Enhanced flux-pinning in fluorine-free MOD YBCO films by chemical doping

Physica C 470 (2010) 1261–1265

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Enhanced flux-pinning in fluorine-free MOD YBCO films by chemical doping W.T. Wang a, M.H. Pu a, Y. Yang a, H. Zhang c, C.H. Cheng b, Y. Zhao a,b,* a Key Laboratory of Magnetic Levitation and Maglev Trains (Ministry of Education of China), Superconductivity R&D Center (SRDC), Mail Stop 165#, Southwest Jiaotong University, Chengdu, Sichuan 610031, China b Superconductivity Research Group, School of Materials Science and Engineering, University of New South Wale, Sydney, 2052 NSW, Australia c Department of Physics, Peking University, Beijing 100871, China

a r t i c l e

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Article history: Available online 15 May 2010 Keywords: Flux-pinning Chemical doping Fluorine-free MOD YBCO film

a b s t r a c t YBCO films without and with dilute cobalt and zinc doping were prepared on (0 0 l) LaAlO3 substrate by non-fluorine metal organic deposition method. Effects of dilute cobalt and zinc doping on biaxial texture, microstructure and flux-pinning properties of YBCO films were investigated. The surface density and smoothness of the doped YBCO films have been distinctly improved compared with that of the pure film. Dilute cobalt- and zinc-doped YBCO films exhibit significantly enhanced Jc values in the magnetic field. The best result is achieved in the cobalt-doped YBCO film. At 77 K, Jc values of cobalt-doped film are 1.7 and 5.4 times higher than that of pure film in 0.5 T and 1.5 T, respectively. These results strongly suggest that dilute cobalt and zinc doping is a promising way to increase the current carrying capability of YBCO films. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Recent years, special attention has been devoted to the preparation and applications of YBCO-based coated conductors (CCS). However, relatively low current carrying capability with an increasing field and temperature prevents the large-scale applications of YBCO CCS in superconducting magnets, generators, motors and etc. To increase the current carrying capability of high temperature superconductors, strong flux-pinning of vortices is essential. As we know, pinning of vortices is optimized when the size of defects is in the range of the superconducting coherence length. Because of the small coherence length of high temperature superconductors, nanosize defects like secondary phases, point defects, dislocations and grain boundaries in a crystal lattice can be potential pinning sites [1–4]. Additionally, pinning centers can be artificially created by various methods except for these naturally grown defects. One example of the artificial method is the use of heavy ion irradiation to produce the column defects to pin the vortex lines [5,6]. Currently, many other artificial pinning centers have been introduced to improve the pinning efficiency by different techniques, including rare earth substitution for yttrium [7,8], addition of second phase particles [9], YBCO multilayer with second phase materials [10,11], decoration of substrate surfaces by nanosize particles [12,13] and target compositional modifications

* Corresponding author at: Key Laboratory of Magnetic Levitation and Maglev Trains (Ministry of Education of China), Superconductivity R&D Center (SRDC), Mail Stop 165#, Southwest Jiaotong University, Chengdu, Sichuan 610031, China. Tel.: +86 28 87600786; fax: +86 28 87600787. E-mail address: [email protected] (Y. Zhao). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.05.089

either by second phase BaZrO3 inclusions [14] or creation of column defects comprised of self-alighted BaZrO3 nanodots and nanorods [15,16]. At present, YBCO films with high performance in the magnetic field can be fabricated by in situ methods like PLD and magnetic sputtering. Goyal et al. [15] have succeeded in producing high Jc YBCO films in the magnetic field in the form of self-assembled stacks of BaZrO3 nanodots and nanorods. Varanasi et al. [17] reported that the maximum of the bulk pinning force (Fp,max) of YBCO films with BaSnO3 nanoparticles can be more than twice the value of Fp,max of regular YBCO films with similar thickness at 77 K and 65 K. In addition, trifluoroacetates metal organic deposition (TFAMOD), as a promising and cost-effective chemical solution approach, can also fabricate YBCO films with relatively high critical current density in the magnetic field. Zhou et al. [18] have successfully fabricated three kinds of doped YBCO films like the introduction of nanoscale Y enriched particles, nanoscale 90° rotated Y1/3Sm2/3Ba2Cu3O7 domains and nanoscale Zr column defects into YBCO layers by different chemical dopings. The pinning force density is found to be larger than the pure YBCO films. However, high cost preparation of pulsed laser deposition (PLD) and fluorine removal as well as potential HF pollution of TFA-MOD may possibly influence the large-scale applications of YBCO-based CCS. Therefore, a kind of novel non-fluorine chemical solution deposition method is highly required for the high performance YBCO film preparation and application. As we all know, many studies have confirmed that incorporation of impurities such as Co, Fe, Ga, Zn, Ni to the Cu sites of YBCO can obviously increase the Jc–H characteristics of high temperature superconductor bulks [19–21]. The enhancement of flux-pinning

