STRUCTURE-PROPERTY CORRELATIONS IN BISMUTH GLASSES

Transcription

1 CHAPTER 4 STRUCTURE-PROPERTY CORRELATIONS IN BISMUTH GLASSES 4.1 Introduction Recently Bajaj et al. (2009a, 2009b, 2009c) reported very detailed density, optical, thermal, and structural and devitrification properties of binary bismuth borate glasses. In this chapter we have presented the results of effects of addition of SiO 2 and Al 2 O 3 on the density, optical and thermal properties of bismuth borate glasses containing 40 and 50- mol% Bi 2 O 3. Although binary Bi 2 O 3 -B 2 O 3 glasses have been studied extensively in the literature [Bajaj et al. (2009a)]. There are very few studies, if any, on bismuth borosilicate, bismuth aluminoborate and bismuth aluminoborosilicate glasses. In this chapter we have discussed the results of characterization studies on bismuth glasses by density, UV-visible absorption spectroscopy, DSC, 11 B and 27 Al MAS-NMR measurements. Glasses of the following composition were prepared and studied: (1) x Bi 2 O 3 - y SiO 2 - (100-x-y) B 2 O 3 (x = 40, 50 and y = 5, 10 & 20 mol%) (2) x Bi 2 O 3 -y SiO 2 -z Al 2 O 3 - (100-x-y-z) B 2 O 3 (x = 50, y = 0, 5, 10 & 20 and z = 2, 4, 6 & 8 mol%) 4.2 Density Fig. 4.1 displays the variation of density of two bismuth borosilicate glass series containing, 40 and 50 mol% Bi 2 O 3, with the increase in SiO 2 concentration from 5 to 20 mol%. The density, molecular weight and molar volume values of crystalline oxide components used for bismuth glass fabrication: Bi 2 O 3, α-al 2 O 3, SiO 2 and B 2 O 3 are given in Table 4.2. The densities and the molar volume data of all the bismuth borosilicate glasses are presented in Table

2 The density of bismuth borosilicate glasses (40 mol% of Bi 2 O 3 ) increases from ± to ± g cm -3 with the addition of SiO 2 (5 to 20 mol%) and in case of 50 mol% Bi 2 O 3 samples the density again increases with SiO 2 incorporation from a value of ± to ± g cm -3, Bajaj et al. (2009) reported density of 6.246±0.001 g cm -3 and 6.874±0.005 g cm -3 for bismuth borate glasses with 40 and 50 mol% Bi 2 O 3 respectively (Table 4.1). The increase in density with silica addition is attributed to the replacement of lighter B 3+ ions with heavier Si 4+ in borosilicate glasses The excess volume, V x is (defined by equations: ) is negative, as all glasses contain tetrahedral borons, while the crystalline B 2 O 3 whose density value was used for calculating the additive crystalline oxide volume in bismuth and lead glasses contains only trigonal borons [Taylor and Cole (1934)]. Table 4.1: Composition, density, molar volume, additive crystalline oxide volume and excess volume in bismuth borate glasses [Bajaj et al. (2009a)]. Sample Code Composition (mol %) Bi 2 O 3 B 2 O 3 Density (gm /cm 3 ) Molar Volume V m (cm 3 mol -1 ) Additive Crystalline Oxide Volume V o (cm 3 ) Excess Volume V x (cm 3 ) Bi40B ± Bi50B ± Table 4.2: Density, molecular weight and molar volume values of crystalline oxide components. Oxide Mol. wt. (amu) Density (g cm -3 ) Molar Volume (cm 3 mol -1 ) Bi 2 O α-al 2 O (rhombohedral) SiO B 2 O

3 7.2 Density (g cm -3 ) mol% Bi 2 O 3 40 mol% Bi 2 O SiO 2 (mol%) Fig. 4.1: Density of bismuth borosilicate glasses as a function of SiO 2 concentration. Table 4.3: Composition, density, molar volume, additive crystalline oxide volume and Sample Code excess volume in bismuth borosilicate glasses. Composition (mol %) Bi 2 O 3 B 2 O 3 SiO 2 Density (gm /cm 3 ) Molar Volume V m (cm 3 mol -1 ) Additive crystalline Oxide Volume V o (cm 3 ) Excess Volume V x (cm 3 ) Bi40BS ± Bi40BS ± Bi40BS ± Bi50BS ± Bi50BS ± Bi50BS ± The effect of increase in density of bismuth borosilicate glasses with SiO 2 incorporation is similar to that found in lead borosilicate glasses (Chapter 3). While lead borosilicate glasses have been earlier investigated by number of other authors, the density data on bismuth borosilicates is new. Recently Zhang et al. (2009) reported density of 99

