ИСТИНА

Barium titanate is known and widely applied in microelectronics due to its outstanding ferroelectric properties. Ceramics based on BaTiO3 serve as component of MLCCs, PTCRs, piezoelectric transducers, pyroelectric sensors and electrooptic devices.Align with solid state reaction and co-precipitation, hydrothermal synthesis is one of the most used methods for BaTiO3 production because of operating convenience and high quality of resulting powder. Hydrothermally obtained BaTiO3 is characterized by spherical morphology of crystals, narrow particle size distribution and performs high sinterability, which is of great importance for the following ceramics processing. Commonly used and extensively investigated reaction media for hydrothermal synthesis are liquid water below the critical point (374C, 22,1 MPa) and supercritical water fluid. Less attention is paid on water vapor below the critical point that also allows conducting the synthesis of BaTiO3 from barium and titanium oxides in mild conditions. The aim of current research is to study the influence of physical state of the reaction medium (vapor/liquid water) on morphology and phase content of BaTiO3 and to investigate sinterability of obtained powders in SPS conditions. Four samples of TiO2 in anatase modification with different degree of crystallinity and varying particle morphology were obtained by hydrolysis of titanium butilate in acid or alkaline conditions in presence of surfactants. These samples and commercial TiO2 in rutile modification were mixed with BaO as starting reagents for BaTiO3 synthesis in hydrothermal conditions at 230°C and 2,94 MPa during 24 h. Two sets of BaTiO3 samples were produced. In the first set BaTiO3 was synthesized using liquid water as reaction medium, in another set the process was conducted in water vapor. In both cases of reaction medium the decrease of BaTiO3crystals size was observed with the lowering of crystallinity of starting TiO2 and hence the growth of its reactivity. Reaction in vapor resulted in smaller BaTiO3 crystals (mean sizes of 40-80 nm) obtained from different TiO2 precursors, than the process in liquid water (mean sizes of 130-200 nm). BaTiO3powders synthesized in vapor inherited the morphology of starting TiO2 samples, while BaTiO3 produced in liquid had no similarity of particle shape with TiO2 powders. This phenomenon points to the difference in mechanisms of BaTiO3 formation in vapor and liquid water. From XRD data, BaTiO3 obtained in liquid water has tetragonal phase modification at room temperature, and BaTiO3 formed in vapor conditions is cubic due to appearance of size effect for nanoscale crystals. In order to develop the most appropriate sintering regime, shrinkage curves of obtained BaTiO3 powders were studied in SPS conditions. On the basis of these results, two-step regime with dwell at 1170°C and 50 MPa was applied. Dwell time varied from 1 to 10 min. The obtained ceramics had density of 98% of theoretical value (6,02 g/cm3) and grain size in range of 250-430 nm. Dielectric permittivity of BaTiO3 ceramics measured at ambient temperature and frequency of 1 MHz increased with the SPS dwell time and reached 2232 for the pellet processed by 10 min. sintering.