Thorium Oxide, "Thoria" (ThO2)

Formula: ThO2

Melting point: 3390ºC

Density: 10 g/cc (exact)

Solubility: Stable in air and water. Dissolves slowly in reducing media.

Thorium oxide, ThO₂, has historically been considered the undisputed king among technical oxides due to its exceptional properties in applications with high thermal and structural demands. Its history is marked by a paradox: it possesses such outstanding qualities that, were it not for the inherent radioactivity of thorium—the actinide element that composes it—its industrial use would be even more widespread than it already is. Despite this drawback, thoria has maintained its privileged position as a dopant of reference in the formulation of ceramic compounds, metallic compounds, and cermets (hybrid materials combining ceramic and metal), thanks to its ability to improve the thermal stability, mechanical strength, and chemical durability of the systems in which it is incorporated.

Since the mid-20th century, when the aerospace, nuclear, and armament industries began to demand materials capable of withstanding extreme conditions, ThO₂ became an essential component in alloys and coatings subjected to elevated temperatures, intense oxidation, and prolonged mechanical stresses. Its extremely high melting point, low thermal conductivity, and resistance to creep positioned it as the ideal stabilizer for refractory metals such as tungsten, molybdenum, and tantalum. In this context, thoria not only stabilizes the crystalline phase of these metals but also acts as a barrier against surface degradation caused by oxygen at critical temperatures.

The radioactivity of thorium, however, has limited its use in civil applications and has driven the search for non-radioactive substitutes, such as cerium oxide (CeO₂) and lanthanum oxide (La₂O₃). These materials, while useful, do not fully match the performance of thoria, which has led to it remaining, in many cases, the first choice when technical performance is prioritized and radiological risks can be managed. Thus, the history of ThO₂ is the story of a material that, due to its properties, should have dominated materials engineering even more widely, but whose actinide nature has imposed ethical and practical limits on its expansion.

In short, thoria represents the technical ideal in its specialty: an oxide that, despite its restrictions, continues to be irreplaceable in certain critical contexts, and whose presence in materials science reflects the delicate balance between extreme performance and technological responsibility.

Thorium oxide, known as thoria, in its pure state, presents as a marmoreal white solid, with an unblemished appearance, considerable hardness, and high density that place it among the most robust oxides known. Its melting point reaches 3390 °C, making it one of the most heat-resistant materials existing in nature and industry. This ability to withstand extreme temperatures without losing structural integrity has consecrated it as the dopant par excellence in the formulation of ceramic compounds, metallic compounds, and cermets. Its incorporation significantly improves the thermal, mechanical, and chemical resistance of the systems in which it is used, allowing them to function stably in aggressive and highly demanding environments.

The obtaining of ThO₂ is carried out through the calcination of minerals containing thorium, an actinide element that, despite its radioactivity, is surprisingly abundant in the Earth's crust. This process involves the separation of thorium from natural mixtures where it is usually found accompanied by rare earths—mainly lanthanides—and other actinides of its family, such as uranium, neptunium, and, to a lesser extent, plutonium. The purification of the oxide requires controlled conditions and specific techniques to ensure the elimination of impurities and the obtaining of a stable and functional product.

The crystalline structure of ThO₂ is highly compact, which contributes to its density and its resistance to creep, a phenomenon that affects many materials when exposed to oxygen at high temperatures. This property, together with its low thermal conductivity, makes it an ideal stabilizer for refractory metals, allowing them to maintain their crystalline phase without undergoing transformations that compromise their performance. In conclusion, thoria not only stands out for its extreme physical properties but also for its ability to radically transform the behavior of the materials that incorporate it, consolidating itself as a technical reference component in advanced materials science.

Thorium oxide, known as thoria, has two fundamental applications, one of which is outside metallurgy but equally relevant from a technological point of view. Firstly, its use as nuclear fuel has been the subject of attention from the energy industry for decades. Although the term “fuel” can be misleading—as thoria does not burn or explode in the conventional sense—its role in energy generation is based on its capacity to act as a source of controlled fission. It is processed into sintered and compact capsules that function as nuclear batteries, offering a more stable and safer alternative to uranium, neptunium, or plutonium. Despite being radioactive and toxic due to its high thorium content, thoria exhibits superior physical and chemical stability compared to its actinide counterparts, making it a less dangerous option in operational terms. Its non-pyrophoric behavior—unlike uranium, which can burn on impact—reinforces its profile as a highly reliable nuclear material.

The second application, directly linked to metallurgy, is widely known by professionals specialized in high-precision welding, especially in the context of the TIG (Tungsten-Inert-Gas) technique. In this type of welding, which employs refractory torches made with tungsten—the metal with the highest melting point known—thoria is used as a dopant to improve the thermal resistance of the assembly. When added in typical proportions of 2% by mass, ThO₂ acts as a phase stabilizer, strengthening the crystalline structure of tungsten and protecting it against the harmful effects of extreme heat, such as deformation, oxidation, and creep. This improvement not only extends the permissible temperature range for the base metal but also prolongs its useful life under intensive working conditions.

The use of thoria in TIG welding has generated some controversy due to its radioactive nature, although in this specific context, the risk is considerably smaller than commonly imagined. The presence of 2% ThO₂ in a tungsten-based alloy—an element that itself acts as a radiation barrier—does not represent a significant danger if handled under controlled conditions. The public perception of radioactivity as an absolute threat has led governments and regulatory bodies to promote the progressive elimination of radioactive substances outside the strictly nuclear sphere. However, in practice, when seeking to manufacture maximum quality parts for critical applications, thoria remains irreplaceable. Although materials like ceria (CeO₂) and lanthana (La₂O₃) have been introduced as replacements, their performance does not fully match that of ThO₂, which has led this oxide to continue being used in environments where technical excellence allows no compromises.

Ultimately, thoria represents a unique case in materials science: a compound that, despite its restrictions, continues to be chosen for its unparalleled properties, both in the energy and metallurgical fields. Its application in TIG welding is a testament to how, even in times of strict regulation, the pursuit of extreme quality keeps this actinide oxide present in contemporary industry.