High-Temperature Proton-Conducting LaNbO 4 -Based Materials: Powder Synthesis by Spray Pyrolysis Tommy Mokkelbost, Øystein Andersen, z Ruth Astrid Str^m, Kjell Wiik, Tor Grande,* and Mari-Ann Einarsrud w Department of Materials Science and Engineering, Norwegian University of Science and Technology, 7491 Norway Submicrometer lanthanum ortho-niobate (LaNbO 4 (LN))-based powders have been prepared by spray pyrolysis of an aqueous solution containing stable La–EDTA and Nb–malic acid com- plexes. The powders had a particle size of B0.1 lm, a narrow particle size distribution, and high purity after calcination above 8001C. The powders possessed excellent sintering properties re- sulting in 498% dense materials at 12001C. The present route is shown to be excellent for the large-scale preparation of high- quality LaNbO 4 -based powders. I. Introduction M ULTICOMPONENT Nb-containing oxide materials are inter- esting in many fields due to their attractive physical and structural properties, e.g.,lanthanum ortho-niobate (LaNbO 4 (LN))-based materials have recently been shown to be proton conductors at high temperatures. 1,2 The state-of-the-art proton conductors, e.g., BaZrO 3 and SrCeO 3 , have one order of mag- nitude higher proton conductivity but are not stable in CO 2 - containing atmospheres and render them useless in contact with proton sources like CH 4 . 3 Because LN-based materials are sta- ble in a reducing atmosphere, they are potential candidates as high-temperature proton conductors. LN crystallizes in a mono- clinic structure, 4 isostructural with fergusonite. At B5001C, transformation to a tetragonal structure isostructural with sche- elite is reported. 5,6 For most applications, polycrystalline materials are needed and synthesis routes to high-quality powders are of significant importance. LN powders have mainly been prepared by the sol- id-state reaction method; 5–9 however,this method generally gives powders with large particle size. Solution-based methods are advantageous for the preparation of high-quality powders with smallparticle size, but for materials with Nb(V), this is challenging due to the high valency and thus fast hydrolysis of the niobium cation. 10 Hence, precursors for preparing aqueous niobium solutions are limited to Nb 2 O 5 , 11 NbCl 5 , 12,13 alkoxides of niobium, 14,15 and niobium ammonium oxalate. 16,17 These precursors can be stabilized by complexing with, e.g.,tartaric acid, 18–20 citric acid, 17 DL -malic acid, 13,18,21 hydrogen peroxide in combination with citric acid, 16,22 making peroxo-carboxylato compounds, 10 or using the polymeric precursor method devel- oped by Pechini. 23 Most of these precursors and some of the complexing agents are not suitable for large-scale powder pro- duction due to, e.g., toxicity or price. The aim of this work was to develop a synthesis route for large-scale preparation of high-quality powder of alkaline ea substituted LN. Spray pyrolysis of aqueous solutions was ch sen as this method is shown to give excellent powder qualit The influence of the synthesis conditions and type of alkalin earth element on the purity, particle size, surface area, and ring behavior of the powders was studied. II. Experimental Procedure (1) Synthesis Powders of pure LN, La 0.995 Sr 0.005 NbO 4 , and La 0.98 A 0.02 NbO 4 where A 5 (Ca, Sr, or Ba) were synthesized by spray pyrolys using standardized aqueous solutionsof a 0.2M La–EDTA complex and 0.16M Nb–malic acid complex. La 1x A x NbO 4 powders are hereby denoted LN-y% A where y 5 x 100. The La–EDTA complex solution was prepared by dissolving lanthanum nitrate, La(NO 3 ) 3 6H 2 O (1 mole) (Fluka, Steinheim, Germany, 499%), in 3 L of distilled water and mixing with ethylenediaminetetraacetic acid, H 4 EDTA (1 mole) (Acros Or- ganics, Geel,Belgium, 99%).Ammonium hydroxide, NH 4 OH (25%) (800 mL), was added to the colloidal solution to form water-soluble La–EDTA complex. pH was adjusted to 7 by adding nitric acid, HNO 3 (65%), drop by drop, and the solution was filtered. A flowchart for the preparation of La–EDTA so- lution is shown in Fig. 1(A). Nb–malic acid complex solution was prepared from ammo nium niobium dioxalate oxide pentahydrate, (NH 4 )NbO (C 2 O 4 ) 2 5H 2 O (1 mole) (H.C.Starck,Goslar,Germany), dis- solved in 4 L of distilled water. NH 4 OH (25 wt%) was dropped while stirring until pH 5 11, precipitatingniobic acid, Nb 2 O 5 nH 2 O. The mixture was heated at 801C for 6 h while stirring, followed by 12 h of aging at ambient temperature. T precipitate was washed three times in NH 4 OH (1%) by centrif- ugation to eliminate oxalate ions. The elimination of oxalate ions is important because lanthanum oxalate has a very low solubility in water. 24 The precipitated niobic acid was dissolved in DL -malic acid solution, MA (0.33M) (Aldrich,Steinheim, Germany, 99%). 13,18,21 The molar ratio of [MA]/[Nb] was 2:1. The mixture was heated at 701C to dissolve Nb 2 O 5 nH 2 O, fil- tered, and the pH was adjusted to 7 by dropping NH 4 OH (25 wt%). A flowchart for the preparation of Nb–malic acid solu- tion is shown in Fig. 1(B). The solutions were standardized by thermogravimetry, mi to provide the correct stoichiometry, and added dried (2501C) Ca(NO 3 ) 2 4H 2 O (Merck, Darmstadt,Germany,499.0%), Sr(NO 3 ) 2 (Merck, 99.0%) or Ba(NO 3 ) 2 (Riedel-de Hae ¨n, Seelze, Germany, 99%). The solutions were spray pyrolyzed using p scale equipment. A two-phase nozzle (diameter 1 mm) with bar pressurized air was used to atomize the solution at a rat L/h. The solutions were atomized directly into a rotating furn at 8401–8501C. The outlet temperature of the furnace was 4 5001C. The as-prepared powders were dry ball milled (yttria- stabilized zirconia balls) for 15 min to reduce the tap density followed by calcination at 8001–12001C for 6 h in air. The R. Cutler—contributing editor Based in part on the thesis submitted by T. Mokkelbost for the Ph.D. degree in material science and engineering. Supported by the Research Council of Norway, Grant No. 1585171431 (NANOMAT). *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: Mari-Ann.Einarsrud@ material.ntnu.no z Present address: FMC Technologies, Kongsberg, Norway. Manuscript No. 22474. Received November 10, 2006; approved May 16, 2007. J ournal J. Am. Ceram. Soc., 90 [11] 3395–3400 (2007) DOI: 10.1111/j.1551-2916.2007.01904.x r 2007 The American Ceramic Society 3395