Borate glassies

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Recently, Silver and Bray (52) were able to differentiate and to estimate the relative proportions of three- and four-coordinated borons in binary borate glasses. This technique was thus adopted by the present author in order to ascertain the presence or absence of four-coordinated boron in liquid B203 (35). Glassy samples were quenched in liquid mercury from temperatures up to 1400°C but no four-coordinated boron was detected. (The lower limit of detectability was estimated to be about 2%.) Experiments were also carried out on liquid B203 up to 500°C but again no four-coordinated boron was found. It thus appeared that at least up to 500 or 600°C, the structural variation of liquid B203 with temperature is not primarily the result of a boron coordination change of from three to four. 

Compared to the silicates, only a few systematic physico-chemical measurements have been carried out on the binary borates. Whereas in silica and silicates the Si04 tetrahedron is the only building unit, both B08 and BCb groups are known to be present in crystalline and glassy borates (52, 60). Structural interpretations similar to those for the silicates are thus more difficult. At present, no satisfactory model exists for the binary borates. Some very interesting results arc, however, available and these will be discussed below. 

At room temperature the X-band EPR spectrum of a non-annealed borate glass exhibits an asymmetric geff 4.3 signal, characteristic of isolated iron ions in the glassy matrix. As Ta increases from 460 to 550 °C, this line... 

Finally, it is interesting whether local relaxation processes of the glassy matrix facilitate ionic diffusion. For phosphate glasses and borate glasses, 31P and nB NMR spin-lattice relaxation studies gave evidence for some coupling of ion and matrix dynamics below Tg. Moreover, it was reported that local rearrange-... 

Depending on the temperature, nickel borates can form crystals and glassy phases with different degrees of polymerisation, which are very often difficult to identify from the crystallographic point of view [66GME]. The numerous water-free and water-... 

The thermal expansion curve of a phase separated sample containing two glassy phases may show two glass transitions, as is shown in Figure 7.11. This type of curve will be observed if the more viscous phase is continuous, and if the immiscibility temperature for the glass lies above the Tg of the more viscous phase. Curves of this type have been observed for lead borate and barium silicate glasses, where the immiscibility temperature is greater than the Tg of either phase. 

Boron surpasses in heat output every element except hydrogen and beryllium. By itself it burns only partially, even under pure oxygen at higher pressure, forming a glassy, low-melting oxide that envelops the unburnt residue. Reactivity in mixtures, however, is complete and borate formation may contribute significantly to heat output. 

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