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Influence of Y2O3 and CeO2 additions on growth of YBCO bulk superconductors
Journal of Crystal Growth 356 (2012) 75–80
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Influence of Y2O3 and CeO2 additions on growth of YBCO bulk superconductors D. Volochova´ n, P. Diko, V. Antal, M. Raduˇsovska´, S. Piovarcˇi Institute of Experimental Physics SAS, Watsonova 47, 040 01 Koˇsice, Slovak Republic
a r t i c l e i n f o
abstract
Article history: Received 8 June 2012 Received in revised form 12 July 2012 Accepted 14 July 2012 Communicated by S. Uda Available online 23 July 2012
The Top-Seeded Melt-Growth process for preparation of bulk YBCO single-grain samples from the nominal composition 1 mol YBa2Cu3Ox, 0.25 mol Y2O3 and 1 wt% CeO2 was analysed by optical and electron microscopy, EDAX microanalyses and thermal analyses. It is shown that 4.2 wt% of CuO is formed after reaction of constituents. During an isothermal dwell at the optimised temperature, the growth of Y123 single-crystal stops due to decreasing peritectic temperature with increasing CuO concentration in the melt. The growth at furnace cooling leads to trapping of droplets of melt into the growing crystal, which finally epitaxially crystallises on Y123 as BaCu2O2 phase. The last part of the sample solidifies as Y123 elongated spheroids with Y211, BaCu2O2 and CuO phases. Additional slow cooling 1 1C/h from the temperature of isothermal growth leads to epitaxial growth of Y123 crystal over the entire sample. Only very narrow part at the rim of the sample contains CuO phase and blocky Y123 crystals are formed there due to self-nucleation. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal morphology A1. Optical microscopy A2. Growth from melt A2. Single crystal growth B1. Cuprates B2. Oxide superconducting materials
1. Introduction The mostly used technology for production of bulk single-grain YBCO superconductors is so called Top-Seeded Melt-Growth (TSMG) process [1–3]. The TSMG process involves heating of the pressed Y1Ba2Cu3Ox powder (with addition of Y2O3 to form excess of Y211 particles or direct addition of Y211 powder) to a temperature above that of the peritectic decomposition (incongruent melting) of the Y123 phase [4]. 2(YBa2Cu3O7–x)(s)¼Y2BaCuO5(s)þBa3Cu5Oy(L)þzO2(g)
(1)
At this temperature, samples nearly retain their original shape. During isothermal undercooling or very slow cooling, the Y123 crystal (also called single-grain or single-domain) growths epitaxially from the seed, located at the top surface of the cylindrical sample, from an undercooled mixture of Y211 particles and a melt. To sustain the growth of the Y123 grains, the dissolution of Y211 particles into the melt is essential to supply Y3 þ ions to the solidification front. During the growth, the Y211 particles which are not consumed by reaction (1) are trapped in to Y123 crystal. It was shown that refinement of Y211 particles effectively improves critical current density [3] and maximum trapped field of these bulks. An addition of 1 wt% CeO2 can retard the growth of n
Corresponding author. Tel.: þ421 55 7922240; fax: þ421 55 7922244. E-mail address: [email protected] (D. Volochova´).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.021
Y211 particles in a partially melted state at temperatures higher than peritectic [5]. An addition of Y2O3 fine powder and creation of Y211 particles during heating and sintering of Y123/Y2O3 mixture with optimised nominal composition 1 mol YBa2Cu3Ox, 0.25 mol Y2O3 and 1 wt% CeO2 [6] simplifies this modified fabrication process but has some consequences on the process due to changes in the phase equilibrium of the system [7]. In this contribution we analyse the influence of Y2O3 and CeO2 additions on the Y123 single-grain growth at optimised temperature of isothermal growth [8].
2. Experimental 1 mol YBa2Cu3Ox (Solvay, particle size 30 mm), 0.25 mol Y2O3 (99.9%) and 1 wt% CeO2 (99.0%) powders (nominal composition, NC) were mixed in appropriate amounts in a mixer for 30 minutes and then intensively milled for 15 min in a friction mill. Powder was uniaxially pressed into the cylindrical pellets of 20 mm in diameter. The samples were treated in a chamber furnace with time/temperature programme optimised for high Y123 crystal quality [8]: heating up at the rate of 100 1C/h to 940 1C/dwell 24 h, heating up at the rate of 100 1C/h to the maximum temperature 1040 1C, dwell time for 10 h, fast cooling to the temperature 1008 1C at the rate of 50 1C/h, slow cooling to the isothermal hold temperature, 996 1C, with cooling rate 1 1C/h, holding at 996 1C for 60 h and finally cooling to the room temperature with furnace. At the temperature 940 1C the liquid
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´ et al. / Journal of Crystal Growth 356 (2012) 75–80 D. Volochova
phase sintering leads to well sintered sample. Following heating up to the maximum temperature 1040 1C, about 30 1C above the peritectic temperature, Tp is believed to be necessary for full homogeneous peritectic melting of the sample. The slow cooling rate from the temperature 1008 1C–12 1C higher than the temperature of isothermal dwell, Tis was used to equilibrate melt composition because the solubility of Y in the melt is a function of temperature [9]. The temperature of isothermal dwell, Tis—996 1C was found as optimum in our previous works [8]. At this temperature medium growth rate and better crystal quality is obtained. Temperature was monitored by a thermocouple positioned close to the sample. The macrostructure of the sample surface was done by a stereo microscope. Samples for microscopic analyses were prepared by grinding and polishing. The microstructure details were analysed with an optical microscope under normal and polarised light. The DTA measurements of pure YBa2Cu3Ox powder, mixed nominal powder with CuO additions in artificial air were applied to determine the characteristic transformation temperatures at heating.
