Yuan Xun Li

Characterization of charge transport and electrical properties in disordered organic semiconductors

The conventional charge transport models based on density- and field-dependent mobility, only having a non-Arrhenius temperature dependence, cannot well describe the current–voltage (J–V) characteristics of hole-only devices made from disordered organic semiconductors. In this paper, we study the charge transport in a large variety of organic semiconductors and provide evidence for the availability of our improved mobility model based on both the Arrhenius temperature dependence and the non-Arrhenius temperature dependence. It is validated with experimental data collected from hole-only devices made of different organic materials in a wide range of operating conditions. Furthermore, we calculate and analyze in detail some important electrical properties of the relevant polymers by using a numerical method, adopting the uneven discretization and Newton iteration methods to solve the coupled equations describing the space-charge-limited current. It is shown that the boundary carrier density has a significant effect on the J–V characteristics and that the numerically calculated carrier density is a decreasing function of the distance to the interface.

Phase Formation, Microstructure and Magnetic Properties of NiCuZn Ferrites with Bi2O3 Additive

The ferrites (Ni0.2Cu0.2Zn0.6)1.02(Fe2O4)0.98 were prepared by standard solid state method and sintered at different temperatures. The effects of different Bi2O3 additives (0<x<5 wt%) on the phase formation, crystal structure and properties were investigated. The results of thermal analysis (DSC) showed that Bi2O3 played an important role in the process of sintering. The magnetization and the permeability were measured at room temperature, with increasing of Bi2O3 amount, the saturation magnetization had no obvious change and the initial permeability decreased from 291.2 to 197.6. SEM micrographs showed that the average grain size decreased with increasing Bi2O3 contents, but more Bi2O3 (x≥4wt %) additives may resulted in the abnormal grain growth.

Synthesis of Bi2O3- B2O3- SiO2- ZnO Glass and their Effects on the Sintering Temperature and Magnetoelectric Properties of Co / Ti Doped M-Type Barium Ferrite

The 25%Bi2O3-30%B2O3-10%SiO2-35%ZnO (molar ratio, abbreviated as BBSZ) glass were successfully prepared by solid-phase method. The amount of BBSZ glass varied from 1wt% to 5 wt% was optimized and its effects on the sintering behavior, structure and magnetoelectric properties of the Co / Ti doped M-type barium ferrite were investigated by HP 4291B vector analysis, scanning electron microscopy (SEM) and VSM magnetometer respectively. The testing results show that the ferrite could be well sintered at 900°C with excellent performances when the 3wt% BBSZ is added. The initiative permeability and cut-off frequency are 13.5 and 800MHz, respectiveley, which suggests that this material is a candidate for application in the fabrication of chip inductors by the technology of low temperature cofired ceramics and ferrites (LTCC).

Dielectric Properties of Li-Doped CaCu3Ti4O12 Ceramics

CaCu3Ti4O12 ceramics doped with 0-2.0 wt% Li2CO3 were prepared by the solid-state reaction, and their electric and dielectric properties were investigated. It is found that these ceramics had the properties of high dielectric constant and comparatively low dielectric loss. At the doping amount of 0.5 wt%, the dielectric constant is kept to be 105 with weak frequency dependence below 105 Hz, and its loss tangent (tan δ) is suppressed below 0.1 between 300 Hz-5 kHz (with the minimum value of 0.06 at 1 kHz from 218 K to 338 K). The impedance spectroscopy analysis confirms that the decrease of dielectric loss is mainly due to the increase of resistance in the grain boundary, which may be related to the influence of Ti4O7 secondary phase. Our result indicates that doping Li2CO3 is an efficient method to optimize the dielectric properties of CaCu3Ti4O12.

Influence of Magnetic Field Thermal Annealing on the High- Frequency Performance of the FeCo-Based Granular Thin Films

A series of FeCoHfO granular films were fabricated by reactive DC magnetron reactive sputtering at varying partial pressure of oxygen and annealed by magnetic field thermal annealing. By using magnetic field annealing method suitably, the soft magnetic properties of FeCoHfO granular thin films are improved obviously. The optimal annealing temperature and annealing time are 350 °C and 20 minute, respectively. The films with desired properties of low coercivity, Hc~2Oe, relatively high saturation magnetization, 4pMs~20.5 kG, high anisotropy field Hk~50Oe, and high electrical resistivity r ~ 1875mWcm and natural ferromagnetic resonant frequency about 3 GHz have been obtained.

