Titanium

Summary

Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in sea water, aqua regia, and chlorine. Titanium was discovered in Cornwall, Great Britain, by William Gregor in 1791 and was named by Martin Heinrich Klaproth after the Titans of Greek mythology. The element occurs within a number of minerals, principally rutile and ilmenite, which are widely distributed in the Earth's crust and lithosphere; it is found in almost all living things, as well as bodies of water, rocks, and soils. The metal is extracted from its principal mineral ores by the Kroll and Hunter processes. The most common compound, titanium dioxide, is a popular photocatalyst and is used in the manufacture of white pigments. Other compounds include titanium tetrachloride (TiCl4), a component of smoke scr

Official source

https://en.wikipedia.org/wiki/Titanium

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Nanostructured Functional Particles and Surfaces

Arthur Olivier Joseph Ganz

Topographic nanostructuring of surfaces is a promising concept, in particular to improve the bio-integration of titanium orthopedic implants. In this context, a new method for the nanostructuring of anodised titanium surfaces was developed. Ordered topographic features in the tens of nanometre in height were created by anodising electropolished titanium in the presence of polymeric particles deposited as monolayers. To do so, an existing electropolishing method for titanium was applied and optimised, allowing the production of extremely smooth starting surfaces. The particle-substrate contact was mesured, modeled and finally modified by plasma, thermal and chemical vapour treatments and its effect on the topography of the anodic oxide layer was investigated. Different types of ordered structures were produced, and characterised by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The influence of the anodisation conditions on the topography and morphology of the surface was studied. To gain deeper understanding of the mechanisms at play, a model experiment using electron beam lithography was designed. Circular masks of increasing diameters were deposited on the surface to simulate the presence of particles of corresponding sizes, and the effect on the topography of the oxide layer was characterised. Some aspects of the structuring phenomena observed with the particles were thus cleared up, in particular the extent of oxide layer growth underneath the masks. Finally, numerical finite element modeling was applied to simulate the initial stages of anodic oxide growth around masks, leading to a better understanding of the formation of some of the topographic features observed. In parallel to this work on surface structuring, mesoporous silica particles containing different functional nanoparticles were produced. They were originally destined to serve as an alternative to the polymeric particles used to create our structurations. Their synthesis process, based on the formation of particles by sol-gel in a miniemulsion, was scaled-up and optimised to obtain narrowly distributed submicron particles with a high micropore volume and a high functional nanoparticle loading. These multifunctional particles were finally found to be unadapted for surface structuring but were assessed for various other applications, depending on the type of nanoparticles incorporated in the silica matrix. In combination with superparamagnetic iron oxide nanoparticles (SPIONS), they were evaluated as potential magnetic drug delivery vehicles. Drug loading and release of the anticancer drug paclitaxel was studied as a model system by simulation and experiment. The release kinetics was found to be very slow, restricted by the large chemical potential difference between the molecule in solution and the molecule adsorbed inside micropores. Chromium doped alumina (ruby) nanoparticles were synthesised and characterised for potential application as near-IR fluorescent markers for biomedical imaging. Their photoluminescent properties were studied and were found to be sufficiently intense for bright-field fluorescence microscopy and confocal laser scanning microscopy. They could however not be incorporated in the multifunctional particles during the gel emulsion synthesis. Manganese doped zinc sulphide quantum dots were also encapsulated in a mesoporous silica matrix and used as the basis material for the production of multicolour fluorescent surfaces. Localised laser thermal treatment of the material was successfully realised, causing a shift in its fluorescence emission wavelength. Multicolour fluorescent images could thus be drawn by programing the laser appropriately.

