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-Crystallinity, crystalline, often transparency and translucency, transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most ...

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, 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 ...

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 (chemistry), salt (ionic compound), a hydration (chemistry), hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a base (chemistry), basic aqueo ...

with sulfuric acid in a

fusion furnace Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole. Fusion may also refer to: Science and technology Physics *Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...

. 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 mill ...

. 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 Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...

* Fluxing agent for

and enamels *Starting material for synthesizing other boron compounds such as boron carbide *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 ...

s) *The inert capping layer in the Liquid Encapsulation Czochralski process for the production of gallium arsenide 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 ...

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 ...

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 compoundsUS NIH hazard information
See NIH.
Material Safety Data Sheet
{{DEFAULTSORT:Boron Trioxide Boron compounds Acidic oxides Glass compositions Sesquioxides