美国经济周期及其对教育政策影响评价

本文简要地回顾了美国经济发展的几个典型时期,分析了不同经济时期的经济状况和相应的教育政策,在此基础探讨了具有一定普遍性的经济状况与教育政策的关系,即经济运行情况最终将会直接影响教育政策变革重点,使教育政策蒙上与经济状况相应的印记.     

农药残留降解技术研究现状与展望

本文对现有农药残留降解技术进行了总结与展望,以期为降低茶叶农药残留提供参考。     

云南有色金属勘查开发技术的发展及展望

从地质勘探、采矿、选矿、冶炼4个方面叙述和分析了云南省有色金属技术发展的现状,提出了技术发展的指导思想、发展思路、发展重点及方向。     

A photochemical method for the preparation of indium oxide and indium-cobalt oxides thin films

Indium oxide and indium-cobalt oxide thin films have been successfully prepared by direct UV irradiation of amorphous films of β-diketonate complexes on Si(1 0 0) substrates. Deposited films were characterized by X-ray diffraction, Auger electron spectroscopy and X-ray photoelectron spectroscopy. The surface morphology of the films, examined by atomic force microscopy and scanning electron microscopy, revealed that mixed indium-cobalt oxide films are much smoother than In₂O₃ films, with rms surface roughness of 7.24 and 26.1 nm, respectively.     

Thermal oxidation of InAs and characterization of the oxide film

The thermal oxidation of InAs and the properties of the resultant oxide films were studied. Uniform InAs oxide films with a resistivity of 10¹² Ω cm were obtained on InAs substrates by low temperature oxidation at around 400°C. The thermal oxide films are mainly composed of polycrystalline In₂O₃ and As₂O₃. Increasing oxidation temperature causes thermal decomposition of the As₂O₃ and the accumulation of elemental arsenic in the oxide film. The oxide resistivity decreases with increasing oxidation temperature, mainly as a result of the thermal decomposition of the As₂O₃. The interface state densities near the midgap of InAs are (2–5) × 10¹¹ cm^-2 eV^-1. A metamorphic layer is formed beneath the thermal oxide of InAs by high temperature oxidation above 500°C.     

A study on the prediction of mechanical properties of titanium alloy based on adaptive fuzzy-neural network

An important trend in material research is to predict mechanical properties for a new titanium alloy before committing experimental resources. Often the prediction of mechanical properties of these alloys changes depending on their chemical composition and processing methods. Therefore, modeling the relationship between composition and property is crucial to the engineering. This study employs an adaptive fuzzy-neural network approach to predict the mechanical properties of titanium alloys. In adaptive fuzzy-neural network, to reduce the complexity of fuzzy models while keeping good model accuracy, a fuzzy clustering algorithm and a back-propagation learning algorithm are introduced to improve the accuracy of the simple model. For purpose of constructing this model, experimental results for 57 specimens with 14 different chemical compositions were gathered from the literature. The chemical composition contents were employed as the inputs while yield strength, tensile strength, elongation and reduction of area, which were employed as the outputs. Thus, the model can be trained by using the prepared training set. After training process, the testing data were used to verify model accuracy. It is found that there is insignificant difference between predict results and experimental value and the maximum relative error is less than 9%. It proved that the predictive performance of the clustering-based adaptive fuzzy-neural network modeling is available and effective in simulating the composition content and predicting the mechanical properties of titanium alloys.     

Predictive equations of the tensile properties based on alloy hardness and microstructure for an A356 gravity die cast cylinder head

One of the main problems in the design of complex Al–Si cast components is the wide variety of mechanical properties in different regions of the castings which is due to the wide range of solidification microstructures, related to the local solidification conditions. There are many papers available on the widely used A356/A357 Al–Si–Mg alloys, however, most experimental data on their tensile or fatigue properties are generally obtained from specimens cast separately or produced under controlled laboratory conditions, that are extremely different from those of industrially cast components. Moreover, most of these data often relate the mechanical properties to only one microstructural parameter, such as solidification defects or secondary dendrite arm spacing, and do not take their simultaneous effect into consideration. For all these reasons, the main problem, in the design phase, is the lack of knowledge of the true local mechanical properties in complex-shaped castings, which often means a conservative approach is necessary, with a consequent increase in thickness and therefore in weight. The aim of this research was to study a complex A356 gravity die cast cylinder head, in order to verify the range of variability of the main microstructural parameters and tensile properties, using specimens directly machined from the casting. The component was heat treated at the T6 condition, and the effect of the delay between quenching and aging on the alloy hardness was also evaluated. Simple experimental equations have been proposed, able to successfully predict the local tensile properties in the casting, when only the most important microstructural parameters and alloy hardness are known. These equations allow the designer to predict the local tensile behaviour without any tensile tests; moreover they can also link the post-processing results of the casting simulation software to the pre-processing phase of the structural ones, with an approach of co-engineered design.     

