全文获取类型
收费全文 | 14536篇 |
免费 | 681篇 |
国内免费 | 113篇 |
专业分类
化学 | 10153篇 |
晶体学 | 141篇 |
力学 | 402篇 |
数学 | 1420篇 |
物理学 | 3214篇 |
出版年
2023年 | 79篇 |
2022年 | 251篇 |
2021年 | 312篇 |
2020年 | 258篇 |
2019年 | 258篇 |
2018年 | 253篇 |
2017年 | 219篇 |
2016年 | 457篇 |
2015年 | 417篇 |
2014年 | 535篇 |
2013年 | 933篇 |
2012年 | 1096篇 |
2011年 | 1235篇 |
2010年 | 731篇 |
2009年 | 600篇 |
2008年 | 949篇 |
2007年 | 866篇 |
2006年 | 817篇 |
2005年 | 723篇 |
2004年 | 645篇 |
2003年 | 544篇 |
2002年 | 504篇 |
2001年 | 315篇 |
2000年 | 270篇 |
1999年 | 195篇 |
1998年 | 120篇 |
1997年 | 124篇 |
1996年 | 151篇 |
1995年 | 126篇 |
1994年 | 124篇 |
1993年 | 89篇 |
1992年 | 92篇 |
1991年 | 73篇 |
1990年 | 88篇 |
1989年 | 69篇 |
1988年 | 52篇 |
1987年 | 34篇 |
1986年 | 29篇 |
1985年 | 68篇 |
1984年 | 46篇 |
1983年 | 39篇 |
1982年 | 45篇 |
1981年 | 42篇 |
1980年 | 32篇 |
1979年 | 43篇 |
1978年 | 34篇 |
1977年 | 34篇 |
1976年 | 35篇 |
1975年 | 27篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
151.
Kim HG Lee CW Yun S Hong BH Kim YO Kim D Ihm H Lee JW Lee EC Tarakeshwar P Park SM Kim KS 《Organic letters》2002,4(22):3971-3974
[formula: see text] A new molecular system, 2,11-dithio[4,4]metametaquinocyclophane containing a quinone moiety, was designed and synthesized. As the quinone moiety can readily be converted into an aromatic pi-system (hydroquinone) upon reduction, the nanomechanical molecular cyclophane system exhibits a large flapping motion like a molecular flipper from the electrochemical redox process. The conformational changes upon reduction and oxidation are caused by changes of nonbonding interaction forces (devoid of bond formation/breaking) from the edge-to-face to face-to-face aromatic interactions and vice versa, respectively. 相似文献
152.
The small gap distance separating a spherical colloidal particle in electrophoretic motion from a planar nonconducting surface is a required parameter for calculating its electrophoretic mobility. In the presence of an externally applied electric field, this gap distance is determined by balancing the van der Waals, electrical double layer interaction, and gravitational forces with a dielectrophoretic (DEP) force. Here, the DEP force was determined analytically by integration of the Maxwell stress over the surface of the particle. The account of this force showed that its previous omission from the analysis always resulted in underpredicted gap distances. Furthermore, the DEP force dominated under conditions of low particle density or high electric field strength and led to much higher gap distances on the order of a few microns. In one particular case, a combination of low particle density and small particle size produced two possible equilibrium gap distances for the particle. However, the particle was unstable in the second equilibrium position when subjected to small perturbations. In general, larger particles had smaller gap sizes. The effects of four other parameters on gap distance were studied, and gap distances were found to increase with lower particle density, higher electric field strength, higher particle and wall zeta potentials, and lower Hamaker constants. Retardation effects on van der Waals attraction were considered. 相似文献
153.
W. Lyoo C. Park J. Yeum B. Ji C. Lee S. Lee J. Lee 《Colloid and polymer science》2002,280(12):1075-1083
To prepare ultrahigh molecular weight (UHMW) poly(vinyl pivalate) (PVPi) with high conversion and high linearity for a precursor of syndiotacticity-rich UHMW poly(vinyl alcohol) (PVA), vinyl pivalate (VPi) was suspension polymerized using a low-temperature initiator, 2,2'-azobis(2,4-dimethylvaleronitrile) (ADMVN), and the effects of polymerization conditions on the polymerization behavior and molecular structures of PVPi and PVA prepared by saponifying PVPi were investigated. Suspension polymerization was slightly inferior to bulk polymerization in increasing the molecular weight of PVA. In contrast, the former was superior in increasing the conversion of the polymer. Suspension polymerization of VPi at 25 °C by controlling various polymerization factors proved to be successful in obtaining PVA of UHMW (number-average degree of polymerization (Pn): 14,700-16,700), high syndiotactic diad content (62%), and of high yield (ultimate conversion of VPi into PVPi: 85-90%). In the case of bulk polymerization of VPi under the same conditions, maximum Pn, conversion of 15,800-17,000, and 25-35% were obtained, respectively. The degree of branching was lower and the Pn and syndiotacticity were higher with PVA prepared from PVPi polymerized at lower temperatures. All PVAs from PVPi suspension-polymerized at 25 °C were fibrous, with a high degree of crystallinity and orientation of the crystallites. 相似文献
154.
