全文获取类型
收费全文 | 7212篇 |
免费 | 329篇 |
国内免费 | 81篇 |
专业分类
化学 | 4297篇 |
晶体学 | 221篇 |
力学 | 194篇 |
综合类 | 4篇 |
数学 | 1049篇 |
物理学 | 1857篇 |
出版年
2022年 | 83篇 |
2021年 | 76篇 |
2020年 | 97篇 |
2019年 | 145篇 |
2018年 | 104篇 |
2017年 | 99篇 |
2016年 | 146篇 |
2015年 | 123篇 |
2014年 | 190篇 |
2013年 | 564篇 |
2012年 | 316篇 |
2011年 | 357篇 |
2010年 | 239篇 |
2009年 | 211篇 |
2008年 | 246篇 |
2007年 | 255篇 |
2006年 | 220篇 |
2005年 | 176篇 |
2004年 | 173篇 |
2003年 | 204篇 |
2002年 | 171篇 |
2001年 | 100篇 |
2000年 | 94篇 |
1999年 | 67篇 |
1997年 | 70篇 |
1996年 | 79篇 |
1995年 | 85篇 |
1994年 | 82篇 |
1993年 | 84篇 |
1992年 | 98篇 |
1991年 | 88篇 |
1990年 | 76篇 |
1989年 | 110篇 |
1988年 | 76篇 |
1987年 | 70篇 |
1986年 | 81篇 |
1985年 | 126篇 |
1984年 | 119篇 |
1983年 | 93篇 |
1982年 | 99篇 |
1981年 | 101篇 |
1980年 | 85篇 |
1979年 | 98篇 |
1978年 | 74篇 |
1977年 | 103篇 |
1976年 | 79篇 |
1975年 | 87篇 |
1974年 | 92篇 |
1973年 | 97篇 |
1972年 | 57篇 |
排序方式: 共有7622条查询结果,搜索用时 46 毫秒
41.
The true potential energy curves for different electronic states of diatomic molecules have been constructed by a simplified and modified form of the Jarmain method. To check the validity of this new procedure potential energy curves are constructed for NO+, VO, Si2, AsO, AsO+, PF+, CO and Bi2. The results are in good agreement with the Jarmain, RKRV and Lakshman and Rao methods. 相似文献
42.
Svein G. Dahl Peter A. Kollman Shashidhar N. Rao U. Chandra Singh 《Journal of computer-aided molecular design》1992,6(3):207-222
Summary The side-chain conformations of psychoactive phenothiazine drugs in crystals are different from those of biologically inactive ring sulfoxide metabolites. This study examines the potential energies, molecular conformations and electrostatic potentials in chlorpromazine, levomepromazine (methotrimeprazine), their sulfoxide metabolites and methoxypromazine. The purpose of the study was to examine the significance of the different crystal conformations of active and inactive phenothiazine derivatives, and to determine why phenothiazine drugs lose most of their biological activity by sulfoxidation. Quantum mechanics and molecular mechanics calculations demonstrated that conformations with the side chain folded over the ring structure had lowest potential energy in vacuo, both in the drugs and in the sulfoxide metabolites. In the sulfoxides, side chain conformations corresponding to the crystal structure of chlorpromazine sulfoxide were characterized by stronger negative electrostatic potentials around the ring system than in the parent drugs. This may weaken the electrostatic interaction of sulfoxide metabolites with negatively charged domains in dopamine receptors, and cause the sulfoxides to be virtually inactive in dopamine receptor binding and related pharmacological tests. 相似文献
43.
Two layered amine-templated cobalt squarates, [C6N2H14]2[Co2(C4O4)3(H2O)4], I, and [C3N2H5]2[Co2(C4O4)3(H2O)4], II, have been prepared under hydrothermal conditions. Both I and II contain chains formed by dimers comprising two cobalt atoms bound to the squarate units, the chains being connected through hydrogen bond interactions. An amine-templated cobalt squarate of the formula [C4N2H12][Co(C4O4)2(H2O)4][H2O]2, III, as well as its Ni, Zn and Cd analogues have been prepared by room temperature reactions. III has a layered architecture wherein the cobalt-squarate monomers are linked by the amine molecules. Co and Zn analogues of [Ni(C4O4)(H2O)2(C3N2H4)] with ligating imidazole units have also been prepared and characterized. 相似文献
44.
D. Choudhury R. C. Borah R. L. Goswamee H. P. Sharmah P. G. Rao 《Journal of Thermal Analysis and Calorimetry》2007,89(3):965-970
Pyrolysis of petroleum refinery sludge has received global acclamation as a clean conversion technique for providing solution
of sludge disposal as well as efficient resource utilization. This communication reports the kinetics study of pyrolysis of
petroleum refinery sludge. Experiments were carried out by means of thermogravimetric analysis at different heating rates
of 5, 10 and 20°C min−1. The pyrolytic reaction is significant in the temperature range of 200–350°C and analysis and evaluation of kinetic parameters
is done in the 100–500°C region of non-isothermal TG curves obtained in nitrogen atmosphere.
