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Modified melt crystallization processes for improved RE-123 based bulk materials (RE=Y, Nd)
PII:
Applied Superconductivity Vol. 6, Nos. 2±5, pp. 61±76, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0964-1807(98)00088-X 0964-1807/98 $ - see front matter
MODIFIED MELT CRYSTALLIZATION PROCESSES FOR IMPROVED RE-123 BASED BULK MATERIALS (RE = Y, Nd) G. KRABBES*$, P. SCHAÈTZLE$, W. BIEGER$ and G. FUCHS$ $IFW Dresden Institute of Solid State and Materials Research, P.O.B. 270016, D-01171 Dresden, Germany (Received 15 January 1998; accepted 15 February 1998) AbstractÐThe considered modi®ed melt texturing processes bene®t by peculiarities of the quaternary phase diagram. Examples are discussed for YBaCuO by applying compositions which represent a mixture of Y123 and Y2O3 instead of Y2BaCuO5 or by processing with reduced oxygen partial pressures. The preferable orientation of single grains is initialized by seeding with Sm123. Large single grains (up to 2 in), high critical current densities (45 kA/cm2), high trapped magnetic ®elds (>0.7 T at 77 K in 1 in discs, and 9.6 T in a minimagnet at 47 K) and eectiveness for levitation (1.8 N per 1 g YBaCuO and 84 N total for 35 mm discs) have been achieved in the modi®ed processed YBaCuO materials. In contrast to Y123, nonstoichiometric metal±metal ratios are typical for the Nd123 superconductor and related solid solutions. The corresponding defects may enhance pinning but on the other hand they deteriorate Tc. The results of phase diagram investigations are used as a tool to control the cation stoichiometry and growth mechanism. This can be experimentally achieved by using an appropriate composition or by the oxygen potential controlled isothermal process. Critical current densities up to 75 kA/ cm2 have been achieved in NdBaCuO materials. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Since the discovery of high temperature superconductors (HTSC), several applications have been suggested which are based on the peculiarities of the magnetization loop in these hard superconductors or on the forces appearing between the bulk HTSC and an external magnetic ®eld. Examples were reported as magnetic bearings, linear frictionless transport systems, permanent magnets, ¯ywheels and motors (see e.g. Refs [1, 2]). The breakthrough to technical applications is depending on the availability of materials of high qualities with respect to jc, the trapped ®eld, irreversibility ®eld and levitation forces. High quality parameters have to be realized in bulks of a sucient size as it is representative for the considered application (i.e. larger 10 10 cm2). Meanwhile, it became clear that materials standards will dier for dierent applications. The stiness required for magnetic bearing is in¯uenced by pinning forces which are mainly determined by the intra-grain jc. On the other hand, high trapped ®elds are determined by the product jc d (d is the diameter of the internal current loop) and its realization may open new applications in engineering. Since the introduction of melt texturing by Jin et al. [3], Salama et al. [4] and Murakami et al. [5], numerous variants have been reported which dier mainly in the thermal processing and the pre-treatment of the precursor in some cases (see e.g. Ref. [6] as summarizing report). We consider crystallization processes which proceed in unconventional parts of the phase diagram whereby the equilibrium conditions near the phase boundary will be in¯uenced. First the experiences with the YBaCuO system will be reported concerning the substitution of the conventional Y211 excess by an admixture of yttria as well as the processing in varying oxygen partial pressures. More extensively, the modi®ed processing of NdBaCuO will be regarded. Nd123 based bulk materials are characterized by remarkably increased irreversibility ®elds compared with Y123, and also the remanence magnetization increases [7]. Furthermore, a dominant peak in the magnetization vs ®eld dependency was reported. Probably, there is an interrelation between this behavior and the peculiarities of the stability ®eld of the RE123 *Corresponding author. 61
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phases. The RE123 phases with large RE3+ ions form solid solutions. Usually it was assumed that the excess RE3+ substitutes Ba2+ on the corresponding sites according to the formula RE1 + yBa2 ÿ yCu3O7 2 d. On the other hand, indications were reported for the occurrence of vacancies on Ba sites to compensate most of the surplus charge carried by the excess RE3+. The occurrence of defects (point defects, clusters of point defects or precipitations) is related to the nonstoichiometry and it is generally assumed to be the source of extra pinning centres in these materials. Although the pinning mechanism is not yet really clear, the experience shows that handling and reproducible processing becomes much more complicated than in the case of YBaCuO. The superconducting properties such as Tc and jc of NdBaCuO depend very sensitively on the deviation from the 1:2:3 stoichiometry in the cation sublattice. Therefore, the stoichiometric ratio in the superconducting phase of the bulk must be controlled during the crystallization process. A method of stoichiometry control by controlling the chemical potentials and using appropriate admixed phases was proposed in Ref. [8]. A dierent method applies an environment containing less than 1% oxygen to stabilize the composition with optimum properties [9]. Previous results indicate that the detailed knowledge of phase diagrams is an important precondition for improving properties and size of single grain materials by controlling both the crystallization (near the liquidus surface) and the postannealing (in solidus and subsolidus ranges) processes. Partial information is reported in very recent references [10, 11]. The present work presents parts of the phase diagram in more detail. The conclusions for processing will be demonstrated for controlling both by appropriate admixtures and using varying oxygen partial pressures in an isothermal mode. PHASE DIAGRAM INFORMATION
The quasiternary projection in Fig. 1 represents the phase compatibility of the Nd±Ba±Cu±O system at 0.21 bar oxygen partial pressure at a ®xed temperature below the eutectic temperature. Compared with the well known Y123, the Nd123 phase (as an example for light RE systems) possess a homogeneity range. According to the Gibbs' phase rule, the chemical potentials (and consequently also the composition of the compounds) are determined, if the RE123 phase coexists under equilibrium conditions with two more condensed phases provided p(O2) and T are set (e.g. 0.21 bar and 8908C in Fig. 1). Otherwise, the composition of the RE123ss (ss = solid solution) is not unambiguously determined in a two-phase equilibrium, i. e. Nd123 + Nd422. The most important information with respect to melt processing, however, comes from the knowledge of the primary crystallization ®eld because it is well known that even the melt texturing process is proceeded by a primary crystallization mechanism [15±17], based on a ``peritectic'' reaction L 211 O2 123
Fig. 1. Subsolidus phase diagram of the Nd±Ba±Cu±O system at 8908C and p(O2) = 0.21 bar according to refs [12±14].
1
Modi®ed melt crystallization processes
63
Fig. 2. The polythermal section Y2BaCuO5±YBa2Cu3O7 ÿ d±`Ba0.375Cu0.625O' at p(O2) = 0.21 bar.
It is very important to note that the arising equilibrium melt in both considered systems is depleted in oxygen. It was shown in Ref. [18] that it corresponds to a formal CuO0.68 stoichiometry in the Y±Ba±Cu±O system at p(O2) = 0.21 bar. The exact value for the Nd±Ba± Cu±O system has not been determined yet. Due to the dierent oxygen contents of solid phases and melt (and/or suspension), thermogravimetry (TG) with very small heating or cooling rates is an useful tool for a careful analysis of melting and crystallization processes in the considered systems. The system Y±Ba±Cu±O The primary crystallization ®eld of YBa2Cu3O7 ÿ d is limited by the equilibrium conditions of the four univariant reactions, the reaction temperature of which is ®xed provided p(O2) is set (T is given in parenthesis for 0.21 bar oxygen partial pressure): 4 a Y2 BaCuO5 bL m1 cO2 1020 C m1 : YBa2 Cu3 O7ÿd 3
2
4 dL e1 eO2 899 C e1 : aYBa2 Cu3 O7ÿd bBaCuO2 cCuO 3
3
4 cY2 BaCuO5 dL p1 eO2 940 C p1 : aYBa2 Cu3 O7ÿd bCuO 3
4
4 cYBa4 Cu3 O9 dL p3 eO2 991 C p3 : aYBa2 Cu3 O7ÿd bBaCuO2 3
5
The `peritectic' reaction Equation (1) represents an univariant equilibrium (m1) for YBaCuO due to the ®xed composition of the 123 phase in this system for all gross compositions corresponding to Y123 or mixtures of Y123 + Y211. The well known section, Fig. 2, represents the reversible decomposition and growth of Y123 at the univariant temperature (10208C at 0.21 bar O2) and composition of the melt at this temperature. Figure 3 shows the fact that this univariant equilibrium is in¯uenced by the oxygen potential and it is limited by the invariant point i1 [19]. The system Nd±Ba±Cu±O The occurrence of solid solutions in the RE±Ba±Cu±O systems with large RE ions complicates both the phase compatibilities as well as the technologies of processing derived from these considerations. The study of the stability region of Nd123ss in ambient air atmosphere including its primary decomposition reactions was the topic of a series of papers of Goodilin et al. (see e.g. Refs [20, 21] and references therein). The maximum extent of Nd3+ substitution on Ba2+ sites (which shall be described by the variable y according to the formula Nd1 + yBa2 ÿ yCu3O7 2 d) approaches y 1 1.0 but only at T = 9958C. This temperature corresponds to the minimum solidus temperature: Nd123ss converts into melt and Nd2CuO4, and Nd123ss with smaller y remains. It was proved that the Nd3+ solubility in the Ba±Cu±O melt is higher than that of Y3+ [22, 23]. Below 9958C, Nd123ss coexists with Nd2CuO4 and
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Fig. 3. The stability ®eld of YBa2Cu3O7 ÿ d in dependence on temperature and oxygen partial pressure including subsequent reactions. When Y2O3 is initially added, the stability limit of Y123 is divided into a solidus line and a `quasi±liquidus' (Y211 + melt) line; both are being located at lower temperatures. Solidus and `quasi-liquidus' lines are the boundaries of the enhanced process window of the MCP process described in Section 3.1.
