Oxidacion Zno

Journal of Alloys and Compounds 457 (2008) 565–570

Investigation on microstructure and performance of Ag/ZnO contact material
C.P. Wu a,∗ , D.Q. Yi a,b , J. Li a,b , L.R. Xiao a,b , B. Wang a,b , F. Zheng a,b
a

School of Materials Science and Engineering, Central South University, Changsha 410083, China b Key Laboratory of Non-ferrous Metals, Ministry of Education, China Received 25December 2006; received in revised form 15 March 2007; accepted 17 March 2007 Available online 30 March 2007

Abstract Ag/ZnO contact material was prepared by gas atomization internal oxidation forging technology (AIOF technology). The granularity, morphology and component of gas atomized Ag/Zn alloy powder were studied; the phase constitution, microstructure and physical performance were analyzed byX-ray diffraction (XRD), optical microscope (OM) and scanning electron microscope (SEM), respectively. Furthermore, the oxidation mechanism of Ag-based alloy was also discussed. The results showed that 82% gas atomized Ag/Zn alloy powders were less than 125 m in diameter of irregular shape. The microstructure was homogeneous with electrical contact resistance equal to 2.10 cm and of 98% relativedensity and hardness of 102.8 HV. The performance was much better than that prepared by conventional powder metallurgy method. The oxidation of Ag-based alloy was considered to be controlled by oxygen diffusion. © 2007 Elsevier B.V. All rights reserved.
Keywords: Ag/ZnO; Contact materials; Microstructure; Internal oxidation; Oxidation mechanism

1. Introduction Owing to their high electricaland thermal conductivities, high resistance to arcing, high welding adhesion resistance, low contact resistance, high hardness and strength, silver metal oxides (Ag/MeO), especially silver cadmium oxide (Ag/CdO), have found wide application in electrical and electronics industries [1–3]. However, toxic cadmium vapor releasing into environment during melting and service has caused serious pollutionproblem. Scientists are engaged to develop non-toxic, environment-friendly materials to replace the conventional Ag/CdO for electrical contact application [4–11]. Being pollution-free, silver tin oxide (Ag/SnO2 ) has been used to replace toxic Ag/CdO contact materials in last two decades. However, Ag/SnO2 material has high temperature rise and is more difficult to processing [12]. In order tocompensate the shortcoming of Ag/SnO2 materials, Ag/ZnO contact material has been developed in 1790s [13]. Like Cd, Zn belongs to

∗

Corresponding author. Tel.: +86 731 8830263; fax: +86 731 8836320. E-mail address: wcpphd@126.com (C.P. Wu).

the II B family in the periodic table. Both Zn and Cd have similar physical and chemical features and close oxide vapor pressure. In addition, theelectrical contact resistance of Ag/ZnO is lower and is a potential candidate to replace Ag/CdO as new contact material [14]. There are several successful methods including powder metallurgy (PM) and internal oxidation (IO) to prepare Ag/ZnO contact materials. Comparing to PM method, IO method offers high density, little arc erosion and long service life. But oxygen diffusion is difficult in IO method,where Zn cannot be fully oxidized. Subsequently, the structure of Ag/ZnO is inhomogeneous. A zinc deficit zone or “light belt” appeared in the middle of casting Ag/Zn alloy which is detrimental for electrical properties [15]. To resolve this problem, Japanese researchers have employed [16] high pressure internal oxidation technology which requiring the use of expensive equipments. Moreover, atomizationinternal oxidation forging (AIOF) technology has been proposed [17]. The merits of AIOF technology include short production time, simple process and low cost. Ag/ZnO electrical contacts produced by AIOF technology have been used in electrical industries. However, the investigation on oxidation mechanism of Ag/Zn alloy is less. Therefore,

0925-8388/$ – see front matter © 2007 Elsevier B.V....