Rubidium and Cesium Toxicology

Characteristics :

Atomic weight: Rb 85.468, Cs 132.905

Rubidium and Cesium are monovalent alkaline metals among those which present the highest number of protons and atomic weight. Their activity increases with the increase in the atomic number, resulting in extending the range of vitrification in particular in the ternary glasses of the M2O-CaO-SiO₂ type comparatively to other alkaline metals which have a smaller atomic weight. Contrary to other alkaline metals, they do not present phase separations by non-miscibility in binary glasses of the Rb2O-SiO₂ and Cs2O-SiO₂ types . The phase separation occurs in the borate binary systems Rb2O-B₂O₃ and Cs2O-B₂O₃.

The ionic radius of alkaline metals increases with the increase in the atomic number, thus the ion Cesium is the largest among those monovalent metals. This characteristic relates to the immobility of its ions in an electric field and its extremely positive effects on the insulating properties of glasses. On another side, Cesium shows the lowest bonding strength, as indicated by the thermal expansion of Cesium glasses, which is the highest among that of glasses containing alkaline metals.

Technology :

Experimental fusions of silica-lime glasses with equivalent molar contents of Rb2O or Cs2O showed that these glasses melt and solidify at temperatures similar to those corresponding respectively to potassium and sodium glasses. Binary glasses with Cesium melt at 800-1000°C and crystallize with difficulty. They do not attack excessively refractory structures and they are not difficult to refine. Their viscosity increases with the atomic number of the alkaline metal used. These two elements improve considerably the fusibility of the binary glasses R2O-SiO₂, and glasses with multiple components.

Cesium volitilizes easily during fusion, in particular if it is introduced in the CsNO₃ form. The resulting losses during heating depend on the rate of the reactivity between CsNO₃ and the other components of the composition. The optimal temperature of softening is 700°C, when CsNO₃ reacts quickly with SiO₂. Great losses by volatilization take place when the temperature of reaction is maintained too high or too.

The reaction of CsNO₃ with B₂O₃ starts at approximately 350°C, where the losses by volatilization of borate glasses are also the lowest. The losses by volatilization of Cesium glasses can be decreased by using finely ground sand.

Effects of Rubidium and Cesium on the properties of glass :

Density and refraction index :

they are increased considerably by Rb and Cs comparatively to other alkaline metals. The density of Silica-Lime-Rubidium glasses varies between 2.6 and 3.0 g/cm³, thus reaching the values characteristic of leaded crystal glasses.

Thermal expansion :

it is increased by these two elements, more than by Na or K, in such a way that for a Cs2O-CaO-SiO₂ glass, containing 30% molar of Cs2O, the dilation coefficient reaches a value that may go to 195 X 10-7 °K-1 (between 20 and 400°C).

Viscosity :

It increases with the increase in the atomic number of the alkaline metal used.

Electric properties :

they are influenced by Rb and Cs on an extreme level. With silico-calcic-alkaline glasses, the substitution of Cs2O by Na₂O increases the electrical resistance in a range of values which exceeds by far that of the effects obtained with lead and barium. In this way, the effects of Rubidium are similar to those of lead and thallium. Since the permittivity of silica-lime-alkaline glasses increases from lithium towards potassium, the three elements potassium, rubidium and cesium present an identical influence. The permittivity of acid borosilicate glasses decreases towards cesium.

Together rubidium and cesium maintain the perfect miscibility which they have with binary glasses even in the case of glasses with multiple components, where they act against the coming of the separation of phases and devitrification. Glasses containing Rb and Cs are slightly hygroscopic.

Reference : Chemical Approach to Glass, by Milos .B Volf, Elsevier 1984.

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Translated by Edouard Bastarache

By Edouard Bastarache

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Typecodes	Article by Edouard Bastarache Edouard Bastarache is a well known doctor that has written many articles on the subject of toxicity of ceramic materials and books on technical aspects of ceramics. He writes in both English and French.

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