Assessing the Influence of the Sourcing Voltage on Polyaniline Composites for Stress Sensing Applications.

Polyaniline (PANI) has recently gained great attention due to its outstanding electrical properties and ease of processability; these characteristics make it ideal for the manufacturing of polymer blends. In this study, the processing and piezoresistive characterization of polymer composites resulting from the blend of PANI with ultra-high molecular weight polyethylene (UHMWPE) in different weight percentages (wt %) is reported. The PANI/UHMWPE composites were uniformly homogenized by mechanical mixing and the pellets were manufactured by compression molding. A total of four pellets were manufactured, with PANI percentages of 20, 25, 30 and 35 wt %. Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), differential thermal analysis (DTA), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to confirm the effective distribution of PANI and UHMWPE particles in the pellets. A piezoresistive characterization was performed on the basis of compressive forces at different voltages; it was found that the error metrics of hysteresis and drift were influenced by the operating voltage. In general, larger voltages lowered the error metrics, but a reduction in sensor sensitivity came along with voltage increments. In an attempt to explain such a phenomenon, the authors developed a microscopic model for the piezoresistive response of PANI composites, aiming towards a broader usage of PANI composites in strain/stress sensing applications as an alternative to carbonaceous materials.

Preparation of La0.7Ca0.3-xSrxMnO3 Manganites by Four Synthesis Methods and Their Influence on the Magnetic Properties and Relative Cooling Power.

Manganites of the family La0.7Ca0.3-xSrxMnO3 were fabricated by four preparation methods: (a) the microwave-assisted sol-gel Pechini method; (b) sol-gel Pechini chemical synthesis; (c) solid-state reaction with a planetary mill; and (d) solid-state reaction with an attritor mill, in order to study the effect of the preparation route used on its magnetocaloric and magnetic properties. In addition, the manganites manufactured by the Pechini sol-gel method were compacted using Spark Plasma Sintering (SPS) to determine how the consolidation process influences its magnetocaloric properties. The Curie temperatures of manganites prepared by the different methods were determined in ~295 K, with the exception of those prepared by a solid-state reaction with an attritor mill which was 301 K, so there is no correlation between the particle size and the Curie temperature. All samples gave a positive slope in the Arrot plots, which implies that the samples underwent a second order Ferromagnetic (FM)⁻Paramagnetic (PM) phase transition. Pechini sol-gel manganite presents higher values of Relative Cooling Power (RCP) than the solid-state reaction manganite, because its entropy change curves are smaller, but wider, associated to the particle size obtained by the preparation method. The SPS technique proved to be easier and faster in producing consolidated solids for applications in active magnetic regenerative refrigeration compared with other compaction methods.

First Order Reversal Curve Study of SmFe2 Melt-Spun Ribbons.

First-order reversal curves (FORC) and the FORC distribution provide a detailed characterization of the relative proportions of reversible and irreversible components of the magnetization of a material, revealing the dominant interactions in the system. Alloys with the nominal composition SmFe2 were obtained by melt-spinning with a cooper wheel velocity of 30 m/s. X-ray powder diffraction analysis showed a greater part consisting of an amorphous phase and a very small amount of SmFe2 crystalline phase with an average crystallite size of 8 nm. A constant acceleration Mössbauer spectrum, measured at room temperature in transmission mode, was fitted to a continuous distribution of effective fields at the nucleus of the amorphous phase (about 84% of the total area), plus two sextets for the non-equivalent sites of Fe in the SmFe2 crystalline phase. 91 first-order reversal curves were collected in a Quantum Design PPMS-VSM with reversal fields from ⁻800 mT to +800 mT and using a calibration field of 850 mT. The obtained FORC diagrams showed a combined effect of a local interaction field and a mean interaction field, and showed that the reversible magnetization is a function of both, the applied magnetic field and the irreversible magnetization.

Effect of Native Defects on Transport Properties in Non-Stoichiometric CoSb3.

