Papers by Author: Yu Bai Pan

Abstract: Lead-free EO ceramics have been received increasing attentions from the view point of green environment in recent years. In this study, lead-free Ba_6-1.5xLa_xTi₂Nb₈O₃₀ (0≤x≤7%) ceramics were synthesized by the conventional solid state reaction sintering method. The effects of lanthanum doping on the microstructure and electrical properties of Ba₆Ti₂Nb₈O₃₀ (BTN) ceramics were investigated and analyzed. The XRD results revealed that the lanthanum doping changed the BTN lattice parameters slightly. The sample grain size, dielectric constants (1kHz) and the temperature Tm gradually decreased with the increasing of lanthanum doped content. The optimized BLTN ceramic sample with x=4 mol % exhibited a good transparency and excellent electrical properties (Ps=6.41 μC/cm², Pr=0.77 μC/cm², and Ec=3.80 kV/cm).

Abstract: The main problems for multiphase materials are the even mixing and the bonding or reacting among two or more phases, as well as the states of their grain boundaries. The mixing of two phases is usually effective by using liquid medium. The technology of second phase coating for the original particles or so call nucleo-shell structure is a well approach for the even mixing. For ceramic matrix multiphase materials low sintering temperature is the effective way to avoid the occurring of reaction between different phases. The design of grain boundary and its stress is important for their bonding states. The thinking for above problems would be described briefly in this paper. The purpose of multiphase materials design is expected that every phase will be playing its role, combining together and then given out a comprehensive performances. So it is necessary to consider the following problems. 1. Even mixing Even mixing between each other material is important for multiphase materials. It’s the better when the processing is carrying on in the liquid state for the even mixing of two phase materials. Using physical or chemical method could be formed the fine solid particles with even dispersion of two or more phases materials. Usually it could be obtained nano-scale particles. The application of nano-technology is necessary. The nano-scale mixing of two phases materials would be greatly increased the contacting surface of each other particles without agglomeration. Of course, if one phase of material is fine solid particle which disperses in another phase of material solution, the even mixing of two phases is obtained similarly. Making a coating to other phase particle is a very good approach for the two phases even mixing. Coating process is attracting more and more attention in the preparation of composite materials mainly due to its predominance in the improvement of the uniformity for different phases [1]. The thickness of coating material determines the amount proportion of two phases which is able to be control. Two examples of so-called “nucleo-shell structure” will be introduced in the following parts. Cermet composites have been widely studied due to their potential for achieving higher toughness and reasonable strength compared with ceramic matrix. But except for numbered systems, such as Co/WC and Ni/TiN, few systems have reached the people’s expectation mainly for the poor wettability between metal and ceramic. Al/Al2O3 cermet is a low density and high strength material, and it has many potential and actual applications in military, industrial and consumer regions. To improve the dispersive uniformity of the two phases, coating aluminum with alumina may be a good candidate since such coatings not only stabilize aluminum dispersions but also make it possible to control inter-particle and particle-matrix interactions [2]. Figure1a-c shows the images

Abstract: Well-dispersed SiO2 slurry with high solid content for aqueous tape casting was obtained by selecting the appropriate dispersant, binder, plasticizer and other additives. However, it is difficult to prepare well-dispersed CNTs/SiO2 slurry because CNTs tend to aggregation and the bulk density disparity of CNTs and SiO2 is obvious. In addition, the contradiction between removing organic additives and retaining the properties of CNTs should be considered. Some methods are provided to solve the as-mentioned difficulties.

Abstract: The effects of high-energy ball milling on SiC powders were studied using a planetary apparatus. Conditions to obtain nanostructured SiC powders with an average crystallite size of 4 nm were determined and powders were characterized by XRD, SEM and TEM analyses. This process was applied to prepare fine powders leading to dense SiC ceramics by sintering at 1900oC for 30 minutes under 30 MPa in argon.

