Low4ost Dilatometer for High Temperature Work L. A. Boschl Gerencia de Desarrollo, Comisi6n Nacional de Energia At6mlca. Avda. del Libertador 8250. Capital Federal (1429), Argentina
J. A. Barbem' Departamento Quimica de Reactores, Comisi6n Nacional de ~nergiaAtbmica, Avda. del Libertador 8250, Capital Federal (1429), Argentina
Dilatometry is an important experimental technique for both physicists and chemists (studies of the phase transformations in steels and many other systems, kinetics of isothermalshrinkage of powder compacts, etc.). Here we show a very low-cost dilatometer that can be built in almost any laboratory just with the help of a good glass shop. The instrument is suitable to measure any solid sample that does not react with silica, and it operates up to 1100 "C. An experimental assembly of the dilatometer is illustrated schematically in Figure 1. The apparatus consists of a vertical quartz pushtube, which is mobile with negligible friction, a suspension system, and a calibrated Pt/F'tlO% Rh thermocouple next to the samole. All is enclosed withina auartz tube that is surrounded by a nichrome-wound furnace with a hot zoneconstant to 1.5 O C alone: the soecimen assemhlv. The yorine-loaded tio of the gauge c o n k s the top of the mobile quartz tube, which lies on the sample, as indicated in Figure 1. Any dimensional change undergone hy the pellet is followed by the mobile pushtuhe and consequently by the transducer. In our apparatus we used either a mechanical dial gauge or an electronic transducer. The former was a Puppistat instrument, manufactured by Carl Mahr. The vertical displacements of the spring-loaded tip of this gauge are indicated in a circle-shape scale with a maximum deflection of 100 pm and a minimum division of 1pm. The electronic transducer, a linear variable differential transformer. has a small dc
output that is proportional to the displacement of the tip. The output is fed to a 100-mV full-scale recorder (with zero suppression), which in our experiments registered approximately a 10-mV change for a 100-pm dimensional change (Fig. 2). Either using one instrument or the other as discussed above, they allow the easy readings of dimensional changes of the sample of the order of micrometers. The equipment can operate under static or dynamic gas atmosphere. In the latter, a very low gas flow ( 4 . 5 mllmin) comes into and goes out of the quartz tube surrounding the dilatometer. The apparatus was developed for a study of the influence of inorganic additives (CunO, CaO, COO, etc.) on the initial stage of the sintering of the fuel pellets of uranium dioxide. The powder compacts undergo a shrinkage during sintering and phase transition^.^^ With a dilatometric study, the process can he identified easily; in the initial phase of an oxide sintering, the dimension change of the sample will be continuous, whereas during a phase transition, an abrupt change in slope will occur in the dimension
zBacmann,J. J.; Cizeron, G. J. Amer. Ceram. Soc. 1988,51,209. Lay. K. W.; Carter. R. E. J. Nucl. Mater. 1969, 30. 74. Bannister, M, J.; Buykx, W. J. J. Nucl. Mater. 1974, 55, 345.
a-
DIAL GAUGE
F U S E D QUARTZ PLATFORM O-RING. FUSEDQUARTZ TUBES
DIAL GAUGE SUPPORT
-
GAS I N L E T
PilPl-1OD/oRh THERMOCOUPLE
GAS OUTLET
-+jr
l INCH U
Figure 1. Dilatometer-schematic.
F U S E E QUARTZ
QUICK-COUPLING
LOO
n,
2m
100
0
Shrinkage ()rm) Figure 2. Shrinkage versus time for a pellet of U02doped with 1% COO. Volume 63
Number 6
August 1986
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versus temperature plot. This dilatometer allows one to follow continuously the dimensional variations of the pellet, providing in this way agreat deal of information of the effect of the additives and the temperature on the sintering kinetics of UOz. Before carrying out any of the experiments with the samples, we increased the temperature of the furnace up to 1100 O C with the dilatometer empty in order to check the zero of
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Journal of Chernlcal Education
the apparatus. The maximum deviation ohserved was 3 pm, which represents less than 1% of the total shrinkage of the pellet in the region 25 OC-1100 OC. By test experiments, the sensitivity of the apparatus was found to he about 1pm. A typical run is shown in Figure 2, where the shrinkage is plotted against time a t different temperatures. More information including detailed operation tests and complete schematics may be obtained by contacting Barbero.
