The effect of segment length on some properties of thermoplastic poly(ester–siloxane)s

A series of novel thermoplastic elastomers, based on poly(dimethylsiloxane) (PDMS) as the soft segment and poly(butylene terephthalate) (PBT) as the hard segment, were synthesized by catalyzed two‐step, melt transesterification reactions of dimethyl terephthalate and methyl esters of carboxypropyl‐terminated poly(dimethylsiloxane)s (M?_n = 550–2170 g mol^?1) with 1,4‐butanediol. The lengths of both the hard and soft segments were varied while the weight ratio of the hard to soft segments in the reaction mixture was maintained constant (57/43). The molecular structure, composition and molecular weights of the poly(ester–siloxane)s were examined by ¹H NMR spectroscopy. The effectiveness of the incorporation of the methyl‐ester‐terminated poly(dimethylsiloxane)s into the copolymer chains was verified by chloroform extraction. The effect of the segment length on the transition temperatures (T_m and T_g) and the thermal and thermo‐oxidative degradation stability, as well as the degree of crystallinity and hardness properties of the synthesized TPESs, were studied. Copyright © 2003 Society of Chemical Industry     

Functionalized oligopolysiloxanes by heterogeneously catalyzed equilibration polymerization reactions

Summary A new general procedure for preparation of functionalized oligopolysiloxanes of predetermined molecular weight is described. It utilizes heterogeneously catalyzed siloxane equilibration polymerization reactions which do not require troublesome and sometimes difficult post-preparative work-up procedures usually encountered with the well known homogeneously catalyzed corresponding reactions. The method is described using as example the preparation of , -telechelic vinyldimethylsiloxy-oligopolydimethylsiloxanes from octamethylcyclotetrasiloxane and 1,3-divinyltetramethyldisiloxane, but reference to the preparations of trimethylsiloxy-, dimethylsiloxy-and carboxypropyldimethylsiloxyoligopolydimethylsiloxanes, oligopolymethylhydridosiloxanes or their copolymers is also made.     

On the critical molecular chain length of polydimethylsiloxane

Because a study of the results reported for the chain dimensions of polydimethylsiloxane (PDMS), critical for the onset of this polymer's non-Newtonian flow behavior, obtained from the viscosity-molecular weight relationships available from the literature, clearly revealed that considerable differences exist between the reported data, a detailed analysis of these data was performed together with an additional examination of 10 new PDMS samples that were selected so as to have molecular weights that would fill the gaps observed in the polymer viscosity–chain length relationship constructed from the accepted literature data. The results obtained were analyzed by using several different procedures integrated into a recently described comparative method that could allow for determination of what is called the most realistic critical value, Z_wc. The latter was determined as 930 PDMS main-chain atoms, which corresponds to this polymer's degree of polymerization of 464.5 and the weight-average molecular weight of 34,500. It is not only shown that after elimination of some clearly erroneous data points from the previously reported relationships the obtained critical chain-length values could very well fit the earlier relationships, but also that appropriate “master” relationships were constructed including 48 pairs of the old and 10 pairs of the new data points. It is suggested that this relationship be accepted as the best-fit viscosity–polymer chain-length dependence for PDMS, and it is pointed out that the obtained PDMS critical chain-length value ranks this polymer's macromolecules as the most flexible of the corresponding long-chain molecules presently known. © 1993 John Wiley & Sons, Inc.     

Transient studies on methane oxidative coupling over alkali-metal promoted titanate catalysts

A transient method to study the role of the different oxygen species involved in oxidative coupling of methane is discussed. The method is applied to two new active catalysts for this process, which are based on alkali metal loaded titanates of lanthanum and nickel and also to a lithium/titania catalyst used in previous works. The results show that the participation of lattice oxygen in the formation of C2 products depends on the Li loading and the type of titanate used.     

Ecological efficiency and thermoeconomic analysis of a cogeneration system at a hospital

This work aims with an approach for cogeneration plants evaluation based on thermoeconomic functional diagram analysis. The second law of thermodynamics is used to develop a methodology to analyse cogeneration systems, based on exergoeconomics evaluation. The thermoeconomic optimisation method developed is applied to allow a better configuration of the cogeneration plant associated to a university hospital. Also ecological efficiency is evaluated. The method was efficient and contributes for thermoeconomics modelling and analysis and can be applied to any sort of thermal system, especially those with combined heat and power in thermal parity.     

