Browsing by Author "Reddy, Kevin Poobalan."

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An experimental investigation of the weak-link problem in granular high-Tc superconductors.
(2002) Reddy, Kevin Poobalan.; Doyle, Terence Brian.; Brookes, H. C.
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.
Thermal evolution of radiation spheres undergoing dissipative gravitational collapse.
(2014) Reddy, Kevin Poobalan.; Govender, Megandren.; Maharaj, Sunil Dutt.
In this study we investigate the physics of a relativistic radiating star undergoing dissipative collapse in the form of a radial heat flux. Our treatment clearly demonstrates how the presence of shear affects the collapse process; we are in a position to contrast the physical features of the collapsing sphere in the presence of shear with the shear-free case. We first consider a particular exact solution found by Thirukkanesh et al [1] which is expanding, accelerating and shearing. By employing a causal heat transport equation of the Maxwell-Cattaneo form we show that the shear leads to an enhancement of the core stellar temperature thus emphasizing that relaxational effects cannot be ignored when the star leaves hydrostatic equilibrium. We also employ a perturbative scheme to study the evolution of a spherically symmetric stellar body undergoing gravitational collapse. The Bowers and Liang [2] static model is perturbed, and its subsequent dynamical collapse is studied in the linear perturbative regime. We find that anisotropic effects brought about by the differences in the radial and tangential pressures enhance the perturbations to the temperature, and that causal and non–causal cases yield identical profiles.