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I NSTITUTE OF P HYSICS P UBLISHING S UPERCONDUCTOR S CIENCE AND T ECHNOLOGY Supercond. Sci. Technol. 16 (2003) 865–870 PII: S0953-2048(03)61071-7 Reactivity betweenNd 1+ x Ba 2 − x Cu 3 O 7 − δ + x /2 and Nd 4 − 2 z Ba 2+2 z Cu 2 − z O 10 − 2 z phases in superconducting NdBaCuOpowders and melt textured bulk samples Marcello Gombos 1 , Vicente Gomis 2 , Anna Esther Carrillo 3 ,Antonio Vecchione 1 , Patrizia Tedesco 4 , Sandro Pace 5 and Xavier Obradors 3 1 INFM-Coherentia Salerno and Universit`a degli Studi di Salerno, Dip. Fisica‘E. R. Caianiello’, Via Salvador Allende, I-84081 Baronissi (SA), Italy 2 Universitat Politecnica de Catalunya, Dpto Fisica Aplicada, Barcelona, Catalunya, Spain 3 ICMAB-CSIC, Campus de la UAB, E-08193 Bellaterra (BCN), Catalunya, Spain 4 I.N.F.M U.d.R. Pavia and Universit`a degli Studi di Pavia, Dip. Fisica ‘A.Volta’,Pavia (PV) Italy 5 INFM UdR Salerno and Universit`a degli Studi di Salerno, Dip. Fisica ‘E. R. Caianiello’,Baronissi (SA) ItalyE-mail:
[email protected] and
[email protected] Received 10 March 2003, in final form 23 May 2003Published 2 July 2003Online at stacks.iop.org/SUST/16/865 Abstract The high temperature reaction between the superconducting Nd123( x )(Nd 1+ x Ba 2 − x Cu 3 O 7 − δ + x/ 2 ) phase and Nd422( z ) (Nd 4 − 2 z Ba 2+2 z Cu 2 − z O 10 − 2 z ),leading to substitutions of Nd in the Ba sites of Nd123, was analysed. Forthis purpose, differential thermal analysis (DTA) was performed on powderswith different Nd422 addition. Measurement results were compared with thevalues, reported in the literature, of the peritectic temperatures in function of Nd123 substitution. The superconducting transition critical temperatureanalysis of melt-textured samples shows that the use of Barium rich Nd422allows significant improvements of the superconducting parameters of produced samples, avoiding the occurrence of the Nd–Ba substitutions. 1. Introduction Although the high temperature RE123 (REBa 2 Cu 3 O 7 − δ ,RE = Yorrareearths)superconductingfamilyhasbeendeeplystudied, the phenomenon of cationic substitution, correlatedto the stoichiometric instability of nonsubstituted phases, hasstill not been analysed in a satisfactory way.The presence of cationic substitution in Nd123 wasobserved for the first time by Yoo and McCallum [1]. Theyobserved that the rates among the constituting elements werevariable, leading to the formula:Nd123 (x) = Nd 1+ x Ba 2 − x Cu 3 O 7 − δ + x/ 2 (1)where x is the variable solubility parameter which lies ina limited range [ x min ; x max ], and the variation of oxygenion number preserves ion charges balancing [2]. Differentexpressions of this variability are reported in the literature,whose equivalence to (1) is discussed in the appendix.However, by increasing x , a monotonic decrease of Nd123superconducting transition temperature [3, 4] occurs, untilsuperconductivity completely disappears for x > 0.4. Onthe other hand, the substituted phases seem to play a rolein magnetic flux pinning [5], determining in bulk samplesthe critical current densities for high magnetic fields, higherthan Y123 samples [6]. Substitutions are favoured by hightemperature and high oxygen pressure treatments. In fact 0953-2048/03/080865+06$30.00 © 2003 IOP Publishing Ltd Printed in the UK 865 M Gombos et al Figure 1. Solubility region of Nd123( x ) and Nd422( z ) in the pseudo-ternary phase diagram of the (1 / 2)Nd 2 O 3 –BaO–CuO system (inset).Bold full segments are the solubility line (up) for Nd123( x ) and (down) for Nd422( z ). Any segment (thin line) passing through the totalcomposition point represents an allowed decomposition in the substituted phases corresponding to the extremes of the segment(decomposition line). The two darker segments are the initial decomposition line and the final one of reaction (3). samples characterized by the minimum values of thesubstitution parameter were achieved by fabrication processesin oxygen reduced atmosphere ( P O 2 0 . 