4 50Bi 2 O 3-10B 2 O 3-40SiO 2 glass as g cm -3. Their glass contained SiO 2 concentration of 40 mol % but still its density ( g cm -3 ) is significantly less than the density (7.162±0.004 g cm -3 ) of our bismuth borosilicate glass with highest SiO 2 content of 20 mol %, although we find from our studies that density increases in bismuth borosilicate glasses on replacing lighter B 3+ with heavier Si 4+ ions. This is in all probability due to the use of alumina crucible by Zhang et al. (2009) for glass melting. Alumina crucibles react unkindly with borosilicate melts and introduce significant Al 3+ impurities that drastically reduce density as demonstrated by our studies on bismuth aluminoborate glasses. We prepared and characterized four bismuth aluminoborate glasses with 50 mol% Bi 2 O 3. We find that on gradually increasing alumina concentration from 2 to 8 mol %, density decreases systematically from a value of ± to ± g cm -3 and the glass molar volume increases with increase in alumina content whereas additive volume decreases (Table 4.4). Similar effects are observed in lead aluminoborate glasses although density decrease in bismuth aluminoborate glasses is greater (Chapter 3). These results indicate that alumina incorporation produces drastic modification in the borate glass structure, probably by a large reduction in the fraction of tetrahedral borons (which are more compact than the three coordinated planar trigonal boron units), and/or by decrease in Al-O coordination number from 6 to 5 and 4. Our 11 B and 27 Al MAS-NMR investigations (discussed later) on these glasses indeed confirm these conclusions. Table 4.4: Composition, density, molar volume, additive crystalline oxide volume and excess volume in bismuth aluminoborate glasses. Sample Code Composition (mol %) Bi 2 O 3 B 2 O 3 Al 2 O 3 Density (gm/cm 3 ) Molar Volume V m (cm 3 mol -1 ) Additive Crystalline Oxide Volume V o (cm 3 ) Excess Volume V x (cm 3 ) Bi50BA ± Bi50BA ± Bi50BA ± Bi50BA ±

5 Fig. 4.2: Density of (a) bismuth aluminoborate and (b) aluminoborosilicate glasses as a function of SiO 2 concentration. We prepared three bismuth aluminoborosilicate glass series each containing 50 mol% Bi 2 O 3 and three SiO 2 concentrations of 5, 10 and 20 mol% and with increasing alumina concentrations of 2 to 8 mol%. In bismuth aluminoborosilicate glasses (5 mol% SiO 2 ) density decreases from 6.965± to 6.872± g cm -3 with the increase in aluminum oxide. A similar trend of decrease in density is found in the other two bismuth aluminoborosilicate series with 10 and 20 mol% SiO 2. For glasses with equal alumina content but higher SiO 2 concentration, density is more. The variation of density with Al 2 O 3 concentration in one bismuth aluminoborate and three bismuth aluminoborosilicate glass series is displayed in Fig. 4.2 and the data is presented in Table

6 Table 4.5: Composition, density, molar volume, additive crystalline oxide volume and excess volume in bismuth aluminoborosilicate glasses. Sample Code Composition (mol %) Bi 2 O 3 B 2 O 3 Al 2 O 3 SiO 2 Density (gm/cm 3 ) Molar Volume (V m cm 3 mol -1 ) Additive Crystalline Oxide Volume V o (cm 3 ) Excess Volume V x (cm 3 ) Bi50BA2S ± Bi50BA4S ± Bi50BA6S ± Bi50BA8S ± Bi50BA2S ± Bi50BA4S ± Bi50BA6S ± Bi50BA8S ± Bi50BA2S ± Bi50BA4S ± Bi50BA6S ± Bi50BA8S ± Earlier Lenior et al. (2008) also reported an increase in density of barium borosilicate glasses with the increase in silica concentration. Similarly Fujino et al. (2004) carried out in situ measurements on density, surface tension and viscosity of several lead borosilicate melts and found that density rises with the increase in silica concentration. Our density results strongly suggest that SiO 2 act as a network former in bismuth borosilicate and bismuth aluminoborosilicate glasses, whereas Al 2 O 3 depolymerizes the glass network by breaking BO 4 into BO 3 units. 11 B and 27 Al MAS NMR measurements are more useful for elucidating the influence of Si and Al ions in the borate network. 102

