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A new improved synthesis of the 110 K bismuth superconducting phase: freeze-drying of acetic solutions
HAmmALs LETTms
Materials Letters 15 ( 1992 ) 149- 155 North-Holland
A new improved synthesis of the 110 K bismuth superconducting freeze-drying of acetic solutions V. Primo, F. Sapiiia, M.J. Sanchis, R. Ibkiez, A. Belt&
phase: and D. Beltran
UEKM, Dg~art~rngnt de ~~~rn~eaInorg&nica, Fa~u~tat de Quimira, ~‘n~~er~it~tde Valhznria, Dr. ‘~~liner 50, 46100 3~~~soi, Valeneia, Spain
Received 2 1 July 1992
Metastability has greatly hindered the separated synthesis of the h&temperature superconducting phases represented as (Bi, _-n Pb,)2SrtCan_IC~n04+2n (n=2, 2-2-l-2, T,xSO K, and n=3,2-2-2-3, T,r;: 110 K). By careful control of the synthetic variables, it becomes possible to obtain the 110 K phase as the only superconducting one through processing of freeze-dried acetic soktions. This technique leads to homogeneously sized (5-10 pm) micaceous platelets of the superconducting material.
1. Introduction
Entropy stabilization of highly deffective hightemperature superconducting phases of the bismuth system, represented by the nominal ideal formula Bi2Sr2Can_1C~n04+2n (n=Z, 3), is widely assumed [ I ,2]. Difficulties in obtaining a given single phase without impurities increase as its region of true thermodynamic stability decreases and, consequently, procedural variables acquire a special relevance. This is particularly the case of the n = 3 (2-2-2-3, T, z I 10 K) phase [ 31 where, moreover, “the weak bonding along the c axis may also contribute to its stronger propensity to form intergrowth products, with sections of varying n values in a single grain” [ 11. The result is that, although its synthesis becomes facilitated by addition of lead [ 41, the different treatments described as appropriate to increase the volume fraction of 2-2-2-3 in the (Bi,Pb)Sr-Ca-Cu-0 system are usually tediously muIti-step and prolonged (until twelve days) [ 5,6] and, very frequently, do not lead to this phase as the only superconducting one in the final material [ 5,7 1, which, in turn, complicates the measurements of superconducting properties other than T, [ 8 1. To overcome some of the well-known inherent Iimitations of the traditional “heat and beat” ce-
ramic procedure [ 91, it seems evident that direct chemical-precursor-based methods (which yield satisfactory results in the case of related simpler oxides [ lo] ) are no longer applicabIe to multimetallic systems such as that we are dealing with. On the other hand, the related solution techniques - intended also to achieve better homogeneity through an intimate mixture at the molecular level - seem not have resulted, in this case, in “substantial improvement of the synthesis, compared to conventional solid-state reaction” [ II]. Indeed, in the reported rapid formation of the 110 K phase through freeze-drying nitrates powder processing - achieved after calcination in air at 830°C for 10 h, pelletizing and subsequent sintering in air at 840°C during 50 h - the final material contains, besides 2-2-2-3, XRD-detectable amounts ofthe n=2 (2-2-l-2), T,x80 K) phase and SrCaCu,Os, Ca&uO, and CuO [ 121. Notwithstanding, freeze-drying has proved to be a very powerful and versatile technique for mild preparation of complex oxides [ 131, including that of the YBACUO-type ones [ 141. Thus, it may be thought that, dealing with metastable phases, there is the abovementioned influence of procedural variables which lies behind the pointed failure of solution techniques. In this context, we have previously discussed in
0147-577x/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
149
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MATERIALS LETTERS
detail the potential advantages of lower mixed carboxylates when looking for basic-metal cuprate precursors [ lo,15 1. In the light of previous results [ 10 1, formate solutions might be considered among the best possible cuprate precursors, but problems related to the stability of bismuth basic formates led us to approach the synthesis of the 110 K phase starting from acetic solutions. By carefully controlling the synthetic conditions, the present study shows the usefulness of the freeze-drying technique for shortening to a great extent the reaction times required for obtaining the 110 K phase as the only superconducting one in the (Bi,Pb)-Sr-Ca-Cu-0 system. The final polycrystalline material is obtained as a finely divided powder in which the micaceous platelets of 22-2-3 show a narrow particle size distribution.
UJ
STARTING
An aqueous solution of metal acetates with a mocomposition Bi : Pb : Sr : Ca: Cu = lar nominal 1.6 : 0.4 : 2.0 : 3.0 : 4.0 was prepared by mixing the following three solutions: Solution A: A suspension of 1.527 g ( 15 mmol) of CaCO, and 1.509 g ( 10 mmol) of SrCOs in = 50 ml of Hz0 was gently heated with stirring while glacial acetic acid was added dropwise until total solution. Solution B: 4.071 g (20 mmol) of CuZ(COOCH3)4*2 HZ0 were solved while stirring in x 50 ml of cold water. Solution C: A suspension of 1.876 g (4 mmol) of Bi203 and 0.457 g (2 mmol) of PbO in x40 ml of glacial acetic acid was gently heated while stirring for 15 min. After addition (while it was warm) of z 10 ml of HzO, a transparent solution resulted. Droplets of the resulting bluish acetic solution were flash frozen by projection on liquid nitrogen and transferred to a Leybold freeze-dryer operating at a pressure of z 10m2 atm. The dried blue powder was transferred to an alumina crucible, placed in a tubular furnace preheated at 500’ C, and tired in air at 865 “C for 10 h (step 1). After slow cooling of the furnace, the samples were pelletized and sintered in air at 870 ’ C for 10 h (step 2 ) . The samples were then reground, pelletized and sintered again at 870’ C for 10 h (step 3). The scheme in fig. 1 summarizes the synthetic procedure. 150
SOLUTION
1
Step 1 at 500°C
step 3 2. Experimental
ACETIC
Bi:Pb:Sr:Ca:Cu = 1.6:0.4:2:3:4
I
870 OC 10 h
Pelletizatio Is
100 %
865 OC IO h
2223
Fig. 1. Scheme of the synthetic procedure for the preparation of the 2-2-2-3 material.
