Characterization and optimization of the coercivity-modifying nitrogenation and re-calcination process for strontium hexaferrite powder synthesized conventionally

Strontium hexaferrite powder synthesized conventionally in-house from strontium carbonate and hematite (Fe2O3) without using additives has been treated in a static nitrogen atmosphere and subsequently calcined in static air. The phase identification studies by means of X-ray diffraction (XRD) and thermal magnetic analysis (TMA) indicated the decomposition of the strontium hexaferrite and the reduction of the resultant iron oxide (Fe2O3) during the reaction with nitrogen. High-resolution scanning electron microscopy (HRSEM) studies show that the reduction occurring during nitrogenation results in the conversion of some of the large grains into much finer sub-grains. Strontium hexaferrite, Fe3O4, and Sr7Fe10O22 were the main phases obtained after reduction. However, weak traces of other phases, such as Fe2O3, were also detected. The hexaferrite phase re-formed on subsequent calcination. The magnetic measurements indicated a significant decrease in the intrinsic coercivity during nitrogenation due to the formation of Fe3O4. However, after a re-calcination process, the remanence and maximum magnetization (i.e., magnetization at 1100 kA/m) exhibited values close to the initial values before treatment, but the value of the intrinsic coercivity was higher than that prior to nitrogenation. Examination of the re-calcined microstructure showed that this could be attributed to the fine grains that originated from the fine sub-grain structures formed in the powder particles during nitrogenation.The optimum time, initial gas pressure, and temperature of nitrogenation and the optimum temperature of re-calcination were investigated using a vibrating sample magnetometer (VSM), XRD, and HRSEM. The optimum temperature for nitrogenation was 950 and 1000 °C for re-calcination. The optimum time and initial nitrogen pressure were 5 h and 1 bar, respectively. The highest intrinsic coercivity obtained after re-calcination was 340 kA/m.     

Comparative effects of the hydrogen and nitrogen gas treatment and re-calcination (GTR) routes on the composition, microstructure, and magnetic properties of conventionally synthesized Sr–hexaferrite

Optimized static hydrogen treated and recalcined (HTR) and static nitrogen treated and recalcined (NTR) Sr–hexaferrite powders synthesized conventionally in-house are compared with one another. The phase identification studies and lattice parameter measurements showed first that the Sr–hexaferrite decomposed, forming iron oxide (Fe2O3), which was then reduced during the static hydrogen or nitrogen treatment, and, second, that the hexaferrite phase was recovered albeit with a small change in the composition (as indicated by the lattice spacings) after the re-calcination treatment in static air. These effects were more pronounced in the hydrogen process than in the nitrogen process. The main effect of this gas-treatment and re-calcination (GTR) process on the microstructure of the Sr–hexaferrite was the transformation of the single-crystal particles into particles with a very fine sub-grain structure during the gas treatment, which resulted in the formation of polycrystalline hexaferrite particles with a much finer grain size during subsequent recalcination, compared to that of the initial hexaferrite powder. This finer structure was responsible for the higher coercivities observed after re-calcination. With regard to the hydrogen and nitrogen processes, the former resulted in a higher degree of oxide reduction and hence a higher coercivity on re-calcination. The coercivity of the initial Sr–hexaferrite increased from 310 kA/m (3.9 kOe) to 400 kA/m (5 kOe) after HTR and to 342 kA/m (4.3 kOe) after NTR. The initial magnetization behavior was also different for the HTR- and NTR-processed powders, with the former exhibiting behavior characteristic of single domains. This was consistent with the grain size being significantly less than the single-domain size (1 ).     

Heat treatment of strontium hexaferrite powder in nitrogen,hydrogen and carbon atmospheres: a novel method of changing the magnetic properties

Strontium hexaferrite powder has been treated in nitrogen, hydrogen and carbon atmospheres. The results show that the phase composition and morphology, and hence, the magnetic properties of the strontium hexaferrite are affected significantly by these gas/vapour treatments. Generally, the coercivity decreased to below 0.8 kOe (regardless of the initial coercivity) and the magnetization at 14 kOe increased significantly, when strontium hexaferrite powder had been treated in a nitrogen, hydrogen or carbon atmosphere. However, it was found that a post-gas treatment of calcination in air, under appropriate conditions, resulted in a recovery of the hexaferrite structure (i.e. it is a reversible reaction). However, the particle/grain sizes of the calcined samples were significantly smaller than those of the non-treated samples, and it is believed that they were single domain particles/grains. In some cases, the coercivity increased by about 400%. The magnetization at 14 kOe and the remanence were either not affected or sometimes increased; magnetic measurements indicated a preferred orientation of the grains.     

