The effect of ball-milling solvent on the decomposition properties of Ba(Pb1-xBix)O-3

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Materials Chemistry and Physics 69 (2001) 226–229

The effect of ball-milling solvent on the decomposition

properties of Ba

(Pb

1 −x

Bi

x

)O

3 M.C. Chang

a,∗

, J.M. Wu

a

, S.Y. Cheng

b

, S.Y. Chen

c

a_{Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC} b_{Materials Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan, ROC}

c_{Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC}

Received 28 January 2000; received in revised form 7 July 2000; accepted 14 July 2000

Abstract

Ba(Pb1−xBi_x)O3compound, which exhibits perovskite structure is a superconducting material. It can be a useful material for resistor when x value equals 0. The conducting behavior of BaPbO3is liable to be affected during aqueous processing. In contrast, the phenomenon is not found in Ba(Pb0_.8Bi0_.2)O3. The milling solvents of water and ethanol have different effect on the stability of perovskite phase. The decomposition of BaPbO3into BaCO3 and PbO2is found when CO32−ion exists. The partial substitution of Bi3+ stabilizes the perovskite phase. The stabilization and decomposition of perovskite phase explain the resistivity change with different milling solvents. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Ba(Pb1−xBi_x)O3; Perovskite; Solvent; Decomposition

1. Introduction

The structure of BaPbO3is cubic perovskite at room tem-perature which was reported by Hoppe and Blinne [1], Wag-ner and Binder [2,3], and Nitta et al. [4], who determined the lattice parametera = 4.27, 4.265 and 4.267 Å, respec-tively. Shannon and Bierstedt [5] reported that BaPbO3was found to be pseudocubic, with a unit cell and space group similar to those of the orthorhombic perovskites CaTiO3, CdSnO3and SrZrO3. Despite the fact that this is a normal valence compound, BaPbO3 exhibits metallic conduction, with a room-temperature resistivity of 8.3 × 10−4 cm and a positive temperature coefficient from 70–420 K [4,6]. Wagner and Binder [2,3] found that BaPbO3 was formed below 1000◦C and decomposed into Ba2PbO4 and other compounds above 1000◦C. BaPbO3 ceramics are useful as a resistor of small temperature coefficient in resistivity [6]. Sleight et al. [7] first discovered superconductivity in Ba(Pb1−xBix)O3 while studying the metal–semiconductor transition in the pseudo-binary system BaPbO3–BaBiO3. Ba(Pb1−xBix)O3 was the first high temperature super-conductor found which contains no transition metals. The semiconductor–metal transition occurred at x = 0.35, with Tc decreasing from 13 K at this composition

∗_{Corresponding author.}

E-mail address: [email protected] (M.C. Chang).

to 9 K at x = 0.05. Khan et al. [8] have investi-gated a semiconductor–metal phase transition in the Ba(Pb1−xBix)O3 system with X-ray diffraction, mag-netic induction and infrared absorption. Superconducting Ba(Pb1−xBix)O3 can also be prepared by RF sputtering. Gilbert et al. [9] reported that a superconducting film was ob-tained by annealing the amorphous film made by sputtering. In the present experiment, we study the effect of ball-milling solvent (water and ethanol) on the electric properties of Ba(Pb1−xBix)O3and the stability of BaPbO3 phase. Water and ethanol are the popular solvents for ball-milling and grinding in fabricating ceramics. From the results of this experiment, use of different solvents (water and ethanol) will result in different electric proper-ties of BaPbO3 ceramics. The phenomenon is connected with the decomposition of BaPbO3 phase. In contrast, the phenomenon is not found in Ba(Pb0.8Bi0.2)O3.

2. Experimental procedures

Raw materials of BaCO3, PbO2 and Bi2O3 with purity higher than 99.5% were used. Both BaPbO3 (composition A) and Ba(Pb0_.8Bi0_.2)O3(composition B) were weighed ac-cording to the composition formula with an extra 5 mol% PbO2added for each composition. Milling process was con-ducted by yittrium stabilized ZrO2balls with either water or 0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.