X-ray powder diffraction patterns were obtained by means of a Rigaku CD 2455D 6 diffractometer using Cu Ku radiation. Microstructure of the samples was studied with a Hitachi S-2500 scanning electron microscope, which allowed EDX analysis of samples for semi-quantitative composition. For TEM observations, samples were ground under acetone and dispersed onto holey C grids in the usual manner. Electron diffraction micrographs were made using a Hitachi H-800 microscope operating at 200 kV equipped with a double-tilt stage. ac resistivity and susceptibility measurements were performed using a Lake Shore Cryotonics Inc. model 7000 ac equipment. The frequency and exciting field amplitude used for susceptibility measurements were 333.3 Hz and 1 Oe, respectively. Resistivity measurements were carried out with a standard fourprobe technique using a frequency of 333.3 Hz. The ac measuring currents were 0.1, 1 and 10 mA.
3. Results and discussion XRD data, EM observations
and ac resistivity
and
susceptibility measurements were used to conclude the formation of 2-2-2-3 as the unique superconducting phase following the procedure reported here. In fig. 2 are shown the X-ray diffraction patterns of samples as resulting from steps 1,2 and 3 (see fig. 1). We have labelled the most prominent diffraction peaks allowing unambiguous phase identification. It can be noted that peaks corresponding to 2-2-2-3 are clearly seen even at the first step of the thermal treatment (i.e. after only 10 h at 865’C). The relative quantity of 2-2-2-3 (with regard to 2-2-l-2) present after each step (see fig. 1) was estimated according to the equation 1 2
IH(002) ( IH(OO2)
+k(OO2)
November 1992
MATERIALS LETTERS
Volume 15, number 3
IH(L15) +
zH(115)
-zL(115)
xl00 >
where zH(O02) and zH(115) are the peak intensities of the (002) and (115) planes of 2-2-2-3, and ILcoo2)
10 h
ones to 2-2-l-2 [ 161. The volume fraction of 2-2-2-3 is considerably increased after the second step ( 10 h at 87O”C), and there is no further evidence of the presence of 2-21-2 (within the detection limit of the XRD technique) at the final stage of the thermal treatment ( 10 additional hours at 870’ C). Further prolonged thermal treatments (as long as 5 times the total reaction time) at 870°C do not result in any appreciable modification of the X-ray pattern. This result, stabilization of 2-2-2-3, which differs from some previous observations about the equilibrium between the 2-2-l-2 and 2-2-2-3 phases [ 17 1, i.e. formally 22-2-34 2-2-l-2 + tCa2Cu03 + 1CuO, must be related in some way with the calcium and copper excess deliberately introduced in the synthesis. As can be noted, this excess remains in the final material as CazCu03 and CuO, as expected from the processing temperatures [ 18 1. A full indexation of the X-ray pattern of the 2-2-2-3 phase is given in table 1. On the other hand, although CazPb04 appears as
and hlt5) the corresponding
Table 1 X-ray powder diffraction data for 2-2-2-3. Reflections marked with an arterisk have been used to determine the cell parameters by least-squaresrefinement (~~5.4056, b=5.4055, c=37.12)
20h
30h
130h
Fig. 2. Evolution of the X-ray diffraction pattern along the synthetic procedure schematized in fig. 1.
III0
20 exp. (deg)
hkl
32 18 18 26 41 34 67 92 60 100 44 44 19 35 54 37 23 23 22 28 28 24
4.81 16.60 17.86 23.43 23.99 24.39 26.25 28.83 31.95 33.20 33.86 35.52 41.26 44.54 47.59 48.09 52.67 54.03 55.11 56.57 58.48 59.82
00 2’ 01 1 01 3 11 1 00 lo* I1 3 I1 5’ 0012’ 11 9’ 20 0’ 00 14 1 Ill’ 20 10 2012’ 22 o* 2014’ 1 1 19 22 10 31 5 31 7 31 9 1023
20 theoret.
