Tungsten carbide (WC) nanopowders synthesized via novel core–shell structured precursors

WC powders with a size in the range of 20–60 nm were prepared through carbothermal reduction of a novel core–shell structured precursor in vacuum. The samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The thermolysis process of the precursor has been investigated by themogravimetric analysis and differential thermal analysis (TG–DSC). The results revealed that the single phase WC nanopowders were synthesized at 980 °C for 1 h. Spectra of XPS indicate that the surface of the specimen mainly consists of W, C and O three species elements only. The effects of experimental parameters and reaction mechanism have been explored. Mainly due to the homogeneous chemical composition of the precursor, the synthesizing temperature was greatly lower than the conventional method.     

Nanostructured Nd:YAG powders via gel combustion: The influence of citrate-to-nitrate ratio

Nanostructured neodymium doped yttrium aluminum garnet (Nd:YAG) powders were synthesized at low temperature by a gel combustion method with citric acid as fuel and nitrate as oxidizer. The method involves exothermic decomposition of an aqueous citrate–nitrate gel. The decomposition is based on a thermally induced anionic redox reaction. A variety of 1.0 at% Nd:YAG powders with different agglomerate structures were obtained by altering the citrate-to-nitrate ratio γ. The gel with γ = 0.277 yielded nanocrystalline Nd:YAG at 800 °C without the formation of any intermediate phase. For other gels nanostructured Nd:YAG powders were obtained at 850 °C. The gel with γ = 0.1 yielded nanostructured Nd:YAG powder with an average particle size of ∼40 nm. The stoichiometric citrate-to-nitrate ratio (γ = 0.277) gave the lowest amount of agglomeration. The decomposition of the gel was investigated by TG–DSC and FTIR. The produced ashes and calcined powders were characterized by XRD, BET and FETEM analysis.     

Effect of annealing process on the growth and surface properties of Au–ZnO nanowire films grown by chemical routes

Au–ZnO nanowire films have been synthesized by chemical routes, electrochemical deposition (ECD) and chemical bath deposition (CBD) techniques, on zinc foil followed by annealing in air at 400 °C. X-ray diffraction patterns reveal formation of the ZnO wurtzite structure along with binary phases Au₃Zn and AuZn₃. Scanning electron microscopy shows the presence of ZnO nanowires having several micrometers in length and less than 120 nm in diameter synthesized by ECD and in the range of 70–400 nm using the CBD technique. During the annealing process, different surface morphologies originating from different catalytic effects of Au atoms/layers were observed. In addition, the effect of synthesis routes on crystalline quality and optical properties were studied by Raman and photoluminescence spectrometers indicating varying concentration of defects on the films. The Raman results indicate that Au–ZnO nanowire film prepared by chemical bath deposition route had better crystalline quality.     

Gas sensing performance of hydrothermally grown CeO2–ZnO composites

The sensors based on cerium oxide–zinc oxide (CeO₂–ZnO) composites were fabricated by using thick-film screen printing of hydrothermally grown powders. The structural, morphological investigations were carried out by using XRD, FESEM and TEM and these studies revealed that the synthesized products were grown in high-density and possessed well-crystallinity. Furthermore, the gas responses were evaluated towards the ethanol, acetone, liquid petroleum gas (LPG) and ammonia gases. The 2 wt% CeO₂–ZnO composite exhibited excellent response of 94% at 325 °C and better selectivity towards ethanol with low response and recovery time as compared to pure ZnO and can stand as reliable sensor element for ethanol sensor related applications.     

Synthesis and characterization of multiferroic BiFeO3 powders fabricated by hydrothermal method

The influence of processing parameters on phase formation and particle size of hydrothermally synthesized BiFeO₃ powders was investigated. BiFeO₃ powder was synthesized by dissolving bismuth nitrate and iron nitrate in KOH solution at temperatures ranging from 150 to 225 °C. X-ray diffraction patterns and scanning electron microscopy observation indicated that rod-like α-Bi₂O₃ phase was formed at initial stage of reaction and dissolved into ions to form thermodynamically stable BiFeO₃ phase. Single-phase perovskite BiFeO₃ has been formed using a KOH concentration of 8 M at a temperature of ≥175 °C in a 6 h reaction period. BiFeO₃ particle growth was promoted by lowering the KOH concentration, or increasing the duration time or reaction temperature. The effects of processing conditions on the formation of crystalline BiFeO₃ powders were discussed in terms of a dissolution–precipitation mechanism. The magnetization of the BiFeO₃ powders at room temperature showed a weak a ferromagnetic nature.     

