Physica C 469 (2009) 1357–1360

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Physica C

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The effects of Sm and Y addition on the properties of YBCO thin filmprepared by a DCA-MOD method

B.J. Kim a, J.H. Yu a, J.B. Lee a, Y.S. Yoo a, J.G. Kim b, H.G. Lee a, G.W. Hong a,b,*

a Korea Polytechnic University, 2121 Jungwang dong, Siheung Shi, Gyunggi-do 429-793, Republic of Koreab ISEM, University of Wollongong, Wollongong, NSW 2522, Australia

a r t i c l e i n f o

Article history:Available online 28 May 2009

PACS:74.78.Bz74.72.Bk74.25.Sv81.20.Fw74.62.Dh

Keywords:Dichloroacetic acidDCA-MODJc

Yttrium-excessSamarium-added

0921-4534/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.physc.2009.05.029

* Corresponding author. Address: Korea Polytechnidong, Siheung Shi, Gyunggi-do 429-793, Republic of Kfax: +82 31 8041 0349.

E-mail address: gwhong@kpu.ac.kr (G.W. Hong).

a b s t r a c t

Off-stoichiometric (Y1REx)Ba2Cu3O7�d (x = 0–0.4, RE = Y or Sm) films have been prepared on LaAlO3 (1 0 0)single-crystal substrates by a metal-organic deposition using dichloroacetic acid as chelating solution.Coating solutions with excess Sm or Y were prepared by dissolving Y, Sm, Ba and Cu acetates in dichlo-roacetic acid and distilled water followed by refluxing, drying to obtain blue gel. The final precursor solu-tion for dip coating was prepared by diluting blue gel in 2-methoxyethanol. Coated films were calcined atlow temperature up to 500 �C in flowing humid oxygen atmosphere. Conversion heat treatment was per-formed at 800 �C for 2 h in flowing Ar gas containing 1000 ppm oxygen with a humidity of 9.45%. The vis-cosities of the precursor solution were increased as the content of added Sm or excess Y increased. Thetransport critical current density (Jc) of (Y1REx)Ba2Cu3O7�d film was enhanced with the addition of excessSm and Y up to 20% and then decreased with further addition. Scanning electron microscope (SEM) obser-vation showed that surface roughness was increased with the addition of excess Y.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction tapes, the MOD process is considered to be a strong candidate as

The development of high-temperature superconductors (HTS)coated conductor with high critical current density (Jc) offers greatpromise for applications of HTS on various superconducting de-vices such as cable, transformer, generators, and motors operatingat 77 K [1–6]. The Jc value of the YBa2Cu3O7�x (YBCO) system doesnot drop so critically as the Bi-system under high magnetic fields[7]. YBCO has more advantages in cost, mechanical strength andlosses in AC applications. YBCO wire comprises several layers ofmetal substrate, buffer layer, superconducting layer, stabilizinglayer from which it is called as a coated conductor. It results in alow material cost due to its low volume ratio of Ag in whole wirevolume. The architecture of YBCO tapes allows to increase itsmechanical strength by a metal substrate and to scribe the tapesfor AC loss reduction. YBCO tapes have been expected for futureapplications though their problems of difficulty in fabrication.

The metal-organic deposition (MOD) process is a non-vacuumprocess for fabrication of films with uniform thickness from start-ing liquid chemical solutions. For fabrication of long-length YBCO

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c University, 2121 Jungwangorea. Tel.: +82 31 8041 0480;

a low cost process. The MOD process using the starting solutioncontaining trifluoroacetate (TFA) salts of yttrium, barium and cop-per could easily yield YBCO films with high Jc. The use of TFA saltsappears to avoid the formation of BaCO3 because the stability ofbarium fluoride is greater than that of barium carbonate and fluo-rine can be removed during the high-temperature anneal (>700 �C)in a humid, low oxygen partial pressure environment. Nonethelessseveral factors maintain interest in a fluorine-free precursor MODapproach. Recently, it has been reported that high quality HTSfilms with Jc > 1 MA/cm2 could be obtained by a chemical solutionmethod using fluorine-free precursors. Yamaguchi et al. [5] usedacetylacetonates as starting materials and Yao et al. [6] utilizedmixed propionic acid/amine solvent for the preparation of precur-sor solutions. These are encouraging results for the scientists whopursue to develop new MOD precursors for the fabrication of highquality HTS films. More recently, Kim et al. [8] also succeeded inpreparing high quality YBCO film using dichloroacetic acid (DCA)as solvent for the preparation of coating solution. Among theseprocesses, DCA-MOD process is very recent one and it is desiredto optimize the processing parameters in order to obtain higherJc over 1 MA/cm2. Previous study using DCA-MOD precursorsshowed possibility of preparing epitaxial YBCO film.

