Structural Inversion and Behavioural Changes as a Function of Composition in Sr-LaAl-BSi Based Glasses

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 increasing 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 Scm to 1.09 × 10 Scm with increasing substitution from x = 10 to x = 30.


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 network formers in the basic structure, and modifier oxides "invert" the network structure by breaking the network bonds and generate non bridging oxygens.The term "invert" was introduced by Trapp and Stevels [1], because the traditional network forming oxides SiO 2 , B 2 O 3 , and P 2 O 5 form continuous molecular/ionic networks in normal conditions; however when the network modifying oxides are in majority on the molar basis, the glasses are inverted structurally compared to conventional glasses as shown in Figure 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 physicalchemical properties of glasses.For example, systematic substitution of PbO by B 2 O 3 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 Q 4  increase of thermal expansion coefficient, deformation and flow temperature [3] due to structural modification.It has been reported [4][5][6][7] that addition of network modifiers (alkali/alkaline earth metal oxides) to borosilicate glasses results in the initial conversion of BO 3 to BO 4 structural units.At higher concentration of modifiers, BO 4 structural units in the glass are replaced by BO 3 -structural units (planar BO 3 structural units with one non bridging oxygen atom).Hence the properties of glasses with different modifier concentrations are different.To understand the criticality of glass science it is pertinent to investigate the structure of glasses and correlate the structure with glass behavior.In this regard many researchers have tried to elucidate the structure of glasses using various spectroscopic techniques [8][9][10][11][12][13][14][15][16][17][18][19][20][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][23][24].These studies highlight the existence of various structural units like trigonally coordinated boron (BO 3 ), tetrahedrally coordinated boron (BO 4 ), silicon atoms with 3 and 4 bridging oxygen atoms, Q n units with Si-O-B/ Si-O-Al linkages, etc. in the glass matrix.These structural units finally govern the properties of the glasses.
In this work structure property correlation of a series of glasses with composition SrO (x wt%), La 2 O 3 (

Experimental
SiO 2 , AR grade from S. D. Fine-Chem.Ltd., India, Al 2 O 3 , AR grade from CDH, India and La 2 O 3 , GR grade from Loba Chemie, India were used as received for preparation of batches.Boric acid (H 3 BO 3 ) (AR grade) from SRL Pvt. Ltd., India was used as the source for B 2 O 3 and SrCO 3 , extra pure grade from Loba Chemie, India was used as the source for SrO.Batch formulations for compositions SrO (x wt%), La 2 O 3 (15 wt%), Al 2 O 3 (15 wt%), B 2 O 3 (40-x wt%) and SiO 2 (30 wt%) [x = 10,15,20,25,30] were calculated considering the gravimetric factors for H 3 BO 3 and SrCO 3 .Raw materials in appropriate proportion for 50g batch size were mixed thoroughly.Total mass was melted in a platinum crucible 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 from 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˚ (2θ) with step size of 0.005˚ using CuKα radiation.Fourier Transform Infrared Spectroscopy (FTIR) was carried out using 1600 Series FTIR of Perkin-Elmer via KBr pellet technique method.Quantitative information about the structural 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 spectroscopy (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, absorbance 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 measured from room temperature to 800˚C by Impedance Spectroscopy using AUTOLAB, ECO CHEMIE, Netherlands.Samples in the form of circular disc were inserted between two platinum disks into an alumina holder 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 calculating the conductivity, taking into account the sample dimensions.