7 4.3 Optical Properties The optical absorption spectra of bismuth borosilicate and aluminoborate glasses are displayed in Figs. 4.3 and 4.4 respectively. Fig. 4.5 and 4.6 display the optical spectra of bismuth aluminoborosilicate glasses. The optical properties of bismuth glasses like density values are similar to that of lead glasses (Chapter 3). We determined the absorption edge, E o and Urbach energy, E values using equations 3.3 and 3.4. The maximum uncertainty in E o and E are 0.1 ev and 0.05 ev respectively. The optical absorption edge values show insignificant changes with increase in silica and alumina concentrations in borosilicate, aluminoborate and aluminoborosilicate glasses (Tables 4.6, 4.7 and 4.8 respectively) These results suggest that there is little or no change in the fraction of non-bridging oxygens, f NBO in each glass series. We observe a very intriguing, strong and rather broad optical absorption band in bismuth aluminoborate glasses just below the optical edge (Fig. 4.4) Bi50BS5 Bi50BS10 Bi50BS20 ln Bi40BS5 Bi40BS10 Bi40BS E(eV) Fig. 4.3: Urbach plots of bismuth borosilicate glasses containing 40 and 50 mol% Bi 2 O 3 and SiO 2 concentration of 5 to 20 mol%. 103

8 Table 4.6: Absorption edge and Urbach energy values in bismuth borosilicate glasses. Sample Code Absorption Edge E o (ev) Urbach Energy ΔE (ev) Bi40BS Bi40BS Bi40BS Bi50BS Bi50BS Bi50BS This band is missing or greatly suppressed in bismuth borosilicate glasses (Fig. 4.3) suggesting that Al 3+ have a role in its origin. This optical band is strongest in bismuth aluminoborate glass with 2 mol % of Al 2 O 3 and is missing or suppressed in bismuth glasses containing SiO 2. Earlier Fujmoto and Nakatsuka (2006) had found a new infrared fluorescence at 1.3 m in bismuth doped silica glass. [Fujmoto and Nakatsuka (2000)]. Later these workers carried out 27 Al NMR studies and concluded that the 6-fold aluminum coordination in bismuth doped silica glass plays an important role in configuring Bi luminescent centers in these glasses [Fujmoto and Nakatsuka (2006)] ln ( ) Bi50BA2 Bi50BA4 Bi50BA6 Bi50BA E (ev) Fig. 4.4: Urbach plots of bismuth aluminoborate glasses. 104

9 Table 4.7: Absorption edge and Urbach energy values in bismuth aluminoborate glasses. Sample Code Absorption Edge E o (ev) Urbach Energy ΔE (ev) Bi50BA Bi50BA Bi50BA Bi50BA Our 27 Al MAS NMR studies on Al 2 O 3 containing bismuth glasses (discussed later) reveal that 6-fold Al coordination decreases with increasing SiO 2 concentration and is maximum in bismuth aluminoborate glasses which contain as high as 28±2 % of 6- fold coordinated Al ions (values given later in Table 4.17). Therefore the strong absorption band found just below the absorption edge in bismuth aluminoborate glasses band is due to significant fraction of Al 3+ ions in 6-fold coordination. On adding SiO 2 in bismuth aluminoborate glasses the concentration of Al 3+ ions in 6-fold coordination decreases, for instance bismuth aluminoborosilicate glass with 20 mol % SiO 2 (50Bi 2 O 3-22B 2 O 3-8Al 2 O 3-20SiO 2, Sample Bi50BA8S20) contains only 4±2 % of Al ions in 6-fold coordination (Table 4.17), and this band is missing in the optical spectra (Fig. 4.6). Similarly the bismuth aluminoborosilicate glass with 10 mol % SiO 2 (50Bi 2 O 3-28B 2 O 3-2Al 2 O 3-10SiO 2, Sample Bi50BA2S10) contains about 15 % of Al ions in octahedral coordination and consequently it exhibits this optical absorption band (Fig. 4.6). This optical absorption band is missing or significantly suppressed in bismuth glasses containing SiO 2. Recently Bajaj et al. (2009) also reported observing very strong optical absorption band in binary Bi 2 O 3 -B 2 O 3 glasses with 40 mol% or higher concentration of Bi 2 O 3. As compared to the binary bismuth borate glasses, the intensity of this band is certainly less, but significant in boroaluminate glasses, and seems to reduce with increase in alumina and silica concentrations. Murata and Mouri (2007) reported a very interesting glass matrix effect on the UV-visible absorption and fluorescence properties of silicate and borate glasses containing Bi 2 O 3. They found that 105

10 Bi 3+ ion containing borate glasses show an optical absorption band around 440 nm, which is completely missing in silicate glasses. Sanz et al. (2006) reported large influence of glass preparation conditions on the optical properties of several oxide glasses containing Bi 2 O 3. Transmission electron microscopy (TEM) studies by these workers concluded that at high bismuth oxide concentration, Bi 3+ ions reduce to Bi 2+. The oxidation state of bismuth ions can critically influence the optical absorption and fluorescence properties of bismuth borate glasses. The optical band can also be due to excitonic transitions in nano crystals that may exist in these glasses at high Bi 2 O 3 concentration of 50 mol % ln ( ) Bi50BA2S5 Bi50BA4S5 Bi50BA6S5 Bi50BA8S E (ev) Fig. 4.5: Urbach plots of bismuth aluminoborosilicate glasses (5 mol% SiO 2 ). 106

