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Formation of CoFe2O4 nanoparticles has been investigated via three hydrothermal synthesis routes with substantially different heating rates. The syntheses were conducted at various temperatures in the interval 100–390 °C using three different rector setups: 1) a conventional Teflon-lined steel autoclave, 2) a custom-designed spiral reactor and 3) a continuous flow reactor. A careful structural analysis was performed by powder X-ray diffraction and the crystallite size was determined by peak profile analysis employing both Rietveld refinement and whole powder pattern modelling. Transmission electron microscopy was used for morphology characterization and estimation of the particle size distribution. Complementarily, the crystallites' size distribution was determined by using the whole powder pattern modelling method. The crystallite sizes are highly dependent on the type of reactor used, and crystallites in the range of 8–15 nm with different morphologies were synthesized. The continuous flow reactor produced a narrow size distribution compared with the other two types of reactors. At room temperature, the as-prepared samples exhibit coercive field values from low (0.2 kOe) to moderate (1.2 kOe). The largest coercive value of 1230 Oe was obtained for the sample synthesized in an autoclave reactor at 240 °C for 1 hour. Zero-field-cooled/field-cooled susceptibility measurements revealed the blocking temperature to be above room temperature and a strong interparticle interaction was confirmed by Henkel plots. The Curie temperatures measured for the nanoparticles are either comparable or lower than that of the bulk, depending on the particle size. [ABSTRACT FROM AUTHOR]

Platelet-shaped nano-crystallites of SrFe12O19 were hydrothermally synthesised and compacted into bulk magnets using Spark Plasma Sintering (SPS). Pellets were obtained with densities ＞90% of the theoretical crystallographic density. Compaction of the as-synthesised powders at 950 °C for 2 minutes causes improved alignment of the crystallites within the pellet, compared with room-temperature compaction. Applying an external magnetic field prior to the SPS compaction further increases the degree of alignment leading to an enhanced magnetic saturation and remanence, resulting in a 25% improvement of the energy product (BHmax), from 21.5 kJ m−3 obtained without magnetic alignment to 27.0 kJ m−3 with the applied magnetic field. Post-annealing of the SPS-pressed pellets improves the magnetic saturation even further leading to an increase of the energy product to 30.4 kJ m−3. Ball milling, on the other hand, diminishes the magnetic remanence, causing a reduction of the energy product. [ABSTRACT FROM AUTHOR]

The hydrothermal synthesis of magnetic strontium hexaferrite (SrFe12O19) nanocrystallites was followed in situ using synchrotron powder X-ray diffraction. For all the studied temperatures, the formation of SrFe12O19 happened through an intermediate crystalline phase, identified as the so-called six-line ferrihydrite (FeOOH). The presence of FeOOH has been overlooked in previous studies on hydrothermally synthesized SrFe12O19, despite the phase having a non-trivial influence on the magnetic properties of the final material. The chemical synthesis was successfully reproduced ex situ in a custom-designed batch-type reactor that resembles the experimental conditions of the in situ setup, while allowing larger quantities of material to be produced. The agreement in phase composition between the two studies reveals comparability between both experimental setups. Hexagonal platelet morphology is confirmed for SrFe12O19 combining Rietveld refinements of powder X-ray diffraction (PXRD) data with transmission electron microscopy (TEM). Room temperature magnetization curves were measured on the nanopowders prepared ex situ. The magnetic properties are discussed in the context of the influence of phase composition and crystallite size. [ABSTRACT FROM AUTHOR]

Magnetically soft zinc-substituted cobalt ferrite ZnxCo1−xFe2O4 (x = 0.4, 0.5 and 0.6) nanocrystallites were successfully synthesized from cheap, abundant materials, using a mild, scalable hydrothermal route. The partial substitution of zinc by cobalt was generally observed to reduce the resulting crystallite sizes and the saturation magnetization. Post-synthesis annealing proved to be an efficient way of inducing crystallite growth to a certain limit, thereby improving the magnetic properties. In the annealing experiments crystallite growth was observed to be extremely dependent on the annealing atmosphere, with the size increasing from dynamic vacuum, to air, argon and helium. As prepared crystallite sizes were found to be between 4.74(1) nm and 5.90(1) nm. Heat treatment caused the growth to increase to anywhere between 7.9 nm and 21.7 nm. The largest crystallite sizes, 35.2(1) nm to 44.9(1) nm, were reached by compaction of the powders prior to heating. The largest magnetizations were generally observed in the largest samples containing the least amount of zinc. The highest observed saturation magnetization was 80.49(1) emu g−1 measured for a sample with 35.2(1) nm sized crystallites of the composition Zn0.35Co0.66Fe1.99O4. [ABSTRACT FROM AUTHOR]

Platelet-shaped nano-crystallites of SrFe12O19 were hydrothermally synthesised and compacted into bulk magnets using Spark Plasma Sintering (SPS). Pellets were obtained with densities ＞90% of the theoretical crystallographic density. Compaction of the as-synthesised powders at 950 °C for 2 minutes causes improved alignment of the crystallites within the pellet, compared with room-temperature compaction. Applying an external magnetic field prior to the SPS compaction further increases the degree of alignment leading to an enhanced magnetic saturation and remanence, resulting in a 25% improvement of the energy product (BHmax), from 21.5 kJ m−3 obtained without magnetic alignment to 27.0 kJ m−3 with the applied magnetic field. Post-annealing of the SPS-pressed pellets improves the magnetic saturation even further leading to an increase of the energy product to 30.4 kJ m−3. Ball milling, on the other hand, diminishes the magnetic remanence, causing a reduction of the energy product. [ABSTRACT FROM AUTHOR]

Magnetically soft zinc-substituted cobalt ferrite ZnxCo1-xFe2O4 (x = 0.4, 0.5 and 0.6) nanocrystallites were successfully synthesized from cheap, abundant materials, using a mild, scalable hydrothermal route. The partial substitution of zinc by cobalt was generally observed to reduce the resulting crystallite sizes and the saturation magnetization. Post-synthesis annealing proved to be an efficient way of inducing crystallite growth to a certain limit, thereby improving the magnetic properties. In the annealing experiments crystallite growth was observed to be extremely dependent on the annealing atmosphere, with the size increasing from dynamic vacuum, to air, argon and helium. As prepared crystallite sizes were found to be between 4.74(1) nm and 5.90(1) nm. Heat treatment caused the growth to increase to anywhere between 7.9 nm and 21.7 nm. The largest crystallite sizes, 35.2(1) nm to 44.9(1) nm, were reached by compaction of the powders prior to heating. The largest magnetizations were generally observed in the largest samples containing the least amount of zinc. The highest observed saturation magnetization was 80.49(1) emu g(-1) measured for a sample with 35.2(1) nm sized crystallites of the composition Zn0.35Co0.66Fe1.99O4.