arXiv subjects

Y. Gagou

Publications and source records attributed to Y. Gagou.

11 recordsLinked to original sources

arXiv2023

Structural, dielectric, ferroelectric and electrical properties of lead-free Ba0.9Sr0.1Ti0.9Sn0.1O3 ceramic prepared by sol-gel method

Lead-free Ba0.9Sr0.1Ti0.9Sn0.1O3 (BSTSn) ceramic was prepared by the sol gel method. X-ray diffraction measurement at room temperature reveals the single-phase formation with tetragonal structure refined in the P4mm space group. Raman spectroscopy analysis at room temperature was used to confirm the structure of BSTSn ceramic sintered at 1420C during 5h. The dielectric measurements were examined in the temperature range [-90 250 C] and the frequency range of [100Hz 1MHz]. The maximum dielectric constant was found to be 5484 (1kHz) at room temperature. The diffuse phase transition characteristic at 1kHz was ascribed by the modified Curie Weiss law. Macroscopic polarization field (P E) hysteresis loops recorded at room temperature (RT) confirm the ferroelectric nature of BSTSn ceramic. Impedance spectroscopy analysis of BSTSn sample in the temperature range 300 380 C reveals the presence of two relaxation contributions originated from grains and grain boundaries. Activation energies of grain and grain boundaries resistances were deduced in order to determine the conduction mechanism of the studied sample.

cond-mat.mtrl-sci↗

arXiv2023

Improvement of the electrocaloric effect and energy storage performances in Pb-free ferroelectric Ba0.9Sr0.1Ti0.9Sn0.1O3 ceramic near room temperature

Perovskite-type Ba0.9Sr0.1Ti0.9Sn0.1O3(BSTSn) ceramic was synthesized by the sol gel method. The P E hysteresis loops were recorded at different temperatures to investigate the ferroelectric and energy storage properties of BSTSn ceramic. Enhanced recoverable energy density and high energy storage efficiency were found to be 58.08 mJ/cm3 and 84.36 %, respectively at room temperature (RT) under a moderate applied electric field of 20 kV/cm. The electrocaloric effect (ECE) in the BSTSn ceramic was explored using two different approaches based on P E hysteresis loops, and pyroelectric current measurements. The largest electrocaloric (EC) temperature change of 0.63 K was determined using the Maxwell relationship obtained near RT under 20 kV/cm. The corresponding EC responsivity value of 0.31 K.mm/kV is one of the highest reported values in lead-free ferroelectrics near RT. This study demonstrates that the BSTSn ceramic is a promising candidate for solid-state cooling and energy storage applications.

cond-mat.mtrl-sci↗

arXiv2020

Enhancing the dielectric, electrocaloric and energy storage properties of lead-free Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics prepared via sol-gel process

The Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) ceramics were successfully prepared by sol-gel process, and sintered at 1420 degree. The effect of sintering times (2, 4 and 6 h) on structural, microstructural, electric properties, energy storage, and electrocaloric effect was systematically examined. X-ray diffraction (XRD) results show that all the samples crystallize in the pure perovskite structure. The morphotropic phase transition from the tetragonal to the orthorhombic phase (T-O) was identified and confirmed by Rietveld refinement. The BCZT ceramic sintered at 1420 degree for 4h possesses a good relative density of 98%, and exhibits optimal properties with a high dielectric permittivity ~16310, a large electrocaloric effect coefficient ~ 0.244Kmm/kV and an energy density storage of ~ 10.61 mJ/cm3.

cond-mat.mtrl-sci↗

arXiv2020

The electrocaloric response in Lanthanum-modified lead zirconate titanate ceramic

Over the past decades, there has been significant interest in new cooling technology based on the electrocaloric effect. The large electrocaloric effect observed in polymeric and inorganic ferroelectric materials made possible development of dielectric cooling devices of a new generation. We report a significant impact of annealing on the electrocaloric effect observed in 12/65/35 PLZT bulk ceramics. The electrocaloric data were obtained by direct measurements. Electrocaloric results confirm the existence of the significant electrocaloric response in this relaxor ferroelectric PLZT composition exceeding previously obtained electrocaloric values in perovskite relaxor ferroelectrics such as PMN-PT and x/65/35 PLZT ceramics.

