Microreactor-assisted synthesis of α-alumina nanoparticles

In this paper, nanosized α-Al₂O₃ powders were synthesized with the aid of a high throughput impinging stream microreactor. It was demonstrated that the as-prepared powders exhibited better dispersity and more homogeneous distribution of particle size than that prepared by conventional parallel flow precipitation method due to the drastic collisions and homogeneous explosive nucleation in microchannel during the precipitation process. The effects of calcinating temperature and time on the morphology, phase composition and particle size of α-Al₂O₃ were systematically studied. Comparatively well dispersed and crystallized α-Al₂O₃ powders with higher sintering activity were obtained by calcinating the as-prepared precursors at 1200 °C for 2 h in air. The specific surface area of α-Al₂O₃ powders was above 20 m² g⁻¹ and average particle size was about 110 nm with higher sintering behavior. From room temperature to 1520 °C, the green compact exhibited shrinkage of 19.34%. This work opens a door for developing ultrafine α-Al₂O₃ powders with uniform size distribution, high crystallinity, and excellent thermal expansion property.     

Reactivity of methane at low temperatures (∼400°C) with ‘refractory’ bonds

The thermal cracking of benzene, toluene, diphenylmethane, biphenyl, diphenyl sulfide, diphenyl ether, phenol and dinaphthol was studied in the presence of inert gases, methane, methane-hydrogen mixtures and Cu-Beta, H-Beta and Cu-ZSM-5 catalysts at various reaction temperatures (350–480°C), pressures (3.5–9 MPa) and times (1–4 h). For some systems methane assists conversion by either methylation (benzene, biphenyl) or debenzylation and methylation (diphenylmethane). In others, where methylated materials are reactants, demethylation may occur at the same time as debenzylation. The activities of the various catalysts are compared. Their effectiveness in conversion or selectivity varies with organic substrate. Two mechanisms of conversion are identified. One involves direct addition of methyl groups, the other includes a disproportionation process probably resulting in carbon deposition.     

A simple plasma reduction for synthesis of Au and Pd nanoparticles at room temperature☆

A simple and fast plasma reduction method is developed for synthesis of Au and Pd metal nanoparticles. The scanning electron microscopy (SEM) analysis indicates a formation of aggregates of Au and Pd nanoparticles with branched structure. The transmission electron microscopy (TEM) image shows that the inclusive nanopar-ticles are al about 5 nm in size. Compared to conventional hydrogen reduction method, plasma method inhibits the agglomeration of metal particles. The room temperature operation is very helpful to limit the nanoparticle size. Most interestingly, plasma reduction produces more flattened metal particles. This plasma reduction does not require the use of any hazardous reducing chemicals, showing the great potential for the fabrication of noble metal nanoparticles.     

Hydrothermal synthesis of ZrO2–Y2O3 solid solutions at low temperature

Ultrafine powders of ZrO₂–Y₂O₃ solid solutions have been synthesized by hydrothermal treatment at 110°C. Zirconia gel, crystalline Y₂O₃ and various mineralizing solutions have been utilized as precursors for the hydrothermal synthesis. Yttria-stabilized zirconia (YSZ) with different Y₂O₃ content and characterized by different crystallite sizes have been produced by changing the hydrothermal treatment temperature, and the nature and concentration of the mineralizer solution. The role of mineralizer solutions on the crystallization-stabilization of zirconia gel at low temperature of hydrothermal treatment is discussed.     

Continuous hydrothermal synthesis of ZnGa2O4:Mn2+ nanoparticles at temperatures of 300–500 °C and pressures of 25–35 MPa

Mn²⁺ doped ZnGa₂O₄ particles were continuously synthesized in subcritical and supercritical water using a flow hydrothermal reaction system. Zn, Ga and Mn nitrates were used as the starting materials. The syntheses were carried out at temperatures from 300 to 500 °C, at pressures from 25 to 35 MPa, at KOH concentration of 0.04 M, and for residence times from 0.13 to 1.13 s. X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric differential thermal analysis (TG-DTA) and photoluminescent spectroscopy were used to characterize the samples. Single phase of ZnGa₂O₄ was achieved at residence times of less than 1 s. The crystallite sizes were in the range from 12 to 21 nm which tend to increase with an increase in the reaction temperature and with a decrease in the reaction pressure. Green emission was observed for the annealed samples under reducing conditions whereas as-synthesized samples did not exhibit photoluminescence.     

