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Yaovi Gagou

Publications and source records attributed to Yaovi Gagou.

12 recordsLinked to original sources
arXiv2023

Enhanced RCP and large inverse magnetocaloric effect of CoFe$_2$O$_4$ nanoparticles synthesized by auto-combustion method

This work focuses on the microstructure, magnetic properties and magnetocaloric effect of CoFe$_2$O$_4$ (CFO) nanoparticles elaborated by sol-gel auto combustion method. The XRD investigation indicates that CFO is crystallized in a cubic spinel structure and the SEM micrograph shows a fine quasi-spherical with an average grain sizes of 160 nm. The temperature dependence of the Raman spectra reveals the ferromagnetic to paramagnetic (FM-PM) transition started from 723 K and the magnetization versus temperature measurements shows the Curie temperature located at T$_{\rm C}$ = 785 K. Large value of magnetocaloric temperature change of $Δ$T =11.2 K with a high RCP of 687.56 J Kg$^{-1}$ are achieved indirectly via the Maxwell approach making our CFO nanopowder suitable candidate for both environmentally friendly magnetic refrigeration and medical applications at ambient temperature.

cond-mat.mtrl-sci↗
arXiv2023

Magnetoelectric coupling in multiferroic CFO/BCTSn core shell nanofibers elaborated by co-axial electrospinning method

Multiferroic CoFe2O4-Ba0.95Ca0.05Ti0.89Sn0.11O3 core-shell nanofibers (CFO@BCTSn NFs) were synthesized by a sol-gel co-axial electrospinning technique. The scanning electron microscope and transmission electron microscope were used to check nanofibers' core-shell structure/configuration. X-ray diffraction and a high-resolution transmission electron microscope were used to confirm the spinel structure of CFO and the perovskite structure of BCTSn. The magnetic character of the resultant CFO@BCTSn NFs was determined by SQUID magnetometry. The piezoelectricity was verified using piezo-response force microscopy, which revealed an entirely covered ferroelectric shell outline, in accordance with SEM and TEM observations. The magnetoelectric (ME) coefficient was measured as a function of the applied external DC magnetic field. The maximum ME coefficient obtained for the CFO@BCTSn NFs was 346 mV cm-1 Oe-1. The high magnetoelectric coupling suggests that CFO@BCTSn NFs could be a promising candidate for magnetic field sensor and magnetoelectric device applications.

cond-mat.mtrl-sci↗
arXiv2023

Room-temperature magnetoelectric effect in lead-free multiferroic $(1-x)$ Ba$_{0.95}$Ca$_{0.05}$Ti$_{0.89}$Sn$_{0.11}$O$_3$-$(x)$CoFe$_2$O$_4$ particulate composites

Multiferroic particulate composites $(1-x)$ Ba$_{0.95}$Ca$_{0.05}$Ti$_{0.89}$Sn$_{0.11}$O$_3$-$(x)$CoFe$_2$O$_4$ with ($x$ = 0.1, 0.2, 0.3, 0.4 and 0.5) have been prepared by mechanical mixing of the calcined and milled individual ferroic phases. X-ray diffraction and Raman spectroscopy analysis confirmed the formation of both perovskite Ba$_{0.95}$Ca$_{0.05}$Ti$_{0.89}$Sn$_{0.11}$O$_3$ (BCTSn) and spinel CoFe$_2$O$_4$ (CFO) phases without the presence of additional phases. The morphological properties of the composites were provided by using Field Emission Scanning Electron Microscopy. The BCTSn-CFO composites exhibit multiferroic behavior at room temperature, as evidenced by ferroelectric and ferromagnetic hysteresis loops. The magnetoelectric (ME) coupling was measured under a magnetic field up to 10 kOe and the maximum ME response found to be 0.1 mV /cm/ Oe for the composition 0.7 BCTSn-0.3 CFO exhibiting a high degree of pseudo-cubicity and large density.

cond-mat.mtrl-sci↗
arXiv2022

Enhanced electrocaloric and energy-storage properties of environment-friendly ferroelectric Ba0.9Sr0.1Ti1-xSnxO3 ceramics

