S. A. Petrov

Synthesis and Study of Isomorphous Miscibility Limits of Crystalline Phases with a Hollandite-Type Tunnel Structure in the Cs2O(MeO)–Al2O3–TiO2 (Me = Ba, Sr) Systems*

The possibility of forming hollandite-type phases during the solid-phase reaction in the Cs2O(MeO)–Al2O3–TiO2 (Me = Ba, Sr) systems is studied. It is revealed that, in the Cs2O–BaO–Al2O3–TiO2 system, the region of existence of CsxBa1 – x/2Al2Ti5O14 solid solutions with a hollandite-type structure lies in the concentration range 0 ≤ x ≤ 0.7. In the SrO–BaO–Al2O3–TiO2 system, no solid solutions with a hollandite-type structure are observed. The mechanism and kinetics of formation of the CsxBa1 – x/2Al2Ti5O14 (0 ≤ x ≤ 0.7) solid solutions are analyzed, and the optimum conditions for synthesis of these solutions are determined.

Synthesis and study of novel catalysts based on hollandite K2Ga2Ti6O16

The results of experimental studies of synthesis of the hollandite phase K2Ga2Ti6O16 using initial mixtures of different dispersion compositions obtained by two methods, i.e., mechanical dispersion (MD) (followed by solid-phase sintering) and sol-gel method (Pechini method, MP), are reported. The catalytic properties of the obtained materials in the reactions of carbon monoxide (CO) and hydrogen (H2) oxidation have been determined. It has been demonstrated that an increase in the catalytic activity in the CO oxidation reaction is observed on the samples obtained using the sol-gel method, in which the hollandite phase content is higher and crystallization is more complete. The samples obtained using the MD method are characterized by a low porosity and activity in comparison with those produced by the Pechini method.

Physicochemical prerequisites of the synthesis of new ionic conductors based on complex oxides with a ramsdellite-type structure

This paper reports on the results of experimental investigations of solid solutions that have a ramsdellite structure and crystallize in the Li2O-Fe2O3-SnO2-TiO2, Li2O-Fe2O3-TiO2, and Li2O-Cr2O3-TiO2 systems. The concentration boundaries of existence of these solid solutions are determined, and their thermal and electrical properties are studied. It is established that the ramsdellite phases crystallizing in the Li2O-Fe2O3-SnO2-TiO2 and Li2O-Fe2O3-TiO2 systems transform into a metastable hexagonal phase at temperatures of ∼ 650–700 and 550°C, respectively. The results of investigations of this phase transformation by differential thermal analysis, differential scanning calorimetry, and high-temperature X-ray diffraction, as well as measurements of the electrical conductivity of the ramsdellite phases in the Li2O-Fe2O3-SnO2-TiO2 system, indicate that this transition is a first-order phase transition. Solid solutions of ramsdellite phases crystallizing in the Li2O-Cr2O3-TiO2 system do not undergo the phase transformation under consideration. These solid solutions are stable during multiple dynamic heating to 1000°C (under heating-cooling conditions at a rate of 10–15°C/min). Long-term heat treatments at 1000°C (for more than 3 h) lead to their decomposition with the formation of ramsdellite phases of the unknown composition and the Li2TiO3 compound. The electrical conductivities of the solid solutions formed by the ramsdellite phases crystallizing in the Li2O-Fe2O3-TiO2 and Li2O-Cr2O3-TiO2 systems at a temperature of 500°C are evaluated to be σ ≈ 10−1.5 S/cm. The transformation of the ramsdellite phases in the Li2O-Fe2O3-TiO2 system into the metastable hexagonal phases is accompanied by a decrease in the electrical conductivity by several orders of magnitude. The electrical conductivity of Li1.9CrxTi3.025 − 0.75xO7 (0 ≤ x ≤ 0.8) solid solutions remains unchanged upon multiple dynamic heating to 1000°C (with subsequent cooling).

Investigation into the formation of phases with a Ba2Ti9O20-type structure in the BaO-TiO2 and BaO-SrO-TiO2 systems

The mechanism of formation of barium titanate Ba2Ti9O20 in the BaO-TiO2 and BaO-SrO-TiO2 systems is investigated using initial mixtures prepared by three methods, namely, mechanical grinding of the initial reactants, coprecipitation from aqueous solutions of salts, and the sol-gel technique. It is established that, irrespective of the preparation procedure, the formation of Ba2Ti9O20 proceeds through the formation of the intermediate phases BaTi4O9 and BaTi5O11. The nature of the intermediate phases is determined by the homogeneity and dispersion of the initial mixture, as well as by the stability of the intermediate phase. The most optimum conditions for the synthesis of Ba2Ti9O20 are provided by the formation of BaTi5O11 as an intermediate phase upon heat treatment of the coprecipitation products in the nanocrystalline state. The metastability and structural defects in the BaTi5O11 intermediate phase encourage a decrease in the temperature of the final heat treatment by 100–150°C in the course of the preparation of Ba2Ti9O20 single-phase ceramics.

