Advances in Condensed Matter Physics
Volume 2021 (2021), Article ID 2321558, 12 pages
https://doi.org/10.1155/2021/2321558

Research Article

The Relationship between the Structure and Thermal Properties of Bi₂O₃-ZnO-B₂O₃ Glass System

Sung-Hung Lan,¹ Chin-Tung Lee,² Yi-Sheng Lai,1,2 Chien-Chon Chen,¹ and Hsi-Wen Yang1,2

¹Ph. D. Program in Materials and Chemical Engineering, National United University, 2, Lienda, Miaoli 360001, Taiwan
²Department of Materials Science and Engineering, National United University, 2, Lienda, Miaoli 360001, Taiwan

Correspondence should be addressed to Hsi-Wen Yang; hwyang@nuu.edu.tw

Received 9 May 2021; Revised 15 June 2021; Accepted 6 August 2021; Published 21 August 2021

Academic Editor: Ram N. P. Choudhary

Copyright © 2021 Sung-Hung Lan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The influence of Bi₂O₃ and melting temperature on the thermal and structural properties of xBi₂O₃-(60-x) ZnO-40B₂O₃ glasses has been investigated in this study. It is expected that these factors can be used to control the degree of reduction of Bi₂O₃, and the relationship between these factors and the color change of the process for bismuth glass is discussed. Due to high-temperature melting, the bismuth-doped borate glasses have changed into dark/black from original transparent yellow and the light transmittance will decrease, so it is not used in optical applications. The thermal properties of glass are measured by a thermomechanical analyzer (TMA), and the glass structure is analyzed by FTIR and XPS. The results show that the glass is mainly composed of [BiO₆] octahedron, [BiO₃] triangle, [BO₄] tetrahedron, and triangle [BO₃] units, and the network of the glass system is mainly bonded by B-O-B, B-O-Zn, B-O-Bi, and Bi-O-Bi. The glass thermal expansion coefficient (CTE) of this glass system increases with the increase of Bi₂O₃ content, and the O1s nuclear electron binding energy shifts to the lower energy direction with the increase of Bi₂O₃ addition. In terms of FTIR, as the melting temperature rises, the B-O-B bonding vibration concentration of [BO₄] inside the borate glass decreases, and the density of B-O-B bonding vibration of [BO₃] increases, Moreover, the increase in melting temperature increases the probability of reducing Bi ions to Bi⁰, reduces the bonding of Bi-O-B, and increases the bonding of B-O-B, and the CTE also slightly decreases.

1. Introduction

Adding PbO to the glass can make the glass have good light transmittance, stable structure, low-temperature glass characteristic temperature, excellent optical properties, thermal properties, and electrical properties [1], but it is easy to cause harmful effects on the environment and the human body. The development of lead-free glass to replace lead-containing glass is the main research direction at present, and the component of lead oxide is bismuth oxide (Bi₂O₃), its Bi³⁺ property is similar to Pb²⁺ of lead oxide (PbO), both have the same electronic configuration and high polarizability [2], and the glass also has low melting temperature, high density, high refractive index, and other characteristics, which can be used to replace lead oxide to develop various lead-free glass products [3, 4]; therefore, bismuth glass is one of the most promising glasses without considering the lead-containing glass [5, 6].

Dietzel [7] derived the Coulomb electrostatic force field and field strength formula in 1942 and pointed out that the structure of borate glass [BO₃] is a plane triangle, and the boron coordination number is 3. If an additional free oxygen atom is provided, BO₃ can bond with the oxygen atom to form a BO₄ tetrahedral with a coordination number of 4. [BO₃] and [BO₄] are usually connected in a ring in glass, which is called a boron ring structure (boroxol group structure), a random network structure formed by bonding with B-O-B. In the cation field strength theory of glass structure theory, because the field strength of Bi₂O₃ is weak, it belongs to the nature of glass as an intermediate agent and cannot form glass alone. Adding Bi₂O₃ to ZnO, SiO₂, B₂O3, and other materials can help the glass network to be more stable and promote the formation of glass structures. S. Bale, T. Maeder, and other authors [8–11] stated, respectively, that the glass intermediate Bi₂O₃ is sometimes presented as [BiO₃] and [BiO₆] structural units in the glass structure or as [BiO₃] triangles or [BiO₆] octahedron and other structural units appear, and Bi₂O₃ can be used as a glass modifier or glass forming agent, depending on the proportion of Bi₂O₃ in the overall glass composition.

