Enhanced Dielectric Properties of Sintered MgFe1.98Nd0.02O4 Nanoparticles.
Summary: Spinal MgFe1.98Nd0.02O4 was prepared by simplified sol-gel method. To measure the dielectric properties samples were sintered from 700-800AdegC in the steps of 50AdegC. The sample's phase purity, crystallographic phase and crystallite size was measured by X-ray diffraction method (XRD). The pellets were analyzed in Scanning Electron Microscope for their surface morphology and grain shape. Dielectric properties were measured from 20 Hz to 3 MHz at room temperature. Samples sintered at 750AdegC, showed highest value of AC conductivity which indicated that the material is suitable for use in sensors. However, minimum value of dielectric loss factor was obtained at 800AdegC which makes it more suitable for antenna applications.
Key words: Dielectric conductivity, High-frequency applications, Rare earths, Sintering temperature.
If certain metal oxides and an oxide of iron (which have two unequal sublattices and are ordered anti-parallel to each other) are alloyed than product is termed as ferrites and possesses ferrimagnetic properties. Each sublattice may be magnetized spontaneously at room temperature [1-6]. They have been widely investigated to extend their domain of applications in electronics e.g. memory cores, magnetic media, gas sensors, high frequency devices, catalysis, etc.  Among various ferrites, the magnesium ferrite (MgFe2O4) is unique due to its low eddy currents and high resistivity values and as a result being used in hyperthermia, sensors, catalysis, adsorption, recording media, and magnetic technologies. Nano particles of MgFe2O4 have also better photo electrical properties [1, 7-11].
Various chemical and physical methods are reported in the literature for the synthesis of Mg ferrites such as citrate method [12, 13], chemical method [14-16], co-precipitation method, [17-19], combustion method [15, 20, 21], High temperature thermal decomposition , Electro-spinning , solid-state reaction method [22, 24], hydrothermal method , sol-gel method [15, 22, 26-28] etc. Among above mentioned routes, Sol-gel method is known to produce most outstanding results for the synthesis of phase pure particles of nanometer size at relatively low temperature.
Effect of small percentages of metal ion substitution such as Al [29, 30], Ca , Co , Cr , Cu , Ge , Ni , Mo and Sn , Si  and Zn  on structure, dielectric and magnetic behavior of MgFe2O4 have been reported. However, not much published work is available on doping of rare earth elements in the MgFe2O4 and their electrical or magnetic properties. Bamzai et al  prepared Dysprosium Dy3+ doped Magnesium ferrite, MgDyxFe2-xO4 (x=0.00 to 0.07). They used solid state method to prepare the material and measured its magnetic properties. Kumar et al  prepared MgGd0.15Fe1.85O4 ferrite with improved magnetic and electrical properties by solid state reaction technique and reported their microstructures, magnetic and electrical properties. It is reported that substitution of rare earth ions produce strains into the spinel structure.
Similarly, Manikandan et. al. [42-45] reported rare earth element doped cobalt ferrite and its opto-magnetic properties. Anu Rana et. al.  reported that dielectric parameters of Gd doped cobalt ferrite are strongly dependent upon Gd3+ ion concentrations and their frequency dependence. Recently, the dielectric properties such as AC conductivity, dielectric loss factor, dielectric constant and tangent loss as a function of Nd concentration on the phase pure Mg ferrite has been reported . Some of these results not reported (Fig. 1) earlier are presented here. The Fig. shows that the values of dielectric constant (Fig 1a), dielectric loss factor (Fig 1b) and AC conductivity (Fig 1c) as a function of Nd concentration follow similar trend. However, Tangent loss shows sharp decrease as a function of Nd concentration up to 0.02% Nd concentration. Further increase in the concentration of Nd doping showed increase in the tangent loss values.
Thus minimum value of tangent loss is seen at 0.02% of Nd . This indicates that this material is suitable for the sensing devices of perfect vacuum, dry air and most pure gases such as helium, nitrogen and energy efficient devices. It is generally known that factors such as sintering temperature, rate of heating, sintering time, rate of cooling, etc.  strongly influence the physical and chemical properties of ferrites. Therefore it was considered worthwhile to investigate and report the dielectric properties dependence of MgFe1.98Nd0.02O4 on sintering temperature in the range of 700 to 800AdegC.
