Effect of the oxide film formed on the electrical properties of Cu-Zn alloy electric contact material

Transcription

1 Effect of the oxide film formed on the electrical properties of Cu-Zn alloy electric contact material Hao-Long Chen *, Ke-Cheng Tseng and Yao-Sheng Yang Department of Electronic Engineering, Kao Yuan University, No.1821, Chung Shan Road, Lu Chu Hsiang, Kaohsiung County 82151, Taiwan, R.O.C. Abstract The Cu-Zn alloy has widely used as electric contact material on the micro-electronic device, automotive connectors, electric machine and electronic instrument, etc. In traditional ideas, an oxide film formed on the surface of contact materials may give rise to an additional resistance. The oxide film formed from heating oxidation of 70 wt% Cu-30wt% Zn alloy contact material in atmosphere at high-temperature and different heating times. The resulting films were analyzed by grazing-incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM) and Auger Electron Spectroscopy (AES). The electrical properties were measured using four probes. The results show that the electrical resistances of oxide films were increased with heating times that dominated, whether ZnO layer grown and covered the Cu-Zn alloy substrate, not all the thickness of oxide film. The relationships among oxide film thickness, microstructures and electrical properties of the Cu-Zn alloy contact materials were discussed in this paper. Keywords: Oxide film, Electrical properties, Electric contact material, Cu-Zn alloy. *Corresponding author: (Hao-Long Chen). Tel: ext.2019 Fax:

2 1. Introduction The electric contact means a releasable junction between two conductors which is apt to carry electric current [1]. Identically, a connector might be described as an electromechanical component which provides a separable interface between two subsystems of an electrical system without an unacceptable effect on the performance of the system [2]. In electrical contact, contact resistance is a key performance characteristic. The total contact resistance is included constriction resistance, film resistance and material bulk resistance [3]. The Cu-Zn alloys has widely used as electric contact material on the micro-electronic device, automotive connectors, electric machine and electronic instrument, etc. Materials were used application to electrical contact are usually constituted of a copper-zinc alloys interesting for its excellent electrical conductivity and an inexpensive materials cost. However, a major difficulty in the use of copper alloy contacts is its easy reactivity to atmospheric environment. Films of oxide or contamination on the surface of contact create an excess of electrical resistance which can cause failure in contact applications [4]. The classical solution is to use a protective coating of noble metals like palladium or gold. In previously report has indicated effect of coating materials on the electrical performance of copper joints [5]. Georges etc. [6] had used Laser surface treatment to modify the structure of the gold coating on the copper alloy contact is to suppress the pores in the gold layer, which are responsible of the corrosion pits and to smooth the surface as the roughness prevent a correct electrical contact. In this study is interested of oxide forming for influence of electrical properties of Cu-Zn alloy contact materials. The Cu-Zn oxide film formed in natural environment, which were composed of Cupric oxide (CuO) and Cuprous oxide (Cu 2 O) and Znic oxide (ZnO). Cupric oxide (CuO) is a p-type semiconductor having a band gap of ev and a monoclinic crystal structure. Cuprous oxide (Cu 2 O) is also a p-type semiconductor having a band gap of 2.0 ev and a cubic crystal structure [7]. Znic oxide (ZnO) is an n-type semiconductor having a band gap of 3.37 ev and a wurtzite-type crystal structure. Cupric oxide (Cu 2 O) is good electrical conductor, its resistivity about 1~10Ωm. In traditional ideas, an oxide film formed on the surface of contact materials may give rise to an additional resistance. The oxide film formed from heating oxidation of Cu- Zn alloy contact material in air atmosphere at high-temperature and different heating times. The relationships among oxide film thickness, microstructures and electrical properties of the Cu-Zn alloy contact materials were discussed in this paper. 2. Experimental method Commercial brass sheet (70wt% Cu-30%wt Zn) was polished with different 2