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M.C. Chang et al. / Materials Chemistry and Physics 69 (2001) 226–229 227

Fig. 1. XRD patterns of compositions A and B after calcining at 850◦C for 2 h: composition A using (a) water and (b) ethanol as milling solvent, composition B using (c) water and (d) ethanol as milling solvent.

ethanol for 20 h. The mixture was dried and calcined in cov-ered Al2O3 crucible and O2 atmosphere at 850◦C for 2 h. The calcined body was ground into powder by ball milling with solvent of water or ethanol. The purity of the ethanol is 95%. After drying, disk type sample of 5 mm diameter and 4 mm thickness was formed by pressing with 50 MPa pres-sure. The sample was sintered in a closed Al2O3 crucible at 900–1100◦C for 30 min. Ohmic contact was made with Ag paste (SR5083, Namics Corporation, Japan) which was fired at 580◦C for 10 min. Multimeter,1 XRD2 and SEM3 were used for resistance measurement, phase identification and microstructure observation, respectively.

3. Results and discussion

3.1. The effects of ball-milling solvent

Perovskite phase was formed after calcining at 850◦C for 2 h for both composition A (BaPbO3) and B (Ba(Pb0.8Bi0.2) O3), as shown in Fig. 1, where either water or ethanol was chosen as milling solvent. The resistivity of sintered body is shown in Fig. 2 for both compositions. The sintering con-ditions were at 900–1100◦C for 30 min. The resistivity of composition B remains almost unchanged despite the kind of solvent used. In contrast, the resistivity of composition A increases to a relatively high value when water is used as milling solvent. Furthermore, the pressed disk melted when sintering was done at temperatures higher than 1050◦C. These results indicate that the use of water produce phase

1_{34401A Multimeter, Hewlett-Packard Company, USA.}

2_{PW1700, Philips Electronic Instruments Co., Eindhoven, Netherlands.} 3_{Cam Scan, Cambridge Scanning Co., Cambridge, UK.}

Fig. 2. The resistivity of sintered body for both compositions with different milling solvent (water and ethanol).

changes in composition A. The evidence can be found in the XRD patterns of specimens sintered at 1000◦C for 0.5 h (Fig. 3). Composition B exhibits pure perovskite phase no matter what milling solvent was used. However, composition A possesses PbO and BaCO3phases in addition to perovskite when water was used as milling solvent. The existence of these second phases may explain the increase of resistivity as shown in Fig. 2. Fig. 4 shows the microstructures of speci-mens sintered at 1000◦C for 0.5 h. The second phases render different microstructures as observed in Fig. 4(a) and Fig. 4(b). The EDS results of Fig. 4(a) are shown in Fig. 5(a) and Fig. 5(b). Fig. 5(a) is the EDS peaks of the segregation of PbO (point P) in Fig. 4(a), and Fig. 5(b) is the EDS peaks of matrix (point M) in Fig. 4(a). The PbO is liable to segregate when water is used. This phenomenon demonstrates that wa-ter has certain effect on the phase formation and electrical conductivity of composition A. The effect cannot be found when Pb is partially substituted by Bi (the composition B).

Fig. 3. XRD results of both compositions sintered at 1000◦C for 0.5 h: composition A using (a) water and (b) ethanol as milling solvent, com-position B using (c) water and (d) ethanol as milling solvent.

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228 M.C. Chang et al. / Materials Chemistry and Physics 69 (2001) 226–229

Fig. 4. Microstructures of both compositions sintered at 1000◦C for 0.5 h: composition A using (a) water and (b) ethanol as milling solvent, composition B using (c) water and (d) ethanol as milling solvent.

Fig. 5. (a) The EDS peaks of the segregation of PbO (point P) in Fig. 4(a); (b) the EDS peaks of matrix (point M) in Fig. 4(a).

After ball-milling in water, the calcined powder of com-position A which has pure perovskite phase (Fig. 1) was changed to a combination of perovskite BaPbO3, BaCO3 and PbO2 as shown in Fig. 6(a). The decomposition of BaPbO3results in the high resistivity of the corresponding sintered specimens. In addition, the resulting PbO2 phase

Fig. 6. XRD patterns of calcined powder after ball-milling with different solvent: composition A using (a) water and (b) ethanol as milling solvent, composition B using (c) water and (d) ethanol as milling solvent.

