<p> The partitioning of niobium to slag and gaseous niobium oxide vapourizing from metal/slag may cause niobium losses and erratic recovery rates in steelmaking practices. Knowledge of the volatility and activities of niobium oxides in slag melts are of great value for both theoretical evaluation and practical applications in niobium microalloyed steels. Because of the multi-valence state of niobium ions in slags, the behaviour of niobium in metallurgical slags is complicated. So far, little systematic attempts have been made and activity data of niobium oxides in slags are extremely scarce. The aim of this study is to determine precise data on the vapour pressures of niobium oxides, and consequently, to obtain information on thermodynamic quantities of niobium oxides in slag melts.</p> <p> The thermodynamic properties of niobium oxide in CaO-SiO2-NbOx and CaOSiO2-Al2O3-NbOx slag melts were determined by employing the transpiration method from 1800-1873K under a controlled atmosphere. To confirm the validity of the transpiration method for the measurement of thermodynamic properties, the binary alloy system silver-gold was chosen for a comparison with the same property which has been measured by other recognized procedures. The agreement with literature results confirmed that the measurement yields reliable results for thermodynamic activity data by the transpiration method.</p> <p> The vapourization of liquid Nb2O5 was studied as a function of partial pressure of oxygen in the system and this confirms that atmosphere control is the essential condition for the vapourization study. The gaseous niobium oxide species was verified to be NbO2; hence, Nb2O5 vapourizes by the reaction Nb2O5(1) = 2NbO2(g)+1/2O2(g). Heat of vapourization was estimated by applying the second law method and comparison with the literature showed a fairly good agreement.</p> <p> The thermodynamic properties of niobium oxide in the slag system of CaO-SiO2-NbOx and CaO-SiO2-Al2O3-NbOx were measured by varying the experimental conditions of slag basicities, slag compositions, temperature and oxygen partial pressures. From the basicity dependency of the activity coefficient for each oxide in this study, it is proposed that niobium oxide behaves as an amphoteric oxide and niobium pentoxide as an acidic oxide. On the other hand, it was observed in the redox equilibrium experiment that NbO2.5 becomes predominant as the slag basicity increases. However, insufficient interaction parameters as well as parameter conversions prevent the application of the regular solution model. The co-relationship between the ionic diameter and ionic energy was discovered and shows good agreement with calcium oxide and silicon oxide. With the interaction parameter and converting parameter attained, the regular solution model shows good agreement for the activity coefficients between measurement and calculation.</p> / Thesis / Doctor of Philosophy (PhD)

Les recherches consacrées aux points fixes à haute température (PFHT) ont été intensifiées depuis 1996, suite à une recommandation du Comité Consultatif de Thermométrie de développer des références au-delà de 2000 °C. Les Laboratoires Nationaux de Métrologie se sont alors engagés dans le développement de PFHT construits à partir d'alliages de métal et de carbone. Ces points fixes ont montré des transitions de phase à des températures reproductibles indépendamment de la proportion de carbone dans le métal. Les projets internationaux qui ont suivis les premiers developpements ont permis d'accroitre la robustesse et la stabilité à long terme des points fixes et ont mis en évidence l'importance des conditions thermiques de mise en œuvre. Aujourd'hui ces points fixes montrent une reproductibilité de l'ordre de 0.1 °C et sont en voie de renforcer l'exactitude des mesures pyrométriques aux plus hautes températures, jusqu'ici limitée par l'incertitude d'extrapolation dans l'Echelle Internationale de Température (EIT-90). Un projet de mesure par voie radiométrique de la température thermodynamique des PFHT les plus étudiés a été récemment mené. Les résulats de ces mesures absolues permettent aujourd'hui d'aborder la future mise-en-pratique du kelvin à haute température, par des méthodes de dissémination directe de la température thermodynamique sans recours à l'EIT-90. Ce document décrit les principales étapes depuis la conception de ces nouvelles références, jusqu'à la mesure de leur température thermodynamique. / Research dedicated to high-temperature-fixed-points (HTFPs) have been intensified since 1996, further to a recommendation of the Consultative Committee for Thermometry to develop references above 2000 °C. National Laboratories of Metrology envolved in the development of HTFPs based on metal-carbon alloys. These fixed points showed phase transitions at reproducible temperatures independently to the proportion of carbon in the metal. The international projects that followed the first developments allowed increasing the robustness and the long-term stability of the fixed points and highlighted the importance of the thermal conditions of implementation. Today these fixed points show a reproducibility of about 0.1 °C and are on the way to strengthen the accuracy of radiation thermometry measurements at high temperatures, up to now, limited by the uncertainty of the International Temperature Scale (ITS-90). A project to measure the thermodynamic temperature of the most studied HTFPs by a radiometric method was recently led. Results of this absolute measurement is a step towards the future mise en pratique of the kelvin at high temperature, by a direct dissemination of the thermodynamic temperature without any link to the ITS-90. This document describes the main steps from the conception of these new references, up to the measurement of their thermodynamic temperature.

