3.4. Tratamientos de datos
3.4.2. Tratamiento digital del giroscopio
2.2.1 Density Measurement in Molten Carbonate Systems.
As a supplement to the high pressure viscosity and density measurements made on molten carbonates, density was measured in selected carbonate systems at a range of temperatures and atmospheric pressure, using the archemedian method for density determination. The compositions studied were 50:50 mole fraction K2C0 3-MgC0 3, and 40:60 K2C0 3-CaC0 3. The starting materials were prepared as ground mixtures of the end-member carbonates which were fused, under CO2, prior to experimentation. During preliminary fusion of the powder mixtures, the resultant melt contained up to 50 volume % vesicles which separated slowly from the melt, thus making the pre-fusion stage necessary to ensure that the density of pure melt and not liquid plus gas was measured.
Experiments were performed in a vertical cylindrical furnace, (Lenton Thermal Designs Ltd.) with the experimental arrangement shown in figure 2.2. The furnace tube was made of recrystallised alumina, with an internal diameter of 49 mm and was surrounded by molybdenum disilicide heater elements. Temperature was controlled using a Pt/Rh thermocouple external to the furnace tube in a feedback loop to a 'Eurotherm 815' temperature controller.
Sample was held in a platinum crucible, within a deep recrystallised alumina crucible. This was supported from below using a 25 mm diameter alumina tube through which the sample thermocouple passed. The top end of the furnace tube was loosely sealed by an aluminium cap with a 5 mm hole for the reference weight wire. The reference weight, which was a 0.4 cm^ piece of 6 mm diameter corrundum rod, was
suspended in the sample using a 100 |Xm palladium wire; the refence weight wire. This was sufficiently heavy to pull the thin palladium wire taught and so eliminate the risk of contact with the furnace walls, or the top cap and the errors due to friction that this would entail. The top end of the palladium wire was connected, via a wire stirrup to a 'Sartorius' top-pan balance set in the less precise (± 0.000Ig) mode. Between the top of the furnace tube and the stirrup connection, there were several layers of thermal baffling, with only a small hole for the palladium wire, intended to suppress instabilities in the measured weight due to the effects of convection in the air above the furnace.
Sample temperature was monitored using a chromel-alumel (K-type) thermocouple which had its weld in contact with the outside of the alumina crucible. All experiments were performed in a CO2 atmosphere in order to maximise the temperature range available for measurements. Carbon dioxide was fed in to the bottom of furnace at a rate of 1 0 cm^ per minute and exhausted through the loose fitting cap at the top.
Experiments were set up with the reference mass sitting on top of the solid sample, with about 1 cm slack on the palladium wire, thus the mass would be submerged under at least 1 cm depth of sample after melting. The furnace was then steadily heated at a rate of 1 0 °C per minute until the sample thermocouple reached the nearest whole multiple of 50 °C, above the temperature at which the balance first registered a weight. This was taken to be the first data point, the temperature was allowed to stabilise and the mass registered by the balance was noted. This value was actually an average of the maxima and minima of five consecutive fluctuations in measured mass. The furnace temperature was then increased by a further 20 °C and the weighing process repeated. These steps were repeated until the decarbonation temperature for the magnesian composition; 600 °C, was reached and to 1000 for the calcic composition.
Top-pan Balance Palladium wire Therm al baffle Fibrous SilicaE A lum ina M olybdenum disilicide
H eating elem ents
Sam ple T herm ocouple Control T herm ocouple Platinum Crucible Furnace tube — — Sampl e R eference M ass um m a C rucible
Figure 2.2 The vertical furnace arrangement used for archimedian measurement o f density of selected
2.2.2 Guinier-Lenne X-ray Camera.
Solid carbonate phases were studied for thermal stability and thermal expansion using a Guinier-lenne heated stage X-ray camera, supplied by Nonius, Holland. This system allows the continuous recording of a powder X-ray diffraction pattern on an image plate during the heating of the sample. X-rays were produced using a Philips water-cooled copper rotating anode tube and were filtered for CuKoti radiation. Accelerating voltage was 45 KeV and beam current was 40 mA.
The general arrangement of the camera system and a block diagram of the heated sample carriage is shown in figure 2.3. The internal thermocouple was calibrated against a K-type thermocouple inserted in the sample space so that the thermocouple weld was in contact with the sample holder. The temperature measured by the internal thermocouple, after the calibration correction, is accurate to ± 5 ° C . Heating was controlled by a motorised thyristor which was set such that the sample temperature increased from 25 °C to 400 ®C over about 40 hours. Although the heating rate due to this was non-linear , the maximum rate was about 15 °C per hour. The temperature measured by the internal thermocouple was plotted on a chart recorder travelling at 10 mm/h. The image plate was moved by a stepper motor at a rate of 2 mm/h; thus the temperature represented by a given point on the image plate could be determined with a precision of ± 2 °C and an estimated accuracy of ± 7 ®C. The distance between the sample and the image plate was such that 1°28 = 2 mm on the image plate. This meant that 20 could be measured to better than 0.2°.
About 15 mg of the powdered sample was dry-pressed into an 8 mm by 4 mm rectangular wire gauze in the holder which consisted of a 1 mm thick sheet of Pt-Ir alloy surrounding the 100 |im Pt wire gauze. This was then inserted into the cavity in the sample carriage as indicated in figure 2.3.