11 Fig. 4.6: Urbach plots of bismuth aluminoborosilicate glasses (10 and 20 mol% SiO 2 ). Table 4.8: Absorption edge and Urbach energy values in bismuth aluminoborosilicate glasses. Sample Code Absorption Edge E o (ev) Urbach Energy ΔE (ev) Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S

12 4.4 Thermal Properties The DSC patterns of two bismuth borosilicate glass series are displayed in Fig The values of the glass transition temperature (midpoint value), crystallization temperature (peak point) and melting temperatures (peak point) of bismuth borosilicate glasses are given in the Table 4.9. The glass transition temperature does not show any significant change within the limits of experimental error ( 2 o C), on adding silica in the first bismuth borosilicate series containing 40 mol% Bi 2 O 3. As compared to pure 40Bi 2 O 3-60B 2 O 3 glass (Table 4.10) T g decreases from a value of 440 to 434 C on adding 5 mol% SiO 2. In the second bismuth borosilicate glass series, T g falls significantly from a value of 434 to 397 o C on increasing SiO 2 concentration from 5 to 20 mol% (sample Bi50BS5 and Bi50BS20, Table 4.9). 0 Q (W g -1 ) Bi50BS5 Bi50BS10 Bi50BS20 Bi40BS5 Bi40BS10 Bi40BS T ( 0 C) Fig. 4.7: DSC spectra of bismuth borosilicate glasses containing 40 and 50 mol% Bi 2 O 3 and SiO 2 concentration of 5 to 20 mol%. 108

13 Table 4.9: Glass transition, crystallization and liquidus temperatures of bismuth borosilicate glasses (maximum uncertainty in temperature is ± 2 o C). Sample T g T c1 T c2 T c3 T m1 T m2 T m3 T m4 Code Bi40BS Bi40BS Bi40BS Bi50BS Bi50BS Bi50BS Table 4.10: Glass transition, crystallization and liquidus temperatures of bismuth borate glasses (maximum uncertainty in temperature is ± 2 o C) [Bajaj et al. (2009a)]. Sample T g T c1 T c2 T c3 T m1 T m2 Code Bi40B Bi50B The glass transition temperature is related to the density of cross-linking, the tightness of the network formers and the co-ordination number of the network forming atoms. The decrease in transition temperature with rise in Bi 2 O 3 content from 40 to 50 mol% in bismuth borosilicates is due to the increase in the number of NBOs and increase in the concentration of Bi-O bonds, which weaken the borosilicate glass network. The DSC patterns of all bismuth borosilicate glasses exhibit exothermic crystallization and endothermic liquidus peaks in the first heating cycle of DSC. These crystallization and melting peaks were completely missing in lead boroaluminate, lead borosilicate and lead 109

14 aluminoborosilicate glasses (chapter 3). Moreover like lead borate glasses, bismuth glasses also do not exhibit crystallization and melting peaks during the second heating cycle of DSC experiment. This indicates the intriguing influence of melt history on the final glass properties. In bismuth aluminoborate glasses, we again observe little or no significant change in the glass transition temperature (Fig. 4.8 and Table 4.11) on increasing alumina concentration from 2 to 8 mol%. The glass transition temperature is constant at 393 C within the limits on experimental error i.e. 2 o C. We again observe crystallization and liquidus peaks in bismuth aluminoborates, which were absent in lead aluminoborate glasses. Further we note that all bismuth aluminoborate glasses show a single, clear glass transition while lead aluninoborate show a second weak glass transition around 465 C. Therefore bismuth glasses are structurally and/or compositionally more homogeneous than lead aluminoborate and lead aluminoborosilicate glasses. The intensity of exothermic crystallization peak decreases with increase in alumina concentration clearly suggesting that alumina stabilizes the glass structure by reducing its devitrification tendency. 1 0 Q (W g -1 ) -1-2 Bi50BA2 Bi50BA4 Bi50BA6 Bi50BA T Fig. 4.8: DSC spectra of bismuth aluminoborate glasses. 110

