physics and chemistry of

ISSN 1753-3562

August 2017 Volume 58 Number 4

Saint Malo, France 9–13 July 2018

PHYSICS AND CHEMISTRY OF GLASSES

European Journal of Glass Science and Technology Part B

Abstract deadline 31 December 2017

NaPO3

Wt. Loss, % 0·332 0·241 0·085 0·016

15

10

5

-100

0

100

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Temperature (°C) (b) 5

α

4

f = 7·125 s-1 Cw, ppm 150 1700 2430 10900 19000

βΟΗ

γ

tanδ × 103

15th International Conference on the Physics of Non-Crystalline Solids & 14th European Society of Glass Science and Technology Conference

Internal Friction Q-1 × 103

(a)

3

βΟΗ

2

γ

βΗ 2Ο

1 0 300

https://pncs-esg-2018.sciencesconf.org

400

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100 years of Society of Glass Technology journals

August 2017

Online Manuscript Submission, Tracking and Peer Review System for

Physics and Chemistry of Glasses

The European Journal of Glass Science and Technology

Volume 58 Number 4

European Journal of Glass Science and Technology B CONTENTS

The European Journal of Glass Science and Technology is a publishing partnership between the Deutsche Glastechnische Gesellschaft and the Society of Glass Technology. Manuscript submissions can be made through Editorial Manager, see the inside back cover for more details. Senior Editor Professor R. J. Hand Regional Editors Professor J. M. Parker Professor L. Wondraczek Professor A. Duran Dr A. C. Hannon Professor Bo Jonson Professor M. Liška Professor Y. Yue Managing Editor D. Moore Assistant Editor S. Lindley

115 Influence of phase separation on structure–property relationships in the (GeSe2–3As2Se3)1−xPbSex glass system Anupama Yadav, Myungkoo Kang, Charmayne Smith, Jason Lonergan, Andrew Buff, Laura Sisken, Karima Chamma, Cesar Blanco, Joe Caraccio, Theresa Mayer, Clara Rivero-Baleine & Kathleen Richardson 127 Recent advances in the chemical strengthening of glass Arun K. Varshneya 133 Percolation, phase separation and crystallisation Christian Bocker & Christian Rüssel 142 Phase separation in melting gels Lisa C. Klein, Kutaiba Al-Marzoki & Andrei Jitianu 150 The thermodynamic origin of compositional nanoheterogeneity in glasses Natalia M. Vedishcheva & Adrian C. Wright 156 The role of water in surface stress relaxation of glass Minoru Tomozawa & Emily M. Aaldenberg 165 Homogeneity of modifier ion distributions and the mixed alkaline earth effect in MgO–CaO–SiO2 silicate glasses using molecular dynamics Laura A. Swansbury & Gavin Mountjoy 171 Preferential bonding in low alkaline borosilicate glasses Doris Möncke, Gregory Tricot, Anja Winterstein, Doris Ehrt & Efstratios I. Kamitsos 180 The role of fluoride in the nanoheterogeneity of bioacti e glasses Jamieson K. Christie & Delia S. Brauer

Open Access The Society of Glass Technology is able to offer authors the Open Access route to publication. This option is now available to our authors, on payment of an appropriate up-front one-off fee to meet the costs of electronic production. The fee will be payable by the author after paper acceptance and prior to publication. The fee is £800 for members of the SGT and DGG; for non-members the fee is £1200. VAT is applicable to residents in the EU. Open Access papers are licenced under the Creative Commons licence BY: https://creati ecommons.org/licenses/by/4.0/

Society of Glass Technology 9 Churchill Way Chapeltown Sheffield S35 2 , UK Tel +44(0)114 263 4455 Fax +44(0)8718754085 Email [email protected] Web http://www.sgt.or The Society of Glass Technology is a registered charity no. 237438. Advertising Requests for display rates, space orders or editorial can be obtained from the above address. Physics and Chemistry of Glasses: European Journal of Glass Science and Technology, Part B ISSN 1753-3562 (Print) ISSN 1750-6689 (Online) The journal is published six times a year at the beginning of alternate months from February. Electronic journals: peer reviewed papers can be viewed by subscribers through Ingenta Select http://www.ingentaconnect.co The editorial contents are the copyright © of the Society. Claims for free replacement of missing journals will not be considered unless they are received within six months of the publication date.

