Effect of doping by low content of yttrium at Ca and Sr sites of Bi(Pb)-2212 superconducting ceramics

Physica B 406 (2011) 1022–1027

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Effect of doping by low content of yttrium at Ca and Sr sites of Bi(Pb)-2212 superconducting ceramics Abderrezak Amira a,n, Y. Boudjadja a, A. Saoudel a, A. Varilci b, M. Akdogan b, C. Terzioglu b, M.F. Mosbah c a

LEND, Faculty of Science and Technology, Jijel University, BP 98 Ouled Aı¨ssa, 18000 Jijel, Algeria Department of Physics, Faculty of Arts and Sciences, Abant Izzet Baysal University, 14280 Bolu, Turkey c LCMI, Faculty of Exact Sciences, Mentouri University, 25017 Constantine, Algeria b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 September 2010 Received in revised form 21 December 2010 Accepted 22 December 2010 Available online 28 December 2010

In this work, a comparative study of the effect of doping by low content of Y3 + between (Ca site) and out of (Sr site) the CuO2 planes of Bi(Pb)-2212 phase is presented. Ceramics of Bi1.6Pb0.4Sr2Ca1  xYxCu2O8 + d (called CY series) and Bi1.6Pb0.4Sr2  xYxCaCu2O8 + d (called SY series) with x ¼0, 0.025, 0.05, 0.075 and 0.1 are elaborated in air by conventional solid state reaction. They are characterized by X-ray diffraction (XRD), Scanning Electronic Microscopy (SEM), density, Vickers microhardness and resistivity measurements. The refinement of cell parameters is done by considering the structural modulation. In comparison with the undoped sample (x ¼ 0), the cell parameters a, b and c are reduced by the doping for both series while the b component of the modulation vector increases. A good correlation between the variations of the bulk density and the Vickers microhardness with x is obtained. For both series, the SEM analysis shows that the doped samples exhibit a reduced grain size than that of the undoped one. The variation of resistivity with temperature shows that all samples exhibit a metallic-like character in the normal state. For all doping levels, the CY series presents higher onset critical transition temperature than that of the undoped sample, which is equal to 85.43 K. The opposite is obtained for SY series. The highest value of this temperature is obtained for x ¼ 0.075 in the doped samples and is about 92.15 and 79.96 K for CY and SY series, respectively. These values may correspond to a near optimally doped state since the slope (dr/dT) of resistivity shows a maximum at the same value of x ¼ 0.075. For both series, when Y3 + is introduced into the samples, the residual resistivity decreases first for x ¼ 0.025 and increases gradually after this value until x ¼ 0.1. & 2010 Elsevier B.V. All rights reserved.

Keywords: Bi(Pb)-2212 phase Doping Structure Mechanical and electrical properties

1. Introduction The Bi(Pb)-2212 phase is obtained for n ¼2 in the general formula (Bi,Pb)2Sr2Can  1CunO2n + 4 + d of the Bi-based superconductors [1–3]. Because of its high stability and the facility of preparation, this compound is one of the most promising superconducting materials for technological applications [4–8]. It is well known that the physical properties such as the critical transition temperature and the critical current density of high Tc cuprates are strongly dependent on the hole carrier density of the CuO2 planes. One of the efficient ways to act on this density is the atomic partial substitutions that are very important for achieving a good understanding of superconductivity in this kind of systems. Beside the substitutions at Bi [9–12], Cu [13–16] and O [17,18] sites of Bi(Pb)-2212, many works have focused on the doping by trivalent rare-earth (RE) elements between the CuO2 planes at Ca site or out of them at the Sr site. In addition to yttrium (Y), the

n

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0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.12.052

