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Nós estudamos as propriedades ópticas dos íons Cr+3 em matriz de GdAlO3 com técnicas de espectroscopia de absorção, excitação, luminescência, todas em função da polarização e o tempo de decaimento ente 1.5K e 70K. Devido à excelente qualidade do cristal usado, novos comportamentos da transição 4A2 &#8594 2E foram obtidos, e com eles um modelo adaptado do trabalho de Tanabe e Sugano é proposto. Para um cristal com alta concentração, as propriedades dos pares foram estudadas, tanto do estado fundamental (4A2 - 4A2) quanto dos estados simplesmente excitados (2E - 4A2; 2T1 - 4A2) e os duplamente excitados (2E - 2E; 2E - 2T1 e 2T1 - 2T1)We have studied the optical properties of Cr+3 íon in GdAlo3 between 1.5K and 70K, with the following spectroscopy technique: absorption, excitation, luminescence, polarization and decay time. Due to the excellent quality of the crystal we obtained new results concerning the transition 4A2 &#8594 2E which leads us to propose a coherent model adapted from Tanable and Sugano work. The properties of Cr+3 pairs have also been studied with a high Cr+3 doped crystal, either in the ground state (4A2 - 4A2) or in the single excited states (2E - 4A2; 2T1 - 4A2) or in the doubly excited states (2E - 2E; 2E - 2T1 e 2T1 - 2T1

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Nós estudamos as propriedades ópticas dos íons Cr+3 em matriz de GdAlO3 com técnicas de espectroscopia de absorção, excitação, luminescência, todas em função da polarização e o tempo de decaimento ente 1.5K e 70K. Devido à excelente qualidade do cristal usado, novos comportamentos da transição 4A2 &#8594 2E foram obtidos, e com eles um modelo adaptado do trabalho de Tanabe e Sugano é proposto. Para um cristal com alta concentração, as propriedades dos pares foram estudadas, tanto do estado fundamental (4A2 - 4A2) quanto dos estados simplesmente excitados (2E - 4A2; 2T1 - 4A2) e os duplamente excitados (2E - 2E; 2E - 2T1 e 2T1 - 2T1)We have studied the optical properties of Cr+3 íon in GdAlo3 between 1.5K and 70K, with the following spectroscopy technique: absorption, excitation, luminescence, polarization and decay time. Due to the excellent quality of the crystal we obtained new results concerning the transition 4A2 &#8594 2E which leads us to propose a coherent model adapted from Tanable and Sugano work. The properties of Cr+3 pairs have also been studied with a high Cr+3 doped crystal, either in the ground state (4A2 - 4A2) or in the single excited states (2E - 4A2; 2T1 - 4A2) or in the doubly excited states (2E - 2E; 2E - 2T1 e 2T1 - 2T1

Three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films

A process for preparing three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films is introduced. Samples are made from commercial FEP films by means of laser cutting, laser bonding, electrode evaporation, and high-field poling. The observed dielectric-resonance spectra demonstrate the piezoelectricity of the FEP sandwiches. Piezoelectric d (33) coefficients up to a few hundred pC/N are achieved. Charging at elevated temperatures can increase the thermal stability of the piezoelectrets. Isothermal experiments for approximately 15 min demonstrate that samples charged at 140A degrees C keep their piezoelectric activity up to at least 120A degrees C and retain 70% of their initial d (33) even at 130A degrees C. Acoustical measurements show a relatively flat frequency response in the range between 300 Hz and 20 kHz.CAPES (Brazil)DAAD (Germany)[D/08/11608]DFG[WA 2688/1-1]E

Temporal evolution of the re-breakdown voltage in small gaps from nanoseconds to milliseconds

A detailed understanding of electric breakdown in dielectrics is of scientific and technological interest. In gaseous dielectrics, a so-called re-breakdown is sometimes observed after extinction of the previous discharge. Although time-dependent re-breakdown voltage is essentially known, its behavior immediately after the previous discharge is not precisely understood. We present an electronic circuit for accurate measurements of the time-dependent re-breakdown voltage in small gaps from tens of nanoseconds to several milliseconds after the previous spark. Results from such experiments are compared with earlier findings, and relevant physical mechanisms such as heating of the gas, decay of the plasma, and ionization of excited atoms and molecules are discussed. It is confirmed that the thermal model is not valid at times below several microseconds

Template-based fluoroethylenepropylene piezoelectrets with tubular channels for transducer applications

We describe the concept, the fabrication, and the most relevant properties of a piezoelectric-polymer system: Two fluoroethylenepropylene (FEP) films with good electret properties are laminated around a specifically designed and prepared polytetrafluoroethylene (PTFE) template at 300 degrees C. After removing the PTFE template, a two-layer FEP film with open tubular channels is obtained. For electric charging, the two-layer FEP system is subjected to a high electric field. The resulting dielectric barrier discharges inside the tubular channels yield a ferroelectret with high piezoelectricity. d(33) coefficients of up to 160 pC/N have already been achieved on the ferroelectret films. After charging at suitable elevated temperatures, the piezoelectricity is stable at temperatures of at least 130 degrees C. Advantages of the transducer films include ease of fabrication at laboratory or industrial scales, a wide range of possible geometrical and processing parameters, straightforward control of the uniformity of the polymer system, flexibility, and versatility of the soft ferroelectrets, and a large potential for device applications e.g., in the areas of biomedicine, communications, production engineering, sensor systems, environmental monitoring, etc.Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq), Brazil(DAAD) Deutscher Akademischer Austausch Dienst, GermanyUniversity of Potsdam, GermanyDeutsche Forschungsgemeinschaft (DFG)[QI 65/1-1

Laminated composite based on polyester geotextile fibers and polyurethane resin for coating wood structures

New environmental laws have restricted the use of hardwood trees in overhead power lines structures, such as, poles and cross-arms, leading companies to seek alternative materials. Reforested wood coated with polymeric resin has been proposed as an environmental friendly solution, with improved electrical properties and protection against external agents, e.g. moisture, ultraviolet radiation and fungi. However, the single thin layer of resin, normally applied on such structures reveal to be inefficient, due to be easily damage during handling. In this paper, we present a composite coating, based on geotextile fibers and polyurethane resin that is suitable for wooden structures. Results obtained from two different tree species (from managed and reforested areas) coated with the composite reveal that the additional layer not only provided a stronger adhesion between wood and ccoating layer but also a further improvement in the electrical properties and better protection against abrasion and moisture