首页 | 官方网站   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   4787篇
  国内免费   671篇
  完全免费   1052篇
数理化   6510篇
  2018年   2篇
  2017年   22篇
  2016年   19篇
  2015年   77篇
  2014年   163篇
  2013年   158篇
  2012年   197篇
  2011年   264篇
  2010年   247篇
  2009年   470篇
  2008年   697篇
  2007年   465篇
  2006年   481篇
  2005年   382篇
  2004年   358篇
  2003年   317篇
  2002年   252篇
  2001年   237篇
  2000年   219篇
  1999年   183篇
  1998年   191篇
  1997年   160篇
  1996年   117篇
  1995年   144篇
  1994年   141篇
  1993年   142篇
  1992年   88篇
  1991年   70篇
  1990年   35篇
  1989年   49篇
  1988年   41篇
  1987年   42篇
  1986年   23篇
  1985年   10篇
  1984年   15篇
  1983年   1篇
  1982年   7篇
  1981年   4篇
  1980年   7篇
  1979年   4篇
  1977年   4篇
  1976年   1篇
  1975年   1篇
  1973年   3篇
排序方式: 共有6510条查询结果,搜索用时 78 毫秒
1.
For three-phase liquid emulsion membranes, a reaction zone model is derived on the basis of the known reaction front model. Differences between the two are discussed in terms of the concepts and some results. The reaction zone model is more generally valid and it simplifies to the earlier model under appropriate conditions.  相似文献   
2.
Atomic-resolution structures of the transmembrane 7--helical domains of 26 G-protein-coupled receptors (GPCRs) (including opsins, cationic amine, melatonin, purine, chemokine, opioid, and glycoprotein hormone receptors and two related proteins, retinochrome and Duffy erythrocyte antigen) were calculated by distance geometry using interhelical hydrogen bonds formed by various proteins from the family and collectively applied as distance constraints, as described previously [Pogozheva et al., Biophys. J., 70 (1997) 1963]. The main structural features of the calculated GPCR models are described and illustrated by examples. Some of the features reflect physical interactions that are responsible for the structural stability of the transmembrane -bundle: the formation of extensive networks of interhelical H-bonds and sulfur–aromatic clusters that are spatially organized as 'polarity gradients' the close packing of side-chains throughout the transmembrane domain; and the formation of interhelical disulfide bonds in some receptors and a plausible Zn2+ binding center in retinochrome. Other features of the models are related to biological function and evolution of GPCRs: the formation of a common 'minicore' of 43 evolutionarily conserved residues; a multitude of correlated replacements throughout the transmembrane domain; an Na+-binding site in some receptors, and excellent complementarity of receptor binding pockets to many structurally dissimilar, conformationally constrained ligands, such as retinal, cyclic opioid peptides, and cationic amine ligands. The calculated models are in good agreement with numerous experimental data.  相似文献   
3.
Tetra-N-substituted 1,10-dioxa-4,7,13,16-tetraazacyclooctadecanes (tetraazacrown ethers) (1-5) have been synthesised. Each compound was incorporated in a PVC membrane that provided the basis for a lead-selective potentiometric electrode. The electrode based on the 4,7,13,16-tetrathenoyl-1,10-dioxa-4,7,13,16- tetraazacyclooctadecane (5) ionophore bearing four thenoyl functional subunits exhibited excellent lead (II) ion selectivity with only Ag+ and Hg2+ significantly interfering. The crystal structure of this host (5) has been determined by single crystal X-ray analysis. Crystal data. C32H36N4O6S4, M 700.9, monoclinic, space group P21/c, a, 10.315(4), b, 13.635(2), c, 14.357(6)Å, 124.63(2)°, V 1661.6(9)Å3, Dc 1.40 g cm-3, Z 2, Cu 29.90 cm-1. Final residuals R, Rw were 0.054, 0.082 for the observed data.  相似文献   
4.
Original hybrid organic-inorganic materials have been synthesised, characterised and used as optical chemical hydrazine sensors. Because of its optical and chemical properties, 4-(dimethyl)aminobenzaldehyde (DMAB) was chosen as indicator. DMAB has been immobilised by physical entrapment in a microporous silica network or by a chemical bonding on a colloidal silica network. The response time of the sensor was essentially governed by the diffusion of hydrazine in the host matrix whereas its life time was dependent on the retention of DMAB. These two features were controlled by the nature and the quantity of the network agent and the catalyst, by the ageing time of the sol, and the drying and thermal treatment of the films.  相似文献   
5.
