- Email: [email protected]
DyBa2Cu3O7−x growth on different polycrystalline Dy2O3 interacting layers
July 1999
Materials Letters 40 Ž1999. 71–77 www.elsevier.comrlocatermatlet
DyBa 2 Cu 3 O 7yx growth on different polycrystalline Dy2 O 3 interacting layers F. Auguste a , N. Vandewalle
b,1
, M. Ausloos b, R. Cloots
a,)
a b
SUPRAS, Institut de Chimie B6, UniÕersite´ de Liege, ` B-4000 Liege, ` Belgium SUPRAS, Institut de Physique B5, UniÕersite´ de Liege, ` B-4000 Liege, ` Belgium Received 18 December 1998; accepted 1 February 1999
Abstract We have considered two types of interacting substrates for testing the melt-textured growth of DyBa 2 Cu 3 O 7yx , i.e., a sintered Dy2 O 3 or a compacted Dy2 O 3 powder. Resulting microstructures are compared. Compacted powders are found to provide the most developed 123 grains. Physical and chemical arguments are given for such findings. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.50.Cj; 81.40.y z; 74.72.Jt; 74.80.Bj Keywords: DyBa 2 Cu 3 O 7yx ; Melt-texturation method; Dysprosium 123 system
1. Introduction Among superconducting ceramics bulk REBa 2Cu 3 O 7yx Žso-called RE-123 where RE s Y, Dy, Ho, etc.. are usually produced by solid-state reactions between RE 2 O 3 , CuO and BaCO 3 oxide powders in a range of temperatures around 10008C depending on the nature of the rare earth ion w1,2x. At the sintering temperature Tp s 10108C for the dysprosium 123 system w1x, the solid Dy-123 phase is transformed into a liquid and a solid following the reaction ‘123 ™ L q 211’. The peritectic decomposition gives thus a liquid phase Žthe so-called L-phase. poor in dysprosium
) 1
Corresponding author E-mail: [email protected].
Žessentially composed of BaO q CuO. in which solid Dy2 BaCuO5 particles, the so-called 211 phase, are dispersed. Below Tp s 10108C, the Žperitectic. recombination process ‘211 q L ™ 123’ is initiated. A slow cooling rate, i.e., typically y18Crh, is imposed down to 9708C w1x for grain growth. In fact, the growth of the 123 phase requires a steady supply of rare earth ion which is provided by the Žslow. partial dissolution of the 211 particles in the liquid phase w3x. Due to the random distribution of nucleation centers, the resulting microstructure is multigranular and is characterized by many grain boundaries where an excess of liquid phase L is often segregated. This is one of the disadvantages of the slow kinetics of the recombination process. As a consequence, unreacted 211 particles are trapped in the crystalline matrix though they are homogeneously distributed w4,5x in
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 5 1 - 8
72
F. Auguste et al.r Materials Letters 40 (1999) 71–77
the 123 grains. In order to obtain large single domains and good grain boundaries for practical applications, several techniques w6,7x have been developed. Among others, seeding techniques have recently received much attention. In the top-seeded melt-texturation method w6x, one Žparallelepipedic. single crystal of, e.g., SmBa 2 Cu 3 O 7yx or NdBa 2 Cu 3 O 7yx is often used as the seed material. This material has a much higher peritectic temperature, i.e., Tp s 10608C for Sm-123 and 11008C for Nd-123, than for example the intended DyBa 2 Cu 3 O 7yx . Therefore at 10108C, the Sm-123 or Nd-123 seed does not decompose but constitutes the main nucleation center for the DyBa 2 Cu 3 O 7yx crystal growth. Furthermore, since the lattice matching is favorable, the growing 123 phase is expected to conserve the crystallographic orientation of the seed. Moreover, the amount of crystal defects is expected to be minimized. This ‘unreactive seed method’ has been shown to be able to produce large melt-textured single domains with sizes up to a few centimeters w7x. Whence, the two main advantages of the seed method are Ži. large single domains can be grown with a size up to a few centimeters wide and Žii. the orientation of the sample can be controlled by adjusting the orientation of the seed before the thermal cycle. Other seeds can be considered. For example, single crystals of Dy2 O 3 can be used for the production of Dy-123 large grains of superconducting materials w8x. Dy2 O 3 single crystals provide a sufficient concentration gradient of Dy 3q in the liquid phase modifying the supersaturation and consequently the growth conditions. Nevertheless, the dysprosium oxide crystal cannot play the specific role of crystalline seed for epitaxy. The Dy2 O 3 seed reacts with the liquid phase and this reaction leads to the formation of a locally high concentration of small 211 particles; these will be easily dissolved and will favor the growth conditions of the 123 phase w8x. We propose to combine the effect of Sm- or Nd-123 single crystal as a seed and Dy2 O 3 in order to facilitate the growth of large grains of the 123 phase with a preferential orientation. The dysprosium oxide can play the role of an interacting layer with 123 phase modifying the supersaturation in the bulk and near the single crystal of Sm- or Nd-123 phase. However, Dy2 O 3 single crystals are very expensive
materials Žobtained from ESCETE, Enschede, The Netherlands.. In order to reduce the cost of such interacting layers, we have investigated the influence provided by the use of two types of polycrystalline Dy2 O 3 materials: both sintered and compacted powders. The present report suggests that the best results are obtained when compacted dysprosium oxide layers are used.
