Effect of Ethylcellulose Addition on Rheological Properties and Dried-Film Structures of Ni Pastes

To enable practical deployment in multilayer ceramic capacitors, optimization of Ni paste requires urgent attention. Adding ethylcellulose (EC) as a binder to Ni paste can significantly alter the rheological properties of the paste and the resulting dried coated film structure. However, the underlying mechanism remains unclear. In this study, we used commercially available EC samples (STD-10, STD-45, and STD-300) with a degree of substitution of ca. 2.5, weight-averaged absolute molar masses (Mw) of 27, 62, and 107 kg/mol, respectively, and molar mass distributions (Mw/Mn) of 1.6–2.0 to prepare Ni pastes by varying the additive amount of EC (wEC). Subsequently, we investigated the particle adsorption properties, rheological properties of the pastes, and structure of the coated dried films. The amount of EC adsorbed on the particle surface monotonically increased over the wEC range of 0.85–3.4 wt%, indicating a multilayer adsorption picture rather than a single-layer one. Rheological measurements of the pastes demonstrated that the shear viscosity and plateau modulus increased as wEC increased, and their dependence on wEC indicated interactions involving EC, such as interparticle bridging via EC chains and entanglement between EC chains, particularly in pastes containing high molar mass ECs (i.e., STD-45 and STD-300). Flow curve measurements revealed shear thickening in the intermediate shear-rate range combined with shear thinning at high shear rates. The structural evaluation of the dried Ni-paste films revealed that both the overall surface roughness and the aggregation size of the small barium titanate particles present in the gaps between the large Ni particles constitute key parameters contributing to the film properties. Considering these multifaceted measurements, we propose conditions for developing an excellent Ni paste and discuss the role of EC and its related mechanism.