Hong J Kim YH Gil JH Cho K Jung JH Han SY 《Rapid communications in mass spectrometry : RCM》2002,16(22):2089-2093
The structural determination of sn-1 and sn-2 hexadecanoic lysophosphatidylcholine (LPC) regioisomers was carried out using fast atom bombardment tandem mass spectrometry (FAB-MS/MS). The collision-induced dissociation (CID) of protonated and sodiated molecules produced diverse product ions due mainly to charge remote fragmentations. Based on the information obtained from the CID spectra of protonated and sodiated molecules, sn-1 and sn-2 hexadecanoic LPC isomers could be discriminated. Especially, the abundance ratio of the diagnostic ion pair [m/z 224/226] in the CID spectra of [M + H](+) ions was shown to be greatly different. Moreover, the CID-MS/MS spectra of sodium-adducted molecules for hexadecanoic LPC isomers showed characteristic product ions such as [M + Na - 103](+), [M + Na - 85](+), and [M + Na - 59](+), by which their regio-specificity can be differentiated. 相似文献
155.
Park JB 《Experimental & molecular medicine》2003,35(5):325-335
Phagocytosis by inflammatory cells is an essential step and a part of innate immunity for protection against foreign pathogens, microorganism or dead cells. Phagocytosis, endocytotic events sequel to binding particle ligands to the specific receptors on phagocyte cell surface such as Fcgamma recptor (FcgammaR), complement receptor (CR), beta-glucan receptor, and phosphatidylserine (PS) receptor, require actin assembly, pseudopod extension and phagosome closure. Rho GTPases (RhoA, Cdc42, and Rac1) are critically involved in these processes. Abrupt superoxide formation, called as oxidative burst, occurs through NADPH oxidase complex in leukocytes following phagocytosis. NADPH oxidase complex is composed of membrane proteins, p22PHOX and gp91PHOX, and cytosolic proteins, p40PHOX, p47PHOX and p67PHOX. The cytosolic subunits and Rac-GTP are translocated to the membrane, forming complete NADPH oxidase complex with membrane part subunits. Binding of imunoglobulin G (IgG)- and complement-opsonized particles to FcgammaR and CR of leukocytes induces apoptosis of the cells, which may be due to oxidative burst and accompanying cytochrome c release and casapase-3 activation. 相似文献
156.
Microwave synthesis,characterization and catalytic properties of titanium-incorporated ZSM-5 zeolite
Taihuan Jin Young Kyu Hwang Do-Young Hong Sung Hwa Jhung Jin-Soo Hwang San-Eon Park Young Ho Kim Jong-San Chang 《Research on Chemical Intermediates》2007,33(6):501-512
Titanium-incorporated ZSM-5 zeolites (Si/Al = 50–200 and Si/Ti = 70) were successfully synthesized in a one-step sol-gel process
under microwave irradiation. The characteristics of Ti-ZSM-5 zeolites were investigated using X-ray power diffraction, UV/Vis-DRS,
FT-IR spectroscopy and solid-state 27Al-NMR to monitor the physico-chemical properties. Simultaneously, the acidic properties were characterized by the NH3-TPD profile. The characterization results revealed that the Ti4+ and Al3+ ions were well incorporated into the framework of Ti-ZSM-5 zeolite. The prepared zeolite was moderately active but selective
in the dehydration of methanol to dimethyl ether. 相似文献
157.
