Dielectric properties of Al-substituted Co ferrite nanoparticles

A series of polycrystalline spinel ferrites with composition, CoFe_2−x Al _x O₄ (0 ≤ x ≤ 1), have been synthesized by sol-gel method. The effect of Al-substitution on structural and dielectric properties is reported in this paper. X-ray diffraction analysis revealed the nanocrystalline nature in the prepared ferrite samples. The particle size, D, decreases with increase in Al-content. The lattice parameter, a and X-ray density, d _x , decreased with increase in Al-content. The dielectric properties for all the samples have been studied as a function of frequency in the range 100 Hz–10 MHz. Dielectric properties such as dielectric constant, ɛ′, dielectric loss, ɛ″ and dielectric loss tangent, tan δ, have been studied for nanocrystalline ferrite samples as a function of frequency. The dielectric constant and dielectric loss obtained for the nanocrystalline ferrites proposed by this technique possess lower value than that of the ferrites prepared by other methods for the same composition. The low dielectric behaviour makes ferrite materials useful in high frequency applications.     

Dielectric behaviour of erbium substituted Mn-Zn ferrites

Dielectric properties such as dielectric constant (ε′) and dielectric loss tangent (tan□δ) of mixed Mn-Zn-Er ferrites having the compositional formula Mn_0.58Zn_0.37Fe_2.05−xEr_x0₄ (where itx = 0.2, 0.4, 0.6, 0.8 and 1.0) were measured at room temperature in the frequency range 1–13 MHz using a HP 4192A impedance analyser. Plots of dielectric constant (ε′) vs frequency show a normal dielectric behaviour of spinel ferrites. The frequency dependence of dielectric loss tangent (tan δ) was found to be abnormal, giving a peak at certain frequency for all mixed Mn-Zn-Er ferrites. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant and dielectric loss tangent. Plots of dielectric constant vs temperature have shown a transition near the Curie temperature for all the samples of Mn-Zn-Er ferrites. However, Mn_0.58Zn_0.37Er_1.0Fe_1.05O₄ does not show a transition. On the basis of these results an explanation for the dielectric mechanism in Mn-Zn-Er ferrites is suggested.     

Preparation and characterization of NIC thermistors based on Fe2+δMn1−x−δNixO4

Single-phase nickel manganese ferrites, Ni_xMn_1−xδFe_2+δO₄, have been prepared by mild heating of solid solutions of nickel-manganese-iron formates obtained by coprecipitation. Dilatometric and X-ray diffraction studies were conducted at different temperatures, showing the different stages of the sintering process of highly densified nickel manganese ferrite ceramics (density, 94%–96%). Granulometric and SEM studies showed that the oxide particles formed agglomerates of 4 μm, with a narrow size distribution. XPS studies showed that Fe²⁺ was oxidized to Fe³⁺ on the ceramic surface. Finally, electrical measurements showed that Fe_2.16Mn_0.074Ni_0.10O₄ presented the minimum resistivity (1340 Ωcm). The sensitivity indices were around 4000 K.     

Mechanochemical synthesis of stoichiometric MgFe2O4 spinel

The influence of long-term milling of a mixture of (1) MgO and α-Fe₂O₃, (2) MgCO₃, and α-Fe₂O₃, and (3) Mg(OH)₂ and α-Fe₂O₃ powders in a planetary ball mill on the reaction synthesis of nanosized MgFe₂O₄ ferrites was studied. Mechanochemical reaction leading to formation of the MgFe₂O₄ spinel phase was followed by electron microscopy, (SEM and TEM), X-ray diffraction and magnetization measurements. The spinel phase was observed first in cases (1) and (2) after 5 h of milling, and its formation was observed in all cases after 10 h. The synthesized MgFe₂O₄ ferrite has a nanocrystalline structure with a crystallite size of about 11, 10, and 12 nm, respectively for cases (1)–(3). Magnetic measurements after 10 h of milling show magnetization values of 19.8 J/(Tkg), 23.5 J/(Tkg) and 13.8 J/(Tkg), respectively for the cases (1)–(3).     

