Impurity-Induced Exaggerated Grain Growth in Mn-Zn Ferrites

The effects of 20 oxide dopants on the microstructure development of Mn-Zn ferrite during sintering were investigated by diffusion-couple-like experiments. Nine oxides enhanced grain growth in ferrite; the effects of TiO₂ dopant were studied in detail. The TiO₂-induced exaggerated grain growth shows a parabolic time dependence, as was also true for the diffusion of Ti into ferrite as shown by X-ray spectrometry. For samples sintered at a given temperature but for different times, the Ti concentrations at the growth fronts of the exaggerated grains are identical. Exaggerated grain growth kinetics and Ti diffusivities have similar activation energies. Furthermore, the mobility of the exaggerated grains is reduced to that of the matrix grains after the TiO₂ layer is removed, showing that the exaggerated grain growth is sustained by Ti diffusion into ferrite. Probable mechanisms for TiO₂-promoted exaggerated grain growth are proposed. Some self-consistent arguments are presented for the observed effects of other dopants. Grain boundary mobilities of undoped Mn-Zn ferrite were measured and are in good agreement with those reported in the literature.     

Interface Reactions between Silicon Dioxide-Lead Oxide Glass and Manganese Zinc Ferrite

The interface reations between SiO₂–PbO melt and Mn-Zn ferrite were studied using electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Intermediate layers were formed at the interface between the glass and the Mn-Zn ferrite which were heated at 800° and 900°C, although those layers were not found in specimens heated at 1000°C. Using EPMA and XRD, the intermediate layers were found to be Pb₂(Mn, Fe)₂Si₂O₉ and Pb₈(Mn, Fe)Si₆O₂₁. The mechanisms of interface reactions are discussed, related to glassforming regions. It was concluded that the interface reaactions between SiO₂–PbO melt and Mn-Zn ferrite are controlled by the dissolution of Zn ions and Mn ions from the Mn-Zn ferrite.     

Effect of Powder Parameters on Grain Growth in Manganese-Zinc Ferrite

The effect of calcination and ball milling on the grain growth in Mn-Zn ferrite is presented. Rates of grain growth and the effect of ball milling on the growth behavior were observed for ferrite powders calcined above and below the recrystallization temperature. It is shown that in addition to particle size and distribution, calcination temperature was a critical factor responsible for the growth behavior of ferrite.     

Quantitative Analysis of Zinc Vaporization from Manganese Zinc Ferrites

Zinc vaporization of Mn-Zn ferrites was quantitatively characterized in terms of oxygen partial pressure P _O2, temperature, grain size and sample geometry. The amount of zinc loss was measured as a function of time at various temperatures by a thermogravimetric method. The weight loss due to irreversible zinc vaporization showed a linear behavior with time and increased exponentially with temperature. The observed weight loss due to zinc evaporation at 1100°C was small, whereas a significant weight change was detected at 1200°C. The weight loss was even greater in a reducing atmosphere ( P _O2= 5 × 10⁻⁵). Below 1300°C, the diffusion of elemental zinc was sufficiently fast to compensate the zinc loss at the interface region, resulting in a linear dependence on time. At temperatures ≥1400°C, the weight change no longer followed the linear dependence and showed a rather parabolic behavior with a concave upward slope. The core shape and the gas flow around ferrite cores were important factors that affected the rate of zinc vaporization, but not the grain size.     

微波法制备纳米铁酸锌及其晶粒生长动力学

纳米铁酸锌广泛应用于催化和材料领域,为了避免传统焙烧法高耗能的缺点,本实验选用高效微波法制备纳米铁酸锌,并与常规焙烧进行对比,研究了铁酸锌晶粒生长动力学。采用FTIR、XRD和SEM对样品进行表征。结果表明,相同焙烧温度和焙烧时间下,微波法制备的铁酸锌比常规制备的样品结晶度高,颗粒大小更均匀。通过谢乐公式计算不同温度下铁酸锌粒径得出,焙烧温度低于500℃时,焙烧方式对铁酸锌粒径影响较大;纳米铁酸锌晶粒生长动力学研究显示,微波焙烧时晶粒的平均生长指数为9.66,低于常规焙烧生长指数(10.6),表明微波焙烧时晶粒的平均生长速率较高,有利于晶粒生长;同时,微波晶粒生长平均活化能为122.1kJ/mol,远低于常规焙烧平均活化能(179.4kJ/mol),说明微波可以降低晶粒生长活化能,且微波的“非热效应”影响晶粒的生长。     

