J.P. Mercurio

Relaxor behaviour of low lead and lead free ferroelectric ceramics of the Na0.5Bi0.5TiO3–PbTiO3 and Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 systems

Abstract Ceramics with compositions belonging to the Na0.5Bi0.5TiO3–PbTiO3 and Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 systems were fabricated by natural sintering of powders prepared by thermal decomposition of adequate precursor solutions. The ferroelectric to paraelectric phase transitions were studied by variable temperature X-ray diffractometry, differential scanning calorimetry and impedance measurement in a wide range of temperature and frequency. In contrast with pure NBT, both the permittivity and dielectric loss of the NBT-rich solid solutions show a strongly temperature and frequency dependent behaviour. The permittivity decreases and its maximum is shifted towards high temperatures as the frequency increases. In the high temperature range, the thermal variation of the permittivity is well described by a law 1 e − 1 e m =C T−T m γ with γ close to 1.5. Such a relaxor-like behaviour is interpreted in terms of cation disorder due to the statistical repartition of (Na,Bi) and Pb (or Na and K). This would be one very rare case of relaxor phenomena correlated with the A-site occupancy in perovskite-like materials.

Sodium-bismuth titanate based lead-free ferroelectric materials

Abstract In search of lead-free ferroelectric ceramics with improved properties, an investigation was carried out on barium and strontium-modified Na0.5Bi0.5TiO3 (NBT) ceramics. Many compositions among the Ba and Sr-modified ceramic materials exhibit diffuse phase transition and are characterised by a strong temperature and frequency dispersion of the permittivity which would be connected with the cation disorder in the A site of the perovskite unit cell.

Temperature behaviour of structural, dielectric and piezoelectric properties of sol-gel processed ceramics of the system LiNbO3-NaNbO3

Abstract Ceramics of the system LiNbO 3 -NaNbO 3 were prepared from sol-gel derived powders in the whole range of compositions and structurally characterized by X-ray diffraction. The impedance of thin ceramic disks was measured as a function of the temperature from 1 kHz to 15 MHz up to 800 °C. An automatic iterative procedure was used to calculate the electromechanical coupling factor of the thickness mode resonance from impedance data. The solid solution limits of Na in LiNbO 3 and Li in NaNbO 3 were found to be 7mole% and 15mole%. As a consequence the materials whose nominal composition is Li 1− y Na x NbO 3 with 0.07 ≤ × ≤ 0.85 contain mixtures of the two limit solid solutions. For ceramics with Na content exceeding 10mole%, two dielectric anomalies at 300 and 400–450 °C are observed as local maximums in the real part of the dielectric permittivity when measurements are made while heating the sample. These are more enhanced as the Na content increases. There is a change of sign in the temperature coefficient of the conductivity of the ceramics giving place to a minimum at low temperature, 100–150 °C, which is not present at high frequency (> 5 MHz). There is a shift in the frequency number of the thickness mode of resonance towards higher values together with a minimum in the electromechanical coupling coefficient at about 350 °C. For high Na content ceramics (> 90mole%) an irreversible decrease to zero of the coupling coefficient is observed instead. The second anomaly in the real part of the dielectric permittivity and the anomalies in the electromechanical behaviour are related with the antiferroelectric-orthorhombic to paraelectric-orthorhombic transition reported for NaNbO 3 at 370 °C. The first anomaly in the real part of the dielectric permittivity and the change in the temperature coefficient of the conductivity are linked with a change in the conductivity mechanisms of localized charge carriers, allowed by a certain degree of structural disorder caused by the Na + substitution of the smaller Li + ion, or viceversa, as reported for similar systems.

Effect of axial pressure on electric properties of Pb-modified Na0.5Bi0.5Tio3 ceramic

Measurements of temperature/frequency dependence (30-500 °C) of electric permittivities of (Na 0.5 Bi 0.5 ) 0.9 Pb 0.1 TiO 3 ceramic under axial pressure (0-1500 bar) as well as hysteresis loops have been carried out. It was shown that this material is sensitive to external stress. This is connected with: (i) the shift of phase transition, diffuseness of permittivity characteristics, and reduction of thermal hysteresis, and (ii) the decrease in the field-polarization hysteresis. The results have been compared with the data for pure Na 0.5 Bi 0.5 TiO 3 .

