sintering temperature with the addition of V2O5 can be explained as follows. The melting point of V2O5 is 690oC, which forms a liquid phase during the sintering process. In liquid phase sintering, the liquid spreads to cover the solid surfaces, works as liquid bridge between particles. This decreases the friction between the MTO particles and exerts a capillary force, which can be used to rearrange the particles more easily and enhance the densification process to get maximum packing [28]. From all the results, it is demonstrated that addition of V2O5 is not only decreases the sintering temperature but also improves the grain growth of the MTO ceramics.
Figure 3.13(b) shows the variation in dielectric constants (εr) of Mg1.07TiO3.07 as a function of V2O5 (x wt%) sintered at different temperatures. The variation in εr is consistent with that of the relative density as a function of V2O5 and as a function of sintering temperature. The improvement in the dielectric constant with addition of V2O5 and sintering temperature are due to uniform grain growth and large grain size. The maximum dielectric constant of 17.3 is observed for x = 1.5 wt% samples, sintered at 1100oC.
The product of quality factor (Q) and resonance frequency (f0) is the tool for evaluating the performance of a dielectric resonator material. Figure 3.14(a) demonstrates the variation in Q×f0 values for Mg1+δTiO3+δceramics as a function of δ concentration (δ = 0 - 0.15) at different sintering temperatures. The Q×f0 exhibits similar behavior as that of relative density and dielectric constant as a function of δ concentration and as well as sintering temperature. Nevertheless, the obtained Q×f0 values were in the range 59 - 161 THz for the Mg1+δTiO3+δ samples, sintered in the sintering temperature range of 1250 - 1400oC. In general, the microwave dielectric losses arise from the two mechanisms (i) intrinsic and (ii) extrinsic losses [29, 30]. Intrinsic losses arise due to the anharmonic forces that mediate the interaction between crystal lattice modes and electromagnetic radiation, which leads to damping of the optical phonons. On the other hand, extended dislocations, grain boundaries, grain size, densification or porosity, oxygen vacancies, secondary phases and sample processing conditions are responsible for the extrinsic losses. At lower δ concentration, the lower Q×f0 values are due to the presence of MgTi2O5 (Q×f0 ~ 47,000 GHz), which deteriorates the microwave dielectric properties of the MTO ceramics. However above δ = 0.07, samples exhibited low Q×f0 values even the secondary phase Mg2TiO4 (Q×f0 ~ 150, 000 GHz) has comparable Q×f0 with MTO. This is attributed to the reduction in grain size and the presence of pores (see Figure 3.9(d)) in the prepared samples. In the present case, secondary phase and grain size significantly influenced the microwave dielectric properties of the MTO ceramics. The maximum Q×f0 value of 161 THz is obtained for the δ = 0.07 sample, sintered at 1350oC is owing to the formation of phase pure MgTiO3 with larger grain size and maximum relative densities. Djuniadi et al. [31] reported that the increase in average grain size reduces the porosity and the grain boundary area which controls the microwave loss of the ceramics. Because, grain boundary is a plane defect as a result microwave dielectric loss increases.
Figure 3.14: Variation in Q×f
Mg1.07TiO3.07 as a function of x wt% at different sintering temperatures.
Figure 3.14(b) shows the V sintered at different temperatures. The
density and dielectric constant with sintering temperature and opposite trend with pure Mg1.07TiO3.07,sintered at 1100
Interestingly, with the addition of of the sample are large. The obtained
Mg1.07TiO3.07 ceramics, sintered in the temperature V2O5, the maximum Q×f0 of 85.6 THz
with lower wt% addition of V
of V5+ with Ti4+ which is confirmed from the XRD results. When a
Ti4+, V5+ ion introduces one extra positive charge. A single positive charge is eliminated from the sample to maintain the electroneutrality and acts as a donor, the react
as [27]
The reduction in oxygen vacancies influence the anharmonic interaction: consequently, there is an increase in Q×f0. Similar reports
addition of V2O5 to (Zr0.8Sn0.2 V2O5 addition, the decrease in
Variation in Q×f0 for (a) Mg1+δTiO3+δas a function of δ concentration and (b) as a function of x wt% at different sintering temperatures.
(b) shows the V2O5 dependent Q×f0 values for Mg1.07TiO
sintered at different temperatures. The Q×f0 values follow the similar trend as that of relative density and dielectric constant with sintering temperature and opposite trend with
sintered at 1100oC exhibits very low Q×f0 value of 42 THz at 9.3 GHz.
addition of V2O5, the Q×f0 values decreases even though the . The obtained Q×f0 values were in the range 30.6 -
sintered in the temperature range of 1050-1200oC. For
of 85.6 THz is obtained at 1100oC. The improvement in the with lower wt% addition of V2O5 is due to reduction in oxygen vacancies by
confirmed from the XRD results. When a V5+ ion substitutes for a ion introduces one extra positive charge. A single positive charge is eliminated from the sample to maintain the electroneutrality and acts as a donor, the reaction can be expressed
o Ti
o V O
V O
V2 5 + •• →2 • +5 The reduction in oxygen vacancies influence the anharmonic interaction: consequently, there
Similar reports are available on the improvement in
0.2)TiO4 and Mg2TiO4 ceramics [26, 32]. With the increase in the addition, the decrease in Q×f0 values can be due to the presence of V -
concentration and (b)
TiO3.07 ceramics, the similar trend as that of relative density and dielectric constant with sintering temperature and opposite trend with x wt%. The value of 42 THz at 9.3 GHz.
though the grain size 85.6 THz for the C. For x = 0.5 wt% of C. The improvement in the Q×f0
is due to reduction in oxygen vacancies by the substitution ion substitutes for a ion introduces one extra positive charge. A single positive charge is eliminated from ion can be expressed
(3.1) The reduction in oxygen vacancies influence the anharmonic interaction: consequently, there are available on the improvement in Q×f0 with the ]. With the increase in the rich liquid phase
at grain boundary and abnormal grain growth, which is confirmed from SEM images and EDS spectra. Ferreira et al. [33] reported that a smaller concentration of pentavalent oxides improves the Q×f0 values in MTO ceramics. The Q×f0 values obtained in this case are much higher than the MTO ceramics added with V2O5 - Bi2O3 [34]. From the above results the excess of Mg doping and V2O5 addition have greatly influenced the microwave dielectric properties of MTO ceramics.