CuO. At a higher temperature, probably between 10458C and 10608C, Nd422 appears instead of Nd2CuO4. According to Ref. [20], Nd123 with y = 0.00 shall be stable in air between 9608C and 10808C. However, as a result of the present investigations, it was impossible to synthesize stoichiometric Nd123 (y = 0.00) in air. Small amounts of BaCuO2 were formed in all solid state preparations, and the approach closest to the stoichiometric composition was found to be ymin10.05 by EDX and WDX analyses in both solid state reaction products and melt-textured samples. Therefore, we suggest that the lower limit of Nd3+ substitution in air equals about ymin10.05. The decomposition temperature decreases by reducing the oxygen partial pressure, and the dependence of the decomposition temperature on the Nd fraction in the Nd123ss phase becomes increasingly pronounced [24, 25]. TG measurements of Nd123 with y = 0.00, 0.05, 0.10, 0.25 and 0.50, respectively, in the present study at p(O2) = 5 10ÿ3 bar and p(O2) = 5 10ÿ4 bar con®rm the remarkable narrowing of the Nd123ss homogeneity range at reduced p(O2), see Fig. 4. Nevertheless, the phase relations become much more complex. BaCu2O2 appears as an additional stable phase at p(O2)E 10ÿ3 bar which in¯uences the decomposition behaviour. In order to study the decomposition mechanism, soaking experiments (see e.g. Ref. [26]) of Nd123
Fig. 4. TG investigations of the stability limits of Nd123ss at p(O2) = 5 10ÿ4 bar and p(O2) = 5 10ÿ3 bar.
Modi®ed melt crystallization processes
65
Table 1. Soaking experiments of Nd123ss with dierent initial composition y0 at several oxygen partial pressures and temperatures p(O2)/bar 0.01
T/8C
y0
Mass loss/%
940
0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50
0.6 1.1 0.6 not deter. not deter. not deter. 4.2 5.0 7.0 16.0 15.2 5.9 51.3 32.7 0.7 0.9 1.2 2.4 6.5 31.6 3.1 12.6 5.8 2.8 5.6 21.5 4.1 27.4 30.8 1.3 2.0 2.5 2.4 6.7 30.4 2.3 6.1 15.7 56.3 50.2 25.8
985 995 1000 1020 0.001
900 930 945 960 985
0.0001
885 915 950 985
No. of EDX points 2 2 2 3 2 3 0 2 4 3 no 4 1 no 2 3 4 3 3 3 1 3 no 1 4 no 4 3 no 2 2 5 1 no no 3 no no no no no
Nd123 Nd123
Nd123 Nd123 Nd123
Nd123 Nd123 Nd123 Nd123 Nd123 Nd123 Nd123
Secondary phases+
yres* 0.047 0.253 0.480 0.004 0.244 0.177 Ð 0.055 0.026 0.082 phase 0.019 0.014 phase 0.065 0.261 0.491 0.065 0.116 0.106 0.030 0.087 phase 0.011 0.106 phase 0.039 0.027 phase 0.042 0.210 0.156 0.015 phase phase 0.047 phase phase phase phase phase
Ð Ð Ð Ð Ð Nd422 Ð Nd422 Ð Nd422 Nd422 Ð Nd422 Nd422 Ð Ð Ð Ð Nd422 Nd422 Ð Nd422 BaCu2O2 Ð Nd422 Nd422 (Nd422) Nd422 Nd422 Ð Nd2CuO4 Nd422 Ð Nd422 Nd422 Nd422 Nd422 Nd422 Nd422 Nd422 Nd422
(Cu2O) Cu2O residue residue residue residue
Nd2CuO4 Nd2CuO4 residue Nd422 residue residue residue residue (Nd422) Nd2CuO4 BaCu2O2 BaCu2O2 residue BaCu2O2 BaCu2O2 residue residue residue
*Calculated from EDX intensities according to yres=I(Nd)/[I(Nd) + I(Ba) + I(Cu)] 6.0±1.0. Phases in brackets inhomogeneously distributed
+
with dierent initial metal stoichiometries (y = 0.00 . . . 0.05, y = 0.25, y = 0.50) have been performed at several oxygen partial pressures and temperatures. The residual body was studied by SEM, the coexisting equilibrium phases were determined by XRD and EDX. An estimate of the total composition of the resulting Nd123ss phase was calculated from EDX measurements using a Nd123 single crystal with y = 0.00 as a standard, prepared by Wolf [27]. Selected results are summarized in Table 1. The results for p(O2) = 1 10ÿ3 bar are visualized in Fig. 5. The compositions formed at 9008C are placed near the initial compositions, indicating the Nd content to remain unchanged for all y = 0.05, 0.25 and 0.50, respectively. The scattering of the points indicates that the con®dence interval of the calculated stoichiometry amounts to about Dy = 0.05. At T e 9308C, a considerable mass loss is observed. Furthermore, Nd422 is detected. The Nd excess in the Nd123ss equilibrium phase is remarkably restricted to ymax<0.2 even if Nd123ss, with y = 0.50, is applied. Furthermore, the microstructural analysis of the residual substance of a soaking experiment starting from Nd123ss with y = 0.50 indicates needle like inclusions within the Nd123ss matrix, the EDX analysis of which corresponds to Nd2CuO4 (compare Fig. 6). At 9008C such inclusions have never been found. This refers to a primary decomposition reaction:
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Fig. 5. Composition of the Nd123ss phase in the residual substance of soaking experiments starting from Nd1 + yBa2 ÿ yCu3Oz with y = 0.00. . .0.05, 0.25 and 0.50 at every considered temperature.
4 1 ÿ DNd123 y1 y0 ÿ d aNd2 CuO4 b third phase r1 Nd123 y y0 3
6
at 9008C
7
at T10.2 completely vanishes and BaCu2O2 is found besides Nd422 and melt. Consequently, several phase equilibria arise at ®xed external conditions depending on the initial metal stoichiometry of Nd123ss. At T = 9458C and p(O2) = 1 10ÿ3 bar, we ®nd e.g. pure Nd123 for a low substitution degree adjoining to a three phase region Nd123 (y = const.)± Nd422-melt with constant composition which is displaced by a two phase region Nd422-melt with variable composition. Finally, for initial y values greater than 0.25, Nd422 coexists with BaCu2O2 and melt. It is really impossible to study all imaginable equilibria including the melt compositions for the entire temperature±oxygen partial pressure range. However, in order to produce Nd123 with superior properties, it is necessary to focus the attention on the preparation of Nd depleted samples.
Fig. 6. Residual solid of a representative soaking experiment revealing Nd123ss, Nd422 and Nd2CuO4 (T = 9308C, p(O2) = 0.001 bar).
Modi®ed melt crystallization processes
67
MODIFIED CRYSTALLIZATION PROCESSES
YBaCuO-processing The dierent methods of melt texturing reported so far (denominated MTG, MPMG, QMG, PMP etc. [6]) dier by the applied precursor materials, by the applied parameters for temperature, temperature-time schedules, temperature gradients and by the equipment, respectively. Modi®ed techniques were used as directional solidi®cation and Bridgman technique, zone melting and top seeding. All methods are conventionally based on the reversible reaction Equation (1) applying an admixture of Y211 phase to Y123 powder or any precursor mixture corresponding to a gross composition corresponding to x parts Y123 + y parts Y211. From the thermodynamic point of view, the crystallization process proceeds with all these `conventional' methods in the vicinity of invariant equilibrium conditions (m1, Equation (2)) for the peritectic reaction. Conventional melt texturing proceeds (in a ®rst approach) on line with the projection through the points Y211 and Y123 in the Gibbs triangle (compare Fig. 2). In contrast, the modi®ed melt crystallization process applied in this study proceeds o line with respect to the Y123±Y211 projection. This was investigated in detail for precursor mixtures made of Y2O3 and Y123. Modi®ed melt crystallization processesÐmixtures of Y123 and yttria. The idea of modi®ed processing is to determine conditions to avoid instabilities of the growth process due to local temperature and concentration ¯uctuations. It will be shown that by admixing a proper system inherent second phase instead of Y211, the crystallization by reaction in Equation (1) can proceed in a variable temperature range. As concluded from the polythermal section Y123±Y2O3 at p(O2) = 0.21 bar in Fig. 7, the `peritectic' reaction which is responsible for the crystallization does not proceed with invariant temperature conditions. This fact opens a remarkable enlarged process window for stable growth conditions. A second important feature of Fig. 7 is that Y2O3 and Y123 cannot coexist in equilibrium. Y2O3 reacts with a part of the Y123 precursor powder pressed into the green body in a solid state reaction in the pre-heating ramp before the solidus line will be achieved. Therefore, Y211 can be formed as small grains which are homogeneously distributed in the pressed pellet. The modi®ed melt crystallization process (MCP) is based on the previous conclusions. The green body is prepared from a mixture of Y123 (provider: Solvay) + Y2O3. The optimized formula contains 0.2±0.4 mol yttria per mol Y123 phase. An admixture of Pt in amounts up to 0.5% is recommended analogously to the conventional process. Sm123 seeds have to be placed appropriately onto the green body to achieve optimum seeding conditions in every desired orientation. Mainly the c-axes orientation perpendicular to the top was the goal of experiments. Seeding permits us to keep the supersaturation in the melt low and consequently the crystallization proceeds near the thermodynamic equilibrium which results in a controlled melt crystallization process.
Fig. 7. The polythermal section Y2O3±YBa2Cu3O7 ÿ d±`Ba0.4Cu0.6O' at p(O2) = 0.21 bar.
