What is the effect of AgSnO₂ contact material on the Electric Vehicles' Power Switch

I. INTRODUCTION

The development of Environmentally friendly electric vehicles with high efficiency, energy saving, low noise and zero emission has become an inevitable trend in the development of automotive industry at home and abroad. All countries in the world have set goals for the development of electric vehicles. China will produce and sell 5 million electric vehicles in 2020. The sales share of electric vehicles will reach 7%-10% in 2020, 15-20%in 2025 and more than 50% in 2030.

The power switch can automatically control and switch, and supply power to the battery management system to ensure the normal operation of the battery management system in the running and charging of the electric vehicle. The power voltage is about DC 40V-60V. Electric vehicles with frequent start-up, acceleration and deceleration will lead to large current. Therefore, the power switch of electric vehicle needs to withstand a current range from several amperes to more than 1000 amperes. General electrical life: under 2A-10A current needs to meet the life cycles of more than 100,000 about 300A needs several hundred. breaking capacity test: It is necessary to cut off the current several dozen life cycles under 1000A-2000A, and the switch can be breaking normally. For the contact material used in power switch, it not only has good electrical conductivity, but also has good wear resistance, better welding resistance, low hardness, small arc and so on. This paper introduces the contact material used in power switch of electric vehicle.

AgMeO material is used in power switch of electric vehicle because of its good electrical conductivity, not easy to be oxidized and material stability. According to the feedback from the market, the common AgMeO materials have the following advantages and disadvantages:

A. AgSnO₂ material
AgSnO₂ material has good welding resistance and wear resistance, and the tendency of forming arc pits under the action of arc is very weak. Therefore, it is widely used in automobile relay function management system, such as steering lights, car seats. In the electric vehicle switch, the AgSnO₂ material 1 on the market has good electrical performance at current below 500A, but it is easy to welding at current close to 1000A. Most contact material manufacturers improve their performance by increasing oxide content, which results in high bulk resistance, high temperature, high hardness and even affects the noise requirements of automobiles, welding still occurs at 1000A.

B. AgZnO material
The thermal stability of ZnO is higher than that of CdO, and its melting point is 1795°C. It is generally considered that AgZnO material has excellent resistance to welding, arc corrosion and low and stable resistance 3. The material can meet the requirement of 1000A about 60 cycles in the switch of electric vehicle, and the arc time is obviously shorter than that of AgSnO₂ material, but the welding occurs around 300A. Contact manufacturers try to improve the interruption performance by adding brittle materials to reduce the strength of the interface. The effect is not significant, and the arc time increases sharply.

C. AgCuO material
Copper oxide has high stability, good bonding with silver matrix interface, the material has good welding resistance, and can meet the needs of 1000A or larger. However, due to the decomposition of CuO under the action of arc, the weak conductivity of Cu2O is produced. It is easy to form oxide film on the contact surface, which leads to larger film resistance, and leads to the risk of temperature rise or non-conduction.

Given the above, the contact materials on the market have potential quality hazards in the use of power switches for electric vehicles. Contact manufacturers are carrying out upgrading research in order to meet the high reliability and quality requirements of power switches for electric vehicles. This paper introduces a kind of silver tin oxide material which meets the market demand.

II. AgSnO₂ CONTACT MATERIAL FOR POWER SWITCHES

This paper mainly introduces a common contact material for low-speed power supply switchgear. The voltage is 50V-60V. Its minimum service current is 2A. The other current classes are about 20A, 100A, 200A, 300A. The cut-off life current is 1000A-2000A.

A. Introduction of Material Preparation and Physical Properties
Composition of material: Ag content is about wt.88%, the remainder is MeO. The MeO are mainly tin oxide and indium oxide as reinforcing phases.

Material preparation: Ag, Sn and In additives were first alloyed to form silver alloy material, then the alloy was oxidized to be AgSnO₂ , pressing and extrusion and then obtained the silver tin oxide material with uniform distribution and high density. By doping, oxygen vacancies are generated to increase the carrier concentration and improve the conductivity of materials.The oxide particles are coarsened by step oxidation, and the uniform distribution structure is adopted to ensure the material's life stability.

B. Mechanical Properties of Materials
The content of silver and indium oxide in the improved material is the same as that in the original material.The physical properties and metallographic structure of the improved material are compared with those of the original material in Tables I and Fig.1.

