J. Cent. South Univ. Technol. (2010) 17: 257-262
DOI: 10.1007/s11771-010-0039-x 
Structure of Bi2O3-ZnO-B2O3 system low-melting sealing glass
HE Feng(何峰), CHENG Jin-shu(程金树), DENG Da-wei(邓大伟), WANG Jun(王俊)
Key Laboratory of Silicate Materials Science and Engineering of Ministry of Education,
Wuhan University of Technology, Wuhan 430070, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2010
Abstract: Bi2O3-ZnO-B2O3 system glass is a kind of lead-free low melting sealing glasses. The structure of Bi2O3-ZnO-B2O3 system low-melting sealing glass was investigated by DSC, FT-IR, XRD and SEM. The results show that with the increase of B2O3 content, the transition temperature Tg and softening temperature Tf of Bi2O3-ZnO-B2O3 system low-melting sealing glasses increase, which leads to the liquid phase precipitation temperature increasing and promotes the structure stability in the glass. With increasing the heat treatment temperature, a large number of liquid phases appear in samples and the sinter efficiency of the samples increases. The FT-IR spectra of the glasses show the presence of some bands that are assigned to vibrations of Bi—O bond from [BO3] pyramidal and [BiO6] octahedral units and B—O from [BO3] and [BO4] units. With the decrease of B2O3 content, the crystallization tendency of the glass increases. In glass samples B1 and B2, crystallization starts at 460 ℃ and 540 ℃, respectively. Both of them precipitate Bi24B2O39 phases.
Key words: Bi2O3-ZnO-B2O3 system glass; low-melting sealing glass; transition temperature; softening temperature
1 Introduction
Sealing glass is the middle layer material that can be used for sealing up glasses, ceramics, metal and other materials. Resulting from the development of modern science and technology, especially with the rapid progress in the areas, such as electronic vacuum technology, micro-electronics technology, laser and infrared technology, high-energy physics, and aerospace industry, size of apparatus is getting smaller and smaller, exactitude of structural devices is enhanced continuously, type of electronic components is getting more and more, shape of products is getting more complex. The requirements of gas-tight and reliability for the seal products and working situation are increased [1].
Sealing materials can generally be divided into organic materials, inorganic materials and metal materials. Organic materials include epoxy resin, silicone rubber and silicone resin. Organic polymer materials are frequently used for low-temperature sealing. Inorganic materials include glass and enamel, which are applied to high-temperature, gas-tight sealing mainly. Metal sealing materials primarily made from Pb-Sn solder are mainly applied to seal of electronics [2]. As a sealing material, glass-type materials have their advantages. They are better than organic polymer materials in air tightness and heat resistance, and also better than metal materials in electrical performance. Thus, sealing glasses have a wide range of applications. In the design of glass on the basis of the composition, two aspects should be taken into account: first, the structure of the basic glass should be stable. Second, the softening point of basic glass should not be very high, expansion coefficient should adapt with that of sealing device, and the basic glass should have better mechanical strength and chemical stability. In addition, with the enhancement of awareness about environmental protection, the glass of lead-free solder made the request [3-5]. Therefore, Bi2O3-ZnO-B2O3 system low melting sealing glasses was investigated, in order to get a new system sealing glass. Recently, CHENG et al [6] studied the structure and crystallization kinetics of Bi2O3-B2O3 glasses and RADU et al [7] employed infrared and Raman spectra studies to investigate the structural units in bismuth based glasses. Despite the fact that Bi2O3 is not a classical glass former, due to high polarizability and small field strength of Bi3+ ions, in the presence of conventional glass formers (such as B2O3, PbO and SiO2) a glass network of [BiOn](n=3, 6) pyramids may be built [8]. However, the structural role played by Bi2O3 in glasses is complicated and poorly understood. BALE et al [9] studied role of Bi2O3 content on physical, optical and vibrational in Bi2O3-ZnO-B2O3 glasses. In all the studies above, the content of Bi2O3 was below 70%. The investigation also extended dielectric properties and structure of these glasses. This work is to investigate the structure of the glass system with the change of B2O3 content which was studied by DSC, FT-IR spectra, XRD and SEM.
2 Experimental
2.1 Glass sample preparation
The compositions are designated by the B2O3 content. Compositions of bismuth borate glasses are given in Table 1. The stoichiometric amounts of B2O3, ZnO, Bi2O3, CuO, Co2O3, BaCO3, and Na2CO3 (AR grade) are thoroughly mixed for 15 min and loaded into a Pt crucible. They were melted in a furnace at 1 200 ℃ for 1 h and then the melting glass was poured down on the preheated stainless die. The glasses were annealed at 320 ℃ for 1 h to eliminate internal stress. Then, the black and metallic luster glass samples with good melting and no defect and stripes were gained.