DCA-MOD researches have performed using the coating solu-tion with a stoichiometric composition of Y:Ba:Cu = 1:2:3. In sev-

1358 B.J. Kim et al. / Physica C 469 (2009) 1357–1360

eral years, it was reported that the presence of finely dispersed sec-ond phase particles in YBCO grains enhanced the current carryingcapacity of YBCO films prepared by TFA-MOD [9,10].

Accordingly in this study, DCA precursor solutions with differentcomposition such as; Y-excess (10–40 at%) and Sm-added (10–40 at%) YBCO films as well as a stoichiometric (Y:Ba:Cu = 1:2:3)YBCO film were prepared in order to investigate the effects of theexcess addition of Y and Sm on the properties of YBCO films pre-pared by a DCA-MOD method.

2. Experimental

Coating solutions with excess amount of Sm or Y (10–40 at%)were prepared by dissolving Y, Sm, Ba and Cu acetates in dichloro-acetic acid (DCA) and distilled water followed by refluxing to ob-tain blue solution. Blue gel was obtained by evaporating waterand acetic acid in the solution using a rotary evaporator. The finalcoating solution was prepared by dissolving blue-colored MO pre-cursor in a dilution solvent of 2-methoxyethanol [11]. The coatingsolution has been adjusted as 2 molarity based on total cation con-centration. Precursor film was coated with a speed of 25 mm/min

Fig. 1. Heat treatment schedules for (a) calcination and (b) conversion.

on LaAlO3 (1 0 0) single-crystal (4 mm width, 12 mm length) usinga dip coater.

Coated films were heat treated following a processing shown inFig. 1a and b. Coated films were calcined at lower temperature upto 500 �C on the heating rate of 2.22 �C/min in flowing humid oxy-gen atmosphere. Conversion heat treatment was carried out at800 �C in moisture-containing flowing mixture gas atmospherewhich containing 1000 ppm oxygen and 9.45% humidity [11].Gas flow rate was 1000 sccm.

The phase formation and texture of the prepared films werecharacterized by X-ray diffraction (XRD) using CuKa radiation.The surface morphology and the thickness of YBCO films were ob-served by field emission scanning electron microscopy (FE-SEM).Critical current (Ic) values were determined with a criteria of1 lV/cm from I–V curves at 77 K and self-field. Jc values were cal-culated by dividing Ic using cross-sectional area of the films.

3. Results and discussion

Fig. 2 shows the XRD patterns for the (Y1REx)Ba2Cu3O7�d (x = 0–0.4, RE = Y or Sm) samples prepared using coating solutions thatthe various amount of Y or Sm was added excessively. Main diffrac-

Fig. 2. The X-ray diffraction patterns of (Y1REx)Ba2Cu3O7�d (x = 0–0.4, RE = Y or Sm)films with the excess addition of (a) Y atoms and (b) Sm atoms.

Fig. 3. SEM images for the YBCO films with the addition of excess Y (10–40%).

Fig. 4. SEM images for the YBCO films with the addition of excess Sm (10–40%).

B.J. Kim et al. / Physica C 469 (2009) 1357–1360 1359

tion peaks from (0 0 1) planes of YBCO crystal are well developedregardless of Y content for the films which were prepared by add-ing excess Y. Lee et al. [12] also resulted that the texture of YBCOfilm was not affected significantly with the addition of excess Yin YBCO film prepared by TFA-MOD process. It is also seen thatmain diffraction peaks from (0 0 1) planes of YBCO crystal are welldeveloped regardless of Sm content for the films which were pre-pared by adding excess Sm. High intensity of (0 0 1) peak is a char-acteristics of the 123 films prepared with light rare earth metalssuch as La, Nd, Sm, etc. XRD peaks from Y2O3 or Sm2O3 phases werehardly detected even for the films that were prepared using a pre-cursor solution containing excess Y or Sm. However, it is seen thatadditional minor XRD peaks appeared at 27.7� and 33.5� for(Y1Smx)Ba2Cu3O7�d (x = 0.1–0.4). Peaks at 27.7� and 33.5� are con-sidered as a XRD peak from Ba–Cu–O [8].

