Thorium dioxide

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].

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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 allows 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 was found to be a carcinogen, sometimes causing cholangiocarcinoma. Today, barium sulfate is the standard X-ray contrast agent. Thoria has the fluorite crystal structure. Few other binary dioxides have this structure: uranium dioxide, hafnium dioxide and cerium dioxide, not to mention plutonium dioxide. The band gap of thoria is about 6 eV.

Applications

Thorium dioxide is used as a stabilizer in tungsten electrodes in TIG welding,electron tubes, and aircraft engines. 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 of laterns, 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 dioxide 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 carbon) and a few compounds (including tantalum carbide) have higher melting points.

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, UK, 1984. See pages 1425, 1456.