Optimisation of CGO suspensions for inkjet-printed SOFC electrolytes

Abstract

A detailed procedure for the preparation of gadolinium doped (10 mol%) cerium (IV) oxide (CGO) suspension for inkjet printing is described in this paper. The optimisation of inkjet printing parameters for the deposition of solid oxide fuel cell electrolytes was also performed using a custom-built drop visualisation system. Additionally, the uniformity of the deposited drop relics on porous substrates was evaluated. The ink used in this study was an evaporative type comprising a solvent mixture of terpineol and methanol, ethyl cellulose and CGO powder. Successful printing of regular drops was achieved after printing optimisation. It has been demonstrated that inkjet printing is a promising technique for high quality membrane fabrication for applications including solid oxide fuel cells. The ink formulation and optimisation procedure would also be applicable for other ceramic ink development.

Introduction

There is an increasing trend of utilising inkjet printing technology for the production of various electro-ceramic devices such as solid oxide fuel cells (SOFC)1, 2 and superconductors.³ Inkjet printing has proved to be a versatile and robust manufacturing technique for creating continuous coatings² and more complex two- and three-dimensional structures,3, 4 offering the capability to produce films of accurately controllable thickness and high-resolution patterns. It is also cost effective, requiring only modest investment and minimising wastage of expensive precursor materials.

SOFC is an area of application of inkjet printing, currently receiving particular attention. Producing a thin and dense electrolyte membrane is a very important requirement for a practical SOFC operated at intermediate temperatures, as described by Steele et al.⁵ However, most conventional non-vacuum techniques such as screen printing and dip coating cannot achieve dense coatings thinner than 10 μm.6, 7 Recently Tomov et al. have shown that it is possible to produce a gas-tight YSZ electrolyte of 5 μm reproducibly by inkjet printing, which sets a new benchmark for an SOFC electrolyte using cost effective methods.²

In order to create an electro-ceramic device using inkjet printing, the first step is to create a stable ink. Ceramic particles with a controlled size distribution can be made into an ink by dispersing them in a fluid system. The ink is then deposited by drop-on-demand (DOD) inkjet printing, producing a green ceramic structure, which is subsequently sintered to obtain the final ceramic. The evaporation type has been widely used in application such as paint, and conductive ink.⁸ The ink is solidified via solvent evaporation, where a volatile solvent is used as the fluid system in which ceramic particles are dispersed. After drops are deposited onto the substrate, evaporation of the solvent takes place, which will leave the solid particles behind. This type of suspension can produce a high packing density of solid particles⁷; hence it is suitable for making dense structures and films. In this study, the ink will be based on the solvent evaporation type of suspension, since making a dense SOFC electrolyte is the primary objective. An evaporative ink system usually consists of three main components: ceramic solid particles, polymeric dispersant and solvent. The solid particle provides the raw material to produce the ceramic. The polymeric dispersant not only serves to stabilise the particles (via steric stabilisation),¹⁶ but also acts as a binder and relieves drying stresses (preventing cracking).⁹ Polymeric dispersant can also be used to tailor the rheology of the ink. In some cases polymeric dispersant are used as a fugitive agent to create porous structures.¹⁰ Finally, the solvent disperses polymer-coated particles and carries them within a fluid system.

In order to successfully produce ceramic films by inkjet printing, the ink must satisfy a numbers of requirements. First, the suspension must be stable (i.e. no sedimentation or excessive agglomeration). Secondly, its rheological properties must be such that they can generate regular drops consistently in the chosen inkjet printing system. This criterion has been expressed in the literature as a constraint on the reciprocal of the Ohnesorge number (Z = Oh⁻¹, 1 < Z < 10):

$$Oh=\frac{\eta }{(\gamma \rho a^{0.5})}$$

where ρ, η and γ are the density, dynamic viscosity and surface tension of the fluid respectively, and a is a characteristic length(after Derby).¹¹ Next, the deposition of the solid must be uniform. One potential problem during the solidification of a drop of an evaporative type ink is known as the “coffee staining” effect. The effect has been studied and well documented on both dense¹² and porous substrates.¹³ The coffee staining effect, explored first by Deegan et al.,¹² represents the formation of a deposit with non-uniform thickness on drying, with more particles deposited near the perimeter. Deegan et al. pointed out that this effect was caused by contact line pinning of a drying drop, followed by evaporation of solvent along the perimeter of the drop where the exposure area is greatest. As a result a driving force is created causing suspension to flow outwards from the centre of the drops in order to compensate for the loss of solvent, creating a thick rim around the perimeter of the drops. Therefore it is important to minimise the coffee staining effect in order to produce well-defined and controllable structures or smooth coatings.

Ideally for the deposition of multiple coatings, the solvent should have a fast evaporation rate. However it has been reported that using a highly volatile solvent would enhance the coffee staining effect since there is a greater driving force.13, 14 One way to minimise the coffee staining effect is to use a solvent mixture incorporating a low vapour pressure solvent with low surface tension.¹⁵ In this way an opposing flow to the evaporation flow, known as Marangoni flow, can be generated. Dou et al.¹³ has exploited this technique and obtained uniform drop and line deposition on a dense substrate; but the coffee staining effect was still observed on both drops and lines when they were deposited on a pre-dried printed layer which represented a porous substrate.

In this study, we explored a number of combinations of solvent systems in order to minimise the coffee staining effect on a porous substrate, while maintaining ink stability and good jetting performance. A systematic approach was taken: first a number of stable suspensions were made, next the printing/jetting parameters were optimised for each ink, and finally the ink was deposited on a porous substrate to study the droplet surface uniformity.