(deg) 4.16 16.57 17.91 16.57 23.97 24.31 26.22 28.86 31.92 33.14 33.81 35.49 41.32 44.52 47.58 48.07 52.61 53.96 55.13 56.57 58.44 59.80
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an intermediate at the first step of the treatment, it is absent in the final material, this being a result which, up to a point, contradicts some recent arguments about the role of calcium plumbate during the formation of 2-2-2-3 [ 19 1. We will return to both this point and the role played by CazCu03 in the synthesis below. Scanning electron microscopy observations of sample I (fig. 3a) show the plate-like morphology characteristic of the bismuth superconducting phases. The microcrystals (typically between 5 and 10 pm) are elongated platelets with no preferred orientation. Among these particles, there appear aggregates of very small grains ( = 1 pm). Semiquantitative analysis by EDX indicates that these aggregates are formed by a mixture of CazCuOJ and CuO, consistently with XRD results. Both the distribution of these impurities and the size and morphology of the superconductor grains within the pellet are significantly homogeneous, mainly when compared with results from conventional solid-state preparations [ 201. It seems then that it is the intimate mixture of the constituent metals achieved at the initial preparative stage which, by allowing that the nucleation processes proceed simultaneously throughout the bulk material, results in the high homogeneity degree of the samples obtained by this technique. ac resistivity measurements on sample I were carried out using 0.1, 1 and 10 mA currents. Within experimental error, there are no significant differences among the measurements performed at these three ac currents. The normalized resistance curve, r(T)=R(T)/R(250 K), obtained for 0.1 mA is shown in the inset of fig. 4. Only one resistive transition, with an onset temperature of 110 K, is observed. The 50% resistive mid-point is 105.5 K and the transition width (lo-90%) is 6.5 K. Zero resistance is obtained at 101 K. Between 250 and = 125 K, the normalized resistivity shows a linear decrease characteristic of metallic behaviour. Whereas this result fully agrees with our above statements on the uniqueness of the superconducting 2-2-2-3 phase in sample I, susceptibility measurements could dissipate any doubt about possible masking of 2-2-l-2 due to percolative effects while checking our own results by observation of shielding effects. Thus, shown in fig. 4 are both the in-phase, x’, and out-of-phase, x”, components of the ac susceptibility of sample I. Two 152
November 1992
steps are clearly distinguishable in x’, with onset temperatures T, = 107 K and T2 = 95 K. The fact that this last is significantly higher than the T, value of 2-2-l-2 (about 80 K) clearly excludes the presence of this phase in the sample. In the same sense, the presence of only one maximum in x”, without any additional effect, associated with the low temperature anomaly in x’ (at T=91 K, i.e. close to that of the inflexion point of x’ ), is clearly indicative of a single superconducting phase. On the other hand, the higher temperature at which hysteresis losses are observed is 100 K, in good agreement with the zero-resistance value. It can be noted also that, when the transition is completed, the diamagnetic signal reaches a constant value of -2.47~ 10e3 emu g-‘, which corresponds, within experimental error, to a perfect exclusion for the sample volumes. Although a more detailed study intended to analyze the magnetic behaviour by critical state models is in course, it can be stated that the high-temperature susceptibility regime corresponds to the onset of microscopic diamagnetic currents inside the grains, whereas the low-temperature one is related to the onset of macroscopic diamagnetic currents in the sample volume due to phase coherence between grains. The electron diffraction patterns along the [ 1001 and [ 1001 zone axes are shown in fig. 3b. These patterns, and others obtained by tilting around a*, b* and c* axes, can be indexed on the basis of an orthorhombicuntilcellofdimensionsa=5.31, b=5.36, ~~35.76. Strong satellite reflections, indicating the presence of an incommensurate superstructure along the a axis, were observed. Direct measurements along this axis give a value of 7.5 for the period of the incommensurate structure. Dealing with the synthesis in itself, although it would be hazardous to extract any mechanistic conclusion at the current stage of our work (and, moreover, the literature suggestions in this matter are questionable, see e.g. ref. [ 191, because of both the lack of chemical evidence about real intermediates and the diversity of physico-chemical variables, whose control is very difficult, involved in the reported syntheses), there are several aspects that deserve further commentary. Thus, we have experimentally verified that any procedural modification implying that lead is not present during the hydrolytic process undergone by bismuth renders very dif-
Volume
15,number 3
MATERIALS
LETTERS
November
1992
Fig. 3. (a) SEM micrographs of sample 1. (b) Electron diffraction patterns of 2-2-2-3 (sample I) along the [ 1001 and [OOl ] zone axis showing incommensurate superstructure along the u axis.
fkult the formation of 2-2-2-3 (i.e. this is a “nonAbelian” synthesis). On the other hand, the presence of an excess of calcium and copper over the stoichiometric requirements not only contributes to the stabilization of 2-2-2-3 after its formation (see above), but also promotes it. This observation, which is in line with some previous results [ 8,20,21], is based on the fact that, in the absence of the men-
tioned excess, thermal treatments identical to the one reported here lead to the majority formation of 2-21-2. Although further study is needed, some insight is provided by the study of the thermal evolution of the freeze-dryed precursor powders. Thus, under O2 atmosphere, the pyrolytic decomposition from the acetate precursor to the oxide mixture is a complex 153
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Volume 15, number 3
thesis of the final homogeneous microcrystals reported here.
G 0.03 ‘1 E 0.02 5 _ 0.01 x v, Om
4. concluding remarks
^M-0.01 ‘3 LE
-0.02 -
-.&-0.03 t 0
/ 2.5
/ 50 T (I$
100
I 12.5
Fig. 4. In-phase, x’, and out-of-phase, x”, components of the ac susceptibility of sample I as a function of temperature. In the inset is shown the temperature dependence of the resistivity.
