Journal of Non-Crystalline Solids 355 (2009) 414–419

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Journal of Non-Crystalline Solids

journal homepage: www.elsevier .com/locate / jnoncrysol

Effect of fluoride ion incorporation on the structural aspects of barium–sodiumborosilicate glasses

R.K. Mishra a, V. Sudarsan b, C.P. Kaushik a, Kanwar Raj a, R.K. Vatsa b, M. Body c, A.K. Tyagi b,*

a Waste Management Division, Mumbai-400085, Indiab Chemistry Division, Bhabha Atomic Research Centre, Trombay, Maharashtra, Mumbai-400085, Indiac Laboratoire de Physique de l’Etat Condense. CNRS UMR 6087, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 August 2008Received in revised form 24 December 2008Available online 21 February 2009

PACS:42.70.Ce67.30.er

Keywords:Glass transitionBorosilicates

0022-3093/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.jnoncrysol.2009.01.002

* Corresponding author.E-mail address: aktyagi@barc.gov.in (A.K. Tyagi).

Barium–sodium borosilicate glasses containing upto 6 wt% fluoride ions were prepared by conventionalmelt quench method and characterized by 19F, 29Si and 11B nuclear magnetic resonance (NMR) tech-niques.19F NMR studies have confirmed the presence of mainly linkages like F–Si(n) or F–B(n) along withF–Ba(n). Their relative concentrations are unaffected by F� content in the glass. Incorporation of fluorideions in the glass is associated with significant reduction in the nonbridging oxygen concentrationattached to silicon, as revealed by the increase in the concentration of Q3 structural units of silicon atthe expense of Q2 structural units. 11B NMR studies have established that the relative concentrationsof BO3 structural units are higher for F� ion containing glasses compared to the one without F� ion incor-poration. The observed increase in the relative concentrations of Q3 structural units of silicon and BO3

structural units with fluoride ion incorporation in the glass has been attributed to the formation ofF–Ba(n) type of linkages, thereby reducing the concentration of network modifying cations for breakingthe Si–O–Si/B–O–B linkages. Formation of such structural units weakens the glass network therebydecreasing the glass transition temperatures.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Borosilicate glasses have potential application in nuclear indus-try as suitable matrix for the immobilization of high-level nuclearwastes [1,2]. India has vast thorium resources amounting to about1/3rd of the world reserve for its energy security on a sustainablebasis. Accordingly, India has an ambitious program to use thoriumin the blanket zone of fast breeder reactors at an appropriategrowth of installed nuclear power in the second stage of nuclearenergy program, which will be followed by introduction of Ad-vanced Heavy Water Reactors (AHWR) based on Th–233U MOX(mixed oxide) fuel in the third stage [3]. Spent fuel from such reac-tors will contain oxides of thorium and uranium as main compo-nents along with fission products like 137Cs, 90Sr, 106Ru etc.During reprocessing of the spent fuel, hydrofluoric acid will beused to bring the solid material into solution. Hence the high-levelwaste obtained will contain significant amount of fluoride ions.This needs to be immobilized in a suitable inert matrix before theirlong term disposal in repositories. Barium–sodium borosilicateglass is one of the suitable candidates for such an immobilization.In a previous study [4] we have reported that unlike sodium boro-

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silicate glasses, barium–sodium borosilicate glasses can accommo-date upto 16 wt% ThO2 without any phase separation/segregation.Based on the 29Si magic angle spinning nuclear magnetic resonance(MAS NMR), it has been confirmed that the significant number ofnonbridging oxygen atoms brought about by BaO incorporationfacilitates the Th4+ incorporation in the glass. This is further sup-ported by the relatively large concentration of nonbridging oxygenatoms present in barium–sodium borosilicate glasses as revealedby the 17O NMR study reported by Zhao et al. [5]. In a related study[6] we have also reported that simultaneous incorporation of boththorium and uranium oxides in barium–sodium borosilicate glassresults in the phase separation of ThO2 when uranium oxide incor-poration exceeds 5 wt% along with 15.87 wt% of ThO2 in the glass.There are numerous investigations on the structural aspects offluoride ion containing silicate/alumino-silicate glasses investi-gated by 19F NMR and XPS techniques [7–11]. Based on these stud-ies it has been concluded that most of the F� ions in the glass existas F–M(n) (fluorine with unknown number of network modifyingcations like Na+, Ca2+, Ba2+ etc.) type of linkages. Higher the fieldstrength of the cation, higher the extent of F–M(n) linkage forma-tion. Further only a negligible amount of F� ions forms Si–F link-ages in the glass. Such linkages in the glass are expected to affectits physico-chemical properties like glass transition temperature,viscosity, thermal expansion coefficient and refractive index.

