Although both A- and B-site cations have the same simple cubic topology in the perovskite structure they typically adopt different patterns of chemical order. As a general rule B-site cations order more readily than A-site cations. When cation ordering does occur, rock salt ordering of B/B′ cations is favored in A2BB′X6 perovskites, whereas layered ordering of A/A′ cations is favored in AA′B2X6 and AA′BB′X6 perovskites. The unexpected tendency for A-site cations to order into layers stems from the bond strains that would result at the anion site if A and A′ cations of different size were to order with a rock salt arrangement. The bonding instabilities that are created by layered ordering are generally offset either by anion vacancies or second order Jahn–Teller distortions of a B-site cation. Novel types of A-site cation ordering can be stabilized by a+a+a+ or a+a+c− tilting of the octahedra.

Cation ordering in perovskites

Fracture toughness, KIC, of a single-phase commercial lead zirconate titanate (PZT) ceramic (Zr/Ti=0.54/0.46) of tetragonal structure (c/a=1.019) was measured using the single edge notched beam method above and below the Curie temperature. Domain switching (poling) under electrical and mechanical loading was examined using X-ray diffraction. Surface grinding, electrical poling, and mechanical poling caused crystallographic texture. Similar texture, indicative of domain switching, was also observed on fracture surfaces of some saples fractured at room temperature. At room temperature, the highest KIC measured was 1.85 MPa·m1/2, while above the Curie temperature it was about 1.0 MPa·m1/2. Cracks emanating from Vickers indents in poled samples were different in the poling and the transverse directions. The difference in crack sizes is explained on the basis of domain switching during crack growth. These results indicate that ferroelastic domain switching (twinning) is a viable toughening mechanism in the PZT materials tested.

Fracture Mechanisms in Ferroelectric‐Ferroelastic Lead Zirconate Titanate (Zr: Ti=0.54:0.46) Ceramics

Linear electro-elastic analysis of a cavity or a crack in a piezoelectric material

Crack propagation in a piezoelectric lead–zirconium–titanate (PZT) material under simultaneous mechanical loading and applied electric fields is studied using the Vickers indentation technique. It is demonstrated experimentally that electric fields can inhibit or enhance crack propagation in piezoelectric materials. Cracks introduced by indentation are observed to propagate less under a positive applied electric field (the polarity of the field was the same as that for poling), whereas under a negative applied electric field, crack propagation is enhanced. Such an effect is observed to be more profound with increasing electric-field strength and decreasing mechanical loading. Attempts are made to compare these experimental observations with the results of various theoretical analyses. A mechanism for the change in crack propagation behavior of the piezoelectric PZT material under applied electric fields is presented.

Crack propagation in piezoelectric ceramics: Effects of applied electric fields

This chapter reviews experimental results that allow one to interpret the essential features of fracture in ferroelectric ceramics under electric and mechanical load. First, crack growth measurements on unpoled and poled ferroelectric ceramics are reviewed, and numerous experiments that demonstrate the existence and relevance of a domain-switching zone are presented. Thereafter the review concentrates on results of fracture experiments with applied electric fields and addresses the controversial outcomes of such experiments. Theoretical fracture mechanical concepts are then introduced. One part of the discussion focuses on electrical boundary conditions used at the crack surface because they decisively determine the predicted energy release rate. The other part of the discussion, which concerns theoretical concepts, discusses the predicted switching zones around cracks and their influence on the stress intensity factor and energy release rate. Finally, this chapter attempts to mirror the tremendou...

Influence of Electric Field and Mechanical Stresses on the Fracture of Ferroelectrics

Dielectric, electromechanical, optical, and mechanical properties of hot-pressed lanthanum-modified PbTiO/sub 3/ ceramics were measured as a function of crystal tetragonality, c/a, in a range of tetragonal-to-cubic phases. The results obtained in this study are summarized as follows: The c/a changes linearly as a function of lanthanum content. Similarly, Curie point changed linearly and remanent polarization, coercive field, and coupling factors decreased with decreasing c/a. The sample with c/a = 1,010 showed the maximum transmittance of 61% wavelengths above 600 nm. The internal stress measured using the microindentation technique decreased with decreasing c/a in the tetragonal region and was zero in the cubic region.

Dielectric, Electromechanical, Optical, and Mechanical Properties of Lanthanum-Modified Lead Titanate Ceramics

Crystal structure and defects of ordered ((Pb1-xCax)TiO3 ceramics have been investigated by transmission electron microscopy. The structure is determined to be tetragonal, belonging to point group 4mm. Pb and Ca are in an ordered fcc arrangement on the A sites. In addition to the chemical ordering, there is ordering due to atomic shuffling or what has been referred to as electrical ordering. Two distinct sets of planar defects are observed in the same region. One set of planar defects are identified as antiphase boundaries that are the results of chemical ordering and the other set are displacement boundaries that are the results of electrical ordering.

Crystal Structure and Defects of Ordered (Pb1-xCax)TiO3

Lanthanum modified lead titanate, (Pb0·775La0·15)TiO3 ceramics with a tetragonality of c/a = 1·024 were hot-pressed and the anisotropy of internal stress generated by applying a poling field were measured using a microindentation technique. In this specimen with a grain size of Gs=3·0 μm, the internal stresses in the directions perpendicular and parallel to the DC poling field were 151 and 35 MN/m2, respectively. The results are explained by the crack-extentional and compressive forces, respectively.

Internal stress anisotropies induced by electric field in lanthanum modified PbTiO3 ceramics

The (Pb1-x, Cax)TiO3, (x=0.12 to 0.5) ceramics were sintered, and the relationship between dielectric, piezoelectric properties and microstructure were classified as a function of Ca content. There existed no phase boundaries over the range of Ca content of 0.12 to 0.5 mol. However, the ratio of kt and kp was drastically changed as a function of Ca content. These changes were related with the appearance of 90° domains and superlattice.

Electrical Properties and Microstructure of Ca Modified PbTiO3Ceramics

The (Pb, La) (Zr, Ti)O3 ceramics were hot-pressed, and a micro-indentation technique was applied to the ceramics before and after poling. Internal stress anisotropies induced by application of dc electric field was found. The fracture toughness increased for the parallel direction to a poling field and decreased for the perpendicular direction due to a compressive and a extensional stress, respectively. The stress anisotropy disappeared by thermal depolarization at above Curie point. The results were explained by a ferroelectric domain rearrangement.

Stress Anisotropy of PLZT Ceramics Induced by Polarization

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