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Chlorosilane

Chlorosilanes are a class of organosilicon compounds in which silicon atoms are bonded to one or more chlorine atoms, with the remaining valences typically occupied by hydrogen or alkyl groups such as methyl.^[1] These reactive substances, related to silane (SiH₄), are essential intermediates in the synthesis of silicon-based polymers and materials.^[2] The production of chlorosilanes primarily occurs through the direct process, also known as the Müller-Rochow synthesis, involving the reaction of elemental silicon powder with methyl chloride (CH₃Cl) gas in the presence of a copper catalyst at temperatures of 250–300°C.^[2] This method yields a mixture of methylchlorosilanes, including the predominant dimethyldichlorosilane (Me₂SiCl₂, 70–90% yield), methyltrichlorosilane (MeSiCl₃), and trimethylchlorosilane (Me₃SiCl), which are subsequently separated by fractional distillation.^[3] Silicon powder is produced by the carbothermic reduction of quartz with carbon in an electric arc furnace, while methyl chloride is generated from hydrochloric acid and methanol.^[3]^[4] Chlorosilanes exhibit high reactivity due to the polar Si–Cl bond, readily hydrolyzing in the presence of water or moist air to form silanols, siloxanes, and hydrogen chloride (HCl) gas, often accompanied by exothermic reactions and potential flammability.^[1] They are corrosive, poisonous upon ingestion or inhalation, and can irritate skin, eyes, and mucous membranes, primarily owing to the HCl produced during hydrolysis; some variants are pyrophoric or flammable.^[1] Commercial chlorosilanes may require distillation to eliminate trace siloxanes from inadvertent hydrolysis.^[2] In applications, chlorosilanes are hydrolyzed and polycondensed to produce silicone polymers, including oils (linear polydimethylsiloxanes for lubricants and cosmetics), gums (for elastomers in seals and gaskets), and resins (cross-linked structures for coatings and adhesives).^[3] They also serve as precursors for high-purity silicon in semiconductors and solar cells, as well as in the manufacture of silsesquioxanes, coatings, and microfluidic devices.^[2]

Definition and Properties

Definition and Classification

Chlorosilanes are a class of reactive chemical compounds containing silicon bonded to one or more chlorine atoms, with the general formula R_n \mathrm{SiCl}_{4-n}, where n ranges from 0 to 3 and R represents hydrogen, alkyl, aryl, or other substituent groups.^[1] These compounds are structurally analogous to silane (\mathrm{SiH_4}), the simplest silicon hydride, in which some or all hydrogen atoms are substituted by chlorine or organic groups.^[1] Chlorosilanes are broadly classified into inorganic and organo types based on the nature of the substituents. Inorganic chlorosilanes, such as silicon tetrachloride (\mathrm{SiCl_4}) and trichlorosilane (\mathrm{HSiCl_3}), contain only silicon, hydrogen, and chlorine atoms with no carbon-silicon bonds.^[5] In contrast, organochlorosilanes incorporate carbon-containing groups, exemplified by methyltrichlorosilane (\mathrm{(CH_3)SiCl_3}), which feature silicon-carbon linkages and serve as key precursors for organosilicon materials.^[5] The discovery and early study of chlorosilanes occurred in the 19th century, with notable contributions from chemists Charles Friedel and James M. Crafts, who in the 1860s synthesized the first organosilicon compounds, such as tetraethylsilane, by reacting silicon tetrachloride with organometallic reagents like diethylzinc.^[6] Nomenclature for chlorosilanes follows IUPAC substitutive rules, treating silane as the parent hydride and prefixing the substituents in alphabetical order, such as trichlorosilane for \mathrm{HSiCl_3} and trichloro(methyl)silane for \mathrm{(CH_3)SiCl_3}.^[7]^[8]