(a)

(b)

(c)

Inte

nsity

(a.u

)

R.K. Mishra et al. / Journal of Non-Crystalline Solids 355 (2009) 414–419 415

As fluoride ions will be invariably present in the waste generatedfrom the thoria based reactors, it is essential to study the effect offluoride ion addition on the structural aspects and thermo-physicalproperties of barium–sodium borosilicate glasses. The results andinferences obtained from this study will be useful for preparingsuitable glass compositions with optimum waste loading charac-teristics for the high level waste generated from thoria based reac-tors. In this view, we have prepared a number of barium–sodiumborosilicate glasses containing up to 6 wt% fluoride ions and charac-terized them using XRD, 19F, 29Si and 11B MAS NMR techniques fortheir structural aspects and DTA for their thermo-physical proper-ties like glass transition temperatures. The results obtained fromthe above techniques are discussed in this manuscript.

2. Experimental

Required amounts of analytical grade NaNO3, Ba(NO3)2, SiO2,H3BO3 were taken so as to get the base glass with a composition(SiO2)0.39(B2O3)0.25(Na2O)0.12(BaO)0.24 which is then mixed with dif-ferent amounts of NaF (Na+ content in the glass was kept constantby proportionately decreasing the amount of NaNO3), wellgrounded and heated at 1000 �C for 4 h in platinum crucible. Thefree flowing melt was quenched between two stainless steel plates.In order to check loss of fluoride ions and silica content from thesamples due to formation of gaseous SiF4, evolved gas analyzeswere performed during melting of the samples and confirmed thatthere is only around 2–3 wt% loss on the fluoride ions from the glass.

X-ray diffraction patterns were recorded using a Philips X’PertPro diffractometer with monochromatized CuKa radiation. 19FNMR spectra were recorded on an Avance 300 Bruker spectrometer(7T) with a Larmor frequency of 282.2 MHz for 19F, using a highspeed CP MAS probe with a 2.5 mm rotor. The external referenceused for isotropic chemical shift measurement was C6F6 (diso(C6F6)vs. CFCl3 = �164 ppm). All 19F MAS NMR chemical shifts are ex-pressed with respect to CFCl3. Spectra were acquired using a Hahnecho sequence with an interpulse delay equal to one rotor period.The recycle delay is 1s, ensuring the relative peak intensities arequantitative. Reconstructions of the 19F NMR spectra were per-formed with the DMFIT software [12] including spinning side-bands, using six adjustable parameters for each NMR line:isotropic chemical shift diso, chemical shift anisotropy daniso, chem-ical shift asymmetry parameter g, line width, relative line intensityand line shape. diso, daniso, g, the relative line intensity and the lineshape were assumed to be independent of the spinning rate. In thisstudy, the diso values and the relative line intensities are the tworelevant parameters. 29Si and 11B MAS NMR patterns of theseglasses were recorded using a Bruker Avance DPX 300 machine.Powdered samples were packed inside 7 mm zirconia rotors andsubjected to a spinning speed of 5 kHz. Typical 90� pulse durationsfor the 29Si and 11B nuclei are 4.5 and 2.09 ls, respectively, withthe corresponding delay times of 6 and 2 s. 11B static NMR patternswere also recorded with same pulse duration and delay times asthat used for MAS NMR experiments. 11B NMR experiments werecarried out with lower pulse durations also (upto 0.3 ls) and theline shapes were found to be identical. Static 11B NMR patternswere simulated using the WINFIT program supplied by Bruker.The chemical shift values for 29Si and 11B NMR spectra are reportedwith respect to tetramethylsilane and BF3–(OC2H5)2, respectively.All the 11B NMR patterns were corrected for the boron nitride(BN) background arising from the Bruker MAS NMR probe.

10 20 30 40 50 60 702θ/°

Fig. 1. XRD patterns for barium–sodium borosilicate glasses containing (a) 2 wt%(b) 4 wt% and (c) 6 wt% F� ions.

3. Results

Fig. 1 shows the XRD patterns of barium–sodium borosilicateglasses containing up to 6 wt% fluoride ions. For glasses containing

upto 4 wt% a broad hump around a 2h value of 25� is observed,characteristic of the amorphous borosilicate network. Howeverabove 4 wt% addition of F� ions in the glass resulted in appearanceof sharp peaks around 2h values 24.93�, 28.83�, 41.2� and 48.75�characteristic of crystalline BaF2 phase.

