Effects of composition and sintering temperature on properties of NiZn and NiCuZn ferrites

Effects of composition and sintering temperature on grain size, porosity and magnetic properties of the NiZn and NiCuZn ferrites were investigated. It was found that the lowest power loss could be obtained with the equimolar composition for both NiZn and NiCuZn ferrites, which could be attributed to the lowest porosity. A slight deficiency or excess of Fe₂O₃ content had no pronounced influence on saturation magnetic flux density (B_s) in our testing range. However, a slight excess of Fe₂O₃ was effective to improve the initial permeability, which could be attributed to decrease of the magnetocrystalline anisotropy. With the increase of sintering temperature, the initial permeability and power loss of the NiZn and NiCuZn ferrites had different development trend, which could be explained by the different variation trend of the grain size and porosity. Power losses of the NiCuZn ferrite samples were lower than that of the NiZn ferrite samples at any sintering temperature. Synthetically, the NiCuZn ferrites had a better performance than the NiZn ferrites in power field use.     

Structural, magnetic and electrical properties of NiCuZn ferrites prepared by microwave sintering method suitable for MLCI applications

Polycrystalline NiCuZn soft ferrites with stoichiometric iron were prepared by a novel microwave sintering method. The powders were calcined, compacted and sintered at 950 °C for 30 min in a microwave sintering furnace. X-ray diffraction patterns confirm the formation of single phase cubic spinel structure. The grain size was estimated using SEM micrographs. The lattice constant is found to increase with increase in zinc concentration. The sintered ferrites have been investigated for their physical, magnetic and electrical properties such as bulk density, X-ray density, porosity, anisotropy constant, initial permeability, saturation magnetization, DC resistivity, dielectric constant and dielectric loss as a function of zinc concentration. Permeability, saturation magnetization, dielectric constant and dielectric loss were found to increase while DC resistivity was found to decrease with the replacement of Zn with Ni. The present series of ferrites are found to posses properties that are suitable for the core materials in multilayer chip inductors.     

Preparation and investigation of low firing temperature NiCuZn ferrites with high relative initial permeability

The effects of CuO and V₂O₅ additions and the particle sizes of precursor materials on the microstructure and relative initial permeability of low firing temperature NiCuZn ferrites were investigated. It was found that additions of CuO and V₂O₅ contribute to the grain growth and densification of matrix in the sintering process, which were favorable for increase in relative initial permeability. The relative initial permeability was also strongly affected by the average particle size of precursor materials. Through using precursor materials of 0.8 μm average particle size and adding 10 mol% CuO and 0.20 mol% V₂O₅, for the low firing temperature NiCuZn ferrite, very high relative initial permeability of 1417 can be achieved at the frequency of 1 MHz.     

Effect of zinc concentration on the structural,electrical and magnetic properties of mixed Mn–Zn and Ni–Zn ferrites synthesized by the citrate precursor technique

Mixed manganese-zinc and nickel-zinc ferrites of composition Mn_0.2Ni_0.8−xZn_xFe₂O₄ where x=0.4$x=0.4$, 0.5 and 0.6 have been synthesized by the citrate precursor technique. Decomposition of the precursor at temperatures as low as 500 °C gives the ferrite powder. The ferrites have been investigated for their electrical and magnetic properties such as saturation magnetization, initial permeability, Curie temperature, AC-resistivity and dielectric constant as a function of sintering temperature and zinc content. Structural properties such as lattice parameter, grain size and density are also studied. The mixed compositions exhibited higher saturation magnetizations at sintering temperatures as low as 1200 °C. While the Curie temperature decreased with zinc content, the permeability was found to increase. The AC-resistivity ranged from 10⁵–10⁷ Ω cm and decreased with zinc content and sintering temperature. The dielectric constants were lower than those normally reported for the Mn–Zn ferrites. Samples sintered at 1400 °C densified to about 94% of the theoretical density and the grain size was of the order of about 1.5 μm for the samples sintered at 1200 °C and increased subsequently with sintering temperature.     