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The melt crystallization experiments were performed within the isothermal zone of a tube furnace. Typical process conditions are: seeding with Sm123 before the fast pre-heating ramp starts, ramping up to 10508C followed by slow cooling 0.5±1 K/h down to 9408C. After the oxidation at 3808C between 200 and 250 h the superconducting properties were measured as reported in Section 4. Low oxygen pressure processing of YBaCuO. Another modi®cation is based on the dependence of the equilibrium temperature of the `peritectic' reaction in Equation (1) on the applied oxygen partial pressure. Figure 3 represents the relationship between log p(O2) and the temperature of the univariant peritectic reaction m1 (above the invariant point i1). This line determines the equilibrium conditions provided that the reaction in Equation (1) proceeds univariantly as it is the case for ingots of pure Y123 or Y123 + Y211 mixtures, respectively. This low oxygen pressure modi®cation was applied to decrease the decomposition temperature which is important for using low melting substrates (e.g. Ag) [28]. The decomposition is ®xed by the parameters p(O2) and temperature. The solidi®cation at lower oxygen partial pressure is controlled by the oxygen diusion to the solidi®cation front. A further decrease of the melting temperature is achieved by the addition of Y2O3 which shifts the decomposition line towards lower temperature. In this case, however, the reaction in Equation (1) no longer proceeds in an univariant equilibrium. Analogously to the MCP process, the unique equilibrium line (Fig. 3) is split into a solidus and a pseudo-liquidus branch according to a two step process with the melting beginning at the solidus line and the complete decomposition of Y123 into melt and Y211 at the liquidus line, respectively. With respect to bulk materials, the low pressure modi®cations become more important for NdBaCuO and related materials and will be discussed there in more detail. Modi®ed processing of NdBaCuO bulk samples The oxygen controlled melt growth (OCMG) method [29] is conventionally accepted to be an appropriate technique to produce NdBaCuO bulk material with promising properties. This process proceeds at a reduced oxygen partial pressure which is kept constant during the solidi®cation. Nd123 of stoichiometric composition (y = 0.00) is stable at reduced p(O2) and its control is supported by the narrowing of the Nd/Ba homogeneity range at reduced p(O2). Analogous to the YBaCuO processing, Nd422 is usually admixed to Nd123 in the precursor material. However, the size of single grains is still restricted to several mm, and the interest in alternative processing arises. In the following sections we will show the signi®cant in¯uence of dierent admixtures on both, the stoichiometry control, even at p(O2) = 0.2 bar and the growth process. Furthermore, an alternative approach is proposed to process NdBaCuO bulk samples by varying the oxygen partial pressure during the crystallisation process. The in¯uence of system inherent admixtures. Despite of the advantageous eect of Y2O3 admixtures on the processing of YBaCuO bulk samples, an analogous application of Nd2O3 for producing NdBaCuO failed completely [8]. Although we succeeded to grow single grains up to 10 10 3 mm3 by conventional melt texturing techniques, the transition temperature remains in the order of 60±70 K even after long time oxygen annealing at 300±3808C. The reason for the resulting properties is attributed to the Nd±Ba homogeneity range in Nd123. The system inherent admixture determines both the equilibrium activities as well as the reaction path. Because Y123 has a neglectible homogeneity range with respect to the 1:2:3 cationic ratio, the resulting stoichiometry as well as the corresponding properties remain practically unchanged by any kind of addition. This is, of course, no longer valid for RE123 compounds with a RE±Ba homogeneity range and it becomes necessary to consider, furthermore, how admixed phases determine the cationic ratio of the growing RE123 compound. The dierent eect of Nd2O3, Nd422 and Nd2BaO4 admixtures on the resulting properties of melt textured Nd123 was predicted and proved in Ref. [8] for the ®rst time. The present study will throw light on the controlling in¯uence of these admixtures on processing in more detail. Figure 8 schematically demonstrates the equilibrium phase relations at several conditions which are relevant for the crystallization process of Nd123 starting from the precursor mixtures (I) Nd123 ÿ + nNd2O3 and (II) Nd123 + nNd2BaO4, respectively. Both precursor mixtures do not coexist at any conditions. Nd123 and Nd2O3 convert, in a subsolidus prereaction, into Nd123
Modi®ed melt crystallization processes
Fig. 8(a)±(d)Ðcaption overleaf.
69
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G. KRABBES et al.
Fig. 8. The in¯uence of the initial precursor composition on the resulting stoichiometry of the crystallized Nd123 phase (schematically): (I) Nd123 + Nd2O3, (II) Nd123 + Nd2BaO4 (a) T 1T(m1), (b) T = T(`liquidus') for precursor (I), (c) T = T(`liquidus') for precursor (II), (d) T = T(solidus) for precursor (I), (e) T = T(solidus) for precursor (II).
with an Nd excess (y>ymin) and Nd422. The reaction of Nd123 with Nd2BaO4 results in Nd123 with the minimum possible Nd content (y = ymin), Nd422 and Nd163 (for details see e.g. Ref. [8]). Figure 8(a) reveals the phase equilibria just below the `liquidus' temperature T(m1) of pure Nd123 according to Equation (8) 4 aNd422 bL m1 cO2 for p O2 1 10ÿ3 bar m1 Nd123 3
8
in the ternary NdO1.5±BaO±CuOx representation. Although Nd123 with y = ymin has become stable, the total compositions of both initial mixtures are still within the Nd422 + L two phase regions. However, the signi®cant distinction is the dierent composition and the copper (or CuOx) activity of the corresponding melts. The copper content of the melt L(I) which is formed from Nd123 and Nd2O3 is much higher than that of the melt L(II) which results from the Nd123 ÿ Nd2BaO4 precursor. Although Nd422 also forms a solid solution, EDX studies of equilibrated Nd123 ÿ Nd422 mixtures indicated a compound with nearly the exact 4:2:2 stoichiometry at any conditions independent of the metal stoichiometry in the Nd123 phase, thus indicating that the equilibrium composition of Nd422 depends quite weakly on the variation of the Cu activity.
Modi®ed melt crystallization processes
71
Figure 8(b) and (c) illustrate the phase relations when the liquidus reaction becomes relevant for the considered precursors. The corresponding temperatures are lower than the peritectic decomposition temperature T(m1) of Nd123 with y = ymin. The actual values depend on the amount of initially admixed secondary phase as well as on the oxygen partial pressure of the environment. The important dierence of both admixtures becomes obvious by comparing both ®gures. Whereas Nd123 with the minimum Nd excess (y = ymin) crystallizes from the suspension formed from mixture (II), the crystallization of the suspension from the initial mixture (I) leads to Nd123 with a ®nite (although small) Nd3+ substitution on Ba2+ sites, y0>ymin. The disadvantageous eect of the Nd2O3 addition also appears in the subsequent proceeding crystallization process: the Nd123 phase, which crystallizes in the range between the liquidus and the solidus temperatures, will be continuously enriched by Nd3+. This becomes obvious by Fig. 8(d). The composition of the ®nally crystallizing Nd123, marked by y1 in Fig. 8(d) distinctly diers from the composition with the minimum Nd content and the growing grain becomes inhomogeneous due to the variation of y during the process (y0Ey Ey1). On the other hand, Nd2BaO4 stabilizes a Nd depleted Nd123 with y = ymin during the entire process as shown in Fig. 8(e). Therefore, Nd2BaO4 should be an appropriate admixture from the thermodynamic point of view. Green pellets consisting of Nd123 (y = 0.00 . . . 0.05) and 0.25 to 0.4 mol Nd2BaO4 per formula unit Nd123 admixed with up to 1 wt% Pt were melt processed in a tube furnace within the isothermal zone. The optimum temperature±time schedule depends on the applied oxygen partial pressure. In air, after a ramping at 11008C, the temperature was decreased at 0.1 K/min from 11008C to 10408C. The samples were oxidized for 4±8 days in ¯owing O2 at 3008C [8]. Wolf et al. [27] prepared single grain NdBaCuO material with high quality (jc,max(3 T, 77 K, Bkc)1 65 kA/cm2, Birr,max111.8 T) using BaCuO2 admixtures. BaCuO2 determines the proceeding mechanism similar to Nd2BaO4. In Fig. 8(e) a possible total composition is marked with `W'. This composition also stabilizes Nd123 with a minimum Nd/Ba ratio. The amount of melt is higher compared with Nd123/Nd2BaO4 precursor material which may lead to a diminished amount of Nd422 inclusions but enhances the risk of melt loss. Substitution of Nd3+ by Y3+. Conventional low oxygen pressure processing (OCMG) and inherent admixed phases have been revealed to control the stoichiometry of the Nd123 phase, however, they do not promote the growth of enlarged single grains. The idea of the study in this part is to combine the stoichiometry control by OCMG with the widthening of the temperature range for the crystallization as it is typical for the liquidus behaviour of solid solutions. The latter is achieved by partial substituting of Nd3+ by Y3+ in Nd123. Synthesis of solid solutions (Nd, Y)Ba2Cu3O7 2 d and melt texturing in air were previously reported in Refs [30, 31]. Yttrium is located on the RE3+ sites and suppresses the substitution of Ba2+ by RE3+. Samples (up to 20 mm diameter) consisting of a (Nd1 ÿ yYy)Ba2Cu3O7 2 d precursor (synthesized by solid state reaction from Nd2O3, Y2O3, CuO and BaCO3) and Nd2BaO4 with 0.5% Pt admixed have been applied for the experiments. The melt crystallization process was performed at p(O2) = 10ÿ2 bar by fast ramping up to 10708C and consecutive slow cooling with 1 K/h from 1070 to 10008C. The oxygen annealing was performed as usual at 3008C. Although the samples consist of several grains, the grain size achieves up to 8 8 5 mm3. The in¯uence of the substitution by Y3+ on the crystallization process was investigated by model experiments performed in a thermogravimetric analyzer (TG) in a ¯owing atmosphere with p(O2) = 10ÿ2 bar. The cuprate melts are signi®cantly depleted by oxygen related to their solid parent compounds. Therefore, the melting process is accompanied by a drastic loss of oxygen whereas an uptake of oxygen (weight gain) indicates the crystallization process. Therefore, the latter can be characterized by the weight change dependent on temperature (or time). These model experiments were performed with small tablets, the composition of which was corresponding to the green body used for the melt crystallization process. Although the decomposition temperature decreases if Nd3+ is substituted by Y3+, the beginning of the solidi®cation does not depend on the Y3+ concentration and was observed at 10108C (Fig. 9). Once beginning, the solidi®cation of the small samples used in these experiments, proceeds very fast in the ®rst stage and is nearly completed immediately in the sample without substitutions. The fast step indicates the initial crystallization occurs with a large supersaturation. In the case
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Fig. 9. TG investigations of NdBaCuO (precursor mixture Nd-123 + Nd2BaO4+Pt), (Nd0.9Y0.1)BaCuO (precursor mixture (Nd0.9 Y0.1)-123 + Nd2BaO4+Pt) and YBCO (precursor mixture Y-123 + Y2O3+Pt) at p(O2) = 0.01 bar.
