Phase stability and magnetic properties of SrFe18O27 W-type hexagonal ferrite

W-type ferrite is a member of the hexagonal ferrite family and a potential permanent magnet material. However, its synthesis conditions are not fully understood yet. Samples were sintered either at 1400°C in air and quenched, or at 1300°C at reduced oxygen partial pressure. The precise stability conditions of this W-type ferrite were investigated in the temperature range of 1200°C-1400°C using thermogravimetry, XRD, and electron microscopy. At 1300°C, the ferrite is stable at oxygen partial pressures of . At more oxidizing conditions, the ferrite decomposes into M-type ferrite and hematite, while at more reducing atmospheres Sr₄Fe₆O₁₃ and magnetite are formed. The nonstoichiometry δ of SrFe_18−δO₂₇ was derived from thermal analysis data at 1300°C as function of oxygen partial pressure and was found to be mainly due to cation vacancies. Magnetization measurements show that this W-type ferrite exhibits M_s = 103 emu/g at T = 4 K, which agrees well with a ferrimagnetic spin arrangement according to Gorter's model. As alternative, Zn-substituted W-ferrite was found to be stable in air at 1200°C with a large M_s = 123 emu/g at 4 K.     

自蔓延高温合成SrFe12O19的工艺研究

介绍利用自蔓延高温合成技术合成SrFe12O19的工艺过程以及对合成产物的分析。采用X射线衍射(XRD)分析产物的物相；采用透射电子显微镜(TEM)观察其微观结构，晶粒形状及尺寸；采用振动样品磁强计(VSM)测试粉末磁性能，Hc为2559．08奥，4π Ms为3468．3高斯。试验结果表明：采用自蔓延高温合成技术可合成高纯度的SrFe12O19，具有良好稳定的磁性能。     

Fast synthesis of SrFe12O19 hexaferrite in a single-mode microwave cavity

The M-type SrFe₁₂O₁₉ hexaferrite has been synthesized by a microwave solid state reaction process – a fast heating process – in a home-made 2.45 GHz single-mode microwave cavity. Starting from SrCO₃ and Fe₂O₃ mixtures (1:6 ratio), cold pressed samples have been heated up in the microwave electric field without needing any susceptor, demonstrating the good coupling of the precursors with this microwave mode in our experimental setup.After optimization of the experimental conditions, the properties of the obtained ceramics, including structure, microstructure and magnetic properties, are compared with those of ceramics synthesized by conventional solid state reaction. With that microwave process, it is found that SrFe₁₂O₁₉ ceramics prepared in less than 30 min exhibit magnetic properties similar to those of the same compound produced by a conventional process. This highlights the potentialities of the technique to synthesize hexaferrite ceramics.     

A novel magnetic photocatalyst Bi3O4Cl/SrFe12O19: Fabrication,characterization and its photocatalytic activity

A novel Bi₃O₄Cl/SrFe₁₂O₁₉ magnetic photocatalyst has been fabricated by a hydrothermal-roasting method. Its structure and performance were studied through XRD, XPS, SEM, UV–vis DRS, PL, EIS and VSM, and the photocatalytic property was tested by photodegradation of RhB irradiation with solar-simulated light. The optimum sample of Bi₃O₄Cl/SrFe₁₂O₁₉(20%) displayed the excellent photocatalytic activity, and the degradation rate was more than 99.7% in 80 min. In addition, the Bi₃O₄Cl/SrFe₁₂O₁₉(20%) showed the quite high magnetic performance, of which the saturation magnetization was 7.65 emu·g⁻¹ and the recovery percent was 92.4%. Meantime, the product revealed the great photocatalytic stability, with the photodegradation leave reaching 84.1% after five cycles. The trapping experiments demonstrated that the hydroxyl radicals, photoproduction holes and superoxide radicals played a critical part in photodegrading of RhB. This research provided a new method to synthesize composite magnetic photocatalyst and studied the possible photodegradation mechanism.     

Hydrothermal synthesis,magnetic properties and characterization of CoFe2O4 nanocrystals

Magnetic cobalt ferrite (CoFe₂O₄) nanocrystals were synthesized via the hydrothermal method and the crystallite size was measured using Sherrer's equation. Instrumental broadening was a significant parameter for determining crystallite size. The effect of annealing time and calcination on crystallite size and magnetic properties was discussed. It was found that the coercivity was highly dependent on the crystallite size. As the crystallite size increased from 61 to 68.2 nm, room temperature coercivity increased from 1488 Oe to 1700 Oe, while high coercivity (5.2 kOe) was achieved at lower temperature (80 K). It was found that the presence of hematite could affect the crystallite size after calcination.     