The region which was formed at furnace cooling is marked with fine porosity. The pores in this layer are much smaller than the standard large pores in the central part of the sample. Close to the sample rim also a nucleation of Y123 grains with high misalignment appears. These grains have some features similar to spheroids formed during furnace cooling already described in our former work [12]. At higher magnification and suitable orientation of the Y123 crystal to the vector of polarised light, besides Y211 particles also white phase can be seen in the region with small pores (Fig. 3). The composition of this phase determined by EDAX is BaCu2O2 (012). In the nucleated Y123 elongated spheroids, the 012
3. Results and discussion Macroscopic image of the sample grown under isothermal conditions at 996 1C for 60 h is presented in Fig. 1. The time for isothermal growth was long enough for Y123 single-grain to grow over the full 10 mm radius of the sample with the growth rate typical for this temperature (0.2 mm/h [8,10,11]). In spite of enough long time the high quality Y123 crystal does not overgrow through the whole sample and it occupies about 50% of the sample volume. The microstructure of the region at the rim of the sample is shown in Fig. 2.
Fig. 1. Top surface of the sample with 60 h isothermal dwell at 996 1C cooled with furnace to the room temperature.
Fig. 3. Particles of BaCu2O2 phase (white) in the part grown during furnace cooling seen under polarised light (a). Detail of the trapped BaCu2O2 phase epitaxially crystallized on Y123 crystal and porosity developed due to solidification shrinkage (b).
Fig. 2. Sample section along the a/c-plane showing a region at the sample rim. The part of the sample crystallized during furnace cooling (FC) is marked. The epitaxially grown layer is followed by the subgrains with increasing misalignment. Later also a nucleation of Y123 spheroids elongated in temperature gradient appears.
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phase is present in the form of blankets between Y123 spheroid branches (Fig. 4(a)). Close to the sample rim the mixture of CuO solidifies and is located at the boundaries of nucleated elongated spheroids (Fig. 4(b)). The question is why at applied isothermal dwell the Y123 crystal does not grow over the entire sample volume. The termination of Y123 single-crystal growth is apparently related to changes in composition of the sample rest during crystal growth. The position of the sample nominal composition in the isothermal cut of the ternary system YO1.5–BaO–CuO is expressed in Fig. 5. The addition of Y2O3 shifts the composition of the sample into the field where phases Y123–Y211 and CuO are in equilibrium. CuO is formed by reaction between Y123 and Y2O3: 12YBa2Cu3O6.5 þ3Y2O3 þO2 ¼10YBa2Cu3O6.5 þ4Y2BaCuO5 þ2CuO (2)
Fig. 5. Isothermal section of CuO–BaO–YO1.5 ternary phase diagram (concentration in molar fraction). Addition of Y2O3 to Y123 shifts the nominal composition (NC) along the line Y123–YO1.5 to the three phase region Y123–Y211–CuO. The compositions with CuO additions studied by DTA are marked on the line NC–CuO.
In 100 g charge (Y123þY2O3) the amount of CuO after reaction (2) is 1.9 g. CuO is formed also by reaction between YBa2Cu3O6.5 and CeO2: 2YBa2Cu3O6.5 þCeO2 þ2O2 ¼3BaCeO3 þY2BaCuO5 þ5CuO
(3)
Addition of 1 g CeO2 to 100 g charge (Y123þY2O3) creates 2.26 g CuO. It means that when reactions (2) and (3) are finished,
Fig. 6. Polythermal section of YO1.5–1/6 Y123–‘‘Ba0,4Cu0,6O’’ along the connecting line of Y123 and Y2O3 phases. Arrows pointing on the peritectic reaction line show the decreasing temperature of peritectic reaction for compositions with different Y2O3 additions. CeO2 addition behaves similarly as higher Y2O3 addition.
there is 4.2 wt% CuO in the charge. This will significantly influence behaviour of the system during heating and cooling. At first, a reaction between CuO and Y123 will appear at 940 1C invariant point [13]: YBa2Cu3Ox þCuO¼L
(4)
as it is clear from the polythermal section of YO1.5–1/6 Y123– ‘‘Ba0,4Cu0,6O’’ along the connecting line of Y123 and Y2O3 phases (Fig. 6), i.e. the area, in which works the addition of Y2O3 to Y123 [7].
