Structural study of bismuth borosilicate, aluminoborate and aluminoborosilicate glasses by 11B and 27Al MAS NMR spectroscopy and thermal analysis

Journal of Non-Crystalline Solids 373–374 (2013) 34–41

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Structural study of bismuth borosilicate, aluminoborate and aluminoborosilicate glasses by 11B and 27Al MAS NMR spectroscopy and thermal analysis Atul Khanna a,⁎, Amanpreet Saini a, Banghao Chen b, Fernando González c, Carmen Pesquera c a b c

Department of Physics, Guru Nanak Dev University, Amritsar 143005, Punjab, India Chemistry & Biochemistry Department, Florida State University, Tallahassee, FL 32306, USA Department of Chemical Engineering and Inorganic Chemistry, University of Cantabria, 39005 Santander, Spain

a r t i c l e

i n f o

Article history: Received 26 December 2012 Received in revised form 1 March 2013 Available online 22 May 2013 Keywords: Bismuth borosilicate; aluminoborate and aluminoborosilicate glasses; 11 B and 27Al MAS NMR; DSC; Thermal stability

a b s t r a c t Glasses of bismuth borosilicates, aluminoborates and aluminoborosilicates were prepared by melt quenching and characterized by density measurements, 11B and 27Al Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) spectroscopy, and Differential Scanning Calorimetry (DSC). Increase in silica concentration increases density without any change in the fraction of tetrahedral borons (N4) in the borosilicate glass network, whereas increase in alumina concentration in aluminoborate and aluminoborosilicate glasses drastically reduces density and N4. 27Al MAS NMR measurements revealed that Al ions are in four, five and six fold coordination with oxygen and about 47% of these are in [5]Al and [6]Al structural units, the concentration of the latter species decreases by adding silica in the glass network. Higher cation field strength of Bi3+ generates very large concentration of non-bridging oxygens, which act as charge balancing centers in the glass network. DSC measurements found that alumina and silica incorporation in bismuth borate glasses do not produce any systematic changes in the glass transition temperature but the tendency towards crystallization decreases steadily. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Borosilicate glasses containing heavy metal oxides such as Bi2O3 and PbO are technologically important materials due to their several useful properties like low melting temperatures, high densities, high refractive indices and large attenuation coefficients for X-ray and gamma radiation. These materials find applications as glass to metal seals, ceramic sealants and transparent nuclear radiation shielding windows [1–4]. Bismuth borosilicate glasses have useful luminescent and non-linear optical properties [5–8]. B2O3 and SiO2 are the two important networking forming constituents of borosilicate glasses. Al2O3 is another component, which can act as both network former and modifier and enhances the glass forming ability of the batch mixture, besides increasing the chemical durability and thermal stability of glasses. The coordination number of network forming ions: B3+, Si4+ and Al3+ with oxygen is an important structural parameter of oxide glasses which determines properties like density, elastic moduli, and chemical and thermal stability. Boron–oxygen coordination is variable and there are both triangularly coordinated boronoxygen BO3 units (denoted as [3]B) and tetrahedrally coordinated units, [BO4]− ([4]B) in borate and borosilicate glasses having alkali, alkaline earth and other metal oxides [9–13]. Al\O coordination is also variable ⁎ Corresponding author. Tel.: +91 183 225 8802x3162; fax: +91 183 225 8820. E-mail address: [email protected] (A. Khanna). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.020

and there are [AlO4]− ([4]Al), AlO5 ([5]Al) and AlO6 ([6]Al) units in glasses [13–17], although crystalline forms of aluminum oxide such as γ and α-alumina do not contain [5]Al and have only [4]Al and [6]Al [18]. The Si\O coordination is fixed at 4, and four oxygens bonded to each Si4+ can be either bridging oxygen (BO) or non-bridging oxygen (NBO) [19]. The presence of NBOs deteriorates the mechanical strength and the chemical durability of glasses. The concentration of NBOs can be measured directly by 17O MAS NMR [20–22] or by X-ray photoelectron spectroscopy (XPS) [23–25], however knowledge of the glass composition, B\O, Al\O and Si\O coordination can also provide quantitative information about fraction of NBOs (fNBO) in glasses by considering the local charge neutrality condition on a glass molecule [13,17]. Qualitative information about changes in fNBO and cation–cation linkages in the glass network can also be obtained from the analysis of the subtle changes in the lineshape and chemical shift of [3]B and [4]B peaks [26,27]. For the purpose of modeling the glass structure and calculating its density, mechanical, thermal and optical properties it is essential to have knowledge about the short range structure of borosilicate and aluminoborosilicate glasses, in particular about the B\O, Si\O and Al\O coordination [28]. The fraction of tetrahedral borons (N4) and Al\O coordination in glasses can be determined by 11B and 27Al Magic Angle Spinning (MAS) and Nuclear Magnetic Resonance (NMR) spectroscopy respectively. It has been reported that there is significant intertetrahedral avoidance between [4]B and [4]Al since both are negatively charged species [13,17]. Cation field strength (defined as the