of Y3+ substitution, the initial fast step is followed by a period of slowly proceeding crystallization which, obviously, is caused by the diusion controlled process occurring in the extended temperature range. In contrast, the crystallization is not signi®cantly observed in the pure YBaCuO system at the applied oxygen pressure indicating the process to be much more in¯uenced by the oxygen diusion in YBaCuO [32]. Although supersaturation is large in the NdBaCuO sample when the crystallization is initialized, the absolute undercooling with reference to the equilibrium temperature (10208C at p(O2) = 10ÿ2 bar) is only DT = 10 K. Thereafter, the crystallization proceeds within the interval of less than 15 K. This interval can signi®cantly be enlarged to 30 K by seeding with a MgO crystal as shown in Fig. 10a. As expected, the undercooling from the beginning of growth becomes undetectably small. The solidi®cation interval is reduced to 20 K in air [Fig. 10b] for pure NdBaCuO. The crystallization proceeds continuously until the solidus is achieved. This behaviour is in contrast to the related YBaCuO experiment. The slow solidi®cation rate following the fast initial period may indicate that the crystallization is much more controlled by the diusion processes than in the Nd system. The ®nal step in the curve is caused by a univariant solidi®cation. Processing at variable oxygen partial pressure. An alternative approach to control crystallization and growth as well as the cation stoichiometry of the resulting Nd123 matrix is a low pressure process in which p(O2) is altered dependent on time instead of the temperature. This proposed `oxygen controlled isothermal processing' (OCIP) starts from an equilibrium suspension at p(O2) 11 10ÿ5 bar and T 1 9908C consisting of Nd422, BaCu2O2 and melt which has been formed from the precursor Nd123 + nNd2BaO4 (+Pt). Now, the oxygen partial pressure is slowly raised above the stability limit of Nd123 at the ®xed temperature [33, 11]. In order to choose optimum conditions, the study was supported by TG experiments. Figure 11 reveals the relative mass change of small pellets of the initial composition equilibrated at 10ÿ5 bar and 9908C when the oxygen partial pressure is suddenly raised to 10ÿ3, 5 10ÿ3 and 10ÿ2 bar, respectively. Whereas there is nearly no eect at 10ÿ3 bar, the crystallization at p(O2) = 10ÿ2 bar proceeds immediately. Only at 5 10ÿ3 bar do we ®nd two small distinct steps indicating a more controlled crystallization. This result is con®rmed by melt processing experiments. After an argon treatment of a Nd123/Nd2BaO4/Pt precursor material at 9808C and changing the atmosphere to 1 10ÿ3 bar oxygen, only a few small single grains of Nd123 are solidi®ed. In a second experiment, the ®nal oxygen content of the atmosphere is raised to 5 10ÿ3 bar. More and larger crystals are obtained. If 1 10ÿ2 bar oxygen is applied, a multicrystalline texture of the sample is achieved. In the OCIP method, the green pellets are isothermally pre-heated in ¯owing argon at 9908C for two days. Then, the oxygen partial pressure is enhanced by small steps up to 6 . . .7 10ÿ3 bar within a period of two days. After holding the samples at these conditions for a further two days, the tablets are oxidized for 4±8 days at 3008C.
Modi®ed melt crystallization processes
73
Fig. 10a. TG investigations of pressed small pellets of NdBaCuO (precursor mixture Nd123 + Nd2BaO4+Pt) with and without a MgO seed at p(O2) = 0.01 bar.
The proposed method may become a useful alternative for the following reason: in contrast to the comparable YBCO crystallization, the Nd3+ diusion through the melt proceeds fast enough to guarantee a sucient amount of Nd3+ at the crystallization front. Therefore, the Nd3+ diusion does not control the rate of the process which is obviously mainly caused by the higher Nd3+ content in the quasi-peritectic melt (see e.g. Ref. [22]). PROPERTIES
In this section, the superconducting properties of the NdBaCuO and YBaCuO materials which were processed by modi®ed methods are summarized. The modi®ed crystallization process (MCP) starting from pressed pellets of Y123 and Y2O3 was applied to produce single YBaCuO monoliths with a size up to 50 mm in diameter, rectangular single grain blocks up to 40 40 15 mm3 or 40 100 15 mm3 rectangular blocks with biaxial oriented multi seeded grains. Trapped magnetic ®elds up to 0.78 T were realized at 77 K. The levitation forces up to 84 N in monoliths with 26±50 mm diameter were measured with a SmCo magnet (d = 25 mm, 0.4 T). In small samples cut from this monolith, critical current densities with 45 kA/cm2 at 0 T and Hirr of 6.5 T were achieved (VSM measurements). The highest trapped ®eld achieved so far with MCP±YBaCuO materials is 9.6 T at 47 K. Using the OCIP method, NdBaCuO grains up to 5 5 2 mm3 have been grown. The properties of several selected samples are summarized in Table 2. The jc(H) curves have been
Fig. 10b. TG investigations of pressed small pellets of NdBaCuO (precursor mixture Nd123 + Nd2BaO4+Pt) and YBCO (precursor mixture Y-123 + Y2O3+Pt) at p(O2) = 0.2 bar.
74
G. KRABBES et al.
Fig. 11. TG investigations of the isothermal crystallization of NdBaCuO from a suspension formed by a Nd123/Nd2BaO4/Pt precursor at 9858C for several oxygen partial pressures.
calculated from VSM measurements with a sweep rate of 16 mT/s using the Bean's model. Tc amounts to usually 94±96 K and the transition is very sharp (usually DTc 1 1 K). The jc(0 T, 77 K) values reach up to 75 kA/cm2. Most samples reveal a plateau of about 20 to 25 kA/cm2 at 1 to 2 T for Bkc. Plateau levels of about 40 kA/cm2 were found in several samples. The irreversibility ®eld amounts more than 8 T for samples which do not exhibit a pronounced peak at 1 to 2 T but a high jc(0 T) value. Larger bulk samples of NdBaCuO were achieved by a melt process (slow cooling with 1 K/h from 1070 to 10008C) with a pressed sample of (Nd0.94 Y0.06)BaCuO (d = 20 mm) admixed with Nd2BaO4 under an oxygen partial pressure of 10ÿ2 bar. The trapped magnetic ®eld was measured with a hall sensor and showed maximum values of 100 mT (Bz=0). The multigrain structure of the sample is shown [Fig. 12a] by the existence of several maxima. Levitation forces up to 6.0 N were measured with a SmCo standard magnet (d = 25 mm, B0=0.4 T) in this sample [Fig. 12b]. Due to the multigrain structure of the bulk sample the zfc curve becomes broad. CONCLUSION
Detailed information on phase diagrams in the subsolidus and liquidus areas have built a fundament for melt crystallization processing of YBaCuO bulk materials of high quality. The processing of NdBaCuO as an example for light RE related HTSC has turned out to be much more dicult. The solid solution like behaviour of RE123 (RE = La, Nd, Sm) and the strong in¯uence of any deviation from the stoichiometric ratio 1:2:3 for RE:Ba:Cu on the superconducting properties requires the control of both the stoichiometry and the conditions of stable growth to obtain large single domains. Forthcoming results on phase equilibria have been contributed and modi®ed processes have been applied to NdBaCuO as (1) controlling chemical potentials by system inherent admixtures, (2) substitutional solid solutions (Y for Nd) and (3)
Table 2. Superconducting properties of selected samples melt processed with the OCIP method Sample
LT99/2 WT08 WT09 WT10 WT16 WT32 WT33 WW74
Susceptibility measurements
Magnetization measurements
Tc/K*
DTc/K$
jc(0 T, 77 K)/ kA/cm2
jc(1.5 T, 77 K, Bkc)/ kA/cm2
Birr/T%
94.9 94.7 94.4 95.6 95.5 93.9 93.1 Ð
1.6 1.2 1.5 0.6 0.6 1.5 0.4 Ð
46.7 Ð 73.7 62.0 72.9 43 39.5 41.4
15.8 Ð 39.8 20.8 19.9 21 16 27.1
6.9 Ð 7.6 18.8} >8 6.8 6.8 6.0
*Tc 1T( 0rel = ÿ 0.5). $DTc 1T( 0rel = ÿ 0.1) ÿ T( 0rel = ÿ 0.9). %m(Birr) < 50 A/cm2. }Estimated
Modi®ed melt crystallization processes
75
Fig. 12a. Trapped magnetic ®eld of a bulk NdBaCuO sample processed under a reduced oxygen atmosphere of p(O2) = 0.01 bar.
Fig. 12b. Levitation force of a NdBaCuO sample (d = 20 mm) processed under a reduced oxygen atmosphere of p(O2) = 0.01 bar.
the oxygen controlled isothermal process (OCIP). The properties, 75 kA/cm2 (77 K) for jc, Birr=8.5 T (77 K) and the levitation force 6 N with a 15 mm multigrain sample are quite promising results with the new material, but they indicate the eort which it remains necessary to bring together the high quality ( jc) with large sizes of single grains.
AcknowledgementsÐThe authors are grateful to Mrs G. Stoever, W. Hoeppner, A. Leistikow and C. Hornauf for experimental help, Dr Verges for VSM characterization, Dr K. Ruck for assisting in thermoanalytic investigations.This work was granted by the Federal Minister of Education, Research and Technology (BMBF), contract no. 13N6934/4.A grant by the Fonds der Chemischen Industrie is gratefully acknowledged. The authors are indebted to Solvay Barium Strontium GmbH Hannover for providing with the Nd123 precursors.
