Journal of Magnetism and Magnetic Materials 262 (2003) 514–519 Properties of the Bi-surplus superconducting Bi2.1ÀxPbxSr2Ca2Cu3Oy compounds N.H Sinh* Cryogenics Laboratory, Faculty of Physics, College of Natural Science, Hanoi National University, 334 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet Nam Abstract Properties of the Bi-surplus superconducting Bi2.1ÀxPbxSr2Ca2Cu3Oy (x ¼ 0:0020:60) compounds have been investigated It is found that Pb plays a very important role in the formation of the superconducting phases with high purity and especially in promoting and enhancing the stability of the 2223 phase in (Bi,Pb)–Sr–Ca–Cu–O compounds By increasing the duration of the heat treatment, the single-phase region will be widened, while the transition temperatures and values of zero-resistance remain nearly unchanged With suitable heat treatment, the 120 K high-TC phase can be synthesized by the solid-state reaction method The superconducting fraction reaches a maximum in the compounds with x ¼ 0:3020:60: Special attention is paid that the superconducting state is destroyed by annealing in vacuum for the compound with x ¼ 0:40: r 2003 Elsevier Science B.V All rights reserved PACS: 74.72.Hs Keywords: Superconductors; High-T C; Heat treatment; Role of Pb; Superconducting fraction Introduction Many studies have been made on the high temperature superconductivity in the 2223 phase of the Bi–Sr–Ca–Cu–O system, but most of the studies have been carried out using samples including a small portion of this phase, because the 2223 phase was difficult to prepare as a single phase Some authors [1–3] pointed out that the addition of Pb to the superconducting (Bi,Pb)–Sr– Ca–Cu–O compounds leads to higher TC values, because Pb might increase the ratio of Cu3+ to *Corresponding author Tel.: +84-4-8585281; fax: +84-48584438 E-mail address: nhsinh@netnam.vn (N.H Sinh) Cu2+ On the other hand, Pb improves the connectivity between regions of the 2223 structure [4,5], and promotes the formation of a 2223 nearly single-phase sample The Bi-based superconductors offer potential advantages in comparison to the Y -based superconductors These include a better resistance to reactions with water or carbon dioxide and being stable in terms of oxygen stoichiometry There are three superconducting phases, denoted as 2201, 2212 and 2223 in the Bi-based high temperature superconductors The structure similarity of these three phases suggests the phase intergrowth of 2212 from 2201 and 2223 from 2212 through an intercalation-diffusion of Ca and Cu atoms over a short distance The addition of Pb enables the 0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved doi:10.1016/S0304-8853(03)00088-X N.H Sinh / Journal of Magnetism and Magnetic Materials 262 (2003) 514–519 synthesis of an almost pure 2223 phase, especially, with a surplus of Bi as in Bi2.1ÀxPbxSr2Ca2Cu3Oy The present work, studies the properties and the formation of the Bi-2223 phase in nominal compositions of Bi2.1ÀxPbxSr2Ca2Cu3Oy 2.1 Relation between formation of the superconducting phases and heat treatment process Samples with nominal compositions of Bi2.1ÀxPbxSr2Ca2Cu3Oy (x ¼ 0:0020:60) were prepared in air by the solid-state reaction method The starting materials were powders of Bi2O3, PbO, SrCO3 and CuO (all of 3N purity) Our aim was to set-up a reproducible procedure for preparing a 2223 phase as pure as possible The powders were manually mixed and ground for 60 and then pressed into pellets of about 20 mm in diameter and mm in thickness Sintering took place in air at 840–860 C during times varying from 24 to 144 h, followed by annealing at 500–520 C for the same time variation Finally, the samples were furnace cooled to room temperature A sample of Bi1.7Pb0.4Sr2Ca2Cu3Oy was quenched after the heat treatment in vacuum and in flowing oxygen The sintering temperature versus time in range of 750–910 C establishes the diagram on the formation of the superconducting phase as presented in Fig This diagram shows the relation between the formation of the superconducting phases and the heat treatment in the Bi-surplus Bi2.1ÀxPbxSr2Ca2Cu3Oy compounds (x ¼0:1020:60) As suggested in Ref [3] the formation of the 2223 phase is associated with the formation of Bi/Pb rich mobile liquid droplets, which migrate over the growing platelets This phenomenon may correspond to a fast chemical reaction with an interconnection of 2212 platelets in the presence of the liquid phase [4] The presence of this liquid phase would enhance significantly the diffusion process of Ca2+ and Cu2+ ions by dissolution of the impurity phases Still the role of Pb, present in Ca2PbO4 and PbO, is important in enhancing the stability and 1000 Liquid