2. Experimental procedure First, we prepared a set of 2 mm thick pretreated DyBa 2 Cu 3 O 7yx pellets according to a standard technique w9x. Next, interacting layers were prepared with a micrometric powder of Dy2 O 3 . The mean radius of Dy2 O 3 particles is roughly 2 mm. Two different types of Dy2 O 3 interacting layers were prepared: Ži. sintered and Žii. pelletized. The pelletized material is obtained by uniaxial pressing. The sintered material is obtained by heating up the pelletized one to 15508C and held at this temperature during 36 h. Then, the sintered material is rapidly cooled down to room temperature. In both cases, the interacting layers were 2 mm thick and the largest size was about 10 mm wide. For the growth processing, the dysprosium oxide interacting layer and the preformed DyBa 2 Cu 3 O 7yx pellet were put close together in a Al 2 O 3 crucible placed at the center of a specially built vertical furnace. A classical thermal cycle was applied. The pellet was rapidly heated Žq1508Crh. up to 10358C and held during 2 h at this temperature. The temperature was next rapidly decreased to 9958C at a rate of y508Crh. We have demonstrated very recently w10x that the nucleation of the 123 phase can be controlled from 9958C to 9808C, when the supersaturation is sufficiently high enough to provide the subsequent growth of the 123 phase. The sample was then slowly cooled down with a rate of y18Crh to 9808C inducing the peritectic recombination reaction. At 9808C, the sample was rapidly cooled down in air to room temperature. Microstructure analysis was performed on several polished samples mounted in epoxy resin, using optical polarized light microscopy as well as Scanning Electron Microscopy ŽSEM.. Around 10 sam-
F. Auguste et al.r Materials Letters 40 (1999) 71–77
73
ples have been produced and examined. The findings presented in Section 3 are selected for discussion purpose but are representative of all cases. A few samples without any Dy2 O 3 interacting layer were also synthetized as a reference for our investigations w11,12x. This kind of sample has been extensively studied and reported previously and we confirm the reports.
3. Results 3.1. Sintered substrate Fig. 1 presents an optical micrograph of the interface between the sintered Dy2 O 3 interacting layer and the 123 phase. There is no texturation initiated by the dysprosium oxide layer. The Dy-123 grain
Fig. 2. A blow up of the interaction layer made of 211 particles between the sintered Dy2 O 3 powder and the Dy-123 melt-textured phase.
Fig. 1. Optical micrography in polarized light of the melt-textured DyBa 2 Cu 3 O 7y d sample with sintered Dy2 O 3 powder as the interacting layer.
mean size is equivalent to the grain sizes observed in samples grown directly on the alumina substrate, i.e., roughly 1 = 1 = 1 mm3. The interaction between the interacting layer and the 123 ceramic is in this case very weak. Black spots observed in the picture are large pores filled up with epoxy. Pores seem to be homogeneously dispersed in the sample. In the Dy123 phase, the size distribution of the 211 particles is similar to classical samples grown without any particular condition. The mean size of the 211 particles is about 5 mm. One should note that the dispersion of the 211 particles is not homogeneous and circular regions, free of 211, are often observed w12x. The mean diameter of these circular regions is about 30 to 40 mm. In addition to these circular regions, large scale inhomogeneities are observed in the density of the 211 particles throughout the grains. A few 211
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F. Auguste et al.r Materials Letters 40 (1999) 71–77
particles are observed near the interface between the sintered Dy2 O 3 interacting layer and the Dy-123 phase. The width, d 211 , of this interaction layer is about 20 mm. The particles in this layer are huge with respect to the mean particle size in the sample, i.e., more than 10 mm instead of 5 mm in the sample. Fig. 2 shows a blow up of this interface. 3.2. Compacted layers Fig. 3 presents the optical micrograph of the interface between the compacted Dy2 O 3 powder used as the interacting layer and the Dy-123 phase. A strong interaction between Dy-123 phase and the dysprosium oxide layer is observed. Indeed, an extended layer of 211 particles is formed at the interface between the compacted Dy2 O 3 powder and the
Fig. 4. A blow up of the interaction layer made of 211 particles between the pelletized Dy2 O 3 powder and the Dy-123 melt-textured phase.