Ferrite grade iron oxides from ore rejects

Iron oxyhydroxides and hydroxides were synthesized from chemically beneficiated high SiO₂/Al₂O₃ low-grade iron ore (57.49% Fe₂O₃) rejects and heated to get iron oxides of 96–99.73% purity. The infrared band positions, isothermal weight loss and thermogravimetric and chemical analysis established the chemical formulas of iron-oxyhydroxides as γ-FeOOH.0.3H₂O; α-FeOOH.0.2H₂O and amorphous FeOOH. The thermal products of all these were α-Fe₂O₃ excepting that of γ-FeOOH.0.3H₂O which gave mainly γ-Fe₂O₃ and some admixture of α-Fe₂O₃. The hydrazinated iron hydroxides and oxyhydroxides, on the other hand, decomposed autocatalytically to mainly γ-Fe₂O₃. Hydrazine method modifies the thermal decomposition path of the hydroxides. The saturation magnetization,J _s, values were found to be in the range 60–71 emu g⁻¹ which are close to the reported values for γ-Fe₂O₃. Mechanism of the γ-Fe₂O₃ formation by hydrazine method is discussed.     

Compositional dependence of the structural and dielectric properties of Li<Subscript>2</Subscript>O–GeO<Subscript>2</Subscript>–ZnO–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

(10Li₂O–20GeO₂–30ZnO–(40-x)Bi₂O₃–xFe₂O₃ where x = 0.0, 3, 6, and 9 mol%) glasses were prepared. A number of studies, viz. density, differential thermal analysis, FT-IR spectra, DC and AC conductivities, and dielectric properties (constant ε′, loss tan δ, AC conductivity, σ _ac, over a wide range of frequency and temperature) of these glasses were carried out as a function of iron ion concentration. The analysis of the results indicate that, the density and molar volume decrease with an increasing of iron content indicates structural changes of the glass matrix. The glass transition temperature T _g and onset of crystallization temperature T _x increase with the variation of concentration of Fe₂O₃ referred to the growth in the network connectivity in this concentration range, while glass-forming ability parameter ΔT decrease with increase Fe₂O₃ content, indicates an increasing concentration of iron ions that take part in the network-modifying positions. The FT-IR spectra evidenced that the main structural units are BiO₃, BiO₆, ZnO₄, GeO₄, and GeO₆. The structural changes observed by varying the Fe₂O₃ content in these glasses and evidenced by FTIR investigation suggest that the iron ions play a network modifier role in these glasses while Bi₂O₃, GeO₂, and ZnO play the role of network formers. The temperature dependence of DC and AC conductivities at different frequencies was analyzed using Mott’s small polaron hopping model and, the high temperature activation energies have been estimated and discussed. The dielectric constant and dielectric loss increased with increase in temperature and Fe₂O₃ content.     

Dispersion of dielectric constant and resistivity of Cu x Zn1−x Fe2O4 samples

Cu _x Zn_1−x Fe₂O₄ samples exhibit dispersion of dielectric constant, tanδ and resistivity in the frequency range of 1 kHz to 50 MHz. The dispersion exhibited is in general accord with Koops’ model. However, the details of the conducting and non-conducting regions must be taken into account when composition tends to change interrelationship between the elementary capacitor resistor circuits. On quenching these samples from 800°C the dielectric constantε ¹ showed an increase for CuFe₂O₄ and Cu_0·8Zn_0·2Fe₂O₄ samples. The dielectric constant of the remaining samples showed no influence on quenching. The compositional variation showed that the dielectric constant has higher value for the ferrite Cu_0·4Zn_0·6Fe₂O₄ The results are explained on the basis of cation transfer.     

Stress sensitivity of inductance in NiMgCuZn ferrites and development of a stress insensitive ferrite composition for microinductors

Three series of NiMgCuZn ferrites were prepared by conventional sintering process. The formation of single phase in these ferrites was confirmed by x-ray diffraction. Initial permeability measurements on these samples were carried out in the temperature range of 30–400°C. The effect of the external applied stress on the open magnetic circuit type coil with these ferrites was studied by applying uniaxial compressive stress parallel to magnetizing direction and the change in the inductance was measured. The variation of ratio of inductance (ΔL/L)% increases upto certain applied compressive stress and there after it decreases, showing different stress sensitivities for different compositions of ferrites studied in the present work. With a view to develop stress insensitive NiMgCuZn ferrite, a low stress sensitivity composition among all the ferrites studied was chosen and different amounts of SiO₂ were added to it and a series of ferrite compositions were prepared. The variation of ratio of inductance (ΔL/L)% with external applied compressive stress was examined. These results show that, 0.05 wt% SiO₂ added Ni_0.3Mg_0.3Cu_0.1Zn_0.3Fe₂O₄ ferrite exhibited stress insensitivity. It was noticed that addition of SiO₂ was found to be effective in reducing the stress sensitivity. This was confirmed from the elastic behaviour studies at room temperature on these ferrite samples. These studies were carried out to develop a ferrite composition for its use as core material for microinductor applications.     