multi-step process showing four partially superimposed exothermic effects (DTA) in the 40-850°C range. The weight loss associated with the first effect (40-240°C) fits nicely to that expected for the decomposition from copper acetate to copper oxide (calculated: 17.5%, experimental: x 18%) [ 22 1, and the weight loss associated with the last effect (580750°C) corresponds well with that expected for calcium carbonate decomposition (calculated: 9.3%, experimental: 8.5%). On the other hand, the total weight loss (48.6O/b)is significantly lower than what might be expected (x 58%) assuming an acetate amount in the precursor equal to that stoichiometrically required from the respective aliquot and formal valence of the cations. This last result should be due in part to the formation of oxocationic species during the bismuth hydrolysis but, moreover, the abovementioned “non-Abelian” character of this synthesis also suggests that acetate groups are bridging in some way cationic species in the precursor. The catalytic role of copper in the pyrolytic degradation of carboxylate groups f lo], the relative proximity and cationic distribution homogeneity (mainly when thinking @bout bismuth and lead) in the “green-body” acetate-precursor, and the increased reactivity of strontium carbonate in the presence of phases such as Bi2Cu04 [ 23 1, may be, all of them, factors contributing to the enhanced reactivity of the intermediate phases resulting in the rapid syn154
November 1992
Whereas freeze-drying acetate powder processing results in the rapid formation of pure 2-2-2-3 single phase in the bismuth system, mechanistic aspects (involving both solution and solid-state chemistry) are currently far to be well understood, which constitutes a limitation when looking for improving further on its synthesis and performances. Notwithstanding, it seems that the simultaneous hydrolysis of bismuth and lead reduces the amount of Ca2Pb04 formed in the course of the reaction, which may favour its subsequent unstabilization against superconducting phases. On the other hand, the presence of an excess of Ca&uOs (and CuO) in the synthesis of the 2-2-2-3 phase clearly plays a kinetic role in its stabilization. Very likely, being the intergrowth leading from 2-2-l-2 to 2-2-2-3 a process which must occur at the interfaces between 2-2-l-2 and Ca2CuO:l, the calcium and copper diffusion becomes greatly enhanced by increasing the nucleation interfaces. Our current work is intended to gain insight into the mechanistic aspects of the processes leading to Z2-2-3, being mainly focused on both the clarification of the role of CasCu03 and its influence on the superconducting properties.
Acknowledgement This work was supported by the Spanish Comision fnterministerial de Ciencia y Tecnologia (C.1.C.Y T. MAT 89.0427 and MAT 90-l 020) and MIDAS Project (Red ElCctrica de Espaiia - UNESA, 89/2017 and 8913799). MJS thanks the Spanish Ministerio de Education y Ciencia for a FPI fellowship. The SCME of the University of Valencia (Spain) is acknowledged for SEM and TEM facilities.
Volume 15, number 3
MATERIALS LETTERS
References [ I] S.A. Sunshine and T.A. Vanderah, in: Chemistry of superconductor materials, ed. T.A. Vanderah (Noyes Publications, New Jersey, 1992), ch. 6. [ 21 A.W. Sleight, Phys. Today, June ( 199 1) 24. [ 31 K. Schulze, P. Majewski, B. Hettich and G. Petzow, Z. Metall. 8 1 ( 1990) 836. [ 41 Y. Masuda, R. Ogawa, Y, Kawate, T. Tateishi and N. Hara, J. Mater. Res. 7 (1992) 292, and references therein. ] Q. Feng, H. Zhang, S. Feng, X. Zhu, K. Wu, Z. Liu and L. Xue, Solid State Commun. 78 (1991) 609. ] H. Sasakura, S. Minamigawa, K. Nakahigashi, M. Kogachi. S. Nokanishi, N. Fukuoka, M. Yoshikawa, S. Noguchi, K. Okuda and A. Yanese, Japan. Appl. Phys. 28 L ( 1989) 1163. ] A. Maqsood, M. Maqsood, M.S. Awan and N. Amin, J. Mater. Sci. 26 ( 199 1) 4893. [S] H.K. Liu, S.X. Dou, S.J. Guo, K.E. Easterling and X.G. Li, J. Mater. Res. 6 ( 199 1) 2287. [ 91 C.N.R. Rao and J. Gopalakrishnan, New directions in solid state chemistry (Cambridge Univ. Press, Cambridge, 1986), ch. 3. [IO] M.J. Sanchis, F. Sapiiia, R. Ibaxiez, A. Belt&n and D. Belt&, Mater. Letters 12 (1992) 409, and references therein. [ 111 P. Barboux, in: Chemistry of superconductor materials, ed. T.A. Vanderah (Noyes Publications, New Jersey, 1992) ch. 7. [ 12 JK. Song, H. Liu, S. Dou and CC. Sorell, J. Am. Ceram. Sot. 73 (1990) 1771.
November 1992
[ 131 N. Ichinose, Y. Ozaki and S. Kashu, Superfine particles technology (Springer, Berlin, 1992) ch. 5. [ 141 D. Belt&t-Porter, E. Martinez-Tamayo, R. Ibatiez, A. Beltran-Porter, J.V. Folaado, E. Escriva, V. Mufioz, A. Segura and J. Martinez-Pastor, Solid State Ionics 32/33 (1989) 1160. 1M.J. Sanchis, P. Gomez-Romero, J.V. Folgado, F. Sapiiia, R. Ibanez, A. Beitran, J. Garcia and D. Beltdn, Inorg. Chem., in press, and references therein. tl6 K. Ohasi, S. Yada, S. Naka, H. Itoh, H. Kitaguchi, J. Takada, Y. Tomii, Y. Kaeda and M. Takano, J. Japan. Sot. Powder Powder Metall. 37 ( 1990) 747. 117 L. Ben-dor, H. Diab and I. Felner. J. Solid State Chem. 88 (1990) 183. 118 R.S. Roth, C.J. Rawn, J.J. Ritter and B.P. Burton. 1. Am. Ceram. Sot. 72 ( 1989) 1545. t19 AK. Sarkar, Y.J. Tang, X.W. Cao, J.C. Ho and G. Kozlowski, Mater. Res. Bull. 27 ( 1992) 1. [20 Q. Wu, Z. Fu, A. Zhang, J. Huang, D. Tang, P. Yao, S. Chu, S. Yi, X. Rong, A. Zhang and X. Cheng, J. Appl. Phys. 71 (1992) 2772. [21 D. Shi, M. Tang, K. Vandervoort and H. Claus, Phys. Rev. 39 (1989) 9091. [22] R. Mehotra and R. Bohra. Metal carboxylates (Academic Press, New York, 1983). ch. 3. [23] D. Beltran, M.T. Caldes, R. Ibaiiez, E. Martinez, E. Escriva and A. Beltran. J. Less-Common Met, 150 ( 1989) 247.
