Influence of chemistry and hot rolling conditions on high permeability non-grain oriented silicon steel

This paper discusses the influence of chemical composition on the final electromagnetic properties in higher permeability material. Furthermore, the effect of the hot rolling practice and the end of austenite transformation temperature range on the hot band microstructure is described. The magnetic polarization J₅₀₀₀ better than 1.7 T, using hot rolling conditions 40 mm transfer bar thickness, finish mill entry temperature 1000 °C, and finishing temperature 800–840 °C and after decarburization heat treatment and grain growth treatment, was obtained.     

Effect of annealing prior to cold rolling on magnetic and mechanical properties of low carbon non-oriented electrical steels

The effects of annealing prior to cold rolling on the microstructure, magnetic and mechanical properties of low-C grain non-oriented (GNO) electrical steels have been investigated. The grain structure of hot-rolled electrical steel strips is modified by annealing at temperatures between 700 and 1050 °C. Annealing at temperatures less than the ferrite to austenite+ferrite transformation temperature on heating (Ac₁) causes a marginal effect on the grain size. However, annealing in the intercritical region at temperatures between Ac₁ and Ac₃ (the ferrite+austenite to austenite transformation temperature on heating) causes rapid decarburization and development of large columnar ferrite grains free of carbide particles. This microstructure leads, after cold rolling and a fast annealing treatment, to carbide free, large ferrite grain microstructures with magnetic and mechanical properties superior to those observed typically in the same steel in the industrially fully processed condition. These results are attributed to the increment in grain size and to the {1 0 0} fiber texture developed during the final annealing at temperatures up to 850 °C. Annealing at higher temperatures, T>Ac₃, results in a strong {1 1 1} fiber texture and an increase of the quantity of second phase particles present in the microstructure, which lead to a negative effect on the final properties. The results suggest that annealing prior to cold rolling offers an attractive alternative processing route for the manufacture of fully processed low C GNO electrical steels strips.     

The influence of shear bands on final structure and magnetic properties of 3% Si non-oriented silicon steel

The presence of shear bands in the deformed material before final annealing is very important for Goss and Cube textures formation in silicon steel [S.C. Paolinelli, M.A. Cunha, J. Magn. Magn. Mater. 255 (2003) pp. 379. [1]; J.T. Park, J.A. Szpunar, Acta Mater., 51 (2003) 3037. [2]]. The increase of the hot-band grain size can increase the number of shear bands, which favor the nucleation of these orientations. In this work, the effect of the hot band grain size variation, promoted by varying the hot rolling finishing temperature, on final structure and magnetic properties was investigated for 3% Si alloy. It was found that the increase of the hot-band grain size increases the occurrence of shear bands and promotes an increase of η fiber fraction and a reduction of γ fiber fraction, improving the magnetic induction. On the other hand, the final grain size is reduced when the hot-band grain size is larger than 190 μm, deteriorating the core loss values in spite of the texture benefits. The reduction of final grain size was explained by the increase of the number of nuclei at the beginning of the recrystallization caused by the increase of shear bands in the deformed material.     

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.     

Low core loss non-oriented silicon steels

Low core loss non-oriented silicon steels are produced with high (Si+Al) content to reduce eddy current losses. However, high alloy content has detrimental effect on mechanical properties, saturation polarization and thermal conductivity. A new generation of medium and, particularly, low core loss non-oriented silicon steels was developed, with lower alloy content than the conventional grades, based on improved purity and texture. The development allowed the production of new low loss grades, with maximum core loss (W_1.5/50) of 2.30 W/kg at 0.50 mm and 1.95 W/kg at 0.35 mm, with high permeability (J₅₀ of 1.7 and 1.72 T, respectively). Texture improvement was based on hot band structure control and higher boundary mobility. Large hot band grain size and low [1 1 0]∥RD fiber fraction in the hot band texture contribute to reduce the intensity of [1 1 1]∥ND and slightly increase the intensity of [0 0 1]∥RD in the final product. Higher grain boundary mobility and/or a two-stage cold rolling process, with an intermediate annealing, increase the fraction of [0 0 1]∥RD and reduce the fraction of [1 1 1]∥ND on recrystallization and lead to favorable texture evolution on grain growth.     

Preparation of non-oriented silicon steel with high magnetic induction using columnar grains

Columnar grains can lead to detrimental surface ridging and an inhomogeneous microstructure, although their {1 0 0}〈0 v w〉 texture is considered desirable due to their good magnetic properties in non-oriented silicon steel. Based on the hereditary tendency of {1 0 0}〈0 v w〉 texture, the effects of lubrication and heating rate on texture and on final magnetic properties were investigated using a cast slab containing 100% columnar grains. Hot rolling with lubrication, normalization at low heating rate, two-stage cold rolling, and final annealing at 1000 °C helped achieve high performance. As a result, a new non-oriented silicon steel with high magnetic induction (B₅₀=1.82 T) and low core loss (P_1.5=2.35 W/kg) was prepared. The possibility of further performance optimization was also discussed.     

