Fluidization of binary mixtures of nanoparticles under the effect of acoustic fields

The present work is focused on the characterization of the aeration behaviour of two different nanopowders, Al₂O₃ and Fe₂O₃, under the application of acoustic fields of different intensities (125–150 dB) and frequencies (50–300 Hz). The two nanopowders showed slightly different behaviours; in particular, Fe₂O₃ is generally characterized by a better fluidization quality. These differences have been explained by referring to nanoparticles aggregates properties. In particular, aggregates size and density and fractal dimension have been numerically evaluated. The fluidization of binary mixtures of the two powders has also been investigated under the application of different acoustic fields (130–135 dB, 120 Hz) and varying the amount of the two powders from 7%wt to 90%wt of Fe₂O₃. The addition of Fe₂O₃ generally has a beneficial effect on the fluidization quality of the binary mixtures, in terms of pressure drop and bed expansion ratio, becoming relevant from an amount of Fe₂O₃ of 33%wt.     

Tensile properties of the Cr–Fe–Ni–Mn non-equiatomic multicomponent alloys with different Cr contents

A series of Fe₄₀Mn₂₈Ni_32 − xCr_x (x = 4, 12, 18, 24 (at.%)) multicomponent alloys was prepared by vacuum arc melting. The Fe₄₀Mn₂₈Ni₂₈Cr₄, Fe₄₀Mn₂₈Ni₂₀Cr₁₂ and Fe₄₀Mn₂₈Ni₁₄Cr₁₈ alloys were ductile single phase fcc solid solutions. The Fe₄₀Mn₂₈Ni₈Cr₂₄ alloy had intermetallic sigma phase matrix and was extremely brittle after homogenization. The tensile properties of the Fe₄₀Mn₂₈Ni₂₈Cr₄, Fe₄₀Mn₂₈Ni₂₀Cr₁₂ and Fe₄₀Mn₂₈Ni₁₄Cr₁₈ solid solution alloys were examined in recrystallized condition with average grain size of ~ 10 μm. The yield strength increased from 210 MPa of the Fe₄₀Mn₂₈Ni₂₈Cr₄ alloy to 310 MPa of the Fe₄₀Mn₂₈Ni₁₄Cr₁₈ alloy. The elongation to fracture of the alloys decreased from 71% to 54%, respectively. Solid solution strengthening by the constitutive elements of the alloys was calculated using Labush approach. Strong solid solution strengthening by Cr was predicted. Gypen and Deruyttere approach was used to estimate solid solution strengthening of the Fe₄₀Mn₂₈Ni_32 − xCr_x alloys. Good correlation between predicted solid solution strengthening and the experimental yield strength values was found.     

Microwave-induced combustion synthesis of Li0.5Fe2.5−xCrxO4 powder and their characterization

Li_0.5Fe_2.5−xCr_xO₄ (0 ≦ x ≦ 1.0) powders with small and uniformly sized particles were successfully synthesized by microwave-induced combustion, using lithium nitrate, iron nitrate, chromium nitrate, and carbohydrazide as the starting materials. The process takes only a few minutes to obtain as-received Cr-substituted lithium ferrite powders. The resultant powders annealed at 650 °C for 2 h and were investigated by thermogravimeter/differential thermal analyzer (TG/DTA), X-ray diffractometer (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and thermomagnetic analysis (TMA). The results revealed that the lattice constant decreases linearly with increasing of Cr content in Li_0.5Fe_2.5−xCr_xO₄ specimens. Moreover, the magnetic properties of Cr-substituted lithium ferrite were also strongly affected by Cr content. The saturation magnetization, remanent magnetization, and coercive force decrease monotonously with increasing of Cr content.     

Microstructure and microwave absorption properties of Fe3O4/dextran/SnO2 multilayer microspheres

Fe₃O₄/dextran/SnO₂ multilayer microspheres have been successfully designed and synthesized by solvothermal and hydrothermal reactions. Dextran worked as a linker between Fe₃O₄ core and SnO₂ shell. It can not only prevent the oxidation of Fe₃O₄ but also be carbonize to another absorber carbon black. The as-synthesized microspheres were about 320 nm in size and well-defined in shape. The maximum reflection loss of Fe₃O₄/dextran/SnO₂ microspheres and paraffin wax composites could reach 20.26 dB at 4.72 GHz, and the bandwidth with a reflection loss less than ? 10 dB was 4.86 GHz with 4 mm in thickness. The excellent microwave absorption properties of the composites were attributed to the special multilayer structures of Fe₃O₄/dextran/SnO₂ microspheres and the effective complementarity between dielectric loss and magnetic loss.     