effects of different elements of the lanthanides series have been tested [19–28]. Most of these studies concerned relatively high doping levels and reported that the superconducting properties are generally improved. Different results of the effect of Y doping at Ca site on the properties of the Pb-free Bi-2212 phase were reported. However, the highest onset critical transition temperature Tc,on of Bi2Sr2Ca1 xYx Cu2O8 + d is seen for x¼0.1 [29], 0.25 [30,31] or 0.35 [32]. Depending on the doping levels, different mechanisms of electrical conduction (from metallic to semiconducting-like) could be observed in the normal state. The effect of the substitution by Y on superconductivity is attributed to the change in hole concentration. It was also reported by Hudakova et al. [33] that doping at Sr site of this phase with a content of x¼0.1 leads to an increase in Tc,on. The authors associated this result to the partial substitution of Y3 + for Bi3 + . Since there is a disagreement between the typical length of the Bi–O bond and the size of the crystal cell in the structure of Bi-2212 phase, the adjustment of layers is achieved by the insertion of additional oxygen into BiO layers. The resulted negative charge is compensated by increasing the oxidation state of CuO2 layers and by substituting overstoichiometric Bi3 + for divalent ions Sr2 + or Ca2 + .

A. Amira et al. / Physica B 406 (2011) 1022–1027

(1 1 11) (0 2 8)

(2 0 2)

(0 2 10) (1 1 13) (0 0 16) (2 2 0) (0 2 12)

(1 1 7)

ψ

x = 0.05

2. Experimental Ceramics of Bi1.6Pb0.4Sr2Ca1  xYxCu2O8 + d (called CY series) and Bi1.6Pb0.4Sr2  xYxCaCu2O8 + d (called SY series) with x ¼0, 0.025, 0.05, 0.075 and 0.1 are elaborated in air by conventional solid state reaction. High purity chemicals of Bi2O3, PbO, SrCO3, CaCO3, CuO and Y2O3 with appropriate quantities are hand milled in an agate mortar and calcined in air at 800 1C for 24 h. The obtained powders are ground and pressed into pellets of 13 mm diameter and 1–2 mm thickness under a pressure of 3 tons at room temperature. They were then submitted to three successive cycles of sintering in air at 820, 835 and 850 1C/30 h with two intermediate grindings and pellitizations. At the last cycle, the samples are cooled to room temperature naturally in the furnace. X-ray diffraction (XRD) is used for phase analysis. The patterns are registered on a Siemens D8-Advance powder diffractometer using CuKa radiation. The microstructural characterization of the samples was done on a JEOL JSM-6390LV scanning electron microscope (SEM). The Vickers microhardness (VHN) is determined using a B3212001 Zwick microhardness tester with an applied force of 2 N. The obtained values correspond to an average of five readings at different regions of each pellet’s surface.

φ φ φ (2 0 0)

x = 0.05

0.185

φφ φ

20

30

0.195 0.190

φ

φ φ

x = 0.025 x=0

0.200 

(0 2 10 ) (1 1 13) (0 0 16) (2 2 0) (0 2 12)

(0 2 6)

(1 1 11) (0 2 8)

(0 0 12)

φ φ φ

Bi1.6 Pb0.4 Sr2Ca1-X YX Cu2 O8+d (CY) Bi1.6 Pb0.4 Sr2-X YX Ca Cu2 O8+d (SY)

0.205

(2 0 2)

ψ

x = 0.075

10

(0 2 0)

(1 1 3)

(101) (0 0 6)

(004)

φ: Bi-2201 ψ: Ca2PbO4

(0 0 8)

Bi1.6Pb0.4Sr2-xYxCu2O8+δ

0.210

(1 1 5) (0 0 10) (1 1 7)

x=0

x = 0.1

microstructural, mechanical and electrical transport properties of Bi(Pb)-2212 phase. The samples are elaborated by a solid solution reaction with a doping level in the range 0 rx r0.1. Beside a relatively similar evolution of structural and microstructural parameters with doping, the bulk density and mechanical properties as well as the onset critical transition temperature Tc,on depends strongly on the type of doping.