cond-mat.mtrl-sci↗

arXiv2020

Structural, dielectric and magnetic studies of (0-3) type multiferroic (1-x) BaTi0.8Sn0.2O3-(x) La0.5Ca0.5MnO3 (0 <x< 1) composite ceramics

Multiferroic composites with the composition (1-x) BaTi0.8Sn0.2O3 - (x) La0.5Ca0.5MnO3 (x =0, 0.3, 0.5, 0.7, 0.9 and 1) were prepared by sol-gel techniques. Rietveld refinements of the composites with 0-3 and 3-0 type connectivities exhibit the formation of pure phases and confirm the fraction of the composite samples. It was found that the composites grain size decreases slightly with increasing the La0.5Ca0.5MnO3 content. Magnetic measurements of the composite ceramics exhibit a gradual increase in saturation and remanent magnetization with increasing La0.5Ca0.5MnO3 content. Dielectric data demonstrate that the composite materials show high values of the dielectric constant in comparison with the parent compound BaTi0.8Sn0.2O3 (BTSO). In addition, an attempt is made to measure the magnetoelectric coupling coefficient and to explain the negative permittivity and losses occurring in these materials at a certain frequency and temperature conditions.

cond-mat.mtrl-sci↗

arXiv2020

Structural, dielectric, ferroelectric and tuning properties of Pb-free ferroelectric Ba0.9Sr0.1Ti1-xSnxO3

A series of Pb-free ferroelectric materials Ba0.9Sr0.1Ti1-xSnxO3 (BSTS-x) with 0 < x < 0.15 was successfully prepared via solid-state reaction method. The effect of Sn substitution on the crystal structure, microstructure, dielectric behavior, ferroelectric and tunable features of BSTS-x ceramics were systematically investigated. Room temperature (RT) x-ray diffraction (XRD) analysis using the Rietveld refinement method reveals that all the synthesized BSTS-x ceramics were well crystallized into single perovskite structure. The results show a tetragonal phase for 0.00 < x < 0.02, which evolves to orthorhombic and tetragonal coexisting phases for 0.05 < x < 0.07. The composition x = 0.10 showed a mixture of tetragonal, orthorhombic and rhombohedral phases at RT, while a single cubic phase is observed for x = 0.15. The crystal phases determined by XRD were confirmed by Raman spectroscopy. Enhanced dielectric permittivity with a maximum value of ε'~ 35000 is observed for x = 0.10 at RT. The ferroelectric behavior of BSTS-x ceramics was investigated through polarization hysteresis loops and tunability measurements. High tunability of 63% at RT and under the low DC-applied electric field of 1.40 kV/cm is achieved for x = 0.10.

cond-mat.mtrl-sci↗

arXiv2020

Electrocaloric response in Lanthanum-modified lead zirconate titanate ceramics

Recent findings of a large electrocaloric (EC) effect in polymeric and inorganic ferroelectric materials open a potential possibility of development of solid-state cooling or heating devices of new generation with better energy efficiency that may be less harmful for the environment. We investigate by using direct measurements, the temperature and electric field dependence of the electrocaloric response in Pb1-xLax(ZryTi1-y)1-x/4O3 bulk ceramics (PLZT) with x=0.06 and 0.12. Here, the properties of the EC response were probed in a part of the PLZT composition phase diagram with low y=0.40 composition, in which the EC effect was not previously studied. Measurement results show the existence of the sizeable EC response in 12/40/60 PLZT sample with the EC temperature change (ΔTEC) of 2.92 K at 430 K and 80 kV/cm. This value exceeds previously obtained ΔTEC values in relaxor ferroelectric x/65/35 PLZT compositions and rivaling the best EC response in lead magnesium niobate-lead titanate ceramics. The electrocaloric responsivity (ΔT/ΔE) value of 0.41x10-6 Km/V determined at a lower electric field of 20 kV/cm and 410 K is comparable to those observed in other perovskite ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2019

Synthesis of La0.5Ca0.5-x xMnO3 nanocrystalline manganites by sucrose assisted auto combustion route and study of their structural, magnetic and magnetocaloric properties

