Investigando as concepções de força em situações do cotidiano ao longo do ciclo da experiência Kellyana

This research explored the construction process of the scientific concept of force among pupils at High School, who were engaged in a didactic sequence organized according to the five phases of Kellyan Cycle of experience: Anticipation, Investment, Encounter, Confirmation or Disconfirmation and Constructive Revision. It also intended to identify conceptions of force applied to everyday life situations. The results of this research showed that pupils, during the Anticipation phase, possessed intuitive conceptions as well as difficulties in differentiating the concepts of orientation and direction of a force. The activities developed during the Cycle aroused pupils´ attention, impelling them to develop hypotheses, to reflect and to establish relationships between experienced situations and the theme under study. This enabled the evolution of their concepts.http://dx.doi.org/10.5007/2175-7941.2008v25n2p287Esta pesquisa teve como objetivo investigar o processo de construção do conceito científico de força entre alunos do 1º ano do Ensino Médio, que participaram de uma seqüência didática organizada de acordo com as cinco fases do Ciclo da Experiência Kellyana: Antecipação; Investimento; Encontro; Confirmação ou Desconfirmação e Revisão Construtiva. Procurou-se identificar as concepções sobre força em situações do cotidiano. Os resultados desta pesquisa mostraram que os alunos, na fase da Antecipação, possuíam conceitos intuitivos, bem como tinham dificuldades de entender os conceitos de direção e sentido de uma força. As atividades desenvolvidas durante o Ciclo despertaram a atenção dos alunos, impulsionando-os a levantar hipóteses, refletir e estabelecer relações das situações vivenciadas com o tema em estudo, o que oportunizou uma contribuição de forma relevante para a evolução dos seus conceitos

Red, green, and blue upconversion luminescence in ytterbium-sensitized praseodymium-doped lead-cadmium-germanate glass

Blue, green, red, and near-infrared upconversion luminescence in the wavelength region of 480 - 740 nm in Pr3+/Yb3+-codoped lead-cadmium-germanate glass under 980 nm diode laser excitation, is presented. Upconversion emission peaks around 485, 530, 610, 645, and 725 nm which were ascribed to the 3P0 - 3HJ (J=4, 5, and 6), and 3P0 - 3FJ (J=2, and 3,4), transitions, respectively, were observed. The population of the praseodymium upper 3P0 emitting level was accomplished through a combination of ground-state absorption of Yb3+ ions at the 2F7/2, energy-transfer Yb3+(2F 5/2) Pr3+(3H4), and excited-state absorption of Pr3+ ions provoking the 1G4 - 3P0 transition. The dependence of the upconversion luminescence upon the Yb3+-concentration and diode laser power, is also examined, in order to subsidize the proposed upconversion excitation mechanism

Infrared-to-visible frequency upconversion in transparent glass ceramics containing trivalent-rare-earth-doped nanocrystals

In this work we report on visible upconversion emission in Er 3+-, and Ho3+-doped PbGeO3-PbF 2-CdF2-based transparent glass ceramics under 980 nm infrared excitation. In erbium-doped vitroceramic samples, blue(410 ran), green(530, and 550 nm) and red(660 nm) emission signals were generated, which were identified as due to the 2H9/2, 2H 11/2, 4S3/2, and 4F9/2 transitions to the 4I15/2 ground-state, respectively. Intense red(650 nm) upconversion emission corresponding to the 5F5 - 5I8 transition and very small blue(490 nm) and green(540 nm) signals assigned to the 5F 2,3 - 5I8 and 4S2, 5F4 - 5I8 transitions, respectively, were observed in the holmium-doped samples. The 540 nm is the dominant upconversion signal in Ho3+-doped vitroceramics under 850 nm excitation. The dependence of the upconversion processes upon pump power and doping concentration are also investigated, and the main routes for the upconversion excitation processes are also identified. The comparison of the upconversion process in transparent glass ceramics and the precursor glass was also examined and the results revealed that the former present higher upconversion efficiencies

Efficient UV-visible upconversion luminescence and thermal effects in terbium-ytterbium codoped fluorogermanate vitroceramic

Cooperative energy-transfer upconversion luminescence in Tb 3+/Yb 3+-codoped PbGeO 3-PbF 2-CdF 2 vitroceramic and its precursor glass under resonant and off-resonance infrared excitation, is investigated. Bright UV-visible emission signals around 384, 415, 438 nm, and 473-490, 545, 587, and 623 nm, identified as due to the 5D 3( 5G 6 → 7F J(J=6,5,4) and 5D 4 → 7F J(J=6,5,4,3) transitions, respectively, were readily observed. The results indicate that cooperative energy-transfer between ytterbium and terbium ions followed by excited-state absorption are the dominant upconversion excitation mechanisms herein involved. The comparison of the upconversion process in a vitroceramic sample and its glassy precursor revealed that the former present much higher upconversion efficiency. The dependence of the upconversion emission upon pump power, temperature, and doping content is also examined