76
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REFERENCES 1. M. Murakami, Appl. Supercond. 1, 1157 (1993). 2. T. Habisreuther, T. Strasser, W. Gawalek, P. GoÈrnert, K. V. Ilushin and L. K. Kovalev, IEEE Transact. Appl. Supercond. 7, 900 (1997). 3. S. Jin, T. H. Tiefel, R. C. Sherwood, M. E. Davis, R. B. van Dover, G. W. Kammlott, R. A. Fastnacht and H. D. Keith, Appl. Phys. Lett. 52, 2074 (1988). 4. K. Salama, V. Selvamanickam, L. Gao and K. Sun, Appl. Phys. Lett. 54, 2352 (1989). 5. M. Murakami, M. Morita, K. Doi and K. Miyamoto, Jpn. J. App. Phys. 28, 1189 (1989). 6. M. Murakami, Melt Processed High Temperature Superconductors, World Science, (1992). 7. M. Murakami, S. I. Yoo, T. Higuchi, N. Sakai, J. Welta, N. Koshizuka and Sh. Tanaka, Jpn. J. Appl. Phys. 33, L 715 (1994). 8. W. Bieger, G. Krabbes, P. SchaÈtzle, L. Zelenina, U. Wiesner, P. Verges and J. Klosowski, Physica C 257, 46 (1996). 9. M. Murakami, N. Sakai, T. Higuchi and S. I. Yoo, Supercond. Sci. Technol. 9, 1015 (1996). 10. G. Krabbes, P. SchaÈtzle, W. Bieger, G. Fuchs, U. Wiesner and StoÈver, IEEE Transact. Appl. Supercond. 7, 1735 (1997). 11. W. Bieger, G. Krabbes, P. SchaÈtzle, A. Leistikow, J. Thomas and P. Verges, Mater. Sci. Engng. B53, 100 (1998). 12. S. I. Yoo and R. W. McCallum, Physica C 210, 147±156 (1993). 13. W. Wong-Ng, L. P. Cook, B. Paretzkin, M. D. Hill and J. K. Stalick, J. Am. Ceram. Soc. 77, 2354±2362 (1994). 14. J. L. MacManus±Driscoll, Adv. Mater. 9, 457±473 (1997). 15. M. J. Cima, M. C. Flemings, A. M. Figueredo, M. Nakade, H. Ishii, H. D. Brody and J. S. Haggerty, J. Appl. Phys. 72, 179 (1992). 16. T. Izumi, Y. Nakamura and Y. Shiohara, J. Mater. Res. 7, 1621 (1992). 17. G. Krabbes, P. SchaÈtzle, W. Bieger, U. Wiesner, G. StoÈver, M. Wu, T. Strasser, A. KoÈhler, D. Litzkendorf, K. Fischer and P. GoÈrnert, Physica C 244, 145±152 (1995). 18. G. Krabbes, U. Wiesner, W. Bieger and M. Ritschel, Z. Metallkd. 85, 70 (1994). 19. G. Krabbes, W. Bieger, U. Wiesner, K. Fischer and P. SchaÈtzle, J. Electron. Mater. 23, 1135 (1994). 20. E. A. Goodilin, N. N. Oleynikov, E. V. Antipov, R. V. Shpanchenko, G. Yu. Popov, G. V. Balakirev and Yu.D. Tretyakov, Physica C 272, 65±78 (1996). 21. E. Goodilin, M. Kambara, T. Umeda and Y. Shiohara, Physica C 289, 251±264 (1997). 22. M. Nakamura, M. Kambara, T. Umeda and Y. Shiohara, Physica C 266, 178±182 (1996). 23. M. Kambara, M. Nakamura, Y. Shiohara and T. Umeda, Physica C 275, 127±134 (1997). 24. M. J. Kramer, A. Karion, K. W. Dennis, M. Park and R. W. McCallum, J. Electron. Mater. 23, 1117±1120 (1994). 25. T. B. Lindemer, E. D. Specht, P. M. Martin and M. L. Flitcroft, Physica C 255, 65±75 (1995). 26. N. Nevriva, P. Holba, S. Durcok, D. Zemanova, E. Pollert and A. Triska, Physica C 157, 334 (1989). 27. Th. Wolf, A-C. Bornarel, H. KuÈpfer, R. Meier-Hirmer and B. Obst, Phys. Rev. B 56, 6308 (1997). 28. K. Fischer, G. Leitner, G. Fuchs, M. Schubert, B. Schlobach, A. Gladun and C. Rodig, Cryogenics 33, 97 (1993). 29. M. Murakami, N. Sakai, T. Higuchi and S. I. Yoo, Supercond. Sci. Technol. 9, 1015±1032 (1996). 30. P. SchaÈtzle, W. Bieger, U. Wiesner, P. Verges and G. Krabbes, Supercond. Sci. Technol. 9, 869±874 (1996). 31. P. SchaÈtzle, A. Berning, W. Bieger and G. Krabbes, Mat. Sci. Engng. B53, 95 (1998). 32. G. Krabbes, P. SchaÈtzle, U. Wiesner and W. Bieger, Physica C , 235±240, 299±300 (1994). 33. W. Bieger, U. Wiesner, G. Krabbes, P. SchaÈtzle, A. Bauer, P. Verges and L. Zelenina, J. Low Temp. Phys. 105, 1445±1450 (1996).
Applied Superconductivity Vol. 6, Nos. 2±5, pp. 61±76, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0964-1807(98)00088-X 0964-1807/98 $ - see front matter
MODIFIED MELT CRYSTALLIZATION PROCESSES FOR IMPROVED RE-123 BASED BULK MATERIALS (RE = Y, Nd) G. KRABBES*$, P. SCHAÈTZLE$, W. BIEGER$ and G. FUCHS$ $IFW Dresden Institute of Solid State and Materials Research, P.O.B. 270016, D-01171 Dresden, Germany (Received 15 January 1998; accepted 15 February 1998) AbstractÐThe considered modi®ed melt texturing processes bene®t by peculiarities of the quaternary phase diagram. Examples are discussed for YBaCuO by applying compositions which represent a mixture of Y123 and Y2O3 instead of Y2BaCuO5 or by processing with reduced oxygen partial pressures. The preferable orientation of single grains is initialized by seeding with Sm123. Large single grains (up to 2 in), high critical current densities (45 kA/cm2), high trapped magnetic ®elds (>0.7 T at 77 K in 1 in discs, and 9.6 T in a minimagnet at 47 K) and eectiveness for levitation (1.8 N per 1 g YBaCuO and 84 N total for 35 mm discs) have been achieved in the modi®ed processed YBaCuO materials. In contrast to Y123, nonstoichiometric metal±metal ratios are typical for the Nd123 superconductor and related solid solutions. The corresponding defects may enhance pinning but on the other hand they deteriorate Tc. The results of phase diagram investigations are used as a tool to control the cation stoichiometry and growth mechanism. This can be experimentally achieved by using an appropriate composition or by the oxygen potential controlled isothermal process. Critical current densities up to 75 kA/ cm2 have been achieved in NdBaCuO materials. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Since the discovery of high temperature superconductors (HTSC), several applications have been suggested which are based on the peculiarities of the magnetization loop in these hard superconductors or on the forces appearing between the bulk HTSC and an external magnetic ®eld. Examples were reported as magnetic bearings, linear frictionless transport systems, permanent magnets, ¯ywheels and motors (see e.g. Refs [1, 2]). The breakthrough to technical applications is depending on the availability of materials of high qualities with respect to jc, the trapped ®eld, irreversibility ®eld and levitation forces. High quality parameters have to be realized in bulks of a sucient size as it is representative for the considered application (i.e. larger 10 10 cm2). Meanwhile, it became clear that materials standards will dier for dierent applications. The stiness required for magnetic bearing is in¯uenced by pinning forces which are mainly determined by the intra-grain jc. On the other hand, high trapped ®elds are determined by the product jc d (d is the diameter of the internal current loop) and its realization may open new applications in engineering. Since the introduction of melt texturing by Jin et al. [3], Salama et al. [4] and Murakami et al. [5], numerous variants have been reported which dier mainly in the thermal processing and the pre-treatment of the precursor in some cases (see e.g. Ref. [6] as summarizing report). We consider crystallization processes which proceed in unconventional parts of the phase diagram whereby the equilibrium conditions near the phase boundary will be in¯uenced. First the experiences with the YBaCuO system will be reported concerning the substitution of the conventional Y211 excess by an admixture of yttria as well as the processing in varying oxygen partial pressures. More extensively, the modi®ed processing of NdBaCuO will be regarded. Nd123 based bulk materials are characterized by remarkably increased irreversibility ®elds compared with Y123, and also the remanence magnetization increases [7]. Furthermore, a dominant peak in the magnetization vs ®eld dependency was reported. Probably, there is an interrelation between this behavior and the peculiarities of the stability ®eld of the RE123 *Corresponding author. 61
62
G. KRABBES et al.
phases. The RE123 phases with large RE3+ ions form solid solutions. Usually it was assumed that the excess RE3+ substitutes Ba2+ on the corresponding sites according to the formula RE1 + yBa2 ÿ yCu3O7 2 d. On the other hand, indications were reported for the occurrence of vacancies on Ba sites to compensate most of the surplus charge carried by the excess RE3+. The occurrence of defects (point defects, clusters of point defects or precipitations) is related to the nonstoichiometry and it is generally assumed to be the source of extra pinning centres in these materials. Although the pinning mechanism is not yet really clear, the experience shows that handling and reproducible processing becomes much more complicated than in the case of YBaCuO. The superconducting properties such as Tc and jc of NdBaCuO depend very sensitively on the deviation from the 1:2:3 stoichiometry in the cation sublattice. Therefore, the stoichiometric ratio in the superconducting phase of the bulk must be controlled during the crystallization process. A method of stoichiometry control by controlling the chemical potentials and using appropriate admixed phases was proposed in Ref. [8]. A dierent method applies an environment containing less than 1% oxygen to stabilize the composition with optimum properties [9]. Previous results indicate that the detailed knowledge of phase diagrams is an important precondition for improving properties and size of single grain materials by controlling both the crystallization (near the liquidus surface) and the postannealing (in solidus and subsolidus ranges) processes. Partial information is reported in very recent references [10, 11]. The present work presents parts of the phase diagram in more detail. The conclusions for processing will be demonstrated for controlling both by appropriate admixtures and using varying oxygen partial pressures in an isothermal mode. PHASE DIAGRAM INFORMATION
The quasiternary projection in Fig. 1 represents the phase compatibility of the Nd±Ba±Cu±O system at 0.21 bar oxygen partial pressure at a ®xed temperature below the eutectic temperature. Compared with the well known Y123, the Nd123 phase (as an example for light RE systems) possess a homogeneity range. According to the Gibbs' phase rule, the chemical potentials (and consequently also the composition of the compounds) are determined, if the RE123 phase coexists under equilibrium conditions with two more condensed phases provided p(O2) and T are set (e.g. 0.21 bar and 8908C in Fig. 1). Otherwise, the composition of the RE123ss (ss = solid solution) is not unambiguously determined in a two-phase equilibrium, i. e. Nd123 + Nd422. The most important information with respect to melt processing, however, comes from the knowledge of the primary crystallization ®eld because it is well known that even the melt texturing process is proceeded by a primary crystallization mechanism [15±17], based on a ``peritectic'' reaction L 211 O2 123
Fig. 1. Subsolidus phase diagram of the Nd±Ba±Cu±O system at 8908C and p(O2) = 0.21 bar according to refs [12±14].
1
Modi®ed melt crystallization processes
63
Fig. 2. The polythermal section Y2BaCuO5±YBa2Cu3O7 ÿ d±`Ba0.375Cu0.625O' at p(O2) = 0.21 bar.
