Effect of surface stoichiometry on Be doping in GaAs by intermittent supply of AsH3 and TEG in an ultra-high vacuum

The effects of surface stoichiometry on Be doping in GaAs grown by molecular layer epitaxy have experimentally been investigated. Be-doped p⁺-GaAs layers were grown on (0 0 1)-oriented GaAs substrates by intermittent supply of AsH₃ and triethylgallium (TEG) in an ultra-high vacuum. Be(MeCp)₂ was used as a p-type dopant gas. The surface stoichiometry before introducing the dopant gas was controlled by changing the AsH₃ and TEG injection sequence and supply time. The doping characteristics were evaluated by secondary ion mass spectroscopy analysis. It was found that doping characteristics of Be-doped GaAs are strongly dependent on the doping sequence and surface stoichiometry. This experimental result and the Be doping mechanism are discussed on the basis of rate law of the surface chemical reaction.     

Compensation mechanisms in nominally undoped semi-insulating InP and comparison with undoped InP grown under stoichiometry control

Nominally undoped semi-insulating InP can be prepared reproducibly by annealing under controlled phosphorus pressure. We present the electrical properties of a large series of undoped InP samples before and after annealing. Spectroscopic investigations show that the specimens are contaminated by iron during annealing, but native defects have to be taken into consideration in order to explain the electrical data. Annealed InP specimens are characterized by electrical and optical measurements and the results are compared to those obtained from InP crystals grown under stoichiometry control by the horizontal gradient freeze technique.     

Advanced oxidation of cork‐processing wastewater using Fenton's reagent: kinetics and stoichiometry

This work evaluates Fenton oxidation for the removal of organic matter (COD) from cork‐processing wastewater. The experimental variables studied were the dosages of iron salts and hydrogen peroxide. The COD removal ranged from 17% to 79%, depending on the reagent dose, and the stoichiometric reaction coefficient varied from 0.08 to 0.43 g COD (g H₂O₂)^?1 (which implies an efficiency in the use of hydrogen peroxide varying from 17% to 92%). In a study of the process kinetics, based on the initial rates method, the COD elimination rate was maximum when the molar ratio [H₂O₂]_o:[Fe²⁺]_o was equal to 10. Under these experimental conditions, the initial oxidation rate was 50.5 mg COD dm^?3 s^?1 with a rate of consumption of hydrogen peroxide of 140 mg H₂O₂ dm^?3 s^?1, implying an efficiency in the use of the hydrogen peroxide at the initial time of 77%. The total amount of organic matter removed by Fenton oxidation was increased by spreading the H₂O₂ and ferrous salt reagent over several fractions by 15% for two‐fractions and by 21% for three‐fractions. Copyright © 2004 Society of Chemical Industry     

Structural studies on Fe-substituted Y-Ba-Cu-O superconductors by Mössbauer spectroscopy

In this paper we report preparation and structural aspects of YBa₂Cu_2·97Fe_0·03O_7?x with different oxygen stoichiometry. The samples were prepared by standard solid-state reaction. The samples were characterized by X-ray powder diffraction and oxygen contents were estimated from iodometric titration method. The superconducting transition temperatureT _c is measured using the standard four-probe technique. Mössbauer studies of YBa₂Cu_2·97Fe_0·03O_7?x with different oxygen stoichiometry were carried out and the results show that Fe substitutes both Cu(2) and Cu(1) sites and clear asymmetry in the doublets except for the tetragonal case. Also our results show the progressive oxygenation inb-axis as oxygen stoichiometry is increased.     

Unleashing Strain Induced Ferroelectricity in Complex Oxide Thin Films via Precise Stoichiometry Control

Strain tuning has emerged as a powerful means to enhance properties and to induce otherwise unattainable phenomena in complex oxide films. However, by employing strain alone, the predicted properties sometimes fail to emerge. In this work, the critical role of precise stoichiometry control for realizing strain‐induced ferroelectricity in CaTiO₃ films is demonstrated. An adsorption controlled growth window is discovered for CaTiO₃ films grown by hybrid molecular beam epitaxy, which ensures an excellent control over the Ti:Ca atomic percent ratio of <0.8% in the films. Superior ferroelectric and dielectric properties are found for films grown inside the stoichiometric growth window, yielding maximum polarization, dielectric constant, and paraelectric‐to‐ferroelectric transition temperatures. Outside this growth window, properties are severely deteriorated and ultimately suppressed by defects in the films. This study exemplifies the important role of precise compositional control for achieving strain‐induced properties. Untangling the effects of strain and stoichiometry on functional properties will accelerate both fundamental discoveries yet to be made in the vast materials design space of strained complex oxide films, as well as utilization of strain‐stabilized phenomena in future devices.     

Hydrogen production dynamic during cheese whey Dark Fermentation: New insights from modelization

The modelization of non methanogenic anaerobic environments can be particularly challenging owing to the variability of the metabolic products. In particular, both hydrogen production and consumption take place at the same time due to the simultaneous occurrence of Dark Fermentation (DF) and homoacetogenis. The goal of this study is to investigate the kinetic and thermodynamic aspects of the biochemical pathways involved in the fermentation of ultrafiltered cheese whey; to this aim, a continuous digester was operated under three different Hydraulic Retention Times (6, 9 and 12 h) and fixed pH (5.5). A mathematical model, based on a variable stoichiometry approach, was implemented and calibrated; the proposed model allowed the determination of the parameters governing the most relevant pathways, namely homoacetogenesis and butyric and ethanol-type DF.A special focus was given to the quantification of the hydrogen turnover rate; the model proved to be an effective tool, in addition to widely adopted techniques such as microbial and isotopic analysis, for obtaining a deeper comprehension of the crucial aspects governing the non-methanogenic process.     

