Ceramic Technology and Processing, King

Milling and Equipment 53

containing the media is rinsed with water and then ultrasonically cleaned with a detergent. Rinse the media with deionized or distilled water, place the media back into the mill with pure water and then roll the media. Repeat this cleaning process before finally oven-drying the media. It will take about two cleaning cycles for the water to run clean.

The cleaning process is exacerbating when it is not practical to remove the grinding media from the mill. An option is to repeatedly run pure water in the mill until it runs clear. Circulating milling systems have additional complications with pipes, fittings, tubes, seals, pumps, and tanks. With the initial assumption that the utmost cleanliness is a prerequisite, the entire system will have to be dismantled and cleaned piece by piece.

8.0 BLUNGERS

A blunger is an impeller that vigorously stirs the slip in a tank. Ordinarily, one would think of it as a mixer rather than milling equipment, but there is an exception. Clay slips are often deagglomerated by blunging with a surfactant. Laboratory blungers can be small, stirring motors with a shaft and a propeller. A blunger could also be a mixer such as is found in the kitchen.8 Additionally, it could be a variable-speed drive with a vertical shaft ending in a disc-shaped impeller. Such a mechanism moves up and down with the help of an air cylinder. A blunger of this type is shown in Figure 3.18.

The intense shear produced in the slip depends on the impeller's peripheral velocity and the slip viscosity. The impingement between rotating particles in the slip rather than the impingement on the impeller causes the deagglomeration. Deagglomeration is enhanced when the slurry contains coarse particles as they act as a grinding media.

54 Ceramic Technology and Processing

Figure 3.18: Blunger (Courtesy of Shar)

Check List

The following check list helps summarize the contents of this chapter.

•Jar mill type: porcelain, high alumina, polyurethane, neoprene, steel, and high purity ceramics

•Contamination from the mill

•Dead spots

•Dumping procedure

Milling and Equipment 55

•Media: type, size, shape, specific gravity, wear rate, fill, and testing procedures

•Mill racks: rotation, variable speed, critical speed, tachometer, idler adjustment, roller/mill friction, containment flanges, mill accidents, enclosures, and timing problems

•Other mill designs: attritors, stirred, vibratory, jet, and planetary

•Clean up

•Blungers

REFERENCES

1.Alan G. King. "Improved Design for Milling of Advanced Ceramics." Bull. Amer. Ceram. Soc. 72[12] 43 (1993).

2.Sweco Process Industries

3.SEPR

4.Personal Communication, Don Fink

5.Dale E. Wittmer. "Alternative Processing Through Turbomilling." Ceramic Bulletin, 67[10] 1670-2 (1988).

6.Lawrence Gamblin, personal communication.

7.Sweco Process Industries

8.Hobart

Slip Preparation Procedures 57

2.0 SELECTION OF MATERIALS

Selection of the starting raw ceramic materials is crucial. It is virtually impossible to obtain the optimum properties if the powders or grain do not have the right characteristics. Both fine and coarse materials will be discussed.

Fine Grained Powders

Four attributes of powders will be discussed in relation to the selection of materials. These are particle size distribution, shape, purity, and flaw sources.

Particle Size Distribution

There are two conflicting consequences to particle size in this range ( 0.1-5.0 μm). These are sinterability and processing characteristics. Very fine particles have a high surface area that can be used to lower sintering temperatures, increase fired density, and produce a small grain size in the fired ceramic. These ceramics are wear resistant and strong. Toward the lower end of the range, the amount of surface makes these powders strongly influenced by absorbed materials and inter particulate forces. They become difficult to process and result in a low green density and a high firing shrinkage.

The surface area is much lower toward the upper end of the size range.. This requires a higher firing temperature, a lower firing density, and a coarser grain when fired. These ceramics are easier to process since surface effects are much less. Typically, the green density is higher and the firing shrinkage is less. Alumina, in the upper part of the size range, can have a glassy bonding phase.

In the lower middle of the range (0.3-0.6 μm), a compromise is reached where processing is manageable. Sintering temperatures are relatively low, firing shrinkage is fair, fired density is near theoretical, and grain size in the ceramic is small (1-3 μm).

Slip Preparation Procedures 59

The material is a TZP ceramic with a void as the fracture origin. The arrow in the figure points to the flaw. Agglomerates are more common origins. The relatively smooth area around the origin is called the mirror; it often helps to locate the origin. At higher magnification, the origin is seen to be a 50 μm void that concentrates stresses. This results in the fracture at a lower strength than the ceramic would have shown if the void was absent.

Consider a tensile test specimen measuring 1 cm x 1 cm x 10 cm. Assuming the grain size to be one μm, there will be about 104 x 104 x 105 grains in the structure. This multiplies out to 1013 grains, where one inclusion 10 μm in diameter can compromise the measured strength.

Figure 4.2 shows the effect of flaw removal on the hypothetical strength distributions.1

Figure 4.2: Effect of Flaw Removal on Strength. Strength increases and becomes tighter in distribution as flaw sources are removed.

