Effect of atmosphere control on magnetic properties of CaO-Al2O3-SiO2-Fe3O4 glass ceramics

The influences of atmosphere during processes of melting and heat treatment, heat treatment temperature, Fe₃O₄ content and basicity on the magnetic properties of magnetite-based glass ceramics were investigated. For sample containing 20 % Fe₃O₄ melted in different atmospheres, the highest saturation magnetisation was realized in 20vol% air + 80 vol% Ar, due to the fact that ratio of Fe³⁺ to Fe²⁺ in melt obtained in this atmosphere was close to 2. However, it was found that the coercivity of glass ceramics was not affected by the melting atmosphere. A high sintering temperature led to the decrease of saturation magnetisation and the increase of coercivity. As increasing Fe₃O₄ content, the main crystal phase transformed from CaSiO₃ to CaFe_0.6Al_1.3Si_1.08O₆ and finally to magnetite phase, accompanied by the increase of saturation magnetisation and coercivity. In addition, the increase of basicity caused the decrease of saturation magnetisation and the increase of coercivity.     

Effects of B2O3–Li2CO3–SiO2–ZnO glass on properties of Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites sintered at low temperatures

In this study, B₂O₃–Li₂CO₃–SiO₂–ZnO (BLSZ) glass was fabricated and employed as a sintering aid for Li_0.43Zn_0.27Ti_0.13Fe_2.17O₄ ferrites. The introduction of BLSZ glass could allow the ferrites to be sintered at low temperatures (~920 °C) while maintaining a pure spinel phase. The results obtained from scanning electron microscopy revealed that BLSZ glass significantly promoted grain growth because of the liquid phase formed during sintering, which resulted in a homogeneous morphology with high densification. The saturation induction and saturation magnetization reached maximum values of 286.7 mT and 78.7 emu/g, respectively, with the addition of an appropriate amount (1.5 wt %) of BLSZ glass. This result can be attributed to the larger grains that were formed with the assistance of the BLSZ glass. Correspondingly, the coercivity and ferromagnetic resonance linewidth reached minimum values of 247.9 A/m and 236.6 Oe, respectively. The main reasons explaining this result are considered to be the homogeneous morphology and the low porosity of the samples. Therefore, it is believed that BLSZ glass can enable co-firing between LiZn ferrites and silver electrodes, which is crucial for the realization of low temperature co-fired ceramic (LTCC) technology.     

Synthesis of magnetite powder from iron ore tailings

Iron ore tailing—a waste material of mineral beneficiation plants, is used as a source of iron for synthesizing magnetite powder. Iron ore tailings containing 15.98% Fe₂O₃, 83.36% SiO₂ and 0.44% Al₂O₃ have been subjected to HCl digestion on a hot plate to extract the entire amount of Fe₂O₃ as FeCl₃. A portion of extracted FeCl₃ solution has been used to convert it to FeCl₂ via metallic iron formation by using NaBH₄ as a reducing reagent. Then, the left out FeCl₃ solution and derived FeCl₂ solutions (from FeCl₃) are mixed in an appropriate molar ratio (2:1) for synthesizing magnetite powder by the addition of alkali solution. The magnetite powder samples have been characterized by means of powder XRD, SEM, vibrating sample magnetometer and laser particle size analyzer. XRD study confirms the formation of magnetite phase. The magnetite particles synthesized in different ways show varying degrees of magnetization behavior which is attributed to the change in their particle size induced by the use of different precipitating reagents.     

Sinter forging of rapidly quenched eutectic Al2O3–ZrO2(Y2O3)-glass powders

Spray dried agglomerates of Al₂O₃–ZrO₂ (1% Y₂O₃) with 4 wt.% borosilicate glass were arc plasma sprayed and rapidly quenched into water. Because of the rapid quenching the particles <25 μm were mostly amorphous. After annealing 1 h at 1200 °C the scale of the microstructure of the particles was on the order 30 nm. Hot forging of this powder yielded dense specimens with the width of the ZrO₂ phase still less than 100 nm. Since the particle size ranged from 5 to 25 μm and the scale of the particle microstructure was <100 nm, densification was controlled by creep of the particles rather than by the typical hot pressing mechanism of diffusion along the neck between particles to fill the pores. Thus, the scale of the microstructure controls densification rather than the particle size. These powders offer an alternate source for manufacturing nanostructured parts and should be more suitable for hot pressing or forging than nanoparticulate powders.     