Fig. 4. BaCu2O2 (white-yellow) phase in the form of blankets between Y123 spheroid branches (a). CuO phase (white-blue) crystallized at the boundaries of nucleated elongated spheroids (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
At this temperature, the first melt in the system is formed (Fig. 7). The second consequence of CuO in the system is that the temperatures of peritectic melting and solidification are changed. DTA analyses of pure Y123 powder and mixture of NC þCuO clearly showed this influence (Fig. 7). The temperature of peritectic decomposition, pronounced by the second endothermic peak, is much lower for the system with Y2O3 and CeO2 addition than that for pure YBa2Cu3Ox (Table 1). The enthalpy of the Y123 peritectic melting, DH123, is lower for mixed powders than for pure powder. This is obviously only apparent because some Y123 was already consumed by reactions (2) and (3), so only 69.6% of Y123 rests in the NC. Taking this into account, the DH123 for the
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NC charge will be 112.3 J/g what is nearly identical with pure Y123. The first endothermic peak corresponds to reaction (4). The sign of the first endothermic peak also in pure YBa2Cu3Ox points that used YBa2Cu3Ox powder has some CuO traces. Important is how the CuO excess in the charge influences a growth of Y123/211 crystal. The TSMG solidification process starts at the seed surface. CuO in the melt is pushed by growing crystal and the concentration of CuO, CCuO, in the rest of the sample is increasing according to the peritectic reaction line (Fig. 6) CCuO (wt%)¼4.2Gs/(Gs Gcr)
(5)
where Gs is the mass of the sample and Gcr is the mass of growing Y123/211 crystal. CCuO increases slowly at the beginning and rapidly at the end of solidification (Fig. 8). For example, when one halve of the sample is solidified (what represents the distance 0.73 R for the sample of cylindrical shape according to Fig. 9) the CuO concentration increases twice (the densities of Y123, Y211 and CuO are nearly equal and close to 6.3 g/cm3). The influence of increasing CuO concentration in the notsolidified rest of the sample on peritectic temperature was examined by DTA measurements and is presented in Fig. 10. As can be seen from the DTA records, the temperature of peritectic melting decreases with increasing CuO concentration. This decrease in peritectic temperature causes that Y123 crystal growth stops at some critical CuO concentration in the rest of the sample. Its further grow can continue only under decreasing temperature. At the beginning of furnace cooling, the Y123 crystal further grows epitaxially but at higher undercooling and consequently with higher growth rate.
Fig. 7. DTA records at heating with 5 1C/min in artificial air for pure YBa2Cu3Ox powder and for mixture YBa2Cu3Ox þY2O3 þ1 wt% CeO2.
Observed higher misalignment of subgrains, reflected in higher contrast under polarised light (Fig. 2), is a consequence of this higher growth rate. At some undercooling, BaCu2O2 (012) phase particles appear in the Y123 crystal (Fig. 3). According to Riches [14] this phase is present in the system due to decomposition of Ba3Cu5Oy (L) melt in to two liquid phases BaCuO2 (L1) and BaCu2O2 (L2). Obviously, due to higher CuO concentration in the melt and higher growth rate, L2 melt droplets can be trapped by growing Y123 crystal. During further cooling L2 crystallises epitaxially on Y123 phase and some porosity develops around due to L2 shrinkage at solidification. Epitaxial growth of BaCu2O2 phase is pronounced by the same crystal orientation of all particles reflected through their behaviour in polarised light. When the temperature for spontaneous nucleation of spheroids, TSNS, is reached, the new crystals with high crystal misalignment appear. They grow in the form of spheroids elongated in the temperature gradient (Fig. 2). The full compositional and morphological image of the sample structures is schematically presented in Fig. 10(a). To grow the Y123/211 crystal as large as possible we realised experimental growth under the same conditions as for the first sample (isothermal growth at 996 1C/60 h) with an addition of slow cooling 1 1C/h to 940 1C. The result is the Y123 crystal overgrown through the entire sample (Fig. 12). The section along the sample axes in a/c-plane is showing that only a narrow region at the sample rim was not grown epitaxially and contains blocky Y123 grains (Fig. 13). It means that during slow cooling the Y123 crystal continues in epitaxial growth and only when the
Fig. 8. Dependence of CuO concentration, CCuO, in the notsolidified part of the sample on the portion of solidified sample, (Gs Gcr)/Gs (Gs—weight of the sample, Gcr—weight of the crystal).
