Sitchai Hunpratub

Dielectric relaxations and dielectric response in multiferroic BiFeO3 ceramics

Single-phase multiferroic BiFeO3 ceramics were fabricated using pure precipitation-prepared BiFeO3 powder. Dielectric response of BiFeO3 ceramics was investigated over a wide range of temperature and frequency. Our results reveal that the BiFeO3 ceramic sintered at 700 °C exhibited high dielectric permittivity, and three dielectric relaxations were observed. A Debye-type dielectric relaxation at low temperatures (−50 to 20 °C) is attributed to the carrier hopping process between Fe2+ and Fe3+. The other two dielectric relaxations at the temperature ranges 30–130 °C and 140–200 °C could be due to the grain boundary effect and the defect ordering and/or the conductivity, respectively.

Investigation of structural, morphological, optical, and magnetic properties of Sm-doped LaFeO3 nanopowders prepared by sol–gel method

Pure orthorhombic phase of La1−xSmxFeO3 (x = 0, 0.1, 0.2, and 0.3) nanoparticles can be obtained by sol–gel method after calcination at 800 °C for 3 h in air. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, X-ray absorption near edge spectroscopy, ultraviolet-visible spectroscopy, and vibrating sample magnetometry were used to study the crystal structure, morphology, oxidation state, functional group, optical, and magnetic properties of samples. Pure orthorhombic phase of perovskite structure is confirmed by X-ray diffraction results. Decreasing lattice parameters, crystallite sizes, and cell volumes with increasing microstrains indicate structure distortion due to the substitution of Sm ions with small ionic radius on the La sites in the orthorhombic structure. Scanning electron microscopy and transmission electron microscopy images show a homogeneous distribution of almost spherical nanoparticles with decreasing average particle sizes ranging from 56.48 ± 3.22 to 23.21 ± 4.40 nm for samples of high Sm content. Fourier transform infrared spectroscopy spectra confirm the Fe–O stretching mode in octahedral FeO6 unit of a perovskite structure. X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy results indicate the oxidation states +3 of La and Fe ions. The optical band gaps are found to decrease from 2.218 to 1.880 eV with increasing Sm content. vibrating sample magnetometry results show the antiferromagnetic behavior of undoped sample and ferromagnetic behavior for doped samples, affecting by structure distortion and particle size reduction. Interestingly, the coercive field is significantly enhanced from 95.07 Oe (x = 0.1) to 13,062.79 Oe (x = 0.3). Curie temperature (Tc) is suggested to be above 400 K.Graphical AbstractThe magnetization curves of La1-xSmxFeO3 (x = 0.0, 0.1, 0.2, and 0.3) nanoparticles prepared by the sol-gel method with the inset show the comparing coercive forces (Hc) of the present work and the previous works, La0.7M0.3FeO3 (M = Al and Ga). Sm-doped LaFeO3 nanoparticles can exhibit ferromagnetic behavior with the significant enhancement of Hc from 95.07 to 13,062.79 Oe.

On preferred Mn site in multiferroic BiFeO3: A view by synchrotron x-ray absorption near edge structure spectroscopy

Synchrotron X-ray Near Edge Structure (XANES) experiments were performed on Mn-doped BiFeO3 samples and compared with ab initio XANES calculations. The Fe and Mn K-edge XANES measurements were carried out on Mn-doped BiFeO3 powders (with 5–40 Mn mole%). Both XANES spectra were obtained in a fluorescent mode at ambient temperature with a Ge(111) double crystal monochromator. From the XANES results, it was clearly identified that the oxidation state of both Fe and Mn ions in BiFeO3 structure was +3. The features of the measured Mn K-edge XANES were consistent with ab initio XANES of Mn on the Fe site and inconsistent with Mn on other sites. The clear agreement between measured and ab initio theoretical XANES spectra was the strongest evidence of Mn substituting for Fe in BiFeO3 structure for low Mn content. More interestingly, at higher Mn content, the presence of a second phase precipitation of BiMnO3 and BiMn2O5 was evident. This clearly indicated the Mn solubility limit in the BiFeO3 structure, while the two trace compounds could also be responsible for the relevant properties reported in Mn-doped BiFeO3 materials.

Magnetic and optical properties of Cu1−xFexO nanosheets prepared by the hydrothermal method

Cu1−xFexO (x = 0, 0.025, 0.05, 0.075, and 0.1) nanosheets can be sucessfully prepared by hydrothermal method at 160 °C for 3 h in a solution of 0.5 M KOH. X–ray diffraction results indicated a monoclinic phase of crystalline CuO nanosheets. Crystallite sizes and lattice constants were determined by Rietveld refinements of the X–ray diffraction peaks. The crystallite sizes significantly decreased from 28.46 to 21.71 nm with the increase of Fe content. Scanning electron microscopy and transmission electron microscopy revealed the plate-like shapes with the estimated width and length in the range of 50–170 and 100–275 nm, respectively. The optical band gaps (Eg) studied by ultraviolet-visible spectroscopy are found to decrease from 3.625 eV for undoped sample to 3.430 eV in Fe–doped CuO sample of x = 0.1. The magnetic properties of samples studied by vibrating sample magnetometry indicated a paramagnetic behavior for CuO nanosheets and ferromagnetic behavior in Fe–doped CuO nanostructures with the magnetization strongly depended on the Fe content. The ferromagnetism of Fe-doped samples was discussed base on the F-center exchange mechanism via oxygen vacancy defects.Graphical AbstractThis figure shows the magnetization of Fe doped CuO nanosheets synthesized at 160 °C for 3 h in 0.5 M KOH with SEM micrograph, showing the plate-like morphology. It can be seen in this figure that ferromagnetic behavior of CuO nanosheets can be induced by doping with Fe ion. Moreover, the optical band gaps of Fe-doped CuO nanosheets decrease with the increase of Fe content, suggesting that Fe-doped CuO nanosheets may be applied as photocatalytic materials. Ferromagnetism is proposed to be originated from the coupling of Fe3+ with the oxygen vacancy promoted in the samples.

Dielectric Properties of 0–3BCTZO/High Calcium Fly Ash Geopolymer Composites

0–3 Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCTZO)/geopolymer composites were fabricated by normal mixing of geopolymer paste and BCTZO ceramic particles. Four composites were prepared with BCTZO concentrations of 0, 30, 50 and 70% wt. of total solid (BCTZO+FA). Phase and morphology of composite samples were characterized. The effect of addition of BCTZO ceramic particles on the dielectric properties of composite samples was also investigated. The results showed that high dielectric constant values in the order of ∼105 at frequency of 103 Hz for all composites were obtained. This was due to space charge polarization mechanism.

Effect of Mn doping on magnetic properties of BiFeO 3 Nanopowders

Structural, spectroscopic and magnetic properties of pure BiFeO3 and Mn-doping BiFeO3 nanoparticles synthesized by precipitation method were studied. The synthesized samples were characterized using X-ray diffraction (XRD), Transmission electron microscopy (SEM), X-ray absorption near edge structure spectroscopy (XANES) and Vibrating sample magnetometry (VSM). No secondary phase or metallic Mn clusters can be detected for pure BiFeO3 and those samples with Mn content up to 10%. The results from VSM reveal that the pure BiFeO3 and Mn-doping BiFeO3 samples exhibit antiferromagnetic behavior at room temperature.