| High pressure sodium vapour lamps are made using high density
alumina tubes as the lamp envelope, alumina ceramic plugs, niobium
metal leads and a specially formulated sealing glass to make the
end regions hermetic. Finite element analysis has been used to show
that the ends develop significant magnitude of stresses. The single
most important geometry factor which controls the success of sealing
is the gap between the envelope tube and the plug. For this reason,
tube ovality tolerance limits are crucial. The chemical factors of
importance are the extent of alumina dissolution w1d subsequent
crystallization of phases such as 12Ca0.7Al20 3 whict. depend upon
the sealing schedule. |
