Interplay between cold densificatio and malic acid addition (C4H6O5) for the fabrication of near-isotropic MgB2 conductors for magnet application

Md.Srr A.Hossn, Crmne Sentore, Yusue Ymuc, Mslv Mustp′c,Dnel Gd, Dp Ptel, Aslm Kn, Jun Ho Km, Andrze J Morws, René Fluer

a School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

b Department of Quantum Matter Physics (DQMP), University of Geneva, Geneva, Switzerland

cKey Laboratory of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China

d School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia

e International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

fUniversity of Osijek, Department of Physics, 31000 Osijek, Croatia

g Institute of Low Temperature and Structure Research PAS, Okólna 2, 50-422 Wroclaw, Poland

h Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

iKing Abdullah Institute for Nanotechnology, King Saud University, Riyadh, 11451, Saudi Arabia

j Australian Institute for Innovative Materials (AIIM), University of Wollongong, Squires Way, North Wollongong, NSW 2500, Australia

k Institute of High Pressure Physics, Polish Academy of Sciences (PAS), Soko?owska 29/37, 01-142 Warsaw, Poland

Abstract The effect of cold high pressure densificatio (CHPD) on anisotropy of the critical current density (Jc) in ? in situ ? single core binary and alloyed MgB2 tapes has been determined as a function of temperatures at 4.2K, 20K and 25K as well as at applied magnetic field up to 19 T.The study includes binary and C4H6O5 (malic acid) doped MgB2 tapes before and after CHPD.It is remarkable that the CHPD process not only improved the Jc values, in particular at the higher magnetic fields but also decreased the anisotropy ratio, Γ=Jc///Jc┴.In binary MgB2 tapes, the anisotropy factor Γ increases with higher aspect ratios, even after applying CHPD.In malic acid (C4H6O5) doped tapes, however, the application of CHPD leads only to small enhancements of Γ, even for higher aspect ratios.This is attributed to the higher carbon content in the MgB2 filaments which in turn is a consequence of the reduced chemical reaction path in the densifie filaments At all applied fiel values, it was found that CHPD processed C4H6O5 doped tapes exhibit an almost isotropic behavior.This constitutes an advantage in view of industrial magnet applications using wires with square or slightly rectangular configuration

Keywords: Magnesium diboride; Cold high pressure densification Anisotropy; Tapes; Critical current density.

1.Introduction

The manufacture and commercialization of industrial MgB2wires and tapes is currently in progress and a large number of works has been published since its discovery in 2001 [1].The perspectives of magnet application are not only based on the promising superconducting and electronic properties of MgB2tapes, but also on their low production costs.There is still much work required to be done in regards to the study and the improvement of the basic material properties, e.g.the grain connectivity and the mass density inside the MgB2filaments At the present day, industrial lengths of MgB2wires and tapes have been manufactured by bothex situandin situtechniques for large scale magnet applications while promising results have been obtained by a third process,Internal Mg Diffusion (IMD) [2].