Table 1 Characteristic temperatures for DTA records, Tp—declination from monotonous curve, Tp peak temperature, and enthalpy of endotermic reactions, DH. Tp (1C)
Tp
Onset—onset
Onset
peak temperature, TpPeak—maximum
Tp
Peak
DH (J/g)
YBa2Cu3Ox þ Y2O3 þ 1 wt% CeO2
1st peak 2nd peak
930 999.4
936.5 1011.3
946.1 1024.50
20.63 78.17
YBa2Cu3Ox
1st peak 2nd peak
not resolved 1008.5
E948.40 1024.9
not resolved 1037.8
not resolved 109.20
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temperature of the sample reached the temperature of spontaneous nucleation of blocky crystals, TSNB, the narrow multicrystalline part at the sample rim was formed. CuOþY211 structure component is also present only at the sample rim and its formation started at higher temperature than TSNB (Fig. 11(b)). 012 phase was not formed during slow cooling. The compositional and morphological image of the sample structures is schematically presented in Fig. 11(b).
4. Conclusions Macrostructural and microstructural examination, thermal analyses and EDAX microanalyses allow us to describe complexity of the Top-Seeded Melt-Growth process at growth of bulk YBCO single-grain samples. It is shown that in standard nominal composition 1 mol YBa2Cu3Ox, 0.25 mol Y2O3 and 1 wt% CeO2 of the charge for the TSMG growth of Y123 crystal, 4.2 wt% of CuO is
Fig. 9. Schematic illustration of the considered sample and Y123 crystal geometry.
Fig. 10. DTA records at heating with 5 1C/min in artificial air for mixture YBa2Cu3Ox þ Y2O3 þ 1 wt% CeO2 with CuO (in wt%).
Fig. 12. Top surface of the sample with 60 h isothermal dwell at 996 1C with an addition of slow cooling 1 1C/h to 940 1C.
Fig. 11. Schematic illustration of the dependence of solidification temperature on the distance from the seed for isothermal growth (a) and isothermal growth and slow cooling (b). TSNS—temperature of self nucleation of Y123 spheroids, TSNB—temperature of self nucleation of blocky Y123 crystals. Solidified phases: 001–CuO, 012–BaCu2O2, 211–Y2BaCuO5, 123–YBa2Cu3Ox.
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trapping of droplets of melt in to the growing crystal, which at further cooling epitaxially crystallises on Y123 as BaCu2O2 phase. Appearance of this phase supports the former results pointing on decomposition of peritectic Ba3Cu5Oy (L) melt in to two liquid phases BaCuO2 (L1) and BaCu2O2 (L2). The last part of the sample solidifies as Y123 elongated spheroids with Y211, BaCu2O2 and CuO phases. Additional slow cooling 1 1C/h from the temperature of isothermal growth leads to epitaxial growth of Y123 crystal over the entire sample. Only very narrow part at the rim of the sample which solidifies last contains CuO phase and blocky Y123 crystals are formed there due to self-nucleation.
Acknowledgement This work was realized within the Framework of the projects: Centre of Excellence of Advanced Materials with Nano- and Submicron Structure (ITMS 26220120019), Infrastructure Improving of Centre of Excellence of Advanced Materials with Nano- and Submicron- Structure (ITMS 26220120035), New Materials and Technologies for Energetics (ITMS 26220220061), Research and Development of Second Generation YBCO Bulk Superconductors (ITMS 26220220041), which are supported by the Operational Programme ‘Research and Development’ financed through the European Regional Development Fund, VEGA Project no. 2/0211/ 10, Project ERANET-ESO and SAS Centre of Excellence: CFNT MVEP. References
Fig. 13. Blocky Y123 crystals (a) and CuO phase solidified at the sample rim (b).
formed after reaction of constituents. During isothermal epitaxial growth of Y123 single-crystal from the seed at optimised temperature, this off stoichiometry CuO is not trapped into the growing crystal and consequently its concentration is increasing in the notsolidified melt. Increasing CuO concentration in the melt lowers peritectic temperature, therefore at some critical CuO concentration the growth of Y123 crystal stops. During furnace cooling from the temperature of isothermal growth the Y123 crystal continues in epitaxial growth but with higher growth rate and higher misalignment of formed subgrains e.g with lower crystal quality. The growth at lower temperatures leads to
