J.M. Martínez-Duart

Electrodeposition of hydroxyapatite coatings in basic conditions

Hydroxyapatite films have been grown in this work by an electrodeposition method involving both physical and chemical processes and presenting several differences with respect to other reported works. Description of the coating formation is based on the evolution of current through the sample placed as positive electrode in the basic electrolyte. The characterisation of hydroxyapatite films is of special importance since the bioactive properties related to HAP have been directly identified with its specific composition (Ca/P ratio) and crystalline structure. This characterisation has been traditionally fulfilled by the use of XRD, FTIR and SEM. Results of a further characterisation of the coatings by TEM and SFM, additional to the analysis by XRD, FTIR and SEM, are presented. Interpretation and comparison of our results with those obtained by other electrodeposition methods lead to arguments in favour of a deposition produced directly from ionic species.

A theory of avalanche breakdown during anodic oxidation

Abstract The breakdown characteristics during anodization of valve metals are interpreted in terms of a recent model. The model quantitatively takes into account, in addition to the oxidation ionic current, the current transported by the electrolyte incorporated into the oxide and the avalanche electronic current. The primary electronic current of the avalanche is attributed to the electrons released by the electrolyte species in the oxide.

Optical gas sensing properties of thermally hydrocarbonized porous silicon Bragg reflectors

In the present work, porous silicon (PS) based Bragg reflectors are fabricated, and the reactive PS surface is passivated by means of thermal carbonization (TC) by acetylene decomposition. The gas sensing properties of the reflectors are studied with different gas compositions and concentrations. Based on the results it can be concluded that thermally carbonized Bragg reflectors provide an easy and inexpensive means to produce chemically stable high quality PS reflectors with good gas sensing properties, which differ from those of unpassivated PS reflectors.

Optical and structural characterization of r.f. sputtered CeO2 thin films

The optical and structural properties of r.f. sputtered CeO2 thin films deposited on Pyrex substrates have been studied as a function of substrate temperature during deposition. The refractive index, n, extinction coefficient, k, and bandgap of the films were calculated from reflectance, R, and transmittance, T, spectra in the wavelength range 340–900 nm. The refractive index of CeO2 films at 550 nm comprises values from about 2.25–2.4 depending on the substrate temperature during deposition. The extinction coefficient was negligible for wavelength values higher than 400 nm. The value obtained for the bandgap was 3.1 eV. The X-ray diffraction patterns showed the same (f c c) cubic structure with preferential orientation depending on substrate temperature during deposition. The scanning force microscope measurements showed that the roughness and grain size of the CeO2 films increase with increasing substrate temperature.

Atomic force microscopy (AFM) morphological surface characterization of transparent gas barrier coatings on plastic films

The relationship between the surface morphology and water vapor diffusivity of barrier coatings on polymeric polyethylene terephthalate (PET) films is studied by atomic force microscopy. The coatings, composed of aluminum oxide and/or silicon oxide, were grown by various deposition methods. It is observed that no correlation exists when the surface roughness is measured over large scanned areas owing to the inhomogeneous coating morphology. However, for scan sizes restricted to representative zones 0.5 μm wide, free from bumps and pinholes, it is found that those films smoother than the PET substrate show a low permeability. This result would support the idea that some other mechanism, different from the existence of surface irregularities (i.e. bumps, pinholes, etc.) and related to the form of growth, should also determine the barrier diffusivity.

