CERAMIC ENERGY: Thick Film Pastes for SOFCs

By knowing what to look for in a thick film paste and in a paste supplier, planar SOFC developers can bring their designs closer to commercialization.

Figure 1. A typical SOFC cross-sectional microstructure.
In the various design approaches to planar solid oxide fuel cells (SOFCs), the screen printing of the functional anode, electrolyte and cathode materials is key to the overall fabrication technology. These multilayer green structures of different materials are assembled and then cofired and sintered into the final SOFC architecture (see Figure 1).

For thick film paste materials applied by screen printing, important characteristics include the dispersion of the functional materials, the paste rheology or visco-elastic properties, the appearance of the deposits, the interface microstructure, and the thermal coefficient of expansion. Each of these variables can have a significant effect on the performance of the final SOFC. The ability to obtain scaled-up quantities of these pastes is also important. It's one thing to produce high-quality pastes in a laboratory, but quite another to consistently maintain this quality in a production environment.

Fortunately, suppliers are responding to these requirements and are developing thick film pastes that meet the needs of SOFC manufacturers. By knowing what to look for in a thick film paste and in a supplier, companies can bring their planar SOFCs that much closer to commercialization.

Figure 2. Low-angle light pictures of good (top) vs. bad (bottom) screen printed dried and fired materials.

Paste Selection Criteria

A variety of thick film pastes are used in SOFC manufacturing. A general list of paste products and examples of materials that can be used in combination with cast ceramic tape to build tailored parts are shown in Table 1.

The rheology or visco-elastic characteristics of the paste should be adjusted for fairly large-area prints and should be thixotropic (shear thinning), have a long and stable screen life, and consistently deliver high-quality prints without drying on the screen. An ideal paste should print at a high speed for mass production. The wet print should be smooth and should level within a few minutes. It should maintain a uniform thickness across the printed area, hold a well-defined edge and not slump or spread out. (The optimal viscosity target would be different for a paste needing to hold very fine lines and spaces.) The paste viscosity must remain stable and ready for production use over several months of shelf life.

Figure 2. (bottom)
Deposits must dry to smooth, uniform and even films of a targeted thickness and green percent solids content. No pinholes, dimples or mesh marks should result after printing, and no "orange peel effects" should develop after drying. The dispersion of the inorganic particles must be complete to form a homogeneous paste with no visible agglomerates in the printed film. Figure 2 compares a high-quality printed film against a poor-quality printed film.

The films should dry on a variety of substrates, including fired ceramic, unfired green ceramic tape or other underlying screen printed layers. However, printing onto green tape requires some paste formulation adjustments because the substrate material will soak up the paste solvent and dry differently from a similar print on a fired ceramic substrate. The dried print should possess sufficient green strength properties to survive subsequent handling operations.

The Supplier's Role

As with many high-tech materials, thick film pastes for SOFCs are often tailored specifically for the SOFC developer. As a result, it is the paste supplier that must ensure that the right materials are selected and the proper processes are followed to achieve the desired outcome. However, knowing what goes on behind the scenes can help companies choose the right supplier and provide the specifications needed to assist in paste development.

Figure 3. A typical three-roll mill for paste.

Material Selection

Key characteristics of the inorganic powders in a properly performing paste are particle size distribution, surface area and surface chemistry. Micron-sized powders generally perform better than the extremely high-surface-area submicron powders available through some combustion spray pyrolyzed production methods. Additionally, the overall particle size distribution will affect the particle packing in the green state, which will affect the densification of the final fired structure. The green density affects the fired thickness and densification dynamics such as the time at peak sintering temperature.

Selection of the binder resin chemistry, solvents, plasticizer, surface tension modifier, dispersant and other additives also plays an important role in paste performance. Conventional ethyl-cellulose-based resin vehicles perform well in most air atmosphere furnace profiles, while acrylic- or vinyl resin-based vehicles are generally selected for inert atmosphere furnace conditions. Caution must be exercised in material selection, since the impurity content of several known poisons (e.g., alkalis) can have a detrimental effect on the triple-phase boundaries within the SOFC microstructure.

