Particle size effects on the oxidation of tungsten carbide nanopowders

The oxidation of tungsten carbide (WC) powders with the average particle size 〈D〉 from 20 to 6000 nm was studied by differential thermal and differential thermogravimetric analyses. WC powders are oxidized, irrespective of their dispersity, to higher oxide WO₃. The conclusion was drawn that at smaller particle sizes of WC powder, the oxidation rate increases, and the temperature of the peak of the exo effect decreases. The functional dependences of the temperature of the peak and the activation energy of oxidation on the particle size were determined.     

渣油中沥青质分子颗粒尺寸及其胶粒模型研究

分别以苯和硝基苯为溶剂测定了大庆、胜利、孤岛和辽河减压渣油沥青质、胶质和芳香分的分子量；依据球形分子模型计算了这些物质的分子尺寸；构筑了渣油中沥青质胶粒（胶团）模型，并依此计算了渣油中沥青质胶粒尺寸。结果表明，以硝基苯为溶剂所测沥青质分子量更能反应沥青质化学结构的实质；就原始沥青质来说，以苯为溶剂测得的沥青质分子直径为3.8 nm～5.0 nm,以氯代苯为溶剂测得的分子直径为3.2 nm～3.7 nm,以硝基苯为溶剂测得的分子直径为2.8 nm～3.2 nm；辽河、胜利与大庆减压渣油中沥青质的胶粒直径为10.0 nm～11.0 nm，孤岛减压渣油中沥青质的胶粒直径为9.0 nm～10.0 nm。     

Crystal structure, morphology and composition of magnetron sputtered tungsten carbide films

Summary It is shown that small variations of the deposition parameters during magnetron sputtering of tungsten carbide thin films may result in drastic changes of film properties. An increasing working gas pressure for example lowers stress and hardness values. Simultaneously, the texture of the WC_1–x cristallites turns from 200 preferential orientation to 111, whereas the composition of the films does not change. In reactive sputtering with a tungsten target there is a narrow range from 2 to 3% C₂H₂ gas admixture to the working gas where the films are stochiometric (WC) and hard, and grain size and morphology are similar to that of non-reactively sputtered films. The generation of different crystallite structures and orientations in the range of 0–3% C₂H₂ admixtures are used to produce a multiphase thin film with extremely low crack propagation.     

Influence of the particle size distribution of potassium heptafluorotantalate on the characteristics of sodium-reduced tantalum powders

Two versions of heterophase sodium-thermic reduction of potassium heptafluorotantalate were studied. One version implies initiation of the reaction in a homogeneous mixture of solid K₂TaF₇ and sodium flux, and the other, supply of solid K₂TaF₇ onto the surface of liquid sodium occurring above the molten halides. The surface area of the tantalum powder was correlated with the K₂TaF₇ particle size. The formation mechanisms for the tantalum powder particles were discussed for different reduction procedures. The conditions for preparation of powders with the specific surface area of ca. 4 m² g⁻¹ were identified.     

Novel evidence for the selective behaviour of tungsten carbide in liquid phase heterogeneous catalytic hydrogenation

Liquid (aqueous) phase catalytic hydrogenation of compounds containing two oxo-groups in -position (biacetyl, glyoxal, alloxan) was studied in the presence of tungsten carbide catalyst. It has been shown that only one of the oxo-groups is affected in the course of the process and is transformed into a >CH–OH group. Comparative studies carried out in the presence of platinum catalyst attest the selective behaviour of tungsten carbide.

---

- - (, , ). , >C=O >CH–OH, . .

     

Conductive composite of polyaniline and tungsten carbide

Conductive polyaniline/tungsten carbide (PANI/TC) composite was synthesized via polymerization of the aniline monomer by (NH₄)₂S₂O₈/H₂SO₄ oxidant system in the presence of an aqueous suspension of TC. The structure, thermal stability and conductivity of PANI/TC composite were studied and the results were also compared with the pure PANI. The results showed that there was a strong interaction between the TC particles and PANI molecular chains. The crystalline structure of TC remained undisturbed upon with interaction with PANI chains. The thermal stability of PANI/TC composite was better than that of pure PANI. The direct current conductivity values of PANI/TC composite decreased slowly as the temperature increased from 25 to 165°C and PANI/TC composite exhibited significantly higher conductivity than the pure PANI.     