Investigation of the mechanism of formation of BaTi4O9 from initial mixtures of different dispersion

The mechanism of solid-phase interaction in synthesis of BaTi4O9 is investigated using initial mixtures characterized by different degrees of dispersion and prepared by three methods: (i) mechanical mixing and grinding of initial components, (ii) coprecipitation from aqueous solutions of salts, and (iii) the citrate-nitrate sol-gel technique. The use of initial mixtures consisting of nanoparticles in the synthesis makes it possible to decrease the sintering temperature by 100–300°C, which ensures the preparation of single-phase samples, and to reduce several times the heat treatment time. Dilatometric investigations of the sintering process in the synthesis of BaTi4O9 from the initial mixtures under study indicate that the maximum change in the linear sizes of samples occurs in the temperature range 1000–1250°C. It is shown that, in this temperature range, the concurrently formed impurity phases undergo decomposition and the growth rate of BaTi4O9 crystals increases. The optimum conditions for synthesis of BaTi4O9 in the form of a powder with a particle size of approximately 77 nm and in the form of a ceramic material are determined, which is necessary for use of this compound as different functional materials.

Kinetics and mechanism of the formation of hollandites in the BaO(Cs2O)-Al2O3-TiO2 system from initial mixtures prepared by different methods

The mechanism and kinetics of formation of solid solutions based on hollandite in the BaO(Cs2O)-Al2O3-TiO2 system are investigated using the initial mixtures prepared by two methods: (i) mechanical grinding and mixing of the initial components and (ii) coprecipitation from aqueous solutions of the salts. It is established that the mechanism of formation of hollandite in the system under investigation depends on the degree of dispersion of the initial mixtures used in the synthesis. When the synthesis is performed with the initial mixture prepared by mechanical grinding and mixing of the initial reactants (the particle size is equal to 50–300 nm), hollandite is formed at temperatures in the range 1100–1250°C in the presence of the accompanying phase Cs2Al2Ti2O8. When the synthesis is performed with the initial mixture prepared by coprecipitation from aqueous solutions of salts (the particle size is equal to 10–12 nm), hollandite is formed at temperatures in the range 850–1050°C. The investigation into the kinetics of formation of the hollandite phase from the above mixtures made it possible to determine the temperature-time conditions for the synthesis of this titanate in the form of a powder with a particle size of approximately 50 nm or in the form of a dense ceramic material with a particle size of ∼200 nm.

Mechanism of the formation of Ba2Ti9O20-based phases in the course of solid-phase interaction in the BaO-TiO2(ZrO2) and Cs2O-BaO-TiO2(ZrO2) systems

The mechanism of solid-phase interaction in the BaO–TiO2(ZrO2) and Cs2O–BaO–TiO2(ZrO2) systems is investigated. It is established that the formation of the Ba2Ti9O20 compound and Ba2Ti9O20-based solid solutions is a multistage process proceeding through the formation of intermediate phases. The solid-phase interaction in the BaO–TiO2(ZrO2) system occurs through the formation of the BaTi4O9 intermediate compound. The Ba2Ti9O20 single-phase product is formed only in the presence of ZrO2 (0.82 mol %) upon heat treatment at a temperature of 1250°C for 5 h. In the Cs2O–BaO–TiO2(ZrO2) system, the BaTi5O11 metastable intermediate phase is formed at the first stage of the solid-phase interaction. The CsxBa2 – x/2Ti9 – yZryO20 single-phase solid solutions are prepared upon heat treatment at 1100°C for 1 h. It is demonstrated that, in the Ba2Ti9O20 structure, cesium can isomorphously substitute for barium with the formation of CsxBa2 – x/2Ti9 – yZryO20 solid solutions (0 ≤ x ≤ 0.8, y = 0 and 0.09).

Electroconductivity of alkali-zinc diphosphates in partial sections of the Zn2P2O7–Li2ZnP2O7–Na2ZnP2O7–K2ZnP2O7 system

Electric properties of diphosphates Na2ZnP2O7, K2ZnP2O7, K2Zn3(P2O7)2, Li12Zn4(P2O7)5, Li2xZn2–xP2O7 (0.30 < x ≤ 0.56), Li2ZnP2O7, and Li4P2O7 with one alkaline cation; LiNaZnP2O7, LiKZnP2O7, Na2–xKxZnP2O7 (0.0 < x ≤ 0.6), NaxK2–xZnP2O7 (0 < x ≤ 0.54), and Na1–xK1 + xZnP2O7 (0 ≤ x ≤ 0.1) with two alkaline cations; and solid solutions LiNa1–xKxZnP2O7 (0 ≤ x ≤ 1) containing three alkaline cations have been studied in system Zn2P2O7–Li2ZnP2O7–Na2ZnP2O7–K2ZnP2O7. It has been demonstrated that the nature of the alkaline cation and the crystal structure affect the electroconductivity of mixed alkali-zinc diphosphates.