# Properties of Ag-SnO2 Contact Materials for Low-Voltage Electrical Appliances with Different Doped Particle Sizes

Authors: Haitao Wang; Mei Zhang; Menglin Yang
Journal: Advances in Materials Science and Engineering
 (2019)

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## Abstract

In this paper, the relationship between additive particle size and properties of Ag-SnO2 contact materials was studied. La2O3 and Fe2O3 were selected as additives for contact materials, and the proportion of additives were determined by wettability experiment results; Ag-SnO2 contact materials were successfully fabricated by the powder metallurgy method, and the physical and electrical contact properties of Ag-SnO2 contact materials with five additive particle sizes were investigated. The results show the influence of additives on the properties of Ag-SnO2 contact materials are disparate, the contact resistance and arc energy are smaller and more stable when the additive particle size is about 500 nm. For further proving this conclusion, the arc-eroded surface morphologies of the doped AgSnO2 contact materials were investigated by scanning electron microscopy (SEM), which indicated that the properties of two kinds of Ag-SnO2 contact materials could be improved significantly by selecting appropriate additive particle size.


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## Body

## 1. Introduction

The environment friendly Ag-SnO2 contact materials have been extensively utilized in low-voltage apparatus (such as relays, circuit breakers, contactors, etc.) that attributes to their wonderful performance [1–5]. However, it also presents some drawbacks, such as unstable contact resistance and poor ductility [6]. Therefore, it is of great significance to further develop the performance improvement of Ag-SnO2 contact materials for low-voltage electrical appliances.Research shows that the contact resistance and temperature rise of Ag-SnO2 contact materials for low-voltage electrical appliances are too high, it can be ascribed to the fact that SnO2 enriched in the contact surface will form an insulating layer after the arc extinguished, which leads to the increase of the contact resistance of Ag-SnO2 contact material. Some efforts have been reported to improve the properties of Ag-SnO2 contact materials, and suitable additives were introduced to modify the electrical contact material [7–9] and through modification of the preparation method to improve the electrical properties [10].The powder metallurgy method [11], widely used in the preparation of Ag-SnO2 contact materials, was introduced in this paper. Powders are critical element in the powder metallurgy method, and the above studies suggest that the particle size of powders strongly influences the properties of contact materials. Therefore, elucidating the optimal particle size of additive and improving the properties of Ag-SnO2 contact materials for low-voltage electrical appliances are worthwhile topics of investigation.


## 2. Experiment

### 2.1. Wettability Experiment

The wetting angle between Ag and SnO2 with different kinds and ratios of additives was measured by the sessile drop method. Most importantly, the metal droplets melting and spreading process on the oxide surface should be observed dynamically with this method. The procedure of the wettability experiment was as follows: the proportions of additives selected in this paper were 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, and 4%, and the powders of SnO2 and additives were dried at 120°C under 0.08 MPa for 30 minutes in vacuum-drying oven and then the dried powders were mixed in a Simoloyer mill for 2 hours, at the rotational speed of 500 r/min for 5 minutes and opposite rotational speed of 600 r/min for 8 minutes. The evenly mixed oxide powders were pressed into oxide-based discs whose diameter was 20 mm and thickness was 2 mm and then sintered in the box-type resistance furnace at 980°C. As shown in Figure 1, take the SEM images of SnO2 doped with La2O3 (500 nm) and Fe2O3 (1000 nm), for example, it can be seen that powders are easy to agglomerate and the powder is uniform and the granularity is accurate in the substrate prepared by the above process, which ensures the accuracy of the experimental results [12]. Each 0.3 g Ag powder was put on each oxide-based disc, then placed in a vacuum furnace, and heated for 30 min to 1050°C. The wetting process is the process of the melted Ag spreading on the oxide-based disc surface. In the heating process, silver was oxidized to become silver oxide at the beginning. When the temperature rose to 100°C, silver oxide began to decompose into silver and oxygen. When the temperature reached 300°C, silver oxide was decomposed completely. So the impact of silver oxide on wetting ability can be ignored in this test.Figure 1
SEM images of (a, b) La2O3 (500 nm) and (c, d) Fe2O3 (1000 nm).
(a)
(b)
(c)
(d)


