Iron nanoparticles produced by high-energy ball milling

In this investigation, the chemical and structural characteristics of Fe nanoparticles synthesized by high-energy ball milling have been explored. After the milling process the nanoparticles were collected using a magnetic field. The structure, morphology and composition of the powders were obtained using high-resolution electron microscopy. HREM images confirmed the nanoparticles’ presence with approximately 2–4 nm in size. It was found that using this method allowed the formation of nanoparticles in a smaller size range than other synthesis methods. Also, it was confirmed by HREM images that the obtained nanoparticles were mainly of the fcc nature and some of them of the MTP type.     

Synthesis and characterization of spinel LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles prepared by high-energy milling and hydrothermal method

Spinel LiMn₂O₄ has been known to be a technologically important, environmental-friendly, and low-cost cathode material used in Li-based rechargeable batteries, and it is also widely available. Nanoparticle spinel LiMn₂O₄ has been synthesized by the top-down, high-energy milling, and hydrothermal methods. SEM images, X-ray diffraction patterns, and neutron high-resolution powder diffraction patterns have confirmed the nanocrystalline nature of the spinel LiMn₂O₄ samples. Raman and Fourier transform infrared (FTIR) measurements show typical absorption and vibration spectra typical for the spinel LiMn₂O₄ showing the formation of various metallic bonds in the sample. The strongest Raman and FTIR signals come from the higher frequency region, with weaker signals appearing in the lower frequency range.     

Enhanced coercivity and remanence of PrCo_5 nanoflakes prepared by surfactant-assisted ball milling with heat-treated starting powder

PrCo5 nanoflakes with strong texture and high coercivity of 8.15 k Oe were prepared by surfactant-assisted ball milling with heat-treated starting powder. The thickness and length of the as-milled nanoflakes are mainly in the ranges of 50–100 nm and 0.5–3 μm, respectively. The x-ray diffraction patterns demonstrate that the heat treatment can increase the single phase and crystallinity of the Pr Co5 compound, and combined with the demagnetization curves, indicate that the single phase and crystallinity are important for preparing high-coercivity and strong-textured rare earth permanent magnetic nanoflakes. In addition, the coercivity mechanism of the as-milled Pr Co5 nanoflakes is studied by the angle dependence of coercivity for an aligned sample and the field dependence of coercivity, isothermal(IRM) and dc demagnetizing(DCD)remanence curves for an unaligned sample. The results indicate that the coercivity is dominated by co-existing mechanisms of pinning and nucleation. Furthermore, exchange coupling and dipolar coupling also co-exist in the sample.     

Preparation of Y<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore using high-energy ball milling and their structural,thermal and conducting properties

Pyrochlore-structured materials are very important materials due to their structural and conducting properties. These properties can be further modified by changing processing conditions. In the present study, pyrochlore (Y₂Ti₂O₇) is synthesized using high-energy ball milling. During various stages of ball milling, the ball-milled powder is taken for investigating the structural and thermal properties. The replacement of Ti₂O₃ by TiO₂ in nominal composition leads to lower ball milling duration to form Y₂Ti₂O₇. Differential thermal analysis showed the single exothermic peak below 800 °C, which indicates formation of disordered pyrochlore phase. The as prepared powders (40-h ball milled) were compacted and heat treated at 1,450 °C for 12 h. The conductivity of sintered sample is found to be one order higher than earlier reported pure Y₂Ti₂O₇ pyrochlore.     

Morphology and magnetic properties of CeCo5 submicron flakes prepared by surfactant-assisted high-energy ball milling

CeCo₅ permanent magnetic alloy has been processed by surfactant assisted high energy ball milling. Heptane and oleic acid were used as the solvent and surfactant, respectively. The amount of surfactant used was 50% by weight of the starting powder. The produced particles were deposited on a piece of copper (4 mm in length and width) under a magnetic field of 27 kOe applied along the copper surface and immobilized by ethyl α-cyanoacrylate. Scanning electron microscope pictures show that the particles are flakes, several μm in length and width and tens of nm in thickness. X-ray diffraction patterns and magnetic measurements prove that the flakes are crystalline with c-axes magnetic anisotropy. The easy magnetization axis is oriented perpendicular to the surface of the flake. A maximum coercivity of 3.3 kOe was obtained for the sample milled for 40 min.     

Mechanochemical reactions in Co<Subscript>80</Subscript>B<Subscript>20</Subscript> and Co<Subscript>60</Subscript>B<Subscript>40</Subscript> mixtures during ball milling and postannealing

The solid-phase interaction of cobalt with boron during high-energy ball milling of Co₈₀B₂₀ and Co₆₀B₄₀ mixtures was studied. It was found that Co₃B boride forms during the mechanical alloying of components in the presence of both modifications of Co with a reduction in the size of blocks of up to 7–15 nm. The transformation of the metastable Co₃B phase into stable Co₂B and CoB borides is observed in the temperature range of 590–700°C.     

