Preparation and Characterization of a Large-Scale YBa2Cu3O7�x

Superconductor Prepared by Plastic Forming without a High-PressureMolding: Effect of Polyvinyl Alcohol (PVA) Addition on the

Superconducting Properties

Makoto Takahashi,w Toshiyuki Miyauchi, Kengo Sawada, Hiroyuki Ishikawa, and Shoji Sato

Department of Applied Chemistry, College of Engineering, Chubu University, Aichi 487-8501, Japan

Masahiro Tahashi and Koichi Wakita

Department of Electronic Engineering, College of Engineering, Chubu University, Aichi 487-8501, Japan

Sadao Ohkido

Department of Natural Science and Mathematics, College of Engineering, Chubu University, Aichi 487-8501, Japan

Motohisa Honda and Asami Murai

Advance Electric Co. Inc., Aichi 480-0304, Japan

Masato Kamiya and Masanori Matubara

Shinko Yogyou Co. Ltd., Gifu 509-5312, Japan

The preparation of large-scale YBa2Cu3O7�x superconductorsamples was investigated. This method is based on plastic form-ing using a slurry consisting of YBa2Cu3O7�x particles and a solsolution made up of multimetallic hydroxide particles (YBa2Cu3(OH)x colloidal particles) and poly(vinyl) alcohol (PVA).The effects of adding PVA on the product, the crystallinity, andthe superconducting properties of the sample were investigated.It was found that PVA acted as a protective colloid in the solsolution and stabilized YBa2Cu3(OH)x colloidal particles, andthat the role of PVA changed from a thickener to a flocculantduring drying so that the formability/workability of the greensheet sample was improved and large samples (about 80 mm� 80 mm� 3 mm) without large cracks were obtained after fir-ing. The samples became superconducting at 91.570.5 K (Tcon)and the full transition temperature (Tcoff) was 88.571.5 K. Thecritical current density (Jc) of the sample prepared from theslurry containing 1 wt% PVA was 7137150 A/cm

2at 77 K.

This Jc value was improved to 2300 A/cm2by heat treatment at

773 K under an oxygen atmosphere.

I. Introduction

OXIDE superconductors such as YBa2Cu3O71,2 and Bi–Sr–

Ca–Cu–O3,4 have been the subject of many experimentaland theoretical studies since their discovery.5 The YBa2Cu3O7�xsuperconductor has many advantages over other high-tempera-ture oxide superconductors, namely, its superconducting transi-tion above 77 K (the boiling point of liquid nitrogen) and largepinning force.6 Several studies on the synthesis and applications

of YBa2Cu3O7�x films and bulk samples have been reported.7–9

The bulk ceramic samples are prepared using several methodssuch as the slip-casting method, the pressing method (sinteringmethod), and the melt-growth method.

To improve the forming performance of the green body andthe homogenization properties of the sintered sample, organic orinorganic forming aids such as polyvinyl alcohol (PVA), poly-vinyl pyrrolidone (PVP), poly ethylene glycol, and alumina col-loid are added to the ceramic powder. It is well known thatresidual carbon, carbide, and impurities exist along the grainboundary and influence the electrical and mechanical properties.Many researchers have reported a correlation between impuri-ties and superconducting properties.10,11 Maiwa et al.12 reportedthe effects of impurity contents in the starting materials on thecritical transition temperature (Tc) and the critical current den-sity (Jc). Wang et al.13 showed a correlation between the carboncontent and Tc. The high Tc and Jc of the large bulk sample ofceramic superconductor show that it was prepared with high-purity starting materials using either the melt-texturation pro-cess14 or the melt-growth method.15 Use of these methodsbecomes costly for the preparation of samples that differ inshape and size. Therefore, a technique for an easy and inexpen-sive preparation of a large bulk superconductor with variousthree-dimensional structures is highly desirable.

