Network connectivity and properties of non-alkali aluminoborosilicate glasses

Journal of Non-Crystalline Solids 481 (2018) 403–408

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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Network connectivity and properties of non-alkali aluminoborosilicate glasses

T

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Jun Xie, Herui Tang, Jing Wang , Mengming Wu, Jianjun Han, Chao Liu State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road, Hongshan, Wuhan, Hubei 430070, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Aluminoborosilicate glasses Network structure NMR Properties

The effects of alkaline earth metal oxides (RO) concentrations on the network structure and properties of nonalkali aluminoborosilicate glasses were studied. With the increasing of RO concentrations from 10 to 20 mol%, the number of Q4 units decreased, while the number of Q3 units increased. The [AlO4] units always predominated in Al groups, and the fraction of [BO4] among B groups increased from 2% to 10%. Meantime, the value of NBO/T increased from 0.48 to 0.70, indicating the weak network connectivity. The depolymerization of T-O-T network resulted in higher coefficient of thermal expansion, lower temperature of transition point, weaker durability in HF acid. While, the increasing in R2 + cations and [BO4] units led to the higher elastic modulus of glasses. The increasing in RO also enhanced the density, dielectric constant and durability in NaOH solution of glasses.

1. Introduction Non-alkali aluminoborosilicate glasses are an ideal substrate material for TFT-LCD (Thin Film Transistor Liquid Crystal Display) and OLED (Organic Light-Emitting Diode) because of their excellent properties, such as low coefficient of thermal expansion (CTE), low density, high elastic modulus, high chemical durability, high thermal stability and high temperature of strain point [1–4]. The local structure of the network-forming cations (Si4 +, B3 + and 3+ Al ) in this glasses is still not clear enough, but it plays an important role in many essential physical and chemical performances [5,6]. The close relationships among compositions, structure and properties of traditional borosilicate glasses has been well studied [7–12]. For example, a good prediction of the network connectivity of boron in sodium borosilicate glasses was provided by Dell and Bray model [13], and the prediction of NBO (non-bridging oxygen) contents by using this model was in good accordance with direct 17O NMR results [14]. Abd El-Moneim et al. proposed that the concentration of tetrahedral boron groups and linkages of B-O-Al became higher with the increasing fraction of network modifier cations with higher field strength, which made the connection of tetrahedral units close [15]. In many boratecontaining glass systems, boron-11 wide-line MAS NMR was used to determine the fraction of [BO4] and [BO3] groups including symmetric and asymmetric trigonal boron groups [16,17]. In the reported paper, the coordination-state of Al3 + in aluminosilicate, aluminoborosilicate glasses were investigated by high-resolution NMR [18–26]. For

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instance, H. Li et al. reported that the increase of [AlO5] resulted in the enhancement of glass elastic modulus [26]. The structure of aluminoborosilicate glasses is more complicated than that of well-modeled borosilicate glasses because of the variable role of Al3 + cations. Non-alkali aluminoborosilicate glasses is also different from that alkali aluminoborosilicate glasses owning to the alkaline earth cations (R2 + = Mg2 +, Ca2 +, Sr2 + and Ba2 +) with high field strength instead of alkali cations (R′+ = Li+, Na+ and K+) [27]. The structural differences between alkali aluminoborosilicate and nonalkali aluminoborosilicate glasses were attempted to understand. LinShu Du et al. proposed the network mixing behavior in K-containing quaternary aluminoborosilicate glasses tend to follow the avoidance of linkages between tetrahedral aluminum and tetrahedral boron groups, while Ca-containing glasses prefer random mixing of Si, B, and Al by high resolution 11B, 27Al and 17O MAS NMR [27]. In this work, the non-alkali aluminosilicate glasses with different RO concentrations were prepared. The chemical environments of networkformer cations were investigated by 29Si, 27Al, 11B NMR. And the density, dielectric constant, elastic modulus, CTE, Tg and chemical durabilities of glasses were characterized. Furthermore, the relationships among compositions, network connectivity and properties of glasses were discussed.

