Thorium dioxide
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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].
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.
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 emmission 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].
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.
References
- N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, UK, 1984. See pages 1425, 1456.