Xi Wei Qi

Luminescence Properties of Spherical Phosphors SrSiO3:Eu3+

Spherical SiO2 particles have been successfully coated with SrSiO3:Eu3+ phosphor layers through a sol-gel process. The resulted core-shell phosphors were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence spectra as well as kinetic decays. The XRD results demonstrate that the SrSiO3:Eu3+ layers begin to crystallize on the SiO2 particles after annealing at 1000oC. The obtained core-shell phosphors have perfect spherical shape. The Eu3+ shows a strong photoluminescence (PL) (dominated by 5D0-7F2 red emission at 612 nm).

Rheological Behavior of Alumina Aqueous Suspension for Extrusion Gelation Freeform Fabrication

Extrusion gelation freeform fabrication (EGFF) is a new solid freeform fabrication technique to fabricate ceramic parts. The study of ceramic suspensions rheological behavior is essential to a better understanding of parts fabrication through EGFF process. In this paper, the effects of dispersant concentrations, solids loading and binder composition on the rheological behavior of alumina suspensions were examined. The optimal amount of dispersant was found to be about 0.1 wt.%. The suspensions with 50 vol.% alumina and 1wt.% sodium alginate show shear thickening behavior. Such behavior is unfavorable for extrusion gelation freeform fabrication. By addition of PVA or styrene-acrylic latex, the rheological behavior of suspensions was changed. The addition of PVA or latex has a great effect on the viscosity of the suspensions and the type of their rheological behavior. The ceramic suspensions with PVA or latex were found to be shear thinning more suitable for EGFF.

Ferromagnetic Properties of Ni0.2Cu0.2Zn0.6Fe2O4-PZT Composites

A series of novel composites with the composition of xNiCuZn-(1-x)PZT (NiCuZn: Ni0.2Cu0.2Zn0.6Fe2O4; PZT: PbZr0.52Ti0.48O3.) were prepared by solid-state reaction methods. The presence of ferromagnetic NiCuZn ferrite phase and ferroelectric PZT phase was confirmed by XRD analysis. It was revealed that composite materials exhibit typical ferromagnetic hysteresis behaviors. With the increase of phase fraction of NiCuZn ferrite, the permeability of composites increases and the natural resonance peak appears. Meanwhile, the maximum of quality factors shifts to the lower frequency.

Preparation and Magnetic Properties of BiFeO3 and NiCuZn Ferrite Multiferroic Composites

A series of xBiFeO3-(1-x) NiCuZn ferrite multiferroic composites have been prepared by solid state reaction method, in which x is 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0. Composites are densified at 700 °C for 3 h. XRD results show that ferroelectric phase and ferromagnetic phase can be co-existed in the composites. The density of composites increases with an increase of BiFeO3 content. SEM results reveal that the bigger grains are BiFeO3 and the smaller grains are NiCuZn ferrite. Composites exhibits the typical magnetic hysteresis loops. Increasing x, the saturation magnetization of composites linearly decrease. The real part of permeability for composites decreases with increasing x. Meanwhile, the peak of imaginary part of permeability shifts toward higher frequency.

Preparation and Characterization of LaAlO3 via Sol-Gel Process

Lanthanum aluminate powders were synthesized by sol-gel process with metal nitrates as raw materials, 2-methoxyethanol/water as solvent and citric acid as chelating agent. The influence of this techniques (the amount of citric acid, pH value, calcination temperature and the ratio of 2-methoxyethanol/water) on the lanthanum aluminate powders were studied. The XRD shows pure LaAlO3 powders could be obtained after calcining at 600-900 °C. The SEM analysis indicates that the LaAlO3 particles are uniform and nanosized with a range of about 40-70 nm, which is slightly larger than the estimated particle size using Scherrer formula.

Fabrication and luminescence properties of Y2SiWO8 : Dy3+ phosphors via sol-gel process

Y2SiWO8:Dy3+ phosphors were prepared through a sol-gel process. XRD and photoluminescence spectra were used to characterize the resulting phosphors. The results indicated that the phosphors crystallized completely at 1000°C. In Y2SiWO8:Dy3+ phosphors, the Dy3+ showed its characteristic yellow emission at 483nm (4F9/2-6H5/2) and 575nm (4F9/2-6H13/2) upon excitation into 275nm.

