Grain boundary weak-link behaviour in superconductors is investigated using a critical
state model (CSM), which has been specifically formulated for the applied field range
o ~ Ha ~ Hc1g , where Hclg is the intra-granular lower critical field. The CSM includes an
expression for the inter-granular critical current density, Jc(H, T), that has been derived
from a percolation model for a random 3D network of weak-links in which the current
Bowing through the individual grain boundary junctions is assumed to be spatially random.
This expression, namely: Jc(H,T) = Jco[exp(-H/Ho)+b], where JCO ) Ho , and
b are characteristic weak-link parameters, includes a field-dependent component and a
field-independent component both of which are microstructurally sensiti ve. Calculated
magnetisation behaviour, obtained from the critical state model, are fitted to experimental
isothermal D.C magnetisation data, j\1(Ha) , for 0 ~ Ha ~ Hclg , 71.5K~ T ~ Tc,
obtained from the two control specimens, namely: Y IBa2Cu307-x and EUIBa2Cu307-x ,
x ~ I, and from a third specimen, which is a mixture of the two control compounds. In
the CSM fits, the temperature dependent characteristic parameters Jco , Ho, and bare
treated as free fitting parameters. These free fitting parameters are subsequently fitted to
specific theoretical models for Jca(T), Ho(T), and b(T) and a comparison is made between
the control specimens and mixed specimen parameters to establish the effect of mixing
on the grain boundary weak-link behaviour. Jca and b are found to be significantly larger
in the mixed specimen as compared with either of the two control specimens. / Thesis (M.Sc.)-University of Natal, Durban, 2002.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/5048 |
| Date | January 2002 |
| Creators | Reddy, Kevin Poobalan. |
| Contributors | Doyle, Terence B., Brookes, H. C. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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