 | |
| Names | |
|---|---|
| IUPAC names Thorium dioxide Thorium(IV) oxide | |
| Other names Thoria Thorium anhydride | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider | |
| ECHA InfoCard | 100.013.842 |
| EC Number |
|
| 141638 | |
| UNII | |
| UN number | 2910 2909 |
| |
| |
| 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 inalkali slightly soluble inacid[1] |
| −16.0·10−6 cm3/mol[2] | |
Refractive index (nD) | 2.200 (thorianite)[3] |
| Structure | |
| Fluorite (cubic),cF12 | |
| Fm3m, No. 225 | |
a = 559.74(6) pm[4] | |
| Tetrahedral (O2−); cubic (ThIV) | |
| Thermochemistry | |
Std molar entropy(S⦵298) | 65.2(2) J K−1 mol−1 |
Std enthalpy of formation(ΔfH⦵298) | −1226(4) kJ/mol |
| Hazards | |
| GHS labelling:[5] | |
  | |
| 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) | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 400 mg/kg |
| Related compounds | |
Otheranions | Thorium(IV) sulfide |
Othercations | Hafnium(IV) oxide Cerium(IV) oxide |
Related compounds | Protactinium(IV) oxide Uranium(IV) oxide |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Thorium dioxide (ThO2), also calledthorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known asthoria, it is mainly a by-product oflanthanide anduranium production.[4]Thorianite is the name of the mineralogical form ofthorium 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 (includingtungsten andcarbon) and a few compounds (includingtantalum carbide) have higher melting points.[6] All thorium compounds, including the dioxide, are radioactive because there are no stableisotopes of thorium.
Thoria exists as two polymorphs. One has afluorite crystal structure. This is uncommon amongbinary dioxides. (Other binary oxides with fluorite structure includecerium dioxide,uranium dioxide andplutonium dioxide.)[clarification needed] Theband gap of thoria is about 6 eV. A tetragonal form of thoria is also known.
Thorium dioxide is more stable thanthorium monoxide (ThO).[7] Only with careful control of reaction conditions can oxidation of thorium metal give the monoxide rather than the dioxide. At extremely high temperatures, the dioxide can convert to the monoxide either by adisproportionation 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]
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 fissileuranium-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 anuclear fuel by breeding it into uranium-233 (see below and refer to the article onthorium for more information on this). The highthermal stability of thorium dioxide allows applications in flame spraying and high-temperature ceramics.
Thorium dioxide is used as a stabilizer intungsten electrodes inTIG welding, electron tubes, and aircraft gas turbine 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 ofelectrons (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 ascerium,lanthanum andzirconium.
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]
Thorium dioxide has almost no value as a commercial catalyst, but such applications have been well investigated. It is a catalyst in theRuzicka large ring synthesis. Other applications that have been explored includepetroleum cracking, conversion ofammonia tonitric acid and preparation ofsulfuric acid.[14]
Thorium dioxide was the primary ingredient inThorotrast, a once-commonradiocontrast agent used forcerebral angiography, however, it causes a rare form of cancer (hepaticangiosarcoma) many years after administration.[15] This use was replaced withinjectable iodine or ingestablebarium sulfate suspension as standardX-ray contrast agents.
Another major use in the past was ingas mantle of lanterns developed byCarl Auer von Welsbach in 1890, which are composed of 99% ThO2 and 1%cerium(IV) oxide. Even as late as the 1980s it was estimated that about half of all ThO2 produced (several hundred tonnes per year) was used for this purpose.[16] Some mantles still use thorium, butyttrium oxide (or sometimeszirconium oxide) is used increasingly as a replacement.
When added toglass, thorium dioxide helps increase itsrefractive index and decreasedispersion. Such glass finds application in high-qualitylenses 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 aslanthanum oxide in almost all modern high-index glasses, as they provide similar effects and are not radioactive.[19]