Glass is that essential material that is taken for granted because it is there at the interface, holding the beer, allowing our finge s to navigate the internet by touch, providing a transparent layer between divergent environments, holding visibly attracti e produce without tainting them. The objects of the Society of Glass Technology are to encourage and advance the study of the history, art, science, design, manufacture, after treatment, distribution and end use of glass of any and every kind. These aims are furthered by meetings, publications, a web site, the maintenance of a library and the promotion of association with other interested persons and organisations. The journals, Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B and Glass Technology: European Journal of Glass Science and Technology Part A are the regular peer reviewed publications, their origins coming from the Journal of the Society of Glass Technology established in 1917; the split in 1960 to form Glass Technology and Physics and Chemistry of Glasses; and the merger in 2006 with the Deutsche Glastechnische Gesellschaft’s Glass Science and Technology in 2006. As the pure science journal Physics and Chemistry of Glasses reports advances in the basic understanding of glass structure and the physical and chemical properties of inorganic glasses of all types. We are moving towards a better understanding of the random nature of glass structure beyond the very shortest range and the influence this has on the properties and applications of glass. Papers appearing in Glass Technology are concerned with glass making, glass fabrication, properties and applications of glasses or glass ceramics and other related topics. Physics and Chemistry of Glasses accepts papers of a more purely scientific interest concerned with glasse and their structure or properties. Thus the subject of a paper will normally determine the journal in which it will be published. Papers on structure of glass, for example will always appear in Physics and Chemistry of Glasses while those on furnace operation will appear in Glass Technology. In some cases, the way in which a subject is discussed determines the appropriate journal and the Editors will advise authors in such cases. Both journals are published six times per year.

Cover: (a) Internal friction loss peaks of sodium phosphate (Na2O–P2O5) glass containing various amounts of water. The peak at ~100°C is due to water in glass. (b) Dynamic mechanical loss characteristics of soda–lime–silica glass with various water contents. There are loss peaks due to OH and H2O in this glass. In both diagrams, the lowest temperature peaks are attributed to sodium motion in the glass. The role of water in surface stress relaxation of glass Minoru Tomozawa & Emily M. Aaldenberg, this issue, pages 156–164.

Editorial Manager Our online manuscript submission, tracking and peer review system uses the Editorial Manager platform: Authors may submit manuscripts and track their progress through the system, hopefully to publication. Reviewers can download manuscripts and submit their opinions to the editor. Editors can manage the whole submission/review/revise/publish process. Publishers can see what manuscripts are in the pipeline awaiting publication. Email is sent automatically to appropriate parties when significant e ents occur. Glass Technology: European Journal of Glass Science and Technology Part A: http://www.editorialmanager.com/gt Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B: http://www.editorialmanager.com/pcg All manuscript preparation software will be supported by Editorial Manager. Files submitted will be converted to Acrobat PDF file format for distribution to editors and revie ers.

SGT100 Turner Legacy Symposium

Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, August 2017, 58 (4), 150–155

The thermodynamic origin of compositional nanoheterogeneity in glasses Natalia M. Vedishcheva* Institute of Silicate Chemistry of the Russian Academy of Sciences, Nab. Makarova 2, St. Petersburg, 199034, Russia

Adrian C. Wright J.J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading, RG6 6AF, UK

In this paper, it is shown that a consideration of oxide glasses as systems formed from nano-scale chemical groupings, which are products of interactions between the oxide components, enables the following aspects of the vitreous state to be described: the chemistry of the melt quenching process and glass crystallisation, the concept of the metastability of glasses, the reason for the lower density of glasses as compared to crystals, in terms of the intermediate range order, and the chemical origin of phase separation in binary glasses.

