Tape-casting

Tape casting is a well known method for making thin, flat and dense ceramics [13-15]. This technique is limited by the thickness (a few millimetres) of the films obtained. The most important applications are in the electronics industry. Simon et al. [16] have described the preparation of flat ceramic membranes. This type of geometry is receiving interest because of the possibility of obtaining a high module compactness (compactness is defined by the ratio of membrane surface area to module volume). 

Tape casting [CHA 94], [MIS 78] produces ceramic sheets of low thickness (25 to 1,000 micrometers) and large surface, whose homogenity, surface quality and mechanical strength in the green state are satisfactory for electrorric applications such as sirbstrates and multilayer capacitors. 

A doctor blade spreads the suspension on a fixed support (glass, stainless steel, plastic film). For a given suspension, the height of the blade and the speed of the reservoir will determine the thickness of the tape. This casting technique is suitable for the realization of high thicknesses (substrates). 

The casting must be performed under constant conditions of temperature and hygroscopy to ensure the maintenance of the rheological and evaporation properties. 

The solvent ensures the suspension of ceramic particles and the solubilization of the various organic components, or their dispersion in the case of emulsions. 

A mixture of solvents is often chosen for the solubihty of the various organic componnds. The azeotrope proportions thus make evaporation with constant composition possible. The most widely used solvents are 2-bntanone/thanol azeotrope (60/40 vol.) and trichloiethylene/ethanol (72/28 vol.). 

The tape-casting method makes possible the fabrication of films in the region of several hundred micrometers thick. The mechanical strength allows the use of such a solid electrolyte as the structural element for devices such as the high-temperature solid oxide fuel cell in which zirconia-based solid electrolytes are employed both as electrolyte and as mechanical separator of the electrodes. 

In solid-state batteries, it is extremely favorable to use the solid electrol)de for mechanical support. Despite the larger thickness, which lowers the relative amount for active material in the battery, the advantages are the absence of pinholes of the solid electrolyte, high electronic resistance, and simple multistack fabrication, since the individual cells may be contacted by their electronically conducting current collectors. 

FIGURE 13.8 (a) Tape casting apparatus with (b) detail of the flow of fluid under the doctor blade. Note opposite flow directions in (a) and (b). 

For the Cross equation, which is valid for monodisperse suspensions of hard spheres, the shear stress, t,, is given by 

All of these rheological expressions (equations 13.16, 13.17, and 13.18) can be used to analyze the flow under the doctor blade in tape casting. Using the momentum balance equation 13.14 and one of the preceding equations for the shear stress, the differential equation which governs the velocity V,., can be determined. For Newtonian fluids, the solution is given by 

When there is insufficient velocity to overcome the yield stress, such as V To /17, the velocity distribution consists of two parts a stagnant zone near the stationary doctor blade and a moving zone near the 

The shape of the velocity distribution is similar to that for Newtonian flow but is truncated to a zone of fluid near the moving belt. 

In order to produce substrates with uniform dielectric and mechanical properties great emphasis has to be placed on the preparation and control of the slip and the casting technique. 

Slips are prepared by ball-milling the appropriate quantities of powder for a number of hours. It is vital that the viscosity of the slip is closely controlled. Because the viscosity is temperature dependent it should be warmed to and maintained at a temperature close to the normal highest ambient. If the slip has too low a viscosity due to excess liquid when at the operating temperature it can be increased by applying a vacuum. This will cause the liquid to evaporate and also promote de-airing. 

The slip should be easily pourable and not exhibit much thixotropy or dilatency, neither should it have a large yield point. The powder must be stable in suspension, ie there should be minimal settling out. Several additives are required in order to produce a castable slip that will yield a handleable cast tape. 

Binders are used to form an adherent layer around the ceramic particles so that when the solvent is removed from the cast tape it has a degree of mechanical strength. Different acrylic polymers are used to this end and usually added at about 3% to 8% by weight. Plasticisers such as polyethylene glycols are added in order to impart flexibility into the dried tape by weight of ceramic. 

The liquid used in forming the slip will preferably have a low boiling point and a low viscosity. Usually a non-aqueous solvent will be chosen that will allow the additives to be dissolved but will be inert to the inorganic powder. 

There are variations on the above approach. For instance, the hopper may move over a stationary carrier film. It is also possible to draw a polymer ribbon through a slip so as to pick up thin layers on both surfaces. 

All of these techniques are important in industry and you should know the language of thick films, e.g., slurry, slip, and paste. 