It is very important to note that the arising equilibrium melt in both considered systems is depleted in oxygen. It was shown in Ref. [18] that it corresponds to a formal CuO0.68 stoichiometry in the Y±Ba±Cu±O system at p(O2) = 0.21 bar. The exact value for the Nd±Ba± Cu±O system has not been determined yet. Due to the dierent oxygen contents of solid phases and melt (and/or suspension), thermogravimetry (TG) with very small heating or cooling rates is an useful tool for a careful analysis of melting and crystallization processes in the considered systems. The system Y±Ba±Cu±O The primary crystallization ®eld of YBa2Cu3O7 ÿ d is limited by the equilibrium conditions of the four univariant reactions, the reaction temperature of which is ®xed provided p(O2) is set (T is given in parenthesis for 0.21 bar oxygen partial pressure): 4 a Y2 BaCuO5 bL m1 cO2 1020 C m1 : YBa2 Cu3 O7ÿd 3
2
4 dL e1 eO2 899 C e1 : aYBa2 Cu3 O7ÿd bBaCuO2 cCuO 3
3
4 cY2 BaCuO5 dL p1 eO2 940 C p1 : aYBa2 Cu3 O7ÿd bCuO 3
4
4 cYBa4 Cu3 O9 dL p3 eO2 991 C p3 : aYBa2 Cu3 O7ÿd bBaCuO2 3
5
The `peritectic' reaction Equation (1) represents an univariant equilibrium (m1) for YBaCuO due to the ®xed composition of the 123 phase in this system for all gross compositions corresponding to Y123 or mixtures of Y123 + Y211. The well known section, Fig. 2, represents the reversible decomposition and growth of Y123 at the univariant temperature (10208C at 0.21 bar O2) and composition of the melt at this temperature. Figure 3 shows the fact that this univariant equilibrium is in¯uenced by the oxygen potential and it is limited by the invariant point i1 [19]. The system Nd±Ba±Cu±O The occurrence of solid solutions in the RE±Ba±Cu±O systems with large RE ions complicates both the phase compatibilities as well as the technologies of processing derived from these considerations. The study of the stability region of Nd123ss in ambient air atmosphere including its primary decomposition reactions was the topic of a series of papers of Goodilin et al. (see e.g. Refs [20, 21] and references therein). The maximum extent of Nd3+ substitution on Ba2+ sites (which shall be described by the variable y according to the formula Nd1 + yBa2 ÿ yCu3O7 2 d) approaches y 1 1.0 but only at T = 9958C. This temperature corresponds to the minimum solidus temperature: Nd123ss converts into melt and Nd2CuO4, and Nd123ss with smaller y remains. It was proved that the Nd3+ solubility in the Ba±Cu±O melt is higher than that of Y3+ [22, 23]. Below 9958C, Nd123ss coexists with Nd2CuO4 and
64
G. KRABBES et al.
Fig. 3. The stability ®eld of YBa2Cu3O7 ÿ d in dependence on temperature and oxygen partial pressure including subsequent reactions. When Y2O3 is initially added, the stability limit of Y123 is divided into a solidus line and a `quasi±liquidus' (Y211 + melt) line; both are being located at lower temperatures. Solidus and `quasi-liquidus' lines are the boundaries of the enhanced process window of the MCP process described in Section 3.1.
CuO. At a higher temperature, probably between 10458C and 10608C, Nd422 appears instead of Nd2CuO4. According to Ref. [20], Nd123 with y = 0.00 shall be stable in air between 9608C and 10808C. However, as a result of the present investigations, it was impossible to synthesize stoichiometric Nd123 (y = 0.00) in air. Small amounts of BaCuO2 were formed in all solid state preparations, and the approach closest to the stoichiometric composition was found to be ymin10.05 by EDX and WDX analyses in both solid state reaction products and melt-textured samples. Therefore, we suggest that the lower limit of Nd3+ substitution in air equals about ymin10.05. The decomposition temperature decreases by reducing the oxygen partial pressure, and the dependence of the decomposition temperature on the Nd fraction in the Nd123ss phase becomes increasingly pronounced [24, 25]. TG measurements of Nd123 with y = 0.00, 0.05, 0.10, 0.25 and 0.50, respectively, in the present study at p(O2) = 5 10ÿ3 bar and p(O2) = 5 10ÿ4 bar con®rm the remarkable narrowing of the Nd123ss homogeneity range at reduced p(O2), see Fig. 4. Nevertheless, the phase relations become much more complex. BaCu2O2 appears as an additional stable phase at p(O2)E 10ÿ3 bar which in¯uences the decomposition behaviour. In order to study the decomposition mechanism, soaking experiments (see e.g. Ref. [26]) of Nd123
Fig. 4. TG investigations of the stability limits of Nd123ss at p(O2) = 5 10ÿ4 bar and p(O2) = 5 10ÿ3 bar.
Modi®ed melt crystallization processes
65
Table 1. Soaking experiments of Nd123ss with dierent initial composition y0 at several oxygen partial pressures and temperatures p(O2)/bar 0.01
T/8C
y0
Mass loss/%
940
0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50 0.0 0.25 0.50
0.6 1.1 0.6 not deter. not deter. not deter. 4.2 5.0 7.0 16.0 15.2 5.9 51.3 32.7 0.7 0.9 1.2 2.4 6.5 31.6 3.1 12.6 5.8 2.8 5.6 21.5 4.1 27.4 30.8 1.3 2.0 2.5 2.4 6.7 30.4 2.3 6.1 15.7 56.3 50.2 25.8
985 995 1000 1020 0.001
900 930 945 960 985
0.0001
885 915 950 985
No. of EDX points 2 2 2 3 2 3 0 2 4 3 no 4 1 no 2 3 4 3 3 3 1 3 no 1 4 no 4 3 no 2 2 5 1 no no 3 no no no no no
Nd123 Nd123
Nd123 Nd123 Nd123
Nd123 Nd123 Nd123 Nd123 Nd123 Nd123 Nd123
Secondary phases+
yres* 0.047 0.253 0.480 0.004 0.244 0.177 Ð 0.055 0.026 0.082 phase 0.019 0.014 phase 0.065 0.261 0.491 0.065 0.116 0.106 0.030 0.087 phase 0.011 0.106 phase 0.039 0.027 phase 0.042 0.210 0.156 0.015 phase phase 0.047 phase phase phase phase phase
Ð Ð Ð Ð Ð Nd422 Ð Nd422 Ð Nd422 Nd422 Ð Nd422 Nd422 Ð Ð Ð Ð Nd422 Nd422 Ð Nd422 BaCu2O2 Ð Nd422 Nd422 (Nd422) Nd422 Nd422 Ð Nd2CuO4 Nd422 Ð Nd422 Nd422 Nd422 Nd422 Nd422 Nd422 Nd422 Nd422
(Cu2O) Cu2O residue residue residue residue
Nd2CuO4 Nd2CuO4 residue Nd422 residue residue residue residue (Nd422) Nd2CuO4 BaCu2O2 BaCu2O2 residue BaCu2O2 BaCu2O2 residue residue residue
*Calculated from EDX intensities according to yres=I(Nd)/[I(Nd) + I(Ba) + I(Cu)] 6.0±1.0. Phases in brackets inhomogeneously distributed
+
with dierent initial metal stoichiometries (y = 0.00 . . . 0.05, y = 0.25, y = 0.50) have been performed at several oxygen partial pressures and temperatures. The residual body was studied by SEM, the coexisting equilibrium phases were determined by XRD and EDX. An estimate of the total composition of the resulting Nd123ss phase was calculated from EDX measurements using a Nd123 single crystal with y = 0.00 as a standard, prepared by Wolf [27]. Selected results are summarized in Table 1. The results for p(O2) = 1 10ÿ3 bar are visualized in Fig. 5. The compositions formed at 9008C are placed near the initial compositions, indicating the Nd content to remain unchanged for all y = 0.05, 0.25 and 0.50, respectively. The scattering of the points indicates that the con®dence interval of the calculated stoichiometry amounts to about Dy = 0.05. At T e 9308C, a considerable mass loss is observed. Furthermore, Nd422 is detected. The Nd excess in the Nd123ss equilibrium phase is remarkably restricted to ymax<0.2 even if Nd123ss, with y = 0.50, is applied. Furthermore, the microstructural analysis of the residual substance of a soaking experiment starting from Nd123ss with y = 0.50 indicates needle like inclusions within the Nd123ss matrix, the EDX analysis of which corresponds to Nd2CuO4 (compare Fig. 6). At 9008C such inclusions have never been found. This refers to a primary decomposition reaction:
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Fig. 5. Composition of the Nd123ss phase in the residual substance of soaking experiments starting from Nd1 + yBa2 ÿ yCu3Oz with y = 0.00. . .0.05, 0.25 and 0.50 at every considered temperature.
4 1 ÿ DNd123 y1 y0 ÿ d aNd2 CuO4 b third phase r1 Nd123 y y0 3
6
at 9008C
7
at T1
Fig. 6. Residual solid of a representative soaking experiment revealing Nd123ss, Nd422 and Nd2CuO4 (T = 9308C, p(O2) = 0.001 bar).
Modi®ed melt crystallization processes
67
MODIFIED CRYSTALLIZATION PROCESSES
YBaCuO-processing The dierent methods of melt texturing reported so far (denominated MTG, MPMG, QMG, PMP etc. [6]) dier by the applied precursor materials, by the applied parameters for temperature, temperature-time schedules, temperature gradients and by the equipment, respectively. Modi®ed techniques were used as directional solidi®cation and Bridgman technique, zone melting and top seeding. All methods are conventionally based on the reversible reaction Equation (1) applying an admixture of Y211 phase to Y123 powder or any precursor mixture corresponding to a gross composition corresponding to x parts Y123 + y parts Y211. From the thermodynamic point of view, the crystallization process proceeds with all these `conventional' methods in the vicinity of invariant equilibrium conditions (m1, Equation (2)) for the peritectic reaction. Conventional melt texturing proceeds (in a ®rst approach) on line with the projection through the points Y211 and Y123 in the Gibbs triangle (compare Fig. 2). In contrast, the modi®ed melt crystallization process applied in this study proceeds o line with respect to the Y123±Y211 projection. This was investigated in detail for precursor mixtures made of Y2O3 and Y123. Modi®ed melt crystallization processesÐmixtures of Y123 and yttria. The idea of modi®ed processing is to determine conditions to avoid instabilities of the growth process due to local temperature and concentration ¯uctuations. It will be shown that by admixing a proper system inherent second phase instead of Y211, the crystallization by reaction in Equation (1) can proceed in a variable temperature range. As concluded from the polythermal section Y123±Y2O3 at p(O2) = 0.21 bar in Fig. 7, the `peritectic' reaction which is responsible for the crystallization does not proceed with invariant temperature conditions. This fact opens a remarkable enlarged process window for stable growth conditions. A second important feature of Fig. 7 is that Y2O3 and Y123 cannot coexist in equilibrium. Y2O3 reacts with a part of the Y123 precursor powder pressed into the green body in a solid state reaction in the pre-heating ramp before the solidus line will be achieved. Therefore, Y211 can be formed as small grains which are homogeneously distributed in the pressed pellet. The modi®ed melt crystallization process (MCP) is based on the previous conclusions. The green body is prepared from a mixture of Y123 (provider: Solvay) + Y2O3. The optimized formula contains 0.2±0.4 mol yttria per mol Y123 phase. An admixture of Pt in amounts up to 0.5% is recommended analogously to the conventional process. Sm123 seeds have to be placed appropriately onto the green body to achieve optimum seeding conditions in every desired orientation. Mainly the c-axes orientation perpendicular to the top was the goal of experiments. Seeding permits us to keep the supersaturation in the melt low and consequently the crystallization proceeds near the thermodynamic equilibrium which results in a controlled melt crystallization process.
