Ca(1-x)Nd2x/3TiO3 and MgTiO3-Ca0.61Nd0.26TiO3 composite ceramics were prepared by the mixed oxide route and characterised in terms of their structure, microstructure and properties. Ceramics sintered at 1450-1500oC achieved better than 95% of the theoretical density. X-Ray diffraction (XRD) revealed that Ca(1-x)Nd2x/3TiO3 ceramics were single phase for all compositions. For x ≤ 0.39 the structure was Pbnm with lattice parameters of a = b = √2ac and c = 2ac and a tilt system of a-a-c+. Compositions with x ≥ 0.48 could be better described by a C2/m structure with lattice parameters of a = b = c = 2ac. Scanning electron microscopy (SEM) revealed that the ceramics had grain sizes in the 5-70 μm range with abnormal grain growth for Nd3+ rich compositions. Images revealed that the twin domains in CaTiO3 were needle shaped and on addition of Nd3+ the domain morphology becomes more complex. The needle domain morphology returns for Ca0.43Nd0.38TiO3. High resolution electron microscopy (HAADF-STEM and electron diffraction) was used to probe cation-vacancy ordering (CVO) in the lattice. It was found that there was no CVO for x < 0.48 whilst at x = 0.48 there was evidence of a transition to a short range CVO. A transition to long range ordering is almost complete for the Ca0.1Nd0.6TiO3. The structural characteristics of Ca(1-x)Nd2x/3TiO3 ceramics as a function of temperature were investigated using in-situ XRD and Raman spectroscopy. All compositions were found to have the same structure across the entire temperature range. The Raman spectroscopy as a function of temperature indicated a possible transition with similar characteristics to a Curie temperature in a ferroelectric ceramic. The transition temperature was dependent on the cation ordering with the ceramics with greatest degree of disorder having the lowest transition temperature. The microwave dielectric properties of the samples were measured by a cavity resonance method in the 2-4GHz range. The relative permittivity (εr) was found to decrease from 180 for CaTiO3 to approximately 80 for Ca0.1Nd0.6TiO3 with an exponential dependence between the composition and the property. The temperature coefficient of resonant frequency (τf) ranged from +770ppmK-1 for CaTiO3 to +200ppmK-1 for Ca0.1Nd0.6TiO3. The Q x f for CaTiO3 was found to be 6000GHz and this increased to a maximum of 13000GHz for Ca0.7Nd0.2TiO3. After the Ca0.7Nd0.2TiO3 composition, the Q x f decreased to approximately 1100GHz for Ca0.1Nd0.6TiO3. The εr and τf were found to be mainly dependent on the composition of the ceramics whilst the Q x f value was more complex being dependent on the width of the twin domains in the grains. CaTiO3 samples fabricated by spark plasma sintering at 1150oC and above achieved better than 95% of the theoretical density. XRD revealed only a single phase with an orthorhombic Pbnm structure at room temperature and a tilt system of a-a-c+. SEM confirmed that the samples were single phase with grain size between 500nm-5μm. Transmission electron microscopy (TEM) of specimens sintered at 1150oC showed evidence of both (011) and (112) type domains. The τf of the ceramics was shown to be dependent on the volume of the unit cell, in agreement with the Bosman-Havinga equations. The ceramic sintered at 1150oC showed improvement in the Q x f value compared to samples prepared by conventional sintering. The structure, microstructure and properties of composite ceramics based on the MgTiO3-Ca0.61Nd0.26TiO3 system were investigated. Optimum properties were achieved at a composition of 0.8MgTiO3-0.2Ca0.61Nd0.26TiO3 with τf = -0.1ppmK-1, Q x f of 39000GHz and εr of 25.4. XRD revealed the presence of 3 phases including Ca0.61Nd0.26TiO3, MgTiO3 and MgTi2O5. The grain size of the ceramics was typically 5μm. The Q x f value was sensitive to the cooling rate and these changes could be related to changes in the vibrational properties of the lattice through changes in the lattice parameters.

The aim of this work was to contribute to a better understanding of the characteristics and applications of (Ba,Sr)Sm2Ti4O12 microwave dielectric ceramics. The major contents are as follows. Firstly, to establish the correct reaction sequence of (Ba,Sr)Sm2Ti4O12, phases present in different calcining temperatures are identified by X-ray diffraction patterns. When different calcining temperatures are used, the source materials BaO (BaCO3), TiO2 and Sm2O3 are consumed at different calcining temperatures; the intermediate phases BaTiO3, BaTi4O9, and Sm2Ti2O7 reveals and consume at different calcining temperatures before the BaSm2Ti4O12 phase starts to reveal. However, the real solid reaction processes are usually more complex, and some intermediate reaction processes might happen. Secondly, in the (Ba1-xSrx)Sm2Ti4O12 system, SrO can be used to substitute the BaO site and improve the microwave dielectric characteristics. In this study, we find that SrO content in the range of 2 ~ 6 mol% is the acceptable composition because of the higher Q*f values and acceptable and values. Thirdly, the CaO-BaO- Li2O-Sm2O3-TiO2(CBLST) ceramics system was studied. In general, a dielectric material with a high has a large . To adjust to close 0ppm/oC, two or more compounds having negative and positive values are employed to form a solid solution or mixed phases in order to obtain the desired dielectric properties. In this study, BaO was used to substitute the CaO site and improved the microwave dielectric characteristics. Finally, the (Ba0.98Sr0.02)Sm2Ti4O12 system was adopted as a case of applications in dielectric resonator antenna. It possessed a low value of -5.96ppm/¢XC, a high value of 79, and a high Q*f value of 7920 GHz (at 3.311GHz). With the loading of a (Ba0.98Sr0.02)Sm2Ti4O12 dielectric resonator (DR), a circular polarization (CP) design of DR antenna through a cross slot of unequal slot lengths in the ground plane of a microstrip line is fabricated. From the results obtained, it is also found that the present proposed CP design has relatively relaxed manufacturing tolerances, as compared to the conventional CP designs that require slight geometrical modifications of the microstrip patch or DR elements. With the loading of a (Ba0.98Sr0.02)Sm2Ti4O12 superstrate layer and a 1W chip resistor, a compact rectangular microstrip antenna with enhanced gain and wider bandwidth can be implemented. The antenna size is reduced to be ~ 6.05% times of a conventional patch antenna, the proposed structure can have an operating bandwidth of more than six times that of a conventional patch antenna, with an almost equal antenna gain level.

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<div>Understanding grain growth in dielectric ceramics is essential to controlling the electrical and mechanical properties necessary to produce ceramic capacitors and sensors. The effect of alloying barium titanate with strontium titanate on the equilibrium crystal shape was investigated in order to determine possible impacts on grain growth. The equilibrium crystal shape was studied through three experimental methods to identify possible changes in grain boundary energy or anisotropy with changing composition.</div><div>The first method was by imaging intergranular pores to observe faceting behavior and relative interfacial energies. Intergranular pores were reconstructed to determine the relative surface energies of the identified facets. The second method was to perform atomic force microscopy on surface facets to collect topography data. The topography data was combined with orientation data obtained by EBSD analysis from the same region, and used to calculate the normal vector of the surface facets. These datasets were plotted in a stereographic projection to study the faceting anisotropy. The third method involved collecting EBSD orientation data and images of surface faceting behavior. The surface faceting behavior of each grain was categorized by type of facet and plotted on a stereographic projection at the corresponding orientation. This allowed for the analysis of faceting transitions and the differentiation of faceted and continuous regions of the equilibrium crystal shape. The analysis of faceting behavior across compositions has implications on grain growth of the barium titanate/strontium titanate system.</div>