As for the coloring phenomenon of bismuth glass, Saitoh et al. [12] and other scholars pointed out that the electronic structure of coloring is related to the Bi valence, originating from Bi³⁺ and Bi⁵⁺, and/or plasma Bi⁰ clusters in colored glass, because Bi³⁺, the energy bandgap of the transition state in the s–p orbital domain between 3.6 and 4.7 eV, is the optical band gap and the visible absorption band between 2.5 and 2.7 eV, which is related to the coloring phenomenon, the ionic reduction behavior of bismuth glass. Gerth and Russel [13] pointed out that in the Bi₂O₃-TiO₂-B₂O₃ glass system the Bi₂O₃ glass increases both the Bi₂O₃ content and the melting temperature, which causes the glass to change from light yellow to brownish-black gradually. Because Bi³⁺ is easily reduced to Bi²⁺, Bi⁺, and Bi⁰ [14, 15], the reduction reaction can be strengthened with the increase of melting temperature and Bi₂O₃ content, which make the color of the glass deep, and the light transmittance decreases [14].

2. Materials and Methods

In this study, x Bi₂O₃-(60-x) ZnO-40 B₂O₃ (x = 10, 20, 30), where 40B₂O₃ is a fixed component, changes x = Bi₂O₃ content at different melting temperatures (900°C, 1000°C, 1100°C, and 1200°C), to explore the influence of bismuth oxide content change and melting temperature change on its structure. The use of reagent grade B₂O₃ (JT Baker, 99.9%), ZnO (JT Baker, 99.7%), and Bi₂O₃ (Sigma-Aldrich, 98%) as the raw material of glass is configured into the following glass component series and melted at the process temperature of 900°C, 1000°C, 1100°C, and 1200°C:

 A1: 10 Bi₂O₃-50 ZnO-40 B₂O₃ A2: 20 Bi₂O₃-40 ZnO-40 B₂O₃ A3: 30 Bi₂O₃-30 ZnO-40 B₂O₃

In the experiment, the required weight of each raw material was calculated according to the molar fraction of the composition, and the raw material powder was mixed into 30 g. After the prepared raw materials are uniformly mixed, they were put into an alumina crucible, melted in an electric furnace at a heating rate of 10°C per minute, heated to different process temperatures, and kept at a constant temperature for 1.5 hours to promote uniform mixing of the glass. The molten glass is taken out of the high-temperature electric furnace and cast on a stainless steel plate preheated to the annealing temperature, in order to eliminate the thermal stress caused by the rapid cooling of the glass, and the glass must be immediately sent to the annealing furnace and slowly cooled down to room temperature.

X-ray diffraction analysis uses Rigaku Rint 2200 type and uses CuKa as the diffraction light source and the filter as Ni filter, the operating voltage is 30 kV, and the current is 20 mA. The scanning speed is set to 8 degrees per minute for analysis, and the finished glass is analyzed by X-ray diffraction (XRD) to confirm that there is no crystal in the glass. Fourier-transform infrared (FTIR) spectroscopy (Perkin Elmer, USA) was used for analysis, and it was carried out at room temperature. The glass powder to be measured was placed in the alumina grind, with a concentration of 1wt % of optical grade KBr powder, and mixed and ground evenly in the bowl, and about 500 mg of the mixed powder was taken, pressed into a translucent round sheet, and then tested. The range of test wavenumber is 450∼2000 cm⁻¹. Through the XPS spectroscopic analysis, the glass test piece is ground into powder and analyzed by a X-ray photoelectron spectrometer. Experimental parameters are as follows: source type: Al K alpha, spot size: 400 μm, analyzer mode: CAE, and pass energy: 50.0 eV. We obtain the O1s and Bi4f XPS lines of each sample and analyze the peaks of the XPS line components. TMA, the thermal mechanical analyzer, is the most used to measure the coefficient of thermal expansion (CTE), which is the average value of the thermal strain versus the temperature change after a material passes through a temperature change curve. The thermal analysis of the glass in this experiment uses a thermomechanical analyzer Perkin Elmer TMA 7, USA. The glass sample was ground into a square test piece with a size of 5 × 5 × 5 (mm) parallel to the top and bottom and placed in the carrier of the TMA instrument at a heating rate of 10°C per minute.