Optical properties of ZnS nanoparticles produced by mechanochemical method

Nanosized zinc sulfide (ZnS) has been synthesized by the mechanochemical route using zinc acetate and sodium sulfide as source materials in a high energy planetary ball mill (HEPBM) with 300 rpm for 2 h. The mechanochemically synthesized powders have been analyzed by X-ray diffraction (XRD) for phase analysis, Field Emission Scanning Electron Microscope (FESEM) for the morphological characterization, UV–vis–NIR spectrophotometer for determining band gap energy and Fluorescence spectroscopy for determining the emission wavelength. The crystallite size of the synthesized ZnS nanoparticles calculated by the Debye–Scherer's formula is in the range 7–9 nm. FESEM morphology shows the fibrous structure of ZnS samples. The value of optical band gap has been found to be in the range 5.2–5.3 eV. Room temperature photoluminescence (PL) spectrum of the samples exhibits a blue light emission using UV excitation wavelength of 280 nm.     

Densification ability of combustion-derived Al2O3 powders

Nanocrystalline Al₂O₃ powders containing different amounts of MgO (0.1–5.0 mol%) or added boehmite (AlOOH) have been synthesized by combustion synthesis from aluminium nitrate and magnesium nitrate, using urea or sucrose as fuels. The as synthesized alumina powders were deagglomerated, compacted by dry pressing and sintered at 1625 °C for 2 h. For comparison purposes, a commercial high purity α-Al₂O₃ powder (ACC) was also processed following the same route. The sintered materials were characterized for bulk density (BD), apparent porosity (AP), and water absorption (WA) capacity, microstructure using SEM, and XRD phase composition. In comparison to boehmite, the MgO had a considerable effect on the densification behaviour of combustion-synthesized powder.     

Combustion synthesis of doped nanocrystalline ZnO powders for varistors applications

Doped nanocrystalline ZnO powders in the size range between 15 and 250 nm were synthesized by chemical combustion method. The powders were characterized for their physical, structural and chemical properties by BET, X-ray diffraction, FESEM, TEM and XPS. These powders were consolidated into dense varistors discs by compaction, sintering and evaluated for their I-V characteristics. Post-calcinations of these powders were found to have great influence on the green density and sinterability. The formations of phases after sintering were confirmed by XRD analysis and EDX. The varistor properties have been studied for different calcination temperatures and compositions. Breakdown voltage as high as 9.5 kV/cm and coefficient of nonlinearity 134 were obtained. Leakage current density was found to be ∼1.29 μA/cm² for a specific composition and condition. These studies demonstrate the feasibility of one step synthesis of doped ZnO nanopowder and their consolidation into ZnO fine grain varistor exhibiting improved performance.     

A CTAB-assisted hydrothermal and solvothermal synthesis of ZnO nanopowders

ZnO nanopowders were synthesized by hydrothermal and solvothermal method by using CTAB as surfactant, and the effects of CTAB on the morphologies of ZnO nanopowders were investigated. The results showed that the presence of CTAB could greatly vary the shape of the ZnO crystals. ZnO nanorods were prepared from the hydrothermal system without CTAB and flowers-like ZnO nanostructures were produced from hydrothermal system with 0.4 M and 0.5 M CTAB. Low concentration of CTAB in ethanol was conducive to the formation of ZnO nanorods, but the concentration continued to increase, the morphology of sample transformed into hexagonal bipyramid, and then transformed into spherical. The synthesis mechanism of ZnO powders with different morphologies has been presented.     

Preparation of titanium nitride ultrafine powders by sol–gel and microwave carbothermal reduction nitridation methods

Titanium nitride ultrafine powders were prepared from tetrabutyl titanate and sucrose by sol–gel and microwave carbothermal reduction methods. The influences of reaction temperature, molar ratio of Ti to C, addition of crystal seeds and amount of NH₄F on the synthesis of titanium nitride were studied. The results show that excess amount of carbon, addition of crystal seeds and NH₄F plays a positive effect on the preparation of TiN at low temperature. The inceptive formation temperature of TiN ultrafine powders is about 800 °C, and pure TiN can be prepared at 1000 °C. Field emission-scanning electron microscopy (FE-SEM) was used to get the micrograph of the TiN powder, it shows that the size of the powders synthesized at 1000 °C is about 0.1–0.5 μm.     

