Thermomechanical properties of fine-grained, 96% dense, polycrystalline BaZrO3 specimens were studied. The average thermal expansion coefficient of BaZrO3 at 25–1000°C was 7.72×10−6°C−1. At 25°C, Young's modulus was 240.5 GPa and the shear modulus was 97.2 GPa. These values decreased by ≈10% with heating to 1015°C. Poisson's ratio was 0.237 and was independent of temperature. The BaZrO3 specimens exhibited a thermal expansion coefficient of about one half, and elastic moduli more than twice, those of polycrystalline high-temperature superconductors. Diffusional creep was observed when BaZrO3 was compressed at 1300–1400°C; the activation energy for steady-state deformation was 460±30 kJ/mole. The mechanism was probably grain-boundary sliding, controlled by diffusion of one of the cations along grain boundaries. Deformation rates below 1000°C were too low to relax thermally generated stresses.

Thermomechanical response of polycrystalline BaZrO3

The velocities of longitudinal and shear ultrasonic waves propagated in very dense (95% of theoretical), highly-textured, ceramic Bi2Sr2CaCu2O8+y (Bi2212) have been measured as functions of temperature (between 10 and 290 K) and hydrostatic pressure at 290 K. The main finding is a marked anisotropy of the elastic stiffness constants and their dependences on pressure. The authors have observed an elastic stiffness constant in the a-b plane that is substantially greater than that in the c direction. This observation is consistent with the crystallographic structure of Bi2212. The anomalous elastic effects and thermal hysteresis, observed previously by several groups in various Bi-based superconductors in the range of 19-240 K, have not been found in the present material. These observations suggest that these anomalies are not an inherent property of Bi-based cuprates, but instead may depend on the oxygen content and microstructure of the ceramic material. An analysis of sound velocity in a polycrystalline array of elastically anisotropic crystallites is presented.

Anisotropic elastic and nonlinear acoustic properties of very dense textured Bi2Sr2CaCu2O8+y

The electronic band structure probed by angle-resolved photoemission spectroscopy on thin epitaxial La2–xSrxCuO4 films under extreme tensile strain shows anomalous features compatible with c-axis dispersion. This result is in striking contrast with the usual quasi-two-dimensional (2D) dispersion observed up to now in most superconducting cuprates, including relaxed and compressively strained La2–xSrxCuO4 films grown under the same conditions. The data were analyzed using a 3D tight-binding dispersion for a body-centered-tetragonal lattice. We relate the enhancement of the c-axis dispersion to the significant displacement of the apical oxygen induced by epitaxial strain.

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Three-dimensional dispersion induced by extreme tensile strain in La_(2−x)Sr_xCuO_4 films

▪ Abstract The layered perovskite cuprate materials are a unique class of superconductors with unusual normal-state and superconducting properties. The common physics to all these materials is that of the underlying CuO2 planes. This review provides a survey of and guide to their physical properties as it relates to the superconductivity of this interesting group of conducting oxides.

The new generation high-temperature superconductors

Ultrasonic velocity measurements on superconducting (x = 0, 0.1 and 0.2) with and non-superconducting were performed at 10 MHz using the pulsed echo overlap ultrasonic system. Pr substitution in the samples caused a systematic increase in the absolute shear and longitudinal velocities. The reduction of oxygen content to , however, caused a decrease in both absolute velocities. The and samples showed a similar per cent change in velocity of about 3.8% (longitudinal) between 80 K and 300 K and 2.3% (shear) between 80 K and 220 K. In addition, a longitudinal velocity hysteresis was observed in both samples between 190 K and 220 K. A smaller per cent change in velocities was observed for the superconducting and non-superconducting samples. The effects of Pr substitution and oxygen depletion on the change of velocity and the 190 K-230 K longitudinal velocity hysteresis were explained in terms of possible oxygen ordering in the materials.

The effects of oxygen content and Pr substitution on ultrasonic velocity in superconductors

The results of ultrasonic, sonic and infrasonic experiments in high-Tc superconductors (HTSC) are reviewed. Some other related fields of investigation are also mentioned.

https://iopscience.iop.org/article/10.1088/0953-2048/6/3/001/pdf

J Dominec

Papers

Ultrasonic and related experiments in high-Tc superconductors