Materials Science Forum Vols. 480-481

Abstract: A new metal-assisted chemical etching method using Na2S2O8 as an oxidant is proposed to form a porous layer on a multicrystalline silicon (mc-Si). This method does not need an external bias and enables formation of uniform porous silicon layers, more rapidly than the conventional stain etching method. A thin layer of Pd is deposited on the mc-Si surface prior to immersion in a solution of HF and Na2S2O8. The characterisations of etched layer formed by this method as a function of etching time were investigated by scanning electron microscopy, X-ray diffraction (XRD), Energy-dispersive X-ray (EDX) and reflectance spectroscopy. It shows that the surface is porous and the etching is independent of grain orientation. In addition, reflectance measurements made with a variety of etching conditions show a lowering of the reflectance from 25 % to 6 % measured with respect to the bare as-cut substrate. However, this result can be improved by changing the experimental conditions (concentration, time, temperature, …).

Abstract: This paper presents the results of studying the thermoluminescent (TL) characteristics of Gadolinium-doped Zirconium oxide (ZrO2:Gd) films. Samples were prepared by the spray pyrolisis method depositing them on different substrates using various concentrations of the dopant and exposing to UV radiation of different wavelengths in the range from 200 to 330 nm. Optimum concentration of the dopant determined on the base of maximum TL response was 12% when films were exposed to 260 nm UV light. At these conditions, ZrO2:Gd films presented a glow curve exhibiting two peaks at 110°C and 290°C, a broad range linear TL response as a function of spectral irradiance and a fading of the order of 11% in a week. No significant differences were observed in the TL characteristics of the films deposited on quartz or glass substrates. These results suggest that ZrO2:Gd films could be used in UV radiation measurement.

Abstract: In this work we present results of Si/SiO2/SiON/SiO2 waveguides fabricated by means of ECR-PECVD. In order to change refraction index and simultaneously to reduce losses related with hydrogen, we have used N2 as precursor gas for controlling the nitrogen to oxygen relation present in the samples. The composition of the samples were carefully controlled by RBS and ERDA analysis. The refractive index and thickness were measured by using a prisma coupler method at a wavelength of 632.8 nm.

Abstract: The materials used in microwave oven cavities must have specific dielectric properties in order to maintain the efficiency of the food heating. Plastics, by their mechanical and chemical properties and low cost, are one of those potential materials. In this study, we present the results of the measurements of complex dielectric constant, ´´ ´ * e e e í − = , in the microwave frequency region, on different plastics: polyoxymethylene (POM), polypropylene (PP) and polybutylene terephtalate (PBT), using the cavity resonant method. We measure the shift in the resonant frequency of the cavity, Df, caused by the insertion of the sample, which can be related to the real part of the complex permitivitty, e´, while the change in the inverse of the quality factor of the cavity, D(1/Q), gives the imaginary part, e´´. The relations are simple when we consider only the first order perturbation in the electric field caused by the sample.

Abstract: Polylactid acid (PLA), as an aliphatic polyester, is one of the most important and studied biodegradable material. The structure, the morphology, the molar mass regulate that biodegrability. During degradation, the process of chain scission occurs through hydrolysis. In this work we have investigated the water penetration through industrial and purified PLA films, in order to understand that degradability. Using different physical measurements, particularly the activation energy of b relaxation, the crystallinity, the water vapour permeability, the glass transition and the ion selective permeability. The complementary of the techniques permits to obtain results that support the physical model presented.

Abstract: Comparative radon investigations of two natural materials, crystalline apatite and amorphous oxide, are presented here. The radon retention in the apatite (sedimentary apatite) is more than 70% of the total radon formed in the solid matrix. The amorphous oxide (gel hydrothermal alteration) retains only 7% of radon. The nature of the material which plays an important role in the radon diffusion is confirmed by heat treatment. The major and interesting result of this heat treatment is increasing of crystallite size. This phenomenon induces improvement on radon retention ability. To study these properties one uses a structure band calculations based on the Linear Combinations of Orbital Atomic (LCAO) method convenient numerically for the ionic systems.

Abstract: In this work, creep data for a polypropylene (PP) were obtained at different levels of tensile stress and temperature, and were then analysed in the light of a model developed, which is based on physical mechanisms (at the molecular scale) responsible for the material’s behaviour, considering that a minimum retardation time does exist. The model yields very good agreement with the experimental data and physically meaningful theoretical parameter values.

Abstract: Tool adhesion wear can be produced by two different ways. On the one hand, direct adhesion wear is caused by the incorporation of tool particles to the chips. On the other hand, indirect adhesion wear is caused by the incorporation of fragment of the workpiece material to the tool. When these fragments are removed, they can drag out tool particles causing tool wear. Indirect adhesion can be localised in the tool edge, giving rise to Built-Up Edge (BUE), or in the tool rake face, giving rise to Built-Up Layer (BUL). In this work microstructural differences between both effects in different machining processes of Al-Cu alloys have been analysed. From these microstructural differences, a model has been proposed for the mechanism of formation of both effects.

Abstract: TiN films with different nitrogen contents were deposited on glass and Si (100) substrates by d. c. magnetron reactive sputtering. The structure of the films was determined by Xray diffraction. It was found that heat treatment at 773 decreased the lattice parameter. A considerable thermal instability of over-stoichiometric films was observed after annealing films in air. This instability enhanced with increasing nitrogen content and is characterized by an abundant appearance of clear and dark disks in Scanning Electron Microscopy (SEM). Also, it was shown that the film instability does not come from an interfacial reaction but was observed when the TiNx layers present a (200) preferred orientation together with high nitrogen content.