3. Results and Discussion

3.1. XRD Diffraction Analysis

A series of glass samples were analyzed by XRD diffraction. In the analysis chart, glass samples such as A1, A2, and A3 were melted at 900°C, 1000°C, 1100°C, and 1200°C, respectively, and all 12 samples were with hump characteristic of typical amorphous materials, without any crystal diffraction peaks, so it can be confirmed that the sample has no crystals. As the amorphous material is a disordered material, the hump, or the second broad hump at the higher incidence angle of XRD, is an indication of the disordered state. When the content of Bi₂O₃ increases, indicating a second wide hump at higher incidence angles of XRD, the internal structure of the material at this content is also amorphous, as shown in Figure 1.

Figure 1: XRD diffraction patterns of A series glass melted test pieces.

3.2. FTIR

In FTIR spectroscopy, based on the research of previous literature, there are four characteristic peaks of glass structure in A series glass, such as [BiO₃], [BiO₆], [BO₃], and [BO₄] (Table 1).

Table 1: Corresponding characteristics and bands of each vibration peak of FTIR of A glass series.

Wavenumber (cm⁻¹)
Vibrational mode
Reference

510
Symmetrical stretching vibration of Bi–O–Bi bond
[1, 6, 16]

600
Symmetrical stretching vibration of the Bi–O–Bi bond
[16–18],

700
750780
B-O bending vibration of [BO₃] structural unit
[16, 19–23]

820
900950
[BO₄] B–O– stretching vibration in the unit
[16, 21, 24, 25]

1000
1015–1021
B–O– stretching vibration of [BO₄] unit
[16, 19, 20, 22, 23]

12501310
B–O– asymmetric stretching vibration of [BO₃] unit
[16, 19, 23, 25]

1365–1472
B–O– asymmetric stretching vibration of [BO₃] unit
[16, 19, 23]

16351650
Bending vibration of OH– bond
[1, 16, 20]

In the FTIR spectrum, there are [BiO₃], [BiO₆], Bi–O–Bi, Bi-O⁻, and the triangular [BO₃] unit of the borate B₂O₃ glass and the B–O⁻ unit of the [BO₄] tetrahedron and absorption peak. In Figure 2, it can be roughly divided into 3 blocks: (1) between 510 cm⁻¹ and 700 cm⁻¹, it is a three-coordinated plane [BiO₃] and a six-coordinated octahedral [BiO₆] structural unit; (2) between 700 cm⁻¹ and 1382 cm⁻¹, it is a plane triangle [BO₃], and a tetrahedral [BO₄] structural unit is the main one; and (3) between 1382 cm⁻¹∼1640 cm⁻¹, the plane triangle [BO₃] and [OH⁻] bond is the main part. Among the three blocks, there are 4 absorption characteristic peak locations, 700 cm⁻¹, 777 cm⁻¹, 1382 cm⁻¹, and 1640 cm⁻¹, which are relatively easy to distinguish. And the others are 510 cm⁻¹, 820 cm⁻¹, and 1000 cm⁻¹, where the absorption characteristic peaks are relatively weak.

Figure 2: FTIR spectra of A1 glass series at different melting temperatures of 900°C, 1000°C, 1100°C, and 1200°C.