Fig. 3. Optical micrography in polarized light of the melt-textured DyBa 2 Cu 3 O 7y d sample with pelletized Dy2 O 3 powder as the interacting layer.
123 phase. The width, d 211 , of the interaction layer is about 200 mm, i.e., 10 times wider than the case of the sintered Dy2 O 3 material. The grain size is large: roughly 3 = 3 = 2 mm3, i.e., more than in the case of interaction with the sintered Dy2 O 3 powder. Contrary to the samples grown in presence of a sintered Dy2 O 3 interacting layer, the pores seem to be localized only near the interaction layer. The only largescale defects to be reported are cracks located far from the interaction layer. A heavy density of 211 particles Žmean size s 3 to 4 mm. can be found throughout the whole sample. It seems that they have been formed from the interaction layer. Circular regions free of 211 particles are again observed. This feature seems to be intrinsic to melt-textured 123 compounds w13x. However, the dispersion of small 211 particles seems to be homogeneous here in
F. Auguste et al.r Materials Letters 40 (1999) 71–77 Table 1 The summary of our findings Intereacting layer
Dy2 O 3 sintered Dy2 O 3 pelletized
volume fraction x structure dispersion of pores size of Dy-123 grains mean diameter of 211 211 size range dispersion of 211 width d 211 of the layer size of 211 in the layer
0.91 polycrystalline homogeneous 1=1=1 mm3 5 mm 0.5–15 mm inhomogeneous 20 mm 10 mm
0.59 polycrystalline at the interaction layer 3=3=2 mm3 4 mm 0.5–12 mm homogeneous 200 mm 30–40 mm
contrast with above samples Žsee Section 3.1.. For completeness, Fig. 4 exhibits a blow up of the interaction layer between a Dy-123 grain and the pelletized Dy2 O 3 powder. The microstructure should be compared to that shown in Fig. 2. The size of the 211 particles in the interaction layer is 30 to 40 mm, thus larger than in the grain. That size is also greater than in the interaction layer shown in Fig. 2.
4. Discussion A summary of the above findings is given in Table 1. The various examined quantities of interest are listed: the relative volume fraction x of the interacting layer, the width d 211 of the interaction layer, the size of 211 particles in the sample and at the Dy2 O 3rDyBa 2 Cu 3 O 7yx interface.
75
In order to explain the observed microstructure chemical and physical interactions between the dysprosium oxide layer and the liquid L-phase can be discussed as follows. First, the interacting layer can be considered as a porous medium in which the liquid L-phase penetrates during the growth processing. The phenomenology of liquid invasion in porous materials is an ‘old problem’ which is not yet solved. It is often approached in statistical physics by percolation-like theories w14,15x. The pores are supposed to be invaded by the viscous phase L over a layer of thickness d due to capillarity effect. Since the density of sintered Dy2 O 3 powder is likely greater than the pelletized Dy2 O 3 one, d for sintered materials should be less than d for pelletized ones. This is schematically illustrated in Fig. 5. The wetting thickness d in the interacting layer can be estimated by considering various physical parameters such as the capillary forces and the mean pore radius r. In view of the slow kinetics quasi equilibrium scaling arguments w15x can be used. Relating Ži. the capillary pressure to Žii. the pressure difference between gasrliquid interfaces in the porous materials through the characteristic length of the interface, we find that d s Ž1 y x .Ž S 2rr .; where S is the interacting layer effective surface and Ž1 y x . is the fraction of pores in the so-called layer Ž x represents the relative volume fraction of the dysprosium oxide layer.. We realize that the relationship d s Ž1 y x .Ž S 2rr . is a crude approximation since the pore size r usually ranges over more than one decade. More elabo-
Fig. 5. A schematic view of the invasion of the liquid L phase Žin grey. in the intereacting layer Žin white.. Both types of materials are illustrated: pelletized and compacted Dy2 O 3 powder. The 211 particles are represented in black.
