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.     

Synthesis of (Mg0.476Mn0.448Zn0.007)(Fe1.997Ti0.002)O4 powder and sintered ferrites by high energy ball-milling process

(Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder prepared by high energy ball-milling process were consolidated by microwave and conventional sintering processes. Phases, microstructure and magnetic properties of the ferrites prepared by different processes were investigated. The (Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder could be prepared by high energy ball-milling process of raw Fe₃O₄, MnO₂, ZnO, TiO₂ and MgO powders. Prefired and microwave sintered ferrites could achieve the maximum density (4.86 g/cm⁻³), the average grain size (15 μm) was larger than that (10 μm) prepared by prefired and conventionally sintered ferrites with pure ferrite phase, and the saturation magnetization (66.77 emu/g) was lower than that of prefired and conventionally sintered ferrites (88.25 emu/g), the remanent magnetization (0.7367 emu/g) was higher than that of prefired and conventionally sintered ferrites (0.0731 emu/g). Although the microwave sintering process could increase the density of ferrites, the saturation magnetization of ferrites was decreased and the remanent magnetization of ferrites was also increased.     

Low temperature fired Ni-Cu-Zn ferrite with Bi4Ti3O12

The Ni-Cu-Zn ferrites with different contents of Bi₄Ti₃O₁₂ ceramics (1-8 wt%) as sintering additives were prepared by the usual ceramic technology and sintered at 900 °C to adapt to the low temperature co-fired ceramic (LTCC) technology. The magnetic and dielectric properties of the ferrite can be effectively improved with the effect of an appropriate amount of Bi₄Ti₃O₁₂. For all samples, the ferrite sintered with 2 wt% Bi₄Ti₃O₁₂ has relatively high density (98.8%) and permeability, while the ferrite with 8 wt% Bi₄Ti₃O₁₂ has relatively good dielectric properties in a wide frequency range. The influences of Bi₄Ti₃O₁₂ addition on microstructure, magnetic and dielectric properties of the ferrite have been discussed.     

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.     

Effects of sintering temperature and Bi2O3 content on microstructure and magnetic properties of LiZn ferrites

The effects of sintering temperature and Bi₂O₃ content on the microstructure and magnetic properties of lithium–zinc (LiZn) ferrites prepared by a conventional ceramic method were investigated. The results show that the densification behavior and grain growth rate were greatly improved by the addition of Bi₂O₃, because a liquid phase sintering occurred during the sintering process at high temperature due to the low-melting point of Bi₂O₃ (825 °C). X-ray diffraction (XRD) patterns of the slightly doped samples did not reveal the appearance of any phase other than spinel LiZn ferrite. However, the secondary phase of perovskite BiFeO₃ was detected for Bi₂O₃ content of more than 0.25 wt%. The studies further show that Bi oxide was present at grain boundary, and promoted the grain growth as reaction center at lower temperature. A high saturation magnetization, squareness ratio, minimum ferromagnetic resonance linewidth and low coercive force were obtained for the sample with 1.00 wt% Bi₂O additive at lower sintering temperature (1100 °C).     

Glass additive influence on the sintering behavior, microstructure and microwave magnetic properties of Cu-Bi-Zn co-doped Co2Z ferrites

The Bi₂O₃-B₂O₃-ZnO-SiO₂ (BB35SZ) glass effects on the sintering behavior and microwave magnetic properties of Cu-Bi-Zn co-doped Co₂Z ferrites were investigated to develop low-temperature-fired ferrites. The glass wetting characteristics on the Co₂Z ferrite surface, X-ray diffractometer, scanning electron microscopy and a dilatometer were used to examine the BB35SZ glass effect on Co₂Z ferrite densification and the chemical reaction between the glass and Co₂Z ferrites. The results indicate that BB35SZ glass can be used as a sintering aid to reduce the densification temperature of Co₂Z ferrites from 1300 to 900 °C. 3(Ba_0.9Bi_0.1O)·2(Co_0.8Cu_0.2O)·12(Fe_1.975Zn_0.025O₃) ferrite with 2 wt% BB35SZ glass can be densified below 900 °C, exhibiting an initial permeability of 3.4. This process provides a promising candidate for multilayer chip magnetic devices for microwave applications.     

