Vapor-Phase Synthesis and Surface Passivation of ZnSe Nanocrystals

The vapor-phase synthesis of II–VI nanocrystals (quantum dots) has several advantages over traditional liquid-phase techniques, including better compatibility with existing operations in the microelectronics industry. On the other hand, it makes size control and surface passivation a more challenging task. In this paper we report the synthesis and surface passivation of luminescent ZnSe nanocrystals by reacting vapors of dimethylzinc:triethylamine (DMZ–TEA) adduct with hydrogen selenide gas (both diluted in hydrogen) in a counterflow jet reactor operating at 300 K and low pressures (120--60 Torr). The two reactants flow into a cylindrical chamber from separate inlets of a vertical opposed-flow configuration. Homogeneous nucleation of ZnSe occurs through an irreversible reaction between the precursors. The nuclei subsequently grow by surface reactions and cluster--luster coalescence to form nanocrystals that exhibit size-dependent luminescence. Surface passivation was achieved by introducing vapors of 1-pentanethiol into the reactor. The thiol vapors do not interfere with the nucleation and growth of the primary nanocrystals, but significantly change the morphology and size of the final coagulated particles collected on transmission electron microscopy (TEM) grids. The chemisorption of thiol vapors on the surface of the nanocrystals also leads to smaller degradation of the photoluminescence (PL) intensity after 1 month. Structural and morphological characterization of the particles was performed using HR-TEM, stoichiometric analysis using electron diffraction and Auger electron spectroscopy (AES), while PL and Raman spectroscopy were used to study their size-dependent optical properties.     

SLURRY FISCHER-TROPSCH/ZSM-5 TWO-STAGE PROCESS OF CONVERTING SYNGAS TO GASOLINE Ⅱ. REACTOR-WAX WITHDRAWAL AND LONG-TERM TEST

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反应堆内中子注量率新型探测器的理论研究

利用核泵浦激光强度与入射中子注量率的相关性,探讨了反应堆活性区核泵浦激光中子探测器的理论可行性.从核泵浦激光机理出发,对该中子探测系统——核泵浦激光3He?Ar?Xe气体体系的能量沉积密度和激光的本征效率进行了深入的理论研究.提出了较完整的理论模型,讨论了该系统对中子注量率的响应函数及其检测灵敏度随工作时间的变化情况,论证了该中子探测系统的理论可行性.     

Characterisation of an exothermic reaction using adiabatic and isothermal calorimetry

A simple esterification reaction is used to demonstrate standard procedures for determining the thermokinetic parameters of an exothermic reaction from adiabatic calorimetric data. The influence of variations in the heat capacity of the sample due to changes in temperature and concentration is explored. Shortcomings in the simple interpretation of adiabatic data are identified and isothermal heatflow calorimetry is used to reveal autocatalytic effects which were not apparent from the adiabatic experiments. A more rigourous interpretation of the adiabatic and isothermal data is outlined and used to predict the conditions which can lead to exothermic runaway in a batch reactor. Mathematical simulation of the conditions in a jacketed reactor is used to demonstrate the importance of developing reliable kinetic expressions before assessing the safety of a batch process.

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Zusammenfassung Anhand einer einfachen Veresterungsreaktion wurden Standardverfahren zur Ermittlung thermokinetischer Parameter exothermen Reaktionen aus adiabatischen kalorimetrischen Daten demonstriert. Dabei wurde der Einfluß von Temperatur und Konzentration auf Änderungen der Wärmekapazität untersucht. Fehler bei der einfachen Interpretation adiabatischer Daten wurden identifiziert und isotherme Wärmeflußkalorimetrie wurde angewendet, um autokatalytische Effekte aufzuzeigen, die sich anhand der adiabatischen Experimente nicht ersehen lassen. Es wurde eine gründlichere Interpretation adiabatischer und isothermer Daten umrissen und verwendet, um die Bedingungen vorherzusagen, die in einem Kesselreaktor zu einem exothermen Davonlaufen der Reaktion führen. Mathematische Simulation der Bedingungen in einem Mantelkessel wurde angewendet, um zu zeigen, von welch großer Bedeutung die Entwicklung zuverlässiger kinetischer Ausdrücke ist, bevor man die Sicherheit einer Reaktion in einem Kesselreaktor beurteilt.

