Classification of submicron Ni particles by heterocoagulation Toyohisa Fujita a, ⁎ , Ryoji Ito a , Chiharu Tokoro c , Jun Sadaki a , Gjergj Dodbiba a , Ryosuke Tsukamoto b , Hiroshi Okuda d , Hiroshi Yamane e a Department of Geosystem Eng., Graduate School of Eng., The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan b Graduate School of Eng., The University of Tokyo, Japan c Department Environment and Resources Eng., Waseda University, Japan d RACE, Research into Artifacts, Center for Engineering, The University of Tokyo, Japan e JFE Mineral Co., Japan Received 11 August 2005; received in revised form 4 July 2006; accepted 17 October 2006 Available online 1 November 2006 Abstract In this work, the size classification of a mixture of submicron and micron size Ni particles dispersed in aqueous solution was investigated. The suspension of Ni particles was allowed to flow, under laminar condition, through a burette filled with an adhesion medium (i.e. substrate). The difference between zeta potentials of larger particles and adhesion medium created a base for the heterocoagulation. Thus, the larger particles move towards the medium assisted by the Saffman lift force and are attracted to its surface. The finer particles, on the other hand, flow off the burette and were collected as a separate fraction. This phenomenon was then explained by numerical simulating the particle's motion and showing that a stronger Saffman lift force will act in the direction of the substrate only for large particles, leaving the smaller ones unaffected as they sediment out. © 2006 Elsevier B.V. All rights reserved. Keywords: Classification; Heterocoagulation; Saffman lift force; Submicron particle; Nickel particles 1. Introduction There are various methods for classification of micrometer- size particles, including the use of hydrocylones, [1]. In this article, the authors are putting forward a cost-effective and efficient method, based on the heterocoagulation theory [2], for size classification of Ni particles. Submicron Ni powders are extensively used in the production of the internal electrode for ceramic chip capacitors, which are used in the production of cellular phones, personal computers, etc. The structure of a ceramic chip capacitor is depicted in Fig. 1(a). It can be seen that the capacitor consists of several layers of the ceramic dielectric material and the internal electrode (Fig. 1(a)). The internal electrode layers, which thickness is 0.8– 1.0 μm, are prepared by sintering of submicron nickel particles of 0.3– 0.5 μm in diameter. Fig. 1(b) shows a SEM image of the cross section of a multilayer capacitor. Generally speaking, Ni particles used in the production of the internal electrodes should be of similar size, in order to create a uniform distance between the electrodes and hence prevent the electric breakdown. Thus, the classification of submicron Ni particles is usually carried out in advance for producing a uniform size fraction. The aim of the classification is to remove few coarse particles (i.e. size larger than 1 μm) from a lot, which contains approximately 10 4 to 10 5 fine particles. The methods employed for removing such large particles involve gravity, inertia, or centrifugal force that is combined with filtration. In this work, the size classification of a mixture of submicron and micron Ni particles dispersed in aqueous solution was investigated. In other words, the authors demonstrate how to remove few large Ni particles from a lot of small ones by utilizing differences in energy barrier between: (a) the large particles and a substrate (i.e. the adhesion medium), and (b) the small particles and the substrate. Two case studies were considered, i.e.: (1) the substrate and particles have the same Powder Technology 173 (2007) 19 – 28 www.elsevier.com/locate/powtec ⁎ Corresponding author. Tel./fax: +81 3 5841 7083. E-mail address: tfujita@geosys.t.u-tokyo.ac.jp (T. Fujita). 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.10.033