Reaction stoichiometry between TiCl3 and NaAlH4 in Ti-doped alanate for hydrogen storage: The fate of the titanium species

In order to understand the final state of the TiCl₃ dopant during the dehydrogenation and rehydrogenation cycles of NaAlH₄, we determined the reaction stoichiometry between TiCl₃ and NaAlH₄ by measuring the amount of hydrogen evolution from NaAlH₄ with the varying TiCl₃ -load. We found that: (i) TiCl₃ reacted with 3 M equivalents of NaAlH₄ during the doping process of ball-milling, (ii) the Ti dopant continued to react with NaAlH₄ during the first dehydrogenation process until total six equivalents of NaAlH₄ were consumed, and (iii) Ti fixed Al, not NaH, so that Al became insufficient during the rehydrogenation process. These findings lead to the conclusion that the reaction stoichiometry between Ti and Al is 1:6, which probably yields TiAl₆ and plays a catalytic role in the hydrogen storage reactions of Ti-doped NaAlH₄.     

Effect of composition of starting materials of Mo–Si on self-propagating high temperature SHS reaction

Abstract

An experimental study of the self-propagating high temperature synthesis of Mo–Si alloys was conducted from elemental powder compacts. Test specimens with seven compositions, including Mo/Si?=?1∶1·25, 1∶1·50, 1∶1·75, 1∶2·00, 1∶2·25, 1∶2·50 and 1∶2·75 respectively, were employed. Experimental evidence showed that a combustion wave featuring a spinning reaction zone can be observed. When the powder compacts are from Mo∶1·25Si to Mo∶1·75Si, the combustion temperature and the propagation velocity of combustion wave increase with increasing silicon in the sample, and the combustion products are composed of MoSi₂, Mo₅Si₃ and Mo. However, when the powder compacts are from Mo∶2·25Si to Mo∶2·75Si, the combustion temperature and the propagation velocity decrease rapidly as the silicon in the compact increases, and the combustion products are composed of MoSi₂ and Si. The sample with Mo/Si?=?1∶2·00 possesses the highest combustion temperature (1628·9 K) and propagation velocity (3·13 mm s^?1). A single-phase MoSi₂ is synthesised by the Mo/Si?=?1∶2·00 sample.     

Stoichiometry of Fully Oxidized PuO2-NdO1.5 and PuO2-YO1.5 Solid Solutions

A new method has been established to calculate sensitivity coefficients of cell parameters based on generalized perturbation theory using the collision probability method. The proposed method does not require the calculation of the changes of collision probabilities due to cross section changes, so it is as powerful as the commonly used generalized perturbation theory in diffusion theory, We demonstrate the validity of the method by comparing the calculated sensitivity coefficients with those obtained from the direct cell calculations. AS an application, we calculate the sensitivity coefficients of neutronic properties in cells with different moderator to fuel volume ratios, and discuss the physical meaning of the difference between the sensitivity coefficients.     

Influence of Growth Conditions on Magnetite Nanoparticles Electro-Crystallized in the Presence of Organic Molecules

Magnetite nanoparticles were synthesized by electrocrystallization in the presence of thiourea or sodium butanoate as an organic stabilizer. The synthesis was performed in a thermostatic electrochemical cell containing two iron electrodes with an aqueous solution of sodium sulfate as electrolyte. The effects of organic concentration, applied potential and growth temperature on particle size, morphology, structure and magnetic properties were investigated. The magnetite nanoparticles were characterized by X-ray diffraction, electron microscopy, magnetometry and Mössbauer spectrometry. When the synthesis is performed in the presence of sodium butanoate at 60 °C, a paramagnetic ferric salt is obtained as a second phase; it is possible to avoid formation of this phase, increase the specific magnetization and improve the structure of the oxide particles by tuning the growth conditions. Room-temperature magnetization values range from 45 to 90 Am²kg⁻¹, depending on the particle size, type of surfactant and synthesis conditions. Mössbauer spectra, which were recorded at 290 K for all the samples, are typical of nonstoichiometric Fe_3−δO₄, with a small excess of Fe³⁺, 0.05 ≤ δ ≤ 0.15.     

High quality lead zirconate titanate films grown by organometallic chemical vapour deposition

Abstract

Organometallic chemical vapour deposition is a suitable technique for the deposition of thin films of oxidic compounds such as lead zirconate titanate, PbZr_xTi_1?xO₃. Above a deposition temperature of about 600°C stoichiometric PbZr_xTi_1?xO₃ films can be grown on platinized silicon wafers within a large process window, independent of the precursor partial pressures and the deposition temperature. This is the result of a self-regulating mechanism. The PbZr_xTi_1?xO₃ films have excellent ferroelectric properties exhibiting high values, up to 60μC/cm², for the remanent polarisation. The value of the coercive field strength varies between 50 and 180 kV/cm, dependent on the composition. Layers with comparable properties can also be grown at lower temperatures, down to 500°C. In this case careful control of the gas-phase composition is required to obtain films with the correct stoichiometry.     

