Conventional calorimetry has always the difficulty of choosing between near to equilibrium working conditions and high thermal
ramp rates. Thus, either the transport phenomena and sample homogeneities are good but the signals become weak due to thermal
losses, or the signals are sharp, but strong gradients across the sample lead to chemical and thermal heterogeneities. The
described pulsed fluidized bed technique, by strongly stirring the sample, allows good sample homogeneities even at high ramp
rates. Moreover, the permanently regenerated cover gas allows as well a good heat transfer towards the thermocouples as a
constant atmosphere composition leading to very precise onset temperatures.
Pure alkali metal preparation is a complex problem: in most available commercial samples, all of them are simultaneously present. Conventional separation techniques are not always effective enough to reach parts per million total impurity levels. However, near the melting point, superficial segregation occurs. A zone melting derived technique coupled with a specifically developed solvent extraction process allows the total impurity content of sodium to be lowered below a few parts per million. The described thermal process, although using chemical reactions, is purely physically steered: it purifies as well potassium containing sodium as sodium containing potassium. 4 alkali metals are considered: Li, Na; K, and Cs.
Solid state purification generally requires efficient diffusion mechanisms in order to allow impurity migration towards the sample surface, from which it can be removed by a suitable mean. Since solid state diffusion just becomes efficient near the melting point, generally high working temperatures are required, resulting in expensive, energy consuming processes. The addition of small amounts of a common liquid solvent of both matrix and impurity results, even at low temperatures, in effective diffusion mechanisms the thermodynamical aspects of which are discussed in this work. Thermal cycling enhances the efficiency of the described process. Its concerns industrial and analytical applications.
Authors:A. Hadj Mebarek, C. Cogneville, S. Helle, and S. Walter
The formation of alkali metal alcoxides by an alcohol reacting on the elemental metal itself cannot be completed under stoichiometric
conditions. As a consequence of solvation, the chemical activity of the reacting alcohol is drastically reduced. Thus, the
reaction cannot undergo completion without a large excess of alcohol with respect to the alkali metal. Moreover, solvation
processes can drop the reaction kinetics down to nearly zero. When an excess of alkali metal is reacted with alcohol, the
heat accumulated by solvation can be suddenly released by an addition of pure alcohol. Extremely dangerous thermal runaways
can be started this way.
Authors:A. Hadj Mebarek, S. Walter, G. Killé, and C. Cogneville
Alkali metal alkoxides can be formed by the direct reaction of alkali metals with the corresponding alcohol. Under certain conditions, however, these reactions become dangerous. One of the reasons for the instability build-up in the reaction mixture is related to the electrochemical behaviour of the heterogeneous medium. Another reason is the instability introduced by the simultaneous presence of oxygen and alkali metal atoms in the reagents. Accelerating rate calorimetry is an excellent way to determine safe working conditions for the handling of such compounds. The hazards that are encountered are discussed by means of some examples.