Advanced ceramics: Combustion synthesis and properties

Fine-particle ceramic powders such as chromites, manganites, ferrites, cobaltites, aluminas (α-Al₂O₃, Cr³⁺/Al₂O₃, zirconia-toughened alumina, mullite and cordierite), ceria, titania, zirconia (t, m, c and PSZ), dielectric oxides (MTiO₃, PZT and PLZT) as well as highT _c cuprates have been prepared by the combustion of redox compounds or mixtures. The combustion-derived oxide materials are of submicron size with a large surface area and are sinteractive.     

Crystal structure and magnetic properties of high-coercivity Sr1 ? xPrxFe12 ? xZnxO19 solid solutions

Sr_1−x Pr _x Fe_{12 − x} Zn _x O₁₉ ferrites with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 have been prepared by solid-state reactions between praseodymium, iron, and zinc oxides and strontium carbonate in air at 1470 K. According to X-ray diffraction results, the samples with x ≤ 0.2 were single-phase and those with 0.3 ≤ x ≤ 0.5 contained, in addition to the magnetoplumbite phase, small amounts of α-Fe₂O₃, ZnFe₂O₄, and PrFeO₃. The mixed-phase samples further fired twice at 1470 K for 4 and 2 h contained no impurity phases at x = 0.3 and contained only α-Fe₂O₃ at x = 0.4 and 0.5. In the composition range 0 ≤ x ≤ 0.3, the a and c cell parameters, unit-cell volume V, and X-ray density ρ_x of the magnetoplumbite phase vary linearly according to the relations a(?) = 5.8869 − 0.0162x, c(?) = 23.027 + 0.449 x, V(?³)= 691.10 + 9.65x, and ρ_x(g/cm³) = 5.102 + 0.230 x. The highest degree of combined heterovalent substitution of Pr³⁺ for Sr²⁺ and Zn²⁺ for Fe³⁺ in the SrFe₁₂O₁₉ ferrite (formation of Sr_1−x Pr _x Fe_{12 − x} Zn _x O₁₉ solid solutions) at 1470 K is x = 0.32−0.36. The saturation magnetization per formula unit (n _s) of the x = 0.1 ferrite exceeds that of SrFe₁₂O₁₉ by 1.7% at 6 K and by 15.2% at 308 K. The 308-K n _s and coercive force (_σ H _c) of the x = 0.2 ferrite exceed those of SrFe₁₂O₁₉ by 7.6 and 8.5%, respectively.     

Effect of forms of B2O3-SiO2 as an additive on magnetic properties of SrFe12O19 ferrites

Strontium ferrites (SrO · 5.5Fe₂O₃) were prepared using hot-rolled mill scale as a source of iron oxides. Calcination was performed at 1200°C for 2 h. The fused B₂O₃-SiO₂, with various mole ratios of B₂O₃ to SiO₂, was added to the calcined ferrites during the milling stage. The ferrites were formed anisotropically. The fused additives are quite effective to enhance the magnetic properties; (BH)_max can reach 3.0 MGOe for the ferrite sintered at 1220°C for 2 h. In general, (BH)_max is independent of the mole ratio of B₂O₃ to SiO₂ and dominantly influenced by the sintering temperature. The addition of 0.3 or 0.5 wt% fused additives showed no significant difference in the magnet quality. The quality of the magnet was decreased with increasing mole ratio of B₂O₃ to SiO₂ when unfused B₂O₃-SiO₂ was used under the same processing conditions.     

