Novel microwave assisted chemical synthesis of Nd₂Fe₁₄B hard magnetic nanoparticles

Design of modern magnetic materials with giant coercivity

The review is devoted to compounds and materials demonstrating extremely high magnetic hardness. The recent advances in the synthesis of modern materials for permanent magnets are considered, and a range of exotic compounds interesting for fundamental research is described. The key details of chemical composition, crystal structure and magnetic microstructure responsible for the appearance of high magnetic anisotropy and giant coercivity are analyzed. The challenges of developing the title materials are noted and strategies for their solution are discussed.

The bibliography includes 389 references.

Investigation on Crystallization and Magnetic Properties of (Nd, Pr, Ce)2Fe14B/α-Fe Nanocomposite Magnets by Microwave Annealing Treatment

In the present work, the structures and magnetic properties of (Nd, Pr, Ce) ₂Fe₁₄B/α-Fe nanocomposite magnets were thoroughly investigated. The microwave annealing was applied to achieve a uniform heating effect and uniform grains. Microwave annealing is more favorable to obtain α-Fe phase than conventional annealing, which leads to the enhanced coercivity of hysteresis loops. The coercivity of nanocomposite magnets was 245 kA/m after annealing at 2000 W for 10 min.

High coercivity Pr2Fe14B magnetic nanoparticles by a mechanochemical method

Nd₂Fe₁₄B nanoparticles are widely used because of their outstanding hard magnetic properties. In fact, Pr₂Fe₁₄B has higher magneto-crystalline anisotropy than Nd₂Fe₁₄B, which makes Pr-Fe-B a promising magnetic material. However, the chemical synthesis route to Pr₂Fe₁₄B nanoparticles is challenging because of the higher reduction potential of Pr³⁺, as well as the complex annealing conditions. In this work, Pr₂Fe₁₄B nanoparticles were successfully synthesized via an efficient and green mechanochemical method consisting of high energy ball milling, annealing, and a washing process. Microstructural investigations revealed that the oxide precursors were uniformly wrapped by CaO and CaH₂, which formed an embedded structure after ball milling. Then, Pr₂Fe₁₄B powder was synthesized via a time-saving annealing process. The impact of the Pr₂O₃ content and the preparation conditions was investigated. The coercivity of the as-annealed powder with 100 wt% Pr₂O₃ excess is 18.9 kOe. After magnetic alignment, the coercivity, remanence, and maximum energy product were: 9.8 kOe, 78.4 emu g⁻¹, and 9.8 MGOe, respectively. The present work provides a promising strategy for preparing anisotropic Pr-Fe-B permanent magnetic materials.

Nd₂Fe₁₄B nanoparticles are widely used because of their outstanding hard magnetic properties.

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.

First-order-reversal-curve analysis of rare earth permanent magnet nanostructures: insight into the coercivity enhancement mechanism through regulating the Nd-rich phase

The coercivity enhancement mechanism of Nd₂Fe₁₄B-based nanostructures with Nd-rich phase is revealed by first-order-reversal-curve diagram, which is that increased Nd-rich phase content leads to optimized magnetic interactions and microstructure.

Rare earth permanent magnetic nanostructures: chemical design and microstructure control to optimize magnetic properties

The routes for the optimization of the magnetic properties of rare earth permanent magnetic nanostructures are discussed, i.e. the control of microstructure, such as size and shape as well as the exchange-coupling interactions.

Insight into the Property Enhancement Mechanism of Chemically Prepared Multi-Main-Phase (Nd,Ce)2Fe14B

Nd₂Fe₁₄B has attracted intensive attention because of its excellent magnetic properties since 1980s. However, large demands for the expensive rare earth (mainly refers to Nd/Pr/Dy) limit its wider applications. Investigations of Ce-doped Nd₂Fe₁₄B have been attempted recently and multi-main-phase (MMP) (Nd,Ce)₂Fe₁₄B provides a promising way for the preparation of high-performance Ce-doped permanent magnets even though the inner mechanism has not been absolutely understood. We synthesized Ce-doped Nd₂Fe₁₄B nanostructures by the chemical method and successfully realized the obvious property enhancement of the MMP sample compared with that of the single-main-phase one. The coercivity of the MMP nanostructures is nearly 4.5 kOe with a remanence ratio of 0.36 before magnetic orientation, which is much larger than that of the SMP sample (1.7 kOe and 0.21), respectively. The property enhancement mechanism of the MMP sample analyzed mainly by first-order reversal curves could be concluded in three aspects: first, the content of α-Fe will be decreased; hence, the difficulty of the magnetic nucleation is increased. Second, the exchange coupling effect between the adjacent magnetic structures will be strengthened significantly. Last, the grain boundary phases with various magnetic features are formed, enhancing the magnetic pinning effect and specially tuning the inner interactions. This work is helpful for the deeper understanding of the property enhancement mechanism in MMP nanomagnets and provides an instructive way for the effective design and preparation of high-performance MMP Ce-doped Nd₂Fe₁₄B nanomagnets.