A. Khanna et al. / Journal of Non-Crystalline Solids 373–374 (2013) 34–41

ratio of ion charge to the square of its radius) of alkali, alkaline earth and other metal ions are reported to critically influence Al\O speciation in aluminoborate and aluminoborosilicate glasses. It has been found that high cation field strength of metal ions promotes the formation of [6] Al and [5]Al in aluminoborate and aluminoborosilicate glasses [15,29,30]. Lead aluminoborate and aluminoborosilicate glasses with small amounts of Al2O3 (2 to 8 mol%) contain high concentration ~20 to 30% of [5]Al and [6]Al [13]. Du and Stebbins determined that 30Na2O–5Al2O3–65B2O3 glass contains 96% of [4]Al, 4% of [5]A and no [6] Al [16]. Bunker et al. analyzed the coordination of boron and aluminum ions in several alkaline earth aluminoborate glasses and concluded that cation field strength plays a central role in determining the concentration of [5]Al and [6]Al units. Magnesium aluminoborate glasses with 10 mol% Al2O3 and 40 mol% MgO have 68% of [5]Al and [6]Al [31]. The effects of cation field strength of Na +, Mg2+ and Ca 2+ on Al speciation was analyzed by Chan et al. by double resonance NMR, who reported that while sodium aluminoborate glasses contain overwhelming fraction of Al3+ as [4]Al, calcium and magnesium glasses possess higher concentration of [5]Al and [6]Al [32]. Bi3+ ions have higher cation field strength than Ca2+, Sr2+, Ba2+and 2+ Pb ions (but lower than Mg 2 +) hence it is interesting to study the short-range structure of bismuth borosilicate, aluminoborate and aluminoborosilicate glasses. It is the objective of this work to study the effects of alumina and silica on the density, boron–oxygen, aluminum–oxygen speciation and thermal properties of bismuth borosilicate, aluminoborate and aluminoborosilicate glasses. The structure and properties of ternary and quaternary glasses have been compared with that of binary bismuth borate glasses studied earlier [12].

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2.2. Density measurements The densities (d) of the glasses were measured by the Archimedes method using dibutylphthalate (DBP) as the immersion fluid. Density measurements were repeated three to four times per sample; Table 1 gives the average of repeated measurements on each glass. Density measurements were done with a precision better than 0.1%, the maximum error was in the range of ±0.001–0.007 g cm −3. 2.3. Differential Scanning Calorimetery (DSC) DSC measurements were carried out on a SETARAM SETYS 16 TG-DSC system in the temperature range of 200–800 °C with a heating rate of 10 °C/min. DSC studies were done on powdered glass samples in platinum pans under ambient air. Sample amounts of 20–50 mg were used to perform the DSC measurements. DSC measurement on each sample was repeated twice to check the reproducibility of glass thermal properties. 2.4.

11

B MAS-NMR

The 11B MAS NMR spectra for the glass samples in this study were collected with a 2.5 mm Bruker MAS probe at room temperature on a Bruker Avance NMR spectrometer operating at 16.4 T corresponding to a Larmor frequency at 224.667 MHz for 11B nuclei. Single pulse acquisition was applied with a spinning rate of 20 kHz and a short RF pulse of 0.4 μs (power of 78.4 kHz) was used with a recycle delay of 5 s. Data were background corrected by subtraction of the empty rotor signal. All the spectra are referenced to the conventional standard, BF3 •O(CH2CH3)2 at 0 ppm.

2. Experimental methods 2.5.

27

Al MAS-NMR

2.1. Glass preparation Glass samples of the following two bismuth borosilicates, one bismuth aluminoborate and four bismuth aluminoborosilicate series were synthesized by melt quenching technique: (1) (2) (3) (4) (5) (6) (7)

40Bi2O3–xSiO2–(60 − x)B2O3 (x = 5, 10 and 20 mol%) 50Bi2O3–xSiO2–(50 − x)B2O3 (x = 5, 10 and 20 mol%) 50Bi2O3–xAl2O3–(50 − x)B2O3 (x = 2, 4, 6 and 8 mol%) 50Bi2O3–2Al2O3–xSiO2–(48 − x)B2O3 (x = 5, 10 & 20 mol%) 50Bi2O3–4Al2O3–xSiO2–(46 − x)B2O3 (x = 5, 10 & 20 mol%) 50Bi2O3–6Al2O3–xSiO2–(44 − x)B2O3 (x = 5, 10 & 20 mol%) 50Bi2O3–8Al2O3–xSiO2–(42 − x)B2O3 (x = 5, 10 & 20 mol%).

The starting materials were high purity H3BO3 (Merck, Germany, ACS grade), SiO2 (quartz) powder (Central Drug House, India, 99.5%), Bi2O3 (Aldrich Inc. USA, 99.9%) and α-Al2O3 (Central Drug House, India, 99.9%). Appropriate amounts of these starting materials were weighed and thoroughly mixed and ground in an agate mortar and pestle for about an hour. The batch mixture was transferred to a platinum crucible (25 cm3), slowly heated to 250 °C in an electric furnace and sintered at this temperature for 24 h. A second sintering at 450 °C for 24 h followed this. The furnace temperature was then slowly raised to 950 °C. This temperature was sufficient to obtain clear, bubble-free melts of all glass compositions listed in Table 1. The platinum crucible containing the melt was occasionally swirled inside the furnace for a few minutes to homogenize the glass melt. The melt was heat-treated at 950 °C for 60 to 90 min before pouring it on a heavy brass block to form button-shaped glasses. The glass sample was quickly transferred to another furnace previously kept at 350 °C and annealed at this temperature for 30 min to reduce thermal stresses generated by rapid cooling. All glass samples were disk shaped about 15 mm diameter and thickness of 2–3 mm.

