Giant anelastic responses in (BaZrO3-ZnO)-BaTiO3 composite materials

Composite materials containing particulate BaTiO3 inclusion have been fabricated with BaZrO3-ZnO as the matrix. Giant anelastic responses (abrupt increase in modulus and damping vs. temperature) have been observed after 75 ◦C aging 24 h in these composites in the temperature region far below the Curie point of BaTiO3. Anomalies disappeared after 185 ◦C aging 4 h. Such responses are attributed to the constraining negative-stiffness effect of the inclusion. An oxygen vacancy mechanism has been discussed which is considered to be responsible for the negative-stiffness effect. The present work is novel in terms of the idea of manipulation of negative stiffness in ferroelastic inclusions to achieve the desired composite properties. Copyright c © EPLA, 2011 Classical composite theory predicts that composite properties, such as stiffness, damping, thermal expansion, etc., cannot surpass those of the constituents. Such theory tacitly assumes that stiffness is always positive. However, such bounds can be exceeded if the composite has a negative-stiffness phase [1]. Negative stiffness entails a reaction force in the same direction as imposed deformation, and has been used in the design of composites with extremely high physical properties [2,3]. To achieve superior composite responses requires good interfacial conditions, including perfect wetting and sufficient strength. It is easy to achieve these in metal matrix composites prepared via casting techniques with well-selected metal matrix, yet much more difficult in the case of ceramic matrix composites which are usually synthesized via the solid-state reaction method. Porous interface and its brittleness nature are inevitable drawbacks for ceramic matrix composites unfavorable for attaining sufficient negative stiffness for the inclusion in order to achieve the desired composite properties. In view of the typical synthesis temperature (usually > 1000 ◦C) and time being used for ceramics, inter-diffusion is another unavoidable disadvantage (this will be discussed later). Up to now, no experimental report is available (a)E-mail: [email protected] on ceramic matrix composites with negative-stiffness phase which attained the desired composite properties. The present work has filled this gap by studying the anelastic properties of ceramic matrix (BaZrO3-ZnO) composites containing inclusions of BaTiO3. In particular, aging-process–activated giant anelastic responses were observed far below 130 ◦C. Such an effect is neither due to matrix behavior, as BaZrO3-ZnO has no transformation in the temperature range considered (20 ◦C to 180 ◦C), nor can be explained via known transformations of the BaTiO3 inclusion as the classical Landau theory predicts a negative bulk stiffness in constrained BaTiO3 only near its three ferroelastic transformation temperatures (cubicto-tetragonal: 130 ◦C; tetragonal-to-orthorhombic: 5 ◦C; and orthorhombic-to-rhombohedral: −75 ◦C). A similar phenomenon (extreme responses near 60 ◦C but only small sigmoid anomalies near 130 ◦C) was also observed in metal matrix composites (Sn-BaTiO3) [3]. In view of its aging process dependence, we explained such phenomena in the context of negative-stiffness effect [4] in BaTiO3 caused by an oxygen vacancy (OV) mechanism [5,6]. To the best of our knowledge, this is the first experimental report of a manipulation (activation and deactivation) of negative stiffness in ferroelastic inclusion via specific aging processes to attain giant anelastic composite responses. Theory [7] also predicts that giant piezoelectric properties