Physical Properties

Chlorosilanes are generally colorless liquids or gases at room temperature, often exhibiting a pungent odor and fuming behavior due to their reactivity with atmospheric moisture.^[9]^[8]^[10] They display high volatility attributable to their low molecular weights and weak intermolecular forces, with boiling points typically ranging from below 0°C for lower homologs like monochlorosilane (-30.4°C) to around 57°C for silicon tetrachloride (SiCl₄), and 8°C for dichlorosilane.^[10]^[11] For instance, trichlorosilane (HSiCl₃) has a boiling point of 31.8°C, making it a liquid under standard conditions but highly prone to evaporation.^[8] These compounds are insoluble in water, instead undergoing rapid hydrolysis to produce hydrochloric acid and silanol intermediates, but they are miscible with many organic solvents such as benzene, ether, and chlorinated hydrocarbons.^[10]^[2] Densities vary but are often greater than that of water; silicon tetrachloride, for example, has a density of 1.48 g/cm³ at 25°C.^[9] Chlorosilanes exhibit limited thermal stability, decomposing at elevated temperatures to yield silicon, hydrogen chloride, and other products, with decomposition onset typically above 400–500°C depending on the substitution.^[12] Lower chlorosilanes, such as chlorosilane (SiH₃Cl), are particularly flammable, with flash points below -90°C and the ability to form explosive mixtures with air.^[13]^[14]

Chemical Properties

Chlorosilanes exhibit high reactivity primarily due to the polarity of the Si–Cl bond, arising from the electronegativity difference between silicon (1.90) and chlorine (3.16), which renders silicon partially positive and electrophilic. This polarity facilitates nucleophilic attack, particularly by water, leading to rapid hydrolysis and release of HCl; for instance, silicon tetrachloride (SiCl₄) reacts violently with moisture to form silicic acid and hydrogen chloride. The Si–Cl bond dissociation energy is approximately 381 kJ/mol, which, while comparable to other covalent bonds, contributes to instability under hydrolytic conditions because of the bond's susceptibility to heterolytic cleavage at the electrophilic silicon center.^[15]^[16] The silicon atom in chlorosilanes acts as a Lewis acid, coordinating with nucleophiles due to its empty d-orbitals and larger atomic size compared to carbon, which allows for greater polarization and back-donation from ligands. In contrast to carbon analogs like CCl₄, where the smaller size and lack of accessible d-orbitals result in lower Lewis acidity and greater kinetic stability, the Si–Cl bond displays charge-shift character with significant ionic contribution, enhancing coordination and reactivity. This difference explains why chlorosilanes readily form adducts with Lewis bases, such as amines or ethers, whereas alkyl chlorides are far less prone to such interactions.^[17] Stability in chlorosilanes increases with the number of alkyl substituents replacing chlorine atoms, as Si–C bonds are less polar and more covalent than Si–Cl bonds, reducing susceptibility to hydrolysis; for example, trimethylchlorosilane ((CH₃)₃SiCl) is more stable than SiCl₄ but still hydrolyzes under moist conditions.^[16]^[5] Certain hydridochlorosilanes, such as dichlorosilane (H₂SiCl₂), exhibit pyrophoricity, igniting spontaneously in air due to exothermic oxidation of the Si–H bond in the presence of oxygen and moisture.^[16]^[5]

Synthesis

Hydrochlorosilanes

Hydrochlorosilanes, particularly trichlorosilane (HSiCl₃), are primarily synthesized through the direct hydrochlorination of metallurgical-grade silicon with hydrogen chloride gas. This process involves reacting powdered silicon with HCl in a fluidized-bed reactor at temperatures of 300–350°C, facilitated by a copper-based catalyst such as copper(I) chloride (CuCl), which promotes the formation of an active Cu₃Si phase. The principal reaction is represented by the equation:

