3.3.1 Effect of milling time on the phase formation of MTO ceramics
To see the effect of ball milling on the initial powders, XRD patterns were recorded for different milling time. Figure 3.1(a) shows the XRD patterns of the MgO and TiO2 oxides milled for 20, 25 and 30 h.
Figure 3.1: (a) XRD patterns of the MgO and TiO2 oxides milled for 20, 25 and 30 h.
The patterns indicate that the intensities of the MgO and TiO2 weaken gradually with increase in ball milling time upto 20 h. As the milling time increases further upto 25 h along with MgO and TiO2 phases MgTi2O5 and Mg2TiO4 phases were also appeared. After 30 h milling, all the peaks of the parent oxides disappear; strong diffraction peaks of MgTiO3
were observed clearly. From the XRD patterns, it is expected that after 30 h of ball milling, surface crystal structures of the oxide particles contain lattice distortion, dislocations and defects. In the mechanochemical synthesis process, the crystal structures of MgO and TiO2
are heavily deformed and even become amorphous which results in a dramatic increase of the internal energy of the solid particles. These highly reactive MgO and TiO2 particles can easily shed ions with diffusion ability at high temperature which promotes the formation of
MgTiO3 [11]. The formation of the heterogeneous phases comprising MgTi2O5 and Mg2TiO4 in the MgO - TiO2 system, mainly due to the differences in the degree of the incipient mechanochemical reaction. It is observed that with increase in milling time the crystallite sizes of the MTO ceramics decreases and are in the range of 30 - 50 nm.
3.3.2 Effect of sintering temperature on crystal structure
Figure 3.1(b) shows the XRD patterns of the MTO ceramics sintered at different sintering temperatures. All the samples exhibited single - phase crystalline MTO peaks which are in agreement with PDF 79-0831. It is interesting to note that all the heterogeneous phases were completely suppressed during the sintering process. Figure 3.1(c) shows the x-ray diffraction pattern along with the Rietveld refinement of MgTiO3 ceramics sintered at 1350oC for 3 h. The Rietveld refinement was performed through Fullprof program [12]. The refinement was carried out by considering R3space group and by refining the lattice parameters, position of the Mg, Ti and O atoms, occupancy, and thermal parameters. The obtained lattice parameters are found to be a = b = 5.0567 (12) Å, c = 13.9056 (9) Å. From the Rietveld refinement, it is confirmed that there is no secondary phase. The values of χ2, RBrag factor and Rf factor were found to be 1.09, 14.26 and 12.53, respectively. The above XRD analysis results are in agreement with those reported by Ferri et al. [2].
Figure 3.1(b): XRD patterns of the MgTiO3 ceramics milled for 30 h and sintered at different sintering temperatures
.
Figure 3.1(c): XRD pattern along with Rietveld refinement of MgTiO3 ceramics sintered at 1350oC. The circles and solid line represent the experimental and the Rietveld refined data, respectively. The blue line at the bottom shows the difference between the experimental and the refined data. The vertical bars correspond to the allowed Bragg’s peaks.
3.3.3 Microstructure
The scanning electron microscopic (SEM) image of the MTO powder milled for 30 h is shown in Figure 3.2(a). It is observed that all the particles exhibited nearly spherical shape and the mean particle size is around 100 nm. The SEM image of the MTO ceramics sintered at different sintering temperatures for 3 h are shown in Figure 3.2(b - d). As the milling time increases, the particles would be finer and highly reactive. When an aggregate of fine grained crystals is heated at high temperatures, recrystallisation takes place and the average grain size increases. As the initial particle size decreases, the driving force for the process increases, because the increased surface area enhances the relative density and the grain size. The bigger grain size reduces the grain boundary area. The average grain size of the MTO ceramics ranged between 1.5 to 8 µm. Further, MTO ceramics sintered at 1350oC showed larger grain sizes. From the SEM pictures, it can be surmised that initial particle size plays an important role in the densification and grain size, both of which lead to the differences in the dielectric properties of MTO ceramics.
Figure 3.2: SEM images of (a) MTO powder milled for 30 h. The MTO ceramics sintered at (b) 1300oC (c) 1350oC (d) 1400oC.
3.3.4 Relative density
The variation in relative density as a function of milling time, sintered at 1350oC is shown in Figure 3.3(a). It is found that the density of the MTO ceramics increases linearly with increasing milling time. The SEM image of the MTO ceramics milled for 30 h is shown in Figure 3.2(a) and it is observed that the particle size of the powders is 100 nm. The initial particle size of the powders is > 2 µm. From the SEM image and Figure 3.2(b), it is clear that mechanochemical synthesis reduces the initial particle size of the MTO ceramics along with the phase formation. The initial particle size plays an important role on the densification of MTO ceramics and it is well known that the microwave dielectric properties are heavily influenced by the density of the ceramics (Figure 3.7). To optimize the sintering temperature of MTO ceramics, the sintering temperature is varied and the variation in densities as a function of sintering temperature is shown in Figure 3.3(b). It is again confirmed that the MTO ceramics exhibited highest density at 1350oC.
Figure 3.3: (a) The variation in relative density as a function of milling time, sintered at 1350oC. (b) Variation in relative density of MTO ceramics as a function of sintering temperature.
It was observed that as the temperature increases, the densities of the MTO ceramics increase up to 1350oC and above that started decreasing. The maximum relative density of 97.1% was obtained for the sample sintered at 1350oC for 3 h. The decrease in relative density at higher sintering temperatures may be due to non - uniform grain growth which is observed from SEM images (Figure 3.2(d)). The decrease in sintering temperature in this study can be attributed to the smaller initial particle sizes. It is well known that fine particles have tremendous surface areas and surface energies. The finer the particles are, the higher is the surface energy which leads to a faster densification rate. The larger grains and higher densities obtained in the present study can be due to uniform mixing of the powders and smaller particle sizes.