EPFL2010

Electronic properties of nano-crystalline titanium dioxide thin films

Alain Bally

Titanium dioxide is a cheap, chemically stable and non-toxic material. However its electrical properties are unstable and it is a modest semiconductor and a mediocre insulator. For several applications it would be interesting to make it either more insulating or more conducting. The goal of this work was to modify the electrical properties of nano-crystalline TiO2 thin films deposited by reactive sputtering and to understand the mechanism leading to these modifications. The principal factors that influence the electrical conductivity are on the one hand the concentration and nature of the chemical impurities incorporated in TiO2, and on the other hand the morphology of the thin films. The study was split into two parts. The fist part describes the modifications of the sputtered material obtained by chemical doping of the TiO2 thin films. Niobium, cerium, iron, and fluorine were incorporated successfully in TiO2. The second part describes the modifications obtained by modifying the reactive gas used during the sputtering process: thus oxygen was substituted with water vapor. Several analysis techniques have been used to characterize the TiO2 thin films. They are essentially divided in four categories. The chemical analyses included electron probe microanalyses, x-ray photoemission spectroscopy, and secondary ion mass spectrometry. The structure and morphology analyses of thin films were carried out with x-ray diffraction and atomic force microscopy. The electrical properties in dc or ac mode were measured between room temperature and 350°C. Finally optical transmission provided information on the electronic states and morphology of the thin films. With an appropriate choice of impurities, titanium dioxide can be made more insulating (TiO2:Ce), more conducting with an n-type electrical conductivity (TiO2:Nb) or p-type electrical conductivity (TiO2:Fe). Chemical doping is also the cause of important structure and morphology changes, for example it can force the transformation from the anatase to the rutile structure. It is shown in particular that dopant atoms generate defects such as oxygen vacancies. These defects impede the variation of the electrical conductivity produced by the impurities, thus, in spite of dopant concentration in the percent range, the variation of the electrical conductivity is an increase or a decrease of three order of magnitude at most when compared with undoped TiO2. Thin films deposited with water vapor as the reactive gas present an increase in the electrical conductivity by height order of magnitude compared to samples prepared with oxygen in similar conditions. No atom other than titanium or oxygen could be detected in the thin films in significant concentration. The dramatic conductivity increase is due to the injection inside TiO2 grains of electrons donated by unsaturated titanium atoms lying on the grain surface. A nanometric grain size is a requirement to make such high doping levels possible. Depending on the impurities or reactive gas selected, titanium dioxide can be made either into a good insulator with a high breakdown field and a high permittivity or into a reasonably conducting, transparent film. The electrical conductivity of the samples prepared for this study cover the range from 10-10 to 103 S m-1.

EPFL1999

This study concerns the evolution of the thermomechanical properties of new high-performance materials. In particular, we focus on the relationship between the microstructure and the mechanical properties of a superalloy based on Co-Ni-Cr. Superalloys are metallic alloys with superior characteristics to conventional alloys, especially in extreme conditions, and find application in all that areas requiring materials with high elastic modulus, yield strength and resistance. The investigated alloy is used with satisfaction since several years, but the origin of its outstanding mechanical properties is still poorly understood, as well as the role of small alloying elements additions on mechanical properties. To understand the influence, on the mechanical properties, of the elements added to the base composition, the study is carried out simultaneously on two variants of the material, differing for the presence of a small amount of beryllium in one them. The variant containing beryllium exhibits superior mechanical properties with respect to the base variant. The microstructural evolution and mechanical properties was monitored by using several complementary techniques. Series of heat treatments were performed on samples from the two variants, and both TEP and hardness were systematically measured. This has allowed us to establish the different stages of microstructural changes with the temperature. The TEP is a parameter very sensitive to the change in matrix's composition. It indicates that exposure of this material to a temperature between 500°C and 600°C produces a precipitation stage (A), involving titanium. Such a precipitation is responsible for a remarkable hardening, as well as an increase in the elastic limit. Mechanical spectroscopy allows us to obtain information about the mobility of structural defects which dissipate energy when undergone to a periodic stress. By developing a phenomenological model based on the ADIF curves, we can affirm that the hardening comes from the pinning of dislocations by A precipitates. A second precipitation stage (B) occurs only on the variant containing beryllium, undergone a complex sequence of heat treatments up to 700°C. The analysis of samples by TEM and APT provided essential support to the interpretation of results, and to characterization of the precipitation B. Precipitates B are nanostructured intermetallic compounds NiBe, analogous to GP zones. Their growth is favored by the segregation of titanium at the precipitate-matrix interface to relieve the coherency strains. The hardening provided by B precipitates, together with the structural hardening due to the cold working and the precipitation hardening due to the heating at 550°C, allows to obtain the maximum hardness values. The peaks on the internal friction spectra, also provide informations about the influence of beryllium on the mobility of grain boundaries. This parameter determines the recrystallization and the relaxation mechanism at high temperature, which can be observed by mechanical spectroscopy. Finally, the work here presented yields an important contribution to the understanding of the performance's origin of a Co-Ni-Cr superalloy. It also points the way for other studies aiming the optimization of production processes, in terms of temperature and time of annealing, to obtain the desired properties. This thesis allowed to discover a new hardening phase (B) previously unknown, which could lead to even higher hardness values.

EPFL2016