Chemical quenching of positronium in Fe2O3/Al2O3 catalysts

Fe₂O₃/Al₂O₃ catalysts were prepared by solid state reaction method using α-Fe₂O₃ and γ-Al₂O₃ nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes τ₃ and τ₄ are attributed to positronium annihilation in two types of pores distributed inside Al₂O₃ grain and between the grains, respectively. With increasing Fe₂O₃ content from 3 wt% to 40 wt%, the lifetime τ₃ keeps nearly unchanged, while the longest lifetime τ₄ shows decrease from 96 ns to 64 ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe₂O₃ content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of τ₄ is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores.     

The effect of P2O5 on the ultra broadband near-infrared luminescence from bismuth-doped SiO2-Al2O3-CaO glass

The effect of P₂O₅ on infrared luminescence properties of bismuth-doped SiO₂-Al₂O₃-CaO (SAC) glass was investigated. Under excitation of 690 and 808 nm LD, two infrared emissions from bismuth ions central at 1100 and 1300 nm were observed, respectively. The addition of P₂O₅ was not only found to lead to the increase of full width at half maximum (FWHM) of two infrared emissions, but also result in intensity variety of the infrared emissions. The intensity of the infrared emission located at 1300 nm is reduced by a factor of 2, while the luminescence at 1110 nm is increased by a factor of 5. We propose that the infrared emissions at 1100 and 1300 nm may originate from different valence Bi ion in glasses. Compared with emission at 1300 nm, the infrared emissions at 1100 nm is more possibly from the transition of lower valent Bi ion.     

Effect of TiO2/Al2O3 film coated diamond abrasive particles by sol-gel technique

The diamond abrasive particles were coated with the TiO₂/Al₂O₃ film by the sol-gel technique. Compared with the uncoated diamonds, the TiO₂/Al₂O₃ film was excellent material for the protection of the diamonds. The results showed that the incipient oxidation temperature of the TiO₂/Al₂O₃ film coated diamonds in air atmosphere was 775 °C, which was higher 175 °C than that of the uncoated diamonds. And the coated diamonds also had better the diamond's single particle compressive strength and the impact toughness than that of uncoated diamonds after sintering at 750 °C. For the vitrified bond grinding wheels, replacing the uncoated diamonds with the TiO₂/Al₂O₃ film coated diamonds, the volume expansion of the grinding wheels decreased from 6.2% to 3.4%, the porosity decreased from 35.7% to 25.7%, the hardness increased from 61.2H_RC to 66.5H_RC and the grinding ratio of the vitrified bond grinding wheels to carbide alloy (YG8) increased from 11.5 to 19.1.     

Synthesis of nanocrystalline Zn0.5Mn0.5Fe2O4 via in situ polymerization technique

Nanocrystalline Zn_0.5Mn_0.5Fe₂O₄ was synthesized through the pyrolysis of polyacrylate salt precursors prepared via in situ polymerization of the metal salts and acrylic acid. The pyrolysis behavior of the polymeric precursors was studied by use of thermal analysis. The as-obtained product was characterized by powder X-ray diffraction (XRD), transmission electron microscope (TEM), electron diffraction (ED) pattern, scanning electron microscopy (SEM) and electron dispersive X-ray (EDX) analysis. The results revealed that the particle size is in the range of 15–25 nm for Zn-Mn ferrites with good crystallinity. Magnetic properties of the sample at 300 K were measured using a vibrating sample magnetometer, which showed that the sample exhibited characteristics of superparamagnetism.     

Surface study of fine MgFe2O4 ferrite powder prepared by chemical methods

To study surface behaviors, MgFe₂O₄ ferrite materials having different grain sizes were synthesized by two different chemical methods, i.e., a polymerization method and a reverse coprecipitation method. The single phase of the cubic MgFe₂O₄ was confirmed by the X-ray diffraction method for both the precursors decomposed at 600-1000 °C except for a very small peak of Fe₂O₃ was detected for the samples calcined at 600 and 700 °C by the polymerization method. The crystal size and particle size increased with an increase in the sintering temperature using both methods. The conductance of the MgFe₂O₄ decreased when the atmosphere was changed from ambient air to air containing 10.0 ppm NO₂. The conductance change, C = G(air)/G(10 ppm NO₂), was reduced with an increase in the operating temperature. For the polymerization method, the maximum C-value was ca. 40 at 300 °C for the samples sintered at 900 °C. However, the samples sintered at 1000 °C showed a low conductance change in the 10 ppm NO₂ gas, because the ratio of the O₂ gas adsorption sites on the particle surface is smaller than those of the samples having a high C-value. The low Mg content on the surface affects the low ratio of the gas adsorption sites. For the reverse coprecipitation method, the particle size was smaller than that of the polymerization method. Although a stable conductance was obtained for the sample sintered at 900 and 1000 °C, its conductance change was less than that of the polymerization method.     