Synthesis of aligned La3+-substituted Sr-ferrites via molten salt assisted sintering and their magnetic properties

M-type hexagonal Sr_1−xLa_xFe₁₂O₁₉ (x=0, 0.05, 0.1, 0.15 and 0.2) ferrites were synthesized in a molten salt matrix to avoid the agglomeration of particles and control grain growth. By eliminating the impurity phase, the influence of rare earth La³⁺ ion substitutions on the lattice structure, morphology and magnetic properties has been investigated. The results show that La³⁺ substitutions have a great influence not only on the grain growth but also on the magnetic orientation. The easy magnetization direction <001> was remarkably enhanced in all substituted powders aligned along a 5 kOe magnetic field. The optimal magnetic properties were obtained via magnetic orientation when the substitution is x=0.15, showing a coercivity of 6.25 kOe, a saturation magnetization of 69.5 emu/g and a remanence ratio of 0.71. This will possibly enable a wide-range application to generate rare earth La³⁺ ion substituted Sr-ferrites for bonded magnets via molten salt assisted sintering.     

Greener processing of SrFe12O19 ceramic permanent magnets by two-step sintering

With an annual production amounting to 800 kilotons, ferrite magnets constitute the largest family of permanent magnets in volume, a demand that will only increase as a consequence of the rare-earth crisis. With the global goal of building a climate-resilient future, strategies towards a greener manufacturing of ferrite magnets are of great interest. A new ceramic processing route for obtaining dense Sr-ferrite sintered magnets is presented here. Instead of the usual sintering process employed nowadays in ferrite magnet manufacturing that demands long dwell times, a shorter two-step sintering is designed to densify the ferrite ceramics. As a result of these processes, dense SrFe₁₂O₁₉ ceramic magnets with properties comparable to state-of-the-art ferrite magnets are obtained. In particular, the SrFe₁₂O₁₉ magnet containing 0.2% PVA and 0.6% wt SiO₂ reaches a coercivity of 164 kA/m along with a 93% relative density. A reduction of 31% in energy consumption is achieved in the thermal treatment with respect to conventional sintering, which could lead to energy savings for the industry of the order of 7.10⁹ kWh per year.     

Influence of post-synthesis NaCl flux treatment on the magnetic properties of jet-milled SrFe12O19 powders

A post-synthesis NaCl flux treatment was carried out on jet-milled strontium hexaferrite, SrFe₁₂O₁₉ powders prepared by conventional high-temperature solidstate reaction starting from SrCO₃ and Fe₂O₃. Microstructural studies reveal that the adverse effects of jet-milling on the particle morphology like jagged edges and ruptured surfaces have been effectively mitigated by annealing at elevated temperatures in the presence of molten NaCl flux. The coercivity values obtained from angle-dependent M versus H measurements revealed that the coercivity mechanism in the jet-milled powders is dominated by reverse nucleation due to the strain induced during milling. Annealing the powders in presence of NaCl flux changes the coercivity mechanism to exclusively domain rotation. This results in a dramatic increase of coercivity from 1.6 kOe for the jet-milled powders to a maximum of 5.3 kOe for the flux-annealed powders. XPS studies show that the NaCl flux treatment has not altered the chemical state of Fe.     

Synthesis,characterization and magnetic properties of polythiophene/SrFe12-x(ZrZn)0.5xO19 nanocomposite as a microwave absorber

Ferrites are an important group of magnetic materials which are used as absorbers. The incorporation of ferrite and conducting polymer achieves great enhancement in microwave absorption properties. The nanocomposites of hexagonal ferrites embedded by conducting polymers such as polypyrrole, polyaniline and polythiophene (PTH) have been paid much attention. In the present study, strontium hexagonal ferrite doped by Zr and Zn with the final formula of SrFe_12-x(ZrZn)_0.5xO19 considering x = 0.9 and embedded by PTH was produced to achieve a nanocomposite with the highest microwave absorbing ability. In this study, after synthesis of SrFe₁₂O₁₉(ZrZn)_0.5xO19 and PTH, the nanocomposite was prepared by in situ polymerization. Wrapping the ferrite particles and PTH chains could form nanocomposite properly, and therefore acceptable interactions were observable between SrFe_12-x(ZrZn)_0.5xO19ferrite particles and PTH polymer chains in the composites. Assessing the X-ray diffraction (XRD) patterns of SrFe_12-x(ZrZn)_0.5xO19, PTH, and PTH/SrFe_12-x(ZrZn)_0.5xO19 nanocomposite indicated that the PTH characteristic peak shifts slightly and its peak intensity reduces, which may be attribute to the coating of PTH polymer chains onto SrFe_12-x(ZrZn)_0.5xO19 particles. We revealed also lower magnetic properties in the obtained nanocomposite. The morphological assessment also suggested that PTH could effectively coat the SrFe_12-x(ZrZn)_0.5xO19 particles. The synergistic effect of SrFe_12-x(ZrZn)_0.5xO19 particle plus PTH leads to microwave absorption percentage higher than 95% by PTH/SrFe_12-x(ZrZn)_0.5xO19 nanocomposite. Overall, nanocomposite creating by coupling interaction between SrFe_12-x(ZrZn)_0.5xO19 particles (x = 0.9) and PTH can effectively lead to achieve the highest rate of absorption of electromagnetic waves.     

