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Friday, April 1, 2011

Superconducting Devices

A major activity of the division has been continuation of basic research in doped MgB2 superconductors. Bulk samples of MgB2 with different dopants like Al, nano-C, nano-SiC and nano-diamonds were synthesized and characterized by thermoelectric power S(T), magneto-resistivity [r(T,H)] and magnetization M(H) in high fields. A unique phenomenological interpolation formula was evolved to fit both positive and negative S(T) dependence observed as a function of Al concentration in Mg1-xAlxB2. The doping of nano-C, nano-SiC and nano-diamonds were shown to significantly enhance the upper critical field Hc(T), the irreversibility field Hirr(T) and the critical current density Jc(B,T) in the MgB2 bulk material.
We study the effect of n-SiC addition on the crystal structure, critical temperature, critical current density and flux pinning in MgB2 superconductor. X-ray diffraction patterns show that all the samples have MgB2 as the main phase with very small amount of MgO, further with n-SiC addition the presence of Mg2Si is also noted and confirmed by SEM & EDS. The Tc value for the pure MgB2 is 18.9K under 8 Tesla applied field, while is 20.8K for the 10-wt % n-SiC doped sample under the same field. This points towards the increment in upper-critical field value with n-SiC addition.
Polycrystalline MgB2-nDx (x= 0 to 0.1) samples are synthesized by solid-state route with ingredients of Mg, B and n-Diamond. The results from magneto-transport and magnetization of nanodiamond doped MgB2-nDx are reported. Superconducting transition temperature is not affected significantly by x up to x = 0.05 and latter decreases slightly for higher x > 0.05.
Thermoelectric power, S(T) of the Mg1- xAlxB2 system has been measured for x = 0.0, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0. XRD, resistivity and magnetization measurements are also presented. It has been found that the thermoelectric power is positive for x ≤ 0.4 and is negative for x ≥ 0.6 over the entire temperature range studied up to 300 K, see Fig.7.1. The thermoelectric power of x ≤ 0.4 samples vanishes discontinuously below a certain temperature, implying existence of superconductivity.
We report the synthesis and variation of superconductivity parameters such as transition temperature Tc, upper critical field Hc, critical current density Jc, irreversibility field Hirr and flux pinning parameter (Fp) for the MgB2-xCx system with nano-Carbon doping up to x=0.20. Carbon substitutes successfully on boron site and results in significant enhancement of Hirr and Jc(H).

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