Type of Document	Dissertation
Author	Song, Honghai
Author's Email Address	hs06d@fsu.edu
URN	etd-07032010-175209
Title	Microscopic observations of quenching and the underlying causes of degradation in Y-Ba-Cu-O coated conductor
Degree	Doctor of Philosophy
Department	Electrical and Computer Engineering, Department of
Advisory Committee	Advisor Name Title Victor DeBrunner Committee Chair Justin Schwartz Committee Co-Chair Jim P. Zheng Committee Member Petru Andrei Committee Member Thomas Baldwin Committee Member David C. Larbalestier University Representative
Keywords	YBCO Coated Conductor Quench and Stability Quench Propagation Mechanism Quench Degradation Mechanism Magneto-optical Imaging Microscopic Observations
Date of Defense	2010-05-03
Availability	unrestricted
Abstract Significant advances have been made in the processing and scale-up of YBa2Cu3O7-δ (YBCO) coated conductor (CC) and sufficient lengths up to 1 km for YBCO coils are now available. This progress is very promising for a wide range of applications, including high field magnets and electric power systems. One of the remaining issues in the transition from long length conductor to functional coils which needs to be addressed, however, is quench protection, which requires a detailed understanding of dynamic thermal and nonlinear electromagnetic behaviors during a quench. These behaviors remain poorly understood. Foremost, to have a complete description of macroscopic behaviors of YBCO using traditional voltage and temperature characterization, from high temperature to 4.2 K, and from short, straight sample to long length coil, we carried out measurements on short straight YBCO CC at 4.2 K and conduction-cooled YBCO CC pancake coils. We found that, for the same fraction of critical current (I/Ic) at 4.2 K, YBCO CCs have similar minimum quench energy (MQE) and normal zone propagation velocity (NZPV) to that of Ag-alloy clad Bi2Sr2CaCu2Ox wires and significantly higher MQE and lower NZPV than MgB2 round wires of similar Ic(4.2 K). In the conduction cooled YBCO coils, the longitudinal NZPV (10 – 40 mm/s) is about one order of magnitude larger than the transverse NZPV (1 – 2 mm/s). Moreover, when coil results are compared to those of a short straight sample at 50 K, the longitudinal propagation in the short sample is significantly faster than the longitudinal propagation in the coil. This is due to transverse heat conduction (transverse propagation) which reduces the temperature gradients in the coil but also slows down the longitudinal propagation. Moreover, we simultaneously observe normal zone propagation during a heater-induced quench using a high-speed, high-resolution CCD camera with magneto-optical imaging (MOI) and monitor the voltage and temperature distribution as a function of time. We for the first time present the real-time, dynamic observation of magnetic field redistribution during a thermal disturbance via magneto-optical imaging. The optical images are converted to a two-dimensional, time-dependent data set that is then analyzed quantitatively. We found that the normal zone propagates non-uniformly in two dimensions within the YBCO layer and that its normal front has a diagonal shape while propagating along the length. Two stages of normal zone propagation are observed. The normal zone propagation velocity at 45 K, I = 50 A (~50% Ic), is determined as 22.7 mm/s using the time-dependent optical light intensity data. If time for current redistribution can be reduced, the quench propagation velocity is likely to be increased. Lastly, to understand the failure mechanisms during quenching, YBCO coated conductors were quenched such that the samples were degraded at different levels and the microstructure was locally evaluated in the degraded zones. To evaluate the microstructures, the Cu and Ag layers were etched from both quenched and unquenched control samples for comparison. In the control samples, the YBCO layer is found to have some porosity and a few distributed particles in and above the YBCO surface. Two types of quenched samples were prepared. One quenched sample showed nearly no reduction in end-to-end Ic after the quench series and the other was somewhat damaged. In the sample without any reduction in Ic, the Ag cap, however, was found to be partially broken, which is likely due to inhomogeneous heat deposit from the quench heater. In the sample with degradation in superconducting properties, although the degradation zone was eroded by the etchant, the reactants became a signature of the Ag delamination where there is degradation. Quench propagation induced damage is believed to originate from pre-existing edge damage in the YBCO CC. The damage propagation starts from the damaged edge was influenced by a thermal gradient along the normal zone. Meanwhile, more damage is likely happen to the other edge of the conductor due to current redistribution starting from both edges. For the first time, we report that quench induced degradation has a dendritic structure, which may come from the thermomagnetic instability in YBCO layer during quenching. The presence of dendritic structure implies that the YBCO has delamination from the Ag/Cu layers. The damage caused by the point defects in the YBCO structure is circular in shape and is not dependent on normal zone propagation. Another interesting feature among the results is that melted Ag particles were found in the degradation zone which implies that the local temperature at defects is very high during quenching.
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