Simplified sol-gel method was used to prepare the Neodymium based Magnesium ferrite, MgFe1.98Nd0.02O4 . The analytical grade magnesium nitrate, iron nitrate and neodymium nitrate were weighed in the stoichiometric ratio and dissolved in the ethylene glycol separately. The salts and ethylene glycol were taken in the molar ratio of 1:14. The homogeneous solution was obtained after continuous stirring of 30 minutes at room temperature. Than the solutions were mixed and temperature of solution was gradually raised to 80AdegC. The continuous stirring resulted thick gel formation. The temperature was further increased to 320AdegC; mixture caught fire and started burning. When flames were extinguished, the product appeared as brown powder. To perform characterizations, the powder was pressed in to pallets. All the pallets (samples) were sintered in a Protherm furnace (model PLF 110/6) for two hours at selected temperatures.
The samples were studied for the structural analysis and ph ase purity by X-ray diffraction (XRD) technique. XRD spectrums were recorded using Cu-K[alpha] radiations (I>> = 1.5406 AAdeg) from 20Adeg to 80Adeg. XRD patterns were recorded from PANalytical X'Pert Pro (MPD) diffractometer. The operating voltage was 40 keV and 30 mA. Ni filter was used to get Cu-K[alpha] radiation. Lattice parameters were calculated using software called "CheckCell". A FESEM of HITACHI SU-1500 was used to study the surface morphology and grain size of the samples. A LCR Analyzer made by Wayne Kerr (Model 6440B) was used to measure frequency dependence of the dielectric properties in the range of 20 Hz to 3 MHz at room temperature for all sintered pallets by two probe methods.
Pressure contacts were of the size of the pallet. To achieve better contacts polished surface of the pallet was used. The dissipation factor (tanI') is found by dividing the energy dissipated by the energy stored in the component. Values of the dielectric constant were calculated us ing the equation :
Iu = Cd/Iu0A
where pallet's cross-sectional area is A, thickness is d, capacitance is C and permittivity of free space is Iu0. Dielectric loss factor (Iu') related to the complex relative permittivity, Iu' is a measure of the loss of energy in the dielectric material through conduction, slow polarization currents and other dissipative phenomena.
The values of dielectric loss tangent, (tan I') and dielectric constant (Iu) at the same frequency range can be used to calculate the AC conductivity by using the following relation :
IAC = I Iu0Iu tan(I').
where IAC is the AC conductivity and I is the angular frequency.
Results and Discussion
XRD patterns for the samples are shown in Fig. 2 at different sintering temperatures. Well defined diffraction peaks and respective peak intensities are increasing with the increase of sintering temperature. All the peaks match with the JCPD card no 01-088-1937 and indexed accordingly for the cubic lattice. Maximum intensity peak in this pattern is (311). Table-1 shows the increase in the crystallite size as a function of sintering temperature. The values of lattice constant are not dependent upon sintering temperature and are not statistically significant. This is in confirmation with the work of Ilhan et. al. , Omer et. al.  and Gabal et. al .
The grain structure and surface morphology of Nd doped magnesium ferrite as a function of sintering temperature was studied in a FE-SEM and is shown in Fig.3. The grain size in all micrograpghs ranges from 100-500 nm. The average value of the grain size is 250 +- 100 nm. It was concluded that substitution of neodymium restricts the grain growth.
Table-1: The changes in the values of average crystallite size and lattice parameter as function of sintering temperature.
700###23 +- 3###8.38+-0.07
750AdegC###27 +- 3###8.39+-0.08
800AdegC###37 +- 4###8.36+-0.08
Fig.4a presents the changes in the values of dielectric constant with the sintering temperature. Two distinct regions may be seen in this Fig. First region lies in between 102-103 Hz and second region is located from 103-106 Hz. The first region marks decrease in the values of dielectric constant with the increase in frequency because charge carriers are aligned along the direction of the applied field to produce a high dielectric constant value. This leads to decrease in the values of dielectric constant at higher frequencies. But in second region the dielectric constant remains almost constant and is independent of the frequency due to the reason that charge carriers cannot orient themselves along the applied field direction. This is the reason that the graph becomes constant at higher frequencies .
