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排序方式: 共有232条查询结果,搜索用时 218 毫秒
181.
Poly(ethylene glycol) (PEG) triblock and diblock amphiphilic block copolymers were synthesized from poly(ethylene glycol) and poly(ethylene glycol) monomethyl ether, respectively. The hydroxyl groups of PEG readily react with 2-(1-octadecenyl) succinic anhydride (OSA) at 140 °C through ring-opening reaction of the succinic anhydride. Both the PEG-OSA diblock and triblock copolymers are produced without use of any solvent or catalyst. The molecular structure of the copolymers was characterized by 1H NMR and FTIR spectroscopy, and the thermal properties by DSC. The behavior of the copolymers in selective and nonselective solvents was studied by 1H NMR spectroscopy in deuterium oxide and d-chloroform. The aggregation of the polymers in water was studied with a particle size analyzer and a transmission electron microscope (TEM) in bright field mode. The results show that the hydrophobic C18 chain with intramolecular succinic anhydride linker can be attached to the hydrophilic PEG chain, an ester bond forming between the blocks. The copolymers exhibit flexible, liquid-like hydrophobic blocks even in water, which is a nonsolvent for OSA. PEG-OSA block copolymers self-organize in water, forming micellar polymer aggregates in nanoscale. 相似文献
182.
Study of immersion silver and tin printed-circuit-board surface finishes in lead-free solder applications 总被引:8,自引:0,他引:8
Minna Arra Dongkai Shangguan Dongji Xie Janne Sundelin Toivo Lepistö Eero Ristolainen 《Journal of Electronic Materials》2004,33(9):977-990
The wetting of I-Ag (immersion silver) and I-Sn (immersion tin) printed-circuit-board (PCB) finishes by Sn/Ag/Cu and eutectic
Sn/Pb solders was studied in this work with Ni/Au (electroless nickel/immersion gold) and organic solderability preservative
(OSP) finishes as baselines. Wetting tests were performed on fresh boards and boards subjected to different preconditioning
treatments that simulated the effects of aging, storage, and multiple reflow cycles. When the boards are fresh, the wetting
of the I-Sn and Ni/Au finishes is better than that on the I-Ag and OSP finishes. However, after the preconditioning treatments,
the wetting of the I-Sn finish degrades the fastest, whereas the wetting of the I-Ag and OSP finishes degrade less through
the different preconditioning treatments. The wetting of the Ni/Au finish remains excellent through all the preconditioning
treatments. The chemical and microstructural changes in the finishes during aging treatments were evaluated using electron
spectroscopy chemical analysis (ESCA), x-ray diffractometry (XRD), and cross-sectioning followed by scanning electron microscopy
(SEM). The results indicate that a single lead-free reflow cycle consumes the I-Sn layer faster than a Sn/Pb reflow cycle
because of the formation of the Sn/Cu intermetallic compound (IMC). Consequently, I-Sn finished boards having an original
Sn thickness of ∼1 μm will not withstand multiple lead-free reflow cycles without significant degradation in wetting but up
to two Sn/Pb reflow cycles are still feasible. The minimum thickness of I-Sn required for adequate wetting was evaluated by
comparing the wetting after different aging treatments. The exposure of I-Sn samples to 85°C/85% relative humidity (RH) conditions
increases the thickness of the Sn-oxide layer, which, above a certain thickness, can degrade wetting. Oxidized copper areas
formed on top of the I-Ag surface after exposure to 85°C/85% RH treatment, and this was considered a major factor influencing
wetting. The formation of sulfides on I-Ag was detected, but their overall quantity remained too small to have a detectable
impact on the wetting. 相似文献
183.
Olli Kursu Marko Riikola Janne Aikio Timo Rahkonen 《Analog Integrated Circuits and Signal Processing》2007,50(1):13-20
This paper presents the structure, test setup and measured results of a polynomial RF predistorter IC fabricated in a 0.35 μm SiGe BiCMOS process. The predistorter is designed for the base station WCDMA band at 2.1 GHz. The predistortion signal is generated by a 5th-degree complex polynomial. Also a squared envelope is generated that can be used as a baseband injection signal to cancel 2nd-order distortion that typically causes memory effects. The performance of the predistorter was measured by driving a three-stage discrete power amplifier chain by a 2-tone test and a 3.84 MHz wide 3GPP WCDMA modulated signal. The 2-tone test showed more than 20 dB IM3 cancellation and the WCDMA signal’s ACPR was improved by 8 dB. 相似文献
184.
Lempinen Janne Muuri Eveliina Lusa Merja Lehto Jukka 《Journal of Radioanalytical and Nuclear Chemistry》2018,316(2):717-723
Journal of Radioanalytical and Nuclear Chemistry - The sorption of inorganic radiocarbon on goethite, hematite and magnetite was studied as a function of carbon concentration, pH and ionic... 相似文献
185.
Elin?Nilsen Mari-Ann?EinarsrudEmail author Janne?Puputti Mika?Lind’n Jean?Le?Bell Mikael?Perander 《Journal of Sol-Gel Science and Technology》2005,35(2):143-150
The preparation and characterization of a composite binder made through a sol-gel route from an amorphous mineral raw material has been studied. The amorphous mineral raw material is alumino-silicate based but contains also alkaline earth and some transition metal oxides. A stable sol can be prepared by dissolving the raw materials in formic acid. The drying and heat treatment of the subsequent gel has been studied by thermogravimetry, nuclear magnetic resonance, infrared spectroscopy as well as X-ray diffraction studies. The different constituents of the raw material have shown to play a major role during the drying and heat treatment of the gels. The binding effect of the binder was evaluated to be good by studying paper-binder composites as well as wetting properties. 相似文献
186.
