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Research Papers

Phase Stability and Sintering Behavior of 10mol%Sc2O31mol%CeO2ZrO2 Ceramics

[+] Author and Article Information
Sergey Yarmolenko, Jag Sankar

Department of Mechanical Engineering, Center for Advanced Materials and Smart Structures, North Carolina A&T State University, 1601 E. Market Street, 242 IRC Building, Greensboro, NC 27411

Nicholas Bernier, Michael Klimov, Jay Kapat

Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32816-2450

Nina Orlovskaya1

Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32816-2450norlovsk@mail.ucf.edu

1

Corresponding author.

J. Fuel Cell Sci. Technol 6(2), 021007 (Feb 24, 2009) (8 pages) doi:10.1115/1.2971126 History: Received June 13, 2007; Revised February 19, 2008; Published February 24, 2009

The phase composition and sintering behavior of two commercially available 10mol%Sc2O31mol%CeO2ZrO2 ceramics produced by Daiichi Kigenso Kagaku Kogyo (DKKK) and Praxair have been studied. DKKK powders have been manufactured using a wet coprecipitation chemical route, and Praxair powders have been produced by spray pyrolysis. The morphology of the powders, as studied by scanning electron microscopy, has been very different. DKKK powders were presented as soft (100μm) spherical agglomerates containing 60100nm crystalline particles, whereas the Praxair powders were presented as sintered platelet agglomerates, up to 30μm long and 34μm thick, which consisted of smaller 100200nm crystalline particles. X-ray diffraction analysis has shown that both DKKK and Praxair powders contained a mixture of cubic (c) and rhombohedral (r) phases: 79% cubic +21% rhombohedral for DKKK powders and 88% cubic +12% rhombohedral for Praxair powders. Higher quantities of the Si impurity level have been detected in Praxair powder as compared to DKKK powder by secondary ion mass spectroscopy. The morphological features, along with differences in composition and the impurity level of both powders, resulted in significantly different sintering behaviors. The DKKK powders showed a more active sintering behavior than of Praxair powders, reaching 93–95% of theoretical density when sintered at 1300°C for 2h. Comparatively, the Praxair powders required high sintering temperatures at 15001600°C. However, even at such high sintering temperatures, a significant amount of porosity was observed. Both DKKK and Praxair ceramics sintered at 1300°C or above exist in a cubic phase at room temperature. However, if sintered at 1100°C and 1200°C, the DKKK ceramics exist in a rhombohedral phase at room temperature. The DKKK ceramics sintered at 1300°C or above exhibit cubic to rhombohedral and back to cubic phase transitions upon heating at a 300500°C temperature range, while Praxair ceramics exist in a pure cubic phase upon heating from room temperature to 900°C. However, if heated rather fast, the cubic to rhombohedral phase transformation could be avoided. Thus it is not expected that the observed phase transitions play a significant role in developing transformation stresses in ScCeZrO2 electrolyte upon heating and cooling down from the operation temperatures.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

SEM images of as received DKKK and Praxair powders

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Figure 2

XRD patterns of (a) DKKK and (b) Praxair powders both at room and high temperatures

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Figure 3

SIMS analysis of DKKK and Praxair powders

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Figure 4

A (a) shrinkage, (b) porosity, and (c) grain size (c) of the DKKK and Praxair ceramics as a function of the sintering temperatures

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Figure 5

SEM micrographs polished and thermally etched DKKK (top line) and Praxair (bottom line) ceramics sintered at different temperatures. The difference in grain color is due to the strong crystal orientation dependence of the intensity of high energy backscattered electrons (BSEs) (30).

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Figure 6

Fracture surfaces of (a) DKKK and (b) Praxair ceramics sintered at 1600°C. Surface termination steps discovered inside the close pores in (c) DKKK and (d) Praxair ceramics.

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Figure 7

Room temperature XRD patterns of DKKK and Praxair ceramics as a function of sintering temperature

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Figure 8

High temperature XRD patterns of DKKK ceramic sintered at 1600°C. Silver peaks were used for internal temperature calibration.

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Figure 9

DSC data and fraction of the cubic phase for DKKK samples sintered at 1600°C upon heating. DSC data are also presented upon cooling of the ceramics.

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Figure 10

(a) Results of the XRD Rietveld multiphase analysis for DKKK samples in the temperature range of 200–600°C. (b) Content of the β phase at 400°C as a function of the grain size of DKKK ceramics.

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Figure 11

AFM images of a thermally etched DKKK ceramic sample sintered at 1600°C before (top) and after (bottom) conversion to the β phase by annealing at 400°C for 12h

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