C. Real

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.

The use of constant rate thermal analysis (CRTA) for controlling the texture of hematite obtained from the thermal decomposition of goethite

Constant rate thermal analysis (CRTA) has been used for studying the decomposition of three synthetic needle-shaped goethite samples. This method controls the reaction temperature such that the reaction rate and partial pressure of water vapour are kept constant at a selected value. The effects of the pressure of water vapour generated during the dehydration of goethite and of the decomposition rate on the porosity of the resulting hematite have been studied. The effect of the particle size of the precursors on the texture of the final products has also been analysed. By this method, acicular particles of α-Fe 2 O 3 with controlled porosity oriented along the c-lattice axis (the long axis of the particle) have been prepared.

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.

Combustion Synthesis of TiN Induced by High-energy Ball Milling of Ti Under Nitrogen Atmosphere

A planetary ball-mill device that enables one to perform solid-gas reactions at constant pressure was developed. Titanium powders were ball milled under nitrogen at a spinning rate of 960 rpm. The influence of the nitrogen pressure on the mechanochemical reactivity of titanium was analyzed at 1.5 and 11 bars. A spontaneous combustion took place during the grinding process, leading to a high yield of TiN for short milling times. The conversion of titanium into titanium nitride was facilitated by increasing the nitrogen pressure. At 11 bars, full conversion was reached for grinding times shorter than 5 h. Titanium nitride obtained in this way exhibited a high sintering activity.

The use of X-ray photoelectron spectroscopy to characterize fine AlN powders submitted to mechanical attrition

Abstract AlN powders have been submitted to mechanical attrition in air. X-ray diffraction (XRD) and transmission electron microscopy (TEM) show that mechanical treatments are producing the fracture of primary particles and the incorporation of microstructural defects. At the same time, Bremsstrahlung excited Auger electron spectroscopy (AES) in conjunction with x-ray photoelectron spectroscopy (XPS) have allowed the surface characterization of the samples. Under mechanical attrition this surface oxidation increases after some minutes but then a steady-state situation is achieved with the formation of a nanometric passivation layer that contains Al(OH)3 and AlOOH species. This degradation layer prevents the material for further oxidation.

Study of mechanochemical phase transformation of TiO2 by EPR. Effect of phosphate

Abstract The evolution of EPR spectra points out that the grinding of TiO 2 generates Ti 3+ associated to oxygen vacancies in the bulk of anatase. The corresponding EPR signal of Ti 3+ located in the surface emerges by outgassing the sample at 200–300°C. If the comminution is in progress a migration of Ti 3+ from the surface to the bulk takes place. The addition of phosphate inhibits the formation of surface Ti 3+ defects through a mechanism that implies its chemisorption on TiO 2 as a bidentate ligand which hinders the surface ionic mobility and, therefore, strongly inhibits the anatase-rutile conversion. In such a case the polymorphic transformation would take place through direct nucleation around the bulk defects.

Influence of the experimental conditions and the grinding of the starting materials on the structure of silicon nitride synthesised by carbothermal reduction

Abstract Silicon nitride has been obtained by carbothermal nitridation of silica using Constant Rate Thermal Analysis (CRTA) method. This method permits to maintain the CO concentration generated in the reaction in a constant value previously selected by the user. The results obtained have shown that the synthesis of α-Si 3 N 4 is improved by increasing the partial pressure of CO in the vicinity of the sample. The previous grinding of the mixture of carbon/silica to be used as raw material in the synthesis of silicon nitride leads to an important reduction of the temperature, at which the carbothermal reduction occurs at the time that an important modification of the particle morphology takes place.

Sample Controlled Reaction Temperature (SCRT): Controlling the Phase Composition of Silicon Nitride Obtained by Carbothermal Reduction

Carbothermal reduction of silica is one of the most common methods of producing Si 3 N 4 powders. The experimental conditions have an important influence on the structure of the final product, especially the balance of α- to β- Si 3 N 4 . The Sample Controlled Reaction Temperature method has permitted to conclude that the phase composition of the silicon nitride is governed by the partial pressure of CO in the close vicinity of the sample. Moreover, the control of this parameter has an important influence on particle size and morphology of the final product.

Use of emanation thermal analysis in characterization of the influence of grinding on textural and structural properties of nanosized titania powders

It is shown that emanation thermal analysis (ETA) is a very sensitive method for studying the textural and structural changes undergone during thermal and/or mechanical treatment of TiO2. It is shown that the activation energy for the bulk diffusion of radon into titania samples annealed at 1100 °C dramatically increases by increasing the prior grinding time of the sample at room temperature. These results demonstrate that grinding of TiO2 prior to the annealing treatment considerably improves sintering and densification of the material.

Polymorphic transformation from body-centered to face-centered cubic vanadium metal during mechanosynthesis of nanostructured vanadium nitride determined by extended x-ray absorption fine structure spectroscopy

The pathway for vanadium nitride (VN) formation obtained by milling treatment has been traced out. At the initial stages of the process, the reactant, vanadium metal, showing body-centered cubic (bcc) structure, becomes highly distorted. Simultaneously, the formation of a small nucleus of the product, VN, takes place. X-ray absorption spectroscopy (XAS) has allowed the quantification of the distortion degree as well as the detection of the VN nucleus in the early stages of their formation, while other standard structural characterization techniques are unable to detect such phenomena. For increasing milling times, apart from the expected increase in the size of the VN nucleus, a polymorphic transformation from bcc to fcc vanadium metal has been recorded. This phase might play a key role in the overall synthesis process and could be a reaction intermediate in other solid state processes involving V metal. The sensitivity of XAS to noncrystalline domains and to highly distorted environments, as well as the ...