K. Kluczewska

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.

Electrical transport in low-lead (1−x)BaTiO3–xPbMg1/3Nb2/3O3 ceramics

Low-lead (1−x)BaTiO3–xPbMg1/3Nb2/3O3 ceramics (x = 0, 0.025, 0.05, 0.075, 0.1, and 0.15) were prepared by the conventional oxide mixed sintering process, and their optical band gap, Seebeck coefficient, ac (σac) and dc (σdc) conductivities as a function of temperature were investigated for the first time. It was found that all samples have p-type conductivity. The low-frequency (20 Hz–2 MHz) ac conductivity obeys a power law σac~ωs, which is characteristic for disordered materials. The frequency exponent s is a decreasing function of temperature and tends to zero at high temperature. σac is proportional to ω0.07–ω0.31 in the low-frequency region and to ω0.51–ω0.98 in the high-temperature region. The temperature dependence of the dc conductivity showed a change in slope around the temperature at which the phase transition appeared. Both ac and dc conductivities showed a thermally activated character and possessed linear parts with different activation energies and some irregular changes. It was found that the hopping charge carriers dominate at low temperature and small polarons and oxygen vacancies dominate at higher temperature. (1−x)BaTiO3–xPbMg1/3Nb2/3O3 ceramics are expected to be promising new candidate for low-lead electronic materials.

Dielectric, thermal and Raman spectroscopy studies of lead-free (Na0.5Bi0.5)1−xSrxTiO3 (x = 0, 0.04 and 0.06) ceramics

ABSTRACT Lead-free (Na0.5Bi0.5)1−xSrxTiO3 (x = 0, 0.04 and 0.06) ceramics with relative densities above 97% were prepared by solid-state synthesis process. Their dielectric, thermal and Raman properties were studied. X-ray diffraction analysis shows perovskite structure with rhombohedral symmetry at room temperature. Sr doping of Na0.5Bi0.5TiO3 (NBT) results in an increase of the dielectric permittivity, diffusing of the permittivity maximum and its shift toward lower temperatures. The temperature of the rhombohedral–tetragonal phase transition indicated by the differential scanning calorimetry (DSC) peak and relaxational dielectric anomaly near the depolarization temperature are also shifted toward lower temperatures. The observed increase and broadening of the permittivity maximum, enhancement of the dielectric relaxation near the depolarization temperature, broadening of the DSC anomaly related to the rhombohedral–tetragonal phase transition and broadening of the Raman bands with increasing Sr content are attributed to the increase of the degree of cationic disorder and evident enhancement of the relaxor-like features in NBT–xST. This enhancement could play a positive role in the improvement of the piezoelectric performance of NBT-based 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.

Influence of PbTiO3 doping on semiconductor behavior of K0.5Bi0.5TiO3 ceramics

ABSTRACT High-quality ceramics of (1 − x)KBT–xPT (x = 0.18 and 0.5) were fabricated by a conventional solid-phase synthesis method. Both materials exhibit the features of perovskite solid solutions. The temperature dependence of dc, ac conductivity (, respectively), the Seebeck coefficient (α), the concentration of charge carriers (n) and their mobility (μ) were investigated. DC conductivity plot for both materials reveals four linear sections with four different activation energies Ea. The ac conductivity exhibits four linear sections for 0.82KBT–0.18PT and three for the second composition. The possible causes of the observed effects are examined. Finally, KBT–PT solid solutions are recommended for use in electronic applications.

Thermal, Raman, dielectric and ferroelectric properties of 0.975BaTiO3-0.025Pb(Zn1/3Nb2/3)O3 ceramic

ABSTRACT A new low-lead content 0.975BaTiO3-0.025PbZn1/3Nb2/3O3 (0.975BT-0.025PZN) ceramic with a high relative density and good mechanical quality was fabricated by the SPS method. X-ray diffraction measurements showed that the obtained specimen possesses a pure perovskite structure. The composition undergoes a sequence of phase transitions as pure BT. Dielectric studies revealed that the dielectric permittivity decreases at its maximum and the phase transitions shift after PZN-doping of BT. In addition, the dielectric dispersion and polarization increases and decreases, respectively. Possible reasons for the observed properties change are discussed. The results show that the investigated ceramic is a promising low-lead material for electronic applications.