A facile and novel way for the synthesis of nearly monodisperse silver nanoparticles

A facile and novel way was reported for the preparation of nearly monodisperse silver nanoparticles with controlled hydrophilic or hydrophobic surface, using trioctylphosphine as the surfactant and stabilizer. The synthesized nanoparticles were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and UV-vis spectroscopy. The monodisperse silver nanoparticles showed a strong surface plasmon resonance band at 402 nm from the UV-vis spectrum.     

A complex approach to the development of the method and equipment for thermal nondestructive testing of CFRP cylindrical parts

Composite materials are being increasingly used in high-tech industries, such as aerospace, automotive manufacture and building inspection. Thermal nondestructive testing (TNDT) has become an accepted method for composite inspection. However, the majority of investigations have dealt with flat or slightly-curved composite components with a thickness of up to 5 mm. Particular studies have been devoted either to NDT modeling with an emphasis on some theoretical issues, or they have been based exclusively on experimental results. There has been some recent interest in the use of composite materials in the nuclear industry. Some critical parts, including centrifuge components, have been made of carbon fiber reinforced polymer (CFRP) composites. The working conditions in a centrifuge include radioactivity and high rotational speed, and the composites used in centrifuges must have very uniform thermal properties and must be free of defects.This paper describes a complex approach to the TNDT of cylindrical parts made of CFRP by starting from thermal properties measurement, theoretical modeling and preliminary experiments, and finishing with the technical requirements for the development of practical equipment capable of operating in both laboratory and industrial conditions.The objects tested were CFRP cylinders with a diameter of 150 mm and a wall thickness of 4–6 mm, and they contained some artificial defects of varying size and depth. Both one- and two-sided test procedures have been analyzed for spot, line and uniform heating. Ultrasonic excitation has also been used as an alternative stimulation technique.In a one-sided test, the depth detection limit has been about 4 mm. Similar results have been observed in the case of ultrasonic stimulation, but the practical implementation of ultrasonic IR thermography to the inspection of cylindrical parts requires further exploration.In a two-sided test, even fairly mild heating resulted in the reliable detection of all defects independent of their size and depth.In all test cases, the highest signal-to-noise ratio occurred after applying the technique of principal component analysis.     

Projection and Contraction Method for the Valuation of American Options

An efficient numerical method is proposed for the valuation of American options via the Black-Scholes variational inequality. A far field boundary condition is employed to truncate the unbounded domain problem to produce the bounded domain problem with the associated variational inequality, to which our finite element method is applied. We prove that the matrix involved in the finite element method is symmetric and positive definite, and solve the discretized variational inequality by the projection and contraction method. Numerical experiments are conducted that demonstrate the superior performance of our method, in comparison with earlier methods.     

A comparison of the deformation of magnesium alloys with aluminium and steel in tension, bending and buckling

A comparison was made between high pressure die cast and wrought magnesium alloys and formed mild steel and aluminium in tensile, bending and buckling deformation. It was found that the energy absorption properties of magnesium alloys were particularly good in bending and buckling, absorbing up to 50% more energy than the aluminium and over 10 times more energy than the mild steel. The primary reason for the good performance of Mg alloys was that the low density means that sections of increased thickness can be made without increasing weight. This is particularly beneficial in bending as the strength of a section in bending is proportional to the square of the thickness. However, it was also observed that the high rate of work hardening of Mg alloys is particularly important and this allows for considerably more energy to be absorbed. A simple analytical strut buckling model has been modified to incorporate work hardening and a good correlation has been observed between this model and the experimental results.