Measurements on Molten Metals

Measurements on molten metals. The maximum bubble pressure method has proved one of the most satisfactory, but sessile drops, and drop-volumes have also been used with success.2 The principal difficulty lies in the proneness of metals to form skins of oxides, or other compounds, on their surfaces and these are sure to reduce the surface tension. Unless work is conducted in a very high vacuum, a freshly formed surface is almost a necessity if the sessile bubble method is used, the course of formation of a surface layer may, if great precautions are taken, be traced by the alteration in surface tension. Another difficulty lies in the high contact angles formed by liquid metals with almost all non-metallic surfaces, which are due to the very high cohesion of metals compared with their adhesion to other substances. 

Urbain, G. Viscosity and density measurements on molten metals Scite 1F in Nat. Phys. Lab. Symp. 9. London Her Majesty s Stationery Office 1959. 

Equation [7.39] is, in fact, approximate since the bubble is flattened by the variations in hydrostatic pressure across its surface and it has a tendency to spread across the thickness of the tube waU. For these reasons, the method is most widely used for the comparison of surface free energies. It is very convenient for the study of the temperature variation of 7 and has been used for measurements on molten metals. The bubble surface is always freshly formed. 

The maximum bubble pressure method is good to a few tenths percent accuracy, does not depend on contact angle (except insofar as to whether the inner or outer radius of the tube is to be used), and requires only an approximate knowledge of the density of the liquid (if twin tubes are used), and the measurements can be made rapidly. The method is also amenable to remote operation and can be used to measure surface tensions of not easily accessible liquids such as molten metals [29]. 

It is thus apparent from what has been discussed so far that the measurement of the surface tension of liquids is an important analysis. The method to use in the measurement of y depends on the system and experimental conditions (as well as the accuracy needed). For example, if the liquid is water (at room temperature), then the method used will be different from that if the system is molten metal (at very high temperature, ca. 500°C). These different systems will be explained in the methods described in this section. 

CAUTION As a safety measure, place a metal pan under the apparatus. If the glass vessel should crack during the experiment the molten mixture would catch fire on con tact with the air sand may be used to extinguish the fire.]... 

For the testing of refractories with reference to their electrical conductivity at higher temperatures a cup of 60 mm. outside diameter, 65 mm. height and having a wall 2.5 mm. thick is made from the material in question and fired. This specimen is placed in a suitable furnace. Electrical contact is provided by the use of molten metal on the inside of the cup and its immersion in a shallow metal bath on the outside. An alternating current, 60 cycle, of 500 volts is employed and the current passing through the material measured by means of a sensitive milliammeter or a dynamometer watt meter. The temperature is determined by means of insulated thermocouples immersed in the molten metal on the inside of the cup. 

A more sophisticated technique involves measuring the damping of the oscillations of a body immersed in a fluid. The oscillating body may be a sphere, a thin disk, or a cylinder. Research instruments of this type can attain better than 0.5% accuracy (0.05% in the best cases) for both vapors and liquids over a wide range of conditions. In a variant known as the oscillating-cup method, the fluid is contained inside a hollow body this method is more often used for high-temperature measurements on fluids such as molten metals or molten salts. 

Pure oxygen-less melts contain no oxide ions in any form, and, therefore, such pure melts cannot serve as donors of O2-. The melts, which are solvents of the second kind, can affect acid-base interaction on their background in two manners by fixation of oxide ions entering in the melt and by solvation of the conjugate acid or base. However, the ionic solvents of the second kind, used in practice for different measurements and applied purposes, contain admixtures of oxide-ion donors, which are formed in the melt from initial admixtures of oxo-anions such as SO4-, COf- or OH-. The second way of appearance of oxide ion admixtures in molten media is characteristic of the melts based on alkali metal halides the process of high-temperature hydrolysis of the said halide melts results in the formation of hydroxide ions and, after their dissociation, of oxide ions ... 

The major problem in using a single wavelength radiation thermometer to measure the surface temperature is the unknown emissivity of the measured surface. The emissivity is the major parameter in the spectral radiance temperature equation (Eq. 16.28) for the temperature evaluation. Objects encountered for temperature measurements are often oxidized metal surfaces, molten metal, or even semitransparent materials. On these surfaces, the emissivity is usually affected by the surface temperature and the manufacturing process for these materials. 

Reactions of considerable extent occur even in very dilute solutions of non-metals in alkali metals. Small concentrations of dissolved non-metals also influence physical properties of the molten metals. Very exact analyses are necessary to define the chemical potentials of non-metals in alkali metals. Oxygen can be removed from sodium, for instance, to such a degree, that only 0.1 to 0.01 wppm remain in solution. Electrochemical cells have the ability to estimate such extremely low concentrations. Carbon in liquid alkali metals, which are in contact with austenitic stainless steels, is in the same range of concentrations. Thus, only activity meters, based on gasanalytical devices or electrochemical cells, are able to measure such low carbon concentrations. There is still need for the development of analytical procedures to estimate nitrogen in alkali metals with the same sensitivity and accuracy. 

Early in their work on molten salt electrolytes for thermal batteries, the Air Force Academy researchers surveyed the aluminum electroplating literature for electrolyte baths that might be suitable for a battery with an aluminum metal anode and chlorine cathode. They found a 1948 patent describing ionicaUy conductive mixtures ofAlCh and 1-ethylpyridinium halides, mainly bromides [6]. Subsequently the salt 1-butylpyridinium chloride -AICI3 (another complicated pseudo-binary) was found to be better behaved than the earlier mixed halide system, so the chemical and physical properties were measured and published [7]. I would mark this as the start of the modern era for ionic liquids, because for the first time a wider audience of chemists started to take interest in these totally ionic, completely nonaqueous new solvents. 

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