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F. Auguste et al.r Materials Letters 40 (1999) 71–77
rated invasion models should be taken into account including the multiple length scales in the porosity w14x. From a chemical point of view, after invasion, the liquid L poor in yttrium reacts with the Dy2 O 3 following L q Dy2 O 3 ™ LX q Ž 1 y z . Dy-211. where LX is a liquid phase rich in dysprosium. Such a chemical reaction locally modifies the concentration of dysprosium in the porous structure. A steady supply of dysprosium takes place in the layer of width d 211 ' d resulting in: Ži. the possible nucleation of 211 particles in the vicinity of the interacting layer forming the 211 layer as observed in Section 3, and Žii. the directional growth of Dy-123 phase in the concentration gradient of dysprosium across the liquid phase. The more porous is the interacting layer, the larger is d for the reaction ‘L q Dy2 O 3’ to proceed, thus the larger is the gradient induced in the liquid phase for the growth. This explains why larger domains of 123 do better grow in presence of a porous Dy2 O 3 interacting layer. Moreover at the rougher Dy2 O 3rDyBa 2 Cu 3 O 7yx interface, the growth of the 211 particles is more favored since the supply of dysprosium is better distributed, whence the larger size of 211 particles at the interface for the compacted materials. This is schematically illustrated in Fig. 5. Notice that the porosimetry of both types of layers is quite hard to control and to measure. Further work could be dedicated to quantify the porosimetry and relate it to microstructure findings in order to improve the interaction process.
5. Conclusion In summary, two different types of Dy2 O 3 powders as interacting layers for the melt-textured growth of DyBa 2 Cu 3 O 7yx have been investigated. The interacting layers are made of sintered and compacted Dy2 O 3 powder. Resulting microstructures have been compared together and with respect to ‘non-interacting’ Dy-123 materials. It has been found that compacted powders provide the best polycrystalline structure in such a interaction process for melt-tex-
Fig. 6. A schematic view of the experimental setup to produce single domain melt-textured 123 material by combining seeding and intereacting techniques.
tured DyBa 2 Cu 3 O 7yx . Physical and chemical arguments have been given. Our present findings suggest a new way to improve the melt-textured 123 compounds towards the processing of larger single domains. Combined seeding and interacting techniques using a Sm- or Nd-123 single crystal and a Dy2 O 3 ‘layer’ are future ways of investigations indeed as schematically shown in Fig. 6.
Acknowledgements N.V. is particularly grateful to FNRS for a research grant. Part of this work is financially supported through the Minister of Education under contract ARC Ž94-99r174. at the University of Liege. `
References w1x S. Jin, G.W. Kammlott, T.H. Tiefel, S.K. Chen, Physica C 198 Ž1992. 333. w2x K. Salama, D.F. Lee, Supercond. Sci. Technol. 7 Ž1994. 177. w3x T. Izumi, Y. Shiohara, J. Mater. Res. 7 Ž1992. 16. w4x C.-J. Kim, S.H. Lai, P.J. McGinn, Mater. Lett. 19 Ž1994. 185. w5x N. Vandewalle, M. Ausloos, N. Mineur, R. Cloots, G.-W. Hong, C.-J. Kim, Supercond. Sci. Technol. 9 Ž1996. 665. w6x M. Morita, S. Takebayashi, M. Tanaka, K. Kimura, K. Miyamoto, K. Savano, Adv. Supercond. 3 Ž1991. 733. w7x L. Gao, Y.Y. Xue, D. Ramirez, Z.J. Huang, R.L. Meng, C.W. Chu, Appl. Phys. Lett. 63 Ž1993. 260. w8x R. Cloots, F. Auguste, A. Rulmont, N. Vandewalle, M. Ausloos, J. Mater. Res. 12 Ž1997. 3199. w9x C. Hannay, R. Cloots, M. Ausloos, Solid State Commun. 83 Ž1992. 349.
F. Auguste et al.r Materials Letters 40 (1999) 71–77 w10x F. Auguste, A. Rulmont, N. Vandewalle, H. Bougrine, J. Mucha, M. Ausloos, R. Cloots, J. Low Temp. Phys. 105 Ž1996. 1439. w11x N. Vandewalle, Supercond. Sci. Technol. 11 Ž1998. 35. w12x N. Vandewalle, M. Ausloos, N. Mineur, R. Cloots, G.-W. Hong, C.-J. Kim, Supercond. Sci. Technol. 9 Ž1996. 665.
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w13x C.-J. Kim, H.-G. Lee, K.-B. Kim, G.-W. Hong, J. Mater. Res. 10 Ž1995. 2235. w14x D. Stauffer, A. Aharony, Introduction to Percolation Theory, Taylor & Francis, London, 1992, 2nd printing. w15x M. Sahimi, Flow and Transport in Porous Media and Fractured Rock, VCH, Weinheim, 1995.
