An investigation on the mechanical and electrical properties of multilayer brittle materials

This dissertation comprises two parts: Part I, Mechanical behavior of multilayer brittle materials and Part II, Fabrication, microstructure and properties of Bi(2223)/Ag superconductor tapes.; In Part I, experimental study was conducted to verify a strengthening model which proposed that weak brittle thin films could be strengthened by stacking together to form a multilayered structure. Al₂O₃ thin film was coated on a quartz substrate, and cracks of various lengths were introduced to the film before mechanical test. Propagation of these cracks was found to be suppressed and all cracks, regardless of length, started to propagate at a critical stress value. To quantify the strengthening effect, the strength of notched thin mica sheets without constraint and the strength of notched mica sheets that were sandwiched between two other unnotched mica sheets were directly measured in uniaxial tensile test. The strength of notched mica sheets was significantly increased due to the constraint from adjacent sheets in the sandwich samples. The strength of the notched mica sheet in the sandwich samples had an approximate power-law relationship with sheet thickness, which was in agreement with the strengthening model given by Yang.; In Part II, silver-sheathed (Bi, Pb)₂Sr₂Ca ₂Cu₃O_y superconductor tapes were fabricated by a powder-in-tube technique. It was found that repeated cold pressing and sintering were beneficial to the increase of the critical current density of Bi(2223)/Ag tapes. Microstructure observation indicated that dense and well-aligned Bi(2223) phase platelets contributed to the high critical current density of the Bi(2223)/Ag superconductor tapes. Transport critical current density measurement in the presence of an external magnetic field revealed that textured Bi(2223)/Ag tape had poor flux pinning around 77 K, while having strong flux pinning at 4.2 K. Hot-pressing was used to densify the superconductor inside a Bi(2223)/Ag tape with the objective of increasing the critical current density of the tape. The highest J_C increase was on a rolled tape whose J_C had been increased by 260%, from 9700 A/cm2 to 35200 A/cm2. Strong links and weak links between grains and colonies of grains form the intergranular paths carrying supercurrent in Bi(2223)/Ag superconductor tapes. The reason why a hot-pressed tape exhibits a higher critical current density at zero magnetic field, and poorer J_C-B characteristics than an original tape might be that hot-pressing increases more weak links than strong links although both types of links have been increased.