We have investigated the preparation and characterization ofthick LiNbO3 films prepared by the sol–gel method. We havereported that in order to prepare a high-quality sample, con-trolling preparation factors such as colloidal particles, impuritytype, and its content in the sol solution, etc., is very impor-tant.16–18 The conventional sol–gel method does not provide anycontrol over the diameter and composition of colloidal particles.In our previous work on the preparation of a personal magneticshield for use in magnetic resonance imaging (MRI) diagnosis,we reported that a large YBa2Cu3O7�x superconducting sheet(about 50 mm� 35 mm� 2 mm), whose the full-transition tem-perature (Tcoff) and Jc were 88.771.4 K and 440 A/cm2 (at 77 K,0 T), respectively, could be prepared using a slurry consistingof YBa2Cu3O7�x particles and a fine YBa2Cu3(OH)x colloid

M. Parans Paranthaman—contributing editor

This work was partly supported by grants from Chubu University (April 2007–March2008).

wAuthor to whom correspondence should be addressed. e-mail: thinfilm@isc.chubu.ac.jp

Manuscript No. 24499. Received April 3, 2008; approved November 20, 2008.

Journal

J. Am. Ceram. Soc., 92 [3] 578–584 (2009)

DOI: 10.1111/j.1551-2916.2008.02900.x

r 2009 The American Ceramic Society

578

solution, without the use of a high-pressure process.19 This re-sult indicates that refined colloidal particles, which have a con-trolled diameter and composition, can be used as a superiorinorganic binder. The fine colloid solution was prepared by sus-pending refined multimetallic hydroxide (YBa2Cu3(OH)x) par-ticles that were separated and purified after the synthesis usingthe conventional sol–gel method.

Cracks on the surface near the peripheral region were ob-served on the sample during the preparation and drying of agreen sheet, when making samples larger than reported previ-ously. This resulted in large cracks and breaks after firing. Theseproblems may be attributed to the following:

Poor green body forming performance due to the low viscos-ity of the slurry;

A crazed surface resulting in varying solvent evaporation ve-locities that generated shrinkage cracks during drying;

A heterogeneous colloid particle diameter, resulting in an in-creasing diameter and decreasing particle density over time.

To solve these problems, organic polymers, such as PVA andPVP, were added to the slurry. It is known that PVA and PVPact as dispersing agents, emulsifying agents, thickeners, floccul-ants, and coating formation suppressants. These materials havebeen widely used in various fields such as chemistry, the ceramicindustry, civil engineering, and cosmetics.20–22 To improve theformability/workability and superconducting properties ofYBa2Cu3O7�x bulk samples, we have investigated the effectsof PVA addition on the composition, crystallinity, and super-conducting properties. We also varied the PVA concentration,because the effects are strongly concentration dependent.

II. Experimental Procedure

The preparation procedure for the YBa2Cu3O7�x bulk sample isshown in Fig. 1. Calcined YBa2Cu3O7�x powder was preparedby firing a mixture of Y2O3 (purity 99.99%), BaCO3 (purity99.9%), and CuO (purity 99.99%) at 1223 K in air. TheYBa2Cu3(OH)x precursor was synthesized by hydrolyzing anethanol solution containing Y(OC3H7)3 (purity 99.9%),Ba(OC3H7)2 (purity 99.9%), and Cu(CH3COO)2 (purity495%)at a molar ratio of 1:2:3. The precursor was refined with abso-lute ethanol after precipitation and filtration. Sol solutions were

prepared by suspending the refined precursor in the solution atvarious PVA concentrations using an ultrasonic cleaner. PVAwith an average molecular weight of 22000 (degree of polymer-ization5 500) was dissolved in warm water (at 401–451C) andthe solution was equilibrated to room temperature.