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.     

Room temperature photoluminescence mechanism of SiOx film after annealing at different temperatures

SiO_x films were deposited on Si(1 0 0) substrates by evaporation of SiO powder. The samples were annealed from room-temperature (RT) to 1100 °C. After the samples were cooled down to RT, photoluminescence (PL) spectra from these samples were measured. It was found that when the annealing temperature Ta is not higher than 1000 °C, there is a PL centered at 620 nm, and with Ta increasing the intensity increases at first and then decreases when Ta is higher than 500 °C. When Ta is no less than 1000 °C another PL peak located at 720 nm appears. Combined with Raman and XRD spectra, we confirm that the latter PL is from Si nanocrystals that start to form when Ta is higher than 1000 °C. PL spectra for Ta<900 °C were discussed in detail and was attributed to defects in the matrix rather than from Si clusters.     

Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

High purity Fe₂O₃/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe₂O₃/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe₂O₄. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe₂O₃/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.     

Influence of deposition temperature on the structural and morphological properties of Be3N2 thin films grown by reactive laser ablation

Be₃N₂ thin films have been grown on Si(1 1 1) substrates using the pulsed laser deposition method at different substrate temperatures: room temperature (RT), 200 °C, 400 °C, 600 °C and 700 °C. Additionally, two samples were deposited at RT and were annealed after deposition in situ at 600 °C and 700 °C. In order to obtain the stoichiometry of the samples, they have been characterized in situ by X-ray photoelectron (XPS) and reflection electron energy loss spectroscopy (REELS). The influence of the substrate temperature on the morphological and structural properties of the films was investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The results show that all prepared films presented the Be₃N₂ stoichiometry. Formation of whiskers with diameters of 100-200 nm appears at the surface of the films prepared with a substrate temperature of 600 °C or 700 °C. However, the samples grown at RT and annealed at 600 °C or 700 °C do not show whiskers on the surface. The average root mean square (RMS) roughness and the average grain size of the samples grown with respect the substrate temperature is presented. The films grown with a substrate temperature between the room temperature to 400 °C, and the sample annealed in situ at 600 °C were amorphous; while the αBe₃N₂ phase was presented on the samples with a substrate temperature of 600 °C, 700 °C and that deposited with the substrate at RT and annealed in situ at 700 °C.     

Multi-peak behavior of photoluminescence of silica particles heat-treated in hydrogen at elevated temperature

Pure nano-partical silica was prepared by sol-gel method, and then was treated in a H₂ ambient at different temperatures. The surface structure and valence bonding of samples were analyzed with IR spectrum and X-ray induced photoelectron spectroscopy, respectively. Its photoluminescence property was studied with fluorescence spectroscopy. The results showed that only one luminescence band at 344 nm exists for silica heat-treated at 450 °C. Silica samples heat-treated at higher temperatures showed completely different luminescence comparing with the samples heat-treated at lower temperatures. Multi-peaks were found in the SiO₂ samples heat-treated at temperatures higher than 700 °C, in which the luminescence peaks at about 379 and 392 nm are similar with the β bands in silica glass originated from the same defect center of two-fold coordinated silicon atoms, and the luminescence bands at about 493 and 528 nm are few reported. The intensity of the luminescence bands increase with the increase of heat treatment temperature from 700 to 900 °C.     

Improvement in magnetic properties of La substituted BaFe12O19 particles prepared with an unusually low Fe/Ba molar ratio

In this study, effect of lanthanum substitution on the phase composition, lattice parameters and magnetic properties of barium hexaferrite has been studied in samples synthesized in ammonium nitrate melt. Samples, prepared with different lanthanum amount and having various initial Fe/(Ba+La) ratios in between 12 and 2 {(Ba_1−xLa_x)·n Fe₂O₃, where 0≤x≤1 and 1≤n≤6)}, are sintered at temperatures from 800 to 1200 °C. The lattice parameters, both a and c, decreases with increasing La amount which results in a decrease of the unit cell volume. The scanning electron microscope micrographs show that the pure and La-substituted sample with x=0.3, both calcinated at 1000 °C, have grain sizes smaller than 1 μm. The coercivities of the La-substituted samples increase with increasing La amount and reaches to a maximum value of 5.73 kOe, when x=0.3. Sintering at higher temperatures (above 1000 °C) decreases the coercivity, resembling a transition from single to multi-domain behavior of the particles, while saturation magnetization of the samples continues to increase due to the increasing grain size. Magnetization measurements of the samples prepared with different Fe/(Ba+La) molar ratios, n's, revealed that the specific saturation magnetization slightly increases with decreasing n, while coercivities fluctuates around 5.5 kOe. However, a sharp increase in the saturation magnetization has been observed in the sample having n=1 and washed in HCl. It was measured as 59.2 emu/g at 15 kOe, which is higher than that of the pure sample (57.5 emu/g). Thus, the magnetic parameters are optimized in the sample Ba_0.7La_0.3Fe₁₂O₁₉ so as to maximize both coercivity and specific saturation magnetization in the HCl-washed sample synthesized by starting with an unusually low Fe/(Ba+La) molar ratio of 2 (or n=1).     

