No abstract

SEM, TEM, SAM and X-ray diffraction analyses have suggested that the bulk YBCO samples sintered with the co-precipitation powders are almost free of second phrases. However, a layer with non-123-stoichiometric compositions is always present near the grain boundaries, which forms the dominant zone of the weak links in these samples. The improved critical current density with reducing field dependence can be obtained by the melt-process. It has produced a multiphase microstructure with highly aligned plate-like 123 grains up to several mm in length and several mu m in thickness, which are parallel to the a-b plane, and the grain plate sometimes is the same width as the sample, and boundaries between parallel plates are usually discontinuous. Such structure is very favourable for the improvement of Jc. However, there are still various structure defects, such as second phases (CuO, BaCuO2, BaCO3 and 211 phase), cracks and some interfaces between adjacent superconducting regions with different c axis direction in the melt-processed samples. They would limit further improvement of the Jc of melt-processed samples.

The coating material of a tool directly affects the efficiency and cost of machining malleable cast iron. However, the machining adaptability of various coating materials to malleable cast iron has been insufficiently researched. In this paper, turning tests were conducted on cemented carbide tools with different coatings (a thick TiN/TiAlN coating, a thin TiN/TiAlN coating, and a nanocomposite (nc) TiAlSiN coating). All coatings were applied by physical vapor deposition. In a comparative study of chip morphology, cutting force, cutting temperature, specific cutting energy, tool wear, and surface roughness, this study analyzed the cutting characteristics of the tools coated with various materials, and established the relationship between the cutting parameters and machining objectives. The results showed that in malleable cast iron machining, the coating material significantly affects the cutting performance of the tool. Among the three tools, the nc-TiAlSiN-coated carbide tool achieved the minimum cutting force, the lowest cutting temperature, least tool wear, longest tool life, and best surface quality. Moreover, in comparisons between cemented-carbide and compacted-graphite cast iron machined under the same conditions, the wear mechanism of the coated tools was found to depend on the cast iron being machined. Therefore, the performance requirements of a tool depend on multiple factors, and selecting an appropriately coated tool for a particular cast iron material is essential.

The morphology, composition and phases of grain boundaries in YBa 2 Cu 3 O x superconductors have been investigated by SEM, TEM and AES. The results show that some impure phase particles present at the grain boundary in the samples sintered at 940°C for 26 hours, which grow to be a shell around the grain as the sintering temperature increases to 970°C. No such particle has been found at the grain boundaries in the sample sintered at 920°C for 26 hours by SEM, Jc is the highest in our studied samples. AES and TEM analyses indicate that the grain boundaries are complex multilayer structures consisting of barium-rich, copper-rich, carbon-rich, yttrium and oxygen-deficient impure phases. The dominant impure phases harmful to Jc at the boundary are the mixtures of BaCO 3, Cu 2 O , BaC 2, Y 2 C 3 and an amorphous material.

The microstructure, composition, morphology and distribution of second phases in the YBCO superconductor have been investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning Auger microprobe (SAM) and optical polarising microscopy (OPM). The vast majority of 123 grains show the 'twin layer' structure (with 5-100 nm width). The twin boundary is usually a clean interface. A few second-phase domains are present between 123 grains (which have an average size of 1 mu m). Some microdomains ( approximately several tens of nm) of non-superconductive phase are present in a 123 grain. The majority of grain boundaries in the authors samples are free of second phases. A few of the boundaries usually contain second-phase particles and amorphous material and have a multilayer structure. SAM data suggest that these boundaries are rich in Ba and C and deficient in Cu and Y. SEM of the boundaries show that the second-phase particles contain barium carbonate, copper oxide and other carbides. The influence of second phases on the superconductivity of materials is also discussed.