Corresponding author. E-mail address: [email protected] (J. Wang).

https://doi.org/10.1016/j.jnoncrysol.2017.11.023 Received 23 August 2017; Received in revised form 3 November 2017; Accepted 10 November 2017 Available online 16 November 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.

Journal of Non-Crystalline Solids 481 (2018) 403–408

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x value

recorded by a Bruker AVANCE III 600 spectrometer. The frequency of Al, 29Si, 11B data were 104.26 MHz, 79.49 MHz, 192.41 MHz respectively. Spinning speeds of 12 kHz, 6 kHz, 18 kHz at magic angle and 4 mm, 7 mm, 6 mm rotors were chosen for 27Al, 29Si, 11B separately. The pulse sequences for 27Al, 29Si, 11B data collection were single pulse, with the relaxation delays of 2s, 5s, 10s.

10 12 14 16 18 20

2.3. Glass properties characterizations

Table 1 Compositions of glass samples (mol%). The molar ratios of MgO:CaO:SrO are 4:3:3. Glasses

R10 R12 R14 R16 R18 R20

Oxide composition SiO2

Al2O3

B2O3

MgO

CaO

SrO

71.0 69.0 67.0 65.0 63.0 61.0

11.0 11.0 11.0 11.0 11.0 11.0

8.0 8.0 8.0 8.0 8.0 8.0

4.0 4.8 5.6 6.4 7.2 8.0

3.0 3.6 4.2 4.8 5.4 6.0

3.0 3.6 4.2 4.8 5.4 6.0

27

Some typical physical and chemical properties of glasses were tested, such as density, coefficient of thermal expansion, elastic modulus, dielectric constant and chemical durability. The densities of glasses were carried out on the basis of Archimedes drainage with an analytical scale by neglecting the buoyancy of air. The thermal expansion curves of glasses were collected on a dilatometer (DIL402C, NETZSCH, German), and the characteristic temperatures of Tg (temperature of glass transition) were determined from the thermal expansion curves. The elastic modulus of glasses was collected on a ceramic experimental system (MTS810 100KN, MTS Inc. America). The dielectric constants of glasses were obtained on an impedance analyzer (HP4294A, Agilent Technologies Inc. America). The glasses were cut into the dimensions 30 mm × 10 mm × 2 mm and optical polished for chemical durability test. Some of the glass sheets were dipped into 10 vol% HF solution at 20 °C for 20 min, and some of the glass sheets were dipped into 5 wt% NaOH solution at 95 °C for 6 h. The values of weight loss ratio (WLR) were calculated by followed formulation (1).

2. Experimental 2.1. Glass preparation and characterization The nominal compositions of glasses were (81 − x) SiO2-11Al2O38B2O3 − xRO (in mol%), and the detailed oxide compositions were listed in Table 1. The oxides were introduced by analytical reagents of SiO2 (≥ 99%), Al2O3 (≥ 99%), B2O3 (≥ 98%), MgO (≥98.5%), CaCO3 (≥ 99%) and SrCO3 (≥ 99%) respectively. The well-mixed glass batches were melted at 1640 °C for ~ 2 h in a Pt-Rh (Pt:Rh = 9:1) crucible to obtain the bubble-free glassy melt. Then the melts were quickly poured onto a preheated foundry iron mould to form glasses. The as-prepared glasses were annealed at 650–700 °C for 2 h to eliminate the thermal stress. It was likely that a significant proportion of boron volatilized at high temperature, and caused deviations from nominal compositions. When taking into account this, each sample was added an additional 15% B2O3 to compensate for the volatilization before melting. For example, the 8 mol% B2O3 of R10 were equivalent to 8.46% in weight of oxides, and additional 8.46 × 15% grams of B2O3 were added to the wellmixed batches per 100 g of oxide. The practical proportions and volatility of B2O3 measured by methods of chemical analysis were listed in Table 2. The test method was followed the national standard method of China. From the results of quantitative analysis, the volatiles of B2O3 in all samples were not exactly the same, but the practical concentrations of B2O3 were close to nominal values. As given in Table 2, the deviations of boron content were no > 5%. So the deviations from nominal compositions were neglected in the calculation to simplify the process of understanding.

