38,352 research outputs found
Development of new proton conducting materials for intermediate temperature fuel cells
No full textThe work in this thesis mainly focuses on the preparation and characterization of several phosphates and solid oxide systems with the aim of developing new proton conducting materials for intermediate temperature fuel cells (ITFCs). Soft chemical methods such as sol-gel methods and conventional solid state methods were applied for the synthesis of these materials.
Aluminum phosphate obtained by a solution method is single phase and belongs to one of the Al(H₂PO₄)₃ allotropies with hexagonal symmetry. The material is stable up to 200°C and decomposes into Al(PO₃)₃ at a higher temperature. The electrical conductivity of pure Al(H₂PO₄)₃ is on the order of 10⁻⁶-10⁻⁷ S/cm, very close to the value for the known proton conductors AlH₃(PO₄)₂•3H₂O and AlH₂P₃O₁₀•2H₂O. Much higher conductivity is observed for samples containing even a trace amount of excess H₃PO₄. It is likely that the conduction path gradually changes from grain interior to the surface as the acid content increases. The conductivity of Al(H₂PO₄)₃-0.5H₃PO₄ exhibited a good stability over the measured 110 hours.
Although tin pyrophosphate (SnP₂O₇) has been reported to show a significantly high conductivity (~10⁻² S/cm) at 250°C in various atmospheres, we observed large discrepancies in the electrical properties of SnP₂O₇ prepared by different methods. Using an excess amount of phosphorous in the synthetic procedure generally produces SnP₂O₇ with much higher conductivity (several orders of magnitude higher) than samples with stoichiometric Sn:P ratios in their synthetic procedure. Solid state ³¹P NMR confirmed the presence of residual phosphoric acid for samples with excess starting phosphorous. Transmission Electron Microscope (TEM) confirmed an amorphous layer covered the SnP₂O₇ granules which was probably phosphoric acid or condensed phases. Thereby, it is quite likely that the high conductivity of SnP₂O₇ results mainly from the contribution of the residual acid. The conductivity of these samples exhibited a good stability over the measured 80 hours.
Based on the observations for SnP₂O₇, we developed a nano core-shell structure based on BPO₄ and P₂O₅ synthesised by solid state methods. The particle size of BPO₄ using this method varied between 10-20 nm depending on the content of P₂O₅. TEM confirmed the existence of an amorphous layer that is homogeneously distributed. The composite exhibits the highest conductivity of 8.8×10⁻² S/cm at 300°C in air for 20% extra P₂O₅ and demonstrates a good stability during the whole measured 110 hours. Polytetrafluoroethylene (PTFE) was introduced into the composites in order to increase malleability for fabrication. The conductivity and mechanical strength were optimized by adjusting the PTFE and P₂O₅ content. These organic-inorganic composites demonstrate much better stability at elevated temperature (250°C) over
conventional SiC-H₃PO₄-PTFE composites which are common electrolytes for phosphoric acid fuel cells (PAFCs). Fuel cells based on BPO₄-H₃PO₄-PTFE composite as the electrolyte were investigated using pure H₂ and methanol as fuels. A maximum power density of 320 mW/cm² at a voltage of 0.31 V and a maximum current density of 1.9 A/cm² at 200°C were observed for H₂/O₂ fuel cells. A maximum power density of 40 mW/cm² and maximum current of 300 mA/cm² 275°C were observed when 3M methanol was used in the cell.
Phosphoric acid was also introduced into materials with internal open structures such as phosphotungstic acid (H₃PW₁₂O₄₀) and heteropolyacid salt ((NH₄)₃PW₁₂O₄₀), for the purpose of acquiring additional connections. The hybrids obtained have a cubic symmetry with enlarged unit cell volume, probably due to the incorporation of phosphoric acid into the internal structures. Solid state ³¹P NMR performed on H₃PW₁₂O₄₀-xH₃PO₄ (x = 0-3) showed additional peaks at high acid content which could not assigned to phosphorus from the starting materials, suggesting a strong interaction between H₃PW₁₂O₄₀ and H₃PO₄. The conductivity of hybrids was improved significantly compared with samples without phosphoric acid. Fourier transform infrared spectra (FT-IR) suggest the existence of large amount of hydrogen bonds (OH••••O) that may responsible for the high conductivity. A H₂/O₂ fuel cell based on H₃PW₁₂O₄₀-H₃PO₄-PTFE exhibited a peak power density of 2.7 mW/cm² at 0.3 V in ambient temperature.
