Influence of a Reduction Process on the Phase Component and Magnetic Properties of NdFeB Magnetic Nanoparticles

NdFeB magnetic nanoparticles with a main phase of Nd₂Fe₁₄B have successfully been synthesized. The preparation includes the following processes. First, the NdFeB intermediate was synthesized by a wet-chemical method. The NdFeB intermediate was annealed at 800 °C, which resulted in the formation of an NdFeB oxide. Then, the NdFeB oxide was reduced into NdFeB nanoparticles by a second-step reduction annealing with a CaH₂ reductant. The second-step reduction annealing temperature was a key factor in preparing the NdFeB nanoparticles. A lower reduction temperature of 900 °C could not completely reduce the NdFeB oxide into Nd₂Fe₁₄B. There were some residual unreduced nonmagnetic phases in the prepared materials, which resulted in obvious decreases of coercivity. A sufficiently high second-stage reduction temperature resulted in an increased reduction, and more of the Nd₂Fe₁₄B phase could be obtained. In this work, nanoparticles with a uniform morphology and an increased Nd₂Fe₁₄B phase could be obtained at an optimum reduction temperature of 940 °C, achieving a high coercivity of 5.4 kOe.

Synthesis of hard magnetic NdFeB composite particles by recycling the waste using microwave assisted auto-combustion and reduction method

The present work provides a facile and energy-saving approach to recycle Nd-Fe-B wastes. Employing the severely oxidized and contaminated sintered Nd-Fe-B scraps as the raw materials, sub-micro sized Nd-Fe-B/α-Fe composite magnetic powders were successfully synthesized by microwave assisted auto-combustion method followed by a reduction diffusion process. In details, the waste magnets were dissolved in HNO₃ solution and both the rare earth oxides and iron oxide were obtained by auto-combustion approach with the help of microwave heating. These oxides were then reduced to achieve Nd-Fe-B alloys with the reducing agent CaH₂. To modify the composition of the final product, Nd(NO₃)₃·6H₂O and H₃BO₃ were added during dissolving and consequently three compositions of the final products were obtained. TEM results demonstrate that the sizes of α-Fe and Nd₂Fe₁₄B grains are below 10 nm and 20 nm, respectively. The magnetic properties of the composite powders obtained from the wastes in this study are comparable to those obtained by other methods using pure reagents. The room temperature coercivity of as-synthesized Nd₁₅Fe₇₇B₈ is higher than 400 kA/m. In addition, an appropriate mass ratio of CaH₂ to Nd-Fe-B oxides is required in order to obtain good magnetic properties.

Preparation of Nd-Fe-B by nitrate-citrate auto-combustion followed by the reduction-diffusion process

The Nd2Fe14B alloy has been successfully synthesized by nitrate-citrate auto-combustion followed by the reduction and diffusion process with low energy consumption. H3BO3, Fe(NO3)3·9H2O, and Nd(NO3)3·6H2O were used as precursors and citric acid was used as the chelating ligand of metal ions. Ammonia water was used to adjust pH to 7. CaH2 was used as a reducing agent for the reduction and diffusion process. NdFeO3 and Fe2O3 were produced during auto-combustion of gel. The combustion process of the gel was investigated by TGA/DTA curve measurements. The phase compositions were studied by XRD measurements. The differences of the overall morphology and magnetic properties were measured by SEM, TEM and vibrating sample magnetometry (VSM) at 300 K. The comparison of the magnetic properties of the reduced samples between the pellet type and the random powder type was done with VSM and it showed better magnetic properties of the pellet type Nd2Fe14B. Making a compact pellet type sample for reduction is more efficient for solid reduction and phase transition for higher coercivity.

Novel microwave assisted chemical synthesis of Nd₂Fe₁₄B hard magnetic nanoparticles

The high coercivity and excellent energy product of Nd2Fe14B hard magnets have led to a large number of high value added industrial applications. Chemical synthesis of Nd2Fe14B nanoparticles is challenging due to the large reduction potential of Nd(3+) and the high tendency for Nd2Fe14B oxidation. We report the novel synthesis of Nd2Fe14B nanoparticles by a microwave assisted combustion process. The process consisted of Nd-Fe-B mixed oxide preparation by microwave assisted combustion, followed by the reduction of the mixed oxide by CaH2. This combustion process is fast, energy efficient and offers facile elemental substitution. The coercivity of the resulting powders was ∼8.0 kOe and the saturation magnetization was ∼40 emu g(-1). After removal of CaO by washing, saturation magnetization increased and an energy product of 3.57 MGOe was obtained. A range of magnetic properties was obtained by varying the microwave power, reduction temperature and Nd to Fe ratio. A transition from soft to exchange coupled to hard magnetic properties was obtained by varying the composition of NdxFe1-xB8 (x varies from 7% to 40%). This synthesis procedure offers an inexpensive and facile platform to produce exchange coupled hard magnets.

Magnetic and ultrasonic investigations on magnetite nanofluids

Magnetite nanofluids of various concentrations have been prepared through co-precipitation method. The structural and magnetic properties of the magnetic nanofluids have been analyzed which respectively revealed their face centered cubic crystal structure and super paramagnetic behavior. Ultrasonic investigations have been made for the nanofluids at different temperatures and magnetic fields. Open- and close-packed water structure is considered to explain the temperature effects. The inter particle interactions of surface modified nanomagnetite particle and the cluster formation are realized through the variations in ultrasonic parameters.

Microwave synthesis of noncentrosymmetric BaTiO3 truncated nanocubes for charge storage applications

Truncated nanocubes of barium titanate (BT) were synthesized using a rapid, facile microwave-assisted hydrothermal route. Stoichiometric composition of pellets of nanocube BT powders was prepared by two-stage microwave process. Characterization by powder XRD, Rietveld refinement, SEM, TEM, and dielectric and polarization measurements was performed. X-ray diffraction revealed a polymorphic transformation from cubic Pm3̅m to tetragonal P4mm after 15 min of microwave irradiation, arising from titanium displacement along the c-axis. Secondary electron images were examined for nanocube BT synthesis and annealed at different timings. Transmission electron microscopy showed a narrow particle size distribution with an average size of 70 ± 9 nm. The remanence and saturation polarization were 15.5 ± 1.6 and 19.3 ± 1.2 μC/cm(2), respectively. A charge storage density of 925 ± 47 nF/cm(2) was obtained; Pt/BT/Pt multilayer ceramic capacitor stack had an average leakage current density of 5.78 ± 0.46 × 10(-8) A/cm(2) at ±2 V. The significance of this study shows an inexpensive and facile processing platform for synthesis of high-k dielectric for charge storage applications.