Materials Letters 40 Ž1999. 71–77 www.elsevier.comrlocatermatlet
DyBa 2 Cu 3 O 7yx growth on different polycrystalline Dy2 O 3 interacting layers F. Auguste a , N. Vandewalle
b,1
, M. Ausloos b, R. Cloots
a,)
a b
SUPRAS, Institut de Chimie B6, UniÕersite´ de Liege, ` B-4000 Liege, ` Belgium SUPRAS, Institut de Physique B5, UniÕersite´ de Liege, ` B-4000 Liege, ` Belgium Received 18 December 1998; accepted 1 February 1999
Abstract We have considered two types of interacting substrates for testing the melt-textured growth of DyBa 2 Cu 3 O 7yx , i.e., a sintered Dy2 O 3 or a compacted Dy2 O 3 powder. Resulting microstructures are compared. Compacted powders are found to provide the most developed 123 grains. Physical and chemical arguments are given for such findings. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.50.Cj; 81.40.y z; 74.72.Jt; 74.80.Bj Keywords: DyBa 2 Cu 3 O 7yx ; Melt-texturation method; Dysprosium 123 system
1. Introduction Among superconducting ceramics bulk REBa 2Cu 3 O 7yx Žso-called RE-123 where RE s Y, Dy, Ho, etc.. are usually produced by solid-state reactions between RE 2 O 3 , CuO and BaCO 3 oxide powders in a range of temperatures around 10008C depending on the nature of the rare earth ion w1,2x. At the sintering temperature Tp s 10108C for the dysprosium 123 system w1x, the solid Dy-123 phase is transformed into a liquid and a solid following the reaction ‘123 ™ L q 211’. The peritectic decomposition gives thus a liquid phase Žthe so-called L-phase. poor in dysprosium
) 1
Corresponding author E-mail: [email protected].
Žessentially composed of BaO q CuO. in which solid Dy2 BaCuO5 particles, the so-called 211 phase, are dispersed. Below Tp s 10108C, the Žperitectic. recombination process ‘211 q L ™ 123’ is initiated. A slow cooling rate, i.e., typically y18Crh, is imposed down to 9708C w1x for grain growth. In fact, the growth of the 123 phase requires a steady supply of rare earth ion which is provided by the Žslow. partial dissolution of the 211 particles in the liquid phase w3x. Due to the random distribution of nucleation centers, the resulting microstructure is multigranular and is characterized by many grain boundaries where an excess of liquid phase L is often segregated. This is one of the disadvantages of the slow kinetics of the recombination process. As a consequence, unreacted 211 particles are trapped in the crystalline matrix though they are homogeneously distributed w4,5x in
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 5 1 - 8
72
F. Auguste et al.r Materials Letters 40 (1999) 71–77
the 123 grains. In order to obtain large single domains and good grain boundaries for practical applications, several techniques w6,7x have been developed. Among others, seeding techniques have recently received much attention. In the top-seeded melt-texturation method w6x, one Žparallelepipedic. single crystal of, e.g., SmBa 2 Cu 3 O 7yx or NdBa 2 Cu 3 O 7yx is often used as the seed material. This material has a much higher peritectic temperature, i.e., Tp s 10608C for Sm-123 and 11008C for Nd-123, than for example the intended DyBa 2 Cu 3 O 7yx . Therefore at 10108C, the Sm-123 or Nd-123 seed does not decompose but constitutes the main nucleation center for the DyBa 2 Cu 3 O 7yx crystal growth. Furthermore, since the lattice matching is favorable, the growing 123 phase is expected to conserve the crystallographic orientation of the seed. Moreover, the amount of crystal defects is expected to be minimized. This ‘unreactive seed method’ has been shown to be able to produce large melt-textured single domains with sizes up to a few centimeters w7x. Whence, the two main advantages of the seed method are Ži. large single domains can be grown with a size up to a few centimeters wide and Žii. the orientation of the sample can be controlled by adjusting the orientation of the seed before the thermal cycle. Other seeds can be considered. For example, single crystals of Dy2 O 3 can be used for the production of Dy-123 large grains of superconducting materials w8x. Dy2 O 3 single crystals provide a sufficient concentration gradient of Dy 3q in the liquid phase modifying the supersaturation and consequently the growth conditions. Nevertheless, the dysprosium oxide crystal cannot play the specific role of crystalline seed for epitaxy. The Dy2 O 3 seed reacts with the liquid phase and this reaction leads to the formation of a locally high concentration of small 211 particles; these will be easily dissolved and will favor the growth conditions of the 123 phase w8x. We propose to combine the effect of Sm- or Nd-123 single crystal as a seed and Dy2 O 3 in order to facilitate the growth of large grains of the 123 phase with a preferential orientation. The dysprosium oxide can play the role of an interacting layer with 123 phase modifying the supersaturation in the bulk and near the single crystal of Sm- or Nd-123 phase. However, Dy2 O 3 single crystals are very expensive
materials Žobtained from ESCETE, Enschede, The Netherlands.. In order to reduce the cost of such interacting layers, we have investigated the influence provided by the use of two types of polycrystalline Dy2 O 3 materials: both sintered and compacted powders. The present report suggests that the best results are obtained when compacted dysprosium oxide layers are used.