Solid oxide proton conductors based on yttrium doped BaZrO₃ were investigated by introducing potassium or lanthanum at the A-sites. The materials were prepared by different methods and were obtained as a single phase with space group Pm-3m (221). The unit cell of these samples is slightly smaller than the undoped one. The upper limit of solid solution formation on the A-sites for potassium is between 5 ~ 10% as introducing more K results in the occurrence of a second phase or impurities such as YSZ (yttrium stabilized zirconium). K doped Barium zirconates showed an improved water uptake capability even with 5% K doping, whereas for La doped ones, water uptake is strongly dependent on particle size and synthetic history. The conductivity of K doped BaZrO₃ was improved by a factor of two (2×10⁻³ S/cm) at 600°C compared with undoped material. Fuel cells based on Pt/Ba₀.₉₅K₀.₀₅Zr₀.₈₅Y₀.₁₁Zn₀.₀₄O[subscript(3-δ)]/Pt under humidified 5% H₂/air conditions gave a maximum power density 7.7 mWcm⁻² at 718°C and an interfacial resistance 4 Ωcm⁻². While for La doped samples, the conductivity was comparable with undoped ones; the benefits of introducing lanthanum at A-sites may not be so obvious as deficiency of barium is one factor that leads to the diminishing conductivity
Iteratively Decoded Variable Length Space-Time Coded Modulation: Code Construction and Convergence Analysis
No full textAn Iteratively Decoded Variable Length Space Time Coded Modulation (VL-STCM-ID) scheme capable of simultaneously providing both coding and iteration gain as well as multiplexing and diversity gain is proposed. Non-binary unity-rate precoders are employed for assisting the iterative decoding of the VL-STCM-ID scheme. The discrete-valued source symbols are first encoded into variable-length codewords that are mapped to the spatial and temporal domains. Then the variable-length codewords are interleaved and fed to the precoder assisted modulator. More explicitly, the proposed VL-STCM-ID arrangement is a jointly designed iteratively decoded scheme combining source coding, channel coding, modulation as well as spatial diversity/multiplexing. As expected, the higher the source correlation, the higher the achievable performance gain of the scheme becomes. Furthermore, the performance of the VLSTCM-ID scheme is about 14.6 dB better than that of the FixedLength STCM (FL-STCM) benchmarker at a source symbol error ratio of 10-4
PHOTO STIMULATED OXYGEN ATOM TRANSFER WITH MO(DIOXO) COMPLEXES ANCHORED ON TIO2
No full textEn este trabajo se presenta la formación de un sistema verde bioinspirado para la Transferencia de Oxígeno (TAO). Se estudió la TAO a Pf3 y etilbenceno empleando oxígeno molecular y luz UV-Vis, el catalizador consiste en complejos del tipo [Mo(=O)2L], L=bipiridil (bipi), bipirazolil (bpz) anclandos sobre TiO2. La TAO se estimula por una corriente electrónica, generada por acción de radiación UV-visible sobre TiO2, que es transferida a la unidad Mo=O para propiciar la TAO. La contribución del anión superóxido O2-, formado por reacción de los e- fotogenerados con el O2, parece jugar un papel importante en la regeneración del centro activo Mo(=O)2 ya que en ausencia de luz la TAO solo es estequiométrica.Se observó que aunque el complejo bipirazolil es un buen agente en la TAO, debido a su propiedad donadora de electrones, el sistema es pobre en la activación de Oxígeno debido a su labilidad a la lixiviación en medio orgánico, para este efecto se obtiene un mejor balance con el ligando Bipi. Con estos sistemas se abre una pers-pectiva para el desarrollo de procesos verdes para la oxidación selectiva de compuestos orgánicosIn this paper, the formation of an Oxygen Transfer (OAT) green bioinspired system was developed. The OAT to Pφ3 and ethylbenzene, using molecular oxygen and UV-Vis light is reported. The catalyst consists of com-plexes of the type [Mo(=O)2L], L=bipyriyll(bipi), lbipyrazolyl(bpz) bound to TiO2. The OAT is stimulated by an electron current, generated by the action of UV-visible light on TiO2, which is transferred to the Mo = O unit to promote the OAT. The contribution of the superoxide anion O2-, formed by the action of the e-photogenerated with O2, seems to play an important role in regenerating the Mo(= O)2 active center since in the absence of light OAT is stoichiometric. Although the bipirazolyl is a good OAT agent due to its electron donating property, the system is poor in the activation of oxygen due to its organic media leaching lability, with the ligand Bipi a better balance is obtained. These systems offer a good opportunity to develop green processes for selective oxidation of organic compounds