聚苯胺/钡铁氧体的制备及性能研究

分别采用柠檬酸溶胶-凝胶法和原位聚合法制备了钡铁氧体粒子和聚苯胺-钡铁氧体复合物。利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、四探针测试仪等分析手段表征了复合材料的结构,观察了其形貌,测试了其性能。结果表明,钡铁氧体粒子为M型六角晶系片状晶相结构,粒径在10μm～50μm。聚苯胺/钡铁氧体复合材料的电导率随钡铁氧体含量的增加而下降,当BaFe12O19质量分数增加到10%时,电导率从8.85×10-2S/cm急剧下降到2.98×10-2S/cm。而后,随着钡铁氧体含量的增加,电导率变化不大。     

A comparative study of spinel ZnFe2O4 ferrites obtained via a hydrothermal and a ceramic route: Structural and magnetic properties

The spinel ZnFe₂O₄ specimens were obtained via a hydrothermal and a ceramic method, respectively, and their structural and magnetic properties were comparatively studied. It was found that all the specimens exhibited a single-phase and mixed spinel structure. The magnetism of specimens synthesized via the hydrothermal method is obviously better than that of specimen prepared via the ceramic method. This can be ascribed to the different occupancy of Fe ions resulted from the loss of Zn during the hydrothermal process.     

Role of exchange coupling between the hard and soft magnetic phases on the absorption of microwaves by SrFe10Al2O19/Co0.6Zn0.4Fe2O4 nanocomposite

(1-x)SrFe₁₀Al₂O₁₉/(x)Co_0.6Zn_0.4Fe₂O₄-(SFAO/CZFO) hard/soft nanocomposite ferrite materials were synthesized by ‘one-pot’ self-propagating combustion route. The co-existence of the two magnetic phases were confirmed by XRD, FESEM, EDS and VSM. The prepared nanocomposite samples were also characterized by TGA/DSC, Raman spectroscopy and VNA. Exchange coupling between the hard and the soft magnetic grains was observed by determining the switching field distribution (SFD) curve. As a result of the competing effects of exchange interaction and dipolar interaction, magnetic parameters were observed to be sensitive to the incorporation of soft magnetic phase into the nanocomposite. Results showed that with the inclusion of soft magnetic phase, exchange coupling behaviour between the hard and the soft ferrite phases had significant influence on the microwave absorption capacity of the samples. Related electromagnetic parameters and impedance matching ratio of the nanocomposite system were discussed. A minimum reflection loss of ?42.9 dB with an absorber thickness of 2.5 mm was attained by the nanocomposite (90 wt%)SrFe₁₀Al₂O₁₉/(10 wt %)Co_0.6Zn_0.4Fe₂O₄ at a matching frequency of 11.45 GHz. This assured the candidacy of SrFe₁₀Al₂O₁₉/Co_0.6Zn_0.4Fe₂O₄ nanocomposite as a promising microwave absorption material in the X-band (8–12 GHz).     

Synthesis and Magnetic Properties of Nearly Monodisperse CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>Nanoparticles Through a Simple Hydrothermal Condition

Nearly monodisperse cobalt ferrite (CoFe₂O₄) nanoparticles without any size-selection process have been prepared through an alluring method in an oleylamine/ethanol/water system. Well-defined nanospheres with an average size of 5.5 nm have been synthesized using metal chloride as the law materials and oleic amine as the capping agent, through a general liquid–solid-solution (LSS) process. Magnetic measurement indicates that the particles exhibit a very high coercivity at 10 K and perform superparamagnetism at room temperature which is further illuminated by ZFC/FC curves. These superparamagnetic cobalt ferrite nanomaterials are considered to have potential application in the fields of biomedicine. The synthesis method is possible to be a general approach for the preparation of other pure binary and ternary compounds.     

纳米SrFe12O19化学法制备的研究进展

综述了纳米SrFe12O19的化学制备方法的研究进展,主要包括有机树脂法,自蔓延高温合成法,金属有机物水解法,共沉淀法,溶胶-凝胶法,水热法,微乳液法,熔盐法等,并列出一部分合成实例.最后指出了纳米SrFe12O19化学制备方法的发展方向.     

