CRYSTAL GROWTH & DESIGN
In Situ Investigation of Phase Equilibria and Growth Mechanisms of Compositions near the Bi2Sr2Ca2Cu3Ox Stoichiometry by High-Temperature Optical Microscopy
2005 VOL. 5, NO. 5 1751-1754
D. Maier*,† and A. B. Kulakov‡ Max-Planck-Institut fuer Festkoerperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany, and Institute of Solid-State Physics, 142432 Chernogolovka, Moscow district, Russia Received December 15, 2004;
Revised Manuscript Received July 22, 2005
ABSTRACT: Phase equilibria and growth mechanisms of compositions near the Bi2Sr2Ca2Cu3Ox stoichiometry were investigated by high-temperature optical microscopy in air atmosphere and 6.5% O2/93.5% Ar atmosphere. The resulting phases were determined by energy-dispersive X-ray spectroscopy. No primary crystallization phase field of Bi2Sr2Ca2Cu3Ox was found. Bi2Sr2Ca2Cu3Ox crystallizes through a peritectic reaction of Bi-rich melt with (Sr,Ca)14Cu24Ox crystals in air or with (Sr,Ca)CuOx crystals in 6.5% O2/93.5% Ar atmosphere at 870 °C. So Bi2Sr2Ca2Cu3Ox can be grown only for initial compositions within the compatibility tetrahedrons Bi2Sr2Ca1Cu2Ox-Bi11(Sr,Ca)9Cu5Ox-(Sr,Ca)14Cu24Ox-CuO (air atmosphere) or Bi2Sr2Ca1Cu2Ox-Bi11(Sr,Ca)9Cu5Ox(Sr,Ca)CuOx-CuO (6.5% O2/93.5% Ar atmosphere) even though the Bi2Sr2Ca2Cu3Ox composition does not fall within these compatibility tetrahedra. 1. Introduction Bi-based high-temperature superconductors of general formula Bi2Sr2Can-1CunO2n+4+δ for n ) 1-3 have been indentified up to now. Numerous papers have been published concerning phase equilibria1 and phaseformation mechanisms.2-6 But most studies were done for the Pb- and Ag-doped Bi2Sr2Ca2Cu3Ox.7-10 Grivel et al. and Komatsu et al. predict nucleation and growth of Bi2Sr2Ca2Cu3Ox,3,4 while Cai et al. predict a layer by layer intercalation model.5 So the growth mechanism is not clear up to now. Because the growth of Bi2Sr2Ca2Cu3Ox11,12 still leads to a multiphase product, a clearer understanding of the growth mechanism and phase diagram is necessary. It has been shown by Aswal et al. that high-temperature optical microscopy (HTOM) is very useful to understand growth processes and to determine the phase diagram.13 The goal of this paper is to show in situ the growth mechanism of the Pb-free Bi2Sr2Ca2Cu3Ox phase and the phase equilibria near the Bi2Sr2Ca2Cu3Ox stoichiometry by HTOM and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX). For better readability, all phases were abbreviated according to Table 1. 2. Experimental Section Samples with different compositions listed in Table 2 were prepared from Bi2O3 (99.9%), SrCO3 (99.9%), CaCO3 (99.99%), and CuO (99.97%) powders. The powders were ground in an agate mortar, calcined at 790 °C for 12 h, and reground. Some samples were analyzed by induction-coupled plasma atomic emission spectroscopy (ICP-AES) and showed about 1 wt % of CO2. The in situ observation was performed in a HTOM OLYMPUS MS-11 equipped with an optical heating system MS-E1S/VMC-1 ULVAC-RICO, Japan. A small amount of about 2 mg of the sample powder was put on a single-crystal MgO substrate (3 × 3 mm2) placed in an alumina crucible of 5 mm in diameter. The MgO substrate was not attacked by the melt. The alumina crucible was in direct contact with the * Corresponding author. E-mail:
[email protected]. † Max-Planck-Institut fuer Festkoerperforschung. ‡ Institute of Solid-State Physics.
Figure 1. Schematic drawing of the high-temperature optical microscope setup. Table 1. Chemical Formulas and Their Corresponding Symbols Used in This Studya chemical formula
abbreviation
chemical formula
abbreviation
Bi2Sr2Ca2Cu3Ox Bi11(Sr,Ca)9Cu5Ox Bi2Sr2CaCu2Ox
2223 119×5 2212
(Sr,Ca)14Cu24Ox (Sr,Ca)CuOx (Sr,Ca)2CuOx
14×24 1×1 2×1
a
All these phases are referenced in ref 1.