### 2.2. Properties Test

Considering the normal types of additive particle size in the market, additive particle sizes of 50 nm, 200 nm, 500 nm, 1μm, and 10 μm were used in the text and the optimum additive ratio was selected according to the results of wettability experiment. The mixed powder of Ag, SnO2, and additives were prepared according to the previous method [11]. Each group of contact material powder is weighed by 10 g, and the powders were compacted under the pressure of 38 MPa for 5 minutes in the mould, sintered at 500°C for 90 minutes in the vacuum sintering furnace, then repressed under the pressure of 58 MPa for 5 minutes, and resintered at 800°C for 90 minutes. Then the surface is polished at the contact pressure of 16 N for 3 minutes. Finally, the contact materials sheet was cut into a diameter of 4.5 mm and thickness of 3.5 mm contact terminal [12].In this paper, the density was measured by the drainage method, and the main theoretical basis is Archimedes law. The electrical conductivity of the Ag-SnO2 contact materials was measured on the polished surface by using a SIGMASCOPE SMP10 conductivity tester. A HXD-1000TM microscope hardness tester was utilized to measure the hardness of Ag-SnO2 contact materials. The final results of the physical properties are an average value of 10 testing points. The JF04C contact material testing system developed by Kunming Noble Metals Research Institute was used to test the electrical contact performance test of the prepared Ag-SnO2 contact materials. The instrument must be readjusted when a new round of electrical performances test begins: the electrical current was set to dc 13 A and voltage of dc 24 V, and the system protection voltage was set to ±40 V. After the test started, the contact pressure was adjusted to 86 cN by adjusting the open drive voltage and the closed drive voltage in the parameter setting, and the electrical contact experiment was performed at room temperature. The contact resistance and arc energy of each sample were collected in real time, and the average value was measured every 100 times during the whole 25000 times in the experiment [12]. The surface morphologies of the doped AgSnO2 electrical contacts after arc erosion were investigated by scanning electron microscopy (Nova NanoSEM 450), and EDS was used to analyze the microarea compositions.


## 2.1. Wettability Experiment

The wetting angle between Ag and SnO2 with different kinds and ratios of additives was measured by the sessile drop method. Most importantly, the metal droplets melting and spreading process on the oxide surface should be observed dynamically with this method. The procedure of the wettability experiment was as follows: the proportions of additives selected in this paper were 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, and 4%, and the powders of SnO2 and additives were dried at 120°C under 0.08 MPa for 30 minutes in vacuum-drying oven and then the dried powders were mixed in a Simoloyer mill for 2 hours, at the rotational speed of 500 r/min for 5 minutes and opposite rotational speed of 600 r/min for 8 minutes. The evenly mixed oxide powders were pressed into oxide-based discs whose diameter was 20 mm and thickness was 2 mm and then sintered in the box-type resistance furnace at 980°C. As shown in Figure 1, take the SEM images of SnO2 doped with La2O3 (500 nm) and Fe2O3 (1000 nm), for example, it can be seen that powders are easy to agglomerate and the powder is uniform and the granularity is accurate in the substrate prepared by the above process, which ensures the accuracy of the experimental results [12]. Each 0.3 g Ag powder was put on each oxide-based disc, then placed in a vacuum furnace, and heated for 30 min to 1050°C. The wetting process is the process of the melted Ag spreading on the oxide-based disc surface. In the heating process, silver was oxidized to become silver oxide at the beginning. When the temperature rose to 100°C, silver oxide began to decompose into silver and oxygen. When the temperature reached 300°C, silver oxide was decomposed completely. So the impact of silver oxide on wetting ability can be ignored in this test.Figure 1
SEM images of (a, b) La2O3 (500 nm) and (c, d) Fe2O3 (1000 nm).
(a)
(b)
(c)
(d)


