Structural Inversion and Behavioural Changes as a Function of Composition in Sr-La-Al-B-Si Based Glasses

doi:10.4236/njgc.2011.12004

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New Journal of Glass and Ceramics, 2011, 1, 21-27

doi:10.4236/njgc.2011 .12004 Published Online July 2011 (http://www.SciRP.org/journal/njgc)

Structural Inversion and Behavioural Changes as a

Function of Composition in Sr-La-Al-B-Si Based

Glasses

Prasanta Kumar Ojha1,2*, Sangram K. Rath1,2, Tapas K. Chongdar1, Nitin M. Gokhale1,

Ajit R. Kulkarni2

1Naval Materials Research Laboratory, Ambernath, Thane, Maharashtra, India; 2Indian Institute of Technology Bombay, Mumbai,

India.

Email: [email protected]

Received April 29th, 2011; revised May 27th, 2011; accepted June 3rd, 2011.

ABSTRACT

A series of glass sealants for solid oxide fuel cell (SOFC) with compositions SrO (x wt%) La2O3 (15 wt%) Al2O3 (15

wt%) B2O3 (40-x wt%) SiO2 (30 wt%) [x = 10, 15, 20, 25 & 30] [SLABS] have been investigated by quantitative Fourier

Transform Infrared Spectroscopy (FTIR). Structural findings from FTIR reveal that with increasing substitution of B2O3

by SrO, even though the B2O3/SiO2 ratio decreases, however the Si-O-non-bridging bond content in the matrix is in-

creasing and glass structure is getting more inverted. UV-Vis Diffused Reflectance Spectroscopy (UV-Vis-DRS) of the

glass series shows that electrical band gap of glasses decreases in the series from 3.07 eV to 2.97 eV with increasing

substitution from x = 10 to x = 30. Conductivities of the glass samples were measured by AC impedance spectroscopy

and found to be increasing from 2.74 × 10-5 Scm-1 to 1.09 × 10-4 Scm-1 with increasing substitution from x = 10 to x =

30.

Keywords: Glass Cera mics, Mu llite Ceramics Thin Tile, Composites, Bending Strength

1. Introduction

Oxide glasses are amorphous materials showing glass

transition behavior and exhibit composition dependent

properties. Basic studies of glasses reveal that the oxides

used to synthesize glasses can be broadly divided into

three groups. Network formers provide the basic network

of glass, intermediate oxides which, though not able to

form the network by themselves, participate with net-

work formers in the basic structure, and modifier oxides

“invert” the network structure by breaking the network

bonds and generate non bridging oxygens. T he term “in-

vert” was introduced by Trapp and Stevels [1], because

the traditional network forming oxides SiO2, B2O3, and

P2O5 form continuous molecular/ionic networks in nor-

mal conditions; however when the network modifying

oxides are in majorit y on the molar ba sis, the glas ses are

invert ed str uctur all y compare d to convent iona l glasse s as

sho wn in Figur e 1. Composition of the glass and type of

additives decide the nature of different structural units

present in the glass which in turn decides the physical-

chemical properties of glasses. For example, systematic

substitution of PbO by B2O3 in ternary lead borosilicate

glasses [2] decreases the thermal expansion coefficient,

and increases the glass transition temperature. This has

been attributed to the formation of Si-O-B linkages and

increase in concentration of Q4 structural units of silicon

(where Qn represents silicon structural units having

n-number of bridging oxygen atoms). Substitution of

PbO by Bi2O3 in PbO-B2O3-SiO2 glasses results in the

Figure 1. Structure of glass inverte d by modifier ions.

Stru ctural Inversion and Behavioural Changes as a Function of Composition in Sr-La-Al-B-Si Based Glasses

increase of thermal expansion coefficient, deformation

and flow temperature [3] due to structural modification.

It has been reported [4-7] that addition of network mod-

ifiers (alkali/alkaline earth metal oxides) to borosilicate

glasses results in the initial conversion of BO3 to BO4

structural units. At higher concentration of modifiers,

BO4 structural units in the glass are replaced by

BO3-structural units (pla nar B O3 s tr uct ur al u nit s wit h o ne

non bridging oxygen atom). Hence the properties of

glasses with different modifier concentrations are differ-

ent. To understand the criticality of glass science it is

pertinent to investigate the structure of glasses and cor-

relate the structure with glass behavior. In this regard

many researchers have tried to elucidate the structure of

glasses using various spectroscopic techniques [8-21].

