Comparative analysis of structural and electromagnetic characteristics have been performed on nano SiC- and polycarbosilane-doped MgB2 samples prepared by the dry and liquid mixing approaches. The total benefit of liquid mixing approach for fabrication of  organic  MgB2 superconductor with excellent electromagnetic performance has been demonstrate

Babic, Emil

Dou, S. X.

Pan, Alexey V

Shcherbakova, Olga V

English

Research Online

University of Wollongong Research Online Faculty of Engineering - Papers (Archive) Faculty of Engineering and Information Sciences 2010 "Organic" MgB2-xCx superconductor with high performance enabled by liquid mixing approach Olga V. Shcherbakova University of Wollongong, olga@uow.edu.au Alexey V. Pan University of Wollongong, pan@uow.edu.au Emil Babic University of Zagreb S. X. Dou University of Wollongong, shi@uow.edu.au Follow this and additional works at: https://ro.uow.edu.au/engpapers  Part of the Engineering Commons https://ro.uow.edu.au/engpapers/4046 Recommended Citation Shcherbakova, Olga V.; Pan, Alexey V.; Babic, Emil; and Dou, S. X.: "Organic" MgB2-xCx superconductor with high performance enabled by liquid mixing approach 2010, 1-6. https://ro.uow.edu.au/engpapers/4046 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au “Organic” MgB2−xCx superconductor with highperformance enabled by liquid mixing approachO. V. Shcherbakova1, A. V. Pan1, E. Babic2, and S. X. Dou11Institute for Superconducting and Electronic Materials, University of Wollongong, NSW2522, Australia2Department of Physics, Faculty of Science, University of Zagreb, 10002 Zagreb, CroatiaE-mail: olga@uow.edu.au (O. V. Shcherbakova)Abstract. Comparative analysis of structural and electromagnetic characteristics have beenperformed on nano SiC- and polycarbosilane-doped MgB2 samples prepared by the dryand liquid mixing approaches. The total benefit of liquid mixing approach for fabricationof “organic” MgB2 superconductor with excellent electromagnetic performance has beendemonstrated.1. IntroductionThe “middle temperature” MgB2 superconductor (with Tc = 39 K) discovered in 2001 [1] hasbeen regarded as a very promising candidate for high current applications in the temperaturerange of 20 to 25 K. The progress in development and fabrication of this material made itsperformance highly efficient for medical applications, such as magnetic resonance imagingsystems (MRI). Extensive investigation and study of MgB2 superconductor has resulted ina significant breakthrough for the performance of this conductor at high fields (required forapplications in high field magnets). This breakthrough has been achieved with introduction ofcarbon into the crystal lattice of MgB2 via chemical doping with nano-C or C-based compounds.Chemical doping is easy and a relatively cheap and effective approach for enhancement ofMgB2 superconducting properties. The high values of critical current density (Jc(Ba), Bais an applied magnetic field), irreversibility (Birr) and upper critical (Bc2) fields of MgB2superconductor achieved by nano SiC and C doping are bright examples of this technique[2, 3, 4, 5, 6, 7]. There are, however, some unavoidable obstacles which reduce the effectivenessof this method.First of all, it is extremely difficult to homogeneously distribute a small amount of nano-dopant among matrix elements. This is because any dry mixing results in the formation of largeparticle agglomerates. This unavoidably results in inhomogeneity of the final MgB2 material,which is not desirable from the applications point of view.Second, nano particles used for chemical doping always have a wide distribution of particlesize. While small particles have better reactivity with matrix elements, large particles have poorreactivity and remain unreacted in the system. This was demonstrated on the samples withnano-SiC doping [8]. Unreacted dopants are less desirable, because they result in reduction ofsuperconducting volume and degrade the current flowing through the superconductor.9th European Conference on Applied Superconductivity (EUCAS 09) IOP PublishingJournal of Physics: Conference Series 234 (2010) 012038 doi:10.1088/1742-6596/234/1/012038c© 2010 IOP Publishing Ltd 1Third, the vast majority of the nano dopants employed have been introduced into the MgB2matrix in passivated state. In this case, the surface of the nano particles is contaminated (byoxide compounds, for example), which degrades their reactivity and, hence, doping effectiveness.That is why for various forms of nano C doping, C substitution for B cannot be achieved at thesame temperature as MgB2 formation and requires a much higher annealing temperature.Therefore, in our group we have developed a new advanced and, at the same