Characterization of Y<Subscript>2</Subscript>BaCuO<Subscript>5</Subscript> nanoparticles synthesized by nano-emulsion method

Nanoscale yttrium–barium–copper oxide (Y₂BaCuO₅, Y211) particles were synthesized using the emulsion method and the solution method. The basic water-in-oil (w/o) emulsion system consisted of n-octane (continuous oil phase), cetyltrimethylammonium bromide (cationic surfactant), butanol (cosurfactant) and water. The composition of the emulsion system was varied and characterized by measuring the conductivity of the solutions and droplet size. The droplet size of emulsion was determined by using the dynamic light scattering method. The water content, cosurfactant content, and surfactant/n-octane ratio affected the droplet size which was in the range of 3–8 nm, and hence the w/o emulsion system was referred to as a nano-emulsion system. A model was used to verify the droplet size. The influence of salt (Y₂(NO₃)₃) content on the droplet size was investigated and the addition of salt reduced the droplet size. The effects of reaction time and temperature on the Y211 particle sizes were also investigated. The particles were characterized using the TEM, SEM, and XRD. Nanoparticles produced by the nano-emulsion method were calcined at 850°C to form the Y211 phase as compared to solid state processing temperature of 1050°C. Based on the TEM analysis, the average diameter of the Y211 particles produced using the nano-emulsion method was in the range of 30–100 nm. The effect of adding 15% Y211 nanoparticles to the superconductor YBCO-123 as flux pinning centers, was investigated, and the transition temperature was reduced by 3 K.     

Novel nanocomposite membranes based on polybenzimidazole and Fe<Subscript>2</Subscript>TiO<Subscript>5</Subscript> nanoparticles for proton exchange membrane fuel cells

In this work, Fe₂TiO₅ nanoparticles were used for improving the proton conductivity, and water and acid uptake of polybenzimidazole (PBI)-based proton exchange membranes. The nanocomposite membranes have been prepared using different amounts of Fe₂TiO₅ nanoparticles and dispersed into a PBI membrane with the solution-casting method. The prepared membranes were then physico-chemically and electrochemically characterized for use as electrolytes in high-temperature PEMFCs. The PBI/Fe₂TiO₅ membranes (PFT) showed a higher acid uptake and proton conductivity compared with the pure PBI membranes. The highest acid uptake (156 %) and proton conductivity (78 mS/cm at 180 °C) were observed for the PBI nanocomposite membranes containing 4 wt% of Fe₂TiO₅ nanoparticles (PFT₄). The PFT₄ composite membrane showed 380 mW/cm² power density and 760 mA/cm² current density in 0.5 V at 180 °C at dry condition. The above results indicated that the PFT₄ nanocomposite membranes could be utilized as proton exchange membranes for high-temperature fuel cells.     

Ultra-sharp α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoflakes: growth mechanism and field-emission

We report the synthesis of single-crystalline α-Fe₂O₃ nanoflakes from a simple Fe–air reaction within the temperatures range of 260–400 °C. The nanoflakes synthesized at the lowest temperature (260 °C) in this work show an ultra-sharp morphology: 5–10 nm in thickness, 1–2 μm in length, 20 nm in base-width and around 5 nm at the tips; successfully demonstrate the promising electron field emission properties of a large-scaled α-Fe₂O₃ nanostructure film and exhibit the potential applications as future field-emission (FE) electron sources and displays (FEDs). The formation and growth of α-Fe₂O₃ nanostructures were discussed based on the surface diffusion mechanism. PACS 79.60.Jv; 79.70.+q; 77.84.Bw     

Immobilized TiO<Subscript>2</Subscript> nanoparticles produced by flame spray for photocatalytic water remediation

Anatase/rutile mixed-phase titanium dioxide (TiO₂) photocatalysts in the form of nanostructured powders with different primary particle size, specific surface area, and rutile content were produced from the gas-phase by flame spray pyrolysis (FSP) starting from an organic solution containing titanium (IV) isopropoxide as Ti precursor. Flame spray-produced TiO₂ powders were characterized by means of X-ray diffraction, Raman spectroscopy, and BET measurements. As-prepared powders were mainly composed of anatase crystallites with size ranging from 7 to 15 nm according to the synthesis conditions. TiO₂ powders were embedded in a multilayered fluoropolymeric matrix to immobilize the nanoparticles into freestanding photocatalytic membranes. The photocatalytic activity of the TiO₂-embedded membranes toward the abatement of hydrosoluble organic pollutants was evaluated employing the photodegradation of rhodamine B in aqueous solution as test reaction. The photoabatement rate of best performing membranes significantly overcomes that of membranes produced by the same method and incorporating commercial P25-TiO₂.     