## 2.2. Properties Test

Considering the normal types of additive particle size in the market, additive particle sizes of 50 nm, 200 nm, 500 nm, 1μm, and 10 μm were used in the text and the optimum additive ratio was selected according to the results of wettability experiment. The mixed powder of Ag, SnO2, and additives were prepared according to the previous method [11]. Each group of contact material powder is weighed by 10 g, and the powders were compacted under the pressure of 38 MPa for 5 minutes in the mould, sintered at 500°C for 90 minutes in the vacuum sintering furnace, then repressed under the pressure of 58 MPa for 5 minutes, and resintered at 800°C for 90 minutes. Then the surface is polished at the contact pressure of 16 N for 3 minutes. Finally, the contact materials sheet was cut into a diameter of 4.5 mm and thickness of 3.5 mm contact terminal [12].In this paper, the density was measured by the drainage method, and the main theoretical basis is Archimedes law. The electrical conductivity of the Ag-SnO2 contact materials was measured on the polished surface by using a SIGMASCOPE SMP10 conductivity tester. A HXD-1000TM microscope hardness tester was utilized to measure the hardness of Ag-SnO2 contact materials. The final results of the physical properties are an average value of 10 testing points. The JF04C contact material testing system developed by Kunming Noble Metals Research Institute was used to test the electrical contact performance test of the prepared Ag-SnO2 contact materials. The instrument must be readjusted when a new round of electrical performances test begins: the electrical current was set to dc 13 A and voltage of dc 24 V, and the system protection voltage was set to ±40 V. After the test started, the contact pressure was adjusted to 86 cN by adjusting the open drive voltage and the closed drive voltage in the parameter setting, and the electrical contact experiment was performed at room temperature. The contact resistance and arc energy of each sample were collected in real time, and the average value was measured every 100 times during the whole 25000 times in the experiment [12]. The surface morphologies of the doped AgSnO2 electrical contacts after arc erosion were investigated by scanning electron microscopy (Nova NanoSEM 450), and EDS was used to analyze the microarea compositions.


## 3. Results and Discussion

### 3.1. Wettability Test

Take the wetting angle of Ag and oxide substrate when the additive content was 1%; for example, the wetting angle are shown in Figure2. And the wetting angle under different proportion of additives and their trends are shown in Figure 3.Figure 2
When the doped particle content was 1%, the wetting angle of (a, b) AgSnO2/La2O3 and (c, d) AgSnO2/Fe2O3.
(a)
(b)
(c)
(d)Figure 3
The wetting angle under different proportion of additives.Young’s equation shows that the smaller the wetting angle is, the better the wettability between molten Ag and SnO2 is. It was reported that the wettability between the second phase and silver, as one of the significant characteristics of the surface melting pool, has a vital effect on the spray resistance of the contact. The antisplash performance is improved due to the enhancement of the wettability, which contributes to the enhanced combination of molten silver and contact surface, and the reduced separation between molten silver and the second phase, maintaining the viscosity of the molten pool, reducing the splashing loss of the silver melt, resulting in the improvement of the resistance of the material to arc erosion.As shown in Figure3 that the wetting angle of AgSnO2/La2O3 contact material decreased first and then increased, but AgSnO2/Fe2O3 increased, with the increase of doped content, the wetting angle of AgSnO2/La2O3 and AgSnO2/Fe2O3 reached the minimum when the doping contents are 2.5% and 1%. Therefore, according to the experimental results of wettability, the fine additive content selected in this paper is shown in Table 1.Table 1
Additive content of Ag-SnO2 contact materials.
Ingredient
Matching
SnO2 : La2O3
9.5 : 2.5
SnO2 : Fe2O3
11 : 1


