Analysis Method of High-Field Pinning Centers in NbTi Wires and MgB2 Wires

Abstract

The main purpose of this article is to show an easier and more accurate method for analyzing high-field pinning centers. Previous methods did not allow for precise analysis and research on high-field pinning centers, they studied only the dominant pinning mechanism, and they required a maximum of pinning force (F pmax) to analyze pinning centers. The main advantage of this method can be used in samples without the maximum pinning force required by other methods to analyze the pinning mechanism. In addition, this method allows us to observe even small changes in the density of individual pinning centers. This method is based on analysis of critical current density values in different ranges of reduced magnetic field. The method of analysis of highfield pinning centers allows primarily to develop methods and directions for creation of high-field pinning centers. The surface pinning centers are strong pinning centers in low magnetic fields and very weak pinning centers in middle and high magnetic fields. Additionally, the analysis of pinning centers shows that dislocations, strains, substitutions in the crystal lattice, and precipitation inside the grains create strong pinning centers in high magnetic fields and very weak pinning centers in low and middle magnetic fields. Research indicates that strains, substitutions in the crystal lattice, and precipitates inside grains form pinning centers that operate in the high magnetic field range.

Relation between Crystal Structure and Transition Temperature of Superconducting Metals and Alloys

“…Again, Nb is assumed to give one electron. This yields a T c(0) of 9.81 K, which is close to the most common experimental value [72,[74][75][76]. One has to note here, however, that the present form of the Roeser-Huber equation does not account for the chemical variation in an alloy: At a Ti-content of 50%, statistically, every second atom in the Nb based bcc lattice is replaced by Ti, which implies a difference in size of the ions, as well a chemical variation (number of electrons).…”

“…There are three martensitic phases (α , α and ω), which are formed during the fast cooling of the NbTi material. All these phases may form precipitates in the final product, as well as Ti precipitates with the hcp structure [72]. The residual resistivity, ρ 0 , depends on the Ti content.…”

“…This implies that for the calculation of T c , we must consider the Nb base lattice with the modifications introduced by the percentage of Ti. This is also manifested by the small range between 30% and 60%, where NbTi shows the best superconducting properties [72]. The problem is that there are not many reports in the literature concerning the crystal parameters of NbTi and the correlation to the superconducting transition temperature [70,72,73].…”

“…This is also manifested by the small range between 30% and 60%, where NbTi shows the best superconducting properties [72]. The problem is that there are not many reports in the literature concerning the crystal parameters of NbTi and the correlation to the superconducting transition temperature [70,72,73]. Thus, we employ here for an estimation the data of a wire manufacturer and the data of [74][75][76], with a = 0.3285 nm.…”

Effect of Heat Treatments under High Isostatic Pressure on the Transport Critical Current Density at 4.2 K and 20 K in Doped and Undoped MgB2 Wires

“…The analysis of the MgB 2 structure shows that the volume decreases by 25% after a reaction [6]. This in turn causes a reduction in the connections between grains and creates surface pinning centers that significantly decrease the critical current density (J c ) in middle and high magnetic fields [7,8]. Doping, e.g., SiC and C, promotes middle and high pinning centers, which increase J c in middle and high magnetic fields [7,[9][10][11][12].…”