Introduction In studies of the vitreous state, it is important to note that glasses are products of chemical interactions between the substances forming the batch, rather than the result of direct melting. This view, which was first proposed as long ago as in the 19th century, is in line with the concepts of the vitreous state, as put forward by Faraday in the 1830–50s, Mendeleev in the 1860s, Turner in the 1920s, Porai-Koshits in the 1930s and Krogh-Moe in the 1960s. Experimentally, the chemical origin of glasses is confirmed by the large negative values of the Gibbs free energies and enthalpies of formation of glasses and glass-forming melts.(1) In Refs 2 and 3, it is shown that the structural changes observed experimentally during the formation of glass networks, viz. BØ3ÆBØ4−ÆBOØ2− or SiØ4ÆSiOØ3−ÆSiO2Ø2− etc., where Ø represents a bridging oxygen atom, are endothermic. Hence, these processes cannot account for the experimentally observed exothermic effect of glass formation in various borate and silicate systems,(1,4) and so they cannot be considered alone. Their progress is associated with another process, viz. the transformations involving the (network modifying) cation–oxygen polyhedra, MOmÆMOn (n>m), as network modifying oxides interact with the network formers. Therefore, binary and multicomponent glasses should be considered as systems formed from a combination of borate and/or * Corresponding author. Email [email protected] Original version presented as an Invited Paper at the Turner Legacy Symposium within the programme of the Society of Glass Technology Centenary Conference and 13th Symposium of the European Society of Glass Science and Technology, Sheffield, UK, 4–8 September 2016. DOI: 10.13036/17533562.58.4.150

silicate basic structural units, and the cation–oxygen polyhedra. This way of approaching the vitreous state is entirely consistent with the concept of cybotactic groupings in both silicate (Porai-Koshits(5)) and borate glasses (Krogh-Moe(6)), and became the basis for the concept of the chemical structure of glasses developed by Shakhmatkin for any system with chemical interaction between its components.(7) Here, glasses are considered as solutions formed from chemical groupings, whose stoichiometry and structure are closely related to those of the crystalline compounds existing in the given system. The thermodynamic concept does not allow the size of chemical grouping to be determined. However, on the basis of small angle x-ray scattering studies of glasses,(8) it can be assumed that the dimensions of these species fall into the nano-scale and, hence, glasses are nano-heterogeneous systems. In a series of paper by the present authors,(2,7,9,10) it is shown that information on the equilibrium concentrations of the chemical groupings, as a function of the glass composition, enables a variety of properties of glasses and their short and intermediate range structure to be calculated, by minimising the Gibbs free energy of the system. A detailed description of the concept of the chemical structure, together with the mathematical formalism for its calculation is given in Ref. 2. In the present paper, this concept is used to describe the processes of quenching glass-forming melts and the crystallisation of glasses, and to explain the lower densities of glasses, as compared to those of crystals. On the basis of information about the enthalpies of formation of binary glasses and crystals, the concept of the metastability of the vitreous state is considered,

150 Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 58 Number 4 August 2017

SGT100 TURNER LEGACY SYMPOSIUM: THERMODYNAMIC COMPOSITIONAL NANOHETEROGENEITY

and the tendency of glasses towards the phase separation is analysed.

1·0 0·9

(1)

Xi

Li*B

2Li*B 3Li*2B

Li*3B

0·7

Chemical reactions that proceed in stable melts (above the melting temperature, Tm) are interactions between the constituent oxides, which lead to the formation of chemical groupings with stoichiometries similar to those of the crystalline compounds existing in a given system

0·6 0·5 0·4 0·3

Li*3B

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3Li*2B

0·1 0·0 0·0

2*1

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0·2

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XLi O

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0· 6

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and interactions between these groupings Li2O.(n−1)B2O3+Li2O.(n+1)B2O3¤2(Li2O.nB2O3)

Li*2B

B2O3

0·8

Chemistry of melt quenching and glass crystallization

Li2O+nB2O3¤Li2O.nB2O3

700 K 1200 K

(2)

From Figure 1, it is seen that, when lithium borate melts are quenched and the temperature decreases from 1200 to 700 K (close to Tg), the fractions of the groupings Li2O.2B2O3 and Li2O.3B2O3 increase, whilst those of B2O3 and Li2O.B2O3 decrease. This assumes the role of the following reactions that lead to the formation of diborate and triborate groupings at the expense of the metaborate grouping and unreacted boron oxide: Li2O.3B2O3+Li2O.B2O3¤2(Li2O.2B2O3)

(3)

Li2O.B2O3+B2O3¤Li2O.2B2O3

(4)

Li2O.2B2O3+B2O3¤Li2O.3B2O3

(5)

Li2O.B2O3+2B2O3¤Li2O.3B2O3

(6)