01 atm), followed bylow temperature oxygenation processes.In the NdBaCuO system, cationic Nd–Ba substitutionsoccurnotonlyinNd123( x )buteveninNd422( z )phase,leadingto the formula [5]:Nd422 (z) = Nd 4 − 2 z Ba 2+2 z Cu 2 − z O 10 − 2 z . (2)Besides formula (2), a few equivalent expressions of Nd422cation substitution are reported in the literature, and also theseare discussed in the appendix. Experimentally measured z variation range at 890 ◦ C is about [ − 0.2; 0.2] (calculated fromthe values reported in [7]). Formula (2) suggests that Ndsubstitutions in Nd422 have a more complex behaviour thanin Nd123.During melt-texturing processes, Nd422 is generallyadded to Nd123 in the precursor powders. This is done inanalogy to the addition of Y211 to Y123, to avoid liquidlossesandtoprovideflux-pinningcentres,necessarytoachievehigher critical current densities J c . However, an excessiveNd422 content lowers J c , in contrast with the Y123 case.In this work, to explain this effect, we suggest areaction involving Nd123 and Nd422 phases that simplyvaries their substitution parameters by rearranging ioniccontents. We verified this hypothesis by performing DTAanalysis on powders as a function of the different Nd422content. Polarizedlightimagesandsuperconductingtransitionmeasurement of melt-textured samples confirm the validity of our suggestion. 2. Theoretical discussion Following (1) and (2), substitutions between Nd and Ba ions may occur both in Nd123 and Nd422. In the presence of both the substituted phases, after a melt-texturing process, thetransition temperature decreases, suggesting a correspondingincrease of the Nd123 substitution parameter x . For thisreason, the following reaction should take place:Nd123 (x 1 ) + n Nd422 (z 1 ) ⇒ a 2 Nd123 (x 2 ) + n Nd422 (z 2 ) (3)where a 2 = ( 5 − x 1 )( 5 − x 2 ) with the condition n(z 1 − z 2 ) = 3 (x 1 − x 2 ) 5 − x 2 .Since x 2 x 1 is always experimentally verified,a preferred direction exists in (3), determined by thethermodynamics of NdBaCuO system. This behaviour maybe explained on the basis of a simple statistical model [8],requiring Nd123 to be in the presence of a reservoir of Nd 3+ ions. In (3) the addition of Nd422 provides excess Nd ions upto the maximum z , consenting to Nd123 the approach towardsits equilibrium state.It is possible to illustrate (3) by the use of the pseudo-ternary phase diagram of the (1 / 2)Nd 2 O 3 –BaO–CuO system(figure 1). In this diagram the extremes of solid solutionlines of Nd123( x ) and Nd422( z ) identify a quadrangular area(solubility region). Each Nd123( x 1 ) and Nd422( z 1 ) mixture isrepresented by a point inside this area determined by its totalcomposition. However, any point can be decomposed again inmixturesofNd123( x )andNd422( z )withdifferentsubstitutionparameters’ values (within the solid solution lines), provided866 Reactivity between Nd123( x ) and Nd422( z ) phases in superconducting NdBaCuO powders and melt textured bulk samples that they correspond to the extreme points of a segment(decomposition line) passing through the initial mixture point.In our work, we used Nd123( x 1 ) and Nd422( z 1 ) powderswith x 1 ≈ 0 (supposed stoichiometric for all calculationpurposes) and z 1 ∈ { 0 ; 0 . 2 } . In the hypothesis x 1 = z 1 = 0,the parameters in (3) will be (for n 5 / 3) x 2 = 5 n/( 15 + n) and a 2 = 1 + n/ 15, while for z 1 = z max = 0 . 2, equation (3)implies x 2 = x 1 ,a 2 = 1 and no change is expected to occur. 