Extensive studies have been reported on the structural

aspects of boroaluminosilicate glasses using techniques

like FTIR, Raman and Magic Angle Spinning-Nuclear

Magnetic Resonance (MAS-NMR) spectroscopy [11,

22-24]. These studies highlight the existence of various

structural units like trigonally coordinated boron (BO3),

tetrahedrally coordinated boron (BO4), silicon a toms with

3 and 4 bridging oxygen atoms, Qn units with Si-O-B/

Si-O-Al linkages, etc. in the glass matrix. T hese structur-

al units finally govern the properties of t he gla sses.

In this work structure property correlation of a series

of glasses with composition SrO (x wt%), La2O3 (15 wt

%), Al2O3 (15 wt%), B2O3 (40-x wt%) and SiO2 (30 wt

%) have been investigated. Glass compositions were se-

lected for their application as sealant in solid oxide fuel

cell (SOFC) wit h B2O3 and SiO2 as the network formers,

Al2O3 as the intermediate oxide and La2O3 and SrO as

the mod ifier o xides. I n the gl ass co mposit ion, co ncentr a-

tion of SiO2, La2O3 and Al2O3 were kept constant and a

systematic substitution of B2O3 by SrO [x = 10, 15, 20,

25, 30] was introduced. This report includes the investi-

gatio n of struc tural modification as a function of compo-

sition by quantitative FTIR and UV-Vis Diffused Ref-

lectance Spectroscopy (UV-Vis-DRS) and correlated

with the electrical conductivity of glasses measured by

AC Impedance spectroscopy.

2. Experimental

SiO2, AR grade from S. D. Fine-Chem. Ltd., India,

Al2O3, AR grade from CDH, India and La2O3, GR grade

from Loba Chemie, India were used as received for

preparation of batches. Boric acid (H3BO3) (AR grade)

from SRL Pvt. Ltd., India was used as the source for

B2O3 and SrCO3, extra pure grade from Loba Chemie,

India was used as the source for SrO. Batch formulations

for compositions SrO (x wt%), La2O3 (15 wt%), Al2O3

(15 wt%), B2O3 (40-x wt%) and SiO2 (30 wt%) [x = 10,

15, 20, 25, 30] were calculated considering the gravime-

tric factors for H3BO3 and SrCO3. Raw materials in ap-

propriate proportion for 50g batch size were mixed tho-

roughly. To tal mass was melted in a platinum crucib le at

1450˚C for 1hr and quenched in a pre heated brass

mould. Glass samples were characterized by differential

thermal analysis (DTA) for their thermal behaviour.

Subsequen- tly glasses were annealed at temperatures

close their glass transition temperatures for removal of

thermal stresses fro m the glass matrices.

To analyze the phases of the melt quenched sample

X-ray diffractometry (XRD) was carried out using XPert

MPD, PAnalytical. Diffraction studies were carried out

in the range of 20˚ - 80˚ (2

) with step size of 0.005˚

using CuKα radiation. Fourier Transform Infrared Spec-

troscopy (FTIR) was carried out using 1600 Series FTIR

of P erkin-El mer via KBr pellet technique method. Quan-

titative informatio n about the structura l groups in glasses

was obtained from the deconvoluted FTIR spectra. In this

report FTIR data are presented in absorbance mode for

ease of deconvolution. The diffused reflectance spec-

troscopy (DRS) of the glass samples were carried out in

UV-Vis-NIR region using Perkin Elmer precisely,

Lambda 35, UV/VIS Spectroscopy. In the DRS, absor-

bance of the sample has been plotted against energy. The

onset wavelength of the optical absorbance has been

considered for band gap energy calculation using the

standard equation, and is presented in the unit of electron

Volt (eV). Electrical conductivity of samples were meas-

ured from room temperature to 800˚C by Impedance

Spectroscopy using AUTOLAB, ECO CHEMIE, Neth-

erlands. Samples in the form of circular disc were in-

serted between two platinum disks into an alumina hold-

er and positioned (spring-loaded) inside a top loading

furnace. Platinum leads attached to the platinum plates

were connected to the impedance analyzer for collecting,

storing and processing of data. Impedance spectra of the

glasses at different temperatures were recorded in the

frequency range 100 Hz to 1 MHz. From the impedance

data resistance of the sample was used for calcula tin g the

conductivity, taking into account the sample di mensions.