Characterization of interface of Al2O3–304 stainless steel braze joint

Sintered Al₂O₃ is brazed with 304 stainless steel (SS) using 97(Ag28Cu)3Ti active filler alloy at 900 and 1000 °C. The interfaces developed during brazing have been systematically characterized and the results show that the thickness of the interfaces is 15–20 μm and 10–15 μm on SS side and Al₂O₃ side respectively. The presence of TiO, Cu₃Ti₃O and FeTi phases at the Al₂O₃ interface and FeTi, and Fe₃₅Cr₁₃Ni₃Ti₇ phases at the SS interface have been confirmed through XRD and TEM studies. Microhardness analysis across the brazed interfaces suggest that the SS interface (300–600 Hv) is harder than that of the SS substrate (200–250 Hv) whereas alumina interface (550–900 Hv) is softer than that of the alumina substrate (∼ 1900 Hv). It is concluded that the various new phases that are formed during brazing are responsible for the variation in the hardness values.     

Microstructures and mechanical property of laser butt welding of titanium alloy to stainless steel

Laser butt welding of titanium alloy to stainless steel was performed. The effect of laser-beam offsetting on microstructural characteristics and fracture behavior of the joint was investigated. It was found that when the laser beam is offset toward the stainless steel side, it results in a more durable joint. The intermetallic compounds have a uniform thickness along the interface and can be divided into two layers. One consists of FeTi + α-Ti, and other consists of FeTi + Fe₂Ti + Ti₅Fe₁₇Cr₅. When laser beam is offset by 0 mm and 0.3 mm toward the titanium alloy side, the joints fracture spontaneously after welding. Durable joining is achieved only when the laser beam is offset by 0.6 mm toward the titanium alloy. From the top to the bottom of the joint, the thickness of intermetallic compounds continuously decreases and the following interfacial structures are found: FeAl + α-Ti/Fe₂Ti + Ti₅Fe₁₇Cr₅, FeAl + α-Ti/FeTi + Fe₂Ti + Ti₅Fe₁₇Cr₅ and FeAl + α-Ti, in that order. The tensile strength of the joint is higher when the laser beam is offset toward the stainless steel than toward the titanium alloy, the highest observed value being 150 MPa. The fracture of the joint occurs along the interface between two adjacent intermetallic layers.     

Doping of iron phosphate glasses with Al2O3, SiO2 or B2O3 for improved thermal stability

The effects of doping 60 P₂O₅–40 Fe₂O₃ (mol%) glasses with 5–10 mol% SiO₂, Al₂O₃ or B₂O₃ on their thermal stability, iron environments and redox were investigated. Thermal stability improved markedly with 5% dopant addition in the order Al₂O₃ > SiO₂ > B₂O₃ ≫ base glass. Solubility of pro rata additions when melted at 1150 °C was 5–10% SiO₂, <5% Al₂O₃, and >10% B₂O₃. It was possible to dissolve 5% Al₂O₃ by replacing Fe₂O₃. These additions generally had little effect on dilatometric measurements and iron environments, however the Fe²⁺/ΣFe redox ratio increased in the order base glass < Al₂O₃ < SiO₂ < B₂O₃. This behaviour was broadly consistent with the effects of glass basicity. The increased thermal stability of these glasses may improve their suitability for applications such as waste immobilisation or sealing.     

Preparation and luminescent properties of Cr3+:Al2O3 nano-powders by low-temperature combustion synthesis

Cr³⁺:Al₂O₃ nano-powders were prepared through low-temperature combustion synthesis (LCS) method by using glucose as a dispersion agent for the first time. The Cr³⁺:Al₂O₃ nano-powders samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and luminescence spectrometer. XRD results showed that pure α-Al₂O₃ phase was obtained for the sample fired at 1100 °C for 0.5 h. TEM results indicated that nano-powders were well dispersed. Luminescence spectrum analysis results indicated that the excitation spectrum of Cr³⁺:Al₂O₃ nano-powders consisted of two bands peaking at 462 nm and 579 nm, respectively, and the emission spectrum consisted of two bands peaking at 692 nm and 668 nm, respectively.     

