Enhanced dielectric permittivity on yttrium doped cobalt ferrite nanoparticles

Accepted Manuscript Enhanced dielectric permittivity on yttrium doped cobalt ferrite nanoparticles A. Franco Jr., H.V.S. Pessoni, T.E.P. Alves PII: DOI: Reference:

S0167-577X(17)30646-8 http://dx.doi.org/10.1016/j.matlet.2017.04.101 MLBLUE 22522

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Materials Letters

Please cite this article as: A. Franco Jr., H.V.S. Pessoni, T.E.P. Alves, Enhanced dielectric permittivity on yttrium doped cobalt ferrite nanoparticles, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.04.101

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Enhanced dielectric permittivity on yttrium doped cobalt ferrite nanoparticles 1 2

A. Franco Jr.,1 , ∗ H.V.S. Pessoni,1 and T.E.P. Alves2, 3 Instituto de Física, Universidade Federal de Goiás, Goiânia-GO, Brazil

Instituto de Quimica, Universidade Federal de Goiás, Goiânia-GO, Brazil

3

Instituto Federal de Goias, Anapolis-GO, Brazil

Nanoparticles (NPs) of CoYx Fe2−x O4 (0≤ x ≤ 0.04) were synthesized by combustion reaction method in order to study the eect of Y3+ ions (x ) on the dielectric properties. It was found that the reletive dielectric permittivity (ε0 ) measured in a broad range frequency 100-2 MHz at room temperature. ε0 increases with the Y3+ ions concentration and decreased with frequency, being 200 (83), and 998 (320) at 100 Hz (1KHz) for x = 0 and x = 0.04, respectively. The experimental data could be tted to the Cole-Cole model and the relaxation time (τ ) was found to increase with Y3+ ions concentration. The results were discussed in the terms of space charge polarization and ions distribution over tetrahedral A-site and octahedral B-sites of the spinel structure and cation vacancies due to the presence of Y3+ ions in the spinel structure of cobalt ferrites.

∗

[email protected]

2 I.

INTRODUCTION

Cobalt ferrite (CoFe2 O4 ) has been one of the most studied ferrite due to their remarkable physical properties such as high cubic magnetocrystalline anisotropy, high coercivity, moderate saturation magnetization, high mechanical and chemical stability, wear resistance, and electrically insulating behavior. This uniques properties make cobalt ferrite suitable compound for several technological applications such as magnetic high-density digital recording media, microwave devices, chemical sensors, catalysis, ferrouid technology, target drug delivery systems and etc.[14]. It is well known that changes in cation distribution between tetrahedral A-and octahedral B -sites aect the structure and physical properties of cubic ferrites [57]. Furthermore chemical composition, particle shape and size which are closely related to the method of synthesis may aect the physical properties of cubic ferrites as well. However, doping with small amounts of transition metals [811] or rare earth ions [1214] have been extensively used to obtain cobalt ferrite compounds with enhanced physical properties. For instance, doping with small amount of non-magnetic ions Zn2+ , and Bi3+ ions increases the saturation magnetization [810, 15]. Nevertheless, doping cobalt ferrites with Cr2+ , Mg2+ , and Gd3+ ions decrease the magnetic properties at room temperature[11, 14]. However, signicant increase in dielectric properties of cobalt ferrite nanoparticles were achieved by doping with Gd3+ ions [14, 16] which were attributed mainly to space charge polarization and the presence of conducting phases on the insulating nanoparticles. On contrary, reduction in the dielectric permittivity nickel ferrite ceramic doped with Y3+ ions have been reported in the literature [17]. Therefore, in the present study we investigated the eect of substitution of Y3+ ions on the dielectric permittivity of cobalt cobalt ferrite nanoparticles (NPs) CoYx Fe2−x O4 with nominal composition of x = 0.0, 0.005, 0.01, 0.02, 0.03 and 0.04 synthesized by combustion reaction method. The combustion reaction is an eective and low cost method for production of several industrially useful materials involving capabilities such as material improvement, energy saving and environmental protection[18]. In addition it is extremely exothermic with a high heat release rate, which can lead to an explosive reaction to maintain a self-propagation high temperature usually with the appearance of a ame followed by the elimination of large amounts of gases. The nal product, ne powder, commonly is achieved without a need of any further calcination or heat treatment. [9] II.