3 grinding paper (400, 600, 800, 1200, 2000 and 4000 grade) and then ultrasonically cleaned with acetone, ethanol, and deionized water sequentially. The samples were dried with high pressure nitrogen gas blowing. The Cu-Zn alloy sheet was loaded onto a ceramic substrate located in the center of a box furnace then heating temperature of 400 o C with different times in ambient atmosphere. The electrical properties of Cu-Zn alloys which the sheet resistance was measured with a four-point probe system in the air atmosphere. Then the crystal structure of the oxide film of Cu-Zn alloy was identified by grazing-incidence X-ray diffraction (GIXRD) using a Rigaku D/MAX 2500 multipurpose X-ray thin film diffractometer with monochromatic high-intensity CuKα radiation (λ= nm). The microstructure and surface morphology were observed with scanning electron microscopy (SEM, Hitachi S3000N, Japan) operated at 30 kv. The surface and in-depth profile oxide films of Cu-Zn alloys were analyzed by Auger Electron Spectroscopy (JEOL, JAMP-9500F Auger Electron Spectroscopy, Japan). 3. Results and Discussion The structural phase evolution of the oxide films formed on the 70wt% Cu-30wt% Zn alloy sheets after heating at 400 o C with different times were investigated by grazing-incidence X-ray diffraction (GIXRD). Figure 1 shows the 70Cu-30Zn alloy substrate shows (111), (200), (220) and (311) orientations. The ZnO start growing at temperature of 400 o C with time 1hour shows (002) and (101) orientations. The ZnO gradually increased with heating time increasing. The orientations of ZnO show (100), (002), (101), (102), (110), (103) and (112) at temperature of 400 o C with 48 hours. The surface and in-depth profile oxide films of Cu-Zn alloys were analyzed by Auger Electron Spectroscopy as Figure 2. The results show atomic concentration of Zn about 35% on the surface of 70Cu-30Zn alloy at unheated sample. The atomic concentration variations of Zn on the surface of 70wt% Cu-30wt% Zn alloy with different heating times are observed increasing from 35% to 48%. In the other words, the atomic concentration variations of Zn on the surface of 70wt% Cu-30wt% Zn alloy increased with heating time increasing. In the same time, the thicknesses of oxide film formed on the surface of 70wt% Cu-30wt% alloy were measured by Auger Electron Spectroscopy. The sputtering rate of ion is about 4.8 nm/min. Figure 3 show the thicknesses of oxide film formed on the surface of 70wt% Cu-30wt% alloy were estimated 58, 130, 144, 154, 250, 163, 182 and 202 nm at heating time 1, 6, 12, 18, 24, 30, 36 and 48 hours, respectively. The thickness of oxide film formed on the surface of 70wt% Cu-30wt% Zn alloy was abruptly decreasing from 250 nm to 163 nm at heating time of 30 hours then increased to 182 nm at heating time of 36 hours. The reason of thickness of oxide film abruptly decreased at 3

4 heating time of 30 hours is still not very clear. Figure 4 shows SEM images of the oxide films grown on Cu-Zn alloy substrates at temperature of 400 o C with different times 1, 30 and 48 hours, respectively. Figure 4(a), the island-shape structure (black point- like) [8] was observed on the oxide layer at heating time 1 hour. As heating time increasing, the ZnO tetrapod nanostructure (the white product) was easy observed [9, 10] and the island-shape still grow up on the Cu-Zn alloy substrate at heating time 30 hours (Figure 4b). As heating time 48 hours, the ZnO tetrapod nanostructure was almost cover the Cu-Zn alloy substrate and island-shape became larger than at heating time 30 hours (Figure 4c). According to XRD patterns, AES depth profile analysis and SEM images, the formation of ZnO layers were confirmed on the surface of the Cu-Zn alloy after heating at 400 o C in air for 48 hours. The surface of Cu-Zn alloy gets to be covered by ZnO layer, because the oxides and the diffusivity of Zn is two to five times higher and the former is more thermodynamically stable than of Cu. Consequently, Cu-oxides forming get to be restrained [8]. Zhu etc. [10] directly heating various Zn contents of brass heating at 500 o C in air for 48 hours found with an increase in the Zn concentrations, the CuO/Cu 2 O peaks gradually became weaker and finally disappeared for the Zn 40 wt% sample. The ZnO structure began to appear from the Zn 10 wt% and dominated from Zn 40wt% brass. Obviously, the formation of ZnO is dominant due to the lower melting point and higher vapor pressure of Zn than those of Cu under the same conditions. In our studied, the 70wt% Cu-30wt% Zn were heated at temperature of 400 o C, the ZnO layer forming were dominated whether has enough time let Zn diffusion from bulk to surface. The increase of the oxidation time favors the Zn segregation from the bulk to surface and thus the formation of ZnO layer. Figure 5 shows the electrical resistance of oxide film formed on 70wt% Cu-30wt% Zn alloy sheet obtained at heating temperature of 400 o C in air with different times. The resistances of oxide films were increasing from to 0.1 mω with heat time from 1 hour to 24 hours. The electrical resistance of oxide films was abruptly decreasing to mω at heating time 30 hours then increased to mω at heating time 48 hours. The reason of the electrical resistance of oxide films was abruptly decreasing, it is suggestion that the thickness of oxide film was abruptly decreasing at heating time 30 hours. But the reason of thickness of oxide film abruptly decreased at heating time of 30 hours is still not very clear. In traditional ideas, an oxide film formed on the surface of contact materials may give rise to an additional electrical resistance. In our studied, the electrical resistances of oxide films were increased with heating times that dominated, whether ZnO layer grown and covered the Cu-Zn alloy substrate, not all the thickness of oxide film. 4

