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THERMAL PROPERTIES
This assembly is dropped into a preheated liquid metal bath, and the time to ex-
plosion is measured as the time to the sound created by the rupture of the blasting
cap or unseating of the plug. The lowest temperature at which a runaway reaction
can be obtained is the Tm. Many tests are required to determine Tm with con-
fidence, because it is necessary to raise and lower the bath temperature across the
apparent Tm, make many separate tests, and allow enough time for a reaction. A
safe failure criterion for 40-mg samples is no explosion in 1000 seconds. We have
never obtained an explosion after 10 000 seconds.
Because the reactions can be violent, the metal-bath enclosure shown in Fig. 2.04
is used. The baffles keep most of the hot metal in the chamber, and the test can be
made behind a shield in a fume hood.
232
Fig. 2.04. Time-to-explosion
test metal-bath assembly.
A. Cartridge heaters (3 each)
B. Top assembly, bolted to base
C. Sample-cell holder (the sample
cell is insulated from the
holder by a band of glass
tape around its top)
D. Sample-cell holder pivot
arm, which allows cell and
holder to be inserted remotely
into the lower assembly
E. Metal-bath container, made
from mild steel for stability
when
metal
containing molten
F. Sample cell
G. Sample cell support
pedestal, whose length is adjusted
length
to the sample cell