φ φ φ

x = 0.025 I (a.u)

(0 2 0) (2 0 0) (0 0 12)

(1 1 3)

ψ

x = 0.075

0

(0 2 6)

x = 0.1

(101) (0 0 6)

(004)

φ: Bi-2201 ψ: Ca2PbO4

(1 1 5) (0 0 10)

Bi1.6Pb0.4Sr2Ca1-xYxCu2O8+δ

(0 0 8)

Recently Sarun et al. [34] studied the effect of Y addition in Pb doped phase. In addition to a pure sample, compositions of Bi1.7Pb0.4Sr2.0Ca1.1Cu2.1YxO8 + d with x Z0.1 are elaborated. The study showed that both Tc,on and the pinning properties are improved by this procedure. The introduction of Y results in lowering of the hole concentration from the overdoped pure (Bi,Pb)-2212 to the optimum value at x ¼0.3 leading to an increase in Tc,on. Y is expected to occupy the Ca or Sr sites because of their comparable ionic radii. In some cases, doping with low content levels of RE elements gives interesting results. For a rate of x varying from 0.025 to 0.1 of ¨ La element, Jin and Kotzler [35] obtained a linear decrease in Tc with x in Pb-free Bi-2212 phase doped at Ca site. When tried in the Pb doped phase, an increase in Tc as well as an enhancement of the superconducting volume fraction is observed [36]. The presence of lead in the samples may play then a different role since it has an effect on the excess oxygen taken up by the double BiO layers. In this work, our purpose is to study and to compare the effect of doping with low content of Y at Ca and Sr sites on structural,

1023

40

50

2θ (°)

0.180 0.000

0.025

0.050

0.075

0.100

X

Fig. 1. XRD patterns of Bi1.6Pb0.4Sr2Ca1  xYxCu2O8 + d (CY) and Bi1.6Pb0.4Sr2  xYxCa Cu2O8 + d (SY) series. The arrows correspond to the modulations lines.

Fig. 2. Variation of b with yttrium content for CY and SY series.

Table 1 Cell parameters (a,b,c,b), cell volume V, agreement factors (Rp,Rwp) and goodness of fit (GOF) of CY and SY series. Bi1.6Pb0.4Sr2Ca1-xYxCu2O8 + d (CY) x ˚ a (A) ˚ b (A)

Bi1.6Pb0.4Sr2  xYxCa1Cu2O8 + d (SY)

0 5.3910(6)

0.025 5.3888(4)

0.05 5.3838(4)

0.075 5.3871(4)

0.1 5.3897(4)

0.025 5.3945(4)

0.05 5.3810(5)

0.075 5.3636(6)

0.1 5.3779(4)

5.3933(7)

5.3926(4)

5.3918(4)

5.3746(4)

5.3757(4)

5.3906(5)

5.3840(4)

5.3515(5)

5.3673(4)

˚ c (A) V (A˚ 3)

30.813(2)

30.790(1)

30.765(1)

30.722(1)

30.713(2)

30.791(2)

30.719(1)

30.562(1)

30.642(1)

895.9(2)

894.7(1)

893.0(1)

889.5(1)

889.8(1)

895.4(1)

889.9(1)

877.2(1)

884.5(1)

b

0.1829(1) (6.91,9.47) 1.68

0.18492(9) (6.79,9.56) 1.75

0.2016(1) (6.28,8.49) 1.46

0.20512(8) (6.80,9.30) 1.64

0.20524(9) (6.08,8.54) 1.54

0.1848(1) (6.10,8.47) 1.48

0.2028(1) (5.98,8.10) 1.45

0.2054(1) (8.35,11.42) 2.04

0.2060(1) (7.04,9.67) 1.73

(Rp,Rwp) % GOF

1024

A. Amira et al. / Physica B 406 (2011) 1022–1027

The electrical resistivity is determined by the four-probe method on a Cryodine CTI-Cryogenics closed cycle cryostat working in the temperature range of 10–300 K. The electrical contacts were realised by application of silver paint on rectangular bars.