2. Experimental procedure First, we prepared a set of 2 mm thick pretreated DyBa 2 Cu 3 O 7yx pellets according to a standard technique w9x. Next, interacting layers were prepared with a micrometric powder of Dy2 O 3 . The mean radius of Dy2 O 3 particles is roughly 2 mm. Two different types of Dy2 O 3 interacting layers were prepared: Ži. sintered and Žii. pelletized. The pelletized material is obtained by uniaxial pressing. The sintered material is obtained by heating up the pelletized one to 15508C and held at this temperature during 36 h. Then, the sintered material is rapidly cooled down to room temperature. In both cases, the interacting layers were 2 mm thick and the largest size was about 10 mm wide. For the growth processing, the dysprosium oxide interacting layer and the preformed DyBa 2 Cu 3 O 7yx pellet were put close together in a Al 2 O 3 crucible placed at the center of a specially built vertical furnace. A classical thermal cycle was applied. The pellet was rapidly heated Žq1508Crh. up to 10358C and held during 2 h at this temperature. The temperature was next rapidly decreased to 9958C at a rate of y508Crh. We have demonstrated very recently w10x that the nucleation of the 123 phase can be controlled from 9958C to 9808C, when the supersaturation is sufficiently high enough to provide the subsequent growth of the 123 phase. The sample was then slowly cooled down with a rate of y18Crh to 9808C inducing the peritectic recombination reaction. At 9808C, the sample was rapidly cooled down in air to room temperature. Microstructure analysis was performed on several polished samples mounted in epoxy resin, using optical polarized light microscopy as well as Scanning Electron Microscopy ŽSEM.. Around 10 sam-
F. Auguste et al.r Materials Letters 40 (1999) 71–77
73
ples have been produced and examined. The findings presented in Section 3 are selected for discussion purpose but are representative of all cases. A few samples without any Dy2 O 3 interacting layer were also synthetized as a reference for our investigations w11,12x. This kind of sample has been extensively studied and reported previously and we confirm the reports.
3. Results 3.1. Sintered substrate Fig. 1 presents an optical micrograph of the interface between the sintered Dy2 O 3 interacting layer and the 123 phase. There is no texturation initiated by the dysprosium oxide layer. The Dy-123 grain
Fig. 2. A blow up of the interaction layer made of 211 particles between the sintered Dy2 O 3 powder and the Dy-123 melt-textured phase.
Fig. 1. Optical micrography in polarized light of the melt-textured DyBa 2 Cu 3 O 7y d sample with sintered Dy2 O 3 powder as the interacting layer.
mean size is equivalent to the grain sizes observed in samples grown directly on the alumina substrate, i.e., roughly 1 = 1 = 1 mm3. The interaction between the interacting layer and the 123 ceramic is in this case very weak. Black spots observed in the picture are large pores filled up with epoxy. Pores seem to be homogeneously dispersed in the sample. In the Dy123 phase, the size distribution of the 211 particles is similar to classical samples grown without any particular condition. The mean size of the 211 particles is about 5 mm. One should note that the dispersion of the 211 particles is not homogeneous and circular regions, free of 211, are often observed w12x. The mean diameter of these circular regions is about 30 to 40 mm. In addition to these circular regions, large scale inhomogeneities are observed in the density of the 211 particles throughout the grains. A few 211
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F. Auguste et al.r Materials Letters 40 (1999) 71–77
particles are observed near the interface between the sintered Dy2 O 3 interacting layer and the Dy-123 phase. The width, d 211 , of this interaction layer is about 20 mm. The particles in this layer are huge with respect to the mean particle size in the sample, i.e., more than 10 mm instead of 5 mm in the sample. Fig. 2 shows a blow up of this interface. 3.2. Compacted layers Fig. 3 presents the optical micrograph of the interface between the compacted Dy2 O 3 powder used as the interacting layer and the Dy-123 phase. A strong interaction between Dy-123 phase and the dysprosium oxide layer is observed. Indeed, an extended layer of 211 particles is formed at the interface between the compacted Dy2 O 3 powder and the
Fig. 4. A blow up of the interaction layer made of 211 particles between the pelletized Dy2 O 3 powder and the Dy-123 melt-textured phase.