3. Results and Discussion

Different batch compositions used to prepare the glasses

and the code for each such composition is enlisted in

Table 1. 50 g of glass was prepared for each composition

by melt quenching the batch under the conditions men-

tioned earlier. For phase analysis the melt quench

sam-ples werAe analyzed by XRD. Figure 2 shows a

representative XRD plo t of the melt quenched glass wi th

composition SLBS-4. XRD plots of all samples show

absence of high intensity peaks with a broad hump ap-

Structural Inversion and Behavioural Changes as a Function of Composition in Sr-La-Al-B-Si Bas ed Gla sses

Table 1. Batches with SrO-La2O3-Al2O3-B2O3-SiO2 compo-

sitions for glass making and their corresponding nomen-

clature.

Glass Code

Glass Composition (wt%)

SrO

SiO

S LABS -3

S LABS -4

S LABS -5

S LABS -6

S LABS -7

pearing each case which is a clear indication of the

amo rphous/glassy nature of the sample. For structure

elucidation all the glasses were characterized through

FTIR spectroscopy. Figure 3 shows FTIR spectra for

different glasses. Each spectrum shows four active infra-

red spectral regions. First broad peak appeared in the

range 400 - 600 cm-1 and is assigned to the bending vi-

bration in SiO4 network. Peak in the range of 600 - 850

cm-1 is attributed to the bending vibration of borate seg-

ments. 850 - 1200 cm-1 segment is attributed to stre tching

vibration of structural groups containing BO4 tetrahedral

and overlaps with SiO4 tetrahedral. These structural

groups consist of BO3 and BO4 units without

non-bridging oxygen (NBOs) ions. Peak in the region

1200 to 1500 cm-1 arises from B-O bond vibration of

BO3 units [1 1, 25-27]. This signifies two types of network

structures in the glass: one consisting of BO3 and BO4

units and the other consisting of SiO4 unit. FTIR spectra

were corrected using two- point baseline correction. The

spectra were normalized to eliminate the concentration

effect of the po wder sample in KBr disc. To get quantita-

tive informatio n about structu ral groups, the spectra were

deconvoluted in to Gaussian bands. Only the 400 - 1600

cm-1 range was considered for deconvolution and least

square method was used to analyze the graphs. A repre-

sentative plot is shown in Figure 4 which illustrates de-

convolution of the FTIR spectrum of SLABS-4 glass.

Data generated by deconvolution of FTIR spectra of

samples include peak position, peak height, FWHM of

the peak, and area under the peak. Peaks were assigned

for characteristic bands and relative area under the peak

was calculated with respect to the total area under all the

peaks. FTIR spectra of all glass samples were deconvo-

luted and the generated data were analysed for structural

findings. Tabl e 2 is a representative table enlisting de-

convoluted data generated from FTIR spectra of

SLABS-3 glass. As glasses are having complicated

structures, several peaks obtained on deconvolution of

FTIR spectra could not be assigned for characteristic

vibrations as shown in the Table 2. The table includes

relative area under peak which gives a quantitative idea

of the corresponding structural group in the glass struc-

ture. Although these glasses are having three network

formers BO3, BO4 and SiO4 units, however, glass com-

positions are changing with respect to wt% of B2O3 in the

glass matrix (B2O3 is decreasing from 30 wt% to 10 wt%

from SLABS-3 to SLABS-7) so it will not be technicall y

proper to compare characteristic peaks due to BO3 and

BO4 in different glasses. Therefore the effect of increas-

ing SrO content as network modifier is compared in dif-

ferent glasses relating to the changes in SiO4 network

structure. From deconvolution data the relative area un-

der Si-O– non bridging oxygen peak (~ 929 cm-1) [12]

was calculated for different glass compositions. A

grap hic al pr e sent a ti on o f t he no n b r id ging ox yge n (N B O)

content in the glass with respect to Sr O content is shown

in Figure 5. The relative area which is a representation

of the NBO content in the glass matrix was found to in-

crease linearly with increase in SrO content. This is due

to SrO being a network modifier; it tends to invert the

struct ure by breaking the network bonds in SiO4 tetrahe-

dra l. I n the br oke n net work, the S r+2 io ns o ccup y i nters ti-

tial positions surrounded by non bridging oxygen ions.

