Santi Maensiri

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.

Magnesium Ferrite (MgFe2O4) Nanostructures Fabricated by Electrospinning

Magnesium ferrite (MgFe2O4) nanostructures were successfully fabricated by electrospinning method. X-ray diffraction, FT-IR, scanning electron microscopy, and transmission electron microscopy revealed that calcination of the as-spun MgFe2O4/poly(vinyl pyrrolidone) (PVP) composite nanofibers at 500–800 °C in air for 2 h resulted in well-developed spinel MgFe2O4nanostuctures. The crystal structure and morphology of the nanofibers were influenced by the calcination temperature. Crystallite size of the nanoparticles contained in nanofibers increased from 15 ± 4 to 24 ± 3 nm when calcination temperature was increased from 500 to 800 °C. Room temperature magnetization results showed a ferromagnetic behavior of the calcined MgFe2O4/PVP composite nanofibers, having their specific saturation magnetization (Ms) values of 17.0, 20.7, 25.7, and 31.1 emu/g at 10 Oe for the samples calcined at 500, 600, 700, and 800 °C, respectively. It is found that the increase in the tendency ofMsis consistent with the enhancement of crystallinity, and the values ofMsfor the MgFe2O4samples were observed to increase with increasing crystallite size.

Synthesis, characterization, and magnetic properties of monodisperse CeO2 nanospheres prepared by PVP-assisted hydrothermal method

Ferromagnetism was observed at room temperature in monodisperse CeO2 nanospheres synthesized by hydrothermal treatment of Ce(NO3)3·6H2O using polyvinylpyrrolidone as a surfactant. The structure and morphology of the products were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and field-emission scanning electron microscopy (FE-SEM). The optical properties of the nanospheres were determined using UV and visible spectroscopy and photoluminescence (PL). The valence states of Ce ions were also determined using X-ray absorption near edge spectroscopy. The XRD results indicated that the synthesized samples had a cubic structure with a crystallite size in the range of approximately 9 to 19 nm. FE-SEM micrographs showed that the samples had a spherical morphology with a particle size in the range of approximately 100 to 250 nm. The samples also showed a strong UV absorption and room temperature PL. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide. The magnetic properties of the samples were studied using a vibrating sample magnetometer. The samples exhibited room temperature ferromagnetism with a small magnetization of approximately 0.0026 to 0.016 emu/g at 10 kOe. Our results indicate that oxygen vacancies could be involved in the ferromagnetic exchange, and the possible mechanism of formation was discussed based on the experimental results.

Dielectric relaxation and dielectric response mechanism in (Li, Ti)-doped NiO ceramics

Giant dielectric permittivity (Li, Ti)-doped NiO (LTNO) ceramics are prepared by a simple PVA sol–gel method. The dielectric properties are investigated as a function of frequency (102–106 Hz) at different temperatures (233–473 K). The concentration of Li has a remarkable effect on the dielectric properties of the LTNO ceramics. The modified Cole–Cole equation, including the conductivity term, is used to describe the experimental dielectric spectra of a high permittivity response with excellent agreement over a wide range of frequencies (103–106 Hz) and temperatures (233–313 K). A frequency dielectric dispersion phenomenon in an LTNO ceramic is also analyzed by impedance spectroscopy. A separation of the grain and grain boundary properties is achieved using an equivalent circuit model. The grain and grain boundary conduction and the dielectric relaxation time of the Li0.05Ti0.02Ni0.93O follows the Arrhenius law associated with estimated activation energies of 0.216, 0.369 and 0.391 eV, respectively. Through the analysis by the modified relaxation model and impedance spectroscopy, it is strongly believed that the high dielectric permittivity response of the LTNO is not only contributed by the space charge polarization (Maxwell–Wagner polarization) mechanism at low frequency regions, but also by the defect-dipole polarization mechanism at high frequency regions.

Structure and Magnetic Properties of Monodisperse Fe3+-doped CeO2 Nanospheres

This work reports the study concerning the structure and magnetic properties of undoped CeO2 and Fe-doped CeO2 (Ce1−xFexO2, 0.01 ≤ x ≤ 0.07) nanospheres with diameters of 100∼200 nm prepared by hydrothermal method using polyvinylpyrrolidone (PVP) as surfactant. The prepared samples were studied by using X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray absorption near-edge structure (XANES), and vibrating sample magnetometry (VSM). The XRD results showed that Fe-doped CeO2 was single-phased with a cubic structure, and with Fe3+ successfully substituting in Ce4+ sites. Raman spectra showed a redshift of F2g mode that caused by the Fe doping. The samples of both undoped CeO2 and Fe-doped CeO2 exhibited room temperature ferromagnetism, and the saturated magnetization (Ms) increased with increasing Fe content until x = 0.05, and then the samples displayed ferromagnetic loops as well as paramagnetic behavior. The roles of Ce3+ and Fe3+ spin electrons are discussed for the ferromagnetism in the Fe-doped CeO2.

