全文获取类型
收费全文 | 20300篇 |
免费 | 3993篇 |
国内免费 | 3456篇 |
专业分类
化学 | 14947篇 |
晶体学 | 422篇 |
力学 | 1305篇 |
综合类 | 279篇 |
数学 | 2378篇 |
物理学 | 8418篇 |
出版年
2024年 | 20篇 |
2023年 | 354篇 |
2022年 | 501篇 |
2021年 | 698篇 |
2020年 | 946篇 |
2019年 | 958篇 |
2018年 | 822篇 |
2017年 | 852篇 |
2016年 | 1154篇 |
2015年 | 1167篇 |
2014年 | 1356篇 |
2013年 | 1708篇 |
2012年 | 2065篇 |
2011年 | 2026篇 |
2010年 | 1526篇 |
2009年 | 1537篇 |
2008年 | 1606篇 |
2007年 | 1356篇 |
2006年 | 1118篇 |
2005年 | 980篇 |
2004年 | 801篇 |
2003年 | 658篇 |
2002年 | 687篇 |
2001年 | 570篇 |
2000年 | 401篇 |
1999年 | 357篇 |
1998年 | 232篇 |
1997年 | 188篇 |
1996年 | 183篇 |
1995年 | 151篇 |
1994年 | 140篇 |
1993年 | 102篇 |
1992年 | 90篇 |
1991年 | 90篇 |
1990年 | 77篇 |
1989年 | 54篇 |
1988年 | 38篇 |
1987年 | 24篇 |
1986年 | 25篇 |
1985年 | 34篇 |
1984年 | 23篇 |
1983年 | 22篇 |
1982年 | 14篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1974年 | 4篇 |
1972年 | 2篇 |
1966年 | 2篇 |
1957年 | 2篇 |
1936年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 941 毫秒
971.
Xiang Ying Shi Jue Lin Ziyang Li Tao Jin-Wang Huang Liang-Nian Ji 《Liquid crystals》2013,40(2):211-214
We observed that the planar aligned nematic liquid crystal (5CB) doped with a volume fraction of 1% of FeTPPCl [5,10,15,20-tetraphenylporphyriniron(III)chloride] or MnTPPCl [5,10,15,20-tetraphenylporphyrinmanganese(III)chloride] dramatically decreased the critical magnetic field for the magnetic field induced Freedericksz transition, while 5CB doped with ZnTPP [5,10,15,20-tetraphenylporphyrinzinc(II)] revealed no such effect, when compared with pure 5CB. In the guest-host (5CB) system, FeTPPCl and MnTPPCl as guests are both strong paramagnetic materials with an interaction through coordination of the -CN group in 5CB onto the metal ion of the porphyrin. As a result, the 5CB molecules are dragged to reorientate under a static magnetic field, while ZnTPP is a diamagnetic material without such a property. This phenomenon concerning magneto-optical components could be useful in liquid crystal displays. 相似文献
972.
Enzymatic reactions are very basic processes in biological systems, and parameters related to enzymatic reactions always provide good indicators for understanding of mechanisms underlined in enzymatic reactions, for better controlling of enzymatic reactions, and for comparison of different enzymes. In this mini-review: first, parameters in enzymatic reactions were briefly reviewed from three different standpoints; second, predictions of parameters in enzymatic reactions without information on enzyme structure were shortly reviewed from viewpoints of geometric approach, graphic approach and compartmental approach; third, predictions of parameters in enzymatic reaction with information on enzyme structure were reviewed from the points of view of modeling, with 19 currently available databases, and 17 software packages and web servers; fourth, the current state of prediction on parameters in enzymatic reaction in biofuel industry with respect to cellulolytic enzymes were reviewed; fifth, the pros and cons for future development were discussed; and finally, a worked example was given in the Appendix to describe the whole procedures of prediction of enzymatic parameters in reactions. 相似文献
973.
