Applications of thorium oxide (thoria) as a refractory: Given that thoria (ThO2) is expensive and slightly radioactive, in which application is thorium oxide particularly used because of its exceptional refractoriness and purity?

Introduction / Context:
Refractories must withstand high temperatures, thermal shock, and chemical attack. Thoria (ThO2) offers extremely high melting point and chemical stability, making it suitable for specialized, high-temperature applications where contamination must be minimized.

Given Data / Assumptions:

• Thoria is costly and slightly radioactive.
• We seek the application that most benefits from its unique properties, justifying the cost and handling concerns.

Concept / Approach:
Thoria has very high refractoriness and low vapor pressure at elevated temperatures. These traits are critical when melting high-purity, reactive, or precious metals where crucible contamination would degrade product quality. General items like insulating bricks or muffles can use cheaper refractories (silica, alumina, mullite) without requiring thoria's premium performance.

Step-by-Step Solution:
Identify high-value application: contamination-sensitive metal melting in research or specialty metallurgy.Match material properties: ThO2 crucibles provide chemical inertness and temperature capability exceeding many alternatives.Conclude: thoria is well suited to crucibles for high-purity metals.

Verification / Alternative check:
Materials handbooks cite thoria’s use in crucibles and as an additive in tungsten electrodes; bulk structural uses are limited due to cost and radioactivity.

Why Other Options Are Wrong:
Seger cones: require calibrated softening behavior, not extreme refractoriness alone. Muffles/insulating bricks: typically alumino-silicates or high-alumina refractories suffice economically.

Common Pitfalls:

• Choosing high-volume applications where cost would be prohibitive.
• Ignoring contamination control as a driver in choosing crucible materials.

Final Answer:
Crucibles for melting high-purity metals