Synthesis and NTC properties of YCr1−xMnxO3 ceramics sintered under nitrogen atmosphere

International audienceYCr1−xMnxO3 (0 ≤ x ≤ 0.8) negative temperature coefficient (NTC) compositions were synthesized by classical solid state reaction at 1200 °C, and sintered under nitrogen atmosphere at 1500 °C and 1600 °C. XRD patterns analysis has revealed that for x ≤ 0.6, the structure consists of a solid solution of an orthorhombic perovskite YCrO3 phase with Mn substitute for Cr. For x ≥ 0.8, a second phase with a structure similar to the hexagonal YMnO3 phase appears. SEM images and calculated open porosity have shown that the substitution of Mn for Cr results in a decrease in porosity. Whatever the sintering temperature, the electrical characterizations (between 25 and 900 °C) have shown that the increase in the manganese content involves the decrease in both resistivity and material constant B (parameter which characterizes the thermal sensitivity of material) when x ≤ 0.6. The magnitude order of the resistivity at 25 °C is of 104–108 Ω cm and activation energies vary from 0.28 to 0.99 eV at low and high temperatures, respectively

Texturation of lead-free BaTiO3-based piezoelectric ceramics

International audienceNowadays, piezoelectric ceramics are integrated in a wide range of devices, in particular in ultrasonic applications (underwater sonar systems, medical imaging, non-destructive testing…). Most of them use Pb(Zr,Ti)O3 (PZT). However, due to health care and environmental problems, lead content must be reduced in such applications [1]. Recent reviews demonstrated that few lead-free materials families can be considered: the alkaline-niobates (K0.5Na0.5NbO3), the alkaline-bismuth-titanates (Na0.5Bi0.5TiO3) and barium titanate based materials (BaTiO3) [2, 3]. One of the limitations of ceramic materials is their isotropic nature. This is the reason why texturation process has been developed in order to improve their properties in particular electromechanical parameters. The aim of the present study is thus to obtain textured BaTiO3 based materials by using the templated grain growth process (TGG) and to measure their piezoelectric properties. Nanosized BaTiO3 powders were prepared by classical solid state route at relatively low temperature. BaTiO3 templates of different morphologies were elaborated by a molten salts process. Dispersing agent, binder and plasticizer, the mixture of the templates and matrix particles was then dispersed in the appropriated solvent using non-aqueous formulation. The slurry was then tape-casted on plastic film. After drying, the green sheet was cut, stacked, thermo compressed and then sintered at the appropriated temperature. This process allowed obtaining highly-oriented materials with a texturation degree around 95% (fig. 1), presenting Kt values around 35-40%, higher than for non-textured BaTiO3 ceramics. Fig. 1: X-ray diffraction diagram of (tetragonal) textured sample BaTiO3 and (tetragonal) non-textured BaTiO3 ceramic as reference

{111}-textured BaTiO 3 ceramics elaborated by Templated Grain Growth using NaNbO 3 templates

International audienceHighly textured {111} BaTiO 3 ceramics were obtained by templated grain growth using NaNbO 3 templates, with Lotgering factor F 111 , around 95 %, one of the best values reported in the literature. Solid solution formation with NaNbO 3 templates changes the tetragonal structure into cubic, with paraelectric behaviour. The relative permittivity is lower than "pure" tetragonal BaTiO 3 , but dielectric losses are improved with better thermal stability