Year

2008

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting & Electronic Materials - Faculty of Engineering

Abstract

In this thesis I emphasized on the organic compound doping (specially carbohydrate group, malic acid, C4H6O5) and heat treatment effects on the superconducting properties of MgB2. I also focused on the basic and fundamental properties of un-doped MgB2 wires in different temperatures for comparison purpose. And finally I have proposed another new dopant which avoids some problems using carbohydrate in some aspects.

Firstly, I have studied the effects of sintering temperature on the phase transformation, lattice parameters, full width at half-maximum (FWHM), strain, critical temperature (Tc), critical current density (Jc) and resistivity (_) in MgB2/Fe wires. All samples were fabricated by the in situ powder-in-tube method (PIT) and sintered within a temperature range of 650–900 _C. I have showed that why I have taken such sintering temperature range by analyzing with differential thermal analysis (DTA). The increased FWHM and decreased Tc at low sintering temperature region suggested the smaller grain size and poor crystallinity. Strain values also higher at low sintering region. That’s why it was observed that wires sintered at low temperature, 650 _C, resulted in higher Jc up to 12 T. The best transport Jc value reached 4200 A cm−2 at 4.2 K and 10 T. This is related to the grain boundary pinning due to small grain size and poor crystallinity due to strain defects. On the other hand, wires sintered at 900 _C had a lower Jc in combination with better crystallinity due to higher Tc.

The effect of carbohydrate doping on lattice parameters, microstructure, Tc, Jc, Hirr, and Hc2 of MgB2 has been studied. In this work I used malic acid, C4H6O5 as an example of 2 carbohydrates as an additive to MgB2. We have described the advantages of carbohydrate doping include homogeneous mixing of precursor powders, avoidance of expansive nanoadditives, production of highly reactive C, and significant enhancement in Jc, Hirr, and Hc2 of MgB2, compared to un-doped samples. The defects due to the C substitution into boron site lead to the enhancement of Hirr and Hc2. The decrease of a-axis lattice parameter and reduction of Tc indicates poor crystallinity due to C substitution. The microstructure was shown both for un-doped and doped samples which were well consistent with FWHM. The Jc for MgB2+30 wt% C4H6O5 sample was increased by a factor of 21 at 5 K and 8 T without degradation of self-field Jc due to C substitution into B sites.

During the evaporation process of the C4H6O5 with B and solvent, freshly and highly reactive C is produced and C substitution for B can take place at the temperature same as the formation temperature of MgB2. By using this chemical route I again evaluated the doping effects of C4H6O5, from 0 to 30 wt% of the total MgB2, on the lattice parameters, lattice strain, amount of carbon (C) substitution, microstructures, weight fraction of MgO, critical temperature (Tc), critical current density (Jc), and irreversibility field (Hirr) of a MgB2 superconductor. The calculated lattice parameters show a large decrease in the aaxis lattice parameter for MgB2 + C4H6O5 samples from 3.0861(6) to 3.0736(1) Å, with even a 10 wt% addition. This is an indication of C substitution into boron sites, with the C coming from C4H6O5, resulting in enhancement of Jc and Hirr. Specifically, the Hirr of the MgB2 + C4H6O5 samples prepared by the chemical solution route reached around 7 T at 20 K, with a Tc reduction of only 1.5 K. In addition, the self-field Jc of the MgB2 + 3 C4H6O5 samples was only slightly reduced at an additive level as high as 30 wt%. The interesting thing I found here is maximum C-substitution and the maximum enhancement of all the superconducting parameters up to 10 wt% addition, after that the improvement rate is saturated. From these data I can claim 10 wt% addition is enough for maximum Csubstitution and enhancement of superconducting properties. However, residual oxygen after evaporation processing contributed to a large amount of MgO in our MgB2 + 30 wt% C4H6O5 samples. These problems can be further controlled by the amount of C4H6O5 additive or different evaporation temperatures.

After the successful doping effects of C4H6O5 into MgB2, then I investigated the behaviour of C4H6O5 as a dopant with different sintering temperatures. All the samples were prepared by the chemical solution route. I report the carbon (C) substitution effects of MgB2 + 10 wt% C4H6O5 on the lattice parameters, critical temperature (Tc), upper critical field (Hc2), and irreversibility field (Hirr) as a function of sintering temperature in the range from 600 to 900 _C. The additive C4H6O5 as the C source resulted in a small depression in Tc, but significantly increased the C substitution level, and hence improved the Hc2 and Hirr performance at a low sintering temperature of 600 _C in conjunction with a short sintering period of 4 h. In addition, the low-temperature sintering process resulted in small grain size and higher impurity scattering compared to a pure MgB2 superconductor which promotes the flux pinning significantly.

Very recently, I have chosen another solid hydrocarbon dopant named pyrene (C16H10) in to MgB2. There are few reasons behind this decision. Firstly we know all the carbohydrates consist of carbon (C), hydrogen (H), and oxygen (O). During the 4 evaporation process of C4H6O5, I noticed that the MgO amount is gradually increased with increasing doping level. So our group suggests such special solid hydrocarbon without O content which may reduce the MgO content within the matrix. In this work, we report on significantly enhanced Jc in MgB2 superconductor that was easily obtained by doping with a hydrocarbon, highly active C16H10, and using a sintering temperature as low as 600 °C. The processing advantages of the C16H10 additive include production of a highly active carbon C source, an increased level of disorder, and the introduction of small grain size, resulting in enhancement of Jc.

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