Table 2. Sample Compositions and Observed Phases number
sample composition
resulting phases
1 2 3 4 5 6 7 8
Bi2Sr2Ca2Cu3Ox Bi2.1Sr1.9Ca2Cu3Ox Bi2Sr1.8Ca2.2Cu3Ox Bi2Sr1.8Ca2.1Cu3.11Ox Bi2Sr2Ca1.8Cu3.2Ox Bi2Sr2.2Ca1.9Cu2.9Ox Bi2.3Sr1.9Ca1.9Cu2.9Ox Bi2.5Sr2Ca1.3Cu3.2Ox
9
Bi2.9Sr1.9Ca1.2Cu3Ox
2212, 119×5,2×1, CuO 2212, 119×5, 2×1,CuO 2212, 119×5, 2×1, CuO 2212, 119×5, 2×1, CuO 2212, 119×5, 2×1, CuO 2212, 119×5, 2×1, CuO 2212, 119×5, 2×1,CuO 2212, 119×5, 14×24 (air atmos.)/ 1×1 (6.5% O2/93.5% Ar atmos.), CuO 2223 with long reaction time 2212, 119×5, 14×24 (air atmos.)/ 1×1 (6.5% O2/93.5% Ar atmos.), CuO 2223 with long reaction time
thermocouple. The temperature could be controlled within 1 °C. The accuracy was determined to (3 °C by melting Au (1063 °C) and Bi2O3 (817 °C). The melting and crystallization
10.1021/cg049576g CCC: $30.25 © 2005 American Chemical Society Published on Web 08/19/2005
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Maier and Kulakov
Figure 2. Sequence of micrographs recorded during cooling of melt with the 2223 initial composition in an 6.5% O2/93.5% Ar atmosphere: (a) after melting and homogenization residual CaO [A] is still present in the melt [B]; (b) dissolution of CaO and growth of 2×1 [C]; (c) finalized growth of 2×1; (d) dissolution of 2×1; (e) nucleation and growth of 2212 [D]; (f) finalized growth of 2212.
Figure 3. Sequence of micrographs recorded during cooling of melt with composition Bi2.5Sr2Ca1.3Cu3.2Ox in an atmosphere of 6.5% O2/93.5% Ar: (a) after melting and homogenization at 980 °C, residual CaO [A] is still present in the melt [B]; (b) dissolution of CaO and growth of 2×1 [C] at 933 °C; (c) finalized growth of 2×1 at 920 °C; (d) dissolution of 2×1 at 918 °C; (e) nucleation and growth of 1×1 [D]; (f) finalized growth of 1×1 at 900 °C, no residual 2×1 is present; (g) nucleation and growth of 2212 [E] at 890 °C; (h) finalized growth of 2212 and residual Bi-rich liquid [F] at 875 °C; (i,j) 20 h hold at 870 °C, 2212 dissolves slowly (marked by an arrow); (k) solidified Bi-rich melt [F] in an at 870 °C quenched sample.
