Boric acid anomaly
The boric acid anomaly is a term from the chemistry of glasses or glass production. It describes the alternation of boron ions in the glass network , the boron trioxide (B 2 O 3 ) contains, by a triple co-ordination to a four-fold and back. This is associated with considerable changes in the properties of borate glasses and glasses containing boron (e.g. borosilicate glasses ), which are based on the conversion of the rather flat, slightly cross-linked boron trioxide structure after adding alkali oxides into more stable, more three-dimensionally cross-linked alkali borates .
Causes and course of the boric acid anomaly
In a pure borate glass, which consists of 100% B 2 O 3 , the basic element of the glass network is a trigonal-aplanar or pyramidal [BO 3 ] 3− group. This means that each boron ion is surrounded by three oxygen ions which are arranged in a triangle shape around the boron ion. The boron protrudes from the plane of the oxygen ions. These triangles are connected to other triangles by the so-called bridging oxygen at their corners and thus form the glass network.
In contrast to a glass network based on SiO 2 , there are mainly two-dimensional cross-links in a borate glass; in addition, each unit cell is only connected to three neighboring unit cells. Despite a higher Dietzel field strength (named after Adolf Dietzel (1902–1993)) of a boron ion compared to a silicon ion, this explains the relatively low melting temperature of pure borate glasses.
The addition of network converters (e.g. Na 2 O) to a pure borate glass does not initially cause the expected separation point formation according to the following simplified scheme, as is known from silicate glasses.
Rather, the presence of the sodium ion causes a coordination change of the boron ion to a fourfold spatial coordination - [BO 4 ] 5− groups are formed. This increases the degree of networking within the glass network. With increasing addition of alkali, the number of boron ions, which are in fourfold coordination, increases. Only from alkali contents above 14 and up to 22 mol% does the further addition of alkali apparently cause the expected separation point formation according to the above equation and the degree of crosslinking within the glass structure decreases again.
In addition to the alkali content, the temperature also has an influence on the boric acid anomaly. As the temperature rises, the number of boron ions which are four-fold coordinated decreases steadily until there are no more four-coordinated boron ions at around 1000 ° C. It can be seen from this that the formation of the border tetrahedron only takes place below 1000 ° C.
Effect of the boric acid anomaly
The increasing cross-linking of the alkali borate glasses causes a wide variety of changes in properties. For example, glasses in which the boric acid anomaly occurs have viscosity maxima in the composition range around 16 mol% Na 2 O for a constant (low) temperature . At the same time, a minimal coefficient of thermal expansion occurs for this glass system . This effect is one of the reasons for the low expansion coefficient of the borosilicate glasses used as laboratory glass. The boric acid anomaly also has an effect on the density, scratch hardness and chemical resistance of the glasses.
Microstructure investigations to clarify the boric acid anomaly
More recent investigations using X-ray photography have shown that the maximum number of BO 4 tetrahedra is only present at approx. 30 mol% Na 2 O. This fact contradicts the observed maxima or minima of properties, which are all around 16 mol% Na 2 O. From approx. 16 mol% Na 2 O, the BO 4 tetrahedra no longer arrange themselves as boroxole groups in the network, but preferentially form tri- and pentaborate rings. These borate rings are much more wide-meshed than the previous structure. Although the number of spatial cross-links increases continuously up to a sodium oxide content of approx. 30 mol%, the number of bonds per unit volume decreases more and more at the same time. This process continues up to approx. 30 mol% Na 2 O, where about half of the boron is then present in a four-fold cross-linked form. With increasing Na 2 O content, the amount of planar BO 3 groups increases again.
Individual evidence
- ↑ Entry on borate glasses. In: Römpp Online . Georg Thieme Verlag, accessed on February 24, 2013.
- ↑ a b Vogel, Werner. Glass chemistry , 3rd edition. Springer-Verlag, 1992. pp. 156 ff., ISBN 3-540-55171-9 .