选择性焊接为PCB设计者提供新的选择

介绍了选择性焊接的概念、特点、分类和使用工艺要点。选择性焊接是现代组装技术的新概念,它的出现促进了SMT(表面贴装技术)的发展,并为PCB设计者提供了新的工艺选择。可以确信选择性焊接将会被更多地应用于电子组装上,成为一种具有竞争力的新焊接技术。     

Avoiding Loss of Catalytic Activity of Pd Nanoparticles Partially Embedded in Nanoditches in SiC Nanowires

Nanoditches from selective etching of periodically twinned SiC nanowires were employed to hinder the migration and coalescence of Pd nanoparticles supported on the nanowires, and thus to improve their catalytic stability for total combustion of methane. The results show that the etched Pd/SiC catalyst can keep the methane conversion of almost 100% while the unetched one has an obvious decline in the catalytic activity from 100 to 82% after ten repeated reaction cycles. The excellent catalytic stability originates from the limitation of the nanoditches to the migration and growth of Pd nanoparticles.     

Different Effects of Fluoride Surface Modification on the Photocatalytic Oxidation of Phenol in Anatase and Rutile TiO2 Suspensions

The effect of calcination temperature on the photocatalytic degradation of phenol with aqueous suspensions of synthetic anatase and rutile TiO₂ under UV light irradiation (λ > 320 nm), was studied in the absence and presence of NaF. The presence of fluoride accelerates the degradation of phenol in anatase TiO₂, with this positive effect increasing at first, before declining with increasing calcined temperature. A negative effect of fluoride was observed for all the rutile TiO₂ suspensions. The selectivity of catechol increased in the presence of fluoride in both anatase and rutile TiO₂ suspensions.     

Novel supramolecular system of amphiphilic hyperbranched polymer with β-cyclodextrin and hyperbranched topography cavities: Synthesis and selective encapsulation

Novel supramolecular system of amphiphilic hyperbranched polymer with hyperbranched poly(β-cyclodextrin) core was designed and synthesized to accomplish a so-called selective encapsulation, where two types of guest molecules can be encapsulated into two types of molecular cavities from β-cyclodextrin (β-CD) and topography structure of hyperbranched polymer, respectively. The double molecular recognition behaviors from β-CD and hyperbranched cavities drive one guest to go into the former, the other guest to the latter. This selective encapsulation was further confirmed via the release profiles and sequences of Levofloxacin lactate (LL) and Phenolphthalein (PP). LL presents a sustained release period followed by an almost non-release stage, while PP releases on a quite slow rate at first, subsequently on the linearly increasing rate. At the early stage, the release of LL dominates in comparison with PP, and then the release rate of PP increases to play a determinate role in the release system. It can be attributed to the existence of two guests in the different molecular cavities with the different microenvironments. The observed selective encapsulation of supramolecular system is a new phenomenon, which is helpful to extend the application of CD-based hyperbranched polymers in supramolecular science and complex drug delivery system.     

磨矿过程中应注意的问题分析及对策研讨

首先就我国选厂磨矿过程中存在的问题作了简单的介绍,然后从理论上分析了这些问题的症结所在,并从工业试验的研究成果提出了相应的对策。     

Selective detection of sulfur derivatives using microfabricated tuning fork-based sensors

The paper describes an integrated sensor system that can selectively and reversibly detect sulfur derivatives in the presence of interferent molecules. This is accomplished by integrating analyte-specific sensing materials with optimized filter materials. Microfabricated quartz tuning fork arrays are used to provide fast, accurate and low-cost transduction of the analyte binding events into electronic signals. The concept is demonstrated for detection of three sulfur derivatives—dimethyl disulfide, ethanethiol and methylsulfide.     

Selective matrix dissolution in an Al-Si eutectic

The irregular structure of an Al-Si eutectic has been chemically modified by adding small amount of Sr (300 ppm). The initially coarse flake structure was converted into technologically preferable fine fibrous morphology. The selective phase dissolution has been investigated using two different electrochemical approaches, derived from superimposed Pourbaix diagrams. Anodisation at 0.3 V (SHE) in 1 M HCl led to pitting and was partially successful after an extended period of time (20 h). Electropolishing in a mixture of perchloric acid and methanol (10:90) at 0 °C proved successful in both the partial and the complete matrix dissolution, without affecting silicon structures.     

Laser polishing of parts built up by selective laser sintering

In this work, a surface finish method for parts built-up by selective laser sintering (SLS) is presented. One of the main drawbacks of the SLS technique is the high surface roughness of resulting parts. Therefore, parts have to be polished to be valid for operation conditions. Polishing processes are usually based on manual abrasive techniques. However, in the present paper, a surface polishing method based on laser irradiation is presented. The laser beam melts a microscopic layer on the surface, which re-solidifies under shielding gas protective conditions, resulting in a smoother surface.Laser-polishing tests for lines, planar surfaces and inclined planes have been performed, with satisfactory results in all the cases. The experimental tests were carried out on sintered test parts with an initial roughness of 7.5–7.8 μm Ra. The tested material is a commercial alloy denominated LaserForm ST-100^©, composed by sintered stainless steel and infiltrated bronze that it is used mainly for the constitution of injection moulds. Experimental results present final surface roughness below 1.49 μm Ra, which represent an 80.1% reduction of the mean roughness. Finally, a complete analysis of test probes and its metallurgical composition is presented. Considering that the material presents a non-homogeneous structure, the polished surfaces present slightly higher hardness values and are more homogeneous than the initial ones. Thus, polished surfaces do not present any heat affected zone or cracks, which could cause failure during the part operation.     

Separation of dark-colored minerals from feldspar by selective flocculation using starch

Feldspar ores usually contain some amounts of impurity dark-colored minerals (DCM), rich in iron. Such ores must be comminuted for the liberation and separation of the said impurities. Ultra-fine particles which are generated during comminution are difficult to treat by conventional mineral processing methods. It is known that iron minerals have a specific affinity for starch. This phenomenon has been successfully exploited in the selective flocculation and flotation of iron ores. In this study, the possibility of separation of DCM from feldspar by selective flocculation using starch was investigated. It was found that DCM also had a specific affinity for starch. Flocculation tests revealed that DCM alone could be flocculated with starch, while feldspar alone could not. Selective flocculation tests of the mixture of DCM and feldspar indicated that selective separation of DCM from feldspar using starch as a flocculant could be possible.     

Iron–ceria–zirconia fluorite catalysts for methane selective oxidation to formaldehyde

Ceria–zirconia catalysts modified by partial substitution of zirconium by iron were investigated for the reaction of the selective oxidation of methane to formaldehyde. The insertion of iron was ensured by the preparation method, based on decomposition of the mixed precursors. The crystalline structure was studied by XRD and the iron state was determined by Mössbauer spectroscopy. The insertion of Fe³⁺ into the lattice of the mixed oxide and the partial substitution of zirconium ions with iron cations up to 25% have not resulted in any phase rejection of the formed iron oxide. The reducibility of Ce⁴⁺ in the modified ternary oxide is enhanced by doping with iron. Despite their low specific surface area, the catalysts are efficient at activating the methane independently of the iron content. The selectivity to formaldehyde strongly depends on the amount of the iron inserted in the mixed oxide.