3.3.5 Raman spectra
Raman spectroscopy is a standard technique which is used to characterize the lattice - vibrational properties of crystals. The Raman spectra of MTO ceramics, sintered at different temperatures for 3 h are shown in Figure 3.4. It is observed that all ten Raman peaks theoretically expected appear distinctly. The results obtained in the present study agree very well with the previously reported data [13, 14]. From the Raman spectrum, it is clear that all atoms are involved in each vibrational mode. The Ag (220 cm−1) and Ag (302 cm−1) modes mainly originated from the vibrations of Mg2+ and Ti4+ ions along the z axis, while the Ag
(394 cm−1), Ag (498 cm−1), and Ag (712 cm−1) modes are mainly contributed by the vibrations of O2- ions. For the Ag (498 cm−1) and Ag (712 cm−1) modes, the six O2- ions show breathing like vibrations, but each of them corresponds to different liberation directions of oxygen octahedra. The Ag (394 cm−1) mode can be primarily represented as the combination of ag(1) and ag(3) that make the mode a nearly isotropic stretching vibration. In the case of Eg modes, the Eg (278 cm-1) mode can be essentially considered as the anti - symmetric breathing vibration of the oxygen octahedron. The Eg (323 cm-1) and Eg (349 cm-1) modes can be approximately described as the twisting of the oxygen octahedron with the vibrations of the Mg2+ and Ti4+ ions parallel to the xy plane. The Eg (481 cm-1) and Eg (636 cm-1) modes are predominantly due to the anti - symmetric breathing and twisting vibrations of the oxygen octahedra with the cationic vibrations parallel to the xy plane. For the Eg (481 cm-1) mode, both Mg2+ and Ti4+ ions are involved in the vibration, while for the Eg (636 cm-1) mode, the cation vibrations are dominated by that of Ti4+ ion [15].
Figure 3.4: Raman spectra of the MTO ceramics sintered at different sintering temperatures.
3.3.6Low frequency dielectric properties
The dielectric constant and dielectric loss of the MTO ceramics sintered at 1350oC for 3 h were measured as a function of temperature and measured at different frequencies were shown in Figure 3.5(a) and 3.5(b), respectively. It is observed that as the temperature
increases both the dielectric constant and dielectric loss is found to increase and also as the frequency increases both the dielectric constant and dielectric loss decreases. The dielectric constant and dielectric losses were in the range of 18.5 to 24.7 and 7×10-5 to 1.5×10-2 in the temperature range of RT to 500oC. At low temperatures, the molecules cannot orient themselves in polar dielectrics. When the temperature rises, the orientation of dipoles is facilitated and this increases dielectric constant. At high temperatures, the dielectric losses caused by the dipole mechanism reach their maximum value and the degree of dipole orientation increases. Apart from dipole losses electrical conduction also increases with increase temperature. These factors would cause the increase in both dielectric constant and dielectric loss of MTO ceramics with increase in temperature [16].
Figure 3.5: (a) Dielectric constant (εr) and (b) loss tangent (tanδ) measured at different frequencies as a function of temperature of MTO ceramics sintered at 1350oC.
3.3.7 Microwave dielectric properties
Figure 3.6 shows the microwave dielectric constant and Q×f0 of MTO ceramics as a function of sintering temperature. Both the dielectric constant and Q×f0 followed the same trend as a function of temperature. The εr values of MTO ceramics ranged between 15.95 - 18.52. The increase in εr values are due to the simultaneous increase in the relative density of the samples because higher relative density means lower porosity. The variation in dielectric constant and Q×f0 values as a function of relative density was shown in Figure 3.7. In both the cases, it was observed that both the dielectric constant and Q×f0 increases with increase
in relative density, especially at higher values of relative density. The product of the quality factor (Q) and resonant frequency (f0) is considered as a tool for evaluating the quality of dielectric materials. The Q×f0 values increased with an increase in sintering temperature up to1350oC above that started decreasing and the values Q×f0 are in the range 61.5 - 162.3 THz. These values are much higher compared to the values reported by Baek et al., [17] and the samples were prepared by the high energy ball milling method. The obtained dielectric properties are εr =17.1 and Q×f0 =100 THz; sintering temperature: 1350oC for 2 h.
Figure 3.6: Variation in dielectric constant and Q×f0 values of MTO ceramics as a function of sintering temperature.
The maximum Q×f0 value is found to be 162.3 THz for the samples sintered at 1350oC for 3 h. Many microwave dielectric loss mechanisms are suggested including intrinsic and extrinsic [18]. Intrinsic losses in crystals arise because of anharmonic lattice forces that mediate the interaction between the crystal and phonons. This leads to damping of the optical phonons and therefore of the microwave field. Losses in ceramics caused by extended dislocations, grain boundaries, inclusions and secondary phases are termed extrinsic. These losses are caused by either dipole relaxations of impurities concentrated at interfaces or relaxations of space charge polarizations present at interfaces. The increase in Q×f0 value is primarily attributed to the increase in uniform grain size and higher densities. The decrease in Q×f0 values at higher temperatures can be attributed to the lower densities and the non-
uniform grain growth. Since the grain boundary is a plane defect, it probably decreases the Q×f0 value. The specimens with large grains are expected to have a high Q×f0 value because growth decreases the grain boundary area. In the present study, it is clear that under favorable conditions, grains grow uniformly to larger sizes leading to the well - densified MTO ceramics behaving like a single crystal which show that the Q×f0 values were strongly dependent both on the uniform grain size and relative density.
Figure 3.7: Variation in dielectric constant and Q×f0 values of MTO ceramics as a function of relative density.