Hydrazine method of synthesis of γ-Fe2O3 useful in ferrites preparation. Part IV – preparation and characterization of magnesium ferrite, MgFe2O4 from γ-Fe2O3 obtained from hydrazinated iron oxyhydroxides and iron (II) carboxylato-hydrazinates

Electro-magnetic properties and microstructural characterization of MgFe₂O₄ synthesized by a ceramic technique at 1000°C from iron oxides, consisting of mainly -Fe₂O₃ and traces of alpha-Fe₂O₃, prepared from iron ore rejects, are compared with the ferrite obtained from commercial alpha-Fe₂O₃. The sources of -Fe₂O₃ are hydrazinated iron (II) carboxylates and iron oxyhydroxides which autocatalytically decompose giving mainly -Fe₂O₃ of uniform particles of 10–30 nm (by scanning electron microscopy (SEM) studies) having high surface area. The ferrite synthesized from such nanoparticle size -Fe₂O₃ gave a porosity of 25% with grains ranging from 0–3 m. On the other hand, MgFe₂O₄ obtained from commercial alpha-Fe₂O₃ grains (of 1–2 m size) gave particles of 0–6 m with a porosity 42%. Saturation magnetization values 922–1168 G are found for MgFe₂O₄ from -Fe₂O₃ source while the alpha-Fe₂O₃ source gave the lowest value, 609. The Curie temperature, T_c, from magnetic susceptibility, initial permeability and resistivity measurements indicated a highest T_c of 737 K for MgFe₂O₄ from alpha-Fe₂O₃, while lower values are found for the ferrite prepared from -Fe₂O₃.     

Structural and dielectric properties of nanocrystalline Nd<Superscript>3+</Superscript> substituted nickel–zinc ferrites

Ferrite samples with general formula Ni_1−xZn_xNd_yFe_2−yO₄ (x = 0, 0.2, 0.4, 0.6, 0.8 and 1; y = 0.01, 0.02 and 0.03) were synthesized by oxalate co-precipitation technique. The X-ray diffraction study confirms the formation of single-phase cubic spinel structure. The lattice constant of the samples increases with increase in zinc content and obeys Vegard’s law. On Nd³⁺ substitution lattice constant of the samples slightly increases except zinc ferrite. The frequency dependence of the dielectric constant, dielectric loss and AC conductivity of the samples were determined in the frequency range from 20 Hz to 1 MHz at room temperature. The experimental results reveal that the dielectric constant and dielectric loss decreases where as AC electrical conductivity increases with increase in frequency. The dielectric loss increases with increase in zinc content whereas it decreases with increase in Nd³⁺ content. There is no appreciable change in permittivity of the samples with increase in Nd³⁺ content. Permeability of all the samples increases with increase in Nd³⁺ content. Because of lower dielectric loss, Nd³⁺ substituted Ni–Zn ferrites are useful in electronic devices.     

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.     

Dielectric properties and electrical conduction in yttrium iron garnet (YIG)

The dielectric properties (dielectric constant and loss) of a single crystal of yttrium iron garnet (Y₃Fe₅O₁₂) were measured in the temperature range 77–725 K and in the frequency range 100 Hz-1 MHz. AC conductivity was derived from dielectric constant and loss. DC conductivity was measured in the temperature range 30–725 K. Thermoelectric power (TEP) was measured from 77–800 K. On the basis of the results, conduction in this garnet is interpreted as due to small polarons. The nature of conduction at different temperature ranges is discussed in the light of existing reports on defect formation.     

Oxidation of ferritic steels dispersion strengthened with TiO2 and Y2O3 oxides in lead melt

We study the structure and composition of scales formed during the contact of Fe–13Cr–2Motype ferritic steels hardened with oxides TiO₂ and Y₂O₃ with oxygen-containing (10⁻³ mass% O) lead melt at 550°C for 1000 h. It is established that a Fe₃ O₄ – Fe (Fe_{1 − x} , Cr _x )₂ O₄ two-layer scale forms. Its upper layer (Fe₃ O₄) grows in the direction of the melt, and the internal layer (Fe (Fe_{1 – x} , Cr _x )₂ O₄) grows in the direction to the matrix. Oxide particles favor an increase in the porosity of the internal sublayer of the scale. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 5, pp. 38 – 44, September–October, 2008.     

