Powder Preparation and Milling Questions and Answers

Answer: C

Aggregates are strongly bonded, often by partial sintering, and normal milling will not separate them. This is why calcination conditions are controlled to avoid forming them in the first place. Calcining hotter or longer than necessary makes matters worse.

Answer: D

Correct answer: the particles are far finer than any practical sieve

The finest practical woven sieve is about 20 micrometres, and modern ceramic powders are often below one. Sieving therefore says nothing about them, and characterisation is by laser diffraction, sedimentation or gas adsorption instead. Sieving survives for granulated feedstock, where the granules are large enough to measure.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: wet milling reaches a finer size, and spray drying produces the free-flowing granules a press requires. Statement 3 is the reverse of the truth - binder is burnt out before sintering, and any that remained would leave carbon and porosity that weaken the body rather than strengthen it.

Answer: B

Correct answer: influences the shrinkage and the final sintered density

A denser green compact has less porosity to remove, so it shrinks less on firing and generally reaches a higher final density. Predicting the fired dimensions therefore depends on the green density, which is why it is measured and controlled rather than merely achieved.

Answer: C

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. Shrinkage depends on local porosity, so a compact denser at one end than the other shrinks unevenly and warps - and the internal stress may crack it. A uniform but slightly lower density is preferable to a higher average with a gradient through it.

Answer: D

Dividing 2.0 by 4.0 gives one half, or 50 per cent of theoretical density. Green relative densities of this order are typical, and the remaining porosity is what sintering must remove. Removing that porosity is what produces the firing shrinkage.

Answer: A

Friction at the wall produces a density gradient through the compact and makes ejection difficult. A stearate or wax reduces both problems and is burnt out with the binder during firing. Neither additive survives into the finished ceramic.

Answer: B

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. In single-action pressing the punch enters from one side and friction against the die wall absorbs part of the load, so the pressure reaching the far end is lower and that end is less dense. Double-action pressing, or a floating die, presses from both sides and halves the gradient.

Answer: C

Correct answer: False

Powder is not a fluid: friction against the die wall and between the particles absorbs load, so the pressure falls with distance from the punch. The gradient is what produces the density variation through the depth, and it is why the aspect ratio of a pressed part is limited.

Answer: D

As particles get finer they deform rather than fracture, and cold welding begins to rebuild them. Contamination also accumulates with time, so extended milling eventually costs more than it gains. Milling time is optimised rather than simply extended.