The 27Al MAS NMR spectra for alumina containing glass samples were collected with a 2.5 mm Bruker MAS probe at room temperature on a Bruker Avance NMR spectrometer operating at 16.4 T corresponding to a Larmor frequency of 182.5 MHz for 27Al nuclei. Single pulse acquisition was applied with a spinning rate of 25 kHz. A short RF pulse of 0.4 μs (power of 65.8 kHz) was used with a recycle delay of 0.5 s. Chemical shifts was referenced to 1 M AlCl3 (aq) at 0 ppm. 3. Results 3.1. Density Density in two bismuth borosilicate glass series containing 40 and 50-mol% Bi2O3 increases with rise in SiO2 concentration (Table 1). In the bismuth aluminoborate glass series containing 50-mol% Bi2O3 density decreases systematically from a value of 7.002 ± 0.003 g cm−3 to 6.894 ± 0.003 g cm−3 as Al2O3 concentration was increased from 2 to 8 mol%. In the four bismuth aluminoborosilicate glass series containing 50 mol% Bi2O3, density decreases with the increase in Al2O3 content from 2 to 8 mol% for three SiO2 concentrations of 5, 10 and 20 mol% as in case of bismuth aluminoborates. Keeping Al2O3 concentration fixed at 2, 4, 6 or 8-mol%, as SiO2 concentration was increased from 5 to 20-mol%, density increases systematically (Table 1). 3.2. Glass transition, crystallization and liquidus temperatures The glass transition temperature (Tg) in binary 40Bi2O3–60B2O3 glass is reported to be 440 °C [12], whereas bismuth borosilicate glass with 5 mol% SiO2 has Tg of 434 °C. Therefore by adding SiO2 into binary bismuth borate glass to form ternary 40Bi2O3–(60 − x) B2O3–xSiO2 glass (x = 5,10 and 20 mol%), glass transition

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Table 1 Composition, density, molar volume and structural properties of glasses. Maximum uncertainty in density was in the range ±0.002 to 0.007 g cm−3. Sample code

Bi40B Bi40BSi5 Bi40BSi10 Bi40BSi20 Bi50B Bi50BSi5 Bi50BSi10 Bi50BSi20 Bi50BAl2 Bi50BAl4 Bi50BAl6 Bi50BAl8 Bi50BAl2Si5 Bi50BAl2Si10 Bi50BAl2Si20 Bi50BAl4Si5 Bi50BAl4Si10 Bi50BAl4Si20 Bi50BAl6Si5 Bi50BAl6Si10 Bi50BAl6Si20 Bi50BAl8Si5 Bi50BAl8Si10 Bi50BAl8Si20

Composition (mol %) Bi2O3

B2O3

Al2O3

SiO2

40 40 40 40 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50

60 55 50 40 50 45 40 30 48 46 44 42 43 38 28 41 36 26 39 34 24 37 32 22

– – – – – – – – 2 4 6 8 2 2 2 4 4 4 6 6 6 8 8 8

– 5 10 20 – 5 10 20 – – – 5 10 20 5 10 20 5 10 20 5 10 20

Density (g cm−3)

Molar volume (Vm)

N4 (±0.01)

Al4 (±0.03)

Al5 (±0.03)

Al6 (±0.02)

fNBO (±0.01)

6.246 6.351 6.446 6.367 6.874 6.985 7.118 7.162 7.002 6.960 6.961 6.894 7.019 7.068 7.079 6.993 6.996 7.208 6.882 6.916 6.969 6.872 6.882 6.888

36.51 35.84 35.24 35.53 38.96 38.26 37.48 37.12 38.33 38.66 38.74 39.21 38.17 36.99 37.64 38.41 38.32 37.06 39.12 38.86 38.42 39.27 39.14 38.97

0.46 0.46 0.46 0.46 0.45 0.43 0.43 0.42 0.41 0.36 0.34 0.33 0.41 0.40 0.38 0.35 0.35 0.30 0.32 0.33 0.29 0.28 0.27 0.25

– – – – – – – – 0.52 0.51 0.52 0.57 0.59 0.69 0.94 0.62 0.74 0.93 0.63 0.70 0.93 0.59 0.78 0.93

– – – – – – – – 0.19 0.19 0.21 0.22 0.23 0.14 0.03 0.22 0.13 0.04 0.25 0.17 0.04 0.27 0.13 0.03

– – – – – – – – 0.28 0.29 0.27 0.19 0.17 0.15 0.02 0.14 0.12 0.02 0.11 0.12 0.02 0.13 0.07 0.04

