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Analysis on the interface stability and morphology evolution rules of the YBCO crystal growth during the unidirectional solidification
Journal of Alloys and Compounds 462 (2008) 428–431
Analysis on the interface stability and morphology evolution rules of the YBCO crystal growth during the unidirectional solidification Haitao Cao ∗ , Rui Hu, Jinshan Li, Hongchao Kou, Lian Zhou The State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Shaanxi, Xi’an 710072, PR China Received 25 April 2007; received in revised form 22 August 2007; accepted 26 August 2007 Available online 30 August 2007
Abstract The unidirectional solidification technology by the zone melting method is performed to obtain the large single domain YBCO. And the interface stability and morphology evolution rules of the metallic crystal growth during the traditional unidirectional solidification are not applicable to the YBCO crystal growth. It was found that the crystallology features of the YBCO crystal itself mainly decide the interface stability theory and the morphology evolution rules of the YBCO crystal growth during the traditional unidirectional solidification. These research results are useful for the study on the mechanism of the YBCO crystal growth. © 2007 Elsevier B.V. All rights reserved. Keywords: Unidirectional solidification; YBCO; Superconductor; Interface stability; Morphology evolution
1. Introduction The preparation of the high temperature superconductive material YBCO is finished by the peritectic solidification. Because the YBCO high temperature superconductor (HTS) belongs to the multiple oxide-ceramic systems, its peritectic reaction is very complex. To the final material performance, the phases and microstructure formed during the solidification process are quite important. The interface stability is the initial position of the formed complicated morphology after the crystal growth and the metal solidification. And the interface stability has an important influence on the solidification morphology evolution and selection after the stable growth interface broke [1]. In this paper, some basic problems in preparing YBCO superconductor by unidirectional solidification, including the peritectic reaction, the interface stability and the control of the interface morphology are discussed preliminarily. Three different growth regimes were given by Cima et al. [2]: single crystal growth, cellular/dendritic growth and equiaxed blocky. This view thinks the interface morphology of the YBCO
∗
Corresponding author. Tel.: +86 29 88493484; fax: +86 29 88460294. E-mail addresses: [email protected], [email protected] (H. Cao). 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.063
crystal growth is similar to the metallic system, which exists the planar/cellular/dendritic transition. But, the phenomenon has not been found in this experiment ahead of the solid/liquid interface and the YBCO crystallography features mainly decide its interface morphology itself. 2. Experimental The precursor powders were prepared by taking nominal compositions of 1 2 3 + 10% 2 1 1. The powder of Y2 O3 used in our work was 99.99% purity, and the powders of BaCO3 and CuO were 99% purity. The sample preparation [3]: first, the raw materials of Y2 O3 , BaCO3 and CuO were mixed thoroughly by taking nominal ratio 1:2:3 and then calcined twice at 900 ◦ C for 12 h with an intermediate grinding. The partial powder taken from the above calcined and reground powder was analysed by XRD. The above procedures were repeated until the powders are pure YBa2 Cu3 O6+x (1 2 3). Then precursor rods were prepared by filling the powders in the mould and subjecting them to cold isostatic pressing. Each rod is about 0.2 cm × 0.2 cm × 12 cm. The rods were sintered at 900 ◦ C for 12 h. Fig. 1 shows a schematic illustration of the zone melting furnace [4]. The zone melting furnace consisted of a vertical furnace in which the sintered rod was suspended. The main heater, which was set inside the backup furnace, heated a partial zone of the rod above the peritectic temperature of 1 2 3 and produced a partial molten zone at the same position. Precursor rod suspended by a clip was lowered into the preheated furnace. Then the rod was pulled upward at 0.2 cm/h for the crystal growth under a temperature of about 1015 ◦ C. Peritectic solidification occurred in the temperature gradient region of about 150 ◦ C/cm (obtained approximately by making the tangent on the actually cooling curve). Thousand
H. Cao et al. / Journal of Alloys and Compounds 462 (2008) 428–431
429
Fig. 3. Morphology schematic illustration of the YBCO crystal interface growth morphology with the triangle cone shape during the unidirectional solidification.
