Low-temperature synthesis of nanosized bismuth ferrite by the soft chemical method

This paper describes research on a simple low-temperature synthesis route to prepare bismuth ferrite nanopowders by the polymeric precursor method using bismuth and iron nitrates. BiFeO₃ (BFO) nanopowders were characterized by means of X-ray diffraction analyses, (XRD), Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy (Raman), thermogravimnetric analyses (TG-DTA), ultra-violet/vis (UV/Vis) and field emission scanning electron microscopy (FE-SEM). XRD patterns confirmed that a pure perovskite BiFeO₃ structure with a rhombohedral distorted perovskite structure was obtained by heating at 850 °C for 4 hours. Typical FT-IR spectra for BFO powders revealed the formation of a perovskite structure at high temperatures due to a metal–oxygen bond while Raman modes indicated oxygen octahedral tilts induced by structural distortion. A homogeneous size distribution of BFO powders obtained at 850 °C for 4 hours was verified by FE-SEM analyses.     

Synthesis of triclinic and hexagonal SmBO3 powders by mechanically activated annealing of Sm2O3 and B2O3 blends

Samarium borate (SmBO₃) powders were fabricated from oxide raw materials by a two-step solid-state synthesis method including mechanical activation and annealing. Blends containing stoichiometric amounts of samarium oxide (Sm₂O₃) and boron oxide (B₂O₃) were mechanically activated in a high-energy ball mill and subsequently annealed in air. Afterwards, mechanically activated and annealed powders were washed with distilled water in order to remove probable unreacted B₂O₃ phase. The effects of mechanical activation duration (15 min, 1 h, 3 h and 9 h) and annealing temperature (700–1250 °C) on the resultant powders were investigated. Compositional, microstructural, physical, thermal and optical properties of the powders obtained throughout the different process steps were characterized by using an X-ray diffractometry (XRD), particle size analysis (PSA), stereomicroscopy (SM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), gas pycnometry, differential scanning calorimetry (DSC), heating stage microscopy (HSM), atomic absorption spectrometry (AAS), Fourier transform infrared (FTIR) spectrometry and ultraviolet-visible spectrophotometry (UV–vis) techniques. Fine-grained and pure SmBO₃ powders were successfully synthesized via a simple, feasible and scalable route, yielding both triclinic and hexagonal crystal structures. Triclinic SmBO₃ powders were synthesized after mechanical activation for 1 h and annealing at 700 °C for 2 h. The polymorphic transformation temperature of SmBO₃ powders from triclinic to hexagonal is about 1080 °C. Due to the effect of mechanical activation, the synthesis of triclinic SmBO₃ phase and its transformation to hexagonal form were found to take place at ∼50–100 °C lower temperatures than those reported in other methods. Mainly hexagonal SmBO₃ powders were obtained after annealing at 1150 °C in the presence of a very small amount of triclinic SmBO₃. The resultant powders showed intense UV absorptions in the range between 1025 and 1150 nm with minimum reflectivity of 0.57% (triclinic SmBO₃ phase) and 0.68% (hexagonal SmBO₃ phase) depending on their crystal structures.     

Synthesis of (Ni,Mn,Co)O4 nanopowder with single cubic spinel phase via combustion method

(Ni,Mn,Co)O₄ nanopowders with single cubic phase were successfully synthesized using combustion methods. Particle size of the as-burnt nanopowders after combustion was about 20 nm. Crystallization behavior of the NMC was investigated using various techniques such as X-ray diffraction (XRD), thermogravimetric (TG), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). Calcination at different temperature from 400 °C to 700 °C provides the powders with increased crystallinity and grain size. However, further increasing temperature above 800 °C for calcination, cubic spinel phase of NMC partly transformed to tetragonal spinel phase, which implies that cubic spinel phase of NMC nanopowder synthesized by combustion method becomes unstable above 800 °C.     

Synthesis and characterization of Ba1−xSrxTiO3 nanopowders by citric acid gel method

Stoichiometric and monophasic Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders were successfully prepared by the citric acid gel method using barium nitrate, strontium nitrate and tetra-n-butyl titanate as Ba, Sr, Ti sources and citric acid as complexing reagent. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared (IR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the thermal decomposition behavior, the crystallization process and the particle size and morphology of the calcined powders. The results indicated that single-phase and well-crystallized Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders with particle size around 80 nm could be obtained after calcining the dried gel at 950 °C for 2 h.     

