The Effect of Silanisation on Microstructural Stability and Magnetic Properties of the Intermetallic Sm 2 (Co, Fe, Cu, Z
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The Effect of Silanisation on Microstructural Stability and Magnetic Properties of the Intermetallic Sm2(Co, Fe, Cu, Zr)17. M. I. Qadeer1, Bruska Azdhar2, S. J. Savage1,3 1 Department of Materials Science and Engineering and 2Department of Fibre and Polymer Technology, The Royal Institute of Technology, SE-100 44, Stockholm, Sweden. 3 Division of Information Systems, Swedish Defence Research Agency (FOI), SE-581 11 Linköping, Sweden. ABSTRACT The effects of silanising using the coupling agent γ-glycidoxpropyltrimethoxysilane on microstructural stability and magnetic properties of Sm-Co powder particles have been investigated. The silanisation provides structural stability by improving the oxidation resistance at 400oC for 10 hours. The untreated particles undergo microchemical changes by redistribution of alloying elements which mainly accumulate in parallel black and grey streaks in the interior of the particles. The silanised particles after heat treatment show coercivity of 836 Oe and the untreated particles show a much lower coercivity of 376 Oe. The difference in magnetic properties of uncoated particles is caused by diffusion of oxygen and microstructural instability. INTRODUCTION Sm2(Co, Cu, Fe, Zr)17 intermetallic compounds have now attained significant technological and commercial importance for their unique magnetic characteristics such as high magnetocrystalline anisotropy and high Curie temperature, which enable applications in sensors, actuators, gyroscopes and accelerometers [1]. The hard magnetic properties are attributed to a complex three phase structure consisting of cellular and lamellar phases. The cellular phase consists of Sm2Co17 surrounded by SmCo5 and the lamellar phase, which is superimposed over many cellular phases, is rich in Zr [1, 2]. The basic mechanism leading to the hard magnetic properties characteristic of Sm-Co magnets is pinning of domain walls at the cell boundaries of the SmCo5 and the lamellar structure which extends over many cells, stabilizes the cell structure [1]. The renewed interest in Sm-Co intermetallics is based on their suitability for applications at temperatures greater than 400oC. Although Sm-Co magnets are relatively less prone to oxidation as compared to their counterpart NdFeB alloys, at elevated temperatures oxidation of Sm-Co leads to loss in coercivity and energy product, BHmax [3]. The oxidation of this alloy results in the formation of a surface scale and subscale, formed due to diffusion of oxygen into the alloy [4, 5, 6]. The subscale formed is far more detrimental to the magnetic properties as compared to the surface oxidation, as many structural changes occur in the subscale and poor magnetic properties are attributed to it [7]. Surface treatment of Sm-Co alloys is therefore necessary to protect these materials for high temperature applications. The surface treatments that can be adopted depend on the manufacturing route, end use temperature and any impact on the environment. Permanent magnetic materials are produced either by sintering or by the poly
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