In the present paper, the attention is centered onin situtapes.The volume occupied by the reacted MgB2phase is smaller than that of the unreacted Mg+2B mixture and thein situwires after reaction contain a certain amount of porosities, typically reaching only 50% [3] of the theoretical density of 2.62g/cm3[4,5].We reported that the CHPD process strongly influence the fina mass density of the filaments resulting in a significan improvement of the transport properties of superconducting MgB2wires [6,7].This is higher than the highest possible density inex situsamples (74%), according to the close packing model [4,5].The densificatio approach is very effective and particularly applicable to all kind of fil aments fabricated by powder metallurgy.Carbon or carbon based additives have been found to have a beneficia effect on the critical current density (Jc) ofin situMgB2wires, as reported by Collings et al.[5].In the most effective additives reported so far,carbon is always present,either as pure C[8,9]or C based compounds, e.g.SiC [10,11], B4C [12,13], various carbohydrates [14,15] and hydrocarbons [16], and even sugar [17].Yeoh et al.[18] also reviewed various possibilities of effective carbon based chemical doping on MgB2.Malic acid (C4H6O5) [7,14] is particularly an interesting candidate among the numerous carbon based additives leading to strong improvements reported in [5,18] and this will be the major focus of the present study.Isotropic conductors are important for any superconducting magnet design, i.e., the values ofJcin both parallel and perpendicular orientation should be similar.Higher values forJc//(parallel fiel direction) have already been reported for alloyed MgB2in the tape forms:H?ssler et al.[19] published for a highly textured MgB2tape alloyed with carbon with a value ofB(104)//=16 T, while Gao et al.[20] reportedB(104)//= 14 T, whereB(104) is the magnetic fieldBat whichJc= 104A/cm2.However,there is no full data reported for the perpendicular fiel direction,B⊥, which might be substantially lower.H?ssler et al.[21] published a reduction ofΓby pure C substitution.The question arises whetherΓcan be reduced by analyzing the behavior ofJcof tapes with fiel directions parallel and perpendicular to the wire or tape axis, definin an anisotropy ratioΓ=Jc//:Jc⊥.The averagecaxis grain alignment in the MgB2filamen is directly reflecte by the value ofΓand has been extensively studied by Fujii et al.[22], Kovac et al.[23-26] and Lezza et al.[27] on a variety ofin situandex situtapes.For anyin situtape,the anisotropy ratioΓis influence by a number of factors: the method of deformation (drawing or swaging or rolling) [23,24], the types of ball milling procedure [22,23,28], the aspect ratiob/aof the tape [23,24],the types of sheath material [23,24] and the size and shape of the filamen [25].It is well established fact for bothin situandex situtapes that the anisotropy ratioΓis the highest for binary MgB2, but decreases with increasing carbon contents in the MgB2lattice [22-24].

The C4H6O5doped MgB2tapes treated with the CHPD technique show significantl higherJcin both parallel and perpendicular directions [7], but the detailed investigation on theJcanisotropy is critical for the magnet design and is,therefore, worth investigating.In the present work, we have determined the effect of CHPD on the anisotropic behavior ofJcin binary and in C4H6O5alloyedin situmonofilamentar MgB2tapes at 4.2, 20 and 25K, at field up to 19 T.The values ofJcas a function of applied magnetic fiel in both parallel and perpendicular direction in binary and C4H6O5alloyed MgB2tapes at 4.2 and 20K have been reported in our previous works [6,7].The major aim of this study was to investigate whether the observed reduction of anisotropy in malic doped tapes compared to binary tapes can uniquely be explained by the higher carbon substitution in the filamen after the application of CHPD technique.

2.Experimental details

The round binary and alloyed wires (? 0.83mm) for the present work were manufactured at Hyper Tech Research,Inc., Columbus, OH, USA.The MgB2wires doped with 10 wt% C4H6O5were fabricated followed by the route previously described by Hossain et al.[7].The sheath material was Monel with Nb barrier, while the reaction conditions of MgB2/Nb/Monel wires were 600°C for 4 h (heating rate 2.5°C/min.).All monofilamentar wires did not contain any Cu for electrical stabilization.These round wires were fla rolled to tapes.In this work, CHPD process was applied on short tapes under the same conditions as in Refs.5 and 6, the sample length being 75mm, the two-wall pressing tool having a length of 60mm.In the present work, the transportJcs of the tapes before and after CHPD have been measured as a function of the applied magnetic fiel up to 19 T either edgeon (//) or face-on (⊥) to the tape surface, atT=4.2K, 20K and 25K.For each applied magnetic fiel theJcanisotropy is define as the ratioΓbetween the critical current density in the direction of parallel magnetic fielJc//and the one in perpendicular magnetic fielJc⊥.A four-probe technique was used to measureJcover the 45mm lengths of conductor in a cryostat with helium fl w and currents up to 250 A.Temperature sensor was placed on a current lead located close to the tape sample.The voltage taps were used on the sample 10mm apart and 0.1 μV cm?1was the voltage criteria for this transport current measurement.The values of irreversibility fieldBirr//andBirr⊥were determined byJc=100 A/cm2as a criterion using an extrapolation of Jcversus B to 100 A cm?2.

Crystal structures of the samples were characterized by xray diffraction (XRD) using Philips PW1730 instrument and lattice parameters(aandc)were calculated using Rietveld refinement Microscopic and morphological studies and analysis were carried out using a JEOL fiel emission gun - scanning electron microscopy (FEG-SEM).A JEOL JEM-3000F, a 300keV high-resolution transmission electron microscopy(HR-TEM) has been used to obtain high-magnificatio images.The filament were extracted from the MgB2tapes treated at 0 and 2GPa to investigate the grain connectivity and defects in the lattice using FEG-SEM and TEM.

