Preparation of a superconducting ceramic of YBa2Cu3O7

The superconducting state[1] was discovered in 1911 and since then it has revolutionized the technological world with various inventions such as magnetic resonance machines, magnets that guide particles in an accelerator (LHC) and even in the railway industry.

In the decade of the  80s the issue of superconductivity took greater relevance with the discovery of superconducting ceramics, based on copper synthesized by Georg Bednorz and Alex Müller who were awarded in 1987 the Nobel Prize[2] due to their contributions to the physics of materials, thus starting the race for superconductors of high critical temperature[3].

There are currently many types of superconductors with a wide variety of chemical compositions.

The _{-d  value} ranges from 0.1 to 0.5

When the compounds cool below their critical temperature, the material enters a superconducting state, due to which the electrical resistance of the material decreases to a point close to zero and a reaction is generated in the presence of an external magnetic field and repels it, this phenomenon is known as the Meissner effect, which is also responsible for the well-known magnetic levitation in superconductors.

Superconductors have limits on the magnetic field they can repel and the current they can conduct which are known as critical magnetic field Bc and critical electric current I_c, when one of these two values is exceeded the superconducting state disappears.[4]

There are two types of superconducting materials:

Type I: Within this classification we usually find metals that are subjected to liquid helium temperatures:

-Hg (Mercury) with a Tc ≈ 4.2 Kelvin

-It is (Tin) with a Tc ≈ 3.7 Kelvin

-Pb (Lead) with a Tc ≈ 7.2 Kelvin

-Zn (Zinc) with a Tc ≈ 0.9 Kelvin

Type II: Among the materials that fall into this classification are:

-ceramics of Cuprates;

La2-XBaCuO with a Tc ≈ 30 Kelvin

YBa2Cu3O7-d with a Tc ≈ 92 Kelvin

-Other;

NbTi (Neobio-Titanium) with a Tc  ≈ 10 Kelvin and Bc of 10 Teslas.

This discipline has searched for decades for a unified equation that explains the superconducting state.

For more than 40 years there was no theory that explained the superconducting state, but with the introduction of the concepts of quantum mechanics came a tool that helped scientists understand this strange state of matter.

Thus arose the theory of Soviet physicists Vitaly Ginzburg and Lev Landau of 1950 which explains the operation of the phase transition that a superconductor undergoes when cooling below its critical temperature.[5]

This theory is complemented by the BCS theory[6] named after its creators  J. Bardeen, L. N. Cooper* ^and J. R. Schrieffer which  managed to explain the behavior of superconductors created until then with the theoretical model of a new quasiparticle called phonon, which corresponds to the collective vibrations of the atoms of a crystalline solid in a medium of Cooper pairs.

Together these two theories manage to explain type I superconductivity well, but it is flawed in trying to explain type II superconductivity.

Finally, we have the theory of Lev Landau's doctoral student; In 1957 Alexei Abrikosóv theorized that knowing that in a type II superconductor there are areas that are superconducting and areas that are not, with this in mind Abrikosóv used the concepts of topology[7] to explain the magnetic field vortices that penetrate the type II superconductor and how they move along the material under the influence of an external magnetic field.[8]

In YBCO ceramics there is a periodic crystal structure of perovskite-orthorhombic type with ordered CuO2 planes which allow us to explain its superconducting state as follows:

YBCO is an anisotropic compound due to its laminar structure. It is formed by an arrangement of conductive planes of copper and oxygen (CuO2), through which the superconducting current flows, separated by charge reserve blocks that allow modifying the number of carriers in the CuO2 planes. This structure gives rise to an anisotropic behavior, both in the normal state where we have a resistivity in the c direction greater than that we have in the ab plane (rc / rab ~ 50) and in the superconducting properties. The amount of current that can circulate parallel to the CuO2 planes (planes ab) is greater than that which can pass through the c-axis.[9]

When performing heat treatments, an important part of the oxygen in the composition is lost, due to this we must perform oxygenations to adjust the stoichiometry.[10]

When performing the synthesis of the material, two different oxygen tanks were used, which may give us a reason for the differences in oxygenation between the two samples.

In sample 1 an old tank of approximately 3 years of use was used while in the second a new tank that was purchased two days before being used was used.

In addition to XRD[11] , it was determined if the material had deficiencies or excess oxygen in its composition by observing its coloration:[12]

Bluish: oxygen deficiency in its composition (presence of cations).

Greenish: Excess oxygen in the sample (presence of anions).

[1] Superconducting state:

It is defined as the state in which a compound exhibits perfect conduction of electric current and shielding against magnetic fields. (REF. Introduction to superconductors: Yesenia Arredondo León, p. VII)

[2] The Nobel Prize in Physics 1987. NobelPrize.org. Nobel Prize Outreach AB 2023. FRI. 30 jun 2023. <https://www.nobelprize.org/prizes/physics/1987/summary/>

[3] It is the temperature below which the transition from normal to superconducting state takes place.

[4] https://wp.icmm.csic.es/superconductividad/superconductividad/parametros-criticos/#:~:text=Corriente%20cr%C3%ADtica%3A%20La%20corriente%20en,material%2C%20la%.

[5] Physics 127c: Statistical Mechanics Superconductivity: Ginzburg-Landau Theory

[6] https://journals.aps.org/pr/abstract/10.1103/PhysRev.108.1175

[7] "Study of geometric figures due to their respective properties and positions, without considering their shape or size" https://etimologias.dechile.net/?topologi.a#:~:text=La%20palabra%20%22topolog%C3%ADa%22%20est%C3%A1%20formada,considerar%20su%20forma%20o%20tama%C3%B1o%22.

[8] TYPE II SUPERCONDUCTORS AND THE VORTEX LATTICE Nobel Lecture, December 8, 2003, by Alexei A. Abrikosóv.

[9] PREPARATION OF SUPERCONDUCTING OXIDE YBa2Cu 3 OR 7-x BY THE SOL-GEL METHOD A. Bustamante, A. Osorio, J.C. González, M. Carhuancho, N. Salas, L. De Los Santos, N. De La Cruz and A. Díaz Faculty of Chemistry and Chemical Engineering. Universidad Nacional Mayor de San Marcos.

[10] https://cdigital.uv.mx/bitstream/handle/123456789/5368/199726P47.pdf?sequence=2&isAllowed=y

Characterization of YBCO superconducting samples. - José Sergio Duran Niconoff.

[11] R-X diffractometry.

[12] Since ancient times, compounds of copper anions and cations have been used for the production of pigments: Eastaugh, N. et al: "Pigment Compendium: A Dictionary and Optical Microscopy of Historical Pigments". Butterworth-Heinemann, 2008.