Interaction of Glass Powder with Al Powder and Zinc Oxide in Aluminum Paste

Abstract

By analyzing the interaction of different glass powders with Al powder and Zinc oxide, the effect of the wetting property of glass powders on the surface morphology of aluminum paste and the adhesion between aluminum paste and Zinc oxide substrate is discussed. The effect of wetting property for different glass powders on Al and Zinc oxide is analyzed by a high-temperature contact angle tester, and the contact angle-temperature and extension radius-temperature curves are determined during the wetting process of the glass powders. The microstructure of the cross-section of the glass powders and of the substrate, and the surface morphology of the aluminum pastes are analyzed by a scanning electron microscope. Adhesion between the aluminum paste and the Zinc oxide substrate is analyzed by a vertical tensile strength meter. The results show that the wetting property of glass powder is an important factor affecting the adhesion and surface morphology of the paste, and it plays a role in preventing excessive oxidation of aluminum paste during sintering at high temperatures.

1. Introduction

Zinc oxide varistor is a widely used electronic component for over-voltage protection and voltage regulation, and currently, the industry usually uses silver and other costly precious metals as electrode materials [1,2]. Electronic aluminum paste is one of the indispensable and important basic raw materials in the microelectronics industry, mainly used for manufacturing electrode materials for electronic components. With the trend of using less costly metals for electrode materials, aluminum paste has gained a wide range of applications in manufacturing of solar cells, capacitors, inductors, and other modern electronic devices. The prepared aluminum electrode must first have a dense surface. A good network structure is formed between the conductive particles, and the resistance is reduced. And to ensure the adhesion between the aluminum electrode and the substrate, the aluminum electrode will not fall off.

Electronic aluminum paste is composed of aluminum powder, inorganic binder glass powder, and organic carrier [3,4]. Glass powder as the binder phase is an important component of electronic pastes, in which the wetting property of glass powder directly affects the mechanical and electrical properties of the paste. Such a property of the glass powder is related to whether the slurry components can be fully dispersed and uniformly form a dense sintered film during sintering, and whether adhesion of the interface can be improved. In order to achieve a good bond between metal and glass, the contact angle of the glass on the metal surface should be minimized. If this angle between the molten glass and the metal surface is greater than 90° it will result in the molten glass not being able to create a tensile movement on the metal surface, which in turn will affect the good bonding between the metal and the glass [5,6,7]. A great deal of research has been conducted by researchers on the effect of molten glass wetting properties on metal surfaces, but it has been limited to materials such as deleterious alloys and silicate glasses. In the field of electronic pastes, the wetting property and reactive interfaces of glass powders and conductive phase metals as well as substrates have been less studied. During the sintering process, a good bond between the glass powder and the aluminum powder is necessary to obtain an excellent performance of the aluminum electrode and the glass provides a good adhesion between the Zinc oxide matrix and the electrode [8,9,10].

This paper focuses on the study of the wetting behavior of molten glass in aluminum paste on the surface of an Al powder press sheet as well as on the surface of a Zinc oxide substrate, to explore the interaction between glass powder, Al powder and Zinc oxide, and provide necessary references for the study of the effect of the wetting property of glass powder on the electronic pastes and their applications.

2. Materials and Methods

2.1. Experimental Materials

Spherical aluminum powder is purchased from Ansteel Group Aluminum Powder Co., Ltd. (An Shan, China), with a D50 (The corresponding particle size when the cumulative particle size distribution percentage of a sample reaches 50%) of 2~3 μm and aluminum content of 99.86%. Three different glass powders, Bi₂O₃-B₂O₃-SiO₂ (G1), Bi₂O₃-B₂O₃-ZnO (G2), and Bi₂O₃-B₂O₃-SiO₂-ZnO (G3), are selected. The compositions are shown in

Table 1. Composition (mol.%) and softening temperature of the glass.

