Abstract:

"The kinetics of the oxygen exchange in cuprate barium-yttrium, alloyed by the samarium"

Kirichenko N.

RUS

The superconductivity is an effect of electric resistance disappearing in some metals, alloyes and chemical compounds under low temperatures.

Cuprate barium-yttrium is one of the high-temperature superconductors. Because of its properties these materials are perspective in different fields of technology. They would be able to be used in powerengineering, chemistry, electric equipment, etc.

The properties of this compound oxide depend on oxygen content in it, the character of the oxygen entry and strenght of oxygen connection in cuprates. So, the aim of this work was to research oxygen behaviour in YBa₂Cu₃O_x, if the oxygen had been substituted by the samarium.

Let's invastigate the properties of the YBa₂Cu₃O_x. There is a structure of the crystal lattice of this material (fig. 1).

Fig. 1 - The YBa₂Cu₃O_x tetragonal-to-orthorhombic structure transition

The crystal YBCO has a metal conductivity under enough high temperature. If the temperature is decreasing there is a transition into dielectrics or superconductors depending on the value x.

The oxygen stoichiometry influences on the superconducting properties of YBa2Cu3O7-x mainly. It is typical for them to exist of two polymorphic structures: there are tetragonal (x > 0,5) and rhombic (orthorhombic) (0 < x < 0,5) structure.

The synthesis of ceramic high-temperature superconducting materials is carried out at the temperature over 900⁰C. The oxygen content at these temperatures is 6,2 atoms per formal one and the quenching of the sample causes the formation of YBa₂Cu₃O_7-x tetragonal nonconducting structure.

When YBa₂Cu₃O_7-x is rich of oxygen it is taking place the transition from tetragonal into orthorhombic structure (x < 0,5). The temperature of this transition depends on oxygen partical pressure.

The phase transition is taken place if the temperature and the content (the oxygen content) of phase have changed. The changing of the structure is insignificant.

When the transition from tetragonal to orthorhombic structure is taking place in addition to the changing of the oxygen stoichiometry the rebuilding of the oxygen underlattice can be found.

As for all mentioned above, the properties of HTSC-materials depend on their oxygen content. So, the scientists need to deal with the purpose to find HTSC of the new generation. The main direction of this research is to build new compound - chains systems (CuO₂)_n with different layer configurations. It could be reached by the selection of new stoichiometric contents, t.e. by the partical substitute of barium to its izoelectronic analogue - Sm.

We have done the following experiments. The cuprate samples YBa_2-zSm_zCu₃O_x were made from the charge of yttrium oxide Y₂O₃, cuprium oxide CuO, samarium oxide Sm₂O₃ and barrium carbon BaCO₃. The compounds were splintered with great care in an agatium plate under an etilliquid layer. The samples were produced according to ceramic technology. The cuprates were being synthesed during 20 hours at 930⁰C with two intermediate crushings. Then the samples were sintered in the air at 940⁰C.

After this we did the thermocyclization. It is the heating and cooling of the samples in turns from 20 to 9000C in the gravimetric plant (fig. 2).

Fig. 2 - The gravimetric plant

The samples (3-4 g) were hung up on the platinum thread inside vertical silica reactor with two sections. As the result of this research of YBa_1,9Sm_0,10Cu₃O_x oxygen content there has been discovered the anomal oxygen relaxation provided by the cuprate thermocyclization. There was some gysterezis at the area of the tetragonal-to-orthorhombic phase transition (fig. 3,4).

Fig. 3, 4 - The gysterezis of the YBa_2-hSm_hCu₃O_x samples

But there wasn't any gysterezis for the samples, which hadn`t been alloyed by samarium.

Because of this we decided to build the cooling and heating curves (fig. 5).

Fig. 5 - The YBa_1,90Sm_0,10Cu_2,995Li_0,005O_x changing of oxygen content

In conclusion we may say, the analysis of the experimental data has confirmed once again the presence of 2 forms of nonstoichiometrical oxygen, which exhibit themselves in the temperature's range (740-840⁰C). We have also done the radiographical analysis of the samples. As a result of it we have got the diffractograms (fig. 6), using which we were able to calculate the crystal lattice parameters (table).

Fig.6 - The different oxygen content samples` diffractograms

Table

The YBa_2-hSm_hCu_2,995Li_0,005O_x crystal lattice parameters