State whether the following is true or false.
Milling for longer always produces a finer powder.
Answer: A
A limiting size is reached where the rate of fracture equals the rate of re-agglomeration and cold welding. Beyond that point the only thing still increasing is the contamination from the media. Knowing where that limit lies saves both time and purity.
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A mill reduces a powder from 50 micrometres to 5 micrometres. By what factor has the specific surface area increased, assuming spherical particles?
Answer: B
Surface area per unit mass varies inversely with diameter, so a tenfold reduction in size gives a tenfold increase in area. Volume and mass scale with the cube, but area per gram scales with the reciprocal of the diameter alone.
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Freeze drying is occasionally used for ceramic powders because it
Answer: C
Removing the liquid by sublimation avoids the capillary forces that pull particles together as a solvent evaporates. It is slow and expensive, so it is reserved for powders where agglomeration would be particularly damaging.
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Assertion (A): Ordinary evaporative drying of a milled slurry tends to undo the dispersion achieved in the mill.
Reason (R): Capillary forces in the receding liquid pull the particles into hard contact.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. As the liquid evaporates, menisci form between neighbouring particles and surface tension pulls them together with considerable force, producing hard agglomerates that the milling had just removed. Freeze drying and spray drying avoid it by removing the liquid without a receding meniscus, or by drying each droplet separately.
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Consider the following statements about milling media.
1. Media of the same composition as the powder limit contamination.
2. Denser media generally give a higher energy input per collision.
3. Media wear has no measurable effect on the batch composition.
Answer: A
The first two are correct. The third is wrong, since wear is measurable and is the reason media are chosen to match the powder wherever the purity requirement is tight. Zirconia media are common where alumina would be too soft.
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The main reason ceramics are made from powders rather than by melting and casting is that
Answer: B
Alumina melts above 2,000 degrees and many other ceramics higher still, which makes melting expensive and the containment difficult. Sintering consolidates the material well below its melting point, which is the whole point of the route.
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State whether the following is true or false.
Sintering consolidates a ceramic below its melting point.
Answer: C
Diffusion joins the particles and eliminates porosity at temperatures typically two thirds of the melting point in kelvin. This is what makes the powder route practical for materials that would otherwise be impossible to melt and cast.
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Zirconia grinding media are often chosen in preference to alumina because they are
Answer: D
Greater density means more energy in each collision and greater hardness means slower wear, so the grind is quicker and cleaner. The cost per kilogram is considerably higher, which is the reason alumina remains in wide use.
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Zirconia media have a density of about 6 grams per cubic centimetre and alumina about 3.6. How much heavier is a zirconia ball of the same size?
Answer: A
Dividing 6 by 3.6 gives about 1.67, so the zirconia ball is roughly two thirds heavier. Energy per collision scales with mass, which is the practical benefit being paid for. Faster grinding also means less time for contamination to accumulate.
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State whether the following is true or false.
The choice of grinding media is purely a matter of cost.
Answer: B
Density, hardness, chemical compatibility with the powder and the acceptable level of contamination all enter the decision. Cost is one consideration among several rather than the governing one. A cheap medium that spoils the batch is no economy at all.
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