The electrocaloric (EC) effect and energy storage properties of eco-friendly ferroelectric Ba0.9Sr0.1Ti1-xSnxO3 (BSTS-x) ceramics prepared by the conventional solid-state reaction method were studied. Significant energy efficiency exceeding 80% was found in our samples. In particular, BSTS-5 and BSTS-10 samples exhibit 92% and 88% efficiency, respectively, over a wide temperature range around room temperature (RT). Direct EC measurement and an indirect method based on the thermodynamic approach were used to characterize the EC effect, and both yielded consistent results. The largest electrocaloric responsivity of 0.73 K mm/kV was obtained for BSTS-0 at 368 K with an adiabatic temperature change DT(EC) of 0.55 K at a low applied electric field of only 7.4 kV/cm. With increasing Sn-content, the EC response of BSTS-x ceramics decreases, while their diffuseness increases over an extensive temperature range, centered at RT. A significant coefficient of performance (COP > 26) was found for our samples. The obtained results demonstrate the possibility of designing eco-friendly materials with higher EC effect and energy efficiency for potential solid-state refrigeration and energy storage devices.

cond-mat.mtrl-sci↗
arXiv2022

Electrocaloric effect and high energy storage efficiency in lead-free Ba$_{0.95}$Ca$_{0.05}$Ti$_{0.89}$Sn$_{0.11}$O$_3$ ceramic elaborated by sol-gel method

Structural, dielectric, ferroelectric, energy storage properties, and electrocaloric effect were studied in lead-free ceramic Ba$_{0.95}$Ca$_{0.05}$Ti$_{0.89}$Sn$_{0.11}$O$_3$ (BCTSn) elaborated by sol-gel method. Phase purity structure was confirmed from X-ray data using Rietveld refinement analysis which revealed the coexistence of tetragonal (P4mm) and orthorhombic (Amm2) symmetries at room temperature. Phase transitions were detected by dielectric and differential scanning calorimetry results. Energy storage properties were determined from P-E hysteresis, and the electrocaloric properties were calculated indirectly via the Maxwell approach. The large value of electrocaloric temperature change of $Δ$T=0.807 K obtained at a relatively small field of 30 kV cm$^{-1}$ and high energy storage efficiency can make BCTSn ceramic a promising candidate for environmentally friendly refrigeration and energy storage applications.

cond-mat.mtrl-sci↗
arXiv2020

Structural, Dielectric and Ferroelectric Properties of Lead-free BCZT Ceramics Elaborated by Low-temperature Hydrothermal Processing

Lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) ceramics have demonstrated excellent dielectric, ferroelectric and piezoelectric properties in comparison to lead-based materials. The synthesis of pure and crystalline BCZT nanopowders at low temperatures of 25, 80 and 160 C was reported previously by using a sol-gel method followed by a hydrothermal route. In this study, the structural, dielectric and ferroelectric properties of sintered BCZT ceramics at 1250 °C for 10h were investigated. XRD measurements revealed the presence of a single perovskite phase at room temperature with the coexistence of the orthorhombic and tetragonal symmetries. The increase of grain size and the ceramic density in BCZT ceramics result in an enhancement of the dielectric and ferroelectric properties of BCZT ceramics. More interestingly, the synthesis temperature of BCZT powders with high dielectric and ferroelectric properties could be decreased to a low temperature of 160 °C, which is about 1200 °C lower when compared with solid-state reaction and 840 °C lower when compared with sol-gel methods. The BCZT ceramics elaborated at 160 °C revealed excellent electrical properties. Hence, the use of low-temperature hydrothermal processing can be encouraging for the synthesis of lead-free ceramics with high dielectric and ferroelectric properties.

cond-mat.mtrl-sci↗
arXiv2019

Structural, dielectric and electrocaloric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1-xSnx)O3 ceramics elaborated by sol-gel method

Ferroelectric ceramics (Ba0.85Ca0.15)(Ti0.9Zr0.1-xSnx)O3 (x=0.00, 0.02, 0.04, 0.06) were prepared by a sol-gel method. Structural investigation revealed the co-existence of tetragonal (P4mm) and orthorhombic (Pmm2) symmetries at room temperature for the undoped ceramic, while only a tetragonal structure (P4mm) was observed for the doped ceramics. Dielectric measurements indicate a dielectric relaxation process at high temperatures which is essentially related to the hopping of oxygen vacancies. Furthermore, a down shifting of the Curie temperature (TC) with increasing Sn4+ doping rate has been revealed. The temperature profiles of the Raman spectra unveiled the existence of polar nanoregions (PNRs) above the Curie temperature in all ceramics. The ferroelectric properties were found to be related to the microstructure. Electrocaloric effect was investigated in this system that revealed an electrocaloric responsivity of 0.225 10-6 K m/V for the composition with x = 0.04 Sn doping, where other remarkable physical properties were also observed.