3. Results and discussion The XRD patterns of the two series of samples are displayed in Fig. 1. The corresponding (h k l) Miller indices belonging to Bi(Pb)2212 main lines are shown in the diagrams. For the undoped

sample (x¼0) and in addition to the lines of the main phase, extra small lines marked with f symbol are detected. They are attributed to Bi-2201 low Tc phase as compared to JCPDS 46-0392 file. This phase is also detected for x ¼0.025 in CY series and disappears completely after this content value of yttrium. On the contrary, it is present in the samples of SY series until x¼0.075. The other secondary phase attributed to Ca2PbO4 phase (JCPDS 46-0334 file) is denoted by c symbol. It is detected at 2y ¼17.761 for x¼0.1 in SY series and for x ¼0.05 and 0.075 in CY series. We have to note that the sample with x ¼0.1 in CY series is the only one for which no impurity phase is observed.

Fig. 3. SEM micrographs of SY series (A) and CY series (B).

A. Amira et al. / Physica B 406 (2011) 1022–1027

It is clearly shown in Fig. 4 that the density of the samples is improved by Y doping for both series until x¼ 0.075 and then decreases for x¼0.1. For all doping values, the grains of samples for CY series are then more connected and well oriented between them that those of SY series [63,64]. The highest values correspond to approximately 77.67% and 77.10% of the theoretical density for CY and SY series, respectively. The variation of this density with Y content correlates well with that of the Vickers microhardness (VHN) as it can be seen in Fig. 5. Indeed, a maximum of VHN is also seen for x¼0.075. For all doping levels, the samples of CY series are mechanically more resistant than those of SY ones. Similarly to our results, Khalil [32] showed that VHN is improved by relatively high doping (xZ0.15) at Ca site of Pb-free Bi-2212 phase. A good correlation between the variations of density and VHN in these series has also been obtained. The Y doping in our case may then have a more efficient effect on reducing the porosity of the samples [19] or on suppressing microcracks and improving their ductility [65]. The variation of resistivity with temperature is shown in Fig. 6. For both series, all samples exhibit a metallic-like character in the normal state. This result was expected in accordance with previous reports. Indeed, it has been shown that the metal-to-insulator

5.10 5.07

d (g /cm3)

5.04 5.01 4.98 4.95 4.92

Bi1.6 Pb0.4 Sr2-XYX Ca Cu2 O8+d (SY) Bi1.6 Pb0.4 Sr2Ca1-XYX Cu2 O8+d (CY)

4.89 0.000

0.025

0.050

0.075

0.100

X Fig. 4. Evolution of density with yttrium content for CY and SY series.

550 525

VHN (M Pa)