Cofiring Behavior of Ferroelectric Ferromagnetic Composites

Cofiring behavior of composites consisting of ferroelectric PMZNT and ferromagnetic NiCuZn ferrite was investigated via X-ray diffractometer (XRD), thermomechanical analysis (TMA) and scanning electron microscopy (SEM). Mismatch of sintered camber consisting of two layered structure of ferroelectric PMZNT and NiCuZn ferrite was observed. Mismatched sintering behavior was modified by adopting PMZNT and NiCuZn powders to form composite materials. The temperature of appearance pyrochlore phase in prepared composite materials is lower than that of sintering pure PMZNT material. The grain size of PMZNT and NiCuZn in prepared composites is smaller than that of sintering pure PMZNT and NiCuZn ferrite, respectively.

BiFeO3 Powders Synthesized by Different Sol-Gel Methods

The multiferroic BiFeO3 powders with perovskite structure were synthesized by sol-gel auto-combustion and conventional sol-gel methods, respectively. As-prepared powders were characterized by XRD, SEM and VSM techniques to investigate phase structure, microstructure and magnetic properties. The results show that the pure phase BiFeO3 powders are obtained successfully by sol-gel auto-combustion after calcining at 650°C for 2h and leaching by HNO3. Compared with conventional sol-gel method which got pure phase BiFeO3 powders at 800°C for 3h, the sol-gel auto-combustion technique can get more pure powders at low temperature, short time and low cost. The particle sizes of the BiFeO3 powders are smaller and well distributed by the sol-gel auto-combustion method than that of the conventional sol-gel method. Furthermore, the magnetic properties at room temperature prepared by the sol-gel auto-combustion methods are significant higher than that of conventional sol-gel method.

Preparation and Mechanical Properties of BiFeO3 Ceramics

Hydrothermal method and sol-gel process were used to synthesize multiferroic BiFeO3 ceramics. X-ray diffraction, scanning electron microscopy, vickers diamond indenter and three-point bending method were used to investigate the effects of methods on the phase structure, microstructures and mechanical properties. Cold isostatic pressing on the ceramics with two different loads (10 MPa, 200 MPa) was used to illustrate the influence of pressure in mechanical properties. The results show that all samples are crystallized in the perovskite phase. A few small traces of impurity are observed at a 2θ of ~28 o, which are found to be those of Bi2Fe4O9. The SEM images depict that samples prepared by sol-gel process are more uniform and the grain size is slightly larger than that of hydrothermal processed samples. The investigations on the hardness and flexural strength demonstrate the ceramics prepared by hydrothermal method have better mechanical properties than that of sol-gel process, and the mechanical properties can be obviously enhanced by increasing pressure.

Effects of Bi3+ Doping Amounts on the Magnetic Properties of FeSmO3 Nanopowders

The samples of FeSmO3 nanopowders were synthesized from the main raw materials of FeCl3 and SmCl3 by coprecipitation process. The samples were prepared by different precipitating agents including Na2CO3,NaOH and triethylamine (C6H15N). When the precipitating agent was NaOH, almost all of the Sm3+ and Fe3+ were precipitated. In this series of experiments, the molar ratios of Sm3+ to Fe3+ were 0.95:1, 1:1, 1.05:1, 1.1:1, 1.15:1 and 1.2:1. And when the molar ratio was 1.2:1, there was the least impurities in the obtained samples. The samples were sintered in the oven at different temperatures including 600°C,700°C, 800°C and 900°C. And only at 900°C, were the FeSmO3 nanopowders made without impurities. The Sm3+ in FeSmO3 was substituted by the Bi3+ in different doping amounts, which urged the preparation of BixFeSm(1-x)O3 nanopowders. And these nanopowders were characterized by XRD, VSM, SEM and fluorescent property technologies. The results showed that the obtained nanopowders were in high purity when x was 5% and it had the highest remanent polarization when x was 15%.