About Magnets and Superconductors of Mr Scanners

The topic of this paper are parts of modern MR devices, in which the magnet windings are located. MR scanner magnets are made of four types of electromagnetic windings: 1) The main magnet, made of superconducting material, creates a variable magnetic field; 2) X coil, made of a resistive material, creates a variable magnetic field, horizontally, from left to right, across scanning tube; 3) Y coil creates varaing magnetic field, vertically, from botom to top; 4) Z coil creates varaing magnetic field, longitudinally, from head to toe, within scanning tube.Superconductors, which create the main magnetic field, should be cooled by liquid helium and liquid nitrogen. Main magnets made of superconductors should use cryostat, with cooling vessels with liquid helium and liquid nitrogen, thermal insulation and other protective elements of magnet system. The types of magnets that exist in the basic configurations of MR scanners are analyzed. Scanners in the form of a closed cylindrical cavity create their own, magnetic, fields by passing current through the solenoid, which is held at the temperature of the superconductor. The superconductors used exclusively are: niobium-titanium (NbTi), niobium-tin (Nb3Sn), vanadium-gallium (V3Ga) and magnesium-diboride (MgB2). Only magnesium diboride is a high temperature superconductor, with a critical temperature Tc = 390K. The three remaining superconductors are low temperature. New high-temperature superconductors have been discovered, as well as room-temperature superconductors. Newly discovered superconducting materials are not used in MR scanners. The magnet structure of the MR scanner is complex. The resonant frequency changes at each point of the field in a controlled manner. The windings of the main magnet made of superconducting material in the form of microsial fibers are built into the copper core. The nonlinear gradient field is created by windings of conductive material. It is added to the main magnetic field. Thus, the resulting magnetic field is obtained.

Fundamental physical and resource requirements for a Martian magnetic shield

Mars lacks a substantial magnetic field; as a result, the solar wind ablates the Martian atmosphere, and cosmic rays from solar flares make the surface uninhabitable. Therefore, any terraforming attempt will require an artificial Martian magnetic shield. The fundamental challenge of building an artificial magnetosphere is to condense planetary-scale currents and magnetic fields down to the smallest mass possible. Superconducting electromagnets offer a way to do this. However, the underlying physics of superconductors and electromagnets limits this concentration. Based upon these fundamental limitations, we show that the amount of superconducting material is proportional to $B_{\rm c}^{-2}a^{-3}$ , where Bc is the critical magnetic field for the superconductor and a is the loop radius of a solenoid. Since Bc is set by fundamental physics, the only truly adjustable parameter for the design is the loop radius; a larger loop radius minimizes the amount of superconducting material required. This non-intuitive result means that the ‘intuitive’ strategy of building a compact electromagnet and placing it between Mars and the Sun at the first Lagrange point is unfeasible. Considering reasonable limits on Bc, the smallest possible loop radius is ~10 km, and the magnetic shield would have a mass of ~ 1019 g. Most high-temperature superconductors are constructed of rare elements; given solar system abundances, building a superconductor with ~ 1019 g would require mining a solar system body with several times 1025 g; this is approximately 10% of Mars. We find that the most feasible design is to encircle Mars with a superconducting wire with a loop radius of ~3400 km. The resulting wire diameter can be as small as ~5 cm. With this design, the magnetic shield would have a mass of ~ 1012 g and would require mining ~ 1018 g, or only 0.1% of Olympus Mons.

First-Principle Studies on the Mechanical and Electronic Properties of AlxNiyZrz (x = 1~3, y = 1~2, z = 1~6) Alloy under Pressure

The elastic and electronic properties of AlxNiyZrz (AlNiZr, Al2NiZr6, AlNi2Zr, and Al5Ni2Zr) under pressure from 0 to 50 GPa have been investigated by using the density function theory (DFT) within the generalized gradient approximation (GGA). The elastic constants Cij (GPa), Shear modulus G (GPa), Bulk modulus B (GPa), Poisson’s ratio σ, Young’s modulus E (GPa), and the ratio of G/B have been studied under a pressure scale to 50 GPa. The relationship between Young’s modulus of AlxNiyZrz is Al5Ni2Zr > AlNiZr > Al2NiZr6 > AlNi2Zr, which indicates that the relationship between the stiffness of AlxNiyZrz is Al5Ni2Zr > AlNiZr > Al2NiZr6 > AlNi2Zr. The conditions are met at 30 and 50 GPa, respectively. What is more, the G/B ratios for AlNiZr, AlNi2Zr, Al2NiZr6, and Al5Ni2Zr classify these materials as brittle under zero pressure, while with the increasing of the pressure the G/B ratios of AlNiZr, AlNi2Zr, Al2NiZr6, and Al5Ni2Zr all become lower, which indicates that the pressure could enhance the brittle properties of these materials. Poisson’s ratio studies show that AlNiZr, AlNi2Zr, and Al2NiZr6 are all a central force, while Al5Ni2Zr is a non-central force pressure scale to 50 GPa. The energy band structure indicates that they are all metal. The relationship between the electrical conductivity of AlxNiyZrz is Al2NiZr6 > Al5Ni2Zr > AlNi2Zr > AlNiZr. What is more, compared with Al5Ni2Zr, AlNi2Zr has a smaller electron effective mass and larger atom delocalization. By exploring the elastic and electronic properties, they are all used as a superconducting material. However, Al5Ni2Zr is the best of them when used as a superconducting material.