Nanoscale Coating of Silicon and Manganese on Ferrimagnetic Yttrium Iron Garnets

A uniform microstructure of Gd-substituted yttrium iron garnet (Y₂Gd₁Fe₅O₁₂) was obtained with additives of Si and Mn using a sol-gel coating process. Tetraethyl orthosilicate (TEOS) and manganese acetate were used as Si and Mn sources for the sol-gel coating procedure, respectively. TEM/EDS analysis confirmed the presence of a relatively uniform nanoscale coating of Si and Mn around each particle. High density and uniform microstructure were obtained after sintering at 1400°C in O₂ atmosphere. The density and grain size of the specimens depended on Mn content. The results were compared with materials fabricated without additives incorporated by the sol-gel coating procedure.     

Preparation of PZT discs for use in an acoustic wave sensor

Ferroelectric ceramics based on lead zirconate titanate with Nb and Mn additives, for use in chemical sensing, have been prepared. The perovskite phase was produced by solid phase reaction of oxides/carbonates. Powders were shaped in the form of discs by uniaxial pressing, and sintered in a controlled atmosphere at several temperature and time conditions.The microstructure of the sintered samples was observed in a scanning electronic microscope (SEM) and the grain size and density were shown to be dependent on the ceramic composition.PZT ceramic discs showed strong radial vibration modes in the kHz range, which could be used for chemical sensing applications both in gaseous and liquid media. Sensitivity was found to be strongly dependent on the density of the ceramics.     

Formation of Sol–Gel In Situ Derived BTO/NZFO Composite Ceramics with Considerable Dielectric and Magnetic Properties

The BT/NZFO composite ceramics derived by sol–gel in situ process were successfully prepared. The phase composition, morphology, and microstructure of the composite ceramics were determined and observed by X‐ray diffractometer (XRD), SEM, and EDS. Results showed that the Ni–Zn ferrite (NZFO) phase started to grow initially and then the BaTiO₃ (BTO) phase grew among the interfaces of NZFO particles at high ferrite content. The observation of microstructure showed that the NZFO phase in large grain size is enwrapped by the BTO phase in small grain size, and the constituent phases existed in the form of solid solutions doped with Fe and Ti, respectively. The densification and microstructure depended on the volume fraction of ferrite (f_NZFO). The appropriate sintering temperature was 1280°C–1300°C at which stable phase structure could be obtained for the BTO/NZFO composite. The maximum permittivity could achieve 86 000, and the initial permeability was as high as 162 when the ceramics was loaded with 95% ferrite and sintered at 1300°C for 12 h. The BT/NZFO composite ceramics exhibited impressive dielectric and magnetic properties, making it a potential candidate for wide applications in the integration of electronic devices.     

Synthesis,microstructure, and magnetic properties of monosized Mn x Zn y Fe3 ? x ? y O4 ferrite nanocrystals

We report the synthesis and characterization of ferrite nanocrystals which exhibit high crystallinity and narrow size distributions. The three types of samples including Zn ferrite, Mn ferrite, and Mn-Zn ferrite were prepared via a non-aqueous nanoemulsion method. The structural, chemical, and magnetic properties of the nanocrystals are analyzed by transmission electron microscopy, X-ray diffraction, X-ray fluorescence, and physical property measurement system. The characterization indicates that the three types of ferrite nanocrystals were successfully produced, which show well-behaved magnetic properties, ferrimagnetism at 5 K and superparamagnetism at 300 K, respectively. In addition, the magnetization value of the ferrites increases with the increasing concentration of Mn.     

