Oxidation, Volatilization, and Redistribution of Molybdenum from TZM Alloy in Air

Abstract

The excellent high temperature strength and thermal conductivity of molybdenum-base alloys provide attractive features for components in advanced magnetic and inertial fusion devices. Refractory metal alloys react readily with oxygen and other gases. Oxidized molybdenum in turn is susceptible to losses from volatile molybdenum trioxide species, (MoO 3 ) m , in air and the hydroxide, MoO 2 (OH) 2 , formed from water vapor. Transport of radioactivity by the volatilization, migration, and re-deposition of these volatile species during a potential accident involving a loss of vacuum or inert environment represents a safety issue. In this report we present experimental results on the oxidation, volatilization and re-deposition of molybdenum from TZM in flowing air between 400 and 800ºC. These results are compared with calculations obtained from a vaporization mass transfer model using chemical thermodynamic data for vapor pressures of MoO 3 (g) over pure solid MoO 3 and an expression for the vapor pressures of MoO 2 (OH) 2 from the literature. Calculations correlate well with experimental data. The volatilization process is dominated by MoO 3 above 550°C and by MoO 2 (OH) 2 , formed from the small ingress of water vapor, at temperatures below 550°C. Partial saturation of gaseous species of (MoO 3 ) near specimen surfaces accounts for observed reductions in volatilization rates at lower flow rates at 700ºC. We have thus demonstrated predictive capabilities of the model to account for volatilization as influenced by temperature, humidity (vapor content), and flow rate.We obtained oxidation rates (mm/h) as indicated by the recession into the base metal. These rates which accounted for the concurrent processes of oxidation and volatilization showed trends similar to other refractory metals, namely, niobium and tantalum.Deposition of MoO 3 downstream at lower temperatures was calculated with a model using saturation ratios of (MoO 3 ) m within segmented regions. Calculated locations of peak distributions and maximums within the temperature gradients generally correlate reasonable well with experimental measurements. Scanning electron microscopy showed that deposits collected in a final filter consisted of small agglomerated particles. We might expect such nucleation, growth and agglomeration of particles to result from the supersaturation of the (MoO 3 ) m upon cooling during transport. Hydroxide molecules also decompose back into MoO 3 (s) and water vapor at low temperatures. These latter two processes and increased surface areas due to extensive crystal growth from (MoO 3 ) m are plausible explanations for differences in peak height and distribution predictions between the model and experiments..The oxidation-driven mobilization data, along with activation calculations determining radioactive isotope inventories, were used to determine airborne dose rates. These calculations showed that site boundary doses from TZM alloy would be one to two orders of magnitude lower than tungsten at comparable temperatures.iii S...

Catalyst-Free Growth of Quasi-Aligned Nanorods of Single Crystal Cu3Mo2O9 and Their Catalytic Properties

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