8 research outputs found
Phase diagram of TTB ferroelectric compounds Pb1âxK2xNb2O6
Substitution of Pb with K in the PbNb2O6 phases leads to a new composition of solid solution with chemical composition Pb1âxK2xNb2O6 with x ranging from 0 to 0.34 in steps of 0.05. Ferroelectric ceramics were synthesized using solid state reaction between the corresponding oxides and carbonates. Powders are pressed and heated into ceramics and their compaction is about 92%. The tetragonal tungsten bronze (TTB) structure at room temperature was confirmed by X-ray diffraction (XRD). The temperature dependence of dielectric permittivity was measured from 35 to 600°C in the 20-106 Hz frequency range. Transition temperature decreases with the lead concentration while from x=0, with Tc=600°C and to x=0.3 with Tc=388°C.  These measurements permit to present a basical phase diagram of this family compound showing the evolution of the characteristic transition temperature Tc versus temperature. The behaviour of Tc is in agreement with theoretical study of the ferroelectric phase transition in TTB using Monte Carlo (MC) simulation. The dielectric properties of these ceramics are similar to those obtained on a single crystal and illustrate the quality of preparative conditions.Substitution of Pb with K in the PbNb2O6 phases leads to a new composition of solid solution with chemical composition Pb1âxK2xNb2O6 with x ranging from 0 to 0.34 in steps of 0.05. Ferroelectric ceramics were synthesized using solid state reaction between the corresponding oxides and carbonates. Powders are pressed and heated into ceramics and their compaction is about 92%. The tetragonal tungsten bronze (TTB) structure at room temperature was confirmed by X-ray diffraction (XRD). The temperature dependence of dielectric permittivity was measured from 35 to 600°C in the 20-106 Hz frequency range. Transition temperature decreases with the lead concentration while from x=0, with Tc=600°C and to x=0.3 with Tc=388°C.  These measurements permit to present a basical phase diagram of this family compound showing the evolution of the characteristic transition temperature Tc versus temperature. The behaviour of Tc is in agreement with theoretical study of the ferroelectric phase transition in TTB using Monte Carlo (MC) simulation. The dielectric properties of these ceramics are similar to those obtained on a single crystal and illustrate the quality of preparative conditions
Dielectric Properties Of Lead Potassium Lithium Niobate (Pb1,85K1,15Li0,15Nb5O15) With Tetragonal Tungsten Bronze (TTB) Type Structure
A new tungsten bronze ceramic oxide, Pb2-xK1+xLixNb5O15 (PKLN) (x =0.15) was prepared by high temperature solid-state reaction route. Structural and electrical properties are investigated using X-ray diffraction and dielectric measurements. Room temperature XRD pattern confirms the formation of the compound with an orthorhombic crystal system. The dielectric permittivity and the loss tangent of the sample have been measured in a frequency range 1Hzâ1MHz and a temperature range 35â550 °C. Studies of dielectric properties show that the compound exhibits an anomaly at 425°C (usually called transition temperature).The electrical parameters of the material were studied using complex impedance spectroscopy showing that the compound exhibits non-Debye of relaxation process. In the paraelectric phase, activation energy was determined and the value is EÏ = 0.68 eV. The present ceramic is promising candidate for high dielectric constant and low loss dielectric ceramic.A new tungsten bronze ceramic oxide, Pb2-xK1+xLixNb5O15 (PKLN) (x =0.15) was prepared by high temperature solid-state reaction route. Structural and electrical properties are investigated using X-ray diffraction and dielectric measurements. Room temperature XRD pattern confirms the formation of the compound with an orthorhombic crystal system. The dielectric permittivity and the loss tangent of the sample have been measured in a frequency range 1Hzâ1MHz and a temperature range 35â550 °C. Studies of dielectric properties show that the compound exhibits an anomaly at 425°C (usually called transition temperature).The electrical parameters of the material were studied using complex impedance spectroscopy showing that the compound exhibits non-Debye of relaxation process. In the paraelectric phase, activation energy was determined and the value is EÏ = 0.68 eV. The present ceramic is promising candidate for high dielectric constant and low loss dielectric ceramic
Enhancement of the electrocaloric effect in the 0.4BCZT-0.6BTSn ceramic synthesized by sol-gel route