Perovskite manganite La0.5Ca0.5-x xMnO3 (LCMO) nanomaterials were elaborated using the sucrose modified auto combustion method. Rietveld refinements of the X-ray diffraction patterns of the crystalline structure confirm a single-phase orthorhombic state with Pbnm space group (No. 62). The Ca-vacancies were voluntarily created in the LCMO structure in order to study their influence on the magnetic behaviour in the system. The magnetic susceptibility was found to be highly enhanced in the sample with Ca-vacancies. Paramagnetic-to-ferromagnetic phase transition was evidenced in both samples around 254 K. This transition is, characterized by a drastic jump of the susceptibility in the sample with Ca-vacancies. The maximum of entropy change, observed for both compounds at magnetic field of 6T was 2.30 J kg-1K-1 and 2.70 J kg-1K-1 for the parent compound and the lacunar one respectively. The magnetocaloric adiabatic temperature change value calculated by indirect method was 5.6 K and 5.2 K for the non-lacunar and Ca-vacancy compound, respectively. The Ca-lacunar La0.5Ca0.5-x xMnO3 (x=0.05) reported in this work demonstrated overall enhancement of the magnetocaloric effect over the LCMO. The technique used to elaborate LCMO materials was beneficial to enhance the magnetocaloric effect and magnetic behaviour. Therefore, we conclude that this less costly environmentally friendly system can be considered as more advantageous candidate for magnetic refrigeration applications then the commonly Gd-based compounds.

cond-mat.mtrl-sci↗

arXiv2019

Structural, dielectric and magnetic properties of multiferroic (1-x) La0.5Ca0.5MnO3-(x) BaTi0.8Sn0.2O3 laminated composites

High performance lead-free multiferroic composites are desired to replace the lead-based ceramics in multifunctional devices applications. Laminated compounds were prepared from ferroelectric and ferromagnetic materials. In this work, we present laminated ceramics compound by considering the ferromagnetic La0.5Ca0.5MnO3 (LCMO) and the ferroelectric BaTi0.8Sn0.2O3 (BTSO) in two different proportions. Compounds (1-x) LCMO-(x) BTSO with x=1 and 0 (pure materials) were synthesized by the sol gel method and x=0.7 and 0.5 (laminated) compounds were elaborated by welding appropriate mass ratios of each pure material by using the silver paste technique. Structural, dielectric, ferroelectric, microstructure and magnetic characterization were conducted on these samples. X-ray scattering results showed pure perovskite phases confirming the successful formation of both LCMO and BTSO. SEM images evidenced the laminated structure and good quality of the interfaces. The laminated composite materials have demonstrated a multiferroic behavior characterized by the ferroelectric and the ferromagnetic hysteresis loops. Furthermore, the enhancement of the dielectric constant in the laminated composite samples is mainly attributed to the Maxwell-Wagner polarization.

cond-mat.mtrl-sci↗

arXiv2015

Ferrielectricity in smectic-C* dechiralisation lines lattices

Recent experiments probing a new ferroelectric liquid crystal (CLF08) confined in cells with planar alignment have shown dielectric and optic anomalies suggesting the onset of ferrielectric ordering within the surface lattice of dechiralisation lines. We present a phenomenological theory describing the corresponding phase transitions sequence SmA->SmC*->Ferri. Phase diagrams, thermodynamic, dielectric and optic properties are worked out and compared with experiments. The anomalies are related to the predicted tristability of the experimental cells under applied electric field. The order parameters of Landau theory are reinterpreted in terms of line positions, allowing the description of the entrance/exit lines behavior, and yielding the prediction and identification of new limit phases within a non-conventional Landau approach.

cond-mat.soft↗

arXiv2015

Electrocaloric effect in Ba(0.2)Ca(0.8)Ti(0.95)Ge(0.05)O(3) determined by a new pyroelectric method

The present letter explores the electrocaloric effect (ECE) in the lead free oxide Ba0.8Ca0.2Ti0.95Ge0.05O3 ceramics (BCTG). The electrocaloric responsivity (dT/dE) was determined by two different methods using the Maxwell relationship (dT/dE)~(dP/dT)_E. In a first well-known indirect method, P-E hysteresis loops were measured in a wide temperature range from which the pyroelectric coefficient p_E=(dP/dT)_E and thus (dT/dE) were determined by derivation of P(T,E) data. In the second novel method the pyroelectric coefficient p_E and consequently the electrocaloric responsivity was 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 an electrocaloric responsivity equal to 0.18 +/- 0.05 10-6 K.m.V-1 was obtained at about 410 K

cond-mat.mtrl-sci↗