Effect of microcracking on the measured moduli of bulk YBa 2 Cu 3 O x
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I. INTRODUCTION
The mechanical properties of high Tc superconductors are of interest because their usefulness in bulk form hinges on their ability to withstand not only the forming operations but also any stresses imposed during use. It is well known that the mechanical properties of bulk YBa2Cu3O, are not desirable; yet, as this study shows, much of the measured mechanical behavior of this material is not intrinsic to the material itself. Instead, the mechanical behavior is dominated by the presence of numerous fine cracks that arise during processing. A commonly measured mechanical property that is sensitive to the presence of cracks is the bulk modulus, K. It is easy to measure, either by isostatic compression or by ultrasonic velocity techniques. Numerous values have been reported for YBa2Cu3O^, but they vary so widely (38-196 GPa)1"6 that it is difficult to determine the intrinsic value for this material. In addition, it was noted7 that oxidizing a bulk, hot-pressed sample to restore its superconducting properties simultaneously causes the modulus to decrease by 36%. A pressure-induced phase change is not a plausible explanation for the pressure-related effects that are observed. Fietz et al.2 measured the change of lattice parameters under pressures up to 14 GPa. Calculations of a bulk modulus from these data would include the effects of a pressure-induced phase change but not microcracking; their calculated bulk modulus is 200 GPa. The fact that measured moduli from the current experiments consistently fall well below those of Fietz et al. strongly suggests that a phenomenon such as microcracking must account for the low moduli observed when measurements are made using techniques that are sensitive to the bulk properties. The goal of this work was to determine the extent to which variation in modulus values was attributable to microcracks in bulk, superconducting YBa2Cu3OA:. To
determine the effect of microcracks, hydrostatic pressure was applied to sealed samples to force the cracks to close. A closed microcrack causes no change in the moduli of the bulk material. Thus moduli would be expected to increase as increasing hydrostatic pressure closed more and more microcracks. If all microcracks could be closed by a given pressure, then at greater pressures the material would behave as an elastic solid with essentially constant moduli. II. EXPERIMENTAL PROCEDURE A. Sample preparation
Moduli determinations were made on samples prepared in a manner similar to that of Loehman et al.1 Y2O3, Ba(NO3)2, and CuO were mixed in the ratio 1:4:6 and reacted at 925 °C for 4 h in air to produce YBa2Cu3Ox powders. These powders were hot pressed under 6.9 MPa at 950 °C for 30 min (in air) to produce a dense disk 5.0 cm in diameter. The hot-pressed disk was then core-drilled to produce cylindrical specimens 19 mm long by 9.5 mm diameter; no water was used in the machining. The cylinder axis was parallel to the direction of hot-pressing. Several of these hot-pressed specimens were subsequently heated in oxygen at 950 °C for 3
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