155
Materials Letters 15 ( 1992 ) 149- 155 North-Holland
A new improved synthesis of the 110 K bismuth superconducting freeze-drying of acetic solutions V. Primo, F. Sapiiia, M.J. Sanchis, R. Ibkiez, A. Belt&
phase: and D. Beltran
UEKM, Dg~art~rngnt de ~~~rn~eaInorg&nica, Fa~u~tat de Quimira, ~‘n~~er~it~tde Valhznria, Dr. ‘~~liner 50, 46100 3~~~soi, Valeneia, Spain
Received 2 1 July 1992
Metastability has greatly hindered the separated synthesis of the h&temperature superconducting phases represented as (Bi, _-n Pb,)2SrtCan_IC~n04+2n (n=2, 2-2-l-2, T,xSO K, and n=3,2-2-2-3, T,r;: 110 K). By careful control of the synthetic variables, it becomes possible to obtain the 110 K phase as the only superconducting one through processing of freeze-dried acetic soktions. This technique leads to homogeneously sized (5-10 pm) micaceous platelets of the superconducting material.
1. Introduction
Entropy stabilization of highly deffective hightemperature superconducting phases of the bismuth system, represented by the nominal ideal formula Bi2Sr2Can_1C~n04+2n (n=Z, 3), is widely assumed [ I ,2]. Difficulties in obtaining a given single phase without impurities increase as its region of true thermodynamic stability decreases and, consequently, procedural variables acquire a special relevance. This is particularly the case of the n = 3 (2-2-2-3, T, z I 10 K) phase [ 31 where, moreover, “the weak bonding along the c axis may also contribute to its stronger propensity to form intergrowth products, with sections of varying n values in a single grain” [ 11. The result is that, although its synthesis becomes facilitated by addition of lead [ 41, the different treatments described as appropriate to increase the volume fraction of 2-2-2-3 in the (Bi,Pb)Sr-Ca-Cu-0 system are usually tediously muIti-step and prolonged (until twelve days) [ 5,6] and, very frequently, do not lead to this phase as the only superconducting one in the final material [ 5,7 1, which, in turn, complicates the measurements of superconducting properties other than T, [ 8 1. To overcome some of the well-known inherent Iimitations of the traditional “heat and beat” ce-
ramic procedure [ 91, it seems evident that direct chemical-precursor-based methods (which yield satisfactory results in the case of related simpler oxides [ lo] ) are no longer applicabIe to multimetallic systems such as that we are dealing with. On the other hand, the related solution techniques - intended also to achieve better homogeneity through an intimate mixture at the molecular level - seem not have resulted, in this case, in “substantial improvement of the synthesis, compared to conventional solid-state reaction” [ II]. Indeed, in the reported rapid formation of the 110 K phase through freeze-drying nitrates powder processing - achieved after calcination in air at 830°C for 10 h, pelletizing and subsequent sintering in air at 840°C during 50 h - the final material contains, besides 2-2-2-3, XRD-detectable amounts ofthe n=2 (2-2-l-2), T,x80 K) phase and SrCaCu,Os, Ca&uO, and CuO [ 121. Notwithstanding, freeze-drying has proved to be a very powerful and versatile technique for mild preparation of complex oxides [ 131, including that of the YBACUO-type ones [ 141. Thus, it may be thought that, dealing with metastable phases, there is the abovementioned influence of procedural variables which lies behind the pointed failure of solution techniques. In this context, we have previously discussed in
0147-577x/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
149
Volume 15, number 3
November 1992
MATERIALS LETTERS
detail the potential advantages of lower mixed carboxylates when looking for basic-metal cuprate precursors [ lo,15 1. In the light of previous results [ 10 1, formate solutions might be considered among the best possible cuprate precursors, but problems related to the stability of bismuth basic formates led us to approach the synthesis of the 110 K phase starting from acetic solutions. By carefully controlling the synthetic conditions, the present study shows the usefulness of the freeze-drying technique for shortening to a great extent the reaction times required for obtaining the 110 K phase as the only superconducting one in the (Bi,Pb)-Sr-Ca-Cu-0 system. The final polycrystalline material is obtained as a finely divided powder in which the micaceous platelets of 22-2-3 show a narrow particle size distribution.
UJ
STARTING
An aqueous solution of metal acetates with a mocomposition Bi : Pb : Sr : Ca: Cu = lar nominal 1.6 : 0.4 : 2.0 : 3.0 : 4.0 was prepared by mixing the following three solutions: Solution A: A suspension of 1.527 g ( 15 mmol) of CaCO, and 1.509 g ( 10 mmol) of SrCOs in = 50 ml of Hz0 was gently heated with stirring while glacial acetic acid was added dropwise until total solution. Solution B: 4.071 g (20 mmol) of CuZ(COOCH3)4*2 HZ0 were solved while stirring in x 50 ml of cold water. Solution C: A suspension of 1.876 g (4 mmol) of Bi203 and 0.457 g (2 mmol) of PbO in x40 ml of glacial acetic acid was gently heated while stirring for 15 min. After addition (while it was warm) of z 10 ml of HzO, a transparent solution resulted. Droplets of the resulting bluish acetic solution were flash frozen by projection on liquid nitrogen and transferred to a Leybold freeze-dryer operating at a pressure of z 10m2 atm. The dried blue powder was transferred to an alumina crucible, placed in a tubular furnace preheated at 500’ C, and tired in air at 865 “C for 10 h (step 1). After slow cooling of the furnace, the samples were pelletized and sintered in air at 870 ’ C for 10 h (step 2 ) . The samples were then reground, pelletized and sintered again at 870’ C for 10 h (step 3). The scheme in fig. 1 summarizes the synthetic procedure. 150
SOLUTION
1
Step 1 at 500°C
step 3 2. Experimental
ACETIC
Bi:Pb:Sr:Ca:Cu = 1.6:0.4:2:3:4
I
870 OC 10 h
Pelletizatio Is
100 %
865 OC IO h
2223
Fig. 1. Scheme of the synthetic procedure for the preparation of the 2-2-2-3 material.