It is remarkable to mention that at low frequency (60 Hz), the value of dielectric constant has increased with the increase in the sintering temperature. This shows a direct relationship between dielectric constant and the sintering temperature due to the space charge polarization, electrons displacement and the charge produced at the insulating grain boundaries. The partial reduction of Fe3+ to the Fe2+ions is the cause of the increase of dielectric constant values due to increase in sintering temperature. The minimum dielectric constant is observed at 700AdegC because of less contribution of Fe2+ ions and decrease in the space charge polarization expected at this temperature in the ferrites [41, 50].
Fig.4b shows the dielectric loss factor as a result of sintering and consists of two regions. The frequency range 10 to 102 Hz defines first region whereas second region lies between 102.5 to106.5 Hz. The dielectric loss factor has sharply decreased in the first region because the required energy is available to align the localized dipoles in the field direction. But in the second region, energy is not sufficient to orientate dipoles in the applied field direction. That's why in high frequency region, the values of dielectric loss factor does not change and remains almost constant . The values of dielectric loss factor have increased with the increase in the values of sintering temperature. At sintering temperature of 800AdegC the loss factor of the sample is smaller than the sample sintered at 700 and 750AdegC due to the increase in grains size and reduction of number of Fe3+ ions which transforms into to Fe2+ions .
The AC conductivity of sintered MgFe1.98Nd0.2O4 is shown in Fig.4c. This plot may also be divided in two regions. The first region is located between 10 to104 Hz. The second region starts from 105 to 106.5Hz. In the first region the AC conductivity of ferrite remains constant up to 104 Hz because at the lower frequencies grain boundary contributes in AC conductivity and low electrons jumping frequency among the metal ions of adjustable valence states. Thus AC conductivity is low at lower frequencies. However, the AC conductivity has sharply increased in the second region above 105 Hz. This is due to the reason that at higher frequencies grain boundaries have contributed in the AC conductivity. When the frequency is increased, the active grain boundaries restrict the electrons to hope among the matching metal ions of adjustable valence states. So AC conductivity increased with increasing in frequency values .
The AC conductivity changed with the sintering temperature. Increase in A C conductivity is dependent upon the microstructural features and also conversion of trivalent Fe3+ ions to Fe2+state. The crystallite size is increasing with the increase in the sintering temperature which resulted diffusion of grain boundaries in turn and leads to increase the AC conductivity.
Dependence of Tangent loss as a function of frequency shows fascinating results. Four regions may be seen in Fig.4d. The first region occurs between 60 to 101.5 Hz. The range of second region is 102 to 102.8 Hz. The third region lies between 103 to 105 Hz and fourth region is above 105 Hz. The Tangent loss decreased in the first and third region, because the energy is sufficient to restrict the dipoles to orientate them in the direction of the applied field. In second region Tangent loss has increased due to the insufficient energy to restrict dipoles. In the fourth region values of Tangent loss has remained constant because polarization has reduced with aggregation of frequency and became constant because external field frequency cannot follow the alternating field. Loss tangent is changing with the sintering temperature.
Thus it may be concluded that the value of dielectric loss is highest at 750AdegC as compared to the other samples sintered at 700 and 800AdegC, because period of relaxation process is similar as that of applied field at this temperature. When the relaxation time is more than the duration of the applied field, losses are small. This is the reason that dielectric losses at 800AdegC are minimal .
The measurement of tangent loss (I') of the magnesium ferrite as a function of Nd doping (Fig 1d) shows that the values tangent loss of MgFe1.98Nd0.02O4 are about 50% less as compared to the other Nd doped (MgFe2-xNdxO4, x=0.00, 0.01, 0.04) samples. It may be due to the improvement in the microstructure of the material. This makes the material suitable for use at high frequency applications. To make more reliable the dielectric properties of the sample MgFe1.98Nd0.02O4 is studied as a function of sintering temperatures between 700 to 800AdegC. The samples showed very motivating results. One of them is enormous reduction in the values of the tangent loss observed at 800AdegC. Maximum AC conductivity is achieved at 750AdegC but above 750AdegC the AC conductivity decreased rapidly so we assume that 750AdegC is the optimum sintering temperature to achieve minimum tangent loss values.
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|Author:||U. Rafiq, M. Hanif, M. Anis-ur-Rehman and A. Ul Haq|
|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Feb 14, 2020|
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