Danqing Yi

Study of reaction process on Ni/4H–SiC contact

Abstract The present study deals with mechanisms of the reaction process of fabricated thin film Ni/SiC contacts by means of XRD, XPS and Raman spectroscopy. After annealing SiC samples sputter coated with Ni at 800 and 950°C in vacuum for 20 min, the dominant silicide is textured Ni2Si. Its formation consists of two stages: initial reaction rate and subsequent diffusion controlled stage. For ultra thin initial Ni layer (∼3–6 nm), islands formation of Ni2Si is observed after heat treatment. Increasing the Ni film thickness prevents this phenomenon. The C released owing to the Ni2Si formation reaction forms a thin graphite layer on the top of the surface and also tends to form cluster inside the reaction layer. The overall degree of graphitisation is higher at 950°C than that at 800°C.

Preparation and properties of alloy 2618 reinforced by submicron AlN particles

An attempt was made to explore the possibility of fabricating AA2618 reinforced by submicron AIN particles. The results show that the addition of AIN particles decreases remarkably the grain size of the alloy. The 2618-AIN dispersion-strengthened alloy has the same strength as AA2618 at room temperature in the T6 state, but exhibits significantly higher strength after isothermal exposure at 200 °C for 100 h. The strength of the dispersion-strengthened alloy at elevated temperatures was greater than that of AA2618.

Microstructures and Mechanical Properties of Hot-Pressed MoSi2-Matrix Composites Reinforced with SiC and Si3N4 Particles

MoSi2-matrix composites reinforced with Si3N4 and SiC particles were fabricated by means of wet-mixing and heat-pressing process. Scanning electron microscope (SEM), X-ray diffractometry (XRD), polarizing microscopy, Vickers hardness tester, with a universal materials testing machine were used to investigate the morphology, grain size, hardness, fracture toughness, and bending strength of the synthesized composites. Notable effects on the bending strength and fracture toughness of MoSi2 caused by the addition of SiC and Si3N4 particles were found. The MoSi2 composite with 20 vol.% SiC and 20 vol.% Si3N4 particles has the highest strength and toughness, which is about 100% and 340%, respectively, higher than that of pure MoSi2. The grain size of MoSi2 decreases gradually with the volume content of SiC and Si3N4 particles increasing from 0% to 40%, and MoSi2-20 vol% SiC-20 vol% Si3N4 composite exhibits the minimum grain size of MoSi2. The relationship between the grain size of MoSi2 and bending strength is not entirely fit with Hall-Petch equation. The strengthening mechanisms of the composite include fine-grain strengthening and dispersion strengthening. The toughening mechanisms of the composite include fine grain, microcracking, crack deflection, crack microbridging, and crack branching.

In-situ synthesis of graphene on surface of copper powder by rotary CVD and its application in fabrication of reinforced Cu-matrix composites

A novel approach has been developed for fabricating graphene (Gr) reinforced Cu composites. First, graphene was synthesized in-situ on surface of micron-sized copper powder through rotary chemical vapor deposition (RCVD). Then, the composite powders were hot pressed into bulk Gr-Cu composite pieces in vacuum. During RCVD, methane flow rate controlled the number of layers in synthesized graphene. Analyses of Raman spectroscopy suggested that graphene with varied layer numbers were formed on Cu powder surface. The as-fabricated graphene exhibited a ‘wrinkle’ type morphology with a homogeneous distribution observed by fielde mission scanning electron microscopy (SEM). After hot pressing, the structure and morphology of graphene were preserved in the Gr-Cu bulk composites, as confirmed by high-resolution transmission electron microscopy (HRTEM). Physical and mechanical properties of these bulk composites were studied. They were nearly fully densified and featured with an increase of more than 30% in hardness, as well as similar electrical conductivity and thermal conductivity as compared with pure Cu. The mechanism therein was discussed. Correspondence to: Danqing Yi, School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, P.R. China,Tel: +86-73188830263; Fax: +86-731-88836320; E-mail: yioffice@csu.edu.cn, danqing@csu. edu.cn