The effect of B2O3 addition to the microstructure and magnetic properties of Ni0.4Zn0.6Fe2O4 ferrite

The effects of 0.01 and 0.1 mol B₂O₃ addition to the microstructure and magnetic properties of a Ni–Zn ferrite composition expressed by a molecular formula of Ni_0.4Zn_0.6Fe₂O₄ were investigated. The toroid-shaped samples prepared by pressing the milled raw materials used in the preparation of the composition were sintered in the range of 1000–1300 °C. The addition of 0.01 mol B₂O₃ increased the grain growth and densification giving rise to reduced intergranular and intragranular porosity due to liquid-phase sintering. The sintered toroid sample at 1300 °C gave the optimum magnetic properties of B_r=170 mT, H_c=0.025 kA/m and a high initial permeability value of μ_i=4000. The increment of the B₂O₃ content to 0.1 mol resulted in a pronounced grain growth and also gave rise to large porosity due to the evaporation of B₂O₃ at higher sintering temperatures. Hence, it resulted in an air-gap effect in the hysteresis curves of these samples.     

Structural characterization of SiGe nanoclusters formed by rapid thermal annealing

This work presents the structural characterization of nanoclusters formed from a-Si:H/Ge heterostructures processed by rapid thermal annealing (RTA) at 1000 °C for annealing times varying between 30 s and 70 s. The a-Si:H layers were grown on electron cyclotron resonance (ECR) using SiH₄ and Ar precursor gases. The Ge layer was grown in an e-beam evaporation system. The structural characterizations were performed by high-resolution X-ray diffractometer (HRXRD) on grazing incidence X-ray reflection mode (GIXRR) and micro-Raman measurements. The average grain size, Ge concentration (x_Ge) and strain were estimated from Lorentzian GIXRR peak fit. The average grain size varied from 3 nm to 7.5 nm and decreased with annealing time. The x_Ge increase with annealing time and varied from 8% to 19%, approximately. The strain calculated for (1 1 1), (2 2 0) and (3 1 1) peaks at 40 s, 50 s, 60 s and 70 s annealing time suggest the geometrical changes in nanoclusters according to process time.     

Early-stage ordering in in-situ annealed Fe51Pt49 films

We evidenced an early-stage ordering (ESO) in Fe₅₁Pt₄₉ film before the appearance of superlattice diffraction (long-range-order, LRO) using 40-nm-thick films prepared by magnetron sputtering onto quartz substrate. The appearance of L1₀ phase for samples deposited at substrate temperatures (T_s) 400 °C and higher was verified by X-ray diffraction. Surface roughness of Fe₅₁Pt₄₉ films, obtained via X-ray specular reflectivity with computational fitting, increases from 3.8 to 11 Å as T_s is increased from 25 to 275 °C. As further increase of T_s to 375 °C, the roughness drops to 3.2 Å and then increases again to 38 Å with T_s up to 700 °C. Measurement on residual strain demonstrates that it is initially compressive at T_s<400 °C. Thereafter the strain transfers to a tensile one and increases in magnitude as increasing T_s up to 700 °C corresponding to LRO transformation. Local atomic rearrangement is observed for samples deposited at T_s>250 °C by using extended X-ray absorption fine structure. Coercivity of films increases from 10 to 460 Oe as T_s increase from 25 to 375 °C (ESO) and then from 460 to 10,700 Oe with T_s 375-700 °C (normal LRO). The worked out quantitative estimation of ESO engages with that of LRO before T_s 400 °C.     