X-ray powder diffraction patterns were obtained by means of a Rigaku CD 2455D 6 diffractometer using Cu Ku radiation. Microstructure of the samples was studied with a Hitachi S-2500 scanning electron microscope, which allowed EDX analysis of samples for semi-quantitative composition. For TEM observations, samples were ground under acetone and dispersed onto holey C grids in the usual manner. Electron diffraction micrographs were made using a Hitachi H-800 microscope operating at 200 kV equipped with a double-tilt stage. ac resistivity and susceptibility measurements were performed using a Lake Shore Cryotonics Inc. model 7000 ac equipment. The frequency and exciting field amplitude used for susceptibility measurements were 333.3 Hz and 1 Oe, respectively. Resistivity measurements were carried out with a standard fourprobe technique using a frequency of 333.3 Hz. The ac measuring currents were 0.1, 1 and 10 mA.
3. Results and discussion XRD data, EM observations
and ac resistivity
and
susceptibility measurements were used to conclude the formation of 2-2-2-3 as the unique superconducting phase following the procedure reported here. In fig. 2 are shown the X-ray diffraction patterns of samples as resulting from steps 1,2 and 3 (see fig. 1). We have labelled the most prominent diffraction peaks allowing unambiguous phase identification. It can be noted that peaks corresponding to 2-2-2-3 are clearly seen even at the first step of the thermal treatment (i.e. after only 10 h at 865’C). The relative quantity of 2-2-2-3 (with regard to 2-2-l-2) present after each step (see fig. 1) was estimated according to the equation 1 2
IH(002) ( IH(OO2)
+k(OO2)
November 1992
MATERIALS LETTERS
Volume 15, number 3
IH(L15) +
zH(115)
-zL(115)
xl00 >
where zH(O02) and zH(115) are the peak intensities of the (002) and (115) planes of 2-2-2-3, and ILcoo2)
10 h
ones to 2-2-l-2 [ 161. The volume fraction of 2-2-2-3 is considerably increased after the second step ( 10 h at 87O”C), and there is no further evidence of the presence of 2-21-2 (within the detection limit of the XRD technique) at the final stage of the thermal treatment ( 10 additional hours at 870’ C). Further prolonged thermal treatments (as long as 5 times the total reaction time) at 870°C do not result in any appreciable modification of the X-ray pattern. This result, stabilization of 2-2-2-3, which differs from some previous observations about the equilibrium between the 2-2-l-2 and 2-2-2-3 phases [ 17 1, i.e. formally 22-2-34 2-2-l-2 + tCa2Cu03 + 1CuO, must be related in some way with the calcium and copper excess deliberately introduced in the synthesis. As can be noted, this excess remains in the final material as CazCu03 and CuO, as expected from the processing temperatures [ 18 1. A full indexation of the X-ray pattern of the 2-2-2-3 phase is given in table 1. On the other hand, although CazPb04 appears as
and hlt5) the corresponding
Table 1 X-ray powder diffraction data for 2-2-2-3. Reflections marked with an arterisk have been used to determine the cell parameters by least-squaresrefinement (~~5.4056, b=5.4055, c=37.12)
20h
30h
130h
Fig. 2. Evolution of the X-ray diffraction pattern along the synthetic procedure schematized in fig. 1.
III0
20 exp. (deg)
hkl
32 18 18 26 41 34 67 92 60 100 44 44 19 35 54 37 23 23 22 28 28 24
4.81 16.60 17.86 23.43 23.99 24.39 26.25 28.83 31.95 33.20 33.86 35.52 41.26 44.54 47.59 48.09 52.67 54.03 55.11 56.57 58.48 59.82
00 2’ 01 1 01 3 11 1 00 lo* I1 3 I1 5’ 0012’ 11 9’ 20 0’ 00 14 1 Ill’ 20 10 2012’ 22 o* 2014’ 1 1 19 22 10 31 5 31 7 31 9 1023
20 theoret.