WLR = (m0 − m1)/ S × 100%

(1)

m0: the mass of ultrasonic cleaning before etching. m1: the mass of ultrasonic cleaning after etching. S: the surface area of each sheet.

3. Results and discussion 3.1. Aluminum-27 NMR The 27Al MAS NMR spectra of glasses with 10–20 mol% RO were showed in Fig. 1. Al (I = 5/2) is one kind of quadrupole nuclei (I > 1/ 2), that the spectra are subjected to a quadrupole expansion even under MAS NMR [26]. However, the position of main peaks was still clearly presented in spectroscopy. The typical chemical shift δiso (ppm) of 27Al NMR spectrum normally centered around 50 ppm–70 ppm (four-coordination, [AlO4]), 30 ppm–40 ppm (five-coordination, [AlO5]) and 0 ppm–20 ppm (six-coordination, [AlO6]), and the predominant of [AlO4] groups among Al groups also reported when (R′2O or RO)/ Al2O3 > 1 [27,25,31]. In many investigated aluminosilicate glasses, δiso of 27Al decreases by about 5 ppm for each [SiO4] connected to [AlO4] [32]. As shown in Fig. 1, the main peaks of each spectrum from our glasses located around 50 ppm, which indicated the vast majority of Al3 + formed [AlO4] groups. The widened signal at lower chemical shift δiso was mainly attributed to the second-order quadrupole effect [22], and the signal from [AlO5] and [AlO6] were not detected. The small and sharp peaks at about − 20 ppm were mainly caused by background signal from the rotor.

2.2. Nuclear magnetic resonance spectroscopy High-resolution solid NMR is one of the most effective methods to study the microstructure of glasses at the atomic scale [28–30]. NMR is a precise detection of atomic selective technology. When the atoms are placed in a stabled strong magnetic field, they can generate a resonance signal with a pulse of particular frequency because of every atomic specie have a nuclei possessing a non-zero spin (the nuclear spin I). The essential information of different atomic chemical environment can be acquired through NMR study. In this work,we have selected the important nucleus of network-former cations: 29Si (I = 1/2), 27Al (I = 5/ 2) and 11B (I = 3/2) for NMR detection. The spectrums of 27Al and 29Si MAS NMR were obtained on a Bruker VANCE III 400 spectrometer (B0 = 9.6 Tesla), while 11B data were Table 2 The quality content of B2O3 in all samples (wt%). All data are calculated only for oxides. Sample NO.

R10

R12

R14

R16

R18

R20

Nominal ratio (%) Proportion before melting (%) Measured proportion after melting (%) Volatility (%)

8.46 9.61 8.69 ± 0.13 9.57

8.45 9.60 8.80 ± 0.15 8.33

8.44 9.58 8.77 ± 0.18 8.46

8.43 9.57 8.75 ± 0.18 8.57

8.42 9.56 8.83 ± 0.12 7.64

8.41 9.55 8.81 ± 0.10 7.75

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Table 3 Curve fitting results of Q4 and Q3 from Glasses

R10 R12 R14 R16 R18 R20

29

Si NMR spectra.

