- Email: [email protected]
Chemical relaxations of ionically conducting glasses
349
Chemical
relaxations
G.D. Chryssikos
of ionically conducting
glasses
and E.I. Kamitsos
Many amorphous solids conduct ions and find applications due to their Despite intensive research on the versatile preparation and good stability. matter. the fundamental mechanism of ionic conduction in &asses remains eIusive. and the existing theories often contradictory [I]. Meanwhile, “...rk resolrrrir~n of iiiffh-twr interprert2rion.s can CN(Y he rnde on r/w hu.si.s of u&pure rheoq of chrge trurrsport p~i~ce.s.sc.s ur the rndwfflur Itrvd. . ..uird thar m do 1hi.s defirtire xfpprriiz~fi .srr:fcrrf rul and orher me need.s srffficiently properl> evidtww...” [2]. In this report. struc&,r-al and relaxational aspects of ionicaIIy conducting glasses are discussed. Structure is viewed in the broadest sense and extends from the local chemical bonding to long range interactions affecting the morpho!o,T of the glass. Relaxations of chemical nature which occur upon melt co0:ir.g are alsc considered, since it is their “freezing” that determines the structure of glass.
2. TI-IE DIELECTRIC
REGIME
Bob Cole entered the field of glassy ionics (“us u nmfce", he used to say) in the mid-eighties. I-Ie spent a few years developing new eqerimental
350
techniques and adapting others to achieve high frequency, high temperature dielectric data acquisition of ionically conducting sohds [S]. Finally, in 1989 he published an updated version of the well known Wang and Angel1 plot of absorptivity versus frequency [4], by filling in data in the GHz frequency range that had remained till then unaccessible (Figure 1) [S]. This work was important for a number ofreasons. First. it was confirming that the log abso@vity vs log frequency plot approaches the slope of one, before joining smoothly the far-inf+ared trace [6], as envisaged by Wong and Angel1 [4]. Seccnd, it was supporting the thesis of Moynihan and coworkers concerning the frequency - temperature superposition of the dielectric modulus spectra (ivi*=I/E*), as we11 as the near coincidence of rhe activation energies deduced from the temperature dependence of dc conductivity and M” maxima 173. All the above were pointing towards universal aspects of relaxations in glasses, such as those developed by Ngai et ul. [S].
I
0
2
4
8 log (f in Hz) 6
10
12
14
F:gurc 1. Absorptivity spectrum of glassy Na,0.3SiO, at various temperatures. taken from ref. IS]. Arrows mark the positions of the imaginary dielectric moduIus maxima.
With reference to figure I, it can be argued that as the Frequency increases above that of ~xi~num M”, the temperatare dependence of the ~so~tivi~ decreases systematically, and eventuaIIy vanishes in the far-inf?ared. increasing frequency implies that the interactions between the material and the electromagnetic radiation become increasingly localized in space 191,and that the response of the system becomes more dependent on its particular chemical characteristics. This is reflected on the way vib~tjonal data are co~only discussed, ie. with Iittle or no reference to structural trends of more general applicability. It is probable that if such genera1 trends cot&l be detected, they would refer to very fundamental concepts such as the vitrification of the quenched melts and the conditions under which this is experimentally feasible.
3. FAR-INFRARED
SPECTROSCOPY
The first vibrational frequency range where universal structural characteristics of ionically conducting glasses should be sought, is the commonIy referred to as far-infrared (ca. 30-400 cm-‘), Pioneering work by Exarhos and Risen [ IO] and continuing research in our laboratory [ 1 If has demonstrated that glasses containing ionic modifiers such as alkalis and alkaline earths, exhibit in this frequency region modes which correspond to the “rattling” of the cations in sites provided by the network. Within the wide spectrum of cationic motions, extending over more than twelve decades in sequent or time, those active in the far-infrared are the most localized ones, since cations do not escape Corn their potential wells acquired upon cooling the melt through the glass transition. The frequency maxima, widths, and integrated intensities of the far-infrared bands are dependent on composition. the cationic mass and the energetics of the cation-site ionic inte~ctions [ 121. Yet. a number of system-independent characteristics of these bands have been identified. The dispersion of glasses in the fm- inked is always heavily damped, and has a character intermediate to relaxations and to true resonances. The frequency maxima observed are In good agreement with the attempt frequencies in the pro-exponential factor of the Arrhenius type dc conductivity and the fx inf?ared condu~~~~ maxima have been considered by Angel1 as limiting values of ionic conductivity at very high temperatures [ 133. Within the coiltext of the above, there is one characteristic of the cationsite vibrations which has resertied special attention. The far-inf?ared baMls of almost all glasses investigated exhibit a charzteristic asvmmetry which can in most cases be accounted for by the existence of two siperimposed Gaussian
E;imre2, Far-intied spectraof 0.3M30 0_7B,O, glasses,takenfrom lef. [ 151. f)ec;or.vo;u%ed componentbands are shown. bandshapes, both due to cation rattling modes 111, 141. Typical examples of such spectra of alkali borate &asses are shown in Figure 2, together with their deconvolution into component bands. The widespread appearance of this behaviour implies that the origin of the observed Gwen cannot be the chemical details of the charge-balancing network anions, which can change enormously with composition and still leave the f&r&Cared profile relatively unvexed 1141. Instead, it was argued that at least two dist~butions of cationsite environments exist in glass and can be probed through the cation-motion bands[ 11, 143. Such .resuIts .have been interpreted as supporting the hypothesis that the glass structure is mi~~heterogeneous and that ionic t-sport occurs mostly through preferred “pathways” [15].