Lu et al. [28 ] re por ted, incr ease in gla ss net work c o nnec-

tivity with decr easing B 2O3/SiO 2 i n S LAB S gl a ss s yst e m.

However, in this case it was observed that even if the

ratio decreases from 1 (SLABS-3) to 0.333 (SLABS-7),

the connectivity decreases with formation of more non

bridging oxygens as the SrO content i ncrea se d. T his ma y

be due to network modifiers having more impact over the

glas s formers s uch as S iO2 and B2O3 [29].

Diffused Reflectance Spectroscopy (DRS) of glass

samples were carried out in the UV-Vis region. Figure 6

shows absorbance of glass samples in the wavelength

range of 200 to 800 nm. From the plot it is o bserved tha t

with decrease in wavelength (i.e. with increase in energy)

glass samples start absorbing radiation below a certain

wavelength. Absorbance increases with a different gra-

dient with increase in energy and remain constant at

higher energies. This signifies an indirect type of band

edge in the glasses. Wavelength of the onset point of

absorbance was used to calculate the band gap of glass

and the plot of band gap with respect to SrO content in

the glass matrix is shown inset of Figure 6. The band

gaps of glasses were calculated from the absorption wa-

velength, using the standard wavelength energy conver-

sion formula and values found to be within 2.97 - 3.07

eV. The band gap decreased with increase in SrO co ntent

in the glass. Generally, the optical absorption of glasses

in the UV-Vis region is determined by the oxygen bond

stren gth in t he gla ss for ming ne twork. Any c hange i n the

status of the oxygen bonding, for instance, formation of

non bridging oxygen (NBO) changes the characteristic

absorption edge. In the present study, the position of the

fundamental absorption edge shifts to higher wavelength

Stru ctural Inversion and Behavioural Changes as a Function of Composition in Sr-La-Al-B-Si Based Glasses

Figure 2. XRD plot of SLAB S-4.

Figure 3. FTIR plots of SLABS glasses (a. SLABS-3, b.

SLABS -4, c. SLABS-5, d. SLABS-6, e. SLABS-7).

Figure 4. A ty pical deconv oluti on spect ra of SLA BS-4 glass

a. ex perimental c ur ve, b. si mulated c urve a n d the co mput ed

Gaussian bands.

(lo wer ene rgy) wit h incr eas in g SrO c onte nt. T he shifts o f

the absorption band to longer wavelength correspond to

the structural modification with generation of more

NBOs which bound an excited electron less tightly than

the bridging oxygen [30]. UV-Vis DRS result supports

the fi ndings in F TIR deconvoluti on st udy.

All the glasses have been characterized for their ther-

mal behaviour and these glasses show glass transitions

within temperature range of 554˚C to 659˚C and dila

510 15 20 25 30 35

Non bridging Si-O- (Relative area %)

S rO c o n ten t (wt %)

Figure 5. Non br idging Si-O– content i n SLABS glasses as a

functio n of SrO content.

300 400 500 600 700 800

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

10 15 20 25 30

2.96

2.98

3.00

3.02

3.04

3.06

3.08

Band gap (eV)

Sr O c o n te n t (w t%)

Absorbance

W avelength (nm )

SLABS-3

SLABS-4

SLABS-5

SLABS-6

SLABS-7

Figure 6. UV-Vis Diffused Reflectance Spectroscopy (UV-

Vis-DRS) of glasses with band gap versus composition

shown inset.