Green sheet samples (about 80 mm� 80 mm� 3 mm) weremade using the slurry prepared by mixing 10 g of YBa2Cu3O7�xpowder (average diameter5 0.1570.10 mm) and 3 ml of 0.05mol/dm3 precursor solution (kneading time: 50–60 min). Afterbeing dried in the dark for 6 days, the sample was fired in anelectric furnace at 1223 K for 15 h in air. The heating rate wasset to about 50 K/h and the cooling rate to about 90 K/h up toabout 440 K.

The size distribution of the precursor particles in the sol so-lution was measured using a laser particle analyzer (OtsukaElectronics Co. Ltd., DLS-510T, Hirakata, Osaka, Japan). Thecrystal structure of the sample was measured by X-ray diffrac-tion (XRD) analysis (Rigaku Co. Ltd., Mini Flex, Akishima,Tokyo, Japan) using CuKa1 (l5 0.154 nm) radiation. The sur-face morphology and composition were determined using scan-ning electron microscopy (SEM) (S-3500N, Hitachi HighTechnology Co. Ltd., Tokyo, Japan). The density of the sam-ple was measured by the Archimedes method. The specific sur-face area of the sample was measured using the BET method(Micromeritic FlowSorb III 2305, Shimadzu Co. Ltd., Kyoto,Japan). The average diameter of the samples used was 4507150mm.

Electrical resistance was measured by the standard DC four-probe method using a 10 mA constant current. Measurementswere carried out in a flow cryostat operated over a temperaturerange of 50–300 K in a vacuum of 10�4 torr. The degrees ofdependence of the Jc on the magnetic field strength were eval-uated using magnetization curves (M–H curves) and the Beanequation [Jc 5 30(DMH/d), in which DMH is the width of M–Hhysteresis, and d is the sample thickness]. The M–H curves weremeasured using a vibrating sample magnetometer (VSM, ToeiIndustry Co., Ltd., VSM-5, Tokyo, Japan) at 77 K.23 The mag-netic field was varied from �1.0 T to 1.0 T (1 T5 10 kOe). Inthis VSM measurement, the magnetization of the sample ismeasured while changing the external magnetic field from 0 to1.0 T. After the demagnetization, the magnetization is measuredagain while the external magnetic field is varied from 0 to �1.0T. Therefore, the Jc value estimates by this method will besmaller than the true value, because initially (H5 0 T), the mag-netization of the sample is zero. In this work, the Jc value wasrepresented as the J value measured at H5 0.018 T.

Y O :BaCO : CuO= 0.5 : 2 : 3 (mol ratio)

Mixing

Burning(1223 K,15hr in air)

Grinding

Y(OC H ) : Ba(OC H ) : Cu(CH COO) = 1 : 2 : 3 (mol ratio) in ethanol

Reflux (348 K, 20 hr in N )

HydrolysisFiltration and Purification

Preparation of sol(0.05 mol dm )

PVA 0~5wt%(on Powder 10g)

Slurry(Mixing ratio Powder 10g : Solution 3ml)

Forming and Drying(Drying condition for Room temperature in Darkness)

Burning(1223 K,15 hr in air)

Measurements

Synthesis of YBa Cu O powder

Preparation of precursorsolution

Fig. 1. Preparation procedure for a YBa2Cu3O7�x slurry and bulksamples.

Time [hr]

Part

icle

dia

met

er [

μ m

]

0

1

2

3

4

5

6

0 50 100 150 200 250

(a)

(b)

Fig. 2. Average diameter of precursor particles as a function of time.The precursor concentration is 0.50� 10�3 mol/dm3. The PVA concen-trations are (a) 0 wt% and (b) 0.01 wt%.

March 2009 Preparation and Characterization of a Large-Scale YBa2Cu3O7�x Superconductor 579

One sintered sample was divided into 16 (or nine) large pieces(Fig. 3(c)). One large divided piece was further divided into twotest pieces. The size of the test piece is approximately 8 mm� 10mm� 3 mm. One test piece was used for the measurement of Tc,and another was used for the measurement of Jc. The Tc and Jcvalues were measured on eight (or five) test pieces for one sin-tered sample. The Tc and Jc values presented for one sinteredsample are averaged for eight or five test pieces. The valuesplotted in the figure show the relationship among Tc, Jc, andPVA concentration, and are averaged over three or five sinteredsamples prepared under the same condition.