Effect of substrate temperature on microstructure and optical properties of nanocrystalline alumina thin films

Aluminum oxide (Al₂O₃) thin films were deposited on silicon (100) and quartz substrates by pulsed laser deposition (PLD) at an optimized oxygen partial pressure of 3.0×10^?3 mbar in the substrate temperatures range 300–973 K. The films were characterized by X-ray diffraction, transmission electron microscopy, atomic force microscopy, spectroscopic ellipsometry, UV–visible spectroscopy and nanoindentation. The X-ray diffraction studies showed that the films deposited at low substrate temperatures (300–673 K) were amorphous Al₂O₃, whereas those deposited at higher temperatures (≥773 K) were polycrystalline cubic γ-Al₂O₃. The transmission electron microscopy studies of the film prepared at 673 K, showed diffuse ring pattern indicating the amorphous nature of Al₂O₃. The surface morphology of the films was examined by atomic force microscopy showing dense and uniform nanostructures with increased surface roughness from 0.3 to 2.3 nm with increasing substrate temperature. The optical studies were carried out by ellipsometry in the energy range 1.5–5.5 eV and revealed that the refractive index increased from 1.69 to 1.75 (λ=632.8 nm) with increasing substrate temperature. The UV–visible spectroscopy analysis indicated higher transmittance (>80%) for all the films. Nanoindentation studies revealed the hardness values of 20.8 and 24.7 GPa for the films prepared at 300 K and 973 K respectively.     

Effect of sintering temperature on the microstructure and transparency of Nd,Y:CaF2 ceramics

1 at% Nd, 3 at% Y doped CaF₂ transparent ceramics were obtained by hot pressing at the sintering temperature varing from 500 to 800 °C under vacuum environment with co-precipitated CaF₂ nanopowders. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed that the obtained nanoparticles were single fluorite phase with grain size around 26 nm. Scanning electron microscopy (SEM) observations of the Nd, Y: CaF₂ ceramics indicated that the mean grain size of the ceramic sintered at 800 °C was about 748 nm. The influence of the temperature on the grain size, microstructure and optical transmittance was investigated. For the ceramic sintered at 800 °C, the transmittance was 85.49% at the wavelength of 1200 nm. The room temperature emission spectra of Nd: CaF₂ and Nd, Y: CaF₂ ceramics were measured and discussed.     

Structural and morphological transformation of NaX zeolite crystals at high temperature

Nearly perfect crystalline zeolite structures could be used as proton exchangeable membranes for fuel cells, potentially offering major advantages over current separation and catalytic processes. They could also be employed as host materials for semiconductor clusters from 1 to 20 nm in diameter to create electronic and optical properties specific to the form of nano-crystals. Well-shaped NaX zeolite octahedral crystals of a large size of 30 μm were synthesized by a hydrothermal method in a mother solution having a 3.5Na₂O:Al₂O₃:2.1SiO₂:593–2000H₂O composition. Thermal treatment of NaX zeolite crystals resulted in the formation of an intermediate amorphous phase at temperature above 800 and 900 °C and a crystalline phase of aluminium silicate (T < 1000 °C). Environmental scanning electron microscopy (ESEM), high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, DTA/TGA and BET analysis were used to characterize the initial materials and the obtained products after various heat treatments.     

Photoluminescent properties of lead zirconate powders obtained by the polymeric precursor method

Lead zirconate powders, obtained by the polymeric precursor method, were annealed for 2 h at temperatures from 300 to 800 °C. The effect of heat treatment on structural defects and photoluminescent behavior was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Fourier transform Raman spectroscopy (FT-Raman), scanning electron microscopy (SEM), and photoluminescence spectroscopy (PL). XRD patterns and FT Raman spectra showed that the structure of the PbZrO₃ powders was orthorhombric. FT-IR spectra exhibited absorption bands at 450 and 860 cm^?1. These were ascribed to ZrO bands and indicate the ZrO₆ octahedral group. SEM micrographs suggested that the annealing temperature allowed structural morphology changes in the samples. PbZrO₃ powders emitted green photoluminescence at room temperature and at lower annealing temperatures but no photoluminescence was observed from the ordered structure. This optical behavior was attributed to structural evolution from disordered to ordered as a function of PbZrO₃ powder annealing. The intensity of the green PL component increased after annealing at 300 °C.     