tometric softening point within 660˚C to 709˚C. More

over, both glass transition temperatures and dilatometric

softening point temperatures show an increasing trend

with i ncrease in substi tution i n the serie s. In thi s work to

elucidate a structure property correlation in the series,

electrical conductivity of the glasses were investigated by

AC impedance spectroscopy from room temperature to

800˚C and conductivity of the samples were calculated

from the resistance value considering the sample dimen-

sions. As the glasses were originally designed for appli-

cation in solid oxide fuel cell (SOFC) sealant so their

conductivity at SOFC operational temperature which is

higher than the glass transition temperatures is of great

concern. Therefore, a plot of conductivity of glass sam-

ples at 800˚C against the SrO content in the sample is

sho wn in Figur e 7. It is ob served that c ond ucti vit y of the

glass samples increased from 2.74 × 10-5 Scm-1 to 1.09 ×

10-4 Scm-1 with increase in SrO content in the glass ma-

Structural Inversion and Behavioural Changes as a Function of Composition in Sr-La-Al-B-Si Bas ed Gla sses

Table 2 . Deconvoluti on data of the FTIR spectra of SLABS-3.

Peak Peak Type

Peak Position

(cm–1) Heig ht FWH H Ar e a

Relative Area

(%) Band Assignment

1 Gaussian 431 0.02 42.89 0.78 0.76

2 Gaussian 453 0.03 45.56 1.24 1.20 Si-O-Si and O-Si-O bending vib

3 Gaussian 470 0.01 37.61 0.59 0.57

4 Gaussian 494 0.01 40.80 0.54 0.53

5 Gaussian 556 0.00 37.98 0.01 0.01

6 Gaussian 683 0.13 78.27 10.55 10.28 S tretchi ng vib of B-O-B

7 Gaussian 803 0.00 36.85 0.03 0.03

8 Gaussian 903 0.07 66.05 4.76 4.64 Non bridging Si-O

9 Gaussian 968 0.09 68.05 6.76 6.58 BO4 stretching vib

10 Gaussian 1028 0.14 67.88 9.83 9.58

11 Gaussian 1090 0.15 69.19 10.98 10.70 Si-O-Si antisym stretching vib

12 Gaussian 1143 0.06 51.15 3.15 3.07

13 Gaussian 1276 0.08 81.89 6.63 6.46

14 Gaussian 1370 0.15 106.99 16.76 16.33

B-O bond vibration of borate group

15 Gaussian 1458 0.15 96.57 15.49 15.10

16 Gaussian 1542 0.13 90.62 12.16 11.85

17 Gaussian 1621 0.03 61.07 2.36 2.30

trix from 10 wt% to 30 wt%. In general, conductivity in

oxide glass matrix depends on two factors, temperature

and number of available charge carriers (i.e. oxide ions)

[31]. In this case all the conductivities are reported at a

constant temperature of 800˚C, thus the temperature ef-

fect is constant for all the glasses. Again, the B2O3 con-

tent in the glass matrices which may be contributing to

the total conductivity of the glasses is changing. So in

this case the changing conductivity of glasses is corre-

lated with the structural changes of SiO4 units only. In-

creasing SrO content in the glass leads to increase in

NBO content due to structural inversion and decreasing

10 15 20 25 30

0. 0

5. 0x10

-5

1. 0x10

-4

1. 5x10

-4

Conductivity

(

S.cm

-1

)

S rO (wt%)

Figure 7. Conductivity of SLABS glasses versus SrO con-

tent in the glas s.

band edge. This makes more carriers available in the SrO

content the conductivity of the glas s increases.

4. Conclusions

Glasses with composition SrO (x wt%) La2O3 (15 wt%)

Al2O3 (15 wt%) B2O3 (40-x wt%) SiO2 (30 wt%) were

prepared for x = 10, 15, 20, 25 and 30. FTIR analysis

show two types of network structures in the glass: one

consisting o f BO3 and BO4 units and the other consisting

of SiO4 unit. Deconvolution of FTIR plots highlights the

structural changes with composition. With increase in

SrO content in the glass matrix, non bridging Si -O– con-

tent inc rea ses eve n thou gh the B2O3/SiO2 decreases. This

structural inversion is reflected in the properties of the

glasses. The band edge of the glass samples measured b y

Uv-Vis DRS show a decreasing band gap with increasing

SrO content. Conductivity of glass samples which is

measured by impedance spectroscopy increases with

increasi ng substitution of B2O3 by SrO.

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