单相钡铁氧体磁性材料的制备及磁性能研究

以硝酸钡、硝酸铁和柠檬酸为原料,采用溶胶-凝胶法制备了单相钡铁氧体(BaFe12O19)纳米粉体,并进一步研究了n(Fe)/n(Ba)、热处理温度对产物组成、形貌以及磁性能的影响。用X射线衍射仪(XRD)、扫描电子显微镜(SEM)和振动样品磁强计(VSM)分别对样品的组成、形貌和磁性能进行了表征。实验结果表明,当煅烧温度不变时,样品的晶粒尺寸随着n(Fe)/n(Ba)的增大而变大,磁性能随n(Fe)/n(Ba)的增大而增强;当n(Fe)/n(Ba)不变时,样品的晶粒尺寸随着煅烧温度的升高而变大。当n(Fe)/n(Ba)=12时,在800℃煅烧2h得到单一晶型的钡铁氧体粉体。1000℃时样品的磁性能最佳,饱和磁化强度(Ms)为70.88A.m2/kg,矫顽力(Hc)为372.89kA/m。     

Preparation of high quality,single domain BaFe12O19 particles by the citrate sol–gel combustion route with an initial Fe/Ba molar ratio of 4

BaFe₁₂O₁₉ particles have been synthesized by citrate sol–gel combustion route in a wide temperature range between 800 and 1200 °C with initial Fe/Ba molar ratios between 12 and 2. Structural, morphological and magnetic properties of the powders have been investigated by XRD, FT-IR, SEM and magnetization measurements. It was observed that both coercivity and specific saturation magnetization increase with annealing at temperatures up to 1100 °C, where a transition from single to multi domain structure occurs. To prevent formation of the hematite phase (α-Fe₂O₃), samples with different Fe/Ba molar ratios between 12 and 2 have been prepared and an intermediate phase, BaFe₂O₄, which may occur in Ba-rich samples has been removed by etching the powders in diluted hydrochloric acid. In this way, it was shown that single domain barium hexaferrite particles having high saturation magnetization, close to the theoretical value, and high coercivity can be synthesized with the initial Fe/Ba molar ratio of 4 in the sol–gel method. The chemical composition of this sample was determined as BaFe_11.80O_19.45 by the EDS analysis and Ba_1.05Fe_11.54O_18.4 using an ICP-MS device. Both are very close to the theoretical formula.     

Magneto-Structural Characterization of Strontium Substituted Lead Hexaferrite

In this work, the magnetic and structural properties of the system Pb_1?x Sr _x Fe₁₂O₁₉ (x=0.1,0.3,0.5,0.7 and 0.9) are reported. The samples were prepared by the traditional ceramic method. All the compounds are isostructural with the strontium hexaferrite (SrFe₁₂O₁₉). X-ray powder diffraction was used to carry out the quantitative analysis of phases and to determinate the crystallographic parameters. It was found that the compound consists of only one phase and that the coercivity, remanence and saturation increased with the strontium content. The initial susceptibility was also obtained and results are discussed in terms of the magnetization mechanisms produced by the effect of the substitution on the hexaferrite. Furthermore, Néel temperature measurements indicate a strengthening of the exchange interactions with increasing strontium content.     

Microwave properties of polymer composites containing combinations of micro- and nano-sized magnetic fillers

We investigated the microwave absorbing properties of composite bulk samples with nanostructured and micron-sized fillers. As magnetic fillers we used magnetite powder (Fe3O4 with low magnetocrystalline anisotropy) and strontium hexaferrite (SrFe12O9 with high magnetocrystalline anisotropy). The dielectric matrix consisted of silicone rubber. The average particle size was 30 nm for the magnetite powder and 6 micro/m for the strontium hexaferrite powder. The micron-sized SrFe12O19 powder was prepared using a solid-state reaction. We investigated the influence of the filler concentration and the filler ratio (Fe3O4/SrFe12O19) in the polymer matrix on the microwave absorption in a large frequency range (1 / 18 GHz). The results obtained showed that the highly anisotropic particles become centers of clusterification and the small magnetite particles form magnetic balls with different diameter depending on the concentration. The effect of adding micron-sized SrFe12O19 to the nanosized Fe3O4 filler in composites absorbing structures has to do with the ferromagnetic resonance (FMR) shifting to the higher frequencies due to the changes in the ferrite filler's properties induced by the presence of a magnetic material with high magnetocrystalline anisotropy. The two-component filler possesses new values of the saturation magnetization and of the anisotropy constant, differing from those of both SrFe12O1919 and Fe3O4, which leads to a rise in the effective anisotropy field. The results demonstrate the possibility to vary the composite's absorption characteristics in a controlled manner by way of introducing a second magnetic material.     