4. THE STRUCTURE
OF THE GLASS
NETWORK
The next question to be addressed concerns the possibility of identifying chemically microheterogeneities in glass. The usehI frequency range is now at the upper limit of the Wong and Angel1 plot, and can be accessed by midinfmred and Raman spectroscopies, both probing the vibrations of the network k 2ding units. The search for suitable systems has led to the alkali metaborates iM20.B,0,, M:alkali), because it was thought that should structural microheterogeneities exist, they could be “decorated” differently by the rich borate chemistry. -4lthough a detailed discusr;ion of the vibrational spectra of M,O.B,O, glasses cannot be attempted here, a number of results are of relevance to the present discussion. First, it was found that the local chemistry of these glasses is dominated by the isomerization reaction B020--B04-
(1)
where 0, 0, denote oxygen atoms bridging two boron centres, or terminal, respectively [ 163. Given the different cross linking ability of the above metaborate triangles (B0,0-) and tetrahedra (Bta,.), the temperature dependence of equation (1) is expected to play an important role in the vitrification of the metaborate melts. More importantly, a number of common glass-forming oxides (e.g. GeO,, As,O, etc) exhibit similar isomerizations between local structures of different coordination to oxygen. In silicate glasses the disproportionation equilibrium
24”OQn-‘+qn+’
(2)
where n denotes the number of bridging oxygen atoms of the silicate tetrahedron Q” ,seems to play a similar role 1171. The chemical diversity at the local structural level, at which glassy and crystalline states do not differ much anyway 1181,explains the very large number of the ways these local units can combine in forming intermediate range groups. At least one dozen of such groups are known in metaborates and more are expected to be found as their investigation continues. Their identification in the provided that adequate reference vibrational spectra is stlrightforward, (crystalline) compounds cm be isolated and studied. As an axample we mention here the identification of the sequence of polyhedra in complicated networks w?zre metaborate triangles and tetrahedra coexist, through the Raman active stietching vibration of B-O- terminal bonds [t9]. Studies of this sort have allowed the chemical characterization of microdomains in glasses. In the
particular case of lithium met&orate (LiBO1) these microdomains exhibit structures similar to those found in a-, p’-, and y-LiBO,, three of the known LiBO, polymorphs, and, hence differ considerably in IzGcing density [Xl. This granular structural picture is not unique to borates and the structure of many glasses can be described in terms of polymoqhic microdomains [2 11.
5. STRLJCTUliAL
ASPECTS
OF VITRIFlCATION
Apart from the above static structural description of glass, it is the dynamic aspects of chemical transformations that influence vitrification. 5pon quenching, the melt is envisaged to move through a number of Iocal minima in the potential energy surface before being trapped in those of the me&stable glassy state. Since, in most cases, the characteristic quenching times involved are too short to aIlow for the acquisition of vibrational data, different approaches are needed to map the chemistry of vitrification. One of these approaches is based on the remark that the glass has stored in its structure the memory of structural events that led to its formation. Devitrification can then be employed to allow the chemical relaxation of the system towards the cIosest-lying crystalline compound. Once the mapping of the devitrification process as a function of temperature and time is complete, the resulting crystalline products can be studied conveniently, compared to the structure of glass and provide answers about the possible chemical pathway(s) of glass formation. An example of such studies is offered by the systematic investigation of devitrification in lithium-sodium metaborate glasses, xNaBO?.( I-x)LiBO, [22]. Three crystalline lithium metaborate poIymorphs, a mixed lithium-sodium met&orate, Li3NaB,08, and the well known ring sodium metabomte compound, have been isolated upon devitrifjring glasses of increasing x. The temperaturecomposition Gomains of these crystalline products for Wx
355
iii
A A
A A A .._
A A
1zL 0.0
0.1
0.2
-0.3
X products of Fig;ue 3. Temperature and composition dependence of the crystallization xNaE302.( l-x)LiBO, glassesGth CKxzW.25.takenfrom ref. [22j. Symbols ir.dicatethe varior;s
crystallinecompounds:Cla-LiE302,0p-LiBO,, S y-LiBO,.O ring-NaBO,. andA LQJaB,O,. Dashedline shows the compositiondependenceof the Tg.