During the quenching process, the temperature decreases too rapidly for Reactions (3)–(6) to reach the equilibrium state. In addition, the increasing viscosity of the (supercooled) liquid slows down the re-arrangement of the groupings and, finally, leads to the freezing-in of Reactions (3)–(6) at temperatures close to Tg. As a result, interactions between the groupings do not proceed to the extent when the total fraction of Li2O.2B2O3, due to Reactions (3) and (4), and that of Li2O.3B2O3, due to Reactions (5) and (6), each tend to unity. Note that if this happened, the melts of both stoichiometric compositions would crystallise immediately upon quenching, at temperatures that would approach the corresponding melting points, Tm, as described by Equation (6) of Ref. 11. According to this equation, the larger the content of a given chemical grouping in the melt, the more insignificantly the temperature, at which the crystallization of this species starts to proceed, differs from the melting temperature of the corresponding compound. In reality, the characteristic chemical groupings in the glasses of the stoichiometric compositions always co-exist with the groupings that are the initial species in reactions

Figure 1. Chemical structure of lithium borate glasses and melts, at 700 and 1200 K, respectively. The ratio of oxides, Li2O and B2O3, in the chemical groupings is denoted as mLi.nB [Colour available online] of the type (3)–(6). Thus, as is seen from Figure 1, the chemical structure of the glass of the diborate composition, 0·33Li2O.0·67B2O3, incorporates the following groupings, whose content at 700 K is given in brackets: Li2O.2B2O3 (85·6%), Li2O.3B2O3 (8·4%), Li2O.B2O3 (5·5%) and B2O3 (0·5%). Crystallisation of this composition occurs when the sample is heat treated, for a sufficient time, at a temperature above Tg, which is high enough for Reactions (3) and (4) to dominate in the metastable melt, thus resulting in a gradual increase in the content of the grouping Li2O.2B2O3 until its fraction tends to unity.

Metastability of the vitreous state The metastability of the vitreous state is a matter of common knowledge in glass science. Glansdorff & Prigogine(12) define a metastable system as “stable with respect to a small perturbation but unstable with respect to larger perturbations.” In Ref. 13, p. 467, the notion of metastability is explained as follows: “The term metastable is used to describe a definite equilibrium, which is nevertheless not the most stable equilibrium at the given temperature; …” However, it is known that glasses can exist for hundreds if not thousands years without losing their important properties such as the transparency, strength, hardness, or colour. In his book,(14) Jones presents an approach to the vitreous state that helps to understand this inconsistency: “It is important to realize that a super-cooled liquid is in internal thermodynamic equilibrium; … If, however, freezing is able to begin, it will inevitably spread throughout the liquid. In the presence of crystals or crystal nuclei, the system is not in internal equilibrium.” From the above, it can be concluded that the notion of metastability applies to glass only when glass is considered with respect to the crystalline state. This is illustrated in Figure 2, which shows the experimental enthalpies

Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 58 Number 4 August 2017

151

SGT100 TURNER LEGACY SYMPOSIUM: THERMODYNAMIC COMPOSITIONAL NANOHETEROGENEITY 0·0

0·1

0·2

0·3

0·4

0·5

∆Hfgl and ∆Hfcr, kJ/mol

X Li O 2

0

∆Hfgl=−(63·3)Li.2B−(9·1)Li.3B−(6·6)Li.B=−79·0 kJ/mol

-20 Glasses

-40 -60 -80

from Equation (8), is

Crystals

-100

Li*3B Li*2B

Li*B

Figure 2. Experimental enthalpies of formation of lithium borate glasses and crystals from oxides.(4) The values of the enthalpies refer to the mole of the type xLi2O.(1−x)B2O3 [Colour available online] of formation from oxides, according to Reaction (1), of lithium borate glasses (∆Hfgl) and crystals (∆Hfcr), these values being referred to the mole of the type xLi2O.(1−x)B2O3.(4) It is seen that glasses form with lower exothermic effects than crystals and, hence, their formation is somewhat less energetically profitable for a given system than the formation of crystalline compounds. It is only in this sense that the vitreous state can be considered as metastable, and the difference between the enthalpies of formation of crystals and glasses, represented by the heat of crystallisation (∆Hcryst), characterises the degree of metastability of a given glass in relation to the relevant crystal. Thus, for the glass of the diborate composition, 0·33Li2O.0·67B2O3, the heat of crystallisation calculated from the experimental enthalpies ∆Hfcr and ∆Hfgl (4) is: ∆Hcryst=∆Hfcr−∆Hfgl=−90·6+77·4=−13·2 kJ/mol

(7)