The refinement of cell parameters is performed by the use of JANA2006 software [37] in the super-space group Bbmb(0 b 1)000, which takes into account the structural modulation along b and c axes [38–42]. The modulation vector is then expressed by q¼ bb* + c*, where b* and c* are the basic vectors of the reciprocal space. Details of the refinement are as follows. The lines intensities are fitted by a pseudo-Voigt function. A 36 terms of Legendre polynoms is used to describe the background and the asymmetry correction is made by the Simpson method. The lines corresponding to modulation are denoted by arrows in Fig. 1. The obtained cell parameters as well as the agreement factors (Rp,Rwp) and the goodness of fit (GOF) are listed in Table 1. The general tendency is a decrease of a, b and c parameters with yttrium content. This result may be explained on the basis of ionic size considerations [30]. ˚ has a low ionic size than those of the Indeed, Y3 + (1.04 A) ˚ and Sr2 + (1.32 A) ˚ cations [43]. Neversubstituted Ca2 + (1.14 A) theless, all these parameters pass by a minimum at x ¼0.075 for SY series. Also for CY series, while c decreases gradually with x, a little re-increase from 0.05 and 0.075 is seen for a and b, respectively. An increase in these parameters with x was also seen earlier in heavily doped samples of Pb-free phase of this series [41,44–47]. It has been pointed out that the bond length of Cu2 + p–O is smaller than that of Cu2 + –O. By increasing Y content, the number of Cu2 + cations is increased leading to an increase in a and b and a decrease in c. For the cell volume V, it decreases for both series until 0.075 and reincreases after this value. It has been reported by many authors [38–42,48–51] that the origin of the incommensurate modulation in Bi-2212 phase is attributed to a mismatch between the crystallographic units of BiO and CuO layers. The modulation along the a axis is associated to incorporation of oxygen atoms in the BiO layers, while the c component is related to the stacking period of the buckled layers or to the phase slip of one modulated layer relative to another. This latter is independent of chemical doping in Bi-2212 phase [52,53]. The Cu atoms in the Bi-based compounds exist in both divalent and trivalent states, and the excess of positive charge is compensated for by incorporation of extra oxygen atoms in the BiO planes. The influence of this excess of oxygen atoms on the structural modulation was confirmed by many groups [54–56]. In addition to the effect of oxygen excess, it has been pointed out that the Bi-based compounds are in general deficient in divalent Ca and Sr atoms [51,57]. A substitution by trivalent Bi atoms on these sites then takes place and results in higher oxygen content of the samples. As a consequence, an increase in the structural modulation is observed [57]. The values of the b component of the modulation vector of our samples, corresponding to a first-order modulation, are listed in Table 1. Its evolution with yttrium content is plotted in Fig. 2. One can see that b increases with x in both series and for each doping level, very close values are obtained. Our result agrees well with previous observations [54–57] because the substitution of trivalent Y3 + for divalent Ca2 + and Sr2 + , like in the case of La3 + and Eu3 + doped Bi-2201 phase [58–60], would cause an increase in oxygen content and thus of the modulation vector. It should be also noted that the obtained b values are relatively low than that ( 0.21) of the Pb-free phase [51,61]. The reason for this may be associated to the fact that our samples are doped with Pb at Bi site. Such a doping is known to reduce the structural modulation of the Bi-2212 compounds [62]. The scanning electron microscopy (SEM) photographs of the samples are shown in Fig. 3. They were taken at the same magnification (  10000) from the surface of the pellets. For both series, the analysis shows that the doped samples exhibit a reduced grain size than that of the undoped one, similarly to the result seen in Eu3 + [20] and La3 + [36] doped phase at Sr and Ca sites, respectively. The doping element may then act as a growth inhibitor that limits the grains size [60]. Also for a fixed yttrium content, the doped samples exhibit very similar microstructures, in grain size and porosity.

1025

500 475 450 425 0.000

Bi1.6 Pb0.4 Sr2-XYX Ca Cu2 O8+d (SY) Bi1.6 Pb0.4 Sr2Ca1-XYX Cu2 O8+d (CY) 0.025

0.050 X

0.075

0.100

Fig. 5. Evolution of the Vickers microhardness (VHN) with yttrium content for CY and SY series.

1026

A. Amira et al. / Physica B 406 (2011) 1022–1027

7 6 5

transition temperature Tc,on of our samples is estimated from these resistivity curves. Its evolution with yttrium content is shown in Fig. 7 for both series. In spite on a little decrease of about 1 K for x¼ 0.025, the CY series presents higher Tc,on than that of the undoped sample, which is equal to 85.43 K. The opposite is obtained for SY series. The highest value of this temperature is obtained for x¼ 0.075 in the doped samples and is about 92.15 and 79.96 K for CY and SY series, respectively. This value of x (0.075) is very close to 0.08 for which Eisaki et al. [67] obtained a highest Tc of 96 K in Pbfree single crystals doped at Ca site and grown in a mixture of Ar and O2 gazes. It has been also shown by Uemoto et al. [68] that the doped Pb-free phase by La at Ca site exhibits higher Tc than that doped at Sr site. For this phase and for high values of x, Pignon et al. [30] obtained a highest Tc of about 87 K for x¼0.3 by Y doping. As suggested by Roeser et al. [69], the increase in Tc by Y doping at Ca site between the CuO2 planes could be understood when considering two doping patterns, one given by the oxygen excess and the other by the doping element (Y). Where the two doping patterns overlap, the point matched locations act as resonating superconducting distance. When this distance decreases, the superconducting transition temperature Tc must increase.