Fig. 7. The polythermal section Y2O3±YBa2Cu3O7 ÿ d±`Ba0.4Cu0.6O' at p(O2) = 0.21 bar.
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The melt crystallization experiments were performed within the isothermal zone of a tube furnace. Typical process conditions are: seeding with Sm123 before the fast pre-heating ramp starts, ramping up to 10508C followed by slow cooling 0.5±1 K/h down to 9408C. After the oxidation at 3808C between 200 and 250 h the superconducting properties were measured as reported in Section 4. Low oxygen pressure processing of YBaCuO. Another modi®cation is based on the dependence of the equilibrium temperature of the `peritectic' reaction in Equation (1) on the applied oxygen partial pressure. Figure 3 represents the relationship between log p(O2) and the temperature of the univariant peritectic reaction m1 (above the invariant point i1). This line determines the equilibrium conditions provided that the reaction in Equation (1) proceeds univariantly as it is the case for ingots of pure Y123 or Y123 + Y211 mixtures, respectively. This low oxygen pressure modi®cation was applied to decrease the decomposition temperature which is important for using low melting substrates (e.g. Ag) [28]. The decomposition is ®xed by the parameters p(O2) and temperature. The solidi®cation at lower oxygen partial pressure is controlled by the oxygen diusion to the solidi®cation front. A further decrease of the melting temperature is achieved by the addition of Y2O3 which shifts the decomposition line towards lower temperature. In this case, however, the reaction in Equation (1) no longer proceeds in an univariant equilibrium. Analogously to the MCP process, the unique equilibrium line (Fig. 3) is split into a solidus and a pseudo-liquidus branch according to a two step process with the melting beginning at the solidus line and the complete decomposition of Y123 into melt and Y211 at the liquidus line, respectively. With respect to bulk materials, the low pressure modi®cations become more important for NdBaCuO and related materials and will be discussed there in more detail. Modi®ed processing of NdBaCuO bulk samples The oxygen controlled melt growth (OCMG) method [29] is conventionally accepted to be an appropriate technique to produce NdBaCuO bulk material with promising properties. This process proceeds at a reduced oxygen partial pressure which is kept constant during the solidi®cation. Nd123 of stoichiometric composition (y = 0.00) is stable at reduced p(O2) and its control is supported by the narrowing of the Nd/Ba homogeneity range at reduced p(O2). Analogous to the YBaCuO processing, Nd422 is usually admixed to Nd123 in the precursor material. However, the size of single grains is still restricted to several mm, and the interest in alternative processing arises. In the following sections we will show the signi®cant in¯uence of dierent admixtures on both, the stoichiometry control, even at p(O2) = 0.2 bar and the growth process. Furthermore, an alternative approach is proposed to process NdBaCuO bulk samples by varying the oxygen partial pressure during the crystallisation process. The in¯uence of system inherent admixtures. Despite of the advantageous eect of Y2O3 admixtures on the processing of YBaCuO bulk samples, an analogous application of Nd2O3 for producing NdBaCuO failed completely [8]. Although we succeeded to grow single grains up to 10 10 3 mm3 by conventional melt texturing techniques, the transition temperature remains in the order of 60±70 K even after long time oxygen annealing at 300±3808C. The reason for the resulting properties is attributed to the Nd±Ba homogeneity range in Nd123. The system inherent admixture determines both the equilibrium activities as well as the reaction path. Because Y123 has a neglectible homogeneity range with respect to the 1:2:3 cationic ratio, the resulting stoichiometry as well as the corresponding properties remain practically unchanged by any kind of addition. This is, of course, no longer valid for RE123 compounds with a RE±Ba homogeneity range and it becomes necessary to consider, furthermore, how admixed phases determine the cationic ratio of the growing RE123 compound. The dierent eect of Nd2O3, Nd422 and Nd2BaO4 admixtures on the resulting properties of melt textured Nd123 was predicted and proved in Ref. [8] for the ®rst time. The present study will throw light on the controlling in¯uence of these admixtures on processing in more detail. Figure 8 schematically demonstrates the equilibrium phase relations at several conditions which are relevant for the crystallization process of Nd123 starting from the precursor mixtures (I) Nd123 ÿ + nNd2O3 and (II) Nd123 + nNd2BaO4, respectively. Both precursor mixtures do not coexist at any conditions. Nd123 and Nd2O3 convert, in a subsolidus prereaction, into Nd123
Modi®ed melt crystallization processes
Fig. 8(a)±(d)Ðcaption overleaf.
69
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Fig. 8. The in¯uence of the initial precursor composition on the resulting stoichiometry of the crystallized Nd123 phase (schematically): (I) Nd123 + Nd2O3, (II) Nd123 + Nd2BaO4 (a) T 1T(m1), (b) T = T(`liquidus') for precursor (I), (c) T = T(`liquidus') for precursor (II), (d) T = T(solidus) for precursor (I), (e) T = T(solidus) for precursor (II).
with an Nd excess (y>ymin) and Nd422. The reaction of Nd123 with Nd2BaO4 results in Nd123 with the minimum possible Nd content (y = ymin), Nd422 and Nd163 (for details see e.g. Ref. [8]). Figure 8(a) reveals the phase equilibria just below the `liquidus' temperature T(m1) of pure Nd123 according to Equation (8) 4 aNd422 bL m1 cO2 for p O2 1 10ÿ3 bar m1 Nd123 3
8
in the ternary NdO1.5±BaO±CuOx representation. Although Nd123 with y = ymin has become stable, the total compositions of both initial mixtures are still within the Nd422 + L two phase regions. However, the signi®cant distinction is the dierent composition and the copper (or CuOx) activity of the corresponding melts. The copper content of the melt L(I) which is formed from Nd123 and Nd2O3 is much higher than that of the melt L(II) which results from the Nd123 ÿ Nd2BaO4 precursor. Although Nd422 also forms a solid solution, EDX studies of equilibrated Nd123 ÿ Nd422 mixtures indicated a compound with nearly the exact 4:2:2 stoichiometry at any conditions independent of the metal stoichiometry in the Nd123 phase, thus indicating that the equilibrium composition of Nd422 depends quite weakly on the variation of the Cu activity.
Modi®ed melt crystallization processes
71
Figure 8(b) and (c) illustrate the phase relations when the liquidus reaction becomes relevant for the considered precursors. The corresponding temperatures are lower than the peritectic decomposition temperature T(m1) of Nd123 with y = ymin. The actual values depend on the amount of initially admixed secondary phase as well as on the oxygen partial pressure of the environment. The important dierence of both admixtures becomes obvious by comparing both ®gures. Whereas Nd123 with the minimum Nd excess (y = ymin) crystallizes from the suspension formed from mixture (II), the crystallization of the suspension from the initial mixture (I) leads to Nd123 with a ®nite (although small) Nd3+ substitution on Ba2+ sites, y0>ymin. The disadvantageous eect of the Nd2O3 addition also appears in the subsequent proceeding crystallization process: the Nd123 phase, which crystallizes in the range between the liquidus and the solidus temperatures, will be continuously enriched by Nd3+. This becomes obvious by Fig. 8(d). The composition of the ®nally crystallizing Nd123, marked by y1 in Fig. 8(d) distinctly diers from the composition with the minimum Nd content and the growing grain becomes inhomogeneous due to the variation of y during the process (y0Ey Ey1). On the other hand, Nd2BaO4 stabilizes a Nd depleted Nd123 with y = ymin during the entire process as shown in Fig. 8(e). Therefore, Nd2BaO4 should be an appropriate admixture from the thermodynamic point of view. Green pellets consisting of Nd123 (y = 0.00 . . . 0.05) and 0.25 to 0.4 mol Nd2BaO4 per formula unit Nd123 admixed with up to 1 wt% Pt were melt processed in a tube furnace within the isothermal zone. The optimum temperature±time schedule depends on the applied oxygen partial pressure. In air, after a ramping at 11008C, the temperature was decreased at 0.1 K/min from 11008C to 10408C. The samples were oxidized for 4±8 days in ¯owing O2 at 3008C [8]. Wolf et al. [27] prepared single grain NdBaCuO material with high quality (jc,max(3 T, 77 K, Bkc)1 65 kA/cm2, Birr,max111.8 T) using BaCuO2 admixtures. BaCuO2 determines the proceeding mechanism similar to Nd2BaO4. In Fig. 8(e) a possible total composition is marked with `W'. This composition also stabilizes Nd123 with a minimum Nd/Ba ratio. The amount of melt is higher compared with Nd123/Nd2BaO4 precursor material which may lead to a diminished amount of Nd422 inclusions but enhances the risk of melt loss. Substitution of Nd3+ by Y3+. Conventional low oxygen pressure processing (OCMG) and inherent admixed phases have been revealed to control the stoichiometry of the Nd123 phase, however, they do not promote the growth of enlarged single grains. The idea of the study in this part is to combine the stoichiometry control by OCMG with the widthening of the temperature range for the crystallization as it is typical for the liquidus behaviour of solid solutions. The latter is achieved by partial substituting of Nd3+ by Y3+ in Nd123. Synthesis of solid solutions (Nd, Y)Ba2Cu3O7 2 d and melt texturing in air were previously reported in Refs [30, 31]. Yttrium is located on the RE3+ sites and suppresses the substitution of Ba2+ by RE3+. Samples (up to 20 mm diameter) consisting of a (Nd1 ÿ yYy)Ba2Cu3O7 2 d precursor (synthesized by solid state reaction from Nd2O3, Y2O3, CuO and BaCO3) and Nd2BaO4 with 0.5% Pt admixed have been applied for the experiments. The melt crystallization process was performed at p(O2) = 10ÿ2 bar by fast ramping up to 10708C and consecutive slow cooling with 1 K/h from 1070 to 10008C. The oxygen annealing was performed as usual at 3008C. Although the samples consist of several grains, the grain size achieves up to 8 8 5 mm3. The in¯uence of the substitution by Y3+ on the crystallization process was investigated by model experiments performed in a thermogravimetric analyzer (TG) in a ¯owing atmosphere with p(O2) = 10ÿ2 bar. The cuprate melts are signi®cantly depleted by oxygen related to their solid parent compounds. Therefore, the melting process is accompanied by a drastic loss of oxygen whereas an uptake of oxygen (weight gain) indicates the crystallization process. Therefore, the latter can be characterized by the weight change dependent on temperature (or time). These model experiments were performed with small tablets, the composition of which was corresponding to the green body used for the melt crystallization process. Although the decomposition temperature decreases if Nd3+ is substituted by Y3+, the beginning of the solidi®cation does not depend on the Y3+ concentration and was observed at 10108C (Fig. 9). Once beginning, the solidi®cation of the small samples used in these experiments, proceeds very fast in the ®rst stage and is nearly completed immediately in the sample without substitutions. The fast step indicates the initial crystallization occurs with a large supersaturation. In the case
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Fig. 9. TG investigations of NdBaCuO (precursor mixture Nd-123 + Nd2BaO4+Pt), (Nd0.9Y0.1)BaCuO (precursor mixture (Nd0.9 Y0.1)-123 + Nd2BaO4+Pt) and YBCO (precursor mixture Y-123 + Y2O3+Pt) at p(O2) = 0.01 bar.