There is a weak absorption peak at the wavenumber of 510 cm⁻¹, which is the symmetric stretching vibration of the Bi-O-Bi bond in the [BiO₃] and [BiO₆] unit. The absorption peak is located at the wavenumber of about 700 cm⁻¹, which is [BO₃] B-O-B vibration. The absorption peak is located at the wavenumber of about 777 cm⁻¹, which is the absorption peak of B-O-B vibration in the [BO₃] unit. It is the B–O– stretching vibration of the [BO₄] tetrahedral unit in the glass structure at the wavenumber of 820 cm⁻¹, the wavenumber at 1000 cm⁻¹ (solid arrow) is the B–O– stretching vibration of the [BO₄] tetrahedral unit, and at the wavenumber of about 1382 cm⁻¹, it is the Bi-O⁻ unit structure in the [BiO₃] structural unit vibration, and the absorption peak at 1640 cm⁻¹ is the vibration of the OH radical. In Figure 2, there is a weak absorption peak at the wavenumber of 510 cm⁻¹, the symmetrical stretching vibration of the Bi-O-Bi bond in the [BiO₃] and [BiO₆] units, and the signal is very weak. At 700 cm⁻¹ and 777 cm⁻¹ (dashed arrow) is the vibration signal of the [BO₃] unit. At 1000°C, 700 cm⁻¹ is more obvious, indicating that the [BO₃] structure concentration is high, and the [BO₄] structure concentration is low. At 777 cm⁻¹ (dashed arrow), the signal is very weak. At 1382 cm⁻¹ absorption peak, the vibration signal of [BO₃] increases as the melting temperature rises, indicating that the structure of the [BO₃] unit rises with the increase in melting temperature. The OH⁻¹ vibrates at 1640 cm⁻¹, and the signal is smaller at 900°C, and when the melting temperature rises, then the signal weakens. In Figures 3 and 4, the vibration signals of [BO₃] at 700 cm⁻¹ and 777 cm⁻¹ (dashed arrow) are stronger than those in Figure 2, indicating that the [BO₃] structure concentration of A2 and A3 is higher, and the [BO₄] structure concentration is low. The signal is very weak at 777 cm⁻¹ (dashed arrow). The vibration signal of [BO₃] at 1382 cm⁻¹ increases as the melting temperature increases, indicating that the concentration of the [BO₃] structure of A2 and A3 increases as the melting temperature rises, and the [BO₄] structure concentration decreases. Since B-O-B bonding energy > Bi-O-Bi bonding energy, it means that B-O-B bonding of [BO4] unit and Bi-O-Bi bonding will relax and decompose due to high temperature. And based on the energy balance trend of the glass system, this system promoted the gradual formation of the Bi-O-B bond and [BO₃] of the intermediate bond energy.

Figure 3: FTIR spectra of A2 glass series at different melting temperatures of 900°C, 1000°C, 1100°C, and 1200°C.

Figure 4: FTIR spectra of A3 glass series at different melting temperatures of 900°C, 1000°C, 1100°C, and 1200°C.

At 1382 cm⁻¹ (B-O⁻ bonding vibration of [BO₃] unit), A1, A2, and A3 series glasses all have obvious absorption peaks. As the melting temperature increases, the strength of the absorption peak is as follows:

 A3900°C  A31000°C  A31100°C  A31200°C A21100°C  A21000°C  A2900°C  A21200°C A11000°C  A11100°C  A11200°C  A1900°C

In the A3 glass series, because the 30 mol % of Bi₂O₃ content is relatively high, it has a relatively sufficient reaction concentration with [BO₃] and [BO₄] unit, so it is clearly observed that as the melting temperature increases, the concentration of B-O⁻ of [BO₃] unit decreases. And A1, A2, and A3 glass systems all tend to shift to high wavenumbers. This shift phenomenon is that as the melting temperature rises, Bi³⁺ is reduced to low-valent ions and partly reduced to Bi⁰ to dissolve and causes the glass color to be darker brown, and when the melting temperature rises, the B-O-B content of [BO₄] in the original borate glass decreases, and the B-O-B concentration of [BO₃] unit increases. Since the B-O-B bond energy is greater than B-O-Bi, the bond length of B-O-B is shorter than that of B-O-Bi; therefore, it tends to move toward a high wavenumber.