Jia-Ming Liu Fei Gao Tian-Long Yang Jian-Hua Lai Zhi-Ming Li 《International journal of environmental analytical chemistry》2013,93(9):613-624
A new catalytic solid substrate-room temperature phosphorimetry (SS-RTP) for the determination of trace arsenic(V) has been established. It is based on the fact that fullerenol (F-ol) emitted strong and stable room temperature phosphorescence (RTP) on nitric acid cellulose membrane (NCM) substrate. H2O2 could oxidise F-ol to cause the quenching of RTP. As(V) could catalyse H2O2 to oxidise F-ol and decrease the RTP signal of F-ol sharply. After adding tween-80 in the system, its ΔI p enhanced 7.7 times compared with the without-tween-80 levels. Under the optimum conditions, the linear dynamic range of this method was 0.016?11.2 ag spot?1 with a detection limit (LD) of 9.3 zg spot?1 (corresponding concentration: 2.3 × 10?17 g mL?1). This sensitive, simple and selective method has been successfully applied to the determination of trace As(V) in human hair and tea samples. The reaction mechanism for SS-RTP is also discussed. 相似文献
974.
Songfei Chen Fei Gao Qibao Wang Zhiguo Su Guanghui Ma 《Colloid and polymer science》2013,291(1):117-126
Two different polymers, poly(vinylbenzyl chloride) and poly(methyl methecrylate-2-hydroxyethyl methacrylate), were respectively employed as the major component of the particle material and amphiphilic polymer methoxy poly (ethylene glycol)-b-poly (D, L-lactide) was employed as an essential additive. Gigaporous microspheres which possess flow-through pores which range from submicron to micron scale were prepared by two-step emulsification, emulsion ripening, and solvent extraction. Parameters on the final morphology were particularly investigated, including the polymer component, emulsion ripening time, and osmotic gradient. The formation mechanism of gigapores was also discussed with consideration of evolution behavior of the double emulsion system. The evolution process was proved crucial to control the final pore structure. Such kind of gigaporous microspheres could be further exploited as a promising chromatographic media for the purification of bioproducts. 相似文献
975.
Cong-Shan Zhou Ping Xu Ke-Wen Tang Xin-Yu Jiang Tao Yang Pan-Liang Zhang 《Chemical Papers》2013,67(2):155-163
Enantioselective extraction of hydrophilic 2-chloromandelic acid (CMA) enantiomers from organic to aqueous phase with hydroxypropyl-??-cyclodextrin (HP-??-CD) as the selector was investigated. Equilibrium of the extraction system was modeled using a reactive extraction model with a homogeneous aqueous phase reaction. The influence of important process variables on the extraction efficiency, such as the type of the organic solvent and ??-cyclodextrin derivatives (??-CDs), concentration of the selector, pH and temperature, was investigated by experiment and modeling. Important parameters of this model were determined experimentally. Results showed that the experimental data agree with the model prediction perfectly and the model was further applied to accurately predict the extraction efficiency influenced simultaneously by pH and the concentration of HP-??-CD. Combining the experiment and the model data, the best extraction conditions were: pH of 2.5, HP-??-CD concentration of 0.05 mol L?1, and temperature of 5°C, providing the enantioselectivity of 1.285 and the performance factor (pf) of 0.011. 相似文献
976.
977.
Yuhui Kou Derong Cao Hongqi Tao Lingyun Wang Jianquan Liang Zhizhao Chen Herbert Meier 《Journal of inclusion phenomena and macrocyclic chemistry》2013,77(1-4):279-289
A detailed study of the reaction conditions revealed that a quantitative cyclocondensation of 1,4-dialkoxy-2,5-bis(alkoxymethyl)-benzenes to pillar[n]arenes can be achieved by catalysis of p-toluenesulfonic acid in CH2Cl2. Major product of this new reaction is in each case a cyclopentamer (n = 5), but small amounts of the pillar[n]arenes with n = 6, 7 and 10 can be obtained as well. Different alkoxy groups in 1- and 4-position lead to regioisomers. All cyclooligomers exist in pillar structures as pair of enantiomers, which show a racemisation at room temperature, which is fast in terms of the NMR time scale. The racemisation process occurs by rotation of the 1,4-phenylene segments in the macrocyclic rings. Pillar[n]arenes exhibit novel host–guest behavior. 相似文献
978.
