Silicon boride
Encyclopedia
Silicon borides are lightweight ceramic
compounds formed between silicon
and boron
. Several stoichiometric silicon boride compounds, SiBn, have been reported: silicon triboride, SiB3, silicon tetraboride, SiB4, silicon hexaboride, SiB6, as well as SiBn (n = 14, 15, 40, etc.). The n = 3 and n = 6 phases were reported as being co-produced together as a mixture for the first time by Henri Moissan
and Alfred Stock
in 1900 by briefly heating silicon and boron in a clay vessel. The tetraboride was first reported as being synthesized directly from the elements in 1960 by three independent groups: Carl Cline and Donald Sands; Ervin Colton; and Cyrill Brosset and Bengt Magnusson. It has been proposed that the triboride is a silicon-rich version of the tetraboride. Hence, the stoichiometry of either compound could be expressed as SiB4 - x where x = 0 or 1. All the silicon borides are black, crystalline materials of similar density: 2.52 and 2.47 g cm−3, respectively, for the n = 3(4) and 6 compounds. On the Mohs scale of mineral hardness
, SiB4 - x and SiB6 are intermediate between diamond
(10) and ruby
(9). The silicon borides may be grown from boron-saturated silicon in either the solid or liquid state.
The SiB6 crystal structure contains interconnected icosahedra (polyhedra with 20 faces), icosihexahedra (polyhedra with 26 faces), as well as isolated silicon and boron atoms. Due to the size mismatch between the silicon and boron atoms, silicon can be substituted for boron in the B12 icosahedra up to a limiting stoichiometry corresponding to SiB2.89. The structure of the tetraboride SiB4 is isomorphous to that of boron carbide
(B4C), B6P, and B6O. It is metastable with respect to the hexaboride. Nevertheless, it can be prepared due to the relative ease of crystal nucleation and growth.
Both SiB4 - x and SiB6 become superficially oxidized when heated in air or oxygen and each is attacked by boiling sulfuric acid
and by fluorine
, chlorine
, and bromine
at high temperatures. The silicon borides are electrically conducting. The hexaboride has a low coefficient of thermal expansion and a high nuclear cross section for thermal neutrons.
The tetraboride was used in the black coating of some of the space shuttle heat shield tiles.
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
compounds formed between silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
and boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
. Several stoichiometric silicon boride compounds, SiBn, have been reported: silicon triboride, SiB3, silicon tetraboride, SiB4, silicon hexaboride, SiB6, as well as SiBn (n = 14, 15, 40, etc.). The n = 3 and n = 6 phases were reported as being co-produced together as a mixture for the first time by Henri Moissan
Henri Moissan
Ferdinand Frederick Henri Moissan was a French chemist who won the 1906 Nobel Prize in Chemistry for his work in isolating fluorine from its compounds.-Biography:...
and Alfred Stock
Alfred Stock
Alfred Stock was a German inorganic chemist. He did pioneering research on the hydrides of boron and silicon, coordination chemistry, mercury, and mercury poisoning...
in 1900 by briefly heating silicon and boron in a clay vessel. The tetraboride was first reported as being synthesized directly from the elements in 1960 by three independent groups: Carl Cline and Donald Sands; Ervin Colton; and Cyrill Brosset and Bengt Magnusson. It has been proposed that the triboride is a silicon-rich version of the tetraboride. Hence, the stoichiometry of either compound could be expressed as SiB4 - x where x = 0 or 1. All the silicon borides are black, crystalline materials of similar density: 2.52 and 2.47 g cm−3, respectively, for the n = 3(4) and 6 compounds. On the Mohs scale of mineral hardness
Mohs scale of mineral hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...
, SiB4 - x and SiB6 are intermediate between diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
(10) and ruby
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...
(9). The silicon borides may be grown from boron-saturated silicon in either the solid or liquid state.
The SiB6 crystal structure contains interconnected icosahedra (polyhedra with 20 faces), icosihexahedra (polyhedra with 26 faces), as well as isolated silicon and boron atoms. Due to the size mismatch between the silicon and boron atoms, silicon can be substituted for boron in the B12 icosahedra up to a limiting stoichiometry corresponding to SiB2.89. The structure of the tetraboride SiB4 is isomorphous to that of boron carbide
Boron carbide
Boron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications...
(B4C), B6P, and B6O. It is metastable with respect to the hexaboride. Nevertheless, it can be prepared due to the relative ease of crystal nucleation and growth.
Both SiB4 - x and SiB6 become superficially oxidized when heated in air or oxygen and each is attacked by boiling sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
and by fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
, chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, and bromine
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
at high temperatures. The silicon borides are electrically conducting. The hexaboride has a low coefficient of thermal expansion and a high nuclear cross section for thermal neutrons.
The tetraboride was used in the black coating of some of the space shuttle heat shield tiles.