increasing the devitrification temperature, correspond reasonably well with those occuzing upon quenching, some chemical aspects of the vitrification chemistry can be revealed. Thus, in the case of the x=0 composition_, a sudden “stiffening” of the structure seems to occ*ur right before the liquid reaches the glass transition, due to the formation of structural entities reminiscent of the condensed tetrahedra-containing LiBO, pofymorphs To the contrary, the x=0.25 meIt seems to undergo vitrification without major changes in the structural sequence of the network above and below Tg. These differences should be manifested in the log viscosity versus recipzxal temperature curves of the two melts, the former being expected to deviate r~~ore from the Arrhenius behaviour than the latter. A prediction of this sort implies that, upon increasing x Corn 0 to 0.25, the xNaBO,.( I-x)LiBO, melts become “stronger” in Termsof AngeIl’s classification [Z3]. Such changes in relative Yrength”, or “G-agiIity”, can be measured in a number of ways, and their experimental confirmation can be used to check the validity of the hypothesis on the simiIarity of vitrification and devitrification chemistries.
356
Acknowledgements: This work ha+ been supported by NHRF. Helpful discussions M.D.Ingram. J.Kapoutsis and A.Patsis are gratefully ackno\vIedged.
with
Dr.
6. REFERENCES 1
2
3 3 5 6 7 s 9 10 21 12 13 14 15 16 17 18 19 20
M.D. Ingram. Ionic conductivity in glsss. Phys. Chem. Glasses. 2s (1987) 215-231. R.H. Cole, Charge motions and dynamics of relaxation. J. Non-Cyst. SoIids. l3l-133 (1991) 1125-1130. R.H. Cole, J.G. Bcrberian. S. Mashimo, G.Chryssikos. A. Bums and E. Tombari. Time domain reflection methods for dielectric mcasurcmcnts to 10 GHz. J. Appl. Phys.. 66 ( 1989) 793-so2. J. Wang and C.A. AngeII. GIass Structure by Spectroscopy. Dekker. New York. 1976, p. 750. A. Burns. G.D. Chryssikos. E. Tombari. R.H. Cole and W.M. Risen Jr.. Dielectric spectra of ionic conducting yIasses to 2 GHz. Phys. Chem. Glasses. 30 ! 1989) 264-270. A. Bums. H.P. Brack and W.&l. Risen Jr.. Dielectric and infrared refIectance studies of inorganic oxide glasses. J. Non-Cryst. SoIids. 13 l-l 33 ( 1991) 991-1000. V. Provenxano. L.P. Boesch. V. VoIterra. C.T. Moynihan and P.B. Macedo. EIcctricaI relaxation in Na,0.3SiO, gIa.ss. J. Am. Ceram. Sot.. 55 ( 1972) 492396. R.W. RendelI and K.L. Ngai. in: K.L. Ngai and G.B. !%‘right (Eds.). Relaxations in complex systems, Government Printing Office. j\‘ashington. DC. 1985. pp.309-335. M.D. Ingram, Relaxarion processes in ionical!y conducting glasses. J. Non-Cqst. Solids, 131-133 (1991) 955-960. G.J. Exarhos and WM. Risen Jr.. Cation vibrations in inorganic oxide glasses. Solid S*ate Commun.. 11 ( 1972) 755-758. E.I. Kamitsrc. M.A. Karakassides and G.D. Chryssikos, Sation-network interactions in binary alkah-borate ylasscs. A far-infrared study. J. Phys. Chern.. 91 ( 1987) 5807. E.I. Kamitsos, Modifying role of alkali-metal cations in borate glass neavorks. J. Phys. Chem.. 93 (1989) 1604-1611. C. Liu and C.A. AngelI. Mid- and far-intired absorption of aIkali borate ylasw and the limit of superionic conductivity, J. Chem. Phys.. 93 ( 1990) 7378-7386. E-1. Kamitsos, G.D. Chryssikos. A.P. Patsis and M.A. Karakassides. ii:iiI ::n conducting borate glasses: evidence for two broad distributions of cation-hosting environments. J. Non-Crya. Solids, 13 I - i3’i ( 199 I ) 1092-l 095. M.D. Ingram, G.D. Chryssikos and E-i. Karnitsos, Evidence from vibrational spectroscopy for cluster and tissue pseudophases in glass. J. Non-Cryst. Solids, 