Development of high-quality ceramic powders for Sr0.9Ca0.1Zn2-W type hexagonal ferrite for permanentmagnet devices

Fine ceramic powders of Sr_0.9Ca_0.1Zn₂ Fe₁₆O₂₇ (Sr_0.9Ca_0.1Zn ₂-W) ferrite, of almost single domain grain size ~1 μm, have been synthesized successfully by sintering for a few hours a mixture of SrCO₃, CaCO₃, ZnO, and α-Fe₂ O₃ in stoichiometric amounts at an effectively low temperature (T_s) of ~1100°C. Carbon dioxide evolves in the reaction Sr(Ca)CO₃+2ZnO+8 α-Fe₂D₃ &lrarr2;Sr(Ca)Zn₂Fe₁₆O₂+C₂ , making fine pores in the sample, which prevent large grain growth of the material and result in a very loose powder. The small ⩽10% substitution of Sr by Ca activates the reaction, but preserves the crystallization of the small-sized grains. This yields magnetic properties useful for most permanent magnet applications, with a promisingly high coercivity H_c~3650 Oe and saturation magnetization M_s~65 emu/g. A considerably better saturation magnetization M_s~85 emu/g can be obtained by sintering the samples at higher T_s~1300°C, but that results in a rather very low coercivity H_c~105 Oe     

Effect of polymer adsorbed layer on magnetic particle dispersion

Experimental and theoretical investigations of dispersion properties of magnetic particles (acicular γ-Fe₂O₃ ) with an adsorbed epoxy-resin layer are presented. The adsorbed polymer amount was 20-110 mg/g and the adsorbed layer thickness was 6-22 nm, depending on the preparation conditions. Dispersion of γ-Fe ₂O₃ particles was evaluated by the sedimentation rate and surface roughness of the coated film. It was clarified that dispersion was improved with increasing adsorption, increasing solvent/epoxy-resin solubility, and decreasing γ-Fe₂O ₃ particle size. Interaction energy, consisting of magnetic attraction, van der Waals attraction, and steric repulsion of the adsorbed layer between two cylindrical magnetic particles, was calculated. The interaction energy curves, which vary with the distance between two particles, had primary minimum, maximum, and secondary minimum. The maximum energy and the secondary minimum energy were strongly related to dispersion stability. Experimental results are explained by the energy curve shapes     

Fe2O3 films for Ξ optics: Raman and XAS characterization

Two different sol-gel syntheses, both starting from inorganic salts as precursors, Fe(NO₃)₃ · 9H₂O or FeCl₃ - 6H₂O, have been employed to prepare, by dip-coating, films of -Fe₂O₃, which are currently investigated for their high non-linear optical susceptibility Ξ⁽³⁾. Thermal treatments at 500°C lead to the -Fe₂O₃ hematite structure for both preparations as indicated by XRD and Raman. Information on the differences in the local order between the two preparations are obtained by X-ray absorption spectroscopy (XAS) measurements.     

Calorimetric characterization of magnetic pigment surfaces

One of the most direct and sensitive ways of studying pigment surface properties is to measure the enthalpy changes which accompany the immersion of pigments in liquids. Different wetting liquids were used to test for H donor and H acceptor properties as well as to probe the polarity of the pigments' surfaces. The measurements were performed in an isoperibol batch calorimeter. Results are presented for γ-Fe ₂O₃, CrO₂, Co-modified γ-Fe₂ O₃, and barium ferrite pigments. It is found that Co-modified γ-Fe₂O₃ pigments are very polar, whereas CrO₂ pigments have low polarities: H donor and H acceptor properties are less pronounced. It is found that barium ferrite pigments act very much like γ-Fe₂O₃ pigments. It is shown that the effects of surface treatment can clearly be monitored by calorimetry     