The influence of sintering temperature on the structure,optical and magnetic properties of Yttrium iron oxide YFeO3 prepared via Lα-alanine assisted combustion method

Single-crystalline YFeO₃ nanoparticles with orthorhombic perovskite structure were synthesized via L_α-alanine assisted combustion approach. The effect of sintering temperature on the lattice and distortion parameters of the YFeO₃ was discussed. The increase of the sintering temperature increased the lattice parameters and crystallite size, while decreased spontaneous strain, cell distortion, orthorhombic distortion and global instability factors. The sintering temperature did not show any significant effect of the atomic percentage of the elemental compositions of the YFeO₃, where the powders showed excellent stoichiometry preserving the Y:Fe ratio (1:1). SEM micrographs showed that the YFeO₃ powders exhibited quasi-rectangular shape. The magnetic properties showed an improvement of the remnant magnetization and coercivity with increasing the sintering temperature, owing to the decrease of distortion of orthorhombic perovskite structure. The diffuse reflectance UV–vis spectrophotometer was employed to investigate the optical behavior and band gap of the sintered YFeO₃ powders at different temperatures. It was noticed that the reflectance bands are blue shifted and the optical band gap is decreased from 2.02 to 1.44 eV. This will gave rise to the possibility to use the developed YFeO₃ orthoferrites as magnetic recoverable catalyst.     

Synthesis of fine dispersed titanium diboride from nanofibrous carbon

This paper presents the experimental data on the synthesis of titanium diboride (TiB₂) fine dispersed powder carried out in laboratory scale. TiB₂ powder was prepared by the reduction of titanium dioxide with boron carbide and nanofibrous carbon in an argon atmosphere. The powders of TiB₂ were characterized by X-ray diffraction (XRD), elemental analyses, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), low-temperature nitrogen adsorption, particle size analysis, simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC). The resulting material contains a single phase – titanium diboride. The particles of the powder were predominantly aggregated. The average size of the particles and the aggregates were 7.4–8.0 µm with a wide size of distribution. The specific surface values of samples obtained were 2.4–5.8 m²/g. The oxidation of titanium diboride began from the temperature of 450 °C. In this work, the optimal synthesis conditions were estimated: the molar ratio was TiO₂:B₄C:C=2:1:3 (according to stoichiometry), the temperature was 1600 °C, the process duration was 20–30 min.     

Structural,multiferroic properties and enhanced magnetoelectric coupling in Sm1−xCaxFeO3

Sm_1−xCa_xFeO₃ where x=0, 0.25, 0.5, 0.75 and 1.0 was synthesized via the sol-gel method at low temperatures in steps of x=0.25. The as-prepared powders were characterized by Thermogravimetric Analysis (TGA), Transmission Electron Microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). The as-prepared powders were pelletized and then sintered at 1000 °C/4 h to obtain single phase material. The sintered ferrite samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS). The XRD data reveal that a pure phase with perovskite structure is obtained for all the compositions and an exothermic peak appears in the DTA curve at 700 °C for x=0.25, suggesting a crystallization process; a similar behaviour is found to occur for the other compositions as well. The band gap values were determined from UV reflection spectra to be in the range of 1.8–2 eV. The structural transition is further confirmed by the changes observed in Raman vibrational modes. The microstructural characteristics of all the phases show particles of different morphology and size. The dielectric constant (ε_r) and dielectric loss (D) were decreased with an increase of frequency (40 Hz to 110 MHz). A huge enhancement in remnant magnetization (M_r) and coercivity (H_c) is observed as Ca³⁺ is increases. The improvement in the magnetic properties of present samples is due to the destruction of spin cycloid with Sm. No Fe⁴⁺ ions are discovered upon substitution of Sm³⁺ by Ca³⁺. Magnetic hysteresis loops of these samples show a significant weak ferromagnetic behaviour. Clear evidence of magneto-electric coupling is shown by the P–E loops measurement by applying an electric field.     

Low temperature synthesis of ultrafine non vermicular α-alumina from aerosol decomposition of aluminum nitrates salts

In this paper, spherical, smooth and unagglomerated ultrafine amorphous powder particles were prepared by ultrasonic spray pyrolysis (USP) of easy-handling aqueous aluminum nitrate salts increasing the precursor solute concentration to 0.5 mol L^?1 and reducing the pyrolysis temperature to 700 °C. The transformation of the USP alumina powders into α-Al₂O₃ was studied using combination of X-ray diffraction, electron microscopy, infrared spectroscopy, BET surface area, thermogravimetry and differential thermal analysis. A downward shift of the onset temperature of α-phase transformation to 900 °C has been detected using a larger precursor solution concentration and performing a milling before calcination due to an increase in the surface density of defects, in surface area and in anisotropic particle shape. Additional post-milling of the low calcined powders allowed the preparation of agglomerate-free pure ultrafine α-Al₂O₃ powder particles (～100 nm, 28 m² g^?1), free of vermicular microstructures.     