Crystal structure,oxygen permeability and stability of Ba0.5Sr0.5Co0.8Fe0.1M0.1O3−δ (M = Fe,Cr, Mn,Zr) oxygen-permeable membranes

Oxygen-permeable ceramic membrane materials of the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) and partially Fe-substituted Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Cr, Mn, Zr) were synthesized by solid-state reaction method. These materials possess purely cubic perovskite structure with the exception of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Mn, Zr), in which minor impurities exist. Oxygen permeability across these dense membrane disks were measured under an air/He oxygen partial pressure gradient in the temperature range of 973–1123 K. The results demonstrated that the oxygen permeation fluxes of the Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Fe, Cr, Mn, Zr) membranes increased in the following order: Fe (BSCFO) > Cr > Zr > Mn. The corresponding activation energies for oxygen permeation of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Fe, Cr, Zr) membranes were calculated to be similar (53 ± 4 kJ/mol), which was remarkably lower than that (99 ± 3 kJ/mol) of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Mn) membrane. In addition, good oxygen permeation stability of the Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Cr) membrane was achieved at the temperature lower than 1123 K. The X-ray diffraction (XRD) and differential thermal analysis (DTA) experiments showed that the structural stability of BSCFO could be significantly improved when Fe ions in the BSCFO material were partially substituted by Cr, Mn or Zr ions.     

Dielectric spectroscopy of polypyrrole–γ–Fe2O3 composites

In situ polymerization of pyrrole was carried out in the presence of γ–Fe₂O₃ (FE) to synthesize polypyrrole–γ–Fe₂O₃ composites (PPy/FE) by chemical oxidation method. The PPy/FE composites have been synthesized with various compositions viz., 10, 20, 30, 40 and 50 wt.% of γ–Fe₂O₃ in pyrrole. The AC conductivity was studied in the frequency range 10²–10⁷ Hz. The dielectric behaviour was also investigated in the frequency range 10²–10⁷ Hz. The dimensions of γ–Fe₂O₃ particles in the matrix have a greater influence on the conductivity values and the observed dielectric values.     

Studies on the structural,electrical and magnetic properties of LaCrO3, LaCr0.5Cu0.5O3 and LaCr0.5Fe0.5O3 by sol–gel method

The structural, electrical and magnetic properties of LaCr_0.5M_0.5O₃ (M = Cr³⁺, Cu²⁺ and Fe³⁺) synthesized by a sol–gel technique were studied. The X-ray diffraction pattern shows the structure to be orthorhombic and the size of the particles is around 100 nm as seen from the TEM images. The effects of Cu²⁺ and Fe³⁺ on the electrical properties of LaCrO₃ were studied using impedance spectroscopy at room temperature (RT). The properties of LaCr_0.5Cu_0.5O₃ were studied over a wide range of temperature from RT to 533 K. A maximum conductivity of 1.7 × 10^?3 S cm^?1 was observed for LaCr_0.5Cu_0.5O₃ at a measured temperature of 533 K. The impedance spectra indicate a negative temperature coefficient of resistance (NTCR) and also imply the conduction is through bulk of the material. The magnetic studies performed using a SQUID magnetometer interpret the antiferromagnetically ordered LaCrO₃ to behave ferromagnetically on the addition of Cu²⁺ and Fe³⁺, and the magnetization was found to be enhanced in the LaCr_0.5Fe_0.5O₃.     

Effect of La2O3, CoO,Cr2O3 and MoO3 nucleating agents on crystallization behavior and magnetic properties of ferromagnetic glass–ceramic in the system Fe2O3·CaO·ZnO·SiO2

Preparation and characterization of ferromagnetic glass ceramic in the system Fe₂O₃·CaO·ZnO·SiO₂ with different nucleating agents was studied. The effect of La₂O₃, CoO, Cr₂O₃ and MoO₃ as nucleating agents was investigated. Differential thermal analysis; X-ray diffraction and transmission electron microscope were used to investigate thermal behavior, sequence of crystallization and microstructure of the samples. XRD analysis for as prepared samples revealed the crystallization of single magnetite phase. Heat treatment at 900 °C/2 h revealed the appearance of minor amounts of calcium silicate, hematite and cristobalite beside magnetite. TEM revealed crystallization of crystallite size in the range 50–100 nm. Lattice parameters, cell volume and crystallite size were stimulated from XRD data. Magnetic properties of quenched samples were measured under 20 kG.     

Capture and release of genomic DNA by PEI modified Fe3O4/Au nanoparticles

Polyethylenimine (PEI) modified Fe₃O₄/Au nanoparticles were synthesized in aqueous solution and characterized by photo correlation spectroscopy (PCS) and vibrating sample magnetometer (VSM). The so-obtained Fe₃O₄/Au-PEI nanoparticles were capable of efficient electrostatic capture of DNA. The maximum amount of genomic DNA captured on 1.0 mg Fe₃O₄/Au-PEI nanoparticles was 90 μg. The DNA release behavior was studied and the DNA recovery from Fe₃O₄/Au-PEI nanoparticles approached 100% under optimal conditions. DNA extraction from mammalian cells using Fe₃O₄/Au-PEI nanoparticles was successfully performed. Up to approximately 43.1 μg of high-purity (OD₂₆₀/OD₂₈₀ ratio = 1.81) genomic DNA was extracted from 10 mg of liver tissue. The results indicated that the prepared Fe₃O₄/Au-PEI nanoparticles could be successfully used for DNA capture and release.     