Hydrothermal processings to produce magnetic particulates

α-Fe₂O₃, γ-Fe₂O₃ , and α-FeOOH powders were used as starting materials to prepare barium hexaferrite hydrothermally. Since strontium hexaferrite and lanthanum-doped calcium hexaferrite have coercivities similar to that of barium ferrite, the hydrothermal synthesis of strontium hexaferrite and lanthanum-doped calcium hexaferrite were also explored. The reaction products obtained with the various starting materials are described. Electron micrographs showed that α-Fe₂O₃, γ-Fe₂O₃, or α-FeOOH dissolved in the solution first, and then barium hexaferrite nucleated and grew from the solution     

Thermomagnetic study of surface cobalt modified iron oxideparticles derived from oxalic precursors

The cobalt modification of pure γ-Fe₂O₃ acicular particles by the so-called epitaxial process has been applied to γ-Fe₂O₃ particles, derived from oxalic precursors. Six elaboration parameters have been taken into account for the slurry heat treatment: cable content, pH, time, temperature, gas flowing into the solution, and surface activation. A thermomagnetic study including in situ annealing treatments and magnetic measurements at various temperatures in the range 20-380°C has been carried out. The results are compared with those of commercial surface-cobalt-modified pigments     

不同晶型Fe2O3催化H2O2湿法脱硝的实验研究

研究了以α-Fe₂O₃、β-Fe₂O₃和γ-Fe₂O₃为催化剂的类Fenton试剂溶液氧化吸收NO的过程,分析了3种Fe₂O₃的晶相结构和表面性质对NO脱除效率的影响机理。脱硝性能测试结果表明：γ-Fe₂O₃的活性最好,在H₂O₂浓度为1.5 mol/L、催化剂浓度为20 mmol/L、pH值为5以及反应温度为55℃等条件下,γ-Fe₂O₃的脱硝率可达87.5%。机理研究表明：3种Fe₂O₃催化H₂O₂分解湿法脱除NO的反应发生在催化剂表面,反应过程中存在氧化还原循环,H₂O₂催化分解的主要产物是·OH。活性差异分析结果表明：Fe₂O₃的晶相结构和表面性质对NO的脱除效果具有显著的影响,γ-Fe₂O₃的活性最高是由于比表面积大、分散性高和表面的Fe²⁺含量更多,而β-Fe₂O₃的活性高于α-Fe₂O₃是由于表面的氧空位含量更多。     

A device for measuring the concentration and dispersion quality ofmagnetic particle suspensions

A technique, called rheomagnetic measurement, for studying the concentration and orientation of magnetic particles through inductance measurement is presented. The particles are oriented in a predominantly extensional flow field, and, because they are magnetic, their orientation can be detected with a weak magnetic sensing field. Because flocs of magnetic particles orient differently in a flow field than primary particles do, this method can be useful in obtaining information about the particle flocculation aspect of dispersion quality. A magnetic sensing field can also be used to detect the particle concentration in a quiescent flow. Experimental data on the effects of particle concentration and milling for rod-like γ-Fe₂O₃ and plate-like Ba-ferrite suspensions are discussed. The results for Ba-ferrite magnetic markedly contrast with those for the rod-like magnetic particles but showed similarity with those for rod-like γ-Fe₂O₃     

Properties of the W-type hexaferritePbZn2Fe16O27

W-type Pb-hexaferrites were prepared by standard ceramic methods. The lattice constants found by refinement were a=0.59140±0.00006 nm and c=3.29209±0.00041 nm. The X-ray density of a typical composition PbZn_1.9Fe_15.3O_25.8 was ρ=5.32 g/cm³ and the Vickers microhardness value h _v=6 kN/mm². A plot of the saturation magnetization versus temperature is given. The extrapolated value of the saturation magnetization (H→∞, T→0) was σ_s=108 emu×g^-1, and the Curie temperature was T_c=600±20 K     

-Fe2O3 thin films prepared by metalorganic deposition (MOD) from Fe(III) 2-ethylhexanoate

