Measurements of Gas Shielding Factors for Cu, Fe, Ni

Knowledge of the vaporization behavior of different metals in dependence of temperature, pressure of the ambient atmosphere as well as its purity, is a crucial issue for high temperature materials. It has even been proven in the literature that the amount of mass loss of a sample due to vaporization strongly depends on the purity of the atmosphere in which the sample is heated. This coincides with the value of partial pressures of specific impurities. The so called gas shielding factor describes the ratio of the evaporation rate in a specific gas atmosphere and the evaporation rate under UHV.

The dependence of the gas shielding factor on the pressure and the purity of the process gas is remarkable. The range can extend over several orders of magnitude [1].

Within the framework of the presented work, vaporization experiments using a HT- Thermobalance with UHV supply have been carried out for the elements Fe, Cu, and Ni under inert gas atmosphere and with defined oxygen partial pressures. Thereby, the oxygen partial pressure has been maintained using an oxygen ion pump. Here, an electric current between +10 and -10 mA is applied to an yttrium-doped zirconia tube operated at a temperature of 600 °C. The current flow results in oxygen being transferred into or out of the ambient gas provided to the experimental set-up with a flow rate of about 2 cm³/min. The ion pump yields accuracy in oxygen flow of 10^-9 l/min and, therefore, enabled precise adjustment of pO₂ within the area of interest.

The gas shielding factor not only depends on the gas purity but also on the investigated material itself. The dependency from the oxygen partial pressure for example is influenced by the partial vapor pressure of the corresponding oxides and their dissociation pressure.

Reference

[1] E. Fromm, METALLURGICAL TRANSACTIONS A, VOLUME 9A , DECEMBER 1978, 1835-1838