W.T. Wang et al. / Physica C 470 (2010) 1261–1265

LAO

LAO

Pure, dilute cobalt- and zinc-doped YBCO films have been prepared on (0 0 l) LAO substrate by non-fluorine PA-MOD method. Typical X-ray diffraction h–2h patterns of the YBCO films are shown in Fig. 1. As can be seen, only (0 0 l) YBCO reflection peaks have been detected except the peaks of single crystal substrate indicating a strong c-axis orientation. To further examine the texture of YBCO films, phi-scan and omega-scan rocking curves have been documented in Fig. 2a and b. Fig. 2a shows a phi-scan of (1 0 3) peak reflection with the full width at half maximum (FWHM) of 0.9°, 1.2° and 1.25° for pure, zinc- and cobalt-doped YBCO films, respectively, indicating a good in-plane alignment. The typical omega-scan rocking curves of pure and chemical doping films around the (0 0 5) diffraction peak are performed to estimate the quality of out-of-plane alignment shown in Fig. 2b. The FWHM of (0 0 5) peak is about 0.3° for all the films indicating a good outof-plane texture. The results show that biaxially-textured pure and dilute impurity doped YBCO films have been achieved. Fig. 3 shows the surface morphologies of doped and un-doped YBCO films. Dense, smooth and crack-free surface morphologies can be observed in these films. A few of particles floated on the film surface may be attributed to the texture degradation with the increase of film thickness during firing procedure. It is interested that YBCO films with cobalt and zinc doping possess denser and smoother surface microstructures compared with that of pure YBCO film. Castano et al. [24] reported that relatively porous film by TFA-MOD will limit the current transport properties. Therefore, we believe that better surface morphologies of doped YBCO films may be one of the reasons for achievement of high critical current densities in the magnetic fields. Temperature dependence of normalized magnetization for doped and un-doped YBCO films is shown in Fig. 4. As can be seen, the superconducting transition of pure YBCO film spans a smaller temperature range than that of doped films elucidating the exis-

Co-doped LAO

LAO

10

(007)

(006)

pure

(005)

Zn-doped

(004)

Pure and dilute cobalt- and zinc-doped YBa2Cu3 x(Cu or Zn)xO7 z films were prepared on commercially available (0 0 l) LAO single crystal substrate by fluorine-free PA-MOD method. The precursor solutions were synthesized by dissolving acetates of cobalt and zinc, in addition to acetates of yttrium, barium and copper, in propionic acid with x of 0.001 and stirring at 60 °C for 2 h. Then polyvinyl butyral was added into the solution with being subsequently subjected to continuously stirring to adjust the viscosity thus obtain the final coating solution. Then the coating solution was coated on LAO using a spin coater with the rotation speed of 6000 r/min and followed by drying at 150–200 °C for 5–10 min. YBCO films were fabricated with the following three processing steps. Firstly, the films were decomposed in the temperature range of 150–500 °C at the rate of 0.5 °C/min in humid Ar/O2 mixture gas. After this step, the Y, Ba, Cu amorphous matrix was obtained. Secondly, YBCO precursor films were directly inserted into the quartz tube furnace at 815 °C dwelling for 2–7 min to experience a shorttime partial melting process, and then fired at 750–800 °C in dry Ar/O2 gas (99.9% Ar) for 1 h to get the tetragonal YBCO phase. The humidified ambient was obtained by passing dry gas through a reservoir of water, in which the humidity can be controlled by water temperature, i.e., the dew point of water. Finally, superconducting YBCO films with orthorhombic phase were prepared by annealing in dry O2 gas at 400–450 °C for several hours. A Philips X’Pert MRD diffractometer with Cu Ka radiation was used to record the h–2h X-ray diffraction patterns, which characterized the phase purity of the YBCO film. The texture analyses including u-scan and x-scan were performed using a Philips MRD equipped with a four crystal monochromator, delivering a pure Cu Ka1 line of wavelength k = 0.15406 nm. The microstructure analyses as well as the cross-sectional investigation of the YBCO layer were performed by using an environmental scanning electron microscope (SEM) equipped with energy dispersive Xray analysis. Superconducting transition as well as magnetic hysteretic loop has been observed by using Quantum-Design SQUID XL (7 T). The Jc value of the YBCO film in self-field at 77 K was determined by the application of the Bean critical state model formula using the M–H curve. The thickness of the YBCO film has been determined by Ambios XP-2 step profiler and cross-sectional SEM micrograph. The thickness of doped and un-doped YBCO films is about 260 nm. The inductive critical current density Jc was determined using the extended Bean model [23], with the formulae Jc = 20DM/ [a(1 a/3b)], where DM is the width of the magnetization (M)

3. Results and discussion

(003)

2. Experimental

loop; a and b are the length and width of the slab (a < b), respectively. In order to avoid the possible influence of shape and size on the values of Jc measurement, all of the samples were cut into same size of 2.0  2.5 mm2. In addition, the results of the inductive critical current density Jc were compared and calibrated with the 4-probe transport measurement to make sure the results of inductive measurement are comparable with those of the transport measurement.

