How Molybdenum Resistivity Changes with Temperature

The same molybdenum electrode has a resistance several times higher when cold than when hot, and a power supply system designed around room-temperature parameters will run into serious trouble. The temperature dependence of resistivity is an unavoidable part of the electrical calculation for an electric melting furnace, and it is also the key to understanding how electrode power is ramped up. Only by reading this curve correctly can every step of the furnace power supply design be understood.

Basic behavior and reference values

Pure molybdenum has a resistivity of about 0.053 μΩ·m at 20°C, a middling figure among good conductors: far below copper and silver, low among the common refractory metals, and also lower than graphite. As temperature rises, resistivity increases approximately linearly, and above 1,000°C it approaches five times the room-temperature value. The table below gives reference values for engineering estimates:

On the standards side, electrode products are controlled to YS/T 1312-2019, Molybdenum Electrodes; molybdenum rods follow ASTM B387 / GB/T 17792; molybdenum plate and discs follow ASTM B386 / GB/T 3876. Acceptance is based on the quality certificate and the measured records.

Actual values vary slightly with purity, oxygen content and fabrication state, so manufacturer-measured data should govern final selection. The cold resistance of a given molybdenum electrode is also the on-site basis for judging whether the furnace wiring is correct.

Effect on furnace operation

A positive temperature coefficient brings a natural benefit: as the electrode heats up, its resistance rises and power converges automatically toward the design value, so the system has a degree of self-stabilization and thermal runaway is unlikely. The trouble appears in the cold state: a newly installed or cold-furnace molybdenum electrode has very low resistance, and applying full voltage directly produces an inrush current several times the rated value, which is unfriendly to both the grid and the transformer. Supply circuits therefore usually use multi-stage voltage regulation or tapped transformers, switched in stage by stage as temperature climbs, so that current density follows the set curve. Cold-state management comes down to three points:

• Before energizing, measure the loop resistance of every molybdenum electrode and compare it with the factory value; if it is abnormally low, check for short circuits and wiring first;
• During startup, bring stages in according to the tap table; never skip taps to rush the ramp;
• Record voltage and current curves during heat-up as the basis for calibrating hot resistance.

Comparison with the graphite route

Graphite is the opposite: its resistivity falls as temperature rises, making it a negative temperature coefficient material with high cold resistance and low hot resistance, so the power ramp strategy differs completely from that of a metal electrode. The greater difficulty is that graphite is a consumable electrode in the melt: its cross-section gradually shrinks, causing resistance to drift continuously and making the control target hard to fix. An electrode has both a predictable positive temperature coefficient and a geometry that changes slowly over its service life, so its electrical parameters can be recalculated over the long term; this is an important reason why the glass electric melting industry favors the molybdenum route wherever current density must be controlled precisely. For power supply design, hot resistance is the basis for setting tap positions and transformer capacity, while cold values are used only to check wiring and circuit integrity.