Microstructural evolution of injection molded gas- and water-atomized 316L stainless steel powder during sintering

The present study investigates the microstructural evolution and densification behavior of water- and gas-atomized 316L stainless steel powder. Dilatometry and quenching studies were conducted to determine the extent of densification and corresponding microstructural changes. Results indicate that water-atomized powder could be sintered to 97% of theoretical density, while gas-atomized powders could be sintered to near-full density. The difference in the densification behavior is examined in terms of the particle morphology, initial green density and the particle chemistry.     

A novel solvothermal method to synthesize one-dimensional chains of indium tin oxide nanoparticles

One-dimensional conductive chains of indium tin oxide (ITO) nanoparticles with high specific surface areas of about 65 m² g^− 1 were synthesized from indium, tin acetylacetone complexes using isopropanol as solvent by a novel solvothermal method and post heat-treatment. When isopropanol was replaced with water as solvent, the non-conductive individual In₂SnO₅ nanoparticles were obtained.     

Facile synthesis of porous hollow iron oxide nanoparticles supported on carbon nanotubes

Porous hollow iron oxide nanoparticles (PHNPs) supported on carbon nanotubes (CNTs) were facilely synthesized by etching Fe@Fe_xO_y/CNT with dilute nitric acid aqueous solution at ambient temperature without the assistance of any surfactants and ligands. The mean diameter of hollow iron oxide nanoparticles was about 17 nm, with a wall thickness of about 4 nm. The formation mechanism of PHNPs is discussed based on the characterization results from TEM, XRD and H₂-TPR. The combination of nanoscale Kirkendall effect and selective acid etching is proposed to be responsible for the formation of CNT supported PHNPs, through a transformation from core/void/shell structures to hollow nanoparticles.     

Hydrothermal synthesis of surface-modified iron oxide nanoparticles

We synthesized surface-modified iron oxide nanoparticles in aqueous phase by heating an aqueous solution of iron sulfate (FeSO₄) at 473 K with a small amount of either n-decanoic acid (C₉H₁₉COOH) or n-decylamine (C₁₀H₂₁NH₂), which is not miscible with water at room temperature. Transmission electron microscopy showed that the addition of n-decanoic acid or decylamine changed the shape of the obtained nanoparticles. X-ray diffraction spectra revealed that the synthesized nanoparticles were in α-Fe₂O₃ or Fe₃O₄ phase while Fourier transform infrared spectroscopy and thermogravimetry indicated the existence of an organic layer on the surface of the nanoparticles. In the synthetic condition, decreased dielectric constant of water at higher temperature increased the solubility of n-decanoic acid or n-decylamine in water to promote the reaction between the surface of iron oxide nanoparticles and the organic reagents. After the synthesis, the used organic modifiers separated from the aqueous phase at room temperature, which may help the environmentally benign synthesis of surface-modified metal oxide nanoparticles.     

Synthesis and characterization of ZnO nanoparticles prepared in gelatin media

Zinc oxide nanoparticles (ZnO-NPs) were synthesized via the sol-gel method in gelatin media. Long-chain gelatin compounds were utilized to terminate the growth of ZnO-NPs and to stabilize them. The ZnO-NPs were characterized by a number of techniques, such as X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), and high-magnification transmission electron microscopy (TEM). The ZnO-NPs calcined at different temperatures exhibited a hexagonal (wurtzite) structure with sizes from 30 to 60 nm. The influence of the calcination temperature on the morphology of ZnO-NPs was also investigated. The results showed that gelatin is an interesting material that can be used as a stabilizer in the sol-gel technique for preparing small ZnO-NPs.     

Synthesis of hollow sphere-like mesoporous silica with reformer naphtha as a swelling agent

Hollow sphere-like and other shaped mesoporous silicas were prepared using reformer naphtha as a swelling agent via a hydrothermal synthesis process. The mesoporous silicas underwent a complicated process in which their morphology was transformed from a red blood cell-like shape to a golf-ball sphere-like shape, and then from a golf-ball-like shape to a hollow sphere-like shape in response to various synthesis temperatures. Under the control of using reformer naphtha, the mesoporous silica was transformed from a wheat-like shape to a sphere-like shape, and then from a sphere-like shape to a hollow sphere-like shape. The effects of temperature and the swelling agent may have contributed to the formation of reformer naphtha microemulsions in the copolymer aqueous solution, followed by the self-assembly of block copolymer micelles and silica around the emulsions.     