EXPERIMENTAL PROCEDURE

Nanoparticles of CoYx Fe2−x O4 (0≤ x ≤ 0.04) with nominal composition x = 0.0, 0.005, 0.01, 0.02, 0.03 and 0.04 were synthesized by the combustion reaction method[18, 19] using analytical grade reagents (manufactured by Aldrich, New Jersey, USA.) of cobalt nitrite, (Co(NO3 )2 · 6H2 O), yttrium nitrate, (Y(NO3 )2 · 5H2 O), iron nitrite, (Fe(NO3 )2 · 9H2 O), and urea, (CO(NH2 )2 ) as fuel without further calcination process. The crystal structure and phase characterization were carried out by X-ray diraction (XRD) using a Shimadzu diractometer (model 6000) with CuKa radiation (λ = 1.5418 Å) for a wide range of Bragg angles (10o ≤ 2θ ≤ 90o ) with a scanning rate of 2º/min at xed time of 3s at room temperature. The dielectric permittivity behavior of all samples was measured using an Agilent 4980A LCR Meter in a wide frequency range of 102 - 2 MHz at room temperature. The value of relative dielectric permittivity ε0 was obtained by applying 1.0 Vac to the opposite silver painted faces of compressed pellet samples forming a parallel plate capacitor type, C = ε At , where C is the capacitance of the pellet, A is the area of the pellet, t is the thickness of the pellet, and ε is the dielectric permittivity of the pellet. In order to improve the electrical contacts silver painted pellets were heated up to ∼ 600C for 30 minutes. III.

RESULTS AND DISCUSSION

Thus, a series of CoYx Fe2−x O4 nanoparticles with nominal composition x = 0.0, 0.005, 0.01, 0.02, 0.03 and 0.04 were successfully synthesized by the combustion reaction method. The XRD patterns of the CoYx Fe2−x O4 nanoparticles with x = 0.0, 0.005, 0.01, 0.02, 0.03 and 0.04 mols obtained at room temperature are displayed in Fig. 1. The diraction peaks correspond to those typically found in cubic inverse spinel structure (JCPDS PDF no. 73-2211 diraction pattern card) of cobalt ferrite compounds. Also it is evident the absence of any shift on the most prominent diraction peaks with the increasing of yttrium content. The lattice parameter a increases with yttrium amount, being 8.373(2)˚ A and 8.383(2)˚ A, for x = 0 and x = 0.04, respectively, roughly following the Vegard's law [20], as shown in the inset of Fig. 1. This is consistent with the dierences in ionic radii of Co2+ ions (r ≈ 0.82 Å), Fe3+ ions (0.67 Å), and Y3+ ions (r ≈ 0.95 Å). The dielectric permittivity behavior of all samples was studied in a broad frequency range of 102 - 2 MHz at room temperature. The powders were compressed into pellet shape and the porosity eect on the dielectric constant was taken in account according to Bruggeman model[21],

8.385

(311) 8.380

8.375

8.370

0.00

0.01

0.02

0.03

0.04

x

(622)

(620)

(533)

(440)

(511)

(422)

(440)

Yttrium ( , mol)

(222)

(220)

0.04

(111)

Intensity (a.u.)

(311)

x

Lattice parameter (Å)

3

0.03

0.02

0.01

0.005

0.00

10

20

30

40

50

2

60

70

80

35.42

(degree)

Figure 1. XRD pattern of CoYx Fe2−x O4 nanoparticles with nominal composition x = 0.0, 0.005, 0.01, 0.02, and 0.03

εds =

ε − εair p , 1−p

(1)

where εds is the dielectric permittivity of dense solid material, p is the porosity of the material, ε is the measured dielectric constant, and εair = 1.0. Thus, the relative dielectric constant (real part), ε0 , was determined and the results are displayed in Fig. 2. It is clear that ε0 is dependent on both frequency and Y doping concentration. For instance, at 100 Hz and 1.0 kHz, ε0 are ∼ 200 and ∼ 83 , ∼ 998 and ∼ 320 for x = 0 and 0.04, respectively, Table 1 . Similar results have been reported for gadolinium doped cobalt ferrites nanoparticles prepared by sol-gel auto combustion method[14, 16]. The observed high values of the dielectric constant at low frequencies and low values at high frequencies indicate large dispersion which is mainly due to the Maxwell-Wagner type of interfacial polarization[22, 23] and is in good agreement with Koop's phenomenological[22]theory; i.e, the dispersion in due to the inhomogeneous double layer dielectric structure. In this model the dielectric medium consists of badly conducting grain boundary and well conducting grains[22, 2426], i.e., the inability of electric dipoles to be tuned with the frequency of the applied electric eld. It is well known that the polarization in cobalt ferrite is due to electron hoping between Fe3+ ↔ Fe2+ ions and