	Bi2O3	B2O3	SiO2	ZnO	Others	Total	Tg (°C)
G1	70	15	6	0	9	100	449
G2	70	15	1	12	2	100	434
G3	70	15	3	6	6	100	464

.

2.2. Experimental Sample Preparation

A total of 600 mg of aluminum powder and 200 mg of glass powder are weighed on an electronic balance, and the aluminum and glass powders are pressed into cylinders with diameters of 15 mm and 6 mm using a mold, respectively. The height of aluminum powder compacts and glass compacts tablet is 2 mm and 3 mm, respectively. The glass compacts are placed on the aluminum powder compacts and Zinc oxide substrate, respectively (Figure 1).

2.3. Characterization

The melting and wetting process of glass powder compacts on aluminum powder compacts and Zinc oxide substrates are studied by a high-temperature contact angle tester (TA-16B01, Tianjin Zhonghuan Electric Furnace Co., Ltd., Tianjin, China). The furnace is heated at a rate of 6 °C/min. Real-time photographs of the glass state on different surfaces are taken by a CCD real-time imaging system. Then the contact angle and extension radius of the glass are measured with professional software (Contact Angle Measurement System developed by Tianjin Zhonghuan Electric Furnace Co., Ltd., Tianjin, China). The sample cross-section morphology and elemental diffusion are characterized by XL30ESEM-TMP type scanning electron microscope.

Aluminum pastes prepared from three kinds of glass powders were sintered using a high-temperature tube sintering furnace at 600 °C, 620 °C, 640 °C, and 660 °C to study the effect of different glass wetting ability on the surface phase appearance and adhesion of the aluminum pastes at different temperatures. A high-temperature tube sintering furnace was used to heat the furnace at a rate of 10 °C/min. The surface morphology of the samples was characterized by a scanning electron microscope type XL30ESEM-TMP. The adhesion of the samples was characterized by measuring the vertical tension between the samples using a spiral tensiometer.

3. Results and Discussion

3.1. Wetting Behavior of Glass Powder Compacts on Aluminum Powder Compacts

Figure 2 shows the photographs of the wetting process of three glass powder platens pressed on a sample of aluminum powder from room temperature to 640 °C using a high-temperature contact angle tester. The sintering process is divided into two phases according to change in the shape of the glass powder platens in the figure. In the first stage (approx. 50–500 °C), the glass compact shrinks but does not change shape. As the glass compacts are directly cold pressed and not sintered at high temperatures, there are many voids inside the glass powder compacts. When the temperature rises, the surface of the glass particles first reacts, and the glass particles shrink to reduce the surface energy. As the temperature continues to rise close to the softening point of the glass, the glass powder softens and deforms and gradually transforms into viscous droplets. The mixing of liquid droplets and solids makes the gap gradually disappear, and the glass powder platen shrinks. In the second stage (550–640 °C), temperature continues to rise, and the viscosity of the glass decreases so that there is a flow phenomenon, and the pressed glass powder begins to take a hemispherical shape. Due to the viscous flow caused by the high-temperature conditions, as the balance between the wetting tension and the surface tension is broken, it starts to turn into a spherical shape. The molten glass starts to diffuse under the wetting tension, enters the aluminum powder platen and starts to wet the aluminum powder platen (glass viscosity is negligible). The wetting process (hemispherical) of G1 and G2 glasses starts at 620 °C and 640 °C, respectively, while that of G3 glass starts at between 550 and 600 °C. From the variation of contact angle (Figure 3) and spreading radius (Figure 4), with the increase of temperature, G3 glass has a smaller contact angle and a larger spreading area on the surface of aluminum powder at 640 °C. The results show that a moderate increase in the content of Zinc oxide in the glass improves the wetting property of the glass to aluminum powder.