Fig. 2 shows the 19F MAS NMR patterns of these glasses contain-ing upto 4 wt% F� ions. NMR line shapes are asymmetric and foundto be same for glasses containing different amounts of F� ions. Thepatterns could be de-convoluted to five individual peaks withchemical shift values around �52, �9, 30, 50, and 116 ppm ascan be seen from Fig. 3. Among these five peaks, 85% of the contri-bution is coming from 30 ppm and 50 ppm peaks (line nos. 3 and 4in Fig. 3). Remaining 15% is the contribution from �52, �9, and116 ppm peaks. Based on the previous 19F MAS NMR studies on dif-ferent types of borosilicate glasses containing F� ions [7–11,13],the peaks around 50, 30, and �9 ppm have been attributed to F–Na(x)Ba(y) type linkages having unknown values of x and y. Struc-tural units having highest value of x appears around �9 ppm andthat having highest value of y appears around 50 ppm. The less in-tense peaks around 116 and �52 ppm can be attributed to F–Ba(n)and F–Na(n) type of linkages. Variation of the relative intensitiesand chemical shift values of all the five peaks are shown inFig. 4. There is no change in the relative concentration and chem-ical shift values of different F structural units suggesting that thelocal environment around F� is identical and does not undergoany change with increase in F� ion concentration in the glass.

Fig. 5 shows the 29Si MAS NMR patterns for theses glasses. Bar-ium–sodium borosilicate glass without any F� ion incorporationshowed a broad asymmetric peak centered around �92 ppm,deconvolution of which assuming each silicon structural units asa Gaussian peak, resulted in two individual peaks around �87and �98 ppm. Based on the previous 29Si MAS NMR studies on bar-ium–sodium borosilicate glasses [4,6] the peak around �98 ppmhas been attributed to the Q3 structural units of silicon and that

Isotropic chemical shift (ppm)-160-120-80-4004080120160200240

4 % F

3 % F

2 % F

1 % F

Fig. 2. 19F MAS NMR patterns for barium–sodium borosilicate glasses containingdifferent amounts of F� ions.

Isotropic chemical shift (ppm)

-160-120-80-4004080120160200240

exp.

cal.

line 3

line 2

line 1

line 4

line 5

Fig. 3. De-convolution of 19F MAS NMR pattern of a representative glass containing2 wt% F� ions, recorded at 30 kHz spinning speed.

Fig. 4. Variation in the (a) relative intensity and (b) chemical shift values of the fivepeaks as a function of glass composition.

-50 -75 -100 -125 -150

-50 -75 -100 -125 -150

-50 -75 -100 -125 -150Chemical shift (ppm)

22%

78%

(c)

23%

77%

(b)

Inte

nsity

(ar

b. u

nits

)

41%

59%

(a)

Fig. 5. 29Si MAS NMR patterns for barium–sodium borosilicate glasses containing(a) 0 wt% (b) 2 wt% and (c) 4 wt% fluoride ions.

416 R.K. Mishra et al. / Journal of Non-Crystalline Solids 355 (2009) 414–419

around �87 ppm to the Q2 structural units of silicon (where Qn

represents the silicon structural units having ‘n’ number of bridg-ing oxygen atoms). The relative concentrations of Q2 and Q3 struc-tural units are represented below each peak. For fluoride ioncontaining glasses, the peak maximum of the 29Si MAS NMR pat-terns shifts to more negative values as can be seen from Fig. 5(b)and (c). The patterns are similar for both 2 and 4 wt% fluorideion containing glasses. De-convolution of both patterns showedthat there is a significant increase in the relative concentration ofQ3 structural units of silicon at the expense of Q2 structural unitsfor F� ion containing glasses compared to the one without anyfluoride ion content.

Fig. 6 shows the 11B MAS NMR patterns for these glasses. Thepatterns essentially consist of a sharp peak with chemical shift va-lue around �1 ppm and a broad peak around 12 ppm. The sharp

50Chemical shift (ppm)

(a)

(c)

(b)

25 0 -25 -50

Fig. 6. 11B MAS NMR patterns for barium–sodium borosilicate glasses containing(a) 0 wt% (b) 2 wt% and (c) 4 wt% fluoride ions.

Fig. 8. Simulated 11B static NMR pattern for a representative barium–sodiumborosilicate glass containing 4 wt% fluoride ions. Blue line: experimental NMRpattern, Violet line: characteristic of BO3 structural units, green line: characteristicof BO4 structural units, red line: overall simulated pattern and the black line isresiduals (difference between the experimental and simulated patterns). (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.).