Structural and dielectric study of theNa0.5Bi0.5TiO3–PbTiO3and K0.5Bi0.5TiO3–PbTiO3systems

A study of the Na 0.5 Bi 0.5 TiO 3 –PbTiO 3 and K 0.5 Bi 0.5 TiO 3 -PbTiO 3 systems has been carried out using X-ray diffraction, differential scanning calorimetry and dielectric measurements. The limits of the rhombohedral (Na 0.5 Bi 0.5 TiO 3 -rich side) and tetragonal (PbTiO 3 -rich side) solid solutions have been determined, as well as the evolution of their lattice parameters as a function of composition and temperature. Ceramic materials have been prepared by natural sintering (1090–1220 °C; 0.5 h) of powders obtained by solid-state reaction (900–1000 °C; 20 h) of the corresponding oxides and carbonates. The dielectric permittivities of these materials have been measured in a wide frequency range for temperatures between 20 and 800 °C. The results showed that they all are ferroelectric at room temperature and exhibit either a diffuse, probably second-order phase transition, or a sharp, first-order-like phase transition from the ferroelectric to the paraelectric state, depending on the composition.

Dielectric properties of the mixed aurivillius phases MIIBi8Ti7O27 (MII = Ca, Sr, Ba and Pb)

Abstract Ceramic materials have been prepared by natural sintering and hot-forging M II Bi 8 Ti 7 O 27 (M II = Ca, Sr, Ba and Pb) powders synthesized using a molten salt technique. The materials are ferroelectric at room temperature and show strong dielectric anisotropy in connection with the crystal structure. In each case, a double anomaly of the dielectric permittivity was found as a function of temperature. The temperatures at which it occurs are dependent on M II . The origin of this phenomenon is assumed to be closely related to a sequence of phase transitions like those already observed in other mixed Aurivillius phases.

Electric Properties of (Na 0.5 Bi 0.5 ) 0.86 Ba 0.14 TiO 3 Single Crystal

Temperature dependence of electric permittivity, pyroelectric current, piezoelectric coefficients (d 33 and d 31 ) and electromechanical coupling factors (k 33 and k 31 ) of (Na 0.5 Bi 0.5 ) 0.86 Ba 0.14 TiO 3 single crystal have been measured. The presented crystal exhibits diffuse phase transition, ferroelectric properties, high piezoelectric coefficient (d 33 , 230 pC/N), and high electromechanical coupling factor (k 33 , 60%). These characteristics make this material interesting for applications.

Microwave dielectric properties of BaNd2(1−xSm2xTi5O14 ceramics

Abstract Microwave dielectric resonators with large dielectric constant (66–72), low loss and temperature stable resonant frequency at about 4 GHz have been obtained by adjusting the Nd Sm ratio in ceramics sintered from BaNd 2(1− x Sm 2 x Ti 5 O 14 solid solutions.

Fabrication of SrRuO3 powders and thin films by metalorganic decomposition

Abstract Pure SrRuO 3 powders and thin films were obtained using sol–gel synthesis from mixtures of strontium acetylacetonate and ruthenium nitrosylnitrate in 2-methoxyethanol. Fast firing conditions of xerogel (20–600°C, 600°C min −1 and then 600–800°C, 5°C min −1 ) lead to the crystallisation of pure SrRuO 3 powder. SrRuO 3 thin films were prepared by spin-coating a stable precursor solution onto (111)Si wafers and (001)SrTiO 3 single crystals. Whereas they show random orientation on Si wafers, the films are strongly (110) oriented when deposited on (001)SrTiO 3 .

Axial Pressure Effect on Dielectric and Ferroelectric Properties of K 0.5 Bi 0.5 TiO 3 Ceramic

A dense ceramic sample of KBT of very good mechanical quality has been prepared by the conventional method. We show that the intermediate phase of KBT has ferroelectric character and that polar regions can exist at temperatures above T m (that of the permittivity maximum). It is found that external stresses increase the phase transition temperatur and decrease the thermal hysteresis. This might indicate a change with nature of the transition from first order to second order, while decrease of the polarization. These changes of properties could be connected with domain walls moving and with switch of the polarization under axial pressure.