STUDY OF DOPING EFFECT ON STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF MgB2 SUPERCONDUCTOR

Abstract

Superconductivity is a quantum mechanical phenomenon observed at macroscopic level. Today superconducting materials have a wide range of commercial and industrial applications in energy production, storage, its distribution, in sensor materials, and most importantly in high field magnets. Superconducting magnets have exclusive use in MRI instruments in medical field, high field R&D instruments like NMR, beam bending, focusing and detecting magnets for high energy particle physics research, energy storage in SMES, confinement of plasma in fusion reactors and the magnetic levitation systems in transportation. Superconductivity, the loss of electrical resistivity of a material below a critical temperature (T c) , is one of the most fascinating phenomena in condensed matter physics. Superconductivity was first observed in Hg in 1911 by Kamerlingh Onnes. Now, after nearly one century of its discovery thousands of materials including simple elemental metals, alloys, binary and multicomponent compounds of metals, ceramics, doped fullerenes and organic molecules are found to be superconducting with transition temperatures ranging from a millikelvin to —150 K. The discovery of superconductivity in MgB2 at —39 K was certainly surprising. This widely available compound was overlooked until 2001 and its transition temperature was found to be higher than that of any other metallic compounds. This material turned out to be interesting for fundamental research and for technical applications. The two-gap nature of superconductivity in MgB2 offers rich physics and it was the first superconductor to confirm this old concept. Clear indications for two energy gaps were found in the specific heat, in tunneling experiments and in magnetic or optical properties. The absence of the weak link problem at grain boundaries, which represents the main drawback of high temperature superconductors, is most important for applications. Rather simple powder-in-tube techniques can be applied for the production of MgB2 based wires or tapes. The material is cheap, has a comparatively high transition temperature and its upper critical field (Hc2) can exceed that of Nb3Sn, if properly alloyed. The MgB2 superconductor is expected to be useful for various electric power applications because of its high transition temperature and low cost. However, the applications of pure MgB2 superconductors face serious obstacles due to its low upper critical magnetic field (Hu) and irreversibility field (H, ). Numerous studies have been performed in order to improve the iii critical current density (Jc), Hc2 and Hu, of MgB2 wire. In general, there are two methods of improving these properties of MgB2 at high magnetic field, i.e., inducing a fine grain size and doping. In the former method, because pinning is known to be mainly of grain boundary origin and there is a small weak-link problem, it is necessary to study the sintering process, which is known to affect the grain size. In the latter method, numerous elements or materials such as Fe, Al, SiC, Ti, B4C, etc., have been studied for doping in MgB2. Among them, SiC is one of the best dopants for the enhancement of the Jc value and C is known to substitute at B sites of MgB2. Further, a substantial amount of research has been conducted to improve the doping effect using various forms of carbon such as amorphous carbon, carbon nanotube, diamond carbon, carbon black, carbohydrates etc., however, no material has been found which fully satisfies the requirements. In this thesis we studied the effect of doping of organic compounds (especially carbohydrates: malic acid, citric acid and oxalic acid), rare earth oxides (Eu203 and Pr6011) and magnetic elements (Mn and Co) on the structural and superconducting properties of MgB2. We also studied the effect of sintering temperatures on the superconducting properties of pure and malic acid-doped MgB2. The present thesis is divided into seven chapters. The first chapter contains an introductory aspects and survey of the field and describes the crystal structure, superconducting properties and various mechanisms of improvement of superconducting properties of MgB2 superconductor. The second chapter describes the prominent experimental techniques for structural, magnetic and electrical characterization employed in the present investigations. These techniques include x-ray diffraction (XRD) for phase identification, field emission scanning electron microscopy (FE-SEM) using secondary electron imaging mode for investigating the surface morphology and energy dispersive x-ray (EDAX) spectroscopy for elemental analysis. Solid-state reaction method used to prepare the samples has also been described in this chapter. Besides this, basic working principle of vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID) used for the measurement of magnetic property of the samples have been described. The methods of determination of transition temperature (Tc), upper critical field (1-Ic2), irreversibility field (H,„) and critical current density (Jc) (Bean's critical state model) have also been described in this chapter. iv The third chapter describes the effect of rare earth oxide (REO) doping on the superconducting properties