A LOW-COST PROCESS FOR THE SYNTHESIS OF NANOSIZE YTTRIA-STABILIZED ZIRCONIA (YSZ) BY MOLECULAR DECOMPOSITION

Abstract

This report summarizes the results of work done during the performance period on this project, between October 1, 2002 and December 31, 2003, with a three month no-cost extension. The principal objective of this work was to develop a low-cost process for the synthesis of sinterable, fine powder of YSZ. The process is based on molecular decomposition (MD) wherein very fine particles of YSZ are formed by: (1) Mixing raw materials in a powder form, (2) Synthesizing compound containing YSZ and a fugitive constituent by a conventional process, and (3) Selectively leaching (decomposing) the fugitive constituent, thus leaving behind insoluble YSZ of a very fine particle size. While there are many possible compounds, which can be used as precursors, the one selected for the present work was Y-doped Na{sub 2}ZrO{sub 3}, where the fugitive constituent is Na{sub 2}O. It can be readily demonstrated that the potential cost of the MD process for the synthesis of very fine (or nanosize) YSZ is considerably lower than the commonly used processes, namely chemical co-precipitation and combustion synthesis. Based on the materials cost alone, for a 100 kg batch, the cost of YSZ made by chemical co-precipitation is >$50/kg, while that of the MD process should be <$$10/kg. Significant progress was made during the performance period on this project. The highlights of the progress are given here in a bullet form. (1) From the two selected precursors listed in Phase I proposal, namely Y-doped BaZrO{sub 3} and Y-doped Na{sub 2}ZrO{sub 3}, selection of Y-doped Na{sub 2}ZrO{sub 3} was made for the synthesis of nanosize (or fine) YSZ. This was based on the potential cost of the precursor, the need to use only water for leaching, and the short time required for the process. (2) For the synthesis of calcia-stabilized zirconia (CSZ), which has the potential for use in place of YSZ in the anode of SOFC, Ca-doped Na{sub 2}ZrO{sub 3} was demonstrated as a suitable precursor. (3) Synthesis of Y-doped Na{sub 2}ZrO{sub 3} and Ca-doped Na{sub 2}ZrO{sub 3} was achieved using a conventional calcination process. The corresponding surface area was {approx}1 to 2 m{sup 2}/g. (4) By leaching with water, nanosize (very fine) YSZ and CSZ powders were synthesized. The corresponding surface area was {approx}65 m{sup 2}/g. This demonstrates the MD concept, namely macroscopic precursor {yields} leaching {yields} very fine (nanosize) product. (5) Crystallite size was determined by TEM ({approx}5 nm). (6) Anode-supported cells, with YSZ and CSZ made by the MD process, were successfully made by a conventional pressing and sintering process. (7) Single cells were made with as-synthesized YSZ and CSZ as a constituent in anode support. (8) A single cell (LSM + YSZ cathode) was tested at 800 C with H{sub 2}/air, with maximum power density of {approx}1.2 W/cm{sup 2}. (9) Dense samples of both YSZ and CSZ made by the MD process were fabricated. (10) Preliminary cost analysis, based on materials cost only, showed that the cost of YSZ powder made by the MD process should be considerably lower than that made by either chemical co-precipitation or combustion synthesis. For an anode-supported cell design, for an assumed power density of 0.5 W/cm{sup 2}, the cost of YSZ made by the MD process is estimated to be {approx}$$5/kW. By contrast, the cost per kW for chemical co-precipitation or combustion synthesis is {approx}$$70/kW and {approx}$$23/kW, respectively. Efforts are currently underway to fabricate 5 cm x 5 cm active anode-supported cells with YSZ made by the MD process.