The activation energy is calculated by iso-conversional method, then other kinetic parameters are determined by considering
single reaction and two reaction global kinetic model. Two-reaction model is found to fit satisfactorily the experimental
results. 相似文献
45.
Although the feasibility of affinity ultrafiltration was demonstrated more than 20 years ago, commercial applications have not developed due to the high cost and practical limitations of the large macroligands needed for highly selective separations. The objective of this study was to examine the use of small charged affinity ligands for protein purification by exploiting electrostatic interactions between the charged complex and an electrically-charged membrane. Experiments were performed using bovine serum albumin and ovalbumin with Cibacron Blue as the affinity ligand. Negatively charged versions of a composite regenerated cellulose membrane were generated by covalent attachment of a sulfonic acid functionality. Binding experiments were used to identify appropriate conditions for protein separations. The selectivity for the separation of BSA and ovalbumin was a function of the solution conditions, Cibacron Blue concentration, and membrane charge, with the addition of Cibacron Blue causing a 30-fold increase in selectivity. A diafiltration process was performed at the optimal conditions, giving a BSA product with a purification factor of more than 90-fold and a yield greater than 90%. These results clearly demonstrate the potential of using a small charged affinity ligand for high resolution protein separations. 相似文献
46.
Trioxalatocobaltates of bivalent metals KM2+[Co(C2O4)3]·x H2O, with M2+ = Ba, Sr, Ca and Pb, have been prepared, characterized and their thermal behaviour studied. The compounds decompose to yield potassium carbonate, bivalent metal carbonate or oxide and cobalt oxide as final products. The formation of the final products of decomposition is influenced by the surrounding atmosphere. Bivalent metal cobaltites of the types KM2+CoO3 and M2+CoO3—x are not identified among the final products of decomposition. The study brings out the importance of the decomposition mode of the precursor in producing the desired end products. 相似文献
47.
Rao DP Sivakumar SV Mandal S Kota S Ramaprasad BS 《Journal of chromatography. A》2005,1069(1):141-151
The separation of propylene-propane mixture is an energy intensive operation commercially practiced using cryogenic distillation. The separation by pressure swing adsorption has been studied as an alternative. A fixed-bed pressure swing adsorption yields the heavy component as a pure product. The product recovery and the productivity are not high. In a moving-bed process, because of the counter-current solid-gas contact, the separation achieved is similar to that of the fractionation by distillation. Although the moving-bed operation offers the upper limit for the performance of a cyclic adsorptive process, due to mechanical complexities in the handling of solids the 'simulated' moving-bed is preferred. By moving the inlet and outlet ports of streams located along the length of the bed, a moving-bed process can be realized in a fixed bed. We describe here a 'moving-port' system which permits injection or withdrawal of the fluid along the axial direction in a fixed bed. A fixed bed embedded with the moving-port systems emulates a simulated moving-bed adsorber. The proposed adsorber can fractionate a binary gas mixture into two product streams with high purities. It is similar to the Sorbex process of UOP but does not have the eluent as an additional separating agent. A parametric study indicates that high purity products and a higher productivity by an order of magnitude can be achieved with simulated moving-beds compared to the fixed beds. 相似文献
48.
Boruah A Rao IN Nandy JP Kumar SK Kunwar AC Iqbal J 《The Journal of organic chemistry》2003,68(12):5006-5008
A cis-proline derived cyclic mimic of a type VI beta-turn is synthesized via a ring-closing metathesis reaction. The solution NMR conformational study indicates that the major conformer of the cyclic peptide adopts a type VIa beta-turn in CDCl(3) and a type VIb beta-turn in DMSO-d(6). 相似文献
49.
The kinetics of the oxidation of oxalic acid by cerium(IV) in sulfuric acid medium has been studied voltammetrically. The specific reaction rate is 132±4.0 M–1s–1 at 25.0 °C. The energy of activation is 62.6±3.0 kJ mol–1. The entropy of activation is –2.7 J mol–1K–1. The specific reaction rate is influenced by complexation and also by ionic strength (). The most likely mechanism has been suggested.
(IV) . 132±4,0 M–1c–1 25,0 °C. 62,6±3,0 ·M–1. –2,7 ·K–1M–1. , (). .相似文献
50.
The polarographic behaviour of uranium and tellurium has been studied in the presence of increasing concentration of complex forming agent viz. ?-caprolactam at pH 4.4 for U(VI) and at pH 10.0 for Te(IV) at μ=0.1 M KNO3. In both cases the waves have been found to be diffusion controlled and irreversible. The values of kinetic parameters (σπa and K°f,h) have been calculated as a function of the ligand concentration. Method is suitable for the quantitative determination of these metals individually and simultaneously with the effect of diverse ions. 相似文献