Studies on the synthesis and sintering of nanocrystalline yttria

Nanocrystalline yttria powders were synthesized from yttrium nitrate by the citrate gel-combustion technique. The auto-ignition of five different gels with fuel to oxidant (citric acid/nitrate) ratios 0.25, 0.5, 0.75, 1.0 and 1.1 was studied. All these mixtures yielded precursors which upon calcination in air at 1073 K yielded yttria. The bulk densities, specific surface area, X-ray crystallite size, size distribution of particles as well as the residual carbon present in these powders were determined. The influence of the fuel to oxidant ratio on the powder properties was analyzed. Scanning electron microscopy (SEM) showed that these powders were porous while the high resolution transmission electron microscopy (HRTEM) revealed that they consist of randomly oriented cuboidal nanocrystallites with an average crystallite size of 25±7 nm. These powders were compacted at 120 MPa without any lubricant or binder and their sinterability was studied. Pellets with a sintered density as high as 98–99% T.D. (theoretical density) could be obtained at a relatively low sintering temperature of 1673 K. Studies on the dependence of the properties of nanocrystalline yttria powders on the composition of the initial mixture used in the citrate gel-combustion as well as the sintering characteristics of these powders are being reported for the first time. Our investigations revealed that an initial mixture comprising equimolar quantities of the nitrate and citric acid yielded a powder with characteristics most suitable for fabricating yttria crucibles.     

Microwave-assisted preparation of submicron-sized FeTiO3 powders

FeTiO₃ powders were prepared from (Fe+Ti) mixed carbonate precursor by the microwave-assisted calcination method. The thermal analysis of the precursor was conducted by TG/DSC. It is demonstrated that the calcination process can be divided into three different stages: the removal of water, the decomposition of precursor and the formation of FeTiO₃. A variety of techniques, such as XRD, FT-IR, SEM and EDS were utilized to characterize the samples. The results indicated that FeTiO₃ powders can be prepared by microwave-assisted calcination for 20 min at 450 °C, while those cannot be obtained by conventional one until reaction time exceeds 120 min at 600 °C. The FeTiO₃ powders prepared by microwave-assisted calcination are nearly spherical with limited aggregation and have a mean particle size of 400–500 nm.     

Effect of propellant on the combustion synthesized Sr-doped LaMnO3 powders

Lanthanum strontium manganite (LSM) powders of composition La_0.7Sr_0.3MnO₃ are good candidates for cathode application in solid oxide fuel cells. This paper reports the synthesis of LSM powders from nitrate precursors by the combustion method, using two different propellants (urea and glycine) and varying the propellant/nitrate ratio. Thermogravimetric analysis (TGA) revealed two or three decomposition stages of the as-synthesized samples, with complete burn out of organics at about 850–900 °C. X-ray diffraction (XRD) patterns showed formation of only LSM phase for the sample synthesized with excess of urea, whereas SrCO₃ and MnCO₃ phases were also found for the samples prepared from glycine. The powder is better crystallized when a homogeneous gel is formed before burning. The crystallite size calculated using the Scherrer equation is in the range of 15–20 nm. Scanning electron microscopy (SEM) revealed the presence of agglomerates, formed by fine particles of different shapes.     

Properties of sol–gel derived Al2O3–15 wt.% ZrO2 (3 mol% Y2O3) nanopowders using two different precursors

An alumina–15 wt.% zirconia (3 mol% yttria) nanopowder was synthesized by sol–gel method using salt and alkoxide as precursors. Al(NO₃)₃·9H₂O, ZrOCl₂·8H₂O and Y(NO₃)₃·6H₂O were used as salt precursors and Al(OC₄H₉)₃ and Zr(OC₄H₉)₄ were used as alkoxide precursors. The dried gels of two precursors were heat treated in the range of 450–1350 °C. The powders produced by alkoxide precursors calcined at 750 °C were in the range of 15–75 nm, while those prepared by salt precursors had the size in the range of 30–90 nm. The former powders had a higher surface area and smaller mean pore diameter compared with the later powder, i.e. 152 m²/g and 5.63 nm comparing with 121 m²/g and 9.79 nm, respectively. Therefore phase transformation in the former occurred in lower temperatures.     

A new direct coagulation casting process for alumina slurries prepared using poly(acrylate) dispersant

Direct coagulation casting (DCC) of concentrated aqueous alumina slurries prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Addition of small amounts of MgO increased the viscosity of the concentrated alumina slurries with time and finally transformed it in to a stiff gel. Sufficient working time for degassing and casting could be achieved by cooling the slurries to a temperature of ∼5 °C after proper homogenization after the addition of MgO. The DCC slip with alumina loading in the range of 50–55 vol% showed relatively low viscosity (0.12–0.36 Pa s at shear rate of 93 s⁻¹) and yield stress (1.96–10.56 Pa) values. The wet coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% had enough compressive strength (45–211 kPa) for handling during mould removal and further drying. The coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% showed linear shrinkage in the range of 4.8–2.3 during drying and 17.1–16.2 during sintering respectively. Near-net-shape alumina components with density >98% TD could be prepared by the DCC process.     

