J. Farjas

Si3N4 single-crystal nanowires grown from silicon micro- and nanoparticles near the threshold of passive oxidation

A simple and most promising oxide-assisted catalyst-free method is used to prepare silicon nitride nanowires that give rise to high yield in a short time. After a brief analysis of the state of the art, we reveal the crucial role played by the oxygen partial pressure: when oxygen partial pressure is slightly below the threshold of passive oxidation, a high yield inhibiting the formation of any silica layer covering the nanowires occurs and thanks to the synthesis temperature one can control nanowire dimensions.

Numerical model of solid phase transformations governed by nucleation and growth: Microstructure development during isothermal crystallization

A simple numerical model which calculates the kinetics of crystallization involving randomly distributed nucleation and isotropic growth is presented. The model can be applied to different thermal histories and no restrictions are imposed on the time and the temperature dependencies of the nucleation and growth rates. We also develop an algorithm which evaluates the corresponding emerging grain size distribution. The algorithm is easy to implement and particularly flexible making it possible to simulate several experimental conditions. Its simplicity and minimal computer requirements allow high accuracy for two- and three-dimensional growth simulations. The algorithm is applied to explore the grain morphology development during isothermal treatments for several nucleation regimes. In particular, thermal nucleation, pre-existing nuclei and the combination of both nucleation mechanisms are analyzed. For the first two cases, the universal grain size distribution is obtained. The high accuracy of the model is stated from its comparison to analytical predictions. Finally, the validity of the Kolmogorov-Johnson-Mehl-Avrami model is verified for all the cases studied.

Mechanical properties of nanometric structures of Si/SiC, C/SiC and C/SiN produced by PECVD

Abstract Nanometric multilayered structures were deposited by PECVD. Deposition on the cathode resulted in amorphous multilayers (a-C/a-SiC/…/a-SiC and a-C/a-SiN/…/a-SiN), whereas nanostructured multilayers (ns-Si/ns-SiC/…/ns-Si) were obtained at the anode. Previous studies on multilayer structures of ns-Si/ns-SiC deposited at room temperature on c-Si wafers by modulated PECVD revealed the mechanical and wear characteristics of these structures, which showed improved adherence to the substrate and blocking of the cracks induced by nanoindentation. Another characteristic of these structures was the absence of oxygen in the SiC layers after exposure to the atmosphere. In the present study, the mechanical characterization of the nanometric multilayer ns-Si/ns-SiC structures, after annealing under an inert atmosphere, has shown an increase in hardness due to: (a) material densification, with an increase in density after dehydrogenation; and (b) crystallization of the layers. Although the films were deposited at low temperature, the need to anneal them to improve their mechanical properties requires the use of temperature-resistant substrates. To avoid the need for post-thermal treatments, we have chosen the deposition of nanometric multilayer structures of a-C/a-SiC and a-C/a-SiN (5–10 nm/layer) using PECVD at room temperature and depositing them on the cathode (higher ion bombardment). The microstructure and morphology of the hybrid amorphous layers were examined by TEM. Hardness and Youngs modulus were measured by the nanoindentation technique. Wear properties were evaluated using a pin-on-disc system. The structures containing approximately 20 layers had better mechanical properties than the corresponding thick monolayers of their components. Their mechanical characteristics, along with their ability to block crack propagation and wear resistance, are useful for applications such as protective coatings for optical (fibers and lenses), electronic and magnetic devices.

Quantification of the bond-angle dispersion by Raman spectroscopy and the strain energy of amorphous silicon

A thorough critical analysis of the theoretical relationships between the bond-angle dispersion in a-Si,Δθ, and the width of the transverse optical Raman peak, Γ, is presented. It is shown that the discrepancies between them are drastically reduced when unified definitions for Δθ and Γ are used. This reduced dispersion in the predicted values of Δθ together with the broad agreement with the scarce direct determinations of Δθ is then used to analyze the strain energy in partially relaxed pure a-Si. It is concluded that defect annihilation does not contribute appreciably to the reduction of the a-Si energy during structural relaxation. In contrast, it can account for half of the crystallization energy, which can be as low as 7 kJ/mol in defect-free a-Si.