Evolution of microstructure and formation mechanism of Nd-Fe-B nanoparticles prepared by low energy consumption chemical method

Nd2Fe14B nanoparticles were successfully prepared by using a low-energy chemical method. The microscopic characteristics and formation mechanisms of the phases were investigated at each stage during the preparation of Nd-Fe-B nanoparticles. The Nd-Fe-B intermediates, Nd-Fe-B oxides and reduced Nd-Fe-B nanoparticles were detected and analyzed by using TEM, STEM, XRD, SEM, VSM and Rietveld calculations. The results showed that the intermediate of Nd-Fe-B consisted of Fe3O4 and Nd and Fe elements surrounded by nitrile organic compounds. The Nd-Fe-B oxide was composed of NdFeO3 (48.619 wt%), NdBO3 (31.480 wt%) and α-Fe (19.901 wt%), which was formed by the reaction among Nd, Fe3O4 and B2O3. NdFeO3 and NdBO3 exhibited a perovskite-like lamellar structure, and the grain size was smaller than that of α-Fe. Nd-Fe-B particles were mainly composed of Nd2Fe14B and α-Fe phases. The small particles of NdFeO3 and NdBO3 and the interstitial position between oxide particles and α-Fe were more favorable for the formation of Nd2Fe14B particles. At the same time, the surface of α-Fe particles can also diffuse to form Nd2Fe14B nanoparticles. The coercivity of Nd-Fe-B particles was 5.79 kOe and the saturation magnetization was 63.135 emu g-1.

Synthesis and reaction mechanism of high (BH)max exchange coupled Nd2(Fe,Co)14B/α-Fe nanoparticles by a novel one-pot microwave technique

Nd–Fe–B based magnets, exhibiting the high energy product, synthesized by cost-effective one pot microwave approach.

Mechanochemical synthesis of high coercivity Nd2(Fe,Co)14B magnetic particles

With increasing demand for magnets in energy conversion systems, the quest for the development and understanding of novel processing routes to produce permanent magnets has become urgent. We report a novel mechanochemical process for the synthesis of Nd₂(Fe,Co)₁₄B magnetic particles with a high coercivity of 12.4 kOe. This process involves the reduction of neodymium oxide, iron oxide, cobalt oxide and boron anhydride in the presence of a calcium reducing agent and a CaO diluent. The formation mechanism of Nd₂(Fe,Co)₁₄B changed with increasing CaO content, and the average crystal size of the Nd₂(Fe,Co)₁₄B particles also increased, resulting in an increase in the coercivity values. The reaction mechanism during milling was revealed through a study of the phase transformations as a function of milling time. It was found that unlike self-propagating reactions, this reduction reaction during milling requires continuous input of mechanical energy to reach a steady state.

High energy product chemically synthesized exchange coupled Nd2Fe14B/α-Fe magnetic powders

The excellent hard magnetic properties of Nd₂Fe₁₄B based magnets have an enormous range of technological applications. Exchange-coupled Nd₂Fe₁₄B/α-Fe magnets were chemically synthesized by a microwave assisted combustion process to produce mixed oxides, followed by a reduction diffusion process to form magnetic nano-composite powder. This synthesis technique offers an inexpensive and facile platform to produce exchange coupled hard magnets. The size dependent magnetic properties were investigated. The formation mechanisms of the oxide powders and the reduction diffusion mechanism were identified. The microwave power was found to play a crucial role in determining the crystallite size. The coercivity of the powder increased with increasing particle size. Room temperature coercivity (H_c) values greater than 9 kOe and magnetization of 110 emu g^-1 was obtained in particles with a mean size of ∼62 nm. An energy product of 5.2 MGOe was obtained, which is the highest reported value for chemically synthesized hard magnetic Nd₂Fe₁₄B/α-Fe powders.

Facile Synthesis of MgO-Modified Carbon Adsorbents with Microwave- Assisted Methods: Effect of MgO Particles and Porosities on CO2 Capture

In this study, magnesium oxide (MgO)-modified carbon adsorbents were fabricated using a nitrogen-enriched carbon precursor by microwave-assisted irradiation for CO₂ capture. The X-ray diffraction (XRD) patterns showed the characteristic diffraction peaks of MgO at 43° and 62.5°, and no impurities were apparent. By changing the microwave reaction time, the spherical structure of the parent material was transformed to a hybrid structure with MgO crystalline particles in a carbon matrix. The morphology evolution and properties of the prepared materials were also investigated using transmission electron microscopy and N₂ adsorption, respectively. On optimising the conditions, the prepared sample attained a high CO₂ uptake of 1.22 mmol/g (5.3 wt.%) under flue gas conditions (15% CO₂ in N₂). It was found that MgO affected the CO₂ capture behaviour by enhancing the fundamental characteristics of the carbon surfaces.

Chemical synthesis of Nd2Fe14B hard phase magnetic nanoparticles with an enhanced coercivity value: effect of CaH2 amount on the magnetic properties

Nd₂Fe₁₄B hard phase magnetic nanoparticles were successfully synthesized using a chemical synthesis route followed by a reduction and diffusion process without consuming a large amount of energy.

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.