4

Q (− 107 ± 2 ppm)

Q3 (− 96 ± 2 ppm)

Area%

Si4 + (Q4)%

Area%

Si4 + (Q3)%

65.5 61.7 58.8 52.8 49.6 41.4

39.1 35.8 33.1 28.8 26.3 21.2

34.5 38.3 41.2 47.2 50.4 58.6

20.6 22.2 23.2 25.8 26.7 30.0

of 29Si NMR were − 110 ppm to − 100 ppm for Q4, − 95 ppm to − 90 ppm for Q3, −90 ppm to − 80 ppm for Q2, − 76 ppm to − 68 ppm for Q1 and −66 ppm to −62 ppm for Q0. While silicon atoms bonding with aluminum or boron atoms generally leads to frequency dispersion. For instance, a substitution of Al atom for Si atom in the linkages Si–O–Si result in the chemical shifts δiso by about 4 ppm in average [24]. In Qn (n = 0, 1, 2, 3, 4) groups, each type of [SiO4] contained number of BO (bridge oxygen) correspond to the value of n. The results of Qn Gaussian fitting from 29Si MAS NMR spectra and both type of [SiO4] proportion were given in Table 3. The number of Si4 +(Q4) was the proportion of Si-4BO (a tetrahedral Si units connected 4 bridge oxygen) among all cations, and Si4 +(Q3) was the same. The fraction of Q4 drastically reduced from 39.1% to 21.2% and the concentration of Q3 increased substantially from 20.6% to 30.0%, with increasing the concentration of mixed alkali-earth oxide from 10 mol% to 20 mol%. When the concentration of RO was higher than 18 mol% (sample R18), the ratio of Q3/Q4 > 1. There was no significant signal of other [SiO4] groups (Q2, Q1 and Q0) was detected in this work.

Fig. 1. Aluminum-27 MAS NMR spectra for glasses.

3.2. Silicon-29 NMR The signal peak range of 29Si MAS NMR spectrum covered from − 120 ppm to −83 ppm (Fig. 2). The sharp peaks at about 60 ppm were mainly caused by background signal from the rotor. For alkali silicate glasses and alkaline earth silicate glasses, reported typical signal

3.3. Boron-11 NMR 11

B MAS NMR has been widely and successfully used to study the tetrahedral boron groups and trigonal boron groups even symmetric trigonal boron groups and asymmetric trigonal boron groups [5,23,26,27]. In order to minimize the impact of quadrupole parameters and isotropic chemical shifts δiso, high-resolution 11B MAS NMR at a high magnetic field strengths are required to obtain relatively accurate amount of [BO4] and [BO3] groups. A high-resolution NMR even can provide extra perspective about structure such as coordination state, bond length and angle, and first and second nearby atoms. 11 B MAS NMR scanning were carried on the three glass samples, R10 ((ΣRO-Al2O3)/B2O3 < 0), R14 (0 < (ΣRO-Al2O3)/B2O3 < 1) and R20 ((ΣRO-Al2O3)/B2O3 > 1). The clearly distinguishable signals from tetrahedral boron [BO4] groups (at ~0 ppm) and trigonal boron [BO3] groups (at ~12 ppm) were detected, and the relative concentration of the [BO4] and [BO3] were calculated by Gaussian fitting (Fig. 3). In fact, the non-zero concentrations of [BO4] groups (approximately 2%) were observed among B groups when RO/Al2O3 < 1 (correspond to the sample NO. R10, x = 10), and the fraction of [BO4] increased with the increasing of RO concentration. It is indicated that not all modifiers interact with Al2O3 to convert [AlO6] to [AlO4] even though ΣRO/ Al2O3 < 1. The fraction of [BO4] in studied glasses maintained in very low range even the ΣRO/(Al2O3 + B2O3) ratio is > 1 (correspond to sample NO. R20, x = 20), which is largely different from that in alkali aluminoborosilicate glasses. Compared with 27Al NMR data, this demonstrates that the conversion efficiency of reaction type (3) is much higher than (2) in non-alkali aluminoborosilicate glasses. It is consistent with the discovery that the addition of alkali or alkaline-earth metal oxide to aluminoborosilicate glasses will promoted the formation of [BO4] and [AlO4] groups from the [BO3] and [AlO6] groups, with a priority for the formation of [AlO4] over [BO4] groups [33]. This mainly result from the higher field strength of alkaline earth cations

Fig. 2. Silica-29 MAS NMR spectra for glasses.

