Silicon carbide – Research on composite material processing

Abstract. Within the paper are presented the leading aspects concerning the intelligent composite materials processing. Achievement of an intelligent structure or material, involves at least two aspects. Firstly, it is necessary to create a composite structure, usually based on epoxidic resins, strengthened by glass fibres. And secondly, a network of sensors and actuators is integrated throughout this structure.

Such an intelligent structure needs to accomplish the following functions:
-sensor function;
-actuator function;
-processing function.

These functions are carried out by the functional proprieties of some special materials, able to develop transferring mechanisms at atomic or molecular level. Analysis of SiC grains found in the Murchison carbonaceous chondrite meteorite has revealed anomalous isotopic ratios of carbon and silicon, indicating an origin from outside the solar system. 99% of these SiC grains originate around carbon-rich Asymptotic giant branch stars. SiC is commonly found around these stars as deduced from their infrared spectra.

Production. The modern method of manufacturing silicon carbide for the abrasives, metallurgical and refractories industries is basically the same as that developed by Acheson. A mixture of pure silica sand and carbon in the form of finely ground coke is built up around a carbon conductor within a brick electrical resistance-type furnace. Electric current is passed through the conductor, bringing about a chemical reaction in which the carbon in the coke and silicon in the sand combine to form SiC and carbon monoxide gas. A furnace run can last several days, during which temperatures vary from 2200° to 2700°C (4000° to 4900°F) in the core to about 1400°C (2500°F) at the outer edge. The energy consumption exceeds 100.000 kilowatt-hours per run. At the completion of the run, the product consists of a core of green to black SiC crystals loosely knitted together, surrounded by partially or entirely unconverted raw material. The lump aggregate is crushed, ground, and screened into various sizes appropriate to the end use.

For advanced electronic applications, large single crystals of SiC can be grown from vapour; the bole can then be sliced into wafers much like silicon for fabrication into solid-state devices.

For reinforcing metals or other
ceramics, SiC fibers can be formed in a number of ways, including chemical vapour deposition and the firing of silicon-containing polymer fibers.

Properties. Silicon carbide exists in at least 70 crystalline forms. Alpha silicon carbide (α-SiC) is the most commonly encountered polymorph. It is formed at temperatures greater than 2000°C and has a hexagonal crystal structure similar to Wurtzite. The beta modification (β-SiC) with a zinc blended crystal structure similar to diamond is formed at temperatures below 2000 °C. Until recently, the beta form has had relatively few commercial uses, although there is now increasing interest in its use as a support for heterogeneous catalysts, owing to its higher surface area compared to the alpha form.

Silicon carbide has a density of 3.2 g/cm³, and its high sublimation temperature (approximately 2700°C) makes it useful for bearings and furnace parts. Silicon carbide does not melt at any known pressure. It is also highly inert chemically. There is currently much interest in its use as a semiconductor material in electronics, where its high thermal conductivity, high electric field breakdown strength and high maximum current density make it more promising than silicon for high-powered devices. In addition, it has strong coupling to microwave radiation which, together with its high sublimation point, permits practical use in heating and casting metals. SiC also has a very low coefficient of thermal expansion (4.0 × 10-6/K) and experiences no phase transitions that would cause discontinuities in thermal expansion.

Applications

Abrasive. In the arts, silicon carbide is a popular abrasive in modern lapidary due to the durability and low cost of the material.

In manufacturing, it is used for its hardness in abrasive machining processes such as grinding, honing, water-jet cutting and sandblasting.

High-temperature applications. Due to its high thermal conductivity, SiC is used as substrate for other semiconductor materials such as gallium nitride. Due to its wide band gap, SiC-based parts are capable of operating at high temperature (over 350 °C), which together with good thermal conductivity of SiC makes SiC devices good candidates for elevated temperature applications. SiC devices also possess increased tolerance to radiation damage, making SiC a desirable material for defense and aerospace applications. Gallium nitride is itself also an alternative material in many applications.

Heating element. References to silicon carbide heating elements exist from the early 20th century when they were produced by Acheson’s Carborundum Co. in the U.S. and EKL in Berlin. Silicon carbide offered increased operating temperatures compared with metallic heaters, although the operating temperature was limited initially by the water-cooled terminals, which brought the electric current to the silicon carbide hot zone.

Circuit elements. Silicon carbide is used for blue LEDs, ultrafast, high-voltage Schottky diodes, MOSFETs and high temperature thyristors for high-power switching.

Currently, problems with the interface of SiC with silicon dioxide have hampered the development of SiC based power MOSFET and IGBTs. Another problem is that SiC itself breaks down at high electric fields due to the formation of extended stacking faults, but this problem may have been resolved relatively recently.

Structural material. In the 1980s and 1990s, silicon carbide was studied on several research programs for high-temperature gas turbines in the United States, Japan, and Europe. The components were intended to replace nickel superalloy turbine blades or nozzle vanes. However, none of these projects resulted in a production quantity, mainly because of its low impact resistance and its low fracture toughness.

Automotive industry. Silicon-infiltrated carbon-carbon composite is used for high performance ceramic brake discs as it is able to withstand extreme temperatures. The silicon reacts with the graphite in the carbon-carbon composite to become carbon fiber reinforced silicon carbide or C/SiC. Silicon carbide was also used for manufacturing the ceramic composite clutch discs for the Carrera GT supercar. Silicon carbide is used in a sintered form for diesel particulate filters.

Armor. Like other hard ceramics, alumina and boron carbide, silicon carbide is used in composite armor and in ceramic plates in bulletproof vests. Dragon Skin, which is produced by Pinnacle Armor, disks of silicon carbide.

Cutting tools. In 1982 at the Oak Ridge National Laboratories, George Wei, Terry Tiegs, and Paul Becher discovered a composite of aluminium oxide and silicon carbide whiskers. This material proved to be exceptionally strong. Development of this laboratory-produced composite to a commercial product took only three years. In 1985, the first commercial cutting tools made from this alumina and silicon carbide whisker-reinforced composite were introduced by the Advanced Composite Materials Corporation (ACMC) and Greenleaf Corporation.

Jewelry. As a Gemstone used in jewelry, silicon carbide is called Moissanite after the jewel’s discoverer Dr. Henri Moissan. Moissanite is similar to diamond in several important respects: it is transparent and hard (9, although a patent states 8.5-9.0 on the Mohs scale compared to 10 for diamond), with a refractive index between 2.65 and 2.69 (compared to 2.42 for diamond). Moissanite is somewhat harder than common cubic zirconia. Unlike diamond, Moissanite is strongly birefringent. This quality is desirable in some optical applications, but not in gemstones. For this reason, Moissanite jewels are cut along the optic axis of the crystal to minimize birefringent effects. It is lighter (density 3.22 vs. 3.56) and much more resistant to heat. This results in a stone of higher lustre, sharper facets and good resilience.

by Ph.D. candidate eng. Elena Irimie


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