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An investigation into the use of silica-supported bases within EOF-based flow reactors

Using a series of silica-supported bases, we demonstrate the synthesis of eight condensation products within an EOF-based flow reactor; in all cases, high yields (>99%) and product purity are obtained.     

Asymmetric catalysis in a micro reactor—Ce, Yb and Lu catalysed enantioselective addition of trimethylsilyl cyanide to benzaldehyde

A T-shaped micro reactor was used for the optimisation of reaction conditions for the enantioselective silylcyanation of benzaldehyde catalysed by lanthanide-pybox complexes. Compared to a conventional batch procedure, higher conversion was observed within shorter reaction time. The micro reactor process involving Lu(III) afforded essentially the same enantioselectivity as the batch process (73 vs 76% ee), whereas the enantioselectivity was lower in the micro reactor for catalysts containing Yb(III) (53 compared to 72%). Ce(III) provided very low selectivity in both types of processes (1 and 11% ee, respectively). A study of the effect of additives showed that the enantioselectivity in the Yb catalysed reaction performed in the micro reactor could be increased to 66%, whereas only a minor improvement, to 78% ee, was observed in the reaction with Lu.     

Continuous fast pyrolysis of biomass at high temperature in a fluidized bed reactor

In order to perform biomass pyrolysis a continuous fluidized bed reactor (bench scale unit) has been assembled. The influence of experimental conditions such as heating-up time and optimum particle diameter is presented. By feeding the biomass (almond shells) directly into the bubbling bed, pyrolysis has been performed at temperatures ranging from 730° to 930°C at constant feed rate (44 g/h). Remarkable increase of H₂ in the product gas is observed when steam rather than an inert carrier such as nitrogen is used thus confirming the potential of this approach.The support of ENEA (Italian Agency for Renewable Energies) is gratefully acknowledged.     

Ethanol production by yeast cells immobilized in open-pore agar

An open-pore agar matrix has been shown to be suitable for the entrapment of microbial whole cells required for use in reactions that involve cell growth and gas evolution. Beads of porous agar with entrapped yeast cells have been used for the continuous fermentation of sugar cane molasses to ethanol, without apparent bead rupture, even after periods of 3 mo of use. The agar gel does not erode during prolonged operation, unlike porous gelatin cross-linked with glutaraldehyde.     

Development and comparison of spectral methods for passive acoustic anomaly detection in nuclear power plants

We have developed spectral signal processing methods for passive acoustic anomaly detection in nuclear power plants. Furthermore, we compared the developed and existing methods by applying them to stationary sounds recorded in a controlled environment. Our new methods show significant improvement, in particular concerning robustness against false alarms. The results also demonstrate that clear detection of a given sound at a given signal-to-noise ratio is highly dependent on the distribution of characteristic frequency content in the spectrum in relation to the background noise and the spectral uncertainty. Since the frequency monitoring principle used here is quite rigid, we stress the need for research on more flexible methods, also taking into account differences between experiments and real reactor systems.     

Multi-objective optimization of oxidative desulfurization in a sono-photochemical airlift reactor

Response surface methodology (RSM) was employed to optimize ultrasound/ultraviolet-assisted oxidative desulfurization in an airlift reactor. Ultrasonic waves were incorporated in a novel-geometry reactor to investigate the synergistic effects of sono-chemistry and enhanced gas-liquid mass transfer. Non-hydrotreated kerosene containing sulfur and aromatic compounds was chosen as a case study. Experimental runs were conducted based on a face-centered central composite design and analyzed using RSM. The effects of two categorical factors, i.e., ultrasound and ultraviolet irradiation and two numerical factors, i.e., superficial gas velocity and oxidation time were investigated on two responses, i.e., desulfurization and de-aromatization yields. Two-factor interaction (2FI) polynomial model was developed for the responses and the desirability function associate with overlay graphs was applied to find optimum conditions. The results showed enhancement in desulfurization ability corresponds to more reduction in aromatic content of kerosene in each combination. Based on desirability approach and certain criteria considered for desulfurization/de-aromatization, the optimal desulfurization and de-aromatization yields of 91.7% and 48% were obtained in US/UV/O₃/H₂O₂ combination, respectively.