### 3.2. Physical Properties

The physical properties of AgSnO2 contact materials are shown in Tables 2 and 3.Table 2
The physical properties of AgSnO2/La2O3 contact material.
Particle size (nm)
Density (g/cm3)
Hardness (HV)
Conductivity (% IACS)
50
9.299
105.357
50.77
200
9.247
105.06
51.56
500
9.130
96.321
56.643
1000
9.212
98.032
54.043
10000
9.259
106.597
52.663Table 3
The physical properties of AgSnO2/Fe2O3 contact material.
Particle size (nm)
Density (g/cm3)
Hardness (HV)
Conductivity (% IACS)
50
9.512
70.109
55.630
200
9.253
89.623
60.053
500
9.030
85.028
58.370
1000
9.078
89.580
53.773
10000
9.414
93.233
61.027The results showed that with the increase of doped particle size, the electrical conductivity of two kinds of Ag-SnO2 contact materials significantly increased first and then decreased, and the density and hardness both decreased first and then increased.This is because fine doping powder can better fill the void in Ag powder, which increases the loose density of composite powder. Consequently, the surface energy of additive increases with the decrease of doped particle size, which contributes to sintering, causing the density, and hardness to increase [13]. It has been reported that electrical conductivity of the metal-based composite depends on the joint effect of the porosity, particle content, and residual stresses around particulates. And the porosity introduced during fabrication process has deteriorating effects on the conductivity by acting as nonconductivity void during electron conduction. In addition, the decrease of particle size leads to a decrease in conductivity by the introduction of more geometrically necessary dislocations and residual stresses. The above results show that the additive particle size has a vital effort on the physical properties of Ag-SnO2 contact materials.


### 3.3. Contact Resistance

Taking the Ag-SnO2/La2O3 and AgSnO2/Fe2O3 contact materials with 50 nm additive particle size, for example, the value of the contact resistance in real time is shown in Figures 4 and 5. The average value and variance of contact resistance of Ag-SnO2/La2O3 and AgSnO2/Fe2O3 contact materials with five additive particle sizes are shown in Tables 4 and 5.Figure 4
The contact resistance of Ag-SnO2/La2O3 contact material with 50 nm doped particle size.Figure 5
The contact resistance of Ag-SnO2/Fe2O3 contact material with 50 nm doped particle size.Table 4
The contact resistance of Ag-SnO2/La2O3 contact material.
Particle size (nm)
Range (mΩ)
Average value (mΩ)
Variance (×10−3)
50
0.27∼2.47
0.66
88.8
200
0.04∼5.36
1.10
499.3
500
0.3∼1.8
0.65
62.2
1000
0.18∼2.08
0.68
116.7
10000
0.36∼4.23
0.85
251.1Table 5
The contact resistance of AgSnO2/Fe2O3 contact material.
Particle size (nm)
Range (mΩ)
Average value (mΩ)
Variance (×10−3)
50
0.26∼2.4
0.86
180.4
200
0.06∼2.72
0.63
88.5
500
0.28∼2.25
0.53
60.8
1000
0.35∼2.74
0.85
135.2
10000
0.19∼2.68
0.79
174.4It can be seen from Figures4 and 5 that the contact resistance of AgSnO2/La2O3 contact material was relatively stable in the first 16000 times, and then followed by large fluctuations during 25000 times of electrical contact tests, which mainly due to the better thermal stability and the high decomposition temperature (2300°C) of La2O3, resulting in increasing the viscosity of the molten pool, improving the wettability between Ag and SnO2 so as to avoid the precipitation of SnO2 powder and reduce the fluctuation of contact resistance. However, with the increase of breaking times, the surface temperature of the contact increases sharply, which contributes to the dramatically increase the molten pool area, which leads to great contact resistance [14]. Fe2O3 dissolves on the surface of the contact at high temperature under the action of electric arc, which contributes to the formation of dense oxide film (Fe3O4), resulting in the increase of membrane resistance between the contacts.Tables4 and 5 show that with the increase of additive particle size, contact resistance decreases first and then increases, and it is found that the contact resistance of AgSnO2/La2O3 and AgSnO2/Fe2O3 contact materials is smaller and more stable when the additive particle size is about 500 nm. Nanoscale additive can improve the wettability between Ag matrix and SnO2; under the same quality condition, the actual contact area between Ag matrix and SnO2 become larger because the fine doped particle has larger surface energy, which to some extent inhibits the accumulation of SnO2 particles on the contact surface, and the contact resistance becomes smaller and more stable [15]. The results of contact resistance give indications for a further improvement of electric contact properties of Ag-SnO2 contact materials with appropriate additive particle size.