0.3–3 GHz magneto-dielectric properties of nanostructured NiZnCo ferrite from hydrothermal process

A simple hydrothermal route with cetyltrimethylammonium bromide was proposed for directly synthesizing single-crystalline NiZnCo ferrite at 160 °C. X-ray diffraction patterns and micrographs indicate that products consist of spinel ferrite nanocrystals. The dielectric constant of NiZnCo ferrite is about 11 and the imaginary part of complex permittivity is 1.3. The saturation magnetization of Ni_0.54Zn_0.48Fe_1.98O₄ increases from 41.36 to 73.9 emu/g for Ni_0.55Zn_0.46Fe_1.98O₄ with a cobalt stoichiometry of 0.01. The real part μ′ of complex permeability for NiZnCo ferrite reaches 3 at 1 GHz. The imaginary part μ″ of NiZnCo ferrite has values higher than 1.2 within 0.7–3 GHz. Through the incorporation of the magnetic fillers, the low dielectric constant of the composites may meet the requirements of impedance matching to achieve maximal absorption of the electromagnetic energy in GHz frequency range.     

Synthesis of Fe–4.6?wt% B alloy via electro-deoxidation of mixed oxides

Fe–4.6 wt% B alloy was synthesized via electro-deoxidation of the mixed oxide precursor. The oxides, Fe₂O₃ and B₂O₃, mixed in suitable proportions were sintered at 900 °C yielding pellets with a two-phase structure; Fe₂O₃ and Fe₃BO₆. The sintered pellets, connected as cathode, were then electro-deoxidized in molten CaCl₂ or in CaCl₂–NaCl eutectic, against a graphite anode at 3.1 V. The electrolysis at 850 °C has successfully yielded a powder mixture of Fe and Fe₂B. Sequence of changes during the electrolysis was followed by interrupted experiments conducted at 850 °C. This has shown that iron is extracted quite early during the electrolysis through the depletion of oxygen from the starting oxide; Fe₂O₃, forming the other iron oxides in the process. Boron follows a more complicated route. Fe₃BO₆, the initial boron-bearing phase, was depleted in the early stages due to its reaction with molten salt. This gave rise to the formation of calcium borate. Boron was extracted from calcium borate in later stages of electrolysis, which appeared to have reacted in situ with the iron forming compound Fe₂B. An erratum to this article can be found at     

Saturation magnetization and structural analysis of Ni0.6Zn0.4NdyFe2?yO4 by XRD, IR and SEM techniques

Compositions having general formula Ni_0.6Zn_0.4Nd _y Fe_2−y O₄ (where y = 0, 0.01, 0.02 and 0.03) were prepared by oxalate co-precipitation method from high purity sulphates. The samples were characterized by XRD, IR and SEM techniques. X-ray diffraction measurements confirmed the formation of single phase cubic spinel structure. Lattice constant increases with rise in Nd³⁺ content and obeys Vegard’s law. Crystallite size of the samples lies in the range 29.98–31.15 nm. The IR spectra shows two strong absorption bands in the frequency range 400–600 cm⁻¹. Further, it shows that Nd³⁺ occupies B-site. SEM studies show that the grain size of the samples decreases with increase in Nd³⁺ content. Saturation magnetization of Nd³⁺ substituted Ni–Zn ferrites is higher than unsubstituted ferrite.     

Spinel ferrites of the quaternary system Cu-Ni-Fe-O: Synthesis and characterization

The decomposition of the freeze dried Cu(II)-Ni(II)-Fe(III) formate precursors at 1000°C in air yields complex oxides Cu_xNi_1−xFe₂O_4±δ (0 ≤ x ≤ 1) with a cubic spinel structure. For x < 0.7, single phase spinels are formed at 1000°C. However, for 0.7 ≤ x ≤ 1, Copper oxide (CuO) is identified as a second phase and the formation of a pure spinel phase requires an increase of the iron content in the mixture. For example, Cu_0.81Ni_0.1Fe_2.09O₄ is a single phase at 1000°C/air. Other single spinel phases Cu_0.5+yNi_0.5−y−zFe_2+zO_4±δ, 0 ≤ (y + z) ≤ 0.5, in the phase triangle Cu_0.5Ni_0.5Fe₂O₄–CuFe₂O₄–Cu_0.5Fe_2.5O₄ have been synthesized under special p(O₂)/T—synthesis conditions. The increase of the iron content requires an increase of the reaction temperature and/or a decrease of the p(O₂) in the reaction gas stream. The oxygen exchange between Cu_0.9Fe_2.1O_4.02 and the reducing gaseous phases shows that the non stoichiometry δ of copper ferrite is only about ±0.03. Significant changes in the oxygen content lead to the separation in different phases. The electrical and magnetic properties of copper ferrite samples depend on their chemical composition and preparation conditions.