[1] M. Morita, S. Takebayashi, M. Tanaka, K. Kimura, K. Miyamoto, K. Savano, Advances in Superconductivity 3 (1991) 733. [2] T. Izumi, Y. Nakamura, Y. Shiohara, Journal of Materials Research 7 (1992) 1621–1628. [3] M. Murakami, S.I. Yoo, T. Higuchi, N. Sakai, J. Weltz, N. Koshizuka, S. Tanaka, Japanese Journal of Applied Physics 33 (1994) L715–L717. ¨ [4] G. Krabbes, W. Bieger, U. Wiesner, K. Fischer, P. Schatzle, Journal of Electronic Materials 23 (1994) 1135–1142. [5] C.J. Kim, S.H. Lai, P.J. McGinn, Materials Letters 19 (1994) 185–191. [6] S. Kracunovska, P. Diko, D. Litzkendorf, T. Habisreuther, W. Gawalek, Physica C 397 (2003) 123–131. ¨ [7] G. Krabbes, W. Bieger, P. Schatzle, U. Wiesner, Superconductor Science and Technology 11 (1998) 144–148. [8] D. Volochova´, P. Diko, M. Raduˇsovska´, V. Antal, S. Piovarcˇi, K. Zmorayova´, ˇ ˇ ikova´, Journal of Crystal Growth 353 (2012) 31–34. M. Sefc [9] Ch. Kraus, M. Sumida, M. Tagami, Y. Yamada, Y. Shiohara, Zeitschrift fur Physik 96 (1994) 207–212. [10] Y. Nakamura, Y. Shiohara, Journal of Materials Research 11 (1996) 2450–2457. [11] Y-H. Shi, W. Yoeh, A.R. Dennis, N. Hari Babu, S. Pathak, Z. Xu, D.A. Cardwell, Journal of Physics: Conference Series 234 (2010) 012039. ˇ ˇ ikova´, K. Zmorayova´, M. Kalmanova´, D. Volochova´, S. Piovarcˇi, [12] P. Diko, M. Sefc Journal of Crystal Growth 338 (2012) 239–243. [13] R. Bayers, B.T. Ahn, Annual Review Of Materials Science 21 (1991) 335–372. [14] J.D. Riches, J.A. Alarco, T. Yamashita, J.C. Barry, Superconductor Science and Technology 11 (1998) 830–836.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Influence of Y2O3 and CeO2 additions on growth of YBCO bulk superconductors D. Volochova´ n, P. Diko, V. Antal, M. Raduˇsovska´, S. Piovarcˇi Institute of Experimental Physics SAS, Watsonova 47, 040 01 Koˇsice, Slovak Republic
a r t i c l e i n f o
abstract
Article history: Received 8 June 2012 Received in revised form 12 July 2012 Accepted 14 July 2012 Communicated by S. Uda Available online 23 July 2012
The Top-Seeded Melt-Growth process for preparation of bulk YBCO single-grain samples from the nominal composition 1 mol YBa2Cu3Ox, 0.25 mol Y2O3 and 1 wt% CeO2 was analysed by optical and electron microscopy, EDAX microanalyses and thermal analyses. It is shown that 4.2 wt% of CuO is formed after reaction of constituents. During an isothermal dwell at the optimised temperature, the growth of Y123 single-crystal stops due to decreasing peritectic temperature with increasing CuO concentration in the melt. The growth at furnace cooling leads to trapping of droplets of melt into the growing crystal, which finally epitaxially crystallises on Y123 as BaCu2O2 phase. The last part of the sample solidifies as Y123 elongated spheroids with Y211, BaCu2O2 and CuO phases. Additional slow cooling 1 1C/h from the temperature of isothermal growth leads to epitaxial growth of Y123 crystal over the entire sample. Only very narrow part at the rim of the sample contains CuO phase and blocky Y123 crystals are formed there due to self-nucleation. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal morphology A1. Optical microscopy A2. Growth from melt A2. Single crystal growth B1. Cuprates B2. Oxide superconducting materials
1. Introduction The mostly used technology for production of bulk single-grain YBCO superconductors is so called Top-Seeded Melt-Growth (TSMG) process [1–3]. The TSMG process involves heating of the pressed Y1Ba2Cu3Ox powder (with addition of Y2O3 to form excess of Y211 particles or direct addition of Y211 powder) to a temperature above that of the peritectic decomposition (incongruent melting) of the Y123 phase [4]. 2(YBa2Cu3O7–x)(s)¼Y2BaCuO5(s)þBa3Cu5Oy(L)þzO2(g)
(1)
At this temperature, samples nearly retain their original shape. During isothermal undercooling or very slow cooling, the Y123 crystal (also called single-grain or single-domain) growths epitaxially from the seed, located at the top surface of the cylindrical sample, from an undercooled mixture of Y211 particles and a melt. To sustain the growth of the Y123 grains, the dissolution of Y211 particles into the melt is essential to supply Y3 þ ions to the solidification front. During the growth, the Y211 particles which are not consumed by reaction (1) are trapped in to Y123 crystal. It was shown that refinement of Y211 particles effectively improves critical current density [3] and maximum trapped field of these bulks. An addition of 1 wt% CeO2 can retard the growth of n
Corresponding author. Tel.: þ421 55 7922240; fax: þ421 55 7922244. E-mail address: [email protected] (D. Volochova´).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.021
Y211 particles in a partially melted state at temperatures higher than peritectic [5]. An addition of Y2O3 fine powder and creation of Y211 particles during heating and sintering of Y123/Y2O3 mixture with optimised nominal composition 1 mol YBa2Cu3Ox, 0.25 mol Y2O3 and 1 wt% CeO2 [6] simplifies this modified fabrication process but has some consequences on the process due to changes in the phase equilibrium of the system [7]. In this contribution we analyse the influence of Y2O3 and CeO2 additions on the Y123 single-grain growth at optimised temperature of isothermal growth [8].
