Although the phenomenon of superconductivity was discovered very early, the foundation of superconductivity theory was established in the 1930s, and the microscopic theory of superconductivity appeared in the 1950s. However, the breakthrough in application was after the 1960s. Then a series of superconducting alloys and compounds such as Nb-Zr and Nb-Ti appeared, gradually forming a new technical field-superconducting technology.
Since the breakthrough of high-temperature superconducting materials in the 1980s, leading to the rise of "superconducting" in the world, the world's research on superconducting materials has entered a new stage for more than half a century. However, due to the rapid development and short time, related theories are still being gradually formed and explored. Whether a new ceramic superconductor with more practical value can be made is still under continuous research. Therefore, this chapter only gives a general introduction to superconducting ceramics.
Superconductor refers to a substance whose resistance suddenly becomes zero when it cools to a low temperature, and at the same time the substance loses magnetic flux and becomes a completely diamagnetic substance. Every kind of superconductor has a certain superconducting transition temperature, that is, the temperature at which a substance transforms from a normal state to a superconducting state is called the superconducting critical temperature (Critical Temperature) and is represented by Tc. The superconducting critical temperature of different superconducting materials is different. The superconducting critical temperature is generally expressed in absolute temperature (K).
To judge whether a material is superconducting, there are two basic characteristics: superconductivity, which refers to the property of the material to lose resistance at low temperatures; fully diamagnetic, which means that the superconductor is in an external magnetic field and the lines of magnetic force cannot penetrate, and the magnetic flux in the superconductor is zero.
In short, the superconductivity of superconductors depends on the temperature, magnetic field, and current density. The upper limits of these conditions are called critical temperature (Tc), critical magnetic field (Hc), and critical current density (Ic). From the practicality of superconducting materials, in the final analysis, the most important thing is how to improve these three physical properties.
2. Classification of superconductors
The classification of superconductors is currently unclear, and it is difficult to classify them. However, as far as known superconductors are concerned, they can be roughly classified as follows:
(1) From the classification of materials, it can be divided into three major categories, namely elemental superconductors, alloy or compound superconductors, and oxide superconductors (namely superconducting ceramics).
(2) According to the low temperature treatment method, it can be divided into liquid helium temperature zone superconductor (below 4.2K), liquid hydrogen temperature zone superconductor (below 20K), liquid nitrogen temperature zone superconductor (below 77K) and normal temperature superconductor.
Elemental superconductors, there are currently more than 20 kinds of known, including Be, Al, Ti, V, Zn, Ga, Ge, Zr, Nb, Mo, Tc, Ru, Cd, In, Sn, La, Ta, W, Re , Os, Ir, Hg, Tl, Pb, Th, U, etc. Among them, the critical temperature of Nb is the highest (9.1K). Alloy or compound superconductors currently have more than 1,000 kinds of Nb3X and V3X with the highest critical temperature (where X can be Ga, Al, Si, or Ge or Sn), and alloy Nb3Ge has the highest critical temperature (23.2K) .
Oxide superconductors (Superconducting Ceramics) are newly developed superconducting materials. The critical temperature of this type of superconducting ceramics is different in reports due to different researchers and different process methods.
lanthanum oxide super-conducting ceramic
3. The crystal structure of superconducting ceramics
The molecular formula of oxide superconducting ceramics YBa2Cu3O7-x, Y can be replaced by other rare earth elements, especially heavy rare earth elements. After replacing Y with Gd, Dy, Ho, Er, Tm, Tb and Lu, the corresponding superconducting single phase or Multiphase materials.
YBa2Cu3O7-x has two phases, one is the tetragonal phase (P4m2) and the other is the orthogonal phase (Pmmm), both of which are derived from the ABO3 perovskite structure.
It has a tetragonal structure at high temperature and an orthogonal structure at low temperature. The transition temperature is between 600 and 700°C, which is an order-disorder transition. The orthogonal phase is a high-temperature superconducting phase, and the tetragonal phase is a semiconductor.
Some of the crystal structure of superconducting structure has been determined, and some have not yet been. According to the analysis of the crystal structure, the reason for the high Tc or the superconductivity lies in the Cu-O layer.
With the in-depth research and development of superconductors, the relationship between its crystal structure and superconductivity will continue to be revealed.