Boron trioxide
Encyclopedia
Boron trioxide is one of the oxides of boron. It is a white, glassy solid with the formula B2O3. It is almost always found as the vitreous (amorphic) form; however, it can be crystallized after extensive annealing
. It is one of the most difficult compounds known to crystallize.
Glassy boron oxide (g-B2O3) is thought to be composed of boroxol rings which are six-membered rings composed of alternating 3-coordinate boron and 2-coordinate oxygen. This view is controversial, however, because no model has ever been made of glassy boron oxide of the correct density containing a large number of six-membered rings. The rings are thought to make a few BO3 triangles, but mostly link (polymerize) into ribbons and sheets. The crystalline form (α-B2O3) see structure in the infobox) is exclusively composed of BO3 triangles. This trigonal, quartz-like
network undergoes a coesite
-like transformation to monoclinic β-B2O3 at several gigapascals and is 9.5 GPa.
with sulfuric acid
in a fusion furnace. At temperatures above 750 °C, the molten boron oxide layer separates out from sodium sulfate
. It is then decanted, cooled and obtained in 96–97% purity.
Another method is heating boric acid above ~300 °C. Boric acid will initially decompose into water steam and metaboric acid (HBO2) at around 170 °C, and further heating above 300 °C will produce more steam and boron trioxide. The reactions are:
Boric acid goes to anhydrous microcrystalline B2O3 in a heated fluidized bed. Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.
Crystallization of molten α-B2O3 at ambient pressure is strongly kinetically disfavored (compare liquid and crystal densities). Threshold conditions for crystallization of the amorphous solid are 10 kbar and ~200 °C. Its proposed crystal structure in enantiomorphic space groups P31(#144); P32(#145) (e.g., γ-glycine) has been revised to enantiomorphic space groups P3121(#152); P3221(#154)(e.g., α-quartz).
of β-B2O3 is rather high (K = 180 GPa). The Vickers hardness of g-B2O3 is 1.5 GPa and of β-B2O3 is 16 GPa.
Annealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
. It is one of the most difficult compounds known to crystallize.
Glassy boron oxide (g-B2O3) is thought to be composed of boroxol rings which are six-membered rings composed of alternating 3-coordinate boron and 2-coordinate oxygen. This view is controversial, however, because no model has ever been made of glassy boron oxide of the correct density containing a large number of six-membered rings. The rings are thought to make a few BO3 triangles, but mostly link (polymerize) into ribbons and sheets. The crystalline form (α-B2O3) see structure in the infobox) is exclusively composed of BO3 triangles. This trigonal, quartz-like
Trapezohedron
The n-gonal trapezohedron, antidipyramid or deltohedron is the dual polyhedron of an n-gonal antiprism. Its 2n faces are congruent kites . The faces are symmetrically staggered.The n-gon part of the name does not reference the faces here but arrangement of vertices around an axis of symmetry...
network undergoes a coesite
Coesite
Coesite[p] is a form of silicon dioxide SiO2 that is formed when very high pressure , and moderately high temperature , are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953. In 1960, coesite was found by Edward C. T...
-like transformation to monoclinic β-B2O3 at several gigapascals and is 9.5 GPa.
Preparation
Boron trioxide is produced by treating boraxBorax
Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...
with 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...
in a fusion furnace. At temperatures above 750 °C, the molten boron oxide layer separates out from sodium sulfate
Sodium sulfate
Sodium sulfate is the sodium salt of sulfuric acid. When anhydrous, it is a white crystalline solid of formula Na2SO4 known as the mineral thenardite; the decahydrate Na2SO4·10H2O has been known as Glauber's salt or, historically, sal mirabilis since the 17th century. Another solid is the...
. It is then decanted, cooled and obtained in 96–97% purity.
Another method is heating boric acid above ~300 °C. Boric acid will initially decompose into water steam and metaboric acid (HBO2) at around 170 °C, and further heating above 300 °C will produce more steam and boron trioxide. The reactions are:
- H3BO3 → HBO2 + H2O
- 2 HBO2 → B2O3 + H2O
Boric acid goes to anhydrous microcrystalline B2O3 in a heated fluidized bed. Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.
Crystallization of molten α-B2O3 at ambient pressure is strongly kinetically disfavored (compare liquid and crystal densities). Threshold conditions for crystallization of the amorphous solid are 10 kbar and ~200 °C. Its proposed crystal structure in enantiomorphic space groups P31(#144); P32(#145) (e.g., γ-glycine) has been revised to enantiomorphic space groups P3121(#152); P3221(#154)(e.g., α-quartz).
Hardness
The bulk modulusBulk modulus
The bulk modulus of a substance measures the substance's resistance to uniform compression. It is defined as the pressure increase needed to decrease the volume by a factor of 1/e...
of β-B2O3 is rather high (K = 180 GPa). The Vickers hardness of g-B2O3 is 1.5 GPa and of β-B2O3 is 16 GPa.
Applications
- Fluxing agent for glassGlassGlass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
and enamelsVitreous enamelVitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C... - Starting material for synthesizing other boronBoronBoron 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...
compounds such as boron carbideBoron carbideBoron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications... - An additive used in glass fibres (optical fibres)
- It is used in the production of borosilicate glassBorosilicate glassBorosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
- The inert capping layer in the LEC process for the production of gallium arsenide single crystal
- As an acid catalyst in organic synthesisOrganic synthesisOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...