Sintering of BaZrO3 and SrZrO3 perovskites: Role of substitutions by yttrium or ytterbium

The modification of perovskite sintering oxides A(II)B(IV)O(3) was studied. The alkaline earth metal plays a role in the densification of perovskite. The effect of the substitution of Zr4+ by a lanthanide Yb3+ in barium zirconate (BZYb) increases its density and impedes BaO evaporation independently of the thermal cycle. A substitution of Zr4+ by Y3+ increases the density of barium zirconate (BZ) only after 4 h at 1650 degrees C. In all cases, the substitutions of Zr4+ by Y3+ or Yb2+ (or) (3+) in the zirconate inhibit grain growth. The substitution in B site by a Ln(3+) leads to the creation of vacancies (A(II)B((1-x))(IV)Ln(x)(III) O3-x/2). To determine the vacancies' role, three different controlled atmospheres were tested. Argon, dry air, and oxygen were used to study the impact on the linear shrinkage of barium zirconate when some Zr4+ are replaced by yttrium or ytterbium (BZY, BZYb) or not substituted (BZ). Strontium zirconate (SZ) is not as chemically stable as barium zirconate (BZ) during the thermal cycle when Yb is used as a substitution element. In effect, the presence of the secondary phase Zr3Yb4O12 associated with the release of oxygen proves the bivalence of ytterbium (Yb2+ or Yb3+). Therefore, ytterbium can be substituted in either A or/and B sites. The study of reactive sintering between lanthanum strontium zirconate (SZYb) and Yb2O3 shows that a slight excess of Yb2O3 around SZYb grains stabilizes the chemical composition of SZYb. Perovskite symmetry and stability were determined by two factors, i.e., tolerance (t) and octahedral factors (r(B)/r(O)). It was demonstrated that the distortion of the perovskite unit cell increases the density of pellets. (C) 2011 Elsevier B.V. All rights reserved.