Une interface de capteur intégré haute température est proposée. L'interface de capteur a une architecture dans le domaine temporel et entièrement différentielle. Cette approche offre l'avantage d'une meilleure stabilité thermique par rapport aux architectures analogiques classiques. L'interface du capteur est basée sur des oscillateurs à verrouillage d'injection (ILO) utilisés comme déphaseurs. Une paire d’ILOs convertit la tension de sortie du capteur en une différence de déphasage qui est ensuite numérisée à l'aide d'un convertisseur temps-numérique. La sortie de l'interface du capteur dépend uniquement du rapport des valeurs de ses paramètres plutôt que de leurs valeurs absolues, ce qui entraîne une faible dépendance à température. L'interface du capteur est fabriquée à l'aide d'une technologie de silicium sur isolant partiellement déplété (PD-SOI) de 0.18μm de XFAB, qui est choisie pour sa robustesse thermique. Les mesures montrent que l’interface de capteur a une variation thermique de 178ppm / ° C sur ± 60mV de pleine échelle d'entrée et une variation thermique de 65ppm / ° C sur ± 40mV de pleine échelle d'entrée sur une large plage de température de fonctionnement étendue de -20 ° C à 220 ° C. / A high temperature integrated sensor interface is proposed. The sensor interface has a fully differential time domain architecture. This approach offers the advantage of better thermal stability compared to typical analog based architectures. The sensor interface is based on Injections Locked Oscillators (ILO) used as phase shifters. A pair of ILOs converts the sensor output voltage into a phase shift difference which is then digitized using a time to digital converter. The sensor interface output depends only on the ratio of its parameters values rather than their absolute values thus leading to a low temperature dependency. The sensor interface is fabricated using a 0.18µm Partially-Depleted Silicon on Insulator technology (PD-SOI) from XFAB which is chosen for its thermal robustness. Measurements show that the sensor interface achieves a thermal variation of 178ppm/°C over ±60mV input full scale and a thermal variation of 65ppm/°C over ±40mV input full scale over a wide operation temperature range extended from -20°C to 220°C.

Neste trabalho é investigado o comportamento de partículas coloidais em cristais líquidos liotrópicos nas diferentes fases nemáticas. Nessas fases, existe uma ordem orientacional de longo alcance das micelas no volume. O regime de ancoramento das micelas na superfície das partículas, que pode ser forte ou fraco, depende diretamente da energia de ancoramento W, do raio R das partículas coloidais, e da constante elástica K do crista/líquido [8]. Em casos de ancoramento fraco, a simetria da ordem orientacional das micelas próxima às superfícies das partículas, bem como no volume, é a mesma nas fases nemáticas calamítica (\'N IND. c\') e discótica (\'N IND. d\'). Utilizando-se partículas de sílica nanométricas, observa-se por técnicas de microscopia óptica, que, para amostras mais concentradas (fração volumétrica \'fi\'\'DA ORDEM DE\'\'10 POT. -3\'), ocorre uma separação de fases, enquanto que, em sistemas mais diluídos (\'fi\'\'DA ORDEM DE\'\'10 POT. -5\'), existe uma organização das partículas no meio líquido cristalino em temperaturas próximas à \'N IND. d\'. Quando partículas de látex de diâmetro 90nm são dispersadas no cristal líquido liotrópico na fase \'N IND. c\'. c, agregados anisométricos tridimensionais são observados. Apesar de esse tipo de agregado não ter sido anteriormente observado, esse comportamento era previsto teoricamente dentro de certas condições que são satisfeitas pelo sistema nemático liotrópico liotrópico + látex. No entanto, esses agregados desaparecem na fase \'N IND. d\'. Os diferentes comportamentos em \'N IND. c\' e \'N IND. d\' estão ligados à organização do meio liquido-cristalino, assim como às interações entre as partículas e entre as partículas e o meio. F oram feitas também medidas diretas de força por meio de um SFA (surface Force Apparatus) em um cristal líquido liotrópico puro confinado entre duas superfícies. Tais medidas mostram que o perfil da força em função da distância entre as superfícies é diferente em \'N IND. d\' e \'N IND. c\'. / In this work, the behavior of colloidal particles dispersed into a nematic lyotropic liquid crystal matrix is investigated. In such systems, there is a long range orientational order of the micelles in the volume. The micelles anchoring on the particles surfaces can be strong or weak, depending of the anchoring strength W, the particles radius R and the liquid crystal elastic constant K. For weak anchoring, the symmetry of the orientational order in the surroundings of the particles surfaces, and also in the volume, remains the same for both calamitic (\'N IND. c\') and discotic (\'N IND. d\') nematic phases. It was observed that high concentrated solution of nanometric silica particles in a lyotropic medium leads to phase segregation. In more diluted systems, the silica particles tend to organize themselves in the nematic medium for temperatures close to \'N IND. d\'. For 90nm latex particles in the lyotropic matrix, 3D anisotropic aggregates was observed in \'N IND. c\' phases. Although it was never observed before, this behavior was theoretically previewed. On the other hand, there are no aggregates in \'N IND. d\' phase. The different behaviors observed on \'N IND. c\' and \'N IND. d\' phases are related to the liquid crystalline medium organization, as well as to the interparticles and medium with particles interactions. It is also shown in this work, by direct measurements in a pure lyotropic liquid crystal confined between two surfaces, that the force profiles in function of the distance between surfaces are different for \'N IND. c\' and \'N IND. d\' phases.