用废旧锌锰干电池制取软磁铁氧体

报道了用废旧南孚牌锌锰电池为主要原料制备软磁铁氧体的方法。具体方法是,先用物理分离方法将锌锰干电池的外壳、集电体及一些塑料件与电池的含锌锰成分分离开来,然后将锌锰氧化物焙烧以除去其中的石墨。用酸溶解获得的锌锰氧化物,并加足够量的铁粉共同反应已得到含Zn、Mn、Fe的水溶液,然后用碱性溶液将它们共沉淀得到前驱体。焙烧前驱体即得到需要的软磁铁氧体。     

Synthesis of nanocrystalline Mn-Zn ferrite powders through thermolysis of mixed oxalates

Nanocrystalline Mn-Zn ferrite powders were synthesized by thermal decomposition of an oxalate precursor. Two polymorphs of a mixed Mn-Zn-Fe oxalate dihydrate were obtained by precipitation of metal ions with oxalic acid: monoclinic α-(Mn, Zn, Fe)₃(C₂O₄)₃·6H₂O is obtained after precipitation and ageing at 90 °C, whereas the orthorhombic β-type is formed after precipitation at room temperature. The morphology of the oxalate crystals can be controlled by the precipitation conditions. The α-polymorph of the mixed oxalate consists of prismatic and agglomerated particles. The β-oxalate forms non-agglomerated crystallites of submicron size. Thermal decomposition of the oxalate at 350 °C in air results in an amorphous product. Nanosize Mn-Zn ferrite powders are formed at 500 °C and a mixture of haematite and spinel is observed at 750 °C. The thermal decomposition of the mixed oxalate is monitored by thermal analysis, XRD and IR-spectroscopy. The morphology of the oxalate particles is preserved during thermal decomposition; the oxide particle aggregates display similar size and shape as the oxalates. The primary particles are much smaller; their size increases from 3 nm to 50 nm after decomposition of the oxalates at 350 and 500 °C, respectively. The powder synthesized by decomposition at 500 °C was sintered at 1150 °C to dense and fine-grained Mn-Zn ferrites.     

Effects of sintering atmosphere on grain morphology of liquid-phase-sintered SiC with Al2O3 additions

The effects of sintering atmospheres of Ar and N₂ on grain morphology were investigated for pressureless liquid-phase-sintered (LPS) SiC with Al₂O₃ additions. When increasing the sintering temperature, the SiC grain size and its aspect ratio increased in both sintering atmospheres. With a 2 mass% Al₂O₃ addition, no distinct difference was observed between the grain morphology of SiC sintered in the Ar atmosphere and that sintered in the N₂ atmosphere. With a 15 mass% Al₂O₃ addition, sintering in a N₂ atmosphere led to retarded grain growth and this resulted in a fine homogeneous microstructure, whereas sintering in an Ar atmosphere enhanced the grain growth compared with that in 2 mass% Al₂O₃. The effects of atmosphere on the grain morphology depend on the amount of Al₂O₃ addition, and this also affects the grain growth process of solution-reprecipitation. The mechanical properties of the SiC are also considered.     

交变磁场诱导自蔓延法制备Mn-Zn铁氧体及其磁性能(英文)

采用交变磁场诱导溶胶凝胶自蔓延法制备Mn0.6Zn0.4Fe2O4型纳米粉体,利用X射线衍射仪、扫描电镜和超导量子干涉仪对不同磁感应强度下制备粉体的物相、形貌和磁性能进行了表征。结果表明:交变磁场可以促进自蔓延反应的燃烧过程,但并不改变燃烧产物的相结构。随磁感应强度增加,Mn0.6Zn0.4Fe2O4粉体的粒径和矫顽力逐渐减小,饱和磁化强度逐渐增加。当磁感应强度为0.25T时,相比未加入交变磁场制备的粉体,Mn0.6Zn0.4Fe2O4粉体的矫顽力下降96%,达到278.6A/m,饱和磁化强度提高了500%,达到49.4A·m/kg。     