of MgB2. The bulk polycrystalline samples with nominal compositions Mgi-x(Eu203)x/2B2 and Mgi-x(Pr60i i)xt6B2 (where x = 0.0, 0.01, 0.02, 0.03, 0.04 and 0.05) have been prepared via standard solid-state reaction route by sintering at 850° C in Ar/H2 atmosphere. The x-ray diffraction (XRD) results of both series of materials reveal the presence of secondary phases MgO, REB6 (RE=Eu/Pr) and Prat besides the main hexagonal phase of MgB2. It has been found that when the value of x increases from 0 to 0.05 the transition temperature (TO decreases from 39 to 37 K. The strain values have been found to increase almost linearly with the nominal REO content in the sample. As a function of REO content we have found a significant increase in the critical current density (Jc) and the irreversibility field (11,7) in the magnetic field range 0-6 T. The observed significant improvement in Jc(H), Ho and 111,, in the REO-doped MgB2 samples, is mainly attributed to the lattice distortions introduced by REO doping. Variations of HC2) Jc, Hirr and flux-pinning force (Fp) with doping concentrations have been studied in this chapter and a correlation between these superconducting properties and structural characteristics of the samples has been found. The fourth chapter describes the effect of carbohydrate doping on structural and superconducting properties of MgB2. In this work we used carbohydrates: malic acid, citric acid and oxalic acid as carbon sources. The advantage of carbohydrate is that they provide ) highly reactive C which substitute for B in MgB2 at the reaction temperature leading to significant improvement in Jc, HC2i Hirr of MgB2 as compared to undoped one. The defects due to C substitution at boron site lead to the enhancement of Hu and Arr.- The first section of this chapter describes the effect of carbohydrates (oxalic acid and citric acid) doping on the superconducting properties of MgB2 samples. The samples have been prepared via standard solid state reaction route with compositions MgB2 + x wt% carbohydrates (x = 0, 5 and 10) by sintering at 850° C in Ar atmosphere. The x-ray diffraction results reveal the formation of dominantly MgB2 with only small amount of impurity phase MgO and substitution of C at the B site of MgB2. Improvements in Hirr, Hu and high field (> 2.5 T) Jc have been observed on C doping in the samples. The second section of chapter 4 describes the effect of sintering temperature on superconducting properties of malic acid-doped MgB2. The malic acid-doped MgB2 samples have been synthesized with the nominal compositions described in the first section by sintering at 800° C and 850° C in Ar atmosphere. In case of the samples sintered at 800° C there is improvement in Jc values of 5 wt% doped sample in entire field region as compared to undoped sample. It has been found that the improvement in the superconducting properties of the samples sintered at 800° C is better than the corresponding samples sintered at 850° C. The correlation between the superconducting properties and structural characteristics of the samples sintered at both temperatures has been described and discussed in this chapter. The fifth chapter describes the effect of co-doping of carbon and rare earth oxide (REO) on superconducting properties of MgB2 superconductor. In this study we fixed the carbon content and varied the content of rare earth oxides in the samples. We used graphite as carbon source. The samples with nominal compositions Mgi_y(REO)y(130.95Co 02, (where y = 0.00, 0.01, 0.03, 0.05, and REO = Eu203 or Pr601 ) have been prepared via solid state reaction route by sintering at — 850° C in Ar atmosphere. The x-ray diffraction (XRD) results of both series of materials reveal the presence of secondary phases MgO and REB6 (RE = Eu/Pr) besides the main hexagonal phase of MgB2. It has been found that when the value of x increases from 0 to 0.05 the transition temperature (Tc) decreases from 38.3 to 35.9 K and 38.3 to 33.3 K, respectively, for Eu203 -doped and Pr601 1- doped samples. For the co-doped samples we have found improvement in Hu and 1-1,„, but no substantial improvement in Jc(H) values as compared to undoped one. It has been found that the (Pr601 1, C) doping is more effective than (Eu203, C) doping in improving the superconducting properties (Ho., and Hc2). The correlation between the structural and superconducting properties of doped MgB2 sample have been studied and discussed in this chapter. The sixth chapter describes the effect of magnetic element (Mn and Co) doping on the superconducting properties of MgB2 superconductor. The polycrystalline Co- and Mn-doped MgB2 samples with nominal composition of Mgi,MxB2 (M = Mn and Co) (x = 0, 0.01, 0.03 and 0.05) have been synthesized by solid state reaction method. The XRD results reveal the presence of dominantly MgB2 phase with small amount of impurity phase MgO, which is most common impurity seen in all preparations. The Tc of all the doped samples decreases with increasing doping content. In present study it has been found that both Mn and Co doping deteriorate the superconducting properties (Tc, Hc2 and Hi,) of MgB2. vi The seventh chapter contains the brief summary of the work presented in the thesis through chapters 3 - 6. The overall comments and recommendations have also been given in this chapter. vii