13 l-l 33 (1991) lOSO-1091. E-i. Kamitsos, G.D. Chryssikos and M.A. Karakassides. New insights into the structure of alkali borate glasses, J. Non-Cryst. Solids, 123 ( 1990) 283-285. S.J. Gurman, Bond ordering in silicate glasses: a critique and a re-solution, J. NonCryst. Solids, 125 (1990) 151-160. P.H. Gaskell, in: L.D. Pye, W.C. LaCourse and H.J. Stevens (Eds.), The physics of non-crystalline solids, Taylor and Francis, London, 1992. pp. 15-21. G-D. Chryssikos, Bond length-Raman Frequency correlations in borate crystals. J.Raman Spectr., 22 ( 1991) 545-650, G.D.Chryssikos, E.I. Kamitsos. A.P.Patsis, M.S. Bitsis and M.A. Karakassides, The
21 22
23
devltrification of lithium metaborate: polymorphism and $aw fomxttion. J. Non-Cryst. Solids, 126 t 1990) 32-5’1. C.:i.L. Goodman. The strwtu~e and properties of &ass and the stmined mixed cluster model. Phys. Churn Gl;tsses. 26 ( i 985) !- 10. G.D. Chryssikos, J.A. Kapoutsis. M.S. Bitsis, E.I. Kamitsos and A.P. Patsis and A.J. Pappin. Chemical aspects of the glass transition in mixed-alkali and mixed-network glasses. Proc. XVI International Congress 3n G&s. Madrid. 1992. in prrs.~. C.A. Angrll. ReIaxation in liquids, polymers and p&tic crystals-strong’fiagile patterns and problems. J. Non-Cryst. Solids. 13 I-1 33 ( 199 I ) 13-3 1.
Chemical
relaxations
G.D. Chryssikos
of ionically conducting
glasses
and E.I. Kamitsos
Many amorphous solids conduct ions and find applications due to their Despite intensive research on the versatile preparation and good stability. matter. the fundamental mechanism of ionic conduction in &asses remains eIusive. and the existing theories often contradictory [I]. Meanwhile, “...rk resolrrrir~n of iiiffh-twr interprert2rion.s can CN(Y he rnde on r/w hu.si.s of u&pure rheoq of chrge trurrsport p~i~ce.s.sc.s ur the rndwfflur Itrvd. . ..uird thar m do 1hi.s defirtire xfpprriiz~fi .srr:fcrrf rul and orher me need.s srffficiently properl> evidtww...” [2]. In this report. struc&,r-al and relaxational aspects of ionicaIIy conducting glasses are discussed. Structure is viewed in the broadest sense and extends from the local chemical bonding to long range interactions affecting the morpho!o,T of the glass. Relaxations of chemical nature which occur upon melt co0:ir.g are alsc considered, since it is their “freezing” that determines the structure of glass.
2. TI-IE DIELECTRIC
REGIME
Bob Cole entered the field of glassy ionics (“us u nmfce", he used to say) in the mid-eighties. I-Ie spent a few years developing new eqerimental
350
techniques and adapting others to achieve high frequency, high temperature dielectric data acquisition of ionically conducting sohds [S]. Finally, in 1989 he published an updated version of the well known Wang and Angel1 plot of absorptivity versus frequency [4], by filling in data in the GHz frequency range that had remained till then unaccessible (Figure 1) [S]. This work was important for a number ofreasons. First. it was confirming that the log abso@vity vs log frequency plot approaches the slope of one, before joining smoothly the far-inf+ared trace [6], as envisaged by Wong and Angel1 [4]. Seccnd, it was supporting the thesis of Moynihan and coworkers concerning the frequency - temperature superposition of the dielectric modulus spectra (ivi*=I/E*), as we11 as the near coincidence of rhe activation energies deduced from the temperature dependence of dc conductivity and M” maxima 173. All the above were pointing towards universal aspects of relaxations in glasses, such as those developed by Ngai et ul. [S].
