JOURNAL OF RARE EARTHS, Vol. 26, No. 3, Jun. 2008, p. 337

Foundation item: Project supported by the National Natural Science Foundation of China (50774018)

Corresponding author: LI Ying (E-mail: liying@mail.neu.edu.cn; Tel.: +86-24-83686978)

Investigation on pumping oxygen characteristics of (Bi2O3)0.73(Y2O3)0.27 solid electrolyte

LI Ying (厉 英), WANG Changzhen (王常珍) (School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China)

Received 29 April 2007; revised 4 December 2007

Abstract: (Bi2O3)0.73(Y2O3)0.27 fine powders prepared by wet chemical precipitation method were cold isostatically pressed to form solid electrolyte tubes, and sintered at 900 ºC for 10 h in the air. Their pumping oxygen characteristics in non-dehydrated Ar gas were investigated, where a ZrO2 (Y2O3 stabilized) oxygen sensor was used to measure the oxygen partial pressure Po2. The results showed that the Po2 value reached magnitudes of 1×10–20–1×10–10 Pa at the applied pumping oxygen voltage of 0.5 V, 1×10–37–1×10–27 Pa at 1.0 V and 1×10–53–1×10–47 Pa at 2.0 V within the temperature range from 550 to 650 ºC. Moreover, no cracks were found in the tested solid electrolyte tubes. Thus, the Bi2O3-Y2O3 system might be used in solid electrolyte oxygen pump for purifying gases.

Keywords: (Bi2O3)0.73(Y2O3)0.27 solid electrolyte; oxygen sensor; oxygen pump; rare earths

Over the last decades, Bi2O3-based materials with high oxygen ionic conductivity have been extensively studied for their potential use as solid electrolyte in fuel cell. The delta face-centered cubic (fcc) phase of pure Bi2O3 has high oxide ion conduction at high temperatures, but it is only stable between 730 and 825 ºC (melting point). During cooling from high temperature, large volume change is accompanied with phase transformation from the delta phase into mono-clinic alpha phase having low electrical conductivity. This drawback has prevented it from being used as oxide ion conductor. However, the δ-Bi2O3 phase can be retained by adding some di-, tri-, and penta-valence state oxides into Bi2O3, and some stabilized high temperature phases may be produced at room temperature.

Takahashi T et al.[1–3] performed conduction measurement on various Bi2O3-based oxide ion conductors in air. For the Bi2O3-Y2O3 system, the fcc phase showed high oxygen ion conduction, and this phase with a composition of 25 mol%~43 mol% Y2O3 was stable over a wide range of tem-perature. For the Bi2O3-Gd2O3 system, high oxygen ion conduction was found in the stable rhombohedral phase with a composition of 10 mol%~30 mol% Gd2O3 below 600 °C, as well as in the stable fcc phase with a concentration higher than 35 mol% Gd2O3 up to –900 °C. For the Bi2O3-BaO system, the mixed monoclinic and rhombohedral phase, and the mixed rhombohedral and perovskite-type phase were present respectively in the composition ranges less than 20

mol% BaO and more than 28 mol% BaO, with the rhombo-hedral phase as the high oxygen ion conduction phase.

Watanabe A et al.[4] investigated the cubic-hexagonal transformation of Bi2–2xY2xO3 (x=0.215–0.235) and the de-pendence of polymorphism on the ionic radius of Ln3+ (Ln=La, Er, Y) in oxide-ion conductor Bi0.775Ln0.225O1.5. Takahashi et al.[5] reported Bi2O3-Y2O3 solid solution with an equilibrium oxygen partial pressure of 10-8 Pa at 600 ºC. Huang et al.[6] investigated Bi2O3-Y2O3 system in a compo-sition range of 15 mol%~50 mol% Y2O3 using solid elec-trolyte galvanic cell electromotive force (emf) method, and showed that the activity of Bi2O3 with 30 mol% Y2O3 was 0.084 at 598 ºC and the equilibrium oxygen partial pressure was 1×10–10–1×10–11 Pa.

To our knowledge, the Bi2O3-Y2O3 system has not found application as solid electrolyte in oxygen pump, although it is cheap to make and has high oxygen ion conductivity over a broad content of Y2O3 and a wide range of oxygen partial pressure. Here, attempt was made to investigate the charac-teristics of pumping oxygen, namely, the removal of oxygen from unpurified gas using Bi2O3-Y2O3 solid electrolyte.

