Variation of dielectric tangent loss with frequency of mullite precursor gels sintered at 1000°C containing increasing concentration iron 

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... density of the lowest 0.002 M (G 1 ) and highest 0.2 M (G 5 ) concentration of doped iron composites were determined. It has been observed that as doping concentration increases density also increases. From X-ray Diffraction and FESEM, it can be seen that mullite content increases with doping concentration. The density of the composite also increases with the increasing concentration, which may be due to the increased mullite content and metal concentration of the samples. From the x-ray diffractograms, it can be seen that the undoped sample shows considerable mullite phase at 1000°C, while for the doped samples, also prominent mullite peaks were obtained & they are changing with the concentrations. Mullite phase increases with increasing concentration of iron ions uptoG 3 (see Fig. 1).Increasing the concentration of iron ion has an important role on mullite growth as can be seen from the relative peak intensities with the increase of iron concentration. The analysis indicates that iron ion positively influenced the mullitization process. However, the efficiency of iron ion is lost as the concentration increases beyond G 3 [13, 14]. Interaction of the iron ions with the alumina and silica component of the gel is responsible for the incorporation of accelerated transforma- tion to mullite phase [13, 14, 19]. A glassy phase has been observed during mullite formation according to the results of x-ray analysis and it increases (glass phase is indicated by ) as the concentration of the doped iron increases. Form G 2 to G 5 samples, apart from mullite other reflections were observed which may be due to the formation of metal aluminates and silicates (Fig. 1). By spectroscopic method, the characteristic stretching and bending modes of vibration of chemical bonds of a sample can be effectively evaluated. 1 % sample was mixed with spectroscopy grade KBr, pelletized to form disc and analyzed by FTIR (FTIR-8400 S, Shimadzu). Mullite gives characteristic bands at wave numbers around 560, 730, 840, 1060, 1130 and 1170 cm -1 [14]. Figure 2 shows the FTIR spectra of the Iron doped sintered gels. All the characteristic bands of mullite- 561 (AlO 6 ), 741(AlO 4 ), 837 (AlO 4 ), 900(AlO 4 stretching mode) and 1141 cm -1 (Si – O stretching mode) appear in all the doped samples. There is no band around 1380 cm -1 or 1630 cm -1 , indicating removal of volatile components. Absence of band around 1170 cm -1 indicates primarily a tetragonal geometry for mullite [23 – 25]. The dielectric constant ( k ) or relative permittivity ( r ) of each sample was calculated from the capacitance using the formula Where, C is the capacitance of the material, d is the thickness of the pellet and A is the area of cross-section. є r and є 0 are the dielectric constant and permittivity of free space respectively [26, 27]. The variation of dielectric constant with frequency of iron doped mullite composites are shown in Fig. 3. From the plot it is clear that in all the cases, dielectric constant decreased with increase in frequency and attained a saturation tendency at 1.5 MHz for a each concentration of doped metal. This behavior of a dielectric may be explained qualitatively by the supposition that the mechanism of the polarization process in mullite-iron nanocomposite is similar to that the conduction process. The electron-hopping model of Heikes and Johnston [28] can explain the electrical conduction mechanism. It is known that the effect of polarization is to reduce the field inside the medium. Therefore, the dielectric constant of a substance may decrease substantially as the frequency is increased [29]. The electronic polarizations can orient them- selves with the electric field at the lower frequency range but at higher frequency, the internal individual dipoles contribut- ing to the dielectric constant can ’ t move instantly. So as frequency of an applied voltage increases, the dipole response is limited and the dielectric constant diminishes [30, 31]. The dielectric loss tan δ of each sample was measured in the frequency range 500 Hz to 1.5 MHz and is graphically shown in Fig. 4. It was found that for all samples, tan δ decreases with increasing frequency and reaches constant value at 1.5 MHz. A.C. conductivity of the samples were then calculated using the formula where f is the frequency in Hz, tan is the dielectric loss factor, є r and є o are the dielectric constant of the material and permittivity of free space respectively [27, 32]. In ln σ a.c. Vs ln f graph a linear increment of a.c. conductivity with frequency for all doping concentrations is observed (Fig. 5).It has been observed that the increment of a.c. conductivity suddenly jumps from G 4 to G 5 .The linearity of the plots follows the frequency dependent part of Jonscher ’ s universal power law which can be represented by the equation Where dc is the dc (or frequency independent) conductivity, σ is a temperature dependent parameter and s lies in the range 0< s <l [32 35]. The linear increase in a.c. is due to the glassy phase in the sintered gels which increases with the increase of concentration of doped metal (Fig. 1). When there is a large amount of glassy phase present in the structure the mobile ions such as Fe 3+ and Al 3+ finds an easy path to move and hence increases the conductivity. Thus, both amount of glassy phase and the concentration of metal ions present in the glassy phase contribute in increasing σ of the sintered gels at 1000°C [36, 37]. The morphology of the mullite particles of lowest (G 1 ) and highest (G 5 ) concentration of the doped metal were investigated by FESEM. The micrograph for G 1 shows almost round shaped particles of mullite of average size 200 nm. Numerous smaller particles can also be seen along with amorphous aggregates [13, 38] (Fig. 6 (a)). G 5 samples shows distinct elongated morphology of mullite particles of size 350 nm embedded in the matrix (Fig. 6 (b)). The mullite content and crystallization in all the G 5 samples were greater than in G 1 composites indicating the catalytic effect of the metal ions at 1000°C [13, 14]. Over the solubility limit, the iron ions are aggregated- forming particles with a magnetic order that are responsible for the hysteresis loops and observed in G 3 , G 4 and G 5 [39 – 41] (Fig. 7 (a, b, c)). The saturation magnetization is 0.225 T and starts around 260 KA/m (Fig. 7 (c)) for 0.20 M. The narrow hysteresis loop (Fig. 7 (a, b, c)) might suggest a nearly superparamagnetic behaviour of the nanocomposite particles [42]. Iron doped mullite composites have been synthesized by sol – gel technique and their phase evolution and dielectric properties have been investigated. The results showed an increase in mullite phase up to G and glassy phase continued to increase with doping concentration. The dielectric constant decreased with frequency for all the samples attaining con- stancy at high frequency, which is normal behaviour for dielectric ceramics. At 1.5 MHz, G 1 mullite composite gave k value of 3.26. A.c. conductivity increased with frequency following Jonscher ’ s power law and was found to depend on the amount of glassy phase and concentration of mobile ions present in the composites. These composites thus having good dielectric properties at high frequency may be suitable for use as electronic ...