Phase Equilibria and Growth Mechanisms of Bi2Sr2Ca2Cu3Ox
Crystal Growth & Design, Vol. 5, No. 5, 2005 1753
Figure 4. SEM micrographs of the sample with composition Bi2.5Sr2Ca1.3Cu3.2Ox grown in an atmosphere of 6.5% O2/93.5% Ar and quenched after it was kept 50 h at 870 °C: (a,c) two different 1×1 crystals [C] surrounded by 2212 [A] are shownsthese 1×1 crystals are partially transformed to 2223 [B]; (b) the framed region of panel a at higher magnification shows the droplets of Bi-rich liquid [D] on the 1×1 crystals, which remained after quenching; (d) the framed region of panel c at higher magnification shows a Bi-rich part [E] on the 1×1 crystal and a part already transformed to 2223. process was recorded by a CCD camera and stored directly to a PC with a TV-grabber card (Figure 1). Each sample powder was heated, either in a gas flow of 10 sccm air or 6.5% O2/93.5% Ar gas to 980 °C with a heating rate of 20 K/min and held 20 min at 980 °C for homogenization. The melt wets the MgO substrate, so no temperature gradients within the melt have to be expected. Then the melt was cooled at a rate of 1 K/min. To conserve the grown phases at a certain growth stage for later SEM/EDX analysis, the sample was quenched by turning off the heater. The crystallization product was characterized by a SEM equipped with a Roentec Edwin-Nt EDX detector. For the standardless quantitative EDX analysis, the PUZAF14 method was used. The overlapping Bi and Sr peaks were deconvoluted by the Bayes theorem.15
3. Results and Discussion Figure 2 shows the recorded series of micrographs for the sample with the initial 2223 composition in an atmosphere of 6.5% O2/93.5% Ar. After melting at 980 °C residual CaO remains in the melt. Even after 2 h of homogenization, the CaO does not melt. At temperatures above 1000 °C, the Bi2O3 begins to evaporate, but the CaO is still present. On cooling, at 950 °C, the CaO dissolves, while 2×1 crystals begin to nucleate and grow. On further cooling, the 2×1 crystals remelt at about 905 °C until platelike 2212 crystals nucleate and grow at 890 °C. The 2212 grows continuously on the extent of the remaining 2×1 crystals. At 865 °C final (eutectic) solidification occurs. No nucleation and growth of 2223 was found. This is in contrast to a growth process where a primary phase field exists, which normaly should be passed on nonequilibrium cooling of the initial 2223 composition. This behavior was also observed for all compositions near the 2223 stoichiometry (See Table 2, compositons 2-7) in air atmosphere or 6.5% O2/93.5% Ar atmosphere.
Figure 3 shows the series of micrographs for the sample with the composition Bi2.5Sr2Ca1.3Cu3.2Ox in an atmosphere of 6.5% O2/93.5% Ar. Here residual CaO also remains after melting at 980 °C. It dissolves during nucleation and growth of 2×1 at 933 °C. At 918 °C, 2×1 dissolves again until 1×1 nucleates and grows at 915 °C. At 890 °C, 2212 nucleates and grows. At 865 °C, final (eutectic) solidification of the Bi-rich liquid to 119×5 and CuO occurs. No nucleation and growth of 2223 was found. The situation is different when the temperature is kept at 870 °C for 20 h. The Bi-rich liquid was absorbed by the 1×1 crystals, while the 2212 crystals begin to dissolve. The growth process was the same in an air atmosphere unless 14×24 crystals nucleate and grow instead of 1×1 crystals. This behavior also was found in sample 9 (Table 2). The SEM/EDX study of the sample grown in 6.5% O2/ 93.5% Ar atmosphere and kept 50 h at 870 °C shows that the 1×1 crystals were transformed peritectically to 2223 (Figure 4). It can be concluded that this happens by the uptake of Bi-rich melt because Bi-rich parts were found on the 1×1 crystals (Figure 4d). The same peritectic reaction was observed for 14×24 crystals when air atmosphere was used. In these experiments, no evidence was found that the formation of 2223 is a nucleation and growth process, as predicted by Grivel et al. and Komatsu et al.3,4 Because there was no nucleation of 2223 found, one can conclude that 2223 only forms through a peritectic reaction of 1×1 crystals with the Bi-rich melt. This peritectic behavior and the absence of a primary phase field also explains why slow growth rates of 0.04 mm/h found by Liang et al.11 are necessary to grow 2223.
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In these experiments, two compatibility tetrahedra, namely, 2212-119×5-2×1-CuO and 2212-119×514×24 (air atmos.)/1×1 (6.5% O2/93.5% Ar atmos.)-CuO were found, but it is only possible to get 2223 from initial compositions in the compatibility tetrahedron 2212-119×5-14×24 (air atmos.)/1×1(6.5% O2/93.5% Ar atmos.)-CuO by a long time reaction step at 870 °C. It is remarkable that the 2223 composition does not fall within this compatibility tetrahedron. 4. Conclusion In-situ growth experiments were performed with a high-temperature optical microscope on compositions near the 2223 stoichiometry. No primary phase field of 2223 was found. The 2223 crystallizes by peritectic reaction of Bi-rich melt with 1×1 crystals in a 6.5% O2/ 93.5% Ar atmosphere or 14×24 crystals in an air atmosphere at 870 °C. The 2223 is only able to grow from initial compositions in the compatibility tetrahedron 2212-119×5-14×24 (air atmos.)/1×1(6.5% O2/ 93.5% Ar atmos.)-CuO even though the 2223 composition does not fall within this compatibility tetrahedron. Acknowledgment. We thank R. Lauck for helpful discussions.
Maier and Kulakov
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