Thorium dioxide

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Thorium dioxide
Image:Thorium dioxide.jpg
General
Systematic name	Thorium dioxide Thorium(IV) oxide
Other names	Thoria
Molecular formula	ThO₂
Molar mass	264.04 g/mol
Appearance	white odorless solid
CAS number	[1314-20-1]
Properties
Density and phase	9.86 g/cm³, solid
Solubility in water	Insoluble
Melting point	3220 °C
Boiling point	4400°C
Structure
Coordination geometry	Octahedral
Crystal structure	Fluorite
Thermodynamic data
Standard enthalpy of formation Δ_fH°_solid	? kJ/mol
Standard molar entropy S°_solid	? J.K⁻¹.mol⁻¹
Hazards
RADIOACTIVE
EU classification	not listed
NFPA 704
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	?
Other cations	Hafnium dioxide Protactinium pentoxide Uranium trioxide Neptunium pentoxide Plutonium dioxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Thorium dioxide (ThO₂), also called thorium(IV) oxide (IUPAC) is a white, crystalline powder. It was formerly known as thoria or thorina. It is produced mainly as a by-product of lanthanide and uranium production^[1].

1 Chemistry
2 Applications
3 References
4 External links

[edit] Chemistry

The compound is radioactive due to the radioactivity of thorium. Thorium dioxide can be used as a nuclear fuel. (Refer to the article of thorium for more information on this application.) The high thermal stability of thorium dioxide has given it applications in flame spraying and high temperature ceramics. Thorium dioxide was the primary ingredient in the X-ray contrast medium Thorotrast. Use of Thorotrast was abandoned when it turned out to be a carcinogen, sometimes causing cholangiocarcinoma. Today, barium sulfate is the standard X-ray contrast agent.

[edit] Applications

Thorium dioxide is used as a stabilizer in tungsten electrodes in TIG welding and electron tubes. As an alloy, thoriated tungsten metal is not easily deformed because the high fusion material thoria augments the high temperature mechanical properties, and thorium helps stimulate the emission of electrons (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.

A major use in the past was in gas mantles, which were frequently composed of 99% ThO₂ and 1% cerium(IV) oxide. Even as late as the 1980's it was estimated that about half of all ThO₂ produced (several hundred tonnes per year) was used for this purpose^[1]. Some mantles still use thorium, but yttrium oxide (or sometimes zirconium oxide) is used increasingly as a replacement.

Thorium dioxide was formerly added to glasses during manufacture to increase their refractive index, producing thoriated glass with up to 40% ThO₂ content. These glasses were used in the construction of high-quality photographic lenses. However, the radioactivity of the thorium caused both a safety and pollution hazard and self-degradation of the glass (turning it yellow or brown over time). Lanthanum oxide has replaced thorium dioxiode in almost all modern high-index glasses.

The melting point of thorium oxide is 3300°C - the highest of all oxides. Only a few elements (including tungsten) and a few compounds (including tantalum carbide) have higher melting points.