Grains and easy experiments for the classroom

Nos últimos anos houve um grande aumento da atividade experimental e teórica no estudo das propriedades dos materiais granulares. Este artigo apresenta uma breve revisão da literatura sobre este tema e sugere experimentos simples que o professor de física pode fazer ou sugerir aos seus estudantes.During last years there was a large experimental and theoretical activity in the study of the properties of granular materials. This article makes a brief revision of the literature on these topics and suggests artless experiments that the physics teacher can perform in the classroom

Comparative study of some dynamic properties of liquids and grains in the classroom

Visando entender as diferenças entre as propriedades dinâmicas dos materiais granulares e as propriedades dinâmicas dos líquidos, foram realizados experimentos usando água e grãos de arroz e açúcar. Os experimentos requerem poucos recursos e foram pensados para que possam ser desenvolvidos com facilidade na sala de aula ou num laboratório de ensino. Os resultados mostraram que o fluxo de grãos difere significativamente do fluxo de líquidos.In order to understand the differences in the dynamic properties between granular materials and liquids, experiments using water, rice and sugar grains were performed. The experiments require few material resources and were thought so as to be easily made in a classroom or in a teaching laboratory. The results showed that grain's flow differs significantly from that of liquids.Banco Santande

Determinación de la adhesión de hidroxiapatita sobre Ti cp utilizando impedancia electroquímica

La espectroscopia de impedancia electroquímica es una de las técnicas más modernas utilizadas para caracterizar las propiedades eléctricas de una película cerámica y su adhesión a la superficie metálica. Este trabajo representa parte de un proyecto en ejecución en el que se propone evaluar la solubilidad de una película de hidroxiapatita (HA) depositada sobre la superficie pulida de Ti cp por el método biomimético, después de varios períodos de inmersión en un medio fisiológico con la composición siguiente (g/L): NaCl 8,74; NaHCO3 0,35; Na2HPO4 0,06; NaH2PO4 0,06. A través de la técnica de impedancia es posible determinar la cantidad de disolución que penetra en la película y, a partir de esto, estimar la solubilidad y el límite de saturación que conduce a la pérdida de adhesión mediante un gráfico que describe el perfil de las variaciones de la capacidad eléctrica en función del tiempo. Las mediciones de impedancia fueron realizadas a potenciales de circuito abierto después de varios períodos de inmersión, en el intervalo de 100 KHz a 10 mHz . Los diagramas de Bode muestran claramente que el recubrimiento provee una protección cada vez menos eficaz con el aumento del tiempo de exposición en disolución fisiológica. La máxima impedancia |z| obtenida, correspondiente a un período de 24 h en el que la protección fue más efectiva, permaneció alrededor de 106Ω · cm2 a 0,01 Hz

Transport and optical properties of resonant tunneling structures

Transport properties of a GaAs/AlAs superlattice-like double barrier diode are studied in this work as a function of the sample temperature. An activation energy of about 60meV obtained from the Arrhenius plot is in good agreement with the confined level in the central well. Numerical simulations also confirm the importance of bound levels in the gammaand X bands for the resonant tunneling process. The enhancement of photoluminescence as the temperature is increased is also studied. This behavior is associated to the transport properties of holes in the collector contact, which control the supply of minority carriers, which tunnel into the well. The description of the observed results requires the modi-cation of simple known models to take into account the two contributions to the pair generation rate in the well, responsible of the photoluminescence at zero and finite bias.331333Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Photophysics of poly(p-phenylene vinylene) films

We report on time resolved photoluminescence (PLRT) measurements in poly(p-phenylene vinylene) (PPV) films irradiated by laser in the presence of air. We observe a PL intensity enhancement and a biexponential decay dynamics of PL signal for all irradiated films. These results can be understood in terms of a chain shortening process due to carbonyl incorporation and formation of an energy profile that extends and migrates into the film and enables efficient spectral diffusion of excited carriers to a non-degraded PPV segments by Förster energy transfer