An experienced supplier will also know how to handle the interrelated roles of functional organic additives in the paste formulation. All additives must be examined for their burnout characteristics in addition to their role in the formulation. A platform of proven products is typically the starting point, and these are fine tuned as the formulation progresses through functional evaluation and testing. The supplier should pay careful attention to printed film characteristics that are critical to quality, such as resistance to slumping (also known as centerline cracking) as the wet print is dried through a defined heating profile.

Green and fired structure and porosity should be tailored by designing the paste formulation and targeted properties (including solids content and particle morphology) specifically to the application. The thermal coefficient of expansion should also be customized. The firing atmosphere and ramp profile can affect the densification dynamics and result in camber issues if thermal coefficients of expansion are mismatched.

Figure 4. A thick film manufacturing process flow diagram.

Processing Parameters

Often first-generation functional inorganic powders are not (as received) ideally suitable for paste production. Some over-dried or highly heat-processed powders require an initial deagglomeration processing step, along with some type of surface modification, before they can be properly dispersed into functional pastes. The dispersion process typically involves a low-shear "wet out" of the powder ingredients, similar to mixing cake batter, followed by high-shear (103-104 sec-1) processing on a three-roll mill (see Figure 3). This crucial step must be controlled by a set schedule of pressure, gap and number of passes through the mill to achieve a consistent final fineness of grind. Additionally, the initial three-roll-mill stage viscosity must be high for the high-shear action of the mill to deagglomerate the powders. The wet-out protocols followed by the supplier in the pretreatment operations can dramatically affect the degree of dispersion achieved during the final milling of the paste, and the energy or pressure expended in the milling operation will affect the ultimate fineness of grind (micron value) achieved.

Figure 5. Some typical thick film paste viscosity curves.
An experienced supplier with a successful track record in SOFC paste formulation will provide both solid initial material choices and the expertise needed for partnering to develop the next generation of SOFC products. Good manufacturing practices require that the paste manufacturing and production processes be well documented and controlled. The process flow diagram in Figure 4 shows a typical sequence of operations. A paste formulation is "locked in," and no variations in approved raw materials or processing are then allowed. Statistical process control (SPC) data should also be collected.

The paste production area should be maintained as a Class 10,000 clean room facility to minimize the risk of particulate contamination, especially of fibers that tend to cause voids in the final fired film. As the batch size is scaled up to larger and larger mill sizes, attention to the dispersion process parameters must be maintained.

Factors Affecting Stability

As mentioned previously, micron-sized powders generally perform better than extremely high-surface-area submicron powders, which can dramatically affect the achievable high-solids loading limits and the resulting paste viscosity. However, in some cases, a pre-targeted paste thickness is required that necessitates the use of a higher-surface-area powder. SOFC developers should be aware that the higher milling energy needed to deagglomerate these powders and stabilize the dispersion can affect the paste stability and shelf life. An incompatibility between the particulate surface chemistry and additives can also slowly destabilize an initially well-dispersed paste.

Figure 5 shows some typical viscosity vs. shear rate curves for screen printable pastes. A different targeted viscosity would be selected for applications requiring syringe-dispensed pastes.

Once the desired paste viscosity is achieved, it must remain stable and not drift higher or lower over time. Ongoing surface reactions can cause agglomeration effects or settling and separation of ingredients. Some of these effects are subtle and very slow to develop, so viscosity stability should be monitored over several months to ensure the quality of the paste.

Continuing Advances

The development of screen printable pastes for SOFC applications is complicated due to the interactions among the ingredients in the formulation and the effects of these interactions on printability. However, with the right experience and knowledge, along with a great deal of persistence, complex functional oxide cell structures can be realized. Choosing the right supplier and working closely with that supplier during paste development is therefore crucial to achieving high-quality results. As the iterative process develops, progress will be made in optimizing cell performance while scaling up batch sizes to achieve the necessary economies of scale and stringent cost targets.

For more information about thick film pastes for SOFCs and other applications, contact Heraeus Inc., Circuit Materials Division, 24 Union Hill Rd., W. Conshohocken, PA 19428; (610) 825-6050; fax (610) 825-7061; e-mail thochheimer@4cmd.com; or visit http://www.4cmd.com.

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