The next generations of multilayer ceramic capacitors are expected to have active ceramic layer thicknesses of 3 μm or less. To retain a reliable ceramic device, it is advantageous to engineer a uniform and fine-grained microstructure. In this study, a dense ceramic (>96% of theoretical) with fine-grained size (<0.2 μm), and permittivity of 2800, meeting the EIA X7R temperature characteristic has been obtained using hydrothermally synthesized BaTiO3 powder with precipitation-coated additives. The temperature coefficient of capacitance (TCC) and the dissipation factor (DF) are investigated as a function of active layer thickness and compared with that of a conventional coarse-grained dielectric of similar composition and fired density. The fine-grained ceramic maintained X7R characteristic in active layers as thin as 3 μm under 1 VAC and 1 kHz test conditions. For equivalent active thickness, the DF of the fine-grained version was consistently lower than the coarse-grained material. These results indicate that an improved voltage coefficient is possible for finer-grained materials under otherwise equivalent conditions. The possible causes for this behavior are also discussed.

Dielectric Properties of Fine-Grained Barium Titanate Based X7R Materials

R-curves of ferroelectric barium titanate have been measured with CT-specimens. Depending on the grain size, the R-curves exhibit an increase from an initial value of 0.5–0.7 MPa√m to a plateau value of 0.7–1.2 MPa√m after a crack length increment of approximately 100–800 μm. The main toughening mechanism is thought to be ferroelastic domain switching leading to the development of a process zone around the crack. The small differences in the initial values of the R-curves are attributed to grain bridging, crack deflection and crack branching associated with large grains. Applying an electric field perpendicular to the specimen plane results in an increased initial fracture toughness and a stronger R-curve behaviour. Time-dependent reorientation of domains in the crack wake causes stress relaxation combined with a strong time dependence of the R-curves. A qualitative process zone model is proposed to explain these effects.

R-curve behaviour of BaTiO3 due to stress-induced ferroelastic domain switching

Mechanical, piezoelectric and dielectric properties of PZT ceramics (Pb 0.98 M 0.02 [(Zr 0.535 Ti 0.465 ) 0.995 ]O 3 , M=Er, Gd, Eu, Nd, La and Bi) have been studied. The fraction of A sites occupied by the different additives is estimated and is found to increase with the ionic radius of the additive ion. The density is high in Bi-, Gd- and Eu-PZT samples. Small amounts of the pyrochlore phase are present in Gd- and Eu-PZT. A maximum strength of 80–100 MPa and K IC of ∼1.5 MPa m 1/2 are obtained in Gd- and Eu-PZT. The R -curve behavior becomes more pronounced in Gd- and Eu-PZT, and it is observed that the strength and K IC increase with the slope of the K IC versus c 1/2 plot. Very good piezoelectric properties, i.e. d 33 of 410 PCN −1 , dielectric constant of 1005, κ p of 0.49 and the hydrophone figure of merit, d h g h , of 2760 are obtained for Nd-PZT. The piezoelectric properties in Bi-PZT are also higher than for the undoped composition, while for the Gd-PZT, these are slightly inferior. For all other compositions, the piezoelectric properties are not affected much.

Effect of rare earth (Er, Gd, Eu, Nd and La) and bismuth additives on the mechanical and piezoelectric properties of lead zirconate titanate ceramics

The effect of heating rate on the sintering behavior and the piezoelectric properties of lead zirconate titanate (PZT) ceramics was investigated. Two different types of PZT (pure and doped with Nb2O5) were sintered at 1150°C for 2 h with a wide range of heating rate (0.5°–100°C/min). The densification of pure PZT was improved significantly by increasing the heating rate. The improvement was attributed to the suppression of PbO volatilization and grain coarsening during heating. In contrast, the densification behavior of a PZT specimen doped with Nb2O5 was not much influenced by the heating rate. These densification behaviors affected the piezoelectric properties of the specimens. The piezoelectric properties of pure PZT were enhanced significantly by increasing the heating rate, while those of doped specimens were improved only moderately.

Effect of Heating Rate on the Sintering Behavior and the Piezoelectric Properties of Lead Zirconate Titanate Ceramics

R-curve behavior of BaTiO3- and PZT ceramics under the influence of an electric field applied parallel to the crack front

Lead zirconate titanate specimens, at composition Zr 48.9±1.0 /Ti 51.1±1.0 , of grain sizes ranging from 0.5 to 15 μm were prepared by hot pressing. The fracture toughness showed a 40% increase in the fine-grained specimens. Poling produced anisotropy in the fracture toughness. A tensor representation for the fracture toughness, now a function of the crack plane normal and the poling direction, is developed. The maximum and minimum values in the toughness tensor are shown to be related to the unpoled toughness by a simple phenomenological relationship. We also find that the piezoelectric coefficient, d 33 , shows a slight increase for the fine-grained specimens, which is in conflict with the general view that smaller-grained materials can sustain vanishingly small values of spontaneous polarization.

Zhiwu Zhang

Papers

Influence of Grain Size on Ferroelastic Toughening and Piezoelectric Behavior of Lead Zirconate Titanate

Influence of Grain Size on Ferroelastic Toughening and Piezoelectric Behavior of Lead Zirconate Titanate.