15 Table 4.11: Glass transition, crystallization and liquidius temperatures of bismuth aluminoborate glasses (maximum uncertainty in temperature is ±2 o C). Sample T g T c1 T c2 T c3 T m1 T m2 T m3 T m4 Code Bi50BA Bi50BA Bi50BA Bi50BA Q (W g -1 ) Bi50BA2S5 Bi50BA4S5 Bi50BA6S5 Bi50BA8S T Fig. 4.9: DSC spectra of bismuth aluminoborosilicate glasses (5 mol% SiO 2 ). We studied the thermal properties of three bismuth aluminoborosilicate glass series with 50 mol% Bi 2 O 3, as a function of alumina concentration in the range of 2 to 8 111

16 mol%. In the first two bismuth aluminoborosilicate series with 5 mol% and 10 mol% SiO 2, the glass transition temperature is nearly constant in the range of o C. In the third series with higher SiO 2 concentration of 20 mol%, T g first decreases from a value of 396 o C to 387 o C (Samples Bi50BA2S20 to Bi50BA4S20) and then increases to 405 o C in the last sample with 8 mol% Al 2 O 3 indicating that the glass network strengthens. Again, as in bismuth aluminoborate glasses we find that samples show crystallization and melting peaks but the crystallization tendency decreases with rise in both alumina and silica contents(figs 4.9 and 4.10 and Table 4.12). Fig. 4.10: DSC spectra of bismuth aluminoborosilicate glasses (10 and 20 mol% SiO 2 ). 112

17 Table 4.12: Glass transition, crystallization and liquidus temperatures of bismuth aluminoborosilicate glasses (maximum uncertainty in temperature is ± 2 o C). Sample T g T c1 T c2 T c3 T m1 T m2 T m3 T m4 Code Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S B MAS NMR To make structure-property correlations in bismuth glasses, as in lead glasses, we carried out high magnetic field 11 B MAS-NMR measurements. The NMR spectra of two bismuth borosilicate glass series with 40 and 50 mol% Bi 2 O 3 are displayed in Figs and 4.12.The signals from BO 3 and BO 4 units, centered around -15 and 0 ppm respectively are very well resolved at this high magnetic field and therefore the relative intensities of the two types of boron-oxygen sites can be determined very accurately. The 11 B MAS NMR spectra of bismuth glasses are very similar to those of lead glasses 113

18 discussed in detail in chapter 3. The fraction of boron in four coordination with oxygens, N 4, was determined in glasses by integrating the areas under [3] B and [4] B peaks. Bi40BS5 Bi40BS10 Bi40BS ppm Fig. 4.11: 11 B MAS-NMR spectra of bismuth borosilicate glasses containing 40 mol% Bi 2 O 3. Chemical shifts are referenced to BF 3 O(CH 2 CH 3 ) 2 at 0 ppm (Magnetic field 16.4T, Bruker Avance NMR spectrometer). In the two bismuth borosilicate series (40 and 50 mol% Bi 2 O 3 ), there is little or no significant change in the fraction of tetrahedral borons (N 4 ) with the increase in SiO 2 concentration, which is also evident from the lineshape and peak position in 11 B NMR spectra (Fig and 4.12). The fractions of NBO, f NBO were calculated using the equations similar to those used for lead borosilicate glasses i.e. f NBO = 2 - (n OSi + n OB ) where n OSi = 4 (C Si /C O ) the average silicon coordination number n SiO is 4 and n OB = n BO (C B /C O ). C Si, C B and C O are the atomic fractions of silicon, boron and oxygen respectively. The fraction of NBOs in bismuth borosilicate glasses increases with increase in silica concentration in the two borosilicate series (Table 4.13). Bajaj et al. 114

19 recently reported very accurate N 4 data in binary bismuth borate glasses. N 4 values in 40Bi 2 O 3-60B 2 O 3 and 50Bi 2 O 3-50B 2 O 3 glasses are reported to be 0.46 and respectively. Using these values, the calculated values of f NBO for these binary bismuth borate glasses are 0.58 and 0.85 respectively. Bi50BS5 Bi50BS10 Bi50BS ppm Fig. 4.12: 11 B MAS-NMR spectra of bismuth borosilicate glasses containing 50 mol% Bi 2 O 3. Chemical shifts are referenced to BF 3 O(CH 2 CH 3 ) 2 at 0 ppm (Magnetic field 16.4T, Bruker Avance NMR spectrometer. 115

20 Fig. 4.13: N 4 variation with SiO 2 concentration in bismuth borosilicate glasses containing 40 and 50 mol% Bi 2 O 3. Table 4.13: N 4 and fraction of NBOs of bismuth borosilicate glasses (40 and 50 mol% Bi 2 O 3 ). Sample Code N 4 (±0.01) f NBO Bi40BS Bi40BS Bi40BS Bi50BS Bi50BS Bi50BS Fig gives the variation of N 4 with SiO 2 concentration in bismuth borosilicate. The invariance of N 4, optical absorption edge and T g values indicate that SiO 2 incorporation does not significantly alter the borate network. This behavior is different from that in lead borosilicates in which N 4 rises significantly on incorporating silica in the borate network. 116

