Yongping Pu

Dependence of phase configurations, microstructures and magnetic properties of iron-nickel (Fe-Ni) alloy nanoribbons on deoxidization temperature in hydrogen

Iron-nickel (Fe-Ni) alloy nanoribbons were reported for the first time by deoxidizing NiFe2O4 nanoribbons, which were synthesized through a handy route of electrospinning followed by air-annealing at 450 °C, in hydrogen (H2) at different temperatures. It was demonstrated that the phase configurations, microstructures and magnetic properties of the as-deoxidized samples closely depended upon the deoxidization temperature. The spinel NiFe2O4 ferrite of the precursor nanoribbons were firstly deoxidized into the body-centered cubic (bcc) Fe-Ni alloy and then transformed into the face-centered cubic (fcc) Fe-Ni alloy of the deoxidized samples with the temperature increasing. When the deoxidization temperature was in the range of 300 ~ 500 °C, although each sample possessed its respective morphology feature, all of them completely reserved the ribbon-like structures. When it was further increased to 600 °C, the nanoribbons were evolved completely into the fcc Fe-Ni alloy nanochains. Additionally, all samples exhibited typical ferromagnetism. The saturation magnetization (Ms) firstly increased, then decreased, and finally increased with increasing the deoxidization temperature, while the coercivity (Hc) decreased monotonously firstly and then basically stayed unchanged. The largest Ms (~145.7 emu·g−1) and the moderate Hc (~132 Oe) were obtained for the Fe-Ni alloy nanoribbons with a mixed configuration of bcc and fcc phases.

Ferroelectric and Magnetic Properties of SrO-B2O3-SiO2 Glass-Doped BiFeO3 Ceramics

BiFeO3 ceramics doped with varying amounts of SrO-B2O3-SiO2 glass, i.e., xwt% (x = 0, 0.2, 0.4, 0.6, 0.8) have been successfully developed via a solid state reaction route. Phase identification using XRD showed that a relatively pure BiFeO3 phase could be obtained at the reaction temperature of 760° for 2h. The sintering temperature of these glass-doped BiFeO3 ceramics was controlled at about 800°, on the basis of the studies of the optimized bulk density, although the bulk density varied with the different doping levels of SrO-B2O3-SiO2 glass. The dielectric constant and dielectric loss of sintered ceramics are affected by the glass doping level, while the BiFeO3 doped with 0.6% SrO-B2O3-SiO2 glass exhibited a comparatively higher dielectric constant value and minimum dielectric loss. At x = 0.6 and 0.8, the glass-doped BiFeO3 ceramics show saturated hysteresis loops with the respective saturation polarizations of 1.2μC/cm2 and 1.4μC/cm2. Too high a glass doping level in BiFeO3 gave rise to weakened magnetic behavior.

Extended dielectric constant step from −80 °C to 336 °C in the BaTiO3–BiYO3–Ba(Fe0.5Nb0.5)O3 system

Polycrystalline 0.8BaTiO3–(0.2 − x)BiYO3–xBa(Fe0.5Nb0.5)O3 (0.8BT–(0.2 − x)BY–xBFN) (x = 0–0.04) ceramics were fabricated via a conventional solid-state reaction method. The phase transition, microstructure and dielectric properties were obtained by X-ray diffraction and dielectric measurements as functions of chemical composition and temperature. A progressive decline in ermax with increasing x leads to nearly temperature-stable dielectric properties over a wide temperature range. For x ≥ 0, dielectric measurements indicate relaxor behavior. Increasing the BFN content effectively improves the dielectric temperature stability of BT ceramics. For x = 0.02, er = 590 ± 15% from −48 °C to 326 °C, and tan δ ≤ 0.02 across the temperature range of −13 °C to 156 °C. For x = 0.04, er = 540 ± 15% from −80 °C to 336 °C, and tan δ ≤ 0.02 across the temperature range of −22 °C to 231 °C. The plot of ac conductivity for x = 0.04 shows the NTCR character of the compounds, and the activation energy of the dc conductivity is not far from the electron from second ionization energy of oxygen vacancies.

Bi4Ti3O12 Addition in the Ultra-Broad Temperature Stability of BaTiO3-Based Ceramics

BaTiO3-Nb2O5-xBi4Ti3O12 ceramics are prepared by a conventional mixed oxide route. From the observations, the proverskite-type solid solutions are formed for x ≤ 0.075, and second phase BaBi4Ti4O15 is found for x > 0.2. The sintering temperature is reduced effectively to 1090°C. A progressive decline in ϵr max with increasing x led to near temperature-stable dielectric properties over a wide temperature range. For x = 0.15, ϵr max = 772(Δϵ/ϵ25 °C≤ 15%), from −55°C to 400°C, and tan δ≤ 0.02 from −55°C to 192°C. This work provides a roadmap to obtain a temperature-stable dielectric ceramic.