Analysis of the phase composition and homogeneity of ferrite lithium-substituted powders by the thermomagnetometry method

In the present work, the possibilities of application of the thermomagnetometry TG(M)/DTG(M) method for estimation of the phase homogeneity of end products of synthesis at different stages of ferritizing annealing are considered on the example of solid-state synthesis of Li_0.5(1?x)Fe_2.5?0.5xZn_xO₄ and Li_0.5(1 + x)Fe_2.5?1.5xTi_xO₄ (x = 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) lithium-substituted ferrospinels. Results of thermomagnetometric analysis are compared with XRD data. It is shown that the resolution of the TG(M)/DTG(M) method on two orders exceeds capabilities of the traditional XRD method. Considerable influence of both intermediate grinding and mixing of samples and concentration of an inducted impurity on the homogeneity of the synthesized powders is confirmed.     

Pyrolysis kinetics and chemical composition of Hazro coal according to the particle size

Summary The relationship between particle size and chemical composition of Hazro coal (origin: SE Anatolia, Turkey) has been examined by elemental analysis and by thermogravimetric pyrolysis. The chemical composition of the coal was determined by grinding sample particles physically and separating according to their size in mm by sieving. Particle size distribution of the coal and chemical composition of these fractions were given. The coal has been non-isothermally pyrolyzed in a thermogravimetric analyzer to determine the kinetic factors. Thermal gravimetric (TG/DTG) experiments were performed from ambient temperature to 800°C under a nitrogen atmosphere at heating rate 10 K min^-1 with five different particle size ranges. Kinetic parameters of the samples were determined using a Coats and Redfern kinetic model, assuming a first-order reaction. Depending on the particle size of the coal samples, the mean activation energy values varied between 49.1 and 84.6 kJ mol^-1. The results discussed indicate that activation energies increase as the particle size decreases.     

Biocidal activity of nanocrystalline silver powders and particles

While the activity of the SMAD powders is lower than that of pure silver nitrate, it has the ability to kill bacteria very effectively and over long periods of time.     

Synthesis of nanocrystalline silicon carbide using the sol-gel technique

Highly disperse silicon carbide is synthesized using a hybrid method comprising the sol-gel step to provide the SiO₂-C starting mixture involving the formation of a transparent gel and carbothermal synthesis under relatively soft conditions, namely, at temperatures of 1200–1500°C under a dynamic vacuum. The elemental and phase compositions of the products and their thermal behavior in air are studied. A relationship is found to exist between the microstructure of the product, on the one hand, and the temperature and time of heat treatment, on the other.     

Comparison of different chemometric and analytical methods for the prediction of particle size distribution in pharmaceutical powders

This work compares the estimation of the particle size distribution of a pharmaceutical powder using near-infrared spectroscopy (NIRS), powder flowability properties, and components concentration. The estimations were made by considering the former data blocks separately and together using a multi-block approach. The powders were based on a formulation of paracetamol as the pharmaceutical active ingredient. The reference method used to determine particle size distribution was sieving. Partial least squares methods were used to estimate the multivariate regression models, and the results were compared in terms of figures of merit. It was shown that the partial least squares methods gave similar prediction errors. Regarding the data blocks used, the NIRS block was proven the most advantageous to estimate the particle size distribution. The prediction error of the NIRS block was similar to the other data blocks with additional advantages such as less generalization problems and the possibility of its use to predict additional physical and chemical properties with an improvement to analysis time. The multi-block approach produced the worst results but nevertheless allowed a deeper understanding of the individual contributions of the data blocks in the prediction of the particle size distribution.     

Effect of a precursor on the phase composition and particle size of the active component of Ni-ZrO2 catalytic systems for the oxidation of methane into syngas

The effect the method of preparation has on the formation, phase composition, porous structure, and catalytic properties of the 10% Ni-ZrO₂ system during the partial oxidation of methane into syngas is studied. Stages of the formation of catalysts from precursors and the effect the method of preparation has on the phase composition, particle size, and catalytic properties of the active component during the partial oxidation of methane into syngas are investigated by means of synchronous TG-DTG/DSC and X-ray diffraction. The synthesis of a catalyst precursor via coprecipitation is shown to yield a monophase system with high nickel dispersion, catalytic activity, and stability.     

Synthesis of tungsten carbide nanocrystals and their electrochemical properties

Tungsten carbide (WC) nanocrystals have been prepared by a solvothermal method with Mg as the reductant and WO₃ and anhydrous ethanol as the precursors. The effects of time and temperature on the synthesis of WC were investigated and a probable formation mechanism was discussed. The obtained WC nanocrystals were characterized by X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy and electrochemical methods. Hexagonal closepacked WC was successfully synthesized when the temperature was as low as 500°C. The content of carbon was more than that of W, indicating that the composition of the treated sample was C and WC only. The diameters of WC nanocrystals were ranged from 40 nm to 70 nm and the nanocrystals were dispersed on carbon films. The electrochemical measurements reveal that WC nanocrystals obviously promote Pt/C electrocatalytic ability for the oxygen reduction reaction. __________ Translated from Chinese Journal of Catalysis, 2008, 29(7) (in Chinese)