Table 1 Components of group B glass samples (mass fraction, %)
The glass samples were cut by J5075/2F round cutter, and the 30 mm×4 mm×4 mm smooth strip samples were prepared for testing coefficient of thermal expansion. Massive glass samples were ground and sieved to less than 76 μm. The glass power was mixed with appropriate amount of PVA, which was used for molding, and then NYL-500-pressure testing machine was used to make “button” samples with d15 mm× 5 mm at 40 kN. “Button” samples were sintered at different temperatures and the heating rate of 5 ℃/min. The target temperatures were from 380 to 580 ℃ for 30 min. Through sintering “button” experiment, the structure and fluxion property were studied.
2.2 Analysis and testing
Powders of the glasses from 150 to 200 μm were used for the differential thermal analysis (DTA) at a heating rate of 10 ℃/min to determine the glass transition temperature (Tg) and the softening temperature (Tf) by STA449C DTA analysis instrument with a heating rate of 5 ℃/min. The expansion of the samples was tested by the horizontal expansion instrument at a heating rate of 5 ℃/min. The test temperature range was from room temperature to 600 ℃.
The sintered “button” samples were ground to 76 μm. The crystallization of the glass powders was analyzed using an X-ray diffractometer (D/max-ⅢA X-ray diffraction apparatus, Japan). The microstructures of the sintered samples were observed by scanning electron microscopy (SEM, SX-40 Japan). Infrared spectra of the powder glass samples were recorded at room temperature in the range of 400-2 800 cm-1 using a Nicolet-60-SXB FT-IR-infrared spectrometer. These measurements were made on glass powder dispersed in KBr pellets.
3 Results and discussion
3.1 Transition temperature (Tg) and softening temperature (Tf)
Fig.1 shows the DSC curves of the glasses. On the basis of the DSC curves, glass transition temperature (Tg) and softening temperature (Tf) values could be obtained. It is found that Tg and Tf increased from 415 to 456 ℃ and from 455 to 513 ℃, respectively, as a function of B2O3 content. This seems to be caused by the increase in glass viscosity since B2O3 plays the role of network modifier [9], and B3+ can be in the form of planar [BO3] and/or tetrahedral [BO4] in glass structure. This indicates that the increase of B2O3 content may induce the liquid phase at higher temperature. With the increase of B2O3 content, the level of glass network connection increases. On the other hand, LI [10] had the view that whether they could form glass or not, the energy of the bond of oxides was one of the most important factors. In all oxides, B2O3 has the largest single bond energy, which is hard to overcome in the form of molten glass process. With the relative increase of B2O3 content, it needs more energy to form glass phase. Therefore, with increasing the content of B2O3, Tg and Tf of the glass samples also increase. Tg of the glass samples will affect the sintering property of glass power.
Fig.1 DSC curves of glasses
The lower the Tg is, the easier glass power sintering will be. The action of Bi2O3 is as the same as that of PbO in the glass structure and it also has the fluxing action to make the glasses melt at very low temperature.
3.2 FT-IR spectra
Bi2O3 plays a very unique role in the glass structure, when high contents of Bi2O3 and B2O3 are used at the same time in one sample. They could form vitreous structure and have special character. Glasses containing with Bi2O3 have five fundamental vibrations in the IR spectral regions at about 830, 715, 620, 450 and 350 cm-1. Boron also has three vibration bands at 1 200- 1 600, 800-1 200 cm-1 and about 700 cm-1. Table 2 summarizes the major absorption bands and their vibration. Obviously, four IR absorption bands were observed in all samples, including 450-520, 680-720, 900-950 and 1 100-1 400 cm-1. Fig.2 illustrates the infrared spectra of the present glass system. As shown in Fig.2, it could be found that there were three main absorption peaks at the vicinity of about 1 180, 710 and 490 cm-1. In the region of 900-950 cm-1 a band identified is due to the vibration of [BO4] structural units. In the region of 1 100-1 400 cm-1 a band identified is due to the stretching vibration of B—O—B in [BO3] triangles and another band at about 710 cm-1 is due to the bending vibration of B—O—B linkages of [BO3] units [11-12]. With the decrease of B2O3 content, the absorption peak intensity decreases. A band due to symmetrical bending vibrations of [BiO6] units was also reported in Refs.[13-15] at about 470 cm-1, and it was caused by stretching vibration of Bi—O bond in the glass [12]. A band due to symmetric stretching vibration of Bi—O band in [BiO3] was also reported in Ref.[15] at about 715 cm-1. Consequently, it can be suggested that the structure of Bi2O3-ZnO-B2O3 glasses consist of four main units including [BiO3], [BiO6], [BO3] and [BO4].
