Boron trioxide or diboron trioxide is the
oxide of boron with the formula . It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called boric oxide
[ or boria.][ It has many important industrial applications, chiefly in ceramics as a ]flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...
for glazes and enamels and in the production of glass
Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...
es.
Structure
Boron trioxide has three known forms, one amorphous and two crystalline.
Amorphous form
The amorphous form (g-) is by far the most common. It is thought to be composed of boroxol rings which are six-membered rings composed of alternating 3-coordinate boron and 2-coordinate oxygen.
Because of the difficulty of building disordered models at the correct density with many boroxol rings, this view was initially controversial, but such models have recently been constructed and exhibit properties in excellent agreement with experiment.[ It is now recognized, from experimental and theoretical studies,][ that the fraction of boron atoms belonging to boroxol rings in glassy is somewhere between 0.73 and 0.83, with 0.75 = 3/4 corresponding to a 1:1 ratio between ring and non-ring units. The number of boroxol rings decays in the liquid state with increasing temperature.][
]
Crystalline α form
The crystalline form (α-) is exclusively composed of BO3 triangles. It crystal structure was initially believed to be the enantiomorphic space groups P31(#144) and P32(#145), like γ-glycine;[ but was later revised to the enantiomorphic space groups P3121(#152) and P3221(#154) in the ]trigonal crystal system
In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crystal ...
, like α-quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...
[
Crystallization of α- from the molten state at ambient pressure is strongly kinetically disfavored (compare liquid and crystal densities). It can be obtained with prologed annealing of the amorphous solid ~200°C under at least 10 kbar of pressure.][
]
Crystalline β form
The trigonal network undergoes a coesite-like transformation to monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
β- at several gigapascals (9.5 GPa).[
]
Preparation
Boron trioxide is produced by treating borax
Borax is a salt (ionic compound), a hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a basic solution. It is commonly available in powder or granular form ...
with sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
in a fusion furnace. At temperatures above 750°C, the molten boron oxide layer separates out from sodium sulfate
Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound with formula Na2SO4 as well as several related hydrates. All forms are white solids that are highly soluble in water. With an annual production of 6 mil ...
. It is then decanted, cooled and obtained in 96–97% purity.[
Another method is heating ]boric acid
Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolve ...
above ~300°C. Boric acid will initially decompose into steam, (H2O(g)) and metaboric acid (HBO2) at around 170°C, and further heating above 300°C will produce more steam and diboron trioxide. The reactions are:
:H3BO3 → HBO2 + H2O
:2 HBO2 → + H2O
Boric acid goes to anhydrous microcrystalline in a heated fluidized bed.[ Carefully controlled heating rate avoids gumming as water evolves.
Boron oxide will also form when diborane (B2H6) reacts with oxygen in the air or trace amounts of moisture:
:2B2H6(g) + 3O2(g) → 2(s) + 6H2(g)
:B2H6(g) + 3H2O(g) → (s) + 6H2(g)][
]
Reactions
Molten boron oxide attacks silicates. Containers can be passivated internally with a graphitized carbon layer obtained by thermal decomposition of acetylene.[
]
Applications
*Major component of borosilicate glass
* Fluxing agent for glass
Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...
and enamels
*Starting material for synthesizing other boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the '' boron group'' it has t ...
compounds such as boron carbide
Boron carbide (chemical formula approximately B4C) is an extremely hard boron– carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders,
as well as numerous industrial applications. With a Vickers ...
*An additive used in glass fibres (optical fibre
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means t ...
s)
*The inert capping layer in the Liquid Encapsulation Czochralski process for the production of gallium arsenide
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated c ...
single crystal
*As an acid catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
in organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
See also
* boron suboxide
*boric acid
Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolve ...
* sassolite
* Tris(2,2,2-trifluoroethyl) borate
References
[L. McCulloch (1937): "A Crystalline Boric Oxide". ''Journal of the American Chemical Society'', volume 59, issue 12, pages 2650–2652. ]
[I.Vishnevetsky and M.Epstein (2015): "Solar carbothermic reduction of alumina, magnesia and boria under vacuum". ''Solar Energy'', volume 111, pages 236-251 ]
External links
National Pollutant Inventory: Boron and compounds
US NIH hazard information
See NIH.
Material Safety Data Sheet
{{DEFAULTSORT:Boron Trioxide
Boron compounds
Acidic oxides
Glass compositions
Sesquioxides