Synthetic powders are preferred over mined minerals mainly because
Answer: A
A mined mineral carries whatever impurities its deposit contains and varies from face to face, whereas a synthesised powder is made to a specification. That control is what advanced ceramics require, and it is bought at a considerably higher price per tonne.
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State whether the following is true or false.
Synthetic ceramic powders are always finer than mined ones.
Answer: B
Fineness depends on the route rather than on whether the powder was synthesised. A solid-state calcination run hot and long gives a coarse, hard-agglomerated powder, while a carefully deflocculated and milled natural clay can be finer than it.
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The classic solid-state route to barium titanate calcines
Answer: C
Mixed and milled carbonate and oxide are held near eleven hundred degrees, the carbonate decomposes and the oxides interdiffuse to the titanate. The other three describe wet chemical or vapour routes that make the same compound by quite different means.
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Assertion (A): Solid-state synthesis usually needs milling after calcination.
Reason (R): The reaction proceeds by interdiffusion at points of contact, so the product forms as hard agglomerates.
Answer: D
Both are true and the reason gives the mechanism: prolonged heating that brings the particles into reactive contact also sinters necks between them, and those necks have to be broken by milling before the powder can be formed.
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The chief disadvantage of the solid-state route is
Answer: A
Mixing is only as good as the milling, so composition varies from grain to grain and the long hot hold coarsens everything. It remains the dominant industrial route because it is cheap and scales, not because the powder is good.
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Match each synthesis route with its defining step.
List I
A. Sol-gel
B. Coprecipitation
C. Hydrothermal
D. Spray pyrolysis
List II
1. reaction in water above its normal boiling point
2. hydrolysis and condensation of an alkoxide
3. atomising a solution into a hot zone
4. adding a base to bring cations down together
Answer: B
Each route is named for the step that controls the product. Recognising that step tells you what the powder will look like: molecular mixing from sol-gel, one particle per droplet from spray pyrolysis, and well-formed crystals from hydrothermal growth.
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Sol-gel synthesis achieves its homogeneity because mixing happens
Answer: C
The cations are dissolved and linked into one oxide network before any solid forms, so no diffusion distance greater than a few nanometres is ever required. Solid-state mixing, by contrast, leaves distances of microns for diffusion to cross.
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Consider the following statements about the sol-gel route.
1. It can produce powders, fibres, films and monoliths from one sol.
2. It gives crystalline product directly without any heat treatment.
3. Alkoxide precursors are considerably more expensive than oxides.
Answer: D
Correct answer: 1 and 3 only
Statements 1 and 3 are correct: one sol can be drawn into fibres, dip-coated as a film, gelled to a monolith or dried to a powder, and alkoxides cost many times what the corresponding oxides or salts do. Statement 2 is wrong - the gel is amorphous and hydrated, and a calcination is needed to remove the residual organics and water and to crystallise the product.
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State whether the following is true or false.
A xerogel and an aerogel differ in how the liquid was removed, not in their chemistry.
Answer: A
A xerogel is dried conventionally and the capillary forces collapse much of the network, while an aerogel is dried supercritically so no meniscus forms and the porous network survives nearly intact. The chemistry of the solid is the same in both cases.
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In coprecipitation the difficulty that most often spoils the product is
Answer: B
If one hydroxide comes down at pH eight and the other at pH ten, the early precipitate is rich in the first cation and the stoichiometry drifts through the run. Adding the reagents simultaneously into a buffered vessel is the standard remedy.
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