Dans le cadre de l’utilisation potentielle d’oxydes mixtes d’actinides au sein des réacteurs nucléaires de 3ème et 4ème générations, des solutions solides de formules générales U1-xThxO2, U1-xCexO2-y, U0,75Nd0,25O1,875, U0,75Gd0,25O1,875 et Th0,75Nd0,25O1,875 ont été préparées par conversion thermique de précurseurs oxalate. Préalablement à l’évaluation de la durabilité chimique des matériaux, une étape de frittage a été entreprise afin d’obtenir des pastilles denses présentant diverses propriétés physico-chimiques et microstructurales d’intérêt (composition, homogénéité, taux de densification, …) L’étude multiparamétrique de la dissolution, conduite en milieux nitrique, sulfurique et chlorhydrique a souligné l’impact important de la composition chimique au sein du matériau sur la durabilité chimique des échantillons. En effet, plusieurs paramètres (ordres partiels par rapport à l’activité en protons, énergie d’activation apparente, …) ont confirmé une modification significative du mécanisme de dissolution prépondérant pour les échantillons enrichis en uranium. Par ailleurs, le rôle important joué par certaines espèces azotées à l’interface solide/solution a également été démontré. L’évolution de l’interface solide/solution (surface réactive, composition) en cours de dissolution a également été suivie à travers une étude operando par Microscopie Electronique à Balayage en mode Environnemental. Cette étude a souligné l’existence de zones préférentielles de dissolution (jonctions triples, joints de grains, porosités inter- et intragranulaires) pour les échantillons les moins riches en uranium ; laquelle s’accompagne d’une forte augmentation de la surface réactive. En raison d’un phénomène prépondérant d’oxydation de l’uranium(IV) à l’interface, la dissolution des échantillons enrichis en uranium apparaît nettement plus homogène. / In the field of the use of actinides mixed oxides as potential fuels for the Gen(III) and Gen(IV) nuclear reactors, solid solutions with general formula U1-xThxO2, U1-xCexO2-y, U0.75Nd0.25O1.875, U0.75Gd0.25O1.875 and Th0.75Nd0.25O1.875 were prepared by thermal conversion of oxalate precursors. Dense pellets exhibiting various physico-chemical and microstructural properties (in terms of composition, homogeneity, densification rate, …) were prepared through sintering then submitted to dissolution tests.The multiparametric study of the dissolution, performed in nitric, sulfuric and hydrochloric media clearly underlined the important effect of the chemical composition on the chemical durability of the samples. Indeed, several parameters (including partial order related to proton activity, apparent activation energy) confirmed the significant modification of the preponderant dissolution mechanism for uranium-enriched samples. Moreover, the role of various nitrogen-based species was evidenced at the solid/solution interface.The evolving of solid/solution interfaces (reactive surface area, composition) during dissolution was monitored by the means of operando ESEM experiments. Preferential dissolution zones (triple junctions, grain boundaries, inter- and intra-granular porosities) were clearly observed for uranium-depleted samples. They induce a significant increase of the reactive surface area even for short progress of the reaction. On the contrary, the dissolution appeared more homogenous for uranium-enriched samples due to the existence of a preponderant mechanism associated to the oxidation of the uranium(IV) at the interface.