time, simplifiedliquid mixing approach [9] for the fabrication of “organic” MgB2−xCx superconductors. Thisapproach results in extremely homogeneous mixing of raw components and enables enhancedperformance of the final MgB2−xCx superconductors.The idea behind the liquid mixing approach can be extended to a wide group of polymerdopants. In this work, one of these materials, polycarbosilane, has been investigated.Polycarbosilane (C2H6Si, PCS later in the text) is a polymer analogous to SiC [10] and canbe expected to combine the benefits of the liquid mixing of carbohydrates [9] and the co-dopingof C and Si, as in nano SiC doping [2, 3]. In addition, we have performed a comparativestudy of samples with nano-SiC and polymer PCS dopants prepared by dry and liquid mixingtechniques. The absolute benefit of liquid mixing approach for formation of “organic” MgB2−xCxsuperconductors is demonstrated.2. ExperimentalBulk samples of pure, 10 wt.% nano-SiC and PCS-doped MgB2 have been prepared in-situby hand mixing of Mg, B and doping materials (dry mixing technique). The 10 wt.% PCS-doped MgB2 bulk sample was fabricated using our new liquid mixing approach [9]. After PCScompound was dissolved in toluene (C7H8), an appropriate amount of B powder was added todopant in the solvent and well mixed with a mortar and pestle by hand. After the liquid mixingstage, the wet slurry of boron, dopant, and solvent was dried in a vacuum chamber at a slightlyelevated temperature (200◦C) overnight to remove the remaining solvent and moisture. Afterdrying, the powder was ground, mixed with Mg powder, and ground again. The Mg+2B+dopantmixture was pressed in stainless steel tube (at pressure of 12 MPa) and encapsulated with Feplugs using a hydraulic press for formation of MgB2−xCx sample in a pellet form. Pressedsamples were heated at the rate of 5 ◦C/min and sintered at 700◦C for 60 min, followed byfurnace cooling to room temperature. Detailed description on fabrication of pure and 10 wt.%nano-SiC doped MgB2 can be found if Ref. [7], and for PCS-doped MgB2 prepared by drymixing in Ref. [10]. In Figures and Table “PCS-dry” and “PCS-liquid” denote PCS-dopedMgB2 samples prepared by dry and liquid mixing techniques, respectively.The lattice parameters and impurities phase fraction of MgB2−xCx samples were estimatedvia Rietveld refinement of X-ray diffraction (XRD) patterns. The Jc(Ba) curves were derivedfrom the height of magnetization loops measured on a Physical Property Measurement System(PPMS, Quantum Design) in fields of up to 8 T, using the critical state model [11]. The magneticcritical current density was calculated using the relation Jc = 204Ma(1− a3b) , where a and b are thedimensions of a bar shaped sample perpendicular to the magnetic field direction (a < b), 4M isthe width of hysteresis loop. All the samples for magnetic measurement were shaped to the samesize of a = 0.05 cm, b=0.2 cm, and c =0.3 cm for consistent comparison. The Bc2(T ) valueswere obtained from resistivity (ρ) versus temperature curves measured by 4-points techniqueusing criteria of Bc2(T ) = 0.9ρ(Ba, T ).3. Results and discussionThe results on structure and phase analysis obtained from Rietveld refinement of XRD patternsare presented in Table 1. In the MgB2 hexagonal cell, the a-lattice parameter is reduced withaddition of C-based dopants (Table 1), while the c-lattice parameter does not change significantly9th European Conference on Applied Superconductivity (EUCAS 09) IOP PublishingJournal of Physics: Conference Series 234 (2010) 012038 doi:10.1088/1742-6596/234/1/0120382(3.524 ± 0.002 nm). The reduction in the a-value is associated with partial carbon substitutionon B sites in the MgB2 crystal lattice. The smaller the a-value, the higher the level of carbonthat has been introduced into the lattice. The actual level of C substitution (x in Mg(B1−xCx)2,Table 1), has been calculated from the change in the a-parameter of the crystal lattice [12].One level (10 wt.%) of doping by analogous compounds (nano-SiC and polymer PCS) andemployed preparation techniques have notable influence on phase and structure formation insamples studied (Table 1). Among samples prepared by dry mixing approach, PCS dopingresults in higher level of C substitution for B, and larger amount of Mg2Si phase, compared tosample with nano-SiC doping. It can be explained by the fact that PCS initially contains higherlevel of C in its compound (C2H6Si). Some C could be introduced by toluene, which also has highamount of carbon (C7H8). In addition, disordered Si and C atoms released after decompositionof PCS at temperature of 300 ◦C [10] appears in a highly reactive (unpassivated) form whenthe MgB2 formation reaction starts at 650◦C, promoting the enhanced incorporation of C onB places in MgB2 lattice during the heat treatment. In case of nano-SiC doping, according todual reaction model proposed by Dou et al.