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   628篇
  免费   20篇
  国内免费   6篇
化学   367篇
晶体学   11篇
力学   19篇
综合类   1篇
数学   84篇
物理学   172篇
  2023年   9篇
  2022年   23篇
  2021年   18篇
  2020年   13篇
  2019年   13篇
  2018年   16篇
  2017年   21篇
  2016年   26篇
  2015年   22篇
  2014年   17篇
  2013年   59篇
  2012年   31篇
  2011年   35篇
  2010年   27篇
  2009年   20篇
  2008年   28篇
  2007年   27篇
  2006年   24篇
  2005年   20篇
  2004年   15篇
  2003年   9篇
  2002年   10篇
  2000年   6篇
  1999年   4篇
  1998年   7篇
  1997年   10篇
  1996年   7篇
  1995年   6篇
  1994年   3篇
  1993年   6篇
  1990年   4篇
  1989年   9篇
  1988年   5篇
  1987年   5篇
  1986年   3篇
  1985年   7篇
  1984年   8篇
  1983年   3篇
  1982年   7篇
  1981年   4篇
  1980年   11篇
  1979年   6篇
  1978年   7篇
  1977年   8篇
  1976年   3篇
  1975年   3篇
  1974年   3篇
  1973年   5篇
  1971年   3篇
  1959年   5篇
排序方式: 共有654条查询结果,搜索用时 15 毫秒
151.

The efficiency of banks has a critical role in development of sound financial systems of countries. Data Envelopment Analysis (DEA) has witnessed an increase in popularity for modeling the performance efficiency of banks. Such efficiency depends on the appropriate selection of input and output variables. In literature, no agreement exists on the selection of relevant variables. The disagreement has been an on-going debate among academic experts, and no diagnostic tools exist to identify variable misspecifications. A cognitive analytics management framework is proposed using three processes to address misspecifications. The cognitive process conducts an extensive review to identify the most common set of variables. The analytics process integrates a random forest method; a simulation method with a DEA measurement feedback; and Shannon Entropy to select the best DEA model and its relevant variables. Finally, a management process discusses the managerial insights to manage performance and impacts. A sample of data is collected on 303 top-world banks for the periods 2013 to 2015 from 49 countries. The experimental simulation results identified the best DEA model along with its associated variables, and addressed the misclassification of the total deposits. The paper concludes with the limitations and future research directions.

  相似文献   
152.
A novel copper(II)-selective electrode based on graphite oxide/imprinted polymer composite was developed for the electrochemical monitoring of copper(II) (Cu2+) ions. The electrode exhibited highly selective potentiometric response to Cu2+ with respect to common alkaline, alkaline earth and heavy metal cations. The composite composition studies indicated that the most suitable composite composition performing the most promising potentiometric properties was 20.0% ionophore (Cu2+-ion imprinted polymer), 10.0% paraffin oil, 5.0% multiwalled carbon nanotubes, and 65.0% graphite oxide. The fabricated electrode exhibited a linear response to Cu2+ over the concentration range of 1.0?×?10??6–1.0?×?10??1?M (correlation coefficient of 0.9998) with a sensitivity of 26.1?±?0.9?mV decade??1. The detection limit of the fabricated electrode was determined to be 4.0?×?10??7?M. The electrode worked well in the pH range of 4.0–8.0. The electrode had stable, reversible and fast potentiometric response (3?s). In addition, the electrode had a lifetime of more than 1 year. The analytical applications of the proposed electrode were performed using as an indicator electrode for the potentiometric titration of Cu2+ with ethylene diamine tetraacetic acid solution and for the determination of Cu2+ of spiked river, dam, and tap water samples. The obtained results for potentiometric titration and water samples were satisfactory.  相似文献   
153.
Methyl 6-(N-benzenesulfonylamino)-N,O-isopropylidene-2,3,6,8-tetradeoxy--d-erythrooct-2-enopyranos-4-uloside, C18H23N1O6S1, (3) crystallizes in the orthorhombic space groupP212121. The unit cell parameters are:a=7.839(2),b=10.111(2),c=24.217(5) Å. The dihydropyranone ring adopts a deformed sofa conformation with the O5 atom deviating 0.586(2) Å from the least-squares plane and the -face of the carbonyl group shielded by the terminal (C8) methyl group. 1,1-O,O-Ethylidene-3,4-dideoxy-d,l-hex-3-enoz-2-ulopyranos-5-ulose, C8H10O5, (6) crystallizes in the monoclinic space groupP21/n. The unit cell parameters are:a=6.310(1),b=18.656(3),c=7.604(1) Å, =106.01(1)°. The dihydropyranone ring adopts a deformed sofa conformation with O6 atom deviating 0.482(1) Å from the leastsquares plane and the hydroxy and dioxolanyl substituents are in pseudoaxial and pseudoequatorial positions, respectively. The average planes of six-membered and five-membered rings are approximately coplanar.  相似文献   
154.