Table 1Nominal compositions of the glass samples.

SiO2 (wt%) B2O3 (wt%) Na2O (wt%) BaO (wt%) F-content (wt%)

39.0 25.0 12.0 24.0 0.038.2 25.1 11.9 23.8 1.037.8 24.8 11.8 23.6 2.037.4 24.6 11.7 23.3 3.037.1 24.3 11.5 23.1 4.0

R.K. Mishra et al. / Journal of Non-Crystalline Solids 355 (2009) 414–419 417

peak has been attributed to boron in tetrahedral configuration andbroad peak to boron in trigonal configuration. As 11B is quadrupo-lar nuclei (I = 3/2), boron in the trigonal coordination lacks cubicsymmetry, and gives rise to a broad peak. Unlike this, cubic natureof the tetrahedrally coordinated boron structural units, results innegligible quadrupolar coupling constant and thereby giving riseto a sharp NMR peak. NMR patterns are identical for all the sam-ples revealing that there is no change in the relative concentrationof BO3 and BO4 structural units. However during magic angle spin-ning, efficiency of BO3 detection relative to BO4 detection is poorand small changes in the relative concentration of BO3 structuralunits may go unnoticed [14]. In order to check this aspect 11B staticNMR patterns were recorded for these samples and are shown inFig. 7. The patterns essentially consist of a relatively sharp peakwith broad shoulders. The sharp peak is characteristic of BO4 struc-tural units and the broad shoulders are characteristic of BO3 struc-tural units. The static NMR patterns were simulated assuming aGaussian line shape with negligible quadrupolar interaction forthe BO4 structural units and a quadrupolar broadened line shapefor the BO3 structural units. No distinction was made betweenthe symmetric and asymmetric BO3 structural units. Similar proce-dure was also employed by Miyoshi et al. [15] to estimate the rel-ative concentration of BO3 and BO4 structural units from 11B NMRpatterns of borosilicate glasses. A representative simulated 11B sta-tic NMR pattern for barium–sodium borosilicate glass containing

300 200 100 0 -100 -200 -300

BO3

BO4

Inte

nsity

(ar

b. u

nits

)

Chemical shift (ppm)

Fig. 7. Static 11B NMR patterns for barium–sodium borosilicate glasses containing 0wt% (black), 2 wt% (green) and 4 wt% (red) fluoride ions. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthis article.).

4 wt% fluoride ions is shown in Fig. 8. The values of chemical shift,relative concentrations of BO3 and BO4 structural units along withthe quadrupole coupling constant (Cq) values of BO3 structuralunits obtained from 11B static NMR patterns are shown in Table2. With incorporation of F� ion in the glass, there is an increasein the relative concentration of BO3 structural units. Further thequadrupolar coupling constant for the BO3 structural units is high-er in barium–sodium borosilicate glass containing fluoride ions,than that in barium–sodium borosilicate glasses without any fluo-ride ions, as can be seen from Table 1.

Fig. 9 shows the DTA patterns for barium–sodium borosilicateglasses containing different amounts of F� ions. From the onsetof slope change the values of glass transition temperatures are ob-tained for these glasses. Barium–sodium borosilicate glass withoutany F� content is characterized by a glass transition temperature of559 �C. With addition of fluoride ions in the glass, the glass transi-tion temperature systematically decrease as can be seen fromFig. 9. No crystallization has been observed from these glasses onheating upto 900 �C.

4. Discussion

Based on the XRD patterns, it has been inferred that barium–so-dium borosilicate glass can retain upto 4 wt% F� ions and that,above this 4 wt% content, crystalline phase of BaF2 is separated

-40

-20

0

200 300 400 500 600 700 800 900

200 300 400 500 600 700 800 900

200 300 400 500 600 700 800 900

-60

-30

0

-60

-40

-20

(c)T

g = 520oC

Tg = 557oC

Temperature ( º C)

Hea

t flo

w (

mW

)

(a)

(b)

Tg = 544oC

Fig. 9. DTA curves for barium–sodium borosilicate glasses containing (a) 0 wt% (b)2 wt% and (c) 4 wt% F� ions. Uncertainties in the Tg values are ±5 �C.

Table 2Relative concentration of BO3 and BO4 structural units along with their chemical shift values (d). Quadrupole coupling constant (Cq) and asymmetry parameter (g) for BO3

structural units are also shown. Errors in the relative concentration of BO3 and BO4 structural units are around 2–3%.