21 However an increase in NBO concentration (Table 4.13) indicates that silica Q 4 units convert into Q 3 units in bismuth borosilicate network as below: 2[SiO 4/2 ] +O 2-2[SiO 3/2 O - ] (4.1) The 11 B MAS NMR patterns of four bismuth aluminoborate glass samples containing 50 mol% Bi 2 O 3 are displayed below in Fig Bi50BA2 Bi50BA4 Bi50BA6 Bi50BA ppm Fig. 4.14: 11 B MAS NMR spectra of bismuth aluminoborate glasses containing 50 mol% Bi 2 O 3.Chemical shifts are referenced to BF 3 O(CH 2 CH 3 ) 2 at 0 ppm (Magnetic field 16.4T, Bruker Avance NMR spectrometer). The line shape and peak positions are constant with increase in Al 2 O 3 concentration from 2 to 8 mol%. The sharper peak due to BO 4 units decreases in height with alumina doping. In the bismuth aluminoborate glass series, N 4 decreases drastically from a value of 0.41 to 0.33 with increase in Al 2 O 3 concentration from 2 to 8 mol%, although f NBO remains constant (Table 4.14 and Fig. 4.15). To determine f NBO in aluminoborates we used the 27 Al MAS-NMR data on Al-O coordination in these glasses (discussed later in section 4.6). The decrease in BO 4 concentration on adding alumina is due to the consumption of O 2- (contributed by Bi 2 O 3 ) by AlO 6 units to form AlO 4 units similar to that in lead aluminoborates. 117

22 Table 4.14: N 4, and fraction of NBOs of the bismuth aluminoborate glasses. Sample Code N 4 (±0.01) f NBO Bi50BA Bi50BA Bi50BA Bi50BA N Al 2 O 3 (mol%) Fig. 4.15: N 4 variation with Al 2 O 3 concentration in bismuth aluminoborate glasses containing 50 mol% of Bi 2 O B MAS NMR spectra of the three bismuth aluminoborosilicate glass series are displayed below in Figs The values of N 4 in bismuth aluminoborosilicate series are given in Table N 4 decreases drastically in the three glass series with constant SiO2 concentration of 5, 10 or 20 mol % but gradually increasing Al 2 O 3 concentration of 2 to 8 mol %. Earlier Yamashita et al. (2003) had studied K, Na, Ba, Sr, Ca and Mg aluminoborosilicate glasses by 11 B NMR and concluded that cations of smaller size and greater charge (higher field strength) causes greater reduction in N 4 for a constant Al/B ratio. On comparing the N 4 data in lead aluminoborosilicate glasses (Table 118

23 3.17) and bismuth aluminoborosilicate glasses (Table 4.15) each containing 50 mol % of metal oxide and 20 mol % SiO 2 we find that bismuth glasses have lower N 4 than lead glasses with equal Al/B ratio. This is due to the smaller radius (0.96Å) and higher charge (3+) of bismuth ions than the Pb 2+ (radius=1.2 Å, charge=2+) which increases the cation field strength (defined as the ratio of ionic charge to its radius). In each of the three bismuth aluminoborosilicate glass series containing constant SiO 2 concentration (5, 10 or 20 mol %) but gradually increasing alumina concentration of 2 to 8 mol % the calculated value of f NBO is nearly constant (Table 4.15), as in bismuth aluminoborates. However a more interesting behavior can be discovered in four bismuth aluminoborosilicate glass series in which alumina concentration is kept constant at 2, 4, 6 or 8 mol % but silica concentration is varied from 5 to 20 mol %. We observe that on incorporating silica in bismuth aluminoborate glasses, N 4 decreases with simultaneous increase in NBO. This trend is different from lead aluminoborosilicate glasses series where silica addition increases N 4. We attribute this decrease in N 4 to the following reaction mechanism: [BO 4/2 ] - + [SiO 4/2 ] [BO 3/2 ] + [SiO 3/2 O - ] (4.2) Bi50BA2S5 Bi50BA4S5 Bi50BA6S5 Bi50BA8S ppm Fig. 4.16: 11 B MAS NMR spectra of bismuth aluminoborosilicate glasses containing 50 mol% Bi 2 O 3 and 5 mol% of SiO 2. Chemical shifts are referenced to BF 3.O (CH 2 CH 3 ) 2 at 0 ppm (Magnetic field 16.4T, Bruker Avance NMR spectrometer). 119