Cette thèse de doctorat traite de la réalisation et la caractérisation métrologique d’un pyromètre à filtre accordable acousto-optique (AOTF). La première partie expérimentale de ce travail a été consacrée à l’étude du fonctionnement et de la caractérisation du filtre AOTF, la seconde, a été dédiée au développement et à l’optimisation du pyromètre AOTF.Contrairement aux pyromètres habituels dont la sélection spectrale est obtenue par un filtre interférentiel, le pyromètre accordable développé dans ce document utilise un filtre acousto-optique adapté à un étalonnage par laser. La longueur d’onde optique de mesure est imposée par la fréquence de l’onde radiofréquence appliquée au filtre acousto-optique, qui ne requiert aucun mouvement mécanique.En contrepartie, le spectre optique diffracté par ce type de filtres, présente d’importants lobes secondaires de diffraction qui dégradent le taux de réjection hors bande du pyromètre. Grace à un montage en double passage dans une configuration optique en œil de chat, les lobes secondaires de diffraction ont été fortement atténués. Le taux de diffusion optique du pyromètre est de l’ordre de 10-5 en valeur relative.La reproductibilité de sa sensibilité spectrale en longueur d’onde est de 0,01 nm et 0,1 % en amplitude sur une semaine. Ces caractéristiques, comparables à celles des meilleurs monochromateurs, permettent l'utilisation du pyromètre AOTF pour des applications métrologiques au meilleur niveau d’exactitude. L'instrument est accordable en longueur d'onde sur un domaine étendu, compris entre 600 nm et 950 nm. Il est également transportable car sa masse est inférieure à 20 kg et sa longueur inférieure à 1 m. Les performances et la caractérisation métrologique du pyromètre ont été validées par la mesure de la température thermodynamique du point du cuivre, en accord avec celle donnée dans l'EIT-90. / This PhD Thesis presents the work that has been done to design and build, along with the evaluation of the radiometric performance, of a radiation pyrometer based on acousto-optic tunable filter (AOTF). Part of this work has been given to the characterization of the acousto-optic tunable filter (AOTF); however, the other part has been dedicated to the development and the optimization of the radiation pyrometer.Unlike conventional pyrometers whose spectral selection is obtained an interference filters, the AOTF that has been used in this work enables direct calibration of the radiation pyrometer using laser sources without any interference effects. This in turns allows direct calibration of the pyrometer against the cryogenic radiometer or through trap detectors with lower uncertainty levels. The spectral range selected by the AOTF is adjusted by changing the frequency applied to the filter without any mechanical movement, which is an advantage compared to grating-based systems.One of the main limitations in using AOTF is the existence of diffraction lobes which appear beyond the in-band spectral range. These lobes result in significant deterioration in the blocking capabilities of the filter, and thus an increased out-of-band signal levels are obtained. However this has been solved by using a double-pass optical configuration, which has reduced these diffraction lobes and kept them within insignificant levels. The resultant out-of-band signal levels have been found within 10-5 of the in-band signal. The reproducibility of wavelength calibration is within 0.01 nm, yet with 0.1 % reproducibility in the amplitude of the in-band signal over a course of one week. The stability of wavelength is comparable to that found in high quality monochromators. This in turns allows the use of our pyrometer in metrological applications that require high level of accuracy. The instrument spectral coverage can be tuned over a wide spectral range, between 600 nm and 950 nm. The pyrometer is also transportable as its mass is less than 20 kg and with dimensions of 90 L x 20 H x 30 W cm. The performance and metrological characterization of the pyrometer have been validated by measuring the thermodynamic temperature of the freezing point of the copper, which was in a very good agreement with that value defined by the International Temperature Scale (ITS-90).