Improved Performance of Au/Fe2O3 Catalysts Promoted with ZrO2 and Nb2O5 in the WGS Reaction under Hydrogen-rich Conditions

Effect of addition of the ZrO₂ and Nb₂O₅ promoters on the activity and stability of the Au/Fe₂O₃ catalysts in the WGS reaction under hydrogen-rich conditions was studied. Results showed that this new catalyst possesses enhanced activity and stability under conditions common in fuel processors. Its CO conversion almost reached the maximum value 99% at 200 °C and maintained better stability compared with unmodified samples within 50 h on-stream. Detailed characterization including BET, XRD, HRTEM, XPS, XRF and H₂-TPR revealed that ZrO₂ and Nb₂O₅ acted as structural promoters and a strong interaction between ZrO₂ and Nb₂O₅ existed. The enrichment of Zr and Nb on the surface kept the gold and magnetite particles apart delaying sintering. More active gold sites, larger surface area and smaller magnetite particles were the main reasons for the enhanced performance.     

Preparation and properties of ferromagnetic glass ceramics and glass fibers based on the composition of SiO2-B2O3-Fe2O3-SrO

The ferromagnetic glass ceramics in the system SiO₂-B₂O₃-Fe₂O₃-SrO were prepared via four different fabrication methods, i.e., fiber-drawing, melt-quenching, natural-cooling, and annealing, without performing any nucleation and crystallization heat treatments. The influences of chemical composition and fabrication method on the spontaneous crystallization of magnetite were investigated by X-ray diffraction, scanning and transmission electron microscopy. The X-ray diffraction patterns show the presence of nanometric magnetite crystals in the glass matrix, and the increasing boron oxide can promote the spontaneous crystallization of magnetite. The estimated size of crystallized magnetite varies between 12 and 50 nm. The magnetic properties of the glass ceramics derived from the four fabrication methods were analyzed using a Vibrating sample magnetometer (VSM), Agilent HP8722ES vector network analyzer and Mössbauer spectra. We find that both the saturation magnetization (M_S) and coercivity (H_jc) depend on the chemical composition and fabrication method. The calorimetric measurements were carried out using Orton Standard Dilatometers.     

Efficient visible light magnetic modified iron oxide photocatalysts

Magnetite iron oxide (Fe₃O₄) nanoparticles were synthesized via simple co-precipitation method using ferrous and ferric ions salts. Fe₃O₄ nanoparticles were modified by silica and titania. Pure and modified nanoparticles were employed for dye degradation under visible light. X-ray diffraction analysis indicated inverse spinel structure of Fe₃O₄ nanoparticles. The particle size of magnetite nanoparticles is decreased due to coating of silica and titania. Scanning and transmission electron microscopy indicated the spherical morphology for all samples. The synthesized Fe₃O₄ nanoparticles were ferromagnetic in nature with highest saturation magnetization value of 1.1034 emu as compared to silica and titania coated samples. Fourier transform infra-red spectra confirmed the incorporation of magnetite nanoparticles with silica and titania. Titania modified magnetite sample showed the highest photocatalytic activity as compared to silica modified magnetite nanoparticles and bare iron oxide under visible light irradiations.     

Influence of Co3O4, Fe2O3 and SiC on microstructure and properties of glass foam from waste cathode ray tube display panel (CRT)

Abstract

In the present study, the effect of temperature and oxidising agents such as Fe₂O₃ and Co₃O₄ on physical and mechanical properties of glass foam is investigated. The glass foam is made of panel glass from dismantled cathode ray tubes and SiC as a foaming agent. In the process, powdered waste glass (mean particle size below 63 μm) in addition to 4 wt-% SiC powder (mean particle size below 45 μm) are combined with Fe₂O₃ and Co₃O₄ (0·4, 0·8 and 1·2 wt-%) have been sintered at 950 and 1050°C. The glass foamed containing 1·2 wt-% Co₃O₄ has good physical properties, with porosity more than 80% and bending strength more than 1·57±0·12 MPa. However, by adding different amounts of Fe₂O₃ in comparison with samples without iron oxide, little changes in porosity and strength are obtained.     