(deg) 4.16 16.57 17.91 16.57 23.97 24.31 26.22 28.86 31.92 33.14 33.81 35.49 41.32 44.52 47.58 48.07 52.61 53.96 55.13 56.57 58.44 59.80
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an intermediate at the first step of the treatment, it is absent in the final material, this being a result which, up to a point, contradicts some recent arguments about the role of calcium plumbate during the formation of 2-2-2-3 [ 19 1. We will return to both this point and the role played by CazCu03 in the synthesis below. Scanning electron microscopy observations of sample I (fig. 3a) show the plate-like morphology characteristic of the bismuth superconducting phases. The microcrystals (typically between 5 and 10 pm) are elongated platelets with no preferred orientation. Among these particles, there appear aggregates of very small grains ( = 1 pm). Semiquantitative analysis by EDX indicates that these aggregates are formed by a mixture of CazCuOJ and CuO, consistently with XRD results. Both the distribution of these impurities and the size and morphology of the superconductor grains within the pellet are significantly homogeneous, mainly when compared with results from conventional solid-state preparations [ 201. It seems then that it is the intimate mixture of the constituent metals achieved at the initial preparative stage which, by allowing that the nucleation processes proceed simultaneously throughout the bulk material, results in the high homogeneity degree of the samples obtained by this technique. ac resistivity measurements on sample I were carried out using 0.1, 1 and 10 mA currents. Within experimental error, there are no significant differences among the measurements performed at these three ac currents. The normalized resistance curve, r(T)=R(T)/R(250 K), obtained for 0.1 mA is shown in the inset of fig. 4. Only one resistive transition, with an onset temperature of 110 K, is observed. The 50% resistive mid-point is 105.5 K and the transition width (lo-90%) is 6.5 K. Zero resistance is obtained at 101 K. Between 250 and = 125 K, the normalized resistivity shows a linear decrease characteristic of metallic behaviour. Whereas this result fully agrees with our above statements on the uniqueness of the superconducting 2-2-2-3 phase in sample I, susceptibility measurements could dissipate any doubt about possible masking of 2-2-l-2 due to percolative effects while checking our own results by observation of shielding effects. Thus, shown in fig. 4 are both the in-phase, x’, and out-of-phase, x”, components of the ac susceptibility of sample I. Two 152
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steps are clearly distinguishable in x’, with onset temperatures T, = 107 K and T2 = 95 K. The fact that this last is significantly higher than the T, value of 2-2-l-2 (about 80 K) clearly excludes the presence of this phase in the sample. In the same sense, the presence of only one maximum in x”, without any additional effect, associated with the low temperature anomaly in x’ (at T=91 K, i.e. close to that of the inflexion point of x’ ), is clearly indicative of a single superconducting phase. On the other hand, the higher temperature at which hysteresis losses are observed is 100 K, in good agreement with the zero-resistance value. It can be noted also that, when the transition is completed, the diamagnetic signal reaches a constant value of -2.47~ 10e3 emu g-‘, which corresponds, within experimental error, to a perfect exclusion for the sample volumes. Although a more detailed study intended to analyze the magnetic behaviour by critical state models is in course, it can be stated that the high-temperature susceptibility regime corresponds to the onset of microscopic diamagnetic currents inside the grains, whereas the low-temperature one is related to the onset of macroscopic diamagnetic currents in the sample volume due to phase coherence between grains. The electron diffraction patterns along the [ 1001 and [ 1001 zone axes are shown in fig. 3b. These patterns, and others obtained by tilting around a*, b* and c* axes, can be indexed on the basis of an orthorhombicuntilcellofdimensionsa=5.31, b=5.36, ~~35.76. Strong satellite reflections, indicating the presence of an incommensurate superstructure along the a axis, were observed. Direct measurements along this axis give a value of 7.5 for the period of the incommensurate structure. Dealing with the synthesis in itself, although it would be hazardous to extract any mechanistic conclusion at the current stage of our work (and, moreover, the literature suggestions in this matter are questionable, see e.g. ref. [ 191, because of both the lack of chemical evidence about real intermediates and the diversity of physico-chemical variables, whose control is very difficult, involved in the reported syntheses), there are several aspects that deserve further commentary. Thus, we have experimentally verified that any procedural modification implying that lead is not present during the hydrolytic process undergone by bismuth renders very dif-
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Fig. 3. (a) SEM micrographs of sample 1. (b) Electron diffraction patterns of 2-2-2-3 (sample I) along the [ 1001 and [OOl ] zone axis showing incommensurate superstructure along the u axis.
fkult the formation of 2-2-2-3 (i.e. this is a “nonAbelian” synthesis). On the other hand, the presence of an excess of calcium and copper over the stoichiometric requirements not only contributes to the stabilization of 2-2-2-3 after its formation (see above), but also promotes it. This observation, which is in line with some previous results [ 8,20,21], is based on the fact that, in the absence of the men-
tioned excess, thermal treatments identical to the one reported here lead to the majority formation of 2-21-2. Although further study is needed, some insight is provided by the study of the thermal evolution of the freeze-dryed precursor powders. Thus, under O2 atmosphere, the pyrolytic decomposition from the acetate precursor to the oxide mixture is a complex 153
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thesis of the final homogeneous microcrystals reported here.
G 0.03 ‘1 E 0.02 5 _ 0.01 x v, Om
4. concluding remarks
^M-0.01 ‘3 LE
-0.02 -
-.&-0.03 t 0
/ 2.5
/ 50 T (I$
100
I 12.5
Fig. 4. In-phase, x’, and out-of-phase, x”, components of the ac susceptibility of sample I as a function of temperature. In the inset is shown the temperature dependence of the resistivity.