In Figure 5(a), at a melting temperature of 900°C and a wavenumber of 1382 cm⁻¹ (B–O– in the [BO₃] unit), as the concentration of Bi₂O₃ increases, the signal of B–O– in the [BO₃] unit increases, showing that the tetrahedral [BO₄] structure is loosened and dissociated, providing oxygen atoms for Bi₂O₃ bonding, and the tetrahedral [BO₄] structure is transformed into a triangle [BO₃], so the signal of B–O– in the [BO₃] unit is enhanced. Between 1000 cm⁻¹ (solid arrow) and 1382 cm⁻¹, it has a tetrahedral [BO₄] structure. It can be observed that as the concentration of Bi₂O₃ increases, the signal of B–O– in the [BO₄] unit decreases, indicating that the network of the [BO₄] unit of the structure drops. With the increase of Bi₂O₃ concentration, [BiO₃] tends to gradually become the glass network formers of A series glass. Furthermore, from the FTIR diagram, it can be clearly judged that the Bi-O bond strength is less than the B-O bond strength, resulting in the glass structure network connection; the strength is, therefore, weakened at the same time, which causes the CTE to increase with the increase of Bi₂O₃ concentration. Because of the high polarizability of bismuth ions in high-content bismuth oxide glass, the octahedron [BiO₆] produces serious distortion and deformation, so the glass structure, a low-coordination [BiO₃] and bismuth-oxygen trigonal bismuth ion, is formed [11].

(a)

(b)

(c)

(d)

Figure 5: FTIR spectra of A series at different melting temperatures of 900°C, 1000°C, 1100°C, and 1200°C.

Therefore, in the FTIR analysis, the structural system can be determined. As the concentration of Bi₂O₃ increases, the characteristic absorption peak intensity of the tetrahedral [BO₄] structural unit weakens, and the [BO₃] structural unit forms a chain or ring with the B-O-B bond of polyborate. The intensity of characteristic absorption peaks of trigonal [BO₃], [BiO₃], and [BiO₆] structural units has been enhanced.

3.3. XPS

In the A glass series, Figure 6 shows O1s spectra of melting at different temperatures of 900°C, 1000°C, 1100°C, and 1200°C. At 900°C, A1 = 530.85 eV, A2 = 530.5 eV, and A3 = 530.39 eV; the same is true at 1000°C, 1100°C, and 1200°C. As the content of Bi₂O₃ increases, the binding energy of the O1s spectrum all shifts to the lower binding energy direction, which is consistent with the observation results of FTIR.

(a)

(b)

(c)

(d)

Figure 6: XPS-O1s spectra of A series glass melted at different temperatures of 900°C, 1000°C, 1100°C, and 1200°C.

In Figure 7, with the increase of melting temperature, the binding energy of O1s energy spectrum of A1, A2, and A3 series glass is observed, except at 1200°C; the others are all shifting to the lower binding energy direction, which is consistent with the observation results of FTIR.

(a)

(b)

(c)

(d)

Figure 7: O1s spectra of A1, A2, and A3 series glasses, respectively, melted at 900°C, 1000°C, 1100°C, and 1200°C.

Figure 8 shows the spectra of Bi4f7/2 and Bi4f5/2-XPS melted at 900°C, 1000°C, 1100°C, and 1200°C for A1, A2, and A3 series glasses, respectively. The binding energy range is from 155 eV to 170 eV. The former is Bi4f7/2(eV) and the latter is Bi4f5/2(eV) signals [26, 27], located at 156.2 eV, 158.9 eV, 159.5 eV, and 161 eV which are the binding energies of Bi⁰, Bi⁺¹, Bi⁺², Bi³⁺, and Bi⁵⁺ ions of Bi4f7/2, respectively [27], and Bi⁰, Bi⁺¹, Bi⁺², Bi³⁺, and Bi⁵⁺ ions binding energy at 162.1 eV, 164 eV, 165 eV, and 166 eV are Bi4f5/2, respectively [27].

(a)

(b)

(c)

(d)

Figure 8: Spectra of Bi4f7/2 and Bi4f5/2-XPS melted at 900°C, 1000°C, 1100°C, and 1200°C for A1, A2, and A3 series glasses, respectively.