3.2. EDS Analysis of the Cross-Section of Glass Powder Compacts and Aluminum Powder Compacts

Figure 5 shows the microstructure of the cross section after the reaction of different glass powder compacts and aluminum powder compacts. Due to the melting effect of the glass on the alumina film on the surface of the aluminum powder, the volume of the aluminum powder particles decreases and the molten glass diffuses between the molten glass and the aluminum powder tablet [11]. The difference in the wetting property of the glass powder to aluminum has an effect on the degree of diffusion. The G1, G2, and G3 glasses all diffuse into the aluminum to different degrees. G3 glass diffuses more uniformly compared to the G1 and G2 glasses, with a diffusion depth of 10~15 μm. The elemental composition of the different regions in Figure 5 is analyzed and the results are shown in

Table 2. The cross-section energy spectrum results of glass powder tablet and aluminum powder tablet (at. %).

Point	O	Al	Si	Zn	Bi	Sb
1	79.73	3.78	3.04	—	13.44	—
2	7.92	60.62	8.91	—	22.55	—
3	38.08	61.38	0.29	—	0.25	—
4	77.21	2.32	6.33	6.75	7.38	—
5	61.46	13.01	7.81	8.28	9.43	—
6	23.57	76.43	—	—	—	—
7	76.34	2.26	6.62	5.39	7.58	1.82
8	55.62	14.67	3.22	3.24	21.84	1.41
9	31.72	68.16	0.12	—	—	—

, and the nine acquisition points of EDS are shown in Figure 5. From Figure 6, we can see that with the increase of temperature and the decrease of surface tension, the viscosity of molten glass decreases, and the fluidity increases. The results given in Table 2 indicate the presence of Si, Zn, Bi, and Sb elements in the aluminum powder compacts, proving that the molten glass has diffused into the aluminum.

In order to further determine the distribution of the elements and the thickness of the reaction layer, an energy spectrum scan is performed on the reaction interface to build an elemental distribution map, and the results are shown in Figure 7. As seen in Figure 7, there is a clear gradient distribution of Bi and Zn elements. The three gradients correspond to the glass powder flake, the reaction layer, and the aluminum powder flake, which is consistent with the results of the EDS point analysis. The three glasses diffuse into the aluminum powder compact to different degrees, and the diffusion of G3 glass is more uniform and continuous compared with that of G1 and G2 glass.

3.3. Wetting Behavior of Glass Powder Compacts on Zinc Oxide Substrates

Figure 8 shows photographs of the wetting process of the glass powder compacts to the Zinc oxide matrix at different temperatures. Between 50 °C and 500 °C, the contraction of G1, G2, and G3 glass powder compacts do not change the general shape. The viscosity of the glass decreases when the temperature continues to increase from 500 °C to 600 °C. The cylindrical shape begins to transform into a hemispherical shape. Due to the viscous flow caused by the high-temperature conditions, the G1 and G3 glasses, in turn, transform into spherical shapes when the balance between the wetting tension and the surface tension is broken. The molten glass starts to diffuse and enter into the Zinc oxide matrix under the effect of wetting tension. The wetting process of G1 and G3 glass (hemispherical) starts at about 600 °C. However, the wetting process of G2 glass starts at about 630 °C. From the variation of contact angle (Figure 9) and spreading radius (Figure 10), G1 glass has a smaller contact angle and larger spreading area on the surface of the Zinc oxide substrate as compared to G2 and G3 glasses. With the change of substrate, the glass likewise shows different wetting property. The results show that the same glass has different wetting property on different substrates and the content of Zinc oxide in the glass also affects the wettability of the glass to the Zinc oxide substrate.

