Mei Juan Li

Microstructure and Properties of Spark Plasma Sintering AlN/BN Ceramics

Aluminum nitride-boron nitride (AlN/BN) composite ceramics were prepared by spark plasma sintering (SPS). The sintering behaviors of AlN/BN composites with 5~15% volume fraction of BN were studied. The influences of BN content, as well as the sintering temperature on the density, microstructure, mechanical strength, thermal conductivity and machinability of the composites were also investigated. The results showed that the full densification of AlN/BN composite ceramics could be realized by SPS technique at the temperature no higher than 1800°C for 3 minutes. The thermal conductivity of AlN/BN composites is in the range of 66~79W/mK, and AlN/BN composites can be cut or drilled by carbides or even steel tools when BN content is 15% volume fraction. The mechanical strength of AlN/BN composites is about 330MPa and is not remarkably affected by the addition of BN. The improvement of mechanical properties of AlN/BN composite ceramics is due to the fine and homogenous microstructure developed in the SPS process.

Calculation of Tape Thickness for Ceramic Tape Casting

Tape casting play a significant role in industrial area, such as multilayered ceramic (MLC) packages, functionally graded materials (FGM), low temperature co-fired ceramics (LTCC) and so on. For the complexity of the rheology for slurry during tape casing process, the control of tape thickness by experience was unstable. Although few numerical and analytical studies on predicting the tape thickness have been done, but these efforts have focused on Newtonian, Bingham, Power law, respectively. There is no unified equation to calculate the tape thickness among different rheological models. In this paper, the calculation results are characterized by wide adaptability; the blade gap, the casting speed and the slurry rheological property are incorporated into calculation; the effect of parameters in the result is studied; Parameter Pnd can be used as a guide to check which mark patterns of the flow velocity profiles in the channel. The results proposed and the experimental measurements from existing publications are in close agreement. Compared with the prediction of the existing models, the calculation results proposed has good agreement with them.

Densification and Thermal Conductivity of Y2O3-Doped AlN Ceramics by Spark Plasma Sintering

AlN ceramics doped with yttrium oxide (Y2O3) as the sintering additive were prepared via the spark plasma sintering (SPS) technique. The sintering behaviors and densification mechanism were mainly investigated. The results showed that Y2O3 addition could promote the AlN densification. Y2O3-doped AlN samples could be densified at low temperatures of 1600-1700oC in 20-25 minutes. The AlN samples were characterized with homogeneous microstructure. The Y-Al-O compounds were created on the grain boundaries due to the reactions between Y2O3 and Al2O3 on AlN particle surface. With increasing the sintering temperature, AlN grains grew up, and the location of grain boundaries as well as the phase compositions changed. The Y/Al ratio in the aluminates increased, from Y3Al5O12 to YAlO3 and to Y4Al2O9. High-density, the growth of AlN grains and the homogenous dispersion of boundary phase were helpful to improve the thermal conductivity of AlN ceramics. The thermal conductivity of 122Wm-1K-1 for the 4.0 mass%Y2O3-doped AlN sample was reached.

In Situ Synthesis of Size-Controlled Silver/Poly(Methyl Methacrylate) Nanocomposite

In this work, monodispersed silver nanoparticles with controllable size have been successfully in situ synthesized in PMMA matrix. NaHS, HCl and poly (vinyl pyrrolidone) (PVP) were used to optimize the nucleation and growth of silver nanocrystalline. UV–vis analysis and transmission electron microscopy (TEM) were used to characterize the size and dispersion of silver nanoparticles in the Ag/PMMA nanocomposites. The results show that silver nanoparticles homogeneously distribute in PMMA/DMF sol and the particle size of silver nanoparticles increase with the increasing of time. The nucleation of Ag atoms can be facilitated through the addition of a trace amount of NaHS to generate Ag2S clusters as heterogeneous nuclei. Introducing a trace amount of Cl- into the reaction system can effectively reduce the growth rate of the nanoparticles and thus generating more uniform silver nanoparticles in PMMA matrix.

