Effect of polymeric surfactant on particulate structure in nickel-terpineol suspensions

Abstract

Price of noble metals (e.g., platinum, palladium, etc.) has risen rapidly over the years, so using base metals (e.g., nickel) as the interconnection material for microelectronic hybrid thick-film components has received much attention recently [1–4]. Taking the fabrication of a multilayer ceramic capacitor as an example, a stabilized nickel (Ni) slurry with a high solids concentration and an appropriate colloidal rheology is necessary for the production of defect-free, internal electrode layers. By tailoring the interparticle potentials in the slurry [5], a stable ink with a desired particulate structure may be attainable. Sanchez-Herencia et al. [6] recently prepared aqueous Ni colloids, in which nickel powders of 27% volume were mixed uniformly with κ-carrageenan in water to form “flowable” suspensions. The Ni colloids were then transformed into compacts with a sufficient “green” strength before being subjected to sintering at elevated temperatures. Bhattacharya et al. [7] examined rheological behaviors of nickel laterite suspensions at various temperatures, concentrations, particle sizes, and pH values. When solids concentration of the laterite slurries reached 20 wt%, a Bingham plastic flow behavior was observed. The present authors [8, 9] also investigated rheological behaviors of Ni suspensions when different polymeric surfactants and liquid solvents were used in the suspension formulation. Minor addition of appropriate organic surfactant was found effective in reducing the suspension viscosity, hence leading to an increased maximum solids concentration allowable for powdered suspensions. Even though the addition of organic surfactant to the rheology of Ni slurries has been studied rather extensively [6–9], a report that addresses the change of suspension structure when the surfactant is introduced into the powdered mixtures is limited [8]. In this study, we intend to compare the structural change of a model Ni-terpineol suspension when a propylene glycol was used as a surfactant. Submicrometer Ni powders (210 H, Inco Co., USA) with particle size in a range of 0.2–0.5 μm, and a specific surface area 4–8 m2/g (vendor specification) were used as the raw material. The Ni particles were extensively agglomerated, as shown in Fig. 1, from the scanning electron microscopy (SEM, JSM-6335F, Jeol, Japan). A commercially available polymeric dispersant (KD-6, ICI Surfactant, USA) consisting of propylene glycol as its major composition was uniformly mixed with reagent-grade α-terpineol (90%, Aldrich Chemical Co., USA) before addition of the powders to form ink slurries. All the powdered slurries were ball-mixed in polyethylene bottles for a period of 24 hr before their viscosity (ηs) being determined by a strain-controlled concentric viscometer (VT550, Gebruder HAAKE Gmbh, Germany) equipped with a sensor system (MV-DIN 53019, HAAKE, Germany) of a cone-cup geometry operated at a constant temperature (25 ◦C). The viscosity measurement was performed with a steady increment of shear rate (γ ) over a range of 1–1000 s−1. An SEM observation revealed that the Ni particles remained almost spherical in shape after the high-shear ball mixing. This indicated that the impact force involved in the mixing process did not deform the powders into flake forms that might substantially hinder the flow behavior of the suspensions. The suspensions all showed a shear-thinning flow character over the shear-rate range examined. This indicated that the suspensions were flocculated in structure and the Ni aggregates remained existing in the carrier liquid, even with the addition of propylene glycol. The extent of the powder aggregation appeared to be dependent on the shear rate applied, namely, the floc reduced in size as the shear rate increased, as revealed from the shear-thinning flow behavior of the suspensions. Yield stress (τy) of the suspensions can be estimated from the Casson model [10]: