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THERMAL PROPERTIES
Fig. 2.02. Pyrolysis apparatus.
A. Carrier gas supply
B. Pressure regulator
C. Flow control needle valve
D. Reference thermal conductivity cell
E. Pyrolysis chamber
F. Combustion tube
G. Active cell
H. Manometer
I. Pressure control needle valve
J. Rotameter
I H
Fig. 2.03. Pyrolysis block.
A. Pyrolysis chamber
B. Nickel plug
C. Carrier gas inlet
D. Carrier gas outlet
E. Cartridge heater wells (2)
F. Helical threads cut in inner body
of block
G. Outer shell of block
H. Cooling jacket inlet
I. Cooling jacket outlet
test material is weighed into a small combustion boat and placed in the pyrolysis
chamber, initially at room temperature. A lo- to lSml/min flow of helium is then
started, and when the air has been swept out, the pyrolysis chamber temperature is
raised at a constant rate, usually lO”C/min. The helium stream carries gases
evolved from the sample through the combustion tube and into the thermal conduc-
tivity cell, G. The two cells, D and G, form two arms of a Wheatstone bridge whose
output varies with the concentration of impurities (decomposition products, etc.) in
the effluent helium stream. The bridge output is fed to one axis of an X-Y recorder,
and the pyrolysis chamber temperature is fed to the other. In this manner, the rate
of gas evolution from the sample as a function of chamber temperature is deter-
mined.
The combustion chamber converts the more complex products, such as undecom-
posed but vaporized explosive, to simple molecules. This increases the bridge sen-
sitivity and also keeps these products from condensing in the cooler parts of the ap-
paratus.
Data Presentation. All the DTA curves were determined at a heating rate of
ll”C/min with granular NaCl as the reference sample. All the pyrolysis curves were
determined at a heating rate of lO”C/min. Gas-solid interactions were minimimized
because gaseous products were swept away from the sample in the pyrolysis ap-
paratus as rapidly as they were formed,
Any possible contribution to the reaction from atmospheric oxygen also was
eliminated, because the carrier gas was helium.
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