### 3.4. Arcing Energy

Taking the Ag-SnO2/La2O3 and AgSnO2/Fe2O3 contact materials with 50 nm additive particle size, for example, the value of the arc energy in real time is shown in Figures 6 and 7. The average value and variance of arc energy of Ag-SnO2/La2O3 and AgSnO2/Fe2O3 contact materials with five additive particle sizes are shown in Tables 6 and 7.Figure 6
The arc energy of Ag-SnO2/La2O3 contact material with 50 nm doped particle size.Figure 7
The arc energy of Ag-SnO2/Fe2O3 contact material with 50 nm doped particle size.Table 6
The arc energy of Ag-SnO2/La2O3 contact material.
Particle size (nm)
Range (mΩ)
Average value (mΩ)
Variance (×10−3)
50
163.0∼184.6
169.6
7.1
200
151.1∼178.2
166.7
29.5
500
161.7∼178.4
168.5
7.3
1000
159.6∼178.9
170.2
8.1
10000
159.8∼182.2
171.6
10.4Table 7
The arc energy of AgSnO2/Fe2O3 contact material.
Particle size (nm)
Range (mΩ)
Average value (mΩ)
Variance (×10−3)
50
157.7∼185.8
171.3
30.9
200
157.1∼173.0
166.8
10.6
500
156.3∼181.9
166.1
14.0
1000
163.1∼192.3
175.0
27.0
10000
158.3∼175.2
169.1
10.9As shown in Figures6 and 7 that the arc burning energy of Ag-SnO2/La2O3 contact material was relatively stable, which is due to the fact that La2O3, as a stable and hardly decomposable additive, can reduce the burning loss of electrical contacts when it is gathered on the surface of electrical contacts and restrains the movement of arc root spots to some extent under the action of electric arc, which leads to the improvement of stability of the arc energy. As shown in Tables 6 and 7, when the doped particle size was 500 nm, the Ag-SnO2/La2O3 and Ag-SnO2/Fe2O3 contact materials have smaller contact resistance and moderate hardness, resulting in lower arc voltage and less burning loss of electrical contact surface, which makes the arc burning energy smaller and more stable. In this case, it can be indicated that the Ag-SnO2/La2O3 and Ag-SnO2/Fe2O3 contact materials have better arc erosion resistance when the doped particle size is about 500 nm.


### 3.5. Microstructure Analysis

The surface microstructure photos of Ag-SnO2/La2O3 and Ag-SnO2/Fe2O3 contact materials with different additive particle sizes were observed by SEM after arc erosion, and the results are shown in Figures 8 and 9.Figure 8
The surface microstructure photos of Ag-SnO2/La2O3 contact material with different additive particle sizes: (a) 50 nm, (b) 200 nm, (c) 500 nm, (d) 1000 nm, and (e) 10000 nm.
(a)
(b)
(c)
(d)
(e)Figure 9
The surface microstructure photos of Ag-SnO2/Fe2O3 contact material with different additive particle sizes: (a) 50 nm, (b) 200 nm, (c) 500 nm, (d) 1000 nm, and (e) 10000 nm.
(a)
(b)
(c)
(d)
(e)Since the arc can be concentrated to an extremely high energy in a small area on the contact surface, the temperature can increase to melting point or even the boiling point of Ag within a few milliseconds. Meanwhile, the smaller sized oxides (such as SnO2, La2O3, and Fe2O3) decompose rapidly under the arc action, and the larger oxides cannot decompose easily, so they are wetted and redistributed by liquid Ag. As shown in Figures 8 and 9, the river-like structure is formed after arc extinction, the black holes are the ablated pits, and the area without arc action is the normal Ag and oxide distribution. EDS results of the black border (mainly gathering of Ag) and SnO2 and white area (mainly Ag and extremely fine metal oxides) were studied, which indicates that oxides will redistributed in the process of liquid Ag solidification. The reason is that the additives are wetted by melted Ag; when Ag freezes, SnO2 will be redistributed to form river-like structure and oxide layer is formed [16].Figures8(c) and 9(c) show that the surface morphologies of Ag-SnO2 contact materials are relatively flat and had less number of holes when the additive particle size was 500 nm, which indicates that SnO2 can be well wetted by liquid Ag and SnO2 can be less sucked out of the metal surface to form an oxide layer. The less oxide layer makes Ag-SnO2 contact materials have a lower contact resistance.


