High-Temperature Proton-Conducting Lanthanum Ortho-Niobate- Based Materials. Part II: Sintering Properties and Solubility of Alkaline Earth Oxides Tommy Mokkelbost, z,y Ingeborg Kaus, z,y Reidar Haugsrud, z Truls Norby, z Tor Grande,* ,y and Mari-Ann Einarsrud* ,w,y y Department of Materials Science and Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway z Department of Chemistry, Centre for Materials Science and Nanotechnology, University of Oslo, N-0318 Oslo, Norway The sintering properties and microstructure of La 1x A x NbO 4 powders (x 5 0, 0.005, and 0.02 and A 5 Ca, Sr, and Ba), pre- pared by spray pyrolysis have been investigated. Dense materials (497%) were obtained by conventional sintering at 12001C and by hot pressing (25 MPa) at 10501C, respectively. Homoge- neous materials were obtained and the average grain size ob- tained by the two densification methods was B2.0 and B0.4 lm, respectively, for the 2% doped materials. Pure lanthanum ortho-niobate (LaNbO 4 ) showed a higher degree of grain growth. In the acceptor-doped materials, secondary phases were observed to inhibit grain growth at 12001C. At 14001C or higher, molten secondary phases in the Ba-doped materials resulted in severe grain growth, causing microcracking during cooling due to crystallographic anisotropy. A low solubility of AO (A 5 Ca, Sr, and Ba) in LaNbO 4 is inferred from the pres- ence of secondary phases, and 1 mol% solubility of SrO in LaNbO 4 was found by electron microprobe analysis. The elec- trical conductivity in wet hydrogen of the materials demonstrat- ed that the main charge carrier was protons up to 10001C and reached a maximum value of B8 . 10 4 S/cm at 9001C. I. Introduction R ECENTLY, relatively high proton conductivity was reported for lanthanum ortho-niobate (LaNbO 4 )-based materials. 1 LaNbO 4 has a monoclinic fergusonite structure and transforms into a tetragonal scheelite structure at around 5001C. 2 LaNbO 4 is expected to be more stable in a CO 2 /H 2 O atmosphere compared with state-of-the-art proton conductors like Sr- and Ba-based cerates and zirconates 3 and is therefore an attractive candidate material as an electrolyte in solid oxide fuel cells and hydrogen sensors. 1 The maximum total conductivity in LaNbO 4 - based materials is a decade lower than the BaCeO 3 - or SrCeO 3 - based perovskites, which reach a proton conductivity of B10 2 S/cm. 4 The proton conductivity is partly dependent on oxygen vacancies; thus, the solubility of AO acceptors in LaNbO 4 is important. Isothermal phase diagrams of La 2 O 3 –SrO–Nb 2 O 5 5,6 and La 2 O 3 –BaO–Nb 2 O 5 7 have been reported, but the solubility of CaO, SrO, and BaO in LaNbO 4 has, to our knowledge, not been studied in detail. From size considerations of the cations, it is most probable that the alkaline earth cations substitute for La in the ortho-niobate. Proton conductivity has also been reported for other lanthanum-based niobate oxides. 8,9 Ceramic powder processing and densification of LaNbO 4 have not been studied extensively so far. The lower proton con- ductivity of acceptor-doped LaNbO 4 calls for thin electrolytes in the micrometer range in order to achieve suitable power densi- ties. A homogenous microstructure is therefore essential to ob- tain sufficient mechanical performance, and high-quality powders with excellent sintering properties are desired. Mokkelbost et al. 10 have recently reported the preparation of high-quality LaNbO 4 -based powders by spray pyrolysis. LaNbO 4 powders have been prepared previously by a solid-state reaction, giving coarse powders resulting in high sintering temperatures, e.g. 15501C. 2 However, powders of LaNbO 4 prepared by chem- ical methods have a lower sintering temperature. 11 The present study reports on the ceramic processing of La 1x A x NbO 4 powders (x 5 0, 0.005, 0.02, and A 5 Ca, Sr, and Ba) prepared by spray pyrolysis. The influence of the con- centration of AO on the sintering behavior and microstructure is reported and the solubility of AO in LaNbO 4 is discussed. The sintering properties of the materials were investigated by con- ventional sintering, while hot pressing (HP) was applied to ob- tain materials with submicrometer grain size. Materials were also sintered at a relatively high temperature in order to investigate the temperature dependence of alkaline earth oxides in LaNbO 4 . Finally, the electrical conductivity of the different materials is investigated with a focus on the effect of secondary phases. II. Experimental Procedure (1) Synthesis Powders of LaNbO 4 , 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 pyrolysis of La EDTA and Nb malic acid solutions and dried A(NO 3 ) 2 as described elsewhere. 10 Stoichiometric materials are denoted LN and LN–y%A where y 5 x  100. Additional compositions of La 1x Sr x NbO 4 , where x 5 0.007, 0.01, 0.04, or 0.08, and La (1x)70.05 Sr x NbO 4 , where x 5 0 or 0.04, were synthesized by evaporating stoichiometric mixtures of solutions of mixed La EDTA and Nb malic acid solutions to a yellowish gel, dried to foam at 2501C, and crushed before and after calcination to 8001C. Samples with 1% La excess and 1% Nb excess, La 0.99 Sr 0.02 NbO 4 and La 0.98 Sr 0.02 Nb 1.01 O 4 , were pre- pared by adding standardized aqueous solutions of La EDTA or Nb malic acid to La 0.98 Sr 0.02 NbO 4 -calcined powders, followed by drying and calcination at 8001C. The LaNbO 4 , La 0.995 Sr 0.005 NbO 4 , and La 0.98 A 0.02 NbO 4 pow- ders prepared by spray pyrolysis were dry milled (yttria-stabilized zirconia media, YSZ) for 15 min to reduce the tap density, fol- lowed by calcination at 8001C for 6 h in a muffle furnace. The calcined powders were ball milled (YSZ) in 100% ethanol for 3–24 h, dried in a rotavapor, ground, and sieved at 500 mm. 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, the American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: Mari-Ann. Einarsrud@material.ntnu.no z Present address: SINTEF Materials and Chemistry, Sem Sælands vei 12, NO-7465 Trondheim, Norway. Manuscript No. 23648. Received August 23, 2007; approved November 9, 2007. J ournal J. Am. Ceram. Soc., 91 [3] 879–886 (2008) DOI: 10.1111/j.1551-2916.2007.02232.x r 2008 The American Ceramic Society 879