Synthesis and Characterization of CaTiO3-LnAlO3(Ln=La, Nd) Ceramics Manufactured by Reaction Sintering Method

CaTiO 3 -LnAlO 3 (Ln=La, Nd) ceramics were manufactured by reaction-sintering for produce low cost and high efficiency materials. Using reaction-sintering method to manufacture these ceramics, which have excellent comprehensive properties. The subtle variations on densification behavior, phase transformation, phase composition, microstructure evolution and performances of CaTiO 3 -LnAlO 3 (Ln=La, Nd) ceramics were studied systematically. The XRD pattern indicates that the Ca 0.61 La 0.39 Al 0.39 Ti 0.61 O 3 phase and Ca 0.7 Nd 0.3 Ti 0.7 Al 0.3 O 3 phase were generated under certain environmental conditions respectively. The ceramics exhibited excellent performance parameters: when ε r is 42.03(45 635), the quality factor is 45500GHz (45 635) In addition, even in the environment of large temperature changes the ceramics can still maintain good performance. In conclusion, the reaction sintering method is an economic, convenient, available preparation means of making the CaTiO 3 ✝ LnAlO 3 (Ln=La, Nd) ceramics and has broad prospects of application and development.


Introduction
The wireless communication industries led by microwave communication technology are developing at a rapid pace. A better and serious request about the performance of the functional ceramics was also put forward by this prospect. [1][2][3].
In this work, using reaction sintering method to manufacture CaTiO3-LnAlO3(Ln=La, Nd) ceramics to simplify experimental procedures and optimize the dielectric properties. Also, the solid-state reaction mechanism of the CaTiO3-LnAlO3(Ln=La, Nd) solid solution was also investigated, which determine the origin of the intermediate phase.

Materials and methods
CaTiO3-LnAlO3(Ln=La, Nd) ceramic samples were made from high purity materials(≥99%) including CaCO3, TiO2, Al2O3, La2O3, and Nd2O3, therefore preprocessing of high purity raw materials is the foremost section in fabrication of high quality dielectric ceramics, especially La2O3, and Nd2O3. Put La2O3, and Nd2O3 into muffle furnace at 900℃ for two hours respectively, and the mole ratios of CaTiO3-LaAlO3 and CaTiO3-NdAlO3 were determined to be 0.675:0.325 and 0.695:0.305, respectively. The powder was blended with zirconia pellets in alcoholic medium for 4 h. After drying in an oven, samples in cylindrical molds with a diameter of 10mm and a height of 5mm were fabricated at 20 MPa. Finally, they have been sintered from 1475℃ to 1575℃ with a gradient of 25℃ for 6 h respectively.
Using the X-ray diffraction (Model X'Pert PRO, PANalytical, Almelo, Holland) with Cu Kα radiation at 40 kV and 40 mA (5°  2θ  80°) to analyze the microstructure and crystalline of the ceramics. The surface topography and crystal size were observed with SEM (Model JSM6380-LV SEM, JEOL, Tokyo, Japan). Performance parameters of specimens were acquired by using the network analyzer (Model E5071 CENA, Agilent Co, California, USA, 300KHz-20GHz). The τf values of the samples were measured from the resonant frequencies at 25 °C to 85 °C and was defined as: where f2 and f1 are the resonance frequencies at 85 °C and 25 °C, respectively. and NdAlO3 phases, and the secondary phase gradually disappeared at 1200°C [11].

Results and discussion
Upon further increase of the temperature to 1450 °C, the main peak of the CTNA ceramics appeared at 2θ = 33.2°, which matches well with the 2θ value of the  are composed of small crystal and the structure is not dense enough, which manifest that low temperature cannot provide enough energy for grain growth [17][18][19]. Raising the temperature again the samples surface becomes more compact and homogeneous and few grain boundaries appears. When the sintering temperature exceeds 1525℃, there were only a few pores in the sample [20,21]. The above results shown that CaTiO3-LnAlO3(Ln=La, Nd) ceramics could keep excellent performance in a wide temperature range. Using EDS to verify the molar ratios of elements in CaTiO3-LnAlO3(Ln=La, Nd) ceramics, which verifies the analysis and conclusion of XDR.  and then its density declines gradually due to the inner defects caused by extra-high temperature. When the temperature gradually increases to the ideal temperature, the ceramic grains grow and discharge the pores inside the ceramic, making microstructure of the ceramic more compact. and reaching the maximum. When the temperature exceeds the optimal temperature, the phenomenon of overburning occurs. The grain growth rate is too fast to discharge the pores inside the ceramic in time, contributing to the decline in the density [22][23][24].
As the sintering temperature increases, the dielectric constant of CaTiO3-LnAlO3(Ln=La, Nd) ceramics reaches a maximum value at the optimum temperature and then decreases. The trend is similar to the relative density. The high density of ceramics is accompanied by high polarizability and thus to an increase in polarization intensity [2,25]. The relationship between the dielectric constant and the dielectric polarizability and molar volume fraction of the materials can be explained clearly by the Clausius-Mossotti formula used in the literature as follows: where  [26].
There are two types of materials losses at microwave frequency: on the one hand the intrinsic loss mainly determined by the lattice vibration mode could influence the properties of materials, on the other hand the external loss determined by the second phase, oxygen vacancies, grain size and densities could also affect the properties of materials [27,28]. In the same system, the excellent performance of materials depends on the grain size and the number of pores. Therefore, with increasing temperature, the  [29,30]. These indicate that the CaTiO3-LnAlO3(Ln=La, Nd) ceramics has achieved excellent performance in a wide temperature range.

Conclusion
The densification behavior, phase transformation, phase composition,               The curves of ρ, εr, Q×f and τf of CaTiO3-LnAlO3(Ln=La, Nd) ceramics as a function of the sintering temperature.