and CO) at concentrations of 1000 ppm, again at OT = 250 oC.
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Finally, Fig.10(d) compares the change in S for the three sensors to LPG under OT = 250 o
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C as the exposure time of the sensors to LPG is varied. After reaching the maximum value
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of S after about 60 s, the gas is turned off and magnitude of S then decreases, eventually reaching almost zero. As noted earlier and evident in Fig. 10 (d), for all three sensors, the
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response time to achieve 90 % of the change in S is about 40 seconds, and the recovery
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time after the gas is turned off is about 60 seconds. However, the changes in S are the largest for the sensor made of sample A of Ni0.8Co0.2Fe2O4 consisting of the smallest crystallites of D = 10.5 nm obtained by annealing at 600oC as compared to the other two sensors consisting of larger crystallites of Ni0.8Co0.2Fe2O4. This higher sensor response of the sensor made of smaller crystallites of sample A is likely due to higher surface area per 13
unit mass of the crystallites in sample A since the total surface area per unit mass of a sample increases inversely with the crystallite size D of the particles present in the sample. Form Fig. 10(c), it is also evident that the response is the highest for LPG as compared to
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the response to H2, NH3, and CO gases. The response time and the recovery time are also essential parameters for gas sensors, From these data it can be concluded that Ni0.8Co0.2Fe2O4 based sensors are promising sensors in gas analysis, the sensor made of the smallest crystallites being the best. 4. Conclusions
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Synthesis of pure phase of Ni0.8Co0.2Fe2O4 nanoparticles of different sizes is
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reported here using the simple evaporation method assisted with egg white as a bio-
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template. The crystallite size D is controlled with the calcination temperature of the
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obtained product, the higher crystallite size yielding larger crystallites. In this method, egg white may control the shape of nanoparticles acting as a binder-cum-gel for its gelling,
D = 10.5 nm, 16.4 nm and 21.9 nm show hysteresis loops with saturation
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samples with
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foaming and emulsifying characters. Magnetic measurements at ambient on the three
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magnetization MS increasing but coercivity HC decreasing with increase in D. These results are interpreted in terms of the core-shell model with a shell of thickness d =0.4 nm
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containing disordered spins not contributing to MS. Results from the frequency dependence
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of the dielectric constant and dielectric loss on the three samples are presented and discussed in terms of contributions of space charge polarization accumulated at the insulating grain boundaries. Experiments on the use of the three samples of Ni0.8Co0.2Fe2O4 nanoparticles as sensors for detecting different gases (LPG, H2, NH3, and CO) show the best performance for the sample with the smallest crystallite size D = 10.5 nm, likely due 14
to its higher surface area per unit mass of the sample. The synthesis procedure described here with egg white as a bio-template is well suited to prepare ferrite samples without secondary phases.
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REFERENCES 1. J.L. Snoek, “New developments in Ferromagnetic Materials” (Elsevier, Amsterdam, 1949). 2. E.W. Gorter, Phillips Res.Repts. 9, 295 (1954).
3. J.Smit and H.P.J. Wijn, Ferrites” (John Wiley and Sons, Inc. New York, 1959).
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4. M.S. Seehra “Magnetic Spinels: Synthesis, Properties and Applications”Intech Publishers, Rijeka, Croatia, 2017.
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5. A.H. Morrish “The physical Principle of Magnetism” (IEEE Press, New York, 2001).
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6. V.G. Harris at al. J. Magn. Magn. Mater 321, 2035 (2009).
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7. R. Zhang, L. Sun, Z. Wang, W. Hao, E. Cao and Y. Zhang, Mater. Res. Bull. 98 (2018) 133-138.
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8. A.L. Tiano, G.C. Papaefthymiou, C.S. Lewis, J. Han, C. Zhang, Q. Liet al. Chem. Mater. 27 (2015) 3572–3592.
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9. S. Joshi, M. Kumar, S. Chhoker, G. Srivastava, M. Jewariya and V. N. Singh, J. Molecular Structure, 1076 (2014) 55-62. 10. M. Kaur, P. Jain, M. Singh, Mater. Chem. Phy. 162 (2015) 332 – 339.
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11. Ch. Srinivas, B.V. Tirupanyam, A. Satish, V. Seshubai, D. L. Sastry, J. Magn. Magn. Mater. 382 (2015) 15 – 19. 12. Ch. Srinivas, B.V.Tirupanyam, S. S. Meena, S. M. Yusuf, Ch. Seshu Babu, K.S. Ramakrishna, D. M. Potukuchi, D. L. Sastry, J. Magn. Magn. Mater. 407(2016)135 – 141. 13. P. Dutta, S. Pal, M.S. Seehra, N. Shah and G.P. Huffman, J. Appl. Phys, 105, 07B501 (2009).
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14. B. D. Cullity,“Elements of X-Ray Diffraction” (Addison-Wesley Publishing Co, 1956).
