METALS AND MATERIALS International, Vol. 13, No. 3 (2007), pp. 207~210 Effect of Reaction Time on Formation of CoNi Particles Prepared via the Polyol Method Young Min Lee 1 , G. W. Qin 2 , Chan Gyu Lee 1, * , Bon Heun Koo 1 , Kew Yul Moon 1 , Y. Shimada 3 , and O. Kitakami 3 1 School of Nano and Advanced Materials Engineering, Changwon National University, 9, Sarim-dong, Changwon-si, Gyeongnam 641-773, Korea 2 Materials Science College of Materials and Metallurgy, Northeastern University, Shenyang 110004, P. R. China 3 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan CoNi alloy particles were synthesized via the polyol process. Variations of the lattice constant, the mean size and composition, and the magnetic properties as functions of reaction time were evaluated by using X-ray diffractometry, field emission scanning electron microscopy, and vibrating sample magnetometer, respectively. The mean size of the obtained CoNi particles did not change from about 500 nm for up to 2 h of reaction time but increased to about 700 nm at a reaction time of 4 h. It was found that the Co composition of the CoNi alloy decreased as the reaction time increased at 180 o C. This indicates that the reduction of Co 2+ is easier than that of Ni 2+ . The magnetic properties showed that the CoNi alloy particles are typical soft magnetic materials. Keywords: CoNi alloy particles, polyol process, reaction time, XRD, FESEM, VSM 1. INTRODUCTION Fine metal particles with controllable shapes, sizes, and compositions have extensive applications in the fields of optics, electronics, and magnetism [1]. Particularly, ferro- magnetic metal-based materials display exceptional proper- ties for microwave applications, especially for higher working frequencies and the broader working frequency band. To apply fine metal particles to practical applications, the primary task is synthesizing particles with definite shapes, tailor-made sizes, and narrow size distributions. The definite composi- tion appeared as alloy particles with crystals or amorphous phases. Well-defined metallic grains can be obtained through chemical routes, including the following: decomposition of organometallic precursors [2], in microemulsions [3,4] sol- vated metallic atoms deposition [5], vapor deposition [6], electrodeposition [7], and reverse micelle solutions [8]. Uzawa et al. reported the preparation of CoNi alloy particles with the ultra fine crystallites using a leaching technology [9]. However, the complexity involved in the above syn- thetic procedures required many steps and the use of unusual chemicals. The polyol process is known to be a suitable method for the synthesis of ferromagnetic metal particles [10-12]. This method allows an accurate and reproducible control of the mean diameter of particles from a few tens of nanometers to a few micrometers and has the advantages of simplicity and low cost compared with physical approaches. In this reac- tion, the liquid polyol acts both as a solvent of the metallic precursors and as a mild reducing medium. Ethylene glycol, propylene glycol, etc., have been previously shown to be good reducing agents for several metals [13,14]. Recently, many attempts to synthesize CoNi particles have been made using fixed reaction times to understand the nucleation and the growth of bimetallic CoNi particles [15,16]. Systematic investigations, including the influence of the reaction time on the formation process, are still required in order to control and understand the formation of bimetallic CoNi particles. In this study, we report on the effect of the reaction time on the formation of CoNi particles prepared by the polyol method. We have found significant changes in the composi- tion of CoNi particles with variation of reaction time. 2. EXPERIMENTAL PROCEDURE The CoNi alloy particles were prepared via the polyol pro- cess as follows: Co(II) and Ni(II) acetate tetrahydrates and sodium hydroxide were dissolved in propylene glycol. For the preparation of the CoNi alloy particles, the concentration of Co(II) and Ni(II) was fixed at 0.1 mol/l with the sodium *Corresponding author: chglee@changwon.ac.kr