Study On The Stability Of MgB₂Superconducting Wires

MgB2superconductor with advantages of simple fabrication techniques, short product period, high critical current density, low cost and suitability to work in cryogenic refrigerator, emerges to be a promising candidate superconductive material in large-scale applications such as magnetic resonance imaging (MRI). Just as for low temperature superconductors (LTS), the success of MgB2in high current applications, such as magnets and power electrical devices, relies on the development of practical conductors with sufficient stability and appropriate cryogenic design of the winding structure for quench protection. Understanding the quench process from initiation to propagation is a key element in this development-design cycle. The work in this thesis is basic on the projecf "Stable Mechanism of Muti-filaments MgB2Superconducting Wires" which is supported by National Nature Science Foundation of China. The quench parameters including minimum quench energy (MQE) and quench propagation velocity of MgB2superconducting wires are studied.This thesis covers two main parts. On the one hand, conductors with different stability are designed by changing the number of filament (mono-,6-,12-and36-filaments), the sheath metal materials (Nb and Fe) and the doped materials (non-, C-and TiC-doped). MgB2short wires are fabricated by the powder-in-tube (PIT) technique. The samples are analyzed by X-Ray diffraction (XRD) and scanning electron microscope (SEM). Magnetic properties, critical current and critical temperature of the conductors are also measured.On the other hand, quench process of superconducting MgB2wires are studied experimentally and numerically, focusing on the estimation of the quench propagation velocity and minimum quench energy. The measurements are taken at self-field in cryogenic refrigerator condition at20K with variable transport currents. Experimentally, energy pulses are deposited into the wire by loading current to the resistive heater. The electric field profiles and their time evolution are measured from multiple voltage taps along the wire. Finite element method (FEM) is used to solve the one-dimensional heat balance equation of the system to study the quench process numerically. The simulation results are in qualitative agreement with experimental ones. We analyze quench process of MgB2wires with different volumetric ratio of copper to MgB2, thermal conductivity, specific heat and resistivity from experiment and theory of stability. We also calculate the quench propagation velocity and minimum quench energy as function of working temperature, thermal conductivity, specific heat and volumetric ratio of copper to MgB2.The experimental results show that MQE decays exponentially as transport current increases from40A to80A, and simultaneously the propagation velocity increases linearly. Numerical results show that MQE is positive correlation to thermal conductivity, critical current, and difference between working temperature and critical temperature, and is negative correlation to transport current and total resistivity, and is proportional to specific heat. The relation between MQE and volumetric ratio of copper to MgB2is not monotonic. There is a moderate volumetric ratio of copper to MgB2ranging between10and20at which region we get higher MQE and better stability.