III. Results

(1) Effect of PVA on the Stability of the Precursor Solution

To estimate the stability of the precursor solution, the particlesize was measured as a function of time using the dynamic light-scattering method. For this measurement, the original precursorsolution had to be diluted 100-fold with ultrapure water because

the highly concentrated opaque solution does not allow a accu-rate measurement. Figure 2 shows the time-dependent size of theprecursor particles measured in the solutions with 0.50� 10�3

mol/dm3 precursor and (a) 0 and (b) 0.01 wt% PVA. InFig. 2(a), in the case without PVA to act as a protective col-loid, the particle diameter rapidly increased 50 h after solutionadjustment and reached over 3 mm, which is the measurementlimit of our apparatus. This result shows that the particle diam-eter of colloid particles in the original precursor solution mayincrease at a faster rate than in the diluted solution. In general,colloid particles grow by collision between themselves and theirgrowth rate depends on their density. In the solution with PVA(Fig. 2(b)), the particle diameter remained constant at 380770nm. This result indicates that PVA acts as a protective colloid,preventing the growth of the precursor particles.

(2) Effect of PVA Addition on the Macroscopic Appearance

Figure 3 shows photographs of the bulk sheet samples preparedfrom the slurry with (a) 0 and (b) 1 wt% PVA. No bulk sheetswith side lengths 46 cm could be prepared from the slurrywithout PVA (see Fig. 3(a)), because many samples werecracked or crazed at their circumference during drying, whichresulted in large cracks after firing. Samples prepared from aslurry with 1 wt% PVA (Fig. 3(b)) showed minimal crackingafter both the drying and the firing process. These results indi-cate that crack generation is reduced by adding PVA to theslurry.

(3) Effect of PVA Concentration on the Product

The X-ray diffraction patterns of the samples prepared from theslurry with various PVA concentrations, i.e., (a) 0, (b) 1, and (c)5 wt%, are shown in Fig. 4. In Fig. 4(a), 18 diffraction peaks areobserved at 2y5 22.81, 27.71, 27.91, 30.61, 32.51, 32.81, 38.51,

(a) 6cm

8cm(b)

Tc, JcTc, JcD

Tc, JcTc, JcTc, Jc

TcTc, JcTc, JcD

D

Tc, JcTc, JcTc, Jc

(c)

Fig. 3. Photographs of the sheet samples. The samples were preparedfrom the slurry with various PVA concentrations: (a) 0 wt% and (b) 1.0wt%. (c) The divided sample for measurements of the superconductingproperties (Tc, Jc) and densities (D). The size of the divided test piece isabout 8 mm� 10 mm� 3 mm.

20 30 40 50 60 70

(a)

(b)

(c)

: Orthorhombic Y-123

(030

)

(130

)(0

31) (050

)(1

31)

(002

)

(161

)(1

32)

(260

)(070

)

(081

)

(200

)

Inte

nsity

[a.

u.]

(120

)(0

21)

(040

)

(151

)(1

60)

(241

)

2θ [degree]

Fig. 4. X-ray diffraction patterns of the sheet samples prepared from aslurry containing YBa2Cu3O7�x powder (10 g) and 0.05 mol/dm3 pre-cursor solution (3 mL) with various PVA concentrations: (a) 0 wt%, (b)1 wt%, and (c) 5 wt%. All samples were fired at 1223 K in air.