Effect of sintering temperature on structural and magnetic properties of NiCuZn and MgCuZn ferrites

The low temperature microwave sintered NiCuZn and MgCuZn ferrites with compositions Ni_0.35Cu_0.05Zn_0.60Fe₂O₄ and Mg_0.35Cu_0.05Zn_0.60Fe₂O₄ were synthesized by conventional mixed oxide method. NiCuZn and MgCuZn ferrite samples obtained showed better sintered densities at 950 and 900 °C, respectively. The scanning electron micrographs of both the ferrite samples appear to be very much similar. The magnitude of initial permeability of MgCuZn ferrite samples is found to be obviously higher than those of NiCuZn ferrite samples at all sintering temperatures. This is mainly due to the fact that MgCuZn ferrite has smaller magnetocrystalline anisotropy constant and magnetostrictive constant. NiCuZn ferrites have higher saturation magnetization than MgCuZn ferrites, which is due to the higher magnetic moment of NiCuZn ferrites. Our results indicate that the microwave sintering method seems to be a potential technique in the MLCI technology.     

Structural and magnetic properties of Li-Cu mixed spinel ferrites

Li_0.5−x/2Cu_xFe_2.5−x/2O₄ (where x=0.0-1.0) ferrites have been prepared by solid-state reaction. X-ray diffraction was used to study the structure of the above investigated ferrites at various sintering temperatures. Samples were sintered at 1000, 1100 and 1200 °C for 3 h in the atmosphere. For the sintering temperature of 1000 °C, Li_0.5−x/2Cu_xFe_2.5−x/2O₄ undergoes cubic to tetragonal transformation for higher Cu content. However, for the sintering temperature of 1100 and 1200 °C, X-ray diffraction patterns are mainly characterized by fcc structure, though presence of tetragonal distortion was found by other temperature dependence of initial permeability curves. The lattice parameter, X-ray density and bulk density were calculated for different compositions. Curie temperature was measured from the temperature dependence of initial permeability curves. Curie temperatures of Li-Cu mixed ferrites were found to decrease with the increase in Cu²⁺ content due to the reduction of A-B interaction. As mentioned earlier, temperature dependence of initial permeability curves was characterized by tetragonal deformation for the samples containing higher at% of Cu. The complex initial permeability has been studied for different samples. The B-H loops were measured at constant frequency, f=1200 Hz, at room temperature (298 K). Coercivity and hysteresis loss were estimated for different Cu contents.     

A study of ultrasonic velocity and attenuation on nanocystalline MgCuZn ferrites

The nanocrystalline MgCuZn ferrites with particle size (∼30 nm) have been synthesized by microwave-hydrothermal (M-H) method at 160 °C/45 min. The powders were densified at 750-900 °C/30 min using microwave sintering method. The sintered samples were characterized using X-ray diffraction and scanning electron microscope. The grain sizes of the sintered samples are in the range of 60-80 nm. The ultrasonic velocities have been measured on MgCuZn ferrites using the pulse transmission method at 1 MHz. The ultrasonic velocity is found to decrease with an increase of temperature. A small anomaly is observed around the Curie temperature, 520 K. The anomaly observed in the thermal variation of longitudinal velocity and attenuation is explained with the help of magneto-crystalline anisotropy constant.     

Structural and magnetic properties of Mg substituted NiCuZn Nano Ferrites

The present paper examines the effect of magnesium substitution on structural and magnetic properties of NiCuZn nano ferrites synthesised by sol-gel method. The prepared samples were characterised by using X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Field Emission Scanning Electron Microscopy (FE-SEM) and Vibration sample magnetometer (VSM) techniques. The phase identification, unit cell parameter and crystallite size was determined using XRD analysis. The lattice constant reduced with increasing Mg content. Room temperature saturation magnetisation and coercivity showed reverse trend with increasing Mg content. Curie temperature (T_c) obtained from the thermo magnetic curves increases with Mg concentration. The initial permeability (μ_i) decreased with increasing Mg content. This is due to reduced magnetisation, grain size and increased magneto-crystalline anisotropy constant. Simultaneously, there is an upward shift of domain wall relaxation frequency with increasing Mg content. Also the permeability is observed to be constant up to 30 MHz frequency range showing compositional stability and quality of the material. The prepared samples were suitable for applications in Multilayer Chip Inductors due to their invariable permeability up to 30 MHz frequency and high thermal stability along with low sintering temperature.     

Influences of Fe-deficiency on electromagnetic properties of low-temperature-fired NiCuZn ferrites

Low-temperature-fired NiCuZn ferrites with the formula Ni_0.45Cu_0.2Zn_0.35Fe_2−xO_4−3/2x with x values ranging from 0.00 to 0.25 in steps of 0.05 and sintered at 900 °C have been investigated in the present work. It was found that the content of Fe-deficiency could obviously influence the microstructure, sintering behavior, saturation magnetization, permeability and permittivity spectra properties of the ferrites. The variations were much different from those of the high-temperature-fired NiZn ferrites. And the corresponding mechanisms involved were discussed in detail. All-around consideration, the NiCuZn ferrite with 0.10 Fe-deficiency in composition had the best performances on sintering behavior and electromagnetic properties.     