NaCl–H2O-assisted preparation of SrTiO3 nanoparticles by solid state reaction at low temperature

An environmentally friendly NaCl–H₂O system was developed to synthesize monodisperse strontium titanate (SrTiO₃) nanoparticles from commercially available raw materials (SrCO₃ and rutile) by solid state reactions. The formation rate of SrTiO₃ was accelerated by the addition of NaCl and water vapor. Single phase SrTiO₃ was obtained by calcination at 700 °C for 2 h in water vapor (H₂O flow rate of 2.0 mL/min) by the addition of 50 wt% NaCl, although 900 °C and 750 °C for 2 h were required to complete the reaction by calcinations in air and air by the addition of 50 wt% NaCl, respectively. The results demonstrate that both NaCl and H₂O played vital roles to accelerate the formation of SrTiO₃ nanoparticles at relatively low temperature. On the basis of experiments and analysis, a rational growth mechanism has been proposed and discussed.     

Spark plasma sintering of ZrO2–Al2O3 nanocomposites at low temperatures aided by amorphous powders

In present work, ZrO₂-5 wt% Al₂O₃ and ZrO₂-10 wt% Al₂O₃ nanocomposites are fabricated through spark plasma sintering. Al₂O₃–ZrO₂ amorphous powders and polycrystal Al₂O₃ powders and are doped in the polycrystalline ZrO₂ powders, respectively. When doped with amorphous powders, the sintering of ZrO₂–Al₂O₃ nanocomposites is promoted, and ZrO₂-5 wt% Al₂O₃ and ZrO₂-10 wt% Al₂O₃ nanocomposites with relative densities of 99% are obtained after spark plasma sintering at 1200 °C; however, when sintering of polycrystalline ZrO₂ and polycrystalline Al₂O₃ powders, the relative densities are merely 93%. The enhanced sinterability is due to the metastability and phase transformation of the amorphous powders, which act as sintering aids. The nanocomposites with near-theoretical density show refined microstructure with homogenous mixture of ZrO₂ and Al₂O₃ grains, which further leads to excellent mechanical properties. This article provides new ideas for low-temperature sintering of nanocomposites via using doping amorphous powders.     

Combustion synthesis and characterization of spherical α-MnMoO4 nanoparticles

In this study, α-MnMoO₄ nanoparticles were prepared by combustion synthesis method. The structural, morphological and electrical characteristics of α-MnMoO₄ were investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, transmission electron microscopy and impedance spectroscopy. The structural parameters were calculated from XRD pattern which confirmed the monoclinic structure of α-MnMoO₄. FT-IR and Raman spectroscopy results revealed the presence of MoO₄ surface functional groups. The TEM and HRTEM investigations evidenced the presence of homogeneous distribution of spherical nanoparticles and high crystallinity of α-MnMoO₄. Moreover, the SAED patterns clearly revealed the polycrystalline nature of the material. The conductivity measurements had inferred the semiconducting property of α-MnMoO₄ and the maximum conductivity of 1.94 × 10^? 6 S/cm was attained at 540 °C.     

Fischer–Tropsch synthesis with ethene co-feeding: Experimental evidence of the CO-insertion mechanism at low temperature

Experiments were performed at both normal and rather extreme Fischer–Tropsch Synthesis (FTS) operating conditions over a typical cobalt-based catalyst, with the aim of exploring if aspects of the reaction mechanism could be elucidated. The results show that CO reacted when co-feeding C₂H₄ with syngas, while CO did not react with H₂ in absence of C₂H₄, under extremely low-temperature conditions (140°C). The adsorbed CO and C₂H₄ may behave as monomers and initiators, respectively, and react with each other to form long chain hydrocarbons. It suggests that the C C bond coupling precedes the C O bond dissociation, which is consistent with the CO-insertion mechanism. C_3–6 product distribution with a feed of H₂/CO/C₂H₄ at low temperature followed the same trends in terms of normal FTS product distribution. The observed FTS-type chain growth reaction that occurs at abnormally low temperatures (140°C) when co-feeding C₂H₄ may provide new insights into the chemistry of FTS.     

Carbon nanotube vertical interconnects fabricated at temperatures as low as 350 °C

Carbon nanotube (CNT) vertical interconnects (vias) were fabricated on conductive substrates at a record-low temperature of 350 °C, using only standard semiconductor manufacturing techniques and materials. CNT growth rates were investigated for both Co and a Co–Al alloy catalysts, and compared to that of Fe. The activation energy of the Co-based catalysts was found to be lower, allowing lower temperature growth. Using Co as catalyst full-wafer CNT test vias were fabricated at 350 °C, and 400 °C, and electrically characterized. Good uniformity was obtained, with no apparent yield-loss compared to higher temperature fabricated CNT vias. A negative thermal coefficient of resistance was observed of −800 ppm/K, which is advantageous for interconnect applications. The resistivity of the vias increases with temperature, up to 139 mΩ cm for 350 °C, but was found to be lower than several values obtained from literature of CNT vias fabricated at higher temperatures.     

