Microstructure and properties of a vacuum-tempered glass with low-temperature-sintered silver paste

A low-temperature sintered silver paste was used for the metallization of tempered glass, which was used to prepare vacuum-tempered glass by soldering with Sn96.5Ag3Cu0.5 paste (SAC305). The effects of the glass content and the sintering temperature on the microstructures, shear strengths, and fracture mechanisms of the bondlines were investigated in detail. The microstructure of the thick silver film and its interface with the tempered glass was characterized. The dissolution and precipitation behaviors of silver in Bi–B–Zn glass were also analyzed. Bi4B2O9 crystals were detected in the microstructure of the thick silver film when the joining temperature was increased above 450 °C, which can strengthen the silver film and the interface with the tempered glass. After soldering at 450 °C for 10 min, an excellent bondline was formed using the low-temperature sintered paste, which had a shear strength of 42.3 ± 2.4 MPa and a leak rate of 7.2 ± 0.4 × 10–3 Pa.cm3/s. The sintered paste had a composition of 80 wt.% silver and 20 wt.% glass powder. Furthermore, heat transfer tests revealed that the vacuum tempered glass had excellent thermal insulation properties. The results showed that the low-temperature sintered silver paste combined with soldering was an effective method to prepare vacuum-tempered glass.


Introduction
Driven by the requirements of energy conservation, environmental protection and sustainable development, vacuum glass has been widely used in buildings, ship doors and windows, incubators, and airtight seals, which is an important development direction in high-efficiency energy-saving [1][2][3][4].
Similar to the structure and function of a Dewar flask, heat convection and conduction have minimal influences on heat transfer because of the vacuum layer. Vacuum glass has excellent thermal insulation properties [1]. Vacuum glass components have two parallel glass plates with a thickness of about 100 lm, and an array of transparent support columns are required to resist the atmospheric pressure between the glass plates [5]. The edges are usually sealed by organic polymer materials, glass paste, soft metal solder, or low-temperature-sintered silver paste [6][7][8]. Then, the residual gas is evacuated through the reserved small exhaust port and sealed. The pressure in the vacuum chamber needs to be kept below 0.1 Pa and the overall structure is required to withstand an atmospheric pressure of 10 tonÁm -2 [9]. Generally, vacuum glass made of ordinary glass restricts its promotion and application because its strength and safety performance cannot meet the application requirements [10]. The strength of tempered glass is 3-5 times higher than ordinary glass. It is a material with excellent safety performance. It can address the problems of ordinary glass when used to prepare vacuum tempered glass [11]. However, one of the most important issues of annealing tempered glass that needs to be considered during this process is the use of sealing materials. To ensure the high strength of tempered glass, it is necessary to reduce the time of the sealing process above 450°C. Organic polymer materials can meet these sealing requirements, but such these materials are prone to aging and cannot maintain long-term airtightness. Low-temperature sealing materials, such as glass solder or soft metals, which could be used to seal the tempered glass either contain lead or require a higher bonding temperature [12]. Lead will harm the human body and the environment. More and more countries and regions have restricted the use of lead [13]. The United Kingdom, the European Union, and Japan have enacted relevant restrictions to explicitly prohibit the use of hazardous substances such as lead in electronics and automobile industries [14,15]. As a consequence, the development of lead-free glass has attracted the attention of many scientific researchers. The development of a low-temperature melting lead-free glass sealant for vacuum tempered glass is required. However, using glass solder to seal tempered glass also results in a mismatch of thermal expansion coefficient. Heydari reported that a glass-ceramic system, consisting of SiO 2 -Al 2 O 3 -BaO-B 2 O 3 -CaO, has a thermal expansion coefficient (CTE) of 10.3 9 10 -6 K -1 , which is close to that of solid oxide fuel cells, and the glass adhered well to yttria-stabilized zirconia [16]. Lin used Bi 2 O 3 -B 2 O 3 -SiO 2 glass solder to join the Li-Ti ferrite and nearly defect-free bondlines were obtained under 100 Pa of applied pressure [17]. Essam et al. studied the glass network structure and thermal stability of Bi 2 O 3 -B 2 O 3 . The glass transition temperature was 390-480°C, and the thermal expansion coefficient was (6-8) 9 10 -6 /°C [18]. Some researchers have proposed replacing glass solders with low-temperature joining materials (such as metals and resins). However, metal matrix solders (Bi-alloys, Zn-Al based alloys, In-Al based alloys, Au-based alloys, Sn-Zn-Cr alloys) have many limitations because of their poor corrosion resistance and high costs [19][20][21][22]. Resins also have disadvantages in impermeability, thermal stability, and aging resistance. In response to this phenomenon, a low-temperature sintered silver paste was prepared by combining the low-temperature meltability of Bi-B-Zn glass and the excellent plasticity of metallic silver. High-quality sealed components could be obtained by preparing a thick layer of silver film on the tempered glass substrate before soldering. The silver paste is a new type of bonding material, which is composed of a metal phase, a binder phase, and an organic carrier [23][24][25][26][27]. The silver paste with glass was used in a crystalline silicon solar cell. The viscous flow of the glass can promote the sintering between the silver particles and react with the anti-reflective layer to provide adhesion to the thick silver film and metalize the substrate surface [28]. Huang used silver paste for metallization and then connected the glass heat dissipation chip, which can achieve good heat dissipation and high bonding strength. This proved that the thick silver film can achieve low temperature soldering after metallization of the glass and aluminum alloy surface [29]. The silver film formed containing 7 wt.% Bi-B-Zn glass powder and sintered at 600°C for 15 min had a low-electrical resistivity of 2.5 lX cm and high strength of 28.5 MPa [30]. The silver paste was printed on the tempered glass substrate by screen printing to form a silver paste layer, and after sintering the metallization thick silver film for soldering was obtained [31,32].
In this work, the low-temperature sintered silver paste was used for surface metallization, and the vacuum-tempered glass was sealed by soldering. The vacuum-tempered glass was prepared with good bondline performance and excellent airtightness. The microstructure and sintering mechanism of the thick silver film were analyzed and the shear fracture mechanism was also clarified. Therefore, the leadfree low-temperature sintered silver paste was combined with soldering as an effective way to prepare vacuum tempered glass, which will replace the leaded glass sealing method.
2 Experimental procedure