3. Experiment preparation Nd123(0) and Nd422( z ) powders were prepared byconventional solid state reaction, mixing stoichiometriccompounds of high purity Nd 2 O 3 (99.99%, Aldrich), CuO(99.995%, Aldrich) and BaCO 3 (99.98%, Aldrich) [9].The Nd123 phase was obtained by calcination of theprecursors by heating at 880 ◦ C for 30 h in air followed byquenching at room temperature. This process, repeated threetimes with two intermediate grindings, gave the orthorhombicphase-pure Nd123 powder.The Nd422(0) and Nd422(0.2) phases were obtainedby a single 30 h calcination cycle, with the same thermalschedule used for Nd123. Nominal composition of powderswas confirmed by energy dispersive spectroscopy (EDS) andx-ray diffraction (XRD).PowdersforDTAanalysiswerepreparedbymechanicallymixing, in a ball mill for 3 h, proper amounts of Nd123 andNd422(0) powders to obtain the desired n = Nd422 / Nd123molar ratios.Precursor powders for our bulk samples were prepared bymixing, withthesameprocedure, twodifferentNd123–Nd422mixtures(Nd123+0.2Nd422(0)andNd123+0.2Nd422(0.2)).The resulting powders were uniaxially pressed into pellets.Pellets of 1 cm diameter and a thickness of about 8 mmwere then solidified by top seeding growth technique, usingMgO crystals as seeds placed on the centre of the top surfaceof samples as explained elsewhere [11, 12]. In order to avoidspurious growth and contaminations, the samples were alwaysplaced on previously sintered Y211 pedestals and placed inalumina crucible.The samples underwent the following thermal treatment:theywereheatedtothemeltingtemperaturebyatwo-stepramp(at 300 ◦ C h − 1 up to 800 ◦ C, then at 60 ◦ C h − 1 to 1135 ◦ C).Afterwards they were held for 5 h at this temperature, to allowthem to melt completely. Then they were cooled, at 60 ◦ C h − 1 ,to 1075 ◦ C (temperature at which the growth is supposed tostart) and held at this temperature for 2 h to start the orienteddomain growth around the seed. The growing rate was thenincreased by cooling, at a cooling rate of 0.6 ◦ C h − 1 , thefurnace to 1060 ◦ C, the temperature at which it was held for1 h to complete the domain growth along the entire pellet.Finally the furnace was cooled to room temperature. All thedirectional solidification processes were performed in air.The samples were subsequently annealed in flowing pureoxygen gas at the rate of 20 l h − 1 . The temperature scheduleconsisted of rapidly heating the sample in the furnace to500 ◦ C in 3 h, holding it at this temperature for 1 h, thencooling it to 300 ◦ C at 8 ◦ C h − 1 , holding it again at this newtemperature for 200 h, and finally cooling down the furnace toroom temperature. Figure 2. DTA study of Nd123( x min )–Nd422(0) mixturescharacterized by different values of the Nd422 / Nd123 per centweight ratio (wt%). Table 1. Nd422 / Nd123 per cent weight ratio (wt%) and theequivalent molar ratio ( n ) versus the lowering of peritectictemperature with respect to zero additioned powders (figure 3).wt%(%) n δ T onset ( ◦ C) δ T peak ( ◦ C)0 0 0 0.01 0.007 9 8.32 0.015 16 12.05 0.037 17 13.210 0.079 19 13.820 0.178 21 16.830 0.305 23 17.740 0.474 26 20.050 0.711 26 20.4 4. DTA analysis on powders StructuraldisorderisalsocorrelatedtoaloweringofNd123 T p .This is clearly shown in the higher part of the existence regionof Nd123 as shown by [10]. Observations of such a decreasein powder mixtures are symptomatic of the occurrence of reaction (3).To observe this phenomenon, a DTA analysis ondifferently additioned powders was performed. The startingpowders are Nd422 additioned Nd123 with a per centNd422 / Nd123 weight ratio, wt% = { 0%, 1%, 2%, 5%, 10%,20%, 30%, 40%, 50% } . This is equivalent to the previouslywritten Nd123 + n Nd422 with n = 729 . 551082 . 