The growth of carbon nanotubes on large areas of silicon substrate using commercial iron oxide nanoparticles as a catalyst

A new approach to chemical vapour deposition (CVD) growth of carbon nanotubes (CNTs) using commercial magnetite nanoparticles, avoiding its in situ synthesis, is reported. Commercial magnetite nanoparticles were used as catalyst material to growth multiwalled carbon nanotubes by chemical vapour deposition onto a silicon substrate of several square centimeters in area. It is shown that the application of an alternating electric field during the deposition of catalytical nanoparticles is an effective technique to avoid their agglomeration allowing nanotube growth. Scanning electron microscopy showed that the nanotubes grow perpendicularly to the substrate and formed an aligned nanotubes array. The array density can be controlled by modifying the deposited nanoparticle concentration.     

A new two-phase system for the preparation of nearly monodisperse silver nanoparticles

Nearly monodisperse silver nanoparticles were prepared in a two-phase water-cyclohexane system. Aqueous silver nitrate was reduced by the product of the reaction of aqueous hydrazine with benzyl aldehyde in cyclohexane to form, in the presence of oleic acid, cyclohexane-soluble silver nanoparticles. The silver nanoparticles were examined by transmission electron microscopy (TEM), UV-visible spectroscopy (UV-vis), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The nanoparticles are relatively monodisperse (diameter less than 6 nm) and have good stability in cyclohexane due to the oleic acid adsorbed on their surfaces. These silver nanoparticles were successfully assembled into a powder with a face-centered-cubic structure by slow evaporation of the solvent. With some change in particle size, these silver nanoparticles could be transferred from cyclohexane to water by the addition of cetyltrimethylammonium bromide.     

Preparation of uncoated iron oxide nanoparticles by thermal decarboxylation of iron hydroxide cetylsulfonyl acetate in solution

A novel method for preparing uncoated iron oxide nanoparticles by thermal decarboxylation of iron hydroxide cetylsulfonyl acetate in solution followed by heating under the protection of nitrogen was presented. The thermal decarboxylation of the precursor and the formation of iron oxide were monitored by FTIR and XRD, the vibrations of alkyl and sulfonyl groups vanished after refluxing in tetraline and uncoated maghemite was obtained after heating treatment at 400 °C. The sizes and morphologies of the obtained samples were studied by TEM. The particles were about 3 nm after refluxing and 8 nm after calcining at 400 °C but agglomerated due to the absence of capping ligands.     

Novel phase-transfer preparation of monodisperse silver and gold nanoparticles at room temperature

In a novel water-cyclohexane two-phase system, the aqueous formaldehyde is transferred to cyclohexane phase via reaction with dodecylamine to form reductive intermediates in cyclohexane; the intermediates are capable of reducing silver or gold ions in aqueous solution to form dodecylamine protected silver and gold nanoparticles in cyclohexane solution at room temperature. The prepared silver and gold nanoparticles are examined by transmission electron microscopy (TEM), UV-Visible spectroscopy (UV-vis), X-photon electron spectroscopy (XPS) and Fourier transfer infrared spectroscopy (FT-IR). It is found that these nanoparticles are monodisperse in size of less than 10 nm and have good stability in cyclohexane due to the adsorbed dodecylamine on nanoparticle surface. Moreover, the synthesis mechanism is revealed via gas chromatography (GC), gas chromatography-mass spectroscopy (GC-MS), nuclear magnetic resonance (NMR) analyses of the solutions during the preparation process.     