Abstract: In this article stable multi-walled carbon nanotubes (MWNTs) aqueous suspension with a 1.0 wt.% concentration was obtained with a very small quantity of dispersant. Precursor of ceramics were synthesized in the suspension and densely deposited on the surface of MWNTs successfully by a simple and effective in-situ precipitation method. The most important advantage for the in-situ composite method is to make MWNTs homogeneously distributed in the matrix. The fully dense compacts were obtained by spark-plasma-sintering (SPS) the in-situ precipitated composite powders at temperature 200 oC lower than that of composite powders made from the traditional mixing method. Furthermore, the microstructure and the mechanical property of the composites are much better than that of traditional method. The in-situ precipitation could be a promising method to fabricate CNTs composites of ceramics matrix especially those hard to sinter.

Abstract: Stable and disperse carbon nanotubes (CNTs) aqueous solution was prepared by using cetyltrimethylammonium bromide (C16TMAB) as dispersing agent and zeta potential of CNTs solution were measured to quantitatively characterize the stability of CNTs in C16TMAB aqueous solution. The CNTs /SiO2 composites were fabricated with composite powders synthesized by rapid Sol-gel method and sintered by hot-pressing technique. The electric and dielectric properties of the composites were measured and the microstructure of CNTs and composites were investigated by Field emission scanning electron microscope (FSEM) and transmission electron microscopy (TEM). The mechanisms had been investigated to explain the effect of the CNT compositions on the electric and dielectric properties of CNTs /SiO2 composites in this paper.

Abstract: Carbon nanotube-mullite (Al2O3/SiO2=3/2) composites have been prepared by hot-pressing the corresponding composite powders, in which the multi-walled carbon nanotubes(1~10 vol%) are homogeneously dispersed between the mullite grains. The microstructure of the specimens has been studied and discussed in relation to the properties of the matrix, the bending strength and the fracture toughness, the dielectric constant and the dissipative factor. Carbon nanotube-mullite composites are potential electromagnetic wave absorbers owing to the percolation of the carbon nanotubes.

Abstract: SiC matrix composites can be reinforced and toughened by the second phase of AlN particles. SiC-AlN composites are mixed by ball milling of the SiC and AlN powders, and then hot pressed under 40 MPa at 1950oC in Ar atmosphere. The object of this paper is to understand the effect of AlN contents on the properties of SiC-AlN particulate composites. AlN content is between 5 and 40wt%. For improving to mechanical properties of siC-AlN composites, the composition range of SiC-AlN composites are between 90SiC-10AlN and 80SiC-AlN20(wt%). Less or more than this ratio would result in a microstructure with much pores or AlN grain-growth, respectively, two phenomena would decrease in the mechanical properties of SiC-AlN composites.

Abstract: Coating process is attracting more and more attention in the preparation of composite materials mainly due to its predominance in the improvement of the uniformity for different phases. The formation of nanosize alumina shells on the aluminum core particles by a wet-chemical based route was investigated and the composite particles were characterized using TG/DSC, XRD, SEM, TEM, XPS. An enhancement of cermet properties can be expected by using these coated particles as initial powders.

Abstract: Silicon carbide is widely used as an important structural material. It is known for its extreme hardness, high temperature antioxygenic properties and good tribological properties. Moreover, SiC is also an intrinsic semiconductor. SiC can react with AlN and form an extensive solid solution at temperatures between 1800 to 21000C, and has excellent mechanical properties. However, in this article, we study the influence of the microstructure and the dielectric properties of SiC ceramics. SiC-AlN solid-solution ceramics were prepared by hot-pressed sintering using Y2O3 as the sintering additive. The size of SiC and AlN powders were 0.6μm and 1.06µm respectively. The content of AlN starting powders was 14vol%. The hot-processing sintered SiC-AlN multiphase ceramics have reached high density at 1950oC in Ar atmosphere under 30MPa.The hot-processed ceramics were subjected to thermal treatments in a range of temperatures between 11000C and 16000C for 3hr. The grain size increased with the annealing temperature. X-ray diffraction profiles show that phase relationships. Scanning electron microscopy (SEM)) was used to determine fracture surface and the local compositions. Dielectric permittivities and dissipation factor of SiC-AlN composites were investigated with the varieties of annealing temperature and the content of AlN particles. Dielectric constants (ε) and Dielectric loss tangents (tanδ) were measured within the microwave frequency range from 40Hz to10MHz.