Table 2 Vibration types of main absorption bands in samples
Fig.2 Infrared spectra of glasses
Fig.3 shows the XRD patterns of B series glasses. It could be found from Fig.3(a) that, when the heat-treatment temperature was 380 ℃, no crystalline phases were separated from parent glasses. This was the notable feature of the vitreous diffraction. As shown in Fig.3(b), an obvious diffraction peak in sample B1 appeared when the sintering temperature was 460 ℃. After inspection, compared with standard PDF card, it could be found the main crystal phase in sample B1 was Bi24B2O39, but glass phase was still preserved in samples B2 to B5. As shown in Fig.3(c), when the glass samples were sintered at 540 ℃, both samples B1 and B2 precipitated the crystal. According to the intensity of diffraction peaks, the intensity diffraction peak of sample B1 was stronger than that of B2, it meant the amount of crystal in sample B1 is more than that in B2. It could be found that the main crystal phase of sample B1 was also Bi24B2O39. Glass phase was still preserved in sample B3 to B5, and there were not crystal phased in them. With the increase of B2O3 content, the connection of the glass network structure increases, which makes the crystal phase more difficult to be separated from parent glass.
Fig.3 XRD patterns of glasses sintered at different temperatures: (a) 380 ℃; (b) 460 ℃; (c) 540 ℃
Fig.4 shows the SEM images of glass samples. It could be found from the photographs that when the glass particles were sintered at 380 ℃, the edges and corners of B1 glass particles began to be smooth, liquid phase appeared and particles had the sign of bonding, but the quantity of liquid was limited. When the glass particles were sintered at 400 ℃, B1 glass particles became very smooth, there were no obvious edges and corners, most of the particles fused together and the porosity reduced dramatically. When the glass particles were sintered at 420 ℃, B1 glass particles melted together completely with some closed stomata in them.
Fig.4 SEM images of samples: (a) B1, 400 ℃; (b) B1, 420 ℃; (c) B2, 400 ℃; (d) B2, 420 ℃; (e) B3, 420 ℃; (f) B3, 440 ℃; (g) B4, 420 ℃; (h) B4, 440 ℃; (i) B5, 420 ℃; (j) B5, 440 ℃
When the glass particles were sintered at 400 ℃, the edges and corners of B2 glass particles became smooth, liquid phase appeared and particles were bonded each other. When the glass particles were sintered at 420 ℃, the particles of B2 glass melted together and the porosity rate reduced dramatically with some closed stomata in the sample. At the same sintering temperature, the edges and corners of B3, B4 and B5 glass particles began to be smooth, liquid phase appeared and particles were closed, sintered and bonded with each other. And when the temperature was higher than 440 ℃, the glass particles completely melted together with some closed stomata in them. As shown in Fig.4 that samples of B3, B4 and B5 were liquefied, and the surface tension began to play a role. Overall, with the increase of B2O3 content, the B series glass samples became hard to be sintered and the temperature of glass particles changing into liquid was increased. The reason was that B2O3 was the forming composition of glass network which had the largest single bond energy to make the entire system of destruction and combination very hard. The crystallization of the system became weakened as its content increased.
In addition, the coefficients of thermal expansion from B1 to B5 are 1.094 5×10-5, 1.083 8×10-5, 1.067 5×10-5, 1.034 7×10-5 and 1.008 5×10-5 ℃-1. With the increase of B2O3 content, the thermal expansion coefficients of the samples decrease. With the increase of B2O3 content, the network of the glass became stable.
4 Conclusions
(1) Both the Tg and Tf of the samples increase as the B2O3 content increases. The sintering efficiency of the glass is observed due to content of liquid phases. With the increase of B2O3 content, the sintering temperature of the samples also increases.
(2) The boron-oxygen network is the form of planar [BO3] and tetrahedral [BO4] in the glass structure. Bi3+ ions are incorporated in the glass net-work as [BiO3] triangles and [BiO6] octahedral units in the glass samples.
(3) With the decrease of B2O3 content, the crystallization tendency of the glass increases. At different sintering temperatures, both samples B1 and B2 precipitate Bi24B2O39 phases. With the increase of B2O3 content, the thermal expansion coefficients of the samples decrease from 1.094 5×10-5 to 1.008 5×10-5.
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Foundation item: Project(50272043) supported by the National Natural Science Foundation of China
Received date: 2009-04-24; Accepted date: 2009-08-07
Corresponding author: HE Feng, Professor; Tel: +86-27-87860801; E-mail: he-feng2002@163.com
(Edited by YANG You-ping)