## 3.1. Wettability Test

Take the wetting angle of Ag and oxide substrate when the additive content was 1%; for example, the wetting angle are shown in Figure2. And the wetting angle under different proportion of additives and their trends are shown in Figure 3.Figure 2
When the doped particle content was 1%, the wetting angle of (a, b) AgSnO2/La2O3 and (c, d) AgSnO2/Fe2O3.
(a)
(b)
(c)
(d)Figure 3
The wetting angle under different proportion of additives.Young’s equation shows that the smaller the wetting angle is, the better the wettability between molten Ag and SnO2 is. It was reported that the wettability between the second phase and silver, as one of the significant characteristics of the surface melting pool, has a vital effect on the spray resistance of the contact. The antisplash performance is improved due to the enhancement of the wettability, which contributes to the enhanced combination of molten silver and contact surface, and the reduced separation between molten silver and the second phase, maintaining the viscosity of the molten pool, reducing the splashing loss of the silver melt, resulting in the improvement of the resistance of the material to arc erosion.As shown in Figure3 that the wetting angle of AgSnO2/La2O3 contact material decreased first and then increased, but AgSnO2/Fe2O3 increased, with the increase of doped content, the wetting angle of AgSnO2/La2O3 and AgSnO2/Fe2O3 reached the minimum when the doping contents are 2.5% and 1%. Therefore, according to the experimental results of wettability, the fine additive content selected in this paper is shown in Table 1.Table 1
Additive content of Ag-SnO2 contact materials.
Ingredient
Matching
SnO2 : La2O3
9.5 : 2.5
SnO2 : Fe2O3
11 : 1


## 3.2. Physical Properties

The physical properties of AgSnO2 contact materials are shown in Tables 2 and 3.Table 2
The physical properties of AgSnO2/La2O3 contact material.
Particle size (nm)
Density (g/cm3)
Hardness (HV)
Conductivity (% IACS)
50
9.299
105.357
50.77
200
9.247
105.06
51.56
500
9.130
96.321
56.643
1000
9.212
98.032
54.043
10000
9.259
106.597
52.663Table 3
The physical properties of AgSnO2/Fe2O3 contact material.
Particle size (nm)
Density (g/cm3)
Hardness (HV)
Conductivity (% IACS)
50
9.512
70.109
55.630
200
9.253
89.623
60.053
500
9.030
85.028
58.370
1000
9.078
89.580
53.773
10000
9.414
93.233
61.027The results showed that with the increase of doped particle size, the electrical conductivity of two kinds of Ag-SnO2 contact materials significantly increased first and then decreased, and the density and hardness both decreased first and then increased.This is because fine doping powder can better fill the void in Ag powder, which increases the loose density of composite powder. Consequently, the surface energy of additive increases with the decrease of doped particle size, which contributes to sintering, causing the density, and hardness to increase [13]. It has been reported that electrical conductivity of the metal-based composite depends on the joint effect of the porosity, particle content, and residual stresses around particulates. And the porosity introduced during fabrication process has deteriorating effects on the conductivity by acting as nonconductivity void during electron conduction. In addition, the decrease of particle size leads to a decrease in conductivity by the introduction of more geometrically necessary dislocations and residual stresses. The above results show that the additive particle size has a vital effort on the physical properties of Ag-SnO2 contact materials.


