Journal of Alloys and Compounds 395 (2005) 166–173 Effect of mechanochemical activation on the synthesis of NaNbO 3 and processing of environmentally friendly piezoceramics Teresa Hungr´ ıa, Lorena Pardo, Alberto Moure, Alicia Castro ∗ Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Received 8 September 2004; accepted 12 October 2004 Available online 5 January 2005 Abstract High-energy milling has been applied to obtain NaNbO 3 ceramic precursor powders and dense ceramics by a one-step thermal treatment. Stoichiometric mixtures of Nb 2 O 5 and different sodium reactants (Na 2 CO 3 , Na 2 O and NaOH) were mechanically activated. The obtained results are discussed regarding the starting reactants and the milling media. The evolution of the powder mixtures and the thermal behavior of the activated samples were investigated by X-ray powder diffraction and thermal analysis. The most crystalline NaNbO 3 powders were obtained when a 2NaOH/Nb 2 O 5 mixture was homogenized using a wet-chemistry technique followed by mechanochemical activation in a planetary mill and treated at 900 ◦ C for 2 h. Dense ceramics were processed from the different precursors by conventional-sintering and hot-pressing and their microstructure and piezoelectric properties characterized. © 2004 Elsevier B.V. All rights reserved. Keywords: Ferroelectrics; High-energy ball milling; X-ray diffraction; Dielectric response 1. Introduction Niobates are a technologically important class of inorganic materials because of their broad range of physical properties and applications. Alkaline niobate ceramics are considered a good alternative for the substitution of commercial piezo- ceramics by lead-free materials, based on highly toxic lead compounds. Sodium niobate (NaNbO 3 ) is antiferroelectric at room temperature [1], but the application of an electric field or the substitution of Li [2] or K [3] for Na, induces a ferroelectric phase that provides piezoelectric activity that is interesting for high frequency devices. It is known that NaNbO 3 presents a large number of structural phases, vari- ants of lower symmetry of the ideal perovskite structure, hav- ing six phase transitions [4] from the non-polar high temper- ature phase to the antiferroelectric room temperature phase and the low temperature ferroelectric one. Sodium niobate is of particular interest because in this antiferroelectric phase are based some solid solutions [5–7] with good ferroelectric ∗ Corresponding author. Tel.: +34 91 334 9000; fax: +34 91 372 0623. E-mail address: acastro@icmm.csic.es (A. Castro). and piezoelectric properties. Recent studies [6,7] are focused on the analysis, involving dielectric and thermal expansion measurements, of the phase transitions in the polar phase of ceramics of Na 1-x Li x NbO 3 compositions. Knowledge of such transitions is a key factor to assess the thermal stability of their piezoelectric properties. On the other hand, the electrical and optical properties of dielectric ceramics strongly depend on the chemical and microstructural homogeneity, which is mainly influenced by the powder synthesis method and sintering process. Alkali metal niobate powders are usually prepared via a solid-state reaction between Nb 2 O 5 and alkali metal carbonates. This classical method involves high temperatures and long reac- tion times, produces the volatilization of the alkali metal, leads to poor compositional homogeneity and provides prod- ucts with large particles [2]. Particularly for piezoceramics the control of the porosity and pore size is a key issue, since the dielectric strength is dependent on them [8] and a high dielectric strength is required during the poling process. Alternative routes need to be developed to facilitate the production of desired materials in a controllable way. Powder synthesis using evaporation [9], sol–gel [10], hydrothermal 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.10.067