\text{Si} + 3\text{HCl} \rightarrow \text{HSiCl}_3 + \text{H}_2

This method yields trichlorosilane as the main product, with hydrogen gas as a byproduct, and is optimized for high conversion rates of silicon, often exceeding 90% under controlled conditions.^[18] In practice, the reaction produces additional hydrochlorosilanes as byproducts, including silicon tetrachloride (SiCl₄) from further chlorination and dichlorosilane (H₂SiCl₂) in minor quantities via competing pathways. Selectivity toward trichlorosilane is influenced by factors such as catalyst loading, HCl flow rate, and temperature; for instance, lower temperatures favor HSiCl₃ over SiCl₄, achieving selectivities up to 96% with advanced Cu₃Si alloys. These byproducts are separated through fractional distillation, with SiCl₄ often recycled in downstream processes to improve overall efficiency.^[19] Industrial production emphasizes achieving semiconductor-grade purity, requiring trichlorosilane with metallic impurities below parts-per-billion levels to ensure high-quality polycrystalline silicon via the Siemens process. Optimization involves purification steps like multi-stage distillation and adsorbent treatments to remove trace contaminants such as boron, phosphorus, and transition metals, enabling yields suitable for photovoltaic and electronics applications. This focus on purity has driven process refinements, including promoter additions like zinc or tin to enhance catalyst stability and reduce byproduct formation.^[20] The hydrochlorination process evolved from early 20th-century efforts in silicon chemistry, with foundational work on chlorosilane reactions dating to the 1920s, but gained industrial prominence in the 1940s through patents on direct chlorination methods. By the 1950s, companies like Union Carbide and Siemens refined the technique for scalable production, integrating it into silicon manufacturing workflows that addressed wartime and postwar demands for high-purity materials. These developments marked a shift from laboratory-scale syntheses to continuous, catalyst-driven operations essential for modern semiconductor supply chains.^[21]

Alkylchlorosilanes

Alkylchlorosilanes, particularly methyl derivatives, are primarily synthesized industrially through the Müller-Rochow process, also known as the direct process. This heterogeneous catalytic reaction involves the direct reaction of elemental silicon powder with chloromethane (CH₃Cl) in the presence of a copper-based catalyst at temperatures of 280–300°C.^[22] The process typically occurs in a fluidized-bed reactor, where the silicon-catalyst mixture is contacted with gaseous chloromethane, facilitating the formation of silicon-carbon bonds.^[23] The primary reaction can be represented as:

\ce{Si + 2 CH3Cl -> (CH3)2SiCl2}

The process yields a mixture where approximately 90% consists of useful methylchlorosilanes: dimethyldichlorosilane ((CH₃)₂SiCl₂, ~80–85%), methyltrichlorosilane ((CH₃)SiCl₃, ~8–12%), and trimethylchlorosilane ((CH₃)₃SiCl, ~3–5%), while the remainder includes disilanes and other minor species that require separation.^[24] The copper catalyst, often promoted with elements like zinc or tin, plays a crucial role in promoting the formation of Si-C bonds by alloying with silicon to create active sites that enable the insertion of alkyl groups.^[22] Variations of the direct process have been explored for synthesizing other alkylchlorosilanes, such as ethyl derivatives, by substituting chloromethane with ethyl chloride (CH₃CH₂Cl) under similar copper-catalyzed conditions. However, these adaptations face significant challenges, including lower product yields, reduced selectivity due to side reactions like dehydrochlorination, and accelerated catalyst poisoning from carbon deposition or impurities in the silicon feedstock.^[25] Catalyst poisoning, often caused by trace elements or coke formation on active copper sites, diminishes the efficiency and requires frequent regeneration or replacement to maintain process viability.^[26]

Reactions

Hydrolysis Reactions

Hydrolysis of chlorosilanes proceeds via a nucleophilic substitution mechanism in which water acts as the nucleophile, attacking the electrophilic silicon center polarized by the electron-withdrawing chlorine atoms, leading to stepwise replacement of chloride ligands with hydroxyl groups to form silanols.^[2] These unstable silanols then undergo condensation reactions, eliminating water to form siloxane (Si-O-Si) bonds and ultimately yielding oligomeric or polymeric siloxanes, depending on the number of chlorine substituents and reaction conditions.^[2] A representative general equation for a trichlorosilane is:

\text{RSiCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{RSi(OH)}_3 + 3\text{HCl}

followed by condensation of the silanol intermediate.^[10] The reaction is highly exothermic, generating significant heat and releasing hydrogen chloride (HCl) fumes, which necessitates controlled conditions to manage corrosion and safety risks.^[10] Rates are influenced by solvent, temperature, and pH; acid catalysis, often from the HCl byproduct itself or added acids, accelerates both hydrolysis and subsequent condensation steps, while base catalysis promotes silanol formation but may alter polymerization pathways.^[2] Hydrolysis typically occurs rapidly upon exposure to moisture or aqueous media, with the process often conducted in mixed organic-aqueous solvents to achieve homogeneity and desired product distributions.^[2] In specific cases, tetrachlorosilane (SiCl₄) hydrolyzes to orthosilicic acid (Si(OH)₄), which dehydrates to form amorphous silica gel, a process exploited for silica production.^[10] Similarly, methyltrichlorosilane (CH₃SiCl₃) undergoes hydrolysis and condensation to produce polymethylsiloxanes, which are branched or crosslinked structures serving as precursors to silicone resins.^[2]