State 900 Unstable Superconducting + Liquid State 2201 + 2212 2212 + 2223 2223 + 2212 (little) Sintering temperature (T °C) Results and discussion 515 Unstable mixed phases 800 2201 2201 + 2212 (little) 700 600 500 No Superconductivity 400 Insulating 300 24 48 72 96 120 144 168 Sintering time (t hours) Fig The formation of the superconducting phases in Bi2.1ÀxPbx Sr2Ca2Cu3Oy compounds promoting the formation of the 2223 phase in the (Bi, Pb)–Sr–Ca–Cu–O system With respect to the solubility, we can suggest that the solubility region of Pb into compositions to form the 2223 single phase is shifted to Bisurplus concentrations with a Bi content smaller than or equal to 2.1 with sintering-temperatures in the range of 840–860 C There is an optimum in the (Bi+Pb):Cu concentration In our case the ratio of 2.1:3 dropped into the 2223 single-phase range 2.2 X-ray powder diffraction Fig shows XPD patterns, which are typical for the prepared samples From the X-ray diffraction patterns, we can see that the samples with x ¼ 0:00 and 0:10 containing two different superconducting phases, which belong to the 2212 and 2223 phases The results reveal that the amount of the 2223 phase increases and that the amount 516 N.H Sinh / Journal of Magnetism and Magnetic Materials 262 (2003) 514–519 Table Lattice parameters (x ¼ 0:0020:60) system of the Bi2.1ÀxPbxSr2Ca2Cu3Oy X (2 2) phase Pb ( a;b (A) ( c (A) ( a; b (A) ( c (A) 0.00 0.10 0.20 0.30 0.40 0.50 0.60 5.410 5.408 5.405 — — — — 30.815 30.825 30.840 — — — — 5.410 5.408 5.405 5.398 5.392 5.384 5.380 37.110 37.150 37.225 37.243 37.258 37.281 37.290 (2 2 3) phase a 37.30 ° c ° 5.42 37.25 37.20 5.40 37.15 5.38 37.10 0.00 0.40 0.80 Concentration Fig Lattice parameters of Bi2.1ÀxPbxSr2Ca2Cu3Oy compounds as a function of Pb-concentration (Pb=0.00–0.60) increasing the Pb concentration No structure transformation due to the high Pb-concentration has been observed in this system Fig XPD patterns for samples of composition: (a) Bi2Sr2Ca2Cu2Oy, and (b) Bi1.7Sr0.4Ca2Cu2Oy 2.3 Resistivity and AC susceptibility of the 2212 phase decreases with increasing calcination temperatures and Pb concentrations The 2223 phase occupies nearly 100% of the sample for x ¼ 0:3020:60: The structure of the 2223 phase is tetragonal as determined and presented in Table The influence of the substituted Pb concentration on the lattice parameters of the single 2223 phase is illustrated in Fig The lattice parameters slightly change by Results of resistance and AC susceptibility measurements are shown in Fig 4a and b It is clearly observed that there are two phases in the samples with x ¼ 0:00 and 0.10 For the samples with x ¼ 0:3020:50 (x ¼ 0:20 and 0.60 was not shown in the figure), the transitions from metallic to superconducting state with decreasing temperature very sharply occur at TC : From these resistance and AC-susceptibility curves, the transition temperature TC has been determined It is N.H Sinh / Journal of Magnetism and Magnetic Materials 262 (2003) 514–519 517 125 1.0 (a) 120 (a) 115 0.6 T (K) R (T)/R (300 K) 0.8 (c) (b) 110 a:x = 0.5 105 b:x = 0.5 c:x = 0.5 0.2 0.1 0.3 0.5 0.7 Concentration 0.0 50 100 150 200 300 250 Fig Dependence of TC Bi2.1ÀxPbxSr2Ca2Cu3Oy on the Pb-concentration in (b) χac (arb.u) 0:x = ( 1:x = 0.1 ( 3:x = 0.3 ( 4:x = 0.4 ( 5:x = 0.5 ( 60 100 140 ) ) ) ) ) -1 180 T (K) Fig (a) Temperature dependence of the linear resistivity, showing a zero-resistance temperature of 100.5, 99.5 and 98 K for the samples with x ¼ 0:30; 0.40 and 0.50, respectively (b) Temperature dependence of the ac susceptibility of Bi2.1ÀxPbxSr2Ca2Cu3Oy compounds confirmed that the substitution of Pb in the range of x ¼ 0:2020:40 for Bi in Bi-surplus compounds of Bi2.1ÀxPbxSr2Ca2Cu3Oy has increased TC : The highest value of TC about 120 K has been reached at x ¼ 0:40: The dependence of TC on the Pbsubstitution concentration (x) is shown in Fig This figure shows a parabolic behaviour of TC as a function of the doping hole concentration Similar behaviours in some other systems of ours have been found and reported in Ref [6] In the Bi-based superconductors, the holes are confined to the O2p orbitals in the CuO2 planes Therefore, the variation of holes by substitution in Bi-based superconducting compounds will lead to variation of TC as a function of holes concentration in CuO2 planes With increasing hole concentration TC reaches to a maximum value and starts to decrease at the higher doping level This character is accompanied by the changing valence of Cu in Bi2.1ÀxPbxSr2Ca2Cu3Oy compounds, also The zero-resistance temperature monotonically decreases from 100.5 K at x ¼ 0:30 to 97 K at x ¼ 0:60: The fraction of 2223 phase increases by increasing the