Bi1.6 Pb0.4 Sr2Ca1-X YX Cu2 O8+d (CY) X=0 0.025 0.05 0.075 X = 0.1

4 3 2

ρ (Ω .cm).10-3

1 0 9 Bi1.6Pb0.4Sr2-xYxCaCu2O8+d (SY)

8

x=0

0.025

7

0.05

0.075

6

0.1

5

5

4

Bi1.6 Pb0.4 Sr2-X YX Ca Cu2 O8+d (SY)

3

1 60

90

120

150

180

210

240

270

300

ρ0 (Ω.cm).10−3

4

2

0

Bi1.6 Pb0.4 Sr2Ca1-X YX Cu2 O8+d (CY)

T (K) Fig. 6. Temperature dependence of resistivity for CY and SY series.

3

2

1

94

0 0.000

92

0.025

90

0.050 X

0.075

0.100

Fig. 8. Evolution of the residual resistivity r0 with yttrium content for CY and SY series.

88 84 82 80

12

Bi1.6 Pb0.4 Sr2Ca1-X YX Cu2 O8+d (CY) Bi1.6 Pb0.4 Sr2-X YX Ca Cu2 O8+d (SY)

78 76 74 72 0.000

0.025

0.050 X

0.075

0.100

Fig. 7. Evolution of Tc,on with yttrium content for CY and SY series.

dρ/dT (Ω .cm.K-1).10-6

TC,On (K)

86

10

8

6

4

Bi1.6 Pb0.4 Sr2Ca1-X YX Cu2 O8+d (CY) Bi1.6 Pb0.4 Sr2-X YX Ca Cu2 O8+d (SY)

transition occurs at x¼0.5 [30] or 0.55 [66] in Pb-free Bi-2212 phase doped at Ca site. Sarun et al. [34] showed that this transition is not seen for xr0.5 in Y added Bi(Pb)-2212 phase where the doping element could either substitute for Ca or Sr. The onset critical

2 0.000

0.025

0.050

0.075

0.100

X Fig. 9. Evolution of dr/dT with yttrium content for CY and SY series.

A. Amira et al. / Physica B 406 (2011) 1022–1027

The residual resistivity r0, obtained by extrapolating the normal state resistivity to T¼0 K, is related to hole concentration, chemical impurity scattering and lattice defects such as vacancies and dislocations [30,60]. Its variation with Y content is plotted in Fig. 8. For both series, when Y3 + is introduced into the samples, r0 decreases first for x¼0.025, and increases gradually after this value until x¼0.1. This resistivity is slightly greater for SY series suggesting that doping creates more defects when Y is substituted out of the CuO2 planes at Sr site. The increase in this resistivity could also be associated with a decrease in the hole concentration and by the reduction in the relaxation time of charge carriers due to a great number of defects created by a random distribution of the doping element [30]. The samples with x ¼0.025 are then more homogeneous among the others. In ceramic samples where the effect of the grain boundaries could not be neglected in transport properties, the slope dr/dT of resistivity may be considered as a parameter that depends on the intrinsic electronic interactions [30]. As can be seen from Fig. 9, the maximum of this slope is seen for x ¼0.075 in both series. This value of x may then correspond to a nearly doping state since the highest onset critical transition temperature Tc,on is seen for this level of doping [70].

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