No abstract

There have been a number of small contradictions in reports of the structure of the recently discovered thallium superconducting materials, in particular the phase TI,Ba,Ca,Cu,O,, hereafter abbreviated to 2223. The structure has been reported to be pseudo-tetragonal or 14/mmm with a c axis of 3.625, 3.61 or 3.59 nm, and an a axis of 0.54, 0.547 or 0.385 nm. We report here results obtained by high-resolution electron microscopy and diffraction which show that these results are not inconsistent with each other. The a axis is around 0.385 nm, and the cia axis ratio and the experimental images are genuinely variable, which we suspect is due to variations in the oxygen stoichiometry. The earlier reports of an a axis of 0.547 nm are due to loss of symmetry caused by intergrowths. We also report evidence for polytypism based around half unit cell blocks involving the local stoichiometries . In all these materials the a axis appears to remain the same but there is a small systematic contraction in the local Cu-Ca-Cu perovskite spacing along the c axis as the number of perovskite units increases, which we suggest could be linked to the superconducting transition temperature.

A new accurate two-probe time domain method is put forward to measure the straight-going component motion error in machine tools. The characteristics of non-periodic and non-closing in the straightness profile error are liable to bring about higher-order harmonic component distortion in the measurement results. However, this distortion can be avoided by the new accurate two-probe time domain method through the symmetry continuation algorithm, uniformity and least squares method. The harmonic suppression is analysed in detail through modern control theory. Both the straight-going component motion error in machine tools and the profile error in a workpiece that is manufactured on this machine can be measured at the same time. All of this information is available to diagnose the origin of faults in machine tools. The analysis result is proved to be correct through experiment.