580 Journal of the American Ceramic Society—Takahashi et al. Vol. 92, No. 3

40.31, 46.51, 47.51, 51.41, 52.51, 54.91, 58.21, 58.71, 62.71, 68.11,and 68.71, corresponding to the (030), (120), (021), (040), (130),(031), (050), (131), (200), (002), (151), (160), (070), (161), (132),(241), (260), and (081) planes of orthorhombic YBa2Cu3O7�x,respectively.24 Figures 4(b) and (c) also show these 18 diffractionpeaks corresponding to orthorhombic YBa2Cu3O7�x. These re-sults indicate that adding PVA to the slurry has no marked in-fluence on the final product in X-ray resolution.

(4) SEM Photographs of the Sheet Samples

Figure 5 shows a SEM photograph of the sheet sample preparedfrom the slurry with the following PVA concentrations: (a) 0, (b)1, and (c) 5 wt%. In the SEM photograph of the sheet sampleprepared from the slurry without PVA (see Fig. 5(a)), manypores were observed on the surface. In comparison,in sampleprepared from the slurry with 1 wt% PVA, the number of poresdecreased and the particles appeared to be more agglomeratedas shown in Fig. 5(b). However, for samples with an increasedPVA concentration of 5 wt%, many large pores were observed(Fig. 5(c)). The SEM results show that the number of pores de-

creased with an increase in the concentration of PVA, reachingits minimum at 1–2 wt% the PVA, and then the number of poresincreased with an increase in the PVA concentration. Therefore,it is predicted that the maximum bulk density of a sheet samplewill be within a PVA concentration range of 1–2 wt%.

(5) Density and Specific Surface Area of the Samples

The relationship between the density and the PVA concentra-tion is shown in Fig. 6(a). The average density of the samplesgradually increased from 4.670.3 g/cm3 to about 5.4 g/cm3 withan increase in the PVA concentration. At 3 wt% PVA, the den-sity reached 5.470.4 g/cm3; this value is about 86% of the re-ported density of the bulk YBa2Cu3O7�x superconductor (6.36g/cm3).25 Above 3 wt% PVA, the density of the sample re-mained close to about 5.4 g/cm3. But the local variation of themeasured density value increased in this PVA concentrationrange.

Figure 6(b) shows the relationship between the specific sur-face area and the PVA concentration. The specific surface areaof the samples decreased from 1.3370.26 to 0.2970.06 m2/g asthe PVA concentration increased; at a PVA concentration near2 wt%, this value became a minimum. Above 2 wt% PVA, thespecific surface area increased with an increase of PVA concen-tration. These results agree with our expectation from the SEMphotographs shown in Fig. 5. The differences between the re-sults in Fig. 6(a) and the observed results are explained as fol-lows: The Archimedes method was used for the densitymeasurement. For this measurement, the water used as the me-dium could not infiltrate completely into the smallest pores inthe sample. Therefore, the measured density was not the truedensity, but the apparent density dap5W/(V1C), where W isthe mass of the sample, V is the true volume of the sample, andC is the volume of the closed and open pores in the sample. Theresulting density measurements were considerably influenced bydegrees of water infiltration into the pores and the density ob-tained by the Archimedes method is not always correct. This isnot a problem for the gas molecules (e.g., N2 and Kr) used forthe specific surface area measurement because they can com-pletely fill the open pores, although not the closed pores. Thespecific surface area measurement reflects more accurately theporous features of the samples than the Archimedes densitymeasurement. Because of this, we concluded that the optimumconcentration of PVA is in the range from 1 to 2 wt%.

(a)

(b)

(c)

Fig. 5. Scanning electron microscopic (SEM) photographs of the sheetsamples prepared from a slurry with various PVA concentrations: (a) 0wt%, (b) 1 wt%, and (c) 5 wt%. Measured samples were the same asthose used in Fig. 4. Magnification: � 1000.

PVA concentration [wt%]

Spec

ific

sur

face

are

a [m

/g]

Den

sity

[g

/cm

3]

(a)

(b)

Fig. 6. (a) PVA concentration dependence on the sample density (b)PVA concentration dependence of the specific surface area of the sam-ple.