2. Experimental 1 mol YBa2Cu3Ox (Solvay, particle size 30 mm), 0.25 mol Y2O3 (99.9%) and 1 wt% CeO2 (99.0%) powders (nominal composition, NC) were mixed in appropriate amounts in a mixer for 30 minutes and then intensively milled for 15 min in a friction mill. Powder was uniaxially pressed into the cylindrical pellets of 20 mm in diameter. The samples were treated in a chamber furnace with time/temperature programme optimised for high Y123 crystal quality [8]: heating up at the rate of 100 1C/h to 940 1C/dwell 24 h, heating up at the rate of 100 1C/h to the maximum temperature 1040 1C, dwell time for 10 h, fast cooling to the temperature 1008 1C at the rate of 50 1C/h, slow cooling to the isothermal hold temperature, 996 1C, with cooling rate 1 1C/h, holding at 996 1C for 60 h and finally cooling to the room temperature with furnace. At the temperature 940 1C the liquid
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´ et al. / Journal of Crystal Growth 356 (2012) 75–80 D. Volochova
phase sintering leads to well sintered sample. Following heating up to the maximum temperature 1040 1C, about 30 1C above the peritectic temperature, Tp is believed to be necessary for full homogeneous peritectic melting of the sample. The slow cooling rate from the temperature 1008 1C–12 1C higher than the temperature of isothermal dwell, Tis was used to equilibrate melt composition because the solubility of Y in the melt is a function of temperature [9]. The temperature of isothermal dwell, Tis—996 1C was found as optimum in our previous works [8]. At this temperature medium growth rate and better crystal quality is obtained. Temperature was monitored by a thermocouple positioned close to the sample. The macrostructure of the sample surface was done by a stereo microscope. Samples for microscopic analyses were prepared by grinding and polishing. The microstructure details were analysed with an optical microscope under normal and polarised light. The DTA measurements of pure YBa2Cu3Ox powder, mixed nominal powder with CuO additions in artificial air were applied to determine the characteristic transformation temperatures at heating.
The region which was formed at furnace cooling is marked with fine porosity. The pores in this layer are much smaller than the standard large pores in the central part of the sample. Close to the sample rim also a nucleation of Y123 grains with high misalignment appears. These grains have some features similar to spheroids formed during furnace cooling already described in our former work [12]. At higher magnification and suitable orientation of the Y123 crystal to the vector of polarised light, besides Y211 particles also white phase can be seen in the region with small pores (Fig. 3). The composition of this phase determined by EDAX is BaCu2O2 (012). In the nucleated Y123 elongated spheroids, the 012
3. Results and discussion Macroscopic image of the sample grown under isothermal conditions at 996 1C for 60 h is presented in Fig. 1. The time for isothermal growth was long enough for Y123 single-grain to grow over the full 10 mm radius of the sample with the growth rate typical for this temperature (0.2 mm/h [8,10,11]). In spite of enough long time the high quality Y123 crystal does not overgrow through the whole sample and it occupies about 50% of the sample volume. The microstructure of the region at the rim of the sample is shown in Fig. 2.
Fig. 1. Top surface of the sample with 60 h isothermal dwell at 996 1C cooled with furnace to the room temperature.
Fig. 3. Particles of BaCu2O2 phase (white) in the part grown during furnace cooling seen under polarised light (a). Detail of the trapped BaCu2O2 phase epitaxially crystallized on Y123 crystal and porosity developed due to solidification shrinkage (b).
Fig. 2. Sample section along the a/c-plane showing a region at the sample rim. The part of the sample crystallized during furnace cooling (FC) is marked. The epitaxially grown layer is followed by the subgrains with increasing misalignment. Later also a nucleation of Y123 spheroids elongated in temperature gradient appears.
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phase is present in the form of blankets between Y123 spheroid branches (Fig. 4(a)). Close to the sample rim the mixture of CuO solidifies and is located at the boundaries of nucleated elongated spheroids (Fig. 4(b)). The question is why at applied isothermal dwell the Y123 crystal does not grow over the entire sample volume. The termination of Y123 single-crystal growth is apparently related to changes in composition of the sample rest during crystal growth. The position of the sample nominal composition in the isothermal cut of the ternary system YO1.5–BaO–CuO is expressed in Fig. 5. The addition of Y2O3 shifts the composition of the sample into the field where phases Y123–Y211 and CuO are in equilibrium. CuO is formed by reaction between Y123 and Y2O3: 12YBa2Cu3O6.5 þ3Y2O3 þO2 ¼10YBa2Cu3O6.5 þ4Y2BaCuO5 þ2CuO (2)
Fig. 5. Isothermal section of CuO–BaO–YO1.5 ternary phase diagram (concentration in molar fraction). Addition of Y2O3 to Y123 shifts the nominal composition (NC) along the line Y123–YO1.5 to the three phase region Y123–Y211–CuO. The compositions with CuO additions studied by DTA are marked on the line NC–CuO.