0.58 0.64 0.67 0.72 0.85 0.89 0.91 0.98 0.84 0.84 0.83 0.82 0.88 0.91 0.98 0.88 0.91 0.98 0.87 0.90 0.97 0.86 0.90 0.97

temperature first decreases but then with further increase in SiO2 concentration to 10 mol% and 20 mol% its value remains nearly constant. Similarly in case of bismuth aluminoborate glasses, Tg is in the temperature range of 390–393 °C and shows no significant change with the increase in Al2O3 concentration from 2 to 8 mol% (Table 2). In case of three bismuth aluminoborosilicate glass series containing 2, 4 and 6 mol% of Al2O3, Tg shows small changes (2–4 °C) with the increase in SiO2 mol%. In the fourth aluminoborosilicate glass series with 8-mol% Al2O3, Tg increases significantly from a value of 394 °C to 405 °C (Table 2). As compared to binary bismuth borate glass 40Bi2O3–60B2O3 [12], the crystallization peaks are significantly broadened and decrease in intensity in all glasses, which indicates that glass stability against devitrification enhances by adding Al2O3 and/or SiO2. Figs. 1 and 2 show the DSC patterns of borosilicate and aluminoborosilicate glasses respectively, during the first heating cycle. Whereas the glass transition temperature was reproducible during the second heating cycle of the DSC, the exothermic crystallization and endothermic melting peaks observed during the first heating cycle were missing during the second heating cycle of DSC scans. 3.3. Fraction of tetrahedral borons, N4 The 11B MAS NMR measurements on all glasses detected two peaks, one sharp peak at ~ 1 ppm due to [4]B and the second broader peak at ~ 16 ppm due to [3]B. Fig. 3 shows 11B MAS NMR spectra of 50Bi2O3–50B2O3 glass (Sample Bi50B) and three bismuth borosilicate glasses containing 5 to 20 mol% of SiO2. The fraction of tetrahedral borons (N4) in glasses was calculated from the ratio of integrated 4 areas under these two peaks i.e. N4=A3AþA . 4 In the first bismuth borosilicate glass series containing 40 mol% Bi2O3, as SiO2 concentration was increased from 5 to 20 mol%, N4 remains constant at 0.46. Similarly in the second bismuth borosilicate series containing 50 mol% Bi2O3, N4 is again constant at 0.42 as SiO2 concentration was raised from 5 to 20 mol%, although this N4 value (0.42) is lower than the value of 0.45 in binary 50Bi2O3–50B2O3 glass. Fig. 4 shows changes in N4 in the two bismuth borosilicate glass series as a function of SiO2 mol%. Using the measured values of N4, the average B\O coordination number, nBO = 3 + N4 and the average oxygen-boron coordination,

nOB = nBO(cB/cO), where cB and cO are molar concentrations of boron and oxygens respectively and taking Si\O coordination, nSiO, to be fixed at 4, average oxygen-silicon co-ordination number, nOSi = 4(cSi/cO) was calculated and these values were then used to determine the fraction of NBOs (fNBO) in the borosilicate network by the formula, fNBO = 2 − (nOSi + nOB). The above calculation of NBOs is based on the condition of overall charge neutrality of a glass molecule and on the assumption that there are no oxygen triclusters and free oxide ions (O2−) in the glass network, and that Bi3+ ions do not participate in the network formation. Oxygen triclusters like OAl3 are known to exist in oxygen deficient aluminate glasses [33], the present glasses contain an excess of oxygens (all glass components are tri-oxides and each Bi3+ contributes 1.5 oxygen ions) and hence triclusters are unlikely to form in bismuth glasses which instead contain high concentration of negatively charged NBOs to neutralize the positive charge of Bi3+ ions. Fig. 4 shows the increase in fNBO with the increase in SiO2 concentration in two bismuth borosilicate glass series. 11 B MAS NMR spectra of one bismuth borate glass and four bismuth aluminoborate glasses are displayed in Fig. 5. N4 was calculated from the area under these two peaks and it decreases from 0.41 to 0.33 on increasing Al2O3 concentration from 2 to 8 mol% (Fig. 6). In the first bismuth aluminoborosilicate glass series containing fixed Al2O3 concentration of 2-mol%, N4 decreases from 0.41 to 0.38 when SiO2 concentration was raised from 5 to 20-mol%, similar decreasing trend was observed in other three bismuth aluminoborosilicate glass series containing 4, 6 and 8-mol% Al2O3 (Table 1). Fig. 7 shows the decrease in N4 and simultaneous large increase in the fNBO from 0.86 to 0.97 in the fourth bismuth aluminoborosilicate glass series having fixed Al2O3 concentration of 8 mol% and variable SiO2 concentration of 5 to 20 mol%. 3.4. Fractions of

[4]

Al,

[5]

Al and

[6]

Al units

The 27Al MAS NMR spectra of the four bismuth aluminoborate glasses with 2, 4, 6 and 8-mol% Al2O3 are shown in Fig. 8. Three broad well resolved peaks centered at about + 61 ppm, + 33 ppm and + 4 ppm were detected and are due to [4]Al, [5]Al and [6]Al units respectively. The spectra were simulated with DMFit software and

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Table 2 Glass transition (midpoint value), crystallization (peak point) and liquidus temperatures (peak point) in bismuth glasses. Crystallization and melting peaks observed during the first heating cycle were missing during the second heating cycle. Maximum uncertainty in temperature values is ±1 °C. Sample code