March 2009 Preparation and Characterization of a Large-Scale YBa2Cu3O7�x Superconductor 581

(6) Effect of Adding PVA on the Superconducting Properties

Tc and transport Jc were measured for each test piece. Figure 7-1shows the temperature dependence of electrical resistance for thesamples prepared from the slurry with various PVA concentra-tions ((a) 0 wt% and (b) 1 wt%). It can be seen that, for bothsamples, the electrical resistance first decreases linearly withtemperature and then begins to decline sharply near 92 K andreaches zero near 89 K. In this case, the Tcon (onset transitiontemperature) was 92 K and Tcoff (offset transition temperature)at which the electric resistance becomes zero was 89 K. There isno visible effect of PVA addition on Tc. The effects of PVA ad-dition become evident in the normal state resistance observed inthe temperature range above Tcon. The normal state resistanceof the sample prepared from the slurry containing 1 wt% PVA issmaller than that of the sample prepared without PVA. Thisdifference can be attributed to the difference in the densities. Theresults in Fig. 6 show that the density of the sample preparedwith 1 wt% PVA is slightly larger than that of the sample pre-pared without PVA. This indicates that the effective cross sec-tion through which the current is passed for the sample preparedwith 1 wt% PVA, through which the current is flowing, is largerfor the 1 wt% PVA sample than for the sample prepared with-out PVA. Because of this the observed resistance of the sampleprepared with PVA became smaller than that of the sampleprepared without PVA. In this work, the transition temperature(Tc) of the sample is represented as Tcoff. The distribution of Tc

values in the large sample measured in Fig. 7-1(b) is shown inFig. 7-2. These results indicate that Tc values were distributed inthe temperature range from 87.0 to 90.0 K and that the averageTc of this sample was 88.571.5 K.

Figure 8 shows the dependences of Tcon (onset transitiontemperature) and Tcoff (offset transition temperature) on thePVA concentration. The Tcon are in the temperature range 92.0–91.3 K, and the average is 91.570.5 K. Tcoff are between 90.5and 86.5 K, and the average is 88.571.5 K. These observedvalues of Tcon and Tcoff were almost equal to the previously re-

ported values.26 Consequently, no clear PVA concentration de-pendence on Tcon or Tcoff was observed.

Figure 9-1 shows the dependence of current density on themagnetic flux density measured at 77 K for the samples preparedfrom the slurry with the PVA concentrations of (a) 0 wt% and(b) 1 wt%. The samples used were the same as those in Fig. 7.The current density of the sample prepared from the slurry with1 wt% PVA is larger than that of the sample without PVA forthe magnetic field range �1–1 T. Moreover, it can be seen thatwith the addition of PVA, the transport Jc increased from 370 to713 A/cm2. This Jc of 713 A/cm2 was about 35% of the reportedJc (about 2000 A/cm2) of the YBa2Cu3O7�x polycrystalline sam-ple produced by the Bridgman method.27 Using PVA improvesJc. From Fig. 9, it is also found that both samples maintainedsuperconductivity at 1.0 T. Because our study aims to synthesizea personal magnetic shield for MRI diagnosis, this result indi-cates that the samples prepared by our proposed method can beused for magnetic shielding in MRI diagnosis. Generally, themagnetic field range used by a commercial MRI equipment is0.4–1.2 T. The distribution of Jc values on the large sample usedin Fig. 9-1(b) is shown in Fig. 9-2. From this result, it is foundthat Jc values were distributed in the range from 587 to890 A/cm2, and that the average Jc of this sample was 7557135 A/cm2.

