Pr-Co doped Barium hexaferrites, having the general formula of Ba1-xPrxFe12-yCoyO19 (where x,y = 0, 0.1, 0.3) have been synthesized by citrate precursor technique. The prepared samples were characterised by using power X-ray diffractometer , dielectric measuring instrument and scanning electron microscope. All the samples under X-ray diffraction study exhibited single hexagonal plane alongwith space group P63/mmc. It was found that substitution led to decrease in lattice parameter (a) and (c). Similar pattern was observed in unit cell volume. The average size of the grains changed with Pr-Co substitution. Dielectric loss and dielectric constant were studied at different frequencies. At lower frequencies, a decrease in dielectric constant was observed whereas at higher frequencies a constant behaviour was recorded which is general trend in ferrites.
Keywords: Hexaferrite, SEM, Dielectric constant.
The domain of ferrite materials and their substituted variants with different cations, prepared by different techniques, because of their compelling physics and potential applications, have recently attracted the attention of condense matter community. As compared to micron sized bulk particles, the nanocrystalline form of hexaferrites shows particularly interesting dielectric and magnetic properties. Among all the hexaferrites, M-types (AFe12O19 , A = Ba, Sr, Pb) have been studied for several decades now. Hexaferrites, due to their low cost, easy manufacturing and importance in the field of electronics and telecommunications are utilized in manufacturing of high quality filters, radiofrequency circuits, transformers, reflection coils, antennas etc [2-3]. In this backdrop the study the dielectric behavior of hexaferrites at different frequencies has assumed importance as it will allow us to generate valuable information especially pertaining to type and proportion of different additives necessary for obtaining high quality materials with significant practical applications . Doping of hexaferrites with cations causes wide variation in their dielectric properties, which will eventually define their practical use in different instruments. Dielectric characteristics can be improved by elemental substitutions to Ba+2 or Fe+3 sites or both. . The two parts of the dielectric constant viz the real and the imaginary were found to be varying by Brahma et al.  while working on the Sb2O3 doped BAFe12O19 .
With the previous studies in background, this investigation, attempted to study the impact of Pr-Co substitution on the structural and dielectric characteristics of barium hexaferrite. The evident relevance of the present study lies in its endeavour to analyse whether or not the dielectric constant of these hexaferrites be increased in comparison with the pristine materials. Additionally, we found no systematic information in available literature on the dielectric characteristic of Pr-Co substituted barium hexaferrite despite our best efforts. As such this investigation is understandably envisaged to significantly better our current understanding of the Pr-Co substitution in barium hexaferrite. This paper hereby presents the observed results and effects of Pr-Co substitution on dielectric properties of barium hexaferrite.
Citrate-precursor method was followed to prepare Pr-Co substituted barium hexaferrite materials of M-type. This method uses analytical grade chemicals like prasodimium nitrate hexa-hydrate, ferric nitrate nona-hydrate, Cobalt nitrate hexa-hydrate, citric acid anhydrous and barium nitrate. Deionised water was used to separately dissolve Metal salts and citric acid and which were then mixed together at 250C with constant stirring. Here anhydrous citric acid works as a fuel. Using ammonia (25%) the pH of the solution was adjusted at 6.5. The resultant solution was kept at 900C till a gel solution was formed. The gel thus formed was heated till combustion occurred, leaving ultimately only loose ashes. Motor and pestle was used to grind these ashes for about 30 min. In order to remove organic moiety the powder was heated at 5000C. Lastly the powder was subjected to calcination for 3 h at 950 0C
D8 Advance Bruker X-ray diffractometer with CuKα (λ=1.5406 Å) radiation was employed to decipher the crystalline phase of the synthesized materials. JOEL Scanning Electron Microscope (Model JSM-6490LV), operating at voltage of 25 kV was used to carry out morphological studies of the synthesized samples. The measurements of dielectric constant as a function of frequency of the applied ac field (range of 20 Hz to 1 MHz) were made by using Agilent 4285A precision LCR meter.
