Synthesis of Ultrafine Fe_3O_4 Powder via a Novel Ageing Process

Ultrafine Fe3O4 powder was successfully synthesized via a novel ageing process from a precursor FeO（OH）, which was the hydrolysate of FeCl3 in the urea solution. The structure of as-synthesized powder was characterized by X-ray diffraction （XRD）, and the morphology of these nanoparticles was investigated using a transmission electron microscope （TEM）. Pure phase Fe3O4 was obtained and the mean diameter of these nanoparticles was about 21 nm.Furthermore, the study indicated that the precursor FeO（OH） played an important role in the formation of Fe3O4 nanoparticles. The mechanism was also discussed.

Microstructure Characterization of Long W Core SiC Fiber

Microstructure of SiC fiber manufactured by chemical vapor deposition （CVD） onto tungsten （W） wire core was investigated by analytical electron microscopy （AEM）. The results reveal that the fiber consists of W core, SiC sheath and C-coating. SiC sheath could be subdivided into two parts according to whether containing C rich stripe, or not. An emphasis was put on W/SiC interfacial reaction products and the transition zone between sub-layers in SiC sheath. The W/SiC interface consists of three layers of reaction production, namely, W2C, W5Si3 and WC. And there are amounts of facet faults existing in （100） face of WC crystalline and two classes of stack faults in WC have been revealed. The formation essence of different sublayers in SiC sheath was also discussed.

Preparation and Interfacial Stability of SiC_f/Ti-6Al-4V Composites Fabricated Using Powder Coated Fiber Method

SiCf/Ti-6Al-4V composites were fabricated by the powder-coated fiber method. The precursor fiber was prepared under the optimized parameter, and the composites were made using the vacuum hot pressure method. The influence of heat exposure time on products of thelinterfacial reaction was investigated using scanning electron microscope （SEM） and analytical transmission electron microscope （TEM） with energy dispersive spectrometer （EDS）. The main products are TiC and Ti5Si3 after vacuum exposing the samples at 700℃ for 50 h. The growth dynamics of interracial reaction products was analyzed quantitatively, which fitted the parabola rule. The activity energy of the reaction was 252 kJ·mol^-1.

Effects of Substrate Temperature on the Growth of Polycrystalline Si Films Deposited with SiH_4+Ar

Polycrystalline silicon （poly-Si） films were deposited using Ar diluted SiH4 gaseous mixture by electron cyclotron resonance plasma-enhanced chemical vapor deposition （ECR-PECVD）. The effects of the substrate temperature on deposition rate, crystallinity, grain size and the configuration of H existing in poly-Si film were investigated. The results show that, comparing with H2 dilution, Ar dilution could significantly decrease the concentration of H on the growing surface. When the substrate temperature increased, the deposition rate increased and the concentration of H decreased monotonously, but the crystallinity and the grain size of poly-Si films exhibited sophisticated trends. It is proposed that the crystallinity of the films is determined by a competing balance of the self-diffusion activity of Si atoms and the deposition rate. At substrate temperature of 200℃, the deposited film exhibits the maximum poly-Si volume fraction of 79%. Based on these results, higher substrate temperature is suggested to prepare the poly-Si films with advanced stability and compromised crystallinity at high deposition rate.

Interfacial Interaction in Coated Carbon Fibre Reinforced Aluminous Mg-based Composites

Four kinds of Mg alloys reinforced with carbon fibres were fabricated by a gas pressure infiltration technique. The fibres were pre-coated a SiO2 layer prior to fabrication. DifFerent microstructures and interactions in the fibre-matrix interface of these composites were observed by transmission electron microscopy （TEM）. The results showed that the interracial interaction strongly depended on the content of Al in the Mg-based matrices. The microstructure of the interface could then be controlled by adjusting the Al content of the Mgbased matrix. In addition, fibres extracted from different Mg-based matrix all had some degradation owing to the interracial reaction and the fibre-matrix interdiffusion.

