Effects of Impurities in Some Refractory Metal-Alkali Metal Systems

Impurities such as oxygen, nitrogen or carbon in either the refractory metal or the alkali metal can have a significant effect on compatibility in refractory metal-alkali metal systems. In some cases the impurity concentration levels sufficient to cause deleterious effects are only several hundred parts per million, and these levels might be present in "high-purity” metals. Results have been reported which show that the presence off nitrogen in lithium greatly influences the solubility of some of the refractory metals. Experiments conducted at relatively low temperatures have also indicated that oxygen in sodium has a detrimental effect on the corrosion resistance of niobium and tantalum. The migration of impurities between solid and liquid metal will occur in almost any system because of the tendency for each element to attain the same chemical potential in the two metals. Existing information indicates the problem of impurity transfer can be considered as the distribution of a common solute between two relatively immiscible solvents. A distribution coefficient can then be defined for each impurity in the system and should be a constant at a specified temperature. Data are presented which compare calculated values off distribution coefficients with experimental values obtained from studies conducted on Nb-O-Li, Nb-O-K and Zr-O-K systems. Experiments are described which show that there is a strong temperature effect on oxygen transfer in the Nb-O-K system. The ramifications of oxygen redistribution on the operation of an engineering system are also discussed.