New BST–silica suspension coating material for dielectric thin films fabricated at low temperatures

Abstract

A new Ba_0·7Sr_0·3TiO₃ (BST)–silica suspension has been developed as a coating material for dielectric thin films. Using a spin on glass (SOG) wet process, a thin layer consisting of nanoparticles of BST and silica was formed on an Al/SiO₂/Si wafer. The BST–silica suspension was made by mixing a BST nanoparticle dispersion with methyl siloxane oligomer. A silicon wafer was coated with the BST–silica suspension using a spin coater and heat treated. The sample was then coated with methyl siloxane oligomer and heat treated again, and finally, a top electrode was applied. The Al/SiO₂/BST–SiO₂/Al/SiO₂/Si stack thus prepared is a metal insulator metal (MIM) capacitor. The electrical properties of the MIM capacitor were evaluated and its capacitance, dissipation factor and voltage coefficient of capacitance were determined to be 1054 pF mm^?2, <0·1 and <100 ppm V^?1 respectively.     

New versatile synthesis for low dimension superparamagnetic YBa2Cu3O7−x nanoparticles

This paper describes the synthesis and characterization of YBa₂Cu₃O_7−x (YBCO) nanoparticles obtained through an environmentally friendly chemistry approach. Y-, Cu- acetates and Ba trifluoroacetate were used for the synthesis of the precursor gel. Moreover, sucrose and pectin reagents were added as chelating agents inducing the formation of small size oxide nanoparticles. The thermal decomposition process of the precursor powder was investigated by thermal analysis correlated with mass spectrometry. The chemical nature, structure and morphology of the particles were investigated by X-Ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Fourier Transform Infrared Spectroscopy. According to XRD analysis the nanoparticles have an orthorhombic structure and the average diameter between 18–30 nm, additionally confirmed by TEM measurements. The superparamagnetic behavior at room temperature of the YBCO nanoparticles has been clearly evidenced by magnetization measurements. Furthermore, the effect of the annealing atmosphere on the magnetic properties has been studied.     

Green gelatine-assisted sol–gel synthesis of ultrasmall nickel oxide nanoparticles

Nickel oxide nanoparticles (NiO NPs) were synthesised using a sol–gel method in a gelatinous medium. Gelatine was used as a size-limiting polymerisation agent for the growth of NiO NPs. X-ray diffraction (XRD) analysis revealed that increasing the calcination temperature increased the crystallite size and decreased the size of the lattice constant. The size-strain plot method (SSP) was used to measure the individual contribution of grain sizes and micro strain on the peak broadening of NiO NPs. Transmission electron microscopy (TEM) showed the ultrasmall size of the NiO NPs with a narrow size distribution(10±0.2 nm). The band gap value of NiO NPs was calculated using ultraviolet–visible (UV–Vis) spectroscopy and decreased with increased calcination temperature.     

The production of β-SiAlON ceramics with low amounts of additive,at low sintering temperature

In the present work nano-sized powder of β-SiAlON was produced using a wet milling process. Different milling times and mediums (methyl ethyl keton, ethanol and toluene as solvents, polyethyleneglicol, oleic acid, sodium tripolyphosphate and polyvinylpyrrolidon as dispersants) were performed for the determination of the most efficient milling system. The powders were produced using a conventional process (the ball to powder ratio was 1:1.5, at 300 rpm, for 1.5 h) having a few hundred nanometer particle size, and these were used as standard powders in this study. The nano-sized β-SiAlON starting powders (<100 nm) were sintered at lower temperatures than that of the conventional powders. The amount of Y₂O₃ in powders (～130 nm), produced by high energy milling process, was fewer than conventional powders (5 wt.%). The results of the powder size, sintering behavior and mechanical properties of this sample were compared to those of the standard powder and its sintered sample. This sample, produced using the nano-powder, was investigated, and densified at 150 °C lower than that of the standard sample. Even though the amount of Y₂O₃ was decreased, the hardness of the samples was better than that of the standard sample.     

Graphitization at low temperatures (600–1200 °C) in the presence of iron implications in planetology

Our objective is to understand how graphite can be formed at “low” temperatures (<1200 °C) in contrast to the high temperature of the industrial processes (∼3000 °C), and from precursors which are non-graphitizable by a thermal treatment alone. Blends of iron and saccharose char were heated between 650 and 1600 °C. The carbons obtained were characterized by SEM, TEM and Raman microspectrometry. Our work confirms that graphite can be formed from non-graphitizable carbons during a heat-treatment in the presence of iron. Carbon and iron migrations, below the eutectic temperature (1150 °C), appear to be a key factor for carbon transformation. Iron migration and graphitization could be favored by nucleation of Fe nanoparticles and surface melting, detected as soon as 900 °C. This allows formation of turbostratic macroporous carbons. Above the eutectic, all iron is liquid and graphitization occurs; it is complete at 1600 °C. Heat-treatment duration, observed over 4 orders of magnitude, favors the structural improvement. Concerning applications in planetology these experimental samples are pertinent experimental analogues of natural carbons from differentiated parent-bodies (with an iron core), and explain how graphite can be formed at temperatures below 1200 °C in these environments.     