Fig. 1. Schematic illustration of the zone melting furnace and the samples are pulling upward through the molten zone heated by the heaters. sixty-seven degree Celsius was the maximum temperature of the partial molten zone.
3. Observations The peritectic alloy system was classified by St. John et al. [5]. Three types according to the growth pattern of the  phase in the phase diagram. It is believed that the growth of  phase in the type C is the most difficult to obtain in all the three types because  phase exists in a quite small region or single component. It means that the solid solubility between the ␣ phase and the  phase is nearly a definite value. The ␣ phase will grow hardly, namely nearly no element diffusion between the ␣ phase and the  phase once the  phase is formed. To the YBCO reaction system, Y2 BaCuO5 (2 1 1) phase will be encapsulated and not grow once 1 2 3 phase is formed. The Ba3 Cu5 O8 –Y2 BaCuO5 quasi-binary phase diagram is shown in Fig. 2 [6]. It can be clearly seen in it that the reaction between the 2 1 1 and liquid (i.e. 2 1 1 + L → 1 2 3) approximately belongs to the type C compared with phase diagram of binary peritectic systems. At this composition, the peritec-
Fig. 2. The Y2 BaCuO5 –Ba3 Cu5 O8 quasi-binary phase diagram.
tic phase (i.e. 1 2 3 phase) is the certain chemical value, so the growth of the 1 2 3 phase is very difficult and the peritectic reaction is quite complex. In addition, the liquidus slope is very steep, and this indicates that it is difficult to achieve high Y supersaturated liquid for high growth rate. It was generally thought that whether the YBCO crystal growth interface ahead of the front keep planar interface during the growth course was decided by the G/R value. The growth front can keep planar interface when the G/R value is greater than the certain value (300 ◦ C h/cm2 ) [7]. Otherwise, the new YBCO crystal would nucleate in the liquid ahead of the growth front, and then it causes planar/cellular/dendritic growth morphology transition. But the interface stability and morphology evolution rules of the metallic crystal growth during the traditional unidirectional solidification are not applicable to the YBCO crystal growth. Because the YBCO crystal growth is controlled not only by the element diffusion, but also by the crystallography features itself. Fig. 3 shows morphology schematic illustration of the YBCO crystal interface growth morphology with triangle cone shape during the unidirectional solidification [8]. The Rmax (the fastest velocity of YBCO crystal growth) is decided by the growth velocity of the crystal plane (1 0 0), (0 1 0) and (0 0 1). So the YBCO crystallology features determine the YBCO growth morphology ahead of the growth front. The 1 2 3 phase will grow along the solidification direction with triangle cone shape under the certain ratio of G/R. It is a facet growth way along the longitudinal section. This phenomenon has been found in the experiment. Fig. 4(a) shows that the solid/liquid interface morphology in the facet growth way is triangle cone shape in microstructure. In macrostructure, it is the planar interface along the longitudinal section. And Fig. 4(b) and (c) is the partial amplification picture, it can be seen clearly from the picture that the solid/liquid interface is very clear and smooth. The transition regions between the triangle cones are not curly but sharp, and these results are also in well accordance with the triangle cone shape theory and verify what mentioned above. This morphology is believed a steady growth interface, and the G/R value here is the critical value. According to the unidirectional solidification constitutional supercooling theory [9], the planar interface morphology growth
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H. Cao et al. / Journal of Alloys and Compounds 462 (2008) 428–431
Fig. 5. The interface growth morphology under the unsteady growth condition (blocky).
If the G/R value meets the condition in the formula (1), YBCO crystal will keeps the growth continuous in the facet growth way, at the same time, the growth interface is steady state; the height of the triangle cone and the space between the triangle cones will get smaller. But if the G/R value increases and exceeds the critical value, the height of the triangle cones and the space between the triangle cones will get larger instead of the planar/cellular/dendritic transition until the steady growth interface is destroyed. At this moment, the growth interface is instable. Fig. 5 shows that the blocky organization structure occurred in the YBCO samples. The blocky structure occurred because that the G/R value deviate away from the equilibrium critical value. The YBCO crystal cannot grow up and keep the growth continuous if the G/R value does not reach the equilibrium critical value, even if the YBCO crystal grains have formed. In the metal systems, the cell spacing (λ) is defined by the balance of the solute concentration differences between the tip and the bottom of the cells, and the curvatures at the tip, as expressed in the following equation [8]: λ ∝ G−m R−n
Fig. 4. (a) The steady growth interface morphology under the certain G/R value; (b) and (c) partial enlarged figure.