-Fe₂O₃ thin films were prepared by metalorganic deposition (MOD) using Fe(III) 2-ethylhexanoate as the metalorganic precursor. A series of experiments were conducted on the metalorganic spin-coated films and their correspondingly annealed samples by employing experimental techniques ranging from thermal gravimetric analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) to optical property characterization. In this way a better understanding has been achieved regarding the decomposition process of the metalorganic precursor, the solid state -Fe₂O₃ film formation and crystallization process, and the relationship between the structure and the optical properties of the prepared films. The conclusions of our experiments are the following. The decomposition of Fe(III) 2-ethylhexanoate is a multistep process that is characterized by distinct transition temperatures and thermogravimetric loss rates. Amorphous -Fe₂O₃ film is formed at an annealing temperature of around 460°C, further annealing at higher temperatures induces the amorphous-to-crystalline phase transition and grain growth. FTIR, XRD and SEM data for structural characterization are correlated and in good agreement. A new FTIR absorption band, peaking at 1085 cm^-1, is assigned to the vibration of crystalline Fe-O mode, therefore this peak is useful in monitoring the amorphous-to-crystalline phase transition of -Fe₂O₃ material. Instead of columnar structure in physical vapour deposition-prepared films a granular structure is typical of MOD prepared films, the grain size is much larger near the surface of the film than near the substrate. Optical characterization shows that the refractive index and extinction coefficient of the -Fe₂O₃ thin films increase with the increase of annealing temperatures. The potential interesting applications of the MOD-prepared -Fe₂O₃ thin films include gas sensor materials, photoelectrodes and storage media.     

Magnetic properties of acicular BaFe12O19particles prepared by microwave plasma sintering

Acicular α-FeOOH particles coated with barium ions were irradiated with microwaves under an oxygen pressure of 20 torr to prepare fine BaFe₁₂O₁₉ particles by a plasma reaction. The fine acicular particles has an aspect ratio of about 13 and a magnetization of 43.5 emu/g and contained small amounts of α-Fe₂O₃ and BaFe₂O₄. The particles were mixed with a binder, and the resulting mixture was formed into a coating under a magnetic field of 7 kOe applied vertically to the surface of the film. The squareness ratio in the direction perpendicular to the surface of the coating film was 0.83 without any correction for the demagnetizing field. Transmission electron microscopy (TEM) showed that the long axes of the BaFe₁₂O₁₉ particles all were aligned parallel to the film base. The cross-sections of the particles were rectangular when observed by TEM. It follows that the easy axes of the particles are perpendicular to their long axes     

苯胺-SrTi(C2O4)2颗粒制备及其电流变性能

用沉淀法制备了苯胺-草酸氧钛锶（SrTi（C₂O₄）₂）颗粒。FTIR分析表明,苯胺已引入到SrTi（C₂O₄）₂颗粒的表面;XRD分析表明,苯胺-SrTi（C₂O₄）₂颗粒为含少量结晶的无定形态。采用SEM观察颗粒的形貌,结果表明,随反应体系中苯胺与钛原子物质的量比n_aniline/n_Ti的增大,苯胺-SrTi（C₂O₄）₂颗粒由近似球状（n_aniline/n_Ti =0）变为多面体状（n_aniline/n_Ti =2）,而后变为棒状与团簇状颗粒的混合体系（n_aniline/n_Ti =3）。以苯胺-SrTi（C₂O₄）₂颗粒为分散相制备颗粒质量分数为66.7%电流变液,电流变性能测试结果表明,在n_aniline/n_Ti =2时,电流变液具有较高的剪切应力和剪切屈服强度、较大的漏电流密度。苯胺在颗粒制备过程中起控制颗粒形貌的作用,在电流变液体系中起极性分子的作用,其对苯胺-SrTi（C₂O₄）₂电流变液性能的影响是两种效应综合作用的结果。     

