Oxygen-Ion Diffusion in a 110 K Phase BiPbSrCaCuO Superconductor

Wen Zhu,† Chu Kun Kuo, and Patrick S. Nicholson*,**

Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7

The oxygen-ion conductivity of a hot-pressed 110 K phase(2223) superconductor was determined by using an O2,Pt/ZrO 2/110 K phase/ZrO2/Pt,O2 cell and impedance spectros-copy at temperature of 600°–800°C and oxygen partialpressures (PO2

) of 0.21–0.001 atm (∼21–0.1 kPa). The oxy-gen-ion diffusivities were calculated via the Nerst–Ein-stein equation and were greater in air than atPO2

= 0.001atm. Decomposition of the 110 K phase at 800°C andPO2

=0.001 atm increased the bulk diffusivity but had a lessereffect on the grain-boundary diffusivities. The activationenergies for oxygen diffusion, at differentPO2

values, havebeen determined.

I. Introduction

THREE superconducting phases (20 K, 80 K, and 110 Kphases) have been identified in the Bi(Pb)SrCaCuO sys-

tem. The stability, superconductivity, and formation kinetics ofthese phases are closely related to their oxygen stoichiome-try1–6 and are dependent on the oxygen pressure and tempera-ture conditions of preparation.7,8 Optimum temperatures (T)and oxygen partial pressures (PO2

) to promote the synthesis ofthe 80 K and 110 K phases have been determined. Oxygendiffusivity in the phases is important to understand oxygentransport for material preparation, to optimize the oxygen con-tent and to improve the superconducting properties. Few stud-ies have been published on oxygen diffusion in the Bi(Pb)Sr-CaCuO superconductor system. Turrillaset al.9 and Vischjageret al.10 used an electrochemical method to study the self-diffusion coefficient of oxygen ions in the BiSrCaCuO super-conductor at 500° and 550°C. Zhou and co-workers11,12inves-tigated oxygen diffusion in the Bi2Sr2Ca2Cu3Ox (Tc 4 110 K,2223) superconductor phase viain situ resistance measure-ments (237.5# T # 333°C) but gave no diffusivity data.Runde and co-workers13,14determined the oxygen diffusivitiesin the Bi2Sr2CaCu2Oy (Tc 4 80 K, 2212) and Bi2Sr2CuOy(Tc 4 20 K, 2201) phases via oxygen tracer measurementsin pure oxygen. No oxygen diffusivities have been reportedfor the 110 K phase at highT and lowPO2

values.Zhu and Nicholson15 used a solid-electrolyte cell—

O2,Pt/ZrO2/80 K phase/ZrO2/Pt,O2—to measure the oxygen-ion conductivity of the 80 K phase. In this cell, ZrO2 acts as anelectron-blocking, oxygen-ion-transporting electrode. The cal-culated diffusivities compared well with values in the literaturefor a lead-doped phase9 but were higher than those reported forlead-free compositions.13 The increased oxygen diffusivity thatwas associated with the lead dopant was attributed to an in-crease of defect concentration when the valence state of leadchanged at high temperatures. In the present investigation, thesolid-electrolyte cell method is extended to the BiSrCaPbCuO

110 K phase. Oxygen-ion transport behavior is reported for the110 K phase at 600°–800°C atPO2

values of 0.21 and 0.001atm.

II. Experimental Procedure

The 110 K phase with the nominal composition Bi1.84Pb0.34-Sr1.91Ca2.03Cu3.06Ow was prepared via a solid-state reaction ofnitrates (the synthesis procedures of which are described else-where).16 Pellets of 110 K powder (1 in. in diameter and∼1 in.high) were cold isostatically pressed at∼290 MPa and then hotpressed in air at 830°C for 40 min at∼30 MPa. The heat-ing/cooling rate was 2°C/min. No phase decomposition wasdetected via X-ray diffraction (XRD) analysis. The density ofthe hot-pressed pellets was∼97% of the theoretical density(6.29 g/cm3).

The cell that was used to measure the oxygen-ion conduc-tivity consisted of two 8-mol%-Y2O3-stabilized ZrO2 (YSZ)pellets∼0.9 mm thick with an electrode of porous platinum onone side. The superconductor sample (6–8 mm thick) wasplaced between the YSZ pellets, and all the contact surfaceswere well-polished. The sandwich assembly was spring loadedin a quartz tube and introduced into a furnace.15 The ac im-pedance was measured (Model HP 4192A LF impedance ana-lyzer, Hewlett–Packard, Palo Alto, CA) over the frequencyrange of 5 Hz–13 MHz, using 50°C steps at 600°–800°C atPO2values of 0.21–0.001 atm (∼21–0.1 kPa). The impedance of anO2,Pt/YSZ/Pt,O2 cell was determined over the same tempera-ture range, to distinguish the impedance contribution of theZrO2 electrodes from that of the cell. Low oxygen pressure wasacquired using oxygen/argon gas mixtures. The measurementtemperature and oxygen pressure range fell in the electrolyticdomain of the Y2O3–ZrO2 electrolyte.17

III. Results and Discussion

Impedance spectra for a cell with a 110 K phase sample atPO2

4 0.21 atm are shown as solid circles in Figs. 1(a)–(e);the frequency increases from right to left. Three impedancecomponents can be discerned in the 600°–650°C impedancespectra; at high frequency, these components disappear withincreasing temperature. The impedance spectra that were mea-sured atPO2

4 0.001 atm vary in a similar fashion but havehigher values (Figs. 2(a)–(e)). The high-frequency arcs in thiscase also disappear at higher temperature (800°C), althoughthey are more pronounced.