Preparation of hollow carbon nanospheres via explosive detonation

Hollow carbon nanospheres were prepared via a rapid detonation technique, by using negative-oxygen balance explosive trinitrotoluene and nickel powder as starting materials and inorganic acid as solvent. The carbon/metal nanocomposite particles precursor with core-shell structure was engendered firstly during detonation, and then the metal nickel core was dissolved through inorganic acid to attain the hollow carbon nanospheres. High-Resolution Transmission Electron Microscope, X-ray diffraction and Raman spectrum were used to characterize the precursor and the as-synthesized samples respectively. The results show that the external diameter of the hollow carbon nanospheres is 25-150 nm and the thickness of the wall is about 2-10 nm. The surface of hollow carbon nanosphere displays multilayer wall in structure with 0.35 nm space between the layers. Based on the experimental results, possible formation mechanism was also proposed.     

Aging behavior of the matrix of aluminum-magnesium-silicon alloy including carbon nanotubes

The mechanical behavior of the non-aged and the aged Al-Mg-Si alloy composites reinforced with CNTs was evaluated with tensile test and Vickers hardness test. The composites showed higher mechanical strength than the pristine Al alloy before the aging, although exhibiting lower mechanical strength after the aging. The aging treatment was ineffective to strengthen the Al-Mg-Si alloy composites reinforced with CNTs. EDS elemental mapping clarified the Mg and O concentration around CNTs. The reduction of Al oxide species formed around CNTs facilitated the Mg concentration. Due to the Mg consumption around CNTs, Mg elements in Al matrix decreased, and resulted in the incomplete matrix strengthening after the aging.     

Phase transformation and low temperature sintering of manganese oxide and scandia co-doped zirconia

The powders of 89 mol% ZrO₂-11 mol% Sc₂O₃ (11ScSZ) doped with various Mn₂O₃ contents were prepared by a co-precipitation method combined with a SCFD (supercritical fluid drying) route. Inhibition of the cubic-rhombohedral phase transformation in both oxidation and reduction atmospheres is achieved for 11ScSZ by the addition of 2.0 mol% Mn₂O₃. The Mn₂O₃ addition can lower the sintering temperature of 11ScSZ ceramics, and the 11ScSZ-2Mn₂O₃ compact can be sintered to nearly full density at 850 °C. 11ScSZ-2Mn₂O₃ ceramic with the cubic structure sintered at 900 °C possesses the conductivity of ∼ 0.10 S cm^− 1 at 800 °C, which is very promising for intermediate temperature solid oxide fuel cell (SOFC) applications.     

Catalytic synthesis of carbon nanofibers and nanotubes by the pyrolysis of acetylene with iron nanoparticles prepared using a hydrogen-arc plasma method

Carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at different temperatures by the catalytic pyrolysis of acetylene with iron nanoparticles prepared using a hydrogen-arc plasma method. The obtained carbon nanomaterials were characterized by transmission electron microscopy and field-emission scanning electron microscopy. An iron nanoparticle was always located at the tip of CNFs or CNTs, whose diameter was approximately identical with the diameter of the iron nanoparticle. The structures of the products were closely related to the reaction temperature, and could be changed from fibers to tubes by simply increasing the temperature. CNFs were obtained at the reaction temperature of 550-650 °C. When the reaction temperature was increased to 710-800 °C, CNTs were obtained.     

Effects of PVP and KCl concentrations on the synthesis of gold nanoparticles using a solution plasma processing

Application of plasma in an aqueous environment, so called Solution Plasma Processing (SPP) is a recent development. Solution plasma processing for the synthesis of gold nanoparticles by reducing HAuCl₄ in the presence of Polyvinylpyrrolidone (PVP) and potassium chloride (KCl) was investigated in this work. Effects of PVP and KCl concentrations on the size and the shape of the gold nanoparticles in SPP were studied using a UV-vis nir spectrophotometer, and high resolution transmission electron microscopy (HRTEM). The results showed that the shape and size of the nanoparticles were affected by various KCl and PVP concentrations. The average particle size of the gold nanoparticles synthesized by SPP decreased with an increase in PVP concentration and the particle shape became more spherical with the addition of KCl. The HRTEM results indicated that gold nanoparticles with a diameter less than 10 nm were single crystals, while particles with a diameter greater than 15 nm were polycrystalline.     