Synthesis and ion conductivity of (Bi2O3)0.75(Dy2O3)0.25 ceramics with grain sizes from the nano to the micro scale

Using (Bi₂O₃)_0.75(Dy₂O₃)_0.25 nano-powder synthesized by reverse titration co-precipitation method as raw material, dense ceramics were sintered by both Spark Plasma Sintering (SPS) and pressureless sintering. According to the predominance area diagram of Bi-O binary system, the sintering conditions under SPS were optimized. (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with relative density higher than 95% and an average grain size of 20 nm were sintered in only 10 min up to 500 °C. During the pressureless sintering process, the grain growth behavior of (Bi₂O₃)_0.75(Dy₂O₃)_0.25 followed a parabolic trend, expressed as D² − D₀² = Kt, and the apparent activation energy of grain growth was found to be 284 kJ mol^− 1. Dense (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with different grain sizes were obtained, and the effect of grain size on ion conductivity was investigated by impedance spectroscopy. It was shown that the total ion conductivity was not affected by the grain size down to 100 nm, however lower conductivity was measured for the sample with the smallest grain size (20 nm). But, although only the δ phase was evidenced by X-ray diffraction for this sample, a closer inspection by Raman spectroscopy revealed traces of α-Bi₂O₃.     

Effects of different sintering temperature and Nb2O5 content on structural and magnetic properties of Z-type hexaferrites

Nb-doped Z-type hexaferrites (Ba₃(Co_0.4Zn_0.6)₂Fe₂₄O₄₁) with composition of Ba₃(Co_0.4Zn_0.6)₂Fe₂₄O₄₁+x Nb₂O₅ (where x=0.0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.2, 1.6 and 2.0 wt%) were prepared by a solid-state reaction method. The effects of different sintering temperature (T_s) and Nb₂O₅ content on the sintering behaviors, phase composing, microstructure, and magnetic properties of the samples were investigated. The results from X-ray diffraction and scanning electron microscopy show that as the amount of Nb₂O₅ additive increases, the major phase changes to Z-phase, Simultaneously, M-phase and a small amount of niobate phase appear. The Nb₂O₅ additive promotes the grain growth as reaction center at lower sintering temperature (1220 °C), but at higher temperature (1260 °C), niobate phase separated out in grain boundaries as secondary phase will restrain abnormal grain growth, so closed pores in grains are not formed. The Nb₂O₅ additive can enhance densification, improve initial permeability of hexaferrites by increasing the grain growth of hexaferrite and the displacement of ions in the sintering process due to the aberration and activation of crystal lattice, which is accompanied by the solubility of Nb⁵⁺ in the hexaferrites. A relative density of 96%, maximum initial permeability (32–33), minimum coercivity (454–455 A/m) and resonance frequency above 400 MHz were obtained for the sample with 0.8 wt% Nb₂O₅ sintered at 1260 °C for 6 h.     

Effects of V2O5 addition on NiZn ferrite synthesized using two-step sintering process

The combined influence of a two-step sintering (TSS) process and addition of V₂O₅ on the microstructure and magnetic properties of NiZn ferrite was investigated. As comparison, samples prepared by the conventional single-step sintering (SSS) procedure were also studied. It was found that with 0.3 wt% V₂O₅ additive, the sample sintered by the two-step sintering process at a high temperature of 1250 °C for 30 min and a lower temperature of 1180 °C for 3 h exhibited more homogeneous microstructure and higher permeability with a high Q-factor. The results showed that the TSS method with suitable additive brought positive improvement of the microstructure and magnetic properties of NiZn ferrite.     

Color improvement of white-light through Mn-enhancing yellow-green emission of SrSi2O2N2:Eu phosphor for white light emitting diodes

Photoluminescence (PL) enhancement of SrSi₂O₂N₂:Eu and the resultant color improvement of white-light were investigated via co-doping Mn with Eu. We observed that a unique absorption of host lattice of SrSi₂O₂N₂ and its visible band emission peaked at around ∼550 nm for SrSi₂O₂N₂:Mn²⁺ in the wavelength range of 450-600 nm. This highly eye-sensitive ∼550 nm-peaked band emission of SrSi₂O₂N₂ doped with Mn²⁺ happens to overlap the 535 nm-peaked band emission of SrSi₂O₂N₂ doped with Eu²⁺, resulting in an intensified photoluminescence in a maximum by 355%. By combining this as-prepared Mn intensified SrSi₂O₂N₂:Eu phosphor with blue InGaN chip, the quality of white-light was improved to 93.3% for color rendering index and 3584 K for correlated color temperature.     