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15. A. Punnoose, H. Magnone, M. S. Seehra and J. Bonevich, Phys. Rev. B 64 (2001), 174420 (8 pages). 16. S. Deshpande, S. Patil, S. Kuchibhatla, and S. Seal, Appl. Phys. Letters. 87, 133113(2005). 17. K. Pisane, E. Despeaux, and M. S. Seehra: J. Magn. Magn. Mater. 384, 148-154 (2015). 18. J.S. Lee, J. M. Cha, H. Y. Yoon, J-K Lee, and Y. K. Kim, Sci. Rept. 5, 12135, (2015). 19. K. L. Pisane, Sobhit Singh, and M. S. Seehra, Appl. Phys. Letters. 110, 222409, (2017). 20. M. Penchal Reddy, G. Balakrishnaiah, W. Madhuri, M. Venkata Ramana, N. Ramamanohar Reddy, K. V. Siva Kumar, V. R. K. Murthy, R. Ramakrishna Reddy, J. Phy. Chem. Sol. 71(2010) 1373 – 1380.
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21. N. Thomas, P.V. Jithin, V.D. Sudheesh, V.Sebastian, Ceram. Int.43 (9) (2017)
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7305 – 7310.
22. E. Ranjit Kumar, R. Jayaprakash, J. Magn. Magn. Mater. 366(2014) 33 – 39.
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23. R. K. Panda, R. Muduli, S. K. Kar, D. Behera, J. Alloy. Compd. 615 (2014) 899–
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905.
(2013).
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24. A. Stanoiu, C.E. Simion, S. Somacescu, Sens. Actuators B: Chem. 186, 687–694
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25. Qinqin Zhao, Dianxing Ju , Xiufeng Song, Xiaolong Deng, Meng Ding, XijinXu, Haibo Zeng, Sens. Actuators B: Chem 229 (2016) 627-634. 26. Qinqin Zhao, Dianxing Ju, Xiaolong Deng, Jinzhao Huang, Bingqiang Cao,
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Xijin Xu, Sci. Rep. 5 (2015) 7874.
27. Qinqin Zhao, Xiaolong Deng, Meng Ding, Lin Gan, Tianyou Zhai and Xijin Xu ,
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Cryst EngComm, 17 (2015) 4394-4401.
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BiographiesE. 16
Dr. Ranjith Kumar, Assistant Professor of Physics. At present time he is at Department of Physics in Dr. N.G.P. Institute of Technology, Coimbatore. Hisresearch activity concerns with the preparation, characterization and developmentof magnetic and semiconducting nanoparticles for sensor applications.
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Figures with captions:
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Fig. 1 XRD spectra of Ni0.8Co0.2Fe2O4 nanoparticles heat-treated at different temperatures:
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(A) 600°C, (B) 750°C, and (C) 900°C.
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Fig.2.For the Ni0.8Co0.2Fe2O4 nanoparticles heat-treated at 900°C, micrographs of SEM is
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shown in (a), TEM in (b) and SAED pattern in (c).
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Fig. 3. Particle size histograms and TEM micrographs the three samples of Ni0.8Co0.2Fe2O4 heat-treated at 600°C, 750°C, and 900°C.
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Fig. 4 Room temperature hysteresis curves of Ni0.8Co0.2Fe2O4 nanoparticles heat- treated
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at (A) 600°C, (B) 750°C and (C) 900°C.
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Fig. 5. Plot of the magnetization (M) vs inverse magnetic field (1/H) for the three samples annealed at (A) 600°C, (B) 750°C, and (C) 900°C. The extrapolation of the linear fits to
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the data for lower 1/H is used to determine the saturation magnetization in the limit of 1/H
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= 0 (H = ∞).
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Fig.6.Plots of saturation magnetization measured at 15 kOe and coercivity against heat-
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treatment temperatures of the samples. The lines joining the points are visual aids.
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Fig. 7. Frequency dependence of dielectric constant of Ni0.8Co0.2Fe2O4 nanoparticles heattreated at (A) 600°C, (B) 750°C, and (C) 900°C. The lines joining the points are visual
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guides.
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Fig.8. Frequency dependence of dielectric loss of Ni0.8Co0.2Fe2O4 nanoparticles heat-
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treated at (A) 600°C, (B) 750°C, and (C) 900°C. The lines joining the points are visual
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guides.
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Fig. 9. Schematics of the gas sensor response measuring setup. A—gas sensor, B—electron gauge block. 1—gas supply, 2—gas output, 3—sample, 4— electrodes, 5—heating wire, 6— alumina tube, 7—substrate, 8—temperature controller.
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Fig. 10. Response of the three sensors made of Ni0.8Co0.2Fe2O4 nanoparticles annealed at 600 oC, 750 oC and 900 oC. In (a), changes in the response parameter S are plotted as the operating temperature (OT) is varied to determine the optimum OT = 250 oC; In (b), the concentration of LPG is varied at OT = 250 oC; In (c), the sensor response is compared for the four gases at OT = 250 oC; and in (d) changes in S are plotted as a function of the
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exposure time using OT = 250 oC and 1000 ppm of LPG. The line connecting the data
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points are visual guides.
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