Which has larger thermal conductivity, fused silica or polycrystalline silica?

• A. Fused silica
• B. Polycrystalline silica
• C. They are equal
• D. Both are noncrystalline

Answer: (B)

The main idea is how crystal structure affects heat transport in insulators. In fused silica, the network is amorphous, so there’s no long-range order and phonons—the main heat carriers—are scattered by the disorder. This greatly reduces their mean free path and lowers thermal conductivity. In contrast, polycrystalline silica contains crystalline grains with an ordered lattice, which lets phonons travel more freely within grains. Grain boundaries do scatter some phonons, but overall the lattice order supports much better phonon transport than in the fully amorphous fused silica, so heat conduction is higher. Noncrystalline would be incorrect because polycrystalline silica is not fully noncrystalline; it has crystalline regions.