Al2O3:Cr3+ microfibers by hydrothermal route: Luminescence properties

Uniform Al₂O₃:Cr³⁺ microfibers were synthesized by using a hydrothermal route and thermal decomposition of a precursor of Cr³⁺ doped ammonium aluminum hydroxide carbonate (denoted as AAHC), and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD indicated that Cr³⁺ doped samples calcined at 1473 K were the most of α-Al₂O₃ phase. SEM showed that the length and diameter of these Cr³⁺ doped alumina microfibers were about 3–9 μm and 300 nm, respectively. PL spectra showed that the Al₂O₃:Cr³⁺ microfibers presented a broad R band at 696 nm. It is shown that the 0.07 mol% of doping concentration of Cr³⁺ ions in α-Al₂O₃:Cr³⁺ was optimum. According to Dexter's theory, the critical distance between Cr³⁺ ions for energy transfer was determined to be 38 Å. It is found that the curve followed the single-exponential decay.     

Analysis of material degradation and life assessment of 25Cr–38Ni–Mo–Ti wrought alloy steel (HPM) for cracking tubes in an ethylene plant

Radiant tubes made of wrought 25Cr–38Ni–Mo–Ti alloy steel (HPM) have been in-service for 76,500 h as cracking tubes in an ethylene plant and they are expected to provide reliable service for 100,000 h (11.4 years) or more. During service, the tube inner surfaces were operated at temperature in the range of 820–835 °C within which thermal cracking process occurred. These aged tubes were assessed to ensure continued safe operation. The assessment of material degradation was carried out using optical microscopy, scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) analysis, X-ray powder diffraction (XRD) analysis, Vickers microhardness measurement and stress rupture test to obtain stress–Larson–Miller parameter (LMP) curves for remaining life prediction. Results showed that microstructural degradation was observed at the inner surface of the radiant tubes marked by the damage of protective oxide film containing Cr₂O₃, Fe₂O₃ and SiO₂. Once this film was removed, carburization occurred and free C atoms involved during cracking of ethylene easily penetrated along austenitic grain boundaries. In addition, carbon diffusion into the tube metal seemed to promote precipitation of Cr₂₃C₆ at grain boundaries and within the grains resulting in a sharp increase in hardness. The outer surface of the radiant tubes, on the other hand, was exposed to higher temperature, typically 1040–1100 °C during operation and creep damage seemed to be the main cause of material degradation. Based on stress rupture test, the remaining life of the radiant tubes is expected to be 21,107 h (2.4 years) consistent with the design life. In the present investigation, factors affecting creep are discussed.     

Physical and spectroscopic properties of multi-component Na2O–PbO–Bi2O3–SiO2 glass ceramics with Cr2O3 as nucleating agent

Transparent glass ceramics, synthesized from melt quenching followed by heat treatment, of the composition 10Na₂O–30PbO–10Bi₂O₃–(50 − x)SiO₂:xCr₂O₃ (mol%), where 0 ⩽ x ⩽ 0.5, were characterized with XRD, DTA, SEM and EDS. Physical and spectroscopic studies, viz., optical absorption, electron paramagnetic resonance (EPR), FTIR and Raman were investigated. The characterization of the host glass ceramic has revealed that the formation of a major phase of sodium silicate along with two minor phases such as lead silicate and bismuth oxide. By integrating Cr₂O₃ to the host glass additional crystal phases viz., NaCrO₂, Na₂Cr₂O₇ and Pb(CrO₄) which are the complexes of Cr³⁺ and Cr⁶⁺ ions were also developed. As the concentration of nucleating agent is increased, a part of the Cr⁶⁺ ions is found to reduce in to Cr³⁺ ions. Spectroscopic studies have revealed that with an increase in the concentration of Cr₂O₃ from 0.1 to 0.5 mol%, there is a gradual increase in the intensity of vibrational modes of various asymmetric structural units of silicate, bismuthate and chromate in the glass ceramic network at the expense of symmetrical structural units. The analysis of the results of these studies has indicated that in the samples containing higher concentration of Cr₂O₃, chromium ions exists predominantly in Cr³⁺ state and occupy the octahedral positions in glass ceramic matrix and such glass ceramic samples are suitable for lasing action.     