Figure 10 shows the relationship between the average Jc mea-sured at 77 K and the PVA concentration in the slurry. Thisresult shows that Jc undergoes a maximum at 1 wt% PVA. Thissuggests that the optimum PVA concentration is near 1 wt%. Itis well known that the Jc of the YBa2Cu3O7�x superconductordepends on the sample density.28 Comparision of this result withthe results in Fig. 6 shows that the increase of the Jc value ob-

Temperature [K]

0

5

10

15

50 100 150 200

(a)

(b)

Res

ista

nce

[m Ω

]

Fig. 7-1. Dependence of resistance on temperature. The samples wereprepared from a slurry with various PVA concentrations: (a) 0 wt% and(b) 1 wt%. All samples were fired at 1223 K in air.

90.089.0

89.587.587.0

89.588.0

88.487.589.488.4

Fig. 7-2. Tc distribution in the sample used in Fig. 7-1(b).

PVA concentration [wt%]

Tc

[K]

70

80

90

100

0 1 2 3 4 5 6

Fig. 8. Relationship among average Tcon (& ), average Tcoff (�), andPVA concentration in the slurry. All samples were fired at 1223 K in air.

0

100

200

300

400

500

600

700

800

–1 –0.5 0 0.5 1

Jc [

A/c

m2 ]

B [T]

(a)

(b)

Fig. 9-1. Dependence of the current density on the magnetic flux den-sity. The samples were prepared from a slurry with various PVA con-centrations: (a) 0 and (b) 1 wt%. All samples were fired at 1223 K in air.

582 Journal of the American Ceramic Society—Takahashi et al. Vol. 92, No. 3

served for the PVA concentration range of 0–1 wt% is due to theincrease in the density, and that the decrease of the Jc value ob-served in the PVA concentration range of 2–5 wt% is due to thedecrease in the density. The reason for the decrease of the sam-ple density can be explained as follows: when a dry sample con-taining excess PVA is fired, CO2 gas and H2O gas are producedabundantly and pass from the inside of the sample to the sur-face, forming numerous pores. Thus, the sample density de-creases and the surface area increases with an increase in thePVA concentration.

The maximum average Jc observed in this study was about713 A/cm2, which is much smaller than the reported maximumJc of the bulk YBa2Cu3O7�x sample (4104 A/cm2). The mainreasons why Jc is much smaller than the reported value are asfollows:

(1) The sample density is about 86 % of the theoreticaldensity (d5 6.36 g/cm3).

(2) The sample is a polycrystal in which the degree of ori-entation to the c-axis is low.

(3) The degree of oxygen deficiency is large.(4) A non-superconducting material exists among grain

boundaries.To increase Jc, the sample prepared from the slurry with 1

wt% PVA was annealed at 773 K in an oxygen atmosphere. TheJc value of the annealed sample increased from the average valueof 713–2300 A/cm2 at 77 K. This Jc (2300 A/cm2) was in agree-ment with the reported Jc (about 2000 A/cm2) of theYBa2Cu3O7�x polycrystalline sample produced by the Bridg-man method.27 These results indicate that the optimum oxygenannealing significantly increases Jc.

IV. Discussion

The purpose of this study was to establish process technologyfor preparing an inexpensive personal magnetic shield for MRIdiagnosis using a high-temperature superconducting materialsuch as YBa2Cu3O7�x. In MRI diagnosis, the shield must besuitable for the body of each patient. Shields prepared using ahigh-temperature superconductor and the usual technologieswill have very high production costs. Existing production tech-nologies require both high temperatures (41300 K) and highpressures (4several MPa), which may require an expensiveequipment. For our proposed method, multimetallic hydroxideparticles (YBa2Cu3(OH)x colloidal particles) can be transformedto oxide particles by firing at a low temperature (o1300 K) un-der atmospheric pressure to be used as a binder. This processdoes not require large expensive equipment.

In this study, we examined the effects of adding PVA to theslurry on the formability/workability and superconductive prop-erties of the samples and obtained the following results:

(1) adding PVA to the precursor solution inhibited thegrowth of the precursor particles,

(2) green sheet samples prepared from a slurry with PVAadded yielded large crack-free samples with an area more thanfourfold larger than that of the same samples prepared from aslurry without PVA,

(3) green samples prepared from a slurry containing PVAhad a surface that did not craze or generate shrinkage cracksduring drying, and

(4) samples prepared from a slurry with 1–2 wt% PVA ex-hibited markedly improved formability/workability and super-conducting properties.

