María J. Sayagués

Monophasic TiyNb1 −yCxN1 −x nanopowders obtained at room temperature by MSR

It is suggested that further improvement of the properties of nitride and carbonitride-based hard alloys and cermets requires systems exclusively constituted of solid solutions. The use of pre-made complex carbonitrides as the raw materials makes it necessary to search for new production methods in preparing the original powders. This work is the first successful attempt to obtain quaternary carbonitride phases in a reliable and easy manner. TiyNb1 − yCxN1 − x powders have been synthesised by milling titanium, niobium and carbon in a nitrogen atmosphere. TiyNb1 − yCxN1 − x phases were formed by a mechanically induced self-sustaining reaction. Nanocrystalline powders with homogeneous chemical composition were obtained. The stoichiometry of complex carbonitrides can be controlled by adjusting the amount of metals and carbon in the starting mixture.

Study of the thermal stability of carbon nitride thin films prepared by reactive magnetron sputtering

Abstract CN x amorphous films have been prepared by reactive magnetron sputtering in a pure N 2 discharge. The films grown on NaCl have been characterised by Fourier transform infrared spectroscopy (IR), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). C/N atomic ratios have been determined by EELS with values in the range 2.0–1.2 for samples grown under different conditions. The thermal stability of the films upon heating in vacuum was followed ‘in situ’ at the transmission electron microscope by EELS. This study has been completed by a thermogravimetric and mass spectrometer analysis of evolved gases upon heating in nitrogen flow and vacuum, respectively. Under these conditions the films are stable up to 1023 K. Above this temperature the films decompose by elimination of nitrogen remaining a carbonaceous residue. The thermal stability of the films upon annealing in air was studied by following the evolution of the X-ray photoelectron spectroscopy (XPS) peaks during heating in air of films grown on steel. Deconvolution analysis of the XPS spectra allows to determine the evolution of the different type of bonds. In particular pure carbon in the films appears more reactive to oxygen than CN and C–N bonds.

Synthesis of needle-like BaTiO3 particles from the thermal decomposition of a citrate precursor under sample controlled reaction temperature conditions

Needle-like BaTiO3 particles with controlled microstructure have been prepared from the thermal decomposition of a metal-organic precursor under sample controlled reaction temperature (SCRT) conditions. In a first step, a mixed barium and titanium citrate precursor consisting of a single phase compound with controlled morphology and stoichiometry have been prepared. The precursor has been converted isomorphologically into BaTiO3 by thermal treatment under SCRT conditions. The so-obtained needle-like particles are constituted of self-assembled nanoparticles. The size of the assembled subunits has been tailored by controlling the reaction rate and the partial pressure of the gases generated during the thermal decomposition of the barium titanium citrate. This microstructural control accomplished by the SCRT method can not be achieved by conventional thermal decomposition of the citrate precursor. Finally, the set of samples obtained with different microstructures has been investigated in terms of stabilization of the cubic phase at room temperature.

Tribochemical effects on CNx films

Abstract CN x thin coatings have been deposited by dc magnetron sputtering using a graphite target in nitrogen atmosphere under different experimental conditions. X-Ray photoelectron spectroscopy, electron-energy loss spectroscopy, nuclear magnetic resonance and Fourier-transformed infrared spectroscopy were used to elucidate the structural chemistry of each film. The tribological behavior has been investigated using a reciprocating pin-on-flat tribometer in a wide range of environmental conditions: ambient air, dry air and nitrogen. Tribochemical effects in relation to the nature of the surrounding atmosphere during friction tests are presented. Strong relationships between the N/C ratio, the nature of C–N bonds and the friction behavior are highlighted and discussed. An optimum in the friction and wear properties was found for a maximum in the CN/CC ratio for the studied set of CN x samples. The XPS/AES analysis of the sliding counterfaces support a destabilization of the CN x network under friction and build-up of a carbon rich tribolayer under steady-state.