Preparation of glass powder
The Bi-B-Zn glass powder was prepared by meltquenching. The raw mixture of Bi 2 O 3 -50H 3 BO 3 -15ZnO was heated to 1300°C at the heating rate of 10°C /min and was kept 30 min in the muffle furnace (HTF 17/5, Carbolite & Gero). Then, the molten glass was poured into deionized water and cooled quickly. The glass particles were subjected to wet ball-milling in ethanol for 200 min at 300 rpm and then passed through a 2500 mesh sieve. Finally, the Bi-B-Zn glass powder with an average size of 2.58 lm (D50) was obtained after drying at 80°C and passing through a 300 mesh sieve.

Preparation of the silver paste
The low-temperature-sintered silver paste was prepared by mixing silver powder, Bi-B-Zn glass powder, and an organic vehicle (ethyl cellulose, soy lecithin, ethyl acetate, antifoaming agent, 1,2propanediol, and terpineol) with the ratio of 71:18:11 (wt. %) using a planetary gravity mixer. The organic additives were used to disperse the solid powder and change the rheological properties of the low-temperature sintered silver paste (Figs. 1, 2).

Sample preparation process
The low-temperature sintered silver paste was printed on tempered glass surfaces, with a size of 20 mm 9 20 mm 9 4 mm and 10 mm 9 10 mm 9 4 mm. The size of the pattern for tempered glass surface metallization with the silver paste was 2 mm 9 2 mm 9 20 lm. The excess organic solvent was removed in a tube furnace, according to the debinding process (Fig. 3b), and the thick silver film was obtained after the sintering process was carried out in a rapid heating slide furnace at 430°C, 450°C, and 470°C for 10 min, respectively. Then, a layer of SAC305 was printed on the surface of the prepared silver film, as shown in Fig. 1.