Corrosión de juntas soldadas empleando láser y brazing de una aleación odontológica AuPdAgIn

La soldadura ha sido utilizada en la Odontología para resolver problemas de adaptación de las prótesis, permitiendo trabajar con sus segmentos, mejorando la distribución uniforme de fuerzas, haciendo mínimos los traumas o fallas en el hueso, en el implante o en la prótesis. En esta investigación, fue estudiada la corrosión de la aleación AuPdAgIn, utilizada en prótesis sobre implantes en un medio de NaCl 0,9 %, aireado, que simulaba las condiciones del ambiente bucal, después de haber sido sometida a dos procesos de soldadura: láser y brazing. El desarrollo de nuevos equipos de soldadura a láser, de menor costo, tamaño y simplicidad técnica, permitió una mayor utilización de este proceso en la confección de prótesis comparada con la brazing, pues utiliza como fuente de calor un haz de luz concentrado de elevada energía que no causa distorsión en la pieza prostética. Estos dos procesos generan diferencias en las microestructuras; para la soldadura a láser, dentrítica refinada debido a la gran velocidad de enfriamiento (5,96 · 103 ºC/s) y en la brazing (80 ºC/s), granular. La región de soldadura a láser tuvo un desempeño mejor, por presentar un potencial de corrosión más positivo a circuito abierto. Las mediciones de impedancia fueron realizadas a potenciales de circuito abierto después de 24 h de inmersión, se aplicó una perturbación de 10 mV y se obtuvieron cinco puntos por década de frecuencia en el intervalo de 100 kHz a 10 mHz

Linear and Second-order Optical Response of the III-V Mono-layer Superlattices

We report the first fully self-consistent calculations of the nonlinear optical properties of superlattices. The materials investigated are mono-layer superlattices with GaP grown on the the top of InP, AlP and GaAs (110) substrates. We use the full-potential linearized augmented plane wave method within the generalized gradient approximation to obtain the frequency dependent dielectric tensor and the second-harmonic-generation susceptibility. The effect of lattice relaxations on the linear optical properties are studied. Our calculations show that the major anisotropy in the optical properties is the result of strain in GaP. This anisotropy is maximum for the superlattice with maximum lattice mismatch between the constituent materials. In order to differentiate the superlattice features from the bulk-like transitions an improvement over the existing effective medium model is proposed. The superlattice features are found to be more pronounced for the second-order than the linear optical response indicating the need for full supercell calculations in determining the correct second-order response.Comment: 9 pages, 4 figures, submitted to Phy. Rev.

Spin polarization of carriers in InGaAs self-assembled quantum rings inserted in GaAs-AlGaAs resonant tunneling devices

In this work, we have investigated transport and polarization resolved photoluminescence (PL) of n-type GaAs-AlGaAs resonant tunneling diodes (RTDs) containing a layer of InGaAs self-assembled quantum rings (QRs) in the quantum well (QW). All measurements were performed under applied voltage, magnetic fields up to 15 T and using linearly polarized laser excitation. It was observed that the QRs’ PL intensity and the circular polarization degree (CPD) oscillate periodically with applied voltage under high magnetic fields at 2 K. Our results demonstrate an effective voltage control of the optical and spin properties of InGaAs QRs inserted into RTDs

Voltage- and light-controlled spin properties of a two-dimensional hole gas in p-type GaAs/AlAs resonant tunneling diodes

We have investigated the spin properties of a two-dimensional hole gas (2DHG) formed at the contact layer of a p-type GaAs/AlAs resonant tunneling diode (RTD). We have measured the polarized-resolved photoluminescence of the RTD as a function of bias voltage, laser intensity and external magnetic field up to 15T. By tuning the voltage and the laser intensity, we are able to change the spin-splitting from the 2DHG from almost 0 meV to 5 meV and its polarization degree from − 40% to + 50% at 15T. These results are attributed to changes of the local electric field applied to the two-dimensional gas which affects the valence band and the hole Rashba spin–orbit effect