3.4. EDS Analysis of the Cross-Section of Glass Powder Compacts with Zinc Oxide Substrates

Figure 11 shows the cross-section microstructure of different glass powder compacts and Zinc oxide matrices after the reaction. The molten glass and the Zinc oxide matrix will diffuse due to the wetting of the molten glass on the surface of the Zinc oxide matrix. Differences in the wetting property of the glass and the Zinc oxide matrix have an effect on the degree of diffusion. In the wetting process of glass on Zinc oxide, the mutual diffusion of glass and Zinc oxide leads to the change of Zinc oxide concentration at the contact interface, which changes the transition temperature, crystallization temperature and crystallization activation energy of glass [12]. The Zinc oxide matrix will grow toward the glass layer, as can be seen in Figure 11. When the surface of the Zinc oxide matrix is G1 glass, the growth of the Zinc oxide matrix is serrated, uniformly dense, and coarse. However, the diffusion depth is shallow, about 20 μm. When the surface of the Zinc oxide matrix is G2 and G3 glass, the growth of the Zinc oxide matrix is also serrated, but sparse and slender. The diffusion depth is, however, deeper, about 30~40 μm. The elemental composition of the different regions in Figure 11 is analyzed and the results are shown in

Table 3. Energy spectrum results for each point of Figure 10 (at. %).

Point	O	Si	Zn	Bi	B	Sb
1	76.67	4.48	—	15.56	3.29	—
2	60.26	—	34.34	—	5.40	—
3	48.15	—	51.85	—	—	—
4	69.91	1.97	—	21.75	6.37	—
5	60.22	0.28	38.41	0.40	0.69	—
6	47.25	—	52.75	—	—	—
7	75.87	3.09	—	14.23	5.71	1.10
8	63.88	—	36.12	—	—	—
9	49.76	—	50.24	—	—	—
10	59.22	2.56	—	17.74	5.45	15.03

, and the ten acquisition points of EDS are shown in Figure 11. From the analysis, we can infer that the fluidity of the molten glass increases with increasing temperature and decreasing surface tension. The EDS analysis of point 8 shows 15.03% Sb content, which is due to the sinking of Sb₂O₃ in G3 glass. The results given in Table 3 indicate the presence of a large amount of Zn and a small amount of glass component in the interlayer between the glass powder compact and the Zinc oxide matrix. The Zinc oxide matrix grows towards the glass layer during heating.

In order to further determine the distribution of the elements and the thickness of the reaction layer, energy spectrum scanning is carried out at the reaction interface to build the elemental distribution map, and the results are shown in Figure 12. These show that there are obvious gradient distributions of Bi and Zn elements, and the three gradients correspond to the glass powder compact, the intermediate layer and the Zinc oxide matrix, which is consistent with the results of EDS point analysis. The growth of the Zinc oxide matrix to the G1 glass layer is more uniform, dense, and coarse compared with the G2 and G3 glass layers, and the adhesion between the G1 glass and the Zinc oxide matrix is significantly better than that of the G2 and G3 glass. During the sintering process of the slurry, the glass is melted with the increase of temperature, and a transition layer is formed between the conductive phase and the matrix. Therefore, the growth of the Zinc oxide matrix to the glass layer can effectively improve the adhesion between the conductive phase and the matrix.