(002)

for dilute impurity doping to the CuO chain is through the lattice deformation as well as the local change in the superconducting condensation energy without large Tc decreases [19]. With respect to the impurity doping to the CuO2 plane, a local suppression of superconductivity has been observed in YBCO single crystals at a scale of 1.5 nm [21] close to the in-plane coherence length of YBCO demonstrating nanosize defects acting as the effective core pinning centers. Up to now, however, there are a few reports about the doping effects of these dilute impurities in YBCO films [26,27], especially for those prepared with fluorine-free MOD. In this study, dilute impurity doping of YBCO was firstly introduced into the metal organic deposition (MOD) YBCO films. Fluxpinning properties of dilute cobalt- and zinc-doped YBCO films were investigated via a newly developed fluorine-free polymer-assisted metal organic deposition (PA-MOD) method [22]. Epitaxial, dense, smooth and crack-free YBCO films were prepared on LaAlO3 (LAO) single crystal substrate. In-field Jc of YBCO films realized the significant improvement by this dilute impurity doping.

Intensity (a.u.)

1262

20

30

40

50

60

2Theta(deg) Fig. 1. Typical h–2h X-ray diffraction patterns of YBCO films with and without dilute impurity doping.

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a

Intensity (a.u.)

FWHM=1.25

FWHM=1.2

YBCO (103)

o

o

Zn-doped

o

FWHM=0.9

50

100

pure

150

200

250

300

2µm

350

Phi(deg)

b

YBCO (005) o

FWHM = 0.33

Intensity (a.u.)

Pure

Co-doped

Zn-doped

Co-doped

o

FWHM = 0.32

Zn-doped

o

FWHM = 0.31

18.8

pure

19.2

19.6

20.0

Omega(deg)

2µm

Fig. 2. Phi-scan (a) and omega-scan rocking curves (b) of YBCO films with and without dilute impurity doping. The FWHM values are shown in the figure.

tence of purer YBCO superconducting phase. Tc values of pure, cobalt- and zinc-doped YBCO films are 90 K, 88.5 K and 87.4 K, respectively (see the inset of Fig. 4). Decrease in the number of hole carriers by impurity doping to the CuO chain should be responsible for the reduced Tc of YBCO film with cobalt doping. Relatively larger Tc decreases for zinc-doped film may be due to the direct zinc substitution for copper in the CuO2 plane where superconductivity primarily occurs. It was noted that non-magnetic zinc introduces a local magnetic moment of nearly 0.7 lD, which breaks the superconducting pairs in the CuO2 plane [25]. In addition, YBCO film with dilute cobalt doping exhibits the largest magnetic magnetization signal. Fig. 5 shows the Jc–B behaviors of doped and un-doped YBCO films at different temperatures with the magnetic field parallel to the c-axis. It is noted that no fishtail effect has been detected in these dilute cobalt- and zinc-doped YBCO films, which is adverse with the results in YBCO bulks [19]. So, the origin of the fishtail needs to be further discussed in different materials. In Fig. 5a, Jc exceeding 3 MA/cm2 at 77 K and self-field has been achieved for cobalt-doped YBCO film more than 2.4 MA/cm2 of pure film as well as 1.6 MA/cm2 of zinc-doped film. According to Fig. 5a and b, Jc of cobalt-doped film is significantly improved in all magnetic fields and temperatures. Additionally, the Jc–B behavior at 50 K of cobalt-doped film is equal to that of pure YBCO film at 30 K indicating a remarkable improvement of Jc at low temperatures by cobalt doping. With respect to the zinc-doped YBCO film, enhanced flux-pinning properties can also be observed in all magnetic fields

Co-doped

2µm Fig. 3. The surface morphology of YBCO films with and without dilute impurity doping.

and temperatures. For cobalt doping to the CuO chain, there is a lattice deformation resulting in oxygen vacancies which locally suppress the superconductivity. When the regions of suppressed superconductivity is at a scale of several nanometers matching with the coherence length of YBCO, significantly improved fluxpinning features can be obtained. And formation of nanosize point

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0.00

0

T (onset) =

-500

3

-0.25

M (emu/cm )

Normalized magnetization

c

-0.50

T (onset) = c

87.4 K

88.5 K

T (onset) = c

90 K

H = 10 Oe

-1000

Zn-doped pure Co-doped

-1500

Zn-doped pure Co-doped

-2000 80.0

82.5

85.0

87.5

90.0

92.5

T (K)