Preparation,spectroscopic characterization and energy transfer investigation of iron-chromium diffusion co-doped ZnSe for mid-IR laser applications

The spectroscopic characterization and energy transfer mechanism of iron-chromium co-doped ZnSe polycrystalline (Cr,Fe:ZnSe) were reported with dimension of 15 mm × 15 mm × 2 mm obtained by controlled post-growth thermal diffusion method. The infrared absorption is characterized by a strong broad-band centered at 1770 nm which can be attributed to the only spin-allowed transition ⁵T₂ → ⁵E within the 3d⁴ shell of Cr²⁺ ions. Photoluminescence spectrum shows a relatively strong broad emission band centered at 4.1 μm with a width of 0.8 μm (FWHM) under 1770 nm excitation at room temperature and reveals effective Cr²⁺ → Fe²⁺ energy transfer process. Room temperature photoluminescence decay about 8 μs was measured. All the results indicate that Cr,Fe:ZnSe could achieve laser operation at 3.7–4.5 μm via Cr²⁺ → Fe²⁺ energy transfer using a more convenient laser pump source in the near IR region.     

Effect of reaction temperature on crystallite size and sensing response of chromium oxide nanoparticles

Nanoparticles of chromium oxide have been synthesized following a precipitation technique at reaction temperatures 5, 27 and 65 °C. Synthesized powders were characterized using X-ray diffraction, transmission electron microscope and Brunauer–Emmett–Teller techniques to carry out structural and morphological analysis. The reaction temperature has been found to be playing a crucial role in controlling particle size. It has been observed that Cr₂O₃ nanoparticles synthesized at 27 °C were smaller as compared to those synthesized at 5 and 65 °C. Chromium oxide samples thus prepared were deposited as thick films on alumina substrates to act as gas sensors and their sensing response to ethanol vapour was investigated at different operable temperatures. It has been observed that all the sensors exhibited optimum response at 250 °C. The investigations revealed that sensing response of Cr₂O₃ nanoparticles synthesized at 27 °C was exceptionally higher than that of Cr₂O₃ nanoparticles synthesized at 5 and 65 °C.     

Variation with added material in the effects of reactive mechanical grinding and hydriding–dehydriding cycling on the hydrogen-storage properties of Mg

Samples Mg–14Ni–6Fe₂O₃, Mg–14Ni–3Fe₂O₃–3Ti, and Mg–14Ni–2Fe₂O₃–2Ti–2Fe were prepared by reactive mechanical grinding, and their hydrogen storage properties were examined. The activated Mg–14Ni–2Fe₂O₃–2Ti–2Fe had the highest hydriding rate, absorbing 4.14 wt% H for 5 min, and 4.27 wt% H for 10 min, and 4.42 wt% H for 60 min at 573 K under 12 bar H₂. The activated Mg–14Ni–3Fe₂O₃–3Ti had the highest dehydriding rate, desorbing 3.81 wt% H for 20 min, 3.98 wt% H for 25 min, and 4.15 wt% H for 60 min. Mg–14Ni–6Fe₂O₃ dehydrided at n = 4 contained Mg, Mg₂Ni, MgO, and Mg(OH)₂. Mg(OH)₂ is considered to be formed by the reactions of MgH₂ or Mg with water vapor. The effects of reactive mechanical grinding and hydriding–dehydriding cycling are the creation of defects and cracks, and the reduction of Mg particle size. The addition of a larger amount of Ti and/or Fe has stronger effects of reactive mechanical grinding, whereas the addition of a larger amount of Fe₂O₃ has greater effects of hydriding–dehydriding cycling.     

Low-temperature sintering and microwave dielectric properties of CaTi1−x(Fe0.5Nb0.5)xO3 ceramics with B2O3 addition

CaTi_1−x(Fe_0.5Nb_0.5)_xO₃ (0 ≤ x ≤ 1) dielectrics were synthesized via the solid state reaction route and structure analysis was performed together with the dielectric characterization. The substitution of Ti⁴⁺ by Fe³⁺/Nb⁵⁺ and developed phase were studied by X-ray diffraction. The dielectric constant and temperature coefficient of resonant frequency decrease rapidly with an increase of x. The influence of 1–5 wt.% B₂O₃ as a sintering additive investigated at CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ solid solutions. The dielectric properties were found to strongly depend on the sintering conditions and contents of B₂O₃ additions. ɛ_r = 52.3, Q × f_o = 2930 GHz and T_f = 13 ppm/°C were obtained for CaTi_0.5(Fe_0.5Nb_0.5)_0.5O₃ specimen 3 wt.% B₂O₃ sintered at 900 °C for 2 h.