Non-isothermal model-free predictions

Suitable thermal treatment of metal organic precursors is a key process to obtain oxide films. To this purpose, non-isothermal model-free predictions are specially suited. In this article we will explore the ability of these methods to provide an accurate prediction of the evolution of the decomposition of yttrium trifluoroacetate, a precursor used in the synthesis of YBaCuO superconducting thin-films. A good agreement has been obtained between the predicted and the measured reaction courses.

Ene reactions between two alkynes? Doors open to thermally induced cycloisomerization of macrocyclic triynes and enediynes

A domino process is described combining an ene reaction between two alkynes and a Diels-Alder cycloaddition of the vinylallene formed. The process accounts for the thermally induced cycloisomerization of macrocyclic triynes and enediynes to give fused tetracycles in a stereoselective manner.

Cell size distribution in a random tessellation of space governed by the Kolmogorov-Johnson-Mehl-Avrami model: Grain size distribution in crystallization

The space subdivision in cells resulting from a process of random nucleation and growth is a subject of interest in many scientific fields. In this paper, we deduce the expected value and variance of these distributions while assuming that the space subdivision process is in accordance with the premises of the Kolmogorov-Johnson-Mehl-Avrami model. We have not imposed restrictions on the time dependency of nucleation and growth rates. We have also developed an approximate analytical cell size probability density function. Finally, we have applied our approach to the distributions resulting from solid phase crystallization under isochronal heating conditions.

Surface analysis of nanostructured ceramic coatings containing silicon carbide nanoparticles produced by plasma modulation chemical vapour deposition

Abstract Ceramic nanometric multilayer structures of nanostructured particles of SiCx:H layers and amorphous Si films were obtained by chemical vapour deposition using modulated rf plasma. This technology has been extensively used for producing ceramic Si-based nanoparticles (SiCxNy) with unique characteristics including spherical morphology, composition and controlled ultrafine particle size in the range 2–100 nm. Hybrid multilayer nanostructures of ceramic coatings containing Si and SiC were produced to study their structural, mechanical and surface properties. Low densities of crystalline nanoparticles were embedded in a-Si matrix during the growth of these structures and they were intercalated between amorphous Si layers. The phase structure, microstructure and morphology of the hybrid multilayered films were examined by transmission electron microscopy and selected area electron diffraction, which revealed the presence and distribution of the nanoparticles in the multilayered structure of the films. The hardness and Youngs modulus were measured by the nanoindentation technique, and the wear properties were evaluated using an improved pin-on-disc system. These results showed that the mechanical properties of the films (hardness, friction, propagation of cracks and wear resistance) were notably enhanced by the presence of the nanoparticles. Potential applications of these coatings based on ceramic multilayers include the production of tough and hard coatings, protective and wear-resistant coatings for mechanical tools, gears and mechanical parts, optical surfaces and fibres, corrosion and high temperature-resistant coatings, as well as inorganic membranes, buffer layers for heterogeneous coatings, and coatings with anisotropic properties.

Is sintering enhanced under non-isothermal conditions?

Several authors have claimed that the sintering rate of a powder compact is enhanced during a heating ramp. Their experimental results show that: (a) at any temperature the shrinking rate increases with heating rate; (b) during a heating ramp between two isothermal periods, the shrinking rate curve reaches a maximum value; and (c) rapid reduction of the shrinking rate is observed when an isothermal period is suddenly reached after a heating ramp. These features were tentatively explained by introducing some new mechanisms, which interfere with mass transport. Here, however, we report that these experimental observations can be explained within the framework of the standard theory of sintering. It is not necessary to include any dependence of the mass transport mechanisms on the heating rate in order to describe the observed behaviour.

The crystallization temperature of silicon nanoparticles

The solid phase crystallization of a-Si:H nanoparticles grown by plasma-enhanced chemical vapour deposition has been studied using differential calorimetry and transmission electron microscopy (TEM). The peak temperature has an activation energy similar to that of the nucleation rate of a-Si:H films (5.3 eV), indicating that crystallization is governed by nucleation. In fact, the results agree with a first-order crystallization kinetics, which allows prediction of the crystallization temperature as a function of the nanoparticle size and the nucleation rate. The nucleation rate is shown to increase exponentially with the oxygen concentration in the nanoparticles.