The effect of native defects originated by a non-stoichiometric variation of composition in CoSb3 on I-V curves and Hall effect was investigated. Hysteretic and a non-linear behavior of the  I-V curves at cryogenic temperatures were observed; the non-linear behavior originated from the Poole-Frenkel effect, a field-dependent ionization mechanism that lowers Coulomb barriers and increases emission of charge carriers, and the hysteresis was attributed to the drastic decrease of specific heat which produces Joule heating at cryogenic temperatures. CoSb3 is a narrow gap semiconductor and slight variation in the synthesis process can lead to either n- or p-type conduction. The Sb-deficient CoSb3 presented an n-type conduction. Using a single parabolic model and assuming only acoustic-phonon scattering the charge transport properties were calculated at 300 K. From this model, a carrier concentration of 1.18 × 1018 cm-3 and a Hall factor of 1.18 were calculated. The low mobility of charge carriers, 19.11 cm²/V·s, and the high effective mass of the electrons, 0.66 m0, caused a high resistivity value of 2.75 × 10-3 Ω·m. The calculated Lorenz factor was 1.50 × 10-8 V²/K², which represents a decrease of 38% over the degenerate limit value (2.44 × 10-8 V²/K²).

Role of vanadium ions, oxygen vacancies, and interstitial zinc in room temperature ferromagnetism on ZnO-V2O5 nanoparticles.

In this work, we present the role of vanadium ions (V+5 and V+3), oxygen vacancies (VO), and interstitial zinc (Zni) to the contribution of specific magnetization for a mixture of ZnO-V2O5 nanoparticles (NPs). Samples were obtained by mechanical milling of dry powders and ethanol-assisted milling for 1 h with a fixed atomic ratio V/Zn?=?5% at. For comparison, pure ZnO samples were also prepared. All samples exhibit a room temperature magnetization ranging from 1.18?×?10-3 to 3.5?×?10-3 emu/gr. Pure ZnO powders (1.34?×?10-3 emu/gr) milled with ethanol exhibit slight increase in magnetization attributed to formation of Zni, while dry milled ZnO powders exhibit a decrease of magnetization due to a reduction of VO concentration. For the ZnO-V2O5 system, dry milled and thermally treated samples under reducing atmosphere exhibit a large paramagnetic component associated to the formation of V2O3 and secondary phases containing V+3 ions; at the same time, an increase of VO is observed with an abrupt fall of magnetization to σ?~?0.7?×?10-3 emu/gr due to segregation of V oxides and formation of secondary phases. As mechanical milling is an aggressive synthesis method, high disorder is induced at the surface of the ZnO NPs, including VO and Zni depending on the chemical environment. Thermal treatment restores partially structural order at the surface of the NPs, thus reducing the amount of Zni at the same time that V2O5 NPs segregate reducing the direct contact with the surface of ZnO NPs. Additional samples were milled for longer time up to 24 h to study the effect of milling on the magnetization; 1-h milled samples have the highest magnetizations. Structural characterization was carried out using X-ray diffraction and transmission electron microscopy. Identification of VO and Zni was carried out with Raman spectra, and energy-dispersive X-ray spectroscopy was used to verify that V did not diffuse into ZnO NPs as well to quantify O/Zn ratios.

Multimedia educativa para el perfeccionamiento del proceso enseñanza-aprendizaje de la asignatura Biologí­a Celular / Educative multimedia to improve the teaching-learning process of the Cellular Biology subject

El presente trabajo propone una Multimedia educativa para perfeccionar el proceso enseñanza-aprendizaje de la asignatura Biología Celular en la carrera de Licenciatura en Tecnología de la Salud, la cual propicia la aplicación de la informática para facilitar la comprensión de los contenidos, debido a la necesidad de incrementar sus niveles de eficiencia. El medio de enseñanza propuesto se sustenta en una concepción didáctica que instruye, educa y desarrolla. La investigación asume el método dialéctico-materialista y emplea el muestreo intencional que aportó riqueza a la información, se apoyó en una gama de métodos y técnicas que, mediante el diagnóstico de necesidades, demostró la falta de motivación e interés por el estudio de la asignatura. El tema, según los criterios de especialistas, resultó adecuado y pertinente en las condiciones actuales en correspondencia con las transformaciones propias de la nueva universidad cubana en los estilos de aprendizaje(AU)

Pierre Fouchart, padre de la Estomatología Moderna / Pierre Fouchart Father of the Modern Estomatology

Se presenta una breve biografía de Pierre Fouchart, a quién se le atribuye ser el fundador de la Odontología Moderna (AU)