Name	Density g/cm³	Resistivity μΩ·cm	Elongation %	Hardness HV0.3
The improved material	≥9.78	2.20-2.30	≥20	85-95
The original material	≥9.80	2.45-2.60	≥18	110-120

Compared with physical properties,the improved material has low resistance and hardness.

Compared with the metallographic structure in Fig.1, the oxidized particles of the improved material are thicker than those of the original material.



C. Analog Electrical Performance
Rivet contacts were made, Specifications are as follows:
R 3X0.5(0.3)+1.5X1.5,F 3X0.5(0.3)+1.75X1, R
6X1.85(0.6)+3X1.7 and F6X1.9(0.6)+3X3.5.

a) 20A Electrical Life Simulated Test
Test condition:
Specifications: R 3X0.5(0.3)+1.5X1.5 and F3X
0.5(0.3)+1.75X1
Load: 40V 20A DC Resistance
pressure: 30g spacing: 1.5mm
Frequency: 1s on 1s off

The surface structures of contact were obtained by SEM, there are many serious cracks on the surface of the original material.

As shown in Figure 3, the electrical life of the improved material has been significantly improved: 95% of the improved material is 163,668 cycles, 92,397 cycles of the original material.
At low current, the welding force of the improved material is slightly better than that of the original material.

■ Surface morphology



■ Electrical life and welding force



b) The welding force of 2000A high current simulating test
Test materials: AgCuO10, the improved AgSnO₂ and the original AgSnO₂.
Using capacitive discharge, 2000A, pulse width about 10ms.
Each material test five groups, 10 closing-breaking operations for each group, and each operation's welding force was measured by a sensor when the contact was broken.


Under high current about 2000A, the material improvement effect is good, and the welding force is less than that of AgCuO10 material.


In the first 10 operations of the same contact spot contact, the welding force decreases with the increase of the number of operations.

III. RESULTS AND ANALYSIS

1) Low material resistance reduces material contact resistance and improves temperature rise of power switch in electric vehicle.

2) Coarse oxide particles obtained by gradient high temperature oxidation process not only improve the welding resistance of materials, but also reduce the hardness of materials and ensure the noise requirements for new energy vehicles.

3) The electrical performance data of 40V 20A DC simulation test: the surface of material is smooth after 170,000 cycles life test, and it is not easy to form cracks on the surface corroded by arc. Therefore, it is better to improve the life cycles and stability of material. This is mainly due to the addition of additives to improve the wettability of oxide and silver matrix, which is not easy to form cracks and reduce secondary are generation.

4) The data of welding force at 2000A high current show that the improved AgSnO₂ material in this project has low welding force, and its welding resistance is much lower than that of original silver tin oxide material and AgCuO10 material with good welding resistance. In this paper introduced AgSnO₂ material, the coarsening of oxide particles improves the welding resistance of materials, and the material contains brittle materials to improve the breaking ability of contacts. By comparing the welding force of AgCuO10 with that of AgCuO10 under 2000A current, the improved AgSnO₂ material introduced in this paper can meet the 1000A-2000A high current interruption requirements of electric vehicles.

IV. CONCLUSION

Silver content is 88%. The reinforcement phase is mainly tin oxide and indium oxide. A small amount of additives are added to improve wettability and resistance. Ag, Sn and In additives are first alloyed to form silver alloy materials, then oxidized by alloys and gradient oxidation to obtain coarse oxide particles. Then through upsetting and extrusion deformation, wire materials are obtained and rivet contacts of required specifications are prepared. The prepared materials is introduced in this paper have good electrical conductivity, low hardness,coarse oxide particles, high density and uniform distribution of structure.

According to the test data of DV 40V 20A and 2000A, the material has better resistance to welding than AgCuO10. The material has low resistance and hardness, and good wettability. It can meet the requirements of electric vehicle power switch, especially improve the resistance to welding, and ensure the requirement of 1000A-2000A high current impact.

The AgSnO₂ material is introduced in this paper has been used in large quantities in the market. The material used in the client can meet the electrical performance requirements of the electric vehicle power switch from small current to high current. The arcing time of this AgSnO₂ material is better than that of AgSnO₂ and AgCuO materials, but not as good as that of AgZnO materials through HD cameras' data. So the material can also be better optimized in reducing arc time.

The importance of the work or suggest applications and extensions.