Characterization
The phase composition of the Bi-B-Zn glass and the products in the brazed tempered glass bondlines were investigated by X-ray diffraction (XRD) using a diffractometer (Rigaku SmartLab D/max2500) with Cu Ka radiation. The experimental scan rate was 4°/ min, and the scanning range selection was 5-90°. The Differential Scanning calorimetry (DSC) curves of Bi-B-Zn glass powder and silver paste were recorded using a thermogravimetric analyzer (Netzch TGA STA 449 F3) and a differential scanning calorimeter (Netzch DSC STA 449 F3). Nitrogen was used as shielding gas, air was the purge atmosphere, the heating rate was 5°C/min, and the temperature range was 25-800°C. The microstructures of and the fracture morphologies of the bondlines were analyzed using a scanning electron microscope (SEM, S4700, Hitachi) equipped with an energy-dispersive spectrometer (EDS). The room temperature shear strength was tested using a shear strength test apparatus (MFM1200-10,031,311-L). The shear speed was 200 lm/s at room temperature and the schematic diagram is shown in Fig. 2. The measurements of shear strength were carried out 3 times with different glass content. The microstructures and the element distribution in the interface of bondlines were analyzed using a transmission electron microscope (TEM, Tslo F200X) equipped with an energy dispersive spectrometer (EDS). TEM samples were prepared using a focused ion beam (FIB, FEI Scios) and a micro-sampling method. The airtightness of the vacuum tempered glass was evaluated using a helium mass spectrometer leak detector. Test conditions A1: test pressure = 310 kPa, pressurization time = 2 h, test environment conditions = 25°C and humidity = 50% RH, pressure = 101 kPa in the corresponding lumen volume: 0.5 cm 3 B V \ 1.0 cm 3 . The eligibility criterion was B 1.0 9 10 -2 Pa.cm 3 /s. The test procedure was as follows: the component to be tested was placed in a sealed chamber with helium gas at a predetermined pressure and time, the helium gas adsorbed on the surface was removed after taking out from the chamber, and transferred the component to a helium mass spectrometer leak detector to measure a leak rate. Three measurements were taken in the experiment and averaged. The thermal insulation performance of the vacuum tempered glass was recorded using a temperature sensor.

Characterization of Bi-B-Zn glass
To sinter the silver paste at low temperatures, the glass used to prepare the silver paste should preferably be amorphous. The X-ray diffraction pattern of the Bi-B-Zn glass powder is shown in Fig. 3a. There were only two broad peaks and no crystal phase peaks detected, which indicated the Bi-B-Zn glass powder was amorphous. The glass transition temperature (T g = 353°C) and the crystallization temperature (T c = 448°C) of Bi-B-Zn glass were measured by a DSC analyzer, and are shown in Fig. 3b.

Preparation and characterization of silver paste
The silver paste was prepared using a planetary gravity mixer by mixing Bi-B-Zn glass powder, silver particles, and an organic carrier with a ratio of   Table 1.
The DSC and thermogravimetric (TG) analysis of the low-temperature sintered silver paste is shown in Fig. 4. On one hand, the addition of organic carriers with different decomposition and volatilization temperatures played a role in adjusting the viscosity, on the other hand, it could prevent the printed pattern from collapsing due to a large amount of volatilization at a given temperature. The TG curve indicated that the weight of the silver paste had significant decreases at 100°C, 200°C, and 300°C, and remained almost unchanged above 300°C. Therefore, the debonding process should be carried out in three temperature sections: at 100°C, 200°C, and 300°C held for 60 min with a rate of 2.5°C/min, respectively.