SEM surface micrographs of YBCO films with addition of excessY (10–40%) are shown in Fig. 3. The surface of added Y film becamerough and grain size was increased with the increase of excess Y.Pores with wide range of size observed. It is seen that the Jc ofYBCO film is decreased with the increase of excess Y. SEM surfacemicrographs of a stoichiometric Y123 film showed similar porosity.It is observed that YBCO film with 20% excess Y showed largeramount of second particles than YBCO film with 30% excess. It isalso seen that YBCO film with 30% excess Y showed more hillocksthan YBCO film doped with smaller amount of excess Y.

Fig. 4 shows the SEM micrographs of the (Y1Smx)Ba2Cu3O7�d

thin films with various amount of excess Sm. YBCO films with ex-cess Sm showed better grain connectivity between grains than astoichiometric YBCO films with no addition of excess Sm. It is alsoable to see that the porosity of film was decreased with the addi-tion of excess Sm to a stoichiometric YBCO film.

Table 1Thickness of (Y1REx)Ba2Cu3O7�d (x = 0–0.4, RE = Y or Sm) films with added (a) Y or (b)Sm (10–40%).

Y123 10% 20% 30% 40%

(a) Excess Y (lm) 0.17 0.22 0.23 0.20 0.18(b) Addition Sm (lm) 0.29 0.19 0.21 0.21 0.33

The thickness of (Y1Smx)Ba2Cu3O7�d films with addition of ex-cess Y or Sm (10–40%) are shown in Table 1. The thickness of filmwas increased with the increase of Y or Sm addition. But, YBCO film

Fig. 5. Variation of Ic and Jc of (Y1REx)Ba2Cu3O7�d (x = 0–0.4, RE = Y or Sm) films withthe excess addition of (a) Y atoms and (b) Sm atoms.

1360 B.J. Kim et al. / Physica C 469 (2009) 1357–1360

with 30% excess Y decreased thickness because of thickness aver-age with rough and grain size.

Effect of Sm and Y addition on critical current (Ic) and criticalcurrent density (Jc) of YBCO films at 77 K and self-field are shownin Fig. 5. It is seen that Jc of YBCO film was enhanced with the addi-tion of excess Sm and Y up to 20% and then decreased with furtheraddition. Highest Jc was measured as 1.62 MA/cm2 for the YBCOfilm with 20% excess Y and as 1.46 MA/cm2 for the YBCO film with20% addition Sm, maximum Jc was obtained when little bit moreamount of excess rare earth atoms compared with the YBCO filmprepared by a TFA-MOD method [12].

4. Conclusions

Off-stoichiometric (Y1REx)Ba2Cu3O7�d (x = 0–0.4, RE = Y or Sm)films have been prepared on LaAlO3 (1 0 0) single-crystal sub-strates by a metal-organic deposition using dichloroacetic acid aschelating solution (MOD-DCA). Main diffraction peaks from(0 0 1) planes of YBCO crystal were well developed regardless ofY content for the films which were prepared by adding excess yt-trium. The increase of the viscosity of the precursor solution withthe addition of excess Y and Sm led to the increase in film thick-ness. The addition of excess Sm led to the densification and bettergrain connectivity of YBCO film. The transport critical current den-sity (Jc) of YBCO film was enhanced with the addition of excess Smand Y up to 20% and then decreased with further addition. The SEMobservation showed that surface roughness was increased with theaddition of excess Y.

Acknowledgments

This research was supported by a Grant from Center for AppliedSuperconductivity Technology of the 21st Century Frontier R&DProgram funded by the Ministry of Education, Science and Technol-ogy and Manpower Development Program for Energy & Resourcesby the Ministry of Knowledge and Economy, Republic of Korea.

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