Glass

SPECIAL SECTION/GLASS MANUFACTURING: A Better Melt

June 1, 2010
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Molybdenum has long been the electrode material of choice for glass melting applications, providing high-temperature strength, good electrical conductivity and corrosion resistance.

Glass melting electrodes are produced from molybdenum.


The glass industry has achieved a high level of purity in its glasses, primarily through the use of molybdenum in melting applications for homogenizing, feeding and shaping. The advancement of electric glass melting was reliant on the discovery of suitable electrode materials. Few materials were found that possessed the mechanical, electrical, process, corrosion and cost requirements for glass melting. It's not surprising, then, that molybdenum has been the choice electrode material for decades with its high-temperature strength, good electrical conductivity and corrosion resistance to most glass compositions.



Precision rotary forges use state-of-the-art technology to reduce the cost and waste associated with machining.

Beneficial Properties

As a refractory metal, molybdenum's physical properties surpass the criteria required for high-quality glass production and exceed all industry standards for efficient glass melting and electric-boosted melting. Molybdenum's high melting point, resistance to sagging at glass bath temperatures of 2500-3100°F (1370-1700°C), high electrical conductivity and resistance to attack by most glasses make molybdenum an ideal material for furnace components.

Glass melting electrodes used in glass melting furnaces are produced from molybdenum. The cylindrical-shaped electrode is inserted through the furnace walls into the molten glass. Special furnaces are constructed with plate-type electrodes that are located slightly above the bottom surfaces. Maximum furnace operating efficiency is achieved through the innate electrical properties, high-temperature strength and rigidity of molybdenum electrodes.

Exceptionally pure molybdenum (99.95%, minimum)* assures outstanding resistance to chemical erosion and degradation while minimizing detrimental glass discoloring. Molybdenum is not suitable for "high lead" crystal, since the lead oxide rapidly corrodes the molybdenum, causing a high molybdenum content to appear in the glass, dulling the crystal.

In addition to glass melting electrodes, other glass processing components, such as stirrers, pumps, bowl liners, wear plates and some molds, are made of molybdenum. Extruded molybdenum pipes are used in plants where the molten glass is transported from one furnace to another.

Molybdenum billets are extruded into the required shape and size.

Production Options

Producing superior glass melting electrodes can be facilitated using a traditional powder metallurgical process and an exclusive electron beam (EB) melting process. For traditional powder metallurgy processing,** molybdenum powders are reduced from chemicals, pressed and sintered for forging, extrusion, or hot and cold rolling. Glass melting electrodes are precision rotary-forged with a state-of-the-art rotary forging technology developed to reduce costs.

The CNC-controlled forging system can produce tapers and steps for the near-net shapes common in glass melting electrodes. The forging system samples power, tonnage and temperature once per second for the piece-to-piece consistency that is crucial to maintaining purity in glass melting. To transport the molten glass, long molybdenum pipes are extruded using a 5000-metric ton extrusion press.

EB melting technology was developed to use recycled molybdenum as a raw material.† the molybdenum feedstock is "bombarded" with electrons in the furnace to melt it. The material melts into a water-cooled copper mold, where it solidifies into a round molybdenum ingot. The ingot is then forged and formed into the desired shape. EB melting is carried out in a high-vacuum atmosphere, so elements with melting points that are lower than molybdenum volatilize, leaving the molybdenum more than 99.99% pure and 100% dense.

Molybdenum electrodes can be centerless ground with machine-finished surfaces to achieve optimum concentricity and straightness. Glass melting electrodes can also be produced in an "as-forged" or hot-worked, chemically cleaned and straightened condition for added cost savings.

Molybdenum rotary targets are produced with extremely low levels of oxygen for solar energy applications.

Solar Energy Applications

The quest to discover new energy sources led to molybdenum's becoming a key player in the creation of sputtering targets for thin film photovoltaic (TFPV) energy applications. Research engineers met the challenge of finding ways to lower the cost-per-watt ratio by developing molybdenum- and nickel-based monolithic "rotatable" targets for copper indium gallium (di)selenide (CIGS), cadmium telluride (CdTe) and amorphous silicon (a-Si) technologies.

Molybdenum rotary targets are produced with extremely low levels of oxygen; these targets are fully dense and feature a refined microstructure that is optimized for improved sputtering. The monolithic round tube design helps cut costs and offers reduced arcing, increased power density and high material utilization compared to flat planar targets.

Solar energy cells are produced by depositing thin layers of materials such as molybdenum on a substrate like glass. Depending on the type of thin film solar cell being manufactured, a variety of photovoltaic materials, substrates and deposition methods can be used.

Utilizing a 5000-metric-ton direct extrusion press,†† molybdenum rotary targets can be extruded with inside diameters of 125 mm (monolithic), 135 mm (bonded) and up to the largest current rotary target requirement. Targets in lengths of 4 m or more are produced for specific applications.

Research and Product Innovation

Product development and research are paramount in discovering new solutions to glass application problems. For glass melting electrodes, R&D teams developed new methods for controlling molybdenum during the production process. This resulted in improved physical properties that yield a superior glass melting electrode. Sputtering target quality was enhanced through careful microstructure control and increased homogeneity for solar cell applications.

State-of-the-art laboratories facilitate material processing and the testing of thin films. In addition, the employment of advanced analytical tools such as electron backscattering diffraction analysis (EBSD), glow discharge mass spectroscopy (GDMS) and ultrasonic scanning has resulted in the elimination of banding in the microstructure, consistent chemical composition and effective non-destructive inspection.

*Available from H.C. Starck.
**Performed at H.C. Starck's extrusion and forging facility in the U.S.
†H.C. Starck's production facility in Germany manufactures MOLYMELT EB glass melting electrodes using its exclusive EB melting process.
††Such as the one at H.C. Starck's U.S. facility.

For additional information regarding the use of molybdenum in glass manufacturing, contact the author at (517) 279-3673 or david.meendering@hcstarck.com.

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