Preparation and Density Control of PMMA Microcellular Foams via Supercritical Carbon Dioxide Foaming

Microcellular polymeric foam is a new class of materials which has been widely used in many industries. The foaming of polymethyl metacrylate (PMMA) using supercritical carbon dioxide (ScCO2) which is inexpensive and environmental friendly has been studied to better understand the foaming process. The pieces of PMMA are put into a saturation vessel of which temperature and pressure are kept constant. Supercritical carbon dioxide (ScCO2) at temperature between 65 °C and 105 °C and pressure between 8 MPa and 16 MPa is used as a foaming agent. After saturation of carbon dioxide, rapid decompression of ScCO2 saturated PMMA yields expanded microcellular foams. The densities of foamed PMMA materials are tested by true density analyzer, while the microstructures of a variety of density foamed PMMA materials are characterized by scaning electron microscopy (SEM). The cell size and cell density are calculated via image analysis. The effect of the process condition on the cell morphologies and mass density of the foam is investigated by considering the solubility of carbon dioxide in PMMA. The relationship between the mass density of foamed PMMA (ρ) and foaming temperature (T) and pressure (P) are respectively certained quantificationally.

Effect of pH on the Microstructure and Purity of Copper-Coated Tungsten Composite Powders Prepared by Electroless Plating

High-performance copper-coated tungsten composite powders were successfully prepared using electroless plating at an appropriate pH in the plating bath. The effect of pH value in the plating bath on the microstructure and purity of the coated Cu layer was studied systematically. With the increase of pH in the bath, the surface roughness, particle size and average grain size of coated Cu increased, and the particle shape gradually changed from round into square block. At pH 12–12.5, coated Cu was highly pure with very little oxygen content (less than 0.09 wt.%). The promotion effect of the increased pH on both main electroless plating reaction and side reactions causes the changes of surface morphology, average grain size of Cu and the oxygen content in the composite powders.

Synthesis and Characterization of Three-Dimensional (3D) Flowerlike CuO by a Simple Chemical Reduction Method

Three-dimensional (3D) flowerlike CuO structures were prepared successfully by reducing copper chloride (CuCl2·2H2O) aqueous solution in the presence of cetyltrimethylammonium bromide (CTAB). The as-prepared CuO structures were characterized by UV-Vis, X-ray powder diffraction (XRD), FESEM and EDS techniques. The flowerlike CuO structures consisted of Salix leaf-like nanostructures. A possible growth mechanism for the formation of 3D flowerlike CuO structure was proposed. The processes of ripening and directed growing of nanoparticles were most important factors to obtain the 3D flowerlike CuO structures.

Fast and Simple Fabrication of RGO/Ag Nanocomposite with Homogenous Dispersion

The RGO/Ag nanocomposite with a homogeneous dispersion of Ag on the surface of RGO has been successfully prepared via situ chemical reduction method using DMF (dimethylformamide) as solvent and reducing agent. The RGO/Ag nanocomposite was characterized by X-ray diffractometer (XRD), Fourier transform-infrared (FTIR) spectra, Fieldemission scanning electron microscope (FESEM) and transmission electron microscope (TEM). It is suggested that in the presence of the PVP (polyvinylpyrrolidone), the electrostatic attraction of Ag+ ions with negative GO sheets lead to the decoration of Ag nanoparticles on the surface of RGO sheets in RGO/Ag nanocomposite.

Study on Microstructure and Property of Diffusion-Bonded Mo-Cu Joints

Mo and Cu Were Bonded Successfully by Means of Vacuum Diffusion Bonding. The Interfacial Structure of the Joints Was Studied by Scanning Electron Microscopy (SEM), Electron Probe Microanalysis (EPMA), Energy Dispersive X-Ray Spectrometer (EDS) and X-Ray Diffraction (XRD), the Mechanical Property Is Tested by Tensile Strength Measurement. The Results Showed that the Differentatoms Diffused to each other in the Bonding Process. A Mo-Cu Solid Solution Was Formed in the Joint and with No Intermetallic Compounds. The Tensile Strength of the Joint Increased with the Increasing of Temperature, however, while the Holding Time Increased, the Strength Increased in the First Stages and then Decreases. It Were Observed that the Fracture Mode of the Joints Was a Brittle Fracture.

Effects of Coated Nano-BN Particles on Microstructure and Properties of BN-AlN Composite

Surface coating has been used to prepare nano-particle BN-AlN composite powder. Nano-BN was synthesized via in-situ reaction, coating on the surface of AlN particles. The effects of coated nano-BN on the microstructure and properties of BN-AlN composites have been investigated. Results indicate that the microstructure and properties of BN-AlN composites are very different from the origin composite powders. Nano-BN particles cause the homogeneous microstructure and reinforce the grain boundary of BN-AlN composites. They exhibit similar regularity of reinforce effect of ceramic nano-particle and enhances remarkably the composites’ mechanical strength. But nano-BN will introduce more grain interfaces, more oxygen-related defects in grain lattice to increase phonon scattering and reduce the thermal conductivity of BN-AlN composites.