Processing Slurries for Tape Casting

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The production of a ceramic slip is, depending on the application, a very multifaceted undertaking, as it is a very complex system consisting of many components. The main component, i.e. the ceramic or metallic powders, are mixed with organic or aqueous solvents as well as binders and additives. The slurry produced is then processed on a Tape casting bench. In the Doctor Blade process, the slurry is applied to a steel strip or plastic carrier tape by means of a doctor blade. In a further process step, the tape thus obtained is first dried, possibly converted, and then fed into a sintering process. The thickness of the tape thus obtained varies between 10 µm and 1.5 mm and could be reduced or increased as required. Applications include battery and capacitor technology as well as catalyst carriers or heat exchanger technology.

Slip components and their influence


All slurry components must be selected in such a way that they do not (chemically) react with each other. The compatibility of the individual reactants must also be maintained.

Furthermore, the reactant/solvent systems must be considered critically. For example, methyl cellulose can be dissolved in water only. A low-polarity solvent cannot be used. Polyvinyl butyrate (PVB), on the other hand, can be dissolved in ethanol with a low water content. If the water content exceeds the solubility of the binder, it would precipitate.

Starting components:


Oxides, metallic nitrides and metal powders are used as slurry starting powders. Since the physical properties (surface area, particle size and particle shape) of the powders are of great importa nce, the powders are first characterized.


The physical properties (surface tension, dielectric constant, melting point, boiling point, viscosity) of the solvents used have a high influence on the slip production / processing. Besides water, alcohols (ethanol, butanol, isopropanol) or ketones (acetone), organic solvents (toluene, xylene) or hexane and trichloroethylene or can be used. (The solvents can be used pure or as azeotropic mixtures). The drying speed of the films depends crucially on the solvent used.


The binder combines the components into a homogeneous mixture and can also influence the permeability or impermeability of the films. Examples of water-based binders are methyl cellulose, polyvinyl alcohol (PVA) or alginates. Other possible solvents for the binder are polymers (polyvinyl butyral (PVP), polymethylacrylates (PMMA), polyvinyl pyrolidone (PVP), etc.).


The use of a disperser provides steric and electrostatic stabilization to the slurry. Steric stabilization is used for solvent-based systems and electrostatic stabilization for water-based systems.


A plasticizer (polyethylene glycol, glycerol, triethylene glycol di-(2-ethylhexannoate), phthalate ester, etc.) is added to the slurry to give the green films flexibility.

Pore-forming agents

For the production of porous films, so-called pore-forming agents are added to the slurry. In addition to polymethyl acrylate, graphite powder and commercially available starch are suitable for this purpose.


The addition of very small amounts (approx. 0.01 - 1 wt.%) of additives increases the processability of the slurry and at the same time reduces defective or faulty areas in the tape.

Additives are further differentiated according to their influence on the physical and chemical properties of the slip. Thus, in addition to wetting/dispersing additives and rehological additives, there are also defoamers, driers, catalysts and preservative additives.

Several additives can also be used in a slurry, but the number should be kept to a minimum.

Products from BYK, Tego, Zschimmer & Schwarz and Münzig have proved particularly successful in this respect.

General formulation and performance

The table below gives a typical example for an alumina-based slurry.

Components Function Weight % Weight in g
Al2O3 Matrix (Powder) 68.3 1250
Ethanol solvent 7.2 132
Methylketon (MEK) Solvent 13.4 256
Polyvinylbutyral (PVB 98) Binder 4 75
Polyetylenglykol (PEG 400) Plastificizer 3 56
Triethylenglykohl-di-(2- ethylhexannoat) (3G8) Plastificizer 3 56
Disperbyk BYK 220S Disperser 0.34 6,25
Al2O3 Millingballs Ø 5mm 625

In order to obtain a homogeneous slurry, the components are filled into a mixing jar or bottle with grinding balls and processed in a tumbling mixer at a medium rotational speed.

The mixing container should occupy at least half, but not more than 75% of the container volume. The ratio of balls to powder should ideally be 1:2.

First, the balls, solvent and dispersant are weighed and mixed thoroughly. Only then is the matrix powder added. After 6 hours in the tumbling mixer, the powder agglomerates should normally be broken up and a homogeneous mixture should have formed. The mixing time mentioned varies depending on the particle size of the matrix powder used. (The Martoxid aluminum oxide (MR 70) used here had a measured particle size value of d50= >5µm. At this stage of the slurry production, a new particle size determination is now carried out. Depending on the binder addition, an increase in viscosity of the slurry could occur, which makes homogeneous mixing difficult or can only be compensated by increasing the energy input. Only then, the binder is added. The batch is now tumbled for additional two hours. The plasticizer is then added to the lump-free mixture and the mixture is tumbled for another hour. Finally, the grinding balls are removed from the slurry with a sieve. The slurry must now be degassed with a vacuum pump, as bubbles could have formed processing. Finally, the slurry should rest for another 48 hours before it can be used.

Film influencing factors

Whether the films produced meet the desired properties or strength requirements depends on the following factors:

Preparation of the slip

Ratio of slurry density/density of cast film

Gap height of the casting shoe

Gap length/width doctor blade

Hydrostatic pressure in the casting shoe

Slip viscosity, belt speed, ventilation

Sintering parameters

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