Observing Figure 8, it can also be clearly seen that when the Bi₂O₃ concentration is 20 mol %, the Bi³⁺ and Bi⁵⁺ ion concentrations of Bi4f7/2 in the A1, A2, and A3 glass systems decrease, and the Bi⁺¹ and Bi⁺² ion concentrations increase. As the Bi⁰ concentration signal increases, bismuth ions tend to be reduced to Bi⁰, and the structure gradually forms a glass unit structure dominated by [BiO₃], [BiO₆], [BO₃], and [BO₄]. With the increase in melting temperature, Bi³⁺ ions can obtain additional heat energy and will also be reduced. Bi ions of different valences, Bi³⁺ ⟶ Bi⁺¹, Bi⁺², or Bi⁰ are precipitated. As the melting temperature rises, the Bi ions are dissolved in the glass easily bond with other atoms in the system to form Bi-O-B and Bi-O-Zn bonded glass structures. Figure 6 shows the XPX-O1s spectra of A series glass melted at different temperatures of 900°C, 1000°C, 1100°C, and 1200°C. As the content of Bi₂O₃ increases, the binding energy of O1s spectrum shifts to the direction of lower binding energy, showing that the high-strength bonds will gradually decompose and in turn will form more lower-strength [BO₃], [BiO₃], and [BiO₆] bonds, and the overall glass structure will become looser.

Figure 9 shows the Bi4f7/2 and Bi4f5/2 energy spectra for (a) A1 glass, (b) A2 glass, and (c) A3 glass at different melting temperatures of 900°C, 1000°C, 1100°C, and 1200°C. As the melting temperature increases, the energy spectra of Bi4f7/2 and Bi4f5/2 of A1, A2, and A3 series glass show that, except for the A3 series glass that turns to low energy at 1200°C, the rest tend to shift to high energy direction, which is consistent with the observation results of FTIR.

(a)

(b)

(c)

(d)

Figure 9: Bi4f7/2 and Bi4f5/2 energy spectra for (a) A1 glass, (b) A2 glass, and (c) A3 glass at different melting temperatures of 900°C, 1000°C, 1100°C, and 1200°C.

According to Sanz et al. [28], when the melting temperature is increased from 1050°C to 1300°C by TEM, the particle size of metallic bismuth nanoparticles also changes from 10 nm increasing to 30 nm, the existence of Bi⁰ nanoparticles can be found in TEM, and some Bi₂O₃ undergoes thermal reduction, which causes the glass to be colored. Figure 10 shows A series glass test piece. A2 series glass is light yellow at 1000°C, and at 1100°C, it is dark brown, forming a strong contrast.

Figure 10: A series glass test piece.

The O1s, Bi4f7/2, and Bi4f5/2 energy spectra of A series glass show that with the increase of Bi₂O₃ content, the binding energy of O1s, Bi4f7/2, and Bi4f5/2 energy spectra shifts to lower binding energy, and with the increase of melting temperature, except for the binding energies of O1s, Bi4f7/2, and Bi4f5/2 energy spectra of A2 series glasses at 1000°C, the remaining binding energies shift to higher binding energies.

3.4. Thermal Properties of Glass

According to Taisuke Inoue, Tsuyoshi Honma, Vesselin Dimitrov, Takayuki Komatsu, and KH Sun et al. [29, 30], the Zn-O bond strength in the ZnO₄ structural unit is 150.06 kJ/mol, and the Bi-O bond in the BiO₆ structural unit is 102.5 kJ/mol. In this experiment, with the increase of Bi₂O₃ content, the glass XPS energy spectrum (Figure 6) shows the binding energies of O1s, Bi4f7/2, and Bi4f5/2; except for O1s, Bi4f7/2, and Bi4f5/2 of A2 series glass at 1000°C, the remaining binding energies all shifted to lower binding energies as the content of Bi₂O₃ increases. And in the FTIR spectrum (Figure 5), the [BiO₃] and [BiO₆] structural units mainly composed of Bi₂O₃ in the glass increase. As the content of Bi₂O₃ increases, the [BO₃] structure concentration increases, the [BO₄] structure concentration decreases, and some B-O-B bonds are bonded with Bi ion to form a B-O-Bi bond with weaker bond energy. Figure 11 shows the glass thermal expansion coefficient (CTE) of A series glass observed with Bi₂O₃ content; due to the weaker bond energy of Bi-O-Bi and Bi-O-B, the bond length is longer, and the thermal expansion coefficient increases with the increase of Bi₂O₃ content.