-0.75

-1.00 30

45

60

75

90

105

T (K) Fig. 4. The temperature dependence of normalized magnetization for YBCO films with and without dilute impurity doping. Inset is the temperature dependence of magnetic moment for all the films.

a

H//c 65 K

2

Jc (MA/cm )

1

pure Co-doped Zn-doped

0.1

0.01

1E-3

77 K

0

1

2

3

4

3

4

B (T)

b

H//c

pure Co-doped Zn-doped

2

Jc (MA/cm )

10

30 K

1

50 K

0

1

2

B (T) Fig. 5. The magnetic field dependence of critical current densities for YBCO films with and without dilute impurity doping at (a) 77 K and 65 K (b) 50 K and 30 K. Give the significantly enhanced in-field Jc values.

defects in CuO2 plane by zinc doping may be responsible for the enhanced flux-pinning properties of zinc-doped YBCO film. Accordingly, effective pinning centers can be introduced by low level cobalt and zinc doping to the copper sites of YBCO films. And these results are in agreement with the former SEM analysis that dense and smooth surface microstructures are important to improve the current densities in the magnetic fields. However, YBCO film doped with cobalt has much higher Jc in all fields compared with that of film doped with zinc. Incorporation of cobalt to the CuO chain will introduce extra electron carriers which will counteract the partial hole carriers and thus reduce the original hole carriers concentration in the CuO2 plane. Fortunately, high superconductivity of cobalt-doped YBCO film can be recovered by annealing in oxygen atmosphere for several hours. On the contrary, zinc doping to the CuO2 plane directly substitutes the copper site resulting in a destruction of superconductivity which can not be recovered by treating process. Furthermore, for zinc doping to the CuO2 plane, there is not distinct lattice deformation with pronounced strain field and change of oxygen stoichiometry because of their matching ion size. Thus compared with oxygen vacancies by cobalt doping, such a replacement is not expected to have a dramatic promotion in current densities. The magnetic field dependence of volume pinning force density at 77 K is shown in Fig. 6. The volume pinning force density is calculated by the definition of Fp = Jc  H. As can be seen in this figure, dilute cobalt- and zinc-doped YBCO films show remarkably improved Fp values in the fields. And cobalt-doped YBCO film exhibits the highest Fp values in all fields. Furthermore, the maximum Fp of cobalt-doped film shifts to much higher magnetic field. These results indicate that YBCO film doped by dilute cobalt has the best flux-pinning properties. To understand the essential doping effect of cobalt and zinc on flux-pinning properties of YBCO films, it is necessary to make further studies such as high resolution transmission electron microscopy observation and transport measurements. Additionally, detailed researches like different doping levels on YBCO films doped with impurities have been carried out in our group.

W.T. Wang et al. / Physica C 470 (2010) 1261–1265

Doctoral Innovation Foundation of Southwest Jiaotong University under Contract No. A0320502050902-2, the Fundamental Research Funds for the Central Universities, and the Australian Research Council under Grant Nos. DP0559872 and DP0881739.

0.8

YBa2Cu3-xRxO7-z 77 K

References

H//c

3

Fp (GN/m )

0.6

1265

0.4

[1] [2] [3] [4]

pure Co-doped Zn-doped

0.2

[5] [6] [7]

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

H (T) Fig. 6. The magnetic field dependence of volume pinning force density Fp for YBCO films with and without dilute impurity doping at 77 K. Fp values are obviously improved by cobalt doping.

[8] [9] [10] [11]

4. Conclusion High performance doped and un-doped YBCO films have been prepared on LAO single crystal substrate via non-fluorine polymer-assisted MOD method. Highly dense and smooth surface microstructures have been observed in dilute cobalt- and zincdoped YBCO films. The films exhibited dramatically enhanced Jc– B characteristics by dilute impurity doping. And YBCO film doped with cobalt shows much higher current densities and volume pinning force densities in all magnetic fields and temperatures elucidating more effective pinning centers introduced by cobalt doping. These results indicate that cobalt and zinc doping to the copper sites of YBCO is a perspective way to essentially improve the current carrying capability of YBCO films. Acknowledgements The authors are grateful for the financial support of Natural Science Foundation of China under Contract Nos. 50672078 and 50872116, National Science Fund for Distinguished Young Scholars under Contract No. 50588201, National High-Tech Program of China (863 Program) under Contract No. 2007AA03Z203, the PCSIRT of the Ministry of Education of China (IRT0751), the Specialized Research Fund for the Doctoral Program of Higher Education (200806130023), Research and Development Foundation of Southwest Jiaotong University under Grant Contract No. 2004A02, the

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