Synthesis of strontium ferrite nanoparticles by coprecipitation in the presence of polyacrylic acid

Strontium ferrite nanoparticles were prepared by coprecipitation in a PAA aqueous solution. The average diameter of the mixed hydroxide precipitates was 3.1 nm. From the thermal analysis by TGA/DTA and the phase analysis by XRD, it was shown that the appropriate molar ratio of Sr/Fe in aqueous solution was 1/8 and the precursor could yield pure strontium ferrite after calcination at above 700°C. The average diameters of the strontium ferrite nanoparticles calcined at 700 and 800°C were 34 and 41 nm, respectively. The magnetic measurements indicated that their saturation magnetization (57-59 emu/g) reached 85-88% of the theoretical one and increased with the decrease of temperature at 5-400 K. Their coercivity values (55-67 Oe) were much lower than those reported earlier, revealing the resultant nanoparticles were superparamagnetic. All the magnetic properties observed reflected the nature of nanoparticles and also concerned with their morphology and microstructure.     

Formation of ternary carbide Fe3Mo3C by mechanical activation and subsequent heat treatment

Ternary carbide, Fe₃Mo₃C was prepared from the powder mixture of Fe/Mo/C = 1/1/1 which was ground for 3 h in a planetary ball mill and subsequently heated at a temperature as low as 700°C, its amount increased with heating temperature. In contrast, when the 1 h-ground and unground samples were heated at 700–1000°C, Mo₂C formed. From the results obtained about the effect of mixing ratio, grinding time and heating temperature of Fe/Mo/C samples on the formation of Fe₃Mo₃C, it was found that the formation of Fe₃Mo₃C strongly depends on the mixing homogeneity and the activated state of the particles of Fe, Mo and C components induced by mechanical grinding. Fe₃Mo₃C obtained belongs to a hard magnet, having saturation magnetization of 0.4 emu g^–1, remanence of 0.13 emu g^–1 and coercivity of 200 Oe.     

Effect of Fuel on the Synthesis and Properties of Poly(methyl methacrylate) Modified SrFe12O19 Nanoparticles

Nanosized strontium hexaferrite (SrFe₁₂O₁₉) has been synthesized by citrate, urea, oxalic, and glycine precursor via a sol-gel route with poly(methyl methacrylate) (PMMA) as a templating agent. Crystal structure, morphology, and magnetic properties of as-synthesized nanoparticles were characterized by XRD, SEM, FT-IR, and VSM techniques. The formation of strontium hexaferrite and its crystallite size in presence of different fuels were compared. The influence of different fuels was reflected on the phase purity, morphology of the final powders as well as the magnetic properties. Magnetic measurements revealed that samples prepared by citric acid and glycine as fuel have high specific saturation magnetization and moderate coercivity, while urea and oxalic acid fuels resulted in low phase purity, and thus inferior magnetic properties.     

Synthesis of barium hexaferrite for magnetic recording media using the KCl flux system

Individual and fine crystals of barium hexaferrite were prepared by a modified flux method using the KCI flux system. Co²⁺-Ti⁴⁺-substituted barium hexaferrite with a homogeneous composition was synthesized at 950° C for 5 h or at 1000° C for 1 h from a mixture of BaCO₃, Fe₂O₃, CoO and TiO₂ with 30 wt% KCI added. Laboratory-prepared fine Fe₂O₃, was preferred because it gave ferrite particles with diameters of 0.2 to 0.4m. Magnetic properties were controlled by the Co-Ti content in hexaferrite crystals. Coercive force and Curie temperature decreased with the degree of Co-Ti substitution with saturation magnetization held at high value. The present process, from which individual and fine barium hexaferrite crystals can be prepared by using the KCI flux system, is recommended as a means of mass-production of ferrite powders with controlled magnetic properties for use in magnetic recording media.     

Effect of grain size and orientation on the initial permeability of 36 wt % Ni-Fe alloys

The effect of grain size and grain orientation on the initial permeability of a 36 wt % Ni-Fe alloy with additions of molybdenum, chromium and copper is reported. The initial permeability was found to increase with annealing temperature between 600° C and approximately 900° C due to the formation of a (1 2 3) [4 1 ¯2] primary recrystallization texture. Increasing the annealing temperature in the range 900 to 1100° C led to progressively lower permeabilities due to the growth of randomly oriented abnormal grains within the textured matrix. It is suggested that an increase in the misorientation between adjacent grains gives rise to an increase in the local magnetostatic energy, leading to much stronger pinning of magnetic domain walls, with a consequent decrease in permeability. Annealing at temperatures above 1100° C tends to increase the permeability, because of the increase in grain size.     