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M.C. Chang et al. / Materials Chemistry and Physics 69 (2001) 226–229 229 in the specimen transforms into PbO phase during

sinter-ing, which explains the melting of the sintered specimen when the sintering temperature is higher than 1050◦C. The crystallization of PbO phase and its segregation on the sur-face after sintering causes the microstructure as shown in Fig. 4(a). When ethanol was used as milling solvent, no second phase was observed as was shown in Fig. 6(b). In the case of composition B, both water and ethanol solvent produce the same results. No obvious decomposition of per-ovskite phase was found. This implies that the substitution of Bi into Pb site stabilizes the perovskite crystal structure.

3.2. The decomposition of BaPbO3

Phase decomposition affects the properties of BaPbO3. It is important to study the decomposition of BaPbO3 in order to obtain good conductivity material. Massicot PbO is a metastable phase which can transform into the stable litharge phase during either ball-milling or uniaxial grind-ing [10,11]. Accordgrind-ing to the experimental results, use of ethanol as milling solvent during the calcined powder grinding does not affect the BaPbO3 perovskite phase in the sintered bodies. In contrast, if water was used as milling solvent in grinding the calcined powder the sintered bodies consist of BaCO3, PbO and BaPbO3 phases. It can then be concluded that water plays an important role on the decomposition of BaPbO3into BaCO3and PbO2.

To elucidate the decomposition process, three experiments were conducted. First, BaPbO3powder was placed in deion-ized water that had been boiled for 2 h and then cooled down. The mixture of water and BaPbO3powder was kept in vacuum for one month. After drying at 120◦C, the crys-tal phase of the treated powder was determined using XRD analysis. The reason for boiling deionized water for 2 h and keeping the water in vacuum is to dry out CO2and to main-tain the near zero CO2content in the water. The XRD result (Fig. 7(a)) shows that only the BaPbO3 perovskite phase exists in the powder.

Secondly, BaPbO3 powder was kept in CO2 atmosphere (about 1 atm) at 120◦C for one month. The XRD result shown in Fig. 7(b) also exhibits only one BaPbO3 phase. Thirdly, BaPbO3powder was soaked in deionized water for two weeks in open air. During the soaking period, forma-tion of white powder was observed. The crystal phases of the soaked powder were identified to be BaPbO3, PbO2and BaCO3(Fig. 7(c)).

Based on these three experiments, coexistence of the water and CO2 is concluded to be the reason of BaPbO3 phase decomposition. Existence of either water alone or CO2alone did not produce decomposition of BaPbO3. The decomposition may be expressed by the following reactions: H2O+ CO2→ 2H++ CO32−

BaPbO3+ 2H++ CO32−→ BaCO3+ PbO2+ H2O

Fig. 7. (a) XRD pattern of BaPbO3 after keeping in preheated water in

vacuum for one month; (b) XRD pattern of BaPbO3after keeping in CO2

atmosphere (about 1 atm) at 120◦C for one month; (c) XRD pattern of BaPbO3 after keeping in water for two weeks in air.

4. Conclusions

1. The resistivity of BaPbO3 changes when water not ethanol is used as milling solvent. The partial decom-position of BaPbO3is the reason of the change of prop-erties.

2. Ba(Pb0.8Bi0.2)O3 shows no change of the properties when either water or ethanol is used as milling solvent. The partial substitution of Bi into Pb site stabilizes the perovskite phase.

3. The decomposition of BaPbO3is attributed to the co-existence of water and CO2. Either water or CO2alone will not produce the decomposition of the BaPbO3 phase.

References

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(1965) 642.

[5] R.D. Shannon, D.E. Bierstedt, J. Am. Ceram. Soc. 53 (1970) 635. [6] H. Ikushima, S. Hayakawa, Solid-State Electron. 9 (1966) 921. [7] A.W. Sleight, J.L. Gillson, P.E. Bierstedt, Solid State Commun. 17

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[8] Y. Khan, K. Nahm, M. Rosenberg, H. Willner, Phys. Stat. Sol. (b) 39 (1977) 79.

[9] L.R. Gilbert, R. Messier, R. Roy, Thin Solid Films 54 (1978) 129. [10] M. Senna, H. Kuno, J. Am. Ceram. Soc. 54 (1971) 259.

[11] D. Lewis, K.O. Northwood, R.C. Reeve, J. Appl. Cryst. 2 (1969) 156.