In-situ monitoring of surface stoichiometry and growth kinetics study of GaN (0001) in MOVPE by spectroscopic ellipsometry

GaN surface stoichiometry and growth kinetics in MOVPE were studied by in-situ spectroscopic ellipsometry. The effect of MOVPE conditions on both the surface stoichiometry and growth kinetics was investigated. The surface stoichiometry, such as N-rich, Ga-rich and Ga-excess surfaces, was monitored, and was drastically changed by the variation of the NH₃ partial pressure. When the TMG supply was interrupted during the growth, the layer-by-layer decomposition/revaporation was observed in H₂/NH₃ ambient. The decomposition rate was measured as a function of the NH₃ flow rate at the conventional epilayer growth temperatures (1050–1140 C). The decomposition rate was decreased with the increase in the N coverage on the GaN surface. it was found that the surface stoichiometry is a very important parameter for the control of the MOVPE growth kinetics.     

RF磁控溅射制备钛酸锶钡BST纳米薄膜

钛酸锶钡(BST)薄膜作为一种高K介质材料在微电子和微机电系统等领域具有广阔的应用前景,人们已对BST薄膜的制备工艺技术和介电性能进行了大量的研究。BST纳米薄膜的制备工艺直接影响和决定着薄膜的介电性能(介电常数、漏电流密度、介电强度等)。对RF磁控反应溅射制备BST纳米薄膜的工艺技术进行了综述。从溅射靶的制备、溅射工艺参数的优化、热处理、薄膜组分的控制,及制备工艺对介电性能的影响等方面,对现有研究成果进行了较全面的总结。     

X射线三轴晶衍射法测量半绝缘GaAs单晶的化学配比

采用X射线三轴晶衍射法,根据As间隙原子对作为过量As在GaAs单晶材料中存在的主要形式的模型,可以无损、高精度测量半绝缘GaAs单晶的化学配比.并探讨了引起晶格变化的原因及其与熔体组分的关系,对于制备高质量GaAs单晶及其光电器件具有重要的意义.     

Systematic study of back pressure and anode stoichiometry effects on spatial PEMFC performance distribution

A segmented cell system was applied to investigate the effects of the anode and cathode back pressure and hydrogen stoichiometry on fuel cell performance in terms of overpotential distributions along the flow field. The segmented cell system was designed with closed loop Hall sensors and a data acquisition system allowing simultaneous spatial electrochemical impedance spectra (EIS) measurements. It was determined that an increase in back pressure for the tested serpentine flow field design results in an improvement of the cell performance and uneven improvement of individual segments’ performance. In general, the performance and the overpotentials become more uniform downstream with an increase in the back pressure due to a decrease in activation and mass transfer losses. Spatial EIS data for the PEMFC operated at different back pressures support the overpotential analysis. Hydrogen stoichiometry variations do not affect the performance of the cell or the individual segments at low current density because there is no significant hydrogen concentration gradient in the flow field. However, at high current densities a reduction in hydrogen stoichiometry produces a slight decrease in performance for inlet segments while outlet segments showed a noticeable performance loss. The decrease in performance is attributed to an increase in mass transfer losses due to nitrogen diffusion from the cathode to the anode. This effect becomes more pronounced for the outlet segments due to a downstream nitrogen accumulation. Under high current density conditions, the cell is locally fuel starved even with a high fuel stoichiometry creating conditions leading to cell degradation by carbon corrosion. More importantly, this local degradation is masked by the overall cell performance which remains largely unaffected.     

A Direct Comparison of the Fragmentation Test and the Microbond Pull-out Test for Determining the Interfacial Shear Strength

Experiments were conducted to compare the fragmentation test with the microbond pull-out test for determining the interfacial shear strength between carbon AS4 fibers and a thermoset matrix consisting of a Di-Glycidyl Ether of Bisphenol A (DGEBA) resin cured with a diamine (meta-phenylenediamine, m-PDA) curing agent. The results indicate that, for the microbond test, diffusion of the rather volatile m-PDA curing agent at early stages of cure leads to low values of interfacial shear strengths when compared with results obtained for the same system with the fragmentation test.

With the microbond test, a distinct relationship between the glass transition temperature of the droplets and their size is noticed. Smaller (< 150 μm) droplets have very low Tg's and are incompletely cured. While changing to a modified curing cycle and/or using a m-PDA-rich curing environment alleviates the diffusion problem, the interfacial shear strength values are still not in good agreement with the fragmentation test results. Microbond data from another system consisting of DGEBA resin cured with a different, less volatile dimaine curing agent indicates that diffusion of the curing agent becomes less severe as the volatility of the curing agent decreases and the corresponding microbond interfacial shear strengths agree better with fragmentation test results.