Tg (°C)

Tc1 (°C)

Tc2 (°C)

Tc3 (°C)

Tm1 (°C)

Tm2 (°C)

Tm3 (°C)

Tm4 (°C)

Bi40B Bi40BSi5 Bi40BSi10 Bi40BSi20 Bi50B Bi50BSi5 Bi50BSi10 Bi50BSi20 Bi50BAl2 Bi50BAl4 Bi50BAl6 Bi50BAl8 Bi50BAl2Si5 Bi50BAl2Si10 Bi50BAl2Si20 Bi50BAl4Si5 Bi50BAl4Si10 Bi50BAl4Si20 Bi50BAl6Si5 Bi50BAl6Si10 Bi50BAl6Si20 Bi50BAl8Si5 Bi50BAl8Si10 Bi50BAl8Si20

440 434 431 433 408 397 392 391 393 393 390 391 392 392 396 391 391 387 394 392 396 394 397 405

540 583 558 – 447 482 482 481 476 485 499 509 500 504 519 501 524 512 532 535 569 519 542 –

– – 616 – 481 527 531 559 – – – 525 557 – 524 – – 545 – – – – – –

–

648 612 645 598 653 621 621 619 569 574 579 583 622 601 603 604 605 631 588 614 559 579 580 575

725 637 688 642 691 639 – 648 599 600 591 618 – 623 622 625 623 694 617 – 616 613 593 675

– 707 – 669 – – – 698 632 620 622 – – – 680 – – – – – 686 – 609 –

– – – 694 – – – – 638 – – – – – 698 – – – – – – – – –

563 – – –

554

–

–

–

CzSimple model [34,35] and the concentration of Al ions in different coordinations was determined from the areas under the peaks and the values in samples are given in Table 1. All glasses contain unusually large quantities of [5]Al and [6]Al; the sum of fractions of [5]Al (Al5) and [6]Al(Al6) units was 0.47 in glass with 2-mol% Al2O3 (Sample Bi50BAl2) which decreases to 0.41 in glass having 8-mol% Al2O3 (Sample: Bi50BAl8). Fig. 9 shows the changes in Al4, Al5 and Al6 in bismuth aluminoborate glasses with changes in Al2O3 molar concentration. The average Al\O coordination number, nAl\O was calculated from 27Al MAS NMR data as: nAl\O = 4 + Al5 + 2Al6, and using the values of nBO from 11B NMR data, the fraction of NBO was determined as fNBO = 2 − (nOB + nOAl). The values of fNBO are given in Table 1 and it is seen that the fNBO does not change significantly with the increase in Al2O3 concentration in bismuth aluminoborate glasses (Fig. 6), this is contrary to bismuth

borosilicate glasses in which silica addition causes large increase in fNBO (Fig. 4). The 27Al MAS NMR patterns of one bismuth aluminoborate and three bismuth aluminoborosilicate glasses with a fixed Al2O3 content of 8 mol% and variable SiO2 concentration of 5 to 20 mol% are shown in Fig. 10. It can be seen that the intensity of NMR peaks due to [5]Al and [6]Al was suppressed with the increase in SiO2 concentration. The Al4, Al5 and Al6 fractions in bismuth aluminoborosilicate glasses are given in Table 1. It was found that on keeping Al2O3 constant at 2-mol% and on gradually increasing SiO2 concentration from 5 to 20-mol%, Al6 decreases drastically from 0.17 to 0.04. Initially by adding 5 mol% SiO2 to bismuth aluminoborate glass containing 2 mol% Al2O3 (Sample Bi50BAl2Si5), only Al6 decreases from 0.28 to 0.17 and both Al5 and Al4 increase, however as SiO2 content was increased to 10 mol% and finally to 20 mol%, both Al6 and Al5 decrease with simultaneous increase in Al4 from 0.59 to 0.69 and then finally to 0.94 (Sample: Bi50BAl2Si20). Similar changes in Al\O speciation were observed in aluminoborosilicate glasses containing 4, 6 and 8-mol% of Al2O3.

Fig. 1. DSC spectra of bismuth borate (Bi40B) and borosilicate glasses with 40-mol% of Bi2O3. The crystallization peaks decrease in intensity, broaden and shift to higher temperatures with the increase in silica concentration due to the decrease in the crystallization tendency.

Fig. 2. DSC spectra of bismuth borate (Sample Bi50B), aluminoborate (Bi50BAl8) and aluminoborosilicate glasses containing 50-mol% Bi2O3 and 8-mol% Al2O3. Crystallization tendency decreases with the incorporation of Al2O3 and SiO2.

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Fig. 3. 11B MAS-NMR spectra of bismuth borate and bismuth borosilicate glasses containing 50 mol% Bi2O3 showing peaks due to [3]B and [4]B units. N4 decreases with the increase in silica concentration and [4]B peak shifts downfield (shielding) due to the formation of Si\O\B linkages.

