Recasting (i.e., corrective feedback in which the teacher paraphrases a learner's incorrect utterance without explicitly labeling it as erroneous) is a frequent phenomenon in classroom discourse. Despite its frequency and naturalness, educators continue to debate its efficacy. At issue is whether learners notice such implicit feedback in order to make use of it. This on-going debate centers on the following question: What makes a teacher's recast noticeable to a student? While most of the studies in the recast literature have emphasized student factors such as working memory and/or developmental readiness (e.g., Havranek & Cesnik, 2001), few studies have explored how teacher factors affect learner perceptions of and receptivity to recasts. This study fills this gap by employing qualitative methods to investigate student perceptions of their teacher recasts. Six classes in Applied English Departments at three different institutes of technology in mid-southern Taiwan participated in this study. Different methods were employed to gather student and teacher data in order to arrive at a more complete understanding of classroom recasts: classroom observations, individual student interviews, group stimulated recall interviews, and teacher interviews. According to student interview data, seven teacher categories (e.g., nativeness, teacher language use, teacher affect, etc.) were found to have a significant impact on students’ noticing of and receptivity to recasts. In particular, the findings indicate that EFL student perceptions of recasts are profoundly affected by teachers’ language use (e.g. phonetic and syntactic features) and teachers' nativeness (e.g., native vs. nonnative). Other factors such as teacher-student rapport also mediate students’ attention to and understanding of feedback. In addition to the student self-report data, classroom observation data of teachers' behavior indicated striking differences as well--native teachers tended to correct more grammatical errors while nonnative teachers corrected more phonological errors. In light of these findings, suggestions for improving student awareness of corrective recasts are given to both native and nonnative teachers. It is hoped that the qualitative categories uncovered in this study will lead to more rigorous, testable hypotheses for future quantitative analysis. / text

Ce travail porte sur l’étude d’alliages à base prédominante employés en prothèse fixée dentaire (Co-Cr, Colado CC® et Ni-Cr, Colado NC®). La microstructure, la structure, la composition chimique, la dureté et le module d’élasticité de ces alliages ont été étudiés. Une analyse du chemin de solidification et du comportement en microségrégation a été réalisée. L’influence des recoulées a ensuite été analysée. Cinq coulées successives ont été réalisées dans différentes proportions d’alliage neuf (0, 25, 50 et 75 et 100%). La microstructure dendritique est constante au cours des recoulées, comme la large taille des grains. La composition chimique des alliages ne varie pas significativement, aussi bien dans les régions dendritiques qu’interdendritiques. La présence de particules d’Al2O3 a été observée sur les échantillons de Ni-Cr. La structure des alliages étudiés a été déterminée : elle est stable quel que soit le nombre de coulées. Les alliages présentent des valeurs de dureté diminuées lorsque les pourcentages de porosité augmentent tout en respectant le cadre normatif. Tous les échantillons testés répondent aux conditions normatives en termes de modules d’élasticité. Les recoulées n’ont pas d’influence sur le chemin de solidification des alliages ni sur les phases en présence. / This study deals with microstructural, structural, chemical and mechanical properties of two predominantly based dental alloys (Co-Cr, Colado CC® and Ni-Cr, Colado NC®). Cast alloys are characterized to analyze microstructure, structure, composition stability, hardness and elastic modulus. Solidification path was determined and microsegregation has been characterized. Influence of recasting has next been studied. Five times recasted alloys in different amounts were processed (0, 25, 50 and 75% of new alloy). It was demonstrated that the examined alloys are characterized by a dendritic microstructure with a large grain size, typical of the cast materials. No notable degenerative changes in microstructure were observed. Al2O3 particles were characterized on Ni-Cr alloy. Occurrence of porosities significantly increase in recast samples but remains clinically acceptable. Alloys demonstrated great compositional stability after 5 recasts. A significant decrease in Vickers hardness values appeared in the fourth recast in both alloys. Alloys showed no significant variation in term of modulus of elasticity, although a slight tendency decrease was observed. The main phase encountered is a face centered cubic solid solution. Secondary phases were identified. Number of recasting demonstrated to have negligible effect on phase identification and on solidification path.

