Modeling of Electromagnetic and Superconducting Properties of HTSC
There are two basic approaches for description of intercrystalline structures, namely (i) the structural unit approach, focuses on the atomic arrangement at the intergranular boundary and (ii) the intergranular boundary dislocation (IBD) approach, based o
- PDF / 1,531,283 Bytes
- 50 Pages / 439.37 x 666.142 pts Page_size
- 102 Downloads / 234 Views
Modeling of Electromagnetic and Superconducting Properties of HTSC
12.1 Modeling of Intercrystalline Dislocations There are two basic approaches for description of intercrystalline structures [77], namely (i) the structural unit approach, focuses on the atomic arrangement at the intergranular boundary and (ii) the intergranular boundary dislocation (IBD) approach, based on the periodic strain field that is observed at many intergranular boundaries. Both models use a coincidence site lattice (CSL) description of the intergranular boundary geometry. While these models are equivalent [88], each one has its own advantages in description of the intercrystalline structure. Nevertheless, today studies of intergranular boundaries in HTSCs focus primarily on the IBD description. The structure of intergranular boundary may be presented, consisting of two different types of parallel conducting channels, disposed along the boundary and defined by dislocation structure (see Fig. 12.1). One of them possesses approximate structure and properties of superconducting crystal, associated with the regions between dislocations. Other channel is normal one or demonstrating a weak superconductivity, and it is compared with dislocation cores or their elastic deformations [663]. When the misorientation angle, h, increases then the dislocation density increases, the interdislocation spacing diminishes which leads to gradual inhibition of the superconducting channels and to forcing of the weak link behavior. Therefore, an understanding of IBD nature is very important for statement of transition from the strong link behavior to the weak one. The IBD models are based on the concept that the grain boundary free energy is particularly low for a certain set of special misorientation relationships, h [hkl]. In this case, the boundaries with h [hkl] values other than special ones relax to a configuration in which sites of a low-energy structure are preserved by a localized rotation of the crystals. The macroscopic value of h [hkl] and its deviation from a low-energy misorientation are produced by the introduction of a regular array of dislocations that rotates one crystal relative to the other on a macroscopic scale. On the microscopic scale, these dislocations separate the sites of boundary with
I. A. Parinov, Microstructure and Properties of High-Temperature Superconductors, DOI: 10.1007/978-3-642-34441-1_12, Springer-Verlag Berlin Heidelberg 2012
595
596
12 Electromagnetic and Superconducting Properties of HTSC
(a) θ ≤ 5°−10° Strong Superconducting Channels
θ ≥ 5°−10°
Normal or Isolating Dislocations
Mutual Overlapping of Cores (Tunnel Barrier)
(θ ≥ 15°−−20°)
(b)
1 Current-Currying Channels with Weak Superconducting Links
Cores of Normal or Insulating Dislocations
(θ ≤ 20°)
(c)
Strong Superconducting Channels
Regions of Strong Links Core of Normal or Insulating Dislocation
Fig. 12.1 Three models explaining intergranular values of Jc in thin YBCO films with [001] direction versus intergranular boundary structure: (a) model of
Data Loading...
