Columbia subject headings:
Niobium--Congresses.
Rare earth metals--Congresses.
Tantalum--Congresses.
Harvard subject headings:
Niobium--Congresses.
Rare earth metals--Congresses
Tantalum--Congresses.
Princeton subject headings:
Niobium--Congresses.
Rare earth metals--Congresses.
Tantalum--Congresses.