Made available in DSpace on 2014-12-17T14:06:57Z (GMT). No. of bitstreams: 1 CarolineDV.pdf: 1162871 bytes, checksum: cfd19877b2a491c559d68283231f1117 (MD5) Previous issue date: 2008-04-09 / A frequently encountered difficulty in oral prosthetics is associated with the loss of metallic alloys during the melting stage of the production of metal-ceramic replacement systems. Remelting such materials could impar their use in oral rehabilitation due to loss in esthetics, as well as in the chemical, physical, electrochemical and mechanical properties. Nowadays, the Ni-Cr-Mo-Ti alloy is widely used in metal-ceramic systems. Manufacturers state that this material can be remelted without significant alterations in its behavior, however little has been established as to the changes in the performance of this alloy after successive remelting, which is common practice in oral prosthetics. Therefore, the objective of this study was to evaluate possible changes in the esthetics and associated properties of metalceramic samples consisting of Ni-Cr-Mo-Ti and dental porcelain. Three to five remelting steps were carried out. The results revealed that Ni-Cr-Mo-Ti can be safely used even after three remelting steps. Further remelting significantly affect the characteristics of the alloys and should not be recommended for the manufacture of metal-ceramic systems / Uma dificuldade frequente encontrada na pr?tese dent?ria est? associada com a perda de ligas met?licas durante o est?gio de fundi??o na confec??o de sistemas metalocer?micos. A refundi??o de alguns materiais pode impedir seu uso na reabilita??o oral devido ? perdas nas propiedades est?ticas, como tamb?m, nas qu?micas, f?sicas, eletroqu?micas e mec?nicas. Atualmente a liga Ni-Cr-Mo-Ti ? bastante usada em sistemas metalocer?micos. O fabricante afirma que esse material pode ser refundido sem causar significantes altera??es na sua qualidade, no entanto pouco foi estudado quanto ?s mudan?as nas propriedades dessa liga ap?s sucessivas fundi??es, como ? comum na pr?tica nos laborat?rios dent?rios. Com base nisso, o objetivo desse estudo foi avaliar poss?veis mudan?as est?ticas e das propriedades das amostras metalocer?micas fabricadas com a liga Ni-Cr-Mo-Ti e porcelana odontol?gica. De tr?s a cinco etapas de fundi??o foram realizadas. Os resultados revelaram que a liga Ni-Cr-Mo-Ti pode ser usada mesmo ap?s subemtida ? tr?s fundi??es. Refundi??es adicionais afetam significantemente as caracter?sticas da liga e n?o s?o recomendadas para a confec??o de sistemas metalocer?micos

Made available in DSpace on 2014-12-17T14:07:07Z (GMT). No. of bitstreams: 1 JulianaCS.pdf: 1007525 bytes, checksum: 74d4bf5035436a1b506b801dce77838f (MD5) Previous issue date: 2006-03-14 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The metalceramic crowns are usually used in dentistry because they provide a resistant structure due to its metallic base and its aesthetics from the porcelain that recovers this structure. To manufacture these crowns, a series of stages should be accomplished in the prosthetic laboratories, and many variables can influence its success. Changes in these variables cause alterations in the metallic alloy and in the porcelain, so, as consequence, in the adhesion between them. The composition of the metal alloy can be modified by recasting alloys, a common practice in some prosthetic laboratories. The aim of this paper is to make a systematic study investigating metalceramic crowns as well as analyzing the effect of recasting Ni-Cr alloys. Another variable which can influence the mechanism of metalceramic union is the temperature used in firing porcelain procedure. Each porcelain has to be fired in a fixed temperature which is determined by the manufacturer and its change can cause serious damages. This research simulate situations that may occur on laboratory procedures and observe their consequences in the quality of the metalceramic union. A scanning eletron microscopy and an optic microscopy were accomplish to analyse the metal-ceramic interface. No differences have been found when remelting alloys were used. The microhardness were similar in Ni-Cr alloys casted once, twice and three times. A wettability test was accomplished using a software developed at the Laborat?rio de Processamento de Materiais por Plasma, on the Universidade Federal do Rio Grande do Norte. No differences were found in the contact angle between the solid surface (metallic substratum) and the tangencial plane to the