## 3.3. Contact Resistance

Taking the Ag-SnO2/La2O3 and AgSnO2/Fe2O3 contact materials with 50 nm additive particle size, for example, the value of the contact resistance in real time is shown in Figures 4 and 5. The average value and variance of contact resistance of Ag-SnO2/La2O3 and AgSnO2/Fe2O3 contact materials with five additive particle sizes are shown in Tables 4 and 5.Figure 4
The contact resistance of Ag-SnO2/La2O3 contact material with 50 nm doped particle size.Figure 5
The contact resistance of Ag-SnO2/Fe2O3 contact material with 50 nm doped particle size.Table 4
The contact resistance of Ag-SnO2/La2O3 contact material.
Particle size (nm)
Range (mΩ)
Average value (mΩ)
Variance (×10−3)
50
0.27∼2.47
0.66
88.8
200
0.04∼5.36
1.10
499.3
500
0.3∼1.8
0.65
62.2
1000
0.18∼2.08
0.68
116.7
10000
0.36∼4.23
0.85
251.1Table 5
The contact resistance of AgSnO2/Fe2O3 contact material.
Particle size (nm)
Range (mΩ)
Average value (mΩ)
Variance (×10−3)
50
0.26∼2.4
0.86
180.4
200
0.06∼2.72
0.63
88.5
500
0.28∼2.25
0.53
60.8
1000
0.35∼2.74
0.85
135.2
10000
0.19∼2.68
0.79
174.4It can be seen from Figures4 and 5 that the contact resistance of AgSnO2/La2O3 contact material was relatively stable in the first 16000 times, and then followed by large fluctuations during 25000 times of electrical contact tests, which mainly due to the better thermal stability and the high decomposition temperature (2300°C) of La2O3, resulting in increasing the viscosity of the molten pool, improving the wettability between Ag and SnO2 so as to avoid the precipitation of SnO2 powder and reduce the fluctuation of contact resistance. However, with the increase of breaking times, the surface temperature of the contact increases sharply, which contributes to the dramatically increase the molten pool area, which leads to great contact resistance [14]. Fe2O3 dissolves on the surface of the contact at high temperature under the action of electric arc, which contributes to the formation of dense oxide film (Fe3O4), resulting in the increase of membrane resistance between the contacts.Tables4 and 5 show that with the increase of additive particle size, contact resistance decreases first and then increases, and it is found that the contact resistance of AgSnO2/La2O3 and AgSnO2/Fe2O3 contact materials is smaller and more stable when the additive particle size is about 500 nm. Nanoscale additive can improve the wettability between Ag matrix and SnO2; under the same quality condition, the actual contact area between Ag matrix and SnO2 become larger because the fine doped particle has larger surface energy, which to some extent inhibits the accumulation of SnO2 particles on the contact surface, and the contact resistance becomes smaller and more stable [15]. The results of contact resistance give indications for a further improvement of electric contact properties of Ag-SnO2 contact materials with appropriate additive particle size.


## 3.4. Arcing Energy

Taking the Ag-SnO2/La2O3 and AgSnO2/Fe2O3 contact materials with 50 nm additive particle size, for example, the value of the arc energy in real time is shown in Figures 6 and 7. The average value and variance of arc energy of Ag-SnO2/La2O3 and AgSnO2/Fe2O3 contact materials with five additive particle sizes are shown in Tables 6 and 7.Figure 6
The arc energy of Ag-SnO2/La2O3 contact material with 50 nm doped particle size.Figure 7
The arc energy of Ag-SnO2/Fe2O3 contact material with 50 nm doped particle size.Table 6
The arc energy of Ag-SnO2/La2O3 contact material.
Particle size (nm)
Range (mΩ)
Average value (mΩ)
Variance (×10−3)
50
163.0∼184.6
169.6
7.1
200
151.1∼178.2
166.7
29.5
500
161.7∼178.4
168.5
7.3
1000
159.6∼178.9
170.2
8.1
10000
159.8∼182.2
171.6
10.4Table 7
The arc energy of AgSnO2/Fe2O3 contact material.
Particle size (nm)
Range (mΩ)
Average value (mΩ)
Variance (×10−3)
50
157.7∼185.8
171.3
30.9
200
157.1∼173.0
166.8
10.6
500
156.3∼181.9
166.1
14.0
1000
163.1∼192.3
175.0
27.0
10000
158.3∼175.2
169.1
10.9As shown in Figures6 and 7 that the arc burning energy of Ag-SnO2/La2O3 contact material was relatively stable, which is due to the fact that La2O3, as a stable and hardly decomposable additive, can reduce the burning loss of electrical contacts when it is gathered on the surface of electrical contacts and restrains the movement of arc root spots to some extent under the action of electric arc, which leads to the improvement of stability of the arc energy. As shown in Tables 6 and 7, when the doped particle size was 500 nm, the Ag-SnO2/La2O3 and Ag-SnO2/Fe2O3 contact materials have smaller contact resistance and moderate hardness, resulting in lower arc voltage and less burning loss of electrical contact surface, which makes the arc burning energy smaller and more stable. In this case, it can be indicated that the Ag-SnO2/La2O3 and Ag-SnO2/Fe2O3 contact materials have better arc erosion resistance when the doped particle size is about 500 nm.