Principle of oxygen pump —When two different equilib-rium oxygen pressures exist at two separated parts of the Bi2O3-Y2O3 solid electrolyte, an electrolysis cell may be formed at certain temperature. The cell type can be ex-pressed as M, PIo2 | Bi2O3(Y2O3 doped) | PIIo2, M

338 JOURNAL OF RARE EARTHS, Vol. 26, No. 3, Jun. 2008

where, PIo2 and PIIo2 (PIIo2> PIo2) represent the oxygen par-tial pressure at the anode electrode and cathode electrode, respectively. The cell reactions read Cathode electrode: O2 (PIIo2) +4e=2O2– Anode electrode: 2O2–−4e=O2 (PIo2) Overall cell: O2 (PIIo2)=O2(PIo2) (1) The reactions continue according to Eq.(1), or alternatively in the opposite direction, depending on the sign and magni-tude of the net voltage across the electrolyte. If a voltage V is applied across a certain solid electrolyte, the voltage loss owing to electrode polarization (η) is V–Eo.c–η=Iion Ωion (2) where, Eo.c. and Iion Ωion are the measured voltages of the open circuit and ohmic voltage drop, respectively.

1 Experimental

1.1 Solid electrolyte preparation

High purity Bi2O3 and Y2O3 powders with molar ratio of 0.73:0.27 were dissolved into equal molar HNO3 (A.R.) so-lution, respectively. After mixing them together, NH4OH solution was added slowly and stirred until the pH value reached 9. The white coprecipitation was filtered, washed, and dried up. It was pressed into tablets and calcined at 900 ºC for 10 h in air. XRD analysis showed that the Bi2O3-Y2O3 solid solution was formed at this stage. The obtained bisque mix-ture was finely ground (<280 mesh) and cold isostatically pressed to form open tubes with inner diameter of 10 mm, outer diameter of 12 mm, and length of 90 mm, followed by sintering at 900 ºC for 10 h in air. The solid electrolyte tubes were strong and dense after sintering.

1.2 Oxygen pump preparation

The experimental setup of the composite oxygen pump system is shown schematically in Fig.1. Ag paste was re-peatedly coated on the inner and outer surfaces of the solid electrolyte tube and dried up until the resistance of the sur-face Ag layers was less than 1 Ω. Ag wire was used as elec-

trode lead. An oxygen sensor was used to monitor continu-ously the oxygen partial pressure of exit gas. It was prepared using a ZrO2 (doped Y2O3) tube as solid electrolyte, the Cr (>99.99 wt.%)-Cr2O3 (S.P.) mixture as reference electrode, and an Ni-Cr wire as electrode lead. Two Pt/Pt-10%Rh thermocouples were placed to control the temperatures around the oxygen pump as well as the oxygen sensor with an accuracy of ±1 ºC. The outside of the sensor tube was wrapped tightly with Ag wire. The voltage to the oxygen pump was supplied by a potentiostat-galvanostatic instru-ment, and the emf of the oxygen sensor was determined with a Keithley 192 type digital voltmeter (1×1010 Ω).

1.3 Experimental procedure

To test the oxygen pump, Ar gas (Ar 99.99% with O2 3–6 µg/g, H2O 13–15 µg/g, and N2 15 µg/g) was used as the ex-perimental gas. Ar gas stream flowed through the oxygen pump, the flowrate of which was controlled by a capillary flowmeter. When voltage was applied to the pump, the emf of the sensor was measured simultaneously. The oxygen partial pressure Po2 of the reference electrode (Cr-Cr2O3 mixture) can be calculated from the following equation[7], ∆Go=−1.5RTlnPrefo2=1159760−222.80T(J) (298–2100 K)

(3) where, the unit of Prefo2 is the standard atmospheric pres-sure.

In this way, the pumping oxygen characteristics of (Bi2O3)0.73(Y2O3)0.27 tubes, such as onset temperature, the relation between the applied voltage and current, Ar gas flowrate and Po2 in Ar gas stream after pumping oxygen, applied voltage and Po2, were investigated. After the pump-ing oxygen experiments, the test tubes were examined by microprobe analysis.