liquid surface (opaque). To analyse if the temperature of porcelain firing procedure could influence the contact area between metal and porcelain, a variation in its final temperature was achieve from 980? to 955?C. Once more, no differences have been found / Durante o processo de confec??o de coroas metalocer?micas, ? necess?rio que todas as informa??es passadas pelo fabricante sejam seguidas de forma criteriosa. Caso isto n?o ocorra, podem acontecer altera??es tanto na liga met?lica como na porcelana e como conseq??ncia, na ades?o entre elas. Um fator que deve ser levado em considera??o, consiste em analisar o efeito das refus?es, uma vez que nos laborat?rios prot?ticos torna-se comum a pr?tica do reaproveitamento da ligas. Outro fator importante ao se preparar coroas metalocer?micas ? a temperatura de sinteriza??o da porcelana. Sabe-se que a temperatura de sinteriza??o pode influenciar na microestrutura, afetando diretamente suas propriedades f?sicas e mec?nicas. Nesse trabalho, um estudo sistem?tico ? realizado com o intuito de investigar a ades?o metal-cer?mica em ligas Ni-Cr fundidas sob diferentes graus de reaproveitamento, assim como analisar o efeito de diferentes temperaturas de sinteriza??o da porcelana. Procura-se aqui simular situa??es poss?veis de ocorrer durante procedimentos laboratoriais prot?ticos e observar suas conseq??ncias na qualidade da ades?o metal-cer?mica. Deste modo, coroas de ligas de n?quel-cromo refundidas em 1 e 2 vezes, foram comparadas com aquelas apenas fundidas. Uma an?lise por microscopia ?ptica e eletr?nica foi realizada para se avaliar o contato entre o metal e o opaco da porcelana. Al?m do mais, testes de microdureza sobre a liga met?lica foram executados. Neste estudo, procurou-se tamb?m avaliar a molhabilidade da porcelana sobre ligas met?licas com diferentes estados de fus?o, al?m de realizar uma sinteriza??o da porcelana em diferentes temperaturas (980?C e 955?C). Como resultado encontrou-se que n?o houve diferen?a na interface entre as liga em diferentes est?gios de fus?o e a porcelana. Com rela??o ? microdureza, os resultados indicaram que, na medida em que se aumenta o n?mero de refus?es, os valores de microdureza n?o se alteram e com rela??o ? molhabilidade n?o houve diferen?a estatisticamente significativa entre as amostras. Tamb?m n?o se encontrou diferen?a entre as ?reas de contato formadas entre a porcelana e a liga fundida quando a porcelana foi sinterizada em diferentes temperaturas, mostrando que uma diminui??o de 25?C na temperatura final de sinteriza??o n?o interfere na uni?o metal-cer?mica

Made available in DSpace on 2014-12-17T14:07:01Z (GMT). No. of bitstreams: 1 HarrisonAD.pdf: 3488100 bytes, checksum: 805801db06d21e5667735d16a5799fe2 (MD5) Previous issue date: 2008-05-30 / The cobalt-chromium alloy is extensively used in the Odontology for the confection of metallic scaffolding in partial removable denture. During the last few years, it has been reported an increasing number of premature imperfections, with a few months of prosthesis use. The manufacture of these components is made in prosthetic laboratories and normally involves recasting, using parts of casting alloy and parts of virgin alloy. Therefore, the objective of the present study was to analyze the mechanical properties of a commercial cobalt-chromium alloy of odontological use after successive recasting, searching information to guide the dental prosthesis laboratories in the correct manipulation of the cobalt-chromium alloy in the process of casting and the possible limits of recasting in the mechanical properties of this material. Seven sample groups were confectioned, each one containing five test bodies, divided in the following way: G1: casting only with virgin alloy; G2: casting with 50% of the alloy of the G1 + 50% of virgin alloy; G3: casting with 50% of the alloy of the G2 + 50% of virgin alloy; G4: casting with 50% of the alloy of the G3 + 50% of virgin alloy; G5: 50% of alloy of the G4 + 50% of virgin alloy; G6: 50% of alloy of the G5 + 50% of virgin alloy and finally the G7, only with recasting alloy. The modifications in the mechanical behavior of the alloy were evaluated. Moreover, it was carried the micro structural characterization of the material by optic and electronic scanning microscopy, and X ray diffraction.and fluorescence looking into the correlatation of the mechanical alterations with structural modifications of the material caused by successive recasting process. Generally the