5 4. Conclusions The electric contact materials of 70wt% Cu-30wt% Zn alloy were heated at 400 o C in air with various times. The relationship of the oxide film formed on the electrical properties of Cu-Zn alloy electric contact material among electrical resistances, microstructure and oxide film thickness were investigated. The surface of 70wt% Cu-30wt% Zn alloy gets to be covered by ZnO layer after heating at 400 o C in air for 48 hours, because the oxides and the diffusivity of Zn is two to five times higher and the former is more thermodynamically stable than of Cu. Consequently, Cu-oxides forming get to be restrained. The formation of ZnO is dominant due to the lower melting point and higher vapor pressure of Zn than those of Cu under the same conditions. The 70wt% Cu-30 wt% Zn were heated at temperature of 400 o C, the ZnO layer forming were dominated whether has enough time let Zn diffusion from bulk to surface. The increase of the oxidation time favors the Zn segregation from the bulk to surface and thus the formation of ZnO layer. The electrical resistances of oxide films were increased with heating times that dominated, whether ZnO layer grown and covered the Cu-Zn alloy substrate, not all the thickness of oxide film. 5

6 Reference [1] R. Holm, Electric Contacts Theory and Application, Springer-Verlag New York Inc., [2] M. Buggy, C. Conlon, Journal of Materials Processing Technology (2004)213. [3] R.S. Mroczkowski, Proceedings of the First Electrical Material Conference of the American Society for materials, Chicago, IL, [4] M.A. Farahat, Proceedings of the Universities Power Engineering Conference, 1988, p.61. [5] M.A. Farahat, E. Gockenbach, M. M. Abdel Aziz, Fourth Middle East Power System Conference January , Assiut University, Egypt, Paper No DIST-2 p [6] C. Georges, H. Sanches, N. Semmar, C. Boulmer-Leborgne, Applied Surface Science 186 (2002) 117. [7] B. Balamurunga, B.R. Metha, Thin Solid Films 396 (2001) 90. [8] Y.M. Chung, M. J. Jung, S. J. Lee, J. G. Han, C. G. Park, S. H. Ahn, J. G. Kim, Surface & Coatings Technology 193 (2005) 243. [9] J. Ling, C. Chun, J. Zhang, Y. Huang, F.J. Shi, X.X. Ding, C. Tang, S.R. Qi, Journal of Solid State Chemistry 178 (2005) 819. [10] Y. Zhu, C. H. Sow, T. Yu, Q. Zhao, P. Li, Z. Shen, A. Yu, J. T.-L. Thong, Advanced Functional Materials 16 (2006)

7 Captions of Figures Figure 1 X-ray diffraction pattern obtained at temperature of 400 o C for oxide film formed on 70wt% Cu-30wt% Zn alloy sheets during heating process in air with different times. Figure 2 AES depth profile of oxide film formed on 70wt% Cu-30wt% Zn alloy sheets prepared at temperature of 400 o C with different times: (a) unheated, (b) 1hour, (c) 30 hours, (d) 48 hours. Figure 3 Thickness of oxide film formed on 70wt% Cu-30wt% Zn alloy sheets prepared at temperature of 400 o C with different times. Figure 4 SEM photographs of oxide film formed on 70wt% Cu-30wt% Zn alloy sheets prepared at temperature of 400 o C with different times: (a) 1hour, (b) 30 hours, and (c) 48 hours. Figure 5 Electrical resistances of 70wt% Cu-30wt% Zn sheets obtained at heating temperature of 400 o C in air with different times. 7

8 Figure 1 X-ray diffraction pattern obtained at temperature of 400 o C for oxide film formed on 70wt% Cu-30wt% Zn alloy sheets during heating process in air with different times. 8

9 (a) (b) (c) (d) Figure 2 AES depth profile of oxide film formed on 70wt% Cu-30wt% Zn alloy sheets prepared at temperature of 400 o C with different times: (a) unheated, (b) 1hour, (c) 30 hours, (d) 48 hours. 9

10 Figure 3 Thickness of oxide film formed on 70wt% Cu-30wt% Zn alloy sheets prepared at temperature of 400 o C with different times. 10

11 (a) (b) (c) Figure 4 SEM photographs of oxide film formed on 70wt% Cu-30wt% Zn alloy sheets prepared at temperature of 400 o C with different times: (a) 1hour, (b) 30 hours, and (c) 48 hours. 11

12 Figure 5 Electrical resistance of oxide film formed on 70wt% Cu-30wt% Zn alloy sheets obtained at heating temperature of 400 o C in air with different times. 12