Table 1Characterization of the monofilamentar binary and alloyed tape with 10wt.% C4H6O5 (Monel sheath, Nb barrier) before and after pressing by 2GPa.

Fig.1.Variation of the critical current density in parallel and perpendicular direction (Jc// and Jc┴) vs.magnetic fiel at 4.2, 20 and 25K for C4H6O5 alloyed MgB2 tapes (Monel sheath, Nb barrier) at 0 and 2GPa.Reaction conditions: 600°C/4 h.

3.Results and discussion

The geometrical changes, the variation of lattice parameters, carbon content andTcas a function of pressure with and without applying CHPD onin situMgB2tapes (binary and doped with 10 wt% C4H6O5) are listed in Table 1.After applying CHPD at 2GPa, the area of (Mg+B) and the fillin factors decreased in both binary and alloyed tapes(Table 1).The aspect ratioa/bof the present malic acid addedin situMgB2tapes showed a variation from 4.34 to 7.60 after applying CHPD up to 2GPa.On the other hand, it did not change much for binary tapes under the same conditions.The variation ofJcas a function ofBat 4.2, 20 and 25K of the malic acid doped unpressed tape (aspect ratioa/b=4.34)and pressed tape at 2GPa (aspect ratioa/b=7.60) for applied magnetic field in parallel and perpendicular direction is shown in Fig.1.A gradual shift of the curvesJc//andJc┴as a function ofBtowards higher values has been observed by the application of pressure.Atp=2GPa, the values ofB(104)//andB(104)┴were increased from 11.3 to 13.2 T, by 1.9 T and 10.6 - 12 T or by 1.4 T, respectively.

Fig.2.Variation of the anisotropy,Γ (Jc///Jc┴)vs.magnetic fiel at(a)4.2K and (b) 20K for binary and C4H6O5 alloyed MgB2 tapes (Monel sheath, Nb barrier) at 0 and 2GPa.Reaction conditions: 600°C/4 h.

The variation of the anisotropy factor,Γvs.Bat 4.2K and 20K for binary and malic doped tapes before and after CHPD is shown in Figs.2(a) and (b).The value ofΓfor the binary tape after CHPD increased with increasing aspect ratio from 3.87 to 6.96, in agreement with Kovac et al.[26].In malic acid doped tapes, we observed thatΓdecreased after CHPD, even though the aspect ratio increased from 4.34 to 7.60.However, all these tapes have been fabricated by the same conditions concerning deformation, heat treatment and sheath materials.The reason behind the present differences can be explained by the combination of two effects that influenc theJcanisotropy: carbon substitution and texture.TheJcanisotropy is more pronounced in the case of binary tapes after CHPD, where the texture effect is more visible.In thein situprocess, the texture is usually introduced by the mechanical deformation of the hexagonal Mg powder.On the other hand,it is well established that the malic acid addition improved the critical current densities at 4.2K and 20K [7,14], due to effective carbon substitution.The texture of the tape is affected by the impurity scattering due to carbon- doping and hence, the anisotropy in malic acid doped tapes is lower than in binary tapes, in good agreement with other researchers [21,26].At both operating temperatures, the striking result for the malic doped tape is that there is a further reduction of anisotropy after pressing at 2GPa.From Table 1, it is clearly seen that the aspect ratio is larger after pressing at 2GPa in malic acid doped tapes: this should cause stronger texture in the filamen and hence the anisotropy factor in these tapes should be higher after CHPD.But the interesting point is that malic acid tapes pressed at 2GPa exhibit lowerTcvalues and a higher carbon-substitution (thealattice parameter is slightly decreased) than the unpressed ones,which certainly causes the reduction of anisotropy.This statement is further supported by Fig.3,showing the XRD patterns of malic acid treated MgB2layers obtained from Nb/Monel sheathed tapes, at 0 and at 2GPa.These XRD patterns indicate that the main phase is well developed hexagonal MgB2,with small peaks corresponding to MgO impurity.The effect of high non-superconducting MgO impurity is detrimental on the transport current carrying capability of MgB2superconductors.Our previous study shows that higher amount of malic or tartaric doping on MgB2enhances the possibility of MgO generation and hence, leads to poor superconducting properties [29].In this work, we have chosen the optimum(10 wt% malic acid) doping level.It is also interesting to notice from XRD peaks that the relative intensity of the (0 0 2) diffraction peaks of malic treated MgB2at 0GPa is higher than that one after 2GPa.This result clearly indicates thatc-axis grain alignment of the malic treated MgB2layer is obtained by the CHPD process,and that the degree of alignment decreases with increasing pressure and additional carbon substitution.TheΓis influence by thisc-axis grain alignment as discussed above.