The fraction of NBOs in aluminoborosilicate glasses were determined from the relation, fNBO = 2 − (nOB + nOSi + nOAl). NBOs increases significantly with the increase in SiO2 concentration in each bismuth aluminoborosilicate glass series. Bismuth aluminoborosilicate glass with 2 mol% Al2O3 and 20-mol% of SiO2 (Sample Bi50BAl2Si20) has extraordinary, 98% of its oxygens in the form of NBOs. Fig. 11 shows the changes in Al4, Al5 and Al6 in bismuth aluminoborosilicate glass series having 8-mol% Al2O3. There is a large decrease in Al5 and Al6 and simultaneous increase in fNBO with the increase in SiO2 concentration. 4. Discussion 4.1. Bismuth borosilicate glasses The increase in density of glass can be either due to the increase in its molecular mass or due to the compaction of the glass structure or both. In two bismuth borosilicate glass series with 40 and 50 mol%

Fig. 4. Variation in N4 and fNBO with SiO2 mol% in the two bismuth borosilicate glass series.

Fig. 5. 11B MAS NMR spectra of bismuth borate and aluminoborate glasses. The intensity and area under the peak due to [4]B decrease significantly by adding Al2O3 and [4]B peak shifts upfield (deshielding) due to avoidance of connectivity between [4]Al and [4]B.

Bi2O3, the increase in density with silica concentration from 5 to 20 mol% is mostly due to the replacement of lighter boron by heavier silicon ions, the borate glass short range structure is not affected since N4 is constant (Table 1 and Fig. 4). The volume of [4]B tetrahedron assuming B IV\O bond length to be 1.49 Å [36], was calculated as 1.69 Å 3, which is close to the volume of [4]Si tetrahedra since Si\O bond length (1.5 Å) is nearly equal to B\O bond length [36]. Clearly in substituting SiO2 into bismuth borate glass, the total number of B\O bonds decreases due to the decrease in the concentration of B2O3, but the B-O network is not perturbed since N4 remains constant in two bismuth borosilicate glass series. A small but systematic increase in the density of borosilicate glasses with SiO2 incorporation indicates that the volume of glass network remains the same because the volume of [4]B and [4]Si tetrahedral is nearly equal, Si ions simply replace some of the boron sites and since silicon ions are heavier, density rises. Although N4 remains invariant in borosilicate glasses by adding silica, the total number of negatively charged [4]B units decreases due to the replacement of B2O3 with SiO2, therefore in order to maintain

Fig. 6. Variation in N4 and fNBO with Al2O3 mol% in bismuth aluminoborate glasses, for comparison the data for binary bismuth borate glass (Bi50B) is also shown.

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Fig. 7. Variation in N4 and fNBO with SiO2 concentration in bismuth aluminoborosilicate glasses containing 8-mol% Al2O3. N4 and fNBO in ternary bismuth aluminoborate glass (Sample Bi50BAl8) are also shown for comparison.

local charge neutrality in the glass network additional negatively charged NBOs are generated by the following reaction: 2−

2½SiO4=2  þ O

−

⇔2½SiO3=2 O 

ð1Þ

39

Fig. 9. Variation in Al4, Al5 and Al6 with Al2O3 mol% in bismuth aluminoborate glasses.

in NBO concentration. The crystallization peaks in bismuth borosilicate glasses decrease in intensity, broaden and shift to higher temperatures with the increase in silica content (Fig. 1), this represents a decrease in the tendency for crystallization of glasses. During the second heating cycle of DSC analysis, while the glass transition temperature was reproducible, crystallization and melting peaks were absent. These findings indicate that melt history affects the glass thermal properties.

where O 2 − are oxide ions contributed by Bi2O3. Subtle changes in lineshape and chemical shifts of NMR peak of [3] B in the two bismuth borosilicate glass series provide insights about structural modifications that occur with the addition of silica in the borate network. Firstly, it can be seen from Fig. 3 that there are small but significant changes in the lineshape of [3]B peak which can be due to the changes in fNBO. Secondly, [4]B peak shifts from +1.28 ppm in bismuth borate glass (Sample Bi50B) to +0.96 ppm by adding 20 mol% silica (Sample Bi50BSi20). The shifting of the [4]B peak towards lower ppm values is due to the formation of linkages of [4]Si with [4]B via bridging oxygens, this in turn increases the shielding of tetrahedrally coordinated borons and causes negative chemical shift [27]. The glass transition temperature, Tg, of 40Bi2O3–60B2O3 glass was 440 °C [12], whereas Tg of bismuth borosilicate with 5 mol% (Sample Bi40BSi5) was lower: 436 °C. This decrease can be due to the formation of Si\O bonds which have somewhat lower bond dissociation energy of 799 kJ/mol compared to B\O bond energy of 809 kJ/mol [36]. However in further increasing the SiO2 content to 10 and then to 20-mol%, Tg remains nearly constant despite the systematic increase

In bismuth aluminoborates glasses, a large decrease in density is not due to the decrease in molecular mass (which actually increases with Al2O3) but is due to the decrease in N4 as confirmed by 11B MAS NMR. The decrease in N4 with alumina incorporation opens the borate network due to reduction in three-dimensional connectivity and increase in the volume due to the formation of bigger Al\O polyhedra. Al3+ ions exist in [4]Al tetrahedral, [5]Al trigonal bi-pyramidal and [6]Al octahedral units, assuming Al\O bond lengths to be fixed at 1.75 Å [36] and all polyhedra to be regular. The volume of [4]Al, [5]Al and [6]Al can be estimated as 2.72 Å3, 4.63 Å3 and 7.12 Å3 respectively. Clearly by adding alumina in place of B2O3, the concentration of smaller [4]B decreases and larger [4]Al, [5]Al and [6]Al are formed, this steadily increases glass volume and decreases density despite increase in the molar mass. A comparison of the peak positions and lineshapes of 11B MAS NMR spectra of bismuth borate and bismuth aluminoborate glasses

Fig. 8. 27Al MAS-NMR spectra of bismuth aluminoborate glasses showing three peaks due to [4]Al, [5]Al and [6]Al.