Mechanical and in vitro testing of aerosol–gel deposited titania coatings for biocompatible applications

The biocompatible properties of sol-gel litania have increased the interest in the mechanical properties of this material in the form of functional coatings for prosthetic applications. In the present work. titania coatings with thicknesses of 1 microm have been prepared using the aerosol gel process. The main objective has been to evaluate the mechanical properties of the coatings and to prove their in-vitro biocompatibility. For this purpose, the hardness and Youngs modulus of the coatings were measured by nanoindentation with loads in the 6-30 mN range. A continuous increase of these magnitudes was observed for the coatings treated at increasing sintering temperatures (150-800 degrees C). The hardness and the Youngs modulus ranged between 15.8-19.5 GPa and 142-186 GPa, respectively. This behaviour has been confirmed by measurements of the plastic energy of deformation in 10 mN full loading unloading tests and by determination of the mean indentation creep under 30 mN loads. The films were additionally characterised by XRD. FTIR and ellipsometry to study the chemical and structural changes produced by sintering. Biocompatibility tests are very conclusive. Cells seeded on aerosol-gel titania coatings grow while adhered onto the surface. These coatings are thus of potential interest for the enhancement of the properties of prosthetic TiAlV alloys.

Silver-based low-emissivity coatings for architectural windows: Optical and structural properties

Abstract Spectrally selective coatings are, nowadays, deposited onto architectural windows to be employed in commercial and residential buildings for the purpose of saving energy. In this work, the fundamental optical and structural properties of two types of low-emissivity silver-based coatings deposited onto glass, Ag and SnO 2 /Ni-Cr/Ag/Ni-Cr/SnO 2 (Vitrage a couches avec une forte reflexion des rayonnements thermiques, European Patent EP 0 506 507 B1) are analyzed. It has been demonstrated that the silver-based multilayer coatings give an efficient heat isolation due to their low emissivity values, thus not deteriorating the optical properties in the visible range given by the glass substrate. From Atomic Force Microscopy measurements it has been determined that the Ag layer has a more homogeneous grain size when is deposited onto the Ni-Cr layer than when is deposited onto glass.

Lattice-mismatch induced-stress in porous silicon films

Abstract We have studied the stress in porous silicon films with different porosities at the interface with the substrate. Micro-Raman spectra were measured along a cleaved cross-section to sample different layer depths. Each spectrum was fit to the phonon confinement model, with the bulk phonon frequency as a free parameter to remove phonon confinement effects. At the interface this frequency increases sharply, indicating a compressive stress on the porous silicon pillars. The stress is due to the lattice mismatch, measured by X-ray diffraction, between the porous film and the bulk silicon substrate. For porosities between 50 and 85% the stress and the lattice mismatch vary, respectively, between 4 and 10 kbar, and between 2.9×10 −3 and 3.5×10 −3 . Finally, from the dependence of stress on the lattice mismatch we obtain a microscopic Youngs modulus of 156 GPa. This magnitude, mostly dependent on atomic bonding, is close to the bulk silicon value and is much larger than the macroscopic modulus, strongly dependent on the porous structure, reported in the literature.

Deposition of silicon oxinitride films from hexamethyldisilizane (HMDS) by PECVD

Abstract Silicon oxinitride films are of great industrial interest due to their singular electrical, optical and mechanical properties. The interest in obtaining these films from liquid sources is greatly increasing because of the lower harmfulness and of the new potential features that can be obtained from the precursor chemical structure. Thin films of silicon carbo-oxi-nitrides have been deposited by PECVD from a liquid source, hexamethyldisilazane, and different mixtures of oxygen and ammonia. We have investigated the effects of the variations in the composition of the reactant gases and in the applied power levels, on the visible and infrared optical and micromechanical properties. The refractive index can be varied continuously from 1.4 to 1.8, the IR absorption in the 8–13 μ m region can be tailored to the desired shape, and the hardness and Youngs modulus modified, depending on the deposition conditions.

Growth of polyaniline films on porous silicon layers

Abstract The electrochemical deposition of electrically active polyaniline films (PANI) onto the surface of porous silicon (PS) layers formed at p- and n-type silicon wafers has been studied using cyclic voltammetry measurements and infrared spectroscopy. The process of PANI deposition is easier on the PS layers formed at n-Si wafers and essentially retarded at PS/p-Si samples, presumably due to a parallel reaction of oxygen evolution. The polymerization reaction starts at the pore bottoms and propagates towards the external surface of the PS layer. Electric conduction of PS/PANI composites is lower than of uncovered PS layers.