Chemical reduction of hematite by sodium borohydride
Well-crystallized hematite was suspended in water and treated at room-temperature (RT) with sodium borohydride. The product of the reaction is a highly magnetic black powder, which is stable at RT. The NaBH4 treatment converts about half of the hematite to an amorphous Fe–B alloy and to a small fraction of sub-micron sized, amorphous metallic-Fe nodules. Heating at 400°C of this composite has resulted in the crystallization and/or oxidation of more than half of the amorphous Fe–B phase to α-Fe and Fe3O4 and B2O3, respectively. After treatment at 800°C, the metallic Fe and the amorphous Fe–B have completely vanished, and the resulting product consists of hematite and FeBO3 embedded in the matrix of α-Fe2O3.
57Fe Mössbauer effect study of well-crystallized goethite (α-FeOOH)
Well-crystallized, natural goethite has been investigated as a function of temperature. The saturation magnetic hyperfine field was found to be 507 kOe and the Néel temperatureTN=(400±2) K. The antiferromagnetic structure is stable in external fields of up to 6O kOe and the spin direction is at right angles with respect to the EFG's principal axis. AtT/TN<0.5, the reduced hyperfine field varies asT2 and in the range 0.80<T/TN<0.97 asT1/3. From the temperature dependence of the isomer shift, the Debije temperature was found to be 440 K from which a relative Mössbauer fraction of 0.96 against hematite was evaluated.
The synthesis of boride powders and some of their magnetic properties
Abstract Boride powders were synthesized by the reduction of cobalt and iron salts by NaBH4. Some of their magnetic properties, i.e. the coercive force and the rectangularity of the hysteresis loop, were investigated. The influence of the NaBH4 concentration, the ratio of Fe2+ to Co2+ and the presence of cation and anion additives in the solution on the coercive force and the rectangularity of the hysteresis loop and also on the boron content of the powders was studied. The boride powders obtained were found suitable for making high density magnetic recording materials.