Anisotropic Thermal Properties of Nanostructured Magnetic, Carbon and Hybrid Magnetic - Carbon Materials

In this dissertation research we investigated thermal properties of three groups of nanostructured materials: (i) magnetic; (ii) reduced graphene oxide films; and (iii) hybrid magnetic -- graphite -- graphene composites. The thermal measurements were conducted using the transient "hot disk" and "laser flash" techniques. The rare-earth free nanostructured SrFe12O19 permanent magnets were produced by the current activated pressure assisted densification technique. The thermal conductivity of the nanostructured bulk magnets was found to range from 3.8 to 5.6 W/mK for the in-plane and 2.36 W/mk to 2.65 W/mK for the cross-plane directions, respectively. The heat conduction was dominated by phonons near the room temperature. The anisotropy of heat conduction was explained by the brick-like alignment of crystalline grains with the longer grain size in-plane direction. The thermal conductivity scales up with the average grain size and mass density of the material revealing weak temperature dependence. Using the nanostructured ferromagnetic Fe3O4 composites as an example system, we incorporated graphene and graphite fillers into magnetic material without changing their morphology. It was demonstrated that addition of 5 wt. % of equal mixture of graphene and graphite flakes to the composite results in a factor of x2.6 enhancement of the thermal conductivity without significant degradation of the saturation magnetization. We investigated thermal conductivity of free-standing reduced graphene oxide films subjected to a high-temperature treatment of up to 1000°C. It was found that the high-temperature annealing dramatically increased the in-plane thermal conductivity, K, of the films from ∼3 W/mK to ∼61 W/mK at room temperature. The cross-plane thermal conductivity, K⊥, revealed an interesting opposite trend of decreasing to a very small value of ∼0.09 W/mK in the reduced graphene oxide films annealed at 1000°C. The obtained films demonstrated an exceptionally strong anisotropy of the thermal conductivity, K/K ⊥ ∼ 675, which is substantially larger even than in the high-quality graphite. The strongly anisotropic heat conduction properties of these films can be useful for the thermal filler applications. The results obtained for the nanostructured magnetic and hybrid materials are important for the renewable energy and electronic applications of permanent magnets.