可磁分离ZnFe2O4-TiO2/还原氧化石墨烯复合材料的制备及光催化性能

以TiO2(P25)、 Fe(NO3)3·9H2O、 Zn(NO3)2·6H2O和氧化石墨烯(GO)为原料,通过一步溶剂热法合成可磁分离的ZnFe2O4-TiO2/还原氧化石墨烯(rGO)复合材料。采用UV-Vis、 Raman、 XRD、 SEM和EDS对ZnFe2O4-TiO2/rGO复合材料进行表征,并研究不同rGO比例的ZnFe2O4-TiO2/rGO对模拟染料废水亚甲基蓝(MB)的光催化降解性能。GO在溶剂热反应过程中,被还原成rGO。由于ZnFe2O4和rGO的加入,不仅使ZnFe2O4-TiO2/rGO实现对可见光的吸收,而且使其具有磁性,便于分离和回收利用。当GO质量分数为5wt%时, ZnFe2O4-TiO2/rGO显现出对MB最佳的光催化活性, 60 min光照后的降解率达到99.1%。通过光催化活性物种捕获实验得出ZnFe2O4-TiO2/rGO复合材料降解MB的过程中,活性物种主要为·OH和·O2-, TiO2导带(CB)中的光生电子(e+)转移到ZnFe2O4的价带(VB),遵循Z型转移机制。光催化剂稳定性实验表明, ZnFe2O4-TiO2/rGO复合材料具有优越的稳定性,可作为太阳光照射下降解有机染料的光催化剂。     

Magnetic properties of GdBa2Cu3O7crystals in high magnetic field

The magnetic properties of single crystals of GdBa₂Cu ₃O₇ were investigated in a magnetic field of up to 20 T applied parallel to the c axis in the 1.7-300 K temperature range. In the superconducting state, the field and temperature dependences of the critical current densities were deduced from the hysteresis of the half-cycle using Bean's critical state model. The Gd³⁺ paramagnetic moment was then studied. Above about 20 K, the M(H) isotherms were found to be given, at different temperatures, by the Brillouin function of the free Gd³⁺ ion. Below 20 K, the average magnetization does not obey the Brillouin law. The normal-state susceptibility was described by the free-ion Curie-Weiss law     

Topotactic conversions of γ-FeOOH and comparative transformation methods of iron oxides in magnetic spinels

The DTA of a natural, well crystallized lepidocrocite sample containing small amounts of goethite, quartz and manganese shows that the conversion of γ-FeOOH to -Fe₂O₃ (through the γ-Fe₂O₃ metastable phase) occurs at 440°C, whereas in pure, synthetic γ-FeOOH the exothermic peak appears at ˜ 500°. Quartz (1.4% SiO₂) is not affected, nor does it hamper the transformation of FeOOH into magnetic spinels, when the sample is reacted with glycerol at reflux temperature followed by hydrolysis of the resulting solid (iron alkoxide) by boiling H₂O. Starting γ-FeOOH and final solid products are characterized by XRD and IR spectroscopy. This reaction is also discussed comparatively with the solid state transformations of iron oxides into magnetic materials by a combination of heating and grinding-pressing cycles of disks containing alkali halides as diluting agents.     

Al2O3颗粒形貌对注凝成型ZrO2/Al2O3陶瓷性能的影响

采用3种不同形貌的Al₂O₃原料对注凝成型制备ZrO₂/Al₂O₃（ZTA）陶瓷工艺中悬浮体的流变性能进行了研究。以低毒的单体N,N-二甲基丙烯酰胺（DMAA）制备了ZrO₂/Al₂O₃坯体和陶瓷。讨论了3种不同形貌的Al₂O₃原浆料的分散剂用量、球磨时间和固含量对浆料流变性的影响。Al₂O₃粉体呈扁平状有利于降低浆料的黏度,Al₂O₃粉体呈棒状对生坯强度的提高有利。制得的3种ZrO₂/Al₂O₃坯体颗粒间结合紧密,抗弯强度分别达到21.45,19.87,25.90 MPa。Al₂O₃粉体呈颗粒状有利于最终陶瓷力学性能的提高,陶瓷的抗弯强度及断裂韧性分别为680 MPa和7.49 MPa·m^1/2,453.1 MPa和6.8 MPa·m^1/2,549.4 MPa和6.34 MPa·m^1/2。     