At 4.2 and 20K, almost isotropic behavior found at lower field in both binary and malic doped tapes, regardless of CHPD condition.From Figs.2(a) and (b), it is interesting that even ata/b=7.60 after pressing at 2GPa, the anisotropy ratiosΓwere quite small at 4.2 and 20K in malic tapes compared to 0 GPa: for example, at 4.2K after 2GPa, it isΓ=Jc///Jc┴= 1.22 and 1.60 at 10 and 14 T, respectively(from Fig.1) for malic doped tapes.These values are considerably higher with tapes treated at 0GPa (a/b=4.34), the values being 1.35 and 1.80 at 10 and 14 T, respectively.The corresponding values for 20K at 2GPa were 1.13 and 1.30 at 5 T and 7 T, respectively, the values for 0GPa being 1.20 and 1.51 at 5 T and 7 T,respectively.Forin situMgB2tapes,these values ofΓare certainly among the lowest ones ever reported[23-24, 27], which is assumed to be the consequence of various parameters during the low energy ball milling e.g.mass of the balls, rotating speed of the planetary mill and milling duration.More C substitution and reduction ofTcafter CHPD played an important role here.From Table 1, it follows that the lattice parametersadecreased and hence the C substitution increased with pressure, as documented by the valuesa=3.0749 A° andc=3.5240 A° atp=0GPa, anda=3.0721 A°andc=3.5236 A° after pressing atp=2GPa.This explains the enhancement ofJcandBirras well as the reduction of anisotropy after CHPD at all fields in agreement with Kovac et al.[23-26] who described a series ofJcanisotropy results correlated with various additives, sheath materials and mechanical deformations.In CHPD treated tapes, the amount of C substitution from malic acid into boron is enhanced.This is due to a reduced reaction path under the same reaction conditions, i.e.600°C/4h.This can be explained by an improved packing factor of the precursor powders in the densifie fil aments, resulting in shorter reaction paths.This phenomena makes the C substitution more effective in doped tapes, leading to a reduction ofTcas well as anisotropy due to poor texturing.

Fig.3.X-ray diffraction patterns for the C4H6O5 alloyed MgB2 tapes(Monel sheath, Nb barrier) after 2GPa for observing the c-axis grain alignment.

This interesting observation can be further supported by the microstructural characterization conducted by SEM and TEM shown in Figs.4(a), (b) and (c).The observed improvement ofJcafter applying CHPD at 2GPa (Fig.1) in malic doped wires [7] is due to the appearance of less voids (Fig.4(a) and(b)) and to additional defects in the lattice (Fig.4(c)).High resolution scanning transmission electron microscope(STEM)investigations revealed that the subgrain lattice is randomly oriented and that many crystal lattice defects, such as stacking faults and dislocations found in subgrains are due to the typical combination of C doping and cold pressure.In our previous work [30], we investigated this interesting behavior of CHPD in binary and malic acid doped wires, based on theTcdistribution as well as on connectivity and the percolation.In this work, the results arising from the analysis of theTcdistribution and those from resistivity measurements were combined and found that the minimum superconducting volume fraction required for the percolation of a superconducting path is strongly reduced in CHPD treated samples.

The effect of operating temperature onΓafter pressing at 2GPa (only with malic acid doped tapes) with similarb/aratio is shown in Fig.5.For all operating temperatures, a sharp and nearly linear increase ofΓhas been observed at lower fields which turns into an exponential one at higher fields The measuredΓvalues increased rapidly with temperature e.g.Γ≈1 (8 T) atT=4.2K,?！?.6 (8 T) atT=20K and?！?.65 (4.5 T) atT=25K.HighJcanisotropy at elevated temperatures can lead to serious problems in designing superconducting magnets for various applications, such as in the case of Bi-2223/Ag tapes or YBCO thin film at 77K, where the total transport current of the coil winding has strongly been reduced by the high radial fiel component [31].

