C. Batista

Synthesis and characterization of VO2-based thermochromic thin films for energy-efficient windows

Thermochromic VO2 thin films have successfully been grown on SiO2-coated float glass by reactive DC and pulsed-DC magnetron sputtering. The influence of substitutional doping of V by higher valence cations, such as W, Mo, and Nb, and respective contents on the crystal structure of VO2 is evaluated. Moreover, the effectiveness of each dopant element on the reduction of the intrinsic transition temperature and infrared modulation efficiency of VO2 is discussed. In summary, all the dopant elements--regardless of the concentration, within the studied range-- formed a solid solution with VO2, which was the only compound observed by X-ray diffractometry. Nb showed a clear detrimental effect on the crystal structure of VO2. The undoped films presented a marked thermochromic behavior, specially the one prepared by pulsed-DC sputtering. The dopants effectively decreased the transition of VO2 to the proximity of room temperature. However, the IR modulation efficiency is markedly affected as a consequence of the increased metallic character of the semiconducting phase. Tungsten proved to be the most effective element on the reduction of the semiconducting-metal transition temperature, while Mo and Nb showed similar results with the latter being detrimental to the thermochromism.

Structural and Morphological Characterization of Magnetron Sputtered Nanocrystalline Vanadium Oxide Films for Thermochromic Smart Surfaces

Nanocrystalline vanadium oxide thin films have been deposited by reactive DC magnetron sputtering onto glass substrates under different processing conditions. Structural analysis and phase identification have been carried out by means of X-ray diffractometry (XRD). The surface morphologies of the different films have been examined by both scanning electron microscopy (SEM) and atomic force microscopy (AFM). The XRD results revealed single and multiple phase oxides such as VO2(B), VO2(M), V2O5, etc. with considerable differences concerning to surface morphologies, as observed by SEM and AFM. The effects the O2/Ar flow ratio, DC current, and working pressure on the phases formed and growth rates is discussed. Moreover, VO2(M) films exhibited different morphologies concerning to grain size and shape as well as dissimilar preference in crystal orientation, as a result of the processing conditions. The optical/thermochromic response of the VO2(M) specimens deposited under different growth rate conditions was evaluated by optical spectrophotometry and related to the respective structural characteristics.

Thermal Decomposition and Solid Characterization of Calcium Oxide in Limestone Calcination

This work is concerning to production of quicklime (CaO) from thermal decomposition of the calcite limestone (CaCO3) using analytical and instrumental techniques (TGA and MS) to evaluate kinetic and thermodynamic effects as well as heat/mass transfer associates with the process operation. On the other hand, experiments of morphologic, structural and textural characterization (XRD, SEM and BET surface area) were carried out in order to evaluate the quality of the quicklime produced. Under experimental studied conditions it was observed that carbon dioxide (CO2) inhibits the thermal decomposition reaction rate. In addition, it was observed that steam (H2O) can catalyze this reaction but it can also cause sintering of the oxide formed. It was also observed that the calcination reaction is greatly limited by mass transfer effects and that the controlled thermal decomposition generates an increase in the solid porosity. The formed CO2 have also increased the sintering phenomena in the oxide structure, resulting in less reactive quicklime.

Microtopographic inspection of laser-glazed thermal barrier coatings

Thermal barrier coatings (TBCs) are extremely useful in gas turbine engines. The quality of these coatings can be improved by subjecting their surface to laser glazing. A reduction in surface roughness, eliminating open porosity on the surface, and the generation of a controlled segmented crack network, can be achieved. TBCs consisting of atmospheric plasma sprayed (APS) ZrO 2 -8%wtY 2 O 3 are subjected to a CO 2 continuous wave laser glazing process to seal their surface porosity, generating an external dense layer. Different amounts of irradiation are applied to the specimens. The microtopograpic inspection of the samples reveal a significant decrease of the surface roughness after laser treatment. However, a network of surface cracks, dependent on the laser scanning speed and track overlapping, are noticed. The cracks have a tendency to be oriented in two perpendicular directions, one in the direction of the laser beam travel direction, the other perpendicular to it. Cracks parallel to the beam scanning direction are found to be at the overlapping zone, in the edge of the subsequent track.

Pulsed DC reactive magnetron sputtering of vanadium dioxide thermochromic thin films

Abstract Vanadium oxides are an important class of materials with a large diversity of physical and chemical properties which derive from a range of single or mixed valences and a large variety of structures. They are already being used in many technological applications such as electrical and optical switching devices, light detectors, temperature sensors, etc. There has been a great interest in a particular phase, VO2(M), due to its thermochromic behaviour near room temperature which allows the development of smart windows with active control of the solar spectrum, for energy efficiency purposes. However, stoichiometric VO2 is difficult to deposit because of a narrow stability range due to the complex vanadium-oxygen reactive system. In this work, vanadium oxide thin films were synthesised on glass substrates by reactive pulsed direct current magnetron sputtering from a vanadium metal target in an O2/Ar atmosphere. Different processing conditions have been chosen in order to evaluate their influence on the crystal phases formed, surface morphologies and thicknesses and optical performance. The films were characterised by X-ray diffractometry in order to examine the crystal structure and identify the phases present in different films. The obtained VO2(M) films were thereafter analysed in terms of surface morphology by scanning electron microscopy and the characteristic reversible semiconductor-metal transition of the VO2 films was evaluate by optical spectrophotometry in the ultraviolet-visible-near infrared.

Reactive DC Magnetron Sputtering of Vanadium Oxide Thin Films

Vanadium oxides are a class of materials with outstanding physical and chemical properties. They find a wide field of technological applications such as optical and electrical switching devices, light detectors, temperature sensors, micro batteries, etc. There are several studies regarding the production of vanadium oxide films by radio-frequency (RF) magnetron sputtering, and with increasing interest on the thermochromic VO2 phase. However, literature with focus on vanadium oxide films deposited by direct current (DC) magnetron sputtering is very limited. In this work, we have successfully deposited vanadium oxide thin films by reactive DC magnetron sputtering under several processing conditions. The effect of substrate type, temperature, and O2/Ar flow ratio on phase formation has been studied. Structural analysis and phase determination have been carried out by X-ray diffractometry (XRD). Some single phase samples were also analysed with respect to surface morphology by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The thermochromic behaviour of single phase VO2(M) films has been evaluated by optical spectrophotometry.

Optimization of the Production of Quicklime by Calcination in Rotary Kilns

It well known that the operational conditions of rotary kilns affect strongly the quality of quicklime produced. This work presents a study about parametric optimization of rotary kilns for quicklime production (CaO) from thermal decomposition of calcite limestone (CaCO3). The approach used is based on the technique of design of experiments (DOE) to identify the most significant variables which affect the quality of oxide and, in addition, to determine better conditions of production. An experimental setup in laboratory scale was build in order to carry out the experiments. Calcination temperature, calcination time, rotation speed, particle mean diameter, heat flow and air flow rate were evaluate using a fractional factorial design. The parametric optimization shows that the raise of time and temperature of calcinations produces an increase of free CaO terms. However this operation is not useful due to sintering process observed in the BET surface area results. A correlation between quicklime reactivity and specific surface area could be established, in way that reactive quicklimes presented bigger areas. Finally, it was possible to determine the best conditions of quicklime production and to estimate a quadratic model by using a central composite design and a like surface response model. It was found that the best region of operation is about 900oC and 45 minutes of calcination time.