Synthesis and Characterization of MgTiO3-CaTiO3 Based Microwave Dielectric Ceramics

Abstract

MgTiO3 and CaTiO3 microwave dielectrics are known to exhibit opposite temperature coefficient of resonant frequency (τf), generally prepared by solid state sintering route. In this study Zn/Co doped MgTiO3 and Sr/La doped CaTiO3 and their composites were fabricated by chemical synthesis route (polymeric precursor). The non isothermal method was applied to study the kinetics and thermodynamics for the thermal decomposition of polymeric precursor of representative samples; Mg0.95Zn0.5TiO3 and (Ca0.8Sr0.2)0.6La0.267TiO3. Thermo gravimetric-differential scanning calorimetry (TG-DSC) was used to analyze the thermal decomposition behavior of the precursors. X-ray diffraction (XRD), Fourier transform scanning electron microscopy (FE-SEM) and Raman techniques were used to study the phase, microstructure and phonon modes of the prepared samples. Vector network analyzer and impedance spectrometer were used to measure the electrical properties of sintered ceramics. In case of Mg0.95Zn0.5TiO3, three dimensional diffusion reaction (Jander); g(α) = [1-(1-α)1/3] 2 was proposed mechanism function for the reaction. By using Flynn-Wall-Ozawa (FWO) model values of the average activation energy, pre exponential factor and order of the reaction were found to be 129.33 kJ/mol, 1.04 × 108 min-1 , and 2.00, respectively. Whereas in the Kissinger-Akahira-Sunose (KAS) model corresponding values were 123.88 kJ/mol, 2.95 × 107 min-1 , and 2.09, respectively. The thermodynamic parameters such as enthalpy (ΔH* ), Gibbs free energy (ΔG* ) and entropy (ΔS* ) of activation calculated at the peak temperature were noted to be 120.46 kJ/mol, 228.69 kJ/mol, and -146.63 J/mol K, respectively. In case of (Ca0.8Sr0.2)0.6La0.267TiO3 phase boundary reaction contraction sphere is the most probable mechanism function for the reaction. The activation energy for thermal decomposition of the precursor was found to be 70.97 kJ/mol and 63.62 xvii kJ/mol using FWO and KAS models respectively. The average values of thermodynamic parameters were; ΔH* = 120.57 kJ/mol, ΔG* = 282.94 kJ/mol and ΔS* = -223.93 J/mol K. The particle size of the synthesized powder was < 100 nm with orthorhombic (Pbnm) symmetry. The sintered ceramic exhibited dielectric constant (εr) = 116.2, quality factor (Q × f) = 15730 and temperature coefficient of resonant frequency (τf) = +231 ppm/°C. Analysis of impedance data obtained in the temperature range 200-500 °C revealed that the ceramic exhibited negative temperature coefficient resistor (NTCR) characteristics. The composites (1-x)Mg0.95Co0.05TiO3-x(Ca0.8Sr0.2)0.6La0.267TiO3; (1-x)MCoT xCSLT and (1-x)Mg0.95Zn0.05TiO3-x(Ca0.8Sr0.2)0.6La0.267TiO3; (1-x)MZT-xCSLT microwave dielectric ceramics were successfully synthesized. The calination of (1- x)MCoT-xCSLT and (1-x)MZT-xCSLT polymeric precursors at 700 °C yielded ceramic powders with nano-sized particles. Rietveld refinement of sintered ceramics revealed that the composites composed of rhombohedral (R3) and orthorhombic (Pbnm) phases. It was observed that the values of εr increases and Q x f decrease as the molar fraction of CSLT increased in both the composites. The comparison reveals that, (1-x)MZT-xCSLT bears relative higher Q x f values than (1-x)MCoT-xCSLT which may be due to the presence of localized 3d electrons in Co2+ and also the ionic polarizability of Zn is higher than that of Co resulting in higher atomic vibrations causing a decrease in the dielectric loss. The ceramics with the composition (0.8)MCoT-(0.2)CSLT sintered at 1200 °C were observed to bear a good combination of microwave dielectric properties; εr = 26.13, Q x f = 54,820 GHz and τf = -3.9 ppm/°C. Similarly, the ceramics (0.8)MZT-(0.2)CSLT sintered at 1200 °C, was observed to possess a εr = 25.17, Q x f = 58,754 GHz and τf = -5.8 ppm/°C. Finally, these compositions are proposed as a suitable candidate for microwave applications.