The copper(I)‐catalyzed dipolar [2+3] cycloaddition reaction of an azide and a terminal alkyne is exploited in the preparation of various europium(III), terbium(III), and dysprosium(III) chelates (Schemes 1–3). By changing the nature of the alkyne and the azide, a wide range of chelates and biomolecule‐labeling reactants were obtained. The photophysical properties (Table) of the synthesized chelates are also discussed. 相似文献
187.
Terhi Saarinen Monika
sterberg Janne Laine 《Colloids and surfaces. A, Physicochemical and engineering aspects》2008,330(2-3):134-142
The adsorption of polyelectrolyte (PE) multilayers and complexes, obtained from both high- and low-charge polyelectrolytes, was studied on silica and on cellulose model surfaces by quartz crystal microbalance with dissipation (QCM-D). The film properties acquired with the different strategies were compared. When polyelectrolytes were added on an oppositely charged surface in sequence to form multilayers both the change in frequency and dissipation increased. The changes in frequency and dissipation were clearly higher if low-charge PEs were used in the multilayer formation. The substrate, silica or cellulose, did not affect the adsorption behaviour of low-charge PEs and only minor differences were seen in the adsorbed amounts and changes in dissipation of high-charge PEs between SiO2 and cellulose. The complexes formed by low-charge PEs had higher changes in frequency and dissipation at low ionic strength on both surfaces, while the complexes formed from high-charge polyelectrolytes adsorbed more at high salt concentration. The complexes of low-charge polyelectrolytes adsorbed more on silica, while the complexes formed by high-charge PEs formed thicker layers on cellulose. The charge ratio had a significant effect on the adsorption and the highest changes in frequency and dissipation were obtained in the anionic/cationic charge ratio of 0.5–0.6. Generally, the multilayers and complexes formed by low-charge polyacrylamides adsorbed highly and formed rather thick layers on both surfaces, unlike the high-charge PEs which formed thin layers using either one of the addition techniques. 相似文献
188.
We report an analysis of the pH-dependent dissociation of a multimeric metalloprotein, xylose isomerase from Streptomyces rubiginosus (XI), by electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Xylose isomerases are industrially significant enzymes that catalyze interconversion of aldose and ketose sugars. XI is biologically active as a approximately 173-kDa tetrameric complex, comprised of four identical approximately 43-kDa subunits and eight metal cations, unequivocally identified as the Mg(2+) cations in this work. ESI FT-ICR mass spectra of XI measured in the pH range of 3.0-6.9 indicated that the dissociation of the intact holo-tetramer is initiated by the loss of all eight Mg(2+) cations at pH 相似文献
189.
Dr. Shaochen Zhou Lotta Gustavsson Dr. Grégory Beaune Dr. Sourov Chandra Prof. Jukka Niskanen Prof. Janne Ruokolainen Dr. Jaakko V. I. Timonen Prof. Olli Ikkala Dr. Bo Peng Prof. Robin H. A. Ras 《Angewandte Chemie (International ed. in English)》2023,62(49):e202312679
Near-infrared (NIR) fluorophores with pH-responsive properties suggest merits in biological analyses. This work establishes a general and effective method to obtain pH-responsive NIR emissive gold nanoclusters by introducing aliphatic tertiary amine (TA) groups into the ligands. Computational study suggests that the pH-responsive NIR emission is associated with electronic structure change upon protonation and deprotonation of TA groups. Photo-induced electron transfer between deprotonated TA groups and the surface Au-S motifs of gold nanoclusters can disrupt the radiative transitions and thereby decrease the photoluminescence intensity in basic environments (pH=7–11). By contrast, protonated TA groups curb the electron transfer and restore the photoluminescence intensity in acidic environments (pH=4–7). The pH-responsive NIR-emitting gold nanoclusters serve as a specific and sensitive probe for the lysosomes in the cells, offering non-invasive emissions without interferences from intracellular autofluorescence. 相似文献
190.
Ultrasound based elasticity imaging techniques have been developed during the past decades. Some of these techniques are based on an internal radiation force stimulation in which a transient or dynamic radiation force is produced by using a single or dual-frequency sonication. In addition, sonication and data acquisition can be implemented using combined or separate transducers. In this simulation study of lesion detection using localized harmonic motion imaging (LHMI), we used a combined phased array designed for simultaneous thermal ablation and lesion detection. In the sonication mode, a focused single-frequency amplitude-modulated sonication is used to induce harmonic motion and in the tracking mode, some of the array elements are used for pulse-echo tracking of the induced displacements. The results showed that the size of the lesion affected the induced displacement around the sonication point. The displacement tracking simulations demonstrated that these changes in the displacement distributions can be detected using only a few of the array elements in the tracking mode but the exact size of the lesion can not be detected accurately. The simulations also showed that two lesions having the radius of 2.5mm can be distinguished if distance between these lesions is at least 2.5mm. 相似文献