The efficiency of polymeric coatings as corrosion protective materials for carbon steel alloy in sea water was evaluated by different methods: electrochemical techniques (open circuit and potentiodynamic measurements), analytical and spectroscopic techniques (inductive coupled plasma, scanning electron microscopy and X‐ray diffraction). The study was made using ethylene propylene diene monomer (EPDM)/poly&!lpar;ethylene&!rpar; (PE) blend of different ratios. The effect of blend ratio on corrosion protection efficiency was investigated. The best results for corrosion control were achieved using samples with 2 mm thickness. The data obtained reveal that applying adhesive on the substrate increases the protective properties of the coatings. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
155.
3-Ethoxycarbonyl-5-methyl-1-(4-methylphenyl)-4-pyrazoloylhydroximoyl chloride (1) reacted with o-phenylenediamine, o-aminothiophenol, o-aminophenol and methyl anthranilate to afford 3-nitrosoquinoxaline, benzothiadiazine, benzoxadiazine, and 3-hydroxyquinazoline, respectively. Imidazo[1,2-a]pyridine, imidazo[1,2-a]pyrimidine and isoxazole derivatives were obtained via the reaction of 1 with 2-aminopyridine, 2-aminopyrimidine and the appropriate active methylene compounds, respectively. Pyrazolo[3,4-d]pyridazines, and pyrrolidino[3,4-d]isoxazolines derivatives were also synthesized. The structures of the newly synthesized compounds were established on the basis of spectral data and alternate synthesis whenever possible.  相似文献   
156.
The title compound, [RuCl2(C25H29N5)(C18H15P)], a transfer hydrogenation catalyst, is supported by an N,N′,N′′‐tridentate pyridine‐bridged ligand and triphenylphosphine. The RuII centre is six‐coordinated in a distorted octahedral arrangement, with the two Cl atoms located in the axial positions, and the pyridine (py) N atom, the two imino N atoms and the triphenylphosphine P atom located in the equatorial plane. The two equatorial Ru—Nimino distances (mean 2.093 Å) are substantially longer than the equatorial Ru—Npy bond [1.954 (4) Å]. It is observed that the NiminoM—Npy bond angle for the five‐membered chelate rings of 2,6‐bis(imino)pyridine‐based complexes is inversely related to the magnitude of the M—Npy bond. The title structure is stabilized by intra‐ and intermolecular C—H...Cl hydrogen bonds, as well as by intramolecular π–π stacking interactions between the aromatic rings belonging to the triphenylphosphine ligand and the dimethylaminophenyl fragment. The intermolecular hydrogen bonds form an R22(12) ring and a zigzag chain of fused centrosymmetric rings running parallel to the [100] direction.  相似文献   
157.
Both osteryoung square wave voltammetry and cyclic voltammetry have been utilized to elucidate and confirm the possible complexation reaction that occur between the various cephalosporin antibiotics and either the toxic, non-essential metal ion, viz. Cd (II), or the essential but toxic (when their concentration exceeds certain level in serum) metal ions, viz. Cu (II) and Zn (II). Voltammetric measurements indicated the existence of 1:1 metal-to-ligand ratio (as in cephalexin and cephapirin complexes), 1:2 ratio (such as in cefamandole, cefuroxime and cefotaxime complexes) and 2:1 ratio in case of ceftazidime complexes. Adsorption behavior was evidenced for Cu (II)-cefuroxime or ceftazidime complexes as well as for those for Zn (II)-cephalexin or cephapirin. This phenomenon could be used for the determination of either the antibiotic or the metal ion using adsorptive stripping voltammetry. Detection limits down to 7x10(-10) M have been easily achieved.  相似文献   
158.
Thermal reaction of butyl N-(p-toluenesulphonyl)iminoacetate and diethyl mesoxalate with ketones leads to the derivatives of X-oxo-α-amino-acids and X-oxo-α-hydroxy -acids, respectively.  相似文献   
159.
Sample processing is a very important component of uncertainty in analytical results. In order to have reliable results, the laboratory sample should be properly processed to obtain statistically homogenous matrix—before the representative test portions are withdrawn for analysis. The use of 14C-labeled compound is preferable because the analyte can be quantified without cleanup. The method is based on surface treatment of cucumber with 14C-chlorpyrifos, determination of 14C-chlorpyrifos activity in the replicate test portions of different size, and determination of the uncertainty of sample processing.  相似文献   
160.
Summary The acid-base properties of Fast Grey R. A. have been studied with reference to the mode of chelation of the dye with different valent metals. The pK values of 3.83, 10.53 and 10.70 corresponding to the three steps of ionization of the free acid were evaluated by the aid of the relation between log [acid]/[salt] and pH values obtained during the course of the potentiometric titration of the free acid with a free base.  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