The use of ZrO2 as an electronic-blocking oxygen-ion con-ductor has been verified for superconducting systems.9,15,18–20

As in prior work, the multicomponent impedance of the cellwas analyzed via a serial-model-equivalent circuit that con-sisted of resistor//capacitor parallel pairs.15 The low-frequencyarc is attributed to the interface of Pt/YSZ electrodes as well asthe electrode/superconductor. The arc at intermediate frequen-cies is attributed to the grain boundaries of the 110 K phase andthe YSZ. The high-frequency arc is considered to be the acresponse of the grains of 110 K phase and ZrO2, because oftheir high conductance and low capacitance.15,21

The impedance of the 110 K phase pellet was estimated bysubtracting the impedance components of the ZrO2 electrodes

J. D. Cawley—contributing editor

Manuscript No. 191004. Received May 15, 1997; approved March 8, 1999.*Member, American Ceramic Society.**Fellow, American Ceramic Society.†Now with IREQ, Hydro Que´bec, Varennes, QC, Canada J3X 1S1.

J. Am. Ceram. Soc., 82 [6] 1617–20 (1999)Journal

1617

Fig. 1. Impedance spectra of the O2,Pt/ZrO2/110 K phase/ZrO2/Pt,O2 cell (d) before and (s) after subtracting the impedance com-ponent from ZrO2 electrodes (PO2

4 0.21 atm).

Fig. 2. Impedance spectra of the O2,Pt/ZrO2/110 K phase/ZrO2/Pt,O2 cell (d) before and (s) after subtracting the impedance com-ponent from ZrO2 electrodes (PO2

4 0.001 atm). In Fig. 2(a), the solidcircles and open circles overlap; in Fig. 2(b), the two circles do notoverlap only at high frequency.

1618 Communications of the American Ceramic Society Vol. 82, No. 6

from the total impedance. Impedance spectra for the 110 Kpellet obtained thereby are shown as open circles in Figs. 1 and2. The impedance of the YSZ electrodes is trivial, compared tothe 110 K phase, and has a negligible effect on the spectra atlow temperatures. This contribution became appreciable onlyfor temperatures >750°C. The observed low-frequency “tail”can be attributed to the response of the YSZ/110 K interface.This response was not detected in the YSZ/80 K phase cell.The grain and grain-boundary conductivities of the 110 Kphase were derived from the high- and medium-frequency arcs(open circles), and the corresponding diffusivities are calcu-lated from the Nernst–Einstein equation;22 i.e.,

D =RTs

F2|Zi|

wheres is the ionic conductivity (in units of (Vzcm)−1), F theFaraday constant (96485 C), |Zi| the charge number,R the gas

constant (8.314 JzKzmol)−1), T the temperature (in Kelvin), andD the diffusivity (in units of cm2/s).

The oxygen-ion diffusivities for the grains and grain bound-aries of the 110 K phase forPO2

values of 0.21 and 0.001 atmare listed in Table I. The oxygen diffusivities (PO2

4 0.21 atm)versus temperature fit a straight line (Fig. 3(a)), and the acti-vation energies for the grains and grain boundaries (Eg andEgb,respectively) are 1.48 and 1.87 eV, respectively. The diffusiv-ity and activation-energy values are similar to those for the 80K phase at <800°C, which suggests that a similar oxygen-transport mechanism is operating on both phases. This result isnot unexpected, considering the polytypical structure of thetwo crystals. An abrupt change of grain diffusivity is observedat 800°C atPO2

4 0.001 atm (Fig. 3(b)). This change is asso-ciated with the decomposition of 110 K phase at lowPO2

.23–25

In a previous study,16 the 20 K phase was the major decom-position product, with a minor amount of Cu2O and other un-identified phases. The decomposition products of the undoped110 K phase were Bi2Sr2CaO6+x, Cu2O, and Ca2−xSrxCuO3 atPO2

values below the 110 K and 80 K phase-stability limits.24

MacManus-Driscoll and coworkers25 reported that the stabilityranges of the two phases for lead-doped and undoped 110 Kphase almost overlapped atT > 750°C. The 110 K phase at800°C andPO2

4 0.001 atm decomposed to (Ca,Sr)2CuO3 andliquid, from which Bi2Sr2CuO6 crystallized. The dramatic in-crease of diffusivity at 800°C suggests that oxygen diffusesmuch more readily in the decomposition products. Therefore,the 800°C value is not included in the activation-energy cal-culation. The grain-boundary diffusivity does not appear to beinfluenced by the decomposition, probably because the struc-ture of the grain boundary is only weakly dependent on thestructure of the bulk grains.

IV. Summary

Oxygen-ion diffusion coefficients in the 110 K supercon-ducting phase have been determined via impedance measure-ments, using a 110 K phase/Y2O3–ZrO2 solid-state cell attemperatures of 600°–800°C and oxygen partial pressures (PO2

)of 0.21 and 0.001 atm. The diffusivities were greater atPO2

40.21 atm than atPO2

4 0.001 atm for the temperature inter-val that was studied. The decomposition of the 110 K phaseat 800°C andPO2

4 0.001 atm increased the grain diffusivitysignificantly. The activation energies for intragranular and in-tergranular diffusion are 1.48 and 1.87 eV forPO2

4 0.21 atmand 0.75 and 3.03 eV forPO2

4 0.001 atm.

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