Conclusions

The findings of this study are summarized as follows

1. High density, high conductivity yttrium doped barium zirconate can be reproducibly obtained under the processing conditions described. Specifically, the particle size of the starting powder must be less than 100 nm, the green body must be prepared using a binder so as to achieve high green density, and the sintering must be carried out under excess barium so as to avoid barium loss. Sintering under oxygen also appears important, but this point requires further investigation.

2. The total conductivity of BaZr0.8Y0.2O3-δ at 600 °C under humidified nitrogen with pH2O = 0.031 atm is 7.9 × 10-3 S/cm, where, total refers to resistance contributions of both the bulk (grain interior) and grain boundary regions of the material.

3. In the absence of excess barium, the high temperature sintering conditions required to achieve good densification of barium zirconate induce measurable changes in stoichiometry and dramatic reductions in proton conductivity. Specifically, barium deficiency was shown to lower the conductivity by two orders of magnitude and it is believed that stoichiometry differences induced by severe processing conditions are the major contributor to the wide variation in literature data for the conductivity of yttrium doped barium zirconate.

3.6 References

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2. A. Manthiram, J. F. Kuo, and J. B. Goodenough. Characterization of oxygen-deficient perovskites as oxide-ion electrolytes. Solid State Ionics62, 225 (1993).

3. R. C. T. Slade, S. D. Flint, and N. Singh. Investigation of protonic conduction in Yb- and Y- doped barium zirconates. Solid State Ionics82, 135 (1995).

4. H.G. Bohn and T. Schober. Electrical conductivity of the high-temperature proton conductor BaZr0.9Y0.1O2.95. J. Am. Ceram. Soc.83, 768 (2000).

5. K. Katahira, Y. Kohchi, T. Shimura, and H. Iwahara. Protonic conduction in Zr-substituted BaCeO3. Solid State Ionics138, 91 (2000).

6. V. P. Gorelov, V. B. Balakireva, Y. N. Kleshchev, and V. P. Brusentsov. Preparation and electrical conductivity of BaZr1-xRxO3 (R = Sc, Y, Ho, Dy, Gd, In). Inorganic Materials37, 535 (2001).

7. M. Laidoudi, I. Abu Talib, and R. Omar. Investigation of the bulk conductivity of BaZr0.95M0.05O3 (M=Al, Er, Ho, Tm, Yb and Y) under wet N2. J. Phys. D: Appl. Phys. 35, 397 (2002).

8. K. D. Kreuer. Proton-conducting oxides. Annu. Rev. Mater. Res.33, 333 (2003).

9. F. M. M. Snijkers, A. Buekenhoudt, J. Cooymans, and J. J. Luyten. Proton conductivity and phase composition in BaZr0.9Y0.1O3-delta. Scripta Mater.50, 655 (2004).

10. W. S. Wang and A. V. Virkar. Ionic and electron-hole conduction in BaZr0.93Y0.07O3 by 4- Probe D.C. measurements. J. Power Sources142, 1 (2005).

11. C. D. Savaniu , J. Canales-Vazquez, and J. T. S. Irvine. Investigation of proton conducting BaZr0.9Y0.1O2.95 : BaCe0.9Y0.1O2.95 core-shell structures. J. Mater. Chem.15, 598 (2005).

12. F. Iguchi, T. Yamada, N. Sata, T. Tsurui, and H. Yugami. The influence of grain structures on the electrical conductivity of a BaZr0.95Y0.05O3 proton conductor. Solid State Ionics (press 2006).

13. K. D. Kreuer. Aspects of the formation and mobility of protonic charge carriers and the stability of perovskite-type oxides. Solid State Ionics125, 285 (1999).

14. P. Babilo and S. M. Haile. Enhanced sintering of yttrium-doped barium zirconate by addition of ZnO. J. Am. Ceram. Soc.88, 2362 (2005).

15. A. Magrez and T. Schober. Preparation, sintering, and water incorporation of proton conducting Ba0.99Zr0.8Y0.2O3: Comparison between three different synthesis techniques. Solid

State Ionics175, 585 (2004).

16. G. Baldinozzi, J. F. Berar, and G. Calvarin. Rietveld refinement of two-phase Zr-doped Y2O3.

Mater. Sci. Forum278-2, 680 (1998).

17. D. Shima and S. M. Haile. The influence of cation non-stoichiometry on the properties of undoped and Gadolinia-doped barium cerate,” Solid State Ionics97, 443 (1997).

18. J. Wu, L. P. Li, W. T. P. Espinosa, and S. M. Haile. Defect chemistry and transport properties of BaxCe0.85M0.15O3. J. Mater. Res.19, 2366 (2004).

19. J. Wu, S. M. Webb, S. Brennan, and S. M. Haile. Dopant site selectivity in BaCe0.85M0.15O3 by extended x-ray absorption fine structure. J. Appl. Phys.97, 054101 (2005).

20. S. M. Haile, D. L. West, and J. Campbell. The role of microstructure and processing on the proton conducting properties of Gadolinium-doped barium cerate. J. Mater. Res.13, 1576 (1998). 21. X. Guo and R. Waser. Space charge concept for acceptor-doped zirconia and ceria and experimental evidences. Solid State Ionics173, 63 (2004).

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Chapter 4: Microstructure and conductivity of the BaZr

Y

O

system,

where x = 0.2, 0.3, 0.4

4.1 Introduction

The inability to obtain high quality data due to processing and nonstoichiometry challenges for the barium zirconate system has not allowed for the material to be fully optimized in terms of microstructure, composition, or defect structure. Chapter 3 established the methodology to produce such high conductivity samples and that approach will be utilized to further characterize BYZ material properties.

Recent researchers have attempted to optimize the grain structure by elongated sintering periods. Iguchi et al.reported average grain size of 1.61 µm after 200 hrs of sintering at 1800 ºC [1]. Although larger grains were obtained, considerably decreasing the grain boundary density, conductivities of the samples were 2–3 orders of magnitude lower than samples prepared with shorter sintering times. Even though the authors state that chemical compositions were not affected by sintering conditions, in such extreme processing conditions, the loss of Ba from the perovskite structure is inevitable.

An alternative approach to improving the performance of doped barium zirconate system is to significantly increase the proton concentration by the introduction of additional dopant. This methodology has raised conflicting opinions on its effectiveness. Neutron diffraction studies by Hempelmann [2] revealed the introduction of dopant ions caused negatively charged defects that act as proton traps. Scherban and Nowick [3] illustrate, by analysis of preexponential terms in the conductivity of Yb doped SrCeO3, that only a fraction of all protons dissolved are mobile carriers. In addition, Davies and Islam [4] completed a series of calculations to derive binding energies of defect clusters. Their simulations indicated proton-dopant associations may occur. In stark contrast, Kreuer [5] suggests that the observed increase in activation energy for the protonic

defect with increased dopant concentration may be related to the general increase in oxygen basicity and the barrier for proton transfer rather than proton-dopant association.

In this chapter we examine the electrical properties of samples prepared with varying grain sizes to clarify the role of grain boundary contributions to the total conductivity. Further, we explore dopant concentration influence on stability, proton concentration, and mobility.