全文获取类型
收费全文 | 10450篇 |
免费 | 445篇 |
国内免费 | 128篇 |
学科分类
数理化 | 11023篇 |
出版年
2024年 | 24篇 |
2023年 | 93篇 |
2022年 | 473篇 |
2021年 | 434篇 |
2020年 | 359篇 |
2019年 | 444篇 |
2018年 | 450篇 |
2017年 | 370篇 |
2016年 | 606篇 |
2015年 | 420篇 |
2014年 | 572篇 |
2013年 | 1068篇 |
2012年 | 816篇 |
2011年 | 774篇 |
2010年 | 558篇 |
2009年 | 451篇 |
2008年 | 496篇 |
2007年 | 439篇 |
2006年 | 311篇 |
2005年 | 318篇 |
2004年 | 193篇 |
2003年 | 192篇 |
2002年 | 160篇 |
2001年 | 83篇 |
2000年 | 55篇 |
1999年 | 64篇 |
1998年 | 41篇 |
1997年 | 52篇 |
1996年 | 48篇 |
1995年 | 35篇 |
1994年 | 39篇 |
1993年 | 20篇 |
1992年 | 37篇 |
1991年 | 33篇 |
1990年 | 30篇 |
1989年 | 36篇 |
1988年 | 33篇 |
1987年 | 34篇 |
1986年 | 30篇 |
1985年 | 37篇 |
1984年 | 35篇 |
1983年 | 21篇 |
1982年 | 28篇 |
1981年 | 19篇 |
1980年 | 23篇 |
1979年 | 20篇 |
1978年 | 18篇 |
1977年 | 13篇 |
1976年 | 15篇 |
1974年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Ibnelwaleed A. Hussein M. Sayem Mozumder Sk. Asrof Ali 《European Polymer Journal》2005,41(10):2472-2482
Rheological properties of hydrophobically modified copolymer of SO2, N,N-diallyl-N-carboethoxymethylammonium chloride and the hydrophobic monomer N,N-diallyl-N-octadecylammonium chloride were studied. The influence of hydrophobe content (HP) and polymer concentration was investigated. Polymers with HP content in the range 1.5-5% were examined and the concentration was varied in the range 2-5 wt%. Both dynamic and steady-shear experiments were performed in ARES rheometer. Copolymers were observed to exhibit typical viscoelastic behavior even with low HP content. Both the dynamic viscosity, η′ and storage modulus, G′, increase with the increase of both the polymer concentration and the HP content of the system. The viscosity of the high HP content polymer showed a strong shear dependency, while G′ was a weak function of frequency and gel-like behavior was observed. The zero-shear viscosity, η0, showed a strong concentration dependency (η0 ∼ ?α; 1.1 < α < 5.9). The concentration dependency of η0 suggests that intermolecular association is dominant in the high HP content polymer. Control of the HP content and polymer concentration of this class of polymers can lead to a wide range of interesting rheological properties. 相似文献
112.
A liquid chromatographic (LC) multiresidue screening procedure was developed for determination of eprinomectin, moxidectin, abamectin, doramectin, and ivermectin in beef liver at 0, 25, 50, and 100 ppb levels. A procedure using low resolution LC/atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) was developed with further purification steps added to the quantitative LC method to confirm residues. Acetonitrile extracts of liver, prior to derivatization for LC analysis, were further purified by using a C8 solid-phase extraction cartridge and an alumina-B cartridge. The purified extract was analyzed by injection into an LC/positive ion APCI MS. Identity of the compound was confirmed by comparison of its retention time and relative intensity data with those of a standard or recovery from a fortified control liver sample. Anthelmintic drugs in acetonitrile extracts of liver containing eprinomectin, moxidectin, abamectin, doramectin, and ivermectin at 25 ppb, the lowest level of fortification used in the LC determinative method, were successfully confirmed. 相似文献
113.
Ali Tavassoli 《Tetrahedron letters》2005,46(12):2093-2096
Trimethyl (2S,3R)- and (2R,3R)-[2-2H1]-homocitrates, 10b and 10c respectively, and dimethyl (2S,3R)- and (2R,3R)-[2-2H1]-homocitric lactones, 11b and 11c respectively, have been synthesised from shikimic acid and [2-2H]-shikimic acid by a route which defines the stereochemistry of the two chiral centres in each compound. The NMR spectra of these products will enable the stereochemistry of the biological reaction catalysed by homocitrate synthase and by the protein from the nifV gene to be elucidated. 相似文献
114.
Aliphatic and aromatic 1,3‐dithiane are oxidized to the corresponding carbonyl compounds in good yields under mild conditions by N,N′‐diiodo‐N,N′‐1,2‐ethanediyl‐bis(p‐toluenesulphonamide) [NIBTS] and silver nitrate. 相似文献
115.
Ali irpan Selmiye Alkan Levent Toppare Yeim Hepuzer Yusuf Yaci 《Journal of polymer science. Part A, Polymer chemistry》2002,40(23):4131-4140
A thiophene‐functionalized methacrylate monomer (3‐methylthienyl methacrylate) was synthesized via the esterification of 3‐thiophene methanol with methacryloyl chloride. The methacrylate monomer was polymerized by free‐radical polymerization in the presence of azobisisobutyronitrile as the initiator. Graft copolymers of poly(3‐methylthienyl methacrylate) (PMTM2) and polypyrrole and of PMTM2 and polythiophene were synthesized by constant‐potential electrolyses. p‐Toluene sulfonic acid, sodium dodecyl sulfate, and tetrabutylammonium tetrafluoroborate were used as the supporting electrolytes. PMTM2‐coated platinum electrodes were used as anodes in the polymerization of pyrrole and thiophene. Moreover, the oxidative polymerization of poly(3‐methylthienyl methacrylate) (PMTM1) was studied with FeCl3 as the oxidant. The self‐polymerization of PMTM1 was also investigated by galvanostatic electrolysis both in dichloromethane and in propylene carbonate. The structures of PMTM1 and PMTM2 were investigated by several spectroscopic and thermal methods. The grafting process was elucidated with conductivity measurements, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and scanning electron microscopy studies. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4131–4140, 2002 相似文献
116.