Oxidation mechanism of MgO–C in air at various temperatures

Abstract

Oxidation of MgO–C refractories containing 20 wt-%graphite was conducted by measuring the weight loss at regular intervals at various temperatures from 800 to 1600°C in air. The rate of decarburisation increased with rise in temperature from 800 to 1400°C and then remained more or less constant from 1400 to 1600°C. The oxidation kinetics were analysed in detail and reaction rate models derived for the temperature range 800–1400°C. The reaction rate was found to be controlled by diffusion of oxygen through the decarburised layer. At higher temperatures (>1400°C), oxidation of graphite also takes place indirectly by the reaction MgO(s) + C(s) → Mg(g) + CO(g). The magnesium vapour thus produced is reoxidised at the outer surface and redeposited as MgO. This leads to a reduction in porosity in the decarburised outer shell and, consequently, a reduction in the rate of oxidation.     

Microstructural and Mössbauer properties of low temperature synthesized Ni-Cd-Al ferrite nanoparticles

We report the influence of Al³⁺ doping on the microstructural and Mössbauer properties of ferrite nanoparticles of basic composition Ni_0.2Cd_0.3Fe_{2.5 - x}Al_xO₄ (0.0 ≤ x ≤ 0.5) prepared through simple sol-gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray, transmission electron microscopy (TEM), Fourier transformation infrared (FTIR), and Mössbauer spectroscopy techniques were used to investigate the structural, chemical, and Mössbauer properties of the grown nanoparticles. XRD results confirm that all the samples are single-phase cubic spinel in structure excluding the presence of any secondary phase corresponding to any structure. SEM micrographs show the synthesized nanoparticles are agglomerated but spherical in shape. The average crystallite size of the grown nanoparticles was calculated through Scherrer formula and confirmed by TEM and was found between 2 and 8 nm (± 1). FTIR results show the presence of two vibrational bands corresponding to tetrahedral and octahedral sites. Mössbauer spectroscopy shows that all the samples exhibit superparamagnetism, and the quadrupole interaction increases with the substitution of Al³⁺ ions.     

Critical temperatures in the synthesis of graphene-like materials by thermal exfoliation–reduction of graphite oxide

We prepared a series of graphene-like materials by thermal exfoliation/reduction of a graphite oxide (GO) at temperatures between 127 °C and 2400 °C. The extent of the exfoliation and reduction of the GO at different temperatures, as well as the impact on the resultant graphene-like materials (TRGs), were studied through their chemical/structural characterization. The main oxygen loss was observed at 127 °C during the blasting of the GO, which produced its exfoliation into monolayer functionalized TRG with hydroxyl groups and minor amounts of epoxy and carboxyl groups. Above 600 °C, the reduction continued smoothly, with oxygen and hydrogen loss and the conversion of hybridised carbon atoms from sp³ into sp². 1000 °C appears to be a critical temperature for the efficiency of the reduction process, as the resulting TRG contained <2% oxygen and 81.5% sp²-carbon atoms. The materials obtained at 2000 °C and 2400 °C were almost oxygen-free and the layers exhibited a dramatic restoration of the pristine graphite structure, as confirmed by the increase in the average size of the sp²-domains. The typical disordered stacking of TRGs increases with temperature, although they can be dispersed yielding monolayers at 127 and 300 °C and stacks of up to 4–6 layers above 1000 °C, as determined by AFM.     

Preparation of nano-sized iron oxide red using δ-FeOOH as seed and iron as raw material in liquid phase at low temperature

Uniform pseudocube nano-iron oxide red (hematite)particles was prepared in the liquid phase by using δ-FeOOH as seed and iron as raw material at low temperature. The effects of such factors as initial pH of reaction, concentration of catalyst, reaction time on the preparation of these hematite particles were investigated. The product was characterized by XRD, IR, FESEM. The results showed that pure pseudocube hematite particles could be prepared under the experimental conditions of solution initial pH 7, β=0.09（β=［Fe²⁺］/［δ-FeOOH］）, air oxidation temperature and time of 100℃ and 48 h respectively. The product crystallized integrally with particle diameter 90—100 nm, and its color was brightly red. This is a new method to prepare high-purity nano-iron oxide red with the characteristics of simple operation, non-pollution and low cost.