Fig. 3. Optical micrography in polarized light of the melt-textured DyBa 2 Cu 3 O 7y d sample with pelletized Dy2 O 3 powder as the interacting layer.
123 phase. The width, d 211 , of the interaction layer is about 200 mm, i.e., 10 times wider than the case of the sintered Dy2 O 3 material. The grain size is large: roughly 3 = 3 = 2 mm3, i.e., more than in the case of interaction with the sintered Dy2 O 3 powder. Contrary to the samples grown in presence of a sintered Dy2 O 3 interacting layer, the pores seem to be localized only near the interaction layer. The only largescale defects to be reported are cracks located far from the interaction layer. A heavy density of 211 particles Žmean size s 3 to 4 mm. can be found throughout the whole sample. It seems that they have been formed from the interaction layer. Circular regions free of 211 particles are again observed. This feature seems to be intrinsic to melt-textured 123 compounds w13x. However, the dispersion of small 211 particles seems to be homogeneous here in
F. Auguste et al.r Materials Letters 40 (1999) 71–77 Table 1 The summary of our findings Intereacting layer
Dy2 O 3 sintered Dy2 O 3 pelletized
volume fraction x structure dispersion of pores size of Dy-123 grains mean diameter of 211 211 size range dispersion of 211 width d 211 of the layer size of 211 in the layer
0.91 polycrystalline homogeneous 1=1=1 mm3 5 mm 0.5–15 mm inhomogeneous 20 mm 10 mm
0.59 polycrystalline at the interaction layer 3=3=2 mm3 4 mm 0.5–12 mm homogeneous 200 mm 30–40 mm
contrast with above samples Žsee Section 3.1.. For completeness, Fig. 4 exhibits a blow up of the interaction layer between a Dy-123 grain and the pelletized Dy2 O 3 powder. The microstructure should be compared to that shown in Fig. 2. The size of the 211 particles in the interaction layer is 30 to 40 mm, thus larger than in the grain. That size is also greater than in the interaction layer shown in Fig. 2.
4. Discussion A summary of the above findings is given in Table 1. The various examined quantities of interest are listed: the relative volume fraction x of the interacting layer, the width d 211 of the interaction layer, the size of 211 particles in the sample and at the Dy2 O 3rDyBa 2 Cu 3 O 7yx interface.
75
In order to explain the observed microstructure chemical and physical interactions between the dysprosium oxide layer and the liquid L-phase can be discussed as follows. First, the interacting layer can be considered as a porous medium in which the liquid L-phase penetrates during the growth processing. The phenomenology of liquid invasion in porous materials is an ‘old problem’ which is not yet solved. It is often approached in statistical physics by percolation-like theories w14,15x. The pores are supposed to be invaded by the viscous phase L over a layer of thickness d due to capillarity effect. Since the density of sintered Dy2 O 3 powder is likely greater than the pelletized Dy2 O 3 one, d for sintered materials should be less than d for pelletized ones. This is schematically illustrated in Fig. 5. The wetting thickness d in the interacting layer can be estimated by considering various physical parameters such as the capillary forces and the mean pore radius r. In view of the slow kinetics quasi equilibrium scaling arguments w15x can be used. Relating Ži. the capillary pressure to Žii. the pressure difference between gasrliquid interfaces in the porous materials through the characteristic length of the interface, we find that d s Ž1 y x .Ž S 2rr .; where S is the interacting layer effective surface and Ž1 y x . is the fraction of pores in the so-called layer Ž x represents the relative volume fraction of the dysprosium oxide layer.. We realize that the relationship d s Ž1 y x .Ž S 2rr . is a crude approximation since the pore size r usually ranges over more than one decade. More elabo-
Fig. 5. A schematic view of the invasion of the liquid L phase Žin grey. in the intereacting layer Žin white.. Both types of materials are illustrated: pelletized and compacted Dy2 O 3 powder. The 211 particles are represented in black.
