平面Z-型软磁铁氧体频率特性的研究

研究分析了Co2Z材料显微结构与磁性能的关系,在普通铁氧体工艺中引入预磁化处理方法,并适当控制二次再结晶现象,成功地研制出高性能的Co2Z型宽频材料,Z-型铁氧体材料的共振频率可达1.8GHz以上,截止频率在1GHz附近,起始磁导率在8-13之间,介电常数相对较低(ε'<30)。     

一种改进的人工蜂群算法

从经典人工蜂群算法机制出发,针对原始算法在初始种群构造、子种群分组、步长更新和种群淘汰方面的不足进行了改进.新算法运用均匀设计理论构造初始种群,提出了一种种群交叉的Z型分组方法,设计了一种对数函数自适应步长代替原来的随机步长,引入了小生境技术及时淘汰陷入局部最优的个体.实验结果表明,改进后的算法有效地解决了人工蜂群算法早熟收敛、搜索速度较慢等问题,并提高了解的精度.     

Influence of particle size on magnetic and electromagnetic properties of hexaferrite synthesised by sol-gel auto combustion route

Magnetoplumbite barium hexaferrite (BaFe_11.8Co_0.2O₁₉) is synthesised through sol-gel auto-combustion method under two pH conditions of precursor solutions (acidic i.e. pH < 1 and neutral i.e. pH = 7). The XRD analysis followed by Reitveld refinement indicates the formation of phase pure samples in both cases but the barium hexaferrite obtained from acidic precursor solution has smaller crystallite sizes. The Transmission Electron Microscopy (TEM) analysis followed by High Resolution Transmission Electron Microscopy (HRTEM) confirms a lower particle size of ～20 nm for barium hexaferrite synthesised from acidic pH precursor solution. The shift in Raman peak (520-540 cm^?1) by 20 cm^?1, represents the whole structural block and further confirms the differences in the distribution of particle sizes due to the method of synthesis. The magnetic studies display a lower coercive field for the samples with smaller particle sizes. This is due to the crystalline size-induced microstrain that controls the magneto-crystalline anisotropy, shape anisotropy and stress anisotropy. The electromagnetic characterisation confirms broader absorption in the range of 8–18 GHz (X-band) with R_L ≤ ?7 db for the entire range for the samples with smaller particle sizes.     

Synthesis of Co-Zr doped nanocrystalline strontium hexaferrites by sol-gel auto-combustion route using sucrose as fuel and study of their structural,magnetic and electrical properties

Sol-gel auto-combustion route using sucrose as fuel has been employed to synthesize nanocrystalline particles of SrZr_xCo_xFe_(12−2x)O₁₉ (0.0≤ x ≤1.0). The characterization of these materials has been done by TGA-DTA, FT-IR, XRD and EDS. SEM and TEM techniques have been used to study the structure and morphology. Magnetic properties have been investigated by VSM and Mössbauer spectroscopy (MS). The influence of calcination temperature on morphology and magnetic properties of samples is studied in a wide temperature range of 500–1100 °C. XRD analysis indicates the formation of pure single phase hexagonal ferrites at 900 °C. The crystallite size calculated using Scherrer equation lies in a narrow range of 21–33 nm. The crystallite size is small enough to obtain a suitable signal to noise ratio in high density recording medium. Substitution of Zr and Co for Fe has been found to have a profound effect on the structural, magnetic and electrical properties. Upon substitution saturation magnetization (M_S) first increases from 62.67 emu/g to 64.84 emu/g (up to x=0.4) followed by a decrease to 49.71 emu/g at x=1.0. There is a slow fall in coercivity (H_C) from 5785.74 (x=0.0) to 1796.51 Oe (x=1.0). Dielectric constant, dielectric loss tangent and AC conductivity in the frequency range 20 Hz to 120 MHz have been studied for all the compositions (x=0–1.0). The composition and frequency dependence of these dielectric parameters has been qualitatively explained.     

Synthesis of red mud derived M-type barium hexaferrites with tuneable coercivity

Hexagonal ferrites can be employed in a multitude of applications, the most common hexaferrites are the M ferrites such as BaFe₁₂O₁₉ (barium hexaferrite, BaM). It is known that if Fe³⁺ is substituted with a combination of Ti⁴⁺/Co²⁺ the coercivity of BaM can be reduced to produce soft M ferrites with easily switchable magnetisation. They can be utilised as powders, films or bulk ceramics, and can be manufactured from a wide variety of synthesis methods. The production of hexaferrites usually requires commercial raw materials, but if an industrial waste can be utilised, this will help to ease waste disposal and storage costs, valorise a waste material and encourage circular economy. In this study, bauxite residue (red mud) from the production of alumina was used to synthesise M-type hexaferrites, using a simple ceramic process. BaCO₃, or BaCO₃+Co₃O₄, were added to the red mud, blended and heated at 1000 °C to produce the M-type hexaferrites. Without cobalt addition up to 81.1 wt% M ferrite was produced, and with Co addition up to 74.3 wt% M ferrite was formed. Without cobalt, the M ferrite phase closely resembled BaFe₉Al₃O₁₉, and was a hard ferrite with a magnetisation of 12–19 A m²/kg for the whole powder (up to 23.6 A m²/kg for the M ferrite phase) and a coercivity of ~290 kA/m. When cobalt was added, secondary titanate phases vanished, and Ti⁴⁺/Co²⁺ partially substituted very soft M ferrite was formed with a low coercivity of ~16 kA/m but a higher magnetisation of 24.5 A m²/kg for the whole powder (up to 34.9 A m²/kg for the M ferrite phase). Therefore, not only can good quality magnetic materials be easily produced from this common waste material, but its magnetic properties can be tuned by varying the 2 + ions added during the process.     