Effect of Y2O3 on the corrosion resistance of two-step sintered Al5Y3O12-MgAl2O4 sidewalls in the aluminum electrolyte

Magnesium–aluminum spinel (MAS) precursor powder was synthesized through a microwave hydrothermal method. The synergistic effects of sintering process and sintering aids on the densification, hardness and corrosion resistance of MAS were revealed. X-ray diffraction analysis (XRD), Archimedes’ drainage method, fully automatic micro-Vickers hardness test and scanning electron microscopy (SEM) were performed to analyze the phase composition, bulk density, hardness microstructure and corrosion depth of the samples, respectively. Results revealed that the best two-step sintering condition is 1650 °C/3 min/1550 °C/20 h. The MAS products obtained under the best condition have clear grain boundaries, uniform particle size distribution, and few pores. When the amount of Y₂O₃ added is 4 wt.%, Y₂O₃ and Al₂O₃ form the second-phase solid solution Al₅Y₃O₁₂, which activates the crystal lattice and benefits the sintering densification of MAS. Under these conditions, the relative density of the MAS composite ceramics prepared is relatively large (95.94 %), the grain size is relatively uniform, the hardness is relatively large (1264 HV), and the corrosion depth is relatively small (94.58 μm).     

Properties of NiFe2O4 ceramics from powders obtained by auto-combustion synthesis with different fuels

Nickel ferrite (NiFe₂O₄) powders derived by auto-combustion synthesis using three different fuels (citric acid, glycine and dl-alanine) have been characterized. The sintering behavior of ceramics using these powders has been compared. Oxygen balance (OB) setting for the chemical reaction is found to regulate the combustion reaction rate. A rapid reaction rate and a high flame temperature are achieved with dl alanine fuel yielding single phase NiFe₂O₄ powder in the as-burnt stage, whereas powders derived with citric acid and glycine fuels show poor crystallinity and necessitate post-annealing. The powder particles are largely agglomerated with a non-uniform distribution in shape and size, and the average particle size is estimated in the range ~ 54–71 nm. Powders derived from dl-alanine fuel show better phase purity, smaller crystallite size, larger surface area and superior sintering behavior. Additional Raman modes discerned for dl-alanine derived powder support a 1:1 ordering of Ni²⁺ and Fe³⁺ at the octahedral sites relating to microscopic tetragonal P4₁22 symmetry expected theoretically for the formation of NiFe₂O₄ with inverse spinel structure. Microstructure of sintered ceramics depends on the precursor powders that are used and sintering at 1200 °C is found to be optimum. Citric acid and glycine derived powders yield high saturation magnetization (M_s~47–49 emu/g), but poor dielectric properties, whereas dl-alanine derived powders yield ceramics with high resistivity (~3.4×10⁸ Ω cm), low dielectric loss (tan δ~0.003 at 1 MHz) and high magnetization (46 emu/g). Dielectric dispersion and impedance analysis show good correlation with the changes in the ceramic microstructure.     

Influence of anionic and non-ionic surfactants on the synthesis of core-shell Fe3O4@TiO2 nanocomposite synthesized by hydrothermal method

Magnetite spinel nanoparticles (Fe₃O₄) coated titanium dioxide has been prepared by the solvo-hydrothermal method for application in dye degradation and wastewater remediation. The core-shell Fe3O4@TiO2 nanoparticles have been synthesized using titanium butoxide (TBT) and ferric chloride as precursors. In this method, firstly, magnetite nanoparticles have been prepared through a solvothermal process using ethylene glycol as a solvent. Then, titanium butoxide was used as a precursor to synthesize Fe₃O₄@TiO₂ core-shell nanoparticles using the hydrothermal method. The surfactants that were added, in separate synthetic processes, were anionic oleic acid and Sodium Dodecyl sulfonate, and non-ionic Polyvinylpyrrolidone and Polyethylene glycol. The effects of the various surfactants on the fabrication of core-shell magnetic nanoparticles were studied. Various characterization methods have been established to examine the morphology and magnetization features of the nanostructured particles, such as XRD, FTIR, TEM, FESEM, UV-spectroscopy, and VSM, etc., which validated the formation of Titania coated magnetite nanoparticles. The TiO₂ shell formation drastically reduces the saturation magnetization of the magnetic nanoparticles. The Oleic acid as a surfactant produces the smallest nanoparticles. The PVP coating is best amongst these surfactants for the retention of saturation magnetization upon coating.     