Other Reactions

Chlorosilanes undergo substitution reactions with organometallic reagents, such as Grignard compounds, to form higher-substituted organosilanes. In these transformations, a chlorine atom is replaced by an organic group, enabling the synthesis of di- or triorganosilanes from monoorganotrichlorosilanes, though selective partial substitution can be challenging. For example, the reaction of methyltrichlorosilane (MeSiCl₃) with methylmagnesium bromide proceeds as follows:

\text{MeSiCl}_3 + \text{MeMgBr} \rightarrow \text{Me}_2\text{SiCl}_2 + \text{MgBrCl}

This type of substitution is commonly employed to produce specialty silanes like dimethyldichlorosilane for further derivatization.^[27] Redistribution reactions allow the equilibration of chlorosilanes under Lewis acid catalysis, redistributing organic substituents and chlorine atoms among silicon centers. A prototypical example involves the disproportionation of methylchlorosilanes catalyzed by aluminum chloride (AlCl₃), where methyltrichlorosilane equilibrates with tetrachlorosilane and dimethyldichlorosilane:

$2 \text{MeSiCl}_3 \rightleftharpoons \text{Me}_2\text{SiCl}_2 + \text{SiCl}_4

This process, often supported on zeolites like ZSM-5 to improve catalyst stability and selectivity, achieves equilibrium conversions influenced by temperature and catalyst loading, with activation energies around 100 kJ/mol for optimal cluster sizes. Such redistributions are key for utilizing byproduct mixtures in organosilicon production, optimizing yields of commercially valuable dichlorosilanes.^[28] Chlorosilanes, particularly trichlorosilane (HSiCl₃), can be converted to monosilane (SiH₄) through disproportionation, a non-hydrolytic reduction pathway essential for producing high-purity silane gas. The reaction proceeds catalytically, typically with tertiary amines or supported metals, via the equilibrium:

$4 \text{HSiCl}_3 \rightleftharpoons \text{SiH}_4 + 3 \text{SiCl}_4

This transformation recycles chlorosilane streams from silicon deposition processes, yielding silane suitable for chemical vapor deposition in semiconductor manufacturing, with equilibrium favoring silane at elevated temperatures above 300°C.^[29]

Applications

Silicone Production

Chlorosilanes, particularly methyl-substituted variants, are essential precursors in the industrial production of silicone polymers, which are widely used materials valued for their thermal stability, flexibility, and water repellency. The core process begins with the hydrolysis of these chlorosilanes in the presence of water, converting the chlorine atoms to hydroxyl groups to form silanols. This step generates byproducts like hydrochloric acid, which often acts as a catalyst in the subsequent condensation phase. During condensation, silanol groups react to eliminate water, forming siloxane bonds (Si-O-Si) that link into cyclic oligomers, linear chains, or branched structures, depending on reaction conditions such as temperature, pH, and catalysts.^[30]^[3]^[31] A key example is the conversion of dimethyldichlorosilane, the primary monomer derived from chlorosilanes, into polydimethylsiloxane (PDMS), the most common silicone polymer. Hydrolysis of dimethyldichlorosilane ((CH_3)_2SiCl_2) yields dimethylsilanediol ((CH_3)_2Si(OH)_2), which then undergoes polycondensation to produce linear or cyclic siloxanes represented as [(CH_3)_2SiO]_n, where n determines the polymer's molecular weight and properties. This reaction mixture can be tailored by blending different chlorosilanes—for instance, incorporating methyltrichlorosilane to introduce branching for resin formation—allowing precise control over the final silicone's viscosity and structure. The hydrolysis-condensation sequence is typically conducted in aqueous or solvent-based systems to manage the exothermic nature and ensure uniform polymerization.^[3]^[2]^[31] On an industrial scale, silicone production integrates the generation of chlorosilanes with downstream purification via fractional distillation, which separates isomers based on boiling points (e.g., dimethyldichlorosilane boils at 70°C) to achieve over 99% purity essential for high-quality polymers. This streamlined approach, often linked to the direct synthesis of chlorosilanes, enables efficient, large-volume output at facilities processing thousands of tons annually, yielding versatile products such as low-viscosity silicone fluids for lubricants, elastomers for seals and gaskets, and heat-resistant resins for coatings. Catalysts like alkali metal hydroxides or acids further optimize yields, with cyclic siloxanes serving as intermediates that can be ring-opened for extended chains in elastomers. Global production emphasizes recycling of hydrochloric acid to minimize waste, supporting sustainable scaling in the multi-billion-dollar silicone industry.^[30]^[32]^[3]