Synthesis of nanocrystalline Mg0.6Cd0.4Fe2O4 ferrite by glycine-nitrate auto-combustion method and investigation of its microstructure and magnetic properties

Nanocrystalline Mg_0.6Cd_0.4Fe₂O₄ ferrite powders were produced by the glycine-nitrate auto-combustion method for the first time. The influence of the different molar ratios of glycine-to-nitrate G.N⁻¹) on the characteristics of the prepared powders was systematically investigated by X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FE-SEM) and vibrating sample magnetometry (VSM). Thermodynamic calculations revealed that the adiabatic flame temperature changes from 598.79 K to 1757.97 K by increasing the G.N⁻¹ ratios from 0.30 to 0.85. The results confirmed that under fuel-lean combustion (G.N⁻¹ = 0.30), Mg_0.6Cd_0.4Fe₂O₄ nanoparticles can be obtained at a significantly lower temperature and shorter synthesis time, compared to other preparation methods like standard ceramic and co-precipitation. The XRD and ICP results showed that the crystallite size of the powders changes in the range of 8–43 nm, and their Cd content notably decreases with increasing the G.N⁻¹ values. The FE-SEM results proved that the porosity and size of the as-prepared ferrite nanoparticles drastically change with variations in the G.N⁻¹ ratio. The evolution of phase, crystallite/particle size, and magnetic properties after annealing was discussed in detail. At the optimized annealing condition, the synthesized Mg_0.6Cd_0.4Fe₂O₄ ferrite offered a high saturation magnetization of 41.70 Am² kg⁻¹ and a coercivity of 1.92 kA m⁻¹, indicating noticeably better soft magnetic properties compared to the same ferrite produced by the other wet chemical methods.     

Microstructure and deposition mechanism of CVD amorphous boron carbide coatings deposited on SiC substrates at low temperature

Amorphous boron carbide (α-B₄C) coatings were prepared on SiC substrates by chemical vapor deposition (CVD) from CH₄/BCl₃/H₂/Ar mixtures at low temperature (900–1050 °C) and reduced pressure (10 kPa). The deposited coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that two kinds of α-B₄C coatings were deposited with different microstructures and phase compositions, and the effect of deposition temperature was significant. When deposited at 1000 °C and 1050 °C, the coatings exhibited a nodular morphology and had a relatively low content of boron. The free carbon was distributed in them inhomogeneously; in contrast, when deposited at 900 °C and 950 °C, the coatings presented a comparatively flat morphology and had a uniform internal structure and high boron content. They did not contain free carbon. At the last of this paper, the pertinent mechanisms resulting in differences in microstructure and phase composition were discussed.     

Effect of fuel type on the microstructure and magnetic properties of solution combusted Fe3O4 powders

Porous magnetite (Fe₃O₄) powders were synthesized by solution combustion method using the glycine and urea at different fuel to oxidant ratios (ϕ). The combustion behavior depended on the fuel type as characterized by thermal analysis. The structure and phase evolution investigated by X-ray diffraction method showed nearly single phase Fe₃O₄ powders which were achieved only by using the glycine fuel at ϕ=1. The specific surface area and porous structures of the as-combusted Fe₃O₄ powders were characterized by N₂ adsorption-desorption isotherms and scanning electron microscopy, respectively. The surface area using the glycine fuel (62.6 m²/g) was higher than that of urea fuel (42.5 m²/g), due to different combustion reactions. Magnetic properties of the as-combusted powders were studied by vibration sample magnetometry which exhibited the highest saturation magnetization of 74 emu/g using the glycine fuel at ϕ=1 on account of its high purity and large crystallite size.     

Synthesis of (K,Na) (Nb,Ta)O3 lead-free piezoelectric ceramic powders by high temperature mixing method under hydrothermal conditions

A facile hydrothermal route via high temperature mixing method was used to synthesize (K, Na) (Nb, Ta)O₃ lead-free piezoelectric ceramic powders. The influence of Ta doping and K⁺/(K⁺ + Na⁺) molar ratios in the starting solution on the resultant powders were investigated by X-ray diffraction, scanning electron microscope, transmission electron microscopy, and selected area electron diffraction. The Ta element was successfully doped into the alkaline niobate structure to form crystalline (K, Na) (Nb, Ta)O₃ lead-free piezoelectric ceramics powder. The microstructure, piezoelectric, ferroelectric, and dielectric properties of the sintered (K, Na) (Nb, Ta)O₃ ceramics from the obtained powders were investigated. The piezoelectric coefficient (d₃₃), electromechanical coupling coefficient (k_p), dielectric constant (?_r), and remnant polarization (P_r) of the sample sintered at 1180 °C show optimal values of 210 pC/N, 34.0%, 2302, and 19.01 μC/cm², respectively.     