Effect of Sintering Conditions and Microstructure on the Magnetostrictive Properties of Cobalt Ferrite

The effects of sintering temperature and dwell time on the magnetostrictive properties of polycrystalline cobalt ferrite synthesized by the conventional ceramic method have been investigated. The present study showed that the magnitude of magnetostriction strongly depends on the microstructure of the final sintered product. The presence of small and uniform grains with less porous structure in the sintered material leads to enhancement of the magnetostrictive strain. Higher magnetostriction is obtained for samples sintered at a relatively lower temperature of 1100°C. These results are further corroborated by analyzing the effect of some additives during sintering on the microstructural and magnetostrictive properties.     

Magneto-Dielectric Properties of Mg–Cu–Co Ferrite Ceramics: I. Densification Behavior and Microstructure Development

The densification, grain growth, and microstructure development of Mg–Cu–Co ferrite ceramics (MgFe_1.98O₄, Mg_{1− x} Cu _x Fe_1.98O₄, with x =0.10–0.30 and Mg_{0.90− x} Co _x Cu_0.10Fe_1.98O₄, with x =0.05–0.20) were studied. The primary objective was to develop magneto-dielectric materials for miniaturization of high frequency and very-high frequency antennas. It was found that magnesium ferrite (MgFe_1.98O₄) is a promising magneto-dielectric material. However, due to its poor densification, it could not be fully sintered at a temperature below 1200°C. High-temperature sintering resulted in undesirable electrical and dielectric properties, due to the formation of Fe²⁺ ions. The poor densification and slow grain growth rate of MgFe_1.98O₄ can be considerably improved by incorporating Cu, due to the occurrence of liquid-phase sintering at a high temperature. A critical concentration of Cu was observed for Mg_{1− x} Cu _x Fe_1.98O₄, above which both densification and grain growth were maximized or saturated. The presence of Co did not have a significant influence on the densification and grain growth of the Mg-based ferrite ceramics.     

Correlating the microstructure of Mn–Zn ferrite with magnetic noise for magnetic shield applications

Many sensitive atomic devices require magnetic shield to reduce external magnetic field interference. Mn–Zn ferrites are optimistic candidates for shielding because they provide a high shielding factor and a low magnetic noise. This study evaluated the magnetic noise of Mn–Zn ferrite magnetic shield based on its grain size. The morphologies of Mn–Zn ferrite samples were characterized to establish a correlation between their grain sizes and magnetic permeability, which can be used to calculate magnetic noises. The correlation was then used to evaluate the shielding performance of another Mn–Zn ferrite, where the magnetic noise of the magnetic shield made from the ferrite was calculated to be 2.53 f ^−1/2fT. The magnetic noise of the magnetic shield was also measured experimentally using a spin-exchange relaxation-free atomic magnetometer. The measured magnetic noise of ferrite shield was 2.61 f ^−1/2fT, while the white noise of the atomic magnetometer was 0.71 fT/Hz^1/2 below 100 Hz. The consistency between the experimental results and predicted values suggests the viability of the proposed approach, providing a convenient, efficient, and precise method for developing ferrite ceramics materials for low-noise magnetic shields.     

Characterization of Ni–Cu–Zn ferrite prepared from industrial wastes

We propose the distillation method to synthesize Ni–Cu–Zn ferrite powder and to recover nitric acid, using scrap iron and the waste solution of electroplating as the starting materials. It was found that the Ni–Cu–Zn ferrite powder prepared from industry wastes also showed the formation of cubic ferrite with a saturation magnetization (M_s) of 55,825 A m² g⁻¹ and an intrinsic coercive force (Hci) of 579 A m⁻¹. For sintered Ni–Cu–Zn ferrite specimen, the toroidal specimen sintered at 950 °C for 2 h presented an maximum initial permeability (μ_i) of 176 at 28.3 MHz, a maximum quality factor (Q) of 32 at 3 MHz. The AC impedance measurements were performed by using impedance analyzer Solartron 1260. The semicircles in the impedance spectra shift to higher frequencies with increasing temperatures. The values of resistance (grain interior, grain boundary, and total) decreased with increasing temperatures. The semicircles of grain boundary and electrode are observed clearly. These data can be used to analyze typical the grain interior and the grain boundary resistance of Ni–Cu–Zn ferrite.     

