Showing papers by "Dalian University of Technology published in 2004"

Bulk metallic glasses

Abstract: Amorphous alloys were first developed over 40 years ago and found applications as magnetic core or reinforcement added to other materials. The scope of applications is limited due to the small thickness in the region of only tens of microns. The research effort in the past two decades, mainly pioneered by a Japanese- and a US-group of scientists, has substantially relaxed this size constrain. Some bulk metallic glasses can have tensile strength up to 3000 MPa with good corrosion resistance, reasonable toughness, low internal friction and good processability. Bulk metallic glasses are now being used in consumer electronic industries, sporting goods industries, etc. In this paper, the authors reviewed the recent development of new alloy systems of bulk metallic glasses. The properties and processing technologies relevant to the industrial applications of these alloys are also discussed here. The behaviors of bulk metallic glasses under extreme conditions such as high pressure and low temperature are especially addressed in this review. In order that the scope of applications can be broadened, the understanding of the glass-forming criteria is important for the design of new alloy systems and also the processing techniques.

Properties of lead-free solder alloys with rare earth element additions

Abstract: Due to the inherent toxicity of lead (Pb), environmental regulations around the world have been targeted to eliminate the usage of Pb-bearing solders in electronic assemblies. This has prompted the development of “Pb-free” solders, and has enhanced the research activities in this field. In order to become a successful solder material, Pb-free alloys need to be reliable over long term use. Although many of these alloys possess higher strength than the traditional Sn–Pb ones, there still exist reliability problems such as electromigration and creep. Also, the solderability of many Pb-free alloys is inferior to that of Sn–Pb and any improvement or replacement will be welcomed by industry. In order to develop new Pb-free solders with better properties, trace amounts of rare earth (RE) elements were selected by some researchers as alloying additions into Sn-based solders. These solder alloys are mainly Sn–Ag, Sn–Cu, Sn–Zn and Sn–Ag–Cu. In general, the resulting RE-doped solders are found to have better performances than their original ones. The improvements include better wettability, creep strength and tensile strength. In particular, the increase in creep resistance in some RE-doped alloys gives creep rupture time increases by over four times for Sn–Ag and seven times for Sn–Cu and Sn–Ag–Cu. Like other Sn-based alloys, their creep rates are controlled by dislocation pipe diffusion in the Sn matrix. Also, it was found that the creep rate of these Sn-based alloys can be represented by a single empirical equation. With the addition of RE elements, solders for bonding on difficult substrates such as on semiconductors, diamond, and optical materials have also been developed. This report summarizes the effect of RE elements on the microstructure, mechanical properties, wetting behavior of certain Pb-free solder alloys. As an illustration of the advantage of RE doping, interfacial studies were carried out for electronic interconnections with RE-doped Pb-free alloys. It was found that the intermetallic compound (IMC) layer thickness and the amount of interfacial reaction were reduced in a Ball Grid Array (BGA) package. These results indicate that RE elements would play an important role in providing better electronic interconnections.

A Highly Selective and Sensitive Fluorescent Chemosensor for Hg2+ in Neutral Buffer Aqueous Solution

Abstract: A selective and sensitive fluorescent chemosensor for Hg2+, which was composed of two aminonaphthalimide fluorophores and a receptor of 2,6-bis(aminomethyl)pyridine, was synthesized through the reaction of 2,6-bis(chloromethyl)pyridine and N-[2-(2-hydroxyethoxy)ethyl]-4-piperazino-1,8-naphthalimide. The chemosensor showed an about 17-fold increase in fluorescence quantum yield upon addition of 1 equiv of Hg2+ in neutral buffer aqueous solution, and the other common metal ions did not notably disturb the detection of Hg2+.

Prediction of chaotic time series based on the recurrent predictor neural network

Abstract: Chaos limits predictability so that the long-term prediction of chaotic time series is very difficult. The main purpose of this paper is to study a new methodology to model and predict chaotic time series based on a new recurrent predictor neural network (RPNN). This method realizes long-term prediction by making accurate multistep predictions. This RPNN consists of nonlinearly operated nodes whose outputs are only connected with the inputs of themselves and the latter nodes. The connections may contain multiple branches with time delays. An extended algorithm of self-adaptive back-propagation through time (BPTT) learning algorithm is used to train the RPNN. In simulation, two performance measures [root-mean-square error (RMSE) and prediction accuracy (PA)] show that the proposed method is more effective and accurate for multistep prediction. It can identify the systems characteristics quite well and provide a new way to make long-term prediction of the chaotic time series.

Application of chaos in simulated annealing

Abstract: Simulated annealing (SA) has been applied with success to many numerical and combinatorial optimization problems in recent years. SA has a rather slow convergence rate, however, on some function optimization problems. In this paper, by introducing chaotic systems to simulated annealing, we propose a optimization algorithm named chaos simulated annealing (CSA). The distinctions between CSA and SA are chaotic initialization and chaotic sequences replacing the Gaussian distribution. Simulation results of typical complex function optimization show that CSA improves the convergence and is efficient, applicable and easy to implement. In addition, we discuss the advantages of CSA, and show the reasons why CSA performs better than SA.

Chemical fixation of carbon dioxide to cyclic carbonates under extremely mild conditions with highly active bifunctional catalysts

Abstract: Chemical fixation of carbon dioxide to cyclic carbonates proceeds effectively under extremely mild temperature and pressure by using a bifunctional nucleophile–electrophile catalyst system of tetradentate Schiff-base aluminum complexes ((Salen)AlX) in conjunction with a quaternary ammonium salt (n-Bu4NY) in the absence of any organic solvent. Electrophilicity of central Al3+ ion and the steric factor of substituent groups on the aromatic rings of (Salen)AlX (electrophile), and nucleophilicity and leaving ability of the anion Y− of n-Bu4NY (nucleophile) have a great effect on the catalytic activity of the bifunctional catalyst.

Synthesis and Characterization of Bifunctionalized Ordered Mesoporous Materials

Abstract: Direct synthesis (co-condensation reaction) and post-synthesis reaction (grafting) are combined for the first time to efficiently fabricate bifunctionalized ordered mesoporous materials (OMMs). Ethylenediamine-containing OMMs (ED-MCM-41) were first synthesized via direct synthesis and then further modified by the phenyl (PH) group in a supercritical fluid (SCF) medium via grafting reaction, resulting in OMMs with ED and PH groups (PH-ED-MCM-41). X-ray diffraction (XRD) patterns, N2 sorption properties, transmission electron microscopy (TEM), 29Si and 13C magic angle spinning (MAS) NMR, chemical analysis, and hydrothermal treatment were used to characterize the bifunctionalized materials. Experiments show that bifunctionalized OMMs can be efficiently prepared by modifying the directly synthesized monofunctionalized OMMs via grafting reaction in a supercritical fluid medium. Both functional groups are distributed uniformly at the surfaces. The advantage of bifunctionalized OMMs over monofunctionalized OMMs was illustrated by introducing thiol groups into ED-MCM-41 materials and the subsequent formation of CdS nanocrystals inside thiol- and ED-functionalized MCM-41 (HS-ED-MC-41). Because of the variety of the functional groups that can be introduced into OMMs by direct synthesis or post-synthesis reaction, it is expected that the present strategy could provide a generally applicable approach to the design of OMMs with two functional groups.