Which of the following materials has a negative coefficient of thermal expansion over a certain temperature range?
- APolymers
- BQuartz
- CInvar alloy
- DZirconium tungstateCorrect
Solution
Concept
Zirconium tungstate (ZrW2O8) shrinks on heating over a wide temperature range, so it has a negative coefficient of thermal expansion.
Step-by-step solution
1. Negative CTE means the material shrinks when heated. 2. Polymers normally expand a lot on heating, so they have a high positive CTE. 3. Quartz (crystalline) has a small positive CTE in common ranges. 4. Invar (Fe-Ni alloy) has a very low but positive CTE near room temperature. 5. Zirconium tungstate contracts on heating from about 0 K to about 1050 K, so it has a negative CTE. 6. Answer is D.
Explanation
Zirconium tungstate is the classic negative thermal expansion material. It contracts when heated over a very wide temperature range. The other options mostly expand on heating.
Why the other options are wrong
A: Polymers expand a lot (positive CTE), though some oriented polymers can be negative along the chain. B: Quartz is small positive in normal ranges. C: Invar is very low but positive near room temperature.
Shortcut
Remember: negative thermal expansion = zirconium tungstate (ZrW2O8).
Common mistake
Choosing Invar because it is known for 'almost zero' expansion. Almost zero is not negative.
Correct answer: D) Zirconium tungstate