D. Mezzane

Lead-free Ba0.8Ca0.2(ZrxTi1−x)O3 ceramics with large electrocaloric effect

The electrocaloric effect was investigated in lead-free Zr doped Ba0.8Ca0.2(ZrxTi1−x)O3 (BCTZ) ceramics synthesized by a conventional sintering process. Room-temperature x-ray diffraction analysis showed that the tetragonal structure is obtained in BCTZ for x  ≤ 0.08 and a pseudo cubic phase for x > 0.08. The dielectric spectroscopy and calorimetry revealed that the Curie temperature decreases as a consequence of Zr doping and that the BCTZ exhibits a first order ferroelectric phase transition. The electrocaloric effect was determined by the calculation of the electrocaloric change of temperature (ΔT) using the Maxwell relation based on the P–E hysteresis loops measured at different temperatures. A large electrocaloric responsivity ΔT/ΔE = 0.34 × 10−6 Km/V was found for x = 0.04, which significantly exceeds of values found so far in other lead-free electrocaloric materials.

On the nature of phase transitions in the tetragonal tungsten bronze GdK2Nb5O15 ceramics

Phase transitions in gadolinium potassium niobate GdK2Nb5O15 (GKN) ceramics have been investigated by x-ray diffraction, dielectric susceptibility, differential scanning calorimetry, and Raman scattering. The results of our complementary studies show that GKN exhibits two phase transitions at Tc1 = 238 °C and Tc2 = 375 °C attributed to the ferroelectric-antiferroelectric-paraelectric (FE-AFE-PE) phase transitions. According to the x-ray diffraction analysis, the FE and PE phases were refined in the polar P4bm and centrosymmetric P4/mbm space groups. For the intermediate phase, the structure is refined in the space group P4nc with doubling of the c unit cell parameter, which is compatible with an AFE phase. This result was confirmed by Raman spectroscopy since new low-frequency lines are activated in the AFE phase due to the Brillouin zone-folding effect, typical for the modulated phases. The presence of the AFE phase between the FE and PE phases and the complex nature of tetragonal tungsten bronze crystal...

Electrocaloric effect in Ba0.2Ca0.8Ti0.95Ge0.05O3 determined by a new pyroelectric method

The present letter explores the electrocaloric effect (ECE) in the lead-free oxide Ba 0.8 Ca 0.2 Ti 0.95 Ge 0.05 O 3 ceramics (BCTG). The electrocaloric responsivity was determined by two different methods using the Maxwell relationship . In a first well-known indirect method, hysteresis loops were measured in a wide temperature range from which the pyroelectric coefficient and thus ξ were determined by derivation of data. In the second novel method the pyroelectric coefficient p E and consequently the electrocaloric responsivity ξ were determined by direct measurements of the pyroelectric currents under different applied electric fields. Within the experimental error, good agreement was obtained between two methods with at about 410 K.

Studies of Diffuse Phase Transition in Ferroelectric Solid Solution Pb1-xK2xNb2O6 (x = 0.1, 0.2, 0.25 and 0.3)

New compositions of Tetragonal Tungsten Bronze (TTB) ferroelectrics Pb1-xK2xNb5O15 (PKN) with x = 0.1, 0.2, 0.25 and 0.3 were synthesized using the solid state reaction method. Structural and electrical properties were examined as a function of the ratio Pb/K using X-ray diffraction, Raman spectroscopy and dielectric measurements. The dielectric permittivity and the loss tangent of each composition have been measured in a frequency range 10 Hz–1 MH and in a temperatures range 25–600°C. It was shown that the Pb substitution induces the diffuse phase transition.

Lead free Ba0.8Ca0.2TexTi1−xO3 ferroelectric ceramics exhibiting high electrocaloric properties

Direct and indirect electrocaloric measurements were performed on the new Tellurium (Te) doped Ba0.8Ca0.2TiO3 (BCT) ceramics. The effects of Te addition on structural, electrical, and electrocaloric properties of BCT ceramics were investigated. The incorporation of the Te element in the BCT induced the decrease of the Curie temperature and the enhancement of the electrocaloric effect. The significant electrocaloric temperature change ΔT = 1.237 K determined by the direct method was obtained at the relatively moderate field of ∼25 kV/cm in Ba0.8Ca0.2Ti(1−x)TexO3 with x = 0.02. The corresponding electrocaloric responsivity ΔT/ΔE = 0.495 × 10−6 K m V−1 is higher than that observed in pure BCT ceramics and is one of the highest reported so far in lead-free ferroelectric materials. The materials coefficient of performance was determined at the phase transition with a maximal value of 14.7.

Ferroelectric Phases in Rare-Earth TTB Ferroelectric Compounds Pb2(1 - x) K(1 + x)GdxNb5O15

A new family of rare-earth ferroelectric Tetragonal Tungsten Bronze (TTB) - type compounds with general formula Pb2(1− x) K1+ x Gd x Nb5O15 was elaborated by solid state reaction. Structural and dielectric properties were investigated using X-ray diffraction and dielectric measurements. The symmetry of the ferroelectric phase strongly depends of the lead concentration. The ferroelectric phase of the compounds with high lead concentration (x < 0.35) was found to be described by the symmetry group Cm2m whereas the compounds with low lead concentration (x > 0.35) have the symmetry space group Pba2. The ferroelectric transition temperature was shown to decrease with the concentration of lead.

Resistive Switching Hysteresis in Thin Films of Bismuth Ferrite

We have studied the resistive switching (RS) phenomenon in series of BiFeO3 thin films of thickness of 40 – 154 nm deposited by PLD technique on conducting Nb-doped substrate of SrTiO3 and with Pt top electrodes. It was found that 154 nm film demonstrates the interface-provided I–V characteristic of Schottky diode when the applied voltage does not exceed the threshold value Vd = 1.3 V. The RS phenomenon appears as the current hysteresis loop during the 0 → Vm → 0 → −Vm → 0 voltage sweep cycle, provided that the maximal stop-voltage Vm is larger than Vd. For thinner films neither diode-like I–V behavior nor substantial RS effect were observed. The results are interpreted in terms of the filamentary model of the mobile oxygen vacancies.

Impedance spectroscopy studies on lead free Ba 1-x Mg x (Ti 0.9 Zr 0.1 )O 3 ceramics

Abstract Ba1-xMgx(Ti0.9Zr0.1)O3 (x = 0.01 and 0.02) ceramics were prepared using the conventional solid state reaction. Rietveld refinement performed on X-ray diffraction patterns indicates that the samples are tetragonal crystal structure with P4mm space group. By increasing Mg content from 1 to 2% the unit cell volume decreased. Likewise, the grains size is greatly reduced from 10 μm to 4 μm. The temperature dependence of dielectric constants at different frequencies exhibited typical relaxor ferroelectric characteristic, with sensitive dependence in frequency and temperature for ac conductivity. The obtained activation energy values were correlated to the proposed conduction mechanisms.