The curves are conceptual but they propose a point of view that has merit. The author took the liberty of redrawing the curves in the

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reference, since the precision of the measurements can increase as the flaws are sequentially removed. This is seen conceptually in that, while the strengths increase, the distributions also become narrower. There are four types of flaws suggested here: soft agglomerates, hard agglomerates, organic inclusions, and surface cracks. The first three result from the starting powder. As each type of flaw is removed, the ceramic becomes stronger and has a tighter distribution. This strategy can be made to work for fine grained ceramics, since substantial progress can be made by removing flaw populations. Milling, classification, calcining, and polishing or annealing techniques are applicable in this regard.

There are very few techniques to control the particle size distribution with these fine powders. The coarse end of the distribution can be truncated by classification. Classification is a process for narrowing the particle size distribution. One such classification process is settling. The settling process, however, is hard to scale up since a large settling pond is needed for volume, and it is difficult to keep the pond free from contamination.

The fine end can also be reduced by sedimentation depending on the size of the particles. Five μm particles will settle out in a reasonable time allowing decanting removal of the fines. Air classification is another technique that will strip off the coarse end of the distribution at the higher end. Fine grained particles are much more difficult to classify since energy from the fluid does not couple efficiently with the particles. The powder supplier provides the distribution. Milling to break up agglomerates is usually the practical technique for processing.

Aspect ratio

An aspect ratio is the ratio of length to width. Two cases where the aspect ratio is crucial are fibers and platelets.

Fibers are difficult to incorporate uniformly in a batch as they form a tangle, with the bonding powder sifting through. A tangle will also prevent sintering to a high density, and the ceramic may have to be hot pressed such as SiC whiskers in an alumina matrix.2

As the particle size decreases, the surface area increases by the square of the radius. Surface chemistry dominates slip rheology; thus

Slip Preparation Procedures 61

making surfactants, solids content, pH, and electrolytes the prominent players. Anything affecting the surface will have a dominant effect. The details of the adsorption layering control rheology. Because of this, it makes a difference about which surface active agent one applies first. This will be discussed later.

Chemical Purity

Fine grained ceramics usually contain powders ranging from 98% to 99.999% purity. For optimum properties, one uses a purer powder. The impurities have three choices: to diffuse into the grains, form separate phases, or to attach to the grain boundaries. Those that diffuse into the grains are not particularly harmful to mechanical properties but may affect optical or electronic properties. Separate impurity phases are often fracture origins that lower the strength of the ceramic. Grain boundary phases are often glassy and will lower the creep resistance of the ceramic. For every generalization, there is always an exception. In this regard, there is the alumina/yttria/silica glass in Si3N4 that is necessary for the ceramic to sinter. Alumina ceramics often use a Bayer alumina and some of them contain as much as 0.4% Na2O (soda). This extra soda forms a glassy phase with other impurities on the grain boundaries.

Check List, Fine Powders

When selecting and working with fine powders, refer to the following list:

• particle size distribution

Less than 0.3 μm - difficult to process 0.3-0.6 μm - preferable for fine ceramics

3.0-5.0 μm - preferable for conventional ceramics,

•flaw removal for fine ceramics,

•equiaxed particles for most applications,

•high purity powders for fine ceramics,

•lower cost powders when necessary, and

•selection of the starting powder crucial in many applications.

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Coarse-Grained Materials

Coarse particles are often called grog. While the term is not exact, it generally refers to eight mesh and coarser. One can obtain coarse grogs in two ways: sintered or fused that are then crushed and sized. Sintered grain has internal grain boundaries making it less stable dimensionally at high temperatures. Since the grain sintering temperature is higher than the use temperature, the sintered grain is fairly stable. Fused grain often consists of single crystal domains, which makes it thermally stable. Fused grain is often more jagged than sintered, making it more difficult to pack densely. Particle packing is an issue with coarse materials, because the ceramics and refractories have little firing shrinkage. The green density is near the fired density. Packing is a result of particle size distribution and the aspect ratio.

Particle size Distribution

There are three lines of thought on packing strategies. For fine materials, one thought is to have spherical particles all of the same size.3 Also, for porous ceramics, a limited range of coarse sizes controls the v/o% of pores, pore size, and permeability. The second strategy is to gap size the mixture where the sizes fall into a sequence where each smaller size just fits into the interstices of the next coarser size; the ratio of diameters is about 10:1. The third strategy is to blend two or three natural distributions. These are designated by the top size and finer (F). One such example is 24F where all the material passes through a 24-mesh screen.

Except for porous ceramics, this latter strategy of distribution is most economical and practical. This will be discussed in the section on Coarse Particle Sized Slips.

Flaws

The coarse grains themselves act as crack sources and since one does not introduce other damaging things in the processing, flaw populations are not a concern. It is unwise to assume that there will not be