of Y3+ substitution, the initial fast step is followed by a period of slowly proceeding crystallization which, obviously, is caused by the diusion controlled process occurring in the extended temperature range. In contrast, the crystallization is not signi®cantly observed in the pure YBaCuO system at the applied oxygen pressure indicating the process to be much more in¯uenced by the oxygen diusion in YBaCuO [32]. Although supersaturation is large in the NdBaCuO sample when the crystallization is initialized, the absolute undercooling with reference to the equilibrium temperature (10208C at p(O2) = 10ÿ2 bar) is only DT = 10 K. Thereafter, the crystallization proceeds within the interval of less than 15 K. This interval can signi®cantly be enlarged to 30 K by seeding with a MgO crystal as shown in Fig. 10a. As expected, the undercooling from the beginning of growth becomes undetectably small. The solidi®cation interval is reduced to 20 K in air [Fig. 10b] for pure NdBaCuO. The crystallization proceeds continuously until the solidus is achieved. This behaviour is in contrast to the related YBaCuO experiment. The slow solidi®cation rate following the fast initial period may indicate that the crystallization is much more controlled by the diusion processes than in the Nd system. The ®nal step in the curve is caused by a univariant solidi®cation. Processing at variable oxygen partial pressure. An alternative approach to control crystallization and growth as well as the cation stoichiometry of the resulting Nd123 matrix is a low pressure process in which p(O2) is altered dependent on time instead of the temperature. This proposed `oxygen controlled isothermal processing' (OCIP) starts from an equilibrium suspension at p(O2) 11 10ÿ5 bar and T 1 9908C consisting of Nd422, BaCu2O2 and melt which has been formed from the precursor Nd123 + nNd2BaO4 (+Pt). Now, the oxygen partial pressure is slowly raised above the stability limit of Nd123 at the ®xed temperature [33, 11]. In order to choose optimum conditions, the study was supported by TG experiments. Figure 11 reveals the relative mass change of small pellets of the initial composition equilibrated at 10ÿ5 bar and 9908C when the oxygen partial pressure is suddenly raised to 10ÿ3, 5 10ÿ3 and 10ÿ2 bar, respectively. Whereas there is nearly no eect at 10ÿ3 bar, the crystallization at p(O2) = 10ÿ2 bar proceeds immediately. Only at 5 10ÿ3 bar do we ®nd two small distinct steps indicating a more controlled crystallization. This result is con®rmed by melt processing experiments. After an argon treatment of a Nd123/Nd2BaO4/Pt precursor material at 9808C and changing the atmosphere to 1 10ÿ3 bar oxygen, only a few small single grains of Nd123 are solidi®ed. In a second experiment, the ®nal oxygen content of the atmosphere is raised to 5 10ÿ3 bar. More and larger crystals are obtained. If 1 10ÿ2 bar oxygen is applied, a multicrystalline texture of the sample is achieved. In the OCIP method, the green pellets are isothermally pre-heated in ¯owing argon at 9908C for two days. Then, the oxygen partial pressure is enhanced by small steps up to 6 . . .7 10ÿ3 bar within a period of two days. After holding the samples at these conditions for a further two days, the tablets are oxidized for 4±8 days at 3008C.
Modi®ed melt crystallization processes
73
Fig. 10a. TG investigations of pressed small pellets of NdBaCuO (precursor mixture Nd123 + Nd2BaO4+Pt) with and without a MgO seed at p(O2) = 0.01 bar.
The proposed method may become a useful alternative for the following reason: in contrast to the comparable YBCO crystallization, the Nd3+ diusion through the melt proceeds fast enough to guarantee a sucient amount of Nd3+ at the crystallization front. Therefore, the Nd3+ diusion does not control the rate of the process which is obviously mainly caused by the higher Nd3+ content in the quasi-peritectic melt (see e.g. Ref. [22]). PROPERTIES
In this section, the superconducting properties of the NdBaCuO and YBaCuO materials which were processed by modi®ed methods are summarized. The modi®ed crystallization process (MCP) starting from pressed pellets of Y123 and Y2O3 was applied to produce single YBaCuO monoliths with a size up to 50 mm in diameter, rectangular single grain blocks up to 40 40 15 mm3 or 40 100 15 mm3 rectangular blocks with biaxial oriented multi seeded grains. Trapped magnetic ®elds up to 0.78 T were realized at 77 K. The levitation forces up to 84 N in monoliths with 26±50 mm diameter were measured with a SmCo magnet (d = 25 mm, 0.4 T). In small samples cut from this monolith, critical current densities with 45 kA/cm2 at 0 T and Hirr of 6.5 T were achieved (VSM measurements). The highest trapped ®eld achieved so far with MCP±YBaCuO materials is 9.6 T at 47 K. Using the OCIP method, NdBaCuO grains up to 5 5 2 mm3 have been grown. The properties of several selected samples are summarized in Table 2. The jc(H) curves have been
Fig. 10b. TG investigations of pressed small pellets of NdBaCuO (precursor mixture Nd123 + Nd2BaO4+Pt) and YBCO (precursor mixture Y-123 + Y2O3+Pt) at p(O2) = 0.2 bar.
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Fig. 11. TG investigations of the isothermal crystallization of NdBaCuO from a suspension formed by a Nd123/Nd2BaO4/Pt precursor at 9858C for several oxygen partial pressures.
calculated from VSM measurements with a sweep rate of 16 mT/s using the Bean's model. Tc amounts to usually 94±96 K and the transition is very sharp (usually DTc 1 1 K). The jc(0 T, 77 K) values reach up to 75 kA/cm2. Most samples reveal a plateau of about 20 to 25 kA/cm2 at 1 to 2 T for Bkc. Plateau levels of about 40 kA/cm2 were found in several samples. The irreversibility ®eld amounts more than 8 T for samples which do not exhibit a pronounced peak at 1 to 2 T but a high jc(0 T) value. Larger bulk samples of NdBaCuO were achieved by a melt process (slow cooling with 1 K/h from 1070 to 10008C) with a pressed sample of (Nd0.94 Y0.06)BaCuO (d = 20 mm) admixed with Nd2BaO4 under an oxygen partial pressure of 10ÿ2 bar. The trapped magnetic ®eld was measured with a hall sensor and showed maximum values of 100 mT (Bz=0). The multigrain structure of the sample is shown [Fig. 12a] by the existence of several maxima. Levitation forces up to 6.0 N were measured with a SmCo standard magnet (d = 25 mm, B0=0.4 T) in this sample [Fig. 12b]. Due to the multigrain structure of the bulk sample the zfc curve becomes broad. CONCLUSION
Detailed information on phase diagrams in the subsolidus and liquidus areas have built a fundament for melt crystallization processing of YBaCuO bulk materials of high quality. The processing of NdBaCuO as an example for light RE related HTSC has turned out to be much more dicult. The solid solution like behaviour of RE123 (RE = La, Nd, Sm) and the strong in¯uence of any deviation from the stoichiometric ratio 1:2:3 for RE:Ba:Cu on the superconducting properties requires the control of both the stoichiometry and the conditions of stable growth to obtain large single domains. Forthcoming results on phase equilibria have been contributed and modi®ed processes have been applied to NdBaCuO as (1) controlling chemical potentials by system inherent admixtures, (2) substitutional solid solutions (Y for Nd) and (3)
Table 2. Superconducting properties of selected samples melt processed with the OCIP method Sample
LT99/2 WT08 WT09 WT10 WT16 WT32 WT33 WW74
Susceptibility measurements
Magnetization measurements
Tc/K*
DTc/K$
jc(0 T, 77 K)/ kA/cm2
jc(1.5 T, 77 K, Bkc)/ kA/cm2
Birr/T%
94.9 94.7 94.4 95.6 95.5 93.9 93.1 Ð
1.6 1.2 1.5 0.6 0.6 1.5 0.4 Ð
46.7 Ð 73.7 62.0 72.9 43 39.5 41.4
15.8 Ð 39.8 20.8 19.9 21 16 27.1
6.9 Ð 7.6 18.8} >8 6.8 6.8 6.0
*Tc 1T( 0rel = ÿ 0.5). $DTc 1T( 0rel = ÿ 0.1) ÿ T( 0rel = ÿ 0.9). %m(Birr) < 50 A/cm2. }Estimated
Modi®ed melt crystallization processes
75
Fig. 12a. Trapped magnetic ®eld of a bulk NdBaCuO sample processed under a reduced oxygen atmosphere of p(O2) = 0.01 bar.
Fig. 12b. Levitation force of a NdBaCuO sample (d = 20 mm) processed under a reduced oxygen atmosphere of p(O2) = 0.01 bar.
the oxygen controlled isothermal process (OCIP). The properties, 75 kA/cm2 (77 K) for jc, Birr=8.5 T (77 K) and the levitation force 6 N with a 15 mm multigrain sample are quite promising results with the new material, but they indicate the eort which it remains necessary to bring together the high quality ( jc) with large sizes of single grains.
AcknowledgementsÐThe authors are grateful to Mrs G. Stoever, W. Hoeppner, A. Leistikow and C. Hornauf for experimental help, Dr Verges for VSM characterization, Dr K. Ruck for assisting in thermoanalytic investigations.This work was granted by the Federal Minister of Education, Research and Technology (BMBF), contract no. 13N6934/4.A grant by the Fonds der Chemischen Industrie is gratefully acknowledged. The authors are indebted to Solvay Barium Strontium GmbH Hannover for providing with the Nd123 precursors.
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