Typescript (photocopy). / Digitized by Kansas Correctional Industries

La gomme arabique est un exsudat d’arbre qui est essentiellement utilisée pour ses propriétés stabilisantes et émulsifiantes. Ce produit naturel est un mélange complexe de protéines et de polysaccharides liés de façons covalentes, qui différent de part leur masse molaire et leur hydrophobicité. Malgré le grand nombre d’études existantes dans littérature sur la structure et les propriétés interfaciales de la gomme arabique, la compréhension de la relation entre la composition interfaciale, la structuration interfaciale et la métastabilité d’émulsions restent mal comprises à ce jour compliquée. La gomme arabique est classiquement décrite comme un mélange de trois fractions : une fraction riche en polysaccharide (arabinogalactan), une fraction composée de conjugués protéine-polysaccharides et enfin des glycoprotéines. Les propriétés émulsifiantes et stabilisantes de la gomme sont attribuées à la fraction de conjugués dont la partie protéinée s’adsorbe à la surface des gouttes d’huiles et les espèces carbonées apportent des répulsions stériques entre les gouttes. Dans cette thèse, nous avons étudié le comportement microscopique des espèces de la gomme en solution et aux interfaces huile/eau. La première étape a consisté à caractériser la structure des molécules de la gomme en solution. Une séparation bi-dimensionnelle de la gomme a été réalisée par exclusion stérique suivie d’une séparation par affinité hydrophobe, ce qui nous a permis de confirmer la nature très hétérogène de la gomme. Des expériences de diffusion des rayons-X et des neutrons aux petits angles sur la gomme arabique et ses fractions nous ont amené à proposer un modèle structural des espèces conjuguées. Nous avons ensuite étudié la composition de films de gomme arabique adsorbés en émulsion en la comparant avec celle de la gomme en solution. Une augmentation du taux de protéine dans le film interfacial a montré le rôle crucial des espèces polypeptidiques sur l’adsorption. Nous avons observé que la composition du film adsorbé diffère de la gomme en solution mais reste hétérogène en taille et en hydrophobicité. Un bilan massique a révélé une forte dépendance entre la formulation d’émulsion (concentration en gomme et paramètres physicochimiques) et la concentration surfacique alors que la composition de l’interface a très peu changé. Ces résultats suggèrent une modification de conformation des espèces adsorbées en fonction des conditions d’émulsification. Dans un troisième temps, nous nous sommes intéressés aux mécanismes de stabilisation provenant de l’adsorption des espèces amphiphiles de la gomme arabique. Pour cela, nous avons développé une méthode permettant de récupérer les espèces adsorbées dans une émulsion huile dans eau. Cette méthode nous a permis de mettre à jour une structuration du film adsorbé. Les espèces récupérées de l’interface ont montré la présence d’une agrégation, dont l’intensité dépend directement du taux de couverture et du taux de protéine de l’interface. La métastabilité d’émulsions stabilisées par de la gomme arabique est ainsi favorisée par l’augmentation de la structuration interfaciale, i.e. quand le taux d’agrégation des espèces adsorbées augmente. Un tel comportement n’a pas encore été reporté dans la littérature et nous pensons qu’il s’agit d’un mécanisme clef dans le cadre d’émulsions stabilisées par la gomme arabique. Enfin, des expériences de diffusion de neutrons aux petits angles (avec un contraste identique entre les phases continue et dispersée) ont révélé des différences de structuration entre deux régimes de concentration surfacique. Ces observations ont été discutées au regard des spectres de diffusion de la gomme en solution. Pour conclure, cette thèse porte sur la relation entre composition et structuration dans des émulsions stabilisées par la gomme arabique et démontre notamment un mécanisme original de stabilisation qui joue un rôle important dans ce système complexe. / Gum arabic, a tree exudate, is essentially used for its binding and emulsifying properties. This natural product is a complex mixture of covalently linked proteins and charged polysaccharides moieties, differing in their molecular mass and hydrophobicity. A large body of literature now exists on the structure and interfacial properties of gum Arabic but a comprehensive description of the relationship between interfacial composition, interfacial structuration and emulsion metastability remains elusive. In the literature, gum Arabic is described as a mixture of three fractions: an arabinogalactan rich polysaccharide fraction, a polysaccharide-protein conjugates fraction and a fraction of glycoproteins. The conjugate fraction is thought to be responsible for the emulsifying and stabilizing properties of the gum, with the protein part adsorbing at oil droplets surface and the carbohydrate moieties providing steric repulsion between droplets. In this work, we have investigated the microscopic behavior of the gum species in solution and at oil/water interfaces. The