Refractory materials — beryllia (BeO) vs. alumina (Al2O3): Beryllia is often chosen for crucibles used to melt uranium and thorium because, above 1900°C, it outperforms alumina in most high-temperature properties. In which single aspect is beryllia NOT superior to alumina for such service?

Introduction / Context:
Beryllia (BeO) and alumina (Al2O3) are advanced ceramic refractories used in extremely high-temperature environments, including crucibles for melting reactive metals like uranium and thorium. This question probes understanding of which performance attribute does not favor beryllia when compared directly with alumina, especially above 1900°C.

Given Data / Assumptions:

• Service temperature exceeds 1900°C.
• Applications include crucibles for very reactive, high-melting metals.
• We compare intrinsic material properties relevant to refractory service.

Concept / Approach:
Beryllia exhibits exceptional thermal conductivity, high melting (fusion) point, and excellent thermal shock resistance, typically surpassing alumina in these regards. Its electrical behavior at high temperature remains that of a very good electrical insulator with favorable dielectric properties. The major downside of BeO is economics and handling: it is more expensive to produce and requires strict health safeguards due to beryllium toxicity during powder handling and machining.

Step-by-Step Solution:
Identify properties where BeO is superior: thermal conductivity (BeO >> Al2O3), refractoriness/fusion point, and high-temperature stability.Evaluate electrical properties: both are good insulators; BeO has excellent dielectric performance at high temperature.Compare cost and HSE requirements: BeO is significantly more expensive and demands stringent safety controls.Therefore, the single aspect where BeO is not superior is cost.

Verification / Alternative check:
Materials handbooks consistently list BeO's thermal conductivity and high-temperature performance above alumina, while highlighting its higher cost and processing hazards.

Why Other Options Are Wrong:

• Electrical conductivity: both are insulating; BeO is not inferior here.
• Thermal conductivity: BeO markedly exceeds alumina.
• Fusion point: BeO has very high refractoriness, not inferior to Al2O3.

Common Pitfalls:

• Confusing electrical conductivity with dielectric loss; BeO remains an insulator.
• Assuming “better property” always implies “better choice” without considering cost and safety.

Final Answer:
cost