The sintering and grain growth behaviour of ceramic–carbon nanotube nanocomposites

The sintering and grain growth behaviour of alumina + 2, 3.5 and 5 wt.% carbon nanotubes (CNTs) and alumina + 2 wt.% carbon black nanocomposites prepared by Spark Plasma Sintering (SPS) were studied. The addition of CNTs to ceramics produces a large reduction in the sintering temperature required for their complete densification and a significant grain size refinement by a previously unreported mechanism. The CNTs form a strong entangled network around the grains, which constrains the normal and abnormal grain growth. An alumina/alumina + 2 wt.% CNT/alumina laminate structure was prepared to demonstrate directly the large grain-growth retardation effect of CNTs. These effects open up the possibility of using CNTs as a sintering aid to control the sintering behaviour and microstructures of ceramics in bulk, laminate and functionally gradient (FGM) form.     

Synthesis and characterization of Ba2YSbO6 nanoparticles through a modified combustion process

Nanoparticles of Yttrium Barium Antimonate (Ba₂YSbO₆), a complex perovskite ceramic oxide have been synthesized using an auto ignition combustion process for the first time. The particle size and properties of the nanocrystals have been characterized by X-ray diffraction, thermo gravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The XRD studies have shown that the as-prepared powder is phase pure Ba₂YSbO₆ and has a complex cubic perovskite (A₂BB'O₆) crystalline structure with lattice constant a = 8.402 Å. Transmission electron microscopy study has shown that the particle size of the as-prepared powder was in the range 20-50 nm. The nanocrystals of Ba₂YSbO₆ synthesized by the combustion technique could be sintered to 97% of the theoretical density at a temperature of 1550 °C for 4 h. The combustion synthesis has a definite advantage that the phase pure Ba₂YSbO₆ nanopowder could be obtained by a single step process without the need of a calcination step.     

Synthesis of iron-doped vanadium-tin oxide nanocrystallites for CO gas sensing

Iron-doped vanadium-tin oxide nanoparticles have been synthesized by a hydrolysis and co-precipitation method from iron(II) acetate, vanadium(III) acetylacetonate and tin tetrachloride. The mixed oxide was characterized as a tetragonal cassiterite structure by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) revealed an electronic interaction between tin, vanadium and iron atoms in the oxide structure. Addition of iron species into the vanadium-tin oxide led to a decrease in the crystallite size and changes in the oxidation states of vanadium and iron cations in the surface region. Based on sensitivity measurements in a semiconductor CO gas sensor, the iron doping resulted in a shift of the maximum sensitivity toward the lower temperature side. A correlation between the surface state and sensor performance is proposed.     

Economical synthesis of Al2O3 nanopowder using a precipitation method

Al₂O₃ nanopowder was synthesized by the precipitation method using inexpensive AlCl₃·6H₂O and Al powder as raw materials. The dried precipitate was heat treated in the range of 60-1200 °C. The influence of heat treatment on crystallization and phase transformation of the precipitate was investigated using X-ray diffractometry (XRD), thermogravimetry and differential thermal analysis (TG-DTA) and Fourier transform infrared spectroscopy (FTIR). Scanning electron microscopy (SEM) reveals that the particle size of the powders lies between 30 and 95 nm.     

Comparison of sintering behavior and piezoelectric properties of (K,Na)NbO3-based ceramics sintered in conventional and microwave furnace

In this study (1 − x) K_0.48Na_0.48Li_0.04Nb_0.96Ta_0.04O₃ − xSrTiO₃ (0.0 ≤ x ≤ 0.10) ceramics were fabricated by sintering in microwave furnace for first time as well as in conventional furnace (either via single step or two-step procedures). Sintering behavior and piezoelectric properties of sintered samples were studied and compared. It was found that two-step sintering decreases sintering temperature effectively and enhances densification compared to single step sintering. Microstructure analysis revealed that, two-step sintering suppresses grain growth and promotes densification. On the other hand, microwave sintering enhanced densification more effectively and reduced sintering time and temperature. The maximum piezoelectric constants of ceramics were measured for those sintered in microwave furnace. Piezoelectric constant of the sample containing 1 mol% SrTiO₃ which was sintered in microwave furnace was measured 310 pC N⁻¹ while by sintering in conventional furnace via single and two-step sintering it was obtained 208 and 278 pC N⁻¹, respectively.