Cystine-capped CdSe@ZnS nanocomposites: mechanochemical synthesis, properties, and the role of capping agent

Cystine-capped CdSe@ZnS nanocomposites were synthesized mechanochemically with the aim to prepare a material which could be used in medicine for biosensing applications. Although synthesized CdSe@ZnS nanocomposites were capped with l-cysteine, cystine was formed from l-cysteine during the milling process. It was proven that water plays the key role in this oxidative transformation. The novel material was characterized by the complex of physico-chemical methods (FTIR, XPS, SEM, EDX, surface area measurements) and CHNS analysis. The leakage of Cd2+ and Zn2+ ions into physiological solution was also studied.

Preparation, characterization and thermal evolution of oxygen passivated nanocrystalline cobalt

Nanocrystalline cobalt powders have been prepared by the inert gas evaporation method. After preparation the materials were passivated by pure oxygen and air exposure. In the present paper we describe the application of different techniques like transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to achieve an exhaustive chemical and structural characterization of the nanocomposite material in its original state (passivated Co powder) and after thermal treatments in vacuum. After passivation the cobalt fcc nanocrystals (typical sizes 2-20 nm) are covered by an amorphous oxide passivation layer with a short range structural order similar to the cubic (fcc) CoO phase. Upon heating, crystallisation of the oxide layer is observed together with the formation of the Co 3 O 4 spinel phase. In situ XPS experiments allow the determination of the relative amounts of oxide and metal as a function of the oxygen dose. The present study is relevant due to the correlation between microstructure and the unusual magnetic properties of this type of material.

preparation and characterization of CdS and ZnS nanosized particles obtained by the inert gas evaporation method

Abstract In the present paper we describe the preparation conditions to obtain CdS and ZnS nanocrystalline samples by the gas phase condensation method. The materials are constituted in both cases by very homogeneous spherical particles with a mean particle diameter of 8 and 15 nm respectively for the ZnS and CdS samples. Characterization has been carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM). High resolution images show that under certain conditions samples can be obtained with single crystallite grains of homogeneous size. Relative band-gap values have been determined from the UV-vis absorption spectra and correlated with quantum size effects for very small crystallite sizes in the nanostructured powders

Phase-pure BiFeO3 produced by reaction flash-sintering of Bi2O3 and Fe2O3

Mixed powders of Bi2O3 and Fe2O3 are shown to yield single-phase, dense nanostructured polycrystals of BiFeO3 in reaction flash sintering experiments, carried out by applying a field of 50 V cm−1 and with the current limit set to 35 mA mm−2. The furnace was heated at a constant rate with the reaction sintering taking place abruptly upon reaching 625 °C. Remarkably, an intermediate bismuth-rich phase of the oxide that forms just before reaching the flash temperature transforms, and at the same time sinters, into single-phase BiFeO3 within a few seconds after the onset of the flash. The BiFeO3 so produced is electrically insulating, a property that is critical to its applications. This one-step synthesis of single-phase polycrystals of complex oxides from their basic constituents, by reaction flash sintering, is a significant development in the processing of complex oxides, which are normally difficult to sinter by conventional methods.

A novel, simple and rapid route to the synthesis of boron cabonitride nanosheets: combustive gaseous unfolding

The ternary compound boron carbonitride (BCN) was synthesized in the form of few-layer nanosheets through a mechanically induced self-sustaining reaction (MSR). Magnesium was used to reduce boron trioxide in the presence of melamine in a combustive manner. The process to form the nanostructured material was very rapid (less than 40 min). The prepared powder was investigated by various techniques such as X-ray diffraction (XRD), Fourier Transform infrared (FTIR), Micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS). The thermal stability and the optical behavior of the BCN nanosheets were also studied by thermal analysis and UV-vis spectroscopy, respectively. The formation mechanism of the nanosheet morphology was described in detail.

Synthesis of TiN/Si3N4 composite powders by mechanically activated annealing

TiN/Si 3 N 4 composite powders were obtained by a process that combines the mechanical activation of titanium and silicon powders at room temperature through high-energy milling with an isothermal annealing in a nitrogen atmosphere to complete the synthetic reaction. Mechanical activation has allowed us to complete the synthesis at 1350 °C only. The β–Si 3 N 4 content in the final powder tends to increase as the milling time is prolonged. The microstructure of the TiN/Si 3 N 4 composite powders has a bimodal character composed of TiN and β–Si 3 N 4 grains and α-Si 3 N 4 nanowires. Diameters of the nanowires range from 10 to 70 nm.