Effects of post-thermal annealing temperature on the optical and structural properties of gold particles on silicon suboxide films

In this work, silicon suboxide (SiO_x) thin films were deposited using a RF magnetron sputtering system. A thin layer of gold (Au) with a thickness of about 10 nm was sputtered onto the surface of the deposited SiO_x films prior to the thermal annealing process at 400 °C, 600 °C, 800 °C and 1000 °C. The optical and structural properties of the samples were studied using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and optical transmission and reflection spectroscopy. SEM analyses demonstrated that the samples annealed at different temperatures produced different Au particle sizes and shapes. SiO_x nanowires were found in the sample annealed at 1000 °C. Au particles induce the crystallinity of SiO_x thin films in the post-thermal annealing process at different temperatures. These annealed samples produced silicon nanocrystallites with sizes of less than 4 nm, and the Au nanocrystallite sizes were in the range of 7-23 nm. With increased annealing temperature, the bond angle of the Si-O bond increased and the optical energy gap of the thin films decreased. The appearance of broad surface plasmon resonance absorption peaks in the region of 590-740 nm was observed due to the inclusion of Au particles in the samples. The results show that the position and intensity of the surface plasmon resonance peaks can be greatly influenced by the size, shape and distribution of Au particles.     

Mechanical alloyed Ho doping in CoFe2O4 spinel ferrite and understanding of magnetic nanodomains

We doped Ho³⁺ in CoFe_1.95Ho_0.05O₄ spinel ferrite by mechanical alloying and subsequent annealing at different temperatures (600-1200 °C). We understood the structural and magnetic properties of the samples using X-ray diffraction, SEM, Thermal analysis (TGA and DTA), and VSM measurement. The samples have shown structural stabilization within cubic spinel phase for the annealing temperature (T_AN)≥800 °C. Thermal activated grain growth kinetics has been accompanied with the substantial decrease in lattice strain. The gain size dependent magnetism is evident from the variation of magnetic moment, remanent magnetization and coercivity of the material. The paramagnetic to ferrimagnetic transition temperature T_C (∼805 K) seems to be grain size independent in the present material. The magnetic nanograins, either single domain/pseudo-single domain (50-64 nm) or multi-domain (above 64 nm) regime, showed superparamagnetic blocking below T_m, which is below T_C (805 K) and also well above the room temperature.     

Fiber laser annealing of nanocrystalline PZT thick film prepared by aerosol deposition

Nanocrystalline PZT thick films (1 mm square and over 10 μm thick) directly deposited onto stainless-steel substrates (PZT/SUS) by aerosol deposition (AD) technique and then annealed using focused laser beam with a fiber laser to suppress thermal damage to the back sides of the PZT/SUS and substrate near the film edge and to retain the dielectric and/or ferroelectric properties of the PZT/SUS. Compared with CO₂ laser annealing, fiber laser annealing suppressed thermal damage to the substrate. Compared with PZT/SUS annealed at 600 °C using an electric furnace, PZT/SUS annealed at 600 °C using a fiber laser showed superior properties, namely, dielectric constant ? > 1200 at a frequency of 100 Hz, remanent polarization P_r > 30 μC/cm², and coercive field strength E_c < 50 kV/cm at a frequency of 10 Hz. Furthermore, the grain growth for the PZT/SUS formed by AD technique and annealed by fiber laser irradiation was occurred within the laser spot size.     

Post-sintering annealing effect on the diffuse phase transition in (Pb1−xBax)(Yb0.5Ta0.5)O3 ceramics

The effect of post sintering annealing on the dielectric response of (Pb_1−xBa_x)(Yb_0.5Ta_0.5)O₃ ceramics in the diffuse phase transition range (x=0.2) has been investigated. The samples are prepared by conventional solid-state reaction method. The samples are sintered at 1300 °C for 2 h and annealed at different temperatures (800, 900 and 1000 °C) for 8 h and at 800 °C for different time durations (8, 12 and 24 h). A significant change in the dielectric response has been observed in all the samples. The dielectric constant increases remarkably and the dielectric loss tangent decreases. The dielectric peaks of the annealed samples are observed to be more diffused with noticeable frequency dispersion compared to the as sintered sample.     

Sintering temperature dependence of room temperature magnetic and dielectric properties of Co0.5Zn0.5Fe2O4 prepared using mechanically alloyed nanoparticles

Co_0.5Zn_0.5Fe₂O₄ nanoparticles were prepared using mechanical alloying (MA) and sintering. The crystallite size, coercivity, retentivity and saturation magnetization were also measured. The frequency dependence of dielectric and the magnetic parameters, namely, real permittivity ε′, loss tanget tan δ, real permeability μ′ and loss factor μ″ were measured at room temperature for samples sintered from 600 to 1000 °C, in the frequency range 10 MHz to 1.0 GHz. The results show that the crystallite size of the resulting products ranges between 16 and 67 nm for as-milled sample and the sample sintered at 1000 °C, respectively. The sample sintered at 1000 °C, measured at room temperature exhibited a saturation magnetization of 37 emu g⁻¹. The values of permittivity remain constant within the measured frequency, but vary with sintering temperature. The permeability values, on the other hand however vary with both the sintering temperature and the frequency, thus, the absolute value of the permeability decreased after the natural resonance frequency.