From this work, the role of PVA can be explained as follows:for the preparation of bulk samples from ceramic powder, thedispersion and aggregation states of the particles and the rheo-logical properties of the slurry affect crack generation. When anorganic polymer, such as PVA or PVP, is used as a binder, thestate of the polymer in the slurry also affects crack generation.Many studies focusing on the stability or the rheological prop-erties of the slurry and the roles of the binding organic polymerhave been reported.

Figure 11 shows a schematic diagram of the roles of PVA ineach step of our process. First, PVA may stabilize colloidal par-ticles in the precursor solution. Indeed, adding PVA to the pre-cursor solution prevented the growth of colloidal particles(Fig. 2). This indicates that PVA at a low concentration worksas a dispersion agent and an emulsifying agent.

Second, for the slurry containing PVA, the solvent waterevaporation increased the PVA concentration. PVA acts as athickener and a flocculant as the concentration increases, im-proving the formability/workability of the green sheet sample.PVA easily adsorbs on the surface of inorganic materials, re-sulting in particle agglomeration.29 Therefore, on increasing thePVA concentration in the slurry, the slurry becomes more co-hesive and the density of the green sample increases.

Third, PVA forms a thin film after drying. Of particular in-terest to the cosmetic field, this PVA thin film exhibits the waterretention effect.30 In our case, the evaporation velocity of waterstrongly depends on the position on the sample surface. ThePVA concentration is higher for areas of high-velocity waterevaporation, causing a film to rapidly form on the surface. Wa-ter evaporation near the PVA thin film then decreases. Thisphenomenon regulates water evaporation, causing uniformevaporation across the sample surface. Thus, no shrinkagecracks are generated during drying and a large sample withoutlarge cracks is obtained after firing.

V. Conclusion

Large-scale YBa2Cu3O7�x superconductor samples (about 80mm� 80 mm� 3 mm) without large cracks were prepared usinga slurry consisting of YBa2Cu3O7�x particles and a sol solution

587.0630.0

790.3670.0890.3

750.5713.2886.5

880.0740.3730.1

Fig. 9-2. Distribution of critical current density (Jc: A/cm2) in the sam-ple used in Fig. 9-1(b). Jc measurement was performed at 77 K.

0

500

1000

0 1 2 3 4 5 6

Jc [

A/c

m2 ]

PVA concentration [wt%]

Fig. 10. Relationship between the average Jc (at 77 K) and the PVAconcentration in the slurry.

March 2009 Preparation and Characterization of a Large-Scale YBa2Cu3O7�x Superconductor 583

made up of multimetallic hydroxide particles (YBa2Cu3(OH)xcolloidal particles) and PVA. The effects of PVA concentrationon the formability/workability and superconducting propertieswere investigated. Using PVA markedly stabilized the colloidalparticles in the precursor solution and improved the formabil-ity/workability of the green sheet sample, so that large sampleswithout large cracks were obtained after firing at 1223 K for 15h in air. The superconducting properties of the samples pre-pared from the slurry containing 1 wt% PVA were the full-transition temperature (Tcoff) 88.571.5 K and the Jc 7137150A/cm2 at 77 K.

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PrecursorSolution

PVA

Con

cent

ratio

n in

the

slur

ry

Hig

hL

ow

Y123 particle Y123 particle

Y123 particle Y123 particle

Y123 particle Y123 particle

Y123 partcle Y123 partcle

Fig. 11. Schematic diagrams showing the roles of PVA in each step.J,precursor (YBa2Cu3(OH)x) particle;�, PVA; ) , evaporation of water.

584 Journal of the American Ceramic Society—Takahashi et al. Vol. 92, No. 3