Synthesis and characterization of magnetite-maghemite nanoparticles obtained by the high-energy ball milling method

We present the process of synthesis and characterization of magnetite-maghemite nanoparticles by the ball milling method. The particles were synthesized in a planetary ball mill equipped with vials and balls of tempered steel, employing dry and wet conditions. For dry milling, we employed microstructured analytical-grade hematite (α-Fe₂O₃), while for wet milling, we mixed hematite and deionized water. Milling products were characterized by X-ray diffraction, transmission electron microscopy, room temperature Mössbauer spectroscopy, vibrating sample magnetometry, and atomic absorption spectroscopy. The Mössbauer spectrum of the dry milling product was well fitted with two sextets of hematite, while the spectrum of the wet milling product was well fitted with three sextets of spinel phase. X-ray measurements confirmed the phases identified by Mössbauer spectroscopy in both milling conditions and a reduction in the crystallinity of the dry milling product. TEM measurements showed that the products of dry milling for 100 h and wet milling for 24 h consist of aggregates of nanoparticles distributed in size, with mean particle size of 10 and 15 nm, respectively. Magnetization measurements of the wet milling product showed little coercivity and a saturation magnetization around 69 emu g^?1, characteristic of a nano-spinel system. Atomic absorption measurements showed that the chromium contamination in the wet milling product is approximately two orders of magnitude greater than that found in the dry milling product for 24 h, indicating that the material of the milling bodies, liberated more widely in wet conditions, plays an important role in the conversion hematite-spinel phase.     

Modification of the “Transcritical” state in Ni<Subscript>75</Subscript>Fe<Subscript>16</Subscript>Cu<Subscript>5</Subscript>Mo<Subscript>4</Subscript> films produced by RF sputtering

The saturation magnetization, the perpendicular and rotational anisotropy constants, and the coercitivity of Ni₇₅Fe₁₆Cu₅Mo₄ thin magnetic films produced by rf sputtering are measured in the initial state and after annealing. A relation between the presence of perpendicular anisotropy and the “transcritical” state in the films is established. It is shown that, after additional thermal treatment, the magnetic softness of the films can be improved.     

Modification of the Al<Subscript>2</Subscript>O<Subscript>3</Subscript> surface by high-energy bismuth ions

This paper reports on an atomic-force microscopy study of the surface of α-Al₂O₃ single crystals irradiated by Bi ions with energies of 710, 557, 269, and 151 MeV. The shape of the radiation defects produced by single ions was established to depend on the ionization energy loss. The threshold ionization density above which the surface topography is observed to change lies in the 27–35 keV/nm interval. Possible mechanisms of defect formation in the thermal-spike model, namely, a phase transition and the creation of thermoelastic stresses in the high-energy ion track, are considered.     

Spin-wave resonances in Eu<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> and EuMn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

Spin-wave resonances have been observed in superlattices arising due to the phase separation and self-organization of charge carriers in Eu_0.8Ce_0.2Mn₂O₅ single crystals. The resonances are found within the 5–80 K temperature range at frequencies close to 30 GHz. Similar resonances with intensities about an order of magnitude lower are also observed in EuMn₂O₅. The latter suggests the existence of charge transfer processes between the manganese ions of different valences in EuMn₂O₅.     

Synthesis and characterization of mono-dispersed Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Er<Superscript>3+</Superscript>-coated SiO<Subscript>2</Subscript> nanoparticles by co-precipitation process

In this paper, a facile co-precipitation process for preparing mono-dispersed core–shell structure nanoparticles is reported. The 110 nm SiO₂ cores coated with an yttrium aluminum garnet (Y₃Al₅O₁₂) layer doped with Er³⁺ were synthesized and the influence of the concentration ratio of [urea]/[metal ions] on the final product was investigated. The structure and morphology of samples were characterized by the X-ray powder diffraction, Fourier transform IR spectroscopy and transmission electron microscopy, respectively. The results indicate that a layer of well-crystallized garnet Y₃Al₅O₁₂:Er³⁺ were successfully coated on the silica particles with the thickness of 20 nm. The near infrared and upconversion luminescent spectra of the SiO₂@Y₃Al₅O₁₂:Er³⁺ powders further confirm that a Y₃Al₅O₁₂:Er³⁺ coating layer has formed on the surface of silica spherical particles.     