Synthesis, Characterization and Thermal Analysis of Rod-Shaped Polyaniline-Barium Ferrites Nanocomposites

The rod-shaped polyaniline (PANI)-barium ferrite nanocomposites were synthesized by in situ polymerization of aniline in the presence of BaFe12O19 nanoparticles with diameters of 60-80 nm. The composites obtained were characterized by infrared spectra (IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The thermal stability and the composition of the composites were investigated by TG-DTG analysis. The results indicate that the thermal stability of the composites is higher than that of the pure PANI which can be attributed to the interactions existed between PANI chains and ferrite particles.

Low Temperature Fired Ferrite/Ceramic Composite Materials and their Permeability Spectra

Ni-Cu-Zn ferrite/CaTiO3 and Ni-Cu-Zn ferrite/BaTiO3 composites which can be applied in low temperature co-fired ceramic (LTCC) technology were synthesized by conventional solid-state reaction lower than 950°C. The complex permeability spectra of the above two composites have been investigated. The contribution of spin rotation and domain wall motion to the permeability spectra was estimated by the numerical fitting of measured data to the relevant formula. Influence of two types of magnetizing processes on the permeability of different composites has been analyzed combining with the variation of microstructures.

Magnetic Properties and Microwave Loss of Polycrystalline M-Type Barium Hexaferrite Thick Films by Tape Casting

M-type barium hexaferrite (BaM) is a promising gyromagnetic material for self-biased microwave\millimeter wave devices because of its large uniaxial magnetocrystalline anisotropy and low microwave loss in high frequency. Due to the limitation of growth conditions, it is difficult to deposit BaM films with enough thickness by PLD, MBE and Magnetron Sputtering for practical application. However, it is demonstrated in present experiment that large area polycrystalline BaM thick films (500μm) with self-biasing (high remanence) and low microwave loss can be successfully fabricated by tape casting. X-ray diffraction and Scanning electron microscopy results indicate that these BaM thick films have highly c-axis oriented crystallographic texture with hexagonal morphology. Magnetic hysteresis loops reveal that samples exhibit excellent properties with a saturate magnetization (4πMs) of 3606G, a high squareness ratio (Mr/Ms) of 0.82. In addition, ferromagnetic resonance (FMR) measurement shows that the FMR linewidth is as small as 431Oe at 48GHz. These parameters ensure these BaM thick films are potentially useful for self-biased microwave\millimeter wave devices such as circulator, phase shifter and filter.

A Magnetic Array Fabricated by a Novel Process Constructed with Photosensitive Polyimide-Ferrite Nanocomposite

The authors report an approach to the fabrication of a periodic magnetic array using photosensitive polyimide-barium ferrite nanocomposite. These patterns are shaped by a some-like imprint technique, under the interactions between the magnetic nanoparticles and permanent magnetic mold. Scanning electron microscopy has been used to characterize the magnetic array and well-defined structures with magnetic arrays are obtained. The diameter of each dot is as small as 52um. The presented concept leads to a realization of a magnetic array, which offers advances in achieving micrometer-scale patterns with a very low cost and simple process.

Structural and Magnetic Properties of Different Temperature Sintering La-Substituted Barium Ferrite

The Ba1-xLaxFe12O19 (0.0≤x≤0.6) ferrite materials were prepared by the standard solid state reaction technique method and sintered at 920 °C with 4 wt% Bi2O3 and at 1150 °C without Bi2O3 in air. The influence of La3+ contents on the microstructure, density, and magnetic properties were studied. The XRD results showed essentially single phase spinel structure. The SEM images of the samples had no obvious relationship with the substituting value. Saturation magnetization continuously decreased from 59.37 emu/g to 57.93 emu/g with sintering temperature from 1150 °C to 920 °C. However, coercive force reached the value of 4547.0 Oe when x=0.6 at 920 °C. The density of the samples also decreased monotonously with increasing x value.