One aspect of the invention comprises a thermal evaporation source comprising an evaporant chamber, a heater for providing heat to the evaporation chamber; and a crucible in thermal communication with the evaporation chamber for containing a volume of evaporant. The evaporant chamber comprises a first material of construction, and the crucible comprises a second material of construction different from the first material of construction and having a lesser porosity with respect to the evaporant than the first material of construction. For example, for a copper evaporant, the evaporant chamber may comprise a sintered material, such as sintered graphite, and the crucible may comprise a pyrolytic material, such as pyrolytic graphite or pyrolytic boron nitride.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
 The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. ADJ-1-30630-12, awarded by the National Renewable Energy Laboratory.