首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   146篇
  免费   5篇
化学   96篇
力学   1篇
数学   15篇
物理学   39篇
  2022年   1篇
  2021年   2篇
  2020年   1篇
  2019年   1篇
  2018年   3篇
  2017年   9篇
  2016年   6篇
  2015年   3篇
  2014年   8篇
  2013年   7篇
  2012年   18篇
  2011年   15篇
  2010年   11篇
  2009年   6篇
  2008年   8篇
  2007年   10篇
  2006年   10篇
  2005年   2篇
  2004年   10篇
  2003年   7篇
  2002年   3篇
  2000年   3篇
  1998年   1篇
  1996年   2篇
  1993年   2篇
  1990年   1篇
  1984年   1篇
排序方式: 共有151条查询结果,搜索用时 31 毫秒
61.
Ozcan A  Ozcan AS 《Talanta》2004,64(2):491-495
This study compares conventional Soxhlet extraction and analytical scale supercritical fluid extraction (SFE) for their yields in extracting of hydrocarbons from arid-land plant Euphorbia macroclada. The plant material was firstly sequentially extracted with supercritical carbon dioxide, modified with 10% methanol (v/v) in the optimum conditions that is a pressure of 400 atm and a temperature of 50 °C and then it was sonicated in methylene chloride for an additional 4 h. E. macroclada was secondly extracted by using a Soxhlet apparatus at 30 °C for 8 h in methylene chloride. The validated SFE was then compared to the extraction yield of E. macroclada with a Soxhlet extraction by using the Student’s t-test at the 95% confidence level. All of extracts were fractionated with silica-gel in a glass column to get better hydrocarbon yields. Thus, the highest hydrocarbons yield from E. macroclada was achieved with SFE (5.8%) when it compared with Soxhlet extractions (1.1%). Gas chromatography (GC) analysis was performed to determine the quantitative hydrocarbons from plant material. The greatest quantitative hydrocarbon recovery from GC was obtained by supercritical carbon dioxide extract (0.6 mg g−1).  相似文献   
62.
In this study, the aim is to describe the influence of electrospinning parameters on the morphology, the water wetting property and dye adsorption property of poly(methyl methacrylate) nanofiber mats. Specifically, the effects of solution concentration, solvent type, applied voltage, distance between the electrodes and particulate reinforcement on the diameter and shape of the nanofibers were investigated. All poly(methyl methacrylate) nanofiber mats contained beaded nanofiber structures. With increasing the polymer solution concentration, the average fiber diameter also increased. Poly(methyl methacrylate) nanofiber mat electrospun from dimethylformamide solution resulted in thicker fibers when compared with the mat electrospun from acetone solution. Increasing the electric potential difference between the collector and the syringe tip did not increase the average fiber diameter. Besides increasing the distance between the electrodes resulted in a decrease in the average fiber diameter. When compared with PMMA nanofiber mat, thicker fibers were obtained with silica nanoparticles reinforced nanofiber mat. According to the water contact angle measurements, all poly(methyl methacrylate) nanofiber mats revealed hydrophobic surface property. PMMA nanofiber mat with the highest water contact angle gave rise to the highest dye adsorption capacity.  相似文献   
63.
In this study, it is aimed to prepare polyvinyl borate (PVB)/titanium dioxide (TiO2) composites through the condensation reaction of polyvinyl alcohol and boric acid in the presence of TiO2 nanoparticles. The scope of this study contains the photocatalytic activity of the prepared composites with varying TiO2 content from 0 to 35 wt.% for the degradation of methylene blue in aqueous medium under UV light irradiation. The structure and morphological properties of the prepared composites were studied with FTIR, TGA, EDX, SEM and TEM analyses. The photocatalytic activity of the prepared samples was analyzed by UV-Vis spectrophotometer measurements. In parallel with TiO2 content up to 30 wt.%, the photocatalytic activity of PVB/TiO2 composites was enhanced and the composites exhibited higher discoloration rate of the model dye, methylene blue.  相似文献   
64.
This study uses Data Envelopment Analysis to examine relationships between provider experience and technical efficiency in stroke treatment patients. Using 214 hospitals grouped according to stroke treatment experience, the study examines how the inputs—average length of stay (ALOS), occupational and physical therapy charges, and all other charges—are combined to treat mild and severe stroke cases. This study finds that, on average, technical efficiency improves with experience. However, although more experienced providers are on average deemed more technically efficient, they tend to have higher charges. The study further illustrates that the gap in case severity between efficient and inefficient providers widens as experience level increases. These findings suggest there is great potential for inefficient providers to alter practice patterns to those similar to efficient providers, in order to significantly reduce aggregate expenditures.  相似文献   
65.
Pencil graphite electrode (PGE) modified with MWCNT and Bi3+ (MWCNT/Bi/PGE) was utilized in simultaneous analysis of Pb2+, Cd2+, and Zn2+. Surface and electrochemical characteristics of MWCNT/Bi/PGE were investigated via SEM, cyclic voltammetry, electrochemical impedance spectroscopy, and FTIR measurements. Even though modification with MWCNT did not improve the electroactive surface area, it significantly decreased the charge transfer resistance. Furthermore, modification with Bi3+ significantly increased the sensitivity. Finally, MWCNT/Bi/PGE exhibited the highest sensitivity and reproducibility compared to PGE and PGE modified with only MWCNT. MWCNT/Bi/PGE provided LOD values of 0.27, 0.43, and 1.63 μg L−1, and linear ranges of 1–80, 5–80, and 10–80 μg L−1 for Pb2+, Cd2+, and Zn2+, respectively. Proposed modification method offers effective electroanalytical performance with low time consumption and cost for the analyst.  相似文献   
66.