Semiconductor and Other Uses

Chlorosilanes play a critical role in the production of high-purity polysilicon, essential for semiconductor wafers in electronics and photovoltaics. Trichlorosilane (HSiCl₃), a key chlorosilane, undergoes pyrolysis in the Siemens process, where it decomposes at high temperatures (around 1100–1200°C) on heated silicon rods to deposit elemental silicon. This reaction follows the stoichiometry:

$2 \mathrm{HSiCl_3} \rightarrow \mathrm{Si} + \mathrm{SiCl_4} + 2 \mathrm{HCl}

The process yields electronic-grade polysilicon rods, which are subsequently purified and sliced into wafers for integrated circuits and solar cells. Additionally, hydrogenation of silicon tetrachloride (SiCl₄), a byproduct, recycles it back to HSiCl₃ via the reaction with hydrogen gas, enhancing process efficiency and reducing waste in closed-loop systems.^[33] In chemical vapor deposition (CVD), silicon tetrachloride (SiCl₄) serves as a primary precursor for epitaxial silicon growth on substrates, enabling the fabrication of high-performance microchips. During epitaxial CVD, SiCl₄ decomposes in a hydrogen atmosphere at temperatures of 1000–1200°C, depositing single-crystal silicon layers with precise control over thickness and doping for transistor channels and interconnects. This method is favored for its ability to produce low-defect films, critical for advanced semiconductor devices like those in CMOS technology.^[34] Beyond semiconductors, chlorosilanes find use in sol-gel processes to synthesize silica materials from industrial residues. Residual chlorosilanes from polysilicon production are hydrolyzed in aqueous slurries, often with lime or surfactants, to form nano-silica particles via condensation and gelation, yielding amorphous SiO₂ for applications in coatings and fillers. In pharmaceuticals, chlorosilanes such as trimethylchlorosilane act as silylating agents to protect reactive hydroxyl groups in organic intermediates, facilitating selective synthesis of complex molecules like antibiotics and steroids by forming stable silyl ethers that are later removed.^[35] Chlorosilanes also serve as precursors for polyhedral oligomeric silsesquioxanes (POSS), hybrid organic-inorganic nanomaterials synthesized through hydrolytic condensation of alkyltrichlorosilanes or similar compounds. These silsesquioxanes are incorporated into polymers and composites for enhanced mechanical, thermal, and optical properties in applications such as aerospace materials and biomedical devices. Additionally, chlorosilanes are employed in surface modification of microfluidic devices, where silanization with compounds like octadecyltrichlorosilane creates hydrophobic or functionalized channels, improving fluid control, biocompatibility, and resistance to biofouling in lab-on-a-chip systems.^[2]^[36]^[37]