A kinetic model for the effects of vanadate on human erythrocyte membrane   总被引:1,自引:0,他引:1  
The effects of vanadate on human erythrocyte membrane have been investigated with stopped-flow and equilibrium fluorescence quenching techniques. The equilibrium study showed a half-quenching concentration (K_(1/2)) of 0.27 mmol·L~(-1). The stopped-flow experiment exhibited a fast rise (t_(1、2)~f~1s) and a slow drop (t_(1/2)~s 1~2 min) in fluorescence. Based on the results and that from the across membrane transport of vanadate, a kinetic model is proposed which suggests that the membrane proteins experience a series of conformational changes before and during the quenching of the intrinsic fluorescence. These changes are induced mainly by three kinds of interactions: (i) the long-distance, non-specific interaction between the vanadate and the erythrocyte membrane surface, (ⅱ) the charge interaction between the vanadate and parts of the membrane proteins, and(ⅲ) the binding of the vanadate to some membrane proteins.  相似文献   
6.
A palladium composite membrane with a large number of defects was repaired using the electroless plating combined with the technique of osmosis. The loose structure of palladium film prepared by the conventional electroless plating was densified. Defects were repaired. Hydrogen selectivity was thus significantly increased without significantly increasing palladium film thickness and reducing hydrogen permeability. Project supported by the Chinese Academy of Sciences (Grant No. KJ951-A1-508) and the National Natural Science Foundation of China (Grant No. 29392003).  相似文献   
7.
The regularities of chemical reactions in solid 8-hydroxyquinoline—chloramine B mixtures were studied under conditions of organic self-propagating high-temperature synthesis (SHS), isothermal reaction, and thermal explosion in the 20–220 °C temperature range. Comprehensive physicochemical analysis and microstructural study of the reaction products were carried out. The temperature of SHS initiation (58 °C), the heat of the reaction (129±9 kJ mol−1), the stoichiometric coefficient (1), the maximum temperature (T max=98–140 °C), and the velocity of SHS wave propagation (u=0.15–0.55 mm s−1) were determined. Depending on the ratio of the reactants (n), a low-temperature non-degeerate stable gasless mode (n≤1,T max=115 °C,E a=42 kcal mol−1) and a high-temperature mode (n>1,T max=140 °C,E a=0.4 kcal mol−1) are possible for SHS. The SHS affords monohydroxy and monochloro derivatives of 8-hydroxyquinoline, benzenesulfonamide, NaCl, NaOH, and H2O. The mechanism of the solid-phase reaction at temperatures below 58 °C includes surface, solid-phase, and gas-phase diffusion; that for SHS is capillary spreading of the hydroxyquinoline melt. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2271–2284, December, 1999.  相似文献   
8.
The features of etching of latent tracks of heavy ions in poly(ethylene terephthalate) up to pore formation were investigated. It was found that the etching process included the following stages: (1) electrolyte penetration into the pores due to capillary forces and removal of monomeric radiolysis products; (2) swelling of the cross-linked (due to secondary electrons) polymer areas around the tracks to give the gel layer; (3) removal of the gel layer and the formation of track membranes with a pore radius of 40–50 ?. The radiation-chemical processes in polymers influence the physicochemical properties of the obtained membranes. The microrelief of the membrane surface was studied by atomic-force microscopy. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1111–1114, June, 1999.  相似文献   
9.
A program is described which enables performing of genetic algorithms for the determination of two positive real parameters. These new types of procedures are tested on a software of determination of flame temperatures previously developed in a fully classic way. The genetic operators used are crossover and mutation. They perform operations on a binary coded form of the parameters. The goal of the present study consists in developing and optimizing a genetic determination of the parameters at a given temperature. We succeed in selecting the general architecture of the procedure and implementing it in our main software of calculation of flame temperature. We have chosen this pyrotechnic field of application because we knew the behaviour of the real parameters, so the debugging operations were easier. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
10.
Modern thermal analysis, microcalorimetry and new emerging combined techniques which deliver calorimetric, microscopic and spectroscopic data offer a powerful analytical battery for the study of pharmaceuticals. These techniques are very useful in all steps of development of new drug products as well as methods for quality control in production. The characterization of raw materials enables to understand the relationships between polymorphs, solvates and hydrates and to choose the proper development of new drug products with very small amount of material in a very short time. Information on stability, purity is valuable for new entities as well as for marketed drug substances from different suppliers. Excipients which vary from single organic or inorganic entity to complexes matrixes or polymers need to be characterized and properly controlled. The thermodynamic phase-diagrams are the basis of the studies of drug-excipients interactions. They are very useful for the development of new delivery systems. A great number of new formulations need proper knowledge of the behaviour of the glass transition temperature of the components. Semi-liquid systems, interactions in aqueous media are also successfully studied by these techniques. This revised version was published online in August 2006 with corrections to the Cover Date.  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