AgSnO₂ 접점 소재의 기술적 배경 및 AgCdO 대체 필요성

Technical Background of AgSnO₂ Contact Materials and the Need to Replace AgCdO

Silver-based metal oxide contacts are critical functional materials continuously exposed to arc erosion, contact resistance variation, welding, and mechanical wear during switching operations. AgCdO has long been used as a standard contact material due to its excellent arc resistance and anti-welding characteristics. However, the toxicity of cadmium and increasingly strict environmental regulations have made the development of alternative materials an essential technological requirement.

AgSnO₂ has emerged as a representative environmentally friendly substitute, combining non-toxic composition with high arc erosion resistance and anti-welding performance. Its application has expanded across low-voltage switching devices and relay systems where AgCdO was traditionally used.

AgSnO₂ contacts exhibit sensitivity to temperature variations, primarily associated with the thermal stability of SnO₂ particles. At elevated temperatures, contact resistance may increase due to microstructural evolution. The performance of AgSnO₂ contacts is strongly influenced by oxide dispersion, interfacial bonding, porosity, and microstructural uniformity. These factors are fundamentally determined by the manufacturing route, with Internal Oxidation (IO) and Powder Metallurgy (PM) being the most widely used industrial processes.

Overview of Internal Oxidation (IO)

Internal oxidation involves oxygen diffusion into an Ag-Sn alloy, selectively oxidizing Sn to form SnO₂ particles within the Ag matrix. Because oxide formation occurs in situ, extremely fine and uniform dispersion of oxide particles can be achieved, resulting in a dense and continuous silver matrix.

Key characteristics of the internal oxidation process include the formation of nanoscale to submicron oxide particles, strong Ag–SnO₂ interfacial bonding, low and stable contact resistance, and excellent resistance to arc erosion and welding due to the dense microstructure.

Nevertheless, increased Sn content may hinder oxygen diffusion, leading to incomplete oxidation and microstructural heterogeneity. Additionally, the process involves long oxidation cycles and limited productivity, while increased hardness can complicate subsequent machining.

Overview of Powder Metallurgy (PM)

Powder metallurgy involves blending Ag powder with SnO₂ powder followed by compaction and sintering to produce a composite material. Modern processing techniques such as mechanical alloying, hot pressing, and vacuum sintering have enhanced dispersion control and overall material performance.

Powder metallurgy offers flexibility in composition design and oxide content adjustment, enabling large-scale production and complex shape forming. The process also facilitates additive incorporation and functional composite design, contributing to cost-effective material solutions.

However, particle agglomeration and limited wettability between SnO₂ and Ag can lead to higher contact resistance and localized temperature rise. Residual porosity may also reduce arc resistance compared to internally oxidized materials.

Reasons Why AgSnO₂ Replaces AgCdO

AgSnO₂ provides comparable or superior performance without the environmental and health concerns associated with CdO. While CdO effectively disperses arc energy, regulatory restrictions on cadmium have accelerated the transition to alternative systems. SnO₂ exhibits high thermal stability and contributes to improved arc erosion resistance under high-current conditions.

AgSnO₂ demonstrates strong resistance to inrush current, particularly in lamp and capacitive loads, while minimizing material transfer and ensuring stable performance in DC circuits. Its enhanced anti-welding behavior prevents contact sticking, although slightly higher initial contact resistance may lead to temperature rise, which is typically mitigated through additives.

Performance and Electrical Characteristics: Internal Oxidation vs Powder Metallurgy

Contacts produced by these two processes show distinct arc behavior. Internally oxidized contacts often exhibit dendritic oxide structures that enhance arc erosion resistance and extend service life under heavy switching loads. Powder metallurgy contacts, on the other hand, benefit from uniformly dispersed oxide particles that distribute the arc root and reduce localized melting.

From a thermal standpoint, powder metallurgy materials tend to maintain relatively stable contact resistance, whereas internally oxidized materials may experience irregular resistance changes due to partially oxidized regions or segregation over long-term use. However, internally oxidized contacts possess strong intergranular bonding, reducing delamination under mechanical stress.

Temperature Sensitivity and Thermal Characteristics

AgSnO₂ contacts are highly sensitive to temperature and load characteristics. Internally oxidized contacts are commonly used in automotive relays and household main switches requiring moderate load handling and high mechanical reliability. Their inherent ductility facilitates fabrication into complex terminal geometries.

Powder metallurgy contacts are frequently applied in high-current industrial contactors and circuit breakers. The uniformly dispersed oxide phase effectively suppresses welding under inrush current conditions typical of lamp or motor loads. In terms of reliability, powder metallurgy offers superior batch uniformity, whereas internal oxidation provides enhanced resistance to delamination and cracking.

AgSnO₂ combines the high thermal conductivity of silver with the heat resistance of SnO₂, resulting in intrinsically stable thermal behavior. However, interfacial resistance and porosity in powder metallurgy materials may cause localized heating and performance degradation in temperature-sensitive applications. Conversely, internally oxidized materials exhibit stable heat dissipation due to a continuous silver matrix and finely dispersed oxide particles, contributing to suppressed temperature rise.

Internally oxidized AgSnO₂ provides superior reliability, low contact resistance, and excellent arc resistance for long-life switching applications, although higher manufacturing cost and limited productivity remain constraints. Powder metallurgy AgSnO₂ offers strong cost competitiveness and design flexibility, making it widely suitable for general-purpose relays and switches, though high-reliability applications may require further alloying and process optimization.