Effects of Ta2O5 addition on the microstructure and temperature dependence of magnetic properties of MnZn ferrites

MnZn ferrites with the chemical formula Mn_0.68Zn_0.25Fe_2.07O₄ have been prepared by the conventional ceramic technique. Toroidal cores were sintered at 1350 °C for 4 h in N₂/O₂ atmosphere with 4% oxygen. Then the influence of Ta₂O₅ addition on the microstructure and temperature dependence of magnetic properties of MnZn ferrites was investigated by characterizing the fracture surface micrograph and measuring the magnetic properties over a temperature ranging from 25 to 120 °C. The results show that, when the Ta₂O₅ concentration is not more than 0.04wt%, the grain size has a slight increase with the increase of Ta₂O₅ concentration, the temperature of secondary maximum peak in the curve of initial permeability versus temperature and the lowest power loss shift to lower temperature. However, excessive Ta₂O₅ concentration (>0.04wt%) results in the exaggerated grain growth and porosity increase, which make the initial permeability and saturation magnetic flux density decrease and the power loss increase at room temperature. Furthermore, the temperature of secondary maximum peak in the curve of initial permeability versus temperature and the lowest power loss shift to about 100 °C.     

Enhancement of microstructure and initial permeability due to Cu substitution in Ni0.50−xCuxZn0.50Fe2O4 ferrites

Structural and magnetic properties of Cu substituted Ni_0.50−xCu_xZn_0.50Fe₂O₄ ferrites (where x=0.0-0.25) prepared by an auto combustion method have been investigated. The X-ray diffraction patterns of these compositions confirmed the formation of the single phase spinel structure. The lattice parameter increases with the increase in Cu²⁺ content obeying Vegard's law. The particle size of the starting powder compositions varied from 22 to 72 nm. The theoretical density increases with increase in copper content whereas the Néel temperature decreases. The bulk density, grain size and permeability increases up to a certain level of Cu²⁺ substitution, beyond that all these properties decrease with increase in Cu²⁺ content. The bulk density increases with increase in sintering temperatures up to 1250 °C for the parent composition, while for substituted compositions it increases up to 1200 °C. Due to substitution of Cu²⁺, the real part of the initial permeability increases from 97 to ∼390 for the sample sintered at 1100 °C and from 450 to 920 for the sample sintered at 1300 °C. The ferrites with higher initial permeability have a relatively lower resonance frequency, which obey Snoek's law. The initial permeability strongly depends on average grain size and intragranular porosity. The saturation magnetization, M_s, and the number of Bohr magneton, n(μ_B)_, decreases up to x=0.15 due to the reduction of the A-B interaction in the AB₂O₄ spinel type ferrites. Beyond that value of x, the M_s and the n(μ_B) values are enhanced. The substitution of Cu²⁺ influences the magnetic parameters due to modification of the cation distribution.     

Phase formation, grain growth and magnetic properties of NiCuZn ferrites

We studied the effects of iron-deficient, stoichiometric and iron-excessive compositions on the phase formation, crystal structure, grain growth and magnetic property of NiCuZn ferrites. As the Fe₂O₃ ratio increased from iron-deficiency 47.0 mol% to iron-excess 54.0 mol%, the X-ray diffraction peaks initially shifted towards lower angle and then moved to higher angle. Correspondingly, an initial increase in lattice parameter followed by a subsequent decrease was observed. The lattice parameter showed a maximum 8.396 Å when the Fe₂O₃ ratio was 49.0 mol%. When the system was iron-deficient, ZnO phase was detected in addition to the spinel phase. However, equimolar and iron-excessive compositions exhibited a single spinel phase. As the content of Fe₂O₃ increased, the grain size, density, saturation induction and initial permeability first increased and then decreased. Core losses at 50 kHz and 150 mT, however, changed in the opposite way. Finally, NiCuZn ferrite with an equimolar composition (50.0 mol%) showed the highest initial permeability (1467), highest saturation induction (361 mT) and lowest core losses (234 kW/m³).     