24 Bi50BA2S10 Bi50BA4S10 Bi50BA6S10 Bi50BA8S ppm Fig. 4.17: 11 B MAS NMR spectra of bismuth aluminoborosilicate glasses containing 50 mol% Bi 2 O 3 and 10 mol% of SiO 2. Chemical shifts are referenced to BF 3 O (CH 2 CH 3 ) 2 at 0 ppm (Magnetic field 16.4T, Bruker Avance NMR spectrometer). Bi50BA2S20 Bi50BA4S20 Bi50BA6S20 Bi50BA8S ppm Fig. 4.18: 11 B MAS NMR spectra of bismuth aluminoborosilicate glasses containing 50 mol% Bi 2 O 3 and 20 mol% of SiO 2. Chemical shifts are referenced to BF 3 O(CH 2 CH 3 ) 2 at 0 (Magnetic field 16.4T, Bruker Avance NMR spectrometer). 120

25 Table 4.15: N 4 and fraction of NBOs of bismuth aluminoborosilicate glasses. Sample N 4 (±0.01) f NBO Code Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S Fig. 4.19: N 4 variation with Al 2 O 3 concentration in bismuth aluminoborosilicate glasses containing 50 mol% Bi 2 O 3 and 5, 10 and 20 mol% SiO

26 Fig and 4.19 display the variation of N 4 with Al 2 O 3 concentration in bismuth aluminoborate and bismuth aluminoborosilicate glasses respectively with increase in alumina concentration. We see that the variation is nonlinear. A comparison with similar curves in lead aluminoborate and lead aluminoborosilicate glasses shows that those curves were linear (Fig and 3.25). These findings indicate that only one type of mechanism is involved in the destruction of N 4 on alumina doping in lead glasses whereas a more complex speciation mechanisms occur on alumina incorporation in bismuth glasses. The fraction of tetrahedral boron coordination (N 4 ) decreases with the increase of Al 2 O 3 (Table 4.14 and 4.15). Using measured quantities for [5] Al (Al 5 ) and [6] Al (Al 6 ) from 27 Al MAS NMR (discussed later), the average Al coordination number, n AlO, is given by 4 + Al 5 + Al 6. Following the logic of the previous cases, the average oxygen aluminum coordination number is n OAl = (c Al /c O )n AlO (assuming there are no tricluster oxygens), and f NBO = 2-(n OAl + n OSi + n OB ). Correlation between T g and f NBO and T g and N 4 are displayed in Figs 4.20 and 4.21 respectively. Fig. 4.20: Correlations between T g and f NBO in bismuth borosilicate glasses containing 40 and 50 mol% Bi 2 O

27 Fig displays the correlations between f NBO and T g values in bismuth borosilicate, bismuth aluminoborate and bismuth aluminoborosilicate glasses. Decrease in T g in bismuth aluminoborate glasses must be due to the decrease in average M-O bond strength where M denotes Bi 3+, Al 3+ and B atoms while decrease in T g with increase in SiO 2 content in borosilicate and alumninoborosilicate glasses is due to increase in NBO and probable decrease in average M-O bond strength of the glass network Al MAS NMR 27 Al MAS NMR experiments were carried out on one bismuth aluminoborate and two aluminoborosilicate glass series (Fig ). Bismuth aluminoborate glasses show an unusually high concentration of [6] Al and [5] Al structural units of %, which is significantly higher than the concentration of [6] Al and [5] Al units in lead aluminoborate glasses. This is due to the higher field strength (ionic charge by its radius) of Bi 3+ ions (radius=0.96 Å) than Pb 2+ ions (radius=1.20 Å) that stabilizes the high coordination of Al ions with oxygens [Wu and Stebbins (2009)]. The fraction of 6 Al and 5 Al units in bismuth aluminoborate glasses are presented in Table [4] Al [5] Al [6] Al Bi50BA2 Bi50BA4 Bi50BA6 Bi50BA ppm 123

28 Fig. 4.21: 27 Al MAS-NMR spectra of bismuth aluminoborate glasses containing 50 mol% Bi 2 O 3. Chemical shifts are referenced to 1 M Al(NO 3 ) 3 (aq) at 0 ppm (Magnetic field 16.4T, Bruker Avance NMR spectrometer). Table 4.16: Fraction of Al 4, Al 5 and Al 6 units in bismuth aluminoborate glasses. Sample Code 4 Al(±0.03) 5 Al(±0.03) 6 Al(±0.02) Bi50BA Bi50BA Bi50BA Bi50BA [4] Al [5] Al [6] Al Bi50BA2S10 Bi50BA4S10 Bi50BA6S10 Bi50BA8S ppm Fig. 4.22: 27 Al MAS-NMR spectra of bismuth aluminoborosilicate glasses containing 50 mol% Bi 2 O 3 and 10 mol% of SiO 2. Chemical shifts are referenced to 1 M Al (NO 3 ) 3 (aq) at 0ppm (Magnetic field 16.4T, Bruker Avance NMR spectrometer). 124