Correlation between Al2O3 particles and interface of Al-Al2O3 coatings by cold spray

Al-Al₂O₃ composite coatings with different Al₂O₃ particle shapes were prepared on Si and Al substrate by cold spray. The powder compositions of metal (Al) and ceramic (Al₂O₃) having different sizes and agglomerations were varied into ratios of 10:1 wt% and 1:1 wt%. Al₂O₃ particles were successfully incorporated into the soft metal matrix of Al. It was found that crater formation between the coatings and substrate, which is typical characteristic signature of cold spray could be affected by initial starting Al₂O₃ particles. In addition, when the large hard particles of fused Al₂O₃ were employed, the deep and big craters were generated at the interface between coatings and hard substrates. In the case of pure soft metal coating such as Al on hard substrate, it is very hard to get proper adhesion due to lack of crater formation. Therefore, the composite coating would have certain advantages.     

Influence of postdeposition annealing on structural properties and electrical characteristics of thin Tm2O3 and Tm2Ti2O7 dielectrics

We describe the structural properties and electrical characteristics of thin thulium oxide (Tm₂O₃) and thulium titanium oxide (Tm₂Ti₂O₇) as gate dielectrics deposited on silicon substrates through reactive sputtering. The structural and morphological features of these films were explored by X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and atomic force microscopy, measurements. It is found that the Tm₂Ti₂O₇ film annealed at 800 °C exhibited a thinner capacitance equivalent thickness of 19.8 Å, a lower interface trap density of 8.37 × 10¹¹ eV⁻¹ cm⁻², and a smaller hysteresis voltage of ∼4 mV than the other conditions. We attribute this behavior to the Ti incorporated into the Tm₂O₃ film improving the interfacial layer and the surface roughness. This film also shows negligible degrees of charge trapping at high electric field stress.     

Effect of Li2O and CoO-doping of CuO/Fe2O3 system on its surface and catalytic properties

Physicochemical, surface and catalytic properties of pure and doped CuO/Fe₂O₃ system were investigated using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), nitrogen adsorption at −196 °C and CO-oxidation by O₂ at 80-220 °C using a static method. The dopants were Li₂O (2.5 mol%) and CoO (2.5 and 5 mol%). The results revealed that the increase in precalcination temperature from 400 to 600 °C and Li₂O-doping of CuO/Fe₂O₃ system enhanced CuFe₂O₄ formation. However, heating both pure and doped solids at 600 °C did not lead to complete conversion of reacting oxides into CuFe₂O₄. The promotion effect of Li₂O dopant was attributed to dissolution of some of dopant ions in the lattices of CuO and Fe₂O₃ with subsequent increase in the mobility of reacting cations. CoO-doping led also to the formation of mixed ferrite Co_xCu_1−xFe₂O₄. The doping process of the system investigated decreased to a large extent the crystallite size of unreacted portion of Fe₂O₃ in mixed solids calcined at 600 °C. This process led to a significant increase in the S_BET of the treated solids. Doping CuO/Fe₂O₃ system with either Li₂O or CoO, followed by calcination at 400 and 600 °C decreased its catalytic activity in CO-oxidation by O₂. However, the activation energy of the catalyzed reaction was not much affected by doping.     

Microwave-assisted synthesis of SrFe12O19 hexaferrites

Ultra-fine and homogeneous SrFe₁₂O₁₉ hexaferrites were synthesized by a microwave-assisted calcination route. The calcined precursors were prepared by a sol-gel auto-combustion method using Fe(NO₃)₃·9H₂O, Sr(NO₃)₂ and citric acid as starting materials. The structures, powder morphology and magnetic properties of the products were characterized by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer. The results showed that microwaves are helpful to reduce the calcination temperature and shorten the calcination time. The ferrites with saturation magnetization, remanence and intrinsic coercivity of 54.80 emu/g, 29.52 emu/g and 5261 Oe, respectively, were obtained in samples calcined at 800 °C for 80 min.     

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).