A thick film will typically have a thickness in the range 10-25 pm thin films are usually 500nm. However, what really distinguishes thick films and thin Aims, more than just their relative thickness, is the way in which they are produced. Thin films are often deposited using vacuum techniques such as sputtering and molecular beam epitaxy (MBE). We describe these techniques and consider acronyms and hyphens in 

In addition to describing some of the methods used to apply thick ceramic films to a substrate, we also describe the tape casting process. Films produced by tape casting are not used as coatings but as self-supporting ceramic sheets (down to 25 pm thick) that are widely used in the production of thick-film circuits. 

Chapter 28. Thick films are deposited from a solution or paste, which must be dried and then often sintered to produce the final coating. 

Band casting and the doctor-blade process are other names. 

Glasses The familiar glass materials are noncrystalline silicates that contain other oxides. In addition to silica (Si02), the two other primary ingredients of a typical soda-lime glass 

Glass-Ceramics Glass-ceramics are initially fabricated as glasses and then, by heat treatment, crystallized to form fine-grained polycrystalline materials. 

Clay Products Clay is the principal component of whitewares (e.g., pottery and tableware) and structural clay products (e.g., building bricks and tiles). Ingredients (in addition to clay) may be added, such as feldspar and quartz these influence changes that occur during firing. 

Refractories Materials that are employed at elevated temperatures and often in reactive environments are termed refractory ceramics. 

Abrasives The abrasive ceramics are used to cut, grind, and polish other softer materials. 

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Equally important as tape casting in the fabrication of multilayer ceramics is thick film processing. Thick film technology is widely used in microelectronics for resistor networks, hybrid integrated circuitry, and discrete components, such as capacitors and inductors along with metallization of MLC capacitors and packages as mentioned above. 

The thickness of the tape is controUed by the sHp characteristics, the height of the doctor blade, the casting rate, and the pressure head of the sHp reservoir behind the doctor blade. SHp viscosities in the range of 1 5 Pa-s (10—50 P) are used to cast tapes at 5—100 cm/s. To achieve the desired strength and flexibUity in the green tape, tape casting slurries contain more binder than those used in sHp casting, as weU as a plasticizer to ensure flexibUity. 

CeogCdo Oj 9, 30 Jim, tape casting Ni-Ce08Cd02O19 and ANLC-1 electrode... 

Interconnects are formed into the desired shape using ceramic processing techniques. For example, bipolar plates with gas channels can be formed by tape casting a mixture of the ceramic powder with a solvent, such as trichloroethylene (TCE)-ethanol [90], Coating techniques, such as plasma spray [91] or laser ablation [92] can also be used to apply interconnect materials to the other fuel cell components. 

Tai L-W and Lessing PA. Tape casting and sintering of strontium-doped lanthanum chromite for a planar solid oxide fuel cell bipolar plate. J. Am. Ceram. Soc. 1991 74 155—160. 

FIGURE 6.9 Schematic diagram of the tape-casting process for producing a ceramic green film. 

Once the structural support layers have been fabricated by extrusion or EPD for tubular cells or by tape casting or powder pressing for planar cells, the subsequent cell layers must be deposited to complete the cell. A wide variety of fabrication methods have been utilized for this purpose, with the choice of method or methods depending on the cell geometry (tubular or planar, and overall size) materials to be deposited and support layer material, both in terms of compatibility of the process with the layer to be deposited and with the previously deposited layers, and desired microstructure of the layer being deposited. In general, the methods can be classified into two very broad categories wet-ceramic techniques and direct-deposition techniques. 

PLD has also been utilized to produce bilayer electrolytes. In one study, aNiO-YSZ (anode support, tape cast)/NiO-SDC (AFL, screen-printed)/ScSZ-SDC (electrolyte bilayer, PLD)/SSC (cathode, screen-printed) cell showed excellent performance (0.5 and 0.9 W/cm2 at 550 and 600°C, respectively), with an OCV of 1.04 V at 600°C, indicating that the PLD technique was successful in depositing a sufficiently dense ScSZ electronic blocking layer to suppress electronic conduction normally observed across single-layer SDC electrolytes, and which typically result in lower OCV values (0.87 V, 600°C) [46, 127],... 

Corbin SF, Lee J, and Qiao X. Influence of green formulation and pyrolyzable particulates on the porous microstructure and sintering characteristics of tape cast ceramics. J. Am. Cer. Soc. 2001 84 41 17. 

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