Fully Dense, Fine-Grained, Doped Zinc Oxide Varistors with Improved Nonlinear Properties by Thermal Processing Optimization

Fully dense, doped ZnO varistors were prepared using an easy two-stage pressureless-sintering method at temperatures as low as 825°C, with a grain size of ∼0.5 μm. After the highly nonohmic ZnO varistors were sintered, their fine microstructure consisted of uniformly sized grains, small spinel grains with partially dissolved manganese and cobalt oxides discontinuously distributed in the fairly wide grain boundaries, and an intergranular layer of bismuth-rich crystalline phase mainly detected at three or four ZnO grain junctions. There were twins near the middle of almost all the ZnO grains. The abnormally high nonlinear properties of the almost nanostructured varistors ( F _B≈ 6–8 kV/mm and α= 270) were attributed to a uniform and very fine microstructure, a high ZnO–ZnO grain direct contacts concentration, and a uniform hybrid layer substructure (grain boundaries and twin boundaries) with different (but probably accumulative) potential barriers.     

共沉淀法锰锌铁氧体的制备及其磁性能

采用共沉淀法制备了锰锌铁氧体前驱体粉末,对其进行压块处理后,通过烧结得到锰锌铁氧体。测试分析了锰锌铁氧体前驱体粉末的粒度,并对锰锌铁氧体的X射线衍射及磁性能进行了测试与分析。结果表明：在适当的条件下,采用化学共沉淀法,可以制得分布均匀的纳米级锰锌铁氧体前驱体微粒。经过烧结的样品为单一的尖晶石相锰锌铁氧体。所制得的锰锌铁氧体具有较高的磁化率,标准的磁滞回线,锰锌铁氧体的磁化率随着含锌量的降低、含铁量的增加而逐渐升高。     

The microstructures, magnetic properties and impedance analysis of Mn–Zn ferrites doped with B2O3

The relationship between the microstructure, magnetic properties and impedance spectroscopy of Mn–Zn ferrites doped with B₂O₃ (up to 0.5 wt.%) has been further investigated. The ferrites were prepared by using a citrate gel processing route. A uniform microstructure with relatively small grains (9.6±0.7 μm) is observed for undoped ferrites (boron-free), which enables good magnetic properties to be achieved (initial permeability μ_i is 2400, power loss P_L is 26.3 kW/m³ at 100 mT).The results on the samples doped with B₂O₃ show that the doping does not benefit the magnetic properties of these gel-derived ferrites, but it promotes grain growth significantly. Discontinuous grain growth at low doping levels (<0.2 wt.%) results in poor magnetic properties. A maximum value of the initial permeability (μ_i: 2600) and a second minimum value (37.2 kW/m³ at 100 mT) in power loss are obtained at the 0.25 wt.% B₂O₃ doping level when the sample has a relatively uniform microstructure with larger grain size (39.5±3.3 μm). With further increases in B₂O₃ doping (0.5 wt.%), the increased porosity and presence of a B-rich phase result in deteriorated magnetic properties. The results of impedance measurement are closely related to the changes in the microstructure which result from these B₂O₃ additions. By using two models for impedance measurement analysis (The Koops’ model and the simple model), the contributions of B₂O₃ to grain boundary resistivity and bulk resistivity can be separated. It is shown that, whilst B₂O₃ has previously been considered to act as only a grain boundary additive, the impedance analysis indicates that both boundary resistivity and grain (bulk) resistivity are increased, thus implying the possible solution of some B₂O₃ within the ferrite spinel structure or an effective change in composition of grain as result of presence of B₂O₃ at the grain boundaries.