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Fig. 4. The thermal expansion of glasses and the example of Tg were from the curve.

Fig. 3. Boron-11 MAS NMR spectra of glasses R10, R14, R20.

than alkali cations. And earth-alkali cations with higher field strength than alkali cations significantly prefer to form NBO rather than [BO4], and declining the priority in formation of [BO4] groups over NBO [13].

BO3 / 2 + 1/ 2O → BO4 / 2

(2)

AlO6 / 4 + 1/ 2O → AlO4 / 2

(3)

3.4. Effect of network-modifier on properties The number of non-bridge oxygen per tetrahedral cations (NBO/T) reflect the integrity of the linkages between the network-former cations. Values of NBO/T for glasses R10, R14, R20 and some important properties data for all glasses were shown in Table 4. According to 29Si, 27Al and 11B NMR results, the value of NBO/T of glasses R10, R14, R20 were calculated by formula (4). The B4 and Al4 were the concentration of tetrahedral B and Al groups, and B3 is the concentration of trigonal B groups. For instance, in glass of R10 (71SiO2-11Al2O3-8B2O3-10RO), NBO/T was about (2 × 10 + 3 × 16 × 0.98 − 22 − 16 × 0.02) / (71 + 22 + 16 × 0.02) = 0.48. The tested properties includes density, DC (dielectric constant), WLRHF (weight loss ratio in HF), WLRNaOH (weight loss ratio in NaOH), CTE (20–300 °C), E(elastic modulus), Tg. The value of NBO/T and all of the properties showed a monotonous trend.

NBO /T = (2R + 3B3–Al 4–B4)/(Si 4 + Al 4 + B4)

Fig. 5. CTE of glasses R10–R20.

density increased from 2.41 g/cm3 to 2.58 g/cm3 and dielectric constant increased from 6.98 to 8.35(Fig. 7). As shown in Fig. 8, there was a significant increase of the weight loss ratio in 10 vol% HF (20 °C, 20 min), while the weight loss ratio in 5 wt% NaOH (95 °C, 6 h) decreased. But when the Q3 was more than Q4, the increased tendency of WLRHF stopped. Some problems about the formation of T-NBO (in studied composition, T represented tetra-coordinated Si, Al and B) have been discussed in relative glasses. For example, the conclusion from some studies of borosilicate glasses have shown that the addition of Na2O lead to formation of network-former cations groups according to the order of [BO4] > Si-NBO > B-NBO [13]. In this work, free-alkali were contained caused the formation of NBO to differ from alkali-containing glasses. Such as the fraction of Si–NBO among SieO bonds and [BO4] among B groups increased in the order of Si-NBO > [BO4], with increasing RO concentration. In Ca-containing aluminoboronsilicates,

(4)

The curves of thermal expansion including information of CTE, Tg were shown in Fig. 4, and the change trend of CTE were shown in Fig. 5. The CTE increased, while Tg decreased with NBO/T increased. As shown in Fig. 6, the elastic modulus of glasses increased with the value of NBO/T increased. With increasing the concentration of RO, the Table 4 NBO/T calculation result of R10, R14, R20 and properties of glasses. Glasses

R10

R12

R14

R16

R18

R20

NBO/T Density (g/cm3) DC WLRHF (mg/cm2) WLRNaOH (mg/cm2) CTE (× 10− 6/K) E (GPa) Tg (°C)