multi-step process showing four partially superimposed exothermic effects (DTA) in the 40-850°C range. The weight loss associated with the first effect (40-240°C) fits nicely to that expected for the decomposition from copper acetate to copper oxide (calculated: 17.5%, experimental: x 18%) [ 22 1, and the weight loss associated with the last effect (580750°C) corresponds well with that expected for calcium carbonate decomposition (calculated: 9.3%, experimental: 8.5%). On the other hand, the total weight loss (48.6O/b)is significantly lower than what might be expected (x 58%) assuming an acetate amount in the precursor equal to that stoichiometrically required from the respective aliquot and formal valence of the cations. This last result should be due in part to the formation of oxocationic species during the bismuth hydrolysis but, moreover, the abovementioned “non-Abelian” character of this synthesis also suggests that acetate groups are bridging in some way cationic species in the precursor. The catalytic role of copper in the pyrolytic degradation of carboxylate groups f lo], the relative proximity and cationic distribution homogeneity (mainly when thinking @bout bismuth and lead) in the “green-body” acetate-precursor, and the increased reactivity of strontium carbonate in the presence of phases such as Bi2Cu04 [ 23 1, may be, all of them, factors contributing to the enhanced reactivity of the intermediate phases resulting in the rapid syn154
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Whereas freeze-drying acetate powder processing results in the rapid formation of pure 2-2-2-3 single phase in the bismuth system, mechanistic aspects (involving both solution and solid-state chemistry) are currently far to be well understood, which constitutes a limitation when looking for improving further on its synthesis and performances. Notwithstanding, it seems that the simultaneous hydrolysis of bismuth and lead reduces the amount of Ca2Pb04 formed in the course of the reaction, which may favour its subsequent unstabilization against superconducting phases. On the other hand, the presence of an excess of Ca&uOs (and CuO) in the synthesis of the 2-2-2-3 phase clearly plays a kinetic role in its stabilization. Very likely, being the intergrowth leading from 2-2-l-2 to 2-2-2-3 a process which must occur at the interfaces between 2-2-l-2 and Ca2CuO:l, the calcium and copper diffusion becomes greatly enhanced by increasing the nucleation interfaces. Our current work is intended to gain insight into the mechanistic aspects of the processes leading to Z2-2-3, being mainly focused on both the clarification of the role of CasCu03 and its influence on the superconducting properties.
Acknowledgement This work was supported by the Spanish Comision fnterministerial de Ciencia y Tecnologia (C.1.C.Y T. MAT 89.0427 and MAT 90-l 020) and MIDAS Project (Red ElCctrica de Espaiia - UNESA, 89/2017 and 8913799). MJS thanks the Spanish Ministerio de Education y Ciencia for a FPI fellowship. The SCME of the University of Valencia (Spain) is acknowledged for SEM and TEM facilities.
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References [ I] S.A. Sunshine and T.A. Vanderah, in: Chemistry of superconductor materials, ed. T.A. Vanderah (Noyes Publications, New Jersey, 1992), ch. 6. [ 21 A.W. Sleight, Phys. Today, June ( 199 1) 24. [ 31 K. Schulze, P. Majewski, B. Hettich and G. Petzow, Z. Metall. 8 1 ( 1990) 836. [ 41 Y. Masuda, R. Ogawa, Y, Kawate, T. Tateishi and N. Hara, J. Mater. Res. 7 (1992) 292, and references therein. ] Q. Feng, H. Zhang, S. Feng, X. Zhu, K. Wu, Z. Liu and L. Xue, Solid State Commun. 78 (1991) 609. ] H. Sasakura, S. Minamigawa, K. Nakahigashi, M. Kogachi. S. Nokanishi, N. Fukuoka, M. Yoshikawa, S. Noguchi, K. Okuda and A. Yanese, Japan. Appl. Phys. 28 L ( 1989) 1163. ] A. Maqsood, M. Maqsood, M.S. Awan and N. Amin, J. Mater. Sci. 26 ( 199 1) 4893. [S] H.K. Liu, S.X. Dou, S.J. Guo, K.E. Easterling and X.G. Li, J. Mater. Res. 6 ( 199 1) 2287. [ 91 C.N.R. Rao and J. Gopalakrishnan, New directions in solid state chemistry (Cambridge Univ. Press, Cambridge, 1986), ch. 3. [IO] M.J. Sanchis, F. Sapiiia, R. Ibaxiez, A. Belt&n and D. Belt&, Mater. Letters 12 (1992) 409, and references therein. [ 111 P. Barboux, in: Chemistry of superconductor materials, ed. T.A. Vanderah (Noyes Publications, New Jersey, 1992) ch. 7. [ 12 JK. Song, H. Liu, S. Dou and CC. Sorell, J. Am. Ceram. Sot. 73 (1990) 1771.
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[ 131 N. Ichinose, Y. Ozaki and S. Kashu, Superfine particles technology (Springer, Berlin, 1992) ch. 5. [ 141 D. Belt&t-Porter, E. Martinez-Tamayo, R. Ibatiez, A. Beltran-Porter, J.V. Folaado, E. Escriva, V. Mufioz, A. Segura and J. Martinez-Pastor, Solid State Ionics 32/33 (1989) 1160. 1M.J. Sanchis, P. Gomez-Romero, J.V. Folgado, F. Sapiiia, R. Ibanez, A. Beitran, J. Garcia and D. Beltdn, Inorg. Chem., in press, and references therein. tl6 K. Ohasi, S. Yada, S. Naka, H. Itoh, H. Kitaguchi, J. Takada, Y. Tomii, Y. Kaeda and M. Takano, J. Japan. Sot. Powder Powder Metall. 37 ( 1990) 747. 117 L. Ben-dor, H. Diab and I. Felner. J. Solid State Chem. 88 (1990) 183. 118 R.S. Roth, C.J. Rawn, J.J. Ritter and B.P. Burton. 1. Am. Ceram. Sot. 72 ( 1989) 1545. t19 AK. Sarkar, Y.J. Tang, X.W. Cao, J.C. Ho and G. Kozlowski, Mater. Res. Bull. 27 ( 1992) 1. [20 Q. Wu, Z. Fu, A. Zhang, J. Huang, D. Tang, P. Yao, S. Chu, S. Yi, X. Rong, A. Zhang and X. Cheng, J. Appl. Phys. 71 (1992) 2772. [21 D. Shi, M. Tang, K. Vandervoort and H. Claus, Phys. Rev. 39 (1989) 9091. [22] R. Mehotra and R. Bohra. Metal carboxylates (Academic Press, New York, 1983). ch. 3. [23] D. Beltran, M.T. Caldes, R. Ibaiiez, E. Martinez, E. Escriva and A. Beltran. J. Less-Common Met, 150 ( 1989) 247.
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