The microstructure of bondlines
A cross-section image of a typical bondline formed after sintering at 450°C for 10 min is shown in Fig. 5. The silver film acted as a metallized layer on the tempered glass surface which was tightly bound to the tempered glass, and the tin alloy solder layer (SAC305) was between the two silver film layers. The following work will focus on the interface and sintering behavior of the thick silver film.
The cross-section images of the bondlines with different glass content (wt%) sintered at 450°C for 10 min are shown in Fig. 6. Two clear interfaces were observed between the silver film and the SAC305 solder Fig. 6a, and between the silver film and the tempered glass substrate Fig. 6d. The results indicated that the glass was crucial for the interfacial bonding between the silver film and the tempered glass substrate, and between the silver film and the SAC305 solder. The silver film was tightly bound to the tempered glass substrate and there was no separation at the interface with either 40 wt% or 30 wt% glass content. More glass exposure on the surface of the silver film caused a non-wetting of the SAC305 solder and the silver film, which led to the separation of the silver film and the SAC305 solder interface Fig. 6a. Compared with the silver films with either 20 wt% or 10 wt% glass content, the transition layer was not noticeable, and the interdiffusion of silver and tin was hindered by a large amount of glass. Although the wettability of the SAC305 solder on the silver film  . 4 a The TGA/DSC curves of the silver paste; b the curves of the debonding process was good and a transition layer was formed, there were some non-connected holes in the silver film, because it was difficult for the glass to fill these holes. Due to the smaller amount of glass flowing to tempered glass, there was an obvious interface separation between the silver film and the tempered glass with only 10 wt% glass content Fig. 6d. This seriously degraded the interface bonding between the silver film and the tempered glass substrate. In summary, the glass content greatly affected the density of the sintered silver film and the bonding strength with the tempered glass substrates. Due to the presence of the glass, silver microparticles were rearranged during the glass softening process. The silver microparticles were not only used in solid-phase sintering, but also diffused through grain boundaries to form a sintered neck. The glass also promoted the sintering of the silver microparticles and molten glass penetrated the gap between the sintering necks through capillary action.
Although the glass content was relatively small, the dissolution in glass and the precipitation process of the silver particles could promote sintering and accelerate the densification process [28,33]. To confirm that silver could dissolve in the Bi-B-Zn glass and precipitate out during the cooling process, the microstructure of the sample was further analyzed by SEM and TEM. There were nanoscale microparticles on the silver film surface. The high-angle annular dark field (HAADF) image near the interface between the glass and silver microparticles, and the elemental distributions of Ag and Bi are shown in Fig. 7. The silver particles gradually sintered together and grew larger during the sintering process. There were no nano-sized silver particles observed in the original material. However, some nanoparticles were detected  on the silver film, so this result indicated that the silver dissolved into the glass and nanocrystals precipitated as reported by Yao et al. [33]. It could be inferred that silver dissolved the Bi-B-Zn glass during sintering and penetrated into the glass grid microstructure. Then, the silver dissolved in the glass and precipitated in a supersaturated state and finally formed silver nanoparticles on the surface of the glass during the cooling process. The silver diffused into the liquid glass and the precipitation process could accelerate the diffusion rate of the silver, which was equivalent to accelerating the sintering growth process of the silver microparticles [34]. The mixed solid and liquid phase sintering process could be divided into three steps: (1) Formation of the liquid phase and rearrangement of particles: the glass softened and the liquid phase formed when the glass powder was heated to the softening temperature. The silver particles were mostly suspended within the liquid phase and moved collectively with the help of the surface tension of the liquid phase; (2) Solid-phase sintering and liquid-phase filling: the contact of the silver particles with each other through diffusion sintering, gradually formed a sintered neck and grew, and finally sintered into a massive silver layer. The molten glass filled some voids because of capillary action and its fluidity, thereby the microstructure of the thick silver film rapidly densified; (3) Dissolution and reprecipitation of the solid phase: the angular and convex portions on the silver particles' surfaces preferentially dissolved into the molten glass. Then the supersaturated silver ions in the molten glass precipitated during the cooling process, and the nanocrystalline silver particles reached the silver particles and accelerated the growth of the silver microparticles. The interface between the glass and the tempered glass, along with the interface between the silver and the glass were characterized by TEM, as shown in Fig. 8. The high angle annular dark-field (HAADF) TEM image of the silver film sintered at 450°C for 10 min is shown in Fig. 8a. The EDS element mappings of the Bi, Zn, Ag, O, and Si elements are shown in Fig. 8b-f, and Fig. 8g-h show the diffraction patterns of regions A and B shown in Fig. 8a, respectively. The amorphous ring of region A indicated that the glass had an amorphous structure with a low degree of a local order near the substrate surface. The bright Bi-rich region indicated that the Bi 4 B 2 O 9 phase (PDF # 70-1458) was crystal structure, which agreed well with the XRD results shown in Fig. 9. According to the crystallization temperature of 448°C from the DSC analysis, Bi 4 B 2 O 9 crystals precipitated during the sintering process when the temperature was above 450°C. The nucleation and growth of Bi 4 B 2 O 9 were mainly by diffusion and aggregation of the corresponding elements during the cooling process. Thermal convection with the external environment and the transition of the glass phase to crystalline could provide enough energy for the crystallization process. The precipitation of an appropriate amount of crystalline phase contributed to the strengthening of the overall structure of the thick silver film, which also explained why the sample had the highest shear strength under the process of heating at 450°C for 10 min.

