In-situ and post annealing effect on the microstructure and the optical properties of black Cu-Co-Mn oxide spinel coating for Parabolic Trough Collector (PTC) applications

The present paper deals with the deposition of Cu-Co-Mn oxides thin film by magnetron sputtering to produce a black selective coating for solar Parabolic Trough Collector (PTC) systems. The effect of temperature was studied between the nominal temperature range of 300-600°C, using both in-situ heating and post annealing. The optical behavior was investigated by advanced spectrophotometry devices in visible, near and infrared spectral ranges. Scanning Electron Microscopy (SEM) was used to show the microstructure, and X-ray Diffraction (XRD) to identify the phases and to compare the samples'pattern prepared at different temperatures.

Selective light induced chemical vapour deposition of titanium dioxide thin films

Estelle Wagner

Light Induced Chemical Vapour Deposition (LICVD) of titanium dioxide thin films is studied in this work. It is shown that this technique enables to deposit locally and selectively a chosen crystalline phase with a precise controlled thickness at low substrate temperature, allowing even the use of polymer substrates. A home made LICVD reactor was set up, consisting of a main chamber in which the substrate was placed on a temperature controlled plate and could be irradiated perpendicularly through a window by a 250 ns long pulse XeCl excimer laser (308 nm) with a controlled fluence (energy per surface area per pulse). A liquid precursor (titanium tetra-isopropoxide) was brought by an oxygen carrier gas flow in the chamber and a nitrogen flow was added to prevent deposition on the window. The total pressure in the chamber was maintained at 10 mbar. Projection of a mask image on the substrate was realized. Titanium dioxide films could be deposited locally by this technique with a thickness ranging from few nanometers to several micrometers. Deposits were realized at substrate temperatures as low as room temperature and with fluences between 1 and 400 mJ/cm2. The deposited films were characterized by profilometry, Atomic Force Microscopy, optical microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, UV-vis spectroscopy and ellipsometry. A detailed and systematic study of the influence of some selected process parameters (substrate temperature and irradiation dose principally) on the deposition rate and material properties was realized on glass and silicon substrates. An empirical equation of the growth rate as a function of the different experimental parameters was derived and was tested successfully to obtain a precise control of the deposited thickness (a precision better than 10 nm was demonstrated for films below 300 nm), with growth rates up to 300 nm/min. From this equation, a deposition mechanism involving both a photolytic reaction and a thermal contribution of the substrate temperature was proposed. The deposited material was shown to consist of TiO2 with a possible very low additional carbon contamination. Depending on the deposition conditions, amorphous, anatase or rutile material was deposited. One parameter explaining the variation of the crystalline state of the material was the variation of the laser induced temperature rise which was simulated theoretically in parallel. Surface temperature rises in the order of several hundreds of degrees were calculated during the laser pulse, varying with the substrate, the fluence and the film thickness. The optical properties of the deposited material vary in a correlated way with the crystalline state of the material, and high indexes of refraction (up to 2.6 at 633 nm) were measured. Selective deposition in the irradiated area was studied. Provided substrate temperature was kept below 150°C, no thermal deposition was observed, and deposits were strictly located in the irradiated area. The edge resolution (around 10 micrometers) was demonstrated to be due to the optical aberrations of the mask projection set up. No effect of substrate temperature and irradiation conditions on the resolution could be evidenced, but resolution decreased with increasing deposited thickness due to optical effects. Thanks to the low substrate temperature required for the deposition process, deposition on thermo- fragile substrates (such as polymers, in particular PMMA) was demonstrated. However, in the case of polymer substrates, the fluence had to be kept below 15 mJ/cm2 not to damage or ablate the polymers, resulting in very low growth rates (in the order of 10-3 nm/pulse). In these conditions, only amorphous films with about 10% of carbon contamination could be obtained. Additionally, films were shown to crack above a certain deposited thickness, which is attributed to thermal effects. As a matter of fact, calculations showed a high temperature rise up to 70°C of the polymer/oxide interface. Deposition on polymers coated with transparent conductive oxide was also demonstrated.

