SOLUTION COMBUSTION SYNTHESIS OF FERROELECTRIC CERAMIC MATERIALS ON THE BASE OF TITANATES K.B. Podbolotov*, A.A. Khort, A.L. Nikolskaia Belarusian State Technological University, Minsk, Sverdlova str., 13a, Belarus *
[email protected]
Ferroelectric ceramics refers to the category of materials capable to reversible spontaneous polarization. That includes some titanates and zirconates of a number of elements such as barium, lead, bismuth, strontium etc. as well as their solid solutions. Thanks to their unique electrophysical properties ferroelectric materials and items made on their base are used as high-voltage dielectrics, capacitor materials and as materials for manufacturing chemical and physical sensors, memory cells, etc. This paper describes investigating the possibility of exothermal synthesis of ceramic ferroelectric materials in the BaO – TiO2, PbO – TiO2 and Bi2O3 – TiO2 systems. Glycine (C2H5NO2), carbamide (СН4N2O) and glycerin (C3H5(OH)3) were selected as reducing agents capable to provide conditions required for the exothermal oxidation-reduction reaction. Barium, lead, bismuth and titanyl nitrate were used as oxidizers and carriers of metal cations and anions. The calculation results obtained by thermodynamical modeling have shown that the combustion temperature of mixtures is 1700 to 2500 K (1427 to 2227°С). However, under the real synthesis conditions the combustion temperature does not exceed 1300°С due to high temperature losses with escaping gases and heat transfer to environment. The titanates and zirconates forming during synthesis are precipitated in solid state. To produce ceramic materials using a high-temperature exothermal synthesis the stoichiometric quantities of raw components were dissolved in distilled water followed by a subsequent mixing with a pre-synthesized solution of titanyl nitrate. The prepared solutions placed in alumina crucibles and put into a muffle furnace with temperature of 500°C. After water has evaporated exothermic inflammation of the solution components with a sharp increase of temperature started. The synthesis duration was in the range of 4 minutes and up to 10 minutes for glycine and glycerin respectively, which is considerably lower compared with the temperatures observed by conventional methods of high-temperature sintering. In order to investigate phase composition and microstructure of synthesized materials they were subjected to X-ray phase analysis and microscopic examination. It has been established that a synthesized ceramic material of the BaO-TiO2 system contains a barium titanate (BaTiO3) of a high-temperature cubic modification as a basic phase in the cases in which glycine was used as a reducing agent. The retention of metastable state at low temperatures can be attributed to the effects of both the synthesis conditions (material quenching during rapid heating cooling) and space-dynamic factors (high mechanical stresses between fine grains prevent crystal rearrangement). The analogues data have been also obtained for materials synthesized in the PbO – TiO2 and Bi2O3 – TiO2 systems. In both cases the using of glycine allows to obtain ceramic materials (PbTiO3 and Bi4Ti3O12, respectively) in high-temperature metastable polymorphic modification. At the same time using carbamide and glycerin as reducing agents results in that the phase composition of synthesized materials contains nitrates of metals and titanium dioxide, while the target product is in the minor quantities. This indicates that the use of these reducing agents with preset synthesis parameters does not result in achievement of the conditions required for proceeding the oxidation-reduction reaction between the components in a reaction system. 131
The synthesized materials have a fine crystal homogeneous structure that is characterized by a low density. The microstructure of synthesized materials contains some crystal elements aggregated in the form of thin plates, dendritic and filamentary formations (seen in Figure).
а b c Figure 1. Micrographs of surfaces of synthesized ceramic materials (a) BaZrO3 (×2000); (b) PbZrO3 (×2000); (с) Bi4Zr3O12 (×10000) Thus, in the course of carried out investigations it has been found that ferroelectric ceramic materials can be produced by the method of exothermal interaction in solutions. It is shown that synthesized materials possess high porosity and a high-developed surface.
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