Thorium dioxide

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Thorium dioxide
Names
IUPAC names
Thorium dioxide
Thorium(IV) oxide
Other names
Thoria
Thorium anhydride
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
ECHA InfoCard
 100.013.842 Edit this at Wikidata
EC Number
  • 215-225-1
141638
UNII
UN number 2910 2909
  • Key: ZCUFMDLYAMJYST-UHFFFAOYSA-N
  • InChI=1S/2O.Th
  • O=[Th]=O
Properties
ThO2
Molar mass 264.037 g/mol[1]
Appearance white solid[1]
Odor odorless
Density 10.0 g/cm3[1]
Melting point 3,350 °C (6,060 °F; 3,620 K)[1]
Boiling point 4,400 °C (7,950 °F; 4,670 K)[1]
insoluble[1]
Solubility insoluble in alkali
slightly soluble in acid[1]
−16.0·10−6 cm3/mol[2]
2.200 (thorianite)[3]
Structure
Fluorite (cubic), cF12
Fm3m, No. 225
a = 559.74(6) pm[4]
Tetrahedral (O2−); cubic (ThIV)
Thermochemistry
65.2(2) J K−1 mol−1
Std enthalpy of
formation
(ΔfH⦵298)
 −1226(4) kJ/mol
Hazards
GHS labelling:[5]
GHS06: ToxicGHS08: Health hazard
Danger
H301, H311, H331, H350, H373
P203, P260, P261, P264, P270, P271, P280, P301+P316, P302+P352, P304+P340, P316, P318, P319, P321, P330, P361+P364, P403+P233, P405, P501
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 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazard RA: Radioactive. E.g. plutonium
2
0
0
Special hazard RA: Radioactive. E.g. plutonium
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
400 mg/kg
Related compounds
Other anions
 Thorium(IV) sulfide
Other cations
 Hafnium(IV) oxide
Cerium(IV) oxide
Related compounds
Uranium(IV) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is mainly a by-product of lanthanide and uranium production.[4] Thorianite is the name of the mineralogical form of thorium dioxide. It is moderately rare and crystallizes in an isometric system. The melting point of thorium oxide is 3300 °C – the highest of all known oxides. Only a few elements (including tungsten and carbon) and a few compounds (including tantalum carbide) have higher melting points.[6] All thorium compounds, including the dioxide, are radioactive because there are no stable isotopes of thorium.

Structure and reactions

Thoria exists as two polymorphs. One has a

plutonium dioxide.)[clarification needed] The band gap of thoria is about 6 eV
. A tetragonal form of thoria is also known.

Thorium dioxide is more stable than

disproportionation reaction (equilibrium with liquid thorium metal) above 1,850 K (1,580 °C; 2,870 °F) or by simple dissociation (evolution of oxygen) above 2,500 K (2,230 °C; 4,040 °F).[8]

Applications

Nuclear fuels

Thorium dioxide (thoria) can be used in nuclear reactors as ceramic fuel pellets, typically contained in nuclear fuel rods clad with zirconium alloys. Thorium is not fissile (but is "fertile", breeding fissile uranium-233 under neutron bombardment); hence, it must be used as a nuclear reactor fuel in conjunction with fissile isotopes of either uranium or plutonium. This can be achieved by blending thorium with uranium or plutonium, or using it in its pure form in conjunction with separate fuel rods containing uranium or plutonium. Thorium dioxide offers advantages over conventional uranium dioxide fuel pellets, because of its higher thermal conductivity (lower operating temperature), considerably higher melting point, and chemical stability (does not oxidize in the presence of water/oxygen, unlike uranium dioxide).

Thorium dioxide can be turned into a nuclear fuel by breeding it into uranium-233 (see below and refer to the article on thorium for more information on this). The high thermal stability of thorium dioxide allows applications in flame spraying and high-temperature ceramics.

Alloys

Thorium dioxide is used as a stabilizer in

thermions). It is the most popular oxide additive because of its low cost, but is being phased out in favor of non-radioactive elements such as cerium, lanthanum and zirconium
.

Thoria-dispersed nickel finds its applications in various high-temperature operations like combustion engines because it is a good creep-resistant material. It can also be used for hydrogen trapping.[9][10][11][12][13]

Catalysis

Thorium dioxide has almost no value as a commercial catalyst, but such applications have been well investigated. It is a catalyst in the

Ruzicka large ring synthesis. Other applications that have been explored include petroleum cracking, conversion of ammonia to nitric acid and preparation of sulfuric acid.[14]

Radiocontrast agents

Thorium dioxide was the primary ingredient in Thorotrast, a once-common radiocontrast agent used for cerebral angiography, however, it causes a rare form of cancer (hepatic angiosarcoma) many years after administration.[15] This use was replaced with injectable iodine or ingestable barium sulfate suspension as standard X-ray contrast agents.

Lamp mantles

Main article: Gas mantle

Another major use in the past was in

zirconium oxide
) is used increasingly as a replacement.

Glass manufacture

Three lenses from yellowed to transparent left-to-right
Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)

When added to

lenses for cameras and scientific instruments.[17] The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal.[18] Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced by rare-earth oxides such as lanthanum oxide in almost all modern high-index glasses, as they provide similar effects and are not radioactive.[19]

References

  1. ^ a b c d e f g Haynes, p. 4.95
  2. ^ Haynes, p. 4.136
  3. ^ Haynes, p. 4.144
  4. ^
    doi:10.1016/S0022-3115(96)00750-7
    .
  5. ^ "Thorium dioxide". pubchem.ncbi.nlm.nih.gov.
  6. ISBN 978-0-19-850340-8
    .
  7. doi:10.1016/j.jnucmat.2016.12.046
    .
  8. doi:10.1021/ja01648a018
    .
  9. ISBN 978-0-471-43623-2
    .
  10. S2CID 137105492
    .
  11. S2CID 95399863
    .
  12. S2CID 92103123
    .
  13. S2CID 96573790
    .
  14. doi:10.1002/14356007.a27_001
  15. ^ Thorotrast. radiopaedia.org
  16. ISBN 978-0-08-022057-4
    .
  17. ISBN 978-0-8493-0485-9
    .
  18. ^ Oak Ridge Associated Universities (1999). "Thoriated Camera Lens (ca. 1970s)". Retrieved 29 September 2017.
  19. ISBN 978-3-527-31097-5
    .

Cited sources
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