3.5. Influence of Glass Powder Wetting Property on the Surface Densification of Aluminum Pastes

After studying the effect of wetting property of different glass powders and aluminum powder compacts, in order to further investigate the effect of the wetting property of the glass powders on the surface densities of the aluminum pastes, three types of aluminum pastes are prepared by mixing three glass powders with aluminum powders at certain ratios. The aluminum pastes are screen-printed on the Zinc oxide substrate and sintered in air at 600 °C, 620 °C, 640 °C, and 660 °C, respectively, to obtain the morphology shown in Figure 13. During the sintering process, with the increase of temperature, the wetting ability of molten glass increases, the fluidity increases, and the flow of aluminum powder particles is driven. Driven by thermal stress, the pores shrink, and the density increases. The wettability of molten glass is different, which leads to the different surface density of aluminum slurry. At 600 °C, the wetting ability of glass to aluminum powder at this temperature is low, and the wetting ability of the three glasses to the aluminum substrate follows G3 > G2 > G1. Hence the densities and dispersions of the surface morphology of the aluminum pastes prepared from the three glasses are the best with the fewest pores in the aluminum paste with the addition of G3 glass, followed by the paste with the addition of G2 glass, and the most pores on the surface of the paste with the addition of G1 glass. As the temperature increases, the wetting property of the glass improves. It can be seen that for aluminum pastes with the same amount of glass added, while the sintering temperature increases, the viscosity of the glass decreases, and fluidity increases. The conductive phase is dispersed more uniformly, its densification improves, and the pores begin to decrease [13,14,15]. However, it can be seen that the conductive phase on the surface of the aluminum paste with G1 glass added is over-oxidized and results in defects at 640 °C and 660 °C. The aluminum paste with G2 glass added is over-oxidized and results in defects at 660 °C, and there are no defects on the surface of the aluminum paste with G3 glass added. This is because the molten glass has a melting effect on the surface of the aluminum powder, and with the change of temperature, the alumina shell produces a certain stress, resulting in the breakage of the alumina film. The glass bonding phase in the paste not only provides adhesion for the conductive phase and the substrate, but the glass bonding phase also prevents the conductive phase from over-oxidation at high temperatures. As the temperature rises, the wetting ability of G1 glass and G2 glass is not as high as that of G3 glass, which has a high wetting property and dispersing ability of aluminum at high temperatures and effectively protects the conductive phase from over-oxidation at high temperatures [16].

3.6. Influence of Glass Powder Wetting Property on Adhesion

The glass powder in the aluminum slurry is used as the bonding phase. During the sintering process, the molten glass has fluidity, and due to the different specific gravity, the molten glass will gradually sink, forming a transition layer between the conductive phase and the matrix, which has a decisive influence on the adhesion between the conductive phase and the matrix [17,18]. We have studied the wetting reaction of different glasses with Zinc oxide substrate. In order to further confirm the effect of the wetting property of glass powder on the adhesion between aluminum paste and Zinc oxide substrate, three types of aluminum paste are prepared by mixing glass powders with aluminum powder at certain ratios, and screen printed on Zinc oxide substrate by screen printing, and repeated with the above to overlap the two substrates as shown in Figure 14. Since all three types of glass start the wetting process at around 600 °C, the sintering temperatures in air are chosen to be 600 °C, 620 °C, 640 °C, and 660 °C for comparison. The results of vertical tension adhesion are shown in Figure 15. With the increase in temperature, the glass begins to wet, and its viscosity decreases, and fluidity increases in the conductive phase. A thicker transition layer is formed between the conductive phase and the substrate, and adhesion increases [19,20,21]. However, since both wetting dispersion of G2 glass on the aluminum and growth of Zinc oxide in the G2 glass are poor, as compared with the G1 and G3 glass, adhesion of aluminum paste with G2 glass is low. The adhesion of the aluminum pastes prepared from G3 glass is greater than that of the aluminum pastes prepared from G1 glass at 600 °C and 620 °C because G3 glass has better wetting properties of aluminum and forms a thicker transition layer than that of the aluminum pastes prepared from G1 glass at 600 °C and 620 °C. When the temperature is raised to 640 °C and 660 °C, the wetting property of aluminum by G1 glass also reaches a certain level and the growth of the Zinc oxide matrix in G1 glass is the best, so the adhesion of the aluminum pastes prepared by G1 glass at 640 °C and 660 °C is greater than that of the aluminum pastes prepared by G3 glass [22].

4. Conclusions

Different glass powders have different wetting effects on Zinc oxide matrix and aluminum powder, and the wetting property of the glass powder affects the surface morphology of the prepared aluminum paste as well as adhesion.

• Those glass powders having better wetting property to Al fully disperse into Al at lower temperatures and form the surface of the conductive film layer with fewer holes and high density and provides adhesion to the paste.
• Those glass powders having better wetting property to Zinc oxide form a transition layer, due to dispersion of the flow with Al at higher temperatures. Due to the diffusive growth of the Zinc oxide matrix towards the glass, better adhesion is provided.
• Those glass powders having better wetting property to Al can better prevent the excessive oxidation of Al to form defects at higher temperatures.