This value of −13·2 kJ/mol is the measure of metastability of the diborate glass with respect to the crystal Li2O.2B2O3. The concept of the chemical structure provides an explanation of the lower exothermic effect in respect of the formation of glasses as compared to crystals. As is shown in Ref. 7, the enthalpy of formation of glasses (∆Hfgl) is calculated as an additive function of the enthalpies of formation from oxides of the crystalline compounds forming in a given system (∆Hfcr,i): ∆Hfgl=∑ni.∆Hfcr,i

(8)

where ni represents the number of moles of the grouping i present in the chemical structure of a given glass. The glass of the diborate composition, 0·33Li2O.0·67B2O3, incorporates the chemical groupings Li2O.2B2O3, Li2O.3B2O3 and Li2O.B2O3, whose numbers of moles ni are determined in the course of the calculation of the chemical structure of the glass.(2) The experimental values, ∆Hfcr,i, for the relevant crystalline compounds are reported in Ref. 4. The enthalpy of formation of the diborate glass, calculated

(9)

where the indices Li.2B, Li.3B and Li.B indicate the types of groupings that make contributions to the enthalpy, ∆Hfgl. Note that the difference between the calculated and experimental enthalpies, −79·0 kJ/mol and −77·4 kJ/mol,(4) respectively, is comparable to the uncertainty of the experimental measurements, which confirms the reliability of the calculations. From Equations (8) and (9), it follows that, due to the interplay between the number of moles of each grouping present in the given glass, ni, and the enthalpies of formation of the relevant crystals, ∆Hfcr,i, the sum of the contributions from all three groupings is less negative than the enthalpy of formation of crystalline Li2O.2B2O3. From Figure 2, it is seen that, although lithium borate glasses form with lower exothermic effects than the corresponding crystals, their enthalpies of formation from the oxides, Li2O and B2O3, are still large (up to −80 kJ/mol), which leads to the formation of stable products (borates). This explains why, under certain conditions (temperatures well below Tg, pressures not exceeding 1 atm, and in the absence of chemical attack), glasses remain unchanged for a very long time.

The density of glasses and crystals It is generally known that the densities of glasses are normally lower than those of crystals of the similar compositions. The concept of the chemical structure enables this observation to be quantitatively explained. In this approach, a justified assumption is made that the chemical groupings introduce into B2O3-containing glasses various superstructural units, Ref. 2, Table 5.1. These units, which characterise the intermediate range order in the glass structure, “comprise well defined arrangements of the basic borate structural units, with no internal degrees of freedom in the form of variable bond or torsion angles”.(15) In lithium borate glasses, the grouping Li2O.2B2O3 introduces diborate units (or rings), the grouping Li2O.3B2O3 brings triborate units, the unreacted B2O3 – approximately equal numbers of boroxol rings and independent BØ3 triangles with all bridging oxygen atoms. The grouping Li2O.B2O3 introduces BØ2O− basic structural units, viz. chains of triangles with one nonbridging oxygen atom. Figure 3 shows all of these units and their content (%) in a mole of the glass of the diborate composition, 0·33Li2O.0·67B2O3. The content is calculated using equations similar to Equations (5.24)–(5.30) of Ref. 2. Since it is known that crystalline Li2O.2B2O3 comprises only diborate groups,(16) it is clear that, in the corresponding glass, a variety of the units can hardly be packed as closely as diborate groups in the crystal. This certainly results



(a) Na2O–SiO2 system

Li2O.2B2O3

Diborate, 100%

model, melts at 1473 K Symbols - glasses, Ref. 29

Glass, 700 K

Diborate, 74·8%

BØ2O¯, 10·1%

Triborate, 14·7%

Boroxol, 0·2%

Qn

Crystal

BØ 3, 0·2%

Figure 3. Superstructural units in crystalline and vitreous lithium diborate, Li2O.2B2O3 [Colour available online]

Phase separation in binary glasses A comparison of the sets of crystalline compounds existing in various glass-forming systems with information on the immiscibility regions in some of them (see Table 1) reveals that the only systems that are prone to phase separation are those in which crystals with a low content of modifying oxides do not form. Usually this implies a content below 20 mol% M2O (MO), which corresponds to the stoichiometry M2O(MO).4B2O3(SiO2). Here, special attention should be paid to the systems M2O–B2O3 (M=Li, Na, K, Rb and Cs) because, in 1968, Shaw & Uhlmann(20) reported the results of studies by electron microscopy, according to which immiscibility regions were found in glasses of all five systems. Later, in 1977, Golubkov et al(21) convincingly demonstrated by small angle x-ray scattering that it is only lithium borate glasses that tend to phase separate and, in addition, this occurs over a considerably narrower region than that reported by Shaw & Uhlmann. The question arises as to the relationship between