Low temperature sintering of sub-stoichiometric Ni-Cu-Zn ferrites: Shrinkage, microstructure and permeability

We have studied sub-stoichiometric Ni-Cu-Zn ferrites with iron deficiency (i.e., <50mol% Fe₂O₃) of composition Ni_0.20Cu_0.20Zn_0.60+zFe_2−zO_4−(z/2) with 0≤z≤0.06. The temperature of maximum shrinkage rate is shifted from T=1000 °C for z=0 towards lower temperatures down to T=900 °C for a sub-stoichiometric ferrite with z=0.02. Dense samples are obtained after firing at 900 °C for z>0 only. Sub-stoichiometric compositions (z>0) do not form single-phase spinel ferrites after sintering at 900 °C, but rather represent mixtures of CuO and a stoichiometric ferrite with slightly modified composition. The formation of small amounts of CuO at grain boundaries is demonstrated by XRD and SEM. The permeability is increased from μ=80 for stoichiometric ferrites (z=0) to μ=660 for z=0.02. The formation of CuO during sintering of sub-stoichiometric ferrites supports densification and is a prerequisite for low temperature firing of multilayer inductors. Addition of 1 wt% Bi₂O₃ as liquid phase sintering aid is required to provide sufficient densification of the stoichiometric ferrite (z=0) at 900 °C. Addition of 0.37 wt% Bi₂O₃ to a sub-stoichiometric ferrite (z=0.02) results in dense samples after firing at 900 °C; however, the microstructure formation is dominated by heterogeneous grain growth.     

Preparation and properties of ferromagnetic Z-type hexaferrite from wet milled mixtures of intermediates

Almost pure Z-type phases of hexaferrites were synthesized by firing preliminarily milled M- and Y-type phases intermediates at 1250 °C. These phases were obtained by calcining the stoichiometric powder mixture precursors at 1080 °C, followed by wet milling in a planetary mill for 1 h and subsequent heating at 1250 °C that increased the fractional crystallization of the Z-phase up to 96%. Addition of 0.2 wt% SiO₂ to the intermediates reduced the milling time necessary for the sintered density required for practical permeability measurement. Z-phase hexaferrite sintered at 1250 °C for 2 h exhibited fairly good high-frequency properties, i.e. an initial real permeability of up to 19.3 below 100 MHz and 8–10 at around 1 GHz.     

Microwave sintering of high-permeability (Ni0.20Zn0.60Cu0.20)Fe1.98O4 ferrite at low sintering temperatures

The (Ni_0.20Zn_0.60Cu_0.20)Fe_1.98O₄ ferrite was sintered using microwave sintering and conventional sintering technique, respectively. It was found that microwave sintering technique can effectively promote the forward diffusion of ions and thus accelerate the sintering process, resulting in the grain growth and the densification of matrix. At the low frequency of 100 kHz, the magnetizing contribution of domain wall motion is predominant, and compact and coarse matrixes are favorable for domain wall motion, giving rise to improvement of relative initial permeability and loss of ferrites. Using microwave sintering technique, for the (Ni_0.20Zn_0.60Cu_0.20)Fe_1.98O₄ ferrite, the relative initial permeability μ_i of about 2000 and the relative loss factor tanδ/μ_i of about 8.7×10⁻⁶ at 100 kHz were achieved at only 980 °C sintering temperature. In addition, the sintering time of ferrites was reduced from 5 to 0.5 h by using microwave sintering technique.     

Effects of SnO2 addition on the microstructure and magnetic properties of NiZn ferrites

The microstructure and magnetic properties of SnO₂-doped NiZn ferrites prepared by a solid-state reaction method have been investigated. Due to its low melting point (∼1127 °C), moderate SnO₂ enhanced mass transfer and sintering by forming liquid phase, which accelerated the grain growth. However, excessive SnO₂ producing much of liquid phase retarded mass transfer and sintering, leading to a decrease in grain size. The diffraction intensity of the samples doped with SnO₂ addition was stronger than that of the sample without addition. The lattice constant initially decreased up to a content of 0.10 wt% and showed an increase at higher content up to 0.50 wt%. The initial permeability (μ_i) initially increased up to a content of 0.15 wt% and showed a decrease at higher content up to 0.50 wt%; however, losses (P_L) measured at 50 kHz and 150 mT changed contrarily. Both saturation induction (B_S) and Curie temperature (T_C) decreased gradually with increasing SnO₂. Finally, the sample doped with 0.10–0.15 wt% SnO₂ showed the higher permeability and lower losses.     