Effect of Fe2O3 doping on sintering of yttria-stabilized zirconia

This paper reports the effect of Fe₂O₃ doping on the densification and grain growth in yttria-stabilized zirconia (YSZ) during sintering at 1150 °C for 2 h. Fe₂O₃ doped 3 mol% YSZ (3YSZ) and 8 mol% YSZ (8YSZ) coatings were produced using electrophoretic deposition (EPD). For 0.5 mol% Fe₂O₃ doping, both 3YSZ and 8YSZ coatings during sintering at 1150 °C has similar densification. However, a significant grain growth occurred in 8YSZ during sintering, whereas grain size remains almost constant in 3YSZ. XRD results suggest that Fe₂O₃ addition substitutionally and interstitially dissolved into the lattice of 3YSZ and 8YSZ. In addition, colour of 3YSZ and 8YSZ changes differently with doping of Fe₂O₃. A Fe³⁺ ion interstitial diffusion mechanism is proposed to explain the densification and grain growth behaviour in the Fe₂O₃ doped 3YSZ and 8YSZ. A retard grain growth observed in the Fe₂O₃ doped 3YSZ is attributed to Fe³⁺ segregation at grain boundary.     

Densification behaviors of Al2O3 ceramics during flash sintering

The densification behaviors of MgO-doped-Al₂O₃ ceramics in the flashing stage and the steady stage were investigated using the classic kinetic model. The results show that the most densification of MgO-doped Al₂O₃ was completed during the flashing stage. The densification mechanism transferred from particle rearrangement resulted from Columbic force among particles under the effect of electrical field in the flashing stage to the lattice diffusion in the steady stage. Therefore, the densification rate in the steady stage dramatically decreased. Additionally, the estimated densification activation energy in the steady stage of flash sintering is 396 kJ/mol, much lower than the activation densification of lattice diffusion measured from conventional sintering, likely due to the effect of electric field/current-induced point defects on the diffusion.     

Fine grained Al2O3/SiC composite ceramic tool material prepared by two-step microwave sintering

Fine-grained Al₂O₃/SiC composite ceramic tool materials were synthesized by two-step microwave sintering. The effects of first-step sintering temperature (T1), content and particle size of SiC on the microstructure and mechanical properties were studied. It was found that the sample with higher content of SiC was achieved with finer grains, and the incorporation of SiC particles could bridge, branch and deflect the cracks, thus improving the fracture toughness. Higher T1 was required for the densification of the samples with higher content of SiC (>5?wt%). The sample containing 3?wt% SiC particles with the mean particle size of 100?nm, which was sintered at 1600?°C (T1) and 1100?°C (T2) for 5?min had the fine microstructure and optimal properties. Its relative density, grain size, Vickers hardness and fracture toughness obtained were 98.37%, 0.78?±?0.31?μm, 18.40?±?0.24?GPa and 4.97?±?0.30?MPa?m^1/2, respectively. Compared to the sample prepared by single-step microwave sintering, although near full densification can be achieved in both two methods, the grain size was reduced by 36% and the fracture toughness was improved by 28% in two-step microwave sintering.     

Effect of powder milling routes on the sinterability and optical properties of transparent Y2O3 ceramics

In ceramic processing, the size distribution of the starting powder to a certain degree is inevitable. It is prerequisite to control the size distribution, which influences the fabrication of a sound green body featuring both smaller pores and a narrower pore structure for full-density sintering facilitated by the easier elimination of pores. The milling process was systematically investigated here to elucidate the effect of powder characteristics on the sinterability and transmittance of Y₂O₃ ceramics. Three types of powder sets having different width of particle size distribution (W_PSD) while keeping the same median size (D₅₀) were prepared by changing the milling condition. By means of narrowing the W_PSD in this research, pore free transparent polycrystalline Y₂O₃ with average grain size of 730 nm was successfully fabricated by hot-pressing at 1500℃, which is 100℃ lower than the previously lowest known sintering temperature.     

