Alkali metal-based glasses

Alkali Metal-Based Glasses/Interconnects Interface... 

The polyions postulated in solution all have known structural analogues in crystalline borate salts. Investigations of the Raman (66) and B nmr (67) spectra of borate solutions have confirmed the presence of three of these species the triborate (3), B303(0H) 4, tetraborate (4), [B405(0H)2 J, and pentaborate (5) Bb06(0H) 4, polyanions. Skeletal structures were assigned based on coincidences between the solution spectra and those solid borates for which definitive structural data are available (52). These same ions have been postulated to be present in alkali metal borate glasses as well. 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

Some elements nsed in SRDs have chemical as well as radiological toxicity. For example, cesium, an alkali metal, will explode if exposed to water. Cesinm hydroxide, a strong base, is qnite corrosive, and can attack glass. Clinicians and responders will need to be aware of the spectrum of risk posed by chemicals nsed in SRDs and other devices (1). 

A glass membrane electrode was used to measure the pH of the silicate solutions. Adsorption of the smaller alkali metal cations, K, Na, and Li, caused an underestimation of the pH. The pH error reached several pH units for the most alkaline Li silicate samples. On the other hand, the errors for Na silicates were around 0.5 pH unit and the errors for larger cations were relatively small. Corrections to the measured pH were made by comparison of the electrode response for the silicate solutions with that for alkali metal hydroxide solutions of known concentration. Since the hydroxide content of the base solutions is known, the interference from cation adsorption was calculated and added to the pH measured for the silicate solutions (16). 

The alkali metals—lithium, sodium, and potassium—are logical choices for anodes in a sulfur-based electrochemical cell. All three have been incorporated into cells, and lithium and sodium remain under serious consideration. The lithium-sulfur combination is the topic of another chapter in this volume and will not be discussed further. Two types of sodium-sulfur cells have been constructed. One type uses thin-walled glass capillaries as a cell divider, and the other uses various sorts of ionically conducting sodium aluminate for this purpose. Of the two, the latter seems to hold the most promise and certainly has generated the most interest and enthusiasm (1). Because of the unique properties of the solid electrolyte cell separator this battery is also probably the most interesting from a purely scientific point of view. 

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