W. M. Stobbs

The origins of high values of the critical current density in the Bi2Sr2Can−1CunOx system: high-Tc superconducting pathways at low angle tilt boundaries

Abstract Low angle tilt boundaries in melt processed samples from the Bi 2 Sr 2 Ca n −1 Cu n O x system exhibit compositionally modulated faceting. Although we see local regions of the low- T c (20 K) (2201) phase at such boundaries which would be highly resistive at liquid nitrogen temperatures, there are also “pathways” crossing the boundary plane made up of the high- T c (2212) and (2223) phases. The characteristics of such low angle tilt grain boundary structures can therefore be modelled to provide a general insight into the correlation between high critical current densities and low texture breadths.

Lower Tc barriers at high angle boundaries in the (BiPb)2Sr2Ca2Cu3Ox system

The structure of high angle boundaries in the (BiPb)2Sr2Ca2Cu3Ox system, which is compositional and structurally modulated along the boundaries, was investigated by high resolution electron microscopy. These compositional variable boundaries tend to facet onto the (001) plane, leaving steps of height equal c/2 for the (BiPb)2Sr2CuOx and (BiPb)2Sr2Ca1Cu2Ox phases with amorphous material at the corners of these steps. These data suggest a weak link is formed at high angle boundaries where there are such lower Tc or insulating phases.

The compositionally modulated faceting of grain boundaries in the Bi2Sr2Ca n -1Cu n O x system

Abstract The ways in which the low-angle tilt boundaries which typify the Bi2Sr2Ca n -1Cu n O x system are compositionally and structurally modulated are described, while the reasons for the behaviour are discussed in relation to the ways a boundary can lower its energy in general.

A Microstructural Study of the Oxidation of Fe-Ni-Cr Alloys I. Protective Oxide Growth

The oxidation behaviour of a range of Fe—Ni~Cr alloys has been studied by correlating ‘edge-on’ tem observations of the retained metal—oxide interface with data obtained by conventional low resolution techniques for studying oxidation. Results are presented for ‘protective’ scale formation on three Fe-20Cr—Ni alloys as oxidized at 600 °C in air and in 1 % CO—CO2. The temporal development of thin layers of chromium-rich sesquioxides in air was followed and it was shown that growth occurs by a process essentially dominated by the outward diffusion of chromium. Results obtained by ‘edge-on’ tem demonstrated that decreased chromium gradients in the alloy and higher chromium concentrations at the metal—oxide interface are associated with some inward development of the scale. These minor microstructural changes are paralleled by increases in the rate constants with a tendency towards cube law kinetics. A reverse trend in the mass gain data, as a function of the nickel content of the alloy was characterized for oxidation in 1 % CO-CO2. Despite this, chemically similar oxides are seen to form although carbides were observed beneath the scales on the higher nickel content alloys. Significantly the trend with increasing nickel content was found to be paralleled by an increasing tendency for inward oxide growth. The iron activity in the alloy of lowest nickel content is sufficiently high for chemically unstable behaviour and the formation of duplex oxide nodules. Mechanisms by which simultaneous inward oxidation and carburization can take place during the formation of both chromium-rich sesquioxides and duplex spinel scales are discussed.