must meet the condition below [9]: G mL (Cs − CL ) ≥ R DL
(1)
where G is the actual temperature gradient, R the solidification velocity, Cs the solute concentration of the solid ahead of the growth front, CL the solute concentration of the liquid ahead of the growth front, DL the solute diffuse coefficient, and mL the liquidus slope in the temperature constitution equilibrium phase diagram.
(2)
The cell spacing of the unidirectional solidified rods decreased with increasing R and/or G. However, the crystallization of the facet growth YBCO is not only limited by the diffusion of the solute elements, but also by the interface kinetics. This point is different from the metal solidification that is only limited by the solute diffusion. Thus, it is too difficult to define the exponent of m and n in the facet-growing YBCO system. So λ in the formula (2) is believed the spacing between the triangle cones. Results discussed above are similar to the constitutional supercooling theory, so the constitutional supercooling theory has guidance meaning to the YBCO HTS oxides crystal growth. It can be concluded that the YBCO crystal structural feature determine the steady growth interface morphology, and the interface morphology is not to be controlled by changing the G/R value, namely that provided the YBCO crystals keep growth continuous, YBCO crystal will grow inevitable in the steady interface morphology of the triangle cone shape in the facet growth way.
H. Cao et al. / Journal of Alloys and Compounds 462 (2008) 428–431
YBCO crystal growth interface morphology is determined by the anisotropy of the crystal growth velocity that are further determined by the interface morphology, growth mechanism and the growth driving force. When the YBCO interface growth kinetics can compensate for the inhomogeneous concentration distribution ahead of the YBCO crystal growth front, YBCO crystal can keep the growth steady and continuous in the facet growth way, if not, YBCO crystal growth will stop and not occur in the transition of the planar/cellular/dendritic growth morphology. 4. Conclusion The interface stability and morphology evolution rules of the metallic crystal growth during the traditional unidirectional solidification are not applicable to the YBCO crystal growth. The YBCO growth morphology ahead of the growth front is determined by the YBCO crystal characteristic itself. The YBCO crystal would grow in the facet growth way with the triangle cone shape under the critical G/R ratio along the longitudinal section. The YBCO phase would grow along the solidification direction with the triangle cone shape under the certain ratio of G/R by the facet growth way longitudinal section. If the G/R value increases and exceeds the critical value, YBCO crystal growth does not occur in the planar/
431
cellular/dendritic transition. The height of the triangle cone and the space between the triangle cones will increase continuously until the steady growth interface is destroyed. At this moment, the growth interface is instable. Acknowledgements This paper is supported by the National Nature Science Foundation of China contact no. 50432050. The authors thank Prof. Y.F. Lu and Dr. J.Q. Feng in the Northwest Institute for Nonferrous Metal Research for their technical supports and helpful discussions. References [1] M. Naiben, Shang Hai, Physics Foundation of Crystal Growth, Science and Technology Publishing Company, January, 1982. [2] M.J. Cima, et al., J. Appl. Phys. 72 (1) (1992) 179. [3] S.K. Chen, L. Zhou, K.G. Wang, et al., Chin. J. Low Temp. Phys. 23 (3) (2001) 193. [4] A. Hayashi, K. Kurachi, et al., Physica C 357 (2001) 669. [5] D.H John St, L.M. Hogan, Acta Metall. 35 (1) (1987) 171. [6] C.J. Kim, et al., Supercond. Sci. Technol. 12 (1999) R27. [7] T. Izumi, Y. Nakamura, Y. Shiohara, Adv. Sapercond. 3 (1991) 429. [8] J. Maeda, Y. Nakamura, et al., Acta Metall. 322 (1999) 151–162. [9] X. Zuyao, L. Pengxing, Shang Hai, Introduction to Material Science, Science and Technology Publishing Company, 1986, p. 155.
