Industrial furnaces: electrical resistor/heating bars used as elements in high-temperature furnaces are commonly made of which refractory material?

Introduction / Context:
Resistive heating elements for industrial furnaces must combine high electrical resistivity at temperature with oxidation resistance, mechanical integrity, and predictable aging. Silicon carbide (SiC) elements—often marketed as “globars”—are a classic choice for continuous service up to the 1400–1600 °C range depending on atmosphere and design.

Given Data / Assumptions:

• We are referring to solid resistor bars/elements that heat by Joule heating.
• Operating atmospheres may be oxidizing at high temperatures.
• Mechanical robustness and dimensional stability are desirable.

Concept / Approach:
SiC forms a protective silica (SiO2) scale in oxidizing atmospheres, limiting further oxidation and enabling long life. Its resistivity increases with temperature initially, then decreases at very high temperatures, enabling controllable power characteristics. Alumina and zirconia are excellent insulators (not conductors) and thus are unsuitable as resistor bars. Graphite is conductive and used as heating elements in inert/reducing atmospheres, but it oxidizes rapidly in air, making it less suitable where oxidizing service is expected without protective measures.

Step-by-Step Solution:
List candidates by electrical behavior: SiC and graphite conduct; alumina/zirconia are insulators.Consider atmosphere: SiC tolerates oxidizing atmospheres better than graphite due to silica scale.Industry practice prefers SiC for resistor bars in many electric kilns/furnaces.Select silicon carbide.

Verification / Alternative check:
Product datasheets for SiC elements specify operating ranges and resistance-versus-temperature curves, aligning with widespread industrial use.

Why Other Options Are Wrong:

• Alumina/zirconia: electrical insulators; not resistor bars.
• Graphite: viable in inert/reducing atmospheres, but not the generic “resistor bars” choice in air-fired kilns.
• Silicon nitride: generally insulating/semiconducting; not standard resistor bar material.

Common Pitfalls:
Assuming all refractories can serve as heaters; overlooking oxidation of graphite in air.

Final Answer:
Silicon carbide