Well‐defined heterotelechelic poly(styrene) carrying thymine/diaminopyridine (DAP) (Mn,SEC = 9300, PDI = 1.04) and Hamilton wedge (HW)/cyanuric acid (CA) (Mn,SEC = 8200, PDI = 1.04) bonding motifs are prepared via a combination of controlled/living radical polymerization and copper catalyzed azide/alkyne “click” chemistry and are subsequently self‐assembled as single chains to emulate—on a simple level—the self‐folding behavior of natural biomacromolecules. Hydrogen nuclear magnetic resonance (1H NMR) in deuterated dichloromethane and dynamic light scattering analyses provides evidence for the hydrogen bonding interactions between the α‐thymine and ω‐DAP as well as α‐CA and ω‐HW chain ends of the heterotelechelic polymers leading to circular entropy driven single chain self‐assembly. This study demonstrates that the choice of NMR solvent is important for obtaining well‐resolved NMR spectra of the self‐assembled structures. In addition, steric effects on the HW can affect the efficiency of the self‐assembly process. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011  相似文献   
67.
Two novel calix[4]arene receptors containing ferrocene units in cone (L1) and 1,3-alternate (L2) conformations have been synthesized from 25,27-dihydroxy-26,28-bis[(3-aminopropyl)oxy]calix[4]arene 4 or 25,26,27,28-tetra[(3-aminopropyl)oxy]calix[4]arene 6 and ferrocenecarboxaldehyde via condensation, respectively. Their structures have been characterized by 1H, 13C, APT, COSY NMR, FTIR, HSMR, and UV–vis spectral data. The electrochemical behavior of L1 and L2 has been investigated in the presence of F?, Cl?, Br?, H2PO4?, CH3COO? anions. Electrochemical studies show that these receptors electrochemically recognize CH3COO?, H2PO4?, and Cl?, anions. Using an UV–vis study, the selectivity to these anions in DMSO solution was confirmed.  相似文献   
68.
A charge trapping memory with 2 nm silicon nanoparticles (Si NPs) is demonstrated. A zinc oxide (ZnO) active layer is deposited by atomic layer deposition (ALD), preceded by Al2O3 which acts as the gate, blocking and tunneling oxide. Spin coating technique is used to deposit Si NPs across the sample between Al2O3 steps. The Si nanoparticle memory exhibits a threshold voltage (Vt) shift of 2.9 V at a negative programming voltage of –10 V indicating that holes are emitted from channel to charge trapping layer. The negligible measured Vt shift without the nanoparticles and the good re‐ tention of charges (>10 years) with Si NPs confirm that the Si NPs act as deep energy states within the bandgap of the Al2O3 layer. In order to determine the mechanism for hole emission, we study the effect of the electric field across the tunnel oxide on the magnitude and trend of the Vt shift. The Vt shift is only achieved at electric fields above 1 MV/cm. This high field indicates that tunneling is the main mechanism. More specifically, phonon‐assisted tunneling (PAT) dominates at electric fields between 1.2 MV/cm < E < 2.1 MV/cm, while Fowler–Nordheim tunneling leads at higher fields (E > 2.1 MV/cm). (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
69.
In the present study, we discussed to synthesis of a new Schiff base with nitro groups and its complexation properties with Fe/Cr(III) salen/salophen capped complexes. For this, 1,3,5-tris (formylphenoxymethyl)benzene (1, TRIPOD) involving aldehyde groups was converted to the Schiff base derivative (2, TNPIM-TRIPOD) using 4-nitroaniline. The synthesized compound 2 were reacted with four new trinuclear Fe(III) and Cr(III) complexes involving tetradenta Schiff bases N,N-bis(salicylidene)ethylenediamine-(salenH2) or bis(salicylidene)-o-phenylenediamine-(salophenH2). Characterization of all compounds was made with elemental analysis, infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), nuclear magnetic resonance (1H-NMR), and magnetic susceptibility measurement. The complexes can also be characterized as low-spin distorted octahedral Fe(III) and Cr(III) bridged by nitro groups.  相似文献   
70.
Percentages of crude oil, protein, fibre and ash of grape seeds obtained from Turkish cultivars were of the ranges 5.40-10.79, 5.24-7.54, 17.6-27.1, and 1.2-2.6, respectively. The highest crude oil, crude protein and crude fibre were determined in Siyah pekmezlik, Karadimrit and Antep grape seeds. The energy values of seeds were established to be between 102.28 and 148.07?kcal?g(-1). Potassium and calcium contents of seed samples were found to be at high levels compared to sodium. The seeds contained 686-967?ppm of Na, 2468-3618?ppm of K and 2373-4127?ppm of Ca. The refractive index, relative density, acidity, saponification value, unsaponifiable matter and iodine value of seed oils were determined to be in the ranges 1.474-1.477 [Formula: see text], 0.909-0.934 25/25°C, 0.74-1.24%, 181-197, 0.91-1.66%, and 126-135, respectively. The main fatty acids were of the ranges 60.7-68.5% linoleic, 16.1-23.4% oleic and 8.0-10.2% palmitic. The highest percentages of linoleic acid (68.5%) was determined in Siyah pekmezlik seed oil.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号