Enhancement of initial permeability due to Mn substitution in polycrystalline Ni0.50−xMnxZn0.50Fe2O4

The structural and magnetic properties of Mn substituted Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.00, 0.10 and 0.20) sintered at various temperatures have been investigated thoroughly. The lattice parameter, average grain size and initial permeability increase with Mn substitution. Both bulk density and initial permeability increase with increasing sintering temperature from 1250 to 1300 °C and above 1300 °C they decrease. The Ni_0.30Mn_0.20Zn_0.50Fe₂O₄ sintered at 1300 °C shows the highest relative quality factor and highest initial permeability among the studied samples. The initial permeability strongly depends on average grain size and intragranular porosity. From the magnetization as a function of applied magnetic field, M(H), it is clear that at room temperature all samples are in ferrimagnetic state. The number of Bohr magneton, n(μ_B), and Neel temperature, T_N, decrease with increasing Mn substitution. It is found that Mn substitution in Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.20) decreases the Neel temperature by 25% but increases the initial permeability by 76%. Possible explanation for the observed characteristics of microstructure, initial permeability, DC magnetization and Neel temperature of the studied samples are discussed.     

On the thermal stability of Co2Z hexagonal ferrites for low-temperature ceramic cofiring technologies

Co₂Z hexaferrite Ba₃Co₂Fe₂₄O₄₁ was prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD studies show that at T<1300 °C different ferrite phases coexist with the M-type hexaferrite as majority phase between 1000 and 1100 °C and the Y-type ferrite at 1230 °C. The Z-type material has its stability interval between 1300 and 1350 °C. Both synthesis routes result in almost single-phase Z-type ferrites after calcination at 1330 °C, intermediate grinding and sintering at 1330 °C. The permeability of Co₂Z-type ferrite of about μ=20 is stable up to several 100 MHz, with maximum losses μ′′ around 700 MHz. Addition of 3 wt% Bi₂O₃ as sintering aid shifts the temperature of maximum shrinkage down to 950 °C and enables sintering of Z-type ferrite powders at 950 °C. However, the permeability is reduced to μ=3. It is shown here for the first time that Co₂Z ferrite is not stable under these conditions; partial thermal decomposition into other hexagonal ferrites is found by XRD studies. This is accompanied by a significant decrease of permeability. This shows that Co₂Z hexagonal ferrite is not suitable for the fabrication of multilayer inductors for high-frequency applications via the low-temperature ceramic cofiring technology since the material is not compatible with the typical process cofiring temperature of 950 °C.     

Dependence of developing magnetic hysteresis characteristics on stages of evolving microstructure in polycrystalline yttrium iron garnet

The microstructure evolution in several polycrystalline yttrium iron garnet samples as a result of a sintering scheme was studied in detail, in parallel with the changes in their magnetic properties. Samples with nanometer sized starting powder were synthesized by employing the High-Energy Ball Milling technique and then sintering toroidal compacts of the milled powder. Nine sintered samples were obtained, each corresponding to a particular sintering from 600 °C to 1400 °C. The samples were characterized for their evolution in crystalline phases, microstructure and magnetic hysteresis-loops parameters. The results showed an increasing tendency of the saturation magnetization and saturation induction with grain size, which is attributed to crystallinity increase and to reduction of demagnetizing fields in the grains. The variation in coercivity could be related to anisotropy field changes within the samples due to grain size changes. In particular, the starting appearance of room temperature ferromagnetic order suggested by the sigmoid-shaped B-H loops seems to be dependent on a sufficient number of large enough magnetic domain-containing grains having been formed in the microstructure. Viewed simultaneously, the hysteresis loops appear to belong to three groups with different magnetism-type dominance, respectively dependent on phase purity and three different groups of grain size distributions.     

Effect of Mn substitution on the magnetic properties of MgCuZn ferrites

A series of Mn substituted MgCuZn ferrites (Mg_0.2Cu_0.2Zn_0.6O) (Fe_2−xMn_xO₃)_0.97 with x=0.00,0.01,0.03,0.05,0.07 were prepared with nanosized precursor powders synthesized by a sol–gel auto-combustion method. All the ceramic samples can be sintered at low temperature (930°C) (below the melt point of Ag (961°C)). The effect of Mn content on microstructures and magnetic properties were investigated. Experiment shows that low temperature sintered MgCuZn ferrites doped with Mn possess higher initial permeability and better grain structure than that of low temperature sintered NiCuZn ferrites prepared by the same method. Therefor, Mn doped MgCuZn ferrites should be ideal materials for high inductance multilayer chip inductor. It is thought that the variation of initial permeability of MgCuZn ferrites with the Mn substitution was attributed to the decrease of magnetostriction constant.