Silicon Carbide Whisker

Silicon carbide whiskers are nano-sized single crystal fibers featuring an indefect-free crystal structure and excellent chemical resistance, used primarily to strengthen metal, ceramic and polymer composite materials and increase fracture toughness.

After calcination, b-SiC whisker displays significant improvement with respect to stacking faults being eliminated and grain growth nuclei emerging, forming large tabular a-SiC grains according to XRD, SEM, and TEM results.

Characteristics

Silicon carbide whiskers are micron-sized single crystal fibers that can transfer stress in response to external loads, providing reinforcement for metal, ceramic and polymer-based composite materials in aerospace, energy and defense industries. Silicon carbide whiskers exhibit excellent wear resistance, corrosion and high temperature oxidation resistance which makes them an excellent reinforcing and toughening agent.

SiC whiskers play an increasingly prominent role in advanced structural ceramics, yet studies on their microstructure under high-temperature treatment remain scarce. We used X-ray diffraction (XRD) and scanning electron microscopy to examine SiC whiskers at temperatures over 2000 degC; raw whiskers showed severe stacking fault annihilation with formation of b-SiC grains whereas partial b-SiC grains underwent phase transformation to hexagonal prism and triangular prism a-SiC grains as seen via XRD and SEM analyses.

As these whiskers can serve as toughening agents, their use as toughening agents is a promising area of research. When added to a ceramic matrix, whiskers form interfacial layers of a-SiC and b-SiC that increase fracture toughness while simultaneously decreasing dissipation of crack propagation energy.

To evaluate the efficacy of this method, two kinds of whisker-matrix composites were produced: one using Silar-SC-9 whiskers and one using Tateho-SCW-1-S whiskers. To assess their efficacy, P-CMOD curves of these composites were examined as functions of whisker dosage; those created with Silar-SC-9 whiskers demonstrated higher CMOD values, signifying that they had effectively dispersed into and filled initial defects within their cement-based matrix matrix.

Composites made with Tateho-SCW-1-S whiskers demonstrated a lower CMOD value, suggesting they did not completely fill the matrix. Scanning electron microscopy analysis of fracture surfaces revealed limited whisker puttout with Silar-SC-9 whiskers but no such phenomenon with Tateho-SCW-1-S whiskers, demonstrating how bridge and pullout processes contribute to improved fracture toughness of composites made with whisker-toughened materials.

Applications

Silicon Carbide (SiC) whiskers have been developed as reinforcement materials to increase mechanical properties in polymer composite materials, specifically nylon composites. As opposed to metallic or ceramic fillers, SiC whiskers offer superior elastic modulus, strength and hardness as well as better chemical stability, corrosion resistance, high temperature oxidation resistance and can even be incorporated into nylon composites for greater abrasion resistance. Different methods have been employed for synthesizing SiC whiskers such as carbothermal reduction, chemical vapor deposition or combustion of waste materials such as rice husks; however quality may differ in terms of size dimensions or purity of their production.

SiC whisker production requires the optimal combination of raw materials: coal-coke, alumina and silica with high carbon contents; rice husks provide another advantageous source due to their abundance and relative low costs. Once this raw material has been assembled it must first be ashed in air to remove excess carbon before reacting in a continuous bulk system at temperatures from 1350 C-1800 C before being purge with inert gas as a protective measure against moisture entering and eroding furnace walls.

Figure 1 depicts a continuous bulk system for producing SiC whiskers. Coked rice hulls are loaded into several cylindrical containers 9 that are then stacked on a tray 10. Each container’s bottom curved portion 11 is made from graphite material to withstand high temperatures encountered when reacting with feed materials, then continuously moved into dehydrating, heating and cooling zones 14-18-19 for reactions before being purging with inert gas to avoid moisture entering conversion furnace and hindering whisker growth.

Once the reaction reaches temperatures between 1200 deg C and 1600 deg C, CO atmosphere is replaced with argon. This results in the production of b-SiC whiskers with desired length and diameter which are then separated using sieves from non-reacted silicon carbide particles before being washed in hot water, dried under vacuum, packaged, and ready for distribution.

Safety

Silicon carbide is an extremely hard, inert material with excellent thermal shock resistance that makes it suitable for applications requiring long endurance like car clutches or ceramic bulletproof vest plates. Furthermore, this property enables silicon carbide production of LEDs and sensors; and may be utilized as part of composite materials including polymers or metal alloys like titanium alloys.

Inhalation of fine dust particles poses a potential health hazard and care should be taken when handling this material. When handling silicon carbide whiskers, for instance, wearing masks and gloves is strongly advised as their particles could enter your respiratory system through inhalation, potentially leading to irritation, inflammation, or even permanent lung damage.

Studies of workers exposed to man-made mineral fibres at work have demonstrated a correlation between exposure and lung pathologies, such as lung cancer, and silicon carbide whiskers – due to their similar size and shape – and exposure. While there have been studies conducted into their potential long-term impact on health, no epidemiological research studies exist regarding long-term exposure.

SiC whiskers were evaluated using a cell viability assay with V79 Chinese hamster lung cells, as expected. A cell viability assay showed that SiCW-3S is more toxic than SiNW; furthermore, the nick translation assay revealed all whiskers tested could break DNA though their rates of cell death differed depending on whether or not they contained granular, crocidolite-like material (SiCW-3 and SiNW).

Study of SiC whisker effects on PA6 composites revealed that adding 2 weight percent SiC whiskers significantly enhanced tensile strength, elongation at break, fracture toughness and ductility – qualities particularly advantageous when designing composite structures that require high levels of deformation during failure.

Production

This invention concerns methods and apparatus for continuously manufacturing silicon carbide whiskers. The process described in the invention requires creating a first reaction zone containing microfine particles of silicon dioxide uniformly mixed with carbon or its precursor, with an approximate ratio of five or greater by weight between silicon dioxide and carbon. Near the first reaction zone is formed a second reaction zone containing a porous fibrous mass of active carbon or an infusible carbon precursor. The ratio between carbon precursor and inactive carbon is such that, upon formation of whiskers from this material, its purity exceeds several percentages compared with conventional solid-state silicon carbide processes.

Researchers discovered that using an alkali metal or alkaline earth metal halide reaction promoter can yield very high yields of silicon carbide whiskers with excellent dispersibility and low bulk density, making the reaction even more cost effective when heated to temperatures between 400deg C to 700deg C as at higher temperatures, crystal formation dominates and reaction speeds drop considerably. Furthermore, rice hulls ashized between 400deg to 700deg C prove most efficient since crystalline forms take precedence at higher temperatures leading to slower reaction speeds overall.

With a large molar ratio of SiO2 to carbon, reaction is limited to producing SiO2. Therefore, nonvolatile impurities remain within unreacted silica rather than polluting silicon carbide whiskers with oxides; consequently resulting in whiskers with very high aspect ratios and minimal defects such as necking or branching.

Silicon carbide whiskers produced through this method can also be easily formed using appropriate methods into plates, bars, tubes, cylinders, spheres, wires or granules for use as reinforcement or toughening materials for ceramics, metals and polymer composites to increase hardness, strength, chemical inertness, oxidation resistance, thermal stability, dimensional stability and tensile strength. Furthermore, it offers a simple yet cost-effective method of producing whiskers of very high quality suitable for application across these fields.