Figure 1 shows the X-ray powder diffraction patterns for the Ba1-xPrxFe12-yCoyO19 (x=y=0, 0.1, 0.3). The diffraction peaks corresponding to the different planes, that is (107), (110), (114), (203), (205), (2011), (217), (220) resemble very well with the general standard pattern for M-type barium hexaferrite. This signifies that the synthesized material has hexagonal structure with a space group P63 /mmc. Table 1 details the lattice parameters calculated for all the prepared samples. It evidently shows that substitution decreases lattice parameter (a) and (c). Similar pattern is illustrated by unit cell volume.
Table 1: Lattice parameters, unit cell volume and grain size of Ba1-xPrxFe12-yCoyO19 (x,y = 0, 0.1,0.3 ) samples
|Composition||a (Å)||c (Å)||Volume (Å3 )||Grain size (nm)|
SEM images of Ba1-xPrxFe12-yCoyO19 (x,y = 0, 0.1,0.3 ) compounds are showed in Figure 2 and the morphology shows that the grains for Ba1-xPrxFe12-yCoyO19 (x,y = 0, 0.1,0.3 ) compounds are well-defined as against moderately agglomerated particles present in the Pr-Co doped BAFe12O19 samples. The average size of the grains observed to vary with Pr-Co substitution. The average size of the grains measured for Ba1-xPrxFe12-yCoyO19 (x,y = 0, 0.1,0.3 ) Compounds are shown in Table 1.
Following is the equation giving the dielectric constant in the complex form in an ac field: ε = ε' - ϳε" Where ε' and ε" are the real and imaginary parts designating the stored and dissipated energy respectively. The dependence of real part of dielectric constant of Ba1-xPrxFe12-yCoyO19 (x,y = 0, 0.1,0.3 ) on frequency within 20 Hz to 1MHz range in an ac field is shown in Figure 3. Same figure clearly illustrates that at lower frequencies dielectric constant decreases whereas a constant behavior is observed at higher frequencies. This state may be because of changes in valence states of ions and space charge polarization resulting from creation of dipoles and higher dielectric constant at lower frequencies is related to heterogeneous conduction in composites . Constant behavior in dielectric constant at higher frequencies can be explained by the inadequacy of electric dipoles to follow the changes in the frequencies brought about by alternating applied electric field .
Figure 3 shows that at any particular frequency dielectric constant decreases with the substitution of Pr-Co. An increase in the resistance of grain leading to decrease in the probability of electrons reaching the grain boundary seems to be the possible reason for such behavior.
The loss in the dielectric constant is actually a measure of lag in the polarization with the applied alternating field. Figure 4 shows the variability of dielectric loss of Ba1-xPrxFe12-yCoyO19 (x,y = 0, 0.1,0.3 ) as a function of frequency. At a particular concentration of dopant, it was observed that the dielectric loss decreased with the increasing frequency and is explained by using Koop’s model . The low frequency domain consequences the higher resistivity (because of grain boundaries), therefore gaining of higher energy forwards the movement of electrons between ions causing higher loss of energy. The higher frequency region in the same way corresponds to low resistivity (because of grain) and smaller loss of energy takes place.
A loss peak is observed at composition x = 0. When the frequency of the applied ac field equals jumping frequency of electrons between Fe2+ and Fe3+ then only peaking nature occurs.
Citrate precursor method was employed to synthesize Pr-Co doped barium hexaferrite. The impact of Pr-Co doping on the structural and dielectric characteristics of the prepared material was examined. X-ray diffraction evidently confirmed phase formation. Substitution caused decrease in lattice parameter (a) and (c). The unit cell volume also showed the similar pattern. The average size of the grains varied with Pr-Co substitution. Doping decreased dielectric constant as well as dielectric loss Dielectric constant and dielectric loss measurements with respect to frequency imply that the conduction in these materials resembles the conduction in ferrites and is similarly a result of polaron hopping.