Preparation of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> · 6SrB<Subscript>2</Subscript>O<Subscript>4</Subscript> magnetic composites by spray pyrolysis

Spray pyrolysis has been used to produce X-ray amorphous precursors with the nominal composition SrFe₁₂O₁₉ · 6SrB₂O₄ in the form of spherical particles 0.3 to 2 μm in diameter. Heat treatment of the precursors at temperatures from 650 to 900°C has produced platelike strontium hexaferrite particles embedded in a SrB₂O₄ matrix. With increasing annealing temperature, the average dimensions of the hexaferrite particles increase from 80 × 20 to 450 × 100 nm and the coercivity of the material rises from 240 to 440 kA/m.     

The influence of the coprecipitation conditions on the low-temperature formation of barium hexaferrite

We have investigated the formation of barium hexaferrite via the coprecipitation method. Various reagent salts and solvents were tested, and the coprecipitates were calcined at 300–800 °C for 1–50 h. The samples were characterized with X-ray powder diffraction, electron microscopy and magnetometry. The coprecipitation conditions had a significant influence on the formation time of the barium hexaferrite, which started to form at as low as 500 °C. The optimum coprecipitation conditions were: ethanol as a solvent and chlorides as reagent salts. Powders with optimum magnetic properties, saturation magnetization 60–63 emu/g and coercivity 4–5 kOe, were obtained by calcining at 600–700 °C.     

纳米晶复合SrM永磁铁氧体的制备和交换耦合作用

采用sol-gel方法制备M型六角锶铁氧体。利用X光衍射、透射电子显微镜和VSM对纳米晶样品进行了研究。当热处理温度小于 80 0℃ ,样品存在复相。在同样条件下 ,压成薄片的样品存在硬磁与软磁SrFe12 O19/γ Fe2 O3 的纳米复合相的磁性交换耦合作用。温度为 80 0℃的薄片样品 ,比饱和磁化强度σS 为 75 .6emu/g ,内禀矫顽力Hcj 为6 0 15Oe ,最大磁能积 (BH) Max 为 1.87MGOe ,而粉末样品相应的分别为 75 .9emu/ g ,6 40 0Oe和 1.5 2MGOe。当热处理温度大于 85 0℃时 ,只有单一M相     

A novel approach for synthesis of M-type hexaferrites nanopowders via the co-precipitation method

M-type hexaferrites; barium hexaferrite BaFe₁₂O₁₉ and strontium hexaferrite SrFe₁₂O₁₉ powders have been successfully prepared via the co-precipitation method using 5 M sodium carbonate solution as alkali. Effects of the molar ratio and the annealing temperature on the crystal structure, crystallite size, microstructure and the magnetic properties of the produced powders were systematically studied. The results indicated that a single phase of barium hexaferrite was obtained at Fe³⁺/Ba²⁺ molar ratio 12 annealed at 800–1,200 °C for 2 h whereas the orthorhombic barium iron oxide BaFe₂O₄ phase was formed as a impurity phase with barium M-type ferrite at Fe³⁺/Ba²⁺ molar ratio 8. On the other hand, a single phase of strontium hexaferrite was produced with the Fe³⁺/Sr²⁺ molar ratio to 12 at the different annealing temperatures from 800 to 1,200 °C for 2 h whereas the orthorhombic strontium iron oxide Sr₄Fe₆O₁₃ phase was formed as a secondary phase with SrFe₁₂O₁₉ phase at Fe³⁺/Sr²⁺ molar ratio of 9.23. The crystallite sizes of the produced nanopowders were increased with increasing the annealing temperature and the molar ratios. The microstructure of the produced single phase M-type ferrites powders displayed as a hexagonal-platelet like structure. A saturation magnetization (53.8 emu/g) was achieved for the pure barium hexaferrite phase formed at low temperature 800 °C for 2 h. On the other hand, a higher saturation magnetization value (M _s = 85.4 emu/g) was obtained for the strontium hexaferrite powders from the precipitated precursors synthesized at Fe³⁺/Sr²⁺ molar ratio 12 and thermally treated at 1,000 °C for 2 h.     