[7], which is based on two stage process described inRef.[9], free carbon on the atomic scale also appears after reaction between Mg and Si, whichtakes place at temperature of about 600 ◦C [14]. However, lower value of x as well as relativelysmall amount of Mg2Si phase in nano-SiC doped sample indicate on worse reactivity of nanoelements, probably due to inhomogeneous particles distribution or their agglomeration.In the case of PCS-doped MgB2 sample prepared by liquid mixing, the x -value and amount ofMg2Si are notably higher compared to the dry-mixed samples. In our previous work Ref.[13], wehave demonstrated that liquid mixing technique enables coating of boron particles with a nano-layer of carbon and silicon mixture. Our results indicate that liquid mixing approach results inmore efficient doping, which assures the maximum surface for reaction and clean interfacesbetween boron and the doping elements in the nano-layer, in contrast to the arrangementobtained by dry-mixing of nano-powders.Table 1. Some structural and superconducting parameters of samples studied.Sample Anneal. a, x Mg2Si, FWHM Tc, Bc2,temp., ◦C nm in MgB2−xCx wt. % (101), degree K (5 K) TMgB2 825 3.0850 - - 0.3340 37.2 19nano-SiC 825 3.0750 0.020 6.2 0.4840 36.0 28PCS-dry 850 3.0746 0.028 9.7 0.5140 35.1 30PCS-liquid 700 3.0741 0.033 11.2 0.5520 34.7 32FWHM (Full Width at Half Maximum) values of (101) peak for samples investigated aresummarized in Table 1. FWHM value can be used for calculation of crystalline size, which isproportional to grain size. The higher FWHM values correspond to smaller grain size. Themicrostructure investigation has shown a general reduction in MgB2 grain size caused by SiCdoping. This is likely due to the fact that Mg2Si particles formed at lower temperature (about600 ◦C [14]) act as nucleation centers for MgB2 grain formation. This point of view togetherwith large FWHM value support the idea that liquid mixing approach enables the formationof small grain size due to homogenious distribution of possible nucleation sites (in the form ofnano C or Mg2Si impurities) as it occurs in PCS-doped MgB2 sample (PCS-liquid in Table 1).Importantly, increasing width of peak and, therefore, FWHM value is caused by severalfactors, including grain size, level of C-substitution, lattice strain caused by lattice defects9th European Conference on Applied Superconductivity (EUCAS 09) IOP PublishingJournal of Physics: Conference Series 234 (2010) 012038 doi:10.1088/1742-6596/234/1/0120383(vacancies, interstitial, substitutions, stacking faults, etc)[15]. Therefore, large FWHM valuesalso indicate on high level of defects in the sample. Indeed, higher level of carbon substitution onboron sites and the appearance of a high level of nano inclusions promote greater crystal latticedistortion and result in formation of denser network of crystal lattice or “secondary” defects(dislocations, stacking faults, nano-domains, ets.) in doped samples. Hence, the highest densityof “secondary” defects is generated in PCS-doped sample fabricated by liquid mixing technique,which has the highest level of C-substitution and largest amount of Mg2Si impurities (Table 1).0 1 2 3 4 5 6 7 810210310410510620 K   Critical current density, J c (×104, A/m2 )Applied magnetic field, Ba (T) MgB2 nano-SiC PCS-dry PCS-liquid5 KFigure 1. Jc(Ba) performance of samples prepared by dry (MgB2; nano-SiC MgB2, denotedas nano-SiC; PCS-doped MgB2, denoted as PCS-dry) and liquid mixing (PCS-doped MgB2,denoted as PCS-liquid) approaches.The critical temperature, Tc, reduces as the level of C-substitution on B site, x, increases inthe samples (Table 1). This behaviour is commonly observed in MgB2−xCx superconductors.Magnetic field dependance of the critical current density, Jc(Ba), and temperaturedependance of the upper critical field, Bc2(T ), for the samples studied are shown in the Fig. 15 10 15 20 25 30 35 405101520253035    MgB2  nano-SiC  PCS dry  PCS liquidUpper critical field, Bc2 (T)Temperature (K)Figure 2. Temperature dependance of the upper critical field, Bc2(T ), for the samples studied.9th European Conference on Applied Superconductivity (EUCAS 09) IOP PublishingJournal of Physics: Conference Series 234 (2010) 012038 doi:10.1088/1742-6596/234/1/0120384and Fig. 2, respectively. As can be seen, in-field Jc(Ba) and lower temperature (T<25 K) Bc2(T )performances for “organic” MgB2−xCx superconductors are significantly higher than those forundoped one. Linear extrapolation of Bc2(T ) curve for PCS-doped sample prepared by liquidmixing yields Bc2(5 K) of 32 T, which is the highest