| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 92 === There were cracks and voids exist in the single grained superconductors as growth and annealing process due to its difference in thermal expansion coefficients and thermal stresses. In practice, the Lorentz force resulting from the interaction between the current inside the superconductors and the external field results in strong stresses. The superconductors usually fractured when the external magnetic field exceeds 10 Tesla because of the huge stresses. The purpose of this experiment is to enhance the mechanical properties of YBCO-superconductors and capable of high external field.
In this experiment, the improvement of the mechanical properties of YBCO-superconductors is fulfilled by coating the commercial resins and carbon fabrics. When the coating process was operated in the atmosphere, the binding strength between the resins and the superconductor was poor. However, the operation in vacuum will enhance the binding strength.
For the un-coated samples, diminished flexural strength reveals while the size of the bulks are larger than the other small ones because of the un-uniform shrinkages occurring inside them. The flexural strength of the samples at 77K is about 140% of that at 300K, but the flexural modulus at low temperature is lower than that of high temperature.
At room temperature, coating resin on bulks was not good for samples at flexural strength, but decreases the flexural modulus. At low temperature, if we can improve the binding strength of the resin and the bulk, coating resin should be good for the strength of YBCO.
The highest flexural strength was obtained by coating the CF-coatings on the samples. Because the carbon fabrics have anisotropism, as the direction of the carbon fabrics are parallel to the external force, the resulting strength of the sample is the highest. At 77K, the flexural strength of the samples with CF-coatings reaches 253.8MPa, which is about 240% of that of the un-coated ones. Some oscillations were observed on the stress vs. strain curve, it means that some disjoint appeared between the bulk and the resin or between the resin and the carbon fabrics.
The samples at both room temperature and low temperature, the sequence of flexural modulus was in the following sequence of the un-coated samples, CF-coatings, and then the ones with resin-coatings.
|
|---|