Figure 11: The glass thermal expansion coefficient (CTE) of A series glass observed with Bi₂O₃ content.

From the literature [29, 30], the structural unit Zn-O bond strength of ZnO₄ is 150.06 kJ/mol, the Bi-O bonding strength in the BiO₆ structural unit is 102.5 kJ/mol, and the bonding strength in B₂O₃ is 497.896 kJ/mole and 372.376 kJ/mole, BEB > BEZn > BEBi (BE, binding energies). Figure 12 shows the glass thermal expansion coefficient (CTE) of A series glass observed at different melting temperatures. The CTE of A2 series glass at 1000°C obviously drops to 5.89510⁻⁶/°C. Figure 7 shows the O1s spectra of A2; 530.88 eV at 1000°C and 530.68 eV at 1100°C show that the bridging oxygen concentration of A2 at 1000°C is higher than that at 1100°C, and the structural strength of A2 is stronger at 1000°C. Figure 9 shows the Bi4f7/2 and Bi4f5/2 spectra of A2; Bi4f5/2 at 1000°C is 164.14 eV and that at 1100°C is 164.06 eV, and Bi4f7/2 at 1000°C is 158.75 eV and that at 1100°C is 158.77 eV, which indicates that the increase of melting temperature leads to the decrease of bridging oxygen concentration of Bi₂O₃, and Bi atoms in [BiO₃] and [BiO₆] tend to be biased toward lower energy B⁰ reduction. In FTIR, 700 cm⁻¹ and 777 cm⁻¹ (dashed arrow) are the vibration signals of the [BO₃] unit. At 1000°C, 700 cm⁻¹ is more obvious, indicating that [BO₃] structure concentration is high, [BO₄] structure concentration is slightly smaller, and the 777 cm⁻¹ (dashed arrow) signal is weak too. The equilibrium of high binding energy shift shows that the structure and strength of A2 series glass are relatively high when the melting temperature is 1000°C, which means that the A2 series glass structure at the time, with the higher intensity of [BO₃] and [BO₄] structures, has a greater concentration, and the overall structural strength increases, causing the CTE of A2 series glass at 1000°C to drop significantly to 5.895 10⁻⁶/°C.

Figure 12: The coefficient of thermal expansion (CTE) of glass A series observed at different melting temperatures.

From the straight view of Figure 12, at the melting temperature of 900°C, CTE A1 = 6.497 10⁻⁶/°C, CTE A2 = 7.297 10⁻⁶/°C, CTE A3 = 8.402 10⁻⁶/°C, and CTE increases with the increase of Bi₂O₃ content. From the horizontal view of Figure 12, CTE A1-900°C = 6.497 10⁻⁶/°C, CTE A1-1000°C = 6.298 10⁻⁶/°C, CTE A1-1100°C = 6.096 10⁻⁶/°C, CTE A1-1200°C = 5.296 10⁻⁶/°C, and CTE decrease as the melting temperature rises, the factor that affects the change of CTE and makes the Bi₂O₃ content change more.

4. Conclusion

In this study, the glass color changed into dark color from transparent yellow with the increase of Bi₂O₃ content and melting temperature, and the dissolution of Bi ions directly affects the color of the glass. The CTE of A2 glass at 1000°C obviously drops to 5.895 10⁻⁶/°C, indicating that the glass structure of A2 series glass at this time has a higher concentration of [BO₃] and [BO₄] structural units with higher strength, and the overall structural strength has increased, causing the CTE of A2 series glass at 1000°C drops significantly. And the CTE of glass increases with the increase of Bi₂O₃ content and decreases as the melting temperature rises, the factor that affects the change of CTE and makes the Bi₂O₃ content changes more.

Data Availability

No data were used to support this study.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this study.

Acknowledgments

The authors gratefully acknowledge the financial support of the Ministry of Science and Technology (MOST), Taipei, Taiwan, and The National Sci-Tech Programs: The Study on Manufacture and Sealing Characterization of Bismuth-Zinc Borate Glasses (MOST 103-2221-E-239-005).

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