first step was to characterize the structure of gum Arabic species in solution. A twodimensional separation of the gum molecules was performed using size exclusion chromatography followed with by hydrophobic interaction separation, which confirmed the highly heterogeneous composition of the gum. Small angle X-ray and neutron scattering measurements on the gum and its fractions led us to propose a structural representation of the gum conjugated moieties. Then the composition of adsorbed gum Arabic films as compared to gum Arabic solutions has been investigated. An increase in the protein rate of the interfacial film showed the crucial role of the polypeptide moieties on the adsorption. The composition of the adsorbed film was shown to differ from the bulk but remained heterogeneous in size and hydrophobicity. A mass balance revealed a strong dependence between the emulsion formulation (gum concentration and physico-chemical parameters) and the surface concentration, while the composition of the interface was only slightly changed. These results suggest that gum Arabic adsorbing species must adopt conformational changes depending on emulsification conditions. In a third stage, we have addressed the stabilization mechanisms resulting from the adsorption of gum Arabic amphiphilic species. For that purpose, we have developed a method to recover the adsorbed species within an oil-in-water emulsion. This method allowed us to unveil a structuration of the adsorbed film. Species recovered from the interface displayed aggregation, the magnitude of which directly depended on the coverage density and protein rate of the adsorbed film. The metastability of emulsions, stabilized with gum Arabic, increased upon promoting interfacial structuration, i.e. when the aggregation rate of adsorbed species was enhanced. Such behavior has not been reported so far in the literature and we believe that it is a key mechanism of gum Arabic-based on emulsions. Finally, small angle neutron scattering experiments (contrast match between the continuous and dispersed phases) disclosed differences of structuration between two regimes of interface coverage. These observations were discussed in the light of the comparison with the scattering spectra of gum Arabic solutions. To conclude, this thesis revolves around the composition/structuration relationship in gum Arabic-stabilized emulsion stabilized and demonstrates that an original mechanism is at play in this complex system.

Neste trabalho apresentamos um estudo teórico da ação surfactante do Te no crescimento epitaxial do InAs sobre o GaAs (001). Os cálculos foram feitos dentro da teoria do funcional densidade com a aproximação de densidade local, e utilizando-se pseudopotenciais ab initio com conservação da norma. Os resultados indicam que a interdifusão do Te, para a superfície de crescimento, reduz a energia livre do sistema, o que está de acordo com a ação surfactante do Te. Porém verificamos, baseado num processo de troca As <-> Te entre primeiros vizinhos, que parte do Te fica preso na interface GaAs/In-As, dando origem à uma interface GaAs/Te/InAs com meia monocamada de Te, também confirmando resultados experimentais. Calculamos da energia de formação da interface GaAslTef InAs, e os resultados indicam que esta estrutura é energeticamente instável quanto à segregação de fase. Por outro lado, uma interface formada por uma monocamada de In-As sobre o GaAs (001), tem a sua energia de formação reduzida, devido à presença do Te. Também verificamos que o Te evita a formação de aglomerados de As na superfície de crescimento, por meio de uma interação repulsiva entre os dímeros de As adsorvidos sobre o Te. Calculamos os mesmos processos de adsorção e troca do As2 sobre o GaAs (001) terminado em Ga, e coberto por uma monocamada de outros elementos da coluna VI, o Se e o S. Os nossos resultados indicam que a adsorção do As2 é um processo exotérmico, porém a troca As <-> {Se, S} não é energeticamente favorável. Quanto a estrutura eletrônica da interface GaAs/Te/ InAs, para uma concentração de meia monocamada de Te, determinamos a descontinuidade da faixa de valência, \"valence-band-offset\" (VBO), e a estrutura de faixas de energia. Utilizando-se os mesmos procedimentos de cálculo e dentro do estudo de interfaces, estudamos a dependência do VBO da interface GaAslAlAs, com a inclusão de uma monocamada de ,Si ou de Ge na região da interface, bem como a dependência do VBO com a ordem de crescimento dos materiais. Verificamos que o aumento do VBO, quando o GaAs é depositado sobre o AlAs (001), é devido à ocupação pelo Si (Ge) de sítios de As ligado às camadas de GaAs da interface. De forma análoga, a redução do VBO quando o AlAs é depositado sobre o GaAs (001), é devido à ocupação pelo Sz (Ge) de sitios de As ligados às camadas de AlAs da interface. / We investigate the initial steps of the surfactant mediated epitaxial growth of InAs over GaAs (001) surface, with Te as a surfactant element. The calculations are performed within the density functional theory, using norm-conserving ab-initio