Thoria (ThO2) as a refractory: Which combined statements about its properties and uses are correct?

Introduction / Context:
Thoria (ThO2) is a premium refractory oxide used when extreme refractoriness and chemical stability are required. Understanding its strengths and limitations guides selection for crucibles, high-temperature windows, and specialized furnace components.

Given Data / Assumptions:

• Thoria’s melting/fusion temperature exceeds 3000°C.
• We are comparing thermal shock resistance, slag resistance, and application use cases.
• Cost and radioactivity constraints influence practical adoption.

Concept / Approach:
Thoria has an exceptionally high fusion point and excellent chemical stability, especially toward basic slags, but like many high-modulus ceramics, it is susceptible to thermal shock due to relatively low fracture toughness and significant thermal expansion mismatch in rapid temperature cycling. Its expense and radioactivity limit usage to niche, high-purity metal processing where contamination must be minimized.

Step-by-Step Solution:

Verification / Alternative check:
Reference data for oxide refractories list ThO2 among the highest melting oxides with strong slag resistance but caution on thermal shock performance.

Why Other Options Are Wrong:

• Choosing any single statement ignores that all are true for thoria.

Common Pitfalls:
Assuming that very high melting point guarantees thermal shock resistance; thermal shock depends on thermal expansion, modulus, and toughness, not only on melting point.

Final Answer:
All of the statements (a), (b) and (c) are correct.