Direct synthesis of encapsulated ultrafine amorphous Fe-B alloy particles with organic compound by chemical reduction

Abstract

It is well known that fine metal particles, especially ultrafine particles, are liable to aggregate, and that it is very difficult to mix them uniformly and maintain them stably in organic compound because they have an extremely large surface area and are quite active [1, 2]. Surface modification or treatment of ultrafine particles serves many important functions such as promoting dispersion and compatibility with other media, increasing stability and changing surface properties and interaction [3, 4]. Therefore, the surface treatment of ultrafine particles is very important for industrial applications. A great many methods of encapsulation, such as adsorption or encapsulation with polymers, surface oxidation and graphited carbon, have been developed [5-7]. Encapsulation of particles with organic compounds including polymers is one of the most effective surface treatments. Most of the encapsulation techniques for the active solid materials, which are usually metal oxides, have been carried out for solids larger than 1 #m or submicrometre [8, 9]. The preparation of the particles and the encapsulation are generally two separate processes. A little work has focused on the direct preparation of coated particles, in which the synthesis and encapsulation of ultrafine particles are undertaken in one step. In recent years, much attention has been devoted to the preparation of ultrafine amorphous alloys by chemical methods [10-15]. For large-scale preparation by this method, practical applications of the materials in magnetic recording, ferrofluids and catalysis are expected, and are becoming attractive. Usually, the chemical method is the reduction of metal ions with borohydride in aqueous solution. In order to prepare more uniform and smaller ultrafine Fe -B particles, we used methyl methacrylate (MMA) as a solvent to dissolve FeC13 and reduced it with KBH4 aqueous solution and 95% ethanol/ KBH 4 solution. It was very interesting to find that MMA can be adsorped on the surface of the particles to form coated particles. Coated particles possess different surface composition, dispersion and stability to uncoated particles. In this letter, the direct synthesis of coated F e B ultrafine amorphous particles is reported. The alloy powders were synthesized by adding 100 ml FeC13 aqueous solution or FeC13/MMA solution, either dropwise or quickly, to 200ml KBH4 aqueous solution or KBH4/95% ethanol solution under ultrasonic conditions. The reaction lasted for 1 h. The starting temperature was 15 °C. The black precipates were filtered out and washed with water, ethanol and acetone, then dried under vacuum at room temperature for 10 h and under N 2 gas atmosphere containing a little air. The reaction conditions and the composition of the powders in Fel00.xBx are listed in Table I. For the sedimentation experiments, 0.5 g solid powder was added to a beaker containing 100 ml solvent (ethanol or water). After ultrasonic treatment for 30 min the solution was added to a 2.5 cm diameter glass cylinder, and the sedimentation velocity of the particles was recorded. The composition of the particles was analysed by the inductively coupled plasma (ICP) method. Transition electron microscopy (TEM) was used to determine the particle size and the morphology of the powders, and scanning electron microscopy (SEM) to observe the particle surface. Selected-area electron diffraction (SAED) and X-ray diffraction (XRD) were used to investigate the crystal states. X-ray photoelectron spectroscopy (XPS) was per-