Ferrite Compacts Research Articles

Microstructure Formation of LiTiZn Ferrite Ceramics during Radiation-Thermal Sintering

The effect of intensification of the compaction rate of ferrite compacts under irradiation conditions with a high-power electron beam both in the heating regime and in the isothermal stage of sintering was established. The compaction mechanisms of the compacts are different at each of these stages. The intensification of compaction at the non-isothermal stage in radiation-thermal conditions is due to processes involving the liquid phase. The role of bismuth oxide in the compaction of the material at the isothermal stage of sintering is unessential, but its influence is significant in recrystallization processes. Under of Ivensen’s phenomenology, compaction curves are explained by the deceleration of annealing of structural defects responsible for the fluidity of the material. Dislocations are the most probable type of defects, satisfying the detected regularities.

In situ synthesis of high density magnetic ferrite spinel (MgFe 2O 4) compacts using a mixture of conventional raw materials and waste iron oxide

A mixture of magnesite ore and waste iron oxide (mill scale) was used to synthesize high density magnesium ferrite compacts. The effect of different mixture composition (weight percentage of magnesite ore to mill scale) as well as the sintering temperatures on the phase change, compressive strength, physical and magnetic properties of sintered compacts was investigated. The results indicated that a single phase ferrite spinel is obtained in a compact produced from a mixture consisting of 40 wt% magnesite ore and 60 wt% mill scale and sintered at different temperatures. On the other hand, this mixture composition produced compacts possessing low porosity and high saturation magnetization of 6% and 31.89 emu/g, respectively when it is sintered at 1550 °C for 2 h.

Nano-crystalline copper ferrites from secondary iron oxide (mill scale)

Nano-crystalline copper ferrite compacts have been synthesized using mill scale fines via conventional oxide ceramic process. The effects of firing temperatures and time on both physical and magnetic properties of the compacts were studied. Archimedes method as well as the universal testing machine was used for determining the physical properties of the compacts. Meanwhile, the types of phase formed and the magnetic properties of the produced samples were investigated using X-ray diffraction, scanning electron microscope and vibrating sample magnetometer. The results indicated that with a firing temperature of 1100 °C, the compact possesses higher compressive (832 kg/cm 2) strength and higher bulk density (3.93 g/cm 3), whereas higher saturation magnetization (45.2 emu/g) and lower coercivity (6.13 Oe) were achieved for compacts fired at 1200 °C. The effect of firing time at optimum firing temperature on physical and magnetic properties of the produced compacts seems to be insignificant. The crystal structure of the copper ferrite was transformed from its tetragonal into a cubic structure accompanying the dissociation of a part of the copper ferrite into delafossite at high firing temperature.

Effect of Cu2+ on the magnetic properties and reducibility of Fe2TiO5

Abstract Ferrites have been studied for several years due to their wide use as magnetic materials for telecommunications, audio and video, power transformers and many other applications. Equimolar mixtures of Fe2O3 and TiO2 were fired in a muffle furnace at 1200 °C for 4 h. Mixed samples were prepared by replacing TiO2 with calculated amounts of CuO (x = 0.2, 0.4, 0.6, 0.8 and 1 mol). The synthesized samples were characterized with X-ray diffraction and their magnetic properties were measured using vibrating-sample magnetometer. The microstructure of the sample was examined using reflected light microscope and scanning electron microscope. The formation of Fe2TiO5, Fe5CuO8, Cu2TiO3 and CuFeO2 phases were detected whereas their magnetic properties increased with increasing the added mole ratio of Cu2+ ions. The isothermal reduction kinetics of synthesized nanocrystallites Ti–Cu mixed ferrite compacts were studied at 500 °C using hydrogen gas. It was found that the reduction rate and the reduction extent increased with increasing the extent of Cu2+ (0.2–1) whereas the maximum reduction extent (100%) was detected for pure Cu ferrite (Cu2+) while the minimum reduction extent (12%) was detected for pure iron titanate (Cu2+ = 0). The magnetic properties showed a drastic improvement upon reduction with hydrogen gas.

Textured Sc-Doped Barium–Ferrite Compacts for Microwave Applications Below 20 GHz

Sc is substituted for Fe in Ba M-type hexaferrite to reduce the anisotropy field and shift the ferromagnetic resonance (FMR) frequency to below 20 GHz. We have produced pure phase BaFe <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_11$</tex> ScO <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_19$</tex> powders having a platelet microstructure. These materials are pressed under an applied magnetic field of 95.5 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$times$</tex> 10 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^5$</tex> A/m. Resulting compacts are sintered and are shown to have a clear uniaxial anisotropy with the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$c$</tex> -axis perpendicular to the surface plane. Hysteresis loops display a squareness of 0.83 with a coercivity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$sim $</tex> 186 kA/m. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$X$</tex> -band FMR power derivative linewidth is measured to be 79.6 kA/m.