Talebi M Ghassempour A Talebpour Z Rassouli A Dolatyari L 《Journal of separation science》2004,27(13):1130-1136
A simple and rapid microwave-assisted extraction (MAE) procedure was developed and optimized for the extraction of paclitaxel (Taxol) from the needles of yew trees Taxus baccata L. grown in Iranian habitats. The samples, immersed in a methanol-water mixture, were irradiated with microwaves in a closed-vessel system. The method was evaluated using a factorial design approach based on parameters such as extraction time, temperature, methanol concentration in water (v/v), and the ratio of grams of sample to 10 mL of solvent. Statistical treatment of the results revealed that the selected parameters were all significant except the extraction time. Optimum conditions would be 1.5 g samples in 10 mL solvent (90% methanol), an extraction temperature of 95 degrees C, and an extraction time of 7 min. The extracts has been analyzed by reverse-phase high-performance liquid chromatography with UV detection (LC/UV) at 227 nm for quantification. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used for confirmation. The main advantage of the proposed MAE method versus conventional solvent extraction (CSE) are the considerable reductions in time (7 min versus 16 h) and in solvent consumption (20 mL versus 150 mL). The MAE procedure yielded extracts that could be analyzed directly without any preliminary clean-up or solvent exchange steps. Both extraction methods show RSDs lower than 10% and lead to comparable recoveries of paclitaxel (87-92%). 相似文献
117.
The complexes formed by photosubstitution of pyrazine (Pz) in octacyanomolybdate(IV) and -tungstate(IV) with 8-hydroxyquinoline have been assigned the formulae [Mo(CN)2(OH)2(Pz)2(OX)] and [W(CN)2(OH)2(Pz)2(OX)·1.5H2O]. Coordination of Pz as an unidentate ligand by donating a lone pair of electron from nitrogen is shown by an absorption peak between 8–11 µ. Mechanism for the thermal decomposition of the complexes has been given. The formation of tungsten metal as residue in case of II has been confirmed by XRD analysis. The kinetic and thermodynamic parameters like activation energy (E
a), pre-exponential factor (A) and entropy of activation (S
#) were calculated employing different integral methods of Doyle, Coats and Redfern and Arrhenius.H for each stage of decomposition was obtained from DSC.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
118.
Yusuf M. Al-Hiari Ali M. Qaisi Mustafa M. El-Abadelah Wolfgang Voelter 《Monatshefte für Chemie / Chemical Monthly》2006,137(2):243-248
Summary. A synthesis of 3-(4-methoxycarbonyl-2,6-dinitrophenyl)indole, its 2,6-diamino analog, and 3-(2-amino-4-trifluoromethyl-6-nitrophenyl)indole
is described. 4-(Trifluoromethyl)phenyl derivatives exhibit higher antibacterial potency than the former 4-(methoxycarbonyl)phenyl
homologs, while 3-(4-trifluoromethyl-2-nitrophenyl)indole was the most active agent in the series, with MIC ≈ 7 μg/cm3 against E. coli and S. aureus. 相似文献
119.
In this paper we, present some MATLAB and GAP programs and use them to find the automorphism group of the Euclidean graph
of the C80 fullerence with connectivity and geometry of I
h
symmetry point group. It is proved that this group has order 120 and is isomorphic to I
h
≊Z2×A5, where Z2 is, a cyclic group of order 2 and A5 is the alternating group on five symbols. 相似文献
120.
Akbar Ali Mohammad Huq Mirza Aminul Nazimuddin Mohamed Rahman Hafazatur Butcher Raymond J. 《Transition Metal Chemistry》2002,27(3):268-273
New cadmium(II) complexes of empirical formulae, [Cd(dpksme)X] (dpksme = anionic form of the Schiff base; X = NCS–, Cl–, I–) and [Cd(dpksme)2] · 0.5MeOH, respectively have been prepared and characterized. The mono-ligated cadmium(II) complexes, [Cd(dpksme)X] are four-coordinate and tetrahedral but the bis-ligand complex, [Cd(dpksme)2] · 0.5MeOH is six-coordinate and octahedral. The crystal and molecular structure of [Cd(dpksme)2] · 0.5MeOH has been determined by X-ray diffraction. The complex has a distorted mer-octahedral structure in which the ligands are coordinated as uninegatively charged tridentate chelating agents via the pyridine nitrogen atoms, the azomethine nitrogen atoms and the thiolate sulfur atoms. The distortion from regular octahedral geometry is ascribed to the restricted bite angles of the ligands. The Schiff base and its cadmium(II) complexes exhibit mild antibacterial activities against Shigella dysenteriae, Bacillus cereus, Staphylococcus aureus and Escherichia coli. They are also mildly fungitoxic against the phytopathogenic fungi, Alternaria alternata and Macrophomina phaseolina. 相似文献