I
0
2
4
8 log (f in Hz) 6
10
12
14
F:gurc 1. Absorptivity spectrum of glassy Na,0.3SiO, at various temperatures. taken from ref. IS]. Arrows mark the positions of the imaginary dielectric moduIus maxima.
With reference to figure I, it can be argued that as the Frequency increases above that of ~xi~num M”, the temperatare dependence of the ~so~tivi~ decreases systematically, and eventuaIIy vanishes in the far-inf?ared. increasing frequency implies that the interactions between the material and the electromagnetic radiation become increasingly localized in space 191,and that the response of the system becomes more dependent on its particular chemical characteristics. This is reflected on the way vib~tjonal data are co~only discussed, ie. with Iittle or no reference to structural trends of more general applicability. It is probable that if such genera1 trends cot&l be detected, they would refer to very fundamental concepts such as the vitrification of the quenched melts and the conditions under which this is experimentally feasible.
3. FAR-INFRARED
SPECTROSCOPY
The first vibrational frequency range where universal structural characteristics of ionically conducting glasses should be sought, is the commonIy referred to as far-infrared (ca. 30-400 cm-‘), Pioneering work by Exarhos and Risen [ IO] and continuing research in our laboratory [ 1 If has demonstrated that glasses containing ionic modifiers such as alkalis and alkaline earths, exhibit in this frequency region modes which correspond to the “rattling” of the cations in sites provided by the network. Within the wide spectrum of cationic motions, extending over more than twelve decades in sequent or time, those active in the far-infrared are the most localized ones, since cations do not escape Corn their potential wells acquired upon cooling the melt through the glass transition. The frequency maxima, widths, and integrated intensities of the far-infrared bands are dependent on composition. the cationic mass and the energetics of the cation-site ionic inte~ctions [ 121. Yet. a number of system-independent characteristics of these bands have been identified. The dispersion of glasses in the fm- inked is always heavily damped, and has a character intermediate to relaxations and to true resonances. The frequency maxima observed are In good agreement with the attempt frequencies in the pro-exponential factor of the Arrhenius type dc conductivity and the fx inf?ared condu~~~~ maxima have been considered by Angel1 as limiting values of ionic conductivity at very high temperatures [ 133. Within the coiltext of the above, there is one characteristic of the cationsite vibrations which has resertied special attention. The far-inf?ared baMls of almost all glasses investigated exhibit a charzteristic asvmmetry which can in most cases be accounted for by the existence of two siperimposed Gaussian
E;imre2, Far-intied spectraof 0.3M30 0_7B,O, glasses,takenfrom lef. [ 151. f)ec;or.vo;u%ed componentbands are shown. bandshapes, both due to cation rattling modes 111, 141. Typical examples of such spectra of alkali borate &asses are shown in Figure 2, together with their deconvolution into component bands. The widespread appearance of this behaviour implies that the origin of the observed Gwen cannot be the chemical details of the charge-balancing network anions, which can change enormously with composition and still leave the f&r&Cared profile relatively unvexed 1141. Instead, it was argued that at least two dist~butions of cationsite environments exist in glass and can be probed through the cation-motion bands[ 11, 143. Such .resuIts .have been interpreted as supporting the hypothesis that the glass structure is mi~~heterogeneous and that ionic t-sport occurs mostly through preferred “pathways” [15].