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 mentioned earlier.For phase analysis the melt quench sam-ples werAe analyzed by XRD. Figure 2 shows a representative XRD plot of the melt quenched glass with composition SLBS-4.XRD plots of all samples show absence of high intensity peaks with a broad hump ap- pearing each case which is a clear indication of the amorphous/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 infrared spectral regions.First broad peak appeared in the range 400 -600 cm -1 and is assigned to the bending vibration in SiO 4 network.Peak in the range of 600 -850 cm -1 is attributed to the bending vibration of borate segments.850 -1200 cm -1 segment is attributed to stretching vibration of structural groups containing BO 4 tetrahedral and overlaps with SiO 4 tetrahedral.These structural groups consist of BO 3 and BO 4 units without non-bridging oxygen (NBOs) ions.Peak in the region 1200 to 1500 cm -1 arises from B-O bond vibration of BO 3 units [11,[25][26][27].This signifies two types of network structures in the glass: one consisting of BO 3 and BO 4 units and the other consisting of SiO 4 unit.FTIR spectra were corrected using two-point baseline correction.The spectra were normalized to eliminate the concentration effect of the powder sample in KBr disc.To get quantitative information about structural 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 representative plot is shown in Figure 4 which illustrates deconvolution 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 deconvoluted and the generated data were analysed for structural findings.is decreasing from 30 wt% to 10 wt% from SLABS-3 to SLABS-7) so it will not be technically proper to compare characteristic peaks due to BO 3 and BO 4 in different glasses.Therefore the effect of increasing SrO content as network modifier is compared in different glasses relating to the changes in SiO4 network structure.From deconvolution data the relative area under Si-O -non bridging oxygen peak (~ 929 cm -1 ) [12] was calculated for different glass compositions.A graphical presentation of the non bridging oxygen (NBO) content in the glass with respect to SrO content is shown in Figure 5.The relative area which is a representation of the NBO content in the glass matrix was found to increase linearly with increase in SrO content.This is due to SrO being a network modifier; it tends to invert the structure by breaking the network bonds in SiO 4 tetrahedral.In the broken network, the Sr +2 ions occupy interstitial positions surrounded by non bridging oxygen ions.Lu et al. [28] reported, increase in glass network connectivity with decreasing B 2 O 3 /SiO 2 in SLABS glass system.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 increased.This may be due to network modifiers having more impact over the glass formers such as SiO 2 and B 2 O 3 [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 observed that with decrease in wavelength (i.e. with increase in energy) glass samples start absorbing radiation below a certain wavelength.Absorbance increases with a different gradient 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 wavelength, using the standard wavelength energy conversion formula and values found to be within 2.97 -3.07 eV.The band gap decreased with increase in SrO content in the glass.Generally, the optical absorption of glasses in the UV-Vis region is determined by the oxygen bond strength in the glass forming network.Any change in 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   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 increase in substitution in the series.In this 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 dimensions.As the glasses were originally designed for application 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 samples at 800˚C against the SrO content in the sample is shown in Figure 7.It is observed that conductivity 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- In this case all the conductivities are reported at a constant temperature of 800˚C, thus the temperature effect is constant for all the glasses.Again, the B 2 O 3 content 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 correlated with the structural changes of SiO 4 units only.Increasing SrO content in the glass leads to increase in NBO content due to structural inversion and decreasing band edge.This makes more carriers available in the SrO content the conductivity of the glass increases.

Conclusions
Glasses with composition SrO (x wt%) La

Figure 1 .
Figure 1.Structure of glass inverted by modifier ions.

Figure 4 .Figure 5 .Figure 6 .
Figure 4.A typical deconvolution spectra of SLABS-4 glass a. experimental curve, b. simulated curve and the computed Gaussian bands.(lower energy) with increasing SrO content.The shifts of 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 findings in FTIR deconvolution study.All the glasses have been characterized for their thermal behaviour and these glasses show glass transitions within temperature range of 554˚C to 659˚C and dila

Figure 7 .
Figure 7. Conductivity of SLABS glasses versus SrO content in the glass.

Table 2 .
The table includes relative area under peak which gives a quantitative idea of the corresponding structural group in the glass structure.Although these glasses are having three network formers BO 3 , BO 4 and SiO 4 units, however, glass compositions are changing with respect to wt% of B 2 O 3 in the glass matrix (B 2 O 3

Table 2 . Deconvolution data of the FTIR spectra of SLABS-3.
2 O 3 (15 wt%) Al 2 O 3 (15 wt%) B 2 O 3 (40-x wt%) SiO 2 (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 of BO 3 and BO 4 units and the other consisting of SiO 4 unit.Deconvolution of FTIR plots highlights the structural changes with composition.With increase in SrO content in the glass matrix, non bridging Si-O -content increases even though the B 2 O 3 /SiO 2 decreases.This structural inversion is reflected in the properties of the glasses.The band edge of the glass samples measured by Uv-Vis DRS show a decreasing band gap with increasing SrO content.Conductivity of glass samples which is measured by impedance spectroscopy increases with increasing substitution of B 2 O 3 by SrO.