Molybdenum Electrodes for Continuous Quartz Glass Melting: High Purity, Hydrogen Atmosphere and Tungsten-Molybdenum Crucibles

Quartz glass is made from high-purity quartz sand, with SiO₂ content typically above 99.9%; semiconductor grades demand more. Melting temperatures are around 1,750–2,000°C, far above ordinary glass, and most operations run under vacuum, hydrogen or an inert atmosphere. Mainstream equipment falls into two categories: the continuous melting furnace, which melts and draws tubing without interruption, holding the batch in a tungsten crucible with tungsten or molybdenum electrodes and hydrogen protection; and the gas-fired furnace, which builds up layers with an oxy-hydrogen flame. Molybdenum electrodes are central to the continuous furnace, immersed in a melt at close to 2,000°C, where their stability bears directly on tube geometry and hydroxyl content.

Purity Is the Starting Point of Acceptance

Purity requirements for molybdenum parts in this field are managed at the ppm level: electrode purity is generally specified above 99.95%, with iron, copper, sodium and potassium accepted element by element. The reason is that metallic impurities in the melt count directly as product defects; a trace of release from the electrode or crucible surface is enough to leave bubbles and dark spots on the tubing. All molybdenum parts are surface-treated before they enter the warehouse, using electropolishing or chemical cleaning to remove oxide layers and processing contamination; this step is optional for ordinary glass electrodes but mandatory for quartz users. Packaging differs too: ordinary electrodes can go in woven bags, while high-purity parts are usually vacuum-sealed in a cleanroom, then packed in an outer clean bag with desiccant.

In a Hydrogen Atmosphere, Failure Is Mainly Deformation and Volatilization

With a controlled atmosphere, oxidation recedes in importance; at 2,000°C the vapor pressure of molybdenum cannot be ignored, and long-term service thins the surface by volatilization while recrystallized grains coarsen, so strength and conductivity degrade slowly. A continuous melting furnace usually has a maintenance cycle of three to six months, with electrodes and tungsten crucibles replaced on that cycle. A furnace change is expensive, because the melt solidifies and must be remelted, and the loss from one shutdown far exceeds the price of an electrode, so users prefer to design life on the long side and set the lower limit of electrode life above the average. The direction of the spin-forming texture on the tungsten crucible affects how bubbles attach to and rise along the wall; matching the inner-wall texture to the outlet design is an interface refined over years of cooperation between continuous melting plants and crucible makers.

Hydroxyl Control Runs Through Raw Material, Atmosphere and Electrode Surface

The hydroxyl content of quartz tubing is the core specification, and the control chain is long: pre-dehydration of the raw material, the dew point of the furnace atmosphere, and adsorbed water on the electrode surface. Exceeding the limit at any link shows up on the infrared transmittance curve. The electrode is therefore not merely a conductor; its surface condition is a process variable as well. The atmospheric dew point and the permitted impurity list should be written into the agreement. Continuous melting and gas melting divide the work clearly: continuous melting is efficient and low-cost, leading in light-source tubing and general semiconductor auxiliaries; gas-melted parts have fewer bubbles and higher purity, leading in optical fiber preforms and high-end optical components, the latter mainly consuming molybdenum fixtures, charging boats and sintering supports.

Selection advice: customize to the furnace type rather than applying standard specifications, because every continuous melting line has a different crucible size and outlet structure. Make the purity certificate, surface treatment record and clean packaging method delivery conditions, and review the process route once a year. The vapor deposition route for synthetic quartz uses no electrodes, and the boundary at which it replaces continuous melting is worth watching.