EPFL2003

Oxygen assisted focused electron beam induced deposition of silicon dioxide

Alexandre Perentes

Focused Electron Beam Induced Deposition (FEBID) is a rapid prototyping technique for the investigation, production and modification of 2 and 3-D nanostructures. The process takes place at room temperature, in the high-vacuum chamber of a Scanning Electron Microscope (SEM). The principle is based on the local FEB decomposition of molecules injected in the chamber and adsorbed on a surface. These molecules, called the precursors, are specifically chosen to contain the element of interest to be deposited. However, the electron induced decomposition results in contaminated materials containing additional parasitic elements from the precursor molecule. These contaminants are incorporated with the element of interest, and downgrade the properties of the materials and prevent therefore their use for many applications. Gas-assisted FEBID, the subject of this thesis, is a promising evolution and solution to the problem that was never extensively studied in literature. It consists in injecting a reactive gas simultaneously to the precursors, which will react in real time with the deposited contaminants to form volatile products, leaving ideally only the interesting material on the surface. This work focused on the Oxygen assisted FEBID of Silicon dioxide (SiO2) and on the understanding of the deposition process. SiO2 it is a high performance material for nano-optic and electronic applications, widely used in research and industry. Modifications were brought to an existing SEM to allow the simultaneous injection of two different and controlled gases, and to allow the in-situ control of growth of optical materials. Si-precursors of three families (alkoxy-, alkyl- and isocyanato-silanes) were compared. The FEBID material could be gradually converted from contaminated Si-materials to pure SiO2, above a [O2]/[precursor] ratio specific to each precursor chemistry and precursor flow. The SiO2 deposited was stoichiometric, C and OH–free, had an estimated density of 2.2, was amorphous and had optical properties close to that of fused silica. Optical structures for nano-optics and plasmonics were produced. Compared to traditional FEBID, O2 assisted FEBID required much lower energy to induce a decomposition reaction, and secondary electrons achieved 90 % of the deposition process in our setup. The electron efficiency could be multiplied by 20 compared to conventional FEBID. O2 assisted FEBID was insensitive to electron density in our setup. The conditions for no C and large deposition rates during O2 assisted FEBID were high molecule reactivity to O2, low sticking coefficient, low acceleration voltage and dwell time (< 6 µs), and large replenishment time (> 60 µs). As function of the precursor chemistry and flow, additional O2 influenced the deposition rate, which could be increased it by a factor of 7. The O2 and precursor molecules co-adsorption resulted in a surface coverage competition, which determined the deposition efficiencies. Limitations due to the residual water in the SEM were demonstrated and characterized. They appeared as Oxygen incorporation in the deposited materials, which influenced the deposition process. Residual water impinging flow was demonstrated to be responsible of the FEBID process deposition efficiency when no Oxygen was injected.

EPFL2007

Influence of the storage conditions on the dimensional changes of heat-cured mortars

Karen Scrivener

Some cementitious materials cured at elevated temperature may expand on subsequent exposure to moisture. Concern has been expressed as to the relevance of small laboratory specimens to field concretes, in particular due to the potential leaching of ions into the storage solution. The work presented here aims at investigating this effect. Mortars cured at elevated temperature in identical fashion were subsequently stored at 90-100% relative humidity (RH) and in water, a LiOH solution, and KOH solutions of various concentrations. The largest and most rapid expansion occurred with mortars immersed in water. The samples stored at 90-100% RH expanded more slowly. Expansion was even slower when subsequent storage was in LiOH or KOH solutions. The same samples were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). These techniques indicated that ettringite had formed over time in all samples, with little apparent relation to the degree of expansion. The composition of the C-S-H gel changed over time, differently for the different storage conditions. © 2001 Elsevier Science Ltd.