Thèses (Ph.D.)--Université de Sherbrooke (Canada), 1998. / Titre de l'écran-titre (visionné le 20 juin 2006). Publié aussi en version papier.

Molybdenum-bearing alloys are widely used in industry because of their excellent corrosion resistance. However, the role of molybdenum in passivation is a subject which has been a matter for discussion and controversy for many years. In the previous work carried out in this laboratory, Professor Castle and Dr Qiu suggested that molybdenum oxide might provide the nuclei for formation of the passive film. This hypothesis is the basis of the present work. In order to find out the evidence for the existence of the molybdenum oxide nuclei, furthermore, to establish a model of passivation for the molybdenum-bearing alloys, the passivation of molybdenum-bearing alloys are investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), especially in-situ AFM, in conjunction with SEM and TEM. During the passivation study, it was found that the chemical composition on anodically polarised surfaces varied with potential. For 516 alloy, the peak value of molybdenum in the surface is at the low potential of the passive region close to the Flade potential. Therefore, molybdenum exerts its greatest function in this potential region. A further AFM study at this potential revealed, surprisingly, that a platelet layer formed on the passivation surface. The dendritic structure on the surface of the platelets presented under the high resolution of STM, which shows the crystallization property of the platelets. These platelets are mainly composed of chromium and molybdenum oxides and they are only found in the passivation of the molybdenum-bearing alloy, so the formation of the platelets may be associated with molybdenum nucleation in passivation. The in-situ AFM studies provide the evidence for the formation and disappearance of the platelets during the passivation of molybdenum-bearing alloys, i.e. the platelets form at the early stage of passivation and they gradually merge into the passive film if they are not disturbed by the environment. Based on the above finding, the role of molybdenum in passivation is proposed as following: molybdenum oxide precipitated on the surface seeds chromium oxide to form platelets at the early stage of passivation and the formed platelet layer prohibits the dissolution of the passive species from the metal. By this way, molybdenum facilitates the formation of the passive film on alloys. Using Fe-Cr-Mo duplex stainless steels, the corrosion of molybdenum-bearing alloys are studied in HCl and the kinetics of the corrosion are traced by in-situ AFM. During corrosion, it was observed that the ferritic phase in duplex stainless steel dissolves more rapidly than the austenitic phase and the dissolution occurs on the austenite preferentially along grain boundaries, sub-grain boundaries and the planes with high energy. By switching the electrochemical condition from active dissolution to passivation and then changing it back, it is found that under the passivation condition after the active dissolution, the corrosion changes from selective dissolution of the crystallographic feature to general corrosion. This situation persists even though the electrochemical condition changes back to the active condition from the passivation condition. Since AFM is a new technique and this is one of the first attempts at applying it to a corrosion study, an in-depth explanation of the images obtained from a corroded rough surface is an important topic on which so far little work has been reported. In this study, therefore, the artifacts and the reality of the structure in AFM images obtained in the corrosion study are discussed.

The geometry of pulsar beams is one of the intrinsic properties of neutron stars, governing the pulse-profile phenomenon and other aspects of pulsar astron- omy. With a number of pulsars in our dataset, their beam geometry is derived from the polarisation position angle (PPA) using the simple polar cap emission and dipole field model. This includes the rotating vector model (RVM), for which the solutions can hardly be constrained or fail to be consistent because of the lim- itations of the model itself. The inconsistencies in the results suggest that the initial PPAs can be strongly perturbed by additional parameters above the emis- sion altitude, such as the plasma medium or rotational aberration effects, after which their characteristic shape is no longer related to the geometry via the RVM. We investigate further into the effects of wave propagation in the pulsar magne- tosphere, and find an indication that, in most cases, the RVM-calculated PPAs are likely to be altered by plasma effects.In recent years, there have been an increasing number of intermittent and mode-switching pulsars observed to have their radio pulse profiles correlated with the change in pulsar spin frequency (ν ̇) (e.g. Lorimer et al. 2012, Lyne et al. 2010). These two phenomena are understood to be related via the states of plasma in the magnetosphere. As one such pulsar, and also one with known geometry and other astonishing behaviour, PSR B1822–09 is studied in terms of the mode- switching properties, the hollow-cone model and the wave propagation in the magnetosphere. We also study the model for explaining the intermittent pulsars PSRs B1931+24, J1841+0500 and J1832+0029, and find it can be consistently applied for PSRs B1822–09 and B0943+10, and other profile-switching pulsars. However, aspects of the conclusions are limited because of the lack of understand- ing of the connection between the radio flux and the states of plasma. We are also able to use the difference in the PPAs between two states of PSR B0943+10 to predict the change in plasma states and ν ̇, which cannot be measured directly from timing analysis as its switching timescale is too short.

Much contemporary political discourse in the US and industrialized west is defective. According to a number of scholars, such as Scott Aikin and Robert Talisse, this defectiveness can plausibly be explicated in terms of a breakdown in political civility. In this thesis, I scrutinize possible explanations for why incivilities are widespread in political discourse. My goal is to defend an explanation according to which citizen incivility in political discourse is blameless. To do this, I appeal to a principle of reciprocity. According to the principle of reciprocity, citizens are not required to maintain civility in political discourse if they have reason to believe that their interlocutors will not reciprocate civility. When applied to contemporary politics, this principle implies that ordinary citizens in democratic societies across the US and industrialised west are often justified in being uncivil in political debate. For these citizens often have no reason to believe that their interlocutors will be civil. If the reciprocity principle is right, then policies aimed at restoring civility in political discourse must be concerned to build citizen trust that others will reciprocate civility. Without this trust, citizens may not see themselves as having reason to be civil. The thesis is divided into two chapters. In chapter 1, I defend the justificatory account of incivility against two competing accounts of political incivility—identity and group theories. These accounts pathologize political incivility as a kind of irrationality, but based on the argument from reciprocity, I argue that political incivilities are often rational and so justified. In chapter 2, I analyse the kind of trust that is necessary to build more civility in political discourse. In particular, I develop and defend a conception of deliberative trust, which is defined as the belief that one’s interlocutor will reciprocally adhere to the norms of civility. / Thesis / Master of Arts (MA) / In this thesis, I scrutinize possible explanations for why incivilities are widespread in contemporary political discourse. By appeal to a principle of reciprocity, my ultimate goal is to defend an explanation according to which citizen incivility in political discourse is blameless. According to the principle of reciprocity, citizens are not required to maintain civility if they have reason to believe that their interlocutors will not reciprocate civility in political discussion. Based on this principle, I argue that ordinary citizens in democratic societies across the US and industrialised west are often justified in being uncivil in political debate. For these citizens often have no reason to believe that their interlocutors will be civil. If the justificatory account of incivility is on the right track, then policies aimed at restoring civility in political discourse must be concerned to build citizen trust that others will reciprocate civility. Without this trust, citizens may not see themselves as having reason to be civil in political debate.

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Thèse doctorat : Electronique et optique : Brest : 2006. / Bibliogr. p.225-236.

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