pseudopotentials, and local density approximation. The results indicate that the interdiffusion of Te atoms to the growth surface reduce the surface free energy, confirming the surfactant action of Te. Moreover we find that, using the first neighbour As <-> 7e exchange process, only partial interdiffusion of Te atoms to the growth surface occurs, giving rise to the GaAs/Te/InAs interface, with half a monolayer of Te atoms. Next we have calculated the stability of GaAs/Te/InAs interface against phase segregation, where the results show that this structure is energetically unstable. On the other hand, the interface with one monolayer of InAs over GaAs (001), present a reduction of the formation energy with the inclusion of Te atoms in this interface. Also we have verified that 7e atoms induce a repulsive interaction between As dimers adsorbed on Te layer, which prevents the formation of clusters of ,4s on the growth surface. The adsorption and exchange process of As2 molecules over GaAs (001), Ga terminated, and covered by ,Se and,S atoms was considered. The results indicate that the adsorption process is energetically favourable, but the subsequent exchange process, As <-> {Se,S}, is not energetically favourable. The electronic structure and the valence-bandoffset (VBO) of GaAs/Te/InAs interface, with half a monolayer of Te atoms was calculated. Using the same calculation procedure, we have determinated the change of VBO of GaAs/ AlAs interface, with an inclusion of Si or Ge atoms in the interface between GaAs and AlAs, and the dependence of VBO with the growth sequence of materials. The results indicate that the increase of VBO, when the GaAs is deposited over AlAs (001), can be attributed to the Si (Ge) atoms which occupy the As sites binding to the GaAs layers of the interface. Otherwise the reduction of VBO, when the AtAs is deposited over GaAs (001), is attributed to the Sz(Ge) atoms which occupy the As sites binding to the AlAs layers of the interface.

Neste trabalho apresentamos um estudo teórico da ação surfactante do Te no crescimento epitaxial do InAs sobre o GaAs (001). Os cálculos foram feitos dentro da teoria do funcional densidade com a aproximação de densidade local, e utilizando-se pseudopotenciais ab initio com conservação da norma. Os resultados indicam que a interdifusão do Te, para a superfície de crescimento, reduz a energia livre do sistema, o que está de acordo com a ação surfactante do Te. Porém verificamos, baseado num processo de troca As <-> Te entre primeiros vizinhos, que parte do Te fica preso na interface GaAs/In-As, dando origem à uma interface GaAs/Te/InAs com meia monocamada de Te, também confirmando resultados experimentais. Calculamos da energia de formação da interface GaAslTef InAs, e os resultados indicam que esta estrutura é energeticamente instável quanto à segregação de fase. Por outro lado, uma interface formada por uma monocamada de In-As sobre o GaAs (001), tem a sua energia de formação reduzida, devido à presença do Te. Também verificamos que o Te evita a formação de aglomerados de As na superfície de crescimento, por meio de uma interação repulsiva entre os dímeros de As adsorvidos sobre o Te. Calculamos os mesmos processos de adsorção e troca do As2 sobre o GaAs (001) terminado em Ga, e coberto por uma monocamada de outros elementos da coluna VI, o Se e o S. Os nossos resultados indicam que a adsorção do As2 é um processo exotérmico, porém a troca As <-> {Se, S} não é energeticamente favorável. Quanto a estrutura eletrônica da interface GaAs/Te/ InAs, para uma concentração de meia monocamada de Te, determinamos a descontinuidade da faixa de valência, \"valence-band-offset\" (VBO), e a estrutura de faixas de energia. Utilizando-se os mesmos procedimentos de cálculo e dentro do estudo de interfaces, estudamos a dependência do VBO da interface GaAslAlAs, com a inclusão de uma monocamada de ,Si ou de Ge na região da interface, bem como a dependência do VBO com a ordem de crescimento dos materiais. Verificamos que o aumento do VBO, quando o GaAs é depositado sobre o AlAs (001), é devido à ocupação pelo Si (Ge) de sítios de As ligado às camadas de GaAs da interface. De forma análoga, a redução do VBO quando o AlAs é depositado sobre o GaAs (001), é devido à ocupação pelo Sz (Ge) de sitios de As ligados às camadas de AlAs da interface. / We investigate the initial steps of the surfactant mediated epitaxial growth of InAs over GaAs (001) surface, with Te as a surfactant element. The calculations are performed within the density functional theory, using norm-conserving ab-initio pseudopotentials, and local density approximation. The results indicate that the interdiffusion of Te atoms to the growth surface reduce the surface free energy, confirming the surfactant action of Te. Moreover we find that, using the first neighbour As <-> 7e exchange process, only partial interdiffusion of Te atoms to the growth surface occurs, giving rise to the GaAs/Te/InAs interface, with half a monolayer of Te atoms. Next we have calculated the stability of GaAs/Te/InAs interface against phase