Cathodoluminescence spectroscopy of deep defect levels at the ZnSe/GaAs interface with a composition-control interface layer

In this work we investigate ZnSe/GaAs heterostructures with an additional 2 nm controlled interfacial layer (CIL) of Se- or Zn-rich composition to modify the band offset. The samples are analyzed as a function of annealing temperature by cathodoluminescence spectroscopy. The as-prepared samples show defect luminescence at ∼ 0.9 eV. With staged annealing at increasing temperatures, both the Zn-rich as well as the Se-rich interfacial layer exhibits luminescence at ∼ 1.9 eV, indicative of defect formation with an onset temperature of ∼400°C. Excitation-dependent spectroscopy provides evidence for defect formation near the interface, which extends into the ZnSe epilayer at higher temperatures. Compared to earlier work, where the threshold temperature for defect formation in bulk samples fabricated under Se-rich growth conditions occurs at temperatures as low as 325°C, the resistance to defect formation has now been improved to that of stoichiometric ZnSe. These results demonstrate that epitaxially grown CILs provide a means to alter ZnSe/GaAs band offsets without degrading the heterojunction’s resistance to defect formation at elevated temperatures.     

Broadening of molecular‐weight distribution in solid‐state polymerization resulting from condensate diffusion

A kinetic model for the solid‐state polymerization of poly(bisphenol A carbonate) in a single particle has been developed and used to investigate the broadening of molecular‐weight distribution as a result of slow condensate diffusion. The model is based on melt‐phase transesterification kinetics and Fickian diffusion of phenol, the condensate, in the amorphous regions of the semicrystalline particle. Model predictions compare favorably to experimental data. When diffusion is slow compared to reaction, a condensate concentration gradient is established. This gradient induces a molecular‐weight gradient, which results in a broadened overall molecular‐weight distribution with an overall polydispersity above the theoretical limit for homogenous step‐growth polymerization. As the mass transfer resistance inside the particle is decreased, the average molecular weight increases faster with time, and the overall polydispersity decreases. A stoichiometric imbalance of end groups decreases the obtainable molecular weight but mitigates the deleterious effects of slow condensate diffusion. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 928–943, 2001     

Robust control of internal states in a polymer electrolyte membrane fuel cell air-feed system by considering actuator properties

Air stoichiometry, pressure, and relative humidity in the air-feed system of a vehicular polymer electrolyte membrane fuel cell (PEMFC) influence efficiency, durability and reliability. It is critical to develop robust control algorithms for these internal states to improve system performance. There is limited extant research on designing robust control algorithms that consider the three internal states as well as the constraints of real actuators, such as an air compressor, a membrane humidifier, and a back-up pressure valve (BPV). This study examines robust control strategies for the three internal states based on adaptive second order sliding mode (ASOSM) and nonlinear proportional-integral (NPI) feedback control algorithms. In the study, control targets are established based on stable properties of the PEMFC system. The study involves proposing and comparing five control strategies that are a combination of NPI and ASOSM algorithms. The following results are obtained: (1) the stable control targets for the three internal states are followed adequately by using an NPI or an ASOSM algorithm and differences only exists in dynamic processes; (2) with respect to the control of air stoichiometry, an NPI algorithm performs better than an ASOSM algorithm as chattering in air stoichiometry can be avoided and the convergence time to the target value is acceptable; (3) with respect to the control of cathodic pressure, an ASOSM algorithm performs better than an NPI algorithm as the overshoots in cathodic pressures can be effectively reduced; (4) with respect to the control of relative humidity, both NPI and ASOSM algorithms lead to a practical bang–bang strategy. The strategy that performs the best among the five strategies is selected, and the robustness of the selected strategy with respect to parameter uncertainties is verified.     

基于指纹图谱和化学计量学的白芍产地溯源和质量评价

目的 建立白芍高效液相色谱(high performance liquid chromatography, HPLC)指纹图谱,并结合化学计量学进行分析与评价。方法 采用HPLC法测定31批不同产地白芍中没食子酸、儿茶素、氧化芍药苷、芍药内酯苷、芍药苷、五没食子酰葡萄糖、苯甲酰芍药苷和丹皮酚8种成分含量,并用中药指纹图谱相似度评价系统进行相似度评价。结合聚类分析、主成分分析和正交偏最小二乘法分析化学计量学方法,对其进行质量评价。结果 所建立的白芍HPLC指纹图谱共标定了11个色谱峰,经对照品比对鉴定出8个色谱峰并进行含量测定;相似度和聚类分析结果显示,不同产地白芍质量存在差异;含量测定结果显示不同产地白芍中8个成分种类并无差异,但含量差异较大,其中山西产地芍药苷、芍药内酯苷平均含量较高,四川产地五没食子酰葡萄糖和丹皮酚平均含量较低;进一步用主成分分析进行综合评价,发现山西和浙江产地白芍质量较优;利用正交偏最小二乘法分析得出白芍总苷、芍药内酯苷和芍药苷为主要差异成分。结论 本研究建立的白芍HPLC指纹图谱结合化学计量学的方法能够快速、科学、准确地应用到白芍的产地溯源和质量评价中。