Role of p O2 in microstructural development and properties of YBa 2 Cu 3 O x superconductors
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An evaluation of the effects of oxygen partial pressure (po2) o n sintering behavior and the resulting microstructure of YBa 2 Cu 3 0 x (YBCO) indicates that sintering kinetics are enhanced at reduced p02. The density of specimens sintered at 910 °C increased from 79 to 94% theoretical when pOl was decreased from 0.1 to 0.0001 MPa. It is believed that increase in density with decrease in pOl is the result of enhanced sintering kinetics, due probably to increased defect concentration, decreased activation energy of the rate-controlling species, and possibly the presence of a small amount of liquid phase. Sintering at 910 °C resulted in a fine-grain microstructure, with an average grain size of = 4 yum. Such a microstructure results in reduced microcracking. Consequently, strength as high as 191 MPa is achieved. Reduced microcracking may have important implications for developing microstructures with improved critical current density.
I. INTRODUCTION YBa 2 Cu 3 0^ (YBCO) is probably the most widely studied high-temperature superconductor. For this material to be useful in commercial applications, it must possess good superconducting and mechanical properties. However, the strength of YBCO processed by conventional routes generally is unacceptably low. The low strength values have been attributed to the fact that the density of bulk YBCO specimens sintered in a 100% oxygen atmosphere is generally low (80-90% theoretical) if sintered at temperatures =920 °C. Sintering at higher temperatures results in higher densities, but is generally associated with degradation in critical current density due to the presence of second phases.1 Furthermore, sintering at higher temperatures also results in grain growth and microcracking.2 Microcracking will result in reduced strength and will also act as weak links and degrade critical current density (/ c ). Improvement in density, as well as mechanical and superconducting properties, has been achieved by making composites of YBCO with silver additions.3"6 Another approach to improving density and properties is the fabrication of monolithic YBCO with the desired microstructure through the control of processing parameters such as powder particle size and heat treatment. A recent study by Chen et al.1 indicates that sintering at low oxygen partial pressure (po2) may improve density at low temperatures. The increased densification is attributed to the formation of liquid phases that occur at decreasing temperatures with decreasing pOl.7 Sintering rates were also observed to increase with decreasing pOl in the temperature regimes where liquid phases are not formed (=£900 °C), probably because of the increased diffusion of cation species. Sintering at lower temperatures has the 2324
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potential for producing specimens with relatively small grains. However, as observed by Bormann and Nolting,8 Dell'Agli et al.,9 and Feenstra et al.,w YBCO becomes unstable at very low pOl and may precipitate undesirable phases, with a consequent degradation in Jc. Therefore, op
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