全文获取类型
收费全文 | 2639篇 |
免费 | 81篇 |
国内免费 | 22篇 |
专业分类
化学 | 1908篇 |
晶体学 | 26篇 |
力学 | 88篇 |
综合类 | 1篇 |
数学 | 353篇 |
物理学 | 366篇 |
出版年
2024年 | 16篇 |
2023年 | 38篇 |
2022年 | 273篇 |
2021年 | 176篇 |
2020年 | 109篇 |
2019年 | 91篇 |
2018年 | 93篇 |
2017年 | 77篇 |
2016年 | 126篇 |
2015年 | 77篇 |
2014年 | 85篇 |
2013年 | 217篇 |
2012年 | 143篇 |
2011年 | 163篇 |
2010年 | 93篇 |
2009年 | 90篇 |
2008年 | 119篇 |
2007年 | 109篇 |
2006年 | 88篇 |
2005年 | 83篇 |
2004年 | 63篇 |
2003年 | 63篇 |
2002年 | 43篇 |
2001年 | 26篇 |
2000年 | 17篇 |
1999年 | 20篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 18篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 17篇 |
1992年 | 18篇 |
1991年 | 12篇 |
1990年 | 9篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1983年 | 11篇 |
1982年 | 11篇 |
1980年 | 10篇 |
1979年 | 12篇 |
1978年 | 12篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 10篇 |
1974年 | 4篇 |
1973年 | 3篇 |
排序方式: 共有2742条查询结果,搜索用时 15 毫秒
41.
4-Aminoperoxybenzoic acid supported on silica gel was found to be a versatile and efficient oxidant for the oxidation of ketones to esters. 相似文献
42.
An efficient synthesis of predominantly (E) symmetrical or unsymmetrical 1,2-difluorostilbenes based on the Suzuki–Miyaura palladium-catalyzed cross-coupling reaction of arylboronic acids with predominantly (E)-1,2-dibromo-1,2-difluoroethene in the presence of Cs2CO3 in toluene is described. The reaction preserved the stereochemistry of the building block and performed in good yield independently of the electron-withdrawing or electron-donating character of the substituents. 相似文献
43.
Formation, crystal structure, polymorphism, and transition between polymorphs are reported for M(thd)3, (M = Al, Cr, Mn, Fe, Co, Ga, and In) [(thd)– = anion of H(thd) = C11H20O2 = 2, 2, 6, 6‐tetramethylheptane‐3, 5‐dione]. Fresh crystal‐structure data are provided for monoclinic polymorphs of Al(thd)3, Ga(thd)3, and In(thd)3. Apart from adjustment of the M–Ok bond length, the structural characteristics of M(thd)3 complexes remain essentially unaffected by change of M. Analysis of the M–Ok, Ok–Ck, and Ck–Ck distances support the notion that the M–Ok–Ck–Ck–Ck–Ok– ring forms a heterocyclic unit with σ and π contributions to the bonds. Tentative assessments according to the bond‐valence or bond‐order scheme suggest that the strengths of the σ bonds are approximately equal for the M–Ok, Ok–Ck, and Ck–Ck bonds, whereas the π component of the M–Ok bonds is small compared with those for the Ok–Ck, and Ck–Ck bonds. The contours of a pattern for the occurrence of M(thd)3 polymorphs suggest that polymorphs with structures of orthorhombic or higher symmetry are favored on crystallization from the vapor phase (viz. sublimation). Monoclinic polymorphs prefer crystallization from solution at temperatures closer to ambient. Each of the M(thd)3 complexes subject to this study exhibits three or more polymorphs (further variants are likely to emerge consequent on systematic exploration of the crystallization conditions). High‐temperature powder X‐ray diffraction shows that the monoclinic polymorphs convert irreversibly to the corresponding rotational disordered orthorhombic variant above some 100–150 °C (depending on M). The orthorhombic variant is in turn transformed into polymorphs of tetragonal and cubic symmetry before entering the molten state. These findings are discussed in light of the current conceptions of rotational disorder in molecular crystals. 相似文献
44.
Ahmed A. K. Mohammed Peter A. Limacher Benoît Champagne 《Journal of computational chemistry》2013,34(17):1497-1507
The finite field method, widely used for the calculation of static dipole polarizabilities or the first and second hyperpolarizabilities of molecules and polymers, is thoroughly explored. The application of different field strengths and the impact on the precision of the calculations were investigated. Borders could be defined and characterized, establishing a range of feasible field strengths that guarantee reliable numerical results. The quality of different types of meshes to screen the feasible region is assessed. Extrapolation schemes are presented that reduce the truncation error and allow to increase the precision of finite field calculations by one to three orders of magnitude. © 2013 Wiley Periodicals, Inc. 相似文献
45.
A.-H. Abdel-Rahman Adel E. El Aassy Ibrahim Y. Ahmed Fadia F. Hamza Mohammed 《Journal of Dispersion Science and Technology》2013,34(8):1128-1135
Synthesis of poly(acrylamidoxime) resin from polyacrylonitrile performed with different crosslinking ratios 2, 5, and 10 wt% of divinylbenzene as crosslinking agent, using methylbenzoate and dioctylphthalate as pore producing solvent, the reaction mixture occurred under nitrogen. Studies carried out on diluted solution from rare earth elements (REEs) concentrate contains impurities as Cu2+, Ni2+, Zn2+, Fe3+, Al3+, Si4+, Th4+, U6+, Ca2+, and K+. Changing some parameters as pH of the solution, time of feeding and type of acid as HCl, HNO3, H2SO4. The adsorption efficiency of resin is in the order pH 6 > pH 5 > pH 4 > pH 2 with excluding pH 6 due to the precipitation of some of REEs with the impurities and complete precipitation of Dy ion during pH adjustment, the adsorption in HNO3 > HCl > H2SO4 media. 相似文献
46.
