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D E F D
THERMAL PROPERTIES
Fig. 2.01. Differential thermal
analysis cell.
A. Stainless steel tube
B. Plug
C. Thermocouple insulator
D. Thermocouple junction
E. Sample compartment
2.6 Differential Thermal Analysis and Pyrolysis Test. Smothers and Chiang6
give a complete discussion of the differential thermal analysis technique, and its
theory and a complete review is given in the Analytical Reviews edition of
Analytical Chemistry.6
Figure 2.01 shows the DTA cell design. The 0.139-in.-o.d., 1.25-in-long stainless
steel hypodermic tube, A, is reamed to accept the 0.115-in.-o.d. thermocouple in-
sulators, C. The relatively low thermal conductivity of stainless steel allows use of
the axial cell arrangement. The plug, B, between the sample and reference sides of
the cell is made by impregnating a small wad of quartz wool with Sauereisen ce-
ment and packing it into the center of the tube. After the cement is dry, the cell is
ignited in a burner flame. The thermocouples, D, made from 2%gauge
ChromellAlumel, are arc-welded against a carbon rod at the clipped end of a single
twist of both wires.
Expendable tube furnaces are a 75-ohm helical coil of Nichrome wire distributed
on a helically grooved, 3-in.-long, 11/16-in.-i.d. Alundum tube. A 21/32-in.-o.d. by
3/8-in.-i.d. by 3-in.-long graphite tube is used as a furnace liner for thermal ballast.
A l/4-in.-o.d. aluminum tube is inserted into the furnace liner but is isolated from it
by asbestos “0” rings at each end. The natural tubing-cutter constrictions at the
ends of the aluminum tube support the thermocouple insulators of the DTA cell
and keep :it from touching the aluminum walls. A 6-in. cube of foamed glass con-
tains and insulates the assembly. The entire assembly is placed in a blast shield box
before a run is started.
The reference thermocouple that indicates cell temperature is connected to the
abscissa terminal of a Moseley Autograf Model 2 X-Y recorder. A Leeds and
Northrup Model 9835-B dc microvolt amplifier amplifies the differential ther-
mocouple output, which is then connected to the ordinate terminal of the X-Y
recorder. An F&M Model 40 linear temperature programmer, which provides a con-
stant heating rate to the cell, is controlled by a thermocouple placed between the
Alundum furnace shell and the graphite liner.
Five- to twenty-milligram samples give the best results, but samples as small as 3
mg can be tested. The differential temperature scale normally used is f 5”C, but
the sensitivity can be increased to record differential temperatures of f 0.5”C.
A deflagration usually does not damage the DTA cell beyond repair. A low-order
explosion will destroy the sample thermocouple, but the thermocouple can be
replaced without changing the zero-line characteristics of the cell. Detonation of a
lo-mg sarnple will destroy the entire assembly, often including the insulation.
Pyrolysis. Figure 2.02 shows the apparatus used to obtain the pyrolysis curves,
and Fig. f!.03 gives details of the pyrolysis block. In this test an -lo-mg sample of
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