Room-temperature ferromagnetism in Co-doped CeO2 nanospheres prepared by the polyvinylpyrrolidone-assisted hydrothermal method

Nanospheres of pure CeO2 and Co-doped CeO2 (Ce1−xCoxO2, 0.01 ≤ x ≤ 0.07) dilute magnetic oxide were prepared by hydrothermal treatment using cerium (III) nitrate, cobalt (III) nitrate, and polyvinylpyrrolidone (PVP) as a surfactant. The prepared samples were studied using x-ray diffraction (XRD), transmission electron microscopy, field-emission scanning electron microscopy (FE-SEM), and UV-Visible spectroscopy. The valence states of Ce and Co ions were determined by using x-ray absorption near edge spectroscopy. The magnetic properties of the samples were studied using vibrating sample magnetometry. The results from XRD indicated that the synthesized samples had a cubic structure without a change in the structure of CeO2 due to Co substitution. FE-SEM micrographs showed that the samples had a spherical morphology. The Co-doped CeO2 showed a red shift of the band gap energy that originates from defects caused by Co substitution. The samples of both CeO2 and Co-doped CeO2 exhibit room temperature ferromagne...

The origin of giant dielectric relaxation and electrical responses of grains and grain boundaries of W-doped CaCu3Ti4O12 ceramics

The origin of giant dielectric relaxation behavior and related electrical properties of grains and grain boundaries (GBs) of W6+-doped CaCu3Ti4O12 ceramics were studied using admittance and impedance spectroscopy analyses based on the brick–work layer model. Substitution of 1.0 at. % W6+ caused a slight decrease in GB capacitance, leading to a small decrease in the low-frequency dielectric constant. Surprisingly, W6+ doping ions have remarkable effects on the macroscopic dielectric relaxation and electrical properties of grains. X-ray photoelectron spectroscopy analysis suggested that the large enhancements of grain resistance and conduction activation energy of grains for the W6+-doped CaCu3Ti4O12 ceramic are caused by reductions in concentrations of Cu3+ and Ti3+ ions. Considering variation of dielectric properties together with changes in electrical properties of the W6+-doped CaCu3Ti4O12 ceramic, correlation between giant dielectric properties and electrical responses of grains and GBs can be describe...

Charge storage mechanisms of manganese oxide nanosheets and N-doped reduced graphene oxide aerogel for high-performance asymmetric supercapacitors

Although manganese oxide- and graphene-based supercapacitors have been widely studied, their charge storage mechanisms are not yet fully investigated. In this work, we have studied the charge storage mechanisms of K-birnassite MnO2 nanosheets and N-doped reduced graphene oxide aerogel (N-rGOae) using an in situ X-ray absorption spectroscopy (XAS) and an electrochemical quart crystal microbalance (EQCM). The oxidation number of Mn at the MnO2 electrode is +3.01 at 0 V vs. SCE for the charging process and gets oxidized to +3.12 at +0.8 V vs. SCE and then reduced back to +3.01 at 0 V vs. SCE for the discharging process. The mass change of solvated ions, inserted to the layers of MnO2 during the charging process is 7.4 μg cm−2. Whilst, the mass change of the solvated ions at the N-rGOae electrode is 8.4 μg cm−2. An asymmetric supercapacitor of MnO2//N-rGOae (CR2016) provides a maximum specific capacitance of ca. 467 F g−1 at 1 A g−1, a maximum specific power of 39 kW kg−1 and a specific energy of 40 Wh kg−1 with a wide working potential of 1.6 V and 93.2% capacity retention after 7,500 cycles. The MnO2//N-rGOae supercapacitor may be practically used in high power and energy applications.

Effects of grain, grain boundary, and dc electric field on giant dielectric response in high purity CuO ceramics

Giant dielectric constant e′ of ∼(2.8–3.7)×104 was observed in high purity CuO (99.999%) ceramics with grain sizes of 4.57±1.71 and 9.57±3.01 μm. The e′ and Ea increase with an increase in grain size due to the different electrical properties in the grains. The high dielectric response observed in the CuO ceramics can be described by the internal barrier layer capacitance model. The resistance of grain boundaries (Rgb) and the dielectric constant of the CuO samples decrease with increasing dc bias due to the decrease in grain boundaries capacitance, whereas the resistance of grains (Rg) remains constant.

The sintering temperature effects on the electrical and dielectric properties of Li0.05Ti0.02Ni0.93O ceramics prepared by a direct thermal decomposition method

We reported the effects of grain size on high dielectric and related electrical properties of Li0.05Ti0.02Ni0.93O (LTNO) ceramics, which were prepared by a direct thermal decomposition method. The analysis of complex impedance indicated that these LTNO ceramics were electrically heterogeneous consisting of conducting grains and insulating grain boundaries (GBs). Interestingly, our results revealed that the dielectric permittivity (e′) increases with the increase in grain size, which can be well described by Maxwell–Wagner relaxation model. Furthermore, we also found that the activation energy required for relaxation process (Ea) and related activation energy of the conductivity in the grain interior (Eg) decreased with the increase in grain size. These results suggested that the different microstructures resulted in chemical change (e.g., oxygen vacancies) inside the grains, leading to the changes in electrical properties of the LTNO ceramics.