Fig.4.(a, b) High resolution scanning electron microscopy (SEM).(c) Dark fiel (DF) scanning transmission electron microscopy (STEM).Investigations revealed a reduced porosity, stacking faults and dislocations, due to the typical combination of C- doping and cold pressure.

Fig.5.Variation of the anisotropy, Γ (Jc///Jc┴) vs.magnetic field at 4.2, 20 and 25K for C4H6O5 alloyed MgB2 tapes (Monel sheath, Nb barrier) after 2GPa.Reaction conditions: 600°C/4 h.

Fig.6.Variation of the irreversibility fiel (Birr) vs.operating temperatures(T) for C4H6O5 alloyed MgB2 tapes (Monel sheath, Nb barrier) at 0 and after 2GPa.Reaction conditions: 600°C/4 h.

The variation of the irreversible field (Birr) with operating temperature at 0 and 2GPa of malic acid doped tapes has been plotted in Fig.6.It has been shown that the expected convergence ofBirr//andBirr⊥when approaching nearTc.At 20K,Birr//andBirr⊥are almost equal at around 12.2 T for 2GPa and at 7.7 T for 0GPa.The difference betweenBirr//andBirr⊥increases at lower temperatures in tapes treated with both 0 and 2GPa pressures.The extrapolation toT=0 yieldsBirr//(0) = 22 T andBirr⊥(0) = 20 T for the tape with 0GPa,the corresponding values for the tape treated at 2GPa being 25 and 23 T, respectively.

It is known[19,32]that up to 30%of the volume are transformed to MgB2after mechanical alloying (Mg+Bmixtures treated with high energy ball milling for long time).After reaction, large values of the anisotropy factorΓare observed,in particular at higher fields due to a strong texturing of the MgB2phase in the tape filaments In this work, no trace of reacted MgB2was observed by X-rays in the mixtures of Mg+Bafter low energy ball milling.However, a degree of texturing was still present in the reacted binary tapes after 2GPa, especially at 4.2K, as evidenced by the differentΓin Fig.2.The anisotropy in tapes based on low energy ball milled powders is originated due to the particular conditions at the interface between MgB2filamen and Nb barrier during the reaction process [23,24].As shown by Lezza et al.[27],the degree of texture is indeed maximum at the MgB2/Nb interface and decreases towards the center of the tape.In our previous articles [6,7,31], we reported that CHPD induces a strong improvement ofJcandBirrin binary and alloyed MgB2wires due to better grain connectivity.

4.Conclusion

In this article, it has been shown that the effects of CHPD on anisotropy in binary tapes as well as in malic acid doped MgB2tapes are correlated.For the pressed binary tape at 4.2 and 20K, the anisotropy ratiosΓwere larger compared to unpressed tapes due to a certain rolling induced degree of texturing in the tape filaments TheΓvalues were reduced for the CHPD treated malic doped tapes densifie at 2GPa at 4.2 and 20K compared to binary and unpressed malic acid doped tapes.Thus, CHPD contributes to the significan anisotropy reduction in MgB2tapes doped with malic acid by substituting a higher amount of C into the lattice due to the shorter reaction path in densifie filaments This reduction of anisotropy may be caused from the random oriented subgrain defects in the lattice.This observation is further supported by the reduction of thec-axis grain alignment in the MgB2layer obtained by both the applied pressure and the additional carbon substitution.The value ofΓis influence by thisc-axis grain alignment,as discussed above.In summary, a signifi cant reduction of the anisotropyΓhas been found in MgB2tapes doped with malic acid after the CHPD process, which may be advantageous for superconducting magnet design and application.

Acknowledgments

This work was supported by the Australian Research Council (Grant No.LP160101784).A.K.thanks the Researchers Supporting Project (RSP- 2019/127), King Saud University,Riyadh, Saudi Arabia for the support.This work was performed in part at the Queensland node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and microfabrication facilities for Australia’s researchers.M.M.acknowledges an internal funding project of the University of Osijek (ZUP-2018).