Effects of surface modification of α-FeOOH powder on the sintering process of ferrite compacts

A surface-modified α-FeOOH (denoted α-FeOOH[S]) was prepared by adsorption of stearate ion onto an α-FeOOH powder. The non-surface-modified α-FeOOH (denoted simply α-FeOOH) and α-FeOOH[S] powders contained particles of almost identical shape and size. When ferrite NiFe2O4 compacts were prepared by sintering the mixed powders of the surface-modified α-FeOOH[S] and NiO at 1573 K for 1 h, the total porosity of the NiFe2O4 compacts was 14.8% less than that found for compacts prepared by sintering the mixed powders of the non-surface-modified α-FeOOH and NiO. Although the morphology of the α-FeOOH and the α-FeOOH[S] was the same, the use of the α-FeOOH[S] powder accelerated densification of the NiFe2O4 compacts. The mixed powder containing α-FeOOH[S] and MO (M=Co, Zn) also formed denser ferrite compacts than when the powder containing the non-surface-modified α-FeOOH was used. The α-Fe2O3 and α-Fe2O3[S] powders were prepared by firing the non-surface-modified α-FeOOH and the α-FeOOH[S] powders, respectively, at 873 K for 1 h. The XPS O 1s and Fe 2p spectra of the α-Fe2O3 and α-Fe2O3[S] indicated that the surface of the α-Fe2O3[S] powder had a greater Fe3O4-like phase with oxygen vacancy than that of the α-Fe2O3 powder. The temperature dependence of the electrical conductivity of the α-Fe2O3 and α-Fe2O3[S] powder compacts also indicated the presence of an Fe3O4-like phase with a mixed-valence state of Fe3+ and Fe2+ on the surface. This Fe3O4-like phase plays an important role in the sintering and densification of ferrite compacts.

Homogeneous grain growth and fast-firing of chemically modified nanocrystalline MnZn ferrites

A chemical method utilizing sol-gel reactions was investigated to uniformly incorporate small amounts of additives of Si and Ca into nanocrystalline Mn 0.6Zn 0.4Fe 2O 4 powders (≈14 nm particle size). Sintering behavior of the chemically modified nanocrystalline MnZn ferrites was studied with regard to the effects of the chemical additives and sintering conditions on densification and grain growth. The nanopowder samples without the additives exhibited abnormal grain growth regardless of heating rate after pressing at a relatively low pressure and sintering at 1200°C, while the chemical addition of 2 wt% SiO 2 and 0.5 wt% CaO was found to improve microstructural characteristics, i.e., homogeneous grain growth and less porosity. Fast-firing did not seem to favorably affect densification of the chemically modified MnZn ferrite nanopowder, but was found to contribute to the suppression of grain growth in the nanocrystalline ferrite compacts.

Sintering of Nanosized MnZn Ferrite Powders

The sintering and microstructural evolution of nanosized MnZn ferrite powders prepared by a hydrothermal method were investigated. The microstructure of sintered ferrite compacts depends strongly on the strength of the agglomerates formed during the compacting of nanosized ferrite powders. It was found that at 700°C the theoretical density of sintered compacts can almost be reached, while above 900°C an increase of porosity was identified. The formation of extra porosity at higher sintering temperatures is caused mainly by the oxygen release which accompanies the dissolution of relatively large grains of residual alpha‐Fe2O3 in the spinel lattice.

Influence of iron oxide reactivity on microstructure development in MnZn ferrites

The reactive sintering of manganese zinc ferrite has been studied using powders prereacted at 600 °C, 900 °C, and 1200 °C. The results show that the calcination temperature has a pronounced influence on the microstructure evolution during sintering. The selfsintering of iron oxide during the heating of MnZn ferrite compacts prepared from prereacted mixtures, shifts the chemical reaction of MnZn ferrite formation to higher temperatures where the final microstructure is developed.

Densification of compacts during hydrostatic pressing

1. In an investigation into the HSP of ferrite compacts produced by prepressing in a rigid die, a marked anisotropy of deformation, consisting in preferential diametral densification (lR >lA), was observed at all pressures pHSP ≥ ppr. 2. During the HSP of ferrite compacts their relative density was found to grow already at pHSP≥ppr. 3. Prepressing pressure ppr and the geometric factor h/d have a marked effect on the mechanism of densification during HSP only at low pressures (pHSP ≥ 2ppr). 4. The existing hydrostatic pressing equations do not provide an accurate description of the mechanism of densification operating during HSP (particularly at low pressures).

Effects of MgF&lt;sub&gt;2&lt;/sub&gt; Addition on Formation of Sintered MgFe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; Ferrite Compacts and Its Magnetic Properties

The effects of MgF2 addition on the formation of sintered ferrite compacts were studied. They were prepared by mixing and sintering at 1300°C for 2h in air the following compositions. MgO; 50-x, MgF2; x, Fe2O3; 50mol% (x=0, 1, 4, 7, 12 and 20). During sintering almost of fluorine sublimed, its amount remained in the ferrite compacts was small, and seems to exist in ferrite lattice. A large amount of MgF2 addition (x≥12) resulted to precipitate the foreign phase on the surface of the ferrite compacts in the form of such as MgO. Sinterability decreased practically with increasing amount of MgF2. This decrease was attributed to a presumable decrease of cation vacancy concentration by addition of MgF2.The effects of MgF2 addition on magnetic properties and crystal lattice constant of magnesium ferrite were studied. Saturation magnetization and crystal lattice constant decreased by addition of MgF2, on the contrary, Curie temperature increased. B-H (magnetic hysteresis) characteristics also changed. Namely, magnetic flux density decreased, while coercive force increased.