Effect of Ar on Polycrystalline Si Films Deposited by ECR-PECVD using SiH_4

In this paper, polycrystalline silicon films were deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition （ECR-PECVD） using SiH4/Ar and SiH4/H2 gaseous mixture. Effects of argon flow rate on the deposition efficiency and the film property were investigated by comparing with H2. The results indicated that the deposition rate of using Ar as discharge gas was 1.5-2 times higher than that of using H2, while the preferred orientations and the grain sizes of the films were analogous. Film crystallinity increased with the increase of Ar flow rate. Optimized flow ratio of SiH4 to Ar was obtained as F（SiH4）： F（Ar）=10：70 for the highest deposition rate.

Interfacial Shear Strength Measurements of SiC Fiber-Reinforced Titanium Composites

A continuous loading push-out test technique was used to measure the interfacial shear strength of SiC fiber reinforced Ti matrix composites. The interfacial shear strength of samples as-fabricated and after heat exposed at 700 and 800℃ for 50 h was successfully determined. It has been found that the interfacial shear strength of the specimen exposed at 700℃ was higher than that of as-prepared and exposed samples at 800℃. The shear strength of the as-prepared samples was about 112.45 MPa, and increased to about 153.77 MPa after heat-treating at 700℃ for 50 h, but decreased to 133.11 MPa after treating at 800℃ for 50 h. Scanning electron microscope （SEM） was used to investigate the interfacial morphology of the samples. The brittle phase was the main products of interface for samples exposed at 800℃, and the interface was easily peeled off.

Measurement of interfacial residual stress in SiC fiber reinforced Ni-Cr-Al alloy composites by Raman spectroscopy

Raman spectroscopy was used to measure Raman spectra of the inner SiC fibers and surface C-rich layers of SiC fibers, composite precursors and SiCf/Ni-Cr-Al composites. The residual stresses of the inner SiC fibers and surface C-rich layers were calculated, and the effect of the(Al + Al_2O_3) diffusion barrier layer on the interfacial residual stress in the composites was analyzed in combination with the interface microstructure and energy disperse spectroscopy(EDS) elements lining maps. The results show that the existence of(Al + Al_2O_3) diffusion barrier improves the compatibility of the SiCf/Ni-Cr-Al interface,inhibits the adverse interfacial reaction, and relieves the residual stress inside SiC fibers and at the interface of composite material. Heat treatment can reduce the residual stress at the interface. As the heat treatment time increases, the residual stress at the interface decreases.

An effective strategy towards construction of CVD SiC fiber-reinforced superalloy matrix composite

In this work,a modified approach for preparing CVD SiC fiber-reinforced superalloy matrix composites was rationally developed.The composites were fabricated by vacuum hot pressing(VHP)process using precursor wires coated with(Al+Al2O3)diffusion barrier layers and GH4169 superalloy coatings.BNi-7 brazing filler metals were introduced on the surface of precursor wires in order to decrease the temperature of the VHP process.It was found that the VHP temperature was reduced by about 100℃,and the melting,diffusion,nucleation and growth processes of BNi-7 fillers at 900?C motivated the recrystallization and plastic flow of the matrix under the increasing pressure,thereby a compact composite composed of intact SiC fibers and fine equiaxial grain structure superalloy matrix was achieved.Meanwhile,the elements were distributed homogeneously among the fibers in the composite and no interfacial reactions occurred.This method provides a new insight for designing and manufacturing high-quality composites in practical engineering.

Effects of Cr and Al Contents on the Preparation of SiC Fiber-Reinforced NiCrAl Alloy Matrix Composite

In this work,the effects of Cr and Al contents on the preparation of SiC fiber-reinforced NiCrAl alloy matrix composites(SiCf/Ni-20Cr-5Al,SiCf/Ni-15Cr-5Al,SiCf/Ni-10Cr-5Al and SiCf/Ni-10Cr-3Al)were thoroughly discussed.The composites were prepared by vacuum hot pressing process using matrix-coated fibers.It was found that Cr solute atoms played a significant role in retarding the recrystallization of NiCrAl alloy matrix,and the Al elements in the form of γ'-Ni3Al phase had a suppression effect on the plastic flow of the matrix.Therefore,the reduction in Cr and Al contents was conductive to the recrystallization and plastic flow of NiCrAl alloy matrix,thereby reduced the size and number of micro-voids in the composite.In addition,this work provides some guidance for designing and manufacturing reasonable SiC fiber-reinforced Ni alloy matrix composites.