72 wt% ( 100% − wt% ) the calculated values are in table 1.The powders were mixed and analysed by means of aPerkin–Elmer DTA analyser. A ramp rate of 10 ◦ C min − 1 wasused in the range of 600–1200 ◦ C.In figure 2, we can observe the progressive displacementof the negative peak towards lower temperatures. This effectis more pronounced for low Nd422 addition, as we can noticeby comparing the peaks displacement δ T related to wt% = 1–5% samples with δ T (wt% = 40%) (table 1). Thisaffirmation is confirmed by the complete set of data shownin figure 3.867 M Gombos et al Figure 3. Positions of peak’s onset and minimum in function of Nd422(0) addition (wt%). Figure 4. Polarized light images of a Nd123 + 0.5Nd422(0) pellet (left), and a Nd123 + 0.5Nd422(0.2) pellet (right), showing precipitatedistribution. A comparison of these data to those of Goodilin et al [10]is possible only for the datum around x 2 = 0.1 (wt% = 30%corresponding by (3) to x 2 ≈ 0.11). In fact, the coincidenceof measured temperature respectively of peak’s minimum oronset for n = 0 has to be postulated to take into accountany eventual difference due to furnace calibration. Byextrapolation of the data in the literature, we obtain a resultdiffering by only about 2 K in defect of onset temperaturevariation and 3 K in excess of peak’s one, so that effectivelyreaction (3) results reasonably explain this effect. 5. Optical microscopy observation andsuperconducting transition analysis onmelt-textured samples As shown in the preparation section, pellets of 1 cm diameterof Nd422 additioned Nd123 characterized by two differentvalues of the z substitution parameter were prepared. Theas-prepared pellets were polished with clothes down to 1 µ mgrain size. Afterwards, they were observed by a polarizedlight microscope to identify their precipitate characteristics.A comparison between the distribution and mean dimensionsof precipitates in Nd422(0) and Nd422(0.2) additioned pellets(figure 4) does not show any difference between them. Thisfact assures us that difference in barium content of the addedphase does not affect growth dynamics. Moreover, since nostructural difference (namely, different distribution of Nd422pinning centres) is present in our samples, any differencein their T c should be attributed to the different value of thesubstitution parameter of Nd123 due to the occurrence of reaction (3).Afterwards, their superconducting transition was studiedby dc susceptibility measurements (figure 5). The samplessynthesized with the addition of Barium rich Nd422 show a T c onset of 92 K, with a transition width of about 4 K. In contrast,samples fabricated with the same thermal treatments, but withthe addition of the stoichiometric Nd422, show a T c onset of about 88 K with a wide transition completing only at about60 K (figure 5).Thederivative with respect to T ofthe susceptibility curveof the Nd422(0) additioned samples (inset in figure 5) showsa superposition of different transitions. The principal ones aretwoquitelargetransitionscentredaround83Kand74K.Sowecanindividuatethepresenceofatleasttwodifferentsubstitutedphases, meaning that the sample is not homogeneous. This isprobably due to the duration of the various thermal treatmentsthat is insufficient to allow all the Nd123 to be involved in868 Reactivity between Nd123( x ) and Nd422( z ) phases in superconducting NdBaCuO powders and melt textured bulk samples1.00.90.80.70.60.50.450 55 60 65 70 75 80 T (K) χ ( T ) ( a . u . ) 85 90 95 100 Figure 5. Susceptibility measurements of a Nd123 + 0.5Nd422(0)pellet, and a Nd123 + 0.5Nd422(0.2) pellet. Inset shows thederivative, with respect to temperature, of the susceptibility curve of the Nd422(0.2) additioned pellets. the reaction. This makes the final substitution state lesspredictableandgenerallydifferentfromtheoneexpectedfrom(3). It is likely that sufficiently longer thermal treatmentswould allow the homogenization of the samples by diffusionmechanisms until reaching the calculated substitution state. 