Safety and Hazards

Health and Environmental Risks

Chlorosilanes pose significant acute health risks primarily due to their rapid hydrolysis in the presence of moisture, releasing hydrogen chloride (HCl), a highly corrosive gas that causes severe chemical burns to the skin and eyes upon contact.^[14] Inhalation of chlorosilane vapors leads to immediate irritation and damage to the respiratory tract, including coughing, wheezing, pulmonary edema, and potentially fatal asphyxiation, with toxicity largely attributable to the generated HCl.^[10] For trichlorosilane (HSiCl₃), acute oral toxicity in rats yields an LD₅₀ of 1030 mg/kg, while 1-hour inhalation LC₅₀ values for trichlorosilanes range from 1257 to 1611 ppm in rats.^[38]^[39] Chronic exposure to chlorosilanes is less well-studied but may result in ongoing respiratory sensitization and irritation from repeated HCl release, potentially leading to long-term lung damage.^[40] While direct carcinogenicity data for chlorosilanes are limited and primarily linked to HCl effects rather than the compounds themselves, downstream silicon compounds like siloxanes have raised concerns for endocrine disruption and reproductive toxicity based on animal studies.^[41] Environmentally, chlorosilanes contribute to risks through their hydrolysis byproduct HCl, which readily dissolves in atmospheric water to form hydrochloric acid and can significantly acidify rainfall, with global sources of HCl accounting for 10-40% of background rain acidity in some regions.^[42] Further reaction products, such as siloxanes derived from chlorosilane hydrolysis, exhibit high environmental persistence, particularly cyclic variants that resist biodegradation and accumulate in sediments and biota. In 2024, the European Union adopted restrictions on cyclic volatile methylsiloxanes (D4, D5, D6) to reduce emissions by up to 90% due to their persistence and bioaccumulation concerns.^[43] Bioaccumulation is a key concern for organochlorosilanes and their siloxane derivatives, with cyclic methylsiloxanes demonstrating potential to concentrate in aquatic organisms and pose toxic effects to ecosystems, including neurotoxicity and reproductive impairment in marine species.^[44]

Handling Precautions

Chlorosilanes must be stored under an inert atmosphere, such as dry nitrogen, in dry conditions to prevent reaction with moisture, using compatible materials like carbon steel or glass-lined steel vessels that are vacuum-resistant and equipped with pressure relief systems.^[14] Temperature should be controlled below 50°C to minimize vapor pressure and flammability risks, particularly for low-boiling compounds like trichlorosilane.^[40] Storage areas require well-ventilated, non-combustible construction with explosion-proof equipment and grounding to avoid static ignition.^[14] During handling and transport, personnel should wear full-body protective suits made from materials like Tychem® BR or TK, chemical-resistant gloves such as Viton, non-vented impact-resistant goggles or face shields, and rubber boots.^[40] Respirators, including NIOSH-approved supplied-air types or self-contained breathing apparatus (SCBA), are essential when handling, especially to protect against hydrogen chloride (HCl) vapors generated from moisture reactivity.^[40] Containers must be grounded and bonded during transfer, and transport follows UN classifications for corrosive and flammable hazards, such as UN 1818 for silicon tetrachloride and UN 1295 for trichlorosilane (classified as 3.2/8 for flammability and corrosivity).^[14]^[8] For spill response, evacuate the area, eliminate ignition sources, and avoid direct water contact, as chlorosilanes react violently with moisture to produce HCl and heat; instead, use water spray from a distance to disperse vapors.^[14] Small spills can be absorbed with dry inert materials like vermiculite or sand, while larger spills require diking followed by application of alcohol-resistant aqueous film-forming foam (AR-AFFF) to suppress vapors, then neutralization of acidic runoff using lime slurry or sodium hydroxide solutions.^[40]^[45] Collected residues must be disposed of as hazardous waste per local regulations.^[40] Chlorosilanes are subject to strict regulations, including OSHA Process Safety Management (PSM) standards for handling and EPA Risk Management Program (RMP) for facilities, with emergency response plans mandatory for spills or leaks.^[14] A notable incident in the 1980s involved a 1981 spill of approximately 3,000 liters (10,000 pounds) of silicon tetrachloride at a chemical plant in California, where a truck ruptured a storage tank, leading to rapid hydrolysis, HCl vapor release, and evacuation of thousands, highlighting the need for robust containment and response protocols.^[46] A more recent incident occurred on July 21, 2020, when trichlorosilane leaked from an underground storage tank, exposing workers and requiring medical treatment for irritation effects.^[47]

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[PDF] enclosed and industrialized bays of Korea - Frontiers
Apr 19, 2023 · Accumulated evidence indicates that several siloxanes have adverse health effects on the nervous, immune, endocrine, and reproductive systems of ...
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[PDF] RECOVERY OF VALUABLE CHLOROSILANE INTERMEDIATES BY ...
Sep 30, 2001 · Another disposal method for DPR waste is hydrolysis. DPR is hydrolyzed (or “quenched”) with water to form a solid byproduct.11, 12 An alkali, ...
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Health effects of silicon tetrachloride. Report of an urban accident
A spill of silicon tetrachloride (SiCl4) at a chemical plant caused the evacuation of several thousand people from an industrial park.Missing: tetrachlorosilane | Show results with:tetrachlorosilane