Synthesis Of SiC Whiskers Research Articles

Effects of aggregate/matrix‐phase ratio on the in‐situ synthesis of SiC whiskers and properties of reaction‐bonded SiC

AbstractReaction‐bonded silicon carbide (RBSC) materials were synthesized by a carbon‐buried firing method, using α‐SiC powders with different sizes as aggregates, while silicon and graphite powders as matrix‐phase materials for in‐situ synthesis of β‐SiC. The effect of the mass ratio of α‐SiC aggregate to β‐SiC matrix‐phase on the in‐situ synthesis of β‐SiC whiskers was investigated and the optimum mass ratio was determined according to the properties of RBSC materials. The phase compositions, microstructure, and properties of the RBSC materials were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscope (TEM), respectively. The results showed that the decreasing aggregate/matrix‐phase ratio could increase the yield and aspect ratio of β‐SiC whiskers by improving the amount of pores and SiO2 content. The β‐SiC whiskers with stacking faults grew along [111] direction. The in‐situ formed β‐SiC whiskers and SiO2 in the matrix‐phase enhanced the hot modulus of rupture (HMOR) of the RBSC materials, and the sample with the aggregate/matrix‐phase ratio of 70:30 showed the highest cold modulus of rupture (CMOR) of 53.3 MPa, high HMOR of 43.3 MPa, and medium water absorption of 13.8%.

Synthesis of SiC whiskers via catalytic reaction method in self-bonded SiC composites

Catalyzed by in-situ formed Fe nanoparticles (NPs), 3C–SiC whiskers were prepared from expanded graphite and Si powders after firing at 1573 K for 3 h in Argon. The density functional theory calculations revealed that Fe catalysts facilitated the formation of SiC nucleus and the epitaxial growth of SiC whiskers via reducing the bonding strength in CC dimer as well as Si–O and C–O bonds. Moreover, using SiC, expanded graphite and silicon powders as starting materials Fe-catalyzed self-bonded SiC composites were fabricated. Lots of SiC whiskers were synthesized in the as-prepared composites, leading to remarkable enhancements in high temperature mechanical behavior, oxidation resistance and cryolite resistance of the self-bonded SiC composites.

Synthesis of SiC whiskers by VLS and VS process

This study investigates the mechanisms of SiC whisker formation in the carbothermal reduction of quartz to SiC in different gas atmospheres. Reduction of quartz by graphite was studied in Ar, H2, and CH4–H2–Ar gas mixture in a laboratory fixed bed reactor. The reduction products were characterised by XRD, SEM and TEM. Whiskers were not formed in the carbothermal reduction of quartz in argon. Two types of SiC whiskers were observed in the carbothermal reduction of quartz in H2 and CH4–H2–Ar gas mixture. In the process of reduction at 1400–1600°C in H2 and at 1200–1600°C in CH4–H2–Ar gas mixture, whiskers with hexagonal shape with diameter 100–800nm and length up to tens of microns were formed by the VLS mechanism under catalytic effect of iron. The whiskers with the characteristics of cylindrical shape and high aspect ratio were synthesized in CH4–H2–Ar gas mixture at 1400–1600°C by VS mechanism.

Impurity phases of silicon dioxide in commercial SiC whiskers produced by VLS method

Silicon dioxide forms two impurity phases in SiC whiskers, namely α-cristobalite and X-ray-amorphous SiO 2. The particles of α-cristobalite form as a result of hydrolysis of silicon compounds with chlorine by water vapor, and these are not bound with SiC surface. During synthesis of SiC whiskers, the layer of X-ray-amorphous silicon dioxide of up to 2.5 nm thickness forms on the surface of whiskers through oxidation of crystal surface by water vapor. Upon drying SiC whiskers at temperature above 120 °C and also long-time storing of crystals in contact with air, the surface of whiskers oxidizes and SiO 2 film forms of up to 3 nm thickness. The mechanism is proposed for low-temperature oxidation of SiC whiskers in the presence of water vapor. It is shown that aggregately stable suspensions of whiskers form in alkaline medium in the presence of SiO 2 film on the surface of crystals.