Synthetic Silicon Carbide

Silicon carbide, more commonly referred to as corundum, is a hard material widely utilized in engineering applications. With its superior strength, durability, chemical inertness and heat resistance properties it makes an excellent material choice for harsh environments and high performance engines.

Multiple methods have been devised for synthesizing SiC, such as the Acheson method, Lely process, and chemical vapor deposition.

Chemical Vapor Deposition

CVD of silicon carbide is an invaluable manufacturing process used for semiconductors, catalysis and energy storage applications. This deposition method utilizes the vapor phase of controlled chemical reactions to deposit thin films on substrates at temperatures up to 1400degC or in plasma form at lower temperatures with still high deposition rates.

The deposited material can take various shapes and sizes; its surface finish may be smooth or textured. Properties of the film may also be adjusted by altering its temperature of deposition. A variety of preparation gases is utilized during deposition such as silane (SiH4), disilane (Si2H6) and tetrachlorosilane, along with carbon precursors like methane (CH4), acetone (C2H6) propane (C3H8) methine/toluene/toluene (C7H8) hexane (C6H14), methyl chloride (CH3) carbon tetrachloride (CCl4) etc. For optimal results in PECVD systems such as plasma-enhanced chemical vapor deposition systems (PECVD).

PECVD involves the delivery of gas at low pressure to a deposition chamber at less than 1.3kPa, where electrical energy is applied in order to activate its flow and generate a glow discharge plasma consisting of electrons, ions, and electronically excited species that breaks apart and vaporizes reactant molecules before reacting with heated substrate to form thin films.

Deposited b-SiC can transmit light in both visible and infrared wavelength ranges of the spectrum. Furthermore, its electrical resistivity must meet or surpass 500 Ohm-cm-cm; 1000 Ohm-cm-cm would be even better. These characteristics make b-SiC produced through bulk synthesis stand out, which invariably remains opaque and absorbs and scatters light at these wavelengths. The present invention focuses on developing a process for producing synthetic b-SiC that is both highly transparent and features desirable mechanical properties, including hardness. Predictive modeling approaches have been increasingly developed that represent transport phenomena and chemistry ranging from pure thermodynamic and kinetic descriptions through mass transport models.

Thermal Decomposition

Chemical reactions required to create synthetic silicon carbide (SiC) involve high temperatures, so care must be taken when conducting them in a well-ventilated area. Proper safety gear such as heat resistant gloves and protective goggles must also be worn during this process, along with proper fume hoods and ventilation ducts to avoid breathing in any vapors that may be released during this reaction process.

At an approximate temperature of 900 degrees Celsius, silicon (Si) is heated until its melting point of about 905 degrees Celsius. At this point, SiC begins decomposing into carbon dioxide and hydrogen gasses which then react with water molecules to form gaseous silicon dioxide compounds like SiO2. When heated further, hydrogen bonds with oxygen molecules present in air to form solid silicon oxycarbide SiO2, which remains after drying and curing.

SiC oxycarbide solid forms hard ceramic blocks when it cools down, providing bulletproof armor against bullets or any other harmful substances. This material offers reliable protection.

Silicon oxycarbide has many uses other than wear resistance, including creating wear- and corrosion-resistant materials. For instance, it can be used as the insulation material inside aluminum electrolytic tanks and copper melting furnaces as well as making rocket nozzles and blades for gas turbines.

Contrary to its natural mineral counterpart, which only appears in trace amounts in certain meteorites and corundum deposits, much of the SiC sold worldwide is produced synthetically using various processes – particularly when cut and sold as Moissanite gems.

Synthetic silicon carbide can be produced through thermal decomposition, and its thermal stability makes it the material of choice for industrial applications that demand higher heat levels and voltage levels.

Thermal decomposition produces larger single crystals than other methods, which can then be cut and polished into desired types of silicon carbide for industrial use. Furthermore, thermal decomposition allows for the creation of different polytypes of silicon carbide that depend on how atomic layers stack themselves; these varieties can then be classified as cubic, hexagonal, or rhombohedral in shape.

Oxidation

Silicon carbide is inert, not reacting with most acids (hydrochloric, sulphuric or hydrofluoric) or bases. However, at temperatures above 900degC it will oxidize in air to produce SiO2, known as dry oxidation. The kinetics and models for dry oxidation have been extensively researched – most notably Deal and Grove’s model that describes both diffusion-controlled mechanisms and surface controlled processes simultaneously using two constants – one parabolic and one linear constant (where one signifies diffusion-controlled mechanisms while the other represents surface processes). [13]

Oxidizing silicon carbide involves several steps. The initial stage involves creating a carbonyl defect at an oxygen-Si bond site and desorbing carbon dioxide. DFT calculations reveal that this step has an activation energy of 350kJ/mol and occurs faster with higher temperatures; its rate drops when present with nitrides.

Following oxidation of carbonyl defects, an oxide film forms which serves as an initiator for further oxidation. Next comes growth of continuous layers of spherulitic crystals known as cristobalite which dispersion in an amorphous matrix locally increases grain boundaries while slowing oxidation rate.

Cristobalite can also be produced using other processes, including Lely’s electric furnace process that combines liquid silicon and carbon. Cristobalite material can be made into various shapes, sizes and densities with impressive thermal and mechanical properties as well as chemical inertness.

Material made from graphene has found wide application, particularly within gas turbines where it replaces nickel superalloy blades and vanes. With its negative temperature coefficient at room temperature and positive at higher temperatures, graphene makes an excellent material choice for high temperature heating elements and various dopants can be added to improve its electrical conductivity.

Physical Vapor Deposition

Silicon Carbide (SiC), due to its combination of desirable physical, chemical, mechanical and electrical properties has become an attractive material system. SiC’s wide tunable band gap, low density and strength combined with thermal conductivity and shock-resistance have contributed significantly to its success and research [1]. SiC remains at the center of intense investigation worldwide [2-3].

Chemical Vapor Deposition (CVD) offers the promise of producing SiC with superior optical transmission, purity, and electrical resistivity that are free-standing thin films fabricated through chemical vapor deposition processes; however, CVD processes typically operate at high temperatures which could compromise film quality.

Researchers are making strides toward developing low-temperature CVD techniques for producing SiC films, using plasma enhanced chemical vapor deposition (PECVD), electron cyclotron resonance CVD, magnetron sputtering and pulsed laser deposition as methods. Precursor selection, gas mixture used during deposition process conditions and substrate temperature can all have significant influences on final film characteristics.

Recently, there has been increasing interest in CVD production of SiC thin films for use in MEMS/NEMS systems and other applications. Unfortunately, conventional CVD methods require temperatures around 1400 to 1500 degrees Celsius – well beyond its melting point – which makes production difficult.

CVD growth of SiC can generate contaminants such as oxygen and nitrogen from its deposition gas source. These adatoms (contaminants) may degrade films over time, leading to discoloration and poor adhesion issues.

Physical Vapor Deposition (PVD) is an alternative to CVD that operates without solvents, eliminating impurities. PVD technology can be used to deposit various metals, alloys and dielectrics.

PVD technology has been employed to deposit bipolar bi2Te3 and bipolar Sb2Te3 films onto polyethylene terephthalate (PET) substrates to produce foldable thermoelectric generators (f-TEGs). These PVD-deposited films exhibit lower internal resistance compared to foldable thermoelectric generators constructed using wrinkle-free PET substrates.