6. Conclusions The analysed data show how reaction (3) betweenstoichiometric Nd422 and quasi-stoichiometric Nd123 mayexplain the observed reduction of T c in highly additionedtextured samples. DTA analyses made on different mixesof these two phases confirm, infact, a progressive lowering,in function of additioning, of peritectic fusion temperaturethat appear to correspond to the one observed in function of substitution parameter reported in the literature. Observationsmade on bulk samples suggest that using substituted Nd422,with high barium content, may significantly lower this effect,while in principle providing the same beneficial effect inmagnetic flux pinning as those of ordinary Nd422. Acknowledgments We thank Regina Ciancio, from Dipartimento di Fisica‘E.R. Caianiello’, Universit`a degli Studi di Salerno, forassistance on susceptibility measurements, and Dr EnricoVaresi, from Laboratorio M.D.M.-I.N.F.M., for usefuldiscussions on formulae. Appendix. Alternative formulae and the mechanismof Nd123 / Nd422 reaction Every formula of the kindNd123 q (y) = Nd 1+ αy Ba 2+ βy Cu 3+ γy O 6 , 5+ ( 3 α/ 2+ β + γ)y + δ ′ (A.1)is equivalently usable to describe Nd123 behaviour, providedthat the mathematical conditions γ = α + β and β = 2 α areverified. Formula (A.1) is in biunivocal correspondence with(1) by the relationNd123 (x) = c Nd123 q (y(x)) ⇔ Nd123 q (y) = c ′ Nd123 (x(y)) (A.2)where c = 1 − α + β 2 α − βx ⇔ c ′ = 1 − (α + β) 3 yy(x) = 3 x( 2 α − β) − (α + β)x ⇔ x(y) = ( 2 α − β)y 3 + (α + β)y (these formulae can be inverted provided that β ∈ [ −∞; min (( 7 α/ 8 ) ; ( 49 α/ 26 )) ] ∪ [max (( 49 α/ 26 ) ; ( 7 α/ 8 )) ;∞ ]; thesolubility limits are y min = y ( x min ) and y max = y ( x max ) for β > 2 α and y min = y ( x max ) and y max = y ( x min ) for β < 2 α ).Analogously, we can represent the Nd422 by any formulaof the kind:Nd422 q (t) = Nd 4+ αt Ba 2+ βt Cu 2+ γt O 10+ ( 3 α/ 2+ β + γ)t (A.3)provided the mathematical conditions γ = α/ 2 and α = 2 β .The biunivocal correspondence with (2) isNd422 (z) = c Nd422 q (t(z)) ⇔ Nd422 q (t) = c ′ Nd422 ( z (t)) (A.4)where c = 1 − α + β 2 β − α = z ⇔ c ′ = 1 − α + β 2 β − αzt(z) = 6 z( 2 β − α) − (α + β)z ⇔ z(t)( 2 β − α)t 6 + (α + β)t with β ∈ [ −∞; min ( 0 ; 4 α/ 7 ) ] ∪ [min ( 0 ; 4 α/ 9 ) ; max ( 0 ; 4 α/ 9 ) ] ∪ [max ( 0 ; 4 α/ 7 ) ;∞ ] being the invertibility condition.We can now rewrite (3) by using two nonstandardformulae for Nd123 and Nd422, which areNd123 M (p) = Nd 1+2 p Ba 2 − p Cu 3+ p O k 1 (A.5)with p ( x ) = 3 x / (5 − x ) and c = 1 − ( x / 5) with respect to (1),andNd422 M (q) = Nd 4+2 q Ba 2 − q Cu 2+ q O k 2 (A.6)with q ( z ) =− 6 z / (4 + z ) and c = 1 + ( z / 4) with respect to (2).By using (A.4) and (A.5), reaction (3) may be rewritten in the simple form a Nd123 M (p 1 ) + b Nd422 M (q 1 ) ⇒ a Nd123 M (p 2 ) + b Nd422 M (q 2 ) (A.7)with a(p 1 − p 2 ) = − b(q 1 q 2 ) .An interesting characteristic of the previousrepresentation of reaction (3) is that, while the substitutionof each single compound changes, there is a parameter thatmay be called the mixture’s total substitution s = ( ap + bq ), which does not vary. Then the increase of p in Nd123(corresponding to an increase of x in the standard formula(1)) corresponds to a decrease of q in Nd422 (correspondingto an increase of z in (2)). The proportionality between thevariations of p and q depends only on the Nd422 / Nd123 ratioin the reaction. Particularly interesting is the case a = b : inthis case, in fact, ( p 1 − p 2 ) =− ( q 1 − q 2 ) = d and the reactionmay be written asNd123 M (p) + Nd422 M (q) ⇒ Nd123 M (p + d) +Nd422 M (q − d) (A.8)thus suggesting that the fundamental mechanism of reactionlies in the exchange of BaO for Nd 2 CuO 4 . In particular,the thermodynamically favoured direction is that with d > 0,meaning that Nd123 provides the BaO while Nd422 gives theNd 2 CuO 4 .869