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rated invasion models should be taken into account including the multiple length scales in the porosity w14x. From a chemical point of view, after invasion, the liquid L poor in yttrium reacts with the Dy2 O 3 following L q Dy2 O 3 ™ LX q Ž 1 y z . Dy-211. where LX is a liquid phase rich in dysprosium. Such a chemical reaction locally modifies the concentration of dysprosium in the porous structure. A steady supply of dysprosium takes place in the layer of width d 211 ' d resulting in: Ži. the possible nucleation of 211 particles in the vicinity of the interacting layer forming the 211 layer as observed in Section 3, and Žii. the directional growth of Dy-123 phase in the concentration gradient of dysprosium across the liquid phase. The more porous is the interacting layer, the larger is d for the reaction ‘L q Dy2 O 3’ to proceed, thus the larger is the gradient induced in the liquid phase for the growth. This explains why larger domains of 123 do better grow in presence of a porous Dy2 O 3 interacting layer. Moreover at the rougher Dy2 O 3rDyBa 2 Cu 3 O 7yx interface, the growth of the 211 particles is more favored since the supply of dysprosium is better distributed, whence the larger size of 211 particles at the interface for the compacted materials. This is schematically illustrated in Fig. 5. Notice that the porosimetry of both types of layers is quite hard to control and to measure. Further work could be dedicated to quantify the porosimetry and relate it to microstructure findings in order to improve the interaction process.
5. Conclusion In summary, two different types of Dy2 O 3 powders as interacting layers for the melt-textured growth of DyBa 2 Cu 3 O 7yx have been investigated. The interacting layers are made of sintered and compacted Dy2 O 3 powder. Resulting microstructures have been compared together and with respect to ‘non-interacting’ Dy-123 materials. It has been found that compacted powders provide the best polycrystalline structure in such a interaction process for melt-tex-
Fig. 6. A schematic view of the experimental setup to produce single domain melt-textured 123 material by combining seeding and intereacting techniques.
tured DyBa 2 Cu 3 O 7yx . Physical and chemical arguments have been given. Our present findings suggest a new way to improve the melt-textured 123 compounds towards the processing of larger single domains. Combined seeding and interacting techniques using a Sm- or Nd-123 single crystal and a Dy2 O 3 ‘layer’ are future ways of investigations indeed as schematically shown in Fig. 6.
Acknowledgements N.V. is particularly grateful to FNRS for a research grant. Part of this work is financially supported through the Minister of Education under contract ARC Ž94-99r174. at the University of Liege. `
References w1x S. Jin, G.W. Kammlott, T.H. Tiefel, S.K. Chen, Physica C 198 Ž1992. 333. w2x K. Salama, D.F. Lee, Supercond. Sci. Technol. 7 Ž1994. 177. w3x T. Izumi, Y. Shiohara, J. Mater. Res. 7 Ž1992. 16. w4x C.-J. Kim, S.H. Lai, P.J. McGinn, Mater. Lett. 19 Ž1994. 185. w5x N. Vandewalle, M. Ausloos, N. Mineur, R. Cloots, G.-W. Hong, C.-J. Kim, Supercond. Sci. Technol. 9 Ž1996. 665. w6x M. Morita, S. Takebayashi, M. Tanaka, K. Kimura, K. Miyamoto, K. Savano, Adv. Supercond. 3 Ž1991. 733. w7x L. Gao, Y.Y. Xue, D. Ramirez, Z.J. Huang, R.L. Meng, C.W. Chu, Appl. Phys. Lett. 63 Ž1993. 260. w8x R. Cloots, F. Auguste, A. Rulmont, N. Vandewalle, M. Ausloos, J. Mater. Res. 12 Ž1997. 3199. w9x C. Hannay, R. Cloots, M. Ausloos, Solid State Commun. 83 Ž1992. 349.
F. Auguste et al.r Materials Letters 40 (1999) 71–77 w10x F. Auguste, A. Rulmont, N. Vandewalle, H. Bougrine, J. Mucha, M. Ausloos, R. Cloots, J. Low Temp. Phys. 105 Ž1996. 1439. w11x N. Vandewalle, Supercond. Sci. Technol. 11 Ž1998. 35. w12x N. Vandewalle, M. Ausloos, N. Mineur, R. Cloots, G.-W. Hong, C.-J. Kim, Supercond. Sci. Technol. 9 Ž1996. 665.
77
w13x C.-J. Kim, H.-G. Lee, K.-B. Kim, G.-W. Hong, J. Mater. Res. 10 Ž1995. 2235. w14x D. Stauffer, A. Aharony, Introduction to Percolation Theory, Taylor & Francis, London, 1992, 2nd printing. w15x M. Sahimi, Flow and Transport in Porous Media and Fractured Rock, VCH, Weinheim, 1995.