Ceramic Technology and Processing, King

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Figure 4.24: Filter Cloth with 20 μm Opening. The material is regular and clear for material to pass through.

Figure 4.25: Debris Recovered from a Slip by Filtering. Even a high purity slip can contain extraneous material.

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There are three materials in the photo: paper towel fibers, wood sticks, and agglomerates. Fibers from paper towels are endemic in the lab. Try the following experiment. Put on a pair of disposable plastic gloves, wash your hands, and dry with a paper towel. Now, rinse the gloves with a squirt bottle containing acetone (inflammable) onto a watch glass and let it dry. There will be lots of fibers on the watch glass. Electronics and pharmaceutical industries are aware of these contaminating problems. People are also a major source of contamination, and this is why we see pictures of people in these industrial labs wearing lots of protective clothing. This is done not to protect the people, but to protect the materials. Pressure filtering will remove the larger (20 μm) contaminates. Careful handling might cope with the smaller contaminating particles if they were not put in by the powder supplier.

Pressure filtering is not difficult. After the filter is set up, one pours in the slip, closes the apparatus, and pressurizes it to about 60 psi. Cleaning up is the problem. After each run, one dismantles the apparatus and thoroughly cleans it. A buildup of dried slip is a source of hard agglomerates and can cause the apparatus to malfunction. The filter cloth cannot be cleaned effectively. It can, however, be used again with the same slip composition if handled and stored carefully.

De-airing

Bubbles are entrained in the slip during milling. If left in, they will result in voids in an article that is slip cast. Small bubbles are difficult to remove except by vacuum de-airing. The apparatus for de-airing is shown in Figure 4.26. The apparatus consists of a vacuum chamber, a polymer liner, and a view port. Quick disconnects and a rubber hose connects the chamber to the vacuum system. There is also a stirrer to mix the slip during de-airing. A view port is necessary for observing the slip during the deairing process. Both the view port and lid have gaskets for sealing. There is no need for clamping as the vacuum exerts enough force to make the seal. The polymer liner is there to help clean up. The vacuum system includes the disconnects, hoses, valving, vacuum gauge, a desiccant cartridge, air filter, and a fore pump. A schematic of this system is shown in Figure 4.27.

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Figure 4.26: Vacuum De-airing Apparatus. Valving provides for de-airing of slips and de-airing of dry powders.

Figure 4.27: Vacuum System, Schematic. A desiccant or cold trap is essential.

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The system is mounted on a roll-around cart with locking casters. It has a variety of functions. For example, the filter protects the vacuum pump when evacuating isopress tooling containing powders. Desiccants that change color when saturated are available and one can reactivate such cartridges in an oven. A cold trap is an option to desiccants. Liquid nitrogen (LN2) is preferred, but one can use mechanical cold traps if LN2 is not readily available. The fore pump must be able to attain a vacuum of at least 2300Pa (17 mm Hg, 29" Hg), for reasons that will be explained.

To operate the de-airing function one is to pour the slip into the liner no more than a one third level, close the vacuum container, start the slow paddle rotation, and pull the vacuum. When the pressure reaches about 2300 Pa, the slip will froth up to three times its volume. This is why the chamber has the paddle above the original fill and the fill is limited to a one third level. There will be a mess if the liner overflows, limiting the quantity of slip that can be de-aired in one fill. The froth will then collapse under its own weight, and continue to bubble for a little while. The slip is now de-aired.

When the binder in the slip forms a leathery skin due to drying in the vacuum, the paddle will stretch the bubble walls and rupture them. This type of slip is difficult to de-air without stirring. An alginate binder system forms such a skin.

After de-airing, one releases the vacuum and lifts out the liner. One should immediately clean the liner as wet slip is a lot easier to remove. Dried slips can be rock hard depending on the binder.

With the operational procedures out of the way, the physics of the process can be discussed. A liquid boils when the pressure above the liquid surface is equal to the vapor pressure of the liquid. Water boils at 100 °C at 101 kPa (760 mm Hg, one atmosphere), or at 2.34 kPa (17.5 mm Hg) at 20 °C. In each case, the vapor pressure of the liquid is equal to the pressure over its surface. Entrained air bubbles act as nuclei for bubbles of water vapor, which greatly increases the bubble volume. This increases the buoyancy of the bubble so the slip froths up with the water vapor constituting the bulk of the gas volume, and the air just going along for the ride. The froth collapses under its own weight, releasing the water vapor and the entrained air pumped out of the system.

Non-aqueous slips can also be vacuum de-aired, but there are complications. Often, these slips contain two or more organic liquids, each

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of which has its own vapor pressure. The liquid with the highest vapor pressure is selectively removed. Also, some plastics are soluble in organic solvents, some plastics will solvent craze, and other plastics will adsorb solvent vapors and swell. These problems are solvable with the judicious selection of construction materials for the apparatus. When using solvents, the preferred cold trap is LN2.

Magnetic Filtering

When one processes a material through equipment that can add ferrous contamination, one then passes this material through a magnetic filter. Dry filters are often made from permanent magnates and the mix is poured through a baffled filter. One can only remove ferromagnetic particles and large particles this way; however, this filtering process can be good enough for some applications. Figure 4.28 depicts a slip filter using electromagnets.

Figure 4.28: Magnetic Filter for Slips. Magnetic field strength depends on extraneous material to be removed. (Courtesy of Eriez Magnetics).

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Field strength selection is dependent on the removal of ferroor para-magnetic contaminates and on the particle size of the magnetic particle. High field strengths are for weakly magnetic and/or small particles. One can best separate ferromagnetic large particles; it is more expensive to facilitate better separations.

