Ceramics based on ree zirconates, titanates, and stannates

Abstract

O 7 was obtained. It was shown that partial substitution of Zr, Ti, and Sn(IV) for each other allows reducing the sintering temperature.In searching for materials for reliable immobilization of highly active nuclear power plant wastes (HAW), the problem of utilizing HAW with a high actinoid content has recently become especially important. This is due to the development of methods of separating HAW into fractions to immobilize them in matrices of the optimum composition. In one of the fractions obtained, the mass content of actinoids consists of tens of percent. Actinoid-containing HAW are also formed in conversion of weapons-grade plutonium [1].High capacity with respect to radionuclides and chemical and radiation stability are the basic requirements for actinoid immobilization matrices. Compounds with the crystal structure of pyrochlore [2], in particular, REE zirconates and titanates, are considered promising materials for actinoids. Another important criterion for assessing matrix material is the technological effectiveness, i.e., the efficiency of industrial production. Uniaxial molding followed by sintering is one widespread method of matrix synthesis as it is distinguished by simplicity of equipment and low costs in comparison to other methods (for example, hot molding or induction melting). However, synthesis of Ln 2 Zr 2 O 7 from the initial oxides requires a long time (tens of hours) and high temperatures (over 1500°C) [3].We know that using metal hydroxides or thermally unstable salts as the initial compounds allows significantly reducing the solid-phase synthesis temperature.We obtained and investigated the properties of a ceramic based on gadolinium zirconate, titanate, and stannate and mixed zirconate-titanate, zirconate-stannate, and titanatestannate compositions. The interest in obtained mixed compositions is due to the attempt to decrease the sintering temperature of pyrochlore ceramics in partial reciprocal substitution of Zr, Ti, and Sn(IV).A modified method based on thermal decomposition and calcinations of polyacrylamide gel containing metal salts was selected as the method for manufacturing the initial powders for synthesis of the ceramic [4]. The initial compounds were previously prepared solutions of gadolinium nitrate, zirconyl nitrate, Sn(IV) citrate, and nitric acid solution of Ti(IV) peroxide complex. Acrylamide was added to the mixed solution of metal salts and mixed until totally dissolved. The total concentration of metals in the solution obtained was 100 g/liter and the concentration of acrylamide was 250 g/liter. To eliminate the interfering effect of oxygen on radiation-induced polymerization of the acrylamide, the surface of the solution was covered with melted paraffin.The solutions were irradiated in a RXM-g-20 gamma unit for 24 h (GIK-7-2 cobalt sources of g radiation, dose rate of 0.15 Gy/sec). During irradiation, radiation-induced polymerization of acrylamide took place with formation of polyacrylamide gel containing the metal salts in its pores.The gel was pyrolyzed in a t...

Heat capacity of rare-earth stannates in the range 350–1000 K

“…Note that, with decreasing lattice parameter a, the oxygen position parameter x in the rare-earth stannates gradually increases from 0.32943 (7) in La 2 Sn 2 O 7 to 0.3397(1) in Lu 2 Sn 2 O 7 [18], whereas the Sn-O 48f bond distance decreases almost linearly with decreasing a in going from La to Gd and varies in a more com- 12 [28] and 25 K [29]. At the same time, this can be done using the Neumann-Kopp (NK) rule [42]: (4) where (j) is the specific heat of a mixed oxide compound, (i) is the specific heat of the ith constituent binary oxide, and m i is the mole fraction of the corresponding binary oxide. To obviate the need for taking into account the difference in molar mass, specific heats are given in Table 3 in J/(g K).…”

“…Research interest in oxide compounds with the pyrochlore structure, A 2 B 2 O 7 , is due to the fact that such compounds possess a variety of properties of practical importance, such as ionic conductivity, superconductivity, luminescence, ferromagnetism, and catalytic activity [1][2][3][4][5][6][7][8][9][10]. In particular, such materials include rare-earth stannates, R 2 Sn 2 O 7 .…”