A novel process was developed to fabricate biodegradable polymer scaffolds for tissue engineering applications, without using organic solvents. Solvent residues in scaffolds fabricated by processes involving organic solvents may damage cells transplanted onto the scaffolds or tissue near the transplantation site. Poly(L-lactic acid) (PLLA) powder and NaCl particles in a mold were compressed and subsequently heated at 180 degrees C (near the PLLA melting temperature) for 3 min. The heat treatment caused the polymer particles to fuse and form a continuous matrix containing entrapped NaCl particles. After dissolving the NaCl salts, which served as a porogen, porous biodegradable PLLA scaffolds were formed. The scaffold porosity and pore size were controlled by adjusting the NaCl/PLLA weight ratio and the NaCl particle size. The characteristics of the scaffolds were compared to those of scaffolds fabricated using a conventional solvent casting/particulate leaching (SC/PL) process, in terms of pore structure, pore-size distribution, and mechanical properties. A scanning electron microscopic examination showed highly interconnected and open pore structures in the scaffolds fabricated using the thermal process, whereas the SC/PL process yielded scaffolds with less interconnected and closed pore structures. Mercury intrusion porosimetry revealed that the thermally produced scaffolds had a much more uniform distribution of pore sizes than the SC/PL process. The utility of the thermally produced scaffolds was demonstrated by engineering cartilaginous tissues in vivo. In summary, the thermal process developed in this study yields tissue-engineering scaffolds with more favorable characteristics, with respect to, freedom from organic solvents, pore structure, and size distribution than the SC/PL process. Moreover, the thermal process could also be used to fabricate scaffolds from polymers that are insoluble in organic solvents, such as poly(glycolic acid). Cartilage tissue regenerated from thermally produced PLLA scaffold. 相似文献
158.
Fabrication of hollow palladium spheres and their successful application to the recyclable heterogeneous catalyst for suzuki coupling reactions 总被引:25,自引:0,他引:25
Novel palladium hollow spheres were synthesized using silica spheres as a template, and they were successfully applied as recyclable heterogeneous catalysts for Suzuki cross coupling reactions. 相似文献
159.
G. J. Dear D. N. Mallett D. M. Higton A. D. Roberts S. A. Bird H. Young R. S. Plumb I. M. Ismail 《Chromatographia》2002,55(3-4):177-184
Summary In this paper we investigate the potential of alkyl-bonded silica monolithic columns for the isolation and identification
of drug-related components in biological fluids. Up to 6 columns have been connected in series to produce a chromatographic
system with up to 40,000 plates. This high-resolution chromatography system has been coupled to both MS and NMR to enable
efficient detection and characterisation of drug-related components in biological fluids. The use of six coupled columns has
been shown to give enhanced resolution over a high quality silica particulate column packed with 3 μm material which exhibits
the same back pressure. The effect of volume and mass load on the performance of monolithic columns for semi-preparative chromatography
of biological fluids has also been investigated. In these studies it was possible to inject up to 100 mL of neat urine with
no loss of chromatographic performance. Furthermore, upon re-testing, the columns showed similar chromatographic performance.
Again several columns were serially connected, producing enhanced resolution in the semi-preparative mode. 相似文献
160.
Heo YS Ryu JM Park SM Park JH Lee HC Hwang KY Kim J 《Experimental & molecular medicine》2002,34(3):211-223
Protein-tyrosine phosphatases (PTPs) constitute a family of receptor-like, and cytoplasmic enzymes, which catalyze the dephosphorylation of phosphotyrosine residues in a variety of receptors and signaling molecules. Together with protein tyrosine kinases (PTKs), PTPs are critically involved in regulating many cellular signaling processes. In this study, diverse compounds were screened for PTP inhibition and selectively screened for inhibitors with the end product inhibition properties. Among phosphate analogues and their derivatives for PTP inhibition, Keggin compounds phosphomolybdate (PM) and phosphotungstate (PT) strongly inhibited both PTP-1B and SHP-1, with K(i) values of 0.06-1.2 micromM in the presence of EDTA. Unlike the vanadium compounds, inhibition potencies of PM and PT were not significantly affected by EDTA. PM and PT were potent, competitive inhibitors for PTPs, but relatively poor inhibitors of Ser/Thr phosphatase. Interestingly, PM and PT did not inhibit alkaline phosphatase at all. The crystal structure of PTP-1B in complex with PM, at 2.0 A resolution, reveals that MoO(3), derived from PM by hydrolysis, binds at the active site. The molybdenium atom of the inhibitor is coordinated with six ligands: three oxo-ligands, two apical water molecules and a S atom of the catalytic cysteine residue. In support of the crystallographic finding, we observed that molybdenium oxides (MoO(3), MoO(2), and MoO(2)Cl(2)) inhibited PTP-1B with IC(50) in the range 5-15 micromM. 相似文献