Analysis on the interface stability and morphology evolution rules of the YBCO crystal growth during the unidirectional solidification Haitao Cao ∗ , Rui Hu, Jinshan Li, Hongchao Kou, Lian Zhou The State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Shaanxi, Xi’an 710072, PR China Received 25 April 2007; received in revised form 22 August 2007; accepted 26 August 2007 Available online 30 August 2007
Abstract The unidirectional solidification technology by the zone melting method is performed to obtain the large single domain YBCO. And the interface stability and morphology evolution rules of the metallic crystal growth during the traditional unidirectional solidification are not applicable to the YBCO crystal growth. It was found that the crystallology features of the YBCO crystal itself mainly decide the interface stability theory and the morphology evolution rules of the YBCO crystal growth during the traditional unidirectional solidification. These research results are useful for the study on the mechanism of the YBCO crystal growth. © 2007 Elsevier B.V. All rights reserved. Keywords: Unidirectional solidification; YBCO; Superconductor; Interface stability; Morphology evolution
1. Introduction The preparation of the high temperature superconductive material YBCO is finished by the peritectic solidification. Because the YBCO high temperature superconductor (HTS) belongs to the multiple oxide-ceramic systems, its peritectic reaction is very complex. To the final material performance, the phases and microstructure formed during the solidification process are quite important. The interface stability is the initial position of the formed complicated morphology after the crystal growth and the metal solidification. And the interface stability has an important influence on the solidification morphology evolution and selection after the stable growth interface broke [1]. In this paper, some basic problems in preparing YBCO superconductor by unidirectional solidification, including the peritectic reaction, the interface stability and the control of the interface morphology are discussed preliminarily. Three different growth regimes were given by Cima et al. [2]: single crystal growth, cellular/dendritic growth and equiaxed blocky. This view thinks the interface morphology of the YBCO
∗
Corresponding author. Tel.: +86 29 88493484; fax: +86 29 88460294. E-mail addresses: [email protected], [email protected] (H. Cao). 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.063
crystal growth is similar to the metallic system, which exists the planar/cellular/dendritic transition. But, the phenomenon has not been found in this experiment ahead of the solid/liquid interface and the YBCO crystallography features mainly decide its interface morphology itself. 2. Experimental The precursor powders were prepared by taking nominal compositions of 1 2 3 + 10% 2 1 1. The powder of Y2 O3 used in our work was 99.99% purity, and the powders of BaCO3 and CuO were 99% purity. The sample preparation [3]: first, the raw materials of Y2 O3 , BaCO3 and CuO were mixed thoroughly by taking nominal ratio 1:2:3 and then calcined twice at 900 ◦ C for 12 h with an intermediate grinding. The partial powder taken from the above calcined and reground powder was analysed by XRD. The above procedures were repeated until the powders are pure YBa2 Cu3 O6+x (1 2 3). Then precursor rods were prepared by filling the powders in the mould and subjecting them to cold isostatic pressing. Each rod is about 0.2 cm × 0.2 cm × 12 cm. The rods were sintered at 900 ◦ C for 12 h. Fig. 1 shows a schematic illustration of the zone melting furnace [4]. The zone melting furnace consisted of a vertical furnace in which the sintered rod was suspended. The main heater, which was set inside the backup furnace, heated a partial zone of the rod above the peritectic temperature of 1 2 3 and produced a partial molten zone at the same position. Precursor rod suspended by a clip was lowered into the preheated furnace. Then the rod was pulled upward at 0.2 cm/h for the crystal growth under a temperature of about 1015 ◦ C. Peritectic solidification occurred in the temperature gradient region of about 150 ◦ C/cm (obtained approximately by making the tangent on the actually cooling curve). Thousand
H. Cao et al. / Journal of Alloys and Compounds 462 (2008) 428–431
429
Fig. 3. Morphology schematic illustration of the YBCO crystal interface growth morphology with the triangle cone shape during the unidirectional solidification.
Fig. 1. Schematic illustration of the zone melting furnace and the samples are pulling upward through the molten zone heated by the heaters. sixty-seven degree Celsius was the maximum temperature of the partial molten zone.