0.48 2.41 ± 0.02 6.98 ± 0.30 2.54 1.75 20.7 73.83 ± 1.21 715

– 2.44 ± 0.01 7.47 ± 0.39 3.01 1.69 30.1 77.37 ± 0.90 703

0.56 2.48 ± 0.02 7.71 ± 0.21 3.36 1.45 33.3 77.58 ± 1.01 690

– 2.50 ± 0.02 7.92 ± 0.19 4.66 1.39 33.6 78.90 ± 1.10 688

– 2.55 ± 0.03 8.04 ± 0.20 5.62 1.24 39.9 81.40 ± 1.28 689

0.70 2.58 ± 0.02 8.35 ± 0.11 5.18 1.02 43.3 82.49 ± 0.80 667

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supplementary modifier cations with high field strength were filled in the network gap at a high coordination state to tighten network. This result showed that NBO/T have less effect on the determination of elastic modulus of glasses than that of R2 + cations. The changes of glasses density and dielectric constant were mainly attributed to the increase of network-modifier cations and decrease of SiO2. In short term, the increase of Ca2 + and Sr2 + with larger molar mass than Si4 + led to increase in density, while the increase of dielectric constants mainly depended on the increase of Mg2 +, Ca2 + and Sr2 + with higher polarization rate rather than Si4 + [35]. So the mole ratio of R/Si largely determined the glasses density and dielectric constant. The weakened resistance of glasses in HF is mainly due to the degree of connection between [SiO4] groups [36,37]. Obviously, the increase of Q3 and decrease of Q4 indicated the disintegration of Si-O-Si network, then the silicone network skeleton was easier to react with HF. But the situation in NaOH solution should be considered that earth-alkali oxides provide protection to glasses after reacting with NaOH, which resulted in alkali resistance increased with increasing RO [38].

Fig. 6. Elastic modulus of glasses R10–R20.

4. Conclusions In this paper, the alkali-free aluminoborosilicate glasses with 10–20 mol% alkaline earth metal oxide (RO) were prepared and their network connectivity was investigated by MAS NMR. With the substitution of RO for silica, the fraction of Q4 units decreased from 39.1% to 21.2%, and Q3 units increased from 20.6% to 30.0%. Furthermore, it resulted in the increasing in number of T-NBO and the higher value of NBO/T, indicating the depolymerization of network connectivity. The tetrahedral Al occupied the majority among Al groups with the varied concentration of RO. The [BO4] always existed and increased from 2% (sample R10) to 10% (sample R20) with RO increased. But the conversion from [BO3] to [BO4] did not keep up with the increase of Si-NBO. It is indicated the formation preference followed the order of SiNBO > [BO4]. Like the alkali-containing glasses, the conversion from [AlO6] to [AlO4] took precedence over [BO3] to [BO4] non-alkali glasses. But the formation of [BO4] started even though the concentration of RO < Al2O3. The increased concentration of [BO4] groups and high field strength cations were good corresponded with the increase of elastic modulus. The more weakness of network connectivity, the higher CTE and lower Tg. With the depolymerization of the network connectivity, the glasses durability in HF acid weakened. But there was an enhanced durability in alkali resistance.

Fig. 7. Dielectric constant and density of glasses R10–R20.

Acknowledgement We are grateful to J.J. Ren for the discussions of NMR results, and to Spectrum ProRyan Test Institution for the 14.1 Tesla spectrometer. This research is supported by National Key Research and Development Program of China (No. 2016YFB0303701), and Hubei Provincial Major Technical Innovation Program of China (NO. 2017AAA117). Appendix A. Supplementary data Fig. 8. Weight loss ratio of glasses in 4 vol% HF for 20 min at 20 °C and 5 wt% NaOH for 6 h at 95 °C.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jnoncrysol.2017.11.023.

non Al-NBO was observed when Si/Al > 1,and insignificant concentration of B-NBO existed [27,34]. The change of CTE, Tg strongly depended on the NBO/T, which reflected the breakage of T-O-T linkages. Therefore, the more weakness of linkages between network-former cations, the higher CTE and lower Tg for glasses. The increase of elastic modulus of glasses could be explained by the following two main reasons: on the one hand, some B groups were transformed from a two-dimensional layered structure ([BO3]) to a three-dimensional structure ([BO4]), on the other hand, the

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