4. THE STRUCTURE
OF THE GLASS
NETWORK
The next question to be addressed concerns the possibility of identifying chemically microheterogeneities in glass. The usehI frequency range is now at the upper limit of the Wong and Angel1 plot, and can be accessed by midinfmred and Raman spectroscopies, both probing the vibrations of the network k 2ding units. The search for suitable systems has led to the alkali metaborates iM20.B,0,, M:alkali), because it was thought that should structural microheterogeneities exist, they could be “decorated” differently by the rich borate chemistry. -4lthough a detailed discusr;ion of the vibrational spectra of M,O.B,O, glasses cannot be attempted here, a number of results are of relevance to the present discussion. First, it was found that the local chemistry of these glasses is dominated by the isomerization reaction B020--B04-
(1)
where 0, 0, denote oxygen atoms bridging two boron centres, or terminal, respectively [ 163. Given the different cross linking ability of the above metaborate triangles (B0,0-) and tetrahedra (Bta,.), the temperature dependence of equation (1) is expected to play an important role in the vitrification of the metaborate melts. More importantly, a number of common glass-forming oxides (e.g. GeO,, As,O, etc) exhibit similar isomerizations between local structures of different coordination to oxygen. In silicate glasses the disproportionation equilibrium
24”OQn-‘+qn+’
(2)
where n denotes the number of bridging oxygen atoms of the silicate tetrahedron Q” ,seems to play a similar role 1171. The chemical diversity at the local structural level, at which glassy and crystalline states do not differ much anyway 1181,explains the very large number of the ways these local units can combine in forming intermediate range groups. At least one dozen of such groups are known in metaborates and more are expected to be found as their investigation continues. Their identification in the provided that adequate reference vibrational spectra is stlrightforward, (crystalline) compounds cm be isolated and studied. As an axample we mention here the identification of the sequence of polyhedra in complicated networks w?zre metaborate triangles and tetrahedra coexist, through the Raman active stietching vibration of B-O- terminal bonds [t9]. Studies of this sort have allowed the chemical characterization of microdomains in glasses. In the
particular case of lithium met&orate (LiBO1) these microdomains exhibit structures similar to those found in a-, p’-, and y-LiBO,, three of the known LiBO, polymorphs, and, hence differ considerably in IzGcing density [Xl. This granular structural picture is not unique to borates and the structure of many glasses can be described in terms of polymoqhic microdomains [2 11.
5. STRLJCTUliAL
ASPECTS
OF VITRIFlCATION
Apart from the above static structural description of glass, it is the dynamic aspects of chemical transformations that influence vitrification. 5pon quenching, the melt is envisaged to move through a number of Iocal minima in the potential energy surface before being trapped in those of the me&stable glassy state. Since, in most cases, the characteristic quenching times involved are too short to aIlow for the acquisition of vibrational data, different approaches are needed to map the chemistry of vitrification. One of these approaches is based on the remark that the glass has stored in its structure the memory of structural events that led to its formation. Devitrification can then be employed to allow the chemical relaxation of the system towards the cIosest-lying crystalline compound. Once the mapping of the devitrification process as a function of temperature and time is complete, the resulting crystalline products can be studied conveniently, compared to the structure of glass and provide answers about the possible chemical pathway(s) of glass formation. An example of such studies is offered by the systematic investigation of devitrification in lithium-sodium metaborate glasses, xNaBO?.( I-x)LiBO, [22]. Three crystalline lithium metaborate poIymorphs, a mixed lithium-sodium met&orate, Li3NaB,08, and the well known ring sodium metabomte compound, have been isolated upon devitrifjring glasses of increasing x. The temperaturecomposition Gomains of these crystalline products for Wx
355
iii
A A
A A A .._
A A
1zL 0.0
0.1
0.2
-0.3
X products of Fig;ue 3. Temperature and composition dependence of the crystallization xNaE302.( l-x)LiBO, glassesGth CKxzW.25.takenfrom ref. [22j. Symbols ir.dicatethe varior;s
crystallinecompounds:Cla-LiE302,0p-LiBO,, S y-LiBO,.O ring-NaBO,. andA LQJaB,O,. Dashedline shows the compositiondependenceof the Tg.
increasing the devitrification temperature, correspond reasonably well with those occuzing upon quenching, some chemical aspects of the vitrification chemistry can be revealed. Thus, in the case of the x=0 composition_, a sudden “stiffening” of the structure seems to occ*ur right before the liquid reaches the glass transition, due to the formation of structural entities reminiscent of the condensed tetrahedra-containing LiBO, pofymorphs To the contrary, the x=0.25 meIt seems to undergo vitrification without major changes in the structural sequence of the network above and below Tg. These differences should be manifested in the log viscosity versus recipzxal temperature curves of the two melts, the former being expected to deviate r~~ore from the Arrhenius behaviour than the latter. A prediction of this sort implies that, upon increasing x Corn 0 to 0.25, the xNaBO,.( I-x)LiBO, melts become “stronger” in Termsof AngeIl’s classification [Z3]. Such changes in relative Yrength”, or “G-agiIity”, can be measured in a number of ways, and their experimental confirmation can be used to check the validity of the hypothesis on the simiIarity of vitrification and devitrification chemistries.
356
Acknowledgements: This work ha+ been supported by NHRF. Helpful discussions M.D.Ingram. J.Kapoutsis and A.Patsis are gratefully ackno\vIedged.
with
Dr.
6. REFERENCES 1
2
3 3 5 6 7 s 9 10 21 12 13 14 15 16 17 18 19 20
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