Investigation on mechanochemical combustion behavior of Mg–V2O5–Co3O4-C reactive system to synthesize VC–Co nanocomposite powder

VC–Co nanocomposite powders were obtained by mechanochemical combustion synthesis from a mixture of V₂O₅, Co₃O₄, C and Mg powders. The synthesized powders were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). VC–Co nanocomposite was directly produced after 10 min milling through a mechanically induced self-sustaining reaction without further heat treatment. TEM analysis showed that a nanostructured powder with a mean particle size of 100 nm was procured in the sample milled for 10 min.     

Electrical resistivity of α-SiC ceramics sintered with Al2O3 or AlN additives

Highly resistive SiC ceramics were prepared by hot pressing α-SiC powders with Al₂O₃-Y₂O₃ additives with a 4:1 molar ratio. X-ray diffraction patterns, Raman spectra, electron probe microanalysis (EMPA), and scanning electron microscopy (SEM) images revealed that the bulk SiC ceramics consisted mostly of micron-sized 6H-SiC grains along with Y₂O₃ and Si clusters. As the additive content increased from 1 to 10 vol%, the electrical resistivity of the ceramics increased from 3.0 × 10⁶ to 1.3 × 10⁸ Ω cm at room temperature. Such high resistivity is ascribed to Al₂O₃ in which Al impurities substituting Si site act as deep acceptors for trapping carriers. More resistive α-SiC ceramics were produced by adding AlN instead of Al₂O₃. The highest resistivity (1.3 × 10¹⁰ Ω cm) was achieved by employing 3 vol% AlN-Y₃Al₅O₁₂ (yttrium aluminum garnet, YAG) as an additive.     

Microstructure and mechanical properties of Al2O3–20 wt%Al2TiO5 composite prepared from alumina and titania nanopowders

In the present work, Al₂O₃–20 wt%Al₂TiO₅ composite was prepared from reaction sintering of alumina and titania nanopowders. The nano-sized raw powders were reconstituted into nanostructured particles by ball milling. Then, the nanostructured reconstituted powders were pressed and pressureless-sintered into bulk ceramics at 1300, 1400, 1500 °C for 2 h. The phase composition and microstructures of reconstituted powders and as-prepared ceramic composites were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope and energy-dispersive spectrometer (EDS). The microstructural analysis of the ceramic showed that the average grain size of the alumina–aluminium titanate composite increases with increasing the temperature. Also, SEM proved the existence of a proper interface between Al₂TiO₅ and Al₂O₃ grains and preferential distribution of aluminium titanate particles in the grain boundaries. XRD analysis indicated the absence of rutile titania in the sintered composite ensuring complete formation of aluminium titanate. The hardness of the samples sintered at 1300, 1400, 1500 °C were 4.8, 6.2 and 8.5 GPa, respectively.     

Strengthening of Al2O3-C slide gate plate refractories with microcrystalline graphite

The novel carbon sources (including nano-carbon black, carbon nanotubes and graphene oxide nanosheets, etc.) have been extensively researched in low carbon Al₂O₃-C refractory systems. In the present work, ultrafine microcrystalline graphite (UMCG) and nickel-loaded ultrafine microcrystalline graphite (NMCG) were added into low carbon Al₂O₃-C slide gate plate refractories to partially replace graphite flake (GF), respectively. The mechanical properties, phase compositions and microstructures were investigated by three-point bending test, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. Also, the reaction mechanisms of in-situ formed ceramic phases were discussed by thermodynamic analysis. The results indicate that the existence of UMCG powders can facilitate the in-situ formation of intertwined ceramic whiskers, leading to increased densification and mechanical properties of low carbon Al₂O₃-C slide gate plate. Moreover, multi-walled carbon nanotubes and ceramic phases intensively interlock with each other in the Al₂O₃-C refractories containing NMCG powders, which results in their better mechanical properties; the cold modulus of rupture are 36.03±0.12 MPa and 32.14±0.17 MPa for the specimens after coking at 1200 °C and 1350 °C, respectively. This work puts forward a practical application for the microcrystalline graphite as a candidate carbon source in Al₂O₃-C slide gate plate refractories.