The impact of alkali elements on the degradation of CIGS solar cells

Unencapsulated CIGS solar cells with high and low contents of sodium (Na) and potassium (K) were simultaneously exposed to damp heat and illumination. The solar cells with a high alkali (Na, K) content exhibited higher initial conversion efficiencies, but degraded severely within 100 h, while the alkali poor samples kept relatively stable performance under damp heat and illumination. The degradation of the samples with a high alkali content resulted in the formation of sodium rich spots on the top ZnO:Al surface of the samples. This is likely caused by light‐induced Na⁺ migration via the grain boundaries in the absorber to the depletion region, where the Na⁺ accumulated. This allowed subsequent Na⁺ transport through the depletion region due to the lowering of the internal electric field caused both by the Na⁺ accumulation and illumination. The migration resulted in the formation of shunt paths, which reduced the shunt resistance and open circuit voltage. Furthermore, ingression of water into the ZnO:Al is expected to be responsible for a slow but steady increase in series resistance for both high and low alkali solar cells. Additionally, sodium migration led to a severe increase of the series resistance in case of alkali rich samples. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of a‐SiOx:H solar cells with very high Voc × FF product

In this study, deposition conditions for making a‐SiO_x:H are investigated systematically in order to obtain a high band gap material. We found that at given optical band gap, a‐SiO_x:H with favorable opto‐electronic properties can be obtained when deposited using low CO₂ flow rates and deposition pressures. We also found that a low radio frequency power density is required in order to limit the effect of ion bombardment on the material properties of i‐a‐SiO_x:H and thereby the solar cell performance. In addition, by decreasing the heater temperature from 300 to 200°C when making the i‐a‐SiO_x:H, the V_oc can be increased. We employed optimized p‐doped and n‐doped a‐SiO_x:H films into the p‐i‐n solar cells, and as a consequence, a high V_oc of over 1 V and high fill factor (FF) are obtained. When depositing on texture‐etched ZnO:Al substrates, a high efficiency a‐SiO_x:H single junction solar cell having a high V_oc × FF product of 0.761 (V_oc: 1.042 V, J_sc: 10.3 mA/cm², FF: 0.73, efficiency: 7.83%) was obtained. The a‐SiO_x:H solar cell shows comparable light degradation characteristics to standard a‐Si:H solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Reconfigurable digital controller for a buck converter based on FPGA

This paper presents a complete digitally controlled dc–dc buck converter performed by FPGA circuitry. All tasks, analog to digital conversion, control algorithm and pulse width modulation, were implemented in the FPGA. This approach enables high-speed dynamic response and programmability by the controller, without external passive components. In addition, the controller’s structure can be easily changed without external components. The applied algorithm enables a switching frequency of 100 kHz.     

Improved frequency characteristics of the randomized PWM boost rectifier

A randomized pulse width modulation (RPWM) algorithm is applied in the control unit of the boost rectifier to achieve improved frequency characteristics in the wide band. First, the introduction of the RPWM switching is reflected in a smaller increase of the total harmonic distortion (THD) factor in the input current. Nevertheless, decrease of the power factor is negligibly small. Second, the power spectrum density (PSD) of the input current is estimated and measured to evaluate the influence of randomization in the high-frequency range. This approach offers an effective and credible prediction method for reduction of conductive electromagnetic interference (EMI) by using the RPWM switching.     

Kinetics, acid sites and deactivation of H-mordenite during the SCR of NOx with CH4

H-mordenites are active for the SCR reaction but they suffer irreversible partial deactivation after being on stream for one hour at 650°C. The reaction orders and activation energies are not significantly affected by deactivation. This indicates that deactivation originates in a decrease in the number of active sites due to dealumination and possible pore blockage. The NO disappearance rate correlates with TPD NH₃ between 300 and 700°C; FTIR confirms these results. ¹²⁹Xe NMR of adsorbed xenon shows that pore blockage occurs and is due to the presence of aluminum species in the main zeolite channels. The overall deactivation process and the role of acid sites is discussed in terms of the current literature. This revised version was published online in July 2006 with corrections to the Cover Date.