Fe/Al2O3复合材料的制备和性能

用石墨埋烧方法制备Fe/Al₂O₃复合材料,对其力学性能和微观结构进行了分析。结果表明:Fe/Al₂O₃复合材料的弯曲强度与断裂韧性均随Al₂O₃含量的升高先升高后降低,当Al₂O₃含量（质量分数）为70%时,其弯曲强度与断裂韧性分别达到602.49 MPa和9.33 MPa·m¹/2,其硬度随Al₂O₃含量先降低后升高。在烧结过程中在Fe颗粒周围形成一种成分为FeO与FeAl₂O₄的壳体,在壳体与Fe颗粒之间存在微裂纹缺陷。壳体的形成和壳体与金属颗粒间的微裂纹钝化了外部应力,从而提高了复合材料的韧性。     

Magnetic properties of Sm2Fe17Nxpowder and bonded magnet

Sm₂Fe₁₇N_x powders were prepared through metallurgical processes such as melting, casting, pulverizing, and gas nitrogenation. Epoxy resin and zinc-bonded magnets were prepared from the powder by a compression molding process. The magnetic properties of magnetically aligned Sm₂Fe₁₇N_x powder were (BH)_max=170 kJ/m³ (21.4 MG-Oe), Br=1.20 T, and H_cJ=547 kA/m (6.87 kOe). Those of an epoxy-bonded magnet were (BH)_max =103 kJ/m³ (13.0 MG-Oe) and Br=0.835 T. H _cJ values in excess of 2 MA/m (25 kOe) were observed in the case of a zinc-bonded magnet     

Magnetic properties of BaFe12O19 smallparticles

The magnetic properties of small BaFe₁₂O₁₉ particles about 50 nm in size have been studied; the emphasis was on small-size effects. The compound formation of the hexagonal structure was made at a relatively low temperature; the small particle size made this result possible. As compared to the bulk value, a significantly reduced saturation magnetization is observed for these particles; this agrees with similar reports made earlier by several authors. To examine these phenomena further, Mossbauer spectra were obtained with high magnetic field applied longitudinally (H_ex=16.4 and 50 kOe). The spectra show the presence of recalcitrant spins where the spin configuration has a noncollinear arrangement that even a 50-kOe magnetic field does not remove. This hard-to-saturate component may lie in the surface layer of the Ba-ferrite small particles. The data also provide evidence that the Fe ions in the bipyramidal (2b) sites undergo magnetic-field-induced oscillations at 4.2 K     

Preparation of nanocrystalline SnO2 thin film coated Al2O3 ultrafine particles by fluidized chemical vapor deposition

Fluidized chemical vapor deposition (FCVD) technology was developed for coating SnO₂ thin film on Al₂O₃ ultrafine particles. TEM and HREM analysis found that SnO₂ films with different structures were deposited by controlling the coating temperature, reactant concentration, etc. Nanocrystalline SnO₂ film was coated at 573.15 K by gas phase reaction of SnCl₄ with H₂O. EPMA and EDS studies indicated that the distribution of SnO₂ inner and outer of the agglomerates was uniform. Nucleation and film deposition were coexisted mechanism during the FCVD coating process. The fraction of SnO₂ in the composite particles increased with increasing coating temperature, SnCl₄ concentration, and coating time. The mass fraction of SnO₂ in the composite particles increased strongly with the ratio of P_H2O and P_SnCl4 at low mole ratio of H₂O with SnCl₄, but increased little under the conditions of excess H₂O with respect to SnCl₄.