3. Observations The peritectic alloy system was classified by St. John et al. [5]. Three types according to the growth pattern of the  phase in the phase diagram. It is believed that the growth of  phase in the type C is the most difficult to obtain in all the three types because  phase exists in a quite small region or single component. It means that the solid solubility between the ␣ phase and the  phase is nearly a definite value. The ␣ phase will grow hardly, namely nearly no element diffusion between the ␣ phase and the  phase once the  phase is formed. To the YBCO reaction system, Y2 BaCuO5 (2 1 1) phase will be encapsulated and not grow once 1 2 3 phase is formed. The Ba3 Cu5 O8 –Y2 BaCuO5 quasi-binary phase diagram is shown in Fig. 2 [6]. It can be clearly seen in it that the reaction between the 2 1 1 and liquid (i.e. 2 1 1 + L → 1 2 3) approximately belongs to the type C compared with phase diagram of binary peritectic systems. At this composition, the peritec-
Fig. 2. The Y2 BaCuO5 –Ba3 Cu5 O8 quasi-binary phase diagram.
tic phase (i.e. 1 2 3 phase) is the certain chemical value, so the growth of the 1 2 3 phase is very difficult and the peritectic reaction is quite complex. In addition, the liquidus slope is very steep, and this indicates that it is difficult to achieve high Y supersaturated liquid for high growth rate. It was generally thought that whether the YBCO crystal growth interface ahead of the front keep planar interface during the growth course was decided by the G/R value. The growth front can keep planar interface when the G/R value is greater than the certain value (300 ◦ C h/cm2 ) [7]. Otherwise, the new YBCO crystal would nucleate in the liquid ahead of the growth front, and then it causes planar/cellular/dendritic growth morphology transition. But the interface stability and morphology evolution rules of the metallic crystal growth during the traditional unidirectional solidification are not applicable to the YBCO crystal growth. Because the YBCO crystal growth is controlled not only by the element diffusion, but also by the crystallography features itself. Fig. 3 shows morphology schematic illustration of the YBCO crystal interface growth morphology with triangle cone shape during the unidirectional solidification [8]. The Rmax (the fastest velocity of YBCO crystal growth) is decided by the growth velocity of the crystal plane (1 0 0), (0 1 0) and (0 0 1). So the YBCO crystallology features determine the YBCO growth morphology ahead of the growth front. The 1 2 3 phase will grow along the solidification direction with triangle cone shape under the certain ratio of G/R. It is a facet growth way along the longitudinal section. This phenomenon has been found in the experiment. Fig. 4(a) shows that the solid/liquid interface morphology in the facet growth way is triangle cone shape in microstructure. In macrostructure, it is the planar interface along the longitudinal section. And Fig. 4(b) and (c) is the partial amplification picture, it can be seen clearly from the picture that the solid/liquid interface is very clear and smooth. The transition regions between the triangle cones are not curly but sharp, and these results are also in well accordance with the triangle cone shape theory and verify what mentioned above. This morphology is believed a steady growth interface, and the G/R value here is the critical value. According to the unidirectional solidification constitutional supercooling theory [9], the planar interface morphology growth
430
H. Cao et al. / Journal of Alloys and Compounds 462 (2008) 428–431
Fig. 5. The interface growth morphology under the unsteady growth condition (blocky).
If the G/R value meets the condition in the formula (1), YBCO crystal will keeps the growth continuous in the facet growth way, at the same time, the growth interface is steady state; the height of the triangle cone and the space between the triangle cones will get smaller. But if the G/R value increases and exceeds the critical value, the height of the triangle cones and the space between the triangle cones will get larger instead of the planar/cellular/dendritic transition until the steady growth interface is destroyed. At this moment, the growth interface is instable. Fig. 5 shows that the blocky organization structure occurred in the YBCO samples. The blocky structure occurred because that the G/R value deviate away from the equilibrium critical value. The YBCO crystal cannot grow up and keep the growth continuous if the G/R value does not reach the equilibrium critical value, even if the YBCO crystal grains have formed. In the metal systems, the cell spacing (λ) is defined by the balance of the solute concentration differences between the tip and the bottom of the cells, and the curvatures at the tip, as expressed in the following equation [8]: λ ∝ G−m R−n
Fig. 4. (a) The steady growth interface morphology under the certain G/R value; (b) and (c) partial enlarged figure.