Pb-concentration up to x ¼ 0:60 and reaches nearly the same values in samples with x ¼ 0:3020:60: 2.4 Variation of properties of Bi1.70Pb0.40Sr2Ca2Cu3Oy compound The sample of x ¼ 0:40 has a highest value of TC (120 K) when it was heat-treated in air We have investigated the properties of this sample by annealing it in vacuum and in flowing oxygen at 720 C, both for 72 h Results have been obtained by measurements of XPD, resistivity, susceptibility and SEM XPD patterns indicate that there is no peak identifying a superconducting phase when the sample is annealed in vacuum The resistivity versus temperature curve of this sample is shown N.H Sinh / Journal of Magnetism and Magnetic Materials 262 (2003) 514–519 518 725 BPSCCO R (Ω) 700 675 (a) 650 50 100 150 200 T (K) 250 300 Fig Temperature dependence of electrical resistivity of Bi1.7Pb0.40Sr2Ca2Cu3Oy compound with annealing in vacuum in Fig The XPD patterns of the otherwise heattreated samples can be identified with the Bi-2223 superconducting phase The calculated lattice ( and c ¼ 37:245 A, ( parameters are a ¼ b ¼ 5:402 A ( resulting in a unit cell volume of V ¼ 1086:86 A3, for the sample annealed in flowing oxygen For comparison, the corresponding values for the ( sample annealed in air are a ¼ b ¼ 5:392 A, ( ( c ¼ 37:285 A and V ¼ 1058:06 A It is shown that the unit cell volume of the air annealed sample is smaller than that of the oxygen annealed sample The value of TC of the oxygen annealed sample has been obtained from resistivity and susceptibility measurements to be about 120 K This value is the same as the one of the air annealed sample These results indicate the important role of oxygen in the creation of superconductivity The superconductivity has been destroyed by oxygen deficiency (annealing in vacuum), but there is no influence on TC by the oxygen excess (annealing in flowing oxygen) Typical SEM pictures of Bi1.7Pb0.4Sr2Ca2Cu3Oy are shown in Fig 7a–c for the samples annealed in vacuum (a), in air (b) and in flowing oxygen (c), respectively The SEM pictures show a clear change of the grain size The grain size and the distribution of grains on the surface of the sample are quite different This indicates the influence of the different annealing conditions on the properties of the sample (b) (c) Fig SEM micrographs showing the surface morphology of the Bi1.7Pb0.40Sr2Ca2Cu3Oy compound with annealing in: (a) air, (b) vacuum, and (c) flowing oxygen In conclusion, the formation process of the 2223 phase in our samples can be shortly described as follows The liquid phase generated by sintering in the powder reacts with 2212 regions via Ca, Cu, and Pb ion diffusion, resulting in the nucleation of the 2223 phase The diffusion process controls the nucleation growth The dependence of TC on N.H Sinh / Journal of Magnetism and Magnetic Materials 262 (2003) 514–519 Pb-concentration in the Bi2.1ÀxPbxSr2Ca2Cu3Oy compounds has a parabolic character, with the highest TC value of about 120 K at x ¼ 0:40: Pb appears to play an important role in the formation of the 2223-superconducting phase A crucial role is found also for oxygen, in the creation and destruction of superconductivity A strong effect of Pb on the properties of Bi2.1ÀxPbxSr2Ca2Cu3Oy compounds has been observed It is suggested that the substitution of most of the Pb ions at the Bi-site, is responsible for the identified behaviour Acknowledgements The author wishes to acknowledge the great interest of Dr Nguyen The Hien and of Mr Do 519 Hong Minh from the Cryogenic Laboratory for their help in editing this paper This work is supported by National Fundamental research program No 421101/2002 References [1] Y.S Sung, E.E Hellstrom, Phys C 252 (1995) 155 [2] Peter Majewski, Stefanic Kaesche, Huang-Iung, Fritz Aldinger, Phys C 221 (1994) 295 [3] K Eaiprasertsak, I.M Tang, W Tontiwttana, Phys Lett A 142 (8, 9) (1989) 519 [4] S.M Green, C Jiang, Y Mei, H.L LuO, C politis, Phys Rev B 38 (1988) 5016 [5] H.K Lue, S.X Dou, N Savvides, J.P Zhou, N.X Tan, A.J Bourdillon, M Kviz, C.C Sorrell, Phys C 157 (1989) 93 [6] N.H Sinh, Phys C 252 (1995) 147 ... relation between the formation of the superconducting phases and the heat treatment in the Bi-surplus Bi2.1ÀxPbxSr2Ca2Cu3Oy compounds (x ¼0:1020:60) As suggested in Ref [3] the formation of the 2223... 2.4 Variation of properties of Bi1.70Pb0.40Sr2Ca2Cu3Oy compound The sample of x ¼ 0:40 has a highest value of TC (120 K) when it was heat-treated in air We have investigated the properties of this... respectively The SEM pictures show a clear change of the grain size The grain size and the distribution of grains on the surface of the sample are quite different This indicates the influence of the different