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1200 0.00 0.005

1000

0.01 0.02

800

0.03

'

0.04

600

Cole-Cole fitting

400 200 0

10

2

10

3

10

4

10

5

10

6

f (Hz) Figure 2. The variation of relative dielectric permittivity (ε0 ) of CoYx Fe2−x O4 with with frequency at 300 K. Solid lines are tting of Eq. 15.

x

= 0.0, 0.005, 0.01, 0.02, 0.03 and 0.04

Table I. Dielectric parameters obtained from the Cole-Cole model (Eq. 2) at room temperature Parameters Sample, x (mol) α τ (ms) ε0 (@100 Hz) ε0 (@1 kHz) 0.00 0.50(1) 10.2(1) 200 83 0.005 0.46(1) 9.1(1) 400 224 0.01 0.47(1) 9.8(1) 590 265 0.02 0.57(1) 12.4(1) 720 245 0.03 0.62(2) 11.5(1) 470 154 0.04 0.56(1) 11.7(1) 998 320 hole hoping between Co3+ ↔ Co2+ ions. Due to the high resistance charge carries reaching the grain boundaries store there producing polarization. However, as the frequency of the applied electric eld is increased, the charge carries no longer follow the frequency of the alternating applied electric eld hence decreasing the polarization. [22, 2426]. The observed increase in ε0 of cobalt ferrite with increasing Y3+ substitution could be attributed to the increasing number of Fe3+ ions at the octahedral sites in the spinel structure. Doping cobalt ferrite with Y3+ ions may cause migration of Co2+ ions to the tetrahedral sites and, to relieve the strain, an equal amount of Fe3+ ions migrate from the tetrahedral sites to octahedral sites, thus increasing the hoping of electrons between Fe3+ ↔ Fe2+ ions in the

5 octahedral sites and enhancing the dielectric constant. Similar results are also reported in literature for cobalt ferrites [69]. Furthermore, the ColeCole model is quite suitable to analyze the large dispersion in ε0 since it considers a distribution of the relaxation time[27], according to

 ε0s − ε0∞ ε (ω) = ε∞ + 2   sinh(β ln(ωτ )) × 1− cosh(β ln(ωτ )) + cos(βπ/2) 0



(2)

where, ε0∞ is the permittivity at high frequency, ε0s is the static permittivity, ω is the angular frequency, τ is the mean relaxation time, and β = (1 − α) , where α reects the distribution width of the relaxation time. Therefore, the ε0 -frequency dependence tting (solid lines) are displayed in Fig. 2. It was found that the relaxation time, τ , and parameter, α increased with the amount of Y doping, being 10.2 and 11.7, 0.51 and 0.60 for x = 0 and 0.04, respectively; showing a high dispersive relaxation, Table 1. Finally, the enhanced relative dielectric permittivity ε0 may cause an increase in the exciton binding energy thus in the optical-electronic properties of the nano-structured yttrium doped cobalt ferrite. IV.

CONCLUSIONS

The presence of Y3+ ions in the cobalt ferrite structure causes a substantial change on the dielectric permittivity (ε0 ) at room temperature. For instance, for x = 0.04 ε0 was found to be approximately one order of magnitude higher than x = 0. In addition ε0 decreased with increasing frequency and the data could be tted to the Cole-Cole model. The relaxation time (τ ) was found to increase with Y3+ ions concentration. The results were discussed in the terms of space charge polarization and also the ions distribution over tetrahedral A-site and octahedral B-sites of the spinel structure and cation vacancies due to the presence of Y3+ ions in the spinel structure of cobalt ferrites. The enhanced relative dielectric permittivity of Y-doped cobalt ferrite nanoparticles attracts the attention in fundamental research and technological applications in the vast eld of microelectronic devices. . ACKNOWLEDGMENTS

We are thankful for the nancial support provided by CNPq under Grant number. One of us (A.F.Jr) is a CNPq fellow under Grant No. 308183/2012-6.

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