segregation, where the results show that this structure is energetically unstable. On the other hand, the interface with one monolayer of InAs over GaAs (001), present a reduction of the formation energy with the inclusion of Te atoms in this interface. Also we have verified that 7e atoms induce a repulsive interaction between As dimers adsorbed on Te layer, which prevents the formation of clusters of ,4s on the growth surface. The adsorption and exchange process of As2 molecules over GaAs (001), Ga terminated, and covered by ,Se and,S atoms was considered. The results indicate that the adsorption process is energetically favourable, but the subsequent exchange process, As <-> {Se,S}, is not energetically favourable. The electronic structure and the valence-bandoffset (VBO) of GaAs/Te/InAs interface, with half a monolayer of Te atoms was calculated. Using the same calculation procedure, we have determinated the change of VBO of GaAs/ AlAs interface, with an inclusion of Si or Ge atoms in the interface between GaAs and AlAs, and the dependence of VBO with the growth sequence of materials. The results indicate that the increase of VBO, when the GaAs is deposited over AlAs (001), can be attributed to the Si (Ge) atoms which occupy the As sites binding to the GaAs layers of the interface. Otherwise the reduction of VBO, when the AtAs is deposited over GaAs (001), is attributed to the Sz(Ge) atoms which occupy the As sites binding to the AlAs layers of the interface.

Tese de mestrado integrado. Engenharia Informática e Computação. Faculdade de Engenharia. Universidade do Porto. 2009

Tese de mestrado integrado. Engenharia Informática e Computação. Faculdade de Engenharia. Universidade do Porto. 2009

Les déficients visuels rencontrent de nombreuses difficultés lors de leur navigation web avec un lecteur d’écran, d’autant plus que les interfaces web deviennent de plus enplus riches de par leurs interactions et leur densité et diversité informationnelles. Cette thèse confronte ainsi 2 approches de l’accessibilité : l’approche dite « exhaustive » quivise à assurer l’accessibilité normative - garantissant l’accès à l’information par l’application de normes - et l’approche dite « holistique » qui vise à assurer l’accessibiliténormative mais également l’accessibilité effective - garantissant l’utilisabilité de l’interface - par la prise en compte du contexte d’utilisation des utilisateurs.Les 2 premières études ont analysé l’activité des utilisateurs déficients visuels qui a donné lieu à un modèle de la navigation web de ces utilisateurs. Ce modèle permet deconcevoir une interface suivant l’approche holistique par la prise en compte de leurs besoins, notamment celui de filtrage des informations non pertinentes et redondantes. Lorsde 3 expérimentations, les déficients visuels ont été confrontés à l'approche exhaustive par la retranscription exhaustive des informations et à l'approche holistique par l’accèsdirect au contenu associé à l’action réalisée. Les résultats montrent le bénéfice substantiel apporté par la version holistique concernant l’allègement de la charge cognitive etl’utilisabilité de l’interface selon les 3 critères : efficacité, efficience et satisfaction. Ainsi, cette thèse fournit des arguments probants pour promouvoir l’approche holistique pourguider la conception d’interfaces et l’élaboration de solutions d’assistance pour cette population. / Visually impaired people face many difficulties when navigating on the web with a screen reader, especially as web interfaces become increasingly rich because of theirdynamic interactions and their informational density and diversity. This thesis compares two approaches to accessibility: the current approach called "exhaustive" which aims toensure normative accessibility - guaranteeing access to information through the application of standards - and the approach called "holistic" which aims to ensure bothnormative and effective accessibility - guaranteeing the usability of the interface - by taking into account the context of its use (difficulties, strategies, needs, etc.).The first 2 studies analysed the activity of visually impaired users resulted in a model of the web navigation of these users. This model allows for designing interfaces followingthe holistic approach by taking into account their needs, especially the filtering of irrelevant and redundant information. In 3 experiments, visually impaired users performedtasks in two versions of websites: a version based on the holistic approach that gives access directly to content associated with the user’s action and a version using theexhaustive approach that transcribes exhaustively all the information present on the screen. The results show substantial benefit provided by the holistic version as regards thecognitive load and the usability of the interface according to 3 criteria: effectiveness, efficiency and satisfaction. Thus, this thesis provides cogent arguments promoting theholistic approach in order to guide the design of interfaces and the development of assistance solutions for screen reader users.