Coating of TiO 2 nanolayer on spherical Ni particles using a novel sol-gel route
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Jong-Heun Leea) Division of Materials Science and Engineering, Korea University, Seoul 136-701, Korea
Yong Kyun Lee and Jae-Young Choi Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea (Received 29 December 2003; accepted 20 February 2004)
A nano-thin TiO2 layer was uniformly coated on spherical Ni particles for applications in multilayer ceramic capacitors via a controlled hydrolysis process using TiCl4, (C2H5)2NH (diethylamine, DEA), and C4H9OH as Ti-source, gradual (OH)− former, and solvent, respectively. At the constant [DEA]/[TiCl4] ⳱10.6, the Ti-hydroxide could be coated uniformly using the dilute source solution ([TiCl4] 艋 0.2 M) while the Ti-hydroxide powder in a separated form was frequently observed at [TiCl4] 艌 0.4 M. A uniform coating was attained at the slower precipitation, and the kinetics of precipitation was controlled by tuning the trace amount of water in the solution. The TiO2-coating not only prevented Ni oxidation but also retarded the sintering between the Ni particles to a large extent.
I. INTRODUCTION
Well-defined oxide coating on the sub-micrometersized powder has become increasingly important in modifying the physicochemical properties of the core particles.1–4 Furthermore, hollow spheres, which can be prepared by the removal of core part after the oxide coating, are quite useful in encapsulation of the product, protection of light-sensitive components, and use as catalysts and fillers.5,6 TiO2 coating and hollow TiO2 spheres are of great importance in the applications of the catalyst, photonic crystals,6,7 and white pigments. Several procedures to coat TiO2 layer have been reported, which include hetero-coagulation, 8 controlled hydrolysis of titanium-alkoxide,9,10 layer-by-layer deposition of oppositely charged polyelectrolytes and titania molecular precursor,11–14 and the infiltration of a Ti source solution into a periodically macroporous active carbon template.15 Recently, a well-defined TiO2 coating on the Ni particles became very important in multilayer ceramic capacitors (MLCC) using Ni electrodes, which are the most common passive components in electronics. The BaTiO3-based active layer and Ni electrode layer became increasingly thinner to achieve a higher capacitance or to
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0213 J. Mater. Res., Vol. 19, No. 6, Jun 2004
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downsize the capacitor. 16 In the highly integrated MLCC, two main parameters that should be controlled in relation to a Ni electrode are the oxidation of Ni and the shrinkage mismatch between BaTiO3 and Ni layer. The green laminate between the active and Ni electrode layers is heat treated in a nitrogen or air atmosphere at 500 °C to burn out the binder. The atmosphere is then changed into highly reducing one (e.g., H2/N2 ⳱ 5%) during sintering between 500 and 1400 °C to protect Ni electrode from oxidation.16 During the binder burnout procedures, the partial oxidation of Ni partic
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