The Relationship between the Structure and Thermal Properties of Bi 2 O 3 -ZnO-B 2 O 3 Glass System

The inﬂuence of Bi 2 O 3 and melting temperature on the thermal and structural properties of xBi 2 O 3 -(60-x) ZnO-40B 2 O 3 glasses has been investigated in this study. It is expected that these factors can be used to control the degree of reduction of Bi 2 O 3 , 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 [BiO 6 ] octahedron, [BiO 3 ] triangle, [BO 4 ] tetrahedron, and triangle [BO 3 ] 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 Bi 2 O 3 content, and the O1 s nuclear electron binding energy shifts to the lower energy direction with the increase of Bi 2 O 3 addition. In terms of FTIR, as the melting temperature rises, the B-O-B bonding vibration concentration of [BO 4 ] inside the borate glass decreases, and the density of B-O-B bonding vibration of [BO 3 ] increases, Moreover, the increase in melting temperature increases the probability of reducing Bi ions to Bi 0 , reduces the bonding of Bi-O-B, and increases the bonding of B-O-B, and the CTE also slightly decreases.


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. e development of lead-free glass to replace leadcontaining glass is the main research direction at present, and the component of lead oxide is bismuth oxide (Bi 2 O 3 ), its Bi 3+ property is similar to Pb 2+ 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 leadcontaining 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 [BO 3 ] is a plane triangle, and the boron coordination number is 3. If an additional free oxygen atom is provided, BO 3 can bond with the oxygen atom to form a BO 4 tetrahedral with a coordination number of 4.
[ BO 3 ] and [BO 4 ] 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 Bi 2 O 3 is weak, it belongs to the nature of glass as an intermediate agent and cannot form glass alone. Adding Bi 2 O 3 to ZnO, SiO 2 , B 2 O 3, 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][9][10][11] 6 ] octahedron and other structural units appear, and Bi 2 O 3 can be used as a glass modifier or glass forming agent, depending on the proportion of Bi 2 O 3 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 Bi 3+ and Bi 5+ , and/or plasma Bi 0 clusters in colored glass, because Bi 3+ , 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 Bi 2 O 3 -TiO 2 -B 2 O 3 glass system the Bi 2 O 3 glass increases both the Bi 2 O 3 content and the melting temperature, which causes the glass to change from light yellow to brownish-black gradually. Because Bi 3+ is easily reduced to Bi 2+ , Bi + , and Bi 0 [14,15], the reduction reaction can be strengthened with the increase of melting temperature and Bi 2 O 3 content, which make the color of the glass deep, and the light transmittance decreases [14].

Materials and Methods
In this study, 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. e 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. e 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. e 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. e range of test wavenumber is 450∼2000 cm −1 . rough 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. e thermal analysis of the glass in this experiment uses a thermomechanical analyzer Perkin Elmer TMA 7, USA. e 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.

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 Bi 2 O 3 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.    3 ] 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:

FTIR.
In the A3 glass series, because the 30 mol % of Bi 2 O 3 content is relatively high, it has a relatively sufficient reaction concentration with [BO 3 ] and [BO 4 ] unit, so it is clearly observed that as the melting temperature increases, the concentration of B-O − of [BO 3 ] unit decreases. And A1, A2, and A3 glass systems all tend to shift to high wavenumbers.
is shift phenomenon is that as the melting temperature rises, Bi 3+ is reduced to low-valent ions and partly reduced to Bi 0 to dissolve and causes the glass color to be darker brown, and when the melting temperature rises, the B In Figure 5( 6 ] produces serious distortion and deformation, so the glass structure, a low-coordination [BiO 3 ] and bismuth-oxygen trigonal bismuth ion, is formed [11]. erefore, in the FTIR analysis, the structural system can be determined. As the concentration of Bi 2 O 3 increases, the characteristic absorption peak intensity of the tetrahedral

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 Bi 2 O 3 increases, the binding energy of the O1 s spectrum all shifts to the lower binding energy direction, which is consistent with the observation results of FTIR.
Observing Figure 8, it can also be clearly seen that when the Bi 2 O 3 concentration is 20 mol %, the Bi 3+ and Bi 5+ ion concentrations of Bi4f 7/2 in the A1, A2, and A3 glass systems decrease, and the Bi +1 and Bi +2 ion concentrations increase. As the Bi 0 concentration signal increases, bismuth ions tend to be reduced to Bi 0 , and the structure gradually forms a glass unit structure dominated by [BiO 3 ], [BiO 6 ], [BO 3 ], and [BO 4 ]. With the increase in melting temperature, Bi 3+ ions can obtain additional heat energy and will also be reduced. Bi ions of different valences, Bi 3+ ⟶ Bi +1 , Bi +2 , or Bi 0 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-O1 s spectra of A series glass melted at different temperatures of 900°C, 1000°C, 1100°C, and 1200°C. As the content of Bi 2 O 3 increases, the binding energy of O1 s spectrum shifts to the direction of lower binding energy, showing that the high-strength bonds will gradually decompose and in turn will form more lowerstrength [BO 3 ], [BiO 3 ], and [BiO 6 ] bonds, and the overall glass structure will become looser. Figure 9 shows the Bi4f 7/2 and Bi4f 5/2 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          Figure 8: Spectra of Bi4f 7/2 and Bi4f 5/2 -XPS melted at 900°C, 1000°C, 1100°C, and 1200°C for A1, A2, and A3 series glasses, respectively. 8 Advances in Condensed Matter Physics melting temperature increases, the energy spectra of Bi4f 7/2 and Bi4f 5/2 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. 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 Bi 0 nanoparticles can be found in TEM, and some Bi 2 O 3 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. e O1 s , Bi4f 7/2 , and Bi4f 5/2 energy spectra of A series glass show that with the increase of Bi 2 O 3 content, the binding energy of O1 s , Bi4f 7/2 , and Bi4f 5/2 energy spectra shifts to lower binding energy, and with the increase of melting temperature, except for the binding energies of O1 s , Bi4f 7/2 , and Bi4f 5/2 energy spectra of A2 series glasses at 1000°C, the remaining binding energies shift to higher binding energies.

ermal 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 ZnO 4 structural unit is 150.06 kJ/mol, and the Bi-O bond in the BiO 6 structural unit is 102.5 kJ/mol. In this experiment, with the increase of Bi 2 O 3 content, the glass XPS energy spectrum ( Figure 6) shows the binding energies of O1 s , Bi4f 7/2 , and Bi4f 5/2 ; except for O1 s , Bi4f 7/2 , and Bi4f 5/2 of A2 series glass at 1000°C, the remaining binding energies all shifted to lower binding energies as the content of Bi 2 O 3 increases. And in the FTIR spectrum ( Figure 5)   From the literature [29,30] Figure 12 shows the glass thermal expansion coefficient (CTE) of A series glass observed at different melting temperatures. e CTE of A2 series glass at 1000°C obviously drops to 5.895×10 −6 /°C. Figure 7 shows the O1 s 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 Bi4f 7/2 and Bi4f 5/2 spectra of A2; Bi4f 5/2 at 1000°C is 164.14 eV and that at 1100°C is 164.06 eV, and Bi4f 7/2 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 Bi 2 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 Bi 2 O 3 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 Bi 2 O 3 content change more.

Conclusion
In this study, the glass color changed into dark color from transparent yellow with the increase of Bi 2 O 3 content and melting temperature, and the dissolution of Bi ions directly affects the color of the glass. e 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 [BO 3 ] and [BO 4 ] 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 Bi 2 O 3 content and decreases as the melting temperature rises, the factor that affects the change of CTE and makes the Bi 2 O 3 content changes more.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this study.