Slip Storage

Often, the slip is stored for later use or for equilibration. Seal the slip storage container and slowly rotate it to prevent settling of the particles. One can use a modified mill rack provided one lowers the speed to about 1-5 rpm. If the speed is too high, the slip will cascade and reintroduce bubbles. For lab use, a one-liter glass jar is a convenient size. There are three problems with glass storage jars: the jar lip is not flat, the slip reacts with the lid liner, and the jar is fragile. Jars with a bump on the lip prevent an air tight seal. This causes the slip to dry around the lip forming moderately hard agglomerates that drop into the slip when the jar is opened. Lap the jar lip with SiC abrasive, coated or loose grain. Lapping the lip flat and smooth is a little tricky. Friction between the lap and the jar produces a torque that will tip the jar when the jar is held up from the lap surface. One can reduce the torque by holding the jar at a low level. When the jar is held too low, the abrasive when in contact with your fingers will remove skin from your fingers and possibly cause them to bleed. Surprisingly, the finger tips are insensitive to this abrasion. The combination of the cold water and vibration probably accounts for this insensitivity. Please accept my testimonial on this.

Lid liners sometimes react with the slip or are softened by it. A good practice is to cut a rubber gasket to fit into the lid. Neoprene is a good choice. This gasket will not react with the slip and will make a good seal with the lapped jar lip, which will prevent drying and the formation of hard agglomerates.

Wrapping the sides of the jar with duct tape will inhibit breakage, or at least hold the fragments together if the jar is broken. This will also increase friction on the mill rack so that the jar rolls smoothly. Tape wrapping also identifies the jar as modified for exclusive slip rolling use.

After taking a slip aliquot from the jar, one should transfer the slip

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to a clean jar. This transfer prevents agglomerates from getting into the slip from the gasket or lid.

Check List, Fine Particle slips

One should observe the following factors when dealing with fine particle slips.

•Slip equilibration

•Pressure filtering

•De-airing apparatus

•De-airing procedure

•Magnetic filtering

•Slip storage apparatus

•Slip storage procedure

Coarse Particle Slips

Often, one augments the coarse fraction of the slip by additions of coarse grain. There are advantages to doing this such as: increased green density, lower firing shrinkage, improved resistance to thermal shock, and improved resistance to erosion. Since one makes most coarse slips by mixing, this discussion is on casting slips.

Particle Packing

Figure 4.29 shows the particle packing of three constituents: coarse, medium, and fine fractions. Each is a screen size cut with a top size and all of the finer material that passes through that screen. For example, 24F (through 24 mesh and all of the finer sizes) is a typical cut. The ratio of the top size diameters for the three cuts is best determined experimentally where the maximum packing density is obtained. Dry packing is measured by the weight of material that will fill a cylindrical container with the top of the fill scraped off level. One can either tap or vibrate the table holding the cylinder. This is tricky as the grain will

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segregate with the fines moving toward the bottom. In this case, one compromises the packing of the various sizes. Since the concern here is with slips, a different technique is called for. As a generalization, the maximum diameter of the constituents is at a ratio of about 10:1-7:1 for each split. Figure 4.29 shows the approximate amount for each.

Figure 4.29: Particle Packing Diagram. Maximum density is in the approximate location shown for three size fractions.

Taking pure alumina as an example, the fine portion will be a Bayer alumina with choices of diameters from 0.6 μm to 4.0 μm. The fine portion has to sinter to make the bond, so it has to be fine. With other materials, low cost fines may not be available and one may have to use a -325-mesh fraction. Micrometer-sized particles are available for a variety

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of ceramics, but they are expensive.

There are theories for optimum particle packing for both precisely defined sizes and for distributions. These are not directly applicable as there are other variables such as particle shape. For example, in one case a higher density for a SiC refractory was obtained using mulled grain. With the sharp corners removed, the particles packed to a higher density. The practical way to optimize dense packing is by experimentation, but mixing to attain that is an important variable.

For coarse slips, it is common practice to increase the amount of coarse grain to increase the solid content. Since coarse particles are impervious and dense, when they are substituted for a volume of slip, there is an increase in density and a net increase in the interstitial water. The technique for doing this is by rolling the slip, as introduced in the following section.

Rolling the slip

It is feasible to increase the solid content to 85% by rolling the slip and adding coarse grain in increments. There are two concurrent processes: coarse solid particles replace fine agglomerates and the packing becomes denser as the slip is rolled. As the slip is rolled in a slowly rotating mill, it becomes less viscous. This is because the particles pack more efficiently while returning the water to the interstices. One can add a little more coarse grain to the batch and keep repeating the process until the slip thickens.

One can postulate an explanation for what is occurring. In the rolling process, the slip is subjected to shear. In shear, the particles are sliding over one another and are being rotated. Velocity gradients in the liquid rotate the particles as the fluid velocity on one side of a grain is greater than that on the other. There are short range domains where the packing density is increased. Figure 4.30 illustrates this idea.

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Figure 4.30: Packing Energy Wells. Particles will work themselves into deeper wells when the shear rate is appropriate.

Particles rest in energy wells in the grain pack. When the shear vector is high enough, the particle can be displaced. By chance, it will drop into another energy well that may be deeper than the former one. By so doing, it is more tightly packed and harder to displace. Shear gradient, viscosity, and particle size decide the energy level available in the shear field. One can increase shear by increasing the mill speed. However, too high a speed will displace all of the particles and may warrant the grain pack to be dug out of the mill with small hand tools. There is an optimum short range of mill speeds for best packing. One can determine this with experimentation.

High solid slips are dilatant, at least in part, while the densely packed domains retain their identity in the slip. Think of slush balls in mud. When these slips are poured from one bucket to another, they writhe. It would not do so without short range ordering.