Fig. 10. 27Al MAS-NMR spectra of bismuth aluminoborosilicate glasses showing the suppression of peaks due to [5]Al and [6]Al units with the increase in silica mol%.

4.2. Bismuth aluminoborate glasses

40

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in bismuth aluminoborate glasses (Table 1 and Fig. 6). The decrease in negative charge in aluminoborate network due to the decrease in the concentration of negatively charged [4]B, is compensated by the formation of the equal number of [4]Al without any significant change in NBO concentration, in fact fNBO decreases slightly with the increase in alumina mol% (Fig. 6). Classical theories of glass structure and XPS measurements show that fNBO should decrease with the increase in alumina concentration in glasses [23]. The DSC patterns of bismuth aluminoborates were similar to bismuth borosilicates and showed that the crystallization and melting peaks decrease in intensity, broaden and shift to higher temperatures with the increase in Al2O3 concentration. Clearly glass crystallization tendency decreases although it does not completely disappear unlike lead aluminoborates in which even 2-mol% of alumina doping completely suppressed crystallization and melting peaks in the DSC spectra [13]. 4.3. Bismuth aluminoborosilicate glasses Fig. 11. Variation in Al4, Al5 and Al6 with SiO2 concentration in bismuth aluminoborosilicate glasses containing 8-mol% Al2O3.

(Fig. 5) provides important insights about structural modifications that occur in the borate network with the addition of alumina. Firstly, the chemical shift of [4]B peak at 1.29 ppm in Bi50B glass displaces continuously to more positive values and becomes 1.55 ppm in sample Bi50BAl8. This deshielding indicates that [4]B units do not link favorably with [4]Al. Similarly the lineshape of [3]B peak remains the same with alumina addition but its chemical shift changes from 15.4 ppm in Bi50B glass to more positive values (deshielding) and was 15.9 ppm in sample Bi50BAl8. The invariance of the line shape of [3]B peak in aluminoborate glasses suggests that fNBO does not change by adding alumina but there is deshielding due to the demixing of Al\O and B\O polyhedra. Interestingly as the concentration of Al2O3 was raised from 2 to 8 mol%, Al4 increases relative to Al5 and Al6 and although volume of [4] Al is smaller than the volumes of [5]Al and [6]Al, density continues to decrease due to the decrease in the concentration of [4]B. The fundamental reason why the concentration of [4]B must decrease by adding Al2O3 is the inter-tetrahedral avoidance: both [4]B and [4]Al are negatively charged species and repel or avoid each other [13,17,31]. α-Al2O3, which was used as a starting material, contains only [6]Al [18], however due to its interaction with O 2− ions (provided by Bi2O3), [6]Al partly convert to [4]Al (and also to [5]Al) by the following reaction mechanisms: 2½AlO6=4  þ O

2−

2−

½AlO6=4  þ O

⇔2½AlO4=2 

−

−

⇔½AlO3=2  O

ð2Þ

−

ð3Þ

where Ox/y denotes x oxygen anions, each coordinated with y network cations, e.g. bridging oxygen when y = 2, and O − denotes non-bridging oxygen (NBO). Since O 2− are consumed by [6]Al units by the Reactions (2) and (3), the following reaction which produces [4]B was significantly suppressed: 2−

2½BO3=2  þ O

−

⇔2½BO4=2  :

ð4Þ [4]

[4]

There is a preference for the formation of Al over B in aluminoborate glasses. Al4 increases from 0.52 to 0.57 as Al2O3 concentration was increased from 2 to 8 mol% due to the increase in the rate of Reaction (2) towards the right side. Since there is no significant change in fNBO in aluminoborate glasses with the increase in Al2O3 mol%, it can be concluded that reaction (3) does not occur. Using the value of N4 = 0.45 in 50Bi2O3–50B2O3 glass [12], we calculated fNBO in this glass to be 0.85 which is close to fNBO (0.84–0.82)