In 100 g charge (Y123þY2O3) the amount of CuO after reaction (2) is 1.9 g. CuO is formed also by reaction between YBa2Cu3O6.5 and CeO2: 2YBa2Cu3O6.5 þCeO2 þ2O2 ¼3BaCeO3 þY2BaCuO5 þ5CuO
(3)
Addition of 1 g CeO2 to 100 g charge (Y123þY2O3) creates 2.26 g CuO. It means that when reactions (2) and (3) are finished,
Fig. 6. Polythermal section of YO1.5–1/6 Y123–‘‘Ba0,4Cu0,6O’’ along the connecting line of Y123 and Y2O3 phases. Arrows pointing on the peritectic reaction line show the decreasing temperature of peritectic reaction for compositions with different Y2O3 additions. CeO2 addition behaves similarly as higher Y2O3 addition.
there is 4.2 wt% CuO in the charge. This will significantly influence behaviour of the system during heating and cooling. At first, a reaction between CuO and Y123 will appear at 940 1C invariant point [13]: YBa2Cu3Ox þCuO¼L
(4)
as it is clear from the polythermal section of YO1.5–1/6 Y123– ‘‘Ba0,4Cu0,6O’’ along the connecting line of Y123 and Y2O3 phases (Fig. 6), i.e. the area, in which works the addition of Y2O3 to Y123 [7].
Fig. 4. BaCu2O2 (white-yellow) phase in the form of blankets between Y123 spheroid branches (a). CuO phase (white-blue) crystallized at the boundaries of nucleated elongated spheroids (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
At this temperature, the first melt in the system is formed (Fig. 7). The second consequence of CuO in the system is that the temperatures of peritectic melting and solidification are changed. DTA analyses of pure Y123 powder and mixture of NC þCuO clearly showed this influence (Fig. 7). The temperature of peritectic decomposition, pronounced by the second endothermic peak, is much lower for the system with Y2O3 and CeO2 addition than that for pure YBa2Cu3Ox (Table 1). The enthalpy of the Y123 peritectic melting, DH123, is lower for mixed powders than for pure powder. This is obviously only apparent because some Y123 was already consumed by reactions (2) and (3), so only 69.6% of Y123 rests in the NC. Taking this into account, the DH123 for the
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NC charge will be 112.3 J/g what is nearly identical with pure Y123. The first endothermic peak corresponds to reaction (4). The sign of the first endothermic peak also in pure YBa2Cu3Ox points that used YBa2Cu3Ox powder has some CuO traces. Important is how the CuO excess in the charge influences a growth of Y123/211 crystal. The TSMG solidification process starts at the seed surface. CuO in the melt is pushed by growing crystal and the concentration of CuO, CCuO, in the rest of the sample is increasing according to the peritectic reaction line (Fig. 6) CCuO (wt%)¼4.2Gs/(Gs Gcr)
(5)
where Gs is the mass of the sample and Gcr is the mass of growing Y123/211 crystal. CCuO increases slowly at the beginning and rapidly at the end of solidification (Fig. 8). For example, when one halve of the sample is solidified (what represents the distance 0.73 R for the sample of cylindrical shape according to Fig. 9) the CuO concentration increases twice (the densities of Y123, Y211 and CuO are nearly equal and close to 6.3 g/cm3). The influence of increasing CuO concentration in the notsolidified rest of the sample on peritectic temperature was examined by DTA measurements and is presented in Fig. 10. As can be seen from the DTA records, the temperature of peritectic melting decreases with increasing CuO concentration. This decrease in peritectic temperature causes that Y123 crystal growth stops at some critical CuO concentration in the rest of the sample. Its further grow can continue only under decreasing temperature. At the beginning of furnace cooling, the Y123 crystal further grows epitaxially but at higher undercooling and consequently with higher growth rate.
Fig. 7. DTA records at heating with 5 1C/min in artificial air for pure YBa2Cu3Ox powder and for mixture YBa2Cu3Ox þY2O3 þ1 wt% CeO2.
Observed higher misalignment of subgrains, reflected in higher contrast under polarised light (Fig. 2), is a consequence of this higher growth rate. At some undercooling, BaCu2O2 (012) phase particles appear in the Y123 crystal (Fig. 3). According to Riches [14] this phase is present in the system due to decomposition of Ba3Cu5Oy (L) melt in to two liquid phases BaCuO2 (L1) and BaCu2O2 (L2). Obviously, due to higher CuO concentration in the melt and higher growth rate, L2 melt droplets can be trapped by growing Y123 crystal. During further cooling L2 crystallises epitaxially on Y123 phase and some porosity develops around due to L2 shrinkage at solidification. Epitaxial growth of BaCu2O2 phase is pronounced by the same crystal orientation of all particles reflected through their behaviour in polarised light. When the temperature for spontaneous nucleation of spheroids, TSNS, is reached, the new crystals with high crystal misalignment appear. They grow in the form of spheroids elongated in the temperature gradient (Fig. 2). The full compositional and morphological image of the sample structures is schematically presented in Fig. 10(a). To grow the Y123/211 crystal as large as possible we realised experimental growth under the same conditions as for the first sample (isothermal growth at 996 1C/60 h) with an addition of slow cooling 1 1C/h to 940 1C. The result is the Y123 crystal overgrown through the entire sample (Fig. 12). The section along the sample axes in a/c-plane is showing that only a narrow region at the sample rim was not grown epitaxially and contains blocky Y123 grains (Fig. 13). It means that during slow cooling the Y123 crystal continues in epitaxial growth and only when the
Fig. 8. Dependence of CuO concentration, CCuO, in the notsolidified part of the sample on the portion of solidified sample, (Gs Gcr)/Gs (Gs—weight of the sample, Gcr—weight of the crystal).