Novel synthesis and characterization of ZnCo<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoflakes grown on nickel foam as efficient electrode materials for electrochemical supercapacitors

ZnCo₂O₄ nanoflakes were directly grown on Ni foam via a two-step facile strategy, involving cathodic electrolytic electrodeposition (ELD) method and followed by a thermal annealing treatment step. The results of physical characterizations exhibit that the mesoporous ZnCo₂O₄ nanoflakes have large electroactive surface areas (138.8 m² g^?1) and acceptable physical stability with the Ni foam, providing fast electron and ion transport sites. The ZnCo₂O₄ nanoflakes on Ni foam were directly used as integrated electrodes for supercapacitors and their electrochemical properties were measured in 2 M KOH aqueous solution. The ZnCo₂O₄ nanoflake electrode exhibits a high capacitance of 1781.7 F g^?1 at a current density of 5 A g^?1 and good rate capability (62% capacity retention at 50 A g^?1). Also, an excellent cycling ability at various current densities from 5 to 50 A g^?1 was obtained and 92% of the initial capacitance maintained after 4000 cycles. The results demonstrate that the proposed synthesis route is cost-effective and facile and can be developed for preparation of electrode materials in other electrochemical supercapacitors.     

Superspin-glass like behavior of nanoparticle La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> obtained by mechanochemical milling

Single-phase perovskite compound La_0.7Ca_0.3MnO₃ was synthesised by a high-energy ball milling in a single step processing. Structure and morphology characterizations revealed nanoparticle nature of this mixed valent manganite with the average particle diameter of 9 nm. Comprehensive set of magnetic measurements showed that the system can be described as an ensemble of interacting magnetic nanoparticles where each particle possesses high magnetic moment, i.e., superspin. Furthermore, magnetic behavior showed contributions from both superspin-glass (SSG) and superparamagnetic (SP) states, and the prevailing properties depended on the experimental conditions. It was established that SSG state dominated in low magnetic fields up to 500 Oe while in higher applied fields suppression of collective behavior occurred and individual characteristics of nanoparticles prevailed. It was also concluded that the applied method of synthesis produced system with high magnetic anisotropy as well as with the large nanoparticle shell whose thickness amounts 30% of a particle diameter.     

Spin-glass dynamics in interacting nanoparticle system La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> obtained by mechanochemical milling

Spin dynamics in mechanochemically obtained nanoparticle manganite La_0.70Ca_0.30MnO₃ was investigated in this study by means of a series of AC and DC magnetic measurements. AC susceptibility indicates the presence of sizeable interparticle interactions, yielding collective magnetic behavior. The related properties were probed by experiments in weak DC field: memory effects were analyzed in both field-cooled (FC) and zero field-cooled (ZFC) regimes, while, after ZFC aging, magnetic relaxation was recorded. The system appears to be sensitive to magnetothermal history, in analogy with spin-glass-like compounds. The analysis of the data indicates the occurrence of slow dynamics in an ensemble of strongly interacting super spins.     

Synthesis and electrochemical characterization of graphene nanoflakes and LiFe<Subscript>0.97</Subscript>Ni<Subscript>0.03</Subscript>PO<Subscript>4</Subscript>/C for lithium-ion battery

The graphene nanoflakes and olivine-type LiFe_0.97Ni_0.03PO₄/C (LFNP3/C) samples have been synthesized as anode and cathode materials, respectively. Physicochemical characterization of the graphene nanoflakes and LFNP3/C material were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM). The XRD patterns reveal the formation of the pure phase of both the synthesized samples. SEM micrographs disclose the formation of spherically shaped nanosized particles for LFNP3/C while graphene shows flake-type morphology. CR2032 half and full coin cells were assembled for electrochemical testing of the synthesized samples. Cyclic voltammetry (CV) results indicate that the graphene-based half-cells, i.e., GN1H and GN2H, possess reduction peak/plateau around 0.17 V while LFNP3/C cathode shows discharging voltage plateau at 3.4 V vs. Li/Li⁺. The discharge capacities were found to be 700, 900, and 153 mAhg^?1 for GN1H, GN2H, and LFNP3/C half-cells vs. Li/Li⁺, respectively. Among full cells, LFPGN1F with γ = 0.75 (mass/capacity balancing factor) shows better charging/discharging profile at each C-rate as compared to LFPGN2F with γ = 0.55. LFPGN1F delivered an initial discharge capacity of around 154 mAhg^?1 at 0.1C and even at a high discharge rate of 1C, it retained ~97% of the discharge capacity as compared to the initial cycle at the same rate.     

Magnetic dynamics of phase separation domains in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript> and Gd<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

Specific features of the magnetic properties and magnetic dynamics of isolated phase separation domains in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ have been investigated. These domains represent 1D superlattices consisting of dielectric and conducting layers with the ferromagnetic orientation of their spins. A set of ferromagnetic resonances of separate superlattice layers has been studied. The properties of the 1D superlattices in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ are compared with the properties of the previously investigated RMn₂O₅ (R = Eu, Tb, Er, and Bi) series. The similarity of the properties for all the RMn₂O₅ compounds with different R ion types is established. Based on the concepts of the magnetic dynamics of ferromagnetic multilayers and properties of semiconductor superlattices, a 1D model of the superlattices in RMn₂O₅ is built.