Vapor phase synthesis of methylpyrazine using aqueous glycerol and ethylenediamine over ZnCr2O4 catalyst: Elucidation of reaction mechanism

A novel method has been developed for the synthesis of methylpyrazine (MP) by using aqueous glycerol and ethylenediamine (EDA) over Zn–Cr catalyst derived from hydrotalcite precursors. The X-ray diffraction analysis of the oven-dried Zn–Cr samples synthesized at various pH ranging from 7 to 11 showed hydrotalcite phase whereas the calcined catalysts displayed ZnO and ZnCr₂O₄ phases. The cyclisation activity of Zn–Cr catalyst prepared at pH ~ 9 demonstrated 99.4% conversion of EDA and 94% of glycerol with ~ 72% selectivity to MP at a reaction temperature of 400 °C. This process demonstrates direct utilization of bio-glycerol for the synthesis of MP.     

Sol–gel synthesis of Pd doped ZnO nanorods for room temperature hydrogen sensing applications

A sol–gel spin coating technique was described for the synthesis of Pd doped ZnO nanorods for hydrogen sensing applications. The nanorods were hexagonal in shape, 50–100 nm in diameter and uniform in distribution. They exhibited homogeneous surface morphology, c-axis orientation and excellent crystalline properties. The synthesized nanorods were used to sense and detect hydrogen in a homemade gas chamber. The fabricated sensor successfully detected as low as 40 ppm hydrogen at room temperature with a very low level of power supply (16.16 μA) under a mixed background. Dynamic and repeated responses were observed with a wide range of hydrogen concentrations (40–360 ppm) at 200 °C. The developed sensor was at least 25 fold more sensitive over the literature documented Pd doped ZnO nanorods in detecting hydrogen at ambient temperature. The simplicity, low-cost, high sensitivity and high stability of the sensor materials suggested that the synthesized Pd doped ZnO nanorods could be used in hydrogen and chemical sensing devices where Pd-mediated catalysis is involved.     

A comparative study of the synthesis of nanocrystalline Yttrium Aluminium Garnet using sol-gel and co-precipitation methods

Nanocrystalline yttrium aluminium garnet (nYAG) powder has been synthesized via sol-gel and co-precipitation methods using nitrate precursors. Thermal evolution and crystallisation kinetics of both the methods were investigated. The optimised calcination condition for the formation of nYAG was also examined. It was found that a complete transformation to nYAG was observed at 925 °C/2 h and 1000 °C/1 h for the coprecipitation and sol-gel samples respectively. An intermediate YAlO₃ phase was formed at 900 °C in all powders regardless of the synthesis methods. The powder morphologies obtained from TEM revealed very similar particle sizes for the two routes (20–30 nm); whilst the extent of agglomeration was higher for the sol-gel method. It was also observed that by controlling the pH in a narrow range, maintaining the precipitate processing temperature and dehydrating excess OH- ions in the precipitates using n-butanol treatment, the extent of agglomeration was further reduced in the co-precipitated nYAG powder.     

表面活性剂对纳米氧化锌粉体分散性的影响

以硝酸锌、碳酸氢铵为原料,通过沉淀反应制备ZnO超细粉体。考察了制备过程中不同种类、添加量的表面活性剂(DBS、PEG、Tw-80)对ZnO粉体平均粒径的影响。试验结果表明:在沉淀过程中添加表面活性剂可抑制前驱体碱式碳酸锌的长大和团聚,制备出的ZnO粒径小,粒径分布范围窄,且Tw-80的分散效果优于DBS和PEG。     

Eggshell-membrane-templated synthesis of hierarchically-ordered NiO–Ce0.8Gd0.2O1.9 composite powders and their electrochemical performances as SOFC anodes

Hierarchically-ordered NiO-Ce_0.8Gd_0.2O_1.9 (GDC) composite anode powders were synthesized using eggshell membranes as biotemplates. The morphology of the as-synthesized powders depended on the kind of Ni precursor and the use of EDTA as a chelating agent. Hierarchically-ordered anode powders were obtained from Ni chloride and Ni acetate precursors with EDTA. The Ni–GDC anode synthesized from Ni chloride precursor with EDTA exhibited the lowest polarization resistance at 800 °C and an activation energy of 0.01 Ω cm² and 0.74 eV, respectively, in humidified H₂. In accordance with the polarization resistance results, the 0.5-mm thick GDC electrolyte-supported single cell with the Ni–GDC anode synthesized from Ni chloride precursor with EDTA showed a maximum power density of 0.34 W cm⁻² at 800 °C with humidified H₂ fuel.