Powder Preparation and Milling Questions and Answers

Answer: C

Correct answer: rotating the jars about their own axes and about a common axis

The jars spin while the carrier disc rotates in the opposite sense, so the media experience centrifugal accelerations many times gravity and strike with correspondingly greater force. The energy input is high enough to cause mechanical alloying and reaction as well as size reduction.

Answer: D

High-energy milling can introduce defects, amorphise a surface layer and in some systems drive reactions between the components. Mechanochemical synthesis exploits exactly this rather than treating it as an unwanted effect.

Answer: A

Atomised droplets dry in a stream of hot air, leaving hollow or dense granules of controlled size. These flow readily into a die and fill it uniformly, which a fine primary powder would never do. Granule size is chosen against the depth and shape of the die.

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 submicron powder is cohesive and will not flow into a die, so the fill is uneven and the compact varies in density. Spray drying builds it into free-flowing granules of tens of micrometres, which fill the die uniformly and then collapse under pressure.

Answer: C

They must break down under pressure so that the primary particles pack together, and granules that survive leave weak boundaries in the fired piece. Granule strength is therefore specified within a range rather than maximised.

Answer: D

A pressed compact of bare powder is far too fragile to survive handling between the press and the kiln. The binder is burnt out during firing and contributes nothing to the finished ceramic. Its only job is to hold the compact together until firing begins.

Answer: A

Correct answer: False

Binder is a temporary aid to forming and is burnt out before sintering begins - a residue of carbon would spoil the colour, the strength and often the electrical properties. Its removal is what the slow low-temperature stage of the firing schedule exists for.

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. The binder decomposes to gas which must escape through a body of low permeability, and heating too quickly generates it faster than it can leave. The pressure cracks or bloats the piece, so the burnout stage is held slow and is often the longest part of the schedule.

Answer: C

The binder supplies green strength and the plasticiser stops the binder film becoming brittle, so the compact tolerates handling and any small deformation. The two are usually specified together. A brittle binder film cracks and the compact loses its strength.

Answer: D

Heating drives off carbon dioxide or water and allows the wanted compound to form by solid-state reaction. Milling afterwards breaks up the hard agglomerates that calcination tends to produce. The second milling is generally shorter than the first.