Bernard Cleyet is correct that Invar (an iron/nickel alloy) is used as a
shadowmask because of its low thermal expansion. On smaller screens and
less flat screens (where the beam intensity is more uniform) minimum
thermal expansion is less critical, but still important, and iron is
used.
I think iron or iron-alloys are also used because (1) they have the
strength to be very thin yet strong, (2)they provide some magnetic
shielding (until they themselves become magnetized).
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.