Sintering and conductivity of Sc-doped CaZrO3 with Fe2O3 as a sintering aid

This paper reports the effect of 0.1–0.5 wt% Fe₂O₃ addition on sintering and electrical properties of CaZr_0.95Sc_0.05O_3-δ ceramics synthesized by combustion method. Addition of the sintering aid was shown to enhance ceramic densification and grain coarsening at a reduced sintering temperature and a shorter holding time (1430 °C, 2 h). Effect of the sintering aid on electrical conductivity of the ceramics was investigated using impedance spectroscopy. The highest total conductivity was achieved for the composition with 0.5 wt% Fe₂O₃; it was about an order of magnitude higher than that of the composition without Fe₂O₃. The effect of Fe₂O₃ addition on the conductivity of the grain interior and grain boundaries has been discussed. It was concluded that ceramic densification, grain coarsening and formation of small amounts of calcium ferrite at the grain boundaries upon Fe₂O₃ addition were responsible for the conductivity enhancement.     

Pore growth during the initial stages of sintering ceramics

Mercury porosimetry was used to measure changes in pore size distribution during initial stage sintering of compacts of submicron size particles of several oxides. Pore growth was observed in MgO and Fe₂O₃, and in Al₂O₃ under certain conditions. Pores can grow by these mechanisms: surface diffusion, particle size distribution effects, particle coalescence, phase transformation, and evaporation/condensation. Surface diffusion may be the mechanism in the case of an alpha alumina. Phase transformation was shown to be the cause when sintering gamma alumina. In the case of magnesia and ferric oxide, particle coalescence appears to be operating. Since pore growth competes with densification for the use of surface energy, it is an important sintering process.     

Microstructure,magnetic, and power loss characteristics of low-sintered NiCuZn ferrites with La2O3-Bi2O3 additives

Low-temperature-sintered Ni_0.5Cu_0.125Zn_0.375Fe_1.98O₄ ferrites co-added with x wt% (x = 0.00-0.25 wt%) La₂O₃ and 0.25 wt% Bi₂O₃ were successfully prepared via conventional solid-phase reaction method. The phase composition, microstructure, magnetic properties, and especially power loss variation of the samples were systematically studied. The results showed that all samples possessed a single spinel phase structure at a sintering temperature of 900°C, exhibiting high degree of densification and uniform grains. The appropriate amount of La₂O₃ additive improved the saturation flux density and permeability of NiCuZn ferrites, simultaneously reducing the coercivity and power loss. The maximum permeability and the lowest power loss were achieved at x = 0.15 wt%. The corresponding sample had the homogeneous microstructure and excellent magnetic properties, being a promising low-temperature co-fired ferrite candidate for magnetic power components.     

Effect of Bi2O3 content on sintering and crystallization behavior of low-temperature firing Bi2O3–B2O3–SiO2 glasses

The sintering behavior of a Pb-free Bi₂O₃–B₂O₃–SiO₂ glass system was examined as a function of Bi₂O₃ content. The glass transition temperature and the crystallization temperature of the glasses decreased with different decreasing gradients as the Bi₂O₃ content increased. The change in temperature affected the sintering behaviors of the glasses. In the case of the 40 mol% Bi₂O₃ addition, large pore accompanied over-firing phenomenon was observed when the sample was sintered over the optimum sintering temperature. However, over-firing was not observed in the sample with 45 mol% of Bi₂O₃ because of the crystallized phases during sintering. When the Bi₂O₃ content was 50–55 mol%, the crystallization temperature became lower than the glass transition temperature, which resulted in the crystallization of glass and it hindered densification.     

Influence of the structure of MgO·nAl2O3 spinel lattices on transparent ceramics processing and properties

It is demonstrated that a complete elimination of pores on sintering is governed not only by the size of the ceramic powder particles and by the homogeneity of their mutual coordination but similarly strongly by the state of the crystal lattice: with different cation disorder at fixed stoichiometry (n = 1) the sintering temperatures may differ by as much as 200 °C at constant powder particle size and equal homogeneity of the green bodies. Additionally, the impact of stoichiometry was investigated over the range between n = 1 and n = 3 with retarded reactive sintering at moderately increased Al₂O₃ concentrations but promoted densification of alumina-rich compositions. Taking advantage of the observed effects, sintered spinel ceramics were derived by reactive sintering of undoped MgO/Al₂O₃ mixtures resulting in an in-line transmittance which equals spinel single crystals of similar composition from 200 nm wave length up to the IR range.