The bonding mechanism
The bright-field image and the high-resolution image of region A are shown in Fig. 10a and c. The diffusion of Si, Zn, Bi, and O at the glass and tempered glass substrate interfaces were measured using scanning  Fig. 10b. It could be seen that the diffusion layer from the glass into the tempered substrate was about 40 nm. The element diffusion curves clearly showed that the contents of Bi, Zn, and O changed slowly at the interface. However, the Si content reduced sharply, which concluded that the Si remained stable in the glass matrix. Bi diffused into the tempered glass substrate, indicating an excellent bond was formed at the interface. A large amount of fine Bi 4 B 2 O 9 grains precipitated at the interface and inhibited grain growth, because of the large nucleation energy exchange and consumption in the supercooled glass and the tempered glass substrate. The supercooled melt and a large number of nucleation sites were provided with the help of the surface. More fine nano-grains were formed because of the high cooling rate. Thermal convection and heat exchange provided the energy for the transition process from amorphous at the interface to crystalline, thus promoting the continuous nucleation of nanocrystals. The X-ray diffraction pattern of the interface was composed of a diffractive halo and a polycrystalline ring, indicating that the interface was mainly composed of a glass phase and a large number of Bi 4 B 2 O 9 grains. After the Bi diffused, which aggregated at the interface and broke through the nucleation barrier, a large number of Bi 4 B 2 O 9 nanocrystals were generated and distributed evenly throughout the interface. A diffusion bond was formed between the glass substrate and the silver film. The precipitation of small grains could hinder the crack propagation in the glass and played a vital role in dispersion strengthening, thereby inhibiting the occurrence of fracture modes at the interface during shear fracture processes, which will greatly increase the adhesion and shear strength of the sintered silver film.
The fracture mode was mainly brittle fracture when glass content was 40 wt%, as shown in Fig. 11e. The cracks mainly extended along the inside of the glass or the surface of the tempered glass substrate because the silver particles were separated by large glass resulted in relatively low sintering dense. More glass existed at silver film and tempered glass substrate interface weakened silver particles on inhibiting crack propagation, which resulted in a lowbonding strength of 14.8 ± 3.8 MPa. The silver film was well bonded to the tempered glass substrate with 30 wt% glass content, as shown in Fig. 11b. Silver particles were peeled off from the glass in the fracture interface and the large brittle fracture mode disappeared. The crack expanded from the glass inside when the tip of the crack met the silver particles, so that the crack penetrated the silver film along the weakest part, and the shear strength could reach 30.1 ± 2.7 MPa. Since the glass content in the thick silver film was high, the overall silver film still exhibited a certain degree of brittleness, but the silver particles were not separated in the glass, and some silver particles had been sintered together. The MPa was obtained with 20 wt% glass content. The silver film and the tempered glass substrate were well bonded and most of the silver particles were peeled from the glass, as shown in Fig. 11c. Without a large brittle fracture formed, and visible bowl-shaped structure on the tempered glass substrate surface. The silver particles were sintered well and the gap between them was filled with glass solder, which promoted the bonding force between the silver film and tempered glass substrate. However, the shear strength had the lowest value of 12.8 ± 1.5 MPa with 10 wt% glass content. There was not enough molten glass solder to fill the interface between the silver film and the tempered glass substrate, which resulted in a lot of holes and left an interface that was not fully bonded, as shown in Fig. 11d. A small amount of glass that existed at the interface was not enough to ensure that the Bi-B-Zn glass presented evenly at all interfaces, which meant that not all fracture interfaces had a bowl-shaped structure, which was described by Fig. 11h.
One of the most important performance metrics of vacuum tempered glass is airtightness. Good air tightness and internal getter could maintain a high vacuum of the vacuum layer for a long duration. This experiment used the GJB 548B-2005 method 1014.2 test condition A1 (fine leak detection) to test the gas leakage rate of the seal component, as shown in Fig. 12a. The leak rate was 7.2 ± 0.4 9 10 -3 Pa.cm 3 Fig. 12b. The experimental results showed that the heat transfer of the single-layer glass was the fastest, and the thermal equilibrium could be reached in 5 min, at a temperature of 200°C. Due to the heat dissipation from the surroundings and the upper surface to the environment, the final upper surface temperature was maintained at 144.5°C. When the double-layer glass and the hollow glass component were selected for the experiment, the heat transfer rate decreased significantly. Thermal equilibrium was reached in 15 min and the temperatures were 115.1°C and 108.5°C, respectively. However, the gas heat conduction and convection heat transfer were negligible since the middle layer was close to the vacuum in the vacuum glass component. The experimental results also showed that the tempered vacuum glass components reached a thermal equilibrium temperature of 82.5°C after 24 min, which had good thermal insulation performance since the main heat conduction mode was radiant heat transfer. Therefore, these results indicated that the vacuum glass components exhibited excellent thermal insulation performance, compared to hollow glass and other glass structures.

Conclusions
A metallization layer was prepared on the tempered glass surface by sintering a silver paste with 80 wt% silver content and 20 wt% Bi-B-Zn 20 glass content at 450°C for 10 min. Bi 4 B 2 O 9 crystals precipitated and a large number of small grains formed at the interface at 450°C, strengthening the thick silver film and enhancing its bonding with the tempered glass substrate. The dissolution and precipitation process of the silver particles in the Bi-B-Zn glass accelerated the diffusion of Ag and promoted the sintering and growth of silver particles. The silver particles were peeled from the glass as the main shear fracture mechanism, and its bonding force with the tempered glass substrate was up to 42.3 ± 2.4 MPa. The leak rate of 7.2 ± 0.4 9 10 -3 Pa.cm 3 /s and excellent thermal insulation properties were obtained using the low-temperature sintered silver paste combined with soft soldering.