Micr owave Dielectric Properties of (1-x)CaTiO 3 -xNd(Mg 1/2 Ti 1/2 )O 3 Ceramics System
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2. Experimental Samples of CaTiO3 and Nd(Mg1/2Ti1/2)O3 were individually synthesized by conventional solid-state methods from high-purity oxide(>99.9%) powders:CaCO3, TiO2, Nd2O3, and MgO. The starting materials were mixed according to the stoichiometry of Nd(Mg1/2Ti1/2)O3 and CaTiO3, and ground in distilled water for 10h in a baling mill with agate balls. Both mixtures were dried and calcined at 1400oC for 5h. The crystalline phases of the calcined powder were identified by x-ray powder diffraction (XRPD) analysis using Cu-Kα radiation from 20o to 60o in 2θ. The calcined powder was mixed to the desired composition (1-x)CaTiO3-xNd(Mg1/2Ti1/2)O3 and re-milled for 5h with PVA solution as a binder. Pellets of 11 mm diameter and 5 mm thickness were pressed by uniaxial pressing. After debinding, these pellets were sintered at temperatures of 1400-1550oC for 5h . The heating and cooling rates were both set at 10oC/min. The microstructure observation of surfaces of the sintered ceramics was performed by scanning electron microscopy (SEM, JEOL JSM 6400, Japan). The crystalline phases of the sintered ceramics were identified using an X-ray diffraction (XRD, D5000 Diffractometer, Seimens, Germany) pattern. The bulk densities of the sintered pellets were measured using the Archimedes method. The dielectric constant ( ε r ) and the quality factor values (Q) at microwave frequencies were measured using the Hakki-Coleman dielectric resonator method as modified and improved by Courtney.12,13) A system combining a HP8757D network analyzer and a HP8350B sweep oscillator was employed in the measurement. The disks were, in turn, placed between two parallel brass plates. The TE011 and TE012 modes were measured using a HP8510B network analyzer. The dielectric constant was calculated from the resonant frequency of the TE011 mode of the cylindrical disk. For the unloaded Q measurement, the conductor loss resulting from the eddy currents around the conductive plate surfaces must be subtracted to obtain the dielectric quality factor. For this purpose, two disks with different heights were prepared, one for the TE011 mode and the other is for the TE012 mode, where the disk for the TE012 mode measurement is twice the height of the disk for the TE011 mode. An identical technique was applied in measuring the temperature coefficient of resonant frequency (τf). The test set was placed over a thermostat in the temperature range from +20°C to +80°C. The τf value (ppm/°C) can be calculated by noting the change in resonant frequency (∆f), f 2− f1 τ f= ， (1) f 1(T 2 − T 1) where f1 and f2 represent the resonant frequencies at T1 and T2, respectively.
3.
Results and Discussion
The (1-x)CaTiO3-xNd(Mg1/2Ti1/2)O3 ceramics were sintered to dense materials. The sintering temperature increased from 1400oC to 1550oC for 4h with increase in x. Fig. 1 shows the X-ray diffraction pattern recorded from (1-x)CaTiO3-xNd(Mg1/2Ti1/2)O3 for various values of x in the range 0 to 1. CaTiO3 is orthorhombic and has a GdFeO3-type perovskite structure and Nd(Mg1/2Ti1/2
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