must meet the condition below [9]: G mL (Cs − CL ) ≥ R DL
(1)
where G is the actual temperature gradient, R the solidification velocity, Cs the solute concentration of the solid ahead of the growth front, CL the solute concentration of the liquid ahead of the growth front, DL the solute diffuse coefficient, and mL the liquidus slope in the temperature constitution equilibrium phase diagram.
(2)
The cell spacing of the unidirectional solidified rods decreased with increasing R and/or G. However, the crystallization of the facet growth YBCO is not only limited by the diffusion of the solute elements, but also by the interface kinetics. This point is different from the metal solidification that is only limited by the solute diffusion. Thus, it is too difficult to define the exponent of m and n in the facet-growing YBCO system. So λ in the formula (2) is believed the spacing between the triangle cones. Results discussed above are similar to the constitutional supercooling theory, so the constitutional supercooling theory has guidance meaning to the YBCO HTS oxides crystal growth. It can be concluded that the YBCO crystal structural feature determine the steady growth interface morphology, and the interface morphology is not to be controlled by changing the G/R value, namely that provided the YBCO crystals keep growth continuous, YBCO crystal will grow inevitable in the steady interface morphology of the triangle cone shape in the facet growth way.
H. Cao et al. / Journal of Alloys and Compounds 462 (2008) 428–431
YBCO crystal growth interface morphology is determined by the anisotropy of the crystal growth velocity that are further determined by the interface morphology, growth mechanism and the growth driving force. When the YBCO interface growth kinetics can compensate for the inhomogeneous concentration distribution ahead of the YBCO crystal growth front, YBCO crystal can keep the growth steady and continuous in the facet growth way, if not, YBCO crystal growth will stop and not occur in the transition of the planar/cellular/dendritic growth morphology. 4. Conclusion The interface stability and morphology evolution rules of the metallic crystal growth during the traditional unidirectional solidification are not applicable to the YBCO crystal growth. The YBCO growth morphology ahead of the growth front is determined by the YBCO crystal characteristic itself. The YBCO crystal would grow in the facet growth way with the triangle cone shape under the critical G/R ratio along the longitudinal section. The YBCO phase would grow along the solidification direction with the triangle cone shape under the certain ratio of G/R by the facet growth way longitudinal section. If the G/R value increases and exceeds the critical value, YBCO crystal growth does not occur in the planar/
431
cellular/dendritic transition. The height of the triangle cone and the space between the triangle cones will increase continuously until the steady growth interface is destroyed. At this moment, the growth interface is instable. Acknowledgements This paper is supported by the National Nature Science Foundation of China contact no. 50432050. The authors thank Prof. Y.F. Lu and Dr. J.Q. Feng in the Northwest Institute for Nonferrous Metal Research for their technical supports and helpful discussions. References [1] M. Naiben, Shang Hai, Physics Foundation of Crystal Growth, Science and Technology Publishing Company, January, 1982. [2] M.J. Cima, et al., J. Appl. Phys. 72 (1) (1992) 179. [3] S.K. Chen, L. Zhou, K.G. Wang, et al., Chin. J. Low Temp. Phys. 23 (3) (2001) 193. [4] A. Hayashi, K. Kurachi, et al., Physica C 357 (2001) 669. [5] D.H John St, L.M. Hogan, Acta Metall. 35 (1) (1987) 171. [6] C.J. Kim, et al., Supercond. Sci. Technol. 12 (1999) R27. [7] T. Izumi, Y. Nakamura, Y. Shiohara, Adv. Sapercond. 3 (1991) 429. [8] J. Maeda, Y. Nakamura, et al., Acta Metall. 322 (1999) 151–162. [9] X. Zuyao, L. Pengxing, Shang Hai, Introduction to Material Science, Science and Technology Publishing Company, 1986, p. 155.