A 21st. Century Perspective on Molybdenum Powder Production by Hydrogen Reduction

Abstract

High purity molybdenum metal powder is produced commercially from hexavalent molybdenum precursors, viz.: ammonium dimolybdate (ADM) or molybdenum trioxide. One conventional process incorporates first-stage and second-stage flowsheet components, with hydrogen gas serving as reductant. This two-stage strategy is employed in order to minimize the formation of volatile molybdenum species that would otherwise be generated at the high temperature required to obtain molybdenum (Mo) in a single stage conversion of the molybdenum precursor. Although molybdenum powder has been produced commercially for over a century, a comprehensive understanding of the kinetic mechanisms and powder characteristics, e.g. oxygen content and particle morphology, is far from being definitive. In fact, it might be argued that the “art” and engineering, in a commercial context, has advanced ahead of the fine-detail science-derived metallurgical process-engineering. Theoretical contributions presented in this paper are focused primarily on the fundamentals of the conversion process associated with second-stage reduction process – MoO2 to Mo and the factors that contribute to the oxygen content of the molybdenum powder product (1000 to 100 ppm(w) O, range). Thus, equilibrium-configuration details concerning both solid and gas phases are addressed, including the volatile hexavalent molybdenum vapor complexes as well as solubility of oxygen in molybdenum. In regard to the role of a chemical vapor-transport mechanism on powder morphology in second-stage conversion of MoO2 to Mo, it is shown that the partial pressure of the prominent molybdenum hydroxide vapor-complex (MoO2(OH)2) is far too low to support such a mechanism. This contention has been corroborated by employing helium to control the partial pressures of hydrogen and water in the gas phase. Secondarily, a limited assessment of the intrinsic rate-controlling mechanisms that can contribute to the residual oxygen-content of the Mo powder product is also provided. Powder morphology, and its concomitant influence on specific surface-area of the Mo powder product, is found to correlate with the oxygen-content determination of the powder produced during second-stage reduction, and according to the processing strategy employed. Consequently, it has been found cogent to “partition” second-stage reduction into: i) a relatively high-rate Primary Reduction Sequence, and ii) a lower rate Deoxidation Sequence.

Structures of seven molybdenum surfaces and their coverage dependent hydrogen adsorption

“…In practical, reduction of MoO 3 powders by hydrogen is one of the most used methods to obtain metallic molybdenum with high purity. 53 Therefore, hydrogen should play an essential role in determining the structure of the metallic Mo catalyst. Generally, the activity of catalysts was always affected by their surface structures and compositions.…”

The Effect of Microstructure of Molybdenum Target<strong> </strong>

“…Molybdenum (Mo) is a typical refractory metal with many special properties, such as high temperature strength, low thermal expansion coefficient and high Young's modulus [1,2]. In recent years, many studies have shown that using Mo thin films to replace or as barrier layers of Ag, Cu and Al thin films can effectively improve the conductivity, anti-electro migration and thermal stability of thin film electrode.…”

Preparation and Influencing Factors of Molybdenum Targets and Magnetron-Sputter-Deposited Molybdenum Thin Films

“…Molybdenum (Mo) is a typical refractory metal with many special properties, such as high temperature strength, low thermal expansion coefficient and high Young's modulus. So, it is widely used in many fields, for examples, electronics, aerospace, energy and lighting [1][2][3][4]. In recent years, studies show that using Mo thin films to replace or as barrier layers of Ag, Cu and Al thin films can effectively improve the conductivity, anti-electromigration and thermal stability of thin-film electrode .…”