results showed alterations in the fracture energy of the alloy after successive recasting, resulting mainly of the increasing presence of pores and large voids, characteristic of the casting material. Thus, the interpretation of the results showed that the material did not reveal significant differences with respect to the tensile strength or elastic limit, as a function of successive recasting. The elastic modulus increased from the third recasting cycle on, indicating that the material can be recast only twice. The fracture energy of the material decreased, as the number of recasting cycles increased. With respect to the microhardness, the statistical analyses showedno significant differences. Electronic scanning microscopy revealed the presence of imperfections and defects, resulting of the recasting process. X ray diffraction and fluorescence did not show alterations in the composition of the alloy or the formation of crystalline phases between the analyzed groups. The optical micrographs showed an increasing number of voids and porosity as the material was recast. Therefore, the general conclusion of this study is that the successive recasting of of Co-Cr alloys affects the mechanical properties of the material, consequently leading to the failure of the prosthetic work. Based on the results, the best recommendadition is that the use of the material should be limited to two recasting cycles / Ligas cobalto-cromo s?o extensivamente utilizadas na Odontologia para a confec??o de arma??es met?licas em pr?teses parciais remov?veis. Durante os ?ltimos anos, tem sido reportado um n?mero crescente de falhas prematuras, com poucos meses de utiliza??o das pr?teses. A fabrica??o desses componentes ? feita em laborat?rios prot?ticos e normalmente envolve a refundi??o, utilizando partes de liga fundida e partes de liga virgem. Portanto, o objetivo do presente estudo foi analisar as propriedades mec?nicas de uma liga comercial de cobalto-cromo de uso odontol?gico ap?s sucessivas refundi??es, buscando informa??es no sentido de orientar os laborat?rios de pr?tese dent?ria na correta manipula??o dessa liga no processo de fundi??o e nos poss?veis limites de refundi??o desse material. Foram confeccionados sete grupos de amostras, cada um contendo cinco corpos de prova, divididos da seguinte maneira: G1: fundi??o apenas com liga virgem; G2: fundi??o com 50% da liga do G1 + 50% de liga virgem; G3: fundi??o com 50% da liga do G2 + 50% de liga virgem; G4: fundi??o com 50% da liga do G3 + 50% de liga virgem; G5: 50% de liga do G4 + 50% de liga virgem; G6: 50% de liga do G5 + 50% de liga virgem e finalmente o G7, s? com liga refundida. Foram avaliadas as principais modifica??es ocorridas no comportamento mec?nico dessa liga por meio de ensaios de tra??o e microdureza. Al?m disso, foi realizada a caracteriza??o microestrutural do material por meio de microscopia ?ptica e eletr?nica de varredura, difra??o e fluoresc?ncia de raios X procurando correlacionar as altera??es mec?nicas com modifica??es estruturais do material ocasionadas pelos sucessivos processos de refundi??o. De forma geral os resultados mostraram altera??es na plasticidade e energia de fratura da liga ap?s sucessivas refundi??es, decorrentes principalmente da crescente presen?a de poros e vazios, defeitos caracter?sticos da fundi??o do material. A interpreta??o dos resultados permitiu concluir que o material n?o apresentou diferen?as significativas em rela??o ? resist?ncia ? tra??o ou ao limite el?stico, em fun??o das sucessivas refundi??es. A energia de fratura do material mostrou tend?ncia ? diminui??o, ? medida que o material foi refundido. Em rela??o ? microdureza, as an?lises estat?sticas mostraram n?o haver diferen?as significativas. A microscopia eletr?nica revelou a presen?a de falhas e defeitos, resultantes dos processos de refundi??es. A an?lise por fluoresc?ncia e difra??o de raios X n?o mostrou altera??es na composi??o da liga ou na forma??o de fases cristalinas entre os grupos analisados. As micrografias ?pticas mostraram um crescente n?mero de vazios e aumento da porosidade ? medida que o material foi refundido. Assim, a conclus?o geral do trabalho ? que as sucessivas refundi??es em ligas de Co-Cr comprometem as propriedades mec?nicas do material, o que pode levar a falhas e consequentemente, ao fracasso do trabalho prot?tico. Com base nos resultados, o mais recomendado ? a utiliza??o de no m?ximo duas refundi??es, para que as propriedades do material n?o sejam comprometidas