Kongkai Zhu Junyan Lu Zhongjie Liang Xiangqian Kong Fei Ye Lu Jin Heji Geng Yong Chen Mingyue Zheng Hualiang Jiang Jun-Qian Li Cheng Luo 《Journal of computer-aided molecular design》2013,27(3):247-256
New Delhi metallo-β-lactamase-1 (NDM-1) has emerged as a major global threat to human health for its rapid rate of dissemination and ability to make pathogenic microbes resistant to almost all known β-lactam antibiotics. In addition, effective NDM-1 inhibitors have not been identified to date. In spite of the plethora of structural and kinetic data available, the accurate molecular characteristics of and details on the enzymatic reaction of NDM-1 hydrolyzing β-lactam antibiotics remain incompletely understood. In this study, a combined computational approach including molecular docking, molecular dynamics simulations and quantum mechanics/molecular mechanics calculations was performed to characterize the catalytic mechanism of meropenem catalyzed by NDM-1. The quantum mechanics/molecular mechanics results indicate that the ionized D124 is beneficial to the cleavage of the C–N bond within the β-lactam ring. Meanwhile, it is energetically favorable to form an intermediate if no water molecule coordinates to Zn2. Moreover, according to the molecular dynamics results, the conserved residue K211 plays a pivotal role in substrate binding and catalysis, which is quite consistent with previous mutagenesis data. Our study provides detailed insights into the catalytic mechanism of NDM-1 hydrolyzing meropenem β-lactam antibiotics and offers clues for the discovery of new antibiotics against NDM-1 positive strains in clinical studies. 相似文献
979.
Jian‐Nan Xiang Li‐Hui Jiang Chao‐Yue Chen Zhi‐ying Fu Jun‐Fei Duan Xiao‐Xiao He 《Journal of carbohydrate chemistry》2013,32(7):595-614
New retinoyl sugar derivatives of 13‐cis‐retinoic acid were synthesized in three ways in this paper in order to enhance pharmacal effects, especially antiproliferative activities of 13‐cis‐retinoic acid. Their structures were confirmed by IR, 1H‐NMR, 13C‐NMR, and MS spectra and their antiproliferative activities were determined in vitro using human cancer lines. Results showed that some compounds possessed potential antitumor activities. 相似文献
980.
Hui Cao Fei Xu Dai-Xi Li Xiao-Gang Zhang Jin-Song Yu 《Research on Chemical Intermediates》2013,39(6):2321-2337
Molecular imprinting is a technology that facilitates the production of artificial receptors toward compounds of interest. In this study, we prepared a series of molecularly imprinted polymers (MIPs) by precipitation polymerization for the purpose of binding specifically to malachite green (MG). The presence of monomer–template solution complexes in non-covalent MIPs systems had been verified by UV-spectrometric detection and molecular dynamics simulations. The synthesized parameters were, respectively, optimized and the optimal conditions for the efficient adsorption property were as follows: template: MG, 1 mmol; functional monomer: methacrylic acid (MAA), 8 mmol; cross-linker: ethylene glycol dimethacryllate, 16 mmol; and porogen: acetonitrile, 30 mL. Fourier transform infrared spectroscopy and nitrogen adsorption experiments were used to characterize the MIPs. Scatchard analysis was used for estimation of the dissociation constants and maximum amounts of binding sites. The polymer based on MAA had two classes of heterogeneous binding sites characterized by two values of K D and B max: K D = 14.10 μmol L?1 and B max = 1.37 μmol g?1 for the higher affinity binding sites, and K D = 384.62 μmol L?1 and B max = 24.77 μmol g?1 for the lower affinity binding sites. The specificity of MIPs on SPE column was evaluated by rebinding the other structurally similar compounds. The results indicated that the imprinted polymers exhibited an excellent stereo-selectivity toward MG. 相似文献