Q4

Q3

Q2

Q0

L1 + L2

0·0

Q1

0·1

0·2

0·3

X Na O

0·4

0·5

0· 6

2

(b) Na2O–B2O3 system Fractions of basic structural units

in a larger molar volume of the diborate glass as compared to that of the diborate crystal. Hence, the density of this glass (d=2·27 g/cm3(17)) is lower than the density of the crystal Li2O.2B2O3 (d=2·44 g/cm3(18,19)).

1·1 1·0 0·9 0·8 0·7 0·6 0·5 0·4 0·3 0·2 0·1 0·0

1·0 0·9 0·8 0·7 0·6 0·5 0·4 0·3 0·2 0·1 0·0 0·0

BO3/2

BO1/2O22BO2/2O-

Experiment: Ref. 5 Ref. 24 Ref. 25 Ref. 25 Ref. 25

(BO4/2)-

Model: Lines

0·1

0·2

0·3

0·4 X Na O

0·5

0·6

0·7

2

Figure 4(a, b). The short range structure in sodium silicate and sodium borate glasses: experimental data (symbols) and calculated (lines) – after Ref. 2. (The references cited in both graphs can be found in Ref. 2) [Colour available online] the absence, in a given system, of crystals with a low M2O(MO) content and a tendency towards phase separation. From Figure 4, it is seen that information about the short range order in sodium silicate glasses does not give any answer because the concentration

Table 1. The sets of crystalline compounds and the regions of phase separation present in various glass-forming systems. For brevity, the compositions of compounds nM2O(MO).mB2O3(SiO2) are denoted nM.mB(Si) System

Crystalline compounds(22)

Regions of the metastable phase separation, mol% M2O (MO)(22,23)

Li2O–B2O3 MgO–B2O3 CaO–B2O3 BaO–B2O3 Li2O–SiO2 Na2O–SiO2 CaO–SiO2 BaO–SiO2 Na2O–B2O3 K2O–B2O3 Rb2O–B2O3 Cs2O–B2O3

Li.3B, Li.2B, Li.B, 2Li.B Mg.2B, 2Mg.B, 3Mg.B Ca.2B, Ca.B, 2Ca.B, 3Ca.B Ba.4B, Ba.2B, Ba.B, 2Ba.B Li.2Si, Li.Si, 2Li.Si (Na.4Si)*, 3Na.8Si, Na.2Si, Na.Si, 3Na.2Si, 2Na.Si Ca.Si, 3Ca.2Si, 2Ca.Si, 3Ca.Si Ba.2Si, 2Ba.3Si, Ba.Si, 2Ba.Si Na.9B, Na.5B, Na.4B, Na.3B, Na.2B, Na.B, 2Na.B K.5B, 5K.19B, K.3B, K.2B, K.B Rb.5B, 5Rb.19B, Rb.3B, Rb.2B, Rb.B Cs.9B, Cs.5B, 3Cs.13B, Cs.3B, Cs.2B, Cs.B

0–8 0–40 0–33 0–18 0–33 0–20 0–40 0–28 No phase separation No phase separation No phase separation No phase separation

* The existence of the compound Na2O.4SiO2 is reported in the Powder Diffraction Files (PDF 2, N12-0102 & N38-0020). However, it is not shown in the phase diagram,(22) which can be considered as an indication of a very low thermodynamic stability of Na2O.4SiO2 towards a dissociation into SiO2 and 3Na2O.8SiO2 or Na2O.2SiO2.