2 Results and discussion

2.1 Structure and density of solid electrolyte

Fig.2 shows the XRD pattern of sintered (Bi2O3)0.73(Y2O3)0.27

Fig.1 Schematic diagram of the experimental setup for oxygen pump

LI Y et al., Investigation on pumping oxygen characteristics of (Bi2O3)0.73(Y2O3)0.27 solid electrolyte 339

Fig.2 XRD pattern of (Bi2O3)0.73(Y2O3)0.27 at room temperature

specimen. It consists of a delta face- centered cubic phase at room temperature.

The density of the specimen was measured to be 7.65 g/cm3. According to the theoretical model for Bi4(1–x)Y4xO6 2 (x=0.10~0.25, is oxygen ion defect)[8], the theoretical den-sity was about 8.029 g/cm3. Thus, the experimental density reached 95.3% relative to the theoretical density.

2.2 Characteristics of oxygen pump

EMF measurements revealed that the onset temperature of the oxygen pump was 520 ºC. Therefore, a temperature range of 550–700 ºC was chosen for the present investiga-tion.

Fig.3 showed the variations of the pumping oxygen cur-rent (Ipump) with applied voltage of pumping oxygen at an Ar gas flowrate of 150 ml/min. Ipump increased with increasing applied voltage of pumping oxygen, and this tendency be-came more pronounced at higher temperatures. It indicated that the conductivity of oxygen ion in the solid electrolyte was enhanced with increasing temperature.

Fig.4 presents the plots of lgPo2 of Ar gas (at a flowrate of 150 ml/min) vs. applied voltage of pumping oxygen. With increasing applied voltage of pumping oxygen, Po2

decreases at a given temperature. Under the same applied

Fig.3 Vpump vs. Ipump at Ar gas flowrate of 150 ml/min

voltage of oxygen, there is little increase in Po2 with the in-crease of temperature, which indicates that oxygen pumping is more effective at lower temperatures. Moreover, electrode polarization and leaking oxygen may be considered as the limiting process for pumping oxygen at very low Po2 and higher temperatures.

Fig.5 shows the plot of Ar gas flowrate vs lgPo2 at 1.0 V and 600 ºC. The pumping oxygen performance was almost unaffected by the test flowrate. This may be caused by the permeability of oxygen at higher Vpump and higher tempera-tures[9].

It was found that the pumping oxygen velocity of (Bi2O3)0.73(Y2O3)0.27 tube was very fast. When Po2 of Ar gas was considerably lower than the equilibrium oxygen pres-sure of the solid electrolyte, the color of the tested tubes gradually became pale, but no cracks appeared. After the pumping oxygen experiments, the tubes were as strong as before. Thus, (Bi2O3)0.73(Y2O3)0.27 can be used in oxygen pump for purifying gases. ZrO2 (Y2O3 doped) is also a better oxygen pumping material[10]. However, the sintering tem-perature range of ZrO2 (Y2O3 doped) is from 1400 ºC to 1500 ºC[11,12], which is considerably higher than that of the sintering temperature of Bi2O3 (Y2O3 doped). Bi2O3 (Y2O3 doped) can be made very easily in laboratories.

Fig.4 Vpump vs lgPo2 at Ar gas flowrate of 150 ml/min

Fig.5 Ar gas flowrate VAr vs lgPo2 at 1.0 V and 600 ºC

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3 Conclusion

(Bi2O3)0.73(Y2O3)0.27 solid electrolyte tubes were prepared and assembled in an oxygen pump device to remove oxygen from non-dehydrated argon gas. The results showed that the onset temperature of the oxygen pump was 520 ºC, and the conductivity of oxygen ion in the solid electrolyte increased with increasing temperature. The oxygen partial pressure Po2 of argon gas reached the magnitudes of 1×10–10–1×10–20 Pa at the applied voltage of pumping oxygen of 0.5 V, 1×10–27–1×10–37 Pa at 1.0 V, and 10–47–10–53 Pa at 2.0 V within the temperature range from 550 to 650 ºC. After pumping oxygen, no cracks were observed in the tested solid electrolyte tubes. The experimental study demonstrated that the desired Po2 values could be obtained by well-con-trolled oxygen pump device. Thus, it highlighted the per-spective of (Bi2O3)0.73(Y2O3)0.27 solid electrolyte for use in oxygen pump to purify gases or for some other purposes.

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