P. Czaja

Effects of PbTiO3 doping on electric properties of Na0.5Bi0.5TiO3 ceramics

ABSTRACT Good quality ceramics of (1−x)Na0.5Bi0.5TiO3(NBT)−xPbTiO3 (PT) (x = 0, 0.03 and 0.05) were prepared by a conventional solid-phase sintering process. The temperature dependence of the dc, ac conductivity ( respectively), the Seebeck coefficient (α), the concentration of charge carriers n and their mobility μ were investigated. The dc conductivity for pure NBT possesses three linear parts with three different activation energies Ea. For 0.97NBT–0.03PT and 0.95NBT--0.05PT ceramics, the dc conductivity also has three linear parts (three different Ea). On the other hand, the ac conductivity possesses three linear parts for pure NBT and four for other compositions. The possible origin of the observed effects is discussed. We suggest that the presented materials can be a good starting point for the development of low-lead electronic ceramics.

Raman spectra and anomalies of dielectric properties and thermal expansion of lead-free (1−x)Na0.5Bi0.5TiO3-xSrTiO3 (x = 0, 0.08 and 0.1) ceramics

ABSTRACT Thermal expansion, Raman and dielectric properties of the lead-free (1−x)Na0.5Bi0.5TiO3-xSrTiO3 (x = 0, 0.08 and 0.1) ceramic solid solutions, fabricated by the conventional solid-state reaction method, were investigated. The Sr-doping results in an increase of the dielectric permittivity, broadening of the permittivity maximum, enhancement of the relaxation near depolarization temperature, broadening of the Raman bands and shift of all anomalies toward lower temperatures. The observed effects are attributed to an increase of the degree of cationic disorder and enhancement of the relaxor-like features. Anomalies in the thermal expansion strain were observed at the temperatures corresponding to the dielectric anomalies but not related to any phase transitions. These anomalies are supposed to follow changes of the averaged unit cell volume in the temperature interval of tetragonal and rhombohedral phase coexistence.

SrTiO3-doping effect on dielectric and ferroelectric behavior of Na0.5Bi0.5 TiO3 ceramics

ABSTRACT Lead-free (Na0.5Bi0.5)1-xSrxTiO3 ceramics (x = 0–0.04) were synthesized by a conventional mixed-oxide technique. The microstructure study showed a dense structure, in good agreement with that of above 96% relative density determined by Archimedes method. X-ray diffraction measurements showed that the obtained specimens possess a pure perovskite structure with rhombohedral symmetry. The dielectric and ferroelectric behavior of these ceramics were examined. The temperature dependence of the dielectric spectra revealed a frequency dependence near the depolarization temperature Td, which is characteristic of a relaxor mechanism. This suggests that the ceramics lacked long-range ferroelectric order about temperature Td, which was evidenced by observation of deformed and pinched hysteresis loops, and significant decrease of remnant polarization Pr and coercive field Ec near this temperature.

Electrical transport in lead-free (Na0.5Bi0.5)1–xSrxTiO3 ceramics (x = 0, 0.01 and 0.02)

ABSTRACT Lead-free (Na0.5Bi0.5)1xSrxTiO3 (x = 0, 0.01 and 0.02) ceramics were manufactured through a solid-state mixed oxide method and their ac (σac) and dc (σdc) electric conductivity were studied. It is shown that the low-frequency (100 Hz–1 MHz) ac conductivity obeys a power law σac ∼ ωs characteristic for disordered materials. Both the dc and ac conductivities have thermally activated character and possess linear parts with different activation energies. The calculated activation energies are attributed to different mechanism of conductivity. Frequency dependence of σdc and exponent s is reasonably interpreted by a correlated barrier hopping model. The NBT-ST system is expected to be a new promising candidate for lead-free electronic materials.