Tejas Patel Goutam Ghosh Shin-ichi Yusa Pratap Bahadur 《Journal of Dispersion Science and Technology》2013,34(8):1111-1118
The effect of different kinds of additives (electrolytes, nonelectrolytes, hydrotropes, and surfactants) on the cloud point (CP) of low molecular weight and narrow dispersed poly(n-isopropylacrylamide) (PNIPAM) synthesized via reversible addition-fragmentation chain transfer (RAFT) controlled radical polymerization was examined. The CP showed a concentration dependent variation and it is greatly modified in the presence of additives. The size of the random polymer coil at 30°C obtained from dynamic light scattering (DLS) measurements is often influenced by the presence of additives. We have explained the effects of different additives on PNIPAM in terms of their interaction with polymer and resultant changes in the coil structure. 相似文献
47.
J. P. Mata P. R. Majhi M. Yamashita A. Khanal K. Nakashima P. Bahadur 《Journal of Dispersion Science and Technology》2013,34(9):1248-1256
The effect of sodium dodecyl sulfate (SDS) on the micellization and aggregation behavior of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) amphiphilic copolymer (Pluronic L64: EO13 PO30 EO13) have been investigated by various techniques like, cloud point, viscosity, isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC), fluorescence spectroscopy, room temperature phosphorescence (RTP), and small angle neutron scattering (SANS). Addition of SDS in L64 solutions shows mark alteration of different properties. We observed synergistic interaction between SDS and Pluronic L64. The changes in the critical micelle concentration (CMC), critical micelle temperature (CMT), cloud point (CP), micelle size, and shape has been correlated and reported in terms of structure dynamics and mechanics. The ITC titrations have been used to explore the different stages of binding and interactions of SDS with L64. The enthalpies of aggregation for copolymer-SDS aggregates binding, organizational change of bound aggregates, and the threshold concentrations of SDS in the presence of copolymer were estimated directly from ITC titration curves. The effect of temperature on enthalpy values has been reported in terms of different aggregation state. Fluorescence and RTP for L64 were used to investigate the change in micellar environment on the addition of SDS at different temperature. Appearance and shifting of SANS peaks have been used to monitor the size and inter micellar interaction on addition of SDS in L64 solution. Cloud point and viscosity elaborate the penetration of SDS molecule in L64 micelle and hence changing the micellar architect. 相似文献
48.
Mohammed Taghi Zafarani-Moattar Roghayeh Majdan-Cegincara 《Colloid and polymer science》2013,291(8):1977-1987
The Fe3O4 nanoparticles and Fe3O4 nanoparticles coated with oleic acid have been dispersed in base fluid of poly(ethylene glycol) (PEG). Stability and particle size distribution of these nanofluids have been studied by result analysis of UV–Vis spectroscopy, zeta potential and dynamic light scattering. Blue shift of UV–Vis spectra has been related to quantum effects such as band gap enlargement with particle size decreasing and also to effect of oleic acid on the ultraviolet wavelength. Flow behavior and suspension structure of Fe3O4 nanoparticles dispersed in PEG have been determined by rheological properties. Viscosity values of Fe3O4-PEG nanofluid as a function of temperature have also been investigated. The chain-like structure of Fe3O4 nanoparticles coated with oleic acid in base fluid of PEG has been verified by measuring the magnetorheological properties. The effect of temperature on magnetorheological properties of Fe3O4 nanoparticles coated with oleic acid has also been investigated in base fluid of PEG. The volumetric properties of Fe3O4-PEG and Fe3O4 coated with oleic acid–PEG nanofluids and PEG–oleic acid solution have also been measured at different temperatures to specify the suspension structure and also interactions of Fe3O4, PEG and oleic acid molecules. 相似文献
49.
A simple, rapid, sensitive, and inexpensive method for spectrophotometric determination of chromium(VI), based on the absorbance of its complex with 1,4,8,11-tetraazacyclotetradecane (cyclam) is presented. The complex showed a molar absorbtivity of 1.5?×?104?L?mol?1?cm?1 at 379?nm. Under optimum experimental conditions, a pH of 4.5 and 1.960?×?103?mg?L?1 cyclam were selected, and all measurements were performed 10?min after mixing. Major cations and anions did not show any interference; Beer's law was applicable in the concentration range 0.2–20?mg?L?1 with a detection limit of 0.001?mg?L?1. The standard deviation in the determination is ±0.5?mg?L?1 for a 15.0?mg?L?1 solution (n?=?7). The described method provides a simple and reliable means for determination of Cr(VI) in real samples. 相似文献
50.
Treatment of the 1,6‐anhydrosugar epoxide 5 with a cyano‐Gilman cuprate [(CuCN (6 eq.), MeLi (12 eq.)] surprisingly led to the open chain rearranged (2S,3R)‐1,2‐dihydroxy‐3,6‐dimethylheptan‐4‐one (7), structurally confirmed by conversion to the corresponding diacetate 8. Another unusual reaction was found by hydrogenation of the 2‐tosyl‐1‐bromosugar 11, leading in one operation to the twofold deoxygenated chiral pyran derivative 14. This procedure might prove to be useful in the rapid deoxygenation of sugar derivatives. 相似文献