Fabrication and magnetic property of BaSm(x)Fe(12-x)O19 (x < or = 0.4) nanofibers

BaSm(x)Fe(12-x)O19 (x < or = 0.4) ferrite nanofibers were prepared by sol-gel method from starting reagents of metal salts and citric acid. These nanofibers were characterized by TG-DTA, FTIR, SEM, XRD and VSM. These results show that the BaSm(x)Fe(12-x)O19 (x < or = 0.4) ferrite nanofibers were obtained subsequently from calcination at 750 degrees C for 1 h. The BaSm(x)Fe(12-x)O19 (x < or = 0.4) microstructure and magnetic property are mainly influenced by chemical composition and heat-treatment temperature. The grain sizes of BaSm0.3Fe11.7O19 ferrite nanofibers are in a nanoscale from 40 nm to 62 nm corresponding to the calcination temperature from 750 degrees C to 1050 derees C. The saturation magnetization of BaSm(x)Fe(12-x)O19 ferrite nanofiber calcined at 950 degrees C for 1 h initially decreases with the Sm content from 0 to 0.3 and then increases with a further Sm content, while the coercivity exhibits a continuous increase from 348 kA x m(-1) (x = 0) to 427 kA x m(-1) (x = 0.4). The differences of magnetic properties are attributed to lattice distortion and enhancement for the anisotropy energy.     

Etching of (0 0 0 1) habit faces and matched cleavages of flux grown strontium hexaferrite crystals

Results of etching (0 0 0 1) planes of flux grown strontium hexaferrite crystals in 85% H₃P0₄ at 120 °C and 37% HCl at 100 °C are presented. Fractography reveals one-to-one correspondence of cleavage patterns on the two matched (0 0 0 1) cleaved planes. Etch patterns including hexagonal, point-bottomed pits with smooth sloping planes, hexagonal but flat-bottomed pits, geometrically centred hexagonal pits with regularly spaced terracing, eccentric hexagonal pits with irregularly spaced terracing, a large flat-bottomed hexagonal pit with a smaller point-bottomed hexagonal pit within it but having different geometrical centres and flat-bottomed pits with a beak at their centres are illustrated. It is explained that they are indicative of normal, inclined, stepped and bending dislocations in strontium hexaferrite crystals. Pits due to impurity inclusions are also explained. The explanations are supported by the results of mismatchings of etch patterns on matched cleavages.     

Magnetic properties of hydrothermally synthesized strontium hexaferrite as a function of synthesis conditions

Fine particles of strontium hexaferrite, SrFe₁₂O₁₉, with a narrow size distribution have been synthesized hydrothermally from mixed aqueous solutions of iron and strontium nitrates under different synthesis conditions. The relationship between the synthesis variables (temperature, time and alkali molar ratio) and the magnetic properties has been investigated. The results have shown that, as the synthesis temperature increases, the saturation magnetization of the particles increases up to a plateau and the coercivity decreases. As the alkali molar ratio R(=OH^–/NO ₃ ^– ) increases, the coercivity decreases and goes through a local minimum, while the saturation magnetization increases and goes through a local maximum. Increasing the synthesis time from 2 h to 5 h has no significant effect on the saturation magnetization, but decreases the coercivity. An anisotropic sintered magnet with a high saturation magnetization value of 67.26 e.m.u g^–1 (4320 G) has been fabricated from the hydrothermally synthesized powders.Relationship between the c.g.s and S.I.units which are used in this paper are as follows: 1 erg = 10^–7 J, 1 e.m.u. cm^–3 = 12.57×10^–7 Wom^–2 (tesla), 1 oersted (Oe) = 79.6 A m^–1, 1 G = 10^–4 tesla (T).     

A study of the evolution of the constituent phases and magnetic properties of hydrogen-treated Sr-hexaferrite during calcination

A hydrogen treatment followed by calcination, has been developed in order to enhance the intrinsic coercivity of Sr-hexaferrite (SrFe₁₂O₁₉). Fully hydrogen-treated Sr-hexaferrite consists of a mixture of 73%, by weight, of Fe and 27% of Sr₇Fe₁₀O₂₂ phases. Calcination of this material to reform the SrFe₁₂O₁₉ phase occurs in two stages. Between room temperature and 600°C, oxygen was absorbed resulting in a large increase in weight with the formation of a mixture of SrFeO_3–x and Fe₂O₃( and ). During the second stage, the intermediate phases reacted to form SrFe₁₂O₁₉ at a temperature of between 700 and 800°C. A partial desorption of oxygen occurred until calcination reached completion at 1000°C. The magnetization at 1100 kA m^–1 and the remanence were similar to those of the untreated material, but, because of a much refined grain size, the intrinsic coercivity was considerably larger, with values around 400 kA m^–1. Grain growth occurs at temperatures > 1000°C, resulting in a decrease in the intrinsic coercivity.