## 3.5. Microstructure Analysis

The surface microstructure photos of Ag-SnO2/La2O3 and Ag-SnO2/Fe2O3 contact materials with different additive particle sizes were observed by SEM after arc erosion, and the results are shown in Figures 8 and 9.Figure 8
The surface microstructure photos of Ag-SnO2/La2O3 contact material with different additive particle sizes: (a) 50 nm, (b) 200 nm, (c) 500 nm, (d) 1000 nm, and (e) 10000 nm.
(a)
(b)
(c)
(d)
(e)Figure 9
The surface microstructure photos of Ag-SnO2/Fe2O3 contact material with different additive particle sizes: (a) 50 nm, (b) 200 nm, (c) 500 nm, (d) 1000 nm, and (e) 10000 nm.
(a)
(b)
(c)
(d)
(e)Since the arc can be concentrated to an extremely high energy in a small area on the contact surface, the temperature can increase to melting point or even the boiling point of Ag within a few milliseconds. Meanwhile, the smaller sized oxides (such as SnO2, La2O3, and Fe2O3) decompose rapidly under the arc action, and the larger oxides cannot decompose easily, so they are wetted and redistributed by liquid Ag. As shown in Figures 8 and 9, the river-like structure is formed after arc extinction, the black holes are the ablated pits, and the area without arc action is the normal Ag and oxide distribution. EDS results of the black border (mainly gathering of Ag) and SnO2 and white area (mainly Ag and extremely fine metal oxides) were studied, which indicates that oxides will redistributed in the process of liquid Ag solidification. The reason is that the additives are wetted by melted Ag; when Ag freezes, SnO2 will be redistributed to form river-like structure and oxide layer is formed [16].Figures8(c) and 9(c) show that the surface morphologies of Ag-SnO2 contact materials are relatively flat and had less number of holes when the additive particle size was 500 nm, which indicates that SnO2 can be well wetted by liquid Ag and SnO2 can be less sucked out of the metal surface to form an oxide layer. The less oxide layer makes Ag-SnO2 contact materials have a lower contact resistance.


## 4. Conclusions

Doped Ag-SnO2 contact materials were prepared by the powder metallurgy method at different additive particle sizes. The effects of the additive particle size on the physical and electrical contact properties of the doped Ag-SnO2 contact materials were investigated.(1)
The doped Ag-SnO2 contact materials exhibit better performance at the optimal additive particle size. The contact resistance and arc energy were lower and more stable than other sizes under approximately 500 nm doped particle size.(2)
When the La2O3 and Fe2O3 as additives were added to Ag-SnO2 contact materials, the microstructure with river-like structure will be formed in the arc action area after arc extinction, which improves the performance of the Ag-SnO2 contact materials, which indicates that the properties of Ag-SnO2 contact materials can be improved significantly by selecting appropriate additive particle size.



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*Source: 7275681-2019-05-23.xml*