15 research outputs found

    Synthesis and physical characterization of poly(cyclohexane carbonate), synthesized from CO2 and cyclohexene oxide

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    Using [Zn(2,6-difluorophenoxide)2]2 (THF)2 as the catalyst, poly(cyclohexane carbonate) (PCHC) was synthesized from CO2 and cyclohexene oxide. The ether content of the polymer was limited to a few mol%. The molecular weight distribution of the linear polycarbonate was broad, with Mn=42 and Mw=252 kg/mol. The recorded Tg was 115°C, which is in excellent agreement with the reported value of 116°C. In spite of its high molar mass, PCHC behaves like a brittle polymer, with an elongation at break of 1–2%. On the other hand, the tensile modulus of PCHC (3600 MPa) is much higher than the corresponding value for bisphenol-A polycarbonate (BP-A PC) (2400 MPa). Like the extremely tough BP-A PC, the PCHC exhibits a ¿-transition around -110°C, the presence of which has been related to toughness. The magnitude of this ¿-transition is lower than the corresponding value for BP-A PC, which indicates that the main chain of PCHC is less flexible than that of BP-A PC. Moreover, the low temperature relaxation of PCHC is probably related to chair–chair transitions of the cyclohexane side group. The brittle behavior of PCHC is expected from the relatively low plateau modulus of PCHC in the melt, from which a relatively high average molecular weight between entanglements (Me) of ca. 15,000 g/mole was estimated, which is in the same order of magnitude as the Me of the brittle polystyrene

    Synthesis and physical characterization of poly(cyclohexane carbonate), synthesized from CO2 and cyclohexene oxide

    No full text
    Using [Zn(2,6-difluorophenoxide)2]2 (THF)2 as the catalyst, poly(cyclohexane carbonate) (PCHC) was synthesized from CO2 and cyclohexene oxide. The ether content of the polymer was limited to a few mol%. The molecular weight distribution of the linear polycarbonate was broad, with Mn=42 and Mw=252 kg/mol. The recorded Tg was 115°C, which is in excellent agreement with the reported value of 116°C. In spite of its high molar mass, PCHC behaves like a brittle polymer, with an elongation at break of 1–2%. On the other hand, the tensile modulus of PCHC (3600 MPa) is much higher than the corresponding value for bisphenol-A polycarbonate (BP-A PC) (2400 MPa). Like the extremely tough BP-A PC, the PCHC exhibits a ¿-transition around -110°C, the presence of which has been related to toughness. The magnitude of this ¿-transition is lower than the corresponding value for BP-A PC, which indicates that the main chain of PCHC is less flexible than that of BP-A PC. Moreover, the low temperature relaxation of PCHC is probably related to chair–chair transitions of the cyclohexane side group. The brittle behavior of PCHC is expected from the relatively low plateau modulus of PCHC in the melt, from which a relatively high average molecular weight between entanglements (Me) of ca. 15,000 g/mole was estimated, which is in the same order of magnitude as the Me of the brittle polystyrene

    III-nitride nanopyramid light emitting diodes grown by organometallic vapor phase epitaxy

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    Nanopyramid light emitting diodes (LEDs) have been synthesized by selective area organometallic vapor phase epitaxy. Self-organized porous anodic alumina is used to pattern the dielectric growth e templates via reactive ion etching, eliminating the need for lithographic processes. (In,Ga)N quantum well growth occurs primarily on the six {1 (1) over bar 01} semipolar facets of each of the nanopyramids, while coherent (In,Ga)N quantum dots with heights of up to similar to 20 nm are incorporated at the apex by controlling growth conditions. Transmission electron microscopy (TEM) indicates that the (In,Ga)N active regions of the nanopyramid heterostructures are completely dislocation-free. Temperature-dependent continuous-wave photoluminescence of nanopyramid heterostructures yields a peak emission wavelength of 617 nm and 605 nm at 300 K and 4 K respectively. The peak emission energy varies with increasing temperature with a double S-shaped profile, which is attributed to either the presence of two types of InN-rich features within the nanopyramids or a contribution from the commonly observed yellow defect luminescence close to 300 K. TEM cross-sections reveal continuous planar defects in the (In,Ga)N quantum wells and GaN cladding layers grown at 650-780 degrees C, present in 38% of the nanopyramid heterostructures. Plan-view TEM of the planar defects confirms that these defects do not terminate within the nanopyramids. During the growth of p-GaN, the structure of the nanopyramid LEDs changed from pyramidal to a partially coalesced film as the thickness requirements for an undepleted p-GaN layer result in nanopyramid impingement. Continuous-wave electroluminescence of nanopyramid LEDs reveals a 45 nm redshift in comparison to a thin-film LED, suggesting higher InN incorporation in the nanopyramid LEDs. These results strongly encourage future investigations of III-nitride nanoheteroepitaxy as an approach for creating efficient long wavelength LEDs

    Gettered GaP Substrates for Improved Multijunction Solar Cell Devices

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    We report on the characterization of gettered p-type GaP substrates for application in high-efficiency multijunction solar cells. A commercial zinc-doped GaP substrate was divided, with one piece soaked in a phosphorus-saturated gallium-aluminum melt at 975A degrees C. Low-temperature continuous-wave photoluminescence indicated a significant decrease in deep-level impurity peaks due to oxygen and zinc-oxygen complexes after gettering in the phosphorus-saturated gallium-aluminum melt. To illustrate what effect this has on minority-carrier diffusion lengths, Au/GaP Schottky solar cells were fabricated on the substrates, and the spectral response of each was examined. A marked increase in response across all wavelengths on the gettered sample indicates an increase in minority-carrier diffusion lengths. To ensure these results were not simply due to an increase in the depletion region width resulting from a change in carrier density, C-V profiling was performed and found only a small change in carrier concentration of the gettered sample
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