低温烧结Cu、Zn掺杂Co2-Y平面六角铁氧体及其频率特性

报道了通过BiO3的掺杂而获得的较低温度（870℃左右）下烧成的Y型平面六角结构软磁铁氧体的烧结工艺,结构特征及其磁导率和介电常数的频率特性,研究发现Cu,Zn掺杂的Co2Y型软磁铁氧体材料（Ba2Co1.2-xZnxCu0.8Fe12O22)在甚高频段具有良好的磁性能和介电性能,且此种材料烧结温度低,易于实现低温烧结,是一种可以用于甚高频段的理想的软磁材料。     

Thermal Instability and Microstructure of Strontium M-type Hexaferrite Nanoparticles Synthesized by Citrate Approach

The dried gel of SrFe12O19, prepared by citrate approach, was investigated by means of infrared spectroscopy （ IR ）, thermogravimetric analysis （ TG ）, differential scanning calorimetry （ DSC ）, X- ray diffraction（ XRD ） techniques, energy dispersive spectroscopy（ EDS ）, and transmission electron microscopy（ TEM ）. The thermal instability and the thermal decomposition of low-temperature strontium M-type hexaferrite crystallized at about 600℃ were confirmed for the first time by XRD method. The decomposition of the low-temperature strontium M-type hexaferrite took place at about 688.6℃ determined by DSC investigation. The low-temperature strontium M-type hexaferrite nanopartieles were decomposed into SrFeO2.5 with an orthorthombic cell and Fe2O3 with a tetragonal cell as well as possibl α-Fe2O3 . The agglomerated particles with sizes less than 200 nm obtained at 800℃ were plesiomorphous to strontium M-type hexaferrite. The thermally stable strontium M-type hexaferrite nanopartieles with sizes less than 100um cotdd take place at 900 ℃ . Up to 1000 ℃ , the phose transformotion to form strontium M-type hexaferrite was ended, the calcinations with the sizes more than 1μm were composed of α-Fe2O3 and strontium M-type hexaferrite. The method of distinguishing γ-Fe2O3 with a spinel structure from Fe2O3 with tetragonal cells by using powder XRD method was proposed. Fe2O3 with tetragonal cells to be crystallized before the crystallization of thermally stable strontium M-type hexaferrite was confirmed for the first time. The reason why α- Fe2O3 as an additional phase appears in the calcinations is the cationic vacancy of stroutium M-type hexaferrite , SrFe12-x□O19 （0≤x ≤0.5）.     

Collapse of the Magnetic Hyperfine Structure of Barium Hexaferrite by Mechanochemical Activation

The collapse of the magnetic hyperfine structure of barium hexaferrite as an effect of mechanochemical activation was investigated. The various inequivalent sites and phases present in the milled material were studied by transmission Mössbauer spectroscopy. Barium hexaferrite nanoparticles were obtained after only 2 h of milling time and were found to coexist with iron oxide particles for longer times of exposure to mechanochemical activation. The milled samples were then subjected to thermal annealing, in order to test the reversibility of the collapse of the hyperfine structure. The thermal annealing performed (700°C, 1 h) was partly successful in restoring the magnetic hyperfine structure of milled hexaferrite. Complementary information was obtained using transmission electron microscopy (TEM) and electron diffraction (ED).     

Nanoparticles of Barium Hexaferrite by Gel to Crystallite Conversion and their Magnetic Properties

A novel method for the preparation of nanoparticles of barium hexaferrite is realized by the gel-to-crystallite (G-C) conversion method. Here, gels of Fe(OH)₃·xH₂O, 70 < x < 110, were reacted with Ba(OH)₂·8H₂O in ethanol/water medium at 80–95°C yielding the precursor, barium iron (III) oxy hydroxide hydrate which is x-ray amorphous but crystalline by electron diffraction (ED). Thermal analyses showed dehydro-xylation of the precursor around 600°C to barium hexaferrite which exhibits ED with spotty ring patterns. Samples heat-treated at 650°C are X-ray crystalline with average particle size of 17 nm, which on recrystallization at 780–930°C gives monocrystallites with spot patterns by ED. By varying the wet chemical conditions, precursors of variable Fe₂O₃/BaO ratios could be prepared which on heat-treatment yield monophasic hexaferrite of Fe₂O₃/BaO ratio ranging from 4.51 to 6. In hyperbarium compositions, annealing at 1350°C leads to ordering of excess barium in anti-BR sites within Ba—O layers of -alumina type unit cells. Nanoparticles of barium hexaferrite with superparamagnetic as revealed by Mössbauer spectra, the temperature vs. magnetization plots and the absence of hysteresis in B-H curves. With increasing temperature of heat treatment, the area under the B-H loop increases continuously, with the magnetization increasing from 2 to 52 emu/g. The conversion from superparamagnetic to ferrimagnetic state is continuous because of the out-diffusion of cation vacancies, created to charge compensate hydroxyl ions, which, in turn, affects Fe³⁺-O^2–-Fe³⁺ superexchange interactions, with the addition of surface and size factors.