The lead-free ferroelectric 0.4Ba _0.85 Ca _0.15 Zr _0.10 Ti _0.90 O _3 â0.6BaTi _0.89 Sn _0.11 O _3 (0.4BCZTâ0.6BTSn) ceramics were successfully prepared by the solâgel process. Raman spectroscopy was used to examine the structural properties of the 0.4BCZT-0.6BTSn sample. The findings indicate that the sample was well crystallized into a single perovskite structure. The phase transitions of the studied sample have been investigated using the DSC technique. The electrocaloric effect (ECE) properties were indirectly determined using the Maxwell approach. Under a relatively low applied electric field of 30 kV cm ^â1 , the results show enhanced electrocaloric temperature change and entropy change of ÎT = 1.32 K and ÎS = 1.41 J/kg.K, respectively. Besides, the electrocaloric responsivity ( Ο _max = 0.45 K·mm/kV) obtained is among the highest reported values in pb-free ferroelectrics near room temperature. These findings demonstrate that the lead-free 0.4BCZTâ0.6BTSn ceramic is a promising candidate for solid-state cooling applications
Dielectric properties and relaxation phenomena in the diffuse ferroelectric phase transition in K
Structural and dielectric properties of Potassium Lithium Niobate polycrystalline ceramic K3Li2Nb5O15 (KLN), having the tetragonal tungsten bronze (TTB) â type structure are studied in the temperature interval 50â550 °C. Special emphasis is given to the diffuse phase transition occurring around 440 °C. Space charge polarization, relaxation phenomena and free charge conductivity have been elucidated using impedance spectroscopy technique. Argand plots have revealed a non Debye and polydispersive type relaxation. In paraelectric phase the Arrhenius activation energy EÏ = 0.533 eV was determined. The structural and dielectric results are compared with two others TTB compounds derived from KLN family: Pb1.85K1.15Li0.15Nb5O15 (PKLN) and GdK2Nb5O15 (GKN)
Dielectric and structural properties of diffuse ferroelectric phase transition in Pb
We studied the structural and dielectric properties of new Tetragonal
Tungsten Bronze (TTB) ceramics
Pb1.85K1.15Li0.15Nb5O15 that was synthesized by
solid-state reaction. We pay a special attention to the diffuse phase
transition (DPT) that occurs close to 425 °C. Using dielectric
measurements in a frequency range of 10Â Hzâ1Â MHz and in the temperature range
30â560 °C, we have shown that the real permittivity close to DPT is well
described by Santos-Eiras phenomenological model. Space-charge
polarization, relaxation phenomena and free charges conductivity have been
analyzed using dielectric spectroscopy impedance and modulus
characterization. Cole-Cole plots show a non-Debye (polydispersive) type
relaxation. In paraelectric phase the Arrhenius activation energy was
determined as EÏ = 0.72Â eV. We demonstrated that frequency
dependence of ac conductivity at different temperatures obeys the
Jonscher's universal law: Ïac = Ïdc + A(Ï)n
Enhanced electrical properties and large electrocaloric effect in lead-free Ba0.8Ca0.2ZrxTi1âxO3 (xâ=â0 and 0.02) ceramics
The effects of 2% Zr introduction in Ba0.8Ca0.2TiO3 (BCT) system on its electrical and electrocaloric properties was investigated. BCT and Ba0.8Ca0.2Zr0.02Ti0.98O3 (BCZT) ceramics synthesized by solid-state processing were crystallized in a pure perovskite phase with a group space P4mm. After Zr insertion, the enhanced dielectric constant was obtained around the Curie temperature (Tc) in BCZT ceramic (Δrâ=â6330 at Tcâ=â388 K) compared to BCT ceramic (Δrâ=â5080 at Tcâ=â388.6 K). Moreover, the large-signal piezoelectric coefficient (dâ33) was improved from 270 to 310 pm/V in BCT and BCZT ceramics, respectively, under a moderate electric field of 25 kV/cm. The electrocaloric effect was determined via indirect and direct methods. In the indirect approach, the electrocaloric temperature change (ÎT) was calculated via Maxwell relation, and the measured ferroelectric polarization P (E, T) extracted from the PâE curves recorded at 24 kV/cm. The maximum values of ÎTâ=â0.68 K and the electrocaloric responsivity ζâ=â0.283 K mm/kV obtained at 385 K in BCZT ceramic were found to be higher than those observed in BCT ceramic (ÎTâ=â0.37 K and ζâ=â0.154 K mm/kV at 387 K). In the direct approach, ÎT was measured utilizing a modified high-resolution calorimeter at 14 kV/cm. As the direct method is more sensitive to the latent heat, it provided larger values for smaller applied field, i.e., ÎTâ=â0.474 and 0.668 K for BCT and BCZT ceramics, respectively. A significant ζ of 0.477 K mm/kV was obtained in BCZT at 385 K and 14 kV/cm that matches the values found in lead-based materials. These results suggest that BCZT lead-free ceramics could have an excellent potential to be used in solid-state refrigeration applications