Powder Preparation and Milling Questions and Answers

Answer: A

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. Calcination is carried out hot enough to complete the reaction, and that same heat begins to bond neighbouring particles together. The result is hard agglomerates which would leave large pores in the fired body, so the calcined powder is milled again to break them.

Answer: B

Mixing at the level of the solution gives a far more intimate combination than any mechanical mixing can, so the calcination that follows is quicker and more complete. Controlling the pH and the sequence of addition is the difficulty.

Answer: C

Hydrolysis and condensation of a precursor build a network in solution, which is dried and calcined to give a very fine and homogeneous powder. The precursors are expensive, so the method is used where purity matters more than cost.

Answer: D

Correct answer: True

Mixing and firing oxide powders is cheap and needs no special equipment, which is why most ceramic powder is made that way. But mixing is only as good as the milling, the reaction proceeds by solid-state diffusion over the distances between particles, and the product is correspondingly less homogeneous than a chemically derived powder.

Answer: A

Nitrogen adsorbed as a monolayer at low temperature gives the accessible area, including that of pores and of fine features. It complements a size distribution rather than duplicating it, since two powders of the same size can differ in area.

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. A gas adsorption measurement counts every accessible surface, including the inside of pores and the roughness of each particle, while a size measurement sees only the outer dimension. A powder can therefore have a large surface area and a coarse size, and the discrepancy is itself informative.

Answer: C

Correct answer: 5,000 square metres

\[ 10 \times 500 = 5{,}000\ \mathrm{m^2}. \] Half a kilogram of powder carrying the area of a football pitch is worth pausing over: it is why fine powders sinter readily, why they adsorb moisture from the air, and why some of them are pyrophoric.

Answer: D

Finer powders sinter more readily, but they also agglomerate more strongly and are harder to pack uniformly. A poorly dispersed fine powder can end up less dense than a coarser one that packed well. Packing uniformity often matters more than nominal fineness.

Answer: A

Both are true and the reason gives the difficulty: sintering removes porosity between particles but not the large voids left between hard agglomerates, so dispersion matters as much as fineness does. Large voids between agglomerates are the hardest porosity to remove.

Answer: B

Weak agglomerates are what dispersion and milling are meant to remove, whereas aggregates are bonded by sintering or chemical bridges and survive. The distinction matters because only one of them can be dealt with by processing.