Table 1 Characteristic temperatures for DTA records, Tp—declination from monotonous curve, Tp peak temperature, and enthalpy of endotermic reactions, DH. Tp (1C)
Tp
Onset—onset
Onset
peak temperature, TpPeak—maximum
Tp
Peak
DH (J/g)
YBa2Cu3Ox þ Y2O3 þ 1 wt% CeO2
1st peak 2nd peak
930 999.4
936.5 1011.3
946.1 1024.50
20.63 78.17
YBa2Cu3Ox
1st peak 2nd peak
not resolved 1008.5
E948.40 1024.9
not resolved 1037.8
not resolved 109.20
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temperature of the sample reached the temperature of spontaneous nucleation of blocky crystals, TSNB, the narrow multicrystalline part at the sample rim was formed. CuOþY211 structure component is also present only at the sample rim and its formation started at higher temperature than TSNB (Fig. 11(b)). 012 phase was not formed during slow cooling. The compositional and morphological image of the sample structures is schematically presented in Fig. 11(b).
4. Conclusions Macrostructural and microstructural examination, thermal analyses and EDAX microanalyses allow us to describe complexity of the Top-Seeded Melt-Growth process at growth of bulk YBCO single-grain samples. It is shown that in standard nominal composition 1 mol YBa2Cu3Ox, 0.25 mol Y2O3 and 1 wt% CeO2 of the charge for the TSMG growth of Y123 crystal, 4.2 wt% of CuO is
Fig. 9. Schematic illustration of the considered sample and Y123 crystal geometry.
Fig. 10. DTA records at heating with 5 1C/min in artificial air for mixture YBa2Cu3Ox þ Y2O3 þ 1 wt% CeO2 with CuO (in wt%).
Fig. 12. Top surface of the sample with 60 h isothermal dwell at 996 1C with an addition of slow cooling 1 1C/h to 940 1C.
Fig. 11. Schematic illustration of the dependence of solidification temperature on the distance from the seed for isothermal growth (a) and isothermal growth and slow cooling (b). TSNS—temperature of self nucleation of Y123 spheroids, TSNB—temperature of self nucleation of blocky Y123 crystals. Solidified phases: 001–CuO, 012–BaCu2O2, 211–Y2BaCuO5, 123–YBa2Cu3Ox.
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trapping of droplets of melt in to the growing crystal, which at further cooling epitaxially crystallises on Y123 as BaCu2O2 phase. Appearance of this phase supports the former results pointing on decomposition of peritectic Ba3Cu5Oy (L) melt in to two liquid phases BaCuO2 (L1) and BaCu2O2 (L2). The last part of the sample solidifies as Y123 elongated spheroids with Y211, BaCu2O2 and CuO phases. Additional slow cooling 1 1C/h from the temperature of isothermal growth leads to epitaxial growth of Y123 crystal over the entire sample. Only very narrow part at the rim of the sample which solidifies last contains CuO phase and blocky Y123 crystals are formed there due to self-nucleation.
Acknowledgement This work was realized within the Framework of the projects: Centre of Excellence of Advanced Materials with Nano- and Submicron Structure (ITMS 26220120019), Infrastructure Improving of Centre of Excellence of Advanced Materials with Nano- and Submicron- Structure (ITMS 26220120035), New Materials and Technologies for Energetics (ITMS 26220220061), Research and Development of Second Generation YBCO Bulk Superconductors (ITMS 26220220041), which are supported by the Operational Programme ‘Research and Development’ financed through the European Regional Development Fund, VEGA Project no. 2/0211/ 10, Project ERANET-ESO and SAS Centre of Excellence: CFNT MVEP. References
Fig. 13. Blocky Y123 crystals (a) and CuO phase solidified at the sample rim (b).
formed after reaction of constituents. During isothermal epitaxial growth of Y123 single-crystal from the seed at optimised temperature, this off stoichiometry CuO is not trapped into the growing crystal and consequently its concentration is increasing in the notsolidified melt. Increasing CuO concentration in the melt lowers peritectic temperature, therefore at some critical CuO concentration the growth of Y123 crystal stops. During furnace cooling from the temperature of isothermal growth the Y123 crystal continues in epitaxial growth but with higher growth rate and higher misalignment of formed subgrains e.g with lower crystal quality. The growth at lower temperatures leads to
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