M.D. Alcalá

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.

Magnetic properties of magnetite prepared by ball-milling of hematite with iron

Magnetic properties of magnetite powder prepared by ball-milling of stoichiometric mixture of hematite and iron in an inert atmosphere are reported. Hysteresis loops, isothermal remanence acquisition curves and temperature dependence of magnetic susceptibility measurements are used to characterise this material and to examine the effects of heating in air and in an argon atmosphere. Ball-milling of hematite with iron during periods ranging from 30 min up to almost 5 h yields magnetite which exhibits high magnetic hardness, characterised by coercive force three times higher than that typical for single-domain natural magnetites. However, the magnetite produced is unstable upon heating in air, reoxidising almost completely to hematite. Heating in an argon atmosphere causes enhancement of typical magnetic parameters, but decreases the magnetic hardness.

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.

Mechanomaking of Fe/Al2O3 and FeCr/Al2O3 nanocomposites powders fabrication

Abstract Nanocomposite powders (Fe or Fe-Cr alloy)/α-Al2O3 (75 and 85 vol.%) were obtained by room-temperature high-energy milling powder mixtures of hematite (and chromium oxide) with aluminum and alumina in a high-capacity mill for 8-10 h. The composition of iron and iron alloys was followed by Mossbauer spectroscopy, while the appearance of other phases was revealed by X-ray diffraction. The powder particles produced are assemblies of grains (10–20 nm in size) with a wide size distribution (from well below 1 μm up to several hundreds) and low porosity (fully dense particles). Both the metallic and ceramic phases have crystallite sizes below 15 nm for all the compositions investigated. Nano-nano type ceramic nanocomposites were, therefore, obtained.

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.

Synthesis of nanocrystalline magnetite by mechanical alloying of iron and hematite

The synthesis of magnetite has been studied by mechanical alloying in an inert atmosphere of a stoichiometric mixture of micrometric particle size iron and hematite powders. The final products have been characterised by chemical analysis, SEM, TEM, XRD, Mössbauer spectroscopy as well as specific surface and magnetic measurements. The magnetite obtained in this way exhibits a high magnetic hardness. The formation of a wüstite layer on the magnetite core, because of the reaction between magnetite and iron contamination coming from the bowls and grinding balls, tends to decrease the coercive force of magnetite. The formation of this phase would be avoided by controlling the grinding time.

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.

Development of a new thermogravimetric system for performing constant rate thermal analysis (CRTA) under controlled atmosphere at pressures ranging from vacuum to 1 bar

A CI electronic microbalance which allowed sample weights up to 5 g to be measured with a maximum sensitivity of 1 μg has been used to develop a new constant rate thermal analysis (CRTA) technique. This permitted a constant rate of change of weight |C| ≥ 10 μg min−1. Moreover, it has been extended to constant rate jump thermal analysis (CRJTA). The microbalance is attached to a conventional system of vacuum and gas storage that allowed work to be carried out under vacuum or under controlled atmosphere. The performance of the equipment was checked by studying both the thermal decomposition of CaCO3 and the recombination reaction of silicon and nitrogen, producing Si3N4.

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.