Dielectric Properties of (1-x)BaTiO3-xBaFe12O19 Composite Ceramics

The composite ceramics of (1-x)BaTiO3-xBaFe12O19 (x = 0.2, 0.4, 0.5 and 0.6) were prepared by solid state method. The phase formation and diphase microstructure of the composite samples were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicated that ferroelectric BaTiO3 can well coexisted with the magnetic phase BaFe12O19. The electric and dielectric properties of the obtained composites were investigated. The electrical conduction and dielectric constant of the composites increased with the increase of BaFe12O19 contents. The high values of conductivity and dielectric constant near 250°C were due to the effect of space charges that played an important role in the composites. (1-x)BaTiO3-xBaFe12O19 composites exhibited a high value of dielectric constant at low frequencies and high temperatures. At x = 0.4, the composites showed usual dielectric dispersion with increasing frequency.

Maxwell-Wanger/Debye Effects and Complex Impedance Studies on Bi4Ti3O12-0.04Fe2O3 Ceramics

Dense ceramics in the solid solution system, Bi4Ti3O12-0.04Fe2O3 (BIT-0.04Fe) ceramics were prepared by conventional solid-state method. And the electrical properties were investigated by the dielectric and impedance spectroscopies. Moreover, the BIT-0.04Fe ceramic exhibit only one loss peak which can be analyzed in terms of the Maxwell-Wagner capacitor model at lower temperature, while the second peak related to the space charge polarization between the ceramic and electrode appeared above 920K. The Arrhenius plot of the total conductivity of BIT-0.04Fe ceramic indicates three different activation energies. The impedance complex plane plots of BIT-0.04Fe ceramic turned into one semicircular arc at 353 K and then to one semicircular arc and a line at 433 K and two single semicircular arcs at 513 K, which were analyzed as three different equivalent circuits.

Enhanced photocatalytic degradation of Rhodamine B by reduced graphene oxides modified Bi12TiO20 under visible light

In this work, Bi12TiO20/reduced graphene oxide (Bi12TiO20/rGO) photocatalysts were successfully synthesized via a simple microwave hydrothermal process. The as-prepared products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible absorption spectroscopy. The photocatalytic activity was investigated using Rhodamine B under visible-light irradiation. The Bi12TiO20/rGO composites presented much higher photocatalytic performance than pure Bi12TiO20. The B-5rGO sample exhibits the highest photocatalytic activity. The high performance could be attributed to the excellent electrical and mechanical properties of graphene sheets. The present study could provide new direction into the research of the Bi12TiO20/rGO composite materials and promote their application.

Study on the relaxation behavior of BaTiO 3 ceramics modified with BiMO 3 (M = Fe, Y, Al)

The 0.92BaTiO3–0.08BiMO3 (M = Fe, Y, Al) solid solutions were prepared via a conventional solid state reaction technique. X-ray diffractometer, Raman spectra, scanning electron microscope, and impedance analyzer were used to examine the effect of three additions on the crystal structure, micromorphology and dielectric properties of 0.92BaTiO3–0.08BiMO3 (M = Fe, Y, Al) solid solutions. The 0.08 BiMO3 (M = Fe, Y, Al) additions totally transform the characteristic sharp phase transition (associated with permittivity maximum) of pure BaTiO3 into a less-distinct phase transition with a broad and strongly dispersive permittivity maximum characteristic of relaxor dielectrics.

PTCR Behavior of CuO-Doped Ba0.96(Bi0.5K0.5)0.04TiO3 Ceramics

As a candidate of a lead-free positive temperature coefficient of resistivity (PTCR) material, Ba0.96(Bi0.5K0.5)0.04TiO3-xCuO (x = 0–0.80%) ceramics were prepared by the solid-state reaction technique. X-ray diffraction results indicated that the CuO doped Ba0.96(Bi0.5K0.5)0.04TiO3 ceramics sintered at 1360°C for 3h in N2 exhibited a tetragonal structure at room temperature. The PTCR characteristics of Ba0.96(Bi0.5K0.5)0.04TiO3-xCuO ceramics were investigated in terms of CuO content. The undoped sample sintered in N2 showed low resistivity and a small resistance jump. With the increasing CuO content, both the Curie temperature and the room-temperature resistivity increased. The Raman scattering spectra showed that the Cu2+ was partly reduced to Cu+ during sintered in N2 which increased the density of the grain surface acceptors and enhanced PTC effect without reoxidation process. In the range of 0.05 ≤ x ≤ 0.20%, the samples displayed a marked PTC effect. When x ≥ 0.40%, CuO and Cu2O in ceramic samples could act as sintering aids to promote grain growth and all the samples showed NTC effect.

Study on Dielectric Properties of SiO2-doped BaTiO3 Ceramics

(1-x)BaTiO3-xSiO2 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol%) ceramics were prepared by microwave sintering method, which nanopowders of BaTiO3 and SiO2 were successfully synthetized by conventional hydrothermal method and Stöber method, respectively. The effect of SiO2 content on microstructure and dielectric properties was investigated. X-ray diffraction result suggested that tetragonality (c/a) of the samples increased with increasing content of SiO2. It was found that there existed an increase in density and grain size of the samples for x≤0.4, which was mainly attributed to the presence of liquid phase during sintering process. BaTiO3 ceramics containing such amount of SiO2 also showed improved dielectric properties.