State whether the following is true or false.
Silicon nitride sinters readily without any additives.
Answer: C
Covalent bonding makes diffusion very slow, so sintering aids that form a liquid phase are normally required. Those additives leave a boundary phase that limits high-temperature performance, which is the standing compromise for the material.
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Consider the following statements about non-oxide ceramics.
1. Silicon carbide conducts heat well for a ceramic.
2. Silicon nitride resists thermal shock unusually well.
3. Both sinter easily without any additives at all.
Answer: D
The first two are correct. The third is wrong for both materials, since strong covalent bonding makes diffusion slow and sintering aids or applied pressure are generally needed. Hot pressing is common for both materials.
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The three polymorphs of silica encountered in ceramics are
Answer: A
Correct answer: quartz, tridymite and cristobalite
These are the three crystalline forms of silica stable over successive temperature ranges, with quartz lowest and cristobalite highest. Each has its own low and high temperature variant, and it is the alpha to beta quartz inversion at 573 degrees that governs firing schedules. The other options are the clay minerals, the alumina and aluminosilicate phases, and the carbonates.
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Assertion (A): Silica refractory bricks are fired for a long time at high temperature before use.
Reason (R): The quartz must be converted into tridymite and cristobalite, which expand less in service.
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 purpose. Quartz undergoes large volume changes as it converts, so a brick still containing it would expand and disrupt itself the first time a furnace was heated. Firing for long periods during manufacture converts the silica to tridymite and cristobalite, whose subsequent expansion in service is far smaller and more gradual - so the brick is dimensionally stable in use.
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State whether the following is true or false.
Silica brick expands on first heating unless it has been fully converted during manufacture.
Answer: C
Correct answer: True
Unconverted quartz expands sharply as it inverts and then as it converts to the higher forms, so a partly converted brick grows in the furnace wall and can spall or push the structure out of shape. That is why the conversion is done deliberately and completely in the works, and why the permanent linear change on reheating is a specification test for silica refractories.
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The sillimanite group minerals used in refractories are
Answer: D
Correct answer: kyanite, andalusite and sillimanite
The three are polymorphs of aluminium silicate with the same composition and different crystal structures, and all three convert to mullite plus silica on firing. They differ in the temperature at which they convert and in how much they expand doing so, which is what decides which is used where - kyanite converting lowest and expanding most.
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Assertion (A): Kyanite is used in refractory bodies partly to offset shrinkage.
Reason (R): It expands substantially when it converts to mullite on firing.
Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the mechanism. Kyanite expands by some fifteen to eighteen per cent as it converts to mullite, and that expansion is used deliberately to offset the shrinkage of the surrounding body during firing. Balancing the two gives a refractory that comes out of the kiln close to the size it went in, which matters when the shapes must fit a furnace lining.
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Magnesia refractories are described as basic, which means they
Answer: B
Basic and acid in refractory practice describe chemical compatibility with the slag rather than pH in water. A basic refractory resists a basic slag and would be attacked rapidly by an acid one, and silica behaves the other way about.
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State whether the following is true or false.
An acid refractory and a basic refractory can be used interchangeably in a furnace.
Answer: C
Each is attacked rapidly by a slag of the opposite character, so the choice follows the process chemistry. Placing them in contact with one another in a lining is also avoided for the same reason. A reaction zone forms wherever an acid and a basic brick meet at temperature.
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Consider the following statements about refractory classification.
1. Silica brick is an acid refractory.
2. Magnesia brick is a basic refractory.
3. The terms refer to the pH of the material in water.
Answer: D
The first two are correct. The third misreads the terminology, which describes compatibility with furnace slags rather than any behaviour in aqueous solution. The two classes must also be kept apart within a single lining.
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