Facile hydrothermal synthesis of cobalt manganese oxides spindles and their magnetic properties

In the present work, CoMnO₃, Co₃O₄ and Mn₂O₃ powders were prepared by using a simple hydrothermal method, and the urea and polyvinyl alcohol (PVA) were served as the precipitator and surface active agent, respectively. The CoMnO₃ and Mn₂O₃ exhibit the same spindle-like structure with the size of several micrometers, while the Mn₂O₃ displays the nanosheet morphology, the as-prepared materials are all high degree of crystallinity. In addition, the magnetic properties discussion indicates that the as-prepared materials exhibit ferromagnetic and antiferromagnetic behaviors at 5 K and 300 K, respectively.     

Effect of organic solvent on the cold sintering processing of SrFe12O19 platelet-based permanent magnets

In this work we have investigated the effect of the solvent during the processing of SrFe₁₂O₁₉ platelet-based permanent magnets by cold sintering process (CSP) plus a post-thermal treatment. Several organic solvents: glacial acetic acid, oleic acid and oleylamine have been analyzed, optimizing the CSP temperatures at 190?270 °C, under pressures of 375?670 MPa and 6?50 wt% of solvent. Modifications in the morphological and structural properties are identified depending on the solvent, which impacts on the magnetic response. Independently of the solvent, the mechanical integrity of ferrite magnets obtained by CSP is improved by a post-annealing at 1100 °C for 2 h, resulting in relative densities around 92 % with an average grain size of 1 μm and a fraction of SrFe₁₂O₁₉ phase >91 %. H_C ≥ 2.1 kOe and M_S of 73 emu/g are obtained in the final sintered ceramic magnets, exhibiting the highest H_C value of 2.8 kOe for the magnet sintered using glacial acetic acid.     

The role of Eu2O3 and CeO2 on the phase formation,infrared emission and magnetic properties of Co0.6Zn0.4Fe2O4 ceramics by a solvothermal method

Nanosized Eu₂O₃ and CeO₂ co-addition CoZn ceramics have been achieved via a hydrothermal method by adjusting the mol ratios of Eu and Ce. The as-prepared samples were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), Fourier transform infrared (FTIR), Vibrating sample magnetometer (VSM) and Infrared emission measurement (IRE-2). The particle morphologies of the as-prepared samples evolve from spherical, to self-assembled nanoparticles, and irregular nanoparticles when the mol ratios (x) of Eu and Ce was changed from 0:10 to 10:0. Correspondingly, the main phases of the as-prepared samples change from both cubic spinel CoFe₂O₄ and CeO₂, pure cubic cerianite CeO₂, to amorphous. Meanwhile, the as-prepared samples appear transformed from a ferromagnetic behavior with a saturation magnetization 66.4 emu/g to a paramagnetic behavior with a saturation magnetization of 0.55 emu/g at turning point x=3.5:6.5. While the infrared emissivity is increasing as the x from 0:10 to 3.5:6.5, reach the maximum at 3.6:6.4, and then remain stable when further increasing x till 10:0. Those may be due to the amorphous tendency rising and the particle sizes gradual decreasing with x increasing from 0:10 to 10:0. What is more important is that the solvothermal method is proved to be an efficient way to prepare CoZn nano-ceramics in this study which may open new pathways to magnetic and far infrared therapy.     

高性能SrLaxFe12—xO19超微粉的合成与表征

本文首次利用柠檬酸法合成了ＳｒＬａｘＦｅ１２－ｘＯ１９超微粉末，用Ｘ射线衍射仪，Ｍｏｓｓｂａｕｅｒ谱，振动样品磁强计，透射电子显微镜等仪器对粉末的结构、性能及形貌进行表征。考察了合成条件对性能的影响。结果表明，Ｌａ＾３＋可以明显提高ＳｒＦｅ１２Ｏ１９的磁学性能，在ｘ＝０．３，即ＳｒＬａ０．３Ｆｅ１１．７Ｏ１９，其粉末的矫顽力达到５１７．４ｋＡ／ｍ（６５００Ｏｅ），比饱和磁化强度σｓ为７１．４３ｅｍ     

用于聚氨酯醇解的磁性固体碱催化剂CaOMgO SrFe12O19

聚氨酯硬泡降解主要选用1,4-丁二醇为主醇解剂,碱金属氢氧化物为助醇解剂,但多元醇对钾钠离子比较敏感,要求钾钠离子的含量少于10×10^-6,否则可能产生凝胶,使产品报废。文章以选用自制的磁性固体碱催化剂不仅具有碱金属氢氧化物的催化效果,同时又避免因钾钠离子过量产生的凝胶,还兼具催化剂重复使用、易于分离等优点。