29 [4] Al Bi50BA2S20 Bi50BA4S20 Bi50BA6S20 [5] Al [6] Al Bi50BA8S ppm Fig. 4.23: 27 Al MAS-NMR spectra of bismuth boroaluminosilicate glasses containing 20 Mol % of SiO 2. Chemical shifts are referenced to 1 M Al(NO 3 ) 3 (aq) at 0 ppm (Magnetic field 16.4T, BRUKER Avance NMR spectrometer). Table 4.17: Fraction of Al 4, Al 5 and Al 6 units in bismuth aluminoborosilicate glasses. Sample Code 4 Al(±0.03) 5 Al(±0.03) 6 Al(±0.02) Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S Bi50BA2S Bi50BA4S Bi50BA6S Bi50BA8S

30 Figs and 4.23 display the 27 Al MAS NMR spectra of two bismuth aluminoborosilicate glasses with 10 mol % and 20 mol % SiO 2 respectively (the spectra for series with 5 mol % SiO 2 is not shown). It is evident from the 27 Al NMR measurements that the resonance peaks intensity for [5] Al and [6] Al units is significantly less in glasses with 20 mol % SiO 2. Our findings confirm the well known role of SiO 2 in suppressing the concentration of [5] Al and [6] Al units [Prasad et al. (2006)].Further in all three series the concentration of AlO 6 units decreases with increase in Al 2 O 3 content, and this decrease happens with the simultaneous increase in concentration of AlO 5 units without any significant change in the concentration of AlO 4 units (Table 4.17). Unusually high concentration of [5] Al and [6] Al units (40 % in Sample Bi50B2AS5) is due to the high cation field strength of Bi 3+ ions. 27 Al NMR studies carried out by Olier et al. (2004) on mixed alkali Na-K and Na-Li aluminoborosilicate glasses revealed that all Al 3+ ions are in tetrahedral coordination with absolutely no [5] Al and [6] Al units. On the contrary 27 Al NMR studies on calcium aluminosilicate glass and melts with as high as 44.4 mol % SiO 2 and 12.5 mol % Al 2 O 3 by Kanehashi and Stebbins (2007) found that five fold and six fold Al units exist in these materials due to higher field strength of Ca 2+ ions. Therefore the single most important parameter that determines the coordination number of Al ions in oxide glasses is the cationic field strength of modifier ions 4.7 Summary Glass samples from three bismuth borosilicate series: xbi 2 O 3 -ysio 2 -(100-xy)B 2 O 3 (x=30, 40, 50 and y=5, 10 and 20 mol %), one bismuth aluminoborate glass series: 50Bi 2 O 3 -xal 2 O 3 -(50-x)B 2 O 3 (x = 2, 4, 6 and 8 mol % ) and three bismuth alumino-borosilicate glass series : 50Bi 2 O 3 -xal 2 O 3 -ysio 2 -(50-x-y)B 2 O 3 ( x =2,4,6 & 8 and y =5, 10 & 20 mol %) were prepared by a melt quenching technique and characterized by density, DSC, 11 B and 27 Al MAS NMR measurements. Density increases while the fraction of tetrahedral borons (N 4 ) did not show any significant changes with increase in silica content in bismuth borosilicate glass series. Alumina doping produces a large reduction in density and N 4 in the bismuth aluminoborate and aluminoborosilicate glasses. 27-Al MAS NMR experiments revealed that Al ions are hexa, penta and tetra coordinated with oxygens in the glass network. [6] Al and [5] Al units 126

31 showed unusually high concentrations of about 47% in bismuth aluminoborate and 40% in aluminoboroslicate glass series containing 5 mol % of silica indicating the special role of high field strength Bi 3+ ions in the stabilization of these structural units. Increase of silica content to 20 mol % however significantly suppresses the concentration of and [5] Al species. Bismuth aluminoborate glasses exhibit a very strong and broad optical absorption band just below the absorption edge. This optical band in aluminoborate glasses correlates with the existence of 6-fold coordinated Al ions in the glass network Addition of silica suppresses the band intensity. Finally, all bismuth borosilicate, aluminoborate and aluminoborosilicate glasses show crystallization and liquidius peaks during the first heating cycle of the DSC run. These crystallization and melting peaks were completely absent during the second heating cycle of DSC indicating a curious influence of melt history on final glass properties. [6] Al 127

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