In four bismuth aluminoborosilicate glass series there is exactly similar variation in N4, i.e. it decreases by 0.03–0.05 on increasing SiO2 concentration from 5 to 20 mol% for a fixed value of Al2O3 (Table 1). The variation in Al\O coordination number is identical in each bismuth aluminoborosilicate glass series i.e. Al4 increases with the increase in SiO2 mol%. The combined fractions of [5]Al and [6]Al units are about 0.40 in glass with 2-mol% of Al2O3 and 5-mol% SiO2 (Sample Bi50BAl2Si5) and become only 0.05 when SiO2 was increased to 20-mol% (Sample Bi50BAl2Si20). This shows that [5]Al and [6]Al are unstable in the presence of Si\O bonds in the glass network [13,29,30]. A noteworthy feature is that whereas in two borosilicate glass series, N4 was almost invariant with SiO2 concentration but in case of bismuth aluminoborosilicates it decreases by 0.03–0.05 with SiO2 addition by the reaction (1). The consumption of O 2 − by Reaction (1) reduces the formation of [4]B units (Reaction (4)), hence in bismuth aluminoborosilicate glasses, Mechanisms (1) and (2) are responsible for the decrease in N4. Zheng et al. have concluded from DSC and 27Al MAS NMR measurements in several sodium–calcium aluminosilicate glasses that the increase in the concentration of [5]Al in glasses enhances their crystallization tendency i.e. reduces glass thermal stability [37]. Our earlier study on lead aluminoborate and aluminoborosilicate glasses found that the addition of even very small amounts of alumina (2-mol%) in lead borate glasses completely suppressed crystallization tendency although lead aluminoborate glasses contain high concentration of [5]Al (~ 20%) [13]. In the present study on bismuth aluminoborate glasses, the addition of alumina and/or silica does not completely suppress crystallization but the exothermic crystallization peaks detected during DSC scans broaden, reduce in intensity and shift to higher temperatures, for instance the crystallization peak shifts from 476 °C to 509 °C with the increase in Al2O3 concentration from 2 to 8 mol% in bismuth aluminoborate glasses despite the small increase in Al5 with Al2O3 concentration from 2 to 8 mol% (Table 1). By adding silica to form bismuth aluminoborosilicate glasses, the crystallization peaks further reduce in intensity and shift to higher temperatures (Fig. 2), this can be attributed due to decrease in Al5, but then the glass composition also gets modified (with the addition of alumina and silica), and the tendency for crystallization is expected to depend significantly on the glass composition. Therefore we do not agree with the conclusions of Zheng et al. that the presence of [5]Al in glasses enhances its crystallization tendency (i.e. reduces glass thermal stability) [37], a more critical factor would be the glass composition and its closeness to that of crystalline phase. It is generally accepted that [5]Al in glasses enhances disorder in network, and that the [5]Al may be responsible for high thermal stability

A. Khanna et al. / Journal of Non-Crystalline Solids 373–374 (2013) 34–41

of amorphous aluminum oxide thin films against crystallization on heat treatment [38,39]. An important finding from thermal analysis is that the crystallization peaks are significantly modified by adding silica and alumina, but unlike lead aluminoborate and lead aluminoborosilicate glasses these peaks do not disappear completely [13]. This is because lead glasses are not single-phase materials, the existence of several competing phases suppresses the tendency for crystallization. Higher cation field strength of Bi 3+ than Pb 2+ causes random mixing of [3]B, [4]B, [4] Si and [4]Al units and helps form a more homogeneous structure in bismuth glasses than in lead glasses [32]. A homogeneous glass structure can crystallize more readily, this finding is consistent with the earlier and recent studies on the ability of a substance to form glassy or crystalline phase [40,41]. Finally, the NBO concentration increases by large amounts with the increase in silica mol% in bismuth borosilicate and bismuth aluminoborosilicate glasses. The calculated fNBO in bismuth borosilicate glass sample: Bi50BSi20 is 0.98, implying that almost all oxygens in the glass network are in the form of NBOs with the charge of −1. This conclusion is demanded by the overall local charge neutrality condition. The calculated fNBO value will be significantly less if bridging oxygens also possess negative charge and/or if free O 2− ions exist in glasses, indeed according to the recent studies by Stebbins et al. bridging oxygens in aluminosilicate glasses may be partially charged species [42], and moreover glasses with high cation field strength metal ions may contain some concentration of free O 2− ions [22]. 5. Conclusion Bismuth borosilicate, aluminoborate and aluminoborosilicate glasses were prepared and structure–property correlations were made by density, 11B, 27Al MAS NMR and DSC measurements. The addition of Al2O3 drastically reduces the concentration of [4]B in bismuth aluminoborate glasses due to the preference of [6]Al over [3]B in reacting with O2− (provided by Bi2O3) for the formation of [4]Al and [4]B respectively. This preference is however not shown by [4]Si in borosilicate glasses. Bismuth aluminoborate glasses contain high concentration of [5]Al and [6]Al (~ 47%), the latter are rapidly destroyed by adding silica due to the change in the dual role of Al 3 + from network modifier and the former in aluminoborate glasses to only network former in bismuth aluminoborosilicate glasses containing high silica concentration. Our study confirms that [5]Al in glasses does not enhance its crystallization tendency as has been concluded in the recent study on soda-lime aluminosilicate glasses [37], but on the contrary alumina and silica addition decreases the tendency of bismuth glasses towards devitrification and promotes its thermal stability against crystallization. Acknowledgments AK thanks the UGC-DAE-Consortium for Scientific Research, Mumbai Centre, Mumbai, India for the financial support. Professor J. W. Zwanziger from Dalhousie University, Halifax, Canada is thanked for providing the MAS NMR facilities.

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