cond-mat.mtrl-sci↗
arXiv2019

Phase transitions, energy storage performances and electrocaloric effect of the lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramic relaxor

Lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) ceramic exhibits excellent dielectric, ferroelectric and piezoelectric properties at the Morphotropic Phase Boundary (MPB). Previously, we demonstrated that the use of the anionic surfactant Sodium Dodecyl Sulfate (SDS, NaC12H25SO4) could enhance the dielectric properties of BCZT ceramic using surfactant-assisted solvothermal processing [1]. In the present study, structural, dielectric, ferroelectric properties, as well as electrocaloric effect and energy storage performances of this BCZT ceramic were thoroughly investigated. X-ray diffraction (XRD) measurements revealed the presence of single perovskite phase at room temperature with the coexistence of orthorhombic and tetragonal symmetries. In-situ Raman spectroscopy results confirmed the existence of all phase transitions from rhombohedral through orthorhombic and tetragonal to cubic symmetries when the temperature varies as reported in undoped-BaTiO3. Evolution of energy storage performances with temperature have been investigated. BCZT ceramic exhibits a high energy storage efficiency of ~80% at 120 °C. In addition, the electrocaloric responsivity was found to be 0.164.10-6 K.m/V at 360 K.

cond-mat.mtrl-sci↗
arXiv2018

Study of the oxidation process of crystalline powder of In 2 S 3 and thin films obtained by Dr Blade method

Good quality In2S3 films were grown by Dr Blade method from a powder synthesized in a chemical bath, and oxidized to obtain In2O3 films and films of intermediate composition. The oxidation process and kinetics are studied by means of thermogravimetric analysis, which shows complex phenomena. The study is completed by scanning electron microscopy and EDX measurements. X-ray diffraction and UV-visible spectroscopy results are also shown. The oxidation process begins significantly above 420 C, and it appears that at least one intermediate crystal phase takes place in the solid solution, suggesting that the replacement of sulfur by oxygen atoms occurs at different temperatures in the different crystallographic sites. The obtained samples show a band gap varying continuously between 1.94 eV and 3.72 eV.

cond-mat.mtrl-sci↗
arXiv2014

Oxidation of In2S3 films to synthetize In2S3(1-x)O3x thin films as a buffer layer in solar cells

In2S3(1-x)O3x is known from preceding studies to have a bandgap varying continuously as a function of x, the reason why this solid solution is potentially interesting in the field of photovoltaics. In this work, we present results on oxidation of In2S3 by heating in air atmosphere to obtain the desired material. The oxidation is accompanied by a mass loss due to the substitution of S by O atoms that is studied by means of thermogravimetric analysis. It appears that the temperature region in which the oxidation occurs is strongly dependent on the texture of deposited films. As-grown films deposited by chemical bath deposition are subjected to nano-oxidation occurring at lower temperature than oxidation of materials that are characterized by a better crystallinity and larger crystallite size. X ray diffraction and scanning electron microscopy (including EDX) were used to get information on the compounds and the texture of films. The main conclusion of the paper opens the perspective of practical applications for producing layers for solar cells.

cond-mat.mtrl-sci↗
arXiv2006

Mechanism of the Phase Change in PbK2LiNb5O15: Dielectric, structural, and Raman scattering studies

Experiments reveal that PbK2LiNb5O15 which belongs to the tetragonal tungsten bronze family presents paraelectric and ferroelectric phases and a complex structural change between them. High and low temperature phases are of symmetry P4/mbm and Pba2 respectively, so that this change is also of ferroelastic type. As presented here, crystallographic results hint at a displacive character of the ferroelectric ordering but show a more complex behaviour, with a clear order-disorder mechanism which accompanies the appearance of ferroelasticity. To complete our knowledge of this material, we have performed Raman experiments which exhibit a low frequency mode, but no clear soft mode is observed.

cond-mat.mtrl-sci↗