Nominal compositions (in wt%) of fluorideions

dBO4 (ppm)(±1)a

Line width(ppm)

% ofBO4

dBO3 (ppm)(±1)a

Cq (MHz)(±0.1)a

% ofBO3

Asymmetry parameter (g)(±0.1)a

0 �1.0 34.5 57.3 23.1 2.3 42.7 0.62.0 �1.1 35.5 48.0 20.0 2.8 52.0 0.154.0 �0.2 35.0 48.0 21.6 2.7 52.0 0.21

a Errors were calculated by carrying out duplicate measurements.

418 R.K. Mishra et al. / Journal of Non-Crystalline Solids 355 (2009) 414–419

out. These results are in conformity with the previous reports [7–9]which suggests that F� ions have got tendency to form clusterswith network modifying cation with higher z/r values. Here Ba2+

has got higher z/r compared to Na+ and hence it is more favorableto form BaF2 phase compared to NaF phase.

19F MAS NMR studies have clearly revealed that the glass sam-ples mainly consist of structural units of type F–Na(x)Ba(y) havingunknown values of x and y. With incorporation of F� ions, thesestructural units and their chemical shift values remained unaf-fected as can be seen from Fig. 2 and 4. As 19F MAS NMR patternsare same for all the samples with different F� ion concentrations, itis inferred that the F� ions form identical structural units with al-most same relative extents in all the glass samples. 29Si MAS NMRpatterns of the glass samples suggest that F� ion incorporation isassociated with conversion of Q2 structural units to Q3 structuralunits. This is understandable as F� ions have tendency to clusterwith cations, it will interact with Ba2+/Na+ ions, Ba2+ being morepreferred due to its higher z/r value. This leads to decrease in thenetwork modifying cations available for network modificationand there by increasing the relative concentration of Q3 structuralunits at the expense of Q2 structural units. However for glass withboth 2 and 4 wt% F� ions the patterns are identical and this hasbeen attributed to the fact that the additionally incorporated F�

ions interacts only with F–Na(x)Ba(y) structural units (clusters) al-

ready present without having any direct interaction silicon struc-tural units.

11B MAS NMR patterns very clearly demonstrate that in theseglasses, boron atoms exist in both BO3 and BO4 structural units.From the 11B static NMR pattern shown in Fig. 7 and from the rel-ative concentration of BO3 and BO4 structural units shown in Table1, it can be inferred that the relative concentration of BO3 struc-tural units are higher for glasses containing F� ions compared tothe one which does not have any F� ions. However, the relativeconcentration of BO3 structural units are unaffected by the changein the F� ion content in the glass. Initial increase in the relativeconcentration of BO3 structural units has been attributed to theless availability of the network modifying cations as they are takenaway by the F� ions to form F–Ba(n) clusters. As observed for 29SiMAS NMR patterns, 11B MAS NMR patterns are identical for both 2and 4 wt% F� ions containing glasses. This is explained again by thefact that the additionally incorporated F� ions interacts only withF–Na(x)Ba(y) structural units (clusters) already present, leavingthe boron structural units unaffected. Increase in the Cq values ofBO3 structural units for F� ion containing glasses has been attrib-uted to the higher chain length of B–O–B linkages due to the lackof BO3 to BO4 conversion, brought about by the decrease in Ba2+

concentration available for network modification.The glass transition temperature (Tg) values have been found to

decrease systematically with incorporation of F� ions in the glass.Glass transition temperature is nothing but onset of structuralrelaxation brought about by the heat treatment. Formation of F–Na(x)Ba(y) structural units (clusters) disturbs the glass network,reduces the plastic flow and thereby weakens the network andleading to decrease in Tg values.

5. Conclusions

Based on XRD studies on barium–sodium borosilicate glassescontaining different amounts of F� ions it has been concluded thatfluoride ion incorporation above 4 wt% results in phase separationof the glass leading to the formation of crystalline BaF2 phase. F�

ions in these glasses mainly exist as F–Na(x)Ba(y) structural unitsand their relative extents are unaffected by variation of F� ion con-tent in the glass. On the opposite, silicon and boron structural unitsare affected by F� ion addition in the glass and this has been ex-plained by the decrease in the number of network modifying cat-ions like Ba2+ ions brought about by the formation of F–Ba(n)type of linkages. Formation of such linkages disturbs the borosili-cate network, reduces the plastic flow and results in the decreaseof Tg values.

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