Physical properties and microstructure characteristics of (1–x)BaTiO3–xCaTiO3 systems

ABSTRACT The study investigates the microstructure, thermal and mechanical properties of (1–x)BaTiO3–xCaTiO3 ((1–x)BT–xCT)) (x = 0.01, 0.04, 0.08) samples. The BT–CT system to be tested was formed as a solid solution of BT and CT and testing was done on fractured sections of the ceramic. Its thermal properties were investigated using the Differential Scanning Calorimeter (DSC), dielectric measurements were taken based on CT content and frequency data in a wide temperature range. Elastic properties of the samples were examined to determine how the actual CT content and the material microstructure should affect the mechanical properties of the investigated material. Parameters calculated for all samples included the Young’s modulus (E), the shear modulus (G), bulk modulus (K), and Poisson ratio (ν), calculated on the basis of longitudinal and transverse wave velocities. Unlike neat BT samples, (1–x)BT–xCT systems exhibit shifts of phase transition temperatures, improved homogeneity, and feature increased dielectric constant and elasticity modulus. The study aims to identify the underlying causes and origins of this behaviour.

High-density lead-free K0.5Bi0.5TiO3 ceramics: preparation, mechanical and dielectric properties

ABSTRACT The preparation of high-density lead-free potassium-bismuth titanate K0.5Bi0.5TiO3 about 96–98% of the calculated density (X-ray), confirmed by the Archimedes method, is presented. All stages of the technological process are explained in detail. The control of microstructure was performed by scanning electron microscopy. The crystal structure and phase content of samples were determined by X-ray diffraction at each stage of sintering. Four types of samples were obtained under various technological conditions. The ultrasonic measurements of the obtained samples were carried out and basic parameters were determined, such as Young and Kirchoff modulus and Poisson’s ratio were determined at room temperature. The dielectric measurements show that the maximum value of dielectric permittivity is shifted towards higher temperatures as the frequency increases. This feature is characteristic of ferroelectric relaxor materials. The presented materials can be a good starting point for the development of lead-free ceramics.

The Influence of Pb(Mg1/3Nb2/3)O3-doping on the thermoelectric properties of BaTiO3 ceramics

ABSTRACT The low-lead (1-x)Pb(Mg1/3Nb2/3)O3-xBaTiO3 (x = 0, 0.025, 0.05, 0.075, 0.10 and 0.15) ceramics were fabricated by a two-step solid-state reaction route. The X-ray diffraction characterization confirms a perovskite crystal structure with tetragonal symmetry at room temperature (except for x = 0.15). SEM images indicate that all ceramic samples are dense and compact, with the grain size of 2–6 μm. Along with the increase of Pb(Mg1/3Nb2/3)O3 content, the electrical conductivity increases, and the Seebeck coefficient and the thermal conductivity decrease. With increasing temperature, σ and α increase and κ decreases. In general, adding Pb(Mg1/3Nb2/3)O3 to BaTiO3 leads to an improvement of the thermoelectric figure of merit ZT. The results are explained on the basis of structural, morphological and compositional changes.

Isothermal depolarization currents of Na0.5Bi0.5TiO3 ceramics

ABSTRACT Na0.5Bi0.5TiO3 ceramics were synthesized by the conventional solid-state reaction method. The influence of polarizing electric field strength, polarization time, and temperature on the depolarization currents of these ceramics was studied. The measurements were made in both the rhombohedral (ferroelectric) and rhombohedral/tetragonal (nonpolar or weakly polar) phases coexistence. The results indicate that depolarization currents follow the Id∼t-s formula. The value of the s parameter depends on the electric field strength, the polarization time, and the temperature. The calculated activation energy is close to that obtained from electric conductivity measurements.

Influence of sintering conditions on structural, thermal, electric and ferroelectric properties of Na0.5Bi0.5TiO3 ceramics

ABSTRACT Na0.5Bi0.5TiO3 ceramics were prepared by a conventional solid-state reaction method and by a hot-pressing route. The influence of sintering conditions on structural, thermal, dielectric and ferroelectric properties of these ceramics was investigated. All obtained samples exhibited a single perovskite phase. It was shown that the sintering conditions significantly influence the properties under investigation. This includes changes in the value of the electric permittivity ϵ and dielectric loss tanδ, a shift of Tm and Td and change of the ferroelectric properties. These effects are mainly related to volatility of the Na and Bi components during the sintering process along with formation of their compensating charge defects, which leads to local structure change.