153


Figure 5. Experimental enthalpies of formation of sodium silicate(24) and sodium borate (after Ref. 4) glasses from oxides. The values of the enthalpies refer to moles of the type Na2O.nB2O3 and Na2O.mSiO2 [Colour available online] dependences of Q4 and Q3 species do not have any specific features when crossing the boundary of the phase separation region, but are as smooth as the concentration dependences of the basic structural units in sodium borate glasses, which are single phase. It is a consideration of the chemical interactions that proceed in course of glass formation in both systems, together with the enthalpies of these reactions, which can help in understanding the nature of phase separation in glasses. Figure 5 shows the enthalpies of formation of sodium silicate and sodium borate glasses over the extended composition region. The enthalpies, ΔHf, refer to the interactions between the oxide components, which proceed in a way similar to that described by Reaction (1). The values of ΔHf refer to moles of the type Na2O.nB2O3 and Na2O.mSiO2. It is seen that both dependences are linear over the medium (Na2O–B2O3) and high alkali (Na2O–SiO2) concentration regions, but they start deviating from linearity at approximately the composition with 20 mol% Na2O in case of borate glasses and 33 mol% Na2O for silicate glasses. This corresponds, respectively, to the stoichiometries Na2O.4B2O3 and Na2O.2SiO2. As the Na2O content decreases below 20 mol% Na2O in borate glasses and below 33 mol% Na2O in silicate glasses, the enthalpies ΔHf remain practically constant in both systems, at least within the limits of the experimental errors. This observation can be explained as follows. The linear segments of both dependences correspond to the acid–base interactions between sodium oxide with boron oxide and silica, which proceed with large exothermic effects. It is seen that these effects notice-

ably increase as the content of glass-forming oxides becomes larger, and this trend is observed until the content of boron oxide and silica becomes close to 80 and 67 mol%, respectively. A gradual decrease in the change of the enthalpies ΔHf with a further increase in the content of B2O3 and SiO2 points to the fact that the chemical groupings Na2O.4B2O3 and Na2O.2SiO2 combine with the excess of the respective glass-forming oxides, giving rise to the formation of the complexes (Na2O.4B2O3).nB2O3 and (Na2O.2SiO2). mSiO2, according to the reactions Na2O.4B2O3+nB2O3¤(Na2O.4B2O3).nB2O3

(10)

Na2O.2SiO2+mSiO2¤(Na2O.2SiO2).mSiO2

(11)

The enthalpies of these reactions are much lower (a few kcal/mol) as compared to the enthalpies of formation of the chemical groupings from oxides (tens of kcal/mol). In addition, as follows from the authors’ estimates, the enthalpies of Reactions (10) and (11) change insignificantly as the values of the coefficients n and m increase. These factors result in practically constant values of the enthalpies ΔHf over the low alkali regions in both systems. The difference between sodium borate and sodium silicate glasses is in the fact that, in the system Na2O–B2O3, three low alkali crystalline compounds form, Na2O.4B2O3, Na2O.5B2O3 and Na2O.9B2O3. Due to this, the complexes (Na2O.4B2O3).nB2O3 are thermodynamically stable with respect to the dissociation into Na2O.4B2O3 and B2O3 because some of them have crystalline analogues. In sodium silicate glasses, the complexes (Na2O.2SiO2).mSiO2 are thermodynami-



cally stable over the region between the compositions with 33 and 20 mol% Na2O, which is due to the existence of the compound 3Na2O.8SiO2, which falls into this concentration interval. (The compound Na2O.4SiO2 is not considered here because of its low thermodynamic stability – see Table 1.) In glasses containing less than 20 mol% Na2O, the complexes (Na2O.2SiO2).mSiO2 have no crystalline analogues, and hence their formation is not energetically profitable for the system. Due to this, they are unstable with respect to the dissociation into Na2O.2SiO2 and SiO2. Similar arguments apply to all of the systems listed in Table 1.

3. 4.

5. 6. 7. 8. 9.

10. 11.

Conclusions It is shown that the concept of the chemical structure provides the basis for a description of melt quenching and glass crystallisation as chemical interactions between nano-scale groupings, which are products of the chemical interactions between the constituent oxides. This concept also allows the well known fact concerning the lower density of glasses, as compared to that of crystals, to be quantitatively explained in terms of their intermediate range order. It is demonstrated, on the basis of the enthalpies of formation of the system Li2O–B2O3, that the metastability of glasses only makes sense when they are considered together with crystals, and it is established that binary glasses tend to phase separate only in the systems where no low alkali/alkaline earth crystalline compounds form.

12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

23.

References 1. Shakhmatkin, B. A. & Vedishcheva, N. M. J. Non-Cryst. Solids, 1994, 171, 1. 2. Vedishcheva, N. M. & Wright, A. C. In: Glass. Selected Properties and

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