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Boron trioxide

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Boron trioxide
Names
Other names
boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria
Boric acid anhydride
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.013.751 Edit this at Wikidata
RTECS number
  • ED7900000
  • InChI=1S/B2O3/c3-1-5-2-4 checkY
    Key: JKWMSGQKBLHBQQ-UHFFFAOYSA-N checkY
  • InChI=1/B2O3/c3-1-5-2-4
    Key: JKWMSGQKBLHBQQ-UHFFFAOYAI
  • O=BOB=O
Properties
B2O3
Molar mass 69.6182 g/mol
Appearance white, glassy solid
Density 2.460 g/cm3, liquid;

2.55 g/cm3, trigonal;
3.11–3.146 g/cm3, monoclinic

Melting point 450 °C (trigonal)
510 °C (tetrahedral)
Boiling point 1860 °C,[1] sublimates at 1500 °C[2]
22 g/L
Solubility partially soluble in methanol
Acidity (pKa) ~ 4
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 0: Will not burn. E.g. waterInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
2
0
1
Lethal dose or concentration (LD, LC):
3150 mg/kg (oral, rat)
Supplementary data page
Boron trioxide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Boron trioxide (or diboron 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.[clarification needed]

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.[4][5] The crystalline form (α-B2O3) (see structure in the infobox[3]) is exclusively composed of BO3 triangles. This trigonal, quartz-like network undergoes a coesite-like transformation to monoclinic β-B2O3 at several gigapascals (9.5 GPa).[6]

Preparation

Boron trioxide is produced by treating borax 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.[2]

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.[7] Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.[8]

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.[9] Its proposed crystal structure in enantiomorphic space groups P31(#144); P32(#145)[10][11] (e.g., γ-glycine) has been revised to enantiomorphic space groups P3121(#152); P3221(#154)[12](e.g., α-quartz).

Hardness

The bulk modulus 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.[13]

Applications

See also

References

  1. ^ High temperature corrosion and materials chemistry: proceedings of the Per Kofstad Memorial Symposium. Proceedings of the Electrochemical Society. The Electrochemical Society. 2000. p. 496. ISBN 1-56677-261-3.
  2. ^ a b Patnaik, P. (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill. p. 119. ISBN 0-07-049439-8. Retrieved 2009-06-06.
  3. ^ a b Gurr, G. E.; Montgomery, P. W.; Knutson, C. D.; Gorres, B. T. (1970). "The Crystal Structure of Trigonal Diboron Trioxide". Acta Crystallographica B. 26 (7): 906–915. doi:10.1107/S0567740870003369.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Eckert, H. (1992). "Structural characterization of noncrystalline solids and glasses using solid state NMR". Progress in Nuclear Magnetic Resonance Spectroscopy. 24 (3): 159–293. doi:10.1016/0079-6565(92)80001-V.
  5. ^ Hwang, S.-J.; Fernandez, C.; Amoureux, J. P.; Cho, J.; Martin, S. W.; Pruski, M. (1997). "Quantitative study of the short range order in B2O3 and B2S3 by MAS and two-dimensional triple-quantum MAS 11B NMR". Solid State Nuclear Magnetic Resonance. 8 (2): 109–121. doi:10.1016/S0926-2040(96)01280-5. PMID 9203284.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. ^ Brazhkin, V. V.; Katayama, Y.; Inamura, Y.; Kondrin, M. V.; Lyapin, A. G.; Popova, S. V.; Voloshin, R. N. (2003). "Structural transformations in liquid, crystalline and glassy B2O3 under high pressure". JETP Letters. 78 (6): 393–397. doi:10.1134/1.1630134.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ Kocakuşak, S.; Akçay, K.; Ayok, T.; Koöroğlu, H. J.; Koral, M.; Savaşçi, Ö. T.; Tolun, R. (1996). "Production of anhydrous, crystalline boron oxide in fluidized bed reactor". Chemical Engineering and Processing. 35 (4): 311–317. doi:10.1016/0255-2701(95)04142-7.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Morelock, C. R. (1961). "Research Laboratory Report #61-RL-2672M" (Document). General Electric.
  9. ^ Aziz, M. J.; Nygren, E.; Hays, J. F.; Turnbull, D. (1985). "Crystal Growth Kinetics of Boron Oxide Under Pressure". Journal of Applied Physics. 57 (6): 2233. doi:10.1063/1.334368.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ^ Gurr, G. E.; Montgomery, P. W.; Knutson, C. D.; Gorres, B. T. (1970). "The crystal structure of trigonal diboron trioxide". Acta Crystallographica B. 26 (7): 906–915. doi:10.1107/S0567740870003369.
  11. ^ Strong, S. L.; Wells, A. F.; Kaplow, R. (1971). "On the crystal structure of B2O3". Acta Crystallographica B. 27 (8): 1662–1663. doi:10.1107/S0567740871004515.
  12. ^ Effenberger, H.; Lengauer, C. L.; Parthé, E. (2001). "Trigonal B2O3 with Higher Space-Group Symmetry: Results of a Reevaluation". Monatshefte für Chemie. 132 (12): 1515–1517. doi:10.1007/s007060170008.
  13. ^ Mukhanov, V. A.; Kurakevich, O. O.; Solozhenko, V. L. (2008). "On the Hardness of Boron (III) Oxide". Journal of Superhard Materials. 30: 71. doi:10.3103/S1063457608010097.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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