Facile syntheses of l-beta-haloalanine derivatives from l-cysteine or l-cystine
No full textl-beta-Haloalanines are physiologically active unnatural amino acids and they are useful intermediates for the synthesis of natural and unnatural amino acids, S-linked glycopeptides, and lanthionines. In general l-beta-haloalanines were prepared predominantly from l-serine via functional group transformation. Here we reported an alternative approach for the preparation of l-beta-haloalanines via halogenation of protected l-cysteine esters which was obtained from l-cysteine or l-cystine, respectively. The mercapto group of protected l-cysteine esters was efficiently transformed to halo groups by triphenylphosphine/N-halosuccinimides. It has been proved to be a versatile desulfurization strategy via this functional group transformation
The Effect of Emoticons in Simplex and Complex Task-Oriented Communication: An Empirical Study of Instant Messaging
No full textControlling the Chirping of Chorus Waves via Magnetic Field Inhomogeneity
No full textWhistler mode chorus waves are coherent electromagnetic emissions in planetary magnetospheres, characterized by rising-tone or falling-tone chirping elements. Understanding the cause of different chirping directions and their properties is an important step in resolving the long-standing problem of nonlinear chorus generation. We report here, for the first time, particle-in-cell simulations of bidirectional chirping of whistler waves in a uniform magnetic field and falling-tone-only chirping in an inhomogeneous field. Combined with previous simulations of rising-tone-only emissions, we demonstrate that the background magnetic field inhomogeneity is not required for chirping of chorus, but it plays a key role in determining the chirping direction. The findings of the present work also unveil the critical role of the dipole geometry of Earth's magnetic field in causing the statistical predominance of rising-tone chorus and the oblique propagation of falling-tone chorus
The Effect of Emoticons in Simplex and Complex Task-Oriented Communication: An Empirical Study of Instant Messaging
No full textTheoretical and numerical studies of chorus waves: A review
No full textTheoretical and numerical models of chorus waves are reviewed in this paper. Specifically, we focus on the nonlinear wave particle interactions and the current understanding of the frequency chirping of rising tone chorus waves. Various other related topics, such as the optimal excitation condition of chorus, the formation of subpackets, and the non-adiabaticity of the nonlinear interaction are also discussed. We end this review paper with a short list of questions of chorus waves that are still under research and debate
Comparación de tecnologías de oxidación (TAO´s) para la degradación del pesticida Mertect
No full textEl presente trabajo tuvo como objetivo realizar la comparación de tecnologías avanzadas de oxidación (TAO´s) para la degradación del pesticida Mertect. Para el diseño experimental del trabajo se empleó un fotoreactor. La degradación del pesticida se determinó mediante espectrofotometría ultravioleta/visible, método que permitió determinar que los procesos fotoquímicos basados en la fotocatálisis heterogénea con dióxido de titanio – TiO2 y los procesos de fotocatálisis homogénea con hierro (III) permiten obtener porcentajes de remoción significativos del pesticida superiores al 99.0% y al 95.0% respectivamente, lo que permite concluir que los procesos de oxidación avanzados son adecuados para la remoción y degradación del pesticida Mertect.</jats:p
Efficient preparation of L-cysteic acid and its esters
No full textL-Cysteic acid and its esters were prepared in good yields from the oxidation of L-cystine by chlorine in water and in alcohols. When the reaction was carried out in alcohols the corresponding esters were produced
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