I.M. Miranda Salvado

Sol–gel coatings for corrosion protection of metals

Sol–gel derived films exhibit a high potential as substitutes for the environmentally unfriendly chromate metal-surface pre-treatment methods. Inorganic sol–gel derived films offer good adhesion between metals and organic paint. However, they cannot provide adequate corrosion protection due to their high crack-forming potential. Introduction of an organic component to an inorganic sol–gel system leads to the formation of thicker, more flexible and functionalized films with enhanced compatibility to different organic top coatings. Incorporation of nanoparticles in the hybrid sol–gel systems increases the corrosion protection properties due to lower porosity and lower cracking potential along with enhancement of the mechanical properties. Furthermore, the incorporation of inorganic nanoparticles can be a way to insert corrosion inhibitors, preparing inhibitor nanoreservoirs for “self-repairing” pre-treatments with controlled release properties.

ZrO2-SiO2 materials prepared by sol-gel

Abstract Gels with composition xZrO2−(100−x)SiO2, x = 10−55, were prepared in different conditions using zirconium acetylacetonate and TEOS as precursors. Gels treated at different temperatures up to 1100°C were characterized by X-ray diffraction, IR spectroscopy and TEM. Preparation conditions determined the subsequent development of crystalline phases following thermal treatment. Monoclinic zirconia segregation dispersed in a silica matrix occurred when the gels were prepared in a strong hydrocloric acid medium. Preparation with a lower acid content favours instead the formation of very small crystals of tetragonal zirconia.

Processing and seeding effects on crystallisation of PZT thin films from sol-gel method☆

Abstract Lead zirconate titanate PZT ( 52 48 ) thin films on alumina substrates were prepared by the sol-gel method. In order to prepare pure perovskite phase films at lower temperature, the study of the processing conditions and of seeding effects on the crystallisation behaviour of the PZT perovskite phase was carried out by X-ray diffraction and scanning electron microscopy analysis. The obtention of the perovskite phase was seen to be highly dependent on the preparation conditions of the sols. The viscosity behaviour of the stock sols was analysed by rheological studies in order to obtain data allowing verification of the stability of the stock solutions. A considerable improvement on the crystallisation of the pure perovskite phase at lower temperatures was observed when PZT seeds were added to the sols.

TiO2SiO2 glasses prepared by the alkoxide route

Materials in the composition range xTiO2 · (100 − x)SiO2 (x = 10 and 30 mol%) were prepared with different ratios H2O/alkoxide EtOH/alkoxide and with different thicknesses. The precipitation of anatase was observed in all samples heat treated at 550 and 850°C. In samples heat treated at 1300°C, anatase and α-cristobalite were observed. Rutile precipitation was observed only in the samples prepared with high water content heat treated at 1300°C. Infra-red studies indicate that TiO2 enters the structure in tetrahedral and octahedral coordination in all samples. Density and Vickers hardness showed a strong dependence on preparation conditions.

Seeding studies in PZT thin films

Lead zirconate titanate (PZT) solid solutions exhibit excellent ferroelectric, piezoelectric, pyroelectric, and electrooptical properties and, therefore, are very attractive for the electronic industry. Low-temperature thermal treatment of the films enhances the incorporation of sol-gel derived PZT thin films into integrated circuits. The nucleation temperature of the perovskite phase and the temperature to get a pure perovskite phase can be lowered using heterogeneous nucleation. In this work, thin films of PZT with a zirconium to titanium ratio of 52/48 have been prepared by the sol-gel method using metal alkoxides. Different types of crystalline seeds were used, and their effects on the perovskite phase crystallization were compared. The crystallographic and morphological properties of the films have been analyzed by X-ray diffraction and scanning electron microscopy.

Small angle neutron scattering study of silica gels: influence of pH

Abstract The technique of Small Angle Neutron Scattering (SANS) has been used to investigate the microstructure, at the nanoscale level, of SiO 2 gels at different processing stages. The samples were prepared by the alkoxide route of the sol–gel process with different pH values. They were measured as xerogels after heat treatment at different temperatures and as wet gels at the gel point and after aging at 60°C. SANS measurements confirmed that pH has a strong influence upon the nanostructure of gels. The present work reports the main SANS results obtained in this investigation and their interpretation.

SANS of ZrO2.SiO2 gels

Abstract Small angle neutron scattering (SANS) measurements have been performed to investigate the nanoscale structure of materials of the system xZrO2·SiO2 with x≤10 mol%, at different processing stages. The materials were prepared by sol–gel using the alkoxides method, in strong acidic conditions. Samples studied as xerogels were heat-treated at 120° C and 850° C; as wet gels at the gel point; and after 4 h aging at 60° C. The samples showed extended linear chains ca. 10 nm long at the gel point. The aged gel has a mass fractal structure with fractal dimension of ca. 1.8. The 120°C heat-treated xerogel shows homogeneous oxide regions with 16 nm average diameter and mass fractal nature. For the 850°C heat-treated xerogel the oxide regions average diameter reduced to about 13 nm and presented a sharp and smooth surface, obeying the Porod law.

Kinetic aspects of the formation of seeded lead zirconate titanate thin films

Abstract The crystallization kinetics and microstructure development of PZT thin films on platinum passivated silicon substrates prepared using different concentration of PZT seeds dispersed in PZT sols are studied. TEM analysis showed that perovskite seeds did not dissolve in the precursor sol but maintained a crystalline state before annealing. It was shown that using perovskite PZT nanopowders as seeds results in the crystallization of a single perovskite phase in the PZT film either at lower temperatures or at shorter annealing times. The seeded PZT films exhibited enhanced crystallization kinetics and the overall activation energy for the perovskite crystallization was reduced from 219 kJ/mol (unseeded) to 174 kJ/mol for 1wt% seeded PZT film and to 146 kJ/mol for 5 wt% seeded films. These results support the statement that the presence of nanometric PZT seed particles lowers the activation energy needed to reach the critical nuclei size for the crystallization process.

Structural and electrical properties of seeded lead zirconate titanate thin films

Abstract Films of lead zirconate titanate (PZT) (52/48) on platinum passivated silicon substrates were prepared from sol precursors with the addition of perovskite PZT seeds (52/48, 0–5 mol.%). The phase evolution and microstructure of the films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. The effect of seeds on the crystallization kinetics is discussed. For the different films X-ray rocking curves were constructed and the crystallographic orientation of the films is discussed, stressing the influence of seeds on the orientation of the films. Ferroelectric properties were measured and compared between films prepared with and without seeds. The addition of perovskite seeds to the PZT precursor sols induced the formation of the pure perovskite phase at lower temperatures in thin films. Although seeded films show less intense preferred orientation, they show superior ferroelectric properties over unseeded ones.

SANS Study of Zirconia-Silica and Titania-Silica Hybrid Materials

New silica-based hybrid materials have been produced by the sol-gel process. Samples with compositions xMO2·(100 − x)SiO2 (with M = Zr, Ti and x ≤ 10 mol%) were prepared with polydimethylsiloxane, silanol terminated, with different molecular weights. In the present work the microstructure of samples prepared with a volume ratio organic/inorganic of 2/3 was investigated by Small Angle Neutron Scattering. The results show that a rather homogeneous hybrid material can be obtained. It is observed that the polymer is well distributed in the inorganic matrix and that the addition of MO2 perturbs its local conformation when it has low molecular weight. The inorganic oxide network in the hybrid was found to develop as in pure inorganic dried gels.