High temperature viscometry

where 0.1875 is a geometric constant, r the radius of the hemisphere, P the applied force, a the penetration depth and t the experimental run time. The viscometer is calibrated against

a lead-silica glass of the National Bureau of Standards (NBS 711) and a sodium-calcium-silica glass of the Deutsche Glastechnische Gesellschaft (DGG 1). 3 mm thick double polished glass disks were used for viscosity determinations. The accuracy of micropenetration viscosimetry is ±log10 0.23 Pa s, the precision of the measurements lies within ± log10 0.06 Pa s.

2.4.2. High-temperature viscometry

High-temperature viscosity determinations were performed in a DelTech® DT-31-RS box furnace heated by MoSi2 elements at air atmosphere. The sample was housed in a crucible, which was inserted into an alumina-silicium ceramic pedestal that ensured the sample was in the “hot zone” of the furnace. The height of the pedestal or the depth of the hole in which the crucible sat could be adjusted. The temperature profile of the final configuration was determined using a S-TYPE (Pt/Pt90-Rh10) thermocouple (shielded with platinum pipe-lid sheath), which was inserted directly into the melt sample. Stable, vertical and radial temperature gradient of 1.8 °C.cm-1 _{were recorded.}

Viscosity was measured at a pressure of 1 bar in air with the concentric cylinder method. The samples were contained in cylindrical Pt80Rh20 crucible, 5.1 cm in height, with a 2.56 cm inner diameter and 0.1 cm wall thickness. The viscometer head, with which the viscosities

5 . 1 5 . 0 1875 . 0 l r Pt = η LVDT SAMPLE Indenter Indentation Al2O3rod LVDT SAMPLE Indenter Indentation Al2O3rod LVDT SAMPLE Indenter Indentation Al2O3rod LVDT SAMPLE Indenter Indentation Al2O3rod LVDT SAMPLE Indenter Indentation Al2O3rod LVDT SAMPLE Indenter Indentation Al2O3rod LVDT SAMPLE Indenter Indentation Al2O3rod

were measured, was a Brookfield model RVDT (Rotary Variable Displacement Transducer) with a full range torque of 7.2 x 10-2 N.m. In this study two types of spindles were used. Both were made from Pt80Rh20 with a circular cross-section and a 0.24 cm diameter stem. For more viscous melts the spindle had a diameter of 1.44 cm, a length of 3.32 cm and 45° conical ends to reduce end effects. For less viscous melts the spindle was 0.24 cm in diameter, 4.63 cm in length and did not have conical ends. The viscometer head drives the spindle at the range of constant angular velocity (0.5 to 100 rpm) and digitally records the torque exerted on the spindle by the sample.

The spindle and head were calibrated for viscosity measurements using NSB SRM 711 lead-silica glass for which the viscosity-temperature relationship is very well known. The precision of viscosity determination for this apparatus (±3% at the 2σ level, Dingwell, 1986) was derived from replication of viscosity determination of NSB SRM 711 involving successive immersions of the spindle and reoccupation of the temperature settings. The sample-bearing crucible was loaded through the bottom of the box furnace into the ceramic pedestal, such that one third of the crucible was within the pedestal. The viscosity spindle was connected to the reading head and lowered by a rack and pinion mechanism into the sample.

The viscometry determinations were initiated by equilibrating the melt sample with air. The rotation speeds of the spindle required to reach equilibration for each sample were 20 or 40 rpm depending on initial viscosity. The equilibration of melt sample was continuously monitored with a chart recorder that recorded the torque measured by the viscometer head as a function of time. For each sample, viscosity determination were initiated at the highest temperature and then made at successively lower temperatures. One hour was sufficient for equilibration of the sample over each 25-50 °C temperature decrease. During the decreasing temperature steps the thermal equilibrium was monitored with the chart recording of the sample viscosity. At the end of each step, when thermal equilibrium was reached, the melt glass were sampled using the “dip” technique (∼150 mg). The samples were quenched in water for further iron oxidation and structural state and compositional investigation.

Torque measurements were made over a range of angular velocities for each sample. The obtained viscosities were independent of angular velocity in all cases. The measurements were continued with decreasing temperatures in steps until crystallization occurred or the limit of the apparatus was achieved. Crystallization during the final cooling step for each sample resulted in erratic viscosity readings and was easily confirmed by inspection of samples recovered at those times.

MoSi2 - element

Pt crucible

The last measurement of viscosity was always a redetermination of the highest temperature determination to test for experimental drift as a consequence of possible movement of pedestal, crucible or spindle, chemical change (loss of water, volatiles) and, last but not least, the decrease of surface inside the crucible by dip technique. No difference was observed between first and last high-temperature determinations. That indicates that no compositional or instrumental drift occurred during the viscosity measurement.

After all measurements the investigated sample was either poured out and quenched on an iron plate or the whole crucible, containing the analysed sample, was rapidly quenched in water. From this glass material cylinders 8 mm in diameter were cored for container based dilatometry or low temperature viscometry (micropenetration).

Torque transducer ϖ ϖ ∆ϑ Torque transducer ϖ

FIGURE 2.6. Schematic diagram of the concentric cylinder apparatus. The heating system Deltech furnace,

position and shape of one of the 6 MoSi2 heating elements is illustrated in the figure. Details of the Pt80Rh20

crucible and the spindle shape are shown on the left. The viscometer Brookfield RVTD stirring head is coupled to the spindle through a hinged connection which can be seen on the photograpf on the left.