Magnetic Submicron Mullite Coatings with Oriented SiC Whiskers.

Addressing the challenge of making ceramic thin films with the in-plane-oriented nanorods, we propose to decorate the nanorods with magnetic nanoparticles and orient them using the external magnetic field. As an illustration, the mullite thin films with embedded and oriented SiC nanorods were synthesized. The SiC nanorods were decorated with the Fe3O4 nanoparticles. A two-step processing route was developed when the nanorods are first oriented in a sacrificial polymer layer. Then, the polymer film with the aligned nanorods was removed by heat-treatment. In the second step, a sol-gel/dip-coating method was applied to produce the mullite composite film. The main challenge was to guarantee that all of the nanorods that were initially randomly distributed in the polymer would have time to rotate toward the field direction before complete solidification of the sacrificial layer. Theoretical and experimental analyses of the orientational distribution of the nanorod axes were conducted to identify a relationship between the polymer viscosity and processing parameters of the system. In contrast to the ferromagnetic nanorods, the rate of rotation of paramagnetic nanorods and their time of alignment are more sensitive to the magnetic field. This methodology allows manufacturing of different ceramic films with aligned nanorods and making nonmagnetic ceramic coating magnetic.