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Platinum Metals Rev., 1980, 24, (3), 100

The Viscosimetry of Molten Alkali Metal Fluoborates

Platinum Components Serve Successfully under the Arduous Conditions Encountered

  • J.H.F.N.
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The alkali metal fluoborates, well established as brazing fluxes, have been under consideration as heat transfer media for use in molten salt breeder reactors. Recently, a study of the viscosity of these compounds at various temperatures has been carried out at the AERE Harwell Research Laboratories. The viscosimeter employed was of the bob and cup type, where a bob (or spindle) is rotated in a cup containing the fluid under investigation.

A prototype constructed from stainless steel was used for one run to establish that the design and dimensions of the viscosimeter were satisfactory, but the highly corrosive nature of the compounds precluded the use of base metals in any part of the apparatus exposed to the melt during the experimental runs.

The general arrangement of the viscosimeter is shown in Figure 1. To avoid spurious heating effects, the motor drive and the sensor for the bob were located as far from the furnace as practicable. The shape of the final bob design is shown in Figure 2.

Fig. 1

The viscosimeter cup and spindle were supported in an electrical resistance furnace by means of a stainless steel frame arranged inside a ceramic tube. The remotely located spindle drive and sensor were protected from radiant heat by a series of heat shields

The viscosimeter cup and spindle were supported in an electrical resistance furnace by means of a stainless steel frame arranged inside a ceramic tube. The remotely located spindle drive and sensor were protected from radiant heat by a series of heat shields

Fig. 2

The viscosimeter spindle, which weighed approximately 60 g, was machined from a solid piece of platinum to a diameter of 1 cm and an overall length of 4.3 cm, while the internal diameter of the cup was 1.25 cm and the depth 12 cm. The finely etched surface texture seen on the platinum resulted from attack by the molten alkali metal fluoborate but it had no effect on the viscosity measurements

The viscosimeter spindle, which weighed approximately 60 g, was machined from a solid piece of platinum to a diameter of 1 cm and an overall length of 4.3 cm, while the internal diameter of the cup was 1.25 cm and the depth 12 cm. The finely etched surface texture seen on the platinum resulted from attack by the molten alkali metal fluoborate but it had no effect on the viscosity measurements

Variation of Viscosity with Temperature for a K2B3F5O3 Melt

Temperature °CViscosity cPlog(cP)
250236004.37
26095103.98
27044903.65
28023703.37
3008542.93
3204032.61
3501742.24
3801022.01
410731.86
50039.31.59
57524.81.39
62517.81.25
65016.61.22
70014.41.16

At an early stage in the work it was proposed to use platinum-coated stainless steel apparatus, and the most suitable method of coating the inside of the cup was considered to be by plasma vapour deposition. However, in practice, this coating was penetrated rapidly by the melt and detached from the steel. For this reason recourse was made to solid platinum.

The bob was machined from rod, while the cup was made by welding a bottom plate onto a seamless platinum tube, and both proved to be entirely satisfactory. After several experimental runs the platinum bob was attacked slightly, revealing the finely etched grain structure which can be seen in Figure 2. However this minor roughening was insufficient to produce any detectable effect on the measured viscosity.

Under the experimental conditions normally employed, it was found that the bob was self-centering in the cup of molten compound. This enabled a thin flexible 13 per cent rhodium-platinum wire to be used to suspend, and provide torsion to, the bob, the high specific mass of which helped rotational stability.

The viscosity of a typical alkali fluoborate, measured over a range of temperatures by means of the apparatus described, is given in the Table.

The ease of fabrication of platinum and its resistance to corrosion at high temperatures has once again made practicable the investigation of an otherwise difficult problem. It is probable that this technique will be of value for the high temperature study of slags, glasses and other aggressive molten salt systems.

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