value among samples studied (Fig. 2).Observed enhancement of Jc(Ba) and Bc2(T ) in studied MgB2−xCx superconductors are dueto formation of additional defects induced by Si and C co-doping. These defects lead to enhancedelectron scattering and, therefore, increase the upper critical field values. On the other hand,dense network of defects with the size of ∼ 2ξ, where ξ is the coherence length, improves pinningof magnetic vortices and, hence, in-field Jc(Ba) performance. The pinning sites induced by nano-SiC and PCS doping include: (i) grain boundaries and (ii) crystal lattice defects, generatedby nano scale Mg2Si impurities and C-substitution for B in MgB2−xCx samples [2, 3, 7, 10].It is clear that PCS-doped MgB2 sample prepared by liquid mixing approach, which enablesformation of the smallest grain size, the highest level of C-substitution and the largest amountof Mg2Si impurities, shows superior Jc(Ba) and Bc2(T ) performances among other samplesstudied, prepared by dry mixing technique.We believe that an outstanding advantage of liquid mixing approach is in the most possiblehomogeneous distribution of dopant and maximal reaction area between boron and dopant.Moreover, liquid mixing approach employed for carbohydrates and polymer C-based materialslead to two-stage process [9]: (i) formation of fresh carbon and (ii) MgB2 reaction, which wasgeneralized to the so-called dual reaction model [7].4. ConclusionWe have performed structural and electromagnetic analysis of “organic” MgB2−xCxsuperconductors with nano-SiC and PCS dopants prepared by conventional dry and new liquidmixing techniques. It is demonstrated that the liquid mixing enables effective doping, whichassures: i) highly homogeneous mixing of host and doping materials; ii) coating of boron particlesby nano-layer of amorphous carbon and silicon mixture, which facilitate C-substitution in MgB2crystal lattice; iii) high level of small and well dispersed within the sample Mg2Si impurityphase; iv) small grain size; and, therefore, v) formation of microstructure with high amountof small optimally distributed defects, which improve pinning and enhance electron scatteringof final material. The liquid mixing approach employed for PCS dopant results in formationof “organic” MgB2−xCx superconductor with excellent electromagnetic performance, which isimportant result of this work. As shown, the PCS doping outperforms nano-SiC doping, whichwas considered as the best dopant so far.References[1] Nagamatsu J, Nakagawa N, Muranaka T, Zenitani Y, and Akimitsu J, 2001 Nature 410 63[2] Dou S X, Pan A V, Zhou S, Ionescu M, Liu H K, and Munroe P R, 2002 Supercond. Sci. Technol. 15 1587[3] Dou S X, Braccini V, Soltanian S, Klie R, Zhu Y, Li S, Wang X L, and Larbalestier D, 2004 J. Appl. Phys.96 7549[4] Sumption M D, Bhatia M, Rindfleisch M, Tomsic M, Soltanian S, Dou S X, and Collings E W, 2005 Appl.Phys. Lett. 86 092507[5] Wilke R H T, Bud’ko S L, Canfield P C, Finnemore D K, Suplinskas R J, and Hannahs S T, 2004 Phys. Rev.Lett. 92 217003[6] Matsumoto A, Kumakura H, Kitaguchi H, Senkowicz B J, Jewell M C, Hellstrom E E, Zhu Y, Voyles P M,and Larbalestier D C, 2006 Appl. Phys. Lett. 89 132508[7] Dou S X, Shcherbakova O V, Yeoh W K, Kim J H, Soltanian S, Wang X L, Senatore C, Flükiger R, DhalleM, Husnjak O, and Babic E, 2007 Phys. Rev. Lett. 98 097002[8] Shcherbakova O, Dou S X, Soltanian S, Wexler D, Bhatia M, Sumption M, and Collings E W, 2006 J. Appl.Phys. 99 08M510[9] Zhou S, Pan A V, Wexler D, and Dou S X, 2007 Adv. Mater. 19 1373[10] Shcherbakova O V, Pan A V, Wexler D, and Dou S X, 2007 IEEE Trans. Appl. Supercond. 17 27909th European Conference on Applied Superconductivity (EUCAS 09) IOP PublishingJournal of Physics: Conference Series 234 (2010) 012038 doi:10.1088/1742-6596/234/1/0120385[11] C. P. Bean, Rev. Mod. Phys. 36, 31 (1964).[12] Avdeev M, Jorgensen J D, Ribeiro R A, Bud’ko S L, and Canfield P C, 2003 Physica C 387 301[13] Shcherbakova O V, Pan A V, Wexler D, and Dou S X, unpublished.[14] Matsumoto A, Kumakura H, Kitaguchi H, and Hatakeyama H, 2003 Supercond. Sci. Technol. 16 926[15] Serquis A, Liao X Z, Zhu Y T, Coulter J Y, Huang J Y, Willis J O, Peterson D E, Mueller F M, Moreno NO, Thompson J D, Nesterenko V F, and Indrakanti S S, 2002 J. Appl. Phys., 92 3519th European Conference